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

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