Eight-Coordinate Zn(II), Cd(II), and Pb(II) Complexes Based on a 1,7-Diaza-12-crown-4 Platform Endowed with a Remarkable Selectivity over Ca(II)

The thermodynamic stability of the Pb(II), Cd(II), Zn(II), and Ca(II) complexes with the dianionic macrocyclic ligand N,N′-bis[(6-carboxy-2-pyridyl)methyl]-1,7-diaza-12-crown-4 (H2bp12c4) has been investigated by pH-potentiometric titrations at 25 °C in 0.1 M KNO3. The stability constants vary in the following order: Cd(II) > Zn(II) ∼ Pb(II) > Ca(II). As a consequence, H2bp12c4 present an important Cd(II)/Ca(II) selectivity, as well as a certain selectivity for Cd(II) over Zn(II). To rationalize these results, a detailed investigation of the structure of these complexes has been carried out both in solid state and in aqueous solution. Furthermore, the [M(bp12c4)] complexes (M = Ca, Zn, Cd, or Pb) were characterized by means of density functional theory (DFT) calculations (B3LYP model) to obtain information on their solution structures and to investigate the possible stereochemical activity of the Pb(II) lone pair. Our results show that in all cases the metal ion is octacoordinated by the ligand, a situation that is particularly rare for Zn(II) complexes. The coordination polyhedra observed in the solid state for the [M(bp12c4)] complexes (M = Zn, Cd, or Ca) is closely related to the conformation adopted by the ligand in the corresponding complex: The Zn(II) complex adopts a Δ(λλλλ) conformation in the solid state, which results in a square antiprismatic coordination, while the Δ(δδδδ) conformation observed for the Cd(II) and Ca(II) analogues yields inverted-square antiprismatic geometries. The X-ray crystal structure of the Pb(II) analogue shows that the metal ion is directly bound to the eight donor atoms of the ligand, but the bond distances of the metal coordination environment indicate an asymmetrical coordination of the cation by the ligand, which is attributed to the stereochemical activity of the Pb(II) lone pair. In aqueous solution the Ca(II), Zn(II), and Cd(II) complexes show rigid C 2 symmetries, while the Pb(II) analogue presents a more flexible structure.