Theoretical Insight into B–C Chemical Bonding in <i>Closo</i>-Borate [B_nH_n−1CH₃]²⁻ (n = 6, 10, 12) and Monocarborane [CB_nH_nCH₃]⁻ (n = 5, 9, 11) Anions

A theoretical investigation of mono-methyl derivatives of closo-borate anions of the general form [BnHnCH3]2– (n = 6, 10, 12) and monocarboranes [HCBnHnCH3]− (n = 5, 9, 11) was carried out. An analysis of the main bonding descriptors of exo-polyhedral B–C bonds was performed using the QTAIM (quantum theory of “Atoms in Molecules”), ELF (electron localisation function), NBOs (natural bond orbitals) analyses and several other approaches for the estimation of B–C bond orders (viz. Laplacian bond order (LBO), fuzzy bond order (FBO) and Mayer and Wiberg formalisms). Based on the data obtained on electron density descriptors, it can be concluded that orbital interaction increases with increasing boron cluster size. The present investigation provides a better understanding of exo-polyhedral B–C bond phenomena in boron cluster systems. The data obtained can be used to estimate B–C bond strength, which can be useful for studies devoted to the synthesis and properties of boron cluster systems

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

A comprehensive study focused on the preparation of disubstituted carboxonium derivatives of closo-decaborate anion [2,6-B10H8O2CC6H5]− was carried out. The proposed synthesis of the target product was based on the interaction between the anion [B10H11]− and benzoic acid C6H5COOH. It was shown that the formation of this product proceeds stepwise through the formation of a mono-substituted product [B10H9OC(OH)C6H5]−. In addition, an alternative one-step approach for obtaining the target derivative is postulated. The structure of tetrabutylammonium salts of carboxonium derivative ((C4H9)4N)[2,6-B10H8O2CC6H5] was established with the help of X-ray structure analysis. The reaction pathway for the formation of [2,6-B10H8O2CC6H5]− was investigated with the help of density functional theory (DFT) calculations. This process has an electrophile induced nucleophilic substitution (EINS) mechanism, and intermediate anionic species play a key role. Such intermediates have a structure in which one boron atom coordinates two hydrogen atoms. The regioselectivity for the process of formation for the 2,6-isomer was also proved by theoretical calculations. Generally, in the experimental part, the simple and available approach for producing disubstituted carboxonium derivative was introduced, and the mechanism of this process was investigated with the help of theoretical calculations. The proposed approach can be applicable for the preparation of a wide range of disubstituted derivatives of closo-borate anions

Protonation of Borylated Carboxonium Derivative [2,6-B10H8O2CCH3]&minus;: Theoretical and Experimental Investigation

The process of protonation of [2,6-B10H8O2CCH3]&minus; was investigated both theoretically and experimentally. The most suitable conditions for protonation of the derivative [2,6-B10H8O2CCH3]&minus; were found. The process of protonation was carried out in the presence of an excess of trifluoromethanesulfonic acid CF3SO3H at room temperature in dichloromethane solution. The structure of the resulting complex [2,6-B10H8O2CCH3*Hfac]0 was established using NMR data and the results of DFT calculations. An additional proton atom Hfac was found to be localized on one of the facets that was opposite the boron atom in a substituted position, and which bonded mainly with one apical boron atom. The main descriptors of the B-Hfac bond were established theoretically using QTAIM and NBO approaches. In addition, the mechanism of [2,6-B10H8O2CCH3]&minus; protonation was investigated