Diiron Oxadithiolate Type Models for the Active Site of Iron-Only Hydrogenases and Biomimetic Hydrogen Evolution Catalyzed by Fe<sub>2</sub>(<i>μ</i>-SCH<sub>2</sub>OCH<sub>2</sub>S-<i>μ</i>)(CO)<sub>6</sub>

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, Fe<sub>2</sub>(<i>μ</i>-SCH<sub>2</sub>OCH<sub>2</sub>S-<i>μ</i>)(CO)<sub>6</sub> (<b>1</b>), was prepared by reaction of (<i>μ</i>-S<sub>2</sub>)Fe<sub>2</sub>(CO)<sub>6</sub> with 2 equiv of Et<sub>3</sub>BHLi followed by direct treatment with excess (ClCH<sub>2</sub>)<sub>2</sub>O or by successive treatment with 2 equiv of CF<sub>3</sub>CO<sub>2</sub>H and excess (ClCH<sub>2</sub>)<sub>2</sub>O in the presence of Et<sub>3</sub>N. Further reaction of <b>1</b> with 1 equiv of Me<sub>3</sub>NO in MeCN at room temperature followed by treatment of the intermediate Fe<sub>2</sub>(<i>μ</i>-SCH<sub>2</sub>OCH<sub>2</sub>S-<i>μ</i>)(CO)<sub>5</sub>L (L = MeCN or Me<sub>3</sub>N) with 1 equiv of Et<sub>4</sub>NCN, PPh<sub>3</sub>, or Cp(CO)<sub>2</sub>FeSPh gave the single models Fe<sub>2</sub>(<i>μ</i>-SCH<sub>2</sub>OCH<sub>2</sub>S-<i>μ</i>)(CO)<sub>5</sub>L<sub>a</sub> (<b>2</b>, L<sub>a</sub> = (CN)(Et<sub>4</sub>N); <b>3</b>, PPh<sub>3</sub>; <b>4</b>, Cp(CO)<sub>2</sub>FeSPh) in 62−93% yields, whereas the in situ treatment of the intermediate Fe<sub>2</sub>(<i>μ</i>-SCH<sub>2</sub>OCH<sub>2</sub>S-<i>μ</i>)(CO)<sub>5</sub>L with 0.5 equiv of 1,4-(CN)<sub>2</sub>C<sub>6</sub>H<sub>4</sub>, (<i>η</i><sup>5</sup>-Ph<sub>2</sub>PC<sub>5</sub>H<sub>4</sub>)<sub>2</sub>Fe (dppf), or (<i>η</i><sup>5</sup>-Ph<sub>2</sub>PC<sub>5</sub>H<sub>4</sub>)<sub>2</sub>Ru (dppr) afforded the double models [Fe<sub>2</sub>(<i>μ</i>-SCH<sub>2</sub>OCH<sub>2</sub>S-<i>μ</i>)(CO)<sub>5</sub>]<sub>2</sub>L<sub>b</sub> (<b>5</b>, L<sub>b</sub><b> = </b>1,4-(CN)<sub>2</sub>C<sub>6</sub>H<sub>4</sub>; <b>6</b>, dppf; <b>7</b>, dppr) in 57−90% yields. However, in contrast to <b>5</b>−<b>7</b>, the double models [Fe<sub>2</sub>(<i>μ</i>-SCH<sub>2</sub>OCH<sub>2</sub>S-<i>μ</i>)(CO)<i><sub>n</sub></i>]<sub>2</sub>L<sub>c</sub> (<b>8</b>, <i>n</i> = 5, L<sub>c</sub> = (Ph<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>OCH<sub>2</sub>)<sub>2</sub>; <b>9</b>, <i>n</i> = 4, L<sub>c</sub> = [(Ph<sub>2</sub>PCH<sub>2</sub>)<sub>2</sub>NCH<sub>2</sub>]<sub>2</sub>) could be prepared by direct reaction of <b>1</b> in toluene at reflux with 0.5 equiv of diphosphine (Ph<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>OCH<sub>2</sub>)<sub>2</sub> and tetraphosphine [(Ph<sub>2</sub>PCH<sub>2</sub>)<sub>2</sub>NCH<sub>2</sub>]<sub>2</sub> in 86% and 56% yields, respectively. <b>1</b>−<b>9</b> were characterized by elemental analysis and spectroscopy, and particularly for <b>1</b>, <b>2</b>, and <b>4</b>−<b>9 </b>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 <b>1 </b>and <b>5</b> was studied, <b>1</b> 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 H<sub>2</sub> production is preliminarily suggested.