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.