The Exceptional Diversity of Homoleptic Uranium–Methyl Complexes

Article abstract of DOI:10.1002/anie.202005138

Homoleptic σ-bonded uranium–alkyl complexes have been a synthetic target since the Manhattan Project. The current study describes the synthesis and characterization of several unprecedented uranium–methyl complexes. Amongst these complexes, the first example of a homoleptic uranium–alkyl dimer, [Li(THF)₄]₂[U₂(CH₃)₁₀], as well as a seven-coordinate uranium–methyl monomer, {Li(OEt₂)Li(OEt₂)₂UMe₇Li}_n were both crystallographically identified. The diversity of complexes reported herein provides critical insight into the structural diversity, electronic structure and bonding in uranium–alkyl chemistry.

Full text of DOI:10.1002/anie.202005138

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Accepted Article
Title: The Exceptional Diversity of Homoleptic Uranium-Methyl
Complexes
Authors: Jeffrey Sears, Dumitru-Claudiu Sergentu, Tessa Baker,
William Brennessel, Jochen Autschbach, and Michael Neidig
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The Exceptional Diversity of Homoleptic Uranium-Methyl
Complexes
Jeffrey D. Sears,[a] Dumitru-Claudiu Sergentu,[b] Tessa M. Baker,[a] William W. Brennessel,[a] Jochen
Autschbach,*[b] and Michael L. Neidig*[a]
Abstract: Homoleptic σ-bonded uranium-alkyl complexes have
been a synthetic target since the Manhattan Project. The current
study describes the synthesis and characterization of several
unprecedented uranium-methyl complexes. Amongst these
complexes, the first example of a homoleptic uranium-alkyl dimer,
[Li(THF)4]2[U2(CH3)10], as well as a seven-coordinate uranium-
methyl monomer, {Li(OEt2)Li(OEt2)2UMe7Li}n were both
crystallographically identified. The diversity of complexes
iron work, additives such as N-methyl-2-pyrrolidone (NMP) also
displayed a similar role as cation complexant.[15,16] Without NMP,
iron-alkyl clusters were observed, which displayed greatly
lessened stability.[15,17] This motivated exploration of reaction of
MeLi with UCl4 in the absence of TMEDA to evaluate uranium-
methyl speciation. Herein, a collection of homoleptic uranium-
methyl species are reported, the diversity of which demonstrates
unique coordination and bonding previously unknown in
reported herein provides critical insight into the structural diversity, organouranium chemistry.
electronic structure and bonding in uranium-alkyl chemistry.
Spanning the early investigations of uranium enrichment
technologies of the 1940s to the current interest of expanding the
fundamental understanding of organoactinide chemistry,
homoleptic actinide-alkyl and -aryl complexes have continued to
be highly valued targets as model complexes for electronic
structure and bonding studies.[1–3] Early success in the synthesis
of homoleptic transition-metal alkyl complexes for refining early
bonding models sparked interest in extending to the f-block. For
example, Wilkinson reported the synthesis of hexamethyltungsten
Figure 1. Selected examples of homoleptic uranium-alkyl complexes.
(WMe6) while also making note of
a
possible neutral
hexamethyluranium complex (UMe6), which was never
The reaction of 6 equiv of MeLi with UCl4 in THF at low
temperature was monitored by 1H NMR revealing resonances
consistent with [(Li(TMEDA))2U(CH3)6] (Figure S1 and S2).[12]
Crystallization attempts to identify these uranium-methyl species
were carried out by the addition of hexane following addition of
MeLi to UCl4 at -70 °C and storage of the resultant solution at -80
°C for 16 h. Light orange crystalline material formed and was
structurally verified.[4,5]
Akin to Wilkinson’s method, we extended our approach in the
synthesis of homoleptic iron-aryl species to the challenge of
uranium-aryls.[6] This resulted in the synthesis of a series of
homoleptic uranium-aryl complexes containing sterically
unencumbered aryl substituents (Ar = Ph, tolyl, and p-Cl-Ph).[7]
This represented the first successful synthesis of homoleptic
uranium-aryl complexes without the advantageous use of
sterically bulky aryl ligands for kinetic stabilization. Prior to this
report, homoleptic uranium-alkyl and -aryl complexes were made
isolable by introducing large steric bulk to the corresponding alkyl
or aryl substituents.[7–14]
identified
by
X-ray
diffraction
(XRD)
as
the
hexamethyluranium(IV) complex, [Li(THF)4][Li(THF)2U(CH3)6] (1),
in approximately 20% isolated yield. Complex 1 crystallizes in the
monoclinic space group P21/n (Figure 2) and is analogous to
[Li(TMEDA)]2[U(CH3)6].[12] Complex
1
exhibits
a
distorted
octahedral coordination environment reflected in the U-C bond
lengths and angles. The U-C distances of the terminal -CH3
groups range from 2.500(5) to 2.533(5) Å while the lithium
coordination to the methyl groups lengthens the corresponding U-
C distances (U1-C5 = 2.615(5) Å and U1-C6 = 2.591(6) Å). Two
examples of angular distortion due to Li coordination are the C4-
U-C5 = 84.65(18)° and C4-U-C6 = 171.66(19)°. The effect of Li
coordination is also reflected in the 1H NMR of 1 in THF-d8, which
displays two singlet resonances at -20.66 and -24.99 ppm with a
relative ratio of 14:4 consistent with approximate C2v symmetry
with the principal axis intersecting the U and Li centers. (Figure
S3). Both the U-C distances and 1H NMR resonances are
consistent with the previously reported [Li(TMEDA)]2[U(CH3)6]
(2.48(1)‒2.600(9) Å and -19.92 ppm, respectively).[12] Magnetic
susceptibility measurements using Evans method NMR analysis
determined complex 1 to have an effective magnetic moment (µeff)
of 2.1(1) B.M. at -80 °C. This is consistent with previously reported
values for homoleptic uranium(IV)-alkyl complexes.[12] The
synthesis of this monomeric species suggests that the addition of
TMEDA only provides enhanced kinetic stabilization and does not
drastically change the accessible uranium-methyl species.
Homoleptic complexes of uranium bearing simple hydrocarbon
substituents (i.e. Me, Et, etc.) have been extremely rare due to
their inherent thermal instability and reactivity. To date, the only
example
is
a
hexamethyluranium(IV)
complex,
[(Li(TMEDA))2U(CH3)6]
(N,N,N’,N’-tetramethylethylenediamine
(TMEDA)).[12] When compared with the homoleptic uranium-
benzyl derivatized complexes, the homoleptic uranium-methyl
complex does not benefit from additional stabilizing effects (i.e.
interaction with the ipso carbon of the benzyl substituents), other
than the use of the cation complexant TMEDA (Figure 1).[12] In our
[a]
J. D. Sears, Dr. Tessa M. Baker, 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)
E-mail: neidig@chem.rochester.edu
[b]
Dr. D.-C. Sergentu and Prof. J. Autschbach
Department of Chemistry
University at Buffalo, State University of New York
312 Natural Sciences Complex, Buffalo, NY 14260 (USA)
E-mail: jochena@buffalo.edu
Supporting information for this article is given via a link at the end
of the document.
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complexes and overall, complex 3 adopts a distorted octahedral
geometry.[13] The four equatorial U-C distances of 2.449(5) Å and
2.452(5) Å and the shorter axial U-C distances of 2.415(5) Å
reflect this symmetry. Due to the increased difficulty of obtaining
enough material for NMR analysis, the 10 K EPR spectrum of a
solid sample of 3 was instead obtained and displayed a
resonance at a g-value of 1.49 consistent with the U(V) oxidation
state (Figure S5).[20] We hypothesize that the formation of 3 under
these highly reducing conditions is consistent with
disproportionation of two U(IV) centers. However, a homolytic U-
CH3 cleavage and subsequent oxidation of a U(IV) center to yield
U(V) could not be ruled out.
Figure 2. X-ray crystal structure of [Li(THF)4][Li(THF)2U(CH3)6] (1) with
thermal ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg): U1-C1 = 2.533(5), U1-
C2 = 2.510(5), U1-C3 = 2.500(5), U1-C4 = 2.504(5), U1-C5 = 2.615(5), U1-
C6 = 2.591(6), U1…Li1 = 3.136(8), C3-U1-C4 = 96.27(19), C3-U1-C2 =
85.60(19), C3-U1-C5 = 178.5(2), C2-U1-C1 = 173.63(18), C1-U1-C6 =
89.9(2).
The accessibility of similar monomeric species in Et2O was
explored as challenges to identify organouranium products were
reported in this solvent as well.[12] Similar to our efforts in THF, 6
equiv of MeLi was added to UCl4 in Et2O at -70 °C. After stirring
for 10 min, pentane was added, and the mixture was filtered over
a cold Celite plug. In addition to the dark filtrate, unreacted UCl4
was observed in the filtrate. The resultant light-yellow solution was
stored at -80 °C. After several days the heptamethyluranium(IV)
complex, {Li(OEt2)Li(OEt2)2UMe7Li}n (2), formed as a pale-green
crystalline material (ca. trace) (Figure 3). Complex 2 crystallized
in the monoclinic space group, P21. Comparison of complex 2 to
a previously reported thorium analog shows a similar distribution
of An–CH3 (An = Th or U) bond lengths (2.519(6)–2.779(5) Å vs.
2.571(9)–2.765(9) Å, respectively).[18] The carbon atoms C1, C3,
C4, and C7 in complex 2 form a plane in which U1, C5 and C6
are positioned below and C2 above. Furthermore, unlike its
thorium analog, 2 oligomerizes in the solid state in which C7
coordinates to Li1 of another molecule of 2 in the extended
crystalline network (Figure S4). To the best of our knowledge,
Figure 3. X-ray crystal structure of {Li(OEt2)Li(OEt2)2UMe7Li}n (2) with thermal
ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): U1-C1 = 2.588(5), U1-C2
= 2.692(5), U1-C3 = 2.779(5), U1-C4 = 2.593(5), U1-C5 = 2.554(5), U1-C6 =
2.594(5), U1-C7 = 2.519(6), C7-U1-C5 = 116.0(2), C7-U1-C1 = 76.65(19), C1-
U1-C2 = 80.88(16), C1-U1-C4 = 144.96(15), C5-U1-C2 = 156.51(18). Note:
Li1 is coordinated to C7 of another molecule of 2 in the extended crystalline
network (Figure S4).
The modest change in reaction conditions which led to the
isolation of 2 and 3 motivated us to explore the crystallization
conditions previously utilized to isolate 1 to determine whether
other uranium-methyl species were accessible. To our surprise,
the modification of the crystallization conditions used for complex
1, by the significant decrease in the volume of hexane used as
co-solvent (1.0 mL vs. 2.5 mL), led to the formation of an
unexpected uranium-methyl dimer, [Li(THF)4]2[U2(CH3)10] (4) as
dark orange crystalline material in 20% isolated yield (Figure 5).
Complex 4 crystallized in the trigonal space group R-3c. The
asymmetric unit consists of one-sixth of the dinuclear uranium
dianion and one-third of a [Li(THF)4]+. The orientation of the
hydrogen atoms could not be determined from the difference
Fourier map, hence, no agostic interaction could be determined
from the solid-state structure. The dianion of 4 consists of two
uranium centers bound to seven and six methyl groups with
capped octahedral and octahedral geometries, respectively. The
asymmetric coordination environment of the dimer is reflected in
the difference between the terminal U–CH3 bonds of the two
uranium centers with the 7-coordinate U center displaying a
significantly shorter terminal U–CH3 bond length of 2.481(6) Å for
three of the four -CH3 groups compared to that of the 6-coordinate
U center of 2.524(5) Å. The unique methyl substituent, C4, has a
distinctly shorter bond length of 2.438(10) Å. This distance is
reminiscent of previously reported homoleptic U(V)-C bond
lengths.[13] The bridging U–CH3 bonds were expectedly the
longest 2.658(3) Å. The U–U separation is comparable to other
reported alkyl bridged complexes (3.6089(3) Å compared to the
complex
2 also represents an unprecedented coordination
environment for homoleptic U(IV) alkyls. Previous attempts to
generate such species proved unsuccessful and coordination of
six alkyl ligands was hypothesized to be an upper limit to the U(IV)
coordination number.[12] The formation of the 7-coordinate
complex 2 may result from the high effective concentration of
MeLi due to the inherent heterogeneity of the reaction in Et2O.
Unfortunately, due to low conversions from UCl4, sufficient
amounts of crystalline material were unable to be produced,
limiting any further characterization.
Due to the intractability of reactions in Et2O even with low
temperature synthetic techniques, the use of crown ethers as
complexants to facilitate the isolation of uranium-methyl species
was explored.[19] The common Li+ complexant, 12-crown-4 was
added to the reaction following addition of MeLi to UI4(1,4-
dioxane)2. Orange microcrystalline material not suitable for XRD
was obtained. Thus, 18-crown-6, was utilized instead. Upon
addition of a solution of 18-crown-6, a light-yellow precipitate
immediately formed. The reaction mixture was filtered, and the
filtrate was re-dissolved in THF. After storage at -80 °C for a week,
a dark red crystalline material formed which was identified as the
hexamethyluranium(V) complex, [Li(18-crown-6)(THF)2][UMe6]
(3) (<10% yield, Figure 4).
closest of 3.616 and 3.629
Å
of previously reported
complexes).[21,22] The 1H NMR of complex 4 in methylenechloride-
d2 is associated with two broad singlet resonances centered at
4.29 and 1.90 ppm, consistent with the two distinct chemical
Complex 3 crystallizes in the triclinic space group P-1, and the
Li atom disorder is modeled over three positions. The U-C
distances are consistent with other reported homoleptic U(V)-alkyl
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Figure 5. X-ray crystal structure of the dianion [Li(THF)4]2[U2(CH3)10] (4) with
thermal ellipsoids drawn at 50% probability level. Hydrogen atoms and
[Li(THF)4]+ cations are omitted for clarity. Selected bond lengths (Å) and
angles (deg): U1-C1 = 2.658(3), U1-C2 = 2.481(6), U1-C3A = 2.524(5), U1-
C4 = 2.438(10), U1…U1A = 3.6089(3), C4-U1-C2 = 81.78(17), C4-U1-C1 =
132.75(6), C2-U1-C1 = 76.27(16), C3B-U1-C3A = 91.5(2), C2-U1-C1B =
145.39(18).
Figure 4. X-ray crystal structure of [Li(18-crown-6)(THF)2][UMe6] (3) with
thermal ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg): U1-C1 = 2.449(5), U1-
C2 = 2.452(5), U1-C3 = 2.415(5), C1-U1-C1A = 180.0, C1-U1-C2 = 87.36(17),
C1A-U1-C2 = 92.64(17), C3-U1-C1 = 90.2(2), C3-U1-C2A = 88.08(17).
17 to 18%. The U center involves in these bonds primarily with
the 5f orbitals (50%) and secondarily with the 6d orbitals (34%).
The U-C(alkyl) bonding of 1 is therefore very similar to the recently
reported U-C(aryl) bonding (%U, %5f, %6d = 20, 47, 43) in the near-
octahedral [U(p-Cl-Ph)6]2- homoleptic complex.[7] This similarity
was expected since 1 and [U(p-Cl-Ph)6]2- also share similar U-C
average bond distances (2.54 vs 2.53 Å). The molecular
backbone of the dianion 4 obeys C3v symmetry with the three-fold
rotation axis defined by collinear U centers and a methyl C atom.
The metal-ligand bonding in 4 is described by seven 2-center-2-
electron s(U-C) bonds involving the peripheral methyl C atoms,
and three 3-center-2-electron s bonds involving the bridging
methyl C atoms. In comparison to 1, the 2-center bonds of 4 show
enhanced covalency, i.e. the corresponding NLMOs have
between 20 and 25% metal contribution. The s(U-C) bond along
the C3 axis has almost even contribution from the U 5f (44%) and
6d (43%) orbitals. The 2-center s(U-C) bonds have larger
contribution from the metal 6d (between 45 and 55%) than from
the metal 5f (31%) orbitals. The 3-center NLMOs are delocalized
over the two metal centers, with a combined 28% U contribution,
and serve as linkers for the U(CH3)4 and U(CH3)3 molecular units.
The U 5f and 6d orbital contributions in these 3-center orbitals are
quite even. The 2-center U-C(alkyl) bonding in 4 is similar to the
recently reported U-C(aryl) bonding (%U=22-23%, %5f = 38-40%,
%6d = 53-55%) in the [U(Ph)6]2- and [U(p-tolyl)6]2- homoleptic
complexes. Again, this similarity was expected as the dianion of
4 and the aryl complexes share U(IV) metal centers with similarly
distorted ligand coordination and similar average metal-ligand
bond distances (2.50 Å U-C(alkyl) in 4 vs. 2.52-2.53 Å U-C(aryl) in the
homoleptic aryl complexes)
environments in the solid state (bridging and terminal -CH3,
respectively). Magnetic susceptibility measurements using Evans
method determined 4 to have an effective magnetic moment of
3.9(4) B.M at -80 °C.
To our knowledge, complex 4 represents the first example of a
homoleptic uranium-alkyl multimeric species. The ability to
access multimeric uranium-alkyl species in the absence of the
cation complexant TMEDA is highly consistent with the removal
of cation-coordinating NMP from reactions of simple iron salts and
Grignard reagents in which iron-alkyl clusters were observed.[15–
17]
This result has led us to extend our working hypothesis that
cation effects in homoleptic chemistry drastically shift metal
speciation and nuclearity in d- and f-block metals. Similar to the
homoleptic iron-alkyls we have previously characterized, the
thermal stability of complexes 1‒4 was extremely limited. As
crystalline solids, all species are stable at -80 °C for at least
several weeks, but readily decompose at elevated temperatures
(-30 °C) within minutes. While the in situ thermal stability was
found to be better for complexes 1 and 4, a tetrauranium-oxo
cluster which incorporates ring-opened THF was obtained
following prolonged stirring at elevated temperatures (see
SI).[23,24] This uranium-methyl cluster was assumed to form
similarly to previous reports of low-valent uranium reductive
cleaving of C-O bonds in ethereal solvents.[25]
The analogous metal oxidation states in complexes 1 and 4 but
distinct differences in nuclearity and coordination environments,
led us to pursue computational studies in order to obtain
additional insight into their electronic structures and metal-ligand
bonding (see SI for computational details). Complete active space
The identification of complexes 1‒4 addresses the challenge of
synthesizing and handling highly unstable homoleptic uranium
alkyl complexes, which has plagued the f-element community
self-consistent field (CAS) calculations, augmented with
a
dynamic correlation treatment via second order perturbation
theory (PT2), and treatment of the spin-orbit coupling (SOC) via
state interaction, revealed complex, orbitally (near-)degenerate
low-energy electronic structures for both 1 and 4, as detailed in
Section 3 of the SI.
Despite the multiconfigurational GSs, geometrical parameters
determined using density functional theory (DFT) were found to
be in good agreement with the solid-state bond metrics, for both
dianions of 1 and 4 (Figure S10). Furthermore, natural localized
molecular orbital (NLMO) bonding analyses of the optimized and
crystal structures displayed similar metal-ligand bonding. For the
crystal structure, the NLMO bonding information is shown in
Figure 6. The backbone of dianion 1 is maintained by six similar
2-center-2-electron s(U-C) bonds with metal weights ranging from
Figure 6. U-C bonding NLMOs (isosurface values of ±0.03 a.u.), with their
respective % metal character and 6d/5f orbital contributions, obtained for the
X-ray crystal structures of the [U2(CH3)6]2- (1) and [U2(CH3)10 2- (4) dianions.
]
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COMMUNICATION
Int. Ed. 2018, 57, 6496–6500.
since the Manhattan Project. As described herein, by utilizing low-
temperature synthesis and cryogenic handling techniques, novel
organouranium compounds are isolable. In particular, complex 4
represents an unprecedented uranium-alkyl species which
changes the paradigm of homoleptic uranium-alkyl chemistry to
now include multimeric uranium-alkyl complexes. Additionally,
theoretical insight into electronic structure and bonding for
complexes 1 and 4 demonstrates the distinct difference in 6d and
5f atomic orbital participation in distorted ligand coordination
environments for homoleptic uranium-alkyls. Altogether, this
report is indicative that after 70 years of research interest, we are
still only at the tip of the iceberg of uranium-alkyl chemistry.
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Acknowledgements
M.L.N. acknowledges support for this work from the U.S.
Department of Energy, Office of Science, Early Career Research
Program under Award DE-SC0016002. D.C.S. and J.A.
acknowledge support from the U.S. Department of Energy, Office
of Basic Energy Sciences, Heavy Element Chemistry program,
under grant DE-SC0001136. We thank the Center for
Computational Research (CCR) at the University of Buffalo for
providing computational resources and the NSF for support for
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contains
the
supplementary crystallographic data for this paper. These data
are provided free of charge by the joint Cambridge
Crystallographic Data Centre and Fachinformationszentrum
Karlsruhe
Access
Structures
service
www.ccdc.cam.ac.uk/structures.
Keywords: uranium • organometallic • homoleptic • synthesis
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COMMUNICATION
Herein, a diverse group of homoleptic uranium-methyl complexes is presented. The
addition of MeLi to UCl4 at low temperature yields an unprecedented homoleptic uranium-
methyl dimer. Overall this complex as well as several other monomeric uranium-alkyl
complexes demonstrate the rich coordination and electronic structure and bonding of
homoleptic uranium-alkyl complexes with sterically unencumbered alkyl substituents such
as -CH3.
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10.1002/anie.202005138
Angewandte Chemie International Edition
COMMUNICATION
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