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

Abstract

A series of flexible porous coordination polymers (PCPs) <b>RE–Co</b>, composed of a Co­(III)–metalloligand [Co­(dcbpy)₃]^3– (<b>Co</b>; 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 <b>RE–Co</b> contains a smaller and heavier RE³⁺ cation than Nd³⁺, the <b>RE–Co</b> crystallized in the cubic <i><i>Fm</i>-3<i>m</i></i> space group, whereas the other three <b>RE–Co</b> with larger RE³⁺ crystallized in the lower symmetrical orthorhombic <i>Fddd</i> 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 <b>RE–Co</b> 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 <b>RE–Co</b> was hardly observed under exposure to CH₃OH vapor, but the <b>RE–Co</b> 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 <i><b>Ru</b></i><b>A</b>, <i><b>Ru</b></i><b>B</b>, <i><b>Ru</b></i><b>C</b>, and <i><b>Ru</b></i><b>D</b>, respectively; tpy = 2,2′:6′,2″-terpyridine, H₂dctpy = 4,4″-dicarboxy-2,2′:6′,2″-terpyridine), into the Co­(III)–metalloligand site of <b>Gd–Co</b> to form the Ru­(II)-doped PCP <i><b>Ru</b></i><b>X@Gd–Co</b> (X = A, B, C, or D). Three Ru­(II)–metalloligands, <i><b>Ru</b></i><b>A</b>, <i><b>Ru</b></i><b>B</b>, and <i><b>Ru</b></i><b>D</b> dopants, were found to be uniformly incorporated into the <b>Gd–Co</b> framework by replacing the original Co­(III)–metalloligand, whereas the doping of <i><b>Ru</b></i><b>C</b> failed probably because of the less number of coordination sites. In addition, we found that the <i><b>Ru</b></i><b>A</b> doping into the <b>Gd–Co</b> PCP had a large effect on vapor-adsorption due to the electrostatic interaction originating from the negatively charged <i><b>Ru</b></i><b>A</b> sites in the framework and the charge-compensating Li⁺ cations in the porous channel