Mechanistic Studies of Fluorinated Cobaloximes
Authors:Rocio Mercado, Michael J. Rose
Mentor:Harry B. Gray, Arnold O. Beckman Professor of Chemistry, California Institute of Technology
The conversion of solar energy to chemical fuels is an important challenge in the field of energy research. The combination of light-absorbing materials with catalytic materials or molecules has gained acceptance as a strategy to generate “Solar Fuels.” Due to its wide availability, the splitting of water into its elemental components is a particularly attractive approach. One of the missing links in this approach is the identification of molecular catalysts for hydrogen generation that are derived from earth abundant metals like iron, nickel or cobalt. In a previous work, a family of cobalt complexes derived from the fluorinated diphenylglyoxime ligand (“dArFgH2”) was synthesized. These complexes, [Co(dArFgH)2(py)2] (1) and [Co(dArFgH-BF2)2(py)2] (2) , are variations of the extensively studied diglyoxime systems [Co(dRgBF2)2L2] (R = methyl, phenyl), which have been shown to catalyze hydrogen evolution at low overpotentials. Similarly, 2 minimizes energy losses by generating hydrogen at negligible thermodynamic overpotentials (similar to Pt) in electrocatalysis experiments. Through cyclic voltammetry, it was determined that hydrogen evolution is occurring in protic solutions at the second reduction potential (-0.88 V vs. Fc/Fc+). Its precursor, 1, was also characterized and studied as a means of better understanding the hydrogen evolution pathway in 2 since both complexes posses a similar electronic structure. This complex exhibits a similar catalytic peak at E = -0.86 V vs. Fc/Fc+. Nevertheless, due to the differences in the rigidity of the ligand framework, the two complexes differ in their electrochemistry and suggest that the asymmetric complex 2 is a better electrocatalyst for hydrogen evolution.