Synthesis, crystal structures, and quantum chemical calculations of Novel Phosphonium Salt-1,5-diphospha-3-phosphonia-tricyclo pentane cations

Abstract

This work deals with reactions between the kinetically stable 2-tert-butyl-1λ3-phospha-alkyne, tBu-C[tbnd]P, and various halodiorganylphosphines (X = Cl, Br). The isolated ionic salts with the 2,4-di-tert-butyl-3,3-diorganyl-1λ3,5λ3-diphospha-3-phosphonia-tricyclo[2.1.0.02,5]pentane cations, [R2C2tBu2P3]⊕ (R = ethyl, isopropyl, methyl, phenyl), were characterized by spectroscopic methods; additionally, the results of X-ray structure analyses were confirmed by quantum chemical calculations with Gaussian 03 program which were performed on the hydrogen substituted phosphonium and phosphenium cations in order to ascertain optimized structural data and relative energies for different isomers. As for the phosphonium cation ([H2P(CH)2P2]⊕) generated from our work the conventional trigonal bipyramidal framework of point group C2v represents the absolute minimum on the potential energy surface. To our surprise this situation is followed by a second one which has to be attributed to the so-called housene structure of point group C1 showing a somewhat higher energy value + 77.9 kJ/mol. Quite a reverse situation is encountered for the phosphenium cation [P(CH)2P2]⊕. Here the pseudo square-based pyramidal nido structure of point group C2v known from Russell's tetrachloroaluminate(III) compound is found to be the only minimum on the potential energy surface. A phosphenium cation with a trigonal bipyramidal framework of point group C2v is higher in energy by only 35.7 kJ/mol, but due to one imaginary frequency it has to be considered the structure of a transition state. The opened housene structure corresponds neither to a minimum nor to a saddle point on the potential energy surface. In the pseudo square-based pyramidal nido structure (point group C2v) of the phosphenium cation [P(CH)2P2]⊕ the s-orbital and all p-orbitals of the apical four-coordinate phosphorus atom are used to form two P–C and two P–P bonds. Further addition of two hydrogen atoms to entail the phosphonium cation [H2P(CH)2P2]⊕ of point group symmetry C2 not only increases the coordination number of the apical phosphorus atom to six but also requires two electrons and two orbitals for P–H bonding. These are no longer available to the bonding system within the nido structure; as a consequence, its energy increases to +246.6 kJ/mol and the cation rearranges to give the conventional structure of ours.