Uncovering a CF3 Effect on X-ray Absorption Energies of [Cu(CF3 )4 ]- and Related Copper Compounds by Using Resonant Diffraction Anomalous Fine Structure (DAFS) Measurements

Understanding the electronic structures of high-valent metal complexes aids the advancement of metal-catalyzed cross coupling methodologies. A prototypical complex with formally high valency is [Cu(CF3 )4 ]- (1), which has a formal Cu(III) oxidation state but whose physical analysis has led some to a Cu(I) assignment in an inverted ligand field model. Recent examinations of 1 by X-ray spectroscopies have led previous authors to contradictory conclusions, motivating the re-examination of its X-ray absorption profile here by a complementary method, resonant diffraction anomalous fine structure (DAFS). From analysis of DAFS measurements for a series of seven mononuclear Cu complexes including 1, here it is shown that there is a systematic trifluoromethyl effect on X-ray absorption that blue shifts the resonant Cu K-edge energy by 2-3 eV per CF3 , completely accounting for observed changes in DAFS profiles between formally Cu(III) complexes like 1 and formally Cu(I) complexes like (Ph3 P)3 CuCF3 (3). Thus, in agreement with the inverted ligand field model, the data presented herein imply that 1 is best described as containing a Cu(I) ion with dn count approaching 10.

Copper-Carbon Homolysis Competes with Reductive Elimination in Well-Defined Copper(III) Complexes

Despite the recent advancements of Cu catalysis for the cross-coupling of alkyl electrophiles and the frequently proposed involvement of alkyl-Cu(III) complexes in such reactions, little is known about the reactivity of these high-valent complexes. Specifically, although the reversible interconversion between an alkyl-CuIII complex and an alkyl radical/CuII pair has been frequently proposed in Cu catalysis, direct observation of such steps in well-defined CuIII complexes remains elusive. In this study, we report the synthesis and investigation of alkyl-CuIII complexes, which exclusively undergo a Cu-C homolysis pathway to generate alkyl radicals and CuII species. Kinetic studies suggest a bond dissociation energy of 28.6 kcal/mol for the CuIII-C bonds. Moreover, these four-coordinate complexes could be converted to a solvated alkyl-CuIII-(CF3)2, which undergoes highly efficient C-CF3 bond-forming reductive elimination even at low temperatures (-4 °C). These results provide strong support for the reversible recombination of alkyl radicals with CuII to form alkyl-CuIII species, an elusive step that has been proposed in Cu-catalyzed mechanisms. Furthermore, our work has demonstrated that the reactivity of CuIII complexes could be significantly influenced by subtle changes in the coordination environment. Lastly, the observation of the highly reactive neutral alkyl-CuIII-(CF3)2 species (or with weakly bound solvent molecules) suggests they might be the true intermediates in many Cu-catalyzed trifluoromethylation reactions.

Dual Oxidation of Epoxides with a High-Valent Cu(III)-CF3 Compound and DMSO to Access 1,2-Diketones

This study reports sequential dehydrogenation and transfer oxygenation of 1,2-diarylepoxides by high-valent phenCu(III)(CF3)3 and DMSO to produce 1,2-diketones. The Cu(III)-CF3 compound serves as a CF3 radical source to abstract the hydrogen atom of the epoxide ring. The resulting ether α-carbon radical undergoes ring-opening rearrangement to give a ketone α-carbon radical intermediate, which is oxygenated by DMSO with the release of Me2S. The combination of a Cu(III)-CF3 compound and DMSO may be exploited to develop other novel oxidation reactions.

Synthesis of a chiral dinuclear Cu(II)-benzothiazolamine complex: evidence of cuprophilic interaction in its structure and exploration of its electrochemical properties and catalytic performance

The synthesis of a chiral dinuclear [Cu(OAc)2(L1)]2 complex (A) and its analogues Cu(OAc)2(L1)2 (B), Cu(OAc)2(L1)PPh3 (C), CuBr(L1)PPh3 (D), and Cu(OAc)2(L2) (E) is described. The X-ray structure of A reveals a cuprophilic interaction (2.65 Å) and shows that L1 behaves as a monodentate ligand. The stereogenic centre in L1 aligns the NH group to form non-covalent interactions with the paddle-wheel acetate groups at variable distances (2.4-2.5 Å and 2.2-2.7 Å). Thermogravimetric analysis confirmed our hypothesis that two equivalents of L1 (B) or a combination of L1 and PPh3 (C) would disrupt the cuprophilic interaction. All complexes, except D, showed irreversible redox waves by cyclic voltammetry. Complexes C and E have lower oxidative peaks (at 10 V s-1) than complex A between +0.40 and +0.60 V. This highlights the influence of ligand(s) on the redox behaviour of Cu(II) complexes. The significance of this electrochemical behaviour was evident in the Chan-Lam (CL) coupling reaction, where 2.5 mol% of A successfully facilitated the formation of a C-N bond. This study showcased the structure, thermal stability, electrochemical properties and catalytic performance of a chiral dinuclear copper(II)-benzothiazolamine complex.

Cross-Coupling Reactions Enabled by Well-Defined Ag(III) Compounds: Main Focus on Aromatic Fluorination and Trifluoromethylation

AgIII compounds are considered strong oxidizers of difficult handling. Accordingly, the involvement of Ag catalysts in cross-coupling via 2e- redox sequences is frequently discarded. Nevertheless, organosilver(III) compounds have been authenticated using tetradentate macrocycles or perfluorinated groups as supporting ligands, and since 2014, first examples of cross-coupling enabled by AgI /AgIII redox cycles saw light. This review collects the most relevant contributions to this field, with main focus on aromatic fluorination/perfluoroalkylation and the identification of AgIII key intermediates. Pertinent comparison between the activity of AgIII RF compounds in aryl-F and aryl-CF3 couplings vs. the one shown by its CuIII RF and AuIII RF congeners is herein disclosed, thus providing a more profound picture on the scope of these transformations and the pathways commonly associated to C-RF bond formations enabled by coinage metals.

High-valent Cu(III)-CF3 compound-mediated esterification reaction

Cu(III)-CF₃ compounds are reported herein as novel coupling reagents to mediate ester synthesis from carboxyl acids and alcohols/phenols. Carboxylic acids are transformed to trifluoromethyl ester and acyl fluoride activated species that interact with each other. The broad substrate scope and late-stage application of this method are demonstrated. This study opens up new opportunities to develop interesting reactions using Cu(III)-CF₃ compounds without transferring a CF₃ group to the products.

[(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2-: Foundations toward the Development of Trifluoromethylations at Unsupported Nickel

Nickel anions [(MeCN)Ni(CF₃)₃]^- and [Ni(CF₃)₄]^2- were prepared by the formal addition of 3 and 4 equiv, respectively, of AgCF₃ to [(dme)NiBr₂] in the presence of the [PPh₄]⁺ counterion. Detailed insights into the electronic properties of these new compounds were obtained through the use of density functional theory (DFT) calculations, spectroscopy-oriented configuration interaction (SORCI) calculations, X-ray absorption spectroscopy, and cyclic voltammetry. The data collectively show that trifluoromethyl complexes of nickel, even in the most common oxidation state of nickel(II), are highly covalent systems whereby a hole is distributed on the trifluoromethyl ligands, surprisingly rendering the metal to a physically more reduced state. In the cases of [(MeCN)Ni(CF₃)₃]^- and [Ni(CF₃)₄]^2-, these complexes are better physically described as d⁹ metal complexes. [(MeCN)Ni(CF₃)₃]^- is electrophilic and reacts with other nucleophiles such as phenoxide to yield the unsupported [(PhO)Ni(CF₃)₃]^2- salt, revealing the broader potential of [(MeCN)Ni(CF₃)₃]^- in the development of "ligandless" trifluoromethylations at nickel. Proof-in-principle experiments show that the reaction of [(MeCN)Ni(CF₃)₃]^- with an aryl iodonium salt yields trifluoromethylated arene, presumably via a high-valent, unsupported, and formal organonickel(IV) intermediate. Evidence of the feasibility of such intermediates is provided with the structurally characterized [PPh₄]₂[Ni(CF₃)₄(SO₄)], which was derived through the two-electron oxidation of [Ni(CF₃)₄]^2-.

C(sp3)-CF3 Reductive Elimination from a Five-Coordinate Neutral Copper(III) Complex

The reductive elimination from a high-valent late-transition-metal complex for the formation of a carbon-carbon or carbon-heteroatom bond represents a fundamental product-forming step in a number of catalytic processes. While reductive eliminations from well-defined Pt(IV), Pd(IV), Ni(III)/Ni(IV), and Au(III) complexes have been studied, the analogous reactions from neutral Cu(III) complexes remain largely unexplored. Herein, we report the isolation of a stable, five-coordinate, neutral square pyramidal Cu(III) complex that gives CH₃-CF₃ in quantitative yield via reductive elimination. Mechanistic studies suggest that the reaction occurs through a synchronous bond-breaking/bond-forming process via a three-membered ring transition state.

The Myth of d8 Copper(III)

Seventeen Cu complexes with formal oxidation states ranging from CuI to CuIII are investigated through the use of multiedge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. Analysis reveals that the metal-ligand bonding in high-valent, formally CuIII species is extremely covalent, resulting in Cu K-edge and L2,3-edge spectra whose features have energies that complicate physical oxidation state assignment. Covalency analysis of the Cu L2,3-edge data reveals that all formally CuIII species have significantly diminished Cu d-character in their lowest unoccupied molecular orbitals (LUMOs). DFT calculations provide further validation of the orbital composition analysis, and excellent agreement is found between the calculated and experimental results. The finding that Cu has limited capacity to be oxidized necessitates localization of electron hole character on the supporting ligands; consequently, the physical d8 description for these formally CuIII species is inaccurate. This study provides an alternative explanation for the competence of formally CuIII species in transformations that are traditionally described as metal-centered, 2-electron CuI/CuIII redox processes.

Copper-Promoted Functionalization of Organic Molecules: from Biologically Relevant Cu/O2 Model Systems to Organometallic Transformations

Copper is one of the most abundant and less toxic transition metals. Nature takes advantage of the bioavailability and rich redox chemistry of Cu to carry out oxygenase and oxidase organic transformations using O₂ (or H₂O₂) as oxidant. Inspired by the reactivity of these Cu-dependent metalloenzymes, chemists have developed synthetic protocols to functionalize organic molecules under enviormentally benign conditions. Copper also promotes other transformations usually catalyzed by 4d and 5d transition metals (Pd, Pt, Rh, etc.) such as nitrene insertions or C-C and C-heteroatom coupling reactions. In this review, we summarized the most relevant research in which copper promotes or catalyzes the functionalization of organic molecules, including biological catalysis, bioinspired model systems, and organometallic reactivity. The reaction mechanisms by which these processes take place are discussed in detail.

Isolation of OH-bridged Ag(i)/Cu(iii) and ion-pair Cu(i)/Cu(iii) trifluoromethyl complexes with monophosphines

An unusual Ag(i)–Cu(iii) trifluoromethyl complex is isolated and characterized, featuring linear Ag(i) and square planar Cu(iii) units that are bridged by a hydroxide ligand.

General and selective syn-carboxylation-trifluoromethylation of terminal alkynes: application to the late-stage modification of dehydrocholic acid

A general and selective syn-carboxylation-trifluoromethylation across the triple bond of terminal alkynes is developed by virtue of a reactive Cu(iii)–CF₃ complex, which produces a broad range of biologically active trifluoromethylated enol esters with excellent regio- and stereoselectivity.

Diverse copper(iii) trifluoromethyl complexes with mono-, bi- and tridentate ligands and their versatile reactivity

Novel square planar (L)Cu^III(CF₃)₃ (L = pyridine, 2,4,6-trimethylpyridine) complexes, which are fluxional in solution and versatile to enable various transformations, are isolated and characterized for the first time.

Synthesis and reductive elimination of arylPd(ii) trifluoromethyl complexes: a remarkable concentration effect on chemoselectivity

A remarkable and unprecedented concentration effect on chemoselectivity is observed for the thermal decomposition of Pd(ii) aryl trifluoromethyl complexes. As the concentration increases, the Ar–CF₃ cross-coupling is suppressed gradually whilst the Ar–Ar homocoupling increases steadily.

Ligand-dependent formation of ion-pair CuI/CuIII trifluoromethyl complexes containing bisphosphines

Novel ion-pair Cu^I/Cu^III trifluoromethyl complexes with representative bisphosphines are isolated and fully characterized to feature square planar [Cu^III(CF₃)₄]⁻ and tetrahedral [L₂Cu]⁺ units. They show unprecedented excellent reactivity for aerobic trifluoromethylation of arylboronic acids in generally good to quantitative yields.