MoN 2 (Mo = [(HIPTNCH 2 CH 2 ) 3 N]Mo, where HIPT = 3,5-(2,4,6-i-Pr 3 C 6 H 2 ) 2 C 6 H 3 ) is the first stage in the reduction of N 2 to NH 3 by Mo. Its reaction with dihydrogen in fluid solution yields 'MoH 2 ', a molybdenum-dihydrogen compound. In this report, we describe a comprehensive EPR and 1/2 H/ 14 N ENDOR study of the product of the reaction between MoN 2 and H 2 that is trapped in frozen solution, 1. EPR spectra of 1 show that it has a near-axial g tensor, g = [2.086, 1.961, 1.947], with dramatically reduced g-anisotropy relative to MoN 2 . Analysis of the g-values reveal that this anion has the Mo(III), [d xz , d yz ] 3 orbital configuration, as proposed for the parent MoN 2 complex, and that it undergoes a strong pseudo-Jahn-Teller (PJT) distortion. Simulations of the 2D 35 GHz 1,2 H ENDOR pattern comprised of spectra taken at multiple fields across the EPR envelope (2 K) show that 1 is the [MoH] − anion. 35 GHz Mims pulsed 2 H ENDOR spectra of 1 prepared with 2 H 2 shows the corresponding 2 H − signal, with a substantial deuterium isotope effect in a iso . Radiolytic reduction of a structural analogue, Mo(IV)H, at 77 K, confirms the assignment of 1. Analysis of the 2D 14 N ENDOR pattern for the ligand amine nitrogen further reveals the presence of a linear N ax -Mo-H − molecular axis that is parallel to the unique magnetic direction (g 1 ). The ENDOR pattern of the three equatorial nitrogens is well-reproduced by a model in which the Mo-N eq plane has undergone a static, not dynamic, PJT distortion, leading to a range of hyperfine couplings for the three N eq . The finding of a nearly axial hyperfine coupling tensor for the terminal hydride bound Mo supports the earlier proposal that the two exchangeable hydrogenic species bound to FeMo-cofactor of the nitrogense turnover intermediate that has accumulated four electrons/protons (E 4 ) are hydrides that bridge two metal ions, not terminal hydrides.
The molybdenum tetrahydride species (Triphos)MoH4PPh3 (Triphos = PhP(CH2CH2PPh2)2) generated from sodium triethylborohydride addition to (Triphos)MoCl3 was found to promote CO2 functionalization to afford acrylate, propionate, and formate species. The formation of (Triphos)MoH4PPh3 occurs via a (Triphos)Mo(H)Cl(PPh3) intermediate followed by dismutation of an unobserved six-coordinate molybdenum(II) dihydride complex. Addition of dihydrogen to the dismuation product mixture affords a nearly quantitative yield of (Triphos)MoH4PPh3. The molybdenum tetrahydride species facilitates CO2 insertion into a metal hydride to produce a formate complex, (Triphos)Mo(H)(κ2-CHO2)(PPh3), with an observed rate constant of [2.9(2)] × 10–4 s–1 (25 °C), which is independent of CO2 pressure. Selective formation of acrylate and propionate carbon dioxide–ethylene coupling products, (Triphos)Mo(H)(κ2-C3H3O2)(PPh3) and (Triphos)Mo(H)(κ2-C3H5O2)(PPh3), was achieved by sequential addition of olefin and heterocumulene to (Triphos)MoH4PPh3. A formally zerovalent TriphosMo(η2-C2H4)3 intermediate was characterized by NMR spectroscopy and computational analysis along the pathway for carbon dioxide–ethylene coupling.
Nanoscale, localized corrosion underpins billions of dollars in damage and material costs each year; however, the processes responsible have remained elusive due to the complexity of studying degradative material behavior at nanoscale liquid−solid interfaces. Recent improvements to liquid cell scanning/transmission electron microscopy and associated techniques enable this first look at the nanogalvanic corrosion processes underlying this widespread damage. Nanogalvanic corrosion is observed to initiate at the near-surface ferrite/cementite phase interfaces that typify carbon steel. In minutes, the corrosion front delves deeper into the material, claiming a thin layer of ferrite around all exposed phase boundaries before progressing laterally, converting the ferrite to corrosion product normal to each buried cementite grain. Over the following few minutes, the corrosion product that lines each cementite grain undergoes a volumetric expansion, creating a lateral wedging force that mechanically ejects the cementite grains from their grooves and leaves behind percolation channels into the steel substructure.
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