Mechanism of the photochemical ligand substitution reactions of fac-[Re(bPY)(CO)3(PR3)]+ complexes and the properties of their triplet ligand-field excited states

We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X(2)bpy)(CO)(3)(PR3)](+) complexes (1) and the properties of their triplet ligand-field ((LF)-L-3) excited states The reason for the photostability of the rhenium complexes [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis, trans-[Re(X(2)bpy)(CO)(2)(PR3)(CH3CN)](+) (2). Isotope experiments clearly showed that the CO ligand trans to the PR3 ligand was selectively substituted. The photochemical reactions proceeded via a dissociative mechanism from the 3LF excited state. The thermodynamical data for the (LF)-L-3 excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer ((MLCT)-M-3) states were obtained from temperature-dependence data for the emission lifetimes and for the quantum yields of the photochemical reactions and the emission. Comparison of 1 with [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) indicated that the (LF)-L-3 states of some 3- and 4-type complexes are probably accessible from the (MLCT)-M-3 state even at ambient temperature, but these complexes were stable to irradiation at 365 nm. The photostability of 3 and 4, in contrast to 1, can be explained by differences in the trans effects of the PR3, py, and Cl- ligands.