Protonation, electrochemical properties and molecular structures of halogen-functionalized diiron azadithiolate complexes related to the active site of iron-only hydrogenases

Diiron complexes [{( mu-SCH2) 2NCH(2)C(6)H(4)X}{Fe( CO)(2)L}(2)] ( L = CO, X = 2-Br, 1; 2-F, 2; 3-Br, 3; L = PMe3, X = 2-Br, 4) were prepared as biomimetic models of the iron-only hydrogenase active site. The N-protonated species [ 1( NH)]+ClO4-, [ 2( NH)]+ClO4- and the mu-hydride diiron complex [ 4( FeHFe)]+PF6- were obtained in the presence of proton acids and well characterized. The protonation process of 4 was studied by in-situ IR and NMR spectroscopy, which suggests the formation of the diprotonated species [ 4( NH)( FeHFe)](2+) in the presence of an excess of proton acid. The molecular structures of 1, [ 1( NH)]+ClO4-, 4 and [ 4( FeHFe)]+PF6- were determined by X-ray crystallography. The single-crystal X-ray analysis reveals that an intramolecular H center dot center dot center dot Br contact ( 2.82 angstrom) in the crystalline state of [ 1( NH)]+ClO4-. In the presence of 1-6 equiv of the stronger acid HOTf, complex 1 is readily protonated on the bridged-N atom and can electrochemically catalyze the proton reduction at a relatively mild potential ( ca. -1.0 V). Complex 4 is also electrocatalytic active at -1.4 V in the presence of HOTf with formation of the mu-hydride diiron species.