ja6b08705_si_003.pdf (7.02 MB)
C–H Activation on Co,O Sites: Isolated Surface Sites versus Molecular Analogs
journal contribution
posted on 2016-10-21, 00:00 authored by Deven P. Estes, Georges Siddiqi, Florian Allouche, Kirill V. Kovtunov, Olga V. Safonova, Alexander
L. Trigub, Igor V. Koptyug, Christophe CopéretThe activation and
conversion of hydrocarbons is one of the most important challenges
in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated
on silica surfaces are known to catalyze such processes. The mechanisms
of these processes are currently unknown but are thought to involve
C–H activation as the rate-determining step. Here, we synthesize
well-defined Co(II) ions on a silica surface using a metal siloxide
precursor followed by thermal treatment under vacuum at 500 °C.
We show that these isolated Co(II) sites are catalysts for a number
of hydrocarbon conversion reactions, such as the dehydrogenation of
propane, the hydrogenation of propene, and the trimerization of terminal
alkynes. We then investigate the mechanisms of these processes using
kinetics, kinetic isotope effects, isotopic labeling experiments,
parahydrogen induced polarization (PHIP) NMR, and comparison with
a molecular analog. The data are consistent with all of these reactions
occurring by a common mechanism, involving heterolytic C–H
or H–H activation via a 1,2 addition across a Co–O bond.