One-electron oxidation of electronically diverse manganese(III) and nickel(II) salen complexes: Transition from localized to delocalized mixed-valence ligand radicals

Article abstract of DOI:10.1021/ja2016813

Ligand radicals from salen complexes are unique mixed-valence compounds in which a phenoxyl radical is electronically linked to a remote phenolate via a neighboring redox-active metal ion, providing an opportunity to study electron transfer from a phenola

Full text of DOI:10.1021/ja2016813

ARTICLE
pubs.acs.org/JACS
One-Electron Oxidation of Electronically Diverse Manganese(III) and
Nickel(II) Salen Complexes: Transition from Localized to Delocalized
Mixed-Valence Ligand Radicals
Takuya Kurahashi and Hiroshi Fujii*
Institute for Molecular Science & Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences, Myodaiji, Okazaki,
Aichi 444-8787, Japan
S Supporting Information
b
ABSTRACT: Ligand radicals from salen complexes are unique
mixed-valence compounds in which a phenoxyl radical is electro-
nically linked to a remote phenolate via a neighboring redox-active
metal ion, providing an opportunity to study electron transfer from a
phenolate to a phenoxyl radical mediated by a redox-active metal ion
as a bridge. We herein synthesize one-electron-oxidized products
from electronically diverse manganese(III) salen complexes in
which the locus of oxidation is shown to be ligand-centered, not
metal-centered, affording manganese(III)ꢀphenoxyl radical spe-
cies. The key point in the present study is an unambiguous assign-
ment of intervalence charge transfer bands by using nonsymmetrical salen complexes, which enables us to obtain otherwise
inaccessible insight into the mixed-valence property. A d4 high-spin manganese(III) ion forms a RobinꢀDay class II mixed-valence
system, in which electron transfer is occurring between the localized phenoxyl radical and the phenolate. This is in clear contrast to a
d8 low-spin nickel(II) ion with the same salen ligand, which induces a delocalized radical (RobinꢀDay class III) over the two
phenolate rings, as previously reported by others. The present findings point to a fascinating possibility that electron transfer could
be drastically modulated by exchanging the metal ion that bridges the two redox centers.
’ INTRODUCTION
number of recent studies have reported complicated systems
exhibiting an intermediate behavior between class II and class III
which are now categorized as borderline class IIꢀIII.1a,c,d
To investigate the extent of the electronic interaction between
two redox centers, an absorption in the visible or near-infrared
region is of prime importance as a systematic marker. Although
class I mixed-valence compounds show no absorption, class II
compounds generally exhibit a broad and weak absorption, called
an IVCT (intervalence charge transfer) band, which originates
from a light-driven process from one redox site to the other (from
RuII to RuIII, for example). In contrast, class III mixed-valence
compounds show a narrow and intense absorption, which is
traditionally called an IVCT band too, but arises from a transition
between delocalized electronic levels and not from intramolecular
charge transfer. Analysis of the IVCT bands by MarcusꢀHush
theory gives parameters that define the thermal electron transfer
barrier.4
Mixed-valence compounds, typically bearing two bridged metal
centers in different formal oxidation states, have played an important
role in the study of electron transfer, which is one of the most
fundamental reactions in physics, chemistry, and biology.1 A proto-
typical example is the pyrazine-bridged RuIIꢀRuIII dimer, which is
known as the CreutzꢀTaube ion.2 A pivotal issue in mixed-valence
chemistry has been the extent of the electronic interaction
between two metal centers, which determines the efficiency of
the intramolecular electron transfer, from RuII to RuIII, for
example. According to the extent of the electronic interaction,
mixed-valence compounds are classified into three categories,
classes I, II, and III, as proposed by Robin and Day.3 In class I
mixed-valence compounds, no electron transfer occurs when two
bridged metal centers are far apart or when their interaction is
symmetry or spin forbidden. In class II mixed-valence com-
pounds, two metal centers have detectable electronic interaction
and electron transfer is occurring between two metal centers. In
class III mixed-valence compounds, two metal centers have such
strong electronic interaction that delocalization occurs and the
formal charge on each metal center is averaged, RuII.5ꢀRuII.5, for
example. Class III mixed-valence compounds are distinct from
class II compounds in delocalization of the unpaired electron
as a consequence of disappearance of the activation barrier for
electron transfer. In addition to the RobinꢀDay classification, a
One of the continuing themes is preparation of mixed-valence
assemblies containing multiple redox centers, which has signifi-
cant implications in understanding long-range electron transfer
in biological systems, as well as in designing molecular conduc-
tive devices.1f,5 A redox-active mononuclear transition-metal
Received: February 23, 2011
Published: May 10, 2011
r
2011 American Chemical Society
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Chart 1. Symmetrical Salen Ligands and Their Abbreviations
Chart 2. Nonsymmetrical Salen Ligands and Their
Abbreviations
center (M) carrying two redox-active ligands (L) generates a
mixed-valence system, LꢀMꢀL•þ, upon one-electron oxidation.
In this system, the L•þ moiety as an electron acceptor is
electronically linked to the remote L moiety as an electron donor
via a neighboring redox-active metal ion, providing an opportu-
nity to study the nature of electron transfer from L to L•þ
mediated by a redox-active metal ion as a bridge. Examples of
such systems include transition-metal complexes with dioxolenes,6
o-phenylenediamines,7 benzene-1,2-dithiolates,8 o-iminophenolate,9
R-iminopyridines,10 and salens.11 To clarify electronic communica-
tions among multiple redox centers, the most important issue is to
precisely assign visible and near-infrared absorptions, which would
possibly contain at least two overlapping transitions, IVCT from L
to L•þ and MLCT (metal-to-ligand charge transfer) from M to L•þ.
However, it has been a very difficult task to address this issue, and
theoretical calculations have been frequently employed as the only
available tool in previous studies.12
nonsymmetrical phenoxyl radicals bearing the phenolate with
different electron-donor abilities (Chart 2). The present study
unambiguously shows that a d4 high-spin manganese(III) ion
forms a class II mixed-valence system between the localized
phenoxyl radical and the phenolate, which is in clear contrast
to the delocalized class III radical over the two phenolate rings
for a d8 low-spin nickel(II) ion with the same salen ligand, as
reported by other groups.11h,j It is also shown that the redox
potential of the salen ligand modulates the electron transfer from
the phenolate to the phenoxyl radical with the nickel(II) ion as a
mediator.
’ RESULTS AND DISCUSSION
Synthesis, Characterization, and One-Electron Oxidation
of Electronically Diverse MnIII(salen)(OTf) and NiII(salen).
Symmetrical and nonsymmetrical salen ligands utilized in this
study are abbreviated in this paper as shown in Charts 1 and 2,
respectively. Manganese(III) salen complexes are synthesized
using triflate as a counterion. We confirmed that nonsymmetrical
salen complexes do not isomerize to two symmetrical salen
complexes in solution using ESI-MS (electrospray ionization
mass spectrometry) (Figures S1 and S2, Supporting Information).
Cyclic voltammetry measurements were carried out at 233 K in
CH2Cl2 containing 0.1 M Bu4NOTf (Figures S3ꢀS6, Supporting
Information), and the electrochemical parameters referenced
versus the ferrocenium/ferrocene couple (Fcþ/Fc) are summar-
izedin Table 1. The first redox potential(E1/21) of manganese(III)
salen complexes is increased in the order MeO (0.46 V) , Ph
(0.68 V) < t-Bu (0.70 V) , Cl (0.86 V) as a substituent on the
phenolate ring. The same trend is observed for nickel(II) salen
We herein report mixed-valence ligand radicals from
manganese(III) salen complexes. To investigate redox matching
between a manganese(III) center and phenolate ligands, we
prepared salen ligands with different redox potentials by varying
the substituent as shown in Chart 1. We have been interested in
active species that are transiently generated from chiral manganese-
(III) salen complexes, which are well-known as the most excellent
asymmetric oxidation catalyst.13 During the course of the studies,14
we have already reported a manganese(III)ꢀphenoxyl radical
species with a sterically hindered salen ligand, although the
mixed-valence property could not be studied due to inability
to reliably assign IVCT transitions.14b The manganese(III)ꢀ
phenoxyl radical species is shown to be readily converted to
manganese(IV)ꢀphenolate species via intramolecular electron
transfer, suggesting the redox potential of the manganese(III)
ion and the phenolate in salen complexes is very close. We thus
believe the mixed-valence property of ligand radicals from
manganese(III) salen complexes is of great interest. However,
the close proximity of the redox levels accompanies the ambiguity
on whether the locus of oxidation is metal-centered or ligand-
centered, which is carefully investigated in the present study. Indeed,
very few examples are known for manganeseꢀphenoxyl radi-
cal complexes,14b,15 in contrast to the abundance of high-valent
manganeseꢀphenolate complexes.16
1
complexes, which exhibit an increased E1/2 value in the order
MeO (0.21 V) < t-Bu (0.36 V) < Cl (0.49 V). Accordingly, upon
one-electron oxidation of nonsymmetrical salen complexes pre-
pared in this study, one of the two phenolates is preferentially
2
1
oxidized in all the cases. Table 1 includes ΔE1/2 (= E1/2 ꢀ E1/2
the electronic interaction between two redox sites.1f
)
values, which have been used for the assessment of the extent of
One-electron-oxidized products are generated by controlled-
potential electrochemical oxidation at 203 K in CH2Cl2 contain-
ing 0.1 M Bu4NOTf in a thin-layer quartz cell for UVꢀvisꢀNIR
absorption measurements. We carefully applied a voltage which
The key point in the present study is precise assignment of the
IVCT band from a phenolate to a phenoxyl radical by use of
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Table 1. Electrochemical Parameters (V) Measured in
CH2Cl2 Containing 0.1 M Bu4NOTf at 233 Ka
Scheme 1. Preparation of One-Electron-Oxidized
Manganese(III) Salen Complexes
1
2
Ea1
Ea2
Ec1
Ec2 E1/2
E1/2
ΔE1/2
b
MnIII(L-OMe)
MnIII(L-t-Bu)
MnIII(L-Ph)
MnIII(L-Cl)
MnIII(L-Ph-p-OMe) 0.58 0.74 0.47
MnIII(L-OMe/t-Bu) 0.52
MnIII(L-OMe/Cl)
MnIII(L-Ph/Cl)
NiII(L-OMe)
0.51 0.80 0.41
0.46
0.75 1.10 0.64 0.97 0.70
0.72 1.00 0.63 0.90 0.68
1.04
0.95
1.09
0.34
0.27
0.23
0.91 1.14 0.81 1.03 0.86
b
0.53
0.47
0.50
0.70
0.21
c
b
b
b
b
0.42
0.56 1.09 0.44
0.75 1.27 0.65
d
0.26
0.16
NiII(L-t-Bu)
NiII(L-Cl)
NiII(L-OMe/t-Bu)
NiII(L-OMe/Cl)
0.42 0.79 0.30 0.66 0.36
0.73
0.66
0.37
0.17
0.64 0.82 0.33 0.49 0.49
b
0.28 0.73 0.17
0.31 0.79 0.21
0.23
0.26
b
a Ea1 and Ea2 are the first and second anodic oxidation peak potentials,
respectively, and Ec1 and Ec2 are the first and second cathodic reduction
1
peak potentials, respectively. E1/2 values are calculated as averaged
values of Ea1 and Ec1, and E1/22 values are calculated as averaged values of
Ea2 and Ec2. ΔE1/2 values are defined as E1/2 ꢀ E1/21. Potentials are
2
referenced versus the ferrocenium/ferrocene couple. Potentials are
determined with cyclic voltammetry (Figure S3ꢀS6, Supporting In-
formation). b Corresponding peaks are not observed. c Two peaks at 0.95
and 1.10 V are observed. d Two peaks at 0.58 and 0.69 V are observed.
is higher than the first anodic oxidation peak (Ea1) but is lower
2
than the second anodic oxidation peak (Ea ) to obtain one-
electron-oxidized products selectively. We monitored UVꢀvis
spectral changes during electrochemical oxidations and checked
the presence of isosbestic points as a measure for the purity of
one-electron-oxidized products (Figures S7ꢀS10, Supporting
Information). We also confirmed the starting complex is regen-
erated with clear isosbestic points upon the electrochemical
reduction. It was shown that one-electron-oxidized products
are cleanly generated from all the manganese(III) and nickel(II)
salen complexes except for MnIII(L-Cl). One-electron-oxidized
products from manganese(III) salen complexes are alternatively
synthesized according to the procedure shown in Scheme 1. The
SbF6ꢀ counterion is the only choice for the isolation of analyticalꢀly
pure solid samples among other counterions such as TfOꢀ, BF4
,
and PF6ꢀ. It was confirmed that the isolated Mn(salen)(SbF6)2
samples that are dissolved in CH2Cl2 containing 0.1 M Bu4NOTf
exhibit UVꢀvis spectra identical to those of the electrochemically
generated samples. One-electron-oxidized products from nickel-
(II) sꢀalen complexes are alternatively synthesized and isolated with
SbF6 as a counterion, according to the procedure reported by
Storr and Stack.11h
Figure 1. 1H NMR spectrum of a 20 mM solution of MnIII(salen•þ)-
Evidence for Manganese(III)ꢀPhenoxyl Radical. Figure 1
2
(SbF6)2 from MnIII(L-t-Bu) in CD2Cl2 at 233 K (top) and H NMR
1
2
shows H and H NMR spectra of Mn(salen)(SbF6)2 from
MnIII(L-t-Bu) and MnIII(L-t-Bu-d4) (Chart 3). In L-t-Bu-d4,
2H atoms are selectively incorporated into the phenolate rings
(80% D) and the tert-butyl groups (7% D).14d Plots of chemical
shifts versus 1/T in the range from 193 to 253 K are linear with
intercepts (1/T = 0) in the diamagnetic region (Figure S11,
Supporting Information). The 1H NMR signals of high intensity
at ꢀ3.6 and ꢀ22.0 ppm are unambiguously assigned as arising
spectrum of a 5 mM solution of MnIII(salen•þ)(SbF6)2 from MnIII(L-t-
Bu-d4) in CH2Cl2 at 233 K (bottom).
NMR signals of the tert-butyl groups appear within the diamag-
netic region (2.6ꢀ4.8 ppm), indicative of a negligibly small
dipolar interaction between the manganese center and the tert-
butyl groups.14d Therefore, the significant paramagnetic shift of
the tert-butyl groups in Mn(salen)(SbF6)2 clearly indicates the
presence of an unpaired electron on the phenolate rings in close
vicinity to the tert-butyl groups. Then the Mn(salen)(SbF6)2
2
from the tert-butyl groups, in comparison with the H NMR
spectrum. In the case of the crystallographically characterized
MnIV(salen)(L)2 complexes (L = CF3CH2O, N3, and Cl), the 1H
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Chart 3
complex is a salen ligand radical complex, MnIII(salen•þ)(SbF6)2.
Interestingly, the 1H and 2H NMR signal that arises from 4/40-H
or 6/60-H of the phenolate rings appears at ꢀ19 to ꢀ24 ppm in
MnIV(salen)(L)2 (L = CF3CH2O, N3, and Cl),14d while the
corresponding signal is largely shifted to ꢀ93.4 ppm in MnIII-
(salen•þ)(SbF6)2, which could be utilized as a diagnosis for the
formation of the salen ligand radical.
We then focused on the 2H NMR signal at the 7/70 position of
the azomethine group as a probe to investigate electronic
structures and prepared MnIII(L-t-Bu-d2) (Chart 3), which is
selectively deuterated at the 7/70 position (99.5% D). As shown
in Figure 2a, the MnIII(salen•þ)(SbF6)2 complex shows a 7/70-D
NMR signal at ꢀ514 ppm, which is more shifted as compared to
that of MnIV(salen)(N3)2 (ꢀ370 ppm), but is less shifted than
that of MnIII(salen)(OTf) (ꢀ574 ppm). Plots of chemical shifts
versus 1/T are linear with intercepts (1/T = 0) in the diamag-
netic region in the case of MnIV(salen)(N3)2 and MnIII(salen)-
(OTf), but in the case of MnIII(salen•þ)(SbF6)2, the intercept is
considerably deviated from the diamagnetic region. The nonzero
intercept of the Curie plot might be due to a thermally accessible
excited state.
We then prepared one-electron-oxidized products from MnIII-
Figure 2. (a) 2H NMR spectra of a 20 mM solution of MnIII(salen•þ)-
(SbF6)2 (top), a 40 mM solution of MnIV(salen)(N3)2 (middle), and a
40 mM solution of MnIII(salen)(OTf) (bottom) from MnIII(L-t-Bu-d2)
in CH2Cl2 at 233 K. (b) 2H NMR Curie plots and linear least-
squares fits to the data for MnIII(salen•þ)(SbF6)2 (red line), MnIV-
(salen)(N3)2 (green line), and MnIII(salen)(OTf) (blue line) from
MnIII(L-t-Bu-d2).
(L-OMe) and MnIII(L-Ph), to investigate the effect of the
1
substituent on the phenolate rings. Figure 3 shows H NMR
spectra of isolated Mn(salen)(SbF6)2 complexes from MnIII
(L-OMe), MnIII(L-t-Bu), and MnIII(L-Ph) at 253 K, a higher
temperature than that employed in Figure 1 due to broadening of
a signal. 1H NMR signals for one of the phenolate protons appear at
ꢀ105, ꢀ85, and ꢀ54 ppm in Mn(salen)(SbF6)2 complexes from
MnIII(L-OMe), MnIII(L-t-Bu), and MnIII(L-Ph), respectively. The
1H NMR signal for the phenolate protons in Mn(salen)(SbF6)2
from MnIII(L-Ph) is least shifted (ꢀ54 ppm), but is still more
shifted than the corresponding signal at ꢀ19.8 ppm in MnIV(salen)-
(Cl)2 (Figure S12, Supporting Information). Then it is indicated
that one-electron oxidation of MnIII(L-OMe) and MnIII(L-Ph)
in the presence of a weakly bound counterion such as TfOꢀ and
complexes derived from MnIII(L-OMe), MnIII(L-t-Bu), and MnIII-
(L-Ph) commonly exhibit a weak signal at g = 5.3ꢀ5.7 and a
broad signal at g = 2.0. The signals at g = 5.3ꢀ5.7 display a six-line
hyperfine splitting due to the I = 5/2 55Mn nucleus. Sharp signals
marked with an asterisk may arise from a free ligand radical that is
not coupled with a manganese(III) ion. A set of broad g ≈ 2 and g
≈ 5 signals are assigned as a rhombic S = 3/2 system (E/D ≈
0.30), which is consistent with the magnetic susceptibility data
for MnIII(salen•þ)(SbF6)2 from MnIII(L-t-Bu) (Curie constant
C = 1.95 cm3 molꢀ1 K, axial zero-field-spitting parameter |D| =
2.2 cmꢀ1, Figure 5). The S = 3/2 system observed for MnIII-
(salen•þ)(SbF6)2 would be accounted for by an antiferromag-
netic coupling between the manganese(III) ion (S = 4/2) and the
phenoxyl radical (S = 1/2). Close inspection of the χmT versus T
SbF6 also generates a MnIII(salen•þ) complex. Additional evi-
ꢀ
dence for the assignment as MnIII(salen•þ) comes from near-
infrared absorptions that are reliably assigned as arising from the
charge transfer transition from the phenolate to the phenoxyl
radical (vide infra). We then compare EPR spectra of isolated
MnIII(salen•þ)(SbF6)2 complexes dissolvedinCH2Cl2 containing
1.0 M Bu4NOTf (Figure 4). The resulting Mn(salen•þ)(OTf)2
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Figure 4. X-band EPR spectra of a 10 mM frozen solution of MnIII-
(salen•þ)(SbF6)2 derived from (a) MnIII(L-OMe), (b) MnIII(L-t-Bu),
and (c) MnIII(L-Ph). Conditions: temperature, 4 K; solvent, frozen
CH2Cl2 containing 1 M Bu4NOTf; microwave frequency, 9.65 GHz;
microwave power, 0.5 mW; modulation amplitude, 7 G; time constant,
163.84 ms; conversion time, 163.84 ms.
Figure 3. 1H NMR spectra of MnIII(salen•þ)(SbF6)2 derived from (a)
MnIII(L-OMe) (30 mM), (b) MnIII(L-t-Bu) (20 mM), and (c) MnIII
(L-Ph) (10 mM) in CD2Cl2 at 253 K.
plot shows a slight upward deviation from the theoretical line at
higher temperature (>250 K), which is in accord with the
2
nonzero intercept of the Curie plot for the H NMR signal. It
is noted that the S = 3/2 spin system is a common feature with the
MnIV(salen)(L)2 complexes (L = CF3CH2O, N3, Cl, and NO3),
as shown by their EPR spectra14d and magnetic susceptibility
data (Figure S13, Supporting Information).
the near-infrared region at 1270 nm (ε = 1400 Mꢀ1 cmꢀ1), as
shown in Figure 6a. Quite interestingly, the MnIII(salen•þ)(OTf)2
complexes from nonsymmetrical MnIII(L-OMe/t-Bu) and
MnIII(L-OMe/Cl), in which one of the MeO groups in MnIII-
(L-OMe) is replaced by t-Bu and Cl, show broad absorptions
of similarly low intensity at 1015 nm (ε = 1100 Mꢀ1 cmꢀ1) and
890 nm (ε = 1200 Mꢀ1 cmꢀ1), respectively (Figure 7). The
absorptions shifted to a higher energy are consistent with the
assignment as a charge transfer band from the phenolate to the
phenoxyl radical, because an excitation energy required for
photoinduced intramolecular electron transfer would increase
in the order L-OMe < L-OMe/t-Bu < L-OMe/Cl (Scheme 2), as
deduced from the redox potentials of MnIII(L-OMe) (0.46 V),
MnIII(L-t-Bu) (0.70 V), and MnIII(L-Cl) (0.86 V) (Table 1). It is
noted that the ligand-to-ligand charge transfer depicted in
Scheme 2 is exactly the IVCT band from a localized RobinꢀDay
class II mixed-valence system, and then the MnIII(salen•þ)-
(OTf)2 complex from MnIII(L-OMe) is unambiguously assigned
as a class II mixed-valence compound.
Ligand Radicals from Manganese(III) Salen Complexes as
Class II Mixed-Valence Compounds. As shown in Figure 6, the
MnIII(salen•þ)(OTf)2 complexes from MnIII(L-OMe), MnIII
(L-t-Bu), and MnIII(L-Ph) exhibit intense absorptions at 455,
435, and 377 nm, respectively, which could be assigned as πꢀπ*
transitions of the coordinated phenoxyl radical.17 The most
important point in defining a mixed-valence property is an
absorption feature in the visible and near-infrared regions. Indeed,
the MnIII(salen•þ)(OTf)2 complexes from MnIII(L-OMe), MnIII-
(L-t-Bu), and MnIII(L-Ph) commonly exhibit characteristic broad
absorptions of similarly low intensity in the near-infrared region
(λmax = 1270, 1480, and 1510 nm, respectively). It is also
interesting to note that the MnIII(salen•þ)(OTf)2 complex from
MnIII(L-Ph) exhibits anadditional intense absorptioninthe visible
region (λmax = 650 nm).
To unambiguously assign these absorptions, we compare the
absorption spectra of MnIII(salen•þ)(OTf)2 from symmetrical
MnIII(L-OMe) with nonsymmetrical MnIII(L-OMe/t-Bu) and
MnIII(L-OMe/Cl) (Figure 7). The MnIII(salen•þ)(OTf)2 complex
from MnIII(L-OMe) exhibits a broad absorption of low intensity in
The MnIII(salen•þ)(OTf)2 complex from MnIII(L-Ph) bear-
ing Ph as a substituent exhibits an additional intense absorption
in the visible region at 650 nm (Figure 6c), which is another point
to be clarified in defining the mixed-valence property of MnIII-
(salen•þ)(OTf)2. We then compare the absorption spectra of
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ꢀ1
Figure 5. χmT (O) and χm (0) for a polycrystalline sample of
MnIII(salen•þ)(SbF6)2 from MnIII(L-t-Buꢀ)1as a function of temperature
T in an applied field of 1 kOe. The χm vs T plots (>30 K) were
analyzed by the CurieꢀWeiss law 1/χm = (1/C)T ꢀ Θ/C with C =
1.95 cm3 molꢀ1 K and Θ = ꢀ1.21 K (red line). The χmT vs T plots were
simulated by julX written by E. Bill with E/D = 0.30, D = 2.2 cmꢀ1, and
g = 2.00 (blue line). For details regarding julX, see http://ewww.
mpi-muelheim.mpg.de/bac/logins/bill/julX_en.php.
MnIII(salen•þ)(OTf)2 from MnIII(L-Ph) and nonsymmetrical
MnIII(L-Ph/Cl) (Figures 6c and 8a). The position of the absorption
at 650 nm is not altered at all. This clearly indicates the intense
absorption in the visible region at 650 nm cannot be an IVCT
band between the phenolate and the phenoxyl radical, but is
assigned as arising from the substructure containing the phenoxyl
radical and the manganese(III) ion. To further investigate the
origin of this absorption, we prepared the MnIII(L-Ph-p-OMe)
complex bearing p-methoxyphenyl groups instead of phenyl
groups. The MnIII(salen•þ)(OTf)2 complex from MnIII(L-Ph-
p-OMe) exhibits an intense absorption at 810 nm (Figure 8b).
The shift to a lower energy suggests that this absorption arises
from a charge transfer from the aromatic substituent on the
phenolate rings to the phenoxyl radical. We already reported that
the MnIII(salen•þ)(OTf)2 complex from the sterically hindered
manganese(III) salen complex bearing mesityl groups exhibits
intense absorptions around 900 nm,14b which is now most
probably assigned as a charge transfer from the mesityl groups
to the phenoxyl radical. It is interesting to note that the near-
infrared broad absorption at 1510 nm in MnIII(salen•þ)(OTf)2
from MnIII(L-Ph) is shifted to 1070 nm in MnIII(salen•þ)(OTf)2
from nonsymmetrical MnIII(L-Ph/Cl) (Figures 6c and 8a),
which clearly indicates that this absorption arises from a charge
transfer from the phenolate to the phenoxyl radical. Thus, the
ligand radicals from the manganese(III) salen complexes bearing
aromatic substituents also belong to RobinꢀDay class II.
Figure 6. Absorption spectra of one-electron-oxidized products (red
line) that are generated electrochemically from (a) MnIII(L-OMe)
(0.8 mM), (b) MnIII(L-t-Bu) (0.8 mM), and (c) MnIII(L-Ph) (0.4 mM)
(black line) at 203 K. Data in the 2230ꢀ2260 and 2330ꢀ2400 nm ranges
were deleted due to intense noises derived from the absorption of the
solvent.
The corresponding ligand-to-ligand charge transfer band in
MnIII(salen•þ)(OTf)2 from MnIII(L-t-Bu) appears at 1480 nm
(ε = 1600 Mꢀ1 cmꢀ1), which is almost the same wavelength as
that observed for MnIII(salen•þ)(OTf)2 from MnIII(L-Ph)
(Figure 6b,c). This is in agreement with the nearly identical
redox potentials for MnIII(L-t-Bu) and MnIII(L-Ph) (Table 1).
In the case of the MnIII(L-OMe) complex, which shows a lower
oxidation potential by 0.20 V than MnIII(L-t-Bu) and MnIII(L-Ph)
(Table 1), the ligand-to-ligand absorption in MnIII(salen•þ)-
(OTf)2 is shifted to higher energy at 1270 nm (Figure 6a).
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We also carried out MarcusꢀHush analysis of the IVCT
bands. According to the MarcusꢀHush theory, the bandwidth
at half-height (Δν1/2, cmꢀ1) for the IVCT transition from a class
II mixed-valence system is predicted by the following equation:
structure is regarded as class II. When the experimental Δν1/2
value is significantly smaller than the theoretical value, the mixed-
valence system is assigned as class III or borderline class IIꢀIII.
The near-infrared absorptions that are commonly observed for
MnIII(salen•þ) prepared in this study are consistent with the
assignment as class II IVCT bands: for [MnIII(L-OMe)]þ, Δν1/2
-
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Δν1=2
¼
16RTðln 2Þνmax
ð1Þ
(theoretical) = 3500 cmꢀ1 versus Δν1/2(experimental) =
4600 cmꢀ1, for [MnIII(L-t-Bu)]þ, Δν1/2(theoretical) =
3300 cmꢀ1 versus Δν1/2(experimental) = 5300 cmꢀ1, and
for [MnIII(L-Ph)]þ, Δν1/2(theoretical) = 3200 cmꢀ1 versus
where R is the gas constant, T is the temperature (K), and νmax is
the band maximum (cmꢀ1). When the experimental Δν1/2 value
is equal to or broader than the theoretical value, the electronic
Figure 8. Absorption spectra of one-electron-oxidized products
(red line) that are generated electrochemically from (a) MnIII(L-Ph/
Cl) (0.8 mM) and (b) MnIII(L-Ph-p-OMe) (0.4 mM) (black line)
at 203 K. Data in the 2230ꢀ2260 and 2330ꢀ2400 nm ranges were
deleted due to intense noises derived from the absorption of the
solvent.
Figure 7. Absorption spectra of one-electron-oxidized products (red
line) that are generated electrochemically from (a) MnIII(L-OMe/t-Bu)
(0.8 mM) and (b) MnIII(L-OMe/Cl) (0.8 mM) (black line) at 203 K.
Data in the 2230ꢀ2260 and 2330ꢀ2400 nm ranges were deleted due to
intense noises derived from the absorption of the solvent.
Scheme 2
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MnIV(salen)(OH) and MnIII(salen•þ)(OH2) using B3LYP den-
sity functional methods.14e The calculation nicely reproduces an
electronic structural change from manganese(IV)ꢀphenolate to
manganese(III)ꢀphenoxyl radical upon the exchange of the
external axial ligand, but in the geometry-optimized MnIII-
(salen•þ)(OH2), the two phenolate rings are structurally iden-
tical, which is erroneously indicative of a fully delocalized class III
ligand radical.
Transition from Class II to Class III Mixed-Valence Systems
in Ligand Radicals from Nickel(II) Salen Complexes. Our
methodology using a series of nonsymmetrical salen complexes is
applied for the analysis of the NiII(salen•þ) complex from NiII
(L-t-Bu), which was previously shown to be a fully delocalized
RobinꢀDay class III mixed-valence compound by Shimazaki,
Storr, and Stack.11h,j One of the best pieces of evidence for this
assignment is an intense near-infrared absorption at 2130 nm
(ε = 2.15 ꢁ 104 Mꢀ1 cmꢀ1). This near-infrared absorption is also
observed for NiII(salen•þ) that is alternatively prepared electro-
chemically from NiII(L-t-Bu) under the low-temperature condi-
tions identical to those employed for the preparation of
MnIII(salen•þ), although the absorption is slightly broadened
(Figure S14, Supporting Information). The experimental Δν1/2
value (790 cmꢀ1) estimated from this near-infrared absorption is
30% of the theoretical value (2700 cmꢀ1) obtained by eq 1,
which is consistent with the assignment as a class III mixed-
valence compound.
A new finding we have made herein is that the NiII(salen•þ)
complex from NiII(L-OMe) exhibits a significantly broad absorp-
tion in the range from 1200 to 2000 nm, in addition to a narrow
absorption at 2140 nm, indicative of its different mixed-valence
properties (Figure 9a). The narrow absorption at 2140 nm may
be assigned as a delocalized class III IVCT band, in comparison
with the absorption spectrum of NiII(salen•þ) from NiII(L-t-Bu),
a delocalized class III mixed-valence compound. This assignment
is justified by complete disappearance of the narrow near-infrared
absorption in the NiII(salen•þ) complex from nonsymmetrical
NiII(L-OMe/t-Bu) and NiII(L-OMe/Cl) (Figure 9b,c), in which
the radical should be localized on the methoxyphenolate moiety
due to the difference in the redox potential. In contrast, the broad
absorption at ∼1950 nm in NiII(salen•þ) from NiII(L-OMe),
which is shifted to a higher energy (1580 and 1400 nm) in
NiII(salen•þ) from nonsymmetrical NiII(L-OMe/t-Bu) and NiII-
(L-OMe/Cl), is reliably assigned as a class II charge transfer
transition of the type shown in Scheme 2. The band shape
analysis by eq 1 is also consistent with the assignment as a
class II IVCT (Δν1/2(theoretical) = 2800 cmꢀ1 versus Δν1/2
-
(experimental) = 2900 cmꢀ1). Then it is clearly shown that the
NiII(salen•þ) complex from NiII(L-OMe) is just on the transi-
tion between class II and class III mixed-valence compounds.
Consistently, the NiII(salen•þ) complex from NiII(L-OMe)
shows an EPR signal (S = 1/2) at g = 2.023, which is close to
the EPR signals at g = 2.021 and 2.016 for localized NiII-
(salen•þ) from nonsymmetrical NiII(L-OMe/t-Bu) and NiII
(L-OMe/Cl), as compared to the EPR signal at g = 2.045 for
delocalized NiII(salen•þ) from NiII(L-t-Bu) (Figure S15, Sup-
porting Information). Very recently, Thomas and co-workers
succeeded in the X-ray crystallographic analysis of the NiII-
(salen•þ)(SbF6) complex from NiII(L-OMe),11k which indeed
shows a nonsymmetrical structure containing quinoid and
phenolate rings, in contrast to a nearly 2-fold symmetric
structure for NiII(salen•þ)(SbF6) from NiII(L-t-Bu), as reported
by Storr and Stack.11h
Figure 9. Absorption spectra of a 0.5 mM solution of one-electron-
oxidized products (red line) that are generated electrochemically from
(a) NiII(L-OMe), (b) NiII(L-OMe/t-Bu), and (c) NiII(L-OMe/Cl)
(black line) at 203 K. Data in the 2230ꢀ2260 and 2330ꢀ2400 nm
ranges were deleted due to intense noises derived from the absorption of
the solvent.
Δν1/2(experimental) = 5700 cmꢀ1. It is thus concluded that
a manganese(III) ion forms a RobinꢀDay class II mixed-valence
system between the phenoxyl radical and the phenolate. We
recently reported a quantum chemical calculation for
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Chart 4
’ CONCLUSION
containing 0.1 M Bu4NOTf at a scan rate of 50 mV sꢀ1 at 233 K under an
Ar atmosphere. The E values were referenced to the E1/2 value of
ferrocene, which was measured under identical conditions. Solid-state
magnetic susceptibility measurements were made by using an MPMS-7
SQUID susceptometer (Quantum Design) operating in the 2ꢀ300 K
temperature range. Well-ground polycrystalline samples were wrapped
in a plastic sheet and were loaded into the sample folder (a drinking
straw). The susceptibilities of the plastic sheet and the sample folder
were measured in the same temperature range and field to provide an
accurate correction for its contribution to the total magnetic suscept-
ibility. Diamagnetic corrections were estimated from Pascal constants.
NMR (500 and 400 MHz) spectra were measured in a borosilicate glass
tube (o.d. = 5 mm) on LA-500 and LA-400 spectrometers (JEOL),
respectively. 1H NMR chemical shifts in CD2Cl2 and CDCl3 were
referenced to CHDCl2 (5.32 ppm) and CHCl3 (7.24 ppm), respectively.
13C NMR chemical shifts in CDCl3 were reported relative to CHCl3
(77.0 ppm). ESI-MS spectra were obtained with an LCT time-of-flight
mass spectrometer equipped with an electrospray ionization interface
(Micromass). Elemental analyses were conducted on a CHN corder
MT-6 (Yanaco). High-resolution mass spectra were measured with the
JMS-777V mass spectrometer (JEOL). High-performance liquid chro-
matography was carried out on an LC-9201 instrument (Japan Analy-
tical Industry) with JAIGEL-1H and JAIGEL-2H using CHCl3 as the
eluent.
The present study unambiguously assigns IVCT and other
transitions in ligand radicals from manganese(III) and nickel(II)
salen complexes, which enables us to obtain otherwise inacces-
sible insight into electron transfer from the phenolate to the
remote phenoxyl radical mediated by a neighboring metal ion.
One of the most important findings is the function of the metal
ion is strikingly different in mediating the electron transfer from
the phenolate to the phenoxyl radical in metal salen complexes
(Chart 4).18 A d4 high-spin manganese(III) ion forms a Robinꢀ
Day class II mixed-valence system, in which electron transfer is
occurring between the localized phenoxyl radical and the pheno-
late. This is in clear contrast to a d8 low-spin nickel(II) ion with
the same salen ligand, which induces a delocalized radical (Robinꢀ
Day class III) over the two phenolate rings. The other point to be
noted is a substituent effect on mixed-valence properties. In the
case of the NiII(salen•þ) complex, the transition from fully
delocalized class III to localized class II occurs upon the exchange
of the t-Bu substituent to the MeO substituent on the salen ligand,
possibly due to the difference in the redox potential. The present
findings point to a fascinating possibility that electron transfer
could be drastically modulated by exchanging the metal ion that
bridges the two redox centers or by adjusting the redox potential of
the electron donor and acceptor relative to the metal ion mediator.
’ ASSOCIATED CONTENT
’ EXPERIMENTAL SECTION: INSTRUMENTATION
S
Supporting Information. Synthetic details and Figures
b
Absorption spectra in the range from 200 to 1100 nm were recorded
on an Agilent 8453 spectrometer (Agilent Technologies) equipped with
a USP-203 low-temperature chamber (UNISOKU). Absorption spectra
in the range from 200 to 3200 nm were recorded on a UV-3150
UVꢀvisꢀNIR spectrophotometer (Shimadzu) equipped with a USP-
203 low-temperature chamber (UNISOKU). Quartz cells (l = 0.05 cm),
which are fitted for the low-temperature chamber, were custom-made by
domestic glassware manufacturers (Eikosha and Agri). Electrochemical
oxidation was conducted using a gold-mesh working electrode, a
platinum-wire counter electrode, and a Ag/AgCl reference electrode,
which are connected to an HA-151 potentiostatꢀgalvanostat (Hokuto
Denko). The gold mesh, silver rod, and platinum wire and sheet were
obtained from Nilaco. Electron paramagnetic resonance (EPR) spectra
were recorded for 30 μL of the 10 mM frozen CH2Cl2 solution
containing 1.0 M Bu4NOTf at 4 K in a quartz tube (o.d. = 5 mm) on
an E500 continuous-wave X-band spectrometer (Bruker) with an
ESR910 helium-flow cryostat (Oxford Instruments). EPR spectra of
one-electron-oxidized NiII(salen) complexes were recorded for 20 μL
of the 5 mM CH2Cl2 solution at room temperature. Cyclic voltammo-
grams were measured with an ALS612A electrochemical analyzer
(BAS). A saturated calomel reference electrode, a glassy carbon working
electrode, and a platinum-wire counter electrode were utilized. Measure-
ments were carried out for the 1 mM solution in dehydrated CH2Cl2
S1ꢀS18. This material is available free of charge via the Internet
at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
hiro@ims.ac.jp
’ ACKNOWLEDGMENT
We thank Mr. Seiji Makita (Institute for Molecular Science)
for elemental analysis and measurements of high-resolution mass
spectrometry. This study was supported by a grant from Japan
Society for the Promotion of Science (Grant-in-Aid for Science
Research, Grant No. 22350030).
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