Selective ion pairing in [Ir(bipy)H-2(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 oc curs 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 MH2 group. The interac tion specificity falls off with increasing anion size. ONIOM (QM/ MM) calcu lations represent the cation structure very well, and natural population an alysis calculations identify the predominant location of the positive charg e 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 calcul ated for the cation confirms the location of the preferred binding site.