Formation of Giant and Small Cyclic Complexes from a Flexible Tripeptide Ligand Controlled by Metal Coordination and Hydrogen Bonds

• Published in: Journal of the American Chemical Society 2019-06, Vol.141 (22), p.8675-8679
• Main Authors: Miyake, Ryosuke, Ando, Akira, Ueno, Manami, Muraoka, Takahiro
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.9b01541

Formation of giant cyclic complexes by the assembly of small, flexible units is demonstrated by connecting 14 artificial tripeptides (1) with 14 Ni­(II) ions. Although tripeptide 1 is very flexible because of the presence of three CH2 groups in the main chain, it formed a tetradecanuclear cyclic complex ([1 14Ni14]28+) with a large cavity (diameter: ca. 2 nm). In this structure, three tripeptides are coordinated to each Ni­(II) center by three different coordination sites in 1, forming a mesh-like structure. Crystal structure analysis and theoretical calculations indicate that the conformation of 1 was controlled by the formation of metal coordination bonds and intramolecular hydrogen bonds. Because of the structural flexibility, the cyclic framework formed both circular and ellipsoidal structures in the crystalline state, depending on the packing structure. In addition, by the conditions of the assembly process, the size of the cavities could be tuned either with a small decrement (dodecanuclear complex [1 12Ni12]24+) or a large decrement (octanuclear complexes [(1-3H+)4Ni8]4+), in which “shrunk” cavities with a 10-fold difference in diameter (