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.