Metal Complexes Containing Redox-Active Ligands in Oxidation of Hydrocarbons and Alcohols: A Review

Ligands are innocent when they allow oxidation states of the central atoms to be defined. A noninnocent (or redox) ligand is a ligand in a metal complex where the oxidation state is not clear. Dioxygen can be a noninnocent species, since it exists in two oxidation states, i.e., superoxide (O2−) and peroxide (O22−). This review is devoted to oxidations of C–H compounds (saturated and aromatic hydrocarbons) and alcohols with peroxides (hydrogen peroxide, tert-butyl hydroperoxide) catalyzed by complexes of transition and nontransition metals containing innocent and noninnocent ligands. In many cases, the oxidation is induced by hydroxyl radicals. The mechanisms of the formation of hydroxyl radicals from H2O2 under the action of transition (iron, copper, vanadium, rhenium, etc.) and nontransition (aluminum, gallium, bismuth, etc.) metal ions are discussed. It has been demonstrated that the participation of the second hydrogen peroxide molecule leads to the rapture of O–O bond, and, as a result, to the facilitation of hydroxyl radical generation. The oxidation of alkanes induced by hydroxyl radicals leads to the formation of relatively unstable alkyl hydroperoxides. The data on regioselectivity in alkane oxidation allowed us to identify an oxidizing species generated in the decomposition of hydrogen peroxide: (hydroxyl radical or another species). The values of the ratio-of-rate constants of the interaction between an oxidizing species and solvent acetonitrile or alkane gives either the kinetic support for the nature of the oxidizing species or establishes the mechanism of the induction of oxidation catalyzed by a concrete compound. In the case of a bulky catalyst molecule, the ratio of hydroxyl radical attack rates upon the acetonitrile molecule and alkane becomes higher. This can be expanded if we assume that the reactions of hydroxyl radicals occur in a cavity inside a voluminous catalyst molecule, where the ratio of the local concentrations of acetonitrile and alkane is higher than in the whole reaction volume. The works of the authors of this review in this field are described in more detail herein.

Copper(ii) complexes of functionalized 2,2':6',2''-terpyridines and 2,6-di(thiazol-2-yl)pyridine: structure, spectroscopy, cytotoxicity and catalytic activity

Six new copper(ii) complexes with 2,2':6',2''-terpyridine (4'-Rⁿ-terpy) [1 (R¹ = furan-2-yl), 2 (R² = thiophen-2-yl), and 3 (R³ = 1-methyl-1H-pyrrol-2-yl)] and 2,6-di(thiazol-2-yl)pyridine derivatives (Rⁿ-dtpy) [4 (R¹), 5 (R²), and 6 (R³)] have been synthesized by a reaction between copper(ii) chloride and the corresponding ligand. The complexes have been characterized by UV-vis and IR spectroscopy, and their structures have been determined by X-ray analysis. The antiproliferative potential of copper(ii) complexes of 2,2':6',2''-terpyridine and 2,6-di(thiazol-2-yl)pyridine derivatives towards human colorectal (HCT116) and ovarian (A2780) carcinoma as well as towards lung (A549) and breast adenocarcinoma (MCF7) cell lines was examined. Complex 1 and complex 6 were found to have the highest antiproliferative effect on A2780 ovarian carcinoma cells, particularly when compared with complex 2, 3 with no antiproliferative effect. The order of cytotoxicity in this cell line is 6 > 1 > 5 > 4 > 2 ≈ 3. Complex 2 seems to be much more specific towards colorectal carcinoma HCT116 and lung adenocarcinoma A549 cells. The viability loss induced by the complexes agrees with Hoechst 33258 staining and typical morphological apoptotic characteristics like chromatin condensation and nuclear fragmentation. The specificity towards different types of cell lines and the low cytotoxic activity towards healthy cells are of particular interest and are a positive feature for further developments. Complexes 1-6 were also tested in the oxidation of alkanes and alcohols with hydrogen peroxide and tert-butyl-hydroperoxide (TBHP). The most active catalyst 4 gave, after 120 min, 0.105 M of cyclohexanol + cyclohexanone after reduction with PPh₃. This concentration corresponds to a yield of 23% and TON = 210. Oxidation of cis-1,2-dimethylcyclohexane with m-CPBA catalyzed by 4 in the presence of HNO₃ gave a product of a stereoselective reaction (trans/cis = 0.47). Oxidation of secondary alcohols afforded the target ketones in yields up to 98% and TON = 630.

Oxidative functionalization of C–H compounds induced by the extremely efficient osmium catalysts (a review)

In recent years, osmium complexes have found applications not only in thecis-hydroxylation of olefins but also very efficient in the oxygenation of C–H compounds (saturated and aromatic hydrocarbons and alcohols) by hydrogen peroxide as well as organic peroxides.