Coordination cages of rhodium and iridium as exoreceptors for alkali metal ions.

Hexanuclear coordination cages of the formula [(C5Me4R)M(C7H3NO4)]6 (M = Rh, Ir; R = Me, H) were obtained by stepwise reaction of [(C5Me4R)MCl2]2 with, first, AgOAc and, then, pyridine-3,5-dicarboxylic acid. Crystallographic analyses show that the cages adopt a distorted octahedral geometry with the pyridine-3,5-dicarboxylates functioning as dianionic, bridging ligands, each of which connects three different (C5Me4R)M fragments. The cages act as exoreceptors for the large alkali metal ions K(+) and Cs(+) but show low affinity for Na(+). Crystallographic and NMR spectroscopic analyses indicate that two metal ions can be coordinated to the surface of the cages. The respective binding sites comprise three carbonyl O-atoms from the bridging pyridine-3,5-dicarboxylate ligand.