Structure and electronic properties of Pd(III) complexes

Palladium K-edge X-ray Absorption Spectroscopy Studies on Controlled Ligand Systems

X-ray absorption spectroscopy (XAS) is widely used across the life and physical sciences to identify the electronic properties and structure surrounding a specific element. XAS is less often used for the characterization of organometallic compounds, especially for sensitive and highly reactive species. In this study, we used solid- and solution-phase XAS to compare a series of 25 palladium complexes in controlled ligand environments. The compounds include palladium centers in the formal I, II, III, and IV oxidation states, supported by tridentate and tetradentate macrocyclic ligands, with different halide and methyl ligand combinations. The Pd K-edge energies increased not only upon oxidizing the metal center but also upon increasing the denticity of the ligand framework, substituting sigma-donating methyl groups with chlorides, and increasing the charge of the overall metal complex by replacing charged ligands with neutral ligands. These trends were then applied to characterize compounds whose oxidation states were otherwise unconfirmed.

Aryl sulfonate anion stabilized aromatic triangular cation [Pd3]+: syntheses, structures and properties

A series of sulfonate anions paired aromatic triangular palladium clusters 3-7, abbreviated as [Pd3]+[ArSO3]-, were synthesized using a simple "one pot" method, and gave excellent isolated yields (90-95%). Their structures and properties have been fully characterized and further investigated by fluorescence, single crystal X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). In varying organic solvents, they presented apparently stronger absorption and emission in MeOH, driven by the combined interactions of hydrogen bonds and polarity. The crystallographic data demonstrated that the methyl orange ion stabilized complex 7 possessed a D3h symmetric metallic core which was still coplanar and almost equilateral, jointly influenced by the giant hindrance and milder donating effect from the sulfonate. The binding energies for Pdn+ 3d5/2 and Pdn+ 3d3/2 measured by XPS presented at 336.55 and 342.00 eV, respectively. These data were much lower than that of a usual Pd2+ 3d and significantly higher than that of a Pd0 species, further proving the unified palladium valence state (+4/3) in the tri-palladium core and its aromaticity featured by the cyclic electron delocalization.

An unusual Pd(III) oxidation state in the Pd-Cl chain complex with high thermal stability and electrical conductivity

The Pd(iii) oxidation state is unusual and unstable since it strongly tends to disproportionate. We synthesized the quasi-one-dimensional (1D) halogen-bridged Pd(iii)-Cl complex [Pd(dabdOH)2Cl]Cl2 (1-Cl; dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol) with multiple hydrogen bonds. From single-crystal X-ray diffraction, the bridging Cl- ions were located at the midpoint of the Pd-Cl-Pd moieties in the 1D chains, indicating that the Pd ions are in a Pd(iii) average valence (AV) state. Moreover, bright spots for the Pd(iii) dz2 orbitals in the upper Hubbard band above the Fermi level were observed every ∼5 Å using scanning tunnelling microscopy. These results clearly indicate that the Pd ions are in a Pd(iii) AV state in 1-Cl. In addition, 1-Cl has the highest thermal stability (470 K) among the Pd(iii) complexes reported and the highest electrical conductivity (0.6 S cm-1 at 300 K) among the 1D Pd-Cl chains reported so far.

Rapid Electrochemical Methane Functionalization Involves Pd-Pd Bonded Intermediates

High-valent Pd complexes are potent agents for the oxidative functionalization of inert C-H bonds, and it was previously shown that rapid electrocatalytic methane monofunctionalization could be achieved by electro-oxidation of Pd^II to a critical dinuclear Pd^III intermediate in concentrated or fuming sulfuric acid. However, the structure of this highly reactive, unisolable intermediate, as well as the structural basis for its mechanism of electrochemical formation, remained elusive. Herein, we use X-ray absorption and Raman spectroscopies to assemble a structural model of the potent methane-activating intermediate as a Pd^III dimer with a Pd-Pd bond and a 5-fold O atom coordination by H_xSO₄^(x-2) ligands at each Pd center. We further use EPR spectroscopy to identify a mixed-valent M-M bonded Pd₂^II,III species as a key intermediate during the Pd^II-to-Pd^III₂ oxidation. Combining EPR and electrochemical data, we quantify the free energy of Pd dimerization as <-4.5 kcal/mol for Pd₂^II,III and <-9.1 kcal/mol for Pd^III₂. The structural and thermochemical data suggest that the aggregate effect of metal-metal and axial metal-ligand bond formation drives the critical Pd dimerization reaction in between electrochemical oxidation steps. This work establishes a structural basis for the facile electrochemical oxidation of Pd^II to a M-M bonded Pd^III dimer and provides a foundation for understanding its rapid methane functionalization reactivity.

Detection and Characterization of Mononuclear Pd(I) Complexes Supported by N2S2 and N4 Tetradentate Ligands

Palladium is a versatile transition metal used to catalyze a large number of chemical transformations, largely due to its ability to access various oxidation states (0, I, II, III, and IV). Among these oxidation states, Pd(I) is arguably the least studied, and while dinuclear Pd(I) complexes are more common, mononuclear Pd(I) species are very rare. Reported herein are spectroscopic studies of a series of Pd(I) intermediates generated by the chemical reduction at low temperatures of Pd(II) precursors supported by the tetradentate ligands 2,11-dithia[3.3](2,6)pyridinophane (N2S2) and N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (^tBuN4): [(N2S2)Pd^II(MeCN)]₂(OTf)₄ (1), [(N2S2)Pd^IIMe]₂(OTf)₂ (2), [(N2S2)Pd^IICl](OTf) (3), [(N2S2)Pd^IIX](OTf)₂ (X = tBuNC 4, PPh₃ 5), [(N2S2)Pd^IIMe(PPh₃)](OTf) (6), and [(^tBuN4)Pd^IIX₂](OTf)₂ (X = MeCN 8, tBuNC 9). In addition, a stable Pd(I) dinuclear species, [(N2S2)Pd^I(μ-tBuNC)]₂(ClO₄)₂ (7), was isolated upon the electrochemical reduction of 4 and structurally characterized. Moreover, the (^tBuN4)Pd^I intermediates, formed from the chemical reduction of [(^tBuN4)Pd^IIX₂](OTf)₂ (X = MeCN 8, tBuNC 9) complexes, were investigated by EPR spectroscopy, X-ray absorption spectroscopy (XAS), and DFT calculations and compared with the analogous (N2S2)Pd^I systems. Upon probing the stability of Pd(I) species under different ligand environments, it is apparent that the presence of soft ligands such as tBuNC and PPh₃ significantly improves the stability of Pd(I) species, which should make the isolation of mononuclear Pd(I) species possible.

Improved Oxidative C-C Bond Formation Reactivity of High-Valent Pd Complexes Supported by a Pseudo-Tridentate Ligand

There is a large interest in developing oxidative transformations catalyzed by palladium complexes that employ environmentally friendly and economical oxidizing reagents such as dioxygen. Recently, we have reported the isolation and characterization of various mononuclear Pd^III and Pd^IV complexes supported by the tetradentate ligands N,N'-dialkyl-2,11-diaza[3.3](2,6)pyridinophane (^RN4, R = ^tBu, ⁱPr, Me), and the aerobically induced C-C and C-heteroatom bond formation reactivity was investigated in detail. Given that the steric and electronic properties of the multidentate ligands were shown to tune the stability and reactivity of the corresponding high-valent Pd complexes, herein we report the use of an asymmetric N4 ligand, N-mehtyl-N'-tosyl-2,11-diaza[3.3](2,6)pyridinophane (^TsMeN4), in which one amine N atom contains a tosyl group. The N-Ts donor atom exhibits a markedly reduced donating ability, which led to the formation of transiently stable Pd^III and Pd^IV complexes, and consequently the corresponding O₂ oxidation reactivity and the subsequent C-C bond formation were improved significantly.

Photoexcited Pd(ii) auxiliaries enable light-induced control in C(sp3)-H bond functionalisation

Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C–H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp³)–H halogenation products rather than C–C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd(iii) species which promotes C–X bond formation via a distinct Pd(iii)/Pd(iv) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp³)–H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C–H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd(iii)/Pd(iv) redox couples induced by visible light irradiation.

Visible light irradiation of 8-aminoquinoline Pd(ii) complexes initiates photoinduced electron transfer with alkyl halides, affording C–H halogenation over C–C bond adducts. A method for inert C(sp³)–H bond halogenation (Br, Cl and I) is reported.

Conductive zigzag Pd(iii)–Br chain complex realized by a multiple-hydrogen-bond approach

Coexistence of zigzag structure and the uncommon Pd(iii) oxidation state in quasi-1D halogen-bridged metal complexes was realized in a conductive Br-bridged Pd chain complex, [Pd(dabdOH)₂Br]SO₄·3H₂O (2), for the first time.

Alkyliodines in High Oxidation State: Enhanced Synthetic Possibilities and Accelerated Catalyst Turn-Over

In contrast to aryliodine(III) compounds, which have matured into a particularly attractive class of oxidants in modern synthesis, the synthetic potential of related alkyliodine(III) derivatives has remained widely underestimated. This is surprising since several unique synthetic possibilities arise directly from the low stability of their central carbon-iodine bond. In this respect, these high-oxidation-state iodine compounds resemble environmentally benign variants of the prominent metal counterparts such as those derived from palladium, nickel and copper. This Concept article summarizes the general reactivity trends in alkyliodine(III) chemistry and discusses selected examples of their strategic use as highly reactive, transient species in organic synthesis and homogeneous catalysis.

Hemilabile bonding of 1-oxa-4,7-dithiacyclononane in cyclometallated palladium(ii) complexes

The synthesis and characterization of a series of cyclometallated complexes of Pd(ii) incorporating the mixed donor ligand 1-oxa-4,7-dithiacyclononane ([9]aneS₂O) are presented in this study. Complexes of the form [Pd(C^N)([9]aneS₂O)](PF₆) (C^N = 2-phenylpyridine (ppy) 1b, 4-(2-pyridyl)benzaldehyde (ppyCHO) 2b, 7,8-benzoquinoline (bzq) 3b, 2-benzothienylpyridine (btp) 4b, 2-phenylbenzothiazole (pbt) 5b), were obtained in high-yield from a simple two-step synthetic scheme. All of these complexes were fully characterized by NMR, ESI-MS, IR, combustion analyses, and most (1b, 2b, 4b, 5b) by X-ray crystallography. Solution ¹H and ¹³C NMR studies of [Pd(C^N)([9]aneS₂O)](PF₆) complexes demonstrate complicated [9]aneS₂O behavior at room temperature. Variable temperature NMR reveals dynamic bonding of the [9]aneS₂O ligand consistent with the presence of both endodentate and exodentate bonding modes. This is in stark contrast to the related [9]aneS₃ (1,4,7-trithiacyclononane) cogeners that demonstrate fluxional endodentate bonding only in solution. X-ray structures reveal only exodenate [9]aneS₂O bonding in this series, unlike the related [9]aneS₃ complexes that show endodenate bonding with an axial PdS interaction. DFT calculations performed on endo and exo [9]aneS₂O bonding forms of 4b, as well as a transition state calculation for interconversion, suggest reasonable access to both bonding forms based on the energy barrier. Natural bond order calculations provide further evidence for a weak axial PdO interaction in the endo form of 4b.

Highly phosphorescent organopalladium(ii) complexes with metal–metal-to-ligand charge-transfer excited states in fluid solutions

Making Pd(ii) brightly shining: mononuclear analogues are non-emissive, but folded dinuclear palladium(ii) diacetylide complexes phosphoresce from MMLCT excited states with quantum yields up to 48%.

Structure and Reactivity of Pd Complexes in Various Oxidation States in Identical Ligand Environments with Reference to C-C and C-Cl Coupling Reactions: Insights from Density Functional Theory

Bonding and reactivity of [(^RN4)Pd ⁿCH₃X]^{( n-2)+} complexes have been investigated at the M06/BS2//B3LYP/BS1 level. Feasible mechanisms for the unselective formation of ethane and methyl chloride from mono-methyl Pd^III complexes and selective formation of ethane or methyl chloride from Pd^IV complexes are reported here. Density functional theory (DFT) results indicate that Pd^IV is more reactive than Pd^III and Pd in different oxidation states that follow different mechanisms. Pd^III complexes react in three steps: (i) conformational change, (ii) transmetalation, and (iii) reductive elimination. In the first step a five-coordinate Pd^III intermediate is formed by the cleavage of one Pd-N_ax bond, and in the second step one methyl group is transferred from the Pd^III complex to the above intermediate via transmetalation, and subsequently a six-coordinate Pd^IV intermediate is formed by disproportion. In this step, transmetalation can occur on both singlet and triplet surfaces, and the singlet surface is lying lower. Transmetalation can also occur between the above intermediate and [(^RN4)Pd^II(CH₃)(CH₃CN) ]⁺, but this not a feasible path. In the third step this Pd^IV intermediate undergoes reductive elimination of ethane and methyl chloride unselectively, and there are three possible routes for this step. Here axial-equatorial elimination is more facile than equatorial-equatorial elimination. Pd^IV complexes react in two steps, a conformational change followed by reductive elimination, selectively forming ethane or methyl chloride. Thus, Pd^III complex reacts through a six-coordinate Pd^IV intermediate that has competing C-C and C-Cl bond formation, and Pd^IV complex reacts through a five-coordinate Pd^IV intermediate that has selective C-C and C-Cl bond formation. Free energy barriers indicate that iPr, in comparison to the methyl substituent in the ^RN4 ligand, activates the cleaving of the Pd-N_ax bond through electronic and steric interactions. Overall, reductive elimination leading to C-C bond formation is easier than the formation of a C-Cl bond.

Electronic versus steric effects of pyridinophane ligands on Pd(iii) complexes

Several new Pd^II and Pd^III complexes supported by electronically and sterically tuned tetradentate pyridinophane ligands ^MeN4^OMe, ^MeN4, and ^tBuN4 were isolated and fully characterized (^MeN4^OMe: N,N'-dimethyl-2,11-diaza[3,3](2,6)-para-methoxypyridinophane; ^MeN4: N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane; ^tBuN4: N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane). Cyclic voltammetry studies, UV-vis and EPR spectroscopy, and X-ray crystallography were employed to reveal that the steric properties of the N-substituents of the ^RN4 ligands have a pronounced effect on the electronic properties of the corresponding Pd^III complexes, while the electronic tuning of the ligand pyridyl groups has a surprisingly minimal effect. An explanation for these observations was provided by DFT and TD-DFT calculations which suggest that the electronic properties of the Pd^III complexes are mainly dictated by their frontier molecular orbitals that have major atomic contributions from the Pd center (mainly the Pd d_z² atomic orbital) and the axial N atom donors.

Unraveling the Role of a Flexible Tetradentate Ligand in the Aerobic Oxidative Carbon-Carbon Bond Formation with Palladium Complexes: A Computational Mechanistic Study

Mechanistic details of the aerobic oxidative coupling of methyl groups by a novel (^MeL)Pd^II(Me)₂ complex with the tetradentate ligand, ^MeL = N, N-dimethyl-2,11-diaza[3.3](2,6)pyridinophane, has been explored by density functional theory calculations. The calculated mechanism sheds light on the role of this ligand's flexibility in several stages of the reaction, especially as the oxidation state of the Pd changes. Ligand flexibility leads to diverse axial coordination modes, and it controls the availability of electrons by modulating the energies of high-lying molecular orbitals, particularly those with major d _z² character. Solvent molecules, particularly water, appear essential in the aerobic oxidation of Pd^II by lowering the energy of the oxygen molecule's unoccupied molecular orbital and stabilizing the Pd^X-O₂ complex. Ligand flexibility and solvent coordination to oxygen are essential to the required spin-crossover for the transformation of high-valent Pd^X-O₂ complexes. A methyl cation pathway has been predicted by our calculations in transmetalation between Pd^II and Pd^IV intermediates to be preferred over methyl radical or methyl anion pathways. Combining an axial and equatorial methyl group is preferred in the reductive elimination pathway where roles are played by the ligand's flexibility and the fluxionality of trimethyl groups.

Improved synthesis of symmetrically & asymmetrically N-substituted pyridinophane derivatives

The N,N'-di(toluenesulfonyl)-2,11-diaza[3,3](2,6)pyridinophane (^TsN4) precursor was sought after as a starting point for the preparation of various symmetric and asymmetric pyridinophane-derived ligands. Various procedures to synthesize ^TsN4 had been published, but the crucial problem had been the purification of ^TsN4 from the larger 18- and 24-membered azamacrocycles. Most commonly, column chromatography or other laborious methods have been utilized for this separation, yet we have found an alternate selective dissolution method upon protonation which allows for multi-gram scale output of ^TsN4·HCl. This optimized synthesis of ^TsN4 also led to the development of symmetric ^RN4 derivatives as well as the asymmetric derivative N-(tosyl)-2,11-diaza[3,3](2,6)pyridinophane (^TsHN4). Using this ^TsHN4 precursor, different N-substituents can be added to create a library of asymmetric ^RR'N4 macrocyclic ligands. These asymmetric ^RR'N4 derivatives expand the utility of the ^RN4 framework in coordination chemistry and the ability to study the electronic, steric, and denticity effects of these pyridinophane ligands on the metal center.

Palladium Complexes of N,N'-Bis(2-aminoethyl)oxamide (H2L): Structural (PdIIL, PdII2L2, and PdIVLCl2), Electrochemical, Dynamic 1H NMR, and Cytotoxicity Studies

The monomeric (PdL·2H₂O) and dimeric (Pd₂L₂·7H₂O) palladium(II) complexes of N,N'-bis(2-aminoethyl)oxamide (H₂L) were isolated, and their structures were established by single-crystal X-ray diffraction. Both compounds display identical cis-(2N_amide + 2N_amine) coordination environments of the metal ion. The dimer, representing a combination of two PdL species with an open lateral chelate ring, has an "open clamshell"-like structure. The intramolecular metal-metal separation in Pd₂L₂ (3.215 Å) is slightly shorter than the sum of the van der Waals radii of the palladium(II) atoms. The dimeric complex is relatively stable to dissociation, and its spectral features in aqueous solutions have been compared to those of the monomeric complex. A ¹H NMR spectroscopic study revealed the presence of the dynamic conformational exchange process assigned to a turning of the dimeric molecule "inside out" with an activation energy of 65 kJ/mol. Cyclic voltammetry of PdL in perchlorate-, chloride-, and sulfate-containing electrolytes revealed two-electron oxidation of the palladium center. For the dimeric complex similar, though irreversible, oxidation to the palladium(IV) state was observed in NaCl electrolyte. At the same time, in NaClO₄ or Na₂SO₄ solutions oxidation of Pd₂L₂ occurs in two distinct steps. The first step is quasi-reversible and can be assigned to the formation of species in an intermediate Pd^IIIPd^III state. Monomeric palladium(IV) complex Pd^IVLCl₂ was generated via chemical oxidation of Pd^IIL by peroxodisulfate in the presence of chloride ions and structurally characterized. The related M^IIL complexes (M = Pd, Ni, Cu) showed low cytotoxicity in human cancer cell lines AGS (gastric adenocarcinoma) and HCT116 (colorectal carcinoma) with IC₅₀ values from 204 to 525 μM, while the proligand H₂L was devoid of antiproliferative activity (IC₅₀ > 1000 μM).

Catalytic Methane Monofunctionalization by an Electrogenerated High-Valent Pd Intermediate

Electrophilic high-valent metal ions are potent intermediates for the catalytic functionalization of methane, but in many cases, their high redox potentials make these intermediates difficult or impossible to access using mild stoichiometric oxidants derived from O₂. Herein, we establish electrochemical oxidation as a versatile new strategy for accessing high-valent methane monofunctionalization catalysts. We provide evidence for the electrochemical oxidation of simple PdSO₄ in concentrated sulfuric acid electrolytes to generate a putative Pd₂^III,III species in an all-oxidic ligand field. This electrogenerated high-valent Pd complex rapidly activates methane with a low barrier of 25.9 (±2.6) kcal/mol, generating methanol precursors methyl bisulfate (CH₃OSO₃H) and methanesulfonic acid (CH₃SO₃H) via concurrent faradaic and nonfaradaic reaction pathways. This work enables new electrochemical approaches for promoting rapid methane monofunctionalization.

Ligand-Promoted Palladium-Catalyzed Aerobic Oxidation Reactions

Palladium-catalyzed aerobic oxidation reactions have been the focus of industrial application and extensive research efforts for nearly 60 years. A significant transition occurred in this field approximately 20 years ago, with the introduction of catalysts supported by ancillary ligands. The ligands play crucial roles in the reactions, including promotion of direct oxidation of palladium(0) by O₂, bypassing the typical requirement for Cu salts or related redox cocatalysts to facilitate oxidation of the reduced Pd catalyst; facilitation of key bond-breaking and bond-forming steps during substrate oxidation; and modulation of chemo-, regio-, or stereoselectivity of a reaction. The use of ligands has contributed to significant expansion of the scope of accessible aerobic oxidation reactions. Increased understanding of the role of ancillary ligands should promote the development of new synthetic transformations, enable improved control over the reaction selectivity, and improve catalyst activity and stability. This review surveys the different ligands that have been used to support palladium-catalyzed aerobic oxidation reactions and, where possible, describes mechanistic insights into the role played by the ancillary ligand.

Redox trends in cyclometalated palladium(ii) complexes

Electrochemical and DFT studies on palladacycles revealed an increase in the metal–metal distance in the complexes leads to higher oxidation potentials.

Geometry Constrained N-(5,6,7-Trihydroquinolin-8-ylidene)arylaminopalladium Dichloride Complexes: Catalytic Behavior toward Methyl Acrylate (MA), Methyl Acrylate-co-Norbornene (MA-co-NB) Polymerization and Heck Coupling

A new pair of plladium complexes (Pd4 and Pd5) ligated with constrained N-(5,6,7-trihydroquinolin-8-ylidene)arylamine ligands have been prepared and well characterized by ¹H-, ¹³C-NMR and FTIR spectroscopies as well as elemental analysis. The molecular structure of Pd4 and Pd5 in solid state have also been determined by X-ray diffraction, showing slightly distorted square planar geometry around the palladium metal center. All complexes Pd1–Pd5 are revealed highly efficient catalyst in methyl acrylate (MA) polymerization as well as methyl acrylate/norbornene (MA/NB) copolymerization. In the case of MA polymerization, as high as 98.4% conversion with high molecular weight up to 6282 kg·mol⁻¹ was achieved. Likewise, Pd3 complex has good capability to incorporate about 18% NB content into MA polymer chains. Furthermore, low catalyst loadings (0.002 mol %) of Pd4 or Pd5 are able to efficiently mediate the coupling of haloarenes with styrene affording up to 98% conversion.

Isolated Organometallic Nickel(III) and Nickel(IV) Complexes Relevant to Carbon-Carbon Bond Formation Reactions

Nickel-catalyzed cross-coupling reactions are experiencing a dramatic resurgence in recent years given their ability to employ a wider range of electrophiles as well as promote stereospecific or stereoselective transformations. In contrast to the extensively studied Pd catalysts that generally employ diamagnetic intermediates, Ni systems can more easily access various oxidation states including odd-electron configurations. For example, organometallic Ni^III intermediates with aryl and/or alkyl ligands are commonly proposed as the active intermediates in cross-coupling reactions. Herein, we report the first isolated Ni^III-dialkyl complex and show that this species is involved in stoichiometric and catalytic C-C bond formation reactions. Interestingly, the rate of C-C bond formation from a Ni^III center is enhanced in the presence of an oxidant, suggesting the involvement of transient Ni^IV species. Indeed, such a Ni^IV species was observed and characterized spectroscopically for a nickelacycle system. Overall, these studies suggest that both Ni^III and Ni^IV species could play an important role in a range of Ni-catalyzed cross-coupling reactions, especially those involving alkyl substrates.

Stable bis(trifluoromethyl)nickel(III) complexes

Organometallic Ni(III) intermediates have been proposed in several Nickel-catalyzed cross-coupling reactions, yet no isolated bis(hydrocarbyl)Ni(III) complexes have been reported to date. Herein we report the synthesis and detailed characterization of stable organometallic Ni(III) complexes that contain two trifluoromethyl ligands and are supported by tetradentate N-donor ligands (R)N4 (R = Me or tBu). Interestingly, the corresponding Ni(II) precursors undergo facile oxidation, including aerobic oxidation, to generate uncommonly stable organometallic Ni(III) complexes that exhibit limited reactivity.