Synthesis, Characterization and Unique Properties of a Novel, Completely Aliphatic TAML Activator and Applications of TAML Catalysis for Oxygen Evolution and Pollution Remediation

• Main Author: Mills, Matthew R.
• Format: Others
• Published: Research Showcase @ CMU 2016
• Online Access: http://repository.cmu.edu/dissertations/683; http://repository.cmu.edu/cgi/viewcontent.cgi?article=1722&context=dissertations

The novel TAML activator 4, which replaces the aromatic moiety of TAML generation 1 ligands with an aliphatic one, is synthesized and its physical and kinetic properties are characterized. This new catalyst is found to have quite low reactivity at pH 7 compared to catalysts of generations 1, 2, and 3. The stability under oxidative conditions is high compared to these same catalysts, however it is determined that the relationship between the rate of oxidation of the azo dye Orange II, measured by the rate constant kII, and catalyst inactivation, measured by the rate constant ki, is similar to that of previous catalysts. This provides evidence that oxidation of the aromatic moiety of the ligand is not a significant contributor to the suicidal inactivation of TAML activators. The unique properties of TAML activator 4 stemming from the increased steric bulk are explored, which include the counterintuitive resistance to acid-induced demetalation and the lack of dimerization in the iron(IV) state. The combination of these properties allow for clean cyclic voltammetry studies from pH 2 to 13, which has revealed a proton-coupled electron-transfer mechanism in the electrochemical oxidation of 4 in water. Additionally the iron(V)oxo derivative of 4, along with a high-spin iron(IV) complex can be prepared in pure water chemically or through bulk electrolysis. The use of the prototype TAML Activator 1a for electrocatalytic oxygen evolution under alkaline conditions is explored, with evidence for homogeneous oxygen evolution along with in-situ electrodeposition onto the electrode surface of a heterogeneous catalyst. The turnover frequency of homogeneous electrocatalytic oxygen evolution 1,000 ± 100 s-1 is the highest reported for iron-based catalysts operating under alkaline condtions. As a 2 heterogeneous catalyst, the 1a-activated electrode exhibits high performance at pH 14 in terms of overpotential, at 400 mV. The combined effects make 1a one of the best ironbased catalysts for catalytic oxygen evolution under aqueous conditions. TAML Activators 1a and 1b are investigated in the catalytic decomposition of 17α- ethinylestradiol (EE2) with hydrogen peroxide from pH 6 to 9. The rate constant for decomposition of EE2, kII, is found to be (8.6 ± 0.5) × 105 M-1 s-1, which is approximately half the rate of horseradish peroxidase-catalyzed oxidation. The ecotoxicity, specifically the estrogenicity, is found to be greatly reduced after TAML/peroxide treatment according to a Yeast Estrogen Screen and an assay using fathead minnows. The TAML/peroxide treatment remains effective in mitigating EE2 in wastewater effluent samples.