CHAPTER 1 INTRODUCTION Transition metal sulfides are very important in inorganic chemistry, catalysis and material science [1]. These compounds have potential industrial applications and have been used in industries as catalysts, photovoltaic materials [2], solid-state lubricants [3-4] and cathode materials for high energy density batteries [5]. The metal disulfides MS2 (M = Mo, W) are particularly useful in the petroleum industry as hydrodesulphurization (HDS) catalysts [6-7] to prevent emissions of sulfur as sulfur oxides present in crude oil during the combustion of fuels. They are also central to hydrotreating catalysis, which includes removal of nitrogen (hydrodenitrogenation, HDN), oxygen (hydrodeoxygenation, HDO) and metals (hydrodemetalation, HDM), from petroleum products. In view of their importance in existing systems and their potential use in future systems, a better understanding of metal sulfide complexes may prove valuable in the design of catalysts. The chemistry of molybdenum and tungsten with sulfur donor ligands is unique when compared to other transition metal ions. The diversity in structural and reactivity characteristics of sulfido complexes of Mo and W is an important reason for the continuing research in this field. The trichalcogenides MX3 (M = Mo, W; X = S, Se) are generally amorphous in nature and have interesting electrochemical and physical properties. The structure of MX3 type of sulfides has been investigated by EXAFS technique [8]. While the disulfide MS2 exhibits two structures; one consisting of discrete bridged sulfido groups and the other a layered structure. In the layered structure of MoS2, each Mo ion is surrounded by six sulfur anions in a trigonal prismatic arrangement resulting in a sandwich-layered structure. These layers are held together by weak van der Waals interactions. The active centres of MoS2-based catalysts are located on the edges of the layers leaving sulfur vacancies. Thus the catalytic activity of bulk or supported MoS2 depends strongly on its dispersion. In addition, the tetrasulfidomolybdate and tetrasulfidotungstate anions are currently of interest since they play a vital part in the domains of bioinorganic chemistry, nutrition physiology and veterinary medicine and have been extensively studied [9,10]. Berzelius [11,12] first investigated the formation of (NH4)2MoS4 and 1
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CHAPTER 1
INTRODUCTION
Transition metal sulfides are very important in inorganic chemistry, catalysis
and material science [1]. These compounds have potential industrial applications and
have been used in industries as catalysts, photovoltaic materials [2], solid-state
lubricants [3-4] and cathode materials for high energy density batteries [5]. The metal
disulfides MS2 (M = Mo, W) are particularly useful in the petroleum industry as
hydrodesulphurization (HDS) catalysts [6-7] to prevent emissions of sulfur as sulfur
oxides present in crude oil during the combustion of fuels. They are also central to
hydrotreating catalysis, which includes removal of nitrogen (hydrodenitrogenation,
HDN), oxygen (hydrodeoxygenation, HDO) and metals (hydrodemetalation, HDM),
from petroleum products. In view of their importance in existing systems and their
potential use in future systems, a better understanding of metal sulfide complexes may
prove valuable in the design of catalysts.
The chemistry of molybdenum and tungsten with sulfur donor ligands is
unique when compared to other transition metal ions. The diversity in structural and
reactivity characteristics of sulfido complexes of Mo and W is an important reason for
the continuing research in this field. The trichalcogenides MX3 (M = Mo, W; X = S,
Se) are generally amorphous in nature and have interesting electrochemical and
physical properties. The structure of MX3 type of sulfides has been investigated by
EXAFS technique [8]. While the disulfide MS2 exhibits two structures; one consisting
of discrete bridged sulfido groups and the other a layered structure. In the layered
structure of MoS2, each Mo ion is surrounded by six sulfur anions in a trigonal
prismatic arrangement resulting in a sandwich-layered structure. These layers are held
together by weak van der Waals interactions. The active centres of MoS2-based
catalysts are located on the edges of the layers leaving sulfur vacancies. Thus the
catalytic activity of bulk or supported MoS2 depends strongly on its dispersion. In
addition, the tetrasulfidomolybdate and tetrasulfidotungstate anions are currently of
interest since they play a vital part in the domains of bioinorganic chemistry, nutrition
physiology and veterinary medicine and have been extensively studied [9,10].
Berzelius [11,12] first investigated the formation of (NH4)2MoS4 and
1
(NI-14)2WS4 by passing hydrogen sulfide gas into an aqueous ammoniacal solution of
molybdates and tungstates in 1826. Sixty years later, Kruss [13], Corleis and Liebigs
[14] reported the synthesis of (NH4)2MS4 (M=Mo,W). However, Muller [10] and
coworkers have pioneered' the chemistry of sulfidomolybdate concentrating on their
ability to behave as bidentate ligands. The salts with organic cations are important,
since they can be used for the synthesis of multi-metal complexes in organic solvents.
The sulfidometalate have a strong and characteristic absorption band in the UV-Vis
region. The reaction in which they are formed and decomposed can be readily
followed by spectrophotometric methods.
When hydrogen sulfide gas is passed into an aqueous solution of an
oxometalate the electronic spectrum changes (as shown in Fig 1.1 for the
molybdenum), and the bands of all the species appear in quick succession.
2- H2S 2- H2S [IV104] [MO3S] [11402S212 -
H2S H2S Liv-n-"633 -II. [MS4]
2-
( where M= Mo, W)
Scheme I
From the existence of isobestic points it follows that only two species can co-
exist in the solution at any one time. The rate of formation of sulfidometalate depends
markedly on the nature of metal atom present and greater the electron density on
oxygen atom. Thus, sulfidomolybdate form more rapidly than the sulfidotungstate,
while higher sulfidomolybdate forms more rapidly and it is difficult to isolate
monosulfidometalate. The reaction time with hydrogen sulfide, temperature,
concentration and counter cation used, are all important factors to be considered for
the preparation and isolation of various sulfido-metalate.
The tetrasulfidometalate are not very stable in aqueous solution, and at
lower pH their decomposition may either be caused by hydrolysis to oxometalate, by
intramolecular redox process or by their marked tendency to form sulfides. The
stability of sulfidometalate decreases with increasing oxygen content and increasing
electron density on the ligands. They decompose in acidic medium forming binary
sulfide such as MoS3 [8].
2
W432'522-02) Mo0S32- 141
MoS42- 14 2)
mo0352-(42)
r . 1403251(J31
,f:i ‘ McOS12-42 ' MY,'
l\'‘,.\ III
Mo0S3211711 t4t)
13
)4002 (43) ... ......
■ e; ; 4
:•71rii0;1°.--1.1t 1 CC
VS
=w
idow
-41A _ceem
7id
affr-..e
rid
300
X(nml 390 460
Figure 1.1 UV/Vis spectra of the H2S gas into ammoniacal molybdate solution as a
function of time.
On heating the ammonium salts of tetrasulfidometalate decompose to give
NH3, H2S, and the corresponding X-ray amorphous trisulfide, which then releases one
mole of chalcogen to form dichalcogenide at higher temperatures.
1.1 Synthesis of tetrasulfidometalates [11 -38]
Berzelius first investigated the reaction of hydrogen sulfide gas into an
ammoniacal solution of ammonium heptamolybdate or tungstate which lead to the
formation of (NH4)2[MoS4] or (NH4)2[WS4] respectively. The (N114)2[MoS4] synthesis
can be achieved in half an hour at 60 °C, while the synthesis of (NI-14)2[WS4] requires
longer duration (8 hrs) of H2S gas passage at 60 °C.
[M042- + NH3 H2S (NH4)2[MS4] (1.1)
60 °C
Holm have reported the synthesis of tetraethylammonium tetrathiometalate
(Et4N)2[MS4] by a metathesis reaction between (NH4)2[MS4] and Et 4NC1 in CH3CN
been synthesized from [MoS4] 2- and can serve as examples, to illustrate the structural
diversity encountered in Mo-S chemistry [41,42]. A variety of structurally diverse
dinuclear, trinuclear and tetranuclear sulfidotungstates like [W2(S2)2(1-S)2(S4)21 2-,
[W2(S)2(1-S)(11 2-52)42- [43], 1 [WAS)2(SH)(11-re-S (n g 1 1 1441 LW(W54)2]2 r4i -2. L - -4„2,21 L .-„
[SW(WS4)2]2 [W3510]2" [46], [(W2S4)(WS4)2] 2- [47] with W in different oxidation
states are reported. The reactions of sulfidometalate with polysulfides are summarized
in Table 1.3.1 [48-57].
7
Metal source
Polysulfide
Final product Ref.
(N1-14)2 [M0S4]
(NH4)6[M07024]
(NH4)6 [M07024]
(NH4)6[M07024]
(NH4)6[M07024]
(NH4)6 [M07 0241
[W04] 2-+ NH4SCN
+ HC1
(NH4)2S3
(NH4)2Sx
(NH4)2Sx
(NH4)2Sx
(NH4)2S„ with NH2OH
(NH4)2Sx + bpy
(NH4)2Sx + bpy
[MoS9]2-
[Mo3S13] 2-
[1‘402S12]2-
[MO202,S81 2-
[Mo4(40)4$314-
[M00(S2)2(bPY)]
[WO(S2)2(bPY)]
48
49-5 1
51-54
5 5-56
54
57
57
Table 1.3.1 Reactions of sulfide/oxidometalate with polysulfide
Unlike oxygen, sulfur has a tendency to form polysulfide complexes
containing S„2- units. The formation of polysulfidometalate depends on the amount of
sulfur available, counter cations and the type of solvent used. The reaction of
ammonium trisulfide (NH4)2S3 with ammonium tetrathiomolybdate in the presence of
tetraalkylammonium chloride leads to the formation of (Et4N)2[Mo 1vS(S4)2]. The
reaction of oxomolybdates with aqueous ammonium polysulfide solution in the
presence of hydroxylamine leads to the formation of several interesting polynuclear
nitrosyl-molybdenum-sulfido clusters [52, 58-60]. The addition of 2,2'-bipyridine
(bpy) to a reaction mixture containing [Mo04 2- or W03+ and S„2- results in the
formation of the discrete mononuclear bis(disulfido) complexes [MO(S2)2(bpy)] [M =
Mo, W] [57].
The reactions of various chalcogenometalates with elemental S to give
polysulfidometalates are listed in Table 1.3.2. The reaction of (Et4N)2[MoS4] with
elemental sulfur gives the well known sulfur ring complex [MoS9] 2- [41] in good yield
as shown below.
2 (NH4)2[MoS4J + 518 S8 [RIME
(1.21)
In this reaction, the oxidation of the coordinated S 2- to (54)2- and the coupled
8
reduction of Mo(VI) to Mo(IV) are brought about by elemental sulfur with an induced
electron transfer pathway, wherein the electron transfer occurs from coordinated (S) 2-
to the Mo metal center. The above reaction can also be achieved by the use of
dibenzyl trisulfide (BzS3Bz) instead of elemental sulfur.
Attempts to exchange the (Et4N) + with (Ph4P)+in (Et4N)2[MoS(S4)2] resulted
in the formation of an altogether different compound (Ph4P)2[Mo2S10] [48]. A
reaction of the oxidotrisulfidomolybdate complex [Mo0S 3]2- with sulfur gives
[MoO(S4)2]2- complex which is also a hydrolysis product of [MoS(S4)2] 2- .
Heating of ammonium tetrasulfidomolybdate, elemental sulfur, and
tetraethylamrnonium bromide in DMF affords the dinuclear Mo(V) complex
[Mo2S12] 2- as shown below.
DMF, 95 °C, Ar 2 (NH4)2[MoS4] + 5/8 S8 (Et4N)[MO2S12) + (NH4)2S + NH4Br (1.22)
(Et4N)Br
Table 1.3.2 Reactions of chalcogenometalate with elemental chalcogen
Chalcogenometalate Chalogen source Final product Ref.
(Et4N)2[MoS4] Sulfur or BzS 3Bz (Et4N)2 [MoS9] 41
(Ph4P)2[MoS4] BzS3Bz (Ph4P)2[MO2S to] 48
Bz = benzyl
(Et4N)2[MoOS3] S (Et4N)2[MOOS8] 48
(Et2NH)2[MoOS3] S (Et2NH)2[MOOS8] 58
(NH4)2[M0S4] S at 95 °C (‘1114)2[MO2S12] 59
(NI-14)2[W Sa] S at 110 °C 0‘1114)2[W2S12] 59
(Et1N)2[MoS4] S [(S2)OM0S2M0(0)(5300]2- 60
(Ph4P)2[M0 Sea] Se (Ph4P)2[MoSed 29
(Ph4As)2[WSe4] Se (P/I4As)2[WSe9] 29
The reaction of ammonium tetrasulfidotungstate with elemental sulfur at
elevated temperatures (110 °C) in DMF leads to the formation of the dimeric W(V)
complex as shown below.
9
0 110 CDMF 2-
[W2S12] + 2 NH4Br (NH4)2S (1.23)
2 (NH4)2 [WS4] + 5/8 S8 2ET,NBr
In the above reaction, it is importanat to note that failure in purging gas such
as Ar or N2 results in significantly reduced yields of the dimer and reisolation of the
starting material. As ammonium sulfide (NH4)2S, one of the products of the reaction,
can combine with sulfur to form polysulfide which can oxidizes the W(V). The above
point gets credence from the fact that [WS4 2- does not show any reactivity with
polysulfide unlike [MoS42--
The reaction of ammonium tetrathiomolybdate with organic disulfides in DMF
at 90 °C results in the dimerisation of the tetrahedral [MoS4] 2- moiety with
simultaneous reduction of Mo(VI) to form [Mo2S8] 2- as shown below [42].
S A/
2- S 2 [MoS412 + PhSSPh
Is/ ve /S
s/v \ s I] + 2 PhS
In the above transformation, the organic disulfide is reduced by two electrons
while the metal center is reduced by one electron. The conversion of [MoS4 2- to the
dimeric [Mo2S8]2- complex can also be effected by the use of diphenyldiselenide at 90 oC or [p-NO2C6H4SSC61-14NO2-p] at room temperature. The tungsten analogue
[W202(11.-S)2(S2)2] 2- has also been prepared by the oxidation of (NH4)2[W0S3] with
elemental iodine [61].
1.4 Tetrasulfidometalates as sulfur transfer reagents in organic synthesis [63 -69]
Several Mo-S complexes like diammonium bis(1.t-disulfido)
in their infrared and Raman spectra. The vibrational spectral analyses of many
tetrasulfidometalates have been done and the bands are assigned [92, 93]. The IR
spectra in the lower energy region (400-500 cm -1) can be useful to distinguish,
between free and coordinated sulfidometalates. The resonance raman effect can be
used as a sensitive probe for the detection of [MS4] 2- ligands and to make distinction
between the different modes of coordination of sulfidometalates. For the free
tetrahedral (Td) [MS4] 2- anion, four characteristic vibrations vi(Ai), v2(E), v3(F2) and
v4(F) are expected. All four vibrations are Raman active while only v3 and v4 are IR
active. Many tetrasulfidomolybdates exhibit a single strong band at around 475 cm 1
while tetrasulfidotungstates exhibit a strong signal around 455 cm1 which can be
assigned to the triply degenerate asymmetric stretching vibration (v3) of the M=S
bond [90, 91]. The lowering of symmetry due to the effect of hydrogen bonding can
be attributed to splitting of signal in the lower energy region.
1.6.2 Structural characterization
X-ray single crystal structures of (NH4)2[MS4] (M = Mo, W) have been
reported and these compounds are shown to be isomorphous with P—K2SO4 [94]. The
alkali metal tetrasulfidometalates A2[MS4] (A = K, Cs, Rb; M = Mo, W) all of which
crystallize in the orthorhombic space group Pnma are isostructural to (NI -14)2[MS4]
(M = Mo, W). The crystal structure of both (NH4)2[MoS4] [95] and (NH4)2[WS4] [96]
15
have been recently reinvestigated considering the importance of H-bonding
interactions in these compounds. The 5••H contacts in the reinvestigated
[N1142[MoS4], range from 2.55 to 3.0 A. The Mo-S bonds are within 0 01 A of those
for (Et4N)2[MoS4] [97].
A comparative study of the structural parameters of several known
tetrasulfidomolybdate (Table 1.6.2) and tetrasulfidotungstates (Table 1.6.3) has been
made with a view to understand the importance of the 5.-H bonding interactions to
induce the elongation of the M-S bond lengths [94-124]. The isolation and structural
characterization of [M54] 2" with different cations ranging from (NH4) +, Rb+,
Ni(tren)21 2+, (enH2)2+, (pipH2)2+ , Me-enH2)2+ etc. indicates the flexibility of the
tetrasulfidometalate anion to exist in different structural environments. In all these
compounds the average value of the S-M-S angles is very close to the ideal tetrahedral
value. All complexes listed in Table 1.6.2 and Table 1.6.3 exhibit cation-anion
interactions in the form of 5...Rb as in Rb2[MS4] or N-H•-•S as in all the other
compounds. In Rb2[WS4] the mean Rb•••S distance has been reported to be 3.5466 A. It is also noted that the difference between the longest and the shortest W-S bond
ranges from of 0.0092 A in (enH2)[WS4] to 0.0542 A in [Ni(tren)2][WS4]. The
compound [Ni(tren)2][WS4] is different compared with the organic ammonium
tetrasulfidotungstates because it shows the shortest W-S distance of 2.1580 A and also
the maximum A = 0.0542 A, even though the shortest S--H contact is observed for
this complex with a distance of 2.73 A. It is to be noted that in [Ni(tren)2][WS4] the N
atom of tren is not protonated unlike in the organic ammonium compounds but linked
to Ni(II). This indicates that the strengths of the N-H•--S contacts in [Ni(tren)2][WS 4] are probably different from those in the organic ammonium tetrasulfidotungstates. In
the organic ammonium tetrasulfidotungstate complexes and the fully protonated
(NI-14)2[WS4] complex, the longest W-S bond lengths scatter in a very small range
from 2.2147 in (pipH2)[WS4]. It appears that the difference between the longest and
shortest W-S distances is an important factor and this difference can probably be
considered as a measure of distortion of the MS4 tetrahedron. Where A values are
more than 0.033 A exhibit a splitting of the M-S vibration indicating that in these
compounds the W54 tetrahedron is distorted.
16
Table 1.6.2 Comparative structural data for tetrasulfidomolybdates