Synthesis, structural characterization and electrochemical studies of [Fe2(μ-L)(CO)6] and [Fe2(μ-L)(CO)5(PPh3)] (L = pyrazine-2,3-dithiolate, quinoxaline-2,3-dithiolate and pyrido[2,3-b]pyrazine-2,3-dithiolate): Towards modeling the active site of [FeFe]–

The reaction of heterocyclic 1,2-ene-dithiol ligands, namely, pyrazine-2,3-dithiol (H2pydt), quinoxaline-2,3-dithiol (H2qdt) and pyrido[2,3-b]pyrazine-2,3-dithiol (H2ppdt) with Fe2(CO)9 yields the ‘[FeFe]–hydrogenase’ model complexes [Fe2{μ-pydt}(CO)6] (1), [Fe2{μ-qdt}(CO)6] (2) and [Fe2{μ-ppdt}(CO)6] (3), respectively. A further reaction of complexes 1, 2 and 3 with PPh3 in the presence of equimolar amount of decarbonylating agent Me3NO in CH3CN at room temperature resulted in the formation of unsymmetrical mono-PPh3-substituted model complexes [Fe2{μ-pydt}(CO)5PPh3] (4), [Fe2{μ-qdt}(CO)5PPh3] (5) and [Fe2{μ-ppdt}(CO)5PPh3] (6), respectively. The complexes 1−6 were well characterized by routine elemental analysis, IR, 1H NMR, 13C NMR spectroscopy and unambiguously characterized by X−ray crystallographic analysis. IR spectroscopy and electrochemical analysis show that an increase of the electron– withdrawing character of the bridging ligands (where electron–withdrawing character is in the order of pydt2− > ppdt2− ≥ qdt2−) leads to a decreased electron density at the iron centers, which yield a milder reduction potential and higher CO stretching frequencies. All the compounds 1−6 are further characterized by electrochemical studies.

Graphical abstractIn an attempt to model the active sites of [FeFe]–hydrogenase, a series of heterocyclic 1,2-ene-dithiolate bridged di-iron complexes have been synthesized and structurally characterized. The electrochemical generation of di-hydrogen from a strong acid p–toluenesulfonic acid (p–HOTs), mediated by complex 1, has been demonstrated.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Reaction of heterocyclic 1,2-ene-dithiols with Fe2(CO)9 yields the title complexes. ► These compounds further react with PPh3 to form mono-PPh3-substituted derivatives. ► The system shows relatively low reduction potentials among related model compounds. ► Electro-catalytic hydrogen generation has been demonstrated.