Pyridine Degradation Intermediates as Models for Hydrodenitrogenation Catalysis: Preparation and Properties of a Metallapyridine Complex

We report the preparation, structure, and reactions of a stable metallapyridine complex of tantalum prepared in the course of model studies of hydrodenitrogenation (HDN) reactions. is isolated upon thermolyzing the η 2 ( N,C )-pyridine complex [η 2 ( N,C )-2,4,6-NC 5 t Bu 3 H 2 ]Ta(OAr) 2 Me ( 2 ) in the presence of THF, while the metallapyridine is isolated when this reaction is carried out in benzene. Complete NMR characterization of 5 ·THF is described, along with its conversion into 6 . The bis(pyridine) is also described. is shown to react with t BuNCO and i PrNCN i Pr to afford the σ (η 1 ) and π (η 3 ) insertion products and Ta[ NC t Bu CHC t Bu CH(η 3 -C( N i Pr) 2 )](OAr) 2 ( 8 ). The molecular structure of the was detemined by X-ray crystallography and shown to adopt a trigonal-bipyramidal configuration with aryl oxide oxygens and the metallacyclic carbon occupying equatorial positions. The TaNC 4 metallacycle is very nearly planar, and discrete single and double bonds are evident around the ring. This π localization clearly favors the imido than a carbene structure. The relevance of these compounds to hydrodenitrogenation catalysis is described.