POSS Difluoroboron Photocatalysts for Sulfide Oxidation

中文版本 / Chinese version: POSS 二氟硼光催化剂：硫醚选择性氧化制亚砜

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, namely 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 demonstrates the feasibility of intramolecular cooperative activity in catalytic reactions and identifies the key factors influencing its effectiveness. The octahedral structure of POSS provides multiple active sites, which enhances catalytic performance.
• Environmental advantages. The catalysts operate under mild conditions, using molecular oxygen as the oxidant and avoiding toxic heavy metals and hazardous reagents, in keeping with the principles of green chemistry.
• Reusability. POSS-tert-BF₂ retained its catalytic activity over multiple cycles with minimal loss of efficiency, which indicates 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 the POSS derivatives. Changes in the DPA absorbance at 391 nm were measured over time using low concentrations of photosensitizer and DPA in order to minimize potential quenching of ¹O₂ by the photocatalyst. The SOQY values 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-BF₂, and prop-npht-BF₂ were 48%, 35%, 46%, 27%, 26%, and 18%, respectively, which highlights the high efficiency of POSS-tert-BF₂ in generating singlet oxygen. A higher singlet oxygen quantum yield, and therefore a higher degree of DPA oxidation, was obtained for the octametallic POSS-tert-BF₂, POSS-sal-BF₂, and POSS-npht-BF₂ than for the monometallic analogues prop-tert-BF₂, prop-sal-BF₂, and prop-npht-BF₂ under the same reaction conditions. This can be attributed to the intramolecular cooperative effect in compounds bearing the POSS moiety, which was investigated further.

Photocatalytic Oxidation of Sulfides to Sulfoxides

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

Key findings from the study

• Oxidation of thioanisole (0.425 mmol in MeOH) reached full conversion in 40 minutes using only 0.5 mol% of POSS-tert-BF₂ under a 150 W medium-pressure mercury lamp.
• The process achieved a turnover number of 1582 and a turnover frequency of 2373 h⁻¹.
• Scale-up experiments gave a 98% yield.
• The external quantum efficiency of the system was 59%, determined by ferrioxalate actinometry.
• Control experiments conducted without light, without photosensitizer, or under anaerobic conditions showed negligible conversion, which confirms the photocatalytic nature of the mechanism.

The conversion of thioanisole was only 30% in pure DCM, whereas under the same reaction conditions in MeOH the conversion increased to 99%. Protic solvents such as MeOH and H₂O are known to stabilize the intermediate involved in the formation of ¹O₂ and thereby to accelerate photooxidation. These differences in conversion also suggest that ¹O₂ can be identified as one of the reactive oxygen species involved in the photooxidation of thioanisole.

Comparing the activity of POSS compounds bearing various substituents on the phenyl ring, we observed that introducing steric hindrance, as in the tert-butyl groups at the 3- and 5-positions of the phenyl ring in POSS-tert-BF₂, significantly increased the reaction efficiency, giving 99% conversion in 40 minutes compared with 70% for POSS-sal-BF₂, which lacks bulky substituents. 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 the efficiency relative to POSS-sal-BF₂, although it remained lower than that of POSS-tert-BF₂. This underscores the critical role of steric hindrance at the 3-position of the phenyl ring in promoting the conversion of thioanisole. POSS-tert-BF₂ therefore exhibits high efficiency in the photocatalytic oxidation of thioanisole and was studied in greater detail.

The reaction progress was monitored in real time by NMR spectroscopy, which revealed that the conversion of thioanisole to methyl phenyl sulfoxide with POSS-tert-BF₂ as the photosensitizer increased steadily over time, reaching complete conversion within 40 minutes in accordance with zero-order kinetics. By 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 demonstrate the superior reaction efficiency and faster catalytic performance of POSS-tert-BF₂ relative to prop-tert-BF₂.

The higher conversion of thioanisole obtained with the octametallic POSS-tert-BF₂ (99%) than with the monometallic analogue prop-tert-BF₂ (65%) under the same reaction conditions supports the occurrence of intramolecular cooperative catalysis in POSS-tert-BF₂. This observation is consistent with the enhancement recorded in the 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 comparable enhancement in efficiency was observed for POSS-sal-BF₂ and POSS-npht-BF₂. Comparing the activity of POSS-npht-BF₂ with that of prop-npht-BF₂, a higher yield of 90% versus 60% was obtained for POSS-npht-BF₂. The activity of POSS-sal-BF₂ is likewise higher than that of prop-sal-BF₂, at 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 bearing various substituents was evaluated. Thioanisole derivatives containing the electron-donating group –CH₃ and the electron-withdrawing groups –CN, –CHO, and –C(O)CH₃ achieved high yields of 99%, 82%, 78%, and 96%, respectively. Bromine-substituted derivatives at the ortho and meta positions also gave high yields of 99%.

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

Full CitationMateusz Janeta and Sławomir Szafert. "Polyhedral oligomeric silsesquioxane difluoroboron complexes as cooperative octo-site catalysts for the photooxidation of sulfides to sulfoxides." Inorganic Chemistry Frontiers, 2025, 12, 4666-4676.
DOI: 10.1039/D5QI00323G
Full text: Inorganic Chemistry Frontiers → RSC Publishing