Self-assembled M6L4-type coordination nanocage with 2,2′-bipyridine ancillary ligands.: Facile crystallization and X-ray analysis of shape-selective enclathration of neutral guests in the cage

Hollow and roughly spherical cage 1 (ca. 2 nm in diameter) is self-assembled from 2,4,6-tri(4pyridyl)-1,3,5-triazine (2) and Pd(diamine)(ONO2)(2) (3). This cage compound enclathrates a variety of neutral organic molecules in an aqueous phase. Unlike cage 1 a, which possesses ancillary ethylenediamine ligands on the metal centers, 2,2'-bipyridine(bipy)-protected cage lb is easily crystallized, making possible the detailed analysis of the enclathration geometry of guests by X-ray crystallographic study. It is found that guests are enclathrated in three different manners, depending upon the shape and the size of the guests: tetrahedral 1:4 complexation, orthogonal 1:2 complexation, and a simple 1:1 complexation. The solution structures elucidated by NMR are in good accordance with the solid structure, showing that the enclathration geometries in the solid state are kept even in solution.