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.

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.

Ionic liquid-stabilized vanadium oxo-clusters catalyzing alkane oxidation by regulating oligovanadates

The specific ionic liquid [TBA][Pic]-stabilized vanadium oxo-clusters exist in the form of a trimer and a dimer and are highly active for catalyzing C–H bond oxidation with H₂O₂ as an oxidant.

Synthesis, Structural Features, and Catalytic Activity of an Iron(II) 3D Coordination Polymer Driven by an Ether-Bridged Pyridine-Dicarboxylate

New iron(II) three-dimensional coordination polymer (3D CP), [Fe(µ3-Hcpna)2]n (1), was assembled under hydrothermal conditions from 5-(4’-carboxyphenoxy)nicotinic acid (H2cpna) as a trifunctional organic N,O-building block. This stable microcrystalline CP was characterized by standard methods for coordination compounds in the solid state (infrared spectroscopy, elemental analysis, thermogravimetric analysis, powder and single-crystal X-ray diffraction). Structure and topology of 1 were examined and permitted an identification of a 3,6-connected framework of the rtl topological type. In addition, compound 1 acts as effective catalyst precursor for oxidative functionalization of alkanes (propane and cyclic C5−C8 alkanes) under homogeneous catalysis conditions, namely for the oxidation of saturated hydrocarbons with H2O2/H+ system to produce ketones and alcohols, and for alkane carboxylation with CO/H2O/S2O82− system to obtain carboxylic acids. The influence of an acid promoter and substrate scope (propane and cyclic C5−C8 alkanes) were investigated.

Tetracopper(II) Cores Driven by an Unexplored Trifunctional Aminoalcohol Sulfonic Acid for Mild Catalytic C–H Functionalization of Alkanes

Three new tetracopper(II) coordination compounds were easily generated from Cu(NO3)2, a trifunctional aminoalcohol sulfonic acid (H3bes, N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid) as a principal building block, and a benzene carboxylic acid as a supporting ligand (i.e., benzoic (Hba), 4-hydroxybenzoic (Hfba), or 3-hydroxybenzoic (Hthba) acid). The obtained microcrystalline products, [Cu4(µ-Hbes)3(µ-H2bes)(µ-L)]·2H2O (L = ba− (1), fhba− (2), and thba− (3)), were fully characterized by FTIR (Fourier-transform infrared spectroscopy), elemental analysis, ESI-MS (Electrospray Ionisation Mass Spectrometry), and single-crystal X-ray diffraction methods. Compounds 1–3 were applied as effective homogeneous catalysts in the oxidative C−H functionalization of alkanes (cycloalkanes and propane). Two different model reactions were explored: (1) mild oxidation of alkanes with hydrogen peroxide to give alcohols and ketones, and (2) mild carboxylation of alkanes with carbon monoxide, water, and potassium peroxodisulfate to give carboxylic acids. For these reactions, effects of different parameters, as well as mechanistic and selectivity characteristics, were studied.

Interplay between H-bonding and interpenetration in an aqueous copper(ii)-aminoalcohol-pyromellitic acid system: self-assembly synthesis, structural features and catalysis

Two new copper(ii) coordination compounds, [Cu(H1.5mdea)2]2(H2pma) (1a) and [{Cu2(μ-Hmdea)2}2(μ4-pma)]n·2nH2O (1b), were self-assembled at different temperatures from the same multicomponent reaction system, comprising copper(ii) nitrate, N-methyldiethanolamine (H2mdea), pyromellitic acid (H4pma), and potassium hydroxide. Products 1a and 1b were isolated as microcrystalline solids and fully characterized and their structures were established by single-crystal X-ray diffraction. Compound 1a features the bis-aminoalcohol(ate) monocopper(ii) units and H2pma2- anions that are multiply interconnected by strong H-bonds into a firm 2D H-bonded layer. Compound 1b reveals the bis-aminoalcoholate dicopper(ii) motifs that are interlinked by the μ4-pma4- spacers into a 3D + 3D interpenetrated metal-organic framework. From a topological perspective, both networks of 1a and 1b are uninodal and driven by similar 4-connected H2pma2- or pma4- nodes, but result in distinct sql and dia topologies, respectively. Compound 1a was applied as an efficient catalyst for two model cycloalkane functionalization reactions: (1) oxidation by H2O2 to form cyclic alcohols and ketones and (2) hydrocarboxylation by CO/H2O and S2O82- to form cycloalkanecarboxylic acids. The substrate scope, effects of various reaction parameters, selectivity and mechanistic features were also investigated.

Extending the family of quinolone antibacterials to new copper derivatives: self-assembly, structural and topological features, catalytic and biological activity

New copper(ii) compounds with quinolone pefloxacin were synthesized and fully characterized; they exhibit notable catalytic activity and promising biological profiles.

Self-assembly generation, structural features, and oxidation catalytic properties of new aqua-soluble copper(ii)-aminoalcohol derivatives

Multidentate aminoalcohols were applied as unexplored building blocks to generate two novel Cu(ii) coordination compounds that act as efficient catalysts for the mild and acid-promoter-free oxidation of C₅–C₈ cycloalkanes.

Stereoselective Alkane Oxidation with meta-Chloroperoxybenzoic Acid (MCPBA) Catalyzed by Organometallic Cobalt Complexes

Cobalt pi-complexes, previously described in the literature and specially synthesized and characterized in this work, were used as catalysts in homogeneous oxidation of organic compounds with peroxides. These complexes contain pi-butadienyl and pi-cyclopentadienyl ligands: [(tetramethylcyclobutadiene)(benzene)cobalt] hexafluorophosphate, [(C₄Me₄)Co(C₆H₆)]PF₆ (1); diiodo(carbonyl)(pentamethylcyclopentadienyl)cobalt, Cp*Co(CO)I₂ (2); diiodo(carbonyl)(cyclopentadienyl)cobalt, CpCo(CO)I₂ (3); (tetramethylcyclobutadiene)(dicarbonyl)(iodo)cobalt, (C₄Me₄)Co(CO)₂I (4); [(tetramethylcyclobutadiene)(acetonitrile)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [(C₄Me₄)Co(bipy)(MeCN)]PF₆ (5); bis[dicarbonyl(B-cyclohexylborole)]cobalt, [(C₄H₄BCy)Co(CO)₂]₂ (6); [(pentamethylcyclopentadienyl)(iodo)(1,10-phenanthroline)cobalt] hexafluorophosphate, [Cp*Co(phen)I]PF₆ (7); diiodo(cyclopentadienyl)cobalt, [CpCoI₂]₂ (8); [(cyclopentadienyl)(iodo)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [CpCo(bipy)I]PF₆ (9); and [(pentamethylcyclopentadienyl)(iodo)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [Cp*Co(bipy)I]PF₆ (10). Complexes 1 and 2 catalyze very efficient and stereoselective oxygenation of tertiary C–H bonds in isomeric dimethylcyclohexanes with MCBA: cyclohexanols are produced in 39 and 53% yields and with the trans/cis ratio (of isomers with mutual trans- or cis-configuration of two methyl groups) 0.05 and 0.06, respectively. Addition of nitric acid as co-catalyst dramatically enhances both the yield of oxygenates and stereoselectivity parameter. In contrast to compounds 1 and 2, complexes 9 and 10 turned out to be very poor catalysts (the yields of oxygenates in the reaction with cis-1,2-dimethylcyclohexane were only 5%–7% and trans/cis ratio 0.8 indicated that the oxidation is not stereoselective). The chromatograms of the reaction mixture obtained before and after reduction with PPh₃ are very similar, which testifies that alkyl hydroperoxides are not formed in this oxidation. It can be thus concluded that the interaction of the alkanes with MCPBA occurs without the formation of free radicals. The complexes catalyze oxidation of alcohols with tert-butylhydroperoxide (TBHP). For example, tert-BuOOH efficiently oxidizes 1-phenylethanol to acetophenone in 98% yield if compound 1 is used as a catalyst.