Synthesis, Characterization, and Reactivity of a Synthetic End-on Cobalt (II) Alkyl Persulfide Complex as a Model Platform for Thiolate Persulfidation

Persulfides (RSS–) are ubiquitous source of sulfides (S2–) in biology, and interactions between RSS– and bioinorganic metal centers play critical roles in biological hydrogen sulfide (H2S) biogenesis, signaling, and catabolism. Here we report the use of contact-ion stabilized [Na(15-crown-5)][tBuSS] (1) as a simple synthon to access rare metal alkyl persulfide complexes and to investigate the reactivity of RSS– with transition metal centers to provide insights into metal thiolate persulfidation includ-ing the fundamental difference between alkyl persulfides and alkyl thiolates. Reaction of 1 with [CoII(TPA)(OTf)]+ afforded the η1-alkyl persulfide complex [CoII(TPA)(SStBu)]+ (2), which was characterized by X-ray crystallography, UV-vis spectros-copy, and Raman spectroscopy. RSS– coordination to the Lewis acidic Co2+ center provided additional stability to the S–S bond as evidenced by a significant increase in the Raman stretching frequency for 2 (vS–S = 522 cm–1, ΔvS–S = 66 cm–1). The effect of persulfidation on metal center redox potentials was further elucidated using cyclic voltammetry, in which the Co2+ → Co3+ oxidation potential of 2 (Ep,a = +89 mV vs SCE) is lowered by nearly 700 mV when compared to the corresponding thiolate complex [CoII(TPA)(StBu)]+ (3) (Ep,a = +818 mV vs SCE), despite persulfidation being an oxidative post-translational modifica-tion. Reactivity of 2 toward reducing agents including PPh3, BH4–, and biologically relevant thiol reductant DTT led to differ-ent S2– output pathways including formation of a dinuclear 2Co-2SH complex [CoII2(TPA)2(µ2-SH)2]2+(4).