Design, synthesis, and carbon-heteroatom coupling reactions of organometallic nickel(IV) complexes

Persistent acyclic Cp*Ir(III) complexes and their reactivities in cross-coupling reactions

Iridium(III) complexes play a prominent role in organometallic chemistry, with significant research efforts directed toward Cp*Ir(III) species, broadly categorized into cyclic and acyclic types. Although studies on these two classes began roughly simultaneously, the development of acyclic Cp*Ir(III) complexes has lagged significantly behind their cyclic counterparts. Herein, we report a general and efficient strategy for synthesizing various persistent aryl Cp*Ir(III)(CO)Cl complexes directly from aryl aldehydes, with in situ generated CO as a stabilizing ligand. These acyclic Cp*Ir(III) complexes showcase exceptional reactivity, undergoing reactions with up to eight classes of nucleophiles to generate diverse diorganoiridium(III) species with remarkable stability. Electrochemical analysis of these complexes provides insights into their reductive elimination processes. Guided by these findings, Cp*Ir(III)-mediated decarbonylative C-C and C-O cross-couplings of aryl aldehydes are successfully developed. This study establishes a robust platform for the exploration of acyclic Cp*Ir(III) complexes, paving the way for further advancements in iridium(III) chemistry.

Photochemistry of Ni(II) Tolyl Chlorides Supported by Bidentate Ligand Frameworks

Herein, we investigate the photoactivity of four NiII tolyl chloride complexes supported by either the new bidentate [2.2]pyridinophane (HN2) ligand or the traditional 4,4'-di-tert-butyl-2,2'-dipyridyl (tBubpy) ligand. Despite a change in the ligand framework, we observe similar quantum yields for the photodegradation of all four NiII complexes, while noting changes in their affinity for radical side reactivity and ability to stabilize the photogenerated mononuclear NiI species. Furthermore, changing from an ortho-tolyl to a para-tolyl group affects the geometry of the complexes and makes the Ni center more susceptible to side reactivity. By leveraging the newly developed HN2 ligand, a bidentate ligand that hinders axial interactions with the Ni center, we limit the radical side reactivity. Time-dependent density functional theory (TDDFT) and complete active space self-consistent field (CASSCF) calculations predict that all four complexes have accessible MLCTs that excite an electron from a Ni-aryl bonding orbital into a Ni-aryl antibonding orbital, initiating photolysis. By decreasing this energy gap and stabilizing the tetrahedral triplet excited state, we increase quantum yields of photoexcitation. Importantly, we characterize the photogenerated mononuclear NiI chloride species using X-band EPR spectroscopy and show that the HN2-supported NiI complexes do not undergo the deleterious dimerization and tetramerization observed for the (bpy)NiICl species. Overall, this study provides valuable insight into how the steric environment around the Ni center affects its photoactivity and demonstrates that such photoactivity is not unique to bipyridyl-supported Ni compounds.

Fundamental Principles in Catalysis from Mechanistic Studies of Oxidative Decarboxylative Coupling Reactions

ConspectusOxidative decarboxylative coupling (ODC) reactions have been recognized as powerful alternatives to traditional cross-coupling reactions due to the ability to generate (hetero)biaryl structures from simple and readily available carboxylic acid precursors. These reactions, however, are underdeveloped due to the requirement for ortho-nitrobenzoate coupling partners and silver salts as oxidants. Our research program has focused on the development of new catalytic ODC reactions, as well as mechanistic studies of these reactions to uncover the origin of these synthetic limitations. As the framework for these studies, we explored two key ODC reactions developed in our group: (1) a Ni-catalyzed decarboxylative arylation reaction that relies on silver as the oxidant and (2) a Cu-catalyzed decarboxylative thiolation reaction capable of operating under aerobic conditions. Our findings, disclosed in this Account, have uncovered the importance of the ortho-substituent and revealed that Ag-based oxidants are also responsible for mediating the decarboxylation and transmetalation steps.Systematic exploration of the decarboxylation of a series of well-defined Ag-benzoate complexes allowed us to probe the importance of the ortho-nitro group in the decarboxylation step. Kinetic measurements of a large series of differently substituted benzoates were found to correlate with the field effect (F) of the ortho-substituent, revealing this feature to be responsible for the enhanced reactivity of these favored benzoates.Our studies of the Ni-catalyzed decarboxylative arylation reaction uncovered an unexpected redox transmetalation step in this system. Synthesis and isolation of the proposed nickelacycle and Ag-aryl intermediates enabled direct study of the fundamental coupling steps. Catalytic and stoichiometric reactions of these complexes, paired with DFT calculations, supported a redox transmetalation step in which the Ag-aryl intermediate transfers the aryl ligand from AgI to NiII with concomitant oxidation to generate a NiIII-bis(aryl) intermediate.Finally, detailed mechanistic studies of our Cu-catalyzed decarboxylative thiolation reaction demonstrated how this catalyst system is able to use O2 as the terminal oxidant. Kinetic studies paired with the synthesis and reactivity of well-defined copper intermediates revealed decarboxylation from a CuI-benzoate resting state, despite the oxidizing reaction conditions which could support higher oxidation state intermediates. We also identified the intermediacy of diphenyl disulfide (PhSSPh) formed from the thiophenol (PhSH) coupling partner under the aerobic Cu-catalyzed conditions. The reaction of PhSSPh with the catalyst proceeds via oxidative transfer of the PhS fragment to CuI that is analogous to that of the redox transmetalation observed in Ni-catalyzed decarboxylative arylation.These studies combined suggest significant implications for ODC reactions more broadly. For example, it appears that silver is important for mediating the decarboxylation step when the catalyst is unable (as is the case with nickel) but is not needed when the catalyst is an efficient decarboxylation mediator on its own (as with copper). Furthermore, silver plays an additional role in mediating an oxidative transfer of the coupling partner in arylation reactions but is not required when the coupling partner is itself oxidizing (such as PhSSPh). We anticipate that these mechanistic insights will facilitate the development of new ODC reactions that operate under milder conditions and with broader substrate scopes.

High-Valent Nickel Complexes Supported by a Functionalized PC(sp3)P Pincer Ligand: Properties and Catalysis

This article presents the synthesis and characterization of a series of robust high-valent organometallic nickel (Ni) complexes stabilized by a functionalized PC(sp3)P pincer ligand. Notably, the nickel center, covalently confined within the 3D ligand framework, demonstrates predictable coordination and redox behavior, coupled with remarkable stability across oxidation states +2, +3, and +4. These states were found to interconvert via one-electron transfer reactions. Among these complexes, the Ni(III)-PC(sp3)P species was identified as an efficient catalyst for the mild and selective hydrosilylation of alkenes, operating through a nonoxidative reaction mechanism.

Synthesis and Characterization of Co Complexes Coordinated by a Tetraanionic Bis(amidateanilido) Ligand

The synthesis and characterization of a square planar CoII complex coordinated by the tetraanionic bis(amidateanilido) L4- ligand, 12-, and its corresponding one-electron oxidized forms 2-, 3, and 4+ are discussed herein. 12- undergoes aerobic oxidation to form 2-, a square planar S = 1 CoIII complex. This aerobic reactivity is attributed to a remarkably negative CoII/CoIII potential, as observed in its cyclic voltammogram, which is among the most negative CoII/CoIII couples reported. This supports the conclusion that the L4- ligation is strongly stabilizing to the oxidized CoIII center. The cyclic voltammogram of 2- also reveals that further oxidations are possible at mild potentials. Indeed, the stoichiometric addition of a suitable chemical oxidant yields a ligand-oxidized S = 1/2 CoIII complex, 3, while the addition of a further equivalent of the oxidant putatively yields 4+. The formation of the ligand oxidized species 3 suggests that bis(amidateanilido) ligation can further behave as a redox-active ligand upon additional oxidation.

Synthesis of Iron(IV) Alkynylide Complexes and Their Reactivity to Form 1,3-Diynes

The isolation of thermally unstable and highly reactive organoiron(IV) complexes is a challenge for synthetic chemists. In particular, the number of examples where the C-based ligand is not part of the chelating ligand remains scarce. These compounds are of interest because they could pave the way to designing catalytic cycles of bond forming reactions proceeding via organoiron(IV) intermediates. Herein, we report the synthesis and characterization, including single-crystal X-ray diffraction, of a family of alkynylferrates(III) and Fe(IV) alkynylide complexes. The alkynylferrates(III) are formed by transmetalation of the Fe(III) precursor [(N3N')FeIII] (N3N'3- is tris(N-tert-butyldimethylsilyl-2-amidoethyl)amine) with lithium alkynylides, and their further one-electron oxidation enables the synthesis of the corresponding Fe(IV) alkynylides. The electronic structure of this family of organometallic Fe(III) and Fe(IV) complexes has been thoroughly investigated by spectroscopic methods (EPR, NMR, 57Fe Mössbauer, X-Ray absorption (XAS) and emission (XES) spectroscopies) and theoretical calculations. While alkynylferrates(III) are sluggish to engage into C-C bond forming processes, the Fe(IV) alkynylides react to afford 1,3-diynes at room temperature. A bimolecular reductive elimination from a bimetallic Fe(IV) intermediate to form the 1,3-diynes is proposed based on the mechanistic investigations performed.

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.

Stereoselective Vinylic C-H Addition via Metallaphotoredox Migration

Geometrically defined allylic alcohols with SE, SZ, RE and RZ stereoisomers serve as valuable intermediates in synthetic chemistry, attributed to the stereoselective transformations enabled by the alkenyl and hydroxyl functionalities. When an ideal scenario presents itself with four distinct stereoisomers as potential products, the simultaneous control vicinal stereochemistry in a single step would offer a direct pathway to any desired stereoisomer. Here, we unveil a metallaphotoredox migration strategy to access stereodefined allylic alcohols through vinylic C-H activation with aldehydes. This method harnesses a chiral nickel catalyst in concert with a photocatalyst to enable a 1,4-Ni migration by using readily accessible 2-vinyl iodoarenes as starting materials. The efficacy of this methodology is highlighted by the precise construction of all stereoisomers of allylic alcohols bearing analogous substituents and the efficient synthesis of key intermediates en route to Myristinin family. Experimental and computational studies have shed light on pivotal aspects including the synergy of metal catalysis and photocatalysis, the driving forces behind the migration, and the determination of absolute configuration in the C-H addition process.

A Crystalline NiX6 Complex

High-valent nickel species are implicated as intermediates in industrially relevant chemical transformations and in the catalytic cycles of metalloenzymes. Although a small number of tetravalent NiX4 complexes have been crystallographically characterized, higher nickel valence states have not been identified. Here we report a stable, crystalline NiX6 complex, Ni(BeCp)6 (1; cyclopentadienyl anion (Cp)), formed by the insertion of zerovalent nickel into three Be-Be bonds. This 16-electron species features an inverted ligand field, is diamagnetic, and exhibits C3v symmetry, on account of the lifting of Ni 4p-orbital degeneracy in this molecular geometry. Single-crystal X-ray diffraction and quantum chemical calculations both reveal a toroidal band of electron density perpendicular to the C3 axis of the complex, which may be attributed to delocalized, multicenter aromatic NiBe6 bonding.

Ligand-enabled Ni-catalysed dicarbofunctionalisation of alkenes with diverse native functional groups

The transition metal-catalysed dicarbofunctionalisation of unactivated alkenes normally requires exogenous strong coordinated directing groups, thus reducing the overall reaction efficiency. Here, we report a ligand-enabled Ni(II)-catalysed dicarbofunctionalisation of unactivated alkenes with aryl/alkenyl boronic acids and alkyl halides as the coupling partners with a diverse range of native functional groups as the directing group. This dicarbofunctionalisation protocol provides an efficient and direct route towards vicinal 1,2-disubstituted alkanes using primary, secondary, tertiary amides, sulfonamides, as well as secondary and tertiary amines under redox-neutral conditions that are challenging to access through conventional methods. The key to the success of this reaction is the use of a bulky β-diketone ligand, which could enable the insertion of alkene to aryl-Ni(II) species, stabilize the alkyl-Ni(II) species and inhibit the homolytic alkyl-Ni(II) cleavage, supporting by both experimental and computational studies. This dicarbofunctionalisation reaction features the use of native directing group, a broad substrate scope, and excellent scalability.

Theoretical study on the concerted catalysis of Ir/Ni for amino radical transfer for C(sp2)-C(sp3) bond formation

Thomas C. Maier's group has reported a synergistic Ir/Ni catalysis method for the synthesis of C(sp2)-C(sp3) bonds through an amino radical transfer (ART) strategy to generate alkyl radicals. This work employed density functional theory (DFT) to investigate the reaction mechanism, including the redox mechanism of Ir complexes in the generation process of amino radicals, analyzed the role and rationale behind alkyl boronic esters becoming dominant reaction pathways in the ART process, and discussed the competitive reaction mechanisms between oxidative addition and radical capture during C(sp2)-C(sp3) cross-coupling with Ni complexes. Through this theoretical calculation study, we aim to provide a theoretical foundation for constructing key carbon radical intermediates using ART and Ni-complex catalyzed free-radical-involved C(sp2)-C(sp3) cross-coupling reactions.

Magnesium-promoted nickel-catalysed chlorination of aryl halides and triflates under mild conditions

In this study, we present a ligand-free nickel(II)-catalyzed halogen exchange of aromatic halides with magnesium chloride. This method effectively facilitates the retro-Finkelstein reaction for a wide range of aryl bromides, iodides and triflates, demonstrating excellent functional group tolerance. Mechanistic studies reveal that magnesium plays a crucial role in the challenging reductive elimination from Ni(II) intermediates.

Nickel-catalyzed, silyl-directed, ortho-borylation of arenes via an unusual Ni(II)/Ni(IV) catalytic cycle

Nickel-catalyzed C-H bond functionalization reactions provide an impressive alternative to those with noble metal catalysts due to their unique reactivity and low cost. However, the regioselective C(sp2)-H borylation reaction of arenes accomplished by nickel catalyst remains limited. We herein disclose a silyl-directed ortho C(sp2)-H borylation of substituted arenes with a Ni(cod)2/PMe3/KHMDS catalyst system. Using readily available starting materials, this protocol provides easy access to ortho-borylated benzylic hydrosilanes bearing flexible substitution patterns. These products can serve as versatile building blocks for the synthesis of sila or sila/borine heterocycles under mild conditions. Control experiments and DFT calculations suggest that a catalytic amount of base prompts the formation of Ni(II)-Bpin-ate complex, likely related to the C(sp2)-H bond activation. This borylation reaction might follow an unusual Ni(II)/Ni(IV) catalytic cycle.

Mechanistic insights into facilitating reductive elimination from Ni(II) species

Reductive elimination is a key step in Ni-catalysed cross-couplings, which is often considered to result in new covalent bonds. Due to the weak oxidizing ability of Ni(II) species, reductive eliminations from Ni(II) centers are challenging. A thorough mechanistic understanding of this process could inspire the rational design of Ni-catalysed coupling reactions. In this article, we give an overview of recent advances in the mechanistic study of reductive elimination from Ni(II) species achieved by our group. Three possible models for reductive elimination from Ni(II) species were investigated and discussed, including direct reductive elimination, electron density-controlled reductive elimination, and oxidation-induced reductive elimination. Notably, the direct reductive elimination from Ni(II) species often requires a high activation energy in some cases. In contrast, the electron density-controlled and oxidation-induced reductive elimination pathways can significantly enhance the driving force for reductive elimination, accelerating the formation of new covalent bonds. The intricate reaction mechanisms for each of these pathways are thoroughly discussed and systematically summarized in this paper. These computational studies showcase the characteristics of three models for reductive elimination from Ni(II) species, and we hope that it will spur the development of mechanistic studies of cross-coupling reactions.

Excited-State Identification of a Nickel-Bipyridine Photocatalyst by Time-Resolved X-ray Absorption Spectroscopy

Photoassisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances the reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein, we report metal-specific studies using transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4'-di-tert-butyl, bpy = bipyridine, o-tol = ortho-tolyl), in solution. We unambiguously confirm via direct experimental evidence that the long-lived (∼5 ns) excited state is a tetrahedral metal-centered triplet state. These results demonstrate the power of ultrafast X-ray spectroscopies to unambiguously elucidate the nature of excited states in important transition-metal-based photocatalytic systems.

Fingerprint of Dipole Moment Orientation of Water Molecules in Cu2+ Aqueous Solution Probed by X-ray Photoelectron Spectroscopy

The electronic structure and geometrical organization of aqueous Cu2+ have been investigated by using X-ray photoelectron spectroscopy (XPS) at the Cu L-edge combined with state-of-the-art ab initio molecular dynamics and a quantum molecular approach designed to simulate the Cu 2p X-ray photoelectron spectrum. The calculations offer a comprehensive insight into the origin of the main peak and satellite features. It is illustrated how the energy drop of the Cu 3d levels (≈7 eV) following the creation of the Cu 2p core hole switches the nature of the highest singly occupied molecular orbitals (MOs) from the dominant metal to the dominant MO nature of water. It is particularly revealed how the repositioning of the Cu 3d levels induces the formation of new bonding (B) and antibonding (AB) orbitals, from which shakeup mechanisms toward the relaxed H-SOMO operate. As highlighted in this study, the appearance of the shoulder near the main peak corresponds to the characteristic signature of shakeup intraligand (1a1 → H-SOMO(1b1)) excitations in water, providing insights into the average dipole moment distribution (≈36°) of the first-shell water molecules surrounding the metal ion and its direct impact on the broadening of the satellite. It is also revealed that the main satellite at 8 eV from the main peak corresponds to (metal/1b2 → H-SOMO(1b1) of water) excitations due to a bonding/antibonding (B/AB) interaction of Cu 3d levels with the deepest valence O2p/H1s 1b2 orbitals of water. This finding underscores the sensitivity of XPS to the electronic structure and orientation of the nearest water molecules around the central ion.

A mesoionic carbene stabilized nickel(II) hydroxide complex: a facile precursor for C-H activation chemistry

We report the synthesis of a new nickel(II) hydroxide complex 2 supported by a rigid, tridentate triazolylidene-carbazolid ligand. The complex can be accessed in high yields following a simple and stepwise extraction protocol using dichloromethane and aqueous ammonium chloride followed by aqeous sodium hydroxide solution. We found that complex 2 is highly basic, undergoing various deprotonation/desilylation reactions with E-H and C-H acidic and silylated compounds. In this context we synthesized a variety of novel, functionalized nickel(II) complexes with trimethylsilylolate (3), trityl sulfide (4), tosyl amide (5), azido (6), pyridine (7), acetylide (8, 9), fluoroarene (10 & 11) and enolate (12) ligands. We furthermore found that 2 reacts with malonic acid dimethyl ester in a knoevennagel-type condensation reaction, giving access to a new enolate ligand in complex 13, consisting of two malonic acid units. Furthermore, complex 2 reacts with acetonitrile to form the cyanido complex 14. The formation of complexes 13 and 14 is particularly interesting, as they underline the potential of complex 2 in both C-C bond formation and cleavage reactions.

Combining Donor Strength and Oxidative Stability in Scorpionates: A Strongly Donating Fluorinated Mesoionic Tris(imidazol-5-ylidene)borate Ligand

Strongly donating scorpionate ligands support the study of high-valent transition metal chemistry; however, their use is frequently limited by oxidative degradation. To address this concern, we report the synthesis of a tris(imidazol-5-ylidene)borate ligand featuring trifluoromethyl groups surrounding its coordination pocket. This ligand represents the first example of a chelating poly(imidazol-5-ylidene) mesoionic carbene ligand, a scaffold that is expected to be extremely donating. The {NiNO}10 complex of this ligand, as well as that of a previously reported strongly donating tris(imidazol-2-ylidene)borate, has been synthesized and characterized. This new ligand's strong donor properties, as measured by the υNO of its {NiNO}10 complex and natural bonding orbital second-order perturbative energy analysis, are at par with those of the well-studied alkyl-substituted tris(imidazol-2-ylidene)borates, which are known to effectively stabilize high-valent intermediates. The good donor properties of this ligand, despite the electron-withdrawing trifluoromethyl substituents, arise from the strongly donating imidazol-5-ylidene mesoionic carbene arms. These donor properties, when combined with the robustness of trifluoromethyl groups toward oxidative decomposition, suggest this ligand scaffold will be a useful platform in the study of oxidizing high-valent transition-metal species.

Synthesis and Characterization of a Stable Nickelocenium Dication Salt

We report the synthesis and characterization of the nickelocenium cations [NiCp2 ]⋅+ and [NiCp2 ]2+ as their [F-{Al(ORF )3 }2 ]- (Cp = C5 H5 ; RF =C(CF3 )3 ) salts. Diamagnetic [NiCp2 ]2+ represents the first example for the isolation of an unsubstituted parent metallocene dication. Both salts were generated by reacting neutral NiCp2 with [NO]+ [F-{Al(ORF )3 }2 ]- in 1,2,3,4-tetrafluorobenzene (4FB). The salts were characterized by single crystal X-ray diffraction (XRD), indicating shorter metal-ligand bond lengths for the higher charged salt. Powder XRD shows the salts to be phase pure, cyclic voltammetry in 4FB gave quasi reversible redox waves at -0.44 (0→1) and +1.17 V (1→2) vs Fc/Fc+ . The 1 H NMR of [NiCp2 ]2+ is a singlet at 8.6 ppm, whereas paramagnetic [NiCp2 ]⋅+ is significantly shifted upfield to -103.1 ppm.

High-Valent Ni and Cu Complexes of a Tetraanionic Bis(amidateanilido) Ligand

High-valent metal species are often invoked as intermediates during enzymatic and synthetic catalytic cycles. Anionic donors are often required to stabilize such high-valent states by forming strong bonds with the Lewis acidic metal centers while decreasing their oxidation potentials. In this report, we discuss the synthesis of two high-valent metal complexes [ML]+ in which the NiIII and CuIII centers are ligated by a new tetradentate, tetraanionic bis(amidateanilido) ligand. [ML]+, obtained via chemical oxidation of ML, exhibits UV-vis-NIR, EPR, and XANES spectra characteristic of square planar, high-valent MIII species, suggesting the locus of oxidation for both [ML]+ is predominantly metal-based. This is supported by theoretical analyses, which also support the observed visible transitions as ligand-to-metal charge transfer transitions characteristic of square planar, high-valent MIII species. Notably, [ML]+ can also be obtained via O2 oxidation of ML due to its remarkably negative oxidation potentials (CuL/[CuL]+: -1.16 V, NiL/[NiL]+: -1.01 V vs Fc/Fc+ in MeCN). This demonstrates the exceptionally strong donating nature of the tetraanionic bis(amidateanilido) ligation and its ability to stabilize high-valent metal centers..

Tris(pyridyl)borates: an emergent class of versatile and robust polydentate ligands for catalysis and materials applications

Tridentate ligands that incorporate pyridyl rather than pyrazolyl groups are emerging as an attractive class of "scorpionate"-type ligands with enhanced electron donation, increased stability, and divergent geometry at the metal centre relative to tris(pyrazolyl)borates originally introduced by Trofimenko. Following our initial reports, the tris(pyridyl)borate (Tpyb) ligand architecture has been adopted by several research groups in pursuit of functional metal complexes that offer new opportunities in catalysis and materials science. While earlier work had been focused on symmetric octahedral complexes, ML₂, which are advantageous as highly robust building blocks in materials sciences, recently introduced new ligand designs provide access to heteroleptic metal complexes with vacant sites that lend themselves to applications in catalysis. Signficant progress has also been made in the post-complexation functionalization of these ligands via electrophilic and nucleophilic substitution reactions at the boron centres, opening up new routes for integration of Tpyb complexes with diverse functional materials while also raising interesting mechanistic questions.

Scrutinizing formally NiIV centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory

Nickel K- and L2,3-edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L2,3-edge XAS reveals that the physical d-counts of the formally NiIV compounds measured lie well above the d6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF62- is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d6 NiIV center. The reactivity of NiIV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers.

Investigating the Mechanism of Ni-Catalyzed Coupling of Photoredox-Generated Alkyl Radicals and Aryl Bromides: A Computational Study

Photoredox catalysis has emerged as an alternative to classical cross-coupling reactions, promoting new reactivities. Recently, the use of widely abundant alcohols and aryl bromides as coupling reagents was demonstrated to promote efficient coupling through the Ir/Ni dual photoredox catalytic cycle. However, the mechanism underlying this transformation is still unexplored, and here we report a comprehensive computational study of the catalytic cycle. We have shown that nickel catalysts can promote this reactivity very efficiently through DFT calculations. Two different mechanistic scenarios were explored, suggesting that two catalytic cycles operate simultaneously depending on the concentration of the alkyl radical.

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.

A Redox Transmetalation Step in Nickel-Catalyzed C-C Coupling Reactions

Ni-catalyzed C-H functionalization reactions are becoming efficient routes to access a variety of functionalized arenes, yet the mechanisms of these catalytic C-C coupling reactions are not well understood. Here, we report the catalytic and stoichiometric arylation reactions of a nickel(II) metallacycle. Treatment of this species with silver(I)-aryl complexes results in facile arylation, consistent with a redox transmetalation step. Additionally, treatment with electrophilic coupling partners generates C-C and C-S bonds. We anticipate that this redox transmetalation step may be relevant to other coupling reactions that employ silver salts as additives.

Protodemetalation of (Bipyridyl)Ni(II)-Aryl Complexes Shows Evidence for Five-, Six-, and Seven-Membered Cyclic Pathways

Protonation of C-M bonds and its microscopic reverse, metalation of C-H bonds, are fundamental steps in a variety of metal-catalyzed processes. As such, studies on protonation of C-M bonds can shed light on C-H activation. We present here studies on the rate of protodemetalation (PDM) of a suite of arylnickel(II) complexes with various acids that provide evidence for a concerted, cyclic transition state for the PDM of C-Ni bonds and demonstrate that five-, six-, and seven-membered transition states are particularly favorable. Our data show that while the rate of protodemetalation of arylnickel(II) complexes scales with acidity for many acids, several are faster than predicted by pKa. For example, while acetic acid and acetohydroxamic acid are much less acidic than HCl, they both protodemetalate arylnickel(II) complexes significantly faster than HCl. Our data also show how in the case of acetohydroxamic acid, a seven-membered cyclic transition state (CH3C(O)NHOH) can be more favorable than a six-membered transition state (CH3C(O)NHOH). Similarly, five-membered transition states, such as for pyrazole, are highly favorable as well. Comparison of transition state polarization (from density functional theory) compares these new nickel transition states to better-studied precious-metal systems and demonstrates how the base can change the polarization of the transition state giving rise to opposing electronic preferences. Collectively, these studies suggest several new avenues for study in C-H activation as well as approaches to accelerate or slow protodemetalation in nickel catalysis.

Palladium Schiff base complex-modified Cu(BDC-NH2) metal-organic frameworks for C-N coupling

In this study, the synthesis of a novel functionalized metal-organic-framework (MOF) [Cu(BDC-NH₂)@Schiff-base-Pd(ii)] catalyst via post-synthetic modification of Cu(BDC-NH₂) is reported. The targeted complex was prepared by chemically attaching N,N'-bis(5-formylpyrrol-2-ylmethyl) homopiperazine via a Schiff base reaction followed by complexation with Pd ions. Afterwards, the synthesized solid was applied as a very effective multifunctional catalyst in C-N coupling reactions. The synthesized compounds were identified by suitable techniques including N₂ isotherms, EDX spectroscopy, FT-IR spectroscopy, XRD, SEM, ICP-OES and TG-DTA. This nanocatalyst was used in C-N cross-coupling reactions, and it showed its usage in a diverse range of different functional groups with good efficiency. The reasons for introducing this catalyst system are its advantages such as considerably high selectivity, almost complete conversion of products, high yields, and convenient separation of catalysts and products. The results indicate that the highest efficiency of the product in the reaction was obtained in the shortest possible time with the use of [Cu(BDC-NH₂)@Schiff-base-Pd(ii)] catalysts. Overall, the high catalytic activity of the [Cu(BDC-NH₂)@Schiff-base-Pd(ii)] catalyst may be due to the obtained high surface area and the synergistic features created between Lewis acidic Cu nodes and Pd ions.

Revised Mechanism of C(sp3)-C(sp3) Reductive Elimination from Ni(II) with the Assistance of a Z-Type Metalloligand

Reductive elimination is a key step in Ni-catalyzed cross-couplings. Compared with processes that proceed from Ni(III) or Ni(IV) intermediates, C(sp³)-C(sp³) reductive eliminations from Ni(II) centers are challenging due to the weak oxidizing ability of Ni(II) species. In this report, we present computational evidence that supports a mechanism in which Zn coordination to the nickel center as a Z-type ligand accelerates reductive elimination. This Zn-assisted pathway is found to be lower in energy compared with direct reductive elimination from a σ-coordinated Ni(II) intermediate, providing new insights into the mechanism of Ni-catalyzed cross-coupling with organozinc nucleophiles. Mayer bond order, Hirshfield charge, Laplacian of the electron density, orbital, and interaction region indicator analyses were conducted to elucidate details of the reductive elimination process and characterize the key intermediates. Theoretical calculations indicate a significant Z-type Ni-Zn interaction that reduces the electron density around the Ni center and accelerates reductive elimination. This mechanistic study of reductive elimination in Ni(0)-catalyzed conjunctive cross-couplings of aryl iodides, organozinc reagents, and alkenes is an important case study of the involvement of Zn-assisted reductive elimination in Ni catalysis. We anticipate that the novel Zn-assisted reductive elimination mode may extend to other cross-coupling processes and explain the unique effectiveness of organozinc nucleophiles in many instances.

Electrogenerated Nickel Catalyst for C-N Cross-Coupling

Arylamines represent a class of compounds widely found in natural products and pharmaceuticals. Among methodologies devoted to their synthesis, nickel-catalyzed amination of aryl halides constitutes one of the most employed conventional strategies. However, C-N cross-couplings often involve elaborated nickel complexes, which are expensive and/or air and moisture sensitive. To circumvent this issue, we herein report an electrochemical method based on a sacrificial anode process to in situ generate a catalytic amount of nickel salts allowing amination of aryl halides. The approach, simple to set up, proceeds under mild reaction conditions and enables access to a large panel of arylamines.

Organic reaction mechanism classification using machine learning

A mechanistic understanding of catalytic organic reactions is crucial for the design of new catalysts, modes of reactivity and the development of greener and more sustainable chemical processes^1-13. Kinetic analysis lies at the core of mechanistic elucidation by facilitating direct testing of mechanistic hypotheses from experimental data. Traditionally, kinetic analysis has relied on the use of initial rates¹⁴, logarithmic plots and, more recently, visual kinetic methods^15-18, in combination with mathematical rate law derivations. However, the derivation of rate laws and their interpretation require numerous mathematical approximations and, as a result, they are prone to human error and are limited to reaction networks with only a few steps operating under steady state. Here we show that a deep neural network model can be trained to analyse ordinary kinetic data and automatically elucidate the corresponding mechanism class, without any additional user input. The model identifies a wide variety of classes of mechanism with outstanding accuracy, including mechanisms out of steady state such as those involving catalyst activation and deactivation steps, and performs excellently even when the kinetic data contain substantial error or only a few time points. Our results demonstrate that artificial-intelligence-guided mechanism classification is a powerful new tool that can streamline and automate mechanistic elucidation. We are making this model freely available to the community and we anticipate that this work will lead to further advances in the development of fully automated organic reaction discovery and development.

NiH-catalyzed anti-Markovnikov hydroamidation of unactivated alkenes with 1,4,2-dioxazol-5-ones for the direct synthesis of N-alkyl amides

The addition of a nitrogen-based functional group to alkenes via a direct catalytic method is an attractive way of synthesizing value-added amides. The regioselective hydroamidation of unactivated alkenes is considered one of the easiest ways to achieve this goal. Herein, we report the NiH-catalyzed anti-Markovnikov intermolecular hydroamidation of unactivated alkenes enabled by using 2,9-dibutylphenathroline (diBuphen) as the ligand. This protocol provides a platform for the direct synthesis of over 90 structurally diverse N-alkyl amides using dioxazolones, which can be easily derived from abundant carboxylic acid feedstocks. This method succeeds for both terminal and internal unactivated alkenes and some natural products. Mechanistic studies including DFT calculations reveal an initial reversible insertion/elimination of the [NiH] to the alkene, followed by the irreversible amidation to furnish the N-alkyl amides. By crossover experiments and deuterium labeling studies, the observed anti-Markovnikov regioselectivities are suggested to be controlled by the sterical environment of the coupling reaction.

Ligand-free nickel catalyzed perfluoroalkylation of arenes and heteroarenes

We describe a "ligand-free" Ni-catalyzed perfluoroalkylation of heteroarenes to produce a diverse array of trfiluoromethyl, pentafluoroethyl and heptafluoropropyl adducts. Catalysis proceeds at room temperature via a radical pathway. The catalytic protocol is distinguished by its simplicity, and its wide scope demonstrates the potential in the late-stage functionalization of drug analogues and peptides.

C-H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles

Five- and six-coordinate cationic bis(phosphine) cobalt(III) metallacycle complexes were synthesized with the general structures, [(depe)Co(cycloneophyl)(L)(L')][BArF4] (depe = 1,2-bis(diethylphosphino)ethane; cycloneophyl = [κ-C:C-(CH2C(Me)2)C6H4]; L/L' = pyridine, pivalonitrile, or the vacant site, BAr4F = B[(3,5-(CF3)2)C6H3]4). Each of these compounds promoted facile directed C(sp2)-H activation with exclusive selectivity for ortho-alkylated products, consistent with the selectivity of reported cobalt-catalyzed arene-alkene-alkyne coupling reactions. The direct observation of C-H activation by cobalt(III) metallacycles provided experimental support for the intermediacy of these compounds in this class of catalytic C-H functionalization reaction. Deuterium labeling and kinetic studies provided insight into the nature of C-H bond cleavage and C-C bond reductive elimination from isolable cobalt(III) precursors.

Effect of Redox-Inactive Metal Ion-Nickel(III) Interactions on the Redox Properties and Proton-Coupled Electron Transfer Reactivity

Mononuclear nickel(II) and nickel(III) complexes of a bisamidate-bisalkoxide ligand, (NMe₄)₂[Ni^II(HMPAB)] (1) and (NMe₄)[Ni^III(HMPAB)] (2), respectively, have been synthesized and characterized by various spectroscopic techniques including X-ray crystallography. The reaction of redox-inactive metal ions (Mⁿ⁺ = Ca²⁺, Mg²⁺, Zn²⁺, Y³⁺, and Sc³⁺) with 2 resulted in 2-Mⁿ⁺ adducts, which was assessed by an array of spectroscopic techniques including X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and reactivity studies. The X-ray structure of Ca²⁺ coordinated to Ni(III) complexes, 2-Ca²⁺T, was determined and exhibited an average Ni-Ca distance of 3.1253 Å, close to the metal ions' covalent radius. XAS analysis of 2-Ca²⁺ and 2-Y³⁺ in solution further revealed an additional coordination to Ca and Y in the 2-Mⁿ⁺ adducts with shortened Ni-M distances of 2.15 and 2.11 Å, respectively, implying direct bonding interactions between Ni and Lewis acids (LAs). Such a short interatomic distance between Ni(III) and M is unprecedented and was not observed before. EPR analysis of 2 and 2-Mⁿ⁺ species, moreover, displayed rhombic signals with g_av > 2.12 for all complexes, supporting the +III oxidation state of Ni. The Ni^III/Ni^II redox potential of 2 and 2-Mⁿ⁺ species was determined, and a plot of E_1/2 of 2-Mⁿ⁺ versus pK_a of [M(H₂O)_n]^m+ exhibited a linear relationship, implying that the Ni^III/Ni^II potential of 2 can be tuned with different redox-inactive metal ions. Reactivity studies of 2 and 2-Mⁿ⁺ with different 4-X-2,6-ditert-butylphenol (4-X-DTBP) and other phenol derivatives were performed, and based on kinetic studies, we propose the involvement of a proton-coupled electron transfer (PCET) pathway. Analysis of the reaction products after the reaction of 2 with 4-OMe-DTBP showed the formation of a Ni(II) complex (1a) where one of the alkoxide arms of the ligand is protonated. A pK_a value of 24.2 was estimated for 1a. The reaction of 2-Mⁿ⁺ species was examined with 4-OMe-DTBP, and it was observed that the k₂ values of 2-Mⁿ⁺ species increase by increasing the Lewis acidity of redox-inactive metal ions. However, the obtained k₂ values for 2-Mⁿ⁺ species are much lower compared to the k₂ value for 2. Such a variation of PCET reactivity between 2 and 2-Mⁿ⁺ species may be attributed to the interactions between Ni(III) and LAs. Our findings show the significance of the secondary coordination sphere effect on the PCET reactivity of Ni(III) complexes and furnish important insights into the reaction mechanism involving high-valent nickel species, which are frequently invoked as key intermediates in Ni-mediated enzymatic reactions, solar-fuel catalysis, and biomimetic/synthetic transformation reactions.

Electron Shuttle in N-Heteroaromatic Ni Catalysts for Alkene Isomerization

Simple N-heteroaromatic Ni(II) precatalysts, (L)NiMe2 (L = bipy, bipym), were used for alkene isomerization. With an original reduction method using a simple borane (HB(Cat)), a low-valent Ni center was formed readily and showed good conversion when a reducing divalent lanthanide fragment, Cp*2Yb, was coordinated to the (bipym)NiMe2 complex, a performance not achieved by the monometallic (bipy)NiMe2 analogue. Experimental mechanistic investigations and computational studies revealed that the redox non-innocence of the L ligand triggered an electron shuttle process, allowing the elusive formation of Ni(I) species that were central to the isomerization process. Additionally, the reaction occurred with a preference for mono-isomerization rather than chain-walking isomerization. The presence of the low-valent ytterbium fragment, which contributed to the formation of the electron shuttle, strongly stabilized the catalysts, allowing catalytic loading as low as 0.5%. A series of alkenes with various architectures have been tested. The possibility to easily tune the various components of the heterobimetallic catalyst reported here, the ligand L and the divalent lanthanide fragment, opens perspectives for further applications in catalysis induced by Ni(I) species.

Interpretation of Shakeup Mechanisms in Copper L-Shell Photoelectron Spectra

We report on an original full ab initio quantum molecular approach designed to simulate Cu 2p X-ray photoelectron spectra. The description includes electronic relaxation/correlation and spin-orbit coupling effects and is implemented within nonorthogonal sets of molecular orbitals for the initial and final states. The underlying mechanism structuring the Cu 2p photoelectron spectra is clarified thanks to a correlation diagram applied to the CuO₄C₆H₆ paradigm. This diagram illustrates how the energy drop of the Cu 3d levels following the creation of the Cu 2p core hole switches the nature of the highest singly occupied molecular orbital (H-SOMO) from dominant metal to dominant ligand character. It also reveals how the repositioning of the Cu 3d levels induces the formation of new bonding and antibonding orbitals from which shakeup mechanisms toward the relaxed H-SOMO operate. The specific nature, ligand → ligand and metal → ligand, of these excitations building the satellite lines is exposed. Our approach finally applied to the real Cu(acac)₂ system clearly demonstrates how a definite interpretation of the XPS spectra can be obtained when a correct evaluation of binding energies, intensities, and relative widths of the spectral lines is achieved.

Quasi-Restricted Orbital Description of the Copper(I) Photoredox Catalytic Cycle

In this computational study, the electronic structure changes along the oxidative and reductive quenching cycles of a homoleptic and a heteroleptic prototype Cu(I) photoredox catalyst, namely [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline) and [Cu(phen)(POP)]+ (POP = bis[2-(diphenylphosphino)phenyl]ether) are scrutinized and characterized using quasi-restricted orbitals (QRO), electron density differences and spin densities. After validating our density functional theory-based computational protocol, the equilibrium geometries and wavefunctions (using QROs and atom/fragment compositions) of the four states involved in photoredox cycle (S0, T1, Dox and Dred) are systematically and thoroughly described. The formal ground and excited state ligand- and metal-centered redox events are substantiated by the QRO description of the open-shell triplet 3MLCT (d9L-1), Dox (d9L0) and Dred (d10L-1) species and the corresponding structural changes, e.g., flattening distortion, shortening/elongation of Cu-N/Cu-P bonds, are rationalized in terms of the underlying electronic structure transformations. Amongst others, we reveal the molecular-scale delocalization of the ligand-centered radical in the a 3MLCT (d9L-1) and Dred (d9L-1) states of homoleptic [Cu(dmp)2]+ and its localization to the redox-active phenanthroline ligand in the case of heteroleptic [Cu(phen)(POP)]+.

Selective Stepwise Arylation of Unprotected Peptides by PtIV Complexes

LPtIV F(Aryl) complexes bearing a bulky bidentate 2-[bis(adamant-1-yl)phosphino]phenoxide ligand (L) demonstrate excellent reactivity and selectivity in the arylation of X-H (X=S, N) bonds of amino acid residues in unprotected peptides under mild, including aqueous, conditions. Stepwise addition of these complexes allowed a convenient one-pot introduction of different aromatic groups in the X-H bonds of Cys and N terminus. PtIV reagents can also be used to further arylate N-H bonds in Lys and Trp providing access to peptides bearing multiple aromatic groups.

Site-Isolated Main-Group Tris(2-pyridyl)borate Complexes by Pyridine Substitution and Their Ring-Opening Polymerization Catalysis

Tris(2-pyridyl)borates are an emerging class of scorpionate ligands, distinguished as exceptionally robust and electron-donating. However, the rapid formation of inert homoleptic complexes with divalent metals has so far limited their catalytic utility. We report site-isolating tris(2-pyridyl)borate ligands, bearing isopropyl, tert-butyl, and mesityl substituents at the pyridine 6-position to suppress the formation of inert homoleptic complexes. These ligands form the first 1:1 complexes between tris(2-pyridyl)borates and Mg²⁺, Zn²⁺, or Ca²⁺, with isopropyl-substituted Tpy^iPrH showing the most generality. Single-crystal X-ray diffraction analysis of the resulting complexes and comparison to density functional theory (DFT) models showed geometric distortions driven by steric repulsion between the pyridine 6-substituents and the hexamethyldisilazide (HMDS^-, ^-N(SiMe₃)₂) anion. We show that this steric profile is a feature of the six-membered pyridine ring and contrasts with more established tris(pyrazolyl)borate and tris(imidazoline)borate scorpionate complexes. Tpy^iPrMg(HMDS) (1) and its zinc analogue are moderately active for the controlled polymerization of l-lactide, ε-caprolactone, and trimethylene carbonate. Furthermore, 1 gives controlled polymerization under more demanding melt-phase polymerization conditions at 100 °C, and block copolymerization of ε-caprolactone and trimethylene carbonate. These results will enable useful catalysis and coordination chemistry studies with tris(2-pyridyl)borates, and characterizes their structural complementarity to more familiar scorpionate ligands.

C-H Bond Activation by a Mononuclear Nickel(IV)-Nitrate Complex

The recent focus on developing high-valent non-oxo-metal complexes for late transition metals has proven to be an effective strategy to study the rich chemistry of these high-valent species while bypassing the synthetic challenges of obtaining the oxo-metal counterparts. In our continuing work of exploring late transition metal complexes of unusually high oxidation states, we have obtained in the present study a formal mononuclear Ni(IV)-nitrate complex (2) upon 1-e^- oxidation of its Ni(III) derivatives (1-OH and 1-NO₃). Characterization of these Ni complexes by combined spectroscopic and computational approaches enables deep understanding of their geometric and electronic structures, bonding interactions, and spectroscopic properties, showing that all of them are square planar complexes and exhibit strong π-covalency with the amido N-donors of the N3 ligand. Furthermore, results obtained from X-ray absorption spectroscopy and density functional theory calculations provide strong support for the assignment of the Ni(IV) oxidation state of complex 2, albeit with strong ligand-to-metal charge donation. Notably, 2 is able to oxidize hydrocarbons with C-H bond strength in the range of 76-92 kcal/mol, representing a rare example of high-valent late transition metal complexes capable of activating strong sp³ C-H bonds.