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]-Hydrogenase

Abstract

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 pydt^2- > ppdt^2- ≥ qdt^2-) 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.