Solvomorphism and catecholase activities of bis(μ-phenoxido) dicopper(II) complexes

Article abstract of DOI:10.1016/j.poly.2013.05.039

Two phenoxido bridged dinuclear Cu(II) complexes, [Cu₂(L ¹)₂(NCO)₂] (1), and [Cu₂(L ²)₂(NCO)₂]·2CH₃OH (2), have been synthesized using the tridentate reduced Schiff-base ligands 2-[(2-dimethylamino-ethylamino)-methyl]-phenol (HL¹) and 2-[(2-diethylamino-ethylamino)-methyl]-phenol (HL²) respectively. Re-crystallization of compound 2 from dichloromethane produced desolvated complex [Cu₂(L²)₂(NCO)₂] (3). The compounds 2 and 3 are pseudopolymorphs or solvomorphs which differ in the crystal system (orthorhombic, Pbca for 2 and monoclinic, P2₁/n for 3). The complexes have been characterized by X-ray structural analyses and spectroscopic methods. In all three complexes Cu(II) is penta-coordinated having the geometry intermediate between distorted square pyramid and trigonal bipyramid with Addison parameter (τ) = 0.33, 0.48 and 0.53 for 1, 2 and 3 respectively. Using 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate, the catecholase activity of complexes 1 and 2 has been studied in methanol solution; compound 2 shows slightly higher catecholase activity (k_cat = 98.4h ¹) than compound 1 (k_cat = 64.2 h ¹).

Full text of DOI:10.1016/j.poly.2013.05.039

Accepted Manuscript
Solvomorphism and catecholase activities of bis(μ-phenoxido)dicopper(II)
complexes
Apurba Biswas, Lakshmi Kanta Das, Ashutosh Ghosh
PII:
S0277-5387(13)00424-5
DOI:
http://dx.doi.org/10.1016/j.poly.2013.05.039
POLY 10158
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
18 March 2013
22 May 2013
Please cite this article as: A. Biswas, L.K. Das, A. Ghosh, Solvomorphism and catecholase activities of bis(μ-
phenoxido)dicopper(II) complexes, Polyhedron (2013), doi: http://dx.doi.org/10.1016/j.poly.2013.05.039
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Solvomorphism and catecholase activities of bis(µ-phenoxido)dicopper(II)
complexes
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Apurba Biswas, Lakshmi Kanta Das, Ashutosh Ghosh*
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Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C.
Road, Kolkata-700 009, India; e-mail: ghosh_59@yahoo.com
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Abstract
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Two phenoxido bridged dinuclear Cu(II) complexes, [Cu2(L1)2(NCO)2] (1), and
[Cu2(L2)2(NCO)2]·2CH3OH (2) have been synthesized using the tridentate reduced Schiff-base
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ligands 2-[(2-dimethylamino-ethylamino)-methyl]-phenol (HL1) and
2-[(2-diethylamino-
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ethylamino)-methyl]-phenol (HL2) respectively. Re-crystallization of compound 2 from
dichloromethane produced desolvated complex [Cu2(L2)2(NCO)2] (3). The compounds 2 and 3
are pseudopolymorphs or solvomorphs which differ in the inclusion of methanol molecules and
in the crystal system (orthorhombic, Pbca for 2 and monoclinic, P21/n for 3). The complexes
have been characterized by X-ray structural analyses and spectroscopic methods. In all three
complexes Cu(II) is penta-coordinated having the geometry intermediate between distorted
square pyramid and trigonal bipyramid with Addison parameter (τ) = 0.33, 0.48 and 0.53 for 1, 2
and 3 respectively. Using 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate, the catecholase
activity of complexes 1 and 2 has been studied in methanol solution; compound 2 shows slightly
higher catecholase activity (kcat = 98.4 h-1) than compound 1 (kcat = 64.2 h-1).
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Keywords: Copper(II); Reduced Schiff base; Crystal structure; Solvomorphs, Catecholase
activity.
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1. Introduction
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The oxidation of organic substrates with molecular oxygen under mild conditions is of
great interest for industrial and synthetic processes both from an economical and environmental
point of view [1]. The synthesis and investigation of functional model complexes for
metalloenzymes with oxidase or oxygenase activity is therefore of great promise for the
development of new and efficient catalysts for oxidation reactions. Catechol oxidase is a type III
active site protein containing copper which catalyzes the oxidation of a wide range of o-
diphenols (catechols) to the corresponding o-quinones in a process known as catecholase activity
[2]. The ability of dicopper complexes to oxidize phenols and catechols is well known. The
extensive studies performed with copper-based model complexes have shown that various
structural factors may affect their catalytic activity, namely the metal-metal distance, type of
exogenous bridging ligand, coordination geometry around the metal ion, and flexibility of the
ligand [3-12]. In the dinuclear complexes, a copper-copper distance of 2.9-3.2 Å has been
suggested as providing maximum catecholase activity owing to the requirement of a steric match
between the substrate and the catalyst [13]. Various type of ligands are used to obtain dinuclear
Cu(II) complexes. Among them, the tridentate N,N,O donor Schiff bases, derived from diamines
and salicylaldehyde derivatives have been used extensively [14-17]. The reduced Schiff-base
ligands have been gaining popularity over the last few years, as a result of their higher stability
and increased flexibility (imparted by –CH2–NH– moieties) compared to that of the
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corresponding Schiff bases which include rigid azomethine (–CH=N–) fragments [7,18-26].
These tridentate N,N,O donors reduced Schiff bases are also found to generate dinuclear
copper(II) complexes similar to the corresponding unreduced Schiff bases but with diferent
amount of distortion around the Cu(II) centre. Such differences play an important role in
catecholase activity of the complexes and therefore we would like to synthesise new dinuclear
Cu(II) complexes using reduced Schiff base ligands.
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During the course of this study, we are able to grow pseudopolymorphs.
Pseudopolymorphism or solvomorphism is the phenomenon wherein a compound is obtained in
crystalline forms that differ in the nature or stoichiometry of included solvent molecules [27-29]
and are particularly important in the pharmaceutical industry where control over properties such
as solubility, bioavailability and stability of the active drug is desirable [30-33]. It has been
found from the literature that the introduction of solvent molecules in the molecular crystals give
rise to a solvated crystals (pseudopolymorphic forms), which often have different structures
compared to the structures of host crystals (devoid of solvent molecules), provides a way of
perturbing the intermolecular interactions and hence the reactivity of molecules in the solid state
[32].
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In this paper we report the syntheses, crystal structures, and catecholase activities of two
phenoxido
bridged
dinuclear
Cu(II)
complexes,
[Cu2(L1)2(NCO)2]
(1),
[Cu2(L2)2(NCO)2]·2CH3OH (2), obtained from the reduced Schiff base ligands 2-[(2-
dimethylamino-ethylamino)-methyl]-phenol (HL1) and 2-[(2-diethylamino-ethylamino)-methyl]-
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phenol (HL2) respectively. The re-crystalisation of compound 2 from dichloromethane produced
the complex [Cu2(L2)2(NCO)2] (3). The compounds 2 and 3 are solvomorphs.
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2. Experimental
The reagents and solvents used were of commercially available reagent quality.
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2.1. Synthesis of the reduced Schiff base ligands ((HL1) 2-[(2-dimethylamino-ethylamino)-
methyl]-phenol and (HL2) 2-[(2-diethylamino-ethylamino)-methyl]-phenol
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The Schiff base ligand was synthesized by refluxing a solution of salicylaldehyde (0.52
mL, 5 mmol) and N,N-dimethylethylenediamine (0.54 mL, 5 mmol) in methanol (30 mL) for one
hour [34]. The solution was cooled to 0ºC and solid sodium borohydride (210 mg, 6 mmol) was
added slowly to this methanolic solution with stirring. After completion of the addition, the
resulting solution was acidified with concentrated HCl (5 ml) and then evaporated to dryness.
The reduced Schiff base ligand HL1 was extracted from the solid mass with methanol and this
methanol solution (ca. 20 mL) was used for preparation of complexes. HL2 was synthesized in
the same way as HL1 using N,N-diethylethylenediamine (0.70 mL, 5 mmol) instead of N,N-
dimethylethylenediamine [12].
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2.2 Synthesis of the complexes [Cu2(L1)2(NCO)2] (1), [Cu2(L2)2(NCO)2]·2CH3OH (2), and
[Cu2(L2)2(NCO)2] (3)
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An extracted methanol solution of HL1 as prepared above was added to a solution of
CuCl2·2H2O (0.850 g, 5.00 mmol) in methanol (20 mL) and an aqueous solution (1 mL) of
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sodium cyanate (0.325 g, 5.00 mmol) was added to this mixture with stirring. The mixture was
stirred for 1 h and filtered. The filtrate was kept undisturbed at room temperature. Green crystals
of 1 suitable for X-ray diffraction were obtained after 3 days on slow evaporation of the solvent.
The green crystals of complex 2 was obtained in the same way by using HL2 instead of HL1. The
re-cystalisation of compound 2 (1 mmol) from dichloromethane produced compound 3.
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Complex 1: (Yield: 1.090 g; 73%) Anal. Calc. for C26H38Cu2N6O4: C, 49.91; H, 6.12; N, 13.43
%. Found: C, 49.65; H, 6.05; N, 13.09 %. λmax (nm), [ε
(dm3 mol-1 cm-1)] (metanol),
max
650(317), 415(1523); IR: ν(N–H), 3220 cm-1, ν(C–N),1596 cm-1 ,
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Complex 2: (Yield: 1.418 g, 79 %), Anal. calc. for C30H50Cu2N6O6: C, 50.19; H, 7.02; N, 11.71
%. found: C, 49.87; H, 6.75; N, 11.47 %. IR (KBr): ν(N–H), 3217 cm-1, ν(C–N), 1596 cm-1;
λmax (nm), [εmax (dm3 mol-1 cm-1)] (methanol), 622 (618), 425 (1588).
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Complex 3: (Yield: 0.523 g, 80 %), Anal. calc. for C28H42Cu2N6O4: C, 51.44; H, 6.48; N, 12.85
%. found: C, 51.12; H, 6.23; N, 12.62 %. IR (KBr): ν(N–H), 3201 cm-1, ν(C–N), 1595 cm-1;
λmax (nm), [εmax (dm3 mol-1 cm-1)] (methanol), 622 (618), 425 (1588).
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2.3. Catalytic oxidation of 3,5-DTBC
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In order to study the catecholase activity of the complexes, 10-4 (M) solutions of 1 and 2
in methanol were treated with 100 equivalents of 3,5-di-tert-butylcatechol (3,5-DTBC) in
methanol under aerobic condition at room temperature. Absorbance vs. wavelength (wavelength
scan) of these solutions was recorded at a regular time interval of 10 min in the wavelength range
300-600 nm. To determine the dependence of rate on substrate concentration and various kinetic
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parameters, a 10-4 (M) solution of complexes was treated with 10, 30, 50, 70, and 100 equivalents
of substrate. The reactions were followed spectrophotometrically by monitoring the increase in
the absorbance at 390 nm (quinone band maxima) as a function of time (time scan).
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2.4. Physical Measurements
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Elemental analyses (carbon, hydrogen and nitrogen) were performed using a Perkin-
Elmer 240C elemental analyzer. IR spectra in KBr (4500-500 cm-1) were recorded using a
Perkin-Elmer RXI FT-IR spectrophotometer. Electronic spectra in methanol (1200-350 nm) were
recorded in a Hitachi U-3501 spectrophotometer. Thermal analysis (TG-DTA) were carried out
on a Mettler Toledo TGA/SDTA 851 thermal analyzer in a dynamic atmosphere of dinitrogen
(flow rate = 30 cm3 min-1). The samples were heated in an alumina crucible at a rate of 5 °C min-
1. Electrochemical studies were done using PAR 273 potentiostat. The measurements were
performed at 300 K in acetonitrile solutions containing 0.2 M TEAP and 10-3 M complex
deoxygenated by bubbling with nitrogen. The working, counter, and reference electrodes used
were a platinum wire, a platinum coil, and a SCE.
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2.5. Crystal Data Collection and Refinement
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Crystal data for the three crystals are given in Table 1. The independent data for 1, 2,
and 3 respectively were collected with MoKα (λ = 0.71073 Å) radiation, using the Bruker
SMART diffractometer. All structures were solved using Patterson method with the SHELXS 97
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
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parameters equivalent to 1.2 times those of the atom to which they were attached. All
calculations were carried out using SHELXS 97 [35], SHELXL 97 [36], PLATON 99 [37],
ORTEP-32 [38] and WinGX system Ver-1.64 [39].
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3. Results and Discussion
3.1. Synthesis of the Complexes
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The condensation of N,N-dimethylethylenediamine and N,N-diethylethylenediamine
with salicylaldehyde afforded the Schiff bases, 2-[(2-dimethylamino-ethylimino)-methyl]-
phenol, 2-[(2-diethylamino-ethylimino)-methyl]-phenol, respectively. Reduction of these Schiff
bases with sodium borohydride readily produced the reduced Schiff bases, HL1, and HL2
(Scheme 1). HL1 on reaction with copper(II) chloride in presence of sodium cyanate in 1:1:1
molar ratios yielded compound 1. Similarly, HL2 on reaction with copper(II) chloride in presence
of sodium cyanate in 1:1:1 molar ratios yielded compound 2. The re-cystalisation of compound 2
from dichloromethane produced compound 3. Compounds 2 and 3 are well crystalline products
and their PXRD patterns match nicely with the simulated ones indicating that the products are
not mixtures of the two compounds (Figures S1 and S2). On the other hand, thermogravimetric
analysis of compound 2 shows that on heating it loses two molecule of methanol (obs. 8.74 %,
calc. 8.85 %) in the temperature range 47-84 ºC (Figure S3); the PXRD patterns (Figure S4) of
the desolvated product is not identical to that of 3 indicating a different crystalline form is
produced upon desolvation. Compound 1 does not contain any solvent molecule and on
recrystallization from acetonitrile or dichloromethane no compound other than 1 results.
7

CH3
N
CH2 NH
O
CH3
NCO
R
R
Cu
Cu
R
R
CH2 NH
OH
N
O
HC N
OH
N
OCN
NaBH4
H3C
HL (R= CH3: HL1
R= CH3CH2: HL2)
NH CH2
N
H3C
CuCl2.2H2O
NaOCN
HL:Cu2+:OCN- = 1:1:1
1)
2)
(
1
)
C24H34Cu2N6O4
N
CH2 NH
N
CH2 NH
O
NCO
Cu
Cu
NCO
Cu
O
O
Re-crystalisation
from
O
OCN
2CH3OH
OCN
Cu
dichloromethane
NH CH2
N
NH CH2
N
3
2
C28H42Cu2N6O4
[C28H42Cu2N6O4].2CH3OH
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Scheme 1. Formation of the complexes
3.2. IR and electronic spectra
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The IR spectra of the complexes 1, 2, and 3 exhibit a moderately strong, sharp peak at
respectively 3220, 3217, and 3201 cm-1, which is ascribed to the N–H stretching vibration,
therefore confirming that the imine group of the Schiff base has been reduced. The complexes 1,
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2, and 3 exhibit ν-CN bands at 2213, 2206, and 2175 cm-1 respectively. The stretching vibrations
are in good agreement with the literature values which show that νas appears at higher wave
number as the MNC angle approaches 180° [40-44].
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Electronic spectra were recorded in methanol solution, which show a single absorption
band at 650, 622, and 622 nm due to the d-d transition for compounds 1, 2 and 3 respectively. At
higher energy region, the ligand to metal charge transfer bands were located at 415 nm for
compound 1 and 425 nm the compounds 2 and 3.
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3.3. Description of structures of complexes 1, 2 and 3
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The structure of [Cu2(L1)2(NCO)2] (1) is a centrosymmetric dimer as shown in Figure 1 with
each metal atom in a five-coordinate environment. Dimensions in the metal coordination sphere
are given in Table 2. In the asymmetric unit, there are two independent half molecules A and B
of the centrosymmetric dimers with equivalent geometries.
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Figure 1. The centrosymmetric structure of 1 with ellipsoids at 30% probability.
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The metal atom is bonded to the tridentate ligand L1 via O(11) at 2.215(2), 2.192(2),
N(19) at 2.001(2), 2.000(3) and N(22) at 2.107(3), 2.101(2) Å, together with a bridging
oxygen atom O(11)a, (a = 1-x, 1-y, 2-z) from a second ligand at 1.940(2), 1.940(2) Å and a
terminal OCN ligand via N(1) at 1.985(3), 1.978(3) Å for 1A and 1B respectively. The two Cu
atoms are separated by 3.1949(5), 3.1624(5) Å and the Cu(1)–O(11)–Cu(1)a angle are 100.30(9),
99.55(9)° for 1A and 1B respectively. The geometry around Cu(II) may be described as square
pyramidal; the deviations of the coordinating atoms N(1), N(19), N(22), O(11)a from the least-
square mean plane through them are -0.390(3), 0.428(2), -0.409(3), 0.371(2) Å respectively for
1A and -0.400(3), 0.441(3), -0.423(3), 0.382(2) Å respectively for 1B. The deviation of Cu(II)
from the same plane is 0.195(1), 0.221(1) Å for 1A and 1B respectively in the direction of
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chelating oxygen atom O(11) which may be considered as axially coordinated. However, the
geometry around Cu(II) can also be described as trigonal bipyramid with O(11), N(1), and N(22)
make up the equatorial plane which together with Cu(II) provides a r.m.s. deviations 0.0372,
0.0371 Å for 1A and 1B respectively. The Addison parameter (τ) [45] of the penta-coordinated
Cu(II) is 0.38 (average) indicating that the geometry is a distorted square pyramid.
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The hydrogen H(19B) on amine nitrogen N(19) forms a hydrogen bond to a oxygen atom
O(3B) with dimensions N···O 3.024(4) Å, ∠N–H···O 154°, H···O 2.18 Å to result in a 2-D
polymeric structure (Figure 2). Another polymeric 2D structure of 1 is formed by the interdimer
C–H/π interactions [between H(23D) and Cg of phenyl ring with dimensions H···Cg = 2.79] and
hydrogen bonding interactions between the hydrogen H(19A) and a oxygen atom O(3A) with
dimensions N···O 3.192(4) Å, ∠N–H···O 131°, H···O 2.53 Å (Figure 3).
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Figure 2. Hydrogen bonding polymeric structure of compound 1 (hydrogen atoms except H19B
omitted for clarity).
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Figure 3. Polymeric 2D structure of 1 formed by the interdimer C–H/π and Hydrogen bonding
interactions (hydrogen atoms except H19A and H23D omitted for clarity).
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The structures of [Cu2(L2)2(NCO)2]·2CH3OH (2) and [Cu2(L2)2(NCO)2] (3) are also
centrosymmetric dimers, shown in Figures 4 and 5 respectively together with the atomic
numbering scheme. Dimensions in the metal coordination sphere are given in Table 2.
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Figure 4. The structure of 2 with ellipsoids at 30% probability. Hydrogen bonding interactions
are shown in dotted lines.
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Figure 5. The centrosymmetric structure of 3 with ellipsoids at 30% probability.
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In both cases the metal atoms are five-coordinate being bonded to three donor atoms of
one ligand L2 together with an oxygen atom O(11)a a = (-x, -y, -z) in 2, and (a = (2-x,1-y,2-z) in
3) of a second ligand L2 and a nitrogen atom N(1) of the monodentate –NCO. Both oxygens of
the two ligands L2 bridge the two copper atoms. Bond lengths in the metal coordination sphere
are Cu(1)–O(11) 2.175(4), 2.182(2) Å, Cu(1)–N(19) 1.987(4), 1.984(2) Å and Cu(1)–N(22)
2.126(4), 2.141(2) Å to the tridentate ligand L2 with Cu(1)–O(11)a 1.939(3), 1.949(2) Å and
Cu(1)–N(1) 1.994(5), 2.056(2) Å to the monodentate cyanate group for complexes 2 and 3
respectively. The deviations of the coordinating atoms N(1), N(19), N(22), O(11)a from the
least-square mean plane through them are -0.449(4), 0.503(4), -0.468(4), and 0.414(3) Å
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respectively in 2. -0.481(2), 0.550(2), -0.509(2), and 0.440(2) Å respectively in 3. The deviation
of Cu(II) from the same plane is 0.257(1) Å in 2 and 0.280(1) Å in 3 in the direction of chelating
oxygen atom O(11) which may be considered as axially coordinated. However, the geometry
around Cu(II) can also be described as trigonal bipyramid with O(11), N(1), and N(22) make up
the equatorial plane which together with Cu(II) provides a r.m.s. deviations 0.0424, 0.0406 Å for
2 and 3 respectively. The Addison parameter (τ) of the penta-coordinated CuII is 0.48 in 2 and
0.53 in 3 indicating that the geometry is a distorted trigonal bipyramid with O(11)a and N(19)
occupying axial positions. The major difference between the two complexes is the Cu(1)-N(1)-
C(2) angles of cyanate group, which is small (139.5°) in 2 compare to that in 3 (172.3°)
indicating that the orientation of the OCN- ligand in both the complexes are not alike.
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Compounds 2 and 3 are the solvomorphs as 2 has an additional methanol molecule
which is uncoordinated but is H-bonded into the lattice, linking the complex molecules together.
A helical like hydrogen bonding interactions of H(19) with O5 and H(5) with O3 (Figure 6a)
lead to the formation of a 2-D supramolecular structure (Figure 7) with dimensions N(19)···O(5)
2.900(7) Å, H(19)···O(5) 2.03 Å, ∠N(19)–H(19)–O(5) 160° and O(5)···O(3) 2.755(8) Å,
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H(5)···O(3) 1.94 Å, ∠O(5)–H(5)–O(3) 170° respectively (Table 3). There are also hydrogen
bonding interactions of H(13) with N(1) and H(21A) with O(3) with dimensions C(13)···N(1)
3.258(8) Å, H(13)···N(1) 2.59 Å, ∠C(13)–H(13)–N(1) 129°, and C(21)···O(3) 3.413(8) Å,
H(21A)···O(3) 2.60 Å, ∠C(21)–H(21A)–O(3) 142° (Figure 6b). In complex 3, a 2-D
supramolecular structure (Figure 8) is formed through hydrogen bonding with dimensions
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N(19)···N(1) 3.213(3) Å, H(19)···N(1) 2.34 Å, ∠N(19)–H(19)–N(1) 162°, and C(21)···O(3)
3.363(5) Å, H(21B)···O(3) 2.51 Å, ∠C(21)–H(21B)–O(3) 147° (Table 3).
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Figure 6. (a) Helical like hydrogen bonding interactions (all hydrogen atoms of the dimeric
molecule except H19 are omitted for clarity). (b) Intra and intermolecular hydrogen bonding
interactions of compound 2 (hydrogen atoms except H13 and H21A omitted for clarity).
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Figure 7. Hydrogen bonding polymeric structure of compound 2 (all hydrogen atoms of the
dimeric molecule except H19 are omitted for clarity).
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Figure 8. Hydrogen bonding 2D structure of compound 3 (hydrogen atoms except H19 and
H21B omitted for clarity).
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3.4. Kinetic Studies for Catecholase Activity
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In most studies of the catecholase activity of model complexes, 3,5-di-tert-butylcatechol
(3,5-DTBC) has been chosen as the substrate. The oxidation product 3,5-di-tert-butylquinone
(3,5-DTBQ) is very much stable and exhibits a maximum absorption at 390 nm in pure
methanol. Prior to a detailed kinetic study, it is necessary to check the ability of the dinuclear
complexes to oxidize 3,5-DTBC. For this purpose, 10-4 (M) solutions of complexes 1 and 2 were
treated with 100 equivalents of 3,5-DTBC under aerobic condition in methanol solution.
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Immediately after addition of substrate 3,5-DTBC to the solutions of the catalysts, the absorption
peak at 390 nm (ε = 1900 M-1 cm-1), which is indicative of an oxidation from 3,5-DTBC
corresponding to quinone (3,5-DTBQ), increased in size thus showing considerable catecholase
activity, as shown in Figure 9 (for complex 2), and S5 (for complex 1), when compared to the
inactive Cu(ClO4)2·6H2O. (Figure S6).
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Figure 9. Increase in absorbance after addition of 100 equivalents of 3,5-DTBC to a methanolic
solution containing complex 2 (10-4 M). The spectra were recorded every 10 min.
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The kinetic study of the oxidation of 3,5-DTBC to 3,5-DTBQ by the complexes was
carried out by monitoring the increase in absorbance at 390 nm by the initial rates method. The
rate constant for a particular complex-substrate mixture was determined from the log[A /(A –
α
α
At)] vs. time plot. To determine the dependence of the rates on the substrate concentration and
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255
various kinetic parameters, solutions of the complexes 1, and 2 were studied using increasing
concentrations of 3,5-DTBC (from 10 to 100 equivalents) under aerobic conditions at a complex
concentration of 10-4 (M). A first-order dependence was observed at low concentrations of the
substrate, whereas a saturation kinetics was found at higher concentrations of the substrate for all
the complexes shown in Figure 10 (for complex 2), and Figure S7 (for complex 1). This
dependence on the substrate concentration indicates a catalyst-substrate binding to be an initial
step in the catalytic mechanism. A treatment on the basis of the Michaelis-Menten approach,
originally developed for enzyme kinetics, was therefore applied and linearized by means of a
Lineweaver-Burk plot to calculate the Michaelis-Menten constant (KM) and the maximum initial
rate (Vmax) [46]. The kcat values can be calculated by dividing the Vmax values by the
concentration of the corresponding complexes.
21

256
257
Figure 10. Plot of initial rates vs. substrate concentrations for aerobic oxidation of 3,5-DTBC by
complex 2. The inset shows the Lineweaver-Burk plot.
258
259
260
261
262
263
264
265
266
The data obtained from the Lineweaver-Burk plot model are used for a comparison of catalytic
activity towards the oxidation of 3,5-DTBC as shown in Table 4. The catecholase activity of
compound 2 (kcat = 98.4 h−1 ) is slightly higher than that of compound 1 (kcat = 64.2 h−1 ). The
cyclic voltammograms of complexes 1 and 2 show irreversible cathodic peaks E(red)1 at – 0.65, -
0.60 V and E(red)2 at -1.11 and -1.02 V respectively (Figure S8). This reduction peaks
tentatively assigned to the one electron process CuIICuII→CuIICuI and CuIICuI→CuICuI. Many
research groups have attempted to correlate the redox properties of the copper(II) complexes
with their catecholase activity [5,47-50]. Casella and co-workers [51] succeeded in calculating
the reaction rates for the two successive steps of the catalytic reaction (a fast stoichiometric
22

267
268
269
270
271
272
273
274
reaction between a dicopper(II) complex and a catechol and a slower catalytic reaction), and
showed a clear dependence of the reaction rate in the first stoichiometric step on the CuII/CuI
reduction potential. As this step involves the electron transfer from the bound catecholate to the
dicopper(II) center, this observation is easily understood. The lower CuII/CuI reduction potential
of complex 2 than that of complex 1 favours the oxidation of 3,5-di-tert-butylcatechol (3,5-
DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) but the overall reaction rates obviously depend
on many factors, i.e. the rate of the reoxidation of the dicopper(I) species by dioxygen, the rate of
the catechol oxidation by the formed peroxo-dicopper intermediate etc.
275
4. Conclusions
276
277
278
279
280
281
282
283
284
285
286
The tridentate reduced Schiff base ligands 2-[(2-dimethylamino-ethylamino)-methyl]-phenol
(HL1) and 2-[(2-diethylamino-ethylamino)-methyl]-phenol (HL2) with Cu(II) and cyanate anions
have affoarded two related phenoxido bridged dinuclear Cu(II) complexes. The coordination
environment of the Cu(II) ions in both the complexes are square pyramidal but with different
degrees of distortion toward a trigoanal bipyramid. Re-crystalisation of compound 2 from
dichloromethane produced another related desolvated phenoxido bridged dinuclear Cu(II)
complex 3 having the environment around the copper ions is an intermediate between the
trigonal bipyramid and a square based pyramid. The compounds 2 and 3 are solvomorphs. The
strong and weak hydrogen bonds between solvent and solute molecules facilitates or retention of
organic solvents in crystals of 2. The slightly higher catecholase activity of compound 2 than 1
can be explained with the help of their respective reduction potensial.
23

287
Acknowledgements
288
289
290
291
292
We thank CSIR, Government of India [Senior Research Fellowship to A. Biswas, Sanction No.
09/028 (0717)/2008-EMR-I and L.K. Das, Sanction No. 09/028(0805)/2010-EMR-I] and DST
FIST for financial support for the X-ray diffraction facility. We are also thankful to Prof.
Shyamal Kumar Chattopadhyay, Bengal Engineering and Science University for cyclic
voltammetry experiments.
293
Appendix A. Supplementary material
294
295
296
297
CCDC 926840, 926841 & 926842 contain the supplementary crystallographic data for 1,
2 and 3, respectively. These data can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit @ccdc.cam.ac.uk.
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