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.
|Number of pages||10|
|Journal||Journal of the Chemical Society. Dalton Transactions|
|Publication status||Published - 14 Feb 2008|
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