Selective Ion Pairing in [Ir(bipy)H2(PRPh2)2]A (A = PF6, BF4, CF3SO3, BPh4, R = Me, Ph): Experimental Identification and Theoretical Understanding

NMR studies of the title compounds show that selective tight ion pairing occurs with the anion binding site being located in an intuitively unexpected region of the cation: on the side of the bipyridyl ligand remote from the metal instead of being closest to the metal near the MH 2 group. The interaction specificity falls off with increasing anion size. ONIOM (QM/MM) calculations represent the cation structure very well, and natural population analysis calculations identify the predominant location of the positive charge at the bipyridyl ring carbons that take part in the inter-ring C−C bond (C-2 and C-2‘). This is fully consistent with tight ion pairing near these and the adjacent carbons, C-3 and C-3‘. The electrostatic potential calculated for the cation confirms the location of the preferred binding site.