A Homoleptic Uranium(III) Tris(aryl) Complex

Article abstract of DOI:10.1021/jacs.6b11182

The reaction of 3 equiv of Li-C₆H₃-2,6-(C₆H₄-4-^tBu)₂ (Terph-Li) with UI₃(1,4-dioxane)_1.5 led to the formation of the homoleptic uranium(III) tris(aryl) complex (Terph)₃U (1). The U-C bonds are reactive: treatment with excess ⁱPrNCNⁱPr yielded the double-insertion product [TerphC(NⁱPr)₂]₂U(Terph) (2). Complexes 1 and 2 were characterized by X-ray crystallography, which showed that the U-C bond length in 2 (2.624(4) ?) is ～0.1 ? longer than the average U-C bond length in 1 (2.522(2) ?). Thermal decomposition of 1 yielded Terph-H as the only identifiable product; the process is unimolecular with activation parameters ΔH^? = 21.5 ± 0.3 kcal/mol and ΔS^? = ?7.5 ± 0.8 cal·mol^-1 K^-1, consistent with intramolecular proton abstraction. The protonolysis chemistry of 1 was also explored, which led to the uranium(IV) alkoxide complex U(OCPh₃)₄(DME) (3·DME).

Full text of DOI:10.1021/jacs.6b11182

Communication
pubs.acs.org/JACS
A Homoleptic Uranium(III) Tris(aryl) Complex
Michael A. Boreen, Bernard F. Parker, Trevor D. Lohrey, and John Arnold*
Department of Chemistry, University of California, Berkeley, and Chemical Sciences Division, Lawrence Berkeley Laboratory,
Berkeley, California 94720, United States
S
* Supporting Information
would leave the ipso carbon of the central aryl ring accessible
ABSTRACT: The reaction of 3 equiv of Li−C6H3-2,6-
(C6H4-4-tBu)2 (Terph−Li) with UI3(1,4-dioxane)1.5 led to
the formation of the homoleptic uranium(III) tris(aryl)
complex (Terph)3U (1). The U−C bonds are reactive:
treatment with excess iPrNCNiPr yielded the double-
insertion product [TerphC(NiPr)2]2U(Terph) (2). Com-
plexes 1 and 2 were characterized by X-ray crystallography,
which showed that the U−C bond length in 2 (2.624(4)
Å) is ∼0.1 Å longer than the average U−C bond length in
1 (2.522(2) Å). Thermal decomposition of 1 yielded
Terph−H as the only identifiable product; the process is
unimolecular with activation parameters ΔH⧧ = 21.5 0.3
kcal/mol and ΔS⧧ = −7.5 0.8 cal·mol−1 K−1, consistent
with intramolecular proton abstraction. The protonolysis
chemistry of 1 was also explored, which led to the
uranium(IV) alkoxide complex U(OCPh3)4(DME) (3·
DME).
for rapid metalation.
14
Terph−Li reacted quickly with UI3(1,4-dioxane)1.5 in
diethyl ether at room temperature to form the homoleptic
uranium(III) aryl complex (Terph)3U (1), as shown in Scheme
1. While initially appearing to form black crystals, thin plates of
Scheme 1. Synthesis of (Terph)3U (1)
1 appear yellow when viewed under a microscope. Similarly,
solutions of 1 are dark yellow, and the UV−vis spectrum of 1
reveals broad absorption features throughout the visible range
(Figure S1). Complex 1 can be synthesized in gram quantities
and 74% yield with a reaction time of 10 min.
As expected, 1 is extremely air- and moisture-sensitive: solid
samples decomposed exothermically when exposed to air. It is
soluble in diethyl ether, hydrocarbon solvents, and hexame-
thyldisiloxane (HMDSO), although slow decomposition was
observed in all solutions of 1 at room temperature. As
monitored by 1H NMR spectroscopy in C6D6, t1/2 = 10.3 0.2
h at 23 °C. Solid samples of complex 1 decomposed nearly to
completion within 3 weeks at room temperature. Solid samples
of 1 can be stored at −40 °C for over a month with no sign of
decomposition.
In both solution and the solid state, the major product of
decomposition was identified as Terph−H,15 with other
paramagnetic and diamagnetic products present in small
quantities. Samples of 1 that were allowed to decompose to
completion in C6D6 showed no sign of deuterium incorpo-
ration into soluble products on the basis of 2H NMR
spectroscopy.
omoleptic σ-bonded alkyl and aryl complexes of uranium
H
have been synthetic targets since the 1940s, originally as
potential volatile species for uranium isotope separation during
the Manhattan Project.1,2 Despite continuing interest in the
nature of U−C σ bonds, homoleptic uranium alkyl complexes
are quite rare,2−7 as most examples are thermally unstable at or
below room temperature. A major breakthrough came in 1989,
when Sattelberger and co-workers reported the first homoleptic
uranium(III) alkyl complex, U[CH(SiMe3)2]3.7 More recently,
Hayton and co-workers have stabilized actinide−aryl bonds
using an aryl ligand with a pendant amine moiety.8,9 Hayton
and co-workers also provided the first structural character-
ization of a thorium(IV) aryl-ate complex, [Th(C6H5)6]2−.10
There is only a single previous report of related uranium aryl
complexes (Li2UR6·(Et2O)8; R = Ph, Me2NCH2C6H4) that
lacks structural details.2 Additionally, little is known about the
reactivity of the An−C bond in these species. Here we report
the synthesis and structural characterization of the first example
of a homoleptic uranium(III) aryl complex. We also describe
preliminary reactivity studies with this complex.
With the intention of synthesizing stable uranium aryl
complexes, we turned to m-terphenyl ligands, which have been
exploited by Power and co-workers to prepare reactive, low-
valent, low-coordinate metal complexes.11−13 Specifically, we
To elucidate the mechanism by which 1 decomposed, the
1
decomposition reaction was monitored by H NMR spectros-
copy at several different temperatures between 23 and 80 °C.
Within this range, decomposition was first order in 1, and a
graph of ln(k/T) versus 1/T was found to be linear. Eyring
focused on the C6H3-2,6-(C6H4-4-tBu)2 (Terph−) group, as
−
we hypothesized that the bulky substituent at the para position
of the flanking aryl groups would prevent aggregation and
confer adequate steric protection to the uranium center, while
the absence of ortho substituents on the flanking aryl groups
Received: October 26, 2016
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
analysis afforded ΔH⧧ = 21.5 0.3 kcal/mol and ΔS⧧ = −7.5
0.8 cal·mol−1 K−1 (Figure S2). The negative value of ΔS⧧, as
well as the lack of deuterium incorporation in Terph−H
formed by decomposition, provides evidence against a
mechanism involving homolytic cleavage of a U−C bond.
Therefore, we propose that decomposition occurs by proton
abstraction from an ortho carbon on a flanking C6H4-4-tBu
group by a neighboring terphenyl ligand. The proposed
transition state for this process is shown in Scheme 2. This
Å and are similar to the U−C(alkyl) bond length of 2.48(2) Å
7
in U[CH(SiMe3)2]3 and the U−C(phenyl) bond length of
2.513(2) Å in (C5Me5)2(hpp)UPh (hpp− = 1,3,4,6,7,8-
hexahydro-2H-pyrimido[1,2-a]pyrimidinato).18 There is a
single close contact in 1 between the uranium center and an
ortho carbon on a flanking aryl ring [U(1)−C(8) = 2.901(3)
Å]. The uranium center is 0.442(2) Å out of the plane of the
coordinating aryl carbon atoms. Deviation from planarity in
three-coordinate uranium(III) complexes has been well-
reported;7,19−22 it can be explained by the polarized ion
model,19 which states that a more pyramidal geometry results in
a more favorable charge−dipole interaction between the metal
and the ligands.
Scheme 2. Proposed Mechanism for Decomposition of
(Terph)3U
1
The H NMR spectrum of 1 displays only five resonances,
indicating that all three ligands are equivalent on the NMR time
t
scale (Figure S4). While the Bu and central aryl ring protons
were assigned solely on the basis of integrations, the two
resonances from the flanking aryl rings at −6.51 and −14.56
ppm correspond to equivalent numbers of protons. On the
basis of the solid-state structure, the protons bonded to the
carbon atoms ortho to the central aryl ring are closer in space
on average to the uranium center. Therefore, we tentatively
assigned these protons to the broadest, most upfield
resonance.16 Additional support for this assignment was
provided by a 1D selective 1H NOESY experiment (Figure S9).
Variable-temperature 1H NMR experiments on 1 showed no
splitting of the five resonances down to −70 °C (Figure S7),
implying that any U(1)−C(8) interaction is weak. Plots of the
1H chemical shifts versus 1/T are linear down to −70 °C for all
five resonances (Figure S8), as expected for a paramagnetic
complex following Curie−Weiss behavior.23 The room-temper-
ature μeff value of 1 was determined by the Evans NMR method
to be 3.6(1)μB, which is toward the higher end of the reported
range for uranium(III) complexes.24
mechanism is similar to the mechanism proposed for the
decomposition of Cp3UR (R = alkyl, vinyl, aryl), in which a
proton is abstracted from a cyclopentadienyl ligand by the R
group.16 Additionally, two ortho C−H bonds of the related
terphenyl ligand C6H3-2,6-(C6H4-4-Me)2 were found to
undergo σ-bond metathesis with methyl substituents in a
tantalum complex to form a κ3-CCC doubly cyclometalated
complex.17
Despite the thermal instability of complex 1, pure samples
were obtained by recrystallization from diethyl ether (Figure
S4). Additionally, X-ray-quality crystals of 1 were formed by
cooling a hexane/HMDSO solution of 1 to −40 °C. The solid-
state structure of 1 is shown in Figure 1. The uranium center
exhibits a three-coordinate trigonal-pyramidal geometry. The
U−C(aryl) bond lengths in 1 range from 2.497(3) to 2.544(3)
With complex 1 successfully isolated and characterized, we
sought to explore its reactivity. On the basis of previous reports
by Evans and co-workers of carbodiimide insertion into
U(IV)−C(phenyl) bonds,25,26 we treated 1 with an excess of
iPrNCNiPr. A rapid reaction ensued, resulting in a green
solution containing [TerphC(NiPr)2]2U(Terph) (2) as the
only product by 1H NMR spectroscopy. Complex 2 is sparingly
soluble in benzene at room temperature but insoluble in other
hydrocarbon solvents, diethyl ether, THF, and pyridine.
Despite the low solubility of 2 in benzene, it was slow to
crystallize out of solution when formed by the reaction of 1
i
1
with PrNCNiPr in benzene. Thus, H NMR spectra of 2
with sufficient concentration were obtained using crude
reaction mixtures (Figure S5).
Complex 2 was also isolated through a one-pot synthesis by
the in situ reaction of 1 with PrNCNiPr in benzene
i
(Scheme 3). Dark-green X-ray-quality crystals of 2 were formed
in 42% yield after 2 weeks. The structure of 2 was determined
by X-ray crystallography, which confirmed that diisopropylcar-
bodiimide inserted into only two of the three U−C bonds in 1
to give the uranium(III) bis(amidinate) aryl complex 2 (Figure
2). Complex 2 has a τ5 value of 0.51, indicating a geometry
halfway between ideal square-pyramidal (τ5 = 0) and trigonal-
bipyramidal (τ5 = 1).27 The U−C(aryl) bond length, 2.624(4)
Å, is considerably longer than those found in complex 1, likely
because of the higher coordination number and increased steric
pressure imposed by the amidinate ligands.
Figure 1. X-ray crystal structure of 1 with 50% probability ellipsoids. H
atoms have been omitted for clarity. Selected bond distances (Å) and
angles (deg): U(1)−C(1) 2.544(3), U(1)−C(27) 2.497(3), U(1)−
C(53) 2.524(3), C(1)−U(1)−C(27) 110.96(9), C(1)−U(1)−C(53)
120.07(9), C(27)−U(1)−C(53) 120.19(9).
In contrast to 1, 2 displayed high thermal stability: solid
samples of 2 under N2 began to decompose only when heated
B
DOI: 10.1021/jacs.6b11182
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Journal of the American Chemical Society
Communication
Scheme 3. One-Pot Synthesis of [TerphC(NiPr)2]2U(Terph)
(2)
U(IV) through mechanisms proposed to involve disproportio-
nation of U(III) intermediates.32
We reasoned that use of a less bulky alcohol or amine would
lead to the corresponding U(IV) tetrakis(alkoxide) or tetrakis-
(amide) complexes as the major products. When 3 equiv of
Ph3COH were added to a toluene solution of 1 at room
temperature, a green-brown solid rapidly precipitated. After
1
recrystallization from DME, H NMR and elemental analysis
data indicated that the light-blue product was the new
uranium(IV) tetrakis(alkoxide) complex U(OCPh3)4(DME)
(3·DME) (Figure S6). The identity of this product was
confirmed by X-ray crystallography (Figure S12). When 1 was
treated with excess HNPh2 in C6D6, 71% conversion to the
33
uranium(IV) tetrakis(amide) complex U(NPh2)4 was ob-
1
served by H NMR spectroscopy (Figure S11).
In conclusion, we have isolated the homoleptic uranium(III)
tris(aryl) complex (Terph)3U (1) in high yield. Its X-ray crystal
structure represents the first full structural characterization of
such a species. Decomposition studies indicated that intra-
molecular proton abstraction is responsible for its thermal
i
instability. In the presence of excess PrNCNiPr, it is
rapidly and cleanly converted to the double-insertion product
[TerphC(NiPr)2]2U(Terph) (2), which, in contrast to 1, is
thermally stable. The U−C bonds are also prone to
protonolysis, yielding uranium(IV) tetrakis(alkoxide) or
tetrakis(amide) products. Use of a bulkier alcohol, HODtbp,
suppressed oxidation of uranium, leading to the uranium(III)
tris(aryloxide) as the major product. These results indicate that
the use of bulky terphenyl ligands in actinide chemistry is a
fruitful endeavor that we aim to develop further in ongoing
studies.
Figure 2. X-ray crystal structure of 2 with 50% probability ellipsoids. H
atoms and Bu groups have been omitted for clarity. Selected bond
distances (Å) and angles (deg): U(1)−C(1) 2.624(4), U(1)−N(1)
2.457(3), U(1)−N(2) 2.534(3), U(1)−N(3) 2.533(3), U(1)−N(4)
2.467(3), N(1)−U(1)−N(2) 53.5(1), N(3)−U(1)−N(4) 52.7(1).
t
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b11182.
■
S
above 211 °C. Traces of amidine28 and arene were detected
when a solution of 2 in C6D6 was heated at 100 °C in a sealed
NMR tube for 16 h. The thermal stability of 2 supports
previous experimental work showing that U−C(aryl) bonds are
not in general expected to be thermodynamically unstable.29
Blocking kinetic pathways to decomposition with bulky ligands
can thus lead to stable U−C(aryl) bonds.
Crystallographic data for 1·1.5C6H14 (CIF)
Crystallographic data for 2·0.5C6H6 (CIF)
Crystallographic data for 3·DME (CIF)
Experimental procedures, UV−vis spectrum of 1, Eyring
plot for decomposition of 1, NMR data, and crystallo-
graphic data (PDF)
The protonolysis chemistry of 1 was also explored (Scheme
4). When 3 equiv of HO-2,6-tBu2C6H3 (HODtbp) was allowed
a
AUTHOR INFORMATION
Corresponding Author
*arnold@berkeley.edu
Scheme 4. Protonolysis Reactivity of (Terph)3U (1)
■
ORCID
John Arnold: 0000-0001-9671-227X
Notes
The authors declare no competing financial interest.
a
The isolated yield is shown for 3·DME. The percent conversion by
1H NMR spectroscopy is shown for the other substrates.
ACKNOWLEDGMENTS
■
This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences Heavy Element Chemistry
Program of the U.S. Department of Energy (DOE) at LBNL
under Contract DE-AC02-05CH11231. The Advanced Light
Source is supported by the Director, Office of Science, Office of
Basic Energy Sciences, of the U.S. Department of Energy under
Contract DE-AC02-05CH11231. The authors acknowledge Dr.
Simon Teat of ALS station 11.3.1 for his assistance with
to react with 1, the uranium(III) tris(aryloxide) complex
U(ODtbp)3 was observed as the major product (51%
conversion) by H NMR spectroscopy. However, traces of
the uranium(IV) tetrakis(aryloxide) complex U(ODtbp)4 and
the inverse sandwich complex [(DtbpO)2U]2(μ-η6,η6-C6D6)
were also observed in the H NMR spectrum (Figure S10).
Alcohols have previously been observed to oxidize U(III) to
30
1
31
1
C
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crystallographic data collection for 3·DME. M.A.B. thanks UC
Berkeley for a graduate research fellowship.
Barnes, C. L., Walensky, J. R. (2016). Chemical Communica-
tions. http://doi.org/10.1039/C6CC08105C.
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NOTE ADDED IN PROOF
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A new U(III) tris(alkyl) complex was just reported: Behrle, A.
C., Myers, A. J., Rungthanaphatsophon, P., Lukens, W. W.,
D
DOI: 10.1021/jacs.6b11182
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