Formation and dynamic behavior of mono- and bimetallic cadmium(II) porphyrin complexes: Allosteric control of coupled intraligand metal migrations

摘要

The complexation behavior of a bis-strapped porphyrin ligand (1) towards Cd~(II) has been investigated by ~1H 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. Coupled motion: An original fluxional behavior has been characterized in a bimetallic Cd~(II) porphyrin complex. It consists of a coupled intraligand migration of the two cations, resembling the motion of spheres in a Newton's cradle device (see figure). The cations switch concomitantly between out-of-plane and hanging-atop coordination modes, while staying on their respective side of the ligand. The motion can be switched on and off by addition and removal of an allosteric effector.