4 research outputs found
Catalyst Activation by Loss of Cyclopentadienyl Ligands in Hydrogen Transfer Catalysis with Cp*Ir<sup>III</sup> Complexes
The activity of the two related complexes
[Cp*IrĀ(IMe)<sub>2</sub>X]ĀBF<sub>4</sub> (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 iridiumĀ(III) complexes bearing mono- or diphosphines
helped us monitor Cp* ligand loss under mild conditions. An unusual
PāC bond cleavage was also noted in a 1,2-bisĀ(dimethylphosphino)Āmethane
(dmpm) ligand. On the basis of these findings, a novel catalyst activation
mechanism is proposed for [(Ī·<sup>5</sup>-C<sub>5</sub>R<sub>5</sub>)ĀIr] transfer hydrogenation based on the lability of the cyclopentadienyl
ligand
Hydrogen-Transfer Catalysis with Cp*Ir<sup>III</sup> Complexes: The Influence of the Ancillary Ligands
Fourteen Cp*Ir<sup>III</sup> 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*IrĀ(IMe)<sub>2</sub>Cl]ĀBF<sub>4</sub> (<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 IridiumāAluminum and RhodiumāAluminum Complexes
We
demonstrate the synthesis and characterization of a new class of late-transition-metalāaluminum
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-2,2ā²-diyl) Complexes with ĻāAccepting Carbon Donor Ligands
Cp*Ir<sup>III</sup> and CpIr<sup>III</sup> complexes
have attracted
interest as catalysts for oxidative transformations, and highly oxidizing
iridium species are postulated as key intermediates in both catalytic
water and CāH bond oxidation. Strongly electron-donating ligand
sets have been shown to stabilize Ir<sup>IV</sup> complexes. We describe
the synthesis and reactivity of complexes containing the CpIrĀ(biphenyl-2,2ā²-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<sup>IV</sup> species has
been observed by X-band EPR for the complex CpIrĀ(biph)Ā(<i>p-</i>CNCH<sub>2</sub>SO<sub>2</sub>C<sub>6</sub>H<sub>4</sub>CH<sub>3</sub>)