Roles of basicity and steric crowding of anionic coligands in catechol oxidase-like activity of Cu(ii)-Mn(ii) complexes

Article abstract of DOI:10.1039/d0dt00952k

Five new heterometallic Cu(ii)-Mn(ii) discrete trinuclear complexes, [(CuL)2Mn(CH3COO)2] (1), [(CuL)2Mn(NO3)2] (2), [(CuL)2Mn(C6H5COO)(H2O)]Cl (3), [(CuL)2Mn((p-OH)C6H5COO)(H2O)]ClO4 (4) and [(CuL)2Mn(HCOO)(H2O)]ClO4 (5), have been synthesized using a met

Full text of DOI:10.1039/d0dt00952k

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Bhunia, J. Mayans, M. Drew and A. Ghosh, Dalton Trans., 2020, DOI: 10.1039/D0DT00952K.
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Douglas W. Stephen et al.
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
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Roles of basicity and steric crowding of anionic coligandsDOI:
i
1
n
0.103
c
9/
a
D0
t
D
e
T
c
00
hol
95
2K
oxidase like activity of Cu(II)-Mn(II) Complexes
a
a
b
c
a,
Sabarni Dutta , Pradip Bhunia , Júlia Mayans , Michael G. B. Drew , Ashutosh Ghosh *
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road,
Kolkata 700 009, India, E-mail address: ghosh_59@yahoo.com
bDepartament de Química Inorgànica I Orgànica, Secció Inorgànica and Institut de Nanosciència I
2
Nanotecnologia (IN UB). Martíi Franqués 1-11. 08028, Barcelona, Spain.
cSchool of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK.
Abstract
Five new heterometallic Cu(II)-Mn(II) discrete trinuclear complexes, [(CuL) Mn(CH COO) ] (1),
2
3
2
[(CuL) Mn(NO ) ] (2), [(CuL) Mn(C H COO)(H O)]Cl (3), [(CuL) Mn((p-OH)C H COO)(H O)]ClO
2 3 2 2 6 5 2 2 6 5 2 4
(
4) and [(CuL) Mn(HCOO)(H O)]ClO (5) have been synthesized using a metalloligand, CuL derived
2
2
4
from an N O donor Schiff base, H L (N,N'-bis(α-methylsalicylidene)-1,3-propanediamine). Single-
2
2
2
crystal structural analyses reveal that all five complexes have a common [(CuL) Mn] core, where two
2
terminal metalloligands, CuL are connected to the central metal ion, Mn(II) via double phenoxido
bridges. Among the complexes, 1 and 2 possess linear structures where the terminal Cu(II) atoms are
bridged to the central Mn(II) atoms by acetate and nitrate ions, respectively along with the double
phenoxido bridges whereas 3, 4 and 5 have bent structures in which the respective anionic co-ligands,
benzoate, p-hydroxybenzoate and formate ions are coordinated only to central Mn(II) in monodentate
fashion along with a water molecule that complete its hexa-coordinated geometry. Among the
complexes, 1, 3, 4 and 5 show quite high bio-mimicking catecholase like activity for the aerial oxidation
of 3,5-di-tert-butylcatechol with turnover numbers (k ) of 139 h , 439 h , 348 h and 730 h−1,
−
1
−1
−1
cat
respectively whereas complex 2 is practically inactive towards this reaction. The presence of the
coordinated water molecule to Mn(II) in the bent complexes, 3−5, appears to be responsible for their
high catalytic activity and the difference in their activity may be attributed to steric crowding due to the
anionic co-ligand whereas the inactivity of 2 seems to be associated with the low basicity of the nitrate
ion. The temperature-dependent dc molar magnetic susceptibility measurements reveal that complexes
−
1
1
−5 are antiferromagnetically coupled with the exchange coupling constants (J) = –8.54 cm , –11.50
1

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−
1
−1
−1
−1
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DOI: 10.1039/D0DT00952K
cm , –19.83 cm , –10.65cm and –10.27 cm for 1, 2, 3, 4 and 5 respectively as is expected from the
Cu−O−Mn bridging angles.
Introduction
Synthesis and characteristic study of heterometallic complexes with N, O donor ligands have been a
point of interest in the field of coordination chemistry for a long time. This is predominantly because of
1
their potential applications in catalytic, magnetic, gas storing and sensing activities. A plethora of
heterometallic Schiff base complexes has been reported in the past with various types of N, O donor
ligands; of which Robson-type macrocycles, bi-compartmental N O donor ligands or mono-
2
4
2
compartmental N O donor ligands deserve special mention. The easy synthetic pathways, involving
2
2
generally one-pot condensation of a diamine with two salicylaldehyde derivatives and their exceptional
complexing ability, boost the popularity of such Schiff base ligands. Recently many Cu(II) complexes of
N O donor Schiff base ligands have been used as ‘metalloligands’ to react with various 3d/4f metal
2
2
3
salts for the synthesis of heterometallic complexes. The anionic part of such metal salts not only
balance the charge of the resultant complex but also plays a crucial role in modulating the nuclearity and
structures of these heterometallic complexes via different modes of coordination to the metal centres.4
These anion dependent structural variations are very important in understanding the intriguing structural
factors that govern the magnetic properties and catalytic activities of the complexes.5
The magnetic coupling of oxo bridged homometallic complexes have been studied extensively and the
6
magneto-structural correlations of these complexes are now well-established. However, such
correlations for 3d-3d' hetero-metallic complexes are not that well documented as these complexes are
7
relatively rare. The N O donor Schiff bases have been used frequently for synthesis of heterometallic
2
4
complexes but these rigid ligands allow only little variations in the bridging angle. On the other hand,
the trinuclear complexes of N O donor Schiff base ligands, synthesised by metalloligand approach are
2
2
quite flexible and the bridging angles in these complexes can be varied in a wide range by changing the
4
anionic co-ligands. Moreover, heterometallic coordination complexes can show interesting magnetic
properties since the combination of different spins in a single cluster generates more magnetic
anisotropy.8
One of the most promising applications of these heterometallic complexes is in the field of catalysis.
Homometallic coordination complexes of 3d metal ions, Cu(II), Ni(II), Co(II/III), Mn(II/III), and
9
Fe(II/III) were used as model catalysts for oxidase type reactions. Among them the catalytic properties
of Cu(II) complexes were studied extensively since many naturally occurring metalloproteins like
2

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cytochrome c oxidase, catechol oxidase, phenoxazinone synthase etc., have copper ions in their active
DOI: 10.1039/D0DT00952K
1
0
sites. However, the catalytic efficiency of these model complexes is significantly less than that of the
native biological enzymes. Therefore, improvement of the catalytic efficiencies of these complexes is a
great challenge and with this aim several modifications were made in the coordination environment of
the closely knit metal ions. Recently, it is found that heterometallic complexes containing Mn(II) ions
1
1
catalyse such oxidation reactions. It has also been observed that a coordinated solvent molecule at the
Mn(II) center considerably enhances the catalytic oxidase activity in comparison to a Mn(II) centre
without the coordinated solvent. Detailed mechanistic study suggested that the presence of labile solvent
molecule enhance substrate-catalyst binding as it can be replaced easily by the substrate, which in turn
increases the efficiency of the catalyst. However, till date no study has been performed to investigate
whether the anionic coligands which are invariably a part of this kind of trinuclear complexes, have any
effect on the catalytic efficiencies of these complexes.
Herein, we report the synthesis of a series of five complexes, [(CuL) Mn(CH COO) ] (1),
2
3
2
[(CuL) Mn(NO ) ] (2), [(CuL) Mn(C H COO)(H O)]Cl (3), [(CuL) Mn((p-OH)C H COO)(H O)]ClO
2 3 2 2 6 5 2 2 6 5 2 4
(
4) and [(CuL) Mn(HCOO)(H O)]ClO (5) and their characterization by single crystal X-ray diffraction.
2
2
4
Complexes 1 and 2 are isostructural having linear geometry being bridged by acetate/nitrate co-anions
while complexes 3, 4 and 5 possess bent structures with a water molecule coordinated to the central
Mn(II) atom. The temperature dependent dc magnetic susceptibility measurements reveal
antiferromagnetic interactions between the Cu(II) and Mn(II) centers in all five complexes. The coupling
constants of the complexes fit well in the magnetostructural correlations of similar phenoxido bridged
Cu(II)-Mn(II) complexes. Apart from complex 2, all the other complexes show catalytic activities
towards catechol oxidase activities. The difference in catalytic activity of the isostructural complexes 1
and 2 has been explained in terms of the basicity of the respective anionic co-ligands acetate and nitrate.
However the prominent differences in the kcat values of 3−5 can be correlated to the effect of steric
crowding on catalytic activities. ESI-mass spectra have been used to get an idea about the probable
intermediates of these catalytic oxidation reactions and thus a possible mechanism is proposed.
Experimental Section
Starting materials
2
-hydroxyacetophenone, 1,3-propanediamine and p-hydroxybenzoic acid were purchased from
Spectrochem India and formic and benzoic acid from Lancaster Synthesis. All the chemicals were of
reagent grade and commercially available. They were used for reactions without further purification.
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Caution! Although not encountered during experiment, perchlorate salts of metal complexes with
DOI: 10.1039/D0DT00952K
organic ligands are potentially explosive. Only a small amount of material should be prepared and it
should be handled with care.
Synthesis of Schiff base ligand (H L) and the “metalloligand” (CuL)
2
The Schiff base ligand, H L and its corresponding copper metalloligand (CuL) have been prepared
2
following the reported method.12 To account briefly, a 20 mL methanolic solution of 2-
hydroxyacetophenone (10 mmol, 1.206 mL) and 1,3-propanediammine (5 mmol, 0.420 mL) was
refluxed for 2 h. The yellow coloured ligand solution so obtained was cooled, and a methanolic solution
of Cu(ClO ) ·6H O (1.852 g, 5 mmol, 10 mL) was added to it with stirring. Triethylamine (1.4 mL, 10
4
2
2
mmol) was then added to this solution and stirring was continued for 20 min. The brown solution thus
obtained was filtered and kept in an open atmosphere for slow evaporation of the solvent. Brownish
green crystals of the precursor metalloligand were collected by filtration after two days.
Synthesis of the complexes [(CuL) Mn(CH COO) ] (1) and [(CuL) Mn(NO ) ] (2)
2
3
2
2
3 2 2
Methanolic solutions of Mn(CH COO) ·4H O (0.245 g, 1 mmol) and Mn(NO ) ·4H O (0.251 g, 2 mL,
3
2
2
3 2
2
1
mmol) were mixed with methanolic solutions of the metalloligand, CuL (0.772 g, 5 mL, 2 mmol) for
complexes 1 and 2 respectively. The mixtures were well stirred and then filtered. Dark green coloured
filtered solutions were carefully layered with diethylether in long tubes for slow diffusion. Deep green
rod shaped crystals of 1 and light green needle shaped crystals of 2 appeared at the junction of the two
solutions within a few days.
Complex 1: Yield 0.688 g (75%). C H Cu MnN O (916.87). Calculated: C, 54.90; H, 5.27; N, 6.10;
4
2
46
2
4
8
–
Cu, 13.83; Mn, 5.98; found: C, 54.87; H, 5.30; N, 6.12; Cu, 13.68; Mn, 5.82. IR (KBr): νs+as(COO ) =
–
1
–1
1
583, 1541, 1383 cm , ν(C=N) = 1599 cm .
Complex 2: Yield 1.351 g (68%) (with respect to CuL). C H Cu Mn N O (1986.83). Calculated:
8
2
101
4
2
12 23
C, 49.46; H, 4.37; N, 9.11; Cu, 13.77; Mn, 5.95 found: C, 49.42; H, 4.40; N, 9.09; Cu, 13.91; Mn, 5.78.

–1
–1
IR (KBr): ν(NO ) = 1293 cm , ν(C=N) = 1599 cm .
3
Synthesis of the complex [(CuL) Mn(C H COO)(H O)]Cl (3)
2
6
5
2
To a methanolic solution of CuL (0.772 g, 5 mL, 2 mmol), a methanolic solution of MnCl ·4H O (0.197
2
2
g, 1 mmol) was added and stirred well. A methanolic solution of benzoic acid (0.122 g, 1 mmol, 5 mL)
was added to this solution followed by triethylamine (138 µL, 1 mmol) and mixed well by stirring. The
4

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resulting solution was layered with diethylether carefully in a long layer tube. Light green X-ray quality
DOI: 10.1039/D0DT00952K
single crystals started to appear at the junction of the two solutions within two days. We could not
prepare X-ray quality single crystals using Mn(ClO ) ·6H O salt hence we used MnCl ·4H O instead.
4
2
2
2
2
Complex 3: Yield 1.469 g (75%) (with respect to CuL). C H Cl Cu Mn N O (1958.68). Calculated:
9
0
90
2
4
2
8
15
C, 59.88; H, 5.03; N, 5.37; Cu, 13.55; Mn, 5.86; found: C, 59.86; H, 4.97; N, 5.35; Cu, 13.43; Mn, 5.71.
–
–1
–1
IR (KBr): νs+as(COO ) = 1579, 1540, 1398 cm , ν(C=N) = 1602 cm .
Synthesis
of
the
complexes
[(CuL) Mn((p-OH)C H COO)(H O)]ClO
4
(4)
and
2
6
5
2
[
(CuL) Mn(HCOO)(H O)]ClO (5)
2
2
4
In case of complexes 4 and 5, same procedure as reported for complex 3 was followed except for using
solid Mn(ClO ) ·6H O (0.330 g, 1 mmol) instead of MnCl ·4H O. A heterometallic Cu(II)-Mn(II)
4
2
2
2
2
solution was prepared as before and separate solutions of p-hydroxybenzoic acid for 4 (0.138 g, 1 mmol,
mL) and formic acid for 5, in methanol (0.046 g, 1 mmol, 5 mL) were taken and deprotonised using
5
triethylamine (138 µL, 1 mmol). The heterometallic Cu(II)-Mn(II) solutions and the acid solutions were
then mixed and filtered. Finally the deep green solutions were layered with diethylether in long tubes for
slow diffusion. Light green and deep green X-Ray quality single crystals of complexes 4 and 5
respectively appeared at the junction of two solutions after a few days.
Complex 4: Yield 0.782 g (71%) (with respect to CuL). C H ClCu MnN O (1101.36). Calculated:
4
5
47
2
4
15
C, 56.60; H, 5.07; N, 5.87; Cu, 13.32; Mn, 5.76; found: C, 56.64; H, 5.10; N, 5.85; Cu, 13.54; Mn, 5.92.
–
–1
–1
IR (KBr): νs+as(COO ) = 1583, 1543, 1397 cm , ν(C=N) = 1599 cm .
Complex 5: Yield 0.586 g (61%) (with respect to CuL). C H ClCu MnN O (961.26). Calculated: C,
3
9
43
2
4
11
4
8.73; H, 4.51; N, 5.83; Cu, 14.75; Mn, 6.37; found: C, 48.74; H, 4.53; N, 5.85; Cu, 14.53; Mn, 6.58. IR
–
–1
–1
(
KBr): νs+as(COO ) = 1580, 1539, 1384 cm , ν(C=N) = 1598 cm .
Physical measurements
Elemental analyses (C, H and N) were performed using a PerkinElmer 2400 series II CHN analyzer. IR

1
spectra in KBr pellets (4000–500 cm ) were recorded using a PerkinElmer RXI FT-IR
spectrophotometer. Electronic spectra were measured in a Hitachi U-3501 spectrophotometer for
estimation of Cu(II)13a and Mn(II) , employing the 'neo-cuproin' and periodate methods, respectively.
Solid-state, variable-temperature and variable-field magnetic data were collected on powdered samples
using a MPMS5 Quantum Design magnetometer operating at 0.03 T in the 300 to 2.0 K range for the
13b
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magnetic susceptibility and at 2.0 K in the 0 to 5 T range for the magnetization measurements.
DOI: 10.1039/D0DT00952K
Diamagnetic corrections were applied to the observed susceptibilities using Pascal’s constants.
X-ray Crystallography
1
4
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. Structure 3 was disordered over a two-fold axis and treated appropriately. The
trinuclear moieties in the other structures were ordered although perchlorate anions in 4 and 5 were
disordered and solvent molecules were given reduced occupancy. All structures were refined using
1
5
2
Shelxl16-6 on F . Details of the crystallographic data of complexes 1−5 are summarized in Table 1.
CCDC reference numbers are 1990112−1990116 for complexes 1−5 respectively.
Electrochemical measurements.
Electrochemical studies of all five complexes 1–5 were performed using a Basi-Epsilon C3 cell
−
1
instrument at a scan rate of 100 mV s within the potential range of 0 to −1.60 V vs. Ag/AgCl and that
of CuL at a range of 0 to −1.40 V. Cyclic voltammograms were carried out using 0.1 M TBAP as the
supporting electrolyte and 1 × 10−3 M complexes in acetonitrile solution which were deoxygenated by
2
argon purging. The 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 the counter electrode. All
experiments were performed in standard electrochemical cells at 25 °C.
Table 1. Crystal data and structure refinement of complexes 1, 2, 3, 4 and 5.
Complex
1
2
3
4
5
Formula
C H Cu MnN C H
Cu Mn C H ClCu
C H ClCu Mn C H ClCu Mn
45 53 2 40 48 2
4
2
46
2
4
41 50.50
2
45 48
2
O8
916.87
N O
MnN O
N O
N O
4 12
6
11.50
4
7.50
4
15
Formula
Weight
1986.83
triclinic
6
1958.68
1101.36
995.30
Crystal
System
monoclinic
monoclinic
monoclinic
monoclinic

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Cc
DOI: 10.1039/D0DT00952K
Space group
P2 /c
푃1
C2/c
P2 /c
1
1
a, Å
b, Å
c, Å
10.3064(15)
11.5974(17)
16.442(2)
90
11.0088(6)
11.5219(7)
19.5333(12)
94.180(2)
94.333(2)
117.861(2)
2167.7(2)
1
20.3123(6)
14.2566(4)
16.4904(5)
90
15.6553(19)
16.993(2)
19.634(2)
90
24.514(2)
10.5637(9)
15.9325(14)
90
α, deg
β, deg
γ, deg
V, Å3
Z
93.429(3)
90
95.126(2)
90
110.755(5)
90
95.259(3)
90
1961.8(5)
2
4756.3(2)
2
4884.3(10)
4
4108.5(6)
4
-3
ρ (g cm )
1.552
1.522
1.368
1.498
1.606
calc
µ (Mo Kα)
1.451
1.326
1.255
1.242
1.462
-
1
(
mm )
F(000)
946
1027
2012
2260
2048
Rint
0.058
43264
0.072
51388
0.066
26560
0.139
81565
0.061
19631
Total no. of
reflns
No.
of
3496
3192
7943
6447
4136
2879
9355
5060
7062
6396
unique
reflns
No. of rflns
with I>
2
σ(I)
a
b
R1, wR2
0.0311, 0.0825 0.0649, 0.1736 0.0631, 0.2286 0.0792, 0.2523 0.0486, 0.1247
7

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DOI: 10.1039/D0DT00952K
c
2
GOF on F
1.11
1.12
1.09
1.05
1.019
R1(all)
0.0359
296
0.0815
295
0.0932
296
0.1646
295
0.0558
100
Temp, K
Residual
electron
density,
Å−3
-0.69, 0.27
-0.58, 1.32
-0.38, 1.01
-0.56, 0.91
-0.73, 0.87
e
a
b
2
2
2 2
4 ½
c
2
2 2
R = ||F | − |F |||F |, wR (F ) = [[w(F − F ) w F ] and GOF = [[w(F − F ) (N –
1
o
c
o
2
o
o
c
o
o
c
obs
Nparams)]½.
Catalytic Oxidation of 3,5-DTBC.
The catalytic properties of bio-mimicking heterometallic complexes 1−5 were examined towards
catechol oxidase activity. The catecholase activity involves oxidation of 3,5-di-tert-butylcatechol (3,5-
DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ). To study this oxidation process, equal volumes of 2 x
−
5
−3
1
0 M acetonitrile solutions of the complexes were treated with 2 x 10 M solutions of 3,5-DTBC at
room temperature and absorbance vs. wavelength scan of the solutions at a range of 300−500 nm were
recorded at a regular interval of 3 min for half an hour. 3,5-DTBQ shows a broad absorption peak at 398
nm. The increase of absorbance maxima at 398 nm was also monitored with respect to time (time scan).
The formation of hydrogen peroxide as a byproduct of this oxidation process was also detected by
iodometric methods as reported previously.16
Results and discussion:
Syntheses of the complexes
The tetradentate N O donor Schiff base ligand, H L was prepared by 1:2 dicondensation of 1,3-
2
2
2
propanediamine with 2-hydroxyacetophenone and its Cu(II) complex was prepared as reported earlier.17
This complex (CuL) has been used as metalloligand for the synthesis of five trinuclear Cu(II)-Mn(II)
complexes as shown in Scheme 1. For complexes 1 and 2, the metalloligand was reacted with
Mn(CH COO) ·4H O and Mn(NO ) ·4H O, respectively in 2:1 ratio. For preparation of complexes 3
3
2
2
3 2
2
and 4, 5, the metalloligand was first reacted with MnCl ·4H O and Mn(ClO )·6H O respectively in 2:1
2
2
4
2
ratio and then these mixtures were treated with corresponding deprotonated acid solution i.e., benzoic
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acid for 3, p-hydroxybenzoic acid for 4 and formic acid for 5 in 1:1 molar ratio. The difference in the
DOI: 10.1039/D0DT00952K
synthetic procedure results in an interesting difference in the structures and composition of synthesized
complexes. In complexes 1 and 2, two same anionic co-ligands are present per trinuclear unit and both
act as bridges between the terminal and central metal ion resulting a linear trinuclear structure; no
solvent molecule is coordinated to any of the metal ions. On the other hand, in 3, 4 and 5 two different
type of anions are present; one of them i.e., the carboxylate ion coordinates only to the Mn(II) centre in
monodentate mode forming a bent trinuclear structure. A solvent molecule is coordinated to Mn(II) to
complete its hexacoordination geometry in each compound. These differences are very important for
catecholase like activity of the complexes.
Scheme 1. Syntheses of complexes 1–5.
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DOI: 10.1039/D0DT00952K
IR spectra of complexes 1–5
All five complexes show the characteristic stretching vibrations for the azomethine (C=N) group around
−
1 18
−1
19
1
600 cm . A sharp peak at 1293 cm appears due to the presence of nitrate ion in complex 2. Two
−
1
−1
sharp peaks at around 1540 cm and 1398 cm appear for the symmetric and asymmetric stretching for
¯
18a
COO in complexes 1, 3, 4 and 5. Moreover, complexes 4 and 5 display a couple of strong intensity
band at ca. 1101 and 1094 cm−1 respectively, confirming the presence of ionic perchlorate18b in them.
The IR spectral data for the complexes for the complexes are given in the supporting information (Fig.
S1−S5, ESI respectively).
Description of the structures
All five structures were found to contain two CuL metalloligands that are bridged by a manganese atom
to provide a MnCu L core. In 1 and 2, the Mn atom occupied a centre of symmetry in an ordered
2
2
structure. In 3, the central Mn atom was imposed on a two-fold axis with a disordered six-coordinate
environment. In 4 and 5, no crystallographic symmetry was imposed on the structures.
Fig. 1. The structure of 1 with ellipsoids at 20% probability. The Mn atom occupies a centre of
symmetry.
The structure of 1 is shown in Fig. 1 with dimensions in Table S1, ESI and contains two metalloligands
which are bridged by a Mn atom occupying a centre of symmetry. In the metalloligands the copper
atoms are bonded to the O N donor atoms of the ligand with distances to O 1.976(2), 1.924(2) Å and to
2
2
N 1.977(2), 2.001(2) Å. Here the Mn atom bridges the two metalloligands by bonding to the four
oxygen atoms at distances of 2.195(2), 2.199(2) Å in an equatorial plane. A second oxygen atom O(41)
1
0

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of these anions is bonded to the metalloligands via the copper atom with Cu-O(41), 2.218(2) Å thus
DOI: 10.1039/D0DT00952K
completing a five-coordinate square pyramidal arrangement around the copper atom. The four donor
atoms in the equatorial plane around the metal show an r.m.s. deviation of 0.093 Å with the copper atom
0
.238(1) Å from the plane in the direction of the axial atom O(41). The Mn atom is six-co-ordinate with
an octahedral geometry in which two monodentate acetate anions occupy axial positions via O(43) at
distances of 2.127(2) Å.
There are two independent molecules in the structure of 2 called A and B with similar structures as
shown in Fig. 2. As in 1, the structures contain the two metalloligands bridged by a manganese atom
occupying a centre of symmetry. Here the anion is nitrate. Also the nitrate bridges the copper and
manganese atoms, with distances Mn(1)-O(43) 2.196(4), Cu(1)-O(41) 2.414(5) Å in 2A and 2.192(4),
2
.465(5) Å in 2B.
The dimensions involving the metals with the macrocycle are as expected (Table S1, ESI). The copper
atoms have square pyramidal environments with the metal atoms 0.114(2), 0.088(2) Å above the
equatorial plane in which the four donor atoms show r.m.s. deviations of 0.070, 0.040 Å respectively.
The weak axial interaction is consistent with the small deviation of the copper atoms from the plane.
There are three solvent methanol molecules in the asymmetric unit, all refined with 50% occupancy.
Dimensions of hydrogen bonding interactions in 2 are given in Table S4.
Fig. 2. The structures of 2A and 2B with ellipsoids at 20% probability. Solvent methanols are not shown
In the structure of 3, shown in Fig. 3, which contains a two-fold axis coordination of the two
metalloligands remains the same as in complexes 1 and 2 with the copper atoms bonded to the four
donor atoms of the macrocycle. However there are no axial atoms and the copper atoms remain four-co-
ordinate with square planar environments. Dimensions are listed in Table S2.The copper atoms are
1
1

Dalton Transactions
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DOI: 10.1039/D0DT00952K
0
.039(3) Å from the plane of their four donor atoms which show a r.m.s. deviation of 0.201 Å. Thus the
o
two MnO moieties from the different metallocycles intersect at 74.9(1) and the MnO Cu rings are
2
2
o
folded with angles of 32.6(2) between MnO and CuO moieties. The two CuO N planes intersect at
2
2
2
2
o
2
6.2(1) . The structure of 3 has crystallographically imposed C2 symmetry and as a result the two
monodentate ligands, water and benzoate bonded to the manganese atom are disordered. To fit with the
imposed symmetry, each monodentate site was refined with 50% occupancy of the two monodentate
ligands. The asymmetric unit also contains two water molecules refined with occupancies of 0.5 and
0
.25. The Mn-O distances are, Mn-O(11) 2.109(4) Å, Mn-O(31) 2.289(4) Å and Mn-O(41) 2.045(14) Å.
The positively charged trinuclear complex is balanced with a chloride which is disordered over two
positions, refined with occupancies 0.375 and 0.125.
Fig. 3. The structure of one molecule of 3 with ellipsoids at 20 % probability. The hydrogen atoms on
the water molecule O1w were not located and are not shown. Only one of the two possible ordered
molecules is shown. Disordered chloride and solvent water molecules are also not shown.
By contrast the structure of 4 and 5 are very similar to 3 but contains no crystallographic symmetry and
is very different from those of 1 and 2, as is apparent from Fig. 4.
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2

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DOI: 10.1039/D0DT00952K
Fig. 4. The structures of 4 (left) and 5 (right) with ellipsoids at 20% probability. In both structures
disordered perchlorate ions is not shown nor are solvent water molecules in 4. Hydrogen bonds are
shown as dotted lines.
Similar to 3, in 4 and 5 the copper atoms are bonded to four O N donor atoms of the ligand and have no
2
2
axial coordination, thus having a square planar geometry. The coordination of the two metalloligands
remains the same as in 1 and 2. The two structures have very similar dimensions which are compared in
Table S3. In 4, the copper atoms are 0.017(3), 0.030(3) Å from the planes with the four donor atoms
showing a slight tetrahedral distortion with r.m.s. deviations of 0.104, 0.072 Å. Equivalent dimensions
in 5 are 0.041(4), 0.021(4), 0.153, 0.095 Å respectively. The two square planes intersect at 26.9(1),
o
2
8.3(2) respectively in 4 and 5. As in 1 and 2, the four oxygen atoms from the two metalloligands
bridge to the manganese but now in a cis arrangement in which the angle between the planes
o
Mn(1),O(11),O(31) and Mn(1), O(41), O(61) is 77.9(2), 77.6(2) in 4 and 5 respectively compared to the
o
co-planarity found in 1 and 2. The MnO Cu rings are folded with angles of 32.4(2), 32.8(2) in 4 and
2
3
4.2(2), 50.2(3) in 5 between MnO and CuO moieties. In 4 the six-coordinate octahedral structure of
2 2
the manganese atom is completed by a water molecule and an oxygen atom from a benzoic acid ligand,
in mutually cis positions. The Mn-O distances to the oxygen atoms of the metalloligands show
significant variations with Mn-O(11) 2.326(6), Mn-O(31) 2.126(6), Mn-O(41) 2.273(5) and Mn-O(61)
2
.118(5) Å. The bond lengths involving the monodentate ligands are Mn(1)-O(71) at 2.079(5) Å, and
Mn-O(1W) at 2.164(6) Å. In 5 the equivalent dimensions are 2.346(6), 2.119(6), 2.234(6), 2.108(7) Å to
the oxygen atoms from the metalloligands and 2.108(7) Å to the formate oxygen O(71) and 2.183(7) Å
1
3

Dalton Transactions
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to the water molecule O(1). These two trinuclear complexes have a positive change which is balanced
DOI: 10.1039/D0DT00952K
by a perchlorate anion which in both structures is disordered and refined with two sets of four oxygen
atoms. In 4, there are in addition six solvent water molecules, all refined with 50% occupancy. The
water molecule bonded to the Mn forms donor hydrogen bonds to perchlorate oxygen as well as two of
these solvent water molecules. The hydrogen atom of the OH moiety on the benzoate anion also forms
a hydrogen bond to a solvent water molecule. In 5, there are hydrogen bonds between the unbonded
formate oxygen O(73) and the coordinated water molecule O(1) and also between O(1) and a methanol
solvent molecule. Dimensions of these hydrogen bonds in 4 and 5 are given in Table S5 and S6
respectively.
It is noticeable from Fig. 4 that the arrangement of the two metalloligands around the manganese atom
approximates to C2 symmetry in both structures although the symmetry is necessarily broken by the fact
that the two monodentate ligands bonded to Mn are different being a water molecule together with a
benzoate in 4 and a formate in 5.
Magnetic properties
The magnetic properties of complexes 1, 2, 3, 4 and 5 on powdered and pressed samples were
investigated in the 2–300 K temperature range and under a field of 0.3 T and plotted as  T vs. T plots

as shown in Fig. 5. The corresponding magnetization plots are included in ESI, Fig. S6.
Fig. 5. Representation of  T as a function of T for complexes 1 (black circles), 2 (red squares), 3 (blue

triangles), 4 (pink hexagons) and 5 (green rhombs). Solid lines represent the best fit for each product.
1
4

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Complexes 1–5 are trinuclear Cu(II)Mn(II)Cu(II) species, showing an overall
DOI: 10.1039/D0DT00952K
antiferromagnetic behavior. The experimental values of  T at room temperature are given in

3
-1
Table 2; all of them are in good agreement with the theoretical value of 5.125 cm ·mol ·K
corresponding to two Cu(II) and one Mn(II) non-interacting cations. All five complexes have a
similar magnetic response where the  T as a function of T curve continuously decays on

3
-1
lowering of temperature tending to values close to 1.75 cm ·mol ·K at 2 K as is expected for a S
3
-1
=
3/2 ground state (1.875 cm ·mol ·K) that confirms the antiferromagnetic interaction, Table 2.
Slight differences in the shape of the curve are noted for complex 3, suggesting larger
antiferromagnetic interactions.
Table 2. Magnetic parameters of complexes 1–5 extracted from the experimental data and the
fitting curves of  T vs. T plots.

3
-1
3
-1
J1 /cm-1
Complex
 T/cm ·mol ·K(300 K)
 T/cm ·mol ·K(2 K)
g


1
2
3
5.06
5.04
4.48
1.82
1.75
1.81
-8.54
2.02
2.10
2.04
-11.50
-19.83
4
5
4.28
4.71
1.72
1.71
-10.65
-10.27
2.02
2.05
The Spin-Hamiltonian H= −2J (Ŝ ·Ŝ +Ŝ ·Ŝ ) in the full temperature range has been used to
Mn Cu1
Mn Cu2
2
0
fit the curves for 1–5 using PHI software. The best fitting parameters, J and global g value for
1
all the complexes are summarized in Table 2.
In the case of the linear complexes 1 and 2, the bridging between the central Mn(II) cation with
the two Cu(II) cations is through the Schiff base ligans (bis-phenoxido bridge) along with a syn-
syn carboxylate bridge. However, the syn-syn carboxylate pathway has negligible influence in the
magnetic response because the axial position of the Cu(II) cations involves a non-magnetic orbital
(dz2) that does not allow magnetic coupling. Complexes 3, 4 and 5 show a similar structure with
1
5

Dalton Transactions
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the bis-phenoxido bridges in cis configuration and without additional carboxylato bridges. The
DOI: 10.1039/D0DT00952K
magnitude of J for 3 is slightly larger, evidencing a stronger antiferromagnetic interaction.
The magneto-structural corrections in these kind of complexes with a bis(phenoxido) bridge
between Cu(II)Mn(II) have been previously studied by us and it was found that the main
contributive factor to the super-exchange interaction in these type of complexes is the
2
1
Cu(II)OMn(II) angle. The reported complexes in the present text also confirm that a greater
antiferromagnetic coupling is associated with larger Cu(II)OMn(II) angles (Fig. 5). However,
looking at the representation of the magneto-structural correlation in Fig. 7, it is clear that relation
between J and the structural parameters is a multifactor problem and parameters other than the
bridging angle should also be taken into account for such correlations. In the case of complexes 1
and 2, there is a carboxylate bridge connecting the paramagnetic centers. This kind of bridge can
present counter-complementarity with the oxo bridges, and though weak, may decrease the
magnetic coupling. Another factor that could affect the magnetic correlations is the shape: some
molecules are linear (where the magnetic coupling is exclusively through the dx2-y2 orbital) while
others are folded (in these cases communicating the magnetic exchange trough the dx2-y2 orbital
and the perpendicular d orbital). Finally, the differences in the electronegativity of the groups
z2
coordinated to manganese, the bond distances, the dihedral angles or the geometry around the
metal ions (in particular the square-planar to tetrahedral distortion for the Cu(II) cations)
contribute to the deviations from the single-factor magnetic correlation. The attempts to correlate
the magnetic coupling with two parameters such as the bond angle/ring torsion or any other pair
of structural parameters, do not improve unequivocally the correlation.
Fig. 6. Representation of the bis(phenoxido) bridge in M(II)OCu(II) complexes. While all the
reported complexes give the value of J supposing a single exchange pathway,  and  angles have been
taken as a mean.
1
6

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DOI: 10.1039/D0DT00952K
Fig. 7. Dependence of the reported J values (considering Spin-Hamiltonian, H=‒2J·S S ) with the
1
2
Mn(II)OCu(II) angle. Black spots represent the previously reported complexes found in the literature
which follow a good magnetostructural correlation, blue spots correspond to the compounds in this work
(Table S7, ESI).
Electrochemistry
Cyclic voltammograms (CVs) of the metalloligand, CuL, and complexes 1−5 were recorded in
−
1
acetontrile solution at a scan rate of 100 mV s with respect to an Ag/AgCl electrode. Metalloligand
CuL shows a reversible red-ox process due to Cu(II)/Cu(I) couple at the electrode surface with E =
c
−
=
−
−
1.15 V and E = −1.04 V (Fig. S7 (left)). The peak separation value ΔE = E – E is −0.11 V and I /I
a
pc
c
a
c
a
1
2
1.09. However, cyclic voltamograms of complexes 1−5 show prominent peaks at E = − 0.77, E =
c
c
1
2
1
2
1
2
1.03 V for 1, E = −0.72, E = −1.06 V for 2, E = −0.75, E = −1.02 V for 3, E = −0.75, E =
c
c
c
c
c
c
1
2
1.14 V for 4 and E = −0.76, E = − 1.05 V for 5, respectively (Fig. S7 (right)). It can be assumed that
c
c
the trinuclear complexes undergo partial dissociation in acetonitrile solution and in this process of
1
reduction of Cu(II) to Cu(I), the first peak E arises due to the undissociated trinuclear species while
c
2
the second peak, E , due to the free metalloligand. Such partial dissociation and appearance of two
c
2
2
peaks in CV of such complexes were also observed earlier . It is noticeable that in the trinuclear
species, the reduction of Cu(II) to Cu(I) takes place at less negative potential compared to CuL as
coordination of the metalloligand to Mn(II) makes Cu(II) more susceptible to reduction.
Catechol Oxidase Studies and Kinetics.
1
7

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The catecholase like activity of complexes 1−5 for areal oxidation of 3,5-di-tert-butylcatechol
DOI: 10.1039/D0DT00952K
2
3
(
3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) was studied in acetonitrile medium under
open atmosphere. The oxidation of 3,5-DTBC produces 3,5-DTBQ according to the reaction
shown in Scheme 2.
Scheme 2. Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ in acetonitrile solution (ACN).
−
5
In order to carry out the experiment, 2 x 10 M acetonitrile solutions of each of the complexes,
−
3
1
−5 was mixed with 2 x 10 M solutions of 3,5-DTBC at room temperature in equal volumes and
the progress of the reaction was studied by performing absorbance versus wavelength scan of the
mixtures at regular intervals of 3 min for half an hour (wave scan). The gradual increase in the
2
4
absorption maxima near 398 nm is observed due to formation of 3,5-DTBQ for complexes 1, 3,
and 5. Complex 2 shows no increase in absorbance maxima for over 30 min, indicating that it is
inactive towards catechol oxidase activity. The absorbance vs. wavelength plot of complexes 1
left) and 3 (right) are shown in Fig. 8 and those of complexes 2, 4 and 5 are shown in the
supporting information (Fig.s S8 and S9 respectively).
4
(
1
8

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Fig. 8. Increase of the quinone band at around 398 nm after mixing equal volumes of acetonitrile
DOI: 10.1039/D0DT00952K
−
3
−5
solutions of 2 x 10 M 3,5-DTBC and 2 x 10 M complex 1 (left) and complex 3 (right),
respectively. The spectra were recorded at 3 min interval for 30 min.
The kinetics of this oxidation reaction of 3,5-DTBC to 3,5-DTBQ in the presence of complexes 1,
3
, 4 and 5 were determined following initial rate method by measuring the growth of the quinone
band at 398 nm as a function of time (timescan). The dependence of oxidation rate on the
substrate concentration was studied by mixing equal volumes of 1 × 10−5 M solution of the

5
4
4
complexes with various concentrations of the substrate (5 × 10 M, 1 × 10 M, 3 × 10 M, 1 ×

3
3
3
3
3
3
2
2
1
0 M, 1.2 × 10 M, 2 × 10 M, 4 × 10 M, 6 × 10 M, 8 × 10 M, 1 × 10 M and 1.2 × 10
M). For all these measurements, the corresponding solution of substrate without catalyst was
taken as reference. The initial rate was determined from the slope of the absorbance vs. time plot
taking the data of first 10 min and considering the molar extinction coefficient of quinone (3,5-
-1
-1 25
DTBQ) as 1630 M cm . The rate constant vs. concentration of 3,5-DTBC data were then
analyzed on the basis of the Michaelis–Menten approach of enzymatic kinetics to obtain the
Lineweaver–Burk (double reciprocal) plot as well as the values of various kinetic parameters such
as Vmax, K , and K . The curves of both the observed rate vs. [substrate] and the Lineweaver-
M
cat
Burk plot for complexes 1 and 3 are depicted in Fig. 9 and the same for complexes 4 and 5 are
shown in the supporting information (Fig.s S10 and S11 respectively).
Fig. 9. Plots of rate vs. substrate concentration for complexes 1 (left) and 3 (right).The insets show
the corresponding Lineweaver-Burk plot.
1
9

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−
1
−V1iew Article Online
The turnover number, K (in h ) of this catechol oxidase reaction is calculated to be 139 h for
cat
DOI: 10.1039/D0DT00952K
−
1
−1
−1
1
, 439 h for 3, 348 h for 4 and 730 h for 5. The kinetic parameters of the complexes are given
in Table 3.
Table 3. Values of parameters K , V and K obtained from Lineweaver Burk plots.
m
max
cat
Complex
Km
Vmax
Kcat
1
3
4
5
1.24 x 10-3
1.97 x 10−3
1.56 x 10-3
1.68 x 10-3
1.93 x 10-7
6.09 x 10−7
4.84 x 10-7
1.01 x 10-6
139 h−1
439 h−1
348 h−1
730 h−1
ESI-Mass Spectrometric Study.
To suggest a probable mechanism for the catechol oxidase activity of complexes 1, 3, 4 and 5 the
ESI-mass spectra of the complexes (Fig.s S12–S16, ESI) and 1:1 (v/v) mixture of complex and
2
3
,5-DTBC (1:10 molar ratio) were recorded in acetonitrile medium (Fig.s S17S20, ESI). The
mass spectra of complexes 1, 3, 4 and 5 show base peak at m/z = 372.07 (cald. = 372.09) which is
+
assignable to [(CuL) + H] . Prominent peaks at m/z = 394.07, 743.16 and 765.16 are common for
+
+
all four complexes and can be assigned to [(CuL) + Na] (cald. = 394.07), [(CuL) + H] (cald. =
2
+
7
43.14), [(CuL) + Na] (cald. = 765.15) respectively. For complex 1, a peak at m/z = 527.08
2
+
(
cald. = 527.06) is seen which can be attributed to [(CuL)Mn(CH COO) + CH CN] . For 3, the
3
3
+
peak at m/z = 565.06 (cald. = 565.06) is assigned to [(CuL)Mn(C H COO)(H O)] . In 4, a peak
6
5
2
+
for [(CuL)Mn((p-OH)C H COO)(H O)] is found at m/z = 581.05 (cald. = 581.05) and for
6
5
2
+
complex 5 the peak at m/z = 489.09 (cald. = 489.09) is assigned to [(CuL)Mn(HCOO)(H O)] .
2
Complexes 1, 3, 4 and 5 were then mixed separately with 3,5-DTBC in the molar ratio of 1 : 102
and after 5 min, again the spectra were recorded. For all four cases, the base peak was found at
+
m/z = 372.07 (cald. = 372.09) which was assigned to [(CuL) + H] . Other common peaks are
found at m/z = 648.16, 649.16 and 243.14 which may be assigned to [(CuL)Mn(3,5-DTBC) + H]+
+
+
(
cald. = 648.16), [(Cu(I)L)Mn(3,5-H DTBC)] (cald. = 649.16) and [3,5-DTBQ + Na] (cald. =
2
2
43.14) respectively. In the ESI-mass spectra of complex 1, the peaks at m/z = 609.10 and 467.04
2
0

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DOI: 10.1039/D0DT00952K
can be assigned to [(CuL)Mn(CH COO) + 3CH CN] (cald. = 609.12) and [(Cu(I)L)Mn +
3
3
+
CH CN] (cald. = 467.04), respectively. The mass spectra of complex 3 shows peaks at m/z =
3
+
5
65.06, 768.20 and 444.83 for [(CuL)Mn(C H COO)(H O)]
(cald.
=
565.06),
6
5
2
+
+
[
(CuL)Mn(C H COO)(3,5-DTBC) + H] (cald. = 768.20) and [(Cu(I)L)Mn(H O)] (cald. =
6
5
2
4
44.83), respectively. For complex 4 peaks at m/z = 622.09 and 444.83 may be assigned to
+
+
[
(CuL)Mn((p-OH)C H COO)(H O) + CH CN] (cald. = 622.09) and [(Cu(I)L)Mn(H O)] (cald. =
6 5 2 3 2
4
44.83), respectively. Similarly for 5, peaks at m/z = 489.02 and 444.83 are assignable to
(CuL)Mn(HCOO)(H O)]+ (cald. = 489.03) and [(Cu(I)L)Mn(H O)] (cald. = 444.83),
+
[
2
2
respectively.
Mechanistic Insight for Complexes 1, 3, 4 and 5
The mechanistic studies of copper based model complexes for catechol oxidase type activities
reveal that there are two routes for conversion of 3,5-DTBC to 3,5-DTBQ. The first mechanism
involves formation of a dicopper(II) catecholate intermediate where the dicopper(II) species
stoichiometrically oxidizes the catecholic substrate to one molecule of quinone and itself reduces
to a dicopper(I) species. This dicopper(I) species reacts with an oxygen molecule to generate a
peroxo dicopper(II) adduct, which then oxidizes a second molecule of the substrate to quinone;
2
6
water is formed as a byproduct in this four electron reduction process. The second mechanism is
2
7
proposed to proceed through formation of a copper(I) semiquinonate radical intermediate which
2
8
reacts with a dioxygen molecule, reducing it to H O by a two electron reduction and the Cu(I)
2
2
in the radical is oxidized to Cu(II). A quinone molecule is produced in the process. In order to get
an idea of which pathway is followed, we were interested to know if H O is released. The
2
2
estimation of H O shows that after 1 h of oxidation, 92%, 94%, 91% and 95% of H O is
2
2
2
2
generated with respect to the formation of 3,5-DTBQ for 1, 3, 4 and 5, respectively as shown in
Fig. S21 (ESI). The results indicate that almost equimolar amount of hydrogen peroxide is formed
with respect to 3,5-DTBQ.
Based on the ESI-mass spectra, we propose a probable mechanistic pathway for the oxidation
reaction catalyzed by complexes 1, 3, 4 and 5. ESI-mass spectra reveals that complexes 1, 3, 4 and
+
+
5
generate active monopositive species, [(CuL)Mn(CH COO)] , [(CuL)Mn(C H COO)(H O)] ,
3 6 5 2
+
+
[
(CuL)Mn((p-OH)C H COO)(H O)] and [(CuL)Mn(HCOO)(H O)] , respectively in the solution.
6 5 2 2
The X-ray crystallographic studies reveal that complexes 3, 4 and 5 contain a water molecule
coordinated to the central metal atom, Mn(II) but in 1 no such solvent molecule is present. As a
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1

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result a subtle difference is proposed in the mechanistic pathways of complexes 1 and 3, 4 and 5.
DOI: 10.1039/D0DT00952K
Schematic representation of catalysis of complex 3 is shown in Scheme 3 and that of 1, 4 and 5
are attached in the supporting information (Scheme S1, S2 and S3, respectively). In the case of
+
complex 1, the acetate ion of cationic species, [(CuL)Mn(CH COO)] is replaced by a
3
mononegative catecholate molecule, forming an intermediate species, [(CuL)Mn(3,5-HDTBC)]+.
This species further loses another hydrogen ion to form a semiquinonate radical complex,
+
[
(CuL)Mn(3,5-DTBC)] as evident from the mass spectra. However, in the case of complex 3, the
active cationic species first loses the labile water molecule and reacts with singly deprotonated
+
3
,5-DTBC to form an intermediate complex [(CuL)Mn(C H COO)(3,5-HDTBC)] . In the next
6
5
+
step, the benzoate ion is lost and semiquinonate radical, [(CuL)Mn(3,5-DTBQ)] similar to
complex 1 is formed. With formation of the semiquinonate radical complex, O is reduced to
2
H O . Following this, the semiquinone is oxidized to quinone and is eliminated from the metal
2
2
+
center with the reduction of Cu(II) to Cu(I) forming a monocationic species [(Cu(I)L)Mn] for 1
+
and [(Cu(I)L)Mn(H O)] for 3, when a water molecule binds to the remaining species of 3. Then
2
+
another 3,5-H DTBC molecule coordinates to this species to form [(Cu(I)L)Mn(3,5-H DTBC)] .
2
2
Finally, the Cu(I) center of the dinuclear active species is oxidized to Cu(II) by aerial O and the
2
+
active species [(CuL)Mn(3,5-DTBC)] is regenerated, with the formation of H O as byproduct as
2
2
shown in Scheme 3.
2
2

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DOI: 10.1039/D0DT00952K
Scheme 3. Proposed Mechanism for Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ by Complex 3.
Structure−Activity Correlation for the Catalytic Activity
Literature reveals that various homometallic Cu(II), Ni(II), Mn(II/III) and Fe(II/III) complexes
can mimic the catechol oxidase activity but in most cases the efficiency is much lower compared
2
9
to the native enzyme. However, recently it has been shown that the efficiency of this reaction
can be increased many fold by using heterometallic complexes as catalyst presumably due to
cooperative activity of two different metal ions. For example, in the CuMn complexes, that
showed very high catalytic activity, the Mn(II) ion seems to do the job of binding with the
3
0
substrate while Cu(II) participate in the redox process. It has also been noted that a labile solvent
molecule, coordinated to the Mn centre is the common feature of the complexes showing the
3
1
enhanced catalytic efficiency of these complexes. The reason behind this phenomenon is that the
2
3

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substrate can easily replace the labile solvent molecule and gets bonded with the catalyst. Among
DOI: 10.1039/D0DT00952K
the present complexes, 3, 4 and 5 have a water molecule coordinated to Mn(II) and hence show
very high catecholase-like activity as expected. However, the k value of 5 is significantly higher
cat
compared to 3 and 4. This may be attributed to the lower steric demand of the formate ion
compared to benzoate/p-hydroxybenzoate that facilitates the coordination of the substrate to the
complex. On the other hand, neither 1 nor 2 contain a coordinated solvent molecule to its Mn
atom and these two compounds are isostructural. Nevertheless, complex 1 shows significant
catalytic activity while 2 is inert towards the catalytic reaction. This difference in behaviour
probably lies in the difference of the basicity of the acetate and the nitrate ions. Acetate being the
stronger base, can accept a proton from catechol molecule and gets detached from metal ions to
make the way for substrate-catalyst binding. The nitrate ion which has considerably low basicity
does not accept a proton easily and thus cannot be replaced by the catechol molecule in the metal
centre. Hence complex 2 is inactive toward the catalytic reaction. To investigate if acetate ion can
−
5
activate complex 2, we mixed a 2 x 10 M acetonitrile solution of complex 2 with equal volume
−
5
of 4 x 10 M solution of sodium acetate in MeOH-AcCN mixture (1:10 v/v). The resulting
−
3
solution was then mixed with 2 x 10 M acetonitrile solution of 3,5-DTBC in equal volumes and
absorbance vs. wavelength scan was performed (Fig. S22, ESI). An increase in absorption maxima
near 398 nm with time clearly indicates that acetate ion indeed makes complex 2 active towards
oxidation of catechol. Another point to be noted here is that although the reduction potential of
Cu(II) to Cu(I) in all the complexes (1–5) are very close, complexes 1, 3, 4 and 5 are catalytically
active with different efficiencies while 2 is catalytically inactive. This implies that coordination
environment and coligands around the manganese centre and not the ease of reduction of copper is
crucial for catalytic activity/efficiency of such heterometallic complexes.
Conclusion
In this current work, we have synthesized five Cu(II)-Mn(II) complexes of an N O donor ligand
2
2
varying the anionic co-ligands. Among the complexes, 1 and 2 contain only one type of anions,
bridging bidentate acetate and nitrate, respectively, as during synthesis only these ions are
available for balancing the charge of the respective complexes. On the other hand, each of
complexes 3−5 which have been prepared by a slightly different procedure, contains two different
−
−
anions: one monodentate carboxylate and one uncoordinated ClO / Cl along with a coordinated
4
solvent molecule to the Mn(II) centre. Thus, we have shown here that by the judicious choice of
the synthetic procedure, the structures and composition of the resulting complexes can be
2
4

Page 25 of 30
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monitored. Among the complexes, 3, 4 and 5 exhibit high catalytic activity for the oxidation of
DOI: 10.1039/D0DT00952K
3
,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone. This is not surprising as it has been observed
previously that if a solvent molecule is coordinated to the Mn(II) in such a heterometallic
complex, it shows high catalytic activity. However, comparing their K values, we have shown
cat
here for the first time that catalytic efficiency of such complexes can be related to the steric
demand of the anionic co-ligand present in them, as complex 5, containing the least steric
demanding formate ion shows considerably higher efficiency than 3 or 4. We have also shown
here for the first time that basicity of the anionic co-ligands can be important for their catalytic
activity as between the isostructural complexes, 1 and 2, only 1, containing acetate ion with higher
basicity than the nitrate ion in 2 is responsive towards the catalytic process. The variable
temperature magnetic study of the complexes confirms intra-molecular antiferromagnetic
coupling between Cu(II) and Mn(II) through the phenoxido bridges and larger is the
Cu(II)−O−Mn(II) angle greater is the magnitude of antiferromagnetic coupling. In short, the
present study reveals that (i) a solvent molecule required for high catalytic activity can be
introduced to the metal complex by modifying the synthetic method and (ii) the steric
requirements and the basicity of the anionic coligands for this type of heterometallic complexes
can be very important for their catalytic efficiency. Thus this study is expected to help in
designing the highly efficient catalysts for such oxidase reactions by judicious selection of co-
anions and synthetic procedure.
Supporting Information (see footnote on the first page of this article):
IR spectrum of complexes 1−5, magnetization of 1–5, UV-vis spectra of complexes 2, 3 and 5
with 3,5-DTBC, Lineweaver-Burk plot of complexes 4 and 5, ESI-mass spectra of complexes
1
−5, complexes 1, 3, 4 and 5 after addition of 3,5-DTBC. UV-vis spectra of H O emission,
2 2
mechanistic pathways of catalytic reaction for 1, 4 and 5, tables of bond parameters of 1–5, tables
of hydrogen bond for 2, 4 and 5, a comparative table of J values. CCDC 1990112 (1), 1990113
(2), 1990114 (3), 1990115 (4) and 1990116 (5) 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 University Grants Commission (UGC), New Delhi for funding the CAS-V,
Department of Chemistry, and University of Calcutta. S. D. and P. B. thank the University Grants
2
5

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DOI: 10.1039/D0DT00952K
Commission (UGC), New Delhi, for Senior Research Fellowship [Sr. No.- 2121410161, Ref No.-
2
1/12/2014(II)EU-V] and [F. No. 16-9(June 2017)/2018(NET/CSIR), Ref. No.:240/(CSIR-UGC
NET JUNE 2017)] respectively. J. M. thanks CICYT, Project CTQ2018-094031-B-100.
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