Structure, Spectroscopy, and Reactivity of a Mononuclear Copper Hydroxide Complex in Three Molecular Oxidation States

Spectroscopic characterization and reactivity studies of a copper(II) iminoxyl radical complex

A CuII complex (1) of a bis-pyridine-dioxime ligand and its one-electron oxidized analog (1-ox) were thoroughly characterized by various spectroscopic techniques, including X-ray absorption spectroscopy. 1-ox was found to be a CuII complex of a ligand iminoxyl radical and represents the first example of such a type. Reorganization energy (λ) of 2.12 eV was determined for the 1-ox/1 couple, which is considerably higher than the type 1 protein and synthetic CuIII/II(OH) complexes.

Identifying Radical Pathways for Cu(I)/Cu(II) Relay Catalyzed Oxygenation via Online Coupled EPR/UV-Vis/Near-IR Monitoring

Copper-catalyzed C─H oxygenation has drawn considerable attention in mechanistic studies. However, a comprehensive investigation combining radical pathways with a metal-catalytic cycle is challenged by the intricate organic radicals and metallic intermediates. Herein, an online coupled EPR/UV-vis/near-IR detecting method is developed to simultaneously monitor both reactive radical species and copper complex intermediates during the reaction. Focusing on copper-catalyzed phenol oxygenation with cumene hydroperoxide, the short-lived alkylperoxyl radical (EPR signal at g = 2.0143) as well as the unexpected square planar Cu(II)-alkoxyl radical complex (near-IR signal at 833 nm) are unveiled during the reaction, in addition to the observable phenoxyl radical in EPR, quinone product in UV-vis, and Cu(II) center in EPR. With a comprehensive picture of diverse intermediates evolving over the same timeline, a novel Cu(I)/Cu(II) proposed relay-catalyzed sequential radical pathway. In this sequence, Cu(II) activates hydroperoxide through Cu(II)-OOR into the alkylperoxide radical, while the reaction between Cu(I) and hydroperoxide leads to Cu(II)(•OR)OH with high H-atom abstracting activity. These results provide a thorough understanding of the Cu(I)/Cu(II) relay catalysis for phenol oxygenation, setting the stage for mechanistic investigations into intricate radical reactions promoted by metallic complexes.

Unusual Stability of an End-on Superoxido Copper(II) Complex under Ambient Conditions

Superoxido copper complexes play an important role as usually short-lived intermediates in biology and chemistry. The unusual stability of an end-on superoxido copper complex observed in an oxygen-enhanced atom transfer radical polymerization (ATRP) led to a detailed mechanistic investigation of the formation of [CuII(Me6tren)(O2⋅-)]+ (Me6tren=tris(2-dimethyl-aminoethyl)amine) under ambient conditions. The persistence of the superoxido copper complex could be explained by a reaction cycle including the peroxido complex [(Me6tren)2CuII 2(O2)]2+ together with [CuI(Me6tren)(DMSO)]+ and [CuII(Me6tren)(OH)]+ in the overall reaction.

Copper-oxygen adducts: new trends in characterization and properties towards C-H activation

This review summarizes the latest discoveries in the field of C-H activation by copper monoxygenases and more particularly by their bioinspired systems. This work first describes the recent background on copper-containing enzymes along with additional interpretations about the nature of the active copper-oxygen intermediates. It then focuses on relevant examples of bioinorganic synthetic copper-oxygen intermediates according to their nuclearity (mono to polynuclear). This includes a detailed description of the spectroscopic features of these adducts as well as their reactivity towards the oxidation of recalcitrant Csp3 -H bonds. The last part is devoted to the significant expansion of heterogeneous catalytic systems based on copper-oxygen cores (i.e. within zeolite frameworks).

Controlling Redox Potential of a Manganese(III)-Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity

A series of mononuclear manganese(III)-hydroxo and -aqua complexes, [MnIII(TBDAP)(OH)2]+ (1), [MnIII(TBDAP)(OH)(OH2)]2+ (2) and [MnIII(TBDAP)(OH2)2]3+ (3), were prepared from a manganese(II) precursor and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials (E1/2) in the order of 1 (-0.15 V) < 2 (0.56 V) < 3 (1.11 V), while pKa values exhibited a reverse trend in the order of 3 (3.87) < 2 (11.84). Employing the Bordwell Equation, the O-H bond dissociation free energies (BDFE) of [MnII(TBDAP)(OH)(OH2)]+ and [MnII(TBDAP)(OH2)2]2+, related to the driving force of 1 and 2 in hydrogen atom transfer (HAT), were determined as 75.3 and 77.3 kcal mol-1, respectively. It was found that the thermodynamic driving force of 2 in HAT becomes greater than that of 1 as the redox potential of 2 increases through protonation from 1 to 2. Kinetic studies on electrophilic reactions using a variety of substrates revealed that 1 is only weakly reactive with O-H bonds, whereas 2 can activate aliphatic C-H bonds in addition to O-H bonds. The reaction rates increased by 1.4 × 104-fold for the O-H bonds by 2 over 1, which was explained by the difference in BDFE and the tunneling effect. Furthermore, 3, possessing the highest redox potential value, was found to undergo an aromatic C-H bond activation reaction under mild conditions. These results provide valuable insights into enhancing electrophilic reactivity by modulating the redox potential of manganese(III)-hydroxo and -aqua complexes through protonation.

Divergent Bimetallic Mechanisms in Copper(II)-Mediated C-C, N-N, and O-O Oxidative Coupling Reactions

Copper-catalyzed aerobic oxidative coupling of diaryl imines provides a route for conversion of ammonia to hydrazine. The present study uses experimental and density functional theory computational methods to investigate the mechanism of N-N bond formation, and the data support a mechanism involving bimolecular coupling of Cu-coordinated iminyl radicals. Computational analysis is extended to CuII-mediated C-C, N-N, and O-O coupling reactions involved in the formation of cyanogen (NC-CN) from HCN, 1,3-butadiyne from ethyne (i.e., Glaser coupling), hydrazine from ammonia, and hydrogen peroxide from water. The results reveal two different mechanistic pathways. Heteroatom ligands with an uncoordinated lone pair (iminyl, NH2, OH) undergo charge transfer to CuII, generating ligand-centered radicals that undergo facile bimolecular radical-radical coupling. Ligands lacking a lone pair (CN and CCH) form bridged binuclear diamond-core structures that undergo C-C coupling. This mechanistic bifurcation is rationalized by analysis of spin densities in key intermediates and transition states, as well as multiconfigurational calculations. Radical-radical coupling is especially favorable for N-N coupling owing to energetically favorable charge transfer in the intermediate and thermodynamically favorable product formation.

Altering the Localization of an Unpaired Spin in a Formal Ni(V) Species

The participation of both ligand and the metal center in the redox events has been recognized as one of the ways to attain the formal high valent complexes for the late 3d metals, such as Ni and Cu. Such an approach has been employed successfully to stabilize a Ni(III) bisphenoxyl diradical species in which there exist an equilibrium between the ligand and the Ni localized resultant spin. The present work, however, broadens the scope of the previously reported three oxidized equivalent species by conveying the approaches that tend to affect the reported equilibrium in CH3 CN at 233 K. Various spectroscopic characterization revealed that employing exogenous N-donor ligands like 1-methyl imidazole and pyridine favors the formation of the Ni centered localized spin though axial binding. In contrast, due to its steric hinderance, quinoline favors an exclusive ligand localized radical species. DFT studies shed light on the novel intermediates' complex electronic structure. Further, the three oxidized equivalent species with the Ni centered spin was examined for its hydrogen atom abstraction ability stressing their key role in alike reactions.

Modulation of the Bonding between Copper and a Redox-Active Ligand by Hydrogen Bonds and Its Effect on Electronic Coupling and Spin States

The exchange coupling of electron spins can strongly influence the properties of chemical species. The regulation of this type of electronic coupling has been explored within complexes that have multiple metal ions but to a lesser extent in complexes that pair a redox-active ligand with a single metal ion. To bridge this gap, we investigated the interplay among the structural and magnetic properties of mononuclear Cu complexes and exchange coupling between a Cu center and a redox-active ligand over three oxidation states. The computational analysis of the structural properties established a relationship between the complexes' magnetic properties and a bonding interaction involving a dx2-y2 orbital of the Cu ion and π orbital of the redox-active ligand that are close in energy. The additional bonding interaction affects the geometry around the Cu center and was found to be influenced by intramolecular H-bonds introduced by the external ligands. The ability to synthetically tune the d-π interactions using H-bonds illustrates a new type of control over the structural and magnetic properties of metal complexes.

Spectral Physics of Stable Cu(III) Produced by Oxidative Addition of an Alkyl Halide

In this paper, we theoretically investigated spectral physics on Cu(III) complexes formed by the oxidative addition of α-haloacetonitrile to ionic and neutral Cu(I) complexes, stimulated by recent experimental reports. Firstly, the electronic structures of reactants of α-haloacetonitrile and neutral Cu(I) and two kinds of products of Cu(III) complexes are visualized with the density of state (DOS) and orbital energy levels of HOMO and LUMO. The visually manifested static and dynamic polarizability as well as the first hyperpolarizability are employed to reveal the vibrational modes of the normal and resonance Raman spectra of two Cu(III) complexes. The nuclear magnetic resonance (NMR) spectra are not only used to identify the reactants and products but also to distinguish between two Cu(III) complexes. The charge difference density (CDD) reveals intramolecular charge transfer in electronic transitions in optical absorption spectra. The CDDs in fluorescence visually reveal electron-hole recombination. Our results promote a deeper understanding of the physical mechanism of stable Cu(III) produced by the oxidative addition of an alkyl halide.

Structural and Spectroscopic Characterization of Copper(III) Complexes and Subsequent One-Electron Oxidation Reaction and Reactivity Studies

The formation of Cu(III) species are often invoked as the key intermediate in Cu-catalyzed organic transformation reactions. In this study, we synthesized Cu(II) (1) and Cu(III) (3) complexes supported by a bisamidate-bisalkoxide ligand consisting of an ortho-phenylenediamine (o-PDA) scaffold and characterized them through an array of spectroscopic techniques, including UV-visible, electron paramagnetic resonance, X-ray crystallography, and ¹H nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy. The Cu-N/O bond distances in 3 are ∼0.1 Å reduced compared to 1, implying a significant increase in 3's overall effective nuclear charge. Further, a Cu(III) complex (4) of a bisamidate-bisalkoxide ligand containing a trans-cyclohexane-1,2-diamine moiety exhibits nearly identical Cu-N/O bond distances to that of 3, inferring that the redox-active o-PDA backbone is not oxidized upon one-electron oxidation of the Cu(II) complex (1). In addition, a considerable difference in the 1s → 4p and 1s → 3d transition energy was observed in the X-ray absorption near-edge structure data of 3 vs 1, which is typical for the metal-centered oxidation process. Electrochemical measurements of the Cu(II) complex (1) in acetonitrile exhibited two consecutive redox couples at -0.9 and 0.4 V vs the Fc⁺/Fc reference electrode. One-electron oxidation reaction of 3 further resulted in the formation of a ligand-oxidized Cu complex (3a), which was characterized in depth. Reactivity studies of species 3 and 3a were explored toward the activation of the C-H/O-H bonds. A bond dissociation free energy (BDFE) value of ∼69 kcal/mol was estimated for the O-H bond of the Cu(II) complex formed upon transfer of hydrogen atom to 3. The study represents a thorough spectroscopic characterization of high-valent Cu complexes and sheds light on the PCET reactivity studies of Cu(III) complexes.

Sterically and Electronically Flexible Pyridylidene Amine Dinitrogen Ligands at Palladium: Hemilabile cis/trans Coordination and Application in Dehydrogenation Catalysis

Ligand design is crucial for the development of new catalysts and materials with new properties. Herein, the synthesis and unique hemilabile coordination properties of new bis-pyridylidene amine (bis-PYE) ligands to palladium, and preliminary catalytic activity of these complexes in formic acid dehydrogenation are described. The synthetic pathway to form cationic complexes [Pd(bis-PYE)Cl(L)]X with a cis-coordinated N,N-bidentate bis-PYE ligand is flexible and provides access to a diversity of PdII complexes with different ancillary ligands (L=pyridine, DMAP, PPh3 , Cl, P(OMe)3 ). The 1 H NMR chemical shift of the trans-positioned PYE N-CH3 unit is identified as a convenient and diagnostic handle to probe the donor properties of these ancillary ligands and demonstrates the electronic flexibility of the PYE ligand sites. In the presence of a base, the originally cis-coordinated bis-PYE ligand adopts a N,N,N-tridentate coordination mode with the two PYE units in mutual trans position. This cis-trans isomerization is reverted in presence of an acid, demonstrating a unique structural and steric flexibility of the bis-PYE ligand at palladium in addition to its electronic adaptability. The palladium complexes are active in formic acid dehydrogenation to H2 and CO2 . The catalytic performance is directly dependent on the ligand bonding mode, the nature of the ancillary ligand, the counteranion, and additives. The most active system features a bidentate bis-PYE ligand, PPh3 as ancillary ligand and accomplishes turnover frequencies up to 525 h-1 in the first hour and turnover numbers of nearly 1000, which is the highest activity reported for palladium-based catalysts to date.

Formation and Reactivity of a Fleeting NiIII Bisphenoxyl Diradical Species

Cytochrome P450s and Galactose Oxidases exploit redox active ligands to form reactive high valent intermediates for oxidation reactions. This strategy works well for the late 3d metals where accessing high valent states is rather challenging. Herein, we report the oxidation of Ni^II (salen) (salen=N,N'-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexane-(1R,2R)-diamine) with mCPBA (meta-chloroperoxybenzoic acid) to form a fleeting Ni^III bisphenoxyl diradical species, in CH₃ CN and CH₂ Cl₂ at -40 °C. Electrochemical and spectroscopic analyses using UV/Vis, EPR, and resonance Raman spectroscopies revealed oxidation events both on the ligand and the metal centre to yield a Ni^III bisphenoxyl diradical species. DFT calculations found the electronic structure of the ligand and the d-configuration of the metal center to be consistent with a Ni^III bisphenoxyl diradical species. This three electron oxidized species can perform hydrogen atom abstraction and oxygen atom transfer reactions.

Mimicking the Cu Active Site of Lytic Polysaccharide Monooxygenase Using Monoanionic Tridentate N-Donor Ligands

In an effort to prepare small molecule mimics of the active site of lytic polysaccharide monooxygenase (LPMO), three monoanionic tridentate N donor ligands comprising a central deprotonated amide group flanked by two neutral donors were prepared, and their coordination chemistry with Cu(I) and Cu(II) was evaluated. With Cu(I), a dimer formed, which was characterized by X-ray crystallography and NMR spectroscopy. A variety of mononuclear and dinuclear Cu(II) species with a range of auxiliary ligands (MeCN, Cl^-, OH^-, OAc^-, OBz^-, CO₃ ^2-) were prepared and characterized by X-ray diffraction and various spectroscopies (UV-vis, EPR). The complexes exhibit structural similarities to the LPMO active site.

A 4H+/4e- Electron-Coupled-Proton Buffer Based on a Mononuclear Cu Complex

In this research article, we describe a 4H⁺/4e^- electron-coupled-proton buffer (ECPB) based on Cu and a redox-active ligand. The protonated/reduced ECPB (complex 1: [Cu(8H⁺/14e^-)]¹⁺), consisting of Cu^I with 2 equiv of the ligand (^catLH₄: 1,1'-(4,5-dimethoxy-1,2-phenylene)bis(3-(tert-butyl)urea)), reacted with H⁺/e^- acceptors such as O₂ to generate the deprotonated/oxidized ECPB. The resulting compound, (complex 5: [Cu(4H⁺/10e^-)]¹⁺), was characterized by X-ray diffraction analysis, nuclear magnetic resonance (¹H-NMR), and density functional theory, and it is electronically described as a cuprous bis(benzoquinonediimine) species. The stoichiometric 4H⁺/4e^- reduction of 5 was carried out with H⁺/e^- donors to generate 1 (Cu^I and 2 equiv of ^catLH₄) and the corresponding oxidation products. The 1/5 ECPB system catalyzed the 4H⁺/4e^- reduction of O₂ to H₂O and the dehydrogenation of organic substrates in a decoupled (oxidations and reductions are separated in time and space) and a coupled fashion (oxidations and reductions coincide in time and space). Mechanistic analysis revealed that upon reductive protonation of 5 and oxidative deprotonation of 1, fast disproportionation reactions regenerate complexes 5 and 1 in a stoichiometric fashion to maintain the ECPB equilibrium.

Effect of Redox-Inactive Metal Ion-Nickel(III) Interactions on the Redox Properties and Proton-Coupled Electron Transfer Reactivity

Mononuclear nickel(II) and nickel(III) complexes of a bisamidate-bisalkoxide ligand, (NMe₄)₂[Ni^II(HMPAB)] (1) and (NMe₄)[Ni^III(HMPAB)] (2), respectively, have been synthesized and characterized by various spectroscopic techniques including X-ray crystallography. The reaction of redox-inactive metal ions (Mⁿ⁺ = Ca²⁺, Mg²⁺, Zn²⁺, Y³⁺, and Sc³⁺) with 2 resulted in 2-Mⁿ⁺ adducts, which was assessed by an array of spectroscopic techniques including X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and reactivity studies. The X-ray structure of Ca²⁺ coordinated to Ni(III) complexes, 2-Ca²⁺T, was determined and exhibited an average Ni-Ca distance of 3.1253 Å, close to the metal ions' covalent radius. XAS analysis of 2-Ca²⁺ and 2-Y³⁺ in solution further revealed an additional coordination to Ca and Y in the 2-Mⁿ⁺ adducts with shortened Ni-M distances of 2.15 and 2.11 Å, respectively, implying direct bonding interactions between Ni and Lewis acids (LAs). Such a short interatomic distance between Ni(III) and M is unprecedented and was not observed before. EPR analysis of 2 and 2-Mⁿ⁺ species, moreover, displayed rhombic signals with g_av > 2.12 for all complexes, supporting the +III oxidation state of Ni. The Ni^III/Ni^II redox potential of 2 and 2-Mⁿ⁺ species was determined, and a plot of E_1/2 of 2-Mⁿ⁺ versus pK_a of [M(H₂O)_n]^m+ exhibited a linear relationship, implying that the Ni^III/Ni^II potential of 2 can be tuned with different redox-inactive metal ions. Reactivity studies of 2 and 2-Mⁿ⁺ with different 4-X-2,6-ditert-butylphenol (4-X-DTBP) and other phenol derivatives were performed, and based on kinetic studies, we propose the involvement of a proton-coupled electron transfer (PCET) pathway. Analysis of the reaction products after the reaction of 2 with 4-OMe-DTBP showed the formation of a Ni(II) complex (1a) where one of the alkoxide arms of the ligand is protonated. A pK_a value of 24.2 was estimated for 1a. The reaction of 2-Mⁿ⁺ species was examined with 4-OMe-DTBP, and it was observed that the k₂ values of 2-Mⁿ⁺ species increase by increasing the Lewis acidity of redox-inactive metal ions. However, the obtained k₂ values for 2-Mⁿ⁺ species are much lower compared to the k₂ value for 2. Such a variation of PCET reactivity between 2 and 2-Mⁿ⁺ species may be attributed to the interactions between Ni(III) and LAs. Our findings show the significance of the secondary coordination sphere effect on the PCET reactivity of Ni(III) complexes and furnish important insights into the reaction mechanism involving high-valent nickel species, which are frequently invoked as key intermediates in Ni-mediated enzymatic reactions, solar-fuel catalysis, and biomimetic/synthetic transformation reactions.

Hydrocarbon Oxidation Depth: H2O2/Cu2Cl4·2DMG/CH3CN System

The oxidation of hydrocarbons of different structures under the same conditions is an important stage in the study of the chemical properties of both the hydrocarbons themselves and the oxidation catalysts. In a 50% H2O2/Cu2Cl4·2DMG/CH3CN system, where DMG is dimethylglyoxime (Butane-2,3-dione dioxime), at 50 °C under the same or similar conditions, we oxidized eleven RH hydrocarbons of different structures: mono-, bi- and tri-cyclic, framework and aromatic. To compare the composition of the oxidation products of these hydrocarbons, we introduced a new quantitative characteristic, “distributive oxidation depth D(O), %” and showed the effectiveness of its application. The adiabatic ionization potentials (AIP) and the vertical ionization potentials (VIP) of the molecules of eleven oxidized and related hydrocarbons were calculated using the DFT method in the B3LYP/TZVPP level of theory for comparison with experimental values and correlation with D(O). The same calculations of AIP were made for the molecules of the oxidant, solvent, DMG, related compounds and products. It is shown that component X, which determines the mechanism of oxidation of hydrocarbons RH with AIP(Exp) ≥ AIP(X) = 8.55 ± 0.03 eV, is a trans-DMG molecule. Firstly theoretically estimated experimental values of AIP(trans-DMG) = 8.53 eV and AIP(cis-DMG) = 8.27 eV.

Bioinspired Di-Fe Complexes: Correlating Structure and Proton Transfer over Four Oxidation States

Metalloproteins with active sites containing di-Fe cores exhibit diverse chemical reactivity that is linked to the precise transfer of protons and electrons which directly involve the di-Fe units. The redox conversions are commonly corroborated by spectroscopic methods, but the associated structural changes are often difficult to assess, particularly those related to proton movements. This report describes the development of di-Fe complexes in which the movements of protons and electrons are pinpointed during the stepwise oxidation of a di-Fe^II species to one with an Fe^IIIFe^IV core. Complex formation was promoted using the phosphinic amido tripodal ligand [poat]^3- (N,N',N″-[nitrilotris(ethane-2,1-diyl)]tris(P,P-diphenylphosphinic amido)) that provided dynamic coordination spheres that assisted in regulating both electron and proton transfer processes. Oxidation of an [Fe^II-(μ-OH)-Fe^III] complex led to the corresponding di-Fe^III species containing a hydroxido bridge that was not stable at room temperature and converted to a species containing an oxido bridging ligand and protonation of one phosphinic amido group to form [Hpoat]^2-. Deprotonation led to a new species with an [Fe^III-(μ-O)-Fe^III] core that could be further oxidized to its Fe^IIIFe^IV analogue. Reactions with phenols suggest homolytic cleavage of the O-H bond to give products that are consistent with the initial formation of a phenoxyl radical─spectroscopic studies indicated that the electron is transferred to the Fe^IV center, and the proton is initially transferred to the more sterically hindered oxido ligand but then relocates to [poat]^3-. These findings offer new mechanistic insights related to the stability of and the reactions performed by di-Fe enzymes.

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.

Ligand-Constraint-Induced Peroxide Activation for Electrophilic Reactivity

μ‐1,2‐peroxo‐bridged diiron(III) intermediates P are proposed as reactive intermediates in various biological oxidation reactions. In sMMO, P acts as an electrophile, and performs hydrogen atom and oxygen atom transfers to electron‐rich substrates. In cyanobacterial ADO, however, P is postulated to react by nucleophilic attack on electrophilic carbon atoms. In biomimetic studies, the ability of μ‐1,2‐peroxo‐bridged dimetal complexes of Fe, Co, Ni and Cu to act as nucleophiles that effect deformylation of aldehydes is documented. By performing reactivity and theoretical studies on an end‐on μ‐1,2‐peroxodicobalt(III) complex 1 involving a non‐heme ligand system, L1, supported on a Sn₆O₆ stannoxane core, we now show that a peroxo‐bridged dimetal complex can also be a reactive electrophile. The observed electrophilic chemistry, which is induced by the constraints provided by the Sn₆O₆ core, represents a new domain for metal−peroxide reactivity.

Mononuclear and Dinuclear Copper Complexes of Tridentate Redox-active Ligands with Tunable H-bonding Donors: Structure, Spectroscopy and H+ /e- Reactivity

In this research article, we describe the synthesis and characterization of mononuclear and dinuclear Cu complexes bound by a family of tridentate redox-active ligands with tunable H-bonding donors. The mononuclear Cu-anion complexes were oxidized to the corresponding "high-valent" intermediates by oxidation of the redox-active ligand. These species were capable of oxidizing phenols with weak O-H bonds via H-atom abstraction. Thermodynamic analysis of the H-atom abstractions, which included reduction potential measurements, pK_a determination and kinetic studies, revealed that modification of the anion coordinated to the Cu and changes in the H-bonding donor did not lead to major differences in the reactivity of the "high-valent" CuY complexes (Y: hydroxide, phenolate and acetate), which indicated that the tridentate ligand scaffold acts as the H⁺ and e^- acceptor.