Constructing Robust Channel Structures By Packing Metallacalixarenes: Reversible Single-crystal-to-single-crystal Dehydration

摘要

Metal-directed self-assembly processes that form well-defined architectures represent an important chapter in supramolecular chemistry. The leading-role actors in such processes are the metal ions, through their stereochemical preferences, and the ligands, through the relative position of the donor atoms. Racks, ladders, grids, helicates, and boxes are examples of inorganic superstructures that are obtained through spontaneous and correct sequential interactions between highly instructed components. Other discrete nanostructures resulting from self-assembly processes involving metal ions are the metallacalixarenes. These are structural analogues of calixarenes that are obtained by assembling metal ions having two available coordination sites with organic ligands providing 120° bond angles (for example, pyrimidine or 4,7-phenanthroline derivatives). The bridging CH_2 groups in classical calixarenes are replaced by the metal ions. Appropriate assembling cations are monochelated square-planar or tetrahedral metal ions, the two other coordination sites being occupied by labile ligands.rnThe particular shape of calixarenes and calixarene-like compounds raises the following question: is it possible to construct channels by stacking these molecules in the crystal on top of each other? An important condition to be fulfilled in the case of metallacalixarenes is to avoid the presence of counterions in the crystal, which occupy the void space and cannot be removed without changing the chemical nature of the material. This is possible by employing appropriate anionic organic ligands that generate neutral crown-like complexes. Such ligands can be dianionic species resulting from deprotonation of 5-substituted derivatives of 1,3-benzenedicarboxylic acid (with predisposed carboxylic groups at 120°, as requested for the construction of metallacalixarenes). As the assembling cation, we chose a copper(II) complex species with two cis positions blocked by N,N,N',N'-tetramethylethylenediamine (tmen).