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1. A five-coordinate Ni( i ) complex supported by 1,4,7-triisopropyl-1,4,7-triazacyclononane

   https://doi.org/10.1039/d2cc02516g  

   Griego, Leonel; Woods, Toby J.; Mirica, Liviu M. (June 2022, Chemical Communications)

   An isolated Ni( ii )-nitrosyl complex supported by the bulky tridentate 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr 3 TACN) ligand was obtained from the reaction of a Ni( ii ) dimethyl complex with NOPF 6 , suggesting the in situ formation of a Ni( i ) species that reacts with the resulting NO product. Use of a π-acceptor ancillary isocyanide ligand led to the isolation and characterization of an uncommon 5-coordinate Ni( i ) complex supported by the iPr 3 TACN ligand and tert -butylisocyanide. 

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2. Synthesis, characterization, and electrocatalytic activity of bis(pyridylimino)isoindoline Cu( ii ) and Ni( ii ) complexes

   https://doi.org/10.1039/D0DT03030A  

   Saha, Soumen; Sahil, Sha Tamanna; Mazumder, Md. Motiur; Stephens, Alexander M.; Cronin, Bryan; Duin, Evert C.; Jurss, Jonah W.; Farnum, Byron H. (January 2021, Dalton Transactions)

   null (Ed.)

   Two NNN pincer complexes of Cu( ii ) and Ni( ii ) with BPI Me − [BPI Me − = 1,3-bis((6-methylpyridin-2-yl)imino)isoindolin-2-ide] have been prepared and characterized structurally, spectroscopically, and electrochemically. The single crystal structures of the two complexes confirmed their distorted trigonal bipyramidal geometry attained by three equatorial N-atoms from the ligand and two axially positioned water molecules to give [Cu(BPI Me )(H 2 O) 2 ]ClO 4 and [Ni(BPI Me )(H 2 O) 2 ]ClO 4 . Electrochemical studies of Cu( ii ) and Ni( ii ) complexes have been performed in acetonitrile to identify metal-based and ligand-based redox activity. When subjected to a saturated CO 2 atmosphere, both complexes displayed catalytic activity for the reduction of CO 2 with the Cu( ii ) complex displaying higher activity than the Ni( ii ) analogue. However, both complexes were shown to decompose into catalytically active heterogeneous materials on the electrode surface over extended reductive electrolysis periods. Surface analysis of these materials using energy dispersive spectroscopy as well as their physical appearance suggests the reductive deposition of copper and nickel metal on the electrode surface. Electrocatalysis and decomposition are proposed to be triggered by ligand reduction, where complex stability is believed to be tied to fluxional ligand coordination in the reduced state. 

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3. Ni-Catalyzed Enantioselective Desymmetrization: Development of Divergent Acyl and Decarbonylative Cross-Coupling Reactions

   https://doi.org/10.1021/jacs.4c14767  

   Hernández-Mejías, Ángel D; Shimozono, Alexander M; Hazra, Avijit; Richter, Sven; Tong, Zhengjia; Langille, Neil F; Quasdorf, Kyle; Parsons, Andrew T; Sigman, Matthew S; Reisman, Sarah E (January 2025, Journal of the American Chemical Society)

   Ni-catalyzed asymmetric reductive cross-coupling reactions provide rapid and modular access to enantioenriched building blocks from simple electrophile precursors. Reductive coupling reactions that can diverge through a common organometallic intermediate to two distinct families of enantioenriched products are particularly versatile but underdeveloped. Here, we describe the development of a bis(oxazoline) ligand that enables the desymmetrization of meso-anhydrides. When secondary benzylic electrophiles are employed, doubly stereoselective acyl cross-coupling proceeds to give ketone products with catalyst control over three newly formed stereogenic centers. Alternatively, the use of primary alkyl halides in the presence of an additional halogen atom transfer catalyst results in decarbonylative alkylation to give enantioenriched β-alkyl acids. Analysis of reaction rates for a range of both catalysts and substrates supports the notion that tuning the different electrophile activation steps with the two catalysts is required for enhanced reaction performance. These studies illustrate how reaction design can diverge a common Ni-acyl intermediate to either acyl or decarbonylative coupling products and highlight how dual ligand systems can be used to engage unactivated alkyl halides in Ni-catalyzed asymmetric reductive coupling. 

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4. Migratory functionalization of unactivated alkyl bromides for construction of all-carbon quaternary centers via transposed tert-C-radicals

   https://doi.org/10.1038/s41467-020-18658-4  

   Zhu, Chuan; Liu, Ze-Yao; Tang, Luning; Zhang, Heng; Zhang, Yu-Feng; Walsh, Patrick J.; Feng, Chao (December 2020, Nature Communications)

   null (Ed.)

   Abstract Despite remarkable recent advances in transition-metal-catalyzed C(sp 3 )−C cross-coupling reactions, there remain challenging bond formations. One class of such reactions include the formation of tertiary -C(sp 3 )−C bonds, presumably due to unfavorable steric interactions and competing isomerizations of tertiary alkyl metal intermediates. Reported herein is a Ni-catalyzed migratory 3,3-difluoroallylation of unactivated alkyl bromides at remote tertiary centers. This approach enables the facile construction of otherwise difficult to prepare all-carbon quaternary centers. Key to the success of this transformation is an unusual remote functionalization via chain walking to the most sterically hindered tertiary C(sp 3 ) center of the substrate. Preliminary mechanistic and radical trapping studies with primary alkyl bromides suggest a unique mode of tertiary C-radical generation through chain-walking followed by Ni–C bond homolysis. This strategy is complementary to the existing coupling protocols with tert -alkyl organometallic or -alkyl halide reagents, and it enables the expedient formation of quaternary centers from easily available starting materials. 

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5. The curious cases of tetrahydrosalen-type ligands interacting with Ni( ii ): structures and ligand-based oxidation reactions

   https://doi.org/10.1039/d3dt02023a  

   Xu, Vivian; Pandey, Bedraj; Jayawardhena, J. P.; Krause, Jeanette A.; Guan, Hairong (August 2023, Dalton Transactions)

   This work centers around the nickel complexes derived from two tetrahydrosalen-type proligands: N , N ′-bis(2-hydroxybenzyl)- o -phenylenediamine (H 2 salophan) and N , N ′-bis(2-hydroxy-3-methylbenzyl)- o -phenylenediamine (H 2 salophan_Me). The reaction of H 2 salophan with Ni(OAc) 2 ·4H 2 O generates a dinuclear complex Ni 2 (Hsalophan) 2 (OAc) 2 or Na[Ni 2 (salophan) 2 (OAc)] when NaOH is added to assist ligand deprotonation. The reaction of H 2 salophan_Me with Ni(OAc) 2 ·4H 2 O, however, yields a mononuclear complex Ni(Hsalophan_Me) 2 . Unlike the corresponding salen-type nickel complexes, these tetrahydrosalen-type complexes are paramagentic and air sensitive (in solution). Oxidation by O 2 or peroxides results in dehydrogenation of the ligand backbone to form the salen-type complexes. 

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