Selective Oxidation of Biomass Molecules via ZnO Nanoparticles Modified Using Charge Mismatch of the Doped Co ions

A charge mismatch between transition-metal-ion dopants and metal oxide nanoparticles (MO NPs) within an engineered complex engenders a significant number of oxygen vacancies (VO) on the surface of the MO NP construct. To elucidate in-depth the mechanism of this tendency, Co ions with different charge states (Co3+ and Co2+) were doped into ZnO NPs, and their atomic structural changes were correlated with their photocatalytic efficiency. We ascertained that the increase of the Zn–O bond distances was distinctly affected by Co3+-ion doping, and, subsequently, the number of VO was noticeably increased. We further investigated the mechanistic pathways of the photocatalytic oxidation of 2,5-hydroxymethylfurfural (HMF), which have been widely investigated as biomass derivatives because of their potential use as precursors for the synthesis of sustainable alternatives to petrochemical substances. To identify the reaction products in each oxidation step, selective oxidation products obtained from HMF in the presence of pristine ZnO NPs, Co3+- and Co2+-ion-doped ZnO NPs were evaluated. We confirmed that Co3+-ion-doped ZnO NPs can efficiently and selectively oxidize HMF with a good conversion rate (∼40%) by converting HMF to 2,5-furandicarboxylic acid (FDCA). The present study demonstrates the feasibility of improving the production efficiency of FDCA (an alternative energy material) by using enhanced photocatalytic MO NPs with the help of the charge mismatch between MO and metal-ion dopants

Hydrogen Bond-Enabled Heterolytic and Homolytic Peroxide Activation within Nonheme Copper(II)-Alkylperoxo Complexes

To explore the reactivity of copper-alkylperoxo species enabled by the heterolytic peroxide activation, room-temperature stable mononuclear nonheme copper­(II)–alkylperoxo complexes bearing a N-(2-ethoxyethanol)-bis(2-picolyl)­amine ligand (HN3O2), [CuII(OOR)­(HN3O2)]+ (R = cumyl or tBu), were synthesized and spectroscopically characterized. A combined experimental and computational investigation on the reactivity and reaction mechanisms in the phosphorus oxidation, C–H bond activation, and aldehyde deformylation reactions by the copper­(II)–alkylperoxo complexes has been conducted. DFT-optimized structures suggested that a hydrogen bonding interaction exists between the ethoxyethanol backbone of the HN3O2 ligand and either the proximal or distal oxygen atom of the alkylperoxide moiety, and this interaction consequently results in the enhanced stability of the copper­(II)–alkylperoxo species. In the phosphorus oxidation reaction, both experimental and computational results indicated that a phosphine-triggered heterolytic O–O bond cleavage occurred to yield phosphine oxide and alcohol products. DFT calculations suggested that (i) the H-bonding between the ethoxyethanol backbone and distal oxygen of the alkylperoxide moiety and (ii) the phosphine binding to the proximal oxygen of the alkylperoxide moiety engendered the heterolytic peroxide activation. In the C–H bond activation reactions, temperature-dependent reactivity of the copper­(II)–alkylperoxo complexes was observed, and a relatively strong activation energy of 95 kcal mol–1 was required to promote the homolytic peroxide activation. A rate-limiting hydrogen atom abstraction reaction of xanthene by the putative copper­(II)-oxyl radical resulted in the formation of the dimeric copper product and the substrate radical that further underwent autocatalytic oxidation reactions to form an oxygen incorporated product. Finally, amphoteric reactivity of copper­(II)–alkylperoxo complexes has been assessed by conducting kinetic studies and product analysis of the aldehyde deformylation reaction

Dioxygen Activation by Mononuclear Nonheme Iron(II) Complexes Generates Iron−Oxygen Intermediates in the Presence of an NADH Analogue and Proton

Dioxygen Activation by Mononuclear Nonheme Iron(II) Complexes Generates Iron−Oxygen Intermediates in the Presence of an NADH Analogue and Proto

Comparison of High-Spin and Low-Spin Nonheme Fe^III–OOH Complexes in O–O Bond Homolysis and H‑Atom Abstraction Reactivities

The geometric and electronic structures and reactivity of an <i>S</i> = 5/2 (HS) mononuclear nonheme (TMC)­Fe^III–OOH complex are studied by spectroscopies, calculations, and kinetics and compared with the results of previous studies of <i>S</i> = 1/2 (LS) Fe^III–OOH complexes to understand parallels and differences in mechanisms of O–O bond homolysis and electrophilic H-atom abstraction reactions. The homolysis reaction of the HS [(TMC)­Fe^III–OOH]²⁺ complex is found to involve axial ligand coordination and a crossing to the LS surface for O–O bond homolysis. Both HS and LS Fe^III–OOH complexes are found to perform direct H-atom abstraction reactions but with very different reaction coordinates. For the LS Fe^III–OOH, the transition state is late in O–O and early in C–H coordinates. However, for the HS Fe^III–OOH, the transition state is early in O–O and further along in the C–H coordinate. In addition, there is a significant amount of electron transfer from the substrate to the HS Fe^III–OOH at transition state, but that does not occur in the LS transition state. Thus, in contrast to the behavior of LS Fe^III–OOH, the H-atom abstraction reactivity of HS Fe^III–OOH is found to be highly dependent on both the ionization potential and the C–H bond strength of the substrate. LS Fe^III–OOH is found to be more effective in H-atom abstraction for strong C–H bonds, while the higher reduction potential of HS Fe^III–OOH allows it to be active in electrophilic reactions without the requirement of O–O bond cleavage. This is relevant to the Rieske dioxygenases, which are proposed to use a HS Fe^III–OOH to catalyze <i>cis</i>-dihydroxylation of a wide range of aromatic compounds

Dioxygen Activation by a Non-Heme Iron(II) Complex: Formation of an Iron(IV)−Oxo Complex via C−H Activation by a Putative Iron(III)−Superoxo Species

Iron(III)−superoxo intermediates are believed to play key roles in oxygenation reactions by non-heme iron enzymes. We now report that a non-heme iron(II) complex activates O2 and generates its corresponding iron(IV)−oxo complex in the presence of substrates with weak C−H bonds (e.g., olefins and alkylaromatic compounds). We propose that a putative iron(III)−superoxo intermediate initiates the O2-activation chemistry by abstracting a H atom from the substrate, with subsequent generation of a high-valent iron(IV)−oxo intermediate from the resulting iron(III)−hydroperoxo species

High-Voltage Symmetric Nonaqueous Redox Flow Battery Based on Modularly Tunable [Ru₂M(μ₃‑O)(CH₃CO₂)₆(py)₃] (M = Ru, Mn, Co, Ni, Zn) Cluster Compounds with Multielectron Storage Capability

Redox flow batteries (RFBs) provide an attractive solution for large-scale energy buffering and storage. This report describes the development of nonaqueous RFBs based on trimetallic coordination cluster compounds: [Ru2M(μ3-O)(CH3CO2)6(py)3] (M = Ru, Mn, Co, Ni, Zn). The all-ruthenium complex exhibited stable battery cycles in anolyte–catholyte symmetric operation, with rarely observed multielectron storage in a single molecule. Moreover, the complex holds modularly tunable synthetic handles for systematic improvements in solubility and redox potentials. An optimized battery stack containing [Ru3(μ3-O)(CH3CO2)6(py)3]+ anolyte and [Ru2Co(μ3-O)(CH3CO2)6(py)3] catholyte yielded stable cycles with a discharge voltage of 2.4 V, comparable to the state-of-the-art nonaqueous RFBs. Explanation for the exceptional stability of the charged states and prediction of systematic tunability of the redox potentials of the cluster compounds were assisted by DFT calculations

Designed Amyloid Fibers with Emergent Melanosomal Functions

Short peptides designed to self-associate into amyloid fibers with metal ion-binding ability have been used to catalyze various types of chemical reactions. This manuscript demonstrates that one of these short-peptide fibers coordinated with CuII can exhibit melanosomal functions. The coordinated CuII and the amyloid structure itself are differentially functional in accelerating oxidative self-association of dopamine into melanin-like species and in regulating their material properties (e.g., water dispersion, morphology, and the density of unpaired electrons). The results have implications for the role of functional amyloids in melanin biosynthesis and for designing peptide-based supramolecular structures with various emergent functions

Ligand Topology Effect on the Reactivity of a Mononuclear Nonheme Iron(IV)-Oxo Complex in Oxygenation Reactions

Mononuclear nonheme iron(IV)-oxo complexes with two different topologies, cis-α-[FeIV(O)(BQCN)]2+ and cis-β-[FeIV(O)(BQCN)]2+, were synthesized and characterized with various spectroscopic methods. The effect of ligand topology on the reactivities of nonheme iron(IV)-oxo complexes was investigated in C–H bond activation and oxygen atom-transfer reactions; cis-α-[FeIV(O)(BQCN)]2+ was more reactive than cis-β-[FeIV(O)(BQCN)]2+ in the oxidation reactions. The reactivity difference between the cis-α and cis-β isomers of [FeIV(O)(BQCN)]2+ was rationalized with the FeIV/III redox potentials of the iron(IV)-oxo complexes: the FeIV/III redox potential of the cis-α isomer was 0.11 V higher than that of the cis-β isomer

Ligand Topology Effect on the Reactivity of a Mononuclear Nonheme Iron(IV)-Oxo Complex in Oxygenation Reactions

Mononuclear nonheme iron(IV)-oxo complexes with two different topologies, cis-α-[FeIV(O)(BQCN)]2+ and cis-β-[FeIV(O)(BQCN)]2+, were synthesized and characterized with various spectroscopic methods. The effect of ligand topology on the reactivities of nonheme iron(IV)-oxo complexes was investigated in C–H bond activation and oxygen atom-transfer reactions; cis-α-[FeIV(O)(BQCN)]2+ was more reactive than cis-β-[FeIV(O)(BQCN)]2+ in the oxidation reactions. The reactivity difference between the cis-α and cis-β isomers of [FeIV(O)(BQCN)]2+ was rationalized with the FeIV/III redox potentials of the iron(IV)-oxo complexes: the FeIV/III redox potential of the cis-α isomer was 0.11 V higher than that of the cis-β isomer