Phosphorine
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Phosphorine (IUPAC name: phosphinine) 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<ref name="Ashe" /> but is otherwise stable when handled using air-free techniques (however, substituted derivatives can often be handled under air without risk of decomposition).<ref name=":0" /><ref>Template:Cite journal</ref> 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.
HistoryEdit
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 ( P(CH2OH)3 and P(SiMe3)3).<ref name=":0">G. Märkl, 2,4,6-Triphenylphosphabenzol in Angewandte Chemie 78, 907–908 (1966)</ref>
Synthesis of Triphenylphosphabenzene
The (unsubstituted) parent phosphorine was reported by Arthur J. Ashe III in 1971 by the reaction of 1,4-dihydro-1,1 dibutylstannabenzene and phosphorus tribromide.<ref name="Ashe">Template:Cite journal</ref><ref>Template:Greenwood&Earnshaw2nd</ref> Ring-opening approaches have been developed from phospholes.<ref name=Mathey/>
Structure, bonding, and propertiesEdit
Structural studies by electron diffraction reveal that phosphorine is a planar aromatic compound with 88% of aromaticity of that of benzene. Potentially relevant to its high aromaticity are the well matched electronegativities of phosphorus (2.1) and carbon (2.5). The P–C bond length is 173 pm and the C–C bond lengths center around 140 pm and show little variation.<ref>László Nyulászi "Aromaticity of Phosphorus Heterocycles" Chem. Rev., 2001, volume 101, pp 1229–1246. {{#invoke:doi|main}}</ref>
File:Bond lengths of group 15 heterobenzenes and benzene.svg Bond lengths and angles of benzene, pyridine, phosphorine, arsabenzene, stibabenzene and bismabenzene |
Although phosphorine and pyridine are structurally similar, phosphorines are far less basic. The pKa of C5H5PH+ and C5H5NH+ are respectively −16.1 and +5.2. The P-oxides are extremely unstable, rapidly adding nucleophiles to a species tetracoordinate at phosphorus. Strongly backbonding Lewis acids (e.g. tungsten pentacarbonyl) can stabilize a dative bond from phosphorus.<ref name=Mathey>Mathey, François (2011). "Phosphorus Heterocycles" in Modern Heterocyclic Chemistry, 1st ed., edited by Álvarez-Builla, Julio; José Vaquero, Juan; and Barluenga, José. Weinheim: Wiley-VCH. §23.3. {{#invoke:doi|main}}.</ref>
Both electrophiles and strong, hard nucleophiles preferentially attack at phosphorus, but the ring aromaticity is sufficiently weak that the result is an addition reaction, and not aromatic substitution.<ref name=Mathey/> Thus for example methyllithium adds to phosphorus in phosphorine whereas it adds to the 2-position of pyridine.<ref>Ashe III, Arthur J.; Smith, Timothy W. "The reaction of phosphabenzene, arsabenzene and stibabenzene with methyllithium." Tetrahedron Letters 1977, volume 18, pp. 407–410. {{#invoke:doi|main}}</ref> Halophosphorines do undergo noble-metal- or zirconocene-catalyzed substitution, and λ5-phosphorines exhibit a much more traditional substitution chemistry.<ref name=Mathey/>
Unlike arsabenzene, phosphorine rarely participates in Diels-Alder-type cycloadditions; when it does, the coupling partner must be an extremely electron-poor alkyne. Phosphorine complexes are tolerable Diels-Alder reactants.<ref name=Mathey/>
Coordination chemistryEdit
Coordination complexes bearing phosphorine as a ligand are known. Phosphorines can bind to metals through phosphorus center. Complexes of the diphospha analogue of 2,2′-bipyridine are known. Phosphorines also form pi-complexes, illustrated by V(η6-C5H5P)2.<ref name=Mathey/>
See alsoEdit
- Six-membered aromatic rings with one carbon replaced by an element from another group: borabenzene, silabenzene, germabenzene, stannabenzene, pyridine, phosphorine, arsabenzene, stibabenzene, bismabenzene, pyrylium, thiopyrylium, selenopyrylium, telluropyrylium