Kinetic study of the complex formation of boric and boronic acids with mono- and diprotonated ligands

The complex formation reactions of boric and boronic acids (RB(OH)(2): R=OH, n-Bu, Ph, and m-NO2Ph) with 4-isopropyltropolone (Hipt) and chromotropic acid (H(2)cht(2-)) have been studied kinetically at various pH. The reactions of H(2)ipt(+) with boronic acids were faster than those of Hipt by a factor of 1.5-11, and fully deprotonated ipt(-) also reacted with m-NO2PhB(OH)(2), but slower than Hipt. The tetrahedral m-NO2PhB(OH)(3)(-) ion did not react with Hipt. Reaction routes for the complexation of boric acid with chromotropic acid could not be specified because of the unexpected problem of proton ambiguity. Mechanism for the reactions of boronic acids with bidentate ligands was discussed in terms of rate determining chelate ring closure. It was concluded that at least one proton is necessary for the OH- in the tetrahedral intermediates to be eliminated smoothly as water, and a doubly hydrogen-bonded intermediate was proposed for the reactions with diprotic ligands. (C) 2002 Elsevier Science B.V. All rights reserved.