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1. Selective Oxidation of Methane to Methanol over Ceria‐Zirconia Supported Mono and Bimetallic Transition Metal Oxide Catalysts

   https://doi.org/10.1002/cctc.202100268  

   Lyu, Yimeng ; Jocz, Jennifer N. ; Xu, Rui ; Williams, Olivia C. ; Sievers, Carsten ( April 2021 , ChemCatChem)

   Several ceria‐zirconia supported mono and bi‐metallic transition metal oxide clusters containing Fe, Cu, and Ni are synthesized by dry impregnation. Through XRD, H₂‐TPR, NH₃‐TPD, pyridine adsorption followed by FTIR spectroscopy and XAS, the well‐dispersed nature of the transition metal oxide clusters is revealed, and the Lewis acidity of the catalysts is assessed. In‐situ FTIR spectroscopy is used to monitor the methane activation on catalyst surfaces. All catalysts activate methane at 250 °C forming methyl, alkyl, and methoxy species on the catalyst surface. By co‐feeding steam and oxygen together with methane, continuous direct oxidation of methane to methanol can be achieved, with the complete oxidation to CO₂as the other reaction path. Methoxy species are found to be a key intermediate for methanol production. Lowering the methane conversion improves the methanol selectivity. By extrapolation, it is estimated that methanol selectivity close to unity can be achieved below a threshold of methane conversion at about 0.002 %. The formation of CuO and NiO mixed metal oxides produces stronger Lewis acid sites and yields higher methanol selectivity.

    

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2. Catalytic Behavior of Mono‐ N ‐Protected Amino‐Acid Ligands in Ligand‐Accelerated C−H Activation by Palladium(II)

   https://doi.org/10.1002/anie.202002484  

   Salazar, Chase A. ; Gair, Joseph J. ; Flesch, Kaylin N. ; Guzei, Ilia A. ; Lewis, Jared C. ; Stahl, Shannon S. ( April 2020 , Angewandte Chemie International Edition)

   Mono‐N‐protected amino acids (MPAAs) are increasingly common ligands in Pd‐catalyzed C−H functionalization reactions. Previous studies have shown how these ligands accelerate catalytic turnover by facilitating the C−H activation step. Here, it is shown that MPAA ligands exhibit a second property commonly associated with ligand‐accelerated catalysis: the ability to support catalytic turnover at substoichiometric ligand‐to‐metal ratios. This catalytic role of the MPAA ligand is characterized in stoichiometric C−H activation and catalytic C−H functionalization reactions. Palladacycle formation with substrates bearing carboxylate and pyridine directing groups exhibit a 50–100‐fold increase in rate when only 0.05 equivalents of MPAA are present relative to Pd^II. These and other mechanistic data indicate that facile exchange between MPAAs and anionic ligands coordinated to Pd^IIenables a single MPAA to support C−H activation at multiple Pd^IIcenters.

    

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3. Catalytic Behavior of Mono‐ N ‐Protected Amino‐Acid Ligands in Ligand‐Accelerated C−H Activation by Palladium(II)

   https://doi.org/10.1002/ange.202002484  

   Salazar, Chase A. ; Gair, Joseph J. ; Flesch, Kaylin N. ; Guzei, Ilia A. ; Lewis, Jared C. ; Stahl, Shannon S. ( April 2020 , Angewandte Chemie)

   Mono‐N‐protected amino acids (MPAAs) are increasingly common ligands in Pd‐catalyzed C−H functionalization reactions. Previous studies have shown how these ligands accelerate catalytic turnover by facilitating the C−H activation step. Here, it is shown that MPAA ligands exhibit a second property commonly associated with ligand‐accelerated catalysis: the ability to support catalytic turnover at substoichiometric ligand‐to‐metal ratios. This catalytic role of the MPAA ligand is characterized in stoichiometric C−H activation and catalytic C−H functionalization reactions. Palladacycle formation with substrates bearing carboxylate and pyridine directing groups exhibit a 50–100‐fold increase in rate when only 0.05 equivalents of MPAA are present relative to Pd^II. These and other mechanistic data indicate that facile exchange between MPAAs and anionic ligands coordinated to Pd^IIenables a single MPAA to support C−H activation at multiple Pd^IIcenters.

    

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4. A DFT Survey of the Effects of d‐Electron Count and Metal Identity on the Activation and Functionalization of C−H Bonds for Mid to Late Transition Metals

   https://doi.org/10.1002/ijch.201700066  

   Moulder, Catherine A. ; Cundari, Thomas R. ( September 2017 , Israel Journal of Chemistry)

   The contribution of metal identity to the activation and functionalization of methane by a series of three‐coordinate imide complexes is evaluatedin silicofor a 3‐by‐3 block of metals from Fe to Pt. Three mechanisms were studied: oxidative addition (OA) to the metal; hydrogen atom abstraction (HAA) by the imide nitrogen; and, [2+2] addition across the metal‐imide bond. In no studied case, was a [2+2] mechanism preferred, perhaps suggesting this mechanism is largely (entirely?) the domain of d⁰imides. There is a diagonal relationship within the nonet of metals studied in that OA is preferred for earlier, heavier (5d) members of the series, transitioning to an HAA mechanism for later, lighter (3d) imides. DFT indicates that important parameters in partitioning between HAA and OA mechanisms include the strength of the metal‐imide π‐bond, the ability of larger metals to accommodate increases in formal oxidation state and coordination number, and the soft acid/base compatibility of larger transition metals with soft hydride and methyl ligands

    

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5. Proton affinity studies of nickel N2S2 complexes and control of aggregation

   https://doi.org/10.1007/s00775-019-01671-4  

   Arnet, Nicholas A. ; Bhuvanesh, Nattamai ; Darensbourg, Marcetta Y. ( June 2019 , JBIC Journal of Biological Inorganic Chemistry)

   The thiolate ligands of [NiFe]-H2ase enzymes have been implicated as proton-binding sites for the reduction/oxidation of H+/H2. This study examines the ligand effect on reactivity of NiN2S2 complexes with an array of acids in methanol solution. UV–Vis absorption spectroscopy is utilized to observe the transformation from the monomeric species to a trimetallic complex that is formed after proton-induced ligand dissociation. Nickel complexes with a flexible (propyl and ethyl) N to N linker were found to readily form the trimetallic complex with acids as weak as ammonium (pKa = 10.9 in methanol). A more constrained nickel complex with a diazacycloheptane N to N linker required stronger acids such as 2,2-dichloroacetic acid (pKa = 6.38 in methanol) to form the trimetallic complex and featured the formation of an NiN2S2H+ complex with acetic acid (pKa = 9.63 in methanol). The most strained ligand, which featured a diazacyclohexane backbone, readily dissociated from the nickel center upon mixture with acids with pKa ≤ 9.63 and showed no evidence of a trimetallic species with any acid. This research highlights the dramatic differences in reactivity with proton sources that can be imparted by minor alterations to ligand geometry and strain. 

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