From Dimeric to Octameric Metallo‐Supramolecular Macrocycles Based on Sterically Congested Ligand–assisted Self‐Assembly with Zn(II), Cd(II), and Fe(II)

Abstract

Two sterically congested 2,2′:6′,2″‐terpyridine‐based ligands LA and LB, composed of asymmetrically contiguous terpyridine units, are designed and synthesized for metallo‐supramolecular architectures. The significant advantage of this design is that the terpyridines in these ligands have different chemical environments and show a selective coordination ability with each other. For ditopic ligand LA, the self‐assembly with Zn(II), Cd(II), and Fe(II) gave the rhombic dimers, which have the same sets of terpyridine signals as ligand LA. The self‐assembly behavior of tritopic ligand LB with Cd(II) and Zn(II) are observed for discrete tetramers under thermodynamic control, whereas ligand LB and Fe(II) are assembled to generate a mixture of tetramer, hexamer, and octamer, which are successfully isolated using regular chromatographic separation. Moreover, the sterically congested ligands and metal ions formed stable intermediates to drive the formation of discrete structures, which is also proved by mixing LB and Cd(II) with a precise stoichiometric ratio of 1:1. These supramolecular complexes are thoroughly characterized by NMR spectroscopy, electrospray ionization‐mass spectrometry, and 2D traveling‐wave ion mobility‐mass spectrometry. This conceptually new design of sterically congested ligands provides a novel strategy for precisely controlled supramolecular complexes with diverse architectures.

Self‐Assembly Methods for Recently Reported Discrete Supramolecular Structures Based on Terpyridine

“…Except for the unmodified terpyridine, Wang group has designed novel sterically congested ligands for geometrically oriented supramolecular self-assembly (Figure 15). [43] Ditopic ligand 51, which exhibits steric constrains via the introduction of an unsymmetrical terpyridine subunit, can afford rhombus dimers 53 via assembly with Cd 2 + , Zn 2 + , and Fe 2 + . Tritopic ligand 52 and metal ions of Cd 2 + and Zn 2 + were assembled to afford tetramers 54, but a mixture of tetramer, hexamer, and octamer was obtained via assembly with Fe 2 + and controlled by kinetics and thermodynamics.…”