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Reports: UNI352868-UNI3: Alternative Clean Energy Production with Reusable Iron Catalysts: Development of Design Parameters for Future, Durable Green Energy Catalysts

Jesse W. Tye, PhD, Ball State University

Hydrogen is the ultimate clean and renewable energy currency. The best systems for H2 utilization are platinum-based hydrogen fuel cells, which can produce clean H2 from water (a weak acid) at mild voltages using renewable energy. However, the scarcity of platinum limits widespread application of these fuel cells. The present study is increasing the fundamental understanding of an important class of inexpensive synthetic iron compounds that model the basic core structure of the active site of the iron-iron hydrogenase enzymes and act as catalysts for H2 production by systematic study of their fundamental chemical and electrochemical properties. We are working toward the ultimate goal of developing practical iron catalysts that function as well as platinum-based fuel cells. We are working to design very stable iron complexes that require lower voltages for H2 production from weak acids.

Figure 1. Comparison of linked and unlinked complexes.

The majority of previous studies have focused on the study of linked complexes of the form (μ-SRS)[Fe(CO)2L]2, where the R group links the two S atoms. Electrochemical studies of the unlinked (μ-SR)2[Fe(CO)2L]2 compounds (Figure 1) in which the iron centers are bridged by two separate, unconnected SR groups remain largely unexplored. The few, published electrochemical studies of the unlinked complexes have analyzed the electrochemical properties of an equilibrium mixture of two isomeric forms and have not determined the electrochemisty of each isolated isomer. We have synthesized and purified 5 alkyl and 5 aryl unlinked (μ-SR)2[Fe(CO)3]2 complexes and have begun to examine their chemistry and electrochemistry. We are continuing our efforts to synthesize one alkyl thiolate complex and one aryl thiolate complex. In the reaction of adamantyl thiol with Fe3(CO)12 in refluxing toluene, Fe2S2(CO)6 is the major product and the desired (μ-S(adamantyl))2[Fe(CO)3]2 product is not observed. We have also had difficulties in the synthesis of either 4-phenylthiophenol or the corresponding disulfide required for the synthesis of our 4-phenylthiophenolate bridged di-iron complex. In our hands, the published procedures for the synthesis of 4-phenylthiophenol from 4-phenylphenol do not yield a sufficient quantity of the required thiol.

Although the formulae of unlinked (μ-SR)2[Fe(CO)3]2 complexes imply a simple structure, two stereoisomers that differ in the orientations of the R groups with respect to the sulfur atoms may form and these isomers are found to interconvert via a dynamic process. When we synthesize the (μ-SR)2[Fe(CO)3]2 complexes, we clearly observe a mixture of anti and syn isomers by NMR spectroscopy for all of the alkyl thiolate complexes, however, we only observed a symmetric, syn isomer for all of our aryl thiolate complexes. Density functional theory (DFT) calculations predict that the anti and syn isomers of the phenyl thiolate complex, (μ-SC6H5)2[Fe(CO)3]2 have the same free energy and predict a free energy barrier of 18.2 kcal/mol for anti/syn isomerization for the (μ-SC6H5)2[Fe(CO)3]2 complex. We are investigating the possibility of detecting the anti isomer at low temperature. We do not yet have empirical data that allows us to determine whether our samples consist of a single syn stereoisomer or a mixture of rapidly interconverting anti and syn stereoisomers. In any case, the examination of the electrochemistry of the isolated anti and syn forms of the aryl thiolate complexes seems unlikely.

For our five alkyl thiolate complexes, we have separated the anti and syn stereoisomers by careful column chromatography, and we have shown that solutions of either the separated anti isomer or separated syn isomer slowly convert back to an equilibrium mixture of the anti and syn forms at and above room temperature. The chemical and physical properties of isolated anti and syn forms of (μ-SCH3)2[Fe(CO)3]2 are shown to be quite different. For increasingly larger alkyl groups, the anti/syn isomerization appears to be progressively faster and have a greater preference for the anti isomer. In addition, the chemical and physical properties of the isolated anti and syn isomers are less distinct. For these reasons, we focused our initial efforts on the study of the chemical and electrochemical properties of the methylthiolate complex, (μ-SCH3)2[Fe(CO)3]2. We are investigating the effects of the solvent and dissolved ions on the anti/syn equilibrium.

We have found that the electrochemistry of even our simplest complex, namely (μ-SCH3)2[Fe(CO)3]2, is more complicated than initially expected. The nature of both the supporting electrolyte and solvent play a critical role in the stability of the reduced species. Separated anti and syn isomers undergo reduction at –2.01 V and –1.98 V vs. ferrocene in dichloromethane solvent. An additional reduction is observed in CH3CN solvent. We are examining the ability of the isolated anti and syn isomers to act as an electrocatalyst for H2 production from acetic acid in CH2Cl2. The cyclic voltammogram of the anti form with acetic acid in CH2Cl2 shows 1 cathodic reduction peak with the weakly-coordinating [NBu4][B(C6F5)4] electrolyte and 3 cathodic reduction peaks with the standard [NBu4][PF6] electrolyte. The combination of the [NBu4][B(C6F5)4] electrolyte and CH2Cl2 solvent offers the best opportunity to observe the inherent electrochemistry of the di-iron complexes.

The apparent similarity of the electrochemistry of the anti and syn forms may reflect an inherent similarity in the properties of the two isomers or may be due to the fact that reduction lowers the barrier to the anti/syn isomerization process converting each isomer into a mixture. DFT computations offer insight into the isomerization process for the neutral as well as the one-electron reduced species. The calculations predict that the anti and syn forms have very similar energies for the neutral complexes (within 0.2 kcal/mol), while the anti form is slightly more stable (2.6 kcal/mol) than the syn form for the one-electron reduced complex. The oxidation level has a much more profound effect on the computed barriers for the anti/syn isomerization process. Our calculations predict a barrier of 23.2 kcal/mol for conversion of the neutral, anti (μ-SCH3)2[Fe(CO)3]2 complex to the syn complex, while a free energy barriers of just 3.1 kcal/mol is predicted for isomerization of the anti forms of the one-electron reduced [(μ-SCH3)2[Fe(CO)3]2]– complex.