Ni(II) Complex of N2O3 Donor Unsymmetrical Ligand and Its Use for the Synthesis of NiII-MnII Complexes of Diverse Nuclearity: Structures, Magnetic Properties, and Catalytic Oxidase Activities

Article abstract of DOI:10.1021/acs.inorgchem.8b00995

A new mononuclear Ni(II) complex [NiL] (1) of an unsymmetrically dicondensed N₂O₃ donor ligand, H₂L (N-α-methylsalicylidene-N′-3-methoxysalicylidene-1,3-propanediamine), has been synthesized. Complex 1 on reaction with Mn(ClO₄)₂·6H₂O and NaN₃ in different molar ratios yielded three novel heterometallic Ni^II-Mn^II complexes, [(NiL)₂Mn(N₃)](ClO₄) (2), [(NiL)₂Mn₂(N₃)₂(μ_1,1-N₃)₂(CH₃OH)₂] (3), and [{(NiL)₂Mn}₂(μ_1,3-N₃)(H₂O)]·(CH₃OH),(ClO₄)₃ (4). The single crystal structure analyses show a trinuclear Ni^II₂Mn^II structure for complex 2 and a tetranuclear Ni^II₂Mn^II₂ structure where two dinuclear Ni^IIMn^II units are connected via μ_1,1-azido and phenoxido bridges for complex 3. Complex 4 possesses a hexanuclear structure where two trinuclear Ni^II₂Mn^II units are connected via a μ_1,3-azido bridge. The temperature-dependent dc molar magnetic susceptibility measurements reveal that complexes 3 and 4 are antiferromagnetically coupled with the exchange coupling constants (J) of -4.97, -0.14, -0.55 cm^-1 for 3 and -3.94 cm^-1 for 4. All complexes 2-4 show biomimetic catalytic oxidase activities. For catecholase like activity, the turnover numbers (K_cat) are 768, 1985, and 2309 h^-1 for complexes 2-4, respectively, whereas for phenoxazinone synthase like activity, the turnover numbers are 3240, 3360, and 13 248 h^-1 for complexes 2-4, respectively. This difference in catalytic efficiencies is attributed to the variations in structures of the complexes and formation of active Ni^II-Mn^II species in solution during catalysis. The mass spectral analyses suggest the probable intermediate formation and cyclic voltammetry measurement suggest the reduction of Ni(II) to Ni(I) during catalytic reaction. The very high catalytic efficiencies for aerial dioxygen activation of all these heterometallic complexes as well as the highest activity of 4 is attributed to the coordinatively unsaturated penta-coordinated geometry or hexa-coordinated geometry with a solvent water molecule around Mn(II).

Full text of DOI:10.1021/acs.inorgchem.8b00995

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Ni(II) Complex of N2O3 Donor Unsymmetrical Ligand and Its Use for
the Synthesis of NiII−MnII Complexes of Diverse Nuclearity:
Structures, Magnetic Properties, and Catalytic Oxidase Activities
Prithwish Mahapatra,† Michael G. B. Drew,‡ and Ashutosh Ghosh*,†
†Department of Chemistry, University College of Science, University of Calcutta, 92 A. P. C. Road, Kolkata 700009, India
‡School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, United Kingdom
S
* Supporting Information
ABSTRACT: A new mononuclear Ni(II) complex [NiL] (1) of an
unsymmetrically dicondensed N2O3 donor ligand, H2L (N-α-methylsalicyli-
dene-N′-3-methoxysalicylidene-1,3-propanediamine), has been synthesized.
Complex 1 on reaction with Mn(ClO4)2·6H2O and NaN3 in different molar
ratios yielded three novel heterometallic NiII−MnII complexes, [(NiL)2Mn-
(N3)](ClO4) (2), [(NiL)2Mn2(N3)2(μ1,1-N3)2(CH3OH)2] (3), and
[{(NiL)2Mn}2(μ1,3-N3)(H2O)]·(CH3OH),(ClO4)3 (4). The single crystal
structure analyses show a trinuclear NiII MnII structure for complex 2 and a
2
tetranuclear NiII MnII structure where two dinuclear NiIIMnII units are
2
2
connected via μ1,1-azido and phenoxido bridges for complex 3. Complex 4
possesses a hexanuclear structure where two trinuclear NiII MnII units are
2
connected via a μ1,3-azido bridge. The temperature-dependent dc molar
magnetic susceptibility measurements reveal that complexes 3 and 4 are
antiferromagnetically coupled with the exchange coupling constants (J) of −4.97, −0.14, −0.55 cm−1 for 3 and −3.94 cm−1 for
4. All complexes 2−4 show biomimetic catalytic oxidase activities. For catecholase like activity, the turnover numbers (Kcat) are
768, 1985, and 2309 h−1 for complexes 2−4, respectively, whereas for phenoxazinone synthase like activity, the turnover
numbers are 3240, 3360, and 13 248 h−1 for complexes 2−4, respectively. This difference in catalytic efficiencies is attributed to
the variations in structures of the complexes and formation of active NiII−MnII species in solution during catalysis. The mass
spectral analyses suggest the probable intermediate formation and cyclic voltammetry measurement suggest the reduction of
Ni(II) to Ni(I) during catalytic reaction. The very high catalytic efficiencies for aerial dioxygen activation of all these
heterometallic complexes as well as the highest activity of 4 is attributed to the coordinatively unsaturated penta-coordinated
geometry or hexa-coordinated geometry with a solvent water molecule around Mn(II).
Heterometallic complexes are often found to be superior to
their homometallic analogues for catalytic activities due to
multimetallic cooperative activities and for potential magnetic
materials (e.g., as single molecule or single chain magnets, etc.)
as the combination of different spins can generate more
magnetic anisotropy and large unpaired spins.26−31 In addition
to the primary ligand, the polynuclear metal complexes usually
contain anionic coligands which not only balance the charge of
the complexes but also coordinate to the metal centers in
different modes to determine the geometry and nuclearity of
the resulting complexes. Among various anionic species, the
INTRODUCTION
■
Multidentate N,O-donor ligands are widely used for synthesis
of homo- and heterometallic polynuclear complexes of 3d and
4f metal ions due to their applications in redox active
biomimetic catalysis, magnetic materials, and optical and
electrical applications.1−7 For further development of these
properties, structural modifications of the complexes are
carried out, usually with the help of new synthesized ligands
or combination of different ligands.8−13 The dicondensation of
1,2-ethanediamine or 1,3-propanediamine with various alde-
hydes or ketones produce N2O2 or N2O4 donor symmetrical
Schiff base ligands, which have been extensively used for
synthesis of homo- and heterometallic complexes.14−21
However, unsymmetrically dicondensed ligands are used very
rarely, presumably due to difficulty in their synthesis and
hydrolytic cleavage/rearrangements in solution phase although
it has been found that asymmetric coordination around the
metal ions can improve significantly the catalytic, magnetic,
and optical properties of the complexes.22−25
−
azide (N3 ) ion is one of the popular choices because of its
diverse coordination modes (e.g., μ1,1, μ1,3, μ1,1,3, etc.) that
allow formation of various 0D, 1D, and 2D coordination
complexes and transmission of ferro- or antiferromagnetic
coupling between the metal centers.32−36 Moreover, the lability
of the coordinated azide ion facilitates the coordination of
Received: April 12, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00995
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Inorganic Chemistry
Article
India. They were used without further purification. The other reagent-
grade quality metal salts and solvents were purchased from available
sources.
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 should
be handled with care.
Synthesis of the Unsymmetrical Schiff-Base Ligand N-α-
Methylsalicylidene-N′-3-methoxysalicylidene-1,3-propanediamine
(H2L). The unsymmetrically dicondensed Schiff base ligand [H2L] was
prepared in our laboratory by following the previously reported
method.49,50
Synthesis of the Complex [NiL] (1). Methanolic solution (10 mL)
of Ni(ClO4)2·6H2O (1.83 g, 5 mmol) was added to the methanolic
solution of ligand H2L (5 mmol, 25 mL). The mixture was stirred for
0.5 h and kept in a desiccator. The deep red colored crystalline
compound appeared in the beaker after 2−3 days on slow evaporation
of methanol. The compound was filtered out from the solution after 4
days, air-dried, and then recrystallized in dichloromethane solvent to
get X-ray diffractable quality crystals.
substrate to the metal center of complex, which is the crucial
step that determines the efficiency of the catalysts.
The naturally occurring metalloenzymes for oxidase
processes (e.g., catechol oxidase, phenoxazinone synthase,
etc.) contain multiple Cu(II) ions in their active sites. As a
result, polynuclear Cu(II) complexes have been studied
extensively as small molecule mimics of such metalloen-
zymes.37−41 The mono- and polynuclear complexes of other
metal ions (e.g., Ni(II), Co(II/III), Mn(II/III), Fe(II/III),
etc.) have also been found to catalyze these oxidase
processes.42−48 Although these synthesized complexes help
us to understand the mechanisms of the processes, their
turnover numbers are far less compared to the native enzymes.
Therefore, improving of the efficiency of the catalysts is a
challenge. Recently, we have found that some heterometallic
Cu2 −MnII complexes of an unsymmetrical Schiff base
II
containing a coordinated solvent molecule at a Mn(II) center
show considerably higher catalytic oxidase activity and one of
them shows highest phenoxazinone synthase activity among all
the model complexes synthesized until now.23,25 As the
catalytic activities of these heterometallic complexes are
dependent on both the metal ions, we would like to synthesize
some heterometallic NiII−MnII complexes of various nuclearity
and check their catalytic oxidase activities on different
substrates in naturally viable condition.
Complex 1. Yield 1.802
g (77%). Anal. Calcd for
C20H22Cl2N2NiO3 (468.0): Calculated C, 51.33; H, 4.74; N, 5.99.
Found C, 51.12; H, 4.65; N, 5.82. IR: ν(CN) = 1630, 1599 cm−1 for
formylimine and acetylimine, respectively.
Synthesis of the Complex [(NiL)2Mn(N3)](ClO4) (2). Complex 1
(0.936 g, 2 mmol) was dissolved in 20 mL of methanol and
Mn(ClO4)2·6H2O (0.361 g, 1 mmol) was added. The solution was
stirred for 15 min and then a 2 mL 1:1 (v/v) H2O-MeOH solution of
NaN3 (0.065 g, 1 mmol) was added dropwise to the solution at room
temperature. Then, the mixture was heated for 15 min and cooled at
room temperature. A small amount of precipitate appeared while
cooling the solution, which was filtered out and the filtrate was kept in
a beaker inside a desiccator. The reddish brown colored X-ray quality
single crystals appeared in 3−4 days, which were collected by filtration
and dried in air.
Recently, we developed a facile method for synthesis of
unsymmetrically dicondensed Schiff bases. Using this method,
we synthesized a new N2O3 donor ligand H2L (where H2L =
N-α-methylsalicylidene-N′-3-methoxysalicylidene-1,3-propane-
diamine) and its Cu(II) complex.49,50 Herein, we report its
mononuclear Ni(II) complex, [NiL] (1). Using complex 1 as
metalloligand, three heterometallic NiII−MnII complexes,
Complex 2. Yield 0.711g (74%). Anal. Calcd for
C38H38ClMnN7Ni2O10 (960.52): Calculated C, 47.52; H, 3.99; N,
10.21. Found C, 47.66; H, 3.96; N, 10.10. IR: ν(CN) = 1625 and
[(NiL)2Mn(N3)](ClO4) (2), [(NiL)2Mn2(N3)2(μ1,1
-
N3)2(CH3OH)2] (3), and [{(NiL)2Mn}2(μ1,3-N3)(H2O)]·
(CH3OH),(ClO4)3 (4), have been synthesized by varying
the proportions of Ni(II)-metalloligand, Mn(ClO4)2·6H2O,
and NaN3. All four complexes have been characterized by
single crystal X-ray crystallography. Complex 2 is a
1599 cm−1 for formylimine and acetylimine, respectively; ν(ClO)
1089 cm−1 and ν(NNN) = 2140 cm−1.
=
Synthesis of the Complex [(NiL)2Mn2(N3)2(μ1,1-N3)2(CH3OH)2] (3).
Complex 1 (0.936 g, 2 mmol) was dissolved in 20 mL of methanol,
and Mn(ClO4)2·6H2O (0.722 g, 2 mmol) was added to it. Then, 4
mL of 1:1 (v/v) H2O-MeOH solution of NaN3 (0.260 g, 4 mmol)
was added to the solution at room temperature. The mixture was
heated for 0.5 h and cooled at room temperature. A small amount of
precipitate that appeared during this process was filtered off and the
filtrate was kept in a beaker inside a desiccator. The light green
colored X-ray quality single crystals appeared in 3−4 days, which were
filtered out and air-dried.
diphenoxido bridged trinuclear NiII Mn species, 3 is a
2
phenoxido and μ1,1-N3 bridged tetranuclear (NiII MnII )
2
2
complex whereas hexanuclear structure of complex 4 is formed
by linking two trinuclear (NiL)2Mn units via a single μ1,3-N3
bridge. The penta-coordination of Mn(II) center in 2 is rather
unusual among this type of complexes, whereas the nuclearity
and connectivity between the metal centers in 3 and 4 are
unprecedented in heterometallic NiII−MnII complexes. All
three heterometallic complexes (2−4) exhibit very high
biomimetic oxidase activities (e.g., catechol oxidase and
phenoxazinone synthase like activities under aerobic con-
ditions). UV−vis spectra, ESI-mass spectra, and cyclic
voltammograms of all the complexes have been studied to
investigate the probable intermediates of these catalytic oxidase
reactions and subsequently to develop a possible mechanism.
We have shown that the catalytic efficiency of the complexes
can be related to the coordinative unsaturation and
coordinated solvent molecule to the Mn(II) centers, and the
number of active NiII−MnII dinuclear species formed in
solution. Variable-temperature dc magnetic susceptibility has
also been measured for complexes 3 and 4.
Complex 3. Yield 0.796g (72%). Anal. Calcd for
C40H46Mn2N16Ni2O8 (1106.2): Calculated C, 43.43; H, 4.19; N,
20.26. Found C, 43.28; H, 4.11; N, 20.18. IR: ν(CN) = 1629 and
1602 cm−1 for formylimine and acetylimine, respectively; ν(NNN)
=
2077 cm−1.
Synthesis of the Complex [{(NiL)2Mn}2(μ1,3-N3)(H2O)]·(CH3OH),
(ClO4)3 (4). This complex was prepared by following the similar
procedure as for the synthesis of complex 2. Here, complex 1 (1.872
g, 4 mmol) in 20 mL of methanol was reacted with Mn(ClO4)2·6H2O
(0.722 g, 2 mmol) and NaN3 (0.065 g, 1 mmol). The solution
mixture was heated for 15 min and cooled at room temperature.
Then, the mixture is filtered and kept in a desiccator. The deep brown
colored X-ray quality single crystals appeared inside the beaker after
2−3 days, which were filtered off and dried in air.
Complex 4. Yield 1.484g (73%). Anal. Calcd for
C77H86Cl3Mn2N11Ni4O26 (2032.56): Calculated C, 45.50; H, 4.26;
N, 7.58. Found C, 45.37; H, 4.35; N, 7.69. IR: ν(CN) = 1629, 1599
cm−1 for formylimine and acetylimine, respectively; ν(ClO) = 1089
cm−1 and ν(NCN) = 2140 cm−1.
EXPERIMENTAL SECTION
Starting Materials. The reagent grade o-hydroxyacetophenone, o-
vaniline, and 1,3-propanediamine were purchased from Spectrochem,
■
B
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Table 1. Crystal Data and Structure Refinement of Complexes 1−4
1
2
3
4
formula
mol. wt.
crystal system
space group
a (Å)
C20H22Cl2N2NiO3
468.00
triclinic
P21/n
11.3069(5)
12.0820(4)
15.0575(5)
90
100.054(4)
90
C38H40ClMnN7Ni2O10
962.58
triclinic
C40H48Mn2N16Ni2O8
1108.24
monoclinic
P21/n
11.2009(14)
17.1273(13)
12.1136(11)
90
98.122(10)
90
2300.6(4)
2
C80H80Cl2Cu4Mn2N14O18
2032.56
triclinic
P1
̅
P1
̅
10.6335(16)
13.3500(14)
15.5541(14)
73.727(9)
71.102(11)
84.121(11)
2005.1(4)
2
13.637(5)
15.774(5)
21.506(5)
95.575(5)
103.573(5)
110.829(5)
4119(2)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
2025.42(13)
4
1.535
Dc (g cm−3
)
1.594
1.600
1.628
μ (mm−1
F (000)
R(int)
)
1.245
968
0.051
1.373
990
0.058
1.412
1140
0.060
1.676
2060
0.0473
total reflections
unique reflections
I > 2σ(I)
12347
5633
4141
0.0726, 0.1412
0.1044, 0.1552
1.072
9784
6943
4777
0.1131, 0.2999
0.1472, 0.3310
1.070
10291
3928
3257
0.0975, 0.1832
0.1196, 0.1917
1.258
19364
13165
6241
0.0641, 0.1666
0.1478, 0.2289
0.884
ab
,
R1, wR2, I > 2σ(I)
ab
,
R1, wR2 (all data)
c
GOF on F2
temp (K)
150
1.055, −1.113
150
2.793, −0.745
150
1.071, −0.450
293
0.771, −0.667
residual electron density (e/Å−3
)
a
b
c
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 (Fo ) = [∑[w(Fo − Fc2)2/∑w Fo ]1/2. GOF = [∑[w(Fo − Fc2)2/(Nobs− Nparams)]1/2
.
2
2
4
2
complexes 2−4, and iodometric estimation of hydrogen
peroxide during the catalytic reaction were studied by reported
procedures.23,55
PHYSICAL MEASUREMENTS
■
Elemental analyses, UV−vis spectra, and IR spectra, variable
temperature molar magnetic susceptibility of complexes 3 and
4, electrochemical measurements, electrospray ionization mass
spectra (ESI-MS positive), and X-ray powder diffractograms of
complexes 2−4 were recorded as reported earlier.23
RESULTS AND DISCUSSION
■
Syntheses of the Complexes. The asymmetrically
dicondensed N2O3 donor Schiff base ligand, N-α-methylsali-
cylidene-N′-3-methoxysalicylidene-1,3-propanediamine (H2L),
was synthesized as reported recently by us.49,50 This ligand was
reacted with Ni(ClO4)2·6H2O in 1:1 molar ratio to yield the
new mononuclear complex [NiL] (1). This complex (1) was
then used as metalloligand to prepare heterometallic Ni(II)−
Mn(II) complexes. Complex 1 on reaction with Mn(ClO4)2·
6H2O in the presence of NaN3 in 2:1:1 molar ratio yielded
complex [(NiL)2Mn(N3)](ClO4) (2) from methanol solution.
However, when the same reactants were allowed to react in
different proportions (i.e., 1:1:2 molar ratios in methanol)
complex [(NiL)2Mn2(N3)2(μ1,1-N3)2)(CH3OH)2] (3) was
obtained as product. However, if the molar ratios of the
same reactants are 4:2:1, then complex [{(NiL)2Mn}2(μ1,3-
N3)(H2O)]·(CH3OH),(ClO4)3 (4) is produced under similar
reaction conditions. It is also noted that both complexes 2 and
4 can be converted to complex 3 by the reaction with excess
Mn(ClO4)2·6H2O and NaN3 in methanol under reflux
condition. The syntheses of complexes 1−4 are shown in
Scheme 1. The similar patterns of the measured powdered
diffractograms of complexes 2−4 and their respective
simulated diffractograms (from Mercury 3.8; Figures S1−S3)
confirms their phase purity.
ICP-OES Measurements. The ICP-OES measurements of
complexes 2−4 were performed using ICPOES, ICAP Duo
6500 from Thermo Scientific. The standard reference Material
(SRM) is used in ICP-OES analysis was procured from NIST,
USA. The ICP-OES instrument was calibrated with multi-
elemental standard (MES 23) having Ni and Mn concentration
of 1000 ppm. Calibration standards from 1 to 100 ppm were
prepared by appropriate dilution of the MES by 2% HNO3.
Emission wavelengths of Ni and Mn are 221.6 and 257.6 nm,
respectively. The samples were prepared by dissolving 2 mg of
each complexes (2−4) in 2 mL of suprapure HNO3 followed
by filtration and diluted to 20 mL with deionized water.
X-ray Crystallographic Data Collection and Refine-
ment. Data for complexes 1−3 were collected as described
elsewhere23 and analyzed with the CrysAlis program.51 Data
for complex 4 were collected with Mo Kα radiation at 293(2)
K using a Bruker diffractometer and analyze with the Saint
program.
The structures were solved using direct methods with the
Shelxs97 program52 and the atoms were refined with same
procedure as reported earlier.23 For 2, the perchlorate anion
and the terminal nitrogen in an azide were disordered.
Absorption corrections for 1−3 were carried out using the
ABSPACK program53 and for 4 using SADABS.54 The
structures were refined using Shelxl2016−1652 on F2. Details
of the crystallographic data are summarized in Table 1.
Catalytic Oxidation of 3,5-DTBC and OAP. The
catecholase activity, phenoxazinone synthase activity of
It is noteworthy that the mononuclear Ni(II) complex of
N2O3 donor unsymmetrically dicondensed Schiff base ligand is
reported here for the first time. In complex 2, two
metalloligands along with an azide ion coordinate to the
central Mn(II) ion to form the trinuclear structure. Complex 4
C
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Scheme 1. Syntheses of Complexes 1−4
is formed by joining Mn atoms of two (NiL)2Mn units by one
μ1,3-bridging azide ion. In contrast, complex 3 is formed by
coordination of one metalloligand to a Mn(II) ion along with
two azide ions; two such (NiL)Mn units are then joined
together exploiting the terminal as well as μ1,1-bridging modes
of azide ion. It is also noteworthy that Ni(II) ion is four-
coordinated square planar in 1, 2, and 4 but is octahedral in 3.
The difference is discernible in the red/brown color of the first
three complexes compared to the light green color of complex
3.
ICP-OES Analysis of Complexes 2−4. The ICP-OES
analysis results of 100 ppm solutions of complexes 2−4 are
shown in Table 2. The presence of Ni and Mn metals in
complexes 2−4 are analyzed and their ratios in these
complexes are also determined. The result reveal that the
Ni/Mn molar ratios in complexes 2 and 4 are 1.9:1 and 2:1,
respectively, whereas in complex 3 the ratio is 1.1:1. These Ni/
Mn molar ratios are closer to ratio of Ni/Mn atoms in crystal
structure of complexes 2−4 obtained from single crystal X-ray
diffraction.
IR Spectra of Complexes 1−4. In the IR spectra of all the
complexes, two strong peaks are observed at 1625 and 1598
cm−1 for 1, 1624 and 1599 cm−1 for both 2 and 4 (Figure S4),
and 1626 and 1598 cm−1 for 3 (Figure S5), due to two
different types of azomethine v(CN) stretches of the
asymmetric Schiff base.49,50 In addition, the spectra of
complexes 2−4 show strong sharp peaks at 2080, 2085, and
2082 cm−1, respectively, for the stretching vibrations of N3
−
ions.23 The spectra of both complexes 2 and 4 also show
strong peaks at 1085 and 1082 cm−1 for the stretching
Table 2. Result of ICP-OES Analysis of Ni (221.6 nm) and
Mn (257.6 nm) in Complexes 2−4
vibrations of ClO4 ions.23,25
−
Description of Structures. The structure of the 1, the
basic metalloligand is shown in Figure 1. The nickel atom has a
distorted square planar environment, being bonded to four
atoms of the ligand L2−. Details of the coordination sphere are
given in Table S1. Ni−O distances of 1.866(3) and 1.886(3) Å
are shorter than Ni−N distances at 1.940(3) and 1.934(3) Å.
Ni (ppm)/
Mn (ppm)
molar
ratio
complex
Ni (ppm)
Mn (ppm)
2
3
4
13.256 0.052
12.224 0.064
14.78 0.072
6.5237 0.0195
10.383 0.064
6.9327 0.0296
0.226:0.119
0.208:0.189
0.252:0.126
1.9:1
1.1:1
2:1
D
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bonded to four oxygen atoms and a terminal azide group. The
geometry is indeterminate with the maximum internal angle
138.1(3)°. The Mn(1)−O(11) and Mn(1)−O(31) bond
lengths of 2.144(8) and 2.145(7) Å are shorter than those of
Mn(1)−O(30), Mn(1)−O(50) at 2.246(8) and 2.254(7) Å.
The Mn(1)−N(1) bond length is 2.076(13) Å. The geometry
around the manganese atom is very irregular and cannot be
described as based on either the square pyramid or trigonal
bipyramid. The assignment of geometry is made more difficult
by the fact that there are close contacts with both O(131) and
O(331) at distances of 2.670(9) and 2.740(9) Å, respectively.
The structure of centrosymmetric tetramer 3 is shown in
Figure 3. Here the metalloligand is bridged to manganese
which forms a centrosymmetric four-membered ring via O(30)
to form a tetramer.
Figure 1. Structure of 1, the basic metalloligand, with ellipsoids at
30% probability. The solvent dichloromethane molecule is not shown.
The equatorial plane has a distinct tetrahedral distortion with
an r.m.s. deviation of the atoms at 0.297 Å. The two aromatic
rings intersect this equatorial plane at an angle of 17.2(1) and
37.6(1)° and each other at 48.8(1)°. The asymmetry observed
here between the two aromatic rings is prevalent in all the
structures reported here, namely, that the ring containing the
substituent −OMe group is far closer to the equatorial plane
than the ring with no substituents but an adjacent −Me group.
The structure of the cation [Mn(N3)(NiL)2]+ (2) is shown
in Figure 2. The two nickel atoms have similar four-coordinate
Figure 3. Structure of the centrosymmetric tetramer [Mn(NiL)]-
(N3)2]2 (3) with ellipsoids at 30% probability.
The nickel atom in this case is six-coordinate with a
distorted octahedral environment. The equatorial plane is as
expected with the metal bonded to the donor atoms from
ligand L2− but with significantly longer bond lengths (Table
S1). The two aromatic rings intersect the equatorial plane at
angles of 16.3(3) and 36.2(3)°, respectively, and each other at
49.6(3)°. In addition there are two axial atoms, an azide
through N(1)* (* = −x, −y, 1 − z) at 2.179(7) Å which
bridges to manganese, and a terminal methanolate at O(1) at
2.134(6) Å. The manganese atom Mn(1) is hexa-coordinated
with a distorted octahedral environment being bonded to three
oxygen atoms from one ligand, O(11) at 2.094(5) Å which
bridges to Ni(1), O(30) at 2.251(5) Å which bridges to both
Ni(1) and Mn(1)*, and O(131) at 2.280(5) Å. The equatorial
plane is completed by an N(1) atom at 2.140(6) Å from an
azide which bridges to Ni(1)*. The four atoms in the
equatorial plane show an r.m.s. deviation of 0.089 Å with the
metal 0.162(3) Å from the plane. In axial positions are O(30)*
at 2.222(5) Å and N(4) at 2.124(7) Å from a terminal azide.
The structure of 4, [Mn(NiL)2(OH2)](N3)[Mn(NiL)2].3+ is
shown in Figure 4. The cation consists of two different
Figure 2. Structure of 2 with ellipsoids at 30% probability. The
perchlorate anion is not shown.
square planar geometries with bond lengths are equivalent to
those found in 1 (Table S1). The equatorial planes around
Ni(1) and Ni(2) have a much smaller tetrahedral distortion
than is observed in 1 with r.m.s. deviations of the atoms in the
equatorial plane at 0.055 and 0.066 Å. The two aromatic rings
intersect this equatorial plane at an angles of 24.2(4), 21.5(5)°
and 44.7(5), 46.8(4)° and each other at 66.9(5), 67.0(4)°,
respectively. The manganese atom is five-coordinate being
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oxygen atoms in an equatorial plane and a nitrogen atom from
the bridging azide in an axial position. O(531) and O(731)
remain unbonded at distances of 2.810(11), 2.777(11) Å. The
equatorial plane shows an r.m.s. deviation of 0.215 Å with the
metal 1.355(3) Å from the plane in the direction of the
bridging azide. The water molecule also participates in a strong
hydrogen bond to a perchlorate oxygen atom O(24) (x, y − 1,
z) with an O···O distance of 2.917(11) Å. The solvent
methanol also forms a hydrogen bond to a perchlorate oxygen
atom O(43) with an O···O distance of 2.982(13) Å.
It is to be noted that we have used the new square planar
Ni(II) complex (1) of the N2O3 donor Schiff base ligand as
metalloligand very effectively to synthesize three novel
heterometallic complexes. In complex 2, two metalloligands
coordinate to the central Mn(II) through the double
phenoxido bridge as was found in many other similar trinuclear
complexes.23,25,49 However, in the present complex, the
Mn(II) is penta-coordinated in contrast to the hexa-
coordination of the similar other complexes. The proximity
of the two −OCH3 groups 2.669(11) and 2.730(11) Å to
Mn(II) seems prevent the coordination to sixth site. The
Figure 4. Structure of cation [Mn(NiL)2(OH2)](N3)[Mn(NiL)2]+3
(4) with ellipsoids at 30% probability. The three perchlorates and
solvent methanol in the asymmetric unit are not shown.
trinuclear units via the two manganese atoms bridged by an
azide. One trinuclear unit contains Ni(1), Ni(2), and Mn(1)
and consists of [Mn(NiL)2(OH2)], the other Ni(3), Ni(4),
and Mn(2) and consists of [Mn(NiL)2]. In the first unit, the
two nickel atoms are bonded to one ligand each with Ni−N
and Ni−O bond lengths are similar to those in 1 and 2 but
with minor differences as shown in Table S1. However, instead
of all four oxygen atoms from the two ligands bridging to
Mn(1), only O(11), O(30), and O(31) are bound. Instead of a
bond to O(50), the distance being 3.319(9) Å, a bond is
formed to O(331) at 2.358(6) Å an arrangement similar to
that found in 3. O(131) is only weakly bonded at 2.628(9) Å.
Instead of bonding to the metal, O(50) forms a strong
hydrogen bond to the water molecule O(1) which is bound to
the metal at 2.189(6) Å with dimensions O···O 2.656(11) Å,
H···O 1.82 Å, O−H···O 163°. The six-coordinate geometry
around Mn(1) is completed by the bridging azide. The
geometry is best considered as a pentagonal bipyramid with
O(30) and O(1) in axial positions, and one equatorial site is
missing. The second unit around Mn(2) has the more
predictable structure [Mn(NiL)2]. Here the two nickel atoms
have the square planar structure each being bonded to one
tetradentate ligand. The four oxygen atoms, two from each
metalloligand, are bonded to the manganese. The coordinate
sphere shows a square pyramidal structure, with these four
structure of tetranuclear NiII −MnII2 complex is rather unique.
2
There are few examples of phenoxido/oxido bridged Ni2−Mn2
complexes where Mn is in a +3 oxidation state56−59 but none
with Mn(II) or with mixed bridge. The μ1,3-azido bridged
hexanuclear structure of complex 4 is also unprecedented.
Although we have previously reported several NiII −MnII
2
complexes of symmetrical N2O2 donor ligand with various
coligands but got only trinuclear complexes. However, recently
we reported a similar dicyanamide bridged hexanuclear
Cu4Mn2 species.25
Magnetic Properties. Temperature-dependent molar
magnetic susceptibility measurements were only carried out
for complexes 3 and 4 because complex 1 has a mononuclear
structure with a diamagnetic square planar Ni(II) atom, and in
complex 2, the phenoxido bridges connect the paramagnetic
Mn(II) center to diamagnetic square planar Ni(II) atoms. The
temperature variations of the molar magnetic susceptibility
product (χMT) with temperature of complexes 3 and 4 are
shown in Figure 5. The room temperature χMT value of
complex 3 is 9.94 cm3 K mol−1, which is slightly lower than the
value of magnetically noninteracting NiII MnII2 system of 10.77
2
cm3 K mol−1 for gNi = 2.1 and gMn = 2 at 300 K. For complex 4,
the room-temperature χMT value is 8.67 cm3 K mol−1, which is
also slightly lower than the value of magnetically non-
Figure 5. Thermal variations of the χMT products for complexes 3 and 4 (left to right respectively). Solid magenta line represents the best fitting to
the appropriate model for 3 and solid red line represents the best fitting to the appropriate model for 4.
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interacting MnII system of 8.75 cm3 K mol−1 for gMn = 2 at
cm−1, J2(Ni−Mn*) = −0.14 cm−1, J3(Mn−Mn*) = −0.55
cm−1, DNi = 4.0 cm−1, R = 2.35 × 10−5 for complex 3; J(Mn−
Mn) = −3.94 cm−1, gMn = 2.0, R = 1.22 × 10−5 for complex 4.
The g values of Ni and Mn are fixed at gNi = 2.1 and gMn = 2.0
during fitting.
2
300 K. In both the complexes, almost similar type of behavior
is observed; a slow decrease of χMT values takes place upon
cooling down to 50 K, on further cooling the χMT values
sharply decrease and reach 1.13 and 1.36 cm3 K mol−1 for
complexes 3 and 4, respectively, at 2 K. The progressive
decreasing nature of χMT values with lowering the temperature
indicates the presence of intramolecular antiferromagnetic
interactions between the metal centers. The molar magnet-
izations values (M/Nβ) up to 5 T magnetic fields are 2.80 and
2.10 μB for complexes 3 and 4, respectively (Figures S6 and
S7) at 2 K, which correspond to the nonzero spin in the
ground states. This nature of the magnetization plots
corroborates the presence of antiferromagnetic coupling
between Ni(II)−Mn(II) atoms in complex 3 and between
Mn(II)−Mn(II) atoms in complexes 3 and 4, respectively. The
variations of molar magnetic susceptibility (χM) with temper-
ature of complexes 3 and 4 are shown in Figures S8 and S9,
respectively.
The magneto-structural correlation of diphenoxido-bridged
tetranuclear NiII −MnII2 complexes is not well established due
2
to the scarcity of such compounds. A CSD search shows that
some NiII −MnIII complexes are reported, but only very few
2
2
of them are characterized both structurally and magneti-
cally.56−59 Until now, nine trinuclear phenoxido bridged NiII −
2
MnII complexes are reported and in most cases with an
additional carboxylato bridge.61−64 In these complexes, a linear
dependency was observed between the magnetic exchange
coupling (JNi−Mn) and the Ni−O−Mn angles of the
diphenoxido-bridge. This correlation indicates that the
magnitude of the antiferromagnetic exchange coupling
increases with increase in Ni−O−Mn phenoxido angles. It is
apparent that the crossover angle for ferro to antiferromagnetic
exchange is approximately 98° for these complexes.61−64
Considering this linear dependency, a correlation is drawn
between JNi−Mn (including J1) values and the Ni−O−Mn
angles, as shown in Figure 7. It is observed that the NiII−MnII
Centrosymmetric tetranuclear NiII −MnII structure com-
2
2
plex 3 is formed by joining two phenoxido bridged dinuclear
NiII−MnII units via phenoxido bridges as well as azido bridges.
In the structure, each Ni(II) is connected to one Mn(II) atom
via double phenoxido bridges and to another Mn(II) via a
mixed phenoxido/azido bridge, while two Mn(II) atoms are
connected via double phenoxido bridges (Figure 6, left).
Figure 6. Basic tetranuclear unit of complex 3 (left) and dinuclear
unit of complex 4 (right) with different magnetic exchange couplings
(J).
Therefore, for the calculation of magnetic interaction
parameters, we have assumed three exchange interactions in
double-cubane core (Figure 6, left): (i) between Ni(II) (S = 1)
and Mn(II) (S = 5/2) ions of each dinuclear units with
exchange coupling (J1), (ii) between the Ni(II) (S = 1) ion of
one unit and Mn(II)* (S = 5/2) ion of other dinuclear unit
with the exchange coupling (J2), and (iii) between two Mn(II)
(S = 5/2) ions with exchange coupling as J3 (Figure 6 left).
The Hamiltonian used for the simulation is written as H =
−J1(S1S2 + S3S4) − J2(S1S3 + S2S4) − J3(S2S3), where S1 = S4 =
SNi and S2 = S3 = SMn. The exchange coupling between
terminal Ni(II) and Ni(II)* ions is taken as zero (JNi−Ni = 0
cm−1) because of the large Ni···Ni distance. In complex 4, the
Ni(II) atoms are diamagnetic (S = 0) with square planar
geometry, so the exchange interactions between Ni(II) (S = 0)
and Mn(II) (S = 5/2) ions are taken as zero. Therefore, for
calculation of the magnetic interaction parameters of complex
4, we have assumed only one exchange interaction between
two μ1,3-azido bridged Mn(II) (S = 5/2) considering exchange
coupling as J (Figure 6 right). The simulations of experimental
magnetic data were carried out using the PHI program taking
account of the molecular field approximations.60 The best fits
lead to the following set of parameters: J1(Ni−Mn) = −4.97
Figure 7. Variation of the magnetic exchange coupling (J1) in double
phenoxido-bridged NiII−MnII complexes with Ni−O−Mn angle.
magnetic exchange coupling value (J1) in complex 3 is in
agreement with this linear dependency of correlation. As mixed
phenoxido and azido bridged NiII−MnII complexes have not
yet been reported, no correlation for J2 can be drawn. A CSD
search reveals that there are a few diphenoxido-bridged
dinuclear MnII complexes, but only a very few have been
2
characterized both structurally and magnetically.65−67 The
exchange interactions between two Mn(II) ions in these
complexes are found to be both ferro- or antiferromagnetic
(Table S2). The general trend for the change of J values with
Mn−O−Mn angles, which varied from 82.2 to 115.8° is not
very clear as other factors (e.g., Mn−O distances, tilting of
MnO2Mn skeleton from the basal plane containing the Mn
atoms, etc.) have considerable influences on coupling. In
complex 3, the bridging angle is 90.9(2)°, and the least-squares
plane of the MnO2Mn skeleton is tilted by 2.71° from the basal
NO2N plane. Thus, a weak antiferromagnetic interaction (J3 =
−0.55 cm−1) between two Mn(II) ions is not surprising.
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There are a few μ1,3-azido bridged Mn(II) complexes that
have been characterized both structurally and magneti-
cally.68−70 In all the complexes, the coupling is antiferromag-
netic and the magnitude of the antiferromagnetic exchange
interaction between Mn(II) ions (JMn−Mn) depends on number
of μ1,3-azido bridges; JMn−Mn increases with increasing the
number of μ1,3-azido bridges. In case of double μ1,3-azido
bridged Mn(II) complexes, the magnitude of antiferromagnetic
interaction is found to be in the range of 6.0−15.4 cm−1.68−70
The difference in J values is related to the dihedral angle (θ)
which is defined by the angle between two Mn−N−N−N
mean planes and Mn−N distances. Higher values of θ and
shorter Mn−N distances favor stronger antiferromagnetic
coupling. In contrast, there are only two single μ1,3-azido
bridged Mn(II) complexes for which the magnetic data are
fitted, and their J values are found to be −2.29 and −3.60
cm−1. The magnitude of J for these two compounds may be
related to the average Mn−N−N angles (δ) of 128.99 and
141.18°, respectively, and Mn−N distances of 2.17 and 2.209
scan at Ea = −0.66 and −0.94 V (Figure 8). All the complexes
1−4 show a common peak at around −0.8 V, which
corresponds to the one electron reduction of Ni(II) to Ni(I)
of the free mononuclear metalloligand (NiL).71−74 Appearance
of two peaks in complexes 2−4 indicates that the presence of
two types of (NiL) species (mononuclear and dinuclear) in the
solution is due to the partly dissociation of complexes 2−4 in
solution. The additional peaks at slightly higher potential than
−0.9 V might be due to the reduction of the (NiL) in the
dinuclear (NiL)Mn or trinuclear ((NiL)2Mn unit. All the
species (NiL), (NiL)Mn and ((NiL)2Mn are assignable in ESI
mass spectra of complexes 2−4. The similar nature of cyclic
voltammograms with comparable peak separation values
(ΔEpc) of complexes 2−4 indicates that the similar type of
species are present in the solution of all the complexes.
Catechol Oxidase Studies and Kinetics. Detailed
Description. The three complexes, 2−4, show catecholase-
like activity. The detailed kinetic studies of this catechol
oxidase activity were carried out as reported earlier.23 The
oxidation reaction of 3,5-ditert-butylcatechol (3,5-DTBC) to
its corresponding o-quinone (3,5-DTBQ) is shown in Scheme
2. The kinetic studies of catecholase-like activity of complexes
Å.68,69 In complex 4, the magnetic exchange coupling (JMn−Mn
)
is −3.94 cm−1. The highest magnitude of J of 4 among the
three single μ1,3-azido bridged Mn(II) complexes is expected
from its highest δ angle of 149.13° (av.) and shortest Mn−N
distance of 2.095 Å.
Electrochemistry. Cyclic voltammograms (CVs) of all
complexes 1−4 were recorded in acetonitrile with respect to an
Ag/AgCl electrode at a scan rate of 100 mV/Sec. On cathodic
scan (0 to −1.6 V), the cyclic voltammogram of complex 1
(Figure 8) shows one wave for the quasi-reversible reduction
Scheme 2. Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ
in Methanol
2−4 were performed in methanol solvent under aerobic
conditions. We also checked the catalytic activity of complex 1,
but it did not show any such activity (Figure S10).
For the detailed kinetic study of catechol oxidase reaction,
we used 5 × 10−5 M methanol solution (considering trinuclear
unit) of complex 2, 5 × 10−5 M methanol solution
(considering tetranuclear unit) of complex 3, and 2.5 × 10−5
M methanol solution (considering hexanuclear unit) of
complex 4. The 1 × 10−2 M methanol solution of 3,5-DTBC
was used as substrate solution. The change in spectral behavior
of complex 4 during wavelength scan in the presence of 3,5-
DTBC is depicted in Figure 9 (left) and of 2−3 are shown in
Figures S11 and S12 (left). Both the experimental rate versus
[substrate] and the Lineweaver−Burk plots for complex 4 are
shown in Figure 9 (right) and 2−3 are depicted in Figures S11
and S12 (right). The turnover numbers (kcat in h−1) of this
catechol oxidase reaction are 768, 1985, and 2309 for
complexes 2−4 respectively. Kinetic parameters of catecholase
activity are listed in Table S3. The kinetic study was also
carried out by varying the catalyst concentrations and a linear
dependence of rate with catalyst concentration was found. The
plot of experimental rate versus [catalyst] for complex 4 is
shown in Figure 10, and those for 2−3 are shown in Figures
S13 and S14.
Figure 8. Cyclic voltammograms (CVs) of 10−3 M solution of
complexes 1−4 in acetonitrile at room temperature and a scan rate of
100 mV/Sec, using TBAP as supporting electrolyte.
process at the surface of electrode. The peak is observed at Ec
= −0.88 V on forward cathodic scan, and on reverse scan the
corresponding peak is observed at Ea = −0.35 V with the peak
70
separation value (ΔEpc = Ecathodic − Eanodic
)
of −0.53 V. The
CV of complex 1 suggests that the mononuclear Ni(II)
complex is reduced to the Ni(I) state through a one electron
process. Both complexes 2 and 3 show two peaks at Ec = −0.82
and − 1.30 V for 2 and Ec = −0.78 and −1.27 V for 3 on
forward cathodic scan, whereas one peak is observed at Ea =
−0.47 and −0.56 V, respectively, for complexes 2 and 3 on the
reverse scan (Figure 8). Complex 4 shows two peaks at Ec =
−0.86 and −1.03 V on forward cathodic scan and on reverse
ESI-Mass Spectrometric Study. To draw the mechanistic
insight of catecholase activity and to determine the complex-
substrate intermediate during the catalytic oxidation reaction
of catechol, we have recorded ESI-MS spectra of complexes 2−
4 and of 1:1 mixtures (v/v) of the complexes and 3,5-DTBC in
methanol solution within 5 min of mixing (Figures S15−S17).
The mass spectra of all complexes 2−4 show a base peak at m/
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Figure 9. Increase in the 3,5-DTBQ band at around 400 nm after the addition of 3,5-DTBC (1 × 10−2 M) to the methanolic solution of complex 4
(2.5 × 10−5 M) (left) and plot of the rate versus substrate concentration (right). Inset shows the corresponding Lineweaver−Burk plot of 4. The
UV-spectra were recorded at 5 m intervals.
form of the intermediate [(NiIIL)Mn(3,5-DTBSQ)]+. In
addition, a peak at m/z = 243.14 (calcd 243.13) is observed
for [3,5-DTBQ+Na]+ species, which is the oxidized product of
3,5-DTBC during the reaction. In summary, the appearance of
the peaks due to [(NiIIL)Mn(3,5-HDTBC)]+ and [(NiIIL)-
Mn(3,5-DTBSQ)]+ species in the spectra of the complex
substrate mixtures are significant for the identification of
probable intermediate species, formed during the oxidative
conversion of 3,5-DTBC to 3,5-DTBQ by aerial oxygen in the
presence of complexes 2−4 as catalysts.
Mechanistic Insight. Various mechanisms are reported in
the literature for catalytic conversion of catechol to their
respective quinone. In most of the cases, homometallic
copper(II), nickel(II), cobalt(II/III), manganese(II/III), and
zinc(II) complexes have been used for this catalytic
reaction.42−48 Very recently, we have shown that some
heterometallic Cu(II)−Mn(II) complexes can catalyze this
reaction with very high efficiency.23 The mechanistic study of
Figure 10. Rate versus catalyst concentration plot of complex 4 for
the oxidation reaction of 3,5-DTBC (10−2 M) in methanol.
this catalytic reaction reveals that the catalysis process goes
through radical mechanism with the formation of a semi-
quinonate intermediate and reduction of Cu(II) to Cu(I).
During this process H2O2 is formed as a byproduct. We had
z = 383.10 (calcd 383.09), which can be assignable to
protonated metalloligand [(NiL) + H]+ (Figures S15−S17).
also shown that few Ni(II)−Mn(II) complexes can also
Another prominent peak is observed at m/z = 405.08 (calcd
catalyze this reaction.75 However, their catalytic efficiency was
405.07) due to the sodiated metalloligand species [(NiL) +
very low, and mechanism of the catalytic reaction was not
studied. To investigate the mechanism of this catalytic reaction
Na]+. Along with these, two other prominent peaks are
observed at m/z = 765.18 (calcd 765.17) for protonated
dinuclear metalloligand [(NiL)2 + H]+ and at m/z = 787.16
(calcd 787.16) for the sodiated dinuclear metalloligand species
[(NiL)2Na]+ (Figures S15−S17).
by the present Ni(II)−Mn(II) complexes, we found that H2O2
is produced for each case, when complexes 2−4 were mixed
with 3,5-DTBC (Figure S22). The quantitative estimations
revealed that about 73.5, 90.2, and 97.4% H2O2 were generated
for complexes 2−4, respectively, after 30 min of oxidation
reaction with respect to the conversion of 3,5-DTBQ from 3,5-
H2DTBC (Table S4).
On the basis of ESI-mass spectra, we propose here a
probable mechanism for the catechol oxidation reaction. First,
the doubly charged species [(NiL)Mn]2+ is produced in
solution for all complexes 2−4. Then, this doubly charged
species reacts with 3,5-DTBC to generate an intermediate
species [(NiIIL)Mn(3,5-HDTBC)]+ in which one of the
hydroxyl group of 3,5-DTBC is deprotonated and coordinated
to the Mn(II) center. This species then readily reacts with
dissolved O2 by releasing another proton of hydroxyl group
After the addition of 3,5-DTBC to the solutions of
complexes 2−4, spectra of all the mixtures show four common
peaks at m/z = 383.10 (calcd 383.09), m/z = 405.08 (calcd
405.07), m/z = 765.18 (calcd 765.17), and m/z = 787.16
(calcd 787.16) (Figures S18−S21). Here also m/z = 383.10
(calcd 383.09) is the base peak. Along with these, one new
peak at m/z = 658.17 (calcd 658.17) is observed in the
mixtures, which can be assignable to [(NiIIL)Mn(3,5-
HDTBC)]+ species (Figures S18−S21). This peak is very
important from the mechanistic point of view as it clearly
demonstrates the presence of the complex-substrate inter-
mediate species. The presence of another important peak at m/
z = 657.16 (calcd 657.17) also indicates the semiquinonate
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Scheme 3. Proposed Mechanism for Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ by Complexes 2−4
and is converted to its radical form (semiquinonate)
[(NiIIL)Mn(3,5-DTBSQ)]+ as is evident from mass spectra.
During this step of reaction, O2 is reduced to H2O2. After that
the semiquinone itself is converted to quinone and is
eliminated from the metal center, leaving a singly charged
dinuclear species [(NiIL)MnII]+ with reduction of Ni(II)
center to Ni(I). Then, this [(NiIL)MnII]+ species coordinates
to another 3,5-DTBC to form [(NiIL)Mn(3,5-H2DTBC)]+
species. Finally, the Ni(I) center of the dinuclear active species
is oxidized to Ni(II) by aerial O2 and regenerates the active
intermediate species [(NiIIL)Mn(3,5-DTBC)]+, with the
formation of H2O2 as the byproduct. The probable mechanism
for catalytic oxidation of 3,5-H2DTBC to 3,5-DTBQ by
complexes 2−4 is shown in Scheme 3.
Detailed kinetic studies of this phenoxazinone synthase like
activity were performed by following the reported method.23,25
The oxidative dimerization reaction of o-aminophenol (OAP)
to aminophenoxazinone (APX) is shown in Scheme 4. The
Scheme 4. Catalytic Oxidation of o-Aminophenol (OAP) to
Amino Phenoxazinone (APX) in Methanol
kinetic studies of this catalytic activity of complexes 2−4 were
also performed in methanol solvent under aerobic conditions.
We also checked this catalytic activity of complex 1, but it was
catalytically inactive.
The detailed kinetic study of this dimerization reaction was
performed by following the above-mentioned procedure as for
Phenoxazinone Synthase Studies and Kinetics. De-
tailed Description. To determine the catalytic activities of
heterometallic complexes 2−4 toward another oxidase
reaction, we have studied phenoxazinone synthase like activity.
J
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Figure 11. Increase in the amino phenoxazinone band at around 425 nm after the addition of o-aminophenol (1 × 10−2 M) to the methanolic
solution of complex 4 (2.5 × 10−5 M) (left) and plot of the rate versus substrate concentration (right). Inset shows the corresponding Lineweaver−
Burk plot of 4. The UV-spectra were recorded at 5 m intervals.
catecholase-like study and using the same concentration of
complexes 1−4 and o-aminophenol in methanol solvent. Here
also, complex 1 does not show any catalytic activity (Figure
S23). The change of the spectral behavior of complex 4 during
wavelength scan in the presence of OAP is shown in Figure 11
(left) and for 2−3 are depicted in Figures S24 and S25 (left).
Both the plots of the observed rate versus [substrate] and the
Lineweaver−Burk plot for complex 4 are shown in Figure 11
(right), and those for 2−3 are depicted in Figures S24 and S25
(right). The turnover numbers (kcat in h−1) of this catalysis are
3240, 3360, and 13248 for complexes 2−4, respectively. The
kinetic study parameters are listed in Table S5. These kcat
values for complexes 2 and 3 are comparable with those of our
earlier reported CuII−MnII complexes, and for 4, the value is
the highest ever for any reported homo- or heterometallic
complexes. Here also the kinetic study was carried out by
varying the catalyst concentration and the rate was found to
vary linearly with catalyst concentration. The plot of
experimental rate versus [catalyst] for complex 4 is shown in
Figure 12, and those for 2−3 are shown in Figures S26−S27.
ESI-Mass Spectrometric Study. To investigate the mecha-
nistic insight of phenoxazinone synthase like activity and
complex-substrate intermediate during the oxidative dimeriza-
tion reaction of o-aminophenol to phenoxazinone, we have
recorded ESI-MS spectra of complexes 2−4 in a 1:1 mixture
(v/v) of the complexes and o-aminophenol in methanol
solvent within 5 min of mixing (Figures S28−S31). The
spectra of the mixture of complexes 2−4 and o-aminophenol
show a base peak at m/z = 383.10 (calcd 383.09) (Figures
S28−S31) and another three common peaks at m/z = 405.08
(calcd 405.07), m/z = 765.18 (calcd 765.17), and m/z =
787.16 (calcd 787.16) (Figures S28−S31). All four of these
peaks are previously observed in the spectra of complexes 2−4.
In addition, two low intensity peaks are observed at m/z =
652.13 (calcd 652.10) and m/z = 649.13 (calcd 649.08), which
can be assignable to dinuclear species [(NiL)Mn(OAP)2+H]+
and [(NiIL)Mn(APX)]+, respectively (Figures S28−S31).
Along with these, another very low intensity peak is observed
at m/z = 469.05 (calcd 469.05), which seems to be due to the
presence of methanol coordinated dinuclear species [(NiIL)-
Mn(CH3OH)]+ in the mixture (Figure S31). In summary, the
peak of intermediate [(NiL)Mn(OAP)2+H]+ indicates the
appearance of complex-substrate intermediate in solution
during the oxidative conversion of o-aminophenol to
phenoxazinone, and the peak of [(NiIL)Mn(APX)]+ species
also indicates the formation of product (APX) during the
reaction catalyzed by complexes 2−4. We also found that no
H2O2 was formed during this oxidative dimerization reaction.
Mechanistic Insight. The mechanism of phenoxazinone
synthase like activity of some homometallic copper(II),
cobalt(II/III), and manganese(II/III) complexes are reported
in literature, where o-aminophenol is converted to phenox-
azinone.37,39,42 Most of the reported complexes used for this
catalysis are homometallic. Recently, we have reported some
Cu(II)−Mn(II) complexes, which show such catalytic
activity.23,25 The investigation on mechanism of this catalysis
using these Cu(II)−Mn(II) complexes reveal that the catalysis
occurred via a radical mechanism with the formation of o-
aminophenolate intermediate with the reduction of Cu(II) to
Cu(I). During this reaction dioxygen is reduced to H2O with
the formation of phenoxazinone. It was noted that the
Figure 12. Rate versus catalyst concentration plot of complex 4 for
the oxidation reaction of o-aminophenol (10−2 M) in methanol.
K
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Scheme 5. Proposed Mechanism for Catalytic Oxidation of OAP to APX by Complexes 2−4
participation of metal ions is very important in this oxidative
dimerization of o-aminophenol to phenoxazinone.
in aerobic condition are reported. Recently, we reported some
Cu(II)−Mn(II) complexes derived from asymmetrically
dicondensed tetradentate Schiff base ligands. These complexes
showed catecholase and/or phenoxazinone synthase like
activities with very high catalytic efficiencies.23,25 It is observed
that the catalytic efficiencies as well as catalytic promiscuity of
these complexes is very much dependent on the coordination
environment around the Mn(II) center. The presence of
coordinated solvent molecules and the coanions in the
complexes influence the catalytic activities considerably.
Earlier, we reported some Ni(II)−Mn(II) complexes which
showed catecholase-like activities, but the efficiency was very
low.75 The reason for the low activity probably lies in the fact
that, in those complexes Mn(II) centers are hexa-coordinated
without any coordinated solvent molecule. On the contrary,
among the present complexes, the Mn(II) centers are penta-
coordinated in 2 and 4 or hexa-coordinated with a solvent
molecule (in 4). Hence, their catalytic activities are very high.
Moreover, mass spectral data reveal that for both the reactions
the active catalytic species is a (NiL)Mn2+ unit which binds the
substrates. This not only explains the high catalytic activity of
complex 3 but also gives a rough idea about their kcat values
(i.e., for catecholase like activity); the considerably lower kcat of
2 compared to values of 3 or 4 may be related to the number
of (NiL)Mn2+ units generated per molecule (one for 2 and two
for 3 and 4). However, the relationship is not totally
straightforward as several other factors (e.g., rate of formation
of the active species from the molecules, ease of coordination
of the substrate to the metal centers, etc.) should also play
On the basis of ESI-mass spectra, we propose a probable
mechanism for this catalytic conversion reaction. First, a
neutral dinuclear species [(NiL)Mn(OAP)2] is formed on
addition of o-aminophenol to the solution of each of
complexes 2−4, in which two hydroxyl groups are coordinated
to the Mn(II) center (Scheme 5). In the next step, two
deprotonated o-aminophenols release protons and form an o-
iminophenolate/o-aminophenolate radical. During this proc-
ess, 1/2 O2 is reduced to H2O. Then, two radicals dimerize to
form amino-phenoxazinone and reduce Ni(II) to Ni(I) with
the formation of another H2O molecule. The product (APX) is
then eliminated from the dinuclear species [(NiIL)Mn]+ and a
O2 molecule is coordinated to the Ni(I) center. This Ni(I)
center further oxidizes to Ni(II) with reduction of 1/2 O2 to
H2O. After that the dinuclear species [(NiIIL)Mn]2+ enters
into the next cycle of catalysis. The existence and relative
stability of these intermediate species are evident by the mass
spectra (Figures S30 and S31). During this total catalytic
conversion process, 3H2O molecules are formed as byproduct
by the reduction of 3/2O2. The possible mechanism of this
catalytic conversion reaction is shown in Scheme 5.
Structure−Activity Correlation for the Observed Relative
Performance of the Catalysts. The homo- and heterogeneous
catalysis by heterometallic complexes with combinations of
different metal ions is one of the interesting topics of research
in recent times.76−82 Until now only a very few heterometallic
catalysts showing homogeneous catalytic oxidase type activities
L
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Accession Codes
important roles. The interplay of various factors is evident in
the highest ever kcat value of 4 for phenoxazine synthase like
activity among all the model complexes synthesized so far.
CCDC 1836634−1836637 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSIONS
■
In order to develop the chemistry of heterometallic
compounds by metalloligand approach, we synthesized a new
mononuclear complex [NiL] (1) with the N2O3 donor
unsymmetrical Schiff base ligands, N-α-methylsalicylidene-N′-
3-methoxysalicylidene-1,3-propanediamine, and used it to
prepare Ni(II)−Mn(II) complexes. Complex 1 on reaction
with Mn(II) perchlorate and N3− in different ratios produced a
trinuclear complex [(NiL)2Mn(N3)](ClO4) (2), a tetranuclear
complex [(NiL)2Mn2(N3)2(μ1,1-N3)2(CH3OH)2] (3), and a
hexanuclear complex [{(NiL)2Mn}2(μ1,3-N3)(H2O)]·
(CH3OH),(ClO4)3 (4). Complex 2 possesses a commonly
found bent trinuclear structure, but unlike those structures, the
Mn(II) is penta-coordinated here. Both the tetranuclear
structure of 3, which consisted of joining of two dinuclear
(NiL)Mn units via both phenoxido and μ1,1-azido bridges, and
the hexanuclear structure of 4, formed by connecting two
trinuclear (NiL)2Mn units by one μ1,3-azido bridge are
unprecedented. The variable coordination modes of the azide
ions and of the unsymmetrical ligand along with the variable
coordination numbers of both Ni(II) and Mn(II) enable the
formation of such diverse structures. In 3, both the Ni(II) and
Mn(II) ions are paramagnetic, and fitting of the data of
variable temperature magnetic measurements yielded three
different coupling constants (J) commensurate with three
types of connectivity between these paramagnetic metal
centers. The antiferromagnetic nature and magnitude of the
coupling constants are as expected from the bridging angles. In
4, the only antiferromagnetic exchange coupling is due to the
μ1,3-N3 bridged Mn(II) centers. Complexes 2−4 show both
catecholase and phenoxazinone synthase like activities. The kcat
values of these processes, especially those of phenoxazinone
synthase like activities, are one of the highest among the
synthetic model complexes. Mass spectral analyses reveal that
for both the reactions the active catalytic species is (NiL)Mn2+
which binds the substrates. Three donor O atoms of the
metalloligand derived from the unsymmetrical N2O3 donor
Schiff base seem to play a very important role in stabilizing the
active species. The differences in the kcat values of the
complexes may be related to the number of (NiL)Mn2+ units
produced per molecule. Moreover, the Mn(II) centers in these
complexes are coordinately unsaturated penta-coordinated or
hexa-coordinated with a solvent molecule, both of which
facilitate the coordination of substrate to the Mn(II) center
and thus have influence on catalytic efficiency.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ghosh_59@yahoo.com.
ORCID
Ashutosh Ghosh: 0000-0003-2026-7565
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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 measure-
ments. 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. We are thankful to
Dibyasree Choudhury and Prof. Susanta Lahiri, Saha Institute
of Nuclear Physics, Kolkata, India for ICP-OES measurements.
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