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)

Abstract

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 (H(2)bp12c4) has been investigated by pH-potentiometric titrations at 25 degrees C in 0.1 M KNO(3). The stability constants vary in the following order: Cd(II) > Zn(II) approximately Pb(II) > Ca(II). As a consequence, H(2)bp12c4 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 Delta(lambdalambdalambdalambda) conformation in the solid state, which results in a square antiprismatic coordination, while the Delta(deltadeltadeltadelta) 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.

Macrocyclic Receptor Showing Improved PbII/ZnII and PbII/CaII Selectivities

“…The protonation constants of bp15c5 as well as the stability constants of its metal complexes formed with Ca II , Zn II , Cd II , and Pb II were determined by potentiometric titration in 0.1  KNO 3 ; the constants and standard deviations are given in Table 1, which also lists the protonation and stability constants reported for the related systems bp12c4, [15] ba15c5 [17] , and ba12c4 [18] (Scheme 1). The experimental titration curves are shown in Figure 1.…”

“…The metal-donor distances are substantially shorter than those observed for the corresponding complex of bp12c4, in which the metal ion is eight-coordinate. [15] The distances between the Cd II ion and the donor atoms of the pendant arms are intermediate between those observed in the bis-and tris(dipicolinate) complexes. [25] The distance between the Cd II ion and the oxygen atom of the crown moiety O(5) is ca.…”

“…On the grounds of our previous experience, [15,22,29] in these calculations the 6-31G(d) basis set was used for the ligand atoms, while for the metals the effective core potential of Wadt and Hay (Los Alamos ECP) included in the LanL2DZ basis set was applied.…”

“…Both our DFT calculations and the X-ray crystal structure show that for this complex two of the donor atoms of the ligand are not coordinated to the metal ion, and therefore a lower stability is expected for the bp15c5 complex when compared to the bp12c4 analogue. [15] However, in the [Cd(bp15c5)] complex the metal ion is endocyclicly coordinated by the ligand, while for the Zn II analogue the metal ion is placed outside the macrocyclic cavity. Indeed, according to our DFT calculations the Zn II ion is only bound to one of the oxygen atoms of the crown moiety [O (5)], while a weak interaction is observed between Zn II and one of the pivotal nitrogen atoms [N(3), see Table 5].…”

“…In previous papers we have reported on the complexation properties of the macrocyclic ligand containing picolinate pendants bp12c4 (Scheme 1) toward divalent metal ions such as Ca II , Zn II , Cd II , and Pb II , [15] and trivalent lanthanide ions. [16] We have shown that bp12c4 forms stable complexes with Zn II , Cd II , and Pb II in aqueous solution, but it does not show important selectivity for any of these three metal ions.…”