Indian Journal of Chemistry Vol. 45A, November 2006, pp. 2406-24 1 I

Synthesis, characterisation and substitution reaction of trans-( diaqua) (N ,N / -ethy lene-bis-salicy lamide )chromi um(III) ion

s C Dash", G S Brahma", R Dash. N N Dasb" & P Mohanty'" *

"Depa rtment of Chem ist ry, Utbl Uni ve rs ity, Bhubaneswar 751 004, Imlia

hDepartment ofChemi su'y, North Orissa Uni versi ty, Baripada 757003, India

Emai l: saratehand radash @red iITmail.com

Reccil'ed 27 March 2006; rel'ised 7 Septelllber 2006

Tmlls-[Cr(Salm)(OI-I2hlNOj has becn synthesized and characte ri zed by v;l ri ous physicochemical methods. The kin eti cs of subst itution of [Cr(Salm)(O I'12)2t w ith different nucleophiles, viz., thiocyanate (SCN') , azide (N3')' imidazole (lmz), pyridine (Py), pyridine-2-carox ylic ac id (pyc-2) and pyridine-3-caroxy lic acid (Pyc-3) has been studied spect rophotomet ri ca lly at 25-45 °C (/ = 0.5 mol dm- 1 (KN03)' 4.0 :s; pH :s; 7.5 ). The reactions take place \'ia outel'sphere

association between the Cr(lll) complex and va riou s nucleophi les followed by transformation of the outer i nLO inner sphere complexes by slow interchange. k'lIl (25 °C 10-1) values are 18.9 1 (SCN'), I 1.50 (Nj ' ), 9.51 (lmz), 14.54 (Py), 17.6 (Pyc-?),

and 18.76 (Pyc-3) and the corresponding !'1!1# (kJ/mol) va lues are 63.1 ± 3.2, 70.6 ± 7.2, 46.4 ± 2.4, 40.7 ± 4.8,28.3 ± 0.2

and 38.33 ± 6.5 , the !'1S# (J K' lmo\,l ) values al'e -95.9 ± 9.5, -73 ± 2 1.5, -144 ± 3, -169 ± 16, -20 I ± 0.6 and - 172 ± 19. The

higher va lue of k'lII in compari son to kex and hi ghl y negative va lu es of activation entropy and wide variation of k'lII by changing nu cleophiles, support I" mechani sm for the substi tution react ions.

IPC Code: InL CI 8 C07F 11100

The transItion metal complexes containing polydentate Schiff's bases are important in chemical, environmental and biological fieldsl's. The complexes are immobilised or embedded on various solid inorganic/organic supports for heterogenisation of homogeneous catalyst to get the combined advantages of both heterogeneous and homogeneous catalysts9,lo. Schiff's base complexes incorporating phenolic and amide function s as chelating moieties in the same li gand are considered as models for executing important biological reactions and mimic the catalytic activites of metalloenzymes ll . Schiff base complexes of trivalent metal ions may also be useful as starting material in synthesis of precursors for bi metall ic catalyst, molecule based magnet, etc. 12

H2Salm, a potential quadridentate li gand, can undergo multiple stages of deprotonation to yield varying charges from 0-2 at different pH. On coordination to Cr(Ill), the ligand is expected to enhance labi lity of Cr(I1l) centre as also observed previously with oxalate and EDT A coordination to Cr(III) 1:1. 14. It has also been observed that the interaction of H2Salm with Fe (III) results in the formation of 1:1 complex invol vi ng Fe(OH2)r,'+ (/d path) and Fe(OH2)sOH2

+ (/" path) 15. Keeping these in mind. trans-lCr(Sa lm)(OH2h JN03 has been synthe­sised and its substitutional behaviour of coordinating

water towards a variety of inorganic and organIc nucleophi les is reported here.

Materials and Methods

Synthcsis of II 2Saim and Cl'(III) complex

The ligand (H2Salm) was prepared by reflu xing ethylenediamine with methyl salicylate and its purity was checked from elemental (C, H. N) and spectral (IH NMR, FT-IR) analyses, and m.pt. (found 184 °C, IS5°C I5). The acid dissociation constants, KI and K2 for the two-step dissociation of H2Saim ----> Salm2-, were found 6.3xlO- IO and 1.25x lO- lO

• respectivel y at 27°C (I = 0.5 mol dm-3 KN03, 40% v/" MeOH!H20) and were comparable with reported values 16.

For preparing rCr(Salm)(OH2)2]NO, complex. a mixture of concentrated methanolic solution of H2Saim ligand, aqueous solution of chromium nitrate hexahydrate {Cr(NO,h6H20} and lithium hydroxide in molar ratio 1: 1: 1.5 was refluxed at SO°C for 5 h. The resulting solution was evaporated to a small volume, to yield the desired complex as bright green solid upon cooling. The crude complex was recrystallised from methanol and water mixture. The complex was found to be readily soluble in methanol­water mixture (MeOH = 10% v/v) at ambient tempe­rature (m. pt. found 275°C).

DASH el af. SYNTHES IS, CHARACTERISATION & SUBSTITUTION REACTION OF [Cr(Salm)(OHl)21NOJ 2407

Kinetic mcasul'cmcnts

The kinetics of substitution on rCr(Salm)(OHJ2r with different nucleophiles were foll owed at 25-45°C (pH = 4-7.5, f = 0 .5 mol dm-] KNO]). Progress of the

reaction was monitored spectrophotometrically at A = 335-340 nm, ensuring pseudo-first order conditions for each run. rH+] of the solutions was adju sted using HNO] and NaOH. Rate constants (kobJ were computed from the relationship In(A= - Al) = kobs x 1 + C (Al and A=. are the absorbance of the reaction system at time ' r' and at equilibrium).

Results and Discussion

Chanlcterisation of [CI·(Salm)(OHzhlNO.\ The e lemental analysis of recrystallised complex

agreed well with the proposed mo lecular formul a. UV-vi s spectrum exhibited absorption maxima at 578 and 337 nm with extinction coefficients (c:) 115 and 4075 dm] mOrl cm - I. respec tively. These two bands

at 578 and 337 nm may be attributed to VI 4A2g (F) -> 4T:'g (F) and V2 4A2g (F) -> 4TI g (F) transition s, respec­ti vely. The hi gh-energy 4A 2g(F) -> 4TIg (P) (v]) tran si­tion is obscured by charge transfer band. Usually. the Cr(III) complexes with octahedral geometry exhibit bands at 459 and 351 nm. respectiveli 7

. The Ff-IR spectrum of the solid spec imen di splays bands at 2821 , 3072 and 3280 nm that are indicative of the coordinated water and also the amide (O=C-N- H) function ls.19

. The strong bands at 6215 and 6477 nm may be due to C=O stretch of the amide and HOH bending, respectively. Strong band at 12136 nm is indicative of outersphere nitrate l8 Besides. bands of

medium intensity at (7230, 8000, 8688, 13 193) nm are al so observed. The methanolic solution (10-] mol dm-3

) of Cr]+ complex has the molar conductivity

(11M) value 83 ohm- I cm:' mol - I. indicating that

complex behaves as 1: 1 electrolyte in the solvent used.

The TG-DTA showed three step decompositi on. Tlie first stage exhibiting weight loss (-8.6%) in the temperature range 100-150°C with a sharp endothermic peak is attributed to loss of coordinated water molecule from Cr(IIl) centre. The overlapping second and third stages weight losses are attributed to the decomposition of organic moiety followed by outersphere nitrate ion. Thi s is also evident from multiple endothermic followed by exothermic peaks due to decomposition of nitrate . Complete decompo­sition of the complex leading to Cr20] was achieved at -500°C.

The value of magnetic moment (~leff = 3.93 BM) agreed well with that expected for spin only moment of Cr(llI) with three unpaired electrons2o . The X-band ESR spectra of the complex. recorded at room temperature, di splayed broad signals with g" ,,=2.0 1 {g",,=1I3(gll +2g-L)} supporting the usual spin state of Cr(III) . Based on the experimental findings, tentati ve structure for the [Cr(Salm)(OH 2ht complex is given in Scheme 1.

Acid dissociation of trans-[Cr(Salm)(OIlz)2t

Fi gure 1 depicts spectral changes for [Cr(Salm ) (OH2hr as a function of pH upon rOH-] of be ing increased from 2.0xlO-] to 1.2x lO- 1 mol dm-] (I = 0.5

NulNu- = N) -, SeN-, Imidazole, pyridine, pyridine-2-carboxylate and pyrine-3-carboxylate

Scheme 1

2408 INDIAN J Cf-IEM. SEC A, NOV EMB ER 2006

mol dm-3 KN03). The increase and margi nal shift of absorbance band at A ",340 nm is attributed to progressive increase in the formation of hydroxo species of the complex with in creasing [OH1 The absorbance data at different pH va lues and at A = 337 nm, were fitted to Eq. (3) for determination of di sso­ciation constant of coordinated water.

Q) u c

05~----------~~--~--------~

·1

E L O· 25 o Vl

-!:l <:{

o·oL-----~------~------~--~ 260 300 360 380

Wavelength, nm

Fig. I - S pectral changcs of [Cr(sallll)(OH 2)2t as a fun ction of fll-I at 2S oC , I Cr(sallll )(O H2)2r T = 5.0x I 0.3 1ll01.dl11·3 with [Ol-I' J = 0.002 ( 1),0005 (2), O.OOS (3),0.0 I (4),0.05 (S), O.OS (6), 0.09 0), and 0.12 11101.dlll·3 (8) .

K O}I

----'-

-----[Cr(Salm)(OH2)(OH)] + H20 .. . (1)

Kd

[Cr(Sa lm)(OH2ht ~ [Cr(Sa lm)(OH2)(OH)1 + H+ ... (2)

where, Eobs and Eo are the molar extincti on coefficients of the Cr(sa lm) complex in presence and absence of alkali . The value of KOII ' deri ved from the intercept (= 6.00 ± 0.13) to slope (= 6.36 ± 0.14 x 10-3

) ratio of the (Ellbs - E,,)I versus [OHT I plot is found to be 943.4. Using this, the value of Kd (= KOII x Kw) is calcul ated and found to be 9.43x l0- 12 (p Kd=11.02 at 1=0.5 mol dm-3 and 27 DC).

Stoichiomeh'Y and reaction Jlnlducts The absorption spectra of titl e complex with

different nucleophiles (after keeping for 3 h at 25°C) showed an increase of absorbance between 300-350 nm, indicating the interaction of nucleophiles with rCr(Salm)(OH2ht. The formation of 1: 1 metal to li gand anati on product was further evident from Job 's curve whi ch showed only one maxi ma at mole fracti ons (Xi)' s of Cr(III) complex 0.5.

Sllhstitlltion reactions of [Cr(Snlm)(II 20ht

E.fIect (~r complex cOllcelltratioll In the first set of kinetic ex periments,

[chromium(III)] complex was va ried in the range 2 . 0x lO-~ to 7.0xl0-4 mol dm-3 keeping the [Nuh (Nu=enterin g nucleophile) , pH and other parameters

Table I - Rate constan ts for substituti on of [C r(sa llll)(OH2)2t by di l'i'crent nuc leoph iles at 25.0 ± 0. 1 0c, 1= 0.5 11101 dlll-3 (KN03) ,

IC r(sallll )h = 2.0 x 1 0-~ 11101 d ill". 2(J!. ( 1'/11) MeOH- H20 l11ed ium"

I nu cleophileJ-r Il11z No' SCN ' Py Pyc-2 Pye-3 (11101 dlll-') 104

"obs (s- I)I>

0.05 1.53±0.07 (I.S) 14S±004 ( 1.5) 2.S5±0.OS (2 .6) 1.94±0. 11 (2 .0) 2. 19±0. 11 (2. 11 ) 3. 13±0.33 (2 .5)

007 2. 19±0.04 (2.2) 2.00±0.07 ( 1.9) 342±0. 11 (3.6) 2 .59±0. 10 (2 .7) 3.I S±0.13 (3. 1) 337±0.26 (3.2 )

0. 10 2.78±0.11 (2.6) 2.66±0.10 (2.6) 4 .57±0.09 (4.6) 346±0.09 (3 .2) 3.9S±0. 10 (4.0) 4 .25±0.20 (4.3)

0.12 31S±007 (3 .2) 307±009 (30) S. 19±0. 12 (52) 3.99±0.16 (4.0) 4.56±009 (4.6) 4 .75±045 (5.0)

0.15 4.02±0. 14 (3.6) 347±0.12 (3 .6) 599±009 (6.0) 4.54±0.14 (4.S) 5.20±0. 11 (5 .3) 5.57±0.82 (5 .S)

0.20 4.SI±0.13 (44) 4 .2 1±0. 11 (44) 7.15±0. 14 0 ·1) 546±0. IS (5 4 ) 6.27±0. 16 (6.2) 6 .58±047 (6.S )

0.25 5.29±0.22 (5. 1) 5.00±0. 12 (5.2) 8.50±0. 13 (8 4 ) 649±0. 12 (66) 74 3±001 (4) 84 2±0.38 (82 )

0.27 541±0. 13 (53 ) 5.3 1 ±0. 12 (5.2) 8.96±0. 13 (88) 6.84±0.07 (6S) 7.86±0.14 (7.6) 8.S3±041 (85 )

0.30 5.55±0.10 (5 .8) 542±0. 14 (54) 9.18±0 10(9.2) 70 1±0. 13 (7.0) 8.65 ± 0.20 (8.65) 9.75 ± 0.91 (9.5 )

"pH. 6.60 ± 0.05. (1111 7.); 4.72 ± 0 .03. (N, '); 609 ± 0.03. (SCN'); 5.25 ± 0.03. (py); 4.88 ± 0.06, (pyc-2); 4 .83 ± 0.04, (pyc-J); bVa lucs in the parenth es is are ca lcul ated I 04k (S- I)

DASH el (II.: SYNTHES IS, C HARACTERISATION & SUBST IT UTION REACT IO N OF ICr(Salm)(OH 2hlN03 2409

constant. The pseudo-first order constants at [Nuh=O.1 mol dm-3 were found I O~ kobs = 2,71 ± 0.08, 4.41 ± 0, 13,2.80 ± 0.07, 3.49 ± 0,06,3.95 ± 0.09 and 4.23 ± 0.21 S·l for N3' , SCN-, Imz (imidazole), py (pyridine), pyc-2 (pyridine-2-carboxylate) and pyc-3 (pyridine-3-carboxylate) , respecti vely. The indepen­dellCe of kob, under identical conditions of pH. temperature and ioni c strength over a range of I Cr(III)] is in agree men t wi th fi rst order dependence of the reaction rate on lCr(IIIHr. The rate law is, therefore, given by Eq. (4),

... (4)

Effects oj [NIIc/eophileh alld {Irh Oil rate (!(allatioll

The observed pseudo-first order rate constants, kobs ' for the anation reaction of [Cr(Salm))(OH2hf by different nucleophiles at varying concentrations are given in Table 1. It is found that the kob, increases with increase in [Nu}r. The plots of kobs ve rsus [Null' are non-linear with practica ll y zero intercept indicating that the backward reaction is in signifi cant under the experimental conditions. Anation reac ti ons of aqua metal ions with various nucleophiles mostl y follow Eigen-Wilkins mechani sm and kobs vs [Nuh plots are found to be non linear. After limiting situation, by increasing [NuJ-r, k"bs does not change, Hence, k"b/[Nulr dec reases at a given pH.

The anation rate of lCr(Salm))(OH2h tfor different li gands at varying pH are given in Table 2. The marginal increase of kobs values with increase in pH is due to increase in the concentrati on of deprotonated form of the nucleophi les. The monodentate protonated li gand s (A H/AH+, A = N3-, imidazole and Py) under­go one-stage deprotonation (Eq. 5) whi Ie bidentate ligands (AH/, A = pyc-2 and pyc-3) undergo two­stage di ssociation (Eqs 6 and 7)

K "I

HA+/HA ~ = AIA' + H+ ... (5)

K~l

H2A+ ~ HA + H+ ~ ... (6)

K", HA ~ A- + H+

~ .. . (7)

The di ssociation constants (K"l) at 25°C in the case ofHN:" HSCN, protonated imidazole, pyridinium ion , pyc-2, and pyc-3 are 4.17x lO-s, 1.26x lO l, 4.90x lO'(' , 9.34x l0·8, 9.33x lO-2 and 8.91 x lO·3, respecti vely while

Table 2 - Effect of p H on the ratc of substitutioll of ICr(sa lm)(O H2h l+ by differcnt Iluc leophiles at 27.0 ± 0. 1 dc. 1 = 0.5 mol dm-3 (KN01), [Cr(salm)(OH2)/h = 2.0 x 10'~ mol dm-3

in 2 'f(, (vlv) McOH- H20 medium, [Ilu c leophilch = 0. 10 mol dm-·l

Nuc leophilcs

Imz

N' ]

py

pyc-2

pye-3

pH

5.00

5.49

5.9S

6 .5 1

7.02

7.4S

402

4.52

4.97

5.37

5.79

4. 11

4.62

5.16

5.59

6 .0 1

6.4S

4.02

4.47

4 .S3

5.22

5 .73

4 .00

4.50

500

5.55

6.00

104 kobs (S- I)

0.96 ± 0.0 1

1.57 ± 0 .09

2.04 ± O.OS

2. 73 ± 0.07

2.97 ±O. II

3. 11 ±O.IO

2.02 ± 0.07

2.49 ± 0.10

2.S I ± 0. 14

3. 12±0. 1I

3.34 ± O.OS

I.SO ± O.OS

2.5 1 ±0.06

3.45 ± 0.0 I

4 .43 ± O.OS

5.2 1 ± 0.1 2

6. IS± 0.22

2.95 ± 0.06

3.46 ± 0.04

393 ±0.12

5.4S ± 0.11

5.02 ± 0.06

I.S5 ± O.OS

2.S2 ± O. IS

3.08 ± 0.14

500 ± 0.27

4.03 ± 0.20

the K,,2 values fo r pyc-2 and pyc-3 are reported to be 6.17xlO-6 and 15.49xlO·6, respectively"l. Thus, under the experimental pH range the first stage di ssociations of pyc-2 and pyc-3 have been completed and the rate change with pH is due to increase of NH deprotonated fo rm of the li gands, For other monodentate ligands, Eg. (5) is valid . The hydrol ys is constant of [Cr(Salm)(OH 2hf is 9.43x I0- 12 at n °e. Since the reactions have been studi ed at pH :S 7.5, the participa­ti on of the conjugate base of the [Cr(Sa lm)(OH2ht in the substituti on reaction is less likely. Keeping thi s in view, the foll owing reaction scheme is suggested :

2410 INDIAN J CH EM, SEC A, NOVEMBER 2006

Tabl e :I - Compari son or ::lIlation ratc constants (1.:,",), outer sDhere compl ex rormation constants (Kos ) and acti vati on paramctcrs for the anation reaction or [Cr(salm)(OH")"t with that of othcr aqua Cr(lll) systems

Sys tems (Metal centre/nuc leophilc)" 1.:,," (5, 1) Kos Llf-/# kJ mo I' 1 LlS.# JK' lmol ' l Rcfs

Cr(HIO)63+/H20 ' 2 ,40x I 0 ,6 (25 °C) 108.6 I 1.6 23

Cr(H2O )63+/gly (3. 34-6 8) x I 0'4 (3 5°C) 51.9 -42. 7 24

Cr(H 20 )61+ ta l a 0 ,58x I 0'4 (35°C) 64.9 -11 3 .6 25,26

Cr(H2O )6)+/ val 2.34x I 0-4 (25 °C) 3 .29±0.1 90,4 -2 1.0 25,26

Cr(H20 )6)+ IL-ori n i th inc I .70x I 0'4 (40°C) IS.9 55 .S - 138 ± 17 27

Cr(H20 )6 3+ Ih ydrox yprol i ne 1.00x I 0 '4 (40°C) 72.9 -9 S.2 25,26

Cr(H20 )6 3+ Itryptophan I. I 7 x I 0 '4 (40"C ) 65.6 - 11 2 ,4 25,26

Cr(H20 )63+/ methionine 2.22x I 0'4 (Yi "(' 1 61.1 - 116,3 25.26

Cr(H20 )63+/glutaminc I .S I x I 0 4 \ 40"(, ) 52.0 - 150 .6 25.26

Cr(H2O )63+/pyc-3 I 0 .24x I 0 '4 (3 5"C) 5,42 28

Cr(H20 )6 J+ Iphen y l alanine I ,85x I 0'4 (3 5°C) 53 .6 -141.7 25 ,2 6

[Cr(Salm)(OH"llt IImz 9.5 I x 10'4 (25°C) 6.15 46,4 ± 2 ,4 -144 ± 3 Thi s work

[Cr(Salml(OH2l"t IN3' I 1.5x 10'4 (25"C) 4.35 70 .6 ± 7.2 -73 ±21.5 Thi s work

l Cr(Salm)(OH")" r ISC N' IS.91 x 10'4 (25°C ) 3.16 63. 1 ± 3 .2 -95.9 ± 9 .5 Thi s work

[Cr(Salm)(OH2)"t Ipy 14.54x I 0 '4 (25°C) 6.70 40.7 ± 4. 8 -169 ± 16 Thi s work

I Cr(Salm)(OH2ht Ipyc-2 17 .6x 10'4 (25°Cl 8 .75 28,3 ± 0.2 -201 ± 0 .6 Thi s work

[Cr(Salm)(OH")"t Ipye-3 18.76x I 0 4 (25°C) 3.00 38.33 ± 6.5 - 172± 19 Thi s work

"gly = glyc ine: ala = a lanine: val = val inc: Imz = imid azole: pyc-3 = pyridinc-3-cal'box yli c ac id: pyc-2 = pyridine-2-carboxylic ac id

HA/HA+ ... (8)

{Cr(Salm)(H20 h+.A/A} ... (9) o s

K, ,,

{Cr(Salm)(H20)/ .A/A' } ---)

[Cr(Salm)(H20)Aj + H20 ... (10)

for which. the rate law is:

or, kobs ... (11)

Eq. (11) can be rearranged to Eq. (1 2)

k - I k - I robs = ~an + . .. (12)

The plots of lIkobs versus lNuh·- 1 were excellent straight lines with finite intercepts and showed the

validity ofEq. (12) . The values of ka n and Kos . derived from intercepts and slopes of such plots are also presented in Table 3.

It is generally accepted that the complex formation by metal ions proceeds by a mechanism in which the rate determining step is the change from outer sphere complex to inner sphere complex, preceded by the formation of outer sphere complex between the metal ion and the ligand. Usually, kan values are compared with water exchange rate constant at [Cr(OH2)6J'+ for prediction of It! or 101 mechanism valid for the reaction . The much higher values of ka n comparable to the isotopic water exchange rate constant of [Cr(OH2)6]'+ (4. 17x lO'6)22 support an 101 mechanism for the anation reaction of [Cr(Salm)(OH2hf with various nucleophiles.

A comparative anation rate constant (kan), outer­sphere association constant (Kos) along with acti vation parameters for different nucleophiles at (aqua) chromium(III) centre are given in Table 3. It is evident that both ka n and Kos compare well with the values reported for similar systems. The variation of kan also suggests that substitution at (aqua)Cr(lJI) centre is sensitive to nature of nucleophi Ie and hence supports an 101 mechanism2J

. The negative values of

DASH (!/ al. : SYNTHESIS. CH ARACTER ISATION & SUBSTITUTION REACTION OF [Cr(Salm)(OH2)2l N03 2411

11s" wi th moderate values of I1H# are typical for Cr(lll) substitution with associative mechanism24

. A plot of I1S" and I1H# of relevant systems with those derived for the present set of nucleophiles results in an isokinetic relationship which further supports a simi lar mechanism prevailing in the present anation reactions of [Cr(Salm)(OH2hr.

Acknowledgement One of the authors (SCD) is thankful to University

Grants Commission , New Delh i for study leave. Assistance from the Department of Science & Technology (Government of India) is gratefu ll y acknowledged .

References Challerjee D. Mitra A. Hamza M S A & Eldik van R. J Chelll Soc !Ja llon Trans. (2002) 962 and refs therein.

2 Zeng W. Li J & Qin S. Inorg Chem COlli 11 III n, 9 (2006) 10.

:I Dyers L. Jr Que S Y. VanDerveer D & Bu X R. lnorg Chim I\ C/(l, 359 (2006) 197.

4 Gupta K G. Abduslkadir H K & Chand S . .I Mol Cnral 1\. 202 (2003) 253 .

5 Luo Y & Lin J. Micropor Mesopor Maler. 86 (2005 ) 23.

6 Chaube V D. Shylesh S & Singh A P. J Mol Catal /1. 241 (2005 ) 79 .

7 Liu J-Y . Li Y-S. Liu J-Y & Li Z-S . .1 Mol Calal 1\: ChCln, 244 (2005 ) 99.

8 Vaidyanathan V G, Weyh crmull er T. Nair B U & Subramanian J . .Ilnorg iJiocil l' lIl. 99 (2005) 2248.

9 Maurya M R, Titinchi S J J, Chand S & Mi shra I M. J Mol COWl A: Chelll , 180 (2002) 20 I: Maurya M R. Titinchi S J J & Chand S. J Mol ('alai A: Chelll , 2 14 (2004) 257.

10 Zhang H. Xiang S. Xi ao J & Li c..1 Mol Catal 1\ : OW1/ .

238 (2005 ) 175 and refs therein.

II Bedioui F. Coord ChI' lli Rev, 144 ( 1995) 39 and refs therein .

12 Bernhardt P V. Bozoglian F, Macpherson B P & Martinez M, Coo rd Chelll Re v, 249 (2003) 1902.

13 Ashley K R. Lei poldt J G & Joshi V K, Inorg Chelll . 19 (1980) 1608.

14 Leipoldt J G & Mayer H, Polyhedron . 6 ( 1987) 1387.

15 Dash A C & Mishra A , lndian.l Chelll , 37 A ( 1998) 961 .

16 Tripathy S & Bhallamisra S, .I Ch elll Soc Dalton Tmn s. 1907 (1993 ).

17 Huheey J E. Kieter E A & Kieter R L. In organic Chemistr.\": Principles, Strucillre {/lui Reacli l'ilv. 4110 Edn (Harper Collins Coll ege Publishers, New York ) 1993.

18 Rao C N R, Chelllical I\pplicalioll of Infrared Speclroscop.\' (Academic Press, London ) 1967.

19 Nakamoto K, Infrared and Ralllan Speclra of Inorganic and coordination cOlllpollnds, Pan B: SIlo Edn (Wiley-Inter­science, New York) 1997.

20 Lee J D. Concise In org(lIIic Chemislrv , 41h Edn (ELB S. Chapman and Hall. London) 1991 .

2 1 Martell A E & Smith R M. Crilical Siabililv Conslanls. Vols I & 2 (Plenum Press, New York) ( 1974).

22 Tobe M L & Burgess J, In organic Reaction Medll/ni,l'lII (Longman. Ed inburgh Gate Harlow England) ( 1997).

23 Jordan R B. Reaction MechanislII of In organic and Organ ollleta llic Systems (Oxford Uni versity Press. New York) ( 1991 ).

24 Bhallacharya S K & Banerjee R N. Polyhedron . 16 ( 1997) 4217.

25 Khan A & Kabir-ud-Din , In l .I Chol/ Kinel. 17 ( 1985) 1263.

26 Khan I A. Kabir-ud-Din & Sahid M . .I Indian Chelll Soc. 69 ( 1992) 864; Khan I A, Kabir-ud-Din & Sahid M, Tran sition Mel ChCln. 12 ( 1987 ) 293 . 557: Khan I A. Shahid M & Kabir-ud-Din, Z Phys G em (Leipz.ig), 27 ( 1990) 101.

27 Mohanty S. Anand S. Brahma G S & Mohanty P . .I Indian G em Soc. 80 (2003 ) S I o.

28 Behera J. Brahma G S & Mohanty P . .I Indian Chelll Soc. 78 (2001 ) 125.