Carbon-Nitrogen Bond Formation by the Reaction of 1,2-Cumulenes with a Ditantalum Complex Containing Side-On- and End-On-Bound Dinitrogen

摘要

Synthetic ammonia serves as the key precursor for the synthesis of many nitrogen-containing chemicals and is produced by the Haber-Bosch process on the 100 million ton scale annually. Given the energy-intensive nature of this reaction, there is considerable interest in the search for alternative methods to generate nitrogen-containing molecules directly from N2. One approach towards this goal is the functionalization of coordinated dinitrogen to form new nitrogen-carbon bonds. Typically, reagents such as alkyl halides or triflates can be employed for this purpose. A major drawback of those conversions is the formation of inorganic salts or transition metal halides as byproducts, rendering these reactions atom-inefficient. However, concerted addition reactions that involve the activated dinitrogen unit and substrates with C—X double or triple bonds (X = C, N, O, S) could potentially lead to catalytic cycles for the production of nitrogen-containing heterocycles without any waste products. Unfortunately, only a very few stoichiometric cycloadditions at coordinated N2 units have been discovered to date. In all cases, highly activated side-on bridging η~2:η~2-N2 complexes of Group 4 metals have been employed to achieve the desired nitrogen-carbon bond formations by reaction with arylalkynes, isocyanates, carbon dioxide, or carbon monoxide (via migratory insertion). The question we posed was: Could a Group 5 ditantalum dinitrogen complex with the side-on/end-on coordination mode engage in cycloaddition-type processes?