Diiron Oxadithiolate Type Models for the Active Site of Iron-Only Hydrogenases and Biomimetic Hydrogen Evolution Catalyzed by Fe₂(μ-SCH₂OCH₂S-μ)(CO)₆

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Abstract

The biomimetic chemistry of single and double oxadithiolatodiiron-containing model compounds for the active site of Fe-only hydrogenases (FeHases) has been systematically studied. The simplest such model, Fe2(μ-SCH2OCH2S-μ)(CO)6 (1), was prepared by reaction of (μ-S2)Fe2(CO)6 with 2 equiv of Et3BHLi followed by direct treatment with excess (ClCH2)2O or by successive treatment with 2 equiv of CF3CO2H and excess (ClCH2)2O in the presence of Et3N. Further reaction of 1 with 1 equiv of Me3NO in MeCN at room temperature followed by treatment of the intermediate Fe2(μ-SCH2OCH2S-μ)(CO)5L (L = MeCN or Me3N) with 1 equiv of Et4NCN, PPh3, or Cp(CO)2FeSPh gave the single models Fe2(μ-SCH2OCH2S-μ)(CO)5La (2, La = (CN)(Et4N); 3, PPh3; 4, Cp(CO)2FeSPh) in 62−93% yields, whereas the in situ treatment of the intermediate Fe2(μ-SCH2OCH2S-μ)(CO)5L with 0.5 equiv of 1,4-(CN)2C6H4, (η5-Ph2PC5H4)2Fe (dppf), or (η5-Ph2PC5H4)2Ru (dppr) afforded the double models [Fe2(μ-SCH2OCH2S-μ)(CO)5]2Lb (5, Lb = 1,4-(CN)2C6H4; 6, dppf; 7, dppr) in 57−90% yields. However, in contrast to 5−7, the double models [Fe2(μ-SCH2OCH2S-μ)(CO)n]2Lc (8, n = 5, Lc = (Ph2PCH2CH2OCH2)2; 9, n = 4, Lc = [(Ph2PCH2)2NCH2]2) could be prepared by direct reaction of 1 in toluene at reflux with 0.5 equiv of diphosphine (Ph2PCH2CH2OCH2)2 and tetraphosphine [(Ph2PCH2)2NCH2]2 in 86% and 56% yields, respectively. 1−9 were characterized by elemental analysis and spectroscopy, and particularly for 1, 2, and 4−9 by X-ray diffraction analysis. The structural features of some model compounds are compared with those of the active site of FeHases. While the cyclic voltammetric behavior of 1 and 5 was studied, 1 was found to be a catalyst for proton reduction of acetic acid to give hydrogen under the corresponding electrochemical conditions. An EECC mechanism for such electrocatalytic H2 production is preliminarily suggested.

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