Phosphorine

• Phosphorin
• Phosphinine

Liquid

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Phosphabenzene is an aromatic heterocycle containing, besides carbon and hydrogen, a phosphorus atom. The molecular formula is C5H5P. Thus, it is the heavier homologs of pyridine. Other common names are phosphinine or Phosphorin. Phosphabenzene has a planar structure having about 88 % of the aromaticity of the benzene. P -C bond length of 173 pm and the C -C bond length in the range of 140 pm.

Phosphabenzene is relatively stable, low air and moisture sensitive and can be stored under an inert atmosphere for long. In contrast, silabenzene example, is very air-and moisture-sensitive and thermolabile. The stability of the title compound is associated with the similar electronegativities of carbon ( 2.5 ) and phosphorus ( 2.1 ).

Representation

For the first time a Phosphabenzolderivat by Gottfried Märkl was synthesized in 1967 by from the corresponding pyrylium salt by reaction with various phosphines such as P ( SiMe3) could be obtained this.

A synthetic route to unsubstituted phosphabenzene was described by Arthur Ashe III in 1971.

Francois Mathey, one of the world's leading phosphorus chemists who Ashe'schen methods developed with the involvement of transition metal complex compounds including palladium - or nickel -catalyzed coupling reactions on.

Properties and reactions

Although phosphabenzene is the heavy homologues of pyridine, there are crucial differences between these compounds. The free electron pair of the N in pyridine is the HOMO, so that pyridine has a good σ - donor properties.

The HOMO and LUMO of phosphabenzene however, the π and π * molecular orbitals, as well as the free electron pair is localized in a lower energy level. For this reason phosphabenzene acts more as a σ -acceptor and π - donor ligand.

Pyridine reacts with nucleophiles predominantly in the C2-position because of the higher electronegativity of the nitrogen atom. Phosphabenzene other hand, forms with nucleophiles λ4 derivatives, which in turn react with electrophiles to λ5 - phosphabenzenes.

Homologous phosphabenzene

Pyridine and phosphabenzene the homologous compounds of heavy elements are known. They all share their aromaticity. The stability decreases as the atomic number of the hetero atom. Subsequently, the bond lengths and angles of the Straight benzenes group 15 of the periodic table are shown (from left to right: pyridine, phosphabenzene, Arsabenzol, Stilbabenzol and Bismabenzol ):

The increasing with increasing atomic number instability is related to the fact that a [4 2 ] cycloaddition preferably already occurs at low temperatures and designed impossible to isolate the pure substances in some cases.

In electrophilic substitution reactions such as halogenation, acylation, etc. There phosphabenzene behaves like other aromatic compounds.

A compilation of properties, synthesis and reactions of the homologs phosphabenzene and Arsabenzol delivered Märkl 1982.

Transition metal

From phosphabenzene many transition metal complexes are known. Among these are also mixed phosphazene / Phosphabenzo ligands studied by Mathey. Phosphabenzene rhodium systems seem to be superior to the conventional catalysts in the hydroformylation of terminal and internal olefins.

The Marburg Organometallchemiker and author Christoph Elschenbroich discusses in his book phosphabenzene more complex compounds, in which the ligand acts both as η1, as well as η6 ligand. So ( η1 - C5H5P ) - Mo ( CO) 5 has been described by Ashe in 1977. Other transition metals will complex such as ( η6 - RnC5H5 - nP ) nCr (CO) 6-n, which was synthesized by Nöth 1973.