Comparative Study of C∧N and N∧C Type Cyclometalated Ruthenium Complexes with a NAD+/NADH Function

Cyclometalated ruthenium complexes having CAN and NAC type coordinating ligands with NAD(+)/NADH function have been synthesized and characterized by spectroscopic methods. The variation of the coordinating position of sigma-donating carbon atom leads to a drastic change in their properties. Both the complex Ru(phbn)(phen)(2))PF6 ([1]PF6) and [Ru(pad)(phen)(2)]PF6 ([2]PF6) reduced to Ru(phbnHH)(phen)(2)]PF6 ([1HH]PF6) and [Ru(padHH)-(phen)(2)]PF6 ([2HH]PF6) by chemical and electrochemical methods. Complex [1]PF6 photochemically reduced to {1HH)PF6 in the presence of the sacrificial agent triethylamine (TEA) upon irradiation of visible light (lambda >= 420 nm), whereas photochemical reduction of [2]PF6 was not successful. Both experimental results and theoretical calculations reveal that upon protonation the energy level of the pi* orbital of either of the ligands phbn or pad is drastically stabilized compared to the nonprotonated forms. In the protonated complex [Ru(padH)(phen)(2)]PF6)(2) {[2H](PF6)(2)}, the Ru-C bond exists in a tautomeric equilibrium with Ru=C coordination and behaves as a remote N-heterocyclic carbene (rNHC) compex; on the contrary, this behavior could not be observed in protonated complex [Ru(phbnH)(phen)(2)](PF6)(2) {[1H](PF6)(2)).