Synthesis, crystal structure, spectral and magnetic studies and catecholase activity of copper(II) complexes with di- and tri-podal ligands

Article abstract of DOI:10.1016/j.ica.2009.09.041

Five complexes of copper(II) acetate with Schiff base ligands based on salicylaldehyde and N,N-dimethylamino)ethyl/propyl amine and their reduced products, have been synthesized and characterized by various spectroscopic methods. The solid state structures of 1, 2 and 3 have been determined using single crystal X-ray diffraction method. The structures of the other two compounds have been proposed on the basis of spectroscopic and physical methods. The compounds 1, 3 and 4 are dinuclear complexes of the tridentate ligands, where the two Cu(II) centers have square pyramidal geometry with bridging acetate or phenoxo groups. Each arm of the tripodand ligand forms a mononuclear, magnetically dilute complex 5 having five coordinated Cu(II) ions. Complex 2 is mononuclear with a square pyramidal stereochemistry. The catalytic performance of the oxidation of 3,5-di-tert-butylcatechol to quinone was studied using UV-Vis absorption spectral methods. Complex 4 exhibits the highest activity with a turnover number of 41 h^-1 while other showed lower rates of oxidation. A kinetic treatment on the basis of Michaelis-Menten model was applied. Ease of removal of the exogenous acetate ligands and easy access to the Cu(II) ions have been seen to affect the activity in the complexes. At the same time presence of two endogenous phenoxo bridges in the dinuclear complexes reduces the activity.

Full text of DOI:10.1016/j.ica.2009.09.041

Inorganica Chimica Acta 363 (2010) 97–106
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, crystal structure, spectral and magnetic studies and catecholase
activity of copper(II) complexes with di- and tri-podal ligands
a,
Vimal K. Bhardwaj a, Núria Aliaga-Alcalde b, Montserrat Corbella b, , Geeta Hundal
*
*
a Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India
b Departament de Química Inorgànica, Universitat de Barcelona (UB), Diagonal, 647, 08028 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 August 2009
Received in revised form 19 September 2009
Accepted 22 September 2009
Available online 3 October 2009
Five complexes of copper(II) acetate with Schiff base ligands based on salicylaldehyde and N,N-dimeth-
ylamino)ethyl/propyl amine and their reduced products, have been synthesized and characterized by var-
ious spectroscopic methods. The solid state structures of 1, 2 and 3 have been determined using single
crystal X-ray diffraction method. The structures of the other two compounds have been proposed on
the basis of spectroscopic and physical methods. The compounds 1, 3 and 4 are dinuclear complexes
of the tridentate ligands, where the two Cu(II) centers have square pyramidal geometry with bridging
acetate or phenoxo groups. Each arm of the tripodand ligand forms a mononuclear, magnetically dilute
complex 5 having five coordinated Cu(II) ions. Complex 2 is mononuclear with a square pyramidal ste-
reochemistry. The catalytic performance of the oxidation of 3,5-di-tert-butylcatechol to quinone was
studied using UV–Vis absorption spectral methods. Complex 4 exhibits the highest activity with a turn-
over number of 41 hꢀ1 while other showed lower rates of oxidation. A kinetic treatment on the basis of
Michaelis–Menten model was applied. Ease of removal of the exogenous acetate ligands and easy access
to the Cu(II) ions have been seen to affect the activity in the complexes. At the same time presence of two
endogenous phenoxo bridges in the dinuclear complexes reduces the activity.
Keywords:
Catecholase oxidase model
3,5-Di-tert-butylcatechol
Kinetics
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
stoichiometric oxidation of catechol substrates by the peroxo-
and oxo-dicopper complexes. Out of these, the second approach
Catechol oxidase is a copper containing, type III active site pro-
tein which shows catecholase activity i.e. oxidation of a broad
range of catechols to quinones through the four-electron reduction
of molecular oxygen to water [1,2]. According to the crystal struc-
ture, the active site of the enzyme in its native state has a strongly
antiferromagnetically coupled, dinuclear copper(II) core where
each Cu(II) is coordinated by three histidine nitrogens and a bridg-
ing OHꢀ ion [3]. In nature this metalloprotein acts as a very effi-
cient catalyst to catalyze a kinetically unfavored reaction of
triplet O2 with organic substrates. Thus studies on the model com-
pounds mimicking the catecholase activity, are very useful and
promising for the development of new, more efficient bioinspired,
environment friendly catalysts, for in vitro oxidation reactions. The
catecholase activity of various mononuclear and binuclear com-
plexes of copper(II) have been investigated in the past twenty five
years [4–41]. According to Reedijk and coworkers [42] four
approaches have been used in the mechanistic studies on these
model compounds: namely substrate binding studies, structure-
activity relationship, kinetic studies on catalytic reactions and
has been more frequently employed by various groups. Under this
approach the relationships between metal–metal distance; elec-
trochemical properties; exogenous bridging ligand; ligand struc-
ture and pH, with the catecholase activity have been exploited.
Non-planar mononuclear as well as binuclear Cu(II) complexes
with CuꢁꢁꢁCu distance in the range 2.9–3.2 Å have been found to
show good catecholase activity. The binuclear complexes have
generally been found to be more reactive than the former, pointing
towards the possibility of a dicopper-catecholate adduct as an
intermediate in the process.
From the reports in literature on various copper(II) model
compounds and also from the known crystal structure of catechol
oxidase, it is clear that the functional models must have free
coordination sites on the metal atom where the substrate can
bind. Therefore the ligands which would have lesser number of
donor atoms are more promising. In this work we report on syn-
thesis of copper(II) complexes which may act as functional mod-
els for the active site of catecholate oxidase. The ligands HL1–
H3L5 (Scheme 1) contain a phenolic group in a correct chelating
position with an imine and an amine nitrogen or with two amine
nitrogens which can offer a N2O2 donor set and a central Cu2O2
core, in the binuclear complexes. The designs of their copper
* Corresponding authors.
E-mail addresses: monte.corbella@qi.ub.es (M. Corbella), geetahundal@yahoo.
com (G. Hundal).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.09.041

98
V.K. Bhardwaj et al. / Inorganica Chimica Acta 363 (2010) 97–106
temperature magnetic measurements were carried on a Johnson–
Matthey magnetic susceptibility balance with Hg[Co(SCN)4] as
standard. Variable temperature magnetic measurements were car-
ried out, at the Unitat de Mesures Magnètiques (Universitat de Bar-
celona), on polycrystalline samples (30 mg) with a Quantum
Design SQUID MPMS-XL magnetometer working in the 2–300 K
range. The magnetic fields were 0.5 and 1.0 T. The diamagnetic cor-
rections were evaluated from Pascal’s constants. R is the agreement
exptl
factor defined as
R
[(vM
)
– (vM) R[(vM)
calcd]2/ exptl]2. 1H and 13C
NMR of ligands HL1 to H3L5 were performed on a JEOL 300 MHz
instrument with TMS as reference. ESI Mass spectra were taken
on a Bruker Esquire3000_00037 instrument in methanol. Solution
electric conductivity measurements were carried out with a Hioki
3532-50 LCR Hi Tester conductivity bridge with a solute concentra-
tion of 1 mM.
2.1. X-ray crystallography
Data for single crystal X-ray structure analyses of complexes 1,
2 and 3 were collected at 295(2) K on a Siemens P4 diffractometer.
The h ꢀ 2h technique was used to measure the intensities, up to a
maximum of 2h = 50° with graphite monochromatised Mo Ka radi-
ator (k = 0.71073 Å). Cell parameters were refined in the h range,
10–12.5°, the data were corrected for Lorentz and polarization fac-
tors and an empirical
w scan absorption correction was applied
[48], using XSCANS. The structures were solved by the direct meth-
ods using SIR97 [49] and refined by full matrix least squares meth-
ods based on F2 using SHELX97 [50]. All non-hydrogen atoms were
refined anisotropically and hydrogen atoms were assigned calcu-
lated positions. All calculations were performed using WINGX [51].
The relevant crystallographic and refinement parameters are given
in Supplementary data Table S1.
Scheme 1. Showing the ligands used in this study.
acetate complexes offer following structural features known to
enhance the activity.
2.2. Ligand syntheses
1. Tridentate ligands to support lower coordination number
around the metal.
2. An endogenous –OH group capable of bridging between two
metal centers, similar to the active site.
Ligands HL1–HL4 were prepared by methods similar to those
reported in the literature [52,53,22], H3L5 was prepared as re-
ported earlier by us [54].
3. Presence of an exogenous acetate ligand facilitating the depro-
tonation of phenolic group and also the formation of binuclear
complexes with short CuꢁꢁꢁCu distance.
2.2.1. [{Cu(L1)}2(l-CH3COO)2] (1)
A methanolic solution of copper (II) acetate (1.99 g, 10 mmol)
was added to a solution of HL1 (1.92 g, 10 mmol) in CHCl3. The solu-
tion was refluxed for 2 h. After the completion of reaction, the solu-
tion was filtered. After the evaporation of the solvent, a dark–green
compound was obtained, washed with methanol and dried under
vacuum. Dark-green colored crystals were obtained by vapor diffu-
sion method of crystallization using CHCl3 as solvent and PET
(petroleum ether) as a precipitant. Yield 62%. m.p. 192 °C. Anal.Calc.
for C26H36Cu2N4O6: C, 49.76; H, 5.78; N, 8.93. Found: C, 49.84; H,
4. Easily replaceable acetate ligand to augment the binding of sub-
strate to the metal.
To analyze the structure-activity relationship for such ligands
we have varied the flexibility of the ligands, nature of nitrogen do-
nor and length and number of podand arms. Although some com-
plexes of L1 and L2 are known, for Cu(II) salts of azide, perchlorate,
chloride, oxalate and triflate anions (some examples are given in
Refs. [43–47]), our complexes are new because anion in all our
complexes is acetate, either coordinating or non-coordinating
and they have different molecular and crystal structures than the
reported ones.
5.83; N, 9.01%. UV–Vis kmax (nm),
e
(Mꢀ1 cmꢀ1) in CH3OH: 647
(100), 370 (3140), 269 (11670). Selected IR (KBr, cmꢀ1): 3475 (m)
m
m
O–H, 3001 (w) maromatic C–H, 2924, 2888 (w) maliphatic C–H, 1625 (s)
C@N, 1598 (s), 1523 (s) masym OCO, 1450 (s) dC–H, 1379 (s), 1327
(m)
msym OCO, 1195 (m)
mC–N, 1149 (m) mC–O
,
l
B = 2.6 B, Conductivity
(nitrobenzene, 1 mM solution at 298 K):
K
= 2.0
X
ꢀ1 cm2 molꢀ1
2. Experimental
(the range for 1:1 electrolytes in nitrobenzene is 20–30). ESI-MS
(CH3OH, m/z): Obsd. 314.3 [M+H+ where M = (L1+Cu+Ac)] Calcd.
314.5, Obds. 253.8 [(L1+Cu)–H] Calcd. 253.5, Obsd. 188.8 [L1–H]
Calcd. 189.0.
All solvents were dried by standard methods. Unless otherwise
specified, chemicals were purchased from commercial suppliers
and used without further purification. TLC was performed on glass
sheets pre-coated with silica gel. The elemental analyses were per-
formed on a Flash EA 1112 elemental analyzer. FTIR spectra were
recorded on a Shimadzu 8400S IR spectrometer for the compounds
in the solid state as KBr discs or as neat samples, in the range 400–
4000 cmꢀ1. The electronic absorption spectra were recorded on a
Shimadzu Phramaspec UV-1700 UV–Vis spectrophotometer. Room
2.2.2. [(CuL2)(CH3COO)] (2)
A methanolic solution of copper (II) acetate (1.99 g, 10 mmol)
was added to a solution of HL2 (2.06 g, 10 mmol) in CHCl3. The
solution was refluxed for 2 h. After the completion of reaction,
the solution was filtered. After the evaporation of the solvent, a
dark–green compound was obtained, washed with methanol and

V.K. Bhardwaj et al. / Inorganica Chimica Acta 363 (2010) 97–106
99
dried under vacuum. Dark-green colored crystals were obtained on
recrystallization from methanol. Yield 70%. m.p. 168 °C. Anal. Calc.
for C14H20CuN2O3: C, 51.29; H, 6.10; N, 8.54. Found: C, 51.35; H,
tion at 298 K):
K
= 3.78
X
ꢀ1 cm2 molꢀ1. ESI-MS (CH3OH, m/z):
Obsd. 1262.2 [M+H+, where M = L5(Cu)3(Ac)3+3H2O] Calc. 1262.5,
Obsd. 1228.6 [M+ where M = H3L5(Cu)3(Ac)3+H2O] Calc. 1228.5,
Obsd. 983.7 [H3L5+Cu+Ac+H2O] Calc. 983.5.
6.09; N, 8.61%. UV–Vis kmax (nm),
(110), 369 (4530), 302 (4550), 271 (13130). Selected IR (KBr,
cmꢀ1): 3002 (w)
aromatic C–H, 2925 (w) aliphatic C–H, 1633 (s) mC@N
1550 (m), 1537 (m) masym OCO, 1444 (m), 1414 (s) msym OCO, 1341
(m), 1325 (s) dC–H, 1236 (m), 1197 (m) C–N, 1147 (m) mC–O
B = 1.99 BM, Conductivity (nitrobenzene, 1 mM solution at
e
(Mꢀ1 cmꢀ1) in CH3OH: 666
m
m
,
2.3. Kinetics of 3,5-di-tert-butylcatchol oxidation
To determine the catecholase activities of the complexes, 10ꢀ4
M solutions of copper complexes in methanol (except complex 5
[in CH3OH:CHCl3 (29:1)]) were treated with 100 equivalents of
3,5-di-tert-butylcatechol (3,5-DTBC) in methanol under aerobic
condition. The UV spectra of solutions were recorded directly after
the addition and subsequently after regular intervals of 2 min and
absorption values at 390–410 nm were measured as a function of
time over a period of 2–3 h. To determine the dependence of the
rates on the substrate concentration and various kinetic parame-
ters, 10ꢀ4 M solutions of copper complexes were treated with 10,
20, 30, 40, 50, 60, 70, 80, 90 and 100 equivalents of 3,5-DTBC in
methanol under aerobic condition.
m
,
l
298 K):
K = 2.54 X
ꢀ1 cm2 molꢀ1 ESI-MS (CH3OH, m/z): Obsd.
328.1 [M+H+ where M = (L2+Cu+Ac)] Calcd. 328.5, Obsd. 267.9
[(L2+Cu)–H] Calcd. 267.5.
2.2.3. [{Cu(CH3COO)}2(l-L3)2] (3)
A methanolic solution of copper (II) acetate (1.99 g, 10 mmol)
was added to a solution of HL3 (1.94 g, 10 mmol) in CHCl3. The
solution was stirred for half an hour, then, a light-green solid
was separated, filtered, washed with methanol and dried under
vacuum. Dark-green colored crystals were obtained by vapor diffu-
sion method of crystallization using CHCl3 as solvent and Petrol
ether as a precipitant agent. Yield 75%. m.p. 198 °C. Anal. Calc. for
C26H40Cu2N4O6: C, 49.44; H, 6.38; N, 8.87. Found: C, 49.37; H,
3. Results and discussion
6.48; N, 7.97%. UV–Vis kmax (nm),
(100), 423 (630), 281 (4540), 235 (5970). Selected IR (KBr, cmꢀ1):
3156 (w, br) mN–H 3120 (w) maromatic 2926, 2867 (w)
aliphatic C–H, 1594 (s), 1570 (s) asym OCO, 1481 (m),1451 (m) dC–H
C–N, 1021 (w) mC–O B = 2.5 BM. Con-
1 mM solution at 298 K):
e
(Mꢀ1 cmꢀ1) in CH3OH: 661
3.1. Characterization of complexes
,
,
All the compounds have been characterized by elemental and
thermal analyses, Molar conductivity measurements, ESI Mass, IR
and UV–Vis spectroscopy. The X-ray crystal structures of 1, 2 and
3 have also been solved. The room temperature magnetic moments
of all the complexes have been measured and variable temperature
magnetic susceptibilities have been calculated for 1 and 3 which
have been confirmed to exist as weak dimers by X-ray diffraction
methods. The molar conductivity measurements show that com-
pounds 1–5 are non-electrolytes [55]. In the mass spectra of com-
pounds 1 and 3 it is not possible to see the molecular peak
corresponding to the dinuclear compounds, however, for com-
pound 4 there is one peak that could be assigned to the molecular
peak for a dinuclear compound. The presence of water molecules in
complex 5 was inferred from mass spectral and thermal analyses.
The IR spectra [56,57] of 1, 2 and 5 show –C@N characteristic
bands at 1625, 1633 and 1608 cmꢀ1, respectively, which are absent
in the reduced products 3 and 4. The latter two complexes show
medium to weak broad bands due to N–H stretching frequency.
The mC@N stretching band shifts to a lower frequency by 10, 26
and 58 cmꢀ1 in 1, 2 and 5 clearly showing its participation in coor-
dination. In complexes 1 and 3 medium to sharp bands in the re-
gion 1598–1523 cmꢀ1 and 1397–1329 cmꢀ1 have been assigned
to asymmetric and symmetric vibrations of coordinated acetate
C–H
m
m
,
1397(m),
m
sym OCO, 1290 (m)
(nitrobenzene,
m
, l
ductivity
K
= 7.8
X
ꢀ1 cm2 molꢀ1 ESI-MS (CH3OH, m/z): Obsd. 315.1 [M+
where M = (L3+Cu+Ac)] Calcd. 315.5.
2.2.4. [{Cu(L4)}2(l-(CH3COO)2] (4)
A methanolic solution of copper (II) acetate (1.99 g, 10 mmol)
was added to a solution of HL4 (2.08 g, 10 mmol) in CHCl3. The
solution was refluxed for 2 h. After the completion of reaction,
the solution was filtered. After the evaporation of the solvent, a
dark-green compound was obtained, washed with methanol and
dried under vacuum. Dark-green colored powder was obtained
on recrystallization from methanol. The compound is hygroscopic
when exposed to air for some time. Yield 68%. m.p. 162 °C. Anal.
Calc. for C28H44Cu2N4O6: C, 50.98; H, 6.72; N, 8.49. Found: C,
51.06; H, 6.79; N, 7.62%. UV–Vis kmax (nm),
690 (109), 383 (1010), 277 (4920). IR (KBr, cmꢀ1): Selected IR (KBr,
cmꢀ1): 3439 (s)
N–H, 3121(w, br) maromatic C–H, 2908 (m, br)
maliphatic C–H, 1581 (s) masym OCO, 1435 (s) msym OCO, 1331 (m) dC–H
1279(m), mC–N, 1054 mC–O lB 1.96 BM, Conductivity (nitrobenzene,
1 mM solution at 298 K):
e
(Mꢀ1 cmꢀ1) in CH3OH:
m
,
,
K
= 3.8 X
ꢀ1 cm2 molꢀ1 ESI-MS (CH3OH,
m/z): Obsd. 659.1 [M+ where M = (L4+Cu+Ac)2] Calc. 659, Obsd.
540.1 [(L4+Cu)2–H ] Calcd. 540, Obsd. 269.9 [(L4+Cu)–H] Calc.
269.5.
groups.
A
large difference
D
m
between masym
and
OCO
msym OCO ꢂ200 cmꢀ1 or more is indicative of a monodentate coordi-
nation through carboxylate groups [58–60]. Complexes 2 and 5
show strong, symmetrical bands in the range 1526–1575 cmꢀ1
2.2.5. [Cu3(L5)(CH3COO)3]. 3H2O (5)
( (msym OCO) with Dm lying in the
masym OCO) and 1414–1458 cmꢀ1
A methanolic solution of copper(II) acetate (0.597 g, 3 mmol)
was added to a solution of H3L5 (0.843 g, 1 mmol) in CHCl3. The
solution was refluxed for 2 h. Once the completion of the reaction,
the solution was filtered and the solvent evaporated, a dark-green
compound was obtained, washed with methanol and dried under
vacuum. Dark-green colored powder was obtained on recrystalliza-
tion from methanol. Yield 68%. m.p. 205 °C. Anal. Calc. for
C57H57Cu3N3O12S3: C, 54.21; H, 4.55; N, 3.33; S, 7.62. Found: C,
range 113–123 cmꢀ1 which indicates bidentate chelating coordina-
tion through acetate group [56,57,61]. The same has been found in
the crystal structure of 2. Dm
is higher 146 cmꢀ1 in case of complex
4, suggesting a bridging bidentate coordination in 4. The complex 1
shows the presence of water molecule in IR as well as in the TGA
but no water molecule was located in the crystal structure of the
compound which may have been lost during crystallization.
The electronic absorption spectra of complexes 1–5 show
strong red shifted (cf. spectra of ligands) charge transfer bands in
the range 369–423 nm (Table 1) which may all be assigned to li-
gand to metal transitions. Compounds 1–5 show weak d–d bands
in the range kmax 647–690 nm in the visible region. The broadness
of the absorption maxima and low intensity are suggestive of d–d
transitions and a high energy shift of the bands, falling below
53.87; H, 4.63; N, 3.01; S, 7.83%. UV–Vis kmax (nm),
in CH3OH:CHCl3 (29:1): 659 (177), 392 (7220). Selected IR (KBr,
cmꢀ1): 3429 (br)
O–H, 3052 (w, br) maromatic C–H, 2917 (w, br)
maliphatic C–H, 1608 (s) C@N, 1575 (s), 1526 (s) masym OCO, 1458
(s),1440 (s) msym OCO, 1379 (m), 1325 (m) dC–H, 1181 (m) mC–N
1147 (m) mC–O B = 2.3 BM, Conductivity (nitrobenzene, 1 mM solu-
e )
(Mꢀ1 cmꢀ1
m
m
,
,
l

100
V.K. Bhardwaj et al. / Inorganica Chimica Acta 363 (2010) 97–106
Table 1
coordinating atoms O1, O2, N1 and N2 define a plane with maxi-
Electronic absorption data for 1–5 (nm).
mum deviation of 0.02 Å. The Cu(II) ion is 0.111 Å out of this basal
plane towards the apical O2i atom. Such a displacement is quite
common in square–pyramidal copper(II) complexes [65–67]. In
the dimer the two square pyramids are sharing one base-to-apex
edge with parallel basal planes. The N1–Cu–N2 angle (83.8(2)°) is
ꢂ7° smaller than the N1–Cu–O1 (91.0(2)°) because the former is
a part of a tight 5-membered chelate ring and the latter is involved
in a 6-membered ring. The 5–membered chelate ring (defined by
Cu, N2, C9, C8 and N1) makes a dihedral of 11.5(1)° with the 6-
membered chelate ring (defined by Cu, N1, C7, C6, C1, and O1).
Both chelate rings are non-planar with the 5-membered chelate li-
gand adopting an ‘open envelope’ conformation due to the disposi-
tion of C9 and N2, being 0.4 and 0.2 Å above and below its plane,
respectively. The five non-metallic members of the six membered
chelate ring are almost in a plane with a maximum deviation of
0.03 Å for C6 and the Cu(II) lies 0.3 Å above this plane, again form-
ing the open end of an ‘envelope like’ conformation. The CuꢁꢁꢁCui
non-bonding distance is 3.405(3) Å which is much too large to sug-
gest any significant interaction.
CT band
d–d band
1
2
3
4
5
370 (sh)
369 (sh)
423 (br)
383 (br)
392 (sh)
647 (br)
666 (br)
661 (br)
690 (br)
659 (br)
700 nm, indicates distortion of the octahedral geometry to square–
pyramidal [62,63]. Complex 5 has a tri-podal ligand with three me-
tal centers. A single d–d band indicates that all three of them are in
a similar five coordinated mode. It could be achieved by coordina-
tion of each Cu(II) with a N of the imine, an O of the phenol, a mol-
ecule of water and a chelating acetate group.
3.2. X-ray crystal structures
3.2.1. [{Cu(L1)}2(l-CH3COO)2] (1)
Crystal data, structure refinement parameters and CCDC num-
bers for three complexes, 1–3 are given in the supplementary data
as Table S1. Fig. 1 shows the final structure of compound 1. It
shows a dinuclear structure due to bridging through O2 of the ace-
tate group. The second oxygen, O3 of the acetate group remains
uncoordinated and is involved only in H-bonding interactions.
The bond lengths C12–O3 1.231(8) and C12–O2 1.298(8) Å also
indicate coordination through single oxygen only. The bond dis-
tances Cu–O2 and Cu–O2i are 1.961(4) and 2.399(4) Å, respec-
tively, which show that the bridging is highly unsymmetrical
through a single oxygen O2 of the acetate group (i = 1 ꢀ x + 2,
ꢀy + 1,ꢀz). Mean Cu–N and Cu–O distances are 2.032(6) and
1.942(4) Å, respectively. Coordination geometry around Cu may
be considered as square pyramidal with amine nitrogen N1, imine
nitrogen N2, phenolic oxygen O1 and acetate oxygen O2 at the
equatorial positions whereas the axial coordination position is
occupied by the bridging O2i atom in the dimer. The geometry
around the copper(II) ion is best described as an ideal square–pyra-
midal with a very small trigonal–bipyramidal distortion parameter
3.2.2. [(CuL2)(l-CH3COO)] (2)
Fig. 2 shows the monomeric structure of the compound 2. The
Cu(II) ion is five coordinated by an imine through N1, N2 and O1
and acetate oxygens O2 and O3. The acetate anion is behaving as
an unsymmetrical bidentate chelating ligand with Cu–O3
1.993(3) and Cu–O2 2.331(3) Å. The bond lengths C13–O3 of
1.257(5) Å and C13–O2 of 1.244(4) Å being equal indicate chelation
through the acetate group. Cu–N (imine) 1.929(3) Å is significantly
shorter than Cu–N(amine) 2.142(3) Å. The stereochemistry around
Cu (II) may be considered as highly distorted square pyramidal
with O1, O3, N1 and N2 at the equatorial positions and O2 at the
axial position. A high value of the trigonal–bipyramidal distortion
parameter
s 0.43 indicates towards a distorted structure. The an-
gles N1–Cu–N2 93.47(12) and N1–Cu–O1 95.04(11)° are similar.
The two six membered chelate rings are having a dihedral angle
of 40.7° and are twisted significantly with respect to the each
other. The chelate ring which contains three methylene groups
and a flexible amine nitrogen N2 is in a chair conformation
whereas the other six membered chelate ring is rigid and almost
planar with maximum deviation of 0.04 Å for the imine nitrogen
N1.
(s
) of 0.06 (
dination angles, 174.6° and 171.0°, respectively). In a perfect
square–pyramidal geometry equals 0, while a value of 1 would
s = (b – a)/60, with a and b being the two largest coor-
s
indicate perfect trigonal–bipyramidal geometry [64]. The four
3.2.3. [(Cu-
Fig. 3 Showing the ORTEP diagram of complex 3 with
oxo group.
l-L3)(CH3COO)]2. (3)
l
–phen-
The final structure of 3 is shown in Fig. 3. This compound forms
a centrosymmetric dimer due to bridging bidentate coordination
through phenolic oxygen O1. The stereochemistry around each
Cu(II) is best described as a distorted square pyramidal with a va-
lue of
s being 0.31. The equatorial positions are occupied by the
amine nitrogen atoms N1 and N2, oxygen O2 of the monodentate
coordinating acetate group and the phenoxo oxygen O1. The axial
position is occupied by the bridging phenoxo oxygen O1 from the
symmetry related molecule. The monodentate coordination
through acetate ion is also ratified by unequal C12–O2 1.284(12)
and C12–O3 1.236(12) Å bond distances. The two square pyramids
in the dimer are sharing one base-to-apex edge with parallel basal
planes. Average Cu–N and Cu–O distances are 2.044 and 1.976 Å,
respectively. The Cu–O(phenoxo) distances Cu–O1i and Cu–O1
are 1.953(6) and 2.222(7) Å, respectively, which shows that the
bridging by phenolic oxygens is highly unsymmetrical (where
i = ꢀx + 1, ꢀy + 2, ꢀz + 2) and the symmetry related phenoxo oxy-
gen O1 is closer to the Cu(II) ion than the one belonging to the ori-
ginal molecule. The CuꢁꢁꢁCui non-bonding distance between the
two metal ions is 3.185 Å, shorter than that found in compound
Fig. 1. ORTEP diagram of complex (1) and the labeling scheme used.

V.K. Bhardwaj et al. / Inorganica Chimica Acta 363 (2010) 97–106
101
Fig. 2. Showing the ORTEP diagram for complex (2) and the labeling scheme used.
Fig. 3. Showing the ORTEP diagram of complex (3) with l–phenoxo group.
1. The N1–Cu–N2 bite angle (84.0°) is ꢂ5° smaller than the N1–Cu–
O1 angle (89.3°) as explained above. The five membered and six
membered rings form a dihedral angle of 70° also larger than found
in 1 giving rise to an ‘open book-like conformation around the Cu–
N1 bond. This is due to the change from a sp2 C and N of 1 to sp3 C
and N in the reduced compound 3 making it more flexible and also
accounting for the distortion of stereochemistry towards trigonal
bipyramid. This tilting of the five membered ring brings methylene
3.3. Magnetic properties
Solid-state, variable temperature (2–300 K) magnetic suscepti-
bility data using 0.5 and 1.0 T fields were collected on polycrystal-
line samples of compounds 1 and 3, respectively. The resulting
data are plotted in Figs. 4 and 5. The data for compound 1 show
a behavior characteristic of very weak, antiferromagnetic coupled
Cu2+ ions. The MT value at 300 K is 0.86 cm3 molꢀ1 K, higher than
v
C8 close to the phenyl ring resulting in a C–Hꢁꢁꢁ
p
interaction with
that expected for two uncoupled S = 1/2 spins assuming g = 2
(0.75 cm3 molꢀ1 K) and it does not vary upon cooling until approx-
imately 14 K, dropping faster then and arriving finally to
0.78 cm3 molꢀ1 K at 2 K (Fig. 4). The experimental magnetic data
were fitted using the equation of Bleaney–Bowers [68] for dinuclear
centroidꢁꢁꢁC8 distance as 3.94 Å.
A discussion of the crystal structures of all three complexes
including H-bonding interactions has been given in supplementary
data (Fig. S1–S3 and Table S2).

102
V.K. Bhardwaj et al. / Inorganica Chimica Acta 363 (2010) 97–106
allel basal planes [69]. The single electron on each copper atom re-
sides mostly of the dx2 type (basal) and consequently, the
interaction between the magnetic orbitals of the two copper atoms
is expected to be negligible [70]. This is the case of compound 1, in
which local geometry for each copper center is an almost perfect
square–pyramid (s = 0.06) [71] showing a very small magnetic
coupling constant. Instead, the environment of the copper ions in
compound 3 shows a higher distortion towards trigonal bipyramid
(s = 0.31). This variation is enough to partially avoid the orthogo-
nality of the magnetic orbitals and the final coupling is antiferro-
magnetic (J = ꢀ54 cmꢀ1) and stronger than for compound 1.
In principle, a coupling between two Cu(II) ions through two
phenolate ligands is expected to be stronger than one due to
methoxides or like in the case of 1, via acetates [72]. Such a dif-
ference in the linkers leads to a Cu–O–Cu bridge angle of 102.22°
for 1 and 99.29° for 3, the latter being lower than the former.
Both compounds show a rectangular Cu2O2 core, with a Cu–Obasal
distance shorter than the Cu–Oapical distance, however, the differ-
ence between both distances is more significant for compound 1
than for 3. The Cu–O bridging distances are 1.961 and 2.399 Å for
1 and 1.951 and 2.221 Å for 3, as well as the CuꢁꢁꢁCu non-bonding
distance being 3.405 and 3.188 Å for compounds 1 and 3, respec-
tively. Nevertheless, the J value found for compound 3 is rela-
0.88
0.86
0.84
0.82
0.80
0.78
ꢀy2
T
M
χ
0
50
100
150
T / K
200
250
300
Fig. 4. Fitting of the
experimental data are shown as black spheres and the red line corresponds to the
theoretical values.
vMT vs T of compound (1) between 2.0 and 300.0 K. The
tively small comparing with what one would expect for
a
coupling through a double phenoxo group bridge [72]. Once
again, the square–pyramidal nature of the copper centers and
their interaction in a base-to-apex fashion (like for 1) would ex-
plain this value perfectly. Analogous results were obtained by
Rodriguez et al. studying a similar dinuclear cluster to compound
3 using an amine bis (phenolate) derivative as a ligand [73]. The
most relevant crystallographic parameters of this core are of the
range of 3 (Table 2) with a final J of ꢀ45 cmꢀ1. Earlier work per-
formed by Salmiya et al. and Cros et al. also showed an antiferro-
magnetic coupling between the two copper centers with a J value
of ꢂ ꢀ20 cmꢀ1 [74,75].
3.4. Catechol oxidase studies and kinetics
All the complexes were subjected to catecholase-mimitic activ-
ities to find their capability to act as catalysts for the oxidation of
alcohols to quinones, Scheme 2 like catechol oxidase. The oxidation
of 3,5-DTBC to corresponding product 3,5-di-tert-butylquinone
(3,5-DTBQ) was followed by the development of a considerably
stable and strong absorption band at 390 nm in methanol [76].
All the complexes 1–5 showed activity towards the oxidation of
catechols (supplementary Fig. S4–S7). The course of a typical reac-
tion with a solution of 4 is shown in Fig. 6.
Immediately after addition of substrate 3,5-DTBC to the solu-
tions of the catalysts 1–5 various LMCT bands of the complexes
disappeared and a new band corresponding to 3,5-DTBQ appeared
at 390–410 nm, as noted by Zippel et al. [10]. They suggested that
this could only be due to the formation of Cu(I) and not due to the
destruction of the complex because the chelating ligands used by
them and here as well, are much stronger than the substrate, sol-
vent or quinine. The subsequent increase in absorption of this band
is linear for all the copper(II) complexes. Thus all of these com-
plexes are showing catecholase activity.
Fig. 5. Fitting of the
vMT vs T of compound (3) between 12.0 and 300.0 K. The inset
shows the MT vs T. The experimental data are shown as black spheres and the red
v
line corresponds to the theoretical values.
copper(II) complexes with the Hamiltonian in the form Y = ꢀJS1S2.
The best fit parameters were found for J = ꢀ0.37 0.05 cmꢀ1
,
g = 2.14 0.01 and R = 2.07 ꢃ 10ꢀ4
.
Temperature-independent
paramagnetism (TIP) was considered equal to 120 ꢃ
10ꢀ6 cm3 molꢀ1
The MT for compound 3 is depicted in Fig. 5. The magnetic
susceptibility value
MT at 300.0 K is 0.78 cm3 molꢀ1 K varying
.
v
v
smoothly and finally dropping to nearly zero at low tempera-
tures (0.02 cm3 molꢀ1 K at 12 K). As before, this dinuclear system
exhibits an antiferromagnetic behavior although this time, there
is a stronger interaction between the copper centers. Indeed, the
vM versus T data also show a maximum at approximately 50 K
which is indicative of a significant coupling between the two
Cu2+ ions (Inset Fig. 5). Using derived Hamiltonian than the de-
scribed above and taking into account impurities in the sample,
The kinetics of the oxidation of 3,5-DTBC was determined by
the method of initial rates by monitoring the growth of the 390–
410 nm band of 3,5-DTBQ as a function of time. A plot of log [A /
a
(A ꢀ At)] versus time the catecholase activity of the complex
a
the best fit were J = ꢀ54.2 0.4 cmꢀ1, g = 2.06 0.01,
q
= 0.046,
was obtained (Fig. 7), and the rate constants for the catalytic oxida-
tion were calculated and given in Table 3. The rate constant values
show that the reactivity of complexes is differing significantly from
each other. Compounds 4 and 1 show significant catecholase activ-
ity. To determine the dependence of the rates on the substrate
TIP = 120 ꢃ 10ꢀ6 cm3 molꢀ1 and R = 3.67 ꢃ 10ꢀ3
.
Compound 1 and 3 exhibit a similar global arrangement, all
copper atoms are pentacoordinated and in each compound, the
two square pyramids are sharing one base-to-apex edge with par-

V.K. Bhardwaj et al. / Inorganica Chimica Acta 363 (2010) 97–106
103
Table 2
Structural and magnetic parameters of compounds with a [Cu2O2]2ꢀ core sharing a base-to-apex edge.
Compound
s
(°)
d(Cu–Ob) (Å)
d(Cu–Oa) (Å)
Cu–O–Cu (°)
CuꢁꢁꢁCu (Å)
J (cmꢀ1
)
References
a
1
3
0.06
0.31
0.45
0.40
0.43
0.43
1.961
1.953
1.959
1.971
1.974
1.936
1.917
1.980
1.917
1.884
2.399
2.221
2.182
2.199
1.998
2.093
1.989
2.106
1.961
2.524
102.22
99.29
99.95
99.65
103.27
101.18
101.76
103.84
88.9
3.405
3.185
3.174
3.19
3.114
3.124
ꢀ0.39
ꢀ54.2
ꢀ45
a
[(Cu(L1)2Cu) (MeCN)2]
[Cu(L2)2Cu][(CuL(L3)Cu)] (ClO4)
[68]
[69]
ꢀ19.9
ꢀ277
[(Cu(L4)(L5)Cu)]ClO4
0.14
0.29
2.742
ꢀ22
[70]
107.2
a
This work. Ob = O basal, Oa = O axial. J values are referred to H = ꢀJS1S2. L1 = N,N-bis(3,4-dimethyl-2-hydroxybenzyl)-N0,N0-dimethylethylenediamine deprotonated,
L2 = N,N-bis(2-hydroxybenzyl)-N0,N0-dimethylethylenediamine deprotonated, L3 = N,N-bis(2-hydroxybenzyl)-N0,N0-dimethylethylenediamine, L4 = 2-((2-((o-Hydroxy-
a-
methylbenzylidene)amino)ethyl)amino)ethanol deprotonated, L5 = 2-((2-((o-Hydroxy-a-methylbenzylidene)amino)ethyl)amino)ethanol. Cu3 (Ref. [74]) shows two subunits
in the cell which distances are different and depicted separately; however, the
s
value is identical in both.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
[1]
[2]
[3]
[4]
[5]
Scheme 2. Showing basic reaction involved in oxidation of catechol.
0
0
concentration and various kinetic parameters, solutions of copper
complexes were treated with different concentrations of 3,5-DTBC
(from 10 to 100 equivalents) under aerobic conditions. At low con-
centrations of 3,5-DTBC a first-order dependence of the substrate
concentration was observed. At higher concentrations, saturation
kinetics was found for all the compounds. The effect is more pro-
nounced for 4 and 1 whereas for the remaining three complexes
the rates are almost independent of the substrate even at lower
concentrations. The dependence on the substrate concentration
indicates a catalyst-substrate binding to be an initial step in the
catalytic mechanism. The rates of reactions obtained for various
3,5-DTBC concentrations were fitted to the Michaelis–Menten
equation (Fig. 8) and linearized by means of Lineweaver–Burk plot
5
10
15
20
Time (min.)
Fig. 7. Showing the course of absorption maxima at 390 nm with time for 100
equiv. of 3,5-DTBC in solutions of 10ꢀ4 M complexes in methanol.
to calculate various kinetic parameters (Table 3) for these
compounds.
Turnover rates ranging from 3.2 to 41.3 hꢀ1 have been found
which are comparable to those reported by Krebs et al. and Neves
et al. [14,15,24] and to an order of two or three times more than
those reported by Neves et al. [25] but are significantly lower than
Fig. 6. Increase of quinone band at 390 nm after addition of 100 equivalents of (3,5-DTBC) to a solution containing complex (4) (10ꢀ4 M) in methanol at 22 °C. The spectra
were recorded after every 2 min.

104
V.K. Bhardwaj et al. / Inorganica Chimica Acta 363 (2010) 97–106
Table 3
Kinetics parameters for the oxidation of 3, 5-DTBC catalyzed by various Cu(II) complexes.
Complex
Rate constant (mꢀ1
)
Vmax (Msꢀ1
)
Km (M)
Ra
kcat (hꢀ1) turnover rate
(1)
(2)
(3)
(4)
(5)
4.63 ꢃ 10ꢀ2
3.65 ꢃ 10ꢀ2
4.17 ꢃ 10ꢀ2
8.12 ꢃ 10ꢀ2
4.07 ꢃ 10ꢀ2
6.44 ꢃ 10ꢀ7
1.14 ꢃ 10ꢀ7
9.02 ꢃ 10ꢀ8
1.148 ꢃ 10ꢀ6
2.089 ꢃ 10ꢀ7
33.12 ꢃ 10ꢀ4
21.28 ꢃ 10ꢀ4
12.25 ꢃ 10ꢀ4
20.01 ꢃ 10ꢀ4
15.68 ꢃ 10ꢀ4
0.9593
0.9960
0.9874
0.9946
0.9976
23.2
4.1
3.2
41.3
7.5
a
Discrepancy value of the Lineweaver–Burk plot.
those reported by Krebs et al., Monzani et al., and Vittal et al.
[10,18,22,31,32]. Keeping in mind the complicated mechanism of
the reaction involved the data obtained from Lineweaver–Burk plot
are suffice for comparing the catecholase activity of the complexes
which follow an order 4 > 1 > 5 > 2 ꢂ 3. The differences in the rela-
tive performance of these catalysts may be reasoned in the light of
following known facts from the literature.
a very strained geometry to the complex and also by helping to
deprotonate the substrate [14].
In the present case since all five complexes 1–5 have acetate
ions and phenoxo groups their activities may be compared. Com-
plex 4 and 1 are both bridged by acetate groups but 4, being a re-
duced product, with one extra –CH2 group is a more flexible
system and hence favors the catalysis phenomenon more than 1.
Complex 3 shows less activity than 4 and 1 though phenoxo group
offers a minor competition to the incoming substrate than the ace-
tates. The inhibition of the activity may be due to the presence of
3.5. Rationale for the observed relative performance of catalysts
two endogeneous phenoxo bridges in
3 as reported earlier
[29,80]. This observation is also supported by the fact that the pre-
viously reported [Cu2(L4)2](PF6)2ꢁ2CH2Cl2 double phenoxo-bridged
complex [22] shows less activity (5.13 hꢀ1) than our compound 4
with bridging acetate groups. The activity of 3 is even less than
the monomeric complex 2. The CuꢁꢁꢁCu distance in the acetate
bridged complex 1 is 3.405(8) Å and is expected to be ꢂ3.40 Å
for 4 (cf. the distances reported for other bidentate bridged com-
plexes [25]) both of which are larger than 3.185 Å found in 3. All
these distances are however comparable with 3.25 Å observed in
the only o-catecholate bridged complex known in the literature
[77]. The mononuclear complex 2 is less efficient than 5 having a
mononuclear complex in each of its three arms for the obvious rea-
son that there is a labile water molecule which is easily replaceable
by the catechol. Thus dinuclear complexes with acetate bridges
have shown more activity than mononuclear complexes. The pres-
ence of two endogenous bridging phenoxo groups reduce the activ-
ity of the dinuclear species up to the extent of a mononuclear one.
Various factors affecting the structure-activity relationship,
have been recognized as CuꢁꢁꢁCu distance, flexibility of the ligand,
type of exogenous ligand and coordination geometry around the
metal ion [42]. Although dimeric complexes are considered to be
more relevant for mimicking the catalytic activity owing to the ac-
tive site of the naturally occurring enzyme, both mononuclear and
dinuclear copper complexes have been found to show significant
catecholase activity. Among the mononuclear complexes non-pla-
nar complexes with intermediate coordination geometry between
trigonal bipyramid and square pyramidal are potential cases while
square–planar complexes show little or no activity [4]. In the dinu-
clear complexes a CuꢁꢁꢁCu distance 2.9–3.2 Å has been suggested to
give the maximum activity [77] owing to the requirement of a ste-
ric match between the substrate and the catalyst. However, many
dinuclear complexes having longer metalꢁꢁꢁmetal distance (up to
7.5 Å) are known to be good catalysts [38,78,79]. At the same time,
the nature of the exogenous bridging ligand in the dimeric com-
plexes also shows a remarkable influence on the activity, owing
to the fact that a weakly coordinating ligand may easily be dis-
placed by the incoming catechol thus enhancing the activity [42].
Neves et al. have reported that in general acetate groups are com-
peting with the substrate for the binding site, decreasing the activ-
ity [24]. A single hydroxo- or phenoxo-bridged complex on the
other hand has been reported to augment the activity by enforcing
3.6. Mechanistic inferences
Despite the clarification of the active sites of catechol oxidase
being known from its crystal structure the catalytic mechanism
is still a much debated issue. Krebs and coworkers [81] have sug-
gested a mechanism where one equivalent of catechol gives a
monodentate coordination to one of the Cu(II) centres in the dicop-
per(II) containing met form of the enzyme. This results in the for-
mation of one molecule of quinone and to the reduced deoxy
dicopper(I) state where one Cu(I) is bonded to a water molecule.
This is followed by binding of dioxygen and catechol to the dicop-
per(I) state with the concomitant removal of solvent water mole-
cule and oxidation of Cu(I) centers to dicopper(II). In the ternary
enzyme-O22ꢀ catechol complex, two-electrons are transferred
from the substrate to the peroxide, followed by the cleavage of
the O–O bond, loss of water, formation of the quinone product
and regeneration of met state of enzyme completing the catalytic
cycle. Though this mechanism is largely accepted there is much
contest on the initial monodentate or bidentate coordination of
catechol to the enzyme. Krebs and coworkers have proposed a
monodentate asymmetric coordination of the substrate whereas
Solomon et al. [82] have suggested a simultaneous coordination
to both the copper centers. Later on many groups (Lindtvedt and
Thuruya [83], Oishi et al. [4], Karlin [77], Demmim [7]) have
stressed upon the didentate bridging coordination of the catechol
to the active site owing to the higher activity of binuclear catalysts.
10
9
8
7
6
5
4
3
2
1
0
[1]
[2]
[3]
[4]
[5]
0
0.002
0.004
0.006
0.008
0.01
0.012
[ 3-5, DTBC ] (M)
Fig. 8. Dependence of the initial rates on substrate concentrations for the oxidation
process promoted by various copper complexes.

V.K. Bhardwaj et al. / Inorganica Chimica Acta 363 (2010) 97–106
105
Another point of concern among various workers is regarding the
production of water or hydrogen peroxide as a dioxygen reduction
product in the catalytic oxidation of 3,5-DTBCH2 by copper(II)
complexes, as reported by Chyn and Urbach [68], Casella et al.
[18,21] and by Reedijk et al. [84]. The latter have shown a dinuclear
complex catalyzing the oxidation by two different pathways; one
proceeding via the formation of semiquinone species with the sub-
sequent production of dihydrogen peroxide as a byproduct and an-
other proceeding via the two-electron reduction of the dicopper(II)
center by the substrate giving two molecules of quinone and one
molecule of water. They also showed that the product quinone it-
self behaves as an inhibitor at later stages of the catalytic
oxidation.
the observation that easy removal of the exogenous ligand from a
less sterically hindered, easily approachable Cu(II) ion, has a pro-
found effect on the activity. It is further corroborated that the pres-
ence of two endogenous phenoxo bridges in the dinuclear complex
diminishes the catalytic activity.
Acknowledgments
G.H. and V.K.B. are thankful to the CSIR, India for research
grant No. 01(2104)/07/EMR-II and research fellowship, respec-
tively. The recording of IR by Dr. Sarvjit Singh at Department
of Chemistry, Himachal Pradesh University, Simla is gratefully
acknowledged. The work performed at the Universitat de Barce-
lona by Dr. Aliaga-Alcalde was supported by ICREA (Institució
Catalana de Recerca i Estudis Avançats) and the Generalitat de
Catalunya, 2005SGR 00593 (Funding Research Project: Spanish
government (MEC) CTQ2006-01759/BQU).
More recently Das and coworkers [85] have shown for the first
time that in acetonitrile the catalytic reaction proceeds via the for-
mation of two enzyme-substrate adducts ES1 and ES2, which were
detected spectroscopically. Although our present work is not
aimed towards the mechanistic studies of the catalytic process
nevertheless curiosity led us to see the effect of change of solvent
on our results, taking a cue from the above. We repeated the cata-
lytic studies in acetonitrile for complexes 4 and 2 of our catalysts,
hoping to find out information regarding two enzyme-substrate
intermediates. The UV–Vis spectra for both are shown in supple-
mentary material. These, do not show any drastic changes immedi-
ately after addition of DTBC in the 200–500 nm region except for
the expected gradual increase in the intensity of band at ꢂ400
nm. Thus unfortunately there is no revelation of any intermediate
adducts. This of course, does not rule out their formation but
merely suggests that these adducts, in our case are too unstable
to be detected even in acetonitrile unlike that reported by Das
et al. However we observed one definite difference between the re-
sults in acetonitrile and methanol i.e. the reaction rates are much
less in the former. Using the method of initial rates the rate con-
stants have been found to be 1.2 ꢃ 10ꢀ2 and 1.1 ꢃ 10ꢀ2/min. for
4 and 2, respectively. This influence of the solvent on the rates of
reaction indicates that acetonitrile is either competing with the
substrate for binding at the copper (II) center or it is stabilizing
the intermediate reduced deoxy Cu(I) state thus slowing the reac-
tion rate. This indirectly hints at the essential requirement of coor-
dination of the substrate to the catalysts for the activity. This is also
corroborated by the observation that the rates of oxidation for 4
and 1 are dependent on the concentration of the substrate at lower
concentrations (Fig. 8). As 4 and 1 are both dinuclear in nature and
are also the most active catalysts among the five complexes hence
bidentate coordination mode of substrate to the catalysts is
evident.
Appendix A. Supplementary material
CCDC 638641, 630907 and 630906 contain the supplementary
crystallographic data for 1, 2 and 3. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2009.09.041.
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4. Conclusions
We have synthesized and characterized five Cu(II) complexes
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