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.