Metalloporphyrins or vanadium (oxo) complexes and their applications to bond activation chemistry

Recent developments in new molecular designs, scaling relationships, and theoretical modeling of mechanisms have rapidly advanced the utility of metalloporphyrins as electrocatalysts in the activation of small molecules, in particular O2 and CO2. The development and improvement of electrocatalysts for the 4H+ /4e− reduction of O2 to H2O, and for the CO2 reduction reaction (CO2RR) to provide a route to turn a greenhouse gas into value-added products, are ongoing challenges. The addition of ancillary groups (e.g., hydrogen bonding, Brønsted acid/base) near the active site of metal-containing catalysts is an effective way to improve the selectivity and kinetics of the oxygen reduction reaction (ORR) and CO2RR. In this regard, iron porphyrins are among the most researched ORR/CO2RR catalysts. Closely related cobalt porphyrin catalysts can function closer to the O2/H2O thermodynamic potential and can reduce CO2 at a lower overpotentials, but they tend to be less selective and follow a different mechanism than iron porphyrins. Here, I explore strategies to extend the ideas about ancillary groups that have been developed for iron porphyrin ORR/CO2RR electrocatalysts to improve the performance of the corresponding cobalt complexes. I describe a series of porphyrin electrocatalysts for ORR/CO2RR that are modified versions of Co(5,10,15,20-tetraphenylporphyrin), where the 2-position of one of the phenyl groups contains -NH2, -N(CH3)2, and -N(CH3)3+. In Chapter 2, the presence of a cationic ancillary group gives rise to a catalyst that is selective for the conversion of O2 to H2O across a wide pH range. Electrostatic groups for cobalt porphyrins enhance the performance of 4e– reduction of O2 to H2O, while protic ancillary groups are important in the performance of iron porphyrin ORR catalysts. In Chapter 3 and 4, homogeneous and heterogeneous CO2RR are studied, respectively. For cobalt porphyrins, protic hydroxyphenyl ancillary groups aid in production of CO, while electrostatic stabilizers of CO2-bound intermediates seem to favor other CO2 reduction products. These studies show that the second sphere interactions for ancillary groups of metalloporphyrins must be carefully designed for different metals and different reactions. In addition, the photochemical activation of carbon-hydrogen bonds by vanadium-dioxo (VV(O)(O)) and vanadium-oxo-peroxo (VV(O)(O2)) diimine complexes is described in Chapter 5. The involvement of electronically excited V complexes as radical reaction initiators is described.