T here has been intense interest in the coordination compounds of unsaturated sulphur donor chelating ligands, dithiocarbamates, and their related molecules from chemists, physicists, biologists and theoreticians alike owing to their interesting chemical properties and possible wide applications." Interest in molecular structural investigations and chemical studies of these metal chelates covers a full gamut of areas ranging from general considerations of metal-sulphur bonding and the formation of four- membered chelate rings to the employment of these ligands in inorganic qualitative analysis,j their practical application in organic synthe~is,~ medi~ine,~ and biol~gy,~ and their uses as vulcanisatiorl accelerators,' floatation agents,"' fungicides, pesticides" radiation protectors,12 antioxidantslbd photostabilisers of polymers.'-' Their role in material science has also been quite significant. The interesting low spin + high spin cross-over phenomenon was first reported in an iron(1II)dithiocarbamate complex.l5 There are several metal dithiocarbamate complexes with bridging sulphur centres whch are known to participate actively in super exchange phenomenon imparting novel magnetic properties to these systems.16 In this chapter, the interesting ligation characteristics of dithiocarbamates and structural features of their various transition metal complexes in general and the coordination chemistry and stereochemistry of copper complexes in particular, along with the scope of the present work which covers the
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T here has been intense interest in the coordination compounds of
unsaturated sulphur donor chelating ligands, dithiocarbamates, and their
related molecules from chemists, physicists, biologists and theoreticians
alike owing to their interesting chemical properties and possible wide
applications." Interest in molecular structural investigations and chemical
studies of these metal chelates covers a full gamut of areas ranging from
general considerations of metal-sulphur bonding and the formation of four-
membered chelate rings to the employment of these ligands in inorganic
qualitative analysis,j their practical application in organic synthe~is,~
medi~ine,~ and b io l~gy ,~ and their uses as vulcanisatiorl accelerators,'
antioxidantslbd photostabilisers of polymers.'-' Their role in material
science has also been quite significant. The interesting low spin + high spin
cross-over phenomenon was first reported in an iron(1II)dithiocarbamate
complex.l5 There are several metal dithiocarbamate complexes with bridging
sulphur centres whch are known to participate actively in super exchange
phenomenon imparting novel magnetic properties to these systems.16 In this
chapter, the interesting ligation characteristics of dithiocarbamates and
structural features of their various transition metal complexes in general and
the coordination chemistry and stereochemistry of copper complexes in
particular, along with the scope of the present work which covers the
relatively unattended aspect of the primary amine derived dithiocarbamates
are discussed.
1.1 Dithiocarbamates as ligands
The ligand system such as dithiocarbamates, xanthates,
dithiophosphates, dithiophosphinates, and dithiocarbimates are all referred
to as 1,l-dithiolates. Various nucleophiles (Z- or 212-) are capable of attacking
carbondisulphide to form 1,l-dithiolates (la) and (lb).
Metal ions can react readily with (la) and (lb) to yield complexes with the
possibility of the two sulphur atoms getting bound to the same metal,
forming a four-membered chelate ring. A wide variety of ligands can be
made available by merely varying Z as shown in Table 1.1.
Nucleophilic attack of secondary or primary amines on CS2 in alkaline
medium is known to generate R2N-CS2-M+, which can be considered as salt of
carbamodithioic acid.'
RzNH + CS2 + MOH -+ RzNCSSM + H20
The free dithiocarbamic acids are relatively unstable and only a very few
have been isolated.'7 The simplest member of the series, I-IlNCiSH can be
obtained as an unstable crystalline solid by acidification of a concentrated
solution of the ammonium salt.1"
Table 1.1. Major types of 1,l-dithioluto ligands
=Composition / Structure I Name 1
RzNCSi
S /- RS-C<t Q Thioxanthate
S
S /-
R--C<. @ Dithiocarboxy late S
ROCSz
S /-
R2NZC<. 0 S
The disubstituted dihocarbamates are more stable, although they
decompose under acidic conditions according to the equation,
Dithiocarbamate
S /-
KO-C;. o S
CS2-
R2PS2
- -
(R0)2152
-
R~AsSZ~
(R0)2AsS<
Xanthate
/ so S=C\
so R S \ /F
P I 0 R' \-i
RO S \ /-
P I 0 RO' \k R S
\ /C
As;. 0 R' S
RO, /.? A?. O
RO' S
Trithiocarbonate
Dithiophosphinate
Dithiophosphate
Dithioarsinate
Dithioarsenate
R ~ N C S ~ ~ carbamodithioate anion popularly known as dithiocarbamate ions
(Dtc), have function which does not, however, behave quite like the
sulphur analogue of carboxylate (COO-) moiety. The difference is brought
about mainly by the N atom which is directly connected to the carbon of CS2.
In a detailed IR study, Chatt et al. showed that a significant contribution from
structure (2c) was necessary in order to describe the electronic structure of
dithiocarbamate~.'~
Their conclusion is based on the presence of a strong absorption peak,
the 'thioureide ion' band, in the 1542-1480 cm-' region of the IR spectrum
which is observed for all of the dithiorarbarnic acid drrivativc.~. The
contribution of resonance form (2c) to the structure of Dtc ligands and
complexes was offered as a possible explanation for the varying antifungal
activities of these compounds.20 What becomes evident from the contribution
from these various resonance structures is that there is an extended
conjugation encompassing at least four atoms. This imparts special property
to the molecule including stability. Evidence for delocalisation over the four-
atom skeleton S2CN of the dithiocarbamate system was observed by
Colapictro et al. in the short C-S and C-N distances of 1.720 and 1.344 A respectively, in the structure of N ~ ( S ~ C N ( C Z & ) ~ ] . ~ H ~ O . ~ ~ The effects of the
conjugated n system are also seen in the hyperfi~w interactions observed in
the NMR spectra of certain alkyl and aryl dithiocarbamate complexes of
iron(II1). The double bond character of the C-N bond in dithiocarbamate
complexes should result in hindered rotation of the NR2 group. This effect
was observed in the Mo(RzDtc).l(NO) complex. 'I'he NMR spectrum of t h s
compound is consistent with non-equivalent alkyl groups on the NR2 moiety
as a result of hindered rotation.22
Dith~ocarbamates are versatile ligands capable of coordinating in a
variety of forms.'' The diverse nature of its ligation characteristics is
presented in (3) where the ligand moiety can act as a monodentate, chelating
bidentate or as a multidentate to two or even three metal centres.
\ s 'i-M \ /S-M \ />\ \ /* \ \ /> N ; e C 4S N-C, M NI;-;C, M / \;/ / \-/ / \- / N-C' / \ S, S , S-M /N -C\ S-M
M M
Almost all transition and non-transition metal ions exhibit strong
coordinating affinity to the dithiocarbamate moiety and there does not seem
to be any ring-strain instability for four-membered cyclic structure of the
resulting complexes. Excellent reviews ar- available on ligation
characteristics of dithiocarbamates and covering structural aspects of the
wide variety of their metal ~omplexes .~ -~ Unlike many other donor atoms the
sulphur centres in Dtc stabilise polynuclear complexes with mixed valence
state. Another important characteristic of the Dtc ligand is its ability to
accommodate metal ions in unusual oxidation stc>tes, especially remarkable
being its potentiality to stabilise transition metal ions in higher oxidation
states.2,' The stability of complexes with unusually high or low oxidation
I
states of the central metal atom depends largely OIL the possibility for charge
levelling by o-bonding and x-back bonding. While in the case of normal
1,l-dithiolates these two electronic affects of the sulphur atoms are
considered to be of the same order of magnitude, in the dithiocarbamato
ligands an additional x-electron flow from the nitrogen atom to the sulphur
atom via a planar delocalised n-orbital system would make them strong
electron donors able enough to accommodate metal ions at higher oxidation
states. In contrast to the transition elements, the main group element
dithiocarbamates often have asymmetrical metal-sulphur bonds due to the
lack of p.-d, interaction. In these compounds the a-bonds are responsible for
metal-sulphur interaction. High oxidation states for these dithiocarbamates
are only found when high electron density is brought upon to the metal by
o-donating groups. For instance, MeSb(R2Dtc)z exists whereas Me2ISb(R2Dtc)z
does n0t.I'
It is not surprising that dithiocarbamato compounds with copper in the
oxidation state +3 are stable; instead it must be regarded as unexpected that
Cu(I) dithiocarbamato complexes exist The latter complexes are not simply
monomeric, but they are tetrameric or polymeric metal cluster compounds.
Obviously, the stability must be attributed to the metal-metal bond rather
than the stabilising effect of the ligand. The same holds good for the
hexameric Ag(1) and dimeric Au(I) dithiocarbamates. In all other
dithiocarbamate complexes in which the metal has a low oxidation state the
existence of this type of compounds is due to other, low-oxidation-number
stabilising ligands. Examples NO+ in Fe(EtzDtc)z(NO)z and CO in
Fe(E tzDtc)(CO)r .>+
Table 1.2. Stable uridcrflcrflon states of some metal ions in their dithiocarbamate cc~npleues
Metal dithiocarbamate species
The strong donating abilities of the Dtc ligands are lost when the
nitrogen is bound to aryl groups as in an aromatic system like diphenyl
dithiocarbamate. The C-N stretching frequency which is located around
1500 cm-1 in the al~phatic dithiocarbamates is seen lowered in the aromatic
dithiocarbamates." It is found that the lone pair of the nitrogen atom in
dithiocarbamate complex becomes progressively more important for the
donation of electrons, the higher the oxidation state of the metal."
The metal-sulphur distances are consistently longer in the
1,l-dithiolato systems than in the bis 1,Zdithiolene (substituted and
unsubstituted ethene-1,2-dithiolates (4a) and benzene-1,2-dithiolates (4b)
complexes.
The reason for this difference is directly related to the electronic
structures of the ligand systems and also the way the ligand molecular
orbitals interact with the metal valence orbitals upon complexation. In
Figure 1.1, simplified pictures of the molecular orbitals of the two basic
ligand systems, viz., ethene-1,2-dithiolate a ~ d dithiocarbamate are
pre~ented.~ The 3n, function in the dithiolene ligands possesses correct
symmetry and appropriate energy to interact strongly with the metal d.
function to produce more stable, extensively delocalised molecular orbitals in
the metal complexes. Moreover, whereas the 3x, functions are filled when
the 1,Zdithiolene ligands are in their classical dianion formulation, the
orbitals are empty when the ligands are in their htghly oxidised dithione
formulation. It is possible, for these x functions to, therefore, serve as
acceptor orbitals, thus giving these ligand systems the x-acid character. This
type of interaction is not possible in complexes of the 1,l-dithiolato ligand
system such as dithiocarbamate because of the change in symmetry of 3n, of
SzCNHi. It is important to note that the x-acid character results primarily
from the 3n, function delocalised over the S-C-C-S backbone and not from
the vacant d orbitals of sulphur. If the use of the sulphur d orbitals were of
greater importance one would expect to see little change in metal-ligand n
bonding in going from the 1,2-dithiolene complexes to the 1,l-dithiolato
system. Thus the dithiocarbamate ligands exhibit little of the x acidity which
would serve to enhance the electrophilicity of, for example, the d9 Cu(Q ions
in their complexes. As a result, while the Cu(I1) dithiocarbamates are subject
to considerable axial ligation with coordinating Lewis bases, the adducts
formed by them are relatively unstable and cannot be isolated.2
a C-N
a a O ' C - N
Energy
Figure 1.1. A simplified representation of the zv nnzolecular orbitals of the hvo basic ligand systems ethene-1,2-dithiolate, s~c&?-, and dithiwarbamate, SzCNHi.
Normal coordinate analysis of some transition metal dithiocarbamates
have been reported.26-z Absorptions of diagnc~stic value occur in three
regions in the IR spectra. The 1450-1550 cm-1 region is associated primarily
with the thioureide vibration and is attributed largely to the v(CN) vibration
of SzC-NR2 bond. An increase in the double bond character of the C-N bond
(2) results in higher frequencies for this vibration.' A nearly linear correlation
is found in a plot of the v(CN) versus the methylene proton resonance from
lH-NMR data (6, ppm) on symmetric Et2Dtc complexes. An increase of the
partial positive charge on the nitrogen results in a desheilding of the
N-bonded methylene proton. But 6(CH2) and v(CN) are not interdependent
in complexes with asymmetric Et2Dtc bonding.2y An observed decrease in
v(CN) in the sequence R = Me > Et > Pr - Bu, is generally paralleled by the
calculated sequence obtained by increasing the po nt mass of the alkyl group
only. To reproduce the observed sequence, however, it was found necessary
to decrease the C-N force constant (f) in the seqLence Me > Et > Pr - Bu.
These results indicate that both electronic and kinematic effects are important
in determining V(CN).'~ According to Chatt et nl. the energy of the v(CN)
band falls roughly into groups according to the probable arrangements of
sulphur atoms around the central metal atom, the order of decreasing
frequency being planar > tetrahedral > octahedral > distorted octahedral >
pyramidal.19h The v(CN) band is known to undergo a blue shift in the
dithiocarbamato complexes with bidentate or multidentate bonding mode,
while for unidentate coordination this stretching is seen to be shifted towards
lower wavenumbers or remains unchanged at the value of the free
dithiocarbamate ammonium salt30
A second region between 950 and 1050 cm-I is associated with v(CS)
vibration and has been used effectively in differentiating between
monodentate and bidentate R2Dtc ligands. Two absorptions in the region of
1050-950 cm-1 is a diagnostic criterion for asymmetrically bound RzDtc
group.3' This criterion that distinguishes mo.~odentate from bidentate
bonding is shown to be valid, provided comparison is made between
complexes containing the same alkyl groups. It appears that the splitting of
the v(CS) vibration also should occur with unsymmetric bidentate bonding.
Monodentate bonding should be assumed only if the splitting exceeds
20 cm-l.Z8 Small splittings (-15 cm-1) of the v(CN) vibration also should occur
for unsymmetrical bidentate or monodentate binding of the RzDtc ligands.
Because of the ~nherently large width of the v(CIQ)-Emd W m a y not be,
however, observed." The absorptions in 300-400 cm-1 region is associated
with M-S vibrations.
By analysing the IR spectra of a numtc?r of bis(dithiocarbamat0)
palladium(II) complexes Sceney and Magee concluded33 that bands appear
and disappear in the region of v(CS) absorption in a completely random
fashion. Their intensity also vary without any apparent order. It appears
therefore, that v(CS) vibrational modes must be highly coupled with other
modes and are very sensitive to environmental changes.
1.2 Structural features of dithiocarbamate complexes
With 'pure' dithiocarbamate complexes in which no other coordinating
ligands are present, four definite structural types are obser~ed .~
(c) (4 (5)
(The four basic structure types observed for 'pure' dithiocarbamate complexes (a) the square planar coordination geometly, (b) the five coordinate dimer (c) the four coordinate dimer (d) the octahedral coordination geometty: (0) metal, (0) sulphur, ( 0 ) carbon, (0) nitrogen)
The first type (5a) is that of an essentially planar coordination
geometry which is found for all the structurally known bis complexes of
Ni(I1). In the complexes Ni(SzCNHz)z, Ni(S2CNEt;)z and Ni(SCN(n-GH7)z)z
the SC and C-N distances average 1.70 and 1.34 A respectively confirming
that the all the canonical forms (2a-2c) contribute to the electronic structures
of the ligands."~s The average SNi-S bond angle of 79" indicates the
magnitude of deviation from perfect square coordination. The average
intraligand S S distance is a very short 2.85 A and the average interligand S S
distance is 3.41 A. These values contrast sharply with the near equality of the
corresponding intra and interligands values in the monomeric bis
1,Zditluolene structures in which the planar arrangement is essentially
square.
Blauuw et RI. synthesised complexes of stoichiometry
AuX(SCN(~-(;HY)~) by the reaction of [Au(SzCN(n-GHY)~)] with various
halogens.17 The structure determination of this complex showed that it
consists of planar Au(LU) cations of [Au(SzCN(nGHr)2)2]+ and linear Au(1)
anion [AuBrz]( ).jX The structure of the interesting monodithiocarbamate
Cu(III) complex [CuBr2(SCN(nGHs)z)] reveals a strictly planar coordination
about the central metal.3y
Two basic types of dimeric structures have been observed for the bis
complexes containing dithiocarbamate ligands systems. The first structure
type is that of the five coordinate dimer observed in dithiocarbamate
complexes of copper(I1). The coordination in both the Cu(I1) dimers,
CUZ[SZCN(C~&)CI)Z]~ and C U ~ [ S Z C N ( ~ - G H ~ ) ~ ] ~ is best described as square
pyramidal.'"*l One of the Cu-S basal distances is slightly but significantly
larger than 2.312 A (of the other three) by 0.03 A and involves the sulphur
which serves as the apical atom to the centrosymmetrically related copper
atom. In the half dimer unit, the Cu atom is displaced out of the plane of the
four basal sulphur atoms by 0.26 A. The dimer linhagcs of 2.81 A (Cu-S) are
very weak and the [Cu(SzCNRz)z]z complex is found lo have a normal
monomeric molecular weight in such non polar solvents as benzene and
The dimeric arrangement found with such Cu(II)(RzDtc)2
complexes is pre-empted in Cu(MePhDtc)z, because of the orientation of the
phenyl rings. These rings are nearly normal to tl e plane of the rest of the
molecule, an orientation that appears to be dictated by the steric interactions
of the adjacent methyl s~bst i tuents .~~
The bridging dimer linkage has shrunk in the zinc complexes
Znz[S2CN(CH?)z)4 and Z~~[SZCN(CZ&)~]~ to a value of 2.383 A while the one
Zn-S basal distarre involving the bridging sulphur has increased frorn
2.342A to a value of 2.851 A.44.45 The primary function of one of the
dithiocarbarnate ligands has clearly changed from that of a chelating agent to
that of a bridging group. The coordination geometry about the Zn atom is
severely distorted and the four shortest Zn-S distances are directed to the
corners of a severely distorted tetrahedron (6).
(6)
[Idealised shucture of the Zn(Me2Dtc)z complex: (0) zinc, (0) sulphur, ( 0 )
carbon, (0) nitrogen]
The apparent strengthening of the dimer linkage is still not great
enough to maintain i t in solution. The molecular weight data indicate the
presence of monomers in non polar solvents such as benzene and
chlorofo~m.'~ The structure of Cdz(SzCNEtz)4 is also seen similar to that for
the zinc dimer.ib The two seemingly different structure types of the dimers
are still closely related to one another. Further, sbuctures in the intermediate
region between the two limiting geometries have also been obse r~ed .~
Structural studies on tris-dithiocarbamate complexes like
Ru(SCNMe2)3, Cr(SzCNMe& and Fe(SzCN(nE&,)& show the different
transition metal ions to possess significantly distorted octahedral
coordination geometries, the distortion being in the direction of triogonal
p r i ~ m . ~ ~ , ~ V o r example in the iron complex the sulphur donors are arranged
in two parallel equilateral triangles, one of which is rotated 32" (twist angle)
relative to the other (As shown in (7) the twist angle 0 behveen the upper and
lower triangles is 60" for octahedron and 0" for triogonal prism)
[Projection of the upper and lower hiangles of a geometry between an octahedron and Lriogonal prism. Definition of the twist angle O]
The distraction can be considered to be resulting from the strain of the
four-membered chelate rings and the relatively small 'bite angle' of the
1,l-dithiolato ligand system.
Metals like titanium, vanadium, thorium and niobium form complexes
of the compositions M(RzDtc)t. The crystal structure of Ti(Et2Dtc)r has been
determi11ed.4~ The complex contains eight coordinate Ti(1V) and chelating
Et~Dtc ligands. The coordination geometry of TiSs core is very close to
dodecahedral.
The monovalent metal ions Cu(I), Ag(I) and Au(1) are known to form
an interesting series of polymeric compounds with the general formula
[M(S2CNR2)In where n is the degree of polymerity. When M is Au(I), n equals
2; when M is Cu(I), n equals 4; and when M is Ag(I), n is found to be 6. In all
these metal clusters the oxidation state of the metal atom is +1.50.51 Cotton has
pointed out the necessity of low formal oxidation states for the metal ion in
order to achieve the formation of the M-M bond.52 Many sulphur containing
ligands favour cluster formation because these ligands can very effectively
delocalise and distribute the charges within the molecules. Along with this
levelling of charges the possibility of inter ligand S-S bonding may also play
an important role. Because of the great interest in cluster compounds and
metal-metal bonding in general, the structural studies of a large number of
complex compounds containing Dtc ligand systems have been made in detail.
The interesting and pertinent features in discussing these structures concern
the number and arrangement of metal atoms in the cluster, the metal-metal
distances, and the coordination of the sulphur atom around each metal atom.
In [Au(S2CNR2)]2 complex, the two Au(1) ions, which are separated by
only 2.76 A, are each coordinated in a linear arr.~ngement by two sulphur
atoms from different Dtc ligands. The two Dtc ligands can be considered to
be serving as bridging ligands between the Au(1) ions. The structure is such
that a twofold axis of symmetry passes through the two Au(1) ions and the
linear AuSz coordination is perpendicular to the metal-metal axis of
symmetry. Another twofold axis passes through the C-N bonds of both of the
Dtc ligands and the overall symmetry for the [AU~(S~CN(~€~H,)~] complex
can be taken as D2 or 222.s3,X The Au-Au distance is shorter than the
corresponding value found in the structure of the metal. Raman studies
indicate a contribution of resonance structures involving the entities shown
in (8).s5
Introduction 16
In the tetrameric Cu(1) compound of molecular formula
[CU~(SZCN(CZH~)Z)~] the Cu(1) ions are located at the corners of a slightly
distorted tetrahedron with Cu-Cu distances ranging from 2.6 to 2.7 A.51 AS
shown in (9) each of the Cu(1) ions is coordinated to sulphur atoms in a nearly
planar triangular arrangement and each sulphur atom coordinates to either
one or two Cu(1) ions.
Hesse also examined the structure of the hexameric
[Ag6(SzcN(C~tt)~)h] c o m p l e ~ . ~ The metal atoms I :Irm a somewhat distorted
octahedron with six comparatively short and six longer edges. The short
edges correspond to metal-metal distances, r hich are comparable or
somewhat longer than those in the metallic phase of silver. The long edges
form two centrosymmetrically related triangles in the silver octahedron.
Outside each of the other six faces of the octahedron one Dtc ligand is
situated, linked to the silver atoms of the face by silver-sulphur coordination,
one of the sulphur atoms is linked to one and the other to two silver atoms.
The silver coordination is threefold but not planar, the metal atoms being
situated 'inside' the plane of the coordinating sulphur atoms.
When the N-substituted alkyl groups are small, the hexameric Ag(I)
complex is very insoluble in organic solvents and appears to have some
polymeric properties. However, when more bulky alkyl substituents are
employed in the Dtc ligands the hexanuclear compounds show little tendency
to form higher polymers.
13 Copper--coordination chemistry
Copper occurs in a range of oxidation states 0 to 4. The Cu(0) and
Cu(1V) states are extremely limited. The Cu(II1) oxidation state is less
uncommon, and has been characterised for a few compounds including
copper dithiocarbamate ~omplexes.l,~,57,58 The Cu(I) and Cu(I1) oxidation
states are the most abundant for the metal. Cu(I1) is the more stable of the two
under normal conditions and form a wide variety of simple compounds and
complexes. The chemistry of Cu(1) is very much less extensive than that of
Cu(I1) and a number of accounts occur which describe the chemistry of
simple compounds of Cu(I) with less emphasis on the formation of its
cornple~es.~~~5~-61 The realisation that a copper(I) species may be involved as
the precursor of the silent partner in the type 111 copper p r ~ t e i n s ~ ~ ~ ~ has
resulted in a renaissance in the coordination chemistry of copper(1)
compounds which is reflected in the amount of space given to the chemistry
of copper(I) and (11) in the 'Advanced Inorganic Chemistry' of Cotton and
W i l k i n s ~ n . ~ ~ In the first edition of the book in 1952, more space was devoted
to copper(II) than to copper(I), while in the fourth edition the space allocation
is reversed.
1.3.1 Copper(11)compleres
Some of the salient aspects of Cu(I1) complexes are worth mentioning
here.
Copper(I1) forms complexes with coordination numbers four, five and
six, the latter being predominant A significant number of 7 and 8 coordinate
geometries also occur. Unlike other first row transition metal ions, the
copper(1I) complexes are characterised by a variety of distortions.65,66
Majority of six coordinate copper(II) complexes involve an elongated
tetragonal or rhombic octahedral structure, with a few involving a
compressed tetragonal structure. The tetrahedral geometry of Cu(I1) ion
always involves a significant compression along the S4 symmetry axis. Only
the square planar geometry is regular for Cu(II) ions, but even here it
involves a slight tetrahedral distortion. Copper(I1) ions with five
coordination rarely possess a regular square pyramidal geometry; it generally
undergoes both an elongation and a triogonal in-plane distortion," or less
frequently, a tetrahedral distortion. The triogonal bipyramidal geometry of
Cu(II) may be regular, but is frequently distorted towards a square pyramidal
stereochemistry.
Generally, Cu(L1) complexes are blue or green due to d-d electronic
transition causing absorption in the 600-900 nm regions. If there is a strong
charge transfer band spreading to the visible region the' complex appears red
or brown. Since copper(I1) ion is subjected to Jahn-Teller distortion and a
regular octahedral complex is not formed, the formal E, and Tzg terms get
splitted. The spectra do not usually correspond to the simple ZE, -+ 2T2g excitation67 but rather to one based on altered multiple states as shown
in (10).
(10)
[Splitting of 2Eg and 2T~g states in Cu(II)]
Tetragonal copper(Il) complexes are expected to show the transitions
~BI, + 2A~, 2B~, + 2B2, and ~BI, 2E, but bands due to these transitions
usually overlap to give often one broad absorption band.m.~~~
Four<oordinate Cu(I1) complexes are common, but the strict
tetrahedral or square planar stereochemistries are rare. Intermediate
stereochemistry of approximate D z ~ symmetry is more usual and four
transitions (between the d-orbitals) may be observed.b7b,7u The spectra of such
complexes often show two or three more or less resolved bands below about
20000 cm I. The polar~sahon properties of these bands have been studied in
some detail in certa~n cases assisting in the assignments of the transitions
1nvolved.67a "
1.3.2 Copper(1~0mplwolion characteristics
The Cu(1) and Cu(II) ions can readily form complexes in which the
cations act as Lewis acids and the ligands as Lewis bases. While Cu(I1) is
generally considered as borderline based acid, Cu(1) clearly behaves as a soft
acid and the nature of stability of the ligand to Cu(I) is that of a soft base with
class (b) behaviour (Table 1.3).72 In general the halides can form a wide range
of complexes, with the C1, Br and I ions predominant, but with very few
examples of the F ion acting as a ligand.73 Among the 0, S, N and P donor
ligands the 0 and N ligands dominate the chemistry of Cu(1I) while S and P
ligands are more frequent in Cu(I) chemistry.74~7~ 'This reversal of ligand role
is also influenced by the reducing properties of many S, I' and I ligands and
the ready reduction of the Cu(I1) ion (Equation 1.2) to the stable Cu(1) species
with these ligands.
Table 1.3. Hard and sop acid-base classification of copper ion and ligands
The hard-soft concept accounts for the following reactions and product
formations."
(a) CuCl~.H20 + KC1 So2(aq'q) ) K[CuC12]
Hard
(b) CuBrz + KBr -% K[CuBrz] lxrll
Border line
C ~ P P ~ ~ ( I I ) (a) Acid
(b) Base
(d) K[Cu(CN)z] Cuz+(aq) + KCN -t Cuz(CN)l excess
son C ~ P P ~ ~ ( I )
F- < C I ~ < B C ~ < l
O < < S = Se = Te
N < < P > A s > S b > B i
F~ > CI > Br- > I
0 ;.> S > Se > Te
N >> P > As> Sb> Bi
The electrode potentials of the reactions (1.1) and (1.2) readily lead to
the disproportionation as in equation (1.3) shown ldow.57
C U ~ + ~ , ) + ~ -+ CU+(,+ Eo = 0.15 V
2Cu+(,,, 4 Cu" + CU~+(.,), EQ = 0.37 V
Consequently the concentration of the Cu+ ion in aqueous solution is
extremely low (K for equation (1.3) being of the order of lo6) compared with
the very hgh stability of the Cu2+(aq) cation. For this reason water is rarely
found as a ligand to Cu(I), but is a common ligand in copper(1I) chemistry.74
The electrode potentials of reactions (1.1-1.3) get readily modified by
complex formation with appropriate ligands (Table 1.4).75
Table 1.4. (a) Reduction potentials of some Cu(II)/Cu(I) couples und (b) [Cu (Il)]/llcu (I)? ratios
(a) -
Cu(ll)lCu(l) Couple
C N ~ -
r c1- In laccasse
In ceruloplasmin
Loxalate < -0.2 I I 'en - HzN(CHz)zNH2; ' M e e n - HzN(CHz)3NHz; *Mesen - HzN(CH&NHz
bipy I:':: Glvcinate
0.15
0.12
-0.01
-0.16
-
-
In this way the concentration of Cu(I) in aqueous solution can be
significantly increased by complex formation such as in [CuC12]- and
[Cu(CN)2] anion by addition of an excess of tht. appropriate ligand. This
strategy can be made use of the preparation of various Cu(1) complexes.
Alternatively, Cu(I) can be brought into nonaqueous solution in which the
solvent is a good ligand for Cu(1) and forms complex. Such complex solution,
(for example, [Cul(NC-Me)r]X), can be used for the preparation of other Cu(1)
complexes by reacting with suitable ligands.
1.3.3 Copper(1) complexes-stereochemistry
Cu(1) can exist as mononuclear, bi-nuclear, tri-nuclear, tetra, penta and
exanuclear complexes. Even octa and decanucl~ar complexes have been
Many of the Cu(1) complexes are polymeric in nature and
possess chain and ribbon structures and infinite three dimensional lattices. In
all these complexes copper has the relatively low coordination numbers of
two, three and four and very rarely five.
In the solid state the stereochemish-y of Cu(1) in its mononuclear
complexes as determined by X-ray crystallography is dominated by four
coordination. The four- coordinate Cu(1) complexes are generally tetrahedral.
A significant number of three and two coordinate complexes are also known;
very few five coordinate complexes exist and six coordination or above is
unknown. This contrasts with the predominance of six coordination in the
chemistry of Cu(I1) and the absence of two or three coordination in the solid
state and with the formation of a significant number of seven and eight
coordinate geometries.65,m
Binuclear complexes form a significant class of Cu(I) complexes
involving bridgmg by one or two, but not three Iigand atoms or groups. In
practice the bridging role is most common for halide ions, especially iodides.
The resulting structures mainly involve a symmetrical arrangement of two
single atom bridges with either trigonal (llb), tetrahedral ( l la) and mixed
8-hydroxy quinoline, HDtc = H2NCSzH and HDtp = dithiophosphoric acid).
Weak reversible adducts were formed with NO, whereas NO: oxidise the
ligands in all complexes. The reaction of Cu(Dtc)z with NO2 proceeds through
Introduction 37 . .
m~xed l~gand complexes w ~ t h the participat~on of NO,-; Cu(N03)z and
d~sulphidc. front the l~gand be~ng the final products.
Salam r t r r l synthes~sed and character~sed some metal chelates of
ethylenedlam~ne-monodrth~ocarbamates. Cu2+, Ni2+, Znzt, Colt, Fez+ and Cd2+
formed complexes of the composition ML2 and Fe3+ and Colt formed ML?
(HL = HzN-CHJ-CH2-NH CSJ H) species.151
Mekil complexes of 1,3-propanediamine rnonodithiocarbamate were
prepared and characterised by Salam et nl. Divalent cations CuZ+, Ni2+, Mn2+,
Znz', Coz', Fez' and Cdz' formed ML2 and trivalent cations Feq+ and Co"'
formed ML.$(HL = H2N ( C H ~ ) Z - N H C S ~ H ) . ~ ~ ~
Enzymatic activity of copper, zinc superoxide dismutase (SOD), a
metalloprotein that catalyses superoxide radical disproportionation, involves
a cyclic Cu(lI)/Cu(l) redox process of the Cu(I1) i?n held at the active centre
in the protein. St.vcral copper (11) dithiocarbamates are known to exhibit
SOD-like activity.'jT.'3 The activity of such complexes depends on the
Cu(II)/Cu(I) rcdox process. Roberto Cao et nl. studied the SOD-like activity
of the copper(I1) complexes of the amino acids glycine, alanine, serine,
aspargine and glutatnic acid. In all these complexes copper is reported to be
existing as Cu(II) and the EPR inactivity is explained in terms of
antiferromagnetic in te ra~t i0ns . l~~
1.5 Scope and objectives
Conipin in 1920 noted the instability of Cu(I1) complexes of
dithiocarbamates derived from primary amines.143 In 1936 Cambi and
Coriselli noticed that Cu(SzCNHR)2 could not be isolated because of its rapid
decomposition to the Cu(I) species by some complicated mechanism. They
verified the presence of isothiocyanate in the decomposition products.ll'
Since 1936 there have been only very few reports, probably less than ten, on
copper complexrs with RHDtc ligands. In most ~f these reports the metal-
ligand ratio is stated to be 1 :2 the oxidation state of the metal being Cu(I1).
These reports contradict with the observation of Compin, and Cambi and
Coriselli.
The present investigation is to establish the true nature of the
interaction between Cu(II) ions and RHNCS;. The products of the reaction
between Cu(I1) ion and RHNCS; ion were carefully isolated and
characterised by elemental analysis, molecular mass determination, various
spectroscopic techniques and thermal analysis. In anticipation that the
dielectric of the reaction medium and the nature of the substituent on the N
atom of RHNCS, would dictate the nature and course of the reaction and
stereochemistry of the reaction products, interaction of Cu(1I) ions with a
range of RHNCS; ions with various alkyl, acyl and heterocyclic substituents
was studied in different solvents. With a view to evolve a strategy to achieve
redox-stabilised copper(I1) complexes of RHNCSz, the Dtc function was
anchored to a polymeric support and its interaction with Cu(l1) was
investigated mainly by electron spin resonance spectroscopy. The present
study broadly covers the following aspects:
(i) Various dithiocarbamates derived from primary amines are prepared
in solution condition by the nucleophilic addition of primary amines
on CSr in alkaline medium. Dithiocarbamates with a variety of
N-substituents-alkyl, simple aryl, heterocyclic, aralkyl, aryl with
either electron donating or electron withdrawing groups-are
prepared with a view to study the effect of N-substituents on the
course and nature of the reaction between Cu(1I) and RHDtc.
(ii) The primary amine-dithiocarbamates generated in solution are
allowed to interact with cupric ion in aqueous medium. The reaction
products, both in solid state and in solution are isolated and purified.
The compounds are systematically analysed by various analyl~cal and
spectral methods which suggest the occuirence of a redox process
during the interaction, forming polymeric copper(1) dithiocrbamates
and thiuram disulphides, RHNCS2-SzCHNK.
(iii) To establish the identity of the oxidised form of the ligands, thiuram
disulphides, formed on interaction of copper(I1) with Dtc, all the
dithiocarbamates are oxidised separately and individually with iodine
solution and their composition and spectral properties compared with
those obtained through copper(I1) interaction.
(iv) Since preparation of dithiocarbamate ligands and their complexation
with metal ions in ethanol medium have been reported, the generation
of various dithiocarbamates, RHDtc and their interaction with
copper(I1) are tried in ethanol medium also. The course of the reaction
is found to be different for N-alkyl and N-aryl substituents though the
occurrence of the expected redox process is confirmed.
(v) Since the reaction medium is found to affect the redox potential of a
process," the interactions are studied in TIIF and/or DMF expecting
that solvents with a lower polarity might stabilise the copper(1I) state.
The products are isolated in the pure state and analysed by various
physicochemical methods including molecular mass determination.
Though redox process does occur, structure and stereochemistry of the
resulting Cu(1) complexes are found to be different from those formed
in aqueous or alcoholic medium. Various oligomeric complexes,
dimers and tetramers, could be isolated.
(vi) Complexation with metal ions is expected to facilitate interesting
reactivity pattern of RHDtc. With this in view electrophilic
substitution, like benzoylation, is attempted on the various oligomeric
and polymeric Cu(1)RHDtc formed during the interaction studies. 'The
difference in the reactivity of the Cu(1) complexes towards
benzoylation is found helpful, to a great extent, to infer the structure
and stereochemistry of the complexes.
(vii) With a view to prepare cupric dithiocarbam:~te by inhibiting the redox
process, the ditluocarbamate function is immobilised by anchoring to a
polymer matrix. Three polymer bound dith ocarbamates derived from
ethylenediamine, p-aminophenol and o-antinophenol are separately
treated with Cuz+(aq). The polymer beads are analysed mainly by EPR
which indicates the redox stabilisation of copper(I1) during the
interaction.
(viii) Attempts are made to generate mixed ligand Cu(1I) complexes
involving N-monosubstituted Dtc on polymer matrix. Each of the
dithiocarbamate functionalised resins is treated separately and
individually with six different Cu(I1) complexes of Schiff bases,
P-diketones and 8-hydroxy quinoline. All the eighteen polymer metal
complexes generated are studied by EPR to characterise the complexes
formed on the polymer matrices.
(ix) To studv the effect of the N-substituent and slereochemistry of the
complex on the mode of decomposition and thermal stability, and also
to confirm the structure and compositiori proposed for the Cu(1)
complexes prepared by interaction of Cu(1.) with RNHCSk), thermal
analysis of the various dimeric, tetrameric and polymeric Cu(I)
dithiocarbamate complexes are carried out by TG and DTG. With the
help of the nine mechanism-based equations proposed by Satava
various kinetic and thermodynamic parameters are calculated from the
thermograms. Suitable mechanisms are proposed for all the major
decomposition stages.
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Introduction 46
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