Hydrolysis mechanism of the cyclohexaborate anion: Unraveling the intricacies

Abstract

$B_6{O}_7{\left(\mathrm{OH}\right)}_6^{2-}$ is a highly polymerized borate anion of three six-membered rings. Limited research on the $B_6{O}_7{\left(\mathrm{OH}\right)}_6^{2-}$ hydrolysis mechanism under neutral solution conditions exists. Calculations based on density functional theory show that $B_6{O}_7{\left(\mathrm{OH}\right)}_6^{2-}$ undergoes five steps of hydrolysis to form H₃BO₃ and $B{\left(\mathrm{OH}\right)}_4^{-}$ . At the same time, there are a small number of borate ions with different degrees of polymerization during the hydrolysis process, such as triborate, tetraborate, and pentaborate anions. The structure of the borate anion and the coordination environment of the bridging oxygen atoms control the hydrolysis process. Finally, this work explains that in existing experimental studies, the reason for the low $B_6{O}_7{\left(\mathrm{OH}\right)}_6^{2-}$ content in solution environments with low total boron concentrations is that it depolymerizes into other types of borate ions and clarifies the borate species. The conversion relationship provides a basis for identifying the possibility of various borate ions existing in the solution. This work also provides a certain degree of theoretical support for the cause of the "dilution to salt" phenomenon.

Keywords: DFT; borate anion species distribution; hexaborate anion; hydrolysis mechanism.