Dinitrogen Activation and Functionalization with Chromium

The study of metal‐dinitrogen complexes has lent valuable insight into the nature of dinitrogen (N2) reduction to ammonia (NH3), known as “nitrogen fixation.” Even so, understanding this difficult transformation continues to be an elusive goal for chemists. The N2 chemistry of chromium (Cr) is exceptional for its diversity, rarity, and richness of depth. Hans Karsch opined in 1977 that “Almost all transition metals are known to form complexes with molecular nitrogen, but in the case of some metals ‐ notably chromium ‐ the examples are few and far between.” Although that sentiment hasn't changed much in 42 years, recent advances in the field of Cr‐N2 chemistry have begun to illuminate that elusive relationship. Herein we cover the theory and experimental basis for chromium's coordination, activation, reduction, and catalysis of N2 to NH3. An emphasis will be placed on work towards understanding catalytic systems for chromium mediated dinitrogen reduction. Chromium has long been one of the most reluctant metals to coordinate dinitrogen. Historical accounts and recent developments of molecular Cr‐N2 complexes are highlighted herein. Exciting new discoveries have firmly shown Cr complexes are cabable of activating N2, and catalyzing the reduction of N2 to NH3.