Coordination Chemistry Reviews 254 (2010) 2492–2504 Contents lists available at ScienceDirect Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr Review Catalytic hydrogen production at cobalt centres Sebastian Losse a,b , Johannes G. Vos c , Sven Rau b,∗ a Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena, August-Bebel-Str. 2, 07743 Jena, Germany b Lehrstuhl für Anorganische und Allgemeine Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 1, 91058 Erlangen, Germany c Solar Energy Conversion SRC, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland Contents 1. Introduction .......................................................................................................................................... 2492 2. Electrocatalysis ....................................................................................................................................... 2493 2.1. Cobalt macrocycles and porphyrins .......................................................................................................... 2493 2.2. Cobalt phthalocyanines ...................................................................................................................... 2494 2.3. Cobalt glyoximes ............................................................................................................................. 2494 2.4. Cobalt cyclopentadienyl and phosphine complexes ......................................................................................... 2496 2.5. Cobalt cage complexes ....................................................................................................................... 2496 3. Photocatalytic hydrogen production with cobalt catalysts ......................................................................................... 2496 3.1. Co complexes of bpy and phen ............................................................................................................... 2496 3.2. Co glyoximes ................................................................................................................................. 2497 3.3. Cobalt cyclam, porphyrin and corrin complexes ............................................................................................. 2499 3.4. Polypyridine, cyclopentadienyl and inorganic cobalt complexes ........................................................................... 2501 3.5. Supramolecular systems ..................................................................................................................... 2501 4. Conclusion and outlook .............................................................................................................................. 2503 Acknowledgments ................................................................................................................................... 2503 References ........................................................................................................................................... 2503 article info Article history: Received 13 January 2010 Accepted 15 June 2010 Available online 23 June 2010 Keywords: Photocatalysis Ruthenium Cobalt Hydrogen abstract The catalytic properties of a wide range of cobalt complexes with respect to proton reduction are dis- cussed. Electrocatalytic as well as photocatalytic systems are addressed and to allow comparison between the different systems reported considerable attention is paid to the reaction conditions used. For the photocatalytic proton reduction a range of ruthenium, iridium and rhenium complexes are discussed as potential photosensitizers. The photocatalytic systems are discussed in detail and issues such as the nature of the sacrificial agent and the solvents used. Both intermolecular and intramolecular photocatal- ysis are considered. The results obtained are considered with respect to the need to the development of sustainable energy sources. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The development of carbon neutral energy sources is of funda- mental importance for the future growth of our civilization. One of the most promising approaches towards this aim is the conversion of solar energy into storable fuels like hydrogen or via reduction of carbon dioxide [1,2]. Within this area two general options may be considered. Firstly to capture sunlight using conventional solar cells and perform the initial charge separation generating an elec- ∗ Corresponding author. Tel.: +49 9131 8527396; fax: +49 9131 8527367. E-mail address: sven.rau@chemie.uni-erlangen.de (S. Rau). trical potential which than can be used to drive electrocatalytical processes [3]. The other concept uses the known ability of certain dyes to generate high energy electrons upon excitation with light. These electrons may be used to perform redox reactions provided the other partners in this reaction possess the appropriate redox properties. Cobalt complexes have been investigated in detail as catalysts for both hydrogen evolution and CO 2 reduction in both approaches during the past three decades. The process of water splitting can be divided into two reactions: (i) water oxidation, to yield O 2 and (ii) water reduction, producing H 2 . Catalysts for oxidizing water are rare but known [3–6]. In this review we will focus on water reduction using electrical potentials or light for the hydrogen evolution at cobalt catalysts. We discuss the catalytic 0010-8545/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2010.06.004