Study of Rh/TiO2–SiO2 system in photolytic water splitting

Titania silicates (TiO₂–SiO₂) of various compositions were prepared by a sol–gel method. RhCl₃ was used in 0.2 wt% of Rh quantity on the gel surfaces, which were subsequently exposed to UV irradiation in water under a 355-W UV lamp. λ > 370 nm. Both Rh [I] and Rh [III] surface complexes were formed, depending on the gels composition. They also exhibited various efficiency in photo water splitting reaction, the photocatalysts possessing as supports titania-silica gels containing 40 and 50 mol % of TiO₂, were the most effective. According to our proposal, the rhodium complexes took part in the oxidative addition and reductive elimination cycle, where two water molecules in photo water splitting reaction, yielding hydrogen – H₂ and two hydroxyl radicals. The two hydroxyls radicals, catalytically converted to oxygen atom and water, in the second cycle of the investigated water splitting reaction. Titanium octahedra and silicon tetrahedra linked by Ti–O–Si bridges were thought to be the grouping responsible for activity of the investigated catalysts. Their largest amounts are on the gels containing 40–58 mol % TiO_2.

Mechanistic insights into cobalt(ii/iii)-catalyzed C-H oxidation: a combined theoretical and experimental study

Cobalt-mediated C-H functionalization has been the subject of extensive interest in synthetic chemistry, but the mechanisms of many of these reactions (such as the cobalt-catalyzed C-H oxidation) are poorly understood. In this paper, possible mechanisms including single electron transfer (SET) and the concerted metalation-deprotonation (CMD) pathways of the CoII/CoIII-catalyzed alkoxylation of C(sp2)-H bonds have been investigated for the first time using the DFT method. CoII(OAc)2 has been employed as an efficient catalyst in our previous experimental study, but the calculated results unexpectedly indicated that the intermolecular SET pathway with CoIII as the actual catalyst might be the most favorable pathway. To support this theoretical prediction, we have explored a series of Cp*CoIII(CO)I2 catalyzed C(sp2)-H bond alkoxylations, extending the application of cobalt-catalyzed functionalization of C-H bonds. Furthermore, kinetic isotope effect (KIE) data, electron paramagnetic resonance (EPR) data, and TEMPO inhibition experiments also support the SET mechanism in both the Co-catalyzed alkoxylation reactions. Thus, this work should support an understanding of the possible mechanisms of the CoII/CoIII-catalyzed C(sp2)-H functionalization, and also provide an example of the rational design of novel catalytic reactions guided by theoretical calculations.