Miguel I. Gonzalez

Methane storage in flexible metal-organic frameworks with intrinsic thermal management.

As a cleaner, cheaper, and more globally evenly distributed fuel, natural gas has considerable environmental, economic, and political advantages over petroleum as a source of energy for the transportation sector. Despite these benefits, its low volumetric energy density at ambient temperature and pressure presents substantial challenges, particularly for light-duty vehicles with little space available for on-board fuel storage. Adsorbed natural gas systems have the potential to store high densities of methane (CH4, the principal component of natural gas) within a porous material at ambient temperature and moderate pressures. Although activated carbons, zeolites, and metal–organic frameworks have been investigated extensively for CH4 storage, there are practical challenges involved in designing systems with high capacities and in managing the thermal fluctuations associated with adsorbing and desorbing gas from the adsorbent. Here, we use a reversible phase transition in a metal–organic framework to maximize the deliverable capacity of CH4 while also providing internal heat management during adsorption and desorption. In particular, the flexible compounds Fe(bdp) and Co(bdp) (bdp2− = 1,4-benzenedipyrazolate) are shown to undergo a structural phase transition in response to specific CH4 pressures, resulting in adsorption and desorption isotherms that feature a sharp ‘step’. Such behaviour enables greater storage capacities than have been achieved for classical adsorbents, while also reducing the amount of heat released during adsorption and the impact of cooling during desorption. The pressure and energy associated with the phase transition can be tuned either chemically or by application of mechanical pressure.

Comprehensive study of carbon dioxide adsorption in the metal–organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)

Analysis of the CO2 adsorption properties of a well-known series of metal–organic frameworks M2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host–guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal–O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M2(dobdc)–CO2 adducts.

Tristability in a Light-Actuated Single-Molecule Magnet

Molecules exhibiting bistability have been proposed as elementary binary units (bits) for information storage, potentially enabling fast and efficient computing. In particular, transition metal complexes can display magnetic bistability via either spin-crossover or single-molecule magnet behavior. We now show that the octahedral iron(II) complexes in the molecular salt [Fe(1-propyltetrazole)6](BF4)2, when placed in its high-symmetry form, can combine both types of behavior. Light irradiation under an applied magnetic field enables fully reversible switching between an S = 0 state and an S = 2 state with either up (M(S) = +2) or down (M(S) = -2) polarities. The resulting tristability suggests the possibility of using molecules for ternary information storage in direct analogy to current binary systems that employ magnetic switching and the magneto-optical Kerr effect as write and read mechanisms.

Electronic Conductivity, Ferrimagnetic Ordering, and Reductive Insertion Mediated by Organic Mixed-Valence in a Ferric Semiquinoid Metal–Organic Framework

A three-dimensional network solid composed of Fe(III) centers and paramagnetic semiquinoid linkers, (NBu4)2Fe(III)2(dhbq)3 (dhbq(2-/3-) = 2,5-dioxidobenzoquinone/1,2-dioxido-4,5-semiquinone), is shown to exhibit a conductivity of 0.16 ± 0.01 S/cm at 298 K, one of the highest values yet observed for a metal-organic framework (MOF). The origin of this electronic conductivity is determined to be ligand mixed-valency, which is characterized using a suite of spectroscopic techniques, slow-scan cyclic voltammetry, and variable-temperature conductivity and magnetic susceptibility measurements. Importantly, UV-vis-NIR diffuse reflectance measurements reveal the first observation of Robin-Day Class II/III mixed valency in a MOF. Pursuit of stoichiometric control over the ligand redox states resulted in synthesis of the reduced framework material Na0.9(NBu4)1.8Fe(III)2(dhbq)3. Differences in electronic conductivity and magnetic ordering temperature between the two compounds are investigated and correlated to the relative ratio of the two different ligand redox states. Overall, the transition metal-semiquinoid system is established as a particularly promising scaffold for achieving tunable long-range electronic communication in MOFs.

Exchange coupling and magnetic blocking in dilanthanide complexes bridged by the multi-electron redox-active ligand 2,3,5,6-tetra(2-pyridyl)pyrazine

The syntheses and magnetic properties of six new compounds featuring the radical-bridged dilanthanide complexes [(Cp*2Ln)2(μ-tppz˙)]+ (Ln = Gd, 1; Tb, 2; Dy, 3; tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) and [(Cp*2Ln)2(μ-tppz˙)]− (Ln = Gd, 4; Tb, 5, Dy, 6) are reported. Cyclic voltammograms for compounds 1–3 reveal that the tppz ligand can reversibly undergo multiple redox changes. Hence, in the two sets of compounds isolated, 1–3 and 4–6, the redox-active ligand tppz exists in the monoanionic (tppz˙−) and trianionic (tppz˙3−) forms, respectively. Substantial LnIII–tppz˙− exchange coupling is found for the cationic tppz˙− radical-bridged species of 1–3, as suggested by a rise in χMT at low temperatures. For the Gd compound 1, fits to the data yielded a coupling constant of J = −6.91(4) cm−1, revealing antiferromagnetic coupling to give an S = 13/2 ground state. Both of the TbIII and DyIII-containing compounds 2 and 3 exhibit single-molecule magnet behavior under zero applied dc field. Importantly, the Dy congener shows a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 2.8 K and magnetic hysteresis below 3.25 K. Interestingly, the coupling constant of J = −6.29(3) cm−1 determined for the trianionic tppz˙3− radical-bridged Gd compound 4 is of similar magnitude to that of the tppz˙−-bridged analogue 1. However, the anionic tppz˙3−-bridged species containing TbIII and DyIII centers, compounds 5 and 6, do not exhibit slow magnetization dynamics under zero and applied dc fields. Computational results indicate a doublet ground state for the bridging tppz˙3− unit, with a different distribution for the spin density orientation towards the LnIII centers. These results have important implications for the future design of molecule-based magnets incorporating exchange-coupled lanthanide-radical species.

Reversible CO Scavenging via Adsorbate-Dependent Spin State Transitions in an Iron(II)-Triazolate Metal-Organic Framework.

A new metal-organic framework, Fe-BTTri (Fe3[(Fe4Cl)3(BTTri)8]2·18CH3OH, H3BTTri =1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene)), is found to be highly selective in the adsorption of CO over a variety of other gas molecules, making it extremely effective, for example, in the removal of trace CO from mixtures with H2, N2, and CH4. This framework not only displays significant CO adsorption capacity at very low pressures (1.45 mmol/g at just 100 μbar), but, importantly, also exhibits readily reversible CO binding. Fe-BTTri utilizes a unique spin state change mechanism to bind CO in which the coordinatively unsaturated, high-spin Fe(II) centers of the framework convert to octahedral, low-spin Fe(II) centers upon CO coordination. Desorption of CO converts the Fe(II) sites back to a high-spin ground state, enabling the facile regeneration and recyclability of the material. This spin state change is supported by characterization via infrared spectroscopy, single crystal X-ray analysis, Mössbauer spectroscopy, and magnetic susceptibility measurements. Importantly, the spin state change is selective for CO and is not observed in the presence of other gases, such as H2, N2, CO2, CH4, or other hydrocarbons, resulting in unprecedentedly high selectivities for CO adsorption for use in CO/H2, CO/N2, and CO/CH4 separations and in preferential CO adsorption over typical strongly adsorbing gases like CO2 and ethylene. While adsorbate-induced spin state transitions are well-known in molecular chemistry, particularly for CO, to our knowledge this is the first time such behavior has been observed in a porous material suitable for use in a gas separation process. Potentially, this effect can be extended to selective separations involving other π-acids.

Selective, tunable O2 binding in cobalt(II)–triazolate/pyrazolate metal–organic frameworks

The air-free reaction of CoCl2 with 1,3,5-tri(1H-1,2,3-triazol-5-yl)benzene (H3BTTri) in N,N-dimethylformamide (DMF) and methanol leads to the formation of Co-BTTri (Co3[(Co4Cl)3(BTTri)8]2·DMF), a sodalite-type metal–organic framework. Desolvation of this material generates coordinatively unsaturated low-spin cobalt(II) centers that exhibit a strong preference for binding O2 over N2, with isosteric heats of adsorption (Qst) of −34(1) and −12(1) kJ/mol, respectively. The low-spin (S = 1/2) electronic configuration of the metal centers in the desolvated framework is supported by structural, magnetic susceptibility, and computational studies. A single-crystal X-ray structure determination reveals that O2 binds end-on to each framework cobalt center in a 1:1 ratio with a Co–O2 bond distance of 1.973(6) Å. Replacement of one of the triazolate linkers with a more electron-donating pyrazolate group leads to the isostructural framework Co-BDTriP (Co3[(Co4Cl)3(BDTriP)8]2·DMF; H3BDTriP = 5,5′-(5-(1H-pyrazol-4-yl)-1,3-phenylene)bis(1H-1,2,3-triazole)), which demonstrates markedly higher yet still fully reversible O2 affinities (Qst = −47(1) kJ/mol at low loadings). Electronic structure calculations suggest that the O2 adducts in Co-BTTri are best described as cobalt(II)–dioxygen species with partial electron transfer, while the stronger binding sites in Co-BDTriP form cobalt(III)–superoxo moieties. The stability, selectivity, and high O2 adsorption capacity of these materials render them promising new adsorbents for air separation processes.

Giant coercivity and high magnetic blocking temperatures for N 2 3− radical-bridged dilanthanide complexes upon ligand dissociation

Increasing the operating temperatures of single-molecule magnets—molecules that can retain magnetic polarization in the absence of an applied field—has potential implications toward information storage and computing, and may also inform the development of new bulk magnets. Progress toward these goals relies upon the development of synthetic chemistry enabling enhancement of the thermal barrier to reversal of the magnetic moment, while suppressing alternative relaxation processes. Herein, we show that pairing the axial magnetic anisotropy enforced by tetramethylcyclopentadienyl (CpMe4H) capping ligands with strong magnetic exchange coupling provided by an N23− radical bridging ligand results in a series of dilanthanide complexes exhibiting exceptionally large magnetic hysteresis loops that persist to high temperatures. Significantly, reducing the coordination number of the metal centers appears to increase axial magnetic anisotropy, giving rise to larger magnetic relaxation barriers and 100-s magnetic blocking temperatures of up to 20 K, as observed for the complex [K(crypt-222)][(CpMe4H2Tb)2(μ−

M2(m-dobdc) (M = Mn, Fe, Co, Ni) Metal–Organic Frameworks as Highly Selective, High-Capacity Adsorbents for Olefin/Paraffin Separations

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