Control of Photochemical, Photophysical, Electrochemical, and Photocatalytic Properties of Rhenium(I) Complexes Using Intramolecular Weak Interactions between Ligands

Intramolecular interactions between ligands have been successfully applied as a novel tool for
controlling various properties of a series of cis,trans-[Re(dmb)(CO)2(PR3)(PR‘3)]+-type complexes (dmb =
4,4‘-dimethyl-2,2‘-bipyridine), in the ground state and in the excited state and in the one-electron reduced
form. For rhenium complexes with two triarylphosphine ligands, P(p-XPh)3, the dmb ligand was sandwiched
by four aryl rings having CH(aryl)−π(pyridine)−π(aryl) interactions. On the other hand, complexes with
one triarylphosphine ligand and one trialkylphosphite ligand, P(OR)3, had π−π and CH−π interactions
between each pyridine ring in the dmb ligand and the aryl group in the P(p-XPh)3. Various properties of
these two series of rhenium complexes were compared with those of complexes having two trialkylphosphite
ligands, which do not interact through space with the dmb ligand. Properties of the complexes associated
mainly with the dmb ligand are strongly affected by the intramolecular interactions:  (1) UV/vis absorptions
to the π−π* and 1MLCT excited states were both red-shifted, but (2) emission from the 3MLCT excited
state was blue-shifted; (3) the lifetime of the 3MLCT excited state was prolonged up to 3-fold; (4) the reduction
potential in the ground state was positively shifted by 110 mV with π−π and CH−π interactions and by
180−200 mV with the CH−π−π interactions. (5) In the excited states, the oxidation power of the complex
was also enhanced by the intramolecular interactions. (6) In the corresponding one-electron-reduced species
cis,trans-[Re(dmb-•)(CO)2(PR3)(PR‘3)], the intramolecular interactions are maintained and strongly affected
their UV/vis spectra. (7) Photocatalysis for CO2 reduction was significantly enhanced only by the CH−π−π
interaction.