1. Molecular Mechanism of Hydrogen-Formation in Fe-Only Hydrogenases
Nicolai Lehnert, Department of Chemistry, University of Michigan, Ann Arbor, MI 48109
Hydrogenase models: [Fe2(pdt)(dppv)2(CO)2(H)]+ terminal and bridging hydride isomers
Spectroscopy. The H-term and m-H isomers of [Fe2(pdt)(dppv)2(CO)2(H)]+ (see Figure) model the key protonated intermediate of the active site of Fe-only hydrogenase. Resonance Raman and IR spectroscopy show distinct differences between the two isomers
in the n(Fe-CO) ( cm-1) and n(C=O) ( cm-1) stretching regions. However, the n(Fe-H) stretch of H-term, which is the proposed catalytically active form for H2 production, is not observed. The overall weak Raman intensity and the strong signals from the phenyl groups in IR pose serious problems in identifying the important n(Fe-H) stretching mode. DFT calculations predict n(Fe-H) at 2000 cm-1 for H-term and at 1294/1351 cm-1 for m-H. Further spectroscopic studies of an analogous compound
where the dppv ligands are replaced by PMe3, [Fe2(edt)(PMe3 )4(CO)2(H)]+, will be performed next to overcome this problem.
DFT calculations. Initial computations show that the m-H isomer is 5 – 12 kcal/mol (depending on the DFT functional) more stable than H-term, in agreement with experiment. This suggests that the decreased reactivity of m-H toward acids could simply be due to the distinctively lower total energy of this complex compared to H-term. A detailed analysis of the MO diagrams of both isomers
indicates that the atomic charge of hydride is similar in these cases. This is surprising, since one would intuitively think that the terminal hydride is a weaker donor. This result implies that the total charge of the bound hydride does not contribute to the difference in reactivity (no charge control). However, the MO diagrams reveal one important difference between the complexes: the H-term isomer has a key molecular orbital (MO <139>, see left) at relatively high energy that shows a strong hydride(1s) contribution of 23%. No corresponding feature is present for the m-H complex (the closest is MO <138>, see left). This indicates a possible orbital control of the reaction of the complexes with acid. This aspect requires further study.
H-term contour plot of MO <139>
m-H contour plot of MO <138>