Syntheses of Ruthenium Complexes of Borane-Functionalized 4,5-Diazafluorenide and their Reactivity
toward Carbon Dioxide
by
Adam Nicola Pantaleo
A thesis submitted in conformity with the requirements for the degree of Master of Science
Department of Chemistry University of Toronto
© Copyright by Adam Nicola Pantaleo 2014
ii
Syntheses of Ruthenium Complexes of Borane-Functionalized
4,5-Diazafluorenide and their Reactivity toward Carbon Dioxide
Adam Nicola Pantaleo
Master of Science
Department of Chemistry
University of Toronto
2014
Abstract
Carbon dioxide is an abundant molecule that is present in the earth’s atmosphere and is a
product of fossil fuel combustion. Given the implication of CO2 in global warming and its
potential use as a cheap C1 synthetic feedstock, research into systems that can efficiently
convert CO2 into value-added substrates such as methanol is currently of high interest and
relevance. In fact, many transition metal complexes are known to effect this conversion in the
presence of reducing and oxophilic reagents such as boranes. Herein, the syntheses of several
ruthenium(II) complexes featuring a borane-derivatized 4,5-diazafluorenide ligand are
presented, and their reactivity with boranes, dihydrogen, carbon dioxide, and aromatic solvents
is discussed. In several cases, reactivity occurs exclusively at the actor diazafluorenide ligand as
opposed to the spectator ruthenium centre. Furthermore, these complexes are able to catalyze
the reduction of carbon dioxide by catecholborane to B-methoxycatecholborane, a direct
precursor to methanol.
iii
Acknowledgments
First and foremost, I would like to thank my supervisor, Prof. Datong Song, for his guidance
and support during my time at the University of Toronto. Datong is a brilliant chemist, and my
work is a reflection of his ideas, inspiration, and knowledge. He was always willing to lend a
hand and an ear to my concerns, both in and out of the lab. In particular, I am thankful for his
suggestions and advice as I explored my career options, and I am grateful for his support as I
pursued several teaching and co-curricular opportunities throughout my degree.
I would also like to thank all past and present members of the Song research group whom I had
the privilege of working with over the last year: Shaolong Gong, Tongen Wang; Charlie Kivi,
Trevor Janes, Yanxin Yang, Vince Annibale, Runyu Tan, Yu Li; Rhys Batcup, Celia Gendron-
Herndon, Tara Cho; Andy Yen, Walter Liang, Ellen Yan, Brian Tsui, Maotong (Albert) Xu,
Xhoana Gjergji, Stefan Jevtic, Pavel Zatsepin, and Cindy Ma. In particular, I would like to
sincerely thank my three undergraduate students – Ellen, Brian, and Albert – whom I had the
privilege of mentoring as they completed summer projects in our lab. They may not know this,
but teaching and interacting with them brought me immense joy and was a highlight of my
graduate experience. I am also grateful for the opportunity to serve as a teaching assistant for the
CHM139 course. It is these experiences and these students that have confirmed my love of
teaching and inspired me to pursue a teaching career.
I would not be here today without the tremendous support I received from my family and
friends. I am also indebted to the Toronto Newman Centre and the wonderful people I met there.
They provided me with countless opportunities to strengthen my faith, serve the Church, and
grow closer to God during my journey through graduate school.
“Wait for the Lord; be strong, and let your heart take courage; wait for the Lord!” – Psalm 27.14
iv
Table of Contents
Acknowledgments .................................................................................................................... iii
Table of Contents ..................................................................................................................... iv
List of Figures .......................................................................................................................... vi
List of Schemes ....................................................................................................................... vii
List of Abbreviations................................................................................................................ xi
1 Introduction ..................................................................................................................... 1
1.1 Carbon Dioxide in Nature, Society, and Academia ........................................................ 1
1.2 Carbon Dioxide Reduction by Boranes and Silanes Catalyzed by Homogeneous
Complexes ............................................................................................................................ 2
1.2.1 Silanes as the Reducing Agent ............................................................................... 2
1.2.2 Boranes as the Reducing Agent .............................................................................. 5
1.2.2.1 Metal-Free Systems ........................................................................................ 9
1.3 Actor Ligands and their Carbon Dioxide Chemistry ......................................................12
1.3.1 The Carbon Dioxide Chemistry of 4,5-Diazafluorenide .........................................13
1.4 Research Goals and Scope of this Thesis ......................................................................16
2 Experimental Section .....................................................................................................18
2.1 General Considerations .................................................................................................18
2.2 In Situ Synthesis of [RuHCl(LH–Bcat)(PPh3)2] (2) .......................................................19
2.3 Synthesis of [RuH(L–Bcat)(N2)(PPh3)2] (4) ..................................................................19
2.3.1 Method A: ClBcat as Borane Source .....................................................................19
2.3.2 Method B: HBcat as Borane Source ......................................................................20
2.4 In Situ Synthesis of catB–N(SiMe3)2 (6) .......................................................................21
2.5 Synthesis of cis,trans-[RuHCl(LH)(PPh3)2] (7) .............................................................21
2.6 Synthesis of [RuH(LH)(N2)(PPh3)2][catBcat] (8) ..........................................................21
2.7 In Situ Synthesis of [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (9) ............................22
2.8 Synthesis of [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10) ............................................................22
2.8.1 Method A: ClBcat as Borane Source .....................................................................22
2.8.2 Method B: HBcat as Borane Source ......................................................................22
2.9 Synthesis of cis,cis,trans-[RuCl2(LH–Bcat)(PPh3)2] (11) ..............................................23
2.10 In Situ Synthesis of [Ru(H2Bcat)(L)(PPh3)2] (12) ..........................................................23
v
2.11 Synthesis of 9-trimethylsilyl-4,5-diazafluorene (13) ......................................................24
3 Results and Discussions ..................................................................................................25
3.1 Reactivity of 4,5-Diazafluorenide Complexes with Boranes and Silanes .......................25
3.1.1 Reactivity of [RuH(L)(N2)(PPh3)2] (1) with Boranes .............................................25
3.1.1.1 Preliminary Reactivity with ClBcat ...............................................................25
3.1.1.2 Synthesis of Zwitterionic Borane Adduct [RuH(L–Bcat)(N2)(PPh3)2] (4) ......29
3.1.1.2.1 ClBcat as Borane Source ...........................................................................29
3.1.1.2.1.1 In Situ Deprotonation by NaH..............................................................29
3.1.1.2.1.2 In Situ Deprotonation by KN(SiMe3)2 ..................................................30
3.1.1.2.1.3 Stepwise Reactions ..............................................................................33
3.1.1.2.1.4 In Situ Deprotonation by Excess 1 .......................................................33
3.1.1.2.2 HBcat as Borane Source ............................................................................36
3.1.1.3 Reactivity of [RuH(L–Bcat)(N2)(PPh3)2] (4) with Aromatic Solvents ............39
3.1.1.4 Syntheses of Borane-Borohydride Complex [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10)
and Borane Adduct [RuCl2(L–Bcat)(PPh3)2] (11)..........................................................44
3.1.1.5 Reactivity of [RuH(L–Bcat)(N2)(PPh3)2] (4) and [Ru(H2Bcat)(L–Bcat)(PPh3)2]
(10) with Dihydrogen ...................................................................................................47
3.1.2 Reactivity of NaL, MeL, and 1 with Me3SiCl ........................................................49
3.1.2.1 Reaction of NaL with Me3SiCl ......................................................................50
3.1.2.2 Reaction of MeL with Me3SiCl .....................................................................51
3.1.2.3 Reaction of 1 with Me3SiCl ...........................................................................51
3.2 Reactivity of 4,5-Diazafluorenide Complexes with CO2 ...............................................52
3.2.1 Stoichiometric Reaction with CO2.........................................................................52
3.2.2 Catalytic Reaction with CO2 .................................................................................54
4 Conclusion and Future Outlook.....................................................................................56
References ...............................................................................................................................58
vi
List of Figures
Figure 1: Partial 1H NMR spectrum (400 MHz, C6D6) of [RuHCl(LH–Bcat)(PPh3)2] (2),
recorded in situ after 1 and ClBcat were allowed to react for 15 minutes.
Figure 2: Partial 1H NMR spectrum (600 MHz, C6D6) of [RuH(L–Bcat)(N2)(PPh3)2] (4).
Figure 3: Close-up of the 1H NMR resonances centered at 8.58 ppm demonstrating the
conversion of 4 to 9 upon standing in benzene-d6 at room temperature: a) after 1 h in solution, b)
after 12 h in solution.
Figure 4: Partial 2H NMR spectrum (92 MHz, Et2O) of a mixture of [RuH(L–Bcat)(N2)(PPh3)2]
(4) and [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (9).
vii
List of Schemes
Scheme 1: Hydrosilation of carbon dioxide to formic acid catalyzed by a copper(I) phosphine
complex.
Scheme 2: Hydrosilation of carbon dioxide to a bis(methoxy)silane derivative catalyzed by an
NHC-carboxylate adduct.
Scheme 3: Generic hydrosilation of a formylsilane to a methoxysilane and a bis(silyl) ether via a
bis(silyl) acetal intermediate. Depending on the system, these reactions can occur with or
without the aid of a catalyst.
Scheme 4: Hydrosilation of carbon dioxide to methane catalyzed by several ion pairs and
transition metal complexes.
Scheme 5: Hydroboration of carbon dioxide to formic acid catalyzed by a copper(I)-NHC
complex. The mechanism involves CO2 insertion into a copper-hydride bond followed by
hydroboration of the formyl intermediate.
Scheme 6: Hydroboration of carbon dioxide catalyzed by a ruthenium(II) polyhydride complex.
The mechanism involves three distinct steps, with formylpinacolborane and formaldehyde as
intermediates.
Scheme 7: Hydroboration of carbon dioxide catalyzed by a nickel(II) PCP pincer complex.
Scheme 8: Hydroboration of carbon dioxide catalyzed by a ruthenium(II) tris(amino)phosphine
frustrated Lewis pair.
Scheme 9: Hydroboration of carbon dioxide catalyzed by a phosphine-borane organocatalyst.
The mechanism involves initial FLP activation of carbon dioxide followed by successive
hydroborations of the activated substrate while it remains coordinated to the catalyst.
Scheme 10: Hydroboration of carbon dioxide catalyzed by the nitrogenous bases TBD and
Me-TBD. TBD directly activates carbon dioxide, while Me-TBD indirectly activates carbon
dioxide via hydroborane activation.
viii
Scheme 11: Generic PNP pincer complex demonstrating concomitant re-aromatization and E–H
bond cleavage (E = H, C, O, N).
Scheme 12: Reversible carbon dioxide activation by a ruthenium(II) PNP pincer complex.
P = PtBu2.
Scheme 13: Carbon dioxide activation by a doubly deprotonated nickel(II) PNP pincer complex.
P = PiPr2.
Scheme 14: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of the
ruthenium(II) diazafluorenide complex 1.
Scheme 15: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of an
electron-rich metal-diazafluorenide complex, followed by in situ deprotonation of the carboxylic
acid and rearrangement to form a dinuclear species with a bridging carboxylate ligand. [M] =
Rh(PPh3)2, Cu(IPr).
Scheme 16: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of N-
methyl-4,5-diazafluorenide.
Scheme 17: Proposed derivatization of the 4,5-diazafluorenide ligand with a boryl moiety,
insertion of carbon dioxide into the C–B bond, and displacement of formylborane.
Scheme 18: Labelling convention for catecholato and 4,5-diazafluorenyl NMR resonances.
Scheme 19: Proposed reactivity between a nucleophilic ruthenium 4,5-diazafluorenide complex
and an electrophilic chloroborane. The 9-position of the ligand is labelled.
Scheme 20: Reaction of 1 (10 mg scale) with ClBcat in benzene-d6.
Scheme 21: Reaction of 1 (50 mg scale) with ClBcat in toluene. [Ru] denotes a ruthenium(II)
species with a protonated LH ligand.
Scheme 22: Proposed series of reactions that occur when 1 and five equivalents of NaH are
combined in THF.
ix
Scheme 23: Reaction of 1 with ClBcat in the presence of KN(SiMe3)2, where 1 and KN(SiMe3)2
were added dropwise to ClBcat.
Scheme 24: Control experiment confirming the formation of the B–N adduct 6 from ClBcat and
KN(SiMe3)2.
Scheme 25: Reaction of 1 with ClBcat in the presence of KN(SiMe3)2, where ClBcat was added
dropwise to complex 1 and KN(SiMe3)2. [Ru] denotes a ruthenium(II) species with a protonated
LH ligand.
Scheme 26: Reaction of ClBcat and 1, followed by reaction of the intermediate product mixture
with KN(SiMe3)2.
Scheme 27: Synthesis of 4 via reaction of two equivalents of 1 with one equivalent of ClBcat.
Scheme 28: Synthesis of 1 via reaction of 7 with KOtBu, supporting the assignment of 7 as an
isomer of [RuHCl(LH)(PPh3)2].
Scheme 29: Proposed reactivity between a nucleophilic ruthenium 4,5-diazafluorenide complex
and an electrophilic borane.
Scheme 30: Synthesis of 4 via reaction of 1 (10 mg scale) with HBcat.
Scheme 31: Reaction of 1 (50 mg scale) with HBcat in toluene.
Scheme 32: Conversion of 4 to 9 by reaction with benzene-d6.
Scheme 33: Proposed reaction of 4 with benzene-d6 via σ-bond metathesis of the aromatic C–D
bond with the Ru–H bond to form a ruthenium phenyl complex.
Scheme 34: Proposed ortho-cyclometalation reaction that leads to the deuteration of the o-PPh3
protons of complex 4. R = H or C6D5, Ar = C6D5.
Scheme 35: Formation of the borane-borohydride complex 10 and protonated borane complex
11 from reaction of 1 with two equivalents of ClBcat. [Ru] denotes a ruthenium(II) species with
a protonated LH ligand.
x
Scheme 36: Proposed formation of 10 from reaction of 4 with HBcat.
Scheme 37: Proposed formation of 10 and 11 from reaction of 4 with excess ClBcat.
Scheme 38: Reaction of 4 and 10 with dihydrogen in benzene-d6.
Scheme 39: Summary of proposed and confirmed reactions of 4, 9, 10, and 12 under a
dihydrogen atmosphere in benzene-d6. Solid arrows indicate reactions that have been confirmed
to take place in situ, while dashed arrows indicate proposed reactions.
Scheme 40: Reaction of NaL with Me3SiCl in THF, followed by aqueous workup.
Scheme 41: Reaction of MeL with Me3SiCl in toluene.
Scheme 42: Reaction of 1 with Me3SiCl in benzene-d6.
Scheme 43: Desired reactivity of 4 with carbon dioxide to form a catecholboryl ester moiety at
the 9-position of the diazafluorenide ligand.
Scheme 44: Catalytic reduction of carbon dioxide by 1 in the presence of HBcat to form
H3COBcat and catBOBcat.
xi
List of Abbreviations
LH 4,5-diazafluorene
L or L- 4,5-diazafluorenide
MeL N-methyl-4,5-diazafluorenide
NMR nuclear magnetic resonance
IR infrared
FT Fourier transform
ppm parts per million
atm atmospheres
COSY correlation spectroscopy
HSQC heteronuclear single quantum coherence
HMBC heteronuclear multiple bond correlation
FLP frustrated Lewis pair
NHC N-heterocyclic carbene
THF tetrahydrofuran
tol toluene
DMF N,N-dimethylformamide
Me methyl
iPr iso-propyl
nBu n-butyl
xii
tBu tert-butyl
Bn benzyl
Cy cyclohexyl
dH2O distilled water
IMes 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene
IPr 1,3-bis(2,6-di-iso-propylphenyl)imidazol-2-ylidene
nacnac 1,3-diketimine
Cp cyclopentadienyl
Cp* pentamethylcyclopentadienyl
DPPP 1,3-bis(diphenylphosphino)propane
TMP tetramethylpiperidine
TBD 1,5,7-triazabicyclo[4.4.0]dec-5-ene
Me-TBD 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene
cat catecholato, (C6H4O2)2-
catB catecholboryl
[catBcat] or [catBcat]- bis(catecholato)borate
ClBcat B-chlorocatecholborane
HBcat catecholborane
HBpin pinacolborane
9-BBN 9-borabicyclo[3.3.1]nonane
xiii
LH–Bcat 9-catecholboryl-4,5-diazafluorene
L–Bcat 9-catecholboryl-4,5-diazafluorenide
1 [RuH(L)(N2)(PPh3)2]
2 [RuHCl(LH–Bcat)(PPh3)2]
3 [RuH2(LH)(PPh3)2]
4 [RuH(L–Bcat)(N2)(PPh3)2]
5 cis,cis-[RuHCl(LH)(PPh3)2]
6 catB–N(SiMe3)2
7 cis,trans-[RuHCl(LH)(PPh3)2]
8 [RuH(LH)(N2)(PPh3)2][catBcat]
9 [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2]
10 [Ru(H2Bcat)(L–Bcat)(PPh3)2]
11 cis,cis,trans-[RuCl2(LH–Bcat)(PPh3)2]
12 [Ru(H2Bcat)(L)(PPh3)2]
13 9-trimethylsilyl-4,5-diazafluorene
1
1 Introduction
1.1 Carbon Dioxide in Nature, Society, and Academia
Carbon dioxide is a non-toxic, abundant gas that exists in the earth’s atmosphere and plays an
important role in both nature and society. In nature, carbon dioxide is essential to the proper
functioning of almost every ecosystem and organism. Plants consume carbon dioxide and
convert it to useful forms of energy using photosynthesis, while many organisms release carbon
dioxide as a by-product of cellular respiration. In society, carbon dioxide is a product of the
combustion of carbon-containing fuels such as wood, coal, natural gas, and gasoline – fuels that
are the foundation of the current energy infrastructure of the modern world.
Although these carbon-containing fuels have played an integral role in the development and
growth of civilization, their widespread use has come with a price: atmospheric levels of carbon
dioxide have been increasing steadily over the last few centuries. This increase is problematic
because carbon dioxide is a greenhouse gas and is a direct cause of climate change, one of the
most serious environmental concerns of today.1
In light of this reality, a significant body of research has emerged over the last few decades on
the fundamental and applied chemistry of carbon dioxide. In particular, the fields of carbon
dioxide sequestration and derivatization are currently receiving much attention in both academia
and industry. The former field, sequestration, relates to the development of materials that can
selectively absorb carbon dioxide and thereby “sequester” it from the atmosphere. There are
many examples in the literature of materials that are able to sequester carbon dioxide. These
materials include ionic liquids,2 zeolites,3 and metal-organic frameworks.4 The latter field,
derivatization, deals with the goal of directly using atmospheric carbon dioxide as a C1 feedstock
for chemical syntheses. Exploring the field of carbon dioxide derivatization may also lead to the
discovery of novel methods for the synthesis and fabrication of many industrially and
economically relevant chemicals.
Perhaps the biggest obstacle to using carbon dioxide as a chemical reagent is its inertness.
Carbon dioxide contains two very strong C=O double bonds and is both kinetically and
thermodynamically stable, as indicated by its presence in the earth’s atmosphere and its
formation in highly exergonic combustion reactions. In order to effectively use carbon dioxide as
2
a C1 feedstock, it must be activated and derivatized in such a way that the overall reaction is
spontaneous. This often requires the use of forcing conditions, reactive reagents such as epoxides
and acetylenes, or complex organic molecules that are expensive to synthesize. Despite this
obstacle, there are several examples in the literature of synthetic methodologies that are able to
convert carbon dioxide into value-added products such as carbonates, carbamates, ureas,
carboxylic acids, esters, isocyanates, and various polymers.5
In addition to these transformations, the literature contains many examples of homogeneous
transition metal complexes that are able to stoichiometrically or catalytically reduce carbon
dioxide to formyl, acetal, or methoxy derivatives. A number of phosphine and bipyridine metal
complexes, as well as complexes bearing macrocyclic ligands, are known to catalyze the
electrochemical reduction of carbon dioxide to carbon monoxide or formate derivatives in
solution.6 Additionally, a number of transition metal complexes can catalyze the reduction of
carbon dioxide in the presence of a stoichiometric amount of reductant such as a borane or
silane. As this area of research is of particular relevance to this thesis, it is discussed further in
the following sections.
1.2 Carbon Dioxide Reduction by Boranes and Silanes Catalyzed by Homogeneous Complexes
One of the earliest examples of using a stoichiometric amount of reductant to reduce carbon
dioxide was the discovery that simple metal complexes such as [RuCl2(PPh3)3] and
[RuH2(PPh3)4] could facilitate the reduction of carbon dioxide by silanes R3SiH to the
corresponding formylsilanes R3SiOC(O)H.7 Although these systems were inefficient, producing
only a few equivalents of reduced species, their discovery set the stage for further research into
these types of reactions.
1.2.1 Silanes as the Reducing Agent
Several years after this discovery, other systems were found that could effect carbon dioxide
reduction in much higher yields than the ruthenium(II) systems described above. For example,
the ruthenium complexes mer-[RuX3(MeCN)3] and [RuX2(MeCN)4] (X = Cl, Br) are active pre-
catalysts for the reduction of carbon dioxide by silanes and can produce formylsilanes in near
quantitative yields.8 A similar system used [RuCl(MeCN)5][RuCl4(MeCN)2], formed in situ from
RuCl3·nH2O and MeCN, as the catalyst.9 Another system that can reduce carbon dioxide to the
3
formyl level utilized a copper hydride complex bearing a bidentate phosphine ligand.10 The
active copper hydride species was formed in situ from Cu(OAc)2 and the phosphine ligand in the
presence of silane. The formylsilane product could then be hydrolyzed to formic acid and silyl
alcohol after the reaction was complete (Scheme 1). A key step in this catalytic cycle is insertion
of carbon dioxide into the Cu–H bond of the active catalyst, followed by hydrosilation of the
resulting Cu–O bond to form the formylsilane and regenerate the catalyst.
Scheme 1: Hydrosilation of carbon dioxide to formic acid catalyzed by a copper(I)
phosphine complex.10
It is also possible to further reduce carbon dioxide with silanes to methoxysilanes. One of the
first reported catalysts for this transformation was the iridium(I) complex [Ir(CN)(CO)(dppe)],
which facilitated the reduction of carbon dioxide by Me3SiH to the methoxysilane Me3SiOMe.11
Following this initial finding, it was discovered that N-heterocyclic carbenes (NHCs) could also
catalyze this reaction. In particular, the CO2 adduct of the carbene IMes {1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene} catalyzed the reduction of carbon dioxide by diphenylsilane
to bis(methoxy)diphenylsilane with turnover numbers as high as 1840 and catalyst loadings as
low as 0.05 mol% (Scheme 2).12
Scheme 2: Hydrosilation of carbon dioxide to a bis(methoxy)silane derivative catalyzed by
an NHC-carboxylate adduct.12
4
In these types of systems, the partially reduced formylsilane intermediate can re-enter the
catalytic cycle or undergo hydrosilation of the C=O bond without the aid of the catalyst to form a
bis(silyl) acetal species (R3SiO)2CH2. This species, in turn, can be further reduced to the
methoxysilane R3SiOMe and the corresponding bis(silyl) ether (R3Si)2O (Scheme 3).
Scheme 3: Generic hydrosilation of a formylsilane to a methoxysilane and a bis(silyl) ether
via a bis(silyl) acetal intermediate. Depending on the system, these reactions can occur with
or without the aid of a catalyst.
Furthermore, many complexes are known to catalyze the hydrosilation of carbon dioxide to
methane, the most reduced form of carbon. Some noteworthy examples of pre-catalysts for this
reaction include [TMPH]+[HB(C6F5)3]-, the frustrated Lewis pair (FLP) formed from reaction of
tetramethylpiperidine (TMP), B(C6F5)3, and H213; the Lewis acidic aluminum cation (Et2Al)+,
isolated as the salt [Et2Al][CH6B11I6]14; the scandium contact ion pair [Cp*2Sc][HB(C6F5)3]
(Cp* = pentamethylcyclopentadienyl)15; several palladium(II) and platinum(II) hydridoborate
complexes of a PSiP pincer ligand16; a cationic iridium(III) PCP pincer complex17; and a
zirconium complex bearing a bis(phenoxide) ligand in the presence of B(C6F5)3 (Scheme 4).18
5
Scheme 4: Hydrosilation of carbon dioxide to methane catalyzed by several ion pairs and
transition metal complexes.13-18
1.2.2 Boranes as the Reducing Agent
In addition to hydrosilation as a carbon dioxide reduction methodology, numerous examples of
borane-mediated reduction of carbon dioxide are known. These reactions are generally analogous
to the silane systems: initial catalyst-mediated conversion of carbon dioxide to a formylborane
can be followed by successive hydroboration steps to form bis(boryl) acetal and methoxyborane
species.
Nozaki and co-workers reported the copper(I)-NHC pre-catalyst [Cu(OtBu)(IPr)] {IPr = 1,3-
bis(2,6-di-iso-propylphenyl)imidazol-2-ylidene}, which facilitated the reduction of carbon
dioxide by pinacolborane (HBpin) to formic acid.19 The active catalyst is a copper hydride
species: insertion of CO2 into the Cu–H bond followed by hydroboration of the copper formyl
intermediate liberates formylpinacolborane and regenerates the catalyst (Scheme 5). This is
analogous to the mechanisms of several of the silane-based systems described in the previous
section. The formylborane can then be hydrolyzed to formic acid after the reaction is complete.
6
Scheme 5: Hydroboration of carbon dioxide to formic acid catalyzed by a copper(I)-NHC
complex. The mechanism involves CO2 insertion into a copper-hydride bond followed by
hydroboration of the formyl intermediate.19
Other systems are able to further reduce the formylborane to bis(boryl) acetal and
methoxyborane species. Sabo-Etienne and co-workers were able to observe all three of these
reduced species when a ruthenium(II) polyhydride complex was used as the catalyst with HBpin
as the reductant.20 Interestingly, they also observed the direct reductive coupling of two CO2
units to form pinBOCH2OC(O)H. A subsequent study provided mechanistic details for this
reaction and elucidated the role of formaldehyde as a reactive intermediate that could be isolated
under certain conditions (Scheme 6).21 The first step of the mechanism is the formation of
formylpinacolborane, analogous to the systems discussed previously, followed by hydroboration
of formylpinacolborane to form a bis(boryl) acetal. This intermediate can then release
formaldehyde, which undergoes rapid hydroboration to form methoxypinacolborane, the fully
reduced product. In this system, all three reduction steps are mediated by the catalyst.
7
Scheme 6: Hydroboration of carbon dioxide catalyzed by a ruthenium(II) polyhydride
complex. The mechanism involves three distinct steps, with formylpinacolborane and
formaldehyde as intermediates.21
8
In many systems, only the fully reduced methoxyborane species is observed. Guan and co-
workers reported a nickel(II) PNP pincer catalyst that was active for the reduction of carbon
dioxide by HBcat to MeOBcat (Scheme 7).22 Unlike the ruthenium system described above,
hydroboration of the formylcatecholborane intermediate to formaldehyde and then to
methoxycatecholborane did not require the catalyst. Several derivatives of this species with
varying R groups on the pincer ligand are also active catalysts.23, 24
Scheme 7: Hydroboration of carbon dioxide catalyzed by a nickel(II) PCP pincer
complex.22
Note that for all of the borane systems described above, a key step is insertion of CO2 into a
metal-hydride bond. An alternate method of activating carbon dioxide involves FLPs, which can
polarize CO2 and make it amenable to hydroboration. Using this method, Stephan and co-
workers reported a ruthenium(II) tris(amino)phosphine complex that catalyzed carbon dioxide
reduction by HBpin to MeOBpin (Scheme 8).25 Three successive hydroborations of the CO2
substrate occurred while it remained bound to the FLP at the Lewis acidic ruthenium centre and
the Lewis basic phosphine donor.
9
Scheme 8: Hydroboration of carbon dioxide catalyzed by a ruthenium(II)
tris(amino)phosphine frustrated Lewis pair.25
1.2.2.1 Metal-Free Systems
In addition to the ruthenium and nickel catalysts described above, several metal-free catalysts
have recently been reported. Fontaine and co-workers discovered an extremely active
organocatalyst, 1-Bcat-2-PPh2-C6H4, that can catalyze the hydroboration of carbon dioxide to
methoxyboranes with turnover numbers reaching 2950.26 This organocatalyst activates carbon
dioxide via an FLP mechanism involving the Lewis acidic borane and Lewis basic phosphine,
with successive hydroborations occurring while the CO2-derived substrate is bound to the
catalyst (Scheme 9). Another metal-free system involving a phosphine-derived carbene-9-BBN
(9-BBN = 9-borabicyclo[3.3.1]nonane) complex was recently reported by Stephan and co-
workers.27
10
Scheme 9: Hydroboration of carbon dioxide catalyzed by a phosphine-borane
organocatalyst. The mechanism involves initial FLP activation of carbon dioxide followed
by successive hydroborations of the activated substrate while it remains coordinated to the
catalyst.26
Remarkably, Cantat and co-workers reported that simple nitrogenous bases, absent of phosphine
or borane groups, were able to catalyze carbon dioxide reduction by 9-BBN to methoxy-9-BBN
11
(Scheme 10).28 The two most active catalysts, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-
methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (Me-TBD), differed in their mechanisms of carbon
dioxide activation. The former activated carbon dioxide via an FLP mechanism involving a
Lewis basic nitrogen donor and the Lewis acidic borane; the latter activated the borane and
facilitated its hydridic attack on carbon dioxide (Scheme 10). In both cases, the formyl-9-BBN
species re-entered the catalytic cycle and was ultimately reduced to the methoxy level. A similar
complex, the bidentate nitrogenous base 1,8-bis(dimethylamino)naphthalene (also known as
Proton Sponge), is also an active catalyst for the reduction of carbon dioxide by BH3·SMe2 to
(MeOBO)3.29
Scheme 10: Hydroboration of carbon dioxide catalyzed by the nitrogenous bases TBD and
Me-TBD. TBD directly activates carbon dioxide, while Me-TBD indirectly activates carbon
dioxide via hydroborane activation.28
12
Furthermore, several complexes are known to catalyze the reduction of carbon dioxide to carbon
monoxide using a variety of reductants, including in situ carbodiphosphoranes,30 silane-boranes
(R2B–SiR3),31 aromatic aldehydes,32 and diboranes (R2B–BR2).
33
1.3 Actor Ligands and their Carbon Dioxide Chemistry
A careful scrutiny of the metal-based catalysts described in the previous sections reveals that in
some cases, most notably the FLP systems, the reduction and functionalization of carbon dioxide
directly involves a ligand in addition to the metal centre. These ligands, which directly
participate in the chemical reaction, are known as actor ligands. This behaviour is atypical, as
ligands in transition metal chemistry are generally unreactive and serve only to tune the steric
and electronic properties about the metal centre where the reaction takes place.34 Complexes
containing actor ligands exhibit diverse and exciting reactivity toward small molecules such as
dihydrogen, dioxygen, carbon dioxide, silanes, alkenes, and alkynes. Of particular relevance to
this thesis are those systems which are able to activate and reduce carbon dioxide.
A significant class of actor ligand complexes is the PNP and PNN pincer complexes synthesized
by Milstein and co-workers. These pincer ligands contain a benzylic methylene group that can be
deprotonated to form a dearomatized version of the ligand, which can then heterolytically cleave
a variety of E–H bonds (E = H, C, O, N) (Scheme 11). The driving force for these reactions is re-
aromatization of the ligand. Iron(II), ruthenium(II), and nickel(II) complexes of these ligands are
known. In the case of dihydrogen, the reversibility of the aromatization-dearomatization process
makes these complexes efficient hydrogenation catalysts. Indeed, these complexes are reported
to catalyze the hydrogenation of bicarbonates and carbon dioxide;35 ureas;36 and carbonates,
carbamates, and alkyl formates under mild conditions.37
Scheme 11: Generic PNP pincer complex demonstrating concomitant re-aromatization and
E–H bond cleavage (E = H, C, O, N).
13
These pincer complexes are also able to activate carbon dioxide. Indeed, carbon dioxide has been
shown to undergo [1,3] addition to a ruthenium(II) PNP pincer complex, forming new C–C and
Ru–O bonds (Scheme 12).38 Analogous reactivity was observed for a related ruthenium(II) PNN
pincer complex synthesized by Sanford and co-workers.39 Furthermore, Milstein and co-workers
reported a doubly-deprotonated nickel(II) PNP pincer complex that reacted with carbon dioxide
to give a tautomerized C–H insertion product (Scheme 13).40
Scheme 12: Reversible carbon dioxide activation by a ruthenium(II) PNP pincer complex.
P = PtBu2.38
Scheme 13: Carbon dioxide activation by a doubly deprotonated nickel(II) PNP pincer
complex. P = PiPr2.40
Other notable complexes that are able to activate carbon dioxide include titanium-phosphorus
and zirconium-phosphorus FLPs,41 a hafnium-phosphorus FLP,42 and a scandium nacnac (1,3-
diketimine) complex.43 Together, these systems demonstrate the versatility of actor ligands in
carbon dioxide activation chemistry.
1.3.1 The Carbon Dioxide Chemistry of 4,5-Diazafluorenide
Over the last several years, our group has extensively studied the chemistry of 4,5-
diazafluorenide, a versatile actor ligand that displays exciting reactivity toward small molecules.
In 2010, we first reported the synthesis of the ruthenium(II) dinitrogen complex
[RuH(L)(N2)(PPh3)2] (1) (L = 4,5-diazafluorenide) and described its reaction with dihydrogen.44
14
Two years later, we reported the formal insertion of carbon dioxide into the 9-position C–H bond
of this complex to form a carboxylic acid (Scheme 14).45 The first step in this reaction is
believed to be C–H insertion to form a carboxylate moiety, which then deprotonates the 9-
position proton to form the carboxylic acid product. Remarkably, this reversible reaction occurs
exclusively at the diazafluorenide ligand and does not directly involve the ruthenium centre.
Subsequently, the isoelectronic rhodium(III) complex [RhH2(L)(PPh3)2] was synthesized and
shown to react with carbon dioxide in the same way.46 Interestingly, the rhodium(I) and
copper(I) complexes [Rh(L)(PPh3)2] and [Cu(L)(IPr)] behaved differently and formed dinuclear
species with bridging carboxylate ligands upon reaction with carbon dioxide.46 This difference in
reactivity was attributed to the more electron-rich metal centres in these complexes, which
rendered the diazafluorenide ligand more basic and allowed for deprotonation of the intermediate
carboxylic acid species (Scheme 15).
Scheme 14: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of the
ruthenium(II) diazafluorenide complex 1.45
15
Scheme 15: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of an
electron-rich metal-diazafluorenide complex, followed by in situ deprotonation of the
carboxylic acid and rearrangement to form a dinuclear species with a bridging carboxylate
ligand. [M] = Rh(PPh3)2, Cu(IPr).46
Together, these results demonstrate that the metal centre is purely a spectator in these reactions,
serving only to tune the electronic properties of the complex. Remarkably, when the metal was
replaced by a formal positive charge introduced by N-methylation of the diazafluorenide ligand,
the resulting metal-free complex displayed the same reactivity toward carbon dioxide as the
metal-containing complexes (Scheme 16).46
Scheme 16: Formal insertion of carbon dioxide into the 9-position ligand C–H bond of N-
methyl-4,5-diazafluorenide.46
16
1.4 Research Goals and Scope of this Thesis
Given these recent discoveries involving diazafluorenide-mediated carbon dioxide activation, our
group was interested in expanding this exciting chemistry. Aware of the prevalence of borane
reagents in many carbon dioxide reduction systems, we aimed to derivatize the diazafluorenide
ligand by appending a boryl moiety at the 9-position. Then, if carbon dioxide could insert into
the newly formed C–B bond, the product would be a value-added formylborane species.
Additionally, as formyl species are key intermediates in many catalytic carbon dioxide reduction
systems, we were excited to see if these derivatized complexes displayed catalytic activity in the
presence of excess borane (Scheme 17). If successful, this would be a rare example of carbon
dioxide reduction occurring at a carbon centre, as opposed to a nitrogen centre,28 an FLP, or a
metal centre. Furthermore, the presence of ruthenium may enhance the catalytic activity, as
metal-based catalysts are generally more active than their metal-free counterparts.
Scheme 17: Proposed derivatization of the 4,5-diazafluorenide ligand with a boryl moiety,
insertion of carbon dioxide into the C–B bond, and displacement of formylborane.
The previously reported ruthenium(II) dinitrogen complex 1 was used as a starting point for this
chemistry. 1 can be synthesized in two steps from [RuHCl(PPh3)3] and 4,5-diazafluorene (LH) in
good yields and very high purity,44 making it a known and reliable starting material.
17
Furthermore, the properties and reactivity of any new borane complexes derived from 1 can be
directly compared to those of 1, making it easy to study the effects of introducing a boryl moiety.
This thesis reports the syntheses of several ruthenium(II) complexes featuring borane-derivatized
4,5-diazafluorenide ligands and their reactivity toward boranes, dihydrogen, carbon dioxide, and
aromatic solvents. Also briefly reported are attempts to synthesize analogous silane-derivatized
diazafluorenide complexes. The discovery of a catalytic system that reduces carbon dioxide to a
methoxyborane species is also discussed. As this work was mainly an exploratory study, many of
the results are preliminary, but they nevertheless reveal the rich chemistry of these systems and
set the stage for further studies on these and related complexes.
18
2 Experimental Section
2.1 General Considerations
Unless otherwise specified, all operations were performed using Schlenk/vacuum line techniques
under a dinitrogen atmosphere or in a dinitrogen atmosphere glovebox from MBraun. Unless
otherwise stated, all chemicals were purchased from commercial sources and used without
further purification. NaH (60% dispersion in mineral oil) was washed with hexanes to remove
the oil, and the reagent was used in its pure form. KN(SiMe3)2 solution (0.5 M in toluene) was
evaporated to dryness in vacuo, and the reagent was used in its solid form with the composition
{KN(SiMe3)2} (C7H8)0.36. Me3SiCl was distilled under dinitrogen prior to use. NaL was prepared
from LH and NaH according to a literature procedure.47 Complex 1 was prepared according to a
literature procedure.44 MeL was prepared according to a literature procedure.46 All glassware was
dried overnight in a 180 °C oven or flame-dried prior to use, except for NMR tubes and J-Young
NMR tubes, which were dried overnight in a 60 °C oven. Celite was dried under vacuum at
180 °C overnight. Molecular sieves were activated by heating at 300 °C under vacuum for three
days. Dihydrogen (grade 5.0) and carbon dioxide (grade 4.0) were purchased from Linde
Canada. Tetrahydrofuran and benzene-d6 were dried over Na/benzophenone, distilled under
dinitrogen, and stored over activated molecular sieves. Toluene, pentane, hexanes, and diethyl
ether were sparged with dinitrogen, passed through a Pure Solv Innovative Technology Grubbs-
type solvent purification system, degassed through one freeze-pump-thaw cycle, and stored over
activated molecular sieves. Chloroform-d was degassed through three freeze-pump-thaw cycles,
dried over activated molecular sieves, and stored over a fresh batch of activated molecular
sieves.
IR spectra were collected on a Perkin-Elmer Spectrum One FT-IR spectrometer. 1H, 2H, 11B, 13C,
and 31P NMR spectra were recorded on Varian Mercury 300 MHz, Varian Mercury 400 MHz,
Bruker Avance III 400 MHz, or Agilent DD2 600 MHz NMR spectrometers. 1H and 13C
chemical shifts are reported in ppm relative to the residual proteo solvent peaks. 11B and 31P
NMR spectra were referenced externally using 15 v/v% BF3·Et2O in CDCl3 and 85% aqueous
H3PO4, respectively. 2H spectra were not referenced. Where possible, peaks were assigned by
comparison to known complexes as well as 1H-1H COSY, 1H-13C HSQC, and 1H-13C HMBC
correlation experiments. The labels o-L, o-L’, m-L, m-L’, p-L, and p-L’ refer to locations on a
19
4,5-diazafluorenyl moiety; the labels o-cat and m-cat refer to locations on a catecholato moiety
(Scheme 18). The labels o-PPh3, m-PPh3, and p-PPh3 refer to the ortho, meta, and para positions
of the phenyl rings in a triphenylphosphine ligand, respectively. The label 4° L refers to
quaternary 4,5-diazafluorenyl carbon atoms. The labels ipso-PPh3 and ipso-cat refer to ipso
carbon atoms (relative to the phosphorus atom) on a triphenylphosphine ligand and ipso carbon
atoms (relative to the oxygen atoms) on a catecholboryl ligand, respectively. Coupling constants
marked with an asterisk (*) deviate slightly from their expected values because of overlap with
minor impurity peaks.
Scheme 18: Labelling convention for catecholato and 4,5-diazafluorenyl NMR resonances.
2.2 In Situ Synthesis of [RuHCl(LH–Bcat)(PPh3)2] (2)
To a colourless solution of ClBcat (1.9 mg, 0.012 mmol, 1.0 equiv.) in 0.5 mL benzene-d6 was
added dropwise a dark purple solution of 1 (10 mg, 0.012 mmol, 1.0 equiv.) in 0.5 mL
benzene-d6. A rapid colour change to red-orange was observed during addition. The solution was
transferred to a J-Young tube, and NMR spectra were recorded 15 minutes after addition.
1H NMR (C6D6, 400 MHz, 25 °C): δ 8.21 (d, 3JH–H = 7.6 Hz, 1H), 8.17 (d, 3JH–H = 7.6 Hz, 1H),
7.50 (d, 3JH–H = 5.2 Hz, 1H), 7.45-7.40 (m), 7.26-6.67 (m), 6.42 (dd, 3JH–H = 7.6 Hz, 3JH–H =
5.2 Hz, 1H), 5.85 (dd, 3JH–H = 7.6 Hz, 3JH–H = 5.2 Hz, 1H), 4.36 (s, 1H), -12.89 (dd, 2JP–H =
19.2 Hz, 1H). 31P{1H} NMR (C6D6, 162 MHz, 25 °C): δ 47.36 (d, 2JP–P = 5.2 Hz), 46.71 (d,
2JP–P = 5.2 Hz).
2.3 Synthesis of [RuH(L–Bcat)(N2)(PPh3)2] (4)
2.3.1 Method A: ClBcat as Borane Source
To a dark purple solution/suspension of 1 (50 mg, 0.061 mmol, 2.0 equiv.) in 2 mL toluene was
added dropwise with stirring a colourless solution of ClBcat (4.7 mg, 0.031 mmol, 1.0 equiv.) in
8 mL toluene. A colour change from purple to red was observed during addition, along with the
formation of a precipitate. The reaction was stirred for four hours, after which the tan-coloured
20
precipitate (complex 7) was removed by filtration and the red supernatant was evaporated to
dryness in vacuo, leaving 32 mg of dark red solids. Crystallization was performed by allowing
pentane to slowly diffuse into a solution of the crude mixture in a 2:1 toluene-THF solution at
–35 °C; a purified sample remained in the supernatant, which was decanted from the precipitated
solids, filtered through Celite, evaporated to dryness in vacuo, triturated with hexanes, and once
again evaporated to dryness in vacuo to give light red solids. During NMR acquisition, slow
conversion to complex 9 was observed; thus, the spectra contained some unidentified peaks that
likely belong to this compound. 1H NMR (C6D6, 600 MHz, 25 °C): δ 8.58 (dd, 3JH–H = 7.8 Hz,
4JH–H = 0.6 Hz, 1H, o-L), 8.39 (dd, 3JH–H = 7.8 Hz, 4JH–H = 0.6 Hz, 1H, o-L’), 8.02 (d, 3JH–H =
4.8* Hz, 1H, p-L), 7.36-7.32 (m, 12H, o-PPh3), 7.30 (dd, 3JH–H = 5.4 Hz, 4JH–H = 3.0 Hz, 2H,
o-cat), 6.99 (dd, 3JH–H = 4.8 Hz, 4JH–H = 0.6 Hz, 1H, p-L’), 6.93 (dd, 3JH–H = 8.4* Hz, 3JH–H =
4.8 Hz, 1H, m-L), 6.90 (dd, 3JH–H = 6.0* Hz, 4JH–H = 3.0 Hz, 2H, m-cat), 6.88-6.82 (m, 18H,
m-PPh3 + p-PPh3), 6.23 (dd, 3JH–H = 7.8 Hz, 3JH–H = 4.8 Hz, 1H, m-L’), -12.60 (t, 2JP–H = 19.2 Hz,
1H, Ru–H). 31P{1H} NMR (C6D6, 243 MHz, 25 °C): δ 48.36 (s). 11B NMR (C6D6, 192 MHz,
25 °C): δ 15.54 (br). 13C NMR (C6D6, 150 MHz, 25 °C): δ 150.37 (ipso-cat), 146.66 (4° L),
145.87 (4° L), 140.48 (o-L’), 137.90 (o-L), 135.17 (4° L), 134.86 (4° L), 133.55 (o-PPh3),
129.53 (PPh3), 128.02 (PPh3), 128.01 (p-L), 127.93 (4° L), 127.28 (p-L’), 121.72 (m-cat), 119.87
(m-L’), 119.52 (m-L), 111.86 (o-cat). The ipso-PPh3 resonance lies somewhere within a cluster
of unidentified low-intensity peaks (133.1-132.6). IR(Nujol): Ru–N2) 2109 (s) cm-1.
2.3.2 Method B: HBcat as Borane Source
To a colourless solution of HBcat (1.5 mg, 0.012 mmol, 1.0 equiv.) in 0.5 mL toluene was added
dropwise a dark purple solution/suspension of 1 (10 mg, 0.012 mmol, 1.0 equiv.) in 1 mL
toluene. A colour change from purple to red was observed during addition. The reaction was
allowed to stand for one hour and was then filtered through Celite. The red supernatant was then
evaporated to dryness in vacuo. 7 mg of dark red solids were obtained, which were purified by
dissolution in a minimal amount of toluene followed by addition of pentane, isolation of the
precipitated solids by filtration, and drying the solids in vacuo. 1H and 31P{1H} NMR data agreed
with those obtained using Method A and demonstrated the formation of 4 in >91% purity.
21
2.4 In Situ Synthesis of catB–N(SiMe3)2 (6)
ClBcat (2.0 mg, 0.013 mmol) and {KN(SiMe3)2} (toluene)0.36 {3.0 mg, 0.013 mmol
KN(SiMe3)2} were dissolved in 0.75 mL benzene-d6. A precipitate immediately formed. The
solution was transferred to an NMR tube, and spectra were recorded shortly afterward.
Approximately 35% unreacted ClBcat remained, likely a result of error in the mass
measurement. Several minor impurities were also present (18% by integration). 1H NMR (C6D6,
400 MHz, 25 °C): δ 6.96 (dd, 3JH–H = 6 Hz, 4JH–H = 2 Hz, 2H, cat), 6.76 (dd, 3JH–H = 6 Hz, 4JH–H =
2 Hz, 2H, cat), 0.32 {s, 18H, –N(SiMe3)2}. 11B NMR (C6D6, 128 MHz, 25 °C): δ 26.90 (br).
2.5 Synthesis of cis,trans-[RuHCl(LH)(PPh3)2] (7)
To a dark purple solution/suspension of 1 (50 mg, 0.061 mmol, 2.0 equiv.) in 2 mL toluene was
added dropwise with stirring a colourless solution of ClBcat (4.7 mg, 0.031 mmol, 1.0 equiv.) in
8 mL toluene. A colour change from purple to red was observed during addition, along with the
formation of a precipitate. The reaction was stirred for four hours, after which the tan-coloured
precipitate was isolated by filtration, washed with 1 mL toluene, and dried in vacuo. (The
supernatant contained complex 4.) 24 mg of solids were isolated. 1H NMR (CDCl3, 400 MHz,
25 °C): δ 8.54 (d, 3JH–H = 4 Hz, 1H, o-L), 8.06 (d, 3JH–H = 8 Hz, 1H, p-L), 7.95 (d, 3JH–H = 8 Hz,
1H, o-L’), 7.40 (dd, 3JH–H = 8 Hz, 3JH–H = 4 Hz, 1H, m-L), 7.32-7.18 (m, 31H, PPh3 + p-L’), 6.69
(dd, 3JH–H = 8 Hz, 3JH–H = 4 Hz, 1H, m-L’), 4.05 (s, 2H, LH), -13.35 (t, 2JP–H = 20 Hz, 1H,
Ru–H). 31P{1H} NMR (C6D6, 162 MHz, 25 °C): δ 46.89 (s).
2.6 Synthesis of [RuH(LH)(N2)(PPh3)2][catBcat] (8)
To a colourless solution of HBcat (7.3 mg, 0.061 mmol, 1.0 equiv.) in 2.5 mL toluene was added
dropwise with stirring a dark purple solution/suspension of 1 (50 mg, 0.061 mmol, 1.0 equiv.) in
5 mL toluene. A colour change from purple to red was observed during addition, along with the
formation of a precipitate. The reaction was stirred for two hours, after which the precipitate was
isolated by filtration and dried in vacuo. 23 mg of tan-coloured solids were obtained. (The
supernatant contained complex 4.) 1H NMR (CDCl3, 300 MHz, 25 °C): δ 8.51 (d, 3JH–H = 6 Hz,
1H, o-L), 7.72 (d, 3JH–H = 6 Hz, 1H, p-L), 7.60 (d, 3JH–H = 6 Hz, 1H, o-L’), 7.31-7.15 (m, 32H,
m-L + p-L’ + PPh3), 6.67-6.64 (m, 4H, cat), 6.60-6.55 (m, 5H, cat + m-L’), 3.68 (s, 2H, LH),
22
-13.36 (t, 2JP–H = 21 Hz, 1H, Ru–H). 31P{1H} NMR (CDCl3, 122 MHz, 25 °C): δ 46.94 (s).
11B NMR (CDCl3, 96 MHz, 25 °C): δ 14.49 (s).
2.7 In Situ Synthesis of [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (9)
During the characterization of complex 4 in benzene-d6, approximately 50% conversion to 9
occurred after 12 hours at room temperature. 1H NMR (C6D6, 600 MHz, 25 °C): identical to 4
(within 0.01 ppm), with the following resonances being absent: 7.36-7.32 (o-PPh3), -12.60
(Ru–H). 31P{1H} NMR (C6D6, 243 MHz, 25 °C): δ 47.86 (s). 2H NMR (Et2O, 92 MHz, 25 °C):
δ 7.25 (s), 7.09 (br).
2.8 Synthesis of [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10)
2.8.1 Method A: ClBcat as Borane Source
To a colourless solution of ClBcat (38 mg, 0.24 mmol, 2.0 equiv.) in 1 mL toluene was added
dropwise with stirring a dark purple solution/suspension of 1 (100 mg, 0.12 mmol, 1.0 equiv.) in
9 mL toluene. The dark-coloured suspension was stirred overnight at room temperature. The
following day, the suspension was filtered, giving a deep red supernatant and light brown solids
(complex 12). The supernatant was evaporated to dryness in vacuo, leaving dark red solids. A
yield for the reaction could not be obtained as aliquots were periodically removed throughout the
reaction to monitor its progress. The crude solids were purified by crystallization: pentane was
allowed to diffuse into a toluene-THF solution of the crude product. Impurities precipitated out
of solution, leaving relatively pure 10 in the supernatant. NMR data are reported below.
2.8.2 Method B: HBcat as Borane Source
To a colourless solution of HBcat (9.3 mg, 0.078 mmol, 2.0 equiv. relative to 1) in 0.5 mL
toluene was added dropwise with stirring a dark purple solution/suspension of a mixture of 1 and
4 (104 mg, 67% 1 + 33% 4, 0.038 mmol 1, 1.0 equiv. 1) in 8 mL toluene. A colour change from
purple to red was observed during addition. The reaction was stirred for one hour and was then
filtered through Celite to remove a small amount of tan-coloured solids. The red supernatant was
evaporated to dryness in vacuo, and after trituration with (2 x 0.5 mL) hexanes, 93 mg of light
red solids were obtained. NMR showed the presence of 80% complex 4 and 20% complex 10.
1H NMR (C6D6, 400 MHz, 25 °C): δ 8.31 (d, 3JH–H = 8 Hz, 2H, o-L), 8.23 (d, 3JH–H = 8 Hz, 2H,
23
p-L), 7.36-7.31 (m, 14H, o-PPh3 + o-cat), 6.91-6.89 (m, 2H, m-cat), 6.87-6.79 (m, 18H, m-PPh3
+ p-PPh3), 6.69 (dd, 3JH–H = 8 Hz, 3JH–H = 8 Hz, 2H, m-L), 6.53 (dd, 3JH–H = 8 Hz, 4JH–H = 4 Hz,
2H, RuH2Bcat), 6.38 (dd, 3JH–H = 8 Hz, 4JH–H = 4 Hz, 2H, RuH2Bcat), -11.50 (br, 2H, RuH2Bcat).
31P{1H} NMR (C6D6, 162 MHz, 25 °C): δ 49.24 (s). 11B NMR (C6D6, 128 MHz, 25 °C): δ 22.43
(br).
2.9 Synthesis of cis,cis,trans-[RuCl2(LH–Bcat)(PPh3)2] (11)
To a colourless solution of ClBcat (38 mg, 0.24 mmol, 2.0 equiv.) in 1 mL toluene was added
dropwise with stirring a dark purple solution/suspension of 1 (100 mg, 0.12 mmol, 1.0 equiv.) in
9 mL toluene. The dark-coloured suspension was stirred overnight at room temperature. The
following day, the suspension was filtered, giving a deep red supernatant (contained complex 10)
and light brown solids. The solids were washed several times with toluene, then dried in vacuo.
48 mg of solids were isolated. The crude solids were purified by crystallization: pentane was
allowed to diffuse into a THF solution of the crude product. Relatively pure 11 precipitated from
solution. 1H NMR (CDCl3, 300 MHz, 25 °C): δ 7.82 (d, 3JH–H = 6 Hz, 2H), 7.60-7.46 (m),
7.25-6.91 (m), 6.56 (dd, 3JH–H = 6 Hz, 3JH–H = 6 Hz, 2H), 4.50 (s, 1H). 31P{1H} NMR (CDCl3,
122 MHz, 25 °C): δ 24.25 (s). 11B NMR (CDCl3, 96 MHz, 25 °C): δ 22.40 (br).
2.10 In Situ Synthesis of [Ru(H2Bcat)(L)(PPh3)2] (12)
Approximately 5 mg of a mixture of 80% complex 4 and 20% complex 10 in 0.25 mL benzene-
d6 was placed in a 3 mm thick-walled J-Young tube. The tube was freeze-pump-thaw degassed
three times, then backfilled with dihydrogen at –196 °C (4 atm H2) and warmed to room
temperature. The tube was then heated at 40 °C for 18 hours. NMR indicated the presence of
37% complex 10 and 63% complex 12. Partial deuteration of the o-PPh3 protons of 12 was
observed. 1H NMR (C6D6, 600 MHz, 25 °C): δ 8.48 (d, 3JH–H = 6.0 Hz, 2H), 7.79 (d, 3JH–H =
6.0 Hz, 2H), 7.27-7.24 (m, < 12H, o-PPh3), 6.85-6.77 (m, 18H), 6.53 (dd, 3JH–H = 5.4* Hz,
3JH–H = 3.6* Hz, 2H), -9.94 (br, 2H, RuH2Bcat). The singlet for the 9-position L proton could not
be found and may be overlapped by other resonances. 31P{1H} NMR (C6D6, 243 MHz, 25 °C):
δ 51.45 (s).
24
2.11 Synthesis of 9-trimethylsilyl-4,5-diazafluorene (13)
To a colourless solution of Me3SiCl (1.1 mL, 8.9 mmol, 15 equiv.) in 3 mL THF in a Schlenk
bomb was added dropwise with stirring via cannula a dark pink solution of NaL (0.59 mmol,
1.0 equiv.) in 15 mL THF. An immediate colour change to royal blue occurred. The bomb was
sealed and the reaction was stirred overnight at room temperature. The following day, the solvent
and excess Me3SiCl were removed by evaporation in vacuo. In the glovebox, the residues were
re-dissolved in THF and filtered. Evaporation of the supernatant in vacuo gave 124 mg of a
mixture of beige solids and magenta solids. These solids were then exposed to air and subject to
a standard organic workup. The solids were dissolved in 5 mL CH2Cl2 and washed with
(2 x 2 mL) dH2O. The aqueous layers were combined and extracted with (2 x 1 mL) CH2Cl2. The
combined organic layers were dried over anhydrous MgSO4, filtered, and evaporated to dryness
using a Rotary evaporator. 76 mg of beige solids were recovered, which consisted of 65% LH
and 35% 13 according to integration of the 1H spectrum. 1H NMR (CDCl3, 400 MHz, 25 °C):
δ 8.71 (m, 2H), 7.81 (d, 2H), 7.27 (d, 2H), 3.77 (s, 1H), 0.04 (s, 9H).
25
3 Results and Discussions
3.1 Reactivity of 4,5-Diazafluorenide Complexes with Boranes and Silanes
3.1.1 Reactivity of [RuH(L)(N2)(PPh3)2] (1) with Boranes
As a first step in expanding the carbon dioxide chemistry of the 4,5-diazafluorenide (L-) ligand,
attempts were made to append a borane moiety at the 9-position of the ligand in the known
complex [RuH(L)(N2)(PPh3)2] (1). Since the L- ligand in this complex is known to be
nucleophilic, it was hypothesized that reaction with an electrophilic haloborane would lead to the
desired borane adduct via an SN2 mechanism. Subsequent deprotonation of this adduct would
then form a complex analogous to 1, where the 9-position proton has been replaced by a borane
(Scheme 19). Since boranes are regularly used as reductants and oxygen acceptors in carbon
dioxide reduction chemistry, and since the parent complex 1 is known to react with carbon
dioxide, this chemistry is both promising and relevant to the field of carbon dioxide activation.
Scheme 19: Proposed reactivity between a nucleophilic ruthenium 4,5-diazafluorenide
complex and an electrophilic chloroborane. The 9-position of the ligand is labelled.
3.1.1.1 Preliminary Reactivity with ClBcat
Dropwise addition of a dark purple solution of complex 1 (10 mg) in benzene-d6 to a colourless
solution of ClBcat (one equivalent) in benzene-d6 (Scheme 20) resulted in a rapid colour change
to red-orange. NMR spectra recorded approximately 15 minutes after mixing indicated the
complete consumption of 1 and ClBcat and the formation of a new product (90%) along with at
least one minor product. Notably, the singlet at 6.44 ppm arising from the 9-position aromatic
proton of 1 disappeared, and a new singlet at 4.36 ppm that integrated to one proton relative to
the o-L proton appeared (Figure 1). This shift suggests that the central five-membered ring of the
ligand is no longer aromatic, yet the 9-position proton is more deshielded than the protons of free
26
LH. For comparison, the 9-position protons of free 4,5-diazafluorene (LH) give rise to a singlet
at 2.98 ppm.48 There is also an overlapped doublet of doublets at -12.89 ppm that integrates to
one proton and two doublets in the 31P spectrum at 47.36 and 46.71 ppm (2JP–P = 5.2 Hz),
suggesting that two inequivalent PPh3 ligands are bound to ruthenium and adopt a cis geometry.
The 11B spectrum contains several low-intensity, broad peaks that do not correspond to ClBcat.
Given this information, the new product is assigned as [RuHCl(LH–Bcat)(PPh3)2] (2). The
catecholato proton signals are likely buried under the intense resonances from the PPh3 ligands.
An alternate formulation for this product is [RuH(LH–Bcat)(N2)(PPh3)2][Cl]; however, given
that the product is soluble in benzene, the former assignment is more probable.
Scheme 20: Reaction of 1 (10 mg scale) with ClBcat in benzene-d6.
Figure 1: Partial 1H NMR spectrum (400 MHz, C6D6) of [RuHCl(LH–Bcat)(PPh3)2] (2),
recorded in situ after 1 and ClBcat were allowed to react for 15 minutes.
Ru–H
27
Continued monitoring of the reaction by NMR showed the slow decomposition of 2 into at least
four other soluble products over 24 hours along with formation of a precipitate. Interestingly,
dihydrogen was observed in the 1H spectrum after two hours. Complex 1 is known to reversibly
react with dihydrogen to give the complex [RuH2(LH)(PPh3)2] (3), where H2 has been
heterolytically split across the acidic ruthenium centre and the basic 9-position of the L- ligand.44
It is conceivable that similar reactivity could occur with the borane adduct 2, which may explain
the evolution of H2 upon decomposition.
When the same reaction was performed in THF and monitored by 31P NMR, at least two
products formed after 30 minutes. The same pattern of decomposition occurred: at least four
products were observed over the course of the reaction, along with the formation of a precipitate.
Interestingly, the reaction behaved rather differently when performed on a larger scale. A
solution of 1 (50 mg) in toluene was added dropwise to a solution of ClBcat (one equivalent) in
toluene. Aliquots were removed periodically and analyzed by 31P NMR. After one hour, 1 was
consumed and 2 had formed, as expected. However, the spectrum also contained a singlet
corresponding to a new product. Over seven hours, complex 2 was mostly converted to the new
product (Scheme 21), resulting in a deep red solution and formation of a precipitate. After
removing the precipitate by filtration and evaporating the supernatant to dryness in vacuo, dark
red solids were obtained. Based on NMR, these crude solids consisted of a new product (66%)
and several impurities (34%). When the spectra of the new product were compared to those of 2,
three important differences emerged: the 1H singlet at 4.36 ppm had vanished, the hydride
resonance was now a triplet, and only one singlet was present in the 31P spectrum. Based on
these data, the new product was assigned as the zwitterionic borane adduct
[RuH(L–Bcat)(N2)(PPh3)2] (4), where the diazafluorenide ligand has been deprotonated and the
PPh3 ligands have adopted a trans geometry.
28
Scheme 21: Reaction of 1 (50 mg scale) with ClBcat in toluene. [Ru] denotes a
ruthenium(II) species with a protonated LH ligand.
Complex 4 can conceivably be formed by deprotonation of 2, concomitant loss of the chloride
ligand, isomerization, and coordination of N2 to fill the vacant coordination site. Analogous
reactivity is observed in the synthesis of 1 from the precursor cis,cis-[RuHCl(LH)(PPh3)2] (5)
and KOtBu.44 Since no external base such as KOtBu was added to the reaction between 1 and
ClBcat, it is conceivable that complex 1 is acting as a base and deprotonating 2 to form a
ruthenium species with a protonated LH ligand. Indeed, this species may be present in the
precipitate that formed during the reaction. Further experiments, discussed in Section 3.1.1.2.1.4,
confirmed this hypothesis as well as the identity of complex 4. Unlike its protonated analogue 2,
complex 4 showed no signs of decomposition after standing for several hours in toluene.
Isolation of complex 2 was not attempted given its instability and the complexity of the reaction.
Instead, attempts were made to directly synthesize, isolate, and characterize the zwitterionic
borane adduct 4 in high purity. The following sections describe several synthetic pathways that
led to the synthesis of 4: in situ deprotonation with external base, stepwise deprotonation with
external base, in situ deprotonation with 1 as internal base, and direct one-step synthesis from
HBcat.
29
3.1.1.2 Synthesis of Zwitterionic Borane Adduct [RuH(L–Bcat)(N2)(PPh3)2] (4)
3.1.1.2.1 ClBcat as Borane Source
3.1.1.2.1.1 In Situ Deprotonation by NaH
Although complex 1 can be used as a base to deprotonate 2 in situ, this is rather inefficient as 1 is
expensive to synthesize, both in terms of time and cost. An external base may be able to
accomplish the same reaction in a much cheaper and more efficient way.
When a solution of ClBcat in benzene-d6 was added to a mixture containing 1 and NaH (one
equivalent each) in benzene-d6, an orange solution formed. The reaction behaved very similarly
to the reaction performed in the absence of base: NMR monitoring indicated that complex 2
formed initially and began to decompose after several hours with the evolution of dihydrogen.
No evidence for a deprotonated L- ligand was observed, suggesting that NaH was unable to
deprotonate 2. The same result was obtained when five equivalents of NaH were used.
The reaction was then repeated in THF with five equivalents of NaH. An orange solution formed
after addition, but after standing for several minutes, a red colour appeared and the evolution of
gas bubbles was observed. After two hours, the solution was once again purple, the colour of the
starting material 1. At this point, the reaction was evaporated to dryness in vacuo, and the crude
residues were dissolved in benzene-d6. NMR spectroscopy indicated the presence of 1 (75%) and
the hydrogen-splitting product 3 (25%). It is conceivable that the initial orange colour arose from
complex 2, which formed in situ. Then, 2 may have been deprotonated by NaH to form complex
4 along with the evolution of dihydrogen. The evolved dihydrogen may then react with complex
4 to re-form 1, which may further react to form complex 3 (Scheme 22). The reactivity of 4
toward dihydrogen is discussed in Section 3.1.1.5 and lends support to this hypothesis. The
volatile HBcat produced in this proposed reaction scheme would have been removed when the
THF solution was evaporated to dryness prior to recording the NMR spectra. Repeating this
experiment in a closed system and monitoring the intermediates via NMR will provide additional
evidence to support this proposed mechanism.
30
Scheme 22: Proposed series of reactions that occur when 1 and five equivalents of NaH are
combined in THF.
3.1.1.2.1.2 In Situ Deprotonation by KN(SiMe3)2
When KN(SiMe3)2, was used in place of NaH, drastically different results were obtained. A dark
purple solution of 1 and KN(SiMe3)2 (one equivalent each) in toluene was added dropwise to a
colourless solution of ClBcat (one equivalent) in toluene with stirring. The solution became red
after addition and was allowed to stir for three hours. A precipitate formed during the reaction,
which was removed by filtration, and the supernatant was evaporated to dryness in vacuo. NMR
analysis of the residues remaining after evaporation indicated the presence of 1, 4, and the B–N
adduct catB–N(SiMe3)2 (6) in roughly a 1:1:1 ratio (Scheme 23). Several minor impurities were
also present.
31
Scheme 23: Reaction of 1 with ClBcat in the presence of KN(SiMe3)2, where 1 and
KN(SiMe3)2 were added dropwise to ClBcat.
Adduct 6 formed from a side reaction between ClBcat and KN(SiMe3)2; this was verified by
performing a control experiment in the absence of 1 (Scheme 24). Adduct 6 has been previously
reported,49 and its literature NMR data agree well with the experimental data. This side reaction
depleted some of the ClBcat and KN(SiMe3)2, which explains the presence of unreacted 1.
Scheme 24: Control experiment confirming the formation of the B–N adduct 6 from ClBcat
and KN(SiMe3)2.
When the order of addition was reversed – a solution of ClBcat was added dropwise to a solution
of 1 and KN(SiMe3)2 – a roughly 1:1 mixture of 4 and 6 was obtained, with no unreacted 1
remaining (Scheme 25). Based on previous results (see Section 3.1.1.1), some of complex 1
likely acted as a base and deprotonated 2 as it formed in situ.
32
Scheme 25: Reaction of 1 with ClBcat in the presence of KN(SiMe3)2, where ClBcat was
added dropwise to complex 1 and KN(SiMe3)2. [Ru] denotes a ruthenium(II) species with a
protonated LH ligand.
This reaction was also performed using a 1:2:2 ratio of 1, ClBcat, and KN(SiMe3)2. It was hoped
that this stoichiometry would prevent 1 from reacting as a base. This was not the case: the
reaction behaved similarly to the 1:1:1 reaction, but produced significantly more impurities.
Efforts were made to purify these reaction mixtures, but were largely unsuccessful. Washing the
crude solids with pentane or hexanes reduced the amount of 6 present to less than 10%, but could
not completely remove it even after several washes. Attempts to remove 6 by sublimation under
vacuum (boiling point = 70 °C at 60 mTorr)49 were also unsuccessful and led to partial
decomposition of complex 4. Finally, recrystallization of the mixtures was performed using
several different solvent combinations. In most cases, unidentified impurities precipitated from
the solution, leaving 4 and 6 in the supernatant.
Most other common laboratory bases, such as methoxide, tert-butoxide, n-butyllithium, and
benzylpotassium, would likely form the adducts MeOBcat, tBuOBcat, nBuBcat, and BnBcat,
respectively, by reaction with ClBcat in a manner analogous to the formation of 6. Indeed, a
control experiment confirmed that ClBcat and KBn react to form at least two new products, one
of which is likely the B–C adduct BnBcat. Thus, these bases are not feasible reagents for the
synthesis of 4.
33
3.1.1.2.1.3 Stepwise Reactions
Since purifying reaction mixtures containing the adduct 6 proved to be problematic, efforts were
made to suppress or avoid its formation during the synthesis of 4. A stepwise reaction was
attempted, where 1 and ClBcat were allowed to react for a short period of time before addition to
a solution of KN(SiMe3)2 (Scheme 26). The first step should consume all of the ClBcat,
preventing its undesired reaction with KN(SiMe3)2. The order of addition is also important: if
complex 1 (and by analogy, complex 4) is ever in excess, it can act as a base and deprotonate the
reaction intermediates.
Scheme 26: Reaction of ClBcat and 1, followed by reaction of the intermediate product
mixture with KN(SiMe3)2.
When the two steps were separated by five minutes, the reaction did not go to completion, and
36% of complex 1 remained. When the interval was increased to 30 minutes, the same result
occurred, with 22% of 1 remaining. The B–N adduct 6 was observed in both cases, along with
several minor impurities. The time intervals between the two steps were chosen in an attempt to
find a compromise between the time required for the reaction between 1 and ClBcat to go to
completion and the known tendency of 2 to decompose in solution.
3.1.1.2.1.4 In Situ Deprotonation by Excess 1
Given the difficulties described in the preceding sections, the concept of using 1 as a base to
deprotonate 2 in situ to form 4 was revisited. A dilute solution of 1 (two equivalents) in toluene
was added dropwise to a stirred solution of ClBcat (one equivalent) in toluene (Scheme 27).
34
After stirring for four hours, a considerable amount of tan-coloured precipitate had formed,
which was isolated by filtration from the red supernatant. The precipitate was shown by NMR to
be cis,trans-[RuHCl(LH)(PPh3)2] (7), an isomer of the known complex cis,cis-
[RuHCl(LH)(PPh3)2] (5).44 Further support for this assignment was obtained by reacting 7 with
KOtBu (Scheme 28), which deprotonated the ligand as expected and formed complex 1,
analogous to the known reactivity of the cis,cis isomer 5.44
Scheme 27: Synthesis of 4 via reaction of two equivalents of 1 with one equivalent of
ClBcat.
Scheme 28: Synthesis of 1 via reaction of 7 with KOtBu, supporting the assignment of 7 as
an isomer of [RuHCl(LH)(PPh3)2].
When the red supernatant was evaporated to dryness in vacuo, dark red solids were obtained.
NMR revealed the formation of the desired borane adduct 4 along with several unidentified
impurities. In an effort to purify the crude reaction mixture, crystallization was attempted using a
variety of methods and solvent combinations. When vapour diffusion was used, two solvent
combinations – toluene/pentane and diethyl ether/pentane – were able to increase the proportion
of 4 in the supernatant by selectively precipitating out the impurities. Although most attempts did
35
not completely remove the impurities, a “pure” sample of 4 was obtained from one attempt
where pentane was slowly diffused into a 2:1 toluene-THF solution of the crude reaction
products at –35 °C. The multinuclear NMR spectra (1H, 11B, 13C, and 31P) obtained of this sample
are in excellent agreement with the assigned structure of 4, namely the borane adduct
[RuH(L–Bcat)(N2)(PPh3)2] (Figure 2). The IR spectrum of 4, recorded as a Nujol mull,
confirmed the presence of the dinitrogen ligand (N–N = 2109 cm-1). For comparison, the N–N
stretching frequency for the dinitrogen ligand in 1 is 2092 cm-1.44
Figure 2: Partial 1H NMR spectrum (600 MHz, C6D6) of [RuH(L–Bcat)(N2)(PPh3)2] (4).
Numerous attempts were made to obtain single crystals of 4 for analysis by X-ray
crystallography, but none were successful. Three-coordinate boron species are known to be
difficult to crystallize. Efforts to form a four-coordinate boron centre, which would be expected
to crystallize much more readily, by using coordinating solvents such as THF and acetonitrile for
recrystallization did not appear to work. It is worth exploring whether reaction of complex 4 with
a fluoride anion source can lead to the formation of a salt of the form
[E][RuH{L–B(F)cat}(N2)(PPh3)2] (E = non-coordinating cation) that is more amenable to
characterization by X-ray crystallography.
36
Complex 4 slowly converted to other products when heated at 40 °C for several days (see
Section 3.1.1.5). It also decomposed in THF over several days at room temperature to form
several unidentified products. It was later determined that 4 also reacts slowly with aromatic
solvents such as benzene and toluene to form a ruthenium(II) aryl complex via a proposed C–H
activation pathway. This reactivity is discussed in Section 3.1.1.3.
3.1.1.2.2 HBcat as Borane Source
In addition to the reactions with ClBcat described above, the feasibility of using HBcat as a
borane source for the synthesis of 4 was explored. Conceivably, this reaction would be analogous
to the reaction with ClBcat, but since hydride is not a good leaving group, an intermediate
containing a four-coordinate boron centre might be expected. Then, this intermediate may
eliminate dihydrogen to form the desired borane adduct (Scheme 29).
Scheme 29: Proposed reactivity between a nucleophilic ruthenium 4,5-diazafluorenide
complex and an electrophilic borane.
When a dark purple solution of 1 (10 mg) in toluene was added dropwise to a colourless solution
of HBcat (one equivalent) in toluene, a red colour emerged. 31P NMR indicated the formation of
4 in high purity after one hour (Scheme 30). Presumably, dihydrogen was evolved but could not
be detected because the reaction was performed in an open system. The red solution was filtered
through Celite to remove a very small amount of solids, and the supernatant was evaporated to
dryness in vacuo. The resulting crude residues were purified by dissolution in a minimal amount
of toluene followed by addition of pentane and isolation of the precipitated solids, which were
shown to be relatively pure 4 (> 91%) by NMR. Further crystallization at low temperature should
be able to give an analytically pure sample. At this scale, this method of synthesizing complex 4
is superior to the route involving ClBcat and a twofold excess of 1 (Section 3.1.1.2.1.4).
37
Scheme 30: Synthesis of 4 via reaction of 1 (10 mg scale) with HBcat.
Unfortunately, the reaction behaved differently at larger scales. When 50 mg of 1 was reacted
with one equivalent of HBcat, considerably more precipitate formed during the reaction. The
precipitate was isolated by filtration and dried in vacuo, and 23 mg of tan-coloured solids were
isolated (see below). The red supernatant was evaporated to dryness in vacuo, giving 37 mg of
dark red solids. NMR spectroscopy confirmed the presence of 4 in the supernatant in
approximately 73% purity. Attempts to further purify this sample by crystallization or washing
with solvent were unsuccessful.
Interestingly, the 1H spectrum of the tan-coloured solids in chloroform-d contained two
resonances at 6.65 and 6.58 ppm, attributable to a catecholato moiety that integrated to four
protons each, double the expected intensity. A singlet at 3.68 ppm that integrated to two protons
was also present, indicative of a protonated LH backbone. A single peak in the 31P spectrum
indicated that the two PPh3 ligands were equivalent. Finally, the 11B spectrum contained a fairly
sharp singlet at 14.49 ppm. Based on these data, the product is assigned as the protonated borate
salt [RuH(LH)(N2)(PPh3)2][catBcat] (8) (Scheme 31).
38
Scheme 31: Reaction of 1 (50 mg scale) with HBcat in toluene.
The [catBcat]- anion has been reported previously in the literature as part of several organic and
metal-containing compounds. For instance, the P–B adduct [(Me2PhP)2BH2][catBcat] displays an
11B singlet at 15.2 ppm and a 1H multiplet at 6.57 ppm for the [catBcat]- anion (CD2Cl2).50
Similarly, the rhodium(III) complex [RhH2(PMe3)4][catBcat] displays an 11B singlet at 15.1 ppm
(THF-d8).50 Additionally, the [catBcat]- anion was observed in situ in a D2O solution of catechol
and boric acid and displayed 1H and 11B resonances of 6.81 ppm and 14.3 ppm, respectively.51
These data are in good correlation with the resonances observed in the spectra of 8, supporting
the presence of the [catBcat]- anion.
In their work with ruthenium(II) PNP pincer complexes, Milstein and co-workers observed
partial decomposition to ionic borate complexes containing a protonated PNP ligand and the
[catBcat]- anion in several reactions involving HBcat.52 They hypothesized that this
decomposition was autocatalytic and triggered by adventitious moisture or by trace amounts of
BH3 in the HBcat. HBcat is known to slowly decompose in solution by disproportionation to
BH3 and B2(cat)3, the latter consisting of two Bcat units bridged through the boron atoms by a
catecholato anion.50 The [catBcat]- anion has also been observed to form during the reaction of
metal complexes with HBcat. Indeed, the complexes [RhH2(PMe3)4][catBcat],50
[RhH2(DPPP)2][catBcat] {DPPP = 1,3-bis(diphenylphosphino)propane},50 and
[(RO)3Ti(catBcat)] (R = 2,6-di-iso-propylphenyl)53 have been reported.
This information provides some insight to the formation of complex 8 during the synthesis of 4.
First, trace amounts of BH3 produced from disproportionation of HBcat may lead to similar
autocatalytic decomposition during the synthesis of 4, analogous to what was observed for
39
Milstein’s system. Indeed, boranes such as BH3 are known to react with similar ligands such as
N-heterocyclic carbenes to form borates or B–H activation products.54 Second, trace amounts of
moisture may hydrolyze the ligated Bcat moiety of 4 to the corresponding catecholborinic acid,
HOBcat, and regenerate complex 1. The HOBcat or additional traces of moisture may then act as
proton sources to protonate the ligand backbone of 1 to form the cation [RuH(LH)(N2)(PPh3)2]+.
Third, the [catBcat]- anion may form by the decomposition of HBcat or by its reaction with trace
moisture. Together, these events may lead to the formation of 8.
Although a suitable method to avoid this competing side reaction could not be found, the borate
salt 8 precipitates from solution during the synthesis and can easily be separated from the
supernatant, which contains complex 4. Since BH3 is known to form adducts with monodentate
ligands, it is possible that the addition of a small amount of such a ligand will sequester any BH3
that forms and suppress the undesired formation of complex 8.
3.1.1.3 Reactivity of [RuH(L–Bcat)(N2)(PPh3)2] (4) with Aromatic Solvents
In the process of running full NMR characterization of a “pure” sample of 4 in benzene-d6 (see
Section 3.1.1.2.1.4), an interesting observation was made: over several hours at room
temperature, the 1H resonances attributed to the hydride and o-PPh3 protons of 4 noticeably
decreased in intensity. After 12 hours, these peaks integrated to only half of their original
intensity, while the remainder of the spectrum appeared unchanged. Upon closer examination, it
was found that each resonance in the spectrum was actually composed of two resonances with
nearly identical chemical shifts: one for 4 and one for a new product (9) (Figure 3). The
difference in chemical shifts between these peaks was less than 0.01 ppm and thus could only be
observed on the high-field 600 MHz spectrometer. The 31P spectrum confirmed the slow
conversion of 4 into 9, as the singlet attributed to 4 (48.36 ppm) slowly decreased in intensity
while a new singlet emerged (47.86 ppm). It was later discovered that complete conversion of 4
to 9 in situ could be achieved after two days at room temperature or after heating at 40 °C
overnight.
40
Figure 3: Close-up of the 1H NMR resonances centered at 8.58 ppm demonstrating the
conversion of 4 to 9 upon standing in benzene-d6 at room temperature: a) after 1 h in
solution, b) after 12 h in solution.
Very similar reactivity was observed when 4 was allowed to stand in toluene-d8 overnight at
room temperature: the hydride and o-PPh3 resonances in the 1H spectrum decreased in intensity,
and the 31P resonance of 4 (48.51 ppm, toluene-d8) decreased in intensity as a new resonance
(48.00 ppm) appeared.
Complex 1 is known to react with benzene-d6 upon heating to give a new product (1’) where
deuterium has been incorporated into the hydride and o-PPh3 protons.55 This reactivity was
elucidated by the observation of two resonances in the 2H spectrum of 1’ attributable to the
hydride and o-PPh3 deuterons, as well as the discovery that the reverse reaction to reform 1
occurs when 1’ is heated in proteo-benzene. This reaction proceeds via reversible C–D activation
of benzene-d6 by 1, which replaces the hydride with a deuteride, along with reversible
cyclometalation of the PPh3 ligands, which deuterates their ortho protons.
41
The 2H spectrum of a sample containing roughly 50% each of 4 and 9 in proteo-Et2O contains a
singlet at 7.25 ppm and a broad singlet at 7.09 ppm, but does not contain any resonances in the
hydride region (Figure 4). One of the resonances is likely attributable to the o-PPh3 deuterons of
9, suggesting a reversible cyclometalation process is indeed occurring. However, it appears that 9
does not contain a deuteride ligand. Given the chemical shift of free benzene (7.24 ppm in
proteo-Et2O), it is possible that the second resonance arises from a deuterated phenyl ligand
coordinated to the ruthenium centre in place of the expected deuteride. Based on this
information, the structure of 9 is tentatively assigned as
[Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (Scheme 32).
Figure 4: Partial 2H NMR spectrum (92 MHz, Et2O) of a mixture of
[RuH(L–Bcat)(N2)(PPh3)2] (4) and [Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2] (9).
42
Scheme 32: Conversion of 4 to 9 by reaction with benzene-d6.
Complex 9 may form by the initial displacement of N2 by benzene-d6 followed by σ-bond
metathesis to form a ruthenium phenyl complex with a coordinated HD ligand, analogous to
what occurs for complex 1. Unlike with 1, the HD ligand is then presumably replaced with N2,
which prevents the reverse reaction from occurring (Scheme 33). The derivatized L–Bcat ligand
likely renders the ruthenium centre of 4 more electron-deficient compared to that of 1, which
may make replacement of coordinated HD with N2 favourable. This increased lability of the HD
ligand may be explained by the fact that coordinated H2 (or HD) binds more weakly to electron-
poor metal centres because of reduced π-backdonation.56 Since the reaction was performed in an
open system, there was no opportunity to spectroscopically observe the evolution of HD.
Scheme 33: Proposed reaction of 4 with benzene-d6 via σ-bond metathesis of the aromatic
C–D bond with the Ru–H bond to form a ruthenium phenyl complex.
A concomitant ortho-cyclometalation process (Scheme 34) may then lead to deuteration of the o-
PPh3 protons in a manner analogous to what occurs for complex 1.
43
Scheme 34: Proposed ortho-cyclometalation reaction that leads to the deuteration of the
o-PPh3 protons of complex 4. R = H or C6D5, Ar = C6D5.
Leitner and co-workers reported a similar reversible C–D activation process for a ruthenium(II)
dihydrogen-dihydrido PNP pincer complex.57 Initial replacement of the coordinated dihydrogen
ligand with benzene-d6 was followed by σ-bond metathesis to form a ruthenium phenyl hydrido
complex with a coordinated HD ligand. These reactions were performed under an argon
atmosphere. In the case of 4 reacting with benzene-d6 under a dinitrogen atmosphere, a similar
mechanism may be at work with the additional step of displacement of the HD ligand by N2.
Further experiments are necessary to confirm these hypotheses. Performing the reaction in a
closed system would allow for the spectroscopic identification of HD. Additionally, heating
samples of 9 in a non-aromatic solvent under a dinitrogen-dihydrogen atmosphere would probe
the reversibility of the reaction. If the identity of 9 is indeed
[Ru(C6D5)(L–Bcat)(N2){P(2,6-D2-C6H3}3)2], it would presumably revert to 4 under these
conditions.
44
3.1.1.4 Syntheses of Borane-Borohydride Complex [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10) and Borane Adduct [RuCl2(L–Bcat)(PPh3)2] (11)
This section describes the discovery of two new complexes that occurred during an attempted
synthesis of complex 2 from complex 1 and ClBcat. In this synthesis, an excess of ClBcat was
used in an effort to prevent 1 from reacting as a base. When a solution of 1 in toluene was added
dropwise to two equivalents of ClBcat in toluene, two major products were obtained (Scheme
35). The first product, which is soluble in toluene, is assigned as the borane-borohydride
complex [Ru(H2Bcat)(L–Bcat)(PPh3)2] (10), a derivative of 4 in which an additional
catecholboryl moiety is coordinated to the ruthenium centre via two bridging hydride ligands.
This is evidenced by a broad resonance in the 1H NMR spectrum (benzene-d6) at -11.48 ppm that
integrates to two protons relative to the two equivalent meta protons of the L- ligand. A second
set of catecholato resonances is also present in the 1H spectrum, further supporting this
assignment.
Scheme 35: Formation of the borane-borohydride complex 10 and protonated borane
complex 11 from reaction of 1 with two equivalents of ClBcat. [Ru] denotes a ruthenium(II)
species with a protonated LH ligand.
For comparison, an analogous borohydride complex, [(tBu-PNP)FeH(η2-BH4)] (PNP = 2,6-
bis(di-tert-butylphosphinomethyl)pyridine), has a broad 1H resonance at -8.40 ppm (benzene-d6)
for the bridging hydride ligand trans to the N-donor of the PNP ligand.35 Two other analogous
45
complexes, [Cp*Fe(LiPr)(H2Bcat) (LiPr =1,3-di-iso-propyl-4,5-dimethylimidazol-2-ylidene)58
and [RuH2(η2-HBcat)(η2-H2)(PCy3)2]
59, also exhibit broad 1H resonances attributable to the
bridging hydride ligands at -15.6 ppm (benzene-d6) and -8.48 ppm (toluene-d8), respectively. For
complex 10, the presence of an analogous broad resonance in the same general region of the 1H
spectrum reinforces its assignment as a borohydride complex.
The second product, which is insoluble in toluene and precipitates from solution, is tentatively
assigned as cis,cis,trans-[RuCl2(LH–Bcat)(PPh3)2] (11). This assignment is based on two striking
observations: the 1H spectrum lacks a hydride resonance and the 31P spectrum (chloroform-d)
contains a singlet at 24.25 ppm. For comparison, the complex cis,cis,trans-[RuCl2(L’)(PPh3)2]
(L’ = 4,5-diazafluoren-9-one) has a 31P chemical shift of 25.61 ppm (methylene chloride-d2).60
The 1H spectrum of 11 also contains a singlet at 4.50 ppm (chloroform-d) integrating to one
proton relative to the two equivalent meta protons of the diazafluorene ligand. For comparison,
the 9-position proton of complex 2, cis,cis-[RuHCl(LH–Bcat)(PPh3)2], has a chemical shift of
4.36 ppm (benzene-d6).
Interestingly, complex 10 was also observed in a separate synthetic attempt involving HBcat.
When a reaction between 1 and HBcat did not go to completion, two equivalents of additional
HBcat per equivalent of unreacted 1 were added in an effort to drive the reaction to completion.
After the reaction was complete, 4 and 10 (80% and 20%, respectively) were present. However,
no complex 11 was observed under these conditions. These results suggest that complex 10
forms from complex 4 in the presence of excess HBcat (Scheme 36).
46
Scheme 36: Proposed formation of 10 from reaction of 4 with HBcat.
In the case where ClBcat was used as the borane source, the HBcat needed to form 10 may have
arisen from reaction of the excess ClBcat with complex 2 (formed in situ from 1 and ClBcat),
which contains a hydride ligand coordinated to ruthenium. This reaction also leads to the
formation of 11. Complex 2 can also react with 1 to form 4 as described previously, and 4 can
react with the HBcat produced to form 10. These reactions are summarized in Scheme 37.
Indeed, metal-hydride complexes are known to react with haloboranes to produce boranes. For
example, reaction of the compound [K][(MeCp)MnH(CO)2] (Cp = cyclopentadienyl) with
ClBcat yielded [(MeCp)Mn(HBcat)(CO)2] and KCl.61
47
Scheme 37: Proposed formation of 10 and 11 from reaction of 4 with excess ClBcat.
3.1.1.5 Reactivity of [RuH(L–Bcat)(N2)(PPh3)2] (4) and
[Ru(H2Bcat)(L–Bcat)(PPh3)2] (10) with Dihydrogen
As described in the previous section, a mixture of 80% complex 4 and 20% complex 10 was
isolated during a synthetic attempt. In order to study the relationship between 4 and 10 as well as
the conversion of 4 to 9, this mixture was gently heated in benzene-d6 under both dinitrogen and
dihydrogen atmospheres in two separate experiments. Under a dinitrogen atmosphere, 4 fully
converted to 9 after heating overnight at 40 °C. Furthermore, a new minor product 12 appeared,
and the ratio between products changed such that the relative amount of 9 decreased and the
relative amounts of 10 and 12 increased. When this reaction was repeated under a dihydrogen
atmosphere (4 atm), complex 9 was fully depleted and 12 was the major species present after 18
hours. The 1H and 31P NMR spectra of 12 were similar to those of 10, with the notable difference
that only one catechol moiety was present in 12. Since a broad hydride resonance that integrated
to two protons was still present (-9.94 ppm in benzene-d6), complex 12 is tentatively assigned as
[Ru(H2Bcat)(L)(PPh3)2], where the 9-position borane moiety has been lost (Scheme 38).
48
Scheme 38: Reaction of 4 and 10 with dihydrogen in benzene-d6.
Since 12 forms readily under a dihydrogen atmosphere, it is hypothesized that 12 forms from 10
by addition of H2 across the 9-position C–B bond, liberating HBcat in the process. In the absence
of dihydrogen but in the presence of benzene-d6, the HD evolved in the conversion of 4 to 9 may
act as the hydrogen source for this transformation. Neither HD nor HBcat were detected during
the experiment, but this is not surprising since both species are reagents in the proposed reaction
pathways. Scheme 39 summarizes these results and the proposed reactions that may be taking
place. Further experiments involving pure samples of 4, 9, 10, and 12 are needed to confirm
these hypotheses.
49
Scheme 39: Summary of proposed and confirmed reactions of 4, 9, 10, and 12 under a
dihydrogen atmosphere in benzene-d6. Solid arrows indicate reactions that have been
confirmed to take place in situ, while dashed arrows indicate proposed reactions.
3.1.2 Reactivity of NaL, MeL, and 1 with Me3SiCl
To continue expanding the chemistry of the 4,5-diazafluorenide ligand in light of its carbon
dioxide chemistry and to complement the work with boranes described above, attempts were also
made to append a silane moiety at the 9-position of the ligand. Like boranes, silanes are
frequently used as reductants and oxygen acceptors in carbon dioxide reduction chemistry. By
following a similar methodology as described for the borane chemistry (Section 3.1.1), several L-
complexes were reacted with Me3SiCl in an effort to synthesize an analogous silane adduct.
50
3.1.2.1 Reaction of NaL with Me3SiCl
When a dark pink solution of NaL in THF, prepared by reacting LH with NaH according to a
literature procedure,47 was added dropwise with stirring to a colourless solution of excess
Me3SiCl (15 equivalents) in THF, a royal blue colour appeared immediately. The reaction was
stirred overnight at room temperature. The next day, the reaction was filtered to remove a white
precipitate (presumably NaCl), and the royal blue supernatant was evaporated to dryness in
vacuo. The 1H spectrum of the crude residues indicated the presence of at least four products in
addition to free LH, which presumably formed by protonation of the highly reactive NaL by
adventitious moisture. Several intense singlets between 0.0 and 0.4 ppm were present, suggesting
that some of the products contained –SiMe3 groups. A similar result was obtained when the
reaction was repeated using 2.7 equivalents of Me3SiCl and heated at 70 °C for three days.
It is possible that the products contained an –SiMe3 group bonded to the 9-position carbon, a
pyridyl nitrogen, or both. In an attempt to simplify the reaction mixture, the crude solids from the
room temperature reaction were exposed to water by performing an aqueous workup in air. Any
products containing C–Si bonds would be expected to survive aqueous conditions, while
products containing N(pyridyl)→Si interactions would likely be unstable. After the aqueous
workup, the only major products remaining were LH (65%) and a product tentatively assigned as
9-trimethylsilyl-4,5-diazafluorene (13) (35%) (Scheme 40). The 1H spectrum (chloroform-d) of
13 contained three aromatic resonances that integrate to two protons each, a singlet at 3.80 ppm
that integrated to one proton, and an intense singlet at 0.04 ppm that integrated to nine protons.
These resonances can be assigned to the aromatic LH protons, the 9-position proton, and the
–SiMe3 group, respectively. Another resonance was present at -0.05 ppm, but its identity remains
unclear.
Scheme 40: Reaction of NaL with Me3SiCl in THF, followed by aqueous workup.
51
Attempts to separate these two products using preparative thin layer chromatography were
unsuccessful. Additionally, after two weeks in air, the mixture of LH and 13 had converted
entirely to LH and unidentified –SiMe3-containing products, suggesting that 13 is unstable.
3.1.2.2 Reaction of MeL with Me3SiCl
To avoid possible undesired reactivity at the pyridyl nitrogen atoms of the L- ligand, the reaction
described in the previous section was attempted using the zwitterionic species N-methyl-4,5-
diazafluorenide (MeL)46 as the L- source. The N-methyl group should sterically block the pyridyl
nitrogen atoms and prevent them from reacting. When a dark blue solution of MeL in toluene
was added dropwise with stirring to a colourless solution of excess Me3SiCl (15 equivalents) in
toluene, no appreciable colour change occurred. No reaction occurred after stirring overnight at
room temperature. There was also no reaction after heating to 80 °C for three days (Scheme 41).
Scheme 41: Reaction of MeL with Me3SiCl in toluene.
3.1.2.3 Reaction of 1 with Me3SiCl
Given that 1 reacts with ClBcat to form the protonated borane adduct 2 in situ (see Section
3.1.1.1), the analogous reaction with Me3SiCl was attempted. Complex 1 and Me3SiCl were
dissolved in benzene-d6 in a 1:1 ratio in a J-Young tube. No reaction occurred after two hours at
room temperature. When the reaction was heated at 80 °C for three hours, 1H NMR indicated the
formation of a complex mixture of products that could not be identified (Scheme 42).
52
Scheme 42: Reaction of 1 with Me3SiCl in benzene-d6.
3.2 Reactivity of 4,5-Diazafluorenide Complexes with CO2
Section 3.1 described the efforts that were made to append a borane moiety at the 9-position of
the 4,5-diazafluorenide ligand in complex 1. The most important result of these studies was the
isolation of the desired complex 4. The ultimate goal of synthesizing 4 was to test its reactivity
with carbon dioxide. In particular, if 4 could catalyze the reduction of CO2 by boranes, then
value-added methoxyboranes could be produced, which are direct precursors to methanol. This
section summarizes the stoichiometric reaction of 4 with carbon dioxide and presents the
discovery of a novel catalytic system involving complex 1 and HBcat.
3.2.1 Stoichiometric Reaction with CO2
It is known that carbon dioxide readily inserts into the 9-position C–H bond of 1 to form a
carboxylic acid.45 In order to determine if complex 4 demonstrated analogous reactivity, namely
insertion of CO2 into the 9-position C–B bond to form a boryl ester (Scheme 43), a benzene-d6
solution of 4 was exposed to one atmosphere of carbon dioxide at room temperature. Shortly
after exposure, a colour change from dark red to dark yellow-brown occurred. The reaction was
monitored by NMR spectroscopy, which showed the complete consumption of 4 within 15
minutes and the formation of a complex mixture of products. After two hours, a small amount of
precipitate had formed.
53
Scheme 43: Desired reactivity of 4 with carbon dioxide to form a catecholboryl ester moiety
at the 9-position of the diazafluorenide ligand.
After standing at room temperature overnight, the composition of the mixture changed, but
several products were still present. In an effort to drive the reaction to a single species, the
reaction was heated at 45 °C for 20 hours. Afterward, almost no peaks remained in the 1H
spectrum, suggesting that a majority of the ruthenium-containing species had precipitated from
solution. Given the small scale of the reaction, isolation of the precipitated solids was not
feasible. Instead, the benzene-d6 was removed in vacuo and the remaining orange solids were
analyzed by NMR and IR spectroscopy.
The orange solids dissolved completely in bromobenzene-d5, and the 1H spectrum recorded in
this solvent showed the presence of several new species that did not correspond to 1, 4, or free
LH. The IR spectrum of the solids contained a peak at 2120 cm-1, suggesting the presence of a
coordinated dinitrogen ligand. Interestingly, the spectrum also contained a broad peak at
1578 cm-1 that was not present in the spectrum of 4. For comparison, the carbonyl stretching
frequency for the carbon dioxide adduct of complex 1, [RuH(L–CO2H)(N2)(PPh3)2], appears at
1609 cm-1.45 This result suggests that carbon dioxide insertion into the C–B bond of 4 may have
occurred.
An attempt to isolate this new species by performing the reaction on a larger scale using toluene
instead of benzene-d6 was unsuccessful. A complex mixture of products was again observed, and
few solids precipitated from solution. Additionally, attempts to spectroscopically detect a
carbonyl group by 13C NMR were unsuccessful. Further experiments, including the isolation and
54
characterization of the new species, are necessary to confirm that carbon dioxide insertion into
the C–B bond of complex 4 has occurred.
3.2.2 Catalytic Reaction with CO2
Although the formation of a stoichiometric carbon dioxide adduct of 4 could not be confirmed,
the potential still remained that 1, 4, or both could catalyze carbon dioxide reduction in the
presence of boranes. To test this possibility, a solution of complex 1 and ten equivalents of
HBcat in benzene-d6 was exposed to carbon dioxide (1.5 atmospheres), and the reaction was
monitored by NMR spectroscopy.
After mixing of 1 and HBcat but before addition of carbon dioxide, the 1H spectrum
demonstrated the complete consumption of 1 and the formation of 4 along with a new product
(4’). This new product displayed a hydride resonance as well as several aromatic resonances
consistent with the presence of an L- ligand. A new singlet also appeared in the 31P spectrum.
This product is distinct from the complexes reported in Section 3.1.
After the addition of carbon dioxide, a drastic colour change from dark red to light yellow
occurred, together with the formation of a small amount of brown precipitate. NMR revealed the
complete consumption of 4 within ten minutes. HBcat and 4’ were still present, along with a
second new product (4’’) that displayed a broad 1H hydride resonance and a singlet in the 31P
spectrum. A new singlet in the 1H spectrum at 4.47 ppm also appeared, which may be indicative
of dihydrogen being evolved in the reaction. The broad hydride resonance suggests the presence
of an (H2Bcat)- moiety coordinated to ruthenium in 4’’.
Hardly any change was observed after two hours at room temperature. However, heating the
reaction at 40 °C for 15 hours revealed an exciting result: all of the HBcat was consumed, and
the 1H, 11B, and 13C spectra clearly indicated the presence of H3COBcat and catBOBcat, the
expected products of carbon dioxide reduction. The only other major product remaining was 4’’,
which contains a symmetric L- backbone, equivalent PPh3 groups, and an (H2Bcat)- group
coordinated to ruthenium. It is likely a derivative of the known borohydride complexes 10 and
12. Scheme 44 summarizes this catalytic reaction.
55
Scheme 44: Catalytic reduction of carbon dioxide by 1 in the presence of HBcat to form
H3COBcat and catBOBcat.
Although this result is very preliminary, it confirms the ability of complexes 1 and/or 4 to act as
pre-catalysts or catalysts for the reduction of carbon dioxide to methoxycatecholborane. The
optimization and characterization of this novel catalytic system is currently underway.
56
4 Conclusion and Future Outlook
This thesis reported the syntheses of several ruthenium(II) complexes featuring borane-
derivatized 4,5-diazafluorenide ligands. Starting from the known ruthenium diazafluorenide
complex 1, reaction with 0.5 equivalents of ClBcat or one equivalent of HBcat afforded the
borane adduct 4 with a catecholboryl moiety appended at the 9-positon of the diazafluorenide
ligand. In the presence of excess borane, this complex is believed to convert to the borane-
borohydride complex 10. Furthermore, 4 was shown to react with benzene-d6 to form the
ruthenium(II) phenyl complex 9 via a proposed C–D bond activation mechanism. Complexes 4,
9, and 10 are also believed to react with dihydrogen, and a preliminary study of this reactivity
was reported.
Notably, 4 reacted with carbon dioxide to give a mixture of products, one of which may be the
product of CO2 insertion into the 9-position C–B bond of the diazafluorenide ligand.
Furthermore, complex 1 is a pre-catalyst for the reduction of carbon dioxide by HBcat, and
complex 4 was observed to form in situ during the catalysis. The reduction products were
MeOBcat and (catB)2O, the former being a direct precursor to methanol.
Given the exploratory nature of this work, there are still many aspects of these systems that
require further study. In particular, three short-term goals are isolation and full characterization
of complex 4, isolation and identification of the proposed carbon dioxide adduct of 4, and
optimization of the catalytic system involving complex 1.
The isolation of an analytically pure sample of 4 was precluded by the presence of trace
impurities that were difficult to remove by standard purification methods and by the reaction of 4
with the aromatic solvents used in the synthesis and NMR experiments. The former problem can
likely be overcome by careful and repeated crystallization of the crude reaction mixture. The
latter problem is harder to resolve, but one possible solution is to perform the synthesis of 4 in
proteo-benzene with the goal of fully converting 4 to the phenyl species
[Ru(C6H5)(L–Bcat)(N2)(PPh3)2]. Another possibility involves replacing the labile dinitrogen
ligand in 4 with a strongly bound ligand such as CO, which should prevent the reaction with
aromatic solvents from occurring.
57
Moving forward, our group is interested in expanding the borane-diazafluorenide chemistry
reported in this thesis. Preliminary results by a graduate student in our group indicate that
complex 1 also reacts with HBpin to form complexes analogous to the HBcat adducts reported in
this thesis, although the reactions occur rather slowly and display slightly different selectivity.
Although HBpin is less reactive than HBcat, it has the advantage of being less prone to
decomposition than HBcat. Attempts to synthesize the analogous 9-BBN and BH3 adducts of 1
are also underway.
Furthermore, a tetrahedral zinc nacnac diazafluorenide complex was recently synthesised by an
undergraduate student in our group. This complex has a fixed coordination sphere about the zinc
centre and does not contain any labile ligands, unlike the octahedral coordination sphere of
complexes 1 and 4 that is prone to isomerization and dissociation of the dinitrogen ligand. This
fixed coordination sphere should preclude any metal-based reactivity, thus simplifying the
system. This zinc complex has been shown to react with HBpin to form an adduct analogous to
complex 4. This system also exhibits catalytic activity for the reduction of carbon dioxide, and
efforts to characterize and optimize this system are currently underway.
58
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