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

Abstract

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, ¹H, ¹¹B and ¹⁹F NMR spectroscopies, mass spectrometry, single-crystal X-ray diffraction and (spectro)electrochemistry. By analysis of <δ¹¹B>, the weighted average ¹¹B chemical shift, a ranking order for the orthocarboranes 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-R₂-7,9-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.