Two new twisted helical nickel(II) and cobalt(III) octahedral monomer complexes: Synthesis and structural characterization

Article abstract of DOI:10.1007/s12039-014-0660-6

Two mononuclear complexes namely [Ni^II(L)] (1) and [Co^III(L)](NO₃) (2) of a hexacoordinating N₄O₂ donor Schiff base ligand were synthesized and characterized by single crystal X-ray studies. In compound 2 the central cobalt is in + 3 oxidation state while 'in' compound 2, the nickel ion is in + 2 oxidation state. The two complexes are isostructural with octahedral coordination environment exhibiting helical twist topology. They also display strong H-bonding as well as CH-π interactions to generate 1D chain.

Full text of DOI:10.1007/s12039-014-0660-6

c
J. Chem. Sci. Vol. 126, No. 6, November 2014, pp. 1647–1653. ꢀ Indian Academy of Sciences.
Two new twisted helical nickel(II) and cobalt(III) octahedral monomer
complexes: Synthesis and structural characterization
MALAY DOLAI and MAHAMMAD ALI∗
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
e-mail: mali@chemistry.jdvu.ac.in
MS received 16 November 2013; revised 21 April 2014; accepted 27 April 2014
Abstract. Two mononuclear complexes namely [NiII(L)] (1) and [CoIII(L)](NO3) (2) of a hexacoordinating
N4O2 donor Schiff base ligand were synthesized and characterized by single crystal X-ray studies. In compound
2 the central cobalt is in +3 oxidation state while ‘in’ compound 2, the nickel ion is in +2 oxidation state. The
two complexes are isostructural with octahedral coordination environment exhibiting helical twist topology.
They also display strong H-bonding as well as CH-π interactions to generate 1D chain.
Keywords. Coordination chemistry; nickel(II); cobalt(III); Schiff base; twisted helicity; supramolecular
interactions.
1. Introduction
recognition, molecular assembly, and crystal packing
of organic materials, nanomaterials and biomolecules.22
In this context, the occurrence of CH· · · π inter-
actions in solid-state networks has been extensively
described,18,19 and their importance as stabilizing
weak forces have been clearly illustrated in molecular
structures.19–22
Hence, in this work, an octahedral complex, with
NiO2N4 chromophore has been investigated (figure 1)
along with an isostructural CoIII complex of the same
ligand (scheme 1).
Over the last few years, there has been an immense
research interest among the chemists towards the inves-
tigations of biological properties of nickel that lead to
several structural and biochemical models.1,2 Variable
oxidation states coupled with multitude of coordination
geometries are the most interesting aspects that have
been identified in the active site of many Ni-containing
enzymes.3 Several biomimetic NiII complexes have
been found to act as structural and functional models of
metalloenzymes,4 such as the Ni-Fe hydrogenase5–7 or
the CO dehydrogenase/acetyl coenzyme-A synthase,8,9
etc.
The importance of helicates is related to the devel-
opment and the understanding of self-assembly in
supramolecular chemistry10 that are frequently per-
ceived in biology and in physics.11,12 Intense research
activities have been focused on this new field in
order to explore and apply its power of design and
control for the selective preparation of complicated
organized chemical architectures. The fundamental
research on helicates has recently led to a new fas-
cinating metallosupramolecular architectures based on
multicomponents self-assemblies such as directional
light-converting devices,13 inorganic two-dimensional
racks, ladders14 and grids,15 intertwined catenanes and
knots,16 and allosteric ionophores.17 On the other hand,
CH–π,18 π–π,19 cation–π,20, anion–π,21 etc. interac-
tions have been found to play crucial roles in molecular
Both [NiIIL].H2O (1) and [CoIIIL].NO3 (2) com-
plexes were prepared by reacting equimolar quantities
of MII and (H2L) (scheme 1) in acetonitrile in presence
of two equivalents of tryethylamine(TEA) as a base.
Here, we are going to discuss the syntheses and struc-
tural characterization of two new NiII and CoIII octa-
hedral complexes that exhibit twisted helicity and H-
bonding as well as CH-π interactions to generate 1D
chain.
2. Experimental
2.1 Materials
Chemicals such as 3-methoxy-salicylaldehyde (vanillin),
N,Nꢁ-bis(3-aminopropyle)-ethylene-diamine(Aldrich),
Co(NO3)2, Ni(NO3)2 and triethylamine (TEA) (Merck,
India) are of reagent grade and used without further
purification. Solvents like methanol, ethanol, acetoni-
trile (Merck, India) were of reagent grade and used as
received.
∗For correspondence
1647

1648
Malay Dolai and Mahammad Ali
2.1b Synthesis of [Ni(L)](H2O)(1): It was prepared
by adding ligand (H2L) (0.442 g, 1.0 mmol.) to a stir-
ring acetonitrile solution (30 mL) of Ni(NO3)2. 6H2O
(0.291 g,1.0 mmol) to give a pale yellow solution, after
a while a solution of TEA (0.202 g, 2.0 mmol) was
added for the deprotonation of ligand which resulted
in a brown solution. It was stirred for additional
20 m and filtered and then kept aside undisturbed for
slow evaporation. After 3 days, the orange rectangu-
lar block-shaped crystals suitable for X-ray diffraction
were formed. Yield: 2.83 g (55%). Anal. Calcd. for
C24H34N4NiO5: C 55.72%, H 6.63%, N 10.83%. Found:
C 55.96%, H 7.05%, and N 10.42 %.
2.1c Synthesis of [Co(L)](NO3)(2): It was prepared
in the same way as for [Ni(L)](H2O) with the excep-
tion that Co(NO3)2.6H2O (0.293 g,1.0 mmol) was
taken instead of Ni(NO3)2. 6H2O. In 3 days, the dark
brown block-shaped crystals suitable for X-ray diffrac-
tion were formed. Yield: 4.37 g (78%). Anal. Calcd.
for C24H32CoN5O7: C 51.33%, H 5.74%, N 12.47 %.
Found: C 50.96%, H 5.75%, and N 12.49 %.
Figure 1. (a) Molecular view of complex 1. All H-atoms
are omitted for clarity. Symmetry code: a) x,y,z; b) -x,-
y,1/2+z; c) 1/2-x,1/2+y,1/2+z; d) 1/2+x,1/2-y,z.
2.1a Synthesis of ligand(H2L): The Schiff base 2.2 Physical measurements
ligand was obtained by condensation of N,N’-bis(3-
Elemental analyses were carried out using a Perkin-
Elmer 240 elemental analyzer. H NMR spectrum was
recorded on a Bruker 300 MHz NMR spectrometer
using trimethylsilane as an internal standard in CDCl3.
Electronic spectra were recorded from Agilent 8453
UV–vis diode-array spectrophotometer.
amino-propyle)ethylenediamine (1.74 g, 10 mmol.)
and o-vanillin (3.04 g, 20 mmol.) in ethanol by
stirring at 60◦C for 4 hours, during which time a
yellow solid separated out from the reaction mix-
ture. The analytically pure solid was dried, and
stored in vacuum over anhydrous CaCl2 for subse-
quent use. Yield: 3.31 g (75%). Anal. Calcd. for
C24H34O4N4(MW = 442.55): C 65.13%, H 7.74%,
N 12.65 %. Found: C 64.96%, H 7.75%, and N
1
2.3 Crystallography
1
12.42 %. H NMR (300 MHz, CDCl3) (figure S1):
Single crystal X-ray data of complexes 1 and 2
were collected on a Bruker SMART APEX-II CCD
diffractometer using graphite monochromated Mo-Kα
radiation (λ = 0.71073Å). Data collection, reduc-
tion, structure solution, and refinement were performed
using the Bruker APEX-II suite (v2.0-2) program.
All available reflections to 2θmaxwere harvested and
corrected for Lorentz and polarization factors with
Bruker SAINT plus. Reflections were then corrected
for absorption, inter frame scaling, and other sys-
tematic errors with SADABS.23 The structures were
solved by the direct methods and refined by means
of full matrix least-square technique based on F2 with
SHELX-97 software package.24 All the non-hydrogen
atoms were refined with anisotropic thermal parame-
ters. All the hydrogen atoms belonging to carbon and
nitrogen atoms were placed in their geometrically ideal-
ized positions, while hydrogen atoms on oxygen atoms
δ 1.24 (s, 2H, imine*, not detectable precisely),
1.64-1.84 (m, 4H, methylene), 2.30–2.40 (d, 4H,
methylene), 2.53–2.74 (d, 4H, methylene), 3.50–3.44
(d, 4H, methylene), 3.90 (s, 6H, methoxy), 6.88–7.28
(d, 2H, H4), (m, 6H, aromatic), 8.28 (s, 2H, azome-
thine), 13.3125 (s, 2H, phenoxo-OH).
Scheme 1. Schematic representation of formation of
complexes.

Twisted helical nickel(II) and cobalt(III) monomer complexes
1649
atoms (namely N1, N2, N3 and N4) from the N,Nꢁ-
bis(3-amino-propyle)ethylenediamine part of the ligand
are coordinated to nickel(II) centre and the two O atoms
(O1 and O3) from the phenolic –OH groups of the
two o-vanillin moieties of ligand, coordinate axially
to the metal centre thereby forming a complete octa-
hedral geometry. The Ni-N bond distances are in the
range 2.020 -2.096 Å and Ni-O bond distances 2.038-
2.040 Å. All the bond distances are found to be almost
identical. Bond distances and bond angles are listed in
table 2a.
The structure of [Co(L)](NO3) (2) is shown in
figure 2. The complex 2 crystallizes in a orthorhom-
bic space group Pccn.The coordination environment
arround the CoIII metal centre is same as that of com-
plex 1. The four N atoms (N1 and N2 and their
symmetry related counter parts) from the N,Nꢁ-bis(3-
aminopropyle)ethylenediamine part of the ligand coor-
dinate to Co1 to form the square plane and two O
atoms (O2 and its symmetry related counter part) from
the phenolic –OH groups of two o-vanillin moieties,
occupy the axial positions giving an octahedral geom-
etry. The Co-N bond distances are in the range 1.933-
1.976 Å while Co-O bond distances are almost equal
(∼1.890 Å). The shorter axial Co-O bond length than
the equatorial Co-N bond length is in conformity with
the results observed previously in cobalt(III) complexes
of comparable ligand environment25 (table 2b).
of non-coordinated water was found on the difference
Furior map, and all of them were constrained to ride on
their parent atoms. Drawings of molecules were gene-
rated with the program DIAMOND-3.0 and PLATON-
1.16. The crystallographic data for 1 and 2 are given in
table 1.
3. Results and Discussion
Synthesis [NiII(L)].H2O and [CoIII(L)](NO3) complexes
were prepared by reacting the respective (NiII and CoII)
metal ions with 1 equivalent of hexacoordinating ligand
H2L in presence of 2 equivalents of TEA in acetoni-
trile. It was interesting to note that CoII ion is oxidized
to CoIII in presence of hard-donor (N4O2) atoms prob-
ably by the atmospheric oxygen as evidenced from the
charge balance of the formed complex and also by BVS
calcualtions (vide infra).
3.1 Structural description
The structure of [Ni(L)](H2O)(1) is shown in figure 1.
The complex 1 crystallizes in a orthorhombic space
group Pna21. The coordination sphere of the hexa-
coordinated NiII metal centre is surrounded by N4O2
atoms of the Schiff base ligand (H2L). The four N
Table 1. Crystallographic data and details of the structure determination for 1-2.
1
2
(CCDC No.-894740 )
(CCDC No.-894739)
Formula
FW
Crystal System
Space group
a/ Å
C24 H32 N4 Ni O4, H2 O
517.24
C24 H32 Co N4 O4, N O3
561.48
Orthorhombic
Pna21 (No. 33)
37.5238(12)
7.5017(2)
8.6868(3)
90
2445.27(13)
4
1.405
0.836
1096
1.1, 32.6
Orthorhombic
Pccn (No. 56)
7.4561(3)
18.4285(6)
19.1192(7)
90
2627.07(17)
4
1.420
0.705
1176
2.1, 27.1
b/ Å
c /Å
α /◦ ,β/◦,γ /◦
V /Å3
Z
ρcalc /g cm−3
]
μ(Mo-Kα)/ mm−1
F(000)
Theta Min-Max [◦]
Index ranges
-56: 56 ; -11: 11 ; -13: 13
-9: 9 ; -23: 23 ; -24: 24
Reflection collected
Independent reflections (Rint)
No. Of Parameter
GOF
35271
7937 (0.038)
361
1.11
0.0509 ((6648)
0.1012
20284
2908 (0.026)
174
1.026
0.0355 (2261)
0.0979
R1 [I> 2σ(I)]
wR2 (all data)
Min. and Max. Resd. Dens./e Å−3
-0.93, 0.38
-0.27, 0.30

1650
Malay Dolai and Mahammad Ali
Table 2. Summary of bond distances and bond angles of complexes 1 and 2.
(i) Complex 1.
Bond lengthes (Å)
Ni1- N1
Ni1- N4
Ni1- O3
Ni1- O1
Bond angles (◦)
2.020(10)
2.057(3)
2.073(2)
2.0819(17)
2.116(3)
2.066(2)
N1- Ni1- N4
N1- Ni1- O3
N4- Ni1- O3
N1- Ni1- O1
N4- Ni1-O1
O3- Ni1-O1
N1- Ni1- N3
N4- Ni1- N3
O3- Ni1- N3
O1- Ni1- N3
N3- Ni1-N2
88.56(9)
93.19(8)
84.15(8)
90.04(8)
92.17(8)
175.04(7)
83.57(10)
169.05(10)
88.69(9)
95.42(9)
88.47(10)
Ni1- N3
Ni1- N2
Bond angles (◦)
N1- Ni1- N2
N4- Ni1- N2
O3- Ni1- N2
O1 - Ni1- N2
169.91(10)
88.56(9)
92.83(8)
84.53(8)
(ii) Complex 2.
Bond lengthes (Å)
Co1- O2
Co1- O2a
Co1- N1
Bond angles (◦)
O2a- Co1- N1a
N1- Co1- N1a
O2- Co1- N2
O2- Co1- N2a
N1- Co1- N2a
N1- Co1- N2
O2a- Co1- N2
O2a- Co1- N2a
N1- Co1- N2a
N1a- Co1- N2a
N2- Co1- N2a
1.8901(12)
1.8901(12)
1.9332(15)
1.9332(15)
1.9742(15)
1.9742(15)
89.55(5)
93.61(6)
85.90(6)
89.95(6)
90.12(6)
174.32(6)
89.95(6)
85.90(6)
90.12(6)
174.32(6)
86.49(6)
Co1- N1a
Co1- N2
Co1- N2a
Bond angles (◦)
O2 -Co1- O2a
O2- Co1- N1
O2- Co1- N1a
O2a- Co1- N1
174.30(5)
89.55(5)
94.36(5)
94.36(5)
Symmetry code: a = 1/2-x,1/2-y,z.
Space-filling representation of [Ni(L)] and [Co(L)]+
clearly demonstrates mononuclear single-stranded heli-
cal structures for both the complexes figures S2 and
S3. It is generally accepted that a central metal ion
which is too small to fit into the cavity of the bound
multidentate ligand may generate such topology. Steric
interactions between the extremities of the strand
induce a slight twisting of the ligand, reminiscent of
that found in organic helicenes.26 Several acyclic polyg-
lycols and linear podands wrap around group I and II
metals27 as exemplified by the carbocyclic antibiotic
monensin which gives the mononuclear single-stranded
helical complex.28 Here the helical twist is favoured
by the attachment of rigid and bulky terminal aromatic
groups (o-vanillin) to comparatively flexible central
N,Nꢁ-bis(3-aminopropyle)ethylenediamine moiety. The
angles between the two planes constructed from two o-
vanillin moieties is found to be 30.92◦ and 36.06◦ for
complexes 1 and 2, respectively which clearly demon-
strate the fact of helical twisting observed in 1 and 2.
In case of complex 1 there is one non-coordinated
water molecule (O1S) that is involved in four-ways
H-bonding interactions namely, N1-H1· · · O1S, N3-
H45· · · O1S, O1S-H55· · · O1 and O1S-H40· · · O3. It
forms two H-bonds with two amine nitrogen atoms
N1, N3 and with two phenolate oxygen atoms (O1 and
O3) belonging to two neighbouring complexes on either
sides (figure 3a). Spiral binding of complex 1 through
H-bonding is shown in figure 3b. In cobalt(III) complex
Figure 2. (a) Molecular view of complex 2. All H-atoms
are omitted for clarity. Symmetry code: a) x,y,z; b) 1/2+x,
-y,1/2-z; c) -x,1/2+y,1/2-z; d) 1/2-x,1/2-y,z; e) -x,-y,-z;
f) 1/2-x,y,1/2+z; g) x,1/2-y,1/2+z; h) 1/2+x,1/2+y,-z.

Twisted helical nickel(II) and cobalt(III) monomer complexes
1651
Figure 3. (a)The quasi 1D chain through bifurcated H-bonding interaction
in complex 1. (b) Space-filling model showing Spiral binding of complex 1
through H-bonding. N1-H1· · ·O1S: H1· · · O1S = 2.11(4)Å, N1-H1 = 0.92(4)
Å, N1· · · O1S = 3.022(4)Å, <N1-H1· · ·O1S = 173(3)◦; O1S-H40· · ·O3:
H40· · · O3 = 2.11(3) Å, O1S-H40 = 0.78(3)Å, O1S· · · O3 = 2.848(4)
Å, <O1S-H40· · ·O3 = 160(3)◦; N3-H45· · ·O1S: H45· · ·O1S = 2.45(4)Å,
N3-H45 = 0.98(4) Å, N3· · · O1S = 3.323(4) Å,<N3-H45· · ·O1S =
148(3)◦; O1S-H55· · ·O1: H55· · ·O1 = 1.97(4)Å, O1S-H55 = 0.87(4) Å ,
O1S· · · O1= 2.817(3) Å, <O1S -H55· · ·O1 = 2.817(3)◦.
Figure 4. The one dimensional CH· · · π(phenyl) interactions (dotted lines
in magenta colors) and supramolecular H-bonding(dotted lines in black colors)
are observed in complex 1.

1652
Malay Dolai and Mahammad Ali
Table 3. C-H· · · π Interaction for Complexes 1 and 2.
interaction
atoms
distances H· · · Cg (Å)
∠C-H· · · Cg (◦)
distances C(H)· · · Cg(Å)
Complex 1
2.884
C-H· · · π (phenyl)
C-H· · · π (phenyl)
C-H· · · π (phenyl)
C-H· · · π (phenyl)
C-H· · · π (phenyl)
C(26)-H(49)· · ·Cg (1)
C(27)-H(27B)· · ·Cg(1)
C(11)-H(11)· · ·Cg(2)
C(29)-H(32)· · ·Cg(2)
C(30)-H(41)· · ·Cg(2)
132.55
134.83
108.44
118.01
131.19
3.582
4.294
3.679
3.999
4.040
3.554
3.277
3.479
3.314
Complex 2
C-H· · · π (phenyl)
C(10)-H(10B)· · ·Cg(3)
3.441
3.265
100.65
129.49
3.744
3.953
C-H· · · π (phenyl)
C(9)-H(9A)· · ·Cg(3)
Cg (1) = C15 C14 C100 C13 C12 C17 and Cg (2) = C18 C19 C20 C21 C1 C2 denotes for complex 1, Cg (3) = C3 C13 C4
C5 C6 C7 denotes for complex 2.
2, there is strong H-bonding interaction between H is observed in complex 1. The bands at shorter wave-
atom on N2 (imine nitrogen) and O3 (non-coordinated lengths (373 and 402 nm) are due to ligand to metal
NO−3 ) atom and extended in 1D fashion. Details of (L→M) charge-transfer transitions.
H-bonding interactions are shown in figure 3 for 1
figure S4 for 2.
On the other hand, the packing of the molecules in
bothcomplexes1and2 iscontrolledbyCH· · · π as well
as by H-bonding interactions between two neighboring
complex units to give compact 1D supramolecular ar-
chitectures. In complex 1, five types of CH· · · π
(phenyl) interactions are prevalent (figure 4). Among
them, three CH· · ·π interactions are established
between two methylenic and one azomethine H-atoms
of one complex with the centroid of the phenyl ring of
its neighboring complex. Similarly, two CH· · · π iner-
actions are established between one methylenic and one
azomethine H atoms and the centroid of the phenyl ring
of other complex. These two centroids belong to two
different neighboring complexes. These CH· · · π inter-
actions fall in the range 2.884 – 3.554 Å. Details of
CH· · · π interactions are given in table 3. In complex
2, only two types of CH-π interactions are present
(figure S5), they are established between methylenic H
atoms (H10B and H9A) and neighboring phenyl ring of
Schiff base with distances 3.441 Å and 3.265 Å.
3.3 BVS calculation
Bond valence sum (BVS) method is very frequently
used to examine the nature of coordination and oxida-
tion state of the central metal atom using the crystal-
lographic data in coordination complexes as well as in
many biological molecules. The most successfully used
empirical equation is:
ꢀ
ꢀ
ꢁ
ꢂ
sij =
exp r◦ − rij /0.37
Where r is a parameter characteristic of the bond and
◦
rij is the crystallographically obtained bond distances.
3.2 UV-vis spectra of complexes 1 and 2
The electronic spectra of complexes 1 and 2 were
recorded in MeCN (figure 5). The electronic transi-
tion appears at ∼373 nm with molar extinction coef-
ficients (ε) 3280 dm3 mol−1 cm−1 for complex 1. In
case of complex 2 there are two electronic transitions
that appear at 633 and 402 nm with ε values 392
and 7540 dm3 mol−1 cm−1, respectively. The bands at
longer wavelengths may be attributed to the d-d tran-
sitions for complex 2 (633 nm) but no d-d transition
Figure 5. UV-vis spectra of complexes 1 and 2 in MeCN.
[c] = 1.0 × 10−3 M for d-d bands at longer wavelengths
(633 nm, green line) for complex 2 and [c] = 1.0 × 10−5
M
for LMCT bands at shorter wavelengths (373 nm, red line for
1 and 402 nm, black line for 2).

Twisted helical nickel(II) and cobalt(III) monomer complexes
1653
8. Panda R, Zhang Y G, McLauchlan C C, Rao P V T,
de Oliveira F A, Munck E and Holm R H 2004 J. Am.
Chem. Soc. 126 6448
ꢀsij represents the charge on the central metal atom.
BVS values13 are: NiII-O = 1.670 Å, NiII-N = 1.647 Å.
The BVS of Ni was calculated to be 2.07 (table S1a)
which unequivocally establishes the +2 oxidation state
of the central Ni atom in complex 1. Similarly BVS
values are: CoIII-O = 1. 79 Å, CoIII-N = 1.68 Å29,30 and
9. Rao P V, Bhaduri S, Jiang J F, Hong D and Holm R H
2005 J. Am. Chem. Soc. 127 1933
10. Lehn J-M 1995 In Supramolecular Chemistry
(Weinheim: VCH)
calculated BVS value of Co is 3.35 (table S1b) which 11. Lindsey J S 1991 New J. Chem. 15 153
12. Cramer F, Chaos and Order 1993 In The Complex
again establishes the +3 oxidation state for the central
Structure of Living Systems (Weinheim:VCH)
13. (a) Piguet C and Bünzli J -C G 1996 Eur. J. Solid State
Inorg. Chem. 33 165; (b) Bünzli J-C G, Froidevaux P
and Piguet C 1995 New J. Chem., 19 661
cobalt atom in complex 2.
4. Conclusion
14. (a) Sleiman H, Baxter P, Lehn J-M and Rissanen K 1995
J. Chem. Soc., Chem. Commun. 715; (b) Chambron J-C,
Dietrich-Buchecker C O, Nierengarten J-F, Sauvage
J P, Solladie’ N, Albrecht-Gary A-M and Meyer M 1995
New J. Chem. 19 409; (c) Hanan G S, Arana C R, Lehn
J-M and Fenske D 1995 Angew.Chem., Int. Ed. Engl.
34 1122; (d) Baxter P, Hanan G S and Lehn J-M 1996
Chem. Commun. 2019
15. Baxter P, Lehn J-M, Fischer J and Youinou M T 1994
Angew.Chem. Int. Ed. Engl. 33 2284
16. (a) Sauvage J-P 1990 Acc. Chem. Res. 23 319;
(b) Dietrich-Buchecker C O and Sauvage J-P 1987
Chem. Rev. 87 795
In conclusion, two mononuclear complexes of a hexa-
coordinating Schiff base ligand with N4O2 donor atoms
have showed helical twist topology. Both complexes
exhibit strong H-bonding and CH· · · π(phenyl) interac-
tions that fall in the range 2.884–3.554 Å for complex
1 and 3.265–3.441 Å for 2 to generate 1D chain. The
bond valence sum (BVS) calculations clearly indicate
that Co is in +3 oxidation state in complex 2 while Ni
is in +2 oxidation state in 1.
17. (a) Nabeshima T, Inada T, Furukawa N, Hosoya T and
Yano Y 1993 Inorg. Chem. 32 1407; (b) Nabeshima T
1996 Coord. Chem. Rev. 148 151
18. Schneider H J 2009 Angew. Chem., Int. Ed. 48 3924
19. Braga D, Giaffreda S L, Grepioni F, Maini L and Polito
M 2006 Coord. Chem. Rev. 250 1267
Supplementary Information
The electronic Supplementary Information can be seen
at www.ias.ac.in/chemsci.
20. Nishio M. 2004 Cryst. Eng. Comm. 6 130
21. Tsuzuki S and Fujii A 2008 Phys. Chem. Chem. Phys.
10 2584
Acknowledgements
22. Tewari A K and Dubey R 2008 Bioorg. Med. Chem. 16
Financial support from CSIR (Ref. 02(2490)/11/EMR-
II) New Delhi, is gratefully acknowledged. M D grate-
fully acknowledges UGC for the Fellowship (UGC-
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