Facile synthesis of a new Cu(ii) complex with an unsymmetrical ligand and its use as an O3 donor metalloligand in the synthesis of Cu(ii)-Mn(ii) complexes: Structures, magnetic properties, and catalytic oxidase activities

Article abstract of DOI:10.1039/c9dt04315b

A new, facile Cu(ii) template method has been employed for the unsymmetrical dicondensation of 1,2-ethylenediamine with salicylaldehyde and o-vanillin. The mononuclear complex, [CuL] (1), thus obtained, has been used as an O₃ donor metalloligand for the synthesis of four new Cu(ii)-Mn(ii) complexes, [(CuL)MnCl₂] (2), [(CuL)Mn(NO₃)₂(CH₃OH)]_n (3), {[(CuL)Mn(benz)(H₂O)]₂·(CuL)₂(ClO₄)₂} (4) and [(CuL)Mn(benz)Cl]₂ (5) (where benz = benzoate). Single-crystal structural analyses reveal that 2 is a dinuclear complex while complex 3 is polymeric with a repeating dinuclear [(CuL)Mn(NO₃)₂(CH₃OH)] unit, linked via the nitrate ion. Both 4 and 5 are discrete tetranuclear complexes, where the dinuclear units [(CuL)Mn(benz)(H₂O)] and [(CuL)Mn(benz)Cl] are connected by double benzoate and double chloride bridges, respectively. In complex 4, two monomeric [CuL] units are cocrystallized with the tetranuclear complex. An important difference in the structure of 4 from the other three complexes is that one solvent water molecule is coordinated to each Mn(ii) ion, which makes complex 4 catalytically very active towards mimicking catecholase and phenoxazinone synthase-like oxidation reactions. The turnover numbers (k_cat) for the aerial oxidation of 3,5-di-tert-butylcatechol and o-aminophenol are 399 h^-1 and 230 h^-1, respectively. The evidence of the intermediate species in the mass spectra indicates possible heterometallic cooperation where the Mn(ii) center helps in substrate binding and Cu(ii) participates in the oxidation reactions with molecular oxygen. Cyclic voltammetry measurements suggest the reduction of Cu(ii) to Cu(i) during the catalytic process. Temperature-dependent dc molar magnetic susceptibility measurements reveal that complexes 2-5 are antiferromagnetically coupled with the exchange coupling constants (J) of J = -13.5 cm^-1 and J = -13.5 cm^-1 for 2 and 3, respectively, J₁ = -12.6 cm^-1 and J₂ = -1.20 cm^-1 for complex 4 and J₁ = -13.24 cm^-1 and J₂ = 0.36 cm^-1 for complex 5 as is expected from the Cu-O-Mn bridging angles.

Full text of DOI:10.1039/c9dt04315b

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Mayans and A. Ghosh, Dalton Trans., 2020, DOI: 10.1039/C9DT04315B.
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14 March 2017
Pages 3073-3412
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Douglas W. Stephen et al.
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
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DOI: 10.1039/C9DT04315B
Facile synthesis of a new Cu(II) complex of an unsymmetrical
ligand and its use as O3 donor metalloligand in the synthesis of
Cu(II)-Mn(II) complexes : Structures, magnetic properties, and
catalytic oxidase activities
Sabarni Duttaa, Júlia Mayansb and Ashutosh Ghosha,*
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 Nanotecnologia (IN2UB). Martíi Franqués 1-11. 08028, Barcelona, Spain.
Abstract
A new, facile Cu(II) template method has been employed for the unsymmetrical
dicondensation of 1,2-ethylenediamine with salicylaldehyde and o-vanillin. The mononuclear
complex, [CuL] (1), thus obtained, has been used as O3 donor metalloligand for the synthesis
of four new Cu(II)-Mn(II) complexes, [(CuL)MnCl2] (2), [(CuL)Mn(NO3)2(CH3OH)]n (3),
{[(CuL)Mn(benz)(H2O)]2∙(CuL)2(ClO4)2} (4) and [(CuL)Mn(benz)Cl]2 (5) (where benz =
benzoate). Single-crystal structural analyses reveal that 2 is a dinuclear complex while
complex 3 is polymeric with a repeating dinuclear [(CuL)Mn(NO3)2(CH3OH)] unit, linked
via nitrate ion. Both 4 and 5 are discrete tetranuclear complexes, where the dinuclear units
[(CuL)Mn(benz)(H2O)] and [(CuL)Mn(benz)Cl] are connected by double benzoate and
double chloride bridges, respectively. In complex 4, two monomeric [CuL] units are
cocrystallized with the tetranuclear complex. An important difference in the structure of 4
from the other three complexes is that one solvent water molecule is coordinated to each
Mn(II) ions, which makes complex 4 catalytically very active towards mimicking catecholase
and phenoxazinone synthase-like oxidation reactions. The turnover numbers (kcat) for the
aerial oxidation of 3,5-di-tert-butylcatechol and o-aminophenol are 399 h−1 and 230 h−1,
respectively. The evidences of the intermediate species in the mass spectra indicate possible
heterometallic cooperation where the Mn(II) center helps in substrate binding and Cu(II)
participates in the oxidation reactions by molecular oxygen. The cyclic voltammetry
measurements suggest the reduction of Cu(II) to Cu(I) during the catalytic process. The
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temperature-dependent dc molar magnetic susceptibility measurements reveal thaDtOcIo: 1m0.1p0l3e9/xCe9sDT04315B
2−5 are antiferromagnetically coupled with the exchange coupling constants (J) of J = –13.5
cm−1 and J = –13.5 cm−1 for 2 and 3, respectively, J1 = –12.6 cm−1 and J2 = –1.20 cm−1 for
complex 4 and J1 = –13.24 cm−1 and J2 = 0.36 cm−1 for complex 5 as is expected from the Cu
−O−Mn bridging angles.
Introduction
A large number of N, O donor Schiff base ligands have been synthesized mostly for their
utility in the synthesis of coordination complexes with various metal ions1. Among these
ligands, arguably the most popular one is the 1:2 condensation product of 1,2-ethanediamine
and salicylaldehyde, popularly known as ‘salen’. Salen is a symmetrical tetradentate N2O2
donor chelating ligand which forms complexes with almost all the metal ions. These
complexes, especially those with the transition metal ions have played a crucial role in the
development of various aspects of chemistry e.g. catalysis of various organic reactions,
magnetic coupling between paramagnetic metal centers culminating in the emergence of
single molecular magnets, metal-organic frameworks for small molecule adsorption and
storage etc2. Apart from the excellent complexing ability, the popularity of the symmetrical
salen type ligands also lies in their easy synthesis, usually by one-pot condensation of one
diamine with two salicylaldehyde derivatives. In contrast, their asymmetrically dicondensed
analogues are rarely reported presumably due to the inherent synthetic challenges of
asymmetrical condensation of the aliphatic diamines,3 although the complexes of such
ligands showed great promises in asymmetric organic reactions, nonlinear optical properties,
catalysis etc4. Therefore, development of a nonhazardous, facile method for the asymmetric
dicondensation is warranted to boost up research in these fields.
The neutral Cu(II) and Ni(II) complexes of N2O2 donor ligands derived from 1,3-
propanediamine and salicylaldehyde derivatives are capable of coordinating to a second
metal ion and thus can be used as a metalloligand5. Using this metalloligand approach, we
have synthesized quite a few heterometallic complexes5a. These complexes provide an
opportunity to study magnetic coupling between the two different metal ions via phenoxido
group with a wide range of bridging angles in heterometallic systems6. Beside the magnetic
study, these heterometallic complexes are also found to be very good model for catalytic
oxidase reactions e.g. catechol oxidase, phenoxazinone activity etc7. Recently, we also
2

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developed a convenient method for the asymmetric dicondensation of 1,3-propanediamine
DOI: 10.1039/C9DT04315B
with two different aromatic carbonyl compounds and thus obtained Cu(II) and Ni(II)
complexes of unsymmetrical N2O2 and N2O3 donor ligands. These complexes are also very
useful as metalloligands and the structures of the derived heterometallic complexes,
especially those from N2O3 donor ligands are found to be quite interesting. Literature shows
that with 1,2-ethanediamine or its derivatives, analogous heterometallic compounds of N2O2
donor ligands are scanty and no complex of unsymmetrical N2O3 donor ligands is reported till
date whereas a plethora of heterometallic complexes of N2O4 donor bicompartmental ligand
are known8. We would like to investigate the effectiveness of mononuclear complexes of
N2O3 donor ligand derived from ethylenediamine as metalloligand.
Herein, we develop a novel and facile Cu(II) template method for unsymmetrical
dicondensation of ethylenediamine and synthesized a mononuclear complex, [CuL] (1) of a
new N2O3 donor unsymmetrical Schiff base, N-salicylidene-N ′ -3-methoxysalicylidene-1,2-
ethylenediamine (H2L) by condensing with salicylaldehyde and o-vanillin. Using complex 1,
four heterometallic complexes ([(CuL)MnCl2] (2), [(CuL)Mn(NO3)2(CH3OH)]n (3),
{[(CuL)Mn(benz)(H2O)]2∙(CuL)2(ClO4)2} (4), [(CuL)Mn(benz)Cl]2 (5)) (where benz =
benzoate ion) have been synthesized by varying the anionic coligands. Complex 1 has been
characterized by elemental analyses and spectral methods. Complexes 2–5 have been
characterized additionally by single crystal X-ray crystallography. Complex 2 is a dinuclear
complex whereas 3 is polynuclear in which its dinuclear units are connected by nitrate ion.
Both complexes 4 and 5 are tetranuclear; in 4, the dinuclear Cu-Mn units are bridged by
benzoate ions while in 5, they are joined by two chloride ions. Among the complexes, only 4
shows catecholase and phenoxazinone synthase like 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. The results of the variable temperature magnetic
susceptibility measurements of complexes 2−5 which show antiferromagnetic interactions
between the Cu(II) and Mn(II) centers are also reported.
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DOI: 10.1039/C9DT04315B
Experimental Section
Starting materials
The reagent grade salicylaldehyde, 1,2-ethylenediamine and benzoic acid were purchased
from Lancaster, o-vanillin and imidazole were purchased from Spectrochem. All the
chemicals were commercially available and were used for reaction without further
purification.
Caution! Although not encountered during experiment, perchlorate salts of metal complexes
with organic ligands are potentially explosive. Only a small amount of material should be
prepared and it should be handled with care.
Synthesis of the mononuclear Cu(II) complex, [CuL] (1) of unsymmetrical Schiff-base
ligand, N-salicylidene-N′-3-methoxysalicylidene-1,2-ethylenediamine (H2L).
We have developed a facile Cu(II) template method for the synthesis of a metalloligand
[CuL] (1) of unsymmetrical Schiff-base ligand, H2L as described below.
To a 5 mL methanolic solution of salicylaldehyde (420 L, 4 mmol),
Cu(ClO4)2·6H2O (1.5 g, 4 mmol) was added and the mixture was stirred for 15 min . A 5 mL
methanolic solution of imidazole (0.544 g, 8 mmol) was added to this mixture followed by
ethylenediamine (246 L, 4 mmol) with stirring. A dark violet compound, [CuL1Im]ClO4
started to separate in 5 min. The stirring was continued for approximately 30 min till the
separation of solid completed. The product was collected by filtration and was dried before
further use. It was subjected to ESI mass spectroscopy (Fig. S1, ESI).
Complex [CuL1Im]ClO4: Yield 0.295 g (75%). C12H15CuN4ClO5 (Mol. Wt.: 394.27)
Calculated: C, 36.56; H, 3.83; N, 14.21. Found: C, 36.54; H, 3.79; N, 14.25. IR (KBr): 1615
cm−1. Mass (m/z) = 226.01 (cald. 226.01) assigned to [CuL1Im]+.
To a 5 mL methanolic solution of the powdered compound, [CuL1Im]ClO4 (0.394 g, 1
mmol), o-vanillin (0.152 g, 1 mmol) and an aqueous solution of NaOH (1 mL, 1.2 mmol)
were added. The mixture was refluxed for an hour and then cooled. A green coloured
compound of the precursor metalloligand [CuL] (1) started to precipitate out on cooling. The
compound was then collected and washed with warm water and dried in a desiccator. This
compound was also subjected to ESI mass spectroscopy (Fig. S2, ESI).
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DOI: 10.1039/C9DT04315B
Complex 1: Yield 0.306 g (85%). C17H16CuN2O3 (359.87). Calculated: C, 56.74; H, 4.48; N,
7.78; found: C, 56.78; H, 4.43; N, 7.80. IR (KBr): ν(C=N) = 1615 cm−1. Mass (m/z) = 360.06
(cald. = 360.06) was assigned to [(CuL) + H]+.
Synthesis of the complexes [(CuL)MnCl2] (2) and [(CuL)Mn(NO3)2(CH3OH)]n (3).
To a 5 mL methanolic solution of the metalloligand [CuL] (0.720 g, 2 mmol), MnCl2·4H2O
(0.197 g, 1 mmol) and Mn(NO3)2·4H2O (0.251 g, 1 mmol) were added for the preparation of
complexes 2 and 3, respectively and stirred for 15 min . For both the compounds, the mixture
was taken in a long tube and layered with diethylether (15 mL) very carefully for slow
diffusion. Red block shaped X-ray quality crystals started to appear at the junction of the two
solutions after a few days. The crystals were collected after two weeks and dried in a
desiccator. They were subjected to ESI mass spectroscopy (Fig. S3 and S4, ESI respectively).
Complex 2: Yield 0.413 g (85%). C17H16Cl2CuMnN2O3 (485.71). Calculated: C, 42.04; H,
3.32; N, 5.77; found: C, 42.02; H, 3.29; N, 5.70. IR (KBr): ν(C=N) = 1630 cm–1. Mass (m/z)
= 506.92 (cald. = 506.91) was assigned to [(CuL)MnCl2 + Na]+.
Complex 3: Yield 0.410 g (72%). C18H19CuMnN4O10 (569.86). Calculated: C, 37.94; H, 3.36;
–
N, 9.83; found: C, 37.97; H, 3.34; N, 9.86. IR (KBr): ν(NO3 ) = 1285 cm–1, ν(C=N) = 1637
cm–1. Mass (m/z) = 575.93 (cald. = 575.93) was assigned to [(CuL)Mn(NO3)2 + Na]+.
Synthesis of the complexes {[(CuL)Mn(benz)(H2O)]2∙(CuL)2(ClO4)2} (4) and
{[(CuL)Mn(benz)Cl]}2 (5).
To synthesize complexes 4 and 5, solid Mn(ClO4)2·6H2O (0.330 g, 1 mmol) and
MnCl2·4H2O (0.195 g, 1 mmol) respectively were added to 5 mL methanolic solution of the
metalloligand, [CuL], (0.719 g, 2 mmol) and mixed well. To both of these mixtures 5 mL
methanolic solutions of benzoic acid (0.244 g, 2 mmol) were added individually and stirred
for 10 min. The resulting solutions were then layered with diethylether (10 mL) carefully.
Brownish red (for complex 4) and greenish red (for complex 5) single crystals suitable for X-
ray diffraction appeared after few days at the junction of the two solutions. The crystals were
collected after fifteen days and dried in a desiccator. They were also subjected to ESI mass
spectroscopy (Fig. S5 and S6, ESI respectively).
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Complex 4: Yield 1.316 g (65%). C82H76Cl2Cu4Mn2N8O26 (2024.49). Calculated: C, 48.65;
DOI: 10.1039/C9DT04315B
H, 3.78; N, 5.53; found: C, 48.60; H, 3.80; N, 5.50. IR (KBr): νs+as(COO–) = 1551, 1404 cm–
1, ν(C=N) = 1633 cm–1. Mass (m/z) = 553.02 (cald. = 553.02) was assigned to
[(CuL)Mn(benz)(H2O)]+.
Complex 5: Yield 0.891 g (78%). C48H42Cl2Cu2Mn2N4O10 (1142.74). Calculated: C, 50.45;
H, 3.70; N, 4.90; found: C, 50.40; H, 3.72; N, 4.94. IR (KBr): νs+as(COO–) = 1552, 1398,
ν(C=N) = 1632 cm–1. Mass (m/z) = 570.98 (cald. = 570.92) was assigned to
[(CuL)Mn(benz)Cl + H]+.
Physical measurements
Elemental analyses (C, H and N) were performed using a Perkin–Elmer 2400 series II CHN
analyzer. IR spectra in KBr pellets (4000–500 cm1) were recorded using a Perkin-Elmer RXI
FT-IR spectrophotometer.
Electrochemical measurements
Electrochemical studies of all five complexes 1–5 were performed using a Basi-Epsilon C3
cell instrument at a scan rate of 100 mV s−1 within the potential range of 0 to −1.40 V vs.
Ag/AgCl. 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
argon purging. The working electrode was a glassy carbon disk (0.32 cm2) 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.
EDX analysis of complexes 2−5
Energy Dispersive X-ray (EDX) analysis of the synthesized complexes were collected using a
Cark Zeiss EVO-18 Scanning electron microscopy (SEM) equipment with Bruker EDS
attaachment at an accelerating voltage of 20 KV.
X-ray Crystallography
Suitable single crystals of complexes 2−5 were mounted on a Bruker-AXS SMART APEX.II
diffractometer equipped with a graphite monochromator and Mo-K ( = 0.71073Å)
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radiation. The crystals were positioned at 60 mm from the CCD. 360 frames were measured
DOI: 10.1039/C9DT04315B
with a counting time of 5s. The structure was solved by direct methods and refined by full-
matrix least squares on F2 using the SHELX-2014 package. Non-hydrogen atoms were
refined with anisotropic thermal parameters. Hydrogen atoms were placed in their
geometrically idealized positions and constrained to ride on their parent atoms. Successful
convergence was indicated by the maximum shift/error of 0.001 for the last cycle of the least
squares refinement. The data of complex 2 was of poor quality. The oxygen atoms of the
perchlorate anion were refined isotropically using disorder model in complex 4. Absorption
corrections were carried out using the SADABS program9. All calculations were carried out
using SHELXS201410, PLATON 9911, ORTEP-3212 and WINGX system ver-1.6413. Data
collection, structure refinement parameters and crystallographic data for complexes 2−5 are
given in Table 1. CCDC reference numbers are 1922283−1922286 for complexes 2−5
respectively.
Catalytic Oxidation of 3,5-DTBC
The catecholase activity of complex 4 was studied using 3,5-di-tert-butylcatechol (3,5-
DTBC) as the substrate in an acetonitrile solution in open atmosphere at room temperature.
The reactions were followed spectrophotometrically by recording the increase in the
absorbance maxima of the quinone band at 395 nm as a function of wavelength (wave scan)
and time (time scan). Hydrogen peroxide was produced as the byproduct of this process. To
detect its formation during the catalytic oxidase reaction, the iodometric method was
followed as reported earlier14.
Phenoxazinone Synthase of OAP
The phenoxazinone synthase activity of complex 4 was studied using o-aminophenol (OAP)
as the substrate in an acetonitrile solution under aerobic conditions at room temperature. Here
also the reactions were followed spectrophotometrically by monitoring the increase in the
absorbance maxima of the aminophenoxazinone (APX) band at 426 nm first with respect to
wavelength (wave scan) and then as a function of time (time scan) in order to complete a
detailed kinetic study of the catalytic process. In this reaction H2O was eliminated as a
byproduct.
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DOI: 10.1039/C9DT04315B
Table 1. Crystal data and structure refinement of complexes 2, 3, 4 and 5.
Complex
Formula
2
3
4
5
C17H16Cl2CuMn C18H20CuMnN4 C82H78Cl2Cu4Mn2 C48H42Cl2Cu2Mn2
N2O3
O10
N8 O26
N4O10
Formula Weight
Crystal System
Space group
485.71
570.87
2026.50
1142.74
monoclinic
monoclinic
monoclinic
triclinic
P21/n
P21/n
P21/c
푃1
a, Å
7.7792(11)
13.3468(19)
18.111(3)
90
11.5779(6)
13.7188(8)
14.4713(8)
90
15.560(2)
8.5983(13)
31.676(5)
90
9.7423(6)
10.1101(6)
11.7562(6)
92.292(3)
90.616(2)
92.587(3)
1155.74(12)
1
b, Å
c, Å
α, deg
β, deg
100.600(4)
90
109.985(4)
90
103.136(3)
90
γ, deg
V, Å3
1848.3(5)
4
2160.1(2)
4
4127.0(10)
2
Z
ρcalc(g cm-3)
µ (Mo Kα) (mm-1)
F(000)
1.745
1.755
1.631
1.642
2.142
1.634
1.459
1.621
976
1160
2068
580
Rint
0.058
0.087
0.089
0.041
Total no. of reflns
42323
17350
3877
40209
7429
13877
No. of unique reflns 3343
No. of rflns with I > 3124
2σ(I)
4080
2286
4256
3267
R1,a wR2b
GOFc on F2
R(all)
0.0946, 0.2730 0.0664, 0.1939 0.0533, 0.1248
0.0588, 0.1700
1.09
1.14
0.0979
1.00
1.01
0.1321
296
0.1214
295
0.0744
Temp, K
Residual
297
296
electron -0.88, 1.98
-0.58, 0.98
-0.45, 0.59
-0.64, 0.88
density, e Å−3
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DOI: 10.1039/C9DT04315B
2
2
4 ½
2
½
aR1 = ||Fo| − |Fc|||Fo|, bwR2 (Fo ) = [[w(Fo − Fc2)2w Fo ] and cGOF = [[w(Fo − Fc2)2(Nobs– Nparams)] .
Results and discussion
Syntheses of the complexes
For the synthesis of asymmetrically dicondensed Schiff bases of diamines, various strategies
were adopted15˗17. The first and the most difficult step for such synthesis is the half
condensation of the diamine with a carbonyl compound. A general method of the half
condensation is to react the diamine with the carbonyl compound in 1:1 molar ratio in a very
dilute solution at low temperature. This method is very useful for various diamines when one
of the carbonyl compounds is acetylacetone or its derivatives15a,15b. When both the carbonyl
compounds are aromatic e.g. salicylaldehyde or its derivatives, this method was reported to
work for some specific diamines like o-phenylenediamine or 1,2-diaminocyclohexane.
However, the half-condensation of 1,2-ethanediamine or 1,3-propanediamine with
salicylaldehyde by this high dilution method invariably resulted in the formation of double
condensation product, salen or salpen, respectively as the major product. For the half
condensation of 1,3-propanediamine, Elder et al. reported a Ni(II) template method
exploiting the octahedral geometry of Ni(II), that is satisfied by the two tridentate half
condensed Schiff base ligands17. Recently, we have modified this method to prepare several
Ni(II) and Cu(II) complexes of unsymmetrical dicondensed ligand. However, by this method
when we tried to get half unit using 1,2-ethanediamine and salicylaldehyde, the Ni(II)
complex of double condensed ligand (salen) was the only product. Rigamonti et al. reported a
Cu(II) template method for half condensation of 1,2-ethanediamine in presence of pyridine18.
This method is effective but the problem is that handling of pyridine is hazardous. Moreover,
in many cases, in slightly alkaline solution, a hydroxide bridged trinuclear Cu(II) complex of
half-condensed ligand is formed19 but that one is very stable and even under prolonged reflux
with a second carbonyl compound, the complex of unsymmetrical tetradentate ligand is not
formed. Thus, this method cannot be used for the preparation of unsymmetrical tetradentate
ligand when such a hydroxide bridged trinuclear Cu(II) complex is formed. We therefore
developed here a new and facile Cu(II) template method. In this method, a mixture of copper
perchlorate, salicylaldehyde, 1,2-ethanediamine and imidazole, taken in 1:1:1:2 molar ratios
was allowed to react in methanol with stirring. The desired complex of half condensed ligand,
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[(CuL1)Im]ClO4, separated as dark violet solid. It was then reacted with o-vanillin to proVdieuwcAerticle Online
DOI: 10.1039/C9DT04315B
the Cu(II) complex of the double condensed unsymmetrical ligand, [CuL] as shown in
Scheme 1. Thus the strategy for the half-condensation that we adopted here is to block one of
the coordination cites of Cu(II) by imidazole so that the half condensed tridentate ligand is
stabilized by coordinating to the other three sites of Cu(II) to form an isolable solid square
planar complex. The second carbonyl compound reacts readily with the free amine group to
form the unsymmetrical tetradentate ligand which encompasses the square plane of the Cu(II)
by removing the imidazole molecule due to chelate effect.
Scheme 1: Synthesis of metalloligand CuL (1).
This mononuclear Cu(II) complex, [CuL] possesses three oxygen atoms (two phenoxido and
one methoxy) all of which are capable of donating lone pair of electrons to a second metal
ion and thus has the potential to be used as metalloligand. We, therefore, reacted [CuL] with
Mn(II) salts of different anions and succeeded to obtain four heterometallic Cu(II)-Mn(II)
complexes, 2−5 as shown in Scheme 2 and characterized them by single crystal X-ray
crystallography.
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DOI: 10.1039/C9DT04315B
Scheme 2: Synthesis of complexes 2−5
IR spectra of Complexes.
All five complexes show the characteristic stretching vibrations for the azomethine (C=N)
group20 around 1630 cm−1. Two sharp peaks at 1554 cm−1 and 1396 cm−1 appear for the
symmetric and asymmetric stretching for COO¯ in complexes 4 and 520. Moreover, complex 4
displays a couple of strong intensity band at ca. 1091 cm−1, confirming the presence of ionic
perchlorate21. The IR spectral data for the complexes for the complexes are given in the
supporting information (Fig. S7−S11, ESI respectively).
EDX Analysis
The chemical analysis results obtained from EDX for the synthesized complexes 2−5 are
represented in Table 2. The EDX profile peaks of the complexes shows a confirmative
identification of all the metal ions that comprise the complexes (2−5) as represented in Fig.
S12 in the supporting information. The result reveals that the Cu/Mn molar ratios in
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complexes 2, 3 and 5 are 1.12:1, 1.02:1 and 1.12:1, respectively, whereas in complex 4 the
DOI: 10.1039/C9DT04315B
ratio is 1.86:1. These Cu/Mn molar ratios are closer to ratio of Cu/Mn atoms in crystal
structure of complexes 2−5 obtained from X-ray diffraction analysis.
Table 2. Result of EDX analysis of Cu (wt%) and Mn (wt%) in complexes 2 and 5.
Complex
Cu(wt%)
52.82
Mn (wt%)
47.18
Molar Ratio (Cu/Mn)
1.12 : 1
2
3
4
5
50.54
49.46
1.02 : 1
65.03
34.97
1.86 : 1
52.82
47.18
1.12 : 1
Description of the structures
2
3
Fig. 1. Structures of complexes 2 and 3 with thermal ellipsoids at 20% probability. The
hydrogen atoms are not shown for clarity.
Complexes 2 and 3
The asymmetric unit of complex 2 contains one [CuL], one Mn(II) metal atom and two
chlorine atoms. The two phenoxido and one methoxy oxygen atoms of the [CuL]
metalloligand coordinate to the Mn(II) atom forming the final dinuclear complex. The Cu(II)
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atom is bonded to N2O2 donor sites (N(18), N(21), O(10), O(29)) of L2− in a nearly peVrifewecArtticle Online
DOI: 10.1039/C9DT04315B
square planar geometry. The four donor atoms show a root mean squared (r.m.s.) deviation of
0.0080 Å with the Cu(II) metal centre 0.039(2) Å away from the plane. The two CuN bond
lengths differ slightly with CuN(18), 1.898(10) Å and CuN(21), 1.920(2) Å. The
phenoxido CuO(10) and CuO(29) bond lengths are 1.897(8) and 1.891(8) Å respectively.
The geometry around the Cu(II) atom is confirmed by the (τ4) index22, which measures the
distortion between a perfect tetrahedron (τ4 = 1) and a perfect square planar geometry (τ4 = 0)
with the formula of τ4 = [360° – (a + b)]/141°, where a and b (in°) being the two largest
angles around the metal center. The τ4 value of Cu(II) atom is 0.0444, indicating that the
geometry is slightly distorted square planar.
The Mn(II) atom is coordinated to two phenoxido oxygen atoms O(10), O(29), one
methoxy oxygen atom O(30) from the metalloligand together with two terminal chlorine
atoms, Cl(1) and Cl(2) to complete a five coordinated geometry. The MnCl(1) and
MnCl(2) bondlengths are 2.299(5) and 2.309(4) Å, phenoxido Mn−O(10) and Mn−O(29)
bond lengths are 2.218(8) Å and 2.091(8) Å, respectively and methoxy Mn−O(30)
bondlength is 2.450(10) Å. The environment of the manganese atom is slightly distorted
square pyramid with Addison parameter (τ5)23 value of 0.081 (τ5 = 0 for a perfect square
pyramid and a τ5 = 1 for a perfect trigonal bipyramid). The r.m.s deviation of the four O3Cl
donor atoms (O(10), O(29), O(30) and Cl(1)) from the basal plane is 0.183 Å with the Mn(II)
atom 0.532(2) Å away from the plane towards the Cl(2) atom as shown in Fig. 1.
Complex 3 possesses polymeric chain structure with a repeating dinuclear unit,
[(CuL)Mn(NO3)2(CH3OH)] connected via nitrate ion as depicted in Fig. 1. The one
dimensional propagation of the chain structure is shown in Fig. 2. The asymmetric dinuclear
unit consists of one [CuL] metalloligand, one Mn(II) metal atom and two nitrate ions. The
Cu(II) metal atom is equatorially coordinated by four donor atoms, N(18), N(21), O(10),
O(29) from the Schiff base ligand. In the complex, two Cu−N bond distances are almost the
same with Cu−N(18), 1.917(7) and Cu−N(21), 1.919(7) Å. The Cu−O bond lengths are also
nearly equal at Cu−O(10), 1.902(4) and Cu−O(29), 1.907(5) Å. The axial site of the Cu(II)
metal centre is coordinated by an oxygen atom of the nitrate ion, O(39a), (where a = −1/2 + x,
1/2 − y, − 1/2 + z) from a neighbouring unit at a distance of 2.643(7) Å. The Addison
parameter (τ5)23, calculated for Cu(II) centre, is 0.0438, indicating a slightly distorted square
pyramidal geometry. The root mean squared (r.m.s.) deviation of the four N2O2 donor atoms
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in the basal plane is 0.044 Å with the Cu(II) metal atom 0.0396(8) Å away froDmOI:t1h0e.10b39a/Csa9DlT04315B
plane towards the axial oxygen atom.
The Mn(II) metal centre is heptacoordinated and has a distorted pentagonal
bipyramidal geometry. The basal pentagonal plane is formed by the two phenoxido oxygen
atoms, O(10), O(29) and a methoxy oxygen atom O(30) from [CuL] along with two oxygen
atoms, O(32), O(34) of a chelated nitrate ion at distances, 2.329(5), 2.121(5), 2.502(4),
2.312(6) and 2.320(6) Å respectively. It is axially coordinated by an oxygen atom O(36) of a
monodentate nitrate ion at 2.361(7) Å, and a solvent methanol molecule, O(40) at a distance
of 2.195(4) Å. The other oxygen atom, O(38) of the nitrate ion is 2.575(10) Å away from the
Mn(1) atom. The O(36)−Mn(1)−O(40) trans angle is 156.0(2)º. The r.m.s deviation of O5
donor basal plane is 0.227 Å with the Mn(II) metal atom at a distance of 0.078(10) Å from
the plane towards the bridging nitrate ion. Selected bond lengths and bond angles of
complexes 2 and 3 are given in Table S1 and S2, ESI.
Fig. 2. 1D chain of complex 3, along the crystallographic c-axis.
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DOI: 10.1039/C9DT04315B
4
Fig. 3. Structure of the cationic part of complex 4 with thermal ellipsoids at 20% probability
and hydrogen bonding interactions between coordinated water molecule of tetranuclear unit
and mononuclear [CuL] unit. The perchlorate ion is not shown for clarity.
Complexes 4 and 5
Complex 4, {[(CuL)Mn(benz)H2O]2∙(CuL)2(ClO4)2} is a cocrystal, comprising of one
tetranuclear unit and one mononuclear [(CuL)] unit as shown in Fig. 3. The tetranuclear unit
consists of two dinuclear [(CuL)Mn(benz)(H2O)] units, bridged by two benzoate ions with a
crystallographic centre of symmetry. In the tetranuclear unit, the Cu(II) metal centre
possesses approximately square planar geometry being coordinated by four donor atoms
(N(18), N(21), O(10), O(29)) from the deprotonated Schiff base ligand, L2− at 1.891(5),
1.922(5) Å, 1.910(3) and 1.885(3) Å, respectively. The r.m.s deviation of the N2O2 donor
atoms in the basal plane is 0.075 Å and the Cu(II) atom is 0.009(7) Å away from the plane.
The τ4 values22 of Cu(II) atom is 0.0667, indicating that the geometry is slightly distorted
square planar.
Here, the Mn(II) atom is hexacoordinated in a distorted octahedral geometry. It is
equatorially bonded by two phenoxido, O(10), O(29), and one methoxy oxygen O(30) atoms
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from metalloligand [CuL], and an oxygen atom of the deprotonated benzoic acid, O(3V2ie)w aArtticle Online
DOI: 10.1039/C9DT04315B
distances of 2.268(3), 2.145(3) Å, 2.478(4) Å and 2.094(3) Å respectively. The two axial
sites of Mn(II) atom are coordinated by a water molecule, O(1) and a bridging oxygen atom,
O(40)a of benzoate ion from the symmetrically related dinuclear counterpart (where a = 1 − x,
1 − y, 1 − z) at a distance of 2.197(4) Å and 2.122(3) Å, respectively. The r.m.s deviation of
O4 donor atoms (O(10), O(29), O(30), O(32)) in the basal plane is 0.0286 Å. The distortion
from octahedral geometry are considerable with the largest distortion indicated by the trans
O(10)–Mn(1)–O(30) angle of 136.64(2)°.
In the discrete mononuclear unit, [CuL], metal atom is in a nearly perfect square
planar geometry being coordinated by four donor atoms from the ligand (N(49), N(52),
O(41), O(60)) with a slight distortion. The Cu2N(49) and Cu2−N(52) bond lengths are
1.938(4) Å and 1.913(4) Å respectively with the Cu2N(49) bond slightly longer than the
other. The Cu2O(41) and Cu2O(60) bond distances are 1.905(3) and 1.893(3) Å
respectively. The r.m.s deviation of the N2O2 donor atoms is 0.136 Å with the Cu(II) atom
0.025(7) Å away from the plane. The τ4 values22 of Cu(II) atom is 0.125, indicating that the
geometry is slightly distorted square planar The cocrystal formation is facilitated by strong
hydrogen bonding interactions between O(41), O(60) and O(61) of [CuL] and H(1A) and
H(1B) of the water molecule, which is coordinated to the Mn(II) centre of tetranuclear unit as
shown in Fig. 3. The O(41)···O(1), O(60)···O(1), O(61)···O(1) distances are 2.804(5),
2.818(4), 2.918(6) Å respectively and H(1B)···O(41), H(1A)···O(60) and H(1A)···O(61)
distances are 1.994(4), 2.237(4) and 2.126(4) Å respectively with O(1) − H(1B)···O(41)
163(4)°, O(1)−H(1A)···O(60) 126(4)° and O(1)−H(1A)···O(61) 156(5)°.
Like 4, complex 5, [(CuL)Mn(benz)Cl]2 also possesses a centrosymmetric tetranuclear
structure, but here two dinuclear [(CuL)Mn(benz)] units are connected to each other by two
chloride ions as shown in Fig. 4. In the asymmetric dinuclear unit, Cu(II) atom is bonded to
two nitrogen atoms, N(18), N(21) and two phenoxido oxygen atoms, O(10), O(29) at
1.911(4), 1.922(4) Å and 1.905(3) and 1.891(4) Å, respectively in the basal plane. One of its
axial sites is coordinated by a bridging chloride Cl(1)a, (where a = 1 − x, − y, 1 − z) which is
also bonded to Mn(II) atom of the symmetry related dinuclear unit in the tetranuclear
structure, making the Cu(II) centre pentacoordinated. The axial CuCl(1)a bond distance is
2.810(2) Å. The Addison parameter (τ5)23 for Cu(II) atom is 0.089 indicating a slightly
distorted square pyramidal geometry. The r.m.s deviation of the four N2O2 donor atoms in the
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basal plane is 0.048 Å. The Cu(II) atom deviates from the plane by 0.113 (7) Å DtoOIw: 1a0.r1d03s9/tCh9eDT04315B
axially bridging chlorine atom.
In the dinuclear unit, Mn(II) atom is bridged to the Cu(II) metal centre of the [CuL]
by two phenoxido oxygen atoms, O(10), O(29) and a methoxy oxygen atom, O(30) like
complexes 2, 3 and 4 at a distance of 2.196(3), 2.108(3) Å and 2.487(4) Å respectively. A
bridging chloride ion Cl(1) is bonded to it at 2.344(2) Å, which is also linked to Cu(II)a atom
of the other dinuclear unit. The Mn(II) atom is further coordinated by two chelating
carboxylato oxygen atoms of benzoate ion, O(32), O(40) at a distance of 2.318(5) and
2.208(5) Å, respectively.The geometry of the Mn(II) atom is very irregular and cannot be
described by the distortion from octahedral geometry. Selected bond lengths and bond angles
of 4 and 5 are given in Table S3 and S4, ESI respectively. The details of hydrogen bonding in
complex 4 are given in Table S5, ESI.
5
Fig. 4. Structure of the complex 5 with thermal ellipsoids at 20% probability. The hydrogen
atoms are not shown for clarity. The symmetry transformations for complex 5 are a = 1 − x, −
y, 1 − z.
A CSD search reveals that quite a few phenoxido bridged Cu(II)-Mn(II) complexes with
various nuclearity are known24. Among these, a large number of complexes are derived from
symmetrical N2O2 and N2O4 donor Schiff bases. The N2O2 donor Schiff bases usually
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produce trinuclear (Cu2Mn)24 or star shaped tetranuclear (Cu3Mn) complexes24a where aVsiewthAerticle Online
DOI: 10.1039/C9DT04315B
complexes of N2O4 donor Schiff bases are mostly dinuclear (CuMn)25. Till date, only two
Cu−Mn complexes of unsymmetrical N2O3 donor ligands are reported26. In both cases, the
ligands were derived from 1,3-propanediamine by condensation with salicylaldehyde/o-
hydroxyacetophenone and o-vanillin and both the complexes were discrete dinuclear like
complex 2 of the present paper. Thus, complexes 2−5 are the first examples of any
heterometallic complexes of unsymmetrical N2O3 donor Schiff base ligand derived from
ethylenediamine. The novelty of the structures is that 3 is the first 1D chain of dinuclear
heterometallic Cu-Mn units bridged by nitrate and the Mn(II) ion is hepta-coordinated which
is rather rare for Mn(II). Complex 4 is the first discrete tetranuclear Cu2Mn2 compounds
formed by the joining of two dinuclear Cu-Mn units with the help of benzoate ion and 5 is
only the second example of a heterometallic tetranuclear Cu2Mn2 complex in which the
dinuclear Cu-Mn units are joined by dichloride bridge. In the other example, an N2O2 donor
unsymmetrical ligand was used and the Mn(II) centre of one unit is joined with the Mn(II)
atom of other unit by dichloro bridge unlike 5 where the chloro bridges join Mn(II) with the
Cu(II). Moreover, Mn(II) was penta-coordinated in that example while in 5 it is hexa-
coordinated.
Electrochemistry
Cyclic voltammograms (CVs) of complexes 1−5 were recorded in acetontrile solution at a
scan rate of 100 mV s−1 with respect to an Ag/AgCl electrode. Complex 1 shows a reversible
red-ox process due to Cu(II)/Cu(I) couple at the electrode surface with Ec = −1.14 V and Ea
= −1.04 V. The peak separation value ΔEpc = Ecathodic – Eanodic [27] is −0.10 V and Ic/Ia =1.09.
Cyclic voltammograms of complexes 2−5 show that the reduction of Cu(II) to Cu(I) takes
place at less negative potential for all four complexes compared to 1, Ec = −1.11 V for 2, Ec =
−1.06 V for 3, Ec = −1.08 V for 4 and Ec = −1.09 V for 5 indicating that reduction of Cu(II)
to Cu(I) facilitates on coordination of the metalloligand to Mn(II). Moreover, in the reverse
scan no prominent peak was found showing that the reversibility of the redox process is lost
on coordination to Mn(II) as represented in Fig. S13 in the supporting information.
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DOI: 10.1039/C9DT04315B
Catechol Oxidase Studies and Kinetics.
The catalytic properties of the metalloligand [CuL] and complexes 2−5 were investigated by
studying their catecholase-like activity. Catecholase activity is the oxidation of diphenolic
compounds to their respective diquinonic forms in open atmospheric conditions28. For
studying the catecholase activity we used 3,5-di-tert-butylcatechol (3,5-DTBC) as the
substrate which can be oxidized to o-quinone (3,5-DTBQ) easily using atmospheric O2 in
acetonitrile medium29. Prior to the detailed kinetic studies, we examined the catalytic
activities of these complexes for catecholase activity according to the reaction shown in
Scheme 3.
Scheme 3. Catalytic oxidation of 3,5-DTBC to 3,5-DTBQ in acetonitrile.
The catecholase activity was measured spectrophotometrically in aerobic conditions. Equal
volumes of acetonitrile (ACN) solutions of 1 x 10−5 M of each of the synthesised complexes
(1−5) and 1 x 10−3 M of the substrate (3,5-DTBC) were mixed at room temperature. The
absorbance versus wavelength scan was performed in a range of 320−520 nm (wavelength
scan) and the data was collected at a regular interval of 5 min. Complexes 1, 2, 3 and 5 didn’t
show any rise in the absorption maxima in one hour (Fig. S14, S15, ESI respectively), which
indicate that they are not catalytically active towards the biomimicking catechol oxidase
activity. Only complex 4 showed a gradual increase in the absorption maxima near 395 nm
due to formation of 3,5-DTBQ, which is shown in Fig. 5.
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DOI: 10.1039/C9DT04315B
Fig. 5. Increase of the quinone (3,5-DTBQ) band around 395 nm after mixing of equal
volumes of acetonitrile solutions of 3,5-DTBC (1 × 10−3 M) with complex 4 (1 × 10−5 M).
The spectra were recorded at 5 min intervals up to 50 min.
The kinetics of this oxidation reaction of 3,5-DTBC to 3,5-DTBQ in the presence of complex
4 was determined by measuring the growth of the quinone band at 395 nm as a function of
time. The dependence of oxidation rate on the substrate concentration was studied by mixing
equal volumes of 1 × 10−5 M solution of the complex 4 with various concentrations of the
substrate (5 × 105 M, 1 × 104 M, 3 × 104 M, 1 × 103 M, 1.2 × 103 M, 2 × 103 M, 4 × 103
M, 6 × 103 M, 8 × 103 M, 1 × 102 M and 1.2 × 102 M). For all these measurements, the
corresponding solution of substrate without catalyst was taken as reference. The initial rate
was determined by linear regression from the slope of the absorbance versus time plot from
the data of first 10 min considering the molar extinction coefficient of quinone (3,5-DTBQ)
as 1630 M-1 cm-1.[30] The rate constant vs concentration of 3,5-DTBC data obtained was 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 value of kinetic parameters such as
kcat, Km and Vmax. The turnover number, kcat (in h−1) of this catechol oxidase reaction is 399.
The parameters Km and Vmax are 8.638 x 104 and 5.537 x 107 respectively. The curves of
both the observed rate vs [substrate] and the Lineweaver-Burk plot for complex 4 are
depicted in Fig. 6.
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DOI: 10.1039/C9DT04315B
Fig. 6. Plot of the rate vs substrate concentration for complex 4. The inset shows the
corresponding Lineweaver−Burk plot.
ESI-Mass Spectrometric Study.
To propose a probable mechanistic pathway of catalytic oxidation of substrate and to
determine the intermediate species of the oxidation reaction, we have studied the ESI−mass
spectra of complex 4 and of a mixture of 1:1 (v/v) of the complex and 3,5-DTBC (1:102 molar
ratio) in acetonitrile medium (Fig. S5, S16 and S17, ESI respectively). The mass spectrum of
4 shows a base peak at m/z = 360.06 (cald. = 360.06) which is assignable to [(CuL) + H]+.
Another prominent peak is seen at m/z = 382.04 (cald. = 382.04) which can be assigned to
[(CuL) + Na]+. A small peak at m/z = 553.02 (cald. = 553.02) is assigned to
[(CuL)Mn(benz)(H2O)]+ (Fig. S16, ESI). After 5 min of addition of 3,5-DTBC, the spectra
was once again recorded. Here also the base peak is at m/z = 360.06 for [(CuL) + H]+ and the
peak for [(CuL)Mn(benz)(H2O)]+ at m/z = 553.02 is detected. A new peak at m/z = 243.14
(cald. = 243.13) is assigned to [3,5-DTBQ + Na]+, the sodiated form of the oxidized product
of 3,5-DTBC (Fig. S16, ESI). Two other peaks at m/z = 657.12 and 779.15 for
[(CuL)Mn(3,5-DTBC) + Na]+ (cald. = 657.11) and [(CuL)Mn(benz)(3,5-HDTBC)]+ (cald. =
779.15) showing the formation of complex-substrate intermediates are detected. The peak at
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m/z = 634.14 (cald. = 634.13) indicates the presence of semiquinonate form, [(CDuOLI:)1M0.1n03V(9i3e/wC,59ArD-tiTcl0e4O3n1li5nBe
DTBQ)]+ as one of the intermediate species (Fig. S17, ESI).
Mechanistic Insight. Several mechanisms are reported for the catalytic oxidation of 3,5-di-
tert-butylcatechol to its corresponding quinone. The copper based biomimetic model
complexes are generally known to follow two possible mechanistic pathways. One pathway
is through the formation of a dicopper(II) catecholate intermediate where the dicopper(II)
species oxidizes the catechol substrate to its corresponding quinone and itself gets reduced to
dicopper(I) species. This species then reacts with an oxygen molecule to generate a
peroxidodicopper(II) adduct, which again oxidizes a second substrate molecule. Water is
formed as a byproduct in this four-electron-reduction process. The second mechanism
proceeds through the formation of an organic radical intermediate, for instance, copper(I)
semiquinone31. Here oxidation of the substrate goes through a two-electron reduction of two
oxygen atoms along with oxidation of copper(I) to copper(II) ion and release of the quinone
molecule and hydrogen peroxide as a byproduct. For complex 4, we found that H2O2 was
produced during the process of conversion of 3,5-DTBC to 3,5-DTBQ and its amount was
found to be 95% (Fig. S18 and S19, ESI respectively). Therefore, it can be assumed that
complex 4 followed the second pathway for oxidation.
On the basis of ESI-mass spectra, we propose a probable mechanism for the catechol
oxidase reaction. First an active monopositive [(CuL)Mn(benz)(H2O)]+ species is generated
in the solution, which reacts with 3,5-DTBC to form an intermediate [(CuL)Mn(benz)(3,5-
HDTBC)]+, where one of the two hydroxyl group of 3,5-DTBC is deprotonated and is
coordinated to the Mn(II) center. This intermediate species then loses the benzoate ion and
readily releases another proton of hydroxyl group and then it is converted to its
semiquinonate radical form, [(CuL)Mn(3,5-DTBQ)]+ as is evident from mass spectra. In this
step of the reaction, O2 is reduced to H2O2. Following this, the semiquinone is oxidized to
quinone and is eliminated from the metal center with the reduction of Cu(II) is to Cu(I). A
water molecule coordinates to the remaining species producing a singly charged dinuclear
species [(Cu(I)L)Mn(H2O)]+. This species further coordinates to another 3,5-DTBC to form
[(Cu(I)L)Mn(3,5-H2DTBC)]+. Finally, the Cu(I) center of the dinuclear active species is
oxidized to Cu(II) by aerial O2 and the active intermediate species [(Cu(II)L)Mn(3,5-
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DTBC)]+ is regenerated, with the formation of H2O2 as byproduct. The probableDmOI:e1c0h.10a3n9i/sCm9DT04315B
for catalytic oxidation of 3,5-H2DTBC to 3,5-DTBQ by complex 4 is shown in Scheme 5.
Scheme 4. Proposed Mechanism for Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ by
Complex 4.
Phenoxazinone synthase studies and kinetics. Detailed Description.
Phenoxazinone synthase activity is an oxidative dimerisation reaction (Scheme 5). The
possible catalytic activities of the complexes 1–5 and the metalloligand for oxidation of o-
aminophenol (OAP) to aminophenoxazinone (APX) in acetonitrile medium were also
examined. For this reaction also only complex 4 responded.
Scheme 5. Catalytic oxidation of o-aminophenol (OAP) to aminophenoxazinone (APX) in
acetonitrile medium.
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APX shows a broad absorbance band near 426 nm in pure acetonitrile medium. A 1 x 10−5 M
DOI: 10.1039/C9DT04315B
solution of complex was treated with 1 x 10−3 M equivalent of 2-aminophenol (OAP) in
acetonitrile medium under aerobic conditions at room temperature for absorbance vs
wavelength scan and the data was recorded at an interval of 3 min. The spectral change for
complex 4 on addition of OAP in the wave scan is shown in Fig. 7. The metalloligand is
found to be inactive in this case too. Also there was no growth in the absorbance maxima for
complexes 2, 3 and 5, as shown in Fig. S20 and S21, ESI respectively. Therefore, only
complex 4 catalyses this oxidase reaction.
Fig. 7. Increase of the amino-phenoxazinone (APX) band around 425 nm after mixing of
equal volumes of acetonitrile solutions of OAP (1 × 10−3 M) with complex 4 (1 × 10−5 M).
The spectra were recorded at 3 min intervals up to 50 min.
We carried out a detailed kinetic study on complex 4 following the above mentioned
procedure for catechol oxidase study. The kinetics of this oxidation reaction of o-
aminophenol to amino-phenoxazinone in presence of complex 4 was determined by
measuring the growth of the APX band at 425 nm as a function of time. The dependence of
oxidation rate on the substrate concentration was studied by treating 1 × 10−5 M solution of
the complex 4 with various concentrations of the substrate (2 × 104 M, 5 × 104 M, 1 × 103
M, 2 × 103 M, 6 × 103 M, 8 × 103 M, 1 × 102 M and 1.2 × 102 M) similar to the previous
process. Here also the corresponding solution of substrate without catalyst was taken as
reference. The initial rate was determined by linear regression from the slope of the
absorbance versus time plot from the data of first 10 min considering the molar extinction
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coefficient of APX as 9300 M1 cm1.[32] The curves of the observed rate vs [suDbOsIt:r1a0t.1e0]39a/Cn9dDT04315B
Lineweaver‒Burk plots for complex 4 are depicted in Fig. 8. The kcat (in h−1) value of this
catalysis is 230. Values of Km and Vmax obtained from the Lineweaver-Burk plot are 3.35 x
103 and 3.19 x 107 respectively.
Fig. 8. Plot of the rate vs substrate concentration for complex 4. The inset shows the
corresponding Lineweaver−Burk plot.
ESI-Mass Spectrometric Study.
To account for the mechanistic pathway of the phenoxazinone activity, we measured the ESI-
MS spectra of complex 4 with OAP in 1:1 (v/v) mixture (Fig. S22 and S23, ESI respectively).
The base peak is obtained at m/z = 360.06 (cald. = 360.06) which is assignable to [(CuL) +
H]+. The peaks found at m/z = 215.08 (cald. = 215.08) and 450.03 (cald. = 450.01) are
assigned to [APX + H]+ and [(Cu(I)L)Mn(H2O)2]+ respectively (Fig. S22, ESI). The peaks at
m/z = 653.06 (cald. = 653.06), 683.02 (cald. = 683.02), 705.98 (cald. = 705.98) can be
assigned to [(CuL)Mn(OAP)2 + Na]+, [(CuL)Mn(benz)(OAP) + K]+ and [(Cu(I)L)Mn(OAP)2
25

Dalton Transactions
Page 26 of 40
View Article Online
+ 2K]+ (Fig. S23, ESI). The presence of these intermediate species gives a probable indication
DOI: 10.1039/C9DT04315B
to the mechanistic pathway and H2O2 is not produced as a byproduct of this reaction.
Mechanistic Insight. Just as catecholase activity, much of phenoxizinone activity has been
reported earlier using homo-metallic Co(II/III), Mn(II/III), Ni(II) complexes as well as
heterometallic Cu(II)-Mn(II) or Ni(II)-Mn(II) complexes as model catalysts33. It was found
that participation of two metal ions is quite important in the conversion of o-aminophenol to
amino-phenoxazinone. From the reports it has been observed that this mechanism proceeds
through radical pathway.
During this catalytic reaction, the active species [(CuL)Mn(benz)H2O]+ loses its water
molecule and an o-aminophenol molecule coordinates to the Mn(II) centre. Further the
benzoate ion is substituted by another o-aminophenol molecule forming a neutral dinuclear
species [(CuL)Mn(OAP)2]. Then a hydrogen ion is lost from the amine (−NH2) group of one
o-aminophenol and the species converts into o-aminophenolate radical. This radical is
unstable and turns into its o-iminophenolate by releasing a proton from another o-
aminophenol and Cu(II) is reduced to Cu(I) centre. During this oxidative process one
molecule of H2O is produced by reduction of 1/2 O2. The existence of this species is
indicated by mass spectra. Subsequently, the o-iminophenolate and o-aminophenolate
combine to form phenoxazinone and it is eliminated from the dinuclear complex leaving
behind a monocationic [(Cu(I)L)Mn(H2O)2)]+ species. Again oxygen molecule gets reduced
to water in this step. The monocationic species further coordinates to two o-aminophenols
and Cu(I) is oxidised to Cu(II) by oxygen molecule to form [(Cu(II)L)Mn(OAP)2] species.
The probable mechanism of this catalytic reaction is given in Scheme 6.
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Page 27 of 40
Dalton Transactions
View Article Online
DOI: 10.1039/C9DT04315B
Scheme 6. Proposed Mechanism for Catalytic Oxidation of o-aminophenol to
aminophenoxazinone by Complex 4.
Structure−Activity Correlation for the Catalytic Activity
Recently, it has been reported that heterometallic Cu-Mn and Ni-Mn complexes of
dicondensed tetradentate Schiff base ligand can show catecholase and phenoxazinone
synthase-like oxidase activities with high efficiency34a. It is important that the heterometallic
core should be stable in solution for such catalytic reactions. The mechanistic exploration
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Page 28 of 40
revealed that nature of the ligands around the metal centres played a very impoDrtOaIn: 1t0.r10o3Vl9iee/wCi9AnrDtiTcl0e4O3n1l5inBe
the catalytic efficiencies of the complexes35. Generally, presence of coordinated solvent
molecule(s) to the metal centres enhances catalytic efficiencies of the complexes35a. Among
the present complexes 2−5, only 4 shows catalytic activities which can be attributed to the
presence of a coordinated water molecule to Mn(II) of it. The labile water molecule is easily
eliminated, leaving the coordination sites open for binding with the substrate (catechol/o-
aminophenol). On the other hand, although there is a coordinated methanol molecule at the
Mn(II) ion of 3, it is inactive for such catalytic oxidase activity. By a close inspection of the
coordination environment of Mn(II) in this complex, one can see that it is hepta-coordinated.
Therefore, even on removal of the methanol molecule, Mn(II) remains hexacoordinated and
the complex-substrate intermediate like complex 4 is not likely to form. Hence, it is
catalytically inactive. It is also to be noted that although the reduction of Cu(II) to Cu(I) is
facilitated in all heterometallic complexes (2−5) compared to 1, only 4 is catalytically active
suggesting that coordination environment around the metal centre and not the ease of
reduction of Cu(II) is crucial for such heterometallic complex to be catalytically active as
evident from the electrochemical study of the complexes. Moreover, the kcat values of
complex 4 for catecholase and phenoxazinone synthase activity are appreciably higher than
most of the homo-metallic synthetic models Cu(II) compounds36. Although, the kcat values of
a few heterometallic Cu(II)-Mn(II) or Ni(II)-Mn(II) complexes for the same oxidation
reactions are significantly higher that of the present complex. (Table S6 ESI)
Magnetic properties
The magnetic properties of complexes 2, 3, 4 and 5 on powdered and pressed samples were
investigated in the 2 300 K temperature range and under a field of 0.03 T and plotted as T vs
T plots.
Complexes 2 and 3, despite their different structures of discrete and polymeric
complexes, present an overall antiferromagnetic behavior with experimental values of T at
room temperature of 4.67 cm3·K·mol−1 and 4.81 cm3·K·mol−1, respectively, which are
slightly lower than the expected value for one Mn(II) and one Cu(II) non-interacting cations
(Fig. S24, ESI). The T vs. T plot for complex 2 decreases with temperature and reaches a
minimum at 2.21 cm3·K·mol−1. In this case, the final decay of the curve at very low
temperature is due to the existence of antiferromagnetic interactions between molecules37,21b
.
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Page 29 of 40
Dalton Transactions
For 3, the curve decays until a minimum of 3.07 cm3·K·mol−1 is reached at 17DKOI: a10n.1d03Vt9ieh/wCe9AnrDtiTcl0e4O3n1li5nBe
experiences a small increase at very low temperature due to the presence of ferromagnetic
interactions between the neighboring molecules in the chain. The compound presents the
magnetic behavior of an isolated molecule, despite the chain structure of the compound,
because the axial position of the Cu(II) ion does not allow magnetic coupling with the
neighboring paramagnetic ion38.
Using the spin-Hamiltonian H = –2J (ŜMn·ŜCu) in the full temperature range, the best fitting
values obtained are: J = –13.5 cm−1 and J = –13.5 cm−1 for 2 and 3 and a global g value of
2.02 for both complexes.
Complex 4 presents a completely different geometry. The molecule has two different
coupling constants and both are antiferromagnetic. The T vs T curve presents a room
temperature experimental value of 9.91 cm3·K·mol−1 , again a value slightly lower than the
theoretical value for non interacting two Mn(II) and two Cu(II) ions (Fig. S24, ESI). For the
calculation of magnetic interaction parameters, we have assumed two exchange interactions
in the tetranuclear core as shown in Fig. 9. The spin-Hamiltonian H = –2J1(ŜMn1·ŜCu1
+
ŜMn2·ŜCu2 )–2J2(ŜMn1·ŜMn2) is used to fit the experimental data and the curve is reproduced
with J1 = –12.6 cm-1 and J2 = –1.20 cm−1 and a global g value of 2.04.
Fig. 9. Tetranuclear unit of complex 4 with different magnetic exchange couplings (J).
Complex 5 presents a T value of 9.77 cm3·K·mol−1 at room temperature (Fig. S24, ESI). On
lowering the temperature, the curve decays gradually until a value of 6.02 cm3·K·mol−1 is
reached and then decays rather abruptly due to the antiferromagnetic interactions between
neighboring molecules, zJ', reaching a final value of 4.04 cm3·K·mol−1. Again, for the
calculation of magnetic interaction parameters, we have assumed two exchange interactions
in the tetranuclear core (Fig. 10.). The spin-Hamiltonian, H = –2J1(ŜMn1·ŜCu1+ ŜMn2·ŜCu2 )–
29