Contribution of the nido-[7,8-C₂B₉H₁₀]^- Anion to the Chemical Stability, Basicity, and ³¹P NMR Chemical Shift in nido-o-Carboranylmonophosphines

The icosahedral dicarboranes and their decapitated anion, 1-R‘-1,2-C₂B₁₀H₁₀ (closo) and [7-R‘-7,8-C₂B₉H₁₀]^- (nido), exert a distict influence at the α position of substituents attached to the cage carbon atom. The closo fragment is electron-withdrawing while the nido anion is electron-releasing. These effects are studied by ³¹P NMR, phosphorus oxidation, and phosphorus protonation in [7-PR₂-8-R‘-7,8-C₂B₉H₁₀]^- species. The ³¹P NMR chemical shift dependence is related to the R alkyl or aryl nature of [7-PR₂-8-R‘-7,8-C₂B₉H₁₀]^-. No direct relationship to the nature of the R substituent on the nido-carboranylmonphosphine toward oxidation has been found. The basicity of the nido-alkylcarboranylmonophosphines is the highest while the lowest corresponds to the nido-arylcarboranylmonophosphines. Interpretation can be carried out qualitatively by considering the electronic properties of the cluster and the nature of the R groups. The influence of R‘ is less relevant. Confirmation of the molecular structure of the oxidated and protonated nido-carboranylmonophosphine compounds was obtained by X-ray diffraction analysis of [NBu₄][7-P(O)Ph₂-8-Ph-7,8-C₂B₉H₁₀] and [7-PH(ⁱPr)₂-8-Me-7,8-C₂B₉H₁₀].