On the Relevance of Mono- and Dinuclear Iron Carbonyl Complexes to the Fixation and Stepwise Hydrogenation of N2

Abstract

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 ΔHR0 (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 increased 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.