Donor atom interactions in phosphoranes, phosphites, and phosphates: Models for phosphoryl transfer intermediates

Hypercoordination by ligand donor action in pentaoxyphosphoranes has been observed many times and is postulated as a model for possible enzyme-substrate intermediates in phosphoryl transfer systems. Donor atoms include those that may be found at enzyme active sites i.e. nitrogen, oxygen and sulfur. The degree of coordination by the donor atom can be measured by the distance between the donor atom and the phosphorus center and by the degree of distortion of the trigonal bipyramid towards an octahedral configuration. Measurements are made from the molecular structures determined by X-ray crystallography. Other techniques used in structure determination include $\sp1$H and $\sp{31}$P NMR. In order to examine this phenomenon further and to try and establish conditions where coordination is at its lower limit three series of compounds are synthesized. They are bicyclic tetraoxyphosphoranes with oxaphosphorinane rings, tetraoxyphosphoranes with amino or chloro acyclic ligands and a third series of related phosphates and phosphites. Two of the tetraoxyphosphoranes with oxaphosphorinane rings have the potential to show ligand donor action but only the sulfur donor does. The oxygen of the sulfone group of the other one remains non-coordinating. Other compounds in this series display different sizes of ring system. All below eight members are in an axial-equatorial position whilst the two non-coordinating eight membered rings are diequatorial. The tetraoxyphosphoranes with amino or chloro ligands are synthesized by the addition of quinones to phosphites. They show coordination but it is of a reduced strength compared to similar pentaoxyphosphoranes. This is attributed to the generally lower electronegativity about the central phosphorus atom and, in the case of the amino compounds, $\pi$-backbonding from the nitrogen to the phosphorus. The phosphites used to synthesize the chloro and amino phosphoranes also show ligand donor action but it is not as strong as in the related phosphoranes. Differences are also highlighted between coordination in phosphites and their related phosphates. Possible applications as biodegradable organophosphorus pesticides are also discussed