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Lookup NU author(s): Dr Valerie Autissier,
Emeritus Professor Bill Clegg,
Dr Ross Harrington,
Professor Richard Henderson
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The kinetics of the equilibrium reaction between [Ni(SC6H 4R-4)2(dppe)] (R= MeO, Me, H, Cl, or NO2; dppe = Ph2PCH2CH2PPh2) and mixtures of [lutH]+ and lut (lut = 2,6-dimethylpyridine) in MeCN to form [Ni(SHC6H4R-4)-(SC6H 4R-4)(dppe)]+ have been studied using stopped-flow spectrophotometry. The kinetics for the reactions with R = MeO, Me, H, or Cl are consistent with a single-step equilibrium reaction. Investigation of the temperature dependence of the reactions shows that AG‡ = 13.6 ± 0.3 kcal mol-1 for all the derivatives but the values of ΔH‡ and ΔS‡ vary with R (R = MeO, ΔH‡ = 8.5 kcal mol-1, ΔS‡ = -16 cal K-1 mol -1; R = Me, ΔH‡ = 10.8 kcal mol-1, ΔS‡ = -9.5 cal K-1 mol-1; R = Cl, ΔH‡ = 23.7 kcal mol-1, ΔS‡ = +33 cal K -1 mol-1). With [Ni(SC6H4-NO 2-4)2(dppe)] a more complicated rate law is observed consistent with a mechanism in which initial hydrogen-bonding of [lutH] + to the complex precedes intramolecular proton transfer. It seems likely that all the derivatives operate by this mechanism, but only with R = NO2 (the most electron-withdrawing substituent) does the intramolecular proton transfer step become sufficiently slow to result in the change in kinetics. Studies with [lutD]+ show that the rates of proton transfer to [Ni(SC6H4R-4)2(dppe)] (R = Me or Cl) are associated with negligible kinetic isotope effect. The possible reasons for this are discussed. The rates of proton transfer to [Ni(SC 6H4R-4)2(dppe)] vary with the 4-R-substituent, and the Hammett plot is markedly nonlinear. This unusual behavior is attributable to the electronic influence of R which affects the electron density at the sulfur.
Author(s): Autissler V, Clegg W, Harrington RW, Henderson RA
Publication type: Article
Publication status: Published
Journal: Inorganic Chemistry
ISSN (print): 0020-1669
ISSN (electronic): 1520-510X
Publisher: American Chemical Society
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