Lone-Pair Activity in Lead(II) Complexes with Unsymmetrical Lariat Ethers

Abstract

We have carried out a study about the structural effect of the lone-pair activity in lead(II) complexes with the unsymmetrical lariat ethers L(7), L(8), (L(8)-H)-, (L(9)-H)-, and (L(10)-H)-. All these ligands are octadentate and differ by the aromatic unit present in their backbones: pyridine, phenol, phenolate, thiophenolate, and pyrrolate, respectively. In these lead(II) complexes, the receptor may adopt two possible syn conformations, depending on the disposition of the pendant arms over the crown moiety fragment. The conformation where the pendant arm holding the imine group is placed above the macrocyclic chain containing two ether oxygen atoms has been denoted as I, whereas the term II refers to the conformation in which such pendant arm is placed above the macrocyclic chain containing the single oxygen atom. Compounds of formula [Pb(L(7))](ClO4)2 (1) and [Pb(L(8)-H)](ClO4) (2) were isolated and structurally characterized by X-ray diffraction analyses. The crystal structure of 1 adopts conformation I and shows the lead(II) ion bound to the eight available donor atoms of the bibracchial lariat ether in a holodirected geometry, whereas the geometry of 2 is best described as hemidirected, with the receptor adopting conformation II. The five systems [Pb(L(7))]2+, [Pb(L(8))]2+, [Pb(L(8)-H)]+, [Pb(L(9)-H)]+, and [Pb(L(10)-H)]+ were characterized by means of density functional theory calculations (DFT) performed by using the B3LYP model. An analysis of the natural bond orbitals (NBOs) indicates that the Pb(II) lone-pair orbital remains almost entirely s in character in the [Pb(L(7))]2+ complexes, whereas in [Pb(L(8)-H)]+, the Pb(II) lone pair is polarized by a certain 6p contribution. The reasons for the different roles of the Pb(II) lone pair in compounds 1 and 2 as well as in the related model compounds are discussed. Our results point to the presence of a charged donor atom in the ligand (such as a phenolate oxygen atom, pyrrolate nitrogen atom, or even thiophenolate sulfur atom) favoring hemidirected geometries.

Effect of Protonation and Interaction with Anions on a Lead(II) Complex with a Lateral Macrobicycle Containing a Phenol Schiff‐Base Spacer

“…On the basis of our previous experience for these calculations we used the standard 6-31G(d) basis set for the ligand atoms, while the LanL2DZ valence and effective core-potential functions were used for Pb. [26] The calculated structure for this complex (Figure 4) resembles the X-ray crystal structure of 1, with the calculated bond lengths of the metal coordination environment in reasonable agreement with those obtained experimentally ( Table 2). The experimental and calculated bond lengths differ by less than 0.08 Å, except the Pb-O(3) distance, which differs by ca.…”

“…In the hemidirected form there is a void in the liganding attributable to the stereochemical activity of the Pb II lone pair that is not found in holodirected geometry. [ 26] www.eurjic.org…”

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

“…This is typical of the so-called hemidirected compounds, in which the lone pair of electrons causes a nonspherical charge distribution around the Pb II cation. [33,34] Indeed, an analysis of the natural bond orbitals (NBOs) shows that the Pb II lone pair possesses a predominant 6s character, but it is polarized by a substantial 6p contribution: [33] A hemidirected geometry has also been observed in the solid state for the [Pb(bp12c4)] analogue. [15] The stability of the [Pb(bp15c5)] complex is somewhat higher than that of the bp12c4 analogue (see Tables 1 and 2), while our DFT calculations provide a minimum energy conformation for the [Pb(bp15c5)] complex that shows a poor complementarity between the donor atoms offered by the ligand and the metal ion.…”

Molecular simulations of metal adsorption to bacterial surfaces

“…Also, Cd tends to form a relatively stable configuration at circumneutral pH, while the Pb behavior is more complex due to its 6s 2 outer shell electronic configuration. The lone pair electrons are often stereochemically active and induce a strong deformation of divalent lead polyhedra (Galy et al, 1975;Esteban-Gó mez et al, 2006). This allowed us to test the applicability of molecular simulations to describe increasingly complex metal-ligand interactions.…”