engleski

Supramolecular biomimetic modelling of the tripodal mononuclear active site

The synthesis, structural characterization, as well as the chemical activity studies of Zn(II), Cu(I) and Cu(II) "bowl complexes”, based on the resorcin[4]arene scaffold with three imidazole-containing coordinating arms grafted at the large rim, are presented. These complexes are biomimetic models of a ubiquitous mononuclear metalloenzyme active site where cofacial triade of amino-acid residues holds the metal ion in the active site. The trisimidazole ligand RIm3 was prepared in a seven-steps procedure starting from resorcinol and hexanal. The complexes of Zn(II), Cu(I) and Cu(II) were prepared by simple reactions of the ligand with stoichiometric amounts of corresponding metal salts. Spectroscopic studies [and X-ray single crystal analysis in case of the Cu(II) acetato complex] 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 exposed to the solvent, in exo position. These two labile sites are occupied by either coordinating 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 acetatocomplexes with the acetate anion irreversibly embedded inside the cavity. Cu(II) acetate complex was characterized by the X-ray single crystal analysis. Its molecular structure features a rigidified resorcinarene bowl, which was constructed by the addition of the four methylene bridges between the eight hydroxyl groups of the octol precursor. The isolated resorcinarene basket reveals an approximate, non-crystallographic, 4mm point symmetry, and can easily host small guest molecules. Three methylimidazole-containing coordination arms at the large rim coordinate the Cu (II) ion. Its coordination sphere is completed by two O atoms from the intra-cavity bound acetate. The electron donors form a distorted square pyramide, where one of the nitrogens is at the appical position. The endo-coordination of the acetate is supported by an extensive network of intramolecular C-H···O and C-H···π interactions. Complex crystallizes in P21/c space group ; a = 32.3310 (4) Å, b=11.5490 (1) Å, c = 21.6020 (2) Å, β = 102.281 (3)°.

biomimetic ; resorcinarene ; single crystal X-ray

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