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

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)₅L_a (2, L_a = (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]₂L_b (5, L_b = 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]₂L_c (8, n = 5, L_c = (Ph₂PCH₂CH₂OCH₂)₂; 9, n = 4, L_c = [(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.