Syntheses, structural properties and catecholase activity of copper(II) complexes with reduced Schiff base N-(2-hydroxybenzyl)-amino acids.

Article abstract of DOI:10.1039/b310262a

A number of dicopper(II) complexes of reduced Schiff base ligands, N-(2-hydroxybenzyl)-amino acids [Cu2L2(H2O)x].yH2O (L = Sgly (1), D-Sala (2), L-Sala (3), DL-Sala (4), Sab2 (5), Sbal (6), Sab4 (7), Sval (8), Shis (9), Styr (10) and Stryp (11), x= 0-2 &

Full text of DOI:10.1039/b310262a

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Syntheses, structural properties and catecholase activity
of copper(II) complexes with reduced Schiff base
N-(2-hydroxybenzyl)-amino acids†
Chang-Tong Yang,a Muthalagu Vetrichelvan,a Xiandong Yang,a Boujemaa Moubaraki,b
Keith S. Murrayb and Jagadese J. Vittal*a
a Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543. E-mail: chmjjv@nus.edu.sg
b School of Chemistry, Monash University, P.O. Box 23, Victoria, 3800, Australia
Received 26th August 2003, Accepted 11th November 2003
First published as an Advance Article on the web 1st December 2003
A number of dicopper() complexes of reduced Schiff base ligands, N-(2-hydroxybenzyl)-amino acids
[Cu2L2(H2O)x]ؒyH2O (L = Sgly (1), -Sala (2), -Sala (3), -Sala (4), Sab2 (5), Sbal (6), Sab4 (7), Sval (8), Shis (9),
Styr (10) and Stryp (11), x = 0–2 & y = 0–2) have been synthesized, and the solid-state structures of 4, 5,10 and 11
have been determined. The compounds 4 and 5 are binuclear in which the Cu() centres have square-pyramidal
geometry with apical sites occupied by aqua ligands. In 10 and 11 one axial site is occupied by water and the other by
an oxygen atom of the carboxylate group from the adjacent dimer through oxygen atoms to form 1D helical polymer.
Variable temperature magnetic measurements of the dimer 4 and helical polymer 10 showed that they are typical for
moderately strong antiferromagnetic coupling. All the complexes show significant catalytic activity on the oxidation
of 3,5-di-tert-butylcatechol. The activity measured in terms of Kcat in the range 199–3800 hϪ1 has been found to
follow the order: 7 > 6 > 8 > 3 > 5 ∼ 2 ∼1 > 4 > 10 >9 > 11. The catalytic activity is found to increase with increasing
the length of the methylene side chain of the amino acid in the reduced Schiff base ligands.
formation of bimetallic species. The structures of binuclear
complexes of H2Sgly, -H2Sala, -H2Sala, ,-H2Sala, H2Sbal,
H2Sab2, H2Sab4, H2Styr and H2Stryp have shown to have
Cu ؒ ؒ ؒ Cu distances in the range of 3 Å. The solid-state
structures of 4, 5, 10 and 11 have been determined by X-ray
crystallography. The catecholase activity of these binuclear
Introduction
The copper containing proteins are classified into three types:
I, II and III. Type III copper proteins such as hemocyanin,
tyrosinase have a strongly coupled binuclear copper active site,
which is antiferromagnetically coupled and therefore EPR
silent in the oxidized state.1–5 Catechol oxidase is another
compounds have been measured and compared. The results of
copper type III enzyme which only catalyses the oxidation of
our investigations are described in this paper in detail.
catechol to quinone without acting on tyrosinase.6 This reaction
is of great importance in medical diagnosis for the determin-
ation of the hormonally active catacholamines adrenaline,
Results and discussion
noradrenaline and dopa.7,8 The copper in the isolated catechol
Compounds 1–3 have been reported before.27,28 and the struc-
oxidases was found to be EPR-silent and was assigned to an
ture of 8 will be published elsewhere.29 A few years ago the
optically active -H2Sala and -H2Sala ligand have been used
to construct multi-dimensional hydrogen bonded network
structures with chiral channels and these hydrated network
structures undergo solid state supramolecular transformation
to coordination polymeric network compounds.27 The basic
building block consists of noncentrosymmetric dimers. In order
to understand the effect of chirality of the ligands used in the
building blocks on the packing, we have employed the ,-Sala
ligand, in which both enantiomers are present in equal amounts
and the Sab2 ligand, which is optically inactive but closely
related to the ,-Sala ligand. Complexation of ,-Sala with
Cu() ions is expected to furnish centrosymmetric dimer
[Cu2(,-Sala)2(H2O)2].
The IR absorption band of the newly synthesized com-
pounds 4–7 and 9–11 found in the range 3389–3471 cmϪ1 con-
firms the presence of lattice or coordinated water for the com-
pounds.30 This is further supported by the weight loss observed
in TG (see experimental section). The sharp band observed in
the region 2895–2976 cmϪ1 may be assigned to ν(NH).31 The
band in the region 1572–1615 cmϪ1 is assigned to the asym-
metric vibration of coordinated carboxylate group [νas(COOϪ)]
and the band in the region 1325–1397 cmϪ1 is attributed to the
symmetric stretching vibration of carboxylate group [νs(CO-
OϪ)]. The large difference between [νas(COOϪ)] and [νs(COOϪ)]
in frequencies (∆ν > 200 cmϪ1) in all complexes is indicative of
a monodentate coordination of carboxylate groups.32–34 This
antiferromagnetically spin-coupled Cu()–Cu() pair.9
The catecholase activity of synthetically prepared copper
compounds with different structural parameters has been
investigated to some extent.10–22 Modeling the enzyme features
requires the control of the interaction of the two metal centres.
As a consequence, the design of the binucleating ligand has to
satisfy a number of conditions: metal–metal distance, steric,
electronic and bridging ligand features, etc. The crystal struc-
ture of catechol oxidase recently reported by Krebs and co-
workers reveals that these proteins have binuclear copper center
and have similar spectroscopic behaviour, and show close func-
tional relationships.23 The crystal structure of catechol oxidase
have enhanced the understanding of the mechanism of cate-
cholase activity of tyrosinase and/or catechol oxidase which
was proposed by Solomon and others.23–26
In this paper, we report the synthesis of a series of binuclear
copper() complexes as potential structural and functional
models for the active sites of catechol oxidase. The ligands con-
sidered in this paper are as follows: H2Sgly, -H2Sala, -H2Sala,
,-H2Sala, H2Sbal, H2Sab2, H2Sab4, H2Sval, H2Shis, H2Styr
and H2Stryp (Scheme 1). These ligands contain correct terminal
and endogenous bridging ligand types that strongly favour the
† Electronic supplementary information (ESI) available: Thermo-
gravimetry curves, UV/vis spectra, a plot of 1/ν vs. 1/[S], structure
diagrams, plots of χ and µeff, and H-bond distances. See http://
www.rsc.org/suppdata/dt/b3/b310262a/
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 1 1 3 – 1 2 1
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Table 1 Electronic absorption and conductivity data for 4–11
Absorption bands/nm
Molar conductivity/S
Complex
CT
d–d
cm2 molϪ1
4
5
361 (sh)
373 (sh)
393 (sh)
380 (sh)
404 (sh)
364 (sh)
393 (sh)
669 (br)a
658 (br)
620 (br)
690 (br)
671 (br)
678 (br)
656 (br)
2b
2
1
1
2
4c
4
3
6
7
9
10
11
39
8
37
27
4
7
a As Nujol mull transmittance. b In DMSO. c In CH3OH.
Description of crystal structures
The solid-state structures of 4, 5, 10 and 11 have been described
in this section in detail. No suitable single crystal for data
collection has been obtained for 6, 7 and 9.
[Cu2(D,L-Sala)2(H2O)2]ؒ2H2O, 4
An ORTEP diagram with numbering scheme is shown in Fig. 1.
The complex is a centrosymmetric binuclear compound with
both  and -Sala ligands coordinated to the Cu() centres in a
mer fashion through phenolate, carbonyl oxygen and the imine
nitrogen atoms. In the dimer, the phenolic oxygen atoms O1
and O1A bridge the two Cu() centres giving a Cu ؒ ؒ ؒ Cu dis-
tance of 2.9742(7) Å. The apical coordination sites are occupied
by water molecules in trans fashion. The coordination geometry
of Cu() ion can be described as an ideal square pyramid as
indicated by the angular structural parameter, τ = 0.013.39–41
The trans geometry of the two aqua ligands has been found
in a number of binuclear Cu() compounds including [{Cu-
42
(HSalacet)H2O}2]ؒ(NO3)2 (Salacet = salicylaldehyde acetyl-
hydrazone) and [Cu2(bhsNO2)2(H2O)2]43 (bhsNO2 = 2-hydroxy-
5-nitro-benzaldehyde benzoylhydrazone) resulting in Cu ؒ ؒ ؒ
Cu of 3.006(3) and 3.041(4) Å, respectively. Equatorial bond
lengths [Cu(1)–O(1) 1.953(2) Å, Cu(1)–O(2) 1.952(2) Å, Cu(1)–
N(1) 1.975(3) Å, Cu(1)–O(1)A 1.940(2) Å] and weakly bound
axial bond length [Cu(1)–O(4) 2.326(2) Å] agree well with
Scheme 1 Reduced Schiff base ligands. The numbers of the corre-
sponding Cu() complexes [Cu2L2(H2O)x]ؒyH2O of these ligands used
in this paper are given in brackets.
42
those reported for [{Cu(HSalacet)H2O}2]ؒ(NO3)2 and [Cu2-
(bhsNO2)2(H2O)2].43
observation was confirmed by X-ray crystal structures of 4 and
5. There are two kinds of [νs(COOϪ)] exhibited in IR spectrum,
respectively. The small difference (∆ν = 154 < 200 cmϪ1
)
between [νas(COOϪ)] and [νs(COOϪ)] in frequencies and large
difference (∆ν = 220 > 200 cmϪ1) between [νas(COOϪ)] and
[νs(COOϪ)] indicate bridging coordination mode of the carb-
oxylate group and terminal coordination mode for the carb-
oxylate group, respectively, as found in 10 and 11. The band
around the 1259–1285 cmϪ1 can be assigned to ν(C–O) of
phenolic group in complexes.31
The electronic spectral data for the complexes as Nujol mull
transmittance as well as the molar conductivity data are given
in Table 1. The UV absorption band observed exhibits a charge
transfer transition (CT) that falls in the range 404–360 nm for
Cu() complexes, which may be assigned to ligand to metal
transition. The CT band in those Schiff base complexes of sali-
cylaldehyde with Gly and acetyllysine35 and Ala, Val, Phe, and
His36 shows a shift to higher energy attributed to the delocaliz-
ation of charge in the conjugated Schiff base ligands. The d–d
transitions generally fall below 700 nm and are more consistent
with square-pyramidal geometries about certain related Cu()
complexes.37 Compound 6 is assumed to have square-planar
geometry with a d–d transition at 620 nm similar to those
observed for the structurally well characterized square-planar
Cu() complexes.37 The molar conductance values in DMSO
and CH3OH indicate that all complexes are non-electrolytes.38
Fig. 1 An ORTEP diagram of 4.
The neutral dimers are packed in the solid state such that the
carbonyl oxygen atoms are very weakly interacting with Cu
with Cu ؒ ؒ ؒ O, 3.076(4) Å which is further assisted by comple-
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mentary N–H ؒ ؒ ؒ O bonds as well as O–H ؒ ؒ ؒ O hydrogen
bonds. This leads to one dimensional hydrogen bonded poly-
mers. The two lattice water molecules are also involved strong
O–H ؒ ؒ ؒ O bonding to form a 2D sheet as shown in Fig. 2.
polymer along the a-axis as shown in Fig. 4. The coordination
geometry of Cu() centres and structure arrangement are simi-
lar to that found in [Cu2(-Sala)2(H2O)]n.27 These coordination
polymers are further supported by O–H ؒ ؒ ؒ O and N–H ؒ ؒ ؒ O
hydrogen bonds in the crystal structure. The water molecule
and the carboxylate group are on the same side of the molecule,
and form H–O–H ؒ ؒ ؒ O–C hydrogen bonds.
Fig. 2 Packing of 4 showing the intermolecular hydrogen bonds.
[Cu2(Sab2)2(H2O)2], 5
The complex is again a centrosymmetric dimer with the di-
anionic Sab22Ϫ ligand bound to Cu(). The phenolate oxygen
bridges the two Cu() centres giving a Cu ؒ ؒ ؒ Cu distance of
3.0049(4) Å. These four donors are approximately planar with
Cu ion displaced slightly towards a more weakly bound water
molecule [Cu(1)–O(4), 2.603(4) Å], which completes the co-
ordination sphere at the axial site. The Cu() geometry can be
described as an ideal square pyramid with τ = 0.003.39–41 The
structural conformation of 5 is similar to 4 as indicated by the
superimposed structure of 5 and 4 (see ESI†).
The packing of 5 in the solid state is similar to that of 4 as
expected. The dimer is slip-stacked together through weak
Cu ؒ ؒ ؒ O᎐C bonds complemented by O–H ؒ ؒ ؒ O hydrogen
᎐
bonds to form 1D hydrogen bonded polymer. The imine H
atom is not participating in the hydrogen bond to O atom of
carboxylate group. In the absence of two lattice water mole-
cules, unlike 4, these hydrogen-bonded polymers interact with
adjacent strands through carbonyl oxygen and hydrogen atoms
of the aqua ligand to form 2D hydrogen bonded sheets.
[Cu2(Styr)2(H2O)]ؒ2H2O, 10
An ORTEP diagram of 10 with numbering scheme is shown in
Fig. 3. The structure consists of the basic dimeric building block
Cu2(Styr)2 in which both Cu() have approximate square pyr-
amidal geometry (τ = 0.075 for Cu(1) and 0.016 for Cu(2))39–41
similar to 4 and 5. However 10 contains two unique Cu()
centres. In Cu(2), the axial site is occupied by a water molecule
(site A) while the Cu(1) (site B) is bridged by an adjacent dimer
through one of the carboxylate oxygen atoms to form helical
Fig. 4 Packing of 10 showing the 1D helical coordination polymers.
The N–H hydrogen atom of the ligand from site A forms a
hydrogen bond to the O(11) of the lattice water. The proton in
the side chain of the ligand O(8) is weakly hydrogen bonded to
the other lattice water O(10). One of the hydrogens in the lattice
water O(11) is involved in strong intramolecular hydrogen-
bonding to the O(9) of the coordinated water. Both hydrogens
in the other lattice water O(10) are involved in strong hydrogen-
bonding to the carbonyl oxygen atom of the adjacent carb-
oxylate group O(2) and lattice water O(11), respectively. One of
the hydrogens in the coordinated water forms a intramolecular
hydrogen-bond to the C᎐O oxygen atom of the carboxylate
᎐
group (O6) and the other hydrogen forms an intermolecular
hydrogen-bond to the O(3) of adjacent carboxylate group as
depicted in Fig. 5.
[Cu2(Stryp)2(H2O)], 11
The crystal structure reveals that the complex is a one-
dimensional coordination polymer with the binuclear copper
Fig. 3 An ORTEP diagram with the atom labeling scheme of 10.
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Fig. 5 A perspective view of hydrogen bonding network in 10.
fragment Cu2(Stryp)2 as a building block. Both copper()
atoms adapt a distorted square pyramidal geometry as indi-
cated by the angular structural parameters τ = 0.096 for Cu(1)
and 0.052 for Cu(2).39–41 The only difference in the coordination
environment between Cu(1) and Cu(2) is the fifth donor site; a
water molecule is coordinated to Cu(1) while the neighboring
carbonyl oxygen atom is coordinated to Cu(2) to form a helical
polymer along a-axis as shown in Fig. 6. Both axial substituents
are on the same side of the dimer. Like [Cu2(Styr)2(H2O)]ؒ2H2O
complex, the coordination geometry of Cu() centres and
structural arrangement are similar to that found for in [Cu2-
(-Sala)2(H2O)]n.27
The coordination polymers are further supported by the
O–H ؒ ؒ ؒ O and N–H ؒ ؒ ؒ O hydrogen bonds in the crystal
structure. Both hydrogens in the coordinated water O(7) are
involved in strong hydrogen-bonding to oxygen atom of the
adjacent carboxylate group O(2) and O(6), respectively. The
N–H hydrogen atom of the side arm of ligand N(13A) forms a
hydrogen bond to the oxygen atom of the carboxylate group
O(5).
Fig. 6 A view of 11 showing the 1D helical coordination polymers.
corresponds to the loss of one aqua ligand and two lattice water
molecules (wt. loss calcd, 7.2%). The anhydrous sample is stable
up to 245 ЊC. The TG curve of 11 shows that the weight loss of
2.5% occurred from room temperature to 180 ЊC corresponds to
the loss of aqua ligand (calcd. wt. loss, 2.4%). The anhydrous
sample is stable up to 253 ЊC.
Thermal dehydration reactions
The TG of 4 exhibited two weight loss regions; the first one in
the range 50–120 ЊC is due to the dehydration and the second
above 245 ЊC is attributed to the decomposition of the dimer.
The observed weight loss of 11.8% for the dehydration process
matches well with the weight loss 12.2% calculated for the loss
of all the four water molecules. It is noted here that two of the
water molecules lost by 120 ЊC are indeed bound to two Cu()
atoms. The loss of metal bound aqua ligands below 120 ЊC
along with two lattice waters is very similar to those observed
during solid state supramolecular transformations.27,44 The
formation of new bonds between Cu and carbonyl oxygen
atoms after dehydration is presumed to be the driving force for
this behaviour, which is expected to form a 1D coordination
polymer based on the packing in the solid state. However, the
anhydrous 4 can easily be rehydrated on standing in air, as
revealed by TG.44 Although the structure of 5 is quite similar to
4 with respect to the aqua ligand, all of the water could only be
removed after heating the sample above 150 ЊC (ESI†). This
behaviour is different from 4. It is worth noting that the 1D
coordination polymers are formed by noncentrosymmetric
binuclear building blocks obtained from either -Sala or
-Sala.27 The formation of such coordination polymers appears
to be enantiomerically unfavourable for the centrosymmetric
dimers. The TG curve of 10 shows a weight loss of 7.2%
occurred from room temperature to 150 ЊC (ESI†). This
Magnetic properties
The room-temperature magnetic moments, per Cu, for the
Cu() complexes are in the range 1.12∼1.50 µB which indicates
moderately strong antiferromagnetic coupling is occurring in
these complexes.45 The magnetic data are compatible with the
structures reported herein. Magnetic susceptibilities were
measured for 4 and 10 over the temperature 4.2–300 K. The
data for 4 gave a maximum in susceptibility close to room tem-
perature as shown in Fig. 7 and a good fit to the Bleaney–
Bowers (Ϫ2JS1S2) model46 for a S = ½ binuclear compound.
The best fit parameters are g = 2.01, 2J = Ϫ326 cmϪ1, TIP =
0.00006 cm3 molϪ1, % monomer = 0.2. Compound 10 is even
more strongly coupled and the best fit values are g = 2.10, 2J =
Ϫ474 cmϪ1, TIP = 0.00006 cm3 molϪ1, % monomer = 2.2. The
larger amount of monomer ‘impurity’ in 10 is evident at low
temperatures where
a large increase in susceptibility is
observed. The J values are in the range commonly observed for
phenoxo bridged copper() compounds.47 The larger value of
J for 10 is in agreement with the larger Cu–O–Cu angle and
larger Cu ؒ ؒ ؒ Cu separation. The very strong antiferromagnetic
exchange coupling has also been observed in structurally
closely related dicopper() centres bridged by phenoxo
ligands.48,49
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Fig. 7 Plots of χ (ᮀ) and µeff (᭺), per Cu, vs. temperature for 4 in a
field of 1 T. The solid lines are the calculated values using the
parameters given in the text.
Catecholase-mimetic activities
Model studies of synthetic analogues have achieved notable
advances in the understanding of the structural and chemical
properties of type III protein.10–22,50–56 In these studies, mono-
or multinuclear complexes have been employed and the proper-
ties of the chelating ligands have been varied with respect to
architecture, number and nature of the donor atoms. Binuclear
copper() complex [Cu2bbpen2](ClO4)2ؒ3MeOH (Hbbpen = 1,5-
bis(2-benzimidazolyl)-3-pentanol) was studied by Krebs et al.
as structural and functional model for catechol oxidase.12 Its
reactions with 3,5-DTBC were investigated using UV-vis and
XAS spectroscopic studies in MeOH solution (as illustrated in
Scheme 2). The oxidation of 3,5-DTBC to the corresponding
o-quinone was followed by the development of strong absorp-
tion band of the product at about 390 nm.
Fig. 8 Absorbance at 390 nm of the solution containing 7 and 3,5-
DTBC (in methanol, 25 ЊC). The spectra have been recorded at various
intervals of time from 10 to 120 min.
Scheme 2 Oxidation of 3,5-DTBC to 3,5-DTBQ.
Before going to the detailed kinetic study, it is necessary to
get an estimation of the ability of the complexes to oxidize
catechol. For this purpose, 10Ϫ4 mol dmϪ3 solutions of 1–11
were treated with 50 equivalents of 3,5-DTBC under the
aerobic condition. The course of the reaction is followed by
UV-vis spectroscopy. The UV-vis spectra of the original solu-
tion directly after the addition and after 5, 15, 20 min and up to
2 h were recorded and corrected for volume changes. The
increase in the absorption at 390 nm, all the complexes 1–11
showed considerable catecholase activity. The course of oxid-
ation of 3,5-DTBC by 7 with the increase in quinone absorp-
tion band at 390 nm is shown in Fig. 8. Since 3,5-DTBQ
showed a characteristic absorption band at 390 nm and the
content of copper() complex is very small, we monitored the
increase in absorbance vs. time at this wavelength. The reactiv-
ity studies were performed in MeOH solution due to the good
solubility of the substrate, 3,5-DTBC and of its product,
DTBQ.
Fig. 9 A plot of 1/ν vs. 1/[S] for 7.
Although the real mechanism of the reaction is complicated,
the data obtained from Lineweaver–Burk plot is sufficient for a
comparison of the catalytic activity.
The catecholase activity follows the order: 7 > 6 > 8 > 3 > 5 ∼
2 ∼ 1 > 4 > 10 >9 > 11. It has been assumed that geometry
around the copper ions and intermetallic distance are the two
key factors that determine the catalytic activity of the com-
plexes.50–54 However, it should be borne in mind that the solid-
state structure need not be retained in solution, and a direct
correlation between the solid-state structure and catalytic
activity need not be expected. For all Cu() complexes, the
Cu ؒ ؒ ؒ Cu distance is likely to be around 3.0 Å which allows a
bridging catechol coordination compatible with the distance
between the two o-diphenol oxygen atoms.50–52,54,55
From the data obtained in Table 2, among all the complexes,
7 shows the highest activity and 11 shows the lowest activity,
from which one can infer that the catecholase activity is
decreasing with increasing bulky substituents and functional
groups with coordinating ability in the side chain of the
reduced Schiff base ligand. It is argued that the square-planar
geometry of copper complex is not ideal for possible catechol-
oxidase catalyst.7–13 The high activity along with a lower KM for
7 can be explained by assuming that the Cu geometry in 7
should not be square-planar. The compounds bis[1,2-O-isopro-
pylidene-6-N-(3-acetylbut-3-en-2-one)amino-6-deoxy-gluco-
The kinetics of the oxidation of 3,5-DTBC was determined
by the method of initial rates by monitoring the growth of the
390 nm band of the product 3,5-DTBQ. In order to determine
the kinetic parameters, the Michaelis–Menten approach which
originally developed for enzyme kinetics, was applied.8,13,23
A
linear relationship for the initial rates and the complexes con-
centration is obtained (ESI†) for 1–11, which show a first order
dependence on the catalyst concentration for these systems. An
example of Lineweaver–Burk plot is given in Fig. 9 for 7. Table 2
contains the results evaluated from Lineweaver–Burk plots.
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Table 2 Kinetic parameters obtained from a Lineweaver–Burk plot for the oxidation of 3,5-DTBC
Complex
kcat/hϪ1
Km/mM
Vmax 106/M sϪ1
1, [Cu2(Sgly)2(H2O)]ؒ2H2O
2, [Cu2(-Sala)2(H2O)]ؒH2O
3, [Cu2(-Sala)2(H2O)]ؒH2O
4, [Cu2(D,L-Sala)2(H2O)2]ؒ2H2O
5, [Cu2(Sab2)2(H2O)2]
563
564
666
365
564
1287
3800
678
244
265
199
23.8
20.9
25.4
14.1
19.3
10.4
8.7
24.0
20.0
20.2
17.3
31.1
29.4
34.8
17.3
27.7
6.5
193.3
26.6
9.7
6, [Cu2(Sbal)2(H2O)2]
7, [Cu2(Sab4)2(H2O)2]ؒ0.5H2O
8, [Cu2(Sval)2(H2O)3]
9, [Cu2(Shis)2(H2O)]ؒH2O
10, [Cu2(Styr)2(H2O)]ؒ2H2O
11, [Cu2(Stryp)2(H2O)]
9.8
8.9
furanoso] dicopper()] and bis[1,2-O-isopropylidene-6-N-(2,2-
bisethylcarboxy ethylene)amino-6-deoxyglucofuranoso] di-
copper()] with similar geometry also show very high catalytic
activity.55
the side arm of the reduced Schiff base ligand has a profound
effect on the catecholase activity.
Experimental
Recently Mukherjee and Mukherjee reported that the
copper() complexes are reduced to copper() complexes
during catalysis.54 It is more apparent by seeing the colour of
the solution changes from dark green to yellow and then to
brownish yellow. Their investigation confirmed that the electron
transfer from catechol to the copper() complex begins after the
formation of a copper–catechol intermediate which could be
prevented by the competitive formation of copper–quinone
complex.57 All the neutral binuclear compounds 1–11 contain
one four-membered ring, Cu2O2 and two six-membered
rings. Further, two more rings formed by the side arms of the
reduced Schiff base ligands depend on the length of the alkyl
chain in the amino acids. Except 6 and 7, five-membered rings
have been formed by all the complexes. It is obvious that the
catalytic activity increases as the length of the side arm of
the reduced Schiff base ligands increases. Two seven-membered
rings and two six-membered rings have been proposed for 6 and
7, respectively. Karlin and Casella have proposed a mechanism
for the oxidation of catechols to O-quinones by binuclear
copper() complexes.55,58–60 According to the proposed mechan-
ism, the second step is the reduction of a dicopper() to a
dicopper() system. Due to seven membered rings in the case of
6 and 7, the formation of a dicopper() system might be faster
than the other complexes and thus account for the higher
catecholase activities of 6 and 7. Further, the compounds with
functional groups with coordinating ability (9–11) show lower
activity as compared to those with non-reactive bulky groups
since these functional groups can occupy the axial positions
(inter or intramolecularly in solution) and thus prevent 3,5-
DTBC to react with Cu() centres. The trend in Kcat among
various Sala complexes is very unexpected and follows the
order 3 > 2 > 4, i.e.,  >  > ,. The difference in activity
between optically active and inactive Cu() complexes may be
attributed to the difference in the symmetry associated with the
structures. But, it is rather surprising to find that the handed-
ness of the ligand has some influence on the rate of oxidation of
3,5-DTBC.
All reagents were commercially available and were used as
received. Reagents used for the physical measurements were
of spectroscopic grade. The yields are reported with respect to
the metal salts. All the syntheses were carried out at room
temperature in air.
The 1H NMR spectra were recorded on a Bruker ACF
300FT-NMR spectrometer using TMS as an internal reference
at 25 ЊC in DMSO and the infrared spectra (KBr pellet) were
recorded using FTS165 Bio-Rad FTIR spectrophotometer
in the range 4000–450 cmϪ1. The electronic transmittance
spectra were recorded on a Shimadzu UV-2501/PC UV-vis
spectrophotometer in nujol mull. Conductance measurements
were made using a Kyoto Electronics CM-115 conductivity
meter using 1 mM solutions. The elemental analyses were
performed in the Microanalytical Laboratory, Department of
Chemistry, National University of Singapore. Water present
in the compounds was determined using a SDT 2980 TGA
Thermal Analyzer with a heating rate of 10 ЊC minϪ1 in a N2
atmosphere using a sample size of 5–10 mg per run. Room-
temperature magnetic susceptibility measurements were carried
out on a Johnson-Matthey Magnetic Susceptibility balance
with Hg[Co(SCN)4] as standard. Corrections for diamagnetism
were made using Pascal’s constants. The reported magnetic
moments are per Cu() ion. Variable temperature magnetic
measurements were made using a Quantum Design MPMS 5
SQUID magnetometer operating in an applied field of 1 T. The
samples were contained in a gel capsule and held in a soda straw
which was fixed to the end of the sample rod.
H2Sbal, H2Sab4 were synthesized according to the reported
literatures.61–62 The syntheses of ligands, N-(2-hydroxybenzyl)-
amino acids preparation, ,-H2Sala, H2Sab2, H2Shisؒ0.5H2O
and H2Styrؒ1.5H2O are described below.
To a solution of the amino acid in H2O (10 mL) containing
NaOH (0.040 g, 1.00 mmol) was added salicylaldehyde (0.122 g,
1.00 mmol) in EtOH (10 mL). The yellow solution was stirred
for 30 min at room temperature prior to cooling in an ice bath.
The intermediate Schiff base solution was carefully adjusted to
pH = 6.0–7.0 with HOAc, then excess solid NaBH4 (0.460 g,
1.20 mmol) was added in portions with gentle and stirring while
the yellow colour slowly discharged. After 10 min the solution
was evaporated and extracted with dry MeOH, then acidified
with concentrated HCl to pH of 5.0–6.0. The resulting colour-
less solid was filtered off, washed with dry MeOH and Et2O,
dried, and recrystallized from H2O/EtOH (1 : 1).
Summary
A series of Cu() complexes with reduced Schiff base ligands
N-(2-hydroxybenzyl)-amino acids have been synthesized and
characterized. Structural, spectroscopic properties of these
compounds have been investigated. These neutral binuclear
Cu() complexes have Cu ؒ ؒ ؒ Cu distance of about 3 Å and
antiferromagnetically coupled at room temperature. All the
neutral dicopper() complexes show medium to significantly
high catalytic influence on the oxidation of 3,5-DTBC to the
corresponding O-quinones. The compounds are both structural
and functional models for type III copper proteins. Our present
systematic investigation shows that the increase in the length of
D,L-H2Sala
Yield: 78 mg (40%). m.p. 241–242 ЊC. Anal. Calcd for C10H13-
NO3: C, 61.6; H, 6.6; N, 7.2. Found: C, 61.5; H, 6.7; N, 7.2%. 1H
NMR (dmso-d6): δ 1.3 (d, 3H, J = 7.1 Hz), 3.2 (q, 1H, J =
7.1 Hz), 3.9 and 4.0 (AB system, 2H, JAB = 13.5 Hz), 6.7–6.8 (m,
D a l t o n T r a n s . , 2 0 0 4 , 1 1 3 – 1 2 1
118

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2H), 7.1–7.2 (m, 2H). IR (KBr, cmϪ1): ν(OH) 3445, ν(NH) 3110,
νas(COOϪ) 1607, νs(COOϪ) 1380, ν(C–O) (phenolic) 1264.
Cu(OAc)2ؒH2O (0.199 mg, 1.00 mmol) in 15 mL of MeOH
gave the light green product 6, which was filtered off and
washed with H2O, MeOH and Et2O before drying under
vacuum. Yield: 0.18 g (66%). Anal. Calcd for C20H26N2O8Cu2:
C, 43.9; H, 4.8; N 4.8; H2O, 6.6. Found: C, 43.7; H, 4.7;
N, 5.1; H2O, 6.7%. IR (KBr, cmϪ1): ν(OH) 3448, ν(NH)
2931, νas(COOϪ) 1572, νs(COOϪ) 1355, ν(phenolic, CO) 1260.
µB = 1.37 BM.
H2Sab2
Yield: 0.136 g (65%). m.p. 231–232 ЊC. Anal. Calcd for C11-
H15NO3: C, 63.2; H, 7.2; N 6.7. Found: C, 63.0; H 7.4; N,
1
6.8%. H NMR (dmso-d6): δ 1.3 (s, 6H), 3.9 (s, 2H), 6.7–6.8
(m, 2H), 7.2–7.3 (m, 2H). IR (KBr, cmϪ1): ν(OH) 3445;
ν(NH) 2919, νas(COOϪ) 1614, νs(COOϪ) 1458, ν(phenolic, CO)
1278.
[Cu2(Sab4)2]ؒ0.5H2O, 7
To the solution of H2Sab4 (0.209 g, 1.00 mmol) and LiOH
(0.048 g, 2.00 mmol) in water (10 mL) was added a solution
Cu(OAc)2ؒH2O (0.200 g, 1.00 mmol) in water (10 mL). The
dark green reaction mixture was filtered and allowed to stand in
air at room temperature for several days to obtain dark green
fine crystals of 7. Yield: 0.162 g (59%). Anal. Calcd for
C22H27N2O6.5Cu2: C, 47.8; H, 4.8; N 4.9; H2O, 1.6. Found: C,
48.0; H, 4.9; N, 5.1; H2O, 1.5%. IR (KBr, cmϪ1): ν(OH) 3421,
ν(NH) 2936, νas(COOϪ) 1599, νs(COOϪ) 1397, ν(phenolic, CO)
1267. µB = 1.36 BM.
H2Shisؒ0.5H2O
Yield: 0.199 g (77%). m.p. 219–220 ЊC. Anal. Calcd for C12H16-
N3O3.5: C, 57.9; H, 5.7; N 15.5; H2O, 3.5. Found: C, 57.9; H 5.6;
N, 15.6; H2O, 3.5%. 1H NMR (D2O): δ 2.92 (d, 2H, J = 7.0 Hz),
3.00 (t, 1H, J = 7.0 Hz), 3.79 and 3.99 (AB system, 2H,
JAB = 13.6 Hz), 6.8 (s, 1H), 6.8–6.9 (m, 2H), 7.1–7.2 (m,
2H), 7.56 (s, 1H). IR (KBr, cmϪ1): ν(OH) 3420; ν(NH) 3116,
νas(COOϪ) 1603, νs(COOϪ) 1380, ν(phenolic, CO) 1275.
H2Styrؒ1.5H2O
[Cu2(Shis)2(H2O)2]ؒH2O, 9
Yield: 0.172 g (55%). m.p. 230–232 ЊC. Anal. Calcd for C16H20-
NO5.5: C, 61.2; H, 6.4; N 4.4; H2O, 8.6. Found: C, 61.3;
H 6.2; N, 4.2; H2O, 8.7%. 1H NMR (D2O): δ 2.9 (d, 2H,
J = 7.0 Hz), 3.4 (t, 1H, J = 7.0 Hz), 3.6 and 3.8 (AB system, 2H,
JAB =13.6 Hz), 6.7–6.8 (m, 4H), 7.0–7.1 (m, 4H). IR (KBr,
cmϪ1): ν(OH) 3420; ν(NH) 3167, νas(COOϪ) 1617, νs(COOϪ)
1374, ν(phenolic, CO) 1255.
To the solution of H2Shis (0.255 g, 1.00 mmol) and LiOH
(0.048 g, 2.00 mmol) in water (10 mL) was added a solution
Cu(OAc)2ؒH2O (0.200 g, 1.00 mmol) in water (10 mL). The light
green reaction mixture was filtered and allowed to stand in
air at room temperature for several days to get a light green
powder of 9. Yield: 0.199 g (57%). Anal. Calcd for C26H32-
N6O9Cu2: C, 44.4; H, 4.6; N 12.0; H2O, 7.7. Found: C, 44.6; H,
4.6; N, 12.0; H2O, 7.9%. IR (KBr, cmϪ1): ν(OH) 3401, ν(NH)
2895, νas(COOϪ) 1595, νs(COOϪ) 1382, ν(phenolic, CO) 1250.
µB = 1.50 BM.
H2Stryp
Yield: 0.211 g (68%). m.p. 235–237 ЊC. Anal. Calcd for C18H18-
N2O3: C, 69.7; H, 5.8; N 9.0. Found: C, 69.9; H 5.9; N, 7.9%.
1H NMR (dmso-d6): δ 2.93 (d, 2H, J = 7.0 Hz), 3.37 (t, 1H,
J = 7.0 Hz), 3.7 and 3.9 (AB system, 2H, JAB =13.6 Hz), 6.6–
6.7 (m, 4H), 6.9–7.4 (m, 5H). IR (KBr, cmϪ1): ν(OH) 3426;
ν(NH) 3111, νas(COOϪ) 1604, νs(COOϪ) 1370, ν(phenolic, CO)
1275.
[Cu2(Styr)2(H2O)]ؒ2H2O, 10
This complex was prepared as same procedure as described for
7 using H2Styrؒ1.5H2O instead of H2SbalؒH2O. Dark green
needle crystals of 10 were obtained. Yield: 0.286 g (76%). Anal.
Calcd for C32H36N2O11Cu2: C, 51.1; H, 4.8; N 3.7; H2O, 7.2.
Found: C, 51.3; H, 4.9; N, 3.6; H2O, 7.2%. IR (KBr, cmϪ1):
ν(OH) 3418, ν(NH) 3160, νas(COOϪ) 1605, νs(COOϪ) 1451 and
1385, ν(phenolic, CO) 1275. µB = 1.12 BM.
[Cu2(D,L-Sala)2(H2O)2]ؒ2H2O, 4
To the solution of ,-H2Sala (0.195 g, 1.00 mmol) and LiOH
(0.048 g, 2.00 mmol) in water (10 mL) was added a solution
Cu(OAc)2ؒH2O (0.200 g, 1.00 mmol) in water (10 mL). The
dark green reaction mixture was filtered and allowed to stand in
air at room temperature for several days to obtain the needle-
like green crystals of 4. Yield: 0.214 g (73%). Anal. Calcd for
C20H30N2O10Cu2: C, 41.0; H, 5.1; N 4.8; H2O, 12.3. Found: C,
41.3; H, 5.0; N, 4.6; H2O, 12.1%. IR (KBr, cmϪ1): ν(OH) 3447,
ν(NH) 2935, νas(COOϪ) 1612, νs(COOϪ) 1386, ν(phenolic, CO)
1259. µB = 1.32 BM.
[Cu2(Stryp)2(H2O)], 11
This complex was prepared as same procedure as described for
5 using H2Styrؒ1.5H2O instead of H2Stryp. Dark green needle
crystals of 11 were obtained. Yield: 0.274 g (72%). Anal. Calcd
for C36H34N4O7Cu2: C, 56.7; H, 4.5; N 7.4; H2O, 2.4. Found: C,
56.8; H, 4.4; N, 7.4; H2O, 2.5%. IR (KBr, cmϪ1): ν(OH) 3451,
ν(NH) 3219, νas(COOϪ) 1619, νs(COOϪ) 1484 and 1381,
ν(phenolic, CO) 1271. µB = 1.27 BM.
[Cu2(Sab2)2(H2O)2], 5
X-ray crystallography
H2Sab2 (0.209 g, 1.00 mmol) and LiOH (0.048 g, 2.00 mmol)
were stirred in water for 15 min, and then the reaction mixture
was filtered. To the filtrate was added a solution of Cu(OAc)2ؒ
H2O (0.200 g, 1.00 mmol) in water (10 mL). The reaction mix-
ture turned deep blue immediately. After filtration, the solution
was allowed to stand in air at room temperature for several days
to furnish needle-like green crystals of 5. Yield: 0.196 g (68%).
Anal. Calcd for C22H30N2O8Cu2: C, 45.7; H, 5.2; N 4.8; H2O,
6.2. Found: C, 45.9; H, 5.3; N, 4.8; H2O, 6.3%. IR (KBr, cmϪ1):
ν(OH) 3421, ν(NH) 2933, νas(COOϪ) 1615, νs(COOϪ) 1386,
ν(phenolic, CO) 1260. µB = 1.35 BM.
The diffraction experiments carried out on a Bruker AXS
SMART CCD diffractometer. The program SMART63 was
used for collecting frames of data, indexing reflection and
determination of lattice parameter, SAINT64 for integration of
the intensity of reflections and scaling, SADABS64 for absorp-
tion correction and SHELXTL65 for space group and structure
determination, least-squares refinements on F 2. There are two
lattice water molecules found in the lattice of 1 and 2, respect-
ively. All the hydrogen atom positions of water molecules were
located and their positional parameters were refined in the
least-squares cycles. Selected crystallographic data and refine-
ment details are displayed in Table 3.
[Cu2(Sbal)2(H2O)2], 6
CCDC reference numbers 209712–209715 (4, 5, 10 and 11).
See http://www.rsc.org/suppdata/dt/b3/b310262a/ for crystal-
lographic data in CIF or other electronic format.
The addition of a solution of H2Sbal (0.200 g, 1.00 mmol) and
LiOH (48 mg, 2.00 mmol) in water (15 mL) to a solution of
D a l t o n T r a n s . , 2 0 0 4 , 1 1 3 – 1 2 1
119

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Table 3 Crystallographic data and structure refinement details for 4, 5, 10 and 11
Complex
4
5
10
11
Formula
Formula weight
C20H30N2O10Cu2
585.54
C22H30N2O8Cu2
577.56
C32H36N2O11Cu2
751.72
C36H34N4O7Cu2
761.75
T/K
Crystal system
Space group
223(2)
223(2)
223(2)
Monoclinic
P21
293(2)
Monoclinic
P21
Triclinic
Triclinic
¯
¯
P1
P1
a/Å
b/Å
c/Å
α/Њ
7.2289(3)
8.9892(3)
9.8982(4)
73.497(2)
70.294(1)
82.701(2)
580.24
7.6081(6)
8.3925(7)
9.7927(8)
112.186(1)
93.542(2)
104.951(2)
550.45(8)
1
10.4143(5)
11.0346(5)
14.7206(7)
90
105.881(1)
90
1627.09(1)
2
1.370
10.3549(3)
10.5453(2)
15.9521(4)
90
104.367(1)
90
1687.42(7)
2
1.316
β/Њ
γ/Њ
V/Å3
Z
1
µ/mmϪ1
1.891
1.986
Reflns collected
Independent reflns
Rint
3349
2047
0.0528
0.0423, 0.1161
5868
1924
0.0362
0.0318, 0.0770
9364
5049
0.0259
0.029, 0.0668
9642
4908
0.0307
0.0387, 0.0846
b
Final R [I > 2σ], R1,a wR2
2
a R1 = Σ||Fo| Ϫ |Fc||/ΣFo|. b wR2 = [Σw(Fo2 Ϫ Fc2)2/Σw(Fo )2]1/2. The absolute structure parameters for 10 and 11 are 0.03(1) and Ϫ0.009(15), respectively.
23 C. Gerdemann, C. Eicken and B. Krebs, Acc. Chem. Res., 2002, 35,
Acknowledgements
183.
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This research was supported by grant R-143-000-153-112 to
JJV from the National University of Singapore and by an
Australian Research Council Discovery grant to K.S.M.
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