Spectroscopic, structural, computational and (spectro)electrochemical studies of icosahedral carboranes bearing fluorinated aryl groups

The icosahedral carboranes 1-C6F5-2-Ph-1,2-closo-C2B10H10 (1), 1-(4'-F3CC6H4)-2-Ph-1,2-closo-C2B10H10 (2), 1,2-(4'-F3CC6H4)(2)-1,2-closo-C2B10H10 (3), 1-(4'-H3CC6F4)-2-Ph-1,2-closo-C2B10H10 (4), 1-(4'-F3CC6F4)-2-Ph-1,2-closo-C2B10H10 (5), 1,2-(4'-F3CC6F4)(2)-1,2-closo-C2B10H10 (6), 1,7-(4'-F3CC6F4)(2)-1,7-closo-C2B10H10 (7) and 1,12-(4'-F3CC6F4)(2)-1,12-closo-C2B10H10 (8), with fluorinated aryl substituents on cage carbon atoms, have been prepared in good to high yields and characterised by microanalysis, H-1, B-11 and F-19 NMR spectroscopies, mass spectrometry, single-crystal X-ray diffraction and (spectro)electrochemistry. By analysis of , the weighted average B-11 chemical shift, a ranking order for the ortho carboranes 1-6 is established based on the combined electron-withdrawing properties of the C-substituents, and is in perfect agreement with that established independently by electrochemical study. In a parallel computational study the effects of a wide range of different substituents on the redox properties of carboranes have been probed by comparison of Delta E values, where Delta E is the energy gap between the DFT-optimised [7,9-R-2-7,9-nido-C2B10](2-) anion and its DFT-optimised basket-shaped first oxidation product. The overall conclusion from the NMR spectroscopic, electrochemical and computational studies is that strongly electron withdrawing substituents significantly stabilise [7,9-nido-C2B10](2-) dianions with respect to oxidation, and that the best practical substituent is 4-F3CC6F4. Thus attention focussed on the reduction of 1,2-(4'-F3CC6F4)(2)-1,2-closo-C2B10H10, compound 6. The sequence 6/[6](-) /[6](2-) appears reversible on the cyclic voltammetric timescale but on the longer timescale of macroelectrolysis the radical anion is only partially stable. EPR study of the electrogenerated monoanions from the ortho-carboranes 1-6 confirms the cage-centred nature of the redox processes. In contrast, the reduction of the meta-and para-carboranes 7 and 8, respectively, appears to be centred on the aromatic substituents, a conclusion supported by the results of DFT calculation of the LUMOs of compounds 6-8. Bulk 2-electron reduction of 6 affords a dianion which is remarkably stable to reoxidation, surviving for several hours in the open laboratory in the absence of halogenated solvents.