Kinetics of sulfur-transfer from titanocene (poly)sulfides to sulfenyl chlorides: rapid metal-assisted concerted substitution

Abstract

The kinetics of sulfur transfer from titanocene (poly)sulfides (^RCp₂TiS₅, Cp₂TiS₄CMe₂, Cp₂Ti(SAr)₂, Cp₂TiCl(SAr)) to sulfenyl chlorides (S₂Cl₂, RSCl) have been investigated by a combination of stopped-flow UV-Vis/NMR reaction monitoring, titration assays, numerical kinetic modelling and KS-DFT calculations. The reactions are rapid, proceeding to completion over timescales of milliseconds to minutes, via a sequence of two S–S bond-forming steps (k₁, k₂). The archetypical polysulfides Cp₂TiS₅ (1a) and Cp₂TiS₄C(Me₂) (2a) react with disulfur dichloride (S₂Cl₂) through rate-limiting intermolecular S–S bond formation (k₁) followed by a rapid intramolecular cyclization (k₂, with k₂ ≫ k₁ [RSCl]). The monofunctional sulfenyl chlorides (RSCl) studied herein react in two intermolecular S–S bond forming steps proceeding at similar rates (k₁ ≈ k₂). Reactions of titanocene bisthiophenolates, Cp₂Ti(SAr)₂ (5), with both mono- and di-functional sulfenyl chlorides result in rapid accumulation of the monothiophenolate, Cp₂TiCl(SAr) (6) (k₁ > k₂). Across the range of reactants studied, the rates are relatively insensitive to changes in temperature and in the electronics of the sulfenyl chloride, moderately sensitive to the electronics of the titanocene (poly)sulfide (ρ_(Ti–(SAr)) ≈ −2.0), and highly sensitive to the solvent polarity, with non-polar solvents (CS₂, CCl₄) leading to the slowest rates. The combined sensitivities are the result of a concerted, polarized and late transition state for the rate-limiting S–S bond forming step, accompanied by a large entropic penalty. Each substitution step {[Ti]–SR′ + Cl–SR → [Ti]–Cl + RS–SR′} proceeds via titanium-assisted Cl–S cleavage to generate a transient pentacoordinate complex, Cl–[Cp₂TiX]–S(R′)–SR, which then undergoes rapid Ti–S dissociation.