The Highly Connected MOFs Constructed from Nonanuclear and Trinuclear Lanthanide-Carboxylate Clusters: Selective Gas Adsorption and Luminescent pH Sensing

The highly odd-numbered 15-connected nonanuclear [Ln₉(μ₃-O)₂­(μ₃-OH)₁₂­(O₂C−)₁₂­(HCO₂)₃] and 9-connected trinuclear [Ln₃(μ₃-O)­(O₂C−)₆­(HCO₂)₃] lanthanide-carboxylate clusters with triangular and linear carboxylate bridging ligands were synergistically combined into Ln-MOFs, [(CH₃)₂­NH₂]₃­{[Ln₉­(μ₃-O)₂­(μ₃-OH)₁₂­(H₂O)₆]­[Ln₃­(μ₃-O)­(H₂O)₃]­(HCO₂)₃­(BTB)₆}·(solvent)_x (abbreviated as JXNU-3, Ln = Gd, Tb, Er; BTB^3– = benzene-1,3,5-tris­(4-benzoate)), displaying a (3,9,15)-connected topological net. The JXNU-3(Tb) exhibits highly selective CO₂ adsorption capacity over CH₄ that resulted from the high localized charge density induced by the presence of the nonanuclear and trinuclear cluster units. In addition, JXNU-3(Tb) with high chemical stability and characteristic bright green color exhibits fluorescent pH sensing, which is pertinent to the different protonation levels of the carboxylate groups of the benzene-1,3,5-tris­(4-benzoate) ligand with varying pH.