Unique properties and reactivity of high-valent manganese-Oxo versus manganese-hydroxo in the salen platform

Article abstract of DOI:10.1021/ic100673b

To gain an understanding of oxidation reactions by Mn^III(salen), a reaction of Mn^III(salen) with m-chloroperoxybenzoic acid in the absence of a substrate Is investigated. UV-vis, perpendicular- and parallel-mode electron paramagnetic resonance, and X-ray absorption spectroscopy show that the resulting solution contains Mn^Iv(salen)(O) as a major product and Mn^Iv(salen)(OH) as a minor product. Mn^Iv(salen)(O) readily reacts with 4-H-2,6-tert-Bu₂C₆H₂OH (homolytic bond dissociation energy of an OH bond, BDE_oH = 82.8 kcal mol^-1), 4-CH₃CO-2,6-tert-Bu₂C₆H ₂OH (BDE_oH = 83.1 kcal mol^-1), and 4-NC-2,6-tert-Bu₂C₆H₂OH (BDE_OH = 84.2 kcal mol^-1) at 203 K, following secondorder rate kinetics. Mn^Iv(salen)(OH) reacts with 4-CH₃CO-2,6-tert-Bu ₂C₆H₂OH (BDE_oH = 83.1 kcal mol _-1) much more slowly under identical conditions than Mn ^Iv(salen)(O) and does not react with 4-NC-2,6-tert-Bu ₂C₆H₂OH (BDEOH=84.2 kcal mol^-1), suggesting that the thermodynamic hydrogen-atom-abstracting ability of Mn ^Iv(salen)(OH) is about 83 kcal mol^-1. The rate constant for reactions of Mn^Iv(salen)(OH) with phenols is not dependent on the concentration of phenols, suggesting that Mn^Iv(salen)(OH) might bind phenols prior to the rate-limiting oxidation reactions. Quantum chemical calculations are carried out for Mn^Iv(salen)(0) and Mn ^Iv(salen)(OH), both of which well reproduce the extended X-ray absorption fine structures as well as the electronic configurations. It is also indicated that protonation of Mn^Iv(salen)(OH) induces a drastic electronic structural change from manganese(IV) phenolate to a manganese(III) phenoxyl radical, which is also consistent with the experimental observation.

Full text of DOI:10.1021/ic100673b

6664 Inorg. Chem. 2010, 49, 6664–6672
DOI: 10.1021/ic100673b
Unique Properties and Reactivity of High-Valent Manganese-Oxo versus
Manganese-Hydroxo in the Salen Platform
Takuya Kurahashi,† Akihiro Kikuchi,‡ Yoshitsugu Shiro,‡ Masahiko Hada,§ and Hiroshi Fujii*,†
†Institute for Molecular Science & Okazaki Institute for Integrative Bioscience, National Institutes of Natural
Sciences, Myodaiji, Okazaki, Aichi 444-8787, Japan, ‡RIKEN SPring-8 Center, Harima Institute, 1-1-1 Kouto,
§
Sayo, Hyogo 679-5148, Japan, and Department of Chemistry, Graduate School of Science,
Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397, Japan
Received April 9, 2010
To gain an understanding of oxidation reactions by MnIII(salen), a reaction of MnIII(salen) with m-chloroperoxybenzoic
acid in the absence of a substrate is investigated. UV-vis, perpendicular- and parallel-mode electron paramagnetic
resonance, and X-ray absorption spectroscopy show that the resulting solution contains MnIV(salen)(O) as a major
product and MnIV(salen)(OH) as a minor product. MnIV(salen)(O) readily reacts with 4-H-2,6-tert-Bu2C6H2OH
(homolytic bond dissociation energy of an OH bond, BDEOH = 82.8 kcal mol-1), 4-CH3CO-2,6-tert-Bu2C6H2OH
(BDEOH = 83.1 kcal mol-1), and 4-NC-2,6-tert-Bu2C6H2OH (BDEOH = 84.2 kcal mol-1) at 203 K, following second-
order rate kinetics. MnIV(salen)(OH) reacts with 4-CH3CO-2,6-tert-Bu2C6H2OH (BDEOH = 83.1 kcal mol-1) much
more slowly under identical conditions than MnIV(salen)(O) and does not react with 4-NC-2,6-tert-Bu2C6H2OH
(BDEOH=84.2 kcal mol-1), suggesting that the thermodynamic hydrogen-atom-abstracting ability of MnIV(salen)(OH)
is about 83 kcal mol-1. The rate constant for reactions of MnIV(salen)(OH) with phenols is not dependent on the con-
centration of phenols, suggesting that MnIV(salen)(OH) might bind phenols prior to the rate-limiting oxidation
reactions. Quantum chemical calculations are carried out for MnIV(salen)(O) and MnIV(salen)(OH), both of which well
reproduce the extended X-ray absorption fine structures as well as the electronic configurations. It is also indicated that
protonation of MnIV(salen)(OH) induces a drastic electronic structural change from manganese(IV) phenolate to a
manganese(III) phenoxyl radical, which is also consistent with the experimental observation.
Introduction
and biological oxidation reactions. The electronic structures
and reactivity of MnIVdO and MnVtO species in various co-
ordination environments have been actively investigated.1-9
However, less is known for a manganese salen complex,
MnIII(salen) (1), which was discovered by Kochi and co-workers
as an excellent epoxidation catalyst10 and was then applied
for catalytic enantioselective epoxidation of unfunctiona-
lized olefins by Jacobsen and Katsuki.11,12 Kochi originally
High-valent manganese-oxo species have been the focus
of extensive investigations because of their key role in synthetic
*To whom correspondence should be addressed. E-mail: hiro@ims.ac.jp.
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pubs.acs.org/IC
Published on Web 06/16/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6665
postulated that reactions of 1 with oxidants such as NaOCl,
PhIO, and m-chloroperoxybenzoic acid (m-CPBA) generate
MnV(salen)(O), which transfers the manganese-bound oxy-
gen atom to olefins and regenerates 1.10 The MnV(salen)(O)
model has been frequently employed in theoretical considera-
tions to explain the selectivity in MnIII(salen)-catalyzed enantio-
selective epoxidation.13 However, the experimental evidence
for the existence of MnV(salen)(O) is still limited, and the best
evidence to date is detection of the ion signal for MnV(salen)-
(O) by electrospray mass spectrometry.14 It has also been
reported that a transient that could be assigned as MnV-
(salen)(O) wasdetectedvialaserflashphotolysis15a and stopped-
flow spectrophotometry.15b However, electron paramagnetic
resonance (EPR) and X-ray absorption spectroscopy (XAS)
studies have shown that reactions of 1 with NaOCl, PhIO,
and m-CPBA in the absence of a substrate result in mixtures
of MnIV(salen) species that were not fully characterized.16
We recently reported the preparation of MnIV(salen)(O)
using a sterically hindered salen ligand (Chart 1), which
was thoroughly characterized using various spectroscopic
techniques.17
Chart 1. Sterically Hindered 1
abstraction by putative MnV(salen)(O) or simply as a con-
sequence of protonation ofMnIV(salen)(O). Suchhigh-valent
metal complexes carrying hydroxide are also an issue of inte-
rest. Green et al. reported that chloroperoxidase compound
II has FeIVOH rather than FeIVdO.18 They proposed that an
axial thiolateligation enhancesthe basicity of FeIVdO, which
is a major driving force for thiolate-ligated compound I
(formally FeVtO) to perform hydrogen-atom abstraction
from inert substrates. Busch et al. prepared and crystallo-
graphically characterized a mononuclear MnIV complex con-
taining a pair of hydroxo ligands in their ultrarigid ethylene
cross-bridged macrocyclic ligand.19 Their MnIVOH complex
shows highly selective hydrogen-atom-abstracting ability,
suggesting that high-valent metal hydroxo species may also
play a role in catalytic oxidation reactions. The reactivities
of MnIVOH and MnIVdO were investigated by comparing
thermodynamic hydrogen-atom-abstracting abilities by means
of the Bordwell/Mayer method20,21 as well as comparing acti-
vation barriers including both activation entropies and enthal-
pies. We also reported the preparation of MnIV(salen)(OH)
in our sterically hindered salen platform (Chart 1).17
We herein report the reaction of our sterically hindered 1
with m-CPBA, an oxidant that is employed for MnIII(salen)-
catalyzed oxidation,22 using the previously reported MnIV-
(salen)(O) and MnIV(salen)(OH) as reference complexes.17 It
is expected that steric bulk incorporated to the salen ligand
prevents dimerization, thus enabling us to focus on mononuclear
high-valent species. UV-vis, perpendicular- and parallel-mode
EPR, and XAS show that the resulting solution contains
MnIV(salen)(O) as a major product and MnIV(salen)(OH) as
a minor product. To compare the reactivities of MnIV(salen)-
(O) and MnIV(salen)(OH), we employ a series of substitu-
ted phenols as substrates for hydrogen-atom-abstraction
reactions,23 which suggest that their reaction pathways may
Another species to be considered is MnIV(salen)(OH), which
might be generated as a consequence of hydrogen-atom
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6666 Inorganic Chemistry, Vol. 49, No. 14, 2010
Kurahashi et al.
signal at g = 8.1 from 1. The signals at g = 5.0, 3.1, and
1.9, which are derived from an S=3/2 spin system of a d3
MnIV system,16c are almost as intense as those of authen-
tic MnIV(salen)(O) and MnIV(salen)(OH), which were
prepared by alternative methods,17 as shown in Figure S1
(Supporting Information). Thus, this solution contains
MnIV species exclusively. Comparison of the EPR spectra
(Figure S1 in the Supporting Information) suggests that
the resulting solution may contain not only MnIV(salen)-
(O) but also MnIV(salen)(OH).
Then, the solution generated by the addition of 1.0 equiv
of m-CPBA and Bu4NOH to 1 at 203 K was investigated
by XAS. An XAS measurement was carried out in pro-
pionitrile, in which the reaction of 1 with 1.0 equiv of
m-CPBA and Bu4NOH generates the same intermediate,
as confirmed by UV-vis and EPR. As seen from the Mn
K-edge X-ray absorption near-edge structure (XANES)
regions (Figure 3a), the preedge height is not increased
before and after the addition of m-CPBA and Bu4NOH,
which is also the case for MnIV(salen)(OH) and MnIV-
(salen)(O).17 Both the preedge and the absorption edge
are shifted to higher energies by 0.1 and 0.7 eV upon the
addition of m-CPBA and Bu4NOH. This shift may corres-
pond to the change in the oxidation state from MnIII to
MnIV. The extended X-ray absorption fine structure
(EXAFS) spectrum of the solution in Figure 3b shows a
peak at a phase-shifted distance R0 ≈ 1 A. This peak
corresponds to the MnIVdO bond (1.58 A), in compa-
rison with our previous EXAFS analysis of MnIV(salen)-
(O).17 However, the intensity of this peak is somewhat
lower than that in authentic MnIV(salen)(O).
This is consistent with the EPR observation that the
solution may contain MnIV(salen)(O) as well as MnIV-
(salen)(OH), which has a longer MnIV-O bond (1.83 A),
as determined by our previous EXAFS analysis.17 Table 1
shows one of the possible curve-fitting simulations in which
both coordination numbers and interatomic distances are
allowed to vary. The simulation suggests that the solu-
tion may contain 70% MnIV(salen)(O) (shells 1-3) and
30% MnIV(salen)(OH) (shells 2 and 3). Scheme 1 sum-
marizes MnIV(salen) species to be formed under certain
conditions.17
Figure 1. UV-vis spectra of a 1 mM solution of 1 (black line), 1 þ 1.0
equiv of m-CPBA (blue line), and 1 þ 1.0 equiv of m-CPBA þ 1.0 equiv of
Bu4NOH (red line) in CH2Cl2 at 203 K.
be considerably different. We also carried out a quantum
chemical calculation for MnIV(salen)(O) and its protonated
species such as MnIV(salen)(OH).
Results and Discussion
Reaction of a Sterically Hindered 1 with m-CPBA. The
reaction of 1 with m-CPBA in the absence of a substrate is
investigated under the low-temperature conditions emp-
loyed for the MnIII(salen)-catalyzed epoxidation using
m-CPBA.22 As shown in Figure 1, upon the addition of
1.0 equiv of m-CPBA to the CH2Cl2 solution of 1 (depic-
ted as a black line) at 203 K, the resulting solution shows
an UV-vis spectrum almost identical with that of the start-
ing 1 (depicted as a blue line). However, the subsequent
addition of 1.0 equiv of Bu4NOH immediately generates a
distinct species with absorption maxima around 350, 435,
and 600 nm (depicted as a red line). The absorption
spectral features are similar to that of MnIV(salen)(O),
which was identified in our previous study.17 Interest-
ingly, when the addition sequence of m-CPBA and Bu4-
NOH is altered, this species is not generated, suggesting
that Bu4NOH might function as a base to remove a pro-
ton from m-CPBA rather than as a trans axial ligand to
activate m-CPBA. In the case of the low-temperature MnIII-
(salen)-catalyzed epoxidation using m-CPBA as an oxidant,22
the use of excess N-methylmorpholine N-oxide (NMO) as
an additive is reported to be critical. Then, the use of NMO
instead of Bu4NOH is also examined for the reaction of 1
with m-CPBA. The addition of 1.0 equiv of NMO results
in exactly the same spectral change, suggesting that both
NMO and Bu4NOH might function in a similar manner
upon activation of m-CPBA at 203 K.
Parallel-mode EPR spectroscopy of 1 shows a signal at
g = 8.1, which displays a six-line hyperfine splitting with
A=38 G, as expected for the I=5/2 55 Mn nucleus (Figure 2).
This signal was assigned as the transition between the
Ms=(2levelofanS=2 spin system of a d4 MnIII system.16c
Upon the addition of 1.0 equiv of m-CPBA at 203 K, the
signal at g=8.1 is not changed, indicating that the MnIII
center remains intact. Upon the subsequent addition of
1.0 equiv of Bu4NOH, perpendicular-mode EPR spec-
troscopy detects signals at g =5.0, 3.1, and 1.9, which
coincide with the disappearance of a parallel-mode EPR
Quantum Chemical Study of MnIV(salen)(O), MnIV-
(salen)(OH), and MnIII(salen•þ)(OH2). In order to provide
a theoretical support for the experimental observation, we
carried out a quantum chemical calculation, using virtually
the same MnIII(salen) model with 1 as shown in Figure 4.
Figure 4 also shows an optimized structure for MnIV(salen)-
(O). Figure 5 illustrates the difference in coordination geo-
metries for MnIV(salen)(O) and its protonation products.
An optimized geometry for MnIV(salen)(O) inTable2shows
a Mn-O3 bond distance of 1.642 A, which is a little bit
longer but is in qualitative agreement with the experimental
distance of 1.58 A derived from the previous EXAFS
analysis.17 Protonation of MnIV(salen)(O) to MnIV(salen)-
(OH) prolongs a Mn-O3 bond distance to 1.797 A, whichis
also consistent with the EXAFS distance of 1.83 A.17 Upon
further protonation of MnIV(salen)(OH), theMn-O3 bond
distance is significantly prolonged to 2.258 A. Ghosh and
Gonzalez reported theoretical studies on high-valent man-
ganese porphyrins, in which MnIVdO bond distances in
[MnIV(porphyrin)(O)(PF6)]-, MnIV(porphyrin)(O)(pyri-
dine), and [MnIV(porphyrin)(O)(F)]- are calculated to be

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6667
Figure 2. X-band EPR spectra of (a) 1, (b) 1 þ 1.0 equiv of m-CPBA, and (c) 1 þ 1.0 equiv of m-CPBA þ 1.0 equiv of Bu4NOH: (left) perpendicular mode;
(right) parallel mode. Conditions: temperature, 5 K; solvent, frozen CH2Cl2 containing 0.1 M Bu4NClO4; microwave frequency, 9.69 (perpendicular) or
9.53 (parallel) GHz; microwave power, 2.000 mW; modulation amplitude, 10 G; modulation frequency, 100 kHz; time constant, 81.92 ms; conversion time,
81.92 ms.
1.668, 1.674, and 1.709 A, respectively.24 The MnIVdO
bond distance in the present case is shorter than those,
possibly because of the absence of a sixth coordination
ligand trans to the oxo ligand. Borovik and co-workers
reported a monomeric MnIVdO complex, in which the
MnIVdO moiety is stabilized by three hydrogen bonds from
amides.8a Their density functional theory (DFT) calcula-
tions showed that hydrogen bonding prolongs the MnIVdO
bond distance to 1.706 A, which is longer than that in MnIV-
(salen)(O) but is shorter than that in MnIV(salen)(OH).
As shown in Table 3, electronic structures of MnIV-
(salen)(O) and MnIV(salen)(OH) are best described as
high-spin d3 MnIV with an electronic configuration of
(dπ)1(dπ)1(dnb)1, which accounts for S = 3/2 EPR signals
for MnIV(salen)(O) and MnIV(salen)(OH).17 In the case
of MnIV(salen)(O), a substantial spin population is found
on the O3 ligand, while spin populations on the dπ orbitals
are decreased as compared with those of MnIV(salen)-
(OH). This is indicative of a Mn(dπ)-to-O3 spin deloca-
lization, which means a π-bonding character for Mn-O3
in MnIV(salen)(O). Protonation of MnIV(salen)(O) pro-
longs a Mn-O3 bond distance as compared with MnIV-
(salen)(O), leading to an increased spin population on the
dσ1 orbital due to a stabilization of this orbital. Proton-
ation of MnIV(salen)(OH) increases a spin population on
the dσ1 orbital more significantly because of a much longer
Mn-O3 bond distance. It is interesting to note that a spin
population with a negative sign is remarkably increased
on the salen ligand upon protonation of MnIV(salen)-
(OH). This indicates a salen ligand radical antiferromag-
netically coupled to the high-spin d4 MnIII, which is nicely
consistent with our previous spectroscopic assignment
as MnIII(salen•þ)(OH2) of an St = 3/2 spin system.17 The
present theoretical calculation as well as the previous ex-
periment indicates conversion from manganese(IV) phe-
nolate in MnIV(salen)(OH) to a manganese(III) phenoxyl
radical in MnIII(salen•þ)(OH2), because protonation of
hydroxo on manganese significantly stabilizes the dσ1 orbital
relative to the π orbital of the salen ligand, leading to
intramolecular electron transfer from the salen ligand to
the MnIV center. Quantum chemical calculations suggest
a lesser degree of a similar salen ligand radical also for
MnIV(salen)(OH), but a possible contribution of MnIII
-
(salen•þ)(OH) was not evident in the previous experiment.17
Ni(salen) has recently attracted considerable attention
because a one-electron-oxidized form of NiII(salen) ex-
hibits valence tautomerism between NiIII(salen) and NiII-
(salen•þ).25 In the case of NiII(salen•þ), DFT calculations
showed that ca. 80% of the unpaired spin is located on
the salen ligand.25b,e,f A similar equilibrium between the
high-valent metal species and the ligand-radical species
was also proposed for a one-electron-oxidized form of
CuII(salen).26
Reactions of MnIV(salen)(O) and MnIV(salen)(OH) with
Substituted Phenols. We investigate the reactivity of the
MnIV(salen) mixture, which is shown to contain MnIV-
(salen)(O) and MnIV(salen)(OH) as above. The MnIV-
(salen) mixture is prepared under the conditions (1 mM)
shown in Figure 1, and the solution of 2,6-di-tert-butyl-
phenol (10 mM) is added under pseudo-first-order con-
ditions at 203 K. The MnIV(salen) mixture is reduced to
MnIII(salen) within seconds, and this reaction is too fast
to follow with an UV-vis spectrometer. To follow the reac-
tion process by MnIV(salen)(O) and MnIV(salen)(OH) in
(25) (a) Shimazaki, Y.; Tani, F.; Fukui, K.; Naruta, Y.; Yamauchi, O.
J. Am. Chem. Soc. 2003, 125, 10512–10513. (b) Rotthaus, O.; Jarjayes, O.;
Thomas, F.; Philouze, C.; Perez Del Valle, C.; Saint-Aman, E.; Pierre, J.-L.
Chem. Eur. J. 2006, 12, 2293–2302. (c) Rotthaus, O.; Thomas, F.; Jarjayes, O.;
;
Philouze, C.; Saint-Aman, E.; Pierre, J.-L. Chem. Eur. J. 2006, 12, 6953–6962.
;
(d) Shimazaki, Y.; Yajima, T.; Tani, F.; Karasawa, S.; Fukui, K.; Naruta, Y.;
Yamauchi, O. J. Am. Chem. Soc. 2007, 129, 2559–2568. (e) Benisvy, L.;
Kannappan, R.; Song, Y.-F.; Milikisyants, S.; Huber, M.; Mutikainen, I.;
Turpeinen, U.; Gamez, P.; Bernasconi, L.; Baerends, E. J.; Hartl, F.; Reedijk, J.
Eur. J. Inorg. Chem. 2007, 637–642. (f) Rotthaus, O.; Jarjayes, O.; Perez Del
Valle, C.; Philouze, C.; Thomas, F. Chem. Commun. 2007, 4462–4464. (g) Storr,
T.; Wasinger, E. C.; Pratt, R. C.; Stack, T. D. P. Angew. Chem., Int. Ed. 2007, 46,
5198–5201. (h) Shimazaki, Y.; Stack, T. D. P.; Storr, T. Inorg. Chem. 2009, 48,
8383–8392.
(26) Storr, T.; Verma, P.; Pratt, R. C.; Wasinger, E. C.; Shimazaki, Y.;
Stack, T. D. P. J. Am. Chem. Soc. 2008, 130, 15448–15459.
(24) Ghosh, A.; Gonzalez, E. Isr. J. Chem. 2000, 40, 1–8.

6668 Inorganic Chemistry, Vol. 49, No. 14, 2010
Kurahashi et al.
Figure 4. Mn(salen) for quantum chemical calculations and the optimized
structure for MnIV(salen)(O).
Figure 5. Calculated coordination geometries for MnIV(salen)(O), MnIV
-
(salen)(OH), and MnIII(salen•þ)(OH2).
Scheme 1. MnIV(salen) Species to be Formed under Certain Conditions
Figure 3. (a) XANES spectra of 1 (black line) and the solution of1 þ 1.0
equiv of m-CPBA þ 1.0 equiv of Bu4NOH (red line). (b) Fourier trans-
forms of the EXAFS data for 1 (black line) and the solution of 1 þ 1.0
equiv of m-CPBA þ 1.0 equiv of Bu4NOH (red line). XAS data were
measured for 1 in the solid state at room temperature and for the 20 mM
solution of 1 þ 1.0 equiv of m-CPBA þ 1.0 equiv of Bu4NOH in frozen
propionitrile at 10 K.
mixture could not be formed under the low-concentration
conditions possibly because the bimolecular reaction of 1
with m-CPBA is slow. Moreover, we attempted to care-
fully dilute a 1 mM solution of the MnIV(salen) mixture
by adding CH2Cl2 at 203 K, but this was not successful
because of decomposition.
Table 1. EXAFS Analysis of the Solution of 1 þ 1.0 equiv of m-CPBA þ 1.0 equiv
of Bu4NOHa
shell
Nb
R (A)c
σ2 (A2)d
ΔE0 (eV)e
1
2
3
0.68
2.32
2.00
1.570
1.796
1.954
0.003
0.009
0.003
13.22
4.16
-5.09
Alternatively, MnIV(salen)(O) is prepared by the reac-
tion of 1 with O3, and MnIV(salen)(OH) is prepared by
one-electron oxidation of 1 and a subsequent ligand
exchange with OH, as described previously.17 Both spe-
cies are successfully prepared under conditions (0.05 mM)
that are appropriate for a kinetic study with substituted
phenols at 203 K. As shown in Figure 6, upon the addi-
tion of 2,6-di-tert-butylphenol, the UV-vis spectrum of
MnIV(salen)(O) (depicted as a red line) is changed with
clear isosbestic points, and the decay of absorption at 600 nm
P
P
a R factor, defined as |yexp(i) - ytheo(i)|/ |yexp(i)|, where yexp and
ytheo are experimental and theoretical data points, respectively, equal to
7.67%. The curve-fitting results are shown in Figure S2 (Supporting
Information). b The total coordination number was fixed at 3 for the sum
of the shells 1 and 2. The coordination number was fixed at 2 for shell 3.
c Interatomic distances estimated from the EXAFS curve fitting. d Debye-
Waller factor. e Shift in photoelectron energy zero.
more detail, we attempted to prepare the MnIV(salen) mix-
ture at lower concentration. However, the MnIV(salen)

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6669
Table 2. Selected Optimized Geometrical Parameters and Relative Energies
MnIV(salen)- MnIV(salen)- MnIII(salen•þ)-
(O)
(OH)
(OH2)
Mn-O3/A
1.642
1.882
1.883
2.003
1.997
1.307
1.307
1.295
1.295
1.430
1.431
0.327
161.78
157.97
0
1.797
1.844
1.833
1.970
1.984
1.318
1.326
1.302
1.299
1.425
1.428
0.308
159.65
161.65
-240.0
2.258
1.864
1.868
1.971
1.968
1.315
1.311
1.300
1.303
1.432
1.431
0.129
174.18
166.27
-504.0
Mn-O1/A
Mn-O2/A
Mn-N1/A
Mn-N2/A
O1-C2/A
O2-C20/A
N1-C7/A
N2-C70/A
C1-C7/A
C10-C70/A
Mn-N2O2 plane/A
O1-Mn-N2/deg
O2-Mn-N1/deg
relative
energies/kcal mol-1
Figure 6. UV-vis spectral changes upon reaction of MnIV(salen)(O)
(5.10 ꢁ 10-5 M) with 2,6-di-tert-butylphenol (5.10 ꢁ 10-4 M) in CH2Cl2
at203K. The spectraat0, 30, 90, and 720 safter the additionof2,6-di-tert-
butylphenol are shown. Inset: Decay of the absorption at 600 nm derived
from MnIV(salen)(O) (5.10 ꢁ 10-5 M) in the presence of 2,6-di-tert-
butylphenol (5.10 ꢁ 10-4 M) in CH2Cl2 at 203 K.
Table 3. Gross Orbital Spin Populationsa
MnIV(salen)(O) MnIV(salen)(OH) MnIII(salen•þ)(OH2)
Mn dσ1
Mn dσ2
Mn dπ
0.170
0.100
0.698
0.663
0.907
2.562
0.430
0.008
0.326
0.159
0.834
0.843
0.875
3.087
0.015
-0.102
0.883
0.173
0.770
0.916
0.923
3.859
0.026
-0.885
2,6-di-tert-butylphenol by MnIV(salen)(OH) prior to the
rate-limiting oxidation reactions (Scheme 2).
Under conditions of excess phenols, an observed rate
constant, kobs, is given by eq 1.
Mn dπ
Mn dnb
total of Mn
O3
salen ligand
k1K½phenolꢀ
kobs
¼
ð1Þ
a The dσ1 and dπ orbitals represent σ and π bonding to O3, and the
dσ2 orbital represents σ bonding to the salen ligand. dnb represents a non-
bonding orbital.
1 þ K½phenolꢀ
The simulation curve (red line) in Figure 7 suggests that
rather strong binding of 2,6-di-tert-butylphenol by MnIV-
(salen)(OH) (K ≈ 5000 M-1) would give observed kinetics
for the reaction of MnIV(salen)(OH) with 2,6-di-tert-
butylphenol. We hypothesize that the ability of MnIV-
(salen)(OH) to strongly bind a phenol might be ascribed
to double hydrogen bonding as shown in Scheme 1. Reac-
tions of MnIV(salen)(OH) with other substituted phenols
(2,6-di-tert-butyl-4-methylphenol, 2,4,6-tri-tert-butylphenol,
and 30,50-di-tert-butyl-40-hydroxyacetophenone) are also
carried out (Figures S7-S9 in the Supporting Information),
and the averaged kobs values, which are independent of the
concentration of phenols, are listed in Table 4. A reaction
of MnIV(salen)(OH) with3,5-di-tert-butyl-4-hydroxybenzo-
nitrile is negligibly slow under identical conditions. However,
the use of a large excess of 3,5-di-tert-butyl-4-hydroxy-
benzonitrile [200 equiv relative to MnIV(salen)(OH)] gen-
erates a stable MnIV species with the absorption maxi-
mum shifted from 720 nm to shorter wavelength, which
precludes the precise determination of the kobs value.
Reactions of MnIV(salen)(O) and MnIV(salen)(OH)
with 2,6-di-tert-butylphenol were monitored by perpen-
dicular- and parallel-mode EPR spectroscopy. Upon
reactions of MnIV(salen)(O) and MnIV(salen)(OH) with
2,6-di-tert-butylphenol at 203 K for 2 h, EPR signals from
MnIV species in a perpendicular mode disappear, while
parallel-mode EPR spectroscopy shows regeneration of
the signals at g=8.1 from MnIII species, which are almost
as intense as that from the starting 1 (Figures S10 and
S11 in the Supporting Information). It is thus indicated
that both MnIV(salen)(O) and MnIV(salen)(OH) are re-
duced mainly to MnIII(salen) upon reactions with 2,6-di-
tert-butylphenol. Perpendicular-mode EPR spectroscopy
follows pseudo-first-order kinetics under conditions of
excess phenol. For data fitting, the data points within 30 s
after the addition of 2,6-di-tert-butylphenol were exclu-
ded because the solution may not be well mixed. A reac-
tion of MnIV(salen)(O) with 2,6-di-tert-butylphenol is
very fast, and thus a minimum amount of phenols [10-
40 equiv relative to MnIV(salen)(O)] is utilized. The pseudo-
first-order rate constants (kobs) vary linearly with the con-
centration of 2,6-di-tert-butylphenol (Figure S3 in the
Supporting Information), yielding a second-order rate
constant, k2=11.7 M-1 s-1. In a similar manner, second-
order rate constants for oxidation reactions of MnIV-
(salen)(O) with other phenols (30,50-di-tert-butyl-40-hydroxy-
acetophenone and 3,5-di-tert-butyl-4-hydroxybenzonitrile)
and 1,4-cyclohexadiene were determined (Figures S4-S6
in the Supporting Information), and the data are sum-
marized in Table 4.
Figure 7a shows a UV-vis spectral change of MnIV-
(salen)(OH) in the presence of 2,6-di-tert-butylphenol. As
indicated by the decay of absorption at 720 nm, MnIV-
(salen)(OH) (depicted as a red line) also reacts with 2,6-di-
tert-butylphenol but at much slower rate as compared
with MnIV(salen)(O). The reactions of MnIV(salen)(OH)
with 2,6-di-tert-butylphenol are also carried out under
conditions of a slight excess of 2,6-di-tert-butylphenol
(10-50 equiv). The kinetic trace is comprised of two com-
ponents, and we utilized the data points in the former
region for estimating the kobs values. In clear contrast
to MnIV(salen)(O), the kobs values do not show a linear
correlation with the concentration of 2,6-di-tert-butyl-
phenol (Figure 7b). This might be due to binding of

6670 Inorganic Chemistry, Vol. 49, No. 14, 2010
Kurahashi et al.
Table 4. Rate Constants for Hydrogen-Atom Abstraction by MnIV(salen)(O) and MnIV(salen)(OH) in CH2Cl2 at 203 Ka
MnIV(salen)(O) k2b/M-1 s-1
MnIV(salen)(OH) kobsc/s-1
1,4-cyclohexadiene
2.3 ꢁ 10-2 ( 0.005
≈0e
4-Me-2,6-tert-Bu2C6H2OH
2,4,6-tert-Bu3C6H2OH
2,4-tert-Bu2C6H3OH
2,6-tert-Bu2C6H3OH
4-CH3CO-2,6-tert-Bu2C6H2OH
4-NC-2,6-tert-Bu2C6H2OH
>100d
2.1 ꢁ 10-2 ( 4.0 ꢁ 10-4
6.8 ꢁ 10-3 ( 1.0 ꢁ 10-4
>100d
>100d
>1.0 ꢁ 10-1
d
11.7 ( 2.1 (kH/kD 5.3)
2.1 ( 0.5
1.7 ( 0.2
1.9 ꢁ 10-3 ( 1.1 ꢁ 10-4 (kH/kD = 9.5)
2.3 ꢁ 10-4 ( 1.1 ꢁ 10-4
≈0e
a Reaction conditions: [MnIV(salen)(O)] or [MnIV(salen)(OH)] = 5.10 ꢁ 10-5 M, [substrate] = 5.0 ꢁ 10-4-2.5 ꢁ 10-3 M. b Second-order rate
constants. c Averaged pseudo-first-order rate constants, which are not dependent on the concentration of substituted phenols. d MnIV(salen)(O) or
MnIV(salen)(OH) are reduced within 20 s under the reaction conditions at 203 K, and thus the rate constants could not be determined precisely. e UV-vis
spectral changes of MnIV(salen)(OH) (5.10 ꢁ 10-5 M) in the presence of the substrate (1.0 ꢁ 10-2 M) are negligibly small.
2,4,6-tri-tert-butylphenol as a substrate, a sharp signal at
g = 2.0 was observed, indicative of the formation of a
phenoxyl radical from 2,4,6-tri-tert-butylphenol (Figure S12
in the Supporting Information).
As shown in Table 4, the oxidation of OH bonds in
substituted phenols by MnIV(salen)(O) proceeds orders of
magnitude faster than the oxidation of CH bonds in 1,4-
cyclohexadiene, although BDEOH (OH bond dissociation
enthalpy) values of substituted phenols (81.0-84.2 kcal
mol-1 27
are even higher than the BDECH (CH bond
)
dissociation enthalpy) values of 1,4-cyclohexadiene (76.0
kcal mol-1).28 Such marked variations in rates between
different classes of substrates (O-H vs C-H) of similar
bond strengths have already been addressed in detail in
other systems.29 MnIV(salen)(O) readily reacts with 3,5-
di-tert-butyl-4-hydroxybenzonitrile (BDEOH =84.2 kcal
mol-1), while MnIV(salen)(OH) reacts with 30,50-di-tert-
butyl-40-hydroxyacetophenone (BDEOH=83.1kcalmol-1
)
much more slowly under identical conditions than MnIV-
(salen)(O) anddoesnotreactwith3,5-di-tert-butyl-4-hydroxy-
benzonitrile. It is thus indicated that the thermodynamic
hydrogen-atom-abstracting ability of MnIV(salen)(OH)
is about 83 kcal mol-1, but the thermodynamic hydrogen-
atom-abstracting ability of MnIV(salen)(O) well exceeds
84 kcal mol-1. The MnIVdO unit shows higher reactivity
than the MnIVOH unit in the anionic salen platform, which
is also the case for Busch’s neutral macrocyclic ligand.19
A significant kinetic isotope effect (kH/kD=5.3 and 9.5,
respectively) was observed for reactions of MnIV(salen)-
(O) and MnIV(salen)(OH) with 2,6-di-tert-butylphenol-d
30
(∼90% deuteration) in D2O-preequilibrated CH2Cl2
(Figures S14 and S15 in the Supporting Information).
(27) BDEOH values of substituted phenols are adopted from the following
references. See also ref 23a. (a) Lucarini, M.; Pedrielli, P.; Pedulli, G. F.;
Cabiddu, S.; Fattuoni, C. J. Org. Chem. 1996, 61, 9259–9263. (b) Bordwell,
F. G.; Cheng, J.-P. J. Am. Chem. Soc. 1991, 113, 1736–1743.
(28) BDECH values are adopted from the recommended data in a
compilation by Luo: Luo, Y.-R. Handbook of Bond Dissociation Energies
in Organic Compounds; CRC Press: Boca Raton, FL, 2003.
Figure 7. (a) UV-vis spectral changes upon the reaction of MnIV(salen)-
(OH) (5.10 ꢁ 10-5 M) with 2,6-di-tert-butylphenol (1.49 ꢁ 10-3 M) in
CH2Cl2 at 203 K. The spectra at 0, 240, 660, and 2100 s after the addition of
2,6-di-tert-butylphenol are shown. Inset: Decay of absorption at 720 nm
derived from MnIV(salen)(OH) (5.10 ꢁ 10-5 M) in the presence of 2,6-di-
tert-butylphenol (1.49 ꢁ 10-3 M) in CH2Cl2 at 203 K. (b) Plot of the
pseudo-first-order rate constant (kobs) vs concentration of 2,6-di-tert-butyl-
phenol ([phenol]). Data are designated by points; the simulation curve is
designated by a red line, K = 5000 M-1 and k1 = 2.2 ꢁ 10-3 s-1 (eq 1).
(29) Roth, J. P.; Yoder, J. C.; Won, T.-J.; Mayer, J. M. Science 2001, 294,
2524–2526.
(30) Reactions of MnIV(salen)(O) and MnIV(salen)(OH) with 2,6-di-tert-
butylphenol-d were carried out in a CH2Cl2 solvent that was washed with
D2O and then dried over 4A molecular sieves because the deuterium atom in
2,6-di-tert-butylphenol-d is readily exchanged with the hydrogen atom in
residual H2O in CH2Cl2. To evaluate the loss of the deuterium atom in a
nonpolar chlorinated solvent by 1H NMR, 2,6-di-tert-butylphenol-d was
dissolved in a CDCl3 solvent that is washed with H2O or D2O and then dried
over 4A molecular sieves (Figure S13 in the Supporting Information).
Although the CDCl3 solvent is dry enough to be a clear solution even at
203 K, the degree of deuteration is significantly lower in H2O-preequili-
brated CDCl3 (∼70%) than in D2O-preequilibrated CDCl3 (∼90%).
detected an additional intense signal at g=2.0 in the case
of MnIV(salen)(O) and a weak signal at g=2.0 with a six-
line hyperfine splitting in the case of MnIV(salen)(OH)
after the reaction with 2,6-di-tert-butylphenol. These
additional signals were not characterized. In the case of

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6671
Scheme 2. Possible Reaction Pathway for the Oxidation of Phenols by MnIV(salen)(OH)
The present kH/kD values are larger than the kinetic deu-
terium isotope effects reported for the oxidation of phenols
by dicopper-dioxygen complexes via a proton-coupled
electron-transfer mechanism (kH/kD = 1.2-1.6)31 but is
comparable with the kinetic deuterium isotope effects
reported for the oxidation of toluene and dihydroanthra-
cene by permanganate via a hydrogen-atom-transfer me-
chanism (kH/kD =6 and 3).32,33 It is also shown that both
MnIV(salen)(O) and MnIV(salen)(OH) oxidize 2,6-di-tert-
butylphenol significantly more slowly than 2,4-di-tert-butyl-
phenol, which has similar OH bond strength but has less
steric demand around OH. These results indicate that
the rate-determining step involves O-H bond cleavage in
both cases.
XAS datum of the solution of 1, m-CPBA, and Bu4NOH in
frozen propionitrile was collected at 10 K. The data collections
were carried out in a fluorescence mode with a Lytle detector,
using a Si(111) double-crystal monochromator. A copper foil
was utilized to calibrate the energy. In each data collection, the
first and final XAS data were carefully compared to confirm no
appreciable X-ray damage on the sample. The XAS data were
Fourier-transformed between k=2 and 12 A-1 and processed in
a standard manner by WinXAS software (version 3.1).34 Theo-
retical EXAFS signals were calculated using FEFF (version 8.4).35
Curve fittings were carried out only for atoms that are coordi-
nated to manganese.
Materials. CH2Cl2 was purchased from Kanto as an anhydrous
solvent and was stored in the presence of 4A molecular sieves.
Propionitrile (99%) was purchased from Sigma-Aldrich and was
used as received. m-CPBA was purchased from Nacalai and was
purified by washing with a phosphate buffer. The purity of m-CPBA
was checked with iodometry. The synthesis of 1 was reported
elsewhere.17 2,6-Di-tert-butylphenol, 2,4-di-tert-butylphenol, and
2,4,6-tri-tert-butylphenol were purchased from Tokyo Chemical
Industry and were used as received. 2,6-Di-tert-butyl-4-methyl-
phenol was purchased from Sigma-Aldrich and was used as recei-
ved. 30,50-Di-tert-butyl-40-hydroxyacetophenone and 3,5-di-tert-
butyl-4-hydoxybenzonitrile were purchased from Alfa Aesar and
were used as received. D2O (99.8% D) was purchased from Acros
and was purified by distillation under an argon atmosphere before
use. 2,6-Di-tert-butylphenol-d was prepared by evaporation of
a solution of 2,6-di-tert-butylphenol (1 g) dissolved in CH3CN
(9 mL) and D2O (1 mL), which was repeated three times. 1H NMR
shows that the degree of deuteration, which is quite sensitive to resi-
dual H2O in a nonpolar chlorinated solvent, is ∼90% in a CDCl3
solvent, which was washed with D2O and then dried over 4A mole-
cular sieves. The k2 and kobs values upon reaction of MnIV(salen)-
(O) andMnIV(salen)(OH) with2,6-di-tert-butylphenol-dwere thus
obtained in a CH2Cl2 solvent that was similarly preequilibrated
with D2O. Tris(2,4-dibromophenyl)aminium hexachloroantimo-
nate was prepared according to the reported method36 and was
assayed with titration by ferrocene. Caution! The perchlorate salts
used in this study are potentially explosive and should be handled in a
small amount with great care.
Conclusion
We herein investigate a reaction of MnIII(salen) withm-CPBA
under low-temperature conditions, which is shown to generate
MnIV(salen)(O) as a major product and MnIV(salen)(OH) as
a minor product, instead of MnV(salen)(O). MnIV(salen)(O)
shows a distinctively high hydrogen-atom-abstracting ability,
as compared to MnIV(salen)(OH). This might be indicative of
an important role of MnIV(salen)(O) in catalytic oxidation
reactions under low-temperature conditions.
Experimental Section
Instrumentation. UV-vis spectra were recorded in a quartz
cell (l=0.1 or 1 cm) on an Agilent 8453 (Agilent Technologies)
equipped with an USP-203 low-temperature chamber (UNISOKU).
EPR spectra were recorded in a quartz cell (d=4 mm) at 5 K on
an EMX Plus continuous-wave X-band spectrometer (Bruker)
with an ESR910 helium-flow cryostat (Oxford Instruments) and
a dual-mode cavity (Bruker). Differential pulse voltammograms
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. Measurements were carried out for the 1 mM solution
in anhydrous CH2Cl2 containing 0.1 M Bu4NClO4 at a scan rate
of 50 mV s-1 at 203 K. The E values were referenced to that of
ferrocene, which was measured under identical conditions. O3
gas was prepared by UV irradiation to the O2 gas, using a PR-
1300 UV ozone generator (ClearWater Tech).
Preparation of the Sample Solution for Characterization. A
solution of 1.0 equiv of m-CPBA was added to a solution of 1 at
203 K. The resulting solution was kept for 5 min at 203 K. Then,
a solution of 1.0 equiv of NMO or Bu4NOH was slowly added.
After 5 min at 203 K, the solution was subjected to physico-
chemical measurements. The experimental conditions are 0.5 mM
in CH2Cl2 for UV-vis in a quartz cell (l = 0.1 cm), 1.0 mM in
CH2Cl2 containing 0.1 M Bu4NClO4 for EPR, and 20 mM in
propionitrile for XAS.
XAS Measurements. Mn K-edge XAS data were obtained at
the SPring-8, beamline BL01B1, under ring conditions of 8 GeV
and ca. 100 mA (Proposal No. 2007A1090). The XAS datum of
1 in the solid state was collected at room temperature, and the
Reactions of MnIV(salen)(O) and MnIV(salen)(OH) with Sub-
stituted Phenols. MnIV(salen)(O) was prepared by passing a
stream of O3 and O2 (ca. 10 mL) through the CH2Cl2 solution
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6672 Inorganic Chemistry, Vol. 49, No. 14, 2010
Kurahashi et al.
of 1 (51 μM, 2.05 mL) in a quartz cell (l=1 cm), and then excess
The 6-311G(d) basis set was utilized for manganese, nitrogen,
and oxygen atoms. The 6-31G(d) and STO-3G basis sets were
utilized for the salen ligand and xylyl substituents, respectively.
The optimized structures were calculated using B3LYP density
functional methods.
O3 gas was purged by passing Ar gas (10 mL) at 203 K. MnIV
-
(salen)(OH) was prepared by the addition of 1.0 equiv of tris(2,4-
dibromophenyl)aminium hexachloroantimonate (0.105 μmol) in
CH2Cl2 (20 μL) and then 1.0 equiv of Bu4NOH (0.105 μmol) in
CH2Cl2 (20 μL) to a CH2Cl2 solution of 1 (51 μM, 2.05 mL) in a
quartz cell (l = 1 cm) at 203 K. After the formation of MnIV
-
Acknowledgment. We thank Dr. Hajime Tanida (JASRI/
SPring-8) for assistance in the measurement of X-ray absorption
spectra. We thank Prof. James M. Mayer for helpful comments.
This work was supported by grants from the Japan Science and
Technology Agency, CREST, and the Japan Science Promotion
Society, the Global COE program.
(salen)(O) or MnIV(salen)(OH) was confirmed by UV-vis,
substituted phenols in CH2Cl2 (20 μL) were added. The resulting
solution was stirred at 203 K, and UV-vis spectral changes were
monitored.
Quantum Chemical Calculations. Quantum chemical calcula-
tions were performed using the Gaussian03 program package.37
Supporting Information Available: Figures S1-S15 and com-
plete ref 37. This material is available free of charge via the
Internet at http://pubs.acs.org.
(37) Frisch, M. J. Gaussian03, revision C.03; Gaussian, Inc.: Wallingford,
CT, 2004.