Catalytically Relevant Organocopper(III) Complexes Formed through Aryl-Radical-Enabled Oxidative Addition.

Stepwise oxidative addition of copper(I) complexes to form copper(III) species via single electron transfer (SET) events has been widely proposed in copper catalysis. However, direct observation and detailed investigation of these fundamental steps remain elusive owing largely to the typically slow oxidative addition rate of copper(I) complexes and the instability of the copper(III) species. We report herein a novel aryl-radical-enabled stepwise oxidative addition pathway that allows for the formation of well-defined alkyl-CuIII species from CuI complexes. The process is enabled by the SET from a CuI species to an aryl diazonium salt to form a CuII species and an aryl radical. Subsequent iodine abstraction from an alkyl iodide by the aryl radical affords an alkyl radical, which then reacts with the CuII species to form the alkyl-CuIII complex. The structure of resultant [(bpy)CuIII(CF3)2(alkyl)] complexes has been characterized by NMR spectroscopy and X-ray crystallography. Competition experiments have revealed that the rate at which different alkyl iodides undergo oxidative addition is consistent with the rate of iodine abstraction by carbon-centered radicals. The CuII intermediate formed during the SET process has been identified as a four-coordinate complex, [CuII(CH3CN)2(CF3)2], through electronic paramagnetic resonance (EPR) studies. The catalytic relevance of the high-valent organo-CuIII has been demonstrated by the C-C bond-forming reductive elimination reactivity. Finally, localized orbital bonding analysis of these formal CuIII complexes indicates inverted ligand fields in σ(Cu-CH2) bonds. These results demonstrate the stepwise oxidative addition in copper catalysis and provide a general strategy to investigate the elusive formal CuIII complexes.

A New Series of Sandwich-Type 5,5'-Biterphenylenes: Synthetic Challenge, Structural Uniqueness and Photodynamics.

A new series of biaryls, bi-linear-terphenylenes (BLTPs), were prepared using the tert-butyllithium-mediated cyclization as the key synthetic step. The three-dimensional structures of the studied compounds were verified using X-ray crystallography and DFT calculations. Tetraaryl(ethynyl)-substituted BLTPs are highly crowded molecules, and the internal rotation around the central C-C bond is restricted due to a high barrier (>50 kcal/mol). These structures contain several aryl/terphenylenyl/aryl sandwiches, where the through-space π-π (TSPP) interactions are strongly reflected in the shielding of 1 H NMR chemical shifts, reduction of oxidation potentials, increasing aromaticity of the central six-membered ring and decreasing antiaromaticity of the four-membered rings in a terphenylenyl moiety based on NICS(0) and iso-chemical shielding surfaces. Despite the restricted C-C bond associated intramolecular TSPP interactions for BLTPs in the ground state, to our surprise, the electronic coupling between two linear terphenylenes (LTPs) in BLTPs in the excited state is weak, so that the excited-state behavior is dominated by the corresponding monomeric LTPs. In other words, all BLTPs undergo ultrafast relaxation dynamics via strong exciton-vibration coupling, acting as a blue-light absorber with essentially no emission.

Electron Dynamics in Alkane C-H Activation Mediated by Transition Metal Complexes.

Alkanes, ideal raw materials for industrial chemical production, typically exhibit limited reactivity due to their robust and weakly polarized C-H bonds. The challenge lies in selectively activating these C-H bonds under mild conditions. To address this challenge, various C-H activation mechanisms have been developed. Yet, classifying these mechanisms depends on the overall stoichiometry, which can be ambiguous and sometimes problematic. In this study, we utilized density functional theory calculations combined with intrinsic bond orbital (IBO) analysis to examine electron flow in the four primary alkane C-H activation mechanisms: oxidative addition, σ-bond metathesis, 1,2-addition, and electrophilic activation. Methane was selected as the representative alkane molecule to undergo C-H heterolytic cleavage in these reactions. Across all mechanisms studied, we find that the CH3 moiety in methane consistently uses an electron pair from the cleaved C-H bond to form a σ-bond with the metal. Yet, the electron pair that accepts the proton differs with each mechanism: in oxidative addition, it is derived from the d-orbitals; in σ-bond metathesis, it resulted from the metal-ligand σ-bonds; in 1,2-addition, it arose from the π-orbital of the metal-ligand multiple bonds; and in electrophilic activation, it came from the lone pairs on ligands. This detailed analysis not only provides a clear visual understanding of these reactions but also showcases the ability of the IBO method to differentiate between mechanisms. The electron flow discerned from IBO analysis is further corroborated by results from absolutely localized molecular orbital energy decomposition analysis, which also helps to quantify the two predominant interactions in each process. Our findings offer profound insights into the electron dynamics at play in alkane C-H activation, enhancing our understanding of these critical reactions.

exo-6b2-Methyl-Substituted Pentabenzocorannulene: Synthesis, Structural Analysis, and Properties.

exo-6b2-Methyl-substituted pentabenzocorannulene (exoPBC-Me) was synthesized by the palladium-catalyzed cyclization of 1,2,3-triaryl-1H-cyclopenta[l]phenanthrene. Its bowl-shaped geometry with an sp3 carbon atom in the backbone and a methyl group located at the convex (exo) face was verified by X-ray crystallography. According to DFT calculations, the observed conformer is energetically more favorable than the endo one by 39.9 kcal/mol. Compared to the nitrogen-doped analogs with intact π-conjugated backbones (see the main text), exo-PBC-Me displayed a deeper bowl depth (avg. 1.93 Å), redshifted and broader absorption (250-620 nm) and emission (from 585 to more than 850 nm) bands and a smaller optical HOMO-LUMO gap (2.01 eV). exo-PBC-Me formed polar crystals where all bowl-in-bowl stacking with close π···π contacts is arranged unidirectionally, providing the potential for applications as organic semiconductors and pyroelectric materials. This unusual structural feature, molecular packing, and properties are most likely associated with the assistance of the methyl group and the sp3 carbon atom in the backbone.

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.

Highly Distorted Multiple Helicenes: Syntheses, Structural Analyses, and Properties.

A series of hexapole helicenes (HHs) and nonuple helicenes (NHs) were prepared from 1,3,5-tris[2-(arylethynyl)phenyl]benzene through two steps, namely, iodocyclization and subsequent palladium-catalyzed annulation with ortho-bromoaryl carboxylic acids. The crucial advantages of this synthetic method are the facile introduction of substituents, high regioselectivity, and efficient backbone extension. Three-dimensional structures of three C1-symmetric HHs and one C3-symmetric NH were elucidated using X-ray crystallography. Unlike most conventional multiple helicenes, the HHs and NHs investigated herein possess a unique structural feature where some double helical moieties share a terminal naphthalene unit. Chiral resolution of a HH and an NH was successfully achieved, and the enantiomerization barrier (ΔH‡) of the HH was experimentally determined to be 31.2 kcal/mol. A straightforward method for predicting the most stable diastereomer was developed based on density functional theory calculations and structural considerations. It was found that the relative potential energies (ΔHrs) of all diastereomers for two HHs and one NH can be obtained using minimal computational effort to analyze the types, helical configurations, numbers, and ΔH(MP-MM)s [= H(M,P/P,M) - H(M,M/P,P)] of the double helicenyl fragments.

Copper (I) Chloride-Catalyzed Photoredox Synthesis of Multifunctionalized Compounds at Room Temperature and Their Antifungal Activities.

A simple visible-light-induced CuCl-catalyzed synthesis was developed for highly functionalized carbon-centered compounds (α-alk/aryloxy-α-diaryl/alkylaryl-acetaldehydes/ketones) at room temperature using benzoquinone, alkyl/aryl alcohol, and alkyl/aryl terminal/internal alkynes. Late-stage functionalized compounds show good antifungal activities, especially against Candida krusei fungal strain, in in vitro experiments (the Broth microdilution method). Moreover, toxicity tests (zebrafish egg model experiments) indicated that these compounds had negligible cytotoxicity. The green chemistry metrics (E-factor value is 7.3) and eco-scale (eco-scale value is 58.8) evaluations show that the method is simple, mild, highly efficient, eco-friendly, and environmentally feasible.

Examination of the Brønsted-Evans-Polanyi relationship for the hydrogen evolution reaction on transition metals based on constant electrode potential density functional theory.

In the search for efficient and inexpensive electrocatalysts for the hydrogen evolution reaction (HER), the hydrogen binding energy is often used as a descriptor to represent the catalytic activity. The success of this approach relies on the Brønsted-Evans-Polanyi (BEP) relationship. In this study, we used constant electrode potential density functional theory calculations to examine this relationship. Eight fcc metals with a low hydrogen adsorption concentration of 1/9 were used as the model systems. We found that the HER kinetic barriers are indeed correlated to the . Both the s of the hollow site and less favourable top site correlate to the kinetic barriers; however, the correlation is better for the latter. This behaviour leads to a set of equations for estimating the HER kinetic barriers with improved accuracy that can be used to predict the HER performance of the materials with a low hydrogen adsorption concentration. This work demonstrates the importance of calculating the of a suitable adsorption site to establish good BEP relationships.

Selective Activation of Propane Using Intermediates Generated during Water Oxidation.

Electrochemical conversion of light alkanes to high-value oxygenates provides an attractive avenue for eco-friendly utilization of these hydrocarbons. However, such conversion under ambient conditions remains exceptionally challenging due to the high energy barrier of C-H bond cleavage. Herein, we investigated theoretically the partial oxidation of propane on a series of single atom alloys by using active intermediates generated during water oxidation as the oxidant. We show that by controlling the potential and pH, stable surface oxygen atoms can be maintained under water oxidation conditions. The free energy barrier for C-H bond cleavage by the surface oxygen can be as small as 0.54 eV, which can be surmounted easily at room temperature. Our calculations identified three promising surfaces as effective propane oxidation catalysts. Our complementary experiments demonstrated the partial oxidation of propane to acetone on Ni-doped Au surfaces. We also investigated computationally the steps leading to acetone formation. These studies show that the concept of exploiting intermediates generated in water oxidation as oxidants provides a fruitful strategy for electrocatalyst design to efficiently convert hydrocarbons into value-added chemicals.

Gold-Catalyzed [3+2]-Annulations of α-Aryl Diazoketones with the Tetrasubstituted Alkenes of Cyclopentadienes: High Stereoselectivity and Enantioselectivity.

This work reports gold-catalyzed [3+2]-annulations of α-diazo ketones with highly substituted cyclopentadienes, affording bicyclic 2,3-dihydrofurans with high regio- and stereoselectivity. The reactions highlights the first success of tetrasubstituted alkenes to undergo [3+2]-annulations with α-diazo carbonyls. The enantioselective annulations are also achieved with high enantioselectivity using chiral forms of gold and phosphoric acid. Our mechanistic analysis supports that cyclopentadienes serve as nucleophiles to attack gold carbenes at the more substituted alkenes, yielding gold enolates that complex with chiral phosphoric acid to enhance the enantioselectivity.

[2,2](5,8)Picenophanedienes: Syntheses, Structural Analyses, Molecular Dynamics, and Reversible Intramolecular Structure Conversion.

This study presents an important and efficient synthetic approach to 5,8-dibromo-2,11-di-tert-butylpicene (3), with multigram scale, which was then converted to a new series of picenophanes (6-10). The tub-shaped [2,2](5,8)picenophanediene 8 with two cis-ethylene linkers was explored using X-ray crystallography. The tub-to-tub inversion proceed through the successive bending of the linkers and the barrier for isopropyl-substituted derivative 10 was experimentally estimated to be 18.7 kcal/mol. Picenophanes with a large π-system and semi-rigid structure exhibited anomalous photophysical properties. The ethano-bridged picenophane shows the weak exciton delocalization while the cis-ethylene-bridged picenophane exhibits dual emission rendered by the weakly delocalized exciton and excimer. With the aid of the ultrafast time-resolved emission spectroscopy, the mechanism of the excimer formation is resolved, showing a unique behavior of two-state reversible reaction with fast structural deformation whose lifetime is around 20 ps at 298 K. This work demonstrates that the slight difference in the bridge of tub-shaped picenophanes renders distinct photophysical behavior, revealing the potential of harnessing inter-moiety reaction in the picenophane systems.

Tailoring the Electrochemical Production of H2 O2 : Strategies for the Rational Design of High-Performance Electrocatalysts.

The production of H2 O2 via the electrochemical oxygen reduction reaction (ORR) presents an attractive decentralized alternative to the current industry-dominant anthraquinone process. However, in order to achieve viable commercialization of this process, a state-of-the-art electrocatalyst exhibiting high activity, selectivity, and long-term stability is imperative for industrial applications. Herein, an in-depth discussion on the current frontiers in electrocatalyst design is provided, emphasizing the influences of electronic and geometric effects, surface structure, and the effects of heteroatom functionalization on the catalytic performance of commonly studied materials (metals, alloys, carbons). The limitations on the performance of the current catalyst materials are also discussed, together with alternative strategies to overcome the impediments. Finally, directions of future research efforts for the discovery of next-generation ORR electrocatalysts are highlighted.

Gold-Catalyzed Iminations of Terminal Propargyl Alcohols with Anthranils with Atypical Chemoselectivity for C(1)-Additions and 1,2-Carbon Migration.

This work reports gold-catalyzed iminations of terminal propargyl alcohols with anthranils or isoxazoles to yield E-configured α-amino-2-en-1-ones and -1-als with complete chemoselectivity. These catalytic iminations occur exclusively with C(1)-nucleophilic additions on terminal alkynes, in contrast to a typical C(2)-route. For 3,3-dialkylprop-1-yn-3-ols, a methyl substituent is superior to long alkyl chains as the 1,2-migration groups toward α-imino gold carbenes. For secondary prop-1-yn-3-ols, phenyl, vinyl, and cyclopropyl substituents are better than hydrogen as the migrating groups, obviating typical gold carbene reactions. DFT calculations have been performed to rationalize the observed C(1)-regioselectivity and the preferable cyclopropyl migration based on gold carbene pathways.

Ligand-Based Reactivity of Oxygenation and Alkylation in Cobalt Complexes Binding with (Thiolato)phosphine Derivatives.

In our efforts to understand the nature of metal thiolates, we have explored the chemistry of cobalt ion supported by (thiolato)phosphine ligand derivatives. Herein, we synthesized and characterized a square-planar CoII complex binding with a bidentate (thiolato)phosphine ligand, Co(PS1″)2 (1) ([PS1″]- = [P(Ph)2(C6H3-3-SiMe3-2-S)]-). The complex activates O2 to form a ligand-based oxygenation product, Co(OPS1″)2 (2) ([OPS1″]- = [PO(Ph)2(C6H3-3-SiMe3-2-S)]-). In addition, an octahedral CoIII complex with a tridentate bis(thiolato)phosphine ligand, [NEt4][Co(PS2*)2] (3) ([PS2*]2- = [P(Ph)(C6H3-3-Ph-2-S)2]2-), was obtained. Compound 3 cleaves the C-Cl bond in dichloromethane via an S-based nucleophilic attack to generate a chloromethyl thioether group. Two isomeric products, [Co(PS2*)(PSSCH2Cl*)] (4 and 4') ([PSSCH2Cl*]- = [P(Ph)(C6H3-3-Ph-2-S)(C6H3-3-Ph-2-SCH2Cl)]-), were isolated and fully characterized. Both transformations, oxygenation of a CoII-bound phosphine donor in 1 and alkylation of a CoIII-bound thiolate in 3, were monitored by spectroscopic methods. These reaction products were isolated and fully characterized. Density functional theory (DFT, the B3LYP functional) calculations were performed to understand the electronic structure of 1 as well as the pathway of its transformation to 2.

Gold(I)-Catalyzed Highly Diastereo- and Enantioselective Cyclization-[4+3] Annulation Cascades between 2-(1-Alkynyl)-2-alken-1-ones and Anthranils.

This work reports gold-catalyzed [4+3]-annulations of 2-(1-alkynyl)-2-alken-1-ones with anthranils to yield epoxybenzoazepine products with excellent exo-diastereoselectivity (dr>25:1). The utility of this new gold catalysis is manifested by applicable substrates over a broad scope. More importantly, the enantioselective versions of these [4+3]-cycloadditions have been developed satisfactorily with chiral gold catalysts under ambient conditions (DCM, 0 °C); the ee levels range from 88.0-99.9 %. With DFT calculations, we postulate a stepwise pathway to rationalize the preferable exo-stereoselection.

Hydroxide Is Not a Promoter of C2+ Product Formation in the Electrochemical Reduction of CO on Copper.

Highly alkaline electrolytes have been shown to improve the formation rate of C2+ products in the electrochemical reduction of carbon dioxide (CO2 ) and carbon monoxide (CO) on copper surfaces, with the assumption that higher OH- concentrations promote the C-C coupling chemistry. Herein, by systematically varying the concentration of Na+ and OH- at the same absolute electrode potential, we demonstrate that higher concentrations of cations (Na+ ), rather than OH- , exert the main promotional effect on the production of C2+ products. The impact of the nature and the concentration of cations on the electrochemical reduction of CO is supported by experiments in which a fraction or all of Na+ is chelated by a crown ether. Chelation of Na+ leads to drastic decrease in the formation rate of C2+ products. The promotional effect of OH- determined at the same potential on the reversible hydrogen electrode scale is likely caused by larger overpotentials at higher electrolyte pH.

Copper-Catalyzed, Chloroamide-Directed Benzylic C-H Difluoromethylation.

We report herein the first catalytic strategy to harness amidyl radicals derived from N-chloroamides for C-C bond formation, allowing for the discovery of the first catalytic benzylic C-H difluoromethylation. Under copper-catalyzed conditions, a wide variety of N-chlorocarboxamides and N-chlorocarbamates direct selective benzylic C-H difluoromethylation with a nucleophilic difluoromethyl source at room temperature. This scalable protocol exhibits a broad substrate scope and functional group tolerance, enabling late-stage difluoromethylation of bioactive molecules. This copper-catalyzed, chloroamide-directed strategy has also been extended to benzylic C-H pentafluoroethylation and trifluoromethylation. Mechanistic studies on the difluoromethylation reactions support that the reactions involve the formation of benzylic radicals via intramolecular C-H activation, followed by the copper-mediated transfer of difluoromethyl groups to the benzylic radicals.

Csp3-Csp3 Bond-Forming Reductive Elimination from Well-Defined Copper(III) Complexes.

Carbon-carbon bond-forming reductive elimination from elusive organocopper(III) complexes has been considered the key step in many copper-catalyzed and organocuprate reactions. However, organocopper(III) complexes with well-defined structures that can undergo reductive elimination are extremely rare, especially for the formation of Csp3-Csp3 bonds. We report herein a general method for the synthesis of a series of [alkyl-CuIII-(CF3)3]- complexes, the structures of which have been unequivocally characterized by NMR spectroscopy, mass spectrometry, and X-ray crystal diffraction. At elevated temperature, these complexes undergo reductive elimination following first-order kinetics, forming alkyl-CF3 products with good yields (up to 91%). Both kinetic studies and DFT calculations indicate that the reductive elimination to form Csp3-CF3 bonds proceeds through a concerted transition state, with a Δ H⧧ = 20 kcal/mol barrier.

Copper-Catalyzed Decarboxylative Difluoromethylation.

We report herein a highly efficient Cu-catalyzed protocol for the conversion of aliphatic carboxylic acids to the corresponding difluoromethylated analogues. This robust, operationally simple and scalable protocol tolerates a variety of functional groups and can convert a diverse array of acid-containing complex molecules to the alkyl-CF2H products. Mechanistic studies support the involvement of alkyl radicals.

Mechanism and kinetics for both thermal and electrochemical reduction of N2 catalysed by Ru(0001) based on quantum mechanics.

The conversion of N2(g) to NH3(g) is an important industrial process that plays a vital role in sustaining the current human population. This chemical transformation relies heavily on the Haber-Bosch process (N2 thermal reduction, N2TR), which requires enormous quantities of energy (2% of the world supply) and extreme conditions (200 atm and 500 °C). Alternatively, N2(g) can be reduced to NH3(g) through electrochemical means (N2ER), which may be a less energy intensive and lower-capital approach since the H atoms come from H2O not H2. However, N2ER efficiency is far from satisfactory. In order to provide the basis for developing a new generation of energy efficient processes, we report the detailed atomistic mechanism and kinetics for N2ER on Ru(0001) along with a comparison to N2TR. We obtained these results using a new electrochemical model for quantum mechanics (QM) calculations to obtain free energy surfaces for all plausible reaction pathways for N2ER under a constant electrode potential of 0.0 VSHE. For both processes, the elementary steps involve several steps of breaking of the NN bonds, hydrogenation of surface N2HX or NHX, and NH3 release. We find similar energetics for the NN cleavage steps for both systems. However, the hydrogenation steps are very different, leading to much lower free energy barriers for N2ER compared to N2TR. Thus, N2ER favors an associative route where successive hydrogen atoms are added to N2 prior to breaking the NN bonds rather than the dissociative route preferred by N2TR, where the NN bonds are broken first followed by the addition of Hs. Our QM results provide the detailed free energy surfaces for N2ER and N2TR, suggesting a strategy for improving the efficiency of N2ER.