Thorium(IV) and uranium(IV) halide complexes supported by bulky β-diketiminate ligands

Article abstract of DOI:10.1021/om3010965

The coordination behavior of the bulky β-diketiminate ligands N,N′-bis(2,6-diisopropylphenyl)pentane-2,4-diiminate (L^Me) and N,N′-bis(2,6-diisopropylphenyl)-2,2-6,6-tetramethylheptane-3,5-diiminate (L^tBu) toward ThX₄(THF)₄ (X = Br, I) and UCl₄ has been investigated. The reaction between K[L^Me] and ThX₄(THF)₄ (X = Br, I) afforded the mono(β-diketiminate)thorium(IV) halide complexes (L^Me)ThX ₃(THF) (X = Br (7), I (8)). The same reaction carried out with the more sterically demanding K[L^tBu] gave (L^tBu)ThBr ₃(THF) (9) and (L^tBu)ThI₃ (11). All attempts to install two β-diketiminate ligands on thorium(IV) were unsuccessful, giving the mono(β-diketiminate)thorium(IV) halide complex and unreacted K[L^Me] or K[L^tBu]. However, complex 9 was shown to react with smaller anions such as K[C₅H₄Me] to give the mixed-ligand methylcyclopentadienyl β-diketiminate complex (L ^tBu)Th(C₅H₄Me)Br₂ (10). Complexes 7-11 represent rare examples of thorium complexes featuring only one β-diketiminate ligand, and complexes 9-11 are the first examples of thorium and halide complexes supported by the L^tBu framework. In a similar manner, both K[L^Me] and K[L^tBu] were shown to react with UCl₄ to give the corresponding mono(β-diketiminate)uranium(IV) chloride complexes (L^Me)UCl₃(THF) (12) and (L ^tBu)UCl₃ (13). Complex 13 represents the first example of a uranium complex featuring the L^tBu framework. Efforts to prepare the bis(β-diketiminate)uranium(IV) complex (L^Me) ₂UCl₂ by reacting 2 equiv of K[L^Me] with UCl₄ led instead to the interesting cationic diuranium complex [{(L^Me)(Cl)U}₂(μ-Cl)₃][Cl] (14). Complexes 7-14 have been characterized by a combination of ¹H and ¹³C{¹H} NMR spectroscopy, elemental analysis, electrochemistry, and UV-visible-near-IR spectroscopy. Several complexes have also been characterized by X-ray crystallography, and a discussion of their structures is presented. NMR spectroscopy and the X-ray structures demonstrate that the β-diketiminate ligand is symmetrically bound to the actinide metal in the L^Me complexes and is asymmetrically bound to the actinide metal in the L^tBu complexes. In all cases the actinide(IV) metal centers lie out of the plane of the β-diketiminate ligand NCCCN backbone by ～1-2 A. The electronic spectroscopy data on K[L^Me], (L^Me)ThI₃(THF) (8), and (L^Me)UCl ₃(THF) (12) suggest relatively weak metal-(β-diketiminate) ligand bonding interactions, although small perturbations in the characteristics of the β-diketiminate π-π* bands with changes in the the metal ion are consistent with some metal-ligand orbital interactions. This new class of mono(β-diketiminate)thorium and -uranium halide complexes promises to provide a robust platform for developing new chemistry of the actinides.

Full text of DOI:10.1021/om3010965

Article
pubs.acs.org/Organometallics
Thorium(IV) and Uranium(IV) Halide Complexes Supported by Bulky
β‑Diketiminate Ligands
Marisa J. Monreal,† Robert J. Wright,†,‡ David E. Morris,† Brian L. Scott,† Jeffery T. Golden,†
Philip P. Power,‡ and Jaqueline L. Kiplinger*,†
†Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
‡Department of Chemistry, University of California, Davis, One Shields Avenue, California 95616, United States
S
* Supporting Information
ABSTRACT: The coordination behavior of the bulky β-diketiminate ligands N,N′-bis(2,6-diisopropylphenyl)pentane-2,4-
diiminate (LMe) and N,N′-bis(2,6-diisopropylphenyl)-2,2−6,6-tetramethylheptane-3,5-diiminate (LtBu) toward ThX4(THF)4 (X
= Br, I) and UCl4 has been investigated. The reaction between K[LMe] and ThX4(THF)4 (X = Br, I) afforded the mono(β-
diketiminate)thorium(IV) halide complexes (LMe)ThX3(THF) (X = Br (7), I (8)). The same reaction carried out with the more
sterically demanding K[LtBu] gave (LtBu)ThBr3(THF) (9) and (LtBu)ThI3 (11). All attempts to install two β-diketiminate ligands
on thorium(IV) were unsuccessful, giving the mono(β-diketiminate)thorium(IV) halide complex and unreacted K[LMe] or
K[LtBu]. However, complex 9 was shown to react with smaller anions such as K[C5H4Me] to give the mixed-ligand
methylcyclopentadienyl β-diketiminate complex (LtBu)Th(C5H4Me)Br2 (10). Complexes 7−11 represent rare examples of
thorium complexes featuring only one β-diketiminate ligand, and complexes 9−11 are the first examples of thorium and halide
complexes supported by the LtBu framework. In a similar manner, both K[LMe] and K[LtBu] were shown to react with UCl4 to
give the corresponding mono(β-diketiminate)uranium(IV) chloride complexes (LMe)UCl3(THF) (12) and (LtBu)UCl3 (13).
Complex 13 represents the first example of a uranium complex featuring the LtBu framework. Efforts to prepare the bis(β-
diketiminate)uranium(IV) complex (LMe)2UCl2 by reacting 2 equiv of K[LMe] with UCl4 led instead to the interesting cationic
1
diuranium complex [{(LMe)(Cl)U}2(μ-Cl)3][Cl] (14). Complexes 7−14 have been characterized by a combination of H and
13C{1H} NMR spectroscopy, elemental analysis, electrochemistry, and UV−visible−near-IR spectroscopy. Several complexes
have also been characterized by X-ray crystallography, and a discussion of their structures is presented. NMR spectroscopy and
the X-ray structures demonstrate that the β-diketiminate ligand is symmetrically bound to the actinide metal in the LMe
complexes and is asymmetrically bound to the actinide metal in the LtBu complexes. In all cases the actinide(IV) metal centers lie
out of the plane of the β-diketiminate ligand NCCCN backbone by ∼1−2 Å. The electronic spectroscopy data on K[LMe],
(LMe)ThI3(THF) (8), and (LMe)UCl3(THF) (12) suggest relatively weak metal−(β-diketiminate) ligand bonding interactions,
although small perturbations in the characteristics of the β-diketiminate π−π* bands with changes in the the metal ion are
consistent with some metal−ligand orbital interactions. This new class of mono(β-diketiminate)thorium and -uranium halide
complexes promises to provide a robust platform for developing new chemistry of the actinides.
of reaction chemistry unique to the actinides3 and a better
INTRODUCTION
■
understanding of covalency in actinide−ligand bonding
Cyclopentadienyl-based ligand sets have dominated the land-
scape of actinide chemistry since Reynolds and Wilkinson
reported the synthesis of the first organometallic actinide
interactions4 and 5f orbital involvement in bonding and
reactivity.5
In recent years, there has been a shift toward using non-
complex, (C5H5)3UCl, in 1956.1 In this context, both the
cyclopentadienyl, monoanionic, multidentate ligand sets with
tris(cyclopentadienyl) ((C5H5)3) and bis(pentamethyl-
cyclopentadienyl) ((C5Me5)2) ligand sets have been enor-
mously successful frameworks for actinide chemistry.2 These
Special Issue: Recent Advances in Organo-f-Element Chemistry
ligand sets support a variety of oxidation states and
coordination environments, which has enabled the discovery
Received: November 14, 2012
© XXXX American Chemical Society
A
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nitrogen donor atoms to stabilize actinide metal centers.4a,6
Several years ago, we initiated studies to investigate the
chemistry of actinide halide complexes supported by β-
diketiminate ligands. We communicated that β-diketiminate
ligands can support monometallic low-coordinate uranium(III)
metal complexes such as (LMe)2UI (1), which was readily
accessed from the reaction between 2 equivalents of K[LMe]
(LMe = (Ar)NC(Me)CHC(Me)N(Ar), Ar = 2,6-iPr2C6H3) and
the trivalent uranium precursor UI3(THF)4.7 As illustrated in
Chart 1, this interesting molecule featured a β-diketiminate
low- and high-valent uranium complexes and enables reaction
chemistry not available for the C5Me5 ligand set6r,s prompted us
to continue to explore the ability of the bulky β-diketiminate
ligands given in Figure 1 to serve as platforms for developing
Figure 1. Bulky β-diketiminate ligands featured in this work: R =
methyl, LMe; R = tert-butyl, LtBu
.
Chart 1. Examples of Known Actinide β-Diketiminate
a
Complexes
new actinide chemistry. In this contribution, we describe the
synthesis and characterization of a series of new uranium(IV)
and thorium(IV) halide complexes supported by the β-
diketiminate ligand sets LMe = (2,6-iPr2C6H3)NC(Me)CHC-
(Me)N(2,6-iPr2C6H3) and LtBu = (2,6-iPr2C6H3)NC(tBu)-
CHC(tBu)N(2,6-iPr2C6H3), which form a new class of starting
materials for actinide chemistry. In addition, we discuss the
electronic and structural consequences of installing the β-
diketiminate ligand on thorium and uranium.
RESULTS AND DISCUSSION
■
Synthesis and Structural Chemistry. Scheme 1 presents
the synthetic methods used and the yields obtained in the
preparation of the thorium(IV) β-diketiminate complexes.
Treatment of a toluene solution of ThX4(THF)4 (X = Br,
Scheme 1. Synthetic Routes to Thorium Bromide and Iodide
a
β-Diketiminate Complexes 7−11
a
Ar = 2,6-iPr2C6H3.
ligand bound to uranium(III) in an unusual η3(N,C,C′)-1-
azaallyl mode with close U···Calkene contacts. In other work, we
reported that the reaction between K[LMe] and the thorium
iodobutoxide complex ThI3[O(CH2)4I](THF)3 afforded the
first mono(β-diketiminate)thorium complex 2.8 The β-
diketiminate ligand was introduced into 5f-element chemistry
by Lappert and co-workers, who reported the thorium
dichloride complex 3 and the mixed-valence [{UVI}2][UIII]2
salt 4, which were prepared from the reaction of ThCl4 and
UCl4 with Li[(SiMe3)NC(Ph)CHC(Ph)N(SiMe3)], respec-
tively.9 More recently, Hayton and co-workers have exploited
the voluminous β-diketiminate framework to kinetically
stabilize a series of pentavalent uranyl complexes, typified by
5 and 6.10 In these systems, the β-diketiminate ligands prevent
+
dimerization of the UO2 fragment by providing steric
protection both in the uranyl equatorial plane and along the
OUO axis.
Our recent illustration that the monoanionic PNP pincer
ligand (PNP = N[2-P(iPr)2-4-Me-C6H3]2 ) supports a range of
Ar = 2,6-iPr2C6H3.
−
a
B
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I)11 with 1 equiv of K[LMe] at room temperature produces the
corresponding thorium(IV) β-diketiminate complexes 7 (X =
Br) and 8 (X = I) as yellow solids in 56% and 39% isolated
yields, respectively. Following workup by filtration through
Celite and crystallization, the complexes were reproducibly
isolated as analytically pure solids and were characterized by a
combination of 1H and 13C{1H} NMR spectroscopy, elemental
analysis, and X-ray crystallography. Alternatively, complex 8 can
be prepared using the thorium(IV) tetraiodide complex
ThI4(DME)2.8 In THF solution, ThI4(DME)2 generates
ThI4(THF)4; subsequent addition of toluene and 1 equiv of
K[LMe] at room temperature affords (LMe)ThI3(THF) (8) in
40% isolated yield.
spectroscopy, elemental analysis, and X-ray crystallography.
Complex 9 contains a coordinated THF, whereas complex 11
does not, which is most likely a consequence of the smaller
ionic radius of bromide (1.96 Å) in comparison to iodide (2.20
Å).12
Unlike their LMe counterparts (LMe)ThX3(THF) (X = Br
(7), I (8)), the 1H NMR spectra of complexes (LtBu)-
ThBr3(THF) (9) and (LtBu)ThI3 (11) reveal that the two
arms of the β-diketiminate are inequivalent in solution.
Consistent with the greater steric demands of the LtBu ligand,
each complex displays two septets (9, 3.71, 2.49 ppm; 11, 3.68,
2.50 ppm), corresponding to the two sets of methine protons
of the isopropyl substituents, and four doublets (9, 1.73, 1.41,
0.98, 0.83 ppm; 11, 1.73, 1.40, 0.97, 0.81 ppm), corresponding
to the four diastereotopic methyl groups of the isopropyl
substituents.
Complexes 7 and 8 represent members of a new class of
thorium halide complexes that feature only one bulky β-
diketiminate ligand (2,6-iPr2C6H3)NC(Me)CHC(Me)N-
(2,6-iPr2C6H3) (LMe). The only other example is the thorium
iodobutoxide complex 2. Efforts were made to prepare the
bis(β-diketiminate) systems; however, addition of 2 equiv of
K[LMe] to ThX4(THF)4 (X = Br, I) yielded only (LMe)-
ThX3(THF) (X = Br (7), I (8)), even upon heating. Given the
large ionic radius of thorium(IV) (1.05 Å)12 and the previously
reported bis(β-diketiminate) complex [(Me3Si)NC(Ph)CHC-
(Ph)N(SiMe3)]2ThCl2 (3),9a this result is somewhat surprising.
However, it underscores the fact that the [(2,6-iPr2C6H3)NC-
(Me)CHC(Me)N(2,6-iPr2C6H3)] (LMe) framework provides a
larger steric profile than Lappert’s [(Me3Si)NC(Ph)CHC(Ph)-
N(SiMe3)] β-diketiminate ligand.
As with the LMe framework, all attempts to install two LtBu
ligands on thorium were unsuccessful and yielded only
(LtBu)ThBr3(THF) (9) or (LtBu)ThI3 (11). This is not
unexpected, given the larger steric profile of the LtBu ligand in
comparison to the LMe ligand. Although no reaction was
observed with K[C5Me5], treatment of (LtBu)ThBr3(THF) (9)
with 1 equiv of K[C5H4Me] in toluene afforded a mixed-ligand
thorium cyclopentadienyl β-diketiminate complex (10) in 74%
isolated yield (Scheme 1). Efforts to replace a second bromine
atom with a C5H4Me group failed, even with prolonged
reaction times and heating. (LtBu)Th(C5H4Me)Br2 (10) is a
unique example of a complex possessing both cyclopentadienyl
and β-diketiminate ligands. To date, cyclopentadienyl β-
diketiminate mixed-ligand complexes have only been reported
for chromium,14 nickel,15 and ytterbium.16 The synthesis of
(LtBu)Th(C5H4Me)Br2 (9) demonstrates that the mono(β-
diketiminate) halide complexes can support organometallic
chemistry.
Because of these promising results obtained for thorium(IV),
we next turned our efforts toward uranium. A stable mono(β-
diketiminate)uranium(IV) trihalide complex would be a useful
addition to the known tris(pyrazolylborate), pentamethylcyclo-
pentadienyl, and PNP systems [HB(3,5-Me2pz)3]-
UCl3(THF),17 (C5Me5)UCl3(THF)2,2b,18 and (PNP)-
UCl3(THF)/(PNP)UCl3(OPMe3)2/(PNP)UCl3(O
PPh3), respectively, which are rare examples of uranium
trihalide complexes supported by a single monoanionic ligand
set. In general, mono(ligand)actinide complexes are coordina-
tively unsaturated and prone to redistribution, but they are
stable as adducts with one or two additional donor
ligands.6j,18,19
1
The H NMR spectra of 7 and 8 are consistent with those
reported for other diamagnetic transition-metal and lanthanide
complexes possessing the methyl-substituted (LMe) β-diketimi-
1
nate ligand. For instance, the H NMR spectrum of complex 7
in C6D6 exhibits multiplets between 7.16 and 7.22 ppm
assignable to the six aromatic protons on the terminal aryl
groups, a singlet at 5.21 ppm for the unique γ-H of the
NCCCN backbone, and two broad singlets at 4.07 and 1.23
ppm corresponding to the α (4H) and β (4H) protons,
respectively, of the coordinated THF. The two diastereotopic
methyl groups of the isopropyl substituents appear as doublets
at 1.17 and 1.66 ppm. A single resonance at 1.64 ppm,
corresponding to the two methyl groups on the NCCCN
backbone, and a single septet at 3.60 ppm, corresponding to the
four isopropyl methine protons, are consistent with a C2v-
symmetric structure relating the halves of the β-diketiminate
ligand.
Complex 8 displays a similar spectrum, but a wholesale shift
of the resonances to downfield is observed. This is a
consequence of the weaker π donating ability of the iodide
versus the bromide ligand. We and others have noted this
phenomenon for a variety of uranium metallocene complex-
es.4b,13 The poorer the π donor (I < Br), the more electron
deficient the actinide metal center, which results in less
shielding and a downfield shift of the auxiliary ligand protons.
In the present case, (LMe)ThX3(THF) (X = Br (7), I (8)), the
auxiliary ligands are the β-diketiminate and the THF.
As outlined in Scheme 2, treatment of UCl4 with 1 equiv of
either K[LMe] or K[LtBu] in THF at room temperature gives the
corresponding mono(β-diketiminate)uranium(IV) chloride
complexes (LMe)UCl3(THF) (12) and (LtBu)UCl3 (13) as
red solids, respectively, in 72% and 66% isolated yields. Unlike
thorium, the uranium chemistry requires THF to dissolve UCl4,
which is insoluble in toluene. Following workup by filtration
through Celite and crystallization, both complexes were
reproducibly isolated as analytically pure solids and were
characterized by a combination of 1H NMR spectroscopy,
elemental analysis, electronic absorption spectroscopy, and X-
ray crystallography.
Under conditions identical with those employed for the
synthesis of complexes 7 and 8, complexes supported by the
more sterically demanding tert-butyl-substituted β-diketiminate
ligands were prepared. As illustrated in Scheme 1, K[LtBu
]
reacts with ThX4(THF)4 (X = Br, I) at room temperature in
toluene to provide (LtBu)ThBr3(THF) (9) and (LtBu)ThI3 (11),
respectively, in 74% and 45% yields. Both complexes were
As noted above, Hayton and co-workers have used the LMe
ligand in the preparation of a variety of uranyl complexes (e.g.,
5 and 6),10 and we have used it as a supporting ligand for
trivalent uranium (e.g., complex 3).7 The synthesis of
1
characterized by a combination of H and 13C{1H} NMR
C
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Article
resonances; the spectrum of complex 12 is representative and is
shown in Figure 2. Complex 12 displays singlets downfield at
13.20 ppm and upfield at −0.89 ppm corresponding to the γ-H
and methyl groups, respectively, of the NCCCN backbone.
Broad singlets for the coordinated THF are observed upfield at
−1.54 and −7.52 ppm. As observed for the thorium
counterparts (complexes 7 and 8), there is a mirror plane
relating both arms of the LMe ligand, as evidenced by the two
doublets at 0.27 and −4.97 ppm for the two diastereotopic
methyl groups of the isopropyl substituents and the singlet at
−6.26 ppm for the four isopropyl methane protons. As with the
Scheme 2. Synthetic Routes to the Uranium Chloride β-
Diketiminate Complexes 12−14
a
1
thorium systems 9−11, the H NMR spectrum of complex 13
clearly shows
̀
the asymmetric binding of the LtBu ligand to the
uranium metal center, with two signals at 28.44 and −13.79
ppm for the four isopropyl methine protons and four sets of
resonances at 9.78, 5.79, −0.77, and −24.22 ppm correspond-
ing to the four sets of diastereotopic methyl groups of the
isopropyl substituents. The γ-H and tert-butyl groups on the
NCCCN backbone appear as singlets at −56.35 and −3.79
ppm, respectively.
We also attempted to prepare the bis(β-diketiminate)-
uranium(IV) complex (LMe)2UCl2. However, as shown in
Scheme 2, reaction of 2 equiv of K[LMe] with UCl4 led instead
to the cationic diuranium complex [{(LMe)(Cl)U}2(μ-Cl)3]-
[Cl] (14), which was isolated as a dark brown solid in a modest
33% yield. The dimer’s mode of formation is unclear; however,
the reaction of (LMe)UCl3(THF) (12) with another 1 equiv of
K[LMe] does not afford [{(LMe)(Cl)U}2(μ-Cl)3][Cl] (14),
suggesting that 12 is not an intermediate in the formation of
14.
a
Ar = 2,6-iPr2C6H3.
(LMe)UCl3(THF) (12) demonstrates that this ligand set can
also be used as a framework for tetravalent uranium, and
(LtBu)UCl3 (13) represents the first example of a uranium
complex featuring the LtBu framework.
Compound 14 joins a small group of diuranium(IV) bridging
chloride (where μ-Cl > 1) complexes reported in the literature.
In 1985, Cotton and Schwotzer reported the cationic
Typical of 5f 2 uranium(IV) species, the H NMR spectra of
1
complexes 12 and 13 exhibit sharp but paramagnetically shifted
Figure 2. 1H NMR spectrum of (LMe)UCl3(THF) (12) in C6D6. Peaks marked with asterisks denote the residual solvent of crystallization hexane.
D
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t
Figure 3. X-ray crystal structures of (LMe)ThBr3(THF) (7), (LtBu)ThBr3(THF) (9; a Bu group on one aryl ring was removed for clarity), and
(LtBu)Th(C5H4Me)Br2 (10).
Table 1. Crystallographic Experimental Parameters for Complexes 7, 9, 10, and 12−14
7
9
10
12
13
14
formula
a (Å)
C41H65Br3N2O3ThU
36.731(9)
12.654(3)
22.999(6)
90
C43H69Br3N2O2Th
25.940(5)
21.300(4)
17.171(3)
90
C41H60Br2N2Th
10.62080(10)
20.3151(2)
18.8606(2)
90
C39H63Cl3N2OU
35.507(8)
12.576(3)
22.923(5)
90
C35H53Cl3N2U
10.2148(4)
10.6827(4)
18.9861(8)
83.799(1)
82.505(1)
61.941(1)
1810.08(12)
2
C68.50H94Cl6N4U2
15.000(3)
19.922(3)
23.849(4)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
124.220(4)
90
115.181(3)
90
105.5221(4)
90
124.222(3)
90
91.273(3)
90
8839(4)
8
8586(3)
8
3920.99(7)
4
8464(3)
8
7125(2)
4
fw
1105.72
C2/c
1117.77
C2/c
972.77
920.29
C2/c
846.17
1662.24
P21/n
space group
T (K)
P21/c
P1
̅
140(2)
0.71073
1.662
141(2)
0.71073
1.729
141(2)
141(2)
0.71073
1.444
141(2)
0.71073
1.553
141(2)
0.71073
1.550
λ (Å)
0.71073
1.648
Dcalcd (Mg m−3
)
μ (mm−1
)
6.123
6.303
5.871
4.054
4.730
4.805
R1 (I > 2σ(I))
0.0425
0.0347
0.0176
0.0602
0.1431
0.0145
0.0390
0.0652
0.1199
wR2 (all data)
0.1051
0.0876
0.0493
compound {[U-(η6-C6Me6)Cl2]2(μ-Cl)3}AlCl4, which is quite
similar to 14. This compound was formed using a variation of
Fischer’s reductive Friedel−Crafts reaction: the authors
combined UCl4, AlCl3, and hexamethylbenzene in refluxing
hexane for 12 h, replaced the solvent with CH2Cl2, and added
zinc; after workup, yellow crystals of the cationic compound
were isolated.20 Subsequently, [UCl3(THF)2]2(μ-Cl)2, reported
by Rebizant and co-workers, was synthesized from UCl4 and
THF in pentane at room temperature.21 In 1994, Scott and
Hitchcock prepared the first tripodal amido complex of
uranium, [U(N′3N)]2(μ-Cl)2 (N′3N = N[CH2CH2N-
(SiMe3)]3), by sublimation of the product of UCl4 and
Li3[N′3N] combined in cold THF, UCl(N′3N)(THF)x (x ≤
1).22 In that same year, Arliguie and co-workers reported a
cation−anion pair in which both ions were multinuclear in
uranium; in this case, the anion was diuranium(IV):
{[UCl2(Cp)]2(μ-Cl3)}−.23 More recently, Coles and colleagues
reported the structures [UCl(N(CH2CH2PR2)2)2]2(μ-Cl2) (R
described the synthesis of [UCl(OiPr)2(DME)]2(μ-Cl)2 as a
ligand redistribution product, which formed in low yield during
an attempt to synthesize a mixed-metal zirconium−uranium-
(IV) complex; this compound is very similar to the Rebizant
structure, with the axial chlorides replaced by isopropoxides and
the THF ligands replaced by DME.25 Finally, the Liddle group
reported several diuranium(IV) bridging chloride complexes,
[UCl2(BIPMTMSH)]2(μ-Cl2) (BIPMTMS = C(PPh2NSiMe3)2),
[UCl(BIPMTMS)UCl(BIPMTMSH)](μ-Cl3), and [UCl-
(BIPMTMS)(THF)]2(μ-Cl2), which were prepared en route to
an arene-bridged diuranium single-molecule magnet.26
On the basis of our unsuccessful attempts to install two bulky
β-diketiminate ligands on Th and U, we conclude that the
combination of steric bulk and rigidity of these particular (β-
diketiminate) ligands prevents the bis(β-diketiminate) Th and
U complexes from forming. In contrast, others have prepared
bis(ligand) Th(IV) and U(IV) complexes with bulky but more
flexible ligands, such as PNP,6s tris(pyrazolylborate),2a,6o,q
cyclopentadienyl,2a,4e,27 amidinate,6b,e,28 and ferrocene dia-
mide.29 However, bis(ligand) complexes can be a dead end
i
= Et, Pr), in which each uranium(IV) has one tridentate and
one bidentate phosphinoamide ligand.24 The Evans group
E
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t
Figure 4. X-ray crystal structures for (LMe)UCl3(THF) (12), (LtBu)UCl3 (13), and [{(LMe)(Cl)U}2(μ-Cl)3][Cl] (14). The Bu groups on the aryl
rings in all three structures and Me groups on the NCCCN backbone of 14 were removed for clarity.
with respect to further reactivity, in that they tend to be
thermodynamically and kinetically stable and can form as the
majority product when the monoligand complex is the desired
target. In comparison to their mono(ligand) counterparts, they
can have reduced reactivity due to blocked access to the
reactive electrophilic actinide center, as well as reduced
electrophilicity at the actinide center due to twice the electron
donation.
Å for complex 12 and 2.578 Å for complex 13. These distances
are comparable to those reported for other structurally
characterized uranium(IV) chloride complexes.31 In the
cationic bimetallic complex 14, however, the terminal uranium
chloride distances are slightly longer, at U−Cl(av) = 2.639 Å.
Also, as is typical for complexes containing both bridging and
terminal ligands, the U−(μ-Cl) distances are longer than the
terminal U−Cl distances: the U−(μ-Cl)(av) distance in 14 is
2.827 Å. This U−(μ-Cl) distance is average for dinuclear U(IV)
complexes with bridging chlorides.32 The U···U distance in
complex 14 is 4.0389(7) Å, which is at the low end of the range
of other similar complexes.33 The U−N(av) distances and N−
U−N angles are nearly the same in the two monometallic
uranium complexes (2.391 Å and 76.9(3)° for 12; 2.3576 Å and
76.79(5)° for 13) and are comparable to those observed for
other known uranium β-diketiminate complexes (e.g.: 5, U−
N(av) = 2.417 Å and N−U−N = 72.4(2)°;10b 6, U−N(av) =
2.48 Å and N−U−N = 72.6(5)°10a). Neither the N−U−N
angles nor the U−N or U−Cl distances show even the small
differences seen in the thorium complexes as a result of moving
from methyl to tert-butyl groups.
X-ray Crystallography. The X-ray crystal structures for the
thorium bromide complexes (LMe)ThBr3(THF) (7), (LtBu)-
ThBr3(THF) (9), and (LtBu)Th(C5H4Me)Br2 (10) are shown
in Figure 3; crystal experimental parameters are given in Table
1. The average Th−Br distances in complex 7 (2.819 Å),
complex 9 (2.841 Å), and complex 10 (2.833 Å) are similar,
spanning a range of only 0.023 Å. These distances are
comparable to those presented by the other reported
thorium(IV) bromide complexes.30 Substitution of a bromide
ligand and a THF ligand (9) with a cyclopentadienyl ligand
does not significantly affect the Th−Br distance in complex 10.
The average Th−N distances also have a narrow range (0.054
Å), 2.444 Å (complex 7), 2.453 Å (complex 9), and 2.498 Å
(complex 10), and provide a close match to those observed in
the previously reported thorium β-diketiminate complexes
[(2,6-iPr2C6H3)NC(Me)CHC(Me)N(2,6-iPr2C6H3)]ThI2[O-
(CH2)4I](THF) (2; Th−N(av) = 2.468 Å)8 and [(Me3Si)-
NC(Ph)CHC(Ph)N(SiMe3)]2ThCl2 (3; Th−N(av) = 2.467
Å).9a Finally, the Th−O distance is 2.467(4) Å in complex 7
and 2.520(3) Å in complex 9, while the Th−C5H4Me(centroid)
distance in complex 10 is 2.542(2) Å. Although these combined
data show that moving from the smaller methyl substituent on
In all the thorium and uranium β-diketiminate complexes,
the actinide metal ion is situated above the plane defined by the
NCCCN ligand backbone. This distance is greater in the
monometallic LtBu complexes (∼1.7−1.9 Å) than in the LMe
complexes (∼0.82−0.89 Å). As noted earlier, this difference is
1
also observed in solution by H NMR spectroscopy, with the
t
appearance of inequivalent Bu groups in the LtBu complexes
but equivalent Me groups in the LMe complexes. Overall, these
compounds can be considered to exhibit electron delocalization
over the β-diketiminate ligand, and interestingly, their An−N
distances compare well to those within actinide bis(ketimide)
complexes.4e,27b,34
t
the NCCCN backbone to the larger Bu does not significantly
lengthen the Th−N, Th−O, or Th−Br bond distances, the tBu
groups do cause the N−Th−N angles to widen: 74.43(15)° in
complex 7, 82.94(11)° in complex 9, and 78.42(7)° in complex
10.
Electrochemistry and Electronic Spectroscopy. To
assess and compare the electronic structures in the mono(β-
diketiminate) complexes to those in similar thorium(IV) and
uranium(IV) systems electrochemical and UV−visible−near-
infrared absorption spectroscopic studies were undertaken.
Voltammetric data of K[LMe] and complex 12 were obtained in
[Pr4N][B{3,5-(CF3)2-C6H3}4]/1,1,1-trifluorotoluene solutions
(see the Supporting Information for details). Complex 12 was
The X-ray crystal structures of the uranium chloride
complexes (LMe)UCl3(THF) (12), (LtBu)UCl3 (13), and
[{(LMe)(Cl)U}2(μ-Cl)3][Cl] (14) are shown in Figure 4;
crystal experimental parameters are given in Table 1. As in the
thorium counterparts, the average actinide−halide distances in
the monometallic uranium complexes are very similar, at 2.594
F
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determined to rapidly degrade in electrolyte solutions of THF,
necessitating the switch to the less coordinating solvent 1,1,1-
trifluorotoluene. The voltammetric behavior of K[LMe] was
fairly straightforward, exhibiting a single irreversible oxidation
wave at ∼+0.55 V (all potentials are referenced to the
[(C5H5)2Fe]+/0 couple) and no observable reductive electro-
chemical processes within the available cathodic potential limit
(∼−2.8 V) of this solvent/electrolyte system. In contrast, the
voltammetric behavior of 12, while reproducible, was quite
complex. Two irreversible oxidation processes were observed at
∼+0.53 and +0.65 V, which we ascribe to a ligand-based
oxidation comparable to that for the free ligand and probable
oxidation of one or more of the bound chloride ions. In
addition, 12 exhibited a total of three irreversible reduction
waves in the range from −1.84 to −2.34 V. On the basis of
previous studies by us and others,4d this is the proper potential
range to anticipate a U(VI)/U(III) reduction process for this
coordination environment, but the appearance of three discrete
reduction steps (and the absence of reductive voltammetry for
the free ligand) suggests there are some coupled chemical
transformations in concert with metal-based reduction that lead
to additional reduction waves. Electrochemical studies of the
analogous Th(IV) complexes (e.g., 8) were not attempted.
The UV−visible−near-infrared absorption spectral data for
K[LMe] and complexes 8 and 12 in toluene solution are
illustrated in Figure 5. There is an excellent correlation in the
The assignment of these electronic transitions merits some
additional discussion because of the insight it may provide into
the electronic structure and metal−ligand interactions in these
β-diketiminate complexes of the tetravalent actinides. In this
regard, a previous experimental and theoretical study of Cu(II)
complexes containing β-diketiminate ligands is particularly
relevant.35 In this study involving (LMe)CuSCPh3, the β-
diketiminate ligand was modeled in density functional theory
calculations as a simplified C3N2H5 group to arrive at a
description of the orbital character and relative energetics of the
highest lying occupied (HOMO) and lowest lying unoccupied
(LUMO) molecular orbitals of the free β-diketiminate ligand.
In particular, the HOMO is defined as a combination of out-of-
plane p orbitals on the two nitrogen atoms and the central
carbon atom, whereas the next lowest lying molecular orbital
(HOMO-1; only 0.2 eV lower in energy) is defined as an in-
plane, out-of-phase combination of p orbitals on the two
nitrogen atoms. The LUMO is described as an out-of-plane
combination of p orbitals on the two nitrogen atoms and the
two α-carbon atoms. Experimentally, for this Cu(II) complex
there are two high-energy π−π* transitions observed at 28500
cm−1 (ε = 16360 M−1 cm−1) and ∼30500 cm−1 (ε = ∼13000
M−1 cm−1) that are assigned to the β-diketiminate localized
HOMO → LUMO and HOMO-1 → LUMO transitions.
These results are in excellent agreement with those observed in
the present study for K[LMe] and complexes 8 and 12 for the
two highest energy bands illustrated in Figure 5, strongly
suggesting that these bands have very similar β-diketiminate
orbital origins in our actinide systems.
As noted above, the higher energy putative π−π* band in our
systems (Figure 5) is nearly insensitive to the metal ion
identity. In contrast, the lower energy putative HOMO →
LUMO band in our systems is quite sensitive to the identity of
the coordinating (Th(IV) vs U(IV)) or counter metal ion (K+).
Recall that the structural data for the actinide(IV) complexes
(vide supra) shows that the metal ion lies substantially out of
the plane of the NCCCN backbone. Thus, direct σ-bonding
interactions between the in-plane ligand orbitals in HOMO-1
and the metal orbitals should be diminished relative to the
expected interaction for coplanarity in metal−ligand bonding.
However, because of the much greater angular distribution of
the frontier f orbitals in these actinides, there should be
potential for relatively more interaction between the out-of-
plane ligand orbitals in the HOMO and the metal f orbitals that
could lead to the observed variability in the behavior of the
lower-lying electronic transition. The relative energies of the
metal f and d orbitals for U(IV) vs Th(IV) with respect to
those of the ligand orbitals are also clearly contributing to the
observed differences in these spectral band energies and
intensities.
A final interesting observation regarding the electronic
spectral data in these mono(β-diketiminate) complexes pertains
to the intensity in the f−f electronic bands in the near-infrared
spectral region for 12. We have reported a strong correlation
for U(IV) and U(V) complexes in the intensity of the f−f
transitions and the extent of covalency in the metal−ligand
bonding.4b,d,e,5a,27b,36 For example, in the bis(ketimide)
complexes of U(IV) for which there is evidence of partial
multiple-bond character in the metal−ketimide bonding, the f−
f transition intensities are typically ∼100−200 M−1 cm−1. For
12 we find the f−f bands have intensities of only ∼5−10 M−1
cm−1. These are among some of the smallest values we have
observed for f−f bands in organouranium complex-
Figure 5. UV−visible−near-infrared electronic absorption spectral
data of K[LMe], (LMe)ThI3(THF) (8), and (LMe)UCl3(THF) (12) in
toluene solution at room temperature.
most intense absorption bands among all three systems in the
UV−visible region. In particular, each system exhibits two
intense high-energy bands above ∼25000 cm−1 (400 nm). The
energetically higher lying of these two bands appears at nearly
identical energies for all three systems (∼31000 cm−1), whereas
the lower lying of the two bands shows substantial variation
among the free ligand anion K[LMe], the Th(IV) complex 8,
and the U(IV) complex 12. As shown in Figure 5, the U(IV)
complex 12 also exhibits two additional broad, low-intensity
features (illustrated by dotted lines from spectral fitting) in the
visible region, attributed to charge transfer transitions (likely
ligand to metal on the basis of the electrochemical data,
indicating relative ease of oxidation of the ligand and reduction
of the U(IV) center) as well as a host of narrow, very low
intensity features in the near-infrared attributable to f−f
3
transitions arising from the H4 manifold of the 5f 2 valence
electronic configuration on the U(IV) metal ion.
G
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es.4b,d,e,5a,13f,27b,36 Thus, although the β-diketiminate ligand may
be formally akin to a bis(ketimide) in its coordination
environment, it is clear that the metal−ligand coordination
geometry induced by the β-diketiminate precludes strong
metal−ligand bonding interactions. This observation reinforces
the value of f−f intensities as a qualitative means to assess
metal−ligand bonding interactions in organoactinide com-
plexes.
solution resistance (typically ∼1000 Ω under the conditions
employed). Solutions were contained in PARC Model K0264
microcells consisting of a ∼3 mm diameter Pt-disk working electrode,
a Pt-wire counter electrode, and a silver-wire quasi-reference electrode.
Scan rates from 20 to 5000 mV/s were employed in the cyclic
voltammetry scans to assess the chemical and electrochemical
reversibility of the observed redox transformations. Half-wave
potentials were determined from the peak values in the square-wave
voltammograms or from the average of the cathodic and anodic peak
potentials in the reversible cyclic voltammograms. Potential calibra-
tions were performed at the end of each data collection cycle using the
ferrocenium/ferrocene couple as an internal standard. Electronic
absorption and cyclic voltammetric data were analyzed using
Wavemetrics IGOR Pro (Version 4.0) software on a Macintosh
platform.
CONCLUSIONS
■
In summary, the first thorium(IV) and uranium(IV) halide
complexes supported by the ligands N,N′-bis(2,6-
diisopropylphenyl)pentane-2,4-diiminate (LMe) and N,N′-bis-
(2,6-diisopropylphenyl)-2,2−6,6-tetramethylheptane-3,5-diimi-
nate (LtBu) have been prepared. The mono(β-diketiminate)
halide complexes (LMe)AnX3(THF) (An = Th, X = Br, I; An =
U, X = Cl) and (LtBu)AnX3(THF) (An = Th, X = Br, I; An = U,
X = Cl) were synthesized, as well as a mixed-ligand
methylcyclopentadienyl β-diketiminate complex, (LtBu)Th-
(C5H4Me)Br2. We anticipate that this new class of actinide
compounds will be useful as starting materials for future
thorium and uranium β-diketiminate reactivity studies.
Interestingly, the corresponding bis(β-diketiminate) halide
complexes could not be prepared, and for uranium, the cationic
diuranium complex [{(LMe)(Cl)U}2(μ-Cl)3][Cl] was isolated
instead. To gain information about the electronic and structural
consequences of installing the β-diketiminate ligand on thorium
General Synthetic Considerations. Unless otherwise noted,
reactions and manipulations were performed at ambient temperatures
in a recirculating Vacuum Atmospheres Model HE-553-2 inert-
atmosphere (He or N2) drybox equipped with a MO-40-2 Dri-Train, a
Vacuum Atmospheres NEXUS model inert-atmosphere (N2) drybox
equipped with a 40CFM Dual Purifier NI-Train, or using standard
Schlenk techniques. Glassware was dried overnight at 150 °C before
use. All NMR spectra were obtained using a Bruker Avance 300 MHz
1
spectrometer. Chemical shifts for H and 13C{1H} NMR spectra are
reported in parts per million (ppm) and were referenced to
tetramethylsilane. 13C{1H} NMR assignments were confirmed through
the use of DEPT-135 experiments. Elemental analyses were performed
at the University of California, Berkeley, Microanalytical facility on a
Perkin-Elmer Series 2400 CHNS Analyzer.
Materials. Unless otherwise noted, reagents were purchased from
commercial suppliers and used without further purification. Celite
(Aldrich), alumina (Brockman I, Aldrich), and 3 Å molecular sieves
(Aldrich) were dried under dynamic vacuum at 250 °C for 48 h prior
to use. All solvents (Aldrich) were purchased anhydrous and were
dried over KH for 24 h, passed through a column of activated alumina,
and stored over activated 3 Å molecular sieves prior to use. Benzene-d6
(Aldrich) and tetrahydrofuran-d8 (Cambridge Isotope Laboratories)
were purified by storage over activated 3 Å molecular sieves prior to
use. Iodine was purchased from Sigma Aldrich and used as received.
K[LMe], K[LtBu], and K[C5H4Me] were prepared as detailed below.
H[(2,6-iPr2C6H3)NC(Me)CHC(Me)N(2,6-iPr2C6H3)],37 H-
1
and uranium, we performed a combination of H and 13C{1H}
NMR spectroscopy, elemental analysis, X-ray crystallography,
and UV−visible−near-IR spectroscopy. Due to the large ionic
radius of the actinide metal centers, in all cases, the
actinide(IV) metal centers lie out of the plane of the β-
diketiminate ligand NCCCN backbone by ∼1−2 Å. The UV−
visible−near-infrared electronic spectroscopic data for com-
plexes (LMe)ThI3(THF) (8) and (LMe)UCl3(THF) (12) reflect
this interesting metal−ligand bonding motif in the mono(β-
diketiminate) halide complexes. Specifically, perturbations in
ligand-based transitions in the UV region are consistent with
the out-of-plane metal binding, while the intensities in the f−f
transitions in the near-IR region for (LMe)UCl3(THF) (12)
demonstrate that the covalent bonding interaction is relatively
weak.
[(2,6-i Pr2 C6 H3 )NC(t Bu)CHC(t Bu)N(2,6-i Pr2 C6 H3 )],3 8
19e
ThBr4(THF)4,11 ThI4(THF)4,11 ThI4(DME)2,8 and UCl4
prepared according to literature procedures.
were
Caution! Depleted uranium (primarily isotope 238U) and natural
thorium (isotope 232Th) are both weak α-emitters with half-lives of
4.47 × 109 and 1.41 × 1010 years, respectively. Manipulations and
reactions should be carried out in monitored fume hoods or in an
inert-atmosphere drybox in a radiation laboratory equipped with α-
and β-counting equipment.
EXPERIMENTAL SECTION
■
Instrumentation and Sample Protocols. Electronic absorption
spectral data were obtained for toluene solutions of complexes over
the wavelength range 300−1600 nm on a Perkin-Elmer Model
Lambda 950 UV−visible−near-infrared spectrophotometer. All data
were collected in 1.0 or 0.1 cm path length cuvettes loaded in a
recirculating Vacuum Atmospheres NEXUS model inert-atmosphere
(N2) drybox equipped with a 40CFM Dual Purifier NI-Train. Sample
concentrations ranged from ∼0.05 to ∼20 mM, to optimize
absorbance in the UV−visible and near-infrared regions. Spectral
resolution was typically 2 nm in the visible region and 4−6 nm in the
near-infrared region. Sample spectra were obtained versus air and
corrected for solvent absorption subsequent to data acquisition.
Cyclic and square wave voltammetric data were obtained in the
Vacuum Atmospheres drybox system described above. All data were
collected using a Princeton Applied Research Corp. (PARC) Model
263 potentiostat under computer control with PARC Model 270
software. All sample solutions were ∼2−3 mM in complex with 0.1 M
[Pr4N][B{3,5-(CF3)2C6H3}4] supporting electrolyte in 1,1,1-trifluor-
otoluene solvent. All data were collected with the positive-feedback IR
compensation feature of the software/potentiostat activated to ensure
minimal contribution to the voltammetric waves from uncompensated
Synthesis of K[LMe]. A 50 mL round-bottomed flask equipped
with a stir bar was charged with H[(2,6-iPr2C6H3)NC(Me)CHC-
(Me)N(2,6-iPr2C6H3)] (1.20 g, 2.87 mmol) and diethyl ether (20
mL). To this clear brown solution was added K[N(SiMe3)2] (0.573 g,
2.87 mmol), causing the solution to become darker brown. The
reaction mixture was stirred at room temperature. After 18 h, the
reaction mixture was filtered through a Celite-padded coarse-porosity
fritted filter, and the volatiles were removed under reduced pressure.
The resulting solid was scraped and transferred to a medium-porosity
fritted filter and washed with hexane (3 × 10 mL). The washed solid
was dried under reduced pressure to give K[LMe] as an off-white solid
(1.268 g, 2.78 mmol, 97%). 1H NMR (C6D6, 298 K): δ 7.06−7.01 (m,
6H, Ar-H), 4.82 (s, 1H, γ-C-H), 3.36 (sept, J = 27 Hz, 4H, CHMe2),
1.89 (s, 6H, C(CH3)), 1.26 (d, J = 6 Hz, 12H, CHMe2), 1.07 (d, J = 6
Hz, 12H, CHMe2).
Synthesis of K[LtBu]. A 250 mL side-arm flask equipped with a stir
bar was charged with H[(2,6-iPr2C6H3)NC(tBu)CHC(tBu)N-
(2,6-iPr2C6H3)] (2.50 g, 4.97 mmol) and THF (75 mL). To this
was added KH (0.400 g, 9.97 mmol). The reaction mixture was stirred
at reflux temperature. After 1 h, the heat was turned off and the
H
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reaction mixture stirred at room temperature. After 18 h, the reaction
mixture was filtered through a Celite-padded coarse-porosity fritted
filter, and the volatiles were removed under reduced pressure. The
resulting yellow solid was scraped and transferred to a medium-
porosity fritted filter and washed with pentane (75 mL). The washed
solid was dried under reduced pressure to give K[LtBu] as a pale yellow
solid (1.90 g, 3.51 mmol, 71%). 1H NMR (C6D6, 298 K): δ 7.00 (d, J
= 9 Hz, 4H, m-Ar-H), 6.82 (t, J = 6 Hz, 2H, p-Ar-H), 4.94 (s, 1H, γ-C-
H), 3.27 (sept, J = 39 Hz, 4H, CHMe2), 1.38 (s, 18H, C(CH3)3), 1.34
(d, J = 9 Hz, 12H, CHMe2), 0.94 (d, J = 6 Hz, 12H, CHMe2).
Synthesis of K[C5H4Me]. A 250 mL side-arm flask equipped with
a stir bar was charged with K[N(SiMe3)2] (9.52 g, 47.7 mmol) and
diethyl ether (200 mL). To this stirred slurry was added portionwise
C5H5Me (4.99 g, 62.2 mmol) over the course of 15−30 min. The
resulting pale yellow slurry was stirred at room temperature. After 18
h, the volatiles were removed under reduced pressure. Hexanes (75
mL) was added to the resulting off-white solid residue, and the mixture
was stirred at room temperature. After 12 h, the slurry was filtered
through a coarse-porosity fritted filter. The solid was washed with
hexanes (3 × 20 mL) and dried under reduced pressure to give
K[C5H4Me] as an off-white solid (5.21 g, 44.1 mmol, 92%). 1H NMR
(THF-d8, 298 K): δ 5.33 (s, 2H, C5H4Me), 5.29 (s, 2H, C5H4Me),
2.09 (s, 3H, CH3).
spectrum acquired at this point showed the two DME ligands
displaced by four THF ligands, with peaks at 4.30 and 1.30 ppm.) A
toluene solution of K[LMe] (0.0550 g in 5 mL) was prepared and
cooled to −35 °C. Cold toluene (6 mL, −35 °C) was added to the
dried reaction mixture, followed by the addition of the cooled toluene
solution of K[LMe]. The reaction mixture was stirred at room
temperature for 1 h, and then the volatiles were removed under
reduced pressure. The dried reaction mixture was extracted with
pentane, and the pentane extract was filtered through a pipet equipped
with a fiberglass plug and packed with Celite. The pentane filtrate was
discarded, and the reaction mixture was then extracted with toluene
and the extract filtered through the same Celite pipet. The toluene
1
filtrate was dried, giving pure 8 (by H NMR; shifts identical with
those listed above) as a yellow solid (0.0544 g, 0.0487 mmol, 40%).
Synthesis of (LtBu)ThBr3(THF) (9). A 125 mL side-arm flask
equipped with a stir bar was charged with K[LtBu] (1.00 g, 1.85 mmol),
ThBr4(THF)4 (1.63 g, 1.94 mmol), and toluene (75 mL). The
reaction mixture was stirred at room temperature. After 15 h, the
reaction mixture was filtered through a Celite-padded coarse-porosity
fritted filter and the volatiles were removed under reduced pressure.
The resulting yellow solid residue was washed with hexanes (2 × 25
mL). The yellow washings were decanted, and the remaining solid was
dried under reduced pressure to give analytically pure 9 as a yellow
solid (1.51 g, 1.44 mmol, 74%). X-ray-quality crystals of 9 were
obtained by recrystallization from a concentrated toluene solution at
−35 °C. 1H NMR (C6D6, 298 K): δ 7.05−7.02 (m, 2H, Ar-H), 6.96−
6.88 (m, 4H, Ar-H), 6.66 (s, 1H, γ-C-H), 4.31 (m, 4H, OCH2), 3.71
(sept, J = 6.9 Hz, 2H, CHMe2), 2.49 (sept, J = 6.9 Hz, 2H, CHMe2),
1.73 (d, J = 6.9 Hz, 6H, CHMe2), 1.41 (d, J = 6.9 Hz, 6H, CHMe2),
1.30 (m, 4H, OCH2CH2), 1.09 (s, 18H, CMe3), 0.98 (d, J = 6.9 Hz,
6H, CHMe2), 0.83 (d, J = 6.9 Hz, 6H, CHMe2). 13C{1H} NMR (C6D6,
298 K): δ 171.28 (CN), 144.32 (Ardipp-Co), 142.45 (Ardipp-Co),
138.20 (Ardipp-Cipso), 126.55 (Ardipp-Cp), 126.10 (Ardipp-Cm), 125.14 (γ-
CH), 44.89 (CMe3), 34.27 (CHMe2), 30.02 (CMe3), 28.29 (CHMe2),
27.39 (CHMe2), 26.87 (CHMe2), 26.82 (CHMe2), 25.17 (CHMe2).
The 13C{1H} resonances for the coordinated THF were not observed.
Anal. Calcd for C39H61Br3N2OTh (mol wt 1045.66): C, 44.80; H,
5.88; N, 2.68. Found: C, 44.91; H, 6.13; N, 2.69.
Synthesis of (LMe)ThBr3(THF) (7). A 125 mL side-arm flask
equipped with a stir bar was charged with K[LMe] (0.300 g, 0.657
mmol), ThBr4(THF)4 (0.546 g, 0.653 mmol), and toluene (60 mL).
The reaction mixture was stirred at room temperature. After 15 h, the
reaction mixture was filtered through a Celite-padded coarse-porosity
fritted filter and the volatiles were removed under reduced pressure.
The resulting yellow solid residue was extracted with hexanes (2 × 25
mL). The yellow extracts were combined and filtered through a Celite-
padded coarse-porosity fritted filter. The yellow filtrate was
concentrated under reduced pressure to ∼20 mL and placed in a
−35 °C freezer to afford 7 as a yellow crystalline (X-ray quality) solid
1
(0.350 g, 0.364 mmol, 56%). H NMR (C6D6, 298 K): δ 7.16−7.22
(m, 6H, Ar-H), 5.21 (s, 1H, γ-C-H), 4.07 (bs, 4H, OCH2), 3.60 (sept,
J = 6.9 Hz, 4H, CHMe2), 1.66 (d, J = 6.9 Hz, 12H, CHMe2), 1.64 (s,
6H, CH3), 1.23 (bs, 4H, OCH2CH2), 1.17 (d, J = 6.9 Hz, 12H,
CHMe2). 13C{1H} NMR (C6D6, 298 K): δ 168.25 (CN), 145.56
Synthesis of (LtBu)(C5H4Me)ThBr2 (10). A 125 mL side-arm flask
equipped with a stir bar was charged with (LtBu)ThBr3(THF) (9;
0.451 g, 0.431 mmol), K[C5H4Me] (0.105 g, 0.888 mmol), and
toluene (55 mL). The reaction mixture was brought to a mild reflux
for 5 min and then cooled to room temperature with stirring. After 12
h, the reaction mixture was filtered through a Celite-padded coarse-
porosity fritted filter and the volatiles were removed under reduced
pressure. The resulting yellow solid residue was washed with hexanes
(2 × 20 mL) and dried under reduced pressure. Analytically pure 10
was obtained by recrystallization from a saturated hexanes/toluene
(Ardipp-Co), 141.42 (Ardipp-Cipso), 128.65 (Ardipp-Cp), 125.72 (Ardipp
-
Cm), 105.42 (γ-CH), 77.63 (OCH2CH2), 30.01 (CHMe2), 26.32
(OCH2CH2), 25.98 (CHMe2), 25.27 (CH3), 25.16 (CHMe2). Anal.
Calcd for C33H49Br3N2OTh (mol wt 961.51): C, 41.22; H, 5.14; N,
2.91. Found: C, 41.00; H, 4.92; N, 2.85.
Synthesis of (LMe)ThI3(THF) (8). Method A: From ThI4(THF)4. A
125 mL side-arm flask equipped with a stir bar was charged with
K[LMe] (0.229 g, 0.501 mmol), ThI4(THF)4 (0.513 g, 0.501 mmol),
and toluene (25 mL). The reaction mixture was stirred at room
temperature. After 15 h, the reaction mixture was filtered through a
Celite-padded coarse-porosity fritted filter and the volatiles were
removed under reduced pressure. The resulting yellow solid residue
was washed with hexanes (2 × 25 mL). The yellow washings were
decanted, and the remaining solid was dried under reduced pressure to
give analytically pure 8 as a yellow solid (0.215 g, 0.649 mmol, 39%).
X-ray-quality crystals of 8 were obtained by slow diffusion of hexanes
1
(80/20) solution at −35 °C (0.310 g, 0.319 mmol, 74%). H NMR
(C6D6, 298 K): δ 7.12−7.09 (m, 2H, Ar-H), 6.97 (m, 4H, Ar-H), 6.73
(m, 2H, C5H4Me), 6.56 (s, 1H, γ-C-H), 5.84 (m, 2H, C5H4Me), 3.41
(sept, J = 6.9 Hz, 2H, CHMe2), 2.89 (s, 3H, C5H4Me), 2.67 (sept, J =
6.9 Hz, 2H, CHMe2), 1.60 (d, J = 6.9 Hz, 6H, CHMe2), 1.42 (d, J =
6.9 Hz, 6H, CHMe2), 1.18 (s, 18H, CMe3), 1.04 (d, J = 6.9 Hz, 6H,
CHMe2), 0.89 (d, J = 6.9 Hz, 6H, CHMe2). 13C{1H} NMR (C6D6, 298
K): δ 174.11 (CN), 146.27 (Ardipp-Cipso), 140.56 (Ardipp-Co), 139.07
1
into a concentrated THF solution at ambient temperature. H NMR
(C6D6, 298 K): δ 7.16−7.28 (m, 6H, Ar-H), 5.31 (s, 1H, γ-C-H), 4.37
(bs, 4H, OCH2), 3.53 (sept, J = 6.9 Hz, 4H, CHMe2), 1.63 (d, J = 6.9
Hz, 12H, CHMe2), 1.60 (s, 6H, CH3), 1.23 (bs, 4H, OCH2CH2), 1.13
(d, J = 6.9 Hz, 12H, CHMe2). 13C{1H} NMR (C6D6, 298 K): δ 168.56
(CN), 143.90 (Ardipp-Co), 142.80 (Ardipp-Cipso), 125.28 (Ardipp-Cm),
105.94 (γ-CH), 29.92 (CHMe2), 26.23 (CH3), 25.41 (CHMe2), 24.97
(CHMe2). The 13C{1H} resonances for the coordinated THF and the
Ardipp-Cp were not observed. Anal. Calcd for C33H49I3N2OTh (mol wt
1102.51): C, 35.95; H, 4.48; N, 2.54. Found: C, 35.67; H, 4.61; N,
2.49.
(Ardipp-Co), 136.67 (C4H4C-Me), 125.69 (Ardipp-Cp), 125.53 (Ardipp-
Cm), 124.38 (Ardipp-Cm), 122.97 (γ-CH), 120.33 (C4H4CMe), 98.49
(C4H4CMe), 44.66 (CMe3), 31.43 (CHMe2), 30.88 (CMe3), 27.69
(CHMe2), 27.21 (CHMe2), 26.86 (CHMe2), 26.44 (CHMe2), 24.29
(CHMe2), 17.19 (C5H4Me). Anal. Calcd for C41H60Br2N2Th (mol wt
972.77): C, 50.62; H, 6.22; N, 2.88. Found: C, 50.91; H, 6.43; N, 2.83.
Synthesis of (LtBu)ThI3 (11). A 125 mL side-arm flask equipped
with a stir bar was charged with K[LtBu] (0.270 g, 0.500 mmol),
ThI4(THF)4 (0.513 g, 0.500 mmol) and toluene (25 mL). The
reaction mixture was stirred at room temperature. After 15 h, the
reaction mixture was filtered through a Celite-padded coarse-porosity
fritted filter and the volatiles were removed under reduced pressure.
The resulting yellow solid residue was washed with hexanes (2 × 15
mL). The yellow washings were decanted and the remaining solid was
Method B: From ThI4(DME)2. A 20 mL scintillation vial was charged
with ThI4(DME)2 (0.1107 g, 0.1203 mmol), THF (5 mL), and a stir
bar. This reaction mixture was stirred for 20 min at −35 °C, and then
the volatiles were removed under reduced pressure. (A 1H NMR
I
dx.doi.org/10.1021/om3010965 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
dried under reduced pressure to give analytically pure 11 as a yellow
solid (0.251 g, 0.225 mmol, 45%). H NMR (C6D6, 298 K): δ 7.04−
X-ray Crystallography. Crystals of 10 and 13 were mounted in a
nylon cryoloop from Paratone-N oil. The data were collected on a
Bruker D8 diffractometer, with an APEX II charge-coupled-device
(CCD) detector and Cryo Industries of America Cryocool G2 low-
temperature device (120 K). The instrument was equipped with a
graphite-monochromated Mo Kα X-ray source (λ= 0.71073 Å) and a
0.5 mm monocapillary. A hemisphere of data was collected using ω
scans, with 10 s frame exposures and 0.5° frame widths. Data
collection and initial indexing and cell refinement were handled using
APEX II software.39 Frame integration, including Lorentz−polar-
ization corrections, and final cell parameter calculations were carried
out using SAINT+ software.40 The data were corrected for absorption
using redundant reflections and the SADABS program.41 Decay of
reflection intensity was not observed, as monitored by analysis of
redundant frames. The structure was solved using direct methods and
difference Fourier techniques. Unless otherwise noted, non-hydrogen
atoms were refined anisotropically and hydrogen atoms were treated as
idealized contributions. Structure solution, refinement, graphics, and
creation of publication materials were performed using SHELXTL
NT.42
The crystal structures of 7, 9, 12, and 14 were determined as
follows: A crystal was mounted in a nylon cryoloop from Paratone-N
oil. The crystal was placed in a Bruker P4/CCD diffractometer and
cooled to 140(2) K using a Bruker LT-2 low-temperature device. The
instrument was equipped with a sealed, graphite-monochromated Mo
Kα X-ray source (λ= 0.71073 Å). A hemisphere of data was collected
using φ scans, with 30 s frame exposures and 0.3° frame widths. Data
collection and initial indexing and cell refinement were handled using
SMART software.43 Frame integration, including Lorentz−polar-
ization corrections, and final cell parameter calculations were carried
out using SAINT software.44 The data were corrected for absorption
using the SADABS program.45 Decay of reflection intensity was
monitored by analysis of redundant frames. The structure was solved
using direct methods and difference Fourier techniques. Unless
otherwise noted, non-hydrogen atoms were refined anisotropically and
hydrogen atoms were treated as idealized contributions. For the
following structures the program PLATON/SQUEEZE was used to
treat contributions from disordered solvent molecules: 7 and 9
(THF); 12 (hexane).46 Structure solution, refinement, graphics, and
creation of publication materials were performed using SHELXTL
NT.42 Additional details of data collection and structure refinement are
given in Table 1.
1
7.02 (m, 2H, Ar-H), 6.93−6.88 (m, 4H, Ar-H), 6.74 (s, 1H, γ-C-H),
3.68 (sept, J = 6.9 Hz, 2H, CHMe2), 2.50 (sept, J = 6.9 Hz, 2H,
CHMe2), 1.73 (d, J = 6.9 Hz, 6H, CHMe2), 1.40 (d, J = 6.9 Hz, 6H,
CHMe2), 1.08 (s, 18H, CMe3), 0.97 (d, J = 6.9 Hz, 6H, CHMe2), 0.81
(d, J = 6.9 Hz, 6H, CHMe2). 13C{1H} NMR (C6D6, 298 K): δ 171.03
(CN), 144.36 (Ardipp-Co), 140.21 (Ardipp-Co), 137.76 (Ardipp-Cipso),
126.20 (Ardipp-Cm), 125.69 (Ardipp-Cp), 124.76 (γ-CH), 44.31 (CMe3),
33.88 (CHMe2), 29.68 (CMe3), 27.92 (CHMe2), 26.94 (CHMe2),
26.46 (CHMe2), 26.45 (CHMe2), 24.81 (CHMe2). Anal. Calcd for
C35H53I3N2Th (mol wt 1114.17): C, 37.72; H, 4.79; N, 2.51. Found:
C, 37.91; H, 4.80; N, 2.36.
Synthesis of (LMe)UCl3(THF) (12). A 50 mL side-arm flask
equipped with a stir bar was charged with K[LMe] (0.200 g, 0.438
mmol), UCl4 (0.148 g, 0.390 mmol), and tetrahydrofuran (20 mL).
The reaction mixture was stirred at room temperature. After 15 h, the
reaction mixture was filtered through a Celite-padded coarse-porosity
fritted filter and the volatiles were removed under reduced pressure.
The resulting red residue was extracted with toluene (2 × 15 mL). The
red extracts were combined and filtered through a Celite-padded
coarse-porosity fritted filter. The volatiles were removed under
reduced pressure, and the resulting red solid was washed with hexanes
(2 × 10 mL). The washings were decanted, and the remaining solid
was dried under reduced pressure to give 12 as a red solid (0.235 g,
0.282 mmol, 72%). X-ray-quality crystals of 12 were obtained by
slowly cooling a hot hexanes solution (over 3 h) to ambient
temperature. 1H NMR (C6D6, 298 K): δ 13.20 (s, 1H, γ-C-H), 4.40 (t,
J = 7.75 Hz, 2H, Ar-Hp), 3.60 (d, J = 7.75 Hz, 4H, Ar-Hm), 0.27 (d, J =
6.0 Hz, 12H, CHMe2), −0.89 (s, 6H, CH3), −1.54 (bs, 4H, CH2O),
−4.97 (d, J = 6.0 Hz, 12H, CHMe2), −6.26 (b, 4H, CHMe2), −7.52
(bs, 4H, CH2CH2O). Anal. Calcd for C33H49Cl3N2OTh•C6H14 (mol
wt 920.32): C, 50.90; H, 6.90; N, 3.04. Found: C, 51.00; H, 6.85; N,
3.15.
Synthesis of (LtBu)UCl3 (13). A 50 mL side-arm flask equipped
with a stir bar was charged with K[LtBu] (0.250 g, 0.462 mmol), UCl4
(0.159 g, 0.419 mmol), and tetrahydrofuran (20 mL). The reaction
mixture was stirred at room temperature. After 15 h, the volatiles from
the reaction mixture were removed under reduced pressure. Toluene
(30 mL) was added to the resulting red residue. After mixing, the
slurry was filtered through a Celite-padded coarse-porosity fritted
filter. The filtrate was collected, and the volatiles were removed under
reduced pressure. The resulting orange residue was washed with hot
hexanes (2 × 15 mL) and dried under reduced pressure to give 13 as a
pale red solid (0.225 g, 0.275 mmol, 66%). X-ray-quality crystals of 13
were obtained by slowly cooling a hot hexanes solution to ambient
ASSOCIATED CONTENT
■
S
* Supporting Information
1
temperature. H NMR (C6D6, 298 K): δ 28.44 (bs, 2H, CHMe2),
CIF files giving crystallographic information for complexes 7, 9,
10, and 12−14 and a figure giving spectral fitting results for the
electronic spectral data for K[LMe], (LMe)ThI3(THF) (8), and
(LMe)UCl3(THF) (12) and electrochemical data for K[LMe]
and (LMe)UCl3(THF) (12). This material is available free of
charge via the Internet at http://pubs.acs.org.
11.59 (m, 2H, Ar-H), 9.78 (s, 6H, CHMe2), 5.79 (s, 6H, CHMe2), 5.59
(m, 4H, Ar-H), −0.77 (s, 6H, CHMe2), −3.79 (s, 18H, CMe3), −13.79
(bs, 2H, CHMe2), −24.22 (s, 6H, CHMe2), −56.35 (s, 1H, γ-C-H).
Anal. Calcd for C35H53Cl3N2U·1/4C6H14 (mol wt 867.74: C, 50.52; H,
6.56; N, 3.23. Found: C, 50.63; H, 6.49; N, 3.15.
Synthesis of [{(LMe)(Cl)U}2(μ-Cl)3][Cl] (14). A 20 mL scintillation
vial equipped with a stir bar was charged with K[LMe] (0.300 g, 0.657
mmol), UCl4 (0.124 g, 0.326 mmol), and tetrahydrofuran (15 mL).
The reaction mixture was stirred at room temperature. After 15 h, the
reaction mixture was filtered through a Celite-padded coarse-porosity
fritted filter and the volatiles were removed under reduced pressure.
The resulting red residue was extracted with hexanes (2 × 10 mL).
The extracts were combined and filtered through a Celite-padded
coarse-porosity fritted filter. The red filtrate was concentrated under
reduced pressure to ∼10 mL and placed in a −35 °C freezer to afford
14 as a dark brown/black crystalline solid (0.081 g, 0.053 mmol, 33%).
X-ray-quality crystals of 14 were obtained by slow (2 days)
evaporation of a toluene solution (5 mL) at ambient temperature.
1H NMR (C6D6, 298 K): δ 18.16 (ν1/2 = 285 Hz), 14.00 (ν1/2 = 48
Hz), 4.11 (ν1/2 = 147 Hz), −7.32 (ν1/2 = 540 Hz), −13.66 (ν1/2 = 198
Hz), −39.31 (ν1/2 = 90 Hz). Anal. Calcd for C58H82Cl6N4U2·1.5C7H8
(mol wt 1662.28: C, 49.49; H, 5.70; N, 3.37. Found: C, 49.62; H, 5.56;
N, 3.18.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: (+1) 505-665-9553. Fax: (+1) 505-667-9905. E-mail:
kiplinger@lanl.gov.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
For financial support, we acknowledge Los Alamos National
Laboratory (Director’s and Frederick Reines PD Fellowships to
M.J.M.), the LANL G. T. Seaborg Institute for Transactinium
Science (Graduate Student Fellowship to R.J.W.; PD Fellow-
ship to M.J.M.), the LANL LDRD program, and the Division of
J
dx.doi.org/10.1021/om3010965 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2010, 13, 812−820. (q) Matson, E. M.; Crestani, M. G.; Fanwick, P.
E.; Bart, S. C. Dalton Trans 2012, 41, 7952−7958. For bis(phosphino)
amidos (PNPs): (r) Cantat, T.; Graves, C. R.; Scott, B. L.; Kiplinger, J.
L. Angew. Chem., Int. Ed. 2009, 48, 3681−3684. (s) Cantat, T.; Scott,
B. L.; Morris, D. E.; Kiplinger, J. L. Inorg. Chem. 2009, 48, 2114−2127.
(7) Wright, R. J.; Power, P. P.; Scott, B. L.; Kiplinger, J. L.
Organometallics 2004, 23, 4801−4803.
Chemical Sciences, Office of Basic Energy Science, Heavy
Element Chemistry program.
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dx.doi.org/10.1021/om3010965 | Organometallics XXXX, XXX, XXX−XXX