Activation of Low-Valent, Multiply M–M Bonded Group VI Dimers toward Catalytic Olefin Metathesis via Surface Organometallic Chemistry

Olefin metathesis is a broadly employed
reaction with applications
that range from fine chemicals to materials and petrochemicals. The
design and investigation of olefin metathesis catalysts have been
ongoing for over half a century, with advancements made in terms of
activity, stability, and selectivity. Immobilization of organometallic
complexes onto solid supports such as silica or alumina is a promising
strategy for catalyst heterogenization, often resulting in increased
activity and stability. Consequently, a broad range of early transition
metal catalysts bearing alkyl, oxide/alkoxide, and amide ligands have
been grafted onto silica and their reactivities investigated. Herein,
we report a series of silica-supported tungsten and molybdenum dimers
(X3MMX3, where M = W and Mo; X = neopentyl,
tert-butoxide, and dimethyl amide) and their reactivities toward catalytic
olefin metathesis. Dynamic nuclear polarization (DNP)-enhanced solid-state
nuclear magnetic resonance (SSNMR), diffuse reflectance infrared Fourier
transform (DRIFT), UV resonance Raman, and X-ray absorption (XAS)
spectroscopies suggest that upon heterogenization the dimers bind
to the surface in a monopodal fashion, with the MM triple
bond remaining intact. These structural assignments were further corroborated
by density functional theory (DFT) calculations. While the homogeneous
dimer counterparts are inert, the supported low-valent alkyl W and
Mo dimers become active for the disproportionative self-metathesis
of propylene to ethylene and butenes and 4-nonene to 4-octene and
5-decene under mild conditions. The lack of activity observed for
the free and supported tert-butoxide and dimethyl
amide dimers likely suggests that the neopentyl groups are necessary
for the formation of a putative alkylidene active species. The difference
in reactivity between the free and supported dimers could be explained
either by the lowering of the activation barrier of the complex through
the electronic effects of the surface or by site isolation of catalytically
relevant reactive intermediates.