Four-step iron(ii) spin state cascade driven by antagonistic solid state interactions.

A four-stepped cascade of Fe(ii) high spin (HS) to low spin (LS) states is demonstrated in a family of 2-D Hofmann materials, [Fe3II(saltrz)6(MII(CN)4)3]·8(H2O) (MII = Pd (1Pd ), Pt (1Pt ); saltrz = (E)-2-(((4H-1,2,4-triazol-4-yl)imino)methyl)phenol). Alongside the fully HS and LS Fe(ii) states, fractional spin state stabilization occurs at HS/LS values of 5/6, 2/3, and 1/6. This unconventional spin state periodicity is driven by the presence of multiple spin crossover (SCO) active Fe(ii) sites which are in subtly distinct environments driven by a network of antagonistic host-host and host-guest interactions. Alternating long- and short-range magnetostructural ordering is achieved over the five distinct spin state ratios HS1.0LS0.0, HS0.833LS0.167, HS0.667LS0.333, HS0.167LS0.833, and HS0.0LS1.0 owing to the flexibility of this 2-D interdigitated lattice topology interconnected by intermolecular interactions. A distinct wave-like spin state patterning is structurally evidenced for each intermediate phase.

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.

Correction: Hydrophobic pillared square grids for selective removal of CO2 from simulated flue gas.

Correction for 'Hydrophobic pillared square grids for selective removal of CO2 from simulated flue gas' by Sameh K. Elsaidi et al., Chem. Commun., 2015, 51, 15530-15533.

Hydrophobic pillared square grids for selective removal of CO2 from simulated flue gas.

Capture of CO2 from flue gas is considered to be a feasible approach to mitigate the effects of anthropogenic emission of CO2. Herein we report that an isostructural family of metal organic materials (MOMs) of general formula [M(linker)2(pillar)], linker = pyrazine, pillar = hexaflourosilicate and M = Zn, Cu, Ni and Co exhibits highly selective removal of CO2 from dry and wet simulated flue gas. Two members of the family, M = Ni and Co, SIFSIX-3-Ni and SIFSIX-3-Co, respectively, are reported for the first time and compared with the previously reported Zn and Cu analogs.

Understanding the Adsorption Mechanism of Xe and Kr in a Metal-Organic Framework from X-ray Structural Analysis and First-Principles Calculations.

Enhancement of adsorption capacity and separation of radioactive Xe/Kr at room temperature and above is a challenging problem. Here, we report a detailed structural refinement and analysis of the synchrotron X-ray powder diffraction data of Ni-DODBC metal organic framework with in situ Xe and Kr adsorption at room temperature and above. Our results reveal that Xe and Kr adsorb at the open metal sites, with adsorption geometries well reproduced by DFT calculations. The measured temperature-dependent adsorption capacity of Xe is substantially larger than that for Kr, indicating the selectivity of Xe over Kr and is consistent with the more negative adsorption energy (dominated by van der Waals dispersion interactions) predicted from DFT. Our results reveal critical structural and energetic information about host-guest interactions that dictate the selective adsorption mechanism of these two inert gases, providing guidance for the design and synthesis of new MOF materials for the separation of environmentally hazardous gases from nuclear reprocessing applications.

Pressure-induced structural phase transformation in cobalt(II) dicyanamide.

In situ synchrotron powder diffraction has been used to probe the pressure-dependent structural properties of the magnetic molecular framework material Co(dca)2 [dca = dicyanamide or N(CN)2(-)]. An orthorhombic (Pmnn) to monoclinic (P21/n) transformation to a high-pressure phase, namely γ-Co(dca)2, occurs at 1.1 GPa. Structural determination of γ-Co(dca)2 shows that the rutile-like topology of the pristine material is retained at high pressures, with the lower symmetry allowing a progression of volume-reducing structural distortions. γ-Co(dca)2 was stable at the maximum pressure measured of 4.2 GPa. Both phases were soft, with bulk moduli (B0) for α-Co(dca)2 and γ-Co(dca)2 of 13.15 (18) and 9.0 (6) GPa, respectively. Modest uniaxial negative linear compressibility (K) of the order of -4 TPa(-1) was observed over the entire measured pressure range.

In situ studies of a platform for metastable inorganic crystal growth and materials discovery.

Rapid shifts in the energy, technological, and environmental demands of materials science call for focused and efficient expansion of the library of functional inorganic compounds. To achieve the requisite efficiency, we need a materials discovery and optimization paradigm that can rapidly reveal all possible compounds for a given reaction and composition space. Here we provide such a paradigm via in situ X-ray diffraction measurements spanning solid, liquid flux, and recrystallization processes. We identify four new ternary sulfides from reactive salt fluxes in a matter of hours, simultaneously revealing routes for ex situ synthesis and crystal growth. Changing the flux chemistry, here accomplished by increasing sulfur content, permits comparison of the allowable crystalline building blocks in each reaction space. The speed and structural information inherent to this method of in situ synthesis provide an experimental complement to computational efforts to predict new compounds and uncover routes to targeted materials by design.

Exploiting high pressures to generate porosity, polymorphism, and lattice expansion in the nonporous molecular framework Zn(CN)2.

Systematic exploration of the molecular framework material Zn(CN)2 at high pressure has revealed several distinct series of transitions leading to five new phases: four crystalline and one amorphous. The structures of the new crystalline phases have been resolved through ab initio structural determination, combining charge flipping and direct space methods, based on synchrotron powder diffraction data. The specific transition activated under pressure depends principally on the pressure-transmitting fluid used. Without fluid or in large molecule fluids (e.g., isopropanol, ethanol, or fluorinert), the high-pressure behavior intrinsic to Zn(CN)2 is observed; the doubly interpenetrated diamondoid framework structure transforms to a distorted, orthorhombic polymorph, Zn(CN)2-II (Pbca) at ~1.50-1.58 GPa with asymmetric displacement of the bridging CN ligand and reorientation of the Zn(C/N)4 tetrahedra. In small molecule fluids (e.g., water, methanol, methanol-ethanol-water), the nonporous interpenetrated Zn(CN)2 framework can undergo reconstructive transitions to porous, non-interpenetrated polymorphs with different topologies: diamondoid (dia-Zn(CN)2, Fd3m, P(trans) ~ 1.2 GPa), londaleite (lon-Zn(CN)2, P6(3)/mmc, P(trans) ~ 0.9 GPa), and pyrite-like (pyr-Zn(CN)2, Pa3, P(trans) ~ 1.8 GPa). Remarkably, these pressure-induced transitions are associated with near 2-fold volume expansions. While an increase in volume with pressure is counterintuitive, the resulting new phases contain large fluid-filled pores, such that the combined solid + fluid volume is reduced and the inefficiencies in space filling by the interpenetrated parent phase are eliminated. That both dia-Zn(CN)2 and lon-Zn(CN)2 phases were retained upon release to ambient pressure demonstrates the potential for application of hydrostatic pressures to interpenetrated framework systems as a novel means to generate new porous materials.

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.

Chemical bonding and magnetic exchange in two-dimensional [M(TCNE)(NCMe)2]X (M = Fe, Mn; X = FeCl4, SbF6) magnets: a pressure study.

Pressure-dependent X-ray diffraction studies reveal the bulk modulus and compression anisotropy of the 2D magnet [Mn(TCNE)(NCMe)(2)]SbF(6). The Raman response of this and the similar [Fe(TCNE)(NCMe)(2)]FeCl(4) layered magnet, shows that the evolution of the a(g) ν(C=C) frequency correlates well with the magnetic exchange and T(c) variations of these materials under pressure. There is a significantly more complex correlation between the a(g) ν(C≡N) frequency and T(c) despite the fact that some unpaired π* electron density (~0.125 e) is localized on each of TCNE nitrile N≡C group. The shortening of the M-NC bond with pressure (<0.5 GPa) does not result in a T(c) increase, which suggests a more complex bond length magnetic exchange relationship.

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.

Importance of halogen···halogen contacts for the structural and magnetic properties of CuX2(pyrazine-N,N'-dioxide)(H2O)2 (X = Cl and Br).

The structural and magnetic properties of the newly crystallized CuX(2)(pyzO)(H(2)O)(2) (X = Cl, Br; pyzO = pyrazine-N,N'-dioxide) coordination polymers are reported. These isostructural compounds crystallize in the monoclinic space group C2/c with, at 150 K, a = 17.0515(7) Å, b = 5.5560(2) Å, c = 10.4254(5) Å, ß = 115.400(2)°, and V = 892.21(7) Å(3) for X = Cl and a = 17.3457(8) Å, b = 5.6766(3) Å, c = 10.6979(5) Å, ß = 115.593(2)°, and V = 950.01(8) Å(3) for X = Br. Their crystal structure is characterized by one-dimensional chains of Cu(2+) ions linked through bidentate pyzO ligands. These chains are joined together through OH···O hydrogen bonds between the water ligands and pyzO oxygen atoms and Cu-X···X-Cu contacts. Bulk magnetic susceptibility measurements at ambient pressure show a broad maximum at 7 (Cl) and 28 K (Br) that is indicative of short-range magnetic correlations. The dominant spin exchange is the Cu-X···X-Cu supersuperexchange because the magnetic orbital of the Cu(2+) ion is contained in the CuX(2)(H(2)O)(2) plane and the X···X contact distances are short. The magnetic data were fitted to a Heisenberg 1D uniform antiferromagnetic chain model with J(1D)/k(B) = -11.1(1) (Cl) and -45.9(1) K (Br). Magnetization saturates at fields of 16.1(3) (Cl) and 66.7(5) T (Br), from which J(1D) is determined to be -11.5(2) (Cl) and -46.4(5) K (Br). For the Br analog the pressure dependence of the magnetic susceptibility indicates a gradual increase in the magnitude of J(1D)/k(B) up to -51.2 K at 0.84 GPa, suggesting a shortening of the Br···Br contact distance under pressure. At higher pressure X-ray powder diffraction data indicates a structural phase transition at ∼3.5 GPa. Muon-spin relaxation measurements indicate that CuCl(2)(pyzO)(H(2)O)(2) is magnetically ordered with T(N) = 1.06(1) K, while the signature for long-range magnetic order in CuBr(2)(pyzO)(H(2)O)(2) was much less definitive down to 0.26 K. The results for the CuX(2)(pyzO)(H(2)O)(2) complexes are compared to the related CuX(2)(pyrazine) materials.

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.

Spectroscopic study of (two-dimensional) molecule-based magnets: [M(II)(TCNE)(NCMe)2][SbF6] (M = Fe, Mn, Ni).

The M-[TCNE] (M = 3d metal; TCNE = tetracyanoethylene) system is one of the most interesting classes of molecule-based magnets, exhibiting a plethora of compositions and structures (inorganic polymer chains, 2D layers, 3D networks, and amorphous solids) with a wide range of magnetic ordering temperatures (up to 400 K). A systematic study of vibrational (both infrared and, for the first time, Raman) properties of the family of new TCNE-based magnets of M(II)(TCNE) (NCMe)(2)[SbF(6)] [M = Mn, Fe, Ni] composition is discussed in conjunction with their magnetic behavior and newly reso-lved crystal structures. The vibrational properties of the isolated TCNE(●-) anion in the paramagnetic Bu(4)N [TCNE(●-)] salt and recently characterized 2D layered magnet Fe(II)(TCNE)(NCMe)(2)[FeCl(4)] are also reported for comparison. Additionally, a linear correlation between ν(C=C) (a(g)) frequency of the TCNE ligand and its formal charge Z (the spin density on the π* orbital), Z = [1571 - ν(C=C) (a(g))]/154.5 [e], is presented. It is shown that monitoring Z by Raman spectroscopy is of great use in providing information that allows understanding the peculiarity of the superexchange interaction in M-[TCNE] magnets and establishing the structure-magnetic properties correlations in this class of magnetic material.

Elucidating the structure of surface acid sites on γ-Al2O3.

Differential pair distribution function analysis was applied to resolve, with crystallographic detail, the structure of catalytic sites on the surface of nanoscale γ-Al(2)O(3). The structure was determined for a basic probe molecule, monomethylamine (MMA), bound at the minority Lewis acid sites. These active sites were found to be five-coordinate, forming distorted octahedra upon MMA binding. This approach could be applied to study the interaction of molecules at surfaces in dye-sensitized solar cells, nanoparticles, sensors, materials for waste remediation, and catalysts.

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.

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.

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.