Self-Assembly of Tetrameric and Hexameric Terpyridine-Based Macrocycles Using Cd(II), Zn(II), and Fe(II)

The self-assembly behavior of a tritopic 2,2':6',2″-terpyridine (tpy) ligand with Cd(II), Zn(II), and Fe(II) has been exploited herein to generate a series of tetrameric and hexameric macrocycles. The main advantage of using such transition metals with an octahedral coordination geometry is their distinct coordination abilities (e.g., binding strength and reversibility). With the same ligand, this study reveals that the supramolecular structural variation between tetrameric and hexameric macrocycle architectures can be precisely controlled using different metal ions with the same coordination geometry. When Cd(II) was used, a tetrameric macrocycle was the only observed structure in the self-assembly, whereas Zn(II) and Fe(II) assembled a mixture of tetrameric and hexameric macrocycles. Because of the high stability of Fe(II) as the coordination center, we successfully isolated tetrameric and hexameric macrocycles using a regular column. In-depth characterization was carried out to establish the proposed structures, including multinuclear NMR (¹H, ¹⁹F, and ¹³C) analysis, electrospray ionization mass spectrometry, and 2D ion-mobility mass spectrometry.