Selective adsorption of SO₂ is realized in a porous metal–organic framework material, and in-depth structural and spectroscopic investigations using X-rays, infrared, and neutrons define the underlying interactions that cause SO₂ to bind more strongly than CO₂ and N₂.

Understanding the mechanism of gas-sorbent interactions is of fundamental importance for the design of improved gas storage materials. Here we report the binding domains of carbon dioxide and acetylene in a tetra-amide functionalized metal-organic framework, MFM-188, at crystallographic resolution. Although exhibiting moderate porosity, desolvated MFM-188a exhibits exceptionally high carbon dioxide and acetylene adsorption uptakes with the latter (232 cm³ g⁻¹ at 295 K and 1 bar) being the highest value observed for porous solids under these conditions to the best of our knowledge. Neutron diffraction and inelastic neutron scattering studies enable the direct observation of the role of amide groups in substrate binding, representing an example of probing gas-amide binding interactions by such experiments. This study reveals that the combination of polyamide groups, open metal sites, appropriate pore geometry and cooperative binding between guest molecules is responsible for the high uptakes of acetylene and carbon dioxide in MFM-188a.

Hydrogen bonds dominate many chemical and biological processes, and chemical modification enables control and modulation of host–guest systems. Here we report a targeted modification of hydrogen bonding and its effect on guest binding in redox-active materials. MFM-300(V^III) {[V^III₂(OH)₂(L)], LH₄=biphenyl-3,3′,5,5′-tetracarboxylic acid} can be oxidized to isostructural MFM-300(V^IV), [V^IV₂O₂(L)], in which deprotonation of the bridging hydroxyl groups occurs. MFM-300(V^III) shows the second highest CO₂ uptake capacity in metal-organic framework materials at 298 K and 1 bar (6.0 mmol g⁻¹) and involves hydrogen bonding between the OH group of the host and the O-donor of CO₂, which binds in an end-on manner, "Equation missing"=1.863(1) Å. In contrast, CO₂-loaded MFM-300(V^IV) shows CO₂ bound side-on to the oxy group and sandwiched between two phenyl groups involving a unique "Equation missing"···c.g._phenyl interaction [3.069(2), 3.146(3) Å]. The macroscopic packing of CO₂ in the pores is directly influenced by these primary binding sites.

Metal–organic frameworks (MOFs) are usually synthesized using a single type of metal ion, and MOFs containing mixtures of different metal ions are of great interest and represent a methodology to enhance and tune materials properties. We report the synthesis of [Ga₂(OH)₂(L)] (H₄L = biphenyl-3,3′,5,5′-tetracarboxylic acid), designated as MFM-300(Ga₂), (MFM = Manchester Framework Material replacing NOTT designation), by solvothermal reaction of Ga(NO₃)₃ and H₄L in a mixture of DMF, THF, and water containing HCl for 3 days. MFM-300(Ga₂) crystallizes in the tetragonal space group I4₁22, a = b = 15.0174(7) Å and c = 11.9111(11) Å and is isostructural with the Al(III) analogue MFM-300(Al₂) with pores decorated with −OH groups bridging Ga(III) centers. The isostructural Fe-doped material [Ga_1.87Fe_0.13(OH)₂(L)], MFM-300(Ga_1.87Fe_0.13), can be prepared under similar conditions to MFM-300(Ga₂) via reaction of a homogeneous mixture of Fe(NO₃)₃ and Ga(NO₃)₃ with biphenyl-3,3′,5,5′-tetracarboxylic acid. An Fe(III)-based material [Fe₃O_1.5(OH)(HL)(L)_0.5(H₂O)_3.5], MFM-310(Fe), was synthesized with Fe(NO₃)₃ and the same ligand via hydrothermal methods. [MFM-310(Fe)] crystallizes in the orthorhombic space group Pmn2₁ with a = 10.560(4) Å, b = 19.451(8) Å, and c = 11.773(5) Å and incorporates μ₃-oxo-centered trinuclear iron cluster nodes connected by ligands to give a 3D nonporous framework that has a different structure to the MFM-300 series. Thus, Fe-doping can be used to monitor the effects of the heteroatom center within a parent Ga(III) framework without the requirement of synthesizing the isostructural Fe(III) analogue [Fe₂(OH)₂(L)], MFM-300(Fe₂), which we have thus far been unable to prepare. Fe-doping of MFM-300(Ga₂) affords positive effects on gas adsorption capacities, particularly for CO₂ adsorption, whereby MFM-300(Ga_1.87Fe_0.13) shows a 49% enhancement of CO₂ adsorption capacity in comparison to the homometallic parent material. We thus report herein the highest CO₂ uptake (2.86 mmol g^–1 at 273 K at 1 bar) for a Ga-based MOF. The single-crystal X-ray structures of MFM-300(Ga₂)-solv, MFM-300(Ga₂), MFM-300(Ga₂)·2.35CO₂, MFM-300(Ga_1.87Fe_0.13)-solv, MFM-300(Ga_1.87Fe_0.13), and MFM-300(Ga_1.87Fe_0.13)·2.0CO₂ have been determined. Most notably, in situ single-crystal diffraction studies of gas-loaded materials have revealed that Fe-doping has a significant impact on the molecular details for CO₂ binding in the pore, with the bridging M–OH hydroxyl groups being preferred binding sites for CO₂ within these framework materials. In situ synchrotron IR spectroscopic measurements on CO₂ binding with respect to the −OH groups in the pore are consistent with the above structural analyses. In addition, we found that, compared to MFM-300(Ga₂), Fe-doped MFM-300(Ga_1.87Fe_0.13) shows improved catalytic properties for the ring-opening reaction of styrene oxide, but similar activity for the room-temperature acetylation of benzaldehyde by methanol. The role of Fe-doping in these systems is discussed as a mechanism for enhancing porosity and the structural integrity of the parent material.

The key requirement for a portable store of natural gas is to maximize the amount of gas within the smallest possible space. The packing of methane (CH₄) in a given storage medium at the highest possible density is, therefore, a highly desirable but challenging target. We report a microporous hydroxyl-decorated material, MFM-300(In) (MFM = Manchester Framework Material, replacing the NOTT designation), which displays a high volumetric uptake of 202 v/v at 298 K and 35 bar for CH₄ and 488 v/v at 77 K and 20 bar for H₂. Direct observation and quantification of the location, binding, and rotational modes of adsorbed CH₄ and H₂ molecules within this host have been achieved, using neutron diffraction and inelastic neutron scattering experiments, coupled with density functional theory (DFT) modeling. These complementary techniques reveal a very efficient packing of H₂ and CH₄ molecules within MFM-300(In), reminiscent of the condensed gas in pure component crystalline solids. We also report here, for the first time, the experimental observation of a direct binding interaction between adsorbed CH₄ molecules and the hydroxyl groups within the pore of a material. This is different from the arrangement found in CH₄/water clathrates, the CH₄ store of nature.