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

Treatment of parent compounds [(mu-SCH(2))(2)X]Fe(2)(CO)(6) (A, X = O; B, X = NBu-t; C, X = NC(6)H(4)OMe-p) with N-heterocyclic carbene I(Mes) (I(Mes) = 1,3-bis(mesityl)imidazol-2-ylidene) generated in situ through reaction of imidazolium salt I(Mes) center dot HCl with n- BuLi or t-BuOK afforded the monocarbene-substituted complexes [(mu-SCH(2))(2)X] Fe(2)(CO)(5)(I(Mes)) ( 1, X = O; 2, X = NBu-t; 3, X = NC(6)H(4)OMe-p). Similarly, the monocarbene and dicarbene-substituted complexes [(mu-SCH(2))(2)NBu-t]Fe(2)(CO)(5)[I*(Mes)(CH(2))(3)I*(Mes)]center dot HBr (4) and [(mu-SCH(2))(2)CH(2)Fe(2)( CO)(5)](2)[mu-I*(Mes)(CH(2))(3)I* (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)(CH(2))(3)I*(Mes)]center dot HBr and parent compound [(mu-SCH(2))(2)CH(2)] Fe(2)(CO)(6) (D) with di-NHC ligand I*(Mes)(CH(2))(3)I*(Mes) (both NHC ligands were generated in situ from reaction of n-BuLi with imidazolium salt [I*(Mes)I*(Mes)(CH(2))(3)I*(Mes)] center dot 2HBr), respectively. The imidazolium salt [I*(Mes)(CH(2))(3)I*(Mes)] center dot 2HBr was prepared by reaction of 1-(mesityl)imidazole with Br(CH(2))(3)Br. All the new model compounds 1-5 and imidazolium salt [I*(Mes)(CH(2))(3)I*(Mes)] center dot 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. (C) 2008 Published by Elsevier B. V.