Colorimetric and fluorometric determination of boron using β-diketone–BF2 compounds containing catechol moiety

Abstract

In this study, we attempted to develop a new boron quantification reagent using β-diketones, because β-diketones often exhibit strong absorption and luminescence and react with boric acid. The luminescence intensity remarkably varied with the addition of boric acid to a solution of β-diketone containing 4-methoxyphenyl groups. However, luminescence intensity gradually decreased with time because of the tautomerization of the enol form of β-diketone to its keto form. Therefore, in this study, we synthesized novel β-diketone–BF2 compounds in which the β-diketone site was cross-linked with BF2 to avoid changes in the luminescence intensity owing to the keto-enol tautomerization. We introduced a catechol moiety as a reaction site with boron and examined their effectiveness in quantifying boron using absorption and emission spectroscopy. Furthermore, the synthesized β-diketone–BF2 compounds (1–9) that contained 4-dimethylaminophenyl (4-NMe2Ph) or 4-methoxyphenyl (4-MeOPh) group and catechol or salicylic acid moiety as the reaction site, were characterized using various analytical methods. No correlation was observed between the pKa values of β-diketone–BF2 compounds and the formation constants for the reactions with boric acid (log K1). Additionally, quantification limit estimation carried out using absorption and emission spectroscopy suggested that β-diketone–BF2 compounds 3 and 7, which contain 4-NMe2Ph group and catechol moiety, can be used to determine boric acid concentrations > ca. 2 × 10–4 M. This study demonstrated the usefulness of the β-diketone-BF2 compound as a colorimetric and fluorescence quantifying reagent for boric acid.