Deboronation of Icosahedral Carboranes

Deboronation of Icosahedral Carboranes

The removal of a boron atom from a carborane icosahedron to yield a nido-carborane is probably the most common reaction carried out in carborane research because carborane icosahedra are commercially available and the nido carborane are precursors to many metallacarboranes and heteroboranes.

However little is known about the mechanism involved in converting closo-C₂B₁₀H₁₂ into their respective nido– carborane monoanions C₂B₉H₁₂^–. Thus here we have

i)                    obtained an intermediate C₂B₁₀H₁₂.L  [L = HN=P(NMe₂)₃] in the conversion of 1,2-C₂B₁₀H₁₂ into 7,8-C₂B₉H₁₂^–,

ii)                   found that it is necessary to have water present in the fluoride-ion deboronation of carboranes and deduced their mechanisms,

iii)                 found that the most common deboronating agents, KOH in ethanol and Bu₄NF in THF, are not suitable for diaryl-meta-carboranes as they give nido-cages with ethoxy or fluoro substituents at the cage boron, but piperidine is a suitable agent and

iv)                    deboronation is facile for carboranes with halogens at cage borons. 

The molecular structures of the three well known nido-C₂B₉H₁₂^– anions were experimentally determined here for the first time. The location of the hydrogen on the open face in the much discussed anion 7,8-C₂B₉H₁₂^– was confirmed by neutron diffraction. Excellent agreements between these solid state geometries and the MP2-optimized geometries are found. The misfit program ‘ofit’ in the SHELXL packages is used to compare experimental and observed geometries here – it gives one misfit value as opposed to a list of bond lengths of each geometry usually reported in the literature for comparison. The lower the misfit value the better the agreement.

It is often necessary to deprotonate the nido-C₂B₉H₁₂^– anions by base (e.g. BuLi, NaH) prior to metallacarborane formation and the deprotonated carborane is usually referred to as C₂B₉H₁₁^2-. Here we conclude from our NMR and ab initio computations that these species are best described as the monoanion MC₂B₉H₁₁^– where the metal atom sits on the open face. The metal atom depends on what base is used – i.e. M = Li if BuLi is used.

14. Fluoride-ion deboronation of p-fluorophenyl-ortho– and –meta-carboranes. NMR evidence for the new fluoroborate, HOBHF₂^–.

      M. A. Fox, J.A.H. MacBride and K. Wade,

      Polyhedron, 1997, 16, 2499-2507.

28. Deboronation of ortho-carborane by an iminophosphorane: crystal structures of the novel carborane adduct nido-C₂B₁₀H₁₂.HNP(NMe₂)₃ and the borenium salt [(Me₂N)₃PNHBNP(NMe₂)₃]₂O²⁺(C₂B₉H₁₂^–)₂

      M.G. Davidson, M. A. Fox, T.G. Hibbert, J.A.K. Howard, A. Mackinnon, I.S. Neretin and K. Wade,

      Chem. Commun., 1999, 1649-1650.

41. Do the discrete dianions C₂B₉H₁₁^2- exist? Characterisation of alkali metal salts of the 11-vertex nido dicarboranes, C₂B₉H₁₁^2-, in solution

      M. A. Fox, A.K. Hughes, A.L. Johnson and M.A.J. Paterson,

      J. Chem. Soc., Dalton Trans., 2002, 2009-2019.

43. Crystal and molecular structures of the nido-carborane anions, 7,9- and 2,9-C₂B₉H₁₂^–

M. A. Fox, A.E. Goeta, A.K. Hughes and A.L. Johnson,

J. Chem. Soc., Dalton Trans., 2002, 2132-2141.