Synthesis and O₂ Reactivity of a Titanium(III) Metal–Organic Framework

Metal–organic frameworks featuring pores lined with exposed metal cations have received attention for a wide range of adsorption-related applications. While many frameworks with coordinatively unsaturated M^II centers have been reported, there are relatively few examples of porous materials with coordinatively unsaturated M^III centers. Here, we report the synthesis and characterization of Ti₃O­(OEt)­(bdc)₃(solv)₂ (Ti-MIL-101; bdc^2– = 1,4-benzenedicarboxylate; solv = <i>N</i>,<i>N</i>-dimethylformamide, tetrahydrofuran), the first metal–organic framework containing exclusively Ti^III centers. Through a combination of gas adsorption, X-ray diffraction, magnetic susceptibility, and electronic and vibrational spectroscopy measurements, this high-surface-area framework is shown to contain five-coordinate Ti^III centers upon desolvation, which irreversibly bind O₂ to form titanium­(IV) superoxo and peroxo species. Electronic absorption spectra suggest that the five-coordinate Ti^III sites adopt a distorted trigonal-bipyramidal geometry that effectively shields nuclear charge and inhibits strong adsorption of nonredox-active gases

A Five-Coordinate Heme Dioxygen Adduct Isolated within a Metal–Organic Framework

The porphyrinic metal–organic framework (MOF) PCN-224 is metalated with Fe^II to yield a 4-coordinate ferrous heme-containing compound. The heme center binds O₂ at −78 °C to give a 5-coordinate heme-O₂ complex. For the first time, this elusive species is structurally characterized, revealing an Fe^III center coordinated to superoxide via an end-on, η¹ linkage. Mössbauer spectroscopy supports the structural observations and indicates the presence of a low-spin electronic configuration for Fe^III. Finally, variable-temperature O₂ adsorption data enable quantification of the Fe–O₂ interaction, providing a binding enthalpy of −34(4) kJ/mol. This value is nearly half of that observed for comparable 6-coordinate, imidazole-bound heme-O₂ complexes, a difference that further illustrates the importance of axial ligands in biological heme-mediated O₂ transport and storage. These results demonstrate the ability of a MOF, by virtue of its rigid solid-state structure, to enable isolation and thorough characterization of a species that can only be observed transiently in molecular form

Single-Crystal-to-Single-Crystal Metalation of a Metal–Organic Framework: A Route toward Structurally Well-Defined Catalysts

Metal–organic frameworks featuring ligands with open chelating groups are versatile platforms for the preparation of a diverse set of heterogeneous catalysts through postsynthetic metalation. The crystalline nature of these materials allows them to be characterized via X-ray diffraction, which provides valuable insight into the structure of the metal sites that facilitate catalysis. A highly porous and thermally robust zirconium-based metal–organic framework, Zr₆O₄(OH)₄­(bpydc)₆ (bpydc^2– = 2,2′-bipyridne-5,5′-dicarboxylate), bears open bipyridine sites that readily react with a variety of solution- and gas-phase metal sources to form the corresponding metalated frameworks. Remarkably, Zr₆O₄(OH)₄­(bpydc)₆ undergoes a single-crystal-to-single-crystal transformation upon metalation that involves a change in space group from <i>Fm</i>3̅<i>m</i> to <i>Pa</i>3̅. This structural transformation leads to an ordering of the metalated linkers within the framework, allowing structural characterization of the resulting metal complexes. Furthermore, Zr₆O₄(OH)₄­(bpydc)₆ yields an active heterogeneous catalyst for arene C–H borylation when metalated with [Ir­(COD)₂]­BF₄ (COD = 1,5-cyclooctadiene). These results highlight the unique potential of metal–organic frameworks as a class of heterogeneous catalysts that allow unparalleled structural characterization and control over their active sites

Highly Selective Quantum Sieving of D₂ from H₂ by a Metal–Organic Framework As Determined by Gas Manometry and Infrared Spectroscopy

The quantum sieving effect between D₂ and H₂ is examined for a series of metal–organic frameworks (MOFs) over the temperature range 77–150 K. Isothermal adsorption measurements demonstrate a consistently larger isosteric heat of adsorption for D₂ vs H₂, with the largest difference being 1.4 kJ/mol in the case of Ni-MOF-74. This leads to a low-pressure selectivity for this material that increases from 1.5 at 150 K to 5.0 at 77 K. Idealized adsorption solution theory indicates that the selectivity decreases with increasing pressure, but remains well above unity at ambient pressure. Infrared measurements on different MOF materials show a strong correlation between selectivity and the frequency of the adsorbed H₂ translational band. This confirms that the separation is predominantly due to the difference in the zero-point energies of the adsorbed isotopologues

CO₂ Dynamics in a Metal–Organic Framework with Open Metal Sites

Metal–organic frameworks (MOFs) with open metal sites are promising candidates for CO₂ capture from dry flue gas. We applied <i>in situ</i> ¹³C NMR spectroscopy to investigate CO₂ adsorbed in Mg₂(dobdc) (H₄dobdc = 2,5-dihydroxyterephthalic acid; Mg-MOF-74, CPO-27-Mg), a key MOF in which exposed Mg²⁺ cation sites give rise to exceptional CO₂ capture properties. Analysis of the resulting spectra reveals details of the binding and CO₂ rotational motion within the material. The dynamics of the motional processes are evaluated via analysis of the NMR line shapes and relaxation times observed between 12 and 400 K. These results form stringent and quantifiable metrics for computer simulations that seek to screen and improve the design of new MOFs for CO₂ capture

Single-Crystal-to-Single-Crystal Metalation of a Metal–Organic Framework: A Route toward Structurally Well-Defined Catalysts

Metal–organic frameworks featuring ligands with open chelating groups are versatile platforms for the preparation of a diverse set of heterogeneous catalysts through postsynthetic metalation. The crystalline nature of these materials allows them to be characterized via X-ray diffraction, which provides valuable insight into the structure of the metal sites that facilitate catalysis. A highly porous and thermally robust zirconium-based metal–organic framework, Zr₆O₄(OH)₄­(bpydc)₆ (bpydc^2– = 2,2′-bipyridne-5,5′-dicarboxylate), bears open bipyridine sites that readily react with a variety of solution- and gas-phase metal sources to form the corresponding metalated frameworks. Remarkably, Zr₆O₄(OH)₄­(bpydc)₆ undergoes a single-crystal-to-single-crystal transformation upon metalation that involves a change in space group from <i>Fm</i>3̅<i>m</i> to <i>Pa</i>3̅. This structural transformation leads to an ordering of the metalated linkers within the framework, allowing structural characterization of the resulting metal complexes. Furthermore, Zr₆O₄(OH)₄­(bpydc)₆ yields an active heterogeneous catalyst for arene C–H borylation when metalated with [Ir­(COD)₂]­BF₄ (COD = 1,5-cyclooctadiene). These results highlight the unique potential of metal–organic frameworks as a class of heterogeneous catalysts that allow unparalleled structural characterization and control over their active sites

Metal Insertion in a Methylamine-Functionalized Zirconium Metal–Organic Framework for Enhanced Carbon Dioxide Capture

The reaction of ZrCl₄ with 2′,3′,5′,6′-tetramethylamino-<i>p</i>-terphenyl-4,4″-dicarboxylic acid (H₂tpdc-4CH₂NH₂·3HCl) in the presence of NaF affords Zr₆O₄(OH)_2.1F_1.9(tpdc-4CH₂NH₂·3HCl)₆ (<b>1</b>), which is a new member of the Zr₆O₄(OH)₄(dicarboxylate linker)₁₂ or UiO-68 family, and exhibits high porosity with BET and Langmuir surface areas of 1910 m²/g and 2220 m²/g, respectively. Remarkably, fluoride ion incorporation in the zirconium clusters results in increased thermal stability, marking the first example of enhancement in the stability of a UiO framework by this defect-restoration approach. Although material <b>1</b> features four alkylamine groups on each organic linker, the framework does not exhibit the high CO₂ uptake that would be expected for reaction between CO₂ and the amine groups to form carbamic acid or ammonium carbamate species. The absence of strong CO₂ adsorption can likely be attributed to protonation at some of the amine sites and the presence of counterions. Indeed, exposure of material <b>1</b> to acetonitrile solutions of the organic bases 1,8-bis­(dimethylamino)­naphthalene (DMAN) or trimethylamine, affords a partially deprotonated material, which exhibits enhanced CO₂ uptake. Exposure of basic amine sites also facilitates the postsynthetic chelation of copper­(I) ([Cu­(MeCN)₄]·CF₃SO₃) to yield material <b>2</b> with an enhanced CO₂ uptake of 4 wt % at 0.15 bar, which is double that of the parent framework <b>1</b>

Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal–Organic Framework mmen-Mg₂(dobpdc)

Two new metal–organic frameworks, M₂(dobpdc) (M = Zn (<b>1</b>), Mg (<b>2</b>); dobpdc^4– = 4,4′-dioxido-3,3′-biphenyldicarboxylate), adopting an expanded MOF-74 structure type, were synthesized via solvothermal and microwave methods. Coordinatively unsaturated Mg²⁺ cations lining the 18.4-Å-diameter channels of <b>2</b> were functionalized with <i>N</i>,<i>N</i>′-dimethylethylenediamine (mmen) to afford Mg₂(dobpdc)­(mmen)_1.6(H₂O)_0.4 (mmen-Mg₂(dobpdc)). This compound displays an exceptional capacity for CO₂ adsorption at low pressures, taking up 2.0 mmol/g (8.1 wt %) at 0.39 mbar and 25 °C, conditions relevant to removal of CO₂ from air, and 3.14 mmol/g (12.1 wt %) at 0.15 bar and 40 °C, conditions relevant to CO₂ capture from flue gas. Dynamic gas adsorption/desorption cycling experiments demonstrate that mmen-Mg₂(dobpdc) can be regenerated upon repeated exposures to simulated air and flue gas mixtures, with cycling capacities of 1.05 mmol/g (4.4 wt %) after 1 h of exposure to flowing 390 ppm CO₂ in simulated air at 25 °C and 2.52 mmol/g (9.9 wt %) after 15 min of exposure to flowing 15% CO₂ in N₂ at 40 °C. The purity of the CO₂ removed from dry air and flue gas in these processes was estimated to be 96% and 98%, respectively. As a flue gas adsorbent, the regeneration energy was estimated through differential scanning calorimetry experiments to be 2.34 MJ/kg CO₂ adsorbed. Overall, the performance characteristics of mmen-Mg₂(dobpdc) indicate it to be an exceptional new adsorbent for CO₂ capture, comparing favorably with both amine-grafted silicas and aqueous amine solutions

Unconventional, Highly Selective CO₂ Adsorption in Zeolite SSZ-13

Low-pressure adsorption of carbon dioxide and nitrogen was studied in both acidic and copper-exchanged forms of SSZ-13, a zeolite containing an 8-ring window. Under ideal conditions for industrial separations of CO₂ from N₂, the ideal adsorbed solution theory selectivity is >70 in each compound. For low gas coverage, the isosteric heat of adsorption for CO₂ was found to be 33.1 and 34.0 kJ/mol for Cu- and H-SSZ-13, respectively. From <i>in situ</i> neutron powder diffraction measurements, we ascribe the CO₂ over N₂ selectivity to differences in binding sites for the two gases, where the primary CO₂ binding site is located in the center of the 8-membered-ring pore window. This CO₂ binding mode, which has important implications for use of zeolites in separations, has not been observed before and is rationalized and discussed relative to the high selectivity for CO₂ over N₂ in SSZ-13 and other zeolites containing 8-ring windows

Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal–Organic Framework mmen-Mg₂(dobpdc)

Two new metal–organic frameworks, M₂(dobpdc) (M = Zn (<b>1</b>), Mg (<b>2</b>); dobpdc^4– = 4,4′-dioxido-3,3′-biphenyldicarboxylate), adopting an expanded MOF-74 structure type, were synthesized via solvothermal and microwave methods. Coordinatively unsaturated Mg²⁺ cations lining the 18.4-Å-diameter channels of <b>2</b> were functionalized with <i>N</i>,<i>N</i>′-dimethylethylenediamine (mmen) to afford Mg₂(dobpdc)­(mmen)_1.6(H₂O)_0.4 (mmen-Mg₂(dobpdc)). This compound displays an exceptional capacity for CO₂ adsorption at low pressures, taking up 2.0 mmol/g (8.1 wt %) at 0.39 mbar and 25 °C, conditions relevant to removal of CO₂ from air, and 3.14 mmol/g (12.1 wt %) at 0.15 bar and 40 °C, conditions relevant to CO₂ capture from flue gas. Dynamic gas adsorption/desorption cycling experiments demonstrate that mmen-Mg₂(dobpdc) can be regenerated upon repeated exposures to simulated air and flue gas mixtures, with cycling capacities of 1.05 mmol/g (4.4 wt %) after 1 h of exposure to flowing 390 ppm CO₂ in simulated air at 25 °C and 2.52 mmol/g (9.9 wt %) after 15 min of exposure to flowing 15% CO₂ in N₂ at 40 °C. The purity of the CO₂ removed from dry air and flue gas in these processes was estimated to be 96% and 98%, respectively. As a flue gas adsorbent, the regeneration energy was estimated through differential scanning calorimetry experiments to be 2.34 MJ/kg CO₂ adsorbed. Overall, the performance characteristics of mmen-Mg₂(dobpdc) indicate it to be an exceptional new adsorbent for CO₂ capture, comparing favorably with both amine-grafted silicas and aqueous amine solutions