Zirconium complexes with dinitrogen

Perhaps chemists will be able to mimic nature without duplicating the iron-sulfur-molybdenum structure. For example, a zirconium complex with tetramethyl cyclopentadiene can bind dinitrogen in a manner that breaks the NON bond, as shown below. Treatment with hydrogen gas results in formation of small amounts of ammonia. Although the yields are too low to make this a viable commercial process, researchers hope to make the process more efficient through chemical modifications and changes in conditions. 

Quite recently, Fryzuk and co-workers observed the N-H bond formation when a dinuclear zirconium complex with a side-on bridging dinitrogen ligand was treated with molecular dihydrogen. However, no ammonia was formed (a) M. D. Fryzuk, J. B. Love, S. J. Rettig, V. G. Young, Science 275 (1997) 1445. (b) H. Basch, D. G. Musaev, K. Morokuma, M. D. Fryzuk, J. B. Love, W. W. Seidel, A. Albinati, T. F. Koetzle, W. T. Klooster, S. A. Mason, J. Eckert, J. Am. Chem. Soc. 121 (1999) 523. 

Pool JA, Lobkovsky E, Chirik PJ. Hydrogenation and cleavage of dinitrogen to ammonia with a zirconium complex. Nature. 2004 427 527-30. 

Reduction of a toluene solution of bis(r>-pentamethylcyclopenta-dienyl)zirconium dichloride with sodium amalgam under dinitrogen yields the permanganate-red colored dinitrogen complex 22 (35)... 

Complex 3 was a real surprise. It contained a side-on bound dinitrogen fragment bridging the two zirconium centres with a N-N bond distance of 1.548(7) A, the longest N-N bond distance in a dinitrogen complex. When we... 

Bis(cyclopentadienyl)zirconium(III) complexes alkynyl-bridged complexes, 4, 749 dinitrogen complexes, 4, 747 electrochemical reduction, 4, 745 phosphido-bridged complexes, 4, 750 Bis(cyclopentadienyl)zirconium(IV) complexes electrochemical reduction, 4, 745 with M-C r/)-bonds, 4, 895 with M-C r/i2-bonds, 4, 887 with M-C r/i3-bonds, 4, 884 with M-H bonds, 4, 878 with M-N bonds, 4, 910 with M-0 bonds, 4, 917 with M-P bonds, 4, 927 with M-S bonds, 4, 930 with M-Si bonds, 4, 925 non-functionalized metallocene halides, 4, 870 ring-functionalized metallocene halides, 4, 874 Bis(cyclopentadienyl)zirconium(III) halides, in organic synthesis, 4, 754... 

Bis(i -cyclopentadienyl)titanium or titanocene, (Tj-C5H5)2Ti (1), and bis(i7-cyclopentadienyl)zirconium or zirconocene, (i7-C5H5)2Zr (2), although frequently referred to in the literature, have never actually been isolated as discrete chemical compounds. However, these molecules have been implicated as highly reactive intermediates in a wide variety of chemical reactions with olefins, hydrogen, carbon monoxide, and dinitrogen. In recent years some discrete, well-characterized bis(7j-cyclopenta-dienyl) and bis(Tj-pentamethylcyclopentadienyl) complexes of low-valent titanium and zirconium have been isolated and studied, and it has become possible to understand some of the reasons for the remarkable reactivity of titanocene- and zirconocene-related organometallics toward small unsaturated molecules. 

The bis(i7-pentamethylcyclopentadienyl)zirconium dinitrogen complex [tj-C5(CH3)5kZrj(N2)3 (22), in solution reversibly releases its dinitrogen in vacuo over several hours (Section III,E). The solutions remaining are said to consist mainly of [T7-C5(CH3)5][T7-C5(CH3)4CH2]ZrH (23) (33). Permethylzirconocene [tj-C5(CH3 )5l,Zr (24) may exist in these solutions in equilibrium with this hydride tautomer (as in the case of titanium). However, thus far permethylzirconocene has not been isolated as a discrete compound or otherwise characterized. It is very likely that permethylzirconocene is less stable and more prone to form complex metallocene hydrides than permethyltitanocene (16). 

Chirik PJ. Dinitrogen functionalization with bis(cyclopentadienyl) complexes of zirconium and hafnium. Dalton Trans. 2007 38 16-25. 

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