Photochemical generation of manganese(IV)-oxo porphyrins by visible light photolysis of dimanganese(III) μ-oxo bis-porphyrins

Article abstract of DOI:10.1016/j.ica.2016.07.037

Visible light photolysis of dimanganese(III) μ-oxo bis-porphyrins, [Mn^III(Por)]₂O, was studied in three porphyrin systems with different electronic structures. Direct conversion of manganese (III) μ-oxo dimers to manganese(IV)-oxo porphyrins plus manganese(III) products has been observed in benzene solution upon light irradiation. The spectral signature of Mn^IV(Por)(O) was further confirmed by production of the same species in the known experiment of the Mn^III(Por)Cl with PhI(OAc)₂. Continuous irradiation of dimanganese(III) μ-oxo bis-porphyrins in the presence of pyridine or triphenylphosphine gave rise to the formation of Mn^II(Por)(Py) or Mn^II(Por)(PPh₃) which were stable to be detected. A photo-disproportionation mechanism similar to that for diiron(III) μ-oxo bis-porphyrins was proposed to explain above photochemical behaviors of the three dimanganese(III) μ-oxo bis-porphyrins.

Full text of DOI:10.1016/j.ica.2016.07.037

Inorganica Chimica Acta 451 (2016) 202–206
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Research paper
Photochemical generation of manganese(IV)-oxo porphyrins by visible
light photolysis of dimanganese(III)
l-oxo bis-porphyrins
⇑
Ka Wai Kwong, Charles M. Winchester, 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
-oxo bis-porphyrins, [MnIII(Por)]2O, was studied in three
-oxo dimers
Article history:
Received 31 May 2016
Received in revised form 18 July 2016
Accepted 19 July 2016
Available online 20 July 2016
Visible light photolysis of dimanganese(III)
porphyrin systems with different electronic structures. Direct conversion of manganese (III)
to manganese(IV)-oxo porphyrins plus manganese(III) products has been observed in benzene solution
upon light irradiation. The spectral signature of MnIV(Por)(O) was further confirmed by production of
the same species in the known experiment of the MnIII(Por)Cl with PhI(OAc)2. Continuous irradia-
l
l
tion of dimanganese(III)
to the formation of MnII(Por)(Py) or MnII(Por)(PPh3) which were stable to be detected. A photo-
disproportionation mechanism similar to that for diiron(III) -oxo bis-porphyrins was proposed to
explain above photochemical behaviors of the three dimanganese(III) -oxo bis-porphyrins.
l-oxo bis-porphyrins in the presence of pyridine or triphenylphosphine gave rise
Keywords:
Dimanganese(III) l-oxo bis-porphyrins
Manganese(IV)-oxo porphyrin
Disproportionation
Visible light
l
l
Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction
(IV)-oxo derivatives are less reactive than manganese(V)-oxo spe-
cies in manganese porphyrin-catalyzed oxidations [16–18].
Photochemistry is particularly intriguing to explore the metal-
oxo chemistry [19,20]. With photochemical production of reactive
metal-oxo transients, one has access to time scales that are much
shorter than the fastest mixing experiments. Furthermore, kinetics
of oxidation reactions of transients of interest are not convoluted
with the kinetics of reactions that form the transients. In this
regard, laser flash photolysis (LFP) techniques have been success-
fully developed for generation and direct kinetic studies of a vari-
ety of high-valent transition metal-oxo species supported by
porphyrin and corrole ligands [21–26]. In so-called photo-induced
ligand cleavage reactions, both porphyrin-manganese(IV)-oxo spe-
cies and porphyrin-manganese(V)-oxo species can be produced as
a function of the identity of the axial ligands. For examples, irradi-
ation of porphyrin-manganese(III) nitrates and chlorates resulted
in homolytic cleavage of the OAX bonds in the ligands to afford
porphyrin-manganese(IV)-oxo species, whereas irradiation of por-
phyrin-manganese(III) perchlorates resulted in heterolytic cleav-
age of OACl bonds to give porphyrin-manganese(V)-oxo cations
[27].
Catalytic oxidation is a pivotal transformation for chemical syn-
thesis in organic laboratories and petrochemical industries [1,2]. In
Nature, the ubiquitous cytochrome P450 enzymes (P450s) can cat-
alyze a wide variety of oxidation reactions with exceptionally high
reactivity and selectivity [3,4]. In the past decades, many transition
metal catalysts bearing a core structure resembling the iron por-
phyrin core of P450s, have been synthesized as models to probe
the sophisticated mechanism of molecular oxygen activation as
well as to invent enzyme-like oxidation catalysts [5–8]. In general,
high-valent transition metal-oxo species have been implicated as
the active oxidizing species [9,10]. In particular, manganese-oxo
intermediates are among the more reactive oxidizing transition
metal derivatives. A variety of these species are employed catalyt-
ically in applied syntheses [5,6], and Nature uses Mn-oxo species in
the production of oxygen in photosynthesis II [11]. Highly reactive
porphyrin-manganese(V)-oxo derivatives [12,13] are proposed
intermediates in catalytic processes that have been known for
decades [14,15]. In contrast, the well-characterized manganese
Notably, the chemistry of cofacial metal-bis-porphyrins has
drawn increased attention owing to the ability of these systems
to utilize molecular oxygen and visible light (sunlight) for organic
oxidations [28,29]. One example is the catalytic aerobic oxidation
by high-valent iron(IV)-oxo species in a process that involves
Abbreviations: Por, porphyrin; 4-CF3TPP, 5,10,15,20-tetrakis(4-trifuoro-
methylphenyl)porphyrin dianion; 4-FTPP, 5,10,15,20-tetrakis(4-fluorophenyl)
porphyrin dianion; TPP, 5,10,15,20-tetraphenylporphyrin dianion.
⇑
Corresponding author at: Department of Chemistry, Western Kentucky Univer-
sity, 1906 College Heights Blvd #11079, Bowling Green, KY 42101-1079, USA.
photo-disproportionation of a diiron(III)-
l
-oxo bis-porphyrin com-
-oxo systems
plex [30,31]. The catalytic efficiency of diiron(III)-
l
E-mail address: rui.zhang@wku.edu (R. Zhang).
http://dx.doi.org/10.1016/j.ica.2016.07.037
0020-1693/Ó 2016 Elsevier B.V. All rights reserved.

K.W. Kwong et al. / Inorganica Chimica Acta 451 (2016) 202–206
203
was improved by employing ‘‘Pacman” ligand designs with organic
spacer-hinges, which can pre-organize two iron centers in a favor-
able co-facial arrangement [32–35]. In a similar fashion, we discov-
sterically hindered porphyrin
l
-oxo dimers which contain rela-
tively large electron-donating substituents (including methyl and
methoxy) at ortho position of the meso-phenyl rings were not suc-
cessful, apparently due to the steric hindrance. As noticed in earlier
reports [37,41], it was found that electron-withdrawing sub-
ered that photolysis of
a bis-corrole-iron(IV) l-oxo dimer
apparently proceeded by the same type of photo-disproportiona-
tion mechanism to give corrole-iron(V)-oxo transient [36,37]. In
addition, we reported a putative porphyrin-ruthenium(V)-oxo spe-
cies generated in a similar photo-disproportionation process that
has shown great potential for aerobic photocatalytic oxidations
[38,39]. Herein, we report direct spectroscopic observation of pho-
tochemical generation of porphyrin-manganese(IV)-oxo com-
plexes by visible light irradiation of bis-porphyrin-manganese(III)
stituents such as CF3 and F on porphyrin ligand favor l-oxo dimer
formation. Presumably, the electron-withdrawing groups could
stabilize the metal complexes in a dimer form by reducing the elec-
tron density of metal atoms. It is noteworthy that compound 1 is
significantly stable in non-polar solvents such as benzene or cyclo-
hexane; however, in CH2Cl2 or acetonitrile solution, complex 1 is
not stable and gradually returns to the monomeric MnIII complex
(Fig. S3 in the Supplementary Information). Apparently, the rela-
tively weak MnAOAMn bond in dimeric complexes can be readily
dissociated by these polar solvents.
l-oxo dimers. The results in the current study support the conclu-
sion that bis-porphyrin-manganese(III) -oxo dimers undergo
l
photo-disproportionation reaction upon light irradiation to gener-
ate a germinal porphyrin-manganese(II)/manganese(IV)-oxo pair
that can be detected and studied in real time (Scheme 1).
2.2. Visible light photolysis of bis-porphyrin-dimanganese(III)
dimers
l-oxo
2. Results and discussions
Visible light irradiation of l-oxo complex 1a in anaerobic ben-
zene from a SOLA engine (120 W) gave rise to the formation of a
transient species 2a with a slightly blue-shifted Soret band at
420 nm. The absorption spectral changes shows that the twin
peaks of 1a located at 425 and 472 nm gradually decreased and a
new peak at 420 nm appeared during the course of irradiation up
to 3.5 min (Fig. 2A). Judging from the spectral changes and kinetic
behavior, the transient species that formed at 420 nm could be
ascribed to the manganese(IV)-oxo porphyrin, i.e. MnIV(4-CF3TPP)
(O). Note that the absorption spectra of 2a overlap with that of
another MnIII porphyrin which apparently gave a Soret band at
472 nm and partial absorption in the range of 370–415 nm. The
spectra signature of the photo-generated 2a as MnIV(4-CF3TPP)
(O) was further confirmed by production of the same species from
chemical oxidation of manganese(III) with PhI(OAc)2 as a mild oxi-
dant (Fig. 2B) [42]. When excess amounts of organic reductants
such as cyclohexene or styrene were added to above solutions of
2a, the UV–visible spectrum of 2a returned to that of manganese
(III) porphyrin, which was recovered in >90% yield (Fig. 3). The
overall reaction sequence is consistent for the behavior expected
for porphyrin-manganese(IV)-oxo species [42]. In a similar fashion,
irradiation of 1b with visible light also produced 2b in another por-
phyrin system (Fig. S4 in Supplementary Information). Although
the complex 1c showed relatively low solubility in benzene solu-
tion, visible light irradiation of 1c gave similar results (data not
shown) as compared to that of 1a and 1b.
2.1. Synthesis and spectral studies of bis-porphyrin-dimanganese(III)
l-oxo complexes (1)
As shown in Scheme 1, three porphyrin systems, 5,10,15,20-
tetrakis(4-trifluoromethylphenyl)porphyrin (4-CF3TPP, a), 5,10,15,
20-tetrakis(4-fluorophenyl)porphyrin (4-FTPP, b) and 5,10,15,
20-tetraphenylporphyrin (TPP, c), were studied in this work. Abbre-
viations used here follow those conventionally established. All
ligands a-c are generally considered as a sterically non-encumbered
porphyrin due to the absence of substituents on the ortho positions
of the meso-phenyl groups. The different aromatic groups on the
porphyrins also result in varying electron demands with the triflu-
oromethylphenyl system being the most electron withdrawing.
Following a known procedure [40], three bis-porphyrin-diman-
ganese(III)
l-oxo complexes (1) were synthesized by reacting a
corresponding MnIII(Por)Cl with NaOH in benzene solutions. The
formed products in three porphyrin systems are characterized by
UV–visible and 1H NMR spectra. The representative spectra of 1a
are shown Fig. 1 and other spectra for 1b and 1c are shown in
Figs. S1 and S2 in the Supplementary Information. All bis-por-
phyrin-dimanganese(III)
l-oxo complexes show distinct UV–vis
absorption characterized by split Soret bands at approximate
425 nm and 474 nm, respectively, consistent with reported values
in the literature [40]. Interestingly, the non-oxidative transforma-
tion of manganese(III) monomer to its
l-oxo dimer 1 was also
identified by featuring paramagnetically shifted pyrrolic protons,
which was slightly more downfield than that of manganese(III)
monomer of MnIII(Por)Cl (Fig. 1B). Attempts to prepare some
2.3. Proposed mechanism
Visible light photolysis of dimer 1 to produce manganese(IV)-
oxo porphyrin 2 can be explained by a photo-disproportionation
mechanism similar to that previously established for the photoly-
Ar
sis of bis-porphyrin-diiron(III) l-oxo complexes [30]. As outlined in
Scheme 2, photo-disproportionation of dimer 1 gives the reactive
manganese(IV)-oxo transient 2 in addition to one molecule of a
porphyrin-manganese(II) species 3 and oxidation of an organic
reductant by 2 gives a second molecule of 3. Species 3 was known
to be unstable in aerobic media and tends to be rapidly oxidized to
the manganese(III) complex (4) [20]. Clearly, photolyses of diman-
N
N
N
Ar
N
N
MnII
Ar
Ar
N
N
N
MnIII
Ar
Ar
Ar
Ar
visible light
3
Ar
Ar
O
Ar
N
O
N
N
MnIII
ganese(III)
manifold similar to that of diiron(III)
diruthenium(IV)- -oxo bis-porphyrins [38] and diiron(IV)-
l
-oxo bis-porphyrins 1 appear to present a reaction
-oxo bis-porphyrins [30],
-oxo
Ar
N
MnIV
N
N
Ar
Ar
l
N
N
l
l
Ar
Ar = 4-CF3C6H4, a
Ar = 4-FC6H4, b
Ar = C6H5, c
Ar
bis-corroles [37]. In light of the current interest in photo-dispro-
portionation reaction, the effects of electronic structure on the
1
2
photolysis of manganese (III) l-oxo bisporphyrins and insights into
the detailed intramolecular electron transfer process involved
deserves further study including the theoretic support from
Scheme 1. Visible light photolysis of bis-porphyrin-manganese(III)
produce germinal porphyrin-manganese(II)/manganese(IV)-oxo pair in three
porphyrin systems.
l-oxo dimers to
a

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K.W. Kwong et al. / Inorganica Chimica Acta 451 (2016) 202–206
2.0
1.5
1.0
0.5
0.0
0.4
B
A
MnIII(4-CF3 -TPP)Cl
0.2
0.0
[MnIII(4-CF3TPP)]2O
300
400
500
600
700
10
0
-10
-20
-30
Wavelength (nm)
Chemical Shift (ppm)
Fig. 1. (A) UV–vis spectra of manganese(III) chloro precursor MnIII(4-CF3TPP)Cl (dashed line) and bis-porphyrins dimanganese(III)
(solid line) in benzene (B) 1H NMR spectrum of MnIII(4-CF3TPP)Cl (top) and [MnIII(4-CF3TPP)]2O (1a, bottom) in CDCl3.
l
-oxo complex [MnIII(4-CF3TPP)]2O (1a)
1.8
O=MnIV
B
O=MnIV
A
MnIII
1.8
1.2
0.6
0.0
MnIII-O-MnIII
1.2
0.6
0.0
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. (A) Time-resolved spectrum of 2a upon irradiation of 1a with a 120-W visible lamp in anaerobic benzene over 3.5 min. at 23 2 °C; (B) UV–visible spectrum of 2a
(solid line) generated by reacting MnIII(4-CF3TPP)Cl (dash line) with PhI(OAc)2 (25 equiv.) in benzene at ambient temperature.
MnIII
O
O
MnIV
X
MnIII
MnIII
O=MnIV
fast
self decay
0.5 M
MnII
visible light
2
3
4
1.2
0.6
0.0
MnIII
0
100
200
1
Substrate
Oxidized substrate
t/s
Scheme 2. A proposed photo-disproportionation mechanism (square represents
the porphyrin ring).
presence of excess pyridine was conducted. Similar to the well-
known photo-disproportionation of diiron(III)-
complexes [30], the -oxo precursor 1a gradually converted to a
l-oxo bis-porphyrin
300
400
500
600
700
l
Wavelength (nm)
stable product 5a with well-anchored isosbestic points (Fig. 4A).
According to previous studies by Hoshino and coworkers [43],
the product 5a formed with kmax at 440 nm was assigned as MnII(4-
CF3TPP)(Py), which was further confirmed by production of the
same species in the known experiment of the MnIII(4-CF3TPP)
(NO2) with excess pyridine [43]. The reactants and products of
the overall photoreaction are matched to their respective absorp-
tion profiles. This photochemical observation of a clean reduction
Fig. 3. Time-resolved spectra over 220 s for reaction of photo-generated 2a with
0.5 M cyclohexene in benzene at 23 2 °C; the inset shows the kinetic trace at
420 nm in the absence (self-decay) and presence of cyclohexene (0.5 M).
computational calculations. These studies are expected to further
our understanding of photo-disproportionation reactions in differ-
ent systems.
of [MnIII(Por)]2O to MnII(Por)(Py) further supports
a photo-
2.4. Photolysis of bis-porphyrin-dimanganese(III)
presence of pyridine and triphenylphosphine
l-oxo dimers in the
disproportionation pathway that allows for the generation of the
manganese(IV)-oxo porphyrin. Of note, similar spectral changes
were also observed upon the photo-disproportionation of
To accentuate the proposed photo-disproportionation
mechanism, irradiation of the thermally stable complex 1a in the
dimanganese(III)-
l-oxo bis-porphyrin (1a) in the presence of
excess Ph3P that gave another stable MnII(4-CF3TPP)(PPh3) (Fig 4B).

K.W. Kwong et al. / Inorganica Chimica Acta 451 (2016) 202–206
205
MnII
MnIII-O-MnIII
MnII
2.4
A
B
MnIII-O-MnIII
1.8
1.2
0.6
0.0
1.8
1.2
0.6
0.0
300
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 4. (A) Time-resolved spectra of MnII(4-CF3TPP)(Py) generated by visible light irradiation of 1a in the presence of excess amount of pyridine (3 mM) over 50 min in
benzene; (B) Time-resolved spectra of MnII(4-CF3TPP)(PPh3) generated by irradiation of 1a in the presence of excess amount of triphenylphosphine (3 mM) over 8 min in
benzene. Visible light was produced from a 120-W solar lamp.
desired dimanganese(III)-l-oxo compounds. The l-oxo-dimer
3. Conclusions
products were collected by vacuum filtration and washed with
water, and dried in air. The [MnIII(Por)]2O products were further
purified by recrystallization in benzene and cyclohexane, then fully
dried in vacuum.
In conclusion, photo-cleavage of bis-porphyrin-manganese(III)
-oxo dimers using visible light proceeds by homolysis of an
MnAO bond to give manganese(IV)-oxo porphyrins that are spec-
troscopically indistinguishable from the species formed by chemi-
cal oxidation of the corresponding porphyrin-manganese(III)
chlorides. In addition, continuous irradiation of dimanganese(III)
l
4.3. General procedure for photolysis of dimanganese(III) bis-
porphyrin [MnIII(Por)]2O
l
-oxo bis-porphyrins in the presence of pyridine or triphenylphos-
phine gave rise to the formation of MnII(Por)(Py) or MnII(Por)(PPh3)
that was also directly observed. The photochemical behavior of
When the benzene solution of 1 at desired concentration (ca.
1.5 Â 10À5 M) was irradiated with a 120 W of visible light at ambi-
ent temperature, the formation of porphyrin-manganese(IV)-oxo
species 2 was complete in ca. 3–4 min monitored by UV–visible
spectroscopy. Reactions of oxo-species 2 with excess cyclohexene
(0.5 M) were conducted in a solution at 23 2 °C, and the observed
rates were monitored by the decay of the Soret absorption band of
the oxo-species 2. The identity of 2 as MnIV(Por)(O) was confirmed
by producing the same species from chemical oxidation of corre-
sponding manganese(III) chloride precursor with 25-fold excess
of PhI(OAc)2 in benzene. When the solution of 1 in the presence
of excess pyridine or triphenyl phosphine (3 mM) was irradiated
with visible light at ambient temperature, the time-resolved forma-
tion of manganese(II) products (3) was monitored by UV–visible
spectroscopy during the course of irradiation up to 50 min.
dimanganese(III)
to a photo-disproportionation mechanism similar to that for diiron
(III) -oxo bis-porphyrins. Further studies to characterize the
observed transients spectroscopically more fully are ongoing in
our laboratory.
l-oxo bis-porphyrins upon irradiation is ascribed
l
4. Experimental
4.1. General considerations
All commercial reagents were of the best available purity and
were used as supplied unless otherwise specified. HPLC grade ben-
zene (>99.9%) was purified by passing through a dry Al2O3 (Grade I
neutral) column prior to use. Free porphyrin ligand including
H2(4-CF3TPP) (a), H2(4-FTPP)(b) and H2TPP(c) were prepared
according to the known methods [44]. The corresponding
manganese(III) chloride complexes MnIII(Por)Cl (Por = a-c) used
for generation of [MnIII(Por)]2O (1) were prepared by literature
methods and characterized fully, matching those reported [45].
UV–vis spectra were recorded on an Agilent 8453 diode array spec-
trophotometer. 1H NMR was performed on a JEOL ECA-500 MHz
spectrometer at 298 K with tetramethylsilane (TMS) as internal
standard. Chemical shifts (ppm) are reported relative to TMS. Visi-
ble light was produced from a SOLA SE II light engine (Lumencor)
configured with a liquid light guide. The output power is 120 W.
Acknowledgments
This work was supported by the National Science Foundation
(CHE 1464886) and Kentucky NSF EPSCoR program (REG 2015).
K. W. Kwong is thankful to the Graduate School of WKU for a
Graduate Student Research Fellowship (GSRF).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2016.07.037.
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