UC Santa Barbara

For the first part of this defense, we wanted to study new abundant-metal homogeneous ammonia oxidation catalysts in order to gain important mechanistic understandings of these molecular-level transformations. Our catalyst of choice was an Fe-phthalocyanine complex, PcFe (Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalocyanine), which we synthesized and with which we subsequently generated PcFe−amine complexes with ammonia and related alkylamines. Treatment of PcFe with an excess hydrogen atom abstractor, 2,4,6-tri-tert-butylphenoxyl radical (tBuArO•), in the presence of NH3 resulted in catalytic H atom abstraction (HAA) and C−N coupling to produce 4-amino-2,4,6-tritert-butylcyclohexa-2,5-dien-1-one and tBuArOH. Exposing FePc to an excess of the trityl (CPh3) variant, 2,6-di-tert-butyl-4-tritylphenoxyl radical (TrArO•), and NH3 did not lead to catalytic ammonia oxidation as similarly reported in a related Ru-porphyrin complex. However, pronounced coordination-induced bond weakening of both α N−H and β C−H in the PcFe-alkylamine congeners, led to multiple HAA events in the presence of tBuArO•, yielding a cyanide or an imine complex depending on which alkylamine was used. Detailed computational studies were also performed to elucidate the mechanisms involved in these transformations.

For the second half of this defense, we explored the selective, electrochemical capture and release of Li+ from seawater using redox-switchable substituted ortho-carboranes (Cb). Reduction of the neutral “closo-Cb” to the 2 e- reduced “nido-Cb” resulted in C–C bond cleavage and subsequent cage opening, increasing the bite angle of the Cb-appended donor groups. This structural change can be reversed by oxidation of nido-Cb back to closo-Cb. To capture Li+ ions, we synthesized and fully characterized a series of Cb-substituted crown ether compounds, which are known to have high affinity and selectivity to Li+. Li+ binding affinity and selectivity was monitored via 1H, 7Li, 23Na NMR spectroscopy, as well as with cyclic voltammetry. These measurements point to significant observed selectivity for Li+ vs. other alkali metals in the nido-Cb form as described in this thesis. Furthermore, we also extended our studies to group 2 metals, specifically cesium and strontium capture/release, for nuclear waste treatment purposes using larger Cb-crown species.