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Proton Transfer to Nickel-Thiolate Complexes. 2. Rate-Limiting Intramolecular Proton Transfer in the Reactions of [Ni(SC6H 4R-4)(PhP{CH2CH2PPh2} 2)]+ (R = NO2, Cl, H, Me, or MeO)

Lookup NU author(s): Dr Valerie Autissier, Professor Athinoula Petrou, Professor Richard Henderson, Dr Ross Harrington, Emeritus Professor Bill Clegg

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Abstract

The protonation of [Ni(SC6H4R-4)(triphos)] + (triphos = PhP{CH2CH2PPh2} 2; R = NO2, Cl, H, Me, or MeO) by [lutH]+ (lut = 2,6-dimethylpyridine) to form [Ni(S(H)C6H 4R-4)(triphos)]2+ is an equilibrium reaction in MeCN. Kinetic studies, using stopped-flow spectrophotometry, reveal that the reactions occur by a two-step mechanism. Initially, [lutH]+ rapidly binds to the complex (K2R) in an interaction which probably involves hydrogen-bonding of the acid to the sulfur. Subsequent intramolecular proton transfer from [lutH]+ to sulfur (k 3R) is slow because of both electronic and steric factors. The X-ray crystal structures of [Ni(SC6H 4R-4)(triphos)]+ (R = NO2, H, Me, or MeO) show that all are best described as square-planar complexes, with the phenyl substituents of the triphos ligand presenting an appreciable barrier to the approach of the sterically demanding [lutH]+ to the sulfur. The kinetic characteristics of the intramolecular proton transfer from [lutH] + to sulfur have been investigated. The rate of intramolecular proton transfer exhibits a nonlinear dependence on Hammett σ+, with both electron-releasing and electron-withdrawing 4-R-substituents on the coordinated thiolate facilitating the rate of proton transfer (NO2 > Cl > H > Me < MeO). The rate constants for intramolecular proton transfer correlate well with the calculated electron dnsity of the sulfur. The temperature dependence of the rate of the intramolecular proton transfer reactions shows that ΔH‡ is small but increases as the 4-R-substituent becomes more electron-withdrawing {ΔH‡ = 4.1 (MeO), 6.9 (Me), 11.4 kcal mol-1 (NO2)}, while ΔS‡ becomes progressively less negative {ΔS‡ = -50.1 (MeO), -41.2 (Me), -16.4 (NO2) cal K-1 mol-1}. Studies with [lutD]+ show that the rate of intramolecular proton transfer varies with the 4-R-substituent {(k3NO2)H/(k 3NO2)D = 0.39; (k3Cl)H/(k3Cl)D = 0.88; (k3Me)H/(k3Me) D = 1.3; (k3MeO)H/(k 3MeO)D = 1.2}.


Publication metadata

Author(s): Autissier V, Zarza PM, Petrou A, Henderson RA, Harrington RW, Clegg W

Publication type: Article

Publication status: Published

Journal: Inorganic Chemistry

Year: 2004

Volume: 43

Issue: 10

Pages: 3106-3115

ISSN (print): 0020-1669

ISSN (electronic): 1520-510X

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/ic0303237

DOI: 10.1021/ic0303237


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