Mechanism of proton transfer to coordinated thiolates: encapsulation of acid stabilizes precursor intermediate

Abstract

Earlier kinetic studies on the protonation of the coordinated thiolate in the square-planar [Ni(SC₆H₄R′-4)(triphos)]⁺ (R′ = NO₂, Cl, H, Me or MeO) by lutH⁺ (lut = 2,6-dimethylpyridine) indicate a two-step mechanism involving initial formation of a (kinetically detectable) precursor intermediate, {[Ni(SC₆H₄R′-4)(triphos)]⋯Hlut}²⁺ (K^R₁), followed by an intramolecular proton transfer step (k^R₂). The analogous [Ni(SR)(triphos)]BPh₄ {R = Et, Bu^t or Cy; triphos = PhP(CH₂CH₂PPh₂)₂} have been prepared and characterized by spectroscopy and X-ray crystallography. Similar to the aryl thiolate complexes, [Ni(SR)(triphos)]⁺ are protonated by lutH⁺ in an equilibrium reaction but the observed rate law is simpler. Analysis of the kinetic data for both [Ni(SR)(triphos)]⁺ and [Ni(SC₆H₄R′-4)(triphos)]⁺ shows that both react by the same mechanism, but that K^R₁ is largest when the thiolate is poorly basic, or the 4-R′ substituent in the aryl thiolates is electron-withdrawing. These results indicate that it is both NH⋯S hydrogen bonding and encapsulation of the bound lutH⁺ (by the phenyl groups on triphos) which stabilize the precursor intermediate.