Ir Pincer Complexes Research Articles

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.

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.

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.

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.

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.

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.

Cp′FeI]2 as convenient entry into iron-modified pincer complexes: bimetallic η6,κ1-POCOP-pincer iron iridium compounds

The synthesis of [(1,2,4-tBu3C5H2)FeI]2, [Cp′FeI]2 (1), and its reaction chemistry are described. Complex 1 shows remarkable stability toward ligand redistribution in solution and gas-phase due to kinetic stabilization with respect to formation. In coordinating solvents the dimer is homolytically cleaved to form paramagnetic mono-solvate 16 valence-electron (VE) adducts [Cp′FeI(L)]. These solvent adducts are labile, and 1 is re-established on exposure to dynamic vacuum. Complex 1 was demonstrated to be a suitable synthon for [Cp′Fe]+ transfer to Ir pincer complexes and successful η6-coordination of this fragment to the arene ring of κ1-metallated POCOP–iridium pincer complexes [(Cp′Fe)(POCOP)Ir(H)(I)] ([5]++) and [(Cp′Fe)(POCOP)Ir(C2H4)] ([7]++) (POCOP = C6H3-2,6-(OPtBu2)2) was accomplished. In these heterometallic organometallic complexes, the π- and σ-electrons of the POCOP-pincer phenyl anion are involved in η6- and κ1-coordination to iron(II) and iridium(III) and iridium(I), respectively. [Cp′Fe]+ coordination alters the charge distribution in the (POCOP)Ir complex by inducing significant charge transfer from the POCOP aryl π-system to the [Cp′Fe]+ fragment, and therefore yielding an electron-poor and more electrophilic iridium center. Density functional theory (DFT) calculations provide a detailed understanding of this effect. Preliminary catalytic studies show that [7]++ is an active catalyst in transfer dehydrogenation between cyclooctane (COA) and tert-butyl ethylene (TBE).

Density Functional Theoretical Study on the N-H Bond Activation by Ir Pincer Complexes

An appropriate pincer ligand can be used to control the thermodynamics and kinetics of the chemical transformation between the two types of complexes.In this study the density functional theoretical (DFT) studies were conducted in order to determine the Gibbs energy of the activation of N-H bond coupling in

Transfer Dehydrogenation with Alumina-Supported Ir Pincer Complexes

Transfer Dehydrogenation with Alumina-Supported Ir Pincer Complexes

Room temperature benzene C–H activation by a new [PSiP]Ir pincer complex

The synthesis and reactivity of coordinatively unsaturated Rh and Ir complexes supported by the new bis(phosphino)silyl pincer ligand [kappa(3)-(2-Cy(2)PC(6)H(4))(2)SiMe](-) ([Cy-PSiP](-)) are reported, including the first example of facile, room temperature intermolecular arene C-H bond activation mediated by a silyl pincer complex.