Influence of the solvent and R groups on the structure of (carboranyl)R₂PI₂ compounds in solution. Crystal structure of the first iodophosphonium salt incorporating the anion [7,8-nido-C₂B₉H₁₀]⁻

The influence of the electron-donor or electron-acceptor capacity of the R groups (R =ⁱPr, Ph, Et) and the solvent on the molecular geometry in solution of adducts of carboranylphosphanes [(carboranyl)ⁱPr₂P, (carboranyl)Ph₂P and (carboranyl)Et₂P] with I₂ in 1 : 1 ratios, has been studied in detail by³¹P(¹H) and¹¹B(¹H) NMR spectroscopies. The more electron-accepting Ph groups make the (carboranyl)Ph₂P less nucleophilic, thus stabilizing the I₂ encapsulated neutral biscarboranylphosphane–diiodine adducts in solution, such as (carboranyl)Ph₂P⋯I–I⋯PPh₂(carboranyl), generating P⋯I–I⋯P motifs. Even in a polar solvent, such as EtOH, the arrangement is preserved. The expected basicity of these carboranylphosphanes is: (carboranyl)ⁱPr₂P > (carboranyl)Et₂P > (carboranyl)Ph₂P. Thus, the comparatively higher basicity of (carboranyl)Et₂P vs. (carboranyl)Ph₂P facilitates a higher transfer of electron density to the I₂ σ*, generating the ionic species, [(carboranyl)Et₂PI]⁺I⁻, even in low polar solvents, such as CH₂Cl₂ and toluene, with no degradation of the cluster. However, in EtOH, the formation of the anionic [7,8-nido-C₂B₉H₁₀]⁻ cluster takes place by removal of one boron atom from the closo cluster. The basicity of (carboranyl)ⁱPr₂P should be the highest, superior to (carboranyl)Et₂P. This is observed in the reaction of these carboranylphosphanes with I₂ in EtOH. Whereas the formation of P four-coordinated molecular “spoke” charge-transfer complexes, (carboranyl)ⁱPr₂P–I–I, are suggested for (carboranyl)ⁱPr₂P in low polarity solvents, ionic species are formed in ethanolic solutions, which deboronate in a few days, faster than (carboranyl)Et₂P, to yield the zwitterionic species. This is attributed to the higher basicity of (carboranyl)ⁱPr₂P vs. (carboranyl)Et₂P. The X-ray crystal structure of [7-PIⁱPr₂-8-Ph-7,8-nido-C₂B₉H₁₀], 2c, obtained from the reaction of 1-PⁱPr₂-2-Ph-closo-1,2-closo-C₂B₁₀H₁₀ with I₂ in EtOH, confirms the formation of the zwitterion. These results prove that minor changes in the nature of the R substituents on the P atom in carboranylphosphanes, along with the solvent in which the reaction takes place, produce major alterations in the geometry of the (carboranyl)R₂P–I–I species in solution, and in their possible further reactivity.