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.

Compounds of formula [Pb(L2)(NCS)2] (1) and [Pb(L4)(SCN)2] (2) (where L2 is the lariat crown ether N,N'-bis(3-aminobenzyl)-4,13-diaza-18-crown-6 and L4 is the Schiff-base lariat crown ether N,N'-bis(3-(salicylaldimino)benzyl)-4,13-diaza-18-crown-6) were isolated and structurally characterized by X-ray diffraction analyses. The X-ray crystal structures of both compounds show the metal ion coordinated to the six donor atoms of the crown moiety, leaving the corresponding pendant arms uncoordinated. The coordination sphere of lead(II) is completed by two thiocyanate groups that coordinate either through their nitrogen (1) or sulfur (2) atoms. The organic receptor adopts a syn conformation in 1, while in 2 it shows an anti conformation. To rationalize these unexpected different conformations of the L2 and L4 receptors in compounds 1 and 2, as well as the different binding modes found for the thiocyanate ligands, we have carried out theoretical calculations at the DFT (B3LYP) level. These calculations predict the syn conformation being the most stable in both 1 and 2 complexes. So, the anti conformation found for 2 in the solid state is tentatively attributed to the presence of intermolecular pi-pi interactions between phenol rings, for which the dihedral angle between the least-squares planes of both rings amounts to 2.6 degrees and the distance between the center of both rings is 3.766 A. On the other hand, the analysis of the electronic structure has revealed that the Pb-ligand bonds present highly ionic character in this family of compounds. They also suggest a greater transfer of electron density from the NCS- ligands when they coordinate through the less electronegative S atom. The Pb-SCN covalent bond formation mainly occurs due to an effective overlap of the occupied 3p z orbitals of the S atoms and the unoccupied 6p z AO of the Pb atom, while the Pb-NCS bonding interaction is primarily due to the overlap of the 6s and 7s AO of Pb with sp(1.10) hybrids of the N donor atoms. Our electronic structure calculations can rationalize the different coordination of the thiocyanate groups in compounds 1 and 2: the simultaneous formation of two Pb-SCN bonds is more favorable for S-Pb-S angles close to 180 degrees , for which the overlap between the occupied 3p z orbitals of the S atoms and the unoccupied 6 pz AO of the Pb atom is maximized.

The coordinative properties towards lead(II) of two lateral macrobicyclic receptors that incorporate either a 1,10-diaza-crown-6 (L 8 ) fragment are reported. Spectrophotometric titrations performed in acetonitrile solution indicate only the formation of mononuclear complexes in solution. The X-ray crystal structures of the two receptors show that the conformation adopted by the ligand is imposed by the presence of intramolecular hydrogenbonding interactions that involve the secondary amine groups and the pivotal nitrogen atoms. The solid-state structure of [Pb(L 7 )(NCS)](SCN)·0.5H 2 O shows that the metal ion is asymmetrically coordinated inside the macrobicyclic cavity. The Pb II ion is coordinated to the nitrogen atom of the pyridine unit, the two secondary amine atoms, two oxygen