Iron-based superconductivity develops near an antiferromagnetic order and out of a bad-metal normal state, which has been interpreted as originating from a proximate Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, resonant inelastic X-ray scattering and neutron scattering to demonstrate that NaFe1−xCuxAs near x≈0.5 exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behaviour persisting above the Néel temperature, indicative of a Mott insulator. On decreasing x from 0.5, the antiferromagnetic-ordered moment continuously decreases, yielding to superconductivity ∼x=0.05. Our discovery of a Mott-insulating state in NaFe1−xCuxAs thus makes it the only known Fe-based material, in which superconductivity can be smoothly connected to the Mott-insulating state, highlighting the important role of electron correlations in the high-Tc superconductivity.
Artificial enzymatic systems are extensively studied to mimic the structures and functions of their natural counterparts. However, there remains a significant gap between structural modeling and catalytic activity in these artificial systems. Herein we report a novel strategy for the construction of an artificial binuclear copper monooxygenase starting from a Ti metal−organic framework (MOF). The deprotonation of the hydroxide groups on the secondary building units (SBUs) of MIL-125(Ti) (MIL = Mateŕiaux de l'Institut Lavoisier) allows for the metalation of the SBUs with closely spaced Cu I pairs, which are oxidized by molecular O 2 to afford the Cu II 2 (μ 2 -OH) 2 cofactor in the MOF-based artificial binuclear monooxygenase Ti 8 -Cu 2 . An artificial mononuclear Cu monooxygenase Ti 8 -Cu 1 was also prepared for comparison. The MOF-based monooxygenases were characterized by a combination of thermogravimetric analysis, inductively coupled plasma−mass spectrometry, X-ray absorption spectroscopy, Fourier-transform infrared spectroscopy, and UV−vis spectroscopy. In the presence of coreductants, Ti 8 -Cu 2 exhibited outstanding catalytic activity toward a wide range of monooxygenation processes, including epoxidation, hydroxylation, Baeyer−Villiger oxidation, and sulfoxidation, with turnover numbers of up to 3450. Ti 8 -Cu 2 showed a turnover frequency at least 17 times higher than that of Ti 8 -Cu 1 . Density functional theory calculations revealed O 2 activation as the rate-limiting step in the monooxygenation processes. Computational studies further showed that the Cu 2 sites in Ti 8 -Cu 2 cooperatively stabilized the Cu−O 2 adduct for O−O bond cleavage with 6.6 kcal/mol smaller free energy increase than that of the mononuclear Cu sites in Ti 8 -Cu 1 , accounting for the significantly higher catalytic activity of Ti 8 -Cu 2 over Ti 8 -Cu 1 .
Catalytic borylation has recently been suggested as a potential strategy to convert abundant methane to fine chemicals. However, synthetic utility of methane borylation necessitates significant improvement of catalytic activities over original phenanthroline- and diphosphine-Ir complexes. Herein, we report the use of metal–organic frameworks (MOFs) to stabilize low-coordinate Ir complexes for highly active methane borylation to afford the monoborylated product. The mono(phosphine)-Ir based MOF, Zr-P1-Ir, significantly outperformed other Ir catalysts in methane borylation to afford CH3Bpin with a turnover number of 127 at 110 °C. Density functional theory calculations indicated a significant reduction of activation barrier for the rate limiting oxidative addition of methane to the four-coordinate (P1)IrIII(Bpin)3 catalyst to form the six-coordinate (P1)IrV(Bpin)3(CH3)(H) intermediate, thus avoiding the formation of sterically encumbered seven-coordinate IrV intermediates as found in other Ir catalysts based on chelating phenanthroline, bipyridine, and diphosphine ligands. MOF thus stabilizes the homogeneously inaccessible, low-coordinate (P1)Ir(boryl)3 catalyst to provide a unique strategy to significantly lower the activation barrier for methane borylation. This MOF-based catalyst design holds promise in addressing challenging catalytic reactions involving highly inert substrates.
We use transport and neutron scattering to study electronic, structural, and magnetic properties of the electron-doped BaFe2−xNixAs2 iron pnictides in uniaxial strained and external stress free detwinned state. Using a specially designed in-situ mechanical detwinning device, we demonstrate that the in-plane resistivity anisotropy observed in the uniaxial strained tetragonal state of BaFe2−xNixAs2 below a temperature T * , previously identified as a signature of the electronic nematic phase, is also present in the stress free tetragonal phase below T * * (< T * ). By carrying out neutron scattering measurements on BaFe2As2 and BaFe1.97Ni0.03As2, we argue that the resistivity anisotropy in the stress free tetragonal state of iron pnictides arises from the magnetoelastic coupling associated with antiferromagnetic order. These results thus indicate that the local lattice distortion and nematic spin correlations are responsible for the resistivity anisotropy in the tetragonal state of stress free iron pnictides, and suggest that resistivity anisotropy, spin excitation anisotropy, and orbital ordering found in the paramagnetic state of uniaxial strained iron pnictides are due to the externally applied uniaxial strain and its coupling to nematic susceptibility.
Herein we report that the Ti8-BDC (MIL-125) (BDC is 1,4-benzenecarboxylate) metal–organic framework (MOF) supports single-site solid NiII-hydride catalyst for the hydrogenolysis of aryl ethers containing α-O-4, β-O-4, and 4-O-5 linkages to exclusively afford hydrocarbons under mild conditions without the addition of a base. The catalytic activity of Ti8-BDC-NiH is highly dependent on the reduction of Ti8(μ2-O)8(μ2-OH)4 nodes. Density functional theory (DFT) calculations revealed two key steps of σ-bond metathesis in the catalytic cycle of Ti8-BDC-NiH catalyzed hydrogenolysis. This work highlights the potential of MOF-supported single-site catalysts in aryl ether bond scission and other processes for the efficient production of biofuels and chemical feedstocks.
Here we report the design of an enzyme-inspired metal–organic framework (MOF), 1-OTf-Ir, by installing strong Lewis acid and photoredox sites in engineered mesopores. Al-MOF (1), with mixed 2,2′-bipyridyl-5,5-dicarboxylate (dcbpy) and 1,4-benzenediacrylate (pdac) ligands, was oxidized with ozone and then triflated to generate strongly Lewis acidic Al-OTf sites in the mesopores, followed by the installation of [Ir(ppy)2(dcbpy)]+ (ppy = 2-phenylpyridine) sites to afford 1-OTf-Ir with both Lewis acid and photoredox sites. 1-OTf-Ir effectively catalyzed reductive cross-coupling of N-hydroxyphthalimide esters or aryl bromomethyl ketones with vinyl- or alkynyl-azaarenes to afford new azaarene derivatives. 1-OTf-Ir enabled catalytic synthesis of anticholinergic drugs Pheniramine and Chlorpheniramine.
Cleavage of strong C−O bonds without breaking C−C/C−H bonds is a key step for catalytic conversion of renewable biomass to hydrocarbon feedstocks. Herein we report multistep sequential engineering of orthogonal Lewis acid and palladium nanoparticle (NP) catalysts in a metal−organic framework (MOF) built from (Al−OH) n secondary building units and a mixture of 2,2′-bipyridine-5,5′dicarboxylate (dcbpy) and 1,4-benzenediacrylate (pdac) ligands (1) for tandem C−O bond cleavage. Ozonolysis of 1 selectively removed pdac ligands to generate Al 2 (OH)(OH 2 ) sites, which were subsequently triflated with trimethylsilyl triflate to afford strongly Lewis acidic sites for dehydroalkoxylation. Coordination of Pd(MeCN) 2 Cl 2 to dcbpy ligands followed by in situ reduction produced orthogonal Pd NP sites in 1-OTf-Pd NP as the hydrogenation catalyst. The selective and precise transformation of 1 into 1-OTf-Pd NP was characterized step by step using powder Xray diffraction, transmission electron microscopy, thermogravimetric analysis, inductively coupled plasma mass spectrometry, infrared spectroscopy, and X-ray absorption spectroscopy. The hierarchical incorporation of orthogonal Lewis acid and Pd NP active sites endowed 1-OTf-Pd NP with outstanding catalytic performance in apparent hydrogenolysis of etheric, alcoholic, and esteric C−O bonds to generate saturated alkanes via a tandem dehydroalkoxylation−hydrogenation process under relatively mild conditions. The reactivity of C−O bonds followed the trend of tertiary carbon > secondary carbon > primary carbon. Control experiments demonstrated the heterogeneous nature and recyclability of 1-OTf-Pd NP and its superior catalytic activity over the homogeneous counterparts. Sequential engineering of multiple catalytic sites in MOFs thus presents a unique opportunity to address outstanding challenges in sustainable catalysis.
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