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

[38].

(N/14)2[MS4] + 2 (Et4N)C1 --> (Et4N)2[MS4) + 2 NH4C1 4. , (1.12)

V,

0

3

McDonald et al reported synthesis of the (Et4N)2[MS4] [16] by reacting a

strong base such as EttNOH with aqueous solution of ammonium

tetrasulfidometalate. The displacement of ammonia by an organic tetraalkylated

ammonium cation leads to the formation of (Et4N)2[MS4] compounds as depicted

below.

(NH4)2[MS4] + 2 (Et4N)OH -f (Et4N)2[MS4] + 2 NH3 + 2 H2O (1.13)

Alonso et al have reported the synthesis of several tetraalkylammonium

tetrathiometalates [17-19] using a slight modification to the above mentioned

synthetic route. In this method, the organic base i.e. tetraalkylammonium hydroxide is

generated in situ by the reaction of tetraalkylammonium halide with sodium

hydroxide which then reacts with (NH4)2[MS4].

R4NX + NaOH R4NOH + NaX (1.14)

In these reactions the alkyl group R can be methyl, ethyl, propyl, butyl,

pentyl, hexyl, heptyl, octyl and X can be cr or Br" .

The tetrasulfido or tetraselenatometalate synthesized by different methods are

listed in Table 1.1 and Table 1.2 [11-38]. The synthesis involves direct reaction of

H2S gas or H2Se into an ammoniacal solution of molybdate/heptamolybdate or

tungstate respectively, or reaction with alkali metal sulfide such as K2S. Neal and

Kollis have reported the synthesis of tetrasulfidometalates and tetraselenometalates of

Mo and W by an oxidative decarbonylation method [20,21]. The high cost, safety and

toxicity of H2Se has prompted the search of other Se reagents. Bis(dimethyloctylsilyl)

selenide has been used as a source of Se instead of H2Se for the synthesis of

tetraselenometalates. In recent years, solid-state reactions have been employed to

avoid the use of solvents during the synthesis of tetrasulfidometalates. This method is

an effective means of synthesizing metal chalcogenides and direct synthesis of

compounds with general formula [MX4] 2- (where M = Nb, Ta; X =S, Se) complexes

[22]. Ibers et al have reported the synthesis of dirubidium tetrathiotungstate Rb2[WS4]

[23]. In this method solid Rb2S3, sulfur and metallic W are heated in a fused-silica

tube under Ar atmosphere at high temperature. (CTA)2[WS4] has been synthesized by

4

a soft reaction route using thioacetamide with sodium tungstate in presence of

cetyltrimethylammonium bromide (CTABr).

Table 1.1.1: Synthetic methodology for tetrasulfido/chalcogenatometalates

Compounds Metal source Chalcogen source

Ref.

(NH4)2 [MS4] [M042- + NH4OH H2S 11-14, 24

(N1-14)2[MoSe4] MoO3 + NH4OH H2Se 25

(•11-14)2[MoSe4] (NH4)6[M07024]• 4H20 H2Se 26

(NH4)2[WSe4] WO3 + NH4OH H2Se 27

K2 [MOS4] M003 + CH3 OH K2S 28

(Ph4P)2[MS4] M(CO)6 + Ph4PBr K2S 20

(Ph4P)2[MSe4] M(CO)6 + Ph4PBr K2Se3 20-21

(Et4N)2[MoSe4] Na2Mo04- 2H20+ Et4NC1 + (dmos)2Se 29

Et3N

(NH4)2[WSe4] (NH4)2[W04] + NH4OH (dmos)2Se 29

Rb2[WS4] Rb2S3 + W S 23

(CTA)2[WS4] Na2WO4 + CTAB CH3CSNH2 30

The inorganic dirubidium tetrathiomolybdate, Rb2[MoS4] [31] has been

prepared in low yields by the reaction of (NH4)2[MoS4], AgI, RbI, elemental sulfur

and ethylenediamine under solvothermal conditions. In recent years, use of

solvothermal methods has resulted in the synthesis of a large number of compounds

ranging from discrete molecular anions to three-dimensional frameworks. The

compounds obtained by this method exhibit fascinating structural features and

interesting physical properties [39-40]. The sulfido complexes of Mo and W

containing inorganic complex cation like [Ni(en)3][MS4], [Mn(en)3][MS4],

[Co2(tren)3][MoS42, [Ni(tren)2][WS4], [Mn(dien)2][MoS4] have been obtained under

solvothermal conditions.

5

Table 1.1.2. Solvothermal synthesis of tetrasulfidometalate.

Compounds Metal source Chalcogen source

Ref.

Rb2[MoS4] RbI + AgI + en S & 31

(NR4)[Mo Sal

[Ni(en)3][MoS4] NiBr2 + en S & 33

(NRo[moS4]

[Ni(en)3][WS4] NiBr2 + en (180°C) (NR4)2[W Si] 34

[Ni(en)3][WS4] NiC12.6H20 + en (130 °C) (NR4)2[W Si] 34

[Mn(en)3][MoS4] MnC12.4H20 + en (NH4)2[MoS4] 34

[Mn(en)3][WS4] MnBr2 + en (NR4)2[1\40 Si] 34

[Co2(tren)3][MoS42 CoMoO4 + tren S 35

[Ni(tren)2][WS4] NiC12.6H20 + Na2W04-2H20 + en

S 36

[Mn(dien)2][MoS4] MnMo04 + dien S 37

1.2 Reactivity of tetrasulfidometalate [9,10]:

The metal-sulfur bond can react either nucleophilically or electrophilically,

resulting in the formation of rich sulfur or polysulfidomtalate and heterometal

aggregates. The sulfidometalate serve as a ligand in complex chemistry with the

generation of multi metal complexes, and possessing versatile co-ordination

behaviour and unique electronic properties of the ligands.

(a) Nucleophilic reactions:

The reaction of sulfidomolybdate with nucleophiles involves abstraction of sulfur

by the nucleophile and simultaneous reduction of the metal centre.Thus it is a

sulfur transfer reaction with a two-electron reduction of the central metal atom. eg .

The reaction of CN with sulfidometalate has significant importance in bio

inorganic chemistry and in the evolution of primitive molybdenum enzymes [9].

Mor-S + Nu Mor-2 + Nu S (1.15)

eg. Mor-S + CN- ► mor-2 + NCS- (1.16)

(b) Electrophilic reactions:

The reaction of sulfidomolybdate with electrophiles involves the transfer of one

6

sulfur to the electrophile without changing the oxidation state of the metal atom. eg .

tetrasulfidomolybdate reacts with acids to form NH3, H2S and MoS3.

Mor-S + E ■ Mor + ES2-

(1.17)

eg. (NH4)2[MoS4] + H÷

► NH3+ H2S + MoS 3 (1.18)

(c) Redox reactions [41,42]:

The redox reactionsof Mo-S systems are induced by internal electron transfer in

the presence of external oxidant. Addition of organic disulfides to MoS4 2- leads to the

oxidation of [MoS4 2- ion by two electrons. The formation of 522- can occur via the

intramolecular reaction of elemental S ° produced in the first reaction with S 2- .

mot (S2)

► Mor-2 (S°) (1.19)

eg. Mot (S2-)2 Me (S22-) (1.20)

1.3 Formation of sulfur rich polysulfidometalates

The sulfur rich polysulfidometalates can be obtained from the reaction of

sulfidometalates or oxidometalates with a) polysufide b) elemental sulfur c) thiol and

d) organic disulfide. The controlled acidification of tetrasulfidomolybdate results in

the formation of a dinuclear Mo(V) compound, while the use of excess acid leads to

the insoluble MS3 as the final product [8].

A variety of thiomolybdate complexes such as [MoS(S4)2] 2-, [1•402(S)2( 1-

S)2(52)2]2 , [MO2(S)2(11-S)2(S4)(S2)] 2 , [Mo2(S)2(II-S)2(S4)2] 2 , [SMo(MoS4)2] 2- have

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)

tetrakis(disulfido)dimolybdate(V) (NH4)2 [Mo2S 12] [62], benzyltriethylammonium

tetrasulfidomolybdate [(PhCH2N(C2H5)3i2-[M0S4], ammonium tetrasulfidomolybdate

(NH4)2[MoS4] have been used as sulfur transfer reagents in organic synthesis for the

preparation of several organo-sulfur compounds [63-69].

Chandrasekaran and coworkers [69] have pioneered the use of

benzyltriethylammonium tetrasulfidomolybdate in organic synthesis and have

demonstrated this complex to be a versatile sulfur transfer reagent for the convenient

synthesis of a variety of organic-sulfur compounds under mild reaction conditions.

(1.24)

10

1,c) 01. ,0 Ogridliowricls

0

RCHISSCHIR4-

Aki orA)4 clEdicb

S.grdstids

The utility of tetrasulfidomolybdate in organic synthesis as sulfur transfer

reagents is presented in Scheme II.

Othamire

Scheme II

13 Formation of heterobimetallic chalcogenides

Muller and coworkers have studied the ligational behaviour of the

sulfidometalates and synthesized several heteronuclear complexes where the central

metal ion can be either a transition metal or a non-transition metal and sulfidometalate

can be any member of the series (MS4_„O n)2- [M = Mo, W; n = 0-2] [10,701 The

tetrahedral [MS4] 2- (M = Mo, W) moiety has been used as a starting material for the

synthesis of bis(sulfidometalato)metal complexes.

A general reaction for the formation of bis(thiometalato)metal complexes can

be written as below:

11

M'24- + 2(XR4 )+ 2(MS4-n Qn) 2-

z-

(1.25)

Where, M'= Fe, Co, Ni, Pd, Pt, Zn, Cd, Hg

M = Mo, W

X = As, P and n= 0, 1, 2

The reaction of bivalent metal salts in the presence of bulky cation with

tetrasulfidometalates in aqueous medium results in the formation of

bis(sulfidometalato) complexes of Ni, Pd and Pt in +2 are square planer, complexes

of Fe, Co and Zn in +2 oxidation states are tetrahedral as shown in figure 1.2.

Fig 1.2: Complexes with square planar, tetrahedral and octahedral geometry.

The tetrasulfidotungstate complex of tin namely [Sn2(WS4)4 4" [71] has been

reported in the literature. In this complex, tetrasulfidotungstate behaves like a

tridentate ligand unlike bidentate in the above examples. It can also behave like a

doubly bridging ligand as in [NH4CuMoS4],, [72].

12

The aqueous reaction of ferrous salts with tetrasulfidometalate in the presence

of tetraphenylphosponium halide results in the formation of a polymeric species of

approximate composition (Ph413)2[Fe(MS4)2] [70]. In coordinating solvents such as

DMF, DMSO or pyridine the [Fe(WS4)2] 2- complex anion form base adducts of the

type [Fe(WS4)2(DMF)21 2-, Wherein Fe(II) has an octahedral coordination sphere with

two bidentate [WS4 2- ligands and two DMF molecules in trans position as shown in

Fig 1.2.

Recently heterobimetallic compound containing f-block element namely Ce

has been reported [73]. In this report, the synthesis of a Ce-W-S complex

[Ce(dm08]2[WS43 .120 (dmf = dimethylformamide) has been accomplished by the

reaction of (NH4)2[WS4] with CeC13.7H20 in DMF. The structure of

[Ce(dmf)8]2[WS43.H20 consists of [WS4 2- tetrahedral anions and dmf coordinated to

Ce31- cations with inclusion of water molecules.

Molybdenum-sulfur compounds are very important in several enzymes, such

as xanthine oxidase, sulfite oxidase, aldehyde oxidase, oxidoreductase enzymes etc.

The tungstoenzymes have been classified into principal families AOR (aldehyde

oxido-reductase) and F(M)DH (formate dehydrogenase, FDH and N-

formylmethanofurano dehydrogenase, FMDH) [74]. Iron-molybdenum-sulfur cluster

is present as the Fe/Mo cofactor of nitrogenase [75-77], which is responsible for the

fixation of nitrogen. Mo occurs in more than thirty enzymes. Zumft have reported the

isolation of [MS42- by mild hydrolysis of Mo-Fe protein from Clostridium

Pasturanium [78]. The synthesis and characterization of iron-thiometalate (Fe-M-S)

(M =Mo, W) complexes as structural models for the Mo sites of nitrogenase has been

the subject of research. The chemistry of Fe-M-S complexes derived from [MS4 2-

anions has been reviewed by Coucouvanis [79] and Averill [80].

Ammonium tetrasulfidomolybdate has been effectively used for the treatment

of metastatic cancer [81-82]. Copper is an essential element serving as an important

cofactor for more than thirty enzymes, many of which are of key importance, such as

superoxide dismutase to combat radical formation, cytochrome c oxidase for cellular

respiration, tyrosine oxidase in pigmentation, or lysyloxidase for connective tissue

maturation [83]. However, a surplus of copper is toxic and leads to radical formation

and oxidation of bio-molecules. Therefore, copper homeostasis is a key requisite for

every organism. Sulfidomolybdates have been demonstrated to be able to remove Cu

13

from the bodies of Cu-poisoned animals [84-86]. The tetrasulfidomolybdate [MoS4 2-,

a good Cu chelator, has been used for the treatment of Wilson's disease with

neurological symptoms [87]. The uptake of tetrasulfidomolybdate by the liver and

removal of copper in the liver of rats has been well documented [88-89].

Tetrasulfidomolybdate has been extensively used as a specific and selective chelator

to remove copper accumulating in the form bound to metallothionein protein in the

livers of patients suffering from Wilson's disease without disruption of the

metabolism of other essential metals such as Zn and Fe [90-91].

Characterization of sulfidometalates

1.6.1 Spectroscopic Investigation

Several physical methods are reported in the literature for the characterization

of complexes containing tetrasulfidometalate. These methods provide useful

information on the structure, nature of the chemical bonding and stability of these

compounds.

i) UV-Visible spectroscopy

The simple tetrasulfidometalates have characteristic high intensity absorption bands in

the UV-Visible region [10]. The actual UV/visible spectra of the sulfidomolybdate

and sulfidotungstate species are shown in Figs. 1.6.1(a) and 1.6.1(b) respectively.

Figure 1.6.1(a) Electronic spectra of

sulfidomolybdates in H2O.

Figure 1.6.1(b) Electronic spectra of

sulfidotungstates in H2O.

14

The electronic spectra of coordinated sulfidometalates are entirely different

from that of the free sulfidometalates. Various physical measurements have

demonstrated that in sulfidometalato complexes with central metal possessing open d-

shells, there are strong metal-ligand interactions. The known complexes of the type

[MWS44,0„)2] 2- (M = Mo, W; M' = Fe, Co, Ni, Pd, Pt) show characteristic

absorption bands whose positions are only roughly comparable to those in free

sulfidometalates [10].

ii) Vibrational spectroscopy

The vibrational spectroscopy is a powerful tool for the inorganic chemist,

which provides valuable structural information. The free tetrasulfidometalates have

characteristic M=S stretching vibrations (v(m_s) 400-500 cm -1 ; v(M-o) 800-1000 cm-1 )

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

Compound Space

group

Mo-S

(long) (A)

Difference

A (A)

Reference

(NH4)2[MoS4] Pnma 2.186 0.015 95

Rb2[MoS4] Pnma 2.1917 0.0135 31

Cs2[MoS4] Pnma 2.1935 0.0126 99

K2[MoS4l Pnma 2.2000 0.0243 100

[Co(dien)2] 2 [MOS4] [M00 S3] P2 1 /n 2.2041 0.1541 101

tmenH2[1■40S4 P21/n 2.1983 0.0289 102

(1,3-pnH2[Moa4] P21/c 2.1882 0.0183 103

(trenH2)[MoS4].H20 P21/c 2.1951 0.0281 103

pipH2[MoS4 P21/c 2.2114 0.0431 103

(1,4-bnH2)[MoS4] P-1 2.1992 0.0243 104

[(EO4N] 2 [MOS4] P-1 2.187 0.016 97

(trans-1,2-cnH)2[MoS4] C2/c 2.1876 0.0125 104

[(Pir)4N] 2[MoS4 C2/c 2.1928 0.0179 103

[Co(tren)3][MoS4] Fdd2 2.1901 0.027 35

[Ni(en)3][MoS4] Pna2i 2.1865 0.0099 35

[Mn(dien)2][Moa4] 1-4 2.1765 0 37

[(n-Bu)41\1]2[MoS4i Fdd2 2.2047 0.0255 106

(trienH2) [MoS4 Pca21 2.200 0.027 107

(DabcoH)(NR4) [MoS4] P2 1 3 2.1854 0.0092 108

DipnH2[MoS4] Pca21 2.1903 0.0186 109

enH2[MoS4] P2 1 2121 2.1846 0.0111 110

N-Me-enH2[MoS41 P21212 1 2.2014 0.0379 109

Several Mo/W-S complexes containing sulfide, disulfide, tetrasulfide bonded

to Mo or W in different oxidation states have been structurally characterized,

indicating the versatile ligational behaviour of the (S) x-2 ( x = 1, 2, 3, 4 etc). The

structures of the homometallic complexes [M359] 2- (M = Mo, W) consists of two

tetrahedral bidentate [MoS4] 2" units bound to a central square pyramidal {MS} 2+ [48].

Similarly the trimetallic complex [W3S8] 2- consists of a central W 2+ bound to [W542-

17

units one on each side [45]. The central W atom has a square planar coordination of S

atoms, while the terminal W atoms have a tetrahedral coordination.

Table 1.6.3 Comparative structural data for tetrasulfidotungstates

Compound Space

group

W-S (long)

(A)

Difference

A (A)

Reference

(NH4)2[WS4] Pnma 2.2090 0.0234 96

Rb2[WS4] Pnma 2.2053 0.0343 23

Cs2[WS4] Pnma 2.2079 0.0164 111

K2[WS4] Pnma 2.1901 0.0143 112

[Mn(en)3][WS4] Pbca 2.189 0.017 113

(1,4-dmpH2)[WS4] Pbca 2.1943 0.0355 114

(PiPell)2[WS4] P21/n 2.202 0.015 115

[(Et)4N]2[WS4] P21/n 2.2030 0.0184 116

tmenH2[WS4] P21/n 2.1995 0.0223 117

[Ni(tren)2][WS4] P21 /c 2.2122 0.0542 118

(pipH2)[WS4] P21/c 2.2147 0.0385 119

(trenH2)[WS4] H2O P21 /c 2.1997 0.0258 119

mipa)2[WS4] C2/c 2.2126 0.0334 120

(trans-1,2-cnH)2[WS4] C2/c 2.1978 0.0104 121

(PPh4)2[WS4] C2/c 2.1952 0.0047 122

(1,4-bnH2)[WS4] P-1 2.2030 0.0231 120

[Ce(DIVfF)8]2[WS4]3.H20 P-1 2.188, 2.178, 2.183

0.015, 0.023, 0.027

73

DipnH2[WS4] Pca2 1 2.2053 0.0173 121

[Ni(en)3][WS4] Pna2 1 2.190 0.028 113

enH2[WS4] P2 1 21 21 2.1943 0.0092 123

N-Me-enH2[WS4 P2 1 21 2 1 2.2064 0.0337 120

In general the M-Stern, bond lengths are shorter than the M-Si n. bonds (term is

terminal, br is bridging). The Mo-S bond length in the polymeric complex

NH4CuMoS4 is 2.19 A. The structure of this complex consists of a tetrahedral

arrangement of S atoms around each Mo and Cu atoms with each tetrahedron sharing

18

two corners with two of its neighbors [124]. The sulfidomolybdate can be visualized

as doubly bridging ligand in this complex.

1.7 Oxidometalate of group VI metals

Group VI metals Mo and W exhibit a wide variety of stereochemistries as well

as a variety of oxidation states. Their chemistry has been described as the most

complex of those of the transition elements by Cotton [125]. The emerging chemistry

of polyoxometalates [126] which exhibit fascinating structures like giant wheels [127,

128], giant wheels linked to chains [129], giant molecular spheres of the Keplerate

type[130, 131], giant molecular baskets [132] etc. is a testimony to the complex

chemistry of Mo. In recent years, several polyoxomolybdates have been structurally

characterised, representative examples being Na7[Mo7024]OH.21H20 [133],

(dapH2)2[Mo8026] [ 134] (claPH2 = 1,3-propanediammonium), (dienH3)2[M09030]

[134] (dienH3 = diethylenetriammonium), K4M0360112(1120)16] [135] and

[La(H20)7A1(OH)6M06018].4H20 [136]. Interestingly all these complexes have been

isolated from acidic media. The formation of the different oxomolybdates is pH

dependent. Acidification of an aqueous alkaline solution of [Mo04 2- results in the

condensation of the tetrahedral MoO4 units giving rise to polynuclear oxomolybdates.

The ultimate product of the acidification is MoO3, while the first product of the

acidification reaction, at a pH below about 6, is the heptamolybdate [137] [M07024] 6-

[m00412- [M07024]6-

[M08026t

Acidification

MoO3 (1.26)

which is made up of exclusively MoO6 octahedra in which all the metals retain the +6

state. The heptamolybdate has been shown to be the predominant intermediate [138]

and has been isolated with a variety of cations from acidic media. From a structural

point of view [M070246- is very interesting in view of its flexibility to exist in

19

different structural environments, with different counter cations [139-146]. Anions

with 8 and 36 Mo atoms are also formed before the increasing acidity suffices to

precipitate the hydrous oxide. Conversely, when MoO3 is dissolved in aqueous alkali

or an organic amine, the resulting solution contains tetrahedral [Mo04 2- ions and

simple or normal molybdates can be crystallized from this solution [147-148].

The [M04] 1" tetrahedron can be used as an acentric building block to create

non-centrosymmetric materials. The reported synthesis and structural characterization

of complexes such as [M(py) 4Cr207] (M= Cu, Zn; py = pyridine), which exhibit non-

linear optical properties [149], is an important reason for the renewed interest in the

chemistry of dichromate complexes associated with organic cations. Dichromates are

well known for their use in organic syntheses to effect a variety of synthetic

transformations [150]. Chromium(VI) based reagents are an important class of

compounds, which are extensively used in organic syntheses to effect a variety of

synthetic transformations [151,152]. The commonly used Cr(VI) reagents are

chromium trioxide (Cr03) used in solvents such as acetic acid or acetic anhydride or

pyridine, a mixture of sodium dichromate and concentrated sulphuric acid and

pyridinium chlorochromate [153,154]. Chromium(VI) reagents in combination with

amines have been widely used for the oxidation of alcohols to the corresponding

carbonyl compounds [155]. It has been shown that the nature of the amine determines

the oxidizing power of the chromate ion and this is inversely related to the donor

strength of the associated amine ligand [155,156]. The clue to the differing oxidizing

property lies in understanding the nature of cation-anion interactions.

Hence, the organic ammonium salts of Cr(VI) and Mo(VI) compounds are

also investigated in the present work.

SCOPE OF THE PRESENT WORK

In recent years, several extended network containing sulfidometalates have

been assembled under solvothermal conditions. In this method metal salt, elemental

sulfur, an organic amine and oxometalate / thiometalate are reacted at a high

temperature and pressure. These reactions lead to formation of unusual products,

20

which exhibit novel as well as interesting structural features. These reactions are

useful for the synthesis of newer sulfidometalate materials. The drawback to these

reactions is the unpredictability of the final product. In addition to this, special

equipment needed such as Teflon vessel, steel autoclave and controlled temperature

oven for their synthesis. In spite of this, the solvothermal method is slowly emerging

as an important synthetic route for the preparation of newer complexes, as a phase

pure products are obtained in good yields.

The aim of the present investigation is to develop simple synthetic strategies

for the convenient synthesis using readily available starting materials. Chemicals such

as molybdic acid/ tungstic acid, ammonia, organic amines, hydrogen sulfide gas,

water etc. have been used. The H2S gas used as a sulfiding agent was generated by the

reaction of dilute HC1 with ferrous sulfide sticks. Several tetrasulfidometalate [MS4 2-

complexes have been reported. And the structural characterization of

tetrasulfidometalate [MS4] 2- complexes stabilized by organic ammonium cations has

been established. Various reaction of the group VI tetrasulfidometalate with organic

amine as described in the scheme IV has been investigated in this study. The property

of organic amines as a structure-directing agent is utilized to give hydrogen bonded

supramolecular networks. The organic amines functions as a hydrogen bond donor

and the anions functions as acceptors in the supramolecular networks. This weak

interaction has been investigated in the structure which influences the structure of the

compounds. It has been reported that the chemical properties of [MoS4] 2- can be

substantially changed by surrounding it with organic ammonium cations. These

compounds serve as a precursor to obtain highly dispersed carbon contaminated

metalsulfides. Hydrogen bonding interactions lead to the distortion of tetrahedron

[MS42" and lengthening of M-S bond distances, and this can be effected by using a

variety of organic amines. Thus H-bonding interactions in the compounds can be fine

tuned by a proper choice of the amine, which differs in its bulkiness and also the

number of potential H-bonding donors attached to nitrogen atoms.

In view of this chemistry of various amines (listed below) with anions such as

sulfidometalate, molydates and thiosulfate has been investigated and discussed in later

chapters.

21

NH2 H3C CH3

iso-propylamine

CH3 NH2

H3C

tert-butylamine

NH2

a-methylbenzylamine

CH3NH2 C2 H5NH2 n-C3 H7 NH2 n-C4 H9 NH2

methylamine ethylamine n-propylamine n-butylamine

NMe3 NEt3

benzyltrimethylammonium benzyltriethylammonium benzylamine

NH

.,

NH2

piperazine

ethylenediamine

NH

N-methyl

\ NH/

N-ethyl piperazine

piperazine

N-ethylamine piperazine

dabco

NH

NH2

hexamine

Diethylenetriamine

NH2

NH

NH2

Scheme III

22