The crystal and molecular structure of 2,4,6-trimethoxy-1,3,5-trimethyl-2,4,6-trioxocyclotriphosphazane

Abstract

Crystals of 2,4,6-trimethoxy-1,3,5-trimethyl-2,4,6-trioxocyclotriphosphazane, N₃Me₃P₃O₃(OMe)₃, are monoclinic, a= 11·32, b= 8·29, c= 7·63 Å, β= 98·3°, space-group P2₁ with two molecules in the unit cell. The atomic positions have been determined from X-ray diffraction data. The six-membered ring of alternate phosphorus and nitrogen atoms has a twisted boat shape with an average P–N bond length of 1·66 Å. The shortening of this value from the accepted P–N single bond length (1·77 Å) indicates some π bonding between phosphorus and nitrogen arising from donation of the nitrogen lone-pair electrons to vacant 3d orbitals on phosphorus. The P–OMe bonds (average 1·56 Å) are also shorter than expected for a single bond. The relative orientations of the 3d orbitals on the phosphorus and 2p orbitals on the nitrogen and oxygen atoms, as deduced from the molecular geometry, are used to set up π systems in the molecule and it is shown that the distorted ring shape in this structure is one which gives good matching of these orbitals. It is also suggested that the orientations of the methoxy-groups are influenced by the requirement of good matching between the p orbitals on oxygen and d orbitals on phosphorus.