Syntheses, Structures, and Catalytic Hydrocarbon Oxidation Properties of N-Heterocycle-Sulfonated Schiff Base Copper(II) Complexes

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.

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.

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.