X-ray crystal structures and DFT calculations of differently charged aminocyclophosphazenes

Molecular structures of hexakis(amino)cyclotriphosphazene, N3P3(NH2)6, neutral and cationic forms of hexakis(ethylamino)cyclotriphosphazene, N3P3(NHC2H5)6, and dicationic form of octakis(ethylamino)cyclotetraphosphazene, N4P4(NHC2H5)8, were established through the X-ray diffraction experiments at 120(2), 165(2) and 110(2) K, respectively. Geometry of the six- and eight-membered cyclophosphazene rings of the molecules are discussed with the help of B3LYP/6-311G∗∗ method/basis DFT calculations of their methylamino analogs and topological analysis of the electron density distribution. In all cases P–N bonds show high polar nature with considerable P → N charge transfer. Change of the P–N bond lengths upon the cycle protonation well correlated with the net charge redistribution at the P and N centers but separately for the endocyclic and exocyclic bonds. Presence of the additional π-bonding covalent contribution, as it is evidenced from the analysis of the bonds ellipticity, has to be suggested to explain these separate correlations. In the cyclotriphosphazene cation the finer details of the endocyclic P–N bond lengths changes can be attributed to the interplay of the variable π-bonding contribution with the change in the bond ionicity. In the crystal all compounds demonstrate large variety of bridged hydrogen bond patterns though the N–H…N and N–H…Cl interactions.