Polyhedron 27 (2008) 2563–2568

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Polyhedron

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Ruthenium monoterpyridine complexes with 2,6-bis(benzoxazol-2-yl)pyridineas an ancillary ligand: Synthesis, structure and spectral studies

Amardeep Singh, Gopal Das, Biplab Mondal *

Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India

a r t i c l e i n f o a b s t r a c t

+ 2+

Article history:Received 6 February 2008Accepted 2 May 2008Available online 28 June 2008

Keywords:RutheniumTerpyridine2,6-Bis(benzoxazol-2-yl)pyridineStructureSpectra

0277-5387/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.poly.2008.05.022

* Corresponding author. Tel.: +91 3612582317; faxE-mail address: biplab@iitg.ernet.in (B. Mondal).

Ruthenium monoterpyridine complexes, [1] and [2] , with 2,6-bis(benzoxazol-2-yl)pyridine as anancillary ligand, L, have been synthesized and characterized by UV–Vis, FT-IR and 1H NMR spectroscopictechniques. The formulations of the complexes were confirmed by the single crystal structure of theirperchlorate salts. In both complexes, the RuII center is hexa-coordinated in a distorted geometry. In com-plex [1]+, the ancillary ligand L behaves as a bidentate ligand; in [2]2+, however, it binds the metal centeras a tridentate ligand. The central pyridine nitrogen of terpyridine (Np,trpy) is in a cis position with respectto the central pyridine nitrogen of the ancillary ligand (Np,benz) in complex [1]+ and in a trans-position incomplex [2]2+. The cis orientation of Np,trpy and Np,benz in complex [1]+ forces L to behave as bidentate. Thequasi-reversible RuII/RuIII couple appears at 0.90 and 1.44 V versus SCE in the case of complex [1]+ and[2]2+, respectively. [1]+, in the presence of aqueous AgNO3, affords [2]2+ through an intramolecular disso-ciative interchange pathway.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Strong metal to ligand charge-transfer transitions, facile elec-tron-transfer properties and long lived 3MLCT excited states ofruthenium–polypyridine complexes make them effective fordesigning photochemical and electrochemical devices [1–7]. In thisdirection, a variety of ruthenium–terpyridine complexes incorpo-rating different kinds of bidentate and tridentate ancillary ligandshave been synthesized in recent years [8–17]. However, ruthe-nium–monoterpyridine chemistry with the tridentate 2,6-bis(ben-oxazol-2-yl)pyridine ligand, structurally analogous to the triimine2,20:60,200-terpyridine, has not yet been explored, though its chem-istry with some transition metals like copper, iron and zinc hasbeen studied well [18,19]. Due to extensive p-delocalization, the ri-gid planar tridentate ligand will afford a well defined structure forits complexes [18,19] and by virtue, like terpyridine, can bind ametal center in a meridional fashion with a bite angle of about 79�.

In the present study, we have synthesized a ruthenium monot-erpyridine complex [1]+ with the tridentate ancillary ligand, 2,6-bis(benzoxazol-2-yl)pyridine (L). The complex [1]+, in presence ofaqueous silver nitrate solution, undergoes an intramolecular disso-ciative interchange type reaction to afford complex [2]2+.

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: +91 3612582349.

2. Experimental

2.1. Materials

The starting complex Ru(trpy)Cl3 [trpy: 2,20:60,200-terpyridine]was prepared according to the reported procedure [20]. 2,20:60,200-terpyridine and 2,6-pyridinedicarboxylic acid were obtained fromAldrich, USA. 2-Aminophenol was purchased from Loba Chemie,India. Other chemicals and solvents were reagent grade and wereused as received. For spectroscopic and electrochemical studiesHPLC grade solvents were used. Commercial tetraethyl ammoniumbromide was converted into pure tetraethyl ammonium perchlo-rate by following the available procedure [21].

2.2. Physical measurements

UV–Vis spectra were recorded with a Perkin Elmer Lambda-25spectrophotometer. FT-IR spectra were taken on a Perkin Elmerspectrophotometer with samples prepared as KBr pellets. Solutionelectrical conductivity was checked using a Systronic 305 conduc-tivity bridge. 1H NMR spectra were obtained with a 400 MHz Var-ian FT spectrometer. Cyclic voltammetric, differential pulsevoltammetric and coulometric measurements were carried outusing a PAR model 273A electrochemistry system. A glassy carbonworking electrode, platinum auxiliary electrode and an aqueoussaturated calomel reference electrode (SCE) were used in a threeelectrode configuration. The supporting electrolyte was [NBu4]ClO4

and the solute concentration was �10�3 M. The half-wave poten-

2564 A. Singh et al. / Polyhedron 27 (2008) 2563–2568

tial E0298 was set equal to 0.5(Epa + Epc), where Epa and Epc are the

anodic and cathodic cyclic voltammetric peak potentials, respec-tively. All experiments were carried out under a dinitrogen atmo-sphere and were uncorrected for junction potentials. Theelemental analyses were carried out with a Perkin Elmer elementalanalyser.

2.3. Crystallography

X-ray diffraction data for the complexes were collected on Bru-ker 3-circle SMART Apex diffractometer with CCD area detectors,using graphite monochromated Mo Ka (k = 0.71073 Å) radiation,with increasing x (width of 0.3� per frame) at a scan speed of3 s/frame. The structures were solved by direct methods and re-fined by full-matrix least squares against F2 for all data, using SHEL-

XTL software [22]. Multi-scan empirical absorption corrections wereapplied to the data using the program SADABS [23]. All the non-hydrogen atoms were refined anisotropically. The hydrogen atomswere located from the difference Fourier maps and refined. Struc-tural illustrations have been drawn with ORTEP-3 for Windows [24].

2.4. Synthesis of the ligand L

A mixture of pyridine 2,6-dicarboxylic acid (835 mg, 5 mmol)and 2-aminophenol (1.09 g, 10 mmol) in 10 ml ortho-phosphoricacid was refluxed for 8 h. It was then cooled down to room temper-ature and poured into 200 ml pre-cooled water with constant stir-ring, which resulted in a dark brown precipitate. The precipitate,on washing with 20% sodium carbonate solution, afforded the darkcolored ligand, L. It was then recrystallized from hot methanolicsolution (yield = 40%).

Elemental Anal. Calc.: C, 72.84; H, 3.54; N, 13.41. Found: C,72.87; H, 3.53; N, 13.39%. 1H NMR data (400 MHz, in CDCl3) dppm:8.52 (d, 2H), 8.10 (t, 1H), 7.85 (d, 2H), 7.73 (d, 2H), 7.46–7.39 (m,4H).

2.4.1. Synthesis of the complex [1](ClO4)RuIII(trpy)Cl3(200 mg; 0.44 mmol) and the ligand, L (144 mg;

0.46 mmol) were dissolved in 20 ml ethanol, and NEt3 (0.7 mmol)was added dropwise to the mixture. It was then heated to refluxwith constant stirring for 4 h. Then the volume of the mixturewas reduced to 5 ml under reduced pressure and a pre-cooled sat-urated aqueous NaClO4 solution was added. The mixture was keptin a refrigerator overnight. The dark precipitate that formed wasfiltered off and washed with ice cold distilled water. The productwas dried in vacuo over P4O10. It was then passed through a silicagel column with a CH2Cl2/CH3CN solvent mixture. The reddishbrown color complex, [1](ClO4) was separated with 4:1CH2Cl2:CH3CN. Yield: �45%, 165 mg. Elemental Anal. Calc. for com-plex [1]ClO4: C, 52.19; H, 2.81; N, 10.74. Found: C, 52.18; H, 2.83;N, 10.76%. 1H NMR data (400 MHz, in DMSO-d6) dppm: 8.80 (d, 1H),8.73 (d, 1H), 8.54 (d, 2H) 8.3 (d, 1H), 8.14 (d, 2H), 8.03 (t, 2H), 7.96(d, 1H), 7.84 (t, 2H), 7.64–7.60 (m, 3H), 7.54 (d, 1H), 7.45 (t, 2H),7.36 (t, 2H), 7.10 (t, 2H).

2.4.2. Synthesis of the complex [2](ClO4)2

The complex [1]ClO4 (100 mg, 0.12 mmol) was dissolved in aminimum volume of acetonitrile. To this, an aqueous AgNO3 solu-tion (0.15 M, 10 ml) was added and then heated to reflux for 1 hwith constant stirring. The reaction mixture was then cool downto room temperature and filtered through a sintered glass crucibleto get rid off the precipitated AgCl. After reducing the volume ofthe clear filtrate to �2 ml, a pre-cooled saturated NaClO4 solution(�2 ml) was added, and the resulting solution was kept in a refrig-erator overnight. The precipitated complex [2](ClO4)2 was filteredoff as a brown microcrystalline material. It was then dried under

vacuum over P4O10. Yield: �68%, 70 mg. Elemental Anal. Calc. forcomplex [2](ClO4)2 � H2O: C, 46.89; H, 3.45; N, 9.66. Found: C,46.21; H, 3.33; N, 9.72%. 1H NMR data (400 MHz, in DMSO-d6)dppm: 9.15 (d, 2H), 8.92 (d, 2H), 8.70 (d, 2H) 8.60 (t, 2H), 7.93 (d,2H), 7.60 (t, 4H), 7.41 (d, 2H), 7.25 (t, 4H), 6.20 (d, 2H).

2.5. Crystallographic data

2.5.1. Complex [1](ClO4)CCDC No. 652460, C34H22Cl2N6 O6Ru, M = 782.55, orthorhombic,

a = 19.7242(3), b = 15.7100(2), c = 19.9264(3) Å, V = 6174.54(15) Å3,space group Pbca, Z = 8, T = 298(2) K, l(Mo Ka) = 0.740 mm�1,F(000) = 3152, Goodness-of-fit = 1.033; final R indices: R1 = 0.0344[I > 2r(I)], wR2 = 0.0801; R indices (all data): R1 = 0.0536,wR2 = 0.0898.

2.5.2. Complex [2](ClO4)2H2OCCDC No. 652461, C34H22Cl2N6O11Ru, M = 862.55, monoclinic,

a = 8.6420(2), b = 21.3292(6), c = 19.0073(5) Å, b = 91.274(2),V = 3502.69(16) Å3, space group P2(1)/c, Z = 4, T = 298(2) K, l (MoKa) = 0.671 mm�1, F(000) = 1736, Goodness-of-fit = 0.971; final Rindices: R1 = 0.0547 [I > 2r(I)], wR2 = 0.1362; R indices (all data):R1 = 0.0823, wR2 = 0.1570.

3. Results and discussion

The tridentate ligand L has been synthesized by a modified re-ported route [25]. The reaction of L with the ruthenium–terpyri-dine precursor Ru(trpy)Cl3 in ethanol initially afforded a darksolution, from which a dark colored solid mass has been isolatedon addition of excess saturated aqueous NaClO4 solution (Scheme1). Chromatographic purification of the crude product on a silicagel column using a CH3CN–CH2Cl2 mixture (1:4) as the eluent re-sults in a reddish complex [1]+ as its perchlorate salt (yield, 45%),followed by other products.

Complex [1]+ exhibits satisfactory elemental analyses. The for-mulation of the complex has been further supported by FT-IRand 1H NMR spectroscopy (Supplementary data, Figs. S1 and S2).The formation of [1]ClO4 has also been authenticated by its singlecrystal X-ray structure.

Complex [1]+, when heated for 1 h with constant stirring in thepresence of aqueous AgNO3 solution, affords complex [2]2+

(Scheme 1). The formation of [2]2+ was authenticated by elementalanalysis, FT-IR and 1H NMR spectroscopy, and by its single crystalX-ray structure.

3.1. Crystal structure analyses

X-ray quality single crystals were grown by slow diffusion of anacetonitrile solution of the complexes into benzene followed bythe slow evaporation technique. The ORTEP plot of [RuL(ter-py)Cl](ClO4), [1]ClO4, is shown in Fig. 1. In the monomeric complex,The Ru center is in a distorted octahedral arrangement with aRuN5Cl coordination environment (Table 1). The largest distortionof the octahedral geometry is due to the geometrical constraints ofthe terpy ligand, which is shown by small N–Ru–N angles (Table 1)[26–29]. The ligand L is bonded to the metal center in a bidentatefashion. Both the central pyridine nitrogen atoms are in a cis orien-tation and the pyridine nitrogen atom of L is trans to the Cl� ion.The nitrogen from one benzoxazole ring is in a trans position withrespect to the central pyridine nitrogen of terpyridine. As a result,the second benzoxazole ring becomes distant from the rutheniumcenter and hence is not coordinated to the metal. The Ru–N dis-tances are in the range 1.962(2)–2.122(19) Å. The average bite an-gles involving terpyridine/L in [1]ClO4 are 79.50(4) and 78.59(13)�.

Ru

NN

N NN

N

O

O

ON

N ON

N

N

N

Cl

RuRu

N

N

N Cl

Cl

Cl

NN

Aq. AgNO3

Stirrring and Heating for 1h

2+

[1]+ [2]2+

+

a) Ethanol

b)

N = terpyridine

NEt3c) Ligandd) reflux for 4 h

Scheme 1.

Fig. 1. ORTEP plot with 50% thermal ellipsoids (top) and an infinite chain formed byC–H� � �Cl interactions in complex [1]ClO4.

Table 1Selected bond lengths (Å) and bond angles (�)

[1] ClO4 [2](ClO4)2. H2O

Ru(1)–N(1) 2.064(2) 2.077(4)Ru(1)–N(2) 1.962(2) 1.986(4)Ru(1)–N(3) 2.069(2) 2.083(4)Ru(1)-N(4) 2.087(2) 2.098(4)Ru(1)–N(5) 2.122(19) 2.006(4)Ru(1)–Cl(1) 2.381(7)Ru(1)–N(6) 2.075(4)

N(1)–Ru(1)–N(2) 79.58(9) 78.83(18)N(1)–Ru(1)–N(3) 159.37(8) 157.48(17)N(1)–Ru(1)–N(4) 103.25(8) 90.69(18)N(1)–Ru(1)–N(5) 99.34(8) 99.94(18)N(1)–Ru(1)–N(6) 93.88(16)N(2)–Ru(1)–N(3) 79.83(8) 78.66(17)N(2)–Ru(1)–N(4) 177.11(8) 104.65(16)N(2)–Ru(1)–N(5) 102.62(8) 177.41(16)N(2)–Ru(1)–N(6) 99.94(16)N(3)–Ru(1)–N(4) 97.33(8) 95.66(16)N(3)–Ru(1)–N(5) 86.33(8) 102.53(17)N(4)–Ru(1)–N(5) 77.66(8) 77.58(17)N(5)–Ru(1)–Cl(1) 170.20(6)N(5)–Ru(1)–N(6) 77.83(16)

A. Singh et al. / Polyhedron 27 (2008) 2563–2568 2565

The bond distances and angles match well with the reported anal-ogous ruthenium–terpy complexes [26–29]. The RuII–Cl distance,2.3815(7) Å, in [1]ClO4 is comparable to that observed in [Ru(tr-py)(biq)Cl]PF6 (biq = 2,20-bisquinoline), 2.378(2) Å [30], but issmaller than that in other RuII complexes with a N5Cl coordinationcore [31–33]. The metal bonded Cl atom forms a weak intermolec-ular C–H� � �Cl type hydrogen bond with the adjacent ligand in thesolid-state, which leads to the formation of an infinite chain alongthe crystallographic a axis (Fig. 1b). One of the benzoxazole rings ofL is in the plane with its central pyridine ring, while the other is inan almost perpendicular orientation. In the crystal structure boththe aromatic rings form p-stacking interactions, which remain inthe same plane in the solid-state. It is interesting to note that theruthenium center is separated from the outside environment andis in buried inside the hydrophobic environment. The counter an-ions are lined up in a zig-zag fashion along the crystallographic aaxis. The structure contains alternate hydrophobic and hydrophiliclayers (Supplementary data, Fig. S3).

The ORTEP plot of [RuL(terpy)](ClO4)2 � H2O, [2](ClO4)2 � H2O, isshown in Fig. 2. In this complex, the RuN6 chromophore bindsthe ruthenium center in a distorted octahedral geometry (Table1). The hydrogen atoms for the solvent water molecule were notlocated in the electron density map. The Cl� ion in the complex[1]ClO4 has been replaced by the nitrogen atom (N6) of one ofthe benzoxazole moieties, which was on the opposite side of theaxial Cl atom in [1]ClO4. Unlike in [1]ClO4, here the pyridine nitro-gen of the ligand is in a trans orientation with respect to the centralpyridine nitrogen of terpyridine. Moreover, the ligand is bonded toRu in a tridentate fashion with both benzoxazole nitrogens in axialpositions. The Ru–N distances in the complex [2](ClO4)2 � H2O arein the range 1.986(4)–2.098(4) Å. The average bite angles involvingterpyridine/L are 79.80(18) and 78.32(18)�. In the solid-state thereare no p-stacking interactions in this complex. Similar to the com-plex [1]ClO4, several weak C–H� � �O type hydrogen bonding interac-tions are present in the solid-state. The metal ion is also buriedinside the hydrophobic core of the molecule. Perchlorate anionsand water molecules form a hydrogen bonded network in complex[2](ClO4)2 � H2O. The oxygen atoms of the benzoxazole unit are inthe periphery of the hydrophobic sphere. In [2](ClO4)2 � H2O, coun-ter anions along with water of crystallization formed the hydro-philic hemisphere, which is in a face-to-face arrangement inalternate layers. Complex [2](ClO4)2 � H2O formed alternate rowsof square boxes in the solid-state along the crystallographic a axis(see Supplementary data, Fig. S4).

3.2. UV–Vis spectroscopic studies

Both the complexes, [1]+ and [2]2+, exhibit strong MLCTtransitions in the visible region in acetonitrile solvent (Fig. 3). Forcomplex [1]+, a band appears at 521 nm (e = 5960 M�1 cm�1) andin the case of complex [2]2+, at 452 nm (e = 4880 M�1 cm�1). The

Fig. 2. ORTEP Plot with 50% thermal ellipsoids (top) and packing diagram (below)of complex [2](ClO4)2 � H2O.

2566 A. Singh et al. / Polyhedron 27 (2008) 2563–2568

intra-ligand p ? p* transitions appear at 318, 272 nm (e = 26600and 17050 M�1 cm�1) and 304, 270 nm (e = 23650 and15060 M�1 cm�1) for complexes [1]+ and [2]2+, respectively. It isinteresting to note that the MLCT band in [RuII(trpy)(bbp)]2+ ap-pears at 475 nm [trpy = 2,20:60,200-terpyridine; bbp = 2,6-bis(benz-imidazoly-2yl)pyridine] [34]. This may be attributed to the

Fig. 3. UV–Vis spectra of [1]+ (solid line) and [2]2+ (dashed line) in acetonitrilesolvent.

weaker p-acceptor properties of the ligand L compared to trpy orbbp [34].

The broad band at 1080–1100 cm�1 followed by a band at�625 cm�1 in the FT-IR spectra of the complexes confirms thepresence of the perchlorate anion (Supplementary data, Fig. S1).The molar conductivities (KM) of complexes [1]+ and [2]2+, in ace-tonitrile solvent are 124 and 216 X�1 cm2 mol�1, respectively;which are consistent with the 1:1 and 1:2 electrolytic natures of[1]+ and [2]2+, respectively. The diamagnetic nature of the com-plexes is indicative of the presence of Ru(II) with a low-spin d6

configuration.

3.3. 1H NMR spectra

The 1H NMR spectra of complexes [1]+ and [2]2+ were recordedin DMSO-d6 solvent [Supplementary data, Figs. S2 and S3]. Both thecomplexes exhibit the calculated number of aromatic signals, over-lapping between 6 and 10 ppm. In both the complexes, the sym-metrical terpyridine moiety shows only six signals as expected.In [1]+, the ancillary ligand, L, due to its bidentate coordinationmode, exhibits all eleven signals; whereas, in [2]2+, as L binds theruthenium center in a symmetrical tridentate fashion, it showsonly six signals. Attempts were not made to assign the individualsignals because of the overlapping nature of the spectra.

3.4. Electrochemistry

The electron-transfer properties of the complexes [1]+ and [2]2+

have been studied in acetonitrile solvent versus SCE. The one elec-tron reversible RuII/RuIII couple appears at 0.90 and 1.44 V in thecase of complex [1]+ and [2]2+, respectively. A representative vol-tammogram of complex [1]+ is shown in Fig. 4. The replacementof the strong donor chloride in [1]+ by the ‘N’-donor from the benz-oxazole derivative in [2]2+ results in the positive shift of the oxida-tion potential by �0.54 V.

The same RuII/RuIII couple appears at 1.30 and 1.11 V in the caseof the complexes [RuII (trpy)2]2+ and [RuII(trpy)(bbp)]2+, respec-tively [34,35]. The stronger ligand field strength of trpy comparedto bbp or L results in the greater stabilization of the Ru(II) dp orbi-tals. On the other hand, poly-imidazole or -oxazole derivatives areknown as better r-donors and weaker p-acceptors than polypyri-dine derivatives. The effect of these factors is reflected in the shiftof the oxidation potential of the RuII/RuIII couple.

The formation of [2](ClO4)2 from [1]ClO4 can take place eithervia a five coordinated intermediate species [intermediate I] (disso-ciative mechanism) or through a dissociative interchange typereaction pathway [intermediate II] (Scheme 2).

However, when the same reaction was carried out in the pres-ence of azide, pyridine or thiocyanate, no incorporation of the

00.51E/V

I

Fig. 4. Cyclic voltammogram of complex [1]+ in acetonitrile, SCE, TBAP supportingelectrolyte, glassy carbon working electrode.

Ru

NN

N NN

N

O

O

ON

N ON

N

N

NCl

Ru

NO

N ON

N

N

NCl

Ru

ON

N ON

N

N

NRu

2+

[1]+ [2]2+

+

#

[II]

#

[I]

- Cl -

- Cl-

Scheme 2.

A. Singh et al. / Polyhedron 27 (2008) 2563–2568 2567

azide, pyridine or thiocyanate was found; which, in turn, indicatesthat the formation of [2](ClO4)2 takes place through thedissociative interchange type reaction pathway rather than a dis-sociative mechanism which involves a pentacoordinatedintermediate.

4. Conclusions

In conclusion, the present study demonstrates the synthesis,characterization, electron-transfer properties and structural as-pects of a set of ruthenium monoterpyridine complexes with 2,6-bis(benzoxazol-2-yl)pyridine as an ancillary ligand. Complex [1]+

undergoes an intramolecular dissociative interchange type substi-tution in the presence of aqueous AgNO3 solution to afford com-plex [2]2+. Complex [2]2+ represents an unique example wherethe ruthenium center is encapsulated by a hydrophobic and ahydrophilic hemisphere, which are in planes perpendicular to eachother.

Acknowledgements

The author would like to acknowledge their sincere gratitude tothe Department of Chemistry, Central Instrumentation Facility, In-dian Institute of Technology Guwahati, India. We are also thankfulto the Department of Science and Technology, India for the FISTfunded X-ray diffractometer.

Appendix A. Supplementary data

CCDC 652460 and 652461 contain the supplementary crystallo-graphic data for [1]ClO4 and [2](ClO4)2H2O. These data can be ob-tained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Cen-tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. FT-IR spectra in KBr pel-lets, 1H NMR spectra, H–H correlation spectra of the complexes[1]+ and [2]2+, the cyclic voltammogram of complex [2]2+ and thepacking diagrams for both complexes. Supplementary data associ-

ated with this article can be found, in the online version, atdoi:10.1016/j.poly.2008.05.022.

References

[1] F. Scandola, C.A. Bignozzi, M.T. Indelli, in: K. Kalyanasundaram, M. Gratzel(Eds.), Photosensitization and Photocatalysis Using Inorganic andOrganometallic Compounds, Kluwer, Dordrecht, 1993, p. 161.

[2] K. Kalyanasundaram, Coord. Chem. Rev. 28 (1989) 2920.[3] J.P. Sauvage, J.P. Collin, J.C. Chambron, S. Guillerez, C. Coudret, V. Balzani, F.

Barigelletti, L. De Cola, L. Flamigni, Chem. Rev. 94 (1994) 993.[4] C.M. Hartshorn, K.A. Maxwell, P.S. White, J.M. DeSimon, T.J. Meyer, Inorg.

Chem. 40 (2001) 601.[5] B. Gholamkhass, K. Koike, N. Negishi, H. Hori, K. Takeuchi, Inorg. Chem. 40

(2001) 756.[6] A. Doveletoglou, S.A. Adeyemi, T.J. Meyer, Inorg. Chem. 35 (1996) 4127.[7] M. Ebadi, A.B.P. Lever, Inorg. Chem. 38 (1999) 467.[8] H. Chao, G. Yang, G.-Q. Xue, H. Li, H. Zang, I.D. Williams, L.-N. Ji, X.-M. Chen,

X. -Y. Li, J. Chem. Soc., Dalton Trans. (2001) 1326.[9] H. Sugimito, K. Tsuge, K. Tanaka, J. Chem. Soc., Dalton Trans. (2001) 57.

[10] Y.-Q. Fang, N.J. Taylor, G.S. Hanan, F. Loiseau, R. Passalacqua, S. Campagna, H.Nierengarten, D.A. Van, J. Am. Chem. Soc. 124 (2002) 7912.

[11] T.-a. Koizumi, T. Tomon, K. Tanaka, Organometallics 22 (2003) 970.[12] M. Shiotsuki, T. Suzuki, K. Iida, Y. Ura, K. Wada, T. Kondo, T. Mitsudo,

Organometallics 22 (2003) 1332.[13] S. Bonnet, J.-P. Collin, J.-P. Sauvage, E. Schofield, Inorg. Chem. 43 (2004) 8346.[14] J. wang, G.S. Hanan, F. Loiseau, S. Campagna, Chem. Commun. (2004) 2068.[15] J.H. Alstrum-Acevedo, M.K. Brennaman, T.J. Meyer, Inorg. Chem. 44 (2005)

6802.[16] S. Patra, B. Sarkar, S. Ghumaan, M.P. Patil, S.M. Mobin, R.B. Sunoj, W. Kaim, G.K.

Lahiri, J. Chem. Soc., Dalton Trans. (2005) 1188.[17] K.J. Arm, G.J.A. Williams, J. Chem. Soc., Dalton Trans. (2006) 2172.[18] S. Ruttimann, C.M. Moreau, A.F. Williams, G. Bernardinelli, A.W. Addison,

Polyhedron 11 (1992) 635.[19] C. Piguet, B. Bocquet, E. Mueller, A.F. Williams, Helv. Chim. Acta 72 (1989) 323.[20] K. Hutchison, J.C. Morris, T.A. Nile, J.L. Walsh, D.W. Thompson, J.D. Petersen, J.R.

Schoonover, Inorg. Chem. 38 (1999) 2516.[21] D.T. Sawyer, A. Sobkowiak, J.L. Roberts Jr., Electrochemistry for Chemists, 2nd

ed., Wiley, New York, 1995.[22] G.M. Sheldrick, SHELXS-97, University of Gottingen, Germany, 1997.[23] G.M. Sheldrick, SADABS: Software for Empirical Absorption Correction,

University of Gottingen, Institut fur Anorganische Chemieder Universitat,Tammanstrasse 4, D-3400 Gottingen, Germany, 1999–2003.

[24] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.[25] A.W. Addison, P.J. Burke, J. Heterocycl. Chem. 12 (1981) 4455.[26] S. Sarkar, B. Sarkar, N. Chanda, S. Kar, S.M. Mobin, J. Fiedler, W. Kaim, G.K.

Lahiri, Inorg. Chem. 44 (2005) 6092.

2568 A. Singh et al. / Polyhedron 27 (2008) 2563–2568

[27] N. Chanda, D. Paul, S. Kar, S.M. Mobin, A. Datta, V.G. Puranik, K.K. Rao, G.K.Lahiri, Inorg. Chem. 44 (2005) 3499.

[28] N. Chanda, S.M. Mobin, V.G. Puranik, A. Datta, M. Niemeyer, G.K. Lahiri, Inorg.Chem. 43 (2004) 1056.

[29] B. Mondal, V.G. Puranik, G.K. Lahiri, Inorg. Chem. 41 (2002) 5813.[30] A.M.E. Boelrijk, J. Reedijk, J. Mol. Catal. 89 (1994) 63.[31] B. Mondal, M.G. walawalkar, G.K. Lahiri, J. Chem. Soc., Dalton Trans. (2000) 4209.[32] C.J. Cathey, E.C. Constable, M.J. Hannon, D.A. Tocher, M.D. Ward, J. Chem. Soc.,

Chem. Commun. (1990) 621.

[33] R.J. Forster, A. Boyle, J.G. Vos, R. Hage, A.H.J. Dijkhuis, R.A.G. DeGraff,J.G. Haasnoot, R. Prins, J. Reedijk, J. Chem. Soc., Dalton Trans. (1990)121.

[34] D. Mishra, A. Barbieri, C. Sabatini, M.G.B. Drew, H.M. Figgie, W.S. Sheldrick, S.K.Chattopadhyay, Inorg. Chim. Acta 360 (2007) 2231.

[35] R.P. Thummel, S. Chirayil, Inorg. Chim. Acta 154 (1988) 77.