Selective oxidation of amines to imines is one of the most studied reactions in the field of heterogeneous catalysis. Cs ion promoted mesoporous manganese oxide (meso Cs/MnO x ) was synthesized using inverse surfactant micelle as a soft template. The chemical and structural properties of the mesoporous manganese oxide material were characterized by powder X-ray diffraction (PXRD), nitrogen sorption, electron microscopy and X-ray photoelectron spectra (XPS). The meso-Cs/MnO x material presented aggregated nanocrystalline nature with monomodal mesoporous size distributions. The catalyst was found to be effective in oxidation of amines to imines under aerobic conditions. The meso Cs/MnO x exhibited oxidation of primary, secondary, cyclic, aromatic, and aliphatic amines to imines, where the conversions reached as high as >99%. The catalyst was also effective in oxidative cross condensation of two different amines to produce asymmetrically substituted imines. Surface active Mn 3+ species along with labile lattice oxygen were found to play an important role in the catalytic activity. Mild reaction conditions (air atmosphere and absence of any oxidative or basic promoters), ease of product separation by simple filtration and significant reusability make this mesoporous manganese oxide material an economical and ecofriendly catalyst for the syntheses of versatile imine derivatives.

Copper oxide supported on mesoporous manganese oxide (meso Cu/MnO x ) was synthesized by an inverse micelle templated evaporation induced self-assembly procedure. Controlled aggregation of nanoparticles and a monomodal size distribution of mesopores with tunable structural properties were observed. The material possessed superior catalytic activity in the aerobic oxidative coupling of terminal alkynes. Excellent conversion (>99% in most cases) and selectivity were observed in both homocoupling and cross-coupling of alkynes using the optimized reaction conditions. Use of air as the sole oxidant, avoidance of any kind of additives, ease of product separation, great functional group tolerability, wide synthetic scope, and superior reusability (up to eighth cycle) are the notable features of our catalytic protocol. While the reaction mechanism was elucidated, a synergistic cooperative effect between the copper and manganese has been established, which is responsible for the superior catalytic activity. The labile lattice oxygen of the meso Cu/MnO x played a vital role in deprotonation of the alkyne proton, as supported by TPD and TGA studies. Moreover, for the first time, we designed model complexes for the active sites of the catalyst by DFT calculations and provided a qualitative description of the coupling mechanism, which supports the experimental findings.

Herein, we report a heterogeneous, aerobic, additive-free and environmentally benign catalytic protocol for oxidative aromatization of saturated nitrogen-heterocycles using a mesoporous manganese oxide material. The aromatized products can be separated by easy filtration and the catalyst is reusable for at least four cycles. Mechanistic investigation provides evidence for radical intermediates, a multi-electron redox cycle between Mn centers, and an oxygen exchange mechanism.

The atomic-level tunability of molecular structures is a compelling reason to develop homogeneous catalysts for challenging reactions such as the electrochemical reduction of carbon dioxide to valuable C1–C n products. Of particular interest is methane, the largest component of natural gas. Herein, we report a series of three isomeric rhenium tricarbonyl complexes coordinated by the asymmetric diimine ligands 2-(isoquinolin-1-yl)-4,5-dihydrooxazole (quin-1-oxa), 2-(quinolin-2-yl)-4,5-dihydrooxazole (quin-2-oxa), and 2-(isoquinolin-3-yl)-4,5-dihydrooxazole (quin-3-oxa) that catalyze the reduction of CO2 to carbon monoxide and methane, albeit the latter with a low efficiency. To our knowledge, these complexes are the first examples of rhenium­(I) catalysts capable of converting carbon dioxide into methane. Re­(quin-1-oxa)­(CO)3Cl (1), Re­(quin-2-oxa)­(CO)3Cl (2), and Re­(quin-3-oxa)­(CO)3Cl (3) were characterized and studied using a variety of electrochemical and spectroscopic techniques. In bulk electrolysis experiments, the three complexes reduce CO2 to CO and CH4. When the controlled-potential electrolysis experiments are performed at −2.5 V (vs Fc+/0) and in the presence of the Brønsted acid 2,2,2-trifluoroethanol, methane is produced with turnover numbers that range from 1.3 to 1.8. Isotope labeling experiments using 13CO2 atmosphere produce 13CH4 (m/z = 17) confirming that methane originates from CO2 reduction. Theoretical calculations are performed to investigate the mechanistic aspects of the 8e–/8H+ reduction of CO2 to CH4. A ligand-assisted pathway is proposed to be an efficient pathway in the formation of CH4. Delocalization of the electron density on the (iso)­quinoline moiety upon reduction stabilizes the key carbonyl intermediate leading to additional reactivity of this ligand. These results should aid the development of more robust catalytic systems that produce CH4 from CO2.

A heterogeneous copper oxide supported on mesoporous manganese oxide (meso Cu/MnO) was explored for Ullmann-type cross-coupling reactions. An inverse micelle-templated evaporation-induced self-assembly method with in situ addition of copper was adopted to synthesize the mesoporous catalyst. Broad substrate scope and excellent functional group tolerability in C-O, C-N, and C-S bond formation reactions were observed using the optimized reaction conditions. The catalytic protocol was ligand free, and the catalyst was reusable without any significant loss of activity. The kinetic and Hammett analyses provided evidence for oxidative addition to a Cu(I) reaction center followed by nucleophilic addition and reductive elimination at the active copper oxide surface. Rate acceleration was observed for aryl halides with electron-withdrawing groups. The Hammett analysis determined ρ = +1.0, indicative of an oxidative addition, whereas the electronic effect in the phenol ring (ρ = -2.9) was indicative of coordination to a metal ion. Theoretically, the oxidative addition of the aryl halides is assisted by the ligand environment of the copper center. Relevant mechanistic implications are discussed on the basis of the experimental and computational results.

Tandem oxidation processes enabling one-pot multistep reactions received great attention as an efficient synthetic methodology for construction of complex molecules from simple substrates by a single operation. We report here tandem oxidative transformations of seven different functional groups (imine, imidazole, cyanide, amide, lactone, ester and olefin) from a single substrate (alcohol) by a single cesium promoted mesoporous manganese oxide catalyst (meso Cs/MnOx). High conversions were obtained with a broad range of substrates including aliphatic long chain alcohols. The catalyst can be reused without any loss of catalytic activity. We also demonstrated a unique multiple esterification reaction from a single aliphatic alcohol under aerobic atmospheric conditions catalyzed by meso Cs/MnOx.