Mechanistic Insights into Concerted C-C Reductive Elimination from Homoleptic Uranium Alkyls

Article abstract of DOI:10.1021/acs.organomet.7b00438

A mechanistic study was carried out to probe concerted C-C reductive elimination from homoleptic uranium(IV) alkyls. The para-chloro uranium(IV) tetrabenzyl derivative, U(CH₂-p-ClC₆H₄)₄ (2-p-Cl), was synthesized by treating UCl₄ with 4 equivalents of KCH₂-p-Cl-Ph (1-p-Cl) at 108 °C, adding a new member to the previously reported family of uranium alkyl complexes U(CH₂C₆H₅)₄ (2-Bn), U(CH₂-p-ⁱPrC₆H₄)₄ (2-p-ⁱPr), U(CH₂-p^tBu-C₆H₄)₄ (2-p-^tBu), U(CH₂-o-OMeC₆H₄)₄ (2-o-OMe), and U(CH₂-m-OMeC₆H₄)₄ (2-m-OMe). Each member of this family readily reacts with the redox-active α-diimine ligand, ^MesDAB^Me (^MesDAB^Me = [MesN = C(Me)C(Me) = NMes]; Mes = 2,4,6-trimethylphenyl), to afford the products from C-C reductive elimination, namely, (^MesDAB^Me)U(CH₂Ph′)₂ and Ph′CH₂CH₂Ph′ (Ph′ = p-ⁱPrC₆H₄, p-^tBuC₆H₄, m-OMeC₆H₄, p-ClC₆H₄). Room-temperature magnetic-susceptibility values, obtained via SQUID magnetometry, show a correlation with an increase in the magnetic moment as the electron-withdrawing character of the substituent increases. Kinetic studies were used to assess the effect of the benzyl substituent on the rate of reductive elimination, showing that reaction rate increases as the electron-withdrawing nature of the substitution increases. Eyring data revealed a large and negative entropy value, indicative of a highly ordered transition state, consistent with the previously reported concerted elimination concluded from crossover experiments.

Full text of DOI:10.1021/acs.organomet.7b00438

Article
pubs.acs.org/Organometallics
Mechanistic Insights into Concerted C−C Reductive Elimination from
Homoleptic Uranium Alkyls
Sara A. Johnson, Robert F. Higgins, Mahdi M. Abu-Omar, Matthew P. Shores,‡
†
‡
†
,†
and Suzanne C. Bart*
†H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47906, United States
‡Department of Chemistry, Colorado State University, Ft. Collins, Colorado 80523, United States
*
S Supporting Information
ABSTRACT: A mechanistic study was carried out to probe concerted
C−C reductive elimination from homoleptic uranium(IV) alkyls. The
para-chloro uranium(IV) tetrabenzyl derivative, U(CH −p-ClC H ) (2-
2
6
4 4
p-Cl), was synthesized by treating UCl with 4 equivalents of KCH −p-
4
2
Cl-Ph (1-p-Cl) at −108 °C, adding a new member to the previously
reported family of uranium alkyl complexes U(CH C H ) (2-Bn),
2
6
5 4
i
i
t
t
U(CH −p- PrC H ) (2-p- Pr), U(CH −p Bu-C H ) (2-p- Bu), U-
2
6
4 4
2
6
4 4
(
CH −o-OMeC H ) (2-o-OMe), and U(CH −m-OMeC H ) (2-m-
2
6
4 4
2
6
4 4
OMe). Each member of this family readily reacts with the redox-active α-
Mes
Me
Mes
Me
diimine ligand,
DAB
(
DAB
= [MesNC(Me)C(Me)
NMes]; Mes = 2,4,6-trimethylphenyl), to afford the products from C−
Mes
Me
i
t
C reductive elimination, namely, ( DAB )U(CH Ph′) and Ph′CH CH Ph′ (Ph′ = p- PrC H , p- BuC H , m-OMeC H , p-
2
2
2
2
6
4
6
4
6
4
ClC H ). Room-temperature magnetic-susceptibility values, obtained via SQUID magnetometry, show a correlation with an
6
4
increase in the magnetic moment as the electron-withdrawing character of the substituent increases. Kinetic studies were used to
assess the effect of the benzyl substituent on the rate of reductive elimination, showing that reaction rate increases as the electron-
withdrawing nature of the substitution increases. Eyring data revealed a large and negative entropy value, indicative of a highly
ordered transition state, consistent with the previously reported concerted elimination concluded from crossover experiments.
INTRODUCTION
■
As a key fundamental organometallic reaction, carbon−carbon
reductive elimination has been studied widely for its synthetic
applications and mechanistic details.
1
−10
This process is well
understood for transition metals, especially late precious metals,
but relatively little is known about such an important bond-
forming step for the actinide elements. This is in part due to
(
1) the multielectron step requirement for reductive elimi-
nation, which is not characteristic of the actinide elements and
2) the paucity of organoactinide compounds with two or more
labile alkyls as compared to transition-metal organometallic
for both resonance structures of the ligand, but the oxidation
12
state is noted by color in the figures). In this case, the
uranium(IV) oxidation state was maintained, relegating the
redox chemistry to the ligand rather than uranium.
Furthermore, the organic product from C−C coupling,
bibenzyl, was also observed spectroscopically.
(
1
1
complexes.
In 2012, we reported the isolation of the first neutral,
homoleptic uranium(IV) alkyl, tetrabenzyluranium. While its
group IV (Ti, Zr, Hf)
been known for decades, the uranium analogue had remained
12
13−15
16
To complement this study, mechanistic experiments were
performed to determine if the reductive elimination proceeded
by a concerted (two-electron) or radical (one-electron)
pathway. Crossover experiments using 2-Bn and U(CD C D )
and thorium counterparts had
elusive. The generation of U(CH Ph) (2-Bn) was significant,
2
4
as it provided a platform to study carbon−carbon bond
formation via reductive elimination at uranium(IV). Such a
process would normally lead to an unstable uranium(II)
bis(benzyl) product, but utilization of a redox-active α-diimine
ligand circumvented this problem by storing two electrons
gained from the reductive elimination reaction (eq 1). Thus,
2
6
5 4
(
2-Bn-d ) confirmed the concerted nature of the reductive
28
12
elimination, which was in sharp contrast to analogous studies
with Zr(CH Ph) and Hf(CH Ph) .
17−19
For these transition
2
4
2
4
metals, radical chemistry was noted, as benzyl migration to one
Mes
Me
of the imine carbons of the DAB was observed during the
Mes
Me
the organometallic product, ( DAB )U(CH C H ) (3-Bn)
2
6
5 2
featured a ligand reduced by two electrons, now in the
dianionic ene-diamide form (note: the label DAB is used
Received: June 12, 2017
Mes
Me
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00438
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
course of the reaction. Following this, reductive elimination
proceeds, with subsequent benzyl migration back to the metal.
Observation of the concerted reductive elimination process
was surprising, given that neutral uranium tetrakis(alkyl)s are
known to decompose via radical pathways in the absence of
2
0,21
ancillary ligands.
Subsequent to these initial results, we
found that inducing reductive elimination from 2-Bn with an
dipp
dipp
iminoquinone ligand ( iq) ( iq = 4,6-di-tert-butyl-2-[(2,6-
Mes
Me
diisopropylphenyl)-iminoquinone]), rather than
DAB ,
is best stored cold (−35 °C) in the solid state. Analysis of 2-p-
resulted in a radical process, again confirmed by crossover
1
Cl by H NMR spectroscopy (C D , Figure S23) shows a
22
6
6
experiments with 2-Bn-d . The dichotomy in reaction
2
8
resonance for the methylene protons at −72.70 ppm, which is
Mes
Me
mechanism as compared to
DAB is attributed to the
i
t
upfield compared to those for 2-p- Pr and 2-p- Bu, which are
31.09 to −32.07 ppm, respectively. Such a drastic shift points
electrochemical properties of the ligands, as the iminoquinone
shows a reversible one-electron wave by cyclic voltammetry,
−
toward a distinct change in the electronics of the methylene
group, which is expected given the relative electron-with-
drawing nature of the p-chloro group as compared to the
electron-donating alkyl substituents.
Mes
Me
22
whereas DAB does not.
We established, building on these preliminary results, a
general synthetic method based on that for 2-Bn, which
facilitated the synthesis of a substituted uranium tetrabenzyl
Magnetic Properties of Uranium Compounds. Given
the different electronic properties in this family of homoleptic
uranium tetrabenzyl compounds, variable-temperature mag-
i
i
t
species, including 2-Bn′ (Bn′ = p-PrBn (2-p- Pr), p- BuBn (2-
t
23
p- Bu), m-OMeBn (2-m-OMe), o-OMeBn (2-o-OMe)).
These species not only add to the small library of known
i
netic-susceptibility measurements were performed for 2-p- Pr,
1
2,24−26
homoleptic uranium alkyls
but also enable further
t
2
-p- Bu, 2-o-OMe, 2-p-Cl, and 2-m-OMe to determine the
mechanistic experiments to understand concerted C−C
reductive elimination from actinides. Herein, we describe our
studies on the influence of electronic effects of benzyl
substitution on concerted carbon−carbon reductive elimination
effect, if any, of the electron-donating nature of the benzyl
substituent on the magnetic properties. Unfortunately, the
parent compound, U(CH Ph) , decomposed too rapidly to
2
4
include in this study. The measured temperature dependencies
Mes
Me
from uranium(IV) facilitated by
DAB . We combine
of the room-temperature magnetic-susceptibility (χ T) prod-
M
spectroscopic, magnetic, and kinetic experiments to examine
how this important organometallic transformation proceeds for
an f-block element.
i
t
ucts for 2-p- Pr, 2-p- Bu, 2-o-OMe, 2-p-Cl, and 2-m-OMe
compounds are shown in Figure 1 (data for individual
RESULTS AND DISCUSSION
Synthesis and Characterization of Uranium Com-
pounds. During consideration of the previously synthesized
■
i
i
tetrabenzyl−uranium family, [U(p- PrBn) ] (2-p- Pr), [U-
4
t
t
(
p- BuBn) ] (2-p- Bu), and [U(m-OMeBn) ] (2-m-OMe)
4
4
were identified as good candidates to complete our mechanistic
study, as they were analogous in coordination number and
geometry to unsubstituted 2-Bn. To expand this family, [U(p-
ClBn) ] (2-p-Cl) was targeted for the electron-withdrawing
4
nature of the p-chloro substituent. First, the necessary
organopotassium reagent, Kp-ClBn (1-p-Cl), was synthesized
23,27
via a modified procedure utilizing Schlosser’s base
by
treating a thawing n-BuLi/hexane mixture with a thawing slurry
of KO Bu in excess p-chlorotoluene. After 5 h of stirring,
Figure 1. Temperature dependence of magnetic susceptibilities for 2-
t
i
t
i
p- Pr, 2-p- Bu, 2-o-OMe, 2-p-Cl, and 2-m-OMe. Data for 2-p- Pr, 2-o-
OMe, and 2-m-OMe were collected at an applied dc field of 5000 Oe,
filtration and workup afforded a light-brown solid. Character-
1
t
ization by H NMR spectroscopy (C D , Figure S22) showed a
while data for 2-p- Bu and 2-p-Cl were collected at an applied dc field
6
6
resonance for the CH protons at 1.11 ppm, while the aryl
of 1000 Oe.
2
protons appeared as multiplets in the range of 6.80−7.30 ppm,
confirming synthesis of the desired substituted benzylpotassium
salt.
complexes are presented as χ T and μ values in the
Supporting Information, Figures S12−S21), and the tabulated
magnetic data for all complexes are presented in Table 1.
M
eff
Subsequently, synthesis of the homoleptic uranium alkyl,
i
t
[
U(p-ClBn) ] , was performed in an analogous manner to the
At 300 K, the χ T values for 2-p- Pr, 2-p- Bu, 2-o-OMe, 2-p-
4
M
3
−1
previously reported uranium tetra(alkyl)s by mixing thawing
Cl, and 2-m-OMe range from 0.95 to 1.10 cm Kmol (μeff
values range from 2.76 to 2.96). These values slowly decrease
upon cooling until approximately 50 K; below this temperature,
12,23
THF solutions of UCl and 4 equivalents of 1-p-Cl (eq 2).
4
Following workup, 2-p-Cl was isolated as a dark-brown solid
but decomposed after prolonged exposure to THF at room
temperature, with p-chlorotoluene as the only organic product
3
−1
the χ T values decrease sharply to 0.11−0.26 cm Kmol (μeff
M
= 0.95−1.23) at 2 K. These magnetic-susceptibility ranges are
1
as observed by H NMR spectroscopy; no evidence for the
on the order of those for tetrahedral U(N(SiMe ) ) (μ
=
3
2 4
eff
28
carbon−carbon-coupled product, (1,2-bis(4-chlorophenyl)-
ethane), is noted. This is in contrast to 2-Bn, which produced
small amounts of the carbon−carbon-coupled bibenzyl in
1.32−2.94) and resemble those reported for tetravalent
t −
25
[U(CH Bu) ] (μ = 2.36−3.09) and UI(N(SiMe ) ) (μ
2
5
eff
3 2 3
eff
29
= 2.16−3.35). The observed temperature dependencies align
12
addition to toluene during decomposition. Optimally, 2-p-Cl
with literature precedent for mononuclear tetravalent uranium
B
DOI: 10.1021/acs.organomet.7b00438
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 1. Magnetic Susceptibility and Magnetization Values
for 2-p- Pr, 2-p- Bu, 2-o-OMe, 2-p-Cl, and 2-m-OMe
bond distances for these complexes also increase with the
increasing electron-withdrawing nature of the substituent,
i
t
23
although not with the same linear dependence. The bond-
length increase would be expected to weaken σ-overlap
between ligands and U frontier orbitals, which would shift
magnetic properties toward those associated with a free U(IV)
ion (i.e., larger susceptibility values). Further discussion is
provided in the Supporting Information. Irrespective of origin,
this clear trend could offer a magnetic handle for tracking
reactivity.
χ T
χ T
μeff
μeff
M
M
M
3
3
3
00 K (cm ·K· 2 K (cm ·K·
1.8 K, 50
kOe (μB)
−
1
−1
mol
)
mol
)
300 K 2 K
i
2
2
2
2
2
-p- Pr
0.96
0.95
1.01
0.99
1.09
0.11
0.16
0.18
0.16
0.19
2.77
2.76
2.84
2.81
2.96
0.95
1.15
1.22
1.15
1.23
0.10
0.25
0.36
0.31
0.49
t
-p- Bu
-p-Cl
-m-OMe
-o-OMe
Reductive Elimination Chemistry. In order to gain
insight into the mechanism for reductive elimination, it was
first important to confirm that the reductive elimination
complexes: ground-state singlets show paramagnetic responses
at higher temperatures due to population of magnetic excited
states. As observed here and previously, complexes containing
U−alkyl fragments show distinctive temperature-dependent
behavior in that the onset of a sharper downturn in χ T (or
μeff) values occurs at a lower temperature as compared to other
U(IV) complexes.
To better understand the magnetic behavior of 2-p- Pr, 2-
p- Bu, 2-o-OMe, 2-p-Cl, and 2-m-OMe, experiments to study
i
t
reaction proceeds in the same manner for 2-p- Pr, 2-p- Bu, 2-
25
m-OMe, and 2-p-Cl as for the parent compound, 2-Bn. In the
Mes
Me
case of 2-Bn, addition of the α-diimine ligand,
DAB ,
resulted in extrusion of the carbon−carbon reductive
elimination product, bibenzyl, with no observation of toluene,
the product of U−C homolytic scission. In the absence of this
M
12
i
ligand, only 15% bibenzyl was noted (85% as toluene),
t
indicating the ligand facilitates reductive elimination. Upon
Mes
Me
i
t
the field dependencies of magnetization were also performed
addition of DAB to a solution of 2-p- Pr, 2-p- Bu, 2-m-
(
Figures S2−S11). Although none of the complexes display
saturation at 50 kOe, the small magnetization values of 0.10−
.49 μ at low temperature (1.8 K) and high field are consistent
with ground-state singlets mixed with paramagnetic excited
states expected for U(IV) ions, as these values typically remain
around 0 μB.
Seeking evidence of the synthetic tunability of electronic
properties and noting that linear free-energy relationships
linking Hammett parameters to spin crossover properties have
been described previously, we explored potential trends in
magnetization, magnetic susceptibility, and temperature-inde-
pendent paramagnetism (TIP)
donation. 2-o-OMe, which was previously established as an
eight-coordinate species arising from ligation to the O atoms,
shows the largest susceptibility and magnetization values at all
temperatures. This can be attributed to the difference in
coordination geometry relative to the rest of the family of
compounds and thus precludes further comparison in this
study. For the remaining compounds, we observe (Figure 2) a
subtle but appreciable correlation between room-temperature
OMe, or 2-p-Cl, both the substituted bibenzyl and the ene-
Mes
Me
diamide uranium(IV) bis(benzyl) complex, ( DAB )U-
i
t
0
(Bn′) (3-Bn′; Bn′ = p-PrBn, p- BuBn, m-OMeBn, p-ClBn)
2
B
12
were detected in each case, supporting that the analogous
reductive elimination chemistry had occurred.
Evidence of the formation of the substituted bibenzyl was
1
confirmed using H NMR spectroscopy by comparison to
previously reported data. Positive identification was also
i
possible using mass spectrometry (2-p- Pr). In each case, it
30
was evident that the carbon−carbon-coupled product from
reductive elimination was formed in significant quantities, with
only trace amounts of the substituted toluene noted. For
31−34
as a function of substituent
i
example, in 2-p- Pr, GC analysis showed ∼85% bibenzyl vs
23
∼15% toluene formation. However, it should be noted that 3-
i
p- Pr produces some of the p-isopropyl toluene through
homolytic scission of the U−C bonds.
magnetic-susceptibility (χ T) values and electron-withdrawing
M
ability of the benzyl substituents (σp/m). We note that the U−C
Isolation of the ene-diamide uranium(IV) bis(benzyl)
complexes, 3-Bn′, proved to be difficult, as these species are
short-lived in both solution and solid state at room temper-
ature. Nevertheless, isolation of the organometallic products
was possible using an independent synthetic route. A
t
representative reaction will be discussed here, that of 2-p- Bu,
since this is the most stable compared to the other members of
the family. To analyze the reductive elimination product, a
t
THF solution of 2-p- Bu was added to a THF solution of
Mes
Me
DAB
while stirring at room temperature. Over the
duration of an hour, the dark-brown reaction mixture
brightened to brick red. After this time, volatiles were removed
in vacuo, and the solid was washed and recrystallized from
diethyl ether and pentane.
Figure 2. Relationship between the 300 K magnetic-susceptibility
i
t
values and Hammett parameters for compounds 2-p- Pr, 2-p- Bu, 2-p-
Cl, and 2-m-OMe. Note that 2-o-OMe was not included in the linear
fit due to the difference in inner coordination sphere.
C
DOI: 10.1021/acs.organomet.7b00438
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Analysis of the product by H NMR spectroscopy (C D , 23
Article
1
benzyl ligand. This drastic change in rate is well-known for
electron-withdrawing substituents and is due to the weakening
of the M−C bond that results from the presence of electron-
withdrawing substituents that contribute to stronger carba-
nionic character of the methylene group.
6
6
°
C) reveals an asymmetric molecule in solution with 20
paramagnetically shifted and broadened resonances in the range
from −80.27 to 82.12 ppm. While structural data could not be
obtained due to the inability to grow suitable single X-ray
quality crystals for diffraction experiments, support of formation
of the 3-Bn′ family was obtained by protonation experiments.
Treating 3-Bn′ with 2 equivalents of cracked cyclopentadiene
This study marks the first of its kind for carbon−carbon
reductive elimination from an actinide center, although the
17−19
analogous reaction has been studied for group IV metals.
(
(
(
CpH) generated the substituted toluene and the bis-
The radical nature of that reaction precludes a direct
comparison here. Rather, the concerted nature of this reaction
makes comparison to late transition metals more appropriate.
While direct bibenzyl elimination has not been widely studied
for transition metals, comparisons can be drawn to systems that
involve reductive elimination of substituted phenyls. The
observed trend in reductive elimination for uranium(IV)
noted here is opposite to that reported for C−C coupling on
late transition metals, including TpNi(CF ) Ar (Tp =
cyclopentadienyl) uranium ene-diamide product, Cp U-
2
Mes
Me
35
DAB ), which has been fully characterized in each
case. Further support for the formation of 3-Bn′ derives from
the analogous iminoquinone chemistry, where the correspond-
dipp
ing radical reductive elimination product, ( ap) U-
2
dipp
(
CH Ph) (THF) ( ap = 4,6-di-tert-butyl-2-[(2,6-di-iso-pro-
2 2 2
pylphenyl)-amidophenolate]), has been crystallographically
2
2
characterized.
3
2
Kinetic Experiments. Pseudo First-Order Experiments. In
order to probe the effects of the electronics of the benzyl
hydrotris(pyrazolyl)borate; Ar = Ph, p-OMePh, p-MePh, p-
37
BrPh, m-CO MePh), which forms the aryl-CF product.
2
3
substituent on the reductive elimination, kinetic experiments
Instead, the effect of electron-withdrawing substituents noted
here for uranium more closely mirrors the C−N coupling of
1
were conducted in benzene-d at 23 °C and assessed using H
6
i
t
38
NMR spectroscopy for 2-Bn, 2-p- Pr, 2-p- Bu, 2-m-OMe, and
aryl halides with diphenylamine, reported by Buchwald.
2
-p-Cl (Figures S24−S31). The reductive elimination reaction
To probe the mechanistic details of the reductive elimination
reaction, further activation parameters were determined on the
basis of the rates listed in Table 1. Due to the sensitive nature
of the homoleptic uranium(IV) benzyl family, variable
is predicted as a second-order reaction, but to accurately
quantify a rate law, pseudo first-order conditions were used. By
using the uranium alkyl compounds in 10-fold excess and
monitoring the loss of the ligand, meaningful kinetics data were
obtainable. Ligand monitoring was used to circumvent errors
that could be introduced from integration of the paramagnetic
uranium species. In light of the entropy of activation data (vide
infra), the reductive elimination reaction is associative and thus
first order in uranium. The rate constants obtained for each
compound are presented in Table 2. The kinetic data show that
i
temperature data was collected for 2-p- PrBn, which is the
most stable under the conditions of the experiment, but is also
the easiest to synthesize and purify. After performing
decomposition control experiments, reductive elimination
i
experiments on 2-p- PrBn were run from −5 to 25 °C, with
1
ligand disappearance once again monitored by H NMR
spectroscopy (Figure S32). The concentration of the ligand was
plotted vs time in order to calculate the kobs values. These
values were used to create an Eyring Plot in order to calculate
Table 2. Rate Constants for Reductive Elimination of 2-
Bn,2-p- Pr,2-p- Bu,2-m-OMe, and 2-p-Cl at 23° C
i
t
‡
ΔH
entropic data (Figure 3). The slope of −3746.8 − R gives
(
)
reaction rate (10‑ k·M ·sec ) Hammett parameter (σm/p
2
‑1
‑1
)
a
t
2
2
2
2
2
-p- Bu
0.85 (± 0.014)
1.04 (± 0.097)
6.48 (± 0.57)
47.2 (± 4.6)
−0.20
−0.15
0
i
-p- Pr
-Bn
-m-OMe
-p-Cl
0.12
0.23
not able to be measured
36
a
σ values taken from Hansch et al.
the rates of the carbon−carbon reductive elimination range
from 0.472 (± 0.046) to 0.0085 (± 0.00014), with the
compounds containing electron-withdrawing substituents
showing substantially faster rates than those featuring
electron-donating substituents.
To more accurately correlate the effect of the substituent
electron donicity to the rate of reductive elimination, the rate
constants were compared to the known Hammett parameter
σm/p for the benzyl ring substituents (Table 2). The correlation
in the table shows that as the electron-withdrawing effect of the
substituent increases, the rate of the reductive elimination
reaction also increases: p-Cl ≫ m-OMe > p-H > p-Pr > p- Bu.
Reductive elimination of the p-chloro derivative is so rapid that
it cannot be measured under these standard conditions. This
elimination process slows considerably when electron-donating
substituents are present on the phenyl ring, including iso-propyl
and tert-butyl substituents. Thus, this system is highly
susceptible to the electronic effects of the substitution on the
i
Figure 3. Eyring plot for 2-p- PrBn.
i
t
‡
an enthalpy of activation value of ΔH =7.45 ± 0.2 kcal/mol.
‡
k
h
ΔS
R
B
The y-intercept of 0.3788 ln
+
gives an entropy value
(
)
‡
of ΔS =−46 ± 0.6 eu. The large magnitude and negative nature
of calculated entropy value is indicative of a highly ordered
transition state. These parameters, in concert with the
12
previously reported crossover experiment, gives further
D
DOI: 10.1021/acs.organomet.7b00438
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Organometallics
Article
support that reductive elimination of bibenzyl and its
derivatives from 2-Bn, 2-p- Pr, 2-p- Bu, 2-m-OMe, and 2-p-Cl
is a concerted process.
On the basis of the experimentally obtained activation
parameters as well as the apparent concerted nature of the
reaction, a mechanism taking into account these features as well
as literature precedent can be proposed (Scheme 1). The initial
Kinetic experiments in corroboration of the mechanism for
any reductive elimination process from actinide centers have
not previously been performed. In our particular case, the
family of homoleptic uranium tetrabenzyls bearing a variety of
substituents are sufficiently stable over a wide temperature
range to be able to monitor the reductive elimination reaction
spectroscopically. Additionally, it is interesting to note that
variation of the electron-donating or electron-withdrawing
character of the phenyl ring does not impact the uranium−
carbon bond substantially, thus concerted reductive elimination
is noted for the entire family. This is important given that
nonbenzyl homoleptic uranium alkyl species are seen to
decompose over a number of pathways, of which homoleptic
U−C scission to produce alkyl radicals is a major contrib-
i
t
Scheme 1. Proposed Mechanism for Carbon−Carbon
Mes
Me
Reductive Elimination for DAB and U(CH Ph)
2
4
20,21
utor.
CONCLUSIONS
■
In summary, we have reported the preparation of a new
homoleptic uranium(IV) tetrabenzyl species, U(CH −p-ClBn)
(2-p-Cl), that extends our existing family of compounds.
step of the reaction is likely coordination of the neutral
Mes
Me
2
4
DAB ligand to U(CH Ph) . While we did not observe
2
4
23
(MesDAB )U(CH Ph) spectroscopically, its existence is
Me
2
4
Magnetic property measurements of this family reveal a linear
free-energy relationship between susceptibility and substituent
electronic properties. Significantly, carbon−carbon reductive
elimination proceeds readily for this family in a concerted
supported by the 1,2-bis(dimethylphosphino)ethane (dmpe)
analogue, (dmpe)U(CH Ph) , which shows all benzyl groups
2
4
4
coordinated in an η -fashion in the presence of a bidentate
1
2
chelator. This ligand association is proposed as the rate-
determining step, which is consistent with the negative and
large entropy of activation obtained from the kinetic experi-
ments. While the hapticity of the benzyl rings is not known
Mes
Me
fashion due to the presence of the α-diimine ligand, DAB .
i
The reaction proceeds analogously if electron-donating ( Pr,
t
Bu, H) or electron-withdrawing (OMe, Cl) groups are
2
substituted on the phenyl ring, with a drastic rate enhancement
noted for electron-withdrawing substituents.
specifically, we hypothesize that their coordination is likely η -
or greater due to the strong inductive effects noted for electron-
withdrawing substituents that are in conjugation.
Demonstration of concerted reductive elimination here
supports that actinides can perform fundamental organo-
metallic reactions analogously to transition metals. However,
there are some stark differences noted for uranium as compared
to the d-block metals. First, concerted carbon−carbon bond
formation occurs, which is in contrast not only to the group IV
Following ligand association, it is likely that the steric
Mes
Me
pressure of
DAB drives the carbon−carbon coupling to
facilitate the reductive elimination, which would be in direct
contrast to stable (dmpe)U(CH Ph) , which has a sterically
2
4
smaller ligand. On the basis of spectroscopic and structural
Mes
Me
17−19
Mes
Me
derivatives with the DAB ligand
but also to late first-
evidence obtained for the product, ( DAB )U(CH Ph) ,
2
2
Mes
Me
row transition-metal systems, where bibenzyl elimination is
coordinated DAB undergoes reduction by two electrons
during this process.
42−46
typically a radical process.
Second, as compared to well
studied platinum(IV) systems that are known to undergo C−C
reductive elimination with aryl groups, using electron-with-
drawing groups on the uranium-benzyl rings promotes an
increased rate of reductive elimination, rather than a slower
one. With the unique features of this system in mind, future
studies will be focused toward expanding the scope and
mechanistic understanding of the reductive elimination reaction
to heteroatoms and other alkyl substituents.
In classical reductive elimination reactions, the two electrons
obtained from the reaction revert back to the metal, decreasing
the oxidation state by 2. However, since divalent uranium is
generally unstable and has only been isolated in specialized
39,40
ligand environments,
it is highly unlikely in this case. In
41
accordance with the basic principles of redox-active ligands,
Mes
Me
the π* orbitals of
DAB are energetically accessible to
house these electrons. Previous work from our laboratory has
demonstrated electrochemically that the one-electron reduction
Mes
Me
Mes
Me 1−
of
DAB
to produce [ DAB ] is an irreversible
EXPERIMENTAL SECTION
■
process, whereas the two-electron reduction to generate
General Considerations. All air- and moisture-sensitive manip-
ulations were performed using standard Schlenk techniques or in an
MBraun inert atmosphere drybox with an atmosphere of purified
nitrogen. The MBraun drybox was equipped with a cold well designed
for freezing samples in liquid nitrogen as well as two −35 °C freezers
for cooling samples and crystallizations. Solvents for sensitive
manipulations were dried and deoxygenated using literature
Mes
Me 2−
22
[
DAB ] is reversible, thus supporting that concerted
two-electron movement is favorable. Furthermore, the lack of
reductive elimination noted for (dmpe)U(CH Ph) suggests
2
4
the role of the empty energetically low-lying orbitals in
Mes
Me
DAB in facilitating the observed carbon−carbon coupling
chemistry. It is highly likely that the uranium acts as a conduit
for electron migration from the benzyl groups to the DAB
4
7
Mes
Me
procedures with a Seca solvent-purification system. Benzene-d and
6
toluene-d8 were purchased from Cambridge Isotope Laboratories,
ligand, rather than a sink where the electrons reside for any
period of time. In conjunction with the magnetic data, it
appears that the rate of reductive elimination increases as the
excited-state population is increased on the uranium center.
Further studies probing this interesting relationship are
underway in our laboratories.
dried with molecular sieves and sodium, and degassed by three freeze−
pump−thaw cycles. THF-d was purchased from Cambridge Isotope
8
Laboratories and used as received. Potassium t-butoxide and n-
butyllithium (2.5 M in hexanes) were purchased from Sigma-Aldrich
and used as received. p-Chlorotoluene was purchased from Sigma-
48
Aldrich and distilled from CaH prior to use. Uranium tetrachloride,
2
E
DOI: 10.1021/acs.organomet.7b00438
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
MesDABMe
,
49
2-Bn, 2-p- Pr, 2-p- Bu, 2-o-OMe, 2-m-OMe were
12
i
t
23
−8.20 (1H), −7.35 (9H), −4.93 (1H), −1.10 (3H), 1.07 (6H), 9.77
(2H), 15.05 (3H), 18.43(1H), 23.63 (1H) 37.14 (1H), 59.82 (1H),
60.04 (3H), 82.12 (6H).
prepared according to literature procedures.
H NMR and C NMR spectra were recorded on a Varian Inova
00 spectrometer operating at 299.992 and 75.424 MHz, respectively.
H NMR spectra for kinetic experiments were recorded on a Varian
Inova 600 spectrometer operating at 599.967 MHz. All chemical shifts
are reported relative to the peak for SiMe , using H (residual)
chemical shifts of the solvent as a secondary standard. The spectra for
paramagnetic molecules were obtained by using an acquisition time of
1
13
3
General Experimental Kinetics. Stock solutions of ferrocene (0.08
1
Mes
Me
M),
DAB (0.08 M), and homoleptic uranium(IV) tetra-alkyl
compound (0.1 M) in benzene-d were prepared. Four samples of the
6
1
uranium compounds were diluted to different concentrations ranging
from 0.02 to 0.07 M. Ferrocene stock solution (20 μL) was added to
each uranium sample as a standard. Four different solutions of known
concentrations were added to four different screw-cap NMR tubes that
were then sealed with tape. Four syringes (50 μL) were filled with 20
4
0
.5 s, thus the peak widths reported have an error of ±2 Hz. For
1
paramagnetic molecules, the H NMR data are reported with the
chemical shift, followed by the peak width at half height, the
integration value and, where possible, the peak assignment.
Mes
Me
μL of DAB and inserted into rubber septa to limit atmospheric
exposure. Once removed from the glovebox, the NMR tubes were
placed in liquid nitrogen to prevent decomposition between
experiments. Kinetics experiments were run in a 600 MHz Varian
NMR spectrometer at a constant temperature as an array experiment.
Four scans were performed per run with a 1 s delay between runs. The
runs covered a 50 k sweep width, a 7 s delay time, and a 0.5 s
acquisition time. Before placement into the instrument, the samples
were thawed in a room-temperature water bath, and then, the ligand
was injected and thoroughly mixed. Data were analyzed via
MestReNova and visualized with KaliedaGraph software.
Magnetic-susceptibility data were collected using a Quantum
Design MPMS XL SQUID magnetometer. All sample preparations
were performed inside a dinitrogen-filled glovebox (MBRAUN
Labmaster 130). Powdered microcrystalline samples were loaded
into polyethylene bags and sealed in the glovebox, inserted into a straw
and transported to the magnetometer under dinitrogen. Ferromagnetic
impurities were checked through a variable field analysis (0 to 10 kOe)
of the magnetization at 100 K: curvature in the M vs H plot between 0
and ∼2000 Oe (Figures S2−S6) indicates the presence of
ferromagnetic impurities. When this behavior was observed,
susceptibility data were collected at magnetic fields where the field
dependence is linear (usually 5000 Oe). Magnetic-susceptibility data
were collected at temperatures ranging from 2 to 300 K. Susceptibility
data reproducibility was assessed through measurements on two
different batches for compound 1, which showed consistency at all
temperatures, with a maximum difference of 0.03 cm ·K·mol at 300
K. Magnetization measurements were collected at 1.8 K while varying
the applied field up to 50 kOe (Figures S7−S11). Data were corrected
for the diamagnetic contributions of the sample holder and bag by
subtracting empty containers; corrections for the sample were
calculated from Pascal’s constants.
Synthesis of Kp-ClBn (1-p-ClBn). A modified procedure for
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00438.
3
−1
Additional experimental procedures, magnetic data, and
spectroscopic data (PDF)
5
0
AUTHOR INFORMATION
Corresponding Author
E-mail: sbart@purdue.edu.
■
23,27
organopotassium reagents, reported by Schlosser,
generate the series of organopotassium reagents. A 250 mL round-
bottom flask was charged with 2.00 g (17.8 mmol) of KO Bu and
was used to
*
t
ORCID
excess p-chlorotoluene (100 mL). The slurry was cooled in the cold
well for ∼20 min or until frozen. While stirring, cold (−35 °C), n-
butyllithium (2.5 M in hexanes, 7.12 mL, 17.8 mmol) was added to the
thawing slurry, producing an immediate color change to light brown.
The solution was allowed to warm to room temperature for 5 h at
which point the mixture darkened to deep brown. The brown solid
was collected using a fritted funnel and washed with a continuous
stream of pentane totaling approximately ∼200 mL to remove any
Mahdi M. Abu-Omar: 0000-0002-4412-1985
Matthew P. Shores: 0000-0002-9751-0490
Suzanne C. Bart: 0000-0002-8918-9051
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
t
remaining substituted toluene and LiO Bu. The solid was dried on the
The authors declare no competing financial interest.
vacuum line and identified as Kp-ClBn (1-pCl). Quantitative yields
1
were obtained. H NMR (THF-d , 25 °C): δ = 1.11 (s, 2H, CH ),
8
2
ACKNOWLEDGMENTS
6
.80−7.30 (m, 4H, CH).
Synthesis of U(p-Cl-Bn) (2-p-Cl). A 20 mL vial was charged with
■
We acknowledge the National Science Foundation (CAREER
grant to S.C.B., CHE-1149875) for funding. M.P.S. and R.F.H.
thank NSF (CHE-1363274) for support of magnetometry
measurements at Colorado State University. Prof. David
McMillin is acknowledged for helpful discussions.
4
0
.050 g of green UCl and 2 mL of THF. A second vial was charged
4
with 4 equivalents of light-brown 1-pCl and 2 mL of THF. Each vial
was frozen in the cold well to −108 °C and mixed upon thawing. The
mixture immediately turned dark brown whereupon THF was
removed in vacuo. The brown residue was triturated in diethyl ether
and filtered to remove KCl. Drying the filtrate produced 2-p-Cl as a
brown solid (55%). Due to the sensitivity and instability of the
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DOI: 10.1021/acs.organomet.7b00438
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DOI: 10.1021/acs.organomet.7b00438
Organometallics XXXX, XXX, XXX−XXX