Coordination chemistry and reactivity of a cupric hydroperoxide species featuring a proximal H-bonding substituent

Appended Lewis Acids Enable Dioxygen Reactivity and Catalytic Oxidations with Ni(II)

We disclose a suite of Ni(II) complexes featuring secondary sphere Lewis acids of varied Lewis acidity and tether lengths. Several of these complexes feature atypical behavior of Ni(II): reactivity with O2 that occurs only in the presence of a tethered Lewis acid. In situ UV-vis spectroscopy revealed that, although adducts are stable at -40 °C, complexes containing 9-borabicyclo[3.3.1]nonane (9-BBN) Lewis acids underwent irreversible oxidative deborylation when warmed to room temperature. We computationally and experimentally identified that oxidative instability of appended 9-BBN moieties can be mitigated using weaker Lewis acids such as pinacolborane (BPin). These insights enabled the realization of catalytic reactions: hydrogen atom abstraction from phenols and room temperature oxygen atom transfer to PPh3.

Repurposing a peptide antibiotic as a catalyst: a multicopper–daptomycin complex as a cooperative O–O bond formation and activation catalyst

A peptide antibiotic, daptomycin, was repurposed to a multicopper catalyst presenting cooperative rate enhancement in O–O bond formation and activation reactions.

Sterically Stabilized End-On Superoxocopper(II) Complexes and Mechanistic Insights into Their Reactivity with O-H, N-H, and C-H Substrates

Instability of end-on superoxocopper(II) complexes, with respect to conversion to peroxo-bridged dicopper(II) complexes, has largely constrained their study to very low temperatures. This limits their kinetic capacity to oxidize substrates. In response, we have developed a series of bulky ligands, Ar₃-TMPA (Ar = tpb, dpb, dtbpb), and used them to support copper(I) complexes that react with O₂ to yield [Cu^II(η¹-O₂^•-)(Ar₃-TMPA)]⁺ species, which are stable against dimerization at all temperatures. Binding of O₂ saturates at subambient temperatures and can be reversed by warming. The onset of oxygenation for the Ar = tpb and dpb systems is observed at 25 °C, and all three [Cu^II(η¹-O₂^•-)(Ar₃-TMPA)]⁺ complexes are stable against self-decay at temperatures of ≤-20 °C. This provides a wide temperature window for study of these complexes, which was exploited by performing extensive reaction kinetics measurements for [Cu^II(η¹-O₂^•-)(tpb₃-TMPA)]⁺ using a broad range of O-H, N-H, and C-H bond substrates. This includes correlation of second order rate constants (k₂) versus oxidation potentials (E_ox) for a range of phenols, construction of Eyring plots, and temperature-dependent kinetic isotope effect (KIE) measurements. The data obtained indicate that reaction with all substrates proceeds via H atom transfer (HAT), reaction with the phenols proceeds with significant charge transfer, and full tunneling of both H and D atoms occurs in the case of 1,2-diphenylhydrazine and 4-methoxy-2,6-di-tert-butylphenol. Oxidation of C-H bonds proved to be kinetically challenging, and whereas [Cu^II(η¹-O₂^•-)(tpb₃-TMPA)]⁺ can oxidize moderately strong O-H and N-H bonds, it is only able to oxidize very weak C-H bonds.

Solid-state and solution characterizations of [(TMPA)Cu(II)(SO3)] and [(TMPA)Cu(II)(S2O3)] complexes: Application to sulfite and thiosulfate fast detection

Sulfite (SO₃^2-) and thiosulfate (S₂O₃^2-) ions are used as food preservative and antichlor agent respectively. To detect low levels of such anions we used Cu(II) complex of the Tris-Methyl Pyridine Amine (TMPA) ligand, denoted L. Formation of [LCu(SO₃)] (1) and [LCu(S₂O₃)] (2) in solution were monitored using UV-Vis, EPR and cyclic voltammetry, while the solid-state X-ray structures of both complexes were solved. In addition, we also evaluated the pH range in which the complexes are stable, and the anions binding affinity values for the [LCu(solvent)]²⁺ (3) parent complex. As a matter of illustration, we determined the sulfite content in a commercial crystal sugar.

Cu-promoted intramolecular hydroxylation of CH bonds using directing groups with varying denticity

In this research article, we describe the Cu-promoted intramolecular hydroxylation of sp² and sp³ CH bonds using directing groups with varying denticity (bi-, tri- and tetradentate) and natural oxidants (O₂ and H₂O₂). We found that bidentate directing groups, in combination with Cu and H₂O₂, led to high hydroxylation yields. On the other hand, tetradentate directing groups did not form the hydroxylation products. Our mechanistic investigations suggest that bidentate directing groups allow for generating reactive mononuclear copper(II) hydroperoxide intermediates while tetradentate systems form dinuclear Cu₂O₂ species that do not oxidize CH bonds. Our findings might shed light on the reaction mechanism(s) by which Cu-dependent metalloenzymes such as particulate methane monooxygenase or lytic polysaccharide monooxygenase oxidize strong CH bonds.

Controlling the Reactivity of Copper(II) Acylperoxide Complexes

The redox state of the metallomonooxygenases is finely tuned by imposing specific coordination environments on the metal center to reduce the activation energy for the generation of active-oxygen species and subsequent substrate oxygenation reactions. In this study, copper(II) complexes supported by a series of linear tetradentate ligands consisting of a rigid 6-, 7-, or 8-membered cyclic diamine with two pyridylmethyl (-CH₂Py) side arms (L6^Pym2, L7^Pym2, and L8^Pym2) are employed to examine the effects of the coordination environment on the reactivity of their acylperoxide adduct complexes. The UV-vis and electron paramagnetic resonance spectroscopic data indicate that the ligand-field splitting between the d_x²-y² and d_z² orbitals of the starting copper(II) complexes increase with an increase of the ring size of the diamine moiety (L6^Pym2 → L7^Pym2 → L8^Pym2). In the reaction of these copper(II) complexes with m-chloroperbenzoic acid (m-CPBA), the L6^Pym2 complex gives a stable m-CPBA adduct complex, whereas the L7^Pym2 and L8^Pym2 complexes are immediately converted to the corresponding m-chlorobenzoic acid (m-CBA) adducts, indicating that the reactivity of the copper(II) acylperoxide complexes largely depends on the coordination environment induced by the supporting ligands. Density functional theory (DFT) calculations on the m-CPBA adduct complexes show that the ligand-field-splitting energy increases with an increase of the ring size of the diamine moiety, as in the case of the starting copper(II) complexes, which enhances the reactivity of the m-CPBA adduct complexes. The reasons for such different reactivities of the m-CPBA adduct complexes are evaluated by using DFT calculations.

Amine-rich quartz nanoparticles for Cu(II) chelation and their application as an efficient catalyst for oxidative degradation of Rhodamine B dye

The study describes the loading of the quartz SiO₂ nanoparticles (NPs) with (3-aminopropyl)triethoxysilane (APTES) linker with simultaneous lengthening of the linker through the terminal amine group by glutaraldehyde (GA). The reactive polyethylenimine (PEI) was introduced to the surface to increase the ability to capture Cu(II) ions. The composite got the abbreviation SiO₂/PEI-Cu(II). The Cu(II) ions were the active center with a peroxo-complex activation state. The composite characterization included scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron-dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), and Brunauer-Emmett-Teller (BET) surface analyzer. The kinetics of the oxidative degradation of Rhodamine B (RhB) dye obeyed the pseudo-first order under flooding conditions. The reaction parameters including the catalyst dose, solution pH, initial concentration of reactants, and temperature got some attention. The obtained results showed that more than 91.7 ± 1% of RhB dye was degraded to CO₂, NH₄⁺, NO₃^-, H₂O, and some inorganic acids after 30 min as confirmed by gas chromatography mass spectrometry and total organic carbon (TOC) measurements. Also, GC-MS spectra for water samples drawn from the reaction in successive periods had suggested a conceivable degradation pathway for RhB by hydroxyl radicals. Degradation starts with de-alkylation then carboxyphenyl removal followed by two successive ring-opening stages. Both the effects of the catalyst recycling and treated water reusability on the reaction rate were studied. The catalyst provided noticeable stability over three consecutive cycles.

Polypyridyl Co complex-based water reduction catalysts: why replace a pyridine group with isoquinoline rather than quinoline?

The electronic effect of the substituent has been fully leveraged to improve the activity of molecular water reduction catalysts (WRCs). However, the steric effect of the substituents has received less attention. In this work, a steric hindrance effect was observed in a quinoline-involved polypyridyl Co complex-based water reduction catalyst (WRC), which impedes the formation of Co(iii)-H from Co(i), two pivotal intermediates for H₂ evolution, leading to significantly impaired electrocatalytic and photocatalytic activity with respect to its parent complex, [Co(TPA)Cl]Cl (TPA = tris(2-pyridinylmethyl)-amine). In sharp contrast, two isoquinoline-involved polypyridyl Co complexes exhibited significantly improved H₂ evolution efficiencies compared to [Co(TPA)Cl]Cl, benefitting mainly from the more basic and conjugated features of isoquinoline over pyridine. The dramatically different influences caused by the replacement of a pyridine group in the TPA ligand by quinoline and isoquinoline fully demonstrates the important roles of both the electronic and steric effects of a substituent. Our results may provide novel insights for designing more efficient WRCs.

Electronic effects on polypyridyl Co complex-based water reduction catalysts

Three new isomeric cobalt complexes of TPA (tris(2-pyridylmethyl)amine) based on methoxy substitution at the ortho, meta and para positions, respectively, were constructed and their photocatalytic proton reduction efficiencies were compared. It was found that there are good linear correlations with the Hammett constants of the substituents for the computed Co–N bond lengths, redox potentials of Co^II/I and Co^I/0 events, and the photocatalytic activities of the complexes. The ortho-substituted Co complex distinguished itself from the others remarkably in all these comparisons, demonstrating the presence of a steric effect besides the electronic effect. For other examined complexes, a stronger electron-donating substituent may lead to a higher hydrogen evolution efficiency, suggesting that the formation of a Co(iii) hydride intermediate is the rate-limiting step.

Three isomeric Co complexes showed a significant substituent electronic effect in photocatalytic hydrogen production.

Copper catalysts for photo- and electro-catalytic hydrogen production

Square planar 1, square pyramidal 2 and trigonal bipyramidal 3 copper complexes are poor catalysts for hydrogen evolution (HER) under photocatalytic conditions, whereas 1 is, or forms, a good and enduring electrocatalyst for HER, but 2 and 3 do not.

Kβ X-ray Emission Spectroscopy as a Probe of Cu(I) Sites: Application to the Cu(I) Site in Preprocessed Galactose Oxidase

Cu(I) active sites in metalloproteins are involved in O₂ activation, but their O₂ reactivity is difficult to study due to the Cu(I) d¹⁰ closed shell which precludes the use of conventional spectroscopic methods. Kβ X-ray emission spectroscopy (XES) is a promising technique for investigating Cu(I) sites as it detects photons emitted by electronic transitions from occupied orbitals. Here, we demonstrate the utility of Kβ XES in probing Cu(I) sites in model complexes and a metalloprotein. Using Cu(I)Cl, emission features from double-ionization (DI) states are identified using varying incident X-ray photon energies, and a reasonable method to correct the data to remove DI contributions is presented. Kβ XES spectra of Cu(I) model complexes, having biologically relevant N/S ligands and different coordination numbers, are compared and analyzed, with the aid of density functional theory (DFT) calculations, to evaluate the sensitivity of the spectral features to the ligand environment. While the low-energy Kβ_2,5 emission feature reflects the ionization energy of ligand np valence orbitals, the high-energy Kβ_2,5 emission feature corresponds to transitions from molecular orbitals (MOs) having mainly Cu 3d character with the intensities determined by ligand-mediated d-p mixing. A Kβ XES spectrum of the Cu(I) site in preprocessed galactose oxidase (GO_pre) supports the 1Tyr/2His structural model that was determined by our previous X-ray absorption spectroscopy and DFT study. The high-energy Kβ_2,5 emission feature in the Cu(I)-GO_pre data has information about the MO containing mostly Cu 3d_x²-y² character that is the frontier molecular orbital (FMO) for O₂ activation, which shows the potential of Kβ XES in probing the Cu(I) FMO associated with small-molecule activation in metalloproteins.

Directed Hydroxylation of sp2 and sp3 C-H Bonds Using Stoichiometric Amounts of Cu and H2O2

The use of copper for C-H bond functionalization, compared to other metals, is relatively unexplored. Herein, we report a synthetic protocol for the regioselective hydroxylation of sp² and sp³ C-H bonds using a directing group, stoichiometric amounts of Cu and H₂O₂. A wide array of aromatic ketones and aldehydes are oxidized in the carbonyl γ-position with remarkable yields. We also expanded this methodology to hydroxylate the β-position of alkylic ketones. Spectroscopic characterization, kinetics, and density functional theory calculations point toward the involvement of a mononuclear LCu^II(OOH) species, which oxidizes the aromatic sp² C-H bonds via a concerted heterolytic O-O bond cleavage with concomitant electrophilic attack on the arene system.

Mechanistic Dichotomy in Proton-Coupled Electron-Transfer Reactions of Phenols with a Copper Superoxide Complex

The kinetics and mechanism(s) of the reactions of [K(Krypt)][LCuO₂] (Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, L = a bis(arylcarboxamido)pyridine ligand) with 2,2,6,6-tetramethylpiperdine- N-hydroxide (TEMPOH) and the para-substituted phenols ^XArOH (X = para substituent NO₂, CF₃, Cl, H, Me, ^tBu, OMe, or NMe₂) at low temperatures were studied. The reaction with TEMPOH occurs rapidly ( k = 35.4 ± 0.3 M^-1 s^-1) by second-order kinetics to yield TEMPO^• and [LCuOOH]^- on the basis of electron paramagnetic resonance spectroscopy, the production of H₂O₂ upon treatment with protic acid, and independent preparation from reaction of [NBu₄][LCuOH] with H₂O₂ ( K_eq = 0.022 ± 0.007 for the reverse reaction). The reactions with ^XArOH also follow second-order kinetics, and analysis of the variation of the k values as a function of phenol properties (Hammett σ parameter, O-H bond dissociation free energy, p K_a, E_1/2) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO₂, CF₃, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe₂ (data for X = H, Me, ^tBu are intermediate between the extremes). Thermodynamic analysis and comparisons to previous results for LCuOH, a different copper-oxygen intermediate with the same supporting ligand, and literature for other [CuO₂]⁺ complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Intramolecular Hydrogen Bonding Enhances Stability and Reactivity of Mononuclear Cupric Superoxide Complexes

[(L)CuII(O2•-)]+ (i.e., cupric-superoxo) complexes, as the first and/or key reactive intermediates in (bio)chemical Cu-oxidative processes, including in the monooxygenases PHM and DβM, have been systematically stabilized by intramolecular hydrogen bonding within a TMPA ligand-based framework. Also, gradual strengthening of ligand-derived H-bonding dramatically enhances the [(L)CuII(O2•-)]+ reactivity toward hydrogen-atom abstraction (HAA) of phenolic O-H bonds. Spectroscopic properties of the superoxo complexes and their azido analogues, [(L)CuII(N3-)]+, also systematically change as a function of ligand H-bonding capability.

Phenylamino derivatives of tris(2-pyridylmethyl)amine: hydrogen-bonded peroxodicopper complexes

A series of copper complexes bearing new 6-substituted tris(2-pyridylmethyl)amine ligands (L^R) appended with NH(p-R-C₆H₄) groups (R = H, CF₃, OMe) were prepared. These ligands are electronically tunable (ΔE_1/2 = 160 mV) and Cu^I(L^R)⁺ complexes react with oxygen to form hydrogen bonded (trans-1,2-peroxo)dicopper species.

Peroxide Activation Regulated by Hydrogen Bonds within Artificial Cu Proteins

Copper-hydroperoxido species (CuII-OOH) have been proposed to be key intermediates in biological and synthetic oxidations. Using biotin-streptavidin (Sav) technology, artificial copper proteins have been developed to stabilize a CuII-OOH complex in solution and in crystallo. Stability is achieved because the Sav host provides a local environment around the Cu-OOH that includes a network of hydrogen bonds to the hydroperoxido ligand. Systematic deletions of individual hydrogen bonds to the Cu-OOH complex were accomplished using different Sav variants and demonstrated that stability is achieved with a single hydrogen bond to the proximal O-atom of the hydroperoxido ligand: changing this interaction to only include the distal O-atom produced a reactive variant that oxidized an external substrate.

"Two-Story" Calix[6]arene-Based Zinc and Copper Complexes: Structure, Properties, and O2 Binding

A new "two-story" calix[6]arene-based ligand was synthesized, and its coordination chemistry was explored. It presents a tren cap connected to the calixarene small rim through three amido spacers. X-ray diffraction studies of its metal complexes revealed a six-coordinate Zn^II complex with all of the carbonyl groups of the amido arms bound and a five-coordinate Cu^II complex with only one amido arm bound. These dicationic complexes were poorly responsive toward exogenous neutral donors, but the amido arms were readily displaced by small anions or deprotonated with a base to give the corresponding monocationic complexes. Cyclic voltammetry in various solvents showed a reversible wave for the Cu^II/Cu^I couple at very negative potentials, denoting an electron-rich environment. The reversibility of the system was attributed to the amido arms, which can coordinate the metal center in both its +II and +I redox states. The reversibility was lost upon anion binding to Cu. Upon exposure of the Cu^I complex to O₂ at low temperature, a green species was obtained with a UV-vis signature typical of an end-on superoxide Cu^II complex. Such a species was proposed to be responsible for oxygen insertion reactions onto the ligand according to the unusual and selective four-electron oxidative pathway previously described with a "one-story" calix[6]tren ligand.

Decoding the Mechanism of Intramolecular Cu-Directed Hydroxylation of sp3 C-H Bonds

The use of copper in directed C-H oxidation has been relatively underexplored. In a seminal example, Schönecker showed that copper and O2 promoted the hydroxylation of steroid-containing ligands. Recently, Baran (J. Am. Chem. Soc. 2015, 137, 13776) improved the reaction conditions to oxidize similar substrates with excellent yields. In both reports, the involvement of Cu2O2 intermediates was suggested. In this collaborative article, we studied the hydroxylation mechanism in great detail, resulting in the overhaul of the previously accepted mechanism and the development of improved reaction conditions. Extensive experimental evidence (spectroscopic characterization, kinetic analysis, intermolecular reactivity, and radical trap experiments) is provided to support each of the elementary steps proposed and the hypothesis that a key mononuclear LCuII(OOR) intermediate undergoes homolytic O-O cleavage to generate reactive RO• species, which are responsible for key C-H hydroxylation within the solvent cage. These key findings allowed the oxidation protocol to be reformulated, leading to improvements of the reaction cost, practicability, and isolated yield.

Formally Copper(III)-Alkylperoxo Complexes as Models of Possible Intermediates in Monooxygenase Enzymes

Reaction of [NBu₄][LCu^IIOH] with excess ROOH (R = cumyl or tBu) yielded [NBu₄][LCu^IIOOR], the reversible one-electron oxidation of which generated novel species with [CuOOR]²⁺ cores (formally Cu^IIIOOR), identified by spectroscopy and theory for the case R = cumyl. This species reacts with weak O-H bonds in TEMPO-H and 4-dimethylaminophenol (^NMe2PhOH), the latter yielding LCu(OPh^NMe2), which was also prepared independently. With the identification of [CuOOR]²⁺ complexes, the first precedent for this core in enzymes is provided, with implications for copper monooxygenase mechanisms.

Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104, 1013-1046 and Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047-1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amount of mechanistic work performed, particularly in the area of organic substrate oxidation, and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper-oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper-oxygen cores discussed.

Copper(I)-Dioxygen Adducts and Copper Enzyme Mechanisms

Primary copper(I)-dioxygen (O₂) adducts, cupric-superoxide complexes, have been proposed intermediates in copper-containing dioxygen-activating monooxygenase and oxidase enzymes. Here, mechanisms of C-H activation by reactive copper-(di)oxygen intermediates are discussed, with an emphasis on cupric-superoxide species. Over the past 25 years, many synthetically derived cupric-superoxide model complexes have been reported. Due to the thermal instability of these intermediates, early studies focused on increasing their stability and obtaining physical characterization. More recently, in an effort to gain insight into the possible substrate oxidation step in some copper monooxygenases, several cupric-superoxide complexes have been used as surrogates to probe substrate scope and reaction mechanisms. These cupric superoxides are capable of oxidizing substrates containing weak O-H and C-H bonds. Mechanistic studies for some enzymes and model systems have supported an initial hydrogen-atom abstraction via the cupric-superoxide complex as the first step of substrate oxidation.

Perturbing the Copper(III)-Hydroxide Unit through Ligand Structural Variation

Two new ligand sets, (pipMe)LH2 and (NO2)LH2 ((pipMe)L = N,N'-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide, (NO2)L = N,N'-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide), are reported which are designed to perturb the overall electronics of the copper(III)-hydroxide core and the resulting effects on the thermodynamics and kinetics of its hydrogen-atom abstraction (HAT) reactions. Bond dissociation energies (BDEs) for the O-H bonds of the corresponding Cu(II)-OH2 complexes were measured that reveal that changes in the redox potential for the Cu(III)/Cu(II) couple are only partially offset by opposite changes in the pKa, leading to modest differences in BDE among the three compounds. The effects of these changes were further probed by evaluating the rates of HAT by the corresponding Cu(III)-hydroxide complexes from substrates with C-H bonds of variable strength. These studies revealed an overarching linear trend in the relationship between the log k (where k is the second-order rate constant) and the ΔH of reaction. Additional subtleties in measured rates arise, however, that are associated with variations in hydrogen-atom abstraction barrier heights and tunneling efficiencies over the temperature range from -80 to -20 °C, as inferred from measured kinetic isotope effects and corresponding electronic-structure-based transition-state theory calculations.

A polypyridyl Co(ii) complex-based water reduction catalyst with double H2 evolution sites

The unique coordination mode of Cl-TMPA and the double H₂ evolution sites of [Co(Cl-TMPA)Cl₂] provide a new strategy to design more effective WRCs.

Photocatalytic hydrogen evolution by Cu(ii) complexes

[Cu(Cl-TMPA)Cl₂] has a more labile Cl ligand and a dangling Cl-substituted pyridyl unit; both account for its high photocatalytic H₂ evolution activity.

Cage-like copper(II) silsesquioxanes: transmetalation reactions and structural, quantum chemical, and catalytic studies

The transmetalation of bimetallic copper-sodium silsesquioxane cages, namely, [(PhSiO1.5 )10 (CuO)2 (NaO0.5 )2 ] ("Cooling Tower"; 1), [(PhSiO1.5 )12 (CuO)4 (NaO0.5 )4 ] ("Globule"; 2), and [(PhSiO1.5 )6 (CuO)4 (NaO0.5 )4 (PhSiO1.5 )6 ] ("Sandwich"; 3), resulted in the generation of three types of hexanuclear cylinder-like copper silsesqui- oxanes, [(PhSiO1.5 )12 (CuO)6 (C4 H9 OH)2 (C2 H5 OH)6 ] (4), [(PhSiO1.5 )12 (CuO)6 (C4 H8 O2 )4 (PhCN)2 (MeOH)4 ] (5), and [(PhSiO1.5 )12 (CuO)6 (NaCl)(C4 H8 O2 )12 (H2 O)2 ] (6). The products show a prominent "solvating system-structure" dependency, as determined by X-ray diffraction. Topological analysis of cages 1-6 was also performed. In addition, DFT theory was used to examine the structures of the Cooling Tower and Cylinder compounds, as well as the spin density distributions. Compounds 1, 2, and 5 were applied as catalysts for the direct oxidation of alcohols and amines into the corresponding amides. Compound 6 is an excellent catalyst in the oxidation reactions of benzene and alcohols.

Elaboration of copper-oxygen mediated C-H activation chemistry in consideration of future fuel and feedstock generation

To contribute solutions to current energy concerns, improvements in the efficiency of dioxygen mediated C-H bond cleavage chemistry, for example, selective oxidation of methane to methanol, could minimize losses in natural gas usage or produce feedstocks for fuels. Oxidative C-H activation is also a component of polysaccharide degradation, potentially affording alternative biofuels from abundant biomass. Thus, an understanding of active-site chemistry in copper monooxygenases, those activating strong C-H bonds is briefly reviewed. Then, recent advances in the synthesis-generation and study of various copper-oxygen intermediates are highlighted. Of special interest are cupric-superoxide, Cu-hydroperoxo and Cu-oxy complexes. Such investigations can contribute to an enhanced future application of C-H oxidation or oxygenation processes using air, as concerning societal energy goals.

Amine oxidative N-dealkylation via cupric hydroperoxide Cu-OOH homolytic cleavage followed by site-specific fenton chemistry

Copper(II) hydroperoxide species are significant intermediates in processes such as fuel cells and (bio)chemical oxidations, all involving stepwise reduction of molecular oxygen. We previously reported a Cu(II)-OOH species that performs oxidative N-dealkylation on a dibenzylamino group that is appended to the 6-position of a pyridyl donor of a tripodal tetradentate ligand. To obtain insights into the mechanism of this process, reaction kinetics and products were determined employing ligand substrates with various para-substituent dibenzyl pairs (-H,-H; -H,-Cl; -H,-OMe, and -Cl,-OMe), or with partially or fully deuterated dibenzyl N-(CH2Ph)2 moieties. A series of ligand-copper(II) bis-perchlorate complexes were synthesized, characterized, and the X-ray structures of the -H,-OMe analogue were determined. The corresponding metastable Cu(II)-OOH species were generated by addition of H2O2/base in acetone at -90 °C. These convert (t1/2 ≈ 53 s) to oxidatively N-dealkylated products, producing para-substituted benzaldehydes. Based on the experimental observations and supporting DFT calculations, a reaction mechanism involving dibenzylamine H-atom abstraction or electron-transfer oxidation by the Cu(II)-OOH entity could be ruled out. It is concluded that the chemistry proceeds by rate limiting Cu-O homolytic cleavage of the Cu(II)-(OOH) species, followed by site-specific copper Fenton chemistry. As a process of broad interest in copper as well as iron oxidative (bio)chemistries, a detailed computational analysis was performed, indicating that a Cu(I)OOH species undergoes O-O homolytic cleavage to yield a hydroxyl radical and Cu(II)OH rather than heterolytic cleavage to yield water and a Cu(II)-O(•-) species.

Copper-peptide complex structure and reactivity when found in conserved His-X(aa)-His sequences

Oxygen-activating copper proteins may possess His-X(aa)-His chelating sequences at their active sites and additionally exhibit imidiazole group δN vs εN tautomeric preferences. As shown here, such variations strongly affect copper ion's coordination geometry, redox behavior, and oxidative reactivity. Copper(I) complexes bound to either δ-HGH or ε-HGH tripeptides were synthesized and characterized. Structural investigations using X-ray absorption spectroscopy, density functional theory calculations, and solution conductivity measurements reveal that δ-HGH forms the Cu(I) dimer complex [{Cu(I)(δ-HGH)}2](2+) (1) while ε-HGH binds Cu(I) to give the monomeric complex [Cu(I)(ε-HGH)](+) (2). Only 2 exhibits any reactivity, forming a strong CO adduct, [Cu(I)(ε-HGH)(CO)](+), with properties closely matching those of the copper monooxygenase PHM. Also, 2 is reactive toward O2 or H2O2, giving a new type of O2-adduct or Cu(II)-OOH complex, respectively.

Mechanistic insights into the oxidation of substituted phenols via hydrogen atom abstraction by a cupric-superoxo complex

To obtain mechanistic insights into the inherent reactivity patterns for copper(I)-O2 adducts, a new cupric-superoxo complex [(DMM-tmpa)Cu(II)(O2(•-))](+) (2) [DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine] has been synthesized and studied in phenol oxidation-oxygenation reactions. Compound 2 is characterized by UV-vis, resonance Raman, and EPR spectroscopies. Its reactions with a series of para-substituted 2,6-di-tert-butylphenols (p-X-DTBPs) afford 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) in up to 50% yields. Significant deuterium kinetic isotope effects and a positive correlation of second-order rate constants (k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a mechanism that involves rate-limiting hydrogen atom transfer (HAT). A weak correlation of (k(B)T/e) ln k2 versus E(ox) of p-X-DTBP indicates that the HAT reactions proceed via a partial transfer of charge rather than a complete transfer of charge in the electron transfer/proton transfer pathway. Product analyses, (18)O-labeling experiments, and separate reactivity employing the 2,4,6-tri-tert-butylphenoxyl radical provide further mechanistic insights. After initial HAT, a second molar equiv of 2 couples to the phenoxyl radical initially formed, giving a Cu(II)-OO-(ArO') intermediate, which proceeds in the case of p-OR-DTBP substrates via a two-electron oxidation reaction involving hydrolysis steps which liberate H2O2 and the corresponding alcohol. By contrast, four-electron oxygenation (O-O cleavage) mainly occurs for p-R-DTBP which gives (18)O-labeled DTBQ and elimination of the R group.

Peroxynitrite chemistry derived from nitric oxide reaction with a Cu(II)-OOH species and a copper mediated NO reductive coupling reaction

New peroxynitrite-copper chemistry ensues via addition of nitric oxide (˙NO(g)) to a Cu(II)-hydroperoxo species. In characterizing the system, the ligand-Cu(i) complex was shown to effect a seldom observed ˙NO(g) reductive coupling reaction. Biological implications are discussed.