Electronic Tunings in Biomimetic Iron Complexes for Small Molecule Activation
By Teera Chantarojsiri
Doctor of Philosophy in Chemistry
University of California, Berkeley
Professor Christopher J. Chang, Chair
The ever-increasing global energy consumption drives the search for sustainable, alternative, carbon-neutral energy sources. Chemists approach these problems taking inspiration from nature, which utilizes various metalloenzymes for different types of small molecule activations, including proton reduction (hydrogenases), water oxidation (oxygen-evolving complexes), methane oxidation (methane monooxygenases), and carbon dioxide fixation (carbon monoxide dehydrogenases). Several small-molecule model systems have been developed to mimic the reactivity and structures of active sites in these enzymes. The main advantage of molecular inorganic catalysts in this vein includes the ability to chemically modify the electronic properties of the molecule, eliciting changes in physical properties and catalytic reactivity. Using traditional synthetic methods, the reactivity of the modified catalysts can be studied in order to elucidate the mechanisms of small molecule transformations. Iron (Fe) complexes are also good candidates due to iron’s high abundance in the earth crust and rich redox reactivity.
For hydrocarbon oxidation, high-valent iron-oxo intermediates play important roles in both chemical and biological oxidations. Their rich reactivity inspires the development of synthetic analogs, especially Fe(IV)-oxo complexes. In contrast to their enzymatic counterparts, however, the vast majority of biomimetic Fe(IV)-oxo complexes have been prepared and studied in organic solution. A series of water-soluble Fe complexes supported by PY5Me2-X ligand with substituents on the axial pyridine (X= -NMe2,-Me, -H and –CF3) were synthesized to study the ligand’s electronic effects their oxidative reactivity. Crystal structures of Fe(II) complexes showed small variation in bond lengths between different derivatives. Mössbauer spectroscopy confirmed the identity of Fe(II) and small variation in electronic properties as the quadrupole splitting increase as the electron-donating ability of the ligand increase. Electrochemical studies of [Fe(L)(PY5Me2-X)]2+ in MeCN and water showed drastic decrease of Fe(III/II) redox potentials (300 mV in MeCN and 200 mV in water when changing substituents from electron-withdrawing (-CF3) to electron-donating group (-NMe2) and linear correlation with Hammett parameters (σp). We observe that Fe(III/II) redox potentials also significantly less positive in aqueous media, resulting from the proton-coupled electron transfer process.
Oxidation in water using outersphere oxidant, cerium ammonium nitrate (CAN), produced clean Fe(IV)-oxo species, verified by UV-vis and Mössbauer Spectroscopy. Fe(IV)-oxo species produced is stable in water at room temperature for 2 hours with an exception of –NMe2 derivatives as its Fe(IV)-oxo species is only detectable transiently at low temperature by mass spectrometry. Labeling study with H218O showed a m/z shift of 1 Da in mass spectrometry of 18O-labeled Fe(IV)-oxos and a peak shift in IR which corresponds to the harmonic oscillator model, proving that the water is the source of oxygen atom. Furthermore, Fe(IV)-oxo can also be generated photochemically using Ru(bpy)32+ as a photosensitizer, K2S2O8 as a sacrificial quencher and 13 W blue CFL light bulb as a light source. Photochemically-generated Fe(IV)-oxo can be used to probe oxidative reactivity by hydrocarbon oxidation in aqueous solution. Three substrates were used to evaluate oxidative reactivity of each derivatives: benzyl alcohol, 4-ethylbenzene sulfonate sodium salt, and 4-styrenesulfonate sodium salt. Second-order rate constants were measured by observing decay of the characteristic Fe(IV)-oxo signature centered at 710 nm in the UV-vis spectra upon substrate oxidation. Among this systematic series, we observe that the [FeIV(O)PY5Me2-X]2+ derivative containing the most electron-withdrawing group, the -CF3 congener, shows the fastest rates of oxidation for both HAT and OAT reactions with the substrates tested. Reaction rate differences also correlate to Hammett parameters (σp) of the ligand’s substituents.
Extending the concept of biomimetic chemistry, Fe complexes that mimic heme enzymes were studied as a CO2 reduction catalyst as Fe porphyrins have been demonstrated to reduce CO2 electrochemically. Square planar Fe salen complexes were synthesized and characterized for their CO2 reactivity. Unlike the porphyrins, these ligands can be synthesized in large scale and easily tuned the electronics by changing substituents. Exploring redox reactivity of Fe salen complexes electrochemically under Ar reveals a Fe(II/I) redox couple for all the complexes in that family that shifts more positive upon substitution with electron withdrawing groups. Upon exposure to CO2 atmosphere, cyclic voltammetry showed current enhancement at the Fe(II/I) and Fe(I/0), which is indicative of a catalytic process for all complexes. Catalytic onsets can also be shifted more positively by using ligand with electron-withdrawing substituents.
Controlled potential electrolysis experiments under a CO2 atmosphere of FePhsalenCl showed remarkable selectivity toward CO production over H2 production, with Faradaic efficiency of 40% and less than 1% respectively. Because no other C1 product was found, we postulated that some product, CO, was trapped at the Fe center, which then proceed to deactivate the catalyst. To prove this hypothesis, CV experiments in the presence of a CO scavenger molecule, [Ni(TMC)](PF6)2, was conducted. The increased catalytic current supports the hypothesis. Furthermore, electrolysis of Fe catalyst with [Ni(TMC)](PF6)2 also increased the amount of CO produced. CV of Fe salen complex under a CO atmosphere also produced a new feature that resembles the redox couple of Fe complex after controlled potential electrolysis. IR spectroscopy of the electrolysis solutions showed small changed of Fe complex upon applied potentials, suggesting that the catalyst remains intact during the reaction.
Taken together, this body of work displays the potential of biomimetic iron catalysts for both oxidative and reductive transformations of small molecules. Understanding the molecular basis through electronic tunings of metal catalysts for these reactions will further aid the development of robust and efficient catalysts in homogeneous catalysis.