Visible light-induced formation of corrole-manganese(V)-oxo complexes: Observation of multiple oxidation pathways

Article abstract of DOI:10.1016/j.jinorgbio.2016.08.004

Two manganese(V)-oxo corroles [Mn^V(Cor)O] that differ in their electronic environments were produced by visible light irradiation of highly photo-labile corrole-manganese(IV) bromates. The corrole ligands under study include 5,10,15-tris(pentafluorophenyl)corrole (TPFC), and 5,10,15-triphenylcorrole (TPC). The kinetics of oxygen transfer atom (OAT) reactions with various organic reductants by these photo-generated Mn^V(Cor)O were also studied in CH₃CN and CH₂Cl₂ solutions. Mn^V(Cor)O exhibits remarkable solvent and ligand effect on its reactivity and spectral behavior. In the more electron-deficient TPFC system and in the polar solvent CH₃CN, Mn^V(Cor)O returned Mn^III corrole in the end of oxidation reactions. However, in the less polar solvent CH₂Cl₂ or in the less electron-deficient TPC system, Mn^IV product was formed instead of Mn^III. Furthermore, with the same substrates and in the same solvent, the order of reactivity of Mn^V(Cor)O was TPC>?TPFC, which is inverted from that expected based on the electron-demand of corrole ligands. Our spectral and kinetic results in this study provide compelling evidence in favor of multiple oxidation pathways, where Mn^V(Cor)O may serve as direct two-electron oxidant or undergo a disproportionation reaction to form a manganese(VI)-oxo corrole as the true oxidant. The choice of pathways is strongly dependent on the nature of the solvent and the corrole ligand.

Full text of DOI:10.1016/j.jinorgbio.2016.08.004

Journal of Inorganic Biochemistry 163 (2016) 39–44
Contents lists available at ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Visible light-induced formation of corrole-manganese(V)-oxo
complexes: Observation of multiple oxidation pathways
⁎
KaWai Kwong, NgoFung Lee, Davis Ranburger, Jonathan Malone, Rui Zhang
Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101-1079, USA
a r t i c l e i n f o
a b s t r a c t
Two manganese(V)-oxo corroles [MnV(Cor)O] that differ in their electronic environments were produced by vis-
ible light irradiation of highly photo-labile corrole-manganese(IV) bromates. The corrole ligands under study in-
clude 5,10,15-tris(pentafluorophenyl)corrole (TPFC), and 5,10,15-triphenylcorrole (TPC). The kinetics of oxygen
transfer atom (OAT) reactions with various organic reductants by these photo-generated MnV(Cor)O were also
studied in CH3CN and CH2Cl2 solutions. MnV(Cor)O exhibits remarkable solvent and ligand effect on its reactivity
and spectral behavior. In the more electron-deficient TPFC system and in the polar solvent CH3CN, MnV(Cor)O
returned MnIII corrole in the end of oxidation reactions. However, in the less polar solvent CH2Cl2 or in the less
electron-deficient TPC system, MnIV product was formed instead of MnIII. Furthermore, with the same substrates
and in the same solvent, the order of reactivity of MnV(Cor)O was TPCN TPFC, which is inverted from that expect-
ed based on the electron-demand of corrole ligands. Our spectral and kinetic results in this study provide com-
pelling evidence in favor of multiple oxidation pathways, where MnV(Cor)O may serve as direct two-electron
oxidant or undergo a disproportionation reaction to form a manganese(VI)-oxo corrole as the true oxidant.
The choice of pathways is strongly dependent on the nature of the solvent and the corrole ligand.
© 2016 Elsevier Inc. All rights reserved.
Article history:
Received 20 April 2016
Received in revised form 2 August 2016
Accepted 4 August 2016
Available online 05 August 2016
Keywords:
Manganese(V)-oxo
Corroles
Visible light
Oxidation
Kinetics
1. Introduction
photolysis (LFP) methods, serve as important mechanistic probes for OAT
reactions [24–26]. Nevertheless, the factors controlling their OAT reactivity
High-valent transition metal-oxo intermediates are of fundamental
importance because of their central roles as active oxidizing species in
enzymatic and synthetic catalytic oxidations [1–4]. Many transition
metal catalysts have been extensively studied as models to probe the
sophisticated oxygen atom transfer (OAT) mechanism as well as to invent
enzyme-like oxidation catalysts [5–8]. In many cases, however, the active
metal-oxo transients are still puzzling due to their high reactivities and/
or low concentrations. Moreover, a high valent metal-oxo species detected
in a reaction might not be the true oxidant but a precursor to the true ox-
idant that is formed in small, undetectable amounts [9]. In this context,
metallocorroles have garnered considerable interest in their catalytic prop-
erties because they are 19-membered analogues of metalloporphyrins,
and have the capacity to access higher metal-oxo species [10–13]. Thus,
a number of metallocorroles including metallocorralizines (corralizines =
meso-N-substituted tetrazocorroles) [14–17] have been synthesized and
explored in a wide variety of catalytic oxidations [14,18–20]. For examples,
(imido)manganese(V) [21,22] and (nitrido)manganese(V/VI) corroles
[23] are stable enough to be well characterized spectroscopically. In partic-
ular, the relatively stable manganese(V)-oxo corroles, abbreviated as Mn-
V(Cor)O, which can be produced either by chemical oxidation or laser flash
with organic substrates are not fully understood, deserving further explo-
ration [27]. For example, a well characterized MnV-oxo corrole produced
from ozone oxidation of the MnIII precursor shows no reactivity toward
olefins at room temperature [19]. Similarly, the MnV-oxo complex contain-
ing the more electron-deficient corralizine ligand is even more inert [14].
Recent studies reported by Gross and co-workers have suggested that
highly electron-demanding corroles-manganese(V)-oxo species directly
transfers oxygen atom, while relatively less electron-demanding
corroles-managese(V)-oxo complexes undergo disproportionation reac-
tion to generate corrole-managnese(VI)-oxo as the active oxidant [28]. In
addition, a study by Yu et al. who measured the pseudo-first order rate
constants of a variety of MnV-oxo corroles reacting with alkenes in differ-
ent solvents indicated that the oxygen atom transfer pathway of these
metal-oxo species is solvent-dependent [29].
Photochemical activation of transition metal complexes to produce
reactive oxidants has been known for decades [30,31]. Notably, the
use of laser flash photolysis methods to generate a variety of high-valent
transition metal-oxo species supported by porphyrin and corrole li-
gands have been well developed [27,32–36]. With photochemical pro-
duction of reactive metal-oxo transients, one has access to time scales
that are much shorter than the fastest mixing experiments. In addition,
the kinetics of oxidation reactions by photo-generated metal-oxo spe-
cies are not convoluted with the rate constants for formation of the re-
active transients by reaction of the excess sacrificial oxidant with the
⁎
Corresponding author at: Department of Chemistry, Western Kentucky University,
1906 College Heights Blvd #11079, Bowling Green, KY 42101-1079, USA.
E-mail address: rui.zhang@wku.edu (R. Zhang).
http://dx.doi.org/10.1016/j.jinorgbio.2016.08.004
0162-0134/© 2016 Elsevier Inc. All rights reserved.

40
W.K. Ka et al. / Journal of Inorganic Biochemistry 163 (2016) 39–44
low-valent metal species [35]. As our ongoing efforts to explore pho-
tochemical approaches to access high-valent metal-oxo species, we
have utilized photo-induced ligand cleavage reactions to produce
trans-dioxoruthenium(VI) porphyrins [37,38] as well as a putative
ruthenium(V)-oxo species [39] that was also found in a photo-dis-
proportionation of the bis-porphyrin-ruthenium(IV) μ-oxo dimer
[40]. Very recently, we communicated a photo-induced entry to
iron(IV)-oxo porphyrin radical cations (compound I models) or
iron(IV)-oxo porphyrin (compound II models) as controlled by the
electronic nature of porphyrin ligands [41]. In the present work, we
report a visible light irradiation of triarylcorrole–manganese(IV)
bromate complexes to form manganese(V)-oxo corroles and direct
kinetic studies of their OAT reactions with organic reductants. Our
kinetic and spectral results illustrate that the photo-generated
manganese(V)-oxo corroles may oxidize the substrate through dif-
ferent oxidation pathways, depending on the nature of corrole ligand
and the solvent.
2.2. General procedure for visible light formation of manganese(V)-oxo
corroles
Photo-labile compounds 3 were prepared in situ by stirring 2 with
excess of Ag(BrO3) in anaerobic CH3CN, and the formation of bromate
products 3 was indicated by the change of UV–vis absorption (Fig. 1).
The resulting solutions 3 were used for photochemical studies immedi-
ately after preparation. (Caution! Bromate salts of metal complexes are
potentially explosive and should be handled with care). When the solution
of 3 with concentrations of ca. 2 × 10−5 M was irradiated with visible
light at ambient temperature, the time-resolved formation of 4 was
complete within 2 min as monitored by UV–visible spectroscopy. The
same oxo species 4 were also produced by chemical oxidation of 1
with 5-fold excess of PhIO or PhI(OAc)2 in CH3CN.
2.3. Instrumentation and kinetic studies
UV–vis spectra were recorded on an Agilent 8453 diode array spec-
trophotometer. 1H NMR was performed on a JEOL ECA-500 MHz spec-
trometer at 298 K with tetramethylsilane (TMS) as internal standard.
Chemical shifts (ppm) are reported relative to TMS. IR spectra were ob-
tained on a Bio-Rad FT-IR spectrometer. Kinetic measurements were
performed on an Agilent 8453 diode array spectrophotometer
interfaced with the Pro-K 2000 Rapid Mixing System (Applied
Photophysics) by using standard 1.0-cm quartz cuvettes. Visible light
was provided from a SOLA SE II light engine (Lumencor) configured
with a liquid light guide. The output power can be adjusted from 6 to
120 W.
Reactions of oxo-species 4 with excess organic substrate (N100
equiv.) were conducted in a solution at 23 2 °C. The approximate con-
centrations of the photo-generated 4 were estimated by assuming 100%
conversion of manganese(IV) precursors in the photochemical reac-
tions. The rates of the reactions which represent the rates of oxo
group transfer from MnV(Cor)O to substrate were monitored by the
decay of the Soret absorption band of the oxo-species 4. The kinetic
traces at λmax of Soret band displayed good pseudo-first-order behavior
for at least four half-lives, and the data was solved to give pseudo-first-
order observed rate constants, kobs. Plots of these values against the con-
centration of substrate were linear in all cases. The second-order rate
constants for reactions of the oxo species with the organic substrates
were solved according to Eq. (1), where k0 is a background rate constant
2. Experimental
2.1. Materials
All commercial reagents were of the best available purity and were used
as supplied unless otherwise specified. HPLC grade acetonitrile (99.93%) and
methylene chloride were distilled over P2O5 prior to use. Iodosylbenzene
(PhIO) was purchased from the TCI America Co. and was used as obtained.
Tris(4-bromophenyl)ammoniumyl hexachloroantimonate was purchased
from Sigma-Aldrich Co and used as such. All reactive substrates, including
all alkenes, ethylbenzene, 4-methoxythioanisole, 4-methylthioanisole, 4-
fluorothioanisole, 4-chlorothioanisole, thioanisole, for mixing kinetic studies
were passed through a dry column of active alumina (Grade I) before use.
Free corrole ligands including H3TPFC [42] and H3TPC [43] and their
manganese(III) neutral complexes MnIII(TPFC) (1a) and MnIII(TPC) (1b)
were prepared by literature methods and characterized by 1H NMR, IR
and UV–vis spectroscopies, matching those reported [19]. Treatment of
manganese(III) species
1
with tris(4-bromophenyl)ammoniumyl
hexachloroantimonate gave known corrole–manganese(IV) chloride salts
2 [24]. Compound 2 were further purified through recrystallization from
CH2Cl2/hexane followed by column chromatography on silica gel to remove
traces of the amine by-product.
A
B
2.0
2.0
1.5
1.0
0.5
0.0
1.5
1.0
0.5
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 1. Axial ligand exchange monitored by UV–visible spectroscopy: (A) MnIV(TPFC)Cl (2a, dashed) and MnIV(TPFC)(BrO3) (3a, solid) in CH3CN; (B) MnIV(TPC)Cl (2b, dashed) and
MnIV(TPC)(BrO3) (3b, solid) in CH3CN.

W.K. Ka et al. / Journal of Inorganic Biochemistry 163 (2016) 39–44
41
they were prepared in situ and immediately used for photochemical re-
actions after preparation.
Irradiation of bromate complex 3 in anaerobic CH3CN with visible
light from a SOLA engine (output power 60 W) resulted in formation
of corrole-manganese(V)-oxo species in two different systems, i.e.
the electron-deficient 4a (Fig. 2A), and the non-electron-deficient
4b (Fig. 2B). By considering the electrophilic nature of the corrole-
manganese(V)-oxo species, one can expect that the order of appar-
ent stability is TPFC b TPC. These time-resolved UV–vis absorption
spectra show decay of photo-labile precursors (3) and growth of
the oxo products (4) with clean isosbestic points at specific wave-
lengths labeled in Fig. 2. As evident, the species 4a displayed split
Soret bands that are distinct with a stronger blue-shifted band. In
comparison to 4a, the absorption spectrum of the less electron-defi-
cient 4b was similar in shape, but with a significantly weaker blue-
shifted Soret band shown as a shoulder. Of note, the spectra of visible
light-generated 4a and 4b matched those reported previously from
ozone or PhIO oxidation of corresponding manganese(III) precursors
1 (see Fig. S1 in the Supplementary material) [19,25]. The formation
of 4a by photochemical oxidation was also observed with a slower
rate compared to that of 4b. In the TPFC system, the oxidation of
MnIV species 3a from the highly electron-deficient corrole is appar-
ently not favored in view of the expected high oxidation potential.
On the contrary, the TPC system has the less electron-withdrawing
aryl groups on the corrole ring; as thermodynamically favored; the
3b undergoes photo-oxidation to form 4b more readily.
Scheme 1. Photochemical formation of corrole-manganese(V)-oxo complexes by visible
light irradiation of manganese(IV) bromates.
found in the absence of added substrate, kox is the second-order rate
constant for reaction with the substrate, and [Sub] is the concentration
of substrate. All second-order rate constants are averages of 2–3 deter-
minations consisting of 3 independent kinetic measurements. Errors in
the rate constants were weighted and are at 2σ.
kobs ¼ k0 þ kox½Subꢀ
ð1Þ
Laser flash photolysis (355 nm) of corrole-MnIV(ClO3) complexes
gave Mn(V)-oxo species by homolytic cleavage of an O\\Cl bond [27].
Photolysis of porphyrin-RuIV(XO3)2 (X = Cl and Br) complexes was
also reported to produce trans-dioxoruthenium(VI) species through
analogous homolytic cleavage of the O-X bond [37,38]. Clearly, visible
light irradiation of bromates 3 represents the same homolysis manifold
that results in one-electron photo-oxidation to afford corrole-
manganese(V)-oxo species 4. Of note, we found that photochemical
cleavages of the bromate complexes were considerably more efficient
and faster than cleavages of chlorate complexes (see Fig. S2 in the Sup-
plementary material). Control experiments showed that no species 4
was formed in the absence of light. As expected, using more intensive
visible light (ca. 120 W) resulted in about four times faster formation
of 4a under same conditions. However, significant photo-degradation
was found for 4b with high intensity light. The use of other solvents
such as CH2Cl2 or EtOAc also gave similar results. However, no oxo for-
mation was observed when CH3OH or THF was used. Presumably, the
weakly binding bromate or chlorate counterions were displaced by
these coordinating solvents.
3. Results and discussions
3.1. Visible light formation of corrole-manganese(V)-oxo complexes
As shown in Scheme 1, two manganese(III) corroles (1) with differ-
ent electronic environments were comparatively studied in this work.
MnIII(TPFC) (1a) and MnIII(TPC) (1b) were both known and synthesized
according to the reported procedure [19,27]. As noticed in early reports
[13,44], compound 1a is significantly stable in solutions; however, in
CH2Cl2, the less electron-deficient 1b can be oxidized into MnIV corrole.
Oxidation of the neutral triarylcorrole–manganese(III) species 1 with
tris(4-bromophenyl)ammoniumyl hexachloroantimonate gave the
corrole–manganese(IV) chloride salts 2 [24]. Facile exchange of the
counterions in 2 with excess amount of Ag(BrO3) gave the correspond-
ing bromate salts 3, and its formation was indicated by the UV–vis spec-
tra. These species 3 were highly photo-labile and attempts to isolate and
spectroscopically characterize 3 more fully were not successful. Thus,
0.6
0.8
B
A
0.7
0.5
375
0.6
0.5
0.4
0.3
370
318
0.4
318
0.3
499
502
0.2
565
580
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. (A) Time-resolved formation spectra of 4a (red line) following irradiation of 3a (7. 0 × 10−6 M) over 80 s with visible light in anaerobic CH3CN solution at 23 °C; (B) Spectra of 4b
(red line) following irradiation of 3b (8. 0 × 10−6 M) over 30 s with visible light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version
of this article.)

42
W.K. Ka et al. / Journal of Inorganic Biochemistry 163 (2016) 39–44
2.0
1.6
1.2
0.8
0.4
MnV
B
MnIV
A
B
MnV
348 nm
465 nm
430 nm
1.2
0.8
0.4
MnIII
346 nm
348 nm
t/s
t/s
300
400
500
600
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. Time-resolved spectra of 4a reacting (A) in CH3CN with cyclohexene (0.5 M) over 1000s. Inset shows kinetic traces for decay of 4a (348 nm) and formation of MnIII (465 nm). (B) In
CH2Cl2 with cyclohexene (0.5 M) over 300 s. Inset shows kinetic traces for decay of 4a (346 nm) and formation of MnIV (430 nm).
3.2. Kinetic and spectral studies
point out that the formation of the MnIV species in these reactions
does not implicate one-electron reactions because the decay reactions
are relatively slow.
Oxidation kinetics of photo-generated oxo species 4 with alkenes,
activated hydrocarbon, and thioanisoles were investigated. In kinetic
studies, solutions containing the oxo 4 were mixed with solutions con-
taining large excesses of organic substrate, and pseudo-first-order rate
constants for decay of the manganese(V)-oxo species were measured
spectroscopically. Early report indicates the rates of reactions of
corrole-manganese(V)-oxo species were dependent on its concentra-
tion [27]. Thus, we produced transients 4 in a similar concentration of
approximate 2. 0 × 10−5 M for all kinetic studies. Fig. 3 shows the
UV–vis spectra changes of 4a in the presence of organic substrate in dif-
ferent solvents. Fig. 3A shows the clean conversion of 4a in CH3CN to re-
generate the MnIII species with a distinct peak at 465 nm. The decrease
in absorbance at 348 nm corresponding to the decay of 4a was accom-
panied by an isosbestic growth at 465 nm characteristic formation of
MnIII species (inset in Fig. 3A). The traces were fit to single-exponential
decay, as expected for reactions under pseudo-first-order conditions.
When CH2Cl2 was used as the solvent instead of CH3CN, Fig. 3B shows
that the species 4a regenerated a MnIV species which was characterized
by its known λmax at 430 nm. This observation is in accord with previous
LFP studies of 4a, which was also found to give MnIV species in CH2Cl2
solutions after the reaction [27]. In contrast, in both solvents reactions
of the less electron-deficient 4b returned MnIV species only and there
was no evidence for accumulation of any MnIII(TPC) during the
reaction(See Fig. S3 in the Supplementary material). It is worth to
Although the decay reactions of species 4 were complex in different
solvents and corrole systems, apparent pseudo-first-order decay rate
constants for 4 increased linearly as a function of substrate concentra-
tion (Fig. 4), thus permitting calculation of apparent second-order rate
constants (kox) for various substrates (Table 1). The trend of the kox
values in Table 1 clearly parallels the substrate reactivity; for examples,
organic sulfides are about 3 or 4 orders of magnitude more reactive than
alkenes or ethylbenzene with the same oxo species. Of note, the kox
values for cyclohexene are in good agreement with the values for oxi-
dant 4a produced from the chemical oxidation method. For the para-
substituted thioanisoles (Y-thioanisoles; Y = 4-MeO, 4-Me, 4-F, and
4-Cl), we observed a significant substituent dependence on the sec-
ond-order rate constants, as reflected in the Hammett plot shown Fig.
4B. A linear correlation (R = 0.99) of logkrel[krel = k(para-Y-
thioanisole)/k(thioanisole)] versus Hammett σ+ substituent constant
[45] gave the slope (ρ+) of the plot as −1.34
0.18, which reflects
the degree of positive charge development on the styrene in the transi-
tion states for the oxo transfer reactions. Similar values were also found
with species 4a generated by the chemical method in different solvent
(EtOAc) [28]. The Hammett correlation results strongly suggest that
the observed manganese(V)-oxo species is likely to be the active oxi-
dant which transfers an oxygen atom to thioanisoles through an electro-
philic mechanism.
Fig. 4. (A) Plots of the observed rate constants (kobs) versus the concentration of para-substituted thioanisoles; (B) Hammett plot for oxidations of para-substituted thioanisoles with 4a in
CH3CN.

W.K. Ka et al. / Journal of Inorganic Biochemistry 163 (2016) 39–44
43
Table 1
the systems apparently are cationic porphyrin–manganese(V)-oxo spe-
cies formed in disproportionation equilibria [9].
Apparent second-order rate constants (kox) for reactions of photo-generated corrole-
manganese(V)-oxo speciesa.
In view of the above significant decaying spectra and the rate con-
stants of photo-generated species 4, we propose that the oxidation of
organic substrates by corrole-manganese(V)-oxo corroles may proceed
via two different pathways as shown in Scheme 2. In the more electron-
deficient corrole system and in the polar solvent CH3CN, the active oxi-
dizing specie is MnV-oxo corrole, and the dominant pathway is the elec-
trophilic reaction between MnV-oxo corrole and substrate (Scheme 2,
path a). In this situation, MnIII corrole was observed in the end of reac-
tion. When the less polar solvent CH2Cl2 and/or the less electron-defi-
cient system were involved, the direct OAT process becomes
progressively less favorable and the disproportionation reaction of spe-
cies 4 (path b) to form MnVI-oxo (5) as the active oxidant gains domi-
nance. As outlined in Scheme 2, photo-disproportionation of 4 gives
the highly reactive manganese(VI)-oxo 5 in addition to one molecule
of a MnIV product; oxidation of an organic reductant by 5 gives a second
molecule of MnIV product. During the course of the reaction, the pro-
posed MnVI-oxo 5 was not accumulated to be detected because it reacts
much faster than it is formed. This mechanism provided a rationaliza-
tion for the inverted reactivity patterns for 4 as well as the final MnIV
product formation observed during the course of reactions. It is worth
noting that remarkable solvent and ligand effects on the participation
of multiple active oxidants were previously known in heme- and
nonheme-manganese(III) catalyzed oxidation reactions [46,47].
Oxo
Substrate
kox(M−1 s−1
)
CH3CN
CH2Cl2
4a
Cyclohexene
Cyclohexeneb
cis-Cyclooctene
PhEt
Styrene
PhSMe
p-MeO-PhSMe
p-Me-PhSMe
p-F-PhSMe
p-Cl-PhSMe
Cyclohexene
cis-Cyclooctene
PhEt
PhSMe
p-MeO-PhSMe
p-F-PhSMe
(3.4 0.2) × 10−3
(3.9 0.4) × 10−3
(2.9 0.2) × 10−3
(1.4 0.1) × 10−4
(3.0 0.1) × 10−3
4.9 0.4
(1.6 0.2) × 10−2
(1.5 0.1) × 10−2
(11.0 0.9) × 10−3
(2.7 0. 3) × 10−4
62.0 0.7
22.0 0.6
6.7 0.5
3.6 0.3
4b
(10.0 1.0) × 10−2
(5.0 0 6) × 10−2
(2.1 0.3) × 10−3
6.0 0.8
85.5 2.4
24.8 1.5
a
In CH3CN or CH2Cl2 with a concentration of 2.0 × 10−5 M at 23 2 °C. The values are
average of 3 runs with 2σ standard deviation.
b
From 4a generated by chemical oxidation of 1a [25].
3.3. Mechanistic consideration for multiple oxidation pathways
One of the noteworthy aspects of the manganese(V)-oxo kinetics is
the fact that a remarkable solvent effect on the reactivity and spectral
behavior of 4a was observed. In addition to decaying back to MnIV
corrole in CH2Cl2 solution, 4a exhibited higher reactivity in CH2Cl2
than that in CH3CN with same substrates. Owing to the electrophilic na-
ture of high-valent metal-oxo species, one typically observes that more
electron-withdrawing ligands give more reactive metal-oxo derivatives.
For the two corrole systems studied here, however, the reactivity order
is inverted with the system of less electron demand, i.e. the
triphenylcorrole complex 4b apparently reacting faster with any given
substrate than the more electron-deficient 4a. These kinetic results
strongly support the previously proposed mechanistic model involving
disproportionation of 4 to give MnIV corrole and a more reactive corrole-
manganese(VI)-oxo cationic species (5) as the predominant oxidants in
these systems [19,27,28]. Importantly, the report of a well-character-
ized corrole–manganese(VI)-nitrido species suggests that such high ox-
idation state manganese species are accessible [23]. In practice, the
concentration of 5 might be too small to permit observation, and the
major species observed spectroscopically is still 4. If complexes 5 are
the actual oxidants, then the equilibrium reactions for formation of
these species should be more favorable for the ligand that has the less
electron demand. Accordingly, the observed kinetics could reflect the
populations of species 5 controlled by the disproportionation equilibri-
um constant which should be larger for the TPC ligand [27]. The mech-
anistic model is similar to that proposed for reactions of neutral
porphyrin–manganese(IV)-oxo species, where the actual oxidants in
4. Conclusions
In conclusion, we report here a new photochemical entry to pro-
duce and study high-valent corrole-manganese(V)-oxo derivatives
by visible light irradiation of the corresponding manganese(IV) bro-
mate complexes. The photochemistry is ascribed to the homolytic
cleavage of the O\\Br bond in the bromate ligand that results in an
one-electron oxidation of the manganese (IV) to generate the
manganese(V)-oxo species. We have observed the remarkable effect
of the nature of the corrole ligand and the solvent on oxidation pathways
of these high-valent corrole-metal-oxo intermediates. Our kinetic and
spectral findings in this work provide compelling evidence to support
multiple oxidation pathways of biomimetic corrole-MnV-oxo species. A
thorough investigation of solvent effects on the reactivities of various
manganese(V)-oxo corroles derived from different electron-rich and
electron-poor substituents are currently underway in our laboratory.
Abbreviations
Cor
corrole
TPFC
TPC
PhIO
LFP
5,10,15-tripentafluorophenylcorrole trianion
5,10,15-triphenylcorrole trianion
iodosylbenzene
laser flash photolysis
OAT
oxygen atom transfer
Acknowledgments
We greatly acknowledge the National Science Foundation (CHE
1464886) and Kentucky NSF EPSCoR program (REG 2015) for support
of this research. K. W. Kwong is grateful to the Graduate School of
WKU for a Graduate Student Research Fellowship (GSRF). D. Ranburger
and J. Malone are thankful to the WKU Office of Research for awarding
internal FUSE grants.
Appendix A. Supplementary data
Scheme 2. Two oxidation pathways of corrole-manganese(V)-oxo species controlled by
the nature of ligands and solvents. The parallelogram represents a corrole structure and
subs stands for organic substrates.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.jinorgbio.2016.08.004.

44
W.K. Ka et al. / Journal of Inorganic Biochemistry 163 (2016) 39–44
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