Different Complexation Properties of Some Hydroxy Keto Heterocycles toward Beryllium(II) in Aqueous Solutions: Experimental and Theoretical Studies

Four heterocycles containing hydroxy and keto functionalities have been tested as chelating agents of beryllium(II).
These are in the order (i) 3-hydroxy-2-methyl-4H-pyran-4-one (maltol, Hma), (ii) 5-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one (kojic acid, Hka), (iii) 3-hydroxy-1,2-dimethyl-4-pyridinone (Hdpp), (iv) 1-(3-hydroxy-2-furanyl)ethanone
(isomaltol, Hima). Although the skeletons of the first three species, with one nitrogen or oxygen heteroatom at the
six-membered ring, are almost superimposable, straightforward synthesis and crystallization is achieved only for
the 1:2 adduct Be(dpp)2, 1. Also the complex Be(ima)2, 2, precipitates in high yield but the ima- ligand has a
different skeletal structure. X-ray determinations of 1 and 2 showed that the Be2+ ion is pseudotetrahedrally coordinated
by two chelating ligands with slightly asymmetric Be−Oalkoxo and Be−Oketo bonds. The complex Be(ma)2 precipitates
in low yields together with large amounts of unreacted Hma while, under the same conditions, no trace of the
analogous species Be(ka)2 has been observed. This paper presents the results of potentiometric and NMR studies
in the aqueous solutions as well as of DFT structural optimizations for all of the free acids, their associated bases,
and the adducts of the type [BeL(H2O)2]+ and BeL2 in the gas phase. It is consistently found that the basicity of
the ligands and the stability of their complexes decrease in the order dpp- > ma- > ka- > ima-. In solution, all
of the anionic ligands form adducts of the type [BeL(H2O)2]+ at low pH values, whereas higher concentrations of
the free anion are required to form 1:2 adducts. The pH, the basicity, and the stability constants of the complexes
as well as the formation of competing beryllium hydroxide species are strictly correlated factors for the obtainment
of the latter type of adduct. The DFT calculations account nicely for the different donor powers of the various
chelates in terms of electronic redistribution and associated energetics.