A ligand insertion mechanism for cooperative NH3 capture in metal-organic frameworks.

Ammonia is a critical chemical in agriculture and industry that is produced on a massive scale via the Haber-Bosch process1. The environmental impact of this process, which uses methane as a fuel and feedstock for hydrogen, has motivated the need for more sustainable ammonia production2-5. However, many strategies that use renewable hydrogen are not compatible with existing methods for ammonia separation6-9. Given their high surface areas and structural and chemical versatility, metal-organic frameworks (MOFs) hold promise for ammonia separations, but most MOFs bind ammonia irreversibly or degrade on exposure to this corrosive gas10,11. Here we report a tunable three-dimensional framework that reversibly binds ammonia by cooperative insertion into its metal-carboxylate bonds to form a dense, one-dimensional coordination polymer. This unusual adsorption mechanism provides considerable intrinsic thermal management12, and, at high pressures and temperatures, cooperative ammonia uptake gives rise to large working capacities. The threshold pressure for ammonia adsorption can further be tuned by almost five orders of magnitude through simple synthetic modifications, pointing to a broader strategy for the development of energy-efficient ammonia adsorbents.

Confinement of atomically defined metal halide sheets in a metal-organic framework.

The size-dependent and shape-dependent characteristics that distinguish nanoscale materials from bulk solids arise from constraining the dimensionality of an inorganic structure1-3. As a consequence, many studies have focused on rationally shaping these materials to influence and enhance their optical, electronic, magnetic and catalytic properties4-6. Although a select number of stable clusters can typically be synthesized within the nanoscale regime for a specific composition, isolating clusters of a predetermined size and shape remains a challenge, especially for those derived from two-dimensional materials. Here we realize a multidentate coordination environment in a metal-organic framework to stabilize discrete inorganic clusters within a porous crystalline support. We show confined growth of atomically defined nickel(II) bromide, nickel(II) chloride, cobalt(II) chloride and iron(II) chloride sheets through the peripheral coordination of six chelating bipyridine linkers. Notably, confinement within the framework defines the structure and composition of these sheets and facilitates their precise characterization by crystallography. Each metal(II) halide sheet represents a fragment excised from a single layer of the bulk solid structure, and structures obtained at different precursor loadings enable observation of successive stages of sheet assembly. Finally, the isolated sheets exhibit magnetic behaviours distinct from those of the bulk metal halides, including the isolation of ferromagnetically coupled large-spin ground states through the elimination of long-range, interlayer magnetic ordering. Overall, these results demonstrate that the pore environment of a metal-organic framework can be designed to afford precise control over the size, structure and spatial arrangement of inorganic clusters.

Coercive Fields Exceeding 30 T in the Mixed-Valence Single-Molecule Magnet (CpiPr5)2Ho2I3.

Mixed-valence dilanthanide complexes of the type (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Gd, Tb, Dy) featuring a direct Ln-Ln σ-bonding interaction have been shown to exhibit well-isolated high-spin ground states and, in the case of the Tb and Dy variants, a strong axial magnetic anisotropy that gives rise to a large magnetic coercivity. Here, we report the synthesis and characterization of two new mixed-valence dilanthanide compounds in this series, (CpiPr5)2Ln2I3 (1-Ln; Ln = Ho, Er). Both compounds feature a Ln-Ln bonding interaction, the first such interaction in any molecular compounds of Ho or Er. Like the Tb and Dy congeners, both complexes exhibit high-spin ground states arising from strong spin-spin coupling between the lanthanide 4f electrons and a single σ-type lanthanide-lanthanide bonding electron. Beyond these similarities, however, the magnetic properties of the two compounds diverge. In particular, 1-Er does not exhibit observable magnetic blocking or slow magnetic relaxation, while 1-Ho exhibits magnetic blocking below 28 K, which is the highest temperature among Ho-based single-molecule magnets, and a spin reversal barrier of 556(4) cm-1. Additionally, variable-field magnetization data collected for 1-Ho reveal a coercive field of greater than 32 T below 8 K, more than 6-fold higher than observed for the bulk magnets SmCo5 and Nd2Fe14B, and the highest coercive field reported to date for any single-molecule magnet or molecule-based magnetic material. Multiconfigurational calculations, supported by far-infrared magnetospectroscopy data, reveal that the stark differences in magnetic properties of 1-Ho and 1-Er arise from differences in the local magnetic anisotropy of the lanthanide centers.

High-Capacity, Cooperative CO2 Capture in a Diamine-Appended Metal-Organic Framework through a Combined Chemisorptive and Physisorptive Mechanism.

Diamine-appended Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks are promising candidates for carbon capture that exhibit exceptional selectivities and high capacities for CO2. To date, CO2 uptake in these materials has been shown to occur predominantly via a chemisorption mechanism involving CO2 insertion at the amine-appended metal sites, a mechanism that limits the capacity of the material to ∼1 equiv of CO2 per diamine. Herein, we report a new framework, pip2-Mg2(dobpdc) (pip2 = 1-(2-aminoethyl)piperidine), that exhibits two-step CO2 uptake and achieves an unusually high CO2 capacity approaching 1.5 CO2 per diamine at saturation. Analysis of variable-pressure CO2 uptake in the material using solid-state nuclear magnetic resonance (NMR) spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals that pip2-Mg2(dobpdc) captures CO2 via an unprecedented mechanism involving the initial insertion of CO2 to form ammonium carbamate chains at half of the sites in the material, followed by tandem cooperative chemisorption and physisorption. Powder X-ray diffraction analysis, supported by van der Waals-corrected density functional theory, reveals that physisorbed CO2 occupies a pocket formed by adjacent ammonium carbamate chains and the linker. Based on breakthrough and extended cycling experiments, pip2-Mg2(dobpdc) exhibits exceptional performance for CO2 capture under conditions relevant to the separation of CO2 from landfill gas. More broadly, these results highlight new opportunities for the fundamental design of diamine-Mg2(dobpdc) materials with even higher capacities than those predicted based on CO2 chemisorption alone.

Selective Adsorption of Oxygen from Humid Air in a Metal-Organic Framework with Trigonal Pyramidal Copper(I) Sites.

High or enriched-purity O2 is used in numerous industries and is predominantly produced from the cryogenic distillation of air, an extremely capital- and energy-intensive process. There is significant interest in the development of new approaches for O2-selective air separations, including the use of metal-organic frameworks featuring coordinatively unsaturated metal sites that can selectively bind O2 over N2 via electron transfer. However, most of these materials exhibit appreciable and/or reversible O2 uptake only at low temperatures, and their open metal sites are also potential strong binding sites for the water present in air. Here, we study the framework CuI-MFU-4l (CuxZn5-xCl4-x(btdd)3; H2btdd = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), which binds O2 reversibly at ambient temperature. We develop an optimized synthesis for the material to access a high density of trigonal pyramidal CuI sites, and we show that this material reversibly captures O2 from air at 25 °C, even in the presence of water. When exposed to air up to 100% relative humidity, CuI-MFU-4l retains a constant O2 capacity over the course of repeated cycling under dynamic breakthrough conditions. While this material simultaneously adsorbs N2, differences in O2 and N2 desorption kinetics allow for the isolation of high-purity O2 (>99%) under relatively mild regeneration conditions. Spectroscopic, magnetic, and computational analyses reveal that O2 binds to the copper(I) sites to form copper(II)-superoxide moieties that exhibit temperature-dependent side-on and end-on binding modes. Overall, these results suggest that CuI-MFU-4l is a promising material for the separation of O2 from ambient air, even without dehumidification.

Understanding 2p core-level excitons of late transition metals by analysis of mixed-valence copper in a metal-organic framework.

The L2,3-edge X-ray absorption spectra of late transition metals such as Cu, Ag, and Au exhibit absorption onsets lower in energy for higher oxidation states, which is at odds with the measured spectra of earlier transition metals. Time-dependent density functional theory calculations for Cu2+/Cu+ reveal a larger 2p core-exciton binding energy for Cu2+, overshadowing shifts in single-particle excitation energies with respect to Cu+. We explore this phenomenon in a Cu+ metal-organic framework with ∼12% Cu2+ defects and find that corrections with self-consistent excited-state total energy differences provide accurate XAS peak alignment.

Quantum chemical modeling of hydrogen binding in metal-organic frameworks: validation, insight, predictions and challenges.

A detailed chemical understanding of H2 interactions with binding sites in the nanoporous crystalline structure of metal-organic frameworks (MOFs) can lay a sound basis for the design of new sorbent materials. Computational quantum chemical calculations can aid in this quest. To set the stage, we review general thermodynamic considerations that control the usable storage capacity of a sorbent. We then discuss cluster modeling of H2 ligation at MOF binding sites using state-of-the-art density functional theory (DFT) calculations, and how the binding can be understood using energy decomposition analysis (EDA). Employing these tools, we illustrate the connections between the character of the MOF binding site and the associated adsorption thermodynamics using four experimentally characterized MOFs, highlighting the role of open metal sites (OMSs) in accessing binding strengths relevant to room temperature storage. The sorbents are MOF-5, with no open metal sites, Ni2(m-dobdc), containing Lewis acidic Ni(II) sites, Cu(I)-MFU-4l, containing π basic Cu(I) sites and V2Cl2.8(btdd), also containing π-basic V(II) sites. We next explore the potential for binding multiple H2 molecules at a single metal site, with thermodynamics useful for storage at ambient temperature; a materials design goal which has not yet been experimentally demonstrated. Computations on Ca2+ or Mg2+ bound to catecholate or Ca2+ bound to porphyrin show the potential for binding up to 4 H2; there is precedent for the inclusion of both catecholate and porphyrin motifs in MOFs. Turning to transition metals, we discuss the prediction that two H2 molecules can bind at V(II)-MFU-4l, a material that has been synthesized with solvent coordinated to the V(II) site. Additional calculations demonstrate binding three equivalents of hydrogen per OMS in Sc(I) or Ti(I)-exchanged MFU-4l. Overall, the results suggest promising prospects for experimentally realizing higher capacity hydrogen storage MOFs, if nontrivial synthetic and desolvation challenges can be overcome. Coupled with the unbounded chemical diversity of MOFs, there is ample scope for additional exploration and discovery.

Strong Axiality in a Dysprosium(III) Bis(borolide) Complex Leads to Magnetic Blocking at 65 K.

Substituted dysprosocenium complexes of the type [Dy(CpR)2]+ exhibit slow magnetic relaxation at cryogenic temperatures and have emerged as top-performing single-molecule magnets. The remarkable properties of these compounds derive in part from the strong axial ligand field afforded by the cyclopentadiene anions, and the design of analogous compounds with even stronger ligand fields is one promising route toward identifying new single-molecule magnets that retain a magnetic memory at even higher temperatures. Here, we report the synthesis and characterization of a dysprosium bis(borolide) compound, [K(18-crown-6)][Dy(BC4Ph5)2] (1), featuring the dysprosocenate anion [Dy(BC4Ph5)2]- with a pseudoaxial coordination environment afforded by two dianionic pentaphenyl borolide ligands. Variable-field magnetization data reveal open magnetic hysteresis up to 66 K, establishing 1 as a top-performing single-molecule magnet among its dysprosocenium analogues. Ac magnetic susceptibility data indicate that 1 relaxes via an Orbach mechanism above ∼80 K with Ueff = 1500(100) cm-1 and τ0 = 10-12.0(9) s, whereas Raman relaxation and quantum tunneling of the magnetization dominate at lower temperatures. Compound 1 exhibits a 100 s blocking temperature of 65 K, among the highest reported for dysprosium-based single-molecule magnets. Ab initio spin dynamics calculations support the experimental Ueff and τ0 values and enable a quantitative comparison of the relaxation dynamics of 1 and two representative dysprosocenium cations, yielding additional insights into the impact of the crystal field splitting and vibronic coupling on the observed relaxation behavior. Importantly, compound 1 represents a step toward the development of alternatives to substituted dysprosocenium single-molecule magnets with increased axiality.

Synthesis and Crystallographic Characterization of a Reduced Bimetallic Yttrium ansa-Metallocene Hydride Complex, [K(crypt)][(µ-CpAn)Y(µ-H)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2), with a 3.4 Å Yttrium-Yttrium Distance.

The reduction of a bimetallic yttrium ansa-metallocene hydride was examined to explore the possible formation of Y-Y bonds with 4d1 Y(II) ions. The precursor [CpAnY(µ-H)(THF)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2) was synthesized by hydrogenolysis of the allyl complex CpAnY(η3-C3H5)(THF), which was prepared from (C3H5)MgCl and [CpAnY(µ-Cl)]2. Treatment of [CpAnY(µ-H)(THF)]2 with excess KC8 in the presence of one equivalent of 2.2.2-cryptand (crypt) generates an intensely colored red-brown product crystallographically identified as [K(crypt)][(µ-CpAn)Y(µ-H)]2. The two rings of each CpAn ligand in the reduced anion [(µ-CpAn)Y(µ-H)]21- are attached to two yttrium centers in a "flyover" configuration. The 3.3992(6) and 3.4022(7) Å Y···Y distances between the equivalent metal centers within two crystallographically independent complexes are the shortest Y···Y distances observed to date. Ultraviolet-visible (UV-visible)/near infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy support the presence of Y(II), and theoretical analysis describes the singly occupied molecular orbital (SOMO) as an Y-Y bonding orbital composed of metal 4d orbitals mixed with metallocene ligand orbitals. A dysprosium analogue, [K(18-crown-6)(THF)2][(µ-CpAn)Dy(µ-H)]2, was also synthesized, crystallographically characterized, and studied by variable temperature magnetic susceptibility. The magnetic data are best modeled with the presence of one 4f9 Dy(III) center and one 4f9(5dz2)1 Dy(II) center with no coupling between them. CASSCF calculations are consistent with magnetic measurements supporting the absence of coupling between the Dy centers.

A Trinuclear Gadolinium Cluster with a Three-Center One-Electron Bond and an S = 11 Ground State.

The recent discovery of metal-metal bonding and valence delocalization in the dilanthanide complexes (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Y, Gd, Tb, Dy) opened up the prospect of harnessing the 4fn5dz21 electron configurations of non-traditional divalent lanthanide ions to access molecules with novel bonding motifs and magnetism. Here, we report the trinuclear mixed-valence clusters (CpiPr5)3Ln3H3I2 (1-Ln, Ln = Y, Gd), which were synthesized via potassium graphite reduction of the trivalent clusters (CpiPr5)3Ln3H3I3. Structural, computational, and spectroscopic analyses support valence delocalization in 1-Ln resulting from a three-center, one-electron σ bond formed from the 4dz2 and 5dz2 orbitals on Y and Gd, respectively. Dc magnetic susceptibility data obtained for 1-Gd reveal that valence delocalization engenders strong parallel alignment of the σ-bonding electron and the 4f electrons of each gadolinium center to afford a high-spin ground state of S = 11. Notably, this represents the first clear instance of metal-metal bonding in a molecular trilanthanide complex, and the large spin-spin exchange constant of J = 168(1) cm-1 determined for 1-Gd is only the second largest coupling constant characterized to date for a molecular lanthanide compound.

Cooperative Carbon Dioxide Capture in Diamine-Appended Magnesium-Olsalazine Frameworks.

Diamine-appended Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks have emerged as promising candidates for carbon capture owing to their exceptional CO2 selectivities, high separation capacities, and step-shaped adsorption profiles, which arise from a unique cooperative adsorption mechanism resulting in the formation of ammonium carbamate chains. Materials appended with primary,secondary-diamines featuring bulky substituents, in particular, exhibit excellent stabilities and CO2 adsorption properties. However, these frameworks display double-step adsorption behavior arising from steric repulsion between ammonium carbamates, which ultimately results in increased regeneration energies. Herein, we report frameworks of the type diamine-Mg2(olz) (olz4- = (E)-5,5'-(diazene-1,2-diyl)bis(2-oxidobenzoate)) that feature diverse diamines with bulky substituents and display desirable single-step CO2 adsorption across a wide range of pressures and temperatures. Analysis of CO2 adsorption data reveals that the basicity of the pore-dwelling amine─in addition to its steric bulk─is an important factor influencing adsorption step pressure; furthermore, the amine steric bulk is found to be inversely correlated with the degree of cooperativity in CO2 uptake. One material, ee-2-Mg2(olz) (ee-2 = N,N-diethylethylenediamine), adsorbs >90% of the CO2 from a simulated coal flue stream and exhibits exceptional thermal and oxidative stability over the course of extensive adsorption/desorption cycling, placing it among top-performing adsorbents to date for CO2 capture from a coal flue gas. Spectroscopic characterization and van der Waals-corrected density functional theory calculations indicate that diamine-Mg2(olz) materials capture CO2 via the formation of ammonium carbamate chains. These results point more broadly to the opportunity for fundamentally advancing materials in this class through judicious design.

Electron Density Analysis of Metal-Metal Bonding in a Ni4 Cluster Featuring Ferromagnetic Exchange.

We present a combined experimental and theoretical study of the nature of the proposed metal-metal bonding in the tetranuclear cluster Ni4(NPtBu3)4, which features four nickel(I) centers engaged in strong ferromagnetic coupling. High-resolution single-crystal synchrotron X-ray diffraction data collected at 25 K provide an accurate geometrical structure and a multipole model electron density description. Topological analysis of the electron density in the Ni4N4 core using the quantum theory of atoms in molecules clearly identifies the bonding as an eight-membered ring of type [Ni-N-]4 without direct Ni-Ni bonding, and this result is generally corroborated by an analysis of the energy density distribution. In contrast, the calculated bond delocalization index of ∼0.6 between neighboring Ni atoms is larger than what has been found for other bridged metal-metal bonds and implies direct Ni-Ni bonding. Similar support for the presence of direct Ni-Ni bonding is found in the interacting quantum atom approach, an energy decomposition scheme, which suggests the presence of stabilizing Ni-Ni bonding interactions with an exchange-correlation energy contribution approximately 50% of that of the Ni-N interactions. Altogether, while the direct interactions between neighboring Ni centers are too weak and sterically constrained to bear the signature of a topological bond critical point, other continuous measures clearly indicate significant Ni-Ni bonding. These metal-metal bonding interactions likely mediate direct ferromagnetic exchange, giving rise to the high-spin ground state of the molecule.

A spin transition mechanism for cooperative adsorption in metal-organic frameworks.

Cooperative binding, whereby an initial binding event facilitates the uptake of additional substrate molecules, is common in biological systems such as haemoglobin. It was recently shown that porous solids that exhibit cooperative binding have substantial energetic benefits over traditional adsorbents, but few guidelines currently exist for the design of such materials. In principle, metal-organic frameworks that contain coordinatively unsaturated metal centres could act as both selective and cooperative adsorbents if guest binding at one site were to trigger an electronic transformation that subsequently altered the binding properties at neighbouring metal sites. Here we illustrate this concept through the selective adsorption of carbon monoxide (CO) in a series of metal-organic frameworks featuring coordinatively unsaturated iron(ii) sites. Functioning via a mechanism by which neighbouring iron(ii) sites undergo a spin-state transition above a threshold CO pressure, these materials exhibit large CO separation capacities with only small changes in temperature. The very low regeneration energies that result may enable more efficient Fischer-Tropsch conversions and extraction of CO from industrial waste feeds, which currently underutilize this versatile carbon synthon. The electronic basis for the cooperative adsorption demonstrated here could provide a general strategy for designing efficient and selective adsorbents suitable for various separations.

Evaluation of the Stability of Diamine-Appended Mg2(dobpdc) Frameworks to Sulfur Dioxide.

Diamine-appended Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks are a promising class of CO2 adsorbents, although their stability to SO2─a trace component of industrially relevant exhaust streams─remains largely untested. Here, we investigate the impact of SO2 on the stability and CO2 capture performance of dmpn-Mg2(dobpdc) (dmpn = 2,2-dimethyl-1,3-propanediamine), a candidate material for carbon capture from coal flue gas. Using SO2 breakthrough experiments and CO2 isobar measurements, we find that the material retains 91% of its CO2 capacity after saturation with a wet simulated flue gas containing representative levels of CO2 and SO2, highlighting the robustness of this framework to SO2 under realistic CO2 capture conditions. Initial SO2 cycling experiments suggest dmpn-Mg2(dobpdc) may achieve a stable operating capacity in the presence of SO2 after initial passivation. Evaluation of several other diamine-Mg2(dobpdc) variants reveals that those with primary,primary (1°,1°) diamines, including dmpn-Mg2(dobpdc), are more robust to humid SO2 than those featuring primary,secondary (1°,2°) or primary,tertiary (1°,3°) diamines. Based on the solid-state 15N NMR spectra and density functional theory calculations, we find that under humid conditions, SO2 reacts with the metal-bound primary amine in 1°,2° and 1°,3° diamine-appended Mg2(dobpdc) to form a metal-bound bisulfite species that is charge balanced by a primary ammonium cation, thereby facilitating material degradation. In contrast, humid SO2 reacts with the free end of 1°,1° diamines to form ammonium bisulfite, leaving the metal-diamine bond intact. This structure-property relationship can be used to guide further optimization of these materials for CO2 capture applications.

Divalent Lanthanide Metallocene Complexes with a Linear Coordination Geometry and Pronounced 6s-5d Orbital Mixing.

A small but growing number of molecular compounds have been isolated featuring divalent lanthanides with 4fn5dz21 electron configurations. While the majority of these possess trigonal coordination geometries, we previously reported the first examples of linear divalent metallocenes Ln(CpiPr5)2 (Ln = Tb, Dy; CpiPr5 = pentaisopropylcyclopentadienyl). Here, we report the synthesis and characterization of the remainder of the Ln(CpiPr5)2 (1-Ln) series (including Y and excluding Pm). The compounds can be synthesized through salt metathesis of LnI3 and NaCpiPr5 followed by potassium graphite reduction for Ln = Y, La, Ce, Pr, Nd, Gd, Ho, and Er, by in situ reduction during salt metathesis of LnI3 and NaCpiPr5 for Ln = Tm and Lu, or through salt metathesis from LnI2 and NaCpiPr5 for Ln = Sm, Eu, and Yb. Single crystal X-ray diffraction analyses of 1-Ln confirm a linear coordination geometry with pseudo-D5d symmetry for the entire series. Structural and ultraviolet-visible spectroscopy data support a 4fn+1 electron configuration for Ln2+ = Sm, Eu, Tm, and Yb and a 4fn5dz21 configuration for the other lanthanides ([Kr]4dz21 for Y2+). Characterization of 1-Ln (Ln = Y, La) using electron paramagnetic resonance spectroscopy reveals significant s-d orbital mixing in the highest occupied molecular orbital and hyperfine coupling constants that are the largest reported to date for divalent compounds of yttrium and lanthanum. Evaluation of the room temperature magnetic susceptibilities of 1-Ln and comparison with values previously reported for trigonal Ln2+ compounds suggests that the more pronounced 6s-5d mixing may be associated with weaker 4f-5d spin coupling.

Porous materials for carbon dioxide separations.

Global investment in counteracting climate change has galvanized increasing interest in carbon capture and sequestration (CCS) as a versatile emissions mitigation technology. As decarbonization efforts accelerate, CCS can target the emissions of large point-source emitters, such as coal- or natural gas-fired power plants, while also supporting the production of renewable or low-carbon fuels. Furthermore, CCS can enable decarbonization of difficult-to-abate industrial processes and can support net CO2 removal from the atmosphere through bioenergy coupled with CCS or direct air capture. Here we review the development of porous materials as next-generation sorbents for CO2 capture applications. We focus on stream- and sector-specific challenges while highlighting case studies within the context of the rapidly shifting energy landscape. We conclude with a discussion of key needs from the materials community to expand deployment of carbon capture technologies.

Probing growth of metal-organic frameworks with X-ray scattering and vibrational spectroscopy.

Nucleation and crystallization arising from liquid to solid phase are involved in a multitude of processes in fields ranging from materials science to biology. Controlling the thermodynamics and kinetics of growth is advantageous to help tune the formation of complex morphologies. Here, we harness wide-angle X-ray scattering and vibrational spectroscopy to elucidate the mechanism for crystallization and growth of the metal-organic framework Co-MOF-74 within microscopic volumes enclosed in a capillary and an attenuated total reflection microchip reactor. The experiments reveal molecular and structural details of the growth processes, while the results of plane wave density functional calculations allow identification of lattice and linker modes in the formed crystals. Synthesis of the metal-organic framework with microscopic volumes leads to monodisperse and micron-sized crystals, in contrast to those typically observed under bulk reaction conditions. Reduction in the volume of reagents within the microchip reactor was found to accelerate the reaction rate. The coupling of spectroscopy with scattering to probe reactions in microscopic volumes promises to be a useful tool in the synthetic chemist's kit to understand chemical bonding and has potential in designing complex materials.

Ambient-Temperature Hydrogen Storage via Vanadium(II)-Dihydrogen Complexation in a Metal-Organic Framework.

The widespread implementation of H2 as a fuel is currently hindered by the high pressures or cryogenic temperatures required to achieve reasonable storage densities. In contrast, the realization of materials that strongly and reversibly adsorb hydrogen at ambient temperatures and moderate pressures could transform the transportation sector and expand adoption of fuel cells in other applications. To date, however, no adsorbent has been identified that exhibits a binding enthalpy within the optimal range of -15 to -25 kJ/mol for ambient-temperature hydrogen storage. Here, we report the hydrogen adsorption properties of the metal-organic framework (MOF) V2Cl2.8(btdd) (H2btdd, bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), which features exposed vanadium(II) sites capable of backbonding with weak π acids. Significantly, gas adsorption data reveal that this material binds H2 with an enthalpy of -21 kJ/mol. This binding energy enables usable hydrogen capacities that exceed that of compressed storage under the same operating conditions. The Kubas-type vanadium(II)-dihydrogen complexation is characterized by a combination of techniques. From powder neutron diffraction data, a V-D2(centroid) distance of 1.966(8) Å is obtained, the shortest yet reported for a MOF. Using in situ infrared spectroscopy, the H-H stretch was identified, and it displays a red shift of 242 cm-1. Electronic structure calculations show that a main contribution to bonding stems from the interaction between the vanadium dπ and H2 σ* orbital. Ultimately, the pursuit of MOFs containing high densities of weakly π-basic metal sites may enable storage capacities under ambient conditions that far surpass those accessible with compressed gas storage.

Fluoroarene Separations in Metal-Organic Frameworks with Two Proximal Mg2+ Coordination Sites.

Fluoroarenes are widely used in medicinal, agricultural, and materials chemistry, and yet their production remains a critical challenge in organic synthesis. Indeed, the nearly identical physical properties of these vital building blocks hinders their purification by traditional methods, such as flash chromatography or distillation. As a result, the Balz-Schiemann reaction is currently employed to prepare fluoroarenes instead of more atom-economical C-H fluorination reactions, which produce inseparable mixtures of regioisomers. Herein, we propose an alternative solution to this problem: the purification of mixtures of fluoroarenes using metal-organic frameworks (MOFs). Specifically, we demonstrate that controlling the interaction of fluoroarenes with adjacent coordinatively unsaturated Mg2+ centers within a MOF enables the separation of fluoroarene mixtures with unparalleled selectivities. Liquid-phase multicomponent equilibrium adsorption data and breakthrough measurements coupled with van der Waals-corrected density functional theory calculations reveal that the materials Mg2(dobdc) (dobdc4- = 2,5-dioxidobenzene-1,4-dicarboxylate) and Mg2(m-dobdc) (m-dobdc4- = 2,4-dioxidobenzene-1,5-dicarboxylate) are capable of separating the difluorobenzene isomers from one another. Additionally, these frameworks facilitate the separations of fluoroanisoles, fluorotoluenes, and fluorochlorobenzenes. In addition to enabling currently unfeasible separations for the production of fluoroarenes, our results suggest that carefully controlling the interaction of isomers with not one but two strong binding sites within a MOF provides a general strategy for achieving challenging liquid-phase separations.

Strong Ferromagnetic Exchange Coupling and Single-Molecule Magnetism in MoS43--Bridged Dilanthanide Complexes.

We report the synthesis and characterization of the trinuclear 4d-4f compounds [Co(C5Me5)2][(C5Me5)2Ln(µ-S)2Mo(µ-S)2Ln(C5Me5)2], 1-Ln (Ln = Y, Gd, Tb, Dy), containing the highly polarizable MoS43- bridging unit. UV-Vis-NIR diffuse reflectance spectra and DFT calculations of 1-Ln reveal a low-energy metal-to-metal charge transfer transition assigned to charge transfer from the singly occupied 4dz2 orbital of MoV to the empty 5d orbitals of the lanthanides (4d in the case of 1-Y), mediated by sulfur-based 3p orbitals. Electron paramagnetic resonance spectra collected for 1-Y in a tetrahydrofuran solution show large 89Y hyperfine coupling constants of A⊥ = 23 MHz and A|| = 26 MHz, indicating the presence of significant yttrium-localized unpaired electron density. Magnetic susceptibility data support similar electron delocalization and ferromagnetic Ln-Mo exchange for 1-Gd, 1-Tb, and 1-Dy. This ferromagnetic exchange gives rise to an S = 15/2 ground state for 1-Gd and one of the largest magnetic exchange constants involving GdIII observed to date, with JGd-Mo = +16.1(2) cm-1. Additional characterization of 1-Tb and 1-Dy by ac magnetic susceptibility measurements reveals that both compounds exhibit slow magnetic relaxation. Although a Raman magnetic relaxation process is dominant for both 1-Tb and 1-Dy, an extracted thermal relaxation barrier of Ueff = 68 cm-1 for 1-Dy is the largest yet reported for a complex containing a paramagnetic 4d metal center. Together, these results provide a potentially generalizable route to enhanced nd-4f magnetic exchange, revealing opportunities for the design of new nd-4f single-molecule magnets and bulk magnetic materials.