Synthesis,Structure Determination,and Spectroscopic/Computational Characterization of a Series of Fe(II)-Thiolate Model Complexes:Implications for Fe-S Bonding in Superoxide Reductases

摘要

A combined synthetic/spectroscopic/computational approach has been employed to prepare and characterize a series of Fe(II)-thiolate complexes that model the square-pyramidal[Fe(II)(N_His)_4(S_Cys)]structure of the reduced active site of superoxide reductases(SORs),a class of enzymes that detoxify superoxide in air-sensitive organisms.The high-spin(S = 2)Fe(II)complexes[(Me_4cyclam)Fe(SC_6H_4-p-OMe)]OTf(2)and[FeL]PF_6(3)(where Me_4cyclam = 1,4,8,11-tetramethylcyclam and L is the pentadentate monoanion of 1-thioethyl-4,8,11-trimethylcyclam)were synthesized and subjected to structural,magnetic,and electrochemical characterization.X-ray crystallographic studies confirm that 2 and 3 possess an N_4S donor set similar to that found for the SOR active site and reveal molecular geometries intermediate between square pyramidal and trigonal bipyramidal for both complexes.Electronic absorption,magnetic circular dichroism(MCD),and variable-temperature variable-field MCD(VTVH-MCD)spectroscopies were utilized,in conjunction with density functional theory(DFT)and semiemperical INDO/S-CI calculations,to probe the ground and excited states of complexes 2 and 3,as well as the previously reported Fe(II)SOR model[(L~8py_2)Fe(SC_6H_4-p-Me)]BF_4(1)(where L~8py_2 is a tetradentate pyridyl-appended diazacyclooctane macrocycle).These studies allow for a detailed interpretation of the S->Fe(II)charge transfer transitions observed in the absorption and MCD spectra of complexes 1 -3 and provide significant insights into the nature of Fe(II)-S bonding in complexes with axial thiolate ligation.Of the three models investigated,complex 3 exhibits an absorption spectrum that is particularly similar to the one reported for the reduced SOR enzyme(SOR_red),suggesting that this model accurately mimics key elements of the electronic structure of the enzyme active site;namely,highly covalent Fe-S pi- and sigma-interactions.These spectral similarities are shown to arise from the fact that 3 contains an alkyl thiolate tethered to the equatorial cyclam ring,resulting in a thiolate orientation that is very similar to the one adopted by the Cys residue in the SOR_red active site.Possible implications of our results with respect to the electronic structure and reactivity of SOR_red are discussed.