A Theoretical and Experimental Investigation of the Reduction of Dinuclear Persulfide-Bridged Ruthenium Complexes.

Previously, the dinuclear persulfide-bridged ruthenium complex [(H2O)(NH3)4Ru(μ-S2)Ru(NH3)4(OH2)]4+ was demonstrated to be a reduction-activated H2S donor, acting as a promising complex for the biological delivery of this gasotransmitter. Building on previous studies, other complexes bearing the [RuSSRu] motif were investigated for H2S release. In this study, a previously reported persulfide-bridged complex [(acac)(Me3TACN)Ru(μ-S2)Ru(Me3TACN)(acac)]2+ was evaluated. The crystal structure of a new crystal form of this complex was determined revealing a new orientation of supporting ligands about the trans-[RuSSRu] motif. Furthermore, resonance Raman spectroscopy revealed symmetric Ru-S and S-S stretching frequencies of 418 and 528 cm-1, respectively. Lastly, the electrochemistry of this complex was probed in aqueous buffer revealing an irreversible reduction at -758 mV vs SCE. The reactivity of [(acac)(Me3TACN)Ru(μ-S2)Ru(Me3TACN)(acac)]2+ was studied in the presence of different biological reductants and unexpectedly remained intact, showing no evidence for S-S bond cleavage or release of H2S. To investigate the difference between this complex and [(H2O)(NH3)4Ru(μ-S2)Ru(NH3)4(OH2)]4+, density functional theory (DFT) calculations were performed, which revealed that [(acac)(Me3TACN)Ru(μ-S2)Ru(Me3TACN)(acac)]2+ possesses unoccupied ligand-based π* orbitals that more readily accept electrons than S-S bond destabilizing σ* orbitals. These computational findings validate our experimental results and provide guiding principles required for future compound design.