High Field11B and13C Nmr Investigations of Aqueous Borate Solutions and Borate-Diol Complexes

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Abstract
11B nuclear magnetic resonance spectra of concentrated aqueous solutions of boric acid and the tetrahydroxyborate B[OH]- 4 ion, or of borax, Na2B4O7, at 128.4 MHz, reveal substantial amounts of triborate species in the pH range 8–10.5 and enable equilibria such as B3O3(OH)4 - + OH- α B3O3(OH)5 -2 and the hydrolytic dissociation of these ions to be studied. Addition of the polyhydroxy-compounds ethane-1,2-diol, propane-1,2-diol, propane- 1,3-diol and glycerol results in the formation of a variety of mono- and bis-chelated complexes, among which are spirocyclic anions with five- and six-membered rings sharing a common boron atom. Complexes in which boron atoms of triborate anions are coordinated by diol ligands are also identified. 11B chemical shifts characteristic of chelated and hydroxylic boron centres in mono- and triborate species are reported, taking into account the trends accompanying a change from trigonal to tetrahedral coordination. 1H and 13C NMR spectra of borale solutions containing the complexes LB(OH)2 - and L2B-, where L represents propane-1,2-diol as a chelating ligand, reveal all the possible stereoisomers of this system