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On the Relevance of Mono‐ and Dinuclear Iron Carbonyl Complexes to the Fixation and Stepwise Hydrogenation of N 2
Author(s) -
Chen Yu,
Hartmann Michael,
Frenking Gernot
Publication year - 2001
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/1099-0682(200106)2001:6<1441::aid-ejic1441>3.0.co;2-g
Subject(s) - chemistry , endothermic process , exothermic reaction , thermochemistry , enthalpy , density functional theory , hydrazine (antidepressant) , medicinal chemistry , inorganic chemistry , computational chemistry , organic chemistry , adsorption , physics , quantum mechanics , chromatography
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 N 2 , only the first step in this reaction sequence (i.e. the reduction of N 2 to N 2 H 2 ) experiences a significant change in its thermochemistry when coordinated to mono‐ or dinuclear iron tetracarbonyl fragments. The reaction enthalpy Δ H R 0 ( T = 0 K) for the endothermic hydrogenation of (CO) 4 Fe−N 2 to give (CO) 4 Fe−N 2 H 2 is lower than for the corresponding metal‐free process by 16.1 kcal mol −1 . The analogous step involving the dinuclear species (CO) 4 Fe−N 2 −Fe(CO 4 ) and (CO) 4 Fe−N 2 H 2 −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) 4 Fe−N 2 H 4 and the subsequent reduction of coordinated hydrazine to (CO) 4 Fe−NH 3 show only relatively small thermodynamic changes compared to the analogous reactions of isolated N 2 H 2 and N 2 H 4 . The reduction of (CO) 4 Fe−N 2 H 2 to (CO) 4 Fe−N 2 H 4 is almost as exothermic as the analogue reaction involving isolated N 2 H 2 , whereas the hydrogenation of (CO) 4 Fe−N 2 H 4 to (CO) 4 Fe−NH 3 is less exothermic by 4.0 kcal mol −1 . Finally, the reduction of (CO) 4 Fe−N 2 H 2 −Fe(CO) 4 and (CO) 4 Fe−N 2 H 4 −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 N 2 H 2 , 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) 4 Fe−L to (CO) 4 Fe−L−Fe(CO) 4 (L = N 2 , N 2 H 2 ). 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 N 2 H 4 , which only exhibits a comparatively weak π‐acceptor character in (CO) 4 Fe−N 2 H 4 .