Bidentate coordination behaviour of a potentially tridentate ligand.
A mononuclear four-coordinate Ni(II) complex supported by two o-iminobenzosemiquinonato units
Atasi Mukherjee & Rabindranath Mukherjee*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India Email: rnm@iitk.ac.in
Received 23 December 2010; accepted 29 December 2010
Using a new N,O,S–potentially tridentate ligand (H2L), a mononuclear NiII complex of composition [Ni (L ) ],II −ɺ 2 (1) has been synthesized. The structure of the complex has been elucidated in the solution state using 1H NMR spectroscopy and in the solid state by X-ray crystallography. X-ray studies reveal that the NiII is surrounded by two o-iminobenzosemiquinonate(1–) π-radical ligands, providing a NiIIN2O2 square planar coordination environment. Notably, the thioether unit in each ligand remains non-coordinated. Cyclic voltammetric measurements of (1) show two one-electron reductive and two one-electron oxidative redox responses, which are ligand-centred in origin. Characterization of electrochemically-generated reduced and oxidized species has also been accomplished.
Keywords: Bioinorganic chemistry, Metal coordinated radicals, Non-innocent ligands, Crystal structures, Nickel
In recent years the determination of molecular structure, investigating the magnetic, spectral and redox properties and correct assignment of electronic structure of metal complexes of o-aminophenolate ligands in its di-deprotonated form have drawn the attention of inorganic chemists due to non-innocent (redox-active) nature of such ligands. From this perspective Wieghardt and co-workers and others have synthesized using this class of ligands a large number of metal complexes of varying coordination number1-12. As a part of our continuing efforts to investigate the properties of metal-coordinated ligand radical species from bioinorganic perspective13,14, in the present work a new ligand of o-aminophenol class appended with a benzylthio substituent in the ortho position of the N-aryl group (H2L) has been synthesized and its coordination behaviour towards nickel(II) has been investigated. It should be appreciated here that these bidentate ligands (H2L is a potentially tridentate ligand) bind a metal ion as an o-aminophenolate monoanion or as an o-amidophenolate dianion, but they can also be oxidized to the o-iminobenzosemiquinonate radical monoanion species, which eventually can be oxidized to the neutral o-iminobenzoquinone form (Scheme 1).
The ligand H2L has been designed to investigate the effect of an additional thioether S donor atom to
o-aminophenolate framework on the electronic structural properties of resulting complexes. Herein we report the synthesis, structural characterization and properties (absorption and EPR spectral and redox) of a neutral mononuclear NiII complex containing two bidentate N,O-coordinated o-iminobenzo- semiquinonate(1–) π-radical ligands.
Materials and Methods
All reagents were obtained from commercial sources and used as received. Solvents were dried/purified as reported previously13,14. The tetra-n-butylammonium perchlorate (TBAP) was prepared and purified as reported earlier13,14.
Elemental analyses were obtained using a Thermo Quest EA 1110 CHNS-O, Italy. IR spectra (KBr, 4000-600 cm–1) were recorded on a Bruker Vector 22 spectrophotometer. Electronic spectra were recorded by using Agilent 8453 diode-array spectrophotometer.
1H NMR spectra (CDCl3) were obtained on either a Bruker WP-80 (80 MHz) or a JEOL JNM LA (400 MHz) spectrophotometer. Chemical shifts are reported in ppm referenced to TMS. X-band EPR spectra were recorded by using Bruker EMX 1444 EPR spectrometer operating at 9.455 GHz. The EPR spectra were calibrated with diphenylpicrylhydrazyl, DPPH (g = 2.0037).
Cyclic voltammetric (CV) experiments were performed at 298 K by using CH instruments, Electrochemical Analyzer/Workstation model 600B series. The cell contained a Beckman M-39273 platinum-inlay working electrode, a Pt wire auxiliary electrode, and a saturated calomel electrode (SCE), as reference electrode. Details of the cell configuration are as described before13,14. For coulometry, a platinum wire-gauze was used as the working electrode. The solutions were ~1.0 mM in complex and 0.1 M in supporting electrolyte, TBAP.
Synthesis of 2-(benzylthio)aniline
The precursor 2-(benzylthio)aniline for the synthesis of H2L was prepared by a modification of the method reported by Hucher et al.15
Sodium (1.68 g, 0.04 mmol) was dissolved in 50 mL of dry C2H5OH with continuous stirring, under
a dinitrogen atmosphere. To it, 2-aminothiophenol (4 g, 32 mmol) followed by benzyl chloride (4.05 g, 32 mmol) in C2H5OH (60 mL) were added and the resulting yellow suspension was heated to 323 K for 2 h. The reaction mixture was then concentrated under reduced pressure and extracted with CH2Cl2
(3 × 20 mL). The organic layer was washed with water (2 × 20 mL) followed by saturated brine solution (2 × 20 mL) and dried over anhydrous Na2SO4. Finally, evaporation of the solvent afforded 2-(benzylthio)aniline as a yellow solid (Yield: 5.96 g, ca. 82%). 1H NMR (80 MHz; CDCl3): δ 6.60–7.52 (m, 9H, aromatic H), 3.96 (s, 2H, –CH2–), 4.30 (br, –NH2, exchangeable).
Synthesis of 2-[2-(benzylthio)phenylamino]-4,6-di-tert- butylphenol (H2L)
To a magnetically stirred mixture of 3,5-di-tert- butylcatechol (2 g, 8.99 mmol) and Et3N (0.09 g, 0.86 mmol) in n-heptane (10 mL), a solution of 2-(benzylthio)aniline (2.04 g, 8.99 mmol) in n-heptane (10 mL) was added dropwise, under a dinitrogen atmosphere. The resulting reaction mixture was refluxed for 5 h and allowed to stir for 12 h at 298 K. It was then concentrated under reduced pressure and extracted with CH2Cl2 (3 × 10 mL). The organic layer was washed with water (2 × 10 mL), followed by saturated brine solution (2 × 10 mL) and dried over anhydrous Na2SO4. Evaporation of the solvent yielded a brown oily material, which was purified by column chromatography on silica gel (5 % v/v, n-hexane/EtOAc) afforded a white crystalline solid (Yield: 2.77 g, ca. 73 %). 1H NMR (80 MHz; CDCl3): δ 6.20–7.70 (m, 11H, aromatic H), 4.02 (s, 2H, –CH2–), 1.44 (s, 9H, 6-tert-butyl), 1.24 (s, 9H, 4-tert-butyl).
Synthesis of [Ni (L ) ],II −ɺ 2 (1)
To a magnetically stirred mixture of H2L(0.20 g, 0.48 mmol) and Et3N (0.10 g, 0.96 mmol) in CH3OH (6 mL), a solution of Ni(NO3)2.6H2O (0.07 g, 0.24 mmol) in CH3OH (5 mL) was added dropwise and the resulting mixture was refluxed for 2 h. The dark green precipitate that formed was collected by filtration, washed with CH3OH and dried in vacuo (Yield: 0.15 g, ca. 69 %). The complex was recrystallized by diffusion of n-hexane into a concentrated solution of the complex in CHCl3. Green crystals that obtained were found to be suitable for X-ray structural studies. Anal. (%): Calcd for C54H62N2NiO2S2 (1): C, 72.59; H, 6.94; N, 3.14.
NH S
OH H2L
N S
O +e-
-e- N S
L2 O
L
-e- +e-
N S
O
L0
0
1 2
NH S
OH
H2L
- H + + H +
NH S
O
HL 1
- H + + H +
(o-aminophenolate monoanion)
(o-amidophenolate dianion) (o-iminobenzosemiquinonate
monoanion)
(o-iminobenzoquinone)
Structure and redox scheme of the ligand H2L employed Scheme 1
Found: C, 72.51; H, 7.01; N, 3.08. 1H NMR (400 MHz; CDCl3): δ 6.72–7.54 (m, 22H, aromatic H), 4.09 (s, 4H, –CH2–), 1.58 (s, 18H, 6-tert-butyl), 1.08 (s, 18H, 4-tert-butyl). IR (KBr, cm–1, selected peaks): 1578 ν(C=N), 1440 ν(C–O). Absorption spectrum (CH2Cl2): λmax/nm (ε, M–1 cm–1)]
290 (20 550), 420 sh (3500), 900 (20 100), 950 (17 200).
Crystal structure determination
Single crystals of suitable dimensions were used for data collection. Diffraction intensities were collected on a Bruker Smart Apex CCD diffractometer, with graphite-monochromated Mo-Kα (λ = 0.71073 Å) radiation at 100(2) K. The data were corrected for absorption. The structure was solved by SIR-97, expanded by Fourier-difference syntheses and refined with the SHELXL-97 package incorporated in WinGX 1.64 crystallographic package16. The position of the hydrogen atoms were calculated by assuming ideal geometries, but not refined. All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least- squares procedures on F2. Pertinent crystallographic parameters are summarized in Table 1.
Results and Discussion
Synthetic aspects and spectral (IR and 1H NMR) properties
The ligand H2L was prepared, following a synthetic methodology adapted from that reported by Chaudhuri, Wieghardt and co-workers2, in good yield from the reaction between 3,5-di-tert-butylcatechol and 2-(benzylthio)aniline in n-heptane, under a dinitrogen atmosphere in the presence of Et3N.
1H NMR spectrum of H2L is presented in Fig. S1 (Supplementary Data). Complex [Ni (L ) ]II −ɺ 2 (1) was isolated as a dark green powder from the reaction between H2L and Ni(NO3)2.6H2O (in molar ratio 2:1) in CH3OH in the presence of Et3N, under refluxing conditions. Complex (1) is readily soluble in CH2Cl2. IR spectrum of complex (1) exhibits the characteristic stretching vibrations at 1578 and 1440 cm–1 for the imino (C=N) and C–O bonds, respectively. There is absence of the frequencies attributable to ν(NH) and ν(OH) stretching vibrations. Complex (1) is diamagnetic (S = 0) as judged from its normal
1H NMR spectrum at ambient temperature (Fig. S2, Supplementary Data). This can be rationalized by considering the fact that the two coordinated ligands are present in their π-radical o-iminobenzo- semiquinonate monoanion state (L )−ɺ (S = 1/2) and
strongly intramolecularly antiferromagnetically coupled. Further proof comes from the crystal structural analysis of complex (1), attesting the presence of two o-iminobenzosemiquinonate(1–) radical (L )−ɺ units in [Ni (L ) ]II −ɺ 2 .
Crystal structure of [Ni (L ) ],II −ɺ 2 (1)
A perspective view of the metal coordination environment in [Ni (L ) ]II −ɺ 2 (1) is shown in Fig. 1 and the selected bond lengths and bond angles are listed in Table 2. The crystal structure reveals that a NiII ion is coordinated by two L−ɺ (see below) ligands. The geometry around the NiII ion is square planar with
Table 1 – Crystal data and structure refinement for complex (1) Empirical formula C54H62N2NiO2S2
Formula weight 893.89
Temp. (K) 100(2)
Wavelength (Å) 0.71073
Crystal system, space group Monoclinic, C2/c (#15)
a (Å) 29.785(6)
b (Å) 16.910(3)
c (Å) 19.789(3)
α (º) 90
β (º) 100.21(6)
γ (º) 90
Volume (Å3) 9809(3)
Z, Calc. density (g/cm3) 8, 1.211 Absorption coeff. (mm–1) 0.522
F(000) 3808
Crystal size (mm) 0.20 × 0.20 × 0.10 Theta range for data
collection (º) 2.41 − 28.31 Reflections collected / unique 31870 / 11998
[R(int) = 0.0795]
Number of observed reflections [I > 2σ(I)]
5950
Absorption correction Empirical (SADABS) Refinement method Full-matrix least-squares
on F2
Data/restraints/parameters 11998 / 0 / 550 Goodness-of-fit on F2 0.926
Final R indices [I>2σ(I)]a,b R1 = 0.0700, wR2 = 0.1608 R indices (all data) a,b R1 = 0.1485, wR2 = 0.1962 Largest diff. peak and
hole (e.Å–3) 0.718 and –0.363
a R1 = Σ(|Fo| – |Fc|)/Σ|Fo|.
b wR2 = {Σ[w(|Fo|2 – |Fc|2)2]/Σ[w(|Fo|2)2]}1/2.
NiIIN2O2 coordination environment. The two nitrogen atoms and two oxygen atoms from two L−ɺ moieties are in mutually trans disposition. Notably, the benzylthio S-atoms [S(1) and S(2)] from two L−ɺ moieties remain uncoordinated to the NiII ion [Ni–S(thioether) distances: 3.566(5) and 3.838(4) Å].
A closely similar situation was encountered before with a CuII complex6. The two short Ni–O bond distances [1.834(2) and 1.840(2) Å] and also the two short Ni–N bond distances [1.857(3) and 1.835(3) Å]
are similar to other closely similar complexes2. The following discussion on the metric parameters associated with the ligand backbone is important to unambiguously assign the correct electronic structural form of the noninnocent part of the ligand coordinated to NiII. Given the noninnocent nature of the iminophenolate part of the ligand1-12 two possibilities regarding their resonating forms exist (structures A and B).
N
O NiII C
N
O NiII C
A B
The C–N bond lengths [1.368(4) Å] observed here are significantly shorter than the Cphenyl–N bond distances2 (1.420 ± 0.005 Å). Similarly, the C–O bond lengths [1.316(4) and 1.314(4) Å] are also shorter than the bond distances (1.35–1.37 Å) for a coordinated catecholate dianion17. The observed bond distances are in the range expected for other
Fig. 1 ― Perspective view of the crystal of [NiII( L−ɺ)2](1). [Only donor atoms are labeled. All the hydrogen atoms are omitted for clarity].
Table 2 – Bond lengths and bond angles in
II 2
[Ni (L ) ]−ɺ (1)
N
O Ni C15
2 1 3
4 5 6
7 9 8 10 11
12 13
14
O
N 1
1
2 2
Bond lengths (Å)
Ni–N(1) 1.857(3) Ni–O(1) 1.834(2)
Ni–N(2) 1.835(3) Ni–O(2) 1.840(2)
O(1)–C(1) 1.316(4) O(2)–C(28) 1.314(4) C(1)–C(2) 1.423(5) C(28)–C(29) 1.425(5) C(2)–C(7) 1.372(5) C(29)–C(34) 1.375(5) C(7)–C(8) 1.420(5) C(34)–C(35) 1.426(5) C(8)–C(13) 1.368(5) C(35)–C(40) 1.355(5) C(13)–C(14) 1.403(5) C(40)–C(41) 1.410(5) C(1)–C(14) 1.425(5) C(28)–C(41) 1.417(5) N(1)–C(14) 1.368(4) N(2)–C(41) 1.368(4) N(1)–C(15) 1.411(4) N(2)–C(42) 1.424(4) C(15)–C(16) 1.387(5) C(42)–C(43) 1.378(5) C(16)–C(17) 1.364(5) C(43)–C(44) 1.368(5) C(17)–C(18) 1.367(6) C(44)–C(45) 1.370(5) C(18)–C(19) 1.383(6) C(45)–C(46) 1.373(5) C(19)–C(20) 1.366(6) C(46)–C(47) 1.403(5) C(15)–C(20) 1.398(5) S(2)–C(47) 1.763(4) S(1)–C(20) 1.790(4) S(2)–C(48) 1.799(4) S(1)–C(21) 1.783(4) C(48)–C(49) 1.519(5) C(21)–C(22) 1.588(7) C(49)–C(50) 1.383(6) C(23)–C(24) 1.367(7) C(50)–C(51) 1.396(6) C(24)–C(25) 1.383(8) C(51)–C(52) 1.354(6) C(25)–C(26) 1.356(8) C(52)–C(53) 1.382(6) C(26)–C(27) 1.377(7) C(53)–C(54) 1.375(6) C(22)–C(27) 1.374(6) C(49)–C(54) 1.375(6) C(2)–C(3) 1.533(5) C(29)–C(30) 1.528(5) C(3)–C(4) 1.524(6) C(30)–C(31) 1.529(5) C(3)–C(5) 1.541(6) C(30)–C(32) 1.525(5) C(3)–C(6) 1.520(5) C(30)–C(33) 1.544(5) C(8)–C(9) 1.536(5) C(35)–C(36) 1.537(5) C(9)–C(10) 1.516(6) C(36)–C(37) 1.528(6) C(9)–C(11) 1.537(6) C(36)–C(38) 1.540(6) C(9)–C(12) 1.549(6) C(36)–C(39) 1.505(6) Bond angles (º)
O(1)–Ni–O(2) 174.69(11) N(1)–Ni–N(2) 178.0(13) O(1)–Ni–N(1) 85.85(11) N(2)–Ni–O(2) 85.85(11)
reported2 o-iminobenzosemiquinonato(1–)-coordinated complexes of NiII. Within the six ring C–C bond distances, two alternating short C=C bonds [1.355(5)–1.375(5) Å] and four comparatively long C–C bonds [1.403(5)–1.426(5) Å] point towards the existence of the o-iminobenzosemiquinonato form.2-12 The metric parameters observed here associated with C–N and C–O bond lengths point towards the oxidation level of the ligand correctly represented by the resonating structures A and B. The six ring carbon C–C bond distances in the other phenyl ring of the ligand are equidistant, as expected for innocent nature of this part of the ligand backbone. Therefore, the ligand oxidation level present in complex (1) is in good agreement with the presence of two monoanionic π-radical ligands L−ɺ. The NiII ion possesses an experimentally determined spectroscopic oxidation state of II (d8).
Absorption spectra
The electronic spectrum of complex (1) exhibits absorptions at 290 (sh) (ε = 20, 550 M–1 cm–1), 420 (sh) (ε = 3500 M–1 cm–1), 620 (sh) (ε = 2000 M–1 cm–1), 720 (sh) (ε=3000 M–1 cm–1), 900 (ε=20 100 M–1 cm–1), and 950 (sh) (ε = 17 200 M–1 cm–1) nm (Fig. 2). The intense absorption bands at 900 and 950 nm are assigned to spin- and dipole-allowed ligand-to-ligand charge-transfer (LLCT) transition. This spectral feature is very similar to those reported for closely related square-planar NiII complex coordinated by o-iminobenzosemiquinonate(1–) π-radical anions2,5,6.
Redox properties
To authenticate the non-innocent character of our chosen ligand, complex (1) was examined by cyclic voltammetry (CV) in CH2Cl2 at a platinum working electrode (scan rate = 100 mVs–1). Complex (1) displays two one-electron reductive responses:
E1/2 = –0.64 V vs SCE (peak-to-peak separation,
∆Ep = 100 mV) and Epc (cathodic peak potential) = –1.42 V vs SCE. The less cathodic response is quasi- reversible and more cathodic one is irreversible in nature. Complex (1) also exhibits two one-electron quasi-reversible oxidative responses at E1/21 = 0.29 V (∆Ep = 190 mV) and at E1/22 = 0.45 V (∆Ep = 110 mV) vs SCE. Given the structural proof of complex (1) having a NiII ion coordinated by two π–radical o-iminosemiquinonate ligand L−ɺ, it is reasonable to assign all the observed redox responses as ligand- centred2-12. The CV scan is displayed in Fig. 3.
Properties of coulometrically-generated reduced and oxidized species
We consider first the reduction process.
Electrolysis at –0.90 V of a solution of complex (1) in CH2Cl2 (containing 0.1 M TBAP as supporting electrolyte) at 298 K caused transfer of 1.06 electron per molecule and the colour of the solution during electrolysis changed from dark green to dark greenish yellow. To check the chemical integrity of this one- electron reduced species, an anodic CV scan was recorded immediately after the controlled-potential electrolysis experiment. CV response (this time it is oxidative) of such a solution was observed at almost
Fig. 2 ― Absorption spectra in CH2Cl2 solution (containing 0.1 M TBAP) of (1) complex (1), (2) its coulometrically-generated one-electron reduced form, and, (3) its coulometrically-generated two-electron oxidized form.
Fig. 3 ― Cyclic voltammogram (100 mV/s) of a 1.0 mM solution of (1) at platinum electrode in CH2Cl2 (0.1 M in TBAP).
[Indicated potentials (in V) are vs SCE].
same potentials as that of complex (1) (Fig. S3). The electronic spectrum of one-electron reduced species of complex (1) in CH2Cl2 exhibits bands at 310 nm (sh) (ε=17 100 M–1 cm–1), 405 nm (ε=16 700 M–1 cm–1), 610 nm (ε = 8400 M–1 cm–1), and 840 nm (ε = 9500 M–1 cm–1) (Fig. 2). The absorptions are attributed to π–π* transitions of the ligand. Similar spectral feature has been reported previously for one- electron reduced species of NiII complexes having o-iminobenzosemiquinonate(1–) π-radical anions2. The electrogenerated one-electron reduced species in CH2Cl2 (containing 0.1 M TBAP) has been further characterized by EPR spectral measurements at 120 K. The X-band EPR spectrum of such reduced species displays a rhombic S = 1/2 signal with significant g anisotropy (Fig. 4). Similar EPR spectra have been observed before for the closely similar species7. Coulometrically-generated two-electron reduced species of complex (1) is not stable enough to allow its redox and spectroscopic characterization, which is understandable given the irreversible nature of more cathodic redox response. Thus we assign the two reductive redox responses as due to the formation of monoanionic [Ni (L )(L )]II −ɺ 2− − which can very well be present in its resonating form: [Ni (L )(L )]II −ɺ 2− − ↔ [Ni (L )(L )] ;I −ɺ −ɺ − the anisotropic feature in the EPR spectrum of the first reduced state (Fig. 4) implies a resonating form and dianionic [NiII(L2–)(L2–)]2–
species (in the absence of any support from experimental and/or theoretical calculation it is reasonable to define the second reduced state as
2 2 2 II(L )(L )]
Ni
[ − − ↔ [NiI(L−ɺ)(L2−)]2− as represented in the following Eq. 1.
[NiII(L )2] +e- [NiII(L )(L2-)]- [NiII(L2-)(L2-)]2- -e-
+e- -e-
[NiI(L )(L )]- [NiI(L )(L2-)]2-
… (1) Now we consider the oxidative processes. The two- electron oxidized species of complex (1) was generated by electrolysis of CH2Cl2 solutions (containing 0.1 M TBAP) of complex (1) (applied potential: 0.70 V, n = 1.93; colour change: dark green to dark reddish brown) at 298 K. Such an oxidized species of complex (1) is stable enough for its spectroscopic characterization (absorption and EPR),
as the CV scan (reductive response) (Fig. S4) recorded of such dark reddish brown solutions remain practically unchanged, at least for 30 min. The electronic spectrum of such a dark reddish brown solution (Fig. 2) in CH2Cl2 exhibits bands at 280 nm (sh) (ε = 9800 M–1 cm–1), 410 nm (ε = 4700 M–1 cm–1) and 540 nm (ε = 3750 M–1 cm–1). These absorptions are assigned to the π–π* transitions associated with the quinone form of the ligand, as observed before for the closely similar species2,6. The electrochemically- generated two-electron oxidized species of complex (1) is EPR silent at 120 K, attesting to S = 0 ground state of this species. This can be rationalized, if it is invoked that the product of two-electron oxidized species is the dication [NiII(L0)(L0)]2+, as reported previously for two-electron oxidized NiII complexes having o-iminobenzosemiquinonate(1–) π-radical anions2.
[NiII(L0)(L0)]2+
[NiII(L )2] -2e-
+2e- … (2)
Conclusions
In an attempt to examine the effect of S coordination, in the present work we have synthesized and characterized a new 2-aminophenol ligand H2L, incorporating a benzylthio moiety. However, this potentially tridentate ligand affords a diamagnetic neutral bis(ligand) NiII complex. Structural characterization has revealed that S atom of
Fig. 4 ― X-band EPR spectrum (CH2Cl2; 298 K) of coulometrically-generated one-electron reduced species of complex (1).
benzylthio unit remains uncoordinated in this complex and hence the NiII ion assumes a square planar geometry, having two bidentate N,O-coordinated o-iminobenzosemiquinonate(1–) π-radical ligands. The title complex reveals rich ligand-centred oxidation and reduction processes.
Absorption spectral signature has been displayed by this complex and its one-electron reduced and its two- electron oxidized counterparts. Our systematic study on the properties of mononuclear NiII complex containing two bidentate monoanionic π-radical ligands and characterization of its one-electron reduced and two-electron oxidized forms, at a reasonable level of confidence, provides further information on the properties of the complexes having o-iminobenzosemiquinonate(1–) π-radical ligands. It should be mentioned here that our primary goal to examine the effect of S coordination due to the presence of –S-CH2-Ph unit in the ligand H2L has remained unfulfilled. Efforts are on to explore the full coordination potential of this new potentially tridentate ligand towards six-coordination favouring metal ions.
Supplementary Data
CCDC 804203 contains the supplementary crystallographic data for complex (1). These data can be obtained free of charge from the Cambridge Crystallographic Data via www.ccdc.cam.ac.uk/
data_request/cif.
Acknowledgement
Financial assistance received from the Department of Science & Technology (DST), Government of India is gratefully acknowledged. RM sincerely acknowledges the award of J C Bose fellowship by DST.
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