Tri- and hexa-nuclear NiII-MnII complexes of a N2O2 donor unsymmetrical ligand: Synthesis, structures, magnetic properties and catalytic oxidase activities

Article abstract of DOI:10.1039/c8dt02962h

A new mononuclear Ni(ii) complex, [NiL] (1) of an unsymmetrically dicondensed N₂O₂ donor ligand, H₂L (where H₂L = N-α-methylsalicylidene-N′-salicylidene-1,3-propanediamine), has been synthesized. Complex 1, on r

Full text of DOI:10.1039/c8dt02962h

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January 2016 Pages 1–398
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Efficient extraction of sulfate from water using
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DOI: 10.1039/C8DT02962H
II
II
Tri- and hexa-nuclear Ni –Mn complexes of a N
2
O donor unsymmetrical
2
ligand: Synthesis, structures, magnetic properties and catalytic oxidase
activities
a
b
,a
Prithwish Mahapatra , Michael G. B. Drew , Ashutosh Ghosh*
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A. P.
C. Road, Kolkata 700009, India, E-mail: ghosh_59@yahoo.com
bSchool of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6
6
AD, UK.
Abstract
A new mononuclear Ni(II) complex, [NiL] (1) of an unsymmetrically dicondensed N
2
O
2
donor ligand,
H
2
L
(where
H
2
L
=
N-α-methylsalicylidene-N΄-salicylidene-1,3-
propanediamine) has been synthesized. Complex 1, on reaction with Mn(II) in presence of
II
II
different
co-anions,
Mn(NCS) ]CH
{(NiL) Mn(H O)}
yielded
(2),
)](ClO
three
[(NiL)
heterometallic
Ni −Mn
)H
complexes,
(3) and
[
[
(NiL)
2
2
3
CN
2
Mn(N )(H O)](ClO
3
2
4
2
O
2
2
2
(μ1,3-N
3
4
)
3
(4). Single crystal structural analyses show that
complexes 2 and 3 contain similar bent trinuclear NiII
Mn structures, with the difference that
II
2
–
–
in complex 2 two SCN ions and in 3, one N
3
and one H
2
O molecule are coordinated to
MnII
Mn(II) centre. Complex 4 possesses a hexanuclear structure, in which two trinuclear NiII
2
units are connected via a single μ1,3-azido bridge. Variable temperature dc molar magnetic
susceptibility measurements reveal that the two Mn(II) centres in complex 4 are
–
1
antiferromagnetically coupled with an exchange coupling constant (J) of –3.73 cm . Among
the three heterometallic complexes, only 3 and 4 show bio-mimetic catalytic oxidase
activities. For catecholase like activity, the turnover numbers (Kcat) of complexes 3 and 4 are

1
9
84 and 2081 h , respectively whereas for phenoxazinone synthase-like activity, the turnover

1
numbers of complexes 3 and 4 are 6351 and 10545 h , respectively. The high catalytic
oxidase activities of complexes 3 and 4 in contrast to the inactivity of complex 2 are
attributed to the coordination of a water molecule to the Mn(II) centre in the former
structures, which facilitate the binding of substrate with catalyst. Mass spectral analyses
suggest the probable formation of a complex-substrate intermediate in these catalytic
reactions and cyclic voltammetry measurements show that Ni(II) is reducible to Ni(I).
1

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DOI: 10.1039/C8DT02962H
Introduction
The ‘complex as ligand’ strategy is widely used for synthesis of heterometallic polynuclear
complexes using the metalloligand of 3d metal ions derived from single or multi
1
compartmental N,O-donor ligands. These polynuclear heterometallic complexes find their
potential uses in the field of bio-relevant catalysis and molecular magnetism with SMM or
2
SCM like properties. A large number of heterometallic complexes have been synthesized
using symmetrically dicondensed N
2
O
2
or N
2
O
4
donor ligands of 1,2-ethanediamine or 1,3-
3
propanediamine with various carbonyl compounds. One of the significant applications of
these heteromatallic complex is to mimic biological enzyme catalyzed reactions. A variety of
homometallic polynuclear complexes of 3d metal ions e.g. Cu(II), Ni(II), Co(II/III),
4
Mn(II/III), Fe(II/III) has been used as catalysts for oxidase type reactions. Among them, the
activity of Cu(II) complexes has been studied extensively, as naturally occurring
metalloenzymes like catechol oxidase, phenoxazinone synthase, cytochrome c oxidase etc.
5
contain multiple Cu(II) ions in their active sites. The catalytic efficiency of these model
complexes is however significantly less than that of the biological enzymes. Recently, we
found that the heterometallic complexes especially those containing Mn(II) ion not only can
catalyse such oxidase reactions but their efficiency can be very high depending upon the
6
coordination environment around Mn(II) atom. For example, we have synthesized some
complexes with unsymmetrical ligands, namely heterometallic Cu(II)−Mn(II) complexes
with N
2
O
2
donors and Ni(II)−Mn(II) complexes with N
2
3
O donors in which unsymmetrical
coordination is observed around the Mn(II) ion with a coordinated solvent molecule unlike
their symmetric analogues where coordination around Mn(II) is symmetrical without a
coordinated solvent molecule.7 The catalytic activities of these complexes with
unsymmetrical ligands are one of the highest among the synthetic complexes whereas those
of the complexes with symmetrical ligands are very low or negligible.
In comparison with the N
2
O
2
donor Cu(II) metalloligands, the number of complexes
derived from such Ni(II) metalloligands is much less and that of unsymmetrically
dicondensed N
2
O
2
donor ligands is very rare - only one heterometallic complex (Ni Cu) has
2
8
been reported till now. The structure, magnetic and catalytic properties of these
heterometallic complexes depend on the combination of metal ions and the anionic co-
ligands.6,7 Among the co-anions, pseudo halides deserve special mention for their different
mode of coordination and ability to construct a variety of coordination complexes with
9
diverse structures and nuclearity. We would like to synthesise some Ni
2
Mn complexes with
2

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the expectation of an unsymmetrical coordination environment around the Mn(II) centres like
DOI: 10.1039/C8DT02962H
their Cu(II) analogues and investigate their magnetic properties and catalytic efficiencies for
oxidase reactions on different substrates in naturally viable conditions.
Herein, we report a new mononuclear Ni(II) complex, [NiL] (1) of unsymmetrical
N
2
O
2
donor ligand H
2
L (where H L = N-α-methylsalicylidene-N΄-salicylidene-1,3-
2
propanediamine), synthesized by our newly developed method. Using complex 1 as
II
II
metalloligand, three heterometallic Ni −Mn complexes [(NiL)
(NiL) Mn(N )(H O)](ClO )H O (3) and [{(NiL) Mn(H O)}
synthesized. These are the first Ni
2
Mn(NCS)
2
]CH
(4) have been
donor
3
CN (2),
[
2
3
2
4
2
2
2
2
(μ1,3-N
3
)](ClO
4
)
3
2
Mn complexes of asymmetrically dicondensed N
2
O
2
Schifff base ligands. All the complexes 14 are characterized by single crystal X-ray
diffraction. Complexes 2 and 3 show simple bent trinuclear NiII
Mn structures, whereas
II
2
complex 4 shows a hexa-nuclear structure in which two trinuclear NiII
Mn units are
II
2
symmetrically connected via single μ1,3-N
3
bridge. Among these complexes, 3 and 4 exhibit
very high catechol oxidase as well as phenoxazinone synthase-like activities under aerobic
conditions. UV-Vis spectra, ESI-mass spectra and cyclic voltammograms of all the
complexes have been studied to determine the possible intermediates or complex-substrate
mixture in these catalytic oxidase reactions and to draw a possible mechanism based on this.
We have shown that the very high catalytic efficiencies of 3 and 4 compared to that of
complex 2 are due to the presence of a coordinating water molecule and one azide ion on the
Mn(II) centers of the complexes. The variable temperature dc magnetic susceptibility has also
been measured for complex 4.
Experimental Section
Starting materials: Salicylaldehyde, o-hydroxyacetophenone and 1,3-propanediamine were
purchased from Spectrochem, India and were of reagent grade. They were used without
further purification. The other metal salts and solvents were of commercially available
reagent grade quality, unless otherwise stated.
Caution! Although not encountered during experiment, perchlorate and azide salts of metal
complexes with organic ligands are potentially explosive. Only a small amount of material
should be prepared and it should be handled with care.
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Synthesis of the complex [NiL] (1). The [Ni(L1)
aminopropylimino)methyl)phenol) was synthesized
bis(salicylaldehyde)nickel(II) dihydrate (3.09 g, 10 mmol) and 1,3-propanediamine (2.10 mL,
5 mmol). Then, this complex (2.055 g, 5 mmol) was dissolved in 25 mL methanol and solid
·2H O] complex (where HL = 2-((E)-(3-
DOI: 10.1039/C8DT02962H
1
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2
2
by
the
reaction
of
2
bis(o-hydroxyacetophenone) nickel(II) dihydrate (1.685 g, 5 mmol) was added and refluxed
for 8 hours. The deep red colour solution was obtained, which was filtered and kept in a
desiccator at room temperature. The deep red coloured X-ray quality single crystals appeared
inside the beaker after 2-3 days on slow evaporation of methanol. The crystals were filtered
out from the solution after 3-4 days and air dried.
Complex 1: Yield 2.754 g (78%). Anal. Calc. for C18
1.24; H, 5.14; N, 7.93; Found C, 61.13; H, 5.08; N, 7.85. IR: ν(C=N) = 1628, 1597 cm for
formylimine and acetylimine, respectively.
H
18
N
2
NiO
2
(353.05): Calculated C,
–
1
6
Synthesis of the complex [(NiL)
was dissolved in 20 mL acetonitrile and Mn(ClO
mixture was stirred for 10 mins and then 4 mL 1:1 (v/v) H
0.152 g, 2 mmol) was added to the solution at room temperature. The stirring was continued
2
Mn(NCS)
2
]CH
3
CN (2). Complex 1 (0.706 g, 2 mmol)
·6H O (0.361 g, 1 mmol) was added. The
O-MeCN solution of NH SCN
4
)
2
2
2
4
(
for 20 mins. A small amount of precipitate appeared during this process which was filtered
off and the filtrate was kept in a beaker inside a desiccator. The reddish brown coloured X-
ray quality single crystals appeared bottom of the beaker after 2-3 days, which were filtered
out after 3 days and dried in air.
Complex 2: Yield 0.655 g (73%). Anal. Calc. for C39
H36.5MnN6.5Ni
2
O
4
2
S (896.72):
Calculated C, 52.24; H, 4.10; N, 10.15; Found C, 52.11; H, 4.04; N, 10.07. IR: ν(C=N) = 1630
and 1598 cm for formylimine and acetylimine, respectively; ν(S=C=N) = 2087, 2057 cm–1.
–
1
Synthesis of the complex [(NiL)
synthesized by following the similar procedure as for the synthesis of 2. Here a 4 mL 1:1
v/v) H O-MeOH solution of NaN (0.130 g, 2 mmol) was used instead of NH SCN. The
2
Mn(N
3
)(H
2
O)](ClO
4
)H
2
O (3). The complex was
(
2
3
4
mixture was filtered and the filtrate was kept in a beaker inside a desiccator. The deep brown
coloured X-ray quality single crystals appeared inside the beaker in 3-4 days, which were
filtered out after 4 days and air dried.
4

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Complex 3: Yield 0.695 g (74%). Anal. Calc. for C36
H40ClMnN
7
Ni
2
O10 (938.50): Calculated
DOI: 10.1039/C8DT02962H
C, 46.07; H, 4.30; N, 10.45; Found C, 45.96; H, 4.22; N, 10.38. IR: ν(C=N) = 1634, 1599 cm–1
for formylimine and acetylimine, respectively; ν(Cl=O) = 1108 cm and ν(N=N=N) = 2062 cm–1.
–
1
Synthesis of the complex [{(NiL)
synthesized by following the similar procedure as for the synthesis of complex 3. Here, only
the amount of NaN (0.065 g, 1 mmol) was half of the amount used for the synthesis of
2
Mn(H
2
O)}
2
(μ1,3-N
3
)](ClO
4
)
3
(4). This complex was
3
complex 3. The mixture was filtered and kept in a beaker inside a desiccator. The brown
coloured X-ray quality single crystals appeared in the beaker after 3-4 days, which were
filtered out and dried in air.
Complex 4: Yield 0.684 g (72%). Anal. Calc. for C72
H76Cl
3
Mn
2
N11Ni
4
O22 (1898.50):
Calculated C, 45.55; H, 4.04; N, 8.12; Found C, 45.342; H, 3.95; N, 8.03. IR: ν(C=N) = 1633,
–
1
–1
1
2
598 cm for formylimine and acetylimine, respectively; ν(Cl=O) = 1105 cm and ν(N=C=N) =
–
1
065 cm .
Physical Measurements
Elemental analyses (C, H and N) were performed using a Perkin–Elmer 2400 series II CHN

1
analyzer. IR spectra in KBr pellets (4000–500 cm ) were recorded using a Perkin-Elmer RXI
FT-IR spectrophotometer. The electronic absorption spectra (1000–240 nm) in acetonitrile
solution were collected in a Hitachi U-3501 spectrophotometer. Temperature-dependent
molar susceptibility for powdered crystalline sample of compound 4 were measured using 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.
The susceptibility data were corrected by Pascal’s diamagnetic contributions. Isothermal
magnetization measurements were performed at 2K up to 5 Tesla magnetic fields. X-ray
powder diffractograms of complexes 24 were recorded on a Bruker D-8 Advance
diffractometer operated at 40 kV voltage and 40 mA current and calibrated with a standard
silicon sample, under Ni-filtered Cu-Kα (α = 0.15406 nm) radiation.
Electrochemical measurements
The electrochemical measurements of all four complexes (1, 2, 3 and 4) were performed
–
1
using Epsilon Basi-C3 Cell instrument at a scan rate of 100–300 mV s within the potential
range of 0 to −1.60 V vs. Ag/AgCl. Cyclic voltammograms were carried out using 0.1 M
5

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tetrabutylamonium perchlorate (TBAP) as supporting electrolyte and 1.0 × 10–3 M of
View Article Online
DOI: 10.1039/C8DT02962H
complexes in acetonotrile solution which are deoxygenated by bubbling with argon. The
2
working electrode was a glassy carbon disk (0.32 cm ) which was polished with alumina
solution, washed with absolute acetone and acetonitrile, and air dried before each
electrochemical run. The reference electrode was Ag/AgCl, with platinum as counter
electrode. All experiments were performed in standard electrochemical cells at 25°C.
ESI-HRMS
Electro spray ionization mass spectrometry (ESI-MS positive) ion mass spectra were
acquired using a Xevo G2-S QTof (Waters) mass spectrometer, equipped with a Z-spray
interface, over a mass range of 100–1200 Da, in a continuum mode. Aqueous sodium formate
was used for Q-oa-Tof calibration. L-Leucine was used as the external mass calibrant lock
+
mass [M+H] = 556.2771 Da. Solutions of compounds were injected at flow rate of 5 µl/min.
All solutions were made in mass-spectrometric grade methanol. The concentrations of
–
6
complexes 2, 3 and 4 as well as complexes + substrates in solutions were the same (10 M)
during acquisitions of mass spectra.
X–ray Crystallographic data collection and refinement
Data for the complexes 2 and 3 were collected with MoKα radiation at 150 K using the
Oxford Diffraction X-Calibur CCD System. The crystals were positioned at 50 mm from the
CCD and 321 frames were measured with counting times of 10s. Data analyses were carried
1
0
out with the CrysAlis program. Data for complexes 1 and 4 were collected with MoKα
radiation at 293(2) K using a Bruker diffractometer. Data analysis was carried out with the
Saint program.
1
1
The structures were solved using direct methods with the Shelxs97 program. The
non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms
bonded to carbon were included in geometric positions and given thermal parameters
equivalent to 1.2 times those of the atom to which they were attached. In 2, one of the
propylene moieties was disordered and the three carbon atoms were refined using two sets of
positions. One solvent NCMe molecule was refined with 50% occupancy. In 3 the hydrogen
atoms bonded to the two water molecules were located in difference Fourier maps and refined
with distance constraints. In 4, hydrogen atoms bonded to water could not be located. The
perchlorate anions were disordered and a model was refined in which three different sites
6

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were located for the chlorine atoms and in all cases the attached oxygen atoms were
DOI: 10.1039/C8DT02962H
disordered over two sites. Tetrahedral geometry for all sets of oxygen atoms was constrained
via distance constraints. Absorption corrections for 2 and 3 were carried out using the
1
2
13
ABSPACK program and for 1 and 4 using SADABS. The four structures were refined
14
2
using Shelxl2016-6 on F . Details of the crystallographic data are summarized in Table 1.
Table 1. Crystallographic parameters for complexes 14.
Complex
Chemical formula
Formula weight
Crystal system
Space group
a(Å)
1
2
3
4
18
C H
18
N
2
NiO
2
39
C H
36.5MnN6.5Ni
2
O
4
S
2
C
36
H
40ClMnN
7
Ni
2
O
10
C
72
H
76Cl
3
2 4 22
Mn N11Ni O
353.05
896.72
938.50
1898.50
monoclinic
C2/c
orthorhombic
Pbca
monoclinic
monoclinic
Cc
1
P2 /n
7.644(5)
17.654(5)
22.775(5)
(90)
17.790(4)
13.086(2)
19.171(5)
116.90(3)
3980(2)
4
25.090(2)
10.1029(6)
15.1546(9)
94.576(6)
3829.1(5)
4
21.8470(7)
13,0063(7)
30.2495(11)
101.600(3)
8419.8(6)
4
b (Å)
c (Å)
β(o)
V (Å3)
3171.3(4)
8
Z
-
3
ρ
calc(g cm )
1.526
1.273
1472
1.496
1.402
1844
1.628
1.435
1932
1.498
1.338
3896
-
1
µ (Mo Kα) (mm )
F(000)
Reflections
collected
34281
20559
11720
67197
Independent
reflections
3004
2184
9030
5774
5830
5182
8649
6584
Reflections with I
>
2σ(I)
Final
0.0377, 0.0867
0.1093, 0.2100
0.0461, 0.0880
0.0686, 0.2006
indices[I>2σ(I) ]
R
1
, wR
2
R
1],wR2[ [all data] 0.0605, 0.0950
0.1635, 0.2323
1.144
0.0547, 0.0913
1.038
0.0801, 0.2215
1.071
GOF c
1.017
Residual electron
Density, e/Å-3
0.260, -0.301
0.998, -0.5394
0.787, -0.648
1.396, -0.661
aR
b
2) = [[w(F 2
2)2w F
4 ½
c
2
2)2(Nobs– Nparams)]½
1
= ||F
o
|−|F
c
|||F
o
|, wR
2
(F
o
o
− F
c
o
] and GOF = [[w(F
o
− F
c
Catalytic oxidation of 3,5-DTBC
The catecholase activity of complexes 3 and 4 was studied using 3,5-di-tertbutylcatechol
3,5-DTBC) as the substrate in an methanolic solution under aerobic conditions at room
(
temperature. The reactions were followed spectrophotometrically by monitoring the increase
in the absorbance maxima of the quinone band at 400 nm as a function of time (time scan).
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To detect the formation of hydrogen peroxide during the catalytic reaction, we followed the
DOI: 10.1039/C8DT02962H
iodometric method as reported earlier.15
Phenoxazinone synthase of OAP
The phenoxazinone synthase activity of complexes 3 and 4 was studied using o-aminophenol
(OAP) as the substrate in a methanolic solution under aerobic conditions at room
temperature. The reactions were followed spectrophotometrically by monitoring the increase
in the absorbance maxima of the amino phenoxazinone (APX) band at 425 nm as a function
of time (time scan).
Results and discussion
Syntheses of the complexes
The mononuclear Ni(II) complex (1) has been synthesized by the reaction of [Ni(L1)
·2H O]
2
2
1
(
where HL = 2-((E)-(3-aminopropylimino)methyl)phenol) with bis(o-hydroxyacetophenone)
nickel(II) dihydrate. This complex [NiL] is used as metalloligand and on reaction with
Mn(ClO ·6H O in presence of NaN and NH SCN in 2:1:2 molar ratio yielded complexes
(NiL) Mn(NCS) ] (2) and [(NiL) Mn(N )(H O)](ClO )·H O (3), respectively. On the other
hand, complex 1 on reaction with Mn(ClO ·6H O in the presence of NaN in 2:1:1 molar
ratio yielded complex[{(NiL) Mn(H O)} )](ClO (4). Complex 4 can be converted
to 3 by reaction with excess NaN . It is noted that the ratios of reagents are same (2:1:2) for
4
)
2
2
3
4
[
2
2
2
3
2
4
2
4
)
2
2
3
2
2
2
(μ1,3-N
3
)
4 3
3
preparation of complexes 2 and 3, but their compositions are different with respect to co-
ligands. The syntheses of the complexes 14 are depicted in Scheme 1. The similarities of the
measured powdered diffractograms of complexes 24 with their respective simulated (from
mercury 3.8) diffractograms (Figures S1S3, ESI) show the phase purity of the complexes.
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DOI: 10.1039/C8DT02962H
N
N
O
N
Ni
O
N
N
N
N
MeOH
Ni
Mn
O
O
O
Mn(ClO)2·6H2O,
OH2
NaN (1:2)
3
Complex 1
O
ClO -
4
Ni
,
O
N
H 2
6
·
N
H O
) 2
2
O
)
l
Mn(ClO) ·6H O,
1
2
2
C
:
1
(
n
(
MeCN
M
N 3
NH SCN (1:2)
4
a
H
Complex 3
N
O
e
M
Excess NaN3
MeOH
N
N
N
N
N
S
Ni
O
N
O
C
Ni
Ni
O
O
O
N
N
N
N
O
Mn
Mn
N
Mn
O
O
O
C
H O
OH2
2
Ni
O
O
N
S
O
Ni
Ni
N
N
N
N
N
MeCN
ClO -
Complex 2
3
4
Complex 4
Scheme 1. Syntheses of the complexes 14.
It is worth mentioning that the mononuclear Ni(II) complex of N
dicondensed Schiff base ligand is reported here for the first time.
2
2
O donor unsymmetrically
IR spectra of complexes 1, 2, 3 and 4

1
The IR spectra of complexes 1–4 show two strong peaks at 1628 and 1597 cm for complex

1
1
1
; 1630 and 1598 cm for complex 2 (Figure S4, ESI); 1634 and 1599 cm for complex 3

1
(
Figure S5, ESI); 1633 and 1598 cm for complex 4 due to two types of azomethine v(C=N)
6
stretching of unsymmetrical Schiff base. In addition, the spectrum of complex 2 shows
strong sharp peaks at 2087 and 2057 cm 1 for SCN stretching. Whereas, the spectra of

–
6

1
complexes 3 and 4 show strong sharp peaks at 2062 and 2065 cm for stretching vibrations
of N –
ion. Along with this, the spectra of both complexes 3 and 4 show strong peaks at 1108
3

1
–
ions.7
and 1105 cm for the stretching vibrations of ClO
4
9

Dalton Transactions
Page 10 of 32
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DOI: 10.1039/C8DT02962H
Description of structures
The structure of the asymmetrical metalloligand NiL (1), is shown in Figure 1 together with
the atomic numbering scheme. Dimensions in the nickel coordination sphere are given in
Table S1, ESI. The metal atom is four-coordinate being bonded to the four donor atoms of
2
–
ligand L with a square planar geometry with bond lengths Ni-O 1.873(2), 1.888(2) Å and
Ni-N 1.920(2), 1.970(2) Å. Least squares plane calculations show the r.m.s. deviation of the
four donor atoms to be 0.061 Å. However there are differences involving the conformations
of the two six membered chelate rings. Atoms O(11), C(12), C(17), C(18), N(19) show an
r.m.s. deviation of 0.107 Å, with the metal 0.799(3)Å from the plane while atoms O(31),
C(30),C(25), C(24), N(23) show an r.m.s. of 0.022 Å with the metal 0.446(4) Å from the
o
plane. The aforementioned planes intersect at an angle of 57.4(1) . In all the complexes the
Ni-O(11) (phenoxido oxygen of o-hydroxyacetophenone) distances are slightly shorter than
the Ni-O(31) (phenoxido oxygen of salicylaldehyde) distances.
Figure 1. The structure of 1 with ellipsoids at 30% probability.
The structure of complex 2 [(NiL)
2
Mn(NCS) ] is shown in Figure 2. The structure contains
2
two NiL metalloligands in which the metals have similar square planar four-coordinate
geometries equivalent to that found in 1. The r.m.s. deviations from the four donor atoms in
the square planes are 0.000, 0.006 around Ni(1) and Ni(2) respectively. However, the metal
atoms are 0.200(5) and 0.033(4) Å from their respective planes.
As in 1, there are differences in the conformation of the two six-membered chelate
rings around the nickel atoms. Ni(1) is 0.741(8) , 0.443(8) Å and Ni(2) 0.925(10), 0.176(10)
Å from the O-C-C-C-N ring atoms while the two rings intersect at angles of 53.1(3), 50.6(4)o
1
0

Page 11 of 32
Dalton Transactions
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respectively. The two metalloligands are connected via a manganese atom which is bonded to
DOI: 10.1039/C8DT02962H
all four oxygen atoms, O(11) and O(31) bridging to Ni(1), and O(41) and O(61) bridging to
Ni(2). The six-coordinate geometry around Mn(3) is completed by N(1) and N(2) from two
thiocyanate ligands. The four Mn-O bond lengths show considerable variation with Mn(3)-
O(11) and Mn(3)-O(41) short at 2.111(5), 2.112(5) Å, while Mn(3)-O(31) and Mn(3)-O(61)
are longer at 2.383(5) and 2.269(6) Å. The two Mn-N (thiocyanate) bond lengths are 2.133(8)
and 2.121(7) Å. The geometry around Mn(3) can be considered to be distorted octahedral
o
with the main distortion being found in the O(41)-Mn(3)-O(11) angle of 145.0(2) . The two
o
square planes around Ni(1) and Ni(2) intersect at an angle of 31.6(3) . Selected bond lengths
and bond angles are given in Table S2, ESI.
Figure 2. The structure of 2 with ellipsoids at 20% probability. The solvent NCMe molecule
is not shown.
+
Complex 3 consists of a [(NiL)
2
Mn(N
3
(H
2
O)] cation, a perchlorate anion and a solvent water
molecule. The structure of the cation which is shown in Figure 3 is very similar to that found
in 2. Thus the structure contains two NiL metalloligands in which the metals contain similar
square planar four-coordinate geometry as found in 1 and 2 with r.m.s. deviations of 0.034,
0
.137Å. As in 1 and 2, there are differences in the conformation of the two six-membered
chelate rings around the nickel atoms. Ni(1) is 0.931(5), 0.174(5) Å and Ni(2) 0.813(5),
.349(5) Å from the O-C-C-C-N ring atoms while the two rings intersect at angles of 50.7(1),
0
1
1

Dalton Transactions
Page 12 of 32
o
View Article Online
4
7.3(1) . Here the two metalloligands are connected via Mn(3) in a similar fashion to that
DOI: 10.1039/C8DT02962H
found in 2 so that it is bonded to all four oxygen atoms, O(11) and O(31) bridging to Ni(1)
and O(41) and O(61) bridging to Ni(2). The six-coordinate geometry around Mn(3) is
completed by N(1) from an azide anion and a water molecule O(1W). As in 2, the four Mn-O
bond lengths show considerable variation, but here Mn(3)-O(11) and Mn(3)-O(41) are short
at 2.133(4), 2.134(4) Å, and Mn(3)-O(31) and Mn(3)-O(61) are longer at 2.243(3) and
2
.371(3) Å. As in 2, the main distortion from octahedral geometry around the Mn(3) atom is
o
the O(41)-Mn(3)-O(11) angle of 144.5(1) . The two square planes around Ni(1) and Ni(2)
o
intersect at an angle of 28.1(1) . The coordinated water molecule O(1W) forms donor
hydrogen bonds to both the solvent water molecule O(2W) and N(3) (x,1-y,-1/2+z).
o
Dimensions for O…O, H…O and O-H…O are 2.738(8), 1.90(3) Å and 168(9) and for
o
O…N, H…N and O-H…N are 2.906(6), 2.06(2) Å and 179(11) . In addition O(2W) forms a
donor hydrogen bond to a perchlorate oxygen O(83) (x, 1-y, -1/2+z) with dimensions
2
.754(9), 2.05(7) Å, 138(9)o.
+
Figure 3. The structure of the [(NiL)
2
Mn(N
3
(H O)] cation 3 with ellipsoids at 20%
2
probability. The accompanying solvent water molecule and perchlorate anion are omitted for
clarity.
1
2

Page 13 of 32
Dalton Transactions
3
+
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DOI: 10.1039/C8DT02962H
The structure of 4 contains of a [((NiL)
2
Mn(H
2
O))
2
(N
3
)] cation with crystallographic C
2
symmetry together with three perchlorate anions. The perchlorates were disordered over three
sites and in addition their oxygen atoms were also disordered over different sites. The cation
is shown in Figure 4.
+
The structure of the cation in 4 contains two [(NiL)
2
Mn(H
2
O)] moieties bridged by
an azide over a twofold axis. The individual moieties have structures very similar to those
found in 2 and 3 in that the two metalloligands contain nickel atoms in square planar four-
coordinate geometry as found in 1, 2 and 3 with the donor atoms showing r.m.s. deviations of
0
.062, 0.135 Å. As in the previous structures, there are differences in the conformation of the
two six-membered chelate rings around the nickel atoms. Ni(1) is 0.940(5), 0.286(5) Å and
Ni(2) 0.817(1), 0.083(6) Å from the O-C-C-C-N ring atoms while the two rings intersect at
o
angles of 56.3(2), 42.6(2) . Here the two metalloligands are connected via Mn(3) in a similar
fashion to that found in 2 and 3. The six-coordinate geometry distorted octahedral
environment around Mn(3) consists of the four oxygen atoms that bridge to the two nickel
atoms together with a nitrogen atom from the bridging azide anion and a water molecule
O(1W). As in 2 and 3, the four Mn-O bond lengths show considerable variation but here
Mn(3)-O(11) and Mn(3)-O(41) are short at 2.118(4), 2.105(4) Å, and Mn(3)-O(31) and
Mn(3)-O(61) are longer at 2.250(3) and 2.263(4) Å. It will be noted that this pattern of bond
lengths is a common feature of all structures and is a consequence of the presence of the
additional methyl group in the asymmetrical metalloligand. Also as in 2 and 3, the main
distortion from octahedral geometry around the Mn(3) atom is the O(41)-Mn(3)-O(11) angle
o
o
of 146.5(1) . The two square planes around Ni(1) and Ni(2) intersect at an angle of 35.5(2) .
While the hydrogen atoms on the coordinated water molecule were not located, it is clear that
hydrogen bonds are formed with perchlorate oxygen atoms as there are close contacts less
than 3.0 Å with oxygen atoms from all three perchlorates.
1
3

Dalton Transactions
Page 14 of 32
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DOI: 10.1039/C8DT02962H
Figure 4 The structure of the cation in 4 which contains crystallographic C symmetry.
2
Hydrogen atoms on the water molecule O(1W) were not located. The perchlorate anions are
omitted for clarity. Ellipsoids are drawn at 20% probability
The structures of complexes 2 and 3 are similar with our earlier reported trinuclear


6
Cu(II)Mn(II) complexes with SCN and N
3
ions respectively. The hexanuclear structure of
complex 4, in which two trinuclear (NiL)
2
Mn units are symmetrically connected by one 1,3-

bridging N
3
ion is unprecedented. We have recently reported a hexanuclear Ni(II)Mn(II)
complex containing an unsymmetrical ligand with an N
2
O
3
donor set, in which one Mn(II)
7
ion is hexa-coordinated and other Mn(II) ion is penta-coordinated. Here, in complex 4 the
coordination geometry of the two Mn(II) ions are same and both are hexa-coordinated.
Magnetic Properties
The temperature dependent molar magnetic susceptibility measurements were carried out
only for complex 4 because complex 1 has a mononuclear structure with a diamagnetic
square planar Ni(II) atom and in both complexes 2 and 3, the paramagnetic Mn(II) centres are
connected to the diamagnetic square planar Ni(II) atoms via phenoxido bridges. The thermal
variation of molar magnetic susceptibility product (χ
M
T) of complex 4 with temperature is
3
-1
depicted in Figure 5 left. The room temperature χ T value of complex 4 is 8.70 cm K mol ,
M
1
4

Page 15 of 32
Dalton Transactions
which is very close to the value of magnetically non-interacting MnII
system of 8.75 cm K
Vie3w Article Online
2
DOI: 10.1039/C8DT02962H
-
1
mol for gMn = 2 at 300 K. The χ
further cooling the χ T value sharply decreases and reaches to 1.51 cm K mol at 2 K. The
gradual decreasing nature of χ T value with decrease in temperature indicates the existence
of intramolecular antiferromagnetic interaction between the metal centres. The corresponding
thermal variation of molar magnetic susceptibility (χ ) of complex 4 is shown in Figure S6,
ESI. The molar magnetizations value (M/N) of complex 4 up to 5 Tesla magnetic fields is
.39 μ (Figure S7, ESI) at 2 K. This nature of the magnetization plot indicates the presence
M
T value slowly decreases upon cooling down to 50 K, on
3
-1
M
M
M
2
B
of antiferromagnetic coupling between structurally equivalent Mn(II)Mn(II) ions in
complex 4.
In complex 4, the four Ni(II) ions are diamagnetic (S = 0) with square planar geometry,
therefore the exchange interactions between Ni(II) (S = 0) and Mn(II) (S = 5/2) ions are
considered as zero. For calculation of magnetic interaction parameters of complex 4, we have
assumed only one exchange interaction between two 1,3-azido bridged Mn(II) (S = 5/2) ions
considering exchange coupling as J (Figure 5 right). The PHI program is used for simulation
16
of experimental magnetic data taking account the molecular field approximations. The best
–
1
–1
fits leads to the following set of parameters: J(MnMn) = –3.73 cm , gMn = 2.0, cm , R =
−
5
1
.25×10 for complex 4.
Only a few 1,3-azido bridged Mn(II) complexes, reported in the literature, have
been characterized both structurally and magnetically.17 In these complexes,
antiferromagnetic coupling is observed and the magnitude of this antiferromagnetic exchange
interaction between Mn(II) ions (JMn–Mn) is found to be dependent on the number of 1,3
-
azido bridges; JMn–Mn increases with increasing the number of 1,3-azido bridges. In the case
of double 1,3-azido bridged Mn(II) complexes, the value of antiferromagnetic coupling is in
−
1 17
the range of 6.0 to 15.4 cm . The change in J values is correlated with the dihedral angle
), defined by the angle between two Mn−N−N−N mean planes and Mn−N distances.
(
Higher values of  and shorter Mn−N distances facilitate stronger antiferromagnetic
interaction between Mn(II) ions. On the contrary, there are only three single 1,3-azido
bridged Mn(II) complexes where magnetic data are fitted and their J values are observed to
−
1
be −2.29, −3.60 and –3.94 cm . The J values of these three complexes may be related to
their corresponding average Mn−N−N angles (), 128.99, 141.18 and 149.13 respectively,
and Mn−N distances, 2.17, 2.209 and 2.095 Å.7,17 In complex 4, the value of magnetic
1
5

Dalton Transactions
Page 16 of 32
exchange coupling (JMn–Mn) is –3.73 cm–1 which is in good agreement with those of the
View Article Online
DOI: 10.1039/C8DT02962H
reported complexes considering its  angle is 145.52 and Mn−N distance is 2.134 Å.
Figure 5. Thermal variations of the χ
represents the best fitting to the appropriate model for 4. Basic dinuclear unit of complex 4
right).
M
T products for complex 4 (left). Solid red line
(
Electrochemistry
The cyclic voltammograms (CVs) of acetonitrile solution of all the complexes 14 are
recorded at a scan rate of 100 mV/Sec with respect to an Ag/AgCl electrode. On cathodic
scan (0 to –1.6 V), one wave is observed in the cyclic voltammograms of complex 1 (Figure
6
) for the quasi-reversible reduction process at the electrode surface. The peak is observed at
= –0.89 V on forward scan and on reverse scan the corresponding peak is observed at E =
E
c
a
1
8
–
0.77 V with the peak separation value (ΔEpc = Ecathodic – Eanodic) of –0.12 V for complex 1.
This redox behaviour in cyclic voltammogram of complex 1 can be assigned as Ni(II/I)
1
9
couple. All the complexes 2, 3 and 4 show two peaks at E
.61, –0.91 V for 3 and E = –0.49, –0.89 V for 4 on forward cathodic scan, whereas only one
peak is observed at E = –0.75, E = –0.74 and –0.77 V for complexes 24 respectively, on
c
= –0.53, –0.90 V for 2, E = –
c
0
c
a
a
the reverse scan (Figure 6). Here the peak separation values (ΔEpc) are –0.15, –0.17 and –0.12
V for complexes 24 respectively. The appearance of two peaks on forward scan in these
complexes suggests the presence of two types of (NiL) species (free metalloligand and
coordinated to Mn(II)) due to the partial dissociation of complexes 24 in solution. The peak
1
6

Page 17 of 32
Dalton Transactions
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around –0.9 V on the forward scan and at –0.75 V on reverse scan is due to quasi-reversible
DOI: 10.1039/C8DT02962H
Ni(II/I) couple originating from the free metalloligand.
Figure 6. Cyclic voltammograms (CVs) of complexes 14 in acetonitrile at room
temperature at a scan rate of 100 mV/Sec, using TBAP as supporting electrolyte.
Catechol oxidase studies and kinetics
Detailed Description
Among the complexes, only 3 and 4 show catecholase-like activity. The detailed kinetic
studies of this catechol oxidase activity were carried out by following the earlier reported
6
procedure. The catalytic oxidation reaction of 3,5-di-tert-butylcatechol (3,5-DTBC) to its
consequent o-quinone (3,5-DTBQ) is depicted in Scheme 2. The kinetic studies of
catecholase-like activity were performed using the methanolic solution of complexes 3 and 4
under aerobic conditions. We also checked the catalytic activity of complexes 1 and 2, but
they are catalytically silent (Figure S8–S9, ESI).
1
7

Dalton Transactions
Page 18 of 32
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DOI: 10.1039/C8DT02962H
Scheme 2. Catalytic oxidation of 3,5-DTBC to 3,5-DTBQ in methanol.
The detailed kinetic study of catechol oxidase reaction was performed using 5 × 10−5
M
−
5
methanolic solution (considering trinuclear unit) of complex 3 and 2.5 × 10 M methanolic
−
2
solution (considering hexanuclear unit) of complex 4. A 1 × 10 M methanolic solution of
,5-DTBC was used as substrate solution for kinetic study. The change in spectral behaviour
3
of complex 3 during wavelength scan in presence of 3,5-DTBC is shown in Figure 7 (left)
and of complex 4 is shown in Figure S10 (left), ESI. Both the experimental rate vs.
[
substrate] and the Lineweaver–Burk plots for complex 3 are depicted in Figure 7 (right) and
−
1
for complex 4 in Figure S10 (right), ESI. The turnover numbers (Kcat in h ) of complexes 3
and 4 for this catechol oxidase reaction are 984 and 2081, respectively. The kinetic study
parameters of catecholase-like activity are listed in Table S3, ESI.
Figure 7. Increase in the 3,5-DTBQ band at around 400 nm after the addition of 3,5-DTBC
−
2
−5
(
1 × 10 M) to the methanolic solution of complex 3 (5 × 10 M) (left) and plot of the rate
vs. substrate concentration (right). Inset shows the corresponding Lineweaver–Burk plot of 3.
The UV-spectra were recorded at 5 m intervals.
1
8

Page 19 of 32
Dalton Transactions
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DOI: 10.1039/C8DT02962H
ESI-Mass spectrometric study
To determine the mechanism of catecholase-like activity and to identify the complex-
substrate intermediate during the catalytic oxidation reaction of catechol, we have recorded
ESI-MS spectra of complexes 23 and of 1:1 methanolic mixtures (v/v) of the complexes and
3
,5-DTBC within 5 min of mixing (Figures S11–S12, ESI). In mass spectra of complexes
2
3, a base peak is observed at m/z = 353.09 (calcd. 353.08), for protonated metalloligand
+
[
(NiL)+H] . Two other prominent peaks are observed at m/z = 375.08 (calcd. 375.06) and m/z
+
=
391.06 (calcd. 391.04) due to the sodiated metalloligand species, [(NiL)+Na] and
+
potassiated metalloligand species, [(NiL)+K] respectively. Along with these, two small
peaks are observed at m/z = 705.18 (calcd. 705.15) and m/z = 727.16 (calcd. 727.13) for
+
protonated dinuclear metalloligand species [(NiL)
2
+H] and sodiated dinuclear metalloligand
+
species [(NiL)
2
Na] respectively (Figures S11–S12, ESI).
After the addition of 3,5-DTBC to the solution of complexes 23, the spectra of the
mixtures of all the complexes show three peaks at m/z = 353.09 (calcd. 353.08), m/z = 375.08
(calcd. 375.06) and m/z = 727.16 (calcd. 727.13) (Figures S13–S15, ESI). Here, in each case
m/z = 353.09 (calcd. 353.08) is the base peak. Along with these, one new peak appears in the
mixtures of complex 3 at m/z = 981.27 (calcd. 981.24), which can be assigned to
II
I
+
[
(Ni L)(Ni L)Mn(3,5-DTBC)] species (Figures S13–S15, ESI). This peak is very important
from the mechanistic point of view as it clearly indicates the formation of a complex-
substrate intermediate. In addition, another peak appears at m/z = 855.17 (calcd. 855.16),
II
I
]+ species (Figures S14–S15, ESI). The
which is assignable to a [(Ni L)(Ni L)Mn(MeOH)
3
presence of another important peak at m/z = 1025.22 (calcd. 1025.21) also demonstrates the
II
I
+
semi-quinonate form of this intermediate [(Ni L)(Ni L)Mn(3,5-DTBSQ)+2Na] in solution
mixture. In all these trinuclear species, one Ni(II) centre is reduced to Ni(I). In addition, a
+
peak is observed at m/z = 243.14 (calcd. 243.13) for [3,5-DTBQ+Na] species, due to the
oxidation of 3,5-DTBC during the reaction (Figures S14–S15, ESI). In summary, the
II
I
+
II
I
]+ and
appearance of the peaks for [(Ni L)(Ni L)Mn(3,5-DTBC)] , [(Ni L)(Ni L)Mn(MeOH)
3
II
I
+
[
(Ni L)(Ni L)Mn(3,5-DTBSQ)+2Na] species in the spectra of the complex substrate
mixtures, suggest the formation of probable intermediate species during the oxidative
conversion of 3,5-DTBC to 3,5-DTBQ by aerial di-oxygen in the presence of complex 3
acting as a catalyst. The spectra of complex 2 and catechol mixture do not show the peaks for
such types of intermediate species.
1
9

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DOI: 10.1039/C8DT02962H
Mechanistic insight
Different types of mechanisms are reported in the literature for the catalytic conversion of
catechol to their respective quinine for the homometallic copper(II), nickel(II), cobalt(II/III),
manganese(II/III) and zinc(II) complexes.4 Recently, we have shown that some
heterometallic Cu(II)Mn(II) complexes can act as catalysts for this reaction with very high
6
catalytic efficiency. In this case, the mechanistic study reveals that the catalysis process
undergoes through radical mechanism with the formation of semi-quinonate intermediate
with the reduction of Cu(II) to Cu(I). During this process H
We have also shown that some Ni(II)Mn(II) complexes with a symmetrical ligand
containing a N donor set can act as catalyst for this reaction, but their catalytic efficiency
was not very high and the mechanism was not explored. Here, we would like to explore the
mechanism of Ni(II)Mn(II) complexes with an unsymmetrical ligand containing a of N
donor set. It is observed that during the catalytic process with each of 3 and 4, H
produced (Figure S16, ESI). The quantitative estimations show that about 84.5 and 92.4 % of
were formed for complexes 3 and 4, respectively, after 30 min of oxidation reaction
2
O
2
is produced as a by-product.
2
O
2
7
a
2
O
2
2
2
O is
H
2
O
2
with respect to the catalytic conversion of 3,5-DTBC to 3,5-DTBQ (Table S4, ESI).
On the basis of ESI-Mass spectra, we propose here a possible mechanism for the
catalytic oxidation reaction (Scheme 3). At first, a doubly charged trinuclear species
2
+
[
(NiL)
doubly charged species to generate an intermediate species [(Ni L)
which one hydroxyl group of 3,5- DTBC is deprotonated and coordinated to the Mn(II)
2
Mn] is formed in the solution of complexes 3 or 4. Then a 3,5-DTBC binds to this
II
+
2
Mn(3,5-HDTBC)] in
centre. This species then readily reacts with dissolved O
2
by releasing another proton of the
II
hydroxyl group and is thus transformed to its radical semiquinonate form [(Ni L)
2
Mn(3,5-
+
DTBSQ)] . During this step, O
2
is reduced to H
2
2
O . Then the semiquinonate redical transfers
II
I
+
its electron in the catalyst to form [(Ni L)(Ni L)Mn(3,5-DTBSQ)] as is evident from mass
spectra (Figures S14–S15, ESI), and itself is oxidised to quinine. After that the quinone is
eliminated from the metal center, leaving a singly charged trinuclear species
II
I
II +
[
(Ni L)(Ni L)Mn ] in which one Ni(II) centre has been reduced to Ni(I). Then this
II
I
II +
trinuclear [(Ni L)(Ni L)Mn ] species coordinates to another 3,5-H
2
DTBC and forms the
DTBC)] species. Finally, the Ni(I) centre of this trinuclear species
is further oxidized to Ni(II) by aerial O , and regenerates the active trinuclear intermediate
Mn(3,5-DTBC)] species, with the formation of H as the by-product. This
II
I
+
[
(Ni L)(Ni L)Mn(3,5-H
2
2
II
+
[
(Ni L)
2
2
O
2
2
0

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DOI: 10.1039/C8DT02962H
possible mechanism for catalytic oxidation reaction of 3,5-H DTBC to 3,5-DTBQ by
2
complexes 34 is shown in Scheme 3.
tBu
HO
HO
+
tBu
N
H+
-
tBu
N
Ni
O
O
O
O
O
Mn
(
H O+N )
2 3
tBu
Complex 3 or 4
HO
Ni
N
H+
N
-
N
N
Ni
tBu
O
/2O +H+
2
1
O
1/2 H2O2
N
O
N
tBu
Mn
Ni
O
O
O
tBu
O
O
O
O
Ni
Mn
N
tBu
N
O.
O
1
/2 H O
-2H+
2
2
Ni
N
Ni(II)
N
Ni(I)
Ni(II)
Ni(I)
+
1
/2O +H
2
N
N
tBu
Ni
O
O
H
N
tBu
O
N
O
O
Ni
Mn
I)
O
.
HO
tBu
O
O
O
(
Mn
Ni
N
O
tBu
N
(I)
Ni
O
N
N
m/z(+) = 981.27
N
N
Ni
O
O
m/z(+2Na) = 1025.22
O
tBu
tBu
HO
Mn
I)
O
+
O
O
(
HO
Ni
tBu
N
tBu
N
m/z(+Solvent) = 855.17
Scheme 3. Proposed mechanism for catalytic oxidation of 3,5-DTBC to 3,5-DTBQ by
complexes 34.
2
1

Dalton Transactions
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DOI: 10.1039/C8DT02962H
Phenoxazinone synthase studies and kinetics
Detailed Description
To investigate the catalytic oxidase activities of heterometallic complexes 34, upon another
substrate, we have studied phenoxazinone synthase-like activity. The detailed kinetic studies
of this phenoxazinone synthase-like activity were carried out by following the previously
reported method.6 The oxidative dimerization of o-aminophenol (OAP) to amino-
phenoxazinone (APX) in presence of catalyst is shown in Scheme 4. The kinetic studies of
this catalytic activity were also performed using methanolic solutions of complexes 34
under aerobic conditions. We have also checked the catalytic activity of complex 1 and 2, but
they were catalytically inactive.
Scheme 4. Catalytic oxidation of o-aminophenol (OAP) to amino phenoxazinone (APX) in
methanol.
The detailed kinetic study of the oxidative dimerization reaction was carried out by following
the above mentioned procedure as for catecholase-like activity, using the same concentration
of complexes 14 and o-aminophenol in methanol solvent. Here also complexes 1 and 2 do
not show the catalytic activity (Figure S17–S18, ESI). The change in spectral behaviour of
complex 3 during the wavelength scan in presence of OAP is depicted in Figure 8 (left) and
of 4 is shown in Figure S19 (left), ESI. Both the plots of observed rate vs. [substrate] and of
Lineweaver–Burk for complex 3 are depicted in Figure 8 (right) and for 4 in Figure S19
−
1
(
right), ESI. The turnover numbers (Kcat in h ) of this catalytic reaction are 6351 and 10545
for complexes 34 respectively. The kinetic study parameters are listed in Table S5, ESI. The
II
II
k
cat value of complex 3 is comparable with those of our previously reported Cu Mn
complexes, and for 4 the value is comparatively higher than other reported homo- or hetero-
metallic complexes.
2
2

Page 23 of 32
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DOI: 10.1039/C8DT02962H
Figure 8. Increase in the amino phenoxazinone band at around 425 nm after the addition of
−
2
−5
o-aminophenol (1 × 10 M) to the methanolic solution of complex 3 (5 × 10 M) (left) and
plot of the rate vs. substrate concentration (right). Inset shows the corresponding Lineweaver–
Burk plot of 3. The UV-spectra were recorded at 5 m intervals.
ESI-Mass spectrometric study
To determine the mechanism of phenoxazinone synthase-like activity and complex-substrate
intermediate during the oxidative reaction of o-aminophenol to phenoxazinone, we have
recorded ESI-MS spectra of complexes 23 in a 1:1 mixture (v/v) of the methanolic solution
of complexes and o-aminophenol within 5 min of mixing (Figures S20–S22, ESI). In the
spectra of the mixture of complex 3 and o-aminophenol a base peak is observed at m/z =
3
3
53.09 (calcd. 353.08) and other three common peaks are observed at m/z = 375.08 (calcd.
75.06), m/z = 705.18 (calcd. 705.15) and m/z = 727.16 (calcd. 727.13) (Figures S21–S22,
ESI). All these four peaks are shown earlier in the spectra of complexes 23. In addition, one
important peak of low intensity are observed at m/z = 1014.14 (calcd. 1014.16), which can be
II
+
assignable to the trinuclear species [(Ni L)
with these, two low intensity peaks appeared at m/z = 859.14 (calcd. 859.16) and m/z =
042.13 (calcd. 1042.12) (Figures S21–S22, ESI), which are possibly due to the presence of
2
Mn(OAP)
2
+K] (Figures S21–S22, ESI). Along
1
II
I
+
I
+
the trinuclear species [(Ni L)(Ni L)Mn(CH
3
OH)
2
(H
2
O)
2
] and [(Ni L)
2
Mn(OAP)
2
+3Na] in
+K]+,
II
the solution mixture. In summary, the peaks of intermediate species [(Ni L)
2
Mn(OAP)
2
I
+
and [(Ni L)
2
Mn(OAP) +3Na] suggest the formation of complex-substrate intermediates in
2
solution during the catalytically oxidative conversion of o-aminophenol to phenoxazinone by
complexes 34. The spectra of complex 2 and the o-aminophenol mixture do not show the
2
3

Dalton Transactions
Page 24 of 32
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peaks for such types of complex-substrate intermediate species. We have also checked that no
DOI: 10.1039/C8DT02962H
H
2
2
O was formed during the oxidative dimerization reaction of o-aminophenol.
Mechanistic insight
The mechanism of phenoxazinone synthase-like activity of some homometallic copper(II),
cobalt(II/III), manganese(II/III) and iron(II/III) complexes has been reported in the literature,
4
showing o-aminophenol to be dimerized to phenoxazinone. Recently we have reported some
6
heterometallic Cu(II)Mn(II) complexes, which show this catalytic oxidase activity. The
mechanistic determination of this catalysis using these Cu(II)Mn(II) complexes shows that
the catalysis occurred via radical mechanism where the o-aminophenolate intermediate was
formed with the reduction of Cu(II) to Cu(I). During this catalytic reaction, aerial oxygen is
reduced to H O via the formation of amino-phenoxazinone. It was also found that the
2
participation of two metal ions is very important in this dimerization reaction of o-
aminophenol to amino-phenoxazinone.
On the basis of ESI-Mass spectra, we propose here a possible mechanism for this
catalytic dimerization reaction. At first, a trinuclear neutral species [(NiL)
2
Mn(OAP) ] is
2
formed during the mixing of o-aminophenol to the methanolic solution of complexes 34, in
which two hydroxyl groups are deprotonated and coordinated to the Mn(II) centre (Scheme
5
). After that, two deprotonated o-aminophenols release protons and form o-
iminophenolate/o-aminophenolate radicals. Then the radicals transfer their electron to the
catalyst and reduce the Ni(II) centers to Ni(I). In the next step, the two radicals dimerize to
form amino-phenoxazinone (APX) as product which is then eliminated from the catalyst
II
I
+
leaving the trinuclear [(Ni L)(Ni L)Mn] species. During this oxidative conversion process
is reduced to 2H O. This trinuclear species further coordinates to two o-aminophenol by
protons and forms the complex-substrate intermediate
] species. The Ni(I) center of this species is further oxidised to Ni(II)
O molecule and enters into the next cycle of catalysis. The
O
2
2
releasing
two
II
I
[
(Ni L)(Ni L)Mn(OAP)
2
with the formation of another H
2
existence and relative stability of intermediate species are assignable in the mass spectra of
complex 3 and o-aminophenol (Figures S21–S22, ESI). During this total oxidative
dimerization process, 3H
2
O molecules are produced as by-product by the reduction of 3/
2
2
O .
2
4

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The probable mechanism for this catalytically conversion reaction by complexes 34 is
DOI: 10.1039/C8DT02962H
shown in Scheme 5.
HO
+
2
Complex 3 or 4
H N
2
-
2 H+
(
N + H O)
3 2
H O
2
1/2 O2
N
N
Ni
O
O
N
N
Ni(I)
Ni(II)
O
O
O
O
NH2
Mn
Ni
O
(
I)
O
O
O
O
NH2
Mn
Ni
N
H N
2
N
O
Ni
+
N
m/z(+K ) = 1014.14
H N
2
N
Ni(II)
+
m/z(+3Na ) = 1037.15
Ni(I)
HO
+
2
H N
2
N
N
N
N
Ni
I)
Ni
(I)
O
O
O
(
O
O
O
O
O
NH
.
Mn
Mn
.
2
H O
2
O2
O
(
I)
Ni
O
Ni
N
N
HN
N
N
N
O
NH2
+
m/z(+3Na ) = 1042.13
m/z(+Solvent) = 859.14
O
Scheme 5. Probable mechanism for catalytic oxidation of OAP to APX by complexes 34.
Structure-Activity correlation for the observed relative performance of the catalysts
The use of heterometallic complexes with combinations of various metal ions in homo- or
heterogeneous catalysis is one of the interesting topics of current research.20 A few
heterometallic catalysts are reported to show homogeneous catalytic oxidase activities under
2
5

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Page 26 of 32
aerobic condition.6,7 Previously, we had reported some Cu(II)Mn(II) complexes of this
View Article Online
DOI: 10.1039/C8DT02962H
unymmetrically dicondensed tetradentate Schiff base ligand. Among these complexes, one
showed both catecholase and phenoxazinone synthase-like activities and another showed only
phenoxazinone synthase-like activity, both with considerably high catalytic efficiencies.6,7
The catalytic efficiencies as well as catalytic promiscuity of these complexes are found to be
dependent on the coordination environment around the Mn(II) centre. The presence of
coordinated solvent molecules at the Mn(II) atom or penta-coordination geometry of Mn(II)
makes the compound catalytically active while hexa-coordination around Mn(II) without any
6
solvent molecule reduces their oxidase activities significantly. Recently, we reported some
Ni(II)Mn(II) complexes of a N
2
3
O donor unsymmetrical ligand and drew the same
7
b
conclusions. The presence of a solvent molecule or coordinative unsaturation around Mn(II)
seems to facilitate binding of substrate to the metal centre and so is an important requirement
for a compound to be catalytically active. Among the three Ni(II)Mn(II) complexes

reported here, in 2 the Mn(II) centre is hexa-coordinated with two coordinating SCN ions
and has been found to be catalytically inactive. On the other hand, in complexes 3 and 4, the

Mn(II) centre is hexa-coordinated with a solvent water molecule and a N
3
ion and as
expected these complexes are proved to be very efficient catalysts for catecholase as well as
phenoxazinone synthase-like reactions. It is to be mentioned here that we have reported quite
a few heterometallic Cu(II)Mn(II) and Ni(II)Mn(II) complexes derived salen type Schiff
base ligands.6,7,21 Among these, in the complexes derived from symmetrically dicondensed
ligands, the anionic coligands coordinate to the Mn(II) centres to complete its hexa-
coordination while in several complexes derived from unsymmetrical ligands, a solvent
molecule is coordinated to the Mn(II) centre. Hence, unsymmetrically dicondensed salen type
ligands are much more potent for the synthesis of highly efficient heterometallic catalysts for
aerobic oxidase type reactions compared to their symmetric analogues.
Conclusions
To investigate the formation, structural aspects and catalytic oxidase activities of
heterometallic Ni(II)Mn(II) complexes of N
prepared a new mononuclear complex, [NiL] (1) with N-α-methylsalicylidene-N΄-
salicylidene-1,3-propanediamine. Complex 1 was then reacted with Mn(ClO ·6H O in
in 2:1:2 molar ratios to produce trinuclear complexes,
2
O
2
donor unsymmetrical ligand, first we
4
)
2
2


presence of SCN and N
3
2
6

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DOI: 10.1039/C8DT02962H
[
(NiL)
proportion of N
{(NiL) Mn(H
2
Mn(NCS)
2
] (2), [(NiL)
2
Mn(N
3
)(H
2
O)](ClO
4
)·H O (3), respectively. At the lower
2

3
ion, the same reactants (2:1:1) yielded a hexanuclear complex,
)](ClO (4) in which two trinuclear [(NiL) Mn(H O)] units are
[
2
2
O)}
2
(μ1,3-N
3
4
)
3
2
2
linked via a μ1,3-azido bridge. The trinuclear units are bent in all three complexes. However,
there is a conspicuous difference in the coordination environment around Mn(II) ion: in 2,


two SCN ions coordinated to the Mn(II) whereas in 3 and 4 one water molecule and one N
3
ion coordinated to the Mn(II) atom. This difference has a drastic effect on their catalytic
oxidase activities as 2 is found to be catalytically inactive but catalytic efficiencies of 3 and 4
are very high, demonstrating that the presence of a solvent molecule is one of the key factors
for such molecules to be catalytically active. The dc magnetic measurement of complex 4
reveals that the two Mn(II) atoms are antiferromagnetically coupled and exchange coupling
constant agrees well with the reported single 1,3-azido bridged Mn(II) complexes.
Supporting Information (see footnote on the first page of this article): IR spectra, Mass
spectra, powder diffractograms and table of bond parameters of complexes 14 as well as
Mass spectra of complexes 23 with DTBC and OAP; Catalytic oxidase activity parameters
and Kcat values of complexes 24. CCDC 1857025 (for 1), 1857026 (for 2), 1857027 (for 3)
and 1857028 (for 4) contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
A. G. thanks the University Grants Commission (UGC), New Delhi for funding the CAS-V,
Department of Chemistry, University of Calcutta. We also thank CRNN, University of
Calcutta, Kolkata, India for the facilities of magnetic measurements. P. M. is thankful to the
Council of Scientific and Industrial Research (CSIR), Government of India for the Senior
Research Fellowship [Sanction no. 09/028(0935)/2014-EMR-I]. We thank the EPSRC (U.K.)
and the University of Reading for funds for the diffractometer.
2
7

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DOI: 10.1039/C8DT02962H
References
1
(a) K. S. Pedersen, J. Bendix and R. Clérac, Chem. Commun., 2014, 50, 4396–4415; (b) K.
Liu, W. Shi and P. Cheng, Coord. Chem. Rev., 2015, 289−290, 74−122; (c) R. Gheorghe, M.
Andruh, J.-P. Costes and B. Donnadieu, Chem. Commun., 2003, 2778–2779; (d) M. C. Das,
S. Xiang, Z. Zhang and B. Chen, Angew. Chem., Int. Ed., 2011, 50, 10510−10520; (e) M.
Andruh, Chem. Commun., 2011, 47, 3025−3042; (f) S. Sakamoto, T. Fujinami, K. Nishi, N.
Matsumoto, N. Mochida, T. Ishida, Y. Sunatsuki and N. Re, Inorg. Chem., 2013, 52,
7
218−7229; (g) M.-G. Alexandru, D. Visinescu, M. Andruh, N. Marino, D. Armentano, J.
Cano, F. Lloret and M. Julve, Chem. –Eur. J., 2015, 21, 5429 –5446; (h) D. Visinescu, A. M.
Madalan, M. Andruh, C. Duhayon, J.-P. Sutter, L. Ungur, W. V. den Heuvel and L. F.
Chibotaru, Chem. –Eur. J., 2009, 15, 11808–11814; (i) W.-K. Dong, J.-C. Ma, L.-C. Zhu, Y.-
X. Sun, S. F. Akogun and Y. Zhang, Cryst. Growth Des., 2016, 16, 6903−6914.
2
4
(a) S. Handa, V. Gnanadesikan, S. Matsunaga, M. Shibasaki, J. Am. Chem. Soc., 2010, 132,
925−4934; (b) M. Andruh, Chem. Commun., 2018, 54, 3559−3577; (c) T. Ueno, T.
Fujinami, N. Matsumoto, M. Furusawa, R. Irie, N. Re, T. Kanetomo, T. Ishida and Y.
Sunatsuki, Inorg. Chem., 2017, 56, 1679−1695; (d) M. Andruh, Chem. Commun., 2007,
2
565–2577; (e) T. Kajiwara, M. Nakano, K. Takahashi, S. Takaishi and M. Yamashita,
Chem. –Eur. J., 2011, 17, 196–205; (f) S. Osa, T. Kido, N. Matsumoto, N. Re, A. Pochaba
and J. Mrozinski, J. Am. Chem. Soc., 2004, 126, 420–421; (g) Y. Ida, S. Ghosh, A. Ghosh, H.
Nojiri and T. Ishida, Inorg. Chem., 2015, 54, 9543−9555; (h) W.-K. Dong, J.-C. Ma, L.-C.
Zhu and Y. Zhang, New J. Chem., 2016, 40, 6998−7010.
3
(a) T. J. Boyle, J. M. Sears, J. A. Greathouse, D. Perales, R. Cramer, O. Staples, A. L.
Rheingold, E. N. Coker, T. M. Roper and R. A. Kemp, Inorg. Chem., 2018, 57, 2402–2415;
b) M. Towatari, K. Nishi, T. Fujinami, N. Matsumoto, Y. Sunatsuki, M. Kojima, N.
(
Mochida, T. Ishida, N. Re and J. Mrozinski, Inorg. Chem., 2013, 52, 6160−6178; (c) S. Roy,
A. Bhattacharyya, S. Purkait, A. Bauzá, A. Frontera and S. Chattopadhyay, Dalton Trans.,
2
016, 45, 15048–15059; (d) J.-P. Costes, G. Novitchi, S. Shova, F. Dahan, B. Donnadieu and
J.-P. Tuchagues, Inorg. Chem., 2004, 43, 7792–7799; (e) Y.-Y. Ge, G.-B. Li, H.-C.
Fang, X.-L. Zhan, Z.-G. Gu, J.-H. Chen, F. Sun, Y.-P. Cai and P. K. Thallapally, Cryst.
Growth Des., 2010, 10, 4987–4994; (f) A. Finelli, N. Hérault, A. Crochet and K. M. Fromm,
Cryst. Growth Des., 2018, 18, 1215–1226; (g) R. Gheorghe, P. Cucos, M. Andruh, J.-P.
2
8

Page 29 of 32
Dalton Transactions
View Article Online
Costes, B. Donnadieu and S. Shova, Chem. –Eur. J., 2006, 12, 187–203; (h) S. Ghosh, Y.
DOI: 10.1039/C8DT02962H
Ida, T. Ishida and A. Ghosh, Cryst. Growth Des., 2014, 14, 2588−2598; (i) S.-S. Zheng, W.-
K. Dong, Y. Zhang, L. Chen and Y.-J. Ding, New J. Chem., 2017, 41, 4966–4973.
4
(a) S. K. Dey and A. Mukherjee, Coord. Chem. Rev., 2016, 310, 80–115; (b) A. Hazari, L.
K. Das, R. M. Kadam, A. Bauzá, A. Frontera and A. Ghosh, Dalton Trans., 2015, 44, 3862–
876; (c) Y. Thio, X. Yang and J. J. Vittal, Dalton Trans., 2014, 43, 3545–3556; (d) B.
3
Chakraborty, S. Bhunya, A. Paul and T. K. Paine, Inorg. Chem., 2014, 53, 4899−4912; (e) L.
I. Simándi, T. M. Simándi, Z. May and G. Besenyei, Coord. Chem. Rev., 2003, 245, 85–93;
(f) J. Adhikary, A. Chakraborty, S. Dasgupta, S. K. Chattopadhyay, R. Kruszynski, A.
Trzesowska-Kruszynska, S. Stepanović, M. Gruden-Pavlović, M. Swart and D. Das, Dalton
Trans., 2016, 45, 12409–12422; (g) I. A. Koval, P. Gamez, C. Belle, K. Selmeczi and J.
Reedijk, Chem. Soc. Rev., 2006, 35, 814−840; (h) A. Panja, M. Shyamal, A. Saha and T. K.
Mandal, Dalton Trans., 2014, 43, 5443–545; (i) L.-H. Li, W.-K. Dong, Y. Zhang, S. F.
Akogun and Li Xu, Appl Organometal Chem., 2017, 31, e3818.
5
(a) C. Citek, S. Herres-Pawlis and T. D. P. Stack, Acc. Chem. Res., 2015, 48, 2424−2433;
(b) E. I. Solomon, R. Sarangi, J. S. Woertink, A. J. Augustine, J. Yoon and S. Ghosh, Acc.
Chem. Res., 2007, 40, 581 − 591; (c) C. Mukherjee, T. Weyhermueller, E. Bothe, E.
Rentschler and P. Chaudhuri, Inorg. Chem., 2007, 46, 9895−9905; (d) K. S. Banu, T.
Chattopadhyay, A. Banerjee, S. Bhattacharya, E. Suresh, M. Nethaji, E. Zangrando and D.
Das, Inorg. Chem., 2008, 47, 7083−7093; (e) C.-T. Yang, M. Vetrichelvan, X. Yang, B.
Moubaraki, K. S. Murray and J. J. Vittal, Dalton Trans., 2004, 113–121; (f) S. Sagar, S.
Sengupta, A. J. Mota, S. K. Chattopadhyay, A. E. Ferao, E. Riviere, W. Lewisf and S.
Naskar, Dalton Trans., 2017, 46, 1249–1259; (g) P. Chakraborty, J. Adhikary, B. Ghosh, R.
Sanyal, S. K. Chattopadhyay, A. Bauza,́ A. Frontera, E. Zangrando and D. Das, Inorg. Chem.,
2
014, 53, 8257−8269.
6
(a) P. Mahapatra, S. Ghosh, S. Giri, V. Rane, R. Kadam, M. G. B. Drew and A. Ghosh,
Inorg. Chem., 2017, 56, 5105−5121; (b) P. Mahapatra, M. G. B. Drew and A. Ghosh, Cryst.
Growth Des., 2017, 17, 6809−6820.
7
2
8
(a) P. Seth, L. K. Das, M. G. B. Drew and A. Ghosh, Eur. J. Inorg. Chem., 2012, 2012,
232–2242; (b) P. Mahapatra, M. G. B. Drew and A. Ghosh, Inorg. Chem., 2018, 57,
338−8353.
2
9