Conformational Flexibility in 2,2‘-Dioxybiphenyl-chloro-cyclotetraphosphazenes and its Relevance to Polyphosphazene Analogues

The reaction of the cyclotetraphosphazene, [N4P4Cl8], with the difunctional reagent, 2,2‘-biphenol, in the presence
of potassium carbonate in acetone produced the spiro-substituted derivatives, 2,2‘-dioxybiphenylhexachlorocyclotetraphosphazene, bis(2,2‘-dioxybiphenyl)tetrachloro-cyclotetraphosphazene, and tris(2,2‘-dioxybiphenyl)dichlorocyclotetraphosphazene. Both cis and trans geometrical isomers of the bis compound are observed. Although
chromatographic separation of these was unsuccessful, a sample of the trans isomer was obtained by fractional
crystallization. The compounds all show non-first-order 31P NMR spectra which were simulated to extract the spectral
parameters. Single-crystal X-ray structures of both the trans bis and the tris compounds show that the
cyclophosphazene rings exhibit conformational flexibility which gives rise to different crystalline forms being obtained
from the same solvent systems. Crystals of trans-bis(2,2‘-dioxybiphenyl)tetrachloro-cyclotetraphosphazene were
obtained in two different space groups:  Pnna (orthorhombic) and P21/n (monoclinic). In the orthorhombic structure,
the dominant (72%) conformation of one phosphazene ring is a chair form, and the other (28%) resembles a boat.
While for the monoclinic structure, the ring is virtually flat with an oval shape. In both cases the dioxybiphenyl
groups are found in R and S configurations in the same molecule and are π stacked in columns (Pnna) or involved
in π−π or π−H interactions (P21/n), thus anchoring the phosphorus atoms of the cyclotetraphosphazenes but still
allowing flexibility in the ring conformations. Three crystalline modifications of tris(2,2‘-dioxybiphenyl)dichloro-cyclotetraphosphazene were obtained:  two in space group P1̄ (triclinic), which contained two molecules of
dichloromethane in the unit cell, and one solvent-free form in space group P21/n (monoclinic). The cyclophosphazene
rings exhibit puckered conformations with the trans-dioxybiphenyl moieties having opposing RS or SR conformations.
DFT calculations were carried out on each of the phosphazene ring conformations in trans-bis(2,2‘-dioxybiphenyl)tetrachlorocyclotetraphosphazene identified from the X-ray diffraction analysis. It is concluded that intermolecular
interactions (i.e., π−π or π−H) between the dioxybiphenyl groups is a factor that modifies the nature of the potential
energy surface between the different conformers. The flexibility of the phosphazene ring is supported computationally
through the calculated low-energy barriers and experimentally through the highly disordered phosphazene ring
conformations observed in the solid state. The results on 2,2‘-dioxybiphenyl-substituted cyclotetraphosphazenes
provide evidence that microcrystalline domains in their 2,2‘-dioxybiphenyl-substituted polyphosphazene analogues
will be generated by similar π−π and π−H interactions.