Rapid phosphorus(III) ligand evaluation utilising potassium selenocyanate
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
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.
Carrier concentration; Crystal structure; Methanol; Nucleophiles; Organic solvents; Phosphorus; Potassium compounds; Rate constants; Spectrophotometry; Bond distances; Phenyl rings; Polar solvents; Reactions rates; Ligands