Three-electron bonded σ/σ* radical cations from mixedly substituted dialkyl sulfides in aqueous solution studied by pulse radiolysis

Abstract

The formation of several radical cations ([Rⁱ,R^j]S ∴ S[Rⁱ,R^j])⁺, ([Rⁱ]₂S ∴ S[R^j]₂])⁺ and ([Rⁱ,R^j]S ∴ S[R^j]₂)⁺ with mixed alkyl substitution in aqueous solution has been investigated by means of pulse radiolysis. The following substituents were involved: R^i,j= H, methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl and octyl. Two methods of generation have been applied: (i)˙OH-induced oxidation of a sulfide and (ii) one-electron reduction of the corresponding sulfoxide in very acidic solution. The 2σ/1σ* three-electron bonded (>S ∴ S<)⁺-type species exhibit optical absorptions with maxima ranging from 420 nm for (Me₂S ∴ SH₂)⁺ to 600 nm for ([Me,Bu^t]S ∴ S[Bu^t]₂)⁺. The actual transition energy can be related to the electron induction by the substituents as concluded from a linear free energy correlation between the respective λ_max and weighted Taft's inductive σ* parameters. For unbranched substituents λ_max(in eV)= 1.40 (σ*)_w+ 2.65. Evidence is also provided for the destabilization of the three-electron bond by steric demands of bulky substituents and by the effect of the substitution pattern on the ‘σ-lone pair’ interaction. The latter becomes apparent by comparing ([Me₂]S ∴ S[Bu^t]₂)⁺(λ_max 545 nm) with ([Me,Bu^t]S ∴ S[Me,Bu^t])⁺(λ_max 510 nm). Kinetically, a number of rate constants have been determined for the forward and back reactions of the equilibrium > S˙⁺+ S < ⇌(>S ∴ S<)⁺. They are typically of the order of 10⁹ dm³ mol^–1 s^–1 and 10⁴–10⁵ s^–1, respectively. Equilibrium constants derived from these kinetic data range from 2.0 × 10⁵ dm³ mol^–1 for (Me₂S ∴ SMe₂)⁺(confirming an earlier measurement) to ⩽ 5 × 10³ dm³ mol^–1 for ([Me,Bu^t]S ∴ S[Bu^t]₂)⁺. Their decrease parallels the total electron-releasing power of the substituents and the steric constraints exerted by them. The decay of the three-electron bonded radical cations includes first-order release of protons, possibly in association with the dissociation of the three-electron bond and second-order processes, presumably disproportionation. By and large, the kinetic stabilities are reflected in the trend in λ_max with shorter lifetimes referring to more red-shifted absorptions.