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

The reaction of heterocyclic 1,2-ene-dithiol ligands, namely, pyrazine-2,3-dithiol (H(2)pydt), quinoxaline-2,3-dithiol (H(2)qdt) and pyrido[2,3-b]pyrazine-2,3-dithiol (H(2)ppdt) with Fe-2(CO)(9) yields the '[FeFe]-hydrogenase' model complexes [Fe-2{mu-pydt}(CO)(6)] (1), [Fe-2{mu-qdt}(CO)(6)] (2) and [Fe-2{mu-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 [Fe-2{mu-pydt}(CO)(5)PPh3] (4), [Fe-2{mu-qdt}(CO)(5)PPh3] (5) and [Fe-2{mu-ppdt}(CO)(5)PPh3] (6), respectively. The complexes 1-6 were well characterized by routine elemental analysis, IR, H-1 NMR, C-13 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. (C) 2011 Elsevier B.V. All rights reserved.