Phosphonitrilic derivatives. Part XV. The structure of hexadecamethoxycyclo-octaphosphonitrile, [NP(OMe)2]8

Abstract

Crystals of hexadecamethoxycyclo-octaphosphonitrile [NP(OMe)₂]₈ are triclinic, a= 8·40, b= 11·07, c= 11·67 Å, α= 62°7′, β= 84° 18′, γ= 76° 14′, Z= 1, space group P. The structure was determined with Mo-K_α counter data by Patterson, electron-density, and least-squares methods, the final R being 0·079 for 1786 observed reflexions. The centrosymmetrical sixteen-membered phosphonitrilic ring consists of two approximately planar and parallel six-atom segments joined by a step. The P–N bond lengths are all equal within the experimental error, their mean being 1·561 Å; a similar statement applies to the P–O and O–C lengths, their means being 1·576 Å and 1·440 Å respectively. The mean N–P–N angle is 116·7 Å, one being significantly larger (121·2°) than the other three; the mean P–N–P angle is 136·7°, one angle being again larger (141·0°) than the others. The P–O–C angles are all 120°. The methoxy-groups cover the ring evenly, their arrangement being such that there is little conflict between steric and π bonding requirements. The molecular structure as a whole supports earlier conclusions as to the importance of the π system involving orbitals with axes perpendicular to the local ring plane.