Structures of the radical P[N(SiMe3)(2)](NPr2i), its dimer, cation and chloro derivative

Treatment of PCl[N(SiMe3)(2)](NPr2i) (1) with potassium-graphite in thf afforded the colourless, crystalline diphosphine {P[N(SiMe3)(2)](NPr2i)}(2) (2) in good yield. Sublimation of 2 in vacuo yielded the yellow phosphinyl radical P[N(SiMe3)(2)](NPr2i) (3), which upon cooling reverted to 2; the latter in C6D6 at 298 K was a mixture of rac and meso diastereoisomers. The yellow, crystalline phosphenium salt {P[N(SiMe3)(2)](NPr2i)}[AlCl4] (4) was obtained from 1 and 1/2Al(2)Cl(6) in CH2Cl2. By single-crystal X-ray diffraction (XRD) the structures of the known compound 1 and of 2 and 4 were determined. The structure of the radical 3, formed by the thermal homolytic dissociation of the diphosphine 2, was determined in the gas phase by electron diffraction (GED), utilising data from UMP2/6-31+G* ab initio calculations. The model of the molecule in the GED structure analysis was described by a set of internal coordinates and an initial set of Cartesian coordinates from ab initio calculations, facilitating the structure analysis. The experimental data were found to be consistent with the presence of a single conformer of the radical in the gas phase. The computed standard homolytic dissociation enthalpy of the P-P bond in the corresponding diphosphine 2, corrected for BSSE, 54 kJ mol(-1), is substantially reduced compared to the dissociation enthalpy of tetramethyl-diphosphine by the reorganisation energies of the fragments that form upon dissociation. The intrinsic energy content of the P-P bond in the diphosphine 2 was estimated to be 286 kJ mol(-1), in agreement with the results of previous work on a series of crowded diphosphines.