Outer Sphere Metal-to-Ligand Charge Transfer in Organometallic Ion Pairs

Abstract

A series of organometallic salts which comprise a fac-(b)ReI(CO)3(py)+ cation (b = 4,4‘,5,5‘-tetramethyl-2,2‘-bipyridine (tmb), 2,2‘-bipyridine (bpy), or 4,4‘-dicarbomethoxy-2,2‘-bipyridine (dmeb); py = pyridine) paired with the Co(CO)4- anion have been prepared and subjected to photophysical study. In nonpolar solvents the salts feature a broad, low-intensity ion-pair charge transfer (IPCT) absorption band. The energy of the IPCT band decreases with the LUMO energy of the diimine ligand, suggesting that the orbital basis of the transition is d (Co) → π* (b). An X-ray crystal structure of [(bpy)ReI(CO)3(py)+][Co(CO)4-] (2a) reveals that the anion occupies a lattice position which is directly below (or above) the plane defined by the bpy ligand, which supports the d (Co) → π* (b) IPCT assignment. Luminescence studies of the salts indicate that the dπ (Re) → π* (b) metal-to-ligand charge transfer (MLCT) excited state is quenched by reductive electron transfer from Co(CO)4-. Nominally IPCT and MLCT excitatio...