Metal Migration Processes in Homo- and Heterobimetallic Bismuth(III)–Lead(II) Porphyrin Complexes: Emergence of Allosteric Newton’s Cradle-like Devices

Metal ion migration in a bis-strapped porphyrin ligand with overhanging carboxylate groups has been investigated in solution. Two types of homobimetallic complexes are generated with Pb­(II) and Bi­(III) cations, which stand on both sides of the macrocycle: (i) a dissymmetric complex with one cation bound to the porphyrin N core and the other cation hung over the N core through bonding with a carboxylate of a strap; (ii) a C2-symmetric complex with both cations coordinated to the N core and to the carboxylate groups of the straps. Variable-temperature NMR studies and 2D rotational Overhauser effect spectroscopy NMR experiments have shown that in the former dissymmetric complexes, the two cations undergo a coupled intramolecular migration resulting in exchange of their coordination modes. Such complexes constitute active states of Newton’s cradle-like devices (NCDs), with the ion migration rate depending on the lability of the metal–ligand interactions [Pb­(II) faster than Bi­(III) NCDs]. On the other hand, the C2-symmetric complexes constitute either an inactive state [with Pb­(II)] or a resting state [with Bi­(III)] of an NCD, since they correspond respectively to a precursor or an intermediate in the motion of the cations. The NCDs are under both allosteric and acid–base control: (i) with Pb­(II), the addition of an allosteric effector such as an acetate anion to the medium allows the conversion of the symmetric form to the dissymmetric one, thus triggering the Newton’s cradle-like motion of the cations; (ii) with Bi­(III), a lifted state was converted to a resting one by the addition of protons and then restored by the addition of a base. As an extension to nondegenerate systems, a heterobimetallic Bi­(III)–Pb­(II) complex was selectively obtained, and it constitutes a frozen lifted state of a dissymmetric NCD. All of these homo- and hetero-NCDs could be successively formed by selective metal ion exchange. These unique findings open the way to novel tristable devices.