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

Synthesis and Characterization of Heterobimetallic Iridium–Aluminum and Rhodium–Aluminum Complexes

We demonstrate the synthesis and characterization of a new class of late-transition-metal–aluminum heterobimetallic complexes via a novel synthetic pathway. Complexes of this type are exceedingly rare. Joint experimental and theoretical data sheds light on the electronic effect of ligands containing aluminum moieties on late-transition-metal complexes

Electron-Rich CpIr(biphenyl-2,2′-diyl) Complexes with π‑Accepting Carbon Donor Ligands

Cp*Ir^III and CpIr^III complexes have attracted interest as catalysts for oxidative transformations, and highly oxidizing iridium species are postulated as key intermediates in both catalytic water and C–H bond oxidation. Strongly electron-donating ligand sets have been shown to stabilize Ir^IV complexes. We describe the synthesis and reactivity of complexes containing the CpIr­(biphenyl-2,2′-diyl) moiety stabilized by a series of strong donor carbon-based ligands. The oxidation chemistry of these complexes has been characterized electrochemically, and a singly oxidized Ir^IV species has been observed by X-band EPR for the complex CpIr­(biph)­(<i>p-</i>CNCH₂SO₂C₆H₄CH₃)