Six-coordinate uranium complexes featuring a bidentate anilide ligand

Article abstract of DOI:10.1016/j.crci.2010.05.001

The synthesis of a new bidentate anilide ligand and four uranium amide complexes utilizing the ligand are reported. The secondary aniline HN[R]Ar _MeL (R = C(CD₃)₂CH₃, Ar _MeL = 2-NMe₂-5-MeC<

Full text of DOI:10.1016/j.crci.2010.05.001

C. R. Chimie 13 (2010) 781–789
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´
Full paper/Memoire
Six-coordinate uranium complexes featuring a bidentate anilide ligand
*
Alexander R. Fox, Jared S. Silvia, Erik M. Townsend, Christopher C. Cummins
Department of Chemistry, Massachusetts Institute of Technology, 77, Massachusetts Avenue, Room 6-435, Cambridge, MA 02139-4307, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
The synthesis of a new bidentate anilide ligand and four uranium amide complexes
utilizing the ligand are reported. The secondary aniline HN[R]ArMeL (R = C(CD3)2CH3,
ArMeL = 2-NMe2-5-MeC6H3) is prepared by condensation of H2NArMeL and acetone-
d6 followed by alkylation of the resulting imine with MeLi. The ligand precursors
(Et2O)Li(N[R]ArMeL) and K(N[R]ArMeL) are prepared through deprotonation of HN[R]ArMeL
Received 1 February 2010
Accepted after revision 3 May 2010
Available online 16 June 2010
Keywords:
Uranium
with n-BuLi and KH, respectively. Treatment of UI3(THF)4 with (Et2O)Li(N[R]ArMeL
)
(2 equiv) provides the uranium(III) -ate complex Li[I2U(N[R]ArMeL)2] (Li[1]), while
treatment of UI3 with three equiv. of K(N[R]ArMeL) provides the neutral uranium(III)
complex U(N[R]ArMeL)3 (2). Both uranium(III) complexes are susceptible to 1e oxidation, as
N ligands
Ligand design
is demonstrated by the syntheses of the uranium(IV) derivatives I2U(N[R]ArMeL)2 (1) and
[U(N[R]ArMeL)3][OTf] ([2][OTf]; OTf = CF3SO3). The spectroscopic and X-ray structural
characterization of all four uranium complexes is described. The structures of 2 and
[2][OTf] exhibit a large degree of steric pressure about the uranium center, effectively
preventing the [2]+ ion from achieving a seven-coordinate structure.
´
ß 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
Amides are one widely explored ancillary ligand class
that has been applied to uranium with great success [8].
Due to their unique position on the periodic table,
actinoids such as uranium present several challenges to
controlling reactivity through ligand design [1]. These
challenges include the relatively large ionic radii that are
encountered in the actinoid series, and the availability of
both d- and f-orbitals to engage in metal-ligand interac-
Uranium amides have been useful for demonstrating novel
stoichiometric reactions, and several applications to
catalysis have been demonstrated [9–11]. We have
pursued low-coordinate uranium complexes supported
by N-alkylanilide ligands of the form N[R]Ar (R = t-Bu,
C(CD3)2CH3, 1-adamantyl; Ar = 3,5-Me2C6H3). These bulky
monodentate ligands have assisted in stabilizing an
example of a uranium-silicon single bond [12] and the
first monometallic uranium nitride complex [13], as well
as examples of h6-arene coordination [14] and cooperative
reduction of dinitrogen [15].
Herein, we present several uranium-containing com-
plexes featuring a new bidentate anilide ligand. We show
that this new ligand is capable of supporting both
homoleptic and heteroleptic uranium(III) and uraniu-
m(IV) derivatives, and we provide crystallographic evi-
dence suggesting that this new ligand has properties that
tions. In the case of uranium, the ionic radii span a wide
3+
range (1.03 A (U ) ꢀ 0.73 A (U6+), for coordination number
of 6), while coordination numbers range from 3 to at least 9
[2,3]. Controlling the coordination sphere of uranium
across multiple oxidation states while minimizing struc-
tural reorganization may be effected by enforcing steric
constraints about the metal center and by managing the
degree of coordinative saturation provided by the ancillary
ligand set(s) [4–7].
˚
˚
can render
protected.
a central uranium ion sterically well-
* Corresponding author.
E-mail address: ccummins@mit.edu (C.C. Cummins).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.05.001

[(ig._1)TD$FIG]
782
A.R. Fox et al. / C. R. Chimie 13 (2010) 781–789
Fig. 1. Synthesis of bis- and tris-anilide derivatives of uranium(III) incorporating the bidentate N[R]ArMeL ligand.
2. Results and discussion
2.2. Uranium(III) complexes supported by the N[R]ArMeL
ligand
2.1. Synthesis of HN[R]ArMeL and its lithium and potassium
derivatives
We found that the number of N[R]ArMeL ligands readily
appended to a uranium(III) center through simple salt
elimination reactions was dependent upon the counter-
cation employed in the ligand precursor. This substitu-
tional selectivity allowed us to access both bis(anilide) and
tris(anilide) systems (Fig. 1).
We sought to develop a bidentate anilide ligand that
provides access to a six-coordinate uranium(III) derivative.
We envisioned that such a system, given the proper
conditions and stericloading, would allow fornovel reaction
chemistry incorporating the reducing nature of uranium(III)
anda limitedabilitytosignificantlyexpandthecoordination
sphere about the uranium center. Appending a simple
dialkylamino residue at the 2-position of the aryl group
Treatment of UI3(THF)4 with (Et2O)Li(N[R]ArMeL
)
(2 equiv) in toluene resulted in formation of (Et2O)x-
Li[I2U(N[R]ArMeL)2] (Li[1]) as a dark purple powder in 60 %
yield following separation from LiI and precipitation of the
product from solution1. The 1H NMR spectrum of Li[1] in
benzene-d6 consists of seven broad singlets found in a
range spanning from +53 to ꢀ59 ppm, while the 2H NMR
spectrum contains one broad feature at +53 ppm. In the
absence of a vertical mirror plane, the two N-methyl
groups of the N[R]ArMeL ligand are rendered inequivalent.
Thus, the number of observed NMR features suggests that
[1]1- maintains C2 or Cs symmetry in solution on the NMR
timescale.
in an N-alkylanilide would provide
a monoanionic
bidentate ligand suitable for achieving this goal.
Accordingly, we prepared the ligand salts (Et2O)Li(N[R]
ArMeL) (R = C(CD3)2CH3); ArMeL = 2-NMe2-5-MeC6H3) and
K(N[R]ArMeL) in good yields via modifications of literature
procedures [16,17]. Briefly, zinc reduction of 2-nitro-N,N-
dimethyl-p-toluidine provided the primary aniline H2NAr-
MeL in 70 % yield. Condensation of H2NArMeL with acetone-
d6 gave the hexadeuterated imine (CD3)2C = NArMeL in 86 %
yield. Treatment of (CD3)2C = NArMeLwith MeLi in Et2O
followed by an aqueous workup led to the isolation of
the secondary aniline HN[R]ArMeL in 91 % yield. Finally,
treatment of HN[R]ArMeL with n-BuLi provided (Et2O)Li(N[-
R]ArMeL) in 89 % yield, while treatment of HN[R]ArMeL with
KH provided KN[R]ArMeL in 92 % yield, both salts being
The reaction of solvate-free UI3 with K(N[R]ArMeL
)
(3 equiv) in thawing THF provided U(N[R]ArMeL 3 (2) in
)
70 % yield following separation from KI and crystalliza-
tion from Et2O. The 1H NMR spectrum of 2 in benzene-
d6 was found to be quite complex and indicative of more
than one ligand environment present in solution, but its
variable temperature 2H NMR spectrum was very
informative (Fig. 2). At 20 8C, the 2H NMR spectrum of
2 in benzene-d6 consists of three broad peaks at +23.6,
ꢀ9.0, and ꢀ25.0 ppm, with the latter two peaks integrat-
ing in an approximate 1:2 ratio. At 40 8C, all three features
are significantly broadened, and by 60 8C they have
coalesced into one broad resonance centered at +4.7 ppm.
This behavior suggests that in solution at 20 8C, 2 exists in
an equilibrium between two separate conformers: a C3-
symmetric species that gives rise to a single peak at
+23.6 ppm, and a Cs-symmetric species that accounts for
[(g._2)TD$FIG]
oi btained as white powders following standard workup.
1
The protocol for the synthesis and isolation of (Et2O)xLi [1] results in a
variable and non-stoichiometric amount of Et2O binding to the lithium
ion,
a consequence of drying the isolated material under dynamic
vacuum. The amount of Et2O that remains bound to the lithium ion after
drying the isolated material has not been rigorously quantified, but it is
assumed to be a small fraction of the total molecular weight and it is
disregarded in subsequent calculations using Li[1].
Fig. 2. 2H NMR spectra (76.8 MHz, benzene-d6) of 2 recorded from 20-
60 8C.

A.R. Fox et al. / C. R. Chimie [(i_.gT)31DIF$]G 3 (2010) 781–789
783
the two features at ꢀ9.0, and ꢀ25.0 ppm. Raising the
temperature of the system leads to more rapid intercon-
version of these two conformers relative to the NMR
timescale, which is reflected by coalescence of the three
NMR features at elevated temperatures.
2.3. Uranium(IV) complexes supported by the
N[R]ArMeL ligand
Both Li[1] and 2 were found to be readily oxidized by 1e
to their corresponding uranium(IV) derivatives (Fig. 3).
Treatment of Li[1] with I2 (0.5 equiv) in Et2O led to
precipitation of the neutral uranium(IV) complex
I2U(N[R]ArMeL)2 as a bright red-orange powder that was
isolated in 89 % yield by filtration. The 1H NMR spectrum of
1 in CDCl3 contains one set of seven well-resolved singlets
ranging from +76.7 to ꢀ53.6 ppm, corresponding to a
single N[R]ArMeL ligand environment, while the 2H NMR
spectrum of 1 displays one peak at +76.7 ppm. Treatment
of 2 with AgOTf (OTf = CF3SO3) in THF at ꢀ35 8C provided
the uranium(IV) salt [U(N[R]ArMeL)3][OTf] ([2][OTf]) in
82 % yield following separation from Ag0 and subsequent
standard workup. As with 1, the 1H NMR spectrum of
Fig. 3. One-electron oxidation reactions with Li[1] and 2.
[(ig._4)TD$FIG]
Fig. 4. ORTEP renderings of (Et2O)(THF)Li[1] (top-left), 1 (bottom-left), 2 (top-right), and [2] [OTf] (bottom-right) generated by PLATON [26]. Ellipsoids are
displayed at 50 % probability; hydrogen atoms and co-crystallized solvent molecules have been omitted for clarity.

A.R. Fox et al. / C. R. Chimie [(i_.gT)51DIF$]G 3 (2010) 781–789
784
[2][OTf] in CDCl3 consists of one set of seven singlets,
indicative of a single ligand environment, however these
spectroscopic features span a much smaller range, from
+7.76 to ꢀ0.87 ppm; the 2H NMR spectrum of [2][OTf] in
CHCl3 consists of a single broad resonance at ꢀ0.35 ppm.
Not conveyed by the NMR spectra of [2][OTf] is the
disposition of the triflate anion, whether it resides in the
inner or outer coordination sphere of the uranium ion.
2.4. X-ray crystallographic investigations
To further understand the coordination mode adopted
by the N[R]ArMeL ligand, we turned to single-crystal X-ray
crystallography. Crystallization of Li[1] from an Et2O/THF
mixture at ꢀ35 8C provided crystals of (Et2O)(THF)Li(
m-
I)2U(N[R]ArMeL 2 ((Et2O)(THF)Li[1]), while crystals of 1, 2,
)
and [2][OTf] were readily obtained by standard crystalli-
zation methods. The structural models obtained from our
crystallographic investigations are presented in Fig. 4, and
selected average interatomic distances are compiled in
Table 1. In all four cases, the uranium centers are nominally
six-coordinate, and the N[R]ArMeL ligands assume the
expected bidentate coordination mode. The U-Nanilide
distances fall in a range that is consistent with other
reported uranium-anilide complexes [12–15,18], and the
U-Namino distances are typical as well [19]. One noteworthy
feature of all four structures is the interactions between
the uranium center and the
backbone. One structural parameter that may be used to
illustrate this interaction is the envelope angle ( envelope),
p-system of the anilide aryl
f
defined as the obtuse dihedral angle created by uranium
center projecting out of the co-planar ligand-based N-C-C-
N array [20]. The average values of fenvelope in (Et2O)(TH-
F)Li[1] and 1 differ by less than one degree, while fenvelope
in 2 to [2][OTf] differ by over 208. This dramatic difference
may be viewed as a consequence of charge separation in
[2][OTf]. In all cases, the U-Cphenylene distances fall in a
range common for the interaction between a uranium ion
and a neutral arene fragment [21]. These interactions are
frequently observed in uranium-anilide complexes, and
are representative of fairly weak intramolecular interac-
tions that nonetheless assist in stabilizing the central ion.
Both (Et2O)(THF)Li[1] and 1 exhibit highly distorted
octahedral coordination about the central uranium ion,
with the iodide ligands occupying cis-equatorial sites. The
uranium centers in (Et2O)(THF)Li[1] and 1 may alterna-
tively be viewed as pseudo-tetrahedrally coordinated by
N1, N2, I1, and I2, with the amino nitrogens N11 and
N21 acting as additional capping ligands. The U-I distances
Fig. 5. Two space-filling representations of the [2]+ ion (C, gray; H, white;
N, blue; U, turquoise) generated by PLATON [26]: top, viewed looking
down at the plane defined by N1, N2, and N3; bottom, viewed looking
down at the plane defined by N11, N21, and N31.
(Et2O)(THF)Li[1] to 1 tracks well with difference in ionic
radii between U3+ and U4+ (0.14 A). Indeed, very little
˚
structural reorganization is observed upon proceeding
from (Et2O)(THF)Li[1] to 1.
The primary coordination sphere in 2 is best described
as a distorted trigonal anti-prism, with one trigonal plane
comprising the anilide nitrogens and the other comprising
the amino nitrogens. The primary coordination sphere in
[2][OTf] is closer to trigonal prismatic, with the the trigonal
planes defined as in 2. Of note is the fact that the triflate
anion in [2][OTf] does not coordinate the uranium center.
Visualizing a space-filling model of the [2]+ ion provides
insight into why the triflate ion in [2][OTf] is relegated to
the outer coordination sphere (Fig. 5). The uranium center
in [2]+ is fully encapsulated in a hydrocarbyl sheath created
by the three N[R]ArMeL ligands. In particular, the R and
NMe2 residues interdigitate, and in doing so block off what
might otherwise be accessible avenues for ligand coordi-
nation. Neutral 2 displays a similar degree of steric
encumbrance at the uranium center, suggesting that it
˚
in (Et2O)(THF)Li[1] and 1 are typical [19], and the ca. 0.13 A
decrease in the average U-I distance on going from
Table 1
˚
Averaged key interatomic distances (A) and envelope angles (8) in
uranium complexes featuring the N[R]ArMeL ligand.
Molecule
U–Nanilide U–Namino U–I
U–Cphenylene fenvelope
(Et2O)(THF)Li[1] 2.368
2.667
2.594
2.752
2.723
3.213 3.108
3.0976 2.970
124.76
123.13
135.21
115.02
1
2.227
2.467
2.242
2
–
–
3.292
2.919
[2][OTf]

A.R. Fox et al. / C. R. Chimie 13 (2010) 781–789
785
may prove challenging to utilize 2 for reaction chemistry
based on inner-sphere redox activity.
from Restek (Bellefonte, PA USA). Combustion analyses
were performed by Columbia Analytical Services, Inc.
(Tucson, AZ USA).
3. Conclusion
4.1.1. Synthesis of H2NArMeL
Herein, we have described two new uranium amide
In a 1 L, three-necked flask fitted with a reflux
condenser, a mixture of 2-nitro-N,N-dimethyl-p-touidine
(53.18 g, 0.295 mol, 1 equiv), NaOH(aq) (36.0 mL of a 20 %
w/w solution), and ethanol (220 mL) was prepared and
magnetically stirred. The mixture was heated to reflux
using a heating mantle. Once a gentle boil was achieved,
the mantle was removed and zinc (90 g) was added in
small portions (ca. 5 g at a time). The rate of zinc addition
was fast enough to maintain the gentle boil. Half way
through the addition (ca. 40 g Zn), the reaction mixture
began to reflux with great vigor (it is imperative to have an
ice bath ready for this eventuality). The reaction mixture
was cooled in an ice bath for ca. 10 min during which time
the reaction mixture maintained a steady reflux. Once the
mixture began to evolve heat less vigorously, the addition
of zinc was continued at ambient temperature, again in
small portions. The addition at this point was done with
great care, as the reaction mixture was prone to refluxing
after each addition. Once all of the zinc was added, the
heating mantle was returned and the reaction mixture was
refluxed for 1.5 h. During the course of the reaction, the
color of solution changed from bright red to dull brown.
The hot reaction mixture was filtered through a bed of
Celite on a sintered glass frit to remove excess zinc and any
insoluble by-products. The filter cake was washed with
ethanol (2 ꢁ 100 mL). The filtrate was taken to near
dryness using a rotary evaporator. The dark brown residue
was taken up in deionized water (500 mL) and extracted
with diethyl ether (5 ꢁ 200 mL). The organic fractions were
combined and dried with MgSO4. The mixture was then
gravity filtered and the ether removed from the filtrate
using a rotary evaporator. The dark brown oil that
remained was distilled under reduced pressure (< 1 torr,
55 8C) to give the product as a nearly colorless oil (30.95 g,
systems based on the bidentate anilide ligand N[R]ArMeL
.
Both bis- and tris-anilide complexes of uranium(III) have
been obtained, and the 1e oxidation of these compounds to
the corresponding uranium(IV) complexes has been
demonstrated. X-ray diffraction experiments confirm the
bidentate nature of the N[R]ArMeL ligand, and in the case of
the tris-anilide derivative reveals a central uranium ion
that is subject to a high degree of steric congestion. The
synthesis of this new chelating ligand is modular with
respect to varying the alkyl and amino substituents, and
our attention now turns to developing similar chelating
ligands with steric properties amenable to accessing
seven-coordination at uranium.
4. Experimental section
4.1. Methods and materials
All manipulations were carried out at room tempera-
ture (23 8C) under an atmosphere of purified dinitrogen in
a Vacuum Atmospheres MO-40 M glove box or with a
vacuum manifold using standard Schlenk techniques.
˚
Celite 545 (EM Science) and 4 A molecular sieves were
dried via storage in a 225 8C oven for 24 h followed by
complete desiccation under dynamic vacuum at 210 8C for
48 h prior to use. Solvents were purified using a commer-
cial Glass Contour solvent purification system constructed
by SG Water USA (Nashua, NH USA), and were stored over
˚
activated 4 A molecular sieves prior to use. Chloroform-d,
benzene-d6, and acetone-d6 were obtained from Cam-
bridge Isotope Laboratories, Inc. (Andover, MA USA);
chloroform-d and benzene-d6 were vacuum distilled from
CaH2, degassed with two freeze-pump-thaw cycles, and
stored over activated 4 A molecular sieves prior to use.
0.206 mol, 70 %). 1H NMR (500 MHz, benzene-d6):
d = 6.84
˚
UI3(THF)4 and UI3 were prepared according to published
procedures [22,23]. 2-nitro-N,N-dimethyl-p-toluidine was
prepared by nitration of N,N-dimethyl-p-toluidine with
NaNO2 and Ce(SO4)2 according to published methods [24].
2-dimethylamino-5-methylaniline (H2NArMeL) was pre-
pared by reduction of 2-nitro-N,N-dimethyl-p-touidine
(d, 2JHH = 10 Hz, 1H, 3- or 4-ArMeLH), 6.58 (d, 2JHH = Hz, 1H,
3- or 4-ArMeLH), 6.29 (s, 1H, 6-ArMeLH), 3.60 (br s, 2H, H2N)),
2.46 (s, 6H, N(CH3)2), 2.18 (s, 3H, ArMeLCH3) ppm. 13C{1H}
NMR (75.5 MHz, benzene-d6): d = 141.76 (s, ArMeL), 138.27
(s, ArMeL), 133.51 (s, ArMeL), 119.20 (s, ArMeL), 118.92 (s,
ArMeL), 116.00 (s, ArMeL), 43.64 (s, N(CH3)2), 21.07 (s,
ArMeLCH3) ppm. GC/MS: 150 (Mꢂ+) m/z.
using
a modification of a literature procedure [16];
H2NArMeL was converted to HN[R]ArMeL via a modified
literature procedure [17]. The latter two procedures are
described below. All other reagents were obtained from
commercial suppliers and used as received. All glassware
was oven-dried at a temperature of 225 8C prior to use.
Solution 1H, 13C, and 2H NMR spectra were recorded on
Varian Mercury-300 or Varian Inova-500 spectrometers at
20 8C. All 1H and 13C NMR spectra were referenced
internally to residual solvent (C6D5H in C6D6, 7.16 ppm;
CHCl3 in CDCl3, 7.27 ppm). All 2H NMR spectra were
referenced internally to naturally abundant solvent
deuterium peaks. GC-MS data were collected using an
Agilent 6890N network GC system with an Agilent
5973 Network mass selective detector and an Rtx-1 column
4.1.2. Synthesis of (CD3)2C = NArMeL
A colorless solution of H2NArMeL (15.0 g, 0.100 mmol)
and acetone-d6 (ca. 50 mL) was stored over 4 A molecular
sieves at 5 8C for 3 d. The solution was then transferred
˚
˚
onto fresh 4 A molecular sieves, and the used sieves were
washed with acetone-d6. The washings were added to the
bulk mixture, which was then stored at 5 8C for another 5 d.
During this time, the solution obtained a faintly red color.
The solution was decanted from the sieves, the sieves were
then washed with acetone-d6, and the washings combined
with the bulk mixture. Unreacted acetone-d6 was recov-
ered by vacuum transfer, leaving behind spectroscopically
pure (CD3)2C = NArMeL (16.93 g, 86.2 mmol, 86 %) as a red

786
A.R. Fox et al. / C. R. Chimie 13 (2010) 781–789
oil. If desired, the oil may be further purified by vacuum
distillation at ca. 50 8C, but it is of sufficient purity to be
taken on to the next step without any significant impact on
ArMeL), 139.63 (s, ArMeL), 133.00 (s, ArMeL), 117.10 (s,
ArMeL), 113.15 (s, ArMeL), 101.53 (s, ArMeL), 52.01 (s,
C(CD3)2CH3), 43.88 (s, N(CH3)2), 31.80 (s, C(CD3)2CH3),
23.50 (s, ArMeLCH3) ppm. 2H NMR (76.8 MHz, benzene):
yield. 1H NMR (500 MHz, benzene-d6):
d = 6.81 (mult, 2H,
3- and 4-ArMeLH), 6.53 (s, 1H, 6-ArMeLH), 2.57 (s, 6H,
N(CH3)2), 2.20 (s, 3H,ArMeLCH3), 1.94 (mult, 0.15H, residual
N = C(CH3)2), 1.44 (mult, 0.15H, residual N = C(CH3)2) ppm.
d
= 1.56 (s, C(CD3)2CH3) ppm.
4.1.5. Synthesis of K(N[R]ArMeL
)
13C{1H} NMR (126 MHz, benzene-d6):
d
= 167.38 (s,
Solid KH (1.62 g, 40.4 mmol, 1.1 equiv) was added to a
200 mL recovery flask containing a stirred solution of
HN[R]ArMeL (7.77 g, 36.6 mmol, 1 equiv) in THF (80 mL).
The flask was capped with a septum and syringe needle
was inserted into the septum to facilitate the escape of
H2(g). The mixture was allowed to stir for 36 h, whereupon
it was filtered through a Celite-padded frit to remove
excess KH. Volatile material was removed under reduced
pressure from the yellow filtrate, yielding a pale yellow
solid. Toluene (60 mL) was added and subsequently
removed under reduced pressure. The solid was then
slurried in Et2O (30 mL) and n-hexane (80 mL). The total
volume was reduced to ca. 30 mL by concentrating the
mixture under reduced pressure, and the pale yellow
precipitate was isolated by filtering the mixture through a
medium frit. The solids were washed with n-pentane
(2 ꢁ 20 mL) and dried under reduced pressure (8.40 g,
N = C(CD3)2), 145.93 (s, ArMeL), 141.73 (s, ArMeL), 131.66
(s, ArMeL), 124.56 (s, ArMeL), 121.39 (s, ArMeL), 118.25 (s,
ArMeL), 42.85 (s, N(CH3)2), 27.86 (mult, N = C(CD3)2), 21.15
(s, ArMeLCH3). 2H NMR (76.8 MHz, benzene):
d = 1.94 (s,
N = C(CD3)2), 1.44 (s, N = C(CD3)2) ppm. GC/MS: 160 (M ꢀ
2CD3) m/z.
4.1.3. Synthesis of HN[R]ArMeL
A solution of (CD3)2C = NArMeL (26.56 g, 0.135 mol,
1 equiv) in Et2O (50 mL) was added slowly to a stirred
thawing solution of MeLi (200 mL, 1.6 M in Et2O, 0.320 mol,
1.6 equiv). The mixture assumed a yellow color and
became cloudy. After stirring the mixture for ca. 14 h, the
mixture was quenched by slow addition to an ice/water
slurry (400 mL). The organic layer was separated and the
aqueous layer was extracted with Et2O (3 ꢁ 200 mL). The
combined organic fractions were dried over Na2SO4.
Volatile materials were removed with the aid of a rotary
evaporator, leaving a pale green oil that was then subjected
to simple distillation under dynamic vacuum. The product
was obtained as a yellow oil that distills at 60 8C under
dynamic vacuum (25.97 g, 0.122 mol, 91 %). 1H NMR
33.5 mmol, 92 %). 1H NMR (300 MHz, pyridine-d5):
d = 6.79
(d, 7 Hz, 1H, 3- or 4-ArMeLH), 6.18 (s, 1H, 6-ArMeLH), 5.97 (d,
8 Hz, 1H, 3- or 4-ArMeLH, 2.85 (s, 6H, N(CH3)2), 2.46 (s, 3H,
ArMeLCH3), 1.65 (s, 3H, C(CD3)2CH3) ppm. 13C{1H} NMR
(75.5 MHz, pyridine-d5):
d = 154.59 (s, ArMeL), 139.63 (s,
ArMeL), 133.00 (s, ArMeL), 117.10 (s, ArMeL), 113.15 (s,
ArMeL), 101.53 (s, ArMeL), 52.01 (s, C(CD3)2CH3), 43.88 (s,
N(CH3)2), 31.80 (s, C(CD3)2CH3), 23.50 (s, ArMeLCH3) ppm.
(300 MHz, benzene-d6):
d
= 6.93 (d, 2JHH = 6 Hz, 1H, 3- or 4-
2
ArMeLH), 6.84 (s, 1H, 8-ArMeLH), 6.56 (d, JHH = 8 Hz, 1H, 3-
or 4-ArMeLH), 5.11 (br s, 1H, NH), 2.45 (s, 6H, N(CH3)2), 2.20
(s, 3H, ArMeLCH3), 1.30 (s, 3H, C(CD3)2CH3 ppm. 13C{1H}
2H NMR (76.8 MHz, pyridine):
ppm.
d = 1.65 (s, C(CD3)2CH3)
NMR (125.8 MHz, benzene-d6):
d = 142.85 (s, ArMeL),
139.48 (s, ArMeL), 134.40 (s, ArMeL), 120.26 (s, ArMeL),
117.41 (s, ArMeL), 114.43 (s, ArMeL), 50.01 (s, C(CD3)2CH3),
44.87 (s, N(CH3)2), 30.31 (s, C(CD3)2CH3), 23.30 (s,
ArMeLCH3) ppm. GC/MS: 212 (Mꢂ+) m/z.
4.1.6. Synthesis of Li[I2U(N[R]ArMeL)2]
Solid (Et2O)Li(N[R]ArMeL) (1.88 g, 6.44 mmol, 2 equiv)
was added to a stirred suspension of UI3(THF)4 (2.922 g,
3.22 mmol, 1 equiv) in thawing toluene. The mixture was
allowed to stir and warm to ambient temperature for 12 h,
during which time the mixture was maintained under
dynamic vacuum to remove solvent and other volatile
materials. A purple residue remained, to which n-hexane
(200 mL) was added. The mixture was filtered through a
Celite-padded frit, and volatile materials were removed
from the filtrate under reduced pressure. Et2O (20 mL) and
n-hexane (50 mL) were added to the residue thus obtained,
creating a solution. Volatile materials were removed under
reduced pressure from this solution, and again Et2O
(20 mL) and n-hexane (50 mL) were added and removed
under reduced pressure. n-Pentane (20 mL) was then
added, creating a purple suspension. The inner walls of the
flask were scraped to dislodge adhered material, and the
suspened solids were isolated by filtering the mixture
through a medium frit. The solids were washed with n-
pentane (10 mL) and then dried under reduced pressure,
4.1.4. Synthesis of (Et2O)Li(N[R]ArMeL
)
n-BuLi (48.0 mL, 1.6 M in hexane, 76.83 mmol,
1.3 equiv) was slowly added to a stirred solution of
HN[R]ArMeL (12.55 g, 59.10 mmol, 1 equiv) in thawing n-
pentane (80 mL). The mixture assumed a faintly yellow
color and a small amount of white precipitate formed. The
mixture was stirred for 30 min before Et2O (20 mL) was
added. The mixture became homogeneous and was
concentrated under reduced pressure to ca. 40 mL and
was cooled in the glove box cold well, leading to the
formation of a large amount of white precipitate. The
product precipitate was isolated by filtering the cold
mixture through a medium frit, then washing the retained
solids with a small amount of cold n-pentane, and finally
drying the solids under reduced pressure (15.38 g,
52.6 mmol, 89 %). 1H NMR (500 MHz, benzene-d6):
2
d
= 6.79 (d, JHH = 8 Hz, 1H, 3- or 4-ArMeLH), 6.78 (s, 1H,
providing the product as a purple powder (1.77 g,
6-ArMeLH), 6.24 (d, 2JHH = 8 Hz, 1H, 3- or 4-ArMeLH), 3.04 (q,
4H, O(CH2CH3)2), 2.35 (s, 3H, ArMeLCH3), 2.20 (s, 6H,
N(CH3)2), 1.56 (s, 3H, C(CD3)2CH3, 0.92 (t, 6H, O(CH2CH3)2)
1.93 mmol, 60 %). Crystals of this material were grown
from a solution Li[1] in a mixture of THF/Et2O stored at
ꢀ35 8C. 1H NMR (300 MHz, benzene-d6), as [(Et2O)x-
ppm. 13C{1H} NMR (75.5 MHz, pyridine-d5):
d
= 154.59 (s,
Li][I2U(N[R]ArMeL)2]: d = 53.00 (br s, 18H, C(CD3)2CH3),

A.R. Fox et al. / C. R. Chimie 13 (2010) 781–789
787
31.60 (br s, 9H, N(CH3)(CH3), 16.17 (br s, 3H, ArMeLH),
ꢀ3.94 (br s, 9H, ArMeLCH3), ꢀ9.09 (br s, 3H, ArMeLH), ꢀ27.45
(br s, 3H, ArMeLH), ꢀ59.52 (br s, 9H, ArMeLCH) ppm. 2H NMR
Celite-padded frit. The filter cake was washed with THF
(20 mL) and volatile materials were removed from the
combined filtrate under reduced pressure. Addition of an
Et2O/n-pentane mixture created a suspension of a brown
(76.8 MHz, benzene):
d = 53.0 (br s, C(CD3)2CH3) ppm.
solid in
a faintly brown supernatant solution. The
4.1.7. Synthesis of U(N[R]ArMeL 3
)
suspended material was isolated by filtering the mixture
through a medium frit and washing the isolated solids with
n-pentane (5 mL) before drying the solids under reduced
pressure. The product was thus obtained as dark orange
precipitate (0.244 g, 82 %). Crystals of this material were
grown from a THF solution of [U(N[R]ArMeL)3][OTf] layered
with Et2O and stored at ꢀ35 8C. 1H NMR (500 MHz, CDCl3):
Solid K(N[R]ArMeL) (1.521 g, 6.072 mmol, 3.4 equiv)
was added to a stirred suspension of UI3 (1.100 g,
1.777 mmol, 1 equiv) in thawing THF (80 mL). The
resulting mixture was stirred for 2.5 h, over which time
the mixture darkened from a purple-blue to black. The
mixture was filtered through a Celite-padded frit to
remove precipitated KI. Volatile materials were removed
under reduced pressure from the filtrate, leaving behind a
dark blue-black residue. Et2O (50 mL) and n-pentane
(50 mL) were added to the residue, and the resulting
mixture was filtered through a Celite-padded frit. Volatile
materials were removed under reduced pressure from the
filtrate, leaving behind a dark blue-black powder. The
powder dissolved in Et2O (20 mL) and the resulting
solution was filtered through a plug of Celite. The filtrate
was stored at ꢀ35 8C, resulting in the product depositing
on the walls of the storage flask as dark blue-black
microcrystals. The product was isolated in three crops by
removing the mother liquor by pipet and drying the
microcrystals under dynamic vacuum (1.067 g,
1.223 mmol, 69 %). Single crystals of this material were
grown from a solution of 2 in a mixture of THF/Et2O/n-
pentane stored at ꢀ35 8C. 2H NMR (76.8 MHz, benzene-
d
= 7.76 (s, 3H, ArMeLH), 7.02 (s, 3H, ArMeLH), 6.81 (s, 3H,
ArMeLH), 3.50 (s, 9H, N(CH3)(CH3), 3.26 (s, 9H, ArMeLCH3),
ꢀ0.35 (s, 18H, NC(CD3)2CH3), ꢀ0.87 (s, 9H, N(CH3)(CH3)
ppm. 2H NMR (76.8 MHz, CHCl3):
C(CD3)2CH3) ppm.
d
= ꢀ0.35 (br s,
4.2. X-ray crystallographic details
Low-temperature diffraction data were collected on a
Siemens Platform three-circle diffractometer coupled to a
Bruker-AXS Smart Apex charge-coupled device (CCD),
performing w- and v-scans. The structures were solved by
either direct methods or Patterson methods, in conjunc-
tion with standard Fourier difference techniques, and
refined on F2 by full-matrix least-squares procedures. A
semi-empirical absorption correction was applied to the
diffraction data for all structures. All non-hydrogen atoms
were refined anisotropically; all hydrogen atoms were
placed at calculated positions and refined isotropically
using a riding model. All software used for diffraction data
processing and crystal structure solution and refinement
are contained in the APEX2 v2008-3.0 program suite
(Bruker AXS). The structure of (THF)(Et2O)Li[1]-0.5(Et2O)
exhibits a disorder that switches the positions of the THF
and Et2O molecules that coordinate the lithium cation;
this disorder refined to 52 %. The refined structural model
of 1 contained highly disordered solvent molecules in
solvent-accessible channels for which no acceptable
model was constructed. Consequently, the SQUEEZE
routine [25] as implemented in PLATON [26] was used
to remove the unassigned electron density. The refined
model for [2][OTf]ꢂ2(THF) has the uranium-containing
fragment placed in a PART -1 function. One consequence
of this model is short non-bonded interatomic distances
between the outer-sphere triflate and anilide residues in
the unit cell adjacent to the triflate ion. It is presumed that
the orientation of the uranium-containing fragment
strictly alternates from one cell to the next, so no short
interatomic contacts actually occur. Summaries of crys-
tallographic data for complexes (Et2O)(THF)Li[1], 1, 2, and
[2][OTf] are given in Table 2. Complete crystallographic
details for these complexes are available in the form of
Crystallographic Information Files (CIF) and can be
obtained free of charge from the Cambridge Crystallo-
graphic Data Centre (depositions 761469-761472) via the
Internet at http://www.ccdc.cam.ac.uk.datarequest/cif
(or directly: Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336
033).
d6):
d
= 23.6 (br s, C3-symmetric conformer), ꢀ9.0 (br s, Cs-
symmetric conformer) and ꢀ25.0 (br s, Cs-symmetric
conformer) ppm.
4.1.8. Synthesis of I2U(N[R]ArMeL 2
)
A
thawing solution of I2 (0.081 g, 0.319 mmol,
0.5 equiv) in Et2O (5 mL) was added dropwise to a stirred
thawing solution of Li[I2U(N[R]ArMeL)2] (0.587 g,
0.624 mmol, 1 equiv) in Et2O (6 mL), resulting in the
immediate formation of a bright orange precipitate. The
mixture was allowed to stir for 30 min before it was
concentrated to half of its initial volume under reduced
pressure. The precipitate was isolated by filtering the
mixture through a medium frit. The solids were washed
with n-pentane (10 mL) and dried under reduced pres-
sure, yielding the product as a bright red-orange powder
(0.507 g, 0.554 mmol, 89 %). Crystals of this material were
grown from a solution of 1 in a mixture of CH2Cl2/THF/
Et2O stored at ꢀ35 8C. 1H NMR (300 MHz, CDCl3):
d = 76.67
(s, 9H, C(CD3)2CH3), 30.21 (s, 3H, ArMeLH), 5.90 (s, 9H,
ArMeLCH3), ꢀ10.60 (s, 9H, N(CH3)(CH3), ꢀ19.46 (s, 3H,
ArMeLH), ꢀ36.21 (s, 3H, ArMeLH), ꢀ53.55 (s, 9H,
N(CH3)(CH3)) ppm. 2H NMR (76.8 MHz, CHCl3):
(s, C(CD3)2CH3) ppm.
d = 76.7
4.1.9. Synthesis of [U(N[R]ArMeL)3][OTf]
A solution of AgOTf (0.0752 g, 0.280 mmol, 1 equiv) in
cold THF (6 mL, ꢀ35 8C) was added to a stirred solution of
U(N[R]ArMeL)3 (0.255 mmol, 0.280 mmol, 1 equiv) in cold
THF (6 mL, ꢀ35 8C). The color of the mixture immediately
went from purple-black to orange-brown. The mixture was
allowed to stir for 1.5 h before it was filtered through a

788
A.R. Fox et al. / C. R. Chimie 13 (2010) 781–789
Table 2
Crystallographic data for the structures presented in this work.
(Et2O)(THF)Li[l]ꢂ0.5(Et2O)
1
2
[2][OTf]-2(THF)
Empirical formulaa
Formula weight (g/mol)
Temperature (K)
C36H65I2LiN4O2.5U
1092.69
C29H42I2N4U
902.47
C39H63N6U
853.98
C48H79F3N6O5SU
1147.26
100(2)
100(2)
100(2)
100(2)
˚
Wavelength (A)
0.71073
0.71073
0.71073
0.71073
Crystal system
Space group
Monoclinic
C2/c
Monoclinic
C2/c
Orthorhombic
Fdd2
Monoclinic
P21/m
Unit cell dimensions
a = 34.656(4),
a
= 90
a = 41.121(4),
a
= 90
a = 24.8112(19),
a
= 90
a = 11.1943(15), a = 90
˚
(A,8)
b = 16.3018(18),
= 105.405(2)
b = 9.7154(8), 96.114(2)
b = 64.566(5),
b
g
= 90
= 90
b = 14.3832,
b
= 92.661(2)
b
c = 15.9409(17),
g
= 90
c = 17.9839,
7143.9(10)
8
g
= 90
c = 9.5274(7),
15,262(2)
16
c = 15.597(2),
2508.6(6)
2
g = 90
3
˚
Volume (A )
8682.3(16)
Z
8
Density (calculated)
(Mg/m3)
1.672
1.678
1.487
1.519
Absorption coefficient
5.195
4240
6.289
3392
4.288
6896
3.338
1168
(mmꢀ1
)
F(000)
Crystal size (mm3)
Theta range for
collection (8)
0.15 ꢁ 0.13 ꢁ 0.05
0.16 ꢁ 0.11 ꢁ 0.05
0.25 ꢁ 0.05 ꢁ 0.05
0.27 ꢁ 0.08 ꢁ 0.05
1.81 to 28.70
1.99 to 28.28
1.26 to 29.57
1.31 to 28.70
Index ranges
ꢀ46 ꢃ h ꢃ 46
ꢀ22 ꢃ k ꢃ 22
ꢀ21 ꢃ l ꢃ 21
ꢀ54 ꢃ h ꢃ 54
ꢀ12 ꢃ k ꢃ l2
ꢀ34 ꢃ h ꢃ 34
ꢀ89 ꢃ k ꢃ 89
ꢀ13 ꢃ l ꢃ 13
ꢀ15 ꢃ h ꢃ 15
ꢀ19 ꢃ k ꢃ 19
ꢀ21 ꢃ l ꢃ 21
ꢀ23 ꢃ l ꢃ 23
Reflections collected
88,114
62,723
76,743
48,855
Independent reflections
Completeness to umax (%)
Absorption correction
11,226 [R(int) = 0.0831]
100.0
8853 [R(int) = 0.0770]
100.0
10,716 [R(int) = 0.0837]
100.0
10,716 [R(int) = 0.0745]
100.0
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Max. and min.
transmission
0.7812 and 0.5096
0.7439 and 0.4327
0.8142 and 0.4136
0.8509 and 0.4660
Refinement method
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
Full-matrix least-
squares on F2
Data/restraints/parameters
Goodness-of-fitb
11,226/672/527
8853/0/310
10,716/1/434
6727/179/595
1.006
1.072
1.022
1.023
Final R indices [I > 2
s
(I)]c
R1 ¼ 0:0363; wR2 ¼ 0:0661
R1 ¼ 0:0710; wR2 ¼ 0:0780
0.980 and ꢀ1.491
R1 ¼ 0:0447; wR2 ¼ 0:1149
R1 ¼ 0:0624; wR2 ¼ 0:1241
1.317 and ꢀ1.748
R1 ¼ 0:0313; wR2 ¼ 0:0556
R1 ¼ 0:0429; wR2 ¼ 0:0602
1.307 and ꢀ0.814
R1 ¼ 0:0284; wR2 ¼ 0:0528
R1 ¼ 0:0413; wR2 ¼ 0:0573
2.037 and ꢀ1.065
R indices (all data)c
Largest diff. peak and
ꢀ3
˚
hole (e A
)
a
b
c
All 2H atoms refined as 1H.
ꢀ
ꢁ
P
1=2
2
2
2
½wðF ꢀF
o
Þ
ꢄ
c
GooF ¼
.
ðnꢀ pÞ
ꢀ
ꢁ
P
P
1=2
2
2
½wðF ꢀF
o
2
2
2
jjF jꢀjF
o
Þ
ꢄ
ꢄ
R1
¼
j ; wR2
¼
; w ¼
; P ¼
o
c
2F þmaxðF ;0Þ
c o
3
c
1
P
P
2
2
2
o
s2ðF ÞþðaPÞ þbP
jF
j
2
o
½wðF
Þ
[5] I. Castro-Rodriguez, K. Olsen, P. Gantzel, K. Meyer, J. Am. Chem. Soc. 125
(15) (2003) 4565.
Acknowledgements
[6] P.L. Arnold, A.J. Blake, C. Wilson, J.B. Love, Inorg. Chem. 43 (26) (2004)
8206.
[7] P.L. Arnold, D. Patel, C. Wilson, J.B. Love, Nature 451 (7176) (2008)
315.
[8] M.F. Lappert, P.P. Power, A. Protchenko, A. Seeber, Metal amide chem-
istry, Wiley and Sons, New York, 2009, pp. 121.
[9] C.J. Burns, M.S. Eisen, In: The chemistry of the actinide and transactinide
elements, Vol.5, Springer, Dordrecht, 2006, pp. 2911.
[10] T. Andrea, M.S. Eisen, Chem. Soc. Rev. 37 (3) (2008) 550.
[11] A.R. Fox, S.C. Bart, K. Meyer, C.C. Cummins, Nature 455 (7211) (2008)
341.
We thank the U.S. National Science Foundation (CHE-
0724158) for supporting this work. Financial support for
EMT was provided by the Amgen Scholars program (http://
www.amgenscholars.eu) with direction and assistance
provided by the Undergraduate Research Opportunities
Program at MIT.
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