Supramolecular Allosteric Cofacial Porphyrin Complexes

Nature routinely uses cooperative interactions to regulate cellular activity. For years, chemists
have designed synthetic systems that aim toward harnessing the reactivity common to natural biological
systems. By learning how to control these interactions in situ, one begins to allow for the preparation of
man-made biomimetic systems that can efficiently mimic the interactions found in Nature. To this end, we
have designed a synthetic protocol for the preparation of flexible metal-directed supramolecular cofacial
porphyrin complexes which are readily obtained in greater than 90% yield through the use of new hemilabile
porphyrin ligands with bifunctional ether−phosphine or thioether−phosphine substituents at the 5 and 15
positions on the porphyrin ring. The resulting architectures contain two hemilabile ligand−metal domains
(RhI or CuI sites) and two cofacially aligned porphyrins (ZnII sites), offering orthogonal functionalities and
allowing these multimetallic complexes to exist in two states, “condensed” or “open”. Combining the ether−phosphine ligand with the appropriate RhI or CuI transition-metal precursors results in “open” macrocyclic
products. In contrast, reacting the thioether−phosphine ligand with RhI or CuI precursors yields condensed
structures that can be converted into their “open” macrocyclic forms via introduction of additional ancillary
ligands. The change in cavity size that occurs allows these structures to function as allosteric catalysts for
the acyl transfer reaction between X-pyridylcarbinol (where X = 2, 3, or 4) and 1-acetylimidazole. For 3-
and 4-pyridylcarbinol, the “open” macrocycle accelerates the acyl transfer reaction more than the condensed
analogue and significantly more than the porphyrin monomer. In contrast, an allosteric effect was not
observed for 2-pyridylcarbinol, which is expected to be a weaker binder and is unfavorably constrained
inside the macrocyclic cavity.