Department of Chemistry, University of Johannesburg, P. O. Box 524, Johannesburg 2006, South Africa; Department of Chemistry, University of the Free State, P. O. Box 339, Bloemfontein 9300, South Africa; University of the Free State, Bloemfontein 9300, South Africa; Sasol Technology Research and Development, P.O. Box 1, Sasolburg, 1947, South Africa
Muller, A., Department of Chemistry, University of Johannesburg, P. O. Box 524, Johannesburg 2006, South Africa, University of the Free State, Bloemfontein 9300, South Africa; Otto, S., Department of Chemistry, University of the Free State, P. O. Box 339, Bloemfontein 9300, South Africa, Sasol Technology Research and Development, P.O. Box 1, Sasolburg, 1947, South Africa; Roodt, A., Department of Chemistry, University of the Free State, P. O. Box 339, Bloemfontein 9300, South Africa
Oxidative addition of SeCN- to tertiary phosphine ligands has been investigated in methanol at 298 K by use of UV-Vis stopped-flow and conventional spectrophotometry. In most cases kobsvs. [SeCN -] plots were linear with zero intercepts corresponding to a rate expression of kobs = k1[SeCN-]. Reactions rates are dependent on the electron density of the phosphorus centre with k 1 varying by five orders of magnitude from 1.34 ± 0.02 × 10-3 to 51 ± 3 mol-1 dm3 s-1 for P(2-OMe-C6H4)3 to PCy3 respectively. Activation parameters range from 27 ± 1 to 49.0 ± 1.3 kJ mol-1 for ΔH‡ and -112 ± 9 to -140 ± 3 J K-1 mol-1 for ΔS ‡ supporting a SN2 mechanism in which the initial nucleophilic attack of P on Se is rate determining. Reaction rates are promoted by more polar solvents supporting the mechanistic assignment. Reasonable linear correlations were observed between log k1vs. pKa, 1JP-Se and χd values of the phosphines. The reaction rates are remarkably sensitive to the steric bulk of the substituents, and substitution of phenyl rings in the 2 position resulted in a decrease in the reaction rate. The crystal structures of SePPh2Cy and SePPhCy2 have been determined displaying Se-P bond distances of 2.111(2) and 2.1260(8) respectively. © The Royal Society of Chemistry 2008.