Rapid phosphorus(III) ligand evaluation utilising potassium selenocyanate

摘要

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_(obs) vs.[SeCN~-] plots were linear with zero intercepts corresponding to a rate expression of k_(obs) = k_1 [SeCN-].Reactions rates are dependent on the electron density of the phosphorus centre with k1 varying by five orders of magnitude from 1.34 ± 0.02 x 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 DELTA H and -112 ± 9 to -140 ± 3 J K~(-1) mol~(-1) for DELTA S 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 k_1 vs.pKa,J_(P_Se) and CHI_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) A respectively.