Homoleptic Aryl Complexes of Uranium (IV)

Article abstract of DOI:10.1002/anie.201905423

The synthesis and characterization of sterically unencumbered homoleptic organouranium aryl complexes containing U?C σ-bonds has been of interest to the chemical community for over 70 years. Reported herein are the first structurally characterized, steric

Full text of DOI:10.1002/anie.201905423

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Accepted Article
Title: Homoleptic Aryl Complexes of Uranium(IV)
Authors: Nikki J Wolford, Dumitru-Claudiu Sergentu, William
Brennessel, Jochen Autschbach, and Michael Neidig
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10.1002/anie.201905423
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Homoleptic Aryl Complexes of Uranium (IV)
Nikki J. Wolforda, Dumitru-Claudiu Sergentub, William W. Brennessela, Jochen Autschbachb*, and Michael L. Neidiga*
Abstract: The synthesis and characterization of sterically
unencumbered homoleptic organouranium aryl complexes containing
U-C σ-bonds has been of interest to the chemical community for over
70 years. Reported herein are the first structurally characterized,
sterically unencumbered homoleptic uranium (IV) aryl-ate species of
the form [U(Ar)6]2- (Ar = Ph, p-tolyl, p-Cl-Ph). Magnetic circular
dichroism (MCD) spectroscopy and computational studies provide
insight into electronic structure and bonding interactions in the U-C σ-
bond across this series of complexes. Overall, these studies solve a
decades long challenge in synthetic uranium chemistry, enabling new
insight into electronic structure and bonding in organouranium
complexes.
Figure 1. Previously reported homoleptic actinide-aryl complexes.
We were motivated to re-explore this longstanding challenge in
the preparation of homoleptic uranium-aryl complexes with non-
sterically encumbered aryl ligands utilizing our low temperature
synthetic capabilities previously successfully applied to similar
synthetic challenges in homoleptic iron-aryl chemistry.[15] Herein,
we report the first homoleptic six-coordinate uranate (IV)
complexes containing only uranium-carbon σ-bonds between a
uranium center and phenyl, p-tolyl and p-Cl-phenyl ligands and,
combined with spectroscopic and theoretical studies, develop
insight into electronic structure and bonding in these unique
complexes.
Originally proposed for applications in isotope separation and
enrichment over 70 years ago,[1-2] homoleptic organometallic
complexes of uranium with simple alkyl or aryl ligands are of
significant research interest for insight into electronic structure
and bonding in uranium organometallic chemistry. Despite the
significant challenges inherent in their isolation and
characterization due to their severe air and thermal sensitivities,[3]
the past 30 years have seen seminal contributions in the
synthesis and characterization of homoleptic uranium-alkyl
complexes from Sattelberger, Hayton, and Bart.[4-11]
Unfortunately, corresponding advances in the isolation and
characterization of homoleptic uranium-aryl complexes have
been exceedingly more rare despite decades of research.
Our initial synthetic studies targeted the isolation of the
uranium(IV) complex, [UPh6]2-. Drop-wise addition of 6 equiv. of
phenyllithium (PhLi) to UCl4 at -80 °C in a Et2O/THF mixture
resulted in an immediate color change from pale green to dark
red. Note that analogous reactivity can be achieved using the
adduct [UCl4(1,4-dioxane)]2 under otherwise identical reaction
conditions.[16] Addition of hexane and subsequent crystallization
at -80 °C afforded highly air and thermally sensitive red needles
suitable for single-crystal X-ray diffraction studies, identified as
[Li(THF)4][(THF)LiU(Ph)6]·1.5THF (1) (Figure 2A) (1H NMR, see
SI). Isolated yields up to 60% have been achieved for crystalline
material of 1. Magnetic susceptibility studies using the Evans
method resulted in an average magnetic moment of 2.2(2) B.M.
at -80 °C consistent with previous measurements of other six-
coordinate U(IV) complexes.[4, 17]
Early attempts to isolate homoleptic uranium aryl complexes
were first reported in 1977 by Sigurdson and Wilkinson.[12] In
these studies, the reaction of uranium tetrachloride (UCl4) with
lithium nucleophiles resulted in the formation of anionic, thermally
unstable uranate (IV) species which could not be isolated and
readily decomposed at or below room temperature. However,
using techniques including NMR and alcoholysis, the general
formula [Li(solvent)]2[UR6] was assigned to the insitu formed
species. In 2016, the first structurally characterized, homoleptic
uranium-aryl complex was reported by Arnold and co-workers – a
homoleptic uranium(III)-aryl species (Figure 1) that takes
advantage of steric protection provided by the bulky Terph–
ligand.[13] Despite this important advance, homoleptic uranium-
aryl complexes beyond the +3 oxidation state and containing non-
sterically encumbering substituents remain an unsolved synthetic
problem. It is of interest to note, however, that Hayton and
coworkers have recently reported two homoleptic thorium aryl-ate
complexes, each with unique geometries about the actinide metal
center (Figure 1).[14]
The anion of 1 consists of a six-coordinate U(IV) center bonded
to six phenyl rings with a high degree of distortion from octahedral
geometry. One of the lithium cations interacts with the π-systems
of three of the ligands, which appears to further augment the
distortion. The three C-U-C angles for the lithium-coordinating
phenyl ligands range from 76.77(2)° to 79.33(2)°, whereas those
for the three independent phenyl rings range from 92.74(19)° to
101.76(2)° (Table 1). A similar distortion with lithium atom
interaction has previously been reported for a hexaphenyl
hafnium complex, [Li(THF)4][(THF)LiHf(Ph)6].[18]
With a successful method identified for the synthesis and
isolation of 1, the para-substituents of the aryl ring were varied in
order to evaluate the effect of the electronic contribution of the
ligands on the nature of the U-C bond. As a starting point, p-
tolyllithium was selected in order to incorporate a more electron-
rich aryl ligand. Drop-wise addition of 6 equiv. of p-tolyllithium to
[UCl4(1,4-dioxane)]2 in a Et2O/THF mixture at -80 °C resulted in
an immediate color change from yellow/orange to red.
Subsequent crystallization at -80 °C afforded air and temperature
sensitive red/orange plates suitable for single-crystal X-ray
[a]
N. J. Wolford, Dr. W. W. Brennessel, and Prof. M. L. Neidig
Department of Chemistry, University of Rochester
B31 Hutchison Hall, 120 Trustee Road
Rochester, NY 14627 (USA)
neidig@chem.rochester.edu
[b]
Dr. D.-C. Sergentu and Prof. J. Autchbach
Department of Chemistry, University at Buffalo, State University of
New York
312 Natural Sciences Complex
Buffalo, NY 14260 (USA)
diffraction
studies
and
identified
as
[(THF)3LiXLi(THF)3][(THF)LiU(p-tolyl)6]·Et2O·0.62THF (2) (Figure
jochena@buffalo.edu
2B) (1H NMR, see SI). Isolated yields of 45% have been achieved
Supporting information for this article is given via a link at the end of
the document.
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Figure 3. (A) X-ray crystal structure of the uranium hexaaryl complex,
[Li(THF)4]2[U(p-Cl-Ph)6], 3, in the distorted octahedral geometry obtained at
193 K. Cations and hydrogen atoms omitted for clarity. Thermal ellipsoids are
shown at 50% probability. (B) Crystal structure of the uranium hexaaryl
complex, [Li(THF)4]2[U(p-Cl-Ph)6],3, in the nearly octahedral geometry
obtained at 243 K. Cations and hydrogen atoms omitted for clarity. Thermal
ellipsoids are shown at 50% probability.
(Figure 3) (1H NMR, see SI). Isolated yields of 70% have been
achieved for crystalline material of 3. Magnetic susceptibility
studies using the Evans method resulted in an average magnetic
moment of 2.5(2) B.M. for 3 at -20 oC.
Compared to complexes 1 and 2, complex 3 is significantly less
distorted from octahedral symmetry as it lacks lithium counterion
interaction with any of the aryl rings. Low temperature X-ray
experiments resulted in a disorder over two positions of the one
crystallographically unique aryl ring. Unfortunately, due to the
limitation of the crystallographic -3 (S6) symmetry and having just
one unique ligand, the exact distortion from octahedral of the six-
coordinate complex is difficult to define. Using the distorted
thorium hexaphenyl complex reported by Hayton and
coworkers[14], the geometry was assigned as that observed in
Figure 3A. The increased thermal stability of 3 (stable at 0 oC for
several hours) enabled X-ray experiments at elevated
temperatures (243 K) to be performed to evaluate the effects of
temperature on the structural distortion of the complex. A
decrease in distortion was observed at elevated experimental
temperatures where the dianion is ordered with respect to the
crystallographic -3 (S6) position and a nearly octahedral geometry
is observed (Figure 3B). While it is of interest whether this same
flexibility also exists in 1 and 2, the thermal instability of these
complexes does not allow for elevated temperature experiments
to be performed.
Figure 2. X-ray crystal structures of the homoleptic uranium-aryl complexes (A)
[Li(THF)4][(THF)LiU(Ph)6]·1.5THF (1) and [(THF)3LiXLi(THF)3][(THF)LiU(p-
tolyl)6]·Et2O·0.62THF (2), X = 0.64Br, 0.36Cl (B). Independent cations and
hydrogens atoms omitted for clarity. Thermal ellipsoids are shown at 50%
probability.
for crystalline material of 2. Magnetic susceptibility studies using
the Evans method resulted in an average magnetic moment of
2.3(2) at -80 °C.
Complex 2 exhibits an analogous distortion away from pure
octahedral symmetry as observed for 1, with one lithium-THF
counterion interacting with the aryl ring system. The average U-C
bond lengths of the two complexes are nearly identical, 2.525(11)
for 1 and 2.518(11) Å for 2. In both uranium aryl complexes, the
aryl rings feature asymmetric U-Cipso-Cortho angles, suggestive of
an agostic interaction. For example, the U-C1-C2 bond angle in
the uranium hexaphenyl complex has a bond angle of 142.8(4)
degrees while the U-C1-C6 bond angle is 100.7(3) degrees, a
difference of greater than 42 degrees. In the hexatolyl complex,
this sample pair of angles differs by nearly 41 degrees (Table S1).
Natural bond orbital (NBO) second-order perturbation theory
analyses of 1 and 2 suggest that agostic interactions contribute to
the structure stabilization by 6.47 and 6.56 kcal/mol, respectively
(Figure S1). This type of agostic interaction has also been
observed by Hayton et al. in the previously reported distorted six-
coordinate thorium phenyl-ate species.[14]
To gain further insight into potential electronic structure effects
of the aryl ligand variations in these homoleptic [U(Ar)6]2-
complexes, magnetic circular dichroism (MCD) spectroscopic
measurements were utilized. Note that the near-infrared (NIR)
region contains the formally Laporte forbidden f®f transitions, but
gains some dipole-allowed character due to low symmetry and
metal-ligand covalency. These transitions are much more intense
in C-term MCD due to the spin-orbit coupling (SOC) acting more
efficiently in the uranium-centered states than in the ligand-based
charge transfer states.[19] All three complexes have very similar
UV-vis MCD spectra (Figure S2). The energy region of 5000-
12000 cm-1 (Figure 4) contains a series of f®f transitions which,
unlike those in the symmetric [UCl6]- complex, are split due to the
decent of symmetry in the distorted six-coordinate complexes.
Complexes 1 and 2 have nearly identical 5 K, 7 T NIR MCD
spectra with three groupings of transitions in the 5000-7000 cm-1
(features I), 8000-9500 cm-1 (features II), and 10,700 cm-1
(features III) energy regions. While the spectrum of complex 3
Noting the structural similarities of complexes 1 and 2, isolation
of an analogous [U(Ar)6]2- complex containing an electron-poor
aryl ring system, p-chlorophenyl, was also targeted. Drop-wise
addition of 6 equiv. of p-chlorophenyllithium (p-Cl-PhLi) to
[UCl4(1,4-dioxane)]2 in Et2O/THF at -80 °C resulted in an
immediate color change from yellow/orange to very dark red,
nearing black. Subsequent crystallization at -80 °C afforded dark
red/black hexagonal blocks, suitable for single-crystal X-ray
diffraction studies and identified as [Li(THF)4]2[U(p-Cl-Ph)6] (3)
Table 1. Selected metrics of 1, 2, and 3.
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Figure 5. PT2+SO calculated (blue) and experimental (black) MCD spectra of
1 in the NIR region. Spin-orbit states describing main MCD features are
labeled according to the expansion in terms of spin-free states of the U4+ free
ion (see Table S2).
To further probe the metal-ligand bonding interactions in this
series of uranium (IV) aryl complexes, additional computational
studies were performed to analyze the metal-ligand orbital
interactions. Structures for the three complexes were optimized
(see SI) starting from the crystal structure coordinates for
complexes 1 and 2, and from a symmetric structure complying
with the S6 symmetry for complex 3. For comparison, the
analogous thorium hexaphenyl complex, [Th(Ph)6]2-, was also
optimized from the previously reported C3 structure (Figure S4,
Table S3). The calculated average M-C bond lengths of
complexes 1-3 agree with the measurements. Averaged Wiberg
Bond Index (WBI) calculations predict similar bonding interactions
for the uranium complexes, with slightly less covalency in 3, likely
due to the electron withdrawing nature of the aryl rings. It is of
interest to note that the WBI values for complexes 1-3 are larger
than that of [Th(Ph)6]2-, suggesting an increase of metal-ligand
covalency for U(IV). Indeed, [Th(Ph)6]2- has less total metal orbital
character (%M) (16%) in the metal-carbon bonding orbitals than
1-3 (20-23%) (Figure 6, Table S4). The contributing metal orbitals
of [Th(Ph)6]2- are primarily 6d in nature (70% 6d, 20% 5f). For
Figure 4. 5 K, 7 T NIR MCD spectra of 1 (top, red), 2 (center, blue), and 3
(bottom, green).
exhibits slight variations in energy and overall number of f-f
transitions (attributed to the reduced distortion from Oh symmetry
in this complex), overall, these are minimal and support very
similar overall electronic structures for all three complexes.
Utilizing the calculated NIR MCD spectra (Figure 5, Figure S3),
the transitions in the experimental NIR MCD spectra can be
assigned. The free ion U(IV) has a 3H spin-free ground state and
3
1
low lying F and G levels (Table S2). The added ligand field of
the aryl ligands of complexes 1, 2, and 3 removes the degeneracy
of the free-ion states, but all three complexes retain a spin-free
ground state derived from the free ion 3H term. Likewise, the SOC
ground state is derived from the free-ion 3H4 ground state (i.e. is
2-
2-
Figure 6. M-C bonding NLMOs (M = Th or U) for Th(Ph)6 and U(aryl)6
complexes 1, 2, and 3 with their respective % metal character and 6d/5f orbital
contributions to the metal character M-C bonding NLMOs. Hydrogen atoms
removed for clarity.
3
of dominant H free-ion parentage), which is split on average by
2500 cm-1 by the ligand field. The excited states that are
responsible for the first group of MCD bands derive mainly from
the free ion 3H5 term, the second group of MCD bands derive
mainly from the free ion 3F3 and 3F4/1G4 terms, and the third group
complexes 1 and 2 the 5f character is increased (38%, 40%)
compared to the Th complex, while for complex 3 the 5f (47%)
and 6d (43%) contributions are evenly split. The increase of 5f
bonding character in complex 3 can be attributed to a better
directionality of the 5f metal orbitals toward the ligand orbitals,
enhancing metal-ligand orbital overlap, facilitated by the
molecular geometry approaching Oh.
3
of MCD bands derive mainly from the free-ion H6 term. Due to
the combined influence of ligand field and SOC, the ion levels with
different S, L and J quantum numbers are mixed and therefore we
only report the dominant ion spin-state contributions in Figures 5
and S3. The analysis of the calculated electronic states confirms
that the similarity of the measured MCD spectra of the different
complexes reflects the similarity of their ground and low-energy
excited electronic states, and the similarity of the ligand field
effects across the series.
The synthesis and structural characterization of homoleptic aryl
uranium complexes has been a longstanding goal in f-element
chemistry which, to date, has been limited to a single homoleptic
uranium(III) complex with sterically encumbered aryl ligands. In
this study we have reported the first isolated and structurally
characterized organouranium complexes containing simple aryl
ligands of the form [U(Ar)6]2- (Ar = Ph, p-tolyl, p-Cl-Ph), and gained
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insight into electronic structure and bonding through
a
combination of advanced MCD and theoretical studies. The
theoretical MCD studies allowed for quantitative analysis of the
similarities in the electronic structure and ligand field splitting
across the series of homoleptic uranium (IV) species. This work
serves as a critical advancement towards the synthesis and
characterization of non-stabilized homoleptic organouranium aryl
species. Moving forward, the effect of oxidation state on the
electronic structure and bonding of these homoleptic uranium aryl
species as well as insight into the decomposition pathways of
these highly sensitive complexes will continue to provide unique
insight into structure and bonding in organouranium chemistry.
Acknowledgements
MLN acknowledges support for this work from the U.S.
Department of Energy, Office of Science, Early Career Research
Program under Award DE-SC0016002. JA acknowledges support
from the U.S. Department of Energy, Office of Basic Energy
Sciences, Heavy Elements program, under grant DE-
SC0001136. We thank the Center for Computational Research
(CCR) at the University of Buffalo for providing computational
resources. The NSF is gratefully acknowledged for support for the
acquisition of an X-ray diffractometer (CHE-1725028).
Keywords: uranium • organometallic • homoleptic • MCD
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Reported herein are the first
structurally characterized and
Nikki J. Wolford, Dumitru-
Claudiu Sergentu, William W.
Brennessel, Jochen Autschbach,
and Michael L. Neidig.
sterically
unencumbered
uranium (IV) homoleptic aryl-ate
species. MCD spectroscopy
Page No. – Page No.
and
provide insight into electronic
structure and bonding
interactions across this series of
complexes providing insight into
organouranium complexes with
exclusively U-C σ-bonds.
computational
studies
Homoleptic Aryl Complexes of
Uranium (IV)
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