SYNTHESIS AND AN AB-INITIO IGLO NMR-STUDY OF 7-VERTEX NIDO-PHOSPHACARBORANES - NIDO-6-R'-3,4-R2-6,3,4-PC2B4H4

The reactions of Na+Li+[nido-R2C2B4H4]2-(R = Et, Bz) with R'PCl2(R'= P h,t-Bu,Me) have been found to yield new phosphacarboranes 1-6 of the g eneral formula R'R2PC2B4H4. Their observed spectroscopic data and the results of an ab initio/IGLO/NMR study indicate that these compounds h ave open-cage 7-vertex nido-6-R'-3,4-R2-6,3,4-PC2B4H4 geometries, base d on a dodecahedron missing one rive-connected vertex, with the carbon and phosphorus atoms occupying positions on the open face. The reacti on of nido-6-Ph-3,4-Et2-6,3,4-PC2B4H4, 1, with lithium metal or sodium naphthalenide in THF gives quantitative reduction of the phosphacarbo rane to form arachno-PhEt2PC2B4H42-, 1(2-). Subsequent reaction of 1(2 -) with PhPCl2 gives a product that the NMR and mass spectral evidence suggest is the diphosphacarborane arachno-Ph2Et2P2C2B4H4. Comparison of the spectral data obtained for arachno-PhEt2PC2B4H4(2-) and arachno -Ph2Et2P2C2B4H4 with the results of ab initio/IGLO/NMR calculations su pport 7-vertex and 8-vertex arachno geometries, respectively, for thes e cage systems.