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Kinetic evidence for intramolecular proton transfer between nickel and coordinated thiolate

Lookup NU author(s): Emeritus Professor Bill Clegg, Professor Richard Henderson

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Abstract

The complexes [Ni(YR)(triphos)]BPh4 {Y = S, R = Ph or Et or Y = Se, R = Ph; triphos = (Ph2PCH2CH2)2PPh} have been prepared and characterized, and the X-ray crystal structure of [Ni(SPh)(triphos)]BPh4 has been solved. In MeCN, [Ni(YR)(triphos)]+ are protonated by [lutH]+ (lut = 2,6-dimethylpyridine) to give [Ni(YHR)(triphos)]2+. Studies on the kinetics of these equilibrium reactions reveal an unexpected difference in the reactivities of [Ni(SPh)(triphos)]+ and [Ni(SEt)(triphos)]+. In both cases, the reactions exhibit a first-order dependence on the concentration of complex. When R = Ph, the dependence on the concentrations of [lutH+] and lut is given by kobs = k1Ph[lutH+] + k-1Ph[lut], which is typical of an equilibrium reaction where k1Ph and k-1Ph correspond to the forward and back reactions, respectively. Analogous behavior is observed for [Ni(SePh)(triphos)]+. However, for [Ni(SEt)(triphos)]+, the kinetics are more complicated, and kobs = {k1k2[lutH+] + (k-2 + k2)}/(k1[lutH+] + k--1[lut]), which is indicative of a mechanism involving two coupled equilibria in which the initial protonation of the thiolate is followed by a unimolecular equilibrium reaction that is assumed to involve the formation of an η2-EtS-H ligand. The difference in reactivity between the complexes with alkyl and aryl thiolate ligands is a consequence of the {Ni(triphos)}2+ site "leveling" the basicities of these ligands. The pKa's of the PhSH and EtSH constituents coordinated to the {Ni(triphos)}2+ are 16.0 and 14.6, respectively, whereas the difference in pKa's of free PhSH and EtSH differ by ca. 4 units. The pKa of [Ni-(SeHPh)(triphos)]+ is 14.4. The more strongly σ-donating EtS ligand makes the {Ni(triphos)}2+ core sufficiently electron-rich that the basicities of the sulfur and nickel in [Ni(SEt)(triphos)]+ are very similar; therefore, the proton serves as a bridge between the two sites. The relevance of these observations to the proposed mechanisms of nickel-based hydrogenases is discussed.


Publication metadata

Author(s): Clegg W, Henderson RA

Publication type: Article

Publication status: Published

Journal: Inorganic Chemistry

Year: 2002

Volume: 41

Issue: 5

Pages: 1128-1135

ISSN (print): 0020-1669

ISSN (electronic): 1520-510X

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/ic0104306

DOI: 10.1021/ic0104306

PubMed id: 11874347


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