The Crystal Structure of Me₃SiNPPh₂CH₂PPh₂ and NMR Investigations on its Proposed Thermal Isomerization

The crystal structure of Me₃Si₃PPh₂CH₂PPh₂ (1) has been determined in order to obtain information about the ground state conformation in the solid, and about unusual steric effects which could give rise to conformational isomers. It was recently proposed that such isomers exist due to restricted rotation about the P=N bond, and are generated by heating the compound to 160 C. The evidence for them was based on the observation of multiple NMR signals in CDC1₃ solutions of samples which had been so treated. The crystal structure yields a P-N-Si bond angle of 150.2(2). a P-N bond length of 1.529(3) Å and a Si-N bond lengths of 1.668(3) Å. It is clear that, as a result of the relatively large P-N-Si angle in 1, there is no steric hindrance to rotation about the P-N bond in this molecule. Attempts to study the energetics of the proposed isomerization by recording the NMR spectra of heated samples showed that the previously reported multiple NMR signals are not generated in this way. However, such signals are generated at ambient temperature under various circumstances, for example in the course of the reaction of a CDC1₃ solution of 1 with methanol to produce HNPPh₂CH₂PPh₂ (2). The observation that it is not necessary to heat 1 in order to generate these signals makes their assignment to rotational isomers rather less likely. In the absence of a reproducible method of producing these signals in a pure sample, the existence of such isomers should be regarded as not yet proven.