The structures and electronic properties of bulky electron-withdrawing phosphines

The crystal structures of the N-carbazolyl phosphines PPh(NC12H8)(2) and P(NC12H8)(3) have been determined. In the latter, large deviations from planarity are observed for one of the nitrogen atoms, which facilitate strong intermolecular C-H-. . .pi and pi(. . .)pi interactions in the supramolecular structure. The compounds PPh3-n(NC12H8)(n) (n = 1-3) react with selenium to give the phosphine selenides SePPh3-n(NC12H8)(n). From the P-31{H-1} NMR spectra, the (1)J(PSe) coupling constants for these compounds are inversely related to the sigma-basicity of the phosphines, which decreases with the increasing number of N-carbazolyl groups. The crystal structure of 0.65 SeP(NC12H8)(3)(.)0.35 P(NC12H8)(3) contrasts with that for P(NC12H8)(3) as all of the nitrogen atoms are planar and pi(. . .)pi interactions are absent. The electronic differences between phenyl and N-pyrrolyl phosphines are borne out by the reaction of the diphosphine Ph2PCH2P(NC4H4)(2) with sulphur, which gives exclusively Ph2P(S)CH2P(NC4H4)(2).