CHAPTER - III

TRIORGANO ANTIMONY (V) DERIVATIVES OF MONOBASIC

BIDENTATE LIGANDS 3.1 INTRODUCTION

Since the initial report on the preparation and characterization of

triorganoantimony (V) dihalides, there has been an appreciable amount of activity

in the preparation and characterization of triorganoantimony (V) complexes with

monodentate and bidentate Ligands(1 9).

X-ray diffraction studies have shown that triphenylantimony (V) dichloride

and trimethylantimony (V) dihalides exists as trigonal bipyramidals with the

phenyl or methyl group in the plane of antimony atom and the halogens at the

apices(9). It has also been demonstrated that triphenyl-and tribenzyl antimony

dichlorides do not conduct an electric current in acetonitrile solutions(10). It has thus

been quite clearly established that both triaryl- and trialkylantimony dihalides

contain a pentacovalent antimony atom.

In addition to the halides , a number of other anion groups like nitrate,

carboxylate and pseudohalides(11) have also been used to prepare similar type of

compounds. Long and coworkers(6), on the basis of IR spectra in KBr pellet

suggested the presence of nitrate ions in trimethylantimony (V) dinitrate . Other

workers(2,3) however, demonstrated that trimethylantimony dinitrate reacts readily

with solid potassium bromide to form nitrate ions, and that triorganoantimony (V)

dinitrate in Nujal mulls or organic solvents give an IR spectrum typical of a

covalent compound with trigonal-bipyramidal geometry. Similarly IR spectra of

trimethylantimony carbonate, chromate, oxalate(2), sulphate(6), diformate, diacetate,

dipropionate, dibutyrate and dibenzoate(3) show that these compounds are also

covalent, i.e., free ions are not present. In all these compounds antimony is

pentacoordinated and molecular structure is trigonal bipyramidal.

56

Some ligands like benzamidoxime(12) can replace both the halogen atoms of

triphenylantimony (V) dihalides and form compound of the type Ph3SbL2 ( where

L is benzamidoxime ion ). In this complex, amino group associated with

benzamidoxime remains uncoordinated and behaves as a unidentate ligand.

Antimony is again pentacoordinated in this case.

Monofunctional bidentate ligands of the type -diketones(13-15), oxinate(16-19)

and Schiff bases(20) have been used to prepare hexacoordinated organoantimony

complexes of the type R3SbXL ( where R is either methyl or phenyl group ; X is

Cl, Br , OH or OCH3 ion and L represents anion of the ligands). The general

method of their preparation was by replacement of one halogen or methoxide group

of triorganoantimony (V) dihalides or dimethoxides with a ligand anion. Goel and

Coworkers(14) synthesized Ph3SbXacac (where X is OH or Cl and acac is

acetylacetone anion ) by treating ( Ph3SbCl)2O or Ph3SbO with acetylacetone.

The most interesting feature associated with these compounds is that

antimony atom has a choice between penta-and hexacoordination depending upon

the monodentate or bidentate function of the ligands.

Hexacoordination as a result of dimerization of triorganoantimony (V)

compounds has also been proposed by Matsumara and coworkers for

triorganoantimony(V) thio-glycolates and glycolates(21).

Gopinathan and coworkers have reported the replacement of both the

bromide atoms of Ph3SbBr2 with monobasic bidentate ligands (HL)(22) to yield

complexes of the type Ph3SbL2. They have however, not assigned any structure to

these complexes. If L is behaving as bidentate ligands, the coordination number of

antimony in these complexes ought to be seven. Other workers have also used

similar bidentate ligands but they have not been able to get Ph3SbL2 , instead they

observed that only one X was replaced by L even though the ratio of Ph3SbBr2;

ligand taken was 1:2(14).

57

It is because of these conflicting reports on coordination behaviour of

antimony in triorganoantimony (V) derivatives that a study has been carried out in

the present investigation on the complexes of triorganoantimony (V) with

potentially bidentate ligands containing of triorganoantimony (V) with potentially

bidentate ligands containing at least one OH group in their structure. The

structures of the bidentate ligands used in the present work are shown in Fig. 3.1

CH3

CH2

H3C

C = O

C = O

CH3

CH2

C2H5O

C = O

C = O

Acetylacetone (acacH) Ethylacetoacetate (eaaH)

(i) (i i)

N

OH

OH

C = O

H

8 Hydroxyquinoline (oxH) Salicylaldehyde (salH)

(i i i) (iv)

OH

C = O

CH3

OH

C = O

H

o Hydroxyacetophenone (hapH) 2 Hydroxy 1 napthaldehyde (hnaH)

(v) (vi)

Fig. 3.1

58

3.2 EXPERIMENTAL

All the complexes were synthesized by the reaction of triorganoantimony (V)

dibromide with sodium salt of the ligands in 1:1 and 1:3 molar ratio in benzene and

in 1: 3 molar ratio in benzene methanol (equal volume) mixture, respectively.

Representative experimental details are given below:

(i) Reaction of triorganoantimony(V) dibromide with sodium salt of

ligand (1:1 molar ratio)

A benzene solution or triorganoantimony(V) dibromide (5 mmol) was added

to a suspension of sodium salt of ligands (prepared by the interaction of 5 mmol

each of sodium and the ligands in dry methanol; the excess of methanol was

removed under reduced pressure ) in benzene. The mixture was refluxed for 2

hours. The sodium bromide formed was filtered off and filtrate was evaporated to

dryness under reduced pressure. Recrystallization from benzene-hexane or benzene-

petroleum ether (40-60 ) mixture gave crystalline product.

All the above reactions are summarized in the Table 3.1.

(ii) Reaction of triorganoantimony (V) dibromide with sodium salt of ligand

(1:3 molar ratio)

(A) A benzene solution (50 ml) of triphenylantimony (V) dibromide (5 mmol)

was added to a suspension of sodium salt of ligands ( 15 mol ) (prepared as

reported in 3.2 (i) ) in dry benzene (100 ml). The mixture was refluxed for

two hours. The sodium bromide and unreacted sodium salt of the ligands

were filtered off and the filtrate was evaporated to dryness under reduced

pressure. The product was recrystallized from benzene- hexane or benzene-

petroleum ether (40-60 C) mixture.

61

(B) Methanolic solutions of the sodium salt of the ligand (15 mmol in 50 ml)

(prepared from 15 mmol each of sodium and the ligand in dry methanol) was

added to the benzene solution of triorganoantimony (V) dibromide (5 mmol

in 50 ml). The mixture was refluxed for 4 hours. The volume of the solvent

was reduced to 50 ml by distilling under reduced pressure. The sodium

bromide and unreacted sodium salt of the ligands were filtered. Benzene

methanol mixture was removed from this filtrate under reduced pressure.

Recrystallization from benzene-hexane or benzene-petroleum ether (40-60)

mixture gave crystalline products. All the above reactions are summarized in

Table 3.2.

3.3 RESULTS AND DISCUSSION

The reaction between R3SbBr2 and the sodium salt of ligand in 1:1 molar

ratio yielded complexes which were formulated on the basis of elemental analysis

as R3SbBrL (Table 3.3). Similar reaction was also carried out in 1:3 molar ratio

(R3SbBr2: sodium salt of ligand) in benzene in order to obtain complexes having

both the bromine replaced by the ligands, but the elemental analysis results

indicated the presence of R3SbBrL. This finding is in conformity with that reported

by Jain and Coworkers(15), where they obtained 1:1 complexes though they had

taken 1:2 molar ratio of reactants (Ph3SbBr2 : sodium salt of diketones).

In the present case, the 1:3 reaction was also carried out in benzene methanol

mixture which again yielded complexes containing only one ligand moiety per

antimony atom, viz. R3Sb(OMe)L. The only difference was that the formation of the

complexes in this case was R3Sb(OMe)L instead of R3SbBrL; the bromine was

replaced by methoxy group in the benzene methanol mixture.

64

All the complexes are crystalline in nature and are quite stable in dry air,

except Me3SbOMe(acac) and Me3SbOMe(eaa), (where acac = acetylacetone and

eaa = ethylacetoacetate anion) which were obtained as viscous liquids. They are,

however, sensitive towards moisture and even atmospheric moisture affects them.

All the complexes are white in colour except those proposed from

8 hydroxyquinoline which are yellow. They are soluble in benzene, chloroform and

acetonitrile but insoluble in carbon tetrachloride. Solubility of phenyl derivatives is

better in comparison to methyl derivatives. Cryosocopic molecular weight

determination of some of these complexes in freezing benzene shows that they are

monomeric in nature. Except a few all the complexes decompose before melting.

Bromine and antimony content of the compounds were determined as

reported in Chapter II. Analytical data, melting point and molecular weight results

are reported in Table 3.3 and 3.4.

3.3.1 IR Spectra:

The IR and Far IR spectra of some of the complexes and their ligands in the

solid state (acetylacetone, ethylacetoacetate and o hydroxyacetophenone in liquid

film) are illustrated in Figs. 3.2 to 3.8, and the important IR absorption bands and

their assignment are summarized in Table 3.5 and 3.6.

A very broad and weak band in the region ~2700 3000 cm 1 in the ligands, is

assigned to the intramolecularly bonded OH stretching. This band is not observed in

the IR spectrum of the complexes which indicates replacement of hydrogen of the

hydroxyl group of the ligand and hence the formation of Sb O bond.

The band present in the region 3040 3060 cm 1 may be assigned to C H

stretching vibration of the ligands. The C

1200 cm 1 and 765 780 cm 1,

respectively for triphenylantimony complexes, while these vibrations for

72

Trimethylantimony(V) complexes, are observed in the region 1170 1200 cm 1 and

765 780 cm 1, respectively(23 25).

In the IR spectrum of acetylacetone (acacH) a doublet of medium intensity at

1710 and 1720 cm 1 is assigned to ketonic CO group. A broad, very strong band

from 1610 1620 cm 1 includes the stretching vibrations of both the functional

groups C=C and enolic CO group(25). In the corresponding complexes only one

strong broad band in the region 1570 1590 cm 1 is observed (Table 3.5) which may

be assigned to the C O stretching mode of the chelated O bonded ligand group.

The lowering in the stretching frequency of the carbonyl group is attributed to the

resonance between C O M and C=O M linkages.

Similarly in the spectrum of ethylacetoacetate (eaaH) a very strong broad

band at 1720 1760 cm 1 may be assigned to the ketonic C=O stretching vibrations

and a medium intensity band at 1630 cm 1 for enolic C=O and C=C

stretching vibrations(26). In the complexes both these bands are replaced by a single

band at ~1590 cm 1, indicating coordination of C=O group of the ligand to the

antimony atom in the complex.

In the complexes of salicylaldehyde, carbonyl (CO) frequency of hydrogen

bonded ligand at 1660 cm 1 is shifted to 1600 and 1630 cm 1 in Ph3SbBr(sal) and

Ph3SbOMe(sal), respectively (Table 3.5), indicating coordination of the ligand

through the CO group. Similar shift is also observed in the complexes with

2 hydroxy 1 napthaldehyde (hnaH) and o hydroxyacetophenone (hapH) (Table

3.5 and 3.6).

The IR spectrum of 8 hydroxyquinoline (oxH) shows a strong band at 1580

cm 1. This may be due to the combination of C=C and C=N stretching vibrations(27).

In the complex this band is shifted to higher frequency by ~20 cm 1 (Table 3.5 and

73

3.6). The shift of (C=N) vibrations suggests the bonding of ligand molecule

through nitrogen to the antimony atom of triorganoantimony(V) moiety(22). This is

further confirmed by the presence of a band at 270 cm 1 which is assigned to

(Sb N)(28), at least in (CH3)3Sb(OMe)(ox). There is a little ambiguity in the case of

Ph3Sb(OMe)(ox) (q.v.).

In the low frequency region of triphenylantimony(V) complexes, the

symmetrical and asymmetrical (ts and tas vibrations) of Ph Sb band are observed in

the region ~265 and ~300 cm 1, respectively and the y vibrations in the range

430 480 cm 1. For trimethylantimony(V) complexes asymmetrical and symmetrical

Sb C stretching frequencies are observed at ~580 cm 1 and ~530 cm 1, respectively.

These assignments have been made on the basis of the analysis reported for

Me3SbBr2 and Ph3SbBr2 (2.28). It may be noted that in case of Ph3SbOMe(ox) the ts

band is overlapping with (Sb N) band.

A sharp band at ~1070 cm 1 in all the complexes of the type R3Sb(OMe)L

might be due to Sb OMe stretching mode(13).

A band of ~400 cm 1 which is present in all the complexes and absent in the

spectra of the ligands, may be assigned to Sb O stretching frequency(13,29).

3.3.2 PMR Spectra:

The PMR spectra of some of these complexes were recorded in CDCl3 or

DMSO d6 at room temperature using TMS as an internal standard and are

illustrated in Figs. 3.9 & 3.10. The important resonance signals have been

summarized in the Table 3.7.

75

The resonance due to the hydroxyl group of the ligands disappears in the

complexes indicating that substitution has taken place through the OH of the

ligands. This supports the IR interpretation given earlier.

The PMR spectrum of Ph3SbOMe(acac) in CDCl3 at room temperature shows

a quartet ( 7.56 7.47 ppm) due to the ortho protons of three equivalent phenyl

groups, a triplet ( 7.30 7.24 ppm) due to the meta and para protons of phenyl

group, a sharp singlet each of methane ( 5.21 ppm), methoxy ( 2.15 ppm) and

methyl ( 1.80 ppm) protons. Integrated intensities due to the phenyl, methane,

methoxy and methyl protons are in 15/1/3/6 ratio confirming coordination of only

one ligand moiety with the antimony atom in the complex.

In the PMR spectrum of R3SbOMe(sal) and R3SbOMe(hna), a downfield

shift of aldehydic proton on coordination (Table 3.7) confirms strong interaction of

antimony atom with the carbonyl oxygen of the ligand ring. Such a downfield shift

is also observed for the COCH3 protons in R3SbOMe(hap) although the shift was

smaller as expected, because the protons are attached to an adjacent carbon in this

case.

The upfield shift of aromatic protons from (8.78 7.14) ppm for oxH to

(8.64 6.52) ppm for Me3SbOMe(ox) indicates strong interaction of metal atom

with the nitrogen of the ligand ring in the complex(30). The Ph3SbOMe(ox) these

signals of the ligand overlap with those of the triphenylantimony(V) moiety and are

observed in the region (8.86 6.71) ppm and hence no definite conclusion can be

drawn from this.

3.3.3 13C NMR Spectra:

In the 13C NMR spectra of acacH (fig. 3.11) different signals are obtained due

to C=O (201 ppm) and C O (190 ppm)(31), but in the triorganoantimony(V)

76

complex of acac, only one signal is obtained at 191 ppm. Similarly two signals due

to the methyl group at 30 and 32 ppm for acacH are replaced by only one signal at

28 ppm in the complex. Further a slight shift is also observed in the CH< signal of

the ligand which changes from 99 ppm to 102 ppm on complexation. Aromatic 13C

NMR signals are observed from 151 127 ppm in the complex. A signal at 57 ppm

which was not present either in the ligand or in the parent triphenylantimony(V)

compound may be assigned to OCH3 carbon.

13C spectra of 8 hydroxyquinoline (oxH) and Ph3SbBr(ox) are shown in the

Fig. 3.12. Signals at 152.3 and 147.7 ppm may be assigned to CH=N and C O

carbon atoms of the ligand ring.(31) These signals are shifted to 148.0 and 146.0 ppm

respectively in the complex indicating coordination of the ligand to antimony. Rest

of the aromatic carbon signals in the ligand are at 138.0, 135.9, 128.4, 127.4, 121.3,

117.7 and 110.5 ppm. In the complex, the aromatic carbon signals associated with

the ligand are mixed up with the carbon signals of triphenylantimony moiety and

individual assignment is not possible.(32 34)

3.4 Structure of the Complexes:

Taking into consideration the IR, PMR and 13C NMR results octahedral

structure is proposed for all the complexes of the type R3Sb(OMe)L in which

antimony(35 38) atom is hexacoordinated. Such complexes having bidentate ligands

may exist in three possible configurations as shown in Fig. 3.13.

77

R O

R O

Sb

R

OMe

X'

X

fac R

(A)

OMe O

R O

Sb

R

R

X'

X

mer R(i)

(B)

R O

MeO O

Sb

R

R

X|

X

mer R(ii)

(C)

Fig. 3.13

For Ph3SbOMe(acac) (Fig. 3.13, R = Ph, X = X| = Me, structures B and C are

the same) only two isomers are possible. Structure A will give rise to single peaks

due to the methyl and methane protons of the acac group whereas two peaks are

expected for the methyl proton of structure B/C. Since only single peaks were

observed, structure (A) is suggested at first sight in which the methoxy group is

trans to a phenyl group. However, at room temperature the possibility of a

configuration in which the methoxy group is cis to the phenyl group (structure B

and C) cannot be ruled out completely. Since structure B and C will also give single

peaks if the molecule has stereochemical non rigid configuration. An X ray single

crystal study would be required for assignment of unambiguous structure of these

molecules.(39 41)

78

The single peak for the methyl group may also arise if the acac acts as a labile

unidentate ligand, but IR spectrum indicates that acac is bidentate.(42)

For complexes containing sal, hap and hns three geometrical isomers are

possible (Fig. 3.13, X X|). Since singlets were observed for aldehydic protons in

the complexes R3SbOMe(sal) and R3SbOMe(hna) and for methyl(43 47) protons of

the COCH3 group in R3SbOMe(hap), structure A is again suggested at the first

instance in this case as well. However, structure B and C cannot be ruled out as

discussed above.

IR and NMR interpretation of R3SbOMe(ox) confirms that in these

complexes oxinate group coordinate through both the coordinating positions N and

O; thus acting as a bidentate ligand. In these complexes antimony in again

hexacoordinated, and similar octahedral structure will be formed.

Similarly on the basis of IR spectral studies, octahedral structure is indicated

for complexes of the type R3SbBrL.(48)

The observation that the reaction of triorganoantimony(V) halide with the

sodium salts of the ligands did not yield 1:2 complexes even though an excess of

the ligand was taken is significant. It may be due to the reluctance of antimony to

show seven coordination, however, as will be seen in the next two chapters

compounds have been obtained in which antimony does show seven coordination.

This anamoly may perhaps be due to the fact that tri and tetradentate ligands

coordinate in a planar fashion and only one molecule of the ligand is required per

antimony atom. In the case of the bidentate ligands, however, two molecules are

required per atom of antimony to achieve seven coordination. It is likely that after

the attachement of one molecule of ligand, the complex so formed resists the

attachement of the other ligand molecule due to steric reasons.(49)

79

The suggested structures of individual complexes are shown below (Fig.

3.14).

C O R

C O R

CH3

HC

H3C

Sb

R

X

(i)

R3SbX (acac)

C O R

C O R

CH3

HC

C2H5O

Sb

R

X

(ii )

R3SbX (eaa)

N

O

Sb

R

XR

R

(ii i)

R3SbX (ox)

O

C O

Y Sb

R

XR

RY

(iv) to (vi)

(iv) Y = Y| = H; R3SbX(sal)

(v) Y = H, Y| = CH3, R3SbX(hap)

(vi) Y = 5,6 Benzo, Y| =H; R3SbX(hna)

Fig. 3.14

************

80

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