Nuclear magnetic resonance studies of rotation about phosphorus–nitrogen bonds in dialkylaminocyclodiphosphazanes

Abstract

The ¹H n.m.r. spectra of dimethylaminocyclodiphosphazanes, X[graphic omitted]R²[X = Cl, Y = NMe₂, R¹= Me, R²= Bu^t, R¹= R²= Bu^t, R₁= R²= Ph; X = Y = NMe₂, R¹= Me, R²= Bu^t, R¹= R²= Bu^t, R¹= R²= C₆H₄Z-p(Z = H, Cl, Me, and OMe)], at, or below, ambient temperatures show that in some cases the dimethylamino-protons are non-equivalent. Measurements of the free energy of activation (ΔG^‡_Tc) in these, in the diethylamino-derivatives, (Et₂N)[graphic omitted]R (R = Bu^t and Ph), and in the di-isopropylamino-derivative, Cl[graphic omitted]Bu^t, indicate that the non-equivalence is due to restricted rotation about the exocyclic phosphorus–nitrogen bonds. Exceptionally high barriers are found for one of the two possible isomeric forms of Cl[graphic omitted]Bu^t and of (Me₂N)[graphic omitted]Bu^t(16.9 and 17.6 kcal mol^–1 respectively), and there are substantial differences in ΔG^‡_Tc(3–6 kcal mol^–1) for pairs of geometrical isomers. The ³¹P chemical shifts of the aminocyclodiphosphazanes, measured by ¹H-{^3lP} double resonance, are temperature dependent, and this dependence is discussed in relation to possible conformational effects.