Research on the use of transition metal (TM) complexes for the reduction of dinitrogen has proliferated in recent years. In particular, great progress has been made in the past decade in the use of pincer-ligated TM complexes in this context. Several such complexes have a pincer-ligated molybdenum core which appears to effect dinitrogen cleavage and subsequent transformation of the resulting nitride ligand to ammonia using protons and electrons. Molybdenum-based trihalide pincer complexes are found to require ‘pre-activation’, by a strong reducing agent, to the pincer-ligated Mo(I) monohalide which then effects dinitrogen splitting to yield a pincer ligated molybdenum(IV) nitride halide.

Previous studies have demonstrated the reduction of molybdenum nitrides to ammonia in the presence of chemical reductants and proton sources, but little is known about the relative reactivity of various nitrides and the detailed sequence of events leading to ammonia formation and regeneration of the active species. This broad gap in mechanistic understanding piqued our interest and we have now investigated such catalytic cycles for ammonia formation. Mechanistic studies of the new complex (^iPrxanPSP)Mo and of Nishibayashi’s (^tBuPN^pyP) Mo systems, including DFT and electrochemical studies, revealed characteristic roles of the halides in splitting dinitrogen. A new pathway leading to the formation of ammonia was elucidated. Dinitrogen binding was observed to aid the reduction of Mo(II) in a key mechanistic step leading to regeneration of the active pincer-ligated Mo(I) fragment.

<b>Scheme 1</b>. Catalytic cycle for the nitrogen splitting and formation of ammonia using chemical reductants and proton sources

Scheme 1. Catalytic cycle for the nitrogen splitting and formation of ammonia using chemical reductants and proton sources

Quinton Bruch

Stony Brook University

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