Interconverting two classes of architectures by reduction of a self-sorting mixture.

Supramolecular complexity finds its ultimate expression in the structures and functions of natural systems. Chemists are emulating these properties individually in the form of selfassembling complexes (e.g., squares, polyhedra) and molecular switches (e.g., catenanes and rotaxanes), respectively. Combinations of the two, that is, stimuliresponsive self-assembled complexes, are rare. Most selfassemblies will disassemble upon stimulation (e.g., pseudorotaxanes 7] and tetrahedra). Other systems switch within the same architectural manifold (e.g., from one pseudorotaxane into another). While the ability to transform between two different self-assembled architectures (pincer and grid, ring and cage, from 2D to 3D) is growing in number, these demonstrations make use of chemical stimuli. Use of redox stimulation, with its macro-to-molecule interface and the ability to cycle many times, is unprecedented. Herein, we demonstrate this capability with the facile and reversible interconversion between two different architectures (Scheme 1) using electrons instead of chemicals. We self-assembled different architectures around a dynamic Cu ion and used ligand reduction to switch between them. The ligand s steric, electrostatic, and electronic (s/p) properties can be used to direct which architecture is formed. Two different ligands were designed to access two Cu complexes. The first ligand, ReB (Scheme 1), is based on a bischelating 3,6-bis(5-methyl-2-pyridyl)-1,2,4,5tetrazine (Me2BPTZ) ligand system, [14] which reduces easily. The Me2BPTZ ligand is mononucleated with the rhenium(I) tricarbonylchloro, {Re(CO)3Cl}, moiety to afford one chelating site, rather than two, thus simplifying the complexity of the self-sorting mixture (Scheme 1). The two nitrogen atoms that make up the open chelating site have very different electronic characters: The pyridyl N is a strong s donor and weak p acceptor while the tetrazyl N is a weak s donor and strong p acceptor. Upon tetrazine-localized reduction of the ReB ligand, the electronic properties of the tetrazine core invert (strong s donor and p donor) while the pyridine unit will be relatively unchanged. The second ligand is a strong s-donating, sterically bulky and redox-innocent phenanthroline-based macrocycle, MC (Scheme 1), which prevents the formation of a bis-macrocycle complex. Using ReB and MC as ligands, only two architectures can be formed in a mixture with Cu ions. The first is a heteroleptic complex wherein the ReB ligand is templated by the Cu ion to thread through the macrocycle to produce a [2]pseudorotaxane, [2]PR (Scheme 1). The second involves two ReB ligands to form a homoleptic complex, G, which resembles a corner piece of grid-type complexes. On electronic grounds the heteroleptic complex, [2]PR, is expected to be more stable: It has a total of three strong sdonor nitrogen atoms and utilizes the strongly stabilizing macrocycle. Upon ligand-based reduction of an appropriate mixture, the reduced form of the grid corner, G (Scheme 1), can be electrostatically stabilized with the installation of two negative charges. In order to test these ideas, a mixture of the three components Cu:ReB:MC was prepared in the ratio 1:2:1 and the mechanism of switching between the two architectures was verified using digital Scheme 1. A self-sorting mixture of two ligands (ReB and MC) around a Cu ion shows redox-driven interconversion between two distinct architectures: pseudorotaxane, [2]PR, and grid-corner complex, G . (White and gray rods are neutral and reduced, respectively.)