5 research outputs found
Iron(II) Active Species in Iron–Bisphosphine Catalyzed Kumada and Suzuki–Miyaura Cross-Couplings of Phenyl Nucleophiles and Secondary Alkyl Halides
While
previous studies have identified FeMes<sub>2</sub>(SciOPP)
as the active catalyst species in iron–SciOPP catalyzed Kumada
cross-coupling of mesitylmagnesium bromide and primary alkyl halides,
the active catalyst species in cross-couplings with phenyl nucleophiles,
where low valent iron species might be prevalent due to accessible
reductive elimination pathways, remains undefined. In the present
study, in situ Mössbauer and magnetic circular dichroism spectroscopic
studies combined with inorganic syntheses and reaction studies are
employed to evaluate the in situ formed iron species and identify
the active catalytic species in iron–SciOPP catalyzed Suzuki–Miyaura
and Kumada cross-couplings of phenyl nucleophiles and secondary alkyl
halides. While reductive elimination to form FeÂ(η<sup>6</sup>-biphenyl)Â(SciOPP) occurs upon reaction of FeCl<sub>2</sub>(SciOPP)
with phenyl nucleophiles, this iron(0) species is not found to be
kinetically competent for catalysis. Importantly, mono- and bis-phenylated
ironÂ(II)–SciOPP species that form prior to reductive elimination
are identified, where both species are found to be reactive toward
electrophile at catalytically relevant rates. The higher selectivity
toward the formation of cross-coupled product observed for the monophenylated
species combined with the undertransmetalated nature of the in situ
iron species in both Kumada and Suzuki–Miyaura reactions indicates
that FeÂ(Ph)ÂXÂ(SciOPP) (X = Br, Cl) is the predominant reactive species
in cross-coupling. Overall, these studies demonstrate that low-valent
iron is not required for the generation of highly reactive species
for effective aryl-alkyl cross-couplings
A Combined Mössbauer, Magnetic Circular Dichroism, and Density Functional Theory Approach for Iron Cross-Coupling Catalysis: Electronic Structure, In Situ Formation, and Reactivity of Iron-Mesityl-Bisphosphines
While
iron-bisphosphines have emerged as effective catalysts for
C–C cross-coupling, the nature of the in situ formed iron species,
elucidation of the active catalysts and the mechanisms of catalysis
have remained elusive. A combination of <sup>57</sup>Fe Mössbauer
and magnetic circular dichroism (MCD) spectroscopies of well-defined
and in situ formed mesityl-ironÂ(II)-SciOPP species combined with density
functional theory (DFT) investigations provides the first direct insight
into electronic structure, bonding and in situ speciation of mesityl-ironÂ(II)-bisphosphines
in the Kumada cross-coupling of MesMgBr and primary alkyl halides
using FeCl<sub>2</sub>(SciOPP). Combined with freeze-trapped solution
Mössbauer studies of reactions with primary alkyl halides,
these studies demonstrate that distorted square-planar FeMes<sub>2</sub>(SciOPP) is the active catalyst for cross-coupling and provide insight
into the molecular-level mechanism of catalysis. These studies also
define the effects of key reaction protocol details, including the
role of the slow Grignard addition method and the addition of excess
SciOPP ligand, in leading to high product yields and selectivities
New Insights into Structure and Luminescence of Eu<sup>III</sup> and Sm<sup>III</sup> Complexes of the 3,4,3-LI(1,2-HOPO) Ligand
We
report the preparation and new insight into photophysical properties
of luminescent hydroxypyridonate complexes [M<sup>III</sup><b>L</b>]<sup>−</sup> (M = Eu or Sm) of the versatile 3,4,3-LIÂ(1,2-HOPO)
ligand (<b>L</b>). We report the crystal structure of this ligand
with Eu<sup>III</sup> as well as insights into the coordination behavior
and geometry in solution by using magnetic circular dichroism. In
addition TD-DFT calculations were used to examine the excited states
of the two different chromophores present in the 3,4,3-LIÂ(1,2-HOPO)
ligand. We find that the Eu<sup>III</sup> and Sm<sup>III</sup> complexes
of this ligand undergo a transformation after in situ preparation
to yield complexes with higher quantum yield (QY) over time. It is
proposed that the lower QY in the in situ complexes is not only due
to water quenching but could also be due to a lower degree of f-orbital
overlap (in a kinetic isomer) as indicated by magnetic circular dichroism
measurements
Spectroscopic Definition of the Copper Active Sites in Mordenite: Selective Methane Oxidation
Two distinct [Cu–O–Cu]<sup>2+</sup> sites with methane
monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing
that this Cu–O–Cu active site geometry, having a ∠Cu–O–Cu
∼140°, is particularly formed and stabilized in zeolite
topologies. Whereas in ZSM-5 a similar [Cu–O–Cu]<sup>2+</sup> active site is located in the intersection of the two 10
membered rings, Cu-MOR provides two distinct local structures, situated
in the 8 membered ring windows of the side pockets. Despite their
structural similarity, as ascertained by electronic absorption and
resonance Raman spectroscopy, the two Cu–O–Cu active
sites in Cu-MOR clearly show different kinetic behaviors in selective
methane oxidation. This difference in reactivity is too large to be
ascribed to subtle differences in the ground states of the Cu–O–Cu
sites, indicating the zeolite lattice tunes their reactivity through
second-sphere effects. The MOR lattice is therefore functionally analogous
to the active site pocket of a metalloenzyme, demonstrating that both
the active site and its framework environment contribute to and direct
reactivity in transition metal ion-zeolites
Iron Phosphine Catalyzed Cross-Coupling of Tetraorganoborates and Related Group 13 Nucleophiles with Alkyl Halides
Iron
phosphine complexes prove to be good precatalysts for the
cross-coupling of alkyl, benzyl, and allyl halides with not only aryl
triorganoborate salts but also related aluminum-, gallium-, indium-,
and thallium-based nucleophiles. Mechanistic studies revealed that
while FeÂ(I) can be accessed on catalytically relevant time scales,
lower average oxidation states are not formed fast enough to be relevant
to catalysis. EPR spectroscopic studies reveal the presence of bisÂ(diphosphine)ÂironÂ(I)
complexes in representative catalytic reactions and related processes
with a range of group 13 nucleophiles. Isolated examples were studied
by Mössbauer spectroscopy and single-crystal X-ray structural
analysis, while the electronic structure was probed by dispersion-corrected
B3LYP DFT calculations. An EPR study on an iron system with a bulky
diphosphine ligand revealed the presence of an <i>S</i> = <sup>1</sup>/<sub>2</sub> species consistent with the formation of a monoÂ(diphosphine)ÂironÂ(I)
species with inequivalent phosphine donor environments. DFT analysis
of model complexes allowed us to rule out a T-shaped FeÂ(I) structure,
as this is predicted to be high spin