A pentanuclear lead(II) complex based on a strapped porphyrin with three different coordination modes.
Résumé
We have previously described Pb(II) and Bi(III) bimetallic complexes with overhanging carboxylic acid strapped porphyrins in which one metal ion is bound to the N-core ("out-of-plane", OOP), whereas the second one is bound to the strap ("hanging-atop", HAT). In such complexes, the hemidirected coordination sphere of a HAT Pb(II) cation provides sufficient space for an additional binding of a neutral ligand (e.g., DMSO). Interestingly, investigations of the HAT metal coordination mode in a single strap porphyrin show that a HAT Pb(II) can also interact via intermolecular coordination bonds, allowing the self-assembly of two bimetallic complexes. In the pentanuclear Pb(II) complex we are describing in this Article, three different coordination modes were found. The OOP Pb(II) remains inert toward the supramolecular assembling process, whereas the HAT Pb(II) cation, in addition to its intramolecular carboxylate and regular exogenous acetate groups, coordinates an additional exogenous acetate. These two acetates are shared with a third lead(II) cation featuring a holo-directed coordination sphere, from which a centro-symmetric complex is assembled. Density functional theory calculations show some electron-density pockets in the vicinity of the hemidirected HAT Pb(II) atoms, which are associated with the presence of a stereochemically active lone pair of electrons. On the basis of the comparison with other HAT Pb(II) and Bi(III) systems, the "volume" of this lone pair correlates well with the bond distance distributions and the number of the proximal oxygen atoms tethered to the post-transition metal cation. It thus follows the order 6-coordinate Bi(III) > 6-coordinate Pb(II) > 5-coordinate Pb(II).