Synthesis, Structure Determination, and Spectroscopic/Computational Characterization of a Series of Fe(II)-Thiolate Model Complexes: Implications for Fe-S Bonding in Superoxide Reductases
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- 作者：Adam T. Fiedler ; Heather L. Halfen ; Jason A. Halfen ; and Thomas C. Brunold
- 刊名：Journal of the American Chemical Society
- 出版年：2005
- 出版时间：February 16, 2005
- 年：2005
- 卷：127
- 期：6
- 页码：1675 - 1689
- 全文大小：410K
- 年卷期：v.127,no.6(February 16, 2005)
- ISSN：1520-5126
文摘
A combined synthetic/spectroscopic/computational approach has been employed to prepare andcharacterize a series of Fe(II)-thiolate complexes that model the square-pyramidal [Fe(II)(NHis)4(SCys)]structure of the reduced active site of superoxide reductases (SORs), a class of enzymes that detoxifysuperoxide in air-sensitive organisms. The high-spin (S = 2) Fe(II) complexes [(Me4cyclam)Fe(SC6H4-p-OMe)]OTf (2) and [FeL]PF6 (3) (where Me4cyclam = 1,4,8,11-tetramethylcyclam and L is the pentadentatemonoanion 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 N4Sdonor set similar to that found for the SOR active site and reveal molecular geometries intermediate betweensquare pyramidal and trigonal bipyramidal for both complexes. Electronic absorption, magnetic circulardichroism (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 probethe ground and excited states of complexes 2 and 3, as well as the previously reported Fe(II) SOR model[(L8py2)Fe(SC6H4-p-Me)]BF4 (1) (where L8py2 is a tetradentate pyridyl-appended diazacyclooctanemacrocycle). These studies allow for a detailed interpretation of the S
Fe(II) charge transfer transitionsobserved in the absorption and MCD spectra of complexes 1-3 and provide significant insights into thenature of Fe(II)-S bonding in complexes with axial thiolate ligation. Of the three models investigated, complex3 exhibits an absorption spectrum that is particularly similar to the one reported for the reduced SOR enzyme(SORred), suggesting that this model accurately mimics key elements of the electronic structure of the enzymeactive site; namely, highly covalent Fe-S 
- and 
-interactions. These spectral similarities are shown toarise from the fact that 3 contains an alkyl thiolate tethered to the equatorial cyclam ring, resulting in athiolate orientation that is very similar to the one adopted by the Cys residue in the SORred active site.Possible implications of our results with respect to the electronic structure and reactivity of SORred arediscussed.
Fe(II) charge transfer transitionsobserved in the absorption and MCD spectra of complexes 1-3 and provide significant insights into thenature of Fe(II)-S bonding in complexes with axial thiolate ligation. Of the three models investigated, complex3 exhibits an absorption spectrum that is particularly similar to the one reported for the reduced SOR enzyme(SORred), suggesting that this model accurately mimics key elements of the electronic structure of the enzymeactive site; namely, highly covalent Fe-S - and -interactions. These spectral similarities are shown toarise from the fact that 3 contains an alkyl thiolate tethered to the equatorial cyclam ring, resulting in athiolate orientation that is very similar to the one adopted by the Cys residue in the SORred active site.Possible implications of our results with respect to the electronic structure and reactivity of SORred arediscussed.