A MoO x /SiO 2 system is an effective catalyst for alkene metathesis; however, the mechanism of the transformation of the surface metal oxide species into active alkylidene sites is not well recognized. In this work, comprehensive density functional theory studies of the initiation mechanisms for alkene metathesis on the MoO x /SiO 2 catalyst have been performed. It is shown that surface silanol groups interacting with Mo species and constituting Brønsted acid sites can play a key role in reduction of the dioxo Mo(VI) species to the mono-oxo Mo(IV) species by alkene, through Mo(VI) alkoxy species, and in subsequent formation of the Mo(VI) alkylidene species. An alternative activation pathway avoiding the reduction step is also possible. The proposed mechanisms of silanol-assisted reduction/initiation with propene are predicted to be more kinetically and thermodynamically accessible than the often assumed pseudo-Wittig mechanism. The silanol-assisted activation of the mono-oxo Mo(IV) species by propene is kinetically preferred over non-silanol−assisted initiation mechanisms, that is, 1,2-hydrogen shift mechanism, allyl mechanism, and oxidative coupling mechanism involving molybdacyclopentane species. The reactivity of the Mo sites is significantly affected by their geometry and the local structure of silica. Our results suggest that only a small fraction of the Mo oxide species with a suitable geometry and neighborhood can be effectively activated by alkenes.

Although silica-supported molybdenum and tungsten oxide systems are widely used in catalysis, the nature of the surface metal oxide species is still not fully recognized. In this work, comprehensive periodic and cluster density functional theory (DFT) studies of the isolated Mo­(VI) and W­(VI) oxide species on dehydrated amorphous silica have been performed to give insight into their heterogeneity. It is shown that the relative stabilities of the metal oxide species strongly depend on their location that influences their geometry and the strain of the dehydrated silica surface. The favorable located monooxo W­(VI) species are clearly more stable than the dioxo W­(VI) species, whereas no strong thermodynamic preference is predicted in the case of the Mo­(VI) species. The relative stability of the monoxo species increases in the order: Cr < Mo < W. However, due to geometrical constraints on the silica surface, formation of the tetragrafted monooxo species is hampered, compared to the digrafted dioxo species and the latter are expected to be in majority. The monografted hydroxy dioxo Mo­(VI) species are unstable. Based on the vibrational frequency analysis, some details regarding the structure of the experimentally observed dioxo species in the MoO x /SiO2 catalyst are proposed, while the results for the WO x /SiO2 system are somewhat ambiguous.

The regioselectivity of metathesis reactions of trisubstituted vinylsilanes H 2 CCHSiR 3 (SiR 3 = SiCl 3 , SiCl 2 Me, SiClMe 2 , SiMe 3 , Si(OEt) 3 ) with the second-generation ruthenium alkylidene complex has been studied theoretically, by density functional theory (DFT), and experimentally. The DFT results indicate that cycloreversion is the rate-determining step and the formation of a thermodynamically stable ruthenium methylidene complex and PhCHCHSiR 3 is generally preferred. However, the regioselectivity of the process can be also governed by the relative stabilities of the ruthenacyclobutane intermediates, which depend on the electronic and steric properties of the SiR 3 substituent. Higher stability of α,β-disubstituted ruthenacyclobutanes in comparison to α,α-disubstituted ruthenacyclobutanes is predicted, in contrast to the corresponding intermediates formed during metathesis of common α-olefins. The stabilizing Ru−C β interaction in the ring is strengthened by the electron-donor SiR 3 substituent at C β . The experiments performed have shown selectivity toward styrene formation for SiR 3 = SiClMe 2 , SiMe 3 , whereas a preference for the formation of ruthenium methylidene and PhCHCHSiR 3 has been observed for SiR 3 = SiCl 3 , SiCl 2 Me, Si(OEt) 3 , in accordance with the theoretical predictions.

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An ecofriendly method of nanosilver obtaining has been studied. The process involves the chemical reduction method carried out in aqueous environment. Silver nitrate (V) was applied as a silver ions source. Raspberry extract was used as a natural source of both reducing and stabilizing agents. The total amount of phenolic compounds was determined by the Folin-Ciocalteu method. Obtained nanoparticles were analyzed by the dynamic light scattering technique so as to determine the particles size and suspension stability which was characterized by an electrokinetic potential. The results confirmed that the size of some nanoparticles was under 100 nm.