Iridium N-heterocyclic Carbene 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.

(1,2,5,6-η)-Cyclo-octa-1,5-diene](1-ethyl-4-isopropyl-1,2,4-triazol-5-yl-idene)(tri-phenylphos-phane)iridium(I) tetra-fluorido-borate di-chloro-methane sesquisolvate.

The synthesis and crystal structure of a new triazole-based N-heterocyclic carbene iridium(I) cationic complex with a tetra-fluorido-borate counter-anion and solvating di-chloro-methane, [Ir(C8H12)(C7H13N3)(C18H15P)]BF4·1.5CH2Cl2, is reported. The IrI center of the cationic complex has a distorted square-planar conformation, formed by a bidentate cyclo-octa-1,5-diene (COD) ligand, an N-heterocyclic carbene, and a triphenylphosphane ligand. There are weak hydrogen-bonding inter-actions between C-H groupings of the iridium complex and F atoms of the [BF4]- counter-ions. The atoms of the COD ligand are disordered over two sets of sites in a 0.65:0.35 ratio and two of the F atoms of the anion are disordered over adjacent sites in a 0.6:0.4 ratio. One of the di-chloro-methane solvent mol-ecules is disordered about an inversion center with 0.5 occupancy.

(1,2,5,6-η)-Cyclo-octa-1,5-diene](4-isopropyl-1-methyl-1,2,4-triazol-5-yl-idene)(tri-phenyl-phos-phane)iridium(I) tetra-fluorido-borate di-chloro-methane 0.8-solvate.

A new triazole-based N-heterocyclic carbene iridium(I) cationic complex with a tetra-fluorido-borate counter-anion, [Ir(C8H12)(C18H15P)(C6H11N3)]BF4·0.8CH2Cl2, has been synthesized and structurally characterized. The central IrI atom of the cationic complex has a distorted square-planar coordination environment, formed by a bidentate cyclo-octa-1,5-diene (COD) ligand, an N-heterocyclic carbene, and a tri-phenyl-phosphane ligand. The crystal structure comprises C-H⋯π(ring) inter-actions that orient the phenyl rings; non-classical hydrogen-bonding inter-actions between the cationic complex and the tetra-fluorido-borate anion are also present. The complex crystallizes in a triclinic unit cell with two structural units and an incorporation of di-chloro-methane solvate mol-ecules with an occupancy of 0.8.

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 .

An N-heterocyclic carbene iridium(III) complex as a potent anti-cancer stem cell therapeutic

Cancer stem cells (CSCs) represent a difficult to treat cellular niche within tumours due to their unique characteristics, which give them a high propensity for resistance to classical anti-cancer treatments and the ability to repopulate the tumour mass. An attribute that may be implicated in the high rates of recurrence of certain tumours. However, other characteristics specific to these cells, such as their high dependence on mitochondria, may be exploited for the development of new therapeutic agents that are effective against the niche. As such, a previously described phosphorescent N-heterocyclic carbene iridium(III) compound which showed a high level of cytotoxicity against classical tumour cell lines with mitochondria-specific effects was studied for its potential against CSCs. The results showed a significantly higher level of activity against several CSC lines compared to non-CSCs. Mitochondrial localisation and superoxide production were confirmed. Although the cell death involved caspase activation, their role in cell death was not definitive, with a potential implication of other, non-apoptotic pathways shown. A cytostatic effect of the compound was also displayed at low mortality doses. This study thus provides important insights into the mechanisms and the potential for this class of molecule in the domain of anti-CSC therapeutics.

(1,2,5,6-η)-Cyclo-octa-1,5-diene](4-isopropyl-1-methyl-1,2,4-triazol-5-yl-idene)(tri-cyclo-hexyl-phosphane-κP)iridium(I) tetra-fluorido-borate di-chloro-methane monosolvate.

The title compound, [Ir(C8H12)(C6H11N3)(C18H33P)]BF4·CH2Cl2, a triazole-based N-heterocyclic carbene iridium(I) cationic complex with a tetra-fluorido-borate counter-anion, crystallizes with one di-chloro-methane solvent mol-ecule per formula unit. The IrI atom of the cationic complex has a distorted square-planar coordination environment, defined by a bidentate cyclo-octa-1,5-diene (COD) ligand, an N-heterocyclic carbene, and a tri-cyclo-hexyl-phosphane ligand. The complex crystallizes in a C-centered monoclinic unit cell and has an unusually high number of eight formula units.

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.

(4-Butyl-1-methyl-1,2,4-triazol-5-ylidene)[(1,2,5,6-η)-cycloocta-1,5-diene](triphenylphosphane)iridium(I) tetrafluoridoborate

A new triazole-based N-heterocyclic cationic carbene iridium(I) complex with a tetra-fluorido-borate counter-anion, [Ir(C8H12)(C7H13N3)(C18H15P)]BF4, has been synthesized and structurally characterized. The IrI atom of the cationic complex has an expected square-planar coordination environment with unexceptional bond lengths. There are several close F⋯H contacts between the cations and the anions in the range 2.36-2.58 Å, stabilizing the orientation of the out-sphere [BF4 -] counter-anion. In the crystal, C-H⋯π(ring) inter-actions are observed that orient the phenyl rings of the tri-phenyl-phosphane ligands.

(4-Benzyl-1-methyl-1,2,4-triazol-5-yl-idene)[(1,2,5,6-η)-cyclo-octa-1,5-diene](tri-phenyl-phosphane-κP)iridium(I) tetra-fluorido-borate.

A new triazole-based N-heterocyclic carbene iridium(I) cationic complex with a tetra-fluorido-borate counter-anion, [Ir(C10H11N3)(C8H12)(C18H15P)]BF4, has been synthesized and structurally characterized. The cationic complex exhibits a distorted square-planar environment around the IrI ion. One significant non-standard hydrogen-bonding inter-action exists between a hydrogen atom on the N-heterocyclic carbene ligand and a fluorine atom from the counter-ion, BF4 -. In the crystal, π-π stacking inter-actions are observed between one of the phenyl rings and the triazole ring. Both inter-molecular and intra-molecular C-H⋯π(ring) inter-actions are also observed.

Synthesis of α-Alkylated Ketones via Selective Epoxide Opening/Alkylation Reactions with Primary Alcohols.

A new method for converting terminal epoxides and primary alcohols into α-alkylated ketones under borrowing hydrogen conditions is reported. The procedure involves a one-pot epoxide ring opening and alkylation via primary alcohols in the presence of an N-heterocyclic carbene iridium(I) catalyst, under aerobic conditions, with water as the side product.

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.

Effect of the ancillary ligand in N-heterocyclic carbene iridium(III) catalyzed N-alkylation of amines with alcohols

A series of air-stable N-heterocyclic carbene (NHC) Ir(III) complexes (Ir1-6), bearing various combinations of chlorine, pyridine and NHC ligands, were assayed for the N-alkylation of amines with alcohols. It was found that Ir3, with two monodentate 1,3-bis-methyl-imidazolylidene (IMe) ligands, emerged as the most active complex. A large variety of amines and primary alcohols were efficiently converted into mono-N-alkylated amines in 53–96% yields. As a special highlight, for the challenging MeOH, selective N-monomethylation could be achieved using KOH as a base under an air atmosphere. Moreover, this catalytic system was successfully applied to the gram-scale synthesis of some valuable compounds.

Design of Iridium N-Heterocyclic Carbene Amino Acid Catalysts for Asymmetric Transfer Hydrogenation of Aryl Ketones

A series of chiral complexes of the form Ir(NHC)2(aa)(H)(X) (NHC = N-heterocyclic carbene, aa = chelated amino acid, X = halide) was synthesized by oxidative addition of α-amino acids to iridium(I) bis-NHC compounds and screened for asymmetric transfer hydrogenation of ketones. Following optimization of the reaction conditions, NHC, and amino acid ligands, high enantioselectivity was achieved when employing the Ir(IMe)2(l-Pro)(H)(I) catalyst (IMe = 1,3-dimethylimidazol-2-ylidene), which asymmetrically reduces a range of acetophenone derivatives in up to 95% enantiomeric excess.

The catalytic activity of new iridium(I) N-heterocyclic carbene complexes for hydrogen transfer reaction of ketones

In this paper, the reaction of [Ir(COD)Cl]2 with in situ prepared Ag–N-heterocyclic carbene (NHC) complexes yields a series of [IrCl(COD)(NHC)] complexes. All compounds were fully characterized by 1H NMR, 13C NMR, and FT–IR spectroscopy. The manuscript focused on the preparation of new Ir–NHC complexes, characterization and catalytic behavior. A series of hydrogenation transfer reactions were performed to reveal the effects of the Ir–NHC complexes. The new Ir–NHC complexes of benzimidazole-2-ylidene are effective catalysts for the transfer of hydrogenation of different ketones, using i-PrOH as the source of hydrogen in the presence of KOH. The reactions were conducted at a substrate/catalyst/base (S/C/base) molar ratio of 1:0.001:2. Although all of the complexes are active catalysts for the transfer hydrogenation of ketones, moderate yields were obtained with acetylnaphthalene and conversion was not observed with very substituted ketones such as 2′,3′,4′,5′,6′-pentamethylacetophenone. It was observed that for transfer hydrogenation reactions Ir–NHC catalysts were more active, compared to Ru–NHC catalyzed studies performed by our team.

α-Alkylation of arylacetonitriles with primary alcohols catalyzed by backbone modified N-heterocyclic carbene iridium(I) complexes.

A series of backbone-modified N-heterocyclic carbene (NHC) complexes of iridium(i) (1d-f) have been synthesized and characterized. The electronic properties of the NHC ligands have been assessed by comparison of the IR carbonyl stretching frequencies of the in situ prepared [IrCl(CO)2(NHC)] complexes in CH2Cl2. These new complexes (1d-f), together with previously prepared 1a-c, were applied as catalysts for the α-alkylation of arylacetonitriles with an equimolar amount of primary alcohols or 2-aminobenzyl alcohol. The catalytic activities of these complexes could be controlled by modifying the N-substituents and backbone of the NHC ligands. The NHC-IrI complex 1f bearing 4-methoxybenzyl substituents on the N-atoms and 4-methoxyphenyl groups at the 4,5-positions of imidazole exhibited the highest catalytic activity in the α-alkylation of arylacetonitriles with primary alcohols. Various α-alkylated nitriles and aminoquinolines were obtained in high yields through a borrowing hydrogen pathway by using 0.1 mol% 1f and a catalytic amount of KOH (5 mol%) under an air atmosphere within significantly short reaction times.

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

Crystal structures of [μ2-(Ra ,Sa ,3aR,7aR)-1,3-bis-(2,7-di-cyclo-hexyl-naphthalen-1-yl)octa-hydro-1H-benzo[d]imidazolidin-2-yl-idene]chlorido-(η4-1,5-cyclo-octa-diene)iridium di-chloro-methane monosolvate and [μ2-(Sa ,Sa ,3aR,7aR)-1,3-bis-(2,7-di-cyclo-hexyl-naphthalen-1-yl)octa-hydro-1H-benzo[d]imidazolidin-2-yl-idene]chlorido-(η4-1,5-cyclo-octa-diene)iridium.

The title compounds, [Ir(C51H64N2)Cl(C8H12)]·CH2Cl2, (I), and [Ir(C51H64N2)Cl(C8H12)], (II), represent the first two examples of hexa-hydro-benzo-imidazole-based N-heterocyclic carbene (NHC) iridium complexes. The diastereomeric complexes differing only in their axial chirality, which could be separated via column chromatography, show noticeable differences in their 1H NMR spectra. Compound (I) crystallizes in the monoclinic system (P21) with two independent complexes and two half-occupied di-chloro-methane mol-ecules in the asymmetric unit, while compound (II) crystallizes in the ortho-rhom-bic system (P212121) with one complex in the asymmetric unit. The fused five-membered N-heterocycles of NHCs show unusually high backbone torsion angles of -34.1 (5) and -30.9 (5)° for (I) and -31.5 (7)° for (II), but the Ir-Ccarbene bond lengths of 2.046 (6) and 2.021 (6) Å for (I) and 2.045 (8) Å for (II) present typical NHC-Ir bond lengths. The solvent mol-ecule in the crystal of (I) was found to be highly disordered and its contribution to the scattering was masked using the solvent-masking routine smtbx.mask in OLEX2 [Dolomanov et al. (2009 ▸). J. Appl. Cryst. 42, 339-341]. The solvent contribution is not included in the reported chemical formula and other crystal data.

Selective mono-N-methylation of nitroarenes with methanol catalyzed by atomically dispersed NHC-Ir solid assemblies

A series of N-heterocyclic carbene-iridium (NHC-Ir) coordination assemblies based on bis-pyrenoimidazolium salts are prepared, and shown to function as efficient solid molecular catalysts in selective mono-N-methylation of nitroarenes with methanol under mild conditions. The atomically dispersed active Ir(I) centers and the large π-conjugation rings endow the solid catalysts with an exceptionally high activity and selectivity for a broad substrate scope. Such solid NHC-Ir coordination assemblies are robust, which can be easily recovered and reused more than 10 runs without significant loss of their catalytic activity and selectivity. When combined with a subsequent formylation using the same solid catalysts under ambient conditions, this novel protocol can afford diverse formamides in excellent yields, further highlighting the applicability of the present solid catalysts for an efficient diversification of nitroarenes to a broad number of functional amines.

Selective Catalytic Dehydrogenative Oxidation of Bio-Polyols to Lactic Acid.

The global demand for lactic acid (LA) is increasing due to its successful application as monomer for the manufacture of bioplastics. Although N-heterocyclic carbene (NHC) iridium complexes are promising molecular catalysts for LA synthesis, their instabilities have hindered their utilization especially in commercial applications. Here, we report that a porous self-supported NHC-iridium coordination polymer can efficiently prevent the clusterization of corresponding NHC-Ir molecules and can function as a solid molecular recyclable catalyst for dehydrogenation of bio-polyols to form LA with excellent activity (97 %) and selectivity (>99 %). A turnover number of up to 5700 could be achieved in a single batch, due to the synergistic participation of the Ba2+ and hydroxide ions, as well as the blockage of unwanted pathways by adding methanol. Our findings demonstrate a potential route for the industrial production of LA from cheap and abundant bio-polyols, including sorbitol.

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.