Redox-coupled structural changes in copper chemistry: Implications for atom transfer catalysis

Non-Aqueous Electrochemical CO2 Reduction to Multivariate C2-Products Over Single Atom Catalyst at Current Density up to 100 mA cm-2

Electrochemical CO2 reduction reaction (CO2-RR) in non-aqueous electrolytes offers significant advantages over aqueous systems, as it boosts CO2 solubility and limits the formation of HCO3 - and CO3 2- anions. Metal-organic frameworks (MOFs) in non-aqueous CO2-RR makes an attractive system for CO2 capture and conversion. However, the predominantly organic composition of MOFs limits their electrical conductivity and stability in electrocatalysis, where they suffer from electrolytic decomposition. In this work, electrically conductive and stable Zirconium (Zr)-based porphyrin MOF, specifically PCN-222, metalated with a single-atom Cu has been explored, which serves as an efficient single-atom catalyst (SAC) for CO2-RR. PCN- 222(Cu) demonstrates a substantial enhancement in redox activity due to the synergistic effect of the Zr matrix and the single-atom Cu site, facilitating complete reduction of C2 species under non-aqueous electrolytic conditions. The current densities achieved (≈100 mA cm- 2) are 4-5 times higher than previously reported values for MOFs, with a faradaic efficiency of up to 40% for acetate production, along with other multivariate C2 products, which have never been achieved previously in non-aqueous systems. Characterization using X-ray and various spectroscopic techniques, reveals critical insights into the role of the Zr matrix and Cu sites in CO2 reduction, benchmarking PCN-222(Cu) for MOF-based SAC electrocatalysis.

Biochemical Behavior, Influence on Cell DNA Condition, and Microbiological Properties of Wool and Wool-Copper Materials

The paper presents the study concerning the preparation and physio-chemical and biological properties of wool-copper (WO-Cu) materials obtained by the sputter deposition of copper onto the wool fibers. The WO-Cu material was subjected to physio-chemical and biological investigations. The physio-chemical investigations included the elemental analysis of materials (C, N, O, S, and Cu), their microscopic analysis, and surface properties analysis (specific surface area and total pore volume). The biological investigations consisted of the antimicrobial activity tests of the WO-Cu materials against colonies of Gram-positive (Staphylococcus aureus) bacteria, Gram-negative (Escherichia coli) bacteria, and fungal mold species (Chaetomium globosum). Biochemical-hematological tests included the evaluation of the activated partial thromboplastin time and pro-thrombin time. The tested wool-copper demonstrated the ability to interact with the DNA in a time-dependent manner. These interactions led to the DNA's breaking and degradation. The antimicrobial and antifungal activities of the WO-Cu materials suggest a potential application as an antibacterial/antifungal material. Wool-copper materials may be also used as customized materials where the blood coagulation process could be well controlled through the appropriate copper content.

Macrocyclic Copper(II) Complexes as Catalysts for Electrochemically Mediated Atom Transfer

Copper-catalyzed electrochemical atom transfer radical addition (eATRA) is a new method for the creation of new C-C bonds under mild conditions. In this work, we have explored the reactivity of an analogous series of N4 macrocyclic CuII complexes as eATRA precatalysts, which are primed by reduction to their monovalent oxidation state. These complexes were fully characterized structurally, spectroscopically, and electrochemically. A spectrum of radical activation reactivity was found across the series [CuI(Me4cyclen)(NCMe)]+ (Me4cyclen = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane), [CuI(Me4cyclam)(NCMe)]+ (Me4cyclam = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), and [CuI(Me2py2clen)(NCMe)]+ (Me2py2clen = 3,7-dimethyl-3,7-diaza-1,5(2,6)-dipyridinacyclo-octaphane). The rate of radical production by [Cu(Me2py2clen)(NCMe)]+ was modest, but rapid radical capture to form the organocopper complex [CuI(Me2py2clen)(CH2CN)] led to a dramatic acceleration in catalysis, greater than seen in any comparable Cu complex, but this led to rapid radical self-termination instead of radical addition.

Electrocatalytic Atom Transfer Radical Addition with Turbocharged Organocopper(II) Complexes

The utility and scope of Cu-catalyzed halogen atom transfer chemistry have been exploited in the fields of atom transfer radical polymerization and atom transfer radical addition, where the metal plays a key role in radical formation and minimizing unwanted side reactions. We have shown that electrochemistry can be employed to modulate the reactivity of the Cu catalyst between its active (CuI) and dormant (CuII) states in a variety of ligand systems. In this work, a macrocyclic pyridinophane ligand (L1) was utilized, which can break the C-Br bond of BrCH2CN to release •CH2CN radicals when in complex with CuI. Moreover, the [CuI(L1)]+ complex can capture the •CH2CN radical to form a new species [CuII(L1)(CH2CN)]+ in situ that, on reduction, exhibits halogen atom transfer reactivity 3 orders of magnitude greater than its parent complex [CuI(L1)]+. This unprecedented rate acceleration has been identified by electrochemistry, successfully reproduced by simulation, and exploited in a Cu-catalyzed bulk electrosynthesis where [CuII(L1)(CH2CN)]+ participates as a radical donor in the atom transfer radical addition of BrCH2CN to a selection of styrenes. The formation of these turbocharged catalysts in situ during electrosynthesis offers a new approach to the Cu-catalyzed organic reaction methodology.

Recent Advances in Metal-Based Molecular Photosensitizers for Artificial Photosynthesis

Artificial photosynthesis (AP) has been extensively applied in energy conversion and environment pollutants treatment. Considering the urgent demand for clean energy for human society, many researchers have endeavored to develop materials for AP. Among the materials for AP, photosensitizers play a critical role in light absorption and charge separation. Due to the fact of their excellent tunability and performance, metal-based complexes stand out from many photocatalysis photosensitizers. In this review, the evaluation parameters for photosensitizers are first summarized and then the recent developments in molecular photosensitizers based on transition metal complexes are presented. The photosensitizers in this review are divided into two categories: noble-metal-based and noble-metal-free complexes. The subcategories for each type of photosensitizer in this review are organized by element, focusing first on ruthenium, iridium, and rhenium and then on manganese, iron, and copper. Various examples of recently developed photosensitizers are also presented.

Organocopper(ii) complexes: new catalysts for carbon–carbon bond formation via electrochemical atom transfer radical addition (eATRA)

Electrochemical generation of a novel organocopper(ii) complex offers a new way to carry out atom transfer radical addition to alkenes under mild conditions with high yields and low catalyst loadings.

Mapping the Pathway to Organocopper(II) Complexes Relevant to Atom Transfer Radical Polymerization

The rare organocopper(II) complex [Cu(Me₆tren)(CH₂CN)]⁺ (Me₆tren = tris(2-(dimethylamino)ethyl)amine) has emerged as an important model of potential byproducts in copper-catalyzed atom transfer radical polymerization. This complex has been generated by controlled potential electrolysis of [Cu(Me₆tren)(NCMe)]²⁺ in the presence of BrCH₂CN. Time-resolved UV-vis and continuous wave and pulse electron paramagnetic resonance (EPR) spectra identified [Cu(Me₆tren)Br]⁺ as an intermediate. Hyperfine sublevel correlation and electron nuclear double resonance spectroscopy of samples at different timepoints reveal signals that are assigned to a C-bound cyanomethylate ligand, with distinct ¹⁴N and ¹H hyperfine coupling constants in comparison with the corresponding N-bound acetonitrile and bromido complexes. The experimental EPR data are supported by density functional theory calculations to understand how the geometries of the species involved produce distinct spectroscopic signatures, and a clear picture of how this unusual organocopper(II) complex is formed has emerged.

Electrochemical Exploration of Active Cu-Based Atom Transfer Radical Polymerization Catalysis through Ligand Modification

The intersection between Cu-catalyzed atom transfer radical polymerization (ATRP) and organometallic mediated radical polymerization (OMRP) has been recently shown to be a result of competition between the Cu^I and Cu^II complexes of polyamine ligands for the same organic free radical. The tetradentate ligands N,N'-bis-2'-pyridylmethyl-ethane-1,2-diamine (L¹) and N,N'-dimethyl-N,N'-bis-2'-pyridylmethyl-ethane-1,2-diamine (L²) form stable Cu complexes which, depending on their oxidation state, can either liberate or complex organic radicals. Herein, we show that this process may be affected by subtle changes to the ligand system. Switching from a tertiary amine (L²) to a secondary amine (L¹) retains ATRP and OMRP activity through a series of cyclic voltammetry measurements in the presence of the initiator bromoacetonitrile.

Multidentate Phenanthroline Ligands Containing Additional Donor Moieties and Their Resulting Cu(I) and Ru(II) Photosensitizers: A Comparative Study

To bind or not to bind: Driven by the motivation to increase the (photo)stability of traditional Cu(I) photosensitizers, multidentate diimine ligands, which contain two additional donor sites, were designed. To this end, a systematic series of four 1,10-phenanthroline ligands with either OR or SR (R = ⁱPr or Ph) donor groups at the 2 and 9 positions and their resulting hetero- and homoleptic Cu(I) complexes were prepared. In addition, the related Ru(II) complexes were also synthesized to study the effect of another metal center. In the following, a combination of NMR spectroscopy and X-ray analysis was used to evaluate the impact of the additional donor moieties on the coordination behavior. Most remarkably, for the homoleptic bis(diimine)copper(I) complexes, a pentacoordinated copper center, corresponding to a (4 + 1)-fold coordination mode, was found in the solid state. This additional binding is the first indication that the extra donor might also occupy a free coordination site in the excited-state complex, modifying the nature of the excited states and their respective deactivation processes. Therefore, the electrochemical and photophysical properties of all novel complexes (in total 13) were studied in detail to assess the potential of these photosensitizers for future applications within solar energy conversion schemes. Finally, the photostabilities and a potential degradation mechanism were analyzed for representative samples.

Reactivity of CuIN4 Flattened Complexes: Interplay between Coordination Geometry and Ligand Flexibility

The relation between redox activity and coordination geometry in Cu^IN₄ complexes indicates that more flattened structures tend to be more reactive. Such a preorganization of the ligand confers to the complex geometries closer to a transition state, which has been termed the "entatic" state in metalloproteins, more recently extending this concept for copper complexes. However, many aspects of the redox chemistry of Cu^I complexes cannot be explained only by flattening. For instance, the role of ligand flexibility in this context is an open debate nowadays. To analyze this point, we studied oxidation properties of a series of five monometallic Cu^I Schiff-base complexes, [Cu^I(L_n)]⁺, which span a range of geometries from a distorted square planar (n = 3) to a distorted tetrahedron (n = 6, 7). This stepped control of the structure around the Cu^I atom allows us to explore the effect of the flattening distortion on both the electronic and redox properties through the series. Experimental studies were complemented by a theoretical analysis based on density functional theory calculations. As expected, oxidation was favored in the flattened structures, spanning a broad potential window of 370 mV for the complete series. This orderly behavior was tested in the reductive dehalogenation reaction of tetrachloroethane (TCE). Kinetic studies show that Cu^I oxidation by TCE is faster as the flattening distortion is higher and the oxidation potentials of the metal are lower. However, the most reactive complex was not the more planar, contradicting the trend expected from oxidation potentials. The origin of this irregularity is related to ligand flexibility and its connection with the atom/electron transfer reaction path, highlighting the need to consider effects beyond flattening distortion to better understand the reactivity of this important class of complexes.

Coordination-Bond-Driven Dissolution-Recrystallization Structural Transformation with the Expansion of Cuprous Halide Aggregate

Metal-organic frameworks (MOFs) with cuprous-halide-aggregates have shown superiority as organic LED (OLED) and semiconductor materials, while engineering MOF flexibility by involving the expansion of cuprous aggregates remains a great challenge. In this particular work, a dissolution-recrystallization structural transformation (DRST) with the dramatic growth of Cu^I-I aggregates, from 2D NJNU-100 to 3D NJNU-101 has been successfully realized. The unsaturated coordination nodes (2-positional nitrogen atoms) in NJNU-100 have been demonstrated to be the driven force for DRST to NJNU-101 via the formation of coordination bonds. The structural transformation process was irreversible and observed with optical microscopy and powder XRD. The expansion of Cu^I-I aggregates was also computational simulated accompanying with the rotation of the neutral tripodal TTTMB ligand (1,3,5-tris(1,2,4-triazol-1-ylmethyl)-2,4,6-trimethylbenzene) and the reduction of Cu^II to Cu^I. Moreover, the intermediate product NJNU-102 was captured by adding the planar molecular anthrancene to shut down the reaction, where only partial 2-positional nitrogen atoms coordinated to the aggregates and the anthrancene was oxidized to anthraquinone. NJNU-102 has further confirmed that DRST involved the breakage and recombination of coordination bonds and the electron transfer. NJNU-100 and NJNU-101 could be applied as semiconductor and OLED materials. This work has provided insights for crystal engineering, especially for the construction of the Cu^I_xX_y aggregates, and illustrated that DRST could be controlled with a rational design (as the unsaturated coordination modes).

The Coordination Behaviour of CuI Photosensitizers Bearing Multidentate Ligands Investigated by X-ray Absorption Spectroscopy

A systematic series of four novel homo‐ and heteroleptic Cu^I photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X=SMe or OMe) were designed. Their solid‐state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emission and X‐ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time‐resolved X‐ray absorption spectroscopy in the picosecond time scale, coupled with time‐dependent density functional theory calculations, provided in‐depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers.

p-Substituted Tris(2-pyridylmethyl)amines as Ligands for Highly Active ATRP Catalysts: Facile Synthesis and Characterization

A facile and efficient two-step synthesis of p-substituted tris(2-pyridylmethyl)amine (TPMA) ligands to form Cu complexes with the highest activity to date in atom transfer radical polymerization (ATRP) is presented. In the divergent synthesis, p-Cl substituents in tris(4-chloro-2-pyridylmethyl)amine (TPMA^3Cl ) were replaced in one step and high yield by electron-donating cyclic amines (pyrrolidine (TPMA^PYR ), piperidine (TPMA^PIP ), and morpholine (TPMA^MOR )) by nucleophilic aromatic substitution. The [Cu^II (TPMA^NR2 )Br]⁺ complexes exhibited larger energy gaps between frontier molecular orbitals and >0.2 V more negative reduction potentials than [Cu^II (TPMA)Br]⁺ , indicating >3 orders of magnitude higher ATRP activity. [Cu^I (TPMA^PYR )]⁺ exhibited the highest reported activity for Br-capped acrylate chain ends in DMF, and moderate activity toward C-F bonds at room temperature. ATRP of n-butyl acrylate using only 10-25 part per million loadings of [Cu^II (TPMA^NR2 )Br]⁺ exhibited excellent control.

Metal Chlorides Grafted on SAPO-5 (MClx/SAPO-5) as Reusable and Superior Catalysts for Acylation of 2-Methylfuran Under Non-Microwave Instant Heating Condition

Highly active metal chlorides grafted on silicoaluminophosphate number 5, MClx/SAPO-5 (M = Cu, Co, Sn, Fe and Zn) catalysts via simple grafting of respective metal chlorides (MClx) onto SAPO-5 are reported. The study shows that thermochemical treatment after grafting is essential to ensure the formation of chemical bondings between MClx and SAPO-5. In addition, the microscopy, XRD and nitrogen adsorption analyses reveal the homogeneous distribution of MClx species on the SAPO-5 surface. Furthermore, the elemental microanalysis confirms the formation of Si–O–M covalent bonds in ZnClx/SAPO-5, SnClx/SAPO-5 and FeClx/SAPO-5 whereas only dative bondings are formed in CoClx/SAPO-5 and CuClx/SAPO-5. The acidity of MClx/SAPO-5 is also affected by the type of metal chloride grafted. Thus, their catalytic behavior is evaluated in the acid-catalyzed acylation of 2-methylfuran under novel non-microwave instant heating conditions (90–110 °C, 0–20 min). ZnClx/SAPO-5, which has the largest amount of acidity (mainly Lewis acid sites), exhibits the best catalytic performance (94.5% conversion, 100% selective to 2-acetyl-5-methylfuran) among the MClx/SAPO-5 solids. Furthermore, the MClx/SAPO-5 solids, particularly SnClx/SAPO-5, FeClx/SAPO-5 and ZnClx/SAPO-5, also show more superior catalytic performance than common homogeneous acid catalysts (H2SO4, HNO3, CH3COOH, FeCl3, ZnCl2) with higher reactant conversion and catalyst reusability, thus offering a promising alternative for the replacement of hazardous homogeneous catalysts in Friedel–Crafts reactions.

Low Reorganization Energy for Electron Self-Exchange by a Formally Copper(III,II) Redox Couple

The rate constant for electron self-exchange (k₁₁) between LCuOH and [LCuOH]^- (L = bis-2,6-(2,6-diisopropylphenyl)carboximidopyridine) was determined using the Marcus cross relation. This work involved measurement of the rate of the cross-reaction between [Bu₄N][LCuOH] and [Fc][BAr₄^F] (Fc⁺ = ferrocenium; BAr₄^F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)) by stopped-flow methods at -88 °C in CH₂Cl₂ and measurement of the equilibrium constant for the redox process by UV-vis titrations under the same conditions. A value of k₁₁ = 3 × 10⁴ M^-1 s^-1 (-88 °C) led to estimation of a value 9 × 10⁶ M^-1 s^-1 at 25 °C, which is among the highest values known for copper redox couples. Further Marcus analysis enabled determination of a low reorganization energy, λ = 0.95 ± 0.17 eV, attributed to minimal structural variation between the redox partners. In addition, the reaction entropy (ΔS°) associated with the LCuOH/[LCuOH]^- self-exchange was determined from the temperature dependence of the redox potentials, and found to be dependent upon ionic strength. Comparisons to other Cu redox systems and potential new applications for the formally Cu^III,II system are discussed.

Mechanistically Guided Predictive Models for Ligand and Initiator Effects in Copper-Catalyzed Atom Transfer Radical Polymerization (Cu-ATRP)

Copper-catalyzed atom transfer radical polymerization (Cu-ATRP) is one of the most widely used controlled radical polymerization techniques. Notwithstanding the extensive mechanistic studies in the literature, the transition states of the activation/deactivation of the growing polymer chain, a key equilibrium in Cu-ATRP, have not been investigated computationally. Therefore, the understanding of the origin of ligand and initiator effects on the rates of activation/deactivation is still limited. Here, we present the first computational analysis of Cu-ATRP activation transition states to reveal factors that affect the rates of activation and deactivation. The Br atom transfer between the polymer chain and the Cu catalyst occurs through an unusual bent geometry that involves pronounced interactions between the polymer chain end and the ancillary ligand on the Cu catalyst. Therefore, the rates of activation/deactivation are determined by both the electronic properties of the Cu catalyst and the ligand-initiator steric repulsions. In addition, our calculations revealed the important role of ligand backbone flexibility on the activation. These theoretical analyses led to the identification of three chemically meaningful descriptors, namely HOMO energy of the catalyst ( EHOMO), percent buried volume ( Vbur%), and distortion energy of the catalyst (Δ Edist), to describe the electronic, steric, and flexibility effects on reactivity, respectively. A robust and simple predictive model for ligand effect on reactivity is thereby established by correlating these three descriptors with experimental activation rate constants using multivariate linear regression. Validation using a structurally diverse set of ligands revealed the average error is less than ±2 kcal/mol compared to the experimentally derived activation energies. The same approach was also applied to develop a predictive model for reactivity of different alkyl halide initiators using R-X bond dissociation energy (BDE) and Cu-X halogenophilicity as descriptors.

The fate of copper catalysts in atom transfer radical chemistry

The pathway of atom transfer radical polymerisation (ATRP) is influenced by the nature of the alkyl bromide initiator (RBr) to the extent that reactions between the radical R˙ and the original copper(i) catalyst can divert the reaction toward different products.

Coordination Chemistry of Cu2+ Complexes of Small N-Alkylated Tetra-azacyclophanes with SOD Activity

A new tetraaza-pyridinophane macrocycle (L1) N-alkylated with two isopropyl and one methyl groups symmetrically disposed has been prepared and its behavior compared with those of the unsubstituted pyridinophane (L3) and the related compound with three methyl groups (L2). The protonation studies show that, first, a proton binds to the central methylated amine group of L1, while, second protonation leads to a reorganization of the protons that are at this stage attached to the lateral isopropylated amines. The X-ray structure of [HL1]⁺ agrees with the UV-vis and NMR studies as well as with the results of DFT calculations. The stability of the Cu²⁺ complexes decreases on increasing the bulkiness of the alkyl substituents of the amine groups. The crystal structures of [CuL1Cl](ClO₄) and [CuL1(H₂O)](ClO₄)₂·H₂O show square pyramidal coordination geometries with the ligands disposed in a bent L-shaped conformation. Kinetic studies indicate that the rates of both complexation and ligand dissociation decrease with the bulkiness of the substituents, so that the stability changes are surely the results of compensating effects, complex formation dominating over complex dissociation. The pH dependence of the rate constants for complex formation cannot be explained by consideration of rapid pre-equilibria involving the different protonated forms of the ligand, and it has been interpreted in terms of a mechanism involving an acid-base equilibrium for a reaction intermediate. NBT SOD studies show that the Cu²⁺ complex of the bulkiest L1 ligand is the one having the highest activity (IC₅₀ = 0.26(5) μM, k_cat = 13.7 × 10⁶ M^-1 s^-1) which can be associated with the poorer σ-donor ability of the tertiary amino groups, and the rigidity of the system, caused by the bulky isopropyl groups.

Synthesis and Characterization of the Most Active Copper ATRP Catalyst Based on Tris[(4-dimethylaminopyridyl)methyl]amine

The tris[(4-dimethylaminopyridyl)methyl]amine (TPMA^NMe2) as a ligand for copper-catalyzed atom transfer radical polymerization (ATRP) is reported. In solution, the [Cu^I(TPMA^NMe2)Br] complex shows fluxionality by variable-temperature NMR, indicating rapid ligand exchange. In the solid state, the [Cu^II(TPMA^NMe2)Br][Br] complex exhibits a slightly distorted trigonal bipyramidal geometry (τ = 0.89). The UV-vis spectrum of [Cu^II(TPMA^NMe2)Br]⁺ salts is similar to those of other pyridine-based ATRP catalysts. Electrochemical studies of [Cu(TPMA^NMe2)]²⁺ and [Cu(TPMA^NMe2)Br]⁺ showed highly negative redox potentials (E_1/2 = -302 and -554 mV vs SCE, respectively), suggesting unprecedented ATRP catalytic activity. Cyclic voltammetry (CV) in the presence of methyl 2-bromopropionate (MBrP; acrylate mimic) was used to determine activation rate constant k_a = 1.1 × 10⁶ M^-1 s^-1, confirming the extremely high catalyst reactivity. In the presence of the more active ethyl α-bromoisobutyrate (EBiB; methacrylate mimic), total catalysis was observed and an activation rate constant k_a = 7.2 × 10⁶ M^-1 s^-1 was calculated with values of K_ATRP ≈ 1. ATRP of methyl acrylate showed a well-controlled polymerization using as little as 10 ppm of catalyst relative to monomer, while side reactions such as Cu^I-catalyzed radical termination (CRT) could be suppressed due to the low concentration of L/Cu^I at a steady state.