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 (S 2- ) in biology, and interactions between RSS - and bioinorganic metal centers play critical roles in biological hydrogen sulfide (H 2 S) biogenesis, signaling, and catabolism. Here, we report the use of contact-ion stabilized [Na(15-crown-5)][ t 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 η 1 -alkyl persulfide complex [Co II (TPA)(SS t Bu)] + ( 2 ), which was characterized by X-ray crystallography, UV-vis spectroscopy, and Raman spectroscopy. RSS - coordination to the Lewis acidic Co 2+ 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 , Δ v S-S = 66 cm -1 ). The effect of persulfidation on metal center redox potentials was further elucidated using cyclic voltammetry, in which the Co 2+ → Co 3+ 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)(S t Bu)] + ( 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 PPh 3 , BH 4 - , and biologically relevant thiol reductant DTT led to different S 2- output pathways, including formation of a dinuclear 2Co-2SH complex [Co II 2 (TPA) 2 (μ 2 -SH) 2 ] 2+ ( 4 ).