Stable luminescent iridium(iii) complexes with bis(N-heterocyclic carbene) ligands: photo-stability, excited state properties, visible-light-driven radical cyclization and CO2 reduction, and cellular imaging

Neutral iridium(iii) complexes bearing BODIPY-substituted N-heterocyclic carbene (NHC) ligands: synthesis, photophysics, in vitro theranostic photodynamic therapy, and antimicrobial activity

The synthesis, photophysics, and photobiological activities of a series of novel neutral heteroleptic cyclometalated iridium(iii) complexes incorporating boron dipyrromethene (BODIPY) substituted N-heterocyclic carbene (NHC) ligands (Ir1-Ir5) are reported. The effect of the substitution position of BODIPY on the NHC ligands, either on C4 of the phenyl ring (Ir1-Ir3) or C5 of the benzimidazole unit (Ir4 and Ir5), and its linker type (single or triple bond) on the photophysical properties was studied. Ir1-Ir5 exhibited BODIPY-localized intense 1IL (intraligand transition)/1MLCT (metal-to-ligand charge transfer) absorption at 530-543 nm and 1,3IL/1,3CT (charge transfer) emission at 582-610 nm. The nanosecond transient absorption results revealed that the lowest triplet excited states of these complexes were the BODIPY-localized 3π,π* states. Complexes Ir4 and Ir5 exhibited blue-shifted 1IL absorption and 1,3IL/1,3CT emission bands compared to the corresponding absorption and emission bands in complexes Ir1 and Ir3. However, replacing the methyl substituents on N3 of benzimidazole in complexes Ir1 and Ir4 with oligoether substituents in Ir3 and Ir5, respectively, did not impact the energies of the low-energy absorption and emission bands in the corresponding complexes. Water-soluble complexes Ir3 and Ir5 have been explored as photosensitizers for in vitro photodynamic therapy (PDT) effects toward human SKMEL28 melanoma cells. Ir3 showed no dark cytotoxicity (EC50 > 300 μM) but good photocytotoxic activity (9.66 ± 0.28 μM), whereas Ir5 exhibited a higher dark cytotoxicity (20.2 ± 1.26 μM) and excellent photocytotoxicity (0.15 ± 0.01 μM). The phototherapeutic indices with visible light (400-700 nm) activation were >31 for Ir3 and 135 for Ir5. Ir3 and Ir5 displayed 1O2 quantum yields of 38% and 22% in CH3CN, respectively, upon 450 nm excitation. Ir5 was more effective at generating reactive oxygen species (ROS) in vitro. Ir5 was also active against Staphylococcus aureus upon visible light activation, with a phototherapeutic index of >15 and EC50 value of 6.67 μM. These photobiological activities demonstrated that these neutral Ir(iii) complexes are promising in vitro PDT reagents, and substitution at C5 on the benzimidazole group of the NHC ligand was superior to C4 substitution on the phenyl ring.

Enantioselective photoredox dehalogenative protonation

We report an enantioselective photoredox dehalogenative protonation as a new type of asymmetric protonation. As a paradigm, with a cooperative catalytic system consisting of a chiral H-bonding catalyst and a dicyanopyrazine-derived chromophore (DPZ) photosensitizer that is irradiated with visible light, a range of cyclic and acyclic ketones with labile chiral secondary C-F, C-Cl and C-Br bonds at the α-position were obtained in high yields with good to excellent enantioselectivities (up to >99% ee) by using a secondary amine as the terminal reductant. Given the ready accessibility of halides, the success of this work should provide inspiration for constructing diverse chiral α-tertiary carbonyls and their variants.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Luminescent iridium(iii) complexes of N-heterocyclic carbene ligands prepared using the ‘click reaction’

Luminescent and electrochemiluminescent N-heterocyclic carbene-combined 1,2,3-triazole and 1,2,3-triazolylidene Ir(iii) complexes have been prepared and their potential as luminescent probes in cell imaging has been evaluated.

Photofunctions of iridium(iii) complexes in vesicles: long-lived excited states and visible-light sensitization for hydrogen evolution in aqueous solution

Utilization of DPPC vesicles allows water-insoluble photoactive Ir(iii) complexes to be dispersed in bulk aqueous solution.

Optimization of the synthesis of quinoline-based neutral cyclometalated iridium complexes via microwave irradiation: design of light harvesting and emitting complexes using bulky quinolines

We report a fast, simple, and robust microwave methodology for the synthesis of quinoline-based neutral heteroleptic cyclometalated iridium complexes: photophysical and computational characterization of the new complexes.

A kinetic resolution strategy for the synthesis of chiral octahedral NHC–iridium(iii) catalysts

The transmetalation reaction of a chiral-bidentate NHC–silver complex to racemic [lr(μ-Cl)(ppy)₂]₂ operates with kinetic resolution leading to chiral octahedral NHC–iridium(iii) complexes and enantio-enriched bis-cyclometalated iridium(iii) complexes.

Selective Photocatalytic C-F Borylation of Polyfluoroarenes by Rh/Ni Dual Catalysis Providing Valuable Fluorinated Arylboronate Esters

A highly selective and general photocatalytic C-F borylation protocol that employs a rhodium biphenyl complex as a triplet sensitizer and the nickel catalyst [Ni(IMes)₂] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the C-F bond activation and defluoroborylation process is reported. This tandem catalyst system operates with visible (blue, 400 nm) light and achieves borylation of a wide range of fluoroarenes with B₂pin₂ at room temperature in excellent yields and with high selectivity. Direct irradiation of the intermediary C-F bond oxidative addition product trans-[NiF(Ar^F)(IMes)₂] leads to very fast decomposition when B₂pin₂ is present. This destructive pathway can be bypassed by indirect excitation of the triplet states of the nickel(II) complex via the photoexcited rhodium biphenyl complex. Mechanistic studies suggest that the exceptionally long-lived triplet excited state of the Rh biphenyl complex used as the photosensitizer allows for efficient triplet energy transfer to trans-[NiF(Ar^F)(IMes)₂], which leads to dissociation of one of the NHC ligands. This contrasts with the majority of current photocatalytic transformations, which employ transition metals as excited state single electron transfer agents. We have previously reported that C(arene)-F bond activation with [Ni(IMes)₂] is facile at room temperature, but that the transmetalation step with B₂pin₂ is associated with a high energy barrier. Thus, this triplet energy transfer ultimately leads to a greatly enhanced rate constant for the transmetalation step and thus for the whole borylation process. While addition of a fluoride source such as CsF enhances the yield, it is not absolutely required. We attribute this yield-enhancing effect to (i) formation of an anionic adduct of B₂pin₂, i.e., FB₂pin₂^-, as an efficient, much more nucleophilic {Bpin^-} transfer reagent for the borylation/transmetalation process, and/or (ii) trapping of the Lewis acidic side product FBpin by formation of [F₂Bpin]^- to avoid the formation of a significant amount of NHC-FBpin and consequently decomposition of {Ni(NHC)₂} species in the reaction mixture.

Phosphorescent Cyclometalated Platinum(II) aNHC Complexes

The synthesis and characterization of the first bidentate C^C* cyclometalated platinum(II) complexes based on abnormal N-heterocyclic carbenes (aNHC) is presented. The aNHC ligand precursors are prepared from benzonitriles and anilines to form 1,2,3-trisubstituted imidazolium salts. The title compounds were synthesized by in situ generation of the silver carbene complex, followed by transmetalation to platinum and subsequent introduction of the β-diketonate ligand. Structural characterization by 2D NMR experiments, as well as solid-state structures unequivocally prove the abnormal binding mode of the aNHC ligands. Additionally, the photophysical properties of the platinum(II) complexes were examined and studied in detail by DFT calculations and cyclic voltammetry experiments. The title compounds proved to be strongly emissive at room temperature in the green to orange region of the visible spectrum, with emission efficiencies of up to 69 %.

Coordination mode-induced isomeric cyclometalated [Ir(tpy)(nbi)Cl](PF6) complexes: distinct luminescence, self-assembly and cellular imaging behaviors

Two isomeric Ir(iii) complexes Ir-O and Ir-R arising from the different coordination mode of a naphthalene-containing ligand, show distinct luminescence, self-assembly ability and cellular imaging behaviors.

Probing Triplet Excited States and Managing Blue Light Emission of Neutral Tetradentate Platinum(II) Complexes

The structural and photophysical properties of tetradentate Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy), and Pt(czpyOczpy) have been experimentally and theoretically explored. Single-crystal diffraction measurements provided accurate structural information. Electrochemical and photophysical characterizations revealed internal electronic energy levels in ground and excited states. (Time-dependent) Density functional theory calculation revealed electron distributions in transition processes of S₀ → S₁ and S₁ → T₁ → S₀. Electronic transition study indicated that Pt(ppzOppz) demonstrated mixed MLCT/LC states and Pt(czpyOczpy) showed MLCT-dominated states in S₁ and T₁. Both Pt(ppzOpopy) and Pt(ppzOczpy) presented strong delocalized spin transition (DST) during intersystem crossing. Upon frame modification of Pt(ppzOczpy), we found that their S₁ and T₁ can be independently manipulated. These blue emitters showed a tunable and narrow emission band (the narrowest fwhm was 19 nm) with luminescence efficiency as high as 86%. The findings of the DST transition mode in the neutral Pt(II) complexes provide guidance for rational design of novel phosphorescent materials.

A benzoic acid terpyridine-based cyclometalated iridium(iii) complex as a two-photon fluorescence probe for imaging nuclear histidine

A series of two-photon active cyclometalated iridium(iii) complexes (Ir1, Ir2 and Ir3) were designed. Ir1 with a two-photon action cross-section of 40 GM in the NIR region has been developed for targeting intracellular histidine. Two-photon micrographs showed that Ir1 could rapidly and selectively light up the nucleus in both fixed and live cells and is capable of displaying nuclear histidine distribution in ultra-detail using a super resolution (SR) technique under stimulated emission depletion (STED) microscopy.

A leap forward in iridium-NHC catalysis: new horizons and mechanistic insights

This review summarises the most recent advances in Ir-NHC catalysis while revisiting all the classical reactions in which this type of catalyst has proved to be active. The influence of the ligand system and, in particular, the impact of the NHC ligand on the activity and selectivity of the reaction have been analysed, accompanied by an examination of the great variety of catalytic cycles hitherto reported. The reaction mechanisms so far proposed are described and commented on for each individual process. Moreover, some general considerations that attempt to explain the influence of the NHC from a mechanistic viewpoint are presented at the end of the review. The first sections are dedicated to the most widely explored reactions that use Ir-NHCs, i.e., hydrogenation and transfer hydrogenation, for which a general overview that tries to compile all the Ir-NHC catalysts hitherto reported for these processes is provided. The next sections deal with hydrogen borrowing, hydrosilylation, water splitting, dehydrogenation (of alcohols, alkanes, aminoboranes and formic acid), hydrogen isotope exchange (HIE), signal amplification by reversible exchange and C-H bond functionalisation (silylation and borylation). The last section compiles a series of reactions somewhat less explored for Ir-NHC catalysts that include the hydroalkynylation of imines, hydroamination, diboration of olefins, hydrolysis and methanolysis of silanes, arylation of aldehydes with boronic acids, addition of aroyl chlorides to alkynes, visible light driven reactions, isomerisation of alkenes, asymmetric intramolecular allylic amination and reactions that employ heterometallic catalysts containing at least one Ir-NHC unit.

Efficient yellow electroluminescence of four iridium(iii) complexes with benzo[d]thiazole derivatives as main ligands

Four yellow iridium(iii) complexes with benzo[d]thiazole derivatives as the main ligands display good device performances with a maximum current efficiency of up to 69.8 cd A⁻¹ and a maximum external quantum efficiency of up to 24.3%.

Phosphorescent cationic iridium(iii) complexes bearing a nonconjugated six-membered chelating ancillary ligand: a strategy for tuning the emission towards the blue

Blue and blue-green-emitting cationic Ir(iii) complexes bearing a nonconjugated N^N ligand have been studied.

Photo- and Electrochemical Valorization of Carbon Dioxide Using Earth-Abundant Molecular Catalysts

The dramatic increase in anthropogenic carbon dioxide emissions in recent decades has forced us to look for alternative carbon-neutral processes for the production of energy vectors and commodity chemicals. Photo- and electrochemical reduction of CO₂ are appealing strategies for the storage of sustainable and intermittent energies in the form of chemical bonds of synthetic fuels and value-added molecules. In these approaches, carbon dioxide is converted to products such as CO, HCOOH and MeOH through proton-coupled electron transfer reactions. The use of earth-abundant elements as components of the catalytic materials is crucial for the large-scale applicability of this technology. This review summarizes the most recent advances related to this issue, with particular focus on studies where molecular metal complexes are used as catalysts. In addition, with the aim of aiding in the design of more robust and efficient non-noble metal-based catalysts, we discuss the lessons learned from the corresponding mechanistic studies.

Artificial Photosynthetic Systems for CO2 Reduction: Progress on Higher Efficiency with Cobalt Complexes as Catalysts

The conversion of CO₂ into fuels or value-added chemicals is currently a field of great research interest. Molecular cobalt catalysts have long been used as mediators of reductive transformations of CO₂ . In this Minireview, the cobalt complex-based photocatalytic and photoelectrocatalytic systems for CO₂ reduction are discussed and summarized, alongside progress on the design of new molecular cobalt catalysts and their performance in photocatalysis.

New cyclometalated Ir(iii) complexes with NCN pincer and meso-phenylcyanamide BODIPY ligands as efficient photodynamic therapy agents

A new class of cyclometalated iridium(iii) with NCN pincer andmeso-phenylcyanamide BODIPY ligands has been synthesized and studied for photodynamic therapy.

Half-sandwich iridium N-heterocyclic carbene anticancer complexes

Half-sandwich iridium N-heterocyclic carbene complexes display potent anticancer activities and are attractive for development as new anticancer agents.

Cyclometalated Ir(iii) complexes based on 2-(2,4-difluorophenyl)-pyridine and 2,2′-(2-phenyl-1H-imidazole-4,5-diyl)dipyridine: acid/base-induced structural transformation and luminescence switching, and photocatalytic activity for hydrogen evolution

1·PF₆and2show luminescence switching behaviors, and serve as photosensitizers for H₂evolution.

Hirshfeld and DFT analysis of the N-heterocyclic carbene proligand methylenebis(N-butylimidazolium) as the acetonitrile-solvated diiodide salt

N-Heterocyclic carbene (NHC) based systems are usually exploited in the exploration of catalytic mechanisms and processes in organocatalysis, and homo- and heterogeneous catalysis. However, their molecular structures have not received adequate attention. The NHC proligand methylenebis(N-butylimidazolium) has been synthesized as the acetonitrile solvate of the diiodide salt, C15H26N4(2+)·2I(-)·CH3CN [1,1'-methylenebis(3-butylimidazolium) diiodide acetonitrile monosolvate], and fully characterized. An interesting cation-anion connection pattern has been identified in the crystal lattice, in which three iodide anions interact simultaneously with the cisoid-oriented cation. A Hirshfeld surface analysis reveals the predominance of hydrogen bonding over anion-π interactions. This particular arrangement is observed in different methylene-bridged bis(imidazolium) cations bearing chloride or bromide counter-anions. Density functional theory (DFT) calculations with acetonitrile as solvent reproduce the geometry of the title cation.