Formation and isomerisation processes of six-co-ordinate phosphorus anions
A series of spirophosphoranes was prepared in order to study different aspects of nucleophilic substitution at five-co-ordinate phosphorus. The phosphoranes were treated with nucleophiles under temperature controlled conditions and the reactions were monitored by 19F and 31P NMR spectroscopy. The half-lives of the resulting phosphorides could be extended by increasing the electron withdrawing properties of the ligands and by introducing ligands with low lying empty orbitals. Wherever possible nucleophilic attack was assisted by the greatest number of antiperiplanar lone pair interactions from the donor atoms (stereoelectronic effects). This led to the kinetically controlled formation of the trans-isomer of some six-co-ordinate bis (bidentate) phosphorus anions, which under thermodynamic conditions produced the more stable cis-isomer (s). In some cases the trans-cis isomerisation proceeded via a dissociative mechanism, this involving a unidentate ligand. Some cis-phosphorides underwent intramolecular isomerisation processes, possibly via trigonal prismatic intermediates. Six-co-ordinate phosphorus anions containing a phosphor-inane ring or non-equivalent bidentate/unidentate ligand donor atoms were short-lived. Increasing steric hindrance by introduction of substituents onto a donor atom disfavoured the formation of phosphorides. Only one equilibrium between a phosphorane and a phosphoride became rapid on the NMR timescale on increasing the temperature; all other equilibria were slow on the NMR timescale. Introduction of an oxazaphospholane ring into a phosphorane decreased the half-life of the generated phosphoride and introduced in some cases a rigidity at the nitrogen centre. A review of 19F and 31P NMR data of chemical shifts and coupling constants of phosphorides anions is given.