Exploring the insertion chemistry of tetrabenzyluranium using carbonyls and organoazides

Article abstract of DOI:10.1021/om400197j

The insertion chemistry of U(CH₂C₆H₅) ₄ (1) was explored with acetone, benzophenone, mesityl azide, and 1-azidoadamantane. Using 2 equiv of acetone affords the double-insertion product U[OC(CH₃)₂

Full text of DOI:10.1021/om400197j

Article
pubs.acs.org/Organometallics
Exploring the Insertion Chemistry of Tetrabenzyluranium Using
Carbonyls and Organoazides
Steven J. Kraft,† Phillip E. Fanwick, and Suzanne C. Bart*
H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47906, United States
S
* Supporting Information
ABSTRACT: The insertion chemistry of U(CH2C6H5)4 (1)
was explored with acetone, benzophenone, mesityl azide, and
1-azidoadamantane. Using 2 equiv of acetone affords the
double-insertion product U[OC(CH3)2(CH2C6H5)]2-
(CH2C6H5)2(THF)2 (2), while using 4 equiv results in the
tri-inserted enolate product U[OC(CH3)2(CH2C6H5)]3[OC-
(CH3)CH2](THF)3 (3). Deuterium labeling experiments
aided in the assignment of 2 and 3. With 4 equiv of
benzophenone, insertion at all U−C bonds is noted, forming
U[OC(C6H5)2(CH2C6H5)]4 (4) and the THF adduct U[OC-
(C6H5)2(CH2C6H5)]4(THF) (4-THF). Addition of 4 equiv of
N3Mes to 1 forms the tetrakis(triazenido)uranium(IV)
complex U[CH2(C6H5)NNN(Mes)-κ2N1,2][CH2(C6H5)-
NNN(Mes)-κ2N1,3]3 (5), while the same reaction with 1-
azidoadamantane generates the uranium(VI) trans-bis(imido)
complex U(NAd)2[CH2(C6H5)NNN(Ad)-κ2N1,3]2(THF) (6).
All species were characterized by 1H NMR and infrared
spectroscopy, with select examples being structurally charac-
terized using single-crystal X-ray diffraction.
s the study of organoactinide complexes for small-
molecule activation has risen in recent years, so has the
use of actinides, specifically uranium, for insertion chemistry.
Because M−C insertion is an essential role in many catalytic
transformations,1−8 understanding this process for uranium is
an important step toward the development of uranium-based
catalysts.
While trivalent uranium compounds readily undergo radical
reactions, uranium(IV) alkyls are a versatile and robust class of
molecules for insertion chemistry. The uranium(IV) dimethyl
species Cp*2UMe2 (Cp* = 1,2,3,4,5-pentamethylcyclopenta-
dienide) readily inserts carbon dioxide,9 acetone,10 carbodii-
mides,11 nitriles,12 diazoalkanes,13 and 1-azidoadamantane,11
while the related tethered species [(η5-C5Me4SiMe2CH2-
κC)2U] reacts similarly with carbon monoxide,14 carbodii-
mides,15 carbon disulfide,16 and 1-azidoadamantane.16
Our laboratory recently reported the synthesis of tetrabenzy-
luranium, U(CH2Ph)4 (1), a homoleptic uranium(IV) alkyl
compound that engages in concerted carbon−carbon reductive
elimination of bibenzyl in the presence of the redox-active α-
azidoadamantane and the characterization of these products
by H NMR and infrared spectroscopies as well as by X-ray
crystallography. The results presented here mark the first
exploration of the reactivity of a uranium tetrakis(alkyl) toward
insertion chemistry.
1
A
EXPERIMENTAL SECTION
■
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 coldwell 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.18 Benzene-d6 was
purchased from Cambridge Isotope Laboratories, dried with molecular
sieves and sodium, and degassed by three freeze−pump−thaw cycles.
Benzophenone was recrystallized from diethyl ether and dried on the
Schlenk line prior to use. Acetone-d6 and 1-azidoadamantane were
purchased from Sigma Aldrich and used without further purification.
Acetone was purchased from Sigma Aldrich, dried over CaCl2, and
distilled. U(CH2C6H5)4 (1),17 1-d28,17 and mesityl azide19 were
prepared according to literature procedures.
diimine MesDABMe
(
MesDABMe = MesNC(Me)C(Me)
NMes; Mes = 2,4,6-trimethylphenyl), highlighting its ability
to perform fundamental organometallic reactions.17 Given the
reactivity of these U−C bonds, this molecule presents a unique
opportunity to study multiple migratory insertions on a single
tetravalent uranium center. Herein, we present the reactivity of
1 toward acetone, benzophenone, mesityl azide, and 1-
1H NMR spectra were recorded on a Varian Inova 300
spectrometer operating at 299.992 MHz. All chemical shifts are
Received: March 11, 2013
Published: May 28, 2013
© 2013 American Chemical Society
3279
dx.doi.org/10.1021/om400197j | Organometallics 2013, 32, 3279−3285

Organometallics
Article
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 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. Solid-
state infrared spectra were recorded using a Perkin-Elmer FT-IR
Spectrum RX I spectrometer. Samples were made by crushing the
solids, mixing with dry KBr, and pressing into a pellet.
While the mixture was stirred, 0.070 g (0.412 mmol) of benzophenone
was added, resulting in a color change from dark red to green within 5
min. After 1 h of stirring, the solvent was removed in vacuo to yield a
green oil assigned as U[OC(C6H5)2(CH2C6H5)]4(THF) (4-THF).
Trituration with diethyl ether and subsequently pentane (approx-
imately 5 mL three times each) resulted in a gray-purple solid assigned
as U[OC(C6H5)2(CH2C6H5)]4 (4). Yield: 0.120 g (0.093 mmol,
91%). X-ray-quality crystals of 4-THF were obtained from cooling a
concentrated solution in THF and pentane (10:1) to −35 °C. Due to
the product being a mixture of 4 and 4-THF, reproducible elemental
analysis was not obtained. 1H NMR (C6D6, 25 °C): δ −5.59 (98, 8H),
0.58 (32, 8H), 2.52 (26, 4H, p-Ar-H), 7.30 (21, 8H), 7.67 (21, 16H),
10.58 (88, 8H), 11.68 (31, 16H).
Single crystals 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.20 The structures were
solved using the structure solution program PATTY in DIRDIF99.21
Refinement was performed on a LINUX PC using SHELX-97.20 The
data were collected at a temperature of 150(1) K.
The capillary gas chromatography/mass spectrometry analyses were
carried out using an Agilent 5975C (Agilent Laboratories, Santa Clara,
CA) mass spectrometer system. The typical electron energy was 70
eV, with the ion source temperature maintained at 250 °C. The
individual components were separated using a 30 m HP-5 capillary
column (250 m i.d. × 0.25 m film thickness). The initial column
temperature was set at 35 °C (for 3 min) and programmed to reach
280 °C with a ramp of 10.0 °C/min. The flow rate was set at 1 mL/
min and the injector set at 250 °C.
Synthesis of U[OC(CH3)2(CH2C6H5)]2(CH2C6H5)2(THF)2 (2). A
20 mL scintillation vial was charged with 0.042 g (0.070 mmol) of 1
and approximately 5 mL of THF. While the mixture was stirred, 8.0 μL
(0.140 mmol) of acetone was added, resulting in a color change from
dark red to brown within 5 min. Upon stirring for 30 min, the solvent
was removed in vacuo, to yield a brown oil assigned as U[OC-
(CH3)2(CH2C6H5)]2(CH2C6H5)2(THF)2 (2). Yield: 0.065 g (0.067
mmol, 97%). The oily nature of 2 precluded reproducible elemental
analysis. 1H NMR (C6D6, 25 °C): δ −213.97 (504, 4H, CH2), −21.88
(126, 4H), −17.79 (61, 2H, p-Ar-H), −17.35 (577, 8H, CH2-THF),
−16.86 (28, 2H, p-Ar-H), −12.53 (44, 4H), −7.68 (517, 8H, CH2-
THF), 8.46 (51, 4H), 25.99 (73, 4H), 83.85 (172, 12H, CH3), 91.09
(161, 4H).
Synthesis of 2-d28. With use of a procedure similar to that for 2,
the product 2-d28 was prepared from 1-d28 and 2 equiv of acetone.
Synthesis of 2-d12. With use of a procedure similar to that for 2,
the product 2-d12 was prepared from 1 and 2 equiv of acetone-d6.
Synthesis of U[OC(CH3)2(CH2C6H5)]3[OC(CH3)CH2](THF)3 (3).
A 20 mL scintillation vial was charged with 0.064 g (0.106 mmol) of 1
and approximately 5 mL of THF. While the mixture was stirred, 31.2
μL (0.425 mmol) of acetone was added, resulting in a color change
from dark red to green-brown within 5 min. Upon stirring for 30 min,
the solvent was removed in vacuo to yield a green-brown oil assigned
as U[OC(CH3)2(CH2C6H5)]3[OC(CH3)CH2](THF)3 (3). Yield:
0.090 g (0.099 mmol, 85%). The oily nature of 3 precluded
Synthesis of U[CH2(C6H5)NNN(Mes)-κ2N1,2][CH2(C6H5)NNN-
(Mes)-κ2N1,3
] (5). A 20 mL scintillation vial was charged with
3
0.093 g (0.154 mmol) of 1 and approximately 5 mL of THF and
cooled to −35 °C. While the mixture was stirred, 0.100 g (0.617
mmol) of mesityl azide was added. Upon stirring for 1 h, a color
change from dark red to bright red occurred. The THF was removed
in vacuo, and the residue was taken up in diethyl ether, filtered, and
dried. A red solid was obtained. Recrystallization from a concentrated
solution of diethyl ether at −35 °C gave orange-red crystals assigned as
U[CH2(C6H5)NNN(Mes)-κ2N1,2][CH2(C6H5)NNN(Mes)-κ2N1,3
]
3
(5). Yield: 0.124 g (0.099 mmol, 65%). X-ray-quality single crystals
were obtained in the same manner. Compound 5 began to decompose
after 12 h at room temperature in the solid state and thus would not
1
survive shipping for elemental analysis. H NMR (C6D6, 25 °C): δ
−27.26 (312, 18H, o-CH3), −1.36 (5, 3H, p-CH3), −1.31 (5, 1H, p-Ar-
H), −0.94 (7, 6H), −0.14 (7, 9H, p-CH3), 0.17 (32, 6H), 6.36 (t, 8
Hz, 3H, p-Ar-H), 6.49 (t, 7 Hz, 6H), 11.21 (26, 6H), 12.64 (21, 2H),
14.78 (21, 2H), 36.81 (71, 2H), 45.60 (645, 6H), 101.30 (429, 2H).
Synthesis of U(NAd)2[CH2(C6H5)NNN(Ad)-κ2N1,3]2(THF) (6). A
20 mL scintillation vial was charged with 0.052 g (0.086 mmol) of 1
and approximately 5 mL of THF. While the mixture was stirred, 0.061
g (0.345 mmol) of 1-azidoadamantane was added and instantaneous
N2 evolution occurred, as indicated by effervescence. Upon stirring for
1 h, a color change from dark red to orange-red occurred. The THF
was removed in vacuo, and the residue was taken up in pentane,
filtered, and dried. An orange-red solid was obtained. Recrystallization
in a concentrated solution of diethyl ether at −35 °C gave orange
crystals assigned as U(N-Ad)2[CH2(C6H5)NNN(Ad)-κ2N1,3]2(THF)
(6). X-ray-quality single crystals were obtained in the same manner.
Yield: 0.063 g (0.065 mmol, 70%). Compound 6 began to decompose
after 12 h at room temperature in the solid state and thus would not
1
survive shipping for elemental analysis. H NMR (C6D6, 25 °C): δ
0.60 (9, 12H), 0.99 (12, 12H), 1.63 (44, 6H), 1.88 (44, 12H), 2.35
(19, 6H), 2.72 (8, 12H), 3.45 (1072, 4H, CH2-THF), 4.87 (111, 4H,
CH2-THF), 6.20 (7, 4H, CH2(C6H5)), 7.10 (t, 9 Hz, 2H, p-Ar-H),
7.34 (t, 8 Hz, 4H, m-Ar-H), 7.96 (d, 6 Hz, 4H, o-Ar-H).
Crossover Experiment of 1 and 1-d28 with 1-Azidoadaman-
tane. A 20 mL scintillation vial was charged with 0.030 g (0.050
mmol) of 1, 0.031 g (0.050 mmol) of 1-d28, and 5 mL of THF. While
the mixture was stirred, 0.071 g (0.400 mmol) of 1-azidoadamantane
was added and instantaneous N2 evolution occurred, as indicated by
effervescence. Upon stirring for 1 h, a color change from dark red to
orange-red occurred, indicating formation of 6. The resulting solution
was filtered through an alumina column using THF to separate the
uranium-containing material from the organic products (pale orange).
Capillary gas chromatography/mass spectrometry analyses showed the
formation of a statistical mixture of bibenzyl (m/z 182), bibenzyl-d7
(m/z 189), and bibenzyl-d14 (m/z 196).
1
reproducible elemental analysis. H NMR (C6D6, 25 °C): δ −101.27
(43, 1H, p-Ar-H), −83.19 (24, 2H, CH2-enolate), −73.93 (29, 2H,
CH2), −61.07 (19, 6H, CH3), −9.26 (d, 7.2 Hz, 2H, o-Ar-H), 0.67 (t,
2H, m-Ar-H), 2.67 (31, 3H, CH3-enolate), 6.60 (562, 24H, CH2-THF)
7.82 (22, 6H, CH3), 9.18 (t, 8 Hz, 1H, p-Ar-H), 9.99 (t, 7 Hz, 2H, m-
Ar-H), 11.63 (t, 7 Hz, 1H, p-Ar-H), 12.83 (t, 7 Hz, 2H, m-Ar-H),
14.89 (d, 7 Hz, 2H, o-Ar-H), 22.15 (d, 7 Hz, 2H, o-Ar-H), 34.60 (17,
2H, CH2), 47.61 (14, 6H, CH3), 55.08 (19, 2H, CH2).
RESULTS AND DISCUSSION
■
Synthesis of 3-d21. With use of a procedure similar to that for 3,
the product 3-d21 was prepared from 1-d28 and 4 equiv of acetone.
Synthesis of 3-d23. With use of a procedure similar to that for 3,
the product 3-d23 was prepared from 1 and 4 equiv of acetone-d6.
Synthesis of U[OC(C6H5)2(CH2C6H5)]4(THF) (4-THF) and U-
[OC(C6H5)2(CH2C6H5)]4 (4). A 20 mL scintillation vial was charged
with 0.062 g (0.103 mmol) of 1 and approximately 5 mL of THF.
The reactivity for U(CH2Ph)4 (1) toward migratory insertion
was first tested with carbonylated substrates, as the oxophilicity
of uranium was presumed to be an important driving force.
Compound 1 was treated with 2 equiv of acetone, which
afforded a dark brown oil after workup. The infrared spectrum
1
of the product did not show a carbonyl absorption. The H
3280
dx.doi.org/10.1021/om400197j | Organometallics 2013, 32, 3279−3285

Organometallics
Article
Scheme 1
NMR spectrum contained 11 resonances ranging from −213.97
to 91.09 ppm, supporting a C2-symmetric molecule in solution
that is consistent with two acetone insertions. A key resonance
appears at −213.97 ppm (4H) for the U−CH2 linkages of the
two unreacted benzyl groups, similar to the case for
Cp*Cp2U(CH2C6H5), which has a U−CH2 resonance assigned
at −229.58 ppm.22 The other diagnostic singlet appears at
83.85 ppm (12H) for equivalent CH3 groups of two inserted
acetone molecules, leading to the assignment of the brown
product as U[OC(CH3)2(CH2C6H5)]2(CH2C6H5)2(THF)2
(2) (Scheme 1).
tains three alkoxide ligands formed from acetone insertions as
well as an enolate ligand.
The two isotopologues U[OC(CH3)2(CD2C6D5)]3[OC-
(CH3)CH2](THF)3 (3-d21) and U[OC(CD3)2(CH2C6H5)]3-
[OC(CD3)CD2](THF)3 (3-d23) were synthesized using 1-d28
1
and acetone-d6, respectively, to elucidate the NMR data. H
NMR spectroscopic analysis of 3-d21 showed five resonances at
−61.07, 7.82, 47.61, 2.67, and −83.19 ppm (6:6:6:3:2). The
first three resonances represent three equivalent CH3 pairs for
the three inserted acetone molecules, while the last two are
assigned as a single CH3 and a CH2, respectively, for the enolate
ligand. As expected, these five resonances derived from the
Further support for the assignment of the acetone resonance
of 2 was gained by deuteration in the preparation of
U[OC(CH3)2(CD2C6D5)]2(CD2C6D5)2](THF)2 (2-d28) and
U[OC(CD3)2(CH2C6H5)]2-(CH2C6H5)2](THF)2 (2-d12). 2-
1
acetone molecules do not appear in the H NMR spectrum of
3-d23. In addition to these resonances, 13 signals are found for
3, characteristic of the three inequivalent benzyl groups and the
three coordinated THF molecules. Detailed assignments are
presented in the Experimental Section. The THF ligands in 3
are labile in solution, and their resonances weaken with
addition of THF-d8 to an NMR tube sample, indicating solvent
exchange on the NMR time scale.
d28 was synthesized using 1-d28 and acetone and shows only the
resonances for inserted acetone methyl groups and coordinated
1
THF in the H NMR spectrum. Using 2 equiv of acetone-d6
with 1 forms the isotopologue, 2-d12, causing the resonance for
1
the acetone methyls at 83.85 ppm to disappear from the H
NMR spectrum in comparison to the spectra for 2 and 2-d28.
All other expected resonances for 2 are present in the H
spectrum of 2-d12. Compound 1 reacts similarly to Tp2UCl-
(CH2SiMe3) and Tp2U(CH3)2, which insert 1 and 2 equiv of
acetone to form Tp2UCl(OC(CH3)2CH2SiMe3) and Tp2U-
(OCMe3)2, respectively.23
The preference for enolate formation over acetone insertion
is attributed to the increased steric bulk at uranium following
acetone insertions. This is supported by the report of the
isolated uranium(III) enolate species Tp*2U(OC(CH2)-
CH3),24 which forms from protonation of the sterically
protected benzyl group in Tp*2UCH2Ph. An enolate
intermediate has also been proposed in the formation of the
lanthanum aldolate Cp*2Ln(OCMe2CH2C(O)Me).25 For 3,
enolate formation is preferred, as adding 3 equiv of acetone to 1
produces only 3 in reduced yield, with no evidence for the tri-
inserted product. Furthermore, there is no evidence for aldolate
formation in the synthesis of 3.
1
Interestingly, the addition of only 1 equiv of acetone to 1
does not result in a single insertion but produces 2 in ∼50%
yield. Because all four benzyl ligands are not activated in the
preparation of 2, two alkyls remain for further reactivity. Thus,
1 was treated with 4 equiv of acetone under the same reaction
conditions, which produced a green-brown oil, 3. The 1H NMR
spectrum of 3 is paramagnetically shifted and broadened with
18 resonances ranging from −101.27 to 55.08 ppm but does
not display the symmetry for the expected tetrainsertion
product. Rather, it appears that a non-insertion pathway is
operative in the synthesis of 3. To test this, 3 was prepared in a
sealed NMR tube; analysis revealed the formation of 1 equiv of
toluene upon acetone addition, supporting protonation of a
benzyl group by the acidic methyl protons. This has been
previously observed for f-block elements,24−26 where sub-
sequent rearrangement forms an enolate ligand. On the basis of
Benzophenone was also used to probe the insertion
chemistry of 1, as it was hypothesized that the lack of acidic
protons would favor insertion. Upon addition of 4 equiv of
benzophenone to a red THF solution of 1, a color change to
green was observed; however, workup resulted in isolation of a
gray-purple solid. IR spectroscopy of the solid showed no
absorptions characteristic of a carbonyl moiety, supporting
benzophenone insertion.
1
This assertion was supported by H NMR spectroscopy,
which showed a paramagnetically shifted and broadened
spectrum with seven resonances ranging from −5.59 to 11.68
ppm. Two resonances at 7.67 and 11.68 ppm (16H)
t h i s , c o m p o u n d
3
i s a s s i g n e d a s U [ O C -
(CH3)2(CH2C6H5)]3[OC(CH3)CH2](THF)3 (3), which con-
3281
dx.doi.org/10.1021/om400197j | Organometallics 2013, 32, 3279−3285

Organometallics
Article
Figure 1. Molecular structures of 4-THF (a), 5 (b), and 6 (c) shown with 30% probability ellipsoids. Solvent molecules and hydrogen atoms have
been omitted for clarity.
correspond to meta and ortho Ar-H protons of benzophenone,
while the signal at 2.52 ppm (4H) is consistent with p-Ar-H
protons of the benzyl substituent. The remaining four
resonances integrate to eight protons each and are not
assignable on the basis of integration. The number of
resonances and symmetry support that the product is the
3282
dx.doi.org/10.1021/om400197j | Organometallics 2013, 32, 3279−3285

Organometallics
Article
result of benzophenone insertion at each U−C bond,
U[OC(C6H5)2(CH2C6H5)]4 (4) (Scheme 1).
supporting that insertion occurred to form 1-benzyl-3-
mesityltriazenido ligands. A symmetric tetrainserted product
analogous to 4 would show a H NMR spectrum with seven
1
Initial attempts to generate a single crystal of 4 suitable for X-
ray diffraction from hydrocarbon solvents were unsuccessful.
However, upon addition of THF to 4, a color change from
gray-purple to green ensued. Removing THF under reduced
pressure regenerated the gray-purple compound, supporting
that the green product is the THF adduct U[OC-
(C6H5)2(CH2C6H5)]4(THF) (4-THF) (Scheme 1). Com-
pound 4 is capable of cycling between colors at least five
times with little decomposition observed.
resonances. However, the spectrum showed 14 paramagneti-
cally shifted and broadened signals ranging from −27.26 to
101.30 ppm.
1
To gain insight into the unexpected H NMR spectrum, the
molecular structure of 5 was determined by X-ray crystallog-
raphy. Single crystals were grown from a concentrated solution
of diethyl ether at −35 °C (Figure 1b; key structural parameters
are given in Table 2). Analysis revealed the structure as the
Cooling a concentrated THF/pentane solution (10:1) of 4-
THF resulted in pale green crystals. Single-crystal X-ray analysis
showed a five-coordinate product, with a THF ligand and four
alkoxides formed by insertion of benzophenone (Figure 1a; key
structural parameters are given in Table 1). The uranium center
Table 2. Structural Parameters for 5
bond
length (Å)
bond
length (Å)
U1−N21
U1−N31
U1−N41
U1−N23
U1−N33
U1−N43
2.410(5)
2.436(5)
2.421(5)
2.485(5)
2.479(5)
2.433(5)
U1−N12
U1−N13
U1−N42
U1−N22
U1−N32
2.516(5)
2.297(5)
2.939(5)
2.953(5)
2.959(6)
Table 1. Structural Parameters for 4-THF
bond
length (Å)
connection
angle (deg)
U1−O1
U1−O2
U1−O3
U1−O4
U1−O51
2.219(6)
2.123(6)
2.118(6)
2.127(5)
2.485(7)
O51−U1−O4
O3−U1−O2
O3−U1−O1
O3−U1−O4
O3−U1−O51
174.8(2)
118.7(3)
111.8(2)
99.6 (2)
85.4(2)
eight-coordinate, distorted bicapped trigonal prismatic tetra-
azenido uranium compound [CH2(C6H5)NNN(Mes)-κ2N1,2]-
U[CH2(C6H5)NNN(Mes)-κ2N1,3]3 (5). One triazenido ligand
is coordinated in a κ2N1,2 fashion, while the other three are
coordinated in a κ2N1,3 fashion. With the two different ligand
hapticities in 5 established, the complicated 1H NMR spectrum
was completely assigned (details in the Experimental Section).
The U−N distances for the three κ2N1,3 ligands in 5, ranging
from 2.410(5) to 2.485(5) Å, are consistent with full
delocalization over the triazenido ligand. In analogy to 1,
Evans recently reported 1-azidoadamantane insertion into the
U−C σ bonds of Cp*2U(Me)2 to generate Cp*2UMe[(Me)-
NNN(Ad)-κ2N1,3].11 Although structural parameters could not
be established for this compound, substitution of the methyl
group yields Cp*2UX[(Me)NNN-(Ad)-κ2N1,3] (X = I, Br,
OTf), all of which have been crystallographically character-
ized.33 This family has U−N distances ranging from 2.371(4)
to 2.460(2) Å, which are on the order of those for 5. The
κ2N1,2-triazenido ligand in 5 has significantly different U−N
distances of 2.297(5) and 2.516(5) Å, suggesting one anionic
and one dative U−N interaction as in the uranium(IV)
hydrazonido compound Cp*2U[η2(N,N′)-RNNCPh2]2,
which has U−N anionic bonds ranging from 2.275(17) to
2.317(8) Å and U−N dative interactions from 2.469(8) to
2.526(16) Å.13
is pseudo trigonal bipyramidal, with U−O distances of
2.219(6), 2.123(6), 2.118(6), and 2.127(5) Å for the anionic
alkoxide ligands. All are on the order for monoanionic U−O
bonds. The last three are similar to U(OCMe2CH2{C-
(NCH2CH2NR)})(N(SiMe3)2)2 (R = 2,6-iPr2C6H3) of
2.122(3) Å27 and [((AdArO)3tacn)UIV(OCPhPh−CPh2O)-
UIV((AdArO)3tacn)] ((AdArO)3tacn = trianion of 1,4,7-tris(3-
adamantyl-5-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclono-
nane) of 2.109(4) Å.28 The U−O51 bond distance of 2.485(7)
Å is consistent with the distances in UCl4(THF)3.29
The benzophenone insertion by 1 is similar to that observed
for ((Me3 Si)2 N)3 UCH3 in the formation of
30
((Me3Si)2N)3UOCPh2CH3 and reminiscent of aldehyde
insertion shown by Eisen and co-workers to be a key step in
thorium-mediated catalytic aldehyde dimerization.31 Thus,
tetravalent U(CH2Ph)4 facilitates four insertions at a single
uranium center, with no evidence for radical chemistry. The
insertion chemistry is driven by replacing four reactive U−C
bonds with four strong U−O bonds. Using 1, 2, or 3 equiv of
benzophenone did not result in the mono-, di-, or tri-inserted
products; instead, compound 4 was formed in lower yields.
The synthesis of 4 by carbonyl insertion diverges from
products formed with uranium(III) centers and benzophenone.
In these systems, C−O reduction to form a benzophenone
radical intermediate is noted, followed by coupling with a
nearby radical species. The result is an oxidized uranium center
with a new U−O anionic bond.28,32 Therefore, the formation of
4 marks a rare example where structural insight is provided and
highlights the important role the uranium oxidation state plays
in the reactivity with carbonylated substrates.
Organoazides were also studied with 1 to determine if
insertion is favored over oxidation. Addition of 4 equiv of
mesityl azide to 1 at −35 °C results in a color change from dark
red to orange-red. After workup and recrystallization, the
product, 5, was obtained as an orange-red solid (Scheme 1).
Although no nitrogen evolution was noted during the reaction,
IR spectroscopy of 5 also did not show an azide absorption,
Formation of 5 once again demonstrates a rare example of
four insertions at a monomeric uranium alkyl species. However,
compound 5 is not thermally stable, decomposing at room
temperature in both the solution and solid states over a 12 h
period, to generate bibenzyl and intractable uranium products.
Exposure to light does not initiate or accelerate this process.
Addition of 1, 2, or 3 equiv of mesityl azide to 1 generates 5 in
lesser yields, similar to the benzophenone case. The insertion
chemistry for N3Mes is in contrast to that observed in the case
of trivalent uranium, where addition of 1 equiv of N3Mes to
Tp*2UCH2Ph and Tp*U(CH2Ph)2 results in oxidation to the
respective uranium(IV) species Tp*2UNMes and Tp*U-
34
1
(CH2Ph)(NMes), with extrusion of /2 equiv of bibenzyl.
Compound 1 was also treated with 4 equiv of 1-
azidoadamantane (AdN3) to determine the generality of the
azide insertion reaction. Upon addition to 1, immediate
3283
dx.doi.org/10.1021/om400197j | Organometallics 2013, 32, 3279−3285

Organometallics
Article
evolution of N2 was noted, supporting the hypothesis that at
least one imido moiety was formed (Scheme 1). Workup
produced an orange-red solid, 6, that is stable for 12 h in both
the solution and solid states but eventually decomposed to a
mixture of organic products, including adamantylamine. Again,
IR spectroscopy showed no azide stretches.
reductive elimination which has been previously established for
this molecule.17
CONCLUSIONS
■
The chemistry of the uranium−carbon bonds of tetrabenzyl-
uranium reported herein highlight the diverse reactivity of
uranium(IV) alkyl species. With no ancillary ligands present, all
four U−C bonds are reactive, readily undergoing insertion,
protonation, or homolytic bond scission, depending on the
substrate, to generate sterically saturated products. In all cases,
the reactivity of the tested substrates with 1 diverges from that
of trivalent uranium. The results presented here establish that
ancillary ligands are not necessary for stabilizing the uranium
center to facilitate insertion chemistry. These studies also show
that uranium(IV) centers can support isolable yet reactive alkyl
groups, facilitating four insertions at a single uranium center,
which has not previously been observed. Thus, tetravalent
organouranium species can perform fundamental reactions that
are key steps for bond formation important to organometallic
transformations.
1
Surprisingly, analysis by H NMR spectroscopy revealed a
diamagnetic spectrum with 12 resonances ranging from 0.60 to
7.96 ppm, indicative of oxidation to uranium(VI). A resonance
assignable to bibenzyl (1 equiv vs ferrocene internal standard)
is also visible. On the basis of the N2 and bibenzyl extrusions,
compound 6 is assigned as the uranium bis(imido) bis-
(triazenido) species, U(NAd)2[CH2(C6H5)-NNN(Ad)-
κ2N1,3]2(THF). Thus, 2 equiv of azide serves to oxidize the
uranium center, while the remaining two benzyls undergo U−C
bond insertion. A key resonance at 6.20 ppm (4H) is assignable
to the methylene CH2 of the newly formed triazenido ligands.
All other resonances are accounted for and assigned
(Experimental Section).
Further characterization of 6 by X-ray crystallography was
employed to establish the hapticity of the triazenido ligands.
Cooling a concentrated solution of 6 in diethyl ether at −35 °C
afforded suitable orange-red crystals (Figure 1c). Structural
parameters (Table 3) support a seven-coordinate uranium
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files giving crystallographic data for 2-THF, 5, and 6. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Table 3. Structural Parameters for 6
bond
length (Å)
bond
length (Å)
AUTHOR INFORMATION
Corresponding Author
U1−N1
U1−N3
U1−N4
U1−N6
connection
2.440(7)
2.507(8)
U1−N7
U1−N8
U1−O1
1.903(10)
1.912(9)
2.499(6)
■
2.446(8)
*E-mail: sbart@purdue.edu.
Present Address
2.482(8)
avg angle (deg)
connection
avg angle (deg)
†Chemical Sciences and Engineering Division, Argonne Na-
tional Laboratory, 9700 S. Cass Ave., Lemont, IL 60439, United
States.
N7−U1−N8
Nax−U1−Neq
173.4(3)
91.2
Neq−U1−Nadjacent
O1−U1−Nadjacent
61.8
87.5
Notes
The authors declare no competing financial interest.
center in a distorted-pentagonal-bipyramidal geometry with two
adamantyl imido ligands and two κ2N1,3-triazenido ligands. The
two adamantyl imido ligands are trans with a near-linear N−U−
N angle of 173.4(3)° and serve as the caps for the bipyramid,
with the remaining five atoms forming the pentagonal plane.
Compound 6 has U−N distances for the triazenido ligands of
2.440(7), 2.507(8), 2.446(8), and 2.482(8) Å, which compare
well with those of 5 and Cp*2UX[(Me)NNN(Ad)-κ2N1,3].33
The two trans imido ligands in 6 have U−N distances of
1.903(10) and 1.912(9) Å, on the order of that for the recently
prepared U(VI) bis(imido) complex [Li(THF)]2[U-
(NtBu)2(NHtBu)4] (1.915(5) Å).35
Compound 6 represents the first example of a uranium(VI)
triazenido species and highlights the striking difference in
insertion vs oxidation with the reactivity of N3Mes and N3Ad
with 1. Furthermore, the formation of 6 marks an additional
synthetic route to access the trans-bis(imido) uranyl analogues
studied by Boncella and co-workers.36−39
A crossover experiment was performed to determine if a
radical mechanism is operative in bibenzyl formation. An
equimolar THF solution of 1 and 1-d28 was treated with 8
equiv of N3Ad. Separation of the organics by filtration over an
alumina column and analysis by GC-MS revealed the formation
of bibenzyl, bibenzyl-d7, and bibenzyl-d14. The observed
crossover supports homolytic cleavage and radical coupling of
benzyl groups from 1, which is in contrast to the concerted
ACKNOWLEDGMENTS
We acknowledge a grant from the National Science Foundation
(CHE-1149875) and Purdue University for funding.
■
REFERENCES
■
(1) Ananikov, V. P.; Beletskaya, I. P. Top. Organomet. Chem. 2013,
43, 1.
(2) Mandai, T. In Migratory insertion reactions of allyl, propargyl, and
allenylpalladium derivatives involving carbon monoxide and related
derivatives; Wiley: Hoboken, NJ, 2002; p 2505.
(3) Gridnev, I. D.; Imamoto, T. Chem. Commun. 2009, 7447.
(4) Franssen, N. M. G.; Walters, A. J. C.; Reek, J. N. H.; de, B. B.
Catal. Sci. Technol. 2011, 1, 153.
(5) Shu, X.-Z.; Shu, D.; Schienebeck, C. M.; Tang, W. Chem. Soc. Rev.
2012, 41, 7698.
(6) Zhang, Y.; Wang, J. Eur. J. Org. Chem. 2011, 1015.
(7) Razavi, A.; Baekelmans, D.; Bellia, V.; De, B. Y.; Hortmann, K.;
Lambrecht, M.; Miserque, O.; Peters, L.; Slawinski, M.; Van, B. S. In
Syndiotactic specific structures, symmetry considerations, mechanistic
aspects; Springer-Verlag: Berlin, 2001; p 267.
(8) Ito, Y.; Suginome, M. In Synthesis of oligomeric and polymeric
materials via palladium-catalyzed successive migratory insertion of
isonitriles; Wiley: Hoboken, NJ, 2002; pp 2705.
(9) Moloy, K. G.; Marks, T. J. Inorg. Chim. Acta 1985, 110, 127.
(10) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.;
Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6650.
3284
dx.doi.org/10.1021/om400197j | Organometallics 2013, 32, 3279−3285

Organometallics
Article
(11) Evans, W. J.; Walensky, J. R.; Ziller, J. W.; Rheingold, A. L.
Organometallics 2009, 28, 3350.
(12) Jantunen, K. C.; Burns, C. J.; Castro-Rodriguez, I.; Da Re, R. E.;
Golden, J. T.; Morris, D. E.; Scott, B. L.; Taw, F. L.; Kiplinger, J. L.
Organometallics 2004, 23, 4682.
(13) Cantat, T.; Graves, C. R.; Jantunen, K. C.; Burns, C. J.; Scott, B.
L.; Schelter, E. J.; Morris, D. E.; Hay, P. J.; Kiplinger, J. L. J. Am. Chem.
Soc. 2008, 130, 17537.
(14) Evans, W. J.; Siladke, N. A.; Ziller, J. W. Chem. Eur. J. 2010, 16,
796.
(15) Siladke, N. A.; Ziller, J. W.; Evans, W. J. J. Am. Chem. Soc. 2011,
133, 3507.
(16) Siladke, N. A.; LeDuc, J.; Ziller, J. W.; Evans, W. J. Chem. Eur. J.
2012, 18, 14820.
(17) Kraft, S. J.; Fanwick, P. E.; Bart, S. C. J. Am. Chem. Soc. 2012,
134, 6160.
(18) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(19) Spencer, L. P.; Altwer, R.; Wei, P.; Gelmini, L.; Gauld, J.;
Stephan, D. W. Organometallics 2003, 22, 3841.
(20) Sheldrick, G. M. Acta Crystallogr. 2008, 112, A64.
(21) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda,
S.; Gould, R. O.; Smits, J. M. M. DIRDIF2008 Program System;
Crystallography Laboratory, University of Nijmegen, Nijmegen, The
Netherlands, 2008.
(22) Kiplinger, J. L.; Morris, D. E.; Scott, B. L.; Burns, C. J.
Organometallics 2002, 21, 5978.
(23) Campello, M. P. C.; Calhorda, M. J.; Domingos, A.; Galvao, A.;
Leal, J. P.; Pires de Matos, A.; Santos, I. J. Organomet. Chem. 1997,
538, 223.
(24) Matson, E. M.; Fanwick, P. E.; Bart, S. C. Organometallics 2011,
30, 5753.
(25) Heeres, H. J.; Maters, M.; Teuben, J. H.; Helgesson, G.; Jagner,
S. Organometallics 1992, 11, 350.
(26) Sternal, R. S.; Sabat, M.; Marks, T. J. J. Am. Chem. Soc. 1987,
109, 7920.
(27) Turner, Z. R.; Bellabarba, R.; Tooze, R. P.; Arnold, P. L. J. Am.
Chem. Soc. 2010, 132, 4050.
(28) Lam, O. P.; Anthon, C.; Heinemann, F. W.; O’Connor, J. M.;
Meyer, K. J. Am. Chem. Soc. 2008, 130, 6567.
(29) Van Der Sluys, W. G.; Berg, J. M.; Barnhardt, D.; Sauer, N. N.
Inorg. Chim. Acta 1993, 204, 251.
(30) Dormond, A.; Aaliti, A.; Elbouadili, A.; Moise, C. J. Organomet.
Chem. 1987, 329, 187.
(31) Sharma, M.; Andrea, T.; Brookes, N. J.; Yates, B. F.; Eisen, M. S.
J. Am. Chem. Soc. 2011, 133, 1341.
(32) Mohammad, A.; Cladis, D. P.; Forrest, W. P.; Fanwick, P. E.;
Bart, S. C. Chem. Commun. 2012, 48, 1671.
(33) Evans, W. J.; Walensky, J. R.; Ziller, J. W. Organometallics 2010,
29, 101.
(34) Matson, E. M.; Crestani, M. G.; Fanwick, P. E.; Bart, S. C.
Dalton Trans. 2012, 41, 7952.
(35) Seaman, L. A.; Fortier, S.; Wu, G.; Hayton, T. W. Inorg. Chem.
2011, 50, 636.
(36) Jilek, R. E.; Spencer, L. P.; Lewis, R. A.; Scott, B. L.; Hayton, T.
W.; Boncella, J. M. J. Am. Chem. Soc. 2012, 134, 9876.
(37) Spencer, L. P.; Yang, P.; Scott, B. L.; Batista, E. R.; Boncella, J.
M. C. R. Chim. 2010, 13, 758.
(38) Spencer, L. P.; Yang, P.; Scott, B. L.; Batista, E. R.; Boncella, J.
M. J. Am. Chem. Soc. 2008, 130, 2930.
(39) Hayton, T. W.; Boncella, J. M.; Scott, B. L.; Batista, E. R.; Hay,
P. J. J. Am. Chem. Soc. 2006, 128, 10549.
3285
dx.doi.org/10.1021/om400197j | Organometallics 2013, 32, 3279−3285