57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

This proposal investigates the interaction of iron(III) with natural organic matter (NOM) by using model systems to explore reactions that modify NOM structure. Two specific transformations were initially targeted, oxidation of phenols and photo-oxidation of carboxylic acids. Initial phenol oxidation experiments were successful, while carboxylate oxidations were hampered by problems detecting and quantifying the organic products. Work during the reporting period has therefor focused on oxidation of phenols by iron(III), specifically the mechanism of oxidation of hydroquinone by a iron(III)-m-oxo dimer supported by tris(pyridyl)methylamine (TPA) ligands. The progress of this work was presented by the PI in a paper at the 2012 ACS National Meeting in Philadelphia. Two undergraduate students also presented posters on this project in the CHED division. A manuscript is currently in progress and will be submitted for publication in Inorganic Chemistry later this year.

Impact

Diiron(III) complexes (presumably m-oxo bridged) have been identified in NOM samples by x-ray absorption spectroscopy and this redox-active moiety is an interesting target for oxidative modification of NOM structure. We chose initially to investigate diiron(III) complexes of the ligand tris(2-pyridylmethyl)amine (TPA) because of the rich history of this ligand supporting diiron(III) complexes that model the active sites of metalloenzymes (e.g. sMMO and RR). Interestingly, the aqueous coordination chemistry of diiron(III) TPA complexes has not been reported. This work is therefor important as both a proof-of-principle experiment to demonstrate the utility of modeling iron(III)-NOM interactions in natural systems, and by its contribution to the coordination chemistry of an important bioinorganic model system.

Progress

The first part of this project involved determining the aqueous speciation of iron(III)-TPA complexes. This was done by non-linear least squares fitting of a speciation model to potentiometric titration data. This process generates formation constants for each species present in the model, which can then be used to quantify the iron complexes present as a function solution conditions (pH, concentration of reagents). In the pH range of interest (4-7), iron(III)-TPA complexes exist as μ-oxo dimers with the general formula [(TPA)FeX]₂O (1a: X = H₂O; 1b: X = H₂O, OH^-; 1c: X = OH^-), i.e. as the diprotic acid 1a with the conjugate bases 1b and 1c. The two pK_a's for 1a were determined to be 4.4 ± 0.4 and 5.0 ± 0.4. Previous work on 1b in the solid-state and acetonitrile solution showed that the H₂O and OH^- ligands form a H-bonded bridge between the two iron atoms. An important result from this project is that UV-vis data suggests that this the bridge does not persist in aqueous solution.

The information gained on the thermodynamics of iron(III)-TPA complexes was then applied to phenol oxidation. 1a-c oxidizes hydroquinone (HQ) to benzoquinone in 95% yield by HPLC. The reaction may be monitored by UV-vis, and pseudo 1^st order kinetics are observed in the presence of excess HQ. Varying [HQ] gave rise to Michaelis-Menten-type pre-equilibrium behavior, and at pH 5.6 K_d and k₂ were measured to be 50 ± 4 mM and 0.61 ± 0.03 s^-1 respectively. A goal of this work is to determine how the protonation state of the metal complex effects HQ oxidation; however, the kinetic parameters above represent aggregate values for the distribution of species 1a-c at pH 5.6. To determine the individual microscopic rate constants (i.e. K_d and k₂ for the individual complexes 1a-c) a three-dimensional data set was generated (k_obs as a function of [HQ] and [H⁺]). These data were fit using the equilibrium constants derived from potentiometric titration data (above). Although additional kinetic data is needed to complete this analysis, K_d and k₂ for 1c has been determined to be 50 ± 10 mM and 0.77 ± 0.07 s^-1 respectively. These data are in agreement with our aggregate rate data, as the iron(III)-TPA speciation at pH 5.6 is dominated by 1c. Determination of the microscopic rate constants for 1a and 1b will provide insight into the mechanism of oxidation of phenols by iron(III)-μ-oxo dimers.

Student impact

This PRF-UNI grant has significantly impacted several undergraduate students working on this project. Two students, Christine Bange and David Strauss, were supported during the summer research period of 2011. David ultimately decided to pursue biochemistry, however Christine continued her work during AY2011-12 and again during the summer of 2012. Funds from the PRF enabled Christine to travel to the 2012 ACS National Meeting in Philadelphia and present a poster on her work. Her undergraduate research experience at Bucknell University has convinced Christine to pursue a Ph.D. in chemistry and she is in the process of applying to graduate programs. Another undergraduate student, Kaitlyn Perez, also used PRF funds to travel to the Philadelphia ACS Meeting and present a poster. Kaitlyn's 2012 summer research stipend was paid from department funds, however she is planning to continue this project next summer and will be supported by PRF funds.