Synthesis, Characterization, and Reactivity of Functionalized Trinuclear Iron–Sulfur Clusters – A New Class of Bioinspired Hydrogenase Models

The air- and moisture-stable iron–sulfur carbonyl clusters Fe3S2(CO)7(dppm) (1) and Fe3S2(CO)7(dppf) (2) carrying the bisphosphine ligands bis(diphenylphosphanyl)methane (dppm) and 1,1′-bis(diphenylphosphanyl)ferrocene (dppf) were prepared and fully characterized. Two alternative synthetic routes based on different thionation reactions of triiron dodecacarbonyl were tested. The molecular structures of the methylene-bridged compound 1 and the ferrocene-functionalized derivative 2 were determined by single-crystal X-ray diffraction. The catalytic reactivity of the trinuclear iron–sulfur cluster core for proton reduction in solution at low overpotential was demonstrated. These deeply colored bisphosphine-bridged sulfur-capped iron carbonyl systems are discussed as promising candidates for the development of new bioinspired model compounds of iron-based hydrogenases.


Further details on electrochemical experiments:
Peak potential separations ΔE p and ratios of anodic and cathodic peak currents observed in the cyclovoltammetric experiments were used as a diagnostic criteria for distinguishing electrochemically reversible, irreversible and quasireversible processes [J. Heinze, Angew. Chem. 96 (1984) 893]. The condition ΔE p = 59/n was applied for the determination of n-values of the corresponding reversible electron transfer steps.
Overpotential of catalytic proton reduction by Fe 3 S 2 (CO) 7 dppm using TFA in acetonitrile at 298K: From this comparison, an overpotential of 540 mV can be obtained.

Current density:
Working electrode (BAS glassy carbon A= 0.0707 cm 2 geometrical surface area) allows for current density (geometrical) calculation at 540 mV overpotential (= half-peak of the catalytic wave, Fig.5): Catalytic current i cat = 30 µA divided by electrode area leads to: 424 µA cm -2 at -1.37 V vs. Fc + /Fc Turnover frequency of the electrocatalyst Fe 3 S 2 (CO) 7 dppm: (pseudo first order constant at high acid concentration excess: k obs = const. = k * conc. of H + ) Region, where catalytic current i cat does no longer increase with scan rate and H + is in excess gives a maximum turnover frequency = k obs (H 2 product elimination is rate limiting step).

Simulations of Electroatalytic Processes
Current-voltage curves measured with the hydrogenase model compound 1 (Fe 3 S 2 (CO) 7 dppm) in acetonitrile and dichloromethane in the absence and presence of various amounts of trifluoroacetic acid were calculated and plotted using the digital simulation software package DigiSim (version 3.05).
In reasonable agreement with the experimental results obtained, a CECEC-type reaction mechanism with subsequent chemical steps (C) and electron transfer steps (E) could be assumed as a plausible scenario for modelling the electrocatalytic response of 1.
The individual steps simulated using DigiSim were as follows: Step outside the catalytic cycle: H2 = irreversible (gaseous hydrogen evolving and leaving the system) Some of the calculated contributions to the catalytic currents under different reaction conditions (solvent, TFA concentrations added) together with additional experimental results for low acid concentration are given in Figures S11 -S14 below:  The simulated CVs shown in Fig. S12 represent the gradual consumption of initially 20 mM TFA during electrocatalytic hydrogen formation catalyzed by complex 1. Compare these simulated data also with the experimental voltammograms shown in Fig. 5 and with the series of CVs recorded with various TFA concentrations from 0.25 mM up to 5mM (Fig. S13). In Figure S14 below, the simulated contributions to the catalytic currents obtained for complex 1 in dichloromethane are shown in the absence and presence of various amounts of TFA. These data can be compared with the experimental results discussed in the paper (Fig. 6). Figure S14. Simulated CVs for a 1.5 mM solution of monoprotonated complex 1H + in dichloromethane in the absence and presence of excess of trifluoroacetic acid (scan rate 0.1 V s -1 ). Acid concentrations are 0 mM, 5, 10, 15, 20, 30, and 40 mM, respectively (compare also with Figure S6 in acetonitrile solution and with the experimental data given in Figure 6 of the paper).