A 11B NMR study of zwitterionic and cationic monoborate complexes with cationic 1,2-diol ligands

The interaction of B(OH)3 with [R3N{CH2CH(OH)CH2OH}] [OH] (R3 = Me3, Me2Et, MeEt2, Me2Bz, Me2{CH2CH2OH}) in a 1:2, 1:1, and 5:1 ratio in aqueous solution has been studied by 11B NMR spectroscopy and the equilibrium species present have been identified. The cationic bischelate [R3N{CH2CH(O)CH2O}2B]+ dominates the 2:1 ratio solutions, zwitterionic monochelate species, [R3N{CH2CH(O)CH2O}B(OH )2], dominate the 1:1 solutions and the pentaborate anion, [B5O6(OH)4]−, dominates the 1:5 solutions. Borate solutions derived from [NMe4][OH] and [Me3NCH2CH2OH][OH] have also been studied by 11B NMR. Spectra of these borate solutions are essentially the same as those observed for borate solutions with alkali metal cations, and structure directing effects (towards the anion) are not observed in borate systems.

Graphical abstractCationic 1,2-diol moieties, derived from substituted tetraorganoammonium cations [R3N{CH2CH(OH)CH2OH}]+ (with [OH]− or [Cl]− counterions), interact with B(OH)3 to yield equilibrium solutions containing (i) zwitterionic monochelate complexes and (ii) cationic bischelate complexes.Figure optionsDownload full-size imageDownload as PowerPoint slide