Ir Pincer Research Articles

C–H functionalization of indole with diazoacetate catalyzed by a CCC-NHC Ir (III) dimer complex

The first catalytic application of a CCC-NHC pincer Ir (III) dimer complex for the C – H functionalization of N-methylindoles with α-aryl-α-diazoacetates at the C-3 position is reported herein. The Ir pincer dimer complex was used as a catalyst at 3 mol% loading. The best reaction conditions involved a combination of catalyst and substrates in a specific order. It resulted in the activation of the C–H bond with the formation of a new C–C bond to generate α-aryl-α-indolyl acetates with more than 99% conversion at room temperature without requiring any additives. Isolated yields of 84–97% were obtained for a range of substrates. Under these catalytic conditions, the unprotected N–H indoles did not react with α-aryl-α-diazoacetate. These catalytic conditions provided an efficient synthetic route to the synthesis of α-aryl-α-indolyl acetates.

Mechanistic Versatility at Ir(PSiP) Pincer Catalysts: Triflate Proton Shuttling from 2-Butyne to Diene and [3]Dendralene Motifs.

The five-coordinate hydrido complex [IrH(OTf)(PSiP)] (1) catalytically transforms 2-butyne into a mixture of its isomer 1,3-butadiene, and [3]dendralene and linear hexatriene dimerization products: (E)-4-methyl-3-methylene-1,4-hexadiene and (3Z)-3,4-dimethyl-1,3,5-hexatriene, respectively. Under the conditions of the catalytic reaction, benzene, and 363 K, the hexatriene further undergoes thermal electrocyclization into 2,3-dimethyl-1,3-cyclohexadiene. The reactions between 1 and the alkyne substrate allow isolation or nuclear magnetic resonance (NMR) observation of catalyst resting states and possible reaction intermediates, including complexes with the former PSiP pincer ligands disassembled into PSi and PC chelates, and species coordinating allyl or carbene fragments en route to products. The density functional theory (DFT) calculations guided by these experimental observations disclose competing mechanisms for C–H bond elaboration that move H atoms either classically, as hydrides, or as protons transported by the triflate. This latter role of triflate, previously recognized only for more basic anions such as carboxylates, is discussed to result from combining the unfavorable charge separation in the nonpolar solvent and the low electronic demand from the metal to the anion at coordination positions trans to silicon. Triflate deprotonation of methyl groups is key to release highly coordinating diene products from stable allyl intermediates, thus enabling catalytic cycling.

Correction: Efficient and selective catalytic hydrogenation of furanic aldehydes using well defined Ru and Ir pincer complexes

Correction for ‘Efficient and selective catalytic hydrogenation of furanic aldehydes using well defined Ru and Ir pincer complexes’ by Rosa Padilla et al., Green Chem., 2020, 22, 6767–6772, DOI: 10.1039/D0GC01543A.

Fast CO2 hydrogenation to formic acid catalyzed by an Ir(PSiP) pincer hydride in a DMSO/water/ionic liquid solvent system

Complex [IrClH{κP,P,Si-Si(Me)(C6H4–2-PiPr2)2}] (1) showed a remarkable catalytic activity for CO2 hydrogenation in a DMSO/H2O solvent system incorporating 1,2-dimethyl-3-butylimidazolium acetate ionic liquid (IL), producing 0.94 M formic acid with initial TOFs up to 1432 h−1 (CO2/H2 = 20/40 bar, 30 °C). While the hydrogenation outcome followed dependences upon gas composition, pressure and temperature similar to those of other efficient systems in DMSO/H2O, the kinetic dependence upon catalyst loading revealed non-linear effects suggestive of relevant IL-catalyst interactions. NMR speciation studies identified two major complexes, [Ir(OCHO)(H){κP,P,Si-Si(Me)(C6H4–2-PiPr2)2}] (2) and [Ir(H)2{κP,P,Si-Si(Me)(C6H4–2-PiPr2)2}(DMSO)] (3), potentially responsible for catalytic cycling though inactive outside the current solvent system.

Efficient and selective catalytic hydrogenation of furanic aldehydes using well defined Ru and Ir pincer complexes

Homogeneous catalyzed hydrogenation of furanic aldehydes to their corresponding alcohols using PNP complexes.

Reversible addition of ethylene to a pincer-based boryl-iridium unit with the formation of a bridging ethylidene.

This report examines reactions of a series of Ir complexes supported by the diarylboryl/bis(phosphine) PBP pincer ligand with ethylene: (PBP)IrH4 (1), (PBP)IrH2(CO) (2), and (PBP)Ir(CO)2 (3). The outcomes of these reactions differ from those typical for Ir complexes supported by other pincer ligands and do not give rise to simple ethylene adducts or products of insertion of Ir into the C–H bond of ethylene. Instead, the elements of ethylene are incorporated into the molecules to result in B–C bonds. In the case of 2 and 3, ethylene addition results in the formation of B/Ir bridging ethylidene complexes 5 and 6. For 6, the addition of ethylene (and the analogous addition of 1-hexene) is shown to be partially reversible. Addition of ethylene to 2 and 3 is remarkable because they are saturated at Ir and yet the net outcome is such that ethylene binds without replacing any ligands already present. A mechanistic inquiry suggests that dissociation of CO from 3 or 6 is necessary in order for the addition or loss of ethylene to proceed.

Iridium Single-Atom Catalyst Performing a Quasi-homogeneous Hydrogenation Transformation of CO2 to Formate

Summary Heterogeneous single-atom catalysts (SACs) with atomically dispersed active metal centers represent an intermediary between heterogeneous and homogeneous catalysis. In this work, a porous organic polymer with aminopyridine functionalities was designed to fabricate a stable, atomically dispersed Ir catalyst. This Ir-based SAC exhibits excellent catalytic activity during the liquid-phase hydrogenation of CO2 to formate. The associated turnover number is as high as 25,135, representing the best performance yet for a heterogeneous conversion of CO2 to formate. Spectral characterization and density functional theory modeling demonstrate that the chemical structure of the Ir single-atom active site is analogous to that of a homogeneous mononuclear Ir pincer complex catalyst. As a result, a catalytic mechanism similar to that over a homogeneous Ir catalyst occurs during CO2 hydrogenation with this quasi-homogeneous Ir-based SAC. This work suggests a promising basis for the design of efficient SACs for the once-dominant homogeneous catalytic processes.

Transition-metal complex-catalyzed alkane functionalization

The selective conversion of simple alkane feedstocks to high-value organic compounds constitutes one of major challenges in organic synthesis. There are three general catalytic methods for alkane functionalizations: reactions via radical intermediates and through carbene/nitrene insertion pathways typically occur preferentially at the secondary and tertiary C−H bonds, whereas transition metal-mediated C−H bond activation holds promise for the functionalization of primary C−H bonds. From a practical point of view, the installation of a functional group at the terminal position of alkanes is of particular attractive for large-scale synthesis of specialty or commodity chemicals. This review focuses on the area of molecular transition-metal-catalyzed alkane transformations through catalytic alkane dehydrogenation. In the first section, the development of iridium pincer and related complexes for alkane dehydrogenation is summarized. Among various catalysts, the PCP type Ir pincer complexes have proven to be most active toward transfer dehydrogenation of alkanes. α-Olefins are the kinetic products at the early stage of the dehydrogenation process, but they can be rapidly isomerized to internal alkenes. Recently, other non-phosphorus-ligated Ir and non-Ir metal dehydrogenation catalysts have also been developed, but they are generally less efficient than the PCP Ir catalysts. The second section first describes two dehydrogenation-based alkane transformations without the incorporation of heteroatom-containing functionality. The dehydroaromatization reaction involves multiple steps of dehydrogenation of linear alkanes to form conjugate trienes, which undergo electrocyclization and another step of dehydrogenation to generate aromatic products. The alkane-alkene coupling reaction employs an Ir pincer catalyst for dehydrogenation and a tantalum catalyst for alkene/alkene coupling, thus providing a method for upgrading light alkanes to higher alkanes that may be suitable to transportation fuel. Following that, the selective installation of functional groups at the terminal positions of alkanes through a dehydrogenation-alkene isomerization-hydrofunctionalization strategy using a dual catalyst system is described. Several combinations between a PSCOP Ir pincer dehydrogenation catalyst and molecular Fe or Rh catalysts for tandem isomerization and hydrofunctionalization of terminal alkenes have been developed for terminal selective alkane silylation, borylation, carbonylation, and aminomethylation. The third section covers alkane metathesis and its application to polyethylene (PE) degradation. The alkane metathesis reaction described here consists of two catalysts, one Ir dehydrogenation catalyst and one olefin metathesis catalyst. The metathesis process can find potential application in upgrading low carbon number n -alkanes to higher-molecular-weight fuel alkanes. A cross-alkane metathesis strategy has been developed for PE degradation. Using excess of low-value light alkanes as the reagent/solvent, PE with molecular weight up to 1.7 million undergoes multiple times of cross-alkane metathesis at 175℃ to produce liquid fuels or high-quality waxes. The catalysts can tolerate the commercial HDPE, LDPE and LLDPE, and enable the efficient degradation of postconsumer PE plastic wastes. Finally, the review discusses the limitations of the known catalytic approaches and future opportunities in this field. Catalyst development will be the central theme of research, and insights into the factors controlling the activity and selectivity gained in mechanistic studies will guide the design of more efficient dehydrogenation catalysts. The cooperative catalysis involving enantioselective catalysts will provide a protocol for synthesis of valuable fine chemicals from simple saturated hydrocarbons. Lastly, we should not overlook the potential of cooperation between photo- or electro-catalysis and transition-metal catalysis, which may allow the exploitation of alkane feedstocks more cleanly and efficiently.

Hydride Source in Ethers Hydrosilylation Reaction Catalyzed by Brookhart's Ir(Ⅲ) Pincer Complex

采用DFT方法对Ir（Ⅲ）螯合物催化乙醚硅氢化生成乙烷和乙基硅醚的反应展开理论研究.反应中[H-Ir-H]，[H-Ir-Si]，[Ir（HSiEt 3 ）]和[Et 3 Si-Ir-（H） 3 ]化合物均为可能的负氢来源.理论研究表明[H-Ir-H]化合物是最优势的负氢来源.通过扭曲-相互作用能分析，发现其他三种可能的负氢来源不优势的原因在于HSiEt 3 或SiEt 3 基团对铱中心的络合.更为重要的，我们发现[H-Ir-H]化合物中适中的Ir-H键解离能，小位阻以及SiEt 3 对醚的络合而产生的促进作用共同使得[H-Ir-H]化合物上的负氢转移相对优势.

Insight into catalytic reduction of CO2 to methane with silanes using Brookhart's cationic Ir(iii) pincer complex.

Using density functional theory computations, we investigated in detail the underlying reaction mechanism and crucial intermediates present during the reduction of carbon dioxide to methane with silanes, catalyzed by the cationic Ir-pincer complex ((POCOP)Ir(H)(acetone)+, POCOP = 2,6-bis(dibutylphosphinito)phenyl). Our study postulates a plausible catalytic cycle, which involves four stages, by sequentially transferring silane hydrogen to the CO2 molecule to give silylformate, bis(silyl)acetal, methoxysilane and the final product, methane. The first stage of reducing carbon dioxide to silylformate is the rate-determining step in the overall conversion, which occurs via the direct dissociation of the silane Si–H bond to the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O bond of a weakly coordinated Ir–CO2 moiety, with a free energy barrier of 29.5 kcal mol−1. The ionic SN2 outer-sphere pathway in which the CO2 molecule nucleophilically attacks at the η1-silane iridium complex to cleave the η1-Si–H bond, followed by the hydride transferring from iridium dihydride [(POCOP)IrH2] to the cation [OC–OSiMe3]+, is a slightly less favorable pathway, with a free energy barrier of 33.0 kcal mol−1 in solvent. The subsequent three reducing steps follow similar pathways: the ionic SN2 outer-sphere process with silylformate, bis(silyl)acetal and methoxysilane substrates nucleophilically attacking the η1-silane iridium complex to give the ion pairs [(POCOP)IrH2] [HC(OSiMe3)2]+, [(POCOP)IrH2] [CH2(OSiMe3)2(SiMe3)]+, and [(POCOP)IrH2] [CH3O(SiMe3)2]+, respectively, followed by the hydride transfer process. The rate-limiting steps of the three reducing stages are calculated to possess free energy barriers of 12.2, 16.4 and 22.9 kcal mol−1, respectively. Furthermore, our study indicates that the natural iridium dihydride [(POCOP)IrH2] generated along the ionic SN2 outer-sphere pathway could greatly facilitate the silylation of CO2, with a potential energy barrier calculated at a low value of 16.7 kcal mol−1.

Iridium Single-Atom Catalyst Laboring a Quasi-Homogeneous Hydrogenation Transformation of CO2 to Formate

Heterogeneous single atom catalysts (SACs) with atomically-dispersed active metal centres represent an intermediary between heterogeneous and homogeneous catalysis. However, the design of SACs for homogeneous processes remains challenging. In the present work, a porous organic polymer with aminopyridine functionalities was designed so as to fabricate a stable, atomically-dispersed Ir catalyst. This Ir-based SAC exhibits excellent catalytic activity during the liquid phase hydrogenation of CO2 to formate. The associated turnover number is as high as 25,135, representing the best performance yet for a heterogeneous conversion reaction. Spectral characterization and density functional theory modelling demonstrate that the architecture of the Ir single atom active site is analogous to that of a homogeneous mononuclear Ir pincer complex catalyst. As a result, a catalytic mechanism similar to that over a homogeneous Ir catalyst occurs during CO2 hydrogenation with this quasi-homogeneous Ir-based SAC. This work suggests a promising basis for the design of efficient SACs for the once-dominant homogeneous catalytic processes.

Theoretical Study of Ir-Catalyzed Chemoselective C1–O Reduction of Glucose with Silane

Density functional theory (DFT) calculations have been performed to study the mechanism of Ir(III) pincer complex (POCOP)Ir(H)(acetone)+ (POCOP = 2,6-bis(dibutylphosphinito)phenyl) catalyzed chemoselective C1–O hydrosilylative reduction of glucose. The mechanisms for reduction of the external and internal C1–O (i.e., C1–Oext and C1–Oint) on the C1-MeO-substituted glucose (i.e., 1Me) and C1–Me2EtSiO-substituted glucose (i.e., 1Si) have been investigated. The calculation results show that both mechanisms proceed with the first concerted silyl transfer and the subsequent C1–Oext or C1–Oint bond cleavage and hydride transfer steps. In the hydride transfer step, the Ir-H moiety acts as the hydride source. The C1–O cleavage is the rate-determining step of the overall mechanism. The C1–Oext reduction is more favorable than C1–Oint reduction for the substrate 1Me, while the C1–Oint reduction is more favorable for 1Si. These results are consistent with the recent experimental outcomes. Analyzing the origin of chem...

O2 activation by metal-ligand cooperation with Ir(I) PNP pincer complexes.

A unique mode of molecular oxygen activation, involving metal-ligand cooperation, is described. Ir pincer complexes [((t)BuPNP)Ir(R)] (R = C6H5 (1), CH2COCH3 (2)) react with O2 to form the dearomatized hydroxo complexes [((t)BuPNP*)Ir(R)(OH)] ((t)BuPNP* = deprotonated (t)BuPNP ligand), in a process which utilizes both O-atoms. Experimental evidence, including NMR, EPR, and mass analyses, indicates a binuclear mechanism involving an O-atom transfer by a peroxo intermediate.

Computational design of an Iridium based catalyst for releasing H2 from hydrogenated BN nanotubes.

Through carefully calibrated density functional studies we predict that Ir pincer complexes, previously known to effectuate simultaneous proton and hydride transfer from ammonia-borane under ambient conditions, are equally efficient catalysts for the concerted dehydrogenation and subsequent release of H2 from hydrogenated boron nitride nanotubes overcoming free energy of activation accessible at room temperature.

Electrocatalytic Reduction of Carbon Dioxide: Let the Molecules Do the Work

This review summarizes the development of electrochemical CO2 reduction catalysts in the UNC Energy Frontier Research Center for Solar Fuels. Two strategies for converting CO2 to CO or formate have been explored. In one, polypyridyl complexes of Ru(II) have been used to reduce CO2 to CO in acetonitrile and in acetonitrile/water mixtures. In the absence of CO2 water is reduced to H2 by these complexes. With added weak acids in acetonitrile with added water and CO2, reduction to syngas mixtures of CO and H2 is observed. A single polypyridyl complex of Ru(II) has been shown to be both a catalyst for water oxidation and CO2 reduction in an electrochemical cell for CO2 splitting into CO and O2. In parallel, Ir pincer catalysts have been shown to act as selective electrocatalysts for reducing CO2 to formate in acetonitrile with added water and in pure water without competition from electrocatalytic H2 production. Details of the catalytic mechanisms of each have also been investigated.

DFT studies of dehydrogenation of ammonia–borane catalyzed by [Ir(ItBu′)2]+: A proton transfer mechanism

The catalytic dehydrogenation of ammonia borane (NH3BH3) with the Ir pincer complex, [Ir(ItBu′)2]+, has been investigated by density functional theory (DFT). A mechanism has been proposed for dehydrogenation process involving three possible pathways: stepwise B–H/N–H activation, concerted B–H/N–H activation and proton transfer mechanism. The facile occurrence of the studied systems was demonstrated by the activation barriers of the rate-determining steps. It was found that the proton transfer pathway is mostly kinetically favorable with the lowest activation barrier of 28.38kcal/mol.

ChemInform Abstract: Selective Catalytic Transfer Dehydrogenation of Alkanes and Heterocycles by an Iridium Pincer Complex.

AbstractA new phosphinothious/phosphinite Ir pincer complex is prepared and successfully applied as catalyst for the transfer dehydrogenation of a variety of heterocycles using 3,3‐dimethylbuten as hydrogen acceptor.

Complexes with Hybrid Phosphorus-NHC Ligands: Pincer-Type Ir Hydrides, Dinuclear Ag and Ir and Tetranuclear Cu and Ag Complexes

Three types of hybrid phosphorus-imidazolium salts, 1-methyl-3-(3-((diphenylphosphino)methyl)benzyl)-1H-imidazol-3-ium hexafluorophosphate (2·PF6), 1-methyl-3-(3-(di-tert-butylphosphinooxy)phenyl)imidazolium iodide (8a), and 3-(3-((diphenylphosphoryl)methyl)phenyl)-1-methyl-1H-imidazol-3-ium iodide (11) have been prepared and used as precursors to phosphine-NHC, phosphinite-NHC, and phosphoryl-NHC metal complexes, respectively. The structure of 11 has been determined by X-ray diffraction. The Ag(I) and Ir(I) complexes of the phosphine-NHC ligand, [Ag(μ-P-NHC,κC,κP)]2(PF6)2 (3) and [Ir(cod)(μ-P-NHC,κC,κP)]2(PF6)2 (4), were obtained and characterized by NMR, ESI-MS, elemental analysis, and X-ray diffraction. Both complexes are dinuclear and dicationic, with two P-NHC ligands bridging the two metal centers. The presence of the P donor led for 3 to an unprecedented structure compared to that of related Ag(I) complexes with trans spanning bis-NHC ligands. Complex 4 is the first example of a dinuclear iridium complex with a hybrid P-NHC ligand. The new hydrido, Ir(III) pincer-type complex [IrH(CNHCCCNHC)(MeCN)]PF6 (7) is suggested to have a square-pyramidal structure. The tetranuclear Ag(I) complex with the phosphinite-NHC ligand, [Ag2(μ3-I)(μ-PO-NHC,κP,κCNHC)]2 (9a) has a cubane-type structure, with alternating silver and iodine apexes and two PO-NHC ligands bridging opposite edges of the Ag4 tetrahedron. The Ir(III) pincer complexes [IrH(I)(PO-NHC,κP,κC,κCNHC)(Me)] (10a) and [IrH(I)(PO-NHC,κP,κC,κCNHC)(n-Bu)] (10b), with Me or n-Bu substituents on the nitrogen atom, respectively, have been prepared and characterized. Ag(I) and Cu(I) complexes with the phosphoryl-NHC ligand are reported and the centrosymmetric structure of the latter, [Cu(OP-NHC,κCNHC)2(μ-I){Cu(μ-I)}]2 (13), was established by X-ray diffraction and consists of a central Cu2(μ-I)2 rhombus connected by single iodide bridges to two Cu(OP-NHC,κCNHC)2 moieties. The Ir(III) hydride pincer complexes 10a,b were tested as catalyst precursors for the C-H bond activation of alkanes. Although their efficiency was significantly lower for transfer dehydrogenation from cyclooctane (coa) to t-butylethylene (tbe) than that of known PCP-Ir systems, these results represent the first attempts to study the catalytic properties of hybrid P-NHC iridium pincer complexes.

Theoretical mechanism studies on the electrocatalytic reduction of CO2 to formate by water-stable iridium dihydride pincer complex

The reaction mechanism for electrocatalytic reduction of CO2 to formate by water-stable iridium dihydride pincer complex is studied using density functional theory (DFT). The reaction pathways are investigated in detail. The results suggest that the reaction proceeds in three steps: insertion of carbon dioxide into the Ir(III) pincer dihydride, elimination of formate ligand from the hydridoformatoiridium complex, and catalyst regeneration. The reduction potential of the electrode reaction is calculated and accords well with the experimental value. The solvent effect of MeCN and water on the reaction is explored. The results indicate that water has an important effect on CO2 transforming to HCOO(-). In addition, it also plays a critical role for regeneration of the catalyst via non-classical intermolecular hydrogen bonding.

Selective electrocatalytic reduction of carbon dioxide to formate by a water-soluble iridium pincer catalyst

A water-soluble Ir PCP-type pincer catalyst was developed to reduce CO2 to formate electrocatalytically in water with high efficiency and selectivity. Formate is the only reduced carbon product, formed in 93% Faradaic yield with no formation of CO. A small fraction of “background” H2 (ca. 7%) is directly produced at the electrode by solvent reduction. Detailed kinetic information relevant to the catalysis was obtained. The high selectivity for formate production over H2 originates from the aqueous stability of Ir dihydride species, the active species for hydride reduction of CO2. Under neutral pH, the Ir pincer complex does not catalyze the reduction of protons to H2 making water a viable solvent for use with this catalyst system. Addition of small amounts (ca. 1%) of acetonitrile reduces the over-potential and renders the catalysis sustainable. Mechanistic studies suggest that acetonitrile is a key ancillary ligand that ionizes formate effectively preventing catalyst deactivation.