Subtle Structural Changes in (CuIIL)2MnII Complexes To Induce Heterometallic Cooperative Catalytic Oxidase Activities on Phenolic Substrates (H2L = Salen Type Unsymmetrical Schiff Base)

Article

pubs.acs.org/IC

II

Subtle Structural Changes in (Cu L) Mn Complexes To Induce

2

Heterometallic Cooperative Catalytic Oxidase Activities on Phenolic

Substrates (H L = Salen Type Unsymmetrical Schiﬀ Base)

2

†

‡

Prithwish Mahapatra, Soumavo Ghosh, Sanjib Giri, Vinayak Rane, Ramakant Kadam,

§

,†

Michael G. B. Drew, and Ashutosh Ghosh*

†

Department of Chemistry, University College of Science, University of Calcutta, 92, A. P. C. Road, Kolkata 700009, India

Radiochemistry division, Bhabha Atomic Research Center, Trombay, Mumbai 400085, India

‡

§

School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, U.K.

*

S Supporting Information

ABSTRACT: A new Cu(II) complex of an asymmetrically

dicondensed Schiﬀ base (H L = N-(2-hydroxyacetophenylidene)-

2

N′-salicylidene-1,3-propanediamine) derived from 1,3-propanedi-

amine, salicylaldehyde, and o-hydroxyacetophenone has been

synthesized. Using this complex, [CuL] (1), as a metalloligand,

two new trinuclear Cu-Mn complexes, [(CuL) Mn(N )(H O)]-

2

3

2

(

ClO )·H O (2) and [(CuL) Mn(NCS) ] (3), have been

4 2 2 2

prepared. Single-crystal structural analyses reveal that complexes

2

+

and 3 both have the same bent trinuclear {(CuL) Mn}

2

structural unit in which two terminal bidentate square-planar

CuL) units are chelated to the central octahedral Mn(II) ion. This

(

structural similarity is also evident from the variable-temperature

magnetic susceptibility measurements, which suggest that compounds 2 and 3 are both antiferromagnetically coupled with

−1

comparable exchange coupling constants (−21.8 and −22.3 cm , respectively). The only diﬀerence between 2 and 3 lies in the

−

coordination around the central Mn(II) ion; in 3, two SCN groups are coordinated to the Mn(II), leaving a neutral complex,

but in 2, one N₃⁻group and one H O molecule are coordinated to give a positively charged species. The presence of such a labile

2

H O coligand makes 2 catalytically active in mimicking two well-known polynuclear copper proteins, catecholase and

2

phenoxazinone synthase. The turnover numbers (k ) for the aerial oxidation of 3,5-di-tert-butylcatechol and o-aminophenol are

cat

−1

1

118 and 6581 h , respectively, values which reﬂect the facility of the heterometallic catalyst in terms of both eﬃciency and

catalytic promiscuity for aerial dioxygen activation. The mechanisms of these biomimetic oxidase reactions are proposed for the

ﬁrst time involving any heterometallic catalyst on the basis of mass spectral analysis, EPR spectroscopy, and cyclic voltammetry.

The evidence of the intermediates indicates possible heterometallic cooperative activity where the substrates bind to a Mn(II)

center and Cu(II) plays the role of an electron carrier for transformation of the phenolic substrates to their respective products

with the reduction of aerial dioxygen.

INTRODUCTION

condensation of 1,3-propanediamine with salicyladehyde and o-

hydroxycaetophenone at low temperature. However, the

■

5

The large range of N,O-donor ligands has already become one

of the most essential tools in present-day research for the

syntheses of metal-containing molecular materials due to their

ability to assemble multiple metal ions in a predetermined

stereochemical environment. Among them, a small group of

ligands termed as Salen type N O -donor symmetrical

method needs rigorous separation of the unsymmetrical Schiﬀ

base from the two symmetrical bases and the yield of the

former is rather low and is highly susceptible to experimental

conditions. As a result, only a handful of mononuclear

complexes of such asymmetric ligands are known and, barring

only three trinuclear Ni(II) complex and one trinuclear Ni(II)-

Cu(II) complex, no other polynuclear complexes of such

1

2

tetradentate Schiﬀ bases has taken a signiﬁcant share of the

library of coordination compounds, presumably due to their

3

−5

2

ligands have been reported.

On the other hand, very

synthetic simplicity. In contrast, their asymmetrically dicon-

recently we have developed a modiﬁed Elder’s method for the

synthesis of unsymmetrical Schiﬀ bases exclusively in a very

densed analogues are rarely reported due to the inherent

synthetic challenges of the asymmetric condensation of the

aliphatic diamine precursor. Costes et al. reported an eﬃcient

6

3

quick and simple manner. Here, we wish to synthesize the

high-dilution method, but this is eﬀective only when one of the₄

carbonyl compounds is acetylacetone or its derivatives.

Received: January 30, 2017

̈

Recently, Oz et al. synthesized an unsymmetrical ligand by

©

XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

mononuclear Cu(II) complex of this unsymmetrical ligand

other reagents and solvents were of commercially available reagent

quality, unless otherwise stated.

Caution! Although not encountered during experiment, perchlorate and

azide salts of metal complexes with organic ligands are potentially

explosive. Only a small amount of material should be prepared, and it

should be handled with care.

H L, which has not yet been reported, and use it as a

2

metalloligand for the synthesis of heterometallic polynuclear

complexes.

In relation to bioinspired and biomimetic catalysts, the

polynuclear complexes are very useful, as the closely assembled

metal centers with speciﬁc stereochemical arrangement can

Synthesis of the Unsymmetrical Schiﬀ Base Ligand N-(2-

Hydroxyacetophenylidene)-N′-salicylidene-1,3-propanedi-

amine (H L). The unsymmetrical di-Schiﬀ base ligand H L was

1a,d

recognize typical molecules, such as natural metalloproteins.

2

A few of these metalloproteins utilize their active sites for the

activation, storage, and transport of aerial dioxygen molecules

in enzymatic pathways: e.g., catechol oxidase (COx),

phenoxazinone synthase (PHS), cytochrome c oxidase, etc.

Catechol oxidase is a dicopper-containing type III active-site

protein that catalyzes the oxidation reaction of a wide range of

o-diphenols (catechols) to the corresponding o-quinones, a

synthesized in our laboratory by the method developed recently by

6

us. First, the complex [Ni(L ) ]·2H O (HL = 2-((E)-(3-

1

2

1

aminopropylimino)methyl)phenol) was prepared by the reaction of

bis(salicylaldehyde)nickel(II) dihydrate (3.09 g, 10 mmol) and 1,3-

propanediamine (2.10 mL, 25 mmol). Then, this complex (4.11 g, 10

mmol) was dissolved in 50 mL of methanol and solid bis(o-

hydroxoacetophenone)nickel(II) dihydrate (3.37 g, 10 mmol) was

added to it and the mixture was reﬂuxed for 2 h. The complex

[Ni (L ) (o-Hap) ] (o-Hap = o-hydroxyacetophenone), which sepa-

7

process known as catecholase activity. Similarly, activity of

2

1

2

PHS (in which the active site contains ﬁve Cu ions) is also

important to understand the relevance of the copper oxidase

model in a biological system where o-aminophenol (OAP) is

rated out during this reaction, was ﬁltered and air-dried. A 5 mmol

portion (3.85 g) of this complex was dissolved in 25 mL of methanol,

and solid dimethylglyoxime (DMG; 2.32 g, 20 mmol) was added to it.

The mixture was reﬂuxed on a water bath for 2 h, cooled, and ﬁ ltered.

The yellow ﬁltrate was subjected to ESI mass spectroscopy (Figure

S30 in the Supporting Information), and the product was found to be

8

oxidized to produce aminophenoxazinone (APX). These two

enzymatic activities have also been modeled in vitro with

several biomimetic copper complexes using aerial oxygen as a

naturally viable oxidant, which is of immense importance in

exclusively the unsymmetrical Schiﬀ base (H L: m/z 297.18, calcd

2

97.16); no signals corresponding to any of the possible symmetrical

9

,10

economic and environmental issues. The catalytic action of a

large number of mono-/polynuclear Mn(II/III), Fe(II/III), and

Co(II/III) as well as polynuclear Ni(II), Cu(II), and Zn(II)

complexes in terms of catecholase and phenoxazinone synthase

Schiﬀ bases were detected (calcd m/z 283.14 and 311.18). The

methanolic solution of the ligand [H L] was used directly for the

2

1

complex preparation. The H NMR spectrum of the ligand was also

recorded (Figure S29 in the Supporting Information).

Synthesis of [CuL] (1). A methanolic solution (10 mL) of

11,12

activity has already been explored extensively.

However,

Cu(ClO ) ·6H O (1.85 g, 5 mmol) was added to a methanolic

until now the COx activity of heterometallic polynuclear

complexes has been rarely examined and to our knowledge

there has been no report on the PHS activity of any such

4

2

solution of H L (5 mmol, 25 mL). The mixed solution was stirred for

2

0

.5 h and kept in a desiccator. X-ray-quality green single crystals

1

3

started to grow in the beaker after 2−3 days on slow evaporation of

methanol. Crystals were ﬁltered from the solution after 5 days and

dried in air.

complex. In fact, except for the mimicking of cytochrome c

1

4a,b

oxidase,

heterometallic systems still remain largely unex-

plored for dioxygen activation in catalytic oxidase reactions and

Complex 1. Yield 1.431 g (80%). Anal. Calcd for C H CuN O

357.89): C, 60.41; H, 5.07; N, 7.83. Found: C, 60.27; H, 5.11; N,

18

2

are limited to only a few β-diketiminate ligand based high-

(

14

valent heterometallic dinuclear complexes. However, explora-

tion of heterometallic complexes as multimetallic cooperative

catalysts is one of the foremost areas in current research trends

on catalysis, yet their potential for catalytic promiscuity in

7.79. Anal. Calcd: Cu, 17.76. Found: 17.63 (determined by SEM-

−

1

EDS). IR: ν

1625 and 1595 cm for formylimine and acetylimine,

CN

respectively.

Synthesis of the Complex [(CuL) Mn(N )(H O)](ClO )·H O (2).

2

3

2

4

2

8c,15

Complex 1 [CuL] (0.716 g, 2 mmol) was dissolved in 20 mL of

methanol, and Mn(ClO ) ·6H O (0.361 g, 1 mmol) was added to the

multiple catalytic transformations has been rarely evaluated.

Herein, we prepare a tetradentate N O donor unsym-

4

2

solution. The solution was stirred for 10 min and then 2 mL of a 1/1

v/v) H O/MeOH solution of NaN (0.130 g, 2 mmol) was added to

metrical Schiﬀ base ligand (H L = N-(2-hydroxyacetophenyli-

2

(

2

3

dene)-N′-salicylidene-1,3-propanediamine) using 1,3-propane-

it dropwise at room temperature. Stirring of the mixture was continued

for 0.5 h. The small amount of precipitate that appeared during this

process was ﬁltered, and the ﬁltrate was kept in a beaker inside a

desiccator. Brown single crystals of X-ray quality appeared inside the

beaker, which were separated by ﬁltration after 3 days and dried in air.

Complex 2. Yield 0.664 g (70%). Anal. Calcd for

C H ClCu MnN O (948.23): C, 45.60; H, 4.25; N, 10.34.

diamine, salicylaldehyde, and o-hydroxyacetophenone. The

ligand H L is used to produce the mononuclear copper(II)

2

complex [CuL] (1) by reacting with Cu(ClO ) ·6H O in

4

2

methanol. Using this [CuL] chelate (1) as a bidentate

asymmetric metalloligand, we have synthesized two new

trinuclear complexes [(CuL) Mn(N )(H O)](ClO )·H O (2)

36 40

2

7

10

2

3

2

4

2

Found: C, 45.52; H, 4.27; N, 10.38. Cu:Mn ratio 2.08:1.05

and [(CuL) Mn(NCS) ]·H O (3). All three complexes have

2

(

2

determined by SEM-EDS). IR: ν

1633 and 1599, ν

CN

NNN

been characterized by X-ray crystal structure determinations.

Among the three complexes, only complex 2 exhibits a novel

catalytic promiscuity in model biomimetic catecholase like

activity and phenoxazinone synthase like activity under an aerial

atmosphere. Cyclic voltammograms, ESI-mass spectra analysis,

and EPR studies have also been performed to investigate the

possible intermediates of the catalytic reactions and con-

sequently to devise a probable mechanism.

−1

062 and 1599, ν

1108 cm .

C1O

Synthesis of the Complex [(CuL) Mn(NCS) ] (3). The

2

compound was prepared following a procedure similar to that for

the synthesis of 2. The only diﬀerence was that here NH SCN (0.152

4

g, 2 mmol) was used instead of NaN . In this case, yellowish-green

3

single crystals of X-ray quality appeared in the beaker after 2−3 days

on the slow evaporation of solvent. Crystals were ﬁltered from the

solution and air-dried.

Complex 3. Yield 0.603 g (68%). Anal. Calcd for

C H Cu MnN O S (886.89): C, 51.46; H, 4.09; N, 9.48. Found:

38

36

2

6

4 2

EXPERIMENTAL SECTION

Starting Materials. Salicylaldehyde, o-hydroxoacetophenone, and

,3-propanediamine were purchased from Spectrochem India and were

■

1

C, 51.42; H, 4.05; N, 9.45. Cu:Mn ratio 7.01:3.56 (determined by

−

1

SEM-EDS). IR: ν

1627 and 1597, ν

2068 and 2054 cm .

CN

NCS

Physical Measurements. Elemental analyses (C, H, and N) were

of reagent grade. They were used without further puriﬁcation. The

performed using a PerkinElmer 2400 series II CHN analyzer. IR

B

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 1. Crystal Data and Structure Reﬁnement Details of Complexes 1−3

1

2

3

formula

mol wt

cryst yst

space group

a, Å

C H Cu N O

C H Cu MnN O , ClO ,O

C H Cu MnN O S

38 36 2 6 4 2

3

6

36

2

4

36 36

2

7

5

4

715.79

944.21

886.89

monoclinic

Cc

monoclinic

P2 /c

1

P2 /n

1

7.317(5)

22.932(5)

9.537(5)

90

25.260(5)

10.146(2)

15.128(2)

90

13.659(4)

18.556(4)

15.412(2)

90

b, Å

c, Å

α, deg

β, deg

γ, deg

V, Å³

105.390(5)

90

95.317(14)

90

96.028(15)

90

1542.9(14)

2

3860.4(12)

4

3884.7(15)

4

Z

−

3

D , g cm

1.541

1.625

1.516

c

μ, mm⁻

F(000)

R(int)

1

1.427

1.550

1.560

740

1924

1812

0.037

0.082

0.105

total no. of reﬂns

10647

2767

9407

17748

no. of unique reﬂns

no. of rﬂns with I > 2σ(I)

4660

6802

2242

3760

4248

a

b

R1, wR2

0.0332, 0.0859

1.03

0.1073, 0.2470

1.13

0.1128, 0.2208

1.18

c

2

GOF on F

R(all)

0.0441

0.1305

150

0.1919

150

temp, K

residual electron density, e Å⁻

293

3

−0.56, 0.23

−0.87, 1.60

−0.49, 0.96

a

R1 = ∑||F | − |F ||/∑|F |. wR2(F ) = [∑[w(F − F ) /∑wF ] . GOF = ∑[w(F_o− F ) /(N_o_b_s− N_p_a_r_a_m_s₎_]1/2

b

2

o

2

o

2

c

2

4

o

1/2

c

2

2 2

c

.

o

c

o

−

1

spectra as KBr pellets (4000−500 cm ) were recorded using a

PerkinElmer RXI FT-IR spectrophotometer. The electronic absorp-

tion spectra (1000−240 nm) in methanol solution were collected with

a Hitachi U-3501 spectrophotometer. Temperature-dependent molar

susceptibilities for powdered crystalline samples of compounds 2 and 3

were measured using a superconducting quantum interference device

vibrating sample magnetometer (SQUID-VSM, Quantum Design)

with an applied ﬁeld of 500 Oe throughout the temperature range 2−

mass range of 100−1200 Da, in a continuum mode. Aqueous sodium

formate was used for Q-oa-ToF calibration. L-Leucine was used as the

+

external mass calibrant lock with mass [M + H] 556.2771 Da.

Solutions of compounds were injected at a ﬂow rate of 5 μL/min. All

solutions were made in mass spectrometric grade methanol. The

concentrations of complexes 2 and 3 as well as complexes + substrates

−6

in solutions were the same (10 M) during acquisitions of mass

spectra.

3

00 K. The susceptibility data were corrected by Pascal’s diamagnetic

contributions. Isothermal magnetization measurements were per-

formed at 2, 3, 5, 7, and 10 K up to 5 T magnetic ﬁeld. H NMR

spectral analysis of the ligand (after puriﬁcation by crystallization from

X-ray Crystallographic Data Collection and Reﬁnement. The

independent reﬂection data were collected with Mo Kα radiation, for 1

at 293 K and for 2 and 3 at 150 K, with the Oxford Diﬀraction X-

Calibur CCD System. The crystals were positioned at 50 mm from the

CCD, and 321 frames were measured with counting times of 10 s.

1

methanol) in CDCl₃was carried out on a 300 MHz instrument

16

(

Bruker), where tetramethylsilane (TMS) was used as an internal

standard. The Cu and Mn contents and their ratios in complexes 2 and

were measured on a ZEISS EVO-MA 10 scanning electron

Data analysis was carried out with the CrysAlis program. The

structures were solved using direct methods with the Shelxs97

1

7

3

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. Absorption corrections for 1−3 were carried out using the

microscope equipped with an EDS (INCA Energy 250 Microanalysis

System). Powdered samples of all three compounds were placed on

pieces of carbon tape before imaging. Powder X-ray diﬀractograms are

recorded on a Bruker D-8 Advance diﬀractometer operated at 40 kV

voltage and 40 mA current and calibrated with a standard silicon

sample, under Ni-ﬁltered Cu-Kα (α = 0.15406 nm) radiation.

Electrochemical Measurements. The electrochemical measure-

ments of all three complexes (1−3) were performed using an Epsilon

Basi-C3 Cell instrument at a scan rate of 100−400 mV s within the

potential range of 0 to −1.80 V vs Ag/AgCl. Cyclic voltammograms

were carried out using 0.1 M tetrabutylamonium perchlorate (TBAP)

as supporting electrolyte and 1.0 × 10 M of complexes in

dichloromethane solution, which were deoxygenated by bubbling

1

8

ABSPACK program. The hydrogen atoms bonded to the

coordinated water molecule and solvent water molecule in 2 could

2

17

not be located. The structures were reﬁned on F using Shelxl97.

Data collection, structure reﬁnement parameters, and crystallographic

data for complexes 1−3 are given in Table 1.

−1

Catalytic Oxidation of 3,5-DTBC. The catecholase activity of

complex 2 was studied using 3,5-di-tert-butylcatechol (3,5-DTBC) as

the substrate in an methanolic solution under aerobic conditions at

room temperature. The reactions were followed spectrophotometri-

cally by monitoring the increase in the absorbance maxima of the

quinone band at 400 nm as a function of time (time scan). To detect

the formation of hydrogen peroxide during the catalytic reaction, we

−3

2

with argon. The working electrode was a glassy-carbon disk (0.32 cm )

that was polished with alumina solution, washed with absolute acetone

and dichloromethane, 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.

ESI-HRMS. Electrospray ionization mass spectrometry (ESI-MS

positive) ion mass spectra were acquired using a Xevo G2-S QTof

Waters) mass spectrometer, equipped with a Z-spray interface, over a

1

9

followed the iodometric method as reported earlier.

Phenoxazinone Synthase of OAP. The phenoxazinone synthase

activity of complex 2 was studied using o-aminophenol (OAP) as the

substrate in a methanolic solution under aerobic conditions at room

temperature. The reactions were followed spectrophotometrically by

monitoring the increase in the absorbance maxima of the amino

(

C

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 1. Syntheses of Complexes 1−3

phenoxazinone (APX) band at 425 nm as a function of time (time

scan).

their respective simulated (from Mercury 3.8) diﬀractograms

(

Figures S32 −S34 in the Supporting Information) indicates the

EPR Spectra. The EPR spectra were recorded with a Bruker EMX

series spectrometer operating at X-band with 100 kHz modulation

frequency. DPPH (g = 2.0036) was used as a g standard. To ensure

good glass formation, 20−30% ethanol was added to the solutions of

the studied compounds in methanol. Typical experimental parameters

were as follows: modulation amplitude 2 G, microwave power 7 mW,

time constant 20.48 ms, conversion time 40 ms.

purity of the complexes.

It is worthwhile to mention that a large number of

mononuclear/polynuclear complexes of symmetrically dicon-

densed salen type Schiﬀ base ligands have been reported during

the last ﬁve decades. Recently, we have shown that Cu(II) and

Ni(II) complexes of such ligands can be conveniently used to

prepare various heterometallic polynuclear complexes by the

“

complex as ligand” strategy. On the other hand, only recently

RESULTS AND DISCUSSION

■

̈

Oz et al. synthesized three homometallic Ni(II) complexes and

one heterometallic Ni(II)/Cu(II) polynuclear complex using

asymmetrically dicondensed N O -donor salen type di-Schiﬀ

Syntheses of the Complexes. Recently we developed a

6

rapid and facile method for the synthesis of tetradentate N O

2

donor unsymmetrical Schiﬀ base ligands as exclusive products

by modiﬁcation of Elder’s method. Following that method, we

prepared here N-(2-hydroxyacetophenylidene)-N′-salicylidene-

base ligands. However, no mononuclear complexes of such

asymmetric ligands have yet been reported to be used as

metalloligands. The reported heterometallic complex was

synthesized by relying on serendipity as two metal ions were

added simultaneously to the ligand solution. For the designed

synthesis of heterometallic complexes, use of the mononuclear

complex is the key component of the “complex as ligand”

strategy. In the present study, we isolated the key Cu(II)

metalloligand derived from an asymmetrically dicondensed

salen type Schiﬀ base ligand and used it to prepare two

heterometallic trinuclear Cu-Mn complexes with azide and

thiocyanate as coligands.

1

,3-propanediamine [H L] and reacted this N O donor ligand

2 2 2

with Cu(ClO ) ·6H O to synthesize its hitherto unknown

4

2

mononuclear Cu(II) complex (1). This complex [CuL] on

−

reaction with Mn(ClO ) ·6H O in the presence of N and

4

2

3

−

SCN ions yielded the complexes [(CuL) Mn(N )(H O)]-

2

3

2

(

ClO )·H O (2) and [(CuL) Mn(NCS) ] (3), respectively

4 2 2 2

Scheme 1). It is to be noted that, although the [CuL]:Mn-

ClO ) ·6H O:(NaN or NH SCN) ratios are the same

4

2

3

4

2:1:2), the compositions of the complexes 2 and 3 are

diﬀerent with respect to the coligands. In addition to elemental

analyses of all three complexes, the weight percent of Cu in

complex 1 and the ratio of the weight percent of Cu and Mn in

complexes 2 and 3 were evaluated from SEM-EDS analyses

Figure S31 in the Supporting Information). The resemblance

of the measured powdered diﬀractograms of complexes 1−3 to

IR Spectra of Complexes 1−3. In the IR spectra of these

complexes (1−3 ), strong peaks at 1625 and 1595 cm⁻for

compound 1 (Figure S1 in the Supporting Information), 1633

and 1599 cm⁻for compound 2 (Figure S2 in the Supporting

1

(

−1

Information), and 1627 and 1597 cm for compound 3

D

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Figure S3 in the Supporting Information) are observed due to

Article

(

The asymmetric unit of 2 consists of a discrete [(CuL) Mn-

2

5

+

azomethine ν

stretching. The appearance of two peaks for

(N )(OH )] cation, a perchlorate anion, and a water molecule.

In the cation, shown in Figure 2, two molecules of the

CN

3

2

the azomethine groups in all of the complexes indicates the

asymmetric nature of the Schiﬀ base. In the spectrum of

−1

compound 2, the strong bands at 2062 and 1108 cm are

−

attributable to the stretching vibrations for N₃and ClO ,

4

respectively. Compound 3 shows strong and sharp peaks at

1

−

20

2

068 and 2054 cm⁻for SCN stretching. The appearance of

−

two bands indicates that the two SCN ions are not identical in

the compound, as was corroborated by a structural analysis.

Description of Structures. Complex 1 possesses a

diphenoxido-bridged centrosymmetric dinuclear structure of

formula [Cu L ], as shown in Figure 1. In the asymmetric unit,

2

Figure 2. Structure of the cation of 2 with thermal ellipsoids at 20%

probability. The perchlorate anion and solvent water molecules are not

shown. The hydrogen atoms on the coordinated water molecule were

not located.

metalloligand [CuL] are bonded to a central Mn(3) atom

through four oxygen bridges. The octahedral coordination of

the Mn(3) atom is completed by an azide anion and a water

molecule. Charge balance is then provided by a perchlorate

anion. A noncoordinated water molecule is also present in the

asymmetric unit. The copper atoms in the metalloligands retain

their four-coordinate square-planar arrangement, being bonded

Figure 1. Structure of the centrosymmetric dimeric complex 1 with

thermal ellipsoids at 20% probability.

the central metal atom Cu(II) is bonded to four donor atoms of

2−

the ligand (L ). Two phenoxido oxygen atoms (O(11) and

O(31)) and two imine nitrogen atoms (N(19) and N(23)) of

the unsymmetrical Schiﬀ base ligand constitute the basal plane

2−

to the four donor atoms of the asymmetric ligand L , but the

square plane is far less distorted with τ values of 0.077 and

5

a

around Cu(II). The phenoxido oxygen O(11) of the o-

0.055. The bond lengths around Cu(1) and Mn(3) are shown

in Table S1 in the Supporting Information. The equatorial

planes show r.m.s. deviations for the donor atoms of 0.148 and

0.046 Å with the metal atoms deviating by 0.033(7) and

0.020(7) Å. The two planes intersect at 28.1(3)°. In 2 there are

short contacts between the water molecule O(1W) and oxygen

atoms, namely a perchlorate oxygen O(72) at 2.753(3) Å and

the coordinated water molecule O(1) (symmetry trans-

formation 1/2 + x, 1/2 + y, z) at 2.744(3) Å, and these are

likely to be hydrogen bonds, although the hydrogen atoms on

O(1W) and O(1) were not located.

hydroxyacetophenone moiety of the Schiﬀ base of another

symmetry-related unit coordinates to one of the axial sites of

Cu(II) to make its geometry pentacoordinated. The Addison

parameter τ = 0.171 (τ = (β − α)/60) indicates a distorted-

5

21

square-pyramidal geometry. In the basal plane, Cu−O

distances are 1.910(2) and 1.935(2) Å and Cu−N distances

are 1.965(2) and 2.030(2) Å, whereas the axial bond length

a

Cu(1)−O(11) (where the superscript “a” denotes the

symmetry transformation 1 − x, −y, 2 − z) is 2.519(2) Å.

The root mean squared (r.m.s.) deviation of the four basally

coordinated atoms of Cu(1) from the mean plane is 0.217 Å,

with the metal atom placed at 0.071(1) Å away from the plane,

The monomeric structure of [(CuL) Mn(NCS) ] (3) is

2

shown in Figure 3. In this case, charge balance is obtained by

two thiocyanate ligands bonded via nitrogen to the Mn atom.

The overall dimensions in the three metal coordination spheres,

shown in Table S1 in the Supporting Information, are very

similar to those in 2 and also to values found in previous

a

toward the O(11) atom. It should be noted that between the

two basal Cu(1)−O

bonds of the [CuL] in complex 1,

phenoxo

the bond to the phenoxo oxygen atom of salicylaldehyde is

slightly longer (0.025 Å) than that to the phenoxo oxygen of o-

hydroxyacetophenone. The relative elongation of the Cu−

13,22

trinuclear structures of this type.

The two independent

O

phenoxo

bonds in salen type asymmetric ligands within a range

copper atoms are both four-coordinate with square-planar

of 0.02−0.03 Å has been discussed earlier in terms of “push−

environments, each being bonded to four donor atoms of the

3c

2−

pull charge asymmetry”. Such diﬀerences in Cu−O

tetradentate asymmetric ligand L . The τ values are 0.053 and

phenoxo

5

bond distances indicate the induction of electronic asymmetry

within the metalloligand, which results because the “−R” eﬀect

of the formylimine group on O(31) is greater than that of the

0.075, respectively. In the two equatorial planes, the four donor

atoms show a tetrahedral distortion with r.m.s. deviations of

0.173 and 0.119 Å with the metal atoms 0.021(3) and 0.041(4)

Å from the plane. The two planes intersect at 38.1(2)°. The

manganese atom is six-coordinate with a distorted-octahedral

environment, being bonded to four oxygen atoms which bridge

to one of the copper atoms together with two nitrogen atoms

2−

acetylimine group on O(11) of ligand L . This also suggests

that, as O(11) is more electron rich than O(31), the former

atom bridges the two Cu centers in 1. The bond lengths and

angles are shown in Table S1 in the Supporting Information.

E

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 3. Structure of 3 with thermal ellipsoids at 20% probability.

Figure 4. Thermal variations of the χ T product for compounds 2 and

M

3

. The solid yellow line represents the best ﬁt to the appropriate model

of monodentate thiocyanate ligands. The two oxygen atoms

O(31) and O(61) that are trans to the thiocyanate ligands form

longer bonds to the metal at 2.217(7) and 2.213(6) Å in

comparison to O(11) and O(41), which are mutually trans,

albeit with an angle of 149.3(3)°, at 2.074(7) and 2.086(7) Å.

The two Mn−Cu distances are 3.209(3) and 3.128(3) Å in 3 in

comparison to 3.209(3) and 3.135(3) Å in 2.

for 2, and the solid blue line represents the best ﬁt to the appropriate

model for 3.

in the Supporting Information), which represents to a nonzero

spin ground state with S = |S − 2S | = 3/2. This behavior

T

Mn

Cu

also suggested the presence of dominant antiferromagnetic

coupling. To evaluate the magnetic interaction parameters, we

used the spin-only Hamiltonian H = −2J[S S + S S ], where S

It is also to be noted that the “electronic push−pull charge

1

2

1

3

1

asymmetry” eﬀect on the elongation of Cu−O

bonds in

= S and S = S = S on the basis of a linear trinuclear

phenoxo

Mn 2 3 Cu

complexes 2 and 3 is prominent, where the more electron rich

O(11) and O(41) form shorter Cu−O bonds and the less

electron rich O(31) and O(61) form longer Cu−O bonds. This

structure. It can be noted that the exchange coupling between

II

the two terminal Cu ions was assumed to be 0 (J

= 0

Cu−Cu

−1

cm ) because of large Cu···Cu separation. Simulations were

carried out using the PHI program with consideration of

molecular ﬁeld approximations. The best ﬁts lead to the

eﬀect also has a considerable impact on Mn−O

bond

phenoxo

23

distances in complexes 2 and 3. The more electron rich O(11)

and O(41) form shorter Mn−O_p_h_e_n_o_x_obonds and the less

−1

following parameters: J

= −21.8 cm , g = 2.08, g_M_n

=

Mn−Cu

cu

−

1

−5

electron rich O(31) and O(61) form longer Mn−O

2.01, zJ′ = 0.032 cm , R = 1.22 × 10 for compound 2; J

phenoxo

−1

bonds. It is worthwhile to point out that all of the previously

reported heterometallic trinuclear complexes derived from

several salen type Schiﬀ bases are structurally analogous to

complex 3, where the divalent central metal ions (s, p, or d

block) are usually octahedral (few are tetrahedral or square-

planar) and are coordinated to the two same coligands (e.g.,

water, azide, thiocyanate, cyanate, dicyanamide, chloride, nitrite,

nitrate, perchlorate, carboxylates, etc.). On the other hand,

complex 2 is the ﬁrst of its kind where the central metal ion of

such a complex possesses two diﬀerent coligands (azide and

water).

= −22.3 cm , g = 2.11, g = 2.00, zJ′ = −0.008 cm , R =

cu

Mn

1.28 × 10⁻for compound 3. The nature and magnitude of the

coupling constants are as expected, considering the Cu−O−Mn

5

22

angles.

Electrochemistry. The cyclic voltammogram (CV) of

complex 1 in dichloromethane (Figure S6 in the Supporting

Information) shows one wave on a cathodic scan (0 to −1.8 V)

for the quasi-reversible reduction process at the electrode

surface. The peak is observed at E = −1.05 V on a forward

c

cathodic scan with respect to an Ag/AgCl electrode, whereas

on reverse scan the corresponding peak observed at E = −0.86

a

24

Magnetic Properties. Temperature variations of the

product of molar magnetic susceptibility and temperature

V with the peak separation value ΔE = E

− E

is

pc

cathodic

anodic

−0.19 V. The cyclic voltammograms of complex 1 suggest that

the mononuclear Cu(II) complex reduced to the Cu(I) state

through a one-electron process. Complex 2 shows two peaks at

E = −0.54 V and E = −1.05 V on a forward cathodic scan and

(

χ_MT) for compounds 2 and 3 are shown in Figure 4. The

3

compounds show room-temperature χ T values of ca. 4.13 cm

M

K mol⁻for 2 and 4.23 cm K mol for 3, which are lower than

1

3

−1

1

2

the value of a magnetically noninteracting Cu Mn system of 5.1

one peak at E = −0.83 V on a reverse scan (Figure 5). Here

2

1

3

−1

22

cm K mol for g_M_n= g_C_u= 2 at 300 K. In both cases, the

the peak separation value (ΔE ) is −0.22 V and the potential

pc

25

behaviors are very similar; a progressive decrease in χ T value

diﬀerence (ΔE = E − E ) between two peaks on a forward

M

12

1

2

3

takes place upon cooling and reaches a plateau of ca. 1.90 cm

scan is −0.51 V. On the other hand, complex 3 also shows two

K mol⁻for 2 and 1.80 cm K mol for 3 around a

1

3

−1

peaks at E = −0.74 V and E = −1.00 V on a forward cathodic

1

2

temperature range of ca. 8−28 K. At very low temperatures

scan and one peak at E = −0.78 V on a reverse scan (Figure S7

1

(

below ca. 5 K), the χ T value further decreases to reach a

in the Supporting Information). In this complex the peak

M

3

−1

3

−1

value of 1.85 cm K mol for 2 and 1.66 cm K mol for 3 at 2

separation value (ΔE ) of −0.22 V is observed, like that for 2.

pc

K. The progressively decreasing nature of χ T with a decrease

Both complexes 2 and 3 show similar CVs with one irreversible

followed by one quasi-reversible wave for the reduction process.

It is evident that complexes 2 and 3 dissociate in solution to

form a mixture of free metalloligand [CuL] as well as

M

in temperature is attributed to the presence of strong

antiferromagnetic interactions. The saturated molar magnet-

ization values at 2 K are 2.90 μ for complex 2 (Figure S4 in the

B

2+

Supporting Information) and 2.84 μ for complex 3 (Figure S5

polynuclear {(CuL) Mn} species. On comparison of the

B

2

F

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

(

3,5-DTBQ) in methanol under an aerial atmosphere according

to the reaction shown in Scheme 2.

Scheme 2. Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ in

Methanol

For this purpose, a 4 × 10⁻M methanolic solution of

complex 2 was treated with 250 equiv of 3,5-DTBC at room

temperature under aerobic conditions. After mixing of 3,5-

DTBC into the solution of complex 2, the progress of the

reaction was followed by continuous recording of the UV−vis

spectra of the mixtures at 5 min time intervals. The gradual

increase of an absorption band at around 401 nm

5

Figure 5. Cyclic voltammograms (CVs) of 10⁻M solution of

complex 2 in dichloromethane at room temperature and diﬀerent scan

rates, using TBAP as supporting electrolyte.

3

cyclic voltammograms (CV) of the three complexes 1−3, the

quasi-reversible peak (observed at E = −1.05, −1.05. and −1.00

V for complexes 1−3, respectively) is common for all, which

resembles the one-electron reduction of Cu(II) to Cu(I) for the

1

0d,20a

corresponding to the formation of 3,5-DTBQ

for

complex 2 is shown in Figure 6. The UV−vis spectra of the

26

free mononuclear metalloligand [CuL]. On the other hand,

the additional wave for the forward cathodic scan appears only

for complexes 2 and 3 (observed at E = −0.54 and −0.74 V for

2

and 3, respectively), which might come from the reduction of

2

+

remaining polynuclear {(CuL) Mn} species. After the

2

+

reduction of Cu(II) to Cu(I) in the {(CuL) Mn} species,

2

Mn(II) with higher ionic potential possibly replaces the Cu(I)

from the Schiﬀ base core and makes the reduction process

irreversible. This also suggests that the Cu center of complex 2

is reduced at lower potential in comparison to complex 3.

Use of the Complexes as Catalysts for Aerial Oxygen

Activation. Literature reports reveal that complexes containing

coordinated solvent molecules or vacant coordination sites in

the metal ions show high catalytic activity, as these situations

facilitate substrate−catalyst binding. In complexes 2 and 3 the

axial sites of Cu(II) are vacant, but it should be noted that in

complex 2 there is a coordinated water molecule on the central

Mn(II). We checked the catecholase activities of all three

complexes and found that only complex 2 is catalytically active.

Moreover, from previous studies it is evident that active sites of

diﬀerent metalloenzymes are often very similar and complexes

mimicking the active sites of one metalloenzyme can perform

multiple catalytic transformations of similar reaction trajecto-

ries. For example, several catalysts that show catecholase like

activity also often exhibit phenoxazinone synthase like activity.

Catechol Oxidase Studies and Kinetics. Detailed

Description. The catecholase like activity of the complexes

was determined by the catalytic oxidation of 3,5- DTBC. This

substrate is most widely used for catecholase activity of

biomimicking coordination compounds. The possible reasons

for its suitability are the low redox potential of 3,5-DTBC,

which facilitates its oxidation to quinone, and its bulky tert-butyl

substituents, which prevent further oxidation reactions or ring

Figure 6. Increase of the quinone band at around 400 nm after the

addition of 250 equiv of 3,5-DTBC to a methanolic solution of

complex 2. The spectra were recorded at 5 min intervals.

solution of complex 2 in methanol showed a drastic change

after the addition of 3,5-DTBC to the solution. The kinetics of

this oxidation reaction of 3,5-DTBC to 3,5-DTBQ in the

presence of complex 2 was determined by measuring the

growth of the quinone band at 401 nm as a function of time.

The rate constant for this complex−substrate mixture was

calculated from a log[A /(A − A )] vs time plot. The

α

t

substrate-dependent oxidation rates and various kinetic

parameters of the substrate concentration in this reaction

were determined by using 4 × 10⁻M solutions of complex 2

mixed with diﬀerent concentrations of 3,5-DTBC under aerobic

conditions. In this case, ﬁrst-order kinetics was followed at low

concentrations of 3,5-DTBC, whereas at higher concentrations

saturation kinetics was observed. The observed reaction rates

versus the substrate concentration data were then analyzed on

the basis of the Michaelis−Menten approach of enzymatic

kinetics to obtain the Lineweaver−Burk (double reciprocal)

plot and values of kinetic parameters V , K , and k . The

5

2

7

opening. The product of this oxidation reaction, 3,5-di-tert-

butylquinone (3,5-DTBQ), is highly stable and shows a

characteristic absorption band maximum at around 401 nm

1

−1

28

(

ε = 1900 M⁻cm ) in a pure methanol solvent. Before the

determination of detailed kinetics studies, we have examined

the catalytic activity of complex 2 for the oxidation of 3,5-di-

tert-butylcatechol (3,5-DTBC) to the corresponding o-quinone

max

M

cat

G

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

curves of both the observed rate vs [substrate] and the

Lineweaver−Burk plot for complex 2 are depicted in Figure 7.

corresponding dinuclear protonated [(CuL)₂+ H]⁺and

+

sodiated species [(CuL) + Na] , respectively. In addition to

2

these, there are several other signals; one of which at m/z

3

84.52 (calcd 384.53) for the doubly charged trinuclear

2+

[(CuL) Mn] species is noteworthy.

2

After the addition of 3,5-DTBC to a solution of complex 2, a

considerable change is observed in the overall mass spectrum

Figures S14 and S15 in the Supporting Information). It shows

(

a base peak at m/z 380.04 (calcd 380.05), which can be

+

assigned to the sodiated metalloligand species [(CuL) + Na] .

The intensity of the peak at m/z 384.52 (calcd 384.53) for

2+

doubly charged trinuclear [(CuL) Mn] species becomes

2

considerably higher in this spectrum. The appearance of two

new peaks at m/z 991.24 (calcd 991.23) and 769.08 (calcd

II

+

I

7

69.07) assignable to [(Cu L) Mn(3,5-DTBC)] and [(Cu L)-

2

II

+

Mn(Cu L)] , respectively, is very important from a mechanistic

point of view, as they clearly establish the complex−substrate

bonding and subsequent reduction of one of the Cu(II) to

Cu(I) in the trinuclear species (vide infra). Another important

peak is observed at m/z 989.24 (calcd 989.21), which indicates

Figure 7. Plot of the rate vs substrate concentration for complex 2.

The inset shows the corresponding Lineweaver−Burk plot.

the formation of the semiquinonate form of the intermediate

+

For both cases the kinetic parameters are given in Table S3 in

^t^h^e^S^u^p^p^o^r^tⁱⁿ^g^Iⁿ^f^o^r^m^a^tⁱ^oⁿ^.^T^h^e^c^a^l^c^u^l^a^tⁱ^oⁿ^o^f^t^h^e^kcat value

^w^a^s^p^e^r^f^o^r^m^e^d^b^y^d ⁱ ^v ⁱ ^d ⁱ ⁿ ^g ^t ^h ^e ^Vmax value by the concentration

[(Cu(II)L) Mn(3,5-DTBSQ)] . In addition, the spectrum

2

shows a peak at m/z 243.12 (calcd 243.13), which can be

+

assigned to the [3,5-DTBQ-Na] ion, the oxidized product of

−1

of the complex 2 (Table S3). The k_c_a_t(in h ) value of this

catalysis is 1118 for 2. The same procedure was applied to test

the catecholase like activity using complexes 1 and 3 as

catalysts, but they did not show such activity (Figure S8 in the

Supporting Information). In the following sections we compare

the ESI-MS and EPR spectra of the two heterometallic

complexes 2 and 3 to gain an insight into the mechanisms of

the reactions.

ESI-Mass Spectrometric Study. To get information about

the complex−substrate intermediates and to draw the

mechanistic inference of catecholase activity during the

oxidation reaction of catechol, we have recorded ESI-MS

spectra of complexes 1−3 and of 1/1 mixtures (v/v) of the

complexes and 3,5-DTBC in methanol solution within 5 min of

mixing (Figures S13 −S15 in the Supporting Information). The

mass spectrum of complex 2 shows a base peak at m/z 358.06

3,5-DTBC.

The mass spectrum of complex 3 (Figure S18 in the

Supporting Information) in methanolic solution shows a base

peak at m/z 358.06 (calcd 358.07) for the protonated

_m_e_t_a_l_l_o_l_i_g_a_n_d_[₍_C_u_L₎₊_H_]+ and at m/z 380.04 (calcd

3

80.05) and m/z 737.13 (calcd 737.12) for the species

+

[(CuL) + Na] and [(CuL) + Na] , respectively, like those

2

of complex 2. In comparison with 2, the presence of a very low

2+

intensity signal for [(CuL) Mn] at m/z 384.52 (calcd 384.53)

2

is noticeable, which is readily abolished after the addition of

3

,5-DTBC.

The spectrum of the mixture of complex 3 and 3,5-DTBC in

methanolic solution (Figure S19 in the Supporting Informa-

tion) shows a base peak at m/z 380.04 (calcd 380.05) for the

+

sodiated metalloligand species [(CuL) + Na] . The relative

intensities of the peaks at m/z 358.07 (calcd 358.07) for

(

calcd 358.07), which can be assigned to protonated metal-

+

[

(CuL) + H] and m/z 737.13 (calcd 737.12) for [(CuL) +

loligand [(CuL) + H] (Figure S13). Another prominent peak

2

+

at m/z 377.03 (calcd 377.08) is observed due to the water

Na] are reduced signiﬁcantly. It is noted that the peaks for

species such as [(Cu L)

DTBSQ)] , and 3,5-DTBQ are not present or are negligible in

this spectrum.

+

II

+

II

coordinated metalloligand species [(CuL·H O) + H] . In

2

Mn(3,5-DTBC)] , [(Cu L)

Mn(3,5-

2

+

addition, two low-intensity peaks at m/z 715.14 (calcd 715.14)

and m/z 737.13 (calcd 737.12) are observed for the

Figure 8. EPR spectra of 0.1 mM solutions of 2 (left) and 3 (right) in methanol at 96 K.

H

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

EPR Spectroscopy. The complexes under study contain Cu²⁺

and Mn²ions, which are both paramagnetic. Hence, electron

paramagnetic resonance can be very useful in understanding the

role of these ions in catecholase activity. However, the distances

between these ions in the complexes are such that they are

expected to be exchange-coupled, and this is indeed established

by the susceptibility measurements. Exchange coupling between

On addition of DTBC, the copper lines showed a strong

decrease in both complexes 2 and 3. Figures S23 and S24 in the

Supporting Information show the EPR spectra of complexes 2/

3 in methanol containing an excess (>100-fold) of DTBC at

diﬀerent times after its addition. It is clear that both behave

similarly after DTBC addition. In fact, the EPR spectra of both

complexes overlap signiﬁcantly after DTBC addition. This

seems to contradict the optical behavior, in which catecholase

activity was seen only with complex 2 and not with 3, and the

mass spectrometric results of the mixture of DTBC + complex

+

the two Cu²(S = 1/2) and Mn will lead to S = 3/2, 5/2, and

+

2+

7

/2 spin states, whose separation will depend on the strength of

29

exchange interaction. Susceptibility measurements reveals

antiferromagnetic coupling and an exchange interaction of less

2+

2, in which the [(CuL) Mn] species is the base peak (unlike

2

−1

than 30 cm , implying that at room temperature the excited

spin states would also be populated along with the ground

states. Thus, we have employed low-temperature EPR experi-

ments with the aim of reducing the contributions of excited

spin states.

3) along with an intense signal for (CuL), indicating that the

trinuclear cluster is suﬃciently present in the mixture for 2.

However, it is important to note that the signals seen in EPR

are predominantly from the broken complexes (as discussed

earlier). Hence, a change in their intensity does not relate to the

catalytic eﬃciency of the intact complex. Since we could not

identify the EPR of the intact complex separately, we directed

our attention toward the search for a new signal which could be

assigned to a new intermediate species (possibly semiquinone

radical).

Figure 8 shows the EPR spectra of complexes 2 and 3 in

methanol recorded at 96 K. The spectra show typical features of

Mn²and Cu isolated ions. Moreover, the relative amount of

Cu lines is greater in the spectrum of complex 3. Now an

exchange-coupled spectrum (ground state S = 3/2) would

possibly not show these features, as the exchange interaction

changes the hyperﬁne interaction of isolated ions. For example,

studies by Krost et al. on a copper manganese bimetallic

+

2+

Figure 9 shows the EPR spectra of complexes 2 and 3 after

addition of DTBC recorded under comparable conditions. One

3

0

system show that the hyperﬁne coupling of copper (∼23 G)

decreases to about one-sixth of its value in typical monometallic

copper complexes (150−190 G). This is understandable, since

Cu−Mn is a six-electron system where each electron now

spends one-sixth of the time around copper nuclei. In the

current scenario we have a trimetallic system where this eﬀect

would again be present if there were signiﬁcant exchange

interaction. However, the observed copper parallel hyperﬁne

couplings are 184 G, which are close to that of monometallic

2+

Cu complexes. In addition, the spectrum of Mn (including

the forbidden transitions and the diﬀerent line width of Mn

lines) can be simulated (simulation not shown) by assuming a

5

/2 spin state.

To further understand the EPR spectra, we synthesized the

following analogues of 2, where the exchange coupling between

Figure 9. EPR spectra of 2 and 3 in methanol at 96 K immediately

after addition of DTBC.

2+

Cu and Mn is absent: namely, (i) Zn−Mn−Zn (4) and (ii)

2+

Cu−Zn−Cu (5). In complex 4, the EPR of isolated Mn can

2

+

be obtained, while in complex 5 the spectrum of Cu can be

obtained (the terminal copper can be considered to be isolated;

Figures S21 and S22 in the Supporting Information show the

EPR spectra of complexes 2/4 and 3/5). It is clear that

complexes 4 and 5 can explain almost all of the spectral features

of complexes 2 and 3. Thus, the spectra of complexes 2/3 can

can observe a new signal close to g ≈ 2 only in the case of

complex 2, and not in 3. This signal could possibly be due to

the semiquinone intermediate radical or a ligand-centered

radical (in case the reduction involves transfer of an electron to

the ligand instead of copper). The reason for the small intensity

of the extra signal might be that the DTBSQ radical is an

intermediate and is continuously decaying to its ﬁnal

diamagnetic product, thereby resulting in a small steady state

concentration.

2

+

2+

be ascribed to isolated (exchange-free) Cu and Mn ions.

One possibility to explain this behavior is that the complex is

not completely stable and breaks partially into constituent ions.

This is supported from the mass spectroscopy data, which

shows the presence of species such as (CuL), (CuL)₂,

Mechanistic Insight. Mechanistic studies with copper-based

model complexes have already established that there are two

possible mechanisms for the oxidation of 3,5-di-tert-butyl-

catechol to its respective quinone. One occurs through the

formation of a dicopper(II) catecholate intermediate where the

dicopper(II) species oxidizes the catecholic substrate to its

respective quinone and itself reduces to a dicopper(I) species.

This dicopper(I) species further reacts with an oxygen molecule

to generate a peroxidodicopper(II) adduct, which then oxidizes

a second molecule of the catechol substrate to quinone; water is

formed as a byproduct through this four-electron-reduction

(

CuL) Mn, etc.

2

The EPR spectra of such exchange-coupled systems have

been studied only in the solid state by employing single crystals.

To the best of our knowledge, we are not aware of such studies

31

in solution. Colacio et al. have attempted to study the EPR of

an exchange-coupled Cu−Mn−Cu system in frozen methanol

at 100 K. However, they too observed features of isolated ions.

The absence of a signal which can be ascribed to the intact

complex may be because the signal is very broad and, coupled

with its small concentration (limited by the solubility), escapes

detection.

32

process. The second mechanism involves the formation of an

I

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

3

organic radical intermediate such as copper(I) semiquinone.

indicating their absence or negligible presence to sustain the

catalytic process. The EPR spectrum also does not show any

signal for a radical after addition of 3,5-DTBC to the solution of

complex 3. On the other hand, the analogous Zn-Mn-Zn

complex 4 does not show any catecholase like activity (Figure

S9 in the Supporting Information) but shows a strong signal

due to the formation of an organic radical, presumably the

semiquinone (3,5-DTBSQ) (Figure S25 in the Supporting

Information). Unlike Cu(II), Zn(II) cannot be reduced to

Zn(I) by the semiqunone and hence is unable to catalyze the

oxidation reaction; therefore, the radical concentration

increased and is responsible for the strong signal.

Its subsequent reaction with an oxygen molecule may result in

the two-electron reduction of two oxygens, leading to the

further oxidation of the copper(I) ion and release of the

34

quinone molecule, with hydrogen peroxide as a byproduct.

On the other hand, a number of studies of the possible

mechanisms of the catechol oxidation reaction catalyzed by

nickel(II), cobalt(II/III), manganese(II/III), and zinc(II)

complexes with the formation of hydrogen peroxide have

been reported. However, it is important to note that the

participation of the metal center in the catechol to quinone

oxidation process is essential. Thus, this type of oxidation

reaction catalyzed by Ni(II), Co(III), Mn(III), or Zn(II)

complexes proceeds through the formation of semiquinonate

intermediate species similar to the case for Cu(II) complexes.

To understand the mechanism of this catalytic reaction for

the present heterometallic Cu(II)−Mn(II) complexes, we

estimated the amount (if any) of H O formed (Figures S10

and S11 in the Supporting Information). It was observed that

H O is produced only when complex 2 is mixed with 3,5-

DTBC. The quantitative estimation reveals that about 95.4%

H O is generated after 30 min of oxidation with respect to the

conversion of 3,5-DTBQ from 3,5-DTBC (Table 2). Thus, it is

evident that nearly an equimolecular amount of H O is formed

From the present study it is apparent that a slight change in

the structure of the heterometallic complexes can be very

crucial for the catecholase like activity. The substrate−complex

binding is essentially the important step in the process. At ﬁrst

sight, one may assume that catechol may simply coordinate to a

vacant site of the metal ion or can easily replace a coordinated

solvent molecule bonded with the metal ion. However, the

process seems not to be that simple, as in the case of 2 no peak

2

II

+

assignable to [(Cu L) Mn(3,5-DTBSQ)] in which a solvent

2

water molecule is replaced by catechol was detected. Instead,

2+

here generation of the dipositive species [(CuL) Mn] is vital

2

for the catalyst−substrate binding. The vacant axial site of

Cu(II), which is a common feature of complexes 2 and 3,

seems to be of no use for coordination of catechol. Another

unique feature of the catalytic activity of 2 is that, although the

change in oxidation states of copper between 2+ and 1+ is the

key for oxidation of catechol to quinone, the catechol does not

bind to it but to Mn(II), which remains redox innocent

throughout the process.

The explanation of the catalytic behavior of complex 2 over

the catalytically nonresponsive complex 3 can be found in their

structural diﬀerence. While both 2 and 3 contain vacant axial

coordination sites of the square-planar Cu centers, only

with respect to the formation of 3,5-DTBQ.

Table 2. Quantitative Estimation of H O Formation (%)

2

with Time (min) during the Catecholase Reaction

time (min)

2

5

10

20

30

H O (%)

11.5

30.8

43.0

81.3

95.4

2

On the basis of ESI-mass and EPR spectra, we propose here a

possible mechanism for catechol oxidation. The doubly charged

species [(CuL) Mn] which is produced in solution (please

see ESI-Mass Spectrometric Study) from the trinuclear complex

reacts with 3,5-DTBC to generate the intermediate species

(CuL) Mn(3,5-DTBC)] , in which one of the hydroxyl

groups of 3,5-DTBC is deprotonated and coordinated to the

Mn(II) center. This species then reacts with dissolved O and is

converted into its radical form (semiquinonate), [(Cu L) Mn-

2

+

2

complex 2 contains a labile coordinated H O molecule,

2

although it is bonded to the central Mn(II). It would seem

2

[

that the presence of this coordinated H O molecule makes all

+

2

the diﬀerence and induces catalytic activity of the hetero₋-

metallic complex. Water is more easily removed than SCN

from the complex in solution; this is why 3,5-DTBC can

coordinate more readily to Mn(II) in complex 2 than in

complex 3. The strong relative diﬀerence in the intensities of

the mass spectral signals for the doubly charged trinuclear

2

II

2

+

(

3,5-DTBSQ)] , as is evident from mass spectra. O is reduced

to H O . The EPR signal of an organic radical (Figure 9),

2

which was found in the mixture of 2 and 3,5-DTBC, is

presumably due to this semiquinone radical (3,5-DTBSQ),

whereas the six-line EPR spectrum indicates that the oxidation

state of Mn(II) remains unaltered during the catalysis (evident

from the EPR spectrum of the Zn-Mn-Zn plus catechol system

shown in Figure S25 in the Supporting Information). The

semiquinone itself is converted to quinone and is eliminated

from the metal center, leaving a singly charged trinuclear

species, [(Cu L)Mn (Cu L)] , the existence of which is

evident from mass spectra. Finally, Cu(I) of this species is

oxidized to Cu(II) by O , regenerating the catalytically active

species [(CuL) Mn] and producing H O as the byproduct.

The regenerated trinuclear species [(CuL) Mn] then

2

+

[

(CuL) Mn] species obtained before and after adding

2

catechol also suggests that the formation of this species in

solution after adding catechol is highly favored (base peak) in 2

but practically vanishes in 3. A signal for the mass of the singly

+

charged [(CuL) Mn] species is also observed in the spectra of

2

a mixture of 3,5-DTBC and complex 2 (Figure S14 in the

Supporting Information), and this provides direct evidence for

the further continuation of this reaction through a Cu(II)/

Cu(I) redox process and the presence of Cu(I) in the trinuclear

I

II

+

Cu Mn unit. Mass spectra of the mixture of 3,5-DTBC and

2

+

complex 3 show peaks only for protonated and sodiated

2

+

metalloligand [CuL], indicating the almost complete dissoci-

2

ation of the basic [(CuL)

Mn]²⁺trinuclear unit (Figures S18

coordinates to another 3,5-DTBC and converts it to 3,5-

DTBQ following the same procedure.

The proposed mechanism may also be extended to explain

the inactivity of the analogous Cu-Mn complex 3. Unlike 2, in

the mass spectra of 3 there is no evidence for species such as

2

and S19 in the Supporting Information). Therefore, the

reduction of Cu(II) to Cu(I) by the semiquinonate radical is

important for this catalytic oxidative conversion of 3,5-DTBC

to 3,5-DTBQ. The probable mechanism of this catalytic

reaction is given in Scheme 3.

2+

[

(CuL) Mn] or bonding of 3,5-DTBC to the metal complex,

2

J

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 3. Proposed Mechanism for Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ by Complex 2

Phenoxazinone Synthase Studies and Kinetics. De-

tailed Description. To establish the catalytic activity of these

heterometallic complexes, toward another oxidase type

reaction, we have studied phenoxazinone synthase like activity.

This catalytic activity for several complexes was determined by

the catalytic oxidation of o-aminophenol (OAP) to amino-

phenoxazinone (APX). OAP easily oxidizes and dimerizes to

form APX according to the reaction shown in Scheme 4. APX

shows a broad absorption peak at 410−425 nm in pure

methanol solvent; this is why OAP is used commonly for this

study. The conversion of o-aminophenol to phenoxazinone is

an oxidative dimerization reaction. Before the determination of

detailed kinetics studies, we examined the catalytic activity of

complexes 1−3 for the oxidation of o-aminophenol (OAP) to

the corresponding amino phenoxazinone (APX) in methanol

solvent under an aerial atmosphere and found that only

complex 2 is catalytically active (Figure S12 in the Supporting

Information).

Scheme 4. Catalytic Oxidation of o-Aminophenol (OAP) to

Phenoxazinone (APX) in Methanol

A detailed kinetic study of this reaction was performed by

following the aforementioned procedure of the catechol oxidase

study. A 1 × 10⁻M methanolic solution of complex 2 is used

4

−2

as a catalyst, and 100 equiv of a 1 × 10 M solution of o-

aminophenol in methanol is used as a substrate solution. The

change in the spectral behavior of complex 2 in the presence of

OAP is shown in Figure 10. The rate of this catalytic reaction

K

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Mn(II)/Mn(III), Cu(II), and Ni(II) systems performing as

functional models of phenoxazinone synthase.

ESI-Mass Spectrometric Study. To investigate the mecha-

nism of phenoxazinone synthase activity and complex−

substrate intermediate during the oxidative dimerization

reaction of o-aminophenol, we have recorded ESI-MS spectra

of 2 in a 1/1 mixture (v/v) of the complex and o -aminophenol

within 4 min of mixing in methanol (Figures S16 and S17 in

the Supporting Information). A spectrum of a mixture of

complex 2 and o-aminophenol shows a base peak at m/z 380.04

(

calcd 380.05) (Figure S16), which can be assigned to the

+

sodiated metalloligand species [(CuL) + Na] . The spectrum

also shows two peaks at m/z 358.06 (calcd 358.07) for the

+

protonated metalloligand species [(CuL) + H] and m/z

7

37.13 (calcd 737.12) for the sodiated dinuclear species

Figure 10. Increase of the APX band at around 425 nm after the

addition of 100 equiv of OAP to a methanolic solution of complex 2.

The spectra were recorded at 4 min intervals.

+

[(CuL) Na] (Figure S16 ). All of these species have previously

2

appeared in the mass spectra of complex 2. In addition, two

low-intensity peaks are observed at m/z 1026.43 (calcd

1026.14) and m/z 1052.45 (calcd 1052.11) (Figure S17),

which indicate the presence of the two trinuclear species

[(Cu L) Mn(OAP) + K] and [(Cu L) Mn(OAP) + 3Na] ,

respectively. The peaks of intermediates [(CuL) Mn(OAP) +

has a linear dependence on catalyst concentration and shows

ﬁrst-order kinetics with respect to o-aminophenol (OAP). The

curves of the observed rate vs [substrate] and Lineweaver−

II

+

I

+

2

Burk plots for complex 2 are depicted in Figure 11. The k (in

cat

2 2

+

K] and [(CuL) Mn(OAP) + 3Na] also indicate the oxidative

2

conversion of o-aminophenol to phenoxazinone catalyzed by

complex 2. We have also checked that no H O was formed

2

during this catalytic conversion reaction.

EPR Spectroscopy. The phenoxazinone synthase like activity

of complexes 2 and 3 with OAP was also followed with EPR in

a manner similar to that for DTBC. Figures S26 and S27 in the

Supporting Information show the EPR spectra of complexes 2

and 3 in the presence of OAP at diﬀerent times after addition of

OAP. One can see that a new signal is formed in both

complexes 2 and 3 after addition of OAP (Figure 12). It is

present in the same region as that in the DTBC reaction. Thus,

one could say that both complexes 2 and 3 show

phenoxazinone synthase like activity with OAP. To see if

they both are equally eﬃcient, we compared the intensity of

this new signal at diﬀerent times. To get this information, the

EPR spectra of the complexes before addition of OAP were

subtracted from those obtained after OAP addition. This gives

us the spectrum of the new radical, which is shown in Figure 12

for both complexes 2 and 3. The spectra show that the intensity

of this new signal is less in complex 3 in comparison to that in

Figure 11. Plot of the rate vs substrate concentration for complex 2.

The inset shows the corresponding Lineweaver−Burk plot.

−1

h ) value of this catalysis is 6581. This value is signiﬁcantly

greater than the k_c_a_tvalues of the reported Co(II)/(III),

Figure 12. Diﬀerence EPR spectra of 2 (left) and 3 (right) in methanol at 96 K. The spectra have been generated by subtracting the EPR spectra of

the complexes after addition of OAP from those obtained before addition of OAP. The magnetic ﬁeld range is reduced so as to make the comparison

clear.

L

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 5. Proposed Mechanism for Catalytic Oxidation of OAP by Complex 2

2

. Thus, one may conclude that complex 2 is more reactive

should be stable enough in solution during the catalytic process,

but no such report regarding a Cu(II)-Mn(II) complex has

toward OAP in comparison to 3.

35

Mechanistic Insight. After a thorough study of ESI-mass

spectra and EPR spectra we have determined a possible

mechanism. When o-aminophenol is added to a solution of

complex 2 , the trinuclear species [(CuL) Mn(OAP) ] is

been found before. Previous reports also indicate the use of

stable heterometallic complexes as eﬃcient synergistic catalysts

which are often derived from multicompartmental ligands (e.g.,

37

N O -donor di-Schiﬀ bases, Robson-type macrocycles) that

2

4

formed (Figure S17 in the Supporting Information). This is a

neutral species in which two hydroxyl groups of two o-

aminophenols are deprotonated and coordinated to the Mn(II)

center. Then the species releases a hydrogen from the amine

increase the liberty of using the catalysts under diﬀerent

38

conditions. In comparison to these complexes, [(ML) M′]

2

type complexes (H L= salen type N O donor di-Schiﬀ bases,

2

M, M′ = diﬀerent metal ions) are much less stable and they

retain only partially their heterometallic core in solution, which

lowers the eﬀective concentration of the catalyst. Such trends

(

−NH ) group of one o-aminophenol and forms a radical. This

2

is relatively very unstable and quickly turns into its

iminophenolate form by releasing hydrogen from another o-

aminophenol. During this process one O is reduced to H O.

39

are also observed for Shibasaki catalysts, which possess a

diphenoxido-bridged heterometallic core similar to that of

2

4

0

The existence and relative stability of this species can be

assigned by mass spectra (Figure S16 in the Supporting

Information). After that, one o-iminophenolate and one o-

aminophenolate combine to form phenoxazinone and eliminate

the trinuclear complex where Cu(II) is reduced to Cu(I).

During this total process 3/2 O is reduced and 3 H O is

[(ML) M′]. In contrast, the inherent structural ﬂexibility

2

associated with these trinuclear complexes might be advanta-

15a,41

geous

to be adaptable with diﬀerent types of substrates in

comparison to the rigid complexes. However, the use of

[(ML) M′] type complexes as catalysts has been largely limited

2

13

to only a very few occasions.

2

II

formed as a byproduct. The probable mechanism of this

catalytic reaction is given in Scheme 5.

Recently, we reported a few heterometallic Ni Mn

2

complexes derived from symmetric salen type tetradentate

Schiﬀ base ligands which showed moderate to weak COx like

activity. However, mechanistic insight into any such hetero-

metallic complex as an oxidase model is not found in the

Structure−Activity Correlation for the Observed Relative

Performance of the Catalyst. There has been increased

interest in synergistic catalysis, on the basis of heterometallic

complexes and clusters with numerous combinations of metal

1

3

II

literature. Interestingly, analogous Cu Mn (or other

2

3

5

II

ions. For example, a Cu-Mn mixed-metal spinel has been

Cu M) heterometallic complexes derived from similar Schiﬀ

2

reported to show increased eﬃciency as a heterogeneous

oxidation catalyst, but no mechanistic insight was provided.

base ligands did not show COx or any other oxidase type

catalytic activity. Complex 3, derived from an asymmetric

36

22a

To pursue a mechanistic investigation into heterometallic

complexes as homogeneous catalysts, the heterometallic core

salen type Schiﬀ base ligand, is also found to be ineﬀective

toward oxidase type catalysis in the present study. Such

M

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

inactivity could be explained by the common structural feature

tion, where two diﬀerent metal ions do diﬀerent jobs to make

the catalyst highly eﬃcient for biomimetic oxidase reactions.

II

of the central Mn(II) center of the basic Cu Mn unit. In all of

2

these cases the coordination sites of the octahedral Mn(II)

center are blocked by two anionic coligands (pseudohalides

CONCLUSIONS

■

−

(

NCO , SCN , N or C N ), nitrate, carboxylates) which do

In the present work, an asymmetrically dicondensed Schiﬀ base

has been synthesized and its mononuclear copper complex

[CuL] has been isolated in a quick and eﬃcient manner. Using

this copper complex as metalloligand, two trinuclear hetero-

metallic complexes (2 and 3) have been synthesized. The

magnetic properties of the solid compounds reveal antiferro-

magnetic coupling, as expected from bridging angles. An

electrochemical investigation suggests that Cu(II) centers are

reducible to Cu(I) in both complexes. The catalytic activities of

the complexes in model biomimetic oxidase processes show

3

2

3

not leave easily to facilitate substrate binding with Mn(II)

centers. On the other hand, as the Cu centers are buried within

the large tetradentate Schiﬀ base (usually in a square-planar

environment), substrate coordination might cause increased

steric crowding in intermediates, which hampers substrate−

catalyst binding. This could be one good reason for the inactive

nature of complex 3 as a catalyst or for the weak to moderate

II

catalytic activity of Ni Mn complexes in comparison to 2.

2

Moreover, the mass signals of compound 3 or [(CuL) Mn] are

2

practically absent (signals appeared for only metalloligands) in

the presence of catechol, suggesting that the substrate leads to

almost complete rupture of the trinuclear cluster. On the other

hand, unlike the case for complex 3, the presence of the labile

water molecule at the Mn(II) center of complex 2 facilitates

substrate−catalyst binding (catechol or OAP) through Mn and

subsequent formation of the corresponding semiquinonate

that only complex 2 with a labile coordinated H O molecule at

2

the Mn(II) center possess very high catalytic activity toward

catecholase and the highest phenoxazinone synthase like

activity among all of the reported synthetic complexes. The

catalytic conversions take place with the formation of H O and

2

H O in catecholase and phenoxazinone synthase like activities,

2

respectively. ESI-mass spectral analysis of complexes 2 and 3

and complex−substrate (3,5-DTBC and o-aminophenol)

−

radical. The inhibitory role of SCN in both catalytic processes

2+

−

mixtures suggest the occurrence of a [(CuL) Mn] cluster as

was also established by adding 2 equiv of SCN to the mixtures

2

the active catalyst, complex−substrate binding, formation of a

Cu(II) radical intermediate, and reduction of Cu(II) to Cu(I)

in both catalytic processes using 2. On the other hand, EPR

spectroscopic analysis suggests the formation of a radical

intermediate. On the basis of ESI-mass and EPR spectral

analyses, probable mechanisms for the catalytic reactions are

proposed where the Mn(II) behaves as a Lewis acid center to

bind the corresponding phenolic substrate, whereas Cu(II)

behaves as a redox-active center that transports an electron

from substrate to oxygen. The simultaneous action of these two

processes on two metal centers leading to a heterometallic

cooperativity of catalytic oxidase transformation of the two

structurally analogous phenolic substrates seems to account for

the very high catalytic activity of this complex. On the other

hand, complex 3 does not show any discernible catalytic

activity, presumably due to its inability to form a complex−

substrate intermediate in solution.

of compound 2 and the individual substrates, upon which

complete quenching of the catalysis was observed (Figure S28

in the Supporting Information), whereas addition of the same

amount of N₃⁻had no eﬀect on the processes.

In all the proposed mechanisms for biorelevant oxidase

models, substrate coordination takes place directly to the redox-

active metal center. However, in the present study it is evident

that the Cu center does the job of electron transfer while the

Mn(II) center remains redox innocent during the course of the

catalysis, acting only as a Lewis acid center for the binding of

substrate. It is interesting to note that the k_c_a_tvalue of complex

2

is more than 10-fold higher than those of the analogous

II

Ni Mn complexes and for phenoxazinone synthase like

2

activity it is more than 20-fold higher than the highest reported

7

a,42

value for synthetic model coordination compounds

S4 in the Supporting Information).

(Table

44−48

This high biomimetic

oxidase type catalytic activity of complex 2 might originate from

the heterometallic cooperative eﬀects of dioxygen activationa

ﬁeld which remains rather unexplored with the use of

ASSOCIATED CONTENT

Supporting Information

■

S

43

*

heterometallic systems.

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.7b00253 . CCDC 1521947 (for 1), 1521948 (for 2),

and 1521949 (for 3) contain 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.

It should also be noted that the previously reported highly

eﬃcient homometallic Cu(II) catalysts as functional models of

catechol oxidase are often derived from multicompartmental

ligands with dinuclear Cu cores and are much more stable than

2

complex 2 (as 2 only partially retains its trinuclear [(CuL) Mn]

2

unit in solution). Thus, the actual catalyst eﬃciency or the

turnover number cannot be directly compared. The turnover

number of 2 for the oxidation of DTBC is calculated from the

amount of the catalyst dissolved and is close to that obtained

from these highly eﬃcient homometallic catalysts reported.

Moreover, to the best of our knowledge only two homometallic

Cu(II) complexes have been reported to show PHS-like

UV, IR, mass, and EPR spectra, cyclic voltammograms,

SEM micrographs, X-Ray powder diﬀractograms, and

bond parameters of complexes 1−3, EPR and mass

spectra of complexes 2 and 3 with 3,5-DTBC and OAP,

and a list of complexes from the literature and their K_c_a_t

values for which catecholase-like activities have been

analyzed (PDF)

8

c

8f

activity. Thus, complex 2 is only the third copper-based and

the ﬁrst heterometallic catalyst showing phenoxazinone

synthase like activity and its turnover number is the highest

among all the model complexes reported so far. Thus, the

present complex, derived from an asymmetric tetradentate

Schiﬀ base ligand, validates the heterometallic cooperative

eﬀects on apparently substrate independent dioxygen activa-

Crystallographic data for 1−3 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for A.G.: ghosh_59@yahoo.com.

■

N

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

ORCID

active site of catechol oxidase: mechanistic studies. Chem. Soc. Rev.

2

006, 35, 814−840. (c) Citek, C.; Herres-Pawlis, S.; Stack, T. D. P.

Ashutosh Ghosh: 0000-0003-2026-7565

Notes

The authors declare no competing ﬁnancial interest.

Low Temperature Syntheses and Reactivity of Cu2O2 Active-Site

Models. Acc. Chem. Res. 2015, 48, 2424−2433. (d) Solomon, E. I.;

Sarangi, R.; Woertink, J. S.; Augustine, A. J.; Yoon, J.; Ghosh, S. O and

2

N O Activation by Bi-, Tri-, and Tetranuclear Cu Clusters in Biology.

2

Acc. Chem. Res. 2007, 40, 581−591.

ACKNOWLEDGMENTS

■

(

8) (a) Barry, C. E.; Nayar, P. G.; Begley, T. P. Phenoxazinone

A.G. thanks the Department of Science and Technology

DST), New Delhi, India, for ﬁnancial support and providing

Synthase: Enzymatic Catalysis of an Aminophenol Oxidative Cascade.

J. Am. Chem. Soc. 1988, 110, 3333−3334. (b) Panja, A.; Shyamal, M.;

Saha, A.; Mandal, T. K. Methylene bridge regulated geometrical

preferences of ligands in cobalt(III) coordination chemistry and

phenoxazinone synthase mimicking activity. Dalton Trans. 2014, 43,

5443−5452. (c) Mukherjee, C.; Weyhermueller, T.; Bothe, E.;

Rentschler, E.; Chaudhuri, P. A Tetracopper(II)-Tetraradical Cuboidal

Core and Its Reactivity as a Functional Model of Phenoxazinone

Synthase. Inorg. Chem. 2007, 46, 9895−9905. (d) Hassanein, M.;

Abdo, M.; Gerges, S.; El-Khalafy, S. Study of the oxidation of 2-

aminophenol by molecular oxygen catalyzed by cobalt(II) phthalo-

cyaninetetrasodiumsulfonate in water. J. Mol. Catal. A: Chem. 2008,

(

the cyclic voltammetry facility (SR/S1/IC/0034/2012) for us

as well as for the DST-FIST funded single-crystal X-ray

diﬀractometer facility at the Department of Chemistry,

University of Calcutta. We also thank the CRNN, University

of Calcutta, Kolkata, India, for SEM-EDS and magnetic

measurements. P.M. is grateful to the Council of Scientiﬁc

and Industrial Research (CSIR), Government of India, for a

Senior Research Fellowship (Sanction no. 09/028(0935)/

2

014-EMR-I)]. S.G. is grateful to the University Grants

Commission (UGC), New Delhi, India, for the Dr. D. S.

Kothari Postdoctoral Fellowship.

2

87, 53−56. (e) Siman

Catalytic activation of dioxygen by oximatocobalt(II) and oximatoiron-

II) complexes for catecholase-mimetic oxidations of o-substituted

́ ́

di, L. I.; Simandi, T. M.; May, Z.; Besenyei, G.

(

REFERENCES

1) (a) Suzuki, M.; Furutachi, H.; O

phenols. Coord. Chem. Rev. 2003, 245, 85−93. (f) Sagar, S.; Sengupta,

S.; Mota, A. J.; Chattopadhyay, S. K.; Ferao, A. E.; Riviere, E.; Lewisf,

W.; Naskar, S. Cubane-like tetranuclear Cu(II) complexes bearing a

Cu4O4 core: crystal structure, magnetic properties, DFT calculations

and phenoxazinone synthase like activity. Dalton Trans. 2017, 46,

■

(

̅

kawa, H. Bimetallic dioxygen

complexes derived from ‘end-off’ compartmental ligands. Coord. Chem.

Rev. 2000, 200-202, 105−129. (b) Vigato, P. A.; Tamburini, S. The

challenge of cyclic and acyclic schiff bases and related derivatives.

Coord. Chem. Rev. 2004, 248, 1717−2128. (c) Andruh, M. Chem.

Commun. 2011, 47, 3025−3042. (d) Lyons, C. T.; Stack, T. D. P. The

challenge of cyclic and acyclic schiff bases and related derivatives.

Coord. Chem. Rev. 2013, 257, 528−540.

1

(

249.

9) (a) Banu, K. S.; Chattopadhyay, T.; Banerjee, A.; Bhattacharya,

S.; Suresh, E.; Nethaji, M.; Zangrando, E.; Das, D. Catechol Oxidase

Activity of a Series of New Dinuclear Copper(II) Complexes with 3,5-

DTBC and TCC as Substrates: Syntheses, X-ray Crystal Structures,

Spectroscopic Characterization of the Adducts and Kinetic Studies.

Inorg. Chem. 2008, 47, 7083−7093. (b) Rey, N. A.; Neves, A.;

Bortoluzzi, A. J.; Pich, C. T.; Terenzi, H. Catalytic Promiscuity in

Biomimetic Systems: Catecholase-like Activity, Phosphatase-like

Activity, and Hydrolytic DNA Cleavage Promoted by a New

Dicopper(II) Hydroxo-Bridged Complex. Inorg. Chem. 2007, 46,

(

2) (a) Sinn, E.; Harris, C. M. Schiff base metal complexes as

1

ligands . Coord. Chem. Rev. 1969, 4, 391−422. (b) Akine, S.;

Nabeshima, T. Cyclic and acyclic oligo(N O ) ligands for cooperative

2

multi-metal complexation. Dalton Trans. 2009, 10395−10408. (c) Das,

M. C.; Xiang, S.; Zhang, Z.; Chen, B. Functional Mixed Metal−

Organic Frameworks with Metalloligands. Angew. Chem., Int. Ed. 2011,

0, 10510−10520.

3) (a) Atkins, R.; Brewer, G.; Kokot, E.; Mockler, G. M.; Sinn, E.

Copper(II) and Nickel(II) Complexes of Unsymmetrical Tetradentate

Schiff Base Ligands. Inorg. Chem. 1985, 24, 127. (b) Rigamonti, L.;

Demartin, F.; Forni, A.; Righetto, S.; Pasini, A. Copper(II) Complexes

of salen Analogues with Two Differently Substituted (Push−Pull)

Salicylaldehyde Moieties. A Study on the Modulation of Electronic

Asymmetry and Nonlinear Optical Properties. Inorg. Chem. 2006, 45,

5

(

3

48−350. (c) Hanson, S. K.; Baker, R. T. Knocking on Wood: Base

Metal Complexes as Catalysts for Selective Oxidation of Lignin

Models and Extracts. Acc. Chem. Res. 2015, 48, 2037−2048. (d) Oller,

́ ́

I.; Malato, S.; Sanchez-Perez, J. A. Combination of Advanced

Oxidation Processes and biological treatments for wastewater

decontaminationA review. Sci. Total Environ. 2011, 409, 4141−

4

166.

(

10) (a) Kar, P.; Haldar, R.; Gom

́

ez-García, C. J.; Ghosh, A.

1

0976. (c) Das, P.; Linert, W. Schiff base-derived homogeneous and

heterogeneous palladium catalysts for the Suzuki−Miyaura reaction.

Coord. Chem. Rev. 2016, 311, 1.

Antiferromagnetic Porous Metal−Organic Framework Containing

II

III

10+

Mixed-Valence [Mn Mn (μ -O) ]

Units with Catecholase

Activity and Selective Gas Adsorption. Inorg. Chem. 2012, 51, 4265−

273. (b) Wegner, R.; Gottschaldt, M.; Gorls, H.; Jager, E. G.; Klemm,

4

2

4

2

(

4) (a) Costes, J. P.; Fernandez-Garcia, M. I. Easy synthesis of ‘half-

4

units’: their use as ligands or as precursors of non-symmetrical Schiff

base complexes. Inorg. Chim. Acta 1995, 237, 57−63. (b) Sarkar, B.;

Bocelli, G.; Cantoni, A.; Ghosh, A. Copper(II) complexes of

symmetrical and unsymmetrical tetradentate Schiff base ligands

incorporating 1-benzoylacetone: Synthesis, crystal structures and

electrochemical behavior. Polyhedron 2008, 27, 693−700.

D. Copper(II) Complexes of Aminocarbohydrate β-Ketoenaminic

Ligands: Efficient Catalysts in Catechol Oxidation. Chem. - Eur. J.

2001, 7, 2143−2157. (c) Panja, A.; Jana, N. C.; Patra, M.; Brandao, P.;

̃

Moore, C. E.; Eichhorn, D. M.; Frontera, A. Valence tautomerism

induced nucleophilic ipso substitution in a coordinated tetrabromo-

catecholate ligand and diverse catalytic activity mimicking the function

of phenoxazinone synthase. J. Mol. Catal. A: Chem. 2016, 412, 56−66.

(d) Das, L. K.; Biswas, A.; Kinyon, J. S.; Dalal, N. S.; Zhou, H.; Ghosh,

A. Di−, Tri−, and Tetranuclear Nickel(II) Complexes with Oximato

Bridges: Magnetism and Catecholase-like Activity of Two Tetranuclear

Complexes Possessing Rhombic Topology. Inorg. Chem. 2013, 52,

11744−11757.

̈

̌ ̌

5) Oz, S.; Acar, N.; Svoboda, I.; Titis, J.; Boca, R.; Atakol, O.

(

Synthesis, structure and magnetic properties of homotrinuclear Ni(II)

complexes with asymmetric Schiff-base ligands. Inorg. Chim. Acta

014, 421, 531−537.

6) Mahapatra, P.; Ghosh, S.; Giri, S.; Ghosh, A. The unusual

intermediate species in the formation of Ni(II) complexes of

unsymmetrical Schiff bases by Elder’s method: Structural, electro-

chemical and magnetic characterizations. Polyhedron 2016, 117, 427−

2

(

(11) (a) Dey, S. K.; Mukherjee, A. Zero-Order Catechol Oxidase

Activity by a Mononuclear Manganese(III) Complex Showing High

Turnover Comparable to Catechol Oxidase Enzyme. ChemCatChem

2013, 5, 3533−3537. (b) Pait, M.; Shatruk, M.; Ray, D. Anion

coordination selective [Mn ] and [Mn ] assemblies: synthesis,

4

36.

(

7) (a) Dey, S. K.; Mukherjee, A. Catechol oxidase and

phenoxazinone synthase: Biomimetic functional models and mecha-

nistic studies. Coord. Chem. Rev. 2016, 310, 80−115. (b) Koval, I. A.;

Gamez, P.; Belle, C.; Selmeczi, K.; Reedijk, J. Synthetic models of the

3

4

structural diversity, magnetic properties and catechol oxidase activity.

O

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Dalton Trans. 2015, 44, 11741−11754. (c) Adhikary, J.; Chakraborty,

Lone Pair−π, π−π, and Anion···π−Hole Interactions. J. Phys. Chem. B

2014, 118, 14713−14726. (b) Mirzaei, M.; Eshtiagh-Hosseini, H.;

A.; Dasgupta, S.; Chattopadhyay, S. K.; Kruszynski, R.; Trzesowska-

Kruszynska, A.; Stepanovic,

́

S.; Gruden-Pavlovic,

́

M.; Swart, M.; Das,

́

Bauza, A.; Zarghami, S.; Ballester, P.; Maguee, J. T.; Frontera, A. On

II

D. Unique mononuclear Mn complexes of an end-off compartmental

Schiff base ligand: experimental and theoretical studies on their bio-

relevant catalytic promiscuity. Dalton Trans. 2016, 45, 12409−12422.

the importance of non covalent interactions in the structure of

coordination Cu(II) and Co(II) complexes of pyrazine- and pyridine-

dicarboxylic acid derivatives: experimental and theoretical views.

CrystEngComm 2014, 16, 6149−6158. (c) Neves, A.; Rossi, L. M.;

Bortoluzzi, A. J.; Szpoganicz, B.; Wiezbicki, C.; Schwingel, E.; Haase,

W.; Ostrovsky, S. Catecholase Activity of a Series of Dicopper(II)

Complexes with Variable Cu−OH(phenol) Moietie. Inorg. Chem.

2002, 41, 1788−1794.

(

d) Gajewska, M. J.; Ching, W. M.; Wen, Y. S.; Hung, C. H. Synthesis,

structure, and catecholase activity of bispyrazolylacetate copper(II)

complexes. Dalton Trans. 2014, 43, 14726−14736.

(

́

12) (a) Hazari, A.; Das, L. K.; Kadam, R. M.; Bauza, A.; Frontera, A.;

Ghosh, A. Unprecedented structural variations in trinuclear mixed

valence Co(II/III) complexes: theoretical studies, pnicogen bonding

interactions and catecholase-like activities. Dalton Trans. 2015, 44,

(20) (a) Hazari, A.; Das, L. K.; Bauza,

́

A.; Frontera, A.; Ghosh, A.

Exploring the coordinative adaptation and molecular shapes of

II

2

II

3

862−3876. (b) Comba, P.; Martin, B.; Muruganantham, A.; Straub, J.

trinuclear Cu M (M = Zn/Cd) complexes derived from salen type

Structure, Bonding, and Catecholase Mechanism of Copper Bispidine

Complexes. Inorg. Chem. 2012, 51, 9214−9225. (c) Das, M.; Nasani,

R.; Saha, M.; Mobin, S. M.; Mukhopadhyay, S. Nickel(II) complexes

with a flexible piperazinyl moiety: studies on DNA and protein binding

and catecholase like properties. Dalton Trans. 2015, 44, 2299−2310.

Schiff bases: structural and theoretical studies. Dalton Trans. 2016, 45,

5730−5740. (b) Ghosh, S.; Aromí, G.; Gamez, P.; Ghosh, A. The

Impact of Anion-Modulated Structural Variations on the Magnetic

Coupling in Trinuclear Heterometallic Cu −Co Complexes Derived

from a Salen-Type Schiff Base Ligand. Eur. J. Inorg. Chem. 2014, 2014,

3341−3349.

II

(

d) Mondal, M.; Guha, P. M.; Giri, S.; Ghosh, A. Deactivation of

catecholase-like activity of a dinuclear Ni(II) complexby incorporation

(21) Addison, A. W.; Rao, T. N.; Reedijk, J.; Rijn, J. V.; Verschoor, G.

C. Synthesis, structure, and spectroscopic properties of copper(II)

compounds containing nitrogen−sulphur donor ligands; the crystal

and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-

2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc., Dalton Trans.

1984, 1349−1356.

of an additional Ni(II). J. Mol. Catal. A: Chem. 2016, 424, 54−64.

(

13) Seth, P.; Das, L. K.; Drew, M. G. B.; Ghosh, A. Synthesis,

Crystal Structure, and Catecholase Activity of Three Trinuclear

II

Heterometallic Ni −Mn Complexes Derived from a Salen-Type

2

Schiff Base Ligand. Eur. J. Inorg. Chem. 2012, 2012, 2232−2242.

14) (a) Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E.

(

(22) (a) Seth, P.; Ghosh, S.; Figuerola, A.; Ghosh, A. Trinuclear

II

Multicopper Oxidases and Oxygenases. Chem. Rev. 1996, 96, 2563−

605. (b) Kaila, V. R. I.; Verkhovsky, M. I.; Wikstrom, M. Proton-

Coupled Electron Transfer in Cytochrome Oxidase. Chem. Rev. 2010,

10, 7062−7081. (c) Garcia-Bosch, I.; Ribas, X.; Costas, M. Well-

Defined Heterometallic and Unsymmetric M O Complexes Arising

heterometallic Cu −Mn complexes of a salen type Schiff base ligand:

anion dependent variation of phenoxido bridging angles and magnetic

coupling. Dalton Trans. 2014, 43, 990−998. (b) Biswas, S.; Naiya, S.;

2

̈

1

Gom

́

ez-García, C. J.; Ghosh, A. Synthesis of the first heterometalic

star-shaped oxido-bridged MnCu complex and its conversion into

trinuclear species modulated by pseudohalides (N3 , NCS and

NCO ): Structural analyses and magnetic properties. Dalton Trans.

2

3

−

from Binding and Activation of O . Eur. J. Inorg. Chem. 2012, 2012,

2

−

1

79−187. (d) Engelmann, X.; Yao, S.; Farquhar, E. R.; Szilvas

́

i, T.;

Kuhlmann, U.; Hildebrandt, P.; Driess, M.; Ray, K. A New Domain of

2012, 41, 462−473.

Reactivity for High-Valent Dinuclear [M(-μ-O) M′] Complexes in

(23) Chilton, N. F.; Anderson, R. P.; Turner, L. D.; Soncini, A.;

Murray, K. S. PHI: A powerful new program for the analysis of

anisotropic monomeric and exchange-coupled polynuclear d- and f-

block complexes. J. Comput. Chem. 2013, 34, 1164−1175.

(24) Ferrando-Soria, J. F.; Fabelo, O.; Castellano, M.; Cano, J.;

Fordham, S.; Zhou, H. C. Multielectron oxidation in a ferromagneti-

cally coupled dinickel(II) triple mesocate. Chem. Commun. 2015, 51,

13381−13384.

2

Oxidation Reactions. Angew. Chem., Int. Ed. 2017, 56, 297. (e) York, J.

T.; Llobet, A.; Cramer, C. J.; Tolman, W. B. Heterobimetallic

Dioxygen Activation: Synthesis and Reactivity of Mixed Cu-Pd and

Cu-Pt Bis(μ-oxo) Complexes. J. Am. Chem. Soc. 2007, 129, 7990−

7

999. (f) Kundu, S.; Pfaff, F. F.; Miceli, E.; Zaharieva, I.; Herwig, C.;

Yao, S.; Farquhar, E. R.; Kuhlmann, U.; Bill, E.; Hildebrandt, P.; Dau,

H.; Driess, M.; Limberg, C.; Ray, K. A High-Valent Heterobimetallic

III

III 2+

[

Cu (μ-O) Ni ] Core with Nucleophilic Oxo Groups. Angew.

(25) Zhou, Y. Y.; Hartline, D. R.; Steiman, T. J.; Fanwick, P. E.;

Uyeda, C. Dinuclear Nickel Complexes in Five States of Oxidation

Using a Redox-Active Ligand. Inorg. Chem. 2014, 53, 11770−11777.

(26) Lan, Y.; Kennepohl, D. K.; Moubaraki, B.; Murray, K. S.;

Cashion, J. D.; Jameson, G. B.; Brooker, S. Coordination Algorithms

2

Chem., Int. Ed. 2013, 52, 5622−5626. (g) Yao, S.; Herwig, C.; Xiong,

Y.; Company, A.; Bill, E.; Limberg, C.; Driess, M. Monooxygenase-

Like Reactivity of an Unprecedented Heterobimetallic {FeO Ni}

2

Moiety. Angew. Chem., Int. Ed. 2010, 49, 7054−7058.

I

4+

(

15) (a) Thevenon, A.; Romain, C.; Bennington, M. S.; White, A. J.

Control Molecular Architecture: [Cu (L2) ] Grid Complex Versus

4 4

II

y+

P.; Davidson, H. J.; Brooker, S.; Williams, C. K. Dizinc Lactide

Polymerization Catalysts: Hyperactivity by Control of Ligand

Conformation and Metallic Cooperativity. Angew. Chem., Int. Ed.

[M (L2) X ] Side-By-Side Complexes (M = Mn, Co, Ni, Zn; X =

2 2 4

Solvent or Anion) and [Fe (L2) ][Cl Fe OFe Cl ]. Chem. - Eur. J.

II

III

3

2003, 9, 3772−3784.

2

016, 55, 8680−8685. (b) Chakraborty, B.; Bhunya, S.; Paul, A.; Paine,

(27) Mukherjee, J.; Mukherjee, R. Catecholase activity of dinuclear

copper(II) complexes with variable endogenous and exogenous bridge.

Inorg. Chim. Acta 2002, 337, 429−438.

T. K. Reactivity of Biomimetic Iron(II)-2-aminophenolate Complexes

toward Dioxygen: Mechanistic Investigations on the Oxidative C−C

Bond Cleavage of Substituted 2-Aminophenols. Inorg. Chem. 2014, 53,

(28) Zippel, F.; Ahlers, F.; Werner, R.; Haase, W.; Nolting, H. F.;

Krebs, B. Structural and Functional Models for the Dinuclear Copper

Active Site in Catechol Oxidases: Syntheses, X-ray Crystal Structures,

Magnetic and Spectral Properties, and X-ray Absorption Spectroscopic

Studies in Solid State and in Solution. Inorg. Chem. 1996, 35, 3409−

3419.

4

899−4912. (c) Chapman, A. M.; Keyworth, C.; Kember, M. R.;

Lennox, A. J. J.; Williams, C. K. Adding Value to Power Station

Captured CO : Tolerant Zn and Mg Homogeneous Catalysts for

2

Polycarbonate Polyol Production. ACS Catal. 2015, 5, 1581−1588.

(

16) CrysAlis; Oxford Diﬀraction Ltd., Abingdon, U. K., 2006.

17) Sheldrick, G. M. Shelxs97, Shelxl97, Programs for Crystallo-

(29) Kahn, O. Molecular Magnetism; VCH: Weinheim, Germany,

graphic solution and refinement. Acta Crystallogr., Sect. A: Found.

Crystallogr. 2008, 64, 112.

1993.

(30) Krost, D. A.; McPherson, G. L. Spectroscopic and Magnetic

Properties of an Exchange Coupled Copper(II)-Manganese(II) Dimer.

J. Am. Chem. Soc. 1978, 100, 987−989.

(

18) ABSPACK; Oxford Diﬀraction Ltd., Oxford, U.K., 2005.

19) (a) Mitra, M.; Manna, P.; Bauza, A.; Ballester, P.; Seth, S. K.;

́

Choudhury, S. R.; Frontera, A.; Mukhopadhyay, S. 3-Picoline

Mediated Self-Assembly of M(II)−Malonate Complexes (M = Ni/

Co/Mn/Mg/Zn/Cu) Assisted by Various Weak Forces Involving

̈

(31) Colacio, E.; Dominguez-Vera, J. M.; Ghazi, M.; Kivekas, R.;

Klinga, M.; Moreno, J. M. A Novel Series of Heterotrinuclear

Complexes Involving Imidazolate Bridging Schiff’s Base Ligands.

P

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Synthesis, Crystal Structure, and Magnetic Properties. Inorg. Chem.

tion of Epoxides and Carbon Dioxide or Anhydrides. Inorg. Chem.

2015, 54, 11906−11915.

1

(

998, 37, 3040−3045.

32) Monzani, E.; Quinti, L.; Perotti, A.; Casella, L.; Gulotti, M.;

(42) Dey, S. K.; Mukherjee, A. Manganese(III) acetate mediated

catalytic oxidation of substituted dioxolene and phenols. J. Mol. Catal.

A: Chem. 2014, 395, 186−194.

Randaccio, L.; Geremia, S.; Nardin, G.; Faleschini, P.; Tabbi, G.

Tyrosinase Models. Synthesis, Structure, Catechol Oxidase Activity,

and Phenol Monooxygenase Activity of a Dinuclear Copper Complex

Derived from a Triamino Pentabenzimidazole Ligand. Inorg. Chem.

(43) Serrano-Plana, J.; Garcia-Bosch, I.; Company, A.; Costas, M.

Structural and Reactivity Models for Copper Oxygenases: Cooperative

Effects and Novel Reactivities. Acc. Chem. Res. 2015, 48, 2397−2406.

(44) Sreenivasulu, B.; Zhao, F.; Gao, S.; Vittal, J. J. Synthesis,

Structures and Catecholase Activity of a New Series of Dicopper(II)

Complexes of Reduced Schiff Base Ligands. Eur. J. Inorg. Chem. 2006,

2006, 2656−2670.

1

(

998, 37, 553−562.

33) (a) Mendoza-Quijano, M. R.; Ferrer-Sueta, G.; Floresal

Aliaga-Alcalde, N.; Gomez-Vidales, V.; Ugalde- Saldívara, V. M.;

́

amo, M.;

́

Gasque, L. Mechanistic insight on the catecholase activity of dinuclear

copper complexes with distant metal centers. Dalton Trans. 2012, 41,

4

985−4997. (b) Ackermann, J.; Meyer, F.; Kaifer, E.; Pritzkow, H.

(45) Banu, K. S.; Chattopadhyay, T.; Banerjee, A.; Bhattacharya, S.;

Zangrando, E.; Das, D. Catechol oxidase activity of dinuclear

copper(II) complexes of Robson type macrocyclic ligands: Syntheses,

X-ray crystal structure, spectroscopic characterization of the adducts

and kinetic studies. J. Mol. Catal. A: Chem. 2009, 310, 34−41.

Tuning the Activity of Catechol Oxidase Model Complexes by

Geometric Changes of the Dicopper Core. Chem. - Eur. J. 2002, 8,

2

47−258. (c) Koval, I. A.; Selmeczi, K.; Belle, C.; Philouze, C.; Saint-

Aman, E.; Gautier-Luneau, I.; Schuitema, A. M.; Vliet, M. V.; Gamez,

P.; Roubeau, O.; Luken, M.; Krebs, B.; Lutz, M.; Spek, A. L.; Pierre, J.

L.; Reedijk, J. Catecholase Activity of a Copper(II) Complex with a

Macrocyclic Ligand: Unraveling Catalytic Mechanisms. Chem. - Eur. J.

(46) Thio, Y.; Yang, X.; Vittal, J. J. Influence of inductive effects and

steric encumbrance on the catecholase activities of copper(II)

complexes of reduced Schiff base ligands. Dalton Trans. 2014, 43,

3

(

545−3556.

2

(

006, 12, 6138−6150.

47) Guha, A.; Banu, K. S.; Banerjee, A.; Ghosh, T.; Bhattacharya, S.;

34) (a) Balla, J.; Kiss, T.; Jameson, R. F. Copper(II) -Catalyzed

Zangrando, E.; Das, D. Bio-relevant manganese(II) compartmental

ligand complexes: Syntheses, crystal structures and studies of catalytic

activities. J. Mol. Catal. A: Chem. 2011, 338, 51−57.

Oxidation of Catechol by Molecular Oxygen in Aqueous Solution.

Inorg. Chem. 1992, 31, 58−62. (b) Selmeczi, K.; Reglier, M.; Giorgi,

M.; Speier, G. Catechol oxidase activity of dicopper complexes with N-

donor ligands. Coord. Chem. Rev. 2003, 245, 191−201.

(48) Yang, C.-T.; Vetrichelvan, M.; Yang, X.; Moubaraki, B.; Murray,

K. S.; Vittal, J. J. Syntheses, structural properties and catecholase

activity of copper(II) complexes with reduced Schiff base N-(2-

hydroxybenzyl)-amino acids. Dalton Trans. 2004, 113−121.

(

35) (a) Buchwalter, P.; Rose,

Catalysis Based on Heterometallic Complexes and Clusters. Chem.

Rev. 2015, 115, 28−126. (b) Gennari, M.; Brazzolotto, D.; Pecaut, J.;

Cherrier, M. V.; Pollock, C. J.; DeBeer, S.; Retegan, M.; Pantazis, D.

A.; Neese, F.; Rouzieres, M.; Clerac, R.; Duboc, C. Dioxygen

́

J.; Braunstein, P. Multimetallic

́

̀

́

Activation and Catalytic Reduction to Hydrogen Peroxide by a

Thiolate-Bridged Dimanganese(II) Complex with a Pendant Thiol. J.

Am. Chem. Soc. 2015, 137, 8644−8653. (c) Fiedler, A. T.; Fischer, A.

A. Oxygen activation by mononuclear Mn, Co, and Ni centers in

biology and synthetic complexes. JBIC, J. Biol. Inorg. Chem. 2017, 22,

4

(

07.

́

36) Behar, S.; Gonzalez, P.; Agulhon, P.; Quignard, F.; Swierczynski,

́

D. New synthesis of nanosized Cu−Mn spinels as efficient oxidation

catalysts. Catal. Today 2012, 189, 35−41.

(

37) (a) Matsunaga, S.; Shibasaki, M. Recent advances in cooperative

bimetallic asymmetric catalysis: dinuclear Schiff base complexes. Chem.

Commun. 2014, 50, 1044−1057. (b) Garden, J. A.; Saini, P. K.;

Williams, C. K. Greater than the Sum of Its Parts: A Heterodinuclear

Polymerization Catalyst. J. Am. Chem. Soc. 2015, 137, 15078−15081.

(

c) Qin, J.; Xu, B.; Zhang, Y.; Yuan, D.; Yao, Y. Cooperative rare earth

metal−zinc based heterometallic catalysts for copolymerization of CO₂

and cyclohexene oxide. Green Chem. 2016, 18, 4270−4275.

(

38) Matsunaga, S.; Shibasaki, M. Multimetallic Schiff Base

Complexes as Cooperative Asymmetric Catalysts. Synthesis 2013, 45,

21−437.

39) Sasai, H.; Suzuki, T.; Itoh, N.; Tanaka, K.; Date, T.; Okamura,

4

(

K.; Shibasaki, M. Catalytic Asymmetric Nitroaldol Reaction Using

Optically Active Rare Earth BINOL Complexes: Investigation of the

Catalyst Structure. J. Am. Chem. Soc. 1993, 115, 10372−10373.

(

40) (a) Ghosh, S.; Mukherjee, S.; Seth, P.; Mukherjee, P. S.; Ghosh,

A. Solvent-templated supramolecular isomerism in 2D coordination

II

2

II

polymer constructed by Ni Co nodes and dicyanamido spacers:

drastic change in magnetic behaviours. Dalton Trans. 2013, 42,

1

́

3554−13564. (b) Das, L. K.; Biswas, A.; Gomez-García, C. J.; Drew,

M. G. B.; Ghosh, A. Isolation of Two Different Ni Zn Complexes with

2

an Unprecedented Cocrystal Formed by One of Them and a

“Coordination Positional Isomer” of the Other. Inorg. Chem. 2014, 53,

4

34−445. (c) Ghosh, S.; Aromí, G.; Gamez, P.; Ghosh, A. Structural

and Magnetic Analysis of Retrosynthetically Designed Architectures

Built from a Triply Bridged Heterometallic (CuL) Co Node and

2

Benzenedicarboxylates. Eur. J. Inorg. Chem. 2015, 2015, 3028−3037.

(

41) Thevenon, A.; Garden, J. A.; White, A. J. P.; Williams, C. K.

Dinuclear Zinc Salen Catalysts for the Ring Opening Copolymeriza-

Q

DOI: 10.1021/acs.inorgchem.7b00253

Inorg. Chem. XXXX, XXX, XXX−XXX

Article Doi

Subtle Structural Changes in (CuIIL)2MnII Complexes To Induce Heterometallic Cooperative Catalytic Oxidase Activities on Phenolic Substrates (H2L = Salen Type Unsymmetrical Schiff Base)

DOI: 10.1021/acs.inorgchem.7b00253

Source and publish data:

Authors:

Article abstract of DOI:10.1021/acs.inorgchem.7b00253

Full text of DOI:10.1021/acs.inorgchem.7b00253

Products guided by the article

R&D Labs maybe for 1916-59-2

Relevant to this article

Hot Product