Stereochemistry in solution of spiro-2,2′-dioxybiphenyl-cyclotriphosphazenes: an NMR spectroscopic study combined with molecular dynamics simulations

Abstract

The stereochemistry in solution of spirocyclophosphazenes 1–3 containing 2,2′-dioxybiphenyl groups, tris(2,2′-dioxybiphenyl)cyclotriphosphazene [N₃P₃(O₂C₁₂H₈)₃] (1) and the chloro derivatives [N₃P₃Cl₂(O₂C₁₂H₈)₂] (2) and [N₃P₃Cl₄(O₂C₁₂H₈)] (3), was studied using high field ¹H, ¹³C and ³¹P NMR spectroscopy (variable temperature experiments) combined with molecular dynamics (MD) simulations. The ambient temperature spectra, after full assignments, showed a unique set of signals (four and six for hydrogens and carbons, respectively) for the corresponding nuclei of all the biphenoxy systems. This isochrony was compatible with symmetry equivalence as well as with fast interconversions on the NMR time scale between the stereoisomers RRR, SSS, RRS, SSR (1), RR, SS, RS, SR (2) and R, S (3). Dynamic NMR studies evidenced that atropoisomers of 1–3 are populated at low temperature (≤ 183 K) bearing out fast racemization of the NP(O₂C₁₂H₈) units in solution at ambient temperature. The MD trajectories, which were long enough (50 ns) to monitor time-averaged processes on the NMR scale, were selectively compatible with DNMR results, and provided insights into the motional properties of 1–3. They significantly complemented the experimental information on the exchange process of 3, due to the inherent limitation of the single use of the NMR technique when NMR active diastereotopic nuclei are absent in the investigated system.