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

Abstract

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 pi stacked in columns (Pnna) or involved in pi-pi or pi-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 P (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., pi-pi or pi-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 pi-pi and pi-H interactions.

Synthesis and Characterization of Side Group–Modified Cyclotetraphosphazene Derivatives

“…24,25 All chemicals and materials were of analytical grade, and were purchased from Chenzhuo Company. The solvents used were purified and dried by standard procedures.…”

Chiral configurations in cyclophosphazene chemistry

“…The structures of a number of BP derivatives of cyclotetraphosphazene have been characterized by X-ray crystallography [194,206]. There are two structures of bis-BP tetramers (189 [206] and 190 [94]) where the BP units exist in the meso configuration, and three structures of tris-BP tetramers (191 [206], 192a,b [194]) where the BP units are necessarily racemic (RRS/SSR etc.).…”

“…There are two structures of bis-BP tetramers (189 [206] and 190 [94]) where the BP units exist in the meso configuration, and three structures of tris-BP tetramers (191 [206], 192a,b [194]) where the BP units are necessarily racemic (RRS/SSR etc.). …”

Supramolecular Structures of Cyclotriphosphazenes

“…It also lacks the familiar paddlewheel arrangement, but adopts a D 3 propeller shape, since its sevenmembered PO 2 C 4 rings are highly twisted [26]. Moreover, the twisted conformation of the biphenyl moieties enables effective crystal packing involving CH?p and p?p interactions [27]. Correspondingly, the related but enantiomerically stable tris-(2,2u-dioxy-1,1u-binaphthyl) derivative does not form clathrates [28].…”