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

Abstract

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 (X 3 MMX 3 , 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.

Electrochemical Investigation of Low-Valent Multiply M≡M Bonded Group VI Dimers: A Standard Chemical Reduction Leads to an Unexpected Product

“…Complexes 1 and 2 were synthesized according to previously reported procedures . Room-temperature cyclic voltammograms (CVs) of 1 (1 mM) using a glassy-carbon working electrode in a dry tetrahydrofuran (THF) solution of 0.1 M [ n Bu 4 N]­[PF 6 ] under a nitrogen atmosphere (Figure S1) yield a single, irreversible feature at −2.91 V (vs Fc/Fc + ) that is hypothesized to correspond to the population of the π* LUMO orbitals of the WW dimer (Figure ).…”

“…The precatalyst 1-SiO 2 underwent an induction period followed by stable activity, consistent with the formation of an active alkylidene species (Figure B, red). In contrast, 2-SiO 2 showcased a maximum conversion at the onset of the experiment, followed by a rapid deactivation (Figure B, blue) …”

“…In recent work, our group has found that, upon grafting the catalytically inert complexes 1 and 2 onto silica, the new materials 1-SiO 2 and 2-SiO 2 became active toward the catalytic disproportionation of propylene to ethylene and 2-butene (Figure B,C) . While a combination of spectroscopic, microscopic, and computational techniques established that the surface binding was the same for both materials (Figure A), the two exhibited different reaction profiles upon exposure to propylene.…”

Silica‐Supported Cationic Tungsten Imido Alkylidene Stabilized by an N‐Heterocyclic Carbene Ligand Boosts Activity and Selectivity in the Metathesis of α‐Olefins

“…[18 -23] SOMC, a molecular approach to generate well-defined heterogeneous catalysts, in which the surface is exploited as a ligand to covalently anchor molecular complexes, has been particularly successfully employed for the preparation of silica-supported Schrock-type metathesis catalysts of the general formula [(�SiO)M(E)(=CHR) (X)] (M = Mo or W, E = NR or O), that display high activity, selectivity and stability with performances often surpassing those of their homogeneous counterparts (Scheme 1). [24][25][26][27][28][29][30][31][32][33] Yet, these metathesis catalysts display pronounced reactivity differences between terminal versus internal olefins, that depend on the metal and the σ-donation ability of the X and E ligands. For example, increased activity in the metathesis of internal olefins is observed for weaker σdonating X-ligands in supported metathesis catalysts bearing a donating imido ligand.…”

Lithium Ion Battery Materials as Tunable, Redox Non-Innocent Catalyst Supports

“…4,5 These support materials, most commonly silica (SiO 2 ) and alumina (Al 2 O 3 ), play a crucial role in stabilizing and site-isolating reactive intermediates on the surface. [6][7][8][9][10][11] While this strategy has been leveraged to great effect, the chemical properties of the support are seldom employed to modulate catalyst reactivity, despite the fact that the surface occupies the inner-coordination-sphere for single-site catalysts, and therefore plays an intimate role in determining the electronic properties of the catalyst. The treatment of supports as chemically dynamic ligands, as opposed to highsurface-area dispersants would allow for the application of lessons learned from the homogeneous catalysis literature, unlocking unique reactivity.…”