Particle Formation during Oxidation Catalysis with Cp* Iridium Complexes

Real-time monitoring of light scattering and UV–vis profiles of four different Cp*Ir^III precursors under various conditions give insight into nanoparticle formation during oxidation catalysis with NaIO₄ as primary oxidant. Complexes bearing chelate ligands such as 2,2′-bipyridine, 2-phenylpyridine, or 2-(2′-pyridyl)-2-propanolate were found to be highly resistant toward particle formation, and oxidation catalysis with these compounds is thus believed to be molecular in nature under our conditions. Even with the less stable hydroxo/aqua complex [Cp*₂Ir₂(μ-OH)₃]­OH, nanoparticle formation strongly depended on the exact conditions and elapsed time. Test experiments on the isolated particles and comparison of UV–vis data with light scattering profiles revealed that the formation of a deep purple-blue color (∼580 nm) is <i>not</i> indicative of particle formation during oxidation catalysis with molecular iridium precursors as suggested previously

Catalyst Activation by Loss of Cyclopentadienyl Ligands in Hydrogen Transfer Catalysis with Cp*Ir^III Complexes

The activity of the two related complexes [Cp*Ir­(IMe)₂X]­BF₄ (X = Cl (<b>1</b>), H (<b>2</b>)) in transfer hydrogenation from isopropyl alcohol to acetophenone was investigated. The results suggest that the commonly accepted monohydride mechanism for transfer hydrogenation mediated by cyclopentadienyl iridium species does not apply to chloride <b>1</b>. We have found evidence that, although the two monodentate NHC ligands are retained in the coordination sphere, the Cp* ligand is completely released under mild conditions in a precatalytic activation step. Synthesis of modified versions of the initial precatalyst <b>1</b> with different cyclopentadienyl and NHC ligands demonstrated that increasing the steric pressure around the iridium center facilitates precatalyst activation and thus enhances the catalytic performance. Study of five new iridium­(III) complexes bearing mono- or diphosphines helped us monitor Cp* ligand loss under mild conditions. An unusual P–C bond cleavage was also noted in a 1,2-bis­(dimethylphosphino)­methane (dmpm) ligand. On the basis of these findings, a novel catalyst activation mechanism is proposed for [(η⁵-C₅R₅)­Ir] transfer hydrogenation based on the lability of the cyclopentadienyl ligand

Hydrogen-Transfer Catalysis with Cp*Ir^III Complexes: The Influence of the Ancillary Ligands

Fourteen Cp*Ir^III complexes, bearing various combinations of N- and C-spectator ligands, are assayed in hydrogen-transfer catalysis from isopropyl alcohol to acetophenone under various conditions to investigate ligand effects in this widely used reaction. The new cationic complexes bearing monodentate pyridine and N-heterocyclic carbene (NHC) ligands were characterized crystallographically and by variable-temperature nuclear magnetic resonance (VT-NMR). Control experiments and mercury poisoning tests showed that iridium(0) nanoparticles, although active in the reaction, are not responsible for the high activity observed for the most active precatalyst [Cp*Ir­(IMe)₂Cl]­BF₄ (<b>6</b>). For efficient catalysis, it was found necessary to have both NHCs in monodentate form; tying them together in a bis-NHC chelate ligand gave greatly reduced activity. The kinetics of the base-assisted reaction showed induction periods as well as deactivation processes, and H/D scrambling experiments cast some doubt on the classical monohydride mechanism

A Versatile Lab to Pilot Scale Continuous Reaction System for Supercritical Fluid Processing

A compact and versatile continuous reaction system for supercritical fluids as mobile phase was realised using commercially available components where possible. All process parameters of particular importance to the specific properties of near- or supercritical fluids such as pressure, temperature, and composition (<i>p</i>, <i>T</i>, <i>x</i>) can be accurately controlled over a wide flow range. The setup is completely automated by the help of computerised control and features a view cell for inline phase behavior observation. Coupling to supercritical fluid chromatography permits sampling under process conditions for reaction monitoring. Potential applications include continuous flow synthesis and catalysis, extractions, and other operations demanding controlled application of compressible gases. Highly efficient continuous flow asymmetric hydrogenation catalysis with integrated product separation is demonstrated

Electrochemical Activation of Cp* Iridium Complexes for Electrode-Driven Water-Oxidation Catalysis

Organometallic iridium complexes bearing oxidatively stable chelate ligands are precursors for efficient homogeneous water-oxidation catalysts (WOCs), but their activity in oxygen evolution has so far been studied almost exclusively with sacrificial chemical oxidants. In this report, we study the electrochemical activation of Cp*Ir complexes and demonstrate true electrode-driven water oxidation catalyzed by a homogeneous iridium species in solution. Whereas the Cp* precursors exhibit no measurable O₂-evolution activity, the molecular species formed after their oxidative activation are highly active homogeneous WOCs, capable of electrode-driven O₂ evolution with high Faradaic efficiency. We have ruled out the formation of heterogeneous iridium oxides, either as colloids in solution or as deposits on the surface of the electrode, and found indication that the conversion of the precursor to the active molecular species occurs by a similar process whether carried out by chemical or electrochemical methods. This work makes these WOCs more practical for application in photoelectrochemical dyads for light-driven water splitting

Precursor Transformation during Molecular Oxidation Catalysis with Organometallic Iridium Complexes

We present evidence for Cp* being a sacrificial placeholder ligand in the [Cp*Ir^III(chelate)­X] series of homogeneous oxidation catalysts. UV–vis and ¹H NMR profiles as well as MALDI-MS data show a rapid and irreversible loss of the Cp* ligand under reaction conditions, which likely proceeds through an intramolecular inner-sphere oxidation pathway reminiscent of the reductive in situ elimination of diolefin placeholder ligands in hydrogenation catalysis by [(diene)­M^I(L,L′)]⁺ (M = Rh and Ir) precursors. When oxidatively stable chelate ligands are bound to the iridium in addition to the Cp*, the oxidized precursors yield homogeneous solutions with a characteristic blue color that remain active in both water- and CH-oxidation catalysis without further induction period. Electrophoresis suggests the presence of well-defined Ir-cations, and TEM-EDX, XPS, ¹⁷O NMR, and resonance-Raman spectroscopy data are most consistent with the molecular identity of the blue species to be a bis-μ-oxo di-iridium­(IV) coordination compound with two waters and one chelate ligand bound to each metal. DFT calculations give insight into the electronic structure of this catalyst resting state, and time-dependent simulations agree with the assignments of the experimental spectroscopic data. [(cod)­Ir^I(chelate)] precursors bearing the same chelate ligands are shown to be equally effective precatalysts for both water- and CH-oxidations using NaIO₄ as chemical oxidant

Cp* Iridium Precatalysts for Selective C–H Oxidation with Sodium Periodate As the Terminal Oxidant

Sodium periodate (NaIO₄) is shown to be a milder and more efficient terminal oxidant for C–H oxidation with Cp*Ir (Cp* = C₅Me₅) precatalysts than ceric­(IV) ammonium nitrate. Synthetically useful yields, regioselectivities, and functional group tolerance were found for methylene oxidation of substrates bearing a phenyl, ketone, ester, or sulfonate group. Oxidation of the natural products (−)-ambroxide and sclareolide proceeded selectively, and retention of configuration was seen in <i>cis</i>-decalin hydroxylation. At 60 °C, even primary C–H bonds can be activated: whereas methane was overoxidized to CO₂ in 39% yield without giving partially oxidized products, ethane was transformed into acetic acid in 25% yield based on total NaIO₄. ¹⁸O labeling was demonstrated in <i>cis</i>-decalin hydroxylation with ¹⁸OH₂ and NaIO₄. A kinetic isotope effect of 3.0 ± 0.1 was found in cyclohexane oxidation at 23 °C, suggesting C–H bond cleavage as the rate-limiting step. Competition experiments between C–H and water oxidation show that C–H oxidation of sodium 4-ethylbenzene sulfonate is favored by 4 orders of magnitude. <i>In operando</i> time-resolved dynamic light scattering and kinetic analysis exclude the involvement of metal oxide nanoparticles and support our previously suggested homogeneous pathway

Modes of Activation of Organometallic Iridium Complexes for Catalytic Water and C–H Oxidation

Sodium periodate (NaIO₄) is added to Cp*Ir^III (Cp* = C₅Me₅^–) or (cod)­Ir^I (cod = cyclooctadiene) complexes, which are water and C–H oxidation catalyst precursors, and the resulting aqueous reaction is investigated from milliseconds to seconds using desorption electrospray ionization, electrosonic spray ionization, and cryogenic ion vibrational predissociation spectroscopy. Extensive oxidation of the Cp* ligand is observed, likely beginning with electrophilic C–H hydroxylation of a Cp* methyl group followed by nonselective pathways of further oxidative degradation. Evidence is presented that the supporting chelate ligand in Cp*Ir­(chelate) precursors influences the course of oxidation and is neither eliminated from the coordination sphere nor oxidatively transformed. Isomeric products of initial Cp* oxidation are identified and structurally characterized by vibrational spectroscopy in conjunction with density functional theory (DFT) modeling. Less extensive but more rapid oxidation of the cod ligand is also observed in the (cod)­Ir^I complexes. The observations are consistent with the proposed role of Cp* and cod as sacrificial placeholder ligands that are oxidatively removed from the precursor complexes under catalytic conditions

Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution

A series of Cp*Ir^III dimers have been synthesized to elucidate the mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed O₂ evolution. The oxidative stability of the precursors toward nanoparticle formation and their oxygen evolution activity have been investigated and compared to suitable monomeric analogues. We found that precursors bearing monodentate NHC ligands degraded to form nanoparticles (NPs), and accordingly their O₂ evolution rates were not significantly influenced by their nuclearity or distance between the two metals in the dimeric precursors. A doubly chelating bis-pyridine–pyrazolide ligand provided an oxidation-resistant ligand framework that allowed a more meaningful comparison of catalytic performance of dimers with their corresponding monomers. With sodium periodate (NaIO₄) as the oxidant, the dimers provided significantly lower O₂ evolution rates per [Ir] than the monomer, suggesting a negative interaction instead of cooperativity in the catalytic cycle. Electrochemical analysis of the dimers further substantiates the notion that no radical oxyl-coupling pathways are accessible. We thus conclude that the alternative path, nucleophilic attack of water on high-valent Ir-oxo species, may be the preferred mechanistic pathway of water oxidation with these catalysts, and bimolecular oxo-coupling is not a valid mechanistic alternative as in the related ruthenium chemistry, at least in the present system