Some aspects of the chemistry of quinquecovalent phosphoranes.
2015-11-19T08:48:27Z (GMT) by
PART 1. A number of cyclic oxyphosphoranes have been prepared and their dynamic n.m.r. spectra investigated. The changes in the spectra with temperature are interpreted in terms of the slowing of certain pseudo-rotations available to the molecule and the energetic data derived from the spectral changes are suggested to be a measure of the relative apicophi1i cities of the groups bonded to phosphorus. The scale of apicophilicity values so obtained is explained in terms of the following properties of the ligand: electronegativity, ?-donor ability, ?-acceptor ability, polarisability and size. From a study of the energy required to place the five-membered ring of various cyclic oxyphosphoranes in the diequatorial position it is concluded that this energy increases considerably with the presence of endocyclic heteroatoms bonded to phosphorus. The increase in ring strain is attributed to the changes in orientation of the donor orbitals of the heteroatoms when the ring is moved from an apical-equatorial to a diequatorial position. The same donor orbital orientation effect has been shown to be present in six-membered cyclic oxyphosphoranes, although in this case it is suggested that the conformation of the six-membered ring may decrease the heteroatom rotation terms compared with the values for five-membered rings. D.n.m.r. data for a series of oxyphosphoranes, where the size of the alkyl groups bonded to phosphorus is varied, have been interpreted in terms of the following two tenets: (a) the apical position of a trigonal bipyramid is the more hindered position; and (b) steric effects in phosphoranes are not appreciable unless there are at least two bulky groups bonded to phosphorus. Evidence has been presented that nucleophilic substitution at the phosphorus of oxyphosphoranes need not necessarily proceed by attack of the nucleophile directly opposite the leaving group. PART 2. A range of phosphoryl compounds has been synthesised and their suitability for use as fire-resistant hydraulic fluids has been assessed by determining their hydrolytic stability, fire resistance and melting point.