Annette-Enrica Surkus

Nanoscale Fe2O3-based catalysts for selective hydrogenation of nitroarenes to anilines.

Lighter Hydrogenation Catalysts Enzymes have evolved to use abundant metals such as iron, cobalt, and nickel for redox catalysis. However, synthetic catalysis has generally relied on the rarer, heavier relatives of these elements: ruthenium, rhodium, iridium, palladium, and platinum (see the Perspective by Bullock). Friedfeld et al. (p. 1076) used high-throughput screening to show that the right cobalt precursor can be activated for asymmetric hydrogenation catalysis by using the traditional ligands developed for the precious metals. Zuo et al. (p. 1080) focused on iron, demonstrating a highly effective asymmetric transfer hydrogenation catalyst that uses a ligand rationally designed after careful mechanistic study. Jagadeesh et al. (p. 1073) prepared supported iron catalysts that selectively reduce nitro substituents on aromatic rings to amines, thereby facilitating the preparation of a wide range of aniline derivatives. An iron oxide catalyst selects nitro groups for reduction in the presence of many other sensitive chemical substituents. [Also see Perspective by Bullock] Production of anilines—key intermediates for the fine chemical, agrochemical, and pharmaceutical industries—relies on precious metal catalysts that selectively hydrogenate aryl nitro groups in the presence of other easily reducible functionalities. Herein, we report convenient and stable iron oxide (Fe2O3)–based catalysts as a more earth-abundant alternative for this transformation. Pyrolysis of iron-phenanthroline complexes on carbon furnishes a unique structure in which the active Fe2O3 particles are surrounded by a nitrogen-doped carbon layer. Highly selective hydrogenation of numerous structurally diverse nitroarenes (more than 80 examples) proceeded in good to excellent yield under industrially viable conditions.

Heterogenized cobalt oxide catalysts for nitroarene reduction by pyrolysis of molecularly defined complexes

Molecularly well-defined homogeneous catalysts are known for a wide variety of chemical transformations. The effect of small changes in molecular structure can be studied in detail and used to optimize many processes. However, many industrial processes require heterogeneous catalysts because of their stability, ease of separation and recyclability, but these are more difficult to control on a molecular level. Here, we describe the conversion of homogeneous cobalt complexes into heterogeneous cobalt oxide catalysts via immobilization and pyrolysis on activated carbon. The catalysts thus produced are useful for the industrially important reduction of nitroarenes to anilines. The ligand indirectly controls the selectivity and activity of the recyclable catalyst and catalyst optimization can be performed at the level of the solution-phase precursor before conversion into the active heterogeneous catalyst. Pyrolysis of defined nitrogen-ligated cobalt acetate complexes onto a commercial carbon support transforms the complexes into heterogeneous Co3O4 materials. These reusable non-noble-metal catalysts are highly selective for the industrially important hydrogenation of structurally diverse and functionalized nitroarenes to anilines.

Photocatalytic Water Reduction with Copper‐Based Photosensitizers: A Noble‐Metal‐Free System

Of noble descent: a fully noble-metal-free system for the photocatalytic reduction of water at room temperature has been developed. This system consists of Cu(I) complexes as photosensitizers and [Fe(3)(CO)(12)] as the water-reduction catalyst. The novel Cu-based photosensitizers are relatively inexpensive, readily available from commercial sources, and stable to ambient conditions, thus making them an attractive alternative to the widely used noble-metal based systems.

Synthesis and Characterization of Iron–Nitrogen-Doped Graphene/Core–Shell Catalysts: Efficient Oxidative Dehydrogenation of N-Heterocycles

An important goal for nanocatalysis is the development of flexible and efficient methods for preparing active and stable core-shell catalysts. In this respect, we present the synthesis and characterization of iron oxides surrounded by nitrogen-doped-graphene shells immobilized on carbon support (labeled FeOx@NGr-C). Active catalytic materials are obtained in a simple, scalable and two-step method via pyrolysis of iron acetate and phenanthroline and subsequent selective leaching. The optimized FeOx@NGr-C catalyst showed high activity in oxidative dehydrogenations of several N-heterocycles. The utility of this benign methodology is demonstrated by the synthesis of pharmaceutically relevant quinolines. In addition, mechanistic studies prove that the reaction progresses via superoxide radical anions (·O2(-)).

A noble-metal-free system for photocatalytic hydrogen production from water.

A series of heteroleptic copper(I) complexes with bidentate PP and NN chelate ligands was prepared and successfully applied as photosensitizers in the light-driven production of hydrogen, by using [Fe3(CO)12] as a water-reduction catalyst (WRC). These systems efficiently reduces protons from water/THF/triethylamine mixtures, in which the amine serves as a sacrificial electron donor (SR). Turnover numbers (for H) up to 1330 were obtained with these fully noble-metal-free systems. The new complexes were electrochemically and photophysically characterized. They exhibited a correlation between the lifetimes of the MLCT excited state and their efficiency as photosensitizers in proton-reduction systems. Within these experiments, considerably long excited-state lifetimes of up to 54 μs were observed. Quenching studies with the SR, in the presence and absence of the WRC, showed that intramolecular deactivation was more efficient in the former case, thus suggesting the predominance of an oxidative quenching pathway.

Hydrogenation using iron oxide–based nanocatalysts for the synthesis of amines

In this protocol, we describe the preparation of nanoscale iron oxide–based materials and their use in the catalysis of different hydrogenation reactions. Pyrolysis of a Fe(OAc)2-phenanthroline complex on carbon at 800 °C under argon atmosphere results in the formation of nanoscale Fe2O3 particles surrounded by nitrogen-doped graphene layers. By applying these catalysts, the hydrogenation of structurally diverse and functionalized nitroarenes to anilines proceeds with excellent selectivity. Furthermore, we have shown that one-pot reductive amination of carbonyl compounds with nitroarenes is also possible in the presence of these iron oxide catalysts. We report herein the synthesis of more than 40 amines, which are important feedstocks and key intermediates for pharmaceuticals, agrochemicals and polymers. The detailed preparation of the catalysts and the procedures for the hydrogenation processes are presented. The overall time required for the catalyst preparation and for the hydrogenation reactions are 35 h and 20–35 h, respectively.

General and selective reductive amination of carbonyl compounds using a core–shell structured Co3O4/NGr@C catalyst

The application of heterogenized non-noble metal-based catalysts in selective catalytic hydrogenation processes is still challenging. In this respect, the preparation of a well-defined cobalt-based catalyst was investigated by immobilization of the corresponding cobalt(II)-phenanthroline-chelate on Vulcan XC72R carbon powder. The formed core–shell structured cobalt/cobalt oxide nanocomposites are encapsulated by nitrogen-enriched graphene layers. This promising cheap heterogeneous catalyst allows for an efficient domino reductive amination of carbonyl compounds with nitroarenes.

A convenient and general ruthenium-catalyzed transfer hydrogenation of nitro- and azobenzenes.

An easily accessible in situ catalyst composed of [{RuCl(2)(p-cymene)}(2)] and terpyridine has been developed for the selective transfer hydrogenation of aromatic nitro and azo compounds. The procedure is general and the selectivity of the catalyst has been demonstrated by applying a series of structurally diverse nitro and azo compounds (see scheme).

Cobalt-based nanocatalysts for green oxidation and hydrogenation processes

This protocol describes the preparation of cobalt-based nanocatalysts and their applications in environmentally benign redox processes for fine chemical synthesis. The catalytically active material consists of nanoscale Co3O4 particles surrounded by nitrogen-doped graphene layers (NGrs), which have been prepared by pyrolysis of phenanthroline-ligated cobalt acetate on carbon. The resulting materials have been found to be excellent catalysts for the activation of both molecular oxygen and hydrogen; in all tested reactions, water was the only by-product. By applying these catalysts, green oxidations of alcohols and hydrogenation of nitroarenes for the synthesis of nitriles, esters and amines are demonstrated. The overall time required for catalyst preparation and for redox reactions is 35 h and 10–30 h, respectively.

Stable and Inert Cobalt Catalysts for Highly Selective and Practical Hydrogenation of C≡N and C═O Bonds

Novel heterogeneous cobalt-based catalysts have been prepared by pyrolysis of cobalt complexes with nitrogen ligands on different inorganic supports. The activity and selectivity of the resulting materials in the hydrogenation of nitriles and carbonyl compounds is strongly influenced by the modification of the support and the nitrogen-containing ligand. The optimal catalyst system ([Co(OAc)2/Phen@α-Al2O3]-800 = Cat. E) allows for efficient reduction of both aromatic and aliphatic nitriles including industrially relevant dinitriles to primary amines under mild conditions. The generality and practicability of this system is further demonstrated in the hydrogenation of diverse aliphatic, aromatic, and heterocyclic ketones as well as aldehydes, which are readily reduced to the corresponding alcohols.