Breaking bonds

Calculations on the possible reaction pathways of dinitrogen cleavage by a supported tantalum catalyst suggest that side-on binding of the substrate is critical

Credit: © 2008 Wiley

The catalytic synthesis of ammonia has huge biological and industrial significance. The method, whether it is nitrogen fixation through enzymes or the Haber–Bosch process, must cleave the immensely strong N≡N bond and usually relies on cooperation between the multiple metal atoms of a catalyst. An exception to this is the reduction of dinitrogen to an amido–imido product (=NH)(NH₂) using a silica-supported mononuclear tantalum catalyst. The catalyst is known to be a mixture of two species — a monohydride [≡(SiO)₂TaH] and a trihydride [≡(SiO)₂TaH₃] — but the mechanism of the process is unclear.

Now, Jun Li and Shuhua Li from Nanjing University in China have carried out¹ a series of density functional theory calculations aimed at understanding dinitrogen bond cleavage at the tantalum centre. They performed geometry optimization and calculations of Gibbs free energy for the possible intermediates and transition states. They found that the reaction was most likely to proceed through the monohydride rather than the trihydride, and that side-on coordination of dinitrogen is more favourable than end-on. The hypothesized reaction using the trihydride is unlikely because it can only proceed via an end-on dinitrogen complex, and the next step in the reaction mechanism — a hydride transfer from tantalum to one of the nitrogen atoms — leads to an intermediate that does not exist in the geometry optimized calculations; it spontaneously returns to the initial structure.

Side-on bonding to the monohydride is more energetically favourable than end-on, and is shown to strongly activate the dinitrogen bond through d → π^* back bonding. This is critical in the hydride transfer steps that follow and results in a strongly exergonic reaction pathway.