Radical reductive elimination from tetrabenzyluranium mediated by an iminoquinone ligand

Article abstract of DOI:10.1021/om4012104

Reductive elimination from U(CH₂Ph)₄ (1-Ph) mediated by 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (^dippiq) was observed, resulting in the formation of (^dippap) ₂U(CH₂Ph)

Full text of DOI:10.1021/om4012104

Article
pubs.acs.org/Organometallics
Radical Reductive Elimination from Tetrabenzyluranium Mediated by
an Iminoquinone Ligand
Ellen M. Matson,† Sebastian M. Franke,‡ Nickolas H. Anderson,† Timothy D. Cook,† Phillip E. Fanwick,†
and Suzanne C. Bart*,†
†H. C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
‡Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich Alexander University, Egerlandstraße 1, 91058 Erlangen,
Germany
S
* Supporting Information
ABSTRACT: Reductive elimination from U(CH2Ph)4 (1-Ph)
mediated by 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)-
imino]quinone (dippiq) was observed, resulting in the
formation of (dippap)2U(CH2Ph)2(THF)2 (2) (dippap = 4,6-
di-tert-butyl-2-[(2,6-diisopropylphenyl)amido]phenolate) and
bibenzyl. The crossover experiment with U(CD2C6D5)4
showed formation of bibenzyl-d7, indicating that reductive
elimination occurs in a stepwise fashion via benzyl radical
extrusion, presumably through an iminosemiquinone tris-
(benzyl) intermediate, (dippisq)U(CH2Ph)3. Synthesis of this
intermediate was attempted by addition of the iminoquinone
ligand to UI3(THF)4 to form (dippisq)UI3 (3), followed by alkylation with 3 equiv of benzylpotassium. However, this only
resulted in the isolation of 2. Reduction of 3 with KC8 afforded the amidophenolate diiodide species (dippap)UI2(THF)2 (4),
maintaining the tetravalent oxidation state of the uranium and reducing the ligand. Attempts at the formation of 2 via addition of
2 equiv of benzylpotassium to 4 resulted in decomposition. The uranium mono(alkyl) (dippap)UI(CH2Ph)(THF)2 (5) was
1
observed upon addition of 1 equiv of benzylpotassium to 4. All products have been characterized by H NMR and electronic
absorption spectroscopy. X-ray crystallography was employed to ascertain ligand reduction in 2, 3, and 5.
Scheme 1. Reductive Elimination by Uranium Complexes
INTRODUCTION
■
Reductive elimination is a key step in carbon−element bond
formation and has been demonstrated to proceed by both
radical (one-electron)1−3 and concerted (two-electron)4 path-
ways. High-valent transition-metal centers are favored for
reductive elimination, as the two-electron-reduced products are
often stable.5 In the case of uranium, Seyam and co-workers
noted that C−C reductive elimination occurs from the fleeting
uranium(VI) species UO2Ph2, forming biphenyl and unstable
uranium(IV) oxide products.6 However, this is not a general
reaction. In the case of UO2R2 (R = iPr, nBu, tBu), β-hydride
elimination and U−C homolytic cleavage pathways are
operative.
More recently, reductive elimination has been demonstrated
for tetravalent uranium compounds in the presence of
additional substrates or oxidants, which avoids formation of
an unstable divalent uranium product. For instance, Evans has
demonstrated that [Cp*2UH2]2 is an effective multielectron
reductant for PhSSPh and PhNNPh (Scheme 1), generating
Cp*2U(SPh)2 and Cp*2U(NPh)2, respectively, as well as for
cyclooctatetraene to form [Cp*(COT)U]2(μ-COT).7 In all
cases, formation of the product proceeds via the reductive
elimination of dihydrogen from [Cp*2UH2]2. Similar reactivity
has been noted in the case of the tuck-in, tuck-over compound
Cp*U[μ-η5:η1:η1-C5Me3(CH2)2](μ-H)2UCp*2, which behaves
as a masked form of [Cp*2U], generating the same products in
the presence of the aforementioned reagents following C−H
reductive elimination to restore the traditional η5-Cp*
coordination.8
Received: December 17, 2013
Published: April 7, 2014
© 2014 American Chemical Society
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Organometallics
Article
1.0 mM. The ferrocenium/ferrocene couple was observed at 0.645 V
(vs Ag/AgCl) under the noted experimental conditions.
Traditional two-electron reductive elimination reactions are
rare for uranium, due to the tendency of this metal to undergo
radical processes. However, redox-active ligands can control
one-electron transfers, facilitating multielectron processes.
These ligands have been widely used on uranium, including
examples by Kiplinger and co-workers9 and our laboratory,10
which have demonstrated the use of α-diimine ligands in the
synthesis of uranium(III) and -(IV) compounds. Schiff base
ligands can be used to store electrons from uranium in a C−C
bond, as shown by Mazzanti and co-workers.11 Andersen and
co-workers12,13 and our recent work14 have demonstrated that
radical bipyridine ligands coordinated to low-valent uranium
centers facilitate two-electron chemistry to generate U−N and
U−O multiple bonds.
Single crystals of (dippap)U(CH2Ph)2(THF)2 (2), (dippisq)-
UI3(THF)2 (3), and (dippap)U(I)(CH2Ph)(THF)2 (5) for X-ray
diffraction were coated with poly(isobutylene) oil in a glovebox and
quickly transferred to the goniometer head of a Rigaku Rapid II image
plate diffractometer equipped with a MicroMax002+ high-intensity
copper X-ray source with confocal optics. Preliminary examination and
data collection were performed with Cu Kα radiation (λ = 1.54184 Å).
Cell constants for data collection were obtained from least-squares
refinement. The space group was identified using the program
XPREP.22 The structures were solved using the structure solution
program PATTY in DIRDIF99.23 Refinement was performed on a
LINUX PC using SHELX-97.22 The data were collected at a
temperature of 150(1) K.
Preparation of (dippap)U(CH2Ph)2(THF)2 (2). A 20 mL
scintillation vial was charged with U(CH2Ph)4 (0.050 g, 0.083
mmol) and approximately 5 mL of THF. The brown-red solution
was cooled to −35 °C. In a separate vial, 1 equiv of dippiq (0.031 g,
0.082 mmol) was dissolved in approximately 2 mL of THF. The ligand
was added to the uranium solution dropwise with vigorous stirring. No
obvious color change observed. After 30 min of stirring, solvents were
removed under reduced pressure. The product, 2, was extracted with
We recently reported that concerted C−C reductive
elimination from U(CH2C6H5)4 (1-Ph) in the presence of
MesDABMe
(
MesDABMe = ArNC(Me)C(Me)NAr, Ar =
2,4,6-trimethylphenyl (Mes)) generates 1 equiv of bibenzyl and
forms (MesDABMe)U(CH2Ph)2, which has a ligand that has
been reduced by two electrons.15 Rather than a typical
reductive elimination, where electrons reside on the metal
center, in this case the electrons are stored in the ligand,
preventing the formation of unstable divalent and zerovalent
products. On the basis of this interesting result, reductive
elimination of U(CH2C6H5)4 with the related iminoquinone
ligand, 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]-
quinone (dippiq), was explored to determine if analogous
chemistry occurs and, if so, by what mechanism. Herein, we
present the results of this study, along with additional
experiments aimed at supporting mechanistic intermediates in
the reductive elimination reaction.
1
ether and recrystallized at −35 °C (0.069 g, 0.073 mmol, 83%). H
NMR (C6D6, 25 °C): δ −19.34 (ω0.5 = 967 Hz, 8H, THF), −12.89
(ω0.5 = 75 Hz, 9H, C(CH)3), −10.08 (ω0.5 = 154 Hz, 12H,
CH(CH3)2), −6.48 (ω0.5 = 261 Hz, 4H), −1.39 (ω0.5 = 44 Hz, 2H),
3.32 (ω0.5 = 165 Hz, 2H) 3.52 (ω0.5 = 27 Hz, 9H, C(CH)3), 4.71 (ω0.5
= 42 Hz, 1H), 6.73 (ω0.5 = 23 Hz, 4H), 15.24 (ω0.5 = 475 Hz, 4H),
20.85 (ω0.5 = 140 Hz, 1H), 23.92 (ω0.5 = 783 Hz, 8H, THF).
Alternative Synthesis of 2. A 20 mL scintillation vial was charged
with (dippisq)UI3(THF)2 (0.100 g, 0.088 mmol) and approximately 4
mL of THF. The solution was cooled to −35 °C. Three equivalents of
benzylpotassium (0.036 g, 0.274 mmol) was dissolved in 10 mL of
THF and added dropwise to the uranium solution, resulting in an
instantaneous color change from brown to red. After filtration over
Celite to remove KI, THF was removed under reduced pressure. The
product, 2, was recrystallized from a concentrated solution of ether
and toluene (4/1) in high yields (0.074 g, 0.078 mmol, 89%). Crystals
suitable for X-ray analysis were grown from a concentrated ether and
toluene mixture (8/1). Anal. Calcd for C34H53N1O3I2U: C, 61.07; H,
7.15; N, 1.48. Found: C, 60.91; H, 6.96; N, 1.37.
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive manip-
ulations were performed using standard Schlenk techniques or in an
MBraun inert-atmosphere drybox under 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
procedures with a Seca solvent purification system.16 Benzene-d6 was
purchased from Cambridge Isotope Laboratories, dried with molecular
sieves and sodium, and degassed by three freeze−pump−thaw cycles.
Elemental analyses were performed by Midwest Microlab, LLC in
Indianapolis, IN, and Complete Analysis Laboratories, Inc., in
Parsippany, NJ. 4,6-Di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]-
quinone (dippiq),17 UI3(THF)4,18,19 U(CH2Ph)4,15 KC8,20 and
KCH2Ph21 were synthesized following literature procedures.
Preparation of (dippisq)UI3(THF)2 (3). A 20 mL scintillation vial
was charged with UI3(THF)4 (0.100 g, 0.110 mmol) and
approximately 10 mL of diethyl ether. To the blue-purple slurry was
added 1 equiv of the brown-red solid dippiq (0.042 g, 0.112 mmol) by
difference, resulting in an instant color change to brown-yellow. After
the mixture was stirred for 30 min, solvents were removed under
reduced pressure, resulting in isolation of the product, 3, in
quantitative yields. Anal. Calcd for C34H53NO3I3U: C, 37.61; H,
4.92; N, 1.29. Found: C, 37.26; H, 4.82; N, 1.24. 1H NMR (C6D6, 25
°C): δ −26.92 (ω0.5 = 29 Hz, 1H), −17.50 (ω0.5 = 103 Hz, 9H,
C(CH)3), −9.37 (ω0.5 = 17 Hz, 2H, o-CH), −1.39 (47, 1H), 1.57
(ω0.5 = 1266 Hz, 12H, CH(CH)2), −11.58 (ω0.5 = 16 Hz, 1H), 13.62
(ω0.5 = 314 Hz, 9H, C(CH)3), 40.61 (ω0.5 = 673 Hz, 8H, THF), 43.80
(ω0.5 = 90 Hz, 2H), 45.62 (ω0.5 = 1224 Hz, 8H, THF).
1H NMR spectra were recorded on a Varian Inova 300
spectrometer operating at 299.992 MHz. All chemical shifts are
1
reported relative to the peak for SiMe4, 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
0.5 s; thus, the peak widths reported have an error of 2 Hz. For
paramagnetic molecules, the 1H NMR data are reported with the
chemical shift, followed by the peak width at half-height in hertz, the
integration value, and (where possible) the peak assignment.
Electronic absorption spectroscopic measurements were recorded at
294 K in sealed 1 cm quartz cuvettes with a Jasco V-6700
spectrophotometer.
Preparation of (dippap)UI2(THF)2 (4). A 20 mL scintillation vial
was charged with (dippisq)UI3(THF)2 (0.100 g, 0.088 mmol) and
approximately 4 mL of THF. One equivalent of KC8 (0.012 g, 0.088
mmol) was added by difference, resulting in a darkening of the
solution. After it was stirred for 10 h, the solution was filtered over
Celite to remove the byproducts KI and graphite. After solvents were
removed under reduced pressure, the product, 4, was isolated as a dark
brown powder (0.064 g, 0.063, 72%). Anal. Calcd for C34H53N1O3I2U:
1
The cyclic voltammograms of the free ligands dippiq and MesDABMe
were recorded in 0.2 M (n-Bu)4NPF6 solution (tetrahydrofuran, N2-
degassed) on a CHI620A voltammetric analyzer with a glassy-carbon
working electrode (diameter 2 mm), a Pt-wire auxiliary electrode, and
a Ag/AgCl reference electrode. The concentration of analyte is always
C, 40.21; H, 5.26; N, 1.38. Found: C, 40.39; H, 5.24; N, 1.23. H
NMR (C6D6, 25 °C): δ −11.49 (ω0.5 = 1252 Hz, 4H, THF), −9.58 (d,
3JHH = 6, 12H, CH(CH3)2), −8.43 (ω0.5 = 213 Hz, 4H, THF), −7.21
(ω0.5 = 494 Hz, 4H, THF), 1.55 (ω0.5 = 21 Hz, 9H, C(CH)3), 3.27
(ω0.5 = 29 Hz, 1H, CH), 5.35 (ω0.5 = 8 Hz, 9H, C(CH)3), 6.50 (ω0.5
=
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Scheme 2. Synthetic Routes and Potential Intermediate for the Generation of Complex 2
395 Hz, 4H, THF), 7.14 (ω0.5 = 4 Hz, 2H), 14.30 (ω0.5 = 11 Hz, 2H,
CH(CH3)), 19.39 (ω0.5 = 47 Hz, 1H, CH).
were obtained at 200 K. Refinement of the data revealed the
formation of (dippap)U(CH2Ph)2(THF)2 (2), which features a
six-coordinate uranium center in a distorted-octahedral
geometry (Figure 1, Table 1). The U1−O1 distance of
Preparation of (dippap)U(I)(CH2Ph)(THF)2 (5). A 20 mL
scintillation vial was charged with (dippap)UI2(THF)2 (0.050 g, 0.053
mmol) and approximately 5 mL of THF and cooled to −35 °C. In a
separate vial, 2 equiv of KCH2Ph was dissolved in 3 mL of THF. The
bright orange solution was added dropwise to the brown uranium
solution with vigorous stirring, resulting in a color change to red. After
3 h at room temperature, the solution was filtered over Celite to
remove KI. Volatiles were removed under reduced pressure. The
product, 5, was recrystallized from a diethyl ether and pentane mixture
(2/1) in good yield (0.030 g, 0.031 mmol, 59%). X-ray-quality crystals
were grown from a toluene:pentane (1/3) solution at −35 °C.
Analysis for C41H60NIO3U•1/2(C7H8): Calcd. C, 62.21; H, 7.60; N,
1
1.58. Found C, 61.97; H, 7.32; N, 1.55. H NMR (C6D6, 25 °C): δ
−68.02 (ω0.5 = 533 Hz), −37.79 (ω0.5 = 520 Hz), −19.67 (ω0.5 = 358
Hz), −17.11 (ω0.5 = 333 Hz), −14.06 (ω0.5 = 742 Hz), −10.35 (ω0.5
535 Hz), −6.34 (ω0.5 = 505 Hz), −3.04 (ω0.5 = 457 Hz), 0.35 (ω0.5
105 Hz), 3.00 (ω0.5 = 157 Hz), 15.00 (ω0.5 = 93 Hz), 29.56 (ω0.5
395 Hz), 39.47 (ω0.5 = 450 Hz), 44.40 (ω0.5 = 407 Hz), 173.44 (ω0.5
585 Hz).
=
=
=
=
RESULTS AND DISCUSSION
■
Reductive elimination studies commenced with the addition of
1 equiv of dippiq to 1-Ph, which resulted in no perceptible color
change (Scheme 2). The reaction mixture was stirred for 15
min before volatiles were removed under reduced pressure.
After workup, analysis of the crude product by 1H NMR
spectroscopy showed a sharp signal at 2.78 ppm, corresponding
to the methylene protons of bibenzyl. Also in the spectrum
were 12 paramagnetically broadened and shifted signals ranging
from −19.34 to 23.92 ppm, consistent with a Cs-symmetric
product. This would be expected for the formation of the
desired amidophenolate uranium bis(benzyl) product (dippap)-
U(CH2Ph)2(THF)2 (2; dippap = 4,6-di-tert-butyl-2-[(2,6-
diisopropylphenyl)amido]phenolate). Signals for the two tert-
butyl groups and the methyl protons of the isopropyl groups
are located at −12.89, 3.52, and −10.08 ppm, respectively.
Additionally, two broad resonances integrating to 8H are
located at −19.34 and 23.92 ppm, corresponding to the protons
of two THF ligands. Four signals for the two chemically
equivalent benzyl groups are also present.
Figure 1. Molecular structure of 2 shown with 30% probability
ellipsoids. Hydrogen atoms and solvent molecules have been removed
for clarity.
2.147(2) Å24−26 and the U1−N2 distance of 2.328(3) Å27−30
are consistent with anionic bonds between uranium and the
heteroatoms. Upon reductive elimination of bibenzyl, two-
electron reduction of the iminoquinone ligand occurred,
generating the dianionic amidophenolate resonance form
(
dippap). Thus, the U−N and U−O bonds in 2 are similar to
those reported for the uranium amidophenolate (dippap)2U-
(THF)2 (U−O = 2.163(3), 2.155(3); U−N = 2.304(3),
2.317(3) Å).31 Elongation of the N−C (1.420(5) Å) and O−C
(1.369(4) Å) distances in comparison to the structural
parameters of the oxidized free ligand (N−C = 1.287(2) Å,
O−C = 1.221(6) Å)17 lends further support for ligand
reduction, as do analogous bond distances to (dippap)2U-
(THF)2. The U−O(THF) distances of 2.510(3) and 2.470(2)
Å are as expected for dative uranium−oxygen interactions.
The U−C distances for the methylene carbons of 2
(2.517(3) and 2.490(4) Å) are within the range of previously
observed U(IV)−benzyl methylene carbon distances (2.456−
2.538 Å).15,32−38 Interesting features of the uranium−benzyl
To unambiguously determine the identity of 2, crystals
suitable for X-ray diffraction were grown from a solution of
diethyl ether and toluene (5/1) at −35 °C. Data collection at
150 K yielded unpublishable data, most likely on account of a
phase transition at that temperature; thus, publishable data
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Table 1. Structural Parameters (Bond Distances in Å and
Angles in deg) for Compounds 2 and 3
distance or angle
2
3
U1−O1
U1−N2
O1−C11
N2−C12
C11−C12
U1−O51
U1−O61
U1−C30
U1−C40
U1−I1
2.147(2)
2.328(3)
1.369(4)
1.401(5)
1.420(5)
2.470(2)
2.510(3)
2.517(3)
2.490(4)
2.154(4)
2.570(5)
1.346(8)
1.350(8)
1.426(8)
2.499(4)
2.484(4)
3.0140(4)
3.0126(4)
3.0136(5)
U1−I2
U1−I3
U1−C30−C31
105.3(3)
U1−C40−C41
95.5(2)
O(X)1−U1−O(X′)1
162.85(9)
(X = 5, X′ = 6)
68.20(15)
(X = 2, X′ = 3)
Figure 2. Electronic absorption spectroscopy of (dippap)U-
(CH2Ph)2(THF)2 (2) (red), (dippap)UI2(THF)2 (4) (blue), and
(
ature. The inset shows the near-infrared region of the spectrum.
Solvent overtones, ranging between 1600 and 1800 nm, have been
removed for clarity.
interactions are the U−C−C bond angles of 105.3(3) and
95.5(2)°, which are significantly smaller than those of η1-
uranium benzyl complexes (116.6(10)−134.2(4)°),39−41 in-
dicating higher hapticity. To determine the hapticity of the
benzyl groups, two parameters, Δ and Δ′, were calculated for
each benzyl substituent in 2, according to literature
dippap)UI(CH2Ph)(THF)2 (5) (green) in THF at ambient temper-
procedures,42 where Δ = [MCo − MCH2] − [MCipso
−
MCH2] and Δ′ = [MCo′ − MCH2] − [MCipso − MCH2]
(Table 2). In these calculations, MCo is the shorter metal-to-
ortho carbon contact length, MCo′ is the longer metal-to-ortho
contact length, MCH2 is the metal-to-methylene carbon bond
length, and MCipso is the metal-to-ipso carbon bond length.
Although the Δ and Δ′ values are within the appropriate range
for η4-benzyl substituents,15,34,42 the U−Co bond distances are
extremely long, suggesting a lower hapticity. This argument is
further supported by the U−C−C angles in 2, which are larger
than those for the η4-benzyl groups reported for 1-Ph
(82.7(4)−89.4(4)°).15
In addition to structural parameters, electronic absorption
spectroscopy was employed to provide insight into the
oxidation state of the uranium center in 2. A spectrum was
collected from 280 to 2100 nm in THF at ambient temperature
(Figure 2). Bands with low molar absorptivities were noted in
the near-infrared region of the spectrum, as is commonly
observed for uranium(IV) complexes.9,10,31,43,44 This spectrum
is reminiscent of those of other tetravalent uranium complexes
with redox-active ligands, such as (dpp-BIAN)2U(THF) (dpp-
acenaphthene))9 and (MesDABMe)2U(THF) and Cp2U-
(
MesDABMe).10 Thus, the electronic absorption spectroscopic
data agree with the formulation of a tetravalent uranium center,
as supported by crystallographic data. No significant features
were observed in the visible region as would be expected for
either the iminoquinone ligand or its one-electron-reduced
iminosemiquinone [isq]− counterpart, further supporting two-
electron reduction of dippiq to the amidophenolate [ap]2−
during formation of 2.
To investigate the mechanism for reductive elimination from
1-Ph to form bibenzyl, a crossover experiment was performed.
To an equimolar ratio of U(CH2C6H5)4 and U(CD2C6D5)4 was
added 2 equiv of dippiq. Following workup, separation of the
organometallic and organic products was achieved by filtration
through a silica gel plug. Analysis of the organics by GC/MS
showed bibenzyl, bibenzyl-d14, and the radical crossover
product bibenzyl-d7 in a statistical mixture (25:25:50) (Figure
S7, Supporting Information), supporting the notion that C−C
reductive elimination occurs by a radical mechanism via U−C
homolytic cleavage.
BIAN
= 1,2-bis((2,6-diisopropylphenyl)imino)-
Table 2. Hapticity Values for 2 and 5
a
b
c
d
e
compd
MCi − MCH2
MCo − MCH2
MCo′ − MCH2
Δ
Δ′
ref
(dmpe)U(CH2Ph)3CH3
0.22
0.34
0.24
0.51
0.69
0.62
0.55
0.89
0.71
1.00
1.28
1.08
0.91
0.94
0.83
1.28
1.53
1.49
0.33
0.56
0.47
0.49
0.59
0.46
0.69
0.61
0.59
0.77
0.83
0.87
34
27
15
Cp*U(CH2Ph)3
U(CH2Ph)4 (1-Ph)
f
(
(
dippap)U(CH2Ph)2(THF)2 (2)
dippap)U(I)(CH2Ph)(THF)2 (5)
this work
this work
a
b
Average metal to ipso carbon bond length minus metal to methylene carbon bond length. Average metal to shorter ortho carbon bond length
c
minus metal to methylene carbon bond length. Average metal to longer ortho carbon bond length minus metal to methylene carbon length.
d
e
f
[MCortho − MCH2] − [MCipso − MCH2]. [MCortho′ − MCH2] − [MCipso − MCH2]. Shortest contacts.
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Article
Radical reductive elimination to form 2 is in contrast to that
noted in the formation of (MesDABMe)U(CH2C6H5)2,15 which
has been established to proceed through a concerted
mechanism. We reasoned that initial coordination of the
iminoquinone ligand to tetrabenzyluranium results in benzyl
radical extrusion with concurrent ligand reduction to form the
monoanionic iminosemiquinone intermediate (dippisq)U-
(CH2Ph)3 (Scheme 2). This species could subsequently eject
an additional benzyl radical to generate 2 and bibenzyl.
This hypothesis is supported by the cyclic voltammogram
(CV) for dippiq recorded in THF, which shows a reversible one-
electron reduction to form the stable monoanionic iminosemi-
quinone ligand (dippisq)− at −0.664 V (vs Ag/AgCl) (Figure S1,
Supporting Information) when sweeping between −1.5 and 1.6
V.45 In comparison, the CV for MesDABMe shows a quasi-
reversible one-electron reduction at −0.839 V, indicating that
[
MesDABMe]− is not stable. With regard to the mechanism for
reductive elimination from tetrabenzyluranium, observation of
the radical pathway for dippiq may be due to the lower reduction
potential, which generates a stable monoanion that facilitates
radical elimination. Because this reduction is irreversible for
MesDABMe, the two-electron concerted elimination is favored.
To determine the viability of (dippisq)U(CH2Ph)3 as an
intermediate in the formation of 2, independent synthesis was
attempted by salt metathesis of (dippisq)UI3 with benzylpotas-
sium. On the basis of the reduction potential of dippiq, we
reasoned that addition of the iminoquinone ligand to
UI3(THF)4 would generate (dippisq)UI3. Exposure of a dark
blue solution of UI3(THF)4 to 1 equiv of (dippiq) results in an
instantaneous color change to yellow-brown. After workup, the
product was isolated as a brown powder in quantitative yield.
1H NMR analysis revealed 10 broad resonances ranging from
−26.92 to 45.62 ppm, supporting the assignment of the
product as the uranium iminosemiquinone species (dippisq)-
UI3(THF)2 (3). Two resonances are visible at −17.50 and
13.62 ppm (9H each), corresponding to the two tert-butyl
groups of the ligand. Additionally, a signal integrating to 12H
corresponding to the symmetric isopropyl methyl substituents
is located at 1.57 ppm. Two broad resonances at 40.61 and
45.62 ppm (8H each) correspond to two coordinated THF
ligands.
Figure 3. Molecular structure of 3 shown with 30% probability
ellipsoids. Hydrogen atoms and solvent molecules have been removed
for clarity.
bis(iminosemiquinone), [(Risq)2UCl]2 (R = tBu, dipp), which
has short U−O distances (2.151(6), 2.150(6) Å) and longer
U−N distances (2.568(7), 2.563(7) Å).31
One-electron ligand reduction was also evident by
comparison of the intraligand bond distances of 3. The C−O
bond distance of 1.346(8) Å has been elongated from that of
the free ligand dippiq (1.221(2) Å),17 indicating reduction of the
CO bond order from 2 to 1, as in complex 2. The C−N
bond distance of 1.350(8) Å in 3 is longer than that of the
oxidized ligand (1.287(2) Å)17 and is within the range of
distances of organometallic complexes with [isq]− radical
ligands, ranging from 1.33 to 1.36 Å.50,51 The dippisq ligand in 3
shows structural parameters similar to those for the [isq]−
radical anion in (tBuisq)2MCl2 (M = Zr, Hf) (1.33−1.36 Å)
(Chart 1).50 The C1−C2 bond length of 1.443(9) Å has been
Chart 1. Comparison of Bond Distances (Å) in 3 with Those
of Iminosemiquinone Complexes (tBuisq)2MCl2 (M = Zr, Hf)
Compound 3 was further characterized by X-ray crystallog-
raphy of brown crystals grown from a concentrated THF
solution layered with pentane at −35 °C. Refinement of the
crystallographic data confirmed the assignment of the product
as (dippisq)UI3(THF)2, which features a seven-coordinate
uranium center in a distorted-capped-octahedral geometry
(Figure 3, Table 1). The U−I distances of 3.0126(4),
3.0136(5), and 3.0140(4) Å are shorter than those reported
for the starting material UI3(THF)4 (3.103(2), 3.119(2), and
3.167(2) Å),18 consistent with the decrease in ionic radius that
accompanies oxidation from U(III) to U(IV). The U−I
distances compare favorably with those reported for
UI4(OEt2) (2.964 Å),46 UI4(dioxanes)2 (2.964 Å),47 and
UI(O(2,6-di-tert-butyl)phenyl)3 (3.011 Å).48 The U−O(THF)
distances of 2.484(4) and 2.499(4) Å are similar to those of the
starting material, which range from 2.48(1) to 2.56(2) Å.18 The
short U1−O1 distance of 2.154(4) Å is consistent with an
anionic interaction, as observed in the structural parameters of
2. An U1−N1 distance of 2.570(5) Å elongated from that of 2
is noted and is outside the range of uranium(IV) amide linkages
(2.161−2.298 Å), suggesting a neutral U−N interaction.27−30,49
These parameters are also consistent with the uranium(IV)
shortened from that of dippisq (1.519(2) Å), as expected.
Structural similarities of the radical ligand in 3 and 4,6-di-tert-
butyl-2-[(tert-butyl)imino]semiquinone (tBuisqH) are also
noted.45
The electronic absorption spectrum of 3, collected from 280
to 2100 nm in THF at ambient temperature, shows weak bands
(30−110 M−1cm−1) at low energy for f−f transitions of the f2
U(IV) center (Figure S6, Supporting Information).31,43,44 At
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higher energies, a broad feature at 763 nm (ε = 627 M−1 cm−1)
for the open-shell iminosemiquinone ligand dominates the
spectrum. The transition falls within the range of bands
observed in the electronic absorption spectra of organometallic
complexes containing a [isq]− ligand, including (tBuisq)2M(X)2
(M = Ti, Zr, Hf; X = F, Cl, Br)50,51 and (Risq)2UCl2 (R = tBu,
Ad, dipp).40
which have magnetic moments of 1.51 (5 K), 1.61 (4 K), and
1.75 μB (4 K), respectively. Thus, the magnetic moment
observed for 3 is due to the magnetic contribution from the
[
dippisq]− radical ligand, supporting its formulation as a charge-
separated uranium(IV) species with an iminosemiquinone
ligand. Contributions from the trivalent uranium tris(iodide)
resonance structure (dippiq)UI3(THF)2 cannot be eliminated on
the basis of the data collected.
Support for the presence of the iminosemiquinone ligand
[
dippisq]− in 3 was obtained using variable-temperature SQUID
With the successful synthesis of 3, alkylation to form
(
dippisq)U(CH2Ph)3 was attempted (Scheme 2). Three
magnetometry. For comparison, compound 2, which has a
closed-shell amidophenolate ligand, [dippap]2−, was also
measured (Figure 4). Typical magnetic moments for uranium-
equivalents of benzyl potassium were added to a THF solution
of 3, resulting in an instantaneous color change to red with
concurrent precipitation of KI. After workup, the isolated red
solid was recrystallized from a concentrated diethyl ether
solution. The 1H NMR spectrum for this product was
recognizable as that for (dippap)U(CH2Ph)2(THF)2 (2), and
1
analysis of the solution also showed /2 equiv of bibenzyl. The
unsuccessful attempt at isolation of (dippisq)U(CH2Ph)3
highlights the reactive nature of this compound and suggests
the possible formation of this species during radical reductive
elimination to produce 2.
Since the formation of 2 is accompanied by the reduction of
the iminosemiquinone ligand to the amidophenolate, the
reduction chemistry of 3 was investigated further. Exposure
of (dippisq)UI3(THF)2 to 1 equiv of KC8 resulted in immediate
darkening of the solution. After filtration and removal of
solvents in vacuo, a brown-yellow solid was isolated (Scheme
Figure 4. Temperature-dependent SQUID magnetization data (1 T)
for compounds 2 (red) and 3 (blue) plotted as magnetic moment
(μeff) vs temperature (T). Data were corrected for diamagnetism, and
reproducibility was checked on multiple independently synthesized
samples.
1
3). Analysis by H NMR spectroscopy showed 11 resonances
broadened and shifted by the paramagnetic uranium center.
Two large resonances integrating to 9H each are located at 1.55
and 5.35 ppm, corresponding to the tert-butyl groups of the
ligand. A large doublet located at −9.58 ppm integrating to
12H is assigned to the methyl groups of the isopropyl groups
on the aryl ring. Four broad singlets (−11.49, −8.43, −7.21,
and 6.50 ppm), integrating to 4H each, indicate coordination of
two THF molecules and support assignment of the product as
(IV) species show a steady drop in μeff as the temperature is
lowered, decreasing from ∼3.0 (300 K) to ∼0.5 μB (4 K),
consistent with a poorly isolated singlet ground state arising
from ligand field effects.43,44,52 Data obtained for compound 2
fit within this established range and show a magnetic moment
of 2.39 μB at 300 K, which decreases to 0.63 μB at 4 K and to
0.47 μB at 2 K. Three independently synthesized solid samples
of 3 were measured over the same range, and the collected data
display a similar variable-temperature behavior. At 300 K, the
magnetic moment is 3.23 μB; however, at low temperature, a
magnetization of 1.50 μB is measured. This high value at low
temperature is typical for charge-separated uranium(IV) species
w i t h r a d i c a l a n i o n i c l i g a n d s , i n c l u d i n g
[((A d ArO)3 tacnUI V (CO2 • − )],52 [((t B u ArO)3 tacn)-
UIV(OC•tBuPh2)],43 and [((tBuArO)3tacn)UIV(η2-NNCPh2)],53
(
dippap)UI2(THF)2 (4).
The dippisq ligand in 3 serves as an electron sink during
reduction to form (dippap)UI2(THF)2, which maintains the
thermodynamically preferred +4 oxidation state of the uranium.
This is confirmed by the electronic absorption spectrum of 4,
which shows loss of the absorbance for the iminosemiquinone
ligand (763 nm), supporting ligand reduction to the
amidophenolate form (Figure 2). Furthermore, weakly intense
but sharp f−f transitions found in the near-IR region of the
spectrum confirm the presence of a tetravalent uranium
center43,44 and are similar to those previously observed for
Scheme 3. Formation and Reactivity of Complex 4
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the family of amidophenolate species (Rap)2U(THF)2 (R =
tBu, Ad, dipp).31
metathesis of 4 highlights the importance of the reductive
elimination reaction in generating uranium dialkyl complexes.
Interestingly, alkylation of 4 with 2 equiv of benzylpotassium
did not result in the formation of 2 via salt metathesis as
expected (Scheme 3). 1H NMR spectroscopy revealed no
resonances consistent for (dippap)U(CH2Ph)2(THF)2; instead,
only a sharp signal for bibenzyl was noted. The observed
decomposition supports that ligand-assisted extrusion of a
benzyl radical is an essential mechanistic step in the formation
of 2. Treating 4 with 1 equiv of KCH2Ph, however, affords the
mono(benzyl) uranium(IV) product (dippap)UI(CH2Ph)-
(THF)2 (5). This formulation is supported by the 19 broad
resonances, ranging from −68.02 to 173.44 ppm, observed in
CONCLUSIONS
■
In summary, the iminoquinone ligand dippiq effectively mediates
carbon−carbon reductive elimination from tetrabenzyluranium
to form 2. The accompanying redox chemistry does not occur
at the uranium center but rather at the ligand, which is reduced
to the amidophenolate dippap, stabilizing the low-valent
products. In contrast to the α-diimine ligand MesDABMe, dippiq
forces a radical reductive elimination pathway, as supported by
a crossover experiment, likely due to the stability of the
iminosemiquinone intermediate. Attempts to isolate the
potential intermediate (dippisq)U(CH2C6H5)3 through alkyla-
tion of 3 led only to formation of 2 and bibenzyl. While the
overall reductive elimination is a two-electron process, the
iminoquinone ligand has a stable iminosemiquinone inter-
mediate as observed in the cyclic voltammogram, facilitating
two radical events to form 2. The studies presented here
highlight the ability of the iminoquinone ligand to mediate
carbon−carbon reductive elimination, support alkylation
chemistry, and alter the mechanism for reductive elimination
by tetrabenzyluranium on the basis of ligand redox potentials.
1
the H NMR spectrum due to asymmetry.
Structural details of 5 were elucidated by X-ray crystallog-
raphy of crystals grown from a concentrated solution of
pentane. Although only non-merohedral twinned crystals could
be obtained, refinement of publishable quality structural data
gave a pseudo-octahedral uranium compound, (dippap)UI-
(CH2Ph)(THF)2 (5) (Figure 5, Table 3). The U1−N120
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, text, and CIF files giving details of the syntheses and
electrochemical, spectroscopic, and crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.C.B.: sbart@purdue.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 5. Molecular structure of 5 shown with 30% probability
ellipsoids. Hydrogen atoms and solvent molecules have been removed
for clarity.
The authors gratefully acknowledge Purdue University and the
National Science Foundation (CHE-1149875). SCB is a
Cottrell Scholar funded by the Research Corporation.
Table 3. Structural Parameters (Distances in Å and Angles in
deg) for 5
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