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.