Oxygen-Tolerant H₂ Production by [FeFe]-H₂ase Active Site Mimics Aided by Second Sphere Proton Shuttle

Abstract

The instability of [FeFe]-Hases and their biomimetics toward O renders them inefficient to implement in practical H generation (HER). Previous investigations on synthetic models as well as natural enzymes proved that reactive oxygen species (ROS) generated on O exposure oxidatively degrades the 2Fe subcluster within the H-cluster active site. Recent electrochemical studies, coupled with theoretical investigations on [FeFe]-Hase suggested that selective O reduction to HO could eliminate the ROS, and hence, tolerance against oxidative degradation could be achieved ( Nat. Chem. 2017, 9, 88-95). We have prepared a series of 2Fe subsite mimics with substituted arenes attached to bridgehead N atoms in the S to S linker, (μ-S(CH)NAr)[Fe(CO)]. Structural analyses find the nature of the substituent on the arene offers steric control of the orientation of bridgehead N atoms, affecting their proton uptake and translocation ability. The heterogeneous electrochemical studies of these complexes physiadsorbed on edge plane graphite (EPG) electrode show the onset of HER activity at ∼180 mV overpotential in pH 5.5 water. In addition, bridgehead N-protonation and subsequent H-bonding capability are established to facilitate the O-O bond cleavage resulting in selective O reduction to HO. This allows a synthetic [FeFe]-Hase model to reduce protons to H unabated in the presence of dissolved O in water at nearly neutral pH (pH 5.5); i.e., O-tolerant, stable HER activity is achieved.

An [FeFe]‐Hydrogenase Mimic Immobilized through Simple Physiadsorption and Active for Aqueous H2 Production

“…The 2p 1/2 peaks corresponding to two different Fe centres (different ligation) are observed at 708.3 and 710.7 eV, values consistent with those obtained for other reported Fe II -CO species. [18,22] The 1s peak of the bipyridine nitrogen atoms is observed at 399.3 eV, which also agrees with similar complexes reported in the literature. [26,27] The cyclic voltammogram (CV) of FeFe' 2CO @EPG (Figure S2) was recorded in aqueous solutions with 100 mM KPF 6 supporting electrolyte at pH~5.5.…”

“…In order to overcome these drawbacks, the immobilization of molecular catalysts on electrode surfaces is one of the most promising strategies. [15][16][17][18][19][20][21][22][23] Here we report electrocatalytic H 2 production in acidic aqueous solutions with a mimic of the [FeFe]-hydrogenase active site (Scheme 1). The previously described complex [L N2S2 (CO)Fe II (CO)Fe II Cp] + (FeFe' 2CO , Scheme 1, with L N2S2 = 2,2'-(2,2'-bipryridine-6,6'-diyl)bis(1,1'-diphenylethane-thiolate) and Cp À = cyclopentadienyl anion) displays one CO terminally bound to the iron center of the L N2S2 Fe II site and a second CO semi-bridging the two centres in favour of the Fe II Cp site.…”

Following Electroenzymatic Hydrogen Production by Rotating Ring–Disk Electrochemistry and Mass Spectrometry**

“…[24] Next, the ability to electrochemically follow H 2 produced by a CpI + Cc-BPEI-functionalized electrode was investigated by rotating ring-disk electrochemistry (RRDE). While Pt electrodes efficiently reduce H + and oxidize H 2 , and RRDE can therefore be employed to follow H 2 production at Pt ring electrodes, [25,26] this technique has not yet been utilized to study mechanisms of H 2 -producing metalloenzymes. Figure 3 a presents a cyclic voltammogram for Cc-mediated H + reduction by CpI, where a reductive catalytic "wave" commencing at around < À0.4 V vs. SHE was observed representing electroenzymatic H + reduction to H 2 at a glassy carbon (GC) disk electrode.…”

The roles of chalcogenides in O2 protection of H2ase active sites

“…On replacement of the -CH 2group in (m-pdt)[Fe(CO) 2 L][Fe(CO) 2 L 0 ] with a secondary amine bridgehead, H 2 production was possible in the presence of dissolved O 2 in water (pH ¼ 5.5). 57,58 It is proposed that the bridgehead N-protonation and subsequent Hbonding capability assists SO-O bond cleavage which accounts for O 2 tolerance, as shown in the Scheme 6b. 58…”

“…57,58 It is proposed that the bridgehead N-protonation and subsequent Hbonding capability assists SO-O bond cleavage which accounts for O 2 tolerance, as shown in the Scheme 6b. 58…”