Ivan da Silva

Unravelling exceptional acetylene and carbon dioxide adsorption within a tetra-amide functionalized metal-organic framework

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.

Observation of Binding and Rotation of Methane and Hydrogen within a Functional Metal–Organic Framework

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.

Amides Do Not Always Work: Observation of Guest Binding in an Amide-Functionalized Porous Metal–Organic Framework

An amide-functionalized metal organic framework (MOF) material, MFM-136, shows a high CO2 uptake of 12.6 mmol g–1 at 20 bar and 298 K. MFM-136 is the first example of an acylamide pyrimidyl isophthalate MOF without open metal sites and, thus, provides a unique platform to study guest binding, particularly the role of free amides. Neutron diffraction reveals that, surprisingly, there is no direct binding between the adsorbed CO2/CH4 molecules and the pendant amide group in the pore. This observation has been confirmed unambiguously by inelastic neutron spectroscopy. This suggests that introduction of functional groups solely may not necessarily induce specific guest–host binding in porous materials, but it is a combination of pore size, geometry, and functional group that leads to enhanced gas adsorption properties.

Modulating Supramolecular Binding of Carbon Dioxide in a Redox-Active Porous Metal-Organic Framework

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(VIII) {[VIII2(OH)2(L)], LH4=biphenyl-3,3′,5,5′-tetracarboxylic acid} can be oxidized to isostructural MFM-300(VIV), [VIV2O2(L)], in which deprotonation of the bridging hydroxyl groups occurs. MFM-300(VIII) 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, =1.863(1) Å. In contrast, CO2-loaded MFM-300(VIV) shows CO2 bound side-on to the oxy group and sandwiched between two phenyl groups involving a unique ···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.

Confinement of Iodine Molecules into Triple-Helical Chains within Robust Metal–Organic Frameworks

During nuclear waste disposal process, radioactive iodine as a fission product can be released. The widespread implementation of sustainable nuclear energy thus requires the development of efficient iodine stores that have simultaneously high capacity, stability and more importantly, storage density (and hence minimized system volume). Here, we report high I2 adsorption in a series of robust porous metal–organic materials, MFM-300(M) (M = Al, Sc, Fe, In). MFM-300(Sc) exhibits fully reversible I2 uptake of 1.54 g g–1, and its structure remains completely unperturbed upon inclusion/removal of I2. Direct observation and quantification of the adsorption, binding domains and dynamics of guest I2 molecules within these hosts have been achieved using XPS, TGA-MS, high resolution synchrotron X-ray diffraction, pair distribution function analysis, Raman, terahertz and neutron spectroscopy, coupled with density functional theory modeling. These complementary techniques reveal a comprehensive understanding of the host–I2 and I2–I2 binding interactions at a molecular level. The initial binding site of I2 in MFM-300(Sc), I2I, is located near the bridging hydroxyl group of the [ScO4(OH)2] moiety [I2I···H–O = 2.263(9) Å] with an occupancy of 0.268. I2II is located interstitially between two phenyl rings of neighboring ligand molecules [I2II···phenyl ring = 3.378(9) and 4.228(5) Å]. I2II is 4.565(2) Å from the hydroxyl group with an occupancy of 0.208. Significantly, at high I2 loading an unprecedented self-aggregation of I2 molecules into triple-helical chains within the confined nanovoids has been observed at crystallographic resolution, leading to a highly efficient packing of I2 molecules with an exceptional I2 storage density of 3.08 g cm–3 in MFM-300(Sc).

New water soluble Pd-imidate complexes as highly efficient catalysts for the synthesis of C5-arylated pyrimidine nucleosides

The direct reactions between the precursors trans-[Pd(imidate)2(SMe2)2] and 1,3,5-triaza-7-phosphaadamantane (PTA) yield new water-soluble palladium(II) complexes trans-[Pd(imidate)2(PTA)2](imidate = succinimidate (suc) 1, maleimidate (mal) 2, phthalimidate (phthal) 3 or saccharinate (sacc) 4. The new complexes were revealed as excellent catalysts for environmentally friendly, efficient Suzuki–Miyaura cross-coupling of synthetically challenging substrates like the antiviral nucleoside analogue 5-iodo-2′-deoxyuridine in water as solvent.

Porous Metal–Organic Polyhedral Frameworks with Optimal Molecular Dynamics and Pore Geometry for Methane Storage

Natural gas (methane, CH4) is widely considered as a promising energy carrier for mobile applications. Maximizing the storage capacity is the primary goal for the design of future storage media. Here we report the CH4 storage properties in a family of isostructural (3,24)-connected porous materials, MFM-112a, MFM-115a, and MFM-132a, with different linker backbone functionalization. Both MFM-112a and MFM-115a show excellent CH4 uptakes of 236 and 256 cm3 (STP) cm–3 (v/v) at 80 bar and room temperature, respectively. Significantly, MFM-115a displays an exceptionally high deliverable CH4 capacity of 208 v/v between 5 and 80 bar at room temperature, making it among the best performing metal–organic frameworks for CH4 storage. We also synthesized the partially deuterated versions of the above materials and applied solid-state 2H NMR spectroscopy to show that these three frameworks contain molecular rotors that exhibit motion in fast, medium, and slow regimes, respectively. In situ neutron powder diffraction studies on the binding sites for CD4 within MFM-132a and MFM-115a reveal that the primary binding site is located within the small pocket enclosed by the [(Cu2)3(isophthalate)3] window and three anthracene/phenyl panels. The open Cu(II) sites are the secondary/tertiary adsorption sites in these structures. Thus, we obtained direct experimental evidence showing that a tight cavity can generate a stronger binding affinity to gas molecules than open metal sites. Solid-state 2H NMR spectroscopy and neutron diffraction studies reveal that it is the combination of optimal molecular dynamics, pore geometry and size, and favorable binding sites that leads to the exceptional and different methane uptakes in these materials.

Environmental influence on Zn-histidine complexes under no-packing conditions

This paper describes a combined structural analysis of the Zn-histidine complex, using two different and complementary techniques, X-ray absorption spectroscopy (XAS) and surface X-ray diffraction, paying special attention to the environmental conditions. The current procedure for investigating macromolecules consists of examining simple molecules that exhibit properties similar to those of the larger ones, whose functionality is totally related to the atomic structure. The detailed study of the bonding structure formed by zinc and histidine amino acids is motivated by the fact that this material serves as a model for metalloproteins, such as in metalloproteinase, acting as active sites in enzymatic or structural functions. For XAS modeling, Zn-histidine complexes were dissolved in several aqueous solutions, over a wide pH range. Correlations among the degree of protonation, the steric impediment, and the multiple combinations of the histidine amino acid have been found. For the diffraction study, high-quality crystals grown by the seeding method in a supersaturated solution have been studied, and the samples for the surface diffraction study were mounted on a cell specially designed for solid-liquid or solid-gas interface analysis. The surface structural model was built from XAS results. In both cases, the obtained structures are compared with the bulk one, showing atomic differences and highlighting the importance of the environment in which the complex is studied.

Synthetically tuned structural variations in CePdxGe2−x (x = 0.21, 0.32, 0.69) towards diverse physical properties

In this work, we have studied the structure and physical properties of a series of intermetallic compounds with the general formula CePdxGe2−x (where, x = 0.21, 0.32, 0.69). It was found that the compound crystallizes in three different phases with stoichiometries: CePd0.32Ge1.68, CePd0.21Ge1.79 and CePd0.69Ge1.31 by varying the Pd to Ge ratio. While CePd0.32Ge1.68 and CePd0.69Ge1.31 crystallize in the hexagonal AlB2 structure type with the space group P6/mmm, CePd0.21Ge1.79 crystallizes in the tetragonal α-ThSi2 structure type with the space group I41/amd. CePd0.69Ge1.31 is in fact an ordered superstructure of CePd0.32Ge1.68 with tripling of the c-lattice. Relative changes in the Pd/Ge ratio also impart substantial variation in their magnetic properties, although Ce is in the trivalent state in both the phases. CePd0.21Ge1.79 shows metamagnetic behavior below 10 K whereas CePd0.69Ge1.31 showed ferromagnetic behavior in the same temperature range. In addition to the metamagnetic behavior, CePd0.21Ge1.79 also shows spin glass behavior at low temperature. DFT calculations were used to obtain ulterior information on the CePd0.69Ge1.31 phase. Self-consistent calculations revealed that the ferromagnetic ordering of the ground state arises from the spins at the Ce and Pd sites. The observed sharp rise in the low temperature resistivity of CePd0.69Ge1.31 is an indication of a pseudo-gap formation or possible Kondo behavior in the electronic density of states, enhancing the scattering of the charge carriers. Heat capacity measurements on CePd0.69Ge1.31 suggest that it falls in the category of medium heavy fermion compounds.

Ligand hierarchy on driving the crystal packing. Effect of supramolecular interactions on solid-state conformations adopted by saccharinate Pd(II) complexes

The potential of saccharinate as a supramolecular organizer in selected palladium complexes has been thoroughly evaluated from the perspective of those relevant interactions operating in organic salts and co-crystals in which saccharinate is involved. With this purpose, molecular and supramolecular structures of a series of bis-saccharinate complexes with the general formula trans-[Pd(sac)2(L)2] [L = pyridine (1), pyridazine (2), PPh3 (3), SMe2 (4) or nicotinamide (6)] containing co-ligands with different polarities and abilities to form intermolecular bonds have been studied by X-ray diffraction. These X-ray diffraction data were either previously reported (1 and 3), or obtained from newly determined crystal structures (2, 4 and 6). The new crystal structures of closely related complexes [Pd(sac)2(SMe2)(OH2)]·dmso (5) and [Pd(succinimide)2(nicot)2] (7) have also been elucidated and provide an additional structural discussion. We have disclosed that the anti-configuration is preferred for saccharinate ligands and that the CO group defines the most important supramolecular interactions in the crystal packing of 1, 2 and 4. The saccharinate ligand drives crystal packing when pyridine (1), pyridazine (2), PPh3 (3), or SMe2 (4) are the co-ligands, but the crystal packing orienting role relies on water (5) or nicotinamide (6 and 7) when these ligands are present. These results have been compared with other previously reported examples. Molecular geometries have been optimized by DFT, with the interesting output of the energetic preference by the syn-configuration. Supramolecular interactions distort conformations from those optimized ones so that complexes with stronger interacting ligands display larger distortions.