Tetrahedral Imidazolate Frameworks with Auxiliary Ligands (TIF-Ax): Synthetic Strategies and Applications

Zeolitic imidazolate frameworks (ZIFs) are an important subclass of metal-organic frameworks (MOFs). Recently, we reported a new kind of MOF, namely tetrahedral imidazolate frameworks with auxiliary ligands (TIF-Ax), by adding linear ligands (Hint) into the zinc-imidazolate system. Introducing linear ligands into the M2+-imidazolate system overcomes the limitation of imidazole derivatives. Thanks to the synergistic effect of two different types of ligands, a series of new TIF-Ax with interesting topologies and a special pore environment has been reported, and they have attracted extensive attention in gas adsorption, separation, catalysis, heavy metal ion capture, and so on. In this review, we give a comprehensive overview of TIF-Ax, including their synthesis methods, structural diversity, and multi-field applications. Finally, we also discuss the challenges and perspectives of the rational design and syntheses of new TIF-Ax from the aspects of their composition, solvent, and template. This review provides deep insight into TIF-Ax and a reference for scholars with backgrounds of porous materials, gas separation, and catalysis.

Improvement of the Proton Conduction of Copper(II)-Mesoxalate Metal-Organic Frameworks by Strategic Selection of the Counterions

Three copper(II)/mesoxalate-based MOFs with formulas (H₃O)[Cu₉(Hmesox)₆(H₂O)₆Cl]·8H₂O (1), (NH₂Me₂)_0.4(H₃O)_0.6[Cu₉(Hmesox)₆(H₂O)₆Cl]·8H₂O (2), and (enH₂)_0.25(enH)_1.5[Cu₆(Hmesox)₃(mesox)(H₂O)₆Cl_0.5]Cl_0.5·5.25H₂O (3) were synthesized (H₄mesox = mesoxalic acid = 2,2-dihydroxypropanedioic acid, en = ethylenediamine). Essentially, all of the compounds display the same anionic network with a different arrangement of the cations, which have a remarkable effect on the proton conduction of the materials, ranging from 1.16 × 10^–4 S cm^–1 for 1 to 1.87 × 10^–3 S cm^–1 for 3 (at 80 °C and 95% RH). These compounds also display antiferromagnetic coupling among the copper(II) ions through both the carboxylate and alkoxido bridges. The values of the principal magnetic coupling constants were calculated by density functional theory (DFT), leading to congruent values that confirm the predominant antiferromagnetic nature of the interactions.

Three copper(II)-mesoxalate metal−organic frameworks were synthesized in the presence of three different cations: hydronium, dimethylammonium, and ethylenediammonium, which neutralize the charge of the anionic networks. Besides the crystallographic characterization and the investigation of the magnetic properties, the compounds show varying proton conductivities depending on the included cations. The proton conductivity increases 1 order of magnitude in the case of compound 3 (1.87 × 10⁻³ S cm⁻¹ at 80 °C and 95% RH), which contains ethylenediammonium cations.

Effect of the apical ligand on the geometry and magnetic properties of copper(ii)/mesoxalate trinuclear units

Three new heterometallic metal-organic frameworks, namely, {(Ph₄P)₂[MnCu₃(Hmesox)₃Br(H₂O)]·H₂O}_n (1), {(Ph₄P)₂[CoCu₃(Hmesox)₃Br]}_n (2) and {(Ph₄P)₂[ZnCu₃(Hmesox)₃Br]·2.5H₂O}_n (3) were prepared and their structure and magnetic properties were investigated (H₄mesox = mesoxalic acid, Ph₄P⁺ = tetraphenylphosphonium). The structure of all the compounds consist of two interpenetrating opposite-chirality supramolecular cationic and polymeric anionic 3-D (10,3)-a networks, which results in chiral compounds. The anionic network is formed from the polymerization of [Cu₃(Hmesox)₃Br]^4- units, working as three connectors, and M(ii) cations, working as three-connecting nodes, M = Mn(ii), Co(ii) and Zn(ii). The Ph₄P⁺ cations build the cationic chiral supramolecular network opposite to the anionic one. Compounds 1 and 2 exhibit long-range magnetic ordering with critical temperatures of 7.2 K and 6.9 K, respectively. However, compound 3 does not display long-range order, but shows ferromagnetic and antiferromagnetic coupling among the Cu(ii) ions. The magnetic interactions are studied by DFT calculations and compared with related Cu(ii)-mesoxalate compounds previously reported.