Gregory J. Halder

Dynamic Interplay between Spin-Crossover and Host−Guest Function in a Nanoporous Metal−Organic Framework Material

The nanoporous metal-organic framework [Fe(pz)Ni(CN)(4)], 1 (where pz is pyrazine), exhibits hysteretic spin-crossover at ambient conditions and is robust to the adsorption and desorption of a wide range of small molecular guests, both gases (N(2), O(2), CO(2)) and vapors (methanol, ethanol, acetone, acetonitrile, and toluene). Through the comprehensive analysis of structure, host-guest properties, and spin-crossover behaviors, it is found that this pillared Hofmann system uniquely displays both guest-exchange-induced changes to spin-crossover and spin-crossover-induced changes to host-guest properties, with direct dynamic interplay between these two phenomena. Guest desorption and adsorption cause pronounced changes to the spin-crossover behavior according to a systematic trend in which larger guests stabilize the high-spin state and therefore depress the spin-crossover temperature of the host lattice. When stabilizing the alternate spin state of the host at any given temperature, these processes directly stimulate the spin-crossover process, providing a chemisensing function. Exploitation of the bistability of the host allows the modification of adsorption properties at a fixed temperature through control of the host spin state, with each state shown to display differing chemical affinities to guest sorption. Guest desorption then adsorption, and vice versa, can be used to switch between spin states in the bistable temperature region, adding a guest-dependent memory effect to this system.

Pressure-Induced Amorphization and Porosity Modification in a Metal−Organic Framework

The impact of modest, industrially accessible pressures (0-1.2 GPa) on the structure and porosity of the zeolitic imidazolate framework Zn(2-methylimidazole)(2), ZIF-8, was investigated using in situ powder X-ray diffraction in combination with sorption measurements for pressure-treated samples. The framework is highly compressible, with a bulk modulus (K = -V partial differential P/partial differential V) of 6.52(35) GPa, the most compressible metal-organic framework (MOF) documented to date. The framework undergoes an irreversible pressure-induced amorphization following compression beyond 0.34 GPa. The pressure-amorphized ZIF-8 remains porous, although the sorption characteristics are distinctly altered compared to the pristine material. As such, pressure can provide a new route to systematically modify the sorption behavior and other functional properties of MOFs, a nontraditional form of postsynthetic modification. Importantly, pressure modification of MOFs is effective at lower pressures than in other porous materials (e.g., zeolites) and, as such, is easily scalable and industrially relevant.

Reversible switching from antiferro- to ferromagnetic behavior by solvent-mediated, thermally-induced phase transitions in a trimorphic MOF-based magnetic sponge system.

Hydrothermal reactions of copper(II) acetate, tetrazolate-5-carboxylate (tzc), and the neutral N-donor spacer ligand 1,3-di(4-pyridyl)propane (dpp) lead in a single reaction vial to the simultaneous formation of three different single-crystalline solvates [Cu(tzc)(dpp)]n·0.5C6H14·0.5H2O (1), [Cu(tzc)(dpp)]n·4.5H2O (2), and [Cu(tzc)(dpp)]n·1.25C6H14 (3). All three structures were characterized by single crystal X-ray diffraction. None of these solvates can be prepared as phase-pure bulk materials, but reaction conditions similar to those used for single crystal synthesis yield a phase-pure polycrystalline bulk material of an additional forth solvate phase [Cu(tzc)(dpp)]n·2H2O (4). Investigations of its thermal properties by in situ temperature-dependent synchrotron-based powder diffraction experiments have shown interesting phase transitions upon heating in a helium stream. Initially, the precursor dihydrate 4 transforms to an anhydrous phase [Cu(tzc)(dpp)]n (6I) via the intermediate monohydrate phase [Cu(tzc)(dpp)]n·H2O (5). Upon further heating, phase 6I transforms to a new anhydrous polymorph 6II, which transforms upon cooling to a further new phase 6III. Thermogravimetric measurements performed in tandem with differential scanning calorimetry as well as infrared spectroscopic investigations are in agreement with these findings. The de/resolvation behavior is accompanied by a dramatic change in their magnetic properties: The dihydrate phase shows antiferromagnetic exchange interactions, whereas ferromagnetic properties are observed for the trimorphic anhydrate system. This magnetic sponge-like behavior can be reversibly cycled upon de/resolvation of the material.

Guest Tunable Structure and Spin Crossover Properties in a Nanoporous Coordination Framework Material

The electronic switching properties of the nanoporous spin crossover framework [Fe(NCS)(2)(bpbd)(2)] x x(guest), SCOF-2, can be rationally manipulated via sorption of a range of molecular guests (acetone, ethanol, methanol, propanol, 1-acetonitrile) into the 1-D channels of this material. Pronounced changes to the spin crossover properties are related directly to the steric and electronic influence of the individual guests: the degree of lattice cooperativity, as reflected in the abruptness of the transition and presence of hysteresis, is strongly influenced by the presence of cooperative host-guest interactions, and the temperature of the transition varies with guest polarity through a proposed electrostatic interaction.

Trapping Guests within a Nanoporous Metal–Organic Framework through Pressure-Induced Amorphization

The release of guest species from within a nanoporous metal-organic framework (MOF) has been inhibited by amorphization of the guest-loaded framework structure under applied pressure. Thermogravimetric analyses have shown that by amorphizing ZIF-8 following sorption of molecular I(2), a hazardous radiological byproduct of nuclear energy production, the pore apertures in the framework are sufficiently distorted to kinetically trap I(2) and improve I(2) retention. Pair distribution function (PDF) analysis indicates that the local structure of the captive I(2) remains essentially unchanged upon amorphization of the framework, with the amorphization occurring under the same conditions for the vacant and guest-loaded framework. The low, accessible pressure range needed to effect this change in desorption is much lower than in tradition sorbents such as zeolites, opening the possibility for new molecular capture, interim storage, or controlled release applications.

Low-energy selective capture of carbon dioxide by a pre-designed elastic single-molecule trap

Single-molecule trap: Easy activation of the water-stable metal-organic framework PCN-200 provides a new route to low-energy selective CO(2) capture through stimuli-responsive adsorption behavior. This elastic CO(2) trapping effect was confirmed by single-component and binary gas-adsorption isotherms and crystallographic determination.

Hysteretic three-step spin crossover in a thermo- and photochromic 3D pillared Hofmann-type metal-organic framework.

The integration of spin crossover (SCO) centers into porousframework materials is leading to the emergence of newfamilies of functional solids that display a range of interestingand potentially useful physicochemical properties. This mate-rialsdesignapproachgivesrisetoauniquemolecularscenarioin which factors that govern the spin switching response (e.g.,temperature, pressure, light, magnetic field, and chemicalenvironment) are newly intertwined with highly cooperativestructure–function relationships, and potentially also with thedynamic host–guest chemistry of the materials.

Single Crystal to Single Crystal Structural Transformations in Molecular Framework Materials

The rapid advance in the synthesis and characterization of molecular frameworks over the past decade has opened an entirely new approach for the generation of nanoporous materials. With this advance has come an increasingly pressing need for the development of new techniques to characterize the guest-dependent structures of these novel and highly complex materials. In this review we highlight some of the relatively rare cases where single crystal diffraction has been used to characterize the flexible structures of molecular frameworks through the investigation of single crystal to single crystal (SC-SC) transformations.

Zero thermal expansion in a flexible, stable framework : tetramethylammonium copper(I) zinc(II) cyanide.

Tetramethylammonium copper(I) zinc(II) cyanide, which consists of N(CH(3))(4)(+) ions trapped within a cristobalite-like metal cyanide framework, has been studied by variable-temperature powder and single-crystal X-ray diffraction. Its coefficient of thermal expansion is approximately zero over the temperature range 200-400 K and comparable with the best commercial zero thermal expansion materials. The atomic displacement parameters, apparent bond lengths, and structure of a low-temperature, low-symmetry phase reveal that the low-energy vibrational modes responsible for this behavior maintain approximately rigid Zn coordination tetrahedra but involve significant distortion of their Cu counterparts.

Dehydration of the nanoporous coordination framework ErIII[CoIII(CN)6]·4(H2O): single crystal to single crystal transformation and negative thermal expansion in ErIII[CoIII(CN)6]

Desorption of bound and unbound water molecules from the nanoporous coordination framework ErIII[CoIII(CN)6].4(H2O) to form the apohost, ErIII[CoIII(CN)6], proceeds via a single crystal to single crystal transformation in which the Er(III) cations change from 8- to 6-coordinate; dehydration results in a striking change in the thermal expansion properties.