Potential models of the interactions between nitrogen-containing heterocycles and the active catalyst sites in heterogeneous hydrodenitrogenation catalysts

Article abstract of DOI:10.1021/om00130a018

At present, there is no general description of the interactions between nitrogen heterocycles and the active catalyst sites in heterogeneous hydrodenitrogenation (HDN) catalysts that promote hydrogenation and then hydrogenolysis of the C-N bond. We have attempted to model such interactions through studies of the reactions of osmium and ruthenium clusters with common HDN model substrates, including quinoline, pyridine, tetrahydroquinoline (THQ), and piperidine. Os₃(CO)₁₂ and Ru₃(CO)₁₂ with quinoline or pyridine lead to complexes of the general formula M₃(μ-H:μ-Y)(CO)₁₀ (M = Ru, Os; Y = C₉H₆N, C₉H₈N, C₅H₈N) or M₃(μ-H:μ-Y)₂(CO)₈ (M = Ru, Os; Y = C₅H₄N, C₉H₆N). The reactions of Os₃(CO)₁₂ with aliphatic heterocyclic amines leads to complexes having stoichiometries of Os₃(μ-H:μ-Y)(CO)₁₀ (Y = C₅H₈N, C₉H₈N, C₉H₆N), Os₃(μ-H)₂(μ-Y)(μ-Y′)(CO)₈ (Y = C₉H₈N, Y′ = C₉H₆N), or Os₃(μ-H:μ-Y)₂(CO)₈ (Y = C₉H₆N), and when the heterocyclic amine is 1,2,3,4-tetrahydroquinoline (THQ), a small amount of free quinoline is also produced. As a general rule, both aliphatic and aromatic nitrogen heterocycles undergo α-metalation during reactions with both homogeneous and heterogeneous metal complexes or particles. In the case of osmium and ruthenium, metalation results in the formation of metalloazocyclobutenes even when the starting heterocycle is saturated. Thus, activation/dehydrogenation of the N-H and α-C-H bonds of secondary amines occurs readily in the systems described here. Reaction pathways for the formation of the various products and their relationships to hydrogenation and dehydrogenation catalysis are proposed.