Scott C. McKellar

Structural studies of metal–organic frameworks under high pressure

Over the last 10 years or so, the interest and number of high-pressure studies has increased substantially. One area of growth within this niche field is in the study of metal-organic frameworks (MOFs or coordination polymers). Here we present a review on the subject, where we look at the structural effects of both non-porous and porous MOFs, and discuss their mechanical and chemical response to elevated pressures.

Stabilization of Scandium Terephthalate MOFs against Reversible Amorphization and Structural Phase Transition by Guest Uptake at Extreme Pressure

Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally nonporous materials. Here we report a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC = 1,4-benzenedicarboxylate) and its nitro-functionalized derivative (Sc2(NO2-BDC)3) and compare it to direct compression behavior in a nonpenetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitro-form are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations, and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against nonpenetrative hydrostatic fluids at high pressure, this study demonstrates a novel high-pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favorable adsorption sites.

The effect of pressure on the post-synthetic modification of a nanoporous metal-organic framework.

Here we report four post-synthetic modifications, including the first ever example of a high pressure-induced post-synthetic modification, of a porous copper-based metal-organic framework. Ligand exchange with a water ligand at the axial metal site occurs with methanol, acetonitrile, methylamine and ethylamine within a single-crystal and without the need to expose a free metal site prior to modification, resulting in significant changes in the pore size, shape and functionality. Pressure experiments carried out using isopropylalcohol and acetaldehyde, however, results in no ligand exchange. By using these solvents as hydrostatic media for high-pressure single-crystal X-ray diffraction experiments, we have investigated the effect of ligand exchange on the stability and compressibility of the framework and demonstrate that post-synthetic ligand exchange is very sensitive to both the molecular size and functionality of the exchanged ligand. We also demonstrate the ability to force hydrophilic molecules into hydrophobic pores using high pressures which results in a pressure-induced chemical decomposition of the Cu-framework.

Perfluorocarbon liquid under pressure: a medium for gas delivery

A novel method for CO2 delivery to a porous material is reported, wherein a perfluorocarbon containing dissolved CO2 has been used as a pressure-transmitting liquid in a high-pressure single-crystal X-ray diffraction experiment. Pressure causes the gas to be squeezed out of the liquid into the host crystal, monitored via a single-crystal to single-crystal phase transition on uptake of CO2.

A high-pressure crystallographic and magnetic study of Na5[Mn(L-tart)2]·12H2O (L-tart = L-tartrate)

The crystal structure and magnetic properties of the compound Na5[Mn(l-tart)2]·12H2O (1, l-tart = l-tartrate) have been investigated over the pressure range 0.34-3.49 GPa. The bulk modulus of 1 has been determined as 23.9(6) GPa, with a compression of the coordination spheres around the Na(+) ions observed. 1 is therefore relatively incompressible, helping it to retain its magnetic anisotropy under pressure.

Pressure-Tuning of Guest Species and Magnetic Response in Mn- and Fe-based Formate Frameworks

The synthesis of a microporous solid that behaves as a magnet at room temperature remains an open challenge. The combination of magnetism and porosity has recently become a hot topic, as reflected in an increasing number of studies.1 Porosity provides a means for tuning magnetic properties in functional materials where the long range ordering temperature (ferroor antiferromagnetic) can be varied depending on the metal-ligand exchange and guest content. Metal formates ([M3(HCOO)6](G); M(II) = Mn, Fe, Co, Ni, G = guest) are a family of metal-organic frameworks (MOFs) which display permanent porosity and guest-dependent magnetic properties.2 For example, exchange between various small organic guest molecules causes the critical ordering temperature (Tc) to vary between 4.8 and 9.7 K for [Mn3(HCOO)6] and 15.6 to 20.7 K for [Fe3(HCOO)6]. The ability to enhance Tc is desirable in order for materials to be applicable in real-world applications. One approach to structure and magnetic property modification is through the application of high pressure. Here, in the first study of a porous magnetic material to resolve both structural and magnetic changes, we have performed a combined high-pressure single-crystal X-ray diffraction (XRD) and high-pressure SQUID magnetometry investigation of [Mn3(HCOO)6] and [Fe3(HCOO)6]. Upon application of pressures <20 kbar to [Mn3(HCOO)6] and [Fe3(HCOO)6], we have observed large changes around the metal coordination spheres, which cause an increase in Tc, dependent on the guest species. By using different solvents (methanol/water, tetrahydrofuran, pentane/isopentane, Fluorinert FC70) as pressure-transmitting liquids, the structural and magnetic response can be correlated not only to the type of metal but to the nature and quantity of guest present in the framework pores. The variation in Tc is caused in part by twisting of the M-O-M angles of the host framework: materials with larger guests induce larger M-O-M angles and a lower Tc under ambient conditions, but this changes with the application of pressure, depending on the pressure-transmitting liquid. Since metal-metal or metal-ligand distances or the bridging group geometry are all sensitive to pressure, this is an extremely effective method for investigating magneto-structural correlations. [1]Kurmoo(2009)Chem. Soc. Rev.,38,1353. [2]Wang et al.(2007)Polyhedron,26,2207

A combined high-pressure diffraction and computational study on scandium MOFs

Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally non-porous materials.1 Here we present a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC=1,4benzenedicarboxylate)2 and its nitro-functionalized derivative (Sc2(NO2-BDC)3)3 and compare it to direct compression behaviour in a non-penetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically-identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitroform are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against non-penetrative hydrostatic fluids at high pressure this study demonstrates a novel high pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favourable adsorption sites.

In-situ Gas Adsorption SC-XRD Study: Understanding Gas Uptake in a Sc-based MOF

In recent years the development of new methods of storing, trapping or separating light gases, such as CO2, CH4 and CO has become of utmost importance from an environmental and energetic point of view. Porous materials such as zeolites and porous organic polymers have long been considered good candidates for this purpose. More recently, the ample spectrum of existing metal organic frameworks (MOFs) together with their functional and mechanical properties have attracted even further interest. The porous channels found in these materials are ideal for the uptake of guests of different shapes and sizes, and with careful design they can show high selectivity. Adsorption properties of MOFs have been thoroughly studied, however obtaining in depth structural insight into the adsorption/desorption mechanism of these materials is challenging. For example, out of the hundreds of MOF structures published to date, there are less than 20 entries currently in the CSD in which the CO2 molecule can be located. Here we present our novel findings using the high-pressure gas cell at the Diamond Light Source on beamline I19, where we have studied the inclusion of CO2, CH4 and CO on the microporous scandium framework, Sc2BDC3 (BDC = benzene-1,4-dicarboxylate) and its amino-functionalised derivative, Sc2(BDC-NH2)3. Here, the different adsorption sites for CO2, CH4 and CO in both frameworks have been determined as a function of increasing gas pressure. These structures, coupled with Density Functional Theory calculations, have helped to elucidate the host-guest interactions governing the different levels of selectivity shown by both Sc2BDC3 and Sc2(BDC-NH2)3. Additionally, gas mixtures have also been studied; in particular CO2/CH4 mixtures of different compositions, explaining the selectivity of the frameworks for CO2 over other gases and showing the great potential of in situ structural experiments for investigation of the potential applications of MOFs.

The effect of solvent and pressure on the post-synthetic modification of a metal-organic framework

Porous metal-organic frameworks (MOFs) have an array of potential applications including gas storage, separation processes and catalysis. As such, hundreds of MOF-themed research papers are now published annually[1], with many reporting synthetic approaches to making more sophisticated, novel frameworks. Recently, this has led to strong interest in the concept of post-synthetic modification (PSM).[2] This has proven to be a very elegant technique in which to modify MOFs after they have been synthesised, since it offers the potential to tune the pore size, topology and functionality while conserving the integrity of the structure, and is an attractive route for covalent modification that is unachievable by established synthetic routes. To date, two main approaches have been used for PSM of MOFs. In the first method, the organic linker is covalently modified by introducing new functional groups. The second method involves first exposing a free site on the metal, usually by removing a ligand. Here we have investigated the PSM behaviour of the porous MOF, STAM¬1 (St Andrews MOF1).[3] We report four new phases of the material, including the first ever example of a pressureinduced PSM. STAM-1 is comprised of copper ‘paddle-wheels’ linked by monomethyl-esterified benzene-1,3,5-tricarboxylic acid ligands, with water molecules axially coordinated on the CuII ions. The ester and water groups yield a framework containing both hydrophobic and hydrophilic channels, respectively. Here we show how singlestep PSM is possible via ligand exchange at the axial metal coordination site with a number of organic solvents, resulting in significant changes in the pore size and functionality. Specifically, hydrophilic channels in native STAM-1 can be converted into discrete hydrophobic pores. A range of organic solvents have also been used as hydrostatic media for high-pressure single-crystal X-ray diffraction experiments. This has allowed us to induce ligand exchange using pressure and investigate the effect of ligand exchange on the stability and compressibility of the framework, demonstrating that post-synthetic ligand exchange is very sensitive to both the molecular size and functionality of the solvent. This work is, to the best of our knowledge, the most extensive study conducted on the importance of hydrostatic media selection for the investigation of porous materials under pressure. We also demonstrate the ability to force hydrophilic molecules into hydrophobic pores using high pressures, and present the first example of a pressure-induced chemical decomposition of a porous material.