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1. Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

   https://doi.org/10.1002/anie.201903638  

   Roberts, Courtney C. ; Camasso, Nicole M. ; Bowes, Eric G. ; Sanford, Melanie S. ( May 2019 , Angewandte Chemie International Edition)

   Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp³)−C and C(sp³)−O bond‐forming reactions at high‐valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral Ni^IVcomplex ligated by tris(pyrazolyl)borate and a cationic octahedral Ni^IIIcomplex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp³)−C(sp²) bond‐forming reductive elimination occurs from both centers, but the Ni^IIIcomplex reacts up to 300‐fold faster than the Ni^IV, depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO⁻), the Ni^IVcomplex exclusively undergoes C(sp³)−OAc bond formation, while the Ni^IIIanalogue forms the C(sp³)−C(sp²) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M−C(sp³) bonds, and thus their relative reactivity towards outer‐sphere S_N2‐type bond‐forming reactions.

    

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2. Binuclear, High‐Valent Nickel Complexes: Ni−Ni Bonds in Aryl–Halogen Bond Formation

   https://doi.org/10.1002/ange.201611572  

   Diccianni, Justin B. ; Hu, Chunhua ; Diao, Tianning ( February 2017 , Angewandte Chemie)

   Metal–metal bonds play a vital role in stabilizing key intermediates in bond‐formation reactions. We report that binuclear benzo[h]quinoline‐ligated Ni^IIcomplexes, upon oxidation, undergo reductive elimination to form carbon–halogen bonds. A mixed‐valent Ni(2.5+)–Ni(2.5+) intermediate is isolated. Further oxidation to Ni^III, however, is required to trigger reductive elimination. The binuclear Ni^III–Ni^IIIintermediate lacks a Ni−Ni bond. Each Ni^IIIundergoes separate, but fast reductive elimination, giving rise to Ni^Ispecies. The reactivity of these binuclear Ni complexes highlights the fundamental difference between Ni and Pd in mediating bond‐formation processes.

    

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3. Aerobic C–C and C–O bond formation reactions mediated by high-valent nickel species

   https://doi.org/10.1039/c9sc03758f  

   Smith, Sofia M. ; Planas, Oriol ; Gómez, Laura ; Rath, Nigam P. ; Ribas, Xavi ; Mirica, Liviu M. ( November 2019 , Chemical Science)

   Nickel complexes have been widely employed as catalysts in C–C and C–heteroatom bond formation reactions. While Ni(0), Ni( i ), and Ni( ii ) intermediates are most relevant in these transformations, recently Ni( iii ) and Ni( iv ) species have also been proposed to play a role in catalysis. Reported herein is the synthesis, detailed characterization, and reactivity of a series of Ni( ii ) and Ni( iii ) metallacycle complexes stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the oxidation of the Ni( ii ) complexes with various other oxidants led to exclusive C–C bond formation in very good yields, the use of O 2 or H 2 O 2 as oxidants led to formation of appreciable amounts of C–O bond formation products, especially for the Ni( ii ) complex supported by an asymmetric pyridinophane ligand containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry. 

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4. Catalytic Ammonia Oxidation to Dinitrogen by a Nickel Complex

   https://doi.org/10.1002/ange.202213462  

   Stephens, David N. ; Szilagyi, Robert K. ; Roehling, Paige N. ; Arulsamy, Navamoney ; Mock, Michael T. ( November 2022 , Angewandte Chemie)

   We report a nickel complex for catalytic oxidation of ammonia to dinitrogen under ambient conditions. Using the aryloxyl radical 2,4,6‐tri‐tert‐butylphenoxyl (^tBu₃ArO⋅) as a H atom acceptor to cleave the N−H bond of a coordinated NH₃ligand up to 56 equiv of N₂per Ni center can be generated. Employing theN‐oxyl radical 2,2,6,6‐(tetramethylpiperidin‐1‐yl)oxyl (TEMPO⋅) as the H‐atom acceptor, up to 15 equiv of N₂per Ni center are formed. A bridging Ni‐hydrazine product identified by isotopic nitrogen (¹⁵N) studies and supported by computational models indicates the N−N bond forming step occurs by bimetallic homocoupling of two paramagnetic [Ni]−NH₂fragments. Ni‐mediated hydrazine disproportionation to N₂and NH₃completes the catalytic cycle.

    

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5. Catalytic Ammonia Oxidation to Dinitrogen by a Nickel Complex

   https://doi.org/10.1002/anie.202213462  

   Stephens, David N. ; Szilagyi, Robert K. ; Roehling, Paige N. ; Arulsamy, Navamoney ; Mock, Michael T. ( November 2022 , Angewandte Chemie International Edition)

   We report a nickel complex for catalytic oxidation of ammonia to dinitrogen under ambient conditions. Using the aryloxyl radical 2,4,6‐tri‐tert‐butylphenoxyl (^tBu₃ArO⋅) as a H atom acceptor to cleave the N−H bond of a coordinated NH₃ligand up to 56 equiv of N₂per Ni center can be generated. Employing theN‐oxyl radical 2,2,6,6‐(tetramethylpiperidin‐1‐yl)oxyl (TEMPO⋅) as the H‐atom acceptor, up to 15 equiv of N₂per Ni center are formed. A bridging Ni‐hydrazine product identified by isotopic nitrogen (¹⁵N) studies and supported by computational models indicates the N−N bond forming step occurs by bimetallic homocoupling of two paramagnetic [Ni]−NH₂fragments. Ni‐mediated hydrazine disproportionation to N₂and NH₃completes the catalytic cycle.

    

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