Proton Reduction and Dihydrogen Oxidation on Models of the [2Fe]_H Cluster of [Fe] Hydrogenases.A Density Functional Theory Investigation

摘要

Density functional theory was used to compare reaction pathways for H_2 formation and H~+ reduction catalyzed by models of the binuclear cluster found in the active site of [Fe] hydrogenases.Terminal H~+ binding to an Fe~I-Fe~I form,foIIowed by monoelectron reduction and protonation of the di(thiomethyl)amine ligand,can conveniently lead to H_2 formation and release,suggesting that this mechanism could be operative within the enzyme active site.However,a pathway that implies the initial formation of Fe~(II)-Fe~(II) mu-H species and release of H_2 from an Fe~(II)-Fe~I form is characterized by only slightly less favored energy profiles.In both cases,H_2 formation becomes less favored when taking into account the competition between CN and amine groups for H~+ binding,an observation that can be relevant for the design of novel synthetic catalysts.H_2 cleavage can take place on Fe~(II)-Fe~(II) redox species,in agreement with previous proposals [Fan,H.-J.;HaII,M.B.J.Am.Chem.Soc.2001,123,3828] and,in complexes characterized by terminal CO groups,does not need the involvement of an external base.The step in H_2 oxidation characterized by larger energy barriers corresponds to the second H~+ extraction from the cluster,both considering Fe~(II)-Fe~(II) and Fe~(II)-Fe~(III) species.A comparison of the different reaction pathways reveals that H_2 formation could involve only Fe~I-Fe~I,Fe~I-Fe~I,and Fe~(II)-Fe~(II) species,whereas Fe~(III)-Fe~(II) species might be relevant in H_2 cleavage.