Resorcinarene-based biomimetic complexes ; in-situ host-guest adduct formation and crystallization of the novel water-soluble species

Biomimetic chemistry aims to create novel chemical species inspired by the natural systems such as metalloenzymes. This translates to the catalysts design, but en-route, very often secrets of the mechanisms used by the natural systems are discovered.[ ] As an example of the supramolecular approach in biomimetic modelling, [ ] the syntheses, structural characterizations, and chemical activity studies of "bowl complexes”, based on the resorcin[4]arene scaffold with three or four imidazole-containing coordinating arms grafted at the large rim, are presented. These complexes are biomimetic models of a ubiquitous mononuclear active site where three (or four) amino-acid residues hold the metal ion, leaving its one or two coordination sites available for the reversible guest (substrate) binding. The trisimidazole ligand RIm3, as well as the novel, tetraimidazole water soluble ligand WRIm4 were prepared, [ ] as well as their complexes with Zn(II), Cu(I)[ ] and Cu(II). In the case of the RIm3, spectroscopic studies and X-ray single crystal analysis revealed a 5-coordinate environment for the Zn(II) and Cu(II) centres provided by three imidazole arms, and two extra donors, one embedded inside the resorcinarene cavity, the other in exo position. These two labile sites are occupied by solvent molecules or residual water, and are readily displaced by carboxylate donors, the position of which (endo or exo) is under tight control of the bowl-cavity. The reaction of RIm3 ligand with Zn(II) or Cu(II) acetates led to the formation of the complexes with the acetate anion irreversibly embedded inside the cavity. Cu(II) acetate complex was characterized by the X-ray single crystal analysis.[ ] The water soluble WRIm4 ligand is particularly resistant to crystallize. The novel approach to prepare suitable single crystal of its complexes/adducts, including the in-situ host-guest adduct formation and concomitant crystallization of its complexes will be presented.