Synthesis and dynamics of [gamma]-alumina supported molybdenum subcarbonyls and model compounds

Molybdenum hexacarbonyl supported on 𝛾-alumina is a precursor to an active olefin metathesis catalyst. Surface sites on the alumina act as coordinating ligands to stabilize various molybdenum subcarbonyl species. The formation of these species can be controlled by appropriate activation conditions. In particular, Mo(CO)₃(ads) can be quantitatively formed upon activation of Mo(CO)₆ on 𝛾-alumina at 100°C in flowing helium. In the reaction, three carbon monoxide ligands are replaced by either surface OH⁻ or O²⁻ depending on the degree of surface hydroxylation. This species has been shown to be active for olefin metathesis.

The intent of this dissertation is twofold. The first part is an investigation of alternate, low temperature synthetic routes to Mo(CO)₃(ads) via ligand displacement reactions using the molybdenum tricarbonyl complexes, Mo(CO)₃(CH₃CN)₃ and Mo(CO)₃(η⁶-C₆H₆). Molybdenum hexacarbonyl can be impregnated onto 𝛾-alumina using an inert hydrocarbon solvent such as pentane or benzene and these solvents are also used to adsorb Mo(CO);(11°·C6H„). However, a polar solvent such as acetone is necessary in the case of Mo(CO)₃(CH₃CN)₃. The resulting surface species using these complexes are characterized by FTIR, reaction stoichiometry and propylene metathesis activity and are compared with results obtained for 𝛾-alumina supported Mo(CO)₆. In order to interpret the results for Mo(CO)₃(CH₃CN)₃, the interaction of acetone with 𝛾-alumina was investigated since acetone is highly reactive with the surface. The results of this in situ FTIR-MS study of acetone on 𝛾-alumina are also reported.

The second part of this dissertation involves using cross polarization-magic angle spinning ¹³C NMR to probe the dynamic natures of Mo(CO)₃(ads) and Mo(CO)₅(ads). Crystalline samples of metal tricarbonyl complexes, which serve as model compounds for Mo(CO)₃(ads), are characterized by variable temperature CP-MAS ¹³C NMR in order to determine the presence of motional processes involving the carbonyl ligands. These findings yield additional information about the nature of molybdenum subcarbonyls on 𝛾-alumina.