Complex formation between boric acid and triethanolamine in aqueous solutions

The complex formation behavior between boric acid and triethanolamine (TEA, H3L) in aqueous solutions was thoroughly examined by B-11 NMR spectroscopy. In chloroform, one B-11 NMR signal appeared due to triethanolamine borate (TEA-B, L-B) at -4.6 ppm, whereas in aqueous solutions two signals appeared due to TEA-B and a new boron complex at -5.8 and -9.5 ppm, respectively, in addition to a signal ascribed to boric acid and borate ion. The area ratios between the signals at -5.8 and -9.5 ppm were almost constant regardless of the molar ratios between boric acid and TEA, suggesting that the new complex is a 1 : 1 complex between boric acid and TEA. Both complexes were stable in aqueous solutions over the pH range of 6.7 to 10.9. Their chemical shift values were constant and independent of the pH value. This also implies that the new complex has a tetrahedral structure around the boron atom, similar to TEA-B. Based on H-1 and C-13 NMR spectroscopy and a preliminary examination of complexation between boric acid and aminoalkanols, we have concluded that the new complex (NB, HL-B(OH)) is a 1 : 1 boron to TEA complex with a bicyclo[3,3,0]structure having a boron-nitrogen bond.