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.本研究在溶液体系中考察了带有悬垂羧酸酯基团的双缚带卟啉配体（bis-strapped porphyrin ligand）中的金属离子迁移行为。成功制备得到两类以铅(II)（Pb(II)）与铋(III)（Bi(III)）阳离子作为金属中心的同双核金属配合物，二者均位于大环两侧：(i) 不对称配合物：一个阳离子结合于卟啉氮核，另一个阳离子通过与缚带的羧酸酯配位悬挂于氮核上方；(ii) C₂对称配合物：两个阳离子同时配位至卟啉氮核与缚带的羧酸酯基团。 变温核磁共振（variable-temperature NMR）与二维旋转奥弗豪泽效应光谱核磁共振（2D rotational Overhauser effect spectroscopy NMR）实验结果表明，在前述不对称配合物中，两个阳离子发生耦合分子内迁移，实现配位模式的互换。此类配合物构成牛顿摇篮式装置（Newton’s cradle-like devices, NCDs）的活性态，其离子迁移速率取决于金属-配体相互作用的易变性：铅(II)基NCD的迁移速率快于铋(III)基NCD。 另一方面，C₂对称配合物分别对应NCD的非活性态（铅(II)体系）与静息态（铋(III)体系），二者分别为阳离子运动过程中的前驱体与中间体。 此类NCD同时受别构与酸碱双重调控：(i) 对于铅(II)体系，向体系中加入乙酸根阴离子这类别构效应剂，可将对称形式转化为不对称形式，从而触发阳离子的牛顿摇篮式运动；(ii) 对于铋(III)体系，质子的加入可将提升态转化为静息态，而加入碱则可使体系恢复至提升态。 作为非简并体系的拓展研究，本研究选择性制备得到异双核Bi(III)-Pb(II)配合物，其构成不对称NCD的冻结提升态。上述同双核与异双核NCD均可通过选择性金属离子交换依次制备得到。 这些独特的研究发现为新型三稳态器件（tristable devices）的开发开辟了道路。