A Ruthenium(II)-Copper(II) Dyad for the Photocatalytic Oxygenation of Organic Substrates Mediated by Dioxygen Activation

Di(pyridin-2-yl)amino-substituted 1,10-phenanthrolines and their Ru(II)-Pd(II) dinuclear complexes: synthesis, characterization and application in Cu-free Sonogashira reaction

Dinuclear complexes bearing Ru(II) photoactive centers are of interest for the development of efficient dual catalysts for many photocatalyzed reactions. Ditopic polypyridine ligands, bis(pyridin-2-yl)amino-1,10-phenanthrolines, containing an additional coordination site (bis(pyridin-2-yl)amine, dpa) at positions 3, 4 or 5 of the 1,10-phenanthroline core (Phen-3NPy2, Phen-4NPy2 and Phen-5NPy2) were synthesized. They were used as bridging ligands to obtain dinuclear complexes [(bpy)2Ru(Phen-NPy2)PdCl2](PF6)2 (Ru(Phen-NPy2)Pd) in good yields via stepwise complexation. In these complexes Ru(II) is coordinated to 1,10-phenanthroline, while Pd(II) is bound to the dpa chelating moiety, as established by NMR spectroscopy and X-ray single crystal analysis. The influence of the position of dpa in the phenanthroline ring on the structural, optical and electrochemical properties of Ru(Phen-NPy2)Pd complexes was studied. The complexes exhibit photoluminescence in argon-saturated MeCN solution with maxima in the range of 615-625 nm, with emission quantum yields ranging from 0.11 to 0.15 for Ru(Phen-NPy2) complexes and from 0.018 to 0.026 for dinuclear Ru(Phen-NPy2)Pd complexes. All the complexes absorb visible light in the range of 370-470 nm with high extinction coefficients and can be considered useful as photocatalysts. The Ru2+/3+ potential in Ru(Phen-NPy2)Pd complexes showed no significant dependence on the dpa position, while the Pd2+/0 reduction potential was significantly lower for Ru(Phen-3NPy2)Pd and Ru(Phen-4NPy2)Pd, than for Ru(Phen-5NPy2)Pd (-0.57 V and -0.72 V vs. Ag/AgCl, KCl(sat.), respectively). The complexes were used as photoactivated precatalysts in Cu-free Sonogashira coupling under blue LEDs (12 W) irradiation. The reaction proceeded roughly three times faster when Ru(Phen-4NPy2)Pd and Ru(Phen-3NPy2)Pd were used as catalyst precursors compared to the mixed catalytic system Ru(bpy)3(PF6)2/(RNPy2)PdCl2.

Photochemically Mediated Toluene Oxidation through a Copper Complex

A method is described to photochemically oxidize toluene selectively to benzaldehyde, an essential compound in the chemical industry. Copper(I) complexes with different ligands were applied in combination with [Ru(bipy)3 ](PF6 )2 and dioxygen as the oxidant. As a result, a "dioxygen adduct" copper complex, for example, a peroxido complex, is formed as the active species. The copper(II) complex obtained after oxidation can be photochemically reduced to the starting copper(I) species, and the process can be repeated continuously. The ligand tris(2-methylpyridyl)amine (tmpa) led to the highest conversion rates.

Tethered photocatalyst-directed palladium-catalysed C-H allenylation of N-aryl tetrahydroisoquinolines

Harnessing radical intermediates in regioselective reactions presents a substantial challenge. Here, we report a novel control strategy through engineering covalently tethered transition metal-photocatalysts that conjoin Pd-phosphine and Ru/Ir photoredox units. This strategy allows us to override the innate regioselectivity of the Pd-catalysed C-H allenylation of N-aryl tetrahydroisoquinolines.

Bioinspired Heterobimetallic Photocatalyst () for Visible-Light-Driven C-H Oxidation of Organic Substrates via Dioxygen Activation

We report a bioinspired heterobimetallic photocatalyst RuIIchrom-FeIIIcat and its relevant applications toward visible-light-driven C-H bond oxidation of a series of hydrocarbons using O2 as the O-atom source. The RuII center absorbs visible light near 460 nm and triggers a cascade of electrons to FeIII to afford a catalytically active high-valent FeIV═O species. The in situ formed FeIV═O has been employed for several high-impact oxidation reactions in the presence of triethanolamine (TEOA) as the sacrificial electron donor.

Sulfoxide and Sulfone Synthesis via Electrochemical Oxidation of Sulfides

The oxidation of diaryl sulfides and aryl alkyl sulfides to the corresponding sulfoxides and sulfones under electrochemical conditions is reported. Sulfoxides are selectively obtained in good yield under a constant current of 5 mA for 10 h in DMF, while sulfones are formed as the major product under a constant current of 10 or 20 mA for 10 h in MeOH. The oxygen of both the sulfoxide and sulfone function is derived from water.

LEAFY protein crystals with a honeycomb structure as a platform for selective preparation of outstanding stable bio-hybrid materials

Well-organized protein assemblies offer many properties that justify their use for the design of innovative bionanomaterials. Herein, crystals of the oligomerization domain of the LEAFY protein from Ginkgo biloba, organized in a honeycomb architecture, were used as a modular platform for the selective grafting of a ruthenium-based complex. The resulting bio-hybrid crystalline material was fully characterized by UV-visible and Raman spectroscopy and by mass spectrometry and LC-MS analysis after selective enzymatic digestion. Interestingly, insertion of complexes within the tubular structure affords an impressive increase in stability of the crystals, eluding the use of stabilizing cross-linking strategies.

Unveiling the origin of photo-induced enhancement of oxidation catalysis at Mo(VI) centres of Ru(II)-Mo(VI) dyads

Photo-induced oxidation-enhancement in biomimetic bridged Ru(ii)-Mo(vi) photo-catalyst is unexpectedly photo-activated in ps timescales. One-photon absorption generates an excited state where both photo-oxidized and photo-reduced catalytic centres are activated simultaneously and independently.

Triazole appending ruthenium(ii) polypyridine complex for selective sensing of phosphate anions through C–H–anion interaction and copper(ii) ions via cancer cells

Selective fluorescence enhancement by H₂PO₄⁻/H₂P₂O₇²⁻ anions and maximum fluorescence quenching by Cu²⁺ ions were attained upon treatment with different types of anions and cations, respectively.

A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water

Using light energy and O₂ for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O₂ by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)₃ ]²⁺ , generating the highly oxidized chromophore and the powerful reductant methyl-viologen radical MV^+. . MV^+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O₂ to O₂ ^.- in an aqueous medium to generate an active iron(III)-(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.

Switchable Synthesis of Aryl Sulfones and Sulfoxides through Solvent-Promoted Oxidation of Sulfides with O2/Air

A practical and switchable method for the synthesis of aryl sulfones and sulfoxides via sulfide oxidation was developed. The chemoselectivities of products were simply controlled by reaction temperature using O₂/air as the terminal oxidant and oxygen source. The broad substrate scope, easy realization of gram-scale production, and the simplification of a sulfide oxidation system render the strategy attractive and valuable.

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 Bpp-based dinuclear ruthenium photocatalyst for visible light-driven oxidation reactions

The photocatalytic oxidation of organic substrates in water using a diruthenium chromophore-catalyst dyad molecule can be tuned by the nature of the bridging ligand.

A luminescent on–off probe based calix[4]arene linked through triazole with ruthenium(ii) polypyridine complexes to sense copper(ii) and sulfide ions

The supramolecular sensor Ru2L was designed by joining a bis-ruthenium(ii) polypyridyl complex with a p-tert-butyl calix[4]arene platform through a 1,2,3-triazole linker and used for sensing of copper(ii) and sulfide ions by fluorescence.

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.

Elucidating light-induced charge accumulation in an artificial analogue of methane monooxygenase enzymes using time-resolved X-ray absorption spectroscopy

We report the use of time-resolved X-ray absorption spectroscopy in the ns-μs time scale to track the light induced two electron transfer processes in a multi-component photocatalytic system, consisting of [Ru(bpy)₃]²⁺/ a diiron(iii,iii) model/triethylamine. EXAFS analysis with DFT calculations confirms the structural configurations of the diiron(iii,iii) and reduced diiron(ii,ii) states.

Bifunctional organic sponge photocatalyst for efficient cross-dehydrogenative coupling of tertiary amines to ketones

A novel bifunctional organic sponge photocatalyst can enable the efficient coupling of tertiary amines with ketones in water. The asymmetric transformation can be also achieved by using this sponge photocatalyst.

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

A molecular chromophore-catalyst assembly containing a chromophore ruthenium(II) center (RuIIchro ) and a catalytic copper(II) center (CuIIcat ) has been prepared easily. The assembly was employed for photocatalytic oxidation of sulfides without sacrificial reagent in the presence of dioxygen under blue light irradiation. Unprecedented turnover number (TON) up to 32 000 was achieved. It was elucidated that an electron transferred from excited state of chromophore RuII*chro to CuIIcat along with generation of CuIcat that was further activated by O₂ . These results demonstrate a promising strategy for efficient cooperative photocatalytic reactions under air using the chromophore-catalyst assembly.

CuAAC-based assembly and characterization of a ruthenium-copper dyad containing a diimine-dioxime ligand framework

The design of molecular dyads combining a light-harvesting unit with an electroactive centre is highly demanded in the field of artificial photosynthesis. The versatile Copper-catalyzed Azide-Alkyne Cycloaddition (CuAAC) procedure was employed to assemble a ruthenium tris-diimine unit to an unprecedented azide-substituted copper diimine-dioxime moiety. The resulting Ru^IICu^II dyad 4 was characterized by electrochemistry, ¹H NMR, EPR, UV-visible absorption, steady-state fluorescence and transient absorption spectroscopies. Photoinduced electron transfer from the ruthenium to the copper centre upon light-activation in the presence of a sacrificial electron donor was established thanks to EPR-monitored photolysis experiments, opening interesting perspectives for photocatalytic applications.

Merging visible-light photoredox and copper catalysis in catalytic aerobic oxidation of amines to nitriles

Visible-light-initiated homogeneous oxidative synthesis of nitriles from amines was accomplished through a combined use of photoredox and copper catalysis.

Light-Induced Activation of a Molybdenum Oxotransferase Model within a Ru(II)-Mo(VI) Dyad

Nature uses molybdenum-containing enzymes to catalyze oxygen atom transfer (OAT) from water to organic substrates. In these enzymes, the two electrons that are released during the reaction are rapidly removed, one at a time, by spatially separated electron transfer units. Inspired by this design, a Ru(II)-Mo(VI) dyad was synthesized and characterized, with the aim of accelerating the rate-determining step in the cis-dioxo molybdenum-catalyzed OAT cycle, the transfer of an oxo ligand to triphenyl phosphine, via a photo-oxidation process. The dyad consists of a photoactive bis(bipyridyl)-phenanthroline ruthenium moiety that is covalently linked to a bioinspired cis-dioxo molybdenum thiosemicarbazone complex. The quantum yield and luminescence lifetimes of the dyad [Ru(bpy)₂(L²)MoO₂(solv)]²⁺ were determined. The major component of the luminescence decay in MeCN solution (τ = 1149 ± 2 ns, 67%) corresponds closely to the lifetime of excited [Ru(bpy)₂(phen-NH₂)]²⁺, while the minor component (τ = 320 ± 1 ns, 31%) matches that of [Ru(bpy)₂(H₂-L²)]²⁺. In addition, the (spectro)electrochemical properties of the system were investigated. Catalytic tests showed that the dyad-catalyzed OAT from dimethyl sulfoxide to triphenyl phosphine proceeds significantly faster upon irradiation with visible light than in the dark. Methylviologen acts as a mediator in the photoredox cycle, but it is regenerated and hence only required in stoichiometric amounts with respect to the catalyst rather than sacrificial amounts. It is proposed that oxidative quenching of the photoexcited Ru unit, followed by intramolecular electron transfer, leads to the production of a reactive one-electron oxidized catalyst, which is not accessible by electrochemical methods. A significant, but less pronounced, rate enhancement was observed when an analogous bimolecular system was tested, indicating that intramolecular electron transfer between the photosensitizer and the catalytic center is more efficient than intermolecular electron transfer between the separate components.

Cooperative photoredox catalysis

Cooperative photoredox catalysis bridges visible-light photoredox catalysis with other types of catalysis like transition-metal catalysis, biocatalysis or electrocatalysis for establishing demanding organic transformations.

Reduction of a tris(picolyl)amine copper(ii) complex by a polymeric flavo-reductase model in water

An artificial reductase, made by incorporation of FMN cofactors into the locally hydrophobic micro-environment of a modified polyethyleneimine, catalytically reduces Cu(ii) complexes in water.