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Two mononuclear iron( ii )-thiolate complexes have been prepared that represent structural models of the nonheme iron enzymes EgtB and OvoA, which catalyze the O 2 -dependent formation of carbon-sulfur bonds in the biosynthesis of thiohistidine compounds. The series of Fe( ii ) complexes reported here feature tripodal N 4 chelates ( L A and L B ) that contain both pyridyl and imidazolyl donors ( L A = (1 H -imidazol-4-yl)- N , N -bis((pyridin-2-yl)methyl)methanamine; L B = N , N -bis((1-methylimidazol-2-yl)methyl)-2-pyridylmethylamine). Further coordination with monodentate aromatic or aliphatic thiolate ligands yielded the five-coordinate, high-spin Fe( ii ) complexes [Fe II ( L A )(SMes)]BPh 4 ( 1 ) and [Fe II ( L B )(SCy)]BPh 4 ( 2 ), where SMes = 2,4,6-trimethylthiophenolate and SCy = cyclohexanethiolate. X-ray crystal structures revealed that 1 and 2 possess trigonal bipyramidal geometries formed by the N 4 S ligand set. In each case, the thiolate ligand is positioned cis to an imidazole donor, replicating the arrangement of Cys- and His-based substrates in the active site of EgtB. The geometric and electronic structures of 1 and 2 were analyzed with UV-vis absorption and Mössbauer spectroscopies in tandem with density functional theory (DFT) calculations. Exposure of 1 and 2 to nitric oxide (NO) yielded six-coordinate FeNO adducts that were characterized with infrared and electron paramagnetic resonance (EPR) spectroscopies, confirming that these complexes are capable of binding diatomic molecules. Reaction of 1 and 2 with O 2 causes oxidation of the thiolate ligands to disulfide products. The implications of these results for the development of functional models of EgtB and OvoA are discussed.





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Dalton Transactions

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Ekanayake, D.M., Fischer, A.A., Elwood, M.E., Guzek, A.M., Lindeman, S.V., Popescu, C.V., Fiedler, A.T. Nonheme Iron-Thiolate Complexes as Structural Models of Sulfoxide Synthase Active Sites. Dalton Transactions. 2020, 49 (48), 17745-17757. DOI: 10.1039/d0dt03403g

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