POSS as a photocatalyst

 

The publication titled "Polyhedral oligomeric silsesquioxane difluoroboron complexes as cooperative octo-site catalysts for the photooxidation of sulfides to sulfoxides" by Mateusz Janeta and Sławomir Szafert, published in Inorganic Chemistry Frontiers on April 17, 2025, presents a study on the development of novel metal-free photocatalysts. These catalysts are based on polyhedral oligomeric silsesquioxanes (POSS) functionalized with difluoroboron complexes. ​

Scheme 1 Synthesis of POSS-sal-BF2, POSS-tert-BF2 and POSS-npht-BF2. Isolated yields in parentheses.

Key Highlights:

• Catalyst Design: The researchers synthesized three new difluoroboron-functionalized POSS complexes—POSS-tert-BF₂, POSS-sal-BF₂, and POSS-npht-BF₂—derived from imine-functionalized POSSs.
• Photocatalytic Performance: These complexes demonstrated exceptional efficiency in the aerobic photooxidation of sulfides to sulfoxides, significantly outperforming their silsesquioxane-free counterparts. Notably, POSS-tert-BF₂ exhibited a high singlet oxygen quantum yield of 48%. ​
• Intramolecular Cooperative Activity: The study highlights the feasibility of intramolecular cooperative activity in catalytic reactions, identifying key factors influencing its effectiveness. The octahedral structure of POSS allows for multiple active sites, enhancing catalytic performance. ​
• Environmental Advantages: The catalysts operate under mild conditions, using molecular oxygen as the oxidant and avoiding toxic heavy metals or hazardous reagents, aligning with green chemistry principles. ​
• Reusability: POSS-tert-BF₂ retained its catalytic activity over multiple cycles, with minimal loss in efficiency, indicating good stability and recyclability. ​

This research underscores the potential of POSS-based difluoroboron complexes as efficient, sustainable, and reusable photocatalysts for the selective oxidation of sulfides to sulfoxides, with implications for pharmaceutical synthesis and environmental applications.

Singlet oxygen quantum yields

The singlet oxygen quantum yield (SOQY, Φ(¹O₂)) was determined by monitoring the photooxidation of DPA in methanol in the presence of POSS derivatives. Changes in DPA absorbance at 391 nm were measured over time using low concentrations of photosensitizer and DPA to minimize potential 1O2 quenching by the photocatalyst. SOQYs were calculated by plotting the change in DPA absorbance against irradiation time. The calculated Φ(¹O₂) values for POSS-tert-BF₂, POSS-sal-BF₂, POSS-npht-BF₂, prop-tert-BF₂, prop-sal-BF2₂and prop-npht-BF₂ were 48%, 35%, 46%, 27%, 26%, and 18%, respectively, highlighting the high efficiency of POSS-tert-BF₂ in generating singlet oxygen. Higher singlet oxygen quantum yield and therefore higher DPA oxidation were obtained for the octametallic POSS-tert-BF₂, POSS-sal-BF₂, and POSS-npht-BF₂ than with the monometallic analogues prop-tert-BF₂, prop-sal-BF₂ and prop-npht-BF₂ under the same reaction conditions, which can be explained by the occurrence of the intramolecular cooperative effect for compounds bearing the POSS moiety, which was further investigated.

Photocatalytic oxidation of sulfides to sulfoxides

Recent studies have showcased the powerful photocatalytic capabilities of POSS-tert-BF₂, a boron difluoride-functionalized polyhedral oligomeric silsesquioxane. This system shows significant singlet oxygen quantum yield and generates multiple reactive oxygen species, making it highly effective in oxidative transformations.

  

✨ Key findings from the study:

• Thioanisole oxidation (0.425 mmol in MeOH) reached full conversion in 40 minutes using just 0.5 mol% of POSS-tert-BF₂ under a 150 W medium-pressure mercury lamp.

• The process achieved a TON of 1582 and a TOF of 2373 h⁻¹.

• Scale-up experiments gave a 98% yield.

• The external quantum efficiency of the system was reported at 59%, based on ferrioxalate actinometry.

• Crucially, control experiments (no light, no photosensitizer, or anaerobic conditions) showed negligible conversion, confirming the photocatalytic mechanism.

The conversion of thioanisole was only 30% in pure DCM. However, under the same reaction conditions using MeOH, the conversion increased to 99%. Protonic solvents, such as MeOH or H₂O, are known to stabilize the intermediate for the formation of ¹O₂, and, in turn, accelerate photooxidation. These differences in conversion also suggest that ¹O₂ can be identified as one of the ROS involved in the photooxidation of thioanisole. Comparing the activity of different POSS compounds with various substitutions on the phenyl ring, we observed that introducing steric hindrance, such as with tert-butyl groups at the 3- and 5-positions of the phenyl ring in POSS-tert-BF₂, significantly increased the reaction efficiency (99% in 40 minutes) compared to POSS-sal-BF₂, which lacks bulky substituents (70% in 40 minutes). Furthermore, introducing steric hindrance at the 5,6-positions of the phenyl ring in POSS-npht-BF₂  by replacing the phenyl ring with a naphthalene ring also improved efficiency relative to POSS-sal-BF₂. However, the efficiency remained lower than that of POSS-tert-BF₂, underscoring the critical role of steric hindrance at the 3-position of the phenyl ring in promoting the conversion of thioanisole. The results indicate that POSS-tert-BF₂ exhibits a high efficiency in photocatalytic oxidation of thioanisole and it was further studied in detail.

The reaction progress was monitored in real time using NMR spectroscopy, which revealed that the conversion of thioanisole to methyl phenyl sulfoxide with POSS-tert-BF₂ as a photosensitizer increased steadily over time, achieving complete conversion within 40 minutes and following zero-order kinetics. In comparison, the use of prop-tert-BF₂ resulted in only 65% substrate conversion after 40 minutes, with full conversion achieved after 60 minutes. These findings clearly highlight the superior reaction efficiency and faster catalytic performance of POSS-tert-BF₂ compared to prop-tert-BF₂. Higher conversion (99%) of thioanisole was obtained with the octametallic POSS-tert-BF₂ than with the monometallic analogue prop-tert-BF₂ (65%) under the same reaction conditions, supporting the occurrence of intramolecular cooperative catalysis with POSS-tert-BF₂. This observation aligns with the observed enhancement in the efficiency of singlet oxygen quantum yields. A similar intramolecular cooperative effect was previously reported for Zn₄@POSS-1, which contains the POSS-1 ligand, in the formation of cyclic carbonates from epoxides.³⁷ A similar enhancement in efficiency was observed for POSS-sal-BF₂ and POSS-npht-BF₂. Comparing the activity of POSS-npht-BF₂ with prop-npht-BF₂, a higher yield of 90% versus 60%, respectively, was obtained for POSS-npht-BF₂. The activity of POSS-sal-BF₂ is also higher than that of prop-sal-BF₂, 70% versus 59%.

Scheme 2 Proposed reaction mechanisms of sulfide-selective oxidation by POSS-tert-BF2 in the presence of O2.

Substrate scope

To assess the versatility of POSS-tert-BF₂ as a photocatalyst in the photooxidation of thioanisole derivatives in methanol, a series of substrates with various substituents were evaluated. Thioanisole derivatives containing electron-donating (–CH₃) and electron-withdrawing (–CN, –CHO, –C(O)CH₃) groups achieved high yields of 99%, 82%, 78%, and 96%, respectively. Bromine-substituted derivatives at ortho and meta positions also exhibited high yields (99%). 

 📚 These results highlight the potential of POSS-based systems in green and efficient photocatalytic oxidation chemistry.

#ChemistryResearch #Photocatalysis #SingletOxygen #POSS #BoronDifluoride #GreenChemistry #MaterialsScience

See more: https://pubs.rsc.org/en/content/articlehtml/2025/qi/d5qi00323g