Review Lessons learned and lessons to be learned for developing homogeneous transition metal complexes catalyzed reduction of N 2 to ammonia Chinnappan Sivasankar * , Sambath Baskaran, Masilamani Tamizmani, Kankanala Ramakrishna Catalysis and Energy Laboratory, Department of Chemistry, Pondicherry University, Puducherry 605 014, India article info Article history: Received 12 August 2013 Received in revised form 6 November 2013 Accepted 19 November 2013 Keywords: Homogeneous catalysis Nitrogen fixation Nitrogenase enzyme Ammonia production Dinitrogen complex abstract The chemistry of N 2 fixation was inaugurated by Allen and Senoff more than 50 years ago and then various research teams made considerable amount of progress on making large number of dinitrogen complexes of transition metals and reduce them to yield ammonia in solution. Mainly two different strategies have been applied to reduce the coordinated dinitrogen to ammonia. In the first strategy, different sources of protons and electrons have been used along with dinitrogen complex in homoge- neous solution to generate ammonia and this approach is seems to be similar to the mechanism of nitrogenase enzyme in nature. In the second strategy, only few attempts were taken to react dinitrogen complex directly with molecular hydrogen in a homogeneous solution to produce ammonia as similar to HabereBosch process. Nevertheless, till now there is no chemical system known in the literature to convert N 2 to ammonia in a catalytic fashion under low temperature and pressure for wide applications. All the above mentioned methods suffer by several serious of problems namely less stability of the catalysts, H 2 by-product formation and compatibility in handling N 2 and H 2 along with single site cat- alysts etc. However, few theoretical reports are available in which authors have shown to use N 2 and H 2 compatibly along with some organic co-catalysts in homogeneous solutions. This review article focuses on the various attempts that have been made in the literature to produce ammonia in a catalytic and stoichiometric fashion in the presence of proton and electron sources (similar to nitrogenase). And also focuses on some important dinitrogen complexes that could react directly with the molecular hydrogen at low temperature and low pressure in the presence and absence of co-catalysts. Ó 2013 Elsevier B.V. All rights reserved. Contents 1. Introduction ....................................................................................................................... 45 2. Dinitrogen complexes that react with H þ generated from acid ........................................ .................................. 45 3. Dinitrogen complexes that react with H þ generated from H 2 ............................................................................. 49 3.1. Experimental and related computational supports ........................................... ..................................... 49 4. Theoretical model complexes for the reduction of N 2 using H 2 ........................................ .................................. 53 4.1. Mo, Ru and Os complexes of triamidoamine ligand ................................................................................ 53 4.2. Fe, Ru and Os complexes of pincer type ligands ............................................ ...................................... 55 4.3. Reduction of N 2 by H 2 using transition metal catalysts and organic co-catalysts ............................ ......................... 55 5. Conclusions and outlook ............................................................................................................ 56 Acknowledgments .................................................................................................................. 57 References ......................................................................................................................... 57 * Corresponding author. Tel.: þ91 413 2654709; fax: þ91 413 2655987. E-mail addresses: siva.che@pondiuni.edu.in, drcsivas@gmail.com (C. Sivasankar). Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.11.024 Journal of Organometallic Chemistry 752 (2014) 44e58