Phosphorine


Phosphorine is a heavier element analog of pyridine, containing a phosphorus atom instead of an aza- moiety. It is also called phosphabenzene and belongs to the phosphaalkene class. It is a colorless liquid that is mainly of interest in research.
Phosphorine is an air-sensitive oil but is otherwise stable when handled using air-free techniques. In contrast, silabenzene, a related heavy-element analogue of benzene, is not only air- and moisture-sensitive but also thermally unstable without extensive steric protection.

History

The first phosphorine to be isolated is 2,4,6-triphenylphosphorine. It was synthesized by Gottfried Märkl in 1966 by condensation of the corresponding pyrylium salt and phosphine or its equivalent 3 and P.
The parent phosphorine was reported by Arthur J. Ashe III in 1971. Ring-opening approaches have been developed from phospholes.

Structure, bonding, and properties

Structural studies by electron diffraction reveal that phosphorine is a planar aromatic compound with 88% of the aromaticity of that of benzene. Potentially relevant to its high aromaticity are the well matched electronegativities of phosphorus and carbon. The P-C bond length is 173 pm and the C-C bond lengths center around 140 pm and show little variation.
Although phosphorine and pyridine are structurally similar, phosphorines are far less basic. The pKa's of C5H5PH+ and C5H5NH+ are respectively -16.1 and 5.2. Methyl lithium adds to phosphorus in phosphorine whereas it adds to the 2-position of pyridine.
Phosphorine undergoes electrophilic substitution reactions like ordinary aromatic compounds: bromination, acylation, and so on.

Coordination chemistry

es bearing phosphorine as a are known. Phosphorines can bind to metals through phosphorus center. Complexes of the diphospha analogue of bipyridine are known. Phosphorines also form pi-complexes, illustrated by V2.