Flexible Coordination Polymers Composed of Luminescent Ruthenium(II) Metalloligands: Importance of the Position of the Coordination Site in Metalloligands

Coordination polymerization reactions between ruthenium­(II) metalloligands [Ru­(n,n′-dcbpy)]4– ([nRu]; n = 4, 5; n,n′-dcbpy = n,n′-dicarboxy-2,2′-bipyridine) and several divalent metal salts in basic aqueous solutions afforded porous luminescent complexes formulated as [Mg­(H2O)6]­{[Mg­(H2O)3]­[4Ru]·4H2O} (Mg2[4Ru]·13H2O), [Mg2(H2O)9]­[5Ru]·10H2O (Mg2[5Ru]·19H2O), {[Sr4(H2O)9]­[4Ru]2·9H2O} (Sr2[4Ru]·9H2O)2, {[Sr2(H2O)8]­[5Ru]·6H2O} (Sr2[5Ru]·14H2O), and {[Cd2(H2O)2]­[5Ru]·10H2O} (Cd2[5Ru]·12H2O). Single-crystal X-ray structural analyses revealed that the divalent metal ions were commonly coordinated by the carboxyl groups of the [nRu] metalloligand, forming porous frameworks with a void fraction varying from 11.4% Mg2[4Ru]·13H2O to 43.9% Cd2[5Ru]·12H2O. M2[4Ru]·nH2O showed a reversible structural transition accompanied by water and methanol vapor adsorption/desorption, while the porous structures of M2[5Ru]·nH2O were irreversibly collapsed by the removal of crystal water. The triplet metal-to-ligand charge-transfer emission energies of M2[4Ru]·nH2O were lower than those of [4Ru] in aqueous solution, whereas those of M2[5Ru]·nH2O were close to those of [5Ru] in aqueous solution. These results suggested that the position of the coordination site in the metalloligand played an important role not only on the structure of the porous framework but also on the structural flexibility involving the guest adsorption/desorption properties.