NEW MODE OF ION SIZE DISCRIMINATION FOR GROUP-2 METALS USING POLY(PYRAZOLYL)BORATE LIGANDS - CONTROL OF STABILITY AND STRUCTURE OF CHELATE COMPLEXES BY INTRALIGAND CONTACT

Selectivity of [H(n)B(pz)4-n]- (n = 0-2; pz = pyrazolyl ring) for group 2 metal ions has been studied by liquid-liquid extraction. The dominant extracted species is [H(n)B(pz)4-n]2M. Bidentate[H2B(pz)2]- is a weak extracting reagent for Be2+ and Mg2+. Hydrotris(pyrazolyl)borate extracts Be2+, Mg2+, and Ca2+ effectively, and Sr2+ to some extent. Although[B(pz)4]-, a potentially tridentate ligand similar to [HB(pz)3]-, extracts Be2+ and Mg2+, the extractability for Ca2+ drops drastically. The different selectivity of these ligands cannot be explained on the basis of basicity of the donor atoms. To explore the origin of the selectivity, X-ray crystal structures of the extracted species have been determined. Complexes of [HB(pz)3]m (1), [HB(pz)3]2Ca (2), and [B(pz)4]2Mg (3) have distorted octahedral geometries with each ligand having tridentate coordination. In complex [B(pz)4]2Be (4), the ligands are bidentate and the geometry about the beryllium atom is distorted tetrahedral. The extracted species of Be2+ with[HB(pz)3]-changes from [HB(pz)3]2Be to [(HB(pz)3)BeOH]3 (5) as the initial concentration of Be2+ increases from 10(-4) to 10(-2) M. Complex 5 consists of bidentate [HB(pz)3]- and a cyclic [BeOH]3(3+) moiety which is the dominant species of beryllium in aqueous solution (pH5). The H-1 NMR spectra show that solution structures of these complexes in CDCl3 are similar to the structures in solid state. Examination of the dimensions of the complexes has revealed that the stability and structure of poly(pyrazolyl)borate complexes of group 2 metals are controlled by intraligand contact between pyrazolyl rings bonded to the central boron atom. This is a novel mode of ion size discrimination by chelating ligands.