title: 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(R)1), followed by an intramolecular proton transfer step (k(R)2). 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(R)1 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.
authors.affiliation: School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK. richard.henderson@ncl.ac.uk.
authors.forename: Ahmed
authors.initials: A
authors.lastname: Alwaaly
authors.forename: William
authors.initials: W
authors.lastname: Clegg
authors.forename: Ross W
authors.initials: RW
authors.lastname: Harrington
authors.forename: Athinoula L
authors.initials: AL
authors.lastname: Petrou
authors.forename: Richard A
authors.initials: RA
authors.lastname: Henderson