N-heterocyclic Carbene Iridium Complexes Research Articles

Optical properties and exciton transfer between N-heterocyclic carbene iridium(III) complexes for blue light-emitting diode applications from first principles.

N-heterocyclic carbene (NHC) iridium(III) complexes are considered as promising candidates for blue emitters in organic light-emitting diodes. They can play the roles of the emitter as well as of electron and hole transporters in the same emission layer. We investigate optical transitions in such complexes with account of geometry and electronic structure changes upon excitation or charging and exciton transfer between the complexes from first principles. It is shown that excitation of NHC iridium complexes is accompanied by a large reorganization energy ∼0.7eV and a significant loss in the oscillator strength, which should lead to low exciton diffusion. Calculations with account of spin-orbit coupling reveal a small singlet-triplet splitting ∼0.1eV, whereas the oscillator strength for triplet excitations is found to be an order of magnitude smaller than for the singlet ones. The contributions of the Förster and Dexter mechanisms are analyzed via the explicit integration of transition densities. It is shown that for typical distances between emitter complexes in the emission layer, the contribution of the Dexter mechanism should be negligible compared to the Förster mechanism. At the same time, the ideal dipole approximation, although giving the correct order of the exciton coupling, fails to reproduce the result taking into account spatial distribution of the transition density. For charged NHC complexes, we find a number of optical transitions close to the emission peak of the blue emitter with high exciton transfer rates that can be responsible for exciton-polaron quenching. The nature of these transitions is analyzed.

N-Heterocyclic Carbene-Iridium Complexes as Photosensitizers for In Vitro Photodynamic Therapy to Trigger Non-Apoptotic Cell Death in Cancer Cells

A series of seven novel iridium complexes were synthetized and characterized as potential photosensitizers for photodynamic therapy (PDT) applications. Among them, four complexes were evaluated in vitro for their anti-proliferative activity with and without irradiation on a panel of five cancer cell lines, namely PC-3 (prostate cancer), T24 (bladder cancer), MCF7 (breast cancer), A549 (lung cancer) and HeLa (cervix cancer), and two non-cancerous cell models (NIH-3T3 fibroblasts and MC3T3 osteoblasts). After irradiation at 458 nm, all tested complexes showed a strong selectivity against cancer cells, with a selectivity index (SI) ranging from 8 to 34 compared with non-cancerous cells. The cytotoxic effect of all these complexes was found to be independent of the anti-apoptotic protein Bcl-xL. The compound exhibiting the best selectivity, complex 4a, was selected for further investigations. Complex 4a was mainly localized in the mitochondria. We found that the loss of cell viability and the decrease in ATP and GSH content induced by complex 4a were independent of both Bcl-xL and caspase activation, leading to a non-apoptotic cell death. By counteracting the intrinsic or acquired resistance to apoptosis associated with cancer, complex 4a could be an interesting therapeutic alternative to be studied in preclinical models.

Silyl-functionalised silver rhodium and iridium N-heterocyclic carbene complexes with promising cytotoxicity and antimicrobial potential

In the present study, we have synthesized Ag, Rh, and Ir complexes of a silyl-substituted NHC ligand in order to investigate their biological properties. Firstly, Ag-NHC ( 2 ) complex was synthesized by the interaction of Ag 2 O with benzimidazolium iodide ( 1 ). Rh- ( 3 ) and Ir-NHC ( 4 ) complexes were synthesized by the transmetalation reaction between Ag-NHC and [M(cod)Cl] 2 (M = Rh or Ir) dimers. The complexes were characterised by NMR spectroscopy and elemental analyses, and crystal structures of Rh- and Ir-NHC ( 3 and 4 ) were also reported. The cytotoxic effects of the complexes were tested on four human cancer cell lines and the antimicrobial activities of the complexes were tested against a panel of nine pathogenic microorganisms. The complexes performed comparable or better activity than standards, cisplatin and AgNO 3 .

Electronic Effects of Support Doping on Hydrotalcite-Supported Iridium N-Heterocyclic Carbene Complexes

The electronic effects of supports on immobilized organometallic complexes impact their activity and lifetime, yet remain poorly understood. Here we describe a systematic study of the support effects experienced by an organometallic complex immobilized on doped hydrotalcite-like materials. To that end, we describe the synthesis and characterization of the first organometallic species immobilized on a palette of doped hydrotalcites via sulfonate linkers. The organometallic species consists of iridium N-heterocyclic carbene (NHC) carbonyl complex ([Na][Ir-(NHC-Ph-SO3)2(CO)2]), a highly active molecular catalyst for transfer hydrogenation of glycerol. The hydrotalcite supports are composed of Al, Mg, and a compatible transition-metal dopant (Fe, Cu, Ni, Zn). The materials were characterized extensively by STEM, XPS, TGA, PXRD, FT-IR, N2 desorption, ICP-AES, TPD, and microcalorimetry to probe the morphology and electronic properties of the support and elucidate structure–property relationships.

Long-Term Generation of Longitudinal Spin Order Controlled by Ammonia Ligation Enables Rapid SABRE Hyperpolarized 2D NMR.

Symmetry breaking of parahydrogen using iridium catalysts converts singlet spin order into observable hyperpolarization. In this contribution, iridium catalysts are designed to exhibit asymmetry in their hydrides, regulated by in situ generation of deuterated ammonia governed by ammonium buffers. The concentrations of ammonia (N) and pyridine (P) provide a handle to generate a variety of stereo-chemically asymmetric N-heterocyclic carbene iridium complexes, ligating either [3xP], [2xP;N], [P;2xN] or [3xN] in an octahedral SABRE type configuration. The non-equivalent hydride positions, in correspondence with the ammonium buffer solutions, enables to extend singlet-triplet or mixing at high magnetic field and experimentally induce prolonged generation of non-equilibrium longitudinal two-spin order. This long-lasting magnetization can be exploited in hyperpolarized 2D-OPSY-COSY experiments providing direct structural information on the catalyst using a single contact with parahydrogen. Separately, field cycling revealed hyperpolarization properties in low-field conditions. Controlling catalyst stereochemistry by introducing small and deuterated ligands, such as deuterated ammonia, simplifies the spin-system. This is shown to unify experimental and theoretically derived field-sweep experiments for four-spin systems.

Oxidation of Sugar Alcohols Catalyzed by a Polymeric N-Heterocyclic Carbene–Iridium Complex

Oxidation of Sugar Alcohols Catalyzed by a Polymeric N-Heterocyclic Carbene–Iridium Complex

Transfer hydrogenation of levulinic acid from glycerol and ethanol using water-soluble iridium N-heterocyclic carbene complexes

The upgrading of biomass derivatives to biofuels and chemicals through transfer hydrogenation (TH) is attractive relative to direct hydrogenation, especially when the hydrogen donors can be sourced renewably. Here we report the first process that uses glycerol, a renewable waste material from biodiesel processing, as a hydrogen donor in the catalytic TH of a biomass-derived platform chemical, levulinic acid, to selectively afford γ-hydroxyvaleric acid (GHV) and lactic acid (LA). GHV can be further converted to γ-valerolactone (GVL), a widely used platform chemical. Levulinic acid can be used directly, without esterification, which is typically needed for transfer hydrogenation. The process is efficiently facilitated by robust iridium N-heterocyclic carbene (NHC) complexes with sulfonate functional groups at low catalyst loading (1–10 ppm), affording quantitative conversion of levulinic acid in the presence of KOH to GHV, with >100,000 TON in 2 h at 150 °C, using 1 ppm catalyst. The most prolific catalyst, [(NHC-SO3-)2(CO)2Ir]Na, can also facilitate transfer hydrogenation from other hydrogen donors, such as 2-propanol, potassium formate, and most notably, ethanol, which can also be derived from renewables. Ethanol is a highly efficient hydrogen donor for levulinic acid using this catalyst, affording >7,000 turnovers in 2 h using 10 ppm catalyst.

Indirect Detection of Short-Lived Hydride Intermediates of Iridium N-Heterocyclic Carbene Complexes via Chemical Exchange Saturation Transfer Spectroscopy

For the first time, chemical exchange saturation transfer (CEST) nuclear magnetic resonance (NMR) is utilized to study short-lived hydride intermediates in the catalytic cycle of an organometallic ...

Conversion of haloform to carbonate by iridium N-heterocyclic carbene complexes and silver(i) oxide.

The reaction between haloform, the complexes [Cp*IrCl2(κC-MeNC3H2NCH2Ar)] (Ar = C6H3F2-2,6, C6H3F-2-Cl-6, C6H3Me2-2,6) and silver(i) oxide proceeded by carbon-halogen bond fission to yield the carbonate complexes [Cp*Ir(κ2O-CO3)(κC-MeNC3H2NCH2Ar)].

Efficient and Versatile Catalytic Systems for the N-Methylation of Primary Amines with Methanol Catalyzed by N-Heterocyclic Carbene Complexes of Iridium

Efficient and versatile catalytic systems were developed for the N-methylation of both aliphatic and aromatic primary amines using methanol as the methylating agent. Iridium complexes bearing an N-heterocyclic carbene (NHC) ligand exhibited high catalytic performance for this type of transformation. For aliphatic amines, selective N,N-dimethylation was achieved at low temperatures (50–90 °C). For aromatic amines, selective N-monomethylation and selective N,N-dimethylation were accomplished by simply changing the reaction conditions (presence or absence of a base with an appropriate catalyst). These findings can be used to develop methods for synthesizing useful amine compounds having N-methyl or N,N-dimethyl moieties.

An N-heterocyclic carbene iridium catalyst with metal-centered chirality for enantioselective transfer hydrogenation of imines.

A cyclometalating N-heterocyclic carbene iridium complex featuring metal-centered chirality was designed and used for the asymmetric transfer hydrogenation (ATH) of imines. Four strongly σ-donating carbon-based substituents (2 carbenes and 2 phenyl moieties), a chirality transfer directly from the stereogenic metal center to the C[double bond, length as m-dash]N bond of substrates, as well as a restriction of catalyst deactivation by steric demanding substituents, render the new complex one of the most efficient catalysts for ATH of cyclic N-sulfonylimines (down to 0.01 mol% cat., 24 examples, 94-98% ee).

A Site-Specific Study of the Magnetic Field-Dependent Proton Spin Relaxation of an Iridium N-Heterocyclic Carbene Complex

Abstract We report a study of proton spin relaxation of an Iridium N-heterocyclic carbene complex [Ir(COD)(IMes)Cl] complex (where COD=1,5-cyclooctadiene, Imes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). This compound is a pre-catalyst of the most efficient complex allowing the signal amplification by reversible exchange (SABRE) effect, relevant for enhancing weak signals in nuclear magnetic resonance (NMR). An important feature of the study is a combination of relaxation measurements over a wide field range with high-resolution NMR detection. As a result, we are able to measure nuclear magnetic relaxation dispersion (NMRD) curves in the field range 0.1 mT–16.4 T (corresponding to the frequency range 4 kHz–700 MHz) for individual protons in the complex under study. This attractive possibility enables determination of the motional correlation times, τc , for the individual protons by analyzing the features in the NMRD curves (increase of the relaxation times) appearing at the magnetic fields where ωτc ≈1 (here ω is the proton Larmor precession frequency at a given field strength). The following correlation times were determined: (1.3±0.1) ns for the protons of imidazol-2-ylidene, (0.96±0.1) ns for the ortho-protons of two phenyl moieties and (0.95±0.2) ns for the protons of methyl groups. Additionally, we report low-field features coming from “strong coupling” of the protons. One should note that such features must not be misinterpreted by associating them with motional features. From the low-field features we obtain consistent estimates for the proton spin-spin interactions. The analysis of motional correlation times is also of importance for interpretation of spin order transfer from parahydrogen to various substrates in transient organometallic complexes (termed the SABRE effect) at high magnetic field.

Experimental and Computational Studies on the Reactivity and Binding Mode of Thiophene with N-Heterocyclic Carbene Iridium Complexes

The reactivity of thiophene (T), 2-methylthiophene (2-MeT), and benzothiophene (BT) with [Ir(cod)(IPr)(L)]BF4 complexes (L = acetone (1), pyridine (2) or dimethylphenylphosphine (3); IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) in the presence of molecular hydrogen has been investigated. Under these conditions the 1,5-cyclooctadiene ligand is hydrogenated to cyclooctane, which renders an unsaturated species (Ir-IPr-L) able to coordinate the thiophene moiety. The coordination mode of T, 2-MeT, and BT depends on the nature of the substrate and the ligand trans to the IPr (L). The reaction of 1 with T and 2-MeT leads to dissociation of the acetone ligand to afford the complexes [Ir(H)2(IPr)(η5-T)]BF4 (4) and [Ir(H)2(IPr)(η2-2-MeT)(κS-2-MeT)]BF4 (5), respectively, but no stable complex is observed on reaction with BT. Analogously to 1, complex 2 does not give a stable complex on reaction with BT, while reaction with 2-MeT yields complex 5 again. Conversely, reaction with T affords a mixture of comp...

Unveiling the Importance of π-Stacking in Borrowing-Hydrogen Processes Catalysed by Iridium Complexes with Pyrene Tags.

This work describes the preparation of a series of pyrene-tagged N-heterocyclic carbene complexes of iridium, and their use in two benchmark borrowing hydrogen reactions: the reduction of ketones by transfer hydrogenation and the β-alkylation of secondary alcohols with primary alcohols. The detailed study of these homogeneously catalysed reactions reveals several important implications regarding the strong influence of the pyrene tags in the catalysts. First, the catalytic activity is partially inhibited by addition of an external amount of pyrene, but only when pyrene-tagged catalysts and aromatic substrates are used. Second, the rate order of the reaction is highly dependent on the nature of the substrates and the ligand. When pyrene-tagged catalysts and aromatic substrates are used, the reaction follows a zero-order dependence on the concentration of the substrate. All other combinations afford a second-order rate in the substrates. And third, the presence or absence of the pyrene functionality in the catalyst also influences the reaction order with respect to the concentration of the catalyst. Pyrene-containing catalysts display a fractional rate order of below 1. Finally, two pyrene-tagged catalysts were supported onto reduced-graphene oxide (rGO), and used as heterogeneous catalysts. While the dimetallic catalyst was effectively recycled 12 times, the monometallic catalyst maintained its activity for only three runs.

Methanol dehydrogenation by iridium N-heterocyclic carbene complexes.

A series of homogeneous iridium bis(N-heterocyclic carbene) catalysts are active for three transformations involving dehydrogenative methanol activation: acceptorless dehydrogenation, transfer hydrogenation, and amine monoalkylation. The acceptorless dehydrogenation reaction requires base, yielding formate and carbonate, as well as 2-3 equivalents of H2. Of the few homogeneous systems known for this reaction, our catalysts tolerate air and employ simple ligands. Transfer hydrogenation of ketones and imines from methanol is also possible. Finally, N-monomethylation of anilines occurs through a "borrowing hydrogen" reaction. Notably, this reaction is highly selective for the monomethylated product.

Iridium(III) N-heterocyclic carbene complexes: an experimental and theoretical study of structural, spectroscopic, electrochemical and electrogenerated chemiluminescence properties.

Four cationic heteroleptic iridium(III) complexes have been prepared from methyl- or benzyl-substituted chelating imidazolylidene or benzimidazolylidene ligands using a Ag(I) transmetallation protocol. The synthesised iridium(III) complexes were characterised by elemental analysis, (1)H and (13)C NMR spectroscopy and the molecular structures for three complexes were determined by single crystal X-ray diffraction. A combined theoretical and experimental investigation into the spectroscopic and electrochemical properties of the series was performed in order to gain understanding into the factors influencing photoluminescence and electrochemiluminescence efficiency for these complexes, with the results compared with those of similar NHC complexes of iridium and ruthenium. The N^C coordination mode in these complexes is thought to stabilise thermally accessible non-emissive states relative to the case with analogous complexes with C^C coordinated NHC ligands, resulting in low quantum yields. As a result of this and the instability of the oxidised and reduced forms of the complexes, the electrogenerated chemiluminescence intensities for the compounds are also low, despite favourable energetics. These studies provide valuable insights into the factors that must be considered when designing new NHC-based luminescent complexes.

Catalytic applications of magnetic nanoparticles functionalized using iridium N-heterocyclic carbene complexes

The development of a synthetic modular methodology for the preparation of catalytic materials based on magnetic nanoparticles with iridium complexes and their application to transfer hydrogenation.

An iridium N-heterocyclic carbene complex [IrCl(CO)2(NHC)] as a carbon monoxide-releasing molecule (CORM)

Four NHC complexes of rhodium and iridium of the general structure [RhI(COD)(NHC)], [IrCl(COD)(NHC)], [IrCl(CO)2(NHC)], and [Ir(N3)(COD)(NHC)] (NHC = N-heterocyclic carbene) were synthesised and characterised, including X-ray structure determination for all three iridium compounds. Release of carbon monoxide from [IrCl(CO)2(NHC)] occurred via a rapid bolus of CO on a very short timescale, independent of whether the experiment was performed in the dark or with UV-light illumination, thus establishing the compound as a novel ligand-exchange triggered CO-releasing molecule (CORM). The azide complex represents the first iridium–NHC complex containing an azide ligand.

EXAFS and DFT studies on iridium catalysts for SABRE

Since it was first developed, Nuclear Magnetic Resonance (NMR) has become a powerful analytical tool that is now used widely in the fields of chemistry, materials science, and medicine. One way to overcome the intrinsic insensitivity of NMR is to use hyperpolarization techniques to produce non-Boltzmann spin-state distributions. One of these techniques is Signal Amplification By Reversible Exchange (SABRE),[1] in which hyperpolarization is achieved by the temporary association of parahydrogen and a substrate in the coordination sphere of a transition metal. The polarization can be transferred from the parahydrogen-derived hydride ligands to the bound substrate via scalar coupling, followed by dissociation of the hyperpolarized substrate into the bulk solution. We have investigated the efficiency of various iridium NHC complexes with aliphatic and aromatic R groups as SABRE catalysts.[2] The used metal centre is a six-coordinate iridium N-heterocyclic carbene complex, with three substrates and two hydrides, in which the exchange rate of substrate and parahydrogen at the metal centre determines the efficiency of the hyperpolarization. As solvent molecules compete with pyridine for coordination to iridium, the sensitivity of SABRE can be enhanced by displacement of solvent molecules by cosubstrates, i.e. proton-poor ligands such as methyltriazole.[3] In this exchange process, several mixed iridium complexes can be considered to exist, which were not all observed by NMR. Therefore, Density Functional Theory (DFT) calculations were performed on these complexes to better understand this phenomenon. While NMR itself is the best source of information on protons and dynamic processes involved in SABRE, we have found that Extended X-ray Absorption Fine Structure (EXAFS) studies in organic solutions provide interesting complimentary information on the complexes involved.

Hydroxyl and ester functionalized N-heterocyclic carbene complexes of iridium(i): efficient catalysts for transfer hydrogenation reactions

Iridium(i)–NHC catalyzed transfer hydrogenation reaction of aldehydes and ketones with up to 10 000 TON has been achieved.