Pentacoordinate 1H-phosphirenes: reactivity, bonding properties, and substituent effects on their structures and thermal stability

The synthesis, reactivity, and bonding properties of several pentacoordinate P-phenyl-substituted 1H-phosphirenes are discussed. X-ray crystallographic analysis of one of them reveals a highly distorted square pyramidal (SP) arrangement around the phosphorus. NMR studies confirm that they retain the SP structure in solution and demonstrate that the endocyclic P-C bonds in the three-membered ring have a very high degree of p character, which results from their being both basal bonds in the SP structure and endocyclic bonds of the three-membered ring. Structural parameters of the three-membered ring of the pentacoordinate phosphirenes obtained by experiment and theoretical calculations are very close to those of a tetracoordinate phosphirenium cation. Thus, by analogy with tetracoordinate phosphirenium cations, it can be considered that a sigma-pi interaction between the sigma orbital of the apical bond and the pi orbital of the C=C bond in the three-membered ring is operative in pentacoordinate phosphirenes. The sigma-pi interaction is found to lower the reactivity of the C=C bond of the three-membered ring. The reactivities of the pentacoordinate phosphirenes are also affected by the substituent on the carbon atom in the three-membered ring.