Steric Control of Selectivity for Lanthanoids in Liquid–Liquid Extraction with Tris- and Tetrakis(pyrazol-1-yl)borate–β-Diketon Mixed-Ligand Systems

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Abstract
Mixed-ligand chelate extraction of trivalent lanthanoid ions (M3+) into benzene with poly(pyrazol-1-yl)borate (HnB(pz)4−n; n = 0, 1) and β-diketone was studied. The β-diketones used were dibenzoylmethane (dbm) and dipivaloylmethane (dpm). Lanthanoid ions were extracted as [M{HnB(pz)4−n}2(β-diketone)]. The logarithmic extraction constants (log Kex) of the [HB(pz)3]− systems were 5—6 higher than those of the [B(pz)4]− systems. The highest separability for lanthanide ions was achieved with the [B(pz)4]−– dbm system. These features of log Kex were principally governed by intra- and interligand steric contact.