Heterogeneous Oxidation of the [B₁₀H₁₀]^2– Anion by Iron(III) Sulfate: Iron(II) Complexes with 2,2'-Bipyridyl and 1,10-Phenanthroline and Macropolyhedral Borane Cluster [B₂₀H₁₈]^2– as a Counterion

In this work, an efficient method for the synthesis of iron(II) complex compounds with the octadecahydro-eicosaborate anion [B₂₀H₁₈]^2– was developed. The method is based on the heterogeneous two-electron oxidation of the parent closo-decaborate anion [B₁₀H₁₀]^2– in acetonitrile using iron(III) sulfate, which is insoluble in the solvent used. The heterogeneous reaction leads to dimerization of the parent anion with the formation of an intermediate compound of probable composition [Fe(CH₃CN)₆][B₂₀H₁₈], which can subsequently undergo complexation reactions. An advantage of the heterogeneous synthesis is the ease of removal of the unreacted oxidant and the by-products formed from the system, while the target product remains in solution and can be used for subsequent complexation. Addition of ligands L (L = bipy, phen) to the reaction mixture yielded iron(II) complexes [FeL₃][B₂₀H₁₈], which were characterized by elemental analysis, ¹¹B NMR and IR spectroscopies. The structure of complex [Fe(phen)₃][B₂₀H₁₈]·4MeCN was determined by X-ray diffraction.