Dinitrogen Functionalization DOI: 10.1002/anie.200605245 Dinitrogen Activation by Group 4 Metal Complexes** Yasuhiro Ohki and MichaelD. Fryzuk* Keywords: coordination modes · dinitrogen · hafnium · nitrogen fixation · zirconium Activating molecular nitrogen by solu- ble metal complexes is a longstanding challenge in inorganic chemistry. [1] This simple, readily available small molecule is notoriously unreactive and generally resists coordination unless certain con- ditions are met. Since the first report of a dinitrogen complex in 1965, [2] much has been learned about bonding modes, methods of preparation, and reactivity patterns. [3–5] Yet what remains elusive is the discovery of a homogeneous cata- lytic process [6] that transforms molecular nitrogen into higher-value nitrogen-con- taining compounds. To be able to di- rectly convert N 2 to N-heterocycles or amines would circumvent the energy- intensive Haber–Bosch process and rep- resent a major breakthrough in energy saving and atom economy. [4] Trying to design a catalytic cycle in- volving dinitrogen from first principles is fraught with difficulty. [6] As already men- tioned, N 2 is not a good ligand and so binding is a problem, especially in the presence of other reagents that may be better donors. Even more troubling is the fact that there are not a lot of known reactivity patterns for a dinitrogen mole- cule coordinated to a metal complex. [5] But things are changing rapidly, and Group 4 metal complexes have figured prominently. When suitably enticed, dinitrogen can bind to metal complexes in a variety of ways that range from end-on to one metal center ([L n MN N]), to a combi- nation of side-on/end-on with two or more metal centers. For the Group4 metals, a particularly prevalent mode of binding is side-on bridging, [5] represent- ed as [(L n M) 2 (m-h 2 :h 2 -N 2 )]. The first example in Group 4 of this motif was the zirconium complex [{(PNP)ZrCl} 2 (m-h 2 :h 2 -N 2 )] (1; PNP = N(SiMe 2 CH 2 PiPr 2 ) 2 ) reported in 1990. [7] Other examples of this coordi- nation mode followed and are shown in Scheme 1, along with the years they Scheme 1. Group 4 metal complexes with side-on bridging dinitrogen ligands. The Me groups on the macrocycle Si atoms in 3 have been omitted. [*] Prof. M. D. Fryzuk Department of Chemistry The University of British Columbia 2036 Main Mall Vancouver BC, V6T1Z1 (Canada) Fax: (+ 1)604-822-2847 E-mail: fryzuk@chem.ubc.ca Dr. Y. Ohki Department of Chemistry Graduate School of Science Nagoya University Furo-cho, Chikusa-ku Nagoya 464-8602 (Japan) [**] Y.O. acknowledges Professor K. Tatsumi for supporting his research leave at The University of British Columbia and thanks the Ministry of Education, Culture, Sports, Science and Technology, Japan for finan- cial support. Highlights 3180 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2007, 46, 3180 – 3183