Synthesis, characterization and electrochemical behavior of some N-heterocyclic carbene-containing active site models of [FeFe]-hydrogenases

Treatment of parent compounds [(μ-SCH2)2X]Fe2(CO)6 (A, X = O; B, X = NBu-t; C, X = NC6H4OMe-p) with N-heterocyclic carbene IMes (IMes = 1,3-bis(mesityl)imidazol-2-ylidene) generated in situ through reaction of imidazolium salt IMes ·HCl with n-BuLi or t-BuOK afforded the monocarbene-substituted complexes [(μ-SCH2)2X]Fe2(CO)5(IMes) (1, X = O; 2, X = NBu-t; 3, X = NC6H4OMe-p). Similarly, the monocarbene and dicarbene-substituted complexes [(μ-SCH2)2NBu-t]Fe2(CO)5[I∗Mes(CH2)3I∗Mes]·HBr (4) and [(μ-SCH2)2CH2Fe2(CO)5]2[μ-I∗Mes(CH2)3I∗Mes] (5, I∗Mes = 1-(mesityl)imidazol-2-ylidene) could be prepared by reactions of parent compound B with the mono-NHC ligand-containing imidazolium salt [I∗Mes(CH2)3I∗Mes] · HBr and parent compound [(μ-SCH2)2CH2]Fe2(CO)6 (D) with di-NHC ligand I∗Mes(CH2)3I∗Mes (both NHC ligands were generated in situ from reaction of n-BuLi with imidazolium salt [I∗MesI∗Mes(CH2)3I∗Mes] · 2HBr), respectively. The imidazolium salt [I∗Mes(CH2)3I∗Mes] · 2HBr was prepared by reaction of 1-(mesityl)imidazole with Br(CH2)3Br. All the new model compounds 1–5 and imidazolium salt [I∗Mes(CH2)3I∗Mes] · 2HBr were fully characterized by elemental analysis, spectroscopy, and X-ray crystallography. On the basis of electrochemical studies of 1 and 2, compound 2 was found to be a catalyst for proton reduction to hydrogen. In addition, an EECC mechanism for this electrocatalytic reaction is preliminarily suggested.