Deactivation of catecholase-like activity of a dinuclear Ni(II) complex by incorporation of an additional Ni(II)

Article abstract of DOI:10.1016/j.molcata.2016.08.012

A new dinuclear Ni(II) complex [Ni₂L₂(PhCOO)(H₂O)₂]ClO₄ (1) has been synthesized by reacting Ni(ClO₄)₂·6H₂O with a reduced Schiff base ligand [(3-dimethylamino-propyla

Full text of DOI:10.1016/j.molcata.2016.08.012

Accepted Manuscript
Title: Deactivation of Catecholase-like Activity of a Dinuclear
Ni(II) Complex by Incorporation of an Additional Ni(II)
Author: Monotosh Mondal Pampa M. Guha Sanjib Giri
Ashutosh Ghosh
PII:
S1381-1169(16)30336-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.08.012
MOLCAA 9999
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
11-6-2016
8-8-2016
12-8-2016
Please cite this article as: Monotosh Mondal, Pampa M.Guha, Sanjib Giri, Ashutosh
Ghosh, Deactivation of Catecholase-like Activity of a Dinuclear Ni(II) Complex by
Incorporation of an Additional Ni(II), Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.08.012
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Deactivation of Catecholase-like Activity of a Dinuclear Ni(II)
Complex by Incorporation of an Additional Ni(II)
Monotosh Mondala,b, Pampa M. Guhaa,c, Sanjib Giria, Ashutosh Ghosha*
a
Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C.
Road, Kolkata-700 009, India.
b
Department of Chemistry, Haldia Government college, Debhog, Purba Medinipur-721657,
India.
c
Department of Chemistry, The Heritage College, 994, Madurdaha, chowbaga Road,
Anandapur, Kolkata – 700107.
*E-mail: ghosh_59@yahoo.com; Fax: +91-33-2351-9755; Tel: +91-94-3334-4484.
1

Graphical abstract
Ph
O
O
O
N
H2O
N
Ni
Ni
N
OH2
O
N
O
OH
OH
O
O2
H2O2
Two new Ni(II) complexes [Ni2L2(PhCOO)(H2O)2]ClO4 (1) and [Ni3L2(PhCOO)4] (2) has been
synthesized by reacting Ni(II), HL1 and benzoate in a different ratio. complex 1, of reduced
Schiff base ligand (which is redox innocent) shows significant catecholase-like activity, but
complex 2 is inactive towards catechol oxidation. From the magnetic point of view, Ni(II)
centres are antiferromagnetically coupled in 1 but ferromagnetically coupled in 2.
Research Highlights
 Synthesis of nickel complexes with reduced Schiff base ligand.
 Mimicking catecholase like activity with 3,5-di-tert-butylcatechol (3,5-DTBC).
 Magneto-structural correlation for mixed bridged Ni(II) complexes.
 DFT calculations support the experimental findings.
Abstract
A new dinuclear Ni(II) complex [Ni2L2(PhCOO)(H2O)2]ClO4 (1) has been synthesized by
reacting Ni(ClO4)2.6H2O with a reduced Schiff base ligand [(3-dimethylamino-propylamino)-
methyl]-phenol (HL1) in presence of benzoic acid in 1:1:1 ratios. Structural analysis reveals that
complex 1 is a di-μ2-phenoxo-bridged dinuclear Ni(II) complex and has an additional syn-syn
benzoate bridge. Enzyme kinetic studies reflect that it (1) is effective in mimicking catecholase
like activity with 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate. Kinetic measurements
suggest that the rate of catechol oxidation follows first order kinetics with respect to the catalyst.
The possible intermediates during the course of reaction are identified from ESI-MS spectra. The
2

characteristic EPR spectra of the complex in the presence of 3,5-DTBC supports the formation of
nickel(II)-semiquinonate as an intermediate species during the catalytic oxidation. When the
synthesis is carried out with Ni(II): HL ratio of 3:2, a different compound [Ni3L2(PhCOO)4] (2)
is produced incorporating an additional Ni(II)-benzoate. 2 possesses a linear trinuclear structure
with the tridentate reduced Schiff base ligand coordinated to the terminal nickel atoms which are
linked to the central Ni(II) by phenoxo and carboxylate bridges. Unlike 1, it does not show any
catalytic activity towards catechol oxidation. Variable temperature magnetic susceptibility
measurements have also been performed for 1 and 2. Complex 1 indicates the presence of
intradimer antiferromagnetic interactions (J = −5.65 cm-1). In contrast to complex 1,
ferromagnetic coupling (J = 3.55 cm-1) is observed in complex 2.
Keywords: Nickel(II)/ Reduced Schiff base/ Crystal structure/ Catecholase activity/ Magnetic
measurements
Introduction
Nature uses transition metals in different biological processes and it drives the quest of the
scientist to understand the fundamental principles of its functionality, which eventually helps to
develop different structural and more importantly functional model systems [1–8]. Transition
metal induced several biological processes are well known over the years. Apart from these,
many new molecules have been also developed over the years showing interesting properties like
antibacterial, antifungal, antimicrobial activity where the transition metal ion performs crucial
role in terms of structural organization and overall functionality [9-12]. Especially the presence
of two transition metal ions in close proximity at the active site of several metallo-proteins and
metallo-enzymes has simulated interest in the synthesis and study of multinuclear transition
metal complexes. There is also continuing interest in mechanistic studies on the catalytic
oxidation of catechols by transition metal complexes because this reaction plays key role in the
metabolism of aromatic compounds [13-34]. Oxidation of a range of o-diphenols (catechols) in
presence of the enzyme catecholase to corresponding quinone is known as catecholase activity.
Although crystal structure determination of the met-form of catecholase revealed that the active
site of the enzyme consists of a hydroxo bridged dicopper(II) centre, many other metal ions e.g
Mn(II/III), Fe(III), Co(II/III), Zn(II) have also been reported in literature that mimic catecholase
3

activity [26-34] Study of the Ni(II) complexes in this field is comparatively rare. Only few
mono- and dinuclear Ni(II) complexes have been studied for the possible catechol oxidase like
properties [13,19-25,66-71]. All the previous findings, mainly with copper-based model
complexes reveal that the metal ion oxidation state, ligand environment, nuclearity of the
complexes, etc. play important role(s) in catalyzing catechol and its derivative to their
corresponding quinones. However, for catecholase-like activity of Ni(II) complexes, no such
clear structural correlations have been drawn because the number of complexes studied for this
purpose are comparatively rare.
We have synthesised a new dinuclear Ni(II) complex, [[Ni2L2(PhCOO)(H2O)2]ClO4]
with N,N,O donor reduced Schiff base ligand (3-dimethylamino-propylamino)-methyl]-phenol
(HL1)). Examination of catecholase-like activity for the complex reveals that complex 1 acts as
catalysts for the areal oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone. A
monomeric nickel(II)-semiquinonate intermediate which is identified through EPR and ESI-MS
measurements is thought to be responsible for this oxygenation reaction. ESI-MS positive
spectra also signify the presence of a bis(-oxo)dinickel(II) intermediate during the aerobic
oxidation of catechol.
We were further curious to observe the structural, magnetic and catalytic effect of excess
Ni(II) to the dinuclear entity. Hence, we added excess Ni(II) to complex 1 in different ratios in
solution and finally succeeded to isolate a trinuclear entity [Ni3L2(PhCOO)4] (2) with Ni(II): HL:
benzoate 3:2:4. Interestingly, unlike 1, complex 2 does not show any catalytic activity towards
catechol oxidation. Magnetic susceptibility measurements indicate antiferromagnetic exchange
interaction mediated between the Ni(II) centers in 1 and ferromagnetic coupling in 2. Further
insight in magneto-structural correlations, we have also performed theoretical calculations to find
the dependence of J on various bridging pathways. Hence, these two compounds demonstrate
nicely the importance of the ratio of different constituents (i,e metal, ligand and carboxylate) in
determining the nuclearity of the complexes and their effect on catalytic activity.
Experimental section
Caution! Perchlorate salts of metal complexes are potentiallyexplosive; and caution should be
exercised when dealing with such derivatives.
4

Materials and General Methods.
Reagents and solvents were purchased from various commercial sources and used without
further purification unless otherwise stated.
Synthesis of the Schiff-base ligand [(3-dimethylamino-propylamino)-methyl]-phenol (HL1)
A methanolic solution of salicyldehyde (0.52 ml, 5 mM) and N,N-dimethyl-1,3-propanediamine
(0.63 ml, 5 mM) has been refluxed for 1 h at 60˚C. The methanolic solution was subsequently
cooled to 0˚C and solid sodium borohydride (210 mg, 6 mM) was added slowly with constant
stirring. After completion of the addition, the resulting reaction mixture was acidified with
concentrated HCl (5 mL) and then evaporated to dryness. The reduced Schiff-base ligand HL1
was extracted from the solid residue with methanol; the methanolic solution obtained (ca. 20
mL) was used for preparation of the coordination compounds.
Synthesis of the complex [Ni2L2(PhCOO)(H2O)2]ClO4 (1)
A methanolic solution of Ni(ClO4)2·6H2O(1.828 g, 5 mM) was added to a methanolic solution
(10 mL) of the reduced Schiff base (HL1) (5 mmol) with constant stirring. After ca.10 min a
methanolic solution of benzoic acid, PhCOOH (0.610 g, 5 mM) was added with slow stirring
followed by addition of triethylamine (0.70 mL, 5 mM). After evaporation of the solvents to
dryness, the resulting green solid was filtered and washed with diethyl ether and dissolved in
CH3CN. Slow evaporation of the resulting green solution gave X-ray quality deep-green single
crystals of compound 1.Yield: 1.37 g.(70%). Anal Calc. for C31H47ClN4Ni2O10: C, 47.22; H,
6.01; N, 7.10. Found: C, 47.25; H, 6.00; N, 7.02. IR (KBr pellet): IR (KBr pellet): ν(O-H)broad,
3465cm-1, ν(N-H), 3275 cm-1, ν(C-N), 1569 cm-1, νasym(C=O), 1610 cm-1, νsym(C=O), 1480 cm-1.
Synthesis of the complex [Ni3L2(PhCOO)4] (2)
Ni(ClO4)2·6H2O(1.828 g, 5 mM), dissolved in 10 mL of methanol, was added to a methanolic
solution (10 mL) of complex 1 (3.94 g, 5 mM) with constant stirring. After ca.25 min a
methanolic solution of benzoic acid, PhCOOH (1.22 g, 10 mM) was added with slow stirring
followed by addition of triethylamine (1.40 mL, 10mM). Removal of solvent results a dark green
solid. The green solid was then washed with diethyl ether and dissolved in CH3CN. Layering the
acetonitrile solution with Et2O result X-ray quality deep-green single crystals of compound 2.
5

Yield: 1.43 g.(80%). Anal Calc. for C52 H58 N4 Ni3 O10: C, 58.09; H, 5.44; N, 5.21. Found: C,
58.10; H, 5.40; N, 5.18. IR (KBr pellet): ν(N-H), 3224 cm-1, ν(C-N), 1564 cm-1 ν(C=O), 1459
cm-1. νasym(C=O), 1600 cm-1, νsym(C=O), 1484 cm-1
Alternative method for the synthesis of compound 2
This deep green compound was also prepared in the same method as described for Complex 1.
Only difference is the proportion of the added reagent - Ni(ClO4)2·6H2O, Schiff base (HL1) and
benzoic acid, PhCOOH (0.488 g, 4 mM) ratio is maintained at 3:2:4 instead of 1:1:1 as in case of
complex 1.
Physical Measurements.
Elemental analyses (C, H and N) were performed using a Perkin-Elmer 240C elemental analyzer.
IR spectra in KBr pellets (4500-500 cm-1) were recorded using a Perkin-Elmer RXI FT- IR
spectrophotometer. Electronic spectra (1500-250 nm) were recorded in a Hitachi U-3501
spectrophotometer. The X-band (9.13 GHz) EPR experiment were carried out using a JEOL JES-
FA 200 instrument at liquid nitrogen temperature (77 K) in methanol. Temperature-dependent
molar susceptibility for powdered sample of 1-2 were measured with a superconducting quantum
interference device vibrating sample magnetometer (SQUID-VSM, Quantum Design) with an
applied field 500 Oe throughout the temperature range 2–300 K. Isothermal magnetizations
measurements were performed at 2K up to 5 Tesla.
For the investigation of catecholase activity of the complexes, complex 1 was treated with 100
equiv. of 3,5-di-tert-butylcatechol (3,5-DTBC) in methanol under aerobic condition at room
temperature.. The reaction was monitored spectrophotometrically with the increase in the
absorbance at 400 nm (Quinone band maxima) as a function of time (time scan).For
determination of hydrogen peroxide during the catalytic reaction, iodometric method was
followed as reported earlier [30].
Computational Methodology
The theoretical calculation based on broken symmetry DFT has been used to estimate the
coupling constant (J) of these complexes and consequently, the J values have been derived from
the Equation 1 as proposed by Ruiz et al [35]. The SCF calculations were accomplished with
consideration of experimental X-ray structures. The hybrid B3LYP functional [36-38] and
6

widely employed TZVP [39] basis set has been considered in all calculations as implemented in
the ORCA package [40].The zeroth-order regular approximation (ZORA) has been included to
describe scalar relativistic effects[41]. RI approximation with auxiliary TZV/J coulomb fitting
basis set and tight SCF convergence criteria (Grid4) have also been incorporated [42].
Ebs  Ehs
2S1S2  S1
2J 
Where S1≥S2
…………… (1)
Crystal Data Collection and Refinement.
Suitable single crystals of each of the two complexes were mounted on a Bruker SMART
diffractometer equipped with a graphite monochromator and MoKα (λ = 0.71073 Å) radiation.
The crystals were positioned 60 mm from the CCD. 360 frames were measured with a counting
time of 5 s. The structures were solved by the Patterson method by using SHELXS 97.
Subsequent difference Fourier synthesis and least-squares refinement revealed the positions of
the remaining non-hydrogen atoms. However, for complex 1, the four oxygen atoms of the
perchlorate anion was refined in a disordered manner in six positions. The non-hydrogen atoms
were refined with independent anisotropic displacement parameters. All calculations were
carried out by using SHELXS 97, [43] SHELXL 97, [44] PLATON 99, [45] ORTEP-32, [46]
and WinGX Ver-1.64 [47]. Data collection and structure refinement parameters as well as the
crystallographic data for the two complexes are given in Table 1. CCDC 852926 (for 1), and
852928 (for 2) contain the supplementary crystallographic data for this paper. These data can be
obtained free of charge from Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Results and discussion
Syntheses of the complexes
Complex 1 was obtained as a consequence of reaction of reduced tridentate Schiff base ligand,
HL1 with nickel (II) perchlorate in methanol, followed by addition of benzoic acid in 1:1:1 molar
ratio. Increase the Ni/L ratio leads to the formation of dinuclear complex 1 alongwith a trinuclear
complex. Finally, when the Ni/L ratio reached 3:2, the pure free trinuclear compound (2) was
obtained. Hence compound 2 has been prepared by addition of methanolic solution of ligand
(HL1) to the methanolic solution of Ni(II) perchlorate and PhCOOH with HL1:Ni:PhCOOH
7

2:3:4 ratio, In an alternative method, complex 2 has also been prepared by addition of Ni(II)
perchlorate to the methanolic solution of complex 1 followed by addition of benzoic acid
(Scheme 1). The deprotonation of the benzoic acid in both cases was achieved with the required
equimolar amounts of triethylamine. For both of the complexes, x-ray quality single crystals
have been obtained by slow evaporation of the acetonitrile solution.
IR and UV-Vis Spectra of Complexes.
A moderately strong, sharp peak due to a N−H stretching vibration, at 3275 and 3224 cm−1 for
complexes 1 and 2, respectively is obtained. In the IR spectra of complex 1, the broad band near
3465 cm-1 may be attributed to the presence of the O-H stretching of the water molecule. In 1,
the strong band at 1610 cm-1 and 1480 cm-1 are likely to be due to the antisymmetric and
symmetric stretching mode of the carboxylate group and similarly in 2, the strong bands at 1600
cm-1 and 1484 cm-1 can be attributed to the antisymmetric and symmetric stretching modes of the
carboxylate group respectively. In complex 1, the characteristic strong peak of the stretching
vibration of uncoordinated perchlorate is observed at 1113 cm-1.
The solid-state reflectance spectra of the complexes show a broad band centered at 1115,
1008 nm for complexes 1 and 2, respectively. These broad bands are well-separated from the
second transition observed at ca. 630, and 642nm for 1 and 2, respectively. All these bands are
the typical ones for octahedral Ni(II) complexes.
Structure of [Ni2L2(PhCOO)(H2O)2]ClO4 (1).
Single-crystal X-ray diffraction studies on the crystals revealed a dimeric structure of 1 with the
molecular formula [Ni2L2(O2CPh)(H2O]+. A view of the molecule is shown in Figure 1. A non
-
coordinated ClO4 anion counterbalances the charge on the cation. In this dinuclear entity, both
the nickel(II) centres are in an octahedral environment being coordinated by two bridging μ2-
phenoxo oxygen atoms O(10) of the Schiff-base ligand, O(2) of a bridging bidentate benzoate
ligand (1κO:2κO’), by the two amino nitrogen atoms N(18) and N(22) of the Schiff base ligand
and an oxygen atom O(1) from a water molecule (trans to the benzoate O(2) atom, Figure 1).
Schiff base ligand (L1) coordinates the metal centre in meridional fashion. The Ni-O (10), Ni-
N(18), and Ni-N(22) bond distances (Table 2) are similar to those found in other related
complexes [48-53]. The two Ni atoms are separated by 3.11(1) Å and the Ni-O(11)-Ni angle is
8

97.56(12)°. A bifurcated H-bond is formed between one of the hydrogen atoms of the two
coordinated water molecule (H5W) and oxygen atom (O8) of noncoordinated perchlorate
molecule. Other two oxygen atoms of perchlorate (O5 and O6) are engaged in weak hydrogen
bonding with H29 of benzoate and H20b of amine forming a 2D porous sheet (Figure S1).
Structure of [Ni3L2(O2CPh)4] (2). Single crystal X-ray diffraction (XRD) study demonstrates
trinuclear structure of complex 2 (Figure 2). Three nickel(II) ions form a linear array. The
coordination geometry around the three nickeI(II) centre is approximately octahedral. Two
terminal nickel(II) ions are coordinated by the two amine nitrogen atoms (N(18) and N(22)) and
a phenoxo oxygen atom O(10) of tridentate reduced Schiff base ligand. The remaining site is
coordinated by O(2) of a bridging bidentate benzoate ligand and two oxygen atoms O(3) and
O(4) of a second chelating bidentate benzoate ligand. The bond lengths and bond angles, as
summerised in Table 3 are similar to the previously reported complexes [54-59]. The tridentate
reduced Schiff base ligand coordinates the metal centre in meridional fashion. The central metal
ion Ni(2) is also octahedrally coordinated by two bridging phenoxo oxygen atoms, O(10) and
O(10’), two oxygen atoms, O(1) and O(1’) from two bridging benzoate groups and two more
oxygen atoms O(3) and O(3’) from the second bridging bidentate benzoate ligand (1κO:2κO’).
Two benzoate ligands show different coordination modes (1κO:2κO’) and (1κO:2κ2O,O’) to the
nickel centre. The long distance between Ni(1) and Ni(2) (3.093(4)Å), indicates the absence of
any bond between the two nickel centers and are similar to those found in other previously
reported complexes [54-59]. The Ni(1)-O(3)-Ni(2) and Ni(1)-O(10)-Ni(2) bridge angles are
91.90(1) and 99.60(9), respectively. The amino N(18)-H forms a weak hydrogen bond with O(4)
of benzoate.
Catecholase-like Activity Studies:
Catecholase like activity of the synthesized metal complexes was studied by using 3,5-di-tert
butylcatechol (3,5-DTBC) as the substrate due to its low redox potential for the quinone–
catechol couple, (Scheme 2), and its bulky substituents, which make further oxidation reactions
such as ring opening slower. The reactions were monitored by the UV-Vis spectroscopic
technique as the oxidised product 3,5- DTBQ shows a maximum absorption at about 400nm. To
9

determine the catecholase activity of complexes 1 and 2, 100 equiv of 3,5-di-tertbutylcatechol
(3,5-DTBC) in methanol were added to 10−4 M solutions of 1 or 2 in methanol under aerobic
condition at room temperature. The progress of the reaction was investigated by UV−vis spectra
of the mixture at 5 min interval times. Complex 1 show gradual increase of an absorption band
around 400 nm in UV−vis spectroscopy but complex 2 does not show any such changes. This
indicates that only complex 1 catalyzes the oxidation of 3,5-DTBC to 3,5-DTBQ. Hence, the
detailed kinetic studies were only performed on complex 1. Figure 3 presents the variation of the
spectral behavior of complex 1 in the presence of 3,5-DTBC. From the figure, it is clear that the
spectra of 1 in methanol solution shows drastic changes immediately after addition of 3,5-DTBC.
To understand the kinetic aspects, the rate was determined,by traditional initial rate methods by
following the growth of the quinone band at 400 nm as a function of time. The rate constant for
a particular complex substrate mixture was determined from the log[Aα/(Aα − At)] vs time plot
(Figure 4). The substrate concentration dependence of the oxidation rates and various kinetic
parameters were determined by using 10−4 M solutions of 1 with different concentrations of 3,5-
DTBC under aerobic conditions. In all cases, a first-order kinetic dependence was observed at
low concentrations of 3,5-DTBC, whereas higher concentrations resulted in saturation kinetics.
The Michaelis−Menten model was applied to analyse the observed rate versus substrate
concentration data of enzymatic kinetics. The Michaelis–Menten constant (KM) and maximum
initial rate (Vmax) were determined by linearization using Lineweaver– Burk plots and values of
kinetic parameters Vmax, KM, and Kcat was also calculated. Both the observed rate vs [substrate]
and Lineweaver−Burk plot for complex 1 are shown in Figure 4. The kinetic parameters for both
cases are listed in Table 4. The kcat values can be calculated by dividing the Vmax values by the
concentration of the corresponding complexes (Table 4). To determine the substrate
concentration dependence of the rate and to determine various kinetic parameters, 10−4
M
solutions of the different complexes were treated with 10, 30, 50, 70, and 100 equivalents of
substrate.
The catalytic action of many dinuclear copper complexes has been developed over the past few
decades.1-12 It has been proposed that reaction goes always via dinuclear active species. On the
contrary, for the Ni(II) systems, previous reports have shown that the dinuclear Ni(II) complexes
retain it’s Ni2 structural core in solution and behaves as an active species in catechol oxidation
10

whereas the mononuclear complexes respond to catechol oxidation in it’s mononuclear active
species form. In both the cases, reaction goes via radical intermediate species. Moreover, most
of the previously reported Ni(II) complexes, showing catecholase-like activity are of Schiff base
ligands and it is presumed that the Schiff base undergoes one electron reduction during the
catalytic cycle. But in this study we have shown here that the Ni(II) complexes of redox innocent
reduced Schiff base can also show significant catecholase-like activity. We have also proposed a
mechanism for the catalytic cycle of the oxidation of 3,5-di-tert butylcatechol . In the present
system, we have shown for the first time that the initial dinuclear core breaks into mononuclear
one and this mononuclear species becomes catalytically active. This phenomena has not been
clearly shown in the previously reported Ni(II) complexes. Hence to rationalize the probable
mechanism (Scheme 3) of catecholase activity of 1, we have investigated the probable complex-
substrate intermediate through ESI-MS, EPR spectroscopy and qualitative as well as quantitative
−
detection of the I3 band (353 nm.) by UV-Vis spectroscopy for indication of formation of H2O2
during the catalytic oxidation procedure.
The binding of 3,5-DTBC to the complexes has been investigated by mass spectrometry
(ESI; positive mode); hence, ESI-MS spectra have been recorded in methanol. ESI-MS for a 1 :
100 mixture of the proportionate complex 1 and 3,5 DTBC, recorded after 5 min of mixing. The
base peak at m/z = 651.05 (100%) observed in the mass spectra of 1 can be assigned to the
cationic species [Ni2(L)2(PhCOO)]+ (Figure S2, Supporting Information). It thus appears that di-
nickel(II) species are maintained in solution. After addition of 3,5- DTBC to methanol solution
of 1, it exhibits two additional major peaks at m/z = 243 and 463 respectively (Figure S3). These
two peaks correspond respectively to the quinone-sodium aggregates [(3,5-DTBQ)Na]+ and
[(3,5-DTBQ)2Na]+[16]. The peak at m/z = 973.49 [B+H]+ and 995.47 [B+Na]+ (Figure S4) could
be due to the formation of complex-substrate aggregate (“B” in Scheme 3). It proves that the
dinuclear unit is capable of binding two units of 3,5- DTBC, which allows its metal-mediated
oxidation to 3,5- DTBQ. Another peak at m/z 486.23 (Figure S5) could be due to the
mononuclear radical intermediate “D” (Scheme 3). Hence it may be expected that the dinuclear
entity first binds with two units of 3,5- DTBC, followed by its cleavage to a mononuclear entity.
The formation of radical intermediate has also been proved by solution EPR spectra, which is
dominated by an isotropic signal around giso=1.99 with a peak-to-peak line width of ca. 40 G
(Figure 5). The g value of the signal is closed to 2.0023 (value for a free electron), a value that is
11

characteristic for organic radicals, such as a phenoxyl-type radical [23]. Based on the above
observations, we have proposed a probable mechanism for the catalytic cycle of the oxidation of
3,5-di-tert butylcatechol and it is predicted that the dioxygen of the atmosphere is reduced to
H2O2 during the oxidation process.
Iodometric method has been followed to detect the formation of hydrogen peroxide
during the catalytic reaction [20]. The formation of H2O2 has been estimated both qualitatively
and quantitatively; estimation of H2O2 clearly shows that, the % of generation of H2O2 at first
gradually increases with time and after 1 h of oxidation, it become saturated with generation of
64.67% of H2O2, generated with respect to the formation of 3,5-DTBQ (Table 5). A plot of % of
generation of H2O2 vs time has been shown in Figure 6. Comparison of the catecholase activity
of mono and dinuclear Ni(II) complexes in MeOH and MeCN solvent has been depicted in Table
S4 [22,23,66-71].
The activity of the dinuclear complex 1 may be attributed to the fact that presence of two weekly
coordinated solvent water molecules can be removed to facilitate catalyst binding whereas
complex 2 being a neutral rigid molecule has no option to interact with catechol and hence does
not show any catalytic activity.
Magnetic properties
The thermal variation of the product of the molar magnetic susceptibility with temperature (χMT)
for compound 1 is displayed in Figure 7a. It is seen that the compound shows room temperature
χMT values of ca. 2.25 cm3 K mol-1 and it is in agreement with a magnetically noninteracted
Ni(II) dimer having spin only value of 2.0 cm3 K mol-1 for g = 2 at 300K. On cooling, χMT value
slowly goes down; however, a more pronounced rapid decrease is seen at very low temperature
and finally the value reaches to 0.025 cm3 K mol-1 at 1.8 K. The nature of the curve indicates for
antiferromagnetic exchange interaction between the Ni(II) centers. Isothermal magnetizations
curve at 2K indicates that the molar magnetization is almost linear up to 5T and attained a
maximum value of 1.53μB as shown in Figure S6a. This linear nature of the magnetization curve
and the smaller saturation value (expected value of ca. 2μB) also imply the presence of dominant
antiferromagnetic exchange interactions. To estimate the strength of magnetic interaction, we
have fitted with relevant van Vleck’s susceptibility equation of the magnetic susceptibility based
on the isotopic Heisenberg Hamiltonian (H =–2JS1S2) as shown in Equation 2 [23]. The best
12

fitted result provides the following parameters: g = 2.16 ± 0.10, J = –5.65 ± 0.04 cm-1 and R =
4.6×10-5. Here, R represents the agreement factor and it can be defined as R =∑ [(χMT)exp
(χMT)calc]2/∑[(χMT)exp]2.
−
Ng2 2 2e2x 10e6x
 MT 

x = J ∕ kT
(2)
k
13e2x 5e6x
The thermal variation of χMT for linear trimeric Ni(II) complex 2 is shown in Figure 7b. The
room temperature χMT value, ca. 3.91 cm3 K mol-1 is close to the expected spin only value for
magnetically non-interacted Ni(II) trimer (3.0 cm3 K mol-1). Upon cooling from room
temperature, χMT slowly increases down to ca. 150K, after that the value rises rapidly to attain a
maximum value of ca.5.71 cm3 K mol-1 at ca. 8K.
Thereafter, at very low temperature an abrupt decrease of χMT value is observed and a value of
ca. 4.33 cm3 K mol-1 is reached at 2 K. This behavior suggests for the presence of a dominant
ferromagnetic coupling. The sharp decrease at very low temperature should be attributed to the
presence of a zero field splitting and/or intertrimer antiferromagnetic interactions [59].
Isothermal molar magnetization at 2K shows that the saturated molar magnetization value at 5T
is 5.5μB as shown in Figure S6b. The intertrimer interaction and ZFS factor might cause the
slight lowering from the ideal value. To calculate the J value, we have considered the relevant
van Vleck’s susceptibility equation corresponding to isotropic spin Hamiltonian H=–
2J(S1S2+S2S3) for linear NiII trimeric system. We have considered molecular field approximation
to take account of the intermolecular interaction and zero field splitting factor (ZFS). It is worth
to note that the molecular field approximation provides combined result for the contribution of
intermolecular interaction and ZFS; therefore it is not possible to extract separate contributions
for both of them. Taking consideration of isotropic spin Hamiltonian with molecular field
approximation, we have fitted the magnetic susceptibility data with Equation 3 and the best fitted
data provide the following parameters: g = 2.25 ± 0.04, J = 3.55 ±0.01 cm-1, zJ′ = –0.025 ±0.001
cm-1 and R = 5×10-5 [59].
2Ng2β2
3 + 42e4x + 15e2x + 18e−2x + 3e−6x
3 + 7e4x + 5e2x + 8e−2x + e−4x + 3e−6x
χtmrimer
=
× [
]
where x = J/kT
3k
13

χtmrimer
χtmotal
=
… … … . (3)
2zJ´χtmrimer
[1 − (
)]
Ng2β2
Magneto-structural correlations
Since last two decades, several magneto-structural correlations have been drawn for different
polynuclar transition metal systems. The correlation for hydroxo or phenoxo bridged Cu(II)
dimer is most common one and well-established. Irrespective of metal ion, all these correlations
show that the exchange interaction (J) has a strong dependence on metal bridging angle(s). For
example, the di-phenoxido bridged Ni(II) dimers show almost linear dependence of J with Ni-O-
Ni bridging angle [60-61]. If the angle is less than a critical value of ~97°, the effective overlap
of magnetic orbitals is less and ferromagnetic interaction occurs due to orthogonality of magnetic
orbital; however, for larger angles the overlap of magnetic orbitals is enhanced and as a result,
antiferromagnetic coupling is observed. Based on theoretical calculation, Alvarez and his
coworkers reported that the out-of-plane shift of the phenoxido rings and the hinge distortion of
the M2O2 core framework can also change the J value [62]. A large out-of plane shift of the
phenoxido rings results in lowering of antiferromagnetic contribution.
For a better insight in magnetic exchange pathway, we adopted spin-flip DFT
calculations using broken symmetry approach to calculate J values as well as spin density maps.
The DFT calculation provides the values of J = –6.74 cm-1 for 1 and +3.63 cm-1 for 2, which
agree well with the experimentally obtained values of J = –5.65 cm-1 and +3.55 cm-1
respectively. The molecular structure of complex 1 shows that the Ni(II) atoms are connected
with each other via two phenoxido bridges and one μ2-1,3 carboxylate bridge. It can be assumed
that the overall exchange coupling (J) is the vector sum of three individual contributions from
three bridges and it is mediated through two distinct exchange pathways viz. (i) two phenoxido
bridges and (ii) μ2-1,3 carboxylate bridge. It has been established that the magnetic exchange
through multiple bridges can add or counterbalance their effects through different interaction
between the HOMOs of each of the bridging ligands with the combinations of the magnetic
orbitals of metal centers and the phenomena are called as orbital complementarity and
countercomplementarity, respectively [63]. In case of complex 1, the effect of orbital
complementarity or countercomplementarity is expected. The individual magnetic exchange for
phenoxido group is expected to be antiferromagnetic as the Ni(II)…Ophenoxido…Ni(II) angle
14

(97.56°) exceeds the critical value of 97°. It is well known that the syn-syn bridging
coordination mode of carboxylate groups usually causes AF coupling. However, the contribution
through carboxylate can also be ferromagnetic due to countercomplementarity effect. To check
the existence of ferro- or antiferromagnetic contribution through syn-syn carboxylate bridge, we
performed DFT calculations on model structure. Initially, we have taken the structure of complex
1 and then a model structure was built by replacing the syn-syn carboxylate group with two non-
bridging water molecules and kept the rest of the structure unaltered. It is worth to note that the
oxygen atoms of carboxylate group for complex 1 and the oxygen atoms in non-bridging water
molecules in model structure are placed in same crystallographic coordinate. The result shows
that the model complex exhibits stronger antiferromagnetic (J =
with complex 1 (J =
6.74 cm-1). The increased antiferromagnetic coupling for the model
̶
12.4 cm-1) interaction compared
̶
compound indicates ferromagnetic contribution of syn-syn carboxylate bridge in the complex.
Therefore, a countercomplementarity effect is working between ferromagnetic interaction of syn-
syn carboxylate group and the antiferromagnetic contribution of di-phenoxido bridging group
and the antiferromagnetic contribution dominates over the ferromagnetic one leading to a weak
negative J.
The spin density mapping can also help us to identify the nature of exchange interaction
through syn-syn carboxylate bridge. If the carbon atom of syn-syn coordinating carboxylate
possesses a negative spin density in high spin state, then ferromagnetic interaction mediates
through spin polarization mechanism, otherwise antiferromagnetic interaction dominates as a
result of spin delocalization. According to MO theory, larger the negative spin density at
carboxylate carbon atom, stronger is the ferromagnetic interaction [64]. The Mulliken’s spin
density at high spin state of carboxylate carbon atom of 1 is calculated to be –0.00505 au and
hence it should provide weak ferromagnetic coupling. Conversely, the dominance of
antiferromagnetic contributions as result of effective spin delocalization through di-phenoxido
bridging group is also noticed from spin density plot and Mulliken’s atomic spin densities as
depicted in Figure S7 and Table S1, respectively. In this context, the spin delocalization is seen
2
2
from density at phenoxido oxygen and it follows a σ-type exchange pathway involving the dx −y
2
and dz orbitals of the Ni(II) ions and the p-orbitals of the phenoxido group. It is also to be noted
that the small value of plane shift of the phenoxido ring (τ =12.2°) and hinge distortion of Ni2O2
plane (β=30.7°) (Figure S9) in this complex might have weakened the AF coupling via
15

phenoxido bridges but their effects are not high enough to make the overall coupling
ferromagnetic [65].
The molecular structure of complex 2 shows that Ni(II) ions are connected with each
other through one phenoxido, one μ2-1,3 carboxylate (1κO:2κO) and one μ2-1,1 carboxylate
((1κO:2κ2O) bridges. Indeed here, both Ni-Ophenoxido-Ni angle (99.59°) and Ni-Oμ2-1,1 carboxylate-Ni
bond angle (91.93°) play a major role on J value through σ-type exchange mechanism.
Additionally, μ2-1,3 carboxylate bridging should also contribute weak ferro- or antiferromagnetic
coupling depending on other structural parameters as mentioned previously. The Figure S8a
shows that spin density located at phenoxido-oxygen atom is equally spread on both sides of Ni
ion which signifies spin delocalization and as well as antiferromagnetic exchange interaction as
is expected from the bridging angle of 99.59°. On the other hand, it can be noticed that spin
density carried by μ2-1,1 carboxylate-oxygen atom is localized and directed towards the terminal
nickel sites and therefore it mediates ferromagnetic exchange as a results of spin localization or
accidental orthogonality of magnetic orbital through a σ-type exchange. The orthogonality of
magnetic orbital can be visualized from singly occupied molecular orbital diagram as shown in
Figure S8b. The dx2-y2 lobe is seen in the central Ni(II) centers, where as dz2 lobe is located at
terminal Ni(II) atoms. The atomic spin density (Table S2) of high spin state pointed out that the
phenoxido oxygen has a larger positive spin density compared to μ2-1,1 carboxylate-oxygen
indicating a larger spin delocalization as well as antiferromagnetic contribution via phenoxido
bridge. In case of μ2-1,3 carboxylate bridge, as we have discussed earlier that the spin density of
central carbon atom decides the nature exchange. The negative spin density at high spin state of
carboxylate carbon atom (–0.00297 au) suggests for weak ferromagnetic coupling through this
bridge.
To check the ferromagnetic contribution through syn-syn carboxylate bridge, we again
performed a DFT calculations on model structure of complex 2, which was built by replacing the
syn-syn carboxylate groups with four non-bridging water molecules and kept the rest of the
structure unaltered. The atomic dispositions of carboxylate oxygen atoms for complex 2 are kept
constant with respect to non-bridging oxygen atoms for coordinated water molecule in model
structure. The calculated J shows a significant lowering (65.8%) of ferromagnetic interaction
(J=1.24 cm-1) in model complex with respect to complex 2 (J=3.63 cm-1). This lowering of J in
16

model structure provides the evidence of weak ferromagnetic contribution transmitted through
syn-syn carboxylate bridge in the complex 2.
The analysis of the magneto-structural data for isostructural triply bridged trinuclear
Ni(II) complexes are listed in Table S3. It is noticed that most of the structurally alike complexes
possess very close phenoxido bridging angle (θ1 ∼96−97°) and μ2-1,1 carboxylate bridging angle
(θ2∼93-94°). The overall J values are positive for all these complexes as the ferromagnetic
contributions of the μ2-1,1 carboxylate and syn-syn carboxylate bridge dominate over the
antiferromagnetic contribution of phenoxido bridge. Among these complexes, antiferromagnetic
coupling is found only in FUSWUM, where a wider phenoxido bridging angle (θ1 = 99.89°) and
μ2-1,1 carboxylate (θ2=96.49°) causes for the dominance of antiferromagnetic interaction over
the ferromagnetic contributions of and syn-syn carboxylate groups. The comparison of complex
2 and FUSWUM indicates that both of them have almost the same phenoxido bridging angle
(~99°) but the Ni(II) centers of complex 2 are ferromagnetically coupled. The difference in
magnetic behavior seems to lie in the θ2 angle which is 91.93° for 2. In case of FUSWUM
complex, both of the μ2-1,1 carboxylate (96.5°) and phenoxido bridge provide individual
antiferromagnetic interaction which dominate over ferromagnetic contribution of syn-syn
carboxylate group. In case of complex 2, both the μ2-1,1 carboxylate (91.9°) and syn-syn
carboxylate groups transmit ferromagnetic interaction which dominate over the
antiferromagnetic contribution of phenoxido bridge.
Conclusions
The reaction of the tridentate reduced Schiff-base ligand, HL1, with nickel(II) perchlorate and
benzoic acid in 1:1:1 molar ratio afforded double phenoxido and an syn-syn carboxylate bridged
dinuclear nickel(II) complex (1). Complex 1 catalyses oxidation of 3,5-ditertiarybutyl catechol to
the corresponding quinone derivative in methanol. Each Ni(II) centre in the complex contains a
coordinated solvent molecule which is removed easily to facilitate binding of catechol to the
metal centres and subsequently breaks into mononuclear species. These catechol bound
mononuclear species seems to take part in the catalytic cycle which passes through radical
formation and H2O2 generation. Most of the previously reported Ni(II) complexes, showing
catecholase-like activity are of Schiff base ligands and it is presumed that the Schiff base
undergoes one electron reduction during the catalytic cycle. We have shown here that the Ni(II)
17

complexes of reduced Schiff base which is redox innocent can also show significant catecholase-
like activity. In a previous system we had the similar observation. However, to get a clearer
insight into the reaction pathway, study of complexes with similar ligand systems and redox
innocent metals like Zn2+ are needed to be studied. Such work is in progress in our laboratory.
Interestingly, the trinuclear compound (2) which contains an additional Ni(II) ion but similar
bridging system (two oxido- and one syn-syn carboxylate) is inactive towards catechol oxidation
activity. The reason seems to lie in the absence of solvent molecule in the coordination sphere
that would have been replaced by catechol to be bonded to the metal centre.
From the magnetic point of view, Ni(II) centres are antiferromagnetically coupled in 1
but ferromagnetically coupled in 2. DFT calculations reproduced well with experimentally
observed J values. In complex 1, both the phenoxido bridges are equivalent and mediate
antiferromagnetic coupling but in 2 one of the phenoxido bridge mediates AF coupling while μ2-
1,1 carboxylato bridge transmits ferromagnetic interaction as expected from the Ni−O−Ni
angles. In both compounds, syn-syn carboxylate bridges are uniquely trying to implant
ferromagnetic coupling through spin polarization mechanism, however its contribution is
suppressed by other bridging groups as supported by DFT calculations.
Acknowledgements
M.M thanks University Grants Commission (UGC), New Delhi for the Junior Research
Fellowship [Sanction no. UGC/731/Jr. Fellow (Sc)]. We are thankful to the Department of
Science and Technology (DST), New Delhi, India, for financial support (SR/S1/IC/0034/2012).
S.G is grateful to the UGC, Government of India for Dr D. S. Kothari fellowship (F.4-
2/2006(BSR)/CH/13-14/0069).Crystallography was performed at the DST-FIST; India funded
Single Crystal Diffractometer Facility at the Department of Chemistry, University of Calcutta.
and DST FIST for financial support for the X-ray diffraction facility. We also thank CRNN,
University of Calcutta, Kolkata, India for magnetic measurement.
Note: Electronic supplementary information (ESI) available: Fig. S1- S9, Table S1-S4, CCDC
1475002 and 1475003 for 1 and 2.
18

References
1. D. Tietze, M. Tischler, S. Voigt, D. Imhof, O. Ohlenschläger, M. Görlach, G. Buntkowsky,
Chem. Eur. J.16 (2010) 7572–7578.
2. S. M. Barnett, K. I. Goldberg, J. M. Mayer, Nat. Chem. 4 (2012) 498–502.
3. S. Shaik, Nat. Chem. 2 (2010) 347–349.
4. D. M. L. D. Bethany, N. Wigington, T. R. Cundari, D. L. Thorn, S. K. Hanson, S. L. Scott, Chem.
Eur. J. 18 (2012) 14981–14988.
5. K. Ariga , J. Li, J. Fei, Q. Ji , J. P. Hill, Adv. Mater. 28 (2016) 1251–1286.
6. C. Copꢀret, A. Comas-Vives, M. P. Conley, D. P. Estes, A. Fedorov, V. Mougel, H. Nagae, F.
Nꢁꢂez-Zarur, P. A. Zhizhko, Chem. Rev. 116 (2016) 323–421.
7. R. Ragg, M. N. Tahir, W. Tremel, Eur. J. Inorg. Chem. 13-14 (2016) 1906–1915.
8. H. Abe, J. Liu, K. Ariga, Mater. Today 19 (2016) 12–18.
9. N. C. Kasuga, K. Sekino, C. Koumo, N. Shimada, M. Ishikawa, K. Nomiya, J. Inorg. Biochem.
84 (2001) 55–65.
10. R. del Campo, J. J. Criado, E. García, M. a. R. Hermosa, A. Jiménez-Sánchez, J. L. Manzano, E.
Monte, E. Rodríguez-Fernández, F. Sanz, J. Inorg. Biochem. 89 (2002) 74–82.
11. A. Vessieres, S. Top, W. Beck, E. Hillard, G. Jaouen, Dalton Trans. (2006) 529–541.
12. P. C. A. Bruijnincx, P. J. Sadler, Curr. Opin. Chem. Biol. 12 (2008) 197–206.
13. S. K. Dey, A. Mukherjee, Coord. Chem. Rev. 310 (2016) 80–115.
14. D. M. Boghaei, M. Behzad, A. Bezaatpour, J. Mol. Cat A: Chem 241 (2005) 1–7.
15. A. Neves, A. J. Bortoluzzi, R. Jovito, R. A. Peralta, B. de Souza, B. Szpoganicz, A. C. Joussef,
H. Terenzi, P. C. Severino, F. L. Fischer, G. Schenk, M. J. Riley, S. J. Smith, L. R. Gahan, J.
Braz. Chem. Soc. 21 (2010) 1201–1212.
16. A. Nevesa, L. M. Rossi, A. J. Bortoluzzi, A. S. Mangrich, W. Haase, R. Werner, J. Braz. Chem.
Soc. 12 (2001) 747–754.
17. A. Biswas, M. G. B. Drew, C. Diaz, A. Bauzá, A. Frontera, A. Ghosh, Dalton Trans 41(2012)
12200–12212.
18. A. Guha, A. Banerjee, R. Mondol, E. Zangrando, D. Das, J. Coord Chem. 64 (2011) 3872– 3886.
19. T. Ghosh, J. Adhikary, P. Chakraborty, P. K. Sukul, M. S. Jana, T. K. Mondal, E. Zangrando, D.
Das, Dalton Trans. 43 (2014) 841–852.
20. M. Das, R. Nasani, M. Saha, S. M Mobin, S. Mukhopadhyay, Dalton Trans. 44 (2015) 2299–
2310.
21. P. K. Basu, M. Mitra, A. Ghosh, L. Thander, C. –H. Lind, R. Ghosh, J. Chem. Sci. 126 (2014)
1635–1640.
19

22. T. Chattopadhyay, M. Mukherjee, A. Mondal, P. Maiti, A. Banerjee, K. S. Banu, S. Bhattacharya,
B. Roy, D. J. Chattopadhyay, T. K. Mondal, M. Nethaji, E. Zangrando, D. Das, Inorg. Chem. 49
(2010) 3121–3129.
23. A. Biswas, L. K. Das, M. G. B. Drew, G. Aromí, P. Gamez, A. Ghosh, Inorg. Chem. 51(2012)
7993–8001.
24. L. K. Das, A. Biswas, J. S. Kinyon, N. S. Dalal, H. Zhou, A. Ghosh, Inorg. Chem. 52 (2013)
11744–11757.
25. F. Karipcin, B. Dede, I. Ozmen, M. Celik, E. Ozkan, J. Chil. Chem. Soc. 59 (2014) 2539–2544.
26. M. Mitra, A. K. Maji, B. K. Ghosh, G. Kaur, A. R. Choudhury, C.-H. Lin, J. Ribas, R. Ghosh,
Polyhedron. 61 (2013) 15–19.
27. P. Kar, R. Haldar, C J Gómez-García, A. Ghosh, Inorg. Chem. 51 (2012) 4265–4273.
28. S. Mukherjee, T. Weyhermüller, E. Bothe, K. Wieghardt, P. Chaudhuri, Dalton Trans. (2004)
3842–3253.
29. K. S. Banu, T. Chattopadhyay, A. Banerjee, M. Mukherjee, S. Bhattacharya, G. K. Patra, E.
Zangrando, D. Das, Dalton Trans. (2009) 8755–8764.
30. M. U. Triller, D. Pursche, W. –Y. Hsieh, V. L. Pecoraro, A. Rompeland, B. Krebs, Inorg. Chem.
42 (2003) 6274–6283.
31. L. I. Simándi, T. M. Simándi, Z. May, G. Besenyei, Coord. Chem. Rev. 245 (2003) 85–93.
32. Z. May, L. I. Simándi, A. Vértes , J. Mol. Cat. 26 (2007) 239–248.
33. M. Mitra, A. K. Maji, B. K. Ghosh, P. Raghavaiah, J. Ribas, R. Ghosh, Polyhedron. 67 (2014)
19–26.
34. A. Hazari, L. K. Das, R. M. Kadam, A. Bauzá, A. Frontera, A. Ghosh, Dalton Trans. 44 (2015)
3862–3876.
35. E. Ruiz, A. Rodriguez-Fortea, J. Cano, S. Alvarez, P. Alemany, J. Comput. Chem., 24 (2003)
982–989.
36. A. D. Becke, J. Chem. Phys. 98, (1993) 5648– 5652.
37. C. T. Lee, W. T. Yang, R. G. Parr, Phys Rev. B. 37 (1988) 785– 789.
38. A. D. Becke, Phys. Rev. A. 38 (1988) 3098–3100.
39. A. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829–5835.
40. F. Neese, ORCA, An ab initio, Density Functional and Semiempirical Program Package,Version
3.0.2, Universität Bonn: Bonn, Germany, 2014.
41. F. Weigend, Phys. Chem. Chem. Phys. 8 (2006) 1057–1065.
42. F. Neese, J. Comput. Chem. 24 (2003) 1740–1747.
20

43. G. M. Sheldrick, SHELXS97, Program for Structure Solution, University of Göttingen,
Göttingen, 1997.
44. G. M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement, University of
Göttingen, Göttingen, 1997.
45. PLATON, Molecular Geometry Program: A. L. Spek, J. Appl. Crystallogr. 36 (2003) 7–13.
46. L. J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565–565.
47. L. J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837–838.
48. S. K. Dey, M. S. El Fallah, J. Ribas, T. Matsushita, V. Gramlich, S. Mitra, Inorg. Chim. Acta. 357
(2004) 1517–1522.
49. H. S. Jena, J. Subramanian, V. Manivannan, Inorg. Chim. Acta. 365 (2011) 177–182.
50. P. Mukherjee, M. G. B. Drew, C. J. Gomez-Garcı´a, A. Ghosh, Inorg. Chem. 48 (2009) 5848–
5860.
51. A. H. J. L. Fim, A. J. Bortoluzzi, B. Szpoganicz, M. de S. Silva, M. A. Novak, M. B. Neto, L. S.
Eberlin, R. R. Catharino , M. N. Eberlin, C. Fernandes, J. Mol. Str. 797 (2006) 154–164.
52. S. C. Sahoo, M. Ray, Chem. Eur. J. 16 (2010) 5004–5007.
53. E. R. T. Tiekink, J. L. Wardell, C. Fernandes, A. Horn, J. M. S. Skakled, Acta Cryst. E66 (2010)
m566–m567.
54. P. Mukherjee, M. G. B. Drew, C. J. Gomez-Garcı´a, A. Ghosh, Inorg. Chem. 48 (2009) 4817–
4827.
55. P. Mukherjee, M. G. B. Drew, C. J. Gomez-Garcı´a, A. Ghosh, Inorg. Chem. 48 (2009) 5848–
5860.
56. R. Biswas, C. Diaz, A. Bauzá, M. B. -Oliver, A. Frontera, A. Ghosh, Dalton Trans. 43 (2014)
6455–6467.
57. R. Biswas, S. Mukherjee, P. Kar, A. Ghosh, Inorg. Chem. 51 (2012) 8150–8160.
58. P. Mukherjee, M. G.B. Drew, V. Tangoulis, M. Estrader, C. Diaz, A. Ghosh, Inorg. Chem.
Commun. 12 (2009) 929–932.
59. S. Giri, R. Biswas, A. Ghosh, S. K. Saha, Polyhedron. 30 (2011) 2717–2722.
60. E. Berti, A. Caneschi, C. Daiguebonne, P. Dapporto, M. Formica, V. Fusi, L. Giorgi, A. Guerri,
M. Micheloni, P. Paoli, R. Pontellini, P. Rossi, Inorg. Chem. 42 (2003) 348–357.
61. R. Biswas, S. Giri, S. K. Saha, A. Ghosh, Eur. J. Inorg. Chem. 17 (2012) 2916–2927.
62. D. Venegas-Yazigi, D. Aravena, E. Spodine, E. Ruiz, S. Alvarez, Coord. Chem. Rev. 254 (2010)
2086–2095.
63. Y. Nishida, S. Kida, J. Chem. Soc., Dalton Trans. (1986) 2633–2640.
64. E. M. Fatila, R. Clérac, M. Jennings, K. E. Preuss, Chem. Commun. 49 (2013) 9431–9433.
21

65. J. Ruiz, A. J. Mota, A. Rodríguez-Diéguez, I. Oyarzabal, J. M. Seco, E. Colacio, Dalton Trans.
41 (2012) 14265–14273.
66. A. Guha, K. S. Banu, S. Das, T. Chattopadhyay, R. Sanyal, E. Zangrando, D. Das, Polyhedron 52
(2013) 669−678.
67. S. Das, P Maiti, T. Ghosh, E. Zangrando, D. Das, Inorg. Chem. Commun. 15 (2012) 266−268.
68. A. Banerjee, A. Guha, P. Maiti, S. Goswami, T. Chattopadhyay, T. K. Mondal, S. Bhattacharya,
E. Zangrando, D. Das, Transition Met. Chem. 36 (2011) 829–839.
69. R. Modak, Y. Sikdar, S. Mandal, S. Chatterjee, A. Bienko, J. Mrozinski, S. Goswami, Inorg.
Chim. Acta 416 (2014) 122–134.
70. J. Adhikary, P. Chakraborty, S. Das, T. Chattopadhyay, A. Bauza, S. K. Chattopadhyay, B.
Ghosh, F .A. Mautner, A. Frontera, D. Das, Inorg. Chem. 52 (2013) 13442–13452.
71. P. Seth, L. K. Das, M. G. B. Drew, A. Ghosh, Eur. J. Inorg. Chem. 2012 (2012) 2232–2242.
22

Figure 1: ORTEP-3 view of the asymmetric unit of 1 with ellipsoids at 30% probability.
23

Figure 2: ORTEP-3 view of the asymmetric unit of 2 with ellipsoids at 30% probability.
24

Figure 3. Increase in absorbance around 401 nm, after addition of 100 equivalents of 3,5-DTBC
to a 10−4 M methanol solution of 1. The spectra were recorded every 5 min.
Figure 4. Plot of the initial rates versus substrate concentration for the oxidation of 3,5-DTBC
catalyzed by 1. The inset shows Lineweaver−Burk plot.
25

Figure 5: EPR spectrum of a methanol solution of 1(10−3 M) after addition of 3,5-DTBC,
recorded at 77K.
Figure 6. Generation of % of H2O2 with time.
26

(a)
(b)
Figure 7. Variation of χMT as a function of temperature at 500 Oe applied field for 1 (left) and 2
(right).
27

Ph
O
Ni(ClO4)2.6H2O/Et3N
O
O
N
H2O
N
C6H5COOH (A)
L:Ni2+:A = 1:1:1
Ni
Ni
N
OH2
N
O
H
(1)
Me
N
N
Me
Ni(ClO4)2.6H2O
C6H5COOH (A)
OH
Et3N
1:Ni2+:A= 1:1:2
HL
NaBH4
Ph
Ph
O
O
Me
Ni(ClO4)2.6H2O/
NEt3
N
N
Me
O
O
O
N
N
C6H5COOH (A)
L:Ni2+:A = 2:3:4
OH
Ni
Ni
Ni
N
N
O
O
O
O
O
Ph
Ph
(2)
Scheme 1: Synthesis of complexes 1 and 2.
28

Ph
O
O
O
N
H2O
N
Ni
Ni
N
OH2
O
N
O
OH
OH
O
O2
H2O2
Scheme 2. Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ in Air-Saturated Methanol solvent.
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