Ruthenium(II) Complexes of a Xanthene-Spanned Dicarbene Ligand

Octahedral ruthenium(II) complexes of a xanthene-di(N-heterocyclic carbene) ancillary ligand (XdC) have been prepared and structurally characterized. Examples catalyze the transfer hydrogenation of ketones {[Ru(CO)I₂(C,O,C'-XdC)] (1) and [Ru(CO)(MeCN)₂(C,O,C'-XdC)]²⁺ (2)} and the selective electrochemical reduction of CO₂ to CO {[Ru(N,N'-bpy)(CO)(C,O,C'-XdC)]²⁺ (3) at 0.40 V overpotential in MeCN-H₂O (1 M)}. The reaction of 1 with KBEt₃H afforded isomers of [(C,C'-XdC)Ru(μ-H)(H)]₂ dimers, which are stable to reductive elimination of the XdC ligand, thereby suggesting similar (XdC)Rh(coligand)(H) species may be viable intermediates in catalyses. The electrochemical reduction of CO₂ involves a double reduction of 3 to 3, which has been characterized by IR-SEC and DFT calculations. The DFT calculations suggest the Ru-O_xanth bond breaks in 3, opening a metal site for CO₂ binding with selectivity over protons enabled by the diffuse nature of the HOMO delocalized over the metal and the bipyridine and carbonyl coligands. The results point to the promise of metal complexes of flexible and hemilabile xanthene-(NHC)₂ ancillary ligands in catalysis.