PYRIDINE DEGRADATION INTERMEDIATES AS MODELS FOR HYDRODENITROGENATIONCATALYSIS - PREPARATION AND PROPERTIES OF A METALLAPYRIDINE COMPLEX

We report the preparation, structure, and reactions of a stable metall apyridine complex of tantalum prepared in the course of model studies of hydrodenitrogenation (HDN) reactions, Monomeric Ta(=(NCBu)-Bu-t=(CH CBu)-Bu-t=CH)OAr)(2)(THF) (5.THF) is isolated upon thermolyzing the et a(2)(N,C)-pyridine complex [eta(2)(N,C)-2,4,6-(NC5Bu3H2)-Bu-t]Ta(OAr)( 2)Me (2) in the presence of THF, while the metallapyridine dimer [Ta(m u-(NCBu)-Bu-t=(CHCBu)-Bu-t=CH)(OAr)(2)](2) (6) is isolated when this r eaction is carried out in benzene. Complete NMR characterization of 5. THF is described, along with its conversion into 6. The bis(pyridine) adduct Ta(=(NCBu)-Bu-t=CHC-Bu-t=CH)(OAr)(2)(py)(2) (5.py) is also desc ribed, Ta(=(NCBu)-Bu-t=(CHCBu)-Bu-t=CH)(OAr)(2)(THF) (5.THF) is shown to react with (BuNCO)-Bu-t and (PrNCNPr)-Pr-i-Pr-i to afford the sigma (eta(1)) and pi (eta(3)) insertion products respectively NCBu)-Bu-t=( CHCBu)-Bu-t=CHC(=(NBu)-Bu-t)O](OAr)(2) (7) and CHCBu)-Bu-t=CH-(eta(3)- C(=(NPr)-Pr-i)(2))](OAr)(2) (8). The molecular structure of the metall apyridine Ta(=(NCBu)-Bu-t=CHC-Bu-t=CH)(OAr)(2)(THF) (5.THF) was detemi ned by X-ray crystallography and shown to adopt a trigonal-bipyramidal configuration with amyl oxide oxygens and the metallacyclic carbon oc cupying equatorial positions. The TaNC4 metallacycle is very nearly pl anar, and discrete single and double bonds are evident around the ring . This pi localization clearly favors the imido form Ta(=(NCBu)-Bu-t=( CHCBu)-Bu-t=CH)(OAr)(2)(THF) rather than a carbene structure, The rele vance of these compounds to hydrodenitrogenation catalysis is describe d.