Functionalization of the Aryl Moiety in the Pincer Complex (NCN)NiIIIBr2: Insights on NiIII-Promoted Carbon–Heteroatom Coupling

Reactions of cyclonickelated complexes with hydroxylamines and TEMPO˙: isolation of new TEMPOH adducts of Ni(II) and their reactivities with nucleophiles and oxidants

This contribution describes a study on the reactivities of the complexes [{κP,κC-(i-Pr)2PO-Ar}Ni(μ-Br)]2, 1a-d (Ar: C6H4, a; 3-Cl-C6H3, b; 3-OMe-C6H3, c; 4-OMe-napthalenyl, d), with hydroxylamines in the presence of TEMPO˙ (TEMPO˙ = (2,2,6,6-tetramethylpiperidinyl-1-yl)oxyl). The results of this study showed that treating 1a-d with a mixture of Et2NOH and TEMPO˙ did not afford the desired oxidation-induced functionalization of the Ni-aryl moiety in 1a-d, giving instead the corresponding κO-TEMPOH adducts [{κP,κC-(i-Pr)2PO-Ar}Ni(Br)(κO-TEMPOH)], 3a-d (TEMPOH = N-hydroxy-2,2,6,6-tetramethylpiperidine). The TEMPOH moiety in these zwitterionic compounds 3 can be displaced by a large excess of acetonitrile (MeCN), 10 equiv. of morpholine, or 1-2 equivalents of imidazole. Although these reactions have given the authenticated products [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(NCMe)], 4a, [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(morpholine)], 5a, and [{κP,κC-(i-Pr)2PO-C6H4}Ni(imidazole)2]Br, 6a, a few other products were also detected by NMR, indicating that the observed reactivities are far more complex than simple substitution of the TEMPOH moiety. Similarly, treating 3a with AgOC(O)CF3 results in the isolation of [{κP,κC-(i-Pr)2PO-C6H4}Ni{OC(O)CF3}(κO-TEMPOH)], 7a, arising from the substitution of the bromo ligand, whereas spectroscopic evidence suggests further reactivity, possibly including displacement of TEMPOH and oxidative decomposition.

Leveraging the Redox Promiscuity of Nickel To Catalyze C-N Coupling Reactions

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.

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.

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

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.

Mechanistic knowledge of photocatalytic nickel reactions is lacking, particularly with regards to the identities and oxidation states of key intermediates. Here the authors report a class of tridentate ligands that enables in-depth study of a representative cross-coupling reaction, wherein evidence for multiple intermediates in a Ni(I/III) cycle is presented.

Aerobic oxidation-functionalization of the aryl moiety in van Koten's pincer complex (NCN)Ni(ii)Br: relevance to carbon–heteroatom coupling reactions promoted by high-valent nickel species

Treating the pincer complex (NCN)NiBr with protic substrates HX (X = OH, OR, or NR2) under aerobic conditions leads to C–X functionalization of the pincer ligand. The crucial importance of aerobic conditions for the success of this coupling reaction implies the formation of high-valent intermediates during the course of the reaction.

Experimental Electrochemical Potentials of Nickel Complexes

Nickel-catalyzed cross-coupling and photoredox catalytic reactions has found widespread utilities in organic synthesis. Redox processes are key intermediate steps in many catalytic cycles. As a result, it is pertinent to measure and document the redox potentials of various nickel species as precatalysts, catalysts, and intermediates. The redox potentials of a transition-metal complex are governed by its oxidation state, ligand, and the solvent environment. This article tabulates experimentally measured redox potentials of nickel complexes supported on common ligands under various conditions. This review article serves as a versatile tool to help synthetic organic and organometallic chemists evaluate the feasibility and kinetics of redox events occurring at the nickel center, when designing catalytic reactions and preparing nickel complexes.

1 Introduction

1.1 Scope

1.2 Measurement of Formal Redox Potentials

1.3 Redox Potentials in Nonaqueous Solution

2 Redox Potentials of Nickel Complexes

2.1 Redox Potentials of (Phosphine)Ni Complexes

2.2 Redox Potentials of (Nitrogen)Ni Complexes

2.3 Redox Potentials of (NHC)Ni Complexes

Photoreductive chlorine elimination from a Ni(iii)Cl2 complex supported by a tetradentate pyridinophane ligand

Herein we report the isolation, characterization, and photoreactivity of a stable Ni^III dichloride complex supported by a tetradentate pyridinophane N-donor ligand. Upon irradiation, this complex undergoes an efficient photoreductive chlorine elimination reaction, both in solution and the solid-state. Subsequently, the Ni^IIICl₂ species can be regenerated via a reaction with PhICl₂.

Cytotoxicity, anti-tumor effects and structure-activity relationships of nickel and palladium S,C,S pincer complexes against double and triple-positive and triple-negative breast cancer (TNBC) cells

Triple-Negative Breast Cancer (TNBC) is a highly aggressive form of breast cancer. The high rate of metastasis associated with TNBC is attributed to its multidrug resistance, making the treatment of this metastatic condition difficult. The development of metal-based antitumor agents was launched with the discovery of cisplatin, followed by the development of related antitumor drugs such as carboplatin and oxaliplatin. Yet, the severe side effects of this approach represent a limitation for its clinical use. The current search for new metal-based antitumor agents possessing less severe side effects than these platinum-based complexes has focused on various complexes of nickel and palladium, the group 10 congeners of platinum. In this work, we have prepared a series of SCS-type pincer complexes of nickel and palladium featuring a stable meta-phenylene central moiety and two chelating but labile thioamide donor moieties at the peripheries of the ligand. We have demonstrated that the complexes in question, namely L1NiCl, L1NiBr, L1PdCl, L2PdCl, and L3PdCl, are active on the proliferation of estrogen-dependent breast tumor cells (MCF-7 and MC4L2) and triple-negative breast cancer (4 T1). Among the complexes studied, the palladium derivatives were found to be much safer anticancer agents than nickel counterparts; these were thus selected for further investigations for their effects on tumor cell adhesion and migration as well. The results of our studies show that palladium complexes are effective for inhibiting TNBC 4 T1 cells adhesion and migration. Finally, the HOMO and LUMO analysis was used to determine the reactivity and charge transfer within the compounds.

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.

Catalytically Relevant Intermediates in the Ni-Catalyzed C(sp2)-H and C(sp3)-H Functionalization of Aminoquinoline Substrates

This Article describes the synthesis and characterization of cyclometalated aminoquinoline Ni^II σ-aryl and σ-alkyl complexes that have been proposed as key intermediates in Ni-catalyzed C-H functionalization reactions. These Ni^II complexes serve as competent catalysts for the C-H functionalization of aminoquinoline derivatives with I₂. They also react stoichiometrically with I₂ to form either aryl iodides or β-lactams within minutes at room temperature. Furthermore, they react with Ag^I salts at -30 °C to afford isolable five-coordinate Ni^III species. The Ni^III σ-aryl complexes proved inert toward C(sp²)-I bond-forming reductive elimination under all conditions examined (up to 140 °C in DMF). In contrast, a Ni^III σ-alkyl analogue underwent C(sp³)-N bond-forming reductive elimination at 140 °C in DMF to afford a β-lactam product. However, despite the ability of this latter Ni^III species to participate in stoichiometric product formation, the complex was not a competent catalyst for β-lactam formation. Overall, these results suggest against the intermediacy of Ni^III species in these C-H functionalization reactions.

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

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 O₂ or H₂O₂ 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.

The stability of group 10 metal POCOP pincer complexes: decomposition/reconstruction pathways of the pincer backbone

Organometallic chemists usually like to use the word robust to describe pincer ligand frameworks in metal pincer complexes. Although most transition metal pincer complexes are thermally stable, the pincer backbone frameworks can still decompose under certain circumstances. In order to explore the stability of the bis(phosphinite) (POCOP) pincer backbone in transition metal pincer complexes, group 10 metal POCOP pincer complexes were exposed to different nucleophilic and electrophilic conditions, respectively. It was found that the POCOP pincer backbone is stable under intermolecular nucleophilic conditions but cannot survive intramolecular nucleophilic attack; the POCOP pincer backbone is also stable under weak electrophilic conditions but the backbone can be completely destroyed by strong Lewis acids such as AlCl3. Possible decomposition/reconstruction pathways of the POCOP pincer ligand framework were proposed.

C-O and C-N Functionalization of Cationic, NCN-Type Pincer Complexes of Trivalent Nickel: Mechanism, Selectivity, and Kinetic Isotope Effect

This report presents the synthesis of new mono- and dicationic NCN-Ni^III complexes and describes their reactivities with protic substrates. (NCN is the pincer-type ligand κ ^N, κ ^C, κ ^N-2,6-(CH₂NMe₂)₂-C₆H₃.) Treating van Koten's trivalent complex (NCN)Ni^IIIBr₂ with AgSbF₆ in acetonitrile gives the dicationic complex [(NCN)Ni^III(MeCN)₃]²⁺, whereas the latter complex undergoes a ligand-exchange reaction with (NCN)Ni^IIIBr₂ to furnish the related monocationic complex [(NCN)Ni^III(Br)(MeCN)]⁺. These trivalent complexes have been characterized by X-ray diffraction analysis and EPR spectroscopy. Treating these trivalent complexes with methanol and methylamine led, respectively, to C-OCH₃ or C-NH(CH₃) functionalization of the Ni-aryl moiety in these complexes, C-heteroatom bond formation taking place at the ipso-C. These reactions also generate the cationic divalent complex [(NCN)Ni^II(NCMe)]⁺, which was prepared independently and characterized fully. The unanticipated formation of the latter divalent species suggested a comproportionation side reaction between the cationic trivalent precursors and a monovalent species generated at the C-O and C-N bond formation steps; this scenario was supported by direct reaction of the trivalent complexes with the monovalent compound (PPh₃)₃Ni^ICl. Kinetic measurements and density functional theory analysis have been used to investigate the mechanism of these C-O and C-N functionalization reactions and to rationalize the observed inverse kinetic isotope effect in the reaction of [(NCN)Ni^III(Br)(MeCN)]⁺ with CH₃OH/CD₃OD.