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1. An immobilized (carbene)nickel catalyst for water oxidation

   https://doi.org/10.1016/j.poly.2024.116880  

   Lu, Zhiyao; Mitra, Debanjan; Narayan, Sri R.; Williams, Travis J. (February 2024, Polyhedron)

   Gerard Parking (Ed.)

   The oxygen evolution reaction (OER) of water splitting is essential to electrochemical energy storage applications. While nickel electrodes are widely available heterogeneous OER catalysts, homogeneous nickel catalysts for OER are underexplored. Here we report two carbene-ligated nickel(II) complexes that are exceptionally robust and efficient homogeneous water oxidation catalysts. Remarkably, these novel nickel complexes can assemble a stable thin film onto a metal electrode through poly-imidazole bridges, making them supported heterogeneous electrochemical catalysts that are resilient to leaching and stripping. Unlike molecular catalysts and nanoparticle catalysts, such electrode-supported metal-complex catalysts for OER are rare and have the potential to inspire new designs. The electrochemical OER with our nickel-carbene catalysts exhibits excellent current densities with high efficiency, low Tafel slope, and useful longevity for a base metal catalyst. Our data show that imidazole carbene ligands stay bonded to the nickel(II) centers throughout the catalysis, which allows the facile oxygen evolution. 

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2. Structurally Diverse Bench‐Stable Nickel(0) Pre‐Catalysts: A Practical Toolkit for In Situ Ligation Protocols**

   https://doi.org/10.1002/anie.202211794  

   Tran, Van_T; Kim, Nana; Rubel, Camille_Z; Wu, Xiangyu; Kang, Taeho; Jankins, Tanner_C; Li, Zi‐Qi; Joannou, Matthew_V; Ayers, Sloan; Gembicky, Milan; et al (January 2023, Angewandte Chemie International Edition)

   Abstract A flurry of recent research has centered on harnessing the power of nickel catalysis in organic synthesis. These efforts have been bolstered by contemporaneous development of well‐defined nickel (pre)catalysts with diverse structure and reactivity. In this report, we present ten different bench‐stable, 18‐electron, formally zero‐valent nickel–olefin complexes that are competent pre‐catalysts in various reactions. Our investigation includes preparations of novel, bench‐stable Ni(COD)(L) complexes (COD=1,5‐cyclooctadiene), in which L=quinone, cyclopentadienone, thiophene‐S‐oxide, and fulvene. Characterization by NMR, IR, single‐crystal X‐ray diffraction, cyclic voltammetry, thermogravimetric analysis, and natural bond orbital analysis sheds light on the structure, bonding, and properties of these complexes. Applications in an assortment of nickel‐catalyzed reactions underscore the complementary nature of the different pre‐catalysts within this toolkit. 

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3. Accelerating ethylene polymerization using secondary metal ions in tetrahydrofuran

   https://doi.org/10.1039/C9DT04288A  

   Xiao, Dawei; Cai, Zhongzheng; Do, Loi H. (December 2019, Dalton Transactions)

   We have prepared a new series of nickel phosphine phosphonate ester complexes that feature two metal-chelating polyethylene glycol (PEG) side arms. Metal binding and reactivity studies in polar solvents showed that they readily coordinate external cations, including alkali (Li + , Na + , K + ), alkaline (Mg 2+ , Ca 2+ ), transition (Sc 3+ , Co 2+ , Zn 2+ ), post-transition (Ga 3+ ), and lanthanide (La 3+ ) metals. Although olefin polymerization reactions are typically performed in non-polar solvents, which cannot solubilize +2 and +3 metal cations, we discovered that our nickel catalysts could promote ethylene polymerization in neat tetrahydrofuran. This advance allowed us, for the first time, to systematically investigate the effects of a wide range of M + , M 2+ , and M 3+ ions on the reactivity of nickel olefin polymerization catalysts. In ethylene homopolymerization, the addition of Co(OTf) 2 to our nickel-PEG complexes provided the largest boost in activity (up to 11-fold, 2.7 × 10 6 g mol −1 h −1 ) compared to that in the absence of external salts. The catalyst enhancing effects of secondary metals were also observed in studies of ethylene and polar olefin ( e.g. , propyl vinyl ether, allyl butyl ether, methyl-10-undecenoate, and 5-acetoxy-1-pentene) copolymerization. Notably, combining either Co 2+ or Zn 2+ with our nickel complexes increased the rates of polymerization in the presence of propyl vinyl ether by about 5.0- and 2.4-fold, respectively. Although further studies are needed to elucidate the structural and mechanistic roles of the secondary metals, this work is an important advance toward the development of cation-switchable polymerization catalysts. 

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4. Optimizing the Cation Binding Pocket in Nickel Phenoxyimine Catalysts Improves Ethylene Polymerization Efficiency

   https://doi.org/10.1021/acs.organomet.4c00260  

   Ruiz_De_Castilla, Lorenzo C; Ganguly, Tuhin; Tahmouresilerd, Babak; Laconsay, Croix J; Wu, Judy I; Do, Loi H (October 2024, Organometallics)

   Cation tuning is a simple yet powerful strategy to modulate the reactivity of polymerization catalysts but the design rules to achieve maximum cation effects are not well understood. In the present work, it was demonstrated that inserting a methylene spacer between a nickel phenoxyimine complex and an M-polyethylene glycol (PEG) (where M = Li+, Na+, K+, or Cs+) unit led up to >70-fold increase in ethylene polymerization activity and 6-fold higher polymer molecular weight relative to that of the first-generation catalysts. It is hypothesized that these effects are due to the exclusive formation of 1:1 over 2:1 nick-el:alkali species and closer proximity of the M-PEG moiety to the nickel center. These results suggest that the successful creation of cation-responsive catalysts requires an understanding of the cation binding stoichiometry as well as the structural and electronic changes associated with its host-guest interactions. 

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5. Nickel(II) complexes with covalently attached quinols rely on ligand-derived redox couples to catalyze superoxide dismutation

   https://doi.org/10.1039/d4dt03331k  

   Boothe, Robert; Oppelt, Julian; Franke, Alicja; Moore, Jamonica L; Squarcina, Andrea; Zahl, Achim; Senft, Laura; Kellner, Ina; Awalah, Akudo L; Bradford, Alisabeth; et al (February 2025, Dalton Transactions)

   Although nickel is found in the active sites of a class of superoxide dismutase (SOD), nickel complexes with non-peptidic ligands normally do not catalyze superoxide degradation, and none has displayed activity comparable to those of the best manganese-containing SOD mimics. Here, we find that nickel complexes with polydentate quinol-containing ligands can exhibit catalytic activity comparable to those of the most efficient manganese-containing SOD mimics. The nickel complexes retain a significant portion of their activity in phosphate buffer and under operando conditions and rely on ligand-centered redox processes for catalysis. Although nickel SODs are known to cycle through Ni(II) and Ni(III) species during catalysis, cryo-mass spectrometry studies indicate that the nickel atoms in our catalysts remain in the +2 oxidation state throughout SOD mimicry. 

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