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).