Synthesis and Reactivity of (Benzoxazol-2-ylmethyl)phosphonic Acid

An efficient three step synthesis of (benzoxazol-2-ylmethyl)phosphonic acid (6-H2) is described along with IR, mass spectrometry (MS), and 1H, 13C, and 31P NMR spectroscopic characterization data, and a single crystal X-ray diffraction structure determination. 6-H2 is unstable in acidic aqueous solutions (pH < 4) undergoing ring-opening to give [(2-hydroxyphenylcarbamoyl)methyl] phosphonic acid (7-H2) that is characterized by IR, MS, and NMR methods. The protonation constants (pKa) for 7-H2 have been measured, and crystal structure determinations for (NH4)(7-H) and K(7-H)·DMF are described. Reactions of NaOH and KOH with 6-H2 in MeOH/H2O solutions led to isolation and crystal structure determinations of the salts [Na(6-H)·H2O]2, K(6-H), Na3(6)(6-H)·H2O, and [K2(6)]2·3H2O. The complexation reactions of 7-H2 with La(III), Nd(III), and Gd(III), as a function of pH, were also examined by titrametric methods, and a model for the 1:1 anion binding with Ln(III) cations is proposed.