(639d) Nature of the First Electron Transfer in Electrochemical Ammonia Activation in a Nonaqueous Medium
We have found that electrochemical ammonia oxidation in acetonitrile occurs via an outer-sphere mechanism where the initial electron transfer is the rate-determining step, likely producing an ammonia radical cation. This reaction is first order in ammonia concentration in the potential windows explored. DFT calculations suggest a radical intermediate and show that there is a large solvent reorganization energy associated with ammonia oxidation (1.94 eV). This large reorganization energy can explain why the apparent transfer coefficient for ammonia oxidation (0.32±0.01) is lower than the expected value of one-half. Understanding the mechanism and intermediates involved in ammonia oxidation is a key step toward the usage of ammonia in electro-organic syntheses to form products with nitrogen bonds, and this work provides a starting place for electrochemical synthesis of compounds containing nitrogen bonds, such as amino acids and benzylic amines. We believe that ammonia oxidation in solvents other than acetonitrile could enable alternative mechanisms that would further enable the use of ammonia in electrochemical reactions.