A Pentanuclear Lead(II) Complex Based on a Strapped Porphyrin with Three Different Coordination Modes

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).