A copper complex of a noninnocent iminophenol-amidopyridine hybrid ligand: Synthesis, characterization, and aerobic alcohol oxidation

Article abstract of DOI:10.1002/ejic.201402687

Reaction of the noninnocent iminophenol-iminopyridine hybrid ligand HL^IPIP, where L^IPIP denotes [2-((E)-{(E)-2-[(E)-pyridin-2-ylmethyleneamino]benzylidene}amino)-4,6-di-tert-butylphenolate], with copper acetate afforded a copper complex, L^APIPCu^II, in which one of the imine functional groups is oxidized to an amide during metal complexation. The new Cu^II complex is capable of catalyzing efficient aerobic alcohol oxidation under mild conditions. The crystal structure of L^APIPCu^II exhibits a square-planar geometry with the Cu^II center coordinated by three nitrogen atoms and one oxygen atom. Electrochemical studies were conducted to evaluate the redox-active behavior of the complex, and the results showed a quasireversible reduction and a ligand-based oxidation process. The neutral species of L^APIPCu^II is EPR active, which is consistent with a paramagnetic electronic ground state (d⁹, S = 1/2), whereas the one-electron oxidized complex was X-band EPR silent. One-electron chemical oxidation of L^APIPCu^II gave a new species that can be attributed to a Cu^II-phenoxyl radical complex. Based on EPR measurements in conjunction with density functional theory calculations, [L^APIPCu^II]⁺ is proposed to have a triplet electronic ground state, exhibiting a weak ferromagnetic interaction between the Cu^II center and the coordinated phenoxyl radical. A new copper complex of a noninnocent iminophenol-pyridine hybrid ligand that is capable of efficient aerobic alcohol oxidation was studied.

Full text of DOI:10.1002/ejic.201402687

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DOI:10.1002/ejic.201402687
A Copper Complex of a Noninnocent Iminophenol-
Amidopyridine Hybrid Ligand: Synthesis,
Characterization, and Aerobic Alcohol Oxidation
Zahra Alaji,[a] Elham Safaei,*[a,b] Linus Chiang,[c]
Ryan M. Clarke,[c] Changhua Mu,[c] and Tim Storr*[c]
Keywords: Ligand design / Biomimetic synthesis / Copper / Homogeneous catalysis / Oxidation
Reaction of the noninnocent iminophenol-iminopyridine
hybrid ligand HLIPIP, where LIPIP denotes [2-((E)-{(E)-2-[(E)-
pyridin-2-ylmethyleneamino]benzylidene}amino)-4,6-di-
tert-butylphenolate], with copper acetate afforded a copper
complex, LAPIPCuII, in which one of the imine functional
groups is oxidized to an amide during metal complexation.
The new CuII complex is capable of catalyzing efficient aero-
bic alcohol oxidation under mild conditions. The crystal
structure of LAPIPCuII exhibits a square-planar geometry with
the CuII center coordinated by three nitrogen atoms and one
oxygen atom. Electrochemical studies were conducted to
evaluate the redox-active behavior of the complex, and the
results showed a quasireversible reduction and a ligand-
based oxidation process. The neutral species of LAPIPCuII is
EPR active, which is consistent with a paramagnetic elec-
tronic ground state (d9, S = 1/2), whereas the one-electron
oxidized complex was X-band EPR silent. One-electron
chemical oxidation of LAPIPCuII gave a new species that can
be attributed to a CuII-phenoxyl radical complex. Based on
EPR measurements in conjunction with density functional
theory calculations, [LAPIPCuII]+ is proposed to have a triplet
electronic ground state, exhibiting a weak ferromagnetic in-
teraction between the CuII center and the coordinated phen-
oxyl radical.
Transition-metal complexes containing phenolate ligands
have attracted significant attention as active site models of
metalloenzymes, which include a transition-metal ion and a
redox-active organic cofactor such as tyrosine.[4c,5] Galact-
ose oxidase (GAO) is the archetypical example, containing
a copper(II)-tyrosyl (phenoxyl) radical as the key reactive
intermediate for the two-electron oxidation of primary
alcohols to aldehydes.[6] Detailed structural, spectroscopic,
and kinetic studies have demonstrated that the two-electron
reaction catalyzed by GAO is performed by an active site
that contains a single copper ion that shuttles through a
one-electron process between CuI/CuII oxidation states,
and, in addition, a copper-coordinated tyrosinate that is ox-
idized to a tyrosyl radical during the catalytic process
(Scheme 1).[7,8]
A great deal of effort has been invested in the design of
biomimetic synthetic model systems of GAO incorporating
redox active ligands for catalytic applications. The selective
oxidation of alcohols in a facile and environmentally
friendly manner is a desirable chemical transformation,[1b]
and, based on the GAO enzyme, copper complexes that in-
clude phenolate groups have led to the discovery of Cu–
phenolate/phenoxyl radical (or related) model systems that
exhibit GAO-like alcohol oxidation activity.[1f,6c,7,10] In the
past decade, Wieghardt and Chaudhuri, Stack, as well as
many other research groups, have published valuable re-
ports of spectroscopic and functional models of GAO
based on noninnocent Cu complexes of aminophenol li-
Introduction
The chemistry of metal complexes with redox active li-
gands is of considerable interest in both bioinorganic and
catalytic chemistry.[1] Complexes of redox active ligands can
be classified in two categories: (a) a metal-ligand radical
[Mn+(L·)], or (b) a high-valent metal [M(n+1)+(L–)] complex,
depending on the relative energies of the redox-active orbit-
als.[2] Oxidized metal complexes are known to exist in each
form, and the factors that control the oxidation states of
these complexes are currently under investigation.[3] Given
that these ligands possess various energetically accessible
redox levels, tuning their redox processes can provide ad-
ditional electron reservoirs for catalysis reactions. Much of
this research is inspired by biological systems, in which li-
gand-radical species have been found to play key roles in a
number of catalytic processes.[4]
[a] Institute for Advanced Studies in Basic Sciences (IASBS),
45137-66731, Zanjan, Iran
E-mail: safaei@iasbs.ac.ir
http://www.iasbs.ac.ir/chemistry/dep=3&ef=en&page=
personalpage.php&id=257
[b] Department of Chemistry, College of Sciences, Shiraz
University,
71454, Shiraz, Iran
[c] Department of Chemistry, Simon Fraser University,
8888 University Drive, Burnaby, BC V5A 1S6, Canada
E-mail: tim_storr@sfu.ca
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402687.
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Scheme 1. The active site and observable forms of GO.[7,9]
gands.[1a,3h,6c,9,11] These reports prompted us to undertake to synthesize. Although not studied further, Cu complex-
an investigation, described herein, of the synthesis and ation likely facilitates the oxidation of the pyridine-imine of
characterization of a new copper complex of an imi- ligand HLIPIP to form a pyridine-amide in LAPIPCuII.
nophenol-amidopyridine hybrid ligand and the associated
catalytic activity towards aerobic alcohol oxidation under
mild conditions (Scheme 2).
X-ray Analysis
Red-brown crystals of the copper complex for X-ray
analysis were obtained from an ethanol/dichloromethane
(1:1) mixture. Crystal data for LAPIPCuII is shown in
Table 1 and selected bond lengths are listed in Table 2.
Table 1. Crystallographic data for LAPIPCuII.
LAPIPCuII
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C27H29CuN3O2
491.09
monoclinic
P1 21/c 1
9.5598(5)
18.2903(9)
14.1699(8)
90
Scheme 2. The structure of HLIPIP and the associated CuII complex
LAPIPCuII.
α [°]
β [°]
107.231(2)
90
2366.4(2)
4
150(2)
1.378
0.952
57791
9306
0.0450
Results and Discussion
γ [°]
V [Å3]
Synthesis and Characterization of the Ligand HLIPIP and
Complex LAPIPCuII
Z
T [K]
ρcalcd. [g/cm3]
μ [mm–1]
The ligand HLIPIP was synthesized through the sequen-
tial condensation of 2-aminobenzylamine with 3,5-DTBQ
and then pyridine-2-carbaldehyde. Next, HLIPIP was treated
immediately with Cu(OAc)2·2H2O to afford the Cu com-
plex LAPIPCuII. IR analysis of LAPIPCuII revealed a sharp
band at 1642 cm–1, attributed to the νC=O stretch of an
amide substituent, and this moiety was confirmed by X-ray
Reflections collected
Significant reflections
R [I Ͼ 2.5σ(I)]
Rw [I Ͼ 2.5σ(I)]
Goodness of fit
0.0999
0.7317
The solid-state structure of LAPIPCuII exhibits a distorted
analysis of the complex (see below). Oxidation of a coordi- square-planar geometry (dihedral angle = 7.9°), with the
nated imine group (coordinated imines are more susceptible CuII ion surrounded by three nitrogen atoms of amidate,
to nucleophilic attack by H2O or OH–) to a carboxamide, pyridine, and an iminophenol (N2, N3, N4), with the final
which can be started with nucleophilic attack by H2O or donor group of a phenolate oxygen atom (O5). (Figure 1).
OH– on the imine, has been well-documented.[12] This pro-
The phenolate C(17)–O(5) bond length (1.322 Å) and the
cess can provide access to metal complexes that are difficult C–C bonds connected to the C(17)–O(5) moiety (1.422 Å
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Table 2. Comparison of experimental and calculated (in parenthe- EPR Analysis of LAPIPCuII
ses) bond lengths (in Å) for the LAPIPCuII and [LAPIPCuII]+ com-
plexes.
A frozen toluene solution of the neutral species LAPIP
-
CuII is EPR active at 20 K (Figure 2), which is consistent
with a paramagnetic electronic ground state (d9, S = 1/2).
The CuII d9 paramagnetic ground state was also confirmed
Bond
LAPIPCuII
[LAPIPCuII]+ [LAPIPCuII]+
(S = 1)
(S = 0)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(4)
Cu(1)–O(5)
C(17)–O(5)
C(22)–N(4)
C(23)–N(4)
C(6)–N(2)
1.955 (1.958)
1.955 (1.972)
1.921 (1.924)
1.901 (1.895)
1.322 (1.326)
1.414 (1.412)
1.284 (1.297)
1.400 (1.400)
(1.976)
(1.976)
(1.951)
(2.005)
(1.273)
(1.392)
(1.305)
(1.385)
(1.974)
(1.978)
(1.953)
(2.006)
(1.272)
(1.391)
(1.305)
(1.385)
1
by the broadening of H NMR peaks of LAPIPCuII (Fig-
ure S3).
Figure 2. EPR spectrum of LAPIPCuII (black) and simulation (red).
Conditions: frequency: 9.3865 GHz; power: 2 mW; modulation fre-
quency: 100 kHz; amplitude: 0.10 G; T = 20 K; concentration 1 mm
in toluene.
The EPR simulation results shown in Table 3 indicate
that the geometry of the neutral copper exhibits slight tetra-
hedral distortion, with rhombic symmetry.[17] The pattern
of g- and A-values also suggests a formal dx2–y2 ground state
for LAPIPCuII, as predicted by the calculations (see above).
Figure 1. Molecular structure of LAPIPCuII. Hydrogen atoms have
been omitted for clarity. Thermal ellipsoids are set at 50% prob-
ability.
Table 3. EPR parameters[a] for LAPIPCuII.
[b]
[b]
[b]
[b]
A1
(Cu)
A2
A3
A1,2,3
(N)
g1
g2
g3
and 1.412 Å) are also in good agreement with previously
reported metal-phenolate structures.[1f,11e,13] The Cu–O(5)
(1.901 Å) bond length is consistent with the value reported
for the Cu–O Tyr (Y272) in galactose oxidase (1.9 Å)[14]
and some of its model complexes (1.904 Å).[6e] The Cu–O
(1.901 Å) and Cu–N (1.921, 1.955, 1.955 Å) bond lengths
are slightly longer than those of the CuII-salen complex
[CuII(Salen)] (Salen = diphenolate form of SalenH2), N,NЈ-
bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-
diamine (Cu–O, 1.886 and 1.887 Å; Cu–N, 1.904 and
(Cu)
(Cu)
LAPIPCuII 2.025 2.046 2.170 29
29
217
14
[a] See experimental section for details, measured in toluene at
20 K. [b] Values in 10–4 cm–1.
Electrochemistry
The redox processes of LAPIPCuII were examined by cy-
1.915 Å).[15] The coordination geometry around the nitro- clic voltammetry (CV) in CH2Cl2. The Cu complex exhibits
gen donor, N(2), is planar, indicating that this nitrogen is two quasireversible one-electron redox processes in the sol-
three-coordinate (sp2 hybridization) and is not proton- vent window (Figure 3 and Table 4).
ated.[16]
LAPIPCuII displays a quasireversible anodic redox pro-
The ligand is thus in the di-anionic form when bound to cess at 0.46 V, indicating a ligand-based oxidation to form
CuII. ESI-MS analysis confirmed the presence of the LAPIP
-
a phenoxyl radical species (see below). The oxidation poten-
CuII complex at m/z 491.2 (Figure S1). DFT calculations of tial for LAPIPCuII is comparable to the first oxidation po-
LAPIPCuII using the B3LYP functional are in good agree- tential reported for a large number of CuII-imino-phenolate
ment with the solid-state data, with the predicted metrical complexes (ca. 0.45 V vs. Fc+/Fc in CH2Cl2), that exhibit
parameters being withinϮ0.02 Å of the experimental val- ligand radical formation.[3h,16a,18] The redox process at
ues (Table 2). The predicted spin density for LAPIPCuII is negative potentials (–1.61 V) could be a CuII/CuI reduction
shown in the Supporting Information (Figure S2) and is process, based on previously reported data.[17a,19] However,
consistent with a dx2–y2 ground state.
we cannot rule out ligand reduction in this work.
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the visible region along with a broad low-energy band cen-
tered at 7400 cm–1 (3200 m–1 cm–1). The feature at
24000 cm–1 (417 nm) is similar to that reported for other
Cu-coordinated phenoxyl radical complexes,[6e,21] and
GAO.[21,22] Whereas the oxidized form is stable in solution
at low temperature, rapid decomposition occurs upon
warming.
Table 5. Electronic absorption properties of the complex.[a]
ν
˜
(εmax, m–1 cm–1)
max
31000 (9800, br), 29200 (7600, sh), 26700 (6300,
sh), 25600 (8700), 24300 (9100), 20900 (6300)
32000 (41400), 24000 (9700), 21500 (7600, sh),
20000 (5300, sh), 17300 (2000, sh), 7400 (2400, br)
LAPIPCuII
[LAPIPCuII]+
[a] For details see the Experimental Section.
Figure 3. Cyclic voltammogram of LAPIPCuII. Conditions: 2.5 mm
complex, 0.1 m NBu4ClO4, scan rate 100 mV/s, CH2Cl2, 298 K.
Table 4. Redox potentials of LAPIPCuII vs. Fc+/Fc.[a] Peak to peak EPR Analysis of [LAPIPCuII]+
separation in parentheses.
We investigated [LAPIPCuII]+ by variable-temperature
EPR spectroscopy to better understand the electronic struc-
1
2
E1/2 [V]
E1/2 [V]
LAPIPCuII
–1.61 (0.21)
0.46 (0.19)
ture of this oxidized complex. The EPR spectrum of [LAPIP
-
CuII]+ and a concentration-matched sample of the neutral
compound LAPIPCuII are shown in Figure 5. The lower
spectral resolution of neutral LAPIPCuII in Figure 5 (mea-
sured in frozen CH2Cl2), compared with the spectra shown
in Figure 2, is due to improved glassing ability of toluene.
The EPR spectrum of [LAPIPCuII]+ exhibits a weak CuII
signal with an intensity of approximately 0.35 in compari-
son to the concentration-matched neutral sample by spin
quantitation. The weak axial CuII signal (gЌ = 2.03, gʈ =
2.20; AЌ = 35ϫ10–4 cm–1, Aʈ = 186ϫ10–4 cm–1) is likely
due to decomposition of the oxidized complex upon trans-
fer for EPR measurements. Variable-temperature EPR mea-
surements of [LAPIPCuII]+ (Figure S5 and S6) show little
change in the EPR spectrum from 110 to 20 K, suggesting
that the oxidized form is either (i) a CuIII-phenolato com-
plex, (ii) a CuII complex antiferromagnetically coupled to
[a] Peak to peak difference for the Fc+/Fc couple at 298 K is 0.11 V.
Interestingly, the LAPIPCuII redox process at negative po-
tential is less reversible at 233 K than at 298 K (Figure S4);
the changes could indicate considerable geometric re-
arrangement upon reduction from CuII to CuI.[19,20]
Electronic Spectroscopy Studies of LAPIPCuII and
[LAPIPCuII]+
The electronic spectrum of LAPIPCuII in CH2Cl2 is
shown in Figure 4, and exhibits a series of bands in the
visible region (Table 5). Oxidation titration experiments
–
using the oxidant [N(C6H4Br)3]+SbF6 from 0Ǟ1 equiv. at
low temperature (198 K) revealed isosbestic points at 24500
and 19900 cm–1, indicating clean conversion from LAPIP
-
CuII into [LAPIPCuII]+. The intense band at 32000 cm–1 is
due to the oxidation by-product N(C6H4Br3)3. Upon oxi-
dation to [LAPIPCuII]+, a number of new bands appear in
Figure 5. EPR spectrum of LAPIPCuII (black), [LAPIPCuII]+ (red),
and simulated spectrum of oxidized decay product (blue). Condi-
tions: frequency: 9.3783 GHz; power: 2 mW; modulation fre-
quency: 100 kHz; amplitude: 6 G; T = 20 K; concentration: 0.3 mm
in CH2Cl2.
Figure 4. Electronic spectra of 0.2 mm CH2Cl2 solutions of LAPIP
-
CuII (black), [LAPIPCuII]+ (red) and spectra during incremental oxi-
dation (gray) with [N(C6H4Br)3]+SbF6 at 198 K.
–
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phenoxyl radical (S = 0), or (iii) a CuII complex ferromag- Catalytic Activity of LAPIPCuII Complex
netically coupled to a phenoxyl radical (S = 1) with large
The synthesized copper complex LAPIPCuII was em-
ployed for the aerobic oxidation of a large number of struc-
turally diverse alcohols to the corresponding aldehydes
(Scheme 4).
zero-field splitting.[23] A large zero-field splitting can result
in EPR transitions outside of the energy range of X-band
EPR.[24] We thus turned to theoretical calculations to gain
further insight into the oxidized complex (see below).
Theoretical Calculations on [LAPIPCuII]+
DFT calculations of [LAPIPCuII]+ were performed by
using the B3LYP functional/TZVP basis set combination
and a polarized continuum model (PCM) for CH2Cl2 (di-
electric ε = 8.94). This species can be described as one of the Scheme 4. Catalysis conditions relevant to Table 8.
following: (i) a CuIII-phenolato complex, (ii) a CuII complex
Our initial studies focused on determining the optimal
conditions (base, solvent) for the conversion of a represen-
tative alcohol (benzyl alcohol) into the corresponding alde-
hyde (Table 6). We have found that the addition of base is
antiferromagnetically coupled to phenoxyl radical (S = 0),
or (iii) a CuII complex ferromagnetically coupled to a phen-
oxyl radical (S = 1). The calculations predict that the S =
2
ˆ
1 triplet solution (S = 2.02) is favored by 0.4 kcal/mol in
essential for alcohol activation. Compared with previous re-
ports,[11g,29] intriguing features of this catalytic system in-
clude the efficient use of oxygen as the oxidant, high yield
and high selectivity, the use of acetonitrile as a relatively
green solvent,[4c,30] and KOH as a low-cost base under mild
conditions (room temperature) (Table 6, entries 5, 9, and
10). Control runs in the absence of catalyst at room tem-
perature yielded no product. It is worth noting that KOH-
mediated synthesis of imine from aerobic oxidation of
alcohol has been reported to take place at high temperature
using toluene as solvent.[31]
2
II
ˆ
comparison to the broken symmetry (BS, S = 1.02; Cu
complex antiferromagnetically coupled to phenoxyl radical)
electronic structure. The singlet solution is predicted to be
approximately 11 kcal/mol higher in energy. The predicted
spin densities for the triplet and BS solutions are shown in
Figure 6, and they result in an exchange coupling J of
140 cm–1, calculated by using the Yamaguchi equation
[Equation (1)], which is applicable for systems from the
strong to weak exchange limit.[25,26]
EHS – EBS
J =
(1)
2
2
ˆ
ˆ
͗S ͘HS – ͗S ͘
BS
Table 6. The effect of base and solvent on the aerobic oxidation of
benzyl alcohol catalyzed by LAPIPCuII.
Entry Base (equiv.)
Solvent
Time [h] Conversion [%]
1
2
3
4
5
6
7
8
9
Cs2CO3 (2)
Cs2CO3 (2)
Cs2CO3 (2)
Cs2CO3 (2)
KOH (2)
toluene
THF
6
6
6
7
4
70
95
37
97
96
2
n-hexane
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Figure 6. Spin density plot of [LAPIPCuII]+, triplet (left), and
broken symmetry solution (right). See the Experimental Section for
calculation details.
NEt3 (2)
6
Na2CO3 (2)
K2CO3 (2)
KOH (3)
KOH (1)
KOH (0.5)
24
24
3
5
5
2
38
98
100
93
The calculations predict a lengthening of the Cu–
10
O(phenoxyl) bond (1.895 to 2.005 Å) upon oxidation 11
(Table 2), matching other reports for coordinated phenoxyl
radicals.[11e,13b,18,27] The increased Cu–O bond length can
A number of outstanding GO-inspired models have been
be attributed to the decline of the electron-donating ability developed for the aerobic oxidation of benzyl alcohol and
of phenoxyl species relative to the phenolato moiety. In ad- these are summarized in Table 7. A mechanistically faithful,
dition, the C–O bond is predicted to shorten upon oxi- catalytic model of GO was first reported in 1998 by Stack
dation from phenolate to phenoxyl (1.326 to 1.273 Å) et al.[6f] The best results have been obtained by Wieghardt
(Table 2). This feature is consistent with a quinoid structure et al. with a very nice set of complexes in which the redox
for the ring and thus a radical character for this moiety chemistry during the catalytic cycle is ligand-based.[10d,28,32]
(Scheme 3).
As inferred from Table 7, our synthesized copper complex
Scheme 3. Canonical forms of phenoxyl radicals.[7]
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is a versatile catalytic system that catalyzes the oxidation of oxidation reaction was measured by variation of total con-
benzyl alcohol to bezaldehyde with a good turnover number centration of LAPIPCu and benzyl alcohol under 1 bar of
of 260 after 6 h (Figure S7).
oxygen in acetonitrile. From these data (Figures S8 and S9)
the reaction was deduced to be first order in both Cu com-
plex and substrate [Equation (2)].
Table 7. Aerobic oxidation of benzyl alcohol catalyzed by galactose
oxidase inspired model complexes.
Rate = k [complex][benzyl alcohol]
(2)
Author
Year
Yield
[%]
TON Time [h] Ref.
The observed first-order dependence of the rate on the
complex and substrate indicate the composition of activated
complex; in other words, one molecule of complex is uti-
lized for the two-electron oxidation of one molecule of
benzyl alcohol.[11c] In a typical anaerobic experimental set-
up, to a mixture of benzyl alcohol (2 m, 1 mL) and KOH
(1 m, 0.28 g) in anhydrous CH3CN (5 mL) was added the
complex (2 mm, 0.005 g) for 12 h. Under anaerobic condi-
tions, the copper complex oxidizes benzyl alcohol to benzal-
dehyde stoichiometrically. Then the reaction was opened
and stirred for 10 h under oxygen atmosphere (1 bar) to re-
[6f]
Stack et al.
Wieghardt et al. 1998
Wieghardt et al. 1999
Thomas et al.
Wieghardt et al. 2004
Chaudhuri et al. 2008
1998
–
63
55
–
–
72
–
400
–
–
220
95
–
20
12
20
48
10
16
7
[10d]
[28]
[1a]
2002
[4c]
[11c]
[4h]
Paine et al.
Safaei et al.
2012
2014
62
–
260[a]
6
this work
[a] TON reaction conditions: benzyl alcohol (2 m, 1 mL), KOH
(1 m, 0.28 g), catalyst complex (2 mm, 0.005 g) in anhydrous
CH3CN (5 mL).
To help elucidate the mechanism, we conducted kinetic generate the active catalyst and lead to catalyzed oxidation
studies using the initial rate method and an experiment of of benzyl alcohol in relatively good yield (100 TON). Un-
anaerobic alcohol oxidation under turnover conditions with fortunately, it was not possible for us to survey the kinetic
benzyl alcohol. The kinetics of the catalytic benzyl alcohol isotope effect as examined in previous reports,[4c,6f,32] how-
Table 8. LAPIPCuII-catalyzed aerobic oxidation of alcohols.
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ever, the lack of observed reactivity towards primary and
secondary alkyl alcohols (Table 8, entries 15–17) at 298 K
shows the limitation of this catalyst towards substrates with
higher Cα-H bond strengths. Similar reactivity trends are
found for GOase, which oxidizes primary alkyl alcohols at
rates approximately 100 times slower than benzylic sub-
strates.[6f] On the basis of the kinetic studies and the anaero-
Experimental Section
Materials and Methods: All chemicals were purchased from Merck
and used as received. Solvents were purified before use by standard
methods whenever necessary. Manipulations were performed under
aerobic conditions. 3,5-Di-tert-butylcyclohexa-3,5-diene-1,2-dione
(3,5-DTBQ) was synthesized according to a modification of a re-
ported procedure.[33] Electronic spectra were obtained with a Cary
bic experiment, we can infer that the mechanism for the 5000 spectrophotometer with a custom-designed immersible fiber-
optic quartz probe with variable path length (1 and 10 mm; Hellma,
Inc.). IR spectra were recorded in the solid state with an FTIR
Bruker Vector 22 spectrophotometer in the 400–4000 cm–1 range.
NMR spectra were recorded at 400 MHz with a Bruker DRX spec-
trometer in CDCl3 solution. The chemical shifts were referred to
TMS using the residual signals from the solvent. Cyclic voltamme-
try (CV) was performed with a PAR-263A potentiometer, equipped
with a Ag wire reference electrode, a glassy carbon working elec-
trode, and a Pt counter electrode with 0.1 m Bu4NClO4 solutions in
CH2Cl2. Ferrocene was used as an internal standard. Mass spectra
(positive ion) were obtained with an Agilent 6210 TOF ESI-MS
instrument. The magnetic measurements were obtained by using
Evan’s method. NMR and EPR spectra were collected with a
Bruker EMXplus spectrometer operating with a premiumX X-
band (ca. 9.5 GHz) Microwave Bridge.
alcohol oxidation is similar to that proposed for the enzyme
galactose oxidase (Scheme 1) and a number of reported
functional model systems.[1a,4c,7,28] Based on the optimized
conditions determined from Table 6 we screened a variety
of alcohol substrates as detailed in Table 8. A number of
primary and secondary benzylic alcohols, including those
with both electron-withdrawing and -donating groups, were
selectively converted into the corresponding benzaldehydes
in excellent yields under the optimal reaction conditions
(Ͼ94%, Table 8, entries 1–13).
No overoxidation of the aldehyde to the carboxylic acid
was observed by GC analysis. These results show that elec-
tronic effects do not seem to have a significant effect on the
final product yields for electron-rich and electron-deficient
benzylic substrates. An allylic alcohol could be effectively
oxidized to afford excellent yield of the corresponding carb-
onyl product (Table 8, entry 14). However, this method was
not applicable to the oxidation of primary and secondary
aliphatic alcohols (Table 8, entries 15–17). In general, het-
eroatom-functionalized alcohols are regarded as highly
challenging substrates for aerobic oxidation due to their
strong coordination ability to transition-metal catalyst sys-
tems, and resulting deactivation. However, we found that
our protocol was effective in the oxidation of 4-(methyl-
thio)benzyl alcohol to afford the expected aldehyde, which
further confirmed the superior capability of LAPIPCuII in
oxidizing similar substrates (Table 8, entry 7).
Low-temperature measurements of frozen solutions were recorded
with a Bruker helium temperature-control system and a continuous
flow cryostat. Samples for X-band measurements were placed in
4 mm outer-diameter sample tubes with sample volumes of approx-
imately 300 μL. EPR spectra were simulated with Easy Spin 3.1.7
software.[34]
Ligand HLIPIP and LAPIPCuII: LAPIPCuII complex was synthesized
by the following template reaction. First, to a solution of 3,5-
DTBQ (0.22 g, 1 mmol) in acetonitrile (4 mL) was added 2-amino-
benzylamine (0.122 g, 1 mmol), and the reaction solution was
stirred for 30 min at the room temperature in the presence of air.
After some minutes, a yellow precipitate appeared, which was fil-
tered and used in the next step. To a stirred suspension of the yel-
low precipitate (0.324 g, 1 mmol) in acetonitrile, was added pyr-
idine-2-carbaldehyde (0.096 mL, 1 mmol). After 1 min of stirring,
Cu(OAc)2·2H2O (0.199 g, 1 mmol) was added to the reaction mix-
ture, then NEt3 (2 equiv.) was also added. The red-brown solution
was stirred at 298 K for 4 h to afford a red-brown precipitate of
LAPIPCuII. The filtrate was left undisturbed to afford red-brown
microcrystals (0.309 g, 63% yield) by evaporation from dichloro-
methane/acetonitrile solution. X-ray-quality dark-red single crys-
tals were grown from a 1:1 solvent mixture of dichloromethane/
Conclusions
A new copper complex of a noninnocent iminophenol-
pyridine hybrid ligand was synthesized and characterized.
It is evident from X-ray crystallography analysis that LAPIP
-
CuII exists as a distorted square-planar CuII structure coor-
dinated by pyridine, iminophenol, and amidate nitrogen
atoms as well as one phenolate. Cyclic voltammetry mea-
ethanol or n-hexane/dichloromethane. IR (KBr): ν
= 2959 (C–
˜
max
H), 1642 (C=O), 1466 (C=N), 802 (=C–H bending) cm–1. ESI-MS:
m/z (%) = 983.3 (40) [(LAPIPCuII)2]+, 491.2 (100) [LAPIPCuII]+ (Fig-
surements showed two quasireversible redox events for the ure S1). C27H29CuN3O2·0.35C6H14: calcd. C 67.05, H 6.55, N 8.06;
found C 66.79, H 6.61, N 7.76. Magnetic susceptibility: μeff
1.56 μB (Evan’s method).
=
Cu complex, suggesting both the formation of a phenoxyl
radical species in the region of the anodic peak and a metal-
centered redox (CuII/CuI) process at low potentials. Chemi-
Oxidation of LAPIPCuII: Under a nitrogen atmosphere at 195 K, an
cal one-electron oxidation of LAPIPCuII yielded a new spe- aliquot of a CH2Cl2 solution of LAPIPCuII (6.7 mm, 150 μL) was
cies that can be assigned as a CuII-phenoxyl radical moiety
added to CH2Cl2 (3.0 mL). Monitored by UV/Vis/NIR, a solution
analogous to the active CuII–tyrosyl radical form of galact-
of [N(C6H3Br2)3]SbF6 in CH2Cl2 (9.9 mm) was added in 10 μL in-
crements, resulting in conversion into [LAPIPCuII]+ after 75 μL of
ose oxidase. EPR studies, in conjunction with theoretical
oxidant had been added.
Catalytic Activity of LAPIPCuII Complex for Alcohol Oxidation
analysis, point to a triplet ground-state for [LAPIPCuII]+,
due to ferromagnetic coupling of the CuII (d9) metal center
and phenoxyl radical. This copper complex is capable of
efficient aerobic oxidation of alcohols under mild condi-
tions.
Alcohol oxidation experiments were carried out in an oxygen atmo-
sphere at room temperature. In a typical experiment, alcohol
(1 mmol), LAPIPCuII (0.024 g, 4 mol-%), and KOH (0.560 g,
Eur. J. Inorg. Chem. 0000, 0–0
7
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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/KAP1
Date: 11-11-14 14:08:23
Pages: 10
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1 mmol) were stirred in oxygen-saturated acetonitrile (5 mL) in a
10 mL two-neck round-bottom flask equipped with an oxygen bal-
loon. The solution was stirred for 4–24 h according to the corre-
sponding alcohol.
At appropriate intervals, 0.5 mL of reaction mixture was withdrawn
and neutralized by addition of a drop of HCl (10% w/v) and dis-
tilled water (1 mL), and then extracted by dichloromethane or ethyl
acetate. The organic phase was dried with anhydrous sodium sulf-
ate and, after filtration, analyzed by gas chromatographic analyses.
The turnover number (TON) was determined as the number of mol
of product per mol of complex. In a standardized TON experimen-
tal set up, to a mixture of benzyl alcohol (2M, 1 mL) and KOH
(1 m, 0.28 g) in anhydrous CH3CN (5 mL) was added the complex
(2 mm, 0.005 g). The reaction mixture was stirred at ambient tem-
perature under an oxygen balloon (1 bar) and the formation of
benzaldehyde product with time was determined by taking samples
at different time points. The oxidation process continues up to 6 h
and then product formation saturates. The final organic products
were quantitatively monitored by gas chromatographic analyses (as
described above).
[4]
Calculations: Geometry optimizations were performed by using
Gaussian 09 (revision D.01),[35] the B3LYP functional,[36] the TZVP
basis set,[37] and a polarized continuum model (PCM) for CH2Cl2
(dielectric ε = 8.94).[38] Broken-symmetry[39] (BS) DFT calculations
were performed with the same functional and basis sets. Frequency
calculations at the same level of theory confirmed that the opti-
mized structures were located at a minimum on the potential en-
ergy surface.
[5]
[6]
CCDC-1004375 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): DFT-computed metrical parameters, cyclic voltammograms.
Acknowledgments
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E. S. gratefully acknowledges the support by the Institute for Ad-
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grant G2014IASBS127. T. S. acknowledges an NSERC Discovery
Grant. Westgrid and Compute Canada are thanked for access to
computational resources.
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Received: July 21, 2014
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42687
/KAP1
Date: 11-11-14 14:08:23
Pages: 10
www.eurjic.org
FULL PAPER
Ligand Design
Z. Alaji, E. Safaei,* L. Chiang,
R. M. Clarke, C. Mu, T. Storr* ...... 1–10
A new copper complex of a noninnocent
iminophenol-pyridine hybrid ligand that is
capable of efficient aerobic alcohol oxi-
dation was studied.
A Copper Complex of a Noninnocent
Iminophenol-Amidopyridine Hybrid Li-
gand: Synthesis, Characterization, and
Aerobic Alcohol Oxidation
Keywords: Ligand design / Biomimetic syn-
thesis / Copper / Homogeneous catalysis /
Oxidation
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim