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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 electrochemistry of each isolated isomer. In the second year, we have continued our efforts to resolve these mixtures into their anti and syn isomeric formers.  We have accomplished these feat for 3 of the 5 alkyl (μ-S-alkyl)2[Fe(CO)3]2 complexes, but we remain unable to isolate the anti and syn forms of any of the aryl (μ-S-aryl)2[Fe(CO)3]2 complexes.  It appears likely that the aryl complexes either rapidly isomerize under ambient conditions or that the anti and syn forms of the aryl complexes have spectroscopic features that are so similar as to be indistinguishable. We are continuing our efforts to synthesize one alkyl thiolate complex and one aryl thiolate complex. In our hands, reactions of either adamantyl thiol or adamantyl disulfide with Fe(CO)5, Fe2(CO)9 or Fe3(CO)12 have failed to yield (μ-S-adamantyl)2[Fe(CO)3]2, giving Fe2S2(CO)6 as the only isolable product following chromatographic workup. In the first year, we 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.  In the second year, we found that the published 4-phenylthiophenol synthesis works quite well on a reduced scale.  We combined the 4-phenylthiophenol product from dozens of these small scale reactions into a sufficient quantity to attempt syntheses of the desired (μ-(4-phenylthiophenolate))2[Fe(CO)3]2 complex.  Unfortunately, all reactions of 4-phenylthiophenol with Fe(CO)5, Fe2(CO)9 or Fe3(CO)12 yielded Fe2S2(CO)6 and Fe3S2(CO)9 as the only isolable products following chromatographic separation.

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 three of 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. In our second year, we have found evidence of the effect of atmospheric oxygen and/or water on the anti/syn isomerization. In cases where a benzene solution of one isomer of our purified complexes have been stored on the the bench top, we see isomerization of our single isomer to an anti/syn mixture over the course of several days.  However, when the same reaction is repeated by removing aliquots of a C6D6 solution from inside of an argon-filled drybox, we saw little to no sign of isomerization even over the course of several weeks.

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 electrochemical studies of our complexes have been slowed in the second year by the departure of our electrochemist colleague, Dr. Daesung Chong for a position in industry.