Mixed-metal MOFs are metal–organic frameworks that contain at least 2 different metal ions as nodes of their frameworks. They are prepared relatively easily by either a one-pot synthesis with a synthesis mixture containing the different metals, or by a post-synthetic ion-exchange method by soaking a monometallic MOF in a concentrated solution of a different (but compatible) metal-ion. More difficult is the accurate characterization of these materials. Is the formed product a mixture of monometallic MOFs or indeed a MOF with different metallic nodes? Are the metals randomly distributed or do they form domains? What is the oxidation state of the metals? How do the metals mutually influence each other, and impact the material's performance? Advanced characterization techniques are required e.g. X-ray absorption spectroscopy, magnetic resonance and electron microscopy. Computational tools at multiple scales are also often applied. In almost every case, a judicious choice of several techniques is required to unambiguously characterize the mixed-metal MOF. Although still in their infancy, several applications are emerging for mixed-metal MOFs, that improve on conventional monometallic MOFs. In the field of gas sorption and storage, especially the stability and affinity towards the target gases can be largely improved by introducing a second metal ion. In the case of flexible MOFs, the breathing behavior, and in particular the pressure at which the MOF opens, can be tailored. In heterogeneous catalysis, new cascade and tandem reactions become possible, with particular focus on reactions where the two metals in close proximity truly form a mixed-metal transition state. The bimetallic MOF should have a clear benefit over a mixture of the respective monometallic MOFs, and bimetallic enzymes can be a huge source of inspiration in this field. Another very promising application lies in the fields of luminescence and sensing. By tuning the lanthanide metals in mixed-metal lanthanide MOFs and by using the organic linkers as antennae, novel smart materials can be developed, acting as sensors and as thermochromic thermometers. Of course there are also still open challenges, as also mixed-metal MOFs do not escape the typical drawbacks of MOFs, such as low stability in moisture and possible metal leaching in liquids. The ease of synthesis of mixed-metal MOFs is a large bonus. In this critical review, we discuss in detail the synthesis, characterization, computational work and applications of mixed-metal MOFs.

Low-cost metal-free catalysts enable O ₂ activation as a key step in many important sustainable processes.

In this study, we present for the first time the immobilization of phosphotungstic acid (H₃PW₁₂O₄₀, HPW) into the nanocages of a dual amino-functionalized ionic liquid (DAIL)-modified MIL-101(Cr) framework under mild conditions. The obtained PW/DAIL/MIL-101(Cr) was evaluated as a catalyst in the oxidation of benzyl alcohol using TBHP in chloroform. In order to obtain insights into the role of the DAIL during catalysis, a DAIL-free catalyst (HPW/MIL-101(Cr)) was also prepared. Both catalysts were fully characterized using several techniques. In comparison with the HPW/MIL-101(Cr) material, PW/DAIL/MIL-101(Cr) showed an enhanced catalytic performance in the selective oxidation of benzyl alcohol. This was mainly attributed to the ability of the DAIL groups to form hydrogen bonds, thus enhancing the accessibility of TBHP. Furthermore, the immobilization of the DAIL groups onto MIL-101(Cr) resulted in increased thermal stability of the obtained PW/DAIL/MIL-101(Cr) which showed stability up to 400 °C. Moreover, the PW/DAIL/MIL-101(Cr) catalyst exhibited good recyclability and selectivity. The catalyst could be reused for at least five cycles with no significant leaching of the tungsten species or loss of crystallinity and activity.

The luminescence properties of two series of heteronuclear lanthanide-based coordination polymers were studied. The heteronuclear compounds were synthesized via mechanosynthesis routes. These series have the general chemical formula [Ln_1−xLn′_x(pzdc)(Hpzdc)(H₂O)₃]_n (Ln = La³⁺ or Ce³⁺, Ln′ = Sm³⁺, Eu³⁺, Tb³⁺, or Dy³⁺, x = 0.050, 0.075 or 0.100) where H₂pzdc stands for 2,3-pyrazinedicarboxylic acid (Hpzdc⁻ = mono-deprotonated and pzdc²⁻ = bis-deprotonated). The heteronuclear compounds that belong to each series are isostructural to a new two-dimensional La(III) coordination polymer and to an already reported homonuclear two-dimensional Ce(III) coordination polymer, [La(pzdc)(Hpzdc)(H₂O)₃]_n and [Ce(pzdc)(Hpzdc)(H₂O)₃]_n, respectively. These two crystal structures are isostructural as well. The La(III) and Ce(III) compounds have been synthesized via conventional heating and mechanosynthesis. The La(III) coordination polymer was structurally characterized by single crystal X-ray diffraction and was shown to consist of 2D sheets that construct a three-dimensional supramolecular architecture via non-covalent interactions i.e. hydrogen bonding. To find the optimal lanthanide-ion concentrations, the systems were doped with 5%, 7.5% or 10% of a spectroscopically active ion (Sm³⁺, Eu³⁺, Tb³⁺, or Dy³⁺) to investigate the luminescence properties.