Three mononuclear octahedral cobalt(III) complexes with salicylaldimine Schiff bases: Synthesis, characterization, phenoxazinone synthase mimicking activity and DFT study on supramolecular interactions

Article abstract of DOI:10.1016/j.poly.2016.02.035

Three mononuclear facial cobalt(III) complexes, [Co(L)₂]ClO₄ (1), [Co(L)(bzan)(N₃)] (2) and [Co(L)(bzan)(NCS)] (3), {HL = 2-(3-(dimethylamino)propyliminomethyl)-6-ethoxyphenol and Hbzan = 1-benzoylacetone} were prepared an

Full text of DOI:10.1016/j.poly.2016.02.035

Accepted Manuscript
Three mononuclear octahedral cobalt(III) complexes with salicylaldimine
Schiff bases: Synthesis, characterization, phenoxazinone synthase mimicking
activity and DFT study on supramolecular interactions
Kousik Ghosh, Sumit Roy, Anindya Ghosh, Abhisek Banerjee, Antonio
Bauzá, Antonio Frontera, Shouvik Chattopadhyay
PII:
S0277-5387(16)00129-7
DOI:
http://dx.doi.org/10.1016/j.poly.2016.02.035
POLY 11851
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
18 December 2015
24 February 2016
Please cite this article as: K. Ghosh, S. Roy, A. Ghosh, A. Banerjee, A. Bauzá, A. Frontera, S. Chattopadhyay, Three
mononuclear octahedral cobalt(III) complexes with salicylaldimine Schiff bases: Synthesis, characterization,
phenoxazinone synthase mimicking activity and DFT study on supramolecular interactions, Polyhedron (2016),
doi: http://dx.doi.org/10.1016/j.poly.2016.02.035
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Three mononuclear octahedral cobalt(III) complexes with
salicylaldimine Schiff bases: Synthesis, characterization,
phenoxazinone synthase mimicking activity and DFT study
on supramolecular interactions
Kousik Ghosha, Sumit Roya,*, Anindya Ghosha, Abhisek Banerjeea, Antonio Bauzáb,
Antonio Fronterab,*, Shouvik Chattopadhyaya,*
aDepartment of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700 032, India
e-mail: shouvik.chem@gmail.com Tel. +91-33- 2414-2941
bDepartament de Química, Universitat de les Illes Balears, Crta de Valldemossa km 7.5, 07122
Palma de Mallorca (Baleares), SPAIN. e-mail: toni.frontera@uib.es
Abstract
Three mononuclear facial cobalt(III) complexes, [Co(L)2]ClO4 (1), [Co(L)(bzan)(N3)]
(2) and [Co(L)(bzan)(NCS)] (3) {HL
=
2-(3-(dimethylamino)propyliminomethyl)-6-
ethoxyphenol and Hbzan = 1-benzoylacetone} were prepared and characterized by elemental and
spectral analysis. X-ray crystal structure determination confirmed the structures of the
complexes. Extended supra-molecular assemblies were generated in all three complexes through
weak noncovalent interactions. Density functional theory (DFT) calculations were employed to
understand and estimate the contribution of each interaction in the formation of the assembly
using several theoretical models. Bader’s theory of “atoms in molecules” (AIM) was also used to
further describe the long range interactions and its role to stabilize different molecular
assemblies. Complexes 2 and 3 are found to be efficient catalysts to perform the aerial oxidation
1

of 2-aminophenol to 2-aminophenoxazine-3-one and therefore may be used as functional models
for copper(II) containing enzyme, phenoxazinone synthase.
Keywords: Cobalt(III); Schiff base; Phenoxazinone synthase mimicking activity; Supramolecular
interactions; DFT study.
1. Introduction
Over the past several decades, many model complexes of cobalt(II) and cobalt(III) have
been prepared and investigated, with particular emphasis on the reactivity of the metal ions in the
trans methylation reaction and the reversible absorption of molecular oxygen [1-4]. Several
cobalt(III) complexes have also been used to artificially model photosynthesis systems [5-7]. The
most common approach to creating an artificial photosynthetic device involves the development
of efficient and robust catalysts for each half reaction of water splitting, the oxidation of water to
O2 and reduction of H+ to H2 [8-10]. Mononuclear series of cobaloxime complexes have widely
been used by several groups as H2 evaluator from aqueous solution. Dinuclear cobalt(III)
complexes may function as mimics of the active biosites such as in methionine amino peptidase
[11] and can show DNA cleavage activity [12]. Cobalt(III) in distorted octahedral and trigonal
bipyramidal geometries have been identified in the metallohydrolase responsible for the removal
of the N-terminal methionine from the protein chain [13]. Cobalt(III) Schiff base complexes have
also drawn considerable attention in the past for their important biological applications [14-18].
On the other hand, Schiff bases themselves have been extensively studied in coordination
chemistry due to their facile syntheses, easily tunable steric and electronic properties and their
applications in different branches of science. Metal complexes of Schiff bases are important
stereochemical models in main group and transition metal coordination chemistry [19-23]. The
2

complexes have wide applications, including bioinorganic chemistry, material science and
magnetism, catalysis, separation and encapsulation, hydrometallurgy, metal clusters, transport
and activation of small molecules etc [24-31]. Among several Schiff bases, salicylaldimine
Schiff bases constitute a special class for their ability to bind more than one metal centres
exploiting the bridging ability of phenoxo oxygen atom in it. Incorporation of additional groups
in the salicylaldimine moieties may increase their denticity thereby making them more
susceptible to form polynuclear complexes or may produce ligands having pendant donor atoms.
Several pendant ligands have been used in a variety of chemical applications, e.g. in enzyme
simulations [32], for the purpose of carrying a radionuclide into a target cell [33], in magnetic
resonance imaging reagents [34], and for progressively more elaborate investigations for their
structure/activity relationship [35-37]. In the present work, we have used three salicylaldimine
Schiff bases to prepare three distorted octahedral cobalt(III) complexes, [Co(L)2]ClO4 (1),
[Co(L)(bzan)(N3)]
(2)
and
[Co(L)(bzan)(NCS)]
(3)
[HL
=2-(3-
(dimethylamino)propyliminomethyl)-6-ethoxyphenol, Hbzan = 1-benzoylacetone]. The structures
have been confirmed by spectroscopy and single crystal X-ray crystallography highlighting such
aspects like phenoxazinone synthase like activity.
The construction of supramolecular networks with interesting non-covalent interactions
as binding forces is an appealing research topic now-a-days [38-42]. Several well established
noncovalent interactions such as, hydrogen bonding, π-stacking, cation⋅⋅⋅π, and C-H⋅⋅⋅π forces,
have been used to govern the organization of multicomponent supramolecular assemblies [43-
46]. Moreover, other less recognized forces, such as, σ⋅⋅⋅π-hole, lone-pair⋅⋅⋅π and anion⋅⋅⋅π
interactions, have also been utilized in many areas of chemistry and biochemistry [47-49]. The
relevance of several noncovalent interactions has been broadly analyzed by means of theoretical
3

and experimental investigations [50-52]. In the present paper, the theoretical study is devoted to
analyze the noncovalent interactions that govern the crystal packing of complexes 1, 2 and 3
focusing our attention to the remarkable C-H⋅⋅⋅π and C-H···H-C interactions. Latter interaction
has been studied in depth by Alvarez and coworkers [53,54]. Although C-H···H-C contacts in
alkanes are among the weakest intermolecular interactions (∼-0.4 kcal/mol for methane dimer),
they are cumulative, resulting in larger dimerization energies in some cases (e.g. long open
chains). For instance the complexation energy computed for the dimer of n-hexane is -4.5
kcal/mol [53]. At this stage, it is worthy to emphasize that supramolecular chemists usually
analyze the non-covalent interactions in the X-ray structures that are shorter than the sum of van
der Waals radii and they have a tendency to ignore all longer interactions in the crystal
structures. Evidently, this is a simplistic view of crystal packing since interactions slightly longer
also have an impact. That is, the difference in attraction (or repulsion) between a contact at the
van der Waals distance and one that is slightly longer is not very much. Taking this in mind, in
this theoretical study we also intend to exemplify this issue by computing the contribution of the
long-range interactions in the assemblies retrieved from the solid state structures of the
complexes reported herein.
2. Experimental section
All chemicals used were purchased from Sigma-Aldrich and were of reagent grade. They
were used without further purification.
Caution!!! Although no problems were encountered in this work, perchlorate salts containing
organic ligands and azide complexes are potentially explosive. Only a small amount of the
material should be prepared and it should be handled with care.
4

2.1. Preparations
2.1.1. Preparation of the ligand, 2-(3-(dimethylamino)propyliminomethyl)-6-ethoxyphenol (HL)
The tetradentate N2O2 donor Schiff base ligand HL was prepared by refluxing N,N-
dimethyl-1,3-diaminopropane (1 mmol, 0.125 mL) with 3-ethoxysalicylaldehyde (1 mmol, 0.166
g) in methanol (20 mL) for ca. 1 h. The ligand was not isolated and the methanol solution was
used directly for the preparation of complexes.
2.1.2. Preparation of [Co(L)2]ClO4 (1)
A methanol solution of cobalt(II) perchlorate hexahydrate (0.5 mmol, 0.182 g) was added
to the methanol solution of the ligand HL and refluxed for 1 h. Diffraction quality dark brown
single crystals of complex 1 were obtained after few days on slow evaporation of the solution in
open atmosphere.
Yield: 0.40 g (61%). Anal. Calc. for C28H42CoN4ClO8 (FW 657.04): C, 51.18; H, 6.44; N,
8.53. Found: C, 51.12; H, 6.38; N, 8.60 %; IR (KBr, cm-1): 1618 (υC=N), 3006-2928 (υCH), 1082
(υClO4).UV-Vis, λ (nm), [εmax (L mol-1 cm-1)] (DMSO), 275 (9.1 × 103), 325 (5.2 × 103), 522
max
(7.1 × 102). Magnetic moment: diamagnetic.
2.1.3. Preparation of [Co(L)(bzan)(N3)] (2)
A methanol solution of cobalt(II) perchlorate hexahydrate (1 mmol, 0.365 g) was added
to the methanol solution of the ligand HL followed by the addition of methanol solution of 1-
benzoylacetone (Hbzan) (1 mmol, 0.162 g) and refluxed for 1 h. An aqueous methanol solution
of sodium azide (1 mmol, 0.065 g) was then added to it and refluxed further for ca. 1 h.
5

Diffraction quality dark brown single crystals of complex 2 were obtained after few days on slow
evaporation of the solution in open atmosphere.
Yield: 0.30 g (65%). Anal. Calc. for C24H30CoN5O4 (FW 511.46): C, 56.36; H, 5.91; N,
13.69. Found: C, 56.30; H, 5.86; N, 13.76 %; IR (KBr, cm-1): 1625 (υC=N), 2026 (υN3), 1556
(υC=O), 2923 (υCH). UV-Vis, λ (nm), [εmax (dm3 mol-1 cm-1)] (CH3CN), 243 (1.4 × 104), 275
max
(1.1 × 104), 328 (4.1 × 103), 494 (2.1 × 102), 703 (1.4 × 102). Magnetic moment: diamagnetic.
2.1.4. Preparation of [Co(L)(bzan)(NCS)] (3)
A methanol solution of cobalt(II) perchlorate hexahydrate (1 mmol, 0.365 g) was added
to the methanol solution of the ligand HL followed by the addition of methanol solution of 1-
benzoylacetone (Hbzan) (1 mmol, 0.162 g) and refluxed for 1 h. An methanol solution of sodium
thiocyanate (1 mmol, 0.081 g) was then added to it and refluxed further for ca. 1 h. Diffraction
quality dark brown single crystals of complex 3 were obtained after few days on slow
evaporation of the solution in open atmosphere.
Yield: 0.30 g (65%). Anal. Calc. for C25H30CoN3O4S (FW 527.51): C, 56.92; H, 5.73; N,
7.9. Found: C, 56.85; H, 5.67; N, 8.04 %; IR (KBr, cm-1): 1623 (υC=N), 2107 (υNCS), 1513 (υC=O),
2982-2888 (υCH). UV-Vis, λmax (nm), [εmax (dm3 mol-1 cm-1)] (CH3CN), 246 (1.1 × 104), 272 (9.9
× 103), 326 (3.5 × 103), 492 (2.4 × 102), 746 (1.2 × 102). Magnetic moment: diamagnetic.
2.2. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were performed on a Perkin-Elmer
240C elemental analyzer. Infrared spectra in KBr (4500-400 cm-1) were recorded using a
PerkinElmer Spectrum Two FTIR spectrophotometer. Electronic spectra (900-200 nm) were
6

recorded on a PerkinElmer Lambda 35 UV-Vis spectrophotometer. Fluorescence spectra in
acetonitrile were obtained on a Hitachi F-7000 Fluorescence spectrophotometer at room
temperature. The magnetic susceptibility measurements were done with an EG & PAR vibrating
sample magnetometer, model 155 at room temperature and diamagnetic corrections were made
using Pascal’s constants. The powder XRD data were collected on a Bruker D8 Advance X-ray
diffractometer using Cu Kα radiation (λ = 1.548 Å) generated at 40 kV and 40 mA. The PXRD
spectra were recorded in a 2θ range of 5-50° using 1-D Lynxeye detector at ambient conditions.
2.3. X-ray crystallography
Single crystals of three complexes having suitable dimensions were used for data
collection using a Bruker SMART APEX II diffractometer equipped with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K. Molecular structures were solved
using the SHELX-97 package [55]. Hydrogen atoms were placed in their geometrically idealized
positions and constrained to ride on their parent atoms. Multi-scan empirical absorption
corrections were applied to the data using the program SADABS [56]. A summary of the
crystallographic data is given in Table 1. Selected bond lengths and bond angles of three
complexes are given in Tables 2 and 3, respectively.
2.4. Hirshfeld Surface Analysis
Hirshfeld surfaces [57-59] and the associated two-dimensional (2D) fingerprint [60-62]
plots were calculated using Crystal Explorer [63], with bond lengths to hydrogen atoms set to
standard values [64]. For each point on the Hirshfeld isosurface, two distances, de (the distance
from the point to the nearest nucleus external to the surface) and di (the distance to the nearest
7

nucleus internal to the surface), are defined. The normalized contact distance (dnorm) based on de
and di is given by
where rivdW and revdW are the van der Waals radii of the atoms. The value of dnorm is
negative or positive depending on intermolecular contacts being shorter or longer than the van
der Waals separations. The parameter dnorm displays a surface with a red-white-blue color
scheme, where bright red spots highlight shorter contacts, white areas represent contacts around
the van der Waals separation, and blue regions are devoid of close contacts. For a given crystal
structure and set of spherical atomic electron densities, the Hirshfeld surface is unique [65] and
thus it suggests the possibility of gaining additional insight into the intermolecular interaction of
molecular crystals.
2.5. Theoretical methods
The calculations of the noncovalent interactions were carried out using the
TURBOMOLE version 7.0 [66] using the BP86-D3/def2-TZVP level of theory. To evaluate the
interactions in the solid state, we have used the crystallographic coordinates. This procedure and
level of theory have been successfully used to evaluate similar interactions [67-70]. The
interaction energies were computed by calculating the difference between the energies of isolated
monomers and their assembly. The interaction energies were corrected for the Basis Set
Superposition Error (BSSE) using the counterpoise method [71]. The “atoms-in-molecules”
(AIM) [72] analysis was performed at the BP86-D3/def2-TZVP level of theory. The calculation
of AIM properties was done using the AIMAll program [73].
8

2.6. Catalytic oxidation of 2-aminophenol
10-4 M solutions of the complexes in acetonitrile were treated with a 10-2 M solution of o-
aminophenol under aerobic conditions at 250C. The reaction was followed
spectrophotometrically by monitoring the increase in the absorbance maxima at ~433 nm. 10-4 M
solutions of the complexes were mixed with at least 10 equivalents of substrate to maintain
pseudo-first order conditions to determine the dependence of rate on the substrate concentration
and various kinetic parameters. The rate of the reaction was derived from the initial rate method.
3. Results and discussion
3.1. Synthesis
The tetradentate N2O2 donor Schiff base ligand, HL, was synthesized by 1:1 condensation
of N,N-dimethyl-1,3-diaminopropane with 3-ethoxysalicylaldehyde in methanol following the
literature method [74-77]. The ligand was not isolated and the methanol solution was used
directly for the preparation of complexes.
A methanol solution of HL was made to react with cobalt(II) perchlorate hexahydrate
under reflux to prepare mononuclear bis-ligand complex, [Co(L)2]ClO4 (1). Cobalt(II) is readily
oxidised to cobalt(III) by areal oxygen in presence of the Schiff base ligands producing strong
crystal field, as was also observed in similar complexes [78]. We have repeated the reaction in
N2 atmosphere but no complex could be isolated. On the other hand, to stop the formation of this
type of bis-ligand complex, we have reacted cobalt(II) with HL in presence of suitable
coordinating anions, e.g. benzoylacetonate (bzan) and azide. The result is the formation of
[Co(L)(bzan)(N3)] (2), where three coordinating sites of cobalt(III) are occupied by the
deprotonated Schiff base. Two oxygen atoms from bzan coordinate cobalt(III) and the remaining
9

site is occupied by a terminal azide. Complex 3 was prepared in a similar method as that of 2,
except that thiocyanate was added (instead of azide), which coordinated the sixth coordinated
site of cobalt(III) leading to the formation of [Co(L)(bzan)(NCS)] (3). The formation of all three
complexes is shown in Scheme 1.
Scheme 1: Preparation of complexes 1, 2 and 3.
3.2. Description of the structures
3.2.1. [Co(L)2]ClO4 (1)
Complex 1 crystallizes in the triclinic space group P1. A perspective view of complex 1
along with the selective atom numbering scheme is shown in Figure 1. The structure
10

determination reveals that the complex consists of a discrete cationic mononuclear unit [Co(L)2]+
along with a non-coordinating perchlorate anion. The cobalt(III) center is in a six-coordinate
octahedral environment, being bonded to two imine nitrogen atoms, N(2) and N(4), two amine
nitrogen atoms, N(1) and N(3), and two phenoxo oxygen atoms, O(1) and O(3), from two
deprotonated Schiff base ligands (L), which occupy facial positions (fac isomer). The closest
conformation of saturated six membered chelate rings, Co(1)-N(1)-C(3)-C(4)-C(5)-N(2) and
Co(1)-N(3)-C(17)-C(18)-C(19)-N(4), are half-chair, with puckering parameters [79] q =
0.697(8)Å, θ =163.3(7)°, φ = 170(3)° and q = 0.744(8)Å, θ =16.1(7)°, φ =345(2)°, respectively.
The N(1)-Co(1)-N(2) and N(3)-Co(1)-N(4) angles are 84.7(3)° and 83.8(3)°, respectively and are
typical of a six membered chelate ring [80]. The Co-Nimine distances {1.919(7)-1.954(8) Å} is
shorter than the Co-Namine distances {2.077(7)-2.123(7) Å} [81].
3.2.2. [Co(L)(bzan)(N3)] (2)
Complex 2 crystallizes in the monoclinic space group P21/c. A perspective view of
complex 2 along with the selective atom numbering scheme is shown in Figure 2. The structure
determination reveals that the complex consists of a discrete mononuclear unit
[Co(L)(bzan)(N3)]. The cobalt(III) center is coordinated by one amine nitrogen atom, N(1), one
imine nitrogen atom, N(2), and one phenoxo oxygen atom, O(3), from one deprotonated Schiff
base ligand (L)-, and two carbonyl oxygen atoms, O(1) and O(2), of a benzoylacetonate ligand
(bzan). The sixth coordination site of cobalt(III) is occupied by a nitrogen atom, N(3) of a
terminal azide to complete its octahedral geometry. The Schiff base ligand coordinates
cobalt(III) in facial fashion. The Co-Nimine distance {1.898(2) Å} is shorter than the Co-Namine
distance {2.055(2) Å}, as were also observed in similar systems [82]. The saturated six
membered chelate ring Co(1)-N(1)-C(13)-C(14)-C(15)-N(2), present an half-chair conformation
11

with puckering parameters [79] q = 0.672(2) Å, θ =10.6(3)°, φ = 335.0(13)°. The N(1)-Co(1)-
N(2) angle is 86.82(9)° and is typical of a six membered chelate ring. The terminal azide is
quasi-linear with the N-N-N angle being 176.0°(3).
3.2.3. [Co(L)(bzan)(NCS)] (3)
Complex 3 crystallizes in the monoclinic space group P21/c. A perspective view of
complex 3 along with the selective atom numbering scheme is shown in Figure 3. The structure
determination reveals that the complex consists of a discrete mononuclear unit
[Co(L)(bzan)(NCS)]. The cobalt(III) center is coordinated by one amine nitrogen atom, N(1),
one imine nitrogen atom, N(2), and one phenoxo oxygen atom, O(3), from one deprotonated
Schiff base ligand (L)- , and two carbonyl oxygen atoms, O(1) and O(2), of a benzoylacetonate
ligand (bzan). The sixth coordination site of cobalt(III) is occupied by a nitrogen atom, N(3) of a
terminal thiocyanate to complete its octahedral geometry. The Schiff base ligand coordinates
cobalt(III) in facial fashion. The Schiff base ligand coordinates cobalt(III) in meridional fashion.
The Co-Nimine distance {1.905(2) Å} is shorter than the Co-Namine distance {2.058(2) Å}, as were
also observed in similar systems [82]. The saturated six membered chelate ring Co(1)-N(1)-
C(13)-C(14)-C(15)-N(2), present an half-chair conformation with puckering parameters [79] q =
0.662(2) Å, θ =169.25(17)°, φ = 158.4(11)°. The N(1)-Co(1)-N(2) angle is 88.29(8)° and is
typical of a six membered chelate ring [83].
3.3. Geometrical preferences
One meridional and five facial geometrical isomers are possible for any octahedral bis-
ligand complexes of asymmetric tridentate ligand (e.g. HL) (Scheme 2). However, in present
12

case, only the facial one having amine nitrogen, imine nitrogen and phenoxo oxygen atoms in
trans positions ((a) in Scheme 2) has been formed exclusively, as confirmed by X-ray analysis.
Scheme 2: Possible facial (a-e) and meridional (f) (geometrical) isomers of complex 1; NA, NI
and O indicate imine nitrogen, amine nitrogen and phenoxy oxygen atoms, respectively.
It is difficult to explain the stability of this particular facial isomer with the help of
electronic factor, because there should be competition between two (π acceptor) imine groups in
the trans disposition for accepting electrons from the same d-orbitals of cobalt(III), minimizing
the back-bonding stabilization. However, two chelate rings {O(1)-C(12)-C(7)-C(8)-N(2)-Co(1)
and Co(1)-N(1)-C(3)-C(4)-C(5)-N(2)} are orthogonal to each other (89.450) in this disposition
and therefore non-bonding interaction in this facial isomer is expected to be low. Besides the
requirement of efficient crystal packing, this seems to be responsible for its exclusive formation.
13

Scheme 3: Possible facial (a-c) and meridional (d) (geometrical) isomers of complexes 2 and 3;
NA, NI and O indicate imine nitrogen, amine nitrogen and phenoxo oxygen atoms, respectively
{X= azide (in 2) or thiocyanate (in 3)}.
On the other hand, one meridional and three facial geometrical isomers are possible for
both complexes 2 and 3 (Scheme 3). However, in present case, only the facial isomers of both
complexes have been formed exclusively, where azide nitrogen atoms are in the opposite site to
the phenoxo oxygen atoms ((d) in Scheme 3), as confirmed by X-ray analysis.
3.4. Powder X-ray diffraction
The experimental powder XRD patterns of the bulk product of all three complexes are in
good agreement with simulated XRD patterns from single crystal X-ray diffraction, confirming
purity of the bulk samples. The simulated patterns were calculated from the single crystal
structural data (cif file) using the CCDC Mercury software. Figure 4 shows the experimental and
simulated powder XRD patterns of complex 3.
14

3.5. Hirshfeld surface analysis
The Hirshfeld surfaces for complexes 1, 2 and 3 are mapped over dnorm, shape index and
curvedness (Figure 5). The surfaces are shown as transparent to allow visualization of the
molecular moiety around which they are calculated. The dominant interaction between C···H /
H···C for all complexes can be seen in the Hirshfeld surfaces as red spots on the dnorm surface in
Figure 5. Other visible spots in the Hirshfeld surfaces correspond to H···H contacts. The small
extent of area and light color on the surface indicate weaker and longer contact other than
hydrogen bonds. The C···H / H···C interactions appear as distinct spikes in the 2D fingerprint
plot (Figure 6). Complementary regions are visible in the fingerprint plots where one molecule
acts as a donor (de ˃ di) and the other as an acceptor (de ˂ di). The fingerprint plots can be
decomposed to highlight contributions from different interaction types, which overlap in the full
fingerprint [84]. The proportions of C···H / H···C interactions comprise 7.8, 20.8 and 23.5 % of
the Hirshfield surfaces for each molecule of complexes 1, 2 and 3 respectively. The C-H
interaction is represented by lower spike (di = 1.69, de = 1.08 Å in 1, di = 1.6, de = 1.1 Å in 2 and
di = 1.54, de = 1.06 Å in 3) and the H···C interaction is also represented by another spike (di =
1.08, de = 1.69 Å in 1, di = 1.1, de = 1.62 Å in 2 and di = 1.06, de = 1.55 Å in 3) by upper spike
and can be viewed as bright red spots on the dnorm surface.
3.6. IR, electronic and fluorescence spectra and magnetic property
In the IR spectra of all three complexes, distinct bands due to azomethine (C=N)
stretching vibrations appears in the region of 1618-1625 cm-1. Sharp bands at 2026 and 2017 cm-
1
indicate the presence of terminal azide and thiocyanate in the IR spectra of complexes 2 and 3,
respectively. The characteristic absorption band for non-coordinated perchlorate anion appeared
at 1082 cm–1 in the IR spectrum of complex 1. Bands in the range of 3006-2806 cm-1 due to alkyl
15

C-H bond stretching vibrations are customarily noticed in the IR spectra of all complexes [85-
88].
Electronic spectra of complexes are recorded in acetonitrile medium at room temperature
in the range 200-800 nm. In all three complexes, the low-spin octahedral ground term is 1A1g and
1
there are two spin-allowed transitions, A1g → 1T1g and 1A1g → 1T2g [89]. The electronic spectra
of 1, 2 and 3 display one low energy absorption band each at 522, 703 and 746 nm, respectively,
attributable to one of the two expected transitions for any low-spin cobalt(III) in octahedral
geometry, as mentioned above. In the high energy region, all complexes show intense absorption
bands around 243-275 nm, corresponding to π-π* transitions, and bands around 325-328 nm,
attributed to the charge transfer transition from the coordinated ligands to the cobalt(III) centres
(LMCT) [90-92]. All complexes show emissions with maxima in the range of 380-385 nm in the
UV region, upon excitation around 325 nm in acetonitrile at room temperature.
All three complexes are diamagnetic as expected for low-spin cobalt(III) complexes [89].
3.7. Theoretical calculation on supramolecular interactions in solid state
In Figure 7A we show the representation of an infinite 1D chain found in the solid state
of complex 1 that is governed by the formation of two equivalent C-H⋅⋅⋅π and C-H···H-C
interactions. We have used a theoretical model that consists of one dimer taken from this infinite
chain (using the crystallographic coordinates). We have computed its interaction energy (see
Figure 7B), which is ∆E1 = -10.3 kcal/mol that can be initially attributed to the contribution of
both C-H⋅⋅⋅π and C-H···H-C interactions (see dashed lines) along with long range. In an effort to
estimate the contribution of the C-H···H-C interaction, we have computed an additional model
where the aromatic ring has been changed by a double bond (see small arrows in Figure 7C). As
a result the interaction is significantly reduced to ∆E1 = -0.5 kcal/mol that is the contribution of
16

the weak C-H···H-C interaction. This value is in good agreement with pervious calculations of
methane dimer. To evaluate the contribution from long-range dispersion interactions between the
bulk of the two molecules, we have used an additional theoretical model (see Figure 7D) where
the methyl groups that participate in both C-H⋅⋅⋅π and C-H···H-C interactions have been replaced
by H atoms (see small arrows in Figure 7D). In this dimer, all interatomic distances are longer
that the sum of the corresponding van der Waals radii. Remarkably, the interaction energy is
significant ∆E3 = -4.9 kcal/mol confirming that the long range interactions are an important part
of the total interaction energy (~47%).
A similar study has been done for complex 2. In Figure 8A we represent a part of the
crystal packing this complex, where an infinite 1D chain formed, that is governed by the
formation of C-H⋅⋅⋅π and C-H···H-C interactions. We have firstly used a theoretical model that
consists of one dimer taken from this infinite chain (using the crystallographic coordinates). In
this dimer, two symmetrically related C-H···H-C interactions are formed involving both ethoxy
arms. Moreover, four C-H⋅⋅⋅π interactions are also established involving aliphatic and aromatic H
atoms. We have used a theoretical model of this self-assembled dimer (see Figure 8B) and the
interaction energy is large ∆E4 = -23.9 kcal/mol. In an effort to estimate the contribution of the
C-H···H-C interaction, we have computed an additional model where the methyl group of each
ethoxy arm has been replaced by a H atom (see small arrows in Figure 8C). As a result the
interaction is slightly reduced to ∆E5 = -21.1 kcal/mol and the difference with ∆E4 is the
contribution of both C-H···H-C interactions (-2.8 kcal/mol). To evaluate the contribution of the
aliphatic C-H⋅⋅⋅π interaction we have used an additional model where the methyl groups that
participate in both C-H⋅⋅⋅π and C-H···H-C interaction has been replaced by H atoms (see Figure
8D). As a result the interaction energy is reduced to ∆E6 = -17.8 kcal/mol and its difference with
17

respect to ∆E5 is the contribution of the aliphatic C-H⋅⋅⋅π interactions (-3.3 kcal/mol). Finally, to
evaluate the contribution of long-range dispersion interactions between the bulk of the two
molecules, we have used an additional theoretical model where the aromatic ring has been
changed by a double bond (see small arrows in Figure 8E). The resulting interaction energy is
∆E7 = -8.4 kcal/mol, which corresponds to the contribution of long range interactions. These
results confirm that focusing excessively on individual short contacts may lead to ignore the
crucial influence of relevant longer-range interactions.
A simpler study has been done for complex 3 since it does not exhibit C-H···H-C
interactions in the solid state. In Figure 9A we represent a self-assembled dimer that is formed in
the solid state, where two types of C-H⋅⋅⋅π are formed involving both ethoxy arms. The
interaction energy of this self-assembled dimer (see Figure 9A) is ∆E3 = -11.0 kcal/mol that
corresponds to the four C-H⋅⋅⋅π and long range interactions. To evaluate the contribution of latter
interactions between the bulk of the two molecules, we have used an additional theoretical model
(see Figure 9B) where the ethyl groups have been replaced by H atoms. As a result the
interaction energy is reduced to ∆E2 = -5.1 kcal/mol. Therefore the contribution of long range
interactions is an important part of the total binding energy (~29%) confirming the crucial role of
these interactions in the solid state.
Finally, we have used the Bader’s theory of “atoms in molecules”, which provides an
unambiguous definition of chemical bonding, to further demonstrate the existence of long range
interactions in complexes 1 and 2. The AIM theory has been successfully used to characterize
and understand a great variety of interactions. In Figure 10 we show the AIM analysis of the
long-range dimers described above in Figures 7D and 8E for 1 and 2, respectively. In both cases
(all interatomic distances are longer that the sum of the van der Waals radii) it can be observed
18

the presence of several bond (red spheres) and ring (yellow spheres) critical points and bond
paths (dashed lines) inter-connecting several atoms thus confirming the existence of the long
range interactions.
3.8. Phenoxazinone synthase like activity
To check the ability of the complexes to oxidize o-aminophenol to 2-aminophenoxazine-
3-one (phenoxazinone synthase mimicking activity), 10-2 M acetonitrile solution of o-
aminophenol were mixed separately with 10-4 M solution of each complex, and the spectra were
recorded for up to 4 h at room temperature. It was found that both complexes 2 and 3 are
showing phenoxazinone synthase mimicking activity, but complex 1 is not. The time dependent
spectral profiles for a period of 4 h after the addition of o-aminophenol in acetonitrile solution of
2 and 3 are shown in Figure 11. The gradual increase of peak (characteristic of the
phenoxazinone absorption) intensity at ~433 nm implies the catalytic conversation of o-
aminophenol to 2-aminophenoxazine-3-one in aerobic conditions. A blank experiment without
catalyst under identical conditions does not show significant growth of the band at ~433 nm.
To understand the extent of the catalytic efficiency, 10-4 M solutions of the complexes were
treated with substrate (o-aminophenol) under pseudo-first order conditions. Time scan at the λ
max
of 2-aminophenoxazine-3-one was carried out for a period of 200 min. The initial rate was
determined from the slope of the absorbance versus time plot. This has been repeated thrice and
the average value was taken. Rate saturation kinetics of complex 3 is clearly indicated in Figure
12 (initial rate of the reaction versus concentration of the substrate plot). This observation
indicates that an intermediate complex substrate adduct is formed at a pre-equilibrium stage and
that the irreversible substrate oxidation is the rate determining step of the catalytic cycle. This
type of saturation rate dependency on the concentration of the substrate may be treated with the
19

Michaelis-Menten model. This may also, upon linearization, give a double reciprocal
Lineweaver-Burk plot which is used to analyze different parameters, viz. Vmax (Maximum
reaction velocity), KM (Michaelis constant), and Kcat (Turnover number). The initial rate versus
substrate concentration plot and Lineweaver-Burk plot for complex 3 are shown in Figures 12
and 13, respectively. KM values for complexes 2 and 3 are 5.08 x 10-4 M and 5.75 x 10-4 M,
respectively. Similarly Vmax values are 4.04 x 10-4 M-1 (for 2) and 4.29 x 10-4 M-1 (for 3). The
Kcat value is obtained by dividing the Vmax by the concentration of the complex used, and are
found to be 4.12 S-1 and 4.29 S-1 for complexes 2 and 3, respectively. A probable mechanistic
pathway for the oxidation of o-aminophenol to 2-aminophenoxazine-3-one is shown in Scheme
4. Initially, o-aminophenol forms adduct with complexes replacing the monodentate coligands,
azide in 2 and thiocyanate in 3. This results in the formation of an o-aminophenol radical by the
reaction with molecular dioxygen regenerating complexes 2 or 3. In the next step, the o-
aminophenol radical may be oxidized to o-benzoquinone monoamine, which, in turn, may be
converted to 2-aminophenoxazine-3-one by the reaction with dioxygen and o-aminophenol
(Scheme 4). It is to be noted here that complex 3 is inactive towards the phenoxazinone synthase
mimicking activity. This may be correlated with its structure. It is a bis-ligand complex and does
not contain any monodentate ligand, which could be replaced with o-aminophenol initiating
catalysis. Similar mechanism has also been proposed by other groups [93,94].
20

Scheme 4: Probable mechanistic pathway for the oxidation of o-aminophenol to 2-
aminophenoxazine-3-one
4. Conclusion
Three new mononuclear octahedral facial cobalt(III) complexes with a Schiff base ligand
have been synthesized and X-ray characterized. The solid state structures show the participation
of the organic ligand in concurrent C-H⋅⋅⋅π and C-H···H-C interactions involving the methoxy
arm and the aromatic ring. The energetic features of these interactions have also been studied by
means of DFT calculations. Moreover, it has been shown that long range interactions that are
usually ignored by supramolecular chemists are very relevant in the stabilization of the different
21

assemblies observed in the solid state of all three complexes. Complexes 2 and 3 are found to
show to phenoxazinone synthase mimicking activity.
Acknowledgement
Crystallographic data of the complexes were collected at the DST-FIST, India-funded
single crystal diffractometer facility at the Department of Chemistry, Jadavpur University.
Appendix A. Supplementary data
CCDC 1408890, 1419342 and 1419343 contain the supplementary crystallographic data
for complexes 1, 2 and 3, respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.poly.2014.11.012.
22

References
[1] J. -P. Costes, G. Cros, M. -H. Darbieu, J. -P. Laurent, Inorg. Chim. Acta 60 (1982) 111-114.
[2] A. Clearfield, R. Gopal, R. J. Kline, M. Sipski, L. O. Urban, J. Coord. Chem. 7 (1978) 163-
169.
[3] D. H. Busch, N. W. Alcock, Chem. Rev. 94 (1994) 585-623.
[4] D. Chen, A. E. Martell, Inorg. Chem. 26 (1987) 1026-1030.
[5] J. L. Dempsey, B. S. Brunschwig, J. R. Winkler, H. B. Gray, Acc. Chem. Res. 42 (2009)
1995-2004.
[6] T. M. McCormick, B. D. Calitree, A. Orchard, N. D. Kraut, F. V. Bright, M. R. Detty, R.
Eisenberg, J. Am. Chem. Soc. 132 (2010) 15480-15483.
[7] W. R. McNamara, Z. Han, C. Yin, W. W. Brennessel, P. L. Holland, R. Eisenberg, Natl.
Acad. Sci. U.S.A. 109 (2012) 15594-15599.
[8] S. Kal, A. S. Filatov, P. H. Dinolfo, Inorg. Chem. 53 (2014) 7137-7145.
[9] W. R. McNamara, Z. Han, P. J. Alperin, W. W. Brennessel, P. L. Holland, R. Eisenberg, J.
Am. Chem. Soc. 133 (2011) 15368-15371.
[10] V. Artero, M. Chavarot-Kerlidou, M. Fontecave, Angew. Chem. Int. Ed. 50 (2011) 7238-
7266.
[11] S. L. Roderick, B. W. Matthews, Biochemistry 32 (1993) 3907-3912.
[12] Y. Nishida, S. Nishino, L. L. Guo, M. Kunita, H. Matsushima, T. Tokii, Inorg. Chem.
Commun. 2 (1999) 609-611.
23

[13] F. B. Johansson, A. D. Bond, U. G. Nielsen, B. Moubaraki, K. S. Murray, K. J. Berry, J. A.
Larrabee, C. J. McKenzie, Inorg. Chem. 47 (2008) 5079-5092.
[14] R. K. Parashar, R. C. Sharma, A. Kumar, G. Mohan, Inorg. Chim. Acta 151 (1988) 201-208.
[15] H. Chen, D. Han, H. Yan, W. Tang, Y. Yang, H. Wang, Polyhedron 12 (1993) 1097-1099.
[16] F. -L. Mao, C. -H. Dai, Synth. React. Inorg., Met-Org., Nano-Met. Chem. 44 (2014) 1059-
1063.
[17] M. Salehi, G. Dutkiewicz, A. Rezaei, A .Amoozadeh, S. Rahmani, G. H.Grivani, M.
Kubicki, J. Chem. Crystallogr. 42 (2012) 871-878.
[18] C. L. Chen, X. F. Zhu, M. X. Li, H. M. Guo, J. Y. Niu, Russ. J. Coord. Chem. 37 (2011)
435-438.
[19] S. Chandra, S. Sangeetika, Spectrochim. Acta A60 (2004) 147-153.
[20] B. J. Hathaway, D. E. Billing, Coord. Chem. Rev. 5 (1970) 143-207.
[21] C. -M. Che, J. -S. Huang, Coord. Chem. Rev. 242 (2003) 97-113.
[22] M. Kojima, H. Taguchi, M. Tsuchimoto, K. Nakajima, Coord. Chem. Rev. 237 (2003) 183-
196.
[23] M. Calligaris, L. Randaccio, in: G. Wilkinson (Ed.), Comprehensive Coordination
Chemistry, Pergamon Press, Oxford., 1987.
[24] B. De Clercq, F. Verpoort, Macromolecules 35 (2002) 8943-8947.
[25] T. Opstal, F. Verpoort, Angew. Chem. Int. Ed. 42 (2003) 2876-2879.
24

[26] B. De Clercq, F. Lefebvre, F. Verpoort, Appl. Catal. A247 (2003) 345-364.
[27] S. L. Lambert, C. L. Spiro, R. R. Gagne, D. N. Hendriekson, Inorg. Chem. 21 (1982) 68-72.
[28] S. Brooker, Coord. Chem. Rev. 222 (2001) 33-56.
[29] V. Amendola, L. Fabbrizzi, C. Mangano, P. Pallavicini, A. Poggi, A. Taglietti, Coord.
Chem. Rev. 219 (2001) 821-837.
[30] P. A. Vigato, S. Tamburini, Coord. Chem. Rev. 248 (2004) 1717-2128.
[31] K. Ghosh, S. Pattanayak, A. Chakravorty, Organometallics 17 (1998) 1956-1960.
[32] E. Kimura, Y. Kodama, T. Koike, M. Shiro, J. Am. Chem. Soc. 117 (1995) 8304-8311.
[33] D. Parker, Chem. Soc. Rev. 19 (1990) 271-291.
[34] D. D. Dischino, E. J. Delaney, J. E. Emswiler, G. T. Gaughan, J. S. Prasad, S. K. Srivastava,
M. F. Tweedle, Inorg. Chem. 30 (1991) 1265-1269.
[35] J. Lewinski, P. Horeglad, M. Dranka, I. Justyniak, Inorg. Chem. 43 (2004) 5789-5791.
[36] D. C. Ware, P. J. Brothers, G. R. Clark, W. A. Denny, B. D. Palmer, W. R. Wilson, Dalton
Trans. (2000) 925-932.
[37] M. Itoh, J. Nakazawa, K. Maeda, K. Kano, T. Mizutani, M. Kodera, Inorg. Chem. 44 (2005)
691-702.
[38] M. Das, K. Harms, S. Chattopadhyay, Dalton Trans. 43 (2014) 5643-5647.
[39] R. Medishetty, R. Tandiana, J. J. Vittal, Cryst. Growth Des. 14 (2014) 3186-3190.
25

[40] J. -J. Liu, Y. -F. Guan, C. Jiao, M. -J. Lin, C. -C. Huang, W. -X. Dai, Dalton Trans. 44
(2015) 5957-5960.
[41] A. Morsali, A. Mahjoub, Polyhedron 23 (2004) 2427-2436.
[42] M. Solimannejad, V. Ramezani, C. Trujillo, I. Alkorta, G. S. -Sanz, J. Elguero, J. Phys.
Chem. A 116 (2012) 5199-5206.
[43] Z. Zhang, J. Li, Y. Yao, S. Sun, Cryst. Growth Des. 15 (2015) 5028-5033.
[44] W. Yang, G. Zhang, W. Weng, B. Qiu, L. Guo, Z. Lin, G. Chen, RSC Adv. 4 (2014) 58852-
58857.
[45] M. Das, S. Chattopadhyay, J. Mol. Struct. 1051 (2013) 250-258.
[46] A. Bhattacharyya, P. K. Bhaumik, M. Das, A. Bauzá, P. P. Jana, K. Harms, A. Frontera, S.
Chattopadhyay, Polyhedron 101 (2015) 257-269.
[47] A. Bauzá, T. J. Mooibroek, A. Frontera, ChemPhysChem 16 (2015) 2496-2517.
[48] J. -J. Liu, Y. -F. Guan, Y. Chen, M. -J. Lin, C. -C. Huang, W. -X. Dai, Dalton Trans. 44
(2015) 17312-17317.
[49] A. Frontera, P. Gamez, M. Mascal, T. J. Mooibroek, J. Reedijk, Angew. Chem. Int. Ed. 50
(2011) 9564-9583.
[50] A. Bhattacharyya, P. K. Bhaumik, A. Bauzá, P. P. Jana, A. Frontera, M. G. B. Drew, S.
Chattopadhyay, RSC Adv. 4 (2014) 58643-58651.
[51] S. Jana, K. Harms, A. Bauzá, A. Frontera, S. Chattopadhyay, Cryst. Growth Des. 15 (2015)
257-267.
[52] S. Cañellas, A. Bauzá, A. Lancho, A. Garcia-Raso, J. J. Fiol, E. Molins, P. Ballesterc, A.
Frontera, CrystEngComm 17 (2015) 5987-5997.
[53] J. Echeverría, G. Aullón, D. Danovich, S. Shaik, S. Alvarez, Nat. Chem. 3 (2011) 323-330.
26

[54] D. Danovich, S. Shaik, F. Neese, J. Echeverría, G. Aullón, S. Alvarez, J. Chem. Theory
Comput. 9 (2013) 1977-1991.
[55] G. M. Sheldrick, Acta Cryst. A64 (2008) 112-122.
[56] G. M. Sheldrick SADABS: Software for Empirical Absorption Correction, University of
Gottingen, Institute fur Anorganische Chemieder Universitat, Gottingen, Germany, 1999-2003.
[57] M. A. Spackman, D. Jayatilaka, CrystEngComm 11 (2009) 19-32.
[58] F. L. Hirshfeld, Theor. Chim. Acta 44 (1977) 129-138.
[59] H. F. Clausen, M. S. Chevallier, M. A. Spackman, B. B. Iversen, New J. Chem. 34 (2010)
193-199.
[60] A. L. Rohl, M. Moret, W. Kaminsky, K. Claborn, J. J. McKinnon, B. Kahr, Cryst. Growth
Des. 8 (2008) 4517-4525.
[61] A. Parkin, G. Barr, W. Dong, C. J. Gilmore, D. Jayatilaka, J. J. McKinnon, M. A.
Spackman, C. C. Wilson, CrystEngComm 9 (2007) 648-652.
[62] M. A. Spackman, J. J. McKinnon, CrystEngComm 4 (2002) 378-392.
[63] S. K. Wolff, D. J. Grimwood, J. J. McKinnon, D Jayatilaka, M. A. Spackman, Crystal
Explorer
2.0;
University
of
Western
Australia:
Perth,
Australia,
2007.
http://hirshfeldsurfacenet.blogspot.com/.
[64] F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen, R. J. Taylor, J. Chem.
Soc., Perkin Trans. 2 (1987) S1-S19.
[65] J. J. Kinnon, M. A. Spackman, A. S. Mitchell, Acta Cryst. B60 (2004) 627-668.
[66] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Lett. 162 (1989) 165-169.
[67] A. Bauzá, A. Terrón, M. Barceló-Oliver, A. García-Raso, A. Frontera, Inorg. Chim. Acta
(2015) DOI: 10.1016/j.ica.2015.04.028.
27

[68] D. Sadhukhan, M. Maiti, G. Pilet, A. Bauzá, A. Frontera, S. Mitra, Eur. J. Inorg. Chem. 11
(2015) 1958-1972.
[69] M. Mirzaei, H. Eshtiagh-Hosseini, Z. Bolouri, Z. Rahmati, A. Esmaeilzadeh, A.
Hassanpoor, A. Bauzá, P. Ballester, M. Barceló-Oliver, J. T Mague, B. Notash, A. Frontera,
Cryst. Growth Des. 15 (2015) 1351-1361.
[70] P. Chakraborty, S. Purkait, S. Mondal, A. Bauzá, A. Frontera, C. Massera, D. Das,
CrystEngComm 17 (2015) 4680-4690.
[71] S. F. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553-566.
[72] R. F. W. Bader, Chem. Rev. 91 (1991) 893-928.
[73] AIMAll (Version 13.05.06), Todd A. Keith, TK Gristmill Software, Overland Park KS,
USA, 2013.
[74] S. Roy, K. Harms, S. Chattopadhyay, Polyhedron 91 (2015) 10-17.
[75] A. Bhattacharyya, P. K. Bhaumik, P. P. Jana, S. Chattopadhyay, Polyhedron 78 (2014) 40-
45.
[76] S. -F. Lou, X. Zheng, X. -Y. Qiu, J. Struct. Chem. 52 (2011) 1127-1130.
[77] R. -H. Hui, P. Zhou, Z. -L. You, J. Struct. Chem. 51 (2010) 1201-1204
[78] S. Chattopadhyay, G. Bocelli, A. Musatti, A. Ghosh, Inorg. Chem. Commun. 9 (2006)
1053-1057.
[79] D. Cremer, J. A. Pople, J. Am. Chem. Soc. 97 (1975) 1354-1358.
[80] J. Müller, C. Würtele, O. Walter, S. Schindler, Angew. Chem. Int. Ed. 46 (2007) 7775-7777.
[81] S. Sen, P. Talukder, S. K. Dey, S. Mitra, G. Rosair, D. L. Hughes, G. P. A. Yap, G. Pilet, V.
Gramlich, T. Matsushita, Dalton Trans. (2006) 1758-1767.
[82] S. Shit, D. Saha, D. Saha, T. N. G. Row, C. Rizzoli, Inorg. Chim. Acta 415 (2014) 103-110.
28

[83] A. Ray, G. M. Rosair, R. Kadam, S. Mitra, Polyhedron 28 (2009) 796-806.
[84] M. A. Spackman, P. G. Byrom, Chem. Phys. Lett. 267 (1997) 215-220.
[85] S. Roy, S. Chattopadhyay, Inorg. Chim. Acta 433 (2015) 72-77.
[86] S. Roy, K. Harms, S. Chattopadhyay, Polyhedron 91 (2015) 10-17.
[87] A. Bhattacharyya, S. Sen, K. Harms, S. Chattopadhyay, Polyhedron 88 (2015) 156-163.
[88] M. Das, S. Chattopadhyay, Polyhedron 50 (2013) 443-451.
[89] S. Chattopadhyay, M. G. B. Drew, A. Ghosh, Eur. J. Inorg. Chem. (2008) 1693-1701.
[90] A. Saha, P. Majumdar, S. Goswami, J. Chem. Soc. Dalton Trans. (2000) 1703-1708.
[91] R. Vafazadeh, M. Kashfi, Bull. Korean Chem. Soc. 28 (2007) 1227-1230.
[92] S. Thakurta, R. J. Butcher, G. Pilet, S. Mitra, J. Mol. Struct. 929 (2009) 112-119.
[93] A. Panja, Polyhedron 80 (2014) 81-89.
[94] A. Panja, M. Shyamal, A. Saha, T. K. Mandal, Dalton Trans. 43 (2014) 5443-5452.
29