The first uranium bis(acyl)phosphide (BAP) complexes
were synthesized
from the reaction between sodium bis(mesitoyl)phosphide (Na(
mes
BAP)) or sodium bis(2,4,6-triisopropylbenzoyl)phosphide
(Na(
tripp
BAP)) and
UI3(1,4-dioxane)1.5. Thermally stable, homoleptic
BAP complexes were characterized by single-crystal X-ray diffraction
and electron paramagnetic resonance (EPR) spectroscopy, when appropriate,
for the elucidation of the electronic structure and bonding of these
complexes. EPR spectroscopy revealed that the BAP ligands on the uranium
center retain a significant amount of electron density. The EPR spectrum
of the trivalent U(
tripp
BAP)
3
has a rhombic signal near g = 2 (g
1 = 2.03; g
2 = 2.01; and g
3 = 1.98) that
is consistent with the EPR-observed unpaired electron being located
in a molecular orbital that appears ligand-derived. However, upon
warming the complex to room temperature, no resonance was observed,
indicating the presence of uranium character.
This perspective provides an introduction to magnetic circular dichroism (MCD) spectroscopy and its efficacy in elucidating both fundamental electronic structure and in situ reaction speciation in d- and f-block organometallics.
As prevalent cofactors in living organisms, iron−sulfur clusters participate in not only the electron-transfer processes but also the biosynthesis of other cofactors. Many synthetic iron−sulfur clusters have been used in model studies, aiming to mimic their biological functions and to gain mechanistic insight into the related biological systems. The smallest [2Fe−2S] clusters are typically used for one-electron processes because of their limited capacity. Our group is interested in functionalizing small iron−sulfur clusters with redox-active ligands to enhance their electron storage capacity, because such functionalized clusters can potentially mediate multielectron chemical transformations. Herein we report the synthesis, structural characterization, and catalytic activity of a diferric [2Fe− 2S] cluster functionalized with two o-phenylenediamide ligands. The electrochemical and chemical reductions of such a cluster revealed rich redox chemistry. The functionalized diferric cluster can store up to four electrons reversibly, where the first two reduction events are ligand-based and the remainder metal-based. The diferric [2Fe−2S] cluster displays catalytic activity toward silylation of dinitrogen, affording up to 88 equiv of the amine product per iron center.
Experimental and computational studies support an inner-sphere radical pathway for iron-catalysed C–H activation/functionalisation with allyl electrophiles.
In
this work, we aim to formally design iron(0) complexes
combined
with a phenanthroline-type ligand (phen) and investigate their utility
in cycloaddition catalysis. Owing to the strong noninnocence of the
phen scaffold, its ligation to reduced iron oxidation states classically
affords particularly unstable species. The reported examples of such
well-defined coordination complexes are thus particularly scarce.
We demonstrate herein that a strategic steric protection of the C4
and C7 positions of the phen ring leads to neutral (N,N)2Fe species, which exhibits an unprecedented thermal and kinetic stability,
amenable to its easy use as an in situ generated
precursor in catalytic processes. The electronic structure of this
noninnocent complex has been fully rationalized, and its promising
catalytic activity in alkyne [2 + 2 + 2] cyclizations is discussed.
Given its intrinsic thermal stability due to the noninnocent behavior
of the (N,N) ligand, (N,N)2Fe appears to be an efficient
dormant state of the catalytic process, precluding deactivation
of iron as nonreactive aggregates.
Iron-catalyzed amino-oxygenation of olefins often uses
discrete
ligands to increase reactivity and broaden substrate scope. This work
is focused on examining ligand effects on reactivity and in situ iron
speciation in a system which utilizes a bisoxazoline ligand. Freeze-trapped 57Fe Mössbauer and EPR spectroscopies as well as SC-XRD
experiments were utilized to isolate and identify the species formed
during the catalytic reaction of amino-oxygenation of olefins with
functionalized hydroxylamines, as well as in the precatalytic mixture
of iron salt and ligand. Experiments revealed significant influence
of ligand and solvent on the speciation in the precatalytic mixture
which led to the formation of different species which had significant
influence on the reactivity. In situ experiments showed no evidence
for the formation of an Fe(IV)-nitrene intermediate, and the isolation
of a reactive intermediate was unsuccessful, suggesting that the use
of the PyBOX ligand led to the formation of more reactive intermediates
than observed in the previously studied system, preventing direct
detection of intermediate species. However, isolation of the seven
coordinate Fe(III) species with three carboxylate units of the hydroxylamine
and spin-trap EPR experiments suggest formation of a species with
unpaired electron density on the hydroxylamine nitrogen, which is
in accordance with formation of a potential iron iminyl radical species,
as recently proposed in literature. An observed increase in yield
when substrates devoid of C–H bonds as well as isolation of
a ring-closed dead-end species with substrates containing these bonds
suggests the identity of the functionalized hydroxylamine can dictate
the reactivity observed in these reactions.
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