Are DFT Methods Able to Predict Reduction Potentials of Ruthenium Nitrosyl Complexes Accurately?

The nitric oxide (NO) molecule is directly related to important physiological and physiopathological processes. Ruthenium tetraammine nitrosyl complexes can release NO from the [RuIINO+]3+/[RuIINO0]2+ reduction potential. Experimentally, well established is the practice of determining the redox potential with relation to a reference redox pair. However, there is no agreement on the best methodology that allows the minimization of uncertainties, both experimental and theoretical results. Here, the reduction potential relative to a reference redox pair is obtained from calculated absolute potentials for the target complex: trans-[Ru(NO)(L)(NH3)4]3+/2+ and reference pair: [Ru(bpy)3]3+/2+. The correlation between the calculated and experimental reduction potentials strongly depend on the DFT functional chosen. The best results were obtained with the GGA functional BP86, which showed deviations lower than 200 mV. The assignment of explicit solvent, in addition to continuum solvent influence, also appears as a relevant factor.