Nonprecious metal M−N−C (M = Fe or Co) catalysts that are effective for the oxygen-reduction reaction in polymerelectrolyte fuel cells have been developed, but no consensus has yet been reached regarding the nature of the M sites in these heterogeneous catalysts that are responsible for the reaction with dioxygen (O 2 ). While multiple studies have developed correlations between Fe distributions in as-prepared catalysts and ORR activity, the direct identification of sites reactive toward O 2 or O 2 -analogue molecules remains a significant challenge. In the present study, we demonstrate a new approach to identifying and characterizing potential Fe active sites in complex ORR catalysts that combines an effective probe molecule (NO (g) ), Mossbauer spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS) with density functional theory (DFT) calculations. Mossbauer spectroscopic studies demonstrate that NO (g) treatment of electrochemically reduced PANI− 57 Fe−C leads to a selective reaction with only a subset of the Fe species present. Nuclear resonance vibrational spectroscopic studies identified new Fe−ligand vibrations associated with the site reactive toward NO (g) . DFT calculations of the vibrational properties of a selection of previously proposed active-site structures suggest that graphene zigzag edge-hosted Fe−N structures may be responsible for the observed vibrational behavior with NO (g) probe molecules. Furthermore, such sites are likely also reactive to O 2 , possibly serving as the ORR active sites in the synthesized materials.

Vanadium catalysts offer unique selectivity in olefin polymerization, yet are underutilized industrially owing to their poor stability and productivity. Reported here is the immobilization of vanadium by cation exchange in MFU-4l, thus providing a metal-organic framework (MOF) with vanadium in a molecule-like coordination environment. This material forms a single-site heterogeneous catalyst with methylaluminoxane and provides polyethylene with low polydispersity (PDI≈3) and the highest activity (up to 148 000 h ) reported for a MOF-based polymerization catalyst. Furthermore, polyethylene is obtained as a free-flowing powder as desired industrially. Finally, the catalyst shows good structural integrity and retains polymerization activity for over 24 hours, both promising attributes for the commercialization of vanadium-based polyolefins.

Iron phosphine complexes prove to be good precatalysts for the cross-coupling of alkyl, benzyl, and allyl halides with not only aryl triorganoborate salts but also related aluminum-, gallium-, indium-, and thallium-based nucleophiles. Mechanistic studies revealed that while Fe(I) can be accessed on catalytically relevant time scales, lower average oxidation states are not formed fast enough to be relevant to catalysis. EPR spectroscopic studies reveal the presence of bis(diphosphine)iron(I) complexes in representative catalytic reactions and related processes with a range of group 13 nucleophiles. Isolated examples were studied by Mossbauer spectroscopy and single-crystal X-ray structural analysis, while the electronic structure was probed by dispersion-corrected B3LYP DFT calculations. An EPR study on an iron system with a bulky diphosphine ligand revealed the presence of an S = 1 / 2 species consistent with the formation of a mono(diphosphine)iron(I) species with inequivalent phosphine donor environments. DFT analysis of model complexes allowed us to rule out a T-shaped Fe(I) structure, as this is predicted to be high spin. ■ RESULTS AND DISCUSSION Iron Phosphine Complexes for the Coupling of BR 4 − Nucleophiles. We previously found that the preformed Special Issue: Catalytic and Organometallic Chemistry of Earth-Abundant Metals

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