New insights into the electronic structure and reactivity of one-electron oxidized copper(II)-(Disalicylidene)diamine Complexes

Article abstract of DOI:10.1021/ic3018503

The neutral and one-electron oxidized Cu(II) six-membered chelate 1,3-Salcn (1,3-Salcn = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,3-cyclohexanediamine) complexes have been investigated and compared with the five-membered chelate 1,2-Salcn (N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)- diamine) complexes. Cyclic voltammetry of Cu(1,3-Salcn) showed two reversible redox waves at 0.48 and 0.68 V, which are only 0.03 V higher than those of Cu(1,2-Salcn). Reaction of Cu(1,3-Salcn) with 1 equiv of AgSbF₆ afforded the oxidized complex which exists as a ligand-based radical species in solution and in the solid state. The X-ray crystal structure of the oxidized complex, [Cu(1,3-Salcn)]SbF₆, exhibited an asymmetric metal binding environment with a longer Cu-O bond and quinoid distortion in the phenolate moiety on one side, demonstrating at least partial ligand radical localization in the solid state. The ligand oxidation is also supported by XPS and temperature dependent magnetic susceptibility. The electronic structure of the [Cu(1,3-Salcn)]⁺ complex was further probed by UV-vis-NIR, resonance Raman, and electron paramagnetic resonance (EPR) measurements, and by theoretical calculations, indicating that the phenoxyl radical electron is relatively localized on one phenolate moiety in the molecule. The reactivity of [Cu(1,3-Salcn)]⁺ with benzyl alcohol was also studied. Quantitative conversion of benzyl alcohol to benzaldehyde was observed, with a faster reaction rate in comparison with [Cu(1,2-Salcn)]⁺. The kinetic isotope effect (KIE = k(H)/k(D)) of benzyl alcohol oxidation by [Cu(1,3-Salcn)]⁺ was estimated to be 13, which is smaller than the value reported for [Cu(1,2-Salcn)]⁺. The activation energy difference between [Cu(1,2-Salcn)]⁺ and [Cu(1,3-Salcn)]⁺ was in good agreement with the energy calculated from KIE. This correlation suggests that the Cu(II)-phenoxyl radical species, characterized for [Cu(1,2-salcn)] ⁺ is more reactive for hydrogen abstraction from benzyl alcohol in comparison to the 1:1 mixture of Cu(III)-phenolate and Cu(II)-phenoxyl radical species, [Cu(1,2-Salcn)]⁺. Thus, the Cu(II)-phenoxyl radical species accelerates benzyl alcohol oxidation in comparison with the Cu(III)-phenolate ground state complex, in spite of the similar activated intermediate and oxidation pathway.

Full text of DOI:10.1021/ic3018503

Article
pubs.acs.org/IC
New Insights into the Electronic Structure and Reactivity of One-
Electron Oxidized Copper(II)-(Disalicylidene)diamine Complexes
Kazutaka Asami,† Kazuaki Tsukidate,† Satoshi Iwatsuki,‡ Fumito Tani,§ Satoru Karasawa,∥ Linus Chiang,⊥
Tim Storr,⊥ Fabrice Thomas,⊗ and Yuichi Shimazaki*,†
†College of Science, Ibaraki University, Bunkyo, Mito 310-8512, Japan
‡Department of Chemistry, Konan University, Higashinada-ku, Kobe 658-8501, Japan
§Institute for Materials Chemistry and Engineering, Kyushu University, Higashi-ku, Fukuoka 812-8581, Japan
∥Graduate School of Pharmaceutical Sciences, Kyushu University, Higashi-ku, Fukuoka 812-8582, Japan
⊥Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada
⊗Dep
́ ́ ́ ́
artement de Chimie Moleculaire-Chimie Inorganique Redox Biomimetique (CIRE) - UMR CNRS 5250, Universite Joseph
Fourier, B. P. 53, 38041 Grenoble cedex 9, France
S
* Supporting Information
ABSTRACT: The neutral and one-electron oxidized Cu(II)
six-membered chelate 1,3-Salcn (1,3-Salcn = N,N′-bis(3,5-di-
tert-butylsalicylidene)-1,3-cyclohexanediamine) complexes
have been investigated and compared with the five-membered
chelate 1,2-Salcn (N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexane-(1R,2R)-diamine) complexes. Cyclic voltammetry
of Cu(1,3-Salcn) showed two reversible redox waves at 0.48
and 0.68 V, which are only 0.03 V higher than those of Cu(1,2-
Salcn). Reaction of Cu(1,3-Salcn) with 1 equiv of AgSbF6 afforded the oxidized complex which exists as a ligand-based radical
species in solution and in the solid state. The X-ray crystal structure of the oxidized complex, [Cu(1,3-Salcn)]SbF6, exhibited an
asymmetric metal binding environment with a longer Cu−O bond and quinoid distortion in the phenolate moiety on one side,
demonstrating at least partial ligand radical localization in the solid state. The ligand oxidation is also supported by XPS and
temperature dependent magnetic susceptibility. The electronic structure of the [Cu(1,3-Salcn)]+ complex was further probed by
UV−vis−NIR, resonance Raman, and electron paramagnetic resonance (EPR) measurements, and by theoretical calculations,
indicating that the phenoxyl radical electron is relatively localized on one phenolate moiety in the molecule. The reactivity of
[Cu(1,3-Salcn)]+ with benzyl alcohol was also studied. Quantitative conversion of benzyl alcohol to benzaldehyde was observed,
with a faster reaction rate in comparison with [Cu(1,2-Salcn)]+. The kinetic isotope effect (KIE = k(H)/k(D)) of benzyl alcohol
oxidation by [Cu(1,3-Salcn)]+ was estimated to be 13, which is smaller than the value reported for [Cu(1,2-Salcn)]+. The
activation energy difference between [Cu(1,2-Salcn)]+ and [Cu(1,3-Salcn)]+ was in good agreement with the energy calculated
from KIE. This correlation suggests that the Cu(II)-phenoxyl radical species, characterized for [Cu(1,2-salcn)]+ is more reactive
for hydrogen abstraction from benzyl alcohol in comparison to the 1:1 mixture of Cu(III)-phenolate and Cu(II)-phenoxyl radical
species, [Cu(1,2-Salcn)]+. Thus, the Cu(II)-phenoxyl radical species accelerates benzyl alcohol oxidation in comparison with the
Cu(III)-phenolate ground state complex, in spite of the similar activated intermediate and oxidation pathway.
Phenoxyl radical formation from tyrosyl residues has been
well established in metalloenzymes such as class I ribonucleo-
tide reductase and prostaglandin endoperoxide synthase.6,7 It
was discovered that the phenoxyl radical can bind to a metal ion
as an open-shell ligand fulfilling the role of an organic radical
cofactor in the metalloenzyme, galactose oxidase (GO), which
is a single copper oxidase catalyzing the two-electron oxidation
of a primary alcohol to the aldehyde (Figure 1).8 A copper(III)
species had once been considered to be a possibility as the
active form of GO,9 but many physicochemical studies revealed
INTRODUCTION
■
Interest in metal-catalyzed reactions in chemical and biological
systems has prompted studies of the redox chemistry of metal
complexes, which has afforded significant insights into the
reaction mechanisms of many useful homogeneous catalytic
reactions and reactions of metalloenzymes.1 In the course of
these studies, a large number of novel complexes have been
prepared,2−5 and it has been increasingly recognized that
radical species formed as a result of redox reactions are
especially important in biological systems. Thus, radicals of
amino acid residues, such as tyrosyl, cysteinyl, glycyl, and
tryptophyl radicals, are now known to form in the course of
certain enzymatic reactions.6,7
the formation of a Cu(II)−phenoxyl radical species.8,10,11
A
Received: August 24, 2012
Published: October 31, 2012
© 2012 American Chemical Society
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backbone 1,2-Salcn and Salen complexes.20,21 The oxidized
Salpn complexes exhibit more localized phenoxyl radical
species, and the Pd analogue demonstrated localization of the
phenoxyl radical on one of the two phenolate moieties and
consequently an unsymmetrical coordination environment.20,21
Oxidation of the copper salen derivative Cu(1,2-Salcn)
results in a high-valent, diamagnetic Cu(III) species in the solid
state. In solution, however, a temperature-dependent equili-
brium exists between a Cu(III)-ligand and a Cu(II)-ligand
radical form, indicating the nearly isoenergetic nature of these
two species.22 The shift of the oxidation locus from the ligand
for [Ni(1,2-Salcn)]+ to the metal for [Cu(1,2-Salcn)]+
matches the trend for the monoanionic Ni- and Cu-
dithiolenes.23 For Ni(1,2-Salcn), the redox-active orbitals of
the ligand are positioned at a higher energy in comparison with
the d-orbital manifold, resulting in ligand oxidation to form
[Ni(1,2-Salcn)]+.19,21 For Cu(1,2-Salcn), occupancy of the
Figure 1. Proposed catalytic alcohol oxidation mechanism of GO.
proposed mechanism which has been generally accepted
indicates that the oxidized form of GO in the Cu(II)-tyrosine
radical state binds the primary alcohol in the fourth equatorial
position and that subsequent hydrogen atom abstraction and
electron transfer lead to formation of the aldehyde via the ketyl
radical intermediate and the reduced form of GO.8 For an
improved understanding of the detailed mechanism of GO and
the properties of the metal complexes with the coordinated
phenoxyl radical, many metal−phenolate complexes have been
synthesized and characterized.12,13
Depending on the relative energies of the redox-active
orbitals, a metal complex with a noninnocent ligand exists in
two limiting descriptions, either a metal−ligand radical
(Mn+(L•)) or a high valent metal (M(n+1)+(L−)) complex.14,15
Oxidized metal salen-type complexes are known to exist in
either form, and the factors that control the locus of oxidation
in these complexes are being actively pursued. The one-electron
oxidized Ni(1,2-Salcn) complex (Figure 2) exists in the ligand
2
2
higher energy metal-centered orbital (dx −y ) results in a Cu(III)
complex upon oxidation.22 On the other hand, Thomas et al.
have reported that a para-methoxy substituted phenolate
derivative of Cu(1,2-Salcn) yields a partially localized Cu(II)-
phenoxyl radical complex upon oxidation.24
In this paper, we report the synthesis of the one-electron
oxidized Cu(1,3-Salcn) complex (1,3-Salcn = N,N′-bis(3,5-di-
tert-butylsalicylidene)-1,3-cyclohexanediamine (Figure 2) and
characterization of the physical properties and reactivity in
comparison with 1,2-Salcn analogues. The 1,3-Salcn ligand has
the same N2O2 donor set as that of the previously studied Salen
and 1,2-Salcn analogues but contains a six-membered chelate
ring. We expected that the 6-membered chelate backbone
would lead to coordination plane distortion and that this
structural change would influence the orbital overlap between
the copper ion and the ligand as seen for the group 10 metal
Salpn complexes.20 Our results show that the oxidized 6-
membered chelate complex can be described as a Cu(II)-
phenoxyl radical species, which differs from the Cu(III)-
phenolate ground state determined for one-electron oxidized
Cu(1,2-Salcn). By a combination of experiments and
calculations we now show that conversion from a 5- to a 6-
membered chelate backbone alters the electronic structure of
the one-electron oxidized copper(II) complexes. Our reactivity
results also show that the Cu(II)-phenoxyl radical species,
[Cu(1,3-Salcn)]+, which exhibits an electronic structure
comparable to the oxidized form of GO, is more reactive
toward alcohol oxidation as compared to [Cu(1,2-Salcn)]+.
EXPERIMENTAL SECTION
■
Materials and Methods. All the chemicals used were of the
highest grade available and were further purified whenever necessary.25
Solvents were also purified before use by standard methods.25 N,N′-
bis(3,5-di-tert-butylsalicylidene)cyclohexane-1,3-diamine (H21,3-
Salcn) was prepared in a manner similar to the ligands H21,2-Salcn
and H2Salen (Figure 2).16 UV−vis−NIR spectra were obtained with a
JASCO V-670 spectrophotometer, equipped with a liquid nitrogen
cooled cryostat, UNISOKU CoolSpeK UV USP-203-A, from −80 to
30 °C. Cyclic voltammetry (CV) was performed on a HOKUTO
DENKO HZ-5000 automatic polarization system, equipped with a Ag
wire reference electrode, a platinum disk working electrode, and a Pt
counter electrode with 0.1 M Bu4NClO4 solutions in CH2Cl2.
Ferrocene was used as an external standard. Electrospray ionization
mass spectra (ESI-MS) were collected with a PE Sciex API 300 mass
spectrometer. All electron paramagnetic resonance (EPR) spectra were
collected using a Bruker EMX plus spectrometer operating with either
an X-band or a Q-Band (∼34 GHz) microwave bridge. For the X-Band
Figure 2. Structures of some salen-type complexes: M(Salen),
M(Salpn), M(1,2-Salcn), and M(1,3-Salcn).
radical form in the solid state and in solution, and yet the
presence of coordinating ligands and/or solvent shifts the locus
of oxidation to the nickel center, forming a Ni(III)
complex.16−20 This result demonstrates that a subtle change
of electronic structure can have a significant effect on the locus
of oxidation in these systems. Building on these results, we have
reported on the electronic structure of one-electron oxidized
group 10 metal Salpn complexes with a 6-membered chelate
backbone.20 The oxidized complexes exhibit subtle differences
in electronic structure and interaction with exogenous ligands
when compared with the oxidized 5-membered chelate
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experiment a dual mode cavity operating both at ∼9.6 GHz (for an
orientation perpendicular to the magnetic field) and ∼9.4 GHz (for an
orientation parallel to the magnetic field) was used. Low temperature
measurements of frozen solutions were conducted 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 ∼400 μL. Samples for
Q-band measurements were loaded in 1-mm outer-diameter sample
tubes containing sample volumes of ∼40 μL. EPR spectra were
simulated with a commercial Bruker XSophe software. The X-ray
photoelectron spectra (XPS) were recorded on a Thermo VG
Scientific Model Theta Probe spectrometer using Al Kα radiation
(1486.6 eV) operated at 15 kV and 3 mA as the X-ray excitation
source. The solid sample was dispersed on an In film, and the carbon
1s binding energy (284.5 eV) was used to calibrate the binding
energy.26 Resonance Raman spectra were measured with a JASCO
NR-1800 triple polychromator equipped with a liquid-nitrogen-cooled
Princeton Instruments CCD detector. Raman shifts were calibrated
with indene, the accuracy of the peak positions of the Raman bands
being 1 cm−1.
Synthesis of Complexes. [Cu(1,3-Salcn)] (1). To a solution of
H21,3-Salcn (0.51 g, 1.0 mmol) in CH2Cl2 (10 mL) was added
Cu(CH3COO)2·2H2O (0.21 g, 1.0 mmol) in C2H5OH (10 mL). The
resulting solution was refluxed for a few minutes and kept standing at
room temperature for 2−3 h to afford a brown precipitate. Brown-
green crystals were obtained by recrystallization from CH2Cl2/MeOH.
Elemental analysis (%) calcd for 1 (C36H52N2O2Cu): C, 71.07; H,
8.62; N, 4.60. Found: C, 70.77; H, 8.62; N, 4.55.
[Cu(1,3-Salcn)]SbF6 ([1]SbF6). To a solution of Cu(1,3-Salcn)
(1) (0.608 g, 1.0 mmol) in CH2Cl2 (15 mL) was added AgSbF6 (0.344
g, 1.0 mmol), and after a few minutes the reaction mixture was filtered.
A few drops of toluene were added to the resulting solution, which was
kept standing overnight below −60 °C to afford green crystals.
Elemental analysis (%) calcd for [1]SbF6 (C36H52N2O2CuSbF6·0.25-
(C7H8)·0.5(CH2Cl2)): C, 50.51; H, 6.09; N, 3.08. Found: C, 50.74; H,
6.26; N 2.99.
Benzyl alcohol-d (C6H5CH2OD) was obtained through the H−D
exchange between C6H5CH2OH and D2O, and was confirmed as >80
atom % deuteration by NMR spectroscopy.
The sample solutions for the kinetic measurements were prepared
by the following two procedures: (A) A 5.0 mM solution of [1]+ was
prepared by filtering the mixture of 5.0 mM CH2Cl2 solution of 1 and
1 equiv of AgSbF6. After the solution of benzyl alcohol in CH2Cl2
([C6H5CH2OH] = 0.1−1.0 M, 2.5 mL) in a 10-mm quartz cell was
adjusted to the appropriate reaction temperature, the reaction was
initiated by adding 100 μL of the 5.0 mM solution of [1]+. From this
procedure, the total concentration of [1]+ in the sample solutions was
0.2 mM, which was much lower than that of benzyl alcohol (0.1−1.0
M). (B) After the 0.2 mM solution of [1]+ in CH2Cl2 (2.3 mL) in a
10-mm quartz cell was adjusted to the appropriate reaction
temperature, the reaction was initiated by adding the calculated
amount of benzyl alcohol directly into the cell. The kinetic data
obtained from procedure (A) were in good agreement with those from
(B). The kinetic measurements for the reaction with α-benzyl-d2
alcohol and benzyl alcohol-d (1.0 M) were carried out by procedure
(B) using freshly purified alcohol as mentioned above.
The change in the absorbance at 440 nm over time for the reaction
of [1]+ with a large excess benzyl alcohol in CH2Cl2 was analyzed with
first-order kinetics within at least three half-lives of the decrease of
[1]+. Therefore, the reaction rate was expressed as a pseudo first-order
rate law, −d[[1]+]/dt = kobs[[1]+], where kobs denotes the conditional
pseudo first-order rate constant. The kobs values were obtained by
applying nonlinear least-squares fitting of At = A∞ + (A0 − A∞)
exp(−kobst) to the data of absorbance change with time, where A0, At,
and A∞ denote the absorbances at the reaction time, t = 0, t, and
infinite time, respectively.
Calculations. Geometry optimizations were performed using the
Gaussian 09 program (Revision A.02),30 the CAM-B3LYP31,32
functional, and the 6-31G(d) basis set on all atoms. Broken-
symmetry33 (BS) density functional theory (DFT) calculations were
performed with the same functional and basis sets. Frequency
calculations at the same level of theory confirmed that the optimized
structures were located at a minimum on the potential energy surface.
Single point calculations were performed using the CAM-B3LYP31,32
functional and the TZVP basis set of Ahlrichs34 on all atoms. The
above calculations were also completed using the B3LYP35,36
functional for comparison. AOMix37 was used for determining atomic
orbital compositions employing Mulliken Population Analysis. The
intensities of the 30 lowest-energy electronic transitions were
calculated by TD-DFT38 with a polarized continuum model (PCM)
for CH2Cl2 (dielectric ε = 8.94).39
[Cu(1,2-Salcn)] (2) and [Cu(1,2-Salcn)]SbF6 ([2]SbF6). These
complexes were synthesized according to the literature.22
X-ray Structure Determination. The X-ray experiment was
carried out on well-shaped single crystals of complex [1]+ on a Rigaku
FR-E CCD area detector with graphite monochromated Mo Kα
radiation (λ = 0.71073 Å). The crystals were mounted on a glass fiber.
To determine the cell constants and orientation matrix, 6 oscillation
photographs were taken for each frame with the oscillation angle of
0.3° and the exposure time of 16 s. Reflection data were corrected for
both Lorentz and polarization effects. The structures were solved by
the heavy-atom method and refined anisotropically for non-hydrogen
atoms by full-matrix least-squares calculations. All the disordered
carbons were also refined anisotropically. Each refinement was
continued until all shifts were smaller than one-third of the standard
deviations of the parameters involved. Atomic scattering factors and
anomalous dispersion terms were taken from the literature.27 All
hydrogen atoms were located at the calculated positions and were
assigned a fixed displacement and constrained to an ideal geometry
with C−H = 0.95 Å. The thermal parameters of calculated hydrogen
atoms were related to those of their parent atoms by U(H) =
1.2Ueq(C). All the calculations were performed by using the
CrystalStructure program package.28 Summaries of the fundamental
crystal data and experimental parameters for structure determination
are given in Supporting Information, Table S1.
Kinetic Measurements. Kinetic measurements for the reaction of
[1]+ with benzyl alcohol were carried out on a Shimadzu UV2550 or a
JASCO V-660 spectrophotometer. The temperature of the sample
solutions was held constant within 0.1 °C using a thermostatted cell
unit. All the sample solutions for UV−vis measurements were
prepared using purified CH2Cl2. Anhydrous benzyl alcohol (99.8%,
Aldrich) was used without further purification. α-Benzyl-d2 alcohol was
synthesized from benzoyl chloride and lithium aluminum deuteride
and was used immediately after purification: The reactions are highly
sensitive to impurities of the deuterated alcohol as reported in ref 29.
RESULTS
■
Characterization and Redox Behavior of Cu(1,3-Salcn)
(1). The 6-membered chelate N2O2-donor ligand H21,3-Salcn
(Figure 2) reacted with copper(II) acetate in C2H5OH to give
Cu(1,3-Salcn) (1) as brownish-green crystals. The absorption
spectrum of complex 1 showed typical bands for a copper(II)-
salen type complexes at 623 (ε = 460 M−1 cm−1) and 387 nm
(ε = 15500 M−1 cm−1). In comparison with complex Cu(1,2-
Salcn) (2), these bands were slightly red-shifted from 569 (ε =
600 M−1 cm−1) and 377 (ε = 11600 M−1 cm−1), respectively.22
The intensity of the 623 nm band of 1 was lower, while that of
the 386 nm band was higher. Such intensity differences are also
observed for the group 10 metal 5- and 6-membered salen type
complexes.20 These spectral differences for 1 in comparison to
2 are likely due to the 6-membered chelate effect. The cyclic
voltammogram of 1 showed two reversible redox waves at E1/2a
= 0.48 and E1/2b = 0.63 V vs Fc/Fc+ in the range of 0−1.0 V
(Figure 3). The potentials were only 0.03 V higher than those
of complex 2, and these redox waves were found to be one
electron processes from the current analysis for each wave.22
This result suggests that one-electron oxidized complex 1 can
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Previously, we have reported that the 6-membered chelate
group 10 metal Salpn complexes showed a more distorted
geometry as compared with the 1,2-Salcn and Salen analogues,
with larger angles between the two cis N−M−O planes in the
Salpn complexes.20 In the case of the oxidized copper(II)
complexes, we did not observe a large distortion of the 6-
membered chelate 1,3-Salcn complex in comparison with the
5-membered chelate 1,2-Salcn complex. For example, the angle
between the N−Cu−O planes in a cis configuration was
estimated to be 12.7°, which is smaller than the distortion in
the 6-membered diimine chelate complex [Ni(Salpn)]SbF6
(21.4°)20 and neutral 5-membered diimine chelate complex 2
(19.8°).22 This may be due to the rigidity of the cyclo-
hexanediamine moiety. However, we have not been able to
completely determine the crystal structure of precursor
complex 1, mainly because of difficulties in the determination
of space group, and therefore, direct comparison of [1]SbF6
with neutral complex 1 could not be made. The structure of
complex 1 obtained at this stage is shown in the Supporting
Information, Figure S1.
On the other hand, angles between the two phenolate planes
of complex [1]+ were estimated to be 41.7(1)°, which is larger
than other 6-membered chelate phenoxyl radical complexes.
For comparison, the angles between the two phenolate planes
in group 10 metal Salpn complexes were estimated to be 28.3°
for [Ni(Salpn)]+, 24.1° for [Pd(Salpn)]+, and 18.0° for
[Pt(Salpn)]+, whose values are smaller than the angle for
[1]+. The large distortion of these two phenolate moieties in
[1]+ from the square plane may lead to the localized electronic
structure because of limited orbital overlap and the associated
effect on radical electron communication through the central
copper ion.
Figure 3. CV of complex 1 in CH2Cl2.
be generated by adding an equimolar amount of chemical
oxidants such as AgSbF6, which have also been used for
oxidation of complex 2.40 Indeed, the addition of an equimolar
amount of AgSbF6 to 1 in CH2Cl2 caused a color change to
dark green, indicating formation of [1]+. The ESI-mass
spectrum of the dark green species exhibited a peak at m/z =
608, and in combination with further characterization results
(vide infra), the one-electron oxidized complex, [1]+, can be
described as a monocationic species, [Cu(1,3-Salcn)]+.
Crystal Structure and Solid State Characterization of
[Cu(1,3-Salcn)]SbF6 ([1]SbF6). When the dark green solution
of [1]+ prepared by AgSbF6 oxidation was kept below −60 °C
overnight in CH2Cl2/toluene, green crystals of the one-electron
oxidized 1,3-Salcn complex, [1]SbF6, could be isolated. The X-
ray crystal structure analysis gave the structure of [1]SbF6,
whose ORTEP view is shown in Figure 4. The structure of
XPS analysis of the Cu 2p3/2 and 2p1/2 binding energies was
used for assessing the metal oxidation state for the neutral and
one-electron oxidized 1,3-Salcn complexes in the solid state
(Supporting Information, Figure S2). The XPS values depend
on the oxidation state of the metal ion, and for the analogous
complex 2, the energy differences between the Cu(II) and
Cu(III) oxidation states have been reported to be about +1.2
eV for both 2p1/2 and 2p3/2 binding energies.22 The XPS values
for 1 and [1]+ are very similar (2p3/2 = 933.7 and 2p1/2 = 953.6
eV for 1 and 2p3/2 = 933.7 and 2p1/2 = 953.7 eV for [1]+), and
[1]+ is therefore assigned as a Cu(II)-ligand radical species.
Magnetic Properties. The magnetic properties of [1]+
have been investigated by multifrequency EPR and temperature
dependent magnetic susceptibility. The 12 K X-band EPR
spectra of a frozen CH2Cl2 solution of [1]+ have been recorded
both in perpendicular and parallel modes. The perpendicular
mode spectrum exhibited very broad signals at about g = 4 and
g = 1.5 assigned to spin triplet (S = 1) resonances with a
mononuclear Cu(II) signal at g = 2 (Supporting Information,
Figure S3) arising from about 5% of neutral complex 1 as a
contaminant in the sample. The very broad EPR signals and
distribution over a large magnetic field suggests that the zero
field splitting parameters are within the range of or slightly
larger than the X-band energy (∼ 0.3 cm−1 at ν = 9.4 GHz).43
Therefore, the spectrum is not satisfactory for accurate
determination of the zero-field splitting parameters.44 On the
other hand, the parallel mode is particularly useful for
characterizing integer electron spin systems as it allows for
greater intensity of the forbidden transitions.45 The parallel
mode X-Band EPR spectrum exhibited a dominant transition at
Figure 4. ORTEP view of Cu(II)-phenoxyl radical complex, [1]SbF6.
complex [1]+ exhibits a pseudo square-planar geometry formed
by two phenolato oxygen atoms and two imino nitrogen atoms.
The Cu coordination sphere geometry is asymmetric, indicating
that one salicylideneimino moiety becomes a weaker donor in
[1]+ with a longer bond to Cu (Cu−O(1) 1.996(3) vs Cu−
O(2) 1.912(3) Å). The C−O bond length of the weakly
donating phenolate moiety is shortened (1.266(5) Å), and the
ring ortho-C−C bonds are lengthened (1.443(6) and 1.422(6)
Å). This quinoid bonding pattern is consistent with radical
localization to one side of the molecule in [1]+, in agreement
with the bonding pattern observed for other localized phenoxyl
radical complexes.18−21,24,41,42 In addition, the SbF6 counter-
−
ion was located close to the quinoid moiety in [1]+; the closest
−
distance was 3.026 Å between the SbF6 and the C−O carbon
atom of the phenoxyl ligand. Association of the counterion with
the phenoxyl ligand was also observed in other unsymmetric
M(II)-phenoxyl radical salen-type complexes.20,24
g ∼ 4 assigned to the ΔMS =
2 transition, which further
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confirms an accessible triplet spin state of [1]+. The intensity of
the EPR spectrum was studied as a function of temperature: An
increase of the intensity of the spin triplet signals was observed
upon decreasing the temperature, suggesting weak ferromag-
netic coupling between the ligand radical and the copper spins.
To gain insight into the zero field splitting parameters, we
conducted EPR measurements at a higher frequency (Q-Band,
ν ∼ 34 GHz) on a frozen CH2Cl2 solution of [1]SbF6.46 The
Q-Band spectrum at 12 K clearly consists of a two-line pattern
centered at g = 2.06, as well as a half-field ΔMS = 2 transition
at g = 4 (Figure 5). This signature is characteristic of a spin
Figure 6. Temperature dependent magnetic susceptibility of [1]SbF6.
Red dots are actual measurements, black line is fitting based on the
Bleaney−Bower’s equation.
ments. Thus, complex [1]+ can be assigned to a Cu(II)-
phenoxyl radical species with a S = 1 ground state exhibiting a
weak ferromagnetic interaction between the Cu(II) center and
the ligand radical.
Theoretical Characterization. Density functional theory
(DFT) calculations of 1 and [1]+ were completed using both
the B3LYP36 and the Coulomb-Attenuated Method (CAM-
B3LYP)31,32 functionals. With the use of the B3LYP functional
the metrical parameters for 1 were reproduced within 0.02 Å
(Supporting Information, Table S2). However, the B3LYP
functional predicted an essentially symmetric complex for [1]+,
in contrast to the experimental metrical parameters. We thus
employed CAM-B3LYP, as this functional has been demon-
strated to be useful for predicting the energies of delocalized
systems,50 as well as long−range charge transfer transitions.51
In the CAM approach the exchange interaction at short-range is
modeled with 0.19 Hartree−Fock (HF) and 0.81 Becke 1988
(B88), while the long−range interaction is modeled with 0.65
HF and 0.35 B88. The intermediate region is described by a
standard error function.31 The triplet and broken symmetry
calculations (vide infra) for [1]+ using the CAM-B3LYP
functional both predicted a large asymmetry in the coordina-
tion sphere, in good correspondence with the X-ray data (Table
1). The predicted Cu−O phenoxyl bond (Cu−O(1)) length
closely matches the X-ray data, and a phenoxyl bonding pattern
is evident in the C−O and ring C−C bonds. The predicted
Cu−O phenolate bond (Cu−O(2)) is shorter than the
experimental value (1.84 Å vs 1.912 Å), which could be
attributed to the localizing effect of the CAM-B3LYP
functional.52 Overall, we have found that the CAM-B3LYP
functional predicts the asymmetric geometric structure for [1]+,
and in addition, matches the experimental data in terms of the
electronic structure.
The energies of the high spin triplet (S = 1) and broken
symmetry (S = 0) electronic states for [1]+ were calculated and
compared to the experimental magnetic data (vide infra). The
high spin triplet solution was determined to be favored by only
0.6 kcal mol−1, matching both the experimental ground state
and the small energy difference determined by solid state
magnetism and EPR. The closed-shell singlet state was
determined to be 18 kcal mol−1 higher in energy in comparison
to the triplet ground state. The spin density distribution for
both the triplet and the broken symmetry solutions (Figure 7)
shows localization of the ligand radical on one side of the ligand
framework in accordance with the experimental data. In
contrast, a delocalized ligand radical structure is predicted if
Figure 5. Q-Band EPR spectrum of a frozen 0.02 M CH2Cl2 solution
of [1]SbF6: The solid black line is the experimental spectrum, and the
dotted red line is a simulation of the triplet system using the
parameters given in the text. The asterisk denotes a paramagnetic
mononuclear copper (S = 1/2) impurity present in the sample.
Modulation Frequency 100 kHz, amplitude 0.5 mT, microwave power
1.5 mW, frequency 34.02 GHz, T = 12 K.
triplet system having zero field splitting (ZFS) parameters
smaller than the Q-Band energy quantum (∼1.1 cm−1).47 The
spectrum could be satisfactorily simulated by using the ZFS
parameters |D| = 0.470 0.035 cm−1, E/D = 0.06 0.02 and
giso = 2.06. We additionally used these parameters to simulate
the X-Band spectra and found a reasonable agreement with the
experimental data (see Supporting Information, Figure S3).
From these EPR results, the triplet spin state of a Cu(II)-
phenoxyl salen complex could be evidenced experimentally.
The ZFS parameters reported here are the first measured
experimentally for Cu(II)-phenoxyl salen complexes.
The temperature dependent magnetic susceptibility of [1]
SbF6 in the solid state exhibited a decrease in the susceptibility
as the temperature was lowered. Consideration of only the
intramolecular interaction leads to the fitting of this plot as a
weak antiferromagnetic S = 0 ground state with the exchange
coupling J/kB = −1.9 cm−1 based on H = −2JSCu·Srad. This
fitting is contradictory to the result of EPR experiments. Final
fitting of the temperature dependent magnetic susceptibility
was based on the Bleaney−Bower’s equation48 considering the
ZFS values:49
2Ng2β2
χ =
kBT[2 + exp(D/kB) + exp(−2J/kB(T − θ))]
(1)
The best-fit parameters are J/kB = 4.1 cm−1 and θ = −3.4
cm−1 using the values from the results of EPR measurements, g
= 2.06 and D/kB = 0.47 cm−1 (Figure 6). These results indicate
that the intramolecular interaction between copper and radical
unpaired electron spins (J/kB) is ferromagnetic, while the
intermolecular interaction (θ) is weakly antiferromagnetic. This
intramolecular coupling value matches with the EPR experi-
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Table 1. Experimental and Calculated (CAM-B3LYP) Coordination Sphere Metrical Parameters for Complexes 1 and [1]+ in Å
compound and method
Cu−O(1)
Cu−O(2)
Cu−N(1)
Cu−N(2)
C−O
1
1.895
1.995
1.987
1.993
1.891
1.912
1.842
1.841
1.955
2.006
1.982
1.922
1.962
1.963
1.925
1.983
1.295/1.298
1.265/1.304
1.259/1.311
1.257/1.311
[1]+ (X-ray)
[1]+ triplet
[1]+ broken symmetry
Figure 7. Spin density plots for [1]+ showing localization of the ligand radical on one side of the ligand framework; (A) triplet and (B) broken
symmetry solutions.
the B3LYP functional is employed (Supporting Information,
Figure S4). A typical phenoxyl radical spin density pattern is
observed for the CAM-B3LYP calculations with positive spin
population at the Ophenoxyl (0.31), Cortho (0.23, 0.33), and Cpara
(0.42) positions of one ring (Figure 7A).
Absorption and Resonance Raman Spectroscopy. The
UV−vis−NIR absorption spectrum of [1]+ in CH2Cl2 exhibits
characteristic new bands at λ (ε/M−1 cm−1) = 29900 (8550),
26000 (7700), 22800 (4550), 19600 (650), 16100 (610),
11900 (920), and 6600 (920) cm−1 (Figure 8). The absorption
spectral features especially of the NIR bands are different from
those of complex [2]+. The NIR bands of [1]+ are of
significantly lower intensity compared to the NIR transitions
for the Cu(III) ground state complex [2]+, which exhibited
intense visible 18600 (6500) and NIR 5600 cm−1 (2900)
LMCT bands.22 Further, the characteristic 22800 cm−1 band
can be ascribed to the π−π* transition of the coordinated
phenoxyl radical.11−13,53,54 Therefore, the absorption spectral
features of [1]+ clearly suggest that complex [1]+ is a Cu(II)-
phenoxyl radical species in solution.
We have previously examined the shape and intensity of the
visible and NIR bands of the oxidized group 10 metal(II) and
Cu(II) salen type complexes to assess the degree of coupling
between the redox-active phenolates.18−22,55 The oxidized
group 10 metal 1,2-Salcn, Salen, and Salpn complexes were
assigned to fully delocalized Class III or Class II/III borderline
systems,18,20,21 based on the Robin−Day classification
system.56−58 Class III systems exhibit intervalence transitions
(IVCT) that are characteristically intense (εmax ≥ 5000 M−1
cm−1) and narrow (bandwidths (Δυ1/2) ≤ 2000 cm−1) and are
solvent independent.59 The NIR band intensity of the Cu(II)-
phenoxyl radical complex [1]+ at 6600 cm−1 (εmax = 920 M−1
cm−1, Δυ1/2 = 1430 cm−1) is much lower and the bandwidth is
larger than the values for the Class III systems of group 10
metal salen-type complexes.20,21 The minimum bandwidth of
complex [1]+ in the high-temperature limit predicted from eq
258−60 can be estimated to be ΔυHTL = 3910 cm−1, which is
larger than the experimental value of Δυ1/2 = 1430 cm−1:
Figure 8. UV−vis−NIR spectra of complexes 1 and [1]+ in CH2Cl2:
Black line, complex 1; red line, complex [1]+. The vertical blue line at
low energy corresponds to the calculated low energy transition at 3790
cm−1 for [1]+ (triplet).
Figure 9. TD-DFT assignment of the calculated low energy transition for [1]+ (triplet). The predicted βHOMO → βLUMO transition is assigned as
a ligand-to-ligand charge transfer (LLCT) band in accord with the Class II intervalence description in the text.
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ΔυHTL 16 ln 2RTυmax
Article
alcohol oxidation. The UV−vis absorption spectral change
revealed that the spectrum of the final product is different from
that of complex 1, since the final product has weak visible bands
at about 500 and 600 nm (Figure 11). Addition of 1 equiv of
=
(2)
Furthermore, the shift in the band energy (εmax) between
toluene and CH2Cl2 was about 800 cm−1 (Supporting
Information, Figure S5, Table S3), which is 10-fold larger
than that of the class III fully delocalized complex [Pt-
(Salen)]+.21 Experimental analysis of complex [1]+ demon-
strates that this derivative is best described as a Class II
intervalence complex.
We used time-dependent DFT (TD-DFT)38 to better
understand the origin of the low energy NIR transition for
[1]+. The predicted band with highest intensity for the triplet
ground state was red-shifted in comparison to the experimental
data (Figure 8), but provides information on the nature of this
transition. The predicted band is predominantly a βHOMO →
βLUMO transition (Figure 9), and is assigned as a phenolate to
phenoxyl intervalence charge transfer (IVCT) transition in
accord with the Class II description for [1]+.
The resonance Raman spectrum (Figure 10) of complex [1]+
exhibited characteristic new intense bands at 1494 cm−1 and
Figure 11. Absorption spectral changes with time for the reaction of
complex [1]+ (0.2 mM) with benzyl alcohol (0.1 M) in CH2Cl2 at 293
K. The spectra were recorded at 9 min intervals.
Et3N to the solution of the final product affords a spectrum
similar to complex 1, indicating that the final product of this
reaction can be assigned to the proton adduct of the Cu(II)
complex, [1H]+. The ESI-MS spectrum of the final product
supported the existence of the proton adduct [1H]+ with the
observation of m/z = 609. The overall stoichiometry of the
reaction observed is expressed as follows:
2[Cu(1, 3‐salcn•)]+ + C6H5CH2OH
→ 2[Cu(H1, 3‐salcn)]+ + C6H5CHO
(3)
The reaction stoichiometry of complex [1]+ with benzyl alcohol
is in good agreement with that of complex [2]+ reported
previously.29
Figure 10. Resonance Raman spectra of complexes 1 and [1]+ (λex
=
413.1 nm) in CH2Cl2 at −60 °C: Black line, complex 1; red line,
In the presence of a large excess of benzyl alcohol,
conversion of [1]+ to [1H]+ can be regarded as a first-order
process, suggesting that only one molecule of complex [1]+ is
involved in the rate-determining step.29 The first-order decay
constant (kobs) of complex [1]+ was not changed by varying the
concentration of complex (kobs = (3.61−4.06) × 10−3 s−1 for
0.1−0.3 mM solution of [1]+ at [C6H5CH2OH] = 1.0 M,
Supporting Information, Table S4), while the rate constant
depended on the benzyl alcohol concentration. A plot of kobs
against the concentration of benzyl alcohol exhibited a linear
complex [1]+.
1581 cm−1, which can be assigned to the phenoxyl radical ν7a
and ν8a bands, respectively.11,61 On the other hand, the
phenolate ν11a band at 1534 cm−1 was also observed but is very
weak, and the 1606 cm−1 band remained in the spectrum of
[1]+. The very weak 1534-cm−1 band intensity may be due to
the different enhancement from resonance Raman of the
neutral species. These results support that one-electron
oxidized complex [1]+ is a Cu(II)-phenoxyl radical that can
be described as having an asymmetric coordination structure
(phenolate and phenoxyl moieties), which is in good agreement
with the NIR spectral data.19
Reactivity of the Oxidized 1,3-Salcn Complex with
Benzyl Alcohol. A CH2Cl2 solution of the localized phenoxyl
radical complex [1]+ is relatively stable at 293 K, with a decay
constant of less than 1.5 × 10−5 s−1 for 0.1−0.3 mM solutions,
the half-life of [1]+ being estimated to be longer than 13 h.
Addition of excess benzyl alcohol to a solution of [1]+ in
CH2Cl2 caused a color change to bright green within one hour.
GC-MS and absorption spectral changes revealed that
formation of the benzaldehyde was in 50% yield relative to
the initial amount of the phenoxyl radical complex. This
reaction was also observed under a nitrogen atmosphere,
indicating that the presence of air did not affect the benzyl
correlation (Supporting Information, Figure S6), that is, kobs
=
k2[C6H5CH2OH], giving the second-order rate constant k2 =
(3.89 0.05) × 10−3 M−1 s−1 at 293 K for the rate-determining
process. We could not determine the association constant of
complex [1]+ with benzyl alcohol because of the linear
correlation. The reactivity characteristics are very similar to
the reaction of complex [2]+ with benzyl alcohol, although the
second-order rate constant of [1]+ is about 4 times larger than
that of complex [2]+ (k2 = 1.02 × 10−3 M−1 s−1 at 293 K was
obtained at [C6H5CH2OH] = 1.0 M, Supporting Information,
Table S4). From the kinetic analyses, the primary alcohol
oxidation by phenoxyl radical complex [1]+ likely occurs by a
mechanism similar to that of [2]+ but with a faster reaction
rate.29
To determine whether C−H bond cleavage is involved in the
rate determining step, we carried out kinetic analyses using α-
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Table 2. Activation Parameters for the Reaction of [1]+ or
[2]+ with Benzyl Alcohol
benzyl-d2 alcohol. Previously the kinetic isotope effect (KIE),
k2(H)/k2(D), for complex [2]+ has been reported to be 19 at
298 K, and the same value was also obtained at 293 K from our
measurements.29 In this study, the KIE value of complex [1]+
was estimated to be 13 at 293 K, which indicates that the C−H
bond scission is included in the rate-determining step. On the
other hand, the reaction rate for C6H5CH2OD was very similar
(k2 = 3.6 × 10−3 M−1 s−1 obtained at [C6H5CH2OD] = 1.0 M,
Supporting Information, Table S4) to that of C6H5CH2OH,
indicating that the benzyl alcohol oxidation proceeds without
the O−H hydrogen abstraction, in the rate determining step.
These observations are in line with general hydrogen
abstraction from alcohols: Hydrogen abstractors react with
a
Complex
ΔH⧧ kcal mol−1
ΔS⧧ cal mol−1 K−1
Ea kcal mol−1
[1]+
[2]+
10.2 0.1
10.7 0.1
−34.8 0.5
10.8 0.2
11.3 0.1
11.7
−35.6 0.3
b
b
11.1 0.4
−34
b
1
a
Determined from Arrhenius analyses. Ref 29.
different from those of [2]+ (Table 2). The difference of the
activation energy (ΔEa) between [1]+ and [2]+ is estimated to
be 0.6
0.2 kcal mol−1 at 293 K. This value is in good
agreement with the C−H activation energy difference estimated
from the KIE values (Δ(ΔEaKIE) = 0.4 kcal mol−1). Thus, the
activation parameter difference between [1]+ and [2]+ is
relatively small, suggesting a similar reaction mechanism for
[1]+ and [2]+, while the C−H hydrogen abstraction by [1]+ is
slightly preferred as compared with complex [2]+.
alcohols (RCH2OH) to give a hydroxyalkyl radical such as
•
̇
RCHOH rather than the alkoxyl radical (RCH2O ), because
the alcohol O−H bond is usually stronger than the C−H bonds
in the same molecule.62 The difference of the C−H hydrogen
abstraction properties between [1]+ and [2]+ can be estimated
from the KIE values. The correlation between KIE and the
activation energy difference between C−H and C−D bonds
(ΔEaKIE) is expressed by the following equation:63
DISCUSSION
■
Metal complexes of tetradentate N2O2 Schiff-base ligands,
Salen and derivatives, have been extensively studied as catalysts
for small molecule transformations. Alteration of both the
diamine backbone and the substituents on the coordinated
phenolate moieties allows the fine-tuning of catalyst activity.
Installation of chirality into salen systems via the diamine
backbone has resulted in the development of efficient catalysts
for a number of important transformations including
asymmetric epoxidation and asymmetric epoxide ring-opening
reactions. The Cu complex of a tetradentate Schiff-base ligand
containing a dinapthylamine backbone exhibited superior
alcohol oxidation activity in comparison to the 1,2-cyclo-
hexanediamine analogue.64 On the other hand, we have
previously reported that the one-electron oxidized Cu(1,2-
salcn) species is a Cu(III)-phenolate in the solid state, while in
a CH2Cl2 solution it exists in an equilibrium between Cu(III)-
phenolate and Cu(II)-phenoxyl radical species.22 The differ-
ence in catalytic activity was attributed to the increased
flexibility of the dinapthylamine backbone, affording access to
the Cu(I) oxidation state in the catalytic cycle.64 In this work
we have investigated the effects of changing the Salcn diamine
backbone from 1,2-diamino- (five-membered chelate) to 1,3-
diaminocyclohexane (six-membered chelate) on the electronic
structure and reactivity of the one-electron oxidized copper(II)
bis-Schiff-base complexes.
KIE
⎡
⎢
⎣
⎤
⎥
⎦
kH
kD
ΔEa
2RT
KIE =
= exp
(4)
Therefore, the activation energy difference between [1]+ and
[2]+ (Δ(ΔEaKIE)) can be estimated according to the following
equation:
Δ(ΔEaKIE) = 2RT[ln(KIE[2]+) − ln(KIE[1]+)]
(5)
From this equation, Δ(ΔEaKIE) was calculated to be 0.4 kcal
mol−1 at 293 K.
As a further step toward a detailed mechanism of benzyl
alcohol oxidation by Cu(II)-phenoxyl radical complex [1]+,
temperature dependence studies were carried out. The Eyring
plots of complexes [1]+ and [2]+ showed linear fits with similar
slopes (Figure 12). The activation parameters of [2]+ were
The copper(II) 1,3-Salcn complex (1) was synthesized by a
procedure similar to that for the Salen complexes. The redox
potential of complex 1 was about 30 mV higher than that of
1,2-Salcn complex (2), the difference being smaller in
comparison with the difference of more than 50 mV between
5-membered and 6-membered chelate group 10 metal
complexes.20,21 Therefore the one-electron oxidized copper(II)
1,3-Salcn complex [1]+ can be generated by addition of 1 equiv
of AgSbF6, a method similar to that employed for formation of
complex [2]+.
Figure 12. Eyring plots of the second-order rate constants (k2) for the
reactions of complex [1]+ (open circle) and [2]+ (solid circle) with
benzyl alcohol. The k2 values were directly obtained from the kobs at
[C6H5CH2OH] = 1.0 M at each reaction temperature.
On the basis of the molecular structure determined for
crystals of [1]SbF6 suitable for X-ray analysis, the changes in
bonding associated with ligand oxidation in the solid state were
investigated. The Cu−N bond lengths increased upon
oxidation of 1, while the Cu−O bonds became asymmetric
and lengthened slightly on average. The asymmetry is also
apparent in the C−O bonds, with the shorter C−O bond on
the same side of the molecule as the longer Cu−O bond. This
determined to be ΔH⧧ = 10.7
0.1 kcal mol−1 and ΔS⧧ =
−35.6 0.3 cal mol−1 K−1 (Table 2), which are in very good
agreement with the reported values (ΔH⧧ = 11.1 0.4 kcal
mol−1 and ΔS⧧ = −34 1 cal mol−1 K−1).29 The activation
parameters of [1]+ were estimated to be ΔH⧧ = 10.2 0.1 kcal
mol−1 and ΔS⧧ = −34.8 0.5 cal mol−1 K−1, which are slightly
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bonding pattern is in good agreement with the structures of an
oxidized Pd(II) salen complex and other reported localized
phenoxyl radical complexes, reflecting the diminished electron-
donating ability of the phenoxyl ligand compared with that of
of the low energy NIR transitions enables further analysis of the
degree of electronic coupling in the oxidized complexes.
Analysis of the low energy bands according to Hush
demonstrates that the experimental bandwidths (Δυ1/2) for
the oxidized complexes are significantly higher than the
predicted minimum bandwidth (ΔυHTL) at the high temper-
ature limit.58−60 The two values Δυ1/2 and ΔυHTL are estimated
to be 1430 and 3910 cm−1, respectively, indicating that complex
[1]+ can be assigned as class II mixed valence complex.55−58
TD-DFT calculations on [1]+ predict a βHOMO → βLUMO
transition in the NIR, which is assigned as a phenolate to
phenoxyl intervalence charge transfer (IVCT) transition. Thus,
the phenoxyl radical complex [1]+ exists as a localized phenoxyl
radical complex. Localization of the radical electron was also
observed in the resonance Raman spectrum of [1]+; both the
ν7a and ν8a phenoxyl Raman bands (1494 cm−1 and 1581 cm−1,
respectively) and the phenolate ν11a band (1534 cm−1) were
observed. The energy and intensity of the two bands match
those reported for localized NiII-phenoxyl radical complexes
with the same tert-butyl substitution.19 In comparison to [1]+,
the oxidized species of the group 10 metal Salpn series exhibit
enhanced radical delocalization across the ligand framework.20
Because of the increased rigidity of the ligand backbone for [1]+
(vs 1,3-propanediamine for Salpn) the angle between the two
redox-active phenolates in [1]+ is about 42°, limiting orbital
overlap and communication between the phenolate and the
phenoxyl moieties. Similar radical localization was observed for
manganese salen phenoxyl radical complexes.66
the phenolate ligand.20,21 The SbF6 counterion is associated
−
with the phenoxyl ligand in the solid state structure of [1]+.
The asymmetry of the Cu−O bonds for [1]+ differs from the
observed symmetric contraction of the M−O bonds for
oxidized Ni and Pt complexes, and can be attributed to
diminished delocalization of the li; and radical for [1]+.19−21 In
comparison to the Cu(III)-phenolate ground state complex
[2]+, [1]+ shows different Cu−O, Cu−N, and C−O bond
lengths and distortion of the coordination plane. The X-ray
crystal structure of complex [2]+ revealed the shortening of
these bond lengths, and the angle between the two cis N−Cu−
O planes is substantially reduced from that of the neutral
complex 2.22 These structural differences between [1]+ and the
Cu(III) complex [2]+ also support that complex [1]+ is a
localized Cu(II)-phenoxyl radical complex.
The phenoxyl radical complex [1]+ was investigated by a
variety of spectroscopic techniques. The XPS of complex [1]+
exhibited similar Cu 2p1/2 and 2p3/2 binding energies in
comparison to the neutral complexes 1 and 2; a more than 1 eV
shift was observed for Cu(III) ground state complex [2]+.
These data support that a Cu(II) valence state is maintained for
[1]+. The X-band and Q-band EPR spectra of complex [1]+
showed a triplet ground state with |D| = 0.470 0.035 cm−1, E/
D = 0.06
parameters are slightly lower than those predicted recently by
CASSCF calculations for a Cu(II)-radical salen complex.24
0.02, and g = 2.06. It is noteworthy that these
The reactivity of complex [1]+ with benzyl alcohol was
investigated and compared with that of the Cu(III)-phenolate
ground state complex [2]+.22,29 Although the reaction rate of
complex [1]+ was about 4-times higher than that of complex
[2]+, the overall oxidation required two molar equivalents of
[1]+ or [2]+, and the rate determining step of [1]+ is
determined to be a bimolecular reaction between benzyl
alcohol and the oxidized complex. The reaction rates of [1]+
did not show saturation behavior, remaining linear over the
substrate range up to 1.0 M, and therefore the association
constant of complex [1]+ with benzyl alcohol could not be
determined. Such a trend has been reported for the reaction
kinetics of complex [2]+,29 suggesting a similar reaction
mechanism for [1]+. The detailed mechanism of the alcohol
oxidation has been discussed previously; complex [2]+ can act
as a one-electron oxidant for benzyl alcohol and oxidize it to the
ketyl radical in the rate determining step. Therefore, the rate
determining step involves both the substrate association with
the oxidized complex and the C−H hydrogen abstraction from
the benzyl alcohol. The large kinetic isotope effect (KIE =
k(H)/k(D) = 13) of complex [1]+ indicates that the rate
determining step involves C−H bond cleavage. The similarity
in the reactivity of [1]+ and [2]+ may be understood from the
activation parameters. Small differences in the parameters
shown in Table 2 suggest that the activated complexes of [1]+
and [2]+ have similar reactivities for oxidation as well as similar
geometries for the substrate association. The similar potentials
correspond with the similar enthalpies and redox potentials for
[1]+ and [2]+ (ΔE = 0.03 V for the first redox wave).
A
possible explanation for the lower |D| value in [1]+ may be the
longer metal-phenoxyl oxygen bond distance for [1]+ in
comparison to the previously reported 5-membered chelate
salen system. On the other hand, the magnitude of |D| is larger
while the rhombicity is smaller than that of a square-pyramidal
Cu(II)-phenoxyl complex with equatorial phenoxyl radical
coordination (|D| = 0.32 cm−1 and E/D = 0.27 cm−1).44 This
difference is ascribed to the changes in the geometry and the
electronic structure between Schiff bases and Mannich bases.
Further, the intensity of the bands attributed to the S = 1
species increased with decreasing temperature, indicating that
complex [1]+ exhibits weak intramolecular ferromagnetic
coupling between the Cu(II) and radical unpaired electron
spins. From the study of the temperature dependent magnetic
susceptibility, the intramolecular exchange interaction constant
was estimated to be J/kB = 4.1 cm−1 based on the H =
−2JSCu·Srad. The parameters for [1]+ suggest the orthogonality
of the copper dx −y and phenoxyl radical π magnetic orbitals.65
The J value is small in comparison to other Cu(II)-salen
phenoxyl radical complexes, which may be due to the elongated
Cu−Ophenoxyl bond.24 However, it is not possible at this stage to
clarify the reason for the very small ferromagnetic coupling
constant and large |D| values, since the Cu(II)-radical
complexes are rare. The results of the computational experi-
ment support that the one-electron oxidized complex, [1]+, can
be assigned as a Cu(II)-phenoxyl radical species. The CAM-
B3LYP calculation predicts a large asymmetry in the
coordination environment with localization of the phenoxyl
radical on one side of the molecule.
2
2
The increased reaction rate for [1]+ in comparison to [2]+
can be attributed to the different electronic structure, i.e.,
Cu(II)-radical for [1]+, and Cu(III) ground state for [2]+ which
is in equilibrium with the Cu(II)-ligand radical in solution at
room temperature as reported previously.22 The constant k2 for
the reaction of [1]+ corresponds to the rate constant of a simple
The electronic spectrum of [1]+ exhibits weak and broad
NIR bands, indicating the formation of a localized phenoxyl
radical complex. The energies and intensities of the NIR bands
for [1]+ are different from those of complex [2]+. Investigation
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bimolecular process between the Cu(II)-radical and
C6H5CH2OH (k2*) in the rate-determining step (k2 = k2*),
while in the reaction of [2]+, k2 should involve an equilibrium
between Cu(III) and Cu(II)-radical species (Kpre= [Cu(II)-
radical]/[Cu(III)]) as expressed by the following equation:63
alcohol can be described as a second order process with the
complexes without saturation kinetics, and the rate determining
step for both complexes involves hydrogen abstraction. Kinetic
isotope analysis suggests that the barrier for C−H bond scission
is lower for [1]+ in comparison to [2]+. The activation
parameters of the reaction of [1]+ with benzyl alcohol indicate
that hydrogen abstraction is slightly more favorable in
comparison to [2]+. The activation energy difference between
[1]+ and [2]+ was estimated to be 0.6 0.2 kcal mol−1, which is
in line with the value from the KIE measurement. This
agreement indicates that the reactivity difference arises from the
hydrogen abstraction mechanism, for which the localized
Cu(II)-phenoxyl radical is more favorable. From these results,
we suggest that the localized phenoxyl radical and possibly
increased ligand flexibility accelerate benzyl alcohol oxidation.
Further studies on the correlation between reactivity and
electronic structure described herein are under way.
k2 = k2*Kpre/(1 + Kpre
)
(6)
Since complex [2]+ has been reported to exist in about 1:1
equilibrium between Cu(III)-phenolate and Cu(II)-phenoxyl
radical species,22 Kpre was estimated to be ≈1, and hence k2 for
the reaction of [2]+ is calculated to be k2 ≈ k2*/2. If the
Cu(III) species does not react with benzyl alcohol, the k2 value
for the reaction of [1]+ is at least double the value for [2]+,
assuming k2* is the same for both systems. In addition, it is
plausible that the higher reactivity of the Cu(II)-radical of [1]+
toward C6H5CH2OH may be due to the increased conforma-
tional flexibility of the 6-membered chelate ring of [1]+ in
comparison to the 5-membered chelate 1,2-cyclohexanediamine
backbone of [2]+.
Hydrogen abstraction acceleration may be one of the reasons
for the active form of GO which contains a Cu(II)-phenoxyl
radical species. However, the reaction rate of primary alcohol
oxidation by GO is at least 104-fold faster than the two
complexes reported herein and other reported Cu(II)-phenoxyl
radical species.67,68 This 104-fold difference in the reaction rate
cannot be explained only by electronic structure differences.
The present findings suggest that the small difference of
geometry and flexibility because of the chelate ring size gives
rise to a significant reaction rate difference. Therefore, many
factors including the redox behavior of the Cu-phenolate
moiety as well as the molecular environment and geometry at
the active site of GO should be considered for mimicking its
reactivity.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystal data for complex [1]+ (Table S1), X-ray crystal
structure of complex 1 (Figure S1), the XPS of complexes 1
and [1]+ (Figure S2), X-band EPR spectra of complex [1]+
(Figure S3), the DFT calculation data (Table S2 and Figure
S4), the solvent dependent NIR spectra (Table S3 and Figure
S5) and the kinetic data for the reactions of oxidized complexes
with benzyl alcohol (Table S4 and Figure S6). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yshima@mx.ibaraki.ac.jp.
■
Notes
The authors declare no competing financial interest.
SUMMARY
■
ACKNOWLEDGMENTS
■
We have investigated and characterized the neutral and one-
electron oxidized copper(II) 1,3-Salcn complex and compared
the electronic structure and reactivity with those of the 1,2-
Salcn analogue. The six-membered chelate 1,3-Salcn complex
exhibits subtle differences in the electronic structure and
oxidation reactivity with benzyl alcohol, in comparison to its 5-
membered chelate analogue complex of 1,2-Salcn. The one-
electron oxidized 1,3-Salcn complex exists as a ligand-based
radical complex in solution and in the solid state, which is in
contrast to the Cu(III)-phenolate ground state of the oxidized
1,2-Salcn complex. The solid state structure of the oxidized
1,3-Salcn complex exhibits an asymmetric coordination
structure as compared with the 5-membered chelate analogue,
which suggests that this complex exists as a localized phenoxyl
radical in the solid state. XPS results support that the central
copper ion is assigned a valence state of +II. EPR and magnetic
susceptibility measurements revealed that this phenoxyl radical
complex exhibits a very weak intramolecular ferromagnetic
interaction between the radical and the copper unpaired
electron spins. Analysis of the NIR bands for [1]+ indicates that
this species is a Class II localized mixed valence complex.
Ligand radical localization is supported by the resonance
Raman measurements, which show both characteristic phenoxyl
radical and phenolate Raman bands. Both [1]+ and [2]+ exhibit
a similar reaction mechanism for benzyl alcohol oxidation, but
the reaction rate of [1]+ is about 4 times faster in comparison to
[2]+ under the same conditions. The oxidation of benzyl
We would like to thank Prof. Tatsuo Yajima in Kansai
University for XPS measurements. This work was supported by
a Grant-in-Aid for Scientific Research (No. 22550055 to Y.S.)
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan, and by the Cooperative Research
Program of Network Joint Research Center for Materials and
Devices (Institute for Materials Chemistry and Engineering,
Kyushu University). T.S. would like to thank NSERC for a
Discovery Grant and Compute Canada and Westgrid for access
to computational resources.
REFERENCES
■
(1) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. Rev. 1996, 96,
2239.
(2) Nam, W., Ed.; Acc. Chem. Res., 2007, 40, 465−634 (Special Issue
on Dioxygen Activation by Metalloenzymes and Models)
(3) (a) Sono, H.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem.
Rev. 1996, 96, 2841. (b) Meunier, B.; de Visser, S. P.; Shaik, S. Chem.
Rev. 2004, 104, 3947.
(4) (a) Baik, M.; Newcomb, M.; Friesner, R. A.; Lippard, S. J. Chem.
Rev. 2003, 103, 2385. (b) Tshuva, E. Y.; Lippard, S. J. Chem. Rev. 2004,
104, 987. (c) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr.
Chem. Rev. 2004, 104, 939. (d) Solomon, E. I.; Brunold, T. C.; Davis,
M. I.; Kemsley, J. N.; Lee., S.-K.; Lehnert, N.; Neese, F.; Skulan, A. J.;
Yang, Y.-S.; Zhou, J. Chem. Rev. 2000, 100, 235. (e) Gunay, A.;
Theopold, K. H. Chem. Rev. 2010, 110, 1060.
(5) (a) Que, L., Jr.; Tolman, W. B. Angew. Chem., Int. Ed. 2002, 41,
1114. (b) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev.
12459
dx.doi.org/10.1021/ic3018503 | Inorg. Chem. 2012, 51, 12450−12461

Inorganic Chemistry
Article
2004, 104, 1013. (c) Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004,
104, 1047.
(6) Stubbe, J.; van der Donk, W. A. Chem. Rev. 1998, 98, 705.
(7) Frey, P. A.; Hegeman, A. D.; Reed, G. H. Chem. Rev. 2006, 106,
3302.
(8) (a) Whittaker, J. W. Met. Ions. Biol. Syst. 1994, 30, 315.
(b) Whittaker, J. W. Chem. Rev. 2003, 103, 2347.
(9) (a) Dyrkacz, G. R.; Libby, R. D.; Hamilton, G. A. J. Am. Chem.
Soc. 1976, 98, 626. (b) Hamilton, G. A.; Adolf, P. K.; de Jersey, J.;
DuBois, G. C.; Dyrkacz, G. R.; Libby, R. D. J. Am. Chem. Soc. 1978,
100, 1899.
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
̈
(10) Clark, K.; Penner-Hahn, J. E.; Whittaker, M. M.; Whittaker, J.
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
W. J. Am. Chem. Soc. 1990, 112, 6433.
(11) McGlashen, M. L.; Eads, D. D.; Spiro, T. G.; Whittaker, J. W. J.
Phys. Chem. 1995, 99, 4918.
(12) Jazdzewski, B. A.; Tolman, W. B. Coord. Chem. Rev. 2000, 200−
202, 633.
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.
(31) Tawada, Y.; Tsuneda, T.; Yanagisawa, S.; Yanai, T.; Hirao, K. J.
Chem. Phys. 2004, 120, 8425.
(32) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004, 393,
(13) (a) Chaudhuri, P.; Wieghardt, K. Prog. Inorg. Chem. 2001, 50,
151. (b) Thomas, F. Eur. J. Inorg. Chem. 2007, 2379. (c) Shimazaki, Y.;
Yamauchi, O. Indian J. Chem., Sect. A 2011, 50, 383 , and references
cited therein.
51.
(33) (a) Noodleman, L. J. Chem. Phys. 1981, 74, 5737. (b) Noodle-
man, L.; Case, D. A. Adv. Inorg. Chem. 1992, 38, 423. (c) Noodleman,
L.; Davidson, E. R. Chem. Phys. 1986, 109, 131.
(14) (a) Chirik, P. J.; Wieghardt, K. Science 2010, 327, 794.
(b) Holland, P. L. Acc. Chem. Res. 2008, 41, 905. (c) de Bruin, B.;
(34) Shafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
(35) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
Hetterscheid, D. G. H.; Koekkoek, A. J. J.; Grutzmacher, H. Prog. Inorg.
Chem. 2007, 55, 247.
̈
(36) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623.
(15) (a) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.;
(37) (a) Gorelsky, S. I. AOMix: Program for Molecular Orbital
Analysis; University of Ottawa: Ottawa, Canada, 2007; http://www.sg-
chem.net/. (b) Gorelsky, S. I.; Solomon, E. I. Theor. Chem. Acc. 2008,
129, 57.
Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
̈
(b) Herebian, D.; Bothe, E.; Bill, E.; Weyhermuller, T.; Wieghardt, K.
̈
J. Am. Chem. Soc. 2001, 123, 10012.
(16) (a) Shimazaki, Y.; Tani, F.; Fukui, K.; Naruta, Y.; Yamauchi, O.
J. Am. Chem. Soc. 2003, 125, 10512. (b) Shimazaki, Y.; Yajima, T.;
Tani, F.; Karasawa, S.; Fukui, K.; Naruta, Y.; Yamauchi, O. J. Am.
Chem. Soc. 2007, 129, 2559.
(38) (a) Casida, M. E. In Recent Advances in Density Functional
Methods; Chong, D. P., Ed.; World Scientific: Singapore, 1995; p 155;
(b) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998,
109, 8218.
(17) (a) Rotthaus, O.; Jarjayes, O.; Thomas, F.; Philouze, C.; Del
Valle, C. P.; Saint-Aman, E.; Pierre, J.-L. Chem.Eur. J. 2006, 12,
2293. (b) Rotthaus, O.; Thomas, F.; Jarjayes, O.; Philouze, C.; Saint-
Aman, E.; Pierre, J.-L. Chem.Eur. J. 2006, 12, 6953. (c) Chiang, L.;
Kochem, A.; Jarjayes, O.; Dunn, T. J.; Vezin, H.; Sakaguchi, M.; Ogura,
T.; Orio., M.; Shimazaki, Y.; Thomas, F.; Storr, T. Chem.Eur. J.
2012, 18 (44), 14117−14127.
(39) (a) Barone, V.; Cossi, M.; Tomasi, J. J. Chem. Phys. 1997, 107,
3210. (b) Barone, V.; Cossi, M.; Tomasi, J. J. Comput. Chem. 1998, 19,
404. (c) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117.
(d) Tomasi, J.; Mennucci, B.; Cances, E. J. Mol. Struct. 1999, 464, 211.
(40) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(41) (a) Benisvy, L.; Blake, A. J.; Collison, D.; Davies, E. S.; Garner,
C. D.; McInnes, E. J. L.; McMaster, J.; Whittaker, G.; Wilson, C. Chem.
Commun. 2001, 1824. (b) Benisvy, L.; Blake, A. J.; Collison, D.;
Davies, E. S.; Garner, C. D.; McInnes, E. J. L.; McMaster, J.; Whittaker,
G.; Wilson, C. Dalton Trans. 2003, 1975.
(18) Storr, T.; Wasinger, E. C.; Pratt, R. C.; Stack, T. D. P. Angew.
Chem., Int. Ed. 2007, 46, 5198.
(19) Storr, T.; Verma, P.; Shimazaki, Y.; Wasinger, E. C.; Stack, T. D.
P. Chem.Eur. J. 2010, 16, 8980.
(42) Verani, C. N.; Gallert, S.; Bill, E.; Weyhermuller, T.; Wieghardt,
̈
(20) Shimazaki, Y.; Arai, N.; Dunn, T. J.; Yajima, T.; Tani, F.;
Ramogida, C. F.; Storr, T. Dalton Trans. 2011, 40, 2469.
(21) Shimazaki, Y.; Stack, T. D. P.; Storr, T. Inorg. Chem. 2009, 48,
8383.
K.; Chaudhuri, P. Chem. Commun. 1999, 1747.
(43) Muller, J.; Kikuchi, A.; Bill, E.; Weyhermuller, T.; Hildebrandt,
̈
̈
K.; Ould-Moussa, L.; Wieghardt, K. Inorg. Chim. Acta 2000, 297, 265.
(44) Bill, E.; Muller, J.; Weyhermuller, T.; Wieghardt, K. Inorg. Chem.
̈
̈
(22) Storr, T.; Verma, P.; Pratt, R. C.; Wasinger, E. C.; Shimazaki, Y.;
1999, 38, 5795.
Stack, T. D. P. J. Am. Chem. Soc. 2008, 130, 15448.
(45) (a) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of
Transition Ions; Oxford University Press: London, U.K., 1970.
(b) Campbell, K. A.; Yikilmaz, E.; Grant, C. V.; Gregor, W.; Miller,
A.-F.; Britt, R. D. J. Am. Chem. Soc. 1999, 121, 4714.
(46) J. Krzystek, J.; Park, J.-H.; Meisel, M. W.; Hitchman, M. A.;
Stratemeier, H.; Brunel, L.-C.; Telser, J. Inorg. Chem. 2002, 41, 4478.
(47) Sharrock, P.; Melnik, M. Can. J. Chem. 1985, 63, 52.
(48) (a) Kahn, O. Molecular Magnetism; Wiley-VCH Publishers:
Weinheim, Germany, 1993. (b) Bleany, B.; Bowers, K. D. Proc. R. Soc.
London 1952, A214, 451.
(49) Yang, F.-A.; Guo, C.-W.; Chen, Y.-J.; Chen, J.-H.; Wang, S.-S.;
Tung, J.-Y.; Hwang, L.-P.; Elango, S. Inorg. Chem. 2007, 46, 578.
(50) Rostov, I. V.; Amos, R. D.; Kobayashi, R.; Scalmani, G.; Frisch,
M. J. J. Phys. Chem. B 2010, 114, 5547.
(51) (a) Jacquemin, D.; Michaux, C.; Perpete, E. A.; Maurel, F.;
Perrier, A. Chem. Phys. Lett. 2010, 488, 193. (b) Jacquemin, D.;
Perpete, E. A.; Scuseria, G. E.; Ciofini, I.; Adamo, C. J. Chem. Theory
Comput. 2008, 4, 123. (c) Mikolajczyk, M. M.; Zalesny, R.;
Czyznikowska, Z.; Toman, P.; Leszczynski, J.; Bartkowiak, W. J. Mol.
Model. 2011, 17, 2143.
(23) (a) Ray, K.; Petrenko, T.; Wieghardt, K.; Neese, F. Dalton Trans.
2007, 1552. (b) Ray, K.; George, S. D.; Solomon, E. I.; Wieghardt, K.;
Neese, F. Chem.Eur. J. 2007, 13, 2783. (c) Ray, K.; Weyhermuller,
T.; Neese, F.; Wieghardt, K. Inorg. Chem. 2005, 44, 5345.
(24) Orio, M.; Jarjayes, O.; Kanso, H.; Philouze, C.; Neese, F.;
Thomas, F. Angew. Chem., Int. Ed. 2010, 49, 4989.
(25) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals; Pergamon Press: Elmsford, NY, 1966.
(26) Chastain, J.; King, R. G. Jr. Handbook of X-ray Photoelectron
Spectroscopy; Physical Electronics, Perkin-Elmer Inc: Eden Prairie, MN,
1995.
(27) Ibers, J. A.; Hamilton, W. C. International Tables for X-Ray
Crystallography; Kynoch: Birmingham, U.K., 1974; Vol. IV.
(28) Crystal Structure Analysis Package; Rigaku Corporation: The
Woodlands, TX, 1985 and 1999.
(29) (a) Pratt, R. C.; Stack, T. D. P. J. Am. Chem. Soc. 2003, 125,
8716. (b) Pratt, R. C.; Stack, T. D. P. Inorg. Chem. 2005, 44, 2367.
(30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
12460
dx.doi.org/10.1021/ic3018503 | Inorg. Chem. 2012, 51, 12450−12461

Inorganic Chemistry
Article
(52) Use of the CAM-B3LYP functional to explain the electronic
structure in a series of symmetric one-electron oxidized Ni-salen-type
complexes also results in localized electronic structures; see ref 17c.
(53) Das, T. N. J. Phys. Chem. A 2005, 109, 3344.
(54) Mukherjee, A.; McGlashen, M. L.; Spiro, T. G. J. Phys. Chem.
1995, 99, 4912.
(55) Verma, P.; Pratt, R. C.; Storr, T.; Wasinger, E. C.; Stack, T. D. P.
Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 18600.
(56) Robin, M. B.; Day, P. Adv. Inorg. Chem. 1967, 10, 247.
(57) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1969, 91, 3988.
(58) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391.
(59) D’Alessandro, D. M.; Keene, F. R. Chem. Soc. Rev. 2006, 35, 424.
(60) Hush, N. S. Coord. Chem. Rev. 1985, 64, 135.
(61) Schnepf, R.; Sokolowski, S.; Muller, J.; Bachler, V.; Wieghardt,
̈
K.; Hildebrandt, P. J. Am. Chem. Soc. 1998, 120, 2352.
(62) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110,
6961.
(63) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd
ed.; MacGraw-Hill: New York, 2002.
(64) (a) Wang, Y.; Stack, T. D. P. J. Am. Chem. Soc. 1996, 118,
13097. (b) Wang, Y.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.;
Stack, T. D. P. Science 1998, 279, 537.
(65) Muller, J.; Weyhermuller, T.; Bill, E.; Hildebrandt, P.; Ould-
̈
̈
Moussa, L.; Glaser, T.; Wieghardt, K. Angew. Chem., Int. Ed. 1998, 37,
616.
(66) Kurahashi, T.; Fujii, H. J. Am. Chem. Soc. 2011, 133, 8307.
(67) Wacher, R. M.; Branchaud, B. P. Biochim. Biophys. Acta 1998,
1384, 43.
(68) Himo, F.; Eriksson, L. A.; Maseras, F.; Siegbahn, P. E. M. J. Am.
Chem. Soc. 2000, 122, 8031.
12461
dx.doi.org/10.1021/ic3018503 | Inorg. Chem. 2012, 51, 12450−12461