On the relevance of mono- and dinuclear iron carbonyl complexes to the fixation and stepwise hydrogenation of N₂

Density functional theory and high-level ab initio calculations are used to evaluate the influence of mono- and dinuclear iron carbonyl complexes to the fixation and stepwise hydrogenation of dinitrogen via diazene and hydrazine to ammonia. In comparison to the reaction of isolated N₂, only the first step in this reaction sequence (i.e. the reduction of N₂ to N₂H₂) experiences a significant change in its thermochemistry when coordinated to mono- or dinuclear iron tetracarbonyl fragments. The reaction enthalpy ΔH_R⁰ (T = 0 K) for the endothermic hydrogenation of (CO)₄Fe-N₂ to give (CO)₄Fe-N₂H₂ is lower than for the corresponding metal-free process by 16.1 kcal mol^-1. The analogous step involving the dinuclear species (CO)₄Fe-N₂-Fe(CO₄) and (CO)₄Fe-N₂H₂-Fe(CO)₄ is even less endothermic than the reduction involving only one iron tetracarbonyl complex by 13.1 kcal mol^-1. In comparison to that, the second and third step of this reduction sequence, namely the conversion of coordinated diazene to (CO)₄Fe-N₂H₄ and the subsequent reduction of coordinated hydrazine to (CO)₄Fe-NH₃ show only relatively small thermodynamic changes compared to the analogous reactions of isolated N₂H₂ and N₂H₄. The reduction of (CO)₄Fe-N₂H₂ to (CO)₄Fe-N₂H₄ is almost as exothermic as the analogue reaction involving isolated N₂H₂, whereas the hydrogenation of (CO)₄Fe-N₂H₄ to (CO)₄Fe-NH₃ is less exothermic by 4.0 kcal mol^-1. Finally, the reduction of (CO)₄Fe-N₂H₂-Fe(CO)₄ and (CO)₄Fe-N₂H₄-Fe(CO)₄ are both predicted to be less exothermic than their mononuclear analogues by 4.0 and 1.1 kcal mol^-1, respectively. Moreover, we find that only N₂ and N₂H₂, which already show a noticeable π-acceptor behavior in their complexes with Fe(CO)₄, experience important structural changes in their corresponding dinuclear complexes, i.e. a shortening of the Fe-N bonds and a lengthening of the N-N bonds on going from (CO)₄Fe-L to (CO)₄Fe-L-Fe(CO)₄ (L = N₂, N₂H₂). This behavior is in line with a slightly -creased π-acceptor ability of these ligands in their respective dinuclear complexes. Such structural changes are absent for N₂H₄, which only exhibits a comparatively weak π-acceptor character in (CO)₄Fe-N₂H₄.