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Journal of the American Chemical Society
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Namita Sharma,† Jieun Jung,†,‡ Kei Ohkubo,†,§,# Yong-Min Lee,† Mohamed E. El-Khouly,†,& Wonwoo Nam,*,†,
and Shunichi Fukuzumi*,†,¶
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†Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
‡Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
§Institute for Advanced Co-Creation Studies, Osaka University, Suita, Osaka 565-0871, Japan
#Institute for Academic Initiatives, Osaka University, Suita, Osaka 565-0871, Japan
&Department of Chemistry, Faculty of Science, Kafrelsheikh University, Kafrelsheikh 33516, Egypt
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School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China
¶Faculty of Science and Engineering, Meijo University, SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi 468-
0073, Japan
Supporting Information Placeholder
We report herein that photoexcitation of a MnIV-oxo complex bind-
ing scandium ions, [(Bn-TPEN)MnIV(O)]2+–(Sc(OTf)3)2 (Bn-TPEN
= N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diamino-ethane)14,17 re-
sulted in the formation of a long-lived photoexcited state with a lifetime
of 6.4 s, which is capable of hydroxylating benzene with water to pro-
duce phenol. This is the first example showing the formation of an ex-
tremely long-lived photoexcited state of a high-valent metal-oxo com-
plex binding a redox-inactive metal ion, which shows a reactivity in ben-
zene hydroxylation.
The [(Bn-TPEN)MnIV(O)]2+ complex binding two Sc3+ ions, [(Bn-
TPEN)MnIV(O)]2+–(Sc(OTf)3)2 (1), was synthesized as reported pre-
viously (see Scheme 1 for the schematic structure).14 No formation of a
long-lived transient absorption species was observed upon photoexcita-
tion of [(Bn-TPEN)MnIV(O)]2+ without Sc3+ (see Figure S1a in Sup-
porting Information (SI)). In sharp contrast to the case of [(Bn-
TPEN)MnIV(O)]2+, which exhibited no long-lived transient absorption
(Figure S1a in SI), nanosecond laser excitation of a deaerated solvent
mixture of CF3CH2OH (TFE)/CH3CN (MeCN) (v/v = 1:1) of 1 re-
sulted in the formation of the long-lived excited state, which has an ab-
sorption band at max = 640 nm (Figure 1). From the decay time profile
of absorbance at 640 nm, the lifetime of the photoexcited state of 1 was
determined to be = 6.4 µs at 298 K (Figure 1, inset). This long-lived
excited state is assigned as the doublet 2E photoexcited state because of
the spin forbidden decay to the quartet ground state as reported for
ABSTRACT: Photoexcitation of a MnIV-oxo complex binding scan-
dium ions ([(Bn-TPEN)MnIV(O)–(Sc(OTf)3)2]2+) in a solvent
mixture of trifluoroethanol and acetonitrile (v/v = 1:1) resulted in
formation of the long-lived photoexcited state, which can hydrox-
ylate benzene to phenol. The photohydroxylation of benzene by
[(Bn-TPEN)MnIV(O)–(Sc(OTf)3)2]2+ was made possible by elec-
tron transfer from benzene to the long-lived 2E excited state of [(Bn-
TPEN)MnIV(O)–(Sc(OTf)3)2]2+ to produce benzene radical cation,
which reacted with water as revealed by laser-induced transient ab-
sorption measurements.
High-valent metal-oxo complexes have been invoked as key intermedi-
ates in the catalytic reduction of dioxygen and the oxidation of water as
well as organic substrates.1-4 In particular, a MnV-oxo intermediate has
been proposed to be a strong oxidant to be able to oxidize water to diox-
ygen at the oxygen-evolving complex (OEC) in Photosystem II (PSII).5-
10
In the OEC, Ca2+ ion acts as an essential cofactor in the manganese-
calcium-oxygen cluster (Mn4CaO5), which is responsible for water oxi-
dation in PSII.5-10 Binding of redox-inactive metal ions such as Ca2+ to
metal-oxo complexes is reported to result in much enhancement of the
electron accepting ability of metal-oxo complexes.11-14 For example,
binding of two Sc3+ ions to MnIV-oxo complexes, MnIV(O)-(Sc3+)2, re-
sulted in the large positive shifts of the one-electron reduction potentials
of the MnIV(O) complexes, converting them to be much stronger oxi-
dants.14 The electron-transfer reactivity of metal-oxo complexes is ex-
pected to be much enhanced in the photoexcited state and further en-
hanced by binding of Sc3+ ions. However, the excited state chemistry of
first-row transition metal-oxo complexes has yet to be explored due to
the extremely short lifetime. There have been only reports on photoex-
cited states of trans-dioxo complexes of rare metals such as rhenium(V),
osmium(VI), technetium(V) and tungsten(V), which exhibited emis-
sion.15 An iron(III) complex has recently been reported to exhibit emis-
sion from a doublet ligand-to-metal charge-transfer state with a lifetime
of 100 picoseconds.16 Thus, it still remains a formidable challenge to ac-
cess long-lived excited states of first-row transition metal complexes
with a lifetime of over one microsecond.
Scheme 1. Schematic structures of (A) [(Bn-TPEN)MnIV(O)]2+ and
(B) [(Bn-TPEN)MnIV(O)]2+–(Sc(OTf)3)2
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