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 [<b>1</b>; (TbtHP)<sub>2</sub>PFc, Fc = ferrocenyl] as a mixture of three diastereomers in 73% yield. In sharp contrast to the previously reported 1,3-dihydrotriphosphanes [(RHP)<sub>2</sub>PR, R = Ph,<i> t</i>-Bu], <b>1</b> was quite stable toward air and moisture either in the solid state or in solution at ambient temperature. The structural characterization of <b>1</b> was achieved by NMR spectra and X-ray crystallographic analysis. In the <sup>31</sup>P{<sup>1</sup>H} NMR spectrum of the mixture of three diastereomers of <b>1</b>, the characteristic two A<sub>2</sub>B and one ABX system were observed as signals assignable to two <i>meso</i> and one <i>dl</i> isomer, respectively. The X-ray crystallographic analysis for a single crystal obtained from the diastereomer mixture of <b>1</b> 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 <b>1</b> in toluene led to the quantitative formation of TbtPH<sub>2</sub> (<b>2</b>) and (<i>E</i>)-TbtPPFc (<b>3</b>), as judged by the <sup>1</sup>H and <sup>31</sup>P NMR spectra. Kinetic studies indicated that the thermolysis of <b>1</b> is a first-order reaction including a unimolecular dissociative process, which was reasonablely supported by theoretical calculations.