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(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 k, varying by five orders of magnitude from 1.34 +/- 0.02 x 10(-3) to 51 +/- 3 mol(-1) dm(3) 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-double dagger and -112 +/- 9 to -140 +/- 3 J K-1 mol(-1) for Delta S-double dagger supporting a S,2 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. pK(a), (1)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)angstrom respectively.