Interaction of dicoordinate phosphorus with boranes: chemistry of 3a,6a-diaza-1,4-diphosphapentalene as masked phosphinidene

Treatment of 3a,6a-diaza-1,4-diphosphapentalene (DDP) with an excess of PhBCl2 yields the corresponding bis(borane) adduct DDP(PhBCl2)2 (14), demonstrating the availability of two lone pairs on the phosphorus center. The reaction between DDP and B(C6F5)3 yields (1 : 1) phosphino-borane complex 16. The free lone electron pair on the pyramidal P atom in 16 participates in the intramolecular non-covalent interactions P(1)⋯F(1) and P(1)⋯F(6) giving additional 3.9 and 2.8 kcal mol−1, respectively, for stabilization of the complex. This through-space interaction appears in the 31P NMR spectrum as large spin–spin coupling constants of 271 and 219 Hz. The addition of water to 16 (1 : 1) leads to the formation of the insertion product 17 having –H2P–O–B(C6F5)3 moiety. The reaction of DDP with BH3·SMe2 proceeds in several stages, which include the insertion of the masked phosphinidene into the B–H and P–H bonds of the intermediate compounds followed by the dehydrocoupling step and formation of diphosphine 18. The last compound exists in solution as a set of stereoisomers.