Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

Article abstract of DOI:10.1021/acs.inorgchem.5b02019

UV-vis spectral titrations of a manganese(III) corrolazine complex [Mn^III(TBP₈Cz)] with HOTf in benzonitrile (PhCN) indicate mono- and diprotonation of Mn^III(TBP₈Cz) to give Mn^III(OTf)(TBP₈Cz(H)) and [Mn^III(OTf)(H₂O)(TBP₈Cz(H)₂)][OTf] with protonation constants of 9.0 × 10⁶ and 4.7 × 10³ M^-1, respectively. The protonated sites of Mn^III(OTf)(TBP₈Cz(H)) and [Mn^III(OTf)(H₂O)(TBP₈Cz(H)₂)][OTf] were identified by X-ray crystal structures of the mono- and diprotonated complexes. In the presence of HOTf, the monoprotonated manganese(III) corrolazine complex [Mn^III(OTf)(TBP₈Cz(H))] acts as an efficient photocatalytic catalyst for the oxidation of hexamethylbenzene and thioanisole by O₂ to the corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of Mn^III(OTf)(TBP₈Cz(H)) and [Mn^III(OTf)(H₂O)(TBP₈Cz(H)₂)][OTf] in the presence of O₂ revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of Mn^III(OTf)(TBP₈Cz(H)) reacted with O₂ with a diffusion-limited rate constant to produce the putative Mn^IV(O₂^?-)(OTf)(TBP₈Cz(H)), whereas the tripseptet excited state of [Mn^III(OTf)(H₂O)(TBP₈Cz(H)₂)][OTf] exhibited no reactivity toward O₂. In the presence of HOTf, Mn^V(O)(TBP₈Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols, accompanied by regeneration of Mn^III(OTf)(TBP₈Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and essentially no KIE (1.1) was observed for the oxidation of mesitylene-d₁₂, suggesting a proton-coupled electron transfer (PCET) mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, Mn^III(OTf)(TBP₈Cz(H)), acts as an efficient photocatalyst for the oxidation of HMB by O₂ to the alcohol.

Full text of DOI:10.1021/acs.inorgchem.5b02019

Article
pubs.acs.org/IC
Photocatalytic Oxygenation of Substrates by Dioxygen with
Protonated Manganese(III) Corrolazine
Jieun Jung,† Heather M. Neu,‡ Pannee Leeladee,‡ Maxime A. Siegler,‡ Kei Ohkubo,†,§
David P. Goldberg,*,‡ and Shunichi Fukuzumi*,†,§,∥
†Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea
‡Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
§Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA and SENTAN, Japan Science
and Technology Agency (JST), Suita, Osaka 565-0871, Japan
∥Faculty of Science and Engineering, Meijo University, ALCA and SENTAN, Japan Science and Technology Agency (JST), Nagoya,
Aichi 468-0073, Japan
S
* Supporting Information
ABSTRACT: UV−vis spectral titrations of a manganese(III)
corrolazine complex [MnIII(TBP8Cz)] with HOTf in benzoni-
trile (PhCN) indicate mono- and diprotonation of
MnIII(TBP8Cz) to give MnIII(OTf)(TBP8Cz(H)) and
[MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf] with protonation
constants of 9.0 × 106 and 4.7 × 103 M−1, respectively. The
protonated sites of MnIII(OTf)(TBP8Cz(H)) and
[MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf] were identified by
X-ray crystal structures of the mono- and diprotonated
complexes. In the presence of HOTf, the monoprotonated
manganese(III) corrolazine complex [MnIII(OTf)(TBP8Cz-
(H))] acts as an efficient photocatalytic catalyst for the
oxidation of hexamethylbenzene and thioanisole by O2 to the
corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of
MnIII(OTf)(TBP8Cz(H)) and [MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf] in the presence of O2 revealed the formation of a
tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of
MnIII(OTf)(TBP8Cz(H)) reacted with O2 with a diffusion-limited rate constant to produce the putative MnIV(O2•−)(OTf)-
(TBP8Cz(H)), whereas the tripseptet excited state of [MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf] exhibited no reactivity toward
O2. In the presence of HOTf, MnV(O)(TBP8Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols,
accompanied by regeneration of MnIII(OTf)(TBP8Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and
essentially no KIE (1.1) was observed for the oxidation of mesitylene-d12, suggesting a proton-coupled electron transfer (PCET)
mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, MnIII(OTf)(TBP8Cz(H)),
acts as an efficient photocatalyst for the oxidation of HMB by O2 to the alcohol.
sively.17−23 High-valent manganese−oxo complexes have
INTRODUCTION
■
attracted special attention because they are postulated as
Oxidation reactions are essential in the metabolism of organic
substrates, and many enzymes are known to catalyze these
reactions in biological systems. High-valent metal−oxo
complexes have been implicated as important reactive
important intermediates for water oxidation in the oxygen-
evolving center (OEC) of photosystem II.24−32
We have shown that a well-characterized manganese(V)−oxo
complex can be prepared by visible light irradiation of a
intermediates in these reactions for heme and nonheme iron
manganese(III) corrolazine [MnIII(TBP8Cz): TBP8Cz =
octakis(p-tert-butylphenyl)corrolazinato3−] in the presence of
toluene derivatives with dioxygen.15,16 Hexamethylbenzene
(HMB) was shown to be oxidized to pentamethylbenzyl
alcohol during this reaction, serving as a proton/electron source
to assist with O2 activation. However, the produced MnV(O)-
enzymes.1−14 To help clarify related oxidation mechanisms and
to control the reactivity, many synthetic high-valent metal−oxo
complexes have been prepared using oxidants such as
iodosylarenes, peroxy acids, and hydrogen peroxide.3−14
Dioxygen (O2) is an ideal reagent for the production of these
metal−oxo complexes and for the subsequent oxygenation of
substrates because of its abundant availability and non-
toxicity.15,16 The mechanisms of oxidation of substrates by
high-valent metal−oxo complexes have been studied exten-
Received: August 31, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(TBP8Cz) was unreactive toward HMB, which prevented the
regeneration of the MnIII starting complex and removed the
possibility for catalytic turnover. Only in the case of the O atom
acceptor PPh3, or 9,10-dihydro-1-methylacridine, which has a
very weak C−H bond, was photocatalysis observed.16,33 The
reactivity of high-valent metal−oxo complexes to oxidize
substrates has been reported to be much enhanced by the
presence of acids,34−42 suggesting that the addition of an H+
source could activate the MnV(O) complex and possibly lead to
catalytic turnover.
We showed that addition of a strong proton source
[H(OEt2)2]+[B(C6F5)4]− (H+[B(C6F5)4]−) allowed for the
photocatalytic oxidation of HMB by MnIII(TBP8Cz) with O2 as
oxidant.43 The addition of 1 and 2 equiv of H+[B(C6F5)4]− to
MnIII(TBP8Cz) formed two new species that were charac-
terized by X-ray diffraction (XRD) as the monoprotonated
[MnIII(H2O)(TBP8Cz(H))][B(C6F5)4] and the diprotonated
[MnIII(H2O)(TBP8Cz(H)2)][B(C6F5)4]2, respectively. The
monoprotonated [MnIII(H2O)(TBP8Cz(H))][B(C6F5)4] is
protonated at the meso-N atom adjacent to the direct
pyrrole−pyrrole bond of the corrolazine ligand. For the
diprotonated [MnIII(H2O)(TBP8Cz(H)2)][B(C6F5)4]2 com-
plex, both H atoms were found to be located on the opposite
meso-N atoms adjacent to the direct pyrrole−pyrrole bond. The
monoprotonated MnIII complex was catalytically active with a
turnover number (TON) of 18 for pentamethylbenzyl alcohol
and 9 for pentamethylbenzaldehyde. Interestingly, the diproto-
nated [MnIII(H2O)(TBP8Cz(H)2)]2+ complex showed no
catalytic activity, indicating that catalytic turnover was highly
dependent on the level of protonation.
In this work we present several new findings on the proton-
assisted, photoactivated oxygenation chemistry catalyzed by
manganese corrolazine. We demonstrate that substitution of
the H+[B(C6F5)4]− proton donor with triflic acid (HOTf) leads
to dramatic changes in catalytic activity and generates new
protonated Mn(III) corrolazine complexes, MnIII(OTf)-
(TBP8Cz(H)) and [MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf],
which were characterized by X-ray diffraction. The crystal
structures reveal that protonation occurs on the meso-N atoms
as seen for H+[B(C6F5)4]−, but significant changes in axial
ligation state are observed with HOTf, including coordination
of the anionic triflate and formation of both 5- and 6-coordinate
species. Coordination of OTf− provides a possible mechanism
for enhancing catalytic reactivity through a proposed MnV(O)-
(TBP8Cz) intermediate.44 In our previous report on proton-
assisted catalysis, we suggested a mechanism that involved a
short-lived tripseptet excited state of the monoprotonated MnIII
complex as a key intermediate. This mechanism was based on
prior photochemical characterization of the parent MnIII
complex. However, direct spectroscopic evidence for the
proposed photochemical excited states of the protonated
MnIII species was absent. Herein, we present femtosecond
laser flash photolysis measurements on both the monoproto-
nated MnIII(OTf)(TBP8Cz(H)) and the diprotonated
[MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf], which reveal shorter
lived excited states (7T1) in the presence of O2 that provide
direct evidence for the proposed mechanism of photocatalytic
oxygenation. We also demonstrate new catalytic activity for the
monoprotonated complex. This complex was shown to be an
excellent catalyst for the selective S-oxygenation of thioanisole
with only O2 and light as reagents.
EXPERIMENTAL SECTION
■
Materials. The starting material MnIII(TBP8Cz) was synthesized
according to published procedures.45 The commercially available
reagents (hexamethylbenzene and trifluoromethane sulfonic acid)
were purchased with the best available purity and used without further
purification. Benzonitrile (PhCN) was purchased with the best
available purity from Wako Pure Chemical Industries, Ltd., dried
according to literature procedures,46 and distilled under Ar prior to
use.
Spectral and Kinetic Measurements. The formation of
protonated species of MnIII(TBP8Cz) and MnV(O)(TBP8Cz) was
examined from the change in the UV−vis spectra of MnIII(TBP8Cz)
(λmax = 695 nm, εmax = 3.5 × 104 M−1 cm−1) and MnV(O)(TBP8Cz)
(λmax = 634 nm, εmax = 2.0 × 104 M−1 cm−1)16 by spectral titration
with HOTf at 298 K using a Hewlett-Packard HP8453 diode array
spectrophotometer. The first and second binding constants of HOTf
with MnIII(TBP8Cz) were determined in PhCN from the UV−vis
spectral change due to the formation of the protonated species
MnIII(OTf)(TBP8Cz(H)) (λmax = 725 nm) and [MnIII(OTf)(H2O)-
(TBP8Cz(H)2)][OTf] (λmax = 745 nm). The first binding constant of
HOTf with MnV(O)(TBP8Cz) was determined in PhCN from the
UV−vis spectral change due to the formation of the protonated
species MnIV(OH)(OTf)(TBP8Cz•+) (λmax = 795 nm).
Crystallization of MnIII(OTf)(TBP8Cz(H)). To a solution of
MnIII(TBP8Cz) (20 mg, 14 μmol) in CH2Cl2 (1.0 mL) was added
Sc(OTf)3 (70 mg, 1.4 × 10−4 mol, 10 equiv) dissolved in CH3CN (0.4
mL). A color change from brown to reddish-brown was observed. The
solution was then filtered through Celite and eluted with CH2Cl2
(∼1.0 mL). X-ray quality crystals were obtained by slow evaporation of
this solution for about 4 weeks. Characterization by elemental analysis
indicated the monoprotonated MnIII(OTf)(TBP8Cz(H))·CH2Cl2;
calcd for C98H107Cl2F3MnN7O3S: C, 71.52; H, 6.55; N, 5.96.
Found: C, 71.65; H, 6.53; N, 6.01.
Crystallization of [MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf]. To a
solution of MnIII(TBP8Cz) (1.5 mg, 1.1 μmol) in CH2Cl2 (0.5 mL)
was added 2 equiv of HOTf. A color change from brown to red was
observed. The solution was transferred to a NMR tube and layered
with n-heptane. X-ray quality crystals were obtained after 1 month.
Elemental analysis was performed for [MnIII(OTf)(H2O)(TBP8Cz-
(H)2)][OTf]·CH2Cl2·C7H16; calcd for C106H126Cl2F6MnN7O7S2: C,
66.51; H, 6.64; N, 5.12. Found: C, 66.28; H, 6.32; N, 5.56.
X-ray Crystallography. MnIII(OTf)(TBP8Cz(H)). All reflection
intensities were measured at 110(2) K using a KM4/Xcalibur
(detector: Sapphire3) with enhance graphite-monochromated Mo
Kα radiation (λ = 0.71073 Å) under the program CrysAlisPro
(Version 1.171.35.11 Oxford Diffraction Ltd., 2011). The same
program was used to refine the cell dimensions and for data reduction.
The structure was solved with the program SHELXS-97 and refined on
F2 with SHELXL-97.47 Analytical numeric absorption corrections
based on a multifaceted crystal model were applied using CrysAlisPro
(Version 1.171.35.11, Oxford Diffraction Ltd., 2011). The temperature
of the data collection was controlled using the system Cryojet
(manufactured by Oxford Instruments). The H atoms (except when
specified) were placed at calculated positions using the instructions
AFIX 43 or AFIX 137 with isotropic displacement parameters having
values 1.2 or 1.5 times Ueq of the attached C atoms. The H atom
attached to N5 was located from difference Fourier maps, and its
atomic coordinates were refined freely. Seven of the eight 4-tert-
butylphenyl groups are either wholly disordered (over two
orientations) or partially disordered around the tert-butyl groups
(over two orientations). The coordinated counterion is also disordered
over two orientations.
MnIII(OTf)(TBP8Cz(H)). Fw = 1560.88, black plate, 0.58 × 0.33 ×
0.06 mm3, monoclinic, P21/n (no. 14), a = 19.9174(5) Å, b =
21.8922(4) Å, c = 20.3238(5) Å, β = 110.584(3)°, V = 8296.1(3) Å3, Z
= 4, Dx = 1.250 g cm−3, μ = 0.247 mm−1, abs. corr. range 0.923−0.987.
There were 45 329 reflections measured up to a resolution of (sin θ/
λ)max = 0.59 Å−1. There were 14 603 reflections unique (Rint = 0.0440),
of which 10 025 were observed [I > 2σ(I)]. There was 1482
B
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
parameters refined using 1634 restraints. R1/wR2 [I > 2σ(I)]: 0.0624/
0.1552. R1/wR2 [all reflns]: 0.0977/0.1734. S = 1.041. Residual
electron density found between −0.53 and 0.85 e Å−3.
tometer. Under the conditions of actinometry experiments, the
actinometer and MnIII(TBP8Cz) absorbed essentially all of the
incident light at λ = 450 nm. The light intensity of monochromatized
light at λ = 450 nm was determined to be 3.7 × 10−8 einstein s−1. The
quantum yields were determined by the amount of generated a
product.
[MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf]. All reflection intensities were
measured at 110(2) K using a SuperNova diffractometer (equipped
with Atlas detector) with Cu Kα radiation (λ = 1.54178 Å) under the
program CrysAlisPro (Version 1.171.36.32 Agilent Technologies,
2013). The same program was used to refine the cell dimensions and
for data reduction. The structure was solved and refined on F2 with
SHELXL-2013.47 Analytical numeric absorption correction based on a
multifaceted crystal model was applied using CrysAlisPro. The
temperature of the data collection was controlled using the system
Cryojet (manufactured by Oxford Instruments). The H atoms were
placed at calculated positions (unless otherwise specified) using the
instructions AFIX 23, AFIX 43, or AFIX 137 with isotropic
displacement parameters having values 1.2 times Ueq of the attached
C or N atoms. The H atoms attached to O1W and O2W/O2W′ were
found from difference Fourier maps, and their coordinates were
refined freely using the DFIX restraints. The tert-butyl group C53 →
C56, the tert-butylphenyl group C77 → C86, and the hydronium
cation are found to be disordered over two orientations, and the
occupancy factors of the major components of the disorder refine to
0.50(3), 0.588(6), and 0.634(4), respectively. The crystal lattice
contains some disordered solvent molecules (DCM and heptane). All
orientations of the three nonfully occupied DCM molecules were
successfully modeled, and the asymmetric unit contains ca. 2.53 DCM
molecules per Mn complex. The heptane molecule was found to be
disordered, and its contribution has been taken out in the final
refinement.48
Femtosecond Laser Flash Photolysis Measurements. Meas-
urements of transient absorption spectra of MnIII(OTf)(TBP8Cz(H))
and [MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf] were performed ac-
cording to the following procedures. An N2- or O2-saturated PhCN
solution containing MnIII(TBP8Cz) (8.4 × 10−5 M) and HOTf (1.7 ×
10−4 or 1.2 × 10−2 M) was excited using an ultrafast source, Integra-C
(Quantronix Corp.), an optical parametric amplifier, TOPAS (Light
Conversion Ltd.), and a commercially available optical detection
system, Helios provided by Ultrafast Systems LLC. The source for the
pump and probe pulses was derived from the fundamental output of
Integra-C (λ = 786 nm, 2 mJ/pulse and fwhm = 130 fs) at a repetition
rate of 1 kHz. Seventy-five percent of the fundamental output of the
laser was introduced into a second-harmonic generation (SHG) unit:
Apollo (Ultrafast Systems) for excitation light generation at λ = 393
nm, while the rest of the output was used for white light generation.
The laser pulse was focused on a sapphire plate of 3 mm thickness, and
then white light continuum covering the visible region from λ = 410 to
800 nm was generated via self-phase modulation. A variable neutral
density filter, an optical aperture, and a pair of polarizers were inserted
in the path in order to generate stable white light continuum. Prior to
generating the probe continuum, the laser pulse was fed to a delay line
that provides an experimental time window of 3.2 ns with a maximum
step resolution of 7 fs. In our experiments, a wavelength at λ = 393 nm
of SHG output was irradiated at the sample cell with a spot size of 1
mm diameter, where it was merged with the white probe pulse in a
close angle (<10°). The probe beam after passing through the 2 mm
sample cell was focused on a fiber optic cable that was connected to a
CMOS spectrograph for recording the time-resolved spectra (λ =
410−800 nm). Typically, 1500 excitation pulses were averaged for 3 s
to obtain the transient spectrum at a set delay time. Kinetic traces at
appropriate wavelengths were assembled from the time-resolved
spectral data. The decay rate of the tripquintet (5T1) obeyed the first-
order kinetics given by eq 1
[MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf]. Fw = 2111.86, dark brown-
black block, 0.35 × 0.30 × 0.21 mm3, triclinic, P-1 (no. 2), a =
18.3393(4) Å, b = 18.9149(3) Å, c = 21.2275(3) Å, α = 91.1721(13)°,
β = 114.7271(17)°, γ = 118.8374(19)°, V = 5625.8(2) Å3, Z = 2, Dx =
1.247 g cm−3, μ = 3.176 mm−1, Tmin−Tmax = 0.445−0.619. There were
72 524 reflections measured up to a resolution of (sin θ/λ)max = 0.62
Å−1. There were 22 105 reflections unique (Rint = 0.0242), of which
20 831 were observed [I > 2σ(I)]. There was 1549 parameters refined
using 987 restraints. R1/wR2 [I > 2σ(I)]: 0.0500/0.1381. R1/wR2 [all
reflns]: 0.0524/0.1403. S = 1.036. Residual electron density found
between −0.57 and 1.25 e Å−3.
Photocatalytic Reactivity. The catalytic reactivity of
MnIII(TBP8Cz) (1.0 × 10−5 M) was examined under a large excess
of a substrate (hexamethylbenzene or thioanisole 0−0.3 M) in the
presence of HOTf (0−1.0 × 10−4 M) in PhCN (2.0 mL). This
solution was transferred to a glass cuvette equipped with a stir bar. The
generated products of the reaction were analyzed by GC-MS. An
aliquot was taken from the product solution and injected directly into
the GC-MS for analysis. All peaks of interest were identified by
comparison of retention times and coinjection with authentic samples.
Mass spectra were recorded on a JEOL JMS-700T Tandem MS
station, and the GC-MS analyses were carried out by using a Shimadzu
GCMS-QP2000 gas chromatograph mass spectrometer. GC-MS
conditions in these experiments were performed as follows: an initial
oven temperature of 60 °C was held for 1 min and then raised 30 °C
min−1 for 7.3 min until a temperature of 250 °C was reached, which
was then held for further 10 min. The products pentamethylbenzyl
alcohol and pentamethylbenzaldehyde were identified by comparison
with a standard sample. The product yields were quantified by
comparison against a known amount of detected products using a
calibration curve consisting of a plot of product mole versus area.
Calibration curves were prepared by using concentrations in the same
range as that observed in the actual reaction mixtures.
ΔAbs = A1 exp(−k1t) + A2
(1)
where A1 and A2 are pre-exponential factors for the absorbance
changes and the final absorbance and k1 is the rate constant of the
decay of the tripquintet (5T1) after irradiation. The slower decay rate
of the tripseptet (7T1) also obeyed the first-order kinetics given by eq
2, where A3 is the final absorbance of 7T1 and k2 is the rate constant of
the decay of 7T1. All measurements were conducted at room
temperature, 298 K.
ΔAbs = A1 exp(−k1t) + A2 exp(−k2t) + A3
(2)
RESULTS AND DISCUSSION
■
Protonation of MnIII(TBP8Cz). Protonation of
MnIII(TBP8Cz) was examined by addition of HOTf to a
PhCN solution of MnIII(TBP8Cz) (1.0 × 10−5 M). The
formation of a new species with a Q band at λmax = 725 nm was
monitored by UV−vis as shown in Figure 1a, where HOTf was
added up to 1 equiv of MnIII(TBP8Cz). A color change from
brown to reddish-brown was observed. This spectral change
was similar to that seen for the addition of 1 equiv of
H+[B(C6F5)4]− and indicated the formation of the monop-
rotonated complex, MnIII(OTf)(TBP8Cz(H)) (1). Addition of
excess HOTf (from 1.0 × 10−5 to 1.0 × 10−2 M) resulted in a
change in the Q-band region to give λmax = 745 nm with
isosbestic points as shown in Figure 2. These data were
consistent with the formation of the diprotonated [MnIII(OTf)-
Quantum Yield Determination. A standard actinometer
(potassium ferrioxalate)49 was used for the quantum yield determi-
nation of photocatalytic oxidation of HMB by O2 with MnIII(TBP8Cz)
in O2-saturated PhCN. Typically, a square quartz cuvette (10 mm i.d.),
which contained an O2-saturated PhCN solution (2.0 mL) of
MnIII(TBP8Cz) (1.0 × 10−5 M) and HMB (0.3 M) in the presence
of HOTf (4.0 × 10−5 M), was irradiated with monochromatic light of
λ = 450 nm from a Shimadzu RF-5300PC fluorescence spectropho-
C
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
MnIII(TBPCz) + H+ + OTf−
8
K1
⇌ MnIII(OTf)(TBPCz(H))
(3)
8
This analysis gave K1 = 9.0 × 106 M−1, indicating strong
binding of H+ to MnIII(TBP8Cz). The second equilibrium
constant (K2) for the protonation equilibrium in eq 4 was
determined to be 4.7 × 103 M−1 by fitting a plot of reciprocal
absorbance difference vs 1/[HOTf] from the titration curve in
Figure 2b (see Figure S1 in the Supporting Information (SI))
MnIII(OTf)(TBPCz(H)) + H+ + OTf−
8
K2
⇌ [MnIII(OTf)(H2O)(TBPCz(H)2)][OTf]
(4)
8
The K1 value for the first protonation to give 1 is much larger
than the K2 value for the second species 2.
The protonated complexes 1 and 2 were successfully
crystallized and characterized by single-crystal X-ray diffraction
(XRD) (Figure 3). The monoprotonated complex was
originally crystallized from reaction of MnIII(TBP8Cz) with
10 equiv of Sc(OTf)3 in CH2Cl2/CH3CN. The strongly Lewis
acidic Sc3+ ion presumably reacts with exogenous H2O to
generate the proton source, and slow evaporation produced X-
ray quality crystals of 1 after 4 weeks. The crystal structure
revealed that monoprotonation occurs at one of the meso-N
atoms (N5(H)) adjacent to the direct pyrrole−pyrrole bond of
the corrolazine ligand and that the manganese center is five
coordinate with an axially ligated triflate (MnIII−O = 2.115(3)
Å). The same crystals were subsequently obtained from
reaction of MnIII(TBP8Cz) with 1 equiv of HOTf and
recrystallization from CH2Cl2/heptane. A unit cell measure-
ment matched that for 1.
Figure 1. (a) UV−vis spectral changes and (b) plot for determination
of the binding constant of the conversion from MnIII(TBP8Cz) (black
line, 1.0 × 10−5 M) to MnIII(OTf)(TBP8Cz(H)) (blue line) by
spectroscopic titration of HOTf (0−2.0 × 10−5 M) monitored at 695
(MnIII(TBP8Cz), black dots) and 725 nm (MnIII(OTf)(TBP8Cz(H)),
blue dots) in a PhCN solution at room temperature.
Dissolution of crystalline 1 in PhCN yields the same UV−vis
spectrum as seen for the in situ protonation of MnIII(TBP8Cz)
by the addition of 1 equiv of HOTf (λmax = 451, 725 nm)
(Figure S2). Interestingly, dissolution of crystalline 1 in CH2Cl2
(Figure S3) gives a UV−vis spectrum with λmax = 443 and 725
nm, similar to what was seen for [MnIII(H2O)(TBP8Cz(H))]-
[B(C6F5)4] (λmax = 446 and 730 nm in CH2Cl2),43 although the
Q band is shifted by 5 nm. A similar red shift in the Q band
occurs upon axial ligation of anionic donors to MnIII(H2O)-
(TBP8Cz) to give [MnIII(X)(TBP8Cz)]− (e.g., X = CN−, F−).44
X-ray quality crystals of the diprotonated complex 2 were
obtained from the reaction of MnIII(TBP8Cz) with 2 equiv of
HOTf and recrystallization from CH2Cl2/heptanes (Figure 3).
Both H atoms were unambiguously located on the opposite
meso-N atoms (N1 and N5). In contrast to the monoproto-
nated complex, the diprotonated complex has a six-coordinate
manganese center with an axially ligated water at 2.144(2) Å
and a weakly bound triflate at 2.702(2) Å.
Dissolution of crystalline 2 in CH2Cl2 yielded a spectrum
with λmax at 454 and 758 nm (Figure S4). Comparing this
spectrum to that seen for the diprotonated [MnIII(H2O)-
(TBP8Cz(H)2)][B(C6F5)4]2 (λmax = 470 and 763 nm)43 in the
same solvent again reveals a red shift of 5 nm in the Q band,
matching the differences seen for the two monoprotonated
spectra. These results indicate that the triflate (OTf−) anion
stays bound in solution for both the mono- and the
diprotonated complexes. When crystalline 2 was dissolved in
PhCN, the UV−vis spectrum matched that for the monop-
rotonated complex, indicating that this solvent is basic enough
to deprotonate the second meso-N atom.
Figure 2. (a) UV−vis spectral changes and (b) plot for determination
of the binding constant of the conversion from MnIII(OTf)(TBP8Cz-
(H)) (blue line, 1.0 × 10−5 M) to [MnIII(OTf)(H2O)(TBP8Cz-
(H)2)][OTf] (red line) by spectroscopic titration of HOTf (from 1.0
× 10−5 to 4.0 × 10−3 M) monitored at 725 (MnIII(OTf)(TBP8Cz-
(H)), blue dots) and 745 nm ([MnIII(OTf)(H2O)(TBP8Cz(H)2)]-
[OTf], red dots) in a PhCN solution at room temperature.
(H2O)(TBP8Cz(H)2)][OTf] (2) and again similar to the
spectral changes observed for H+[B(C6F5)4]−.
The monoprotonation equilibrium constant (K1) for 1 was
determined by fitting of the titration curves in Figure 1b,
according to the equilibrium expression in eq 3
D
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Displacement ellipsoid plots (50% probability level) and chemical structures for (a) [MnIII(OTf)(TBP8Cz(H))] and (b)
[MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf] at 110(2) K.
Photocatalytic Oxygenations with O2 by MnIII(TBP8Cz)
in the Presence of HOTf. Previously it was shown that the
addition of H+[B(C6F5)4]− to MnIII(TBP8Cz) allowed for
photoactivated catalysis of HMB under ambient conditions but
only with modest turnover numbers and selectivity.43
Replacement of H+[B(C6F5)4]− with the strong acid HOTf
led to the photocatalytic oxygenation of HMB by O2 and
MnIII(TBP8Cz) in PhCN.
The addition of HOTf (2 equiv) to MnIII(TBP8Cz) in PhCN
gave a spectrum (λmax = 451 and 725 nm) indicative of 1.
Addition of HMB followed by photoirradiation initiated the
catalytic reaction, which occurred over 6 h with slow bleaching
of the solution (Figure S5 in SI). Regarding the bleaching of the
catalyst, we followed the reaction by UV−vis, which showed
only the slow loss of the ground state complex. The catalytically
active excited state was only present to a small extent at any
time, and therefore, the slow decomposition of the ground state
(which is in large excess) did not change the effective
concentration of the catalytically active excited state to a large
extent. Analysis of the reaction mixture by removal of aliquots
and analysis by GC showed a steady increase in the amount of
oxidized products produced over time. Thus, there is an
optimized concentration of HOTf for the photocatalytic
oxygenation of HMB by O2 with 1. The reason why the
photocatalytic oxygenation is prohibited in the presence of
large concentrations of HOTf is discussed in relation with the
Scheme 1. Catalytic Oxygenation of Hexamethylbenzene by
O2 with MnIII(TBP8Cz) and HOTf under Photoirradiation
Conditions
Figure 4. (a) Plots of the amount of produced pentamethylbenzyl alcohol for photocatalytic oxygenation of HMB (0−4.0 × 10−2 M) by
MnIII(TBP8Cz) (1.0 × 10−5 M) in the presence of HOTf (4.0 × 10−5 M) in an O2-saturated PhCN solution. (b) Plots of the amount of produced
pentamethylbenzyl alcohol for photocatalytic oxygenation of HMB (2.0 × 10−2 M) by MnIII(TBP8Cz) (1.0 × 10−5 M) in the presence of HOTf (4.0
× 10−5 M) in a PhCN solution vs the oxygen concentration (0−8.5 × 10−3 M) under photoirradiation (white light) for 1 h at room temperature.
E
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
photocatalytic reaction mechanism (vide infra). The production
of oxidized products seemed only limited by the catalyst
stability. The final turnover number (TON) was 563 for the
alcohol PMB−OH and 9 for the aldehyde PMB−CHO (Figure
S6 in SI) with a quantum yield of 0.125%. The catalytic
turnover is dramatically increased by HOTf in comparison to
H+[B(C6F5)4]−, which gave only TON(PMB−OH) = 18 and
TON(PMB−CHO) = 9.43 In addition, the HOTf reaction is
much more selective for production of the alcohol product.
When the HMB was replaced by deuterated HMB, the
reactivity was lower. The kinetic isotope effect (KIE) was
determined to be 3.3 with linear increase of PMB−OH vs time,
indicating H atom abstraction is the rate-determining step in
the overall photocatalytic reaction.
There have been some reports involving iron complexes
reacting with O2 to give catalytic sulfoxidation of thioether
substrates, although they often involved poorly controlled
radical-type pathways.50 The photoactivation of diiron(III)−μ-
oxo-bridged porphyrins has also lead to catalytic sulfoxidation
where dioxygen is used to regenerate the μ-oxo complex.51
Dioxygen has been utilized by iron complexes for catalytic
sulfoxidation by the formation of a peracid oxidant.52 However,
there remains few examples of sulfoxidation mediated by O2
and well-defined transition metal catalysts that avoid coad-
ditives or radical-type pathways. When HMB was replaced by
thioanisole in the photocatalytic reaction with MnIII(OTf)-
(TBP8Cz(H)), a single S-oxygenated product, methylphenyl-
sulfoxide (PhS(O)Me), was obtained (Scheme 2). Excellent
The amount of PMB−OH produced after 1 h photo-
irradiation of a PhCN solution of MnIII(TBP8Cz) with HMB,
O2, and HOTf was proportional to concentrations of HMB
(Figure 4a) and O2 (Figure 4b). Thus, the rate-determining
step in the photocatalytic oxygenation of HMB by O2 may be
the reaction of the photogenerated species from 1 with O2 and
HMB. If HOTf is added in excess (10 equiv), 2 is formed as
seen by UV−vis. Photoirradiation of this complex in the
presence of HMB/O2 shows no oxidized products. This result
is consistent with previous observations on the diprotonated
MnIII complex generated from H+[B(C6F5)4]−, which also
showed no catalytical activity.43 The amount of PMB−OH
produced by 30 min photoirradiation with a xenon lamp
increased with increasing concentration of HOTf to reach a
maximum and then decreased with further increase in
concentration of HOTf as shown in Figure 5 because of the
formation of the diprotonated complex 2, which shows no
catalytic activity for oxygenation of HMB by O2 in the presence
of a large excess of HOTf (Figure S6b).
Scheme 2. Catalytic Oxygenation of Thioanisole by O2 with
MnIII(TBP8Cz) and HOTf under Photoirradiation
Conditions
TONs (902) and conversion (98%) of PhSMe to PhS(O)Me
were found (Figure S7a), whereas a negligible amount of
PhS(O)Me (TON = 14) was obtained in the absence of HOTf
(Figure S7b). Control experiments confirmed that no
corresponding products were observed without photoirradia-
tion or without the MnIII complex (see Figure S8 in SI). These
results show that MnIII(TBP8Cz) is an efficient and a selective
catalyst for the sulfoxidation of thioanisole and requires only
air, light, and an H+ source without the need for a coreductant.
Femtosecond Transient Absorption Measurements.
In order to clarify the photodynamics of 1 and 2, femtosecond
laser flash photolysis measurements were performed in the
absence and presence of O2 in PhCN. The MnIII ion is a high-
spin (S = 2) species, and coupling between the metal d
electrons and the π electrons of the corrolazine ring leads to a
singquintet (5S0) ground state derived from the lowest excited
ring (π,π*) singlet for 1 and 2 due to the coupling between
unpaired electrons of the metal with the π electrons of the
5
7
corrolazine ring. A “tripmultiplet” manifold (3T1, T1, T1) is
derived from the lowest ring (π,π*) triplet.53 Femtosecond
laser excitation of 1 leads to a new maximum absorption at 610
nm, which can be assigned to the tripquintet (5T1) excited state
(Figure 6). The absorption due to the tripquintet (5T1)
decayed in a few picoseconds with a rate constant of 3.1 × 109
s−1, and there was no effect of O2 on the rate due to the fast
intersystem crossing to the tripseptet excited state (7T1) of 1
with a longer lifetime (Figure 6). An extremely rapid
intersystem crossing process from the singquintet excited
state (5S1) to the tripquintet excited state (5T1) resulting from
the presence of unpaired electrons has been seen for a first-row
paramagnetic complex, MnIII.54 For example, in MnIII
porphyrins, the existence of two tripmultiplet levels was
suggested where a tripquintet (5T1) relaxes to a long-lived
tripseptet (7T1), which requires a spin conversion to go back to
the quintet ground state. The absorption at 610 nm due to the
tripseptet excited state (7T1) decayed significantly faster in O2-
saturated PhCN (Figure 6c) as compared with the decay in the
absence of O2 (Figure 6b). The rate constant of the reaction of
7T1 of 1 with O2 was determined to be 5.0 × 109 M−1 s−1,
Figure 5. Plots of the amount of produced pentamethylbenzyl alcohol
for photocatalytic oxygenation of HMB (2.0 × 10−2 M) by
MnIII(TBP8Cz) (1.0 × 10−5 M) in the presence of HOTf (0−1.0 ×
10−4 M) under photoirradiation (white light) for 30 min in an O2-
saturated PhCN solution at room temperature.
Photocatalytic Sulfoxidation by MnIII(TBP8Cz) in the
Presence of HOTf. In our previous studies we focused
exclusively on C−H functionalization with toluene derivatives.
In the current work we wanted to determine if photocataytic
oxygenation could be obtained with an O-atom acceptor
substrate, and the thioether substrate PhSMe was examined.
F
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
109 s−1 at 745 nm in both the absence and the presence of O2.
In contrast to the case of MnIII(TBP8Cz) and 1, the decay rate
constant for the tripseptet excited state (7T1) of 2 was slower
(2.2 × 109 s−1 at 745 nm) and showed no O2 dependence.
Thus, the tripseptet excited state (7T1) of 2 does not react with
O2, in line with the observation that 2 is not catalytically active.
We hypothesize that the one-electron oxidation potential of 7T1
of 2 may be too high for reaction with O2.
Oxidation Potentials of MnIII(TBP8Cz), MnIII(OTf)-
(TBP8Cz(H)), and [MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf]. The
effects of protonation of MnIII(TBP8Cz) on the one-electron
oxidation potentials were examined by cyclic voltammetry
measurements of MnIII(TBP8Cz), 1, and 2, and the peak
potentials (referenced to SCE) are shown in Figure 8. The first
Figure 6. (a) Transient absorbance spectral changes (red after 10 ps,
green 100 ps, blue 500 ps, and black 3000 ps) after photoexcitation of
MnIII(OTf)(TBP8Cz(H)) in PhCN. Time profile of the generation
and decay of {MnIII(OTf)(TBP8Cz(H))}* at λ = 610 nm under (b)
N2 and (c) O2. Black lines are exponential fittings given in eqs 1 and 2.
7
which is the same as that of the reaction of T1 of neutral
MnIII(TBP8Cz) with O2.16 This reaction should produce the
putative superoxo complex [MnIV(O2•−)(OTf)(TBP8Cz(H))].
Femtosecond laser flash photolysis measurements of 2 were
also performed to characterize the excited state of the
diprotonated species in the absence and presence of O2 in
PhCN, as shown in Figure 7. The decay rate constant of the
intersystem crossing from the tripquintet excited state (5T1) to
the tripseptet excited state (7T1) was determined to be 9.4 ×
Figure 8. Cyclic voltammograms of (a) MnIII(TBP8Cz) (upper, 1.0 ×
10−3 M) in the absence of HOTf and in the presence of HOTf (below,
1.0 × 10−3 M) and (b) MnIII(TBP8Cz) (1.0 × 10−3 M) in the presence
of HOTf (0.1 M) in PhCN containing 0.1 M TBAP. Scan rate = 0.10
V s−1.
one-electron oxidation potential of MnIII(TBP8Cz) was
determined to be Eox = 0.78 V vs SCE in PhCN, which was
shifted to the positive direction by the addition of HOTf: Eox =
1.04 V for 1 and Eox = 1.36 V for 2. However, the second one-
electron oxidation potential was not positively shifted by the
addition of HOTf: Eox = 1.23 V for MnIII(TBP8Cz) and Eox
=
1.20 V for 1 probably because of the counteranion effect of
OTf−. Although the energies of 7T1 of MnIII(TBP8Cz), 1, and 2
have yet to be determined because of the absence of
phosphorescence, the Eox value of 2 in the ground state,
which is 0.32 V higher than that of 1, is consistent with the
observation that the 7T1 of 2 is unreactive toward O2 in
contrast to 1.
Reaction Mechanism. On the basis of the study of the
photocatalytic oxygenation of HMB by O2 with 1 and
photodynamics of 1 (vide supra), the mechanism of photo-
catalytic oxygenation of HMB by O2 with 1 is proposed as
shown in Scheme 3.
In the presence of 1 equiv of HOTf, MnIII(TBP8Cz) is
converted to the monoprotonated complex 1, which is then
excited to the tripquintet state {MnIII(OTf)(TBP8Cz(H))}*
Figure 7. (a) Transient absorbance spectral changes (red after 10 ps,
green 100 ps, blue 500 ps, and black 3000 ps) after photoexcitation of
[MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf] in PhCN. Time profile of
the generation and decay of {[MnIII(OTf)(H2O)(TBP8Cz(H)2)]-
[OTf]}* at λ = 745 nm under (b) N2 and (c) O2. Black lines are
exponential fittings given in eqs 1 and 2.
5
(5T1) upon photoirradiation. The T1 of 1 is converted rapidly
by intersystem crossing (ISC) to the tripseptet excited state
7
(7T1). Electron transfer from T1 to O2 occurs to produce the
superoxo complex [MnIV(O2•−)(OTf)(TBP8Cz(H))], which
G
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
presence of a large excess of HOTf (10 equiv HOTf). In such a
case, the involvement of singlet oxygen is unlikely, because the
formation of singlet oxygen may not be affected by the acid. In
addition, the efficient photocatalytic oxygenation of thioanisole
by O2 with MnIII(TBP8Cz) and HOTf in PhCN (Scheme 2)
also suggests no involvement of singlet oxygen, because the
reactions of alkyl and aryl sulfides with singlet oxygen in aprotic
solvents such as PhCN are known to be sluggish.56
Scheme 3. Mechanism of Photocatalytic Oxygenation of
HMB by 1 with O2
Protonation of MnV(O)(TBP8Cz). To further study the
proposed final step in the catalysis to regenerate the MnIII
resting state, the reaction of isolated MnV(O)(TBP8Cz) with
acid was studied. Protonation of MnV(O)(TBP8Cz) was
examined by reaction of MnV(O)(TBP8Cz) with HOTf in
PhCN as shown in Figure 9a. The broadening and decrease in
abstracts a hydrogen atom from HMB to produce the
hydroperoxo complex [MnIV(OOH)(OTf)(TBP8Cz(H))] and
pentamethylbenzyl radical, in competition with the back
electron transfer (k−et) to regenerate the ground state 1 and
O2. C−H activation by a metal superoxo species has been
demonstrated with chromium and copper nonheme model
complexes.55 In our case, the subsequent homolytic O−O bond
cleavage and combination with benzyl radical to yield PMB−
OH is accompanied by generation of high-valent MnV(O)-
(OTf)(TBP8Cz(H)), followed by conversion to MnIV(OH)-
(OTf)(TBP8Cz•+).43 This species, in the presence of excess H+,
is proposed to react with another equivalent of substrate to
produce PMB−OH, regenerate 1, and close the catalytic cycle.
When 1 is further protonated to 2 in the presence of a large
excess of HOTf no catalytic activity is observed. The tripseptet
excited state (7T1) of 2 is favored to go back to the ground state
rather than react with O2, possibly due to the higher oxidation
potential for 2 as compared with that of 1 (vide supra) as
shown in Scheme 4. As a result, the photocatalytic oxygenation
of HMB by O2 with MnIII(TBP8Cz) does not occur in the
Figure 9. (a) UV−vis spectral changes and (b) plot for the conversion
of MnV(O)(TBP8Cz) (blue line, 1.0 × 10−5 M) to MnIV(OH)(OTf)-
(TBP8Cz•+) (red line) by spectroscopic titration of HOTf (0−2.0 ×
10−5 M) monitored at 634 (MnV(O)(TBP8Cz), blue dots) and 795
nm (MnIV(OH)(OTf)(TBP8Cz•+), red dots) in PhCN at 298 K.
the intensity of the Soret band at 420 nm and the Q band at
634 nm together with the appearance of a relatively weak band
in the near-IR region at 795 nm is characteristic of the
formation of a porphyrinoid π-radical cation.43 The new
spectrum matches that observed previously upon the addition
of the Lewis or Brønsted acids (ZnII(OTf)2, B(C6F5)3, H+) to
the MnV(O) complex.42,43 These acids stabilize a high-spin
triplet (S = 1) (or quintet S = 2) state with an electronic
configuration best described as a manganese(IV) corrolazine π-
radical cation.42 Protonation of the terminal oxo ligand should
weaken the Mn−O π-bonding and destabilize the MnV
oxidation state, favoring an MnIV(O)(π-cation-radical) config-
uration. The formation of MnIV(OH)(OTf)(TBP8Cz•+) (3)
(λmax = 795 nm) was monitored by UV−vis spectroscopy as
shown in Figure 9, where HOTf was added up to 1 equiv. The
equilibrium constant for the addition of acid to MnV(O)-
(TBP8Cz) was determined to be 1.0 × 107 M−1 by a plot of
absorbance vs concentration of HOTf in Figure 9b. No further
spectral changes were observed in the presence of a large excess
of HOTf (Figure S9).
Scheme 4. Mechanism of Photocatalytic Oxygenation of
HMB by 2 with O2
The addition of HMB to 3 and monitoring the reaction by
UV−vis showed slow decay of 3 and growth of MnIII over 14 h.
In the presence of excess of HOTf, however, 3 is reduced by
HMB to produce 1 (Figure S10b in SI) and PMB−OH (yield
85%) by GC-MS. Rates of the oxidation of HMB by
MnV(O)(TBP8Cz) in the presence of excess HOTf were
determined from an increase in absorbance at 725 nm due to 1
in PhCN at 298 K, obeying pseudo-first-order kinetics in the
presence of a large excess of HMB and HOTf (Figures S11 and
S12 in SI). The observed pseudo-first-order rate constants are
proportional to concentrations of HOTf and HMB to afford
the second-order rate constants from the slopes of the linear
H
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
plots as 1.4 × 10−3 (Figure 10a) and 5.3 × 10−4 M−1 s−1
(Figure 10b), respectively.
coordination has a significant influence on the catalytic activity
of the system. The dynamics of the photoexcited states were
similar for both acids, and the same photoexcited state
mechanism can be invoked for both H+[B(C6F5)4]− and
HOTf. Oxidation of HMB by MnIV(OH)(OTf)(TBP8Cz•+)
with excess acid occurs to yield pentamethylbenzyl alcohol,
accompanied by regeneration of MnIII(OTf)(TBP8Cz(H)) to
complete the catalytic cycle. The PCET oxidation of HMB by
MnIV(OH)(OTf)(TBP8Cz•+) is enhanced with increasing
concentration of HOTf. In the presence of a large excess of
HOTf, however, MnIII(OTf)(TBP8Cz(H)) is converted to the
diprotonated complex [MnIII(OTf)(H2O)(TBP8Cz(H)2)]-
7
[OTf], when T1 of [MnIII(OTf)(H2O)(TBP8Cz(H)2)][OTf]
cannot react with O2 to produce the superoxo complex because
of the high oxidation potential of [MnIII(OTf)(H2O)(TBP8Cz-
(H)2)][OTf] and thereby the catalytic cycle is stopped. It
would be of interest in future efforts to examine other acids in
order to elucidate the effect of the H+ source and conjugate
base on the catalysis.
Figure 10. (a) Plot of the observed pseudo-first-order rate constant
(kobs) vs concentrations of HOTf for the oxidation of HMB (2.0 ×
10−2 M) by MnV(O)(TBP8Cz) (1.0 × 10−5 M) in the presence of
HOTf (from 2.0 × 10−5 to 1.0 × 10−4 M) in PhCN at 298 K. (b) Plot
of the observed pseudo-first-order rate constant (kobs) vs concen-
trations of HMB for the oxidation of HMB (0−8.0 × 10−3 M) by
MnV(O)(TBP8Cz) in the presence of HOTf (3.0 × 10−5 M) in PhCN
at 298 K.
ASSOCIATED CONTENT
* Supporting Information
■
S
When HMB was replaced by mesitylene (C6H3(CH3)3), 3
was also reduced by mesitylene in the presence of excess HOTf.
A very small deuterium kinetic isotope effect (KIE) close to
unity (KIE = 1.1) was observed when C6H3(CH3)3 was
replaced by the deuterated compound (C6D3(CD3)3) as shown
in Figure 11. The KIE close to unity indicates that the oxidation
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02019.
UV−vis absorption spectral data, product analysis data,
kinetic analyses, and X-ray crystallographic data (PDF)
(CIF)
(CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: dpg@jhu.edu.
■
*E-mail: fukuzumi@chem.eng.osaka-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid (No. 20108010 to
S.F. and 23750014 to K.O.), the NSF (CHE0909587,
CHE121386 to D.P.G.), the NIH (GM101153 to D.P.G.),
and by a Grant-in-Aid (20108010) and an ALCA project (to
S.F.) from JST, Japan.
Figure 11. Plots of pseudo-first-order rate constants vs concentrations
of mesitylene (C6H3(CH3)3) (black) and mesitylene-d12
(C6D3(CD3)3) (blue) in the oxidation of C6H3(CH3)3 and
(C6D3(CD3)3) by MnV(O)(TBP8Cz) (3.0 × 10−5 M) in the presence
of HOTf (1.5 × 10−4 M) in PhCN at 298 K.
REFERENCES
■
(1) (a) In Cytochrome P450: Structure, Mechanism, and Biochemistry,
3rd ed.; Ortiz de Montellano, P. R., Ed.; Kluwer Academic/Plenum
Publishers: New York, 2005. (b) In Metal-Oxo and Metal-Peroxo
Species in Catalytic Oxidations; Meunier, B., Ed.; Springer-Verlag:
Berlin, 2000. (c) Terner, J.; Gold, A. J. Am. Chem. Soc. 2007, 129,
16279−16280. (d) Guengerich, F. P. Chem. Res. Toxicol. 2001, 14,
611−650. (e) Denisov, I. G.; Makris, T. M.; Sligar, S. G.; Schlichting, I.
Chem. Rev. 2005, 105, 2253−2278.
(2) (a) Coelho, P. S.; Brustad, E. M.; Kannan, A.; Arnold, F. H.
Science 2013, 339, 307−310. (b) Zhang, R.; Newcomb, M. Acc. Chem.
Res. 2008, 41, 468−477. (c) Meunier, B.; de Visser, S. P.; Shaik, S.
Chem. Rev. 2004, 104, 3947−3980. (d) Franke, A.; Hessenauer-
Ilicheva, N.; Meyer, D.; Stochel, G.; Woggon, W.-D.; van Eldik, R. J.
Am. Chem. Soc. 2006, 128, 13611−13624.
of mesitylene as well as HMB by 3 proceeds via proton-coupled
electron transfer from mesitylene and HMB to 3 rather than
hydrogen atom transfer from mesitylene and HMB to 3.
CONCLUSIONS
■
In conclusion, MnIII(OTf)(TBP8Cz(H)) is capable of the
photocatalytic oxygenation of HMB and PhSMe under ambient
conditions and was found to be a more robust, efficient, and
selective catalyst than the monoprotonated [MnIII(H2O)-
(TBP8Cz(H))][B(C6F5)4]. Thus, changing the acid from
H+[B(C6F5)4]− to an acid with a different conjugate base
resulted in a dramatic change in catalytic reactivity. The major
difference in conjugate bases is the ability of OTf− to
coordinate to the Mn center and suggests that OTf−
(3) (a) Nam, W. Acc. Chem. Res. 2007, 40, 522−531.
(b) Czernuszewicz, R. S.; Mody, V.; Czader, A.; Gałęzowski, M.;
Gryko, D. T. J. Am. Chem. Soc. 2009, 131, 14214−14215. (c) Shaik, S.;
Hirao, H.; Kumar, D. Acc. Chem. Res. 2007, 40, 532−542. (d) Schmidt,
I
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
A.-C.; Heinemann, F. W.; Lukens, W. W., Jr.; Meyer, K. J. Am. Chem.
Soc. 2014, 136, 11980−11993.
(4) (a) Collman, J. P.; Slaughter, L. M.; Eberspacher, T. A.; Strassner,
T.; Brauman, J. I. Inorg. Chem. 2001, 40, 6272−6280. (b) Crestoni, M.
E.; Fornarini, S. Inorg. Chem. 2005, 44, 5379−5387. (c) Zhou, M.;
Balcells, D.; Parent, A. R.; Crabtree, R. H.; Eisenstein, O. ACS Catal.
2012, 2, 208−218.
(5) (a) Edwards, N. Y.; Eikey, R. A.; Loring, M. I.; Abu-Omar, M. M.
Inorg. Chem. 2005, 44, 3700−3708. (b) Tahsini, L.; Bagherzadeh, M.;
Nam, W.; de Visser, S. P. Inorg. Chem. 2009, 48, 6661−6669. (c) Lai,
S.; Lepage, C. J.; Lee, D. G. Inorg. Chem. 2002, 41, 1954−1957.
(6) (a) Ramdhanie, B.; Telser, J.; Caneschi, A.; Zakharov, L. N.;
Rheingold, A. L.; Goldberg, D. P. J. Am. Chem. Soc. 2004, 126, 2515−
2525. (b) Ensing, B.; Buda, F.; Gribnau, M. C. M.; Baerends, E. J. J.
Am. Chem. Soc. 2004, 126, 4355−4365. (c) Gupta, R.; Lacy, D. C.;
Bominaar, E. L.; Borovik, A. S.; Hendrich, M. P. J. Am. Chem. Soc.
2012, 134, 9775−9784. (d) Yoshizawa, K. Acc. Chem. Res. 2006, 39,
375−382.
(7) (a) Kumar, D.; Hirao, H.; Shaik, S.; Kozlowski, P. M. J. Am.
Chem. Soc. 2006, 128, 16148−16158. (b) Comba, P.; Maurer, M.;
Vadivelu, P. Inorg. Chem. 2009, 48, 10389−10396. (c) Kumar, D.;
Derat, E.; Khenkin, A. M.; Neumann, R.; Shaik, S. J. Am. Chem. Soc.
2005, 127, 17712−17718.
(15) Prokop, K. A.; Goldberg, D. P. J. Am. Chem. Soc. 2012, 134,
8014−8017.
(16) Jung, J.; Ohkubo, K.; Prokop-Prigge, K. A.; Neu, H. M.;
Goldberg, D. P.; Fukuzumi, S. Inorg. Chem. 2013, 52, 13594−13604.
(17) (a) Goldberg, D. P. Acc. Chem. Res. 2007, 40, 626−634.
(b) Conradie, J.; Swarts, J. C.; Ghosh, A. J. Phys. Chem. B 2004, 108,
452−456. (c) Venkataramanan, N. S.; Premsingh, S.; Rajagopal, S.;
Pitchumani, K. J. Org. Chem. 2003, 68, 7460−7470. (d) Sugimoto, H.;
Kitayama, K.; Ashikari, K.; Matsunami, C.; Ueda, N.; Umakoshi, K.;
Hosokoshi, Y.; Sasaki, Y.; Itoh, S. Inorg. Chem. 2011, 50, 9014−9023.
(18) (a) Meier-Callahan, A. E.; Gray, H. B.; Gross, Z. Inorg. Chem.
2000, 39, 3605−3607. (b) Vanover, E.; Huang, Y.; Xu, L.; Newcomb,
M.; Zhang, R. Org. Lett. 2010, 12, 2246−2249. (c) Chung, L. W.; Lee,
H. G.; Lin, Z.; Wu, Y.-D. J. Org. Chem. 2006, 71, 6000−6009.
(d) Harischandra, D. N.; Lowery, G.; Zhang, R.; Newcomb, M. Org.
Lett. 2009, 11, 2089−2092.
(19) (a) McGown, A. J.; Kerber, W. D.; Fujii, H.; Goldberg, D. P. J.
Am. Chem. Soc. 2009, 131, 8040−8048. (b) Kang, M.-J.; Song, W. J.;
Han, A.-R.; Choi, Y. S.; Jang, H. G.; Nam, W. J. Org. Chem. 2007, 72,
6301−6304. (c) Nam, W.; Lee, Y.-M.; Fukuzumi, S. Acc. Chem. Res.
2014, 47, 1146−1154.
́
(20) (a) Mitic, N.; Clay, M. D.; Saleh, L.; Bollinger, J. M., Jr.;
Solomon, E. I. J. Am. Chem. Soc. 2007, 129, 9049−9065. (b) Chen, H.;
Lai, W.; Shaik, S. J. Phys. Chem. Lett. 2010, 1, 1533−1540.
(c) Chiavarino, B.; Cipollini, R.; Crestoni, M. E.; Fornarini, S.;
Lanucara, S.; Lapi, A. J. Am. Chem. Soc. 2008, 130, 3208−3217.
(21) (a) Pan, Z.; Newcomb, M. Inorg. Chem. 2007, 46, 6767−6774.
(b) Chung, L. W.; Li, X.; Hirao, H.; Morokuma, K. J. Am. Chem. Soc.
2011, 133, 20076−20079. (c) Pan, Z.; Wang, Q.; Sheng, X.; Horner, J.
H.; Newcomb, M. J. Am. Chem. Soc. 2009, 131, 2621−2628.
(22) (a) Pestovsky, O.; Bakac, A. J. Am. Chem. Soc. 2003, 125,
14714−14715. (b) Betley, T. A.; Wu, Q.; Van Voorhis, T.; Nocera, D.
G. Inorg. Chem. 2008, 47, 1849−1861.
(8) (a) Bart, S. C.; Anthon, C.; Heinemann, F. W.; Bill, E.; Edelstein,
N. M.; Meyer, K. J. Am. Chem. Soc. 2008, 130, 12536−12546.
(b) Abbina, S.; Bian, S.; Oian, C.; Du, G. ACS Catal. 2013, 3, 678−
684. (c) Comba, P.; Maurer, M.; Vadivelu, P. J. Phys. Chem. A 2008,
112, 13028−13036. (d) Lacy, D. C.; Gupta, R.; Stone, K. L.; Greaves,
J.; Ziller, J. W.; Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc. 2010,
132, 12188−12190.
(9) (a) Jensen, M. P.; Costas, M.; Ho, R. Y. N.; Kaizer, J.; Payeras, A.
M. I.; Munck, E.; Que, L., Jr.; Rohde, J.−U.; Stubna, A. J. Am. Chem.
̈
Soc. 2005, 127, 10512−10525. (b) Tooyama, Y.; Braband, H.;
Spingler, B.; Abram, U.; Alberto, R. Inorg. Chem. 2008, 47, 257−
(23) (a) Taguchi, T.; Gupta, R.; Lassalle-Kaiser, B.; Boyce, D. W.;
Yachandra, V. K.; Tolman, W. B.; Yano, J.; Hendrich, M. P.; Borovik,
A. S. J. Am. Chem. Soc. 2012, 134, 1996−1999. (b) Harischandra, D.
N.; Zhang, R.; Newcomb, M. J. Am. Chem. Soc. 2005, 127, 13776−
13777.
264. (c) Fernandes, A. C.; Fernandes, J. A.; Romao, C. C.; Veiros, L.
̃
F.; Calhorda, M. J. Organometallics 2010, 29, 5517−5525.
(10) (a) Slep, L. D.; Mijovilovich, A.; Meyer-Klaucke, W.;
Weyhermuller, T.; Bill, E.; Bothe, E.; Neese, F.; Wieghardt, K. J.
̈
Am. Chem. Soc. 2003, 125, 15554−15570. (b) Krebs, C.; Fujimori, D.
G.; Walsh, C. T.; Bollinger, J. M., Jr. Acc. Chem. Res. 2007, 40, 484−
492. (c) Martinho, M.; Banse, F.; Bartoli, J.−F.; Mattioli, T. A.;
Battioni, P.; Horner, O.; Bourcier, S.; Girerd, J.−J. Inorg. Chem. 2005,
44, 9592−9596.
(11) (a) Gunay, A.; Theopold, K. H. Chem. Rev. 2010, 110, 1060−
1081. (b) Yi, C. S.; Zeczycki, T. N.; Guzei, I. A. Organometallics 2006,
25, 1047−1051. (c) Bakac, A. J. Am. Chem. Soc. 2000, 122, 1092−
1097. (d) Czernuszewicz, R. S.; Mody, V.; Zareba, A. A.; Zaczek, M.
B.; Gałez̧ owski, M.; Sashuk, V.; Grela, K.; Gryko, D. T. Inorg. Chem.
2007, 46, 5616−5624.
(12) (a) Konar, S.; Clearfield, A. Inorg. Chem. 2008, 47, 3489−3491.
(b) King, A. E.; Nippe, M.; Atanasov, M.; Chantarojsiri, T.; Wray, C.
A.; Bill, E.; Neese, F.; Long, J. R.; Chang, C. J. Inorg. Chem. 2014, 53,
11388−11395. (c) Vasbinder, M. J.; Bakac, A. Inorg. Chem. 2007, 46,
2921−2928.
(13) (a) Bigi, J. P.; Harman, W. H.; Lassalle-Kaiser, B.; Robles, D. M.;
Stich, T. A.; Yano, J.; Britt, R. D.; Chang, C. J. J. Am. Chem. Soc. 2012,
134, 1536−1542. (b) Lovell, T.; Han, W.−G.; Liu, T.; Noodleman, L.
J. Am. Chem. Soc. 2002, 124, 5890−5894. (c) Price, J. C.; Barr, E. W.;
Tirupati, B.; Bollinger, J. M., Jr.; Krebs, C. Biochemistry 2003, 42,
7497−7508. (d) Leeladee, P.; Jameson, G. N. L.; Siegler, M. A.;
Kumar, D.; de Visser, S. P.; Goldberg, D. P. Inorg. Chem. 2013, 52,
4668−4682.
(24) (a) McEvoy, J. P.; Brudvig, G. W. Chem. Rev. 2006, 106, 4455−
4483. (b) Umena, Y.; Kawakami, K.; Shen, J.-R.; Kamiya, N. Nature
2011, 473, 55−60. (c) Vaddypally, S.; Kondaveeti, S. K.; Zdilla, M. J.
Inorg. Chem. 2012, 51, 3950−3952. (d) Suga, M.; Akita, F.; Hirata, K.;
Ueno, G.; Murakami, H.; Nakajima, Y.; Shimizu, T.; Yamashita, K.;
Yamamoto, M.; Ago, H.; Shen, J.-R. Nature 2015, 517, 99.
(25) (a) Lansky, D. E.; Goldberg, D. P. Inorg. Chem. 2006, 45, 5119−
5125. (b) Cho, K.; Leeladee, P.; McGown, A. J.; DeBeer, S.; Goldberg,
D. P. J. Am. Chem. Soc. 2012, 134, 7392−7399. (c) Prokop, K. A.; Neu,
H. M.; de Visser, S. P.; Goldberg, D. P. J. Am. Chem. Soc. 2011, 133,
15874−15877. (d) Leeladee, P.; Goldberg, D. P. Inorg. Chem. 2010,
49, 3083−3085.
(26) (a) Zhang, R.; Newcomb, M. J. Am. Chem. Soc. 2003, 125,
12418−12419. (b) Kumar, A.; Goldberg, I.; Botoshansky, M.;
Buchman, Y.; Gross, Z. J. Am. Chem. Soc. 2010, 132, 15233−15245.
(c) Parsell, T. H.; Behan, R. K.; Green, M. T.; Hendrich, M. P.;
Borovik, A. S. J. Am. Chem. Soc. 2006, 128, 8728−8729.
(27) (a) Shirin, Z.; Hammes, B. S.; Young, V. G., Jr.; Borovik, A. S. J.
Am. Chem. Soc. 2000, 122, 1836−1837. (b) Zdilla, M. J.; Abu-Omar,
M. M. J. Am. Chem. Soc. 2006, 128, 16971−16979.
(28) (a) Fukuzumi, S.; Fujioka, N.; Kotani, H.; Ohkubo, K.; Lee, Y.-
M.; Nam, W. J. Am. Chem. Soc. 2009, 131, 17127−17134. (b) Lee, J.
Y.; Lee, Y.-M.; Kotani, H.; Nam, W.; Fukuzumi, S. Chem. Commun.
2009, 704−706. (c) Prokop, K. A.; de Visser, S. P.; Goldberg, D. P.
Angew. Chem. 2010, 122, 5217−5221; Angew. Chem., Int. Ed. 2010, 49,
5091−5095. (d) Fukuzumi, S.; Kotani, H.; Prokop, K. A.; Goldberg, D.
P. J. Am. Chem. Soc. 2011, 133, 1859−1869.
(14) (a) Liu, S.; Mase, K.; Bougher, C.; Hicks, S. D.; Abu-Omar, M.
M.; Fukuzumi, S. Inorg. Chem. 2014, 53, 7780−7788. (b) Zall, C. M.;
Clouston, L. J.; Young, V. G., Jr.; Ding, K.; Kim, H. J.; Zherebetskyy,
D.; Chen, Y.−S.; Bill, E.; Gagliardi, L.; Lu, C. C. Inorg. Chem. 2013, 52,
(29) (a) Yin, G. Acc. Chem. Res. 2013, 46, 483−492. (b) Zhang, R.;
Horner, J. H.; Newcomb, M. J. Am. Chem. Soc. 2005, 127, 6573−6582.
(c) De Angelis, F.; Jin, N.; Car, R.; Groves, J. T. Inorg. Chem. 2006, 45,
9216−9228. (c) Groysman, S.; Villagran
́
, D.; Nocera, D. G. Inorg.
Chem. 2010, 49, 10759−10761.
J
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
4268−4276. (d) Arunkumar, C.; Lee, Y.-M.; Lee, J. Y.; Fukuzumi, S.;
Nam, W. Chem. - Eur. J. 2009, 15, 11482−11489.
́
(45) (a) Lansky, D. E.; Mandimutsira, B.; Ramdhanie, B.; Clausen,
M.; Penner-Hahn, J.; Zvyagin, S. A.; Telser, J.; Krzystek, J.; Zhan, R.;
Ou, Z.; Kadish, K. M.; Zakharov, L.; Rheingold, A. L.; Goldberg, D. P.
Inorg. Chem. 2005, 44, 4485−4498. (b) Ramdhanie, B.; Stern, C. L.;
Goldberg, D. P. J. Am. Chem. Soc. 2001, 123, 9447−9448.
(c) Mandimutsira, B. S.; Ramdhanie, B.; Todd, R. C.; Wang, H.;
Zareba, A. A.; Czernuszewicz, R. S.; Goldberg, D. P. J. Am. Chem. Soc.
2002, 124, 15170−15171.
(30) (a) Mukhopadhyay, S.; Gandhi, B. A.; Kirk, M. L.; Armstrong,
W. H. Inorg. Chem. 2003, 42, 8171−8180. (b) Kurahashi, T.; Kikuchi,
A.; Shiro, Y.; Hada, M.; Fujii, H. Inorg. Chem. 2010, 49, 6664−6672.
(c) Pecoraro, V. L.; Hsieh, W.−Y. Inorg. Chem. 2008, 47, 1765−1778.
(31) (a) Lansky, D. E.; Kosack, J. R.; Narducci Sarjeant, A. A.;
Goldberg, D. P. Inorg. Chem. 2006, 45, 8477−8479. (b) Wang, S. H.;
Mandimutsira, B. S.; Todd, R.; Ramdhanie, B.; Fox, J. P.; Goldberg, D.
P. J. Am. Chem. Soc. 2004, 126, 18−19. (c) Lewis, R. A.; Wu, G.;
Hayton, T. W. Inorg. Chem. 2011, 50, 4660−4668.
(46) In Purification of Laboratory Chemicals; Armarego, W. L. F.;
Perrin, D. D., Eds.; Pergamon Press: Oxford, 1997.
(47) (a) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112−122. (b) Sheldrick, G. M. Acta Crystallogr., Sect. C:
Struct. Chem. 2015, 71, 3−8.
(32) (a) Shen, D.; Miao, C.; Wang, S.; Xia, C.; Sun, W. Org. Lett.
́ ̀
2014, 16, 1108−1111. (b) Gomez-Hortiguela, L.; Cora, F.; Catlow, C.
̈
(48) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7−13.
(49) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
235, 518−536.
R. A. ACS Catal. 2011, 1, 18−28. (c) Krewald, V.; Neese, F.; Pantazis,
D. A. J. Am. Chem. Soc. 2013, 135, 5726−5739.
(33) Jung, J.; Ohkubo, K.; Goldberg, D. P.; Fukuzumi, S. J. Phys.
Chem. A 2014, 118, 6223−6229.
(50) Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Chem. Rev. 2005, 105,
2329−2363.
(34) (a) Morimoto, Y.; Kotani, H.; Park, J.; Lee, Y.-M.; Nam, W.;
Fukuzumi, S. J. Am. Chem. Soc. 2011, 133, 403−405. (b) Fukuzumi, S.;
Morimoto, Y.; Kotani, H.; Naumov, P.; Lee, Y.-M.; Nam, W. Nat.
Chem. 2010, 2, 756−759. (c) Fukuzumi, S.; Ohkubo, K.; Morimoto, Y.
Phys. Chem. Chem. Phys. 2012, 14, 8472−8484. (d) Fukuzumi, S.;
Ohkubo, K. Coord. Chem. Rev. 2010, 254, 372−385.
(51) Pistorio, B. J.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc.
2002, 124, 7884−7885.
(52) Zhou, X.-T.; Ji, H.-B. Catal. Commun. 2014, 53, 29−32.
(53) Kim, Y.; Choi, J. R.; Yoon, M. J. Phys. Chem. B 2001, 105,
8513−8518.
(54) (a) Humphrey, J. L.; Kuciauskas, D. J. Am. Chem. Soc. 2006, 128,
(35) (a) Chen, J.; Lee, Y.-M.; Davis, K. M.; Wu, X.; Seo, M. S.; Cho,
K.-B.; Yoon, H.; Park, Y. J.; Fukuzumi, S.; Pushkar, Y. N.; Nam, W. J.
Am. Chem. Soc. 2013, 135, 6388−6391. (b) Yoon, H.; Lee, Y.-M.; Wu,
X.; Cho, K.-B.; Sarangi, R.; Nam, W.; Fukuzumi, S. J. Am. Chem. Soc.
2013, 135, 9186−9194. (c) Li, F.; Van Heuvelen, K. M.; Meier, K. K.;
3902−3903. (b) Spanig, F.; Ruppert, M.; Dannhauser, J.; Hirsch, A.;
̈
̈
Guldi, D. M. J. Am. Chem. Soc. 2009, 131, 9378−9388. (c) Gonca
̧ lves,
P. J.; De Boni, L.; Borissevitch, I. E.; Zílio, S. C. J. Phys. Chem. A 2008,
112, 6522−6526. (d) Krokos, E.; Spanig, F.; Ruppert, M.; Hirsch, A.;
̈
Guldi, D. M. Chem. - Eur. J. 2012, 18, 1328−1341.
Munck, E.; Que, L., Jr. J. Am. Chem. Soc. 2013, 135, 10198−10201.
̈
(55) (a) Cho, J.; Sarangi, R.; Nam, W. Acc. Chem. Res. 2012, 45,
1321−1330. (b) Peterson, R. L.; Himes, R. A.; Kotani, H.; Suenobu,
T.; Tian, L.; Siegler, M. A.; Solomon, E. I.; Fukuzumi, S.; Karlin, K. D.
J. Am. Chem. Soc. 2011, 133, 1702−1705. (c) Cho, J.; Woo, J.; Nam,
W. J. Am. Chem. Soc. 2010, 132, 5958−5959. (d) Nemes, A.; Bakac, A.
Inorg. Chem. 2001, 40, 746−749.
(36) Park, Y. J.; Ziller, J. W.; Borovik, A. S. J. Am. Chem. Soc. 2011,
133, 9258−9261.
(37) (a) Park, Y. J.; Cook, S. A.; Sickerman, N. S.; Sano, Y.; Ziller, J.
W.; Borovik, A. S. Chem. Sci. 2013, 4, 717−726. (b) Lee, Y.-M.; Bang,
S.; Kim, Y. M.; Cho, J.; Hong, S.; Nomura, T.; Ogura, T.; Troeppner,
́ ́
O.; Ivanovic- Burmazovic, I.; Sarangi, R.; Fukuzumi, S.; Nam, W.
(56) (a) Clennan, E. L.; Greer, A. J. J. Org. Chem. 1996, 61, 4793−
4797. (b) Bonesi, S. M.; Albini, A. J. Org. Chem. 2000, 65, 4532−4536.
(c) Baciocchi, E.; Chiappe, C.; Fasciani, C.; Lanzalunga, O.; Lapi, A.
Org. Lett. 2010, 12, 5116−5119.
Chem. Sci. 2013, 4, 3917−3923. (c) Yin, G.; Danby, A. M.; Kitko, D.;
Carter, J. D.; Scheper, W. M.; Busch, D. H. J. Am. Chem. Soc. 2008,
130, 16245−16253.
(38) (a) Park, J.; Morimoto, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S.
Inorg. Chem. 2014, 53, 3618−3628. (c) Park, J.; Lee, Y.-M.; Nam, W.;
Fukuzumi, S. J. Am. Chem. Soc. 2013, 135, 5052−5061.
(39) (a) Du, H.; Lo, P.-K.; Hu, Z.; Liang, H.; Lau, K.-C.; Wang, Y.
N.; Lam, W. W. Y.; Lau, T. C. Chem. Commun. 2011, 47, 7143−7145.
(b) Yiu, S.-M.; Man, W.-L.; Lau, T.-C. J. Am. Chem. Soc. 2008, 130,
10821−10827. (c) Lam, W. W. Y.; Yiu, S.-M.; Lee, J. M. N.; Yau, S. K.
Y.; Kwong, H.-K.; Lau, T.-C.; Liu, D.; Lin, Z. J. Am. Chem. Soc. 2006,
128, 2851−2858. (d) Yiu, S.-M.; Wu, Z.-B.; Mak, C.-K.; Lau, T.-C. J.
Am. Chem. Soc. 2004, 126, 14921−14929.
(40) (a) Herbert, D. E.; Lionetti, D.; Rittle, J.; Agapie, T. J. Am.
Chem. Soc. 2013, 135, 19075−19078. (b) Smeltz, J. L.; Lilly, C. P.;
Boyle, P. D.; Ison, E. A. J. Am. Chem. Soc. 2013, 135, 9433−9441.
(c) Asao, N.; Sato, K. Org. Lett. 2006, 8, 5361−5363.
(41) Tsui, E. Y.; Tran, R.; Yano, J.; Agapie, T. Nat. Chem. 2013, 5,
293−299.
(42) (a) Leeladee, P.; Baglia, R. A.; Prokop, K. A.; Latifi, R.; de
Visser, S. P.; Goldberg, D. P. J. Am. Chem. Soc. 2012, 134, 10397−
́ ́
10400. (b) Baglia, R. A.; Durr, M.; Ivanovic-Burmazovic, I.; Goldberg,
̈
D. P. Inorg. Chem. 2014, 53, 5893−5895. (c) Zaragoza, J. P. T.; Baglia,
R. A.; Siegler, M. A.; Goldberg, D. P. J. Am. Chem. Soc. 2015, 137,
6531−6540.
(43) Neu, H. M.; Jung, J.; Baglia, R. A.; Siegler, M. A.; Ohkubo, K.;
Fukuzumi, S.; Goldberg, D. P. J. Am. Chem. Soc. 2015, 137, 4614−
4617.
(44) (a) Neu, H. M.; Quesne, M. G.; Yang, T.; Prokop-Prigge, K. A.;
Lancaster, K. M.; Donohoe, J.; DeBeer, S.; de Visser, S. P.; Goldberg,
D. P. Chem. - Eur. J. 2014, 20, 14584−14588. (b) Neu, H. M.; Yang,
T.; Baglia, R. A.; Yosca, T. H.; Green, M. T.; Quesne, M. G.; de Visser,
S. P.; Goldberg, D. P. J. Am. Chem. Soc. 2014, 136, 13845−13852.
K
DOI: 10.1021/acs.inorgchem.5b02019
Inorg. Chem. XXXX, XXX, XXX−XXX