Improved Selectivity for Cu(II) of Methyl-Substituted Poly(pyrazolyl)borates, [HB(3-Mepz)3]- and [B(3-Mepz)4]-, through Steric Contact

The separation of first-series transition metal and Cd^2+ ions has been studied by solvent extraction using methyl substituted poly(pyrazolyl)borates. These ligands did not extract all of the studied metal ions at a lower pH compared to the parent ligands, and the selectivity was high for Cu(II) and low for Fe(II), although the extracted species were A2M complexes as well (A-: poly(pyrazolyl)borate, M^2+: metal ion) for all the four ligands. An examination of the structures of the nine complexes determined the present and related ones, which were previously determined elsewhere, revealed that 3-methyl groups of the pyrazolyl rings are arrayed around a metal ion, and that the distances between the methyl groups are smaller than the sum of van der Waals radii of the methyl groups. This suggests that the interligand contact of 3-methyl groups decreases the stability of the complexes. Since the geometry in the copper complexes with these ligands is square bipyramid, the increase in steric energy caused by the interligand contact seems to be smaller than that for those complexes having similar ionic radii. Moreover, the spin state of the iron complexes with these ligands is high-spin due to interligand contact. Thus, while the interligand contact causes a general decrease in the stability of all the complexes, the effect is smallest for Cu(II) and largest for Fe(II), resulting in a unique selectivity.