(464g) Alkene Dimerization on Grafted Ni Cations at Sub-Ambient Temperatures: Mechanism, Site Requirements, and Solvation Effects By Intrapore Liquids

The oligomerization of small alkenes (ethene, propene, butenes) occurs at high rates and with >98% initial selectivity to primary dimers at subambient temperatures (240-260 K) on Ni(II) cations grafted within Al-MCM-41 mesopores. The active sites consist of (Ni-OH)⁺ isolated species that replace protons in the aluminosilicate structures; they do not require the aluminoxane activators that must be present in large excess for dimerization turnovers in organometallic catalysts. These active sites deactivate rapidly in the absence of an intrapore liquid phase, but become stable for all alkenes (half-lives > 500 h) when the pressure of the alkene reactants (or of added inert alkanes), the temperature, and the diameter of the mesopores combine to cause the abrupt condensation of these molecules within the mesoporrous network that contains the active Ni species. Such stabilization reflects the preferential stabilization of the late transition states that mediate the desorption of bound oligomers, thus preventing their subsequent growth during the initial surface sojourn that forms them. Density functional theory shows that (Ni-OH)⁺ active structures act as acid-base site pairs to stabilize the transition state for the formation of the C-C bond in dimerization events; this previosuly unrecognized mechanism exploits concerted interactions of the two alkene reactants with the acid and base centers in (Ni-OH)⁺ at the bimolecular C-C bond formation transition states. Such transition states and binding modes account for the uniquely high reactivity of these materials and for the observed differences in dimerization turnover rates among alkenes of different chain length. In addition, the previously unreported effects of the formation of an intrapore liquid phase protect such active centers from fast deactivation, leading to practical lifetimes, to high reactivity and selectivity, and to stable materials that allow more rigorus and accurate mechanistic inquiries.