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.