Kinetics and mechanism of CO substitution of [η5-CpFe(CO)3]PF6 with PPh3 in the presence of substituted pyridine N-oxide

Abstract

Abstract The kinetics of substitution reactions of [ η -CpFe(CO) 3 ]PF 6 with PPh 3 in the presence of R-PyOs have been studied. For all the R-PyOs (R = 4-OMe, 4-Me, 3,4-(CH) 4 , 4-Ph, 3-Me, 2,3-(CH) 4 , 2,6-Me 2 , 2-Me), the reactions yeild the same product [ η 5 -CpFe(CO) 2 PPh 3 ]PF 6 , according to a second-order rate law that is first order in concentrations of [ η 5 -CpFe(CO) 3 ]PF 6 and of R-PyO but zero order in PPh 3 concentration. These results, along with the dependence of the reaction rate on the nature of R-PyO, are consistent with an associative mechanism. Activation parameters further support the bimmolecular nature of the reactions: ΔH ≠ = 13.4 ± 0.4 kcal mol −1 , ΔS ≠ = −19.1 ± 1.3 cal k −1 mol −1 for 4-PhPyO; ΔH ≠ = 12.3 ± 0.3 kcal mol −1 , ΔS ≠ = 24.7 ±1.0 cal K −1 mol −1 for 2-MePyO. For the various substituted pyridine N -oxides studied in this paper, the rates of reaction increase with the increasing electron-donating abilities of the substituents on the pyridine ring or N -oxide basicities, but decrease with increasing 17 O chemical shifts of the N -oxides. Electronic and steric factors contributing to the reactivity of pyridine N -oxides have been quantitatively assessed.