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.

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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.