Phosphorine

Phosphorine is a heavy benzene containing a phosphorus atom instead of a CH moiety, so it is considered to be a heavier element analog of pyridine. It is also called phosphinine or phosphabenzene. Phosphorine is a planar aromatic compound with 88% of the aromaticity of that of benzene. The P-C bond length is 173 pm and the C-C bond lengths center around 140 pm and show little variation.

Phosphorine is generally stable against air and moisture and can be handled without special inert atmosphere equipment, so it is different from silabenzene, which is usually not only air- and moisture-sensitive but also thermally unstable without extensive steric protection. This stability of phosphorine comes from the close electronegativities of phosphorus (2.1) and carbon (2.5). The physical and chemical properties of metal complexes bearing phosphorine as a ligand as well as phosphorine itself have been studied extensively.

History
The first phosphorine is 2,4,6-triphenylphoshorine, which was synthesized by Gottfried Mërkl in 1969 from the corresponding pyrylium salt and phosphorus sources, such as phosphine, P(CH2OH)3, and P(SiMe3)3.

Unsubstituted phosphorine, which was reported by Arthur J. Ashe III in 1971, is a distillable liquid that is somewhat air-sensitive but stable against hydrolysis. In 1990s, François Mathey developed a methodology for the synthesis of functionalized phosphorines using transition metal mediated reactions including palladium- or nickel-catalyzed coupling reactions.

Properties and reactions
Although phosphorine is a heavier element analog of pyridine, the electronic structure is very different from that of pyridine. The lone pair of pyridine is its HOMO, so pyridine has good σ-donating ability. The HOMO and LUMO of phosphorine, on the other hand, are its π and π* orbitals, respectively, and the lone pair is located at the lower energy level. Thus phosphinine works as an σ-acceptor and π-donor ligand rather than an σ-donor ligand. Phosphorine with a 0.55 positive charge on phosphorus and pyridine with a -0.67 negative charge on nitrogen also show different reactivities against nucleophiles. Pyridine reacts with nucleophiles at the C-2 position due to the higher electronegativity of the nitrogen atom. Nucleophiles, however, attack the phosphorus atom of phosphorine, producing λ4-phosphorine anion, which reacts with electrophiles to give λ5-phosphorine. Lately it was reported that anionic λ4-phosphine coordinates to transition metals as a π-donor ligand.



Phosphorine undergoes electrophilic substitution reactions like ordinary aromatic compounds: bromination, acylation, and so on.