Cameron J. Kepert

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.

Hard magnets based on transition metal complexes with the dicyanamide anion, {N(CN)2}-,

We present the crystal structures and magnetic properties of a series of magnetic compounds, MII{N(CN)2}2, where M=Cu (1), Ni (2), Co (3) and Fe (4), and [Mn{N(CN)2}2(C2H5OH)2]Z·(CH3)2CO (5). In the isostructural compounds 1–4, the dicyanamide anion is triply coordinating through its three nitrogen atoms. It bridges the metal ions to form infinite 3D metal-organic frameworks with a rutile-type structure. The framework contains doubly bridged M(–Nâ–·C–N–Câ–·N–)2 ribbons that link approximately orthogonally through the amide nitrogen atoms. The Jahn–Teller distortion in 1 has a strong influence on the packing arrangement (M–N bond lengths: 1.98 and 2.47 A for 1 and 2.10 and 2.15 A for 3). On lowering the temperature the bond distances in 1 remain unchanged except for a decrease of the M–Namide length to 2.45 A. Magnetic data for 1 obey the Curie–Weiss law (Θ=-2.1 K). 2 and 3 are ferromagnets with Curie temperatures (TC) of 9 and 21 K and are characterized by hysteresis loops of 710 and 7975 Oe at 2 K, remnant magnetization, magnetization approaching the expected saturation (gS) of 2 and 3 µB in high field, absorptive component (χ″) in the AC magnetization and λ peak in the heat capacity data. 4 is similarly characterized and shows behaviour that is characteristic of a canted antiferromagnet: the Weiss constant is temperature dependent (+3 K in the range 200–300 K), there is a sharper peak than for 1 or 2 in the AC magnetization and the isothermal magnetization at 3 K increases monotonically to ≈1.3 µB (expected to be 4 µB for ferromagnetic alignment of the spins) in a field of 8 T. Its coercive field (17800 Oe) is the largest observed for any metal-organic compound and exceeds those of alloys of SmCo5 and Nd2Fe14B. The maximum energy product (B · H) is the highest for 3 and is comparable to alloys of Sm–Co. We attribute the large coercive field to a combination of single ion and particle shape anisotropies. 5 is paramagnetic at high temperature with Θ=-3 K. Below 16 K it behaves as a canted antiferromagnet with a very weak resultant spontaneous magnetization.

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.

Dynamic Photo‐Switching in Metal–Organic Frameworks as a Route to Low‐Energy Carbon Dioxide Capture and Release

For post-combustion carbon dioxide capture technology to realize widespread viability, the energy costs must be drastically reduced. Current adsorbent technologies that rely on pressure, temperature, or vacuum swings consume as much as 40% of the production capacity of a power plant, most of which is associated with the liberation of CO2 from the capture medium. Ultimately this penalty, or parasitic energy load, must be brought closer to the thermodynamic minimum of about 4% to avoid prohibitive cost increases. Given that the triggers for release of adsorbed carbon dioxide, such as vacuum and heating, are so energy intensive, 3] requiring energy from the power plant, there is strong motivation to develop new release triggers that do not require extra energy from the plant, using renewable energy sources such as the sun. In conjunction with this, adsorbents with maximum gas sorption efficiency can further reduce the costs compared to the conventional energy-intensive CO2 gas separation process. Light, and in particular concentrated sunlight, is an extremely attractive stimulus for triggering CO2 release. If used with an adsorbent material that strongly absorbs sunlight concomitant with the desorption of large amounts of CO2, it may be possible to drastically reduce the energy costs. Perhaps the most attractive adsorbent candidates are metal–organic frameworks (MOFs), because of their large adsorption capacities, and the potential for incorporation of light-responsive organic groups within the pore structure. MOFs are an important class of 3D crystalline porous materials comprised of metal centers and organic ligands, joined periodically to establish a crystalline porous array. The large internal surface areas can be used to adsorb unprecedented quantities of gases, with particular interest in hydrogen, methane, 8] and carbon dioxide emergent. 7b,h,9] Methods for the incorporation of light-responsive groups within MOFs include the use of pendant groups pointing into the pores, and filling of pores with light-responsive guest molecules. The responsive groups within these materials may then alter their conformation when exposed to filtered light which results in a change in adsorption capacity, as reported thus far for static conditions. The responsive groups within these MOFs can be statically set to one position or another. For use in photoswing carbon dioxide capture, MOFs that can respond dynamically, or to the broadband radiation found in sunlight whilst loaded with adsorbed gas, are ideal. This will increase the speed of operation and lower the energy costs (see Figure 1)

Hydrogen adsorption in HKUST-1: a combined inelastic neutron scattering and first-principles study.

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 (Cu3(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 H2-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 H2 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 H2 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.

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.

Desolvation of a Novel Microporous Hydrogen-Bonded Framework: Characterization by In Situ Single-Crystal and Powder X-ray Diffraction**

Despite significant structural rearrangement upon desolvation of a three-dimensional molecular framework of hexaaquacobalt cations and redox-active functionalized tetrathiafulvalene anions (see the picture; the area filled with water molecules is shown in gray), monocrystallinity and microporosity are retained. X-ray analyses show that a unique combination of hydrogen bonds and π⋅⋅⋅π interactions within the framework gives this material a structural flexibility not seen in zeolites or their analogues.

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.

Organosilane functionalization of halloysite nanotubes for enhanced loading and controlled release

The surfaces of naturally occurring halloysite nanotubes were functionalized with γ-aminopropyltriethoxysilane (APTES), which was found to have a substantial effect on the loading and subsequent release of a model dye molecule. APTES was mostly anchored at the internal lumen surface of halloysite through covalent grafting, forming a functionalized surface covered by aminopropyl groups. The dye loading of the functionalized halloysite was 32% greater than that of the unmodified sample, and the release from the functionalized halloysite was dramatically prolonged as compared to that from the unmodified one. Dye release was prolonged at low pH and the release at pH 3.5 was approximately three times slower than that at pH 10.0. These results demonstrate that organosilane functionalization makes pH an external trigger for controlling the loading of guest on halloysite and the subsequent controlled release.

The first example of a coordination polymer from the expanded 4,4 `-bipyridine ligand [Ru(pytpy)(2)](2+) (pytpy=4 `-(4-pyridyl)2,2 `: 6 `,2 ``-terpyridine)

The complex cation ligand [Ru(pytpy)2]2+ (pytpy = 4′-(4-pyridyl)-2,2′∶6′,2″-terpyridine) is an expanded 4,4′-bipyridine; we describe the first example of a coordination polymer in which [Ru(pytpy)2]2+ plays the role of a bridging bidentate ligand.