Heterogenization of a Water-Insoluble Molecular Complex for Catalysis of the Proton-Reduction Reaction in Highly Acidic Aqueous Solutions

摘要

Our long-held interest in the resiliency of electrochemical functionalities upon surface immobilization has herded us from directly chemisorbed electroactive moieties [1, 2], to anchor group-leashed redox-active couples [3] and to surface-tethered enzyme-inspired molecular catalysts [4-6]. The latter represent the most intricate because the electrocat-alytic activities involve mixed-valence states and may require certain entatic (fractionally rotated) configurations [6, 7]. In this regard, we recently investigated the proton-reduction electrocatalysis by hydrogenase-inspired di-iron complexes at polycrystalline and (111)-faceted Au electrodes [4-6]. One complex, (μ-S2C3H6)[Fe(CO)3][Fe(CO)2PPh3], was devoid of a surface anchor group and, hence, was present only in solution; the other complex, (μ-S2C3H6)[Fe(CO)3][Fe(CO)2(PPh2(CH2)2SH)] (I), was bound to the surface through its mercapto group that allowed the catalytic di-iron moiety to remain to be pendant. The electrochemistry experiments had to be performed in acetonitrile, with n-Bu4NBF4 as a supporting electrolyte, because the complexes are insoluble in aqueous solutions. No activity was displayed by the homogeneous compound; dramatic catalysis was shown by the heterogenized complex. Earlier computational studies had indicated that the iron hydride intermediate is formed at a terminal site on a partially rotated iron [7, 8]; evidently, such entatic process is not hindered even at a densely packed surface-sequestered catalyst. We have now expanded the work to explore the extensibility of the heterogenization protocol to highly acidic aqueous solutions (0.5 M H2SO4). The results are described in this letter..