Robust Cooperative Photo-oxidation of Sulfides without Sacrificial Reagent under Air Using a Dinuclear RuII -CuII Assembly

Phenylbenzothiazole-Based Platinum(II) and Diplatinum(II) and (III) Complexes with Pyrazolate Groups: Optical Properties and Photocatalysis

Based on 2-phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me2N-pbt), mononuclear [Pt(pbt)(R'2-pzH)2]PF6 (R'2-pzH = pzH 1a, 3,5-Me2pzH 1b, 3,5-iPr2pzH 1c) and diplatinum (PtII-PtII) [Pt(pbt)(μ-R'2pz)]2 (R'2-pz = pz 2a, 3,5-Me2pz 2b, 3,5-iPr2pz 2c) and [Pt(Me2N-pbt)(μ-pz)]2 (3a) complexes have been prepared. In the presence of sunlight, 2a and 3a evolve, in CHCl3 solution, to form the PtIII-PtIII complexes [Pt(R-pbt)(μ-pz)Cl]2 (R = H 4a, NMe2 5a). Experimental and computational studies reveal the negligible influence of the pyrazole or pyrazolate ligands on the optical properties of 1a-c and 2a,b, which exhibit a typical 3IL/3MLCT emission, whereas in 2c the emission has some 3MMLCT contribution. 3a displays unusual dual, fluorescence (1ILCT or 1MLCT/1LC), and phosphorescence (3ILCT) emissions depending on the excitation wavelength. The phosphorescence is lost in aerated solutions due to sensitization of 3O2 and formation of 1O2, whose determined quantum yield is also wavelength dependent. The phosphorescence can be reversibly photoinduced (365 nm, ∼ 15 min) in oxygenated THF and DMSO solutions. In 4a and 5a, the lowest electronic transitions (S1-S3) have mixed characters (LMMCT/LXCT/L'XCT 4a and LMMCT/LXCT/ILCT 5a) and they are weakly emissive in rigid media. The 1O2 generation property of complex 3a is successfully used for the photooxidation of p-bromothioanisol showing its potential application toward photocatalysis.

KuQuinone as a Highly Stable and Reusable Organic Photocatalyst in Selective Oxidation of Thioethers to Sulfoxides

A chemoselective photocatalytic system to perform thioether oxidation to sulfoxide is presented. The light-induced oxidation process is here promoted by a metal-free quinoid catalyst, namely 1-hexylKuQuinone (KuQ). Reactions performed in a fluorinated solvent (i.e., HFIP), using O₂ as the oxidant, at room temperature, lead to complete thioanisole conversion to methyl phenyl sulfoxide in 60 min. Remarkably, the system can be recharged and recycled without a loss of activity and selectivity, reaching turnover numbers (TONs) higher than 4000. Excellent catalytic performances and full selectivity have also been obtained for the photocatalytic oxidation of substituted thioanisole derivatives, aliphatic, cyclic, and diaryl thioethers. Likewise, the oxidation of heteroaromatic organosulfur compounds can be accomplished, with longer reaction times.

Noble metal-free bis-tridentate benzimidazole zinc(II) and iron(II) complexes for selective CO2 photoreduction

Three noble metal-free metal complexes [Fe(Me-bzimpy)₂]²⁺ (Fe1), [Fe(bzimpy)₂]²⁺ (Fe2) and [Zn(Me-bzimpy)₂]²⁺ (Zn1) were synthesized and studied in the visible light-driven CO₂ reduction, where ligands bzimpy and Me-bzimpy were 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridine and 2,6-bis(1H-benzo[d]imidazol-2-yl)pyridine, respectively. It was found that Fe1 displayed the best photocatalytic performance with a turnover number (TON) of 878 and high selectivity up to 99.2% towards CO generation in the presence of an organic thermally activated delayed fluorescence (TADF) photosensitizer, which was more than 10 times that of Fe2 (TON_CO = 63) and Zn1 (TON_CO = 53). This is attributed to the much higher stability of Fe1 upon reduction, as proved by the cyclic voltammograms of the three complexes. These results highlight the cooperation of ligands and metals in molecular metal complexes for CO₂ photoreduction.

Aerobic Photocatalysis: Oxidation of Sulfides to Sulfoxides

Sulfoxides constitute one of the most important functional groups in organic chemistry found in numerous pharmaceuticals and natural products. Sulfoxides are usually obtained from the oxidation of the corresponding sulfides. Among various oxidants, oxygen or air are considered the greenest and most sustainable reagent. Photochemistry and photocatalysis is increasingly applied in new, as well as traditional, yet demanding, reaction, like the aerobic oxidation of sulfides to sulfoxides, since photocatalysis has provided the means to access them in mild and effective ways. In this review, we will summarize the photochemical protocols that have been developed for the oxidation of sulfides to sulfoxides, employing air or oxygen as the oxidant. The aim of this review is to present: i) a historical overview, ii) the key mechanistic studies and proposed mechanisms and iii) categorize the different catalytic systems in literature.

Unimolecular cooperative metallaphotocatalysis with conjugately bridged Ir-Ni complexes and its applications in organic coupling reactions

Recent advances successfully upgraded the unique cooperative partnership between the two distinct metals in photocatalysis. Herein we are reporting design, synthesis and comprehensive study of series of heteroleptic Ir(III) complexes...

Z-Scheme In2 S3 /NU-1000 Heterojunction for Boosting Photo-Oxidation of Sulfide into Sulfoxide under Ambient Conditions

Photocatalytic oxidation of sulfide into sulfoxide has attracted extensive attention as an environmentally friendly strategy for chemical transformations or toxic chemicals degradation. Herein, we construct a series of In₂ S₃ /NU-1000 heterojunction photocatalysts, which can efficiently catalyze the oxidation of sulfides to form sulfoxides as the sole product under LED lamp (full-spectrum) illumination in air at room temperature. Especially, the sulfur mustard simulant, 2-chloroethyl ethyl sulfide (CEES), can also be photocatalytically oxidized with In₂ S₃ /NU-1000 to afford nontoxic 2-chloroethyl ethyl sulfoxide (CEESO) selectively and effectively. In contrast, individual NU-1000 and In₂ S₃ show very low catalytic activity on this reaction. The significantly improved photocatalytic activity is ascribed to the constructing of an efficient Z-scheme photocatalysts In₂ S₃ /NU-1000, which exhibits the enhancement of light harvesting, the promotion of photogenerated electron-hole separation, and the retention of high porosity of the parent MOF. Moreover, mechanism studies in photocatalytic oxidation reveal that the superoxide radical (^. O₂ ^- ) and singlet oxygen (¹ O₂ ) are the main oxidative species in the oxidation system. This work exploits the opportunities for the construction of porous Z-scheme photocatalysts based on the photoactive MOFs materials and inorganic semiconductors for promoting catalytic organic transformations. More importantly, it provides a route to the rational design of efficient photocatalysts for the detoxification of mustard gas.

Electrophilic Chlorine from Chlorosulfonium Salts: A Highly Chemoselective Reduction of Sulfoxides

Herein, we describe a selective late-stage deoxygenation of sulfoxides based on a novel application of chlorosulfonium salts and demonstrate a new process using these species generated in situ from sulfoxides as the source of electrophilic chlorine. The use of highly nucleophilic 1,3,5-trimethoxybenzene (TMB) as the reducing agent is described for the first time and applied in the deoxygenation of simple and functionalized sulfoxides. The method is easy to handle, economic, suitable for gram-scale operations, and readily applied for poly-functionalized molecules, as demonstrated with more than 45 examples, including commercial medicines and analogues. We also report the results of competition experiments that define the more reactive sulfoxide and we present a mechanistic proposal based on substrate and product observations.

Synthesis of an anthraquinone-containing polymeric photosensitizer and its application in aerobic photooxidation of thioethers

Work on the synthesis of a polymeric photosensitizer and its application in the photooxidation of thioethers is reported herein. Firstly, the polymeric photosensitizer was designed and synthesized by the reaction of anthraquinone-2-carbonyl chloride (AQ-2-COCl) with poly(2-hydroxyethyl methacrylate) (PHEMA). Then, the visible light-induced photooxidation of thioethers under aerobic conditions was investigated. The results revealed that the reaction yielded sulfoxides highly chemoselectively in excellent yields with good substrate tolerance. Importantly, AQ-PHEMA could be easily recovered and reused more than 20 times without significant loss of the catalytic activity.

Work on the synthesis of a polymeric photosensitizer and its application in the photooxidation of thioethers is reported herein.

A long-lived Donor-Acceptor fluorescent probe for sequential detection of Cu2+ and biothiols

A new long-lived Donor-Acceptor (D-A) fluorophore based on carbazolyl dicyanobenzene was developed as an ON-OFF-ON multifunctional fluorescent probe 1 for sequential detection of Cu²⁺ and biothiols (Cys, Hcy and GSH). The fluorescence of probe 1 can be significantly and selectively quenched by Cu²⁺. Meanwhile, the fluorescence lifetime decreased from 2.1 μs to 18.5 ns. The limit of detection was determined to be 33.6 nM. Upon addition of biothiols (Cys, Hcy and GSH), the generated ensemble 1-Cu²⁺ displayed a "turn-on" fluorescent response at 555 nm and an obvious recovery in fluorescence lifetime and UV-vis absorption within 1 min. The limit of detection for Cys, Hcy and GSH were calculated by fluorescence titration experiments to be 0.19, 0.21 and 0.29 μM, respectively. The ensemble 1-Cu²⁺ was further successfully applied in bioimaging.

Merging an organic TADF photosensitizer and a simple terpyridine–Fe(iii) complex for photocatalytic CO2 reduction

An efficient earth-abundant photocatalytic system composed of an organic TADF photosensitizer and a simple terpyridine–Fe(iii) complex was developed for CO₂ reduction.

Coordination Booster-Catalyst Assembly: Remote Osmium Outperforming Ruthenium in Boosting Catalytic Activity

Presented herein is a set of bimetallic and trimetallic "coordination booster-catalyst" assemblies in which the coordination complexes [Ru^II (terpy)₂ ] and [Os^II (terpy)₂ ] acted as boosters for enhancement of the catalytic activity of [Ru^II (NHC)(para-cymene)]-based catalytic site. The boosters accelerated the oxidative loss of para-cymene from the catalytic site to generate the active catalyst during the oxidation of alkenes and alkynes into corresponding aldehydes, ketones and diketones. It was found that the boosting efficiency of the [Os^II (terpy)₂ ] units was considerably higher than its congener [Ru^II (terpy)₂ ] unit in these assemblies. Mechanistic studies were conducted to understand this unique improvement.

Cyclometalated Ir-Zr Metal-Organic Frameworks as Recyclable Visible-Light Photocatalysts for Sulfide Oxidation into Sulfoxide in Water

Aerobic photo-oxidation of sulfide into sulfoxide in water is of great interest in green chemistry. In this study, three highly stable Ir(III)-Zr(IV) metal-organic frameworks (Ir-Zr MOFs), namely Zr₆-Irbpy (bpy is 2,2'-bipyridine), Zr₆-IrbpyOMe (bpyOMe is 4,4'-dimethoxy-2,2'-bipyridine), and Zr₆-Irphen (phen is 1,10-phenanthroline), are constructed by using [Ir(pqc)₂(L)₂]Cl complexes (where pqc is 2-phenylquinoline-4-carboxylic acid and L is an ancillary ligand bpy, bpyOMe, or phen) as linkers and Zr₆ cluster as nodes. The constructed Ir-Zr MOFs present high catalytic activity on aerobic photo-oxidation of sulfide into sulfoxide under visible light irradiation in water at room temperature. Moreover, the reaction is high chemoselectivity and functional group tolerance. The catalyst can be readily recycled and reused at least 10 times without loss of catalytic activity. Mechanism studies demonstrate that superoxide radical is the reactive oxygen species in the sulfoxidation, which is generated by electron transfer from the excited triplet photosensitizer ³[Ir-Zr-MOF]* to O₂. The high activity of photocatalytic sulfoxidation in water may be attributed to the stabilization of the persulfoxide intermediate by hydrogen bond formation with water solvent, which accelerates the conversion of persulfoxide into sulfoxide and prevents further oxidation of sulfoxide into sulfone. This work provides a new strategy for the green synthesis of sulfoxides under ambient conditions.

Efficient Generation of Singlet Oxygen and Photooxidation of Sulfide into Sulfoxide via Tuning the Ancillary of Bicyclometalated Iridium(III) Complexes

With 2-phenylquinoline (pq) as a cyclometalated ligand, a series of cationic Ir(III) complexes [Ir(pq)₂(L1)₂](PF₆) (L1 is pyridine (1a), 4-methoxypyridine (1b), 4-dimethylaminopyridine (1c), and 4-acetylpyridine (1d)) and [Ir(pq)₂(L2)](PF₆) (L2 is 2,2'-bipyridine (1e), 2,2'-bipyrimidyl (1f), 4,4'-dimethyl-2,2'-bipyridine (1g), and 4,4'-dimethoxy-2,2'-bipyridine (1h)) were synthesized and characterized. The influence of the metal-based highest occupied molecular orbital on triplet-state lifetime, triplet-state quantum yield, and ¹O₂ generation quantum yield as well as aerobic photo-oxidation of sulfide into sulfoxide was evaluated via tuning the ancillary ligand of Ir(pq)₂ complexes. The results revealed that 1h with chelate ancillary ligand bearing electron-donating group possesses a high ¹O₂ generation quantum yield (0.90) and photocatalytic activity for sulfide oxidation with high chemoselectivity and a low catalyst loading (0.5 mol %) under mild conditions. Moreover, one-pot two-step procedure for preparation of enantiopure sulfoxides, including aerobic photo-oxidation of sulfide using 1h as a photosensitizer and chiral resolution of sulfoxide via a chiral-at-metal strategy, was also developed.

Coupling photoredox and biomimetic catalysis for the visible-light-driven oxygenation of organic compounds

Recent advances in the development of coupled photoredox systems for the oxygenation of organic compounds are reviewed.

A supramolecular assembly bearing an organic TADF chromophore: synthesis, characterization and light-driven cooperative acceptorless dehydrogenation of secondary amines

A noble-metal-free chromophore–catalyst supramolecular assembly, which contains an organic thermally activated delayed fluorescence (TADF) chromophore and cobaloximes, has been designed and synthesized for efficient light-driven acceptorless dehydrogenation of secondary amines.

Visible light sensitizer-catalyzed highly selective photo oxidation from thioethers into sulfoxides under aerobic condition

We report herein a visible light sensitizer-catalyzed aerobic oxidation of thioethers, affording sulfoxides in good to excellent yields. The loading of the catalyst was as low as 0.1 mol%. The selectivity was excellent. Mechanism studies showed both singlet oxygen and superoxide radical anion were likely involved in this transformation.