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

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 hydrogenbonded precursor, {[Ni(S2CR)(triphos)](+)center dot center dot center dot HCl} (K-1(R)) and this is followed by the intramolecular proton transfer (kR(2)) 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.