Na+ and Ca2+ ion selective pyridylcarboxylate rings of Pd(II) and Pt(II).

A series of dinuclear Pd(II) and Pt(II) with central rectangular cavities have been self-assembled using pyridyl carboxylates as spacers and phosphines [1,1'-bis(diphenylphosphino)ferrocene (dppf) and triphenylphosphines] as supporting ligands. Some of these function as metallacrowns by capturing adventitious alkali or alkali earth metals ions. Two of the Pt(II) rings bind selectively to Na(+) whereas the Pd(II) rings prefer Ca(2+). Such endocyclic association with the metal ion is facilitated by the converging carbonyl pendants and supported by coordinated triflates. Affinity and selectivity are achieved through stereo-conformational control of the pendant carbonyls and the ring which are isomerically dependent on the spacer. These intermetallic complexes are characterized by X-ray single-crystal crystallography and solution spectroscopy (NMR and ESI-MS). A varied range of local geometries of Na(+) and Ca(2+) are revealed.