Carbene Complexes Of Iridium Research Articles

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.

Iridium Carbene and Carbamoyl Complexes Supported by a Tetradentate Pyridine‐Carboxamide Ligand

The nucleophilic attack of an IrIII isocyanide complex bearing a tetradentate pyridine‐carboxamide ligand with amines to give carbene and carbamoyl complexes has been studied. The treatment of Na[Ir(bpb)Cl2] {bpb2– = dianion of 1,2‐bis(2‐pyridinecarboxamido)benzene} with PR3 (R = p‐tolyl) in refluxing glycol gave [Ir(bpb)(PR3)Cl] (1). Chloride abstraction of 1 with silver tosylate (AgOTs) led to isolation of a mixture of [Ir(bpb)(PR3)(OTs)] (2a) and [Ir(bpb)(PR3)(H2O)](OTs) (2b) that reacted with 2,6‐dimethylphenyl isocyanide (CNxyl) and 4‐tert‐butylaniline to give [Ir(bpb)(PR3)(CNxyl)](OTs) (3) and [Ir(bpb)(PR3)(4‐tBuC6H4NH2)](OTs) (4), respectively. The reaction of 4 with AgOTf (OTf– = triflate) yielded an Ir–Ag dimeric complex, [Ir(bpb)(PR3)(4‐tBuC6H4NH2){Ag(H2O)(µ‐OTf)}]2(OTf)2 (5). Whereas nucleophilic attack of 3 with benzylamine (BnNH2) afforded the carbene complex [Ir(bpb)(PR3){C(NHBn)(NHxyl)}](OTs) (6), that with tBuNH2 yielded the carbamoyl complex [Ir(bpb)(PR3){C(O)NHxyl}] (7). The IrIV–IrIII redox potentials for Ir(bpb) complexes have been determined using cyclic voltammetry. The crystal structures of 2a, 2b, and 5–7 have been determined.

Synthesis, Structures, and Electronic Properties of O- and S-Heterocyclic Carbene Complexes of Iridium, Copper, Silver, and Gold

O- and S-heterocyclic carbenes (OHCs, SHCs) are shown experimentally and computationally to be stronger π acceptors than NHCs and lack, of course, substituents at the heteroatoms. These different e...

Chelating di(N-heterocyclic carbene) complexes of iridium(III): Structural analysis, electrochemical characterisation and catalytic oxidation of water

The synthesis of novel iridium(III) complexes of general formula [IrClCp∗(diNHC)](PF6), with chelating di(N-heterocyclic carbene) ligands (diNHC) is reported. The obtained complexes differ for the wingtip substituents at the nitrogen atom of the heterocycles (Me, t-Bu, Cy, n-oct), the linker between the carbene donors (-CH2-, -CH2CH2-, -CH2CH2CH2-) and for the type of heterocyclic rings (imidazole and benzimidazole). The new complexes have been characterised in solution with NMR spectroscopy, ESI-MS spectrometry and cyclic voltammetry and, in the solid state, by single crystal X-ray structure analysis. These techniques allow to evaluate the influence of the dicarbene ligand on the electronic properties of the metal centre. The complexes have been successfully tested as water oxidation catalysts using Ce(IV) as chemical oxidant.

Fischer Carbene Complexes of Iridium(I) for Application in Catalytic Transfer Hydrogenation.

New examples of the very rare class of iridium(I) Fischer carbene complexes (FCCs) are reported from the facile transmetalation from group 6 FCCs. Postcomplexation modification of either the carbene ligand or the ancillary coligands results in a tunable IrI metal center, for unprecedented application as a (pre)catalyst in a benchmark transfer hydrogenation reaction. The introduction of an aminocarbene ligand with a pendant N-donor moiety capable of hemilabile coordination yielded the best catalytic results with turnover frequencies reaching 445 h–1 and requiring 0.1 mol % catalyst and 0.5 mol % base loading, respectively.

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.

Mesoionic Carbene-Iridium Complex Catalyzed Ortho-Selective Hydrogen Isotope Exchange of Anilines with High Functional Group Tolerance.

Mesoionic carbene-iridium complex 1a has been investigated in the hydrogen isotope exchange (HIE) reaction of anilines. By employing 1 mol % of 1a as catalyst, anilines were selectively deuterated at the ortho-position with high deuteration levels. High ortho-selectivity was observed for anilines with various competing directing groups, which is in contrast with catalytic results of Kerr's catalysts.

Efficient methanol carbonylation to methyl acetate catalyzed by a cyclic(alkyl)(amino)carbene iridium complex

An efficient cyclic(alkyl)(amino) carbene iridium complex (C-2) was developed for methanol carbonylation to methyl acetate (MA) directly.

High‐Efficiency Deep‐Blue‐Emitting Organic Light‐Emitting Diodes Based on Iridium(III) Carbene Complexes

Adv. Mater. 2018, 30, 1804231 In the published communication, the nitrogen atoms in the molecular structure diagram of R3 were in the incorrect places. The updated version of Figure 1a, with the corrected structure of molecule R3 is shown here:

High-Efficiency Deep-Blue-Emitting Organic Light-Emitting Diodes Based on Iridium(III) Carbene Complexes.

High-efficiency pure blue phosphorescent organic light-emitting diodes (OLEDs) remain one of the grand challenges, principally because the emissive complexes employed either do not possess sufficiently high photoluminescence quantum yields or exhibit unsatisfactory Commission International de l'Éclairage (CIE) coordinates. Here two deep-blue-emitting homoleptic iridium(III) complexes are reported and OLEDs are demonstrated with CIE coordinates of (0.15, 0.05) and maximum external quantum efficiency of 13.4%, which decreases slightly to 12.5% at 100 cd m-2 . They represent examples of the most efficient OLEDs surpassing the CIEy requirement of the National Television System Committee (NTSC) and the European Broadcasting Union (EBU). Emitter orientation contributes to the excellent device performance.

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.

Nonradiative Decay and Stability of N-Heterocyclic Carbene Iridium(III) Complexes

Devices based on deep-blue emitting iridium(III) complexes with N-heterocyclic carbene (NHC) ligands have recently been shown to give excellent performance as phosphorescent organic light-emitting diodes (PHOLEDs). To facilitate the design of even better deep-blue phosphorescent emitters, we carried out density functional theory (DFT) calculations of the lowest triplet (T1) potential-energy surfaces upon lengthening the iridium-ligand (Ir-C) bonds. Relativistic time dependent DFT calculations demonstrate that this changes the nature of T1 from a highly emissive metal-to-ligand charge transfer (3MLCT) state to a metal centered (3MC) state where the radiative decay rate is orders of magnitude slower than that of the 3MLCT state. We identify the elongation of an Ir-C bond on the NHC group as the pathway with the lowest energy barrier between the 3MLCT and 3MC states for all complexes studied and show that the barrier height is correlated with the experimentally measured nonradiative decay rate. This suggests that the thermal population of 3MC states is the dominant nonradiative decay mechanism at room temperature. We show that the 3MLCT → 3MC transition is reversible, in marked contrast to deep-blue phosphors containing coordinating nitrogen atoms, where the population of 3MC states breaks Ir-N bonds. This suggests that, as well as improved efficiency, blue PHOLEDs containing phosphors where the metal is only coordinated by carbon atoms will have improved device lifetimes.

N‐Alkylation of Aqueous Ammonia with Alcohols Leading to Primary Amines Catalyzed by Water‐Soluble N‐Heterocyclic Carbene Complexes of Iridium

AbstractA new catalytic system for the N‐monoalkylation of aqueous ammonia with a variety of alcohols was developed. Water‐soluble dicationic complexes of iridium bearing N‐heterocyclic carbene and diammine ligands exhibited high catalytic activity for this type of reaction on the basis of hydrogen‐transfer processes without generating harmful or wasteful byproducts. Various primary amines were efficiently synthesized by using safe, inexpensive, and easily handled aqueous ammonia as a nitrogen source. For example, the reaction of 1‐(4‐methylphenyl)ethanol with aqueous ammonia in the presence of a water‐soluble N‐heterocyclic carbene complex of iridium at 150 °C for 40 h gave 1‐(4‐methylphenyl)ethylamine in 83 % yield.

Cyclometalated N-heterocyclic carbene iridium(iii) complexes with naphthalimide chromophores: a novel class of phosphorescent heteroleptic compounds.

A series of cyclometalated N-heterocyclic carbene complexes of the general formula [Ir(C^N)2(C^C:)] has been prepared. Two sets of compounds were designed, those where (C^C:) represents a bidentate naphthalimide-substituted imidazolylidene ligand and (C^N) = ppy (3a), F2ppy (4a), bzq (5a) and those where (C^C:) represents a naphthalimide-substituted benzimidazolylidene ligand and (C^N) = ppy (3b), F2ppy (4b), bzq (5b). The naphthalimide-imidazole and naphthalimide-benzimidazole ligands 1a,b and the related imidazolium and benzimidazolium salts 2a,b were also prepared and fully characterized. The N-heterocyclic carbene Ir(iii) complexes have been characterized by NMR spectroscopy, cyclic voltammetry and elemental analysis. Moreover, the molecular structures of one imidazolium salt and four Ir(iii) complexes were determined by single-crystal X-ray diffraction. The structures provide us with valuable information, most notably the orientation of the naphthalimide chromophore with respect to the N-heterocyclic carbene moiety. All compounds are luminescent at room temperature and in a frozen solvent at 77 K, exhibiting a broad emission band that extends beyond 700 nm. The presence of the naphthalimide moiety changes the character of the lowest excited state from 3MLCT to 3LC, as corroborated by DFT and TD-DFT calculations. Remarkably, replacing imidazole with a benzimidazole unit improves the quantum yields of these compounds by decreasing the knr values which is an important feature for optimized emission performance. These studies provide valuable insights about a novel class of N-heterocyclic carbene-based luminescent complexes containing organic chromophores and affording metal complexes emitting across the red-NIR range.

Unveiling the role of ancillary ligands in acceptorless benzyl alcohol dehydrogenation and etherification mediated by mesoionic carbene iridium complexes.

We synthesized a set of triazolylidene iridium(iii) complexes [IrCp*(C^N)L]n+ (Cp* = pentamethylcyclopentadienyl, C^N = C,N-bidentate coordinating pyridyl-triazolylidene) containing different neutral or anionic ancillary ligands L and evaluated their impact on the catalytic activity in alcohol conversion. We demonstrate that these ancillary ligands have a strong influence on the catalytic selectivity and direct whether the iridium center preferentially catalyzes either the dehydrogenation or the dehydration of benzyl alcohol. Ligand exchange experiments provide a direct correlation of ligand lability with catalytic activity and selectivity. These results underline the relevance of ancillary ligands and provide a rational approach to tailor the catalytic activity of the iridium center towards aldehyde formation (loss of H2) or etherification (elimination of H2O).

Bis-Cyclometalated Iridium Complexes with Chelating Dicarbene Ancillary Ligands

In this work, we report that covalent postsynthetic modification can be used for the preparation of a class of bis-cyclometalated iridium complexes featuring Chugaev-type chelating dicarbene ligands. Bis-cyclometalated iridium complexes with electron-deficient aryl isocyanide ancillary ligands react with hydrazine to form neutral dicarbene complexes. The neutral iridium carbene complexes have a basic site that can be protonated by strong acid, permitting access to complexes in two protonation states and allowing an additional layer of control over the key properties. These new Chugaev-type iridium complexes exhibit blue phosphorescence at both room temperature and 77 K. Compared to their bis-isocyanide precursors, the electrochemical and photophysical properties of these new complexes are substantially perturbed, demonstrating the concept that the electronic structure and excited state dynamics can be controlled by ancillary ligand modification. Furthermore, the emission spectra and excited-state dynamics...

Chiral Oxazolidine‐Fused N‐Heterocyclic Carbene Complexes of Rhodium and Iridium and Their Utility in the Asymmetric Transfer Hydrogenation of Ketones

The catalytic potential of new N‐heterocyclic carbene ligands, derived from a chiral fused bicyclic ring scaffold with restricted rotation along the C–N bond bearing the chiral auxiliary, has been explored in the transition‐metal‐mediated asymmetric transfer hydrogenation reactions of ketones. In particular, the chiral oxazolidine‐fused N‐heterocyclic carbene precursors (3S)‐3‐R‐6‐methyl‐7‐phenyl‐2,3‐dihydroimidazo[5,1‐b]oxazol‐6‐ium iodide [R = sec‐butyl (1f), i‐butyl (2f), isopropyl (3f)] were synthesized from commercially available optically pure amino acids in a multistep sequence that avoids tedious chiral resolution. The reactions of the chiral imidazolium iodide salts 1f–3f with Ag2O yielded the corresponding silver complexes 1g–3g, which were treated with [(COD)MCl]2 (M = Rh, Ir; COD = 1,5‐cyclooctadiene) to afford the rhodium(I) and iridium(I) complexes (1h–3h and 1i–3i, respectively). The rhodium(I) and iridium(I) complexes conveniently catalyze the asymmetric transfer hydrogenation of acetophenones for a wide variety of substrates ranging from electron‐rich ones such as 4‐methylacetophenone, 3,4‐dimethylacetophenone, 4‐tert‐butylacetophenone, and 4‐(methylthio)acetophenone to electron‐deficient ones such as 4‐bromoacetophenone, 4‐chloroacetophenone, 4‐fluoroacetophenone, 4‐nitroacetophenone, and 3‐fluoroacetophenone in moderate‐to‐good yields (ca. 18–95 %) but with low enantioselectivities (ca. 4–41 % ee).

Synthesis of Indoles and Pyrroles Utilizing Iridium Carbenes Generated from Sulfoxonium Ylides

AbstractMetal carbenes can undergo a myriad of synthetic transformations. Sulfur ylides are potential safe precursors of metal carbenes. Herein, we report cascade reactions that involve carbenoids derived from sulfoxonium ylides for the efficient and regioselective synthesis of indoles and pyrroles. The tandem action of iridium and Brønsted acid catalysts enables rapid assembly of the heterocycles from unmodified anilines or readily accessible enamines under microwave irradiation. The key mechanistic steps are the catalytic transformation of the sulfoxonium ylide into an iridium–carbene complex, followed by N−H or C−H functionalization of an aniline or enamine, respectively, and a final acid‐catalyzed cyclization. The present method was successfully applied to the synthesis of the densely functionalized pyrrole subunit of atorvastatin.

Synthesis of Indoles and Pyrroles Utilizing Iridium Carbenes Generated from Sulfoxonium Ylides.

Metal carbenes can undergo a myriad of synthetic transformations. Sulfur ylides are potential safe precursors of metal carbenes. Herein, we report cascade reactions that involve carbenoids derived from sulfoxonium ylides for the efficient and regioselective synthesis of indoles and pyrroles. The tandem action of iridium and Brønsted acid catalysts enables rapid assembly of the heterocycles from unmodified anilines or readily accessible enamines under microwave irradiation. The key mechanistic steps are the catalytic transformation of the sulfoxonium ylide into an iridium-carbene complex, followed by N-H or C-H functionalization of an aniline or enamine, respectively, and a final acid-catalyzed cyclization. The present method was successfully applied to the synthesis of the densely functionalized pyrrole subunit of atorvastatin.

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).