Syntheses, crystallographic characterization, catecholase activity and magnetic properties of three novel aqua bridged dinuclear nickel(II) complexes

Article abstract of DOI:10.1016/j.ica.2014.03.008

Three novel μ₂-aqua bridged dinuclear Ni(II) complexes, [Ni₂(HL¹)₄(H₂O)] (1), [Ni ₂(HL²)₄(H₂O)] (2) and [Ni ₂(HL³)₄(H₂O)] (3) have been synthesized using Schiff base ligands derived from 5-amino-1-pentanol and salicylaldehyde (H₂L¹), 5-bromo salicylaldehyde (H ₂L²) and 3-methoxy salicylaldehyde (H₂L ³), respectively. They are characterized by a variety of physical techniques including elemental analysis, infrared and UV-Vis spectroscopy, NMR, cyclic voltammetry, variable temperature magnetic measurements and single crystal X-ray diffraction. X-ray crystallographic analysis reveals that all the three complexes possess distorted octahedral environment with a bridging aqua ligand. Complexes 1 and 3 exhibit 3D supramolecular architecture whereas 2 demonstrates a 2D netlike arrangement along ab plane. Catecholase activity of the dinuclear Ni(II) systems are investigated using 3,5-di-tert-butylcatechol as the substrate. The complexes are efficient catalysts with turnover numbers 1.87 × 10⁴, 1.79 × 10⁴ and 1.38 × 10 ⁴ h^-1 for 1, 2 and 3, respectively. The probable structures of the intermediates formed during the process are proposed on the basis of kinetic and spectral analyses. The magnetic data disclose that weak ferromagnetic couplings are transmitted between the metal centers and the magnetic properties are correlated with the structural features around the Ni(II) centers.

Full text of DOI:10.1016/j.ica.2014.03.008

Accepted Manuscript
Syntheses, crystallographic characterization, catecholase activity and magnetic
properties of three novel aqua bridged dinuclear nickel (II) complexes
Ritwik Modak, Yeasin Sikdar, Senjuti Mandal, Sudipta Chatterjee, Alina
Bieńko, Jerzy Mroziński, Sanchita Goswami
PII:
S0020-1693(14)00153-4
http://dx.doi.org/10.1016/j.ica.2014.03.008
ICA 15908
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
20 September 2013
24 February 2014
9 March 2014
Please cite this article as: R. Modak, Y. Sikdar, S. Mandal, S. Chatterjee, A. Bieńko, J. Mroziński, S. Goswami,
Syntheses, crystallographic characterization, catecholase activity and magnetic properties of three novel aqua
bridged dinuclear nickel (II) complexes, Inorganica Chimica Acta (2014), doi: http://dx.doi.org/10.1016/j.ica.
2014.03.008
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1
Syntheses, crystallographic characterization, catecholase activity
and magnetic properties of three novel aqua bridged dinuclear
nickel (II) complexes
Ritwik Modak a , Yeasin Sikdar a , Senjuti Mandal a , Sudipta Chatterjee b , Alina Bieńko c,
Jerzy Mroziński c, Sanchita Goswami a, *
a Department of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata – 700009, India
b Department of Chemistry, Serampore College, West Bengal, India
c Faculty of Chemistry, University of Wroclaw, 14 F. Joliot - Curie, 50-383 Wroclaw, Poland
Abstract
Three novel µ2-aqua bridged dinuclear Ni(II) complexes, [Ni2(HL1)4(H2O)] (1), [Ni2(HL2)4(H2O)] (2) and
[Ni2(HL3)4(H2O)] (3) have been synthesized using Schiff base ligands derived from 5-amino-1- pentanol and
salicylaldehyde (H2L1), 5-bromo salicylaldehyde (H2L2) and 3-methoxy salicylaldehyde (H2L3) respectively. They
are characterized by a variety of physical techniques including elemental analysis, infrared and UV-Vis
spectroscopy, NMR, cyclic voltammetry, variable temperature magnetic measurements and single crystal X-ray
diffraction. X-ray crystallographic analysis reveals that all the three complexes possess distorted octahedral
environment with a bridging aqua ligand. Complexes 1 and 3 exhibit 3D supramolecular architecture whereas 2
demonstrates a 2D netlike arrangement along ab plane. Catecholase activity of the dinuclear Ni(II) systems are
investigated using 3, 5-di-tertbutylcatechol as the substrate. The complexes are efficient catalysts with turnover
numbers 1.87 x 104, 1.79 x 104 and 1.38 x 104 h-1 for 1, 2 and 3 respectively. The probable structures of the
intermediates formed during the process are proposed on the basis of kinetic and spectral analyses. The magnetic
data disclose that weak ferromagnetic couplings are transmitted between the metal centers and the magnetic
properties are correlated with the structural features around the Ni(II) centers.
Keywords: Dinuclear Ni(II) complex; Aqua bridged; Crystal structure; Catechol oxidation; Ferromagnetic
*Corresponding author. Tel.:033-23509937; fax: 910-33-351-9755.
E-mail address: sgchem@caluniv.ac.in (S.Goswami).

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1. Introduction
Dinuclear Ni(II) complexes bridged by water, hydroxide ion or oxo group emerge as an interesting class of
compounds because of their relevance to the active site of the hydrolytic enzyme urease [1-5]. In the literature,
octahedrally coordinated polynuclear Ni(II) complexes are an interesting class of compound because of a variety of
molecular structures and magnetic interactions [6]. Again, magnetic study of various di-, tri- and polynuclear Ni(II)
complexes with different bridging groups like carboxylates and the µ-1,1-azido bridges derives its interest from their
potential to use as molecular magnetic materials [7,8]. The bridging abilities of the phenolate oxygen atom have
been exploited extensively for the purpose of designing dinuclear species featuring generally antiferromagnetic
behaviour, though a couple of complexes showing ferromagnetic coupling have also been described in the literature
[4e, 9-11]. A large number of structural parameters affect the super-exchange mechanism in these systems, such as
the Ni–O–Ni bridging angle, Ni–O bond distance and dihedral angle between the Ni–O–Ni plane and the remaining
coordination plane [6c,12]. Among the several reports of double phenoxide bridged Ni(II) dimers [8d,8e,6c,13], the
presence of an aqua bridge in dinuclear Ni(II) complexes is relatively scanty [4e,12b-d,14].
It is well documented that bimetallic skeletons exist at active sites of many enzymes and play an essential
role in biological systems [15]. There are a couple of recent reports describing phenoxide bridged dinickel(II)
complexes that serve to mimic dinuclear biological active sites. For instance, phenoxo bridged dinickel(II)
complexes are developed with labile coordination site(s) for the hydrolysis of phosphate esters [16]. Recently, Das et
al and Biswas et al have reported catalytic activity of a dinuclear nickel(II) complex towards the oxidation of 3,5-di-
tert-butylcatechol under aerobic condition [17].
Therefore, the chemistry of bridged dinuclear nickel complexes is extensively investigated due to their
potential applications in bioinorganic chemistry, magneto chemistry, materials chemistry and catalysis. In order to
explore these possibilities, we have carried out a thorough investigation of a series of dinickel(II) complexes,
[Ni2(HL1)4(H2O)](1), [Ni2(HL2)4(H2O)](2) and [Ni2(HL3)4(H2O)](3) using Schiff base ligands derived from 5-
amino-1- pentanol and salicylaldehyde(H2L1), 5-bromo salicylaldehyde(H2L2) and 3-methoxy salicylaldehyde(H2L3)
respectively.
2. Experimental section
2.1. Materials
5-amino-1-pentanol, salicylaldehyde, 5-bromo salicylaldehyde and 3-methoxy salicylaldehyde were procured from
Sigma-Aldrich. Solvents were used as received commercially.
2.2. Methods
Elemental (C, H and N) analyses were performed on a Perkin-Elmer 2400 II analyzer. IR spectra were recorded in
1
the region 400–4000 cm-1 on a Bruker-Optics Alpha– T spectrophotometer with samples as KBr disks. H NMR
spectra were recorded in the CDCl3 solvent on a Bruker AV 300 MHz Supercon NMR system dual probe. Cyclic
voltammetric (CV) measurements were done using a BASi Epsilon-EC electrochemical analyzer. The concentration
of the supporting electrolyte, tetrabutylammonium perchlorate(TBAP) was 0.1 M, while that of the complex was 1
mmol. Cyclic voltammetry measurements were carried out in DMF solution at 295 K with a glassy carbon disk
working electrode and the scan rate was 100 mVs-1. Electronic spectra were obtained by using a Hitachi U-3501
spectrophotometer. Electrospray ionization mass (ESI-MS positive) spectra were recorded on a MICROMASS
Q-TOF mass spectrometer. Magnetic measurements in the temperature range 1.8-300 K were performed using
a Quantum Design SQUID-based MPMSXL-5-type magnetometer. The SQUID magnetometer was calibrated
with the palladium rod sample (Materials Research Corporation, measured purity 99.9985%). The
superconducting magnet was generally operated at a field strength ranging from 0 to 5 T. Measurements were

3
made at a magnetic field of 0.5 T. Corrections are based on subtracting the sample – holder signal and
contribution χD estimated from the Pascal constants [18]. Magnetization measurements were conducted at 2 K
in the magnetic field from 0 to 5 T. EPR measurement was done by Magnettech GmbH MiniScope MS400
spectrometer equipped with a temperature controller TC H03 (microwave frequency was measured with a frequency
counter FC400) at77 K.
2.3. Syntheses of the ligands
The ligands H2L1, H2L2, H2L3 (Scheme 1) were prepared by refluxing 5-Amino-1- pentanol(1.032 g, 10 mmol) with
salicylaldehyde (1.08 mL, 10mmol), 5-Bromo-2-hydoxybenzaldehyde (2.01 g, 10 mmol) and 2-hydroxy-5- methoxy
benzaldehyde (1.521 g, 10 mmol) respectively in methanol (40 mL) for 30 minutes. The volatile components were
removed in vacuo to give the bidentate Schiff base ligands H2L1, H2L2 and H2L3 which were subsequently used for
complex formation.
H2L1: Yield 87%. Anal. Found (Calc.) (%) for C12H17N1O2 (MW = 207.125 g/mol): C, 69.35(69.53); H, 8.39 (8.27);
N, 6.54 (6.76).1H NMR ( 300 MHz, CDCl3): δ 8.17 (s, 1H, CH=N), 7.20 – 7.15 (m, 1H, Ar–H), 7.12–7.09 (m, 1H,
Ar–H), 6.87 (d, J = 8.4 Hz, Ar–H), 6.76 – 6.71 (m, 1H, Ar–H), 3.52 – 3.42 (m, 4H, CH2 N, CH2OH), 1.63 – 1.53 (m,
2H, CH2CH2OH), 1.50–1.43(m,2H, CH2CH2N), 1.37–1.30 (m,2H,CH2CH2CH2OH).
H2L2: Yield 83%. Anal. Found (Calc.) (%) for C12H16N1O2Br (MW = 285.036 g/mol): C, 50.40 (50.37); H, 5.61
1
(5.64); N, 4.95 (4.89). H NMR ( 300 MHz, CDCl3): δ 8.37 (s, 1H, CH=N), 8.23 (d, 1H, J = 2.7 Hz, Ar–H), 8.20 –
8.16 (dd, 1H, J1 = 3 Hz, J2 = 9.3 Hz, Ar–H), 6.94 (d, 1H, J = 9 Hz, Ar–H), 3.71 – 3.66 (m, 4H, CH2N, CH2OH), 1.84
– 1.75 (m, 2H, CH2CH2OH), 1.68 – 1.59 (m, 2H, CH2CH2N), 1.55 – 1.45 (m, 2H, CH2CH2CH2OH).
H2L3: Yield 84%. Anal. Found (Calc.) (%) for C12H19N1O3 (MW = 237.136 g/mol): C, 65.73 (65.8); H, 8.29 (8.07);
N, 6.01 (5.90).1H NMR ( 300MHz, CDCl3): δ 8.22 (s, 1H, CH=N), 6.90 – 6.87 (m, 2H, Ar–H), 6.73 (d, 1H, J = 2.4
Hz, Ar–H), 3.73 (s, 3H, CH3O), 3.61 – 3.52 (m, 4H, CH2N, CH2O), 1.72–1.60(m, 2H, CH2CH2OH), 1.57–1.53 (m,
2H, CH2CH2N), 1.46 – 1.38 (m, 2H, CH2CH2CH2OH).
2.4. Syntheses of the complexes
2.4.1. Synthesis of [Ni2(HL1)4(H2O)] (1)
5 mL methanolic solution of NiCl2.6H2O (0.832 g, 3.5 mmol) was added to 20 mL methanolic solution of H2L1
(1.45 g, 7 mmol) under stirring condition. The green color was intensified when methanolic solution of triethylamine
(0.707 g, 7 mmol) was added slowly to the mixture. Then the resulting mixture was stirred for 3 h. The resulting
dark green solution was evaporated to dryness and recrystallised from minimum amount of DMF. Good X-ray
diffraction quality single crystals were obtained after 2 days.Yield 70%. Anal. Found (Calc.) (%) for C48H66N4Ni2O9
(1) (MW = 964.45 g/mol): C, 59.836 (60.025); H, 7.031 (6.926); N, 5.688 (5.833). IR (KBr-cm-1): ν(O-
H),3325;ν(C=N),1630;ν(C-O,1°alcohol),1031; ν(C-O,phenoxide),1195.
2.4.2. Synthesis of [Ni2(HL2)4(H2O)] (2)
In a manner similar to that for 1, NiCl2.6H2O (0.832 g, 3.5 mmol), H2L2 (2.0 g, 7 mmol) and triethylamine
(0.707 g, 7 mmol) gave complex 2. Yield 69%. Anal. Found (Calc.) (%) for C48H62Br4N4Ni2O9 (2) (MW = 1276.03
g/mol): C, 45.293 (45.180); H, 4.735 (4.897); N, 4.467 (4.390). IR(KBr- cm-1): ν(O-H), 3496; 2930; ν(C-
O,1°alcohol),1038; ν(C=N),1630; ν(C-O,phenoxide), 1214.
2.4.3. Synthesis of [Ni2(HL3)4(H2O)] (3)
In a similar way, NiCl2.6H2O (0.832 g, 3.5 mmol), H2L3 (1.66 g, 7 mmol) and triethylamine (0.707 g, 7 mmol)
gave complex 3.Yield 64%. Anal. Found (Calc.) (%) for C52H74N4Ni2O13 (3) (MW = 1112.52 g/mol): C, 57.836
(57.800); H, 6.811 (6.092); N, 5.225 (5.185). IR(KBr-cm-1): ν(O-H), 3402; ν(sp2C-H), 2859; ν(aromatic C-H),
2927; ν(C=N), 1632; ν(C-O, 1°alcohol), 1027; ν(C-O,phenoxide), 1216.

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2.5. Crystal data collection and refinement
The crystallographic data of the compounds are summarized in Table 1. Diffraction data were collected on a
Nonius APEX-II diffractometer with CCD-area detector at 150 K using graphite-monochromated Mo Kα radiation
(k = 0.71073 Å). Crystal structure was determined by direct methods and subsequent Fourier and difference Fourier
syntheses, followed by full-matrix least-squares refinements on F2 using SHELXL-97 [19a]. Absorption correction
was done by Multiscan Absorption Correction method. The contribution of the electron density associated with
disordered solvent molecules were removed by the SQUEEZE subroutine in PLATON [19b]. Prior to SQUEEZE,
all non-hydrogen atoms were made anisotropic and all hydrogen atoms were inserted at their calculated positions,
riding on the atoms to which they are attached. The hydrogen atoms were refined isotropically, while the non
hydrogen atoms were refined anisotropically.
3. Results
3.1. Synthesis of ligand and metal complexes
The Schiff base ligands H2L1, H2L2, H2L3were synthesized by adopting a modified procedure described by
Darensbourg et.al. [20a], taking respective aldehyde and 5-amino-1-pentanol under refluxing condition in MeOH
and characterized by FTIR and1H NMR spectroscopy. (Figs. S1a, S1b and S1c in the supporting information).The
preparation of the nickel compounds of the present series followed almost the same method (Scheme 2). Treatment
of these ligands with NiCl2.6H2O afforded dark green block shaped crystals 1, 2 and 3 after recrystallization from
DMF suitable for X-ray diffraction study. The complexes retain their integrity both in solid and solution phase.
3.2. Spectroscopic characterization
3.2.1. FTIR spectroscopy
The infrared spectra of the three compounds are very similar (Figs. S2a, S2b and S2c in the supporting
information). Focus is mainly on the stretching vibrations of the azomethine C=N and C-O(phenoxide) of the
ligands because of the possibility to infer their coordination modes. A diagnostic sharp absorption at ca. 1630-1632
cm-1 is due to the C=N stretching of the Schiff base ligand. The medium intensity bands at ca. 1195-1216 cm-1 are
assigned to the C-O (phenoxide) stretching. The O-H stretching vibrations are observed at 3325 cm-1, 3496 cm-1 and
3402 cm-1 respectively.
3.2.2. UV-Visible spectroscopy
The absorption bands in the electronic spectra, taken in methanol solution, exhibit very similar spectral patterns
for all the three complexes. They depict a distinct band at 617 nm, 631 nm and 620 nm for 1, 2 and 3 respectively
which may be assigned to the spin allowed d-d transition 3T1g←3A2g. Another broad band centered at 1067 nm for 1,
3
2 and 3 is assignable to the spin allowed d-d transition T2g←3A2g (Fig. S6a in supporting information). The
electronic spectra of 1, 2 and 3 resemble the spectra of the six coordinate Ni(II) complexes depicted in literature
[20(b)]. The solid state reflectance spectra of the complexes show a broad band centred at 1054 nm, 1039 nm and
1056 nm for complexes 1, 2 and 3 respectively assignable to the spin allowed d-d transition 3T2g ← 3A2g. The bands
3
for T1g ←3A2g transition appear at 619 nm, 606 nm and 621 nm respectively for complexes 1, 2 and 3. The solid
state spectrum further shows a very small, sharp feature at 1193, 1191 and 1200 nm region due to spin forbidden
1E←3A2 transition. The intense bands appeared at 347 nm, 365 nm and 410 nm for 1, 2 and 3 respectively are due to
L → M charge transfer transition. (Fig. S6b in the supporting information). All these spectral features for 1−3 are in
agreement with their crystal structures described below.
3.3. Description of the solid state structures of complexes 1, 2 and 3
The crystal structures of complexes 1-3 are determined by the X-ray diffraction study. The atom numbering
scheme for these complexes is provided in Scheme 3 and the selected bond lengths and bond angles are gathered in
Table 2.

5
The crystal structure of 1 consists of a discrete dinuclear unit [Ni2(HL1)4(H2O)] containing two independent nickel
atoms, Ni1 and Ni2, surrounded by four Schiff base ligands that involve their imine nitrogen and phenoxide oxygen
atoms for bonding to the metal centers (Fig. 1a). Out of these four Schiff bases, the phenoxide oxygen atoms of two
Schiff bases are involved in bridging the two nickel centers. In addition to that the two nickel centers are further
capped by an aqua ligand resulting in a triple oxygen bridged Ni2O3 species. Each nickel(II) center bonds with two
imine nitrogen atoms, three phenoxide oxygen atoms from the Schiff bases and an aqua ligand forming a distorted
octahedral environment. The two nickel atoms are separated by 2.988 Å [21]. The two phenoxide bridging angles,
Ni(1)–O(3)–Ni(2) and Ni(1)–O(2)–Ni(2) are 92.86(10)°and 92.57(10)˚ respectively, and the water bridge angle
Ni(1)–O(1)–Ni(2) is 80.11(11)°. It is worth noting that the double phenoxide bridge connecting both Ni(II) ions is
asymmetric because each Ni(II) ion is closer to its own phenoxide oxygen atom (Ni1 – O3= 2.005(2) Å) than to the
phenoxide oxygen atom of another Schiff base (Ni1 – O2 =2.131(3) Å) as observed in other similar phenoxide
bridged species [14c]. The bridging water molecule is slightly asymmetrically bonded between the two metal atoms
[Ni(1)–O(1) 2.334(3)Å; Ni(2)–O(1) 2.310(3)Å]. These distances are rather longer compared to those found in other
aqua-bridged dinuclear Ni(II) complexes, where it lies in the range of 2.09–2.25 Å [14b,22a,22b]. Argument in
support of the unusually longer bond lengths can be provided in terms of bond angle of Ni(2)-O(1)-Ni(1) as
80.11(11)° which is smaller than other reported complexes [14b]. Therefore, angle strain may play an important role
in lengthening the Ni – O (water) bonds. Again O(1) participates in hydrogen bonding with a pendant –OH group of
a Schiff base ligand which can also rationalize the lengthening of the Ni – O (water) bonds. The Ni – Nimine bond
distances range from 2.030(3) to 2.065(4)Å; the Ni – Ophonexo distances are in the range of 1.987(3) - 2.131 Å similar
to those found in related systems [8e, 13c, 13e, 20c].
The four pendant -OH groups present in 1 play significant role in stabilizing the crystal lattice by participating in
extensive hydrogen bonding (Table 3). Each of the four pendant –OH groups offers a bifurcated hydrogen bonding
provision to its neighbour [22c]. Here two different kinds of hydrogen bondings are operative. Two of the pendant –
OH groups are engaged in H-bonding interaction with pendant –OH groups of adjacent moieties via O(7)-
H(7A)···O(9) =2.732(9)Å and O(9)-H(9)···O(7) =2.908(10) Å. As a result of this interaction, one pendant arm
coming from four neighbouring dinickel moieties each congregates at the center to form a rectangular core in the
packing diagram (Fig. 1b). Other two pendant arms are actively involved in strong H-bonding with the phenoxide
oxygen atoms and the bridging water molecule (O(8)-H(8)···O(4) = 2.627(4)Å, O(1)-H(49)···O(6) = 2.778(5)Å,
O(6)-H(6)···O(5) = 2.659(4) and O(1)-H(51)···O(8) = 2.749(5)Å). Combined effect of these H-bonding interactions
result in the final architecture as a 3D supramolecular network for 1 as depicted in Fig. 1c.
As depicted in Fig. 2, the dimeric structure contains a crystallographic two-fold axis through the bridging water
molecule O(1) and can be expressed as [Ni(HL2)2(1/2H2O)]2 consisting of a Ni2O3 core like complex 1. The
bridging water molecule symmetrically binds the two nickel atoms with Ni – Owater bond length of 2.300(4)Å and the
Ni atoms are separated by 2.971 Å with phenoxide bridging angle, Ni(1) – O(3) – Ni(1_a) = 92.50(9)° and the water
bridge angle Ni(1) – O(1)– Ni(1_a) is 80.46(15)°. The average Ni – Ophenoxo bond length is 2.045 Å while the Ni –
Nimine bond distance is 2.050(3) Å.
For complex 2, the H-bonding pattern is quite different from 1 (Table 3). Unlike 1, here only two pendant –OH
groups participate in H- bonding. Here each dinuclear unit connects two neighbouring units via two pendant –OH
groups (O(4)-H(15)···O(2) = 2.684(4)Å). Again this unit is connected to other two adjacent moieties utilizing the
bridging aqua ligand and phenoxide oxygen (O(1)-H(31)···O(4) = 2.745(4)Å) (Fig. S3a). This type of
interconnection propagates to afford a two dimensional net like architecture along ab plane (Fig.S3b).
The coordination features of 3 closely resemble those of 1 and 2. Similar to complex 1, the dinuclear structure of
complex 3 can be formulated as [Ni2(HL3)4(H2O)] with nickel(II) ions located in distorted octahedral environment as
shown in Fig. 3. The two Ni atoms are separated by 2.984 Å with two phenoxide bridging angles, Ni(1)–O(10) –
Ni(2) of 93.26(12)° and Ni(1) – O(4) – Ni(2) of 92.81(12)° and the water bridge angle Ni(1) – O(13) – Ni(2) of
79.69(11)°. The Ni–Nimine, Ni–Ophenoxo distances are in the ranges 2.049Å - 2.030Å and 1.975Å - 2.128Å
respectively and the average Ni – Owater length is 2.328 Å.
Unlike complex 1 and 2, here three pendant –OH groups are involved in H –bonding to propagate the network.
The phenoxide oxygen and the bridging aqua ligand undergo extensive H – bonding interaction with two of the
pendant –OH groups of another moiety (O(6) – H(60)···O(7) = 2.644(5) Å, O(9) – H(90)···O(1) = 2.651 (5), O(13)
– H(10)···O(9) = 2.738 (5) Å and O(13) – H(20) ···O(6) = 2.727 (6) Å) (Fig.S4a). The third pendant arm is utilized
in H – bonding with –OCH3 group (O(3) – H(30)···O(8) = 2.850 (6) Å) generating as a whole a 3D supramolecular
layer architecture (Fig.S4b).

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3.4. Catecholase activity
A couple of recent reports on catecholase activity promoted by dinickel (II) complexes led to this investigation of
interaction between 3,5-di-tertbutylcatechol (3,5-DTBCH2) and the dinuclear nickel center in 1, 2 and 3. A detailed
kinetic investigation of catechol oxidation reaction was therefore undertaken. In most of the cases, the study of
catecholase activity is performed with 3, 5 DTBCH2 as a model substrate as its low redox potential makes it easy to
oxidize and the bulky tertbutyl substituents prevent further oxidation reactions such as ring-opening [23]. The
oxidized product 3, 5- di-tertbutylquinone shows maximum absorption at 402 nm in pure methanol. For studying the
catecholase activity of the synthesized complexes, 10−4 M methanolic solutions of the three complexes were treated
with 10−2 M 3, 5-di-tertbutylcatechol, under aerobic conditions. All the three dinuclear nickel(II) complexes display
significant catalytic activity towards oxidation of 3, 5-di-tertbutylcatechol (3, 5 DTBCH2) to 3,5- di-
tertbutylquinone (3,5-DTBQ) in methanolic medium. Complex 1 has a broad peak at 407 nm which upon addition of
3,5 DTBCH2 gradually disappear with progress in time. Ultimately it gets vanished and subsequently a peak is
generated at 402 nm characteristic of 3,5-DTBQ. The spectral outputs are depicted in Fig. 4a.(See supporting
information Figs. S5a-f for 2 and 3). The kinetics of the 3, 5-DTBCH2 oxidation was determined by the initial rates
method by monitoring the increase of the product 3,5-DTBQ at constant temperature of 20 °C. The rate constant for
a particular complex-substrate couple was eluicidated from absorbance vs time plot by employing initial rate
method. The substrate concentration dependence of the oxidation rate was examined under aerobic conditions, using
1 × 10−4 M solutions of 1, 2, and 3 and increasing amounts of 3,5-DTBCH2 from 0.005 (M) to 0.02 (M). In all cases,
first order dependence was observed at low substrate concentrations, whereas saturation kinetics was found at higher
substrate concentrations. The substrate concentration dependence suggests that the initial step of the catalytic cycle
is the binding of the substrate to the catalyst. Michaelis−Menten kinetics was applied to analyze the data obtained,
and the Michaelis−Menten constant (KM) and maximum initial rate (Vmax) were determined by linearization using
Lineweaver−Burk plots. The turnover number (kcat) values can be calculated by dividing the Vmax values by the
concentration of the corresponding complexes. Different kinetic parameters for catecholase activity are listed in
Table 4. It is seen from the Table 4 that the catecholase activity of the complexes are almost similar. The observed
kcat values in the range of 1.38 – 1.87 x 104 h-1indicate that the three complexes are very efficient catalysts for
catechol oxidation as compared with similar systems reported earlier [17,24] (Table 5). Biswas et al. have critically
assessed the catalytic activity of the complexes on basis of the mode of binding the two active Ni(II) centers by the
ligand. It is argued that a binucleating ligand that can hold the two metal centers, offer more feasibility towards
electron shuttling between the metal centres [8d]. In a similar line of argument, it can be stated that since in the
present work, the nickel centers are held by a binucleating ligand as well as a bridging water molecule, the systems
possess the potentiality to act as efficient catalysts. As it is evident that in the oxidation of 3,5-DTBC to 3,5-DTBQ
aerial oxygen can be reduced to H2O2 or H2O,in order to clarify this fact we performed a reported procedure for the
detection of H2O2 in the catalytic process which clearly demonstrated the generation of H2O2 in the mixture of 3,5-
DTBC and catalyst(Complex 1/2/3).
As stated earlier, it can be presumed that the initial step of the catalytic cycle is the binding of the substrate to the
catalyst. It is important to mention here that for dicopper complexes behaving as catalysts, Krebs and co-workers
had proposed a monodentate asymmetric coordination of the substrate and Solomons and co-workers coined the idea
of simultaneous coordination of the substrate to both metal centers in the bidentate bridging mode [15]. In order to
get an insight regarding the probable structure of the substrate-catalyst species, ESI-MS (positive) studies for the
complexes are carried out in methanol maintaining [Complex]:[3,5-DTBCH2] as 1:100 (see supporting information
Figs. S7a-b for representative example of 1). The adduct having a peak at m/z 1001.03 amu (m/z calcd 1001.33) is
clearly attributable to the species {[Ni2(HL1)3(L1)(µ2-H2O)]- + 2Na+}+ confirming that the dinuclear form is
maintained in the solution. After addition of 3,5-DTBCH2, new peaks are generated in the mass spectral output. The
ESI-MS (positive) is recorded immediately after addition of 3, 5 DTBCH2 and again after 20 minutes interval. The
spectral pattern now displays two important sets of peaks at m/z 1201.37 and 1186.30 amu consistently in both the
cases. Keeping the propositions of Krebs et al and Solomon et al in mind, it can be deduced that the substrate binds
to the catalyst forming two step intermediates IA and IB which can be attributed to the substrate-catalyst
intermediates as {[Ni2(HL1)4(µ1-H2O)(3,5-DTBCH)] + H+ + Na+}+ (m/z calcd 1202.48, exptl 1201.37) and
{[Ni2(HL1)4(3,5-DTBC2-)] + 2H+ + Na+}+ (m/z calcd 1187.49, exptl 1186.30) respectively, as depicted in Scheme
4.

7
3.5. Results of electrochemical studies and EPR
The cyclic voltammograms of complexes 1-3 in DMF are similar in appearance. They display irreversible reductive
responses at Epc = −1.79 V, -2.03 V and -2.36 V respectively (Fig. S8, supporting information), which is attributed
to the NiII/NiI couple. The oxidation peak corresponding to the conversion of NiII to NiIII is observed at around +0.7
V for all the three complexes. Prior works also demonstrated the similar potential ranges for NiII/NiI and NiII/NiIII
couples [8d,21c,25,26,27].
To monitor the solution behaviour during catecholase action, methanol solution of the mixture of complex
1 and 3, 5-DTBC is investigated and the cyclic voltammetric output, taken after 20 minutes, exhibits two reductive
responses at -1.37V and -0.78V as illustrated by the representative voltammogram in Fig.5. It is easily presumable
that since catechol to quinone oxidation is a two electron process, therefore, complexes containing two nickel
centers that turn between Ni(II) and Ni(I) during the cycle are desirable. Therefore, the appearance of two reduction
peaks can be interpreted by considering participation of both the nickel centers as follows NiIINiII
NiIINiI
NiINiI [26]. Our mass spectral output is also along the same line with the proposition of involvement of both
nickel centers. However, no oxidation peak corresponding to the 3,5-DTBC to 3,5-DTBQ conversion is observed in
methanol - a feature usually encountered in acetonitrile solvent. A very recent publication by Das et al[27] have
also reported similar cyclic voltammetric profile.
In the attempt to further characterize the products, X-band EPR spectroscopy is a useful tool as recently
demonstrated by Das et al. [27]. The species generated after mixing complex 1 and 3,5-DTBC (1:100) in methanol,
exhibits a sharp EPR signal at g =2.004, signifying the formation of semiquinone type S =1/2 organic radical (Fig.6
). As Fig.6 depicts, the presence of anisotropy in the signal strongly suggests that the radical interacts with the metal
centres. But whether a concerted mechanism involving both the metal centres and the radical is operative or not is
still a subject of intensive research.
Comparison to other dinickel (II) catalysts
By far the most efficient catalysts showing catecholase activity are reported by Das and co-workers which
presents kcat value upto 3.24 x 104 h-1[28]. In other publications they have come up with a couple of other efficient
dinickel(II) catalysts that have been run under similar conditions to our studies and show comparable results to our
catalysts [17,24,28]. In a recent characterization of nickel based catalysts, they have tested eight dinickel(II)
complexes and found the kcat values in the order of ~102 h-1. Ghosh et al. used three dinuclear nickel (II) complexes
and 3,5-DTBCH2 to monitor the conversion in acetonitrile and the highest kcat value was 81.7 h-1 [27]. Thus, a
comparison of kcat values shows that our complexes 1, 2 and 3 are among the best performing dinickel(II) based
catalysts showing catecholase activity (Table 5).
Furthermore, the authors obtained good assignments of the fragments by assuming the existence of dinuclear
species providing indirect evidence for metal mediated oxidation of 3,5-DTBCH2. They have also observed an
isotropic signal at g = ~ 2.00 in X-band EPR spectrum corresponding to an organic radical reinforcing the
proposition of radical pathway. By combining the analyses of ESI- MS, cyclic voltammetry and X-band EPR, our
findings are also consistent with literature reports.
3.6. Magnetic studies
The study of the magnetic susceptibility data for the examined complexes has been performed within the
temperature range of 1.80 – 300 K. The experimental data, plotted as thermal variations of the χM and χMT, are
shown in Figs. 7a-c (1-3). The three samples show very similar properties. The χM curve starts at 0.00814 cm3mol-1
(1), 0.008 cm3mol-1 (2), 0.0076cm3mol-1 (3) at room temperature (300K) and increases in uniform way to 1.147
cm3mol-1(1), 0.972 cm3mol-1(2), 1.245 cm3mol-1 (3) at 1.8 K. Plots of χMT vs T for all compounds show typical
ferromagnetic behaviour : an increase in the effective magnetic moment with decreasing temperature. At 300K they
show χMT values of 2.38, 2.40 and 2.43 cm3mol-1K for 1, 2 and 3, respectively. This quantity increases up to
maximum values of 3.09 cm3mol-1K at 10 K for 1, 2.77 cm3mol-1K at 9 K for 2 and 3.02 cm3mol-1K at 9 K for 3.
Below this temperature, χMT decreases gradually indicating the contribution of zero – field splitting of ground state
(S = 2) and/or antiferromagnetic intermolecular interactions between the dinuclear entities.
At the first step to interpreted quantitatively magnetic data, we applied standard dimeric equation for Ni(II) with
only two parameters χ (g, J) [29]. The fit was acceptable only at high temperature range (much above the maximum
on χMT vs T curve). For 1 – 3 Ni(II) dimers where the maximum is resolved, the situation requires the addition of
higher order effect. In such case the variable – temperature susceptibility data were analysed using the isotropic

8
Ginsberg model from the Hamiltonian [30], which includes a crystal field splitting term (D) characteristic for axially
distorted octahedral symmetry and a molecular field term (zJ’). The equation will be referred to as χ (g, J, D, zJ’).
ˆ
=
−
+
−
−
ˆ ˆ
- 2JS S2
ˆ
ˆ ˆ
2zJ'Si Si
ˆ
2
2
D(S1
)
g
i BHSi
Η
S2
1
z
z
The molar magnetic susceptibility for a dinuclear Ni2+ complex is given in supplementary part.
The fitting are much better using more parameters in the equations described above. However at low temperature the
very small discrepancies were visible. Probably it can be due to the presence of two different bridge between Ni(II)
ions, which will be described later.
Least-square fits to the experimental data were obtained for J = 2.46 cm-1, D = -2.11 cm-1, zJ’ = − 0.25 cm-1, g
= 2.091 and R = 2.37×10-4 for (1), J = 1.86 cm-1, D = -3.56 cm-1, zJ’ = − 0.37 cm-1, g = 2.12 and R=3.04×10-4 for
(2), J = 1.73 cm-1, D = -3.84 cm-1, zJ’ = − 0.39 cm-1, g = 2.10 and R = 1.25 × 10-4 for (3). Our calculations shown
that the parameters D and zJ’ are strongly correlated with each other but are only weakly correlated with g and J.
The value of the D-parameter for the {NiN2O4} chromophore spans the interval for the other Ni(II) centers [31].
However, the identification of its sign is rather problematic. The best technique for such a target is the high-
frequency/high-field electron spin resonance.
The magnetic properties of the examined compounds can be interpreted in terms of the two different bridging
groups (-O, H2O) and the geometrical consequences of the twisted bridging arrangement. The double phenoxide Ni
– O – Ni bridge angles are lower than 100º (around 93º for all three complexes) and presented as an almost ideal
structure for weak ferromagnetism and dominate the overall exchange process.
The water Ni – O – Ni bridge angles are around 80º, therefore, it is reasonable to assume that any exchange via
this bridge is not likely to be ferromagnetic, and in fact, it might be expected to propagate a weak antiferromagnetic
contribution. The smaller water bridge angle than in most reported complexes, is a result of the long Ni – O bond
length. Antiferromagnetic behavior was typical for hydroxide bridge binuclear copper(II) compounds [31] and
phenoxide bridged binuclear nickel(II) complexes [10b, 33]. Such assumptions remain in good compliance with ab
initio theoretical calculations on binuclear Ni – (µ-O) – Ni model compounds, which show strong correlation
between the value of exchange coupling J and Ni-O-Ni angle: the coupling should be antiferromagnetic for angles
below 85º and above 102º, but weakly ferromagnetic in between [32,34]. The geometry of the dimers determines the
sign and magnitude of intradimer exchange. Consideration of the electronic states of weakly interacting metal
centres have revealed, that the bridging ligand is most effective in supporting antiferromagnetic exchange
interactions between the two metal ions if the bridge has available molecular orbitals that can interact with the
bonding combination of the metals’d-orbitals [35,36]. The antiferromagnetic contribution reduces when the
environment around metal atom changes from planar to tetrahedral geometry. Almost all reported dinuclear
complexes derived from the macrocyclic ligand show antiferromagnetic coupling in a slightly distorted [Ni -(O)2 –
Ni] planar unit with an inversion center, where Ni – O – Ni angle is close to 99.5 – 105.7, with antiferromagnetic
coupling, J, lying between -17.0 and -67.1 cm-1 [10b, 31]. The substantial variation of the exchange parameter was
the result of stereochemical changes (different axial ligands). The extent of antiferromagnetic interaction increases
along this series as progressive increase in the intramolecular Ni ···Ni distance (3.10 – 3.206 Å) as well as bridge
angle. Complexes 1, 2, 3 are different from most of the reported similar complexes for several reasons: the double
phenoxide bridge connecting both Ni(II) ions is asymmetric (with one shorter Ni – O bond, and one similar as
observed in other phenoxo bridged species), all contains an additional superexchange pathway involving the water
bridge, which is perpendicular to the Ni2O2 pathway, as a result the Ni – O – Ni angles are comparatively lower
(close to 93º) with shorter Ni ···Ni distance [2.988 Å for (1), 2.971 Å for (2), 2.984 Å for (3)] (see Table 6). In
addition, the presence of an aqueous bridge causes an additional antiferromagnetic contribution to magnetic effects,
thus weakening the dominant ferromagnetic interactions. This feature explain why the J value is positive and the
lowest reported so far for these Ni(II) dinuclear complexes with two phenoxo bridging ligand.
The magnetization per formula unit adopts a value of 3.81M.B. for 1, 3.65 M.B. for 2 and 3.84 for 3 somewhat
about 5T and T = 2.0 K [see the right inset in Figs. 7a-c].These values are a little bit lower that those characteristic
for completely polarized dimer of Ni2+ (S = 2), provides evidence for a parallel alignment of the spin of the nickel
ions in the magnetic field. These small discrepancies is due to weak coupling between the magnetic centers and

9
evidences a sizable zero–field splitting for Ni(II) ions.
Below 9 K, χMT value decreases due to the combined effect of zero–field splitting, saturation and/or intermolecular
antiferromagnetic couplings. In this procedure, a zJ’ constant has been used to account for intermolecular
interactions. Additionally, according to the crystal structure, in all three complexes oxygen atoms (from water)
forms bifurcated hydrogen bonding pattern with a pendant –OH group of Schiff base ligand (O1-H49···O6 =
2.778(5) Å (1), O1-H31···O4 = 2.745(4) Å (2), O13-H10···O9= 2.738(5) Å (3)) and phenoxide oxygen atoms of
another unit. These contacts may create a next magnetic exchange pathway between the Ni ions of neighbouring
dimers in the crystal network. The small magnitude of this interaction is a result of the rather long Ni ···Ni
separation (8.779 (8) Å for 1, 8.5987 (11) Å for 2 and 7.7009 (15) Å for 3).The obtained very small value of zJ’ = -
0.25 cm-1 for 1, zJ’ = - 0.37cm-1 for 2 and zJ’ = - 0.39 cm-1 for 3 in comparison to large value of DNi parameter
indicates a dominant zero-field splitting effect within the S=1 local ground states of each Ni(II) ions.
4. Conclusion
We have synthesized and characterized three dinuclear Ni(II) complexes, [Ni2(HL1)4(H2O)](1), [Ni2(HL2)4(H2O)]
(2) and [Ni2(HL3)4(H2O)](3) using Schiff base ligands derived from 5-amino-1- pentanol and salicylaldehyde
(H2L1), 5-bromo salicylaldehyde (H2L2) and 3-methoxy salicylaldehyde (H2L3) respectively. All of them contain
bridging aqua ligand - a feature observed only in a couple of dinuclear Ni(II) complexes in literature. All of them
describe distorted octahedral structures and the dispositions of the pendant –OH groups are such so that they remain
relaxed inside the crystal lattice providing a scope for H-bonding. Notably, the presence of coordinated bridging
water molecules also leads to the formation of extensive H-bonding interactions resulting in 3D supramolecular
network in 1 and 3, and a 2D netlike arrangement in 2. The coordinated water molecules having Ni(II) – O(water)
bond lengths in the range of 2.3 Å, which is slightly longer than expected, enables these dinuclear Ni(II) complexes
to act as efficient catalysts for the conversion of 3, 5-di-tertbutylcatechol (3,5-DTBCH2) to 3, 5- di-tertbutylquinone
(3,5-DTBQ) in methanolic medium. The cyclic voltammetry and X-band EPR studies are carried out in order to
monitor the solution behaviour during catecholase action. The magnetic data suggest that weak ferromagnetic
couplings exist between the Ni(II) centers. Simple magnetic considerations which are based on structural knowledge
are employed to account for the weak magnetic interactions. From the structural and magnetic parameters, the Ni –
O – Ni bond angle seems to be a considerable factor affecting the magnitude of magnetic couplings.
Acknowledgement
Financial support from the University Grants Commission for a RFSMS fellowship (Sanction No.
UGC/740/RFSMS) to R. Modak, DST for a junior research fellowship to Y. Sikdar (Sanction No.
SERB/F/1585/2012-13, project No. SR/FT/CS-107/2011) and University Grants Commission for a junior research
fellowship to S. Mandal (Sanction No. UGC/749/Jr. Fellow(Sc.)) is gratefully acknowledged. DST-FIST is
acknowledged for providing the X-ray diffraction facility at the Department of Chemistry, University of Calcutta.
This research was supported by the National Science Centre (Poland) under grant No 2011/01/B/ST5/01624. We
sincerely thank Dr. Prasanta Ghosh of R. K. Mission Residential College, Narendrapur, Kolkata, for his help in EPR
measurement.
Appendix A. Supplementary material
CCDC reference numbers of crystals 1−3 are, 917991,935266 and 917992 respectively. These data can be obtained
free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk.
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[22] (a) S. Mohanta, K.K. Nanda, R. Werner, W. Haase, A.K. Mukherjee, S. Dutta, K. Nag, Inorg. Chem. 36 (1997)
4656;
(b) H. Adams, S. Clunas, D.E. Fenton, Chem. Commun. 2002, 418;
(c) I. Rozas, I. Alkorta, J. Elguero, J. Phys. Chem. A. 102 (1998) 9925.
[23] J. Mukherjee, R. Mukherjee, Inorg. Chim. Acta 337 (2002) 429.
[24] 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.
[25] N. Muresan, K. Chlopek, T. Weyhermuller, F. Neese, K. Wieghardt, Inorg. Chem. 46 (2007) 5327.
[26] K.S. Bharathi, S. Sreedaran, A.K. Rahiman, K. Rajesh, V. Narayanan, Polyhedron 26 (2007) 3993.
[27] T. Ghosh, J. Adhikary, P. Chakraborty, P.K. Sukul, M.S. Jana, T.K. Mondal, E. Zangrando, D. Das, Dalton
Trans. 43 (2014) 841.

12
[28] 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.
[29] R.J. Butcher, C.J. O’Connor, E. Sinn, Inog. Chem. 18 (1079) 492.
[30] A.P. Ginsberg, R.L. Martin, R.W. Brookes, R.C. Sherwood, Inorg. Chem. 11 (1972) 2884.
[31] J. Titiš, R. Boča, Inorg. Chem. 49 (2010) 3971.
[32] V.H. Crawford, H.W. Richardson, J.R. Wasson, D.J. Hodgson, W. E Hatfield, Inorg.Chem. 15 (1976) 2107.
[33] Z. Xu, L.K. Thompson, V.A. Milway, L. Zhao, T. Kelly, D.O. Miller, Inorg. Chem. 42 (2003) 2950.
[34] C. Wang, K. Fink. V. Staemmler, Chemical Physics 192 (1995) 25.
[35] P.J. Hay, J.C. Thibeault, R.J. Hoffmann, J. Am. Chem. Soc. 97 (1975) 4884.
[36] O. Kahn, Molecular Magnetism, first ed., VCH Publishers, New York, 1993.
Figure Captions
Fig. 1a. Crystal structure of 1. Hydrogen atoms except aqua bridged are removed for clarity.
Fig. 1b. Rectagular core formed by four neighbouring dinickel moieties in 1.Blue broken line represents the H-
bonding.
Fig. 1c. 3-dimensional supramolecular architecture formed in 1.
Fig. 2. Crystal structure of 2. Hydrogen atoms except aqua bridged are removed for clarity.
Fig. 3. Crystal structure of 3. Hydrogen atoms except aqua bridged are removed for clarity.
Fig. 4a. UV-Vis spectrum of 1 at 15 minutes time interval after addition of 3,5-DTBCH2.
Fig. 4b. Plot of the initial rates versus substrate concentration for the oxidation of 3,5-DTBCH2 catalyzed by 1. The
inset shows Lineweaver−Burk plot.
Fig. 5. Cyclic voltammogram of methanolic solution 3,5-DTBC and Complex 1 (as representative).
Fig. 6. X-band EPR spectrum of mixture of 3,5-DTBC and Complex 1 (as representative).
Fig. 7a. Thermal dependence of (○)χM and (●)χMT for 1.The inset show thermal dependence of inverse magnetic
susceptibility (left) and field dependence of the magnetization (right). The solid lines are the calculated curves.
Fig. 7b. Thermal dependence of (○)χM and (●)χMT for 2.The inset show thermal dependence of inverse magnetic
susceptibility (left) and field dependence of the magnetization (right). The solid lines are the calculated curves.
Fig. 7c. Thermal dependence of (○)χM and (●)χMT for 3.The inset show thermal dependence of inverse magnetic
susceptibility (left) and field dependence of the magnetization (right). The solid lines are the calculated curves.

13
Scheme 1. Structure of the ligands used in the study
Scheme 2. Synthetic outline of the complexes

14
Coordination core of the complex
Bond lengths/Å
Bond Angles/deg
Ni(1)-O(1)
Ni(1)-O(2)
Ni(1)-O(3)
Ni(1)-O(4)
Ni(1)-N(2)
Ni(1)-N(3)
Ni(2)-O(1)
Ni(2)-O(2)
Ni(2)-O(3)
Ni(2)-O(5)
Ni(2)-N(1)
Ni(2)-N(4)
2.334(4)
Ni(1)-O(1)-Ni(2) 80.11(11)
Ni(1)-O(2)-Ni(2) 92.57(10)
Ni(1)-O(3)-Ni(2) 92.86(10)
2.131(3)
2.005(2)
1.988(2)
2.048(3)
2.030(3)
2.310(3)
2.002(2)
2.118(3)
1.987(3)
2.065(3)
2.036(3)
Ni(1)-O(1)
Ni(1)-O(2)
Ni(1)-O(3)
2.300(4)
1.980(2)
2.109(2)
Ni(1)-O(1)-Ni(1_a) 80.46(15)
Ni(1)-O(3)-Ni(1_a) 92.50(9)
Ni(1)-N(24) 2.048(3)
Ni(1)-O(3_a) 2.002(2)
Ni(1)-N(23_a) 2.050(3)
Ni(1)-O(1)
Ni(1)-O(4)
Ni(1)-O(10) 2.101(3)
Ni(1)-O(13) 2.367(4)
1.975(3)
1.990(3)
Ni(1)-O(13)-Ni(2) 79.69(11)
Ni(1)-O(10)-Ni(2) 93.26(12)
Ni(1)-O(4)-Ni(2) 92.81(12)
Ni(1)-N(1)
Ni(1)-N(2)
Ni(2)-O(4)
Ni(2)-O(7)
2.033(4)
2.049(4)
2.128(3)
1.988(4)
Ni(2)-O(10) 2.003(3)
Ni(2)-O(13) 2.289(3)
Ni(2)-N(3)
Ni(2)-N(4)
2.030(4)
2.048(4)
Symmetry transformations used to generate equivalent atoms: _a: 1-x, y, 1/2-z
Scheme 3. Bond length and bond angles of the metal centre of 1, 2 and 3 respectively.

15
Scheme 4. Plausible intermediate (IA, IB) formation during the coupling of dinuclear Ni(II) complex and 3,5-
DTBCH2(charge and counterions are omitted for clarity).

Table(s)
Table 1.
Crystal data and structure refinement for 1 – 3.
Formula
C48 H66 N4 Ni2 O9 (1)
960.43
C48 H62 Br4 N4 Ni2 O9 (2)
1276
C52 H74 N4 Ni2 O13, C H4 O (3)
M
Crystal System
Space Group
a/Å
1112.52
Monoclinic
P 21/c
12.2617(11)
31.6040(29)
15.2099(14)
90.000
Monoclinic
P 21/c
21.5729(13)
11.7043(7)
19.6413(13)
90.00
Monoclinic
C 2/c
17.6987(8)
15.5343(8)
22.5002(12)
90.00
b/ Å
c/ Å
α(°)
β(°)
92.863(2)
90.00
4953.2(5)
4
1.292
0.816
108.205(2)
90.00
5876.5(5)
4
1.429
3.408
107.020(1)
90.000
5636.0(9)
4
1.187
0.727
γ(°)
V/ Å3
Z
Dc/g cm-3
µ/mm-1
F(000)
2052
2568
2320
R(int)
0.0456
0.0649
0.0473
total Reflections
unique reflections
R1, wR2
T/K
7745
6352
0.0560, 0.1408
150(2)
9351
4602
0.0565, 0.1439
150(2)
8849
6182
0.0597, 0.1754
150(2)

Table(s)
Table 2.
Selected bond distances (Å ) and angles (deg)in the metal coordination spheres for1-3.
1
2
3
Ni(1)-O(4)
Ni(1)-O(3)
Ni(1)-N(3)
Ni(1)-N(2)
Ni(1)-O(2)
Ni(1)-O(1)
Ni(2)-O(5)
Ni(2)-O(2)
Ni(2)-N(4)
Ni(2)-N(1)
Ni(2)-O(3)
1.988(2)
2.005(2)
2.030(3)
2.048(3)
2.131(3)
2.334(3)
1.987(3) Ni(1)-Ni(1_a)
2.002(2)
2.036(3)
Ni(1)-O(2)
1.980(2)
2.003(2)
2.048(3)
2.050(3)
2.109(2)
2.300(4)
Ni(1)-O(1)
1.975(3)
1.990(3)
2.033(4)
2.049(4)
2.101(3)
2.368(3)
2.289(3)
1.988(3)
2.002(3)
2.029(4)
2.049(4)
Ni(1)-O(3_a)
Ni(1)-N(24)
Ni(1)-N(23_a)
Ni(1)-O(3)
Ni(1)-O(1)
2.9709(9)
Ni(1)-O(4)
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-O(10)
Ni(1)-O(13)
Ni(2)-O(13)
Ni(2)-(O7)
Ni(2)-O(10)
Ni(2)-N(3)
Ni(2)- N4
2.065(4)
2.118(3)
Ni(2)- O(1)
Ni(2)- O(13)
2.310(3)
2.289(3)
Ni(2)- O4
2.128(3)
O(1)-Ni(1)- O(2) 74.70(10)
O(1)-Ni(1)-O(3) 76.16(11)
O(1)-Ni(1)-O(4) 91.41(11)
O(1)-Ni(1)-N(2) 88.32(12)
O(1)-Ni(1)-N(3) 171.96(12)
O(2)-Ni(1)-O(3) 77.55(10)
O(2)-Ni(1)-O(4) 93.22(10)
O(2)-Ni(1)-N(2) 160.79(12)
O(2)-Ni(1)-N(3) 97.46(12)
O(3)-Ni(1)-O(4) 166.04(10)
O(3)-Ni(1)-N(2) 89.87(12)
O(3)-Ni(1)-N(3) 100.71(12)
O(4)-Ni(1)-N(2) 96.18(13)
O(4)-Ni(1)-N(3) 90.77(12)
N(2)-Ni(1)-N(3) 99.13(13)
O(1)-Ni(2)-O(2) 77.65(11)
O(1)-Ni(1)-O(2)
O(1)-Ni(1)-O(3)
O(1)-Ni(1)-N(24) 168.91(10)
O(1)-Ni(1)-O(3_a) 92.63(9)
O(1)-Ni(1)-O(23_a) 92.58(9)
92.67(8)
74.52(9)
O(1)-Ni(1)-O(4 ) 166.86(13)
O(1)-Ni(1)-N(1) 91.16(14)
O(4)-Ni(1)-N(1) 99.50(12)
O(1)-Ni(1)-N(2) 94.95(15)
O(4)-Ni(1)-N(2) 91.75(14)
N(1)-Ni(1)-N(2) 95.17(14)
O(1)-Ni(1)-O(10) 92.73(12)
O(4)-Ni(1)-O(10) 77.98(11)
O(10)-Ni(1)-N(1) 99.76(12)
O(10)-Ni(1)-N(2) 163.05(14)
O(1)-Ni(1)-O(13) 92.54(13)
O(4)-Ni(1)-O(13) 76.09(11)
O(13)-Ni(1)-N(1) 172.97(12)
O(13)-Ni(1)-N(2) 90.45(13)
O(10)-Ni(1)-O(13)74.10(11)
O(7)-Ni(2)-O(10) 167.92(13)
O(2)-Ni(1)-O(3)
92.63(9)
O(2)-Ni(1)-N(24) 90.98(10)
O(2)-Ni(1)-O(3_a) 167.30(9)
O(2)-Ni(1)-N(23_a) 76.48(9)
O(3)-Ni(1)-N(24) 94.86(10)
O(3)-Ni(1)-O(3_a) 78.22(8)
O(3)-Ni(1)-N(23_a) 164.60(10)
O(3_a)-Ni(1)-N(24) 98.51(10)
N(23_a)-Ni(1)-N(24) 97.42(10)
O(3_a)-Ni(1)-N(23_a)90.74(10)
Ni(1)-O(1)-Ni(1_a) 80.46(15)

Table(s)
Table 3.
Hydrogen bonding distances (A˚) for 1-3.
1
2
3
O(6)-H(6)···O(5)
2.659(4)
O(4)-H(15)···O(2) 2.684(4)
O(1)-H(31)···O(4) 2.745(4)
O(13)-H(10)···O(9) 2.738(5)
O(13)-H(20)···O(6) 2.727(6)
O(3)-H(30)···O(8) 2.850(6)
O(6)-H(60)···O(7) 2.644(5)
O(9)-H(90)···O(1) 2.651(5)
O(7)-H(7A)···O(9) 2.732(9)
O(8)-H(8)···O(4)
O(9)-H(9)···O(7)
2.627(4)
2.908(10)
O(1)-H(49)···O(6) 2.778(5)

Table(s)
Table 4.
Different kinetic parameters for catecholase activity of 1-3.
Complex
Vmax (M S-1)
KM (M)
Kcat(h-1)
1.87 x 104
1.79 x 104
1.38 x 104
5.19 ± 0.017×10-4
4.97 ± 0.035 ×10-4
3.86 ±0.029 × 10-4
6.05 ± 0.021 ×10-3
8.14 ± 0.011×10-3
5.78 ±0.031×10-3
1
2
3

Table(s)
Table 5.Comparison of Kcat value for the oxidation of 3,5-DTBCH2 to 3,5-DTBQ by different dinuclear Ni(II) Schiff base ligand
complexes.
Complex
Kcat(h-1)
Reference
I n
In
Methanol Acetonitrile
1.87 x 104
1.79 x 104
n.d.
n.d.
Present work
Present work
1
2
1.38 x 104
n.d.
n.d.
Present work
8.(d)
3
∑ Ni2L12(NCS)2
64.1 ±4.1
∑ Ni2L22(NCS)2
∑ Ni2L32(NCS)2
n.d.
n.d.
51.1 ± 6.2
81.7 ±4.7
8.(d)
8.(d)
24
*[Ni2(LH2)(H2O)2(OH)(NO3)](NO3)3 1.4 x 104 Inactive
$[Ni2L(NO3)(H2O)3]NO3
#[Ni2(L2)(N3)3(H2O)2]
#[Ni2(L4)(N3)3(H2O)2]
1.5 x 103
n.d.
n.d.
n.d.
17
27
27
2
1.728×10
2.641×10
2
£ [Ni2(L1)(NCS)3(H2O)2]
3.60 x 103 1.08 x 104
28
28
£ [Ni2(L2)(CH3COO)(NCS)2(H2O)] 2.16 x 104 3.24 x 104
£ [Ni2(L3)(NCS)3] 1.3 x 103 2.95 x 103
28
∑
L1= 2-[1-(3-methylamino-propylamino)-ethyl]-phenolato
L2= 2-[1-(2-dimethylamino-ethylamino)-ethyl]-phenolato
L3= 2-[1-(3-dimethylamino-propylamino)-ethyl]-phenolato
* LH2= 2,6-bis(N-ethylpiperazine-iminomethyl)-4-methyl-phenolato
$ L= [1+2] condensation of 1,3-diaminopropane and 2,6-diformyl-4-tert-butylphenol)
# L2= 4-tert-Butyl-2-[(2-dimethylamino-ethylamino)-methyl]-6-[(2-dimethylamino-ethylimino)-methyl]-
phenolato
L4= 4-tert-Butyl-2,6-bis-[(2-pyridin-2-yl-ethylimino)-methyl]-phenolato
£ L1= 2,6-bis(N-ethylpyrrolidineiminomethyl)-4-methylphenolato
L2= 2,6-bis(N-ethylpiperidineiminomethyl)-4-methylphenolato
L3 = 2,6-bis{N-ethyl-N-(3-hydroxypropyliminomethyl)}-4-methylphenolato

Table(s)
Table 6. Comparison of magnetostructural data for 1 - 3.
Complex
J, cm-1
g
D, cm-1 zJ’, cm-1
Ni···Ni, Å
2.988 Å
2.971 Å
2.984 Å
Ni–O–Ni, deg
phenoxide
92.57(10)
92.86(10)
92.50(9)
Ni–O–Ni, deg
water
[Ni2(HL1)4(H2O)] (1)
[Ni2(HL2)4(H2O)] (2)
[Ni2(HL3)4(H2O)] (3)
2.46 2.091
-2.11
-3.56
-3.84
− 0.25
− 0.37
− 0.39
80.11(11)
1.86
1.73
2.12
2.10
80.46(15)
79.69(11)
93.26(12)
92.81(12)

Figure(s)
Fig. 1a. Crystal structure of 1. Hydrogen atoms except aqua bridged are removed for clarity.

Figure(s)
Fig. 1b. Rectangular core formed by four neighbouring dinickel moieties in 1.Blue broken line represents the H-bonding.

Figure(s)
Fig. 1c. 3-dimensional supramolecular architecture formed in 1.

Figure(s)
Fig. 2. Crystal structure of 2. Hydrogen atoms except aqua bridged are removed for clarity.

Figure(s)
Fig. 3. Crystal structure of 3. Hydrogen atoms except aqua bridged are removed for clarity.

Figure(s)
2
Complex only
3,5-DTBC
0
300
400
500
Wavelength(nm)
Fig. 4a. UV-Vis spectrum of 1 at 15 minutes time interval after addition of 3,5-DTBCH2.

Figure(s)
4000
3500
3000
2500
2000
1500
1000
500
Y =1815.06759+ 10.99076 * X
R2, linear regression coefficient=0.9955
0
50
100
150
200
1 / [Concentration] (M-1
)
Fig. 4b. Plot of the initial rates versus substrate concentration for the oxidation of 3,5-DTBCH2 catalyzed by 1. The
inset shows Lineweaver−Burk plot.

Figure(s)
0.00012
0.00010
0.00008
0.00006
0.00004
0.00002
0.00000
-0.00002
-0.00004
-0.00006
-0.00008
0.5
0.0
-0.5
-1.0
-1.5
Potential(V)
Fig. 5. Cyclic voltammogram of methanolic solution of 3,5-DTBC and Complex 1 (as
representative example).