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

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 ([1₁₄Ni₁₄]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 [1₁₂Ni₁₂]²⁴⁺) or a large decrement (octanuclear complexes [(1-3H⁺)₄Ni₈]⁴⁺), in which "shrunk" cavities with a 10-fold difference in diameter (<0.2 nm) were formed by tuning the tripeptide conformation through additional metal coordination to the tripeptide framework. Dynamic light scattering and mass spectrometry studies indicated that the giant cyclic complexes were also present in the solution state.