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]center dot 4H(2)O} (Mg-2[4Ru]center dot 13H(2)O), [Mg-2(H2O)(9)][5Ru]center dot 10H(2)O (Mg-2[5Ru]center dot 19H(2)O), {[Sr-4(H2O)(9)][4Ru](2)center dot 9H(2)O} (Sr-2[4Ru]center dot 9H(2)O)(2), {[Sr-2(H2O)(8)][5Ru]center dot 6H(2)O} (Sr-2[5Ru]center dot 14H(2)O), and {[Cd-2(H2O)(2)][5Ru]center dot 10H(2)O} (Cd-2[5Ru]center dot 12H(2)O). 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% Mg-2[4Ru]center dot 13H(2)O to 43.9% Cd-2[5Ru]center dot 12H(2)O. M-2[4Ru]center dot nH(2)O showed a reversible structural transition accompanied by water and methanol vapor adsorption/desorption, while the porous structures of M-2[5Ru]center dot nH(2)O were irreversibly collapsed by the removal of crystal water. The triplet metal-to-ligand charge-transfer emission energies of M-2[4Ru]center dot nH(2)O were lower than those of [4Ru] in aqueous solution, whereas those of M-2[5Ru]center dot nH(2)O 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.