STRUCTURE ET ACIDITE DE COMPOSES A ATOME DE BORE ET DE PHOSPHORE HYPERCOORDONNES

Article abstract of DOI:10.1016/S0022-328X(00)98891-0

Boric acid reacts with α-hydroxy acids, oxalic acid, 3-methylcatechol, phenylhydroxamic acid, citric and tartaric acids, in DMF solution leading to spiro compounds.Tartaric acid borates are dimeric derivatives similar to antimony emetics.Spiran structure has been established by NMR spectroscopy (11B, 13C, 1H).With acetonic acid borates, we observed a borane <*> spiro-borate equilibrium in poorly basic solvents (THF, CH3CN).The pK_as of these compounds, determined by potentiometric titration with triethylamine, correspond to strong acids (1.5 < pK_a < 2, in DMF, -2.5 < pK_a < -0.3 in DMSO).A pentaoxyspirophosphorane containing two α-hydroxyacid ligands and showing a tautomeric equilibrium with a six coordinated phosphorus compound, similar to boron spirans, has been studied by use of the same methods.Its pK_a again correspond to that of a strong acid (pK_a = 1.8 in DMF, -0.3 in DMSO).In both, boron spiran and pentaoxyphosphorane systems, an important decrease in the acidity is observed when the α-hydroxy acid ligand is replaced by the ethanediol moiety.