Licensed to -yl : electrocatalytic proton-coupled electron transfer with phenoxyl and metalloverdazyl radicals
Keywords: Humanities and Sciences.
Chemistry.

Chapter One: The current transition from fossil fuels to renewable energy sources requires new energy storage strategies to maximize available solar energy and relieve undue stress on grid infrastructure. Electrochemical solutions to this storage problem, although they are in the early stages of grid-scale deployment, are limited in part by their low charge and energy densities and the expense of the metal ion charge carriers. Redox flow batteries based on neat organic liquids like isopropanol and acetone would be inexpensive and energy-dense by comparison, but the chemical technology to electrochemically interconvert alcohols and carbonyls selectively, rapidly, and efficiently is underdeveloped. New electrocatalytic systems must be realized to meet this challenge. Proton-coupled electron transfer (PCET) mediators can minimize overpotential, avoid undesired hydrogen evolution, and control product selectivity of such electrocatalysts by intercepting or generating key metal hydride intermediates. Coordination-induced bond weakening is a significant theme in many of these PCET-mediated processes, and understanding this phenomenon is key to the development of new mediators. Chapter Two: Electron-rich phenols, including α-rac-tocopherol, 2,4,6,-tri-tert- butylphenol, and butylated hydroxy-toluene, are effective electrochemical mediators for the electrocatalytic oxidation of alcohols by an iridium amido dihyride complex IrN. Addition of phenol mediators leads to a decrease in the onset potential of catalysis from −0.65 V vs Fc+/0 under unmediated conditions to −1.07 V vs Fc+/0 in the presence of phenols. Mechanistic analysis suggests that oxidative turnover of the iridium amino trihydride IrH to IrN 1 proceeds through two successive hydrogen atom transfers (HAT) to 2 equivalents of phenoxyl that are generated transiently at the anode. Isotope studies and comparison to known systems are consistent with initial homolysis of an Ir--H bond being rate-determining. Turnover frequencies up to 14.6 s--1 and an average Faradaic efficiency of 93% are observed. The mediated system shows excellent chemoselectivity in bulk oxidations of isopropanol and 1,2-benzenedimethanol in THF and is also viable in neat isopropanol. Chapter 3: Attempts to expand the phenoxyl mediation platform developed in the previous chapter to other candidate transition metal electrocatalysts are met with mixed results. 4-aminophenol is found to be an effective PCET mediator for the electrocatalytic oxidation of isopropanol with the ruthenium transfer hydrogenation catalyst RuH. This reaction proceeds with an overpotential of only 680 mV in THF. Mediators that were effectively paired with the iridium catalyst IrH in the previous chapter were not suitable for use with RuH, and 4-aminophenol was not viable with IrH. This demonstrates the importance of tailoring the thermochemistry of the PCET mediator to that of the metal hydride catalyst. Attempts to use phenoxyl mediators with the iridium diamine electrocatalyst IrN2H2+ proved unsuccessful. IrN2H2+ is too acidic and doubly deprotonates in the presence of the strong bases needed to turn over the phenoxyl mediator. With no abstractable hydrogen atoms available, PCET mediation is infeasible. Attempts to mediate electrocatalytic benzyl alcohol oxidation with the manganese pincer catalyst MnN led to the discovery that the 2,4,6-tri-tert-butylphenol is competent as a standalone catalyst for this reaction. While the inclusion of MnN enhances the turnover number, the phenol is capable of abstracting hydrogen atoms directly from the alcohol substrate, similar to a N- hydroxyphthalimide oxidation. These investigations indicate that the phenoxyl platform is not universally generalizeable, and that the inherent reactivity of PCET mediators must be examined carefully. Chapter 4: Chelation of a persistent verdazyl radical ligand Vd to Ru(acac)2 results in significant coordination-induced weakening of the ligand's N-H bond in the resulting complex RuVd. Electrochemical measurements provide full thermodynamic square schemes for both the free and bound verdazyls. Coordination lowers the N-H pKa by 5.9, the BDFE by 7.8 kcal/mol, and the hydricity by 22.1 kcal/mol. DFT calculations are in excellent agreement with these experimental results. Examination of the frontier molecular orbitals suggests that the ruthenium center stabilizes the anionic conjugate base RuVd- and relieves the antiaromatic condition experienced by the free anion Vd-. This depresses the pKa and the homolytic and heterolytic bond energies. RuVd exhibits reductive electrocatalytic PCET behavior. In the presence of acetic acid, the trityl radical ·CAr3 (Ar = p-tert-butylphenyl) is catalytically reduced to the corresponding triarylmethane via multi-site concerted proton-electron transfer from RuVdH. Chapter 5: The synthesis, structure, and reactivity of a series of cyclopentadienone and hydroxycyclopentadienyl 4,4'-dimethylbipyridine (dmbpy) iridium complexes (C5Tol2Ph2O)(dmbpy)IrCl 1, [(C5Tol2Ph2OH)(dmbpy)IrCl][OTf] 2 (C5Tol2Ph2O)(dmbpy)IrH 3, and [(C5Tol2Ph2OH)(dmbpy)IrH][OTf] 4 are described. The Ir(I) complexes 1 and 3 are active catalyst precursors for transfer hydrogenation of aldehydes, ketones, and N-heterocycles with HCO2H/Et3N under mild conditions. Model studies implicate the cationic iridium hydride, [(C5Tol2Ph2OH)(dmbpy)IrH][OTf] 4 as a key intermediate, as 4 reacts readily with acetone to generate isopropanol. Selectivity over hydrogenation of alkenes is enhanced compared to other Shvo-type catalysts, and only modest C=C hydrogenation observed when adjacent to polarizing functional groups. Catalytic hydrogenation likely proceeds by a metal-ligand bifunctional mechanism similar to related cyclopentadienone complexes. Chapter 6: Cobaltocene has been long-known to release hydrogen by reductive protonation, stemming from its ability, once protonated, to be reduced a second time producing a net--hydride carrier. Given moderately acidic conditions however, this hydride equivalent can be diverted to a cyclopentadienone-iridium complex that is highly selective for carbonyl reduction, but incompetent as a stand-alone electrocatalyst. This hydride transfer mediation scheme enables highly efficient (Faradaic efficiency > 90 %) electrohydrogenation of acetone, an interesting target as a liquid organic hydrogen carrier. Hydride--transfer mediation presents an under-explored opportunity to access metal hydrides that are resistant to formation by stepwise electron/proton.