Copper(II) complexes of Schiff-base and reduced Schiff-base ligands: Influence of weakly coordinating sulfonate groups on the structure and oxidation of 3,5-DTBC

Article abstract of DOI:10.1002/ejic.200500638

The copper(II) complexes of the Schiff-base ligands H₂Sams and H₂Saes and the reduced Schiff-base ligands H₂Sam and H₂Sae formed between salicylaldehyde and aminomethanesulfonic acid or 2-aminoethanesulfonic aci

Full text of DOI:10.1002/ejic.200500638

FULL PAPER
Copper(II) Complexes of Schiff-Base and Reduced Schiff-Base Ligands:
Influence of Weakly Coordinating Sulfonate Groups on the Structure and
Oxidation of 3,5-DTBC
Bellam Sreenivasulu,[a] Muthalagu Vetrichelvan,[a] Fei Zhao,[b] Song Gao,*[b] and
Jagadese J. Vittal*[a]
Keywords: Biomimetic studies / Copper / Schiff-base complexes / Organosulfonate ligands / Catecholase activity / Magnetic
properties
The copper(II) complexes of the Schiff-base ligands H2Sams
and H2Saes and the reduced Schiff-base ligands H2Sam and
hydrogen-bonded 2D structure respectively. All these com-
plexes have been investigated for their catecholase activity
H2Sae formed between salicylaldehyde and aminometh- and activity measurements have been compared with those
anesulfonic acid or 2-aminoethanesulfonic acid (taurine)
have been synthesized in moderate yields. The solid-state
of dinuclear copper(II) complexes of similar ligands obtained
with carboxylate analogues of the corresponding sulfonic
structures of the five dinuclear complexes, [Cu2(Sams)2(H2O)2] acids; these studies show that 4 has significantly higher ac-
(1), [Cu2(Sam)2(H2O)2]·H2O (2), [Cu2(Saes)2(H2O)2]·2H2O (3),
[Cu2(Sae)2]·2H2O (4), and [Cu2(Sae)2(DMF)2]·2DMF (5), have
been determined by X-ray crystallography, showing that the
CuII centers have distorted square-pyramidal geometry. The
Schiff-base copper complexes 1 and 3 have hydrogen-
bonded 2D sheet structures while the reduced Schiff-base
complexes 4 and 5 display a 2D coordination network and a
tivity. Further, a strong antiferromagnetic interaction be-
tween CuII ions in dimeric complexes 1 [J = –9.04(2) cm–1], 3
[J = –272(4) cm–1), and 4 [J = –237(4) cm–1] has been ob-
served.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
hydrogen-bond donor–acceptor functionalities, these li-
gands can form conformationally flexible five- and six-
membered rings upon complexation. Several copper com-
plexes of reduced Schiff-base ligands formed between sali-
cylaldehyde and amino acids were explored to serve as
models for the intermediate species in biological racemiza-
tion and transamination reactions.[3] Making use of these
advantages, we employed such reduced Schiff-base ligands
to synthesize dinuclear CuII and ZnII complexes that exhibit
interesting structural transformations in the solid state.[4]
Further, in our recent report, we showed that incorporation
of an additional carboxylate group in the side arm of the
ligand N-(2-hydroxybenzyl)--glutamic acid formed be-
tween salicylaldehyde and -glutamic acid resulted in a
novel structural display of a spiral staircase conformation.[5]
Copper() complexes of the reduced Schiff-base ligand de-
rived from salicylaldehyde and histidine have been found to
self-assemble to form a capsule incorporating four pyridine
molecules.[6]
Introduction
In the past decades, significant progress has been made
in understanding the coordination chemistry of copper()
complexes of various Schiff-base ligands.[1] Most of the
model studies of the metal complexes of Schiff-base ligands
containing salicylaldehyde and amino acids have focused
upon the binding mode of these ligands.[2] X-ray crystal
structures of complexes thus obtained demonstrated that
the Schiff-base ligand acts as a tridentate moiety, coordinat-
ing through the phenolato oxygen, imine nitrogen, and car-
boxylate oxygen.
Our research group is interested in tridentate reduced
Schiff-base ligands, specifically N-(2-hydroxybenzyl)amino
acids, as they are more flexible because of the reduction of
the C=N bond of the Schiff base, and help to overcome
the ligand instability. Further, in addition to their potential
[a] Department of Chemistry, National University of Singapore,
3 Science Drive 3, Singapore 117543, Singapore
Fax: +65-6779-1691
Various dicopper active sites found in copper-containing
metalloenzymes have similar structural aspects with three
histidine donors for each of the two Cu centers, which are
separated by a distance of about 3.5 Å, and catechol ox-
idase as one of the prominent members of the type III cop-
per proteins that catalyze the two-electron oxidation of or-
tho-diphenols to the corresponding quinones.[7] Conse-
E-mail: chmjjv@nus.edu.sg
[b] College of Chemistry and Molecular Engineering, Peking Uni-
versity,
Beijing 100871, P.R. China
Fax: +86-10-6275-1708
E-mail: gaosong@pku.edu.cn
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2005, 4635–4645
DOI: 10.1002/ejic.200500638
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4635

B. Sreenivasulu, M. Vetrichelvan, F. Zhao, S. Gao, J. J. Vittal
FULL PAPER
quently, quite a number of dinuclear copper complexes have duced Schiff-base complex [Cu2(Sam)2(H2O)2]·H2O (2) was
been investigated as biomimetic catalysts for catechol oxi- obtained by treating a copper() nitrate trihydrate solution
dation by employing the most common and convenient with H2Sam generated in situ. On the other hand, reduced
model substrate, 3,5-di-tert-butylcatechol (3,5-DTBC), Schiff-base H2Sae was synthesized first and then complexed
which can be oxidized to 3,5-di-tert-butylquinone (3,5- with copper() acetate monohydrate to obtain [Cu2(Sae)2]·
DTBQ).[8] With regard to this, we have recently reported a 2H2O (4). The complex [Cu2(Sae)2(DMF)2]·2DMF (5) re-
series of dinuclear copper() complexes of the tridentate sulted when 4 was recrystallized from the DMF/acetone
and binucleating reduced Schiff-base ligands and their func- solvent mixture. We were unable to isolate N-(2-hy-
tional relationship with catecholase activity.[9] Apart from droxybenzyl)aminopropanesulfonic acid or prepare its CuII
the reduced Schiff-base ligands with carboxylate donor complex in situ as in the case of the H2Sam ligand. Com-
group, their sulfonic acid analogues are expected not only plexes 1–4 show absorption bands at 3400, 3443, and
to improve solubility in aqueous media but also form inter- 3410 cm–1 corresponding to the presence of water mole-
esting supramolecular architectures by modifying the con- cules. The C=N stretching frequencies are observed at
nectivity at the metal centers as well as the hydrogen-bond- 1629 cm–1 in 1 and 1617 cm–1 in 3. As the C=N bond is
ing pattern.[10] However, the coordination chemistry of reduced in 2, 4, and 5, the N–H stretching frequencies ap-
transition metals with a similar ligand system containing peared in the range of 2920–2935 cm–1. The frequencies
organosulfonate species is not well documented.[11] Despite characteristic of the S–O stretching modes are observed in
the available literature showing the organosulfonate anions the range 1000–1380 cm–1.[14]
as good hydrogen-bonding acceptors to form strongly hy-
drogen-bonded networks,[12] there are few metal-based bands at 678 and 688 nm followed by intense bands at 367
supramolecular arrays in the literature.[13]
and 322 nm respectively. The high intensity can be attrib-
The electronic spectra of 1 and 3 in water display weak
Encouraged by our previous results, we were prompted uted to the delocalization of charge in the conjugated Schiff
to synthesize such tridentate N-(2-hydroxybenzyl)amino- base. Copper() complexes of Schiff bases and related li-
methane/ethanesulfonic acids (Scheme 1), and their corre- gands exhibit weak bands in the range 620–680 nm which
sponding dinuclear copper() complexes of both Schiff-base are due to d-d transitions.[15] For an octahedral geometry,
2
2
and reduced Schiff-base ligands. In order to evaluate these the expected Eg to T2g transition takes place at around
complexes for the effect of the sulfonate donor group, com- 800 nm. This band will undergo a significant blue shift
pared to the corresponding carboxylate analogues, on the when octahedral geometry distorts to square pyramidal and
crystal structures and the consequent influence on the cate- square planar structure.[16] For the reduced Schiff-base cop-
cholase activity, herein we report and highlight different per() complexes with square pyramidal geometry, the d-d
structural features between Schiff-base and reduced Schiff- transitions and charge transfer transitions generally occur
base dicopper() complexes, and their catecholase activity. in the ranges 650–720 and 360–450 nm respectively.[9,17] In
Variable temperature magnetic studies have also been dis- the case of 2, 4, and 5 the absorption bands at 717–736 nm
cussed for the CuII complexes containing syn-syn O–S–O- correspond to d-d transitions and a strong band at 405–
bridged 1 and 3 and phenoxo-bridged 4, which can mediate 420 nm is due to ligand-to-metal charge transfer.
magnetic interaction effectively within the dimeric Cu unit.
TG analysis of 1 showed a weight loss of 6.8% (calcu-
lated 6.1%) for the loss of two aqua ligands in the tempera-
ture range 159–192 °C. In 2, the loss of three water mole-
cules, including two aqua ligands and a lattice water, was
indicated by a weight loss of 8.1% (calculated 8.4%) in the
temperature range 148–183 °C. Similarly, a weight loss of
11.7% (calculated 11.0%) corresponding to two aqua li-
gands and two lattice water molecules has been found to
occur at 30–155 °C in 3. TG weight loss of 5.7% (calculated
5.8%) agrees well with the loss of two lattice water mole-
cules in 4.
Description of X-ray Crystal Structures
Scheme 1. Ligands employed for complexation.
Selected crystallographic data and refinement details are
displayed in Table 1.
Crystal Structure of [Cu2(Sams)2(H2O)2] (1)
Results and Discussion
Dark green prismatic single crystals of 1 suitable for sin-
Copper complexes 1 and 3 were prepared by treating gle-crystal X-ray crystallographic studies were obtained
copper() nitrate trihydrate solution with the Schiff bases from the filtrate of the reaction mixture on slow evapora-
formed in situ between salicylaldehyde and aminometh- tion. Compound 1 is a centrosymmetric dimer with copper
anesulfonic acid and/or aminoethanesulfonic acid. The re- centers assuming square pyramidal geometry (τ = 0.16).[18]
4636
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 4635–4645

Copper() Complexes of Schiff-Base and Reduced Schiff-Base Ligands
FULL PAPER
Table 1. Crystallographic data and structure refinement details for 1, 3, 4, and 5.
Complex
1
3
4
5
Formula
Formula mass
T [K]
C16H18Cu2N2O10S2
589.52
223(2)
C9H13CuNO6S
326.8
223(2)
C18H23Cu2N2O9S2
602.6
223(2)
C30H46Cu2N6O12S2
873.93
223(2)
Wavelength, λ [Å]
Crystal system
Space group
0.71073
0.71073
triclinic
P1
0.71073
monoclinic
P21/n
0.71073
monoclinic
P21/n
triclinic
¯
¯
P1
a [Å]
b [Å]
c [Å]
α [°]
7.0092(7)
9.0004(8)
9.0516(8)
97.012(2)
109.829(2)
107.720(2)
495.24(8)
1
6.9079(7)
9.226(1)
10.439(1)
109.042(2)
101.162(2)
104.906(2)
579.0(1)
1
10.8222(7)
8.7872(6)
11.9314(8)
90
108.024(1)
90
1079.0(1)
2
1.855
13.2137(9)
9.7141(7)
14.944(1)
90
99.130(1)
90
1893.9(2)
2
1.532
β [°]
γ [°]
V [Å3]
Z
Dcalcd [gcm–3]
1.977
2.420
1.875
2.086
µ [mm–1]
2.220
1.299
Reflections collected
Independent reflections
Rint
2880
1754
0.0233
1.024
0.0346
0.0822
4727
3168
0.0174
1.034
0.0380
0.0848
6007
1902
0.0195
0.741
0.0226
0.0612
14925
5330
0.0238
1.013
0.0399
0.1023
Gof
[a]
Final R[I Ͼ 2σ], R1
[b]
wR2
2
2 2
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = [Σw(Fo – Fc ) /Σw(Fo2)2]1/2
.
At each copper center, the basal plane of the square pyra-
The dimers are packed in the solid state to give a hydro-
mid is occupied by three oxygen atoms, one each from the gen-bonded 2D structure in the ac plane (Figure 2). Two
phenolate group [Cu1–O1, 1.898(2) Å], the sulfonate group types of complementary hydrogen bonds are present: the
[Cu1–O2, 2.019(2) Å], and the aqua ligand [Cu1–O5, first one is between the phenoxo oxygen and one of the
1.932(2) Å]; and nitrogen [Cu1–N1, 1.936(3) Å] from the hydrogen atoms of the aqua ligand from the adjacent dimer
imine group. The apical sites of the CuII centers have anti along the c-direction; the next one is between the free oxy-
configuration and are occupied by the oxygen atoms [Cu1– gen atom of the sulfonate group with another hydrogen
O3A, 2.393(3) Å; Cu1A–O3, 2.393(3) Å] of the sulfonate atom of the aqua ligand, normal to (100) plane. In addition,
group from the neighboring molecule as shown in Figure 1. these 2D sheets are also supported by C–H···O hydrogen-
Selected bond lengths and bond angles are given in Table 2. bonding interactions, C(3)–H(3)···O(4), 2.53 Å; C(7)–H(7)···
O(4), 2.52 Å, which are considered medium strong when
compared with the shorter and stronger C–H···O interac-
Figure 1. An ORTEP diagram of 1.
Unlike the complexes obtained with reduced Schiff bases
displaying the phenoxo-bridged dinuclear CuII centers with
the usual Cu···Cu distance of about 3 Å, H2Sams gave rise
to the dinuclear copper core with Cu···Cu separation of
5.12 Å without bridging phenolate moiety. Instead, the cen-
trosymmetric dinuclear core forms an interesting eight-
membered ring consisting of Cu1, O2, S1, O3, Cu1A, O2A,
S1A, and O3A (Figure 1). Further, compared with the phe-
nolato-bridged structures formed by the reduced Schiff-
base CuII compounds, formation of an entirely different
structure of 1 may be attributed to kinetic factors, as several
phenolato-bridged dinuclear complexes have been reported
in the literature.[19–21]
Figure 2. Hydrogen-bonded 2D sheet structure of 1 in ac plane.
4637
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

B. Sreenivasulu, M. Vetrichelvan, F. Zhao, S. Gao, J. J. Vittal
FULL PAPER
Table 2. Selected bond lengths [Å] and angles [°] in 1, 3, 4, and 5.
Compound 1
Cu(1)–O(1)
Cu(1)–O(5)
Cu(1)–N(1)
Cu(1)–O(2)
Cu(1)–O(3)[a]
O(3)–Cu(1)[a]
N(I)–C(7)
1.898(2)
1.932(2)
1.936(3)
2.019(2)
2.393(3)
2.393(3)
1.291(4)
1.452(4)
S(1)–O(2)–Cu(1)
S(1)–O(3)–Cu(1)[a]
O(1)–Cu(1)–O(5)
O(1)–Cu(1)–N(1)
O(5)–Cu(1)–N(1)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(3)[a]
O(5)–Cu(1)–O(3)[a]
N(1)–Cu(1)–O(3)[a]
117.7(1)
131.6(1)
91.3(1)
92.2(1)
165.8(1)
175.2(1)
96.82(1)
89.1(1)
N(1)–C(8)
Cu(1)···Cu(1)[a] 5.12
104.2(1)
Figure 3. An ORTEP diagram of 3.
Compound 3
and thus lowers the τ-value to 0.03. As observed in 1, the
dinuclear core in 3 also shows the axial coordination mode
by sulfonato oxygen atoms in the centrosymmetric dimer.
The introduction of a –CH2 group in the H2Saes ligand
changed a five-membered ring to a six-membered ring and
hence increased the Cu···S distances (3.01 Å in 1 vs 3.15 Å
in 3). This is also reflected in the widening of the Cu(1)–
O(2)–S(1) angle from 117.7° in 1 to 131.3° in 3 and an in-
crease in the Cu···Cu separation to 5.33 Å.
Compound 3 also forms hydrogen-bonded sheets similar
to 1 (Figure 4). However, a water molecule is strongly hy-
drogen bonded between the phenoxo oxygen atom and the
aqua ligand. The second hydrogen atom of the lattice water
is disordered and found to be hydrogen bonded to another
lattice water molecule as well as to other oxygen atoms of
the sulfonate group. Table 3 shows the hydrogen bond pa-
rameters.
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–N(1)
Cu(1)–O(5)
Cu(1)–O(3)[a]
C(7)–N(1)
1.886(2)
S(1)–O(2)–Cu(1)
S(1)–O(3)–Cu(1)[a]
O(1)–Cu(1)–O(2)
N(1)–Cu(1)–O(5)
O(1)–Cu(1)–N(1)
O(2)–Cu(1)–N(1)
O(1)–Cu(1)–O(5)
O(2)–Cu(1)–O(5)
O(1)–Cu(1)–O(3)[a]
O(2)–Cu(1)–O(3)[a]
N(1)–Cu(1)–O(3)[a]
O(5)–Cu(1)–O(3)[a]
131.3(1)
134.76(1)
168.87(8)
166.92(9)
94.08(9)
97.04(8)
85.74(9)
83.56(8)
87.32(8)
89.54(7)
103.30(8)
89.76(8)
1.967(2)
1.968(2)
1.989(2)
2.410(2)
1.282(3)
1.479(3)
2.410(2)
N(1)–C(8)
O(3)–Cu(1)[a]
Cu(1)···Cu(1)[a] 5.33
Compound 4
Cu(1)–O(1)[a]
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–O(2)
Cu(1)–O(4)[b]
O(1)–Cu(1)[a]
O(4)–Cu(1)[c]
Cu(1)–O(1)[a]
C(7)–N(1)
1.931(1)
S(1)–O(2)–Cu(1)
S(1)–O(4)–Cu(1)[c]
O(1)[a]–Cu(1)–O(1)
O(1)[a]–Cu(1)–N(1)
O(1)–Cu(1)–N(1)
O(1)[a]–Cu(1)–O(2)
O(1)–Cu(1)–O(2)
120.58(8)
132.93(9)
78.09(6)
172.15(7)
94.31(6)
95.32(6)
152.72(7)
1.972(1)
1.986(1)
2.002(1)
2.330(1)
1.931(1)
2.330(1)
1.931(1)
1.494(3)
1.476(3)
2.330(1)
O(1)[a]–Cu(1)–O(4)[b] 94.63(6)
O(1)–Cu(1)–O(4)[b]
N(1)–Cu(1)–O(4)[b]
O(2)–Cu(1)–O(4)[b]
O(2)–Cu(1)–Cu(1)[a]
99.55(6)
84.60(6)
107.40(6)
129.59(5)
N(1)–C(8)
O(4)–Cu(1)[c]
Cu(1)···Cu(1)[a] 3.031(5)
O(4)[b]–Cu(1)–Cu(1)[a] 99.17(4)
Compound 5
Cu(1)–O(1)[a]
Cu(1)–O(1)
Cu(1)–N(1)
C(7)–N(1)
1.958(1)
1.981(1)
1.991(1)
1.496(3)
O(1)[a]–Cu(1)–O(1)
77.96(7)
95.31(7)
172.38(7)
156.95(8)
O(1)[a]–Cu(1)–O(2)
O(1)[a]–Cu(1)–N(1)
O(1)–Cu(1)–O(2)
Cu(1)···Cu(1)[a] 3.06
[a] Symmetry transformations used to generate equivalent atoms
–x + 1, –y, –z in 1 and 3; –x + 1, –y + 1, –z + 1 in 4 and 5. [b] –x
+ 3/2, y – 1/2, –z + 3/2. [c] –x + 3/2, y + 1/2, –z + 3/2.
tions of about 2.0 Å available in the literature.[22] Hydrogen
bond parameters in 1 are tabulated in Table 3.
Crystal Structure of [Cu2(Saes)2(H2O)2]·2H2O (3)
Figure 4. Hydrogen-bonded 2D sheets in 3.
Compound 3 is also a binuclear complex of formula [Cu2-
(Saes)2(H2O)2]·2H2O with a crystallographic center of in-
version as in 1. An ORTEP diagram of 3 with numbering
scheme is shown in Figure 3. Selected bond lengths and
Crystal Structure of [Cu2(Sae)2]·2H2O (4)
When the C=N double bond is reduced, the complex-
bond angles are given in Table 2. Each CuII center has ation behavior of the Sae dianion is completely different
square pyramidal geometry.
from that of the Saes dianion. The Sae ligand with flexible
The ligand connectivity in 3 is very similar to that found backbone behaves like other reduced Schiff-base ligands in
in 1 except that this Saes dianion forms two six-membered terms of forming a phenolato-bridged CuII dimer having
rings. The ring expansion is due to an additional methylene the expected connectivity of the donor atoms.[4,9] A perspec-
group in the H2Saes ligand, which reduces the ring strain tive view of 4 exhibiting the coordination environments
4638
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 4635–4645

Copper() Complexes of Schiff-Base and Reduced Schiff-Base Ligands
Table 3. Hydrogen bond lengths [Å] and angles [°] in 1, 3, 4, and 5.
FULL PAPER
Complex D–H
d(D–H)
d(H–A)
ЄDHA
d(D–A)
A
Symmetry
1
O5–H5A
0.80(4)
0.78(4)
0.94
1.98(4)
1.85(3)
2.53
171(4)
178(6)
147
2.775(5)
2.626(4)
3.358(5)
3.453(4)
2.761(3)
2.666(4)
2.927(7)
3.108(4)
2.987(5)
3.144(2)
3.007(3)
3.438(3)
3.406(3)
3.482(4)
O4
O1
O4
O4
O4
O6
O6
O3
O1
O2
O3
O4
O4
O4
–x, –y, –z
O5–H5B
C3–H3[a]
C7–H7[a]
O5–H5A
O5–H5B
O6–H6A
O6–H6B
O6–H6B
N1–H1[a]
N1–H1
1 – x, –y, 1 – z
1 – x, 1 – y, 1 – z
1 – x, 1 – y, –z
–x, –y, –z
–x, –y, –z
–x, –y, 1 – z
3
4
5
0.94
2.52
172
0.81(3)
0.81(2)
0.81(3)
0.81(4)
0.81(4)
0.92
0.92
0.98
0.94
0.97
2.00(3)
1.87(2)
2.17(3)
2.45(4)
2.31(5)
2.32
2.20
2.20
2.54
2.57
156(3)
166(4)
157(5)
140(4)
142(4)
144
145
165
153
156
x, y, 1 + z
1 – x, –y, 1 – z
3/2 – x, –1/2 + y, 3/2 – z
3/2 – x, 1/2 + y, 3/2 – z
3/2 – x, 1/2 + y, 3/2 – z
1 – x, 1 – y, 1 – z
1 – x, 1 – y, 1 – z
C9–H9B[a]
C10–H10[a]
C12–H12A[a]
[a] The hydrogen atoms were placed in calculated positions.
around the CuII atoms is shown in Figure 5. Selected bond
lengths and bond angles in 4 are shown in Table 2. The
phenolato oxygen atoms, a nitrogen atom from the imine
group, and an oxygen atom of the sulfonate group form the
basal square. The apical site in each CuII center is occupied
by another oxygen atom of the sulfonate group from the
neighboring dimer. This interdimer connectivity leads to
the formation of a 2D (4, 4) network structure as shown in
Figure 6. The two axial oxygen atoms have trans geometry
due to the crystallographic center of inversion present at
the center of the Cu2O2 ring. Such geometry has also been
observed in several CuII dimers having nonchiral reduced
Schiff-base ligands.[9] Interestingly the infinite 2D network
structure with (4, 4) net formed in 4 is the first of its kind
observed in the CuII and ZnII complexes containing re-
duced Schiff-base ligands although these nets are ubiqui-
tous in the literature.[23]
Figure 6. A portion of the 2D structure in 4.
intermolecular N–H···O hydrogen bond between the NH2
and sulfonate groups in the solid state and in solution has
been utilized as a crystal engineering tool.[12] The properties
and the functions of the N–H···O hydrogen bonds involving
–
coordinated –SO3 groups in a dinuclear calcium complex
have been reported.[24] As in 1, medium strong C–H···O hy-
drogen bonds have also been observed in the solid state.
Crystal Structure of [Cu2(Sae)2(DMF)2]·2DMF (5)
Complex 5 is formed upon recrystallization of 4 from
strongly coordinating solvent DMF. The axial sulfonate
groups in 4 are replaced by DMF molecules in 5 and hence
discrete dinuclear compounds are formed. Two more DMF
molecules were found in the crystal lattice. The crystal
packing of 5 generated 2D hydrogen-bonded polymeric
¯
structures parallel to the (101) planes. The solid-state struc-
ture exhibits N–H···O hydrogen bonding between one of
the sulfonate oxygen and NH hydrogen atoms (Table 3).
Magnetic Properties of 1, 3, and 4
The temperature dependences of χm and χmT for crystal-
line samples of 1, 3, and 4 are presented in Figure 7. For
compound 1, χm increases upon cooling and reaches a
Figure 5. An ORTEP diagram of 4.
The N–H proton is weakly hydrogen bonded to one of maximum at 16 K; the data above 30 K can be well fitted
the sulfonate oxygen atoms (N1–H1···O2, 2.324 Å). The with the Curie–Weiss law [χm = C/(T–θ)], giving C =
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FULL PAPER
0.802 cm3 mol–1K, and θ = –12.7(5) K. The maximum and trace paramagnetic impurity (ρ). Overall, the data suggest
negative Weiss constant suggests an antiferromagnetic a strong antiferromagnetic coupling between the adjacent
coupling between CuII ions. The susceptibility data in the CuII ions in 3 and 4. Based on the structure of 3, two CuII
2–330 K range were fitted with a Cu2 dimer model (H = ions are bridged by two sulfonate O–S–O bridges in syn-
–2JS1S2), giving the intradimer coupling J = –9.04(2) cm–1, syn mode with Cu···Cu separation at 5.32 Å. Although the
and g = 2.002(3) with R = 2.3×10–4 (R = Σ[(χm)obs
(χm)calc]2/Σ[(χm)obs]2).
–
distance is quite large compared to the phenoxo-bridged
Cu2 in 4 with a distance of 3.03 Å, the basal planes of the
two CuII ions are almost parallel, which might favor the
overlap of dx2–y2 orbitals of CuII ions within the dimer. On
the other hand, the coupling between the dinuclear moiety
in the 2D layer of 4 is negligible, as expected, because of
both the anti–syn sulfonate bridging mode and the orthogo-
nal relationship of the planes of the neighboring Cu2O2.[4d]
Therefore, the magnetic data of 3 and 4 can be analyzed by
fitting the susceptibilities to an equation calculated using
an H = –2JS1S2 Hamiltonian for a dimer plus impurity of
monomer model. The parameters have their usual mean-
ings. As shown in Figure 7b and 7c, quite good fits were
obtained for 3: J = –272(4) cm–1, g = 2.05(5), ρ = 0.0164;
Na = 22×10–6 with R = 7.4×10–3; and for 4: J =
–237(4) cm–1, g = 2.21(3), ρ = 0.0163; Na = 149×10–6 with
R = 2.4×10–3.
Catecholase Activity
Many copper-containing metalloenzymes, characterized
by the dinuclear active site with two copper atoms operating
within their convenient intermetallic distance, are involved
in distinct processes in living systems among which the type
III copper protein catechol oxidase catalyzes the oxidation
of a wide range of ortho-di-phenols to ortho-di-quinones.[7,8]
Hence, dinuclear copper complexes with two metal atoms
in close proximity have received a great deal of attention,
particularly in relation to their potential uses as bimetallic
catalysts, mimicking the activity of enzymes.[8,25–27] Such di-
copper() complexes exhibiting the catecholase activity are
probably crucial for a better understanding of the oxygena-
tion reactions mediated by catechol oxidase.[28–32] The CuII
complexes of Schiff-base and reduced Schiff-base ligands
studied here have structurally distinct features. Hence we
have evaluated their ability to oxidize catechols to quinones
by employing 3,5-DTBC, a common and convenient model
substrate. Figure 8 shows the course of oxidation of 3,5-
DTBC with time in the presence of 4. The results are shown
in Table 4.
A linear relationship between the initial rates and the
concentration of the complexes has been obtained for 2–4,
which shows a first-order dependence on the catalyst con-
centration for these systems. An example of the Li-
neweaver–Burk plot[29f] is given in Figure 9 for 4. The data
Figure 7. The temperature dependences of χm and χmT in the range
of 2–350 K for a) 1, b) 3, and c) 4. The solid lines are fits using a
Cu2 dimer model.
Compounds 3 and 4 show similar behavior; on lowering obtained from the Lineweaver–Burk plot model is em-
the temperature, χm decreases gradually, and there is an in- ployed for a comparison of catalytic activity.
crease below about 100 K while the χmT value even at 330
It is obvious that catalytic ability towards the oxidation
or 350 K is quite small compared with the expected value of 3,5-DTBC has been found to follow the order: 4 Ͼ 2 Ͼ
of 0.75 cm3 mol–1 K for two noninteracting CuII ions. Upon 3. The least activity observed in the case of Schiff-base com-
cooling it decreases rapidly to a value close to zero at 2 K. plex 3 can be attributed to the inefficient binding of the
The increase of χm at low temperature is probably due to substrate to the catalyst caused by the formation of the
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Copper() Complexes of Schiff-Base and Reduced Schiff-Base Ligands
Table 4. Kinetic parameters for the activity of complexes 2–4.
FULL PAPER
Complex
kcat [h–1]
Km [mM]
Vmax [10–6 s1]
[Cu2(Sam)2(H2O)2]·H2O, 2 in MeOH
[Cu2(Saes)2(H2O)2]·2H2O, 3 in MeOH
[Cu2(Sae)2]·2H2O, 4 in MeOH
1140 ( 14)
133 ( 15)
4612 ( 28)
5048 ( 42)
5.4 ( 0.2)
14.3 ( 0.1)
4.3 ( 0.1)
3.2 ( 0.1)
41 ( 1)
5 ( 1)
163 ( 2)
178 ( 3)
[Cu2(Sae)2]·2H2O, 4 in MeOH/H2O (95:5)
respectively, were found to exhibit higher activity, which has
been attributed to the larger ring size due to a longer alkyl
chain in the side arm of the corresponding ligands. It has
been proposed that the formation of seven-membered rings
in [Cu2(Sab4)2(H2O)2] and six-membered rings in [Cu2-
(Sbal)2(H2O)2] resulted in higher activity, with Kcat values
of 3800 and 1280 h–1 respectively. The lower activity (Kcat
= 563 h–1) in [Cu2(Sgly)2(H2O)] due to the shorter glycine
side-chain forming five-membered rings appears to support
this view. In the present set of complexes, 2 contains amino-
methanesulfonate, which is the sulfonic acid analogue of
glycine, and 4 has aminoethanesulfonate, which is the sul-
fonic acid analogue of β-alanine. Based on the connectivity
observed in the structures of CuII complexes containing
Schiff bases, as well as chemical, spectroscopic, and cata-
lytic data, the structure of 2 is expected to be similar to that
of [Cu2(Sgly)2(H2O)]. As a consequence, these five-mem-
bered rings formed by the side arm of the ligand in 2 would
account for the lower catalytic activity as compared to 4.
It is important to emphasize here that the Kcat value of
4612 h–1 observed in 4, under similar experimental condi-
tions, is much higher than that observed for 2 and 3 and is
Figure 8. Oxidation of 3,5-DTBC by 4 monitored by UV/Vis spec-
troscopy.
significantly greater than even the highest activity (Kcat
=
3800 h–1) reported for [Cu2(Sab4)2(H2O)2].[9] Quite remark-
ably, complex 4 has also considerably superior catalytic ac-
tivity compared to its carboxylate analogue [Cu2(Sbal)2-
(H2O)2] (Kcat = 1287 h–1).[9] It should be noted that for the
five-coordinate dicopper() complexes to act as active cata-
lysts, dissociation of the axial bonds should easily occur so
that a free coordination site will be readily available for the
binding of the substrate in a bridging mode.[31,32] A confor-
mationally flexible ligand backbone can easily assist the
conformational changes in the dinuclear core and hence the
effective binding of the substrate. Compared to the strongly
coordinating carboxylate oxygen atoms as in [Cu2(Sab4)2-
(H2O)2] or [Cu2(Sbal)2(H2O)2], the sulfonate groups are
weakly coordinating and can be readily dissociated in solu-
tion to accommodate the substrate.[11,13] The combined ef-
fect of the sufficiently longer alkyl side arm and the weakly
coordinating sulfonate donor group facilitate easy binding
of the substrate, especially in 4, and hence enhance its cata-
lytic activity. Further, the presence of a weakly coordinating
sulfonate donor group in 2 makes it a more efficient catalyst
(Kcat = 1140 h–1) than its carboxylate analogue [Cu2-
(Sgly)2(H2O)] (Kcat = 563 h–1).
Figure 9. Lineweaver–Burk plot for catalysis by 4.
eight-membered ring with largely separated CuII ions. Thus,
the larger Cu···Cu distance of 5.33 Å in 3 is highly unfavor-
able for the efficient binding of the substrate. Furthermore,
very distinct structural differences between 3 and 4 account
for the difference in their activity.
The catecholase activity of copper() compounds with
different structural parameters has been investigated and
compared to some extent by varying the properties of che-
lating ligands with respect to their conformation, and the
Our attempts to evaluate the catecholase activity of 2, 3,
number as well as the nature of the donor atoms.[8] In our and 4 in pure water have been unsuccessful because of the
earlier report,[9] the reduced Schiff-base complexes [Cu2- insolubility of the product 3,5-DTBQ in water. Hence we
(Sab4)2(H2O)2] and [Cu2(Sbal)2(H2O)2], containing 4-ami- tried to employ various ratios of MeOH/H2O in order to
nobutanoic acid and 3-aminopropanoic acid (β-alanine) investigate the solvent influence on the activity. Nonethe-
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B. Sreenivasulu, M. Vetrichelvan, F. Zhao, S. Gao, J. J. Vittal
FULL PAPER
less, reliable data could only be collected in 95:5 of MeOH/ gand complements our earlier observation that the ring size
H2O for 4. From this result, it has been found that the ac- of the side arm in these CuII complexes has profound influ-
tivity of 4 increased by about 10% in the presence of water. ence on the kinetics of the oxidation of 3,5-DTBC. Further,
the complexes described here with a weakly coordinating
sulfonate group in the ligands have exhibited higher cate-
cholase activity than that of the corresponding carboxylate
Summary
analogues with strongly coordinating carboxylate groups.
Several CuII complexes of the Schiff-base and reduced These findings are in agreement with the available data in
Schiff-base ligands formed between salicylaldehyde and the literature.[28–32]
aminomethane/ethanesulfonic acid have been synthesized
and characterized by spectroscopic methods, and the solid-
state structures have been determined by X-ray crystallogra-
phy.
Experimental Section
Materials: All reagents were purchased from commercial sources
The salient structural features displayed by the Schiff-
base copper complexes 1 and 3 provided an opportunity to
perform comparative studies with those of the reduced
Schiff-base complexes 2 and 4. Copper centers in 1 and 3
assumed square pyramidal geometry with the apical posi-
tion preferably occupied by the sulfonate oxygen atoms.
While the reduced Schiff-base complexes 4 and 5 form a
phenoxo-bridged dinuclear Cu2O2 core with intermetallic
distances of 3.03 and 3.06 Å, the Schiff-base complexes 1
and 3 resulted in the formation of eight-membered rings
with Cu···Cu distances of 5.12 and 5.33 Å, respectively.
Apart from the expected dinuclear copper complexes with
a Cu2O2 core, there are also reports of mononuclear copper
complexes formed preferentially from Schiff bases contain-
ing amino acid residues.[33] But strikingly, because of the
rigid C=N double bonds, H2Sams and H2Saes with sulfonic
group offered the formation of eight-membered sulfonato-
bridged dinuclear copper centers. Furthermore, 1 and 3
generated a hydrogen-bonded 2D structure unlike the 2D
(4, 4) coordination polymeric network containing channels
in 4. Recrystallization of 4 in DMF/acetone mixture gave
rise to 5 with the solvent DMF coordinated CuII centers.
DMF, being a strong coordinating solvent, is able to dis-
place the axial oxygen atoms of sulfonate groups in 4 and
convert a (4, 4) net to a 2D hydrogen-bonded structure in
5.
The crystal structures of 4 and 5 exhibit N–H···O hydro-
gen bonds from the metal-coordinated imine N–H and the
sulfonate O while 1 and 3 illustrate a 2D hydrogen-bonding
pattern involving the O(w)–H and sulfonate O atoms. These
results will further enhance the structural insight into the
transition-metal coordination chemistry of organosulfon-
ates and their derivatives.[11–13,24] Variable temperature mag-
netic studies show that the syn–syn sulfonate O–S–O brid-
ges in 3 can mediate antiferromagnetic interaction between
CuII ions quite effectively, as well as the expected strong
antiferromagnetic coupling through phenoxo bridges in 4.
While investigating complexes 2–4 as synthetic functional
models for the catechol oxidase, 4 has been found to show
significantly higher activity than 2 and 3. Comparative
structural studies made between the Schiff-base complexes
and the corresponding reduced Schiff-base complexes have
provided more insight into the understanding of their cata-
lytic activity. The trend observed in this study using dif-
and used as received without any further purification.
1
Physical Measurements: The H NMR spectra were recorded with
a Bruker ACF300 FT-NMR instrument using TMS as an internal
reference by using appropriate deuterated solvents and the IR spec-
tra were recorded using an FTS165 Bio-Rad FTIR spectrometer in
the range 4000–450 cm–1. The absorption spectra were recorded
with a Shimadzu UV-2501/PC UV/Vis spectrophotometer in aque-
ous solution. ESI-MS spectra were recorded with a Finnigan MAT
LCQ mass spectrometer. The elemental analyses were performed in
the Microanalytical Laboratory, Department of Chemistry
National University of Singapore. Water present in the compounds
was determined by an SDT 2960 TGA thermal analyzer with a
heating rate of 10 °Cmin–1 under nitrogen using a sample size of
about 10 mg per run. Variable temperature magnetic studies were
made using a Quantum Design MPMS-XL5 SQUID magnetome-
ter operating in an applied field of 5 kOe. The experimental suscep-
tibilities were corrected for the diamagnetism (Pascal’s tables).
Ligand
N-(2-Hydroxybenzyl)aminoethanesulfonic Acid (H2Sae): Salicylal-
dehyde (0.29 g, 2.50 mmol) in MeOH (5 mL) was added to a solu-
tion of aminoethanesulfonic acid (0.31 g, 2.50 mmol) in MeOH
(10 mL) containing NaOH (0.11 g, 2.50 mmol). The yellow solu-
tion was stirred for about 45 min at room temperature prior to
cooling in an ice bath. The intermediate Schiff base that formed
was reduced with an excess of NaBH4 (0.10 g, 2.65 mmol). The
yellow color slowly discharged, and after half an hour the solution
was acidified with acetic acid to pH 3–5. Then the solvent was
reduced to half and Et2O was added to get the product, which was
filtered off, washed with Et2O, dried, and stored in a desiccator, as
H2Sae is very hygroscopic. Because of its highly hygroscopic nature,
elemental analysis results of H2Sae were not consistent and hence
are not provided. Yield: 0.35 g (60%). MS (EI): m/z (%) = 257.0
1
[(L2– + Na)]. H NMR ([D6]DMSO): δ = 7.31 and 6.92 ppm (m, 4
H, aromatic), 4.21 (s, 2 H, benzylic –CH2), 3.32 (t, 2 H, –CH2),
3.21 (t, 2 H, –CH2), 2.01 (s, –SO3H). 13C NMR: δ = 157.1 (=Car–
O), 115.5 (C2ar), 128.2 (C3ar), 120.8 (C4ar), 129.5 (C5ar), 124.4
(C6ar), 49.4 (–CH2SO3H), 48.9 (benzylic –CH2), 44.0 (–CH2CH2).
IR (KBr): ν = 3365 (OH), 2942 (NH), 1401 and 1115 (SOO–), 1030
˜
(SO), 1267 (phenolic CO) cm–1.
Complexes
[Cu2(Sams)2(H2O)2] (1): Salicylaldehyde (0.1 mL, 0.94 mmol) was
added to the clear solution containing aminomethanesulfonic acid
(0.10 g, 0.94 mmol) and NaOH (0.04 g, 0.94 mmol) in aqueous
MeOH (20 mL, 1:1, v/v) to obtain the yellow Schiff base. After
stirring for 30 min, copper nitrate trihydrate in MeOH (0.23 g,
0.94 mmol) was directly added in portions and the resulting green
ferent types of functional groups in the side arm of the li- product was further stirred for 30 min and filtered, washed with
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Copper() Complexes of Schiff-Base and Reduced Schiff-Base Ligands
FULL PAPER
water, MeOH, Et2O and dried under vacuum. Dark green prismatic
single crystals suitable for X-ray diffraction were obtained from the
tronic spectra at 25 °C by time-dependent UV/Vis spectroscopy.
For this purpose, 10–4  solutions of complexes 1–4 were treated
with 50 equiv. of 3,5-DTBC under aerobic conditions. The UV/Vis
spectra of the original solution directly after the addition and after
filtrate after one day. Yield: 0.2 g (67%). IR (KBr): ν = 3400 (OH),
˜
1629 (C=N), 1305 and 1128 (SOO–), 1247 (phenolic CO) cm–1. UV/
Vis (H2O): λmax (ε, –1 cm–1): 678 nm (200), 367 nm (13135). MS 10, 20, and 30 min were recorded and corrected for volume changes
(ESI): m/z (%) = 276.7 [CuL]+, 553.5 [Cu2L2]. C16H14Cu2N2-
S2O8(H2O)2 (589.5): calcd. C 32.6, H 3.1, N 4.7, S 10.9, H2O 6.1;
found C 32.5, H 3.0, N 4.4, S 10.8, H2O 6.8 (from TG weight loss).
up to 2 h.
The kinetics of oxidation of 3,5-DTBC were measured at 25 °C by
the method of initial rates by monitoring the growth of the absorp-
tion band at 390 nm of the product 3,5-DTBQ. The complex (2 mg)
was added to the 3,5-DTBC solution (25 mL) of concentrations
1.0×10–3–1.5×10–2  so that the concentration of the complex was
maintained at 10–4 . During the first 10 min of the reaction, the
development of the absorption band at 390 nm was monitored. In
order to determine the kinetic parameters, the Michaelis–Menten
approach was applied.[8,34]
[Cu2(Sam)2(H2O)2]·H2O (2): Salicylaldehyde (0.1 mL, 0.94 mmol)
in MeCN (10 mL) was added to a clear solution of aminometh-
anesulfonic acid (0.10 g, 0.94 mmol) and NaOH (0.08 g,
1.88 mmol) in MeOH (10 mL) and the resulting yellow Schiff base
solution was stirred for 30 min. The mixture was then cooled in an
ice bath followed by the addition of NaBH4 (0.02 g, 0.47 mmol).
The yellow color of the Schiff base slowly discharged after 20 min
and the clear solution was maintained at a pH of 4–6 by adding a
few drops of CH3COOH. Copper nitrate trihydrate (0.23 g,
0.94 mmol) was added to this acidic solution to facilitate in situ
complexation. The dark green product obtained after stirring for
1 h was filtered off, washed with MeOH (2 mL), Et2O (5 mL), and
The reactivity studies were performed in methanol solution because
of the good solubility of the substrate, 3,5-DTBC, and of its prod-
uct 3,5-DTBQ. Also, because of the considerable solubility of 4 in
water, investigations have also been attempted in a MeOH/H2O
mixture for comparison. Attempts to record the activity of 2 and 3
in MeOH/H2O were unsuccessful because of the solubility problem
producing unreliable data. Preliminary qualitative studies on the
activity of 1 have also been unsuccessful owing to its insolubility
in common solvents.
then dried under vacuum. Yield: 0.22 g (66%). IR (KBr): ν = 3450
˜
(OH), 2920 (NH), 1380 and 1147 (SOO–), 1037 (SO), 1279 (pheno-
lic CO) cm–1. MS (ESI): m/z (%) = 278.7 [CuL]+, 557.8 [Cu2L2],
580.7 [Cu2L2Na]+. UV/Vis (MeOH) λmax (ε, –1 cm–1): 717 nm
(190), 405 nm (870). C16H18Cu2N2S2O8(H2O)3 (611.6): calcd. C
31.4, H 4.0, N 4.7, S 10.5, H2O 8.4; found C 31.7, H 4.1, N 4.6, S
10.8, H2O 8.0 (from TG weight loss).
X-ray Crystallography: The diffraction experiments were carried
out on a Bruker AXS SMART CCD diffractometer. The program
SMART[35a] was used for collecting frames of data, indexing reflec-
tion and determination of lattice parameter, SAINT[35a] for integra-
tion of the intensity of reflections and scaling, SADABS[35b] for
absorption correction, and SHELXTL[35c] for space group and
structure determination, least-squares refinements on F2. Positional
and thermal parameters of the non-hydrogen atoms were refined
in the least-squares cycles. The hydrogen atom positions and indi-
vidual U’s were refined for the water molecules in 1. One of the
hydrogen atoms of the lattice water in 3 was found to be disordered.
The option DFIX was used in the model to idealize the geometry.
No hydrogen atoms could be located for the lattice water in 4. The
methyl carbon atom of the lattice DMF was found to be disordered
(0.65/0.35) in 5. Only isotropic thermal parameters were refined for
the minor component of the disorder.
[Cu2(Saes)2(H2O)2]·2H2O (3): Compound 3 was prepared by a
method similar to that described for 1 by using 2-aminoethanesul-
fonic acid. Yield: 0.2 g (71%). IR (KBr): ν = 3443 (OH), 1617
˜
(C=N), 1287 and 1173 (SOO–), 1244 (phenolic CO) cm–1. UV/Vis
(H2O): λmax (ε,

–1 cm–1): 688 nm (160), 322 nm (5000).
C18H18Cu2N2S2O8(H2O)4 (653.6): calcd. C 33.1, H 4.0, N 4.3, S
10.0, H2O, 11.1; found C 33.3, H 4.1, N 4.4, S 10.1, H2O 11.9 (from
TG weight loss).
[Cu2(Sae)2]·2H2O (4): Copper acetate (0.08 g, 0.40 mmol) in
MeOH (10 mL) was added to a solution of the ligand H2Sae
(0.09 g, 0.40 mmol) in MeOH (20 mL) and the resultant solution
stirred for about 6 h. The green precipitate formed was filtered,
washed with MeOH and Et2O, and dried under vacuum. Slow
evaporation of the dark green clear aqueous solution of 4 furnished
dark green single crystals after 4–5 d. Yield: 0.06 g (55%). IR
CCDC-266070 (for 1), CCDC-266071 (for 3), CCDC-266072 (for
4), and CCDC-266073 (for 5) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
(KBr): ν = 3410 (OH), 2935 (NH), 1312 and 1181 (SOO–), 1020
˜
(SO), 1255 (phenolic CO) cm–1. UV/Vis (H2O): λmax (ε, –1 cm–1):
736 nm (190), 420 nm (855). MS (ESI): m/z (%) = 291.0 [(CuL)–],
522.0 [(CuL2)–], 584.2 [(Cu2L2)–], 875.0 [(Cu3L3)–], 1106.0 [(Cu3-
L4)–], 1397.7 [(Cu4L5)–], 1689.6 [(Cu5L6)–]. C18H22Cu2N2S2-
O8 (H2O) (602.18): calcd. C 34.7, H 4.2, N 4.5, S 10.3, H2O 5.8;
found C 34.3, H 4.2, N 4.6, S 10.3, H2O 5.9 (from TG weight loss).
Supporting Information (see also the footnote on the first page of
this article): Molecular and packing diagrams of 5, thermogravime-
try of 1–5, UV/Vis plots and Lineweaver–Burk plots for 2–4 (total
number of pages: 7).
[Cu2(Sae)2(DMF)2]·2DMF (5): The filtered and clear dark green
solution of 4 (0.04 g) in DMF (4 mL) was slowly diffused into ace-
tone (10 mL) in a small beaker. Dark green crystals suitable for the
single-crystal X-ray diffraction studies were obtained after a week.
Acknowledgments
Yield: 0.02 g (50%). IR (KBr): ν = 2928 (NH), 1387 and 1150
˜
We gratefully acknowledge the financial support from NUS (R143-
000-252-112) and from NSFC (20125104, 20490210).
(SOO–), 1023 (SO), 1260 (phenolic CO) cm–1. UV/Vis (MeOH)
λmax (ε, –1 cm–1): 731 nm (195), 412 nm (660). MS (ESI): m/z
(%) = 292.2 [(CuL)–], 522.3 [(CuL2)–], 584.2 [(Cu2L2)–], 875.0
[(Cu3L3)–]. C30H46Cu2N6S2O12 (873.9): calcd. C 41.2, H 5.3, N 9.6,
S 7.0; found C 41.8, H 5.1, N 9.2, S 7.2.
[1] See for example: a) Y. Nakao, K. Sakurai, A. Nakahara, Bull.
Chem. Soc. Jpn. 1967, 40, 1536; b) K. S. Bai, D. L. Leussing,
J. Am. Chem. Soc. 1967, 89, 6126; c) M. J. O’Connor, R. E.
Ernst, J. E. Schoenborn, R. H. Holm, J. Am. Chem. Soc. 1968,
90, 1744; d) L. Casella, M. Gullotti, J. Am. Chem. Soc. 1981,
103, 6338; e) L. Casella, M. Gullotti, G. Pacchionic, J. Am.
Catalytic Activity for the Oxidation of 3,5-DTBC and Kinetic Mea-
surements: Catecholase activity of complexes for the oxidation of
3,5-DTBC by complexes 2–4 was measured by recording their elec-
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B. Sreenivasulu, M. Vetrichelvan, F. Zhao, S. Gao, J. J. Vittal
FULL PAPER
Chem. Soc. 1982, 104, 2386; f) M. R. Wagner, F. Ann Walker,
Inorg. Chem. 1983, 22, 3021; g) R. D. Gillard, R. Wootton, J.
Chem. Soc. B 1970, 364.
[19]
[20]
C. S. Chan, L. L. Koh, P.-H. Leung, J. D. Ranford, K. Y. Sim,
Inorg. Chim. Acta 1995, 236, 101.
P. Li, N. K. Solanki, H. Ehrenberg, N. Feeder, J. E. Davies,
J. M. Rawson, M. A. Halcrow, J. Chem. Soc., Dalton Trans.
2000, 1559.
N. R. Sangeetha, B. Kavitha, G. Rajeev, P. Chandane Kumar,
M. Vadivelu, S. Pal, Polyhedron 1999, 18, 1425.
a) P. D. J. Grootenhuis, J. van Eerden, P. J. Dijkstra, S. Hark-
ema, D. N. Reinhoudt, J. Am. Chem. Soc. 1987, 109, 8044; b)
T. Steiner, Chem. Commun. 1997, 727; c) T. Steiner, G. R. Desi-
raju, Chem. Commun. 1998, 891; d) G. R. Desiraju, Acc. Chem.
Res. 2002, 35, 565.
[2]
a) K. Tatsumoto, A. E. Martell, J. Am. Chem. Soc. 1981, 103,
6203; b) K. Tatsumoto, A. E. Martell, R. J. Motekaitis, J. Am.
Chem. Soc. 1981, 103, 6197; c) H. M. Dawes, J. M. Waters,
T. N. Waters, Inorg. Chim. Acta 1982, 66, 29; d) I. Bkouche-
Waksman, J. M. Barbe, A. Kvick, Acta Crystallogr. Sect. B
1988, 44, 595; e) R. Hamalainen, U. Turpeinen, Acta Crys-
tallogr. Sect. C 1985, 41, 1726; f) K. Korhonen, R. Hama-
lainen, U. Turpeinen, Acta Crystallogr. Sect. C 1984, 40, 1175;
g) K. Nakagima, M. Kojima, K. Foriumi, K. Saito, J. Fujita,
Bull. Chem. Soc. Jpn. 1989, 62, 760; h) V. Kettmann, E. Fre-
sova, Acta Crystallogr. Sect. C 1993, 49, 1932.
L. L. Koh, J. D. Ranford, W. T. Robinson, J. O. Stevenson,
A. L. C. Tan, D. Wu, Inorg. Chem. 1996, 35, 6466.
a) J. D. Ranford, J. J. Vittal, D. Wu, Angew. Chem. Int. Ed.
Engl. 1998, 37, 1114; b) J. D. Ranford, J. J. Vittal, D. Wu, X.
Yang, Angew. Chem. Int. Ed.. 1999, 38, 3498; c) B. Sreenivas-
ulu, J. J. Vittal, Cryst. Growth Des. 2003, 3, 635; d) C. T. Yang,
B. Moubaraki, K. S. Murray, J. D. Ranford, J. J. Vittal, Inorg.
Chem. 2001, 40, 5934.
B. Sreenivasulu, J. J. Vittal, Angew. Chem. Int. Ed. 2004, 43,
5769.
M. A. Alan, M. Nethaji, M. Ray, Angew. Chem. Int. Ed. 2003,
42, 1940.
K. D. Karlin, Z. Tyeklar (Eds.), Bioinorganic Chemistry of Cop-
per, Chapman & Hill, New York, 1993.
a) R. Than, A. A. Feldman, B. Krebs, Coord. Chem. Rev. 1999,
182, 211; b) C.-J. Lee, C.-H. Kao, H.-H. Wei, Y.-H. Liu, G.-H.
Lee, Y. Wang, J. Inorg. Biochem. 2001, 84, 171; c) J. Reim, B.
Krebs, J. Chem. Soc., Dalton Trans. 1997, 3793.
C.-T. Yang, M. Vetrichelvan, X. D. Yang, M. Moubaraki, K. S.
Murray, J. J. Vittal, Dalton Trans. 2004, 1, 113.
a) Y. Wang, L. Feng, Y. Li, C. Hu, E. Wang, N. Hu, H. Jia,
Inorg. Chem. 2002, 41, 6351, and references cited therein; b)
V. A. Russell, M. C. Etter, M. D. Ward, Chem. Mater. 1994, 6,
1206.
a) J. Cai, C.-H. Chen, X.-L. Feng, C.-Z. Liao, X.-M. Chen, J.
Chem. Soc., Dalton Trans. 2001, 2370; b) M. E. Thompson,
Chem. Mater. 1994, 6, 1168; c) D. Grohol, F. Ging, A.
Clearfield, Inorg. Chem. 1999, 38, 751, and references therein.
a) V. A. Russell, M. C. Etter, M. D. Ward, J. Am. Chem. Soc.
1994, 116, 1941; b) V. A. Russell, C. C. Evans, W. Li, M. D.
Ward, Science 1997, 276, 575.
a) G. K. H. Shimizu, G. D. Enright, J. A. Ripmeester, Can. J.
Chem. 1999, 77, 313; b) G. K. H. Shimiju, G. D. Enright, C. I.
Ratcliffe, K. F. Pretson, J. L. L. Reid, J. A. Ripmeester, Chem.
Commun. 1999, 1485; c) A. P. Cote, G. K. H. Shimizu, Inorg.
Chem. 2004, 43, 6663; d) A. P. Cote, G. K. H. Shimizu, Chem.
Eur. J. 2003, 9, 5361; e) J. Cai, Coord. Chem. Rev. 2004, 248,
1061.
J. Cai, C.-H. Chen, C.-Z. Liao, J.-H. Yao, X.-P. Hu, X.-M.
Chen, J. Chem. Soc., Dalton Trans. 2001, 1137.
a) H. S. Maslen, T. N. Waters, Coord. Chem. Rev. 1975, 17, 137;
b) S. Karunakaran, J. Kandaswamy, J. Chem. Soc., Dalton
Trans. 1995, 1851.
a) L. Sacconi, M. Ciampolini, J. Chem. Soc. 1964, 276; b)
A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, New
York, 1968.
[21]
[22]
[23]
a) B. F. Hoskins, R. Robson, J. Am. Chem. Soc. 1990, 112,
1546; b) R. W. Gable, B. F. Hoskins, R. Robson, J. Chem. Soc.,
Chem. Commun. 1990, 1677; c) M. J. Zaworotko, Chem. Com-
mun. 2001, 1; d) B. Moulton, M. J. Zaworotko, Chem. Rev.
2001, 101, 1629; e) A. N. Khlobystov, A. J. Blake, N. R.
Champness, D. A. Lemenovskii, A. G. Majouga, N. V. Zyk, M.
Schröder, Coord. Chem. Rev. 2001, 222, 155; f) B. Kumar, M.
Fujita, J. Chem. Soc., Dalton Trans. 2000, 3805; g) L. Carlucci,
G. Ciani, D. M. Proserpio, New J. Chem. 1998, 22, 1319; h)
S. R. Batten, R. Robson, Angew. Chem. Int. Ed. Engl. 1998, 37,
1460; i) S. R. Batten, B. F. Hoskins, R. Robson, Chem. Eur. J.
2000, 6, 156; j) S. R. Batten, Cryst. Eng. Comm. 2001, 3, 67.
A. Onoda, Y. Yamada, M. Doi, T.-A. Okamura, N. Ueyama,
Inorg. Chem. 2001, 40, 516.
a) T. N. Sorrell, Tetrahedron 1989, 45, 3; b) P. A. Vigato, S.
Tamburini, D. Fenton, Coord. Chem. Rev. 1990, 106, 25.
a) C. Gerdermann, C. Eicken, B. Krebs, Acc. Chem. Res. 2002,
35, 183; b) T. Klabunde, C. Eicken, J. C. Sacchettini, B. Krebs,
Nat. Struct. Mol. Biol. 1998, 5, 1084; c) C. Eicken, F. Zippel, K.
Buldt-Karentzopoulos, B. Krebs, FEBS Lett. 1998, 436, 293.
a) E. I. Solomon, U. M. Sundaram, T. E. Machonkin, Chem.
Rev. 1996, 96, 2563; b) A. Sanchez-Ferrer, J. N. Rodriguez-Lo-
pez, F. Garcia-Canovas, F. Carcia-Carmona, Biochim. Biophys.
Acta 1995, 1247, 1; c) C. Eicken, B. Krebs, J. C. Sacchettini,
Curr. Opin. Struct. Biol. 1999, 9, 677.
a) P. L. Holland, W. B. Tolman, Coord. Chem. Rev. 1999, 190–
192, 855, and references cited therein; b) T. Plenge, R. Dillinger,
L. Santagostini, L. Casella, F. Z. Tuczek, Z. Anorg. Allg. Chem.
2003, 629, 2258; c) T. M. Simandi, Z. May, I. C. Szigyarto, L. I.
Simandi, Dalton Trans. 2005, 365.
a) N. Oishi, Y. Nishida, K. Ida, S. Kida, Bull. Chem. Soc. Jpn.
1980, 53, 2847; b) P. Gentschev, N. Möller, B. Krebs, Inorg.
Chim. Acta 2000, 300, 442; c) S. Parimala, K. N. Gita, M. Kan-
daswamy, Polyhedron 1998, 17, 3445; d) M. R. Malachowski,
B. T. Dorsey, M. J. Parker, M. E. Adams, R. S. Kelly, Polyhe-
dron 1998, 17, 1289; e) M. R. Malachowski, H. B. Huynh, L. J.
Tomlinson, R. S. Kelly, J. W. Furbee, J. Chem. Soc., Dalton
Trans. 1995, 31; f) R. Wegner, M. Gottschaldt, H. Gorl, E.-G.
Jager, D. Klemm, Chem. Eur. J. 2001, 7, 2143; g) M. Thirumav-
alavan, P. Akilan, M. Kandaswamy, Supramol. Chem. 2004, 16,
137.
[3]
[4]
[5]
[6]
[7]
[8]
[24]
[25]
[26]
[27]
[28]
[29]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[30]
a) F. Zippel, F. Ahlers, R. Werner, W. Haase, H.-F. Nolting, B.
Krebs, Inorg. Chem. 1996, 35, 3409; b) J. Reim, B. Krebs, J.
Chem. Soc., Dalton Trans. 1997, 3793; c) L. M. Berreau, S. Ma-
hapatra, J. A. Halfen, R. P. Houser, V. G. Young, W. B. Tol-
man, Angew. Chem. Int. Ed. 1999, 38, 207; d) E. Monzani, G.
Battaini, A. Perotti, L. Casella, M. Gullitti, L. Santogostini, G.
Nardin, L. Randaccio, S. Geramia, P. Zanello, G. Opromolla,
Inorg. Chem. 1999, 38, 5359; e) E. Monzani, L. Quinti, A. Per-
otti, L. Casella, M. Gullitti, L. Randaccio, S. Geramia, G. Nar-
din, P. Faleschini, G. Tabbi, Inorg. Chem. 1998, 37, 553; f) S.
Torelli, C. Belle, S. Hamman, J. L. Pierre, Inorg. Chem. 2002,
41, 3983; g) A. A. Mukherjee, I. Rudra, S. G. Naik, S. Ramase-
sha, M. Nethaji, A. R. Chakravarty, Inorg. Chem. 2003, 42,
5660.
[16]
[17]
[18]
a) A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier,
Amsterdam, 1984; b) C. T. Yang, B. Moubaraki, K. S. Murray,
J. D. Ranford, J. J. Vittal, Inorg. Chem. 2001, 40, 5934.
a) A. W. Addison, T. N. Rao, J. Reedijk, J. V. Rijn, G. C. Ver-
schoor, J. Chem. Soc., Dalton Trans. 1984, 1349; b) G. Murphy,
C. O. Sullivan, B. Murphy, B. Hathaway, Inorg. Chem. 1998,
37, 240; c) D. S. Marlin, M. M. Olmstead, P. K. Mascharak,
Inorg. Chem. 2001, 40, 7003; d) C. T. Yang, J. J. Vittal, Inorg.
Chim. Acta 2003, 344, 65, and references therein.
[31]
a) J. Mukherjee, R. Mukherjee, Inorg. Chim. Acta 2002, 337,
429; b) K. D. Karlin, M. S. Nasin, B. I. Cohen, R. W. Cruse, S.
Kaderli, A. D. Zuberbuhler, J. Am. Chem. Soc. 1994, 116, 1324;
4644
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 4635–4645

Copper() Complexes of Schiff-Base and Reduced Schiff-Base Ligands
FULL PAPER
c) G. Battaini, E. Monzani, L. Casella, L. Santagostini, R. J.
Pagliarin, J. Biol. Inorg. Chem. 2000, 5, 262.
[34] K. D. Karlin, A. D. Zuberbuhler in Bioinorganic Catalysis
(Eds.: J. Reedjik, E. Bouwman), M. Dekker, New York, 1999,
pp. 469–534.
[35] a) SMART & SAINT Software Reference Manuals, Version
6.22, Bruker AXS Analytic X-ray Systems, Inc., Madison, WI,
2000; b) G. M. Sheldrick, SADABS, Software for Empirical
Absorption Correction, University of Göttingen, Germany,
2000; c) SHELXTL Reference Manual, Version 5.1, Bruker
AXS, Analytic X-ray Systems, Inc., Madison, WI, 1997.
Received: July 19, 2005
[32] a) M. Merkel, N. Moller, M. Piacenza, S. Grimme, A. Rompel,
B. Krebs, Chem. Eur. J. 2005, 11, 1201; b) I. A. Koval, D. Pur-
sche, A. F. Stassen, P. Gamez, B. Krebs, J. Reedijk, Eur. J. In-
org. Chem. 2003, 1669; c) I. A. Koval, M. Huisman, A. F.
Stassen, P. Gamez, M. Lutz, A. L. Spek, Eur. J. Inorg. Chem.
2004, 591; d) N. Moller, M. Luken, M. B. Krebs, J. Inorg. Bi-
ochem. 2001, 86, 344; e) J. Ackermann, F. Meyer, E. Kaifer, H.
Pritzkow, Chem. Eur. J. 2002, 8, 247.
[33] See for example: a) R. J. Butcher, G. M. Mockler, O. McKern,
Acta Crystallogr. 2003, E59, m61; b) R. J. Butcher, G. M.
Mockler, O. McKern, Acta Crystallogr. 2003, E59, m20.
Published Online: October 17, 2005
Eur. J. Inorg. Chem. 2005, 4635–4645
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4645