Synthesis, structures and catecholase activity of a new series of dicopper(II) complexes of reduced Schiff base ligands

Article abstract of DOI:10.1002/ejic.200600022

A series of dinuclear Cu^II complexes of reduced Schiff bases from substituted salicylaldehydes and amino acids have been synthesized and characterized. They are: [Cu₂(RScp11)₂-(H ₂O)₂] {H₂R

Full text of DOI:10.1002/ejic.200600022

FULL PAPER
DOI: 10.1002/ejic.200600022
Synthesis, Structures and Catecholase Activity of a New Series of Dicopper(II)
Complexes of Reduced Schiff Base Ligands
Bellam Sreenivasulu,[a] Fei Zhao,[b] Song Gao,[b] and Jagadese J. Vittal*[a]
Keywords: Biomimetic studies / Copper / Reduced Schiff-base complexes / Tridentate ligands / Catecholase activity /
Magnetic properties
A series of dinuclear CuII complexes of reduced Schiff bases
from substituted salicylaldehydes and amino acids have been
synthesized and characterized. They are: [Cu2(RScp11)2-
(H2O)2] {H2RScp11 = 1-[(2-hydroxy-5-R-benzyl)amino]cyclo-
that [Cu2(Scp11)2(MeOH)2] (1a), [Cu2(ClScp11)2(DMF)-
(H2O)]·MeCN (2a), [Cu2(MeScp11)2(MeOH)2]·2MeOH (3a),
[Cu2(ClSch11)2(MeOH)2]·2MeOH (6a), [Cu2(ClSch12)2]·
2MeOH (9a), and [Cu2(Diala4)2(DMSO)2]·2DMSO·2acetone
pentane-1-carboxylic acid; R = H (1), Cl (2), CH3 (3), OH (4)}, (12a) have 1D hydrogen-bonded polymeric structures while
[Cu2(RSch11)2(H2O)x] {H2RSch11 1-[(2-hydroxy-5-R-ben- 4 has a 3D hydrogen-bonded network structure. Complex 8
=
zyl)amino]cyclohexane-1-carboxylic acid; R = H and x = 1 (5),
R = Cl and x = 2 (6), R = CH3 and x = 2 (7)}, [Cu2(RSch12)2-
(H2O)2] {H2RSch12 = 2-[(2-hydroxy-5-R-benzyl)amino]cyclo-
displays a 2D coordination polymeric network structure. The
complexes 1–13 have been investigated as functional models
for the catechol oxidase by employing 3,5-di-tert-butylcate-
chol as a model substrate. Electron-withdrawing substituents
reduced the activity while electron-donating substituents en-
hanced the activity. Variable-temperature magnetic studies
conducted on compound 8 suggest the presence of strong
inter-dimer antiferromagnetic coupling.
hexane-1-carboxylic acid;
[Cu2(ClSch12)2]·2H2O (9)}, [Cu2(Diala5)2(H2O)2]·H2O [H3-
Diala5 N-(2,5-dihydroxybenzyl)- -alanine] (11), [Cu2-
(Diala4)2(H2O)2]·H2O [H3Diala4 = N-(2,4-dihydroxybenzyl)-
-alanine] (12), and [Cu2(Diala3)2(H2O)2]·H2O [H3Diala3 = N-
(2,3-dihydroxybenzyl)- -alanine] (13). They were isolated
R = H (8), CH3 (10) and
=
L
L
L
and characterized by chemical and spectroscopic methods.
Single crystal X-ray crystallographic studies have revealed
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
relevant dinuclear active sites through synthetic analogues
basically involves the judicious design of binucleating li-
gands to meet the conditions such as bridging mode con-
trolling the metal–metal distance, steric-electronic proper-
ties, and geometry around the metal centers.[4] Conse-
quently, dinuclear copper(II) complexes of the binucleating
ligands have been extensively documented as bimetallic cat-
alysts due to their potential ability to mimic the functions
of the so-called catechol oxidase.[2,4–5]
Our research group has been interested in the coordina-
tion chemistry of reduced Schiff-base ligands, N-(2-hy-
droxybenzyl)-amino acids, which have the potential to form
dinuclear complexes through a bridging phenolate group.
In addition, these ligands can also afford the choice of coor-
dination environments that determine the nature of metal
ions that can be bound within the closest proximity. Apart
from the studies illustrating interesting solid-state supramo-
lecular transformation of CuII and ZnII complexes,[6] and
the novel helical stair-case structure in the NiII complex,[7]
our research group has reported several dicopper(II) com-
plexes derived from reduced Schiff bases as functional mod-
els for catechol oxidase.[8] Very recently, we have also dem-
onstrated that the dicopper(II) complexes containing the
weakly coordinating sulfonate donor group are more active
Introduction
Among the well-known representatives of Type III cop-
per proteins, catechol oxidase with active dicopper(II) sites
is a ubiquitous enzyme in living systems for catalyzing the
oxidation of a wide range of ortho-diphenols to ortho-diqui-
nones. The subsequent auto polymerization of the highly
active quinones into polyphenolic catechol melanins is con-
sidered to be responsible for the defense mechanism ob-
served in plants against pathogens or pests.[1] In fact, the
two copper atoms of dicopper(II) bio-active centers present
in different metalloenzymes are found to act cooperatively
within the proximity of ca. 3.5 Å with each CuII center co-
ordinated by three histidine donors.[2,3] As confirmed by the
recent X-ray crystal structure analysis, catechol oxidase in
the met oxidized form contains dicopper centers with a
Cu···Cu distance of 2.9 Å.[3] Modeling the features of bio-
[a] Department of Chemistry, National University of Singapore,
Science Drive 3, Singapore 117543
Fax: +65-6779-1691
E-mail: chmjjv@nus.edu.sg
[b] College of Chemistry and Molecular Engineering, Peking Uni-
versity,
Beijing 100871, P. R. China
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Synthesis, Structures and Catecholase Activity of a New Series of Dicopper(II) Complexes
FULL PAPER
catalysts towards the oxidation of 3,5-DTBC compared to standing of the parameters affecting the catecholase ac-
their carboxylate analogues.[9]
tivity. In this paper, we present the salient structural fea-
In this paper, we report a new series of dicopper(II) com- tures and the details of catecholase activity of the dicop-
plexes of reduced Schiff-base ligands formed between vari- per(II) complexes of various reduced Schiff-base ligands
ous substituted salicylaldehydes and aminocyclopentane/cy- containing the substituted phenolate moiety. Further, vari-
clohexanecarboxylic acids and -alanine (Scheme 1). Our able temperature magnetic studies of a selected complex
previous studies discussing the effect of chelating side have also been discussed.
arms[8] and the weakly coordinating sulfonate group[9] on
the activity, prompted us to investigate and evaluate the ac-
tivity of these closely related yet distinct series of dicop-
per(II) complexes with substituted phenyl rings under sim-
Results and Discussion
ilar experimental conditions. Despite the plethora of reports
Copper complexes 1–13 have been synthesized in good
on catecholase activity of various dicopper(II) complexes, yields by the complexation of CuII salts with the corre-
studies modulating the activity in terms of electronic prop- sponding reduced Schiff-base ligands according to the pro-
erties via substituted bridging phenolates have not been well cedures described in the experimental section. Owing to
documented.[10] Such an investigation of the structure-reac- their poor solubility in common solvents except in DMF
tivity patterns is essential to enhance the mechanistic under- and DMSO, our attempts to obtain single crystals of the
Scheme 1. Ligands and schematic diagram of dicopper(II) complex.
Eur. J. Inorg. Chem. 2006, 2656–2670
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B. Sreenivasulu, F. Zhao, S. Gao, J. J. Vittal
FULL PAPER
bulk as such were not successful. However, the single crys- perature below 120 °C for 11 and 12, and 130 °C for 13 also
tals obtained were different from the bulk due to lattice and suggests similar behavior of metal bound aqua ligands as
coordinated solvents in the case of [Cu2(Scp11)2(MeOH)2] in 8. Thus, in the light of the observed thermal dehydration
(1a), [Cu2(ClScp11)2(DMF)(H2O)]·MeCN (2a), [Cu2(Me- behavior, UV/Vis, IR, and ESI-MS studies the molecular
Scp11)2(MeOH)2]·2MeOH (3a), [Cu2(ClSch11)2(MeOH)2]· formulae of the bulk have been proposed.
2MeOH (6a), [Cu2(ClSch12)2]·2MeOH (9a), and [Cu2-
(Diala4)2(DMSO)2]·2DMSO·2acetone (12a); however,
complexes 4 and 8 afforded single crystals that were the
Solution Studies
same as the bulk during single crystal growth.
The structural behavior of the complexes 1–13 in solu-
tion has been investigated using UV/Vis, ESI-MS spectro-
scopic techniques.
The IR absorption bands of the newly synthesized com-
plexes 1–13 in the range of 3398–3458 cm–1 indicate the
presence of coordinated or lattice water molecules in the
compounds.[11a,11c] This can be further supported by the
weight loss observed in thermo gravimetric (TG) analysis.
UV/Vis Spectroscopy
Electronic spectra of the complexes 1–13 recorded in
The sharp bands observed in the range of 2927–2960 cm–1 DMF solution and Nujol mull are shown in Table 1. The
are due to ν(N–H). The absorption bands observed in the absorption bands observed in the range 620–703 nm corre-
region of 1597–1640 and 1360–1460 cm–1 are due to the spond to d-d transitions and the strong bands at 358–
asymmetric [νasCOO–] and symmetric [νsCOO–] stretching 402 nm are due to ligand-to-metal charge transfer. For an
frequencies of the carboxylate group, respectively. The dif- octahedral geometry the expected 2Eg to T2g transition
2
ference (∆ν) between the asymmetric and symmetric takes place at around 800 nm. This band will undergo a
stretching frequencies of carboxylate groups can be used to significant blue shift when the octahedral geometry distorts
determine their binding mode in the metal complexes.[11a] to a square pyramidal and square planar structure.[13] For
In general, ∆ν for monodentate carboxylate is greater than the reduced Schiff base copper(II) complexes with square
200 cm–1 and for a bridging carboxylate ∆ν is less than pyramidal geometry, the d-d transitions and charge transfer
200 cm–1.[12] This observation has been confirmed for the transitions generally occur in the range 620–720 and 360–
2D coordination polymeric structure in 8 showing the 450 nm, respectively.[8–9,14a–14d]
bridging (∆ν = 115) mode of carboxylate.
The medium intensity bands occurring in the range 356–
The initial weight loss in TG of 4 in the temperature 401 nm are due to phenolate-to-copper(II) charge trans-
range of 134–177 °C corresponds to the two water mole- fer.[14a–14d] As the present complexes contain the para sub-
cules coordinated to the CuII ions, which is corroborated stituents on the bridging phenolate, definitive assignment
by the crystal structure of 4. The complex 8 showed weight of CT bands can be provided based on the electronic effects
loss for all six water molecules in the temperature range 80– induced by the substituents. Electron-donating groups are
119 °C and the crystal structure of 8 contains two aqua expected to decrease the Lewis acidity of the copper center
ligands and four lattice water molecules. The loss of metal thus shifting the phenoxo-to-copper CT band to lower ener-
bound water molecules along with lattice waters is similar gies while electron-withdrawing groups are expected to in-
to the observed behavior during the solid-state supramolec- crease the Lewis acidity and shift the phenolate-to-copper
ular transformations via thermal dehydration.[6a–b] The TG CT bands to higher energies.[10a,14d] Accordingly, we ob-
of the bulk 1–3, 5–7, and 9–10 showed weight loss in the serve the electron-donating para –CH3 group shifting the
temperature range of 60–168 °C due to the loss of two LMCT band to lower energy and the electron-withdrawing
metal-bound aqua ligands. The observed dehydration tem- para –OH and –Cl groups causing the shift of the LMCT
Table 1. Summary of the UV/Vis spectroscopic data of 1–13.
Complex
Absorption bands
(DMF)
(Nujol)
d-d (ε)[a]
CT(ε)
d-d
CT
[Cu2(Scp11)2(H2O)2], 1
705 (150)
702 (170)
703 (170)
703 (1010)
703 (1890)
704 (180)
702 (1860)
704 (530)
623 (240)
702 (180)
703 (160)
702 (210)
703 (210)
380 (1780)
356 (1340)
394 (2050)
364 (1520)
380 (2800)
370 (1190)
400 (1410)
381 (1430)
368 (1680)
401 (2440)
364 (1740)
368 (2120)
372 (920)
695
690
697
696
698
691
696
700
628
697
695
704
702
378
360
392
370
388
378
398
382
370
399
368
364
379
[Cu2(ClScp11)2(H2O)2], 2
[Cu2(MeScp11)2(H2O)2], 3
[Cu2(OHScp11)2(H2O)2], 4
[Cu2(Sch11)2](H2O), 5
[Cu2(ClSch11)2(H2O)2], 6
[Cu2(MeSch11)2(H2O)2], 7
[{Cu2(Sch12)2}2Cu2(Sch12)2(H2O)2]·4H2O, 8
[Cu2(ClSch12)2]·2H2O, 9
[Cu2(MeSch12)2(H2O)2], 10
[Cu2(Diala5)2(H2O)2]·H2O, 11
[Cu2(Diala4)2(H2O)2]·H2O, 12
[Cu2(Diala3)2(H2O)2]·H2O, 13
[a] (–1 cm–1).
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Synthesis, Structures and Catecholase Activity of a New Series of Dicopper(II) Complexes
FULL PAPER
band to higher energy. On the basis of the crystal structure Table 2. Selected bond lengths and bond angles for the complexes.
of 9a, the geometry around the copper(II) centers in 9 is
assumed to have a square-planar geometry. The observed
Complex 1a
Cu(1)–O(2)
Cu(1)–O(1)[a]
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–O(4)
1.945(2)
1.958(2)
1.961(2)
1.983(3)
2.213(2)
O(2)–Cu(1)–O(1)[a]
102.78(9)
d-d transitions in 9 at 623 nm also indicate square planar
CuII centers. This behavior is similar to that recorded for
the structurally well-characterized square planar copper(II)
complexes.[13] The electronic spectra of 1–13 do not change
much in DMF solution and Nujol mull, and appear to indi-
cate that the coordination number and geometry observed
in the solid state are retained in solution.
O(1)–Cu(1)–Cu(1)[a] 39.44(6)
N(1)–Cu(1)–Cu(1)[a] 131.92(8)
C(1)–O(1)–Cu(1)[a]
C(1)–O(1)–Cu(1)
133.7(2)
118.4(2)
Cu(1)–Cu(1)[a] 3.0245(7)
Cu(1)[a]–O(1)–Cu(1) 101.04(9)
Complex 2a
Cu(1)–O(4)
Cu(1)–N(1)
Cu(1)–O(2)
Cu(1)–O(1)
Cu(1)–O(8)
Cu(1)–Cu(2)
1.949(2)
1.962(3)
1.964(3)
2.002(3)
2.230(3)
3.0428(6)
O(4)–Cu(1)–N(1)
O(4)–Cu(1)–O(2)
N(1)–Cu(1)–O(2)
O(4)–Cu(1)–O(1)
N(1)–Cu(1)–O(1)
O(2)–Cu(1)–O(1)
68.2(1)
102.4(1)
82.5(1)
79.2(1)
93.5(1)
165.7(1)
ESI-MS Studies
The structural behavior of the complexes 1–13 in solu-
tion has been investigated by ESI mass spectral analysis.
ESI mass spectra of the complexes were recorded in MeOH
except for 2, 7, and 9 for which the spectra were recorded
in a 1:1 solvent mixture of MeOH and DMSO because of
a solubility problem. As indicated by the masses of both
positive and negative ions in the spectra, all the complexes
are found to exist in solution mainly as dicopper(II) species.
For example, the positive ion ESI masses observed for 8
shows the existence of dicopper(II) species predominantly
in solution as [Cu2(Sch12)2Na]+ (643, 100). The other less
intense peaks found were [Cu(Sch12)Na]+ (333, 27);
[Cu3(Sch12)3Na]+ (954, 55); [Cu4(Sch12)4Na]+ (1267, 25);
[Cu5(Sch12)5Na]+ (1574, 20). Our attempts to obtain con-
sistent electrochemical data using CV were unsuccessful ow-
ing to the solubility problem. Nonetheless, it maybe noted
that the concentration levels in methanolic solutions were
just enough for the activity studies.
Complex 3a
Cu(1)–O(2)
Cu(1)–O(1)[a]
Cu(1)–O(1)
Cu(1)–N(1)
1.939(2)
1.944(2)
1.960(2)
1.961(2)
O(2)–Cu(1)–O(1)[a]
O(2)–Cu(1)–O(1)
O(1)[a]–Cu(1)–O(1)
O(1)[a]–Cu(1)–N(1)
102.02(7)
174.04(7)
79.33(7)
171.71(7)
Cu(1)–Cu(1)[a] 3.0056(5)
O(2)–Cu(1)–Cu(1)[a] 141.57(5)
O(1)–Cu(1)–N(1)
93.79(7)
Complex 4
Cu(1)–O(2)
Cu(1)–O(1)
Cu(1)–O(1)[a]
Cu(1)–N(1)
Cu(1)–O(4)
1.920(2)
1.930(2)
1.934(2)
1.973(2)
2.660(3)
O(2)–Cu(1)–O(1)
O(2)–Cu(1)–O(1)[a]
O(1)–Cu(1)–O(1)[a]
O(2)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)
O(1)[a]–Cu(1)–N(1)
O(1)[a]–Cu(1)–O(4)
178.97(8)
102.10(8)
78.48(8)
84.57(9)
94.67(9)
165.21(9)
105.32(8)
Cu(1)–Cu(1)[a] 2.9923(7)
Complex 6a
General Description of Crystal Structures
Cu(1)–O(2)
Cu(1)–O(1)[a]
Cu(1)–N(1)
Cu(1)–O(1)
O(1)–Cu(1)[a]
1.940(2)
1.951(1)
1.968(2)
1.972(1)
1.951(1)
O(2)–Cu(1)–O(1)[a]
O(2)–Cu(1)–N(1)
O(1)[a]Cu(1)–N(1)
O(2)–Cu(1)–O(1)
O(1)[a]–Cu(1)–O(1)
103.62(6)
83.76(7)
169.91(7)
173.62(6)
78.49(6)
The crystal structures of complexes 1a, 2a, 3a, 4, 6a, 8,
9a, and 12a were determined by single-crystal X-ray crystal-
lographic analysis. In all these complexes, the reduced
Schiff-base ligands display a tridentate coordination mode
while bridging the two copper ions through phenolate oxy-
gen atoms forming Cu2O2 cores with a Cu···Cu distance of
ca. 3 Å. The crystal structure of 9a displayed CuII centers
with a square-planar geometry while the other complexes
contain square pyramidal CuII centers.
In all the square pyramidal complexes, the basal plane of
the square pyramid is completed by the coordination of
each CuII to the two bridging phenolate oxygen atoms, im-
ine nitrogen, and carboxylate oxygen. The Cu–O bond
lengths due to the coordination of each CuII to the bridging
phenolate and carboxylate oxygen atoms in the basal plane
fall in the range of 1.934(2)–2.002(3) and 1.876(2)–
1.982(7) Å, respectively, while the bond lengths due to Cu–
N (imine nitrogen) are observed in the range of 1.950(2)–
2.004(8) Å. The apical position is occupied by solvents. A
crystallographic center of inversion is present in the dimeric
structure of 1a, 3a, 4, 6a, and 9a. Selected bond lengths
and angles as well as hydrogen bond parameters have been
compiled in Table 2 and Table 3 respectively.
Cu(1)–Cu(1)[a] 3.0382(5)
O(2)–Cu(1)–Cu(1)[a] 142.78(5)
Complex 8
Cu(1)–O(2)
Cu(1)–O(1)
Cu(1)–O(1)[a]
Cu(1)–N(1)
Cu(1)–O(6)
Cu(2)–O(5)
Cu(2)–O(9)
Cu(2)–O(4)[b]
O(1)–Cu(1)[a]
O(4)–Cu(2)[b]
Cu(3)–O(7)[c]
O(7)–Cu(3)[c]
CuI–Cu(1A)
1.91(1)
1.95(1)
1.97(1)
1.98(1)
2.35(1)
1.93(2)
2.51(1)
1.96(1)
1.97(1)
1.96(1)
1.96(1)
1.96(1)
3.074(4)
O(2)–Cu(1)–O(1)[a]
O(1)–Cu(1)–O(1)[a]
O(1)a–Cu(1)–N(1)
O(1)a–Cu(1)–O(6)
O(5)–Cu(2)–O(4)[b]
O(4)[b]–Cu(2)–O(4)
O(4)[b]–Cu(2)–N(2)
94.9(5)
76.8(6)
154.7(6)
111.8(5)
96.8(5)
76.1(5)
161.0(6)
C(15)–O(4)–Cu(2)[b] 133.4(10)
Cu(2)[b]–O(4)–Cu(2) 103.9(5)
O(7)[c]–Cu(3)–O(7)
O(7)[c]–Cu(3)–N(3)
78.1(6)
163.4(6)
Cu(3)[c]–O(7)–Cu(3) 101.9(6)
O(7)[c]–Cu(3)–O(11) 95.0(5)
Cu(3)–Cu(3)[c] 3.056(4)
Complex 9a
Cu(1)–O(2)
Cu(1)–O(1)
Cu(1)–O(1)[a]
Cu(1)–N(1)
1.876(2)
1.926(2)
1.949(2)
1.950(2)
O(2)–Cu(1)–O(1)
O(2)–Cu(1)–O(1)[a]
O(1)–Cu(1)–O(1)[a]
O(2)–Cu(1)–N(1)
O(1)a–Cu(1)–N(1)
164.48(9)
95.72(8)
76.90(9)
95.98(9)
165.17(9)
Cu(1)–Cu(1)[a] 3.0350(7)
With respect to the ligands, 1a, 2a, 3a, and 4 contain a
1-aminocyclopentanecarboxylate side arm but different
O(2)–Cu(1)–Cu(1)[a] 132.51(6)
Eur. J. Inorg. Chem. 2006, 2656–2670
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B. Sreenivasulu, F. Zhao, S. Gao, J. J. Vittal
FULL PAPER
Table 2. (continued)
substituents on the 5-position of the benzene ring; H in 1a,
Cl in 2a, CH3 in 3a, and OH in 4a. All these complexes, 1a,
2a, 3a, and 4 display CuII centers with a distorted square
pyramidal geometry (τ = 0.168, 0.043, 0.038, 0.228, and
0.003, respectively).[15] The common 1-aminocyclopen-
tanecarboxylate side arm of the ligands in 1a, 2a, 3a, and
4 resulted in the formation of five-membered rings. The
complex 1a as a methanol adduct crystallized with two
methanol molecules in the monoclinic system with space
group C2/c with a Cu···Cu separation of 3.0245(7) Å. The
apical positions of the square pyramid of the two CuII
atoms are occupied by methanol molecules in trans fashion
(Figure 1). Complementary MeOH···O=C (O4–H4···O2)
and N–H···O=C (N1–H1···O3) intermolecular hydrogen
bonding generates a 1D polymeric structure in 1a in the
solid state (Figure 2).
Complex 12a
Cu(1)–O(5)
Cu(1)–O(1)
Cu(1)–N(2)
Cu(1)–O(4)
Cu(1)–O(9)
Cu(1)–Cu(2)
1.982(7)
1.999(7)
2.004(8)
2.005(7)
2.300(7)
3.0225(8)
O(5)–Cu(1)–O(1)
O(5)–Cu(1)–N(2)
O(1)–Cu(1)–N(2)
O(5)–Cu(1)–O(4)
O(1)–Cu(1)–O(4)
N(2)–Cu(1)–O(4)
O(5)–Cu(1)–O(9)
O(1)–Cu(1)–O(9)
N(2)–Cu(1)–O(9)
O(4)–Cu(1)O(9)
102.3(3)
84.1(3)
169.1(3)
168.9(3)
77.4(3)
94.7(3)
97.8(3)
93.4(3)
94.5(3)
93.3(3)
[a] Symmetry transformations used to generate equivalent atoms:
–x+1, –y+1, –z+1 (1a); –x+1, –y+1, –z+1 (3a); –x+1, –y+1,
–z+1 (4); –x+1, –y+1, –z+1 (6a); –x+1, –y+1, –z+1 (8);
–x+1, –y+1, –z+1 (9a). [b] –x, –y, –z (8). [c] –x+1, –y+1,
–z (8).
Table 3. Hydrogen bond parameters.
D–H
d(D–H)
d(H···A)
ЄDHA
d(D···A)
A
Symmetry
Compound 1a
N1–H1
O4–H4
0.80(3)
0.72(4)
2.35(4)
1.97(4)
148(4)
174(5)
3.053(3)
2.691(4)
O3
O2
x–1, y, z–1/2
x–1, y, z–1/2
Compound 2a
O7–H7A
O7–H7B
0.73(6)
0.81(6)
2.26(4)
1.92(4)
162(6)
177(5)
2.961(9)
2.728(4)
N4S
O3
x+1, y, z
x–1, y–1, z–1
Compound 3a
N1–H1
O4–H4
O5–H5
0.87(3)
0.76(4)
0.78(6)
2.10(3)
2.05(4)
2.14(5)
159(3)
163(3)
156(5)
2.932(3)
2.785(3)
2.864(4)
O5
O3
O2
–x,–y+1,-z+1
x+1, y, z
Compound 4
N1–H1
0.84(3)
0.68(4)
0.75(4)
0.72(4)
0.98
2.39(3)
2.10(4)
2.20(4)
2.05(4)
2.58
143(3)
162(4)
146(4)
173(4)
144
3.100(3)
2.753(3)
2.849(4)
2.770(4)
3.420(3)
O3
O3
O5
O4
O2
x–1, y–1, –z
x–1, y–1, –z
x–2, –y, 1 –z
x+1, y, z
O4–H4A
O4–H4B
O5–H5
C13–H13B
x–1, y–1, –z
Compound 6a
N1–H1
O4–H4
O5–H5
1.01(3)
0.71(3)
0.75(3)
1.92(3)
2.08(3)
2.11(3)
167(2)
171(4)
169(3)
2.905(3)
2.775(2)
2.851(3)
O5
O3
O2
x–1, y, z
–x+2, –y+1, –z+1
Compound 8
N1–H1
N2–H2
N3–H3
O11–H11C
O11–H11D
O12–H12C
O13–H13C
O13–H13D
0.81(17)
0.8(2)
0.85(18)
0.9(3)
0.9(2)
0.9(3)
2.38(16)
2.4(2)
2.13(19)
1.9(3)
2.0(2)
1.9(3)
157(19)
164(18)
170(15)
165(26)
164(27)
168(18)
153(12)
161(12)
3.14(1)
3.18(2)
2.97(2)
2.83(3)
2.84(2)
2.75(2)
2.88(3)
2.89(3)
O5
O8
O12
O12
O6
O2
O9
O3
0.9(3)
0.9(3)
2.0(3)
2.02(18)
Compound 9a
N1–H1
O1S–H1S
0.84(4)
0.79(4)
2.03(4)
1.89(4)
171(3)
166(5)
2.866(4)
2.667(4)
O1S
O3
–x+1, –y+1, –z
Compound 12a
N1–H1
N2–H2
O7–H7
O8–H8A
0.92
0.92
0.83
0.83
2.11
2.08
1.87
1.93
153
146
164
154
2.96(1)
2.89(1)
2.68(1)
2.70(1)
O11
O12
O3
x+1, y+1, z
x–1, y–1, z
O6
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Synthesis, Structures and Catecholase Activity of a New Series of Dicopper(II) Complexes
FULL PAPER
square pyramidal geometry around each CuII center (Fig-
ure 3) with a Cu···Cu distance of 2.9924(8) Å. Intermo-
lecular hydrogen bonds generated by the hydroxy group on
the phenolate moiety with the aqua ligand (O4–H4B···O5,
O5–H5···O4) and carboxylate oxygen atom (O4–H4A···O3)
and by the imine hydrogen with another carboxylate oxygen
(N1–H1···O3) have resulted in a 3D hydrogen-bonded net-
work in 4 as shown in Figure 4.
Figure 1. A perspective view of the dimer 1a (symmetry equivalent
positions: –x+1, –y+1, –z+1).
Figure 3. A perspective view of the dimer 4 (symmetry equivalent
positions: –x+1, –y+1, –z+1).
Figure 2. A view showing a portion of the 1D hydrogen-bonded
structure in 1a.
In 2a the two copper atoms in the dimer are separated
by a distance of 3.0428(6) Å. The apical site of the square
pyramid is occupied by DMF [Cu(1)–O(8), 2.230(3) Å] with
Cu(1) and the aqua ligand [Cu(2)–O(7), 2.299(3) Å] with
Cu(2) in trans to each other. Each dimer is associated with
two molecules of acetonitrile solvent in the asymmetric
unit. There are very weak interactions between the carbonyl
oxygen atoms and the copper atoms [Cu(1)···O3(1 – x, 1 –
y, 1 – z), 2.885 Å and Cu(2)···O6(1 – x, y, z), 2.922 Å] that
Figure 4. Packing diagram of 4.
¯
The compound 6 crystallized in triclinic space group P1
are about the sum of the van der Waals radii, 2.9 Å. These with two methanol molecules in the crystal lattice and 6a is
Cu···O interactions generate a 1D polymer along the b-axis isomorphous to 3a, i.e. the arrangement of molecules in the
sustained by hydrogen bonds between carboxylate oxygen crystal lattice, but not isostructural. The hydrogen bonding
and the aqua ligand, O(7) H(7B)···O(3). Further O(7)– pattern in 6a is also similar to that observed in 3a. The
H(7A)···N(4S) hydrogen bonds have also been observed.
solid-state crystal packing along the b axis revealed that the
The complex 3a displays square pyramidal geometry at complex is a 1D polymer that resulted from weak Cu···O
each CuII center with a Cu···Cu distance of 3.0056(5) Å. interactions and intermolecular hydrogen bonding. Further,
The apical sites of the square pyramid at each CuII center carboxylate oxygen atoms of the neighboring dimeric units
are occupied by methanol molecules in trans fashion with maintain weak interactions with each CuII ion (Cu···O,
a Cu···O distance of 2.59 Å. Packing of dimers along the a 2.95 Å) in syn-anti mode. In addition to these interactions,
axis displays a 1D hydrogen-bonded polymer in 3a sup- crystal packing also reveals that N–H hydrogen atoms are
ported by intermolecular hydrogen bonds generated be- involved in the intermolecular hydrogen bonding to meth-
tween imine H atoms and methanol oxygen [N–H···O(5)], anolic oxygen atoms [N(1)–H(1)···O(5)] and the methanolic
methanolic hydrogen, and carboxylate oxygen atoms [O(4)– hydrogen atoms maintain intermolecular hydrogen bonds
H(4)···O(3) and O(5)–H(5)···O(2)] as well as weak interac- to carboxylate oxygen atoms [O(4)–H(4)···O(3) and O(5)–
tions of neighboring carboxylate oxygen atoms with each H(5)···O(2)].
CuII ion (Cu···O, 2.885 Å) in syn-anti fashion.
In the crystal structure of 4 the two lattice water mole- space group P1, there are three independent Cu –Sch12
cules are in close interaction with CuII atoms [Cu–O, units and each one is near an inversion center (1/2, 1/2,
2.66(3) Å] occupying the apical positions to complete the 1/2), (0,0,0), and (1/2, 1/2, 0) as illustrated in Figure 5. Of
In the crystal structure of 8 for Z = 3 in the triclinic
II
¯
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FULL PAPER
these Cu3 has an aqua ligand in the apical position. The
The dimer 12a crystallized with four DMSO molecules
O9 of the carboxylate group from this unit is bonded to of which two are bonded at the apical positions of each
Cu2 with the Cu2–O9 distance of 2.51(1) Å. Similarly, O6 CuII in the dimer and two are hydrogen-bonded to the im-
occupies the apical axial position of the Cu1 [Cu1–O6, ine hydrogen atoms through N–H···O interactions. The two
2.36(1) Å] with a distorted square pyramidal geometry. On DMSO molecules are disposed in anti fashion probably due
the other hand, the oxygen atom of the third carboxylate to the bulkiness of the axial substitution. This causes the
group, O3, is only involved in hydrogen bonding to a hydro- dimer to have a pseudo-center of inversion which is unprec-
gen atom, H13D of lattice water. The axial bonding of the edented in these types of structure having chiral reduced
carbonyl oxygen atoms O6 and O9 from neighboring di- Schiff-base ligands.[6] A ribbon-like 1D hydrogen-bonded
meric units to Cu1 and Cu2 centers produces a 2D coordi- polymeric structure is produced by complementary O–
nation polymeric structure and the interdimer connectivity H···O bonds between phenolic hydrogen and oxygen atoms
leads to the formation of a 2D (4, 4) network structure as of the carboxylate group. All these polymers are aligned
shown in Figure 6 and these (4, 4) nets are well known in parallel to the (1ı0) plane. Acetone molecules filled the
¯
coordination polymeric structures.[16]
empty cavities between these strands.
Magnetic Studies of [{Cu2(Sch12)2}2Cu2(Sch12)2(H2O)2]·
4H2O (8)
It is evident from the previous section that only com-
plexes 4 and 8 furnished the single crystals that represent
the structures of the bulk. Further, the inter-dimer connec-
tivity in 8 generated an interesting 2D (4, 4) coordination
polymeric network structure. Hence, we investigated the
variable temperature magnetic moments for 8 in order to
understand the inter-dimer magnetic coupling interactions
mediated by syn-anti carboxylate bridging.
The temperature dependences of χm and χmT for crystal-
line samples of 8 per Cu2 unit are presented in Figure 7. On
lowering the temperature, χm decreases gradually, there is
an increase below ca. 70 K; while the χmT value even at
330 K (0.383 cm3 mol–1 K) is quite small compared with the
expected value 0.75 cm3 mol–1 K for two noninteracting
CuII ions, upon cooling it decreases rapidly to a value close
to zero at 2 K. The increase of χm at low temperature
should be due to trace paramagnetic impurity (ρ). Overall,
the data suggest a strong antiferromagnetic coupling be-
tween the adjacent Cu ions in 8. The coupling between the
dinuclear moiety in the 2D layer of 8 is negligible, as ex-
pected, because of both the syn-anti carboxylate bridging
Figure 5. Perspective view of the unit cell contents in 8. The lattice
water molecules are omitted for clarity.
Figure 6. Portion of packing diagram of 8 showing the 2D connec-
tivity. The hydrogen atoms and lattice water molecules have been
omitted for clarity.
In this series, the only compound that displays a square-
planar geometry at the CuII atoms is 9a. The unit cell con-
tains two methanol molecules involved in hydrogen bonding
via imine nitrogen [N(1)–H(1)···O(1S)] and carboxylate oxy-
gen [O(1S)–H(1S)···O(3)], which lead to the formation of a
1D polymeric structure. The hydrogen bond lengths ob-
served in the solid-state packing of 9a are found to vary
between 1.90 and 2.38 Å. Table 3 contains selected hydro-
gen bond parameters. These hydrogen bonds are considered
to be normal as compared to the available literature on N–
H···O and O–H···O hydrogen bond parameters.[17]
Figure 7. The temperature dependences of χm and χmT in the range
of 2–350 K for 8. The solid lines represent the fit using a Cu2 dimer
model.
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Synthesis, Structures and Catecholase Activity of a New Series of Dicopper(II) Complexes
FULL PAPER
mode and the orthogonal relationship of the planes of the
neighboring Cu2O2.[14b] Therefore, the magnetic data of 8
can be analyzed by fitting the susceptibilities to an equation
calculated using an H = –2JS1S2 Hamiltonian for a dimer
plus impurity of the monomer model.[14e–14f] The param-
eters have their usual meanings. As shown in Figure 7, a
reasonable fit was obtained with the results:
J =
–230(3) cm–1, g = 2.29(2), ρ = 0.0056; Nα = 4.9×10–6 with
R = 1.4×10–3 {R = ∑[(χm)obs – (χm)calc]2/∑[(χm)obs]2}.
Catecholase Biomimetic Studies
In order to gain deeper insight into the various param-
eters that determine the copper-mediated substrate oxi-
dations and bimetallic reactivity both in natural metalloen-
zymes and in synthetic analogues, quite a number of mono-
and dinuclear copper(II) complexes have been investigated
as biomimetic catalysts for catechol oxidation.[18–23] In all
these investigations, a common and convenient model sub-
strate, 3,5-DTBC was used due to its low redox poten-
tials.[20] Some of the crucial factors dictating the catecholase
activity can now be highlighted based on many investi-
gations correlating various structural parameters of the di-
copper(II) complexes with their activity; viz., the distance
between the CuII centers,[24] the nature of the bridging
group between the copper ions,[25] electronic properties of
the complexes,[10,26] and the geometric changes of the di-
copper core.[27] Furthermore, the factors such as the
number of donor sites, nature of the donors, and the rigid-
ity of the ligand or the bridging moiety imposing strain in
the complex have also been found to play a vital role.[28]
However, the studies correlating the activity with electronic
effects due to substituents on the bridging phenolate have
not been well explored.[10] Since the present series of com-
plexes contain both electron-donating and -withdrawing
substituents on the phenyl ring we have attempted to inves-
tigate their activity.
Figure 8. Oxidation of 3,5-DTBC by 8 monitored by UV/Vis spec-
troscopy. Higher concentrations were used only to emphasize the
bands for the oxidation process.
Figure 9. Lineweaver–Burk plot for the catalysis by 8.
Table 4. Kinetic parameters for the oxidation of 3,5-DTBC by the
complexes 1–13.[a]
On the basis of the chemical and spectroscopic charac-
terizations of the complexes 1–13 and the crystal structures
of selected compounds, all these complexes are charac-
terized by active Cu2O2 centers with a Cu···Cu distance of
ca. 3 Å, which is favorable for the efficient binding of the
substrate. Minor changes observed in the UV/Vis spectra
recorded in DMF solution compared to Nujol mull indicate
that the coordination number and geometry of 1–13 are
retained in solution. Further, ESI-MS studies confirm the
existence of copper(II) dimers in solution. Subsequently, all
the complexes exhibit significant activity due to the pres-
ence of active dicopper(II) cores. The course of oxidation of
3,5-DTBC catalyzed by 8 is shown in Figure 8. The kinetic
parameters have been determined by applying the Micha-
elis–Menten approach. A linear relationship for the initial
rates and the concentration of the complexes has been ob-
tained for 1–13, which shows a first-order dependence of
Complex Substituent (R) Kcat
Km
[mM]
Vmax
[h–1]
[106 Ms–1]
1
–H
–Cl
–CH3
–OH
–H
–Cl
–CH3
–H
850(28)
694(32)
1003(59)
773(30)
863(28)
785(49)
1080(30)
2006(44)
883(33)
3120(28)
595(27)
513(21)
460(14)
22.3(4)
26.2(3)
15.4(6)
20.6(7)
18.5(5)
20.3(8)
14.6(3)
9.2(1)
17.7(3)
4.3(1)
21.4(4)
28.6(4)
31.3(3)
30.2(6)
22.4(1)
34.3(6)
26.7(3)
30.2(1)
24.5(5)
36.1(2)
68.5(4)
29.3(3)
104.6(6)
22.3(7)
19.4(8)
17.6(5)
2
3
4
5
6
7
8
9
–Cl
10
11
12
13
–CH3
para –OH
meta –OH
ortho –OH
[a] The values were obtained from three measurements.
the rate on the catalyst concentration. An example of a It is obvious from Table 4 that the activity of 1–4 follows
Lineweaver–Burk plot[18f] is given in Figure 9 for 8. Kinetic the order 3 Ͼ 1 Ͼ 4 Ͼ 2 with 3 showing higher activity
parameters of the complexes are given in Table 4.
(Kcat = 1003 h–1). Further, the activity of 2 has noticeably
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B. Sreenivasulu, F. Zhao, S. Gao, J. J. Vittal
FULL PAPER
decreased due to the strong electron-withdrawing nature of be noted that the exact Cu···Cu separation in solid-state
Cl when compared to the activity of 4 containing the –OH structures need not be retained in the solution, and direct
group.[29] Thus, with respect to the substituents on the correlation between the Cu···Cu distance and catalytic ac-
bridging phenolate, the activity of the corresponding com- tivity need not be expected. However, based on the infor-
plexes followed the order: CH3 Ͼ H Ͼ OH Ͼ Cl. A similar mation regarding the geometries and molecular species ob-
trend (7 Ͼ 5 Ͼ 6) has been observed (Table 4) for CuII tained from the solution studies involving the techniques
complexes of RSch11 ligands, 5–7. Among 8–10, the such as UV/Vis spectroscopy and ESI-MS at least some
methyl-substituted 10 exhibited significantly higher activity correlations can be achieved.
(Kcat = 3120 h–1) while chloro-substituted 9 with CuII in a
square-planar geometry showed a lower activity (Kcat
Recently, Wegner et al. reported the most active catalyst
among the known functional model complexes so far, with
=
883 h–1) following a similar trend. Earlier investigations on a turnover number or Kcat of 9927 h–1.[5h] When compared
the activity of copper(II) systems have shown that the mainly with the turnover numbers of 1188 h–1, 2804 h–1,
square planar copper(II) complexes are weak catechol- and 5470 h–1, reported by Monzani et al.,[31–32] Meyer et
oxidase catalysts.[19,22,24d,28,31] However, reports are also al., [27] and Mukherjee et al.[25a] respectively, and with the
available on square planar dicopper(II) complexes showing other lower to moderate turnover values,[5,10,18–19,22–30] the
very high activity.[18f,25] On the same grounds, 9 with a observed turnover rates of 460 to 3120 h–1 may suggest
similar geometry exhibiting significant activity can be an- moderate to high catalytic activity of the present complexes.
other example for the latter.
However, all activity studies on the model complexes syn-
The six-membered chelate rings facilitate strain-free sub- thesized so far have only achieved turnover numbers of
strate binding compared to the five-membered rings[9,18f,28] about several thousand-fold lower when compared to the
by offering a flexible and relaxed coordination sphere at turnover number of native enzymes[31,26a] (for example cate-
CuII centers thus enhancing the activity of 8 compared to chol oxidase from sweet potatoes has been found to be
the activities of 1 (Kcat = 850 h–1), and 5 (Kcat = 863 h–1) highly active with a Kcat of 2293 s–1 especially towards cate-
forming five-membered rings. Furthermore, it is also the chol).[33] The current high activity in 10 (Kcat = 3120 h–1)
formation of six-membered rings that significantly en- observed among the present model catalysts has been found
hanced the activity of 10 (Kcat = 3120 h–1) compared to 3 to be ca. 2600-fold slower towards the oxidation of 3,5-
(Kcat = 1003 h–1) or 7 (Kcat = 1080 h–1) which contain five- DTBC when compared to the natural enzyme. Finally, these
membered rings. It may be noted that these complexes (3, findings suggest that significant differences in the catalytic
7, and 10) have para methyl phenyl rings.
In the case of 11–13 containing Diala ligands, the trend dicious choice of different sets of binucleating ligands.
in Kcat is 11 Ͼ 12 Ͼ 13 (Table 4). The activity of 11 (Kcat
595 h–1) with a para-hydroxy bridging phenolate is found to
ability of dicopper(II) complexes can be induced by the ju-
=
be higher than that of 12 (Kcat = 513 h–1) and 13 (Kcat
=
460 h–1) containing meta- and ortho-hydroxy bridging phe-
nolate moieties, respectively. These variations in the activity
maybe attributed to the differences in the steric
Summary
A new series of dicopper(II) complexes 1–13 containing
match[18,27,28] caused by the respective changes in the posi- reduced Schiff-base ligands formed between different sub-
tion of the –OH group on the bridging phenolate. Further, stituted salicylaldehydes and amino acids have been synthe-
comparing the activity (Kcat = 666 h–1), under similar exper- sized and characterized by physicochemical and spectro-
imental conditions, of our previously reported unsubsti- scopic methods. On the basis of the single-crystal X-ray
tuted complex [Cu2(-Sala)2(H2O)][8] with the activity (Kcat crystallographic studies of selected compounds, it can be
= 595 h–1) of the present para hydroxy-substituted 11 it is suggested that all the complexes contain Cu2O2 cores with
obvious that the decrease in the activity of 11 can be attrib- a Cu···Cu distance of ca. 3 Å. In view of the interesting
uted to the effect of the electron-withdrawing -OH group 2D coordination polymeric structure, variable temperature
on the phenyl ring of the ligand.
The kinetic data for 1–13 presented in Table 4 shows that strong intradimer antiferromagnetic coupling.
the substitution of phenolate by electron-donating substitu- The catecholase activity of the complexes 1–4, 5–7, 8–10,
magnetic studies carried out on 8 suggested the presence of
ents increased the activity while the substitution by elec- and 11–13 containing 1-aminocyclopentane-1-carboxylate,
tron-withdrawing groups decreased the activity. In the ab- 1-aminocyclohexane-1-carboxylate, 2-aminocyclohexane-1-
sence of electrochemical data for 1–13, it is difficult to con- carboxylate, and -alanine side chains, respectively, in the
clude that the catalytic efficiency is solely governed by the corresponding reduced Schiff-base ligands has been evalu-
electronic influence of the substituents except envisaging a ated and systematically compared. It has been found that
rough correlation. Moreover, no clear and direct correlation the presence of electron-withdrawing groups decreased the
has been established so far between the rates of reaction, activity while the electron-donating groups enhanced the
redox potentials, and electronic effects of the com- activity of the complexes. Under similar experimental con-
plexes.[10,26,28] However, other factors such as geometry, ste- ditions the activity shown by the current series of dicop-
ric factors, and interactions between the substrate and cata- per(II) complexes can be considered moderate to high as
lyst can also influence the overall efficiency. It should also observed in our previous results.[8–9]
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Synthesis, Structures and Catecholase Activity of a New Series of Dicopper(II) Complexes
FULL PAPER
pentane-1-carboxylic acid and 5-hydroxysalicylaldehyde. Yield:
0.75 g (63%). m.p. 263–264 °C. C13H17NO4 (251.6): calcd. C 62.1,
Experimental Section
Materials and Physical Measurements
H 6.8, N 5.5; found C 62.0, H 6.9, N 5.6. IR (KBr): ν = 3398 m
˜
(OH), 2960 m (NH), 1582 s υas(COO–), 1390 s υs(COO–), 1276 s
(phenolic CO) cm–1. 1H NMR (CD3OD): δ = 1.73–2.11 (m, 8 H,
-C5H8), 3.83 (s, 2 H, benzylic), 6.43–6.51 (m, 3 H, Ar-H). 13C NMR
(CD3OD): δ = 24.7, 26.1, 36.0, 37.6, and 40.9 (-C5H8), 72.7 (ben-
zylic), 117.6, 118.7, 119.6, 126.3, 148.2, and 159.6 (aromatic), 184.1
(-COOH). ESI-MS: m/z (%) = 251.
All the chemicals were purchased from commercial sources and
used as received for syntheses. All the solvents used were of reagent
grade. The yields are reported with respect to the metal salts.
1
The H NMR spectra were recorded with a Bruker ACF 300FT-
NMR spectrometer using TMS as an internal reference at 25 °C
and the infrared spectra (KBr pellet) were recorded using a FTS
165 Bio-Rad FTIR spectrometer in the range 4000–400 cm–1. The
electronic transmittance spectra were recorded with a Shimadzu
UV-2501/PC UV/Vis spectrometer in DMF and Nujol mull. ESI-
MS spectra were recorded using a Finnigan MAT LCQ Mass Spec-
trometer using the syringe-pump method. The elemental analyses
were performed in the Micro Analytical Laboratory, Department
of Chemistry, National University of Singapore. Water present in
the compounds was determined using a SDT 2960 TGA Thermal
Analyzer with a heating rate of 10 °Cmin–1 using a sample size of
7–10 mg per sample run. Room-temperature magnetic suscep-
tibility measurements were carried out with a Johnson–Mathey
Magnetic Susceptibility balance with Hg[Co(SCN)4] as the stan-
dard. Variable temperature magnetic studies were made using a
Quantum Design MPMS-XL5 SQUID magnetometer operating in
an applied field of 5 kOe. Corrections for diamagnetism were made
using Pascal’s constants. Optical rotation was measured on the
specified solutions in a 0.1-dm cell at 27 °C using a Perkin–Elmer
model 341 polarimeter. The concentration of the solutions was
5.5 mgmL–1 for both the ligands (in MeOH) and complexes (in
DMSO). The optical rotation of the ligands was recorded at the
wavelength of 365 nm where as for the complexes the measure-
ments were conducted at the wavelength of the D line (578 nm) of
sodium.
1-[(2-Hydroxybenzyl)amino]cyclohexane-1-carboxylic Acid
(H2Sch11): This compound was prepared from 1-aminocyclohex-
ane-1-carboxylic acid and salicylaldehyde. Yield: 1.06 g (90%). m.p.
296–297 °C. C14H19NO3 (249.3): calcd. C 67.4, H 7.6, N 5.6; found
C 67.4, H 7.7, N 5.5. IR (KBr): ν = 2941 m (NH), 1577 s
˜
υas(COO–), 1388 s υs(COO–), 1272 s (phenolic CO) cm–1. 1H NMR
(CD3OD): δ = 1.43–2.03 (m, 10 H, -C6H10), 3.70 (s, 2 H, benzylic),
6.41–7.02 (m, 4 H, Ar-H). 13C NMR (CD3OD): δ = 23.0, 27.0,
34.5, 37.1, 48.1, and 48.4 (-C6H10), 64.3 (benzylic), 117.7, 118.3,
127.0, 129.3, 130.0, and 162.5 (aromatic), 182.8 (-COOH). ESI-MS:
m/z (%) = 249.2
1-[(5-Chloro-2-hydroxybenzyl)amino]cyclohexane-1-carboxylic Acid
(H2ClSch11): This compound was prepared from 1-aminocyclohex-
ane-1-carboxylic acid and 5-chlorosalicylaldehyde. Yield: 0.88 g
(66%). m.p. 290–291 °C. C14H18ClNO3 (283.7): calcd. C 59.3, H
6.4, N 4.9; found C 59.0, H 6.5, N 4.9. IR (KBr): ν = 3462 m
˜
(OH), 2940 m (N–H), 1568 s υas(COO–), 1495 s υs(COO–), 1273 s
1
(phenolic CO) cm–1. H NMR (CD3OD): δ = 1.41–2.03 (m, 10 H,
-C6H10), 3.70 (s, 2 H, benzylic), 6.60–7.01 (m, 4 H, Ar-H). 13C
NMR (CD3OD): δ = 23.4, 26.9, 34.3, 37.0, 46.1, and 48.2,
(-C6H10), 64.61 (benzylic), 119.6, 121.7, 128.2, 128.9, 129.4, and
161.5 (aromatic), 182.2 (-COOH). ESI-MS: m/z (%) = 283.7.
1-[(2-Hydroxy-5-methylbenzyl)amino]cyclohexane-1-carboxylic Acid
(H2MeSch11): This compound was prepared from 1-aminocyclo-
hexane-1-carboxylic acid and 5-methylsalicylaldehyde. Yield: 0.81 g
(66%). m.p. 295–296 °C. C15H21NO3 (263.3): calcd. C 68.4, H 8.0,
Ligands
All the ligands except H3Diala5, H3Diala4, and H3Diala3 have
been synthesized using the same molar quantities according to the
procedure described for H2Scp11 in our earlier report.[6c]
N 5.3; found C 67.9, H 8.0, N 5.4. IR (KBr): ν = 3450 m (OH),
˜
2932 m (NH), 1624 s υas(COO–), 1381 s υs(COO–), 1262 s (phenolic
1
CO) cm–1. H NMR (CD3OD): δ = 1.40–2.17 (m, 10 H, -C6H10),
1-[(5-Chloro-2-hydroxybenzyl)amino]cyclopentane-1-carboxylic Acid
(H2ClScp11): This compound was prepared from 1-aminocyclo-
pentane-1-carboxylic acid and 5-chlorosalicylaldehyde. Yield:
0.92 g (73%). m.p. 271–272 °C. C13H16ClNO3 (269.7): calcd. C
2.38 (CH3), 3.71 (s, 2 H, benzylic), 6.60–6.88 (m, 4 H, Ar-H). 13C
NMR (CD3OD): δ = 20.5 (Ar–CH3), 23.3, 26.8, 34.3, 37.0, 47.2,
and 48.2, (-C6H10), 64.4 (benzylic), 117.0, 125.3, 128.8, 129.7,
130.3, and 156.8 (aromatic), 182.71 (-COOH). ESI-MS: m/z (%) =
263.3.
57.8, H 5.9, N 5.2; found C 57.2, H 6.0, N 5.7. IR (KBr): ν =
˜
2966 m (NH), 1566 s υas(COO–), 1384 s υs(COO–), 1276 s (phenolic
CO) cm–1. 1H NMR (CD3OD): δ = 1.83–2.28 (m, 8 H, -C5H8), 4.12
(s, 2 H, benzylic), 6.83–7.36 (m, 3 H, Ar-H). 13C NMR (CD3OD):
δ = 25.9, 37.3, 47.5, 48.1, and 48.4 (-C5H8), 72.9 (benzylic), 119.4,
122.3, 127.4, 129.1, 129.5, and 160.6 (aromatic), 182.7 (-COOH).
EI-MS: m/z (%) = 269.2.
2-[(2-Hydroxybenzyl)amino]cyclohexane-1-carboxylic Acid
(H2Sch12): This compound was prepared from 2-aminocyclohex-
anecarboxylic acid and salicylaldehyde. Yield: 1.05 g (89%). m.p.
238–239 °C. C14H19NO3 (249.3): calcd. C 67.4, H 7.7, N 5.6; found
C 67.0, H 7.6, N 5.5. IR (KBr): ν = 2853 m (N–H), 1580 s
˜
υas(COO–), 1461 s υs(COO–), 1263 s (phenolic CO) cm–1. 1H NMR
(CD3OD): δ = 1.38–2.07 (m, 8 H, -C6H10), 2.62 (t, 1 H, HC–N),
3.01 (t, 1 H, HC–COO–), 4.06 (s, 2 H, benzylic), 6.53–7.05 (m, 4
H, Ar-H). 13C NMR (CD3OD): δ = 24.0, 25.1, 28.2, 29.2, 48.1,
and 48.4 (-C6H10), 57.4 (benzylic), 116.4, 119.1, 125.8, 129.4, 129.6,
and 164.5 (aromatic), 182.3 (-COOH). ESI-MS: m/z (%) = 249.3.
1-[(2-Hydroxy-5-methylbenzyl)amino]cyclopentane-1-carboxylic
Acid (H2MeScp11): This compound was prepared from 1-aminocy-
clopentane-1-carboxylic acid and 5-methylsalicylaldehyde. Yield:
0.92 g (76%). m.p. 272–273 °C. C14H19NO3 (249.3): calcd. C 67.4,
H 7.6, N 5.6; found C 67.0, H 7.7, N 5.9. IR (KBr): ν = 3447 m
˜
(OH), 2956 m (NH), 1571 s υas(COO–), 1383 s υs(COO–), 1277 s
(phenolic CO) cm–1. 1H NMR (CD3OD): δ = 1.89–2.11 (m, 8 H,
-C5H8), 2.35 (s, Ar–CH3), 3.62 (s, 2 H, benzylic), 6.53–6.83 (m, 3
H, Ar-H). 13C NMR (CD3OD): δ = 20.6 (Ar–CH3), 24.3, 26.1,
37.6, 47.4, and 48.2 (-C5H8), 72.8 (benzylic), 118.4, 126.0, 127.1,
129.6, 130.6, and 160.3 (aromatic), 184.0 (-COOH). ESI-MS: m/z
(%) = 249.2.
2-[(5-Chloro-2-hydroxybenzyl)amino]cyclohexane-1-carboxylic Acid
(H2ClSch12): This compound was prepared from 2-aminocyclohex-
anecarboxylic acid and 5-chlorosalicylaldehyde. Yield: 0.18 g
(64%). m.p. 233–234 °C. C14H18ClNO3 (283.7): calcd. C 59.3, H
6.4, N 4.9; found C 59.1, H 6.4, N 4.8. IR (KBr): ν = 2940 m (NH),
˜
1579 s υas(COO–), 1449 s υs(COO–), 1268 s (phenolic CO) cm–1. 1H
NMR (CD3OD): δ = 1.34–2.10 (m, 8 H, -C6H10), 2.61 (t, 1 H, HC–
N), 2.98 (t, 1 H, HC–COO–), 3.96 (s, 2 H, benzylic), 6.54–7.01 (m,
1-[(2,5-Dihydroxybenzyl)amino]cyclopentane-1-carboxylic Acid
(H2(OH)Scp11): This compound was prepared from 1-aminocyclo-
Eur. J. Inorg. Chem. 2006, 2656–2670
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2665

B. Sreenivasulu, F. Zhao, S. Gao, J. J. Vittal
FULL PAPER
3 H, Ar-H). 13C NMR (CD3OD): δ = 21.4, 22.2, 25.5, 26.3, 44.5, Complexes
and 45.4 (-C6H10), 55.3 (benzylic), 117.0, 117.4, 117.7, 124.4, 126.3,
and 161.5 (aromatic), 179.2 (-COOH). ESI-MS: m/z (%) = 283.3.
The compound [Cu2(Scp11)2(H2O)2] (1) was synthesized according
to the procedure described earlier.[6c] Weight loss (60–115 °C) as
per TGA: 5.1% [calculated for 2H2O: 5.7%]. µB = 1.55 B. M. Slow
diffusion of methanol into a slightly turbid solution of 1 in water
afforded single crystals of [Cu2(Scp11)2(MeOH)2] (1a).
2-[(2-Hydroxy-5-methylbenzyl)amino]cyclohexane-1-carboxylic Acid
(H2MeSch12): This compound was prepared from 2-aminocyclo-
hexanecarboxylic acid and 5-methylsalicylaldehyde. Yield: 60 %.
m.p. 285–286 °C. C15H21NO3 (263.3): calcd. C 68.4, H 8.0, N 5.3;
[Cu2(ClScp11)2(H2O)2] (2): A clear solution containing of
H2Clscp11 (0.13 g, 0.5 mmol) and NaOH (0.04 g, 1.0 mmol) in
methanol (20 mL) was obtained after stirring the mixture for
15 min. To this was added solid copper(II) acetate monohydrate in
portions (0.10 g, 0.5 mmol). After stirring for another 2 h, a green
solid separated out and was filtered off, washed with methanol
(4 mL), diethyl ether (4 mL), and then in air. Yield: 0.31 g (88%).
C26H32Cl2Cu2N2O8 (698.5): calcd. C 44.7, H 4.6, N 4.0; found C
found C 68.1, H 8.0, N 5.0. IR (KBr): ν = 2941 m (N–H), 1632 s
˜
¯
υas(COO–), 1459 s υs(COO–), 1272 s (phenolic CO) cm–1. 1H NMR
(CD3OD): δ = 1.34–2.17 (m, 8 H, -C6H10), 2.37 (Ar–CH3), 2.57 (t,
1 H, HC–N), 2.97 (t, 1 H, HC–COO–), 3.96 (s, 2 H, benzylic), 6.55–
6.87 (m, 3 H, aromatic). 13C NMR (CD3OD): δ = 20.5 (CH3), 25.1,
28.0, 29.1, 37.0, 47.1, and 48.4 (-C6H10), 57.1 (benzylic), 117.4,
117.8, 123.4, 127.4, 130.2, and 159.3 (aromatic), 182.0 (-COOH).
ESI-MS: m/z (%) = 263.4.
45.1, H 4.7, N 3.7. IR (KBr): ν = 3400 m (OH), 2955 m (NH),
˜
1619 s υas(COO–), 1375 s υs(COO–), 1266 s (phenolic CO) cm–1
.
The ligands H3Diala5, H3Diala4, and H3Diala3 were synthesized
as described below.
Weight loss (65–145 °C) as per TGA: 5.5% [calculated for 2H2O:
5.2%]. µB = 1.64 BM. Slow diffusion of acetonitrile into a saturated
solution of 2 in DMF furnished dark green single crystals of
[Cu2(ClScp11)2(DMF)(H2O)]·MeCN (2a).
N-(2,5-Dihydroxybenzyl)-L-alanine (H3Diala5): A mixture of -ala-
nine (0.9 g, 10.1 mmol) and NaOH (0.4 g, 10.1 mmol) in methanol
(20 mL) was stirred for 20 min to get a clear solution. To this was
added 2,5-dihydroxybenzaldehyde (1.4 g, 10.1 mmol). After stirring
for 1 h, the resulting yellow solution was cooled in an ice bath and
then treated with NaBH4 (0.42 g, 11.11 mmol). The yellow color
slowly disappeared. After 45 min, the pH of the solution was
brought to 5 by adding acetic acid. Upon stirring for 40 min, a
pale-brown solid separated out. Filtration followed by washing
with methanol (2 × 5 mL), diethyl ether (2 × 5 mL), and then dry-
ing under vacuum afforded pure H3Diala5. Yield: 1.82 g (85%).
m.p. 268–270 °C. C10H13NO4 (211.2): calcd. C 56.8, H 6.2, N 6.6;
[Cu2(MeScp11)2(H2O)2] (3): To a clear solution of H2MeScp11
(0.12 g, 0.5 mmol) and NaOH (0.04 g, 1.0 mmol) in a mixture of
methanol (10 mL) and water (10 mL) was added copper(II) acetate
monohydrate (0.10 g, 0.5 mmol) in portions. A green solid obtained
after stirring for 2 h was filtered off, washed with water (2 × 2 mL),
methanol (2 × 2 mL) and diethyl ether (2 × 2 mL) and then dried
in air. Yield: 0.28 g (85%). C28H38Cu2N2O8 (657.7): calcd. C 51.1,
H 5.8, N 4.3; found C 51.2, H 5.9, N 4.1. IR (KBr): ν = 3399 m
˜
(OH), 2960 m (NH), 1621 s υas(COO–), 1369 s υs(COO–), 1264 s
(phenolic CO) cm–1. Weight loss (40–102 °C) as per TGA: 5.9%
[calculated for 2 H2O: 5.5 %]. µB = 1.43 BM. Slow diffusion of
methanol into a saturated solution of 3 in DMSO afforded single
crystals of [Cu2(MeScp11)2(MeOH)2]·2MeOH (3a).
found C 56.3, H 6.3, N 6.5. IR (KBr): ν = 3438 m (OH), 2487 m
˜
(NH), 1635 s υas(COO–), 1511 s υs(COO–), 1285 s (phenolic
CO) cm–1. 1H NMR ([D6]DMSO): δ = 1.31 (m, 3 H, CH3), 3.21
(m, 1 H, -CH), 3.92 (m, 2 H, benzylic), 6.61–6.72 (m, 3 H, Ar-H).
13C NMR: δ = 15.7 (CHCH3), 45.5 (benzylic), 56.2 (-CHCH3),
116.0, 116.2, 116.9, 120.3, 148.6, and 149.6 (aromatic), 171.3
(-COOH). ESI-MS: m/z (%) = 211.4. [α]2D7: 7.4 (c = 5.5 mgmL–1,
MeOH).
[Cu2(OHScp11)2(H2O)2] (4): A suspension of H2(OH)Scp11
(0.04 g, 0.16 mmol) in methanol (6 mL) was allowed to slowly dif-
fuse into a clear solution of copper(II) acetate monohydrate (0.03 g,
0.16 mmol) in water (3 mL) in a test tube. Dark brownish green
single crystals of 4 were obtained after 2 days. Yield: 76% (0.08 g).
C26H34Cu2N2O10 (661.6): calcd. C 47.2, H 5.2, N 4.2; found C 47.1,
N-(2,4-Dihydroxybenzyl)-L-alanine (H3Diala4): This ligand was
synthesized according to the same procedure as H3Diala4 except
that 2,4-dihydroxybenzaldehyde was used instead of 2,5-dihydroxy-
benzaldehyde. Yield: 1.65 g (77%). m.p. 288–290 °C. C10H13NO4
(211.2): calcd. C 56.8, H 6.2, N 6.6; found C 56.3, H 6.3, N 6.5. IR
H 5.2, N 4.2. IR (KBr): ν = 3481 m (OH), 2954 m (NH); 1625 s
˜
υas(COO–), 1366 s υs(COO–), 1264 s (phenolic CO) cm–1. Weight
loss (134–177 °C) as per TGA: 5.8% [calculated for 2H2O: 5.4%].
µB = 1.41 BM.
(KBr): ν = 3267 m (OH), 3085 m (NH); 1623 s υ (COO–), 1522 s
˜
as
υs(COO–), 1216 s (phenolic CO) cm–1. H NMR ([D6]DMSO): δ =
1
[Cu2(Sch11)2]·(H2O) (5): To a clear solution of H2Sch11 (0.12 g,
0.50 mmol) and NaOH (0.04 g, 1.0 mmol) in methanol (20 mL)
was added a solution of copper(II) acetate monohydrate (0.10 g,
0.50 mmol) in methanol (5 mL) and the solution was stirred for 3 h.
The resulting green product was filtered, washed with methanol (2
× 2 mL), diethyl ether (2 × 2 mL), and then dried in air. Yield:
0.06 g (90%). C28H36Cu2N2O7 (639.7): calcd. C 52.6, H 5.7, N 4.4;
1.29 (d, 3 H, -CH3), 3.20 (m, 1 H, -CH), 3.91 (s, 2 H, benzylic),
7.01–6.21 (m, 3 H, Ar-H). 13C NMR: δ = 15.4 (CHCH3), 45.7
(benzylic), 56.6 (-CHCH3), 116.1, 116.2, 117.0, 120.2, 148.7, and
149.5 (aromatic), 172.1 (-COOH). ESI-MS: m/z (%) = 211.3. [α]2D7:
8.0 (c = 5.5 mgmL–1, MeOH).
N-(2,3-Dihydroxybenzyl)-L-alanine (H3Diala3): This ligand was
found C 52.4, H 5.7, N 4.3. IR (KBr): ν = 3435 m (OH), 2930 m
˜
prepared according to the same procedure as H3Diala3 except that
2,3-dihydroxybenzaldehyde was used instead of 2,5-dihydroxy-
benzaldehyde. Yield: 1.41 g (66%). m.p. 262–264 °C. C10H13NO4
(211.2): calcd. C 56.8, H 6.2, N 6.6; found C 56.7, H 6.0, N 6.4. IR
(N–H), 1640 s υas(COO–), 1373 s, υs(COO–), 1272 s (phenolic
CO) cm–1. Weight loss (33–99 °C) as per TGA: 2.7% [calculated
for H2O: 2.8%]. µB = 1.57 BM.
(KBr): ν = 3367 m (OH), 3085 m (NH), υ (COO–) 1623 s, 1522 s [Cu2(ClSch11)2(H2O)2] (6): To a clear solution of H2ClSch11
˜
as
υs(COO–), 1216 s (phenolic CO) cm–1. H NMR ([D6]DMSO): δ =
(0.14 g, 0.50 mmol) and NaOH (0.04 g, 1.0 mmol) in methanol
(20 mL) was added copper(II) acetate monohydrate (0.10 g,
1
1.30 (d, 3 H, -CH3), 3.26 (m, 1 H, -CH), 3.96 (s, 2 H, benzylic),
6.78–6.62 (m, 3 H, aromatic). 13C NMR: δ = 15.5 (CHCH3), 45.8 0.5 mmol) and the solution was stirred for 1.5 h. The green product
(benzylic), 56.7 (-CHCH3), 116.1, 116.2, 116.4, 120.6, 148.3, and
149.1 (aromatic), 170.5 (-COOH). ESI-MS: m/z (%) = 211.2. [α]2D7:
8.4 (c = 5.5 mgmL–1, MeOH).
was filtered off, washed with methanol (2 × 2 mL), diethyl ether (2
× 2 m L ) , a n d t h e n d r i e d i n a i r. Yi e l d : 0 . 3 2 g ( 8 8 % ) .
C28H36Cl2Cu2N2O8 (726.6): calcd. C 46.3, H 5.0, N 3.9; found C
2666
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 2656–2670

Synthesis, Structures and Catecholase Activity of a New Series of Dicopper(II) Complexes
FULL PAPER
46.7, H 5.0, N 3.5. IR (KBr): ν = 3447 m (OH), 2937 m (N–H), N 4.6; found C 40.2, H 4.8, N 4.2. IR (KBr): ν = 3406 m (OH),
˜
˜
1622 s υas(COO–), 1382 s υs(COO–), 1266 s, (phenolic CO) cm–1
.
2920 m (NH), 1611 s υas(COO–), 1462 s υs(COO–), 1299 s (phenolic
Weight loss (65–168 °C) as per TGA: 4.4% [calculated for 2H2O:
4.9%]. µB = 1.32 BM. Slow diffusion of methanol into a saturated
CO) cm–1. [α]2D7: 97.4 (c = 5.5 mgmL–1, DMSO). Weight loss (24–
101 °C) as per TGA: 8.6 % [calculated for 3 H2O: 9.0 %]. µB
=
solution of 6 in DMSO afforded dark green single crystals of 1.51 BM.
[Cu2(ClSch11)2(MeOH)2]·2MeOH (6a).
[Cu2(Diala4)2(H2O)2]·H2O (12): To the clear solution of copper
acetate (0.20 g, 1 mmol) in methanol (20 mL) H3Diala4 (0.21 g,
1 mmol) was added directly and stirred. The green precipitate that
formed after 10 min was subjected to further stirring for 3.5 h and
then filtered off, washed with methanol, Et2O, and dried in air.
Yield: 0.36 g (61%). C20H28Cu2N2O11 (599.5): calcd. C 40.1, H 4.7,
[Cu2(MeSch11)2(H2O)2] (7): A clear solution of copper(II) acetate
monohydrate in methanol (5 mL) was added to a clear solution
containing a mixture of H2MeSch11 (0.26 g, 1.0 mmol) and NaOH
(0.08 g, 2.00 mmol) in methanol (20 mL). Upon stirring this reac-
tion mixture for 1.5 h, the resulting greenish product obtained was
filtered, washed with methanol (4 mL), diethyl ether (4 mL), and
then dried in air. Yield: 0.46 g (67 %). C30H42Cu2N2O8 (685.7):
N 4.6; found C 40.1, H 4.5, N 4.4. IR (KBr): ν = 3406 m (OH),
˜
2920 m (NH), 1611 s υas(COO–), 1462 s υs(COO–), 1299 s (phenolic
CO) cm–1. [α]2D7: 32.4 (c = 5.5 mgmL–1, DMSO). Weight loss (23–
calcd. C 52.5, H 6.2, N 4.1; found C 53.2, H 6.2, N 4.1. IR (KBr): ν
˜
= 3436 m (OH), 2932 m (NH), 1624 s υas(COO–), 1381 s υs(COO–),
1262 s (phenolic CO) cm–1. Weight loss (68–127 °C) as per TGA:
5.5% [calculated for 2H2O: 5.2%]. µB = 1.40 BM.
106 °C) as per TGA: 8.2 % [calculated for 3 H2O: 9.0 %]. µB
=
1.25 BM. Slow diffusion of acetone into a saturated solution of 12
in DMSO afforded dark-green single crystals of [Cu2(Diala4)2-
(DMSO)2]·2DMSO·2acetone (12a).
[{Cu2(Sch12)2}2Cu2(Sch12)2(H2O)2]·4H2O (8): The ligand H2Sch21
(0.12 g, 0.50 mmol) along with NaOH (0.04 g, 1.0 mmol) was
added to a solvent mixture of water (15 mL) and acetonitrile
(15 mL) and stirred for 15 min to obtain a clear solution. To this
was added copper(II) nitrate trihydrate (0.12 g, 0.50 mmol) and the
solution was stirred for another 3.5 h. The clear dark-green solu-
tion obtained was filtered. Dark green single crystals were formed
from the clear filtrate upon slow evaporation for 4–5 days. Yield:
0.26 g (79%). C84H114N6O24Cu6 (1973.1): calcd. C 51.1, H 5.8, N
[Cu2(Diala3)2(H2O)2]·H2O (13): To a clear solution of copper ace-
tate (0.40 g, 2 mmol) in methanol (30 mL) H3Diala3 (0.42 g,
2 mmol) was added directly and stirred. The dark-green solution
was stirred for 5 h. The mixture was reduced to a small volume and
then precipitated with excess of diethyl ether. The green product
was dried in air. Yield: 0.82 g (68 %). C20H28Cu2N2O11 (599.5):
calcd. C 40.1, H 4.7, N 4.6; found C 40.0, H 4.6, N 4.4. IR (KBr): ν
˜
= 3406 m (OH), 2920 m (NH), 1611 s υas(COO–), 1462 s υs(COO–);
1299 s (phenolic CO) cm–1. [α]2D7: 40.7 (c = 5.5 mgmL–1, DMSO).
Weight loss (80–131 °C) as per TGA: 8.6% [calculated for 3H2O:
9.0%]. µB = 1.91 BM.
4.3; found C 51.4, H 5.7, N 4.2. IR (KBr): ν = 3429 m (OH),
˜
2928 m (NH), υas(COO–) 1597 s 1360 s υs (COO–), 1265 s (phenolic
CO) cm–1. Weight loss (64–119 °C) as per TGA: 5.9% [calculated
for 6H2O: 5.5%]. µB = 1.31 BM.
Catecholase Activity and Kinetics Measurements: The catecholase
activity of the complexes 1–13 described here in has been measured
by the reaction with model substrate 3,5-DTBC at 25 °C. The
growth of absorption maximum at λmax = 390 nm, characteristic of
the product of oxidation 3, 5-DTBQ, was measured as a function
of time. For this purpose, 10–4 moldm–3 solutions of 1–13 were
treated with 50 equivalents of 3,5-DTBC. The reaction was carried
out in methanol because of the satisfactory solubility of 3,5-DTBC
and 3,5-DTBQ. The course of the reaction was followed by UV/
Vis spectroscopy. The UV/Vis spectra of the original solution di-
rectly after the addition and after 10, 20, 30, 40, 50, 60, 75, 90, 105,
and 120 min were recorded and corrected for volume changes. The
increase in the absorption due to the formation of DTBQ at
390 nm indicated significant catalytic activity for all the complexes.
Kinetic parameters on the rate of catechol oxidation were deter-
mined by reaction of 110–3 to 1.310–2  solutions of DTBC with
10–4 solutions of complexes in methanol. A kinetic treatment on
the basis of the Michaelis–Menten approach was applied and the
results were evaluated from Lineweaver–Burk double reciprocal
plots. The errors in the kinetic parameters were obtained from at
least three measurements.
[Cu2(ClSch12)2]·2H2O (9): To a clear solution of H2ClSch12
(0.14 g, 0.5 mmol) and NaOH (0.04 g, 1.0 mmol) in methanol
(10 mL) was added water (10 mL) followed by the addition solid
copper(II) nitrate trihydrate (0.12 g, 0.5 mmol) and the mixture was
stirred for 1.5 h. The dark green product obtained was filtered,
washed with water (2 × 2 mL), methanol (2 × 2 mL) and diethyl
ether (4 mL) and then dried in air. Yield: 0.31 g (85 %).
C28H36Cl2Cu2N2O8 (726.6): calcd. C 46.3, H 5.0, N 3.9; found C
47.0, H 5.2, N 4.0. IR (KBr): ν = 3396 m (OH), 3115 m (NH),
˜
1603 s υas(COO–), 1384 s υs(COO–), 1264 s (phenolic CO) cm–1
.
Weight loss (80–146 °C) as per TGA: 5.8% [calculated for 2H2O:
5.0%]. µB = 1.25 BM. A clear saturated solution of 9 in the solvent
mixture of methanol/dichloromethane (1:1) afforded single crystals
of [Cu2(ClSch12)2]·2MeOH, 9a upon slow evaporation for 3 days.
[Cu2(MeSch12)2(H2O)2] (10): A clear solution of H2MeSch12
(0.26 g, 1.0 mmol) and NaOH (0.08 g, 2.0 mmol) in methanol
(20 mL) was obtained on stirring the mixture for 15 min. To this
was added solid copper(II) nitrate trihydrate (0.24 g, 1.0 mmol) in
one portion. The reaction mixture was stirred for another 1 h. The
greenish product obtained was filtered, washed with methanol,
Et2O, and then dried in air. Yield: 0.48 g (70%). C30H42Cu2N2O8
(685.7): calcd. C 52.5, H 6.17, N 4.1; found C 52.4, H 6.25, N 4.1.
X-ray Crystallography: The diffraction experiments were carried
out with a Bruker AXS SMART CCD diffractometer. The program
SMART[34a] was used for collecting frames of data, indexing reflec-
tions, and determining lattice parameters, SAINT[34a] for integra-
tion of the intensity of reflections and scaling, SADABS[34b] for
absorption correction and SHELXTL[34c] for space group and
structure determination, least-squares refinements on F2. All the
hydrogen atom positions of the imine groups, methanol, and water
molecules were located and their positional parameters were re-
fined in the least-squares cycles. One of the carbon atoms in 2a
(C12 with occupancies 0.65/0.35) and 3a (C11 with occupancies
IR (KBr): ν = 3458 m (OH), 2936 m (NH), 1601 s υas(COO–),
˜
1396 s υs(COO–), 1262 s (phenolic CO) cm–1. Weight loss (60–
107 °C) as per TGA: 5.2 % [calculated for 2 H2O: 5.3 %]. µB
=
1.34 BM.
[Cu2(Diala5)2(H2O)2]·H2O (11): To a clear solution of copper(II)
acetate monohydrate (0.20 g, 1 mmol) in water (20 mL) H3Diala5
(0.21 g, 1 mmol) was added directly and stirred for 3 h. The green
solid obtained after stirring for 3 h was filtered, washed with meth-
anol (2 × 2 mL), diethyl ether (2 × 2 mL), and then dried in air.
Yield: 0.38 g (63%). C20H28Cu2N2O11 (599.5): calcd. C 40.1, H 4.7, 0.6/0.4) were found to be disordered. Selected crystallographic data
Eur. J. Inorg. Chem. 2006, 2656–2670
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2667

B. Sreenivasulu, F. Zhao, S. Gao, J. J. Vittal
FULL PAPER
Table 5. X-ray crystallographic data and structure refinement details.
Complex
1a
2a
3a
4
Formula
Formula wt.
T [K]
C28H38Cu2N2O8
657.68
223(2)
C31H40Cl2Cu2N4O8
794.65
223(2)
C32H50Cu2N2O10
749.82
223(2)
C26H34Cu2N2O10
661.63
223(2)
Crystal system
Space group
monoclinic
C2/c
triclinic
P1
triclinic
P1
triclinic
P1
¯
¯
¯
a [Å]
b [Å]
c [Å]
α [°]
16.5064(8)
14.5462(7)
14.1803(7)
90
124.930
90
2791.4(2)
4
1.577
11.2530(6)
12.3363(7)
13.9093(8)
107.662(1)
105.555(1)
98.783(1)
1713.9(2)
2
7.7219(5)
10.4139(7)
11.599(28)
106.958(1)
106.312(1)
96.710(1)
836.0(1)
1
7.7024(8)
8.8514(9)
9.835(1)
76.240(2)
89.220(2)
81.352(2)
643.7(1)
1
β [°]
γ [°]
V [Å3]
Z
µ [mm–1]
1.451
1.331
1.716
Reflections collected
Independent reflections
Rint
8047
2460
0.0313
1.010
0.0372
0.0902
9753
5943
0.0196
1.034
0.0479
0.1212
4902
2948
0.0135
1.054
0.0316
0.0828
3806
2266
0.0164
1.053
0.0337
0.0831
GooF
[a]
Final R [IϾ2σ], R1
[b]
wR2
Complex
6a
8
9a
12a
Formula
Formula wt.
C32H48Cl2Cu2N2O10
817.7
C28H38Cu2N2O8
657.68
C30H40Cl2Cu2N2O8
754.62
C34H58Cu2N2O14S4
974.14
T [K]
Wavelength [Å]
Crystal system
Space group
223(2)
0.71073
223(2)
0.71073
223(2)
0.71073
223(2)
0.71073
triclinic
P1
triclinic
triclinic
triclinic
¯
¯
¯
P1
P1
P1
a [Å]
b [Å]
c [Å]
α [°]
7.7614(3)
10.8014(4)
11.8232(4)
108.395(2)
107.969(2)
95.718(2)
873.10(6)
1
12.3573(11)
14.1055(14)
14.1822(15)
104.195(2)
110.017(2)
104.945(3)
2087.2(4)
3
7.4698(9)
9.9411(12)
11.2134(14)
72.522(3)
79.596(3)
88.363(3)
780.9(2)
1
9.5244(7)
10.8149(8)
12.0257(9)
96.841(2)
102.547(2)
111.801(2)
1095.0(1)
1
β [°]
γ [°]
V [Å3]
Z
µ [mm–1]
1.429
1.582
1.586
1.225
Reflections collected
Independent reflections
Rint
7693
5003
0.0206
0.921
0.0398
0.0856
17670
10780
0.0239
1.019
0.0376
0.0734
6432
4305
0.0216
1.084
0.0521
0.1146
6278
4545
0.0162
1.078
0.0444
0.1136
GooF
[a]
Final R [I Ͼ 2σ], R1
[a]
wR2
2
2 2
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = [Σw(Fo – Fc ) /Σw(Fo2)2]1/2
.
and refinement details are displayed in Table 5. The structure of
12a can also be solved in the centrosymmetric space group P1 with
Acknowledgments
¯
the methyl group of the alanine part of the ligand disordered. How-
ever, the optical activity of 12 containing the optically active Diala4
ligand is comparable to those of 11 and 13 and therefore, the chiral
space group was retained.
We gratefully acknowledge the financial support from NUS (R143-
000-252-112), and from NSFC (20125104, 20490210).
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2668
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Eur. J. Inorg. Chem. 2006, 2656–2670

Synthesis, Structures and Catecholase Activity of a New Series of Dicopper(II) Complexes
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