Arylazoimidazole compounds of rhenium nitrosyl: Synthesis, spectral characterization and reactivity

Polyhedron 26 (2007) 1217–1221 www.elsevier.com/locate/poly

Arylazoimidazole compounds of rhenium nitrosyl: Synthesis, spectral characterization and reactivity Pampa Pratihar a, Anup Kumar Dasmahapatra b, Chittaranjan Sinha a

a,*

Department of Chemistry, Jadavpur University, Kolkata 700032, West Bengal, India b Department of Chemistry, Midnapore College, Midnapore 721101, India Received 10 September 2006; accepted 12 October 2006 Available online 24 October 2006

Abstract Reductive nitrosylation of ReO4  in aqueous alkaline medium by NH2OH Æ HCl furnishes the Re(NO)3+ moiety which reacts with arylazoimidazoles to give hitherto unknown arylazoimidazole (RaaiR 0 ) complexes of rhenium nitrosyl, [Re(NO)(OH)3(RaaiR 0 )]. The complexes are non-electrolytes in CH3CN. They exhibit t(NO) at ca. 1700 cm1 and are magnetically active. The ESR profiles in the polycrystalline state at 298 K show Ægavæ  2.0. A spin forbidden ESR transition (DMs = 2, g  4) is observed at <1600 Gauss. A well defined sextet due to the metal hyperfine structure is observed. The complexes exhibit a moderately intense visible band at 440– 450 nm which may be ascribed to a metal-to-ligand charge transfer transition, along with intraligand charge transfer, p ! p* and n ! p*, transitions at <400 nm. Cyclic voltammetry exhibits quasireversible to irreversible metal oxidation and ligand reductions. The reaction of [Re(NO)(OH)3(RaaiR 0 )] with camphor in alkaline medium has assisted C–N bond fusion by synthesizing a camphorquinone monoxime complex.  2006 Elsevier Ltd. All rights reserved. Keywords: Reductive nitrosylation; Re(NO)3+-azoimidazole; Magnetism; Cyclic voltammetry; C–N bond fusion

1. Introduction Nitric oxide (NO) is one of the important signalling and regulatory molecules in living organisms. The key role of NO in human cardiovascular, nervous systems and in the immune response to pathogen invasion has resulted in further interest in nitrosyl transition metal complexes [1–6]. Therefore, attention has been focused on the synthesis, structure, spectroscopic properties and reactivity of coordinated NO groups. Reactions of coordinated NO have been employed in catalyses, production of organo-nitrogen compounds and pollution control. Metal–nitrosyl complexes (M–NO) have been synthesised by direct substitution or addition reaction of NO [7,8] or NOX [9] with a suitable metal complex, acid hydro-

*

Corresponding author. Fax: +91 33 24146584. E-mail address: [email protected] (C. Sinha).

0277-5387/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.10.029

lysis of metal nitrites (M–NO2) [10] or reductive nitrosylation of tetraoxometallates [MO4]n by alkaline NH2OH [11]. Recently, we have described the chemistry of Ru– NO complexes of arylazoimidazoles which have been synthesized by nitrite hydrolysis of [Ru(NO2)2(RaaiR 0 )2] (RaaiR 0 = 1-alkyl-2-(arylazo)imidazoles) [10]. The reactivity of coordinated NO has encouraged us to explore the nitrosyl chemistry of other transition metals. In this work we describe the Re–NO chemistry of arylazoimidazoles (RaaiR 0 ). Ligands belonging to the azoimine, AN@NAC@NA, group of p-acidic N,N 0 -chelating systems and their coordination chemistry is presently an active area of research [12–19]. The azoimine function has the specialty to stabilize lower oxidation states of metal ions and tune the redox and spectroscopic properties of the metal centre [12–15]. The richness of the chemistry of azoimine with ruthenium and osmium has prompted us to develop the parallel chemistry of the rhenium nitrosyl framework. Rhenium nitrosyls with

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a N,N 0 -chelating system like bpy (2,2 0 -bipyridine), phen (1,10-phenanthroline) have been reported [11,20]. The scarcity of the rhenium nitrosyl core with the azoimine function has created profound interest in studying the reactivity of the Re(NO)3+ moiety towards arylazoheterocycles. In order to explore the rhenium–nitrosyl chemistry of RaaiR 0 we have used the reductive nitrosylation process. In an aqueous alkaline medium ReO4  undergoes reductive nitrosylation by NH2OH Æ HCl, furnishing the Re(NO)3+ moiety in solution. The moiety is isolable as M[Re(NO)(OH)4] (M = Ph4P or Ph4As) salts or as [Re(NO)(OH)3(L-L 0 )] (L-L = diimine ligand like bpy or phen) [11]. Addition of RaaiR 0 to this solution has separated a brown red solid complex of the composition [Re(NO)(OH)3(N,N 0 )] (N,N 0 ) = azoimine ligand, RaaiR 0 where R = H; R 0 = H (Pai-H; 2-(phenylazo)imidazole, 1a), Me (Pai-Me; 1-methyl-2-(phenylazo)imidazole, 1b), Et (Pai-Et; 1-ethyl-2-(phenylazo)imidazole, 1c): R = Me; R 0 = H (Tai-H; 2-(p-tolylazo)imidazole, 2a), Me (Tai-Me; 1-methyl-2-(p-tolylazo)imidazole, 2b), Et (Tai-Et; 1-ethyl2-(p-tolylazo)imidazole, 2c)]. The spectroscopic characterisation and reactivity of the compounds are reported in this work. 2. Experimental 2.1. Materials The starting material, K[ReO4], was of extra-pure quality and obtained from Aldrich. The AR grade solvents used for physicochemical studies were further purified by a literature method [21]. Water of high purity was obtained by distilling deionized water from K[MnO4]. The ligands, 2(arylazo)imidazoles (RaaiH) and 1-alkyl-2-(arylazo)imidazoles (RaaiR 0 ) were prepared by the reported procedure [15]. 2.2. Physical measurements UV–Vis spectra were recorded on a Lambda 25, Perkin– Elmer UV–Vis spectrophotometer. IR spectra (KBr disk) were recorded on RX-1 Perkin–Elmer FT-IR (4000– 450 cm1) and JASCO Model 420 FT-IR (4000– 200 cm1) spectrophotometers. Electrical conductivity in a MeCN solution was measured with a Systronics 304 digital conductivity meter with a solution concentration of ca. 103 mol dm3. Magnetic susceptibilities were obtained from a vibrating sample magnetometer PAR 155 model. Microanalytical (C, H, N) data were collected on a Perkin–Elmer 2400C elemental analyzer. ESR spectra were measured in MeCN solution at 298 K using a Bruker ESR spectrophotometer model EMX 10/12, X-band ER 4119 MS cylindrical resonator. All pH measurements were made with an E.C. (India) digital pH meter (model 5651). Electrochemical measurements were carried out with the use of a computer controlled EG&G PARC VersaStat model 250 Electrochemical instrument using a Pt-disk working

electrode and Pt-wire auxiliary electrode. The solution was IR compensated and the results were collected at 298 K. The reported results are referenced to SCE in MeCN and are uncorrected for junction potentials. [n-Bu4N][ClO4] was used as the supporting electrolyte. 2.2.1. Preparation of tri-hydroxo-nitrosyl-{N(1)-methyl-2(phenylazo)-imidazole}rhenium(II) hydrate, [Re(NO)(OH)3(Pai-Me)] Æ H2O, (1b) generation of the Re(NO)3+ moiety A mixture of K[ReO4] (0.11 g, 0.38 mmol) and NH2OH Æ HCI (0.36 g, 5.25 mmol) was dissolved in 25 ml water. The solution was heated to ca. 80 C for 10 min with constant stirring. Solid NaOH beads were gradually added to the stirred solution until it turned red and the pH increased to ca. 10.5. The red solution was stirred at 80 C for half an hour and cooled. NH2 OH  HCI

ReO ReðNOÞ ƒ! 4 ðaqÞ ƒƒƒƒƒ 

3þ

Excess OH

ð1Þ

2.2.2. Preparation of [Re(NO)(OH)3(Pai-Me)] The pH of the red solution was lowered to ca. 5–6 by adding 1:1 HCl. To an ethanolic solution of 1-methyl-2(phenylazo)imidazole (Pai-Me) (0.13 g, 0.70 mmol) the red solution was added dropwise whilst stirring. Immediately a buff coloured precipitate appeared. The solution was stirred for ca. 10 min and the precipitate was filtered, washed with water, ethanol and diethyl ether, and finally dried over fused CaCl2 under vacuum. The yield was 0.12 g (67%). All the other complexes were prepared by the same procedure and yield varied from 60% to 70%. 3þ

ReðNOÞ ðaqÞ þ Pai-Me ! ½ReðNOÞðOHÞ3 ðPai-MeÞ ð2Þ Anal. Calc. for C9H13N5O5Re (1a): C, 23.60; H, 2.84; N, 15.34. Found: C, 23.68; H, 2.80; N, 15.43%. UV–Vis spectral data in MeCN (kmax) (nm) (103) 2 (dm3 mol1 cm1): 470 (2.30), 375 (8.48). FT-IR data (KBr) (m, cm1): 1700 (NO), 1605 (C@N), 1437 (N@N). Anal. Calc. for C10H15N5O5Re (1b): C, 25.44; H, 3.18; N, 14.84. Found: C, 25.53; H, 3.24; N, 14.93%. UV–Vis spectral data in MeCN (kmax) (nm) (103) 2 (dm3 mol1 cm1): 465 (1.20), 368 (9.23). FT-IR data (KBr) (m, cm1): 1697 (NO), 1600 (C@N), 1444 (N@N). Anal. Calc. for C11H17N5O5Re (1c): C, 27.18; H, 3.50; N, 14.42. Found: C, 27.32; H, 3.40; N, 14.52%. UV– Vis spectral data in MeCN (kmax) (nm) (103) 2 (dm3 mol1 cm1): 460 (2.72), 370 (15.80). FT-IR data (KBr) (m, cm1): 1700 (NO), 1600 (C@N), 1438 (N@N). Anal. Calc. for C10H15N5O5Re (2a): C, 25.44; H, 3.18; N, 14.84. Found: C, 25.40; H, 3.14; N, 14.80%. UV–Vis spectral data in MeCN (kmax) (nm) (103) 2 (dm3 mol1 cm1): 463 (1.90), 372 (9.30). FT-IR data (KBr) (m, cm1): 1698 (NO), 1600 (C@N), 1445 (N@N). Anal. Calc. for C11H17N5O5Re (2b): C, 27.18; H, 3.50; N, 14.42. Found: C, 27.27; H, 3.43; N, 14.50%. UV–Vis spectral data in MeCN (kmax) (nm)

P. Pratihar et al. / Polyhedron 26 (2007) 1217–1221

(103) 2 (dm3 mol1 cm1): 446 (2.30), 368 (10.70). FT-IR data (KBr) (m, cm1): 1698 (NO), 1600 (C@N), 1438 (N@N). Anal. Calc. for C12H19N5O5Re (2c): C, 28.82; H, 3.80; N, 14.01. Found: C, 28.70; H, 3.72; N, 14.12%. UV– Vis spectral data in MeCN (kmax) (nm) (103) 2 (dm3 mol1 cm1): 462 (7.20),383 (13.80). FT-IR data (KBr) (m, cm1): 1689 (NO), 1598 (C@N), 1444 (N@N). 3. Results and discussion 3.1. Synthesis An aqueous solution of K[ReO4] was treated with NH2OH Æ HCl and NaOH to prepare a red solution of the Re(NO)3+ moiety (Solution-A). An ethanolic solution of 1-alkyl-2-(arylazo)imidazole (RaaiR 0 ) was added dropwise to Solution-A whilst stirring at room temperature. Brown red complexes of the composition [Re(NO) (OH)3(RaaiR 0 )], namely [Re(NO)(OH)3(Pai-H)] Æ H2O (1a), [Re(NO)(OH)3(Pai-Me)] Æ H2O (1b), {Re(NO)(OH)3(Pai-Et)] Æ H2O (1c), [Re(NO)(OH)3(Tai-H)] Æ H2O (2a), [Re(NO)(OH)3(Tai-Me)] Æ H2O (2b) and [Re(NO)(OH)3(Tai-Et)] Æ H2O (2c), were separated. NO N

NO N

OH

N

R

R

Re N N

Re N N

OH

N

OH

OH OH

1219

sharp and symmetrical t(NO) vibrational band at ca. 1700 cm1. The t(NO) stretch suffers a blue shift by 10– 15 cm1 relative to the corresponding bpy and phen complexes [11,20]. The complexes exhibit a moderately intense band at 1435–1445 cm1 which may be ascribed to t(N@N) vibrations. The said frequency is also blue shifted by 40– 65 cm1 in the complexes relative to the free ligand values. The chelated azoimine function receives electrons by dp(M) ! p*(azo) means which usually lowers the t(N@N) stretch [17–19,22]. In the present series of complexes two p-acidic groups, NO+ and N@N, are competing for same dp(Re(II)) electrons. The stronger p-acidic NO+ accepts p-electrons more efficiently than the azoimine grouping and probably weakens the p back donation that raises the t(N@N) stretching frequency. Other bands at 790, 775, 700–600 cm1 corresponding to the imidazole moiety are observed. The solution electronic spectra of the complexes were recorded in the range 900–200 nm in MeCN. The complexes show absorption bands in the ranges 320–330, 365–385 and 440–450 nm (Fig. 1). The transitions below 400 nm are of high intensity and are ascribed to intraligand charge-transfer, p ! p* and n ! p* transitions [15,16] of the chelated ligand (RaaiR 0 ). The longer wavelength transition may correspond to the combination of MLCT (dp(Re) ! p*(azoimine)) and b2 ! e transitions [23]. The said bands also appear in the complexes but are blue shifted by ca. 55 and 35 nm, respectively.

OH

3.3. Magnetic properties and e.s.r data

Pai-H (1a), Pai-Me (1b), Pai-Et (1c); Ta i-R/

Ta i-H (2a), Tai-Me (2b), Tai-Et (2c)

The complexes were filtered, washed and recrystallised from a 2-methoxy-ethanol-EtOH mixture. The composition and molecularity of the complexes have been supported by microanalytical data. Molar conductance measurements in MeCN suggest non-conductivity of the complexes. Thermogravimetric (TG and DTA) analyses of the compounds reveal that one water molecule in each case is very loosely held, and this is released at 80–85 C. Although three different isomers are possible, with [Re(NO)(OH)3(RaaiR 0 )] having an unsymmetrical N,N 0 chelating ligand, we have not been able to separate them. 3.2. IR and electronic spectra The isolated complexes are all monomeric as is evident from the microanalytical data. The hydroxo group and lattice held water molecules together show a single and slightly broad t(OH) IR band at ca. 3400 cm1, but the o(H2O) band is apparently occluded in the low energy tail of the t(NO) vibrational band. All the complexes exhibit a

The complexes exhibit a leff value of ca. 1.8 BM at 298 K. A slightly higher value than the spin-only value expected for a single electron [Re(II), d5, low-spin octahedral case] may be due to the negative value of the spin– orbit coupling constant and orbital non-degeneracy of the singly occupied molecular orbital (s.o.m.o) involved.

4

1.5x10 -1

Pai-R/

[Re(NO)(OH)3(Tai-R/)]

-1

[Re(NO)(OH)3(Pai-R/)]

Molar Absorptivity (M cm )

Me

4

1.0x10

3

5.0x10

0.0 300

400

500

600

Wavelength(nm)

Fig. 1. Electronic spectra of [Re(NO)(OH)3(Pai-H)] [Re(NO)(OH)3(Pai-Et)] (—) in MeCN solution.

(  ÆÆ)

and

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P. Pratihar et al. / Polyhedron 26 (2007) 1217–1221 NO N

/

NO OH

HO

OH

Re OH HO Structure-I

HO

OH

Re

N

N

OH N

Re

/ N

OH

/

Structure-II

Table 1 E.s.r. parameters of the complexes at 298 K

NO

N Structure-III

Compounda

gs b

gk c

g? c

gavd

ÆAæRe(G)

1a 1b 1c 2a 2b 2c

2.00 2.01 2.00 2.00 2.02 2.00

1.98 1.97 1.96 1.96 1.98 1.98

2.07 2.04 2.11 2.05 2.09 2.03

2.04 2.03 2.06 2.02 2.05 2.01

340–620 350–640 330–610 330–620 320–610 340–630

a

The e.s.r spectra (Fig. 2) are difficult to explain in terms of molecular symmetry. There may be three (I–III) structures; of them, structure-I is asymmetric and II, III posses Cs symmetry. The axial e.s.r. spectra may be more appropriately described by structure II or III than that of I. A well defined sextet is observed which arises from hyperfine coupling with the spin-active 185Re and 187Re nuclei [both have I = 5/2, Fig. 2] [24].

In the polycrystalline state. gs represents the g value of the derivative curve in the polycrystalline state. c For spin paired d5 system g^ > gk and gk <2. d gav = ð2g? þ gk Þ=3. b

3.4. Electrochemical studies Cyclic voltammetry was carried out in a MeCN solution using a Pt-working electrode in the potential range 1.8 V to 1.8 V in the presence of [NBu4][ClO4] as the supporting electrolyte. The cyclic voltammogram shows quasireversible to irreversible responses at 0.7–0.8 (couple I), 0.2–0.3 (couple II), 0.1 to 0.2 (couple III), 0.5 to 0.6 (couple IV) and 1.3 to 1.4 V (couple V) (Table 1, Fig. 3). The couples I and II do not have a well defined cathodic peak on scan reversal from positive to negative. Separate voltammetric drawings at variable scan rates for couples I and II exhibit similar behaviour. This suggests that the redox performances are independent of scan speed or asking potential. At slow scan rates (10– 50 mV s1)DEP remains almost constant, as do the EPa and the EPc values. However, the peak-to-peak separation

Fig. 2. X-band ESR spectrum of [Re(NO)(OH)3(Tai-Me)], polycrystalline, 298 K.

Fig. 3. Cyclic voltammogram of [Re(NO)(OH)3(Tai-Et)] (2c) in MeCN using a Pt-working electrode, SCE reference and a Pt-wire auxiliary electrode in the presence of [n-Bu4N][ClO4] as the supporting electrolyte.

of the redox couples III–V is largely dependent on the scan rate and increases from 120 mV at 10 mV s1 to 600 mV at 1000 mV s1. This observation suggests a low heterogeneous electron-transfer rate constant which has been influenced by the applied potential. The arylazoimidazoles (RaaiR 0 ) do not show any oxidation but irreversible reductive responses, in general two quasireversible couples at 0.8 and 1.3 V are ascribed to [AN@NA]/ [AN'NA] and [AN'NA]/[ANANA]=, respectively [22]. Thus, couples IV and V are reductive responses of the chelated azoimine ligand. The anodic shift of these two couples (IV and V) compared to the free ligand values may be due to electron drifting to Re(II) from the coordinated ligand. The first three couples (couples I, II and III) may be assigned to [Re(NO)] centred redox responses. Couple I is assigned to [Re(NO)(OH)3(RaaiR 0 )]+/[Re(NO)(OH)3(RaaiR 0 )] [11]. Couples II and III are assigned to successive one electron reductions of the coordinated NO+ unit; [Re–NO]+ ! [Re–NO] (couple II), [Re–NO] ! [Re–NO] (couple III). These three couples are irreversible in nature indicating the instability of the reduced species.

P. Pratihar et al. / Polyhedron 26 (2007) 1217–1221

4. Reactivity of [Re(NO)(OH)3(Tai-Me)] The nitrosyl complexes exhibit a high degree of electrophilic character [25] of the coordinated NO ligand. The electrophilic behaviour of these new complexes has been investigated by reacting with camphor (cmp), a compound having an active methylene group. An acetonitrile solution of [Re(NO)(OH)3(Tai-Me)] reacted smoothly with camphor in the presence of NaOMe, and the solution colour changed from brown-red to red-violet. The progress of the reaction was followed by monitoring m(NO), which decreased steadily from 1700 cm1, and the composition of the reaction mixture was followed by thin-layer chromatography, collecting portions of the reaction solution from time-to-time. The product analyses are in accordance with reaction (3). The reaction involves electrophilic addition of the coordinated NO to the activated

1221

tate N,N 0 chelating type. The complexes exhibit t(NO) at ca. 1700 cm1, which upon reaction with camphor in alkaline medium vanishes with appearance of a new impression at 1220 cm1. This and other spectroscopic properties suggest a C–N coupling reaction of the coordinated NO and formation of a camphoroxime complex. The precursors [Re(NO)(OH)3(azoimidazole)] are magnetically active and show Ægavæ  2.0. Cyclic voltammetry exhibits irreversible metal oxidation, and NO+ and azo reductions. Acknowledgments One of us (A.D.M.) thanks the University Grants Commission (UGC), New Delhi for financial support under a minor research scheme. C.S. thanks CSIR, New Delhi for funding. Miss Pampa Pratihar also thanks UGC for a fellowship. References

[Re(NO)(OH)3(Pai-Me)] +

O O

Re(OH)2(Pai-Me) N

3

O

ð3Þ –CH2– group, forming a bound oxime (camphorquinone monoxime). Deprotonation of the active methylene (–CH2–) leads to the generation of a carbanion which subsequently attacks the electrophilic NO+, with the synthesis of camphorquinone monoxime. The disappearance of m(NO) (1700 cm1) and the growth of m(N ! O) at 1220 cm1 strongly support the reaction. Microanalytical, spectroscopic and electrochemical results of the camphorquinone monoxime complex (3) suggest a structure of the complex which supports this coupling reaction of the coordinated NO. The compound shows a very complex pattern of proton signals in the aliphatic region, 0.8–1.0, 1.28–1.40, 2.20, 2.40 and 4.00 ppm. Camphor –CH2– and –CH– may be assigned to 0.8–1.0 and 1.28–1.40 ppm. 9-Me and 1-Me of the coordinated Tai-Me appear at 2.35 and 4.11 ppm, respectively. In the aromatic region (6.00–9.00 ppm) imidazole protons 4-H and 5-H appear as broad singlets at ca. 7.18 and 6.92 ppm, respectively. Broadening of the signals may be due to rapid proton exchange with solvent (CHCl3 from CDCl3). p-Tolyl protons (7-H to 11-H) appear at 8.32 (J = 7.5 Hz, 7, 11-H, doublet) and 7.44 ppm (J = 7.5 Hz, 8, 10-H, doublet). The cyclic voltammogram shows a peak, EPa, at 0.62 V, which suggests a Re(III)/ Re(II) couple, and ligand reductions at 0.73 (DEp = 140 mV) and 1.3 V (DEp = 220 mV).

[1] [2] [3] [4] [5] [6]

[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

5. Conclusion Rhenium–nitrosyl complexes of arylazoimidazioles are synthesized by reductive nitrosylation of ReO4  in aqueous alkaline medium by NH2OH Æ HCl. The ligands are biden-

[23] [24] [25]

K.K. Pandey, Coord. Chem. Rev. 51 (1983) 69;. J.A. McCleverty, Chem. Rev. 104 (2004) 403. K.M. Miranda, Coord. Chem. Rev. 249 (2005) 433. B. Machura, Coord. Chem. Rev. 249 (2005) 2277. G.B. Richter-Addo, P. Legzdins, Metal Nitrosyls, Oxford University Press, New York, 1992. B.L. Wescott, J.H. Enemark, in: A.B.P. Lever, E.I. Solomon (Eds.), Transition Metal Nitrosyls in Inorganic Electronic Structure and Spectroscopy, vol. 2, Wiley& Sons, New York, 1999, Chap 7. R.W. Adams, J. Chatt, N.E. Hooper, G.L. Leigh, J. Chem. Soc., Dalton Trans. (1974) 1075. P. Bandyopadhyay, S. Rakshit, Indian J. Chem. 11 (1973) 496. F. Zingales, A. Trovati, F. Cariati, P. Uguagliati, Inorg. Chem. 10 (1971) 507. P. Byabartta, S.k. Jasimuddin, B.K. Ghosh, C. Sinha, A.M.Z. Slawin, J.D. Woollins, New J. Chem. 26 (2002) 1415. R.G. Bhattacharyya, P.S. Roy, A.K. Dasmahapatra, J. Chem. Soc., Dalton Trans. (1988) 793. D. Dutta, A. Chakravorty, Inorg. Chem. 22 (1983) 1085. G.K. Lahiri, S. Goswami, L.R. Falvello, A. Chakravorty, Inorg. Chem. 26 (1987) 3365. B.K. Ghosh, A. Mukhopadhyay, S. Goswami, S. Ray, A. Chakravorty, Inorg. Chem. 23 (1984) 4633. G.K. Routh, S. Pal, D. Das, C. Sinha, A.M.Z. Slawin, J.D. Woollins, Polyhedron 20 (2001) 363, and references therein. P.K. Santra, P. Byabarta, S. Chattopadhyay, G. Mostafa, L.R. Falvello, C. Sinha, Eur. J. Inorg. Chem. (2002) 1124, and references therein. S.K. Jasimuddin, P. Byabarta, C. Sinha, G. Mostafa, T.H. Lu, Inorg. Chim. Acta 357 (2004) 2015. U.S. Ray, B.G. Chand, A.K. Dasmahapatra, G. Mostafa, T.-H. Lu, C. Sinha, Inorg. Chem. Commun. 6 (2003) 634. J. Dinda, S. Senapoti, T. Mondal, A.D. Jana, M. Chiang, T.-H. Lu, C. Sinha, Polyhedron 25 (2006) 1125. S. Banerjee, S. Bhattacharyya, B.K. Dirghangi, M. Menon, A. Chakravorty, Inorg. Chem. 39 (2000) 6. A.I. Vogel, A Text Book of Practical Chemistry, third ed., Longman, New York, 1959. T.K. Misra, D. Das, C. Sinha, P. Ghosh, C.K. Pal, Inorg. Chem. 37 (1998) 1672. J.H. Enemerk, R.D. Feltham, Coord. Chem. Rev. 13 (1974) 339. B.A. Goodman, J.B. Raynor, Adv. Inorg. Chem. Radiochem. 13 (1970) 135. T. Suganuma, A. Tanada, H. Tomizawa, M. Tanaka, E. Miki, Inorg. Chim. Acta 320 (2001) 2341.