Mechanisms and rates of proton transfer to coordinated carboxydithioates: studies on [Ni(S2CR){PhP(CH2CH2PPh2)2}]+ (R = Me, Et, Bun or Ph)

Abstract

The complexes [Ni(S₂CR)(triphos)]BPh₄ (R = Me, Et, Buⁿ or Ph; triphos = PhP{CH₂CH₂PPh₂}₂) have been prepared and characterised. X-ray crystallography (for R = Et, Ph, C₆H₄Me-4, C₆H₄OMe-4 and C₆H₄Cl-4) shows that the geometry of the five-coordinate nickel in the cation is best described as distorted trigonal bipyramidal, containing a bidentate carboxydithioate ligand with the two sulfur atoms spanning axial and equatorial sites, the other axial site being occupied by the central phosphorus of triphos. The reactions of [Ni(S₂CR)(triphos)]⁺ with mixtures of HCl and Cl⁻ in MeCN to form equilibrium solutions containing [Ni(SH(S)CR)(triphos)]²⁺ have been studied using stopped-flow spectrophotometry. The kinetics show that proton transfer is slower than the diffusion-controlled limit and involves at least two coupled equilibria. The first step involves the rapid association between [Ni(S₂CR)(triphos)]⁺ and HCl to form the hydrogen-bonded precursor, {[Ni(S₂CR)(triphos)]⁺⋯HCl} (K^R₁) and this is followed by the intramolecular proton transfer (k^R₂) to produce [Ni(SH(S)CR)(triphos)]²⁺. In the reaction of [Ni(S₂CMe)(triphos)]⁺ the rate law is consistent with the carboxydithioate ligand undergoing chelate ring-opening after protonation. It seems likely that chelate ring-opening occurs for all [Ni(S₂CR)(triphos)]⁺, but only with [Ni(S₂CMe)(triphos)]⁺ is the protonation step sufficiently fast that chelate ring-opening is rate-limiting. With all other systems, proton transfer is rate-limiting. DFT calculations indicate that protonation can occur at either sulfur atom, but only protonation at the equatorial sulfur results in chelate ring-opening. The ways in which protonation of either sulfur atom complicates the analyses and interpretation of the kinetics are discussed.