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Browsing by Author "Kepert, CJ"

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Continuous negative-to-positive tuning of thermal expansion achieved by controlled gas sorption in porous coordination frameworks
(Springer Nature, 2018-11-19) Auckett, JE; Barkhordarian, AA; Ogilvie, SH; Duyker, SG; Chevreau, H; Peterson, VK; Kepert, CJ
Control of the thermomechanical properties of functional materials is of great fundamental and technological significance, with the achievement of zero or negative thermal expansion behavior being a key goal for various applications. A dynamic, reversible mode of control is demonstrated for the first time in two Prussian blue derivative frameworks whose coefficients of thermal expansion are tuned continuously from negative to positive values by varying the concentration of adsorbed CO2. A simple empirical model that captures site-specific guest contributions to the framework expansion is derived, and displays excellent agreement with the observed lattice behaviour. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License.
Coordination frameworks: host-guest chemistry and structural dynamics
(Australian Institute of Nuclear Science and Engineering (AINSE), 2012-11-07) Ogilvie, SH; Duyker, SG; Peterson, VK; Kepert, CJ
Coordination frameworks employ metal ions possessing well defined coordination geometries and an extensive range of molecular bridging ligands with a vast array of functional groups to produce microporous materials with a range of interesting and useful properties. My PhD research is focused towards characterising the structural behaviour of three of these properties in metal-cyanide and metal-imidazolate materials: 1) gas adsorption; 2) metal insertion and; 3) anomalous thermal expansion. Neutron diffraction and scattering are central to all three areas and provide essential information that cannot be readily obtained from other techniques. This is largely due to their sensitivity to light atoms, important for determining the location of light atoms (e.g. CO2 and Li+ ions); their highly penetrating nature, allowing the use of highly specialised sample environments; and their inelastic scattering to provide information on host-guest binding energetics. Gas Adsorption: The primary goal is to elucidate the packing and ordering behaviours of carbon dioxide. These frameworks contain a variety of functional groups which have a known affinity for interaction with CO2, making them suitable for the selective separation of gas mixtures commonly found as flue gas streams of combustion power sources. Metal insertion: The goal is to develop structural understandings of the redox-insertion of lithium and sodium into metal-cyanide phases for the development of new battery electrode materials. Recent work from our group has shown very high reversible loadings of Li into these materials, with in-situ NPD at OPAL used to determine the structures during insertion. Anomalous thermal expansion: Our group has previously investigated the anomalous thermal expansion behaviour in a range of coordination frameworks. Using both NPD and INS, the goal of this project is to develop an even greater understanding of guest influence on these temperature dependent structural behaviours.
Dual-supramolecular contacts induce extreme Hofmann framework distortion and multi-stepped spin-crossover
(Royal Society of Chemistry, 2021-01-13) Ahmed, M; Brand, HEA; Peterson, VK; Clegg, JK; Kepert, CJ; Price, JR; Powell, BJ; Neville, SM
An extended nitro-functionalised 1,2,4-triazole ligand has been used to induce considerable lattice distortion in a 2-D Hofmann framework material via competing supramolecular interactions. Single crystal X-ray diffraction analyses on [Fe3(N-cintrz)6(Pd(CN)4)3]·6H2O (N-cintrz: (E)-3-(2-nitrophenyl)acrylaldehyde) reveal a substantial deviation from a regular Hofmann structure, in particular as the intra- and inter-layer contacts are dominated by hydrogen-bonding interactions rather than the typical π-stacking arrays. Also, the 2-D Hofmann layers show an assortment of ligand conformations and local FeII coordination environments driven by the optimisation of competing supramolecular contacts. Temperature-dependent magnetic susceptibility measurements reveal a two-step spin crossover (SCO) transition. Variable temperature structural analyses show that the two crystallographically distinct FeII centres, which are arranged in stripes (2[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio) within each Hofmann layer, undergo a cooperative HS ↔ HS/LS ↔ LS (HS = high spin, LS = low spin) transition without periodic spin-state ordering. The mismatch between crystallographic (2[thin space (1/6-em)]:[thin space (1/6-em)]1) and spin-state (1[thin space (1/6-em)]:[thin space (1/6-em)]1) periodicity at the HS[thin space (1/6-em)]:[thin space (1/6-em)]LS step provides key insight into the competition (frustration) between elastic interactions and crystallographically driven order. © The Royal Society of Chemistry 2021
Elucidating negative thermal expansion in MOF-5
(American Chemical Society, 2010-9-09) Lock, N; Wu, Y; Christensen, M; Cameron, L; Peterson, VK; Bridgeman, AJ; Kepert, CJ; Iversen, BB
Multi-temperature X-ray diffraction studies show that twisting, rotation, and libration cause negative thermal expansion (NTE) of the nanoporous metal−organic framework MOF-5, Zn4O(1,4-benzenedicarboxylate)3. The near-linear lattice contraction is quantified in the temperature range 80−500 K using synchrotron powder X-ray diffraction. Vibrational motions causing the abnormal expansion behavior are evidenced by shortening of certain interatomic distances with increasing temperature according to single-crystal X-ray diffraction on a guest-free crystal over a broad temperature range. Detailed analysis of the atomic positional and displacement parameters suggests two contributions to cause the effect: (1) local twisting and vibrational motion of the carboxylate groups and (2) concerted transverse vibration of the linear linkers. The vibrational mechanism is confirmed by calculations of the dynamics in a molecular fragment of the framework. © 2010, American Chemical Society
Guest adsorption in the nanoporous metal–organic framework Cu3(1,3,5-benzenetricarboxylate)2: combined in situ x-ray diffraction and vapor sorption
(American Chemical Society, 2014-07-23) Peterson, VK; Southon, PD; Halder, GJ; Price, DJ; Bevitt, JJ; Kepert, CJ
The structure of the nanoporous metal–organic framework Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) with a variety of molecular guests was studied in situ using single crystal X-ray diffraction. By collecting crystal structure data for a series of guests within the same host crystal, insights into the molecular interactions underpinning guest adsorption processes have been gained. Adsorption behaviors are influenced strongly by both enthalpic and entropic thermodynamic, as well as interpore steric (size-exclusion) effects, and we note correlations between guest attributes and these effects. Vapor adsorption measurements revealed a guest uptake capacity inversely proportional to guest size. Correspondingly, structural results show that guests reside in the smallest pores accessible to them. Interpore steric effects for larger guests cause these to be excluded from the smallest pores, and this corresponds to lower total uptake. Both hydrophilic and lipophilic small guests adsorb favorably into the 5 Å diameter smallest pore of the material, with the number of guests in these pores dependent on guest size and their location, in turn dependent upon both guest–guest interactions and competition between hydrogen-bonding interactions at the apertures of the smallest pore and lipophilic interactions at the center of the smallest pore. Hydrophilic guests with lone electron pairs interact preferentially with the coordinatively unsaturated Cu sites of the desolvated framework, with the number of these depending on steric interactions between neighboring bound guests and guest flexibility. Guest coordination at the Cu sites has a significant effect on the framework structure, increasing the Cu···Cu distance in the dinuclear unit, with the Cu3(BTC)2 unit cell being smaller when guests that do not coordinate with the Cu are present, and in the case of cyclohexane, smaller than for the desolvated framework. Overall, our comprehensive structural study reconciles Cu3(BTC)2 adsorption properties with the underlying guest–host and guest–guest interactions that gives rise to these. © 2014, American Chemical Society.
Guest removal and external pressure variation induce spin crossover in halogen-functionalized 2-D Hofmann frameworks
(American Chemical Society, 2020-09-27) Brennan, AT; Zenere, KA; Brand, HEA; Price, JR; Bhadbhade, MM; Turner, GF; Moggach, SA; Valverde-Muñoz, FJ; Real, JA; Clegg, JK; Kepert, CJ; Neville, SM
The effect of halogen functionalization on the spin crossover (SCO) properties of a family of 2-D Hofmann framework materials, [Fe(II)Pd(CN)4(thioX)2]·2H2O (X = Cl and Br; thioCl = (E)-1-(5-chlorothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine) and thioBr = (E)-1-(5-bromothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine)), is reported. Inclusion of both the chloro- and bromo-functionalized ligands into the Hofmann-type frameworks (1Cl·2H2O and 2Br·2H2O) results in a blocking of spin-state transitions due to internal chemical pressure effects derived by the collective steric bulk of the halogen atoms and guest molecules. Cooperative one-step SCO transitions are revealed by either guest removal or the application of external physical pressure. Notably, removal of solvent water reveals a robust framework scaffold with only marginal variation between the solvated and desolvated structures (as investigated by powder and single crystal X-ray diffraction). Yet, one-step complete SCO transitions are revealed in 1Cl and 2Br with a transition temperature shift between the analogues due to various steric, structural, and electronic considerations. SCO can also be induced in the solvated species, 1Cl·2H2O and 2Br·2H2O, with the application of physical pressure, revealing a complete one-step SCO transition above 0.62 GPa (as investigated by magnetic susceptibility and single crystal X-ray diffraction measurements). © 2020 American Chemical Society
Hierarchical spin-crossover cooperativity in hybrid 1D chains of Fe(II)-1,2,4-triazole trimers linked by [Au(CN)2]− bridges
(John Wiley & Sons, Inc, 2021-02-04) Ezzedinloo, L; Zenere, KA; Xie, Z; Ahmed, M; Scottwell, S; Bhadbhade, MM; Brand, HEA; Clegg, JK; Hua, C; Sciortino, NF; Parker, LC; Powell, BJ; Kepert, CJ; Neville, SM
Foremost, practical applications of spin-crossover (SCO) materials require control of the nature of the spin-state coupling. In existing SCO materials, there is a single, well-defined dimensionality relevant to the switching behavior. A new material, consisting of 1,2,4-triazole-based trimers coordinated into 1D chains by [Au(CN)2]− and spaced by anions and exchangeable guests, underwent SCO defined by elastic coupling across multiple dimensional hierarchies. Detailed structural, vibrational, and theoretical studies conclusively confirmed that intra-trimer coupling was an order of magnitude greater than the intramolecular coupling, which was an order of magnitude greater than intermolecular coupling. As such, a clear hierarchy on the nature of elastic coupling in SCO materials was ascertained for the first time, which is a necessary step for the technological development of molecular switching materials. © 2021 Wiley-VCH GmbH
Host–guest adsorption behavior of deuterated methane and molecular oxygen in a porous rare-earth metal–organic framework
(Cambridge Core, 2014-11-17) Ogilvie, SH; Duyker, SG; Southon, PD; Peterson, VK; Kepert, CJ
The yttrium-based metal–organic framework, Y(btc) (btc = 1,3,5-benzenetricarboxylate), shows moderate uptake of methane (0.623 mmol g−1) and molecular oxygen (0.183 mmol g−1) at 1 bar and 308 K. Neutron powder-diffraction data for the guest-free, CD4-, and O2-loaded framework reveal multiple adsorption sites for each gas. Both molecular guests exhibit interactions with the host framework characterised by distances between the framework and guest atoms that range from 2.83 to 4.81 Å, with these distances identifying interaction most commonly between the guest molecule and the carboxylate functional groups of the benzenetricarboxylate bridging ligand of the host. © 2014, International Centre for Diffraction Data.
Hydration as a trigger for new properties in inorganic materials
(Australian Institute of Nuclear Science and Engineering, 2016-11-29) Duyker, SG; Peterson, VK; Kearley, GJ; Studer, AJ; Kepert, CJ; Hill, JA; Howard, CJ; Goodwin, AL
The humble water molecule binds to metal ions strongly enough that it can have a significant distortive influence on the coordination geometry, yet weakly enough that it can be readily removed, thus providing scope for reversible chemical switching between structural forms. When this principle is applied in 3D coordination frameworks, the unique topological constraints of the framework can lead to new behaviours. Examples from our work will be presented, including anomalous mechanical properties enabled by unsaturated coordination spheres,[1] and new kinds of symmetry breaking transformations triggered by (de)hydration[2] (see figure).
Hydrogen adsorption in HKUST-1: a combined inelastic neutron scattering and first-principles study
(Institute of Physics, 2009-05-20) Brown, CM; Liu, Y; Yildirim, T; Peterson, VK; Kepert, CJ
Hydrogen adsorption in high surface area nanoporous coordination polymers has attracted a great deal of interest in recent years due to the potential applications in energy storage. Here we present combined inelastic neutron scattering measurements and detailed first-principles calculations aimed at unraveling the nature of hydrogen adsorption in HKUST-1 (Cu-3(1,3,5-benzenetricarboxylate)(2)), a metal-organic framework (MOF) with unsaturated metal centers. We reveal that, in this system, the major contribution to the overall binding comes from the classical Coulomb interaction which is not screened due to the open metal site; this explains the relatively high binding energies and short H-2-metal distances observed in MOFs with exposed metal sites as compared to traditional ones. Despite the short distances, there is no indication of an elongation of the H-H bond for the bound H-2 molecule at the metal site. We find that both the phonon and rotational energy levels of the hydrogen molecule are closely similar, making the interpretation of the inelastic neutron scattering data difficult. Finally, we show that the orientation of H-2 has a surprisingly large effect on the binding potential, reducing the classical binding energy by almost 30%. The implication of these results for the development of MOF materials for better hydrogen storage is discussed. © 2009, Institute of Physics
Identification of bridged CO2 binding in a Prussian blue analogue using neutron powder diffraction
(Royal Society of Chemistry, 2013-01-01) Ogilvie, SH; Duyker, SG; Southon, PD; Peterson, VK; Kepert, CJ
Neutron powder diffraction measurements were carried out on the evacuated and CO2-loaded Prussian blue analogue, Fe3[Co(CN)6]2, identifying two distinct CO2 adsorption sites: site A, in which CO2 uniquely bridges between two bare-metal sites, and site B, in which it interacts in a face capping motif. The saturation of site A at low loadings of CO2 demonstrates the favourable nature of the interaction of CO2 with bare-metal sites within the material. © 2013, Royal Society of Chemistry.
Inelastic neutron scattering of H-2 adsorbed in HKUST-1
(Elsevier, 2007-10-31) Liu, Y; Brown, CM; Neumann, DA; Peterson, VK; Kepert, CJ
A series of inelastic neutron scattering (INS) investigations of hydrogen adsorbed in activated HKUST-1 (Cu-3(1,3,5-benzenetricarboxylate)(2)) result in INS spectra with rich features, even at very low loading (< 1.0 H-2:Cu). The distinct inelastic features in the spectra show that there are three binding sites that are progressively populated when the H-2 loading is less than 2.0 H-2:Cu, which is consistent with the result obtained from previous neutron powder diffraction experiments. The temperature dependence of the INS spectra reveals the relative binding enthalpies for H-2 at each site. © 2007, Elsevier Ltd.
Interpenetration as a mechanism for negative thermal expansion in the metal–organic framework Cu3(btb)2 (MOF-14)
(Wiley, 2014-04-01) Wu, Y; Peterson, VK; Luks, E; Darwish, TA; Kepert, CJ
Metal–organic framework materials (MOFs) have recently been shown in some cases to exhibit strong negative thermal expansion (NTE) behavior, while framework interpenetration has been found to reduce NTE in many materials. Using powder and single-crystal diffraction methods we investigate the thermal expansion behavior of interpenetrated Cu3(btb)2 (MOF-14) and find that it exhibits an anomalously large NTE effect. Temperature-dependent structural analysis shows that, contrary to other interpenetrated materials, in MOF-14 the large positive thermal expansion of weak interactions that hold the interpenetrating networks together results in a low-energy contractive distortion of the overall framework structure, demonstrating a new mechanism for NTE. © 2014, WILEY‐VCH Verlag GmbH & Co. KGaA.
Local vibrational mechanism for negative thermal expansion: a combined neutron scattering and first-principles study
(Wiley-VCH Verlag Berlin, 2010-01-12) Peterson, VK; Kearley, GJ; Wu, Y; Ramirez-Cuesta, AJ; Kemner, E; Kepert, CJ
Ask the locals: dynamic deformation of the dicopper tetracarboxylate paddlewheel unit within a metal-organic framework from square-prismatic to distorted occurs at very low energies. This deformation, which contributes strongly to the negative thermal expansion of this system, is a local vibration induced by a redistribution of electron density at the CuO junctions. © 2010, Wiley-VCH Verlag Berlin
Lowering the energetic landscape for negative thermal expansion in 3D-linker metal–organic frameworks
(ACS Publications, 2023-11-30) Chen, C; Maynard-Casley, HE; Duyker, SG; Barbarao, R; Kepert, CJ; Evans, JD; Macreadie, LK
Tuning the coefficient of thermal expansion (CTE) of functional materials is paramount for their practical implementation. The multicomponent nature of metal–organic frameworks (MOFs) offers an opportunity to finely adjust negative thermal expansion (NTE) properties by varying the metal ions and linkers used. We describe a new strategy to adjust the NTE by using organic linkers that include additional rotational degrees of freedom. Specifically, we employ cubane-1,4-dicarboxylate and bicyclo[1.1.1]pentane-1,3-dicarboxylate to form the MOFs CUB-5 and 3DL-MOF-1, respectively, where each linker has low torsional energy barriers. The core of these nonconjugated linkers is decoupled from the carboxylate functionalities, which frees the relative movement of these components. This results in enhanced NTE compared to the analogous, conjugated system; VT-PXRD results were used to calculate the CTE for 3DL-MOF-1 (αL = −13.9(2) × 10–6 K–1), and CUB-5 (αL = −14.7(3) × 10–6 K–1), which is greater than the NTE of MOF-5 (αL = −13.1(1) × 10–6 K–1). These results identify a new route to enhanced NTE behaviors in IRMOF materials influenced by low energy molecular torsion of the linker. © American Chemical Society
Metal-organic frameworks with exceptionally high methane uptake: where and how is methane stored?
(Wiley-VCH Verlag Berlin, 2010-05-03) Wu, H; Simmons, JM; Liu, Y; Brown, CM; Wang, XS; Ma, S; Peterson, VK; Southon, PD; Kepert, CJ; Zhou, HC; Yildirim, T; Zhou, W
Metal–organic frameworks (MOFs) are a novel family of physisorptive materials that have exhibited great promise for methane storage. So far, a detailed understanding of their methane adsorption mechanism is still scarce. Herein, we report a comprehensive mechanistic study of methane storage in three milestone MOF compounds (HKUST-1, PCN-11, and PCN-14) the CH4 storage capacities of which are among the highest reported so far among all porous materials. The three MOFs consist of the same dicopper paddlewheel secondary building units, but contain different organic linkers, leading to cagelike pores with various sizes and geometries. From neutron powder diffraction experiments and accurate data analysis, assisted by grand canonical Monte Carlo (GCMC) simulations and DFT calculations, we anambiguously revealed the exact locations of the stored methane molecules in these MOF materials. We found that methane uptake takes place primarily at two types of strong adsorption site: 1) the open Cu coordination sites, which exhibit enhanced Coulomb attraction toward methane, and 2) the van der Waals potential pocket sites, in which the total dispersive interactions are enhanced due to the molecule being in contact with multiple “surfaces”. Interestingly, the enhanced van der Waals sites are present exclusively in small cages and at the windows to these cages, whereas large cages with relatively flat pore surfaces bind very little methane. Our results suggest that further, rational development of new MOF compounds for methane storage applications should focus on enriching open metal sites, increasing the volume percentage of accessible small cages and channels, and minimizing the fraction of large pores. © 2010, Wiley-VCH Verlag Berlin
Negative thermal expansion in LnCo(CN)6 (Ln=La, Pr, Sm, Ho, Lu, Y): mechanisms and compositional trends
(John Wiley and Sons, 2013-04-09) Duyker, SG; Peterson, VK; Kearley, GJ; Ramirez-Cuesta, AJ; Kepert, CJ
Negative thermal expansion (NTE) is a comparatively rare phenomenon that is found in a growing number of materials.1 The discovery of new NTE materials and the elucidation of mechanisms underpinning their behavior is important both in extending the field and enabling tailored thermal expansion properties. NTE has been found throughout a broad family of cyanide coordination frameworks,2 arising from thermal population of low-energy transverse vibrations of the cyanide bridges, which reduce the average metal–metal distances, and thus the lattice parameters, with increasing temperature. More complex mechanisms have been established in metal–organic framework materials, in which both local and long-range modes contribute to NTE.3 The low-energy dynamics of metal-based materials are often modeled in terms of rigid unit modes (RUMs), wherein the metal-centered polyhedra are treated as rigid, with only the linkage being flexible. © 2013, Wiley-Vch Verlag GmbH & Co.
Negative thermal expansion in the metal-organic framework material Cu-3(1,3,5-benzenetricarboxylate)(2)
(Wiley-VCH Verlag Berlin, 2008-08-08) Wu, Y; Kobayashi, A; Halder, GJ; Peterson, VK; Chapman, KW; Lock, N; Southon, PD; Kepert, CJ
The metal–organic framework [Cu3(btc)2] displays negative thermal expansion (NTE) over a broad temperature range. This property arises from two coincident mechanisms, each of which are unique for NTE systems: the concerted transverse vibration of triangular organic linkers, and the local dynamic distortion of dinuclear metal centers within the framework lattice. © 2008, Wiley-VCH Verlag Berlin
Neutron diffraction and in situ gas-loading investigations of functional MOFs for energy-relevant gas separations
(Australian Institute of Nuclear Science and Engineering (AINSE), 2012-11-08) Duyker, SG; Peterson, VK; Ogilvie, SH; Turner, DR; Hill, MR; D'Alessandro, DM; Kepert, CJ
Intense research is currently directed towards realising metal-organic frameworks (MOFs) for industrially-applied gas separation and storage due to their unique structural properties, including: robustness; thermal and chemical stability; unprecedented internal surface area; and high void volume. A particular focus of current research is the development of MOFs for the separation of CO, from the other components of flue gas in fossil-fuelled power plants. The use of NPD to study gas adsorption in framework materials is a relatively new but growing field. Structural measurements, which show the arrangement of both the host and guest, allow derivation of the nature of the host-guest interaction, and the host's response to the guest. The capability to perform these measurements, with accurate gas dosing and temperature control, has recently been realised at ANSTO's Bragg Institute. Using these techniques, we have investigated the adsorption mechanisms of a number of gases in selected new and established MOFs that display impressive selectivity for specific gases. The location and orientation of industrially-relevant gases including D2, 02, CO2, and CD4, within their crystal structures provide insights into the modes of binding, which will help to tune the materials' performance and benefit the design and development process for the next generation of materials.
Phase diagram, chemical stability and physical properties of the solid-solution Ba(4)Nb(2-x)Ta(x)O(9)
(Academic Press INC Elsevier, 2011-10-01) Dunstan, MT; Southon, PD; Kepert, CJ; Hester, JR; Kimpton, JA; Ling, CD
Through the construction of the Ba4Nb2âˆ’xTaxO9 phase diagram, it was discovered that the unique high-temperature Î³ phase is a thermodynamic intermediate between the low-temperature Î± phase (Sr4Ru2O9-type) and a 6H-perovskite. Refined site occupancies for the Î³ phase across the Ba4Nb2âˆ’xTaxO9 solid-solution indicate that Nb preferentially occupies the tetrahedral sites over the octahedral sites in the structure. When annealed in a CO2-rich atmosphere, all of the phases studied absorb large amounts of CO2 at high temperatures between âˆ¼ 700 and 1300 K. In situ controlled-atmosphere diffraction studies show that this behaviour is linked to the formation of BaCO3 on the surface of the material, accompanied by a Ba5(Nb,Ta)4O15 impurity phase. In situ diffraction in humid atmospheres also confirms that these materials hydrate below âˆ¼ 1273 K , and that this plays a critical role in the various reconstructive phase transitions as well as giving rise to proton conduction.(C) 2011 Elsevier Inc.

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