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

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, (η 5-Ph2PC5H4)2Fe (dppf), or (η 5-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.

Electron and Spin Density Topology of the H‐Cluster and Its Biomimetic Complexes

“…All of the calculations were performed starting from the crystal structure coordinates for [Fe 2 (CO) 6 (μ-SCH 2 -NHCH 2 S)], [20] [Fe 2 (CO) 6 (μ-SCH 2 OCH 2 S)], [21] [Fe 2 (CO) 6 (μ-SCH 2 CH 2 CH 2 S)], [22] [Fe 2 (CO) 4 (CN) 2 (μ-SCH 2 NHCH 2 S)], [23] and 1-(pdt) were analyzed using (i) the coordinates of the crystal structure of the analogous [2Fe]-biomimetic model and (ii) the coordinates that were obtained after the optimization of the geometry. Throughout the text, relaxed structures correspond to molecules that have been obtained by the optimization of the geometry of the crystal structure in the gas phase, whereas fixed structures correspond to complexes where the atoms are fixed at their crystallographic positions (single point calculations).…”

Biomimetic hydrogen generation catalyzed by triironnonacarbonyl disulfide cluster

“…The suggested electrochemical-electrochemical-chemicalchemical (EECC) mechanism is based on the results described above and similar reported examples. 16 As 9 . However, more studies (computational, bulk electrolysis, and spectroelectrochemical) are required to characterize the species involved and ascertain the mechanism.…”

“…These observations are indicative of electrocatalytic reduction of proton to molecular hydrogen. 16 The overpotential was determined to be -0.78 V using the standard reduction (E acid. 17 The overpotential and potential for the reduction of acetic acid to hydrogen (E cat ) by Fe 3 S 2 (CO) 9 are contained in Table 2 alongside those of some diironcarbonyl models of [Fe-Fe] hydrogenase.…”

Design of Biomimetic Models Related to the Active Sites of Fe-Only Hydrogenase

“…The energy difference, Δ CO , therefore can be treated as an approximate gauge of electronic asymmetry between two Fe subunits and act a compliment to the ψ and Φ angles. For the complexes with inequivalent phosphine substitution, the Δ CO value is commonly greater than 100 cm -1 Song et al, 2005;. On the other hand, smaller energy difference of 70-90 cm -1 is observed for the symmetrical diphosphine species (Gao et al, 2007;Li et al, 2007).…”