Photolysis of Aqueous [(NH3)5Os(μ-N2)Os(NH3)5]~(5+): Cleavage of Dinitrogen by an Intramolecular Photoredox Reaction

摘要

The lack of reactivity of dinitrogen which complicates its chemical conversion has been a challenge to chemists for many decades. This difficulty is based on the extreme stability of the nitrogen-nitrogen triple bond. The huge energy difference between HOMO and LUMO (23 eV) makes N2 rather redox inert. Moreover, the conversion of N2 into simple species, such as ammonia or nitride, requires the transfer of six electrons. Such multi-electron transfer processes are generally associated with large activation barriers. Nevertheless, the reduction of N2 to NH3 occurs in nature through the utilization of the enzyme nitrogenase as catalyst. This conversion also takes place in the Haber-Bosch process, however, extreme conditions are required. Accordingly, it is of considerable interest to accomplish this reaction at ambient conditions. In principle, catalysis can be replaced by a photochemical procedure. The activation energy is then supplied by light. In favorable cases the photoactivation is selective and avoids interfering processes. Moreover, light may not only provide the activation energy but also the energy for an endothermic reaction which does not occur in catalysis at lower temperatures.