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

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Abstract
We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X2bpy)(CO)3(PR3)]+ complexes (1) and the properties of their triplet ligand-field (3LF) excited states. The reason for the photostability of the rhenium complexes [Re(X2bpy)(CO)3(py)]+ (3) and [Re(X2bpy)(CO)3Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis,trans-[Re(X2bpy)(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 3LF excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer (3MLCT) 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(X2bpy)(CO)3(py)]+ (3) and [Re(X2bpy)(CO)3Cl] (4) indicated that the 3LF states of some 3- and 4-type complexes are probably accessible from the 3MLCT 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.