Isomerism and conformation of (N-silyl)bis(diphenylphosphino)amines and bis(N-silyl)diphenylphosphinoamines

Lithium bis(diphenylphosphino)amide (2) reacts with chlorotriorgano- and dichlorodiorganosilanes to form either N-silylbis(diphenylphosphino)amides (3) or the isomeric P-diphenylphosphinodiphenylphosphine(silylimides) (4), or an equilibrium mixture of both. The nature of the product depends strongly on the nature of the substituents attached to silicon. ²⁹Si and ³¹P NMR spectroscopy were used to differentiate between the two structures. In N,N-bis(trimethylsilyl)aminodiphenylphosphine the two silyl groups were found by NMR spectroscopy to be non-equivalent at low temperature in solution, and this is attributed to hindered rotation about the PN bond, with an in-plane (Si₂NP) orientation of the lone pair of electrons at phosphorus as the ground state. The reaction of 2 with (CH₃)₂PCl does not yield triphosphinoamine species; instead, a P-phosphinophosphinimine (5) is obtained, which is closely related to the silyl analogues of formula 4.