Different Mechanisms of Photochemical Re−Me and Re−Et Bond Homolysis in [Re(R)(CO)3(4,4‘-dimethyl-2,2‘-bipyridine)]. A Time-Resolved IR Spectroscopic Study Ranging from Picoseconds to Microseconds

The photochemistry of two metal-alkyl complexes, [Re(R)(CO)3(dmb)] (R = methyl (Me), ethyl (Et)), was investigated by IR spectroscopy in the ν(CO) spectral region, time-resolved over an exceptionally broad temporal range, from picoseconds to microseconds. Optical excitation of [Re(Et)(CO)3(dmb)] in MeCN produces within the first two picoseconds the radicals [Re(MeCN)(CO)3(dmb)]• and Et•, together with an excited state, which undergoes a slower (∼90 ps) conversion to the same radicals. The reactive excited state was identified as 3MLCT (metal-to-ligand charge transfer) with an admixture of a 3SBLCT (sigma bond-to-ligand charge transfer) character. In CH2Cl2 solution, this excited state reacts with the solvent molecules to produce the radical anion [Re(Cl)(CO)3(dmb)]•- with ∼130 ps kinetics. A series of slower reactions follows, forming [Re(CH2Cl2)(CO)3(dmb)]• and, ultimately, [Re(Cl)(CO)3(dmb)]. The photochemical mechanism of [Re(Me)(CO)3(dmb)] is different. Irradiation populates a 3MLCT excited state, which undergoes two parallel reactions: a thermally activated Re−Me bond homolysis and decay to the ground state. At room temperature, both reactions have the same time constant, ∼34 ns, giving the photochemical quantum yield of about 0.5 and 3MLCT excited-state lifetime of 17 ns. The same reaction mechanism operates in CH2Cl2 and MeCN. The photoreactivity and excited-state behavior of both complexes are interpreted using qualitative potential energy surfaces. The striking difference between the ethyl and methyl complex is caused by different relative energies of the optically excited 1MLCT state, the dissociative 3SBLCT state, and the 3MLCT state along the Re−R coordinate.