Rapid phosphorus(III) ligand evaluation utilising potassium selenocyanate

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 k_obsvs. [SeCN ^-] plots were linear with zero intercepts corresponding to a rate expression of k_obs = 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 S_N2 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. pK_a, ¹J_P-Se and χ_d 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.