Conformation, Bonding, and Flexibility in Short-Chain Linear Phosphazenes

A number of linear, short-chain phosphazenes have been prepared as structural models for three classes of phosphazene linear high polymers. X-ray diffraction results from studies of OP(C1₂)NPC1₃(7), OP(C1₂)NP(Cl₂)NPCl₃(10), [C1₃PNP-(C1₂)NP(Cl₂)NPCl₃]+PC1₆^-(14), OP(OPh)₂NP(OPh)₃ (8), OP(NHPh)₂NP(NHPh)₃ (9), and OP(NHPh)₂NP(NHPh)₂NP(NHPh)₃(12) suggest different values for the bond angles and bond lengths than have been used in the past in structural studies of the high polymers. The P—N bond lengths in the short-chain species differ by 0.07 Å or less within each molecule, and planar skeletal conformations are preferred, especially cis—trans planar. The evidence suggests that, although the molecules are stabilized by electron delocalization, the conformations originate from intramolecular nonbonding interactions. The short phosphazene chains stack in the crystal lattice in a parallel arrangement analogous to that expected in polymer microcrystallites. Comparisons between the ³¹P NMR shifts of the short chain species and the corresponding high polymers revealed a close similarity between the electronic and structural environments of the middle units in the short chain species and in the repeating units of the high polymers.