Diatomic PN – trapped in a cyclo-tetraphosphazene

Abstract

(Me₃Si)₂NPCl₂, which is formally capable of eliminating two equivalents of Me₃SiCl, is shown to be a suitable starting material to prepare highly reactive PN species by successive elimination of Me₃SiCl. Me₃SiCl elimination can be triggered either thermally and/or by addition of a Lewis acid such as GaCl₃, thus leading to the formation of a highly labile aminochlorophosphenium cation in [(Me₃Si)₂NPCl][GaCl₄] and iminophosphenium salt [Me₃Si–NP][GaCl₄] upon warming to ambient temperatures. This work describes the synthesis and characterization of a cyclo-tetraphosphazane in [PN(dmb)]₄ (5) (dmb = 2,3-dimethyl-1,3-butadiene) obtained by thermal elimination of Me₃SiCl from (Me₃Si)₂NPCl₂ at 120 °C in toluene solution. The reactive intermediate Me₃SiNPCl was trapped with dmb to form the cyclic phosphazane Me₃SiN(dmb)PCl, which eventually oligomerizes to give 5. In the presence of dmb or chd (chd = 1,3-cyclohexadiene) (Me₃Si)₂NP(OTf)₂ reacts by eliminating Me₃SiOTf to yield the spirocyclic phospholenium salts [Me₃SiN(dmb)P(dmb)][OTf] (7) and [Me₃SiN(chd)P(chd)][OTf] (8), of which the solid state structures were successfully determined. 7 decomposes when exposed to moisture to give an unprecedented cyclic ammonium phosphinoxide [P(O)H(dmb)₂NH₂][OTf] (9). Tetraphosphazane 5 is shown to be a versatile ligand in transition metal chemistry. It coordinates in an η³-fashion in {[PN(dmb)]₄Mo(CO)₃} (5·Mo) and is able to coordinate a second metal fragment, exemplified by the formation of the ditungsten complex{[PN(dmb)]₄W₂(CO)₇} (5·W₂) with a semi-bridging carbonyl ligand. All compounds were structurally characterized and the bonding situation was investigated by density functional theory and natural bond orbital analysis (NBO).