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Formation and Dynamic Behavior of Mono‐ and Bimetallic Cadmium(II) Porphyrin Complexes: Allosteric Control of Coupled Intraligand Metal Migrations
Author(s) -
Le Gac Stéphane,
Fusaro Luca,
Dorcet Vincent,
Boitrel Bernard
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201302178
Subject(s) - bimetallic strip , porphyrin , chemistry , crystallography , metal , ligand (biochemistry) , ion , allosteric regulation , metal ions in aqueous solution , stereochemistry , photochemistry , receptor , organic chemistry , biochemistry
The complexation behavior of a bis‐strapped porphyrin ligand ( 1 ) towards Cd II has been investigated by 1 H and 113 Cd NMR spectroscopy with the help of X‐ray diffraction structures. The presence of an overhanging carboxylic acid group on each side of the macrocycle is responsible for the instantaneous insertion of the metal ion(s) at room temperature, and allows the formation of bimetallic species with unusual coordination modes at the origin of unique dynamic behaviors. In the absence of base, a C 2 ‐symmetric bimetallic complex ( 1Cd 2 ) is readily formed, in which the porphyrin acts as a bridging ligand. Both Cd II ions are bound to the N core and to a COO − group of a strap. In contrast, the presence of a base induces a two‐step binding process with the successive formation of mono and bimetallic species ( 1 Cd and 1 Cd ⋅CdOAc ). Formally, a Cd II ion is first inserted into the N core and experiences a strong out‐of‐plane (OOP) displacement due to the binding of an overhanging carbonyl group in an apical position. A second Cd II ion then binds exclusively to the strap on the opposite side, in a so‐called hanging‐atop (HAT) coordination mode. These two complexes display a fluxional behavior that relies on intraligand migration processes of the metal ion(s). In 1 Cd , the Cd II ion exchanges between the two equivalent overhanging apical ligands by funneling through the porphyrin ring. In 1 Cd ⋅CdOAc , the two Cd II ions exchange their coordination mode (HAT↔OOP) in a concerted way while staying on their respective side of the macrocycle, in a so‐called Newton’s cradle‐like motion. The intramolecular pathway was notably evidenced by variable temperature 113 Cd heteronuclear NMR experiments. This coupled motion of the Cd II cations is under allosteric control; the addition of an acetate anion (the allosteric effector) to the “resting” C 2 ‐symmetric complex 1Cd 2 affords the dissymmetric complex 1 Cd ⋅CdOAc and triggers equilibrium between its two degenerate states. The rate of the swinging motion further depends on the concentration of AcO − , with a higher concentration leading to a slower motion. As compared with the related Pb II and Bi III bimetallic complexes, the Newton’s cradle‐like motion proceeds faster with the smaller Cd II ion. These results open the way to novel multistable devices and switches.