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1. Deciphering the mechanism of the Ni-photocatalyzed C‒O cross-coupling reaction using a tridentate pyridinophane ligand

   https://doi.org/10.1038/s41467-022-28948-8  

   Na, Hanah ; Mirica, Liviu M. ( March 2022 , Nature Communications)

   Photoredox nickel catalysis has emerged as a powerful strategy for cross-coupling reactions. Although the involvement of paramagnetic Ni(I)/Ni(III) species as active intermediates in the catalytic cycle has been proposed, a thorough spectroscopic investigation of these species is lacking. Herein, we report the tridentate pyridinophane ligands^RN3 that allow for detailed mechanistic studies of the photocatalytic C–O coupling reaction. The derived (^RN3)Ni complexes are active catalysts under mild conditions and without an additional photocatalyst. We also provide direct evidence for the key steps involving paramagnetic Ni species in the proposed catalytic cycle: the oxidative addition of an aryl halide to a Ni(I) species, the ligand exchange/transmetalation at a Ni(III) center, and the C–O reductive elimination from a Ni(III) species. Overall, the present work suggests the^RN3 ligands are a practical platform for mechanistic studies of Ni-catalyzed reactions and for the development of new catalytic applications.

    

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2. Electronic structures and spectroscopic signatures of diiron intermediates generated by O 2 activation of nonheme iron( ii )–thiolate complexes

   https://doi.org/10.1039/d1dt02286e  

   Ekanayake, Danushka M. ; Pham, Dao ; Probst, Andrew L. ; Miller, Joshua R. ; Popescu, Codrina V. ; Fiedler, Adam T. ( October 2021 , Dalton Transactions)

   The activation of O 2 at thiolate–ligated iron( ii ) sites is essential to the function of numerous metalloenzymes and synthetic catalysts. Iron–thiolate bonds in the active sites of nonheme iron enzymes arise from either coordination of an endogenous cysteinate residue or binding of a deprotonated thiol-containing substrate. Examples of the latter include sulfoxide synthases, such as EgtB and OvoA, that utilize O 2 to catalyze tandem S–C bond formation and S -oxygenation steps in thiohistidine biosyntheses. We recently reported the preparation of two mononuclear nonheme iron–thiolate complexes (1 and 2) that serve as structural active-site models of substrate-bound EgtB and OvoA ( Dalton Trans. 2020, 49 , 17745–17757). These models feature monodentate thiolate ligands and tripodal N 4 ligands with mixed pyridyl/imidazolyl donors. Here, we describe the reactivity of 1 and 2 with O 2 at low temperatures to give metastable intermediates (3 and 4, respectively). Characterization with multiple spectroscopic techniques (UV-vis absorption, NMR, variable-field and -temperature Mössbauer, and resonance Raman) revealed that these intermediates are thiolate-ligated iron( iii ) dimers with a bridging oxo ligand derived from the four-electron reduction of O 2 . Structural models of 3 and 4 consistent with the experimental data were generated via density functional theory (DFT) calculations. The combined experimental and computational results illuminate the geometric and electronic origins of the unique spectral features of diiron( iii )-μ-oxo complexes with thiolate ligands, and the spectroscopic signatures of 3 and 4 are compared to those of closely-related diiron( iii )-μ-peroxo species. Collectively, these results will assist in the identification of intermediates that appear on the O 2 reaction landscapes of iron–thiolate species in both biological and synthetic environments. 

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3. Leveraging the Redox Promiscuity of Nickel To Catalyze C–N Coupling Reactions

   https://doi.org/10.1021/acs.joc.3c02353  

   Taylor, Olivia R. ; Saucedo, Paul J. ; Bahamonde, Ana ( January 2024 , The Journal of Organic Chemistry)

   This perspective details advances made in the field of Ni-catalyzed C–N bond formation. The use of this Earth abundant metal to decorate amines, amides, lactams, and heterocycles enables direct access to a variety of biologically active and industrially relevant compounds in a sustainable manner. Herein, different strategies that leverage the propensity of Ni to facilitate both one- and two-electron processes will be surveyed. The first part of this Perspective focuses on strategies that facilitate C–N couplings at room temperature by accessing oxidized Ni(III) intermediates. In this context, advances in photochemical, electrochemical, and chemically mediated processes will be analyzed. A special emphasis has been put on providing a comprehensive explanation of the different mechanistic avenues that have been proposed to facilitate these chemistries; either Ni(I/III) self-sustained cycles or Ni(0/II/III) photochemically mediated pathways. The second part of this Perspective details the ligand designs that also enable access to this reactivity via a two-electron Ni(0/II) mechanism. Finally, we discuss our thoughts on possible future directions of the field. 

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4. C–H bond activation via concerted metalation–deprotonation at a palladium( iii ) center

   https://doi.org/10.1039/D3SC00034F  

   Bouley, Bailey S. ; Tang, Fengzhi ; Bae, Dae Young ; Mirica, Liviu M. ( April 2023 , Chemical Science)

   Herein we report the direct observation of C–H bond activation at an isolated mononuclear Pd( iii ) center. The oxidation of the Pd( ii ) complex ( Me N4)Pd II (neophyl)Cl (neophyl = –CH 2 C(CH 3 ) 2 Ph; Me N4 = N , N ′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF 6 ) yields the stable Pd( iii ) complex [( Me N4)Pd III (neophyl)Cl]PF 6 . Upon the addition of an acetate source, [( Me N4)Pd III (neophyl)Cl]PF 6 undergoes Csp 2 –H bond activation to yield the cyclometalated product [( Me N4)Pd III (cycloneophyl)]PF 6 . This metalacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd( iii ) complexes. The C–H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest that either a pre-association step such as an agostic interaction may be rate limiting, or that the C–H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ 3 ligand coordination and that the flexible ligand structure is important for this transformation. Overall, this study represents the first example of discrete C–H bond activation occurring at a Pd( iii ) center through a concerted metalation–deprotonation mechanism, akin to that observed for Pd( ii ) and Pd( iv ) centers. 

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5. Transamidation of Amides and Amidation of Esters by Selective N–C(O)/O–C(O) Cleavage Mediated by Air- and Moisture-Stable Half-Sandwich Nickel(II)–NHC Complexes

   https://doi.org/10.3390/molecules26010188  

   Buchspies, Jonathan ; Rahman, Md. Mahbubur ; Szostak, Michal ( January 2021 , Molecules)

   null (Ed.)

   The formation of amide bonds represents one of the most fundamental processes in organic synthesis. Transition-metal-catalyzed activation of acyclic twisted amides has emerged as an increasingly powerful platform in synthesis. Herein, we report the transamidation of N-activated twisted amides by selective N–C(O) cleavage mediated by air- and moisture-stable half-sandwich Ni(II)–NHC (NHC = N-heterocyclic carbenes) complexes. We demonstrate that the readily available cyclopentadienyl complex, [CpNi(IPr)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), promotes highly selective transamidation of the N–C(O) bond in twisted N-Boc amides with non-nucleophilic anilines. The reaction provides access to secondary anilides via the non-conventional amide bond-forming pathway. Furthermore, the amidation of activated phenolic and unactivated methyl esters mediated by [CpNi(IPr)Cl] is reported. This study sets the stage for the broad utilization of well-defined, air- and moisture-stable Ni(II)–NHC complexes in catalytic amide bond-forming protocols by unconventional C(acyl)–N and C(acyl)–O bond cleavage reactions. 

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