ZnII-Salphen Complexes as Versatile Building Blocks for the Construction of Supramolecular Box Assemblies

Zn II -salphen complexes are readily accessible and interesting supramolecular building blocks with a large structural diversity. Higher-order supramolecular assemblies, such as mo- lecular boxes based on a bis-Zn II -sal- phen building block and various ditopic bipyridine ligands, have been construct- ed by means of supramolecular, coordi- native Zn II -Npyr interactions. The use of bipyridine ligands of differing sizes enables the construction of structures with predefined box diameters. The features of the 2:2 box assemblies were investigated in detail by (variable tem- perature) NMR spectroscopy, UV-visi- ble spectroscopy, NMR titrations, and X-ray crystallographic studies. The spectroscopic studies reveal a high as- sociation constant for the Zn II -sal- phen-pyridyl motif, which lies in the range 10 5 -10 6 m 1 . The strong interac- tion between the Zn II center and pyri- dine donors was supported by PM3 cal- culations that showed a relatively high Lewis acid character of the metal center in the salphen complex. Titra- tion curves monitored by UV-visible show a cooperative effect between the two bipyridine ligands upon complexa- tion to the bis-Zn II template, suggesting the formation of 2:2 complexes. The crystal structures of two supramolec- ular boxes have been determined. In both examples such a 2:2 assembly is present in the solid state, and the box size is different because they consist of different building blocks. Interestingly, the box assemblies line up in the solid state to form porous channels that are potentially useful in a number of appli-