Recent Advances in One-Electron-Oxidized CuII -Diphenoxide Complexes as Models of Galactose Oxidase: Importance of the Structural Flexibility in the Active Site

Homo- and Heterogeneous Benzyl Alcohol Catalytic Oxidation Promoted by Mononuclear Copper(II) Complexes: The Influence of the Ligand upon Product Conversion

The catalytic properties of three copper complexes, [Cu(en)2](ClO4)2 (1), [Cu(amp)2](ClO4)2, (2) and [Cu(bpy)2](ClO4)2 (3) (where en = ethylenediamine, amp = 2-aminomethylpyridine and bpy = 2,2'-bipyridine), were explored upon the oxidation of benzyl alcohol (BnOH). Maximized conversions of the substrates to their respective products were obtained using a multivariate analysis approach, a powerful tool that allowed multiple variables to be optimized simultaneously, thus creating a more economical, fast and effective technique. Considering the studies in a fluid solution (homogeneous), all complexes strongly depended on the amount of the oxidizing agent (H2O2), followed by the catalyst load. In contrast, time seemed to be statistically less relevant for complexes 1 and 3 and not relevant for 2. All complexes showed high selectivity in their optimized conditions, and only benzaldehyde (BA) was obtained as a viable product. Quantitatively, the catalytic activity observed was 3 > 2 > 1, which is related to the π-acceptor character of the ligands employed in the study. Density functional theory (DFT) studies could corroborate this feature by correlating the geometric index for square pyramid Cu(II)-OOH species, which should be generated in the solution during the catalytic process. Complex 3 was successfully immobilized in silica-coated magnetic nanoparticles (Fe3O4@SiO2), and its oxidative activity was evaluated through heterogenous catalysis assays. Substrate conversion promoted by 3-Fe3O4@SiO2 generated only BA as a viable product, and the supported catalyst's recyclability was proven. Reduced catalytic conversions in the presence of the radical scavenger (2,2,6,6-tetrametil-piperidi-1-nil)oxil (TEMPO) indicate that radical and non-radical mechanisms are involved.

Diversity of oxidation state in copper complexes with phenolate ligands

The phenoxyl radical binding copper complexes have been widely developed and their detailed geometric and electronic structures have been clarified. While many one-electron oxidized CuII-phenolate complexes have been reported previously, recent studies of the Cu-phenolate complexes proceed toward elucidation of the complexes with other oxidation states, such as the phenoxyl radical binding CuI complexes and CuIV-phenolate complexes in the formal oxidation state. This Perspective focuses on new aspects of the properties and reactivities of various Cu-phenolate and Cu-phenoxyl radical complexes with emphasis on the relationship between geometric and electronic structures.

Cytosolic distribution of copper in the gills of field-collected oysters with different copper bioaccumulation

Oysters can hyper-accumulate copper (Cu) without apparent toxicity, but the mechanism of sequestering excessive cytosolic Cu in oysters remains unclear. We here investigated the Cu distribution in the cytosolic proteins (CPs) in the gills of oysters (Crassostrea hongkongensis) through size-exclusion chromatography coupled to inductively coupled plasma mass spectrometry (SEC-ICP-MS). Oysters collected from the southern coast of China contained a gradient of gill Cu concentrations ranging from 132 to 3540 μg g-1 (dry weight), with 7-41 % of Cu distributed in the CPs fraction. The CPs-Cu concentrations were 8.6 times higher in oysters with high Cu concentrations compared to low concentrations. In the CPs, Cu was dispersed with a broad range of molecular weight, suggesting the involvement of various cytosolic proteins in Cu binding. Among the 10 major Cu peaks, peaks 2 (>600 kDa) and peak 8 (18 kDa) contained substantial Cu and showed obvious differences in response to the variation of CPs-Cu levels. Peak 8 contained metallothionein-like proteins that decreased their role in Cu binding as CPs-Cu concentrations increased. LC-MS/MS analysis revealed that peak 2 contained macromolecular protein complexes (MPCs), which played a critical role in binding excess Cu. The comparison with other bivalve species further suggested that sequestering excess CPs-Cu in MPCs was a special strategy employed by oysters in response to high Cu accumulation. This study provides valuable insights into the mechanism of hyper-accumulation and sequestration of Cu in oysters and helps to better understand Cu biomonitoring by oysters.

Simultaneous Generation of Methyl Esters and CO in Lignin Transformation

Lignin is an abundant renewable carbon source. Due to its complex structure, utilization of lignin is very challenging. Herein, we describe an efficient strategy for the simultaneous utilization of lignin, in which the methoxy groups in lignin react with carboxylic acids to generate methyl carboxylates and the other alkyl and phenyl carbons react with oxygen to predominantly form CO that can be used directly in carbonylation reactions. The method was applied to the methylation of various functionalized aryl and alkyl carboxylic acids, including natural compounds, to produce valuable chemicals, including pharmaceuticals. No solid or liquid residues remain after the reaction. Mechanistic studies demonstrate that a well-ordered C-C and C-O bond activation sequence takes place to realize total transformation of lignin. This work opens a way for transformation of the entire lignin polymer into valuable products, exemplified by the synthesis of the pharmaceutical, Ramipril, on a gram scale.

Biomimetic catalytic aerobic oxidation of C-sp(3)-H bonds under mild conditions using galactose oxidase model compound CuIIL

Developing highly efficient catalytic protocols for C-sp(3)-H bond aerobic oxidation under mild conditions is a long-desired goal of chemists. Inspired by nature, a biomimetic approach for the aerobic oxidation of C-sp(3)-H by galactose oxidase model compound CuIIL and NHPI (N-hydroxyphthalimide) was developed. The CuIIL-NHPI system exhibited excellent performance in the oxidation of C-sp(3)-H bonds to ketones, especially for light alkanes. The biomimetic catalytic protocol had a broad substrate scope. Mechanistic studies revealed that the CuI-radical intermediate species generated from the intramolecular redox process of CuIILH2 was critical for O2 activation. Kinetic experiments showed that the activation of NHPI was the rate-determining step. Furthermore, activation of NHPI in the CuIIL-NHPI system was demonstrated by time-resolved EPR results. The persistent PINO (phthalimide-N-oxyl) radical mechanism for the aerobic oxidation of C-sp(3)-H bond was demonstrated.

Non-innocent redox behavior of CuII-p-dimethylaminophenolate complexes: formation and characterization of the CuI-phenoxyl radical species

Cu complexes with p-dimethylaminophenolate ligands were synthesized by the reaction of Cu^II ions with the ligands under inert gas atmosphere and characterized. The complexes showed a valence state change from Cu^II-phenolate to Cu^I-phenoxyl radical on loss of the coordinated solvent. The Cu^I-phenoxyl radical species showed the characteristic properties and reactivities.

Ratiometric pH Sensing, Photophysics, and Cell Imaging of Nonaromatic Light-Emitting Polymers

Here, four nontraditional fluorescent polymers (NTFPs) of varying N,N-dimethyl-2-propenamide (DMPA) and butyl prop-2-enoate (BPE) mole ratios, i.e., 2:1 (NTFP1), 4:1 (NTFP2), 8:1 (NTFP3), and 16:1 (NTFP4), are prepared via random polymerization in water. The maximum fluorescence enhancement of NTFP3 makes it suitable for ratiometric pH sensing, Cu(II) sensing, and pH-dependent cell imaging of Madin-Darby canine kidney (MDCK) cells. The oxygen donor functionalities of NTFP3 involved in binding and sensing with Cu(II) ions are studied by absorption, emission, electron paramagnetic resonance, Fourier transform infrared (FTIR), and O1s/Cu2p X-ray photoelectron spectroscopies (XPS). The spectral responses of the ratiometric pH sensor within 1.5-11.5 confirm 22 and 44 nm red shifts in absorption and ratiometric emission, respectively. The striking color changes from blue (436 nm) to green (480 nm) via an increase in pH are thought to be the stabilization of the charged canonical form of tertiary amide, i.e., -C(O^-)═N⁺(CH₃)₂, realized from the changes in the absorption/fluorescence spectra and XPS/FTIR analyses. The through-space n-π* interactions in the NTFP3 aggregate, N-branching-associated rigidity, and nonconventional intramolecular hydrogen bondings of adjacent NTFP3 moieties in the NTFP3 aggregate contribute to aggregation-enhanced emissions (AEEs). Here, structures of NTFP3, NTFP3 aggregate, and Cu(II)-NTFP3; absorption; n-π* interactions; hydrogen bondings; AEEs; and binding with Cu(II) are ascertained by density functional theory, time-dependent density functional theory, and reduced density gradient calculations. The excellent limits of detection and Stern-Volmer constants of NTFP3 are 2.24 nM/0.14234 ppb and 4.26 × 10³ M^-1 at pH = 6.5 and 0.95 nM/0.06037 ppb and 4.90 × 10³ M^-1 at pH = 8.0, respectively. Additionally, the Stokes shift and binding energy of NTFP3 are 13,636 cm^-1/1.69 eV and -4.64 eV, respectively. The pH-dependent MDCK cell imaging ability of noncytotoxic NTFP3 is supported via fluorescence imaging and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay.

π-π Stacking Interaction of Metal Phenoxyl Radical Complexes

π-π stacking interaction is well-known to be one of the weak interactions. Its importance in the stabilization of protein structures and functionalization has been reported for various systems. We have focused on a single copper oxidase, galactose oxidase, which has the π-π stacking interaction of the alkylthio-substituted phenoxyl radical with the indole ring of the proximal tryptophan residue and catalyzes primary alcohol oxidation to give the corresponding aldehyde. This stacking interaction has been considered to stabilize the alkylthio-phenoxyl radical, but further details of the interaction are still unclear. In this review, we discuss the effect of the π-π stacking interaction of the alkylthio-substituted phenoxyl radical with an indole ring.

Room-Temperature Cu(II) Radical-Triggered Alkyne C-H Activation

A dimeric Cu(II) complex [Cu(II)2L2(μ2-Cl)Cl] (1) built from an asymmetric tridentate ligand (2-(((2-aminocyclohexyl)imino)methyl)-4,6-di-tert-butylphenol) and weakly coordinating anions has been synthesized and structurally characterized. In dichloromethane solution, 1 exists in a monomeric [Cu(II)LCl] (1') (85%)-dimeric (1) (15%) equilibrium, and cyclic voltammetry (CV) and electron paramagnetic resonance (EPR) studies indicate structural stability and redox retention. Addition of phenylacetylene to the CH2Cl2 solution populates 1' and leads to the formation of a transient radical species. Theoretical studies support this notion and show that the radical initiates an alkyne C-H bond activation process via a four-membered ring (Cu(II)-O···H-Calkyne) intermediate. This unusual C-H activation method is applicable for the efficient synthesis of propargylamines, without additives, within 16 h, at low loadings and in noncoordinating solvents including late-stage functionalization of important bioactive molecules. Single-crystal X-ray diffraction studies, postcatalysis, confirmed the framework's stability and showed that the metal center preserves its oxidation state. The scope and limitations of this unconventional protocol are discussed.

Elaboration on the Electronics of Salen Manganese Nitrides: Investigations into Alkoxy-Substituted Ligand Scaffolds

The ligand electronics of salen manganese nitride complexes directly influence the locus of oxidation and, thus, the reactivity of the resulting oxidized species. This work investigates the influence of tert-butoxy, isopropoxy, and methoxy substituents on the electronics of salen manganese nitride species and includes the first documentation of the para Hammett value for the tert-butoxy substituent (σ_para = -0.13 ± 0.03). Each alkoxy-substituted complex undergoes metal-based oxidation to form manganese(VI), and the kinetics of bimolecular homocoupling to form N₂ were assessed by cyclic voltammetry. Bis-oxidation of the manganese complexes was investigated at low temperature using cyclic voltammery and UV-vis-near-IR spectroscopy, and in combination with theoretical calculations, plausible electronic structures of the dications are provided.

Formation of Ni(II)-phenoxyl radical complexes by O2: a mechanistic insight into the reaction of Ni(II)-phenol complexes with O2

A reaction of Ni(ClO4)2·6H2O with a tripodal ligand having two di(tert-butyl)phenol moieties, H2tbuL, and 1 equivalent of triethylamine in CH2Cl2/CH3OH (1 : 1, v/v) under N2 gave a NiII-(phenol)(phenolate) complex, [Ni(HtbuL)(CH3OH)2]ClO4. The formation of the NiII-phenoxyl radical complex by O2 was observed in the reaction of this complex in the solid state. On the other hand, the NiII-phenoxyl radical complex [Ni(Me2NL)(CH3OH)2]ClO4 was obtained by the reaction of H2Me2NL having a p-(dimethylamino)phenol moiety with Ni(ClO4)2·6H2O in a similar procedure under O2, through the oxidation of the NiII-(phenol)(phenolate) complex. However, a direct redox reaction of the NiII ion could not be detected in the phenoxyl radical formation. The results of the reaction kinetics, XAS and X-ray structure analyses suggested that the O2 oxidation from the NiII-(phenol)(phenolate) complex to the NiII-phenoxyl radical complex occurs via the proton transfer-electron transfer (PT-ET) type mechanism of the phenol moiety weakly coordinated to the nickel ion.

Distorted copper(ii) radicals with sterically hindered salens: electronic structure and aerobic oxidation of alcohols

The sterically hindered salen ligands featuring biphenyl and tetramethyl putrescine linkers were synthesized and chelated to copper. The resulting complexes CuL^bp,tBu, CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe were structurally characterized, showing a significanty tetrahedrally distorted metal center. The complexes show two reversible oxidation waves in the range 0.2 to 0.8 V vs. Fc⁺/Fc. A further reduction wave is detected in the range -1.4 to -1.7 V vs. Fc⁺/Fc. It is reversible for CuL^bp,tBu and CuL^bp,OMe and assigned to the Cu^II/Cu^I redox couple. One-electron oxidation of CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe was performed chemically and electrochemically. It is accompanied by a quenching of the EPR resonances. Phenoxyl radical formation was established by X-Ray diffraction on the cations [CuL^bp,OMe]⁺ and [CuL^pu,OMe]⁺, whereby the coordination sphere is elongated upon oxidation with quinoidal distributions of bond distances. The cations exhibit a NIR band of moderate intensity in their optical spectrum, supporting their classification as class II mixed-valent radical species according to the Robin Day classification. The proposed electronic structures are supported by DFT calculations. The cations [CuL^bp,OMe]⁺, [CuL^pu,tBu]⁺ and [CuL^pu,OMe]⁺ were active towards aerobic oxidation of the unactivated alcohol 2-phenylethanol, with TON numbers up to 58 within 3 h.