The quest for beryllium peroxides.

ABSTRACT

There is no experimental proof documented in the literature for the existence of any beryllium peroxide compound. All recent pertinent preparative attempts described in this work, using a range of beryllium salts with various peroxides as reagents under mild conditions, were equally unsuccessful. (1)H and (9)Be NMR investigations of aqueous solutions containing beryllium salts and hydrogen peroxide in a broad pH range also gave no definite evidence for the presence of peroxoberyllates as components of the manifold equilibria in such solutions. Quantum chemical calculations have therefore been carried out to delineate the energetics and structures of various beryllium peroxide model compounds. Standard Hartree-Fock and density functional methods were employed at various levels of sophistication. The series of prototypes considered consists of [BeOH](+), Be(OH)(2), Be(OH)(OOH), Be(OOH)(2), [Be(O(2))(2)](2-), [BeO(2)(OH(2))(2)], and [Be(2)(O(2))(2)(OH(2))(4)] (all in the gas phase). Surprisingly, the triatomic cation [BeOH](+) has been found to have a linear structure. All the Be-O(peroxide) bonds are found to be rather long, suggesting weaker bonding compared to the Be-O bonds in aquo, hydroxo, or oxo complexes. Hydrogen peroxide or anions derived therefrom are therefore not able to compete successfully with water (hydroxide anions) in aqueous solution. In the mononuclear beryllium peroxide molecules, the peroxide groups form chelating units at tetrahedrally 4-coordinate metal atoms. The binuclear compound [Be(2)(O(2))(2)(OH(2))(4)] has a puckered six-membered-ring structure, close to the standard chair conformation. A significant lengthening of the O-O bonds upon coordination to the Be(2+) centers has been calculated, but it is unlikely that the polarization of the peroxide group by the high positive charge density at Be(2+) is significant to cause an intrinsic instability of beryllium peroxides. All structures represent distinct local minima on the potential energy surface and are predicted to be (meta)stable species in nonaqueous media. The field of aluminum peroxides is a similar gray area on the map of metal and metalloid peroxides and is reminiscent of the well-established "diagonal-relation" of Be and Al in the periodic table of the elements.