CO 2 þ2Hþþ2e!COþH 2 O ð 19 ÞRe bpyðÞðÞCO 3 Cl
þ2e!½ReðbpyÞðCOÞ 3
þCl ð 20 ÞRe bpyðÞðÞCO 3
þCO 2! Re bpyðÞðÞCO 3 CO 2
ð 21 ÞRe bpyðÞðÞCO 3 CO 2
þCO 2!
Cl
Re bpyðÞðÞCO 3 Cl
þCOþCO 32 ð 22 ÞIn photoreactions, however, in principle, only one-electron trans-
fer occurs with one photon and two electrons cannot be simulta-
neously inserted into the complex, as can be achieved
electrochemically. The OER species offac-[Re(LL)(CO) 3 (PR 3 )]þ-
type complexes are especially stable, and their accumulation
can be detected even during photocatalytic CO 2 reduction ( 65 ).
This allows the reaction between this OER species and CO 2 to
be studied in detail. For example, the OER species offac-[Re
(dmb)(CO) 3 {P(OEt) 3 }]þ decayed slowly according to a second-
order kinetics under an Ar atmosphere, but became faster
according to pseudo-first-order kinetics under CO 2
(k 1 ¼5.6 10 ^4 M–^1 s–^1 at [CO 2 ]¼0.139 M) ( 65 ). This rate con-
stant increased with higher CO 2 concentration. The first reduc-
tion potentials (E1/2red) of rhenium complexes, which is
attributable to the reduction of bpy ligand reduction (bpy/bpy),
the reaction rates of the corresponding OER species and CO 2 ,
and the CO production quantum yields of known photocatalysts
are summarized in Table VIII. It is noteworthy thatk 1 is rela-
tively large for complexes withE1/2red<1.4V, and they function
well as photocatalysts for CO 2 reduction. This potential is impor-
tant as complexes withE1/2red>1.4V have low photocatalytic
abilities and react slowly with CO 2.
The three rhenium complexes with anionic ligands, fac-[Re
(bpy)(CO) 3 (L)] ( 12 :L¼NCS, 8 : CN,1a: Cl), show similar
photophysical and electrochemical characteristics (Table IX).
However, their photocatalytic abilities differ significantly. Spe-
cifically, while 12 is a better photocatalyst than 1a, 8 shows
absolutely no photocatalytic behavior. The reason for 8 not to
show photocatalytic behavior is that elimination of the CN
ligand from the OER species does not proceed. As elimination
of the Clligand from the OER species of1a(Eq. 23) is relatively
fast, only a minute accumulation of the OER species is observed
under standard photocatalytic conditions. This elimination
reaction was also observed by spectroelectrochemistry (71,86– 88 ).
RHENIUM(I) DIIMINE COMPLEXES 173