Ammonia Synthesis—Homogeneous

Transformations of molecular nitrogen into ammonia promoted by transition-metal complexes are summarized. Treatment of transition-metal salts, mostly those of Groups 4, 5, and 6 metals, with strong reducing agents under N2 in aprotic media gives ammonia after hydrolysis of the reaction mixtures, while amorphous V(II) and Mo(III) hydroxides as well as V(II) catecholate complexes react with N2 in protic media to afford ammonia and/or hydrazine. The coordinated N2 in numerous mononuclear and multinuclear N2 complexes can be converted into ammonia and/or hydrazine by treatment with protic acids. Among these complexes, N2 complexes of Mo and W with tertiary phosphine coligands are outstanding because the coordinated N2 is transformed into ammonia and/or hydrazine in high yields by protonation and the detailed mechanism is revealed by isolation of several intermediates. Furthermore, a tungsten N2 complex reacts with a ruthenium H2 complex at 55°C to form ammonia, although the reaction is stoichiometric. Keywords: ammonia; hydrazine; nitrogen fixation; reduction of coordinated dinitrogen; homogeneous system; transition metal dinitrogen complex