Supramolecular complexation of alkali cations through mechanochemical reactions between crystalline solids

The organometallic zwitter-ion [Co-III(eta(5)-C5H4COOH)(eta(5)-C5H4COO)] reacts quantitatively as a solid polycrystalline phase with a number of crystalline alkali salts MX (M = K+, Rb+, Cs+, NH4+; X = Cl-, Br-, I-, PF6-, although not in all cation/anion permutations) to afford supramolecular complexes of the formula [Co-III(eta(5)-C5H4COOH)(eta(5)-C5H4COO)](2).M+X-. In some cases, the mechanochemical complexation requires kneading of the two solids with a catalytic amount of water. The characterization of the solid-state products has been achieved by a combination of X-ray single-crystal and powder-diffraction experiments. The hydrogen-bonding interactions have been investigated by solid-state NMR spectroscopy. The mechano-chemical reactions imply a profound solid-state rearrangement accompanied by breaking and forming of O-H...O hydrogen-bonding interactions between the organometallic molecules. All compounds could also be obtained by solution crystallization of the inorganic salts in the presence of the organometallic unit. The solid-state complexation of alkali cations by the organometallic zwitterion has been described as a special kind of solvation process taking place in the solid state.