Heterobimetallic Porphyrin Complexes Displaying Triple Dynamics: Coupled Metal Motions Controlled by Constitutional Evolution

A bis-strap
porphyrin ligand (1), with an overhanging
carboxylic acid group on each side of the macrocycle, has been investigated
toward the formation of dynamic libraries of bimetallic complexes
with Hg­(II), Cd­(II), and Pb­(II). Highly heteroselective metalation
processes occurred in the presence of Pb­(II), with Hg­(II) or Cd­(II)
bound out-of-plane to the N-core and “PbOAc”
bound to a carboxylate group of a strap on the opposite side. The
resulting complexes, 1Hg·PbOAc and 1Cd·PbOAc, display three levels of dynamics. The first is strap-level (interactional dynamics), where the PbOAc moiety
swings between the left and right side of the strap owing to a second
sphere of coordination with lateral amide functions. The second is ligand-level (motional dynamics), where 1Hg·PbOAc and 1Cd·PbOAc exist as two degenerate
states in equilibrium controlled by a chemical effector (AcO–). The process corresponds to a double translocation of the metal
ions according to an intramolecular migration of Hg­(II) or Cd­(II)
through the N-core, oscillating between the two equivalent overhanging
carbonyl groups, coupled to an intermolecular pathway for PbOAc exchanging
between the two equivalent overhanging carboxylate groups (N-coreup ⇆ N-coredown coupled to strapdown ⇆ strapup, i.e., coupled motion #1 in the abstract
graphic). The third is library-level (constitutional dynamics), where a dynamic constitutional evolution of the system was achieved
by the successive addition of two chemical effectors (DMAP and then
AcO–). It allowed shifting equilibrium forward and
backward between 1Hg·PbOAc and the corresponding homobimetallic complexes 1Hg2·DMAP and 1Pb·PbOAc. The latter displays
a different ligand-level dynamics, in the form of an intraligand coupled
migration of the Pb­(II) ions (N-coreup ⇆ strapup coupled to strapdown ⇆ N-coredown, i.e., coupled motion #2 in the abstract graphic). In addition,
the neutral “bridged” complexes 1HgPb and 1CdPb, with the metal ions on opposite sides both bound to
the N-core and to a carboxylate of a strap, were structurally characterized.
These results establish an unprecedented approach in supramolecular
coordination chemistry, by considering the reversible interaction
of a metal ion with the porphyrin N-core as a new source of self-organization
processes. This work should provide new inspirations for the design
of innovative adaptative materials and devices.