Synthesis and Structure of a Stable 1,3-Dihydrotriphosphane and Its Thermal Decomposition Leading to the Formation of the Corresponding Phosphine and Diphosphene

Treatment of dichloroferrocenylphosphine with two molar amounts of a lithium phosphide bearing a 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as Tbt) group afforded the corresponding 1,3-dihydro-2-ferrocenyltriphosphane [1; (TbtHP)2PFc, Fc = ferrocenyl] as a mixture of three diastereomers in 73% yield. In sharp contrast to the previously reported 1,3-dihydrotriphosphanes [(RHP)2PR, R = Ph, t-Bu], 1 was quite stable toward air and moisture either in the solid state or in solution at ambient temperature. The structural characterization of 1 was achieved by NMR spectra and X-ray crystallographic analysis. In the 31P{1H} NMR spectrum of the mixture of three diastereomers of 1, the characteristic two A2B and one ABX system were observed as signals assignable to two meso and one dl isomer, respectively. The X-ray crystallographic analysis for a single crystal obtained from the diastereomer mixture of 1 revealed its molecular structure, having P−P bond lengths of 2.2304(12) and 2.2322(12) Å and a P−P−P bond angle of 96.17(5)°, although the configuration could not be determined. Thermolysis of 1 in toluene led to the quantitative formation of TbtPH2 (2) and (E)-TbtPPFc (3), as judged by the 1H and 31P NMR spectra. Kinetic studies indicated that the thermolysis of 1 is a first-order reaction including a unimolecular dissociative process, which was reasonablely supported by theoretical calculations.