The Mechanism of Halogenation of Decahydro-closo-Decaborate Dianion by Hydrogen Chloride

The mechanism of the halogenation of decahydro-closo-decaborate dianion [B10H10]2− by HCl via the electrophile-induced nucleophilic substitution (EINS) was explored at M06/6-311++G(d,p) level of DFT theory in acetonitrile (MeCN) taking into account non-specific solvent effect (SMD model). The dihydrogen-bonded (DHB) complexes are the first and important reaction intermediates of the EINS process since they determine the principal direction of HCl attack and lead to the proton transfer to the most reactive and elongated B–H bond. Upon the successive replacement of H− with Cl− in the closo-borane structure, a gradual increase in the electron deficiency of closo-borane and the electrophilicity of boron atoms are observed. The lack of stabilization of the η2-H2 complexes for the subsequent stages of the reaction is associated with an increased electrophilicity of boron atoms in substituted closo-boranes. An increased electrophilicity of boron atoms in substituted closo-boranes and higher activation energy of each subsequent stage during the EINS process hamper the reaction and completely stop the chlorination on 3rd or 4th reactions steps. Thus, the data obtained indicate that for the selective synthesis of halogenated products [B10H(10 – x)Clx]2− (x = 5–7) it is necessary to use an approach alternative to the simple acid-initiated nucleophilic substitution. This could be the activation of the bond by Lewis superacids or transition metal catalysis.