In search for future energy supplies the application of hydrogen as an energy carrier is seen as a prospective issue. However, the implementation of a hydrogen economy is suffering from several unsolved problems. Particularly challenging is the storage of appropriate amounts of hydrogen. In this context the utilization of carbon dioxide-formic acid for hydrogen storing is discussed.

Dedicated to Professor Wolfgang A. Herrmann on the occasion of his 60th birthday coupling reactions · homogeneous catalysis · iron · oxidation · reductionThe development of sustainable, more efficient, and selective organic synthesis is one of the fundamental research goals in chemistry. In this respect, catalysis is a key technology, since approximately 80 % of all chemical and pharmaceutical products on an industrial scale are made by catalysts-even more in the case of modern processes (ca. 90 %). In particular, organometallic compounds have become an established synthetic tool for both fine and bulk chemicals and several hundreds of molecular, defined pre-catalysts are commercially available for chemists around the world. The reactivity and selectivity of the active catalyst are widely influenced by the choice of the central metal and by the design of surrounded ligands. During the last decades, manifold transition-metal catalysts especially based on precious metals such as palladium, rhodium, iridium, and ruthenium have been proven to be efficient for a large number of applications. However, the limited availability of these metals as well as their high price ( Figure 1) and significant toxicity makes it desirable to search for more economical and environmentally friendly alternatives. A possible solution of this problem could be the increased use of catalysts based on first row transition metals, such as iron, copper, zinc, and manganese. Especially iron offers significant advantages compared with precious metals, since it is the second most abundant metal in the earth crust (4.7 wt %). Various iron salts and iron complexes are commercially accessible on a large scale or easy to synthesize. Furthermore, iron compounds are relatively nontoxic. In contrast to man-made precious-metal catalysts, iron takes part in various biological systems as essential key element, for example, in metalloproteins for the transport or metabolism of small molecules (oxygen, nitrogen, methane, etc.) and electron-transfer reactions (Figure 2). Thanks to the facile change of oxidation state and the distinct Lewis acid character, iron catalysts allow in principle a broad range of synthetic transformations, for example,

The formation of oxygen-carbon bonds is one of the fundamental transformations in organic synthesis. In this regard the application of palladium-based catalysts has been extensively studied during recent years. Nowadays it is an established methodology and the success has been proven in manifold synthetic procedures. This tutorial review summarizes the advances on palladiumcatalysed C-O bond formation, means hydroxylation and alkoxylation reactions.

The first electron-rich N-heterocyclic silylene (NHSi)-iron(0) complexes are reported. The synthesis of the starting complex is accomplished by reaction of the electron-rich Fe(0) precursor [(dmpe)2Fe(PMe3)] 1 (dmpe =1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC(N(t)Bu)2) 2 to give, via Me3P elimination, the corresponding iron complex [(dmpe)2Fe(←:Si(Cl)L)] 3. Reaction of in situ generated 3 with MeLi afforded [(dmpe)2Fe(←:Si(Me)L)] 4 under salt metathesis reaction, while its reaction with Li[BHEt3] yielded [(dmpe)2Fe(←:Si(H)L)] 5, a rare example of an isolable Si(II) hydride complex and the first such example for iron. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analyses. DFT calculations further characterizing the bonding situation between the Si(II) and Fe(0) centers were also carried out, whereby multiple bonding character is detected in all cases (Wiberg Bond Index >1). For the first time, the catalytic activity of a Si(II) hydride complex was investigated. Complex 5 was used as a precatalyst for the hydrosilylation of a variety of ketones in the presence of (EtO)3SiH as a hydridosilane source. In most cases excellent conversions to the corresponding alcohols were obtained after workup. The reaction pathway presumably involves a ketone-assisted 1,2-hydride transfer from the Si(II) to Fe(0) center, as a key elementary step, resulting in a betaine-like silyliumylidene intermediate. The appearance of the latter intermediate is supported by DFT calculations, and a mechanistic proposal for the catalytic process is presented.

Fuelling the economy: In the search for sustainable and renewable energy systems, the hydrogen economy could be an alternative if several requirements can be fulfilled in the future. One major challenge is still the search for appropriate hydrogen‐storage systems. The potential application of carbon dioxide as a hydrogen carrier is discussed in this Highlight.

Dedicated to Professor Wolfgang A. Herrmann on the occasion of his 60th birthday Noteworthy efforts have been devoted to the development of efficient catalytic asymmetric reductions employing benign and environmentally available biometals such as iron, zinc, and copper. The preparation of enantiomerically pure secondary alcohols is of special significance because these intermediates constitute valuable building blocks for the manufacture of pharmaceuticals, agrochemicals, and advanced materials.[1] Catalytic asymmetric hydrogenation of prochiral ketones is the most direct route to optically active alcohols, [2] however, hydrosilylation of carbon-carbon and carbonheteroatom bonds is a promising alternative to asymmetric hydrogenation because of the mild conditions and operational simplicity.[3]The earliest reports on hydrosilylation appeared three decades ago, [4] and known asymmetric hydrosilylations of prochiral ketones rely on precious metals such as rhodium, [5] ruthenium, [6] and iridium. [7] Less expensive metals such as titanium, [8] zinc, [9] tin, [10] and copper [11] have also been explored. Each method has its virtues as well as its limitations. The limitations include either the cost of the metal catalyst, the toxicity of the residual metal in the product, the operational difficulties (e.g. low temperatures ranging from À50 to À70 8C), or the use of complex ligand systems.Recently, we started a program to develop more sustainable catalysts by replacing precious metals with nonprecious metals. In accord with the concept of "cheap metals for noble tasks", [12] the possible uses of iron catalysts are especially attractive.[13] Iron is the second most abundant metal available and plays an important role in biology.[14] Despite the many advantages and recent attention [15] to iron catalysis, it remains undeveloped compared to other transition metals (e.g. Ru, Rh, Pd, and Ir etc.), particularly in the field of asymmetric catalysis. To the best of our knowledge there is only one report by Nishiyama ând Furuta [16] on the development of iron-catalyzed asymmetric hydrosilylation. They used multidentate nitrogen ligands and reported enantioselectivities of up to 79 %. The scope of this work can be expanded; herein, we disclose our results on an improved and general ironcatalyzed asymmetric hydrosilylation of ketones (Table 1).Our recent study on the hydrosilylation of aldehydes revealed that Fe(OAc) 2 in the presence of electron-rich phosphine ligands and hydrosilanes forms an active catalyst.[17] On the basis of these findings we turned our attention to the asymmetric reduction of ketones.Initially, several chiral ligands were tested for the reduction of acetophenone to 1-phenylethanol by using a given set of conditions and selected phosphines (Table 1 and Figure 1). Privileged ligands such as (S)-2,2'-bis(diphenyl-, (S,S)-l-benzyl-3,4-bis-(diphenylphosphino)pyrrolidine ((S,S)-deguphos), and binaphthyl derived systems, such as L1 and L2, gave good to excellent conversions of acetophenone (68-99 %), but poor enan...

The nickel‐catalyzed decomposition of formic acid to yield molecular hydrogen and the nickel‐catalyzed hydrogenation of bicarbonate as a carbon dioxide mimic have been examined. Well‐defined nickel complexes modified by a PCP‐pincer ligand, especially nickel hydride and nickel formate complexes, revealed catalytic activity with turnover numbers of up to 626 (decomposition) and 3000 (hydrogenation). Thus, a formal hydrogen storage and release cycle performed by a well‐defined nickel catalyst was accomplished.

Ende des Schattendaseins? Die leicht verfügbaren, kaum toxischen und billigen Eisenverbindungen bieten sich für die homogene Katalyse an, ihr Anwendungsbereich ist überraschenderweise aber deutlich geringer als bei anderen Übergangsmetallen. Dieses Highlight fasst einige vielversprechende Ansätze aus der Redox‐ und Kupplungschemie zusammen, die das Potenzial von Eisenkatalysatoren unterstreichen.