Mixed-metal metal-organic frameworks. Abednatanzi Sara,Gohari Derakhshandeh Parviz,Depauw Hannes,Coudert François-Xavier,Vrielinck Henk,Van Der Voort Pascal,Leus Karen Chemical Society reviews 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. 10.1039/c8cs00337h

Selective Catalytic Chemistry at Rhodium(II) Nodes in Bimetallic Metal-Organic Frameworks. Shakya Deependra M,Ejegbavwo Otega A,Rajeshkumar Thayalan,Senanayake Sanjaya D,Brandt Amy J,Farzandh Sharfa,Acharya Narayan,Ebrahim Amani M,Frenkel Anatoly I,Rui Ning,Tate Gregory L,Monnier John R,Vogiatzis Konstantinos D,Shustova Natalia B,Chen Donna A Angewandte Chemie (International ed. in English) We report the first study of a gas-phase reaction catalyzed by highly dispersed sites at the metal nodes of a crystalline metal-organic framework (MOF). Specifically, CuRhBTC (BTC =benzenetricarboxylate) exhibited hydrogenation activity, while other isostructural monometallic and bimetallic MOFs did not. Our multi-technique characterization identifies the oxidation state of Rh in CuRhBTC as +2, which is a Rh oxidation state that has not previously been observed for crystalline MOF metal nodes. These Rh sites are active for the catalytic hydrogenation of propylene to propane at room temperature, and the MOF structure stabilizes the Rh oxidation state under reaction conditions. Density functional theory calculations suggest a mechanism in which hydrogen dissociation and propylene adsorption occur at the Rh sites. The ability to tailor the geometry and ensemble size of the metal nodes in MOFs allows for unprecedented control of the active sites and could lead to significant advances in rational catalyst design. 10.1002/anie.201908761

Template-Directed Approach Towards the Realization of Ordered Heterogeneity in Bimetallic Metal-Organic Frameworks. Kim Daeok,Coskun Ali Angewandte Chemie (International ed. in English) Controlling the arrangement of different metal ions to achieve ordered heterogeneity in metal-organic frameworks (MOFs) has been a great challenge. Herein, we introduce a template-directed approach, in which a 1D metal-organic polymer incorporating well-defined binding pockets for the secondary metal ions used as a structural template and starting material for the preparation of well-ordered bimetallic MOF-74s under heterogeneous-phase hydrothermal reaction conditions in the presence of secondary metal ions such as Ni and Mg in 3 h. The resulting bimetallic MOF-74s were found to possess a nearly 1:1 metal ratio regardless of their initial stoichiometry in the reaction mixture, thus demonstrating the possibility of controlling the arrangement of metal ions within the secondary building blocks in MOFs to tune their intrinsic properties such as gas affinity. 10.1002/anie.201702501

Electronic Properties of Bimetallic Metal-Organic Frameworks (MOFs): Tailoring the Density of Electronic States through MOF Modularity. Dolgopolova Ekaterina A,Brandt Amy J,Ejegbavwo Otega A,Duke Audrey S,Maddumapatabandi Thathsara D,Galhenage Randima P,Larson Bryon W,Reid Obadiah G,Ammal Salai C,Heyden Andreas,Chandrashekhar Mvs,Stavila Vitalie,Chen Donna A,Shustova Natalia B Journal of the American Chemical Society The development of porous well-defined hybrid materials (e.g., metal-organic frameworks or MOFs) will add a new dimension to a wide number of applications ranging from supercapacitors and electrodes to "smart" membranes and thermoelectrics. From this perspective, the understanding and tailoring of the electronic properties of MOFs are key fundamental challenges that could unlock the full potential of these materials. In this work, we focused on the fundamental insights responsible for the electronic properties of three distinct classes of bimetallic systems, MM'-MOFs, MM'-MOFs, and M(ligand-M')-MOFs, in which the second metal (M') incorporation occurs through (i) metal (M) replacement in the framework nodes (type I), (ii) metal node extension (type II), and (iii) metal coordination to the organic ligand (type III), respectively. We employed microwave conductivity, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, pressed-pellet conductivity, and theoretical modeling to shed light on the key factors responsible for the tunability of MOF electronic structures. Experimental prescreening of MOFs was performed based on changes in the density of electronic states near the Fermi edge, which was used as a starting point for further selection of suitable MOFs. As a result, we demonstrated that the tailoring of MOF electronic properties could be performed as a function of metal node engineering, framework topology, and/or the presence of unsaturated metal sites while preserving framework porosity and structural integrity. These studies unveil the possible pathways for transforming the electronic properties of MOFs from insulating to semiconducting, as well as provide a blueprint for the development of hybrid porous materials with desirable electronic structures. 10.1021/jacs.7b01125