UC Santa Barbara

Abstract: 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.