Metal-catalyzed hydrocarbon oxygenations in solutions: the dramatic role of additives: a review

A Family of Hexacopper Phenylsilsesquioxane/Acetate Complexes: Synthesis, Solvent-Controlled Cage Structures, and Catalytic Activity

Convenient self-assembly synthesis of copper(II) complexes via double (phenylsilsesquioxane and acetate) ligation allows to isolate a family of impressive sandwich-like cage compounds. An intriguing feature of these complexes is the difference in the structure of a pair of silsesquioxane ligands despite identical (Cu6) nuclearity and number (four) of acetate fragments. Formation of particular combination of silsesquioxane ligands (cyclic/cyclic vs condensed/condensed vs cyclic/condensed) was found to be dependent on the synthesis/crystallization media. A combination of Si4-cyclic and Si6-condensed silsesquioxane ligands is a brand new feature of cage metallasilsesquioxanes. A representative Cu6-complex (4) (with cyclic silsesquioxanes) exhibited high catalytic activity in the oxidation of alkanes and alcohols with peroxides. Maximum yield of the products of cyclohexane oxidation attained 30 %. The compound 4 was also tested as catalyst in the Baeyer-Villiger oxidation of cyclohexanone by m-chloroperoxybenzoic acid: maximum yields of 88 % and 100 % of ϵ-caprolactone were achieved upon conventional heating at 50 °C for 4 h and MW irradiation at 70 or 80 °C during 30 min, respectively. It was also possible to obtain the lactone (up to 16 % yield) directly from the cyclohexane via a tandem oxidation/Baeyer-Villiger oxidation reaction using the same oxidant.

DFT Studies of the Activity and Reactivity of Limonene in Comparison with Selected Monoterpenes

Nowadays, the effective processing of natural monoterpenes that constitute renewable biomass found in post-production waste into products that are starting materials for the synthesis of valuable compounds is a way to ensure independence from non-renewable fossil fuels and can contribute to reducing global carbon dioxide emissions. The presented research aims to determine, based on DFT calculations, the activity and reactivity of limonene, an organic substrate used in previous preparative analyses, in comparison to selected monoterpenes such as cymene, pinene, thymol, and menthol. The influence of the solvent model was also checked, and the bonds most susceptible to reaction were determined in the examined compounds. With regard to EHOMO, it was found that limonene reacts more easily than cymene or menthol but with more difficultly than thymol and pienene. The analysis of the global chemical reactivity descriptors "locates" the reactivity of limonene in the middle of the studied monoterpenes. It was observed that, among the tested compounds, the most reactive compound is thymol, while the least reactive is menthol. The demonstrated results can be a reference point for experimental work carried out using the discussed compounds, to focus research on those with the highest reactivity.

Experimental and Theoretical Investigation of the Mechanism of the Reduction of O2 from Air to O22- by VIVO2+-N,N,N-Amidate Compounds and Their Potential Use in Fuel Cells

The two-electron reductive activation of O2 to O22- is of particular interest to the scientific community mainly due to the use of peroxides as green oxidants and in powerful fuel cells. Despite of the great importance of vanadium(IV) species to activate the two-electron reductive activation of O2, the mechanism is still unclear. Reaction of VIVO2+ species with the tridentate-planar N,N,N-carboxamide (ΗL) ligands in solution (CH3OH:H2O) under atmospheric O2, at room temperature, resulted in the quick formation of [VV(═O)(η2-O2)(κ3-L)(H2O)] and cis-[VV(═O)2(κ3-L)] compounds. Oxidation of the VIVO2+ complexes with the sterically hindered tridentate-planar N,N,N-carboxamide ligands by atmospheric O2 gave only cis-[VV(═O)2(κ3-L)] compounds. The mechanism of formation of [VV(═O)(η2-O2)(κ3-L)(H2O)] (I) and cis-[VV(═O)2(κ3-L)] (II) complexes vs time, from the interaction of [VIV(═O)(κ3-L)(Η2Ο)2]+ with atmospheric O2, was investigated with 51V, 1H NMR, UV-vis, cw-X-band EPR, and 18O2 labeling IR and resonance Raman spectroscopies revealing the formation of a stable intermediate (Id). EPR, MS, and theoretical calculations of the mechanism of the formation of I and II revealed a pathway, through a binuclear [VIV(═O)(κ3-L)(H2O)(η1,η1-O2)VIV(═O)(κ3-L)(H2O)]2+ intermediate. The results from cw-EPR, 1H NMR spectroscopies, cyclic voltammetry, and the reactivity of the complexes [VIV(═O)(κ3-L)(Η2Ο)2]+ toward O2 reduction fit better to an intermediate with a binuclear nature. Dynamic experiments in combination with computational calculations were undertaken to fully elucidate the mechanism of the O2 reduction to O22- by [VIV(═O)(κ3-L)(Η2Ο)2]+. The galvanic cell {Zn|VIII,VII||Id, [VIVO(κ3-L)(H2O)2]+|O2|C(s)} was manufactured, demonstrating the important applicability of this new chemistry to Zn|H2O2 fuel cells technology generating H2O2 in situ from the atmospheric O2.

Fe(III)-Based Phenylsilsesquioxane/Acetylacetonate Complexes: Synthesis, Cage-like Structure, and High Catalytic Activity

Unprecedented iron-based silsesquioxane/acetylacetonate complexes were synthesized. The intriguing cage-like structure of compounds is alkaline metal-dependent: the Fe2Li2 complex includes condensed Si6-silsesquioxane and four acetylacetonate ligands; the Fe4Na4 complex exhibits two cyclic Si4-silsesquioxane and eight acetylacetonate ligands, while the Fe3K3 complex features two cyclic Si3-silsesquioxane and six acetylacetonate ligands. The latter case is the very first observation of small trimeric silsesquioxane ligands in the composition of cage-like metallasilsesquioxanes. The Fe4Na4-based complex exhibits a record high activity in the oxidation of inert alkanes with peroxides (55% yield of oxygenates in cyclohexane oxidation). It also acts as a catalyst in the cycloaddition of CO2 with epoxides, leading to cyclic carbonates in good yields (58-96%).

Tuning Reactivity of Zr-Substituted Keggin Phosphotungstate in Alkene Epoxidation through Balancing H2O2 Activation Pathways: Unusual Effect of Base

The Zr-monosubstituted Keggin-type dimeric phosphotungstate (Bu4N)8[{PW11O39Zr(μ-OH)(H2O)}2] (1) efficiently catalyzes epoxidation of C═C bonds in various kinds of alkenes, including terminal ones, with aqueous H2O2 as oxidant. Less sterically hindered double bonds are preferably epoxidized despite their lower nucleophilicity. Basic additives (Bu4NOH) in the amount of 1 equiv per dimer 1 suppress H2O2 unproductive decomposition, increase substrate conversion, improve yield of heterolytic oxidation products and oxidant utilization efficiency, and also affect regioselectivity of epoxidation, enhancing oxygen transfer to sterically hindered electron-rich C═C bonds. Acid additives produce a reverse effect on the substrate conversion and H2O2 efficiency. The reaction mechanism was explored using a range of test substrates, kinetic, and spectroscopic tools. The opposite effects of acid and base additives on alkene epoxidation and H2O2 degradation have been rationalized in terms of their impact on hydrolysis of 1 to form monomeric species, [PW11O39Zr(OH)(H2O)x]4- (1-M, x = 1 or 2), which favors H2O2 homolytic decomposition. The interaction of 1 with H2O2 has been investigated by HR-ESI-MS, ATR-FT-IR, and 31P NMR spectroscopic techniques. The combination of spectroscopic studies and kinetic modeling implicated the existence of two types of dimeric peroxo complexes, [Zr2(μ-η2:η2-O2){PW11O39}2(H2O)x]]8- and [{Zr(μ-η2-O2)}2(PW11O39)2(H2O)y]10-, along with monomeric Zr (hydro)peroxo species that begin to dominate at a high excess of H2O2. Both dimeric μ-η2-peroxo intermediates are inert toward alkenes under stoichiometric conditions. V-shape Hammett plots obtained for epoxidation of p-substituted styrenes suggested a biphilic nature of the active oxidizing species, which are monomeric Zr-hydroperoxo and peroxo species. Small basic additives increase the electrophilicity of the catalyst and decrease its nucleophilicity. HR-ESI-MS has identified a dimeric, most likely, bridging hydroperoxo species [{PW11O39Zr}2(μ-O)(μ-OOH)]9-, which may account for the improved epoxidation selectivity and regioselectivity toward sterically hindered C═C bonds.

Photocatalytic cyclohexane oxidation and epoxidation using hedgehog particles

Inorganic particles are effective photocatalysts for the liquid-state production of organic precursors and monomers at ambient conditions. However, poor colloidal stability of inorganic micro- and nanoparticles in low-polarity solvents limits their utilization as heterogeneous catalysts and coating them with surfactants drastically reduces their catalytic activity. Here we show that effective photo-oxidation of liquid cyclohexane (CH) is possible using spiky particles from metal oxides with hierarchical structure combining micro- and nanoscale structural features engineered for enhanced dispersibility in CH. Nanoscale ZnO spikes are assembled radially on α-Fe₂O₃ microcube cores to produce complex 'hedgehog' particles (HPs). The 'halo' of stiff spikes reduces van der Waals attraction, preventing aggregation of the catalytic particles. Photocatalysis in Pickering emulsions formed by HPs with hydrogen peroxide provides a viable pathway to energy-efficient alkane oxidation in the liquid state. Additionally, HPs enable a direct chemical pathway from alkanes to epoxides at ambient conditions, specifically to cyclohexene oxide, indicating that the structure of HPs has a direct effect on the recombination of ion-radicals during the hydrocarbon oxidation. These findings demonstrate the potential of inorganic photocatalysts with complex architecture for 'green' catalysis.

Peroxidative Oxidation of Cyclohexane Using 3d Metal Complexes with Hydrazone-Derived Ligands as Catalysts: Exploring (Un)Conventional Conditions

Two tetranuclear and two mononuclear Cu(II) complexes with arylhydrazones of malononitrile derived ligands (compounds 1–2 and 3–4, respectively), one trinuclear Co(II/III) complex with an arylhydrazone of acetoacetanilide (5) and one tetranuclear Zn(II) complex of 3-(2-carboxyphenyl-hydrazone)pentane-2,4-dione (6) were screened as potential catalysts in the peroxidative oxidation of cyclohexane by aqueous H2O2 in acetonitrile. The best results were attained in the presence of pyrazine-2-carboxylic acid (PCA) with 1 (26% yield, TON = 52.0) and with 2 (24%, TON = 48.0) after 4 h at 40 °C. In the presence of complexes 5 and 6, no oxygenated products were detected in the studied conditions. The employment of non-conventional conditions like supercritical carbon dioxide (scCO2) as reaction medium or microwave (MW) irradiation was assessed for complexes 1 and 2. After 6 h in acetonitrile–scCO2, at 50 °C and with HNO3 as promoter, only 17% yield was achieved using 1 as catalyst, and 21% using 2. Total yields of oxygenates up to 14 (with 1) and 13% (2) and TOFs of 56.0 and 52.0 h−1, respectively, were obtained working under MW irradiation at 70 °C and for the much shorter time of 0.5 h.

Cagelike Octacopper Methylsilsesquioxanes: Self-Assembly in the Focus of Alkaline Metal Ion Influence-Synthesis, Structure, and Catalytic Activity

A family of unusual octacopper cage methylsilsesquioxanes 1-4 were prepared and characterized. Features of their cagelike (prismatic) structure were established using X-ray diffraction studies. Effects of distortion of prismatic cages 1-4 due to variation of (i) additional alkaline metal ions (K, Rb, or Cs), (ii) combination of solvating ligands, and (iii) nature of encapsulating species were found. Opportunities for the design of supramolecular 1D extended structures were found. These opportunities are based on (i) formate linkers between copper centers (in the case of Cu₈K₂-based compound 2) or (ii) crown ether-like contacts between cesium ions and siloxane cycles (in the case of Cu₈Cs₂-based compound 4). Cu₈Cs₂-complex 4 was evaluated in the catalysis of alkanes and alcohols. Complex 4 exhibits high catalytic activity. The yield of cyclohexane oxidation products is 35%. The presence of nitric acid is necessary as a co-catalyst. The oxidation of alcohols with the participation of complex 4 as a catalyst and tert-butyl hydroperoxide as an oxidizer also proceeds in high yields of up to 98%.

Hybrid Silsesquioxane/Benzoate Cu7-Complexes: Synthesis, Unique Cage Structure, and Catalytic Activity

A series of phenylsilsesquioxane-benzoate heptacopper complexes 1-3 were synthesized and characterized by X-ray crystallography. Two parallel routes of toluene spontaneous oxidation (into benzyl alcohol and benzoate) assisted the formation of the cagelike structure 1. A unique multi-ligation of copper ions (from (i) silsesquioxane, (ii) benzoate, (iii) benzyl alcohol, (iv) pyridine, (v) dimethyl-formamide and (vi) water ligands) was found in 1. Directed self-assembly using benzoic acid as a reactant afforded complexes 2-3 with the same main structural features as for 1, namely heptanuclear core coordinated by (i) two distorted pentameric cyclic silsesquioxane and (ii) four benzoate ligands, but featuring other solvate surroundings. Complex 3 was evaluated as a catalyst for the oxidation of alkanes to alkyl hydroperoxides and alcohols to ketones with hydrogen peroxide and tert-butyl hydroperoxide, respectively, at 50 °C in acetonitrile. The maximum yield of cyclohexane oxidation products as high as 32% was attained. The oxidation reaction results in a mixture of cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone. Upon the addition of triphenylphosphine, the cyclohexyl hydroperoxide is completely converted to cyclohexanol. The specific regio- and chemoselectivity in the oxidation of n-heptane and methylcyclohexane, respectively, indicate the involvement of of hydroxyl radicals. Complex 3 exhibits a high activity in the oxidation of alcohols.

A Combined Experimental and Theoretical Investigation of Oxidation Catalysis by cis-[VIV(O)(Cl/F)(N4)]+ Species Mimicking the Active Center of Metal-Enzymes

Reaction of V^IVOCl₂ with the nonplanar tetradentate N₄ bis-quinoline ligands yielded four oxidovanadium(IV) compounds of the general formula cis-[V^IV(O)(Cl)(N₄)]Cl. Sequential treatment of the two nonmethylated N₄ oxidovanadium(IV) compounds with KF and NaClO₄ resulted in the isolation of the species with the general formula cis-[V^IV(O)(F)(N₄)]ClO₄. In marked contrast, the methylated N₄ oxidovanadium(IV) derivatives are inert toward KF reaction due to steric hindrance, as evidenced by EPR and theoretical calculations. The oxidovanadium(IV) compounds were characterized by single-crystal X-ray structure analysis, cw EPR spectroscopy, and magnetic susceptibility. The crystallographic characterization showed that the vanadium compounds have a highly distorted octahedral coordination environment and the d(V^IV–F) = 1.834(1) Å is the shortest to be reported for (oxido)(fluorido)vanadium(IV) compounds. The experimental EPR parameters of the V^IVO²⁺ species deviate from the ones calculated by the empirical additivity relationship and can be attributed to the axial donor atom trans to the oxido group and the distorted V^IV coordination environment. The vanadium compounds act as catalysts toward alkane oxidation by aqueous H₂O₂ with moderate ΤΟΝ up to 293 and product yields of up to 29% (based on alkane); the vanadium(IV) is oxidized to vanadium(V), and the ligands remain bound to the vanadium atom during the catalysis, as determined by ⁵¹V and ¹H NMR spectroscopies. The cw X-band EPR studies proved that the mechanism of the catalytic reaction is through hydroxyl radicals. The chloride substitution reaction in the cis-[V^IV(O)(Cl)(N₄)]⁺ species by fluoride and the mechanism of the alkane oxidation were studied by DFT calculations.

Highly distorted N₄−oxidovanadium(IV), cis-[V^IV(=O)(Cl/F)(N₄)]⁺, compounds mimicking the distorted coordination environment of the metal-ion in metal-enzymes’ active center catalyze oxidation of alkanes. The structural distortion results in significant deviation of the cw EPR parameters from the expected ones and equilibrium between six- and five- coordinate species in solution. DFT studies show that distortion, steric hindrance, and the coordinated halogen define the catalytic mechanism of oxidation of alkanes by cis-[V^IV(=O)(Cl/F)(N₄)]⁺ compounds.

Mononuclear Oxidovanadium(IV) Complexes with BIAN Ligands: Synthesis and Catalytic Activity in the Oxidation of Hydrocarbons and Alcohols with Peroxides

Reactions of VCl3 with 1,2-Bis[(4-methylphenyl)imino]acenaphthene (4-Me-C6H4-bian) or 1,2-Bis[(2-methylphenyl)imino]acenaphthene (2-Me-C6H4-bian) in air lead to the formation of [VOCl2(R-bian)(H2O)] (R = 4-Me-C6H4 (1), 2-Me-C6H4 (2)). Thes complexes were characterized by IR and EPR spectroscopy as well as elemental analysis. Complexes 1 and 2 have high catalytic activity in the oxidation of hydrocarbons with hydrogen peroxide and alcohols with tert-butyl hydroperoxide in acetonitrile at 50 °С. The product yields are up to 40% for cyclohexane. Of particular importance is the addition of 2-pyrazinecarboxylic acid (PCA) as a co-catalyst. Oxidation proceeds mainly with the participation of free hydroxyl radicals, as evidenced by taking into account the regio- and bond-selectivity in the oxidation of n-heptane and methylcyclohexane, as well as the dependence of the reaction rate on the initial concentration of cyclohexane.

A Novel Family of Cage-like (CuLi, CuNa, CuK)-phenylsilsesquioxane Complexes with 8-hydroxyquinoline Ligands: Synthesis, Structure, and Catalytic Activity

The first examples of metallasilsesquioxane complexes, including ligands of the 8-hydroxyquinoline family 1–9, were synthesized, and their structures were established by single crystal X-ray diffraction using synchrotron radiation. Compounds 1–9 tend to form a type of sandwich-like cage of Cu₄M₂ nuclearity (M = Li, Na, K). Each complex includes two cisoid pentameric silsesquioxane ligands and two 8-hydroxyquinoline ligands. The latter coordinates the copper ions and corresponding alkaline metal ions (via the deprotonated oxygen site). A characteristic (size) of the alkaline metal ion and a variation of characteristics of nitrogen ligands (8-hydroxyquinoline vs. 5-chloro-8-hydroxyquinoline vs. 5,7-dibromo-8-hydroxyquinoline vs. 5,7-diiodo-8-hydroxyquinoline) are highly influential for the formation of the supramolecular structure of the complexes 3a, 5, and 7–9. The Cu₆Na₂-based compound 2 exhibits high catalytic activity towards the oxidation of (i) hydrocarbons by H₂O₂ activated with HNO₃, and (ii) alcohols by tert-butyl hydroperoxide. Studies of kinetics and their selectivity has led us to conclude that it is the hydroxyl radicals that play a crucial role in this process.

Acetone Factor in the Design of Cu4-, Cu6-, and Cu9-Based Cage Coppersilsesquioxanes: Synthesis, Structural Features, and Catalytic Functionalization of Alkanes

The present study describes a new feature in the self-assembly of cagelike copperphenylsilsesquioxanes: the strong influence of acetone solvates on cage structure formation. By this simple approach, a series of novel tetra-, hexa-, or nonacoppersilsesquioxanes were isolated and characterized. In addition, several new complexes of Cu₄ or Cu₆ nuclearity bearing additional nitrogen-based ligands (ethylenediamine, 2,2'-bipyridine, phenanthroline, bathophenanthroline, or neocuproine) were produced. Single-crystal X-ray diffraction studies established molecular architectures of all of the synthesized products. Several coppersilsesquioxanes represent a novel feature of cagelike metallasilsesquioxane (CLMS) in terms of molecular topology. A Cu₄-silsesquioxane complex with ethylenediamine (En) ligands was isolated via the unprecedented self-assembly of a partly condensed framework of silsesquioxane ligands, followed by the formation of a sandwich-like cage. Two prismatic Cu₆ complexes represent the different conformers─regular and elliptical hexagonal prisms, "cylinders", determined by the different orientations of the coordinated acetone ligands ("shape-switch effect"). A heterometallic Cu₄Na₄-sandwich-like derivative represents the first example of a metallasilsesquioxane complex with diacetone alcohol ligands formed in situ due to acetone condensation reaction. As a selected example, the compound [(Ph₆Si₆O₁₁)₂Cu₄En₂]·(acetone)₂ was explored in homogeneous oxidation catalysis. It catalyzes the oxidation of alkanes to alkyl hydroperoxides with hydrogen peroxide and the oxidation of alcohols to ketones with tert-butyl hydroperoxide. Radical species take part in the oxidation of alkanes. Besides, [(Ph₆Si₆O₁₁)₂Cu₄En₂]·(acetone)₂ catalyzes the mild oxidative functionalization of gaseous alkanes (ethane, propane, n-butane, and i-butane). Two different model reactions were investigated: (1) the oxidation of gaseous alkanes with hydrogen peroxide to give a mixture of oxygenates (alcohols, ketones, or aldehydes) and (2) the carboxylation of C_n gaseous alkanes with carbon monoxide, water, and potassium peroxodisulfate to give C_n+1 carboxylic acids (main products), along with the corresponding C_n oxygenates. For these reactions, the effects of acid promoter, reaction time, and substrate scope were explored. As expected for free-radical-type reactions, the alkane reactivity follows the trend C₂H₆ < C₃H₈ < n-C₄H₁₀ < i-C₄H₁₀. The highest total product yields were observed in the carboxylation of i-butane (up to 61% based on i-C₄H₁₀). The product yields and catalyst turnover numbers (TONs) are remarkable, given an inertness of gaseous alkanes and very mild reaction conditions applied (low pressures, 50-60 °C temperatures).

Novel Copper(II) Complexes with Dipinodiazafluorene Ligands: Synthesis, Structure, Magnetic and Catalytic Properties

The reactions of CuX₂ (X = Cl, Br) with dipinodiazafluorenes yielded four new complexes [CuX₂L₁]₂ (X = Cl (1), Br (2), L₁ = (1R,3R,8R,10R)-2,2,9,9-Tetramethyl-3,4,7,8,9,10-hexahydro-1H-1,3:8,10-dimethanocyclopenta [1,2-b:5,4-b’]diquinolin-12(2H)-one) and [(CuX₂)₂L₂]n (X = Cl (3), Br (4), L₂ = (1R,3R,8R,10R,1’R,3’R,8’R,10’R)-2,2,2’,2’,9,9,9’,9’-Octamethyl-1,1’,2,2’,3,3’,4,4’,7,7’,8,8’,9,9’,10,10’-hexadecahydro-1,3:1’,3’:8,10:8’,10’-tetramethano-12,12’-bi(cyclopenta [1,2-b:5,4-b’]diquinolinylidene). The complexes were characterized by IR and EPR spectroscopy, HR-ESI-MS and elemental analysis. The crystal structures of compounds 1, 2 and 4 were determined by X-ray diffraction (XRD) analysis. Complexes 1–2 have a monomeric structure, while complex 4 has a polymeric structure due to additional coordinating N,N sites in L₂. All complexes contain a binuclear fragment {Cu₂(μ-X)_2×2} (X = Cl, Br) in their structures. Each copper atom has a distorted square-pyramidal coordination environment formed by two nitrogen atoms and three halogen atoms. The Cu-N_ax distance is elongated compared to Cu-N_eq. The EPR spectra of compounds 1–4 in CH₃CN confirm their paramagnetic nature due to the d⁹ electronic configuration of the copper(II) ion. The magnetic properties of all compounds were studied by the method of static magnetic susceptibility. For complexes 1 and 2, the effective magnetic moments are µ_eff ≈ 1.87 and 1.83 µ_B (per each Cu²⁺ ion), respectively, in the temperature range 50–300 K, which are close to the theoretical spin value (1.73 µ_B). Ferromagnetic exchange interactions between Cu(II) ions inside {Cu₂(μ-X)₂X₂} (X = Cl, Br) dimers (J/k_B ≈ 25 and 31 K for 1 and 2, respectively) or between dimers (θ′ ≈ 0.30 and 0.47 K for 1 and 2, respectively) were found at low temperatures. For compounds 3 and 4, the magnetic susceptibility is well described by the Curie–Weiss law in the temperature range 1.77–300 K with µ_eff ≈ 1.72 and 1.70 µ_B for 3 and 4, respectively, and weak antiferromagnetic interactions (θ ≈ −0.4 K for 3 and −0.65 K for 4). Complexes 1–4 exhibit high catalytic activity in the oxidation of alkanes and alcohols with peroxides. The maximum yield of cyclohexane oxidation products reached 50% (complex 3). Based on the data on the study of regio- and bond-selectivity, it was concluded that hydroxyl radicals play a decisive role in the oxidation reaction. The initial products in reactions with alkanes are alkyl hydroperoxides.

Diastereomeric dinickel(II) complexes with non-innocent bis(octaazamacrocyclic) ligands: isomerization, spectroelectrochemistry, DFT calculations and use in catalytic oxidation of cyclohexane

Diastereomeric dinickel(II) complexes with bis-octaazamacrocyclic 15-membered ligands [Ni(L1-3-L1-3)Ni] (4-6) have been prepared by oxidative dehydrogenation of nickel(II) complexes NiL1-3 (1-3) derived from 1,2- and 1,3-diketones and S-methylisothiocarbohydrazide. The compounds were characterized by elemental analysis, ESI mass spectrometry, and IR, UV-vis, 1H NMR, and 13C NMR spectroscopy. Single crystal X-ray diffraction (SC-XRD) confirmed the isolation of the anti and syn isomers of bis-octaazamacrocyclic dinickel(II) complexes 4a and 4s, the syn-configuration of 5s and the anti-configuration of the dinickel(II) complex 6a. Dimerization of prochiral nickel(II) complexes 1-3 generates two chiral centers at the bridging carbon atoms. The anti-complexes were isolated as meso-isomers (4a and 6a) and the syn-compounds as racemic mixtures of R,R/S,S-enantiomers (4s and 5s). The syn-anti isomerization (epimerization) of the isolated complexes in chloroform was disclosed. The isomerization kinetics of 5a was monitored at five different temperatures ranging from 20 °C to 50 °C by 1H NMR spectroscopy indicating the clean conversion of 5a into 5s. The activation barrier determined from the temperature dependence of the rate constants via the Eyring equation was found to be ΔH‡ = 114 ± 1 kJ mol-1 with activation entropy ΔS‡ = 13 ± 3 J K-1 mol-1. The complexes contain two low-spin nickel(II) ions in a square-planar coordination environment. The electrochemical behavior of 4a, 4s, 5s and 6a and the electronic structure of the oxidized species were studied by UV-vis-NIR-spectroelectrochemistry (SEC) and DFT calculations indicating the redox non-innocent behavior of the complexes. The dinickel(II) complexes 4a, 4s, 5s and 6a/6s were investigated as catalysts for microwave-assisted solvent-free oxidation of cyclohexane by tert-butyl hydroperoxide to produce a mixture of cyclohexanone and cyclohexanol (KA oil). The best value for KA oil yield (16%) was obtained with a mixture of 6a/6s after 2 h of microwave irradiation at 100 °C.

A Keggin-type polyoxometalate-based metal-organic complex as a highly efficient heterogeneous catalyst for the selective oxidation of alkylbenzenes

The direct oxidation of C-H bonds in organic materials into necessary oxygen-containing compounds under mild conditions has attracted increasing attention. A Keggin-type polyoxometalate-based metal-organic complex (MOC), [CuII4Cu^I(H₂trz)₄(C₂O₄)(H₂O)₄(H₃PW_11.18CuII0.82O₄₀)]·8H₂O (1) (H₃trz = 1,2,4-triazole), was designed and synthesized under hydrothermal conditions, and was structurally characterized by single crystal X-ray diffraction, PXRD, IR spectroscopy, TGA, and XPS. Complex 1 is a 3D 4,6,8-connected architecture derived from [Cu₅(H₂trz)₄(C₂O₄)₂(H₂O)₂]_n units and [PW_11.18Cu_0.82O₄₀]^6- anions, which can catalytically oxidize various types of alkylbenzenes. Gas chromatographic analysis showed that complex 1 as a heterogeneous catalyst could effectively catalyze the oxidation of diphenylmethane with 93% conversion and 99% selectivity within 6 h. In addition, the conversion for the catalytic oxidation of ethylbenzene was 96% with 99% selectivity. Compared with some reported catalysts, complex 1 exhibited a better catalytic effect and lower reaction time. Meanwhile, the catalytic oxidation of other benzyl derivatives with complex 1 was also investigated, which indicated that complex 1 possessed excellent catalytic performance.

Organometallic 3d transition metal NHC complexes in oxidation catalysis

The development of processes for the selective oxidation of hydrocarbons is a major focus in catalysis research. Making this process simultaneously environmentally friendly is still challenging. 3d transition metals are...

Homogeneous oxidation of C–H bonds with m-CPBA catalysed by a Co/Fe system: mechanistic insights from the point of view of the oxidant

A Co/Fe system efficiently catalyses the oxidation of C–H bonds with m-CPBA. The nitric acid promoter hampers the m-CPBA homolysis, suppressing the free radical activity. Experimental and computational data evidence a concerted oxidation mechanism.

Remote Amino Acid Recognition Enables Effective Hydrogen Peroxide Activation at a Manganese Oxidation Catalyst

Precise delivery of a proton plays a key role in O₂ activation at iron oxygenases, enabling the crucial O−O cleavage step that generates the oxidizing high‐valent metal–oxo species. Such a proton is delivered by acidic residues that may either directly bind the iron center or lie in its second coordination sphere. Herein, a supramolecular strategy for enzyme‐like H₂O₂ activation at a biologically inspired manganese catalyst, with a nearly stoichiometric amount (1–1.5 equiv) of a carboxylic acid is disclosed. Key for this strategy is the incorporation of an α,ω‐amino acid in the second coordination sphere of a chiral catalyst via remote ammonium‐crown ether recognition. The properly positioned carboxylic acid function enables effective activation of hydrogen peroxide, leading to catalytic asymmetric epoxidation. Modulation of both amino acid and catalyst structure can tune the efficiency and the enantioselectivity of the reaction, and a study on the oxidative degradation pathway of the system is presented.

A novel o-vanillin Fe(III) complex catalytically active in C-H oxidation: exploring the magnetic exchange interactions and spectroscopic properties with different DFT functionals

The novel complex [Fe^IIICl(L)₂(H₂O)] (1) was synthesized by interaction of iron(III) chloride with ethanol solution of o-vanillin (HL) and characterized by IR, UV/Vis spectroscopy, thermogravimetry and single crystal X-ray diffraction analysis. The molecules of 1 in the solid state are joined into supramolecular dimeric units, where a set of strong hydrogen bonds predefines the structure of the dimer according to the "key-lock" principle. From the Hirshfield surface analysis the contribution of π⋯π stacking to the overall stabilization of the dimer was found to be negligible. Broken symmetry DFT calculations suggested the presence of long-range antiferromagnetic interactions (J = -0.12 cm^-1 for H = -JS₁S₂ formalism) occurring through the Fe-O⋯O-Fe pathway, as evidenced by the studies of the model dimers where the water molecules were substituted by acetonitrile and acetone ones. The benchmark studies using a set of literature examples and various DFT functionals revealed the hybrid-GGA B3LYP as the best one for prediction of Fe^III⋯Fe^III antiferromagnetic exchange couplings of small magnitude. Magnetic susceptibility measurements confirmed antiferromagnetic coupling between the metal atoms in 1 with a coupling constant of -0.35 cm^-1. Catalytic studies demonstrated that 1 acts as an efficient catalyst in the oxidation of cyclohexane with hydrogen peroxide in the presence of nitric acid promoter and under mild conditions (yield up to 37% based on the substrate), while tert-butylhydroperoxide (TBHP) and m-chloroperoxybenzoic acid (m-CPBA) as oxidants exhibit less efficiency. Combined UV/TDDFT studies evidence the structural rearrangement of 1 in acetonitrile with the formation of [Fe^IIICl(L)₂(CH₃CN)] species. The TDDFT benchmark using nine common DFT functionals and two model compounds (o-vanillin and [Fe^III(H₂O)₆]³⁺ ion) support the hybrid meta-GGA M06-2X functional as the one most correctly predicting the excited state structure for the Fe(III) complexes, under the conditions studied.

Vanadium(IV) Complexes with Methyl-Substituted 8-Hydroxyquinolines: Catalytic Potential in the Oxidation of Hydrocarbons and Alcohols with Peroxides and Biological Activity

Methyl-substituted 8-hydroxyquinolines (Hquin) were successfully used to synthetize five-coordinated oxovanadium(IV) complexes: [VO(2,6-(Me)₂-quin)₂] (1), [VO(2,5-(Me)₂-quin)₂] (2) and [VO(2-Me-quin)₂] (3). Complexes 1-3 demonstrated high catalytic activity in the oxidation of hydrocarbons with H₂O₂ in acetonitrile at 50 °C, in the presence of 2-pyrazinecarboxylic acid (PCA) as a cocatalyst. The maximum yield of cyclohexane oxidation products attained was 48%, which is high in the case of the oxidation of saturated hydrocarbons. The reaction leads to the formation of a mixture of cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone. When triphenylphosphine is added, cyclohexyl hydroperoxide is completely converted to cyclohexanol. Consideration of the regio- and bond-selectivity in the oxidation of n-heptane and methylcyclohexane, respectively, indicates that the oxidation proceeds with the participation of free hydroxyl radicals. The complexes show moderate activity in the oxidation of alcohols. Complexes 1 and 2 reduce the viability of colorectal (HCT116) and ovarian (A2780) carcinoma cell lines and of normal dermal fibroblasts without showing a specific selectivity for cancer cell lines. Complex 3 on the other hand, shows a higher cytotoxicity in a colorectal carcinoma cell line (HCT116), a lower cytotoxicity towards normal dermal fibroblasts and no effect in an ovarian carcinoma cell line (order of magnitude HCT116 > fibroblasts > A2780).

Time-Dependent Self-Assembly of Copper(II) Coordination Polymers and Tetranuclear Rings: Catalysts for Oxidative Functionalization of Saturated Hydrocarbons

This study describes a time-dependent self-assembly generation of new copper(II) coordination compounds from an aqueous-medium reaction mixture composed of copper(II) nitrate, H₃bes biobuffer (N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid), ammonium hydroxide, and benzenecarboxylic acid, namely, 4-methoxybenzoic (Hfmba) or 4-chlorobenzoic (Hfcba) acid. Two products were isolated from each reaction, namely, 1D coordination polymers [Cu₃(μ₃-OH)₂(μ-fmba)₂(fmba)₂(H₂O)₂]_n (1) or [Cu₂(μ-OH)₂(μ-fcba)₂]_n (2) and discrete tetracopper(II) rings [Cu₄(μ-Hbes)₃(μ-H₂bes)(μ-fmba)]·2H₂O (3) or [Cu₄(μ-Hbes)₃(μ-H₂bes)(μ-fcba)]·4H₂O (4), respectively. These four compounds were obtained as microcrystalline air-stable solids and characterized by standard methods, including the single-crystal X-ray diffraction. The structures of 1 and 2 feature distinct types of metal-organic chains driven by the μ₃- or μ-OH^- ligands along with the μ-benzenecarboxylate linkers. The structures of 3 and 4 disclose the chairlike Cu₄ rings assembled from four μ-bridging and chelating aminoalcoholate ligands along with μ-benzenecarboxylate moieties playing a core-stabilizing role. Catalytic activity of 1-4 was investigated in two model reactions, namely, (a) the mild oxidation of saturated hydrocarbons with hydrogen peroxide to form alcohols and ketones and (b) the mild carboxylation of alkanes with carbon monoxide, water, and peroxodisulfate to generate carboxylic acids. Cyclohexane and propane were used as model cyclic and gaseous alkanes, while the substrate scope also included cyclopentane, cycloheptane, and cyclooctane. Different reaction parameters were investigated, including an effect of the acid cocatalyst and various selectivity parameters. The obtained total product yields (up to 34% based on C₃H₈ or up to 47% based on C₆H₁₂) in the carboxylation of propane and cyclohexane are remarkable taking into account an inertness of these saturated hydrocarbons and low reaction temperatures (50-60 °C). Apart from notable catalytic activity, this study showcases a novel time-dependent synthetic strategy for the self-assembly of two different Cu(II) compounds from the same reaction mixture.

Novel Oxidovanadium Complexes with Redox-Active R-Mian and R-Bian Ligands: Synthesis, Structure, Redox and Catalytic Properties

A new monoiminoacenaphthenone 3,5-(CF₃)₂C₆H₃-mian (complex 2) was synthesized and further exploited, along with the already known monoiminoacenaphthenone dpp-mian, to obtain oxidovanadium(IV) complexes [VOCl₂(dpp-mian)(CH₃CN)] (3) and [VOCl(3,5-(CF₃)₂C₆H₃-bian)(H₂O)][VOCl₃(3,5-(CF₃)₂C₆H₃-bian)]·2.85DME (4) from [VOCl₂(CH₃CN)₂(H₂O)] (1) or [VCl₃(THF)₃]. The structure of all compounds was determined using X-ray structural analysis. The vanadium atom in these structures has an octahedral coordination environment. Complex 4 has an unexpected structure. Firstly, it contains 3,5-(CF₃)₂C₆H₃-bian instead of 3,5-(CF₃)₂C₆H₃-mian. Secondly, it has a binuclear structure, in contrast to 3, in which two oxovanadium parts are linked to each other through V=O···V interaction. This interaction is non-covalent in origin, according to DFT calculations. In structures 2 and 3, non-covalent π-π staking interactions between acenaphthene moieties of the neighboring molecules (distances are 3.36–3.40 Å) with an estimated energy of 3 kcal/mol were also found. The redox properties of the obtained compounds were studied using cyclic voltammetry in solution. In all cases, the reduction processes initiated by the redox-active nature of the mian or bian ligand were identified. The paramagnetic nature of complexes 3 and 4 has been proven by EPR spectroscopy. Complexes 3 and 4 exhibited high catalytic activity in the oxidation of alkanes and alcohols with peroxides. The yields of products of cyclohexane oxidation were 43% (complex 3) and 27% (complex 4). Based on the data regarding the study of regio- and bond-selectivity, it was concluded that hydroxyl radicals play the most crucial role in the reaction. The initial products in the reactions with alkanes are alkyl hydroperoxides, which are easily reduced to their corresponding alcohols by the action of triphenylphosphine (PPh₃). According to the DFT calculations, the difference in the catalytic activity of 3 and 4 is most likely associated with a different mechanism for the generation of ^●OH radicals. For complex 4 with electron-withdrawing CF₃ substituents at the diimine ligand, an alternative mechanism, different from Fenton’s and involving a redox-active ligand, is assumed.

Cu6- and Cu8-Cage Sil- and Germsesquioxanes: Synthetic and Structural Features, Oxidative Rearrangements, and Catalytic Activity

This study reports intriguing features in the self-assembly of cage copper(II) silsesquioxanes in the presence of air. Despite the wide variation of solvates used, a series of prismatic hexanuclear Cu₆ cages (1-5) were assembled under mild conditions. In turn, syntheses at higher temperatures are accompanied by side reactions, leading to the oxidation of solvates (methanol, 1-butanol, and tetrahydrofuran). The oxidized solvent derivatives then specifically participate in the formation of copper silsesquioxane cages, allowing the isolation of several unusual Cu₈-based (6 and 7) and Cu₆-based (8) complexes. When 1,4-dioxane was applied as a reaction medium, deep rearrangements occurred (with a total elimination of silsesquioxane ligands), causing the formation of mononuclear copper(II) compounds bearing oxidized dioxane fragments (9 and 11) or a formate-driven 1D coordination polymer (10). Finally, a "directed" self-assembly of sil- and germsesquioxanes from copper acetate (or formate) resulted in the corresponding acetate (or formate) containing Cu₆ cages (12 and 13) that were isolated in high yields. The structures of all of the products 1-13 were established by single-crystal X-ray diffraction, mainly based on the use of synchrotron radiation. Moreover, the catalytic activity of compounds 12 and 13 was evaluated toward the mild homogeneous oxidation of C₅-C₈ cycloalkanes with hydrogen peroxide to form a mixture of the corresponding cyclic alcohols and ketones.

Oxido- and Dioxido-Vanadium(V) Complexes Supported on Carbon Materials: Reusable Catalysts for the Oxidation of Cyclohexane

Oxidovanadium(V) and dioxidovanadium(V) compounds, [VO(OEt)L] (1) and [Et3NH][VO2L] (2), were synthesized using an aroylhydrazone Schiff base (5-bromo-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H2L). They were characterized by elemental analysis, Fourier-transform infrared spectroscopy (FT-IR), (1H and 51V) nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI-MS) and single crystal X-ray diffraction analyses. Both complexes were immobilized on functionalized carbon nanotubes and activated carbon. The catalytic performances of 1 and 2, homogenous and anchored on the supports, were evaluated for the first time towards the MW-assisted peroxidative oxidation (with tert-butylhydroperoxide, TBHP) of cyclohexane under heterogeneous conditions. The immobilization of 1 and 2 on functionalized carbon materials improved the efficiency of catalytic oxidation and allowed the catalyst recyclability with a well-preserved catalytic activity.

Oxidation of Organic Compounds with Peroxides Catalyzed by Polynuclear Metal Compounds

The review describes articles that provide data on the synthesis and study of the properties of catalysts for the oxidation of alkanes, olefins, and alcohols. These catalysts are polynuclear complexes of iron, copper, osmium, nickel, manganese, cobalt, vanadium. Such complexes for example are: [Fe2(HPTB)(m-OH)(NO3)2](NO3)2·CH3OH·2H2O, where HPTB-¼N,N,N0,N0-tetrakis(2-benzimidazolylmethyl)-2-hydroxo-1,3-diaminopropane; complex [(PhSiO1,5)6]2[CuO]4[NaO0.5]4[dppmO2]2, where dppm-1,1-bis(diphenylphosphino)methane; (2,3-η-1,4-diphenylbut-2-en-1,4-dione)undecacarbonyl triangulotriosmium; phenylsilsesquioxane [(PhSiO1.5)10(CoO)5(NaOH)]; bi- and tri-nuclear oxidovanadium(V) complexes [{VO(OEt)(EtOH)}2(L2)] and [{VO(OMe)(H2O)}3(L3)]·2H2O (L2 = bis(2-hydroxybenzylidene)terephthalohydrazide and L3 = tris(2-hydroxybenzylidene)benzene-1,3,5-tricarbohydrazide); [Mn2L2O3][PF6]2 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). For comparison, articles are introduced describing catalysts for the oxidation of alkanes and alcohols with peroxides, which are simple metal salts or mononuclear metal complexes. In many cases, polynuclear complexes exhibit higher activity compared to mononuclear complexes and exhibit increased regioselectivity, for example, in the oxidation of linear alkanes. The review contains a description of some of the mechanisms of catalytic reactions. Additionally presented are articles comparing the rates of oxidation of solvents and substrates under oxidizing conditions for various catalyst structures, which allows researchers to conclude about the nature of the oxidizing species. This review is focused on recent works, as well as review articles and own original studies of the authors.

Novel H-Bonded Synthons in Copper Supramolecular Frameworks with Aminoethylpiperazine-Based Ligands. Synthesis, Structure and Catalytic Activity

New Schiff base complexes [Cu2(HL1)(L1)(N3)3]∙2H2O (1) and [Cu2L2(N3)2]∙H2O (2) were synthesized. The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis. The HL1 ligand results from the condensation of salicylaldehyde and 1-(2-aminoethyl)piperazine, while a new organic ligand, H2L2, was formed by the dimerization of HL1 via a coupling of two piperazine rings of HL1 on a carbon atom coming from DMF solvent. The dinuclear building units in 1 and 2 are linked into complex supramolecular networks through hydrogen and coordination bondings, resulting in 2D and 1D architectures, respectively. Single-point and broken-symmetry DFT calculations disclosed negligible singlet-triplet splittings within the dinuclear copper fragments in 1 and 2. Catalytic studies showed a remarkable activity of 1 and 2 towards cyclohexane oxidation with H2O2 in the presence of nitric acid and pyridine as promoters and under mild conditions (yield of products up to 21%). Coordination compound 1 also acts as an active catalyst in the intermolecular coupling of cyclohexane with benzamide using di-tert-butyl peroxide (tBuOOtBu) as a terminal oxidant. Conversion of benzamide at 55% was observed after 24 h reaction time. By-product patterns and plausible reaction mechanisms are discussed.

Iron-Catalyzed Oxidation of 1-Phenylethanol and Glycerol With Hydrogen Peroxide in Water Medium: Effect of the Nitrogen Ligand on Catalytic Activity and Selectivity

The iron(II) complexes [Fe(bpy)₃](OTf)₂ (bpy = 2,2'-bipyridine; OTf = CF₃SO₃) (1) and [Fe(bpydeg)₃](OTf)₂ (bpydeg = N⁴,N⁴-bis(2-(2-methoxyethoxy)ethyl) [2,2'-bipyridine]-4,4'-dicarboxamide) (2), the latter being a newly synthesized ligand, were employed as catalyst precursors for the oxidation of 1-phenylethanol with hydrogen peroxide in water, using either microwave or conventional heating. With the same oxidant and medium the oxidation of glycerol was also explored in the presence of 1 and 2, as well as of two similar iron(II) complexes bearing tridentate ligands, i.e., [Fe(terpy)₂](OTf)₂ (terpy = 2, 6-di(2-pyridyl)pyridine) (3) and [Fe(bpa)₂](OTf)₂ (bpa = bis(2-pyridinylmethyl)amine) (4): in most reactions the major product formed was formic acid, although with careful tuning of the experimental conditions significant amounts of dihydroxyacetone were obtained. Addition of heterocyclic amino acids (e.g., picolinic acid) increased the reaction yields of most catalytic reactions. The effect of such additives on the evolution of the catalyst precursors was studied by spectroscopic (NMR, UV-visible) and ESI-MS techniques.

2-Methylimidazole Copper Iminodiacetates for the Adsorption of Oxygen and Catalytic Oxidation of Cyclohexane

The mixed-ligand copper(II) iminodiacetates [Cu(ida)(2-mim)(H₂O)₂]·H₂O (1), [Cu(ida)(2-mim)₂]·2H₂O (2), [Cu(ida)(2-mim)(H₂O)]_n·4.5nH₂O (3), and [Cu₂(ida)₂(2-mim)₂]_n·nH₂O (4) (H₂ida = iminodiacetic acid, 2-mim = 2-methylimidazole) were obtained from neutral or alkaline solutions at different temperatures. The novel complex 4 contains very small holes with diameters of 2.9 Å, which can adsorb O₂ selectively and reversibly between 1.89 to 29.90 bars, compared with the different gases of N₂, H₂, CO₂, and CH₄. This complex is stable up to 150 °C based on thermal analyses and XRD patterns. The four complexes show catalytic activities that facilitate the conversion of cyclohexane to cyclohexanol and cyclohexanone with hydrogen peroxide in a solution. The total conversion is 31% for 4.

Coordination Affinity of Cu(II)-Based Silsesquioxanes toward N,N-Ligands and Associated Skeletal Rearrangements: Cage and Ionic Products Exhibiting a High Catalytic Activity in Oxidation Reactions

An unusual skeletal rearrangement of piperazine into ethylenediamine has been observed for the first time as a result of an attempt to synthesize a piperazine-linked metal-organic framework (MOF) using cage Cu(II),Na-phenylsilsequixane as a potential building block. Instead of the expected "metallasilsesquioxane-based MOF", a Cu₆ complex 1 coordinated both by silsesquioxane and ethylenediamine ligands was isolated. An effort to reproduce this result via direct interaction of Cu-phenylsilsequioxane and ethylenediamine surprisingly afforded two other types of complexes, copper-sodium 2 and copper 3 ionic products. Cationic components in both products 2 and 3 are represented by (i) copper and sodium ions (in the case of 2) or (ii) copper ions exclusively (in the case of 3) coordinated by ethylenediamine ligands. Both complexes 2 and 3 include Si₆-based condensed silsesquioxane fragments serving as anionic components of the products. Symptomatically, the types of the Si₆-frameworks in 2 and 3 are drastically different. More specifically, the Si₆ unit in 2 is an unprecedented distorted silsesquioxane skeleton consisting of two condensed tetramembered rings. Structural features of compounds 1-3 were established by single crystal X-ray diffraction. Compound 2 was found to catalyze the oxidation of cyclohexane to cyclohexanol and cyclohexanone with H₂O₂ (a mixture of these products was obtained after adding PPh₃ to the reaction solution) as well as the transformation of cyclohexanol to cyclohexanone under the action of tert-butyl hydroperoxide.

Modelling a ‘histidine brace’ motif in mononuclear copper monooxygenases

A mononuclear copper complex bearing a ‘histidine brace’ is synthesised and characterised as an active-site model of mononuclear copper monooxygenases such as lytic polysaccharide monooxygenases (LPMOs) and particulate methane monooxygenase (pMMO).