Guest-induced assembly of PdII-linked coordination nanotubes

Abstract

This article reviews a coordination approach to discrete tubular structures. Families of pyridine-based molecular strands assemble into large tubular molecules through metal-coordination, in particular, cis-protected square-planar metals, [enPdII]2+ (en = ethylenediamine), are very useful building blocks to link these molecules. Pyridine-based oligo(3,5-pyridine) ligands efficiently self-assemble into well-defined tube frameworks having a variety of shapes and sizes upon treatment with [enPd(NO3)2] in the presence of rod-like organic guests as templates. The length of the tube can be strictly controlled according to the numbers of pyridine nuclei involved in the component ligand. These nanotubes collapse when the template molecule is removed. A hexapyridine ligand, in which two tripyridine units are linked by an alkyl spacer, forms a complex with the PdII building block leading to the formation of a stable nanotube with an empty cavity without templates. With the aid of a 3.0 nm template strand molecule, a hexapyridine ligand consisting of two tripyridine units and a biphenyl linker assembles into a 3.5-nm nanotube, in which the four ligands are held together by twelve PdIIions. Facile preparation as well as the stability of the tube, even after the template is removed makes possible the hollow discrete assembly of the tubes. A dodecapyridine ligand possessing four tripyridine podands on a benzenetetracarboxylate scaffold, six PdII ions, and a template molecule generate a mono end-capped tube, which converts into a double open tube upon crystallization. Once formed, there is no route back to the end-capped tube. The structures are not in rapid equilibrium and are kinetically trapped during self-assembly. These tubular structures have been confirmed by NMR, CSI-MS observations, and X-ray crystallographic analysis. Studies on the dynamic behavior of rodlike guests accommodated in the nanotubes are also discussed. © 2007 The Chemical Society of Japan.