The N≡N Triple Bond Activation by Transition Metal Complexes

The activation of the N≡N triple bond requires the coordination of N2 molecule to transition metal centers. It is predicted that the stronger M-N2 bond the easier N≡N triple bond utilization, which could occur via various ways including protonation, nucleophilic addition, hydrogenation and coordination of another transition metal center. As example, we report the density functional (B3LYP) studies of the reaction mechanism of model complex A1, [p2n2]Zr(μ-η2-N2)Zr[p2n2] where [p2n2]=(PH3)2(NH2)2 with a H2 molecule. It was shown that reaction with the first H2 molecule proceeds via 21 kcal/mol barrier at the “metathesis-like” transition state, A2, and produces the diazenido-μ-hydride complex, A7(B1). Complex A7(B1) is the only experimentally observed product of the reaction A1+H2 reaction, and separated by nearly 55–60 kcal/mol barriers from the energetically more (by about 40–50 kcal/mol) favorable hydrazono A13, [p2n2]Zr(μ-NH2)(μ-N)Zr[p2n2] and hydrado A17. [p2n2]Zr(μ-NH)2Zr[p 2 n 2] complexes. The addition of the second H2 molecule to complex A1 (the addition of the first H2 to A1) take place with a 19.5 kcal/mol barrier, which is 1.2 kcal/mol smaller than that for the first H2 addition reaction. Since the addition of the first H2 molecule to A1 is known to occur at laboratory conditions, one predicts that the addition of the second hydrogen molecule to A1 should also be feasible. Furthermore, the complex A17, the thermodynamically most stable but kinetically not accessible product of the first H2 addition reaction to A1 could be obtained with the aid of the second reacting H2 molecule. We predict that addition of the second (even third) hydrogen molecule to complex [p2n2]Zr(μ-η2-N2)Zr[p2n2], A1 should be feasible under appropriate laboratory conditions. We encourage experimentalists to check our theoretical prediction.