single-electron transfer with a strong oxidation power. Further,
by choosing appropriate ligands, these complexes can act as elec-
trochemical catalysts that drive the two-electron reduction of
CO 2 ( 58 ). These are the reasons why rhenium(I) complexes
can “single-handedly” work as photocatalysts. The high
photocatalytic abilities of some rhenium(I) complexes toward
CO 2 reduction have received great attention.
As the utilization of CO 2 using solar energy is a promising
technique toward resolving both serious problems of global
warming and the exhaustion of fossil fuels, development of
photocatalysts as a key player for this technique is desired.
Although many studies regarding photocatalytic systems simul-
taneously using Ru(bpy) 32 þ(bpy¼2,2^0 -bipyridine) as a photosen-
sitizer and metal complexes, enzymes, and metal colloids as a
catalyst, which can convert photochemical single-electron trans-
fer into multielectron reduction, and also about semiconductor
photocatalysts, have been reported, there still exist problems
such as low-reaction efficiencies and product selectivities. Rhe-
nium complexes, however, are some of the few photocatalysts
that allow both a high reaction efficiency and product selectivity.
This section gives an overview about the eminent photocatalyses
of rhenium complexes.
A. MONONUCLEARRHENIUM(I) COMPLEXES
Almost 30 years ago, Lehn et al.(59,60) first reported the
photocatalytic reduction of CO 2 by a rhenium(I) complex. They
found thatfac-Re(bpy)(CO) 3 Cl (1a) could efficiently produce CO
from CO 2 using TEOA as a sacrificial reductant. The quantum
yield (FCO) of this reaction was 0.14, and 1a was the most
efficient photocatalyst for CO 2 reduction at the time. The other
outstanding feature of this photocatalytic reaction is its high-
product selectivity. That is, the product produced using1a is
almost exclusively CO, while most other photocatalytic systems
yield H 2 and/or formic acid as side products (Table VII).
The identity of the ancillary monodentate ligand greatly
influences on the photocatalytic ability of the rhenium complexes.
fac-[Re(bpy)(CO) 3 {P(OEt) 3 }]þ (3a) is a superior photocatalyst
(FCO¼0.38) to1a( 61 ). However, the quantum yield of CO produc-
tion reduces to 0.05 iffac-[Re(bpy)(CO) 3 (PPh 3 )]þ(3f) is employed
( 62 ).fac-[Re(bpy)(CO) 3 (py)]þ (2a), with a pyridine ligand, does
not function effectively as a photocatalyst ( 29 ). The complexfac-
Re(bpy)(CO) 3 (NCS) ( 12 ), with an anionic ancillary ligand, is a good
168 HIROYUKI TAKEDAet al.