Steric effects of catalysts and substrates on catechol oxidation catalysis: Cyclodimeric copper(II) complexes containing 1,4-bis(dimethyl(quinolin-3-yl)silyl)benzene

Article abstract of DOI:10.1016/j.ica.2015.12.026

Reaction of CuX₂ with new ligand, 1,4-bis(dimethyl(quinolin-3-yl)silyl)benzene (L), in alcohol affords C₂-symmetric double-alkoxy-supported cyclodimeric copper(II) complexes, [Cu(μ-OR)(L)]₂(X)₂ (R = Me, Et,

Full text of DOI:10.1016/j.ica.2015.12.026

Inorganica Chimica Acta 443 (2016) 51–56
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Inorganica Chimica Acta
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Steric effects of catalysts and substrates on catechol oxidation
catalysis: Cyclodimeric copper(II) complexes containing
1,4-bis(dimethyl(quinolin-3-yl)silyl)benzene
Hyeun Kim a, Haeri Lee a, Tae Hwan Noh a, Jongki Hong b, Ok-Sang Jung a,
⇑
a Department of Chemistry, Pusan National University, Pusan 609-735, Republic of Korea
b College of Pharmacy, Kyung Hee University School of Medicine, 1, Hoegi-dong, Dongdaemun-gu, Seoul 130-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 October 2015
Received in revised form 19 December 2015
Accepted 22 December 2015
Available online 29 December 2015
Reaction of CuX2 with new ligand, 1,4-bis(dimethyl(quinolin-3-yl)silyl)benzene (L), in alcohol affords
C2-symmetric double-alkoxy-supported cyclodimeric copper(II) complexes, [Cu(l-OR)(L)]2(X)2 (R = Me,
Et, Pr; Xꢀ = ClO4ꢀ, BFꢀ4 ) in high yields. These cyclodimeric species with intracyclic Cuꢁ ꢁ ꢁCu distances of
i
2.92–2.98 Å show the significant catalytic effects on the catechol oxidation catalysis in chloroform in
the order [Cu(l-OMe)(L)]2(X)2 > [Cu(l-OEt)(L)]2(X)2 > [Cu(l
-OiPr)(L)]2(X)2. Furthermore, the catalytic
efficiency is strongly substrate-dependent in the order 4-BuCat > 4-ClCat > 3,5-DBuCat > Cat. Such nota-
ble catalytic effects potentially can be explained by the steric hindrance of both catalysts and substrates.
Ó 2015 Elsevier B.V. All rights reserved.
Keywords:
Catechol oxidation
Copper
Cyclodimers
Steric hindrance
Substrates
1. Introduction
[19,22,29,30]. Although general relationships between the dicopper
structures and their catalytic activities have been elucidated, the
Catechol oxidases are omnipresent type III dicopper-active site
enzymes that catalyze the aerial oxidation of versatile catechol spe-
cies to the corresponding o-quinones along with the reduction of
oxygen to water [1–8]. The catechol oxidation reaction is of
considerable importance to medical diagnosis of hormonally active
catecholamines such as adrenaline, noradrenaline, and 3,4-dihy-
droxyphenylalanine [9–12]. Thus, desirable synthesis and catalysis
of unique dicopper analogues have been hot issues in the bioinor-
ganic field for the two decades. Some dicopper complexes have
shown considerably effective activity according to the nature of
bridging coligands, ligands, electrochemical potentials, and pH
[5,12–18].The number and structure of bridging coligands between
the copper centers in a dicopper complex are particularly significant
factors in the catechol oxidation catalysis [19–34]. Furthermore, an
appropriate proximity of two copper centers is a known require-
ment for facilitation of the binding of the two hydroxyl moieties of
catechol substrates prior to electron transfer [25–27]: the Cuꢁ ꢁ ꢁCu
(2.9–3.2 Å) distances of dicopper species have contributed to effi-
cient catalytic activities via an appropriate match between the sub-
strates and catalysts [13,20,28], and some double-alkoxy-bridge
between Cuꢁ ꢁ ꢁCu have exhibited substantial catalytic activity
systematic steric factors effecting catechol oxidation catalysis
remain unexplored. In the meantime, in some of our previous stud-
ies, some horseshoe-type silicon-containing bis(pyridyl) ligands
have been usefully employed in the construction of metallacy-
clodimeric species [31–34]. In the present study, a systematic series
of new double-alkoxy-bridged cyclodimeric copper(II) complexes,
i
[Cu(l
-OR)(L)]2(X)2 (R = Me, Et, Pr; L = 1,4-bis(dimethyl(quinolin-
3-yl)silyl)benzene; Xꢀ = ClO4ꢀ, BFꢀ4 ) were synthesized and character-
ized as active sites of catechol oxidation in order to investigate steric
hindrance of catalysts and substrates on the catalysis. This paper
reports the synthesis and structural properties of the unique dou-
ble-alkoxy-bridged cyclodimeric species along with the significant
steric effects of the catalysts and substrates on the catechol oxida-
tion catalysis. To our knowledge, this paper presents a landmark
research on the systematic steric effects of both catalysts and cate-
chol substrates on oxidation reactions.
2. Experimental
2.1. Materials and physical measurements
All chemicals including copper(II) perchlorate hexahydrate,
copper(II) tetrafluoroborate, 1,4-phenylenebis(chlorodimethylsi-
lane), 3-bromoquinoline and 3,5-di-tert-butylcatechol were pur-
⇑
Corresponding author. Tel.: +82 51 510 2591; fax: +82 51 516 7421.
E-mail address: oksjung@pusan.ac.kr (O.-S. Jung).
http://dx.doi.org/10.1016/j.ica.2015.12.026
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

52
H. Kim et al. / Inorganica Chimica Acta 443 (2016) 51–56
chased from Sigma–Aldrich, and were used without further purifi-
cation. Elemental microanalyses (C, H, N) were performed on
crystalline samples at the KBSI Pusan Center using a Vario-EL III
analyzer. Infrared spectra were obtained on a Nicolet 380 FT-IR
spectrophotometer using samples prepared as KBr pellets. 1H
(300 MHz) and 13C (75 MHz) NMR spectra were recorded on a
Varian Mercury Plus 300. The absorption spectra were recorded
on the UV–Vis spectrophotometer S-3150. Thermal analyses were
undertaken under N2 gas at a scan rate of 10 °C/min using a Labsys
TGA-DSC 1600. ESI-TOF-MS data were obtained using the
ACQUITYÒ UPLC system (Waters, Milford, MA, USA) coupled with
the QTOF instrument (SYNAPTTM G2; Waters, Milford, MA, USA).
Cyclic voltammograms were recorded using the Potentiostat,
Kosentech model PI-1000 (Korea). A three electrodes system consist-
ing of glassy carbon (area = 0.07 cm2), Ag/AgCl, and platinum wire as
working, reference, and counter electrodes, respectively, was used.
(dec.). Anal. Calc. for C64H74N6O11Si4Cl2Cu2: C, 54.38; H, 5.28; N,
5.95. Found: C, 54.30; H, 5.34; N, 5.88%. IR (KBr pellet, cmꢀ1):
1708 (m), 1618 (w), 1575 (w), 1349 (m), 1251 (m), 1099 (s),
1088 (s, ClOꢀ4 ), 867 (m), 800 (s), 782 (s), 752 (m), 676 (m),
622 (m), 511 (w). m/z = 449.1867 ([L+H+]+), 511.1081
([Mꢀ2EtOꢀꢀ2ClOꢀ4 +2H+]2+), 610.0598 ([Mꢀ2EtOꢀ]2+), 626.0945
([MꢀCu2++4H+]2+), 1265.1166 ([MꢀEtOꢀ]+), 1311.1284 ([M+H+]+).
2.5. Synthesis of [Cu(
l
-OiPr)(L)]2(ClO4)2ꢁ3MeCNꢁ2H2O
An acetonitrile solution (15 mL) of copper(II) perchlorate hex-
ahydrate (55 mg, 0.15 mmol) was carefully layered onto a iso-pro-
panol solution (5 mL) of
L (67 mg, 0.15 mmol), resulting in
formation of dark-brown crystals in 5 days. Yield, 68 mg (64%).
M.p. 176 °C (dec.). Anal. Calc. for C68H83N7O12Si4Cl2Cu2: C, 54.42;
H, 5.57; N, 6.53. Found: C, 54.40; H, 5.49; N, 6.55%. IR (KBr pellet,
cmꢀ1): 1708 (m), 1618 (w), 1575 (w), 1349 (m), 1251 (m), 1099
(s), 1088 (s, ClOꢀ4 867 (m), 800 (s), 782 (s), 752 (m), 676 (m),
622 (m), 511 (w). m/z = 449.1856 ([L+H+]+), 511.1087
2.2. Synthesis of 1,4-bis(dimethyl(quinolin-3-yl)silyl)benzene (L)
To a solution of 3-bromoquinoline (2.71 mL, 19.9 mmol) in dry
diethyl ether (65 mL) under a nitrogen gas atmosphere, n-butyl-
lithium (8.2 mL of 2.5 M solution in n-hexane, 20.5 mmol) was
added dropwise at ꢀ78 °C. The resulting mixture was allowed to
0 °C, at which temperature it was stirred for 30 min. Then, 1,4-phe-
nylenebis(chlorodimethylsilane) (2.63 g, 10 mmol) was slowly
added to the above yellow suspension at ꢀ78 °C, and then the reac-
tion mixture was stirred at room temperature for 12 h. Distilled
water (40 mL) was subsequently added, and the organic layer
was separated. The organic solution was washed with water sev-
eral times and then dried over anhydrous magnesium sulfate. Eva-
poration of the solvent produced a reddish-brown oil, which was
then purified by column chromatography using a mixture of ethyl
acetate and n-hexane (v/v = 3:7) as an eluent. The resulting solu-
tion was evaporated thus affording crystalline solid 1,4-bis
(dimethyl(quinolin-3-yl)silyl)benzene (L) in a 64% yield (2.75 g).
M.p. 93 °C. Anal. Calc. for C28H28N2Si2: C, 74.95; H, 6.29; N, 6.24.
Found: C, 74.90; H, 6.21; N, 6.23%. 1H NMR (CDCl3, ppm): 8.96 (s,
2H), 8.28 (s, 2H), 8.09 (d, J = 8.2 Hz, 2H), 7.79 (d, J = 8.2 Hz, 2H),
7.72 (t, J = 8.2 Hz, 2H), 7.55 (t, J = 8.2 Hz, 2H), 7.51 (s, 4H), 0.67 (s,
12H, SiMe2). 13C NMR (CDCl3, ppm): 154.25, 148.30, 143.29,
138.48, 133.74, 130.67, 130.09, 129.28, 127.95, 126.64, 0.27. IR
(KBr pellet, cmꢀ1): 1618 (w), 1575 (w), 1349 (m), 1253 (m), 1132
(s), 1093 (w), 966 (w), 867 (m), 815 (s), 782 (s), 755 (s), 678 (m),
511 (m), 489 (m).
([Mꢀ2iPrOꢀꢀ2ClOꢀ4 +2H+]2+), 610.0598 ([Mꢀ2iPrOꢀ]2+), 1076.2798
i
([MꢀCu2+ꢀ2ClOꢀ4 +H+]+),
1217.1342
([Mꢀ PrOꢀꢀCu2++2H+]+),
1339.1205 ([M+H+]+).
2.6. Synthesis of [Cu(
l
-OMe)(L)]2(BF4)2ꢁ2Me2CO
A methanol solution (10 mL) of copper(II) tetrafluoroborate
(35 mg, 0.15 mmol) was carefully layered onto a mixture solution
of acetone and tetrahydrofuran (15 mL) of L (67 mg, 0.15 mmol),
resulting in formation of dark-brown crystals suitable for X-ray
single crystallography in 4 days. Yield, 68 mg (64%). M.p. 202 °C
(dec.). Anal. Calc. for C64H74B2N4O4F8Si4Cu2: C, 55.85; H, 5.42; N,
4.07. Found: C, 55.60; H, 5.39; N, 4.02%. IR (KBr pellet, cmꢀ1):
1708 (m), 1618 (w), 1575 (w), 1349 (m), 1251 (m), 1099 (s),
1032 (s, BFꢀ4 ), 867 (m), 800 (s), 782 (s), 752 (m), 676 (m), 511 (w).
2.7. Catechol oxidation reactions
Pyrocatechol (Cat), 4-chlorocatechol (4-ClCat), 4-tert-butylcate-
chol (4-BuCat), and 3,5-di-tert-butylcatechol (3,5-DBuCat) were
employed as the substrates for catalytic oxidation reactions. Each
cyclodimeric copper(II) catalyst (0.015 mmol) was treated with
each substrate (0.06 mmol) in the 1:4 mol ratio in 10 mL of chloro-
form at room temperature under aerobic conditions. The catalytic
yields were monitored by reference to the UV–Vis spectra after
10-times dilution of the resulting mixture. All the reactions were
run at least five times and the yields were averaged.
2.3. Synthesis of [Cu(
l
-OMe)(L)]2(ClO4)2ꢁ2MeCNꢁ2H2O
An acetonitrile solution (15 mL) of copper(II) perchlorate
hexahydrate (55 mg, 0.15 mmol) was carefully layered onto a
methanol solution (5 mL) of L (67 mg, 0.15 mmol), resulting in
the formation of dark-brown crystals suitable for X-ray single crys-
tallography in 5 days. Yield, 78 mg (63%). M.p. 206 °C (dec.). Anal.
Calc. for C62H72N6O12Si4Cl2Cu2: C, 53.05; H, 5.17; N, 5.99. Found:
C, 52.98; H, 5.12; N, 6.02%. IR (KBr pellet, cmꢀ1): 1708 (m), 1618
(w), 1575 (w), 1349 (m), 1251 (m), 1099 (s), 1088 (s, ClOꢀ4 ), 867
(m), 800 (s), 782 (s), 752 (m), 676 (m), 622 (m), 511 (w).
m/z = 449.1867 ([L+H+]+), 511.1078 ([Mꢀ2MeOꢀꢀ2ClOꢀ4 +2H+]2+),
2.8. Crystallographic structure determinations
X-ray data for [Cu(
(L)]2(ClO4)2ꢁ2MeCNꢁH2O, and [Cu(
collected on a Bruker SMART automatic diffractometer with gra-
phite-monochromated Mo K radiation (k = 0.71073 Å) and a
l
-OMe)(L)]2(ClO4)2ꢁ2MeCNꢁ2H2O, [Cu(
l-OEt)
l
-OMe)(L)]2(BF4)2ꢁ2Me2CO were
a
CCD detector at ꢀ25 °C. Thirty-six frames of two dimensional
diffraction images were collected and processed to obtain the
cell parameters and orientation matrix. The data were corrected
for Lorentz and polarization effects. The absorption effects were
corrected using the multi-scan method (SADABS) [35]. The structures
were solved using the direct method (SHELXS 97) and refined by
full-matrix least squares techniques (SHELXL 2014/7) [36,37]. The
non-hydrogen atoms were refined anisotropically, and the
hydrogen atoms were placed in calculated positions and refined
only for the isotropic thermal factors.
610.0598
([Mꢀ2MeOꢀ]2+),
626.0932
([MꢀMeOꢀ+H+]2+),
1221.1208 ([MꢀCu2++3H+]+), 1238.1340 ([M+H+]+).
2.4. Synthesis of [Cu(
l
-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O
An acetonitrile solution (15 mL) of copper(II) perchlorate hex-
ahydrate (55 mg, 0.15 mmol) was carefully layered onto an ethanol
solution (5 mL) of L (67 mg, 0.15 mmol), resulting in formation of
dark-brown crystals in 5 days. Yield, 63 mg (60%). M.p. 184 °C

H. Kim et al. / Inorganica Chimica Acta 443 (2016) 51–56
53
1339.1205 could be assigned to [Mꢀ2iPrOꢀ]2+ and [M+H+]+, respec-
tively. The mass peaks were isotopically resolved and found to be
in good agreement with their calculated theoretical distributions.
The thermogravimetric analysis (TGA) confirmed that all of the
skeletons are stable up to 176 °C. Evaporation of the solvate
molecules was accompanied by a drastic collapse of the skeletal
structure. The interesting feature here was the dependence
of the thermal stability of each structure on the bulkiness of
3. Results and discussion
3.1. Synthetic aspects and characterizations
The new ligand, 1,4-bis(dimethyl(quinolin-3-yl)silyl)benzene
(L), was synthesized to a moderate yield by reaction of 1,4-pheny-
lenebis(chlorodimethylsilane) with 3-bromoquinoline. Self-assem-
bly of CuX2 (Xꢀ = BF4ꢀ and ClOꢀ4 ) with L produced dark-brown
bridged alkoxy moiety: the collapse temperatures of [Cu(l-OMe)
crystals, [Cu(l-OMe)(L)]2(X)2 suitable for X-ray crystallography,
(L)]2(ClO4)2ꢁ2MeCNꢁ2H2O,
[Cu(
l
-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O,
as shown in Scheme 1. Even though the crystals contain solvate
molecules, they were reasonably stable under aerobic conditions
at room temperature. All were insoluble in common organic sol-
vents such as toluene, benzene, tetrahydrofuran, and acetone, but
were partially dissociated in dimethyl sulfoxide (Me2SO) and N,
N0-dimethylformamide. Formation of the products is not signifi-
cantly affected by the change of reactant mole ratio or concentra-
tion, which indicates that the products are favorable skeletal
species. The unique ligand structure should be ascribed to the con-
struction of stable double-alkoxy-supported cyclodimeric species,
as will be discussed in detail. Not surprisingly then, it was found
that the combined effects of the stable L conformer and the cop-
per(II) geometry, along with the solvent coordination, might be a
significant driving force behind the formation of the discrete cyclo-
dimeric species. Their compositions and structures were confirmed
by elemental analyses, IR, thermal analysis, ESI-MS, and single
crystal X-ray diffraction measurements. The characteristic broad
bands at 1032 and 1088 cmꢀ1 reflect the stretching frequencies
corres-ponding to BFꢀ4 and ClO4ꢀ, respectively. The ESI-MS data of
[Cu(l
-OiPr)(L)]2(ClO4)2ꢁ3MeCNꢁ2H2O, and [Cu(
-OMe)(L)]2(BF4)2 ꢁ
l
2Me2CO were 206, 184, 176, and 202 °C, respectively (Appendix
A), indicating that the bulky alkoxy moiety is indebted to their
thermal instability.
3.2. X-ray crystal structures
Single crystal X-ray characterizations have provided cyclodi-
meric species, [Cu(
(L)]2(ClO4)2ꢁ2MeCNꢁH2O, and [Cu(
l
-OMe)(L)]2(ClO4)2ꢁ2MeCNꢁ2H2O, [Cu(
l-OEt)-
l
-OMe)(L)]2(BF4)2ꢁ2Me2CO. Their
crystal structures are depicted in Figs. 1 and S2 (Appendix A). The
crystal parameters and procedural information corresponding to
the data collection and structure refinement are listed in Table 1
and their relevant bond lengths and angles are listed in Table 2.
The geometry of each copper(II) ion is a square-planar N2O2 coor-
dination arrangement with two quinolinyl units from two L and
with two bridged alkoxy groups (Cu–N = 1.97(2) – 2.01(1) Å; Cu–
O = 1.87(1) – 1.92(1) Å), resulting in the formation of cyclodimeric
copper(II) complexes. In the formation of the cyclodimeric species,
both the horseshoe-type conformation of L and the N-position of
the quinolin-3-yl group play an important role. The two ClOꢀ4 or
BFꢀ4 anions act as a fifth and a sixth ligand rather than a counteran-
ion (Cuꢁ ꢁ ꢁO = 2.95(2) – 3.02(2) Å; Cuꢁ ꢁ ꢁF = 2.87(2) – 2.93(3) Å), and
thus the geometry around the copper(II) cation can be described
as a pseudo-octahedral arrangement. The intracyclic Cuꢁ ꢁ ꢁCu dis-
[Cu(l-OR)(L)]2(ClO4)2 revealed bridged alkoxy components and
ligands-dissociated
species. For [Cu( -OMe)(L)]2(ClO4)2-
l
ꢁ2MeCNꢁ2H2O, peaks at 449.1867, 610.0598, 626.0932, and
1283.1340, assigned to [L1+H+]+, [Mꢀ2MeOꢀ]2+, [MꢀMeOꢀ+H+]2+
,
and [M+H+]+, respectively, were observed. For [Cu(
l
-OEt)(L)]2-
(ClO4)2ꢁ2MeCNꢁH2O,
the
ethoxy-dissociated
species of
[Mꢀ2EtOꢀ]2+ and [MꢀEtOꢀ]+ as well as [M+H+]+ were detected at
tances are 2.980(2) Å, 2.917(3) Å, and 2.963(4) Å for [Cu(
(L)]2(ClO4)2ꢁ2MeCNꢁ2H2O, [Cu( -OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O, and
[Cu(
-OMe)(L)]2(BF4)2ꢁ2Me2CO, respectively.
As depicted in Fig. 1, the formation of the cyclodimeric species
is partly attributed to the intermolecular interaction
ꢁ ꢁ ꢁ
between the quinolinyl groups (3.75(6) Å and 0.0(3)° for [Cu(
OMe)(L)]2(ClO4)2ꢁ2MeCNꢁ2H2O, 3.45(3) – 3.50(5) Å and 0.0(3)° for
[Cu(
-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O, and 3.62(7) – 3.7(1) Å and 0.0
(5)° for [Cu( -OMe)(L)]2(BF4)2ꢁ2Me2CO). The dihedral angles corre-
sponding to the intracyclodimeric interaction between the
ꢁ ꢁ ꢁ
quinolinyl groups attached to the copper(II) ions were determined
to be 61.2(3), 61.8(4), and 62.6(6)° for [Cu( -OMe)(L)]2(ClO4)2-
-OMe)
l-OMe)
610.0598, 1265.1166, and 1311.1284, respectively. In the case of
[Cu(l
-OiPr)(L)]2(ClO4)2ꢁ3MeCNꢁ2H2O, peaks at 610.0598 and
l
l
p
p
+
l-
n-BuLi
l
l
p
p
l
ꢁ2MeCNꢁ2H2O, [Cu(
l
-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O, and [Cu(
l
CuX2
(L)]2(BF4)2ꢁ2Me2CO, respectively.
ROH
HX
3.3. Catechol oxidation catalysis
Pyrocatechol (Cat), 4-chlorocatechol (4-ClCat), 4-tert-butylcate-
chol (4-BuCat), and 3,5-di-tert-butylcatechol (3,5-DBuCat) were
employed as the substrates for the catalytic catechol oxidation
reactions, which involves binding of the catechol to the vacant site
on the dicopper(II) center, oxidation of the catechol by molecular
oxygen, and subsequent release of the product [1,14]. Beside their
catalytic importance, their oxidized species, o-quinones are non-
innocent intramolecular electron-transfer ligands, exhibiting an
absorption band at around 390 nm [38,39]. Thus, the catalytic effi-
ciencies were monitored according to the absorption bands of the
UV–Vis spectra. Each cyclodimeric copper(II) catalyst was treated
with each catechol substrate in the 1:4 [catalyst]:[catechol] mole
ratio in 10 mL of chloroform at room temperature under aerobic
conditions. After each catechol substrate was added to the
Scheme 1. Synthetic procedure and catechol oxidation catalysis.

54
H. Kim et al. / Inorganica Chimica Acta 443 (2016) 51–56
Table 1
Crystal data and structural refinements for [Cu(
l
-OMe)(L)]2(Cl-O4)2ꢁ2MeCNꢁ2H2O, [Cu(
-OMe)(L)]2(ClO4)2-2MeCNꢁH2O
62H70N6O11Si4Cl2Cu2
l
-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O, and [Cu(
l
-OMe)(L)]2(BF4)2ꢁ2Me2CO.
[Cu(
l
[Cu(
l-OEt)(L)]2(ClO4)2-2MeCNꢁH2O
[Cu( -OMe)(L)]2(BF4)2-2Me2CO
l
Formula
C
C64H72N6O11Si4Cl2Cu2
1411.61
monoclinic
P21/c
11.2166(7)
11.2105(7)
26.050(2)
99.639(3)
3229.4(3)
1.452
C64H74B2N4O4F8Si4Cu2
1376.33
monoclinic
P21/c
11.495(1)
11.513(1)
26.161(3)
102.191(6)
3384.2(6)
1.351
Mw (g molꢀ1
)
1385.58
monoclinic
P21/c
11.5130(7)
11.5756(7)
26.252(2)
101.171(3)
3432.3(4)
1.341
Crystal system
Space group
a (Å)
b (Å)
c(Å)
b (°)
V (Å3)
r
Z
l
(g cmꢀ3
)
2
2
2
(mmꢀ1
)
0.827
0.880
0.769
F(000)
1440
1468
1428
Rint
0.1056
100.0
1.054
0.0974
0.2766
0.1127
100.0
1.005
0.1742
0.1296
100.0
1.059
0.1179
Completeness (%)
Goodness-of-fit (GoF) on F2
R1 [I > 2
r
(I)]a
wR2 (all data)b
0.4203
0.3016
a
R1
wR2 = (
=
R
||Fo| ꢀ |Fc||/
R|Fo|.
b
R
[w(F2o ꢀ F2c)2]/
R .
[w(F2o)2])1/2
Table 2
Selected bond lengths (Å) and angles (°) for [Cu(
l
-OMe)(L)]2(ClO4)2ꢁ2MeCNꢁH2O, [Cu(
l
-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O, and [Cu(
l
-OMe)(L)]2(BF4)2ꢁ2Me2CO.
-OMe)(L)]2(BF4)2ꢁ2Me2CO
[Cu( -OMe)(L)]2(ClO4)2ꢁ2MeCNꢁH2O
l
[Cu(
l-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O
[Cu(l
Cu(1)–O(1)
Cu(1)–O(1)1
Cu(1)–N(1)
Cu(1)–N(2)1
Cu(1)ꢁ ꢁ ꢁCu(1)1
Cuꢁ ꢁ ꢁOClO3
N(1)–Cu(1)–N(2)1
O(1)–Cu(1)–O(1)1
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)1
Cu(1)–O(1)–Cu(1)1
1.895(5)
1.914(4)
1.998(5)
1.995(6)
2.978(1)
Cu(1)–O(1)
Cu(1)–O(1)2
Cu(1)–N(1)
Cu(1)–N(2)2
Cu(1)ꢁ ꢁ ꢁCu(1)2
Cuꢁ ꢁ ꢁOClO3
N(1)–Cu(1)–N(2)2
O(1)–Cu(1)–O(1)2
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)2
Cu(1)–O(1)–Cu(1)2
1.95(1)
1.94(1)
2.01(1)
2.03(1)
2.916(3)
2.92(2), 2.99(2)
Cu(1)–O(1)
Cu(1)–O(1)3
Cu(1)–N(1)
Cu(1)–N(2)3
Cu(1)ꢁ ꢁ ꢁCu(1)3
Cuꢁ ꢁ ꢁFBF3
N(1)–Cu(1)–N(2)3
O(1)–Cu(1)–O(1)3
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)3
Cu(1)–O(1)–Cu(1)3
1.918(8)
1.902(6)
1.997(8)
2.01(1)
2.959(2)
2.81(1), 2.97(1)
2.95(1), 3.02(1)
94.7(2)
77.2(2)
93.7(2)
171.6(2)
102.8(2)
96.1(5)
82.8(4)
90.4(4)
172.8(5)
97.2(7)
94.5(4)
78.5(3)
93.6(3)
171.6(3)
101.5(3)
1
2
3
ꢀx + 1, ꢀy, ꢀz.
ꢀx + 2, ꢀy + 2, ꢀz.
ꢀx + 1, ꢀy + 1, ꢀz.
chloroform suspension of the catalytic reaction system, the absorp-
tion intensity, at about 390 nm gradually increased (Appendix A).
< Cat (311.64 mV) (Appendix A). The order of the oxidation poten-
tial for 3,5-DBuCat was significantly different from that of its
catalytic efficiency. Such a fact indicates that the steric effect of
3,5-DBuCat is a significant factor in its oxidation reaction. The slow
catalytic reaction rate of Cat might be ascribed to its structural
properties, according to which it is dimeric in solid state [41] in
addition to its high oxidation potential. After the catalytic reaction
completed, the reaction solution included a trace free ligand and
partially dissociated copper(II) fragments, indicating that oxidation
can be conducted by marginally solubilized copper(II) fragments.
This result suggests that the reaction mechanism proceeds via a
mechanism based on the distance of two neighbor copper(II) ions
[3,5,42]. Thus, the repeated catalysis diminished with the quantity
of the solid catalyst. Furthermore, the oxidation catalysis of 3,5-
DBuCat using general oxidation catalyst Ag2O as a heterogeneous
catalyst did not show any significant steric effect of the substrate:
39%, 30%, and 4% conversion of 3,5-DBuCat, 4-BuCat, and Cat,
respectively, after 1 day (Appendix A). Of course, the heteroge-
neous catalysis via Ag2O was much slower than the catalysis using
the present cyclodimeric species.
The catalytic effect of [Cu(
lar to that of [Cu(
cated that the polyatomic anions are not significant factors in the
l
-OMe)(L)]2(BF4)2ꢁ2Me2CO is very simi-
l
-OMe)(L)]2(ClO4)2ꢁ2MeCNꢁ2H2O, which indi-
catalytic reaction (Appendix A). Thus, further catalytic reactions
i
using a series of [Cu(
l
-OR)(L)]2(ClO4)2 (R = Me, Et, Pr) catalysts
were carried out in order to scrutinize the steric effects. [Cu(l-
OMe)(L)]2(ClO4)2ꢁ2MeCNꢁ2H2O showed much higher catalytic
activity than did [Cu(
l
-OiPr)(L)]2(ClO4)2ꢁ3MeCNꢁ2H2O. The
catalytic rate of [Cu(
l
-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O, meanwhile,
was the median value between those two (Figs. 2 and 3). That is,
the catalytic effects on the heterogeneous catechol oxidation
catalysis in chloroform were in the order [Cu(
[Cu( -OEt)(L)]2(X)2 > [Cu(
-OiPr)(L)]2(X)2.
For all of the catalysts including [Cu(
l
-OMe)(L)]2(X)2 >
l
l
l
-OMe)(L)]2(ClO4)2-
ꢁ2MeCNꢁ2H2O, the substrates were oxidized in the order 4-
BuCat > 4-ClCat > 3,5-DBuCat > Cat. The catalytic efficiencies were
compared with the corresponding potential measurements. Signif-
icantly, the catalytic oxidation rate of 3,5-DBuCat is contrary to the
general trend [40]. That is, the oxidation reaction of 3,5-DBuCat
required 5 h for completion, which rate can be illustratively com-
pared with that of 4-BuCat, which took only 3 h. In supplemental
experimentation, oxidation potentials were in the order of 3,5-
DBuCat (94.09 mV) < 4-BuCat (197.0 mV) < 4-ClCat (213.39 mV)
For the present cyclodimeric species, the double-alkoxy-
bridged moiety with appropriate Cuꢁ ꢁ ꢁCu distance (2.917(3) –
2.980(2) Å) has played a key role in efficient catechol oxidation.
The number and nature of the bridging alkoxy moieties between
the copper(II) centers as well as L seem to be additional reasons

H. Kim et al. / Inorganica Chimica Acta 443 (2016) 51–56
55
Cl1
O4
B1
(a)
(c)
(b)
O3
F2
F4
Si1
Si1
Si2
F3
N2
Si2
O2
N2
F1
N1
O5
N1
Cu1
Cu1
O1
O1
Fig. 3. Plot showing comprehensive catalytic oxidation yields of 4-BuCat, 4-ClCat,
3,5-DBuCat, and Cat toward the corresponding benzoquinones using [Cu(l-OMe)
(L)]2(ClO4)2ꢁ2MeCNꢁH2O (red lines), [Cu(l-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O (blue lines),
and [Cu(
l
-OiPr)2(L)]2(ClO4)2ꢁ3MeCNꢁ2H2O (green lines) as catalysts. The [catalyst]:
[catechol] ratio was 1:4 in CHCl3 at room temperature. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
remains unclear. However, we did observe one definite steric hin-
drance on the oxidation catalysis. The coordination environments
of [Cu(l-OMe)(L)]2(ClO4)2ꢁ2MeCNꢁ2H2O and [Cu(l-OMe)(L)]2-
(BF4)2ꢁ2Me2CO are similar for except for counteranion, accordingly,
the two catalysts provide similar catalytic efficiency. Therefore, it
can be posited that the enhanced reactivity of the present com-
plexes is related to their cyclodimeric skeletal structures. 3,5-DBu-
Cat, in particular, is significantly slow relative to the other
substrates, indicating the greater relevance of steric hindrance
compared with electronic effect.
Fig. 1. ORTEP drawings of [Cu(
(L)]2(BF4)2ꢁ2Me2CO (b), and molecular packing diagram showing intermolecular
interactions between quinolinyl groups highlighted by dashed lines. Hydro-
l-OMe)(L)]2(ClO4)2ꢁ2MeCNꢁH2O (a) and [Cu(l-OMe)
pꢁ ꢁ ꢁp
gen atoms, solvate molecules, and counteranions (in (c)) were omitted for clarity.
100
4. Conclusion
The new 1,4-bis(dimethyl(quinolin-3-yl)silyl)benzene ligand
can be a valuable tectonic for the doubly-alkoxy-supported cyclo-
dimeric species with appropriate Cuꢁ ꢁ ꢁCu distances for oxidation of
catechols. This series of cyclodimeric copper(II) complexes are a
good model system for catecholase mimetics that show significant
steric effects of both cyclodimeric copper(II) catalysts and sub-
strates via the bulky quinolinyl moieties attached to the copper
(II) active sites. The catalytic efficiency of these cyclodimeric com-
plexes is superior to that of the majority of systems described in
the literature; currently, additional research aiming to elucidate
the role of the cyclodimeric species in the reactivity of these com-
plexes is underway. Further experiments on the system, including
the electronic effects of ligands and coligands will provide more
detailed information on the enormous potentials of catalytic prop-
erties such as adsorption–desorption of small solvent molecules.
75
0.6
3 h
50
25
0
0.4
0.2
0.0
350
400
450
Wavelength (nm)
0
2
4
6
Time (h)
Fig. 2. Plot showing catalytic yields of 4-BuCat using [Cu(
l
-OMe)(L)]2(ClO4)2ꢁ
2MeCNꢁH2O (red), [Cu(l-OEt)(L)]2(ClO4)2ꢁ2MeCNꢁH2O (blue), and [Cu(l
-OiPr)(L)]2
(ClO4)2ꢁ3MeCNꢁ2H2O (green) as catalysts, according to the UV–Vis absorption bands
corresponding to the oxidized species, 4-BuBQ. The [catalyst]:[catechol] ratio was
1:4 in CHCl3 at room temperature. Inset: UV–Vis spectra showing oxidation using
Acknowledgment
[Cu(
l-OMe)(L)]2(ClO4)2ꢁ2MeCNꢁH2O. (For interpretation of the references to color in
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korean Government [MEST]
(2013R1A2A2A07067841).
this figure legend, the reader is referred to the web version of this article.)
for the considerable catalytic activity. The Cuꢁ ꢁ ꢁCu distance is very
similar to the corresponding 2.96 Å distance that provides the most
efficient catecholase activity [20]. Despite the present successful
characterization of the active sites of catechol oxidation based on
the crystal structures and steric effects, the catalytic mechanism
Appendix A. Supplementary material
CCDC 1062311, 1062312, 1062313 contains the supplementary
crystallographic data for [Cu(
l-OMe)(L)]2(ClO4)2ꢁ2MeCNꢁH2O, [Cu

56
H. Kim et al. / Inorganica Chimica Acta 443 (2016) 51–56
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l
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