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

DOI: 10.1039/C4DT03543G DOI Help

Authors: Ahmed Alwaaly (Newcastle University) , William Clegg (Newcastle University) , Richard A. Henderson (Newcastle University) , Mike Probert (Newcastle University) , Paul Waddell (Newcastle University)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Dalton Transactions , VOL 44 (7) , PAGES 3307-3317

State: Published (Approved)
Published: January 2015
Diamond Proposal Number(s): 6749

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} (KR1) and this is followed by the intramolecular proton transfer (kR2) 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.

Subject Areas: Chemistry

Instruments: I19-Small Molecule Single Crystal Diffraction

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