inorganic chemistry

complexes intercalated between layers of Hectorite function as
hydrogen-generating photocatalysts ( 75 ).
Reactions using rhenium complexes as redox photosensitizers
for hydrogen generation have also been reported. The coexistence
of Co complexes(76,77) or Fe complexes ( 78 ) as catalyst induced
efficient hydrogen generation.

B. REACTIONMECHANISM

The lowest excited state of the rhenium complexes used in
photocatalytic reactions is almost without exception the^3 MLCT
state. As stated inSection II, the^3 MLCT state has a long lifetime
and is reduced into an OER species by a suitable reductant
(Eq. 18).

ReIðÞLLðÞCO 3 X

hinþ

þD!

hn

ReIðÞLL
ðÞCO 3 X

hiðÞþn
 1

þDþ;

D¼TEOA; TEA ð 18 Þ

The^3 MLCT excited state of the rhenium complexes is a more pow-
erful oxidizing agent than the ground state. While the first reduc-
tion wave of1ain the ground state isE1/2red¼
1.35V vs. SCE
( 79 ), in the^3 MLCT excited state, it becomes *E1/2red¼þ1.15V,
with the oxidation power enhanced by the excitation energy
(2.50 eV) ( 80 ). Therefore, the^3 MLCT excited state is reductively
quenched by TEOA (E 0 ¼þ0.80 V vs. SCE ( 28 )) or TEA (E1/2ox¼
þ1.15 V vs. SCE ( 81 )). For 1aand 3a, the rate constants of
quenching by TEOA are kq¼8.0 107 and 1.1 109 M
^1 s
^1 ,
respectively. The initial process of this photocatalytic reaction
has been studied in detail by Kutal et al.(82,83),
Kalyanasundaramet al.( 28 ), and Turneret al.( 84 ) by laser flash
photolysis. The formation of the OER species, fac-[ReI(bpy
)
(CO) 3 X] (X¼Cl, Br), has been clearly demonstrated.
Because the potential of the one-electron reduction of CO 2 is

1.9 V (vs. NHE), neither the^3 MLCT excited state nor the
OER species of rhenium complexes can reduce CO 2 with a single
electron through outer-sphere electron transfer. As shown in
Eq. (19), however, the potential for obtaining CO by two-electron
reduction of CO 2 shifts positively to
0.53 V (vs. NHE: these
potentials of CO 2 reduction are close to the values in CH 3 CN
vs. SCE ( 80 )). Such two-electron reduction of CO 2 has been
reported to proceed efficiently using rhenium(I) complexes as
electrochemical catalyst (Eqs. 20–22) (79,85).

172 HIROYUKI TAKEDAet al.