Variations of structures and phenoxazinone synthase-like activity of the complexes based on (CuII)2MnII node and dicyanamide spacer

Article abstract of DOI:10.1021/acs.cgd.7b01407

Three new heterometallic Cu(II)?Mn(II) complexes, [((CuL)₂Mn)₂(μ_1,5-N(CN)₂)(CH₃CN)₂](ClO₄)₃ (1), [(CuL)₂Mn(N(CN)₂)₂]·(H₂O) (2), and [(CuL)₂Mn-(μ_1,5-N(CN)₂)₂]_n (3), have been synthesized using a Cu(II)metalloligand of an asymmetrically dicondensed Schiff base ligand (where H₂L = N-α-methylsalicylidene-N′-salicylidene-1,3-propanediamine). Complex 1 was formed when the ratio of [CuL]/Mn(ClO₄)₂/NaN(CN)₂ was 2:1:1, whereas complexes 2 and 3 were obtained with a 2:1:2 ratio of the same reactants on varying the reaction conditions. Single-crystal structural analyses reveal that complex 1 possesses a hexanuclear structure in which two (Cu^II)₂Mn^II units are connected by one μ_1,5-N(CN)₂^? bridge, 2 is a discrete trinuclear species with two terminally coordinated N(CN)₂^? ions to the Mn(II), whereas complex 3 is a polymeric form of 2 with μ_1,5-N(CN)₂^? bridges between Cu(II) and Mn(II) centers. The thermal variations of dc magnetic susceptibilities suggest that all three complexes (1?3) are antiferromagnetically coupled with comparable exchange coupling constants (?25.4, ?22.8, and ?22.0 cm^?1, respectively) which are expected from the Cu?O?Mn angles. All the complexes show biomemitic phenoxazinone synthase-like activity for the aerial oxidation of o-aminophenol to amino phenoxazinone. The turnover numbers (k_cat) for the process are 4966, 2021, and 1107 h^?1 for complexes 1?3 respectively. The mass spectral evidence on intermediates suggests that the cooperative activity of the two different metal ions, i.e., coordination of substrate to Mn(II) and shuttling of oxidation state of Cu between I and II, is possibly operative in the oxidation process. The highest catalytic activity of 1 is attributed to the presence of one coordinating solvent molecule to Mn(II).

Full text of DOI:10.1021/acs.cgd.7b01407

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Article
Variations of Structures and Phenoxazinone Synthase-like Activity of
the Complexes Based on (CuII)2MnII node and Dicyanamide Spacer
Prithwish Mahapatra, Michael G. B. Drew, and Ashutosh Ghosh
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01407 • Publication Date (Web): 08 Nov 2017
Downloaded from http://pubs.acs.org on November 11, 2017
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Variations of Structures and Phenoxazinone Synthaseꢀlike Activity of the
Complexes Based on (CuII)2MnII Node and Dicyanamide Spacer
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Prithwish Mahapatra†, Michael G. B. Drew§, Ashutosh Ghosh*,†
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†Department of Chemistry, University College of Science, University of Calcutta, 92, A.
P.C. Road, Kolkata 700009, India, Eꢀmail: ghosh_59@yahoo.com
§School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading
RG6 6AD, UK.
Abstract
Three
new
heterometallic
Cu(II)ꢀMn(II)
complexes,
[{(CuL)2Mn}2(ꢁ1,5ꢀN(CN)2)
(CH3CN)2](ClO4)3 (1), [{(CuL)2Mn}2(ꢁ1ꢀN(CN)2)]·(H2O)4 (2) and [{(CuL)2Mn}2(ꢁ1,5ꢀN(CN)2)]n
(3) have been synthesized using Cu(II)ꢀmetalloligand of an asymmetrically dicondensed Schiff
base ligand (where H2L= NꢀαꢀmethylsalicylideneꢀN΄ꢀsalicylideneꢀ1,3ꢀpropanediamine).
Complex 1 was formed when the ratio of [CuL] : Mn(ClO4)2 : NaN(CN)2 was 2:1:1 whereas
complexes 2 and 3 were obtained with a 2: 1: 2 ratio of the same reactants on varying the
reaction conditions. Singleꢀcrystal structural analyses reveal that complex 1 possesses a
−
hexanuclear structure in which two (CuII)2MnII units are connected by one ꢁ1,5ꢀN(CN)2 bridge, 2
−
is a discrete trinuclear species with two terminally coordinated N(CN)2 ions to the Mn(II)
−
whereas complex 3 is a polymeric form of 2 with ꢁ1,5ꢀN(CN)2 bridges between Cu(II) and
Mn(II) centers. The thermal variations of dc magnetic susceptibilities suggest that all three
complexes (1−3) are antiferromagnetically coupled with comparable exchange coupling
1
constants (−25.4, −22.8 and −22.0 cm− , respectively) which are expected from the Cu−O−Mn
angles. All the complexes show biomemitic phenoxazinone synthaseꢀlike activity for the aerial
oxidation of oꢀaminophenol to amino phenoxazinone. The turnover numbers (kcat) for the process
1
are 4966, 2021 and 1107 h− for complexes 1−3 respectively. The mass spectral evidence on
intermediates suggests that the cooperative activity of the two different metal ions i.e.
coordination of substrate to Mn(II) and shuttling of oxidation state of Cu between I and II is
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possibly operative in the oxidation process. The highest catalytic activity of 1 is attributed to the
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presence of one coordinating solvent molecule to Mn(II).
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Introduction
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Heterometallic polynuclear complexes of N, O donor ligands are of growing interest to
the chemists mainly due to their potential applications in catalysis and in molecular
magnetism.1−5 The presence of different metal ions in one cluster may drastically improve
catalytic activity as a result of multimetallic cooperative activities whereas the presence of unlike
spin in one cluster can induce ingenious magnetic properties such as ferrimagnetism, SMM
etc.6−11 Various types of N, O donor ligands have been used for the synthesis of polynuclear
heterometallic complexes of 3dꢀ3d' and 3dꢀ4f metal ions.12−17 Among them, symmetrically
dicondensed Schiff base ligands are arguably the most common ones, but their asymmetrically
dicondensed analogues have been used only rarely. One of the common strategies for the
synthesis small heterometallic clusters is to react the Cu(II) or Ni(II) chelate of the Schiff base
with the salts of another 3d or 4f metal ions. The anionic part of such metal salts not only balance
the charge of the resulting complex but can also coordinate to the metal ions in various ways to
modulate the structures as well as the magnetic and catalytic properties of the clusters
radically.18−22 The anions such as N3 , SCN−, N(CN)2 , RCO2 etc. which have various possible
−
−
−
coordination modes have been used to synthesize complexes with diverse structures. Among
−
them, dicyanamide (N(CN)2 ) deserves special mention as the long, bent shaped, flexible anion
containing three donor Nꢀatoms can produce complexes of varied nuclearity, shape and isomeric
form.23−26 Recently, we were able to isolate rather rare supramolecular isomers in the solid state
with some heterometallic Cu(II)ꢀCd(II), Cu(II)ꢀCo(II) and Ni(II)ꢀCo(II) nodes of symmetrical
−
salen type N2O2 donor Schiff base ligands and N(CN)2 spacers; of which isomeric Ni(II)ꢀCo(II)
species showed intriguing differences in magnetic properties.27−31
One of the most important applications of the polynuclear complexes is in the field of
biomimetic catalytic model enzyme systems. A large number of homometallic complexes of
some redox active metals like Cu(II), Mn(II/III), Co(II/III), Fe(II/III), Ni(II) etc. have been
extensively used for catalytic oxidase type reactions e.g. catechol oxidase, phenoxazinone
synthase, cytochrome c oxidase etc.32−37 It has been shown that in those processes the activity
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and efficiency of these homometallic complexes as catalysts depend largely on the participation
of metal ions into the oxidationꢀreduction reaction. The participation of metal ions in the
oxidation process depends very subtly on the coordination number and nature of the ligands in its
coordination sphere.38−42 These catalytic processes have been investigated extensively by various
groups but till now the catalysts that have been used in these processes are confined mostly to
homometallic complexes. Recently, we have shown that the heterometallic complexes of Schiff
base ligands can also show this type of biomimetic catalytic oxidase reactions.43 The advantage
of heterometallic complexes as catalyst lies in the fact that the cooperative effect of the metal
ions in terms of coordination to the substrate and redox behavior can potentially enhance the
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II
catalytic activity.44−50 In fact, recently we synthesized a heterometallic Cu2 ꢀMnII complex of an
unsymmetric Schiff base and found that it is not only catalytically activity for such oxidase
reactions but its activity is the highest among all the model complexes synthesized so far.43 Most
of the reported synthetic functional model complexes are Cu(II) based as the naturally occurring
metalloenzymes for oxidase process e. g. catechol oxidase, phenoxazinone synthase etc. contains
Cu(II). The presence of redox active Mn(II) ion along with Cu(II) in a complex is expected to
modify the redox behavior of the complex as a whole. Moreover, the intrinsically different
coordination behavior of the metal ions can facilitate the coordination of substrate to the complex
in favorable conditions.44−50
Herein, we report the synthesis and crystal structures of three heterometallic Cu(II)ꢀ
Mn(II) complexes [{(CuL)2Mn}2(ꢁ1,5ꢀN(CN)2)(CH3CN)2](ClO4)3 (1), [{(CuL)2Mn}2(ꢁ1ꢀ
N(CN)2)]·(H2O)4 (2) and [{(CuL)2Mn}2(ꢁ1,5ꢀN(CN)2)]n (3) using the Cu(II) metalloligand of a
unsymmetrical Schiff base ligand H2L (where H2L= NꢀαꢀmethylsalicylideneꢀN΄ꢀsalicylideneꢀ1,3ꢀ
propanediamine). Complex 1 possesses a hexanuclear structure in which one dicyanamide ion is
present per two trinuclear units whereas 2 is a discrete trinuclear species with two dicyanamide
ions per trinuclear unit. The molecular formula of 3 is same as that of 2 but here the trinuclear
units are connected by the ꢁ1,5ꢀN(CN)2 ion to form a 2D polymer. Thus 2 and 3 are coordination
polymerization isomers: 2 is the monomeric form of polymer 3. All three complexes exhibit
model biomimetic phenoxazinone synthase like activity under aerial conditions, but their
catalytic efficiencies are distinctly different. UVꢀVis spectra, ESIꢀmass spectra analysis and
cyclic voltammograms studies have been performed to determine the possible intermediates of
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this catalytic reaction and consequently to develop a probable mechanism. The dc magnetic
susceptibility has also been measured for all these complexes.
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Experimental Section
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Starting materials: Salicylaldehyde, oꢀhydroxoacetophenone and 1,3ꢀpropanediamine were
purchased from Spectrochem, India and were of reagent grade. They were used without further
purification. The other metal salts and solvents were of commercially available reagent grade
quality, unless otherwise stated.
Caution! Although not encountered during experiment, perchlorate salts of metal complexes
with organic ligands are potentially explosive. Only a small amount of material should be
prepared and it should be handled with care.
Synthesis of the unsymmetrical Schiffꢀbase ligand NꢀαꢀmethylsalicylideneꢀN΄ꢀsalicylideneꢀ
1,3ꢀpropanediamine (H2L) and metalloligand (CuL). The unsymmetrically diꢀcondensed
Schiff base ligand [H2L] and the metalloligand [CuL] were synthesized in our laboratory by
following the methods reported earlier.43
Synthesis of the complex [{(CuL)2Mn}2(ꢁ1,5ꢀN(CN)2)(CH3CN)2](ClO4)3 (1). Metalloligand
[CuL] (0.716 g, 2 mmol) was dissolved in 20 mL acetonitrile and Mn(ClO4)2·6H2O (0.361 g, 1
mmol) was added to the solution. The solution was stirred for 15 mins and then 4 mL 1:1 (v/v)
H2OꢀMeCN solution of NaN(CN)2 (0.089 g, 1 mmol was added drop wise to the solution at room
temperature. The mixture was stirred for 20 min. A small amount of precipitate appeared during
this process, which was filtered off and the filtrate was kept in a beaker inside a desiccator. The
deep brown Xꢀray diffractable quality single crystals appeared in 3ꢀ4 days, which were collected
by filtration and air dried.
Complex 1: Yield 0.786g (76%). Anal. Calc. for C82H82Cl3Cu4Mn2N15O20 (2068.06): Calculated
C, 47.62; H, 4.00; N, 10.16; Found C, 47.54; H, 4.16; N, 10.01. IR: ν(C=N) = 1630, 1599 cm–1;
ν(Cl=O) = 1089 cm–1 and ν(N=C=N) = 2182, 2242 and 2322 cm–1.
Synthesis of the complex [{(CuL)2Mn}2(ꢁ1ꢀN(CN)2)]·(H2O)4 (2). The complex was
synthesized following a similar procedure as for the synthesis of 1. Only difference is that here
the Mn(ClO4)2·6H2O:NaN(CN)2 ratio is changed to 1:2 and methanol was used as solvent
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instead of acetonitrile. The brown colored Xꢀray quality single crystals appeared in 2ꢀ3 days,
which were separated by filtration and air dried.
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Complex 2: Yield 0.706 (73%). Anal. Calc. for C40H36Cu2MnN10O8 (966.83): Calculated C,
49.69; H, 3.75; N, 14.49; Found C, 49.85; H, 3.81; N, 14.46. IR: ν(C=N) = 1628 and 1600 cm–1;
ν(N=C=N) = 2152, 2216 and 2265 cm–1.
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Synthesis of the complex [{(CuL)2Mn}2(ꢁ1,5ꢀN(CN)2)]n (3). This complex was synthesized by
following a similar procedure as for the synthesis of 2. Only difference is that here the solution
mixture was refluxed for 1 hour and then cooled at room temperature. The cooled solution is
filtered and kept in a desiccator. The deep green Xꢀray quality single crystals appeared at the
bottom of the beaker after 4ꢀ5 days, which were collected by filtration and air dried.
Complex 3: Yield 0.641g (71%). Anal. Calc. for C40H36Cu2MnN10O4 (902.83): Calculated C,
53.21; H, 4.02; N, 15.51; Found C, 53.32; H, 4.14; N, 15.59. IR: ν(C=N) = 1628 and 1600 cm–1;
ν(N=C=N) = 2152, 2216 and 2265 cm–1.
Physical Measurements
Elemental analyses (C, H and N) were performed using a Perkin–Elmer 2400 series II CHN
analyzer. IR spectra in KBr pellets (4000–500 cm−1) were recorded using a PerkinꢀElmer RXI
FTꢀIR spectrophotometer. The electronic absorption spectra (1000–240 nm) in acetonitrile
solution were collected in a Hitachi Uꢀ3501 spectrophotometer. Temperatureꢀdependent molar
susceptibility for powdered crystalline sample of compounds 1−3 were measured using a
superconducting quantum interference device vibrating sample magnetometer (SQUIDꢀVSM,
Quantum Design) with an applied field 500 Oe throughout the temperature range 2–300 K. The
susceptibility data were corrected by Pascal’s diamagnetic contributions. Isothermal
magnetization measurements were performed at 2K up to 5 Tesla magnetic fields.
Electrochemical measurements
The electrochemical measurements of all three complexes (1, 2 and 3) were performed using
Epsilon BasiꢀC3 Cell instrument at a scan rate of 50–200 mV s–1 within the potential range of 0
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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 × 10ꢀ3 M of complexes in acetonotrile
solution which are deoxygenated by bubbling with argon. The working electrode was a glassy
carbon disk (0.32 cm2) which was polished with alumina solution, washed with absolute acetone
and acetonotrile, and air dried before each electrochemical run. The reference electrode was
Ag/AgCl, with platinum as counter electrode. All experiments were performed in standard
electrochemical cells at 25°C.
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ESIꢀHRMS
Electro spray ionization mass spectrometry (ESIꢀMS positive) ion mass spectra were acquired
using a Xevo G2ꢀS QTof (Waters) mass spectrometer, equipped with a Zꢀspray interface, over a
mass range of 100–1200 Da, in a continuum mode. Aqueous sodium formate was used for Qꢀoaꢀ
Tof calibration. LꢀLeucine was used as the external mass calibrant lock mass [M+H]+ =
556.2771 Da. Solutions of compounds were injected at flow rate of 5µl/min. All solutions were
made in massꢀspectrometric grade methanol. The concentrations of complexes 1, 2 and 3 as well
as complexes + substrates in solutions were the same (10ꢀ6 M) during acquisitions of mass
spectra.
X–ray Crystallographic data collection and refinement
The crystal structure data for 1, 2 and 3 were collected with MoKα at 150K using the Oxford
Diffraction XꢀCalibur CCD System. The crystals were positioned at 50 mm from the CCD and
321 frames were measured with counting times of 10s. Data analysis was carried out with the
CrysAlis program.51 The structures were solved using direct methods with the Shelxs97
program.52,53 The nonꢀhydrogen 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 were carried out using the ABSPACK program.54 In the structure of 1, one
perchlorate was disordered over a centre of symmetry and the bridging ꢀNCNCN was disordered
around a centre of symmetry. In 2, the terminal ꢀNCNCN was disordered and there were two
water molecules both refined with 25% occupancy. The structures were refined using Shelxl16ꢀ
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6.53 The crystallographic data of complexes 1−3 are given in Table 1. Bond distances (Å) and
angles (°) parameters around the metal centers of complexes 1−3 are given in Table S1.
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Phenoxazinone synthase of OAP
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The Phenoxazinone synthase activity of complex 1−3 was studied using oꢀaminophenol (OAP)
as the substrate in acetonotrile solution under aerobic conditions at room temperature. The
reactions were followed spectrophotometrically by monitoring the increase in the absorbance
maxima of the amino phenoxazinone (APX) band at 425 nm as a function of time (time scan).
Table 1. Crystal data and structure refinement of complexes 1, 2 & 3.
1
2
3
C78H76Cu4Mn2N13O8,
3(ClO4),2(C2H3N)
Formula
C40H36Cu2MnN10O4,H2O
C40H36Cu2MnN10O4
Mol. Wt.
Crystal System
Space Group
a/Å
2068.02
Triclinic
Pꢀ1
920.82
Monoclinic
C2/c
902.81
Monoclinic
C2/c
11.1797(10)
13.4481(11)
15.7107(12)
68.133(7)
80.785(7)
77.469(7)
2131.7(3)
1
25.517(7)
10.095(2)
16.111(4)
90
16.354(1)
10.4444(6)
24.6627(18)
90
b/Å
c/Å
α
/°
/°
100.15(3)
90
112.921(8)
90
β
γ
/°
V/Å3
4085.0(17)
4
3880.0(5)
4
Z
Dc/g cmꢀ3
µ/mmꢀ1
1.611
1.497
1.546
1.442
1.393
1.463
F (000)
1056
1884
1844
R(int)
0.051
0.116
0.051
Total Reflections
Unique reflections
14971
8489
8850
11865
4518
4386
6940
2180
3058
I>2
σ
(I)
a
b
R1 , wR2
GOFc on F2
0.0920, 0.2396
1.008
0.0719, 0.1984
0.926
0.1062, 0.2827
1.082
R (all)
0.1520
0.1430
150
0.1295
150
Temp (K)
150
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Residual electron
Density, e/Åꢀ3
ꢀ0.918, 1.891
ꢀ0.616, 0.776
ꢀ1.065, 4.353
2
2
4 ½
2
½
aR1 = ∑||Fo|−|Fc||/∑|Fo|, bwR2 (Fo ) = [∑[w(Fo − Fc2)2/∑w Fo ] and cGOF = [∑[w(Fo − Fc2)2/(Nobs– Nparams)]
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Results and Discussion:
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Syntheses of the complexes
The unsymmetrical ligand, NꢀαꢀmethylsalicylideneꢀN΄ꢀsalicylideneꢀ1,3ꢀpropanediamine (H2L)
and its Cu(II)ꢀmetalloligand, [CuL] were synthesized by the methods as reported earlier by us.43
The metalloligand [CuL] on reaction with Mn(ClO4)2·6H2O in presence of NaN(CN)2 in 2:1:1
molar ratio yielded complex [{(CuL)2Mn}2(ꢁ1,5ꢀN(CN)2) (CH3CN)2](ClO4)3 (1) from acetonitrile
solution. Complex [{(CuL)2Mn}2(ꢁ1ꢀN(CN)2)]·(H2O)4 (2) was obtained by reacting the same
reactants in 2:1:2 molar ratio in methanol and keeping the resulting solution at room temperature.
However, when this reaction mixture was put under reflux for about 1h and the solution was left
at room temperature, a different complex [{(CuL)2Mn}2(ꢁ1,5ꢀN(CN)2)]n (3) was produced
exclucively (Scheme 1). Complex 2 is a trinuclear (CuL)2Mn species in which two terminal
dicyanamide ions are coordinated to Mn(II). When the solution is refluxed, the Nꢀatom in the
other end of dicyanamide ion coordinates to the axial position of the Cu(II) atom of the
neighboring trinuclear unit to yield the 2D polymer, 3. Thus 3 is the polymerized isomer of
momomer 2. 3 can also be prepared by dissolving 2 in methanol and then putting the solution
under reflux for 1h. The similarities of the measured powdered diffractograms of complexes 1−3
with their respective simulated (from mercury 3.8) diffractograms (Figure S3−S5, ESI) show the
phase purity of the complexes.
IR spectra of complexes 1, 2 and 3.
The IR spectra of each of the complexes show two strong peaks at 1630 and 1599 cm−1 for 1
(Figure S1, ESI), 1628 and 1600 cm−1 for both 2 and 3 (Figure S2, ESI) due to two types of
azomethine v(C=N) stretchings of the unymmetrical Schiff base.43 In addition, the spectrum of
complex 1 shows strong and sharp peaks at 2182, 2242, 2322 cm−1 for the stretching vibrations
of N(CN)− ion and at 1089 cm−1 for the stretching vibrations of ClO4 ion, which indicate the
–
presence of two different anions in 1. Compounds 2 and 3 also show similar type of strong and
sharp peaks at 2152, 2216 and 2265 cm−1 for v(N(CN)−) stretching vibrations.27−31
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N
N
N
Cu
O
N
O
O
N
Cu
N
N
O
Mn
Mn
O
N
N
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N
O
O
Cu
O
Cu
N
N
N
ꢀ
3ClO4
Complex 1
Mn(ClO4)2.6H2O,
NaN(CN)2 (1:1)
MeCN
MeOH
Cu(ClO4)2.6H2O
N
N
N
N
Cu
O
O
OH
HO
Mn(ClO4)2.6H2O,
NaN(CN)2 (1:2)
MeOH
H
O
e
M
N
N
N
N
Cu
N
N
N
O
Cu
N
O
N
Cu
O
Reflux, 1h
MeOH
N
N
N
O
Mn
Mn
N
O
O
Cu
O
N
O
Cu
N
N
Cu
N
N
N
N
N
N
Complex 3
Complex 2
Scheme 1. Syntheses of complexes 1−3.
Description of Structures:
The structure of 1 contains [Mn2Cu4L4(NCNCN)(NCMe)2]3+ dimeric cations shown in Figure 1
together with discrete perchlorate anions and a solvent NCMe, the latter refined with 50%
occupancy. Of the two independent perchlorate anions, one is ordered while the other is
disordered with the chlorine atom on a centre of symmetry. The overall dimensions in the three
metal coordination spheres are shown in Table S1.
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The trinuclear unit is formed by the chelation of the two [CuL] metalloligands to the Mn centre
through the Oꢀatoms. The two independent copper atoms are both four coꢀordinate with square
planar environments, each being bonded to four donor atoms of the tetradentate ligand L2ꢀ. The
four atoms in the equatorial plane show r.m.s. deviations of 0.112, 0.203 Å. The two metal atoms
are 0.227(4), 0.221(4) Å from the planes and the two planes intersect at 29.8(2)o. The manganese
atom is sixꢀcoordinate with a distorted octahedral environment being bonded to four oxygen
atoms which are bridging to copper atoms and in addition to two nitrogen atoms, N(81) from an
acetonitrile and N(1) from a ꢀNCNCNꢀ ligand which bridges in a disordered manner around a
centre of symmetry to another manganese atom. The two oxygen atoms O(31) and O(61) which
are trans to nitrogen atoms, namely N(1) and N(81), form longer bonds to the metal atom than
O(11) and O(31) which are mutually trans. Of these two long bonds, Mn(1)ꢀO(61), trans to
N(81) is longer at 2.285(4)Å than Mn(1)ꢀO(31), trans to N(1) at 2.227(4)Å.
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Figure 1. The structure of 1, [Mn2Cu4L4(NCNCN)(NCMe)2]3+ with ellipsoids at 50%
probability. Anions and solvent molecules are not shown. The ꢀNCNCNꢀ bridge is disordered,
only one set of positions is shown.
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The structure of 2, MnCu2L2 (NCNCN)2 is shown in Figure 2 together with the atomic
numbering scheme. The structure contains twoꢀfold symmetry with the Mn(2) atom on the axis.
This complex has a similar structure to many previous structures of this type.27−31 The two Cu(1)
atoms are fourꢀcoordinate with square planar environments while the Mn(2) atom is sixꢀ
coordinate with a highly distorted octahedral environment. Cu(1) is bonded to the tetradentate
ligand through O(11) at 1.904(4), N(19) at 1.994(5), N(23) at 1.939(5) and O(31) at 1.921(4)Å.
The Mn(2) atom, positioned on the 2ꢀfold axis is bonded to O(11) at 2.133(4), O(31) at 2.298(4)
and N(1) at 2.144(6)Å .The disparity in the MnꢀO bond lengths is due to the fact that O(31) is
trans to the NCNCN ligand with an angle of 169.3(2)o while the two O(11) atoms are mutually
trans, though with an angle subtended at the metal of 142.3(2)o . An interesting feature of this
particular structure is that the NCNCN ligand is monodentate and terminal with N(1) bonded to
the metal. The two atoms C(4) and N(5) are disordered over two sites and the latter forms weak
contacts with the disordered solvent water molecules which are refined with 25% occupancy.
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Figure 2. The structure of 2 with ellipsoids at 30% probability.
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The structure of 3 (Figure S6, ESI) also shows twoꢀfold symmetry and is similar to that found in
2 as shown in Figure 2 except for the fact that the two NCNCN ligands bridge to two copper
atoms in different dimers to form a 2ꢀD polymer as shown in Figure 3.
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The coordination sphere of the copper atom is a square pyramid with N(5) as the axial atom
forming a CuꢀN bond of 2.326(7)Å. Remaining bond lengths to copper are similar to those
found in 2, namely O11 at 1.937(4), O(31) at 1.949(6), N(19) at 2.013(9) and N(23) at
1.999(8)Å. The four atoms in the equatorial plane show an r.m.s. deviation of 0.166Å with a
slight tetrahedral distortion. The metal atom is 0.178(3)Å from this plane in the direction of N(5).
The coordination sphere around Mn(2) is similar to that found in 2 with unique distances to
O(11) 2.127(5), O(31) 2.229(5) and N(1) 2.170(7)Å.
Figure 3. The 2D structure of 3. Hydrogen atoms are omitted for clarity.
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It is worth mentioning here that we have synthesized previously a few compounds using
trinuclear heterometallic nodes derived from symmetrical diꢀSchiff base ligand H2L1 or H2L2
(where H2L1
=
N,N’ꢀbis(salicylidene)ꢀ1,3ꢀpropanediamine and H2L2
=
N,N’ꢀbis(αꢀ
methylsalicylidene)ꢀ1,3ꢀpropanediamine) and dicyanamide spacer. It is interesting to note that all
these complexes of H2L2 are discrete trinuclear with terminally coordinated dicyamanide ion. On
the contrary, in all such complexes of H2L1 dicyanamide ion is invariably bridging. Moreover,
the different bridging mode (ꢁ1,5 and ꢁ1,3,5) and conformational flexibility of the dicyanamide ion
resulted in supramolecular isomerism in the complexes ranging from 0Dꢀtoꢀ1Dꢀtoꢀ2D27−31 and
2Dꢀtoꢀ3D.30 In the present paper, the asymmetric Schiff base ligand which is prepared by
condensation of the salicylaldehyde with the ꢀNH2 group at one end of 1,3ꢀpropanediamine and
of the oꢀhydroxyacetophenone with its ꢀNH2 group at the other end yielded complex 2 in which
dicyanamide is terminally coordinated like the complexes of H2L2 and complex 3 where
dicyanamide is ꢁ1,5ꢀbridging like the complexes of H2L1. Thus, unlike its symmetrical analogues,
the asymmetrically condensed diꢀSchiff base enables the isolation of both trinuclear monomer
(2) with terminal dicyanamide and its polymer (3) with ꢁ1,5ꢀbridging dicyanamide by changing
the reaction conditions. It is also to be noted that complex 1 of the present paper is an
unprecedented single dicyanamide bridged heterometallic hexanuclear compound. There is only
one similar complex28 but that one is more common double dicyanamide bridged. The single
dicyanamide bridge allows the solvent acetonitrile to coordinate to the Mn(II) to complete its
hexaꢀcoordination , which makes it catalytically much more active than the other two.
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Magnetic Properties
The thermal variations of the molar magnetic susceptibility product with temperature
(χMT) of complexes 1−3 are depicted in Figure 4. The room temperature χMT values are 4.61,
4.60 and 4.27 cm3 K molꢀ1 for complexes 1−3 respectively, which are lower than the value of
magnetically nonꢀinteracting Cu2Mn system of 5.1 cm3 K molꢀ1 for gMn = gCu = 2 at 300 K.43,55−57
In all the complexes, the behavior is very similar; a progressive decrease of χMT value takes
place upon cooling and reaches to a plateau of ca. 2.10, 2.03 and 1.92 cm3 K molꢀ1 for 1−3,
respectively at the temperature range of ca. 8−28 K. On further cooling at very low temperature
(below ca. 5 K), the χMT value further decreases to reach a value of 1.93, 1.61 and 1.74 cm3 K
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molꢀ1 for complexes 1−3, respectively at 2 K. The progressive declining nature of χMT value with
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decrease in temperature indicates the presence of strong intramolecular antiferromagnetic
7
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interactions. The saturated molar magnetizations values (M/Nβ) at 2K are 3.30, 2.80 and 3.17 ꢁB
9
for complexes 1−3 respectively (Figure S7−S9, ESI), which represent the nonꢀzero spin ground
states with ST = |SMn − 2SCu| = 3/2. This behavior also suggests the presence of dominant
antiferromagnetic coupling between Cu(II) and Mn(II) in the complexes. In the trinuclear CuIIꢀ
MnIIꢀCuII structure of the complexes 1−3, two terminal Cu atoms are connected to the central Mn
atoms through either very similar (in 1) or identical (in 2 and 3) double phenoxido bridge and the
two terminal Cu(II) ions are not connected via any bridge. Therefore, to calculate the magnetic
interaction parameters, we have assumed a simple symmetrical linear trimeric model, where a
central Mn(II) ion (S = 5/2) and two terminal Cu(II) ions (S = 1/2) interact through the same
exchange coupling (J, Figure 5 left) and used a Hamiltonian H = −J[S1S2 + S1S3], where S1 =
SMn and S2 = S3 = SCu . The exchange coupling between the two terminal CuII ions can be taken
as zero (JCu–Cu = 0 cm−1) because of the large Cu....Cu distances. Simulations were carried out
using the equation (I).55 The best fitting data lead to the following parameters: JMnꢀCu = –25.4±0.1
cm–1, gMnꢀCu = 2.13±0.10, θ = –0.183±0.024 (K), R = 1.16×10−6 for compound 1; JMnꢀCu = –
22.8±0.2 cm–1, gMnꢀCu = 2.10±0.12, θ = –0.491±0.028 (K), R = 1.43×10−6 for compound 2; JMnꢀCu
= –22.0±0.1 cm–1, gMnꢀCu = 2.02±0.08, θ = –0.105±0.010 (K), R = 8.87×10−6 for compound 3.
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⁄
⁄
ꢗꢔ ꢃꢕꢖ
⁄
ꢃ
ꢃ
ꢓꢔ ꢃꢕꢖ
ꢙꢔ ꢕꢖꢛ
(I)
ꢁꢂ ꢄ
ꢏꢐꢑ ꢒ
ꢏꢘꢅ ꢒ
χꢀ =
ꢋ × ꢌꢍꢎꢏꢐꢑ ꢒ
⁄
⁄
⁄
ꢓꢔ ꢃꢕꢖ
ꢗꢔ ꢃꢕꢖ
ꢙꢔ ꢕꢖ
ꢇ
ꢅꢆ ꢈꢉꢊ
ꢚꢏꢐ ꢒ
ꢏꢐ ꢒ
ꢏꢅ ꢒ
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Figure 4. Thermal variations of the χMT products for complexes 1−3 (left to right respectively).
Solid red line represents the best fitting to the appropriate model for 1, solid blue line represents
the best fitting to the appropriate model for 2 and solid pink line represents the best fitting to the
appropriate model for 3.
The magnetoꢀstructural correlations of double phenoxidoꢀbridged MnIIꢀCuII reveal that
the sign and magnitude of magnetic exchange coupling (J) depend mostly upon the Cu–O and
Mn–O bond distances, the Cu–O–Mn bond angles and the dihedral angle in the central CuO2Mn
unit. 43,55−57 Till date, eleven double phenoxidoꢀbridged CuIIꢀMnIIꢀCuII trinuclear complexes have
been characterized both structurally and magnetically. Correlating the structural parameters with
the J values of these complexes, it appears that the strength of the antiferromagnetic exchange
increases with increasing Mn–O bond distance (Figure S10, ESI), with decreasing Cu–O bond
distance (Figure S11, ESI) and with increasing Cu–O–Mn bridging angle (Figure S5 right).
However, the figures (Figure S10–S11, ESI) show that although the general trends for the
changes of J values with Mn–O or Cu–O bond distance are discernible, the values are highly
scattered. On the other hand, barring only few points, the correlation of J with Cu–O–Mn
bridging angle is quite obvious and the J values of the present complexes (1–3) having average
Cu–O–Mn bridging angles of 99.16°, 99.11° and 99.61° respectively, fit well in this correlation
diagram (Figure 5 right).
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Figure 5. Basic trinuclear unit of complexes 1−3 (left). Variation of the magnetic exchange coupling
(J) in trinuclear CuIIꢀMnIIꢀCuII double phenoxidoꢀbridged complexes with Cu–O–Mn angle (right).
Crystal Growth & Design
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Electrochemistry
The cyclic voltammograms (CV) of complexes 1−3 in acetonitrile (Figure S12 in ESI) show two
waves on cathodic scan (0 to –1.8 V) for the quasiꢀreversible reduction process at the electrode
surface. The peaks are observed at Ec = –0.88 and –0.99 V for 1; Ec = –0.93 and –1.07 V for 2;
Ec = –0.89 and –1.02 V for 3 on forward cathodic scan with respect to an Ag/AgCl electrode at a
scan rate of 50 mV/Sec. Whereas, on reverse scan the corresponding peaks are observed at Ea = –
0.67, –0.74 and –0.73 V for complexes 1−3 respectively. The peak separation values (∆Epc =
58
Ecathodic – Eanodic
)
are –0.32, –0.33 and –0.29 V for complexes 1−3 respectively. These two
peaks on forward scan indicate that presence of two types of (CuL) species in solution. The
peaks at around –1.0 V are for the reduction of Cu(II) to Cu(I) of isolated metallolihand (CuL)43
and another peaks at slightly lower potential (–0.9 V) are for the (CuL) in trinuclear (CuL)2Mn
unit. Both the species (CuL) and (CuL)2Mn are assignable in ESI mass spectra of the complexes.
The potential differences (
and –0.13 V for complexes 1−3 respectively. The similar peak separation values (
potential difference values ( E12) for all the complexes 1−3 indicate that the presence of similar
type of species in solution phase.
∆
E12 =E1 –E2)59 between two peaks on forward scan are –0.11, –0.14
∆
Epc) and
∆
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Phenoxazinone synthase studies and kinetics
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Detailed Description
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All three complexes, 1−3 exibit phenoxazinone synthaseꢀlike activity. The detailed kinetic
studies of this catalytic oxidase activity were performed as reported previously.43 The oxidative
dimerisation reaction of oꢀaminophenol to phenoxazinone is shown in Scheme 2. We have
examined the phenoxazinone synthaseꢀlike activity of the complexes in acetonitrile solvent under
aerobic condition.
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N
NH2
NH2
Catalyst, MeCN
3/2O2
O
O
OH
3H2O
oꢀaminophenol
Phenoxazinone
Scheme 2. Catalytic oxidation of oꢀaminophenol (OAP) to phenoxazinone (APX) in acetonitrile.
For the detailed kinetic study of this reaction, we used 5 × 10−5 M acetonitrile solution
(considering hexanuclear unit) of complex 1 and 1 × 10−4 M acetonitrile solution (considering
trinuclear unit) of both complex 2 and 3. The 1 × 10−2 M acetonitrile solution of oꢀaminophenol
is used as substrate solution. During wavelength scan the change in spectral behavior of complex
1 in the presence of OAP is shown in Figure 5 (left) and 2−3 are shown in Figures S13–S14
(left). The observed rate vs. [substrate] and the Lineweaver–Burk plots for complex 1 is depicted
in Figure 6 (right) and 2−3 are shown in Figures S13–S14 (rightt). The turnover numbers (kcat in
h−1) of this catalysis are 4966, 2021 and 1107 for complexes 1−3 respectively.
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Figure 6. Increase in the APX band at around 424 nm after the addition of OAP (1 × 10−2 M) to
the acetonitrile solution of complex 1 (5 × 10−5 M) (left) and plot of the rate vs. substrate
concentration (right). Inset shows the corresponding Lineweaver–Burk plot of 1. The UVꢀspectra
were recorded at 5 minute intervals.
ESIꢀMass spectrometric study
To determine the mechanistic insight of phenoxazinone synthase activity and complexꢀsubstrate
intermediate during the catalytic oxidase reaction of oꢀaminophenol, we have recorded ESIꢀMS
spectra of all three complexes and of a 1:1 mixture (v/v) of the complex and oꢀaminophenol
within 5 min of mixing in acetonitrile (Figures S15−S17, ESI). In the spectra of all the
complexes 1−3 a base peak is observed at m/z = 380.06 (calcd. 380.05) attributable to the
sodiated metalloligand species [(CuL)+Na]+. Along with this, another two peaks are observed at
m/z = 358.08 (calcd. 358.07) for protonated metalloligand [(CuL)+H]+ and at m/z = 737.13
(calcd. 737.12) for the sodiated dinuclear species [(CuL)2Na]+ (Figure S15, ESI). Spectra of the
mixture of complexes 1−3 and oꢀaminophenol also show three common peaks at m/z = 380.06
(calcd. 380.05), m/z = 358.07 (calcd. 358.07) and m/z = 737.13 (calcd. 737.12) (Figure S16,
ESI). All of these species have been found in the mass spectra of complexes 1−3, but here the
base peak is the protonated species that appears at m/z = 358.08 (calcd. 358.07). Along with
these peaks another two low intensity peaks are observed at m/z = 1026.32 (calcd. 1026.14) and
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m/z = 1052.33 (calcd. 1052.11) (Figure S17, ESI), assignable to the trinuclear species
[(Cu(II)L)2Mn(OAP)2+K]+ and [(Cu(I)L)2Mn(OAP)2+3Na]+ respectively. In addition, another
two peaks are observed at m/z = 1042.51 (calcd. 1042.16) and m/z = 1096.20 (calcd. 1096.13),
which indicate the presence of two dioxygen coordinated trinuclear species
[(Cu(I)L)2Mn(O2)(OAP)2+Na]+ and [(Cu(II)L)2Mn(O2)(CH3CN)(OAP)2+K]+ respectively. In
summary, in spectra of complex substrate mixture, the appearance of the peaks due to
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[(CuL)2Mn(OAP)2+K]+, [(CuL)2Mn(OAP)2+3Na]+,
[(Cu(I)L)2Mn(O2)(OAP)2+Na]+ and
[(CuL)2Mn(O2)(CH3CN)(OAP)2+K]+ is noteworthy as there can be identified as the probable
intermediate species formed during the oxidative conversion of
oꢀaminophenol to
phenoxazinone by dioxygen in presence of the complexes 1−3 as catalysts.
Mechanistic insight
On the basis of ESIꢀMass spectra, we propose a possible mechanism for the catalytic conversion.
At first, on addition of oꢀaminophenol into the solution of each of the complexes (1−3), a neutral
trinuclear species [(CuL)2Mn(OAP)2] in which two deprotonated hydroxyl groups of two oꢀ
aminophenols are coordinated to Mn(II) centre, is formed (Figure S17, ESI) (Scheme 3). In the
next step, Cu(II) is reduced to Cu(I) with concomitant formation of the oꢀiminophenolate/oꢀ
aminophenolate radical. The existence and relative stability of these intermediate species can be
assigned by mass spectra (Figure S16, ESI). The areal O2 then oxidizes Cu(I) centers to Cu(II)
and the coordinated organic radicals to phenoxazinone. The trinuclear species which is released
react again with aminophenol to enter into the next cycle. During this catalytic conversion
process 3/2O2 molecules are reduced to 3H2O. The probable mechanism of this catalytic
conversion is shown in Scheme 3.
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N
N
Cu
O
6
7
N(CN)2
O
Mn
NCCH3
O
8
9
N
N
HO
O
Cu
Cu
O
2
N
+
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N
O
O
O
O
H2N
NH2
Mn
O
(CH3CN + N(CN)2)
Cu
N
ꢀ2 H+
H2N
N
Cu(II)
Cu(I)
m/z(+K+) = 1026.32
N
ꢀ2 H+
N
Cu
O
O
Mn
O
O
N
Cu
N
N
Cu
(I)
N
O
O
O
O
O
O
NH
.
Mn
NH2
O
N
O
.
(I)
Cu
2H2O
N
HN
N
O2
N
m/z(+3Na+) = 1052.45
N
Cu
O2
Mn
O
O
O
O
O
O
.
.
NH
H2O
1/2 O2
Cu
N
HN
N
Cu(I)
Cu(II)
H3CCN
+ CH3CN
+ O2
m/z(+K+) = 1096.20
Scheme 3. Proposed mechanism for catalytic oxidation of OAP by complex 1.
StructureꢀActivity correlation for the observed relative performance of the catalysts
Recently, there has been growing interest in homoꢀ and heterogeneous catalysis, based on
heterometallic complexes with various combinations of metal ions.60−63 However, till date only
very few heterometallic catalyst are reported which show homogeneous catalytic oxidase type
activities. Recently, we reported some heterometallic Cu(II)ꢀMn(II) and Ni(II)ꢀMn(II) complexes
derived from salen type tetradentate Schiff base ligands, which showed almost nil to very high
catalytic oxidase activities.43,64 It has been proposed that catalytic efficiency as well as catalytic
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promiscuity of the complexes depend very much on the coordinated solvent molecule and
anionic coligands to the Mn(II) centre. Here, we present three heterometallic Cu(II)ꢀMn(II)
complexes using the same ligand and anionic coligand, yet we found that the catalytic efficiency
on phenoxazinone synthase like activity of the complexes (1−3) is distinctly different. Complex
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1 in which one CH3CN solvent is coordinated to the Mn(II) center of trinuclear CuII MnII unit
2
shows highest catalytic activity (kcat = 4966 h−1 ), the lowest activity (kcat =1107 h−1) is recorded
with the polymeric complex 3 whereas the kcat (2021 h−1) value of discrete trinuclear unit 2 where
two dicyanamide ions are coordinated the Mn(II) is in between. The difference can be explained
considering the fact that the aminophenol substrate can coordinate easily to the Mn(II) centre in
1 by replacing the labile solvent acetonitrile molecule. Replacement of the coordinated
dicyanamide ligand is more difficult than acetonitrile; hence the kcat value of 2 is lower than that
of 1. On the other hand, the catalytically active trinuclear unit is produced on dissociation of the
polymeric complex 3 making it least active. As is proposed in the mechanism the redox active
Cu centers do the job of electron transfer while the Mn(II) center is coordinated by the substrate
but remains redox innocent during the course of the catalytic reaction. Thus the reaction rate is
controlled by the ease of coordination of the substrate to the Mn(II) centre of the trinuclear unit.
It is also to be noted that the kcat value of 1 is more than any of the reported homometallic model
complexes reported so far for phenoxazinone synthaseꢀlike activity. There is only one reported
complex (CuII −MnII) showing a higher kcat value than 1 and that species is also heterometallic.
2
Therefore, one may presume that due to the cooperative effect the heterometallic model
complexes can be more efficient for such oxydase reactions than homometallic complexes.
Conclusions
To investigate subtle structural differences on magnetic properties and catalytic activities of
heterometallic complex, we have synthesized three CuII ꢀMnII complexes by changing the
2
reaction conditions and proportion of dicyanamide. The metalloligand, CuL, derived from an
−
asymmetrically dicondensed Schiff base on reaction with Mn(II) perchlorate and N(CN)2 in
2:1:1 ratio resulted in a ꢁ1,5 dicyanamide bridged hexanuclear complex, [{(CuL)2Mn}2(ꢁ1,5ꢀ
−
N(CN)2)(CH3CN)2](ClO4)3 (1). Higher proportion of N(CN)2 in the same reaction mixture
replaces all the perchlorate ions to produce two isomeric complexes 2 and 3 having molecular
unit [{(CuL)2Mn}2(N(CN)2)]. Complex 2, the monomeric unit of polymer 3 has been obtained at
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ambient temperature and is convertible to 3 under reflux. The unsymmetric Schiff base seems
enable the isolation of these two isomeric forms. The magnetic properties of the solid complexes
1−3 show antiferromagnetic coupling as are expected from phenoxido bridging angles. All three
complexes show quite high phenoxazinone synthaseꢀlike activity but their catalytic efficiency is
very different. The highest catalytic activity of complex 1 can be attributed to the presence of
coordinating acetonitrile solvent to the Mn(II) centre. Interestingly, the catalytic activity of
monomeric unit 2 is considerably higher than that of polymer 3 indicating that the two
complexes are distinguishable even in solutions. ESIꢀMass spectral analyses suggest the
formation of substrate coordinated [(CuL)2Mn] cluster as the radical intermediate, and reduction
of Cu(II) to Cu(I) in the catalytic process for all three complexes. The turnover number of
complex 1 is higher than any of the model homometallic complexes and lowers than only one
model complex which is also heterometallic. The observation vindicates the efficacy of
heterometallic cooperative effect in the catalytic process i.e. for the present system, Mn(II) center
binds the oꢀaminophenol while Cu(II) performs the redox activity.
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Supporting Information (see footnote on the first page of this article): IR, Mass spectra and
table of bond parameters of complexes 1−3 as well as Mass spectra of complexes 1 with OAP;
Catalytic activity parameter and Kcat values of complexes 1−3. CCDC–1567136 (for 1), –
1567137 (for 2), and –1567138 (for 3) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
A.G. thank the Department of Science and Technology (DST),New Delhi, India, for financial
support and providing cyclic voltammetry facility (SR/S1/IC/0034/2012) for us as well as for the
We also thank CRNN, University of Calcutta, Kolkata, India for magnetic measurement. P. M. is
thankful to the Council of Scientific and Industrial Research (CSIR), Government of India for
the Senior Research Fellowship [Sanction no. 09/028(0935)/2014ꢀEMRꢀI]. We thank the EPSRC
(U.K.) and the University of Reading for funds for the diffractometer.
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