Sugars to control ligand shape in metal complexes: conformationally constrained glycoligands with a predetermination...

In coordination chemistry, ligand shape can be used to tune properties, such as metal selectivity, coordination number, electronic structure, redox potential, and metal center stereochemistry including coordination helicates formation, and also to generate cavities for encapsulation. The results presented in this article indicate that two epimeric glycoligands (3 and 4) based on the conformationally restrained xylo- and ribo-1,2-O-isopropylidenefurano scaffolds are preorganized in water through pi-pi stacking due to hydrophobic interactions, as evidenced from excimer observation. The structure obtained in the solid state for one of the Cu(II) complexes (5) is chiral, with an original helical chirality arising from the coiling of the two ligands around the Cu-Cu axis. It shows an unusual double-deck type structure, with pi-pi interaction between two triazoyl-pyridyl rings and with a small cavity between the two Cu(II) ions able to host a bridging water molecule, as suggested by electron paramagnetic resonance. The Cu(II) complex from the epimeric ligand (6) shows similar properties with a mirror-image CD spectrum in the d-d region of the Cu(II). There is a predetermination of chirality at the metal center by the glycoligand induced by the C3 configuration, 6 and 5 being pseudoenantiomers. Interestingly, the stereochemistry at the metal center is here controlled by the combination of pi-stacking and chiral backbone.