Ir Cl Research Articles

Synthesis, characterization, and structure determination of Cp*Ir(dpms)Cl

Bipyridyl ligands are commonplace in catalysis. Structurally similar to this ligand class with unique properties is the novel di-(2-pyridyl)methanesulfonate (dpms) ligand, which is prepared and reacted with [Cp*IrCl2]2 to afford Cp*Ir(dpms)Cl (1) in high yield. Its single-crystal X-ray structure indicates an exo–(κ2) conformation of the ligand, with the sulfonate group directed away from the iridium center. Halogen exchange by treatment of 1 with NaI gives the iodide derivative, Cp*Ir(dpms)I (2). Abstraction of the halogen from 1 using AgPF6 generates [Cp*Ir(dpms)]PF6 (3), which was not found to activate the CH bonds of benzene.

A Heterogeneous Iridium Catalyst for the Hydroboration of Pyridines.

Sulfated zirconium oxide (SZO) capped with silylium-like ions reacts with (cod)Ir(py)Cl (cod = 1,5-cyclooctadiene; py = pyridine) to form [Ir(cod)py][SZO] (1) and Me3SiCl. 1 can also be formed in reactions of phosphonium functionalized SZO and [Ir(cod)(OSi(OtBu)3]2, which forms [Ir(cod)P(tBu)2Ph][SZO] (2), followed by reaction with pyridine to form 1. FTIR and 15N{1H} MAS NMR spectroscopy are consistent with coordination of pyridine in 1 to an electrophilic iridium. 1 is moderately active in the dearomative hydroboration of pyridine. The primary product of this reaction is 1,2-dihydropyridine, which converts to the 1,4-dihydropyridine product at long reaction times. 1 catalyzes the dearomative hydroboration of a variety of substituted pyridines and is also reactive toward pyrazines and N-methylimidazole.

Configurational Flexibility of a Triaryl-Supported SBS Ligand with Rh and Ir: Structural Investigations and Olefin Isomerization Catalysis

Here, we report Rh and Ir complexes containing a triaryl SBS pincer ligand flanked by neutral thiomethyl donor groups. Oxidative addition of the B–H bond in the diazaborole proligand H(MeSBSMe) to [Ir(COD)Cl]2 or [Rh(COE)Cl]2 yielded the chloride-bridged dimers [(MeSBSMe)MH(μ-Cl)]2, where M = Ir (1) or Rh (2). Addition of CO to 1 and 2 yielded unstable products that were difficult to isolate, but crystals of monomeric (MeSBSMe)IrH(CO)Cl (3) recovered from the reactions with 1 revealed a change in the MeSBSMe coordination mode from mer to fac. Treating 1 and 2 with LiN(SiMe3)2 yielded mer-(MeSBSMe)IrH[N(SiMe3)2] (4) and the unusual RhI–RhIII dimer (MeSBSMe)Rh(μ-H)[(MeS(μ-B)(μ-SMe)]Rh[N(SiMe3)2] (5) with both mer and fac MeSBSMe. 4 and 5 did not exhibit any alkane transfer hydrogenation reactivity when tested with tert-butylethylene and cyclooctane, but they are highly active for alkene isomerization with 1-hexene. Optimized isomerization reactions showed the highest turnover number (TON) with 4 at 60 °C after 16 h (TON = 10 000), and both catalysts are effective even when tested at room temperature with similar loadings (TON = 600). Collectively, these data highlight the reactivity and inherent coordinative flexibility of the MeSBSMe ligand for comparison to more well-established PBP complexes.

Ligand-to-metal ratio controls stereoselectivity: Highly functional-group-tolerant, iridium-based, (E)-selective alkyne transfer semihydrogenation

Herein, we present ( E )-selective transfer semihydrogenation of alkynes based on an iridium complex in situ generated from [Ir(COD)Cl] 2 and an unsymmetrical (bearing two different phosphorous centers), ferrocene-based phosphine ligand utilizing formic acid as a hydrogen donor. Interestingly, a ligand-to-metal ratio may be used to control the stereoselectivity of the semihydrogenation process: a ratio of 1:1 iridium to ligand led to the formation of ( Z )-alkene as a major product, whereas a ratio of 1:2 gave exclusively ( E )-alkene. The latter 1:2 catalytic system is distinguished by its unprecedented chemoselectivity and exceptional stereoselectivity, which is substantiated by the broad scope of tested substrates, including natural product derivatives. The intriguing difference in catalytic activity between unsymmetrical and symmetrical ferrocene-based ligands was attributed to divergent coordination and steric hindrance. The presented methodology constitutes a solution to the common limitations of the known catalytic systems for semihydrogenation. • Excellent stereoselectivity and functional-group tolerance • Stereoselectivity controlled by metal-to-ligand ratio • Catalytic activity affected by ligand symmetry Olefins are ubiquitous organic compounds which contain unsaturated double C–C bonds, and they may be found in many natural products and pharmaceuticals. Alkenes isomerism plays a vital role in the biological activity of drugs, to give one example. Thus, selective methods of their synthesis are highly coveted. Herein, we provide an unprecedented alternative to the other known catalytic systems for alkynes semihydrogenation, of which stereoselectivity may be controlled by the metal-to-ligand ratio. The ( E )-selective variant is distinguished by excellent stereo- and chemoselectivity. Reducible and problematic functionalities are tolerated under reductive conditions, and this makes our methodology a perfect tool for the late-stage functionalization of organic compounds. The environmentally friendly character of this catalytic system is proven by its efficiency and chemoselectivity, which allow one to avoid the use of protecting groups and allow the application of formic acid as a green and safe hydrogen donor. An iridium-based catalytic system for ( E )-selective transfer semihydrogenation of alkynes was developed. A unique feature of this method is the possibility of stereoselectivity control by the metal-to-ligand ratio. The catalytic system is distinguished by high functional-group tolerance, making it a valuable tool for organic synthesis. The observed intriguing dependence of ligand symmetry on catalytic activity may pave the way for efficient development of catalysts.

Cationic Iridium(I) NHC‐Phosphinidene Complexes and Their Application in Hydrogen Isotope Exchange Reactions

AbstractThe reaction of the dimeric iridium complex [Ir(cod)Cl]2 (cod=1,5‐cyclooctadiene) with the N‐heterocyclic carbene‐phosphinidene adduct (IDipp)PH (1, IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) afforded the neutral bimetallic iridium(I) complex [μ‐{(IDipp)PH}{Ir(cod)Cl}2] (2), and further addition of IDipp(PH) (1) yielded the corresponding monometallic iridium(I) complex [{(IDipp)PH}Ir(cod)Cl] (3). Dechlorination of these complexes with sodium tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (NaBArF4) yielded their cationic derivatives [{μ‐(IDipp)PH}{Ir(cod)}2{μ‐Cl}][BArF4] (4) and [{(IDipp)PH}Ir(cod)(PPh3)][BArF4] (5). All complexes were structurally characterized and showed the expected square‐planar coordination geometry and significant polarization of the P−C bond of the NHC‐phosphinidene adduct upon metal coordination. The complexes 4 and 5 were tested for their applicability in catalytic H/D exchange reactions, and both complexes showed high activity for a broad range of aromatic substrates, ranging from ketones, amides, esters, nitro compounds and heterocycles.

Mono- and Bis(iminoxolene)iridium Complexes: Synthesis and Covalency in π Bonding.

N-(2,6-Diisopropylphenyl)-4,6-di-tert-butyl-o-iminobenzoquinone (Diso) reacts with the (cyclooctadiene)iridium chloride dimer to form a monoiminoxolene complex, (Diso)Ir(cod)Cl. Reaction of 2 equiv of the iminoquinone with chlorobis(cyclooctene)iridium dimer affords the bis-iminoxolene (Diso)2IrCl. This five-coordinate complex adopts a distorted square pyramidal structure with an apical chloride ligand and undergoes halide exchange to form an air-stable iodide complex. (Diso)2IrCl can be reduced by one electron to form neutral, square planar (Diso)2Ir, while oxidation with PhICl2 gives octahedral trans-(Diso)2IrCl2. The cis isomer can be prepared by air oxidation of (Diso)2IrCl; cis/trans isomerization is not observed even on prolonged heating. Structural and spectroscopic features of the complexes are consistent with the presence of strong, covalent π bonding between the metal and the iminoxolene ligands, with structural data suggesting between 45 and 60% iridium character in the π bonding orbitals, depending on the ancillary ligands. The spectroscopic similarity of (Diso)2Ir and (Diso)2IrCl to their cobalt congeners suggests that the first-row metal complexes likewise have appreciably covalent metal-iminoxolene π bonds.

Stepwise Access of Emissive Ir(III) Complexes Bearing a Multi-Dentate Heteroaromatic Chelate: Fundamentals and Applications.

Three multi-dentate coordinated chelates LnH2 (n = 1, 2, and 3), comprising a linked 1-(pyridin-2-yl)ethylbenzene and one pyrazolyl pyridine unit and showing either tridentate or tetradentate coordination modes, are successfully designed and synthesized. Dinuclear Ir(III) complexes [Ir(κ4-Ln)(μ-Cl)]2 bearing tetradentate coordinated κ4-Ln chelate (2a, n = 1; 2b, n = 2; 2c, n = 3) were next obtained en route from the respective intermediate [Ir(κ3-LnH)Cl(μ-Cl)]2 bearing the tridentate coordinated κ3-LnH chelate (1a, n = 1; 1b, n = 2; 1c, n = 3). Next, mononuclear Ir(III) complexes Ir(κ4-Ln)(thd) (3a, n = 1; 3b, n = 2; 3c, n = 3) with the tetradentate chelate were obtained upon treatment of 2 with 2,2,6,6-tetramethyl-3,5-heptanedione (thd)H in the presence of K2CO3. Concurrently, methylation of 2c in the presence of MeI and nBu4NCl afforded tridentate Ir(κ3-L3HMe)Cl3 (4) and, next, can be converted to tetradentate Ir(κ4-L3Me)Cl2 (5) by further cyclometalation and HCl elimination in refluxing diethylene glycol monoethyl ether solution. The Ir(III) complexes 3a, 4, and 5 were unambiguously identified using spectroscopic methods, together with single-crystal X-ray structural analyses on Ir(III) derivatives 3a, 4, and 5. Their photophysical and ,electrochemical properties and device fabrication properties were also investigated and compared with results from theoretical studies.

Iridium-catalyzed hydroacylation reactions of C1-substituted oxabenzonorbornadienes with salicylaldehyde: an experimental and computational study.

An experimental and theoretical investigation on the iridium-catalyzed hydroacylation of C1-substituted oxabenzonorbornadienes with salicylaldehyde is reported. Utilizing commercially available [Ir(COD)Cl]2 in the presence of 5 M KOH in dioxane at 65 °C, provided a variety of hydroacylated bicyclic adducts in up to a 95% yield with complete stereo- and regioselectivity. The mechanism and origins of selectivity in the iridium-catalyzed hydroacylation reaction has been examined at the M06/Def2TZVP level of theory. The catalytic cycle consists of three key steps including oxidative addition into the aldehyde C–H bond, insertion of the olefin into the iridium hydride, and C–C bond-forming reductive elimination. Computational results indicate the origin of regioselectivity is involved in the reductive elimination step.

Low Ir-content Ir/Fe@NCNT bifunctional catalyst for efficient water splitting

In order to reduce the cost of electrocatalysts and increase the exposure of the Ir active sites while ensuring the stability of the catalyst, a N-doped carbon nanotube (NCNT) is applied as a conductive support to confine the Ir clusters for avoiding them growing up via a modified method based on pyrolysis of a mixture of melamine, ferric chloride and iridium trichloride. It is found that Ir species in the as-obtained Ir(20)/[email protected] composite exist in two forms, Ir nanoclusters (1–2 nm) dotted on the wall of NCNT and the Ir atomically scattered on the Fe nanoparticles wrapped in the NCNT. Although the Ir content of Ir(20)/[email protected] is extremely low (∼4 wt% Ir), the composite catalyst delivers excellent activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with an exceptionally low overpotential of 4.7 mV/11 mV for HER and 300 mV/270 mV for OER to drive 10 mA cm−2 in 0.5 M H2SO4/1.0 M KOH electrolyte respectively, which exceeds the commercial Pt/C (20 wt% Pt) and IrO2 benchmarks. In addition, it has much higher mass activity for OER at 1.55 V (1.78 A mg−1Ir) than those of the referenced catalysts in acid. The cell voltage of the two-electrode system assembled by Ir(20)/[email protected] for total water splitting in acidic and alkaline media are only 1.520 V and 1.510 V to afford 10 mA cm−2 separately, lower than that of Pt/C||IrO2 and with a good stability. Our work provides a construction method of low-content precious metal composite catalysts which can be applied in OER and overall water splitting field.

Enantioselective Iridium‐Catalyzed Reductive Coupling of Dienes with Oxetanones and N‐Acyl‐Azetidinones Mediated by 2‐Propanol

AbstractCyclometallated iridium‐PhanePhos complexes generated in situ from [Ir(cod)Cl]2 and (R)‐PhanePhos catalyze 2‐propanol‐mediated reductive couplings of 2‐substituted dienes with oxetanone and N‐acyl‐azetidinones to form branched homoallylic oxetanols and azetidinols with excellent control of regio‐ and enantioselectivity without C−C cleavage of the strained ring via enantiotopic π‐facial selection of transient allyliridium nucleophiles. This work represents the first systematic study of enantioselective additions to symmetric ketones.

Enantioselective Iridium-Catalyzed Reductive Coupling of Dienes with Oxetanones and N-Acyl-Azetidinones Mediated by 2-Propanol.

Cyclometallated iridium-PhanePhos complexes generated in situ from [Ir(cod)Cl]2 and (R)-PhanePhos catalyze 2-propanol-mediated reductive couplings of 2-substituted dienes with oxetanone and N-acyl-azetidinones to form branched homoallylic oxetanols and azetidinols with excellent control of regio- and enantioselectivity without C-C cleavage of the strained ring via enantiotopic π-facial selection of transient allyliridium nucleophiles. This work represents the first systematic study of enantioselective additions to symmetric ketones.

Hydroboration and Diboration of Internal Alkynes under Iridium Catalysis.

Here we demonstrate the feasibility and efficiency of simple iridium-based catalytic systems in the synthesis of multisubstituted alkenyl boronates from internal alkynes with high selectivities. A variety of alkynes were smoothly decorated with HBpin under a mild [Ir(cod)Cl]2/dppm/acetone condition to afford trisubstituted alkenyl boronic esters with up to >99:1 regioselectivity. The diboration reaction could effectively occur in the presence of [Ir(cod)Cl]2/DCM. Plausible mechanisms were provided to illustrate these two catalytic processes, in which the intrinsic functional group of the alkyne was supposed to be important in facilitating these reactions as well as the regioselectivity.

Tritium hydrogen‐isotope exchange with electron‐poor tertiary benzenesulfonamide moiety; application in late‐stage labeling of T0901317

The multifunctional radioligand [3 H]T0901317 ([3 H]1) has been employed as a powerful autoradiographic tool to target several receptors, such as liver X, farnesoid X, and retinoic acid-related orphan receptor alpha and gamma subtypes at nanomolar concentrations. Although [3 H]1 is commercially available and its synthesis via tritiodebromination has been reported, the market price of this radioligand and the laborious synthesis of corresponding bromo-intermediate potentially preclude its widespread use in biochemical, pharmacological, and pathological studies in research lab settings. We exploit recent reports on hydrogen-isotope exchange (HIE) reactions in tertiary benzenesulfonamides where the sulfonamide represents an ortho-directing group that facilitates CH activation in the presence of homogenous iridium(I) catalysts. Herein, we report a time- and cost-efficient method for the tritium late-stage labeling of compound 1-a remarkably electron-poor substrate owing to the tertiary trifluoroethylsulfonamide moiety. Under a straightforward HIE condition using a commercially available Kerr-type NHC Ir(I) complex, [(cod)Ir (NHC)Cl], the reaction with 1 afforded a specific activity of 10.8Ci/mmol. Additionally, alternative HIE conditions using the heterogeneous catalyst of Ir-black provided sufficient 0.72 D-enrichment of 1 but unexpectedly failed while repeating with tritium gas.

Azo-oximate metal-carbonyl to metallocarboxylic acid via the intermediate Ir(III) radical congener: quest for co-ligand driven stability of open- and closed-shell complexes.

The redox non-innocent behavior of the diaryl-azo-oxime ligand LNOH1 has been accentuated via the synthesis of metastable anion radical complexes of type trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 (CO is trans to azo group of the ligand) by the oxidative coordination reaction of 1 with Vaska's complex. The stereochemical role of co-ligands vis-à-vis the interplay of π-bonding has been found to be decisive in controlling the aptitude of the coordinated redox non-innocent ligand to accept or reject an electron. This has been clarified via the isolation of quite a few complexes as well as the failure to synthesize some others. The oxidized analogues of type trans-[Ir(LNO-)Cl(CO)(PPh3)2]+2+ (CO and azo group of the ligand are trans) as well as its cis isomer cis-[Ir(LNO-)Cl(CO)(PPh3)2]+3+ (CO and azo group of the ligand are cis) have been structurally characterized but the radical anion congener of the latter could not be synthesized. Furthermore, the closed shell complexes [Ir(LNO-)Cl2(PPh3)2] 4 and [Ir(LNO-)2Cl(PPh3)] 5 have been well characterized by diffraction as well as spectral techniques but their corresponding azo anion radical complexes could not be isolated and this is attributed to the trans influence of ancillary ligands. The anion radical complexes trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 may be rapidly transformed to the metallocarboxylic acids trans-[Ir(LNO-)Cl(CO2H)(PPh3)2] 6via a proton-coupled electron transfer (PCET) process, thereby demonstrating the role of odd electron over the coordinated ligand framework to trigger metal-mediated carbonyl to carboxylic acid functionalization. Complexes 6 are further stabilized via intramolecular -CO2H⋯ON- (carboxylic acid⋯oximato) H-bonding. The optoelectronic properties as well as the origin of transitions in the complexes were analyzed by TD-DFT and theoretical analysis, which further disclose that the odd electron in trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 is primarily azo-oxime centric with very low contribution from the iridium center.

Iridium(iii) bis(thiophosphinite) pincer complexes: synthesis, ligand activation and applications in catalysis

Iridium(iii) bis(thiophosphinite) complexes of the type [(RPSCSPR)Ir(H)(Cl)(py)] (RPSCSPR = κ3-(2,6-SPR2)C6H3) (R = tBu, iPr, Ph) can be prepared from the ligand precursors 1,3-(SPR2)C6H4 by C–H activation at Ir [Ir(COE)2Cl]2 or [Ir(COD)Cl]2.

Iridium-catalyzed asymmetric transfer hydrogenation of aromatic ketones with a cinchona alkaloid derived NNP ligand.

An iridium complex generated in situ from [Ir(COD)Cl]2 and a cinchona alkaloid derived NNP ligand has been developed for the asymmetric transfer hydrogenation of aromatic ketones. In this study, 30 aromatic ketones and heteroaryl ketones were hydrogenated to produce valuable chiral alcohols with up to 99% ee using i-PrOH as the hydrogen source and the solvent. The easily prepared Ir(L8)(COD)Cl also exhibited excellent activity and enantioselectivity in asymmetric transfer hydrogenation of aromatic ketones with a high S/C ratio (up to 2000).

A recyclable covalent triazine framework-supported iridium(iii) terpyridine complex for the acceptorless dehydrogenative coupling of o-aminobenzyl alcohols with ketones to form quinolines

A complex Ir(tpy)@CTF, which is synthesized by the coordinative immobilization of [Ir(tpy)Cl3] on a covalent triazine framework, was proven to be a recycle catalyst for the acceptorless dehydrogenative coupling of o-aminobenzyl alcohols with ketones.

Cinchona-Alkaloid-Derived NNP Ligand for Iridium-Catalyzed Asymmetric Hydrogenation of Ketones.

Most ligands applied for asymmetric hydrogenation are synthesized via multistep reactions with expensive chemical reagents. Herein, a series of novel and easily accessed cinchona-alkaloid-based NNP ligands have been developed in two steps. By combining [Ir(COD)Cl]2, 39 ketones including aromatic, heteroaryl, and alkyl ketones have been hydrogenated, all affording valuable chiral alcohols with 96.0-99.9% ee. A plausible reaction mechanism was discussed by NMR, HRMS, and DFT, and an activating model involving trihydride was verified.

A Dipolar Cyclization/Fragmentation Strategy for the Catalytic Asymmetric Synthesis of Chiral Eight-Membered Lactams

A Dipolar Cyclization/Fragmentation Strategy for the Catalytic Asymmetric Synthesis of Chiral Eight-Membered Lactams

Development of Titania-supported Iridium Catalysts for the Acceptor-less Dehydrogenative Synthesis of Benzoxazoles

Iridium catalysts supported on anatase with high surface area showed excellent activities for the acceptor-less dehydrogenation synthesis of benzoxazoles from 2-aminophenol and primary alcohols. The catalytic activity greatly depended on the titania supports, iridium precursors, and loading of iridium species. Catalysts supported on anatase JRC (Japan Reference Catalyst)-TIO-10 showed the highest activity for the dehydrogenative reaction. Preparation of catalysts using [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene) as an iridium precursor resulted in higher activity than using Ir(acac)3 (acac = acetylacetonate). Various primary alcohols were reacted to give corresponding benzoxazoles in high to moderate yields. The catalyst could be recycled without significant loss of activity, and no leaching of iridium species occurred into the solution. The hot filtration test strongly suggested that the catalysis occurs on the catalyst surface. Highly dispersed iridium species of less than 2 nm in diameter, which could be reduced at low temperature, were responsible for the excellent activity.