phosphine ligands and the diimine ligand (see Section II.C). This
weak interaction between the ligands red-shifts the MLCT absorp-
tion band, but blue-shifts the emission from the^3 MLCT excited
state. The excited state lifetime becomes longer (t¼1.0ms), and
the oxidation power of the excited state is enhanced. These
changes in the complexes’properties are all positive in terms of
promoting the photocatalytic reaction.
Aiming at enhancing the efficiency of the photocatalytic reac-
tion using rhenium complexes, CO 2 reduction under various
conditions were considered. Under a pressurized CO 2 atmo-
sphere (2.45 MPa), the photocatalytic ability of fac-Re(bpy)
(CO) 3 Cl increases as compared to atmospheric pressure and the
turnover number (TNCO¼the number of CO molecules pro-
duced/the number of rhenium complex molecules used) become
5.1 times greater( 68 ). At a pressure of 1.36 MPa,fac-[Re(bpy)
(CO) 3 {P(Oi-Pr) 3 }]þshowed a 3.8-fold increase in TNCO( 69 ). The
explanation given for such increase in the durability of rhenium
complexes as photocatalysts is the suppression of the decomposi-
tion of the rhenium complexes by side reactions. From the same
perspective, photocatalytic CO 2 reduction using rhenium
complexes in supercritical or liquid CO 2 , which has been given
great attention as a clean solvent, has been considered. The com-
plex employed in this study wasfac-[Re(bpy)(CO) 3 {P(OC 6 H 13 ) 3 }]
(BArF) ( 70 ), which has a fluorinated counter anion BArF
(tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) to increase solu-
bility in liquid CO 2. CO production in liquid CO 2 at 7.0 MPa
was confirmed to have a TNCOof 2.2 using this complex.
Further, gas–solid state reactions are possible with solid
catalysts that have fac-Re(bpy)(CO) 3 Cl or fac-[Re(bpy)
(CO) 3 (py)]þembedded in the pores of porous materials such as
zeolite (NaY) and mesoporous silica (AlMCM-41). CO production
using these catalysts with adsorbed water and CO 2 has been con-
firmed (72,73). From the fact that CO 2 reduction proceeds with-
out the addition of a reducing agent, the AlMCM-41 framework
has been suspected as serving as the reducing agent.
As noted above, in most cases, in which rhenium(I) complexes
are employed as a photocatalyst, CO 2 reduction to CO selectively
proceeds, and hydrogen and formic acid are barely formed even
under conditions associated with significant quantities of proton
donors such as water and TEOA. However, only the following
two cases of hydrogen-generating photocatalytic reactions using
rhenium complexes have been reported. In ether solvents such
as THF, irradiation of fac-Re(bpy)(CO) 3 Br under an Ar atmo-
sphere in the presence of TEA generated hydrogen catalytically
(TNH2¼ 7 – 10) ( 74 ). Also, it has been reported that rhenium
RHENIUM(I) DIIMINE COMPLEXES 171