Nickel(II/IV) Manifold Enables Room-Temperature C(sp3)-H Functionalization

Insights into the mechanism of 3d transition-metal-catalyzed directed C(sp3)-H bond functionalization reactions

The growing interest in the catalytic activity of earth-abundant 3d transition-metals has led to the development of new and more sustainable methods for C-H bond functionalization reactions. However, this is an emerging field which involves considerable mechanistic complexity as the mode of action of 3d transition metals differs markedly from the well-studied mechanisms of precious metals. In this review, we present an overview of the research efforts in Ni-, Cu-, Fe- and Co-catalyzed directed C(sp3)-H bond functionalization reactions, covering design principles and mechanistic discussions, along with potential applications and limitations. To conclude, the unresolved challenges and future viewpoints are highlighted. We aspire for this review to serve as a relevant and valuable reference for researchers in this swiftly progressing field, helping to inspire the development of more original and innovative strategies.

C-C and C-O bond formation reactivity of nickel complexes supported by the pyridinophane MeN3C ligand

The pyridinophane ligands RN3CX (X = H, Br) are well-established scaffolds that facilitate and stabilize nickel oxidative addition complexes to the proximal C(aryl)-X bond. In this study, we report the synthesis, detailed characterization, and reactivity of a series of NiII and NiIII complexes supported by the MeN3CX ligand. Our findings demonstrate that NiII complexes can be oxidized to readily yield well-defined NiIII species. Excitingly, the Ni-disolvento complexes exhibit catalytic trifluoroethoxylation to generate the C-O coupled product. In addition, the NiIII-halide complex undergoes transmetallation with a Grignard reagent and subsequent C-C reductive elimination, while the β-hydride elimination side reaction is suppressed, outperforming its NiII analogue.

Advances in accessing rare oxidation states of nickel for catalytic innovation

Nickel catalysis has experienced a renaissance over the past two decades, driven by its ability to access diverse oxidation states (0 to +4) and unique reactivity. This review consolidates the advancements in nickel chemistry, providing an overview of ligands that stabilize specific nickel oxidation states. The stability, reactivity, and catalytic applications of Ni0 sources, including in situ generation from air- and moisture-stable NiII precursors, are discussed, along with the roles of NiI and NiIII intermediates in catalytic cycles. The progress in synthesizing and utilizing NiIV complexes highlights their emerging importance in catalysis. Advances in spectroscopic and theoretical tools have enhanced the understanding of nickel's complex catalytic behavior.

Synthesis, Structure, and Redox Reactivity of Ni Complexes Bearing a Redox and Acid-Base Non-innocent Ligand with NiII, NiIII, and NiIV Formal Oxidation States

A series of Ni complexes bearing a redox and acid-base noninnocent tetraamido macrocyclic ligand, H4-(TAML-4) {H4-(TAML-4) = 15,15-dimethyl-5,8,13,17-tetrahydro-5,8,13,17-tetraaza-dibenzo[a,g]cyclotridecene-6,7,14,16-tetraone}, with formal oxidation states of NiII, NiIII, and NiIV were synthesized and characterized structurally and spectroscopically. The X-ray crystallographic analysis of the Ni complexes revealed a square planar geometry, and the [Ni(TAML-4)] complex with the formal oxidation state of NiIV was characterized to be [NiIII(TAML-4•+)] with the oxidation state of the NiIII ion and the one-electron oxidized TAML-4 ligand, TAML-4•+. The NiIII oxidation state and the TAML-4 radical cation ligand, TAML-4•+, were supported by X-ray absorption spectroscopy and density functional theory calculations. The reversible interconversions between [NiII(TAML-4)]2- and [NiIII(TAML-4)]- and between [NiIII(TAML-4)]- and [NiIII(TAML-4•+)] were demonstrated in spectroelectrochemical measurements as well as in chemical oxidation and reduction reactions. The reactivities of [NiIII(TAML-4)]- and [NiIII(TAML-4•+)] were then investigated in hydride transfer reactions using NADH analogs. Hydride transfer from 9,10-dihydro-10-methylacridine (AcrH2) to [NiIII(TAML-4•+)] was found to proceed via electron transfer (ET) from AcrH2 to [NiIII(TAML-4•+)] with no deuterium kinetic isotope effect (kH/kD = 1.0(2)). In contrast, hydride transfer from AcrH2 to [NiIII(TAML-4)]- proceeded much more slowly via a concerted proton-coupled electron transfer (PCET) process with kH/kD = 7.0(5). In the latter reaction, an electron and a proton were transferred to the NiIII center and the TAML-4 ligand, respectively. The mechanisms of the ET by [NiIII(TAML-4•+)] and the concerted PCET by [NiIII(TAML-4)]- were ascribed to the different redox potentials of the Ni complexes.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

Characterization and Reactivity of Copper(II) and Copper(III) σ-Aryl Intermediates in Aminoquinoline-Directed C-H Functionalization

Over the past decade, numerous reports have focused on the development and applications of Cu-mediated C-H functionalization reactions; however, to date, little is known about the Cu intermediates involved in these transformations. This paper details the observation and characterization of CuII and CuIII intermediates in aminoquinoline-directed C(sp2)-H functionalization of a fluoroarene substrate. An initial C(sp2)-H activation at CuII occurs at room temperature to afford an isolable anionic cyclometalated CuII complex. This complex undergoes single-electron oxidation with ferrocenium or AgI salts under mild conditions (5 min at room temperature) to afford C(sp2)-C(sp2) or C(sp2)-NO2 coupling products. Spectroscopic studies implicate the formation of a transient diamagnetic CuIII-σ-aryl intermediate that undergoes either (i) a second C(sp2)-H activation at CuIII followed by C-C bond-forming reductive elimination or (ii) reaction with a NO2- nucleophile and C(sp2)-NO2 coupling.

Nickel-Carbon Bond Oxygenation with Green Oxidants via High-Valent Nickel Species

Described herein is the synthesis of the Ni^II complex (^tBuMe₂tacn)Ni^II(cycloneophyl) (^tBuMe₂tacn = 1-tert-butyl-4,7-dimethyl-1,4,7-triazacyclononane, cycloneophyl = -CH₂CMe₂-o-C₆H₄-) and its reactivity with dioxygen and peroxides. The new ^tBuMe₂tacn ligand is designed to enhance the oxidatively induced bond-forming reactivity of high-valent Ni intermediates. Tunable chemoselectivity for Csp²-O vs Csp²-Csp³ bond formation was achieved by selecting the appropriate solvent and reaction conditions. Importantly, the use of cumene hydroperoxide and meta-chloroperbenzoic acid suggests a heterolytic O-O bond cleavage upon reaction with (^tBuMe₂tacn)Ni^II(cycloneophyl). Mechanistic studies using isotopically labeled H₂O₂ support the generation of a high-valent Ni-oxygen species via an inner-sphere mechanism and subsequent reductive elimination to form the Csp²-O bond. Kinetic studies of the exceptionally fast Csp²-O bond-forming reaction reveal a first-order dependence on both (^tBuMe₂tacn)Ni^II(cycloneophyl) and H₂O₂, and thus an overall second-order reaction. Eyring analysis further suggests that the oxidation of the Ni^II complex by H₂O₂ is the rate-determining step, which can be modulated by the presence of coordinating solvents. Moreover, computational studies fully support the conclusions drawn from experimental results. Overall, this study reveals for the first time the ability to control the oxidatively induced C-C vs C-O bond formation reactions at a Ni center. Importantly, the described system merges the known organometallic reactivity of Ni with the biomimetic oxidative transformations resembling oxygenases and peroxidases, and involving high-valent metal-oxygen intermediates, which is a novel approach that should lead to unprecedented oxidative catalytic transformations.

Mechanistic Versatility at Ir(PSiP) Pincer Catalysts: Triflate Proton Shuttling from 2-Butyne to Diene and [3]Dendralene Motifs

The five-coordinate hydrido complex [IrH(OTf)(PSiP)] (1) catalytically transforms 2-butyne into a mixture of its isomer 1,3-butadiene, and [3]dendralene and linear hexatriene dimerization products: (E)-4-methyl-3-methylene-1,4-hexadiene and (3Z)-3,4-dimethyl-1,3,5-hexatriene, respectively. Under the conditions of the catalytic reaction, benzene, and 363 K, the hexatriene further undergoes thermal electrocyclization into 2,3-dimethyl-1,3-cyclohexadiene. The reactions between 1 and the alkyne substrate allow isolation or nuclear magnetic resonance (NMR) observation of catalyst resting states and possible reaction intermediates, including complexes with the former PSiP pincer ligands disassembled into PSi and PC chelates, and species coordinating allyl or carbene fragments en route to products. The density functional theory (DFT) calculations guided by these experimental observations disclose competing mechanisms for C–H bond elaboration that move H atoms either classically, as hydrides, or as protons transported by the triflate. This latter role of triflate, previously recognized only for more basic anions such as carboxylates, is discussed to result from combining the unfavorable charge separation in the nonpolar solvent and the low electronic demand from the metal to the anion at coordination positions trans to silicon. Triflate deprotonation of methyl groups is key to release highly coordinating diene products from stable allyl intermediates, thus enabling catalytic cycling.

Photoinduced remote regioselective radical alkynylation of unactivated C-H bonds

A method for the remote regioselective alkynylation of unactivated C(sp³)-H bonds in diverse aliphatic amides by photogenerated amidyl radicals has been developed. The site-selectivity is dominated via a 1,5-hydrogen atom transfer (HAT) process of the amide. Mild reaction conditions and high regioselectivity are demonstrated in this methodology.

MOF-stabilized perfluorinated palladium cages catalyze the additive-free aerobic oxidation of aliphatic alcohols to acids

Extremely high electrophilic metal complexes, composed by a metal cation and very electron poor σ-donor ancillary ligands, are expected to be privileged catalysts for oxidation reactions in organic chemistry. However, their low lifetime prevents any use in catalysis. Here we show the synthesis of fluorinated pyridine-Pd 2+ coordinate cages within the channels of an anionic tridimensional metal organic framework (MOF), and their use as efficient metal catalysts for the aerobic oxidation of aliphatic alcohols to carboxylic acids without any additive. Mechanistic studies strongly support that the MOF-stabilized coordination cage with perfluorinated ligands unleashes the full electrophilic potential of Pd 2+ to dehydrogenate primary alcohols, without any base, and also to activate O 2 for the radical oxidation to the aldehyde intermediate. This study opens the door to design catalytic perfluorinated complexes for challenging organic transformations, where an extremely high electrophilic metal site is required.

A new C-anionic tripodal ligand 2-{bis(benzothiazolyl)(methoxy)methyl}phenyl and its bismuth complexes

A new tripodal C-anionic ligand, 2-{bis(benzothiazolyl)(methoxy)methyl}phenyl (L), was stably generated by the reaction of the ligand precursor (L'), the corresponding bromide (2-BrC6H4)(MeO)C(C7H4NS)2 (C7H4NS = 2-benzothiazolyl), with nBuLi at -104 °C in the presence of TMEDA (N,N,N',N'-tetramethylethylenediamine). The ligand lithium salt reacted with BiCl3 to give a 2 : 1 complex L2BiCl. A 1 : 1 complex LBiCl2 was obtained in good yield by the redistribution reaction between L2BiCl and BiCl3. X-ray diffraction analysis revealed that the ligand L coordinated in an expected κ3-C,N,N' coordination mode in LBiCl2, while it coordinated in κ3-C,N,O and κ2-C,O coordination modes in L2BiCl. The ligand precursor reacted with BiX3 (X = Cl, Br) to give 1 : 1 complexes L'BiX3 and was found to act as a neutral tripodal C(π),N,N-ligand.

Mediator-Enabled Electrocatalysis with Ligandless Copper for Anaerobic Chan-Lam Coupling Reactions

Simple copper salts serve as catalysts to effect C-X bond-forming reactions in some of the most utilized transformations in synthesis, including the oxidative coupling of aryl boronic acids and amines. However, these Chan-Lam coupling reactions have historically relied on chemical oxidants that limit their applicability beyond small-scale synthesis. Despite the success of replacing strong chemical oxidants with electrochemistry for a variety of metal-catalyzed processes, electrooxidative reactions with ligandless copper catalysts are plagued by slow electron-transfer kinetics, irreversible copper plating, and competitive substrate oxidation. Herein, we report the implementation of substoichiometric quantities of redox mediators to address limitations to Cu-catalyzed electrosynthesis. Mechanistic studies reveal that mediators serve multiple roles by (i) rapidly oxidizing low-valent Cu intermediates, (ii) stripping Cu metal from the cathode to regenerate the catalyst and reveal the active Pt surface for proton reduction, and (iii) providing anodic overcharge protection to prevent substrate oxidation. This strategy is applied to Chan-Lam coupling of aryl-, heteroaryl-, and alkylamines with arylboronic acids in the absence of chemical oxidants. Couplings under these electrochemical conditions occur with higher yields and shorter reaction times than conventional reactions in air and provide complementary substrate reactivity.

Synthesis and Characterization of Bidentate (P^N)Gold(III) Fluoride Complexes: Reactivity Platforms for Reductive Elimination Studies

A new family of cationic, bidentate (P^N)gold(III) fluoride complexes has been prepared and a detailed characterization of the gold-fluoride bond has been carried out. Our results correlate with the observed reactivity of the fluoro ligand, which undergoes facile exchange with both cyano and acetylene nucleophiles. The resulting (P^N)arylgold(III)C(sp) complexes have enabled the first study of reductive elimination on (P^N)gold(III) systems, which demonstrated that C(sp² )-C(sp) bond formation occurs at higher rates than those reported for analogous phosphine-based monodentate systems.

Photo-induced copper-catalyzed alkynylation and amination of remote unactivated C(sp3)-H bonds

A method for remote radical C-H alkynylation and amination of diverse aliphatic alcohols has been developed. The reaction features a copper nucleophile complex formed in situ as a photocatalyst, which reduces the silicon-tethered aliphatic iodide to an alkyl radical to initiate 1,n-hydrogen atom transfer. Unactivated secondary and tertiary C-H bonds at β, γ, and δ positions can be functionalized in a predictable manner.

Nickel-Catalyzed [4 + 2] Annulation of Nitriles and Benzylamines by C-H/N-H Activation

Nickel-catalyzed [4 + 2] annulation of benzylamines and nitriles via C-H/N-H bond activation, providing straightforward atom-economic access to a wide variety of multisubstituted quinazolines, is reported. Mechanistic investigation revealed that the in situ formed amidines from the coupling of benzylamines and nitriles direct the nickel catalyst to activate the ortho-C-H bond of the phenyl ring of the benzylamine.

Dialkyl Ether Formation at High-Valent Nickel

In this article, we investigated the I2-promoted cyclic dialkyl ether formation from 6-membered oxanickelacycles originally reported by Hillhouse. A detailed mechanistic investigation based on spectroscopic and crystallographic analysis revealed that a putative reductive elimination to forge C(sp3)-OC(sp3) using I2 might not be operative. We isolated a paramagnetic bimetallic NiIII intermediate featuring a unique Ni2(OR)2 (OR = alkoxide) diamond-like core complemented by a μ-iodo bridge between the two Ni centers, which remains stable at low temperatures, thus permitting its characterization by NMR, EPR, X-ray, and HRMS. At higher temperatures (>-10 °C), such bimetallic intermediate thermally decomposes to afford large amounts of elimination products together with iodoalkanols. Observation of the latter suggests that a C(sp3)-I bond reductive elimination occurs preferentially to any other challenging C-O bond reductive elimination. Formation of cyclized THF rings is then believed to occur through cyclization of an alcohol/alkoxide to the recently forged C(sp3)-I bond. The results of this article indicate that the use of F+ oxidants permits the challenging C(sp3)-OC(sp3) bond formation at a high-valent nickel center to proceed in good yields while minimizing deleterious elimination reactions. Preliminary investigations suggest the involvement of a high-valent bimetallic NiIII intermediate which rapidly extrudes the C-O bond product at remarkably low temperatures. The new set of conditions permitted the elusive synthesis of diethyl ether through reductive elimination, a remarkable feature currently beyond the scope of Ni.

Merging Two Functions in a Single Rh Catalyst System: Bimodular Conjugate for Light-Induced Oxidative Coupling

A single molecular rhodium catalyst system (PC2-Cp^#Rh^III) bearing two functional domains for both photosensitization and C-H carbometalation was designed to enable an intramolecular redox process. The hypothesized charge-transfer species (PC2^•--Cp^#Rh^IV) was characterized by spectroscopic and electrochemical analyses. This photoinduced internal oxidation allows a facile access to the triplet state of the key post-transmetalation intermediate that readily undergoes C-C bond-forming reductive elimination with a lower activation barrier than in its singlet state, thus enabling catalytic C-H arylation and methylation processes.

Computational Analysis of Mesomerism in para-Substituted mer-NCN Pincer Platinum(II) Complexes, Including its Relationships with Hammett σp Substituent Parameters

Density Functional Theory studies of square-planar Pt^II pincer structures, (4-Z-NCN)PtCl ([4-Z-NCN]^- =[4-Z-2,6-(Me₂ NCH₂ )₂ C₆ H₂ -N,C,N]^- , Z=H, NO₂ , CF₃ , CO₂ H, CHO, Cl, Br, I, F, SMe, SiMe₃ , tBu, OH, NH₂ , NMe₂ ), enable characterisation of mesomerism for the pincer-Pt interaction. Relationships between Hammett σ_p substituent parameters of Z and DFT data obtained from NBO6 and AOMix computation are used to probe the interaction of the 5d_yz orbital of platinum with π-orbitals of the arene ring. Analogous computation for 2,6-(Me₂ CH₂ )₂ C₆ H₃ Z (Z=H, CF₃ , CHO, Cl, Br, I, F, SMe, SiMe₃ , tBu, OH, NH₂ ) and (4-H-NCN)PtZ allows an estimation of the relative substituent effects of "(CH₂ NMe₂ )₂ PtZ" on π-delocalisation in the pincer system.

Focusing on the Catal. of the Pd- and Ni-Catalyzed Hirao Reactions

The Hirao reaction involving the phosphinoylation or phosphonation of aryl halides by >P(O)H reagents is a P–C bond forming transformation belonging to the recently very hot topic of cross-couplings. The Pd- or Ni-catalyzed variations take place via the usual cycle including oxidative addition, ligand exchange, and reductive elimination. However, according to the literature, the nature of the transition metal catalysts is not unambiguous. In this feature article, the catalysts described for the Pd(OAc)₂-promoted cases are summarized, and it is concluded that the “(HOY₂P)₂Pd(0)” species (Y = aryl, alkoxy) is the real catalyst. In our model, the excess of the >P(O)H reagent served as the P-ligand. During the less studied Ni(II)-catalyzed instances the “(HOY₂P)(−OY₂P)Ni(II)Cl⁻” form was found to enter the catalytic cycle. The newest conclusions involving the exact structure of the catalysts, and the mechanism for their formation explored by us were supported by our earlier experimental data and theoretical calculations.

High-Valent NiIII and NiIV Species Relevant to C-C and C-Heteroatom Cross-Coupling Reactions: State of the Art

Ni catalysis constitutes an active research arena with notable applications in diverse fields. By analogy with its parent element palladium, Ni catalysts provide an appealing entry to build molecular complexity via cross-coupling reactions. While Pd catalysts typically involve a M⁰/M^II redox scenario, in the case of Ni congeners the mechanistic elucidation becomes more challenging due to their innate properties (like enhanced reactivity, propensity to undergo single electron transformations vs. 2e⁻ redox sequences or weaker M–Ligand interaction). In recent years, mechanistic studies have demonstrated the participation of high-valent Ni^III and Ni^IV species in a plethora of cross-coupling events, thus accessing novel synthetic schemes and unprecedented transformations. This comprehensive review collects the main contributions effected within this topic, and focuses on the key role of isolated and/or spectroscopically identified Ni^III and Ni^IV complexes. Amongst other transformations, the resulting Ni^III and Ni^IV compounds have efficiently accomplished: i) C–C and C–heteroatom bond formation; ii) C–H bond functionalization; and iii) N–N and C–N cyclizative couplings to forge heterocycles.

Reactivity of a formal Cu(iii)-alkyl species toward aniline: a DFT investigation

In this work, through theoretical investigation of the reactivity between a formal Cu(iii)-alkyl species and aniline, we demonstrated the possibility of a S_N2-like C–N coupling mechanism.