Fig. 3). However, [RuII(dmb) 3 ]^2 þ (dmb¼4,4^0 -dimethyl-2,2^0 -
bipyridine) strongly absorbs light in the visible region and has a
long excited state lifetime of 881 ns and therefore can be employed
as a redox photosensitizer. Because the OER species
[RuII(dmb) 2 (dmb)]þ, produced by reductive quenching of its
(^3) MLCT excited state by a reductant such as 1-benzyl-1,4-
dihydronicotinamide (BNAH), has a strong reducing power of
1.77 V vs. Ag/AgNO 3 , it can reduce rhenium complexes such as
fac-Re(dmb)(CO) 3 Cl (E1/2¼1.78 V vs. Ag/AgNO 3 ),fac-[Re(dmb)
(CO) 3 {P(OEt) 3 }]þ (E1/2¼1.60 V vs. Ag/AgNO 3 ), and fac-[Re
(dmb)(CO) 3 (py)]þ(E1/2¼1.64 V vs. Ag/AgNO 3 ) by electron trans-
fer to give the corresponding OER species. In fact, irradiation of a
1:1 mixture of [Ru(dmb) 3 ]^2 þandfac-Re(dmb)(CO) 3 Cl using>500-
nm light in the presence of BNAH photocatalytically reduced CO 2
to CO (FCO¼0.062, TNCO¼101) ( 96 ).
D. RUTHENIUM(II)–RHENIUM(I) SUPRAMOLECULARPHOTOCATALYSTS
Aiming to promote electron transfer from the OER species of
[Ru(dmb) 3 ]^2 þto the rhenium complex, supramolecular complexes
connecting these two molecules have been synthesized
(Scheme 8). The photocatalytic reaction ofRuC3(OH)ReClthat
has [Ru(dmb) 3 ]^2 þ and fac-Re(dmb)(CO) 3 Cl connected with a
CH 2 CH(OH)CH 2 group between each one of diimine ligands
was reported first. The CO 2 reduction photocatalytic ability of
this supramolecular complex is higher than the mixture of the
corresponding mononuclear complexes, reachingFCO¼0.12 and
TNCO¼170 ( 96 ).
Photocatalysis using these supramolecular complexes shows a
strong dependence on the number of carbon atoms within the
bridge and as can be seen from the results of RuC2Re
(FCO¼0.13, TNCO¼180), RuC4Re (FCO¼0.11, TNCO¼120),
andRuC6Re(FCO¼0.11, TNCO¼120). Bridging by a C 2 H 4
group gave the highest CO 2 reduction photocatalytic ability
( 97 ). The effect of the monodentate ligand of the rhenium unit
is also important. The complex with a P(OEt) 3 ligandRuC3
(OH)ReP(OEt) 3 showed superior photocatalytic behavior
(FCO¼0.21, TNCO¼232), but on the contrary,RuC3(OH)Repy
exhibited reduced durability (TNCO¼97) ( 98 ).
Supramolecular photocatalysts connecting several rhenium
complexes to a ruthenium complex and connecting several ruthe-
nium complexes to one rhenium complex have also been
synthesized. The photocatalytic abilities toward CO 2 reduction
of each were RuRe2 (TNCO¼190), Ru2Re (TNCO¼110)
(99,100),RuRe3(FCO¼0.093, TNCO¼240) ( 96 ).
178 HIROYUKI TAKEDAet al.