Cooperative N–H and CH2 Skeleton Effects on the Catalytic Activities of Bimetallic Ru(II)–NNN Complexes: Experimental and Theoretical Study

Bimetallic ruthenium(II) complexes bearing a bis(pyrazolylimidazolylpyridine) ligand bridged by a rotatable single C–C bond or methylene linker were synthesized, structurally characterized, and exhibited diverse catalytic activities for the transfer hydrogenation (TH) reactions of ketones in refluxing isopropyl alcohol. Both the unprotected NH functionality and bridging methylene moiety demonstrated an acceleration effect on such TH reactions. Combination of the NH and CH2 skeleton functionalities into the bimetallic Ru(II)–NNN complexes remarkably enhanced the catalytic activities of the complex catalysts. Density functional theory calculations have suggested that the difference in the catalytic activities of these Ru(II)–NNN complexes is attributed to the inherent nucleophilic character of the coordinative nitrogen atoms in the bis(NNN) ligand, and the metal–metal interaction resulted from the number of net natural bond orbital charges on these nitrogen atoms.