Guest-Adaptable Spin Crossover Properties in a Dinuclear Species Underpinned by Supramolecular Interactions.

Molecular crystals with guest-adaptable crystalline structures and properties are comparatively rare owing to their inherent reduced structural stability and malleability to support molecular variation. To overcome this intrinsic challenge, here we introduce structural stabilizing supramolecular interactions into a dinuclear material and henceforth demonstrate a dynamic structural and spin crossover property interchange between solvated (A·3MeOH) and desolvated (A·Ø) products (A = [FeII2( o-NTrz)5(NCS)4]; 4-( o-nitrobenzyl)imino-1,2,4-triazole). Relatively uncommon for molecular species, the guest molecules in A·3MeOH are evolved (A·Ø) via a single-crystal to single-crystal transformation with affiliated phase transition resulting in a reversible transformation from one- to two-step spin crossover (SCO) transition character. We additionally present the water-saturated product (A·3H2O), which distinctly shows an abrupt one-step SCO character with a 22 K wide thermal hysteresis loop. Detailed structure-property analysis highlights that the substantial structural malleability and guest-adaptable SCO properties of this dinuclear species are afforded by the supportive, yet flexible, supramolecular interaction pathways derived from the ligand functionalization.

Guest-Responsive Elastic Frustration "On-Off" Switching in Flexible, Two-Dimensional Spin Crossover Frameworks.

In this study we exploit the flexible nature of porous coordination polymers (PCPs) with integrated spin crossover (SCO) properties to manipulate the multistability of spin-state switching profiles. We previously reported the two-dimensional Hofmann-type framework [Fe(thtrz)2Pd(CN)4]·EtOH,H2O (1·EtOH,H2O), N-thiophenylidene-4 H-1,2,4-triazol-4-amine), displaying a distinctive two-step SCO profile driven by extreme elastic frustration. Here, we reveal a reversible release mechanism for this elastic frustration via stepwise guest removal from the parent phase (1·EtOH,H2O → 1·H2O → 1·Ø). Parallel variable temperature structural and magnetic susceptibility measurements reveal a synergistic framework flexing and "on-off" switching of multistep SCO character concomitant with the onset of guest evacuation. In particular, the two-step SCO properties in 1·EtOH,H2O are deactivated such that both the partially solvated (1·H2O) and desolvated (1·Ø) phases show abrupt and hysteretic one-step SCO behaviors with differing transition temperatures (1·H2O: T1/2↓: 215 T1/2↑: 235 K; 1·Ø: T1/2↓: 170 T1/2↑: 182 K). This "on-off" elastic frustration switching is also reflected in the light-induced excited spin state trapping (LIESST) properties of 1·EtOH,H2O and 1·Ø, with nonquantitative (ca. 50%, i.e., LS ↔ 1:1 HS:LS) and quantitative (ca. 100%, LS ↔ HS) photoinduced spin state conversion achieved under light irradiation (510 nm at 10 K), respectively. Conversely, the two-step SCO properties are retained in the water saturated phase 1·3H2O but with a subtle shift in transition temperatures. Comparative analysis of this and related materials reveals the distinct roles that indirect and direct guest interactions play in inducing, stabilizing, and quantifying elastic frustration and the importance of lattice flexibility in these porous framework architectures.

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, [FeII(bztrz)2(PdII(CN)4)]·n(guest) (bztrz = (E)-1-phenyl-N-(1,2,4-triazol-4-yl)methanimine, 1·n(guest)), 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- (1·(H2O,EtOH)), two- (1·3H2O) and three-stepped (1·âˆ¼2H2O) transitions and SCO-deactivation (1)). 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.

Exploiting Pressure To Induce a "Guest-Blocked" Spin Transition in a Framework Material.

A new functionalized 1,2,4-triazole ligand, 4-[(E)-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 [FeIIPd(CN)4(R-1,2,4-trz)2]·nH2O (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 [FeIIPd(CN)4(thiome)2]·2H2O (A·2H2O) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in A·2H2O 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 A·2H2O (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 A·2H2O under hydrostatic pressure compared to a one-step SCO in A 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.

Investigating the Case of Titanium(IV) Carboxyphenolate Photoactive Coordination Polymers.

The reactivity of 2,5-dihydroxyterephthalic acid (H4DOBDC) with titanium(IV) precursors was thoroughly investigated for the synthesis of metal-organic frameworks under solvothermal conditions. Four crystalline phases were isolated whose structures were studied by a combination of single-crystal or powder X-ray diffraction and solid-state NMR. The strong coordination ability of the phenolate moieties was found to favor the formation of isolated TiO6 octahedra bearing solely organic ligands in the resulting structures, unless hydrothermal conditions and precondensed inorganic precursors are used. It is worth noting that these solids strongly absorb visible light, as a consequence of the ligand-to-metal charge transfer (LMCT) arising from Ti-phenolate bonds. Preliminary photocatalytic tests suggest that one compound, namely, MIL-167, presents a higher activity for hydrogen evolution than the titanium carboxylate MIL-125-NH2 but that such an effect cannot be directly correlated with its improved light absorption feature.

Defect engineering of UiO-66 for CO2 and H2O uptake - a combined experimental and simulation study.

Defect concentrations and their compensating groups have been systematically tuned within UiO-66 frameworks by using modified microwave-assisted solvothermal methods. Both of these factors have a pronounced effect on CO2 and H2O adsorption at low and high pressure.

Exploration of the mechanical behavior of metal organic frameworks UiO-66(Zr) and MIL-125(Ti) and their NH2 functionalized versions.

The structural behaviour under mechanical stimuli of two metal organic frameworks, UiO-66(Zr) and MIL-125(Ti) and their amino-functionalized derivatives has been investigated by high-pressure powder X-ray diffraction up to 3.5 GPa. All these solids showed a gradual pressure-induced reversible decrease of their crystallinity and UiO-66(Zr)_NH2 material has been revealed as one of the most resilient MOFs reported so far corresponding to a very high bulk modulus. The mechanical behaviors of these MOFs have been correlated to their chemical and geometric features including the metal-oxygen coordination number, the nature of the organic linker, the porosity as well as their crystal density.

Impact of the Nature of the Organic Spacer on the Crystallization Kinetics of UiO-66(Zr)-Type MOFs.

The influence of the constitutive dicarboxylate linkers (size, functional group) over the crystallization kinetics of a series of porous Zr metal-organic frameworks with the UiO-66 topology has been investigated by in situ time-resolved energy dispersive X-ray diffraction (EDXRD). Both large aromatic spacers (2,6-naphthalene-, 4,4'-biphenyl- and 3,3'-dichloro-4,4'-azobenzene-dicarboxylates) and a series of X-functionalized terephthalates (X=NH2 , NO2 , Br, CH3 ) were investigated in dimethylformamide (DMF) at different temperatures and compared with the parent UiO-66. Using different crystallization models, rate constants and further kinetic parameters (such as activation energy) have been extracted. Finally, the impact of the replacement of the toxic DMF by water on the crystallization kinetics was studied through the synthesis of the functionalized UiO-66-NO2 solid.

Spin crossover intermediate plateau stabilization in a flexible 2-D Hofmann-type coordination polymer.

The abrupt and hysteretic two-step spin crossover in a new triazole-based 2-D Hofmann-type complex shows a record breaking 120 K intermediate plateau (IP) region stabilized by negative cooperative interactions.

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·xH2O, 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.

Cytotoxicity of nanoscaled metal-organic frameworks.

A series of fourteen porous Metal-Organic Frameworks (MOFs) with different compositions (Fe, Zn, and Zr; carboxylates or imidazolates) and structures have been successfully synthesised at the nanoscale and fully characterised by XRPD, FTIR, TGA, N2 porosimetry, TEM, DLS and ζ-potential. Their toxicological assessment was performed using two different cell lines: human epithelial cells from foetal cervical carcinoma (HeLa) and murine macrophage cell line (J774). It appears that MOF nanoparticles (NPs) exhibit low cytotoxicity, comparable to those of other commercialised nanoparticulate systems, the less toxic being the Fe carboxylate and the more toxic being the zinc imidazolate NPs. The cytotoxicity values, higher in J774 cells than in HeLa cells, are mainly function of their composition and cell internalisation capacity. Finally, cell uptake of one of the most relevant Fe-MOF-NPs for drug vectorisation has been investigated by confocal microscopy studies, and indicates a faster kinetics of cell penetration within J774 compared to HeLa cells.

Evidence of photoinduced charge separation in the metal-organic framework MIL-125(Ti)-NH2.

Herein, we describe the photochemical behavior of the porous metal-organic framework MIL-125(Ti)-NH(2), built up from cyclic Ti(8)O(8)(OH)(4) oxoclusters and 2-aminoterephthalate ligands. While MIL-125(Ti)-NH(2) does not emit upon excitation at 420 nm, laser flash photolyses of dry samples (diffuse reflectance) or aqueous suspensions (transmission) of the solid have allowed detecting a transient characterized by a continuous absorption from 390 to 820 nm decaying in the sub-millisecond timescale, which is quenched by oxygen. This transient has been attributed to the charge-separation state. Firm evidence for this assignment was obtained by lamp irradiation of aqueous suspensions of MIL-125(Ti)-NH(2) in the presence of electron-donor (N,N,N'N'-tetramethyl-p-phenylenediamine) or electron-acceptor (methylviologen) probe molecules, which has allowed the visual detection of the corresponding radical ions, in agreement with the occurrence of photoinduced charge separation in MIL-125(Ti)-NH(2).

CH4 storage and CO2 capture in highly porous zirconium oxide based metal-organic frameworks.

A series of porous Zr oxoclusters-based MOFs was computationally explored for their gas storage/capture performances. The highly porous UiO-67(Zr) and UiO-68(Zr) solids show exceptionally high CH(4) and CO(2) adsorption capacities under operating conditions that make these thermal, water and mechanical resistant materials very promising for physisorption-based processes.

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, CO(2) 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 CO(2) uptake was observed with increasing RH, up to 105 mg g(-1) CO(2) 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 CO(2) in the water-filled mesopores of MIL-100(Fe).

Reverse shape selectivity in the liquid-phase adsorption of xylene isomers in zirconium terephthalate MOF UiO-66.

Powder, agglomerates, and tablets of the microporous zirconium(IV) terephthalate metal-organic framework UiO-66 were evaluated for the selective adsorption and separation of xylene isomers in the liquid phase using n-heptane as the eluent. Pulse experiments, performed at 313 K in the presence of n-heptane, revealed the o-xylene preference of this material, which was further confirmed by binary and multicomponent breakthrough experiments in the presence of m- and p-xylene, resulting in selectivities at 313 K of 1.8 and 2.4 with regards to m-xylene and p-xylene, respectively. Additionally, because p-xylene is the less retained isomer, UiO-66 presents a selectivity pattern that is reverse of that of the xylenes' molecular dimension with respect to shape selectivity. The shaping of the material as tablets did not significantly change its selectivity toward the o-xylene isomer or toward p-xylene, which was the less retained isomer, despite a loss in capacity. Finally, the selectivity behavior of UiO-66 in the liquid n-heptane phase makes it a suitable material for o-xylene separation in the extract (heavy product) or p-xylene separation in the raffinate (light product) by simulated moving bed technology.

Toward understanding the influence of ethylbenzene in p-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 n-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 n-heptane as the eluent. MIL-125(Ti) is a para-selective material suitable at low concentrations to separate p-xylene from the other xylene isomers. Pulse experiments indicate a separation factor of 1.3 for p-xylene over o-xylene and m-xylene, while breakthrough experiments using a diluted ternary mixture lead to selectivity values of 1.5 and 1.6 for p-xylene over m-xylene and o-xylene, respectively. Introduction of ethylbenzene in the mixture results however in a decrease of the selectivity.

p-Xylene-selective metal-organic frameworks: a case of topology-directed selectivity.

Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal-organic frameworks: MIL-125(Ti) ([Ti(8)O(8)(OH)(4)(BDC)(6)]), MIL-125(Ti)-NH(2) ([Ti(8)O(8)(OH)(4)(BDC-NH(2))(6)]), and CAU-1(Al)-NH(2) ([Al(8)(OH)(4)(OCH(3))(8)(BDC-NH(2))(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.