On the relevance of mono- and dinuclear iron carbonyl complexes to the fixation and stepwise hydrogenation of N-2

Density functional theory and high-level ab initio calculations are used to evaluate the influence of mono- and dinuclear iron carbonyl complexes to t he fixation and stepwise hydrogenation of dinitrogen via diazene and hydraz ine to ammonia. In comparison to the reaction of isolated N-2, only the fir st step in this reaction sequence (i.e. the reduction of N-2 to N2H2) exper iences a significant change in its thermochemistry when coordinated to mono - or dinuclear iron tetracarbonyl fragments. The reaction enthalpy DeltaH(R )(0) (T = 0 K) for the endothermic hydrogenation of (CO)(4)Fe-N-2 to give ( CO)(4)Fe-N2H2 is lower than for the corresponding metal-free process by 16. 1 kcal mol(-1). The analogous step involving the dinuclear species (CO)(4)F e-N-2-Fe(CO4) and (CO)(4)Fe-N2H2-Fe(CO)(4) is even less endothermic than th e 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 seq uence, namely the conversion of coordinated diazene to (CO)(4)Fe-N2H4 and t he subsequent reduction of coordinated hydrazine to (CO)(4)Fe-NH3 show only relatively small thermodynamic changes compared to the lanalogous reaction s of isolated N2H2 and N2H4. The reduction of (CO)(4)Fe-N2H2 to (CO)(4)Fe-N 2H4 is almost as exothermic as the analogue reaction involving isolated N,H ,, whereas the hydrogenation of (CO)(4)Fe-N2H4 to (CO)(4)Fe-NH3 is less exo thermic by 4.0 kcal mol(-1). Finally, the reduction of (CO)(4)Fe-N2H2-Fe(CO )(4) and (CO)(4)Fe-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 N-2 and N2H2, which already show a noticeable n-acceptor behavior in their complexes with Fe(CO)(4), experience important structural changes in their corresponding dinuclear complexes, i.e. a shor tening of the Fe-N bonds and a lengthening of the N-N bonds on going from ( CO)(4)Fe-L to (CO)(4)Fe-L-Fe(CO)(4) (L = N-2, N2H2). This behavior is in li ne with a slightly increased pi -acceptor ability of these ligands in their respective dinuclear complexes. Such structural changes are absent for N2H 4, which only exhibits a comparatively weak K-acceptor character in (CO)(4) Fe-N2H4.