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

Abstract

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 C2-symmetric bimetallic complex (1Cd2 ) 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" C2-symmetric complex 1Cd2 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.

Sunlight‐Driven Formation and Dissociation of a Dynamic Mixed‐Valence Thallium(III)/Thallium(I) Porphyrin Complex

“…Thus far, two types of double translocation processes have been characterized, which correspond to an equilibrium between two degenerate forms of the complex, as exemplified in Figure 1 with lead(II)‐based metalloporphyrins. Of particular interest is the motion of the metal ions triggered by an allosteric effector (e.g., AcO − ) promoting the HAT coordination mode 5b,d. Furthermore, switching between the two types of motions as depicted in Figure 1 can be readily achieved by a dynamic constitutional evolution of the system triggered by a chemical effector 5e.…”

113Cd as a Probe in NMR Studies of Allosteric Host‐Guest‐Ligand Complexes of Porphyrin Cage Compounds

“…113 Cd meso ‐tetraphenylporphyrins ( 113 CdTPP ) have been the subject of research in the early years of 113 Cd NMR spectroscopy [16–18] . More recently, studies of metal‐migration processes in allosteric Newton's cradle‐like molecular devices [17] made use of 113 Cd centers to identify their coordinative interactions with a porphyrin with the help of 113 Cd NMR spectroscopy [19] …”

HgII‐Mediated TlI‐to‐TlIII Oxidation in Dynamic PbII/Tl Porphyrin Complexes

“…These features underlie new functions of the porphyrin skeleton, which could become part of a molecular device in which metal‐to‐porphyrin exchange plays a key role in obtaining switchable states. Typically, molecular devices inspired by the compartmentalized motion of spheres in a macroscopic two‐ball Newton's cradle can be foreseen (Scheme b) . This would involve the translocation of a metal ion from a strap to the N ‐core (M1‐strap1→M1‐N‐core) coupled with the opposite process on the other side of the ligand (M2‐N‐core→M2‐strap2), and thus, gives rise to two different states.…”

“…Typically,m olecular devices [13] inspired by the compartmentalized motion of spheres in am acroscopic two-ball Newton's cradle can be foreseen (Scheme 1b). [8,9] This would involve the translocation of am etal ion [14] from as trap to the N-core (M1-strap1!M1-N-core) coupled with the opposite process on the others ide of the ligand (M2-N-core!M2-strap2), andt hus, gives rise to two different states. Ideally,s uch ap rocess could be controlled by changing the oxidation degree of one of the metal ions, which would trigger ap referential binding at the level of the strap or the N-core.…”

“…For instance, porphyrins that are decorated with one or two straps, which delivers so‐called overhanging carboxylic acid groups from each side of the N ‐core, have shown a great versatility in the formation of bimetallic species with large metal ions propitious for out‐of‐plane complexation, such as Cd II , Hg II , Tl I/III , Pb II , and Bi III . Fast and quantitative metalation at room temperature, control of the nuclearity and of the out‐of‐plane stereoselectivity, coupled translocation of two metal ions, dynamic constitutional evolution of homo/heterobimetallic species, as well as unexpected reactivities were recently evidenced. These features underlie new functions of the porphyrin skeleton, which could become part of a molecular device in which metal‐to‐porphyrin exchange plays a key role in obtaining switchable states.…”