Mechanism for hydrolysis of double six‐membered ring tetraborate anion

Abstract

[B4O5(OH)42−] is a representative borate anion with a double six‐membered ring structure, but there is limited knowledge about the hydrolysis mechanisms of [B4O5(OH)42−]. Density functional theory‐based calculations show that the tetraborate ion undergoes three‐step hydrolysis to form [B(OH)4−] and an ring intermediate, [B3O2(OH)6−]. Other new structures, such as linear trimer, branched tetraborate, analogous linear tetraborate, are observed, but they are not stable in neutral systems and change to ring structures. [B3O2(OH)6−] hydrolyzes to [B(OH)4−] and [B(OH)3] in the last two steps. The structure of borate anion and the coordination environment of the bridge oxygen atom control the hydrolysis process. [B4O5(OH)42−] always participates in the hydrolysis reaction, even with a decrease in concentration. [B3O3(OH)4−], [B(OH)4−], and [B(OH)3] have different roles in “water‐poor” and “water‐rich” zones. Concentration and pH of solution are the key factors that affect the distribution of borate ions.

Hydrolysis mechanism of the cyclohexaborate anion: Unraveling the intricacies

“…Cheng et al pointed out that when the hydrolysis reaction of Al(OR) 3 occurs, water molecules directly combine with the OR group to form an intermediate in hydrogen bonds 48 . Zhou et al reported that when the hydrolysis reaction occurs between the double six‐membered ring pentaborate ion and the double six‐membered ring tetraborate ion , water molecules also directly react with borate ions first 34,35 . The anions form hydrogen bonds, and then the water molecules break the bonds again, and the dissociated hydroxyl groups and hydrogen form bonds with the corresponding boron and oxygen, respectively.…”

“…Therefore, the intermediate IM1 must absorb energy to reach the transition state. Studies have shown that hydrogen bonds make an important contribution to the change in total energy when the structure of polyborate ions changes during hydrolysis 34,35 . It can be seen that hydrogen bonds play an important role in the energy changes of many reactions.…”

“…48 , water molecules also directly react with borate ions first. 34,35 The anions form hydrogen bonds, and then the water molecules break the bonds again, and the dissociated hydroxyl groups and hydrogen form bonds with the corresponding boron and oxygen, respectively. So far, there has been no report on the phenomenon that water molecules first break bonds during the initial stage of the hydrolysis reaction of polyborate ions, and the released hydrogen atoms form new water molecule with adjacent hydroxyl groups.…”

“…Researchers have used this method to conduct extensive research on the mechanism of silicate polymerization 30–33 . Studies on the hydrolysis mechanism of borate ions in borate solutions have reported pentaborate ions and tetraborate ions 34,35 . However, there are few reports on the hydrolysis mechanism of the complex hexaborate ion.…”

“…[30][31][32][33] Studies on the hydrolysis mechanism of borate ions in borate solutions have reported pentaborate ions and tetraborate ions. 34,35 However, there are few reports on the hydrolysis mechanism of the complex hexaborate ion. At the same time, hexaborate ion is an important key ion in the "dilution to salt" phenomenon, which is crucial to the explanation of this phenomenon.…”