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

摘要

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 macrocyde: (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 C_2-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 spectros-copy 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 C_2-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.