Mechanistic Insights into Electrochemical Dinitrogen Reduction from Studies of Molybdenum PSP Pincer Complexes

Pincer-ligated molybdenum complexes are among the most effective molecular catalysts for the reduction of dinitrogen to ammonia, with the reductive splitting of N2 to generate a pair of metal nitride complexes being proposed as a key step in many cases. Reported herein is a detailed electrochemical study of N2 binding and splitting by a molybdenum complex supported by a diphosphino-thioether (PSP) pincer ligand. Cyclic voltammetry of (PSP)MoCl3 exhibits reversible reductions indicating slow chloride dissociation and a lack of N2 binding, and electrolysis did not result in nitride formation. The corresponding bromide complex, (PSP)MoBr3, was therefore synthesized and characterized by spectroscopy, crystallography, and electrochemical methods. Analysis of scan-rate-dependent cyclic voltammetry of the bromide complex was used to determine rate constants for bromide dissociation. Although the first reduction of (PSP)MoBr3 occurs at encouragingly mild potentials with concomitant Br– dissociation, subsequent binding of N2 requires an additional reduction that occurs at quite negative potentials. Controlled potential electrolysis at the second reduction resulted in successful splitting of N2 to form the nitride complex (PSP)Mo(N)Br. However, depending on the applied potential and the atmosphere (N2 vs Ar), electrolysis is accompanied by varying levels of decomposition that results in deposition of a passivating Mo-based material on the electrode surface. This work provides insight into how pincer ligand donor ability connects to electrochemical mechanisms of N2 splitting and highlights how surface analysis can provide valuable insight into complex stability and the importance of appropriate electrochemical conditions for N2 activation.