Formation and Dynamic Behavior of Mono and Bimetallic Cadmium(II) Porphyrin Complexes: Allosteric Control of Coupled Intraligand Metal Migrations.

The complexation behavior of a bis-strapped porphyrin ligand (1 = I) towards CdII was studied by 1H and 113Cd 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 temp., and gave bimetallic species with unusual coordination modes at the origin of unique dynamic behaviors. In the absence of base, a C2-sym. bimetallic complex (1Cd2) is readily formed, in which the porphyrin acts as a bridging ligand. Both CdII 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 (1Cd and 1Cd·CdOAc). Formally, a CdII 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 CdII 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 1Cd, the CdII ion exchanges between the two equiv. overhanging apical ligands by funneling through the porphyrin ring. In 1Cd·CdOAc, the two CdII ions exchange their coordination mode (OOP) in a concerted way while staying on their resp. side of the macrocycle, in a so-called Newton's cradle-like motion. The intramol. pathway was notably evidenced by variable temp. 113Cd heteronuclear NMR expts. This coupled motion of the CdII cations is under allosteric control; the addn. of an acetate anion (the allosteric effector) to the "resting" C2-sym. complex 1Cd2 affords the dissym. complex 1Cd·CdOAc and triggers equil. between its two degenerate states. The rate of the swinging motion further depends on the concn. of AcO-, with a higher concn. leading to a slower motion. As compared with the related PbII and BiIII bimetallic complexes, the Newton's cradle-like motion proceeds faster with the smaller CdII ion. These results open the way to novel multistable devices and switches. [on SciFinder(R)]