that a rhenium hydride complex is an intermediate in the path-
way to this formate complex ( 60 ), but this remains speculative.
Although the structure of the“CO 2 adduct,”which is an impor-
tant intermediate in the photocatalysis of the rhenium
complexes, has not yet been confirmed, several proposals have
been made. As candidates, a CO 2 -bridged binuclear complex
(CO) 3 (dmb)Re CO 2 Re(dmb)(CO) 3 ( 16 , dmb¼4,4^0 -dimethyl-
2,2^0 -bipyridine) and a rhenium carboxylate complex Re(bpy)
(CO) 3 (COOH) ( 17 ) were synthesized.
Fujitaet al.( 89 – 91 ) reported that 16 is produced by irradiating
the rhenium dimer complex, (CO) 3 (dmb)ReRe(dmb)(CO) 3 , in
THF under a CO 2 atmosphere. This reaction has been reported
to proceed by the reaction between CO 2 and the 17-electron spe-
cies [Re(dmb)(CO) 3 ] generated by light-induced homolysis of the
dimer (see Section III.B for homolysis of the Re Re bond). The
photoreaction of (CO) 3 (dmb)Re CO 2 Re(dmb)(CO) 3 with CO 2
quantitatively yielded CO and CO 32 .
17 is synthesized by reacting Re(bpy)(CO) 4 þwith OH. This reac-
tion is reversible, producing Re(bpy)(CO) 4 þthrough the elimination
of OH from Re(bpy)(CO) 3 (COOH) (Eq. 26). When exposed to
light, Re(bpy)(CO) 4 þreleased one CO ligand (see Section III.C for
the photochemical reaction of this complex) and was thereby con-
verted efficiently intofac-[Re(bpy)(CO) 3 S]þ(S¼solvent) ( 92 – 94 ).
Re bpyðÞðÞCO 4þ
þOH>Re bpyðÞðCOÞ 3 ðCOOHÞ
ð 26 ÞThe formation of the formate complex 14 as an alternate CO 2
adduct has been noted above. No CO is directly produced from
this complex, however, and also due to its low photocatalytic abil-
ity, 14 itself is seen as a“dead end”product rather than a reac-
tion intermediate (60,62). Recently, Fujita et al.( 95 ) have
proposed a possibility that there is a switch between the two
“CO 2 adducts”as the photocatalytic reaction proceeds. That is,
a mechanism in which dimer 16 is the main intermediate early
in the photocatalytic reaction, but due to the increase in Hþcon-
centration as the reaction proceeds, the carboxylate complex 17
becomes the intermediate ( 95 ). However, no report detecting
either“CO 2 adduct”in photocatalytic reaction solution has been
published, indicating a need for further research.
C. MULTICOMPONENTSYSTEMS
As stated above, CO 2 reduction proceeds by the OER species of
the rhenium complex serving two roles. That is, (1) serving as a
single-electron donor and (2) ligand elimination to generate a
176 HIROYUKI TAKEDAet al.