Surface Structural-Chemical Characterization of Single-Site Supported Organometallic Catalysts

Single-site organometallic catalysts supported on solid inorganic or organic substrates are making an important contribution to heterogeneous catalysis. The large majority of supports currently used in industry are inorganic materials (SiO₂, Al₂O₃, MgCl₂), with silica being the most important. Single-site supported catalysts are most commonly prepared by molecular-level anchoring/chemisorption, in which a molecular precursor undergoes reaction with the surface while maintaining most of the ligand sphere of the parent molecule. Chemisorption of discrete organometallic complexes on solid supports yields catalysts with well-defined active sites, greater thermal stability than the homogeneous analogues.

We have previously developed a series of site-isolated, Earth-abundant transition metals (V³⁺, Cr³⁺, Mn²⁺, Co²⁺ and Ni²⁺) supported on a catechol-functionalized porous organic polymer (CatPOP), which demonstrated exceptionally high chemoselectivity for alkyne semi- hydrogenation, such as diphenylacetylene to stilbene. Among the series of precatalysts, the early- first-row metals V and Cr exhibit markedly high semi-hydrogenation activity and selectivity. At 1 mol% metal loading, the [(CatPOP)V(Mes)(THF)] hydrogenates alkynes 40 times faster than the corresponding heavier analogs (e.g., Mn²⁺, Co²⁺ and Ni²⁺). The high catalytic activity of [(CatPOP)V(Mes)(THF)] is attributed to the ability of the CatPOP matrix to stabilize low- coordinate, and monomeric organovanadium(III) sites. In order to verify the potential contribution of the redox-active CatPOP backbone to the observed reactivity, we recently incorporated these organovanadium(III) species on a redox-innocent silica (SiO₂) support, which is less expensive and more stable under relatively high temperature (50-120 °C) conditions, compared to CatPOP. The [V(Mes)₃(THF)] is grafted on dehydrated SiO₂ to give supported, site-isolated organovanadium(III) centers. The structure of the V sites have been fully elucidated through a series of spectroscopic and surface characterization techniques, which include solution-phase 1H NMR spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and thermo-gravimetric analysis (TGA). EPR and XPS characterization of the [(SiO₂)V(Mes)(THF)] reveals that the supported V sites are at the trivalent state. In addition, the UV-Vis spectrum of [(SiO₂)V(Mes)(THF)] suggests that the vanadium sites are isolated and monomeric on the surface. Liquid- and gas-phase hydrogenation of alkynes and alkenes were carried out using catalyst [(SiO₂)V(Mes)(THF)] and V₂O₅/SiO₂. The V³⁺-based catalyst exhibits high activity, while less than 5% conversion is achieved using the V₂O₅/SiO₂ catalyst control, suggesting that V³⁺ centers are the active sites. In addition, the reactivity of [(SiO₂)V(Mes)(THF)] is comparable to [(CatPOP)V(Mes)(THF)], indicating that isolated V³⁺ sites maintain their nature on bulk SiO₂ support.