9 research outputs found
Particle Formation during Oxidation Catalysis with Cp* Iridium Complexes
Real-time monitoring of light scattering and UVâvis
profiles
of four different Cp*Ir<sup>III</sup> precursors under various conditions
give insight into nanoparticle formation during oxidation catalysis
with NaIO<sub>4</sub> as primary oxidant. Complexes bearing chelate
ligands such as 2,2â˛-bipyridine, 2-phenylpyridine, or 2-(2â˛-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*<sub>2</sub>Ir<sub>2</sub>(Îź-OH)<sub>3</sub>]ÂOH,
nanoparticle formation strongly depended on the exact conditions and
elapsed time. Test experiments on the isolated particles and comparison
of UVâvis data with light scattering profiles revealed that
the formation of a deep purple-blue color (âź580 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<sup>III</sup> Complexes
The activity of the two related complexes
[Cp*IrÂ(IMe)<sub>2</sub>X]ÂBF<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 iridiumÂ(III) complexes bearing mono- or diphosphines
helped us monitor Cp* ligand loss under mild conditions. An unusual
PâC bond cleavage was also noted in a 1,2-bisÂ(dimethylphosphino)Âmethane
(dmpm) ligand. On the basis of these findings, a novel catalyst activation
mechanism is proposed for [(Ρ<sup>5</sup>-C<sub>5</sub>R<sub>5</sub>)ÂIr] 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*IrÂ(IMe)<sub>2</sub>Cl]ÂBF<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
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
O<sub>2</sub>-evolution activity, the molecular species formed after
their oxidative activation are highly active homogeneous WOCs, capable
of electrode-driven O<sub>2</sub> 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<sup>III</sup>(chelate)ÂX] series of homogeneous oxidation
catalysts. UVâvis and <sup>1</sup>H 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)ÂM<sup>I</sup>(L,Lâ˛)]<sup>+</sup> (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, <sup>17</sup>O NMR, and resonance-Raman spectroscopy
data are most consistent with the molecular identity of the blue species
to be a bis-Îź-oxo di-iridiumÂ(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)ÂIr<sup>I</sup>(chelate)]
precursors bearing the same chelate ligands are shown to be equally
effective precatalysts for both water- and CH-oxidations using NaIO<sub>4</sub> as chemical oxidant
Cp* Iridium Precatalysts for Selective CâH Oxidation with Sodium Periodate As the Terminal Oxidant
Sodium periodate (NaIO<sub>4</sub>) is shown to be a
milder and
more efficient terminal oxidant for CâH oxidation with Cp*Ir
(Cp* = C<sub>5</sub>Me<sub>5</sub>) precatalysts than cericÂ(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 (â)-ambroxide and sclareolide proceeded selectively,
and retention of configuration was seen in <i>cis</i>-decalin
hydroxylation. At 60 °C, even primary CâH bonds can be
activated: whereas methane was overoxidized to CO<sub>2</sub> in 39%
yield without giving partially oxidized products, ethane was transformed
into acetic acid in 25% yield based on total NaIO<sub>4</sub>. <sup>18</sup>O labeling was demonstrated in <i>cis</i>-decalin
hydroxylation with <sup>18</sup>OH<sub>2</sub> and NaIO<sub>4</sub>. A kinetic isotope effect of 3.0 Âą 0.1 was found in cyclohexane
oxidation at 23 °C, suggesting CâH bond cleavage as the
rate-limiting step. Competition experiments between CâH and
water oxidation show that CâH 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 CâH Oxidation
Sodium
periodate (NaIO<sub>4</sub>) is added to Cp*Ir<sup>III</sup> (Cp*
= C<sub>5</sub>Me<sub>5</sub><sup>â</sup>) or (cod)ÂIr<sup>I</sup> (cod = cyclooctadiene) complexes, which are water and CâH
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 CâH hydroxylation
of a Cp* methyl group followed by nonselective pathways of further
oxidative degradation. Evidence is presented that the supporting chelate
ligand in Cp*IrÂ(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)ÂIr<sup>I</sup> 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<sup>III</sup> dimers have been synthesized to elucidate the
mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed
O<sub>2</sub> 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 O<sub>2</sub> evolution
rates were not significantly influenced by their nuclearity or distance
between the two metals in the dimeric precursors. A doubly chelating
bis-pyridineâpyrazolide 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 (NaIO<sub>4</sub>) as the oxidant, the dimers provided significantly
lower O<sub>2</sub> 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