Structural Flexibility and Selective Guest Accommodation in Two CuIIMetal-Organic Coordination Frameworks

Porous Lanthanide Metal-Organic Frameworks Using Pyridine-2,4-dicarboxylic Acid as a Linker: Structure, Gas Adsorption, and Luminescence Studies

Two types of new lanthanide coordination networks, [Ln₃(pdc)₄(Hpdc)(H₂O)₃]·8H₂O [H₂pdc = pyridine-2,4-dicarboxylic acid; Ln = Ce (1), Pr (2), Sm (3), Eu (4); type A) and [Tb₅(pdc)₄(Hpdc)]·3H₂O (type B), have been synthesized using a hydrothermal synthetic method. The former type A compound has an unfamiliar architecture, even basically comprised of primitive cubic (pcu) topology, and demonstrate porous gas adsorption behavior, giving several hundreds of square meters per gram surface areas after evacuation of the water molecules, while the latter type B compound does not show any porous properties. Most interestingly, the solvothermal synthetic method using N,N-dimethylformamide (DMF) as a solvent gives compounds that crystallize in a structure analogous to that of type A for Ln = La-Ho, probably formulated as [Ln₃(pdc)₄(Hpdc)·(DMF)_m]·nDMF. These compounds also exhibit large surface areas after evacuation of the DMF molecules and also moderate amounts of hydrogen gas uptake at 77 K. The luminescence properties were investigated for Eu and Tb analogues at elevated temperatures, at which an unusual increase in the emission intensity was observed upon the release of solvents, and discussed based on their porous structure.

An implanted paramagnetic metallofullerene probe within a metal-organic framework

Paramagnetic endohedral metallofullerene can be used as a molecular probe because of its sensitive electron spin characters, one of which is to sense its surroundings. Metal-organic framework (MOF) materials have significant applications in selective adsorption owing to their porous structures. Herein, we report a Sc₃C₂@C₈₀ spin probe implanted in MOF-177 to detect the unusual host-guest interaction between the guest molecules of metallofullerene and the host pores of the MOF. Paramagnetic Sc₃C₂@C₈₀ molecules were incorporated into the pores of MOF-177 via absorption method, and there was strong π-π interaction between oleophilic metallofullerene and aromatic framework. The electron paramagnetic resonance (EPR) signals of Sc₃C₂@C₈₀ in MOF-177 exhibit anisotropic properties caused by the restricted motion of implanted Sc₃C₂@C₈₀. This unusual host-guest interaction between Sc₃C₂@C₈₀ and MOF-177 is gradually strengthened with decreasing temperature as revealed by the EPR signals. In addition, the gas desorption from the MOF-177 pores under subatmospheric pressure can weaken the host-guest interaction and lead to slightly enhanced Sc₃C₂@C₈₀ EPR signals. Furthermore, the changes in the host-guest interaction between Sc₃C₂@C₈₀ and MOF-177 at different temperatures and pressures exhibit reversibility, as shown by cycling EPR measurements. These results will inspire material design and applications of fullerene and MOF complexes.