Redox reactions of copper complexes with fluorinated oxygen-donating ligands

• Main Author: Hannigan, Steven Francis
• Language: en_US
• Published: 2018
• Subjects: Chemistry
• Online Access: https://hdl.handle.net/2144/27073

Three Cu complexes varying in oxidation state from +1 to +3 were prepared from CuII starting materials and the hexafluoro-α-cumyl alkoxide ligand. These species include a targeted CuII tris-alkoxide, K[Cu(OC(C6H5)(CF3)2)3] (3); a trimeric Cu(I) complex, {K(18C6)}[K2{Cu(OC(C6H5)(CF3)2)2}3] (4); and a rare organocuprate CuIII complex, {K(18C6)}[Cu(OC(C6H4)(CF3)2)2] (5), which is stable at RT and has formed via ortho¬ metalation of two C-H bonds present. The three complexes have been structurally characterized, and 4 and 5 were studied using NMR spectroscopy to gain insight into the mechanism of formation. Using the bidentate perfluoropinacolate ligand (pinF)2-, four CuI complexes of the form K[(R3P)Cu(pinF)] or {K(18C6)}[(R3P)Cu(pinF)], with PR3 = PPh3 (9, 11) or PCy3 (10, 12) have been synthesized, and structurally and spectroscopically characterized. Complexes 9-12 reduce O2 at -78°C to form a symmetric trinuclear {Cu3O2} species, SyTpinF, which has been characterized by EPR, cryo-MS, and stopped-flow spectroscopy. Kinetic formation and decay constants in conjunction with DFT calculations revealed evidence for an asymmetric, trimeric, intermediate species, AsTpinF, which forms prior to SyTpinF. The trimeric core catalytically oxidizes para-hydroquinone to benzoquinone as a form of oxidase chemistry. The CuII complex, K2[Cu(pinF)2] (14), has been evaluated for stability at reducing potentials in organic and aqueous media by cyclic voltammetry and UV-vis. The electrochemical reactivity of 14 was examined in basic conditions, which showed that 14 is a precatalyst for the reduction of the NOx substrates NO3- and NO2-. The catalytic onset potential of NOx reduction coincides with the reduction of a transient CuI species to Cu0 on the glassy carbon based electrode. Controlled potential electrolysis with concomitant 15N-NMR studies have shown near quantitative reduction of NO3- to NO2- and NH4+ products or reduction of NO2- to NH4+.