inorganic chemistry

it reacts with CO 2 to generate the corresponding“CO 2 adduct”
(Eq. 24). Two-electron reduction of CO 2 proceeds due to a second
one-electron reduction of this CO 2 adduct by another molecule of
the OER species (Eq. 25). At this point,1ais regenerated by
recoordination of Cl
.

ReðbpyÞðCOÞ 3



þCO 2 !“CO 2 adduct” ð 24 Þ

“CO 2 adduct”þ RelðÞbpy
ðÞCO 3 Cl

i


!

Cl


COþ2 RelðÞbpyðÞCO 3 Cl

h h i

ð 25 Þ

The photocatalytic reaction proceeds via a similar mechanism
with complex 12 (Scheme 6). However, the elimination reaction
of the SCN
ligand from the OER species of 12 is slower than
that of the Cl
ligand from1a. Thus, the photocatalytic reaction
proceeds with OER species accumulated to some extent in the
solution. Therefore, it is believed that the high photocatalytic
activity of 12 arises as a result of the rapid electron transfer from
the OER species to the“CO 2 adduct”( 64 ).
If the lifetime of the OER species is relatively long, an inner fil-
ter effect is observed, as the OER species is able to absorb over a
wide spectrum, ranging from the ultraviolet region to the visible
region. Due to the inner filter effect, the ratio of photons
absorbed by the initial complex decreases, lowering the apparent
quantum yield of CO 2 reduction under high light intensity. Fur-
ther, photolysis of the OER species proceeds under such
conditions, also lowering TNCO, in many cases.
However, the rhenium complexes for which the corresponding
OER species rapidly lose the monodentate ligand, such as1a,3f,
and 2a, have been reported to be converted into fac-[Re(bpy)
(CO) 3 (OCHO)] ( 14 ) during the photocatalytic reaction. Due to its
low CO 2 reduction photocatalysis efficiency (FCO¼0.05), the pho-
tocatalysis diminishes as this complex forms(29,62). It is proposed

NCS –

CO

NCS– NCS–

[Re(bpy)(CO) 3 ]• [Re(bpy)(CO) 3 ]+

CO 2 CO

[CO 2 adducts]

CO

CO

N Re

N

NCS –

CO

CO

CO

N Re

N

NCS

CO

CO

CO

N Re

N

NCS

CO

CO

CO

Re

N

• – N
  
  
  • –
  
  
  
  

SCHEME6. Reaction mechanism of the OER species offac-[Re(bpy)
(CO) 3 (NCS)] ( 12 ) and CO 2.

RHENIUM(I) DIIMINE COMPLEXES 175