Synthesis, Characterization, and Reactivity of a Synthetic End-On Cobalt(II) Alkyl Persulfide Complex as a Model...

Persulfides (RSS^-) are ubiquitous source of sulfides (S^2-) 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)][BuSS] (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, including the fundamental difference between alkyl persulfides and alkyl thiolates. Reaction of 1 with [Co^II(TPA)(OTf)]⁺ afforded the η¹-alkyl persulfide complex [Co^II(TPA)(SSBu)]⁺ (2), which was characterized by X-ray crystallography, UV-vis spectroscopy, 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 (v_S-S = 522 cm^-1, Δv_S-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 (E^p,a = +89 mV vs SCE) is lowered by nearly 700 mV when compared to the corresponding thiolate complex [Co^II(TPA)(SBu)]⁺ (3) (E^p,a = +818 mV vs SCE), despite persulfidation being generally seen as an oxidative post-translational modification. The reactivity of 2 toward reducing agents including PPh₃, BH₄^-, and biologically relevant thiol reductant DTT led to different S^2- output pathways, including formation of a dinuclear 2Co-2SH complex [Co^II₂(TPA)₂(μ²-SH)₂]²⁺(4).