Organometallic Iridium Research Articles

Organometallic Iridium-Complex-Functionalized Nanographene Catalysts for Low-Temperature Hydrogenation of Carbonyl Derivatives.

An organometallic iridium (Ir)-complex-functionalized nanographene catalyst Ir-PyPh-GC was prepared via a two-step strategy involving amide ligand modification and metal Ir coordination. Ir-PyPh-GC showed ultrahigh hydrogenation capability, good recyclability, and selectivity for carbonyl derivatives (ketones, aldehydes, and quinones) at a low temperature (40 °C). The as-prepared Ir-complex-based catalyst is less expensive, making it feasible for industrial application.

The anticancer application of half-sandwich iridium(III) ferrocene-thiosemicarbazide Schiff base complexes.

Ferrocenyl derivatives and organometallic iridium(III) complexes have been prospective substitutes for platinum-based anticancer drugs. Eight half-sandwich iridium(III) ferrocene-thiosemicarbazide (Fc-TSC) Schiff base anticancer complexes were prepared in this study. These complexes displayed a dimeric structure and exhibited a particular fluorescence due to the "enol" orientation of the TSC pro-ligand. An energy-dependent pathway of the uptake mechanism was ascertained, which ended in the lysosome and led to lysosome damage and apoptosis. Flow cytometry confirmed that the complexes could block the cell cycle (G1 phase) and improve the levels of intracellular reactive oxygen species, indicating an anticancer mechanism of oxidation. Then, a lysosomal-mitochondrial anticancer pathway was verified through western blotting. In vivo toxicity assays confirmed that these complexes showed better anti-migration ability and less toxicity in comparison to cisplatin. Thus, these complexes provide a new strategy for the design of non-platinum organometallic anticancer drugs.

Tuning the Organelle-Specific Imaging and Photodynamic Therapeutic Efficacy of Theranostic Mono- and Trinuclear Organometallic Iridium(III) Complexes.

The organelle-specific localization of mononuclear and trinuclear iridium(III) complexes and their photodynamic behavior within the cells are described herein, emphasizing their structure-activity relationship. Both the IrA2 and IrB2 complexes possess a pair of phenyl-benzothiazole derived from the -CHO moieties of mononuclear organometallic iridium(III) complexes IrA1 and IrB1, which chelates IrCp*Cl (Cp* = 1,2,3,4,5-pentamethylcyclopentadiene) to afford trinuclear complexes IrA3 and IrB3. Insights into the photophysical and electrochemical parameters of the complexes were obtained by a time-dependent density functional theory study. The synthesized complexes IrA2, IrA3, IrB2, and IrB3 were found to be nontoxic to human MCF7 breast carcinoma cells. However, the photoexcitation of complexes using LED light could effectively trigger intracellular reactive oxygen species (ROS) generation, leading to cell death. Furthermore, to check the organelle-specific localization of IrA2 and IrB2, we observed that both complexes could selectively localize in the endoplasmic reticulum. In contrast, trinuclear IrA3 and IrB3 accumulate in the nuclei. The photoexcitation of complexes using LED light could effectively trigger intracellular reactive oxygen species (ROS) generation, leading to cell death.

Bis-oxazoline derivatives as ancillary ligands for bis-cyclometalated iridium complexes

Organometallic iridium complexes with two cyclometalated ligands (C^N) and one bis-oxazoline derived ancillary ligand (L^X), i.e. (C^N)2Ir(L^X), are reported. The C^N ligands are 1-phenylpyrazoline (ppz), 2-(4,6-difluorophenyl)pyridine (F2ppy), 2-phenylpyridine (ppy), 1-phenylisoquinoline (piq). The box ligand is (4S-+)-phenyl-α-[(4S)-phenyloxazolidin-2-ylidene]-2-oxazoline-2-acetonitrile. The emission of these complexes span across the visible and into the near-ultraviolet region of the electromagnetic spectrum with moderate to high photoluminescence quantum yields (ΦPL = 0.45–1.0). These complexes were found to emit from a metal-ligand to ligand charge transfer (ML'LCT) state and have lifetimes (1.3–2.1 μs), radiative rates (105 s−1), and nonradiative rates (104–105 s−1) comparable to state-of-the-art iridium emitters. The (ppy)2Ir(BOX-CN) complexes were resolved into the Δ- and Λ- diastereomers using differences in their solubility and additionally characterized by x-ray crystallography, stability, and chiroptic studies. The high ΦPL of these isomers results in the best to date brightness for circularly polarized luminescence (CPL) from iridium complexes (7.0 M−1 cm−1), with dissymmetry factors of −0.57 × 10−3 and +1.9 × 10−3 for 3Δ and 3Λ, respectively. The significant difference in CPL magnitude between 3Δ and 3Λ likely arises from interligand interactions (edge-to-face arrangement versus strong π-π interaction) for the pendant phenyl ring of the BOX-CN ligand which differ for the two isomers.

Iridium(III) complexes based on cyanomethane and cyanamide ligands with luminescence quenching properties for Fe(III) sensing and biological activities

Two organometallic iridium(III) complexes, Ir(ppy)2Cl(NCCH3) (1) and Ir(ppy)2Cl(NCNH2) (2), were accidentally synthesized from the reaction between the chloro-bridged dimer of (ppy)2Ir(μ-Cl)2Ir(ppy)2 and 2-aminobenzothiazole (abt, C7H6N2S) ligand refluxed in dichloromethane under Ar gas. In the obtained complexes, ppy is 2-phenylpyridine, NCCH3 is cyanomethane or acetonitrile, and NCNH2 is the cyanamide ligand. Complex 1 was received instead of the expected Ir(ppy)2Cl(abt) complex, after the reaction completed for 10 h. Acetonitrile entered as a ligand to the coordination sphere of Ir(III) during the precipitation stage. The reaction was repeated, refluxing for 20 h. This time, the abt ligand fragmented into the cyanamide molecule yielding complex 2 instead of the Ir(ppy)2Cl(abt) complex. Single-crystal X-ray analysis revealed the distorted octahedral geometries of both title complexes. The N and C donors of ppy were in their typical trans- and cis- orientations, respectively. The cyanomethane and cyanamide ligands coordinated via the N-atom. Both complexes exhibited similar photo-physical properties. Complexes 1 and 2 emitted green luminescence between 520 and 525 nm when excited with 325 and 350 nm, respectively, at room temperature in dichloromethane. The respective relative quantum efficiencies were 0.06 and 0.08, compared to coumarin 6 in ethanol. The luminescent emission of both complexes was selectively turn-offtoward increasing Fe3+ concentrations. The 1:1 binding stoichiometry between the complex and Fe3+ ion was determined by Job's plot. Benesi-Hildebrand plots were applied to determine the binding constants (Kb) of 5.86 × 104 and 9.50 × 104 M−1 for complexes 1 and 2, respectively. The complex 2 displayed in vitro cytotoxicity toward HCC1937 and MDA-MB-231 human breast cancer cells with IC50 values of 13.5 and 72.8 µM, respectively. These values indicated 2–3 folds higher sensitivity than cisplatin. In addition, complex 1 showed inhibitory activity against the Gram-positive bacteria Staphylococcus aureus (SA) and methicillin-resistant Staphylococcus aureus (MRSA), with MIC/MBC values of 32/32 and 32/64 μg. mL−1, respectively.

Comparative investigation on interaction mechanism and native conformation of human serum albumin with organometallic iridium(III) complexes via spectroscopic and electrochemical approaches

Organometallic iridium(III) complexes, as high-efficient phototherapic drugs for cancer therapy and tumor treatment, have attracted widespread interest in diverse areas. To further clarify the biological functions of iridium(III) complexes, the interaction mechanism and native conformation of human serum albumin (HSA) affected by three organometallic iridium(III) complexes containing curcuminoid ligands were systematically and comparably investigated via various spectroscopic and electrochemical approaches. The results revealed that three iridium(III) complexes reacted with HSA tightly at its Sudlow's site I under the binding forces of van der Waals interaction, protonation, and hydrogen bond, leading to the hypochromic effect in the absorption spectrum of HSA and the static quenching in the intrinsic fluorescence of HSA. Three iridium(III) complexes disturbed the native conformation of HSA by reducing its α-helical content, finally reducing the melting temperature and the thermal stability of HSA. In comparison, the iridium(III) complex containing curcuminoid ligand with bigger spatial steric-hindrance effect and stronger chemical polarity was prone to binding with HSA more tightly and subsequently affecting its native conformation and biological function more prominently. This research finding reveals the interaction mechanisms of organometallic iridium(III) complexes with HSA and evaluates the function of curcuminoid ligands during their binding interactions, which pays the way for the targeted design and the potential application of novel iridium(III) complexes in biological fields.

Understanding AIE and ACQ phenomenon of organometallic iridium(III) complexes by simple cationization engineering

Understanding the relationship between structure and properties is critical to the development of solid-state luminescence materials with desired characteristics and performance optimization. In this work, we elaborately designed and synthesized a pair of mononuclear iridium(III) complexes with similar structures but different degrees of cationization. [Ir2-f][2PF6] with two counterions is obtained by simple N-methylation of the ancillary ligand of [Ir1-f][PF6] which is a classic cationic iridium(III) complex. Such a tiny modification results in tremendously different optical properties in dilute solutions and powders. [Ir1-f][PF6] exhibits weak light in solution but enhanced emission in solid-state as well as poly(methyl methacrylate) matrix, indicative of its aggregation-induced emission (AIE) activity. On the sharp contrary, [Ir2-f][2PF6] is an aggregation-caused quenching (ACQ) emitter showing strong emission in the isolated state but nearly nonemissive in aggregation states. Benefiting from the appealing characteristics of mechanochromic luminescence and AIE behavior, [Ir1-f][PF6] has been successfully applied in reversible re-writable data recording and cell imaging. These results might provide deep insights into AIE and ACQ phenomenon of iridium(III) complexes and facilitate the development of phosphorescent materials with promising properties.

Organometallic Iridium Complexes with Glucose Based Phosphite Ligands

New organometallic iridium compounds with phosphorus modified glucose ligands containing isopropylidene protection group or bearing uracil, thymine, and 5-fluorouracil (3,5,6-bicyclophosphite-1,2-O-isopropylidene-α-d-glucofuranoside, 3,5,6-bicyclophosphite-1-β-D-glucofuranosyluracil, 3,5,6-bicyclophosphite-1-β-D-glucofuranosylthymine, 3,5,6-bicyclophosphite-1-β-D-glucofuranosyl-5-flurouracil) were prepared. The structure of the new complexes was confirmed by the spectroscopic technique (1H, 31P{1H} NMR) and mass spectrometry, and purity by elemental analysis. The molecular structure of the complex with the isopropylidene protection group was established by the X-ray analysis. The antiproliferative activity of the new iridium compounds was evaluated against several cancer cell lines of human origin, and all compounds showed low toxicity independent of the pyrimidine base nature, attached to the sugar unit.

Dinuclearization strategy of cationic iridium(iii) complexes for efficient and stable flexible light-emitting electrochemical cells

A simple and feasible di-nuclearization strategy of organometallic iridium(iii) complex used to achieve high-efficiency and long-lasting flexible light-emitting electrochemical cells concurrently is demonstrated.

Sustainable Ir-Photoredox Catalysis by Means of Heterogenization.

A heterogenized iridium catalyst was employed to perform photoredox catalysis for a collection of mechanistically orthogonal reactions using very low quantities of iridium (0.01-0.1 mol %). The heterogenized construct consists of an organometallic iridium coordination complex bonded to an aluminum metal oxide solid-state support via an anchoring group. The solid-state support allows for easy recovery and reusability of the catalyst. Evaluation of the catalytic activity was performed with five different reactions, showing broad applicability and demonstrating the general potential for a heterogenized strategy. Moreover, the heterogenized catalyst was shown to be reusable up to five times and also mediated the reactions with much higher efficiency than the original processes by employing the corresponding homogeneous catalyst. As a result of the low catalyst loadings employed, the feasibility of reusage, and faster reaction times, this catalyst offers a more sustainable option when precious metal catalysts are used in organic synthesis. Finally, the catalyst was successfully applied to a gram-scale reaction, showing it is susceptible to scalability.

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds.

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (LCNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(LCNC)2](PF6), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(LCNC)2](PF6) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(LCNC)2](PF6), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Iridium catalyst detection by laser induced breakdown spectroscopy

Monopropellant catalytic thrusters have been used for decades in space systems due to their simplicity and reliability. However, the degradation or loss of the catalyst is a major lifetime limiting effect, both for hydrazine fueled systems as well as for newer hydrazine-replacement fueled systems. Currently there is no in-situ diagnostic technique to measure the health status of the catalyst in such devices. Herein, we report a gas-phase diagnostic technique for measuring iridium, a common catalyst material for monopropellant thrusters. Laser induced breakdown spectroscopy was employed to quantify iridium in the gas phase by thermally vaporizing various organometallic iridium compounds to known concentrations within a test cell and observing the resulting iridium optical emission lines that had no interfering lines in the region from other possible emitting species (such as nitrogen, oxygen, and carbon). A limit of detection for the iridium was determined to be 6.21 μmol/L (197 ppm by volume), and this, to our knowledge, is the first report on the quantitative analysis of gas-phase iridium by laser induced breakdown spectroscopy.

Organometallic Iridium Complexes of (Z)-1-Phenyl-2-(4′,4′-dimethyl-2′-oxazolin-2′-yl)-eth-1-en-1-ate: Structural Aspects, Reactivity and Applications in the Catalytic Dehydrogenation of Alkanes

Abstract The treatment of [IrCl(cod)]2 with (Z)-1-phenyl-2-(4′,4′-dimethyl-2′-oxazolin-2′-yl)-eth-1-en-1-ol (HL) in the presence of base yields the first Ir complex of this ligand class: Ir(κ2-N,O-L)(cod) (3). Complex 3 is reactive with MeI or HSnPh3 to yield the oxidative addition products 4 (trans-Ir(Me)I(κ2-N,O-L)(cod)) and 5 (cis-IrH(SnPh3)(κ2-N,O-L)(cod)), respectively. All three of these derivatives have been fully characterised including via single crystal X-ray diffraction data. Complex 3 is generally resistant to cod ligand substitution but shown to be reactive with CO (g) to give Ir(κ2-N,O-L)(CO)2 (6). In addition, 3 is demonstrated to be a dehydrogenation catalyst for the conversion of C8H16 into cyclooctene and H2 under acceptor-free conditions.

Optical and Electrical Measurements Reveal the Orientation Mechanism of Homoleptic Iridium-Carbene Complexes.

Understanding and controlling the driving forces for molecular alignment in optoelectronic thin-film devices is of crucial importance for improving their performance. In this context, the preferential orientation of organometallic iridium complexes is in the focus of research to benefit from their improved light-outcoupling efficiencies in organic light-emitting diodes (OLEDs). Although there has been great progress concerning the orientation behavior for heteroleptic Ir complexes, the mechanism behind the alignment of homoleptic complexes is still unclear yet. In this work, we present a sky-blue phosphorescent dye that shows variable alignment depending on systematic modifications of the ligands bound to the central iridium atom. From an optical study of the transition dipole moment orientation and the electrically accessible alignment of the permanent dipole moment, we conclude that the film morphology is related to both the aspect ratio of the dye and the local electrostatic interaction of the ligands with the film surface during growth. These results indicate a potential strategy to actively control the orientation of iridium-based emitters for the application in OLEDs.

A phosphorescent iridium complex as a probe for diatom cells’ viability

Diatoms are unicellular photosynthetic algae that autonomously fabricate a porous organized biosilica shell refined in billion years of evolution. They represent an inexhaustible source of low cost, biocompatible mesoporous silica. Despite the major advances in the genomic field, studies on diatom cell biology are still hampered by a lack of cellular tools. In particular, cell staining assays of diatoms viability are still limited or not well performant. Here we provide a phosphorescent organometallic iridium complex (Ir-Fcx) suitable to act as staining agent to detect diatoms viability.

Mechanistic Insights into C–H Borylation of Arenes with Organoiridium Catalysts Embedded in a Microporous Metal–Organic Framework

Organometallic iridium catalysts can be used in conjunction with bispinacolatodiboron (B2Pin2) to effect the borylation of a variety of substrates such as arenes, alkanes, heteroarenes, and oxygena...

Lysosome-targeted chemotherapeutics: Anticancer mechanism of N-heterocyclic carbene iridium(III) complex

N-heterocyclic carbenes-modified half-sandwich iridium(III) complex [(η5-C5Me4C6H4C6H5)Ir(C^C)Cl]PF6 (C1) (where C^C is a N-heterocyclic carbene ligand) can effectively prevent the proliferation of human cervical cancer cells. Here, this study aims to investigate the in-deep anticancer effects of this complex on non-small cell lung cancer cells and explore the underlying molecular mechanism. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay showed that iridium(III) complex had potent cytotoxicity studies towards non-small cell lung cancer cells (A549), human lung squamous cells (L78), human cervical cancer cells (Hela) and human bronchial epithelial cells (BEAS-2B). Colocalization and cellular uptake studies were analyzed by confocal microscopy. Notably, C1 targeted lysosomes and entered the cancer cells partially through an energy-dependent pathway, inducing the release of cathepsins and other proteins. These proteins regulated lysosomal-mitochondrial dysfunction, thus leading to the release of cytochrome c (cyt c), which amplified apoptotic signals by activating many downstream pathways such as caspase pathways to promote cell apoptosis. The results showed that the inhibitory mechanism of this organometallic iridium(III) complex may involve caspase-associated apoptosis initiated by the lysosomal-mitochondrial pathway.

Targeted cell imaging properties of a deep red luminescent iridium(iii) complex conjugated with a c-Myc signal peptide.

A nuclear localisation sequence (NLS) peptide, PAAKRVKLD, derived from the human c-Myc regulator gene, has been functionalised with a long wavelength (λ ex = 550 nm; λ em = 677 nm) cyclometalated organometallic iridium(iii) complex to give the conjugate Ir-CMYC. Confocal fluorescence microscopy studies on human fibroblast cells imaged after 18-24 h incubation show that Ir-CMYC concentrations of 80-100 μM promote good cell uptake and nuclear localisation, which was confirmed though co-localisation studies using Hoechst 33342. In comparison, a structurally related, photophysically analogous iridium(iii) complex lacking the peptide sequence, Ir-PYR, showed very different biological behaviour, with no evidence of nuclear, lysosomal or autophagic vesicle localisation and significantly increased toxicity to the cells at concentrations >10 μM that induced mitochondrial dysfunction. Supporting UV-visible and circular dichroism spectroscopic studies show that Ir-PYR and Ir-CMYC display similarly low affinities for DNA (ca. 103 M-1), consistent with electrostatic binding. Therefore the translocation and nuclear uptake properties of Ir-CMYC are attributed to the presence of the PAAKRVKLD nuclear localisation sequence in this complex.

Organometallic Iridium(III) Complex Sensitized Ternary Hybrid Photocatalyst for CO2 to CO Conversion.

A series of heteroleptic iridium(III) complexes functionalized with two phosphonic acid (-PO3 H2 ) groups (dfppy IrP, ppy IrP, btp IrP, and piq IrP) were prepared and anchored onto rhenium(I) catalyst (ReP)-loaded TiO2 particles (TiO2 /ReP) to build up a new IrP-sensitized TiO2 photocatalyst system (IrP/TiO2 /ReP). The photosensitizing behavior of the IrP series was examined within the IrP/TiO2 /ReP platform for the photocatalytic conversion of CO2 into CO. The four IrP-based ternary hybrids showed increased conversion activity and durability than that of the corresponding homo- (IrP+ReP) and heterogeneous (IrP+TiO2 /ReP) mixed systems. Among the four IrP/TiO2 /ReP photocatalysts, the low-energy-light (>500 nm) activated piq IrP immobilized ternary system (piq IrP/TiO2 /ReP) exhibited the most durable conversion activity, giving a turnover number of ≥730 for 170 h. A similar kinetic feature observed through time-resolved photoluminescence measurements of both btp IrP/TiO2 and TiO2 -free btp IrP films suggests that the net electron flow in the ternary hybrid proceeds dominantly through a reductive quenching mechanism, unlike the oxidative quenching route of typical dye/TiO2 -based photolysis.

Dual visible/NIR emission from organometallic iridium(III) complexes

A series of four substituted benzo[g]quinoxaline species have been synthesised and utilised as cyclometalating ligands for iridium(III). The ligands (L1-L4) were synthesised and isolated in good yield following the condensation of 2,3-diaminonaphthalene with benzil and three of its derivatives. The substituent modulated electronic properties of L1-L4 were dominated by intraligand π−π* transitions, with the fluorescence profile demonstrating vibronic features attributed to the highly conjugated nature of the chromophore. Iridium(III) complexes of the form [Ir(L)2(bipy)]PF6 were synthesised from L1-L4 in two steps. The electronic properties of the complexes reveal absorption in the UV-vis. region with spin forbidden metal-to-ligand charge transfer (MLCT) transitions possibly contributing at longer wavelengths to ca. 600 nm. Steady state luminescence (aerated, room temperature) on solutions of the complexes showed dual emissive properties in the visible and near-infra red (NIR) regions. Firstly, a vibronically structured emission in the visible region (ca. 525 nm) was attributed to ligand centred fluorescence (lifetime < 10 ns). Secondly, a broad emission peak in the NIR (ca. 950 nm) which extended to around 1200 nm was observed with corresponding lifetimes of 116–162 ns, indicative of triplet excited state emission.