Transformations relevant to hydrodenitrogenation as modeled by reactions of trimethylphosphine molybdenum compounds

Heterocyclic nitrogen compounds that contain five-membered pyrrole or six-membered pyridine rings are some of the most resistant compounds towards hydrodenitrogenation (HDN). Molybdenum is an important component of current hydrodenitrogenation catalysts, but the coordination chemistry of molybdenum with such compounds has not been well defined. In an effort to obtain information concerned with hydrodenitrogenation, the reactions of Mo(PMe3) 6 with a variety of heterocyclic nitrogen compounds (e.g., pyrrole, indole, carbazole, pyridine, pyrazine, pyrimidine, triazine, quinoline, isoquinoline, quinoxaline, quinazoline, pyrazole, acridine, and phenazine) have been studied to provide structural models for the coordination of these heterocycles to the molybdenum centers of hydrodenitrogenation catalysts. Among them is the first structurally characterized d 4 eta2-coordination pyrazolyl complex of molybdenum, namely [eta2- pzBut2 ]Mo(PMe3)4H, which shows a displacement of the hydride ligand from the pseudo-C2 axis as indicated by X-ray crystallography. After employing density functional theory (DFT) studies, this displacement of the hydride ligand is attributed to the intramolecular steric interactions, specifically from the pair of the "cis" PMe3 ligands.;Complexes derived from heterocyclic nitrogen compounds with more than one ring exhibit thermally and photochemically induced inter-ring haptotropic rearrangements (IHR) between the carbocyclic and the heterocyclic coordination modes, and the reactivity of the heterocyclic nitrogen ligand towards hydrogenation depends critically on its coordination mode. For example, [eta6-(C 5N)-quinoline]Mo(PMe3)3 is readily hydrogenated by H2 to yield 1,2,3,4-tetrahydroquinoline, whereas the isomeric species [eta6-C6)-quinoline]Mo(PMe3) 3 is stable under comparable conditions. Further, Mo(PMe3) 4H4 has been demonstrated as a homogeneous molybdenum catalyst for hydrogenating quinoline, isoquinoline, and quinoxaline with selective hydrogenation of the heterocyclic nitrogen ring, although the reaction rate is very slow.;An investigation of the reactivity of various eta6-arene compounds (eta6-ArH)Mo(PMe3)3 (ArH = benzene, naphthalene, and anthracene) towards H2 shows that the facility of oxidative addition of H2 depends critically on the nature of the arene, with only the anthracene derivative being reactive to generate (eta4-anthracene)Mo(PMe3)3H 2, indicating an "anthracene effect". DFT calculations have suggested that this reactivity difference is not properly rationalized by simply focusing on resonance stabilization energies of the arenes, but is primarily a consequence of the fact that the Mo-(eta4-ArH) bonding interaction increases in the sequence benzene < naphthalene < anthracene, while the Mo-(eta 6-ArH) bonding interaction exhibits an opposing trend. (Abstract shortened by UMI.)...