Systematic Syntheses and Metalloligand Doping of Flexible Porous Coordination Polymers Composed of a Co(III)–Metalloligand

A series of flexible porous coordination polymers (PCPs) RE–Co, composed of a Co­(III)–metalloligand [Co­(dcbpy)3]3– (Co; H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) and lanthanide cations (RE3+ = La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Er3+), was systematically synthesized. X-ray crystallographic analysis revealed that the six carboxylates at the top of each coordination octahedron of Co­(III)–metalloligand were commonly bound to RE3+ cations to form a rock-salt-type porous coordination framework. When RE–Co contains a smaller and heavier RE3+ cation than Nd3+, the RE–Co crystallized in the cubic Fm-3m space group, whereas the other three RE–Co with larger RE3+ crystallized in the lower symmetrical orthorhombic Fddd space group, owing to the asymmetric 10-coordinated bicapped square antiprism structure of the larger RE3+ cation. Powder X-ray diffraction and vapor-adsorption isotherm measurements revealed that all synthesized RE–Co PCPs show reversible amorphous–crystalline transitions, triggered by water-vapor-adsorption/desorption. This transition behavior strongly depends on the kind of RE3+; the transition of orthorhombic RE–Co was hardly observed under exposure to CH3OH vapor, but the RE–Co with smaller cations such as Gd3+ showed the transition under exposure to CH3OH vapors. Further tuning of vapor-adsorption property was examined by doping of Ru­(II)–metalloligands, [Ru­(dcbpy)3]4–, [Ru­(dcbpy)2Cl2]4–, [Ru­(dcbpy)­(tpy)­Cl]−, and [Ru­(dcbpy)­(dctpy)]3– (abbreviated as RuA, RuB, RuC, and RuD, respectively; tpy = 2,2′:6′,2″-terpyridine, H2dctpy = 4,4″-dicarboxy-2,2′:6′,2″-terpyridine), into the Co­(III)–metalloligand site of Gd–Co to form the Ru­(II)-doped PCP RuX@Gd–Co (X = A, B, C, or D). Three Ru­(II)–metalloligands, RuA, RuB, and RuD dopants, were found to be uniformly incorporated into the Gd–Co framework by replacing the original Co­(III)–metalloligand, whereas the doping of RuC failed probably because of the less number of coordination sites. In addition, we found that the RuA doping into the Gd–Co PCP had a large effect on vapor-adsorption due to the electrostatic interaction originating from the negatively charged RuA sites in the framework and the charge-compensating Li+ cations in the porous channel.