Computational Studies of Substituted Phenylboronic Acids in Common Electrolyte Solvents

Boronic acids and boronates are known as redox shuttles and film-forming additives. For example, 3,5-bis(trifluoromethyl)phenylboronic acid is reduced at a higher potential than that of PC-solvated Li+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Li}^{+}$$\end{document} ion because of its lower LUMO energy level. Theoretical (molecular modelling) studies of the HOMO and LUMO energies of several phenylboronic acids and -boronates showed that the LUMO energies of all boronates were significantly lower than the LUMO energies of the commonly used carbonate electrolytes, both in vacuo and in solution, making them good candidates as electrolyte additives. The preferred conformation in vacuo and in solution of the boronate ester groups was ‘in–out’, with dihedral angles between the aromatic ring and boronate group varying between 29.5∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$29.5{^{\circ }}$$\end{document} and 33.6∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$33.6{^{\circ }}$$\end{document}. In contrast, the preferred conformations of the phenylboronic acids were found to be always coplanar and ‘out–out’, with dihedral angles close to 0∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0{^{\circ }}$$\end{document}. We speculated that back-bonding and intermolecular hydrogen bonding played a role. In this study, therefore, we investigated the role of intermolecular hydrogen bonding and solvation in this phenomenon, using HF and DFT methods.