Guest Programmable Multistep Spin Crossover in a Porous 2‑D Hofmann-Type Material

The spin crossover (SCO) phenomenon defines an elegant class of switchable materials that can show cooperative transitions when long-range elastic interactions are present. Such materials can show multistepped transitions, targeted both fundamentally and for expanded data storage applications, when antagonistic interactions (i.e., competing ferro- and antiferro-elastic interactions) drive concerted lattice distortions. To this end, a new SCO framework scaffold, [Fe^II(bztrz)₂(Pd^II(CN)₄)]·<i>n</i>(guest) (bztrz = (<i>E</i>)-1-phenyl-<i>N</i>-(1,2,4-triazol-4-yl)­methanimine, <b>1·</b><i><b>n</b></i><b>(guest)</b>), has been prepared that supports a variety of antagonistic solid state interactions alongside a distinct dual guest pore system. In this 2-D Hofmann-type material we find that inbuilt competition between ferro- and antiferro-elastic interactions provides a SCO behavior that is intrinsically frustrated. This frustration is harnessed by guest exchange to yield a very broad array of spin transition characters in the one framework lattice (one- (<b>1·(H</b>_<b>2</b><b>O,EtOH)</b>), two- (<b>1·3H</b>_<b>2</b><b>O</b>) and three-stepped (<b>1·∼2H</b>_<b>2</b><b>O</b>) transitions and SCO-deactivation (<b>1</b>)). This variety of behaviors illustrates that the degree of elastic frustration can be manipulated by molecular guests, which suggests that the structural features that contribute to multistep switching may be more subtle than previously anticipated

A Joint Experimental/Computational Exploration of the Dynamics of Confined Water/Zr-Based MOFs Systems

A joint modeling (molecular dynamics simulations)/​experimental (broadband dielectric spectroscopy) approach was conducted to investigate the water adsorption in the UiO-66­(Zr) MOF, and its functionalized versions bearing acidic polar groups (−COOH or 2-COOH per linker). It was first pointed out that the proton conduction measured at room temperature increases with (i) the water uptake and (ii) the concentration of the free acidic carboxylic functions. This trend was further analyzed in light of the preferential arrangements of water within the pores of each MOF as elucidated by molecular dynamics simulations. Indeed, it was revealed that the guest molecules preferentially (i) form interconnected clusters within the UiO-66­(Zr)­s cages and generate a H-bond network responsible for the proton propagation and (ii) strongly interact with the −COOH grafted functions, resulting in the creation of additional charge carriers in the case of the hydrated functionalized solids. Broadband dielectric spectroscopy shed light on how these water configurations impact the local dynamics of both the water molecules and the MOF frameworks. The dielectric relaxation investigation evidenced the existence of one or two relaxation processes, depending on the nature of the UiO-66­(Zr) framework and its hydration level. Compared to the dielectric behavior of water confined in a large variety of media, it was thus concluded that the fastest process corresponds to the dynamics of the water molecules forming clusters, while the slowest process is due to the concerted local motion of water/ligand entities

Guest Programmable Multistep Spin Crossover in a Porous 2‑D Hofmann-Type Material

The spin crossover (SCO) phenomenon defines an elegant class of switchable materials that can show cooperative transitions when long-range elastic interactions are present. Such materials can show multistepped transitions, targeted both fundamentally and for expanded data storage applications, when antagonistic interactions (i.e., competing ferro- and antiferro-elastic interactions) drive concerted lattice distortions. To this end, a new SCO framework scaffold, [Fe^II(bztrz)₂(Pd^II(CN)₄)]·<i>n</i>(guest) (bztrz = (<i>E</i>)-1-phenyl-<i>N</i>-(1,2,4-triazol-4-yl)­methanimine, <b>1·</b><i><b>n</b></i><b>(guest)</b>), has been prepared that supports a variety of antagonistic solid state interactions alongside a distinct dual guest pore system. In this 2-D Hofmann-type material we find that inbuilt competition between ferro- and antiferro-elastic interactions provides a SCO behavior that is intrinsically frustrated. This frustration is harnessed by guest exchange to yield a very broad array of spin transition characters in the one framework lattice (one- (<b>1·(H</b>_<b>2</b><b>O,EtOH)</b>), two- (<b>1·3H</b>_<b>2</b><b>O</b>) and three-stepped (<b>1·∼2H</b>_<b>2</b><b>O</b>) transitions and SCO-deactivation (<b>1</b>)). This variety of behaviors illustrates that the degree of elastic frustration can be manipulated by molecular guests, which suggests that the structural features that contribute to multistep switching may be more subtle than previously anticipated

In Situ Energy-Dispersive X‑ray Diffraction for the Synthesis Optimization and Scale-up of the Porous Zirconium Terephthalate UiO-66

The synthesis optimization and scale-up of the benchmarked microporous zirconium terephthalate UiO-66­(Zr) were investigated by evaluating the impact of several parameters (zirconium precursors, acidic conditions, addition of water, and temperature) over the kinetics of crystallization by time-resolved in situ energy-dispersive X-ray diffraction. Both the addition of hydrochloric acid and water were found to speed up the reaction. The use of the less acidic ZrOCl₂·8H₂O as the precursor seemed to be a suitable alternative to ZrCl₄·<i>x</i>H₂O, avoiding possible reproducibility issues as a consequence of the high hygroscopic character of ZrCl₄. ZrOCl₂·8H₂O allowed the formation of smaller good quality UiO-66­(Zr) submicronic particles, paving the way for their use within the nanotechnology domain, in addition to higher reaction yields, which makes this synthesis route suitable for the preparation of UiO-66­(Zr) at a larger scale. In a final step, UiO-66­(Zr) was prepared using conventional reflux conditions at the 0.5 kg scale, leading to a rather high space-time yield of 490 kg m^–3 day^–1, while keeping physicochemical properties similar to those obtained from smaller scale solvothermally prepared batches

How Water Fosters a Remarkable 5-Fold Increase in Low-Pressure CO₂ Uptake within Mesoporous MIL-100(Fe)

The uptake and adsorption enthalpy of carbon dioxide at 0.2 bar have been studied in three different topical porous MOF samples, HKUST-1, UiO-66­(Zr), and MIL-100­(Fe), after having been pre-equilibrated under different relative humidities (3, 10, 20, 40%) of water vapor. If in the case of microporous UiO-66, CO₂ uptake remained similar whatever the relative humidity, and correlations were difficult for microporous HKUST-1 due to its relative instability toward water vapor. In the case of MIL-100­(Fe), a remarkable 5-fold increase in CO₂ uptake was observed with increasing RH, up to 105 mg g^–1 CO₂ at 40% RH, in parallel with a large decrease in enthalpy measured. Cycling measurements show slight differences for the initial three cycles and complete reversibility with further cycles. These results suggest an enhanced solubility of CO₂ in the water-filled mesopores of MIL-100­(Fe)

Toward Understanding the Influence of Ethylbenzene in <i>p</i>-Xylene Selectivity of the Porous Titanium Amino Terephthalate MIL-125(Ti): Adsorption Equilibrium and Separation of Xylene Isomers

The potential of the porous crystalline titanium dicarboxylate MIL-125­(Ti) in powder form was studied for the separation in liquid phase of xylene isomers and ethylbenzene (MIL stands for Materials from Institut Lavoisier). We report here a detailed experimental study consisting of binary and multi-component adsorption equilibrium of xylene isomers in MIL-125­(Ti) powder at low (≤0.8 M) and bulk (≥0.8 M) concentrations. A series of multi-component breakthrough experiments was first performed using <i>n</i>-heptane as the eluent at 313 K, and the obtained selectivities were compared, followed by binary breakthrough experiments to determine the adsorption isotherms at 313 K, using <i>n</i>-heptane as the eluent. MIL-125­(Ti) is a <i>para</i>-selective material suitable at low concentrations to separate <i>p</i>-xylene from the other xylene isomers. Pulse experiments indicate a separation factor of 1.3 for <i>p</i>-xylene over <i>o</i>-xylene and <i>m</i>-xylene, while breakthrough experiments using a diluted ternary mixture lead to selectivity values of 1.5 and 1.6 for <i>p</i>-xylene over <i>m</i>-xylene and <i>o</i>-xylene, respectively. Introduction of ethylbenzene in the mixture results however in a decrease of the selectivity

Exploiting Pressure To Induce a “Guest-Blocked” Spin Transition in a Framework Material

A new functionalized 1,2,4-triazole ligand, 4-[(<i>E</i>)-2-(5-methyl-2-thienyl)­vinyl]-1,2,4-triazole (thiome), was prepared to assess the broad applicability of strategically producing multistep spin transitions in two-dimensional Hofmann-type materials of the type [Fe^IIPd­(CN)₄(R-1,2,4-trz)₂]·<i>n</i>H₂O (R-1,2,4-trz = a 4-functionalized 1,2,4-triazole ligand). A variety of structural and magnetic investigations on the resultant framework material [Fe^IIPd­(CN)₄(thiome)₂]·2H₂O (<b>A·2H</b>_<b>2</b><b>O</b>) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in <b>A·2H</b>_<b>2</b><b>O</b> due to a combination of guest and ligand steric bulk effects. The water molecules can be reversibly removed with retention of the porous host framework and result in the emergence of an abrupt and hysteretic one-step spin transition due to the removal of guest internal pressure. A spin transition can, furthermore, be induced in <b>A·2H</b>_<b>2</b><b>O</b> (0–0.68 GPa) under hydrostatic pressure, as evidenced by variable-pressure structure and magnetic studies, resulting in a two-step spin transition at ambient temperatures at 0.68 GPa. The presence of a two-step spin crossover (SCO) in <b>A·2H</b>_<b>2</b><b>O</b> under hydrostatic pressure compared to a one-step SCO in <b>A</b> at ambient pressure is discussed in terms of the relative ability of each phase to accommodate mixed HS/LS states according to differing lattice flexibilities

<i>p</i>-Xylene-Selective Metal–Organic Frameworks: A Case of Topology-Directed Selectivity

Para-disubstituted alkylaromatics such as <i>p</i>-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal–organic frameworks: MIL-125(Ti) ([Ti₈O₈(OH)₄(BDC)₆]), MIL-125(Ti)-NH₂ ([Ti₈O₈(OH)₄(BDC-NH₂)₆]), and CAU-1(Al)-NH₂ ([Al₈(OH)₄(OCH₃)₈(BDC-NH₂)₆]) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity