Zeolite Activation of Organometallics:Anchoring and Decarbonylation Kinetics of Mo(CO)_6 in Dehydrated Na_(56)Y Zeolite

摘要

Quantitative temperature-dependent kinetic studies are reported of thermal reactions of Mo(CO)_6 that has been newly encapsulated in the alpha-cages of a Na_(56)Y zeolite host.Under static vacuum,Mo(CO)_6 undergoes parallel anchoring and decarbonylation reactions,the latter yielding (OC)_3Mo-Na_(56)Y directly and the former resulting in the development of a characteristic six-band spectrum in the C-O stretching region.Kinetic and spectroscopic evidence was obtained for there being two different forms of (OC)_3Mo-Na_(56)Y.The activation parameters are unusual,with very low enthalpies and extremely negative entropies,viz.DELTAH~(not =)=40+-3 kJ mol~(-1) and DELTAS~(not =)=-185+-9 J K~(-1) mol~(-1) for anchoring and decarbonylation.This contrasts with the typically high enthalpic and positive entropic factors reported for corresponding CO dissociative reactions in xylene (DELTAH~(not =)=133+-6 kJ mol~(-1) and DELTAS~(not =)=+28 +-16 J K~(-1) mol~(-1)) or even the quite unusual values DELTAH~(not =)=61+-5 kJ mol~(-1) and DELTAS~(not =)=-139 +-13 J K~(-1) mol~(-1) reported previously for ~(13)CO exchange in Mo(CO)_6-Na_(56)Y.It is concluded that the inner surface of Na_(56)Y provides different activating environments for different reactions,behaving as multidentate anionic "zeolate" nucleophiles,with varying numbers of O~(2-) ions participating in highly ordered transition states.A common intermediate,with three O~(2-) ions participating in the anchoring and decarbonylation process,is proposed,one with two O~(2-) ions being proposed for the ~(13)CO exchange reaction.The importance of temperature-dependent time-resolved infrared spectroscopy in elucidating such intimate mechanistic details is emphasized.