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

摘要

The icosahedral carboranes 1-C_6F_5-2-Ph-1,2-closo- C_2B_(10)H_(10) (1), 1-(4′-F_3CC _6H_4)-2-Ph-1,2-closo-C_2B_(10)H _(10) (2), 1,2-(4′-F_3CC_6H_4) 2-1,2-closo-C_2B_(10)H_(10) (3), 1-(4′-H_3CC_6F_4)-2-Ph-1,2-closo-C _2B_(10)H_(10) (4), 1-(4′-F_3CC _6F_4)-2-Ph-1,2-closo-C_2B_(10)H _(10) (5), 1,2-(4′-F_3CC_6F_4) _2-1,2-closo-C_2B_(10)H_(10) (6), 1,7-(4′-F_3CC_6F_4)_2-1,7-closo- C_2B_(10)H_(10) (7) and 1,12-(4′-F _3CC_6F_4)_2-1,12-closo-C _2B_(10)H_(10) (8), with fluorinated aryl substituents on cage carbon atoms, have been prepared in good to high yields and characterised by microanalysis, ~1H, ~(11)B and ~(19)F NMR spectroscopies, mass spectrometry, single-crystal X-ray diffraction and (spectro)electrochemistry. By analysis of <δ~(11)B>, the weighted average ~(11)B 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 ΔE values, where ΔE is the energy gap between the DFT-optimised [7,9-R _2-7,9-nido-C_2B_(10)]~(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-C_2B10]~(2-) dianions with respect to oxidation, and that the best practical substituent is 4-F_3CC _6F_4. Thus attention focussed on the reduction of 1,2-(4′-F_3CC_6F_4)_2-1,2-closo- C_2B_(10)H_(10), 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.