Nitrogen nuclear magnetic resonance spectroscopy. Part 9. Cyclophosphazenes and related phosphorus–nitrogen compounds

Abstract

Nitrogen-14 n.m.r. shifts determined by wide-line methods are reported for the cyclophosphazenes (NPX₂)_n(n= 3, X = NMe₂, OMe, F, Cl, Br, or NCS; n= 4, X = OMe, F, or Cl) and for the acyclic compounds P(NMe₂)₃, As(NMe₂)₃, P(NMe₂)₃S, and Cl₃PNPCl₂O. The cyclophosphazene nitrogen resonances span a range of 90 p.p.m., and are at fairly high field, as for example for S₄N₄, in contrast to those of aromatic azines and the cyclothiazenium ion [S₄N₃]⁺ at medium field. The effects of ring size and perhaps of puckering are fairly small, and the chemical shifts of the ring nitrogens are not very different from those in the acyclic compounds with similar substituents on phosphorus. The shielding of the ring nitrogens in the cyclotriphosphazenes reflects the combined inductive and conjugative effects of the substituent X on phosphorus. Thus the sequence of the (NPX₂)₃ resonances resembles that observed for acyclic nitrogen groups in which the X substituents are attached directly to nitrogen, and is the same for nitrogen and phosphorus shielding, if the latter is corrected for the variation in the diamagnetic contribution of the ligand X.