Synthesis, structure, spectroscopy and redox energetics of a series of uranium(IV) mixed-ligand metallocene complexes

Article abstract of DOI:10.1016/j.crci.2010.04.008

A series of uranium(IV) mixed-ligand amide-halide/pseudohalide complexes (C₅Me₅)₂U[N(SiMe₃)₂](X) (X = F (1), Cl (2), Br (3), I (4), N₃ (5), NCO (6)), (C ₅Me₅)₂

Full text of DOI:10.1016/j.crci.2010.04.008

C. R. Chimie 13 (2010) 790–802
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Full paper/Memoire
Synthesis, structure, spectroscopy and redox energetics of a series of
uranium(IV) mixed-ligand metallocene complexes
*
*
Robert K. Thomson, Brian L. Scott, David E. Morris , Jaqueline L. Kiplinger
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
A
series of uranium(IV) mixed-ligand amide–halide/pseudohalide complexes
Received 2 March 2010
Accepted after revision 1 April 2010
Available online 18 June 2010
(C5Me5)2U[N(SiMe3)2](X) (X = F (1), Cl (2), Br (3), (4), N3 (5), NCO (6)),
I
(C5Me5)2U(NPh2)(X) (X = Cl (7), N3 (8)), and (C5Me5)2U[N(Ph)(SiMe3)](X) (X = Cl (9), N3
(10)) have been prepared by one electron oxidation of the corresponding uranium(III)
amide precursors using either copper halides, silver isocyanate, or triphenylphosphine
gold(I)azide. Agostic Uꢀ ꢀ ꢀH–C interactions and h3-(N,C,C0) coordination are observed for
these complexes in both the solid-state and solution. There is a linear correlation between
Keywords:
Uranium
Amide
the chemical shift values of the C5Me5 ligand protons in the 1H NMR spectra and the UIV
III reduction potentials of the (C5Me5)2U[N(SiMe3)2](X) complexes, suggesting that there
is a common origin, that is overall -/ -donation from the ancillary (X) ligand to the metal,
/
Halide
U
Pseudohalide
s
p
Oxidative functionalization
Agostic interactions
h3-Coordination
Electronic structure
Voltammetry
contributing to both observables. Optical spectroscopy of the series of complexes 1–6 is
dominated by the (C5Me5)2U[N(SiMe3)2] core, with small variations derived from the
identity of the halide/pseudohalide. The considerable
p-donating ability of the fluoride
ligand is reflected in both the electrochemistry and UV-visible-NIR spectroscopic behavior
of the fluoride complex (C5Me5)2U[N(SiMe3)2](F) (1). The syntheses of the new trivalent
uranium amide complex, (C5Me5)2U[N(Ph)(SiMe3)](THF), and the two new weakly-
coordinating electrolytes, [Pr4N][B{3,5-(CF3)2C6H3}4] and [Pr4N][B(C6F5)4], are also
reported.
Mixed-ligand
Metallocenes
UV–visible-near-IR spectroscopy
Weakly-coordinating electrolytes
X-ray crystallography
ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
oxidative functionalization chemistry of trivalent uranium
with copper(I) [8,9] and gold(I) [10] compounds to provide
Mixed-ligand metallocene complexes of the type
(C5Me5)2U(X)(Y) serve as important starting materials
for organometallic actinide chemistry, but access to this set
of molecules has historically been limited to very specific
reaction chemistries [1–7]; consequently, very little is
known about the electronic structure and redox energetics
for this rare class of compounds. We have been developing
simple and mild methods for synthesizing UIV mixed-
ligand metallocene complexes of the type (C5Me5)2U(X)(Y)
(where X = halogen, azide, acetylide, etc.; Y = amide). Since
uncontrolled oxidation and ligand redistribution are not
observed with the Au- or Cu-based UIII ! UIV oxidation
procedures, they provide attractive synthetic routes for the
preparation of a variety of mixed-ligand metallocene
complexes. For example, reaction of the trivalent uranium
complex (C5Me5)2U(NPh2)(THF) with copper(I) iodide or
(Ph3P)Au-N3 affords the corresponding tetravalent urani-
um amide-iodide and -azide complexes (C5Me5)2U(N-
Ph2)(I) [9] and (C5Me5)2U(NPh2)(N3) [11], respectively
(Eq. (1)).
* Corresponding author.
E-mail addresses: demorris@lanl.gov (D.E. Morris), kiplinger@lanl.gov
(J.L. Kiplinger).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.04.008

R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
791
(1)
During the course of these studies, it became evident
that the amide mixed-ligand framework provided the
opportunity to examine a new homologous structural
series from which we can draw new inferences regarding
uranium metal–ligand bonding. This falls within the
broader context of our continuing efforts to map out
trends in the redox energetics and electronic structure of
metallocene complexes of the early actinides and correlate
these trends with structural chemistry [12–16]. In
particular, the amide complexes represent an important
end-member along the nitrogenous ligand series: imide
(A) [17–20], ketimide (B) [21–24], hydrazonato (C) [2,25–
27], amide (D) [8,9,28], as illustrated in Fig. 1.
complex (C5Me5)2U[N(SiMe3)2] [29] with various Cu, Ag,
and Au reagents afforded the series of related complexes
(C5Me5)2U[N(SiMe3)2](X) (X = F (1), Cl (2), Br (3), I (4), N3 (5),
and NCO (6)). Specifically, the UIV amide–halide complexes
(C5Me5)2U[N(SiMe3)2](F) (1), (C5Me5)2U[N(SiMe3)2](Cl) (2),
(C5Me5)2U[N(SiMe3)2](Br) (3), and (C5Me5)2U[N(SiMe3)2](I)
(4) were obtained by oxidation of (C5Me5)2U[N(SiMe3)2]
using an excess (5 equiv.) of the appropriate Cu halide salt.
Separation of the insoluble Cu salts from the reaction
products was accomplished by filtration through Celite.
Following crystallization, the UIV amide–halide complexes
were isolated in moderate to high yields (up to 93%)
as crystalline red solids. The amide–azide complex
(C5Me5)2U[N(SiMe3)2](N3) (5) was prepared in a similar
manner using (Ph3P)Au-N3 as the oxidant [11]. The
complexes were characterized by a combination of 1H
NMR spectroscopy, elemental analyses and X-ray crystal-
lography.
The amide–isocyanate derivative (C5Me5)2U[N(Si-
Me3)2](NCO) (6) was synthesized in 71% isolated yield
by oxidation of (C5Me5)2U[N(SiMe3)2] with AgNCO.
Oxidations using copper halides occur over 12 h with an
excess of the oxidant (5 equiv.), however use of excess
AgNCO or reaction times longer than 2–3 h lead to
decomposition of the uranium isocyanate product. This
is most likely attributable to the much stronger oxidizing
power of AgI over CuI [30].
The uranium–amide bonding interaction should have
the lowest degree of
p-donation from the nitrogen lone-
pair among this series, and this should be reflected in both
spectral and electrochemical data. In this contribution, we
report the synthesis and structural characterization of a
series of mixed-ligand tetravalent uranium complexes
of the type (C5Me5)2U(X)(Y) (Y = N(SiMe3)2, NPh2,
N(Ph)(SiMe3); X = halide/pseudohalide) from the reaction
of Cu, Ag, and Au reagents with the corresponding UIII
amide precursors. Further, we describe the electrochemi-
cal and spectroscopic characterization of a series of related
UIV complexes, (C5Me5)2U[N(SiMe3)2](X), from this
rare class of uranium complexes. We also report the
synthesis of the new trivalent uranium amide complex,
(C5Me5)2U[N(Ph)(SiMe3)](THF), and the two new weakly-
coordinating electrolytes, [Pr4N][B{3,5-(CF3)2C6H3}4] and
[Pr4N][B(C6F5)4].
The molecular structures of the amide–chloride
(C5Me5)2U[N(SiMe3)2](Cl) (2) (left) and amide–isocyanate
(C5Me5)2U[N(SiMe3)2](NCO) (6) (right) complexes are
shown in Fig. 2. Compound 6 represents a rare example
of a structurally characterized actinide isocyanate com-
plex, of which there are only two other known complexes
[31,32]. Both complexes 2 and 6 feature a bent-metallo-
cene framework with the amide and chloride (for 2) or
isocyanate (for 6) ligands contained within the metallo-
cene wedge. For (C5Me5)2U[N(SiMe3)2](Cl) (2), the U(1)–
2. Results and discussion
2.1. Synthesis and structural characterization
2.1.1. (C5Me5)2U[N(SiMe3)2](X) complexes
Oxidative functionalization provides convenient access
to mixed-ligand metallocene complexes of the type
(C5Me5)2U(X)(Y) (where Y is an amide ligand). As summa-
ri ized in Scheme 1, reaction of the trivalent uranium amide
[(g._1)TD$FIG]
˚
˚
Cl(1) (2.606(3) A) and U(1)–N(1) (2.268(4) A) bond lengths
are typical of UIV–Cl [3,5,33,34] and UIV–Namide [5,8,9,34–
37] bond lengths.
Fig. 1. Uranium metallocene complexes with electronically different nitrogen-based-donor ligands: imide (A), ketimide (B), hydrazonato (C), mixed-amide
halide/pseudohalide (D).

[(Schme_1)TD$FIG]
792
R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
Scheme 1. Mixed-ligand UIV metallocene complexes accessed through oxidative functionalization.
[(ig._2)TD$FIG]
Fig. 2. Molecular structures of (C5Me5)2U[N(SiMe3)2](Cl) (2) (left) and (C5Me5)2U[N(SiMe3)2](NCO) (6) (right) with thermal ellipsoids projected at the 30%
˚
probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (A) and angles (8) for 2: U(1)–N(1) 2.268(4), U(1)–Cl(1) 2.606(3),
U(1)–C(24) 3.158, U(1)–Si(2) 3.335, Cl(1)–U(1)–N(1) 89.39(14), U(1)–N(1)–Si(1) 130.7(3), U(1)–N(1)–Si(2) 111.2(3), Si(1)–N(1)–Si(2) 118.2(3), U(1)–
˚
C(24)–Si(2) 78.2, C(24)–Si(2)–N(1) 107.3(6), N(1)–U(1)–C(24) 63.2. Selected bond lengths (A) and angles (8) for 6: U(1)–N(1) 2.241(11), U(1)–N(2)
2.354(16), N(2)–C(13) 1.10(2), C(13)–O(1) 1.29(2), U(1)–C(15) 3.05, U(1)–Si(10) 3.31, N(1)–U(1)–N(2) 89.2(5), U(1)–N(2)–C(13) 166.4(17), N(2)–C(13)–O(1)
174(3), U(1)–N(1)–Si(1) 128.3(6), U(1)–N(1)–Si(10) 113.2(6), Si(1)–N(1)–Si(10) 118.5(6), U(1)–C(15)–Si(10) 80.7, C(15)–Si(10)–N(1) 103.9(10), N(1)–U(1)–
C(15) 62.0.
For (C5Me5)2U[N(SiMe3)2](NCO) (6), the isocyanate
ligand is bound to the uranium in a linear fashion through
with those observed for the other reported uranium
isocyanate complexes (2172–2185 cmꢁ1) [31,32].
the
N
donor, with
˚
a
U(1)–N(2) bond distance of
A short U–C contact between the methyl group of one of
the SiMe3 groups of the [N(SiMe3)2] ligand and the
uranium center is present in both complexes (2: U(1)–
2.354(16) A, U(1)–N(2)–C(13) bond angle of 166.4(17)8,
and N(2)–C(13)–O(1) bond angle of 174(3)8. These metrical
parameters compare well with those reported for [(Ad-
ArO)3tacn]U(NCO) (U–N = 2.389(6) A, U–N–C = 171.2(6)8,
N–C–O 178.2(9)8)[31] and [(Me2N)3PO]2UO2(NCO)2 (U–
˚
˚
C(24) = ꢂ3.14 A and 6: U(1)–C(15) = ꢂ3.05 A). This agostic
Uꢀ ꢀ ꢀH–C interaction generates a planar U(1)–C(24)–Si(2)–
˚
N(1) ring (S = 359.88) in 2 and U(1)–C(15)–Si(1)–N(1) ring
˚
N = 2.336(5) A, U–N–C = 160.2(5)8, N–C–O = 179.1(7)8)
(S = 359.88) in 6. Further support is provided by the
[32]. Additionally, the IR spectrum for complex 6 showed
a diagnostic NCO stretch at 2194 cmꢁ1, which is consistent
different U–N–Si geometric parameters in both complexes
2 and 6. In (C5Me5)2U[N(SiMe3)2](Cl) (2), the U(1)–N(1)–

R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
793
Si(2) angle of 111.2(3)8 is significantly smaller than the
U(1)–N(1)–Si(1) angle of 130.7(3)8. Similarly, in
(C5Me5)2U[N(SiMe3)2](NCO) (6), the U(1)–N(1)–Si(10) an-
gle is 113.2(6)8 and the U(1)–N(1)–Si(1) angle is 128.3(6)8.
Agostic Uꢀ ꢀ ꢀH–C interactions have been previously
reported for the structurally related amide-iodide
(C5Me5)2U[N(SiMe3)2](I) (4) [9] and amide–azide
(C5Me5)2U[N(SiMe3)2](N3) (5) derivatives [11], which have
downfield with variation of the halide/pseudohalide from
I ꢂ Br ! Cl ! NCO ! N3 ! F. Thus, the better the -donor,
p
the more electron-rich the uranium center, which yields a
weaker agostic Uꢀ ꢀ ꢀH–C interaction and subsequently a
downfield shift of the Si–Me protons. The chemical shift of
analogous protons within the (C5Me5)2U[N(SiMe3)2](X)
series track with the electron density at the UIV center and
this is in good agreement with the electrochemical data
which suggest a similar trend in ease of reduction across
the series (vide infra).
˚
˚
U–C contacts of ꢂ3.28 A and ꢂ3.23 A, respectively. Similar
interactions have been observed for other uranium
complexes bearing the [N(SiMe3)2] ligand. Three Uꢀ ꢀ ꢀH–
˚
C interactions (U–Cave = ꢂ3.31 A) were proposed to exist in
2.1.2. (C5Me5)2U(NPh2)(X) complexes
the UIV complex [(Me3Si)2N]3U–S(2,6-Me2C6H3) [38],
Under conditions identical to those employed for
the synthesis of (C5Me5)2U[N(SiMe3)2](Cl) (2) and
while the UIII metallocene complex (C5Me5)U[N(SiMe3)2]2
featured two Uꢀ ꢀ ꢀH–C interactions (U–Cave = ꢂ2.83 A) [39].
(C5Me5)2U[N(SiMe3)2](N3)
(5),
the
UIII
complex
˚
Decreased U–N–Si angles were also hallmarks for Uꢀ ꢀ ꢀH–C
interactions in these complexes.
(C5Me5)2U(NPh2)(THF) [8] was smoothly oxidized with
CuCl and (Ph3P)Au-N3 to afford the corresponding UIV
amide–chloride (C5Me5)2U(NPh2)(Cl) (7) and UIV amide–
azide (C5Me5)2U(NPh2)(N3) (8) complexes, respectively
(Scheme 1). Following workup by filtration through Celite
and crystallization, 7 and 8 were reproducibly isolated as
analytically pure solids and characterized by a combina-
tion of 1H NMR spectroscopy, elemental analyses and X-ray
crystallography.
The molecular structure of the amide–chloride complex
(C5Me5)2U(NPh2)(Cl) (7) is given in Fig. 3 and reveals the
typical bent-metallocene framework with the amide and
chloride ligands contained within the metallocene wedge.
The structure of (C5Me5)2U(NPh2)(Cl) (7) is similar to that of
(C5Me5)2U[N(SiMe3)2](Cl) (2) and the U–Cl and U–Namide
bonddistances are comparable. In complex 7, there isa close
U–C contact between the uranium center and one of the
ortho carbon atoms of one of the phenyl rings of the [NPh2]
Interestingly, these solid-state agostic Uꢀ ꢀ ꢀH–C inter-
actions are also maintained in solution. For example, the
1H NMR spectrum for (C5Me5)2U[N(SiMe3)2](Cl) (2) shows
three different SiMe3 environments at
ꢁ107.98 in a 3:2:1 ratio. The considerably upfield shifted
methyl signal at
d 7.42, 4.31, and
d
ꢁ107.98, which integrates to three
protons, is consistent with restricted rotation of the SiMe3
group due to the presence of an agostic Uꢀ ꢀ ꢀH–C interaction
between one of the Si–Me groups and the uranium center.
The two methyl groups not involved in the agostic
interaction appear as a single resonance at
the freely rotating SiMe3 group is present at
C5Me5 groups are equivalent and give rise to a singlet at
d
4.31, while
d
7.42. The
d
10.29. A similar four signal pattern is observed for the
entire series of complexes (C5Me5)2U[N(SiMe3)2](X) (X = F
(1), Cl (2), Br (3), I (4), N3 (5), NCO (6)), and these data are
summarized in Table 1.
˚
˚
ligand (U(1)–C(32) = ꢂ3.41 A, U(1)–H(32) = ꢂ2.87 A). This
interaction also generatesa nearly planar U(1)–N(1)–C(27)–
C(32) ring, with the sum of angles being 359.08.
There is a relationship between the chemical shift for
like protons (i.e. the C5Me5 resonances) and the identity of
the ancillary (X) ligand in the paramagnetic 5f2
(C5Me5)2U[N(SiMe3)2](X) framework. The data in Table 1
show that the chemical shift of the C5Me5 group moves
upfield with variation of the halide/pseudohalide ligand
from I ! Br ! Cl ! NCO ! N3 ! F. In effect, the better the
Unlike the Uꢀ ꢀ ꢀH–C interaction which persists in
solution for the isostructural iodide complex (C5Me5)2U(N-
Ph2)(I) [9], the 1H NMR spectrum for (C5Me5)2U(NPh2)(Cl)
(7) shows that in solution the NPh2 ligand is freely rotating
and appears as a single broad resonance at
sharp singlet at 12.73 for the C5Me5 protons. In contrast,
(C5Me5)2U(NPh2)(I) has a highly upfield shifted Ar–H
proton (
d 15.53 with a
p
-donor, the more electron-rich the uranium center,
d
which results in a larger shielding and an upfield shift of
the auxiliary ligand protons. Similar trends have been
observed for other paramagnetic trivalent [40], tetravalent
[41,42] and pentavalent [15,16,43,44] uranium systems.
Not surprisingly, the data in Table 1 also demonstrate that
the chemical shift of the Si–Me group engaged in the
agostic Uꢀ ꢀ ꢀH–C interaction with the uranium center is
sensitive to the nature of the (X) group and moves
d
ꢁ146.83) relative to the other proton signals [9].
This observation that the Uꢀ ꢀ ꢀH–C interaction in
(C5Me5)2U(NPh2)(Cl) (7) is weaker than that in
(C5Me5)2U(NPh2)(I) is consistent with the trend noted
above for the (C5Me5)2U[N(SiMe3)2](X) complexes: the
better the
p-donor, the more electron-rich the uranium
center, which yields a weaker Uꢀ ꢀ ꢀH–C interaction. These
Table 1
1H NMR spectroscopic data for (C5Me5)2U[N(SiMe3)2](X).
Complex
d
C5Me5
5.95
d
SiMe3
d
SiMe2
d
SiMe
(C5Me5)2U[N(SiMe3)2](F) (1)
(C5Me5)2U[N(SiMe3)2](Cl) (2)
(C5Me5)2U[N(SiMe3)2](Br) (3)
(C5Me5)2U[N(SiMe3)2](I) (4)
(C5Me5)2U[N(SiMe3)2](N3) (5)
(C5Me5)2U[N(SiMe3)2](NCO) (6)
ꢁ0.78
7.42
8.41
9.22
4.84
6.12
ꢁ0.33
4.31
4.51
4.83
3.33
5.15
ꢁ58.26
10.29
11.40
12.84
8.69
ꢁ107.98
ꢁ109.75
ꢁ109.70
ꢁ102.36
ꢁ105.78
8.92
Spectra collected in C6D6 with chemical shifts given in parts per million (ppm) relative to SiMe4 (0.00).

[(ig._3)TD$FIG]
794
R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
plexes, respectively (Scheme 1). Following workup by
filtration through Celite and crystallization, 9 and 10 were
reproducibly isolated as analytically pure solids in 80%
isolated yield and characterized by a combination of 1H
NMR spectroscopy, elemental analyses and X-ray crystal-
lography.
The molecular structures of (C5Me5)2U[N(Ph)(Si-
Me3)](Cl) (9) (left) and (C5Me5)2U[N(Ph)(SiMe3)](N3) (10)
(right) are provided in Fig. 4. Both complexes 9 and 10
feature a bent-metallocene framework with the amide and
chloride(for 9) orazide(for 10) ligands contained within the
metallocene wedge. The structure of (C5Me5)2U[N(Ph)-
(SiMe3)](Cl) (9) is similar to that of (C5Me5)2U[N(SiMe3)2]-
(Cl) (2) and (C5Me5)2U(NPh2)(Cl) (7), with the U–Cl and
U–Namide bond distances being comparable. Likewise, the
azide ligand in (C5Me5)2U[N(Ph)(SiMe3)](N3) (10) is bound
in a linear fashion with a U(1)–N(1)–N(2) bond angle of
176.8(6)8 and a N(1)–N(2)–N(3) bond angle of 177.9(10)8
and is akin to the structurally related and previously
characterized azide complexes (C5Me5)2U[N(SiMe3)2](N3)
(5) [11], (C5Me5)2U(NPh2)(N3) (8) [11], and (C5Me5)2U-
(O-2,6-iPr2C6H3)(N3) [10].
Fig. 3. Molecular structure of (C5Me5)2U(NPh2)(Cl) (7) with thermal
ellipsoids projected at the 50% probability level. Hydrogen atoms have
˚
been omitted for clarity. Selected bond lengths (A) and angles (8): U(1)–
N(1) 2.286(5), U(1)–Cl(1) 2.6099(15), U(1)–C(32) 3.410, U(1)–H(32)
2.870, Cl(1)–U(1)–N(1) 98.93(13), U(1)–N(1)–C(21) 121.8(4), U(1)–N(1)–
C(27) 122.3(4), C(21)–N(1)–C(27) 115.9(5), U(1)–C(32)–C(27) 71.9,
C(32)–C(27)–N(1) 119.9(5), C(32)–U(1)–N(1) 44.9.
The amide ligands in both (C5Me5)2U[N(Ph)(SiMe3)](Cl)
(9) and (C5Me5)2U[N(Ph)(SiMe3)](N3) (10) also exhibit short
U–C contacts, but these interactions are considerably
different from that of (C5Me5)2U(NPh2)(Cl) (7) in that the
data also suggest that the Uꢀ ꢀ ꢀH–Caryl interaction in
(C5Me5)2U(NPh2)(Cl) (7) is weaker than the Uꢀ ꢀ ꢀH–C
interaction observed for (C5Me5)2U[N(SiMe3)2](Cl) (2).
Close U–C contacts have been observed previously
between uranium and aryl amides. For example, the
rings formed by U(1)–N(1)–C(21)–C(22) (S = 343.58) for 9
and U(1)–N(4)–C(24)–C(25) ( = 345.28) for 10 are non-
S
bridged amide system {[(Me3Si)2N]2U(
Me3C6H2))}2 exhibits several close U–Cmesityl interactions
m
-NH(2,4,6-
planar. Furthermore, these interactions result in a marked
decrease in the U–N–Cipso angle, with U(1)–N(1)–
C(21) = 105.32(16)8 for 9 and U(1)–N(4)–C(24) = 103.6(5)8
for 10 versus U(1)–N(1)–C(27)/C(21) = 122.3(4)/121.8(4)8
for 7.
The [N(Ph)(SiMe3)] ligand is structurally analogous to a
benzyl (CH2Ph) ligand and is coordinated to the uranium
metal center in an h3-(N,C,C0)-fashion in (C5Me5)2U[N(Ph)
(SiMe3)](Cl) (9) and (C5Me5)2U[N(Ph)(SiMe3)](N3) (10). The
strongest secondary interaction occurs between the urani-
um and Cipso of the phenyl ring of the coordinated
˚
with U–C distances of ꢂ3.10–3.39 A [45]. A close U–Cipso
˚
contact of ꢂ2.94 A with one of the phenyl rings of the NPh2
ligand in the hydrotris(3,5-dimethylpyrazolyl)borate ura-
nium complex [U(TpMe2)Cl2(NPh2)] has also been reported
[35]. This interaction also results in a decrease in the U–N–
Cipso angle to 105.5(8)8 for the interacting phenyl group
versus 140.2(8)8 for the non-interacting group [35].
2.1.3. (C5Me5)2U[N(Ph)(SiMe3)](X) complexes
˚
Given the interesting solid-state and solution agostic
Uꢀ ꢀ ꢀH–C interactions seen for the (C5Me5)2U[N(Si-
Me3)2](X) and (C5Me5)2U(NPh2)(X) systems, the new
trivalent uranium amide complex, (C5Me5)2U[N(Ph)(Si-
Me3)](THF) was prepared to determine which type of
Uꢀ ꢀ ꢀH–C interaction (SiMe3 or phenyl) would be dominant.
As shown in Eq. (2), reaction of (C5Me5)2U(I)(THF) with
[N(Ph)(SiMe3)] ligand (9: U–Cipso = ꢂ2.98 A; 10: U–
˚
C
ipso = ꢂ2.94 A) with a weaker unsymmetrical secondary
interaction takingplacebetween theuraniumand oneof the
ortho carbons atoms of the phenyl ring of the [N(Ph)(SiMe3)]
ligand (9: U–C(22) = ꢂ3.18 A, U–C(26) = ꢂ4.16 A; 10: U–
˚
˚
˚
˚
C(25) = ꢂ3.05 A, U–C(29) = ꢂ4.14 A).
For benzyl complexes, two parameters have been
defined to quantify the benzyl ligand-to-metal interaction,
(THF)K[N(Ph)(SiMe3)]
afforded
(C5Me5)2U[N(Ph)(Si-
Me3)](THF) as a green crystalline solid in 65% isolated
yield.
D
and D0, where
D
= [MCo ꢁ MCH2] ꢁ [MCipso ꢁ MCH2] and
D0 = [MCo0 ꢁ MCH2] ꢁ [MCipso ꢁ MCH2], where MCo is the
shorter metal-to-Cortho contact, MCo’ is the longer metal-
to-Cortho contact, MCH2 is the metal-to-methylene carbon
bond length, and MCipso is the metal-to-Cipso bond length
(2)
[22,46–48]. For the f-block metals,
D
and D0 have been
shown to have comparable values for an h4-benzyl-to-
metal bonding interaction [22,48], with larger differences
between
metal interaction [47].
D
and D0 being indicative of an h3-benzyl-to-
As with the other UIII amide complexes,
(C5Me5)2U[N(Ph)(SiMe3)](THF) is readily oxidized with
CuCl and (Ph3P)Au-N3 to form the corresponding UIV
amide–chloride (C5Me5)2U[N(Ph)(SiMe3)](Cl) (9) and UIV
amide–azide (C5Me5)2U[N(Ph)(SiMe3)](N3) (10) com-
By extension to the [N(Ph)(SiMe3)] ligand,
D
and D0 can
be similarly defined for phenyl amide ligands:
= [MCo ꢁ MN] ꢁ [MCipso ꢁ MN] and D0 = [MCo0 ꢁ MN] ꢁ
[MCipso ꢁ MN], where MCo is the shorter metal-to-Cortho
D

[(ig._4)TD$FIG]
R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
795
Fig. 4. Molecular structures of (C5Me5)2U[N(Ph)(SiMe3)](Cl) (9) (left) and (C5Me5)2U[N(Ph)(SiMe3)](N3) (10) (right) with thermal ellipsoids projected at the
˚
50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (A) and angles (8) for 9: U(1)–N(1) 2.275(2), U(1)–Cl(1)
2.5991(7), U(1)–C(22) 3.179, U(1)–C(21) 2.979, U(1)–C(26) 4.155, Cl(1)–U(1)–N(1) 89.12(6), U(1)–N(1)–Si(1) 137.08(12), U(1)–N(1)–C(21) 105.32(16),
˚
C(21)–N(1)–Si(1) 117.31(17), U(1)–C(22)–C(21) 68.99, C(22)–U(1)–N(1) 49.54, C(22)–C(21)–N(1) 119.63. Selected bond lengths (A) and angles (8) for 10:
U(1)–N(1) 2.243(6), U(1)–N(4) 2.268(7), N(1)–N(2) 1.201(9), N(2)–N(3) 1.133(10), U(1)–C(25) 3.053, U(1)–C(24) 2.937, U(1)–C(29) 4.141, N(1)–U(1)–N(4)
85.6(2), U(1)–N(1)–N(2) 176.8(6), N(1)–N(2)–N(3) 177.9(10), U(1)–N(4)–Si(1) 135.5(3), U(1)–N(4)–C(24) 103.6(5), C(24)–N(4)–Si(1) 119.8(5), U(1)–C(25)–
C(24) 71.85, C(25)–C(24)–N(4) 118.3(7), C(25)–U(1)–N(4) 51.40.
Table 2
Bond lengths for uranium arene interactions for uranium phenyl amide complexes.
e
Complex
MCipso ꢁ MNa
MCo ꢁ MNb
MCo0 ꢁ MNc
Dd
D0
[U][N(Ph)(SiMe3)](Cl) (9)
[U][N(Ph)(SiMe3)](N3) (10)
[U4(NPh)6I4(py)6]ꢀ5py
0.70
0.67
0.48
0.90
0.79
0.59
1.88
1.87
1.57
0.20
0.12
0.11
1.18
1.20
1.09
a
Uranium-to-ipso carbon bond length minus uranium-nitrogen bond length.
Uranium-to-shorter ortho carbon bond length minus uranium-nitrogen bond length.
Uranium-to-longer ortho carbon bond length minus uranium-nitrogen bond length.
[(MCo ꢁ MN) ꢁ (MCipso ꢁ MN)].
b
c
d
e
[(MCo0 ꢁ MN) ꢁ (MCipso ꢁ MN)]. [U] = (C5Me5)2U.
0
contact, MCo is the longer metal-to-Cortho contact, MN is
For (C5Me5)2U[N(Ph)(SiMe3)](N3) (10), the resonances for
ortho-aryl, para-aryl, and meta-aryl protons appear at
the metal-to-Namide bond length, and MCipso is the metal-
to-Cipso bond length. Table 2 presents the
and D0 values
determined for (C5Me5)2U[N(Ph)(SiMe3)](Cl) (9) and
(C5Me5)2U[N(Ph)(SiMe3)](N3) (10). The metrical param-
eters exhibited by complexes 9 and 10 (large difference in
and D0 values) support the assignment of h3-hapticity
d
D
0.30, ꢁ5.82 and ꢁ10.12, respectively. The C5Me5 and SiMe3
protons are sharp singlets at d 8.45 and 1.31, respectively.
2.2. Electrochemical studies
D
for the [N(Ph)(SiMe3)] ligands in these complexes. These
compare well with the
other h3-(N,C,C0)-coordinated phenyl imide indentified in
the uranium(IV) cluster complex [U4(NPh)6I4(py)6]ꢀ5py
reported by Berthet et al. [49].
The 1H NMR spectrum for (C5Me5)2U[N(Ph)(SiMe3)](Cl)
(9) suggests a rigid structure is maintained in solution as
evidenced by two distinct resonances for the ortho-aryl
Voltammetric data (cyclic and square wave) were
obtained for complexes 1–3, 5, and
in ꢂ0.1 M
D
and D0 values determined for the
6
[Pr4N][B{3,5-(CF3)2C6H3}4]/THF solution at a Pt working
electrode. Similar studies were also undertaken for the
iodide complex (4), but the data clearly indicated rapid
decomposition of this complex in the solvent/supporting
electrolyte solution that precluded extraction of redox
potential data. All stable complexes exhibited a reversible
UV/UIV oxidation wave and a reversible UIV/UIII reduction
wave within the accessible potential range. Half-wave
potentials were determined in all cases from the peaks in
the square-wave voltammograms, and these values are
reported in Table 3 versus the ferrocenium/ferrocene
internal standard. Typical cyclic voltammograms obtained
at 200 mV/s are illustrated in Fig. 5.
protons at
protons at
d
8.12 and
d 0.29, and singlets for the C5Me5
d
9.47 and the non-interacting SiMe3 group at
d
5.78, with the para-aryl and meta-aryl protons appearing at
5.90 and
ꢁ10.03, respectively.
The azide complex 10 exhibits a very similar 1H NMR
d
d
spectrum, however, the aryl ring does not appear to be as
tightly bound as in the chloride derivative 9. This
observation is again consistent with the trends noted
There are two notable points in these data. The first
pertains to the electron-donating ability of the ancillary (X)
ligand. Previous studies have clearly shown that the redox
potentials for the uranium metal-based redox couples in
above: the better the
p-donor (the azide ligand), the more
electron-rich the uranium center, which yields a weaker
Uꢀ ꢀ ꢀH–C interaction, or in this case a weaker U–C contact.

796
R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
Table 3
Summary of voltammetric dataa for (C5Me5)2U[N(SiMe3)2](X) complexes in ꢂ0.1 M [Pr4N][B{3,5-(CF3)2C6H3}4]/THF solution.
b
Complex
E1/2[UV/UIV
]
E1/2[UIV/UIII
]
j
DE1/2
j
(V)
(C5Me5)2U[N(SiMe3)2](F) (1)
(C5Me5)2U[N(SiMe3)2](Cl) (2)
(C5Me5)2U[N(SiMe3)2](Br) (3)
(C5Me5)2U[N(SiMe3)2](N3) (5)
(C5Me5)2U[N(SiMe3)2](NCO) (6)
ꢁ0.06
0.10
ꢁ2.35
ꢁ2.04
ꢁ1.97
ꢁ2.15
ꢁ2.06
2.29
2.14
2.09
2.11
2.15
0.12
ꢁ0.04
0.09
a
E1/2 values in volts versus [(C5H5)2Fe]+/0
E1/2j = jE1/2[UV/UIV] ꢁ E1/2[UIV/UIII]j.
.
b
j
D
bent-metallocene complexes are very sensitive to the
electrostatic perturbation induced at the metal by the
ancillary ligand(s) [12,15,16,21,22]. In particular, the more
strongly electron-donating ancillary ligands destabilize
the UIV/UIII reduction process, shifting it to more negative
potential values. Similarly, the more strongly electron-
donating ligands stabilize the UV oxidation state leading to
a concomitant negative shift in the UV/UIV oxidation wave.
In the present study, these redox data suggest that the
electrostatic perturbation at the UIV center by the ancillary
ligands follows the trend F ꢃ N3 > NCO > Cl > Br. Based on
the relative change in the potential of the UIV/UIII couple,
the influence of the fluoride ligand is significantly larger
than that of the other ancillary ligands, suggesting that the
series of (C5Me5)2UIV(ketimide)2 complexes [21,22]. This
implies that the influence of the wedge ligands on the metal-
based redox properties is comparable for both reduction and
oxidation processes; the UIII state is destabilized to the same
extent that the UV state is stabilized.
The previously observed trend in the chemical shift of
the C5Me5 protons by 1H NMR spectroscopy was evaluated
through comparison to the electrochemical data collected
for complexes 1–6. An excellent linear correlation is
observed between the chemical shift of the C5Me5 protons
and the UIV/UIII reduction potential, and is illustrated in
Fig. 6. As expected, the (C5Me5)2U[N(SiMe3)2](X) com-
plexes bearing weaker electron donors (X = Cl (2), Br (3), I
(4)) that gave downfield shifted signals for their C5Me5
protons exhibit more positive reduction potentials owing
to the greater ease with which they are reduced. The
complexes bearing stronger donors (X = N3 (5), NCO (6), F
(1)) are more difficult to reduce and exhibit more negative
reduction potentials and more upfield shifted C5Me5
proton signals in their 1H NMR spectra. As illustrated by
Fig. 6, the fluoride derivative (C5Me5)2U[N(SiMe3)2](F) (1)
is considerably more electron rich than even the azide (5)
and isocyanate (6) species, suggesting a far greater degree
fluoride ion interacts with the metal center in both a
s- and
p
-donating capacity to amplify its influence on the metal-
based redox energetics. The electrostatic influence of these
ancillary ligands at the metal center is also reflected in the
1H NMR spectroscopic chemical shift data described
below.
The second observation from these redox data is that
the potential separation between the two metal-based UIV
couples (j E1/2j, Table 3) is very similar to that seen across
D
a very large range of UIV bent-metallocene complexes
reported by us [12]. The average separation for the six
of p-donation from the fluoride anion.
complexes investigated here (j
D
E1/2 ave
j
= 2.16 ꢄ 0.03 V) is
2.3. Spectroscopy
nearly identical to that found for the previously reported
[
The UV–visible-near IR electronic absorption spectral
data for complexes 1–6 in toluene solution at room
temperature are illustrated in Fig. 7. The data in both the
UV–visible region (Fig. 7, top) and the near IR region (Fig. 7,
bottom) are similar across this entire series of complexes
suggesting that the (C5Me5)2U[N(SiMe3)2] structural core
provides a highly conserved ligand-field environment that
determines the gross features for most of the low-lying
electronic transitions. The similarity in the characteristics
(moderately intense, rather broad) of the bands found in
the visible spectral region for all six complexes, and in
particular the very minor influence of the halide ion on
these bands, suggests that these are likely charge-transfer
(ligand-to-metal and/or metal-to-ligand) transitions
as seen in similar UIV bent-metallocene systems
[12,21,22,24]. The moderate intensity in these bands, if
charge-transfer in character, is consistent with metal f-
orbital/ligand orbital parentage in the ground- and
excited-states since the intensity scales with orbital
overlap and is therefore an indicator of covalency in
bonding as described previously [12,21,22,24,50,51].
The consistency in spectral behavior for these six
complexes is also found in the NIR spectral region in which
Fig. 5. Cyclic voltammograms obtained at 200 mV/s scan rate for ꢂ2–
5 mM solutions of (C5Me5)2U[N(SiMe3)2](X) (X = F (1), Cl (2), Br (3), N3 (5),
and NCO (6)) in ꢂ0.1 M [Pr4N][B{3,5-(CF3)2C6H3}]/THF at
a Pt disk
electrode.

[(ig._6)TD$FIG]
R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
797
in the U–N bonds [12,21,22,24]. Thus, consistent with our
maturing description of these f–f spectral intensities as
markers for metal–ligand covalent bonding, these new
spectral data for the (C5Me5)2U[N(SiMe3)2](X) complexes
indicate that they possess less covalent bonding than
found in the UIV ketimides (or UIV and UV imides), but
greater than that seen in simple UIV halide or alkyl/aryl
complexes [12,15,16,21,22,24,52].
3. Summary
In summary, a series of UIV amide–halide and amide-
pseudohalide complexes have been prepared by one-
electron oxidations of the corresponding UIII amide
precursors (C5Me5)2U[N(SiMe3)2], (C5Me5)2U(NPh2), and
(C5Me5)2U[N(Ph)(SiMe3)] using Cu, Ag, and Au salts.
Characterization of these complexes in the solid-state
revealed close U–C contacts between the ancillary amide
ligands and the uranium center that are consistent with
agostic Uꢀ ꢀ ꢀH–C interactions (for the (C5Me5)2U[N(Si-
Fig. 6. Linear correlation between 1H NMR chemical shift of C5Me5
protons and UIV/UIII reduction potential for (C5Me5)2U[N(SiMe3)2](X)
(X = F (1), Cl (2), Br (3), N3 (5), NCO (6)).
Me3)2](X) and (C5Me5)2U(NPh2)(X) systems) and h3
-
[(ig._7)TD$FIG]
(N,C,C0)-coordination (for the (C5Me5)2U[N(Ph)(SiMe3)](X)
systems). The series of complexes (C5Me5)2U[N(Si-
Me3)2](X) (X = F (1), Cl (2), Br (3), I (4), N3 (5), NCO (6))
was examined by electrochemistry and spectroscopy. The
reduction potentials for these complexes were directly
correlated to the chemical shift values of the C5Me5 protons
in their 1H NMR spectra. Not only does this make 1H NMR
spectroscopy a useful gauge for relative electron density at
the uranium center in these complexes, but it also suggests
that there is a common origin, namely overall
s- and p-
donation from the ancillary (X) ligand to the metal,
contributing to both observables. Electronic spectroscopy
of these complexes is supportive of the (C5Me5)2U[N(-
SiMe3)2] core dominating the low-lying electronic transi-
tions for these complexes. Transitions between electronic
states derived from the 5f2 valence electron configuration
are considerably weaker than those observed for related
UIV ketimide and imide complexes, indicating less
covalency for the mixed amide–halide and amide-pseu-
dohalide metallocene complexes presented here, but
greater covalency than that observed for simple dihalide
or dialkyl/aryl complexes of uranium(IV).
4. Experimental
4.1. General information
Unless otherwise specified, all reactions and manipula-
tions were performed in either a recirculating Vacuum
Atmospheres NEXUS model inert atmosphere (N2) drybox
equipped with a 40CFM Dual Purifier NI-Train, or using
standard Schlenk techniques. Glassware was dried in an
oven at 150 8C overnight prior to use. All NMR spectra were
collected using a Bruker Avance 300 MHz NMR spectrom-
eter. Chemical shifts for 1H NMR spectra are reported in
parts per million (ppm) and referenced to residual proton
solvent impurities calibrated against external TMS. Infra-
red spectra were collected on a Nicolet Avatar 370 DTGS
spectrophotometer. UV-visible-NIR spectra were recorded
Fig. 7. Electronic absorption spectral data for (C5Me5)2U[N(SiMe3)2](X)
(X = F (1), Cl (2), Br (3), I (4), N3 (5), and NCO (6)) in toluene solution at
room temperature. Top: UV–visible spectral region. Bottom: near IR
spectral region.
the electronic transitions are attributable to states derived
from the ground (3H4) and higher-lying ligand-field
manifolds that result from the 5f2 valence electronic
configuration. The intensity in these bands is on average
ꢂ2–3 times less than that found in (C5Me5)2UIV(ketimide)2
complexes for which multiple-bond character is observed

798
R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
using a Perkin-Elmer Lambda 950 spectrophotometer.
Mass spectrometric (MS) analyses were performed at the
University of California, Berkeley Mass Spectrometry
Facility, using a VG Prospec (EI) mass spectrometer.
Elemental analyses were performed at either the Univer-
sity of California, Berkeley Microanalytical Facility, on a
Perkin-Elmer Series II 2400 CHNS analyzer, Columbia
Analytical Services, or Midwest Microlab LLC. Single crystal
X-ray diffraction data were collected using a Bruker APEX2
diffractometer. Structural solution and refinement were
accomplished using the SHELXL97 suite of software [53–
55]. Details regarding data collection are provided in the
CIF files.
in complex with 0.1 M [Pr4N][B{3,5-(CF3)2C6H3}4] support-
ing electrolyte in THF solvent. The advantageous proper-
ties of quarternary ammonium fluoroarylborate salts like
this as electrolytes for voltammetric studies in low
dielectric constant solvents have been noted in several
recent reports [57,58], and the origin of this advantageous
effect has been demonstrated to be directly related to the
greater dissociation of the cation/anion pair in low
dielectric media such as THF as a result of the more highly
delocalized charge in the fluoroaryl borate anions [59].
Furthermore, these anions appear to be more inert towards
fluoride abstraction chemistry. We have found that the n-
propyl ammonium salts are superior to the n-butyl
ammonium salts by ꢂ2ꢅ in reducing solution resistance,
with the [B{3,5-(CF3)2C6H3}4]ꢁ salt slightly better than the
[B(C6F5)4]ꢁ salt in this regard. All data were collected with
the positive-feedback IR compensation feature of the
software/potentiostat activated to ensure minimal contri-
bution to the voltammetric waves from uncompensated
Except where otherwise noted, reagents were purchased
from commercial suppliers and used without further
purification. [D6]Benzene (Aldrich, anhydrous) and
[D8]THF (Cambridge Isotopes, anhydrous) were purified
˚
by storage over 4 A molecular sieves under N2 prior to use.
˚
Celite (Aldrich), alumina (Aldrich, Brockman I), and 4 A
molecular sieves (Aldrich) were dried under dynamic
vacuum at 250 8C for 48 h prior to use. All solvents (Aldrich)
were purchased anhydrous and dried over KH for 24 h,
passed through a column of activated alumina, and stored
solution resistance (typically ꢂ500
V under the conditions
employed). Solutions were contained in PARC Model
K0264 microcells consisting of a ꢂ3 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 voltammo-
grams. Potential calibrations 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 Wave-
metrics IGOR Pro (Version 4.0) software on a Macintosh
platform.
˚
over activated 4 A molecular sieves prior to use. The
following compounds were prepared according to literature
procedures: (C5Me5)2UI(THF) [39], (C5Me5)2U[N(SiMe3)2]
[29], (C5Me5)2U(NPh2)(THF) [8], (Ph3P)Au-N3 [10], C and
[56], and (C5Me5)2U[N(SiMe3)2](I) (4) [9].
Caution! Depleted uranium (primary isotope 238U) is a
weak
a
-emitter (4.197 MeV) with a half-life of 4.47 ꢅ 109
years. Manipulations and reactions should be carried out in
monitored fumehoods or in an inert atmosphere drybox in
a radiation laboratory equipped with
equipment.
a- and b-counting
Caution! While we have not observed any explosive
behavior with (Ph3P)Au-N3 or (C5Me5)2U[N(SiMe3)2](N3)
(5), all azide complexes are potentially shock sensitive and
should be handled with care on an appropriate scale, using
personal protection precautions.
4.3. Synthesis of [Pr4N][B{3,5-(CF3)2C6H3}4]
In a fumehood, a 500-mL Erlenmeyer flask was charged
with Na[B{3,5-(CF3)2C6H3}4] (8.6 g, 9.7 mmol) and metha-
nol (100 mL), giving an auburn colored solution. A colorless
solution of [Pr4N][Br] (3.1 g, 11.8 mmol) in methanol
(100 mL) was added with stirring. The resulting pale
yellow solution was stirred at room temperature for 22 h,
after which time deionized water (150 mL) was added to
the reaction mixture. This immediately generates a thick
white precipitate and the reaction becomes warm to the
touch. The resulting warm suspension was stirred at room
temperature for 10 min and the white solid was collected
by filtration using a coarse-porosity fritted filter and
washed with deionized water (4 ꢅ 150 mL). Each time, the
vacuum was disconnected, water added, and the solid was
mixed/mashed using a spatula, and then the vacuum
reapplied to remove the water. The resulting sticky white
solid was then transferred to a 125-mL side-arm flask and
dried under vacuum for 24 h. The crude reaction product
was taken up into CH2Cl2 (40 mL) and the resulting amber
colored solution was filtered through a Celite-padded
coarse-porosity fritted filter to remove particulates. The
4.2. 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 0.1 cm path length cuvettes loaded in the
recirculating Vacuum Atmospheres drybox system dis-
cussed above. Samples were typically run at two dilutions,
ꢂ0.5 and ꢂ20 mM, to optimize absorbance in the UV–
visible and near-infrared, respectively. Spectral resolution
was typically 2 nm in the visible region and 4–6 nm in the
near-infrared. 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 Perkin-
Elmer Princeton Applied Research Corporation (PARC)
Model 263 potentiostat under computer control with PARC
Model 270 software. All sample solutions were ꢂ2–3 mM
filtrate was collected and transferred to
a 125-mL

R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
799
Erlenmeyer flask and layered with diethyl ether (40 mL)
followed by hexanes (60 mL). The flask was capped with
watch glass and the mixture was allowed to slowly
evaporate for 2 days at room temperature during which
time the product crystallized from solution. The superna-
tant was decanted and the solid was washed with
hexanes (2 ꢅ 50 mL) and air-dried to give [Pr4N][B{3,5-
(CF3)2C6H3}4] as an off-white crystalline solid (8.6 g,
8.2 mmol, 84%). Note: Drying under reduced pressure for
24 h removes coordinated water. 1H NMR (CDCl3, 25 8C,
mixture was stirred at room temperature. After 18 h, the
reaction mixture was filtered through a medium-porosity
fritted filter. The white solid collected was then dried under
reduced pressure for 5 h to give (THF)K[N(Ph)(SiMe3)]
(7.03 g, 25.5 mmol, 84%). 1H NMR ([D8]THF, 25 8C,
300 MHz):
d 6.98 (m, 2 H, m-Ar), 6.63-6.50 (m, 3 H, o-Ar
and p-Ar), 3.57 (br, 4 H, O(CH2CH2)2), 1.72 (br, 4 H,
O(CH2CH2)2), 0.22 (s, 6 H, SiMe2), 0.06 (s, 3 H, SiMe).
4.6. Synthesis of (C5Me5)2U[N(Ph)(SiMe3)](THF)
300 MHz):
(m, 8 H, N-CH2), 1.60 (m, 8 H, CH2-CH2-CH3), 0.94 (t, 12 H,
CH3). 19F NMR (CDCl3, 25 8C, 282 MHz):
CF3). Anal. Calcd for C44H40BF24Nꢀ2(H2O) (molecular
weight 1085.6 g molꢁ1): C, 48.68; H, 4.09; N, 1.29. Found:
C, 48.85; H, 3.83; N, 1.16.
d 7.69 (br s, 8 H, Ar-H), 7.54 (br s, 4 H, Ar-H), 2.93
A 125-mL side-arm flask equipped with a magnetic stir
bar was charged with (C5Me5)2UI(THF) (0.250 g,
0.353 mmol), toluene (35 mL), and THF (10 mL). To this
stirred green solution was added (THF)K[N(Ph)(SiMe3)]
(0.097 g, 0.353 mmol) as a solid, and the resulting cloudy
green–grey solution was stirred at room temperature.
After 18 h, the green–brown solution was filtered through
a Celite-padded coarse-porosity fritted filter, and the
volatiles were removed under reduced pressure. The
green-brown residue was extracted into hexane and
filtered through a Celite-padded coarse-porosity fritted
filter, and the volatiles were removed from the dark green
filtrate under reduced pressure to give (C5Me5)2U[N(Ph)
(SiMe3)](THF) as a dark green solid (0.170 g, 0.229 mmol,
d
ꢁ62.81 (s, 24F,
4.4. Synthesis of [Pr4N][B(C6F5)4]
In a fumehood, a 500-mL Erlenmeyer flask was charged
with Li[B(C6F5)4]ꢀ2.5(Et2O) (7.7 g, 8.8 mmol) and deionized
water (100 mL), giving a pale yellow solution. A colorless
solution of [Pr4N][Br] (2.9 g, 11 mmol) in methanol
(150 mL) was added with stirring, immediately producing
a thick white precipitate. The resulting suspension was
stirred at room temperature for 4 h and the white solid was
collected by filtration using a coarse-porosity fritted filter
and washed with deionized water (5 ꢅ 150 mL). Each time,
the vacuum was disconnected, water added, and the solid
was mixed/mashed using a spatula, and then the vacuum
reapplied to remove the water. The resulting sticky white
solid was then transferred to a 125-mL side-arm flask and
dried under vacuum for 24 h. The crude reaction product
was taken up into CH2Cl2 (40 mL) and the resulting pale
yellow solution was filtered through a Celite-padded
coarse-porosity fritted filter to remove particulates. The
65%). 1H NMR ([D6]benzene, 25 8C, 300 MHz):
d 8.12 (s, 1 H,
Ar), 3.59 (s, 4 H, O(CH2CH2)2), ꢁ4.31 (t, J = 7 Hz, 1 H, p-Ar),
ꢁ7.65 (s, 30 H, C5Me5), ꢁ10.84 (d, J = 5 Hz, 2 H, o-Ar),
ꢁ21.79 (s, 9 H, SiMe3), ꢁ46.30 (br, 1 H, Ar). Satisfactory
elemental analysis results could not be obtained due to the
thermally sensitive nature of this material.
4.7. Synthesis of (C5Me5)2U[N(SiMe3)2](Cl) (2)
A 125-mL side-arm flask equipped with a magnetic stir
bar was charged with (C5Me5)2U[N(SiMe3)2] (0.293 g,
0.438 mmol) and toluene (50 mL). To this stirred green-
grey solution was added CuCl (0.217 g, 2.19 mmol) as a
solid, resulting in an immediate color change to dark red.
The resulting solution was stirred at room temperature.
After 15 h, the reaction mixture was filtered through a
Celite-padded coarse-porosity fritted filter, and the Celite
plug was washed with toluene until the washings went
colorless. The filtrate was collected and the volatiles were
removed under reduced pressure. The resulting red solid
was extracted into hexanes and filtered through a Celite-
padded coarse-porosity fritted filter, and the Celite plug
was washed with hexanes until the washings went
colorless. The red colored filtrate was collected and the
volatiles were removed under reduced pressure to give 2 as
dark red crystals (0.239 g, 0.339 mmol, 77%). 1H NMR
filtrate was collected and transferred to
a 250-mL
Erlenmeyer flask and layered with diethyl ether (40 mL)
followed by hexanes (60 mL). The flask was capped with a
watch glass and the mixture was allowed to evaporate for 2
days at room temperature during which time the product
crystallized from solution. The supernatant was decanted
and the solid was washed with hexanes (2 ꢅ 50 mL)
and dried under reduced pressure for 24 h to give
[Pr4N][B(C6F5)4] as
7.7 mmol, 88%). 1H NMR (CDCl3, 25 8C, 300 MHz):
(m, 8 H, N-CH2), 1.63 (m, 8 H, CH2-CH2-CH3), 1.01 (t, 12 H,
CH3). 19F NMR (CDCl3, 25 8C, 282 MHz):
a
white crystalline solid (6.7 g,
d
2.99
d
ꢁ133.02 (br s, 8 F,
o-F), ꢁ163.20 (t, 4 F, p-F), ꢁ167.16 (m, 8 F, m-F). Anal. Calcd
for C36H28BF20N (mol. wt. 865.4 gmolꢁ1): C, 49.96; H, 3.26;
N, 1.62. Found: C, 49.95; H, 3.27; N, 1.61.
([D6]benzene, 25 8C, 300 MHz):
d 10.29 (s, 30 H, C5Me5),
4.5. Synthesis of (THF)K[N(Ph)(SiMe3)]
7.42 (s, 9 H, SiMe3), 4.31 (s, 6 H, SiMe2), ꢁ107.98 (s, 3 H,
SiMe). Anal. Calcd for
C26H48ClNSi2U (mol. wt.
A 125-mL side-arm flask equipped with a magnetic stir
bar was charged with HN(Ph)(SiMe3) (5.00 g, 30.2 mmol)
and hexanes (50 mL). To this stirred clear solution,
K[N(SiMe3)2] (6.03 g, 30.2 mmol) was added as a solid at
room temperature, giving a white suspension. Approxi-
mately 10 mL of THF was added to the suspension to aid in
the dissolution of K[N(SiMe3)2]. The cloudy white reaction
704.32 g molꢁ1): C, 44.34; H, 6.87; N, 1.99. Found: C,
44.07; H, 6.68; N, 1.77.
4.8. Synthesis of (C5Me5)2U[N(SiMe3)2](Br) (3)
A 125-mL side-arm flask equipped with a magnetic stir
bar was charged with (C5Me5)2U[N(SiMe3)2] (0.250 g,

800
R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
0.374 mmol) and toluene (50 mL). To this stirred green–
grey solution was added CuBr (0.268 g, 1.87 mmol) as a
solid, resulting in an immediate color change to dark red.
The resulting solution was stirred at room temperature.
After 15 h, the reaction mixture was filtered through a
Celite-padded coarse-porosity fritted filter, and the Celite
plug was washed with toluene until the washings went
colorless. The filtrate was collected and the volatiles were
removed under reduced pressure. The resulting red solid
was extracted into hexanes and filtered through a Celite-
padded coarse-porosity fritted filter, and the Celite plug
was washed with hexanes until the washings went
colorless. The red colored filtrate was collected and the
volatiles were removed under reduced pressure to give 3 as
a dark red solid (0.145 g, 0.194 mmol, 52%). 1H NMR
padded coarse-porosity fritted filter, and the Celite plug
was washed with hexanes until the washings went
colorless. The red colored filtrate was collected and the
volatiles were removed under reduced pressure to give 7 as
a dark red solid (0.440 g, 0.617 mmol, 86%). 1H NMR
([D6]benzene, 25 8C, 300 MHz): d 15.53 (br, 10 H, Ar), 12.73
(s, 30 H, C5Me5). Satisfactory elemental analysis results
could not be obtained for this compound despite several
attempts. This is most likely due to thermal instability
during analysis.
4.11. Synthesis of (C5Me5)2U[N(Ph)(SiMe3)](Cl) (9)
A 125-mL side-arm flask equipped with a magnetic stir
bar was charged with (C5Me5)2U[N(Ph)(SiMe3)](THF)
(0.328 g, 0.440 mmol) and toluene (50 mL). To this stirred
green solution was added CuCl (0.131 g, 1.32 mmol) as a
solid, resulting in an immediate color change to dark red.
The resulting solution was stirred at room temperature.
After 15 h, the reaction mixture was filtered through a
Celite-padded coarse-porosity fritted filter, and the Celite
plug was washed with toluene until the washings went
colorless. The filtrate was collected and the volatiles were
removed under reduced pressure. The resulting red solid
was extracted into hexanes and filtered through a Celite-
padded coarse-porosity fritted filter, and the Celite plug
was washed with hexanes until the washings went
colorless. The red colored filtrate was collected and the
volatiles were removed under reduced pressure to give 9 as
a dark red solid (0.250 g, 0.353 mmol, 80%). 1H NMR
([D6]benzene, 25 8C, 300 MHz):
d 11.40 (s, 30 H, C5Me5),
8.41 (s, 9 H, SiMe3), 4.51 (s, 6 H, SiMe2), -109.75 (s, 3 H,
SiMe). Anal. Calcd for C26H48BrNSi2U (0.5-C6H14) (mol. wt.
748.77 g molꢁ1): C, 43.99; H, 7.00; N, 1.77. Found: C, 44.38;
H, 6.68; N, 1.95.
4.9. Synthesis of (C5Me5)2U[N(SiMe3)2](NCO) (6)
A 125-mL side-arm flask equipped with a magnetic stir
bar was charged with (C5Me5)2U[N(SiMe3)2] (0.250 g,
0.374 mmol) and toluene (50 mL). To this stirred green-
grey solution was added AgNCO (0.067 g, 0.449 mmol) as a
solid, resulting in a color change to dark red. The resulting
solution was stirred at room temperature. After 2 h, the
reaction mixture was filtered through a Celite-padded
coarse-porosity fritted filter, and the Celite plug was
washed with toluene until the washings went colorless.
The filtrate was collected and the volatiles were removed
under reduced pressure. The resulting red solid was
extracted into hexanes and filtered through a Celite-
padded coarse-porosity fritted filter, and the Celite plug
was washed with hexanes until the washings went
colorless. The red colored filtrate was collected and the
volatiles were removed under reduced pressure to give 6 as
dark red crystals (0.190 g, 0.267 mmol, 71%). 1H NMR
([D6]benzene, 25 8C, 300 MHz): d 9.47 (s, 30 H, C5Me5), 8.12
(s, 1 H, o-Ar), 5.78 (s, 9 H, SiMe3), 0.29 (d, J = 5 Hz, 1 H, o-Ar),
ꢁ5.90 (t, J = 7 Hz, 1 H, p-Ar), ꢁ10.03 (s, 2 H, m-Ar). Anal.
Calcd for C29H44ClNSiU (mol. wt. 708.23 g molꢁ1): C, 49.18;
H, 6.26; N, 1.98. Found: C, 48.94; H, 5.99; N, 1.62.
4.12. Synthesis of (C5Me5)2U[N(Ph)(SiMe3)](N3) (10)
A 125-mL side-arm flask equipped with a magnetic stir
bar was charged with (C5Me5)2U[N(Ph)(SiMe3)](THF)
(0.280 g, 0.376 mmol) and toluene (50 mL). To this stirred
green solution was added (Ph3P)AuN3 (0.181 g, 0.376
mmol) as a solid, resulting in an immediate color change to
dark red. The resulting solution was stirred at room
temperature. After 15 h, the reaction mixture was filtered
through a Celite-padded coarse-porosity fritted filter, and
the Celite plug was washed with toluene until the
washings went colorless. The filtrate was collected and
the volatiles were removed under reduced pressure. The
resulting red solid was extracted into hexanes and filtered
through a Celite-padded coarse-porosity fritted filter, and
the Celite plug was washed with hexanes until the
washings went colorless. The red colored filtrate was
collected and the volatiles were removed under reduced
pressure to give 10 as a dark red solid (0.215 g, 0.301 mmol,
([D6]benzene, 25 8C, 300 MHz): d 8.91 (s, 30 H, C5Me5), 6.09
(s, 9 H, SiMe3), 5.15 (s, 6 H, SiMe2), ꢁ105.09 (s, 3 H, SiMe). IR
(Nujol, cmꢁ1):
Calcd for
n
2194 (s) (NCO asymmetric stretch). Anal.
C27H48N2OSi2U (0.5-C6H14) (mol. wt.
710.88 g molꢁ1): C, 47.79; H, 7.35; N, 3.72. Found: C,
47.66; H, 7.01; N, 3.20.
4.10. Synthesis of (C5Me5)2U(NPh2)(Cl) (7)
A 125-mL side-arm flask equipped with a magnetic stir
bar was charged with (C5Me5)2U(NPh2)(THF) (0.537 g,
0.717 mmol) and toluene (50 mL). To this stirred green
solution was added CuCl (0.355 g, 3.59 mmol) as a solid,
resulting in an immediate color change to dark red. The
resulting solution was stirred at room temperature. After
15 h, the reaction mixture was filtered through a Celite-
padded coarse-porosity fritted filter, and the Celite plug
was washed with toluene until the washings went
colorless. The filtrate was collected and the volatiles were
removed under reduced pressure. The resulting red solid
was extracted into hexanes and filtered through a Celite-
80%). 1H NMR ([D6]benzene, 25 8C, 300 MHz):
d 8.45 (s, 30
H, C5Me5), 1.31 (s, 9 H, SiMe3), 0.30 (s, 2 H, o/m-Ar), ꢁ5.82
(t, J = 7 Hz, 1 H, p-Ar), ꢁ10.12 (s, 2 H, o/m-Ar). IR (Nujol,
cmꢁ1):
C
n 2087 (s) (N3 asymmetric stretch). Anal. Calcd for
29H44N4SiU (0.5-C7H8) (mol. wt. 714.80 g molꢁ1): C,
51.30; H, 6.36; N, 7.36. Found: C, 51.26; H, 6.56; N, 6.48.

R.K. Thomson et al. / C. R. Chimie 13 (2010) 790–802
801
4.13. X-ray crystallography
Chemical Sciences, Office of Basic Energy Science, Heavy
Element Chemistry program.
CIF files representing the X-ray crystal structures of 2, 6,
7, 9, and 10 have been submitted to the Cambridge
Crystallographic Database as submission numbers
767534–767538. Crystallographic data and parameters
are listed below:
References
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ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
(C5Me5)2U[N(SiMe3)2](Cl) (2): C26H48ClNSi2U, triclinic,
P-1, lattice constants a = 8.574(2), b = 9.585(2), c =
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18.103(4),
V = 1424.9(6) A , Z = 2,
a
= 97.632(2),
b
= 92.113(3),
g = 104.352(2),
3
(Mo-K ) = 5.888 mmꢁ1
, umax =
˚
m
a
28.21, 6473 [Rint = 0.0491] independent reflections mea-
sured, of which 5607 were considered observed with
I > 2
s
(I); max. residual electron density 4.860 and
3
˚
ꢁ4.100 e/A ; 248 parameters, R1 (I > 2
s(I)) = 0.0762;
wR2 (all data) = 0.1898;
(C5Me5)2U[N(SiMe3)2](NCO) (6): C27H48N2OSi2U, hexag-
onal,
c = 16.1546(14),
(Mo-K ) = 5.613 mmꢁ1
P63/m,
lattice
constants
a = 17.7701(8),
3
˚
g
= 120.00, V = 4417.8(5) A , Z = 6,
m
,
u
max = 25.68,
2915
a
[Rint = 0.0698] independent reflections measured, of
which 2578 were considered observed with I > 2
s
(I);
˚
3
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ger, Inorg. Chem. 49 (2010) 1995.
max. residual electron density 2.283 and ꢁ1.377 e/A ;
193 parameters, R1 (I > 2s(I)) = 0.0431; wR2 (all da-
ta) = 0.1099;
(C5Me5)2U(NPh2)(Cl) (7): C35.5H44ClNU, monoclinic, P21/
n, lattice constants a = 10.5907(11), b = 20.993(2),
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Angew. Chem., Int. Ed. 47 (2008) 2993.
3
˚
c = 13.7785(15),
b
= 90.1140(10), V = 3063.3(6) A , Z = 4,
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30.
m
(Mo-K ) = 5.410 mmꢁ1
,
u
max = 27.75, 6147 [Rint
=
a
0.0839] independent reflections measured, of which
4050 were considered observed with I > 2
s
(I); max.
˚
3
residual electron density 1.004 and ꢁ0.760 e/A ; 326
parameters, R1 (I > 2s(I)) = 0.0359; wR2 (all da-
ta) = 0.0946;
(C5Me5)2U[N(Ph)(SiMe3)](Cl)
(9):
C29H44ClNSiU,
monoclinic, P21/c, lattice constants a = 9.4785(9),
b = 17.1420(16),
c = 18.1000(17),
b = 101.6620(10),
3
(Mo-K ) = 5.787 mmꢁ1
, umax =
˚
V = 2880.2(5) A , Z = 4,
m
a
28.33, 6851 [Rint = 0.0380] independent reflections mea-
sured, of which 5943 were considered observed with
I > 2
s
(I); max. residual electron density 0.845 and
3
˚
ꢁ0.627 e/A ; 311 parameters, R1 (I > 2
s(I)) = 0.0216;
wR2 (all data) = 0.0497;
(C5Me5)2U[N(Ph)(SiMe3)](N3) (10): C29H44N4SiU, mono-
clinic, P21/c, lattice constants a = 12.500(3), b = 14.823(3),
3
˚
c = 15.590(3),
b
= 93.757(3), V = 2882.4(10) A , Z = 4,
m
(Mo-K ) = 5.697 mmꢁ1
,
u
max = 25.32, 5257 [Rint
=
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a
0.0851] independent reflections measured, of which
3852 were considered observed with I > 2
s
(I); max.
˚
3
residual electron density 1.958 and ꢁ1.159 e/A ; 329
parameters, R1 (I > 2s(I)) = 0.0420; wR2 (all da-
ta) = 0.1096.
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For financial support of this work, we acknowledge the
LANL G.T. Seaborg Institute for Transactinium Science
(postdoctoral fellowship to R.K.T.) and the Division of

802
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