Active-site models for iron hydrogenases: Reduction chemistry of dinuclear iron complexes

Reduction of Fe-2(mu-S2C3H6)(CO)(6) (1) in tetrahydrofuran with 1 equiv of decamethylcobaltocene (Cp*Co-2) affords a tetranuclear dianion 2. The IR spectra of samples of 2 in solution and in the solid state exhibit a band at 1736 cm(-1), suggestive of the presence of a bridging carbonyl (CO) ligand. X-ray crystallography confirms that the structure of 2 consists of two Fe2 units bridged by a propanedithiolate moiety formulated as [Fe-2(mu-S2C3H6)(CO)(5)(SCH2CH2CH2-mu-S)Fe-2(mu-CO)(CO)(6)](2-). One of the Fe-2 units has a bridging CO ligand and six terminal CO ligands. The second subunit exhibits a bridging propanedithiolate moiety. One CO ligand has been replaced by a terminal thiolate ligand, replicating the basic architecture of Fe-only hydrogenases. The reduction reaction can be reversed by treatment of 2 with 2 equiv of [Cp2Fe][PF6], reforming complex 1 in near-quantitative yield. Complex 2 can also be oxidized by acids such as p-toluenesulfonic acid, regenerating complex 1 and forming H-2.