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

We have carried out a study about the structural effect of the lone-pair activity in lead(II) complexes with the
unsymmetrical lariat ethers L7, L8, (L8−H)-, (L9−H)-, and (L10−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(L7)](ClO4)2 (1) and [Pb(L8−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(L7)]2+, [Pb(L8)]2+, [Pb(L8−H)]+, [Pb(L9−H)]+, and [Pb(L10−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(L7)]2+ complexes,
whereas in [Pb(L8−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.