Non-Innocence in 2Fe2S Clusters for the Purpose of Molecular Hydrogen Production

Abstract

The production of molecular hydrogen as a fuel source to replace traditional carbon-based sources is key to the world's environment and the national security of the United States. Production of molecular hydrogen by energy efficient means with abundant (inexpensive) materials is a recently emerged field throughout science. One approach is to look to nature for inspiration, and this is indeed what our research group has done by mimicking the active site of [FeFe]-hydrogenases. Unfortunately all current models operate at a potential considerably more negative than the thermodynamic potential of hydrogen production. This work attempts to address this deficiency by modulating the reduction/oxidation potentials of the 2Fe2S core by binding it to non-innocent ligands. Non-innocent ligands are ligands which have redox behavior coupled to the reduction/oxidation events of the metal center to which they are bound. This includes ligands which are chromophores, potentially allowing for the reduction of excited states at a less negative potential than the ground state of the complex. One group of catalysts examined in this work is a series of substituted (μ-S₂-1,4-quinone) Fe₂ (CO)₄L₂ complexes where L= CO, or PR₃. The electronic communication between the disulfide ligands and the metal centers where catalysis occurs dictates the reactivity of a catalytic complex, and thus knowledge of this interaction is vital to construct more efficient catalysts for hydrogen production. These complexes are found to go through two separate one-electron reductions analogous to their parent 1,4-quinone compounds but with much less negative reduction potentials. The once-reduced quinone complex is relatively stable and gives the ability to study the electronic communication of the radical species via electron paramagnetic spectroscopy (EPR). Delocalization to the iron centers is demonstrated visibly by the ³¹P hyperfine splitting in the EPR spectrum of a phosphine-substituted derivative. Modeling these EPR spectra with DFT calculations indicates about 20% spin electron delocalization from the semiquinone anion radical to the iron centers, and changing the functionality of the quinone gives the ability to tune spin density at the metal centers. In the presence of excess acid, the electrochemically produced semiquinone reacts to form the hydroquinone derivatives which subsequently form molecular hydrogen. One possible way of lowering the overpotential of hydrogen producing catalysts is to use a chromophore to capture the energy contained in light, and then transfer the energy to the catalyst active site. One example of a chromophore, which also happens to be capable of binding metal centers is 2-phenylazopyridine. In order to study the interplay between this chromophore and 2Fe2S catalysts, of the study of two compounds, 1,2-(µ-benzenedithiolato)-2'-phenylazopyridinediiron-tetracarbonyl and 1,3-(µ-propanedithiolato)-2'-phenylazopyridinediiron-tetraacarbonyl, was undertaken. The UV-Vis spectra of both complexes show an intense absorption with a molar extinction coefficient in the range of a ligand-to-metal charge transfer; however the wavelength of maximum absorption does not show a dependence on solvent polarity for either complex. Time-dependent-DFT calculations predict the UV-Vis spectra well and show the transition to indeed be a ligand-to-metal charge transfer. DFT calculations show that the difference between the energy of the ground state and excited state has little variance with solvent polarity. Additionally molecular orbital correlation diagrams are constructed to illustrate the relative orbital energies and intramolecular interactions of the complex. Of particular interest is that the LUMO of 2-phenylazopyridine is relatively low lying in energy, and mixes to comprise approximately 65% of the LUMO of each complex. Modeling the electronic structure of a non-innocent ligand when not ligated can aid in understanding the electronics of a complex containing the ligand. With the aim of eventually synthesizing a series of [FeFe]-hydrogenase mimics with non-innocent 4,4'-bipyridine 3,3'-dithiolato ligands; the behavior upon reduction of a series of three 4,4'-bipyridine 3,3'-disulfide compounds has been investigated by means of cyclic voltammetry and DFT calculations. These complexes contain two possible redox active sites; the disulfide bond and the bipyridine/bipyridinium ring. The three compounds show distinct cyclic voltammograms. The nonmethylated compound goes through an irreversible two-electron reduction followed by a two electron oxidation. The monomethylated compound undergoes two quasi-reversible one-electron reductions, while the dimethylated compound undergoes four reversible one-electron reductions. DFT calculations show that both reductions of the nonmethylated compound are centered around the disulfide bond with S-S cleavage occurring after insertion of the second electron. The LUMO of the monomethylated and dimethylated compounds is centered on the bipyridine rings, and consequently that is where initial reduction occurs. Evidence of intramolecular electron transfer is observed after the first reduction of the monomethylated and the second reduction of the dimethylated compound. The experimental cyclic voltammograms can be simulated in agreement with the proposed DFT mechanisms. Non-innocent ligands show rich and complex redox behavior which allow for modulating the redox potentials of catalytic 2Fe2S centers; understanding the interaction between the two redox centers allows for better design of catalytic systems, and is a promising approach.