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

Abstract

The mechanism of the halogenation of decahydro-closo-decaborate dianion [B 10 H 10 ] 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 η 2 -H 2 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 [B 10 H (10 -x) Cl x ] 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.

Theoretical Insight on the Formation Mechanism of a Trisubstituted Derivative of Closo-Decaborate Anion [B10H7O2CCH3(NCCH3)]0

“…In the case of migration to an apical position (B10_TSmigr_ap), the energy barrier was equal to 49.1 kJ/mol for wB97X-D3/def2-TZVPP or 57.2 kJ/mol for DLPNO-CCSDT/cc-pvdz. [28,34]. H fac can migrate to three different boron atoms: two equatorial boron atoms and one apical boron atom.…”

“…Proton migration led to the formation of an intermediate in which the dihydrogen H 2 fragment coordinated to one boron atom of the cluster cage. The nature of the interaction between the boron atom and the H 2 fragment has been discussed in detail in several articles [28,29]. As in the case of transition state structures, the anion [B 10 H 7 O 2 CCH 3 (H 2 )] − has three possible isomers: H 2 _B 1 , H 2 _B 3 and H 2 _B 4 (Figure 3).…”

“…Brønsted and Lewis acids can act as an electrophile inducer [26,27]. For closo-decaborate anions, the most common inducers are CF 3 COOH and CF 3 SO 3 H. The mechanism of the EINS process has been described in detail in several articles [28][29][30]. This process involves several steps.…”

Recent Achievements on Functionalization within closo‐Decahydrodecaborate [B10H10]2− Clusters

“…Halogenation of the closo‐decaborate anion has been extensively studied due to the importance and versatility of halo‐decaborate derivatives though most of which found it exceedingly difficult to get a selective reaction that leads only to a mono‐substituted derivative [21b,46] . In fact, direct halogenation of closo‐decaborate with dihalogens (chlorine, bromine, iodine) and recently with HCl yields a mixture of derivatives with uncontrolled degrees of substitution [B 10 H 10‐y X y ] 2− [21c,47] . The derivatives were purified and isolated via either chromatography or electrophoresis.…”

Synthesis of Disubstituted Carboxonium Derivatives of Closo-Decaborate Anion [2,6-B10H8O2CC6H5]−: Theoretical and Experimental Study

“…However, despite the widespread occurrence of EINS process in laboratory practice, its mechanism has been studied in only a few papers. The mechanism of chlorination of closo -borates with the HCl molecule has been investigated in several publications [ 54 , 55 ]. This paper reports on comprehensive investigations of the key stages of the EINS process.…”