On the interaction of cyclohexanone azine with PBr3. Crystal structure of tris(4-bromo-3a,6a-diaza-1,4-diphosphapentalen-1-yl)amine

The interaction of cyclohexanone azine with phosphorus(III) bromide without a solvent has been studied. It has been shown that the main reaction product is cyclohexane-annelated 1,4-dibromo-3а,6а-diaza-1,4-diphosphapentalene (2, DDP-Br2, 57 %). The by-products of the synthesis are tris (4-bromo-3а,6а-diaza-1,4-diphosphapentalen-1-yl)amine (3, 8 %) and ammonium bromide. The molecular structure of compound 3 has been studied by X-ray analysis. The crystal cell contains two pairs of enantiomers and eight THF solvate molecules, four of which are disordered over two positions. The central nitrogen atom N(7) in compound 3 forms three bonds with the phosphorus atoms of the DDP fragments P(1), P(3), P(5), located in the same plane. The nitrogen atoms of the heteropentalene fragments (N(2), N(4), N(6)) that are located closer to the center of the molecule have a planar configuration (with the sum of bond angles 359.62, 359.50, 359.38°, respectively). In its turn, for the N(1), N(3), N(5) atoms, a greater deviation from the planar structure is observed (the sum of bond angles is 353.85, 353.71, 353.96°, respectively). The peripheral phosphorus-nitrogen bonds (P(2)-N(1) 1.689(2), P(4)-N(3) 1.691(3), P(6)-N(5) 1.690(2) Å) are shorter than the corresponding bonds in neighboring five-membered rings (P(1)-N(2) 1.744(2), P(3)-N(4) 1.738(3), P(5)-N(6) 1.733(2) Å). The difference in the phosphorus-bromine bond lengths P(2)-Br(1) 2.4805(8), P(4)-Br(2) 2.4675(8), and P(6)-Br(3) 2.4836(8) Å can be caused by the presence of various short contacts Br···H between the adjacent molecules of compound 3, as well as between compound 3 and the THF solvate molecules. It is assumed that compound 3 is formed during the side reaction of DDP dibromide (2) with ammonium bromide, which appeared in the reaction mixture as a result of the partial decomposition of cyclohexanone azine upon prolonged heating with phosphorus bromide.