Facile reduction of a uranyl(VI) β-ketoiminate complex to U(IV) upon oxo silylation

Article abstract of DOI:10.1021/ic200387n

Reaction of the uranyl β-ketoiminate complex UO₂( ^tBuacnac)₂ (1) (^tBuacnac = tBuNC(Ph)CHC(Ph)O) with Me₃SiI, in the presence of Ph₃P, results in the reductive silylation of the uranyl moiet

Full text of DOI:10.1021/ic200387n

ARTICLE
pubs.acs.org/IC
Facile Reduction of a Uranyl(VI) β-Ketoiminate Complex to U(IV) Upon
Oxo Silylation
Jessie L. Brown, Charles C. Mokhtarzadeh, Jeremie M. Lever, Guang Wu, and Trevor W. Hayton*
Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, California 93106, United States
S Supporting Information
b
ABSTRACT: Reaction of the uranyl β-ketoiminate complex
UO2(tBuacnac)2 (1) (tBuacnac = tBuNC(Ph)CHC(Ph)O) with
Me3SiI, in the presence of Ph3P, results in the reductive
silylation of the uranyl moiety and formation of the U(V) bis-
silyloxide complex [Ph3PI][U(OSiMe3)2I4] (2). Subsequent
reaction of 2 with Lewis bases, such as 2,20-bipyridine (bipy),
1,10-phenanthroline (phen), and tetrahydrofuran (THF), re-
sults in a further reduction of the metal center and isolation of the U(IV) complexes U(OSiMe3)2I2(bipy)2 (3), U(OSiMe3)2I2-
(phen)2 (4), and [U(OSiMe3)2I(THF)4][I3] (5), respectively.
’ INTRODUCTION
Me3SiI, in the presence of Ph3P, results in the reductive silylation
of the uranyl moiety and formation of [Ph3PI][U(OSiMe3)2I4]
(2). Interestingly, complex 2 reacts with Lewis bases to undergo
further reduction, ultimately generating a U(IV) bis-silyloxide
complex and iodine.
Reductive silylation of the uranyl moiety (UO22þ) is one of
the few synthetic procedures available for functionalizing this
normally unreactive fragment.1ꢀ3 This reaction involves silyla-
tion of either one or both uranyl oxo ligands with concomitant
reduction of the uranium center to U(V) or U(IV). For example,
Arnold and co-workers observed reductive silylation of UO2-
(THF)(K2L) (L = “Pacman” polypyrrolic macrocycle) by HN-
(SiMe3)2, forming the U(V) silyloxide [UO(OSiMe3)(THF)-
Fe2I2L] as the final product.1,4ꢀ6 During the reaction,
HN(SiMe3)2 acts as both the reductant and the silylating reagent.
We recently reported a similar transformation, namely, that
addition of excess Me3SiI to UO2(Aracnac)2 (Aracnac = ArNC-
(Ph)CHC(Ph)O, Ar = 3,5-tBu2C6H3) results in the formation of
the U(V) bis-silyloxide U(OSiMe3)2I2(Aracnac).2 This material
is formed by reduction of the uranium center concomitant with
silylation of both oxo ligands. We postulated that coordination of
Me3Siþ to the uranyl oxo ligand substantially increases the
uranium oxidation potential, allowing it to oxidize the iodide
anion. Consistent with this hypothesis we observed the forma-
tion of I2 in the reaction mixture. Ephritikhine and co-workers
have also observed the reductive silylation of uranyl. Specifically,
the addition of excess of Me3SiX (X = Br, Cl, I) to either
UO2(OTf)2 or UO2I2(THF)3, in acetonitrile, results in com-
plete removal of the oxo ligands, and the formation of the U(IV)
complexes, UX4(MeCN)4.3 In each of these examples, reductive
silylation is driven by the formation of strong SiꢀO bonds in
combination with the presence of weaker SiꢀX bonds (either
SiꢀI or SiꢀN).7 However, the presence of an easily oxidizable
substrate (to generate the U(V) center) also plays a role in the
transformations.
’ RESULTS AND DISCUSSION
We have been exploring the ability of the diphenyl-β-ketoi-
minate ligand platform to coordinate the UO2 moiety,8,9 as
2þ
this ligand class creates a bulky, substitutionally inert ligand
environment that prevents further coordination of Lewis bases to
the uranyl equatorial plane. Moreover, the strongly electron-
donating properties of this ligand set appear to activate the uranyl
oxo ligands toward functionalization.8 To increase the electron-
donating capability of the ligand, and consequently the nucleo-
philicity of the oxo ligands, we have endeavored to install an
electron-donating tert-butyl substituent on the nitrogen atom of
the diphenyl-β-ketoimine. The synthesis of β-ketoimines is
normally achieved by condensation of a β-diketone with a
primary amine.10ꢀ14 However, the reaction of tert-butylamine
with dibenzoylmethane does not result in formation of a
β-ketoimine, presumably because of an unfavorable steric inter-
action between the tert-butyl substituent and the phenyl ring of
the dibenzoylmethane. As a result, an alternate synthetic route
for the preparation of a tert-butyl substituted diphenyl-β-ketoi-
mine was utilized, namely, reaction of excess tert-butylamine with
diphenylpropynone, in the presence of 10 mol % Zn(OTf)2.14
After heating the reaction mixture to reflux for 48 h, tBuNHC-
(Ph)CHC(Ph)O, (tBuacnac)H, could be isolated as a yellow
1
solid in 86% yield (Scheme 1). The H NMR spectrum of
(
tBuacnac)H in C6D6 (see the Supporting Information, Figure
Herein we report another example of reductive silylation in
uranyl and explore the reactivity of the functionalized product. In
particular, the reaction of the uranyl β-ketoiminate complex
S12) exhibits a singlet at 1.00 ppm, assignable to the methyl
Received: February 23, 2011
Published: May 05, 2011
UO2(tBuacnac)2 (1) (tBuacnac = BuNC(Ph)CHC(Ph)O) with
t
r
2011 American Chemical Society
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Scheme 1
Scheme 2
groups of the tBu moiety, while the amine proton is observed at
12.32 ppm.
Reaction of (tBuacnac)H in Et2O with 0.5 equiv of UO2(N-
{SiMe3}2)2(THF)2 results in the deposition of UO2(tBuacnac)2
(1) an orange microcrystalline solid in 67% yield (Scheme 1).
Complex 1 is partly soluble in Et2O and toluene, but reasonably
1
soluble in tetrahydrofuran (THF) and CH2Cl2. Its H NMR
spectrum in C6D6 exhibits a singlet at 1.73 ppm, assignable to the
methyl groups of the tBu moiety, while the proton attached to the
γ-carbon of the ketoiminate ring is observed at 5.81 ppm
(Supporting Information, Figure S14). The IR spectrum of 1
(in Nujol) exhibits a vibration at 907 cmꢀ1, assignable to
νasym(UdO), while the Raman spectrum exhibits a strong
vibration at 823 cmꢀ1, assignable to νsym(UdO) (Supporting
Information, Figures S1 and S6, respectively). Surprisingly, the
latter assignment is higher that that observed for UO2(Aracnac)2
(Ar = 3,5-tBu2C6H3) [νsym(UdO) = 812 cmꢀ1] (Supporting
Information, Figure S7), despite the stronger donating capacity
of the tert-butyl substituent in 1.
Figure 1. Molecular structure of UO2(tBuacnac)2 (1 2CH2Cl2) with
3
50% probability ellipsoids. CH2Cl2 and hydrogen atoms omitted for
clarity. Selected bond lengths (Å) and angles (deg): U1ꢀO1 = 1.770(3),
U1ꢀO2 = 2.228(2), U1ꢀN1 = 2.531(3), O1ꢀU1ꢀO1* = 180,
O2ꢀU1ꢀN1 = 75.28(9), O2ꢀU1ꢀN1* = 104.72(9).
Crystals of 1 suitable for X-ray analysis were grown from a
dichloromethane/hexane solution. Complex 1 crystallizes in the
The cyclic voltammogram of 1 in CH2Cl2 reveals a reversible
reduction feature at E1/2 = ꢀ1.46 V (vs Fc/Fcþ), which we
attribute to the U(VI)/U(V) redox couple (Figure 2). This
reduction potential lies between those reported previously for
the other uranyl β-ketoiminate complexes. For example, UO2-
(Aracnac)2 (Ar = 3,5-tBu2C6H3) exhibits a U(VI)/U(V) reduc-
tion potential at ꢀ1.35 V, while UO2(Aracnac)2 (Ar = 2,
4,6-Me3C6H2) exhibits a U(VI)/U(V) reduction potential
at ꢀ1.52 V.8 In addition, the observed potential for 1 is
smaller than that seen for [UO2(salan-tBu2)(py)] (ꢀ1.81 V),15
but similar to most other uranyl Schiff-base complexes.15,16
Overall, the cyclic voltammetry data, in combination with the
Raman spectroscopic results, suggests that coordination of the
tert-butyl-substituted β-ketoiminate ligand does not result in a
more electron-rich uranium center than that observed with
the aryl-substituted β-ketoiminates, likely because the longer
triclinic space group P1 as a dichloromethane solvate, 1 2CH2Cl2.
3
Its solid-state molecular structure is shown in Figure 1, while the
crystallographic details are found in Table 1. Complex 1 exhibits
a slightly distorted octahedral geometry about the uranium
center in which two β-ketoiminate ligands occupy the equatorial
plane. As observed in other uranyl β-ketoiminate complexes, the
N-tert-butyl substituents in 1 exhibit a trans configuration.8 Its
UꢀO(oxo) bond length (U1ꢀO1 = 1.770(3) Å) and OꢀUꢀO
bond angle (O1ꢀU1ꢀO1* = 180°) are typical of uranyl(VI)
complexes. Interestingly, the UꢀN bond length in 1 (U1ꢀN1 =
2.531(3) Å) is longer than the comparable bond length in
UO2(Aracnac)2 (U1ꢀN1 = 2.449(6) Å),8 likely because of the
greater steric demand of the tert-butyl substituent.
Solution-phase cyclic voltammetry was carried out on 1 to
ascertain its redox properties (Supporting Information, Table S1).
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Table 1. X-ray Crystallographic Data for Complexes 1 2CH2Cl2, 2, 3 CH2Cl2, and 5
3
3
1 2CH2Cl2
2
3 CH2Cl2
5
3
3
empirical formula
crystal habit, color
crystal size (mm)
crystal system
space group
volume (Å3)
a (Å)
UO4C40H44N2Cl4
block, orange
0.20 ꢁ 0.20 ꢁ 0.05
triclinic
UO2C24H33Si2P1I5
block, black
0.2 ꢁ 0.2 ꢁ 0.30
triclinic
UO2C27H36N4Si2I2Cl2
block, orange
0.8 ꢁ 0.3 ꢁ 0.2
monoclinic
C2/c
UO6C22H50Si2I4
block, orange
0.20 ꢁ 0.10 ꢁ 0.05
triclinic
P1
P1
P1
996.1(4)
9.5091(19)
9.552(2)
11.408(2)
98.382(3)
97.715(3)
99.983(3)
1
1879(3)
3533(2)
20.377(7)
9.122(3)
19.007(6)
90.00
1907.3(12)
9.519(4)
12.725(5)
16.355(6)
89.944(6)
74.313(6)
89.970(6)
2
9.452(7)
9.961(8)
20.940(16)
103.327(12)
90.511(13)
101.222(12)
2
b (Å)
c (Å)
R (deg)
β (deg)
90.883(6)
90.00
γ (deg)
Z
4
formula weight (g/mol)
density (calculated) (Mg/m3)
996.60
1313.18
1065.49
1212.43
1.661
2.321
2.003
2.111
absorption coefficient (mmꢀ1
)
4.384
8.553
6.591
7.583
F000
490
1186
2000
1124
total no. reflections
unique reflections
Rint
8559
15723
15034
16397
3968
7383
3848
7615
0.0487
0.1135
0.0740
0.0803
final R indices [I > 2σ(I)]
R1 = 0.0333
wR2 =0.0651
1.261 and ꢀ0.772
1.015
R1 = 0.0494
wR2 = 0.1216
3.346 and ꢀ4.095
1.004
R1 = 0.0389
wR2 = 0.0892
3.311 and ꢀ1.381
1.088
R1 = 0.0483
wR2 = 0.1224
1.869 and ꢀ2.275
0.973
largest diff. peak and hole (eꢀ Åꢀ3
)
GOF
UꢀN bonds found in 1 result in relatively poor UꢀN orbital
overlap.
of the phosphonium cation, [Ph3PI]þ, are similar to those ob-
served previously.17ꢀ21
1
To ascertain the nucleophilicity of the uranyl oxo ligands in 1
we have explored its reactivity with electrophiles. Addition of
excess Me3SiI to 1 in toluene produces a deep red solution.
Subsequent addition of Ph3P to the reaction mixture results in
the deposition of a black solid. Recrystallization of this solid from
CH2Cl2 at ꢀ25 °C affords the U(V) bis-silyloxide complex,
[Ph3PI][U(OSiMe3)2I4] (2), in 40% yield (eq 1).
The H NMR spectrum of complex 2 in CD2Cl2 exhibits a
broad resonance at 4.03 ppm, assignable to the Me3Si protons of
1
the oxo-derived silyloxide ligands, while the H resonances of
the [Ph3PI]þ moiety are observed at 7.38 ppm, 7.61 ppm, and
7.77 ppm, assignable to the ortho, meta, and para positions, res-
pectively. Interestingly, a second singlet, at 2.17 ppm, is observed
in the 1H NMR spectrum, which is also assignable to an SiMe3
resonance. This peak is present in all samples of 2, and is always
observed in a 1:3 ratio with respect to the resonance at 4.03 ppm.
This ratio does not change appreciably upon cooling to ꢀ60 °C.
Given this, we suggest that the two resonances are indicative of
the presence of cis and trans isomers in solution. Alternately, this
behavior could be due to iodide dissociation from complex 2;
however, addition of 2 equiv of [Ph3PI][I] to an NMR sample of
2 does not affect the two SiMe3 resonances, arguing that I-
dissociation is not occurring. Complex 2 slowly decomposes in
Complex 2 crystallizes in the triclinic space group P1 as a
discrete anion/cation pair, and its solid-state molecular structure
is shown in Figure 3. Further crystallographic details for 2 can be
found in Table 1. The uranium center in the [U(OSiMe3)2I4]ꢀ
anion exhibits an octahedral geometry, in which four iodide
ligands occupy the equatorial plane, while the oxo-derived silyl-
oxide ligands occupy the axial positions (O1ꢀU1ꢀO1* = 180°).
The UꢀO(SiMe3) bond length (U1ꢀO1 = 1.990(6) Å) is
similar to those in previously reported U(V) silyloxide com-
plexes,1,2 and is much longer than the UdO bond lengths typi-
cally observed for uranyl. In addition, the Si1ꢀO1 (1.688(8) Å)
and UꢀI (U1ꢀI1 = 2.9841(19) Å, U1ꢀI2 = 2.999(2) Å) bond
lengths in 2 are comparable with those reported for U(OSiMe3)2I2-
(Aracnac) (Ar = 3,5-tBu2C6H3).2 Finally, the metrical parameters
1
solution, and upon standing in CD2Cl2 for 24 h, its H NMR
spectrum reveals the appearance of a new resonance at 0.09 ppm,
attributable to the formation of Me3SiOSiMe3, while the reso-
nances assignable to 2 are decreased in intensity.
The magnetic moment of 2 (μeff = 1.4 μB at 295 K), deter-
mined via Evans’ method,22 is much lower than the theoretical
value of 2.54 μB,23 but is comparable to the magnetic moments of
other U(V) complexes,24 including U(OSiMe3)2I2(Aracnac)
(μeff = 1.52 μB at 298 K).2 Finally, the NIR spectrum for complex
2 is consistent with the presence of a 5f1 ion (Supporting
Information, Figure S9).15,25,26
We also explored the mechanism of formation of complex 1.
An in situ 1H NMR spectrum of the reaction mixture in C6D6,
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Figure 2. Room temperature cyclic voltammogram for 1 in CH2Cl2 (vs Fc/Fcþ, 0.1 M [NBu4][PF6] as supporting electrolyte).
U6þ ion more oxidizing. As a consequence, one iodide anion is
oxidized by uranium, forming 0.5 equiv of iodine and an inter-
mediate U(V) species, presumably U(OSiMe3)2I3. The iodine
then reacts with Ph3P to form the iodophosphonium salt,
[Ph3PI][I],17 which is subsequently trapped by U(OSiMe3)2I3 to
give the final product, complex 2. Because only 0.5 equiv of I2 is
generated in the reduction to U(V), anda full equiv of I2 is required
to form [Ph3PI][I], the maximum yield of 2 can only be 50%.
We have endeavored to explore the reactivity of complex 2
with Lewis bases to better understand the coordination chem-
istry of the uranyl-derived U(V) silyloxides. Addition of 2 equiv
of 2,20-bipyridine (bipy) to a suspension of 2 in toluene results in
a color change to orange, concomitant with the deposition of a dull
yellow solid. The yellow solid was determined to be [Ph3PI]-
1
[I] by both H and 31P NMR spectroscopy, while a U(IV) com-
plex, U(OSiMe3)2I2(bipy)2 (3), could be isolated from the super-
natant in good yield as an orange microcrystalline powder (Scheme 2).
Similarly, addition of 1,10-phenanthroline (phen) to a toluene suspen-
sion of 2 generates U(OSiMe3)2I2(phen)2 (4), in moderate yields.
The reductant in these transformations is likely iodide, which would
result in the formation of 0.5 equiv of I2 over the course of the reactions.
Single crystals of 3 suitable for X-ray diffraction analysis were
grown from a 1:4 mixture of CH2Cl2 and hexane. Complex 3
crystallizes in the monoclinic space group C2/c as a dichloro-
Figure 3. Molecular structure of [Ph3PI][U(OSiMe3)2I4] (2) with
50% probability ellipsoids. Hydrogen atoms omitted for clarity. Selected
bond lengths (Å) and angles (deg): U1ꢀO1 = 1.990(6), U1ꢀI1 =
2.9841(19), U1ꢀI2 = 2.999(2), O1ꢀSi1 = 1.688(8), P1ꢀI5 = 2.408(3),
O1ꢀU1ꢀO1* = 180, O1ꢀU1ꢀI1 = 90.4(2).
before addition of Ph3P, revealed the presence of a broad singlet
at 5.03 ppm (Supporting Information, Figure S15), suggesting
the formation of a uranium silyloxide complex. However, all
attempts to isolate this intermediate species have proven un-
successful because of its high solubility. Also observed in this
spectrum are two resonances at ꢀ0.24 ppm and þ1.48 ppm, in a
1:1 ratio, assignable to the Me3Si and tBu protons, respectively, of
tBuNC(Ph)CHC(Ph)OSiMe3. This is in agreement with the
independently prepared material, formed via reaction of Na-
methane solvate, 3 CH2Cl2, and its solid-state molecular struc-
3
ture is shown in Figure 4. Complex 3 exhibits a dodecahedral
geometry, in which the two orthogonal trapezoids are defined by
O1ꢀN2ꢀN1ꢀI1 and O1*ꢀN2*ꢀN1*ꢀI1*.27 The UꢀO-
(silyloxide) bond length (U1ꢀO1 = 2.084(4) Å) in 3 is longer
than that observed in 2, in accord with the larger radius of the
U4þ ion and the higher coordination number. Interestingly, the
O1ꢀU1ꢀO1* bond angle in 3 (115.5(2)°) is substantially
smaller than the OꢀUꢀO angle observed in either 2 (O1ꢀ
U1ꢀO1* = 180°) or a typical uranyl complex. The UꢀI bond
length (U1ꢀI1 = 3.2435(8) Å) in 3 is significantly elongated
relative to 2 (U1ꢀI1 = 2.9841(19) Å), while the UꢀN bond
lengths in 3 (2.646(5) Å and 2.670(5) Å) are typical of other
reported UꢀN(bipy) bond distances.28ꢀ31
(
tBuacnac) with 2 equiv of Me3SiCl.13 Thus, 4 equiv of Me3SiI are
formally required for the formation of 2, as 2 equiv are required
for silylation of each oxo ligand and 2 equiv are required for
silylation of each β-ketoiminate ligand. The extra steric bulk of
the tert-butyl substituted β-ketoiminate likely makes ligand
abstraction more facile than its aryl-substituted counterpart,2
accounting for the loss of both tBuacnac ligands from the product.
As observed in previous silylation reactions, the uranium
center in 1 is reduced to U(V). We suspect that the oxo ligand
silylation alters the redox potential of the U6þ center, making the
1
The H NMR spectrum of complex 3 in CD2Cl2 exhibits a
broad resonance at 52.14 ppm, assignable to the Me3Si pro-
tons of the silyloxide ligands, while the proton at the 3-posi-
tion of the bipyridine ring is observed as a broad singlet
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Figure 4. Molecular structure of U(OSiMe3)2I2(bipy)2 (3 CH2Cl2)
3
with 50% probability ellipsoids. CH2Cl2 and hydrogen atoms omitted
for clarity. Selected bond lengths (Å) and angles (deg): U1ꢀO1 =
2.084(4), U1ꢀI1 = 3.2435(8), O1ꢀSi1 = 1.639(4), O1ꢀU1ꢀO1* =
115.5(2), I1ꢀU1ꢀI1* = 98.72(3), U1ꢀO1ꢀSi1 = 165.4(3).
Figure 5. Molecular structure of [U(OSiMe3)2I(THF)4][I3] (5) with
50% probability ellipsoids. Hydrogen atoms omitted for clarity. Selected
bond lengths (Å) and angles (deg): U1ꢀO1 = 2.065(6), U1ꢀO2 =
2.080(6), O1ꢀSi1 = 1.670(6), O2ꢀSi2 = 1.659(6), U1ꢀI1 =
3.1445(13), U1ꢀO6 = 2.472(7), O1ꢀU1ꢀO2 = 173.8(3), U1ꢀ
O1ꢀSi1 = 170.0(4), O1ꢀU1ꢀO6 = 90.4(2), O1ꢀU1ꢀI1 = 93.95(18).
at ꢀ28.68 ppm (Supporting Information, Figure S17). The
remaining protons of the bipyridine moiety are observed
at ꢀ19.63 ppm, ꢀ3.01 ppm, and ꢀ2.75 ppm. Not surprisingly,
iodide ligand occupy the equatorial plane. The UꢀO(silyloxide)
bond distances (U1ꢀO1 = 2.065(6) Å and U1ꢀO2 = 2.080(6) Å)
are nearly identical to those observed in 3. In addition, the
metrical parameters of the triiodide anion, I3ꢀ, are similar to
previously reported values.37 Most importantly, the presence of
the I3ꢀ anion in 5 indicates that I2 is being formed upon addition
of THF, and confirms that iodide can reduce U(V) to U(IV)
upon addition of Lewis bases to complex 2.
1
the H NMR spectrum of 4 in CD2Cl2 (Supporting Informa-
tion, Figure S18) is similar to that of 3, while the NIR spectra
of complexes 3 and 4 are consistent with other U(IV) com-
plexes.32ꢀ34 Finally, the effective magnetic moment of 3 (2.7 μB
at 295 K),22 is greater than that observed for 2, and suggestive of a
5f2 electronic configuration.23,35,36
A 1H NMR spectrum (in CD2Cl2) of the material isolated by
crystallization reveals a broad singlet at 58.10 ppm, which is
similar to one of the resonances observed in the in situ experi-
ment. We have assigned this resonance to the silyloxide groups of
complex 5. As was observed in the in situ experiment, the
material is thermally unstable, and upon standing the resonance
at 58.10 ppm decreases in intensity, while the resonance assign-
able to Me3SiOSiMe3 increases. Clearly the decomposition of
5 is driven by formation of hexamethyldisiloxane, presumably
formed by scrambling of the Me3Siþ groups. It is also possible
that its relatively high sensitivity may be due to the ability of
uranium(IV) iodide complexes to ring-open THF.38 Unfortu-
nately, further attempts to characterize complex 5 were compli-
cated by its thermal instability and its similar solubility with
[Ph3PI][I]. Consequently, we could not determine a yield.
However, owing to the fact that only 0.5 equiv of I2 is generated
in the reaction, the maximum yield of 5 is 50%.
The reactivity of complex 2 with monodentate Lewis bases
was also explored. For instance, addition of 30 equiv of THF to a
CD2Cl2 solution of 2 results in the immediate formation of a
bright orange solution. This new material exhibits a singlet in its
1H NMR spectrum at 63.2 ppm, assignable to the methyl groups
of the silyloxide ligand. The chemical shift of this resonance
suggests reduction of the uranium center to U(IV), as was
observed for complexes 3 and 4. Also present in these samples
are small amounts of Me3SiOSiMe3, as revealed by the presence
of a resonance at 0.12 ppm. This peak was confirmed to be
Me3SiOSiMe3 by comparison with an authentic sample. On
standing at room temperature, the signal at 63.2 ppm decreases in
intensity, while the Me3SiOSiMe3 signal increases. New signals at
1
58.5 ppm and 53.4 ppm also begin to appear in the H NMR
spectrum. Similar results are observed upon dissolution of 2 into
THF-d8. However, under these conditions the disappearance of
the signal at 63.2 ppm and the appearance of Me3SiOSiMe3
occur at a much faster rate.
Crystallization of the solution formed upon dissolution of 2 in
THF, by layering with hexane, generates a mixture of orange
needles and yellow needles. The yellow needles were identified as
[Ph3PI][I] by comparison of the 31P and 1H NMR spectra with
an authentic sample, while the orange needles were identified as
[U(OSiMe3)2I(THF)4][I3] (5) by X-ray crystallography
(Scheme 2). Complex 5 crystallizes in the triclinic space group
P1 as a discrete anion/cation pair, and its solid-state molecular
structure is shown in Figure 5. The uranium center in complex 5
exhibits a pentagonal bipyramidal geometry in the solid-state.
The two silyloxide ligands occupy the axial coordination sites
(O1ꢀU1ꢀO2 = 173.8(3)°), while four THF molecules and one
’ SUMMARY
Reaction of UO2(tBuacnac)2 with excess Me3SiI, in the pre-
sence of Ph3P, results in reductive silylation of the uranyl moiety
to afford the U(V) bis-silyloxide complex, [Ph3PI][U(OSiMe3)2I4].
Addition of Lewis bases to [Ph3PI][U(OSiMe3)2I4] induces a
second one-electron reduction and formation of a U(IV) bis-
silyloxide complex. Normally the 6þ uranyl ion is a challenge to
reduce to U(V) or U(IV);39 however, our results reveals that a
rich redox chemistry can be accessed by silylation of the uranyl
moiety. This is because the oxo-derived silyloxide ligands are not
as adept as oxo ligands at stabilizing a 6þ or 5þ charge on
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uranium, an observation which is further highlighted by the fact
that the reductant in the process, Iꢀ, is a relatively mild reducing
agent. In addition, the formation of U(OSiMe3)2I2(bipy)2 from
uranyl parallels the synthesis of UX4(MeCN)4 (X = Cl, Br, I),
reported by Ephritikhine and co-workers,3 and it is likely that
[U(OSiMe3)2I4]ꢀ and U(OSiMe3)2I2(bipy)2 are analogous to
the intermediates generated along the pathway to UX4(MeCN)4.
Ultimately, the observed redox chemistry reveals the importance
of functionalizing the uranyl oxo ligands to access the 4þ state,
an observation which may have implications for the bioremedia-
tion of uranyl in the environment or the separation of uranyl in
spent nuclear waste.
7.23 (s, 2H, aryl CH), 8.09 (s, 2H, aryl CH), 12.32 (s, 1H, NH).
13C{1H} NMR (C6D6, 25 °C, 125 MHz): δ 32.1 (Me), 54.0 (CMe),
96.0 (γ-C), 128.0, 128.7, 128.8, 129.0, 129.1, 131.1, 138.8, 141.4, 166.8
(β-CN), 188.1 (β-CO). EIþ MS: m/z 279. UVꢀvis (CH2Cl2, 6.4 ꢁ
10ꢀ5 M): 355 nm (ε = 11600 L molꢀ1 cmꢀ1).
tBuNC(Ph)CHC(Ph)OSiMe3. To a Et2O solution (3 mL) of
tBuNHC(Ph)CHC(Ph)O (53 mg, 0.19 mmol) was added NaN-
(SiMe3)2 (35 mg, 0.19 mmol). The solution immediately turned pale
pink. Me3SiCl (50 μL, 0.39 mmol) was then added to the reaction
mixture, and the solution underwent a color change to yellow. After 2 h
of stirring, the solution was filtered through a Celite column (0.5 cm
ꢁ2.0 cm) supported on glass wool, and all volatiles were removed in vacuo
to provide a yellow oil. The product was extracted into hexanes (3 mL) and
refiltered through a Celite column (0.5 cm ꢁ2.0 cm) supported on glass
wool. The volatiles were removed in vacuo to give a yellow oil (54 mg, 81%
yield). 1H NMR (C6D6, 25 °C, 500 MHz): δ ꢀ0.16 (s, 9H, SiMe), 1.56 (s,
9H, Me), 6.16 (s, 1H, γ-CH), 6.92 ꢀ 7.42 (s, 6H, aryl CH), 7.56 (s, 2H,
arylCH), 8.12 (s, 2H, aryl CH). 13C{1H} NMR (C6D6, 25 °C, 125 MHz):
δ 1.0 (SiMe), 31.3 (Me), 35.5 (CMe), 107.3 (γ-C), 126.6, 129.0, 129.2,
129.3, 129.5, 139.5, 142.8, 152.6 (β-CN), 160.5 (β-CO). One aryl CH
resonance was not observed. EIþ MS: m/z 351.
’ EXPERIMENTAL SECTION
General Procedures. All reactions and subsequent manipulations
were performed under anaerobic and anhydrous conditions under either
high vacuum or an atmosphere of nitrogen or argon. THF, hexanes,
Et2O, and toluene were dried using a Vacuum Atmospheres DRI-SOLV
solvent purification system. CH2Cl2 and CD2Cl2 were dried over 3 Å
molecular sieves for 24 h before use. C6D6 was dried over activated 4 Å
molecular sieves for 24 h before use. UO2(N{SiMe3}2)2(THF)2 was
synthesized according to the previously reported procedure.40 UO2-
(Aracnac)2 (Ar = 3,5-tBu2C6H3) was also synthesized according to the
previously reported procedure.8 This complex exhibits a νsym(UdO) at
812 cmꢀ1 (see Supporting Information, Figure S7). All other reagents
were purchased from commercial suppliers and used as received.
NMR spectra were recorded on a Varian UNITY INOVA 400 or
Varian UNITY INOVA 500 spectrometer. 1H and 13C{1H} NMR
spectra were referenced to external SiMe4 using the residual protio
solvent peaks as internal standards (1H NMR experiments) or the
characteristic resonances of the solvent nuclei (13C NMR experiments).
31P{1H} NMR spectra were referenced to external 85% H3PO4. IR
spectra were recorded on a Mattson Genesis FTIR spectrometer. Raman
spectra were recorded on a Nicolet 6700 FTIR spectrometer with a NXR
FT Raman Module. UVꢀvis/NIR experiments were performed on a
UV-3600 Shimadzu spectrophotometer. Magnetic moments were de-
termined using the Evans’ method.22 Elemental analyses were per-
formed by the Micro-Mass Facility at the University of California,
Berkeley.
UO2(tBuacnac)2 (1). To a Et2O solution (3 mL) of BuNHC-
t
(Ph)=CHC(Ph)O (80 mg, 0.287 mmol) was added UO2(N-
{SiMe3}2)2(THF)2 (104 mg, 0.141 mmol). The solution immediately
turned orange, concomitant with the deposition of orange solid. After 24
h of stirring, the supernatant was decanted off, and the orange powder
was washed with Et2O (2 ꢁ 5 mL) and dried in vacuo (78 mg, 67%
yield). Analytically pure material was grown from a solution of dichlor-
omethane and hexane. Anal. Calcd for UO4N2C38H40 2CH2Cl2: C,
3
1
48.21; H, 4.45; N, 2.81 Found: C, 49.16; H, 4.34; N, 3.05. H NMR
(C6D6, 25 °C, 500 MHz): δ 1.73 (s, 18H, Me), 5.81 (s, 2H, γ-CH), 6.99
(br s, 6H, aryl CH), 7.11 ꢀ 7.18 (br s, 4H, aryl CH), 7.20 (br s, 6H, aryl
CH), 8.17 (br s, 4H, aryl CH). 13C{1H} NMR (CD2Cl2, 25 °C, 125
MHz): δ 33.6 (Me), 102.9 (γ-C), 127.6, 127.9, 128.4, 129.0, 130.6,
131.1, 139.7, 146.5, 171.8 (β-CN), 173.4 (β-CO). The (CMe) reso-
nance was not observed. UVꢀvis (CH2Cl2, 3.1 ꢁ 10ꢀ5 M): 343 nm (ε =
18900 L molꢀ1 cmꢀ1). IR (KBr pellet, cmꢀ1): 1608(m), 1588(s),
1562(s), 1481(s), 1459(s), 1396(w), 1383(m), 1367(m), 1341(s),
1303(w), 1276(m), 1241(w), 1227(m), 1197(s), 1135(w), 1077(w),
1059(m), 1027(m), 1003(w), 973(w), 907(s, νasym(UdO)), 857(w),
815(w), 758(s), 705(s), 694(s), 626(w), 594(w), 554(m), 532(m),
502(m). Raman (cmꢀ1): 823 (s, νsym(UdO)).
[Ph3PI][U(OSiMe3)2I4] (2). To an orange suspension of 1 (135 mg,
0.163 mmol) in toluene (3 mL) was added Me3SiI (240 μL, 1.90 mmol).
The solution immediately turned deep red. After stirring for 60 min, the
solution was filtered through a Celite column (0.5 cm ꢁ2.0 cm)
supported on glass wool. Triphenylphosphine (44 mg, 0.17 mmol)
was then added to the reaction mixture. After 15 min of stirring the
deposition of black microcrystalline material was observed. The solid
was isolated and extracted into dichloromethane (3 mL). Storage of
this solution at ꢀ25 °C for 24 h resulted in the deposition of black
crystals suitable for X-ray analysis (85 mg, 40% yield). The reaction
can also be performed in the presence of 0.5 equiv of Ph3P; however,
the isolated yield is slightly reduced (34%). Anal. Calcd for
UO2C24H33Si2P1I5: C, 21.95; H, 2.53; N, 0.00. Found: C, 22.43;
H, 2.20; N, <0.2. 1H NMR (CD2Cl2, 25 °C, 500 MHz): δ 2.17
(s, 18H, Me), 4.03 (s, 18H, Me), 7.38 (q, 6H, JHH = 7.7 Hz, ortho
CH), 7.61 (br s, 6H, meta CH), 7.77 (t, 3H, JHH = 7.1 Hz, para CH).
31P{1H} NMR: (CD2Cl2, 25 °C, 202 MHz): δ 11.6 (br s). UVꢀ
vis/NIR (CH2Cl2, 4.5 ꢁ 10ꢀ3 M): 1102 nm (ε = 10 L molꢀ1
cmꢀ1), 1478 nm (ε=6.6 Lmolꢀ1 cmꢀ1), 1570 nm (ε=11Lmolꢀ1 cmꢀ1),
1664 nm (ε = 4.0 L molꢀ1 cmꢀ1). IR (KBr pellet, cmꢀ1): 1480(w),
1437(m), 1367(w), 1351(w), 1252(s), 1164(m), 1125(s), 1102(s),
1067(s), 997(m), 924(m), 851(s), 812(s), 756(s), 725(s), 689(s),
Cyclic Voltammetry Measurements. CV experiments were
performed with a CH Instruments 600c Potentiostat, and the data were
processed using CHI software (version 6.29). All experiments were
performed in a glovebox using a 20 mL glass vial as the cell. The working
electrode consisted of a platinum disk embedded in glass (2 mm
diameter), the counter electrode was a platinum wire, and the reference
electrode consisted of AgCl plated on Ag wire. Solutions employed
during CV studies were typically 1 mM in the metal complex and 0.1 M
in [Bu4N][PF6]. All potentials are reported versus the [Cp2Fe]0/þ
couple. For all trials, ip,a/ip,c = 1 for the [Cp2Fe]0/þ couple, while ip,c
√
increased linearly with the square root of the scan rate (i.e., v). Redox
couples which exhibited behavior similar to the [Cp2Fe]0/þ couple were
thus considered reversible.
tBuNHC(Ph)=CHC(Ph)O. To a yellow solution of diphenylpropy-
none (1.0972 g, 0.0053 mol) in toluene (20 mL) was added tert-
butylamine (6.0 mL, 0.057 mol) and Zn(OTf)2 (200 mg, 0.55 mmol).
After heating to reflux for 48 h, the volatiles were removed in vacuo to
provide a pale yellow solid. This material was extracted into Et2O
(20 mL) and filtered through a Celite column (0.5 cm ꢁ2.0 cm)
supported on glass wool. The volatiles were removed in vacuo to give a
pale yellow solid which was washed with hexane (3 ꢁ 5 mL). 1.2833 g,
86% yield. 1H NMR (C6D6, 25 °C, 500 MHz): δ 1.00 (s, 9H, Me), 5.82
(s, 1H, γ-CH), 7.07 (br s, 3H, aryl CH), 7.10 ꢀ 7.20 (m, 3H, aryl CH),
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ARTICLE
543(w), 522(w), 501(w), 465(w). Evans method (CD2Cl2, 0.028 M,
22 °C): μeff = 1.4 μB.
powder determined to be [Ph3PI][I] by comparison with the authentic
material (10 mg, 63% yield).
U(OSiMe3)2I2(bipy)2 (3). To a black suspension of 2 (56 mg,
0.043 mmol) in toluene (3 mL) was added bipyridine (14 mg,
0.090 mmol) in toluene (1 mL). Over 40 min, the solution undergoes
a color change to orange, concomitant with the deposition of yellow
solid. After 40 min, the solution was filtered through a Celite column
(0.5 cm ꢁ2.0 cm) supported on glass wool. The supernatant was
concentrated in vacuo to 2 mL. Storage of this solution at ꢀ25 °C for
24 h resulted in the deposition of orange microcrystalline powder. The
product was isolated by decanting off the supernatant and drying in
vacuo (36 mg, 85% yield). Crystals suitable for X-ray analysis were
grown from a solution of dichloromethane and hexane (1:4 v/v). Anal.
Calcd for UO2C26H34N4Si2I2: C, 31.78; H, 3.49; N, 5.70. Found: C,
32.79; H, 3.78; N, 4.98. 1H NMR (CD2Cl2, 25 °C, 500 MHz): δ ꢀ28.68
(s, 4H, aryl CH), ꢀ19.63 (s, 4H, aryl CH), ꢀ3.01 (s, 4H, aryl CH), ꢀ2.75
(s, 4H, aryl CH), 52.14 (s, 18H, Me). UVꢀvis/NIR (CH2Cl2, 4.9 ꢁ 10ꢀ3
M): 690 nm (ε = 18 L molꢀ1 cmꢀ1), 736 nm (ε = 19 L molꢀ1 cmꢀ1),
790 nm (ε = 21 L molꢀ1 cmꢀ1), 928 nm (ε = 17 L molꢀ1 cmꢀ1), 1004 nm
(ε =15L molꢀ1 cmꢀ1), 1070 nm (ε =47L molꢀ1 cmꢀ1), 1156 nm (ε = 35
L molꢀ1 cmꢀ1), 1334 nm (ε = 48 L molꢀ1 cmꢀ1), 1388 nm (ε = 25
L molꢀ1 cmꢀ1), 1452 nm (ε = 26 L molꢀ1 cmꢀ1), 1670 nm (ε = 6.4
L molꢀ1 cmꢀ1). IR (KBr pellet, cmꢀ1): 1598(m), 1563(w), 1526(w),
1510(w), 1495(w), 1475(w), 1437(s), 1316(m), 1244(s), 1174(w),
1156(m), 1103(m), 1064(m), 1045(w), 1012(s), 928(s), 877(s), 864(s),
834(s), 763(s), 736(s), 695(m), 644(m), 627(w), 468(m), 419(m). Evans
method (CD2Cl2, 0.010 M, 22 °C): μeff = 2.7 μB.
[U(OSiMe3)2I(THF)4][I3] (5). Complex 2 (36 mg, 0.027 mmol) was
dissolved in THF (2 mL), immediately forming an orange solution. After
5 min the solution was filtered through a Celite column (0.5 cm
ꢁ2.0 cm) supported on glass wool and layered with hexane (4 mL).
Storage of this solution at ꢀ25 °C for 24 h resulted in the cocrystalli-
zation of yellow needles and orange needles. The total mass of material
collected was 25.8 mg. The yellow needles were determined to be
[Ph3PI][I] by comparison with the spectral properties of the indepen-
dently prepared material.17 The orange needles were determined to be
[U(OSiMe3)2I(THF)4][I3] (5) by X-ray crystallography. Because of
similar solubility properties of [Ph3PI][I] and complex 5, isolation of
pure samples of 5 proved unsuccessful. As such, characterization was
performed on the mixture of [Ph3PI][I] and complex 5. 1H NMR
(CD2Cl2, 25 °C, 500 MHz): δ 7.40 (br m, 6H, aryl CH), 7.59 (br m, 6H,
aryl CH), 7.74 (br m, 3H, aryl CH), 58.10 (s, 18H, Me). 31P{1H} NMR
(CD2Cl2, 25 °C, 202 MHz): δ ꢀ1.11 (br s). 31P{1H} NMR (THF-h8,
25 °C, 202 MHz): δ ꢀ20.8 (br s). IR (KBr pellet, cmꢀ1): 1591(w),
1562(w), 1544(w), 1528(w), 1510(w), 1500(w), 1477(w), 1460(w),
1437(m), 1336(w), 1313(w), 1253(m), 1183(w), 1165(w), 1122(s),
1101(m), 1053(s), 1039(s), 1015(s), 996(s), 926(w), 877(s), 844(s),
748(m), 727(s), 688(s), 618(w), 608(w), 540(s), 518(w), 499(s),
445(w), 421(w).
X-ray Crystallography. Data for 1 2CH2Cl2, 2, 3 CH2Cl2, and 5
3
3
were collected on a Bruker 3-axis platform diffractometer equipped with
a SMART-1000 CCD detector using a graphite monochromator with a
Mo KR X-ray source (R = 0.71073 Å). The crystals were mounted on a
glass fiber under Paratone-N oil, and all data were collected at 150(2) K
using an Oxford nitrogen gas cryostream system. A hemisphere of data
was collected using ω scans with 0.3° frame widths. Frame exposures of
25, 10, 10, and 35 s were used for complexes 1 2CH2Cl2, 2, 3 CH2Cl2,
The yellow solid produced during the reaction was extracted into
dichloromethane (2 mL) and layered with hexanes. Storage of this
solution at ꢀ25 °C for 24 h resulted in the deposition of yellow needles
(13 mg, 60% yield). These were determined to be [Ph3PI][I] by
1
3
3
comparison with the authentic material. H NMR (CD2Cl2, 25 °C,
and 5, respectively. Data collection and cell parameter determination
was conducted using the SMART program.41 Integration of the data
frames and final cell parameter refinement was performed using the
SAINT software.42 Absorption correction of the data was carried out
empirically based on reflection ψ-scans. Subsequent calculations were
carried out using SHELXTL.43 Structure determination was done using
direct methods and difference Fourier techniques. All hydrogen atom
positions were idealized, and rode on the atom of attachment. Structure
solution, refinement, graphics, and creation of publication materials
were performed using SHELXTL.43 Complex 3 contains a disordered
molecule of CH2Cl2, which was modeled over two positions in a 50:50
ratio. In addition, complex 5 contains two disordered THF ligands, each
of which were modeled over two positions, in a 50:50 ratio. Finally, one
silyloxide ligand in 5 was found to be disordered between two positions
about the silicon atom, also in a 50:50 ratio. Idealized hydrogen atoms
were not assigned to the disordered carbon atoms. A summary of
relevant crystallographic data for 1 2CH2Cl2, 2, 3 CH2Cl2, and 5 is
500 MHz): δ 7.60 (br m, 12H, aryl CH), 7.71 (br s, 3H, aryl CH, JHH
=
6.4 Hz). 31P{1H} NMR: (CD2Cl2, 25 °C, 202 MHz): δ ꢀ19.6 (br s).
U(OSiMe3)2I2(phen)2 (4). To a black suspension of 2 (40 mg,
0.031 mmol) in toluene (3 mL) was added phenanthroline (11 mg,
0.061 mmol) in toluene (1 mL). After 10 min the solution undergoes a
color change to orange concomitant with the deposition of yellow solid.
The solution was filtered through a Celite column (0.5 cm ꢁ2.0 cm)
supported on glass wool. The volatiles were removed in vacuo leaving a
dark red-orange oil. This material was extracted into dichloromethane
(2 mL) and filtered through a Celite column (0.5 cm ꢁ2.0 cm)
supported on glass wool. The supernatant was then layered with diethyl
ether (2 mL). Subsequent storage of this solution at ꢀ25 °C for 24 h
resulted in the deposition of orange microcrystalline solid. The product
was isolated by decanting off the supernatant and drying in vacuo
(18 mg, 56% yield). Anal. Calcd for UO2C30H34N4Si2I2: C, 34.96; H,
1
3.33; N, 5.43. Found: C, 35.02; H, 3.35; N, 4.99. H NMR (CD2Cl2,
3
3
25 °C, 500 MHz): δ ꢀ6.20 (s, 4H, aryl CH), ꢀ1.58 (s, 4H, aryl CH),
0.75 (s, 4H, aryl CH), 45.64 (s, 18H, Me). One of the aryl resonances
was not observed. UVꢀvis/NIR (CH2Cl2, 3.7 ꢁ 10ꢀ3 M): 682 nm
(ε = 32 L molꢀ1 cmꢀ1), 800 nm (ε = 27 L molꢀ1 cmꢀ1), 920 nm (ε =
31 L molꢀ1 cmꢀꢀ11), 1006 nm (ε = 34 L molꢀꢀ11 cmꢀ1), 1068 nm (ε =
46 L molꢀ1 cm ), 1180 nm (ε = 52 L mol cmꢀ1), 1334 nm (ε =
56 L molꢀ1 cmꢀ1), 1376 nm (ε = 45 L molꢀ1 cmꢀ1), 1666 nm
(ε = 17 L molꢀ1 cmꢀ1). IR (KBr pellet, cmꢀ1): 1590(w), 1517(w),
1543(w), 1519(m), 1498(w), 1476(w), 1473(w), 1460(w), 1453(w),
1423(m), 1348(w), 1246(m), 1223(w), 1143(m), 1103(m), 997(w),
930(s), 865(s), 846(s), 777(w), 752(m), 727(s), 690(w), 637(w),
543(w), 579(w), 500(m), 418(m).
presented in Table 1.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic details (as
b
CIF files) for 1 2CH2Cl2, 2, 3 CH2Cl2, and 5, tabulated cyclic
voltammetry data for 1 2CH2Cl2, and spectral data 1 2CH2Cl2,
2, 3 CH2Cl2, 4, and 5. This material is available free of charge via
the Internet at http://pubs.acs.org.
3
3
3
3
3
’ AUTHOR INFORMATION
The yellow solid produced during the reaction was extracted into
dichloromethane (2 mL) and layered with hexane (2 mL). Storage of
this solution at ꢀ25 °C for 24 h resulted in the deposition of dull yellow
Corresponding Author
*E-mail: hayton@chem.ucsb.edu.
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ARTICLE
’ ACKNOWLEDGMENT
(31) Karmazin, L.; Mazzanti, M.; Bezombes, J.-P.; Gateau, C.;
Pecaut, J. Inorg. Chem. 2004, 43, 5147–5158.
(32) Monreal, M. J.; Diaconescu, P. L. Organometallics 2008,
27, 1702–1706.
(33) Cohen, D.; Carnall, W. T. J. Phys. Chem. 1960, 64, 1933–1936.
(34) Morss, L. R.; Edelstein, N. M.; Fuger, J.; Katz, J. J. The Chemistry
of the Actinide and Transactinide Elements; Springer: Dordrecht, The
Netherlands, 2006.
We thank the University of California, Santa Barbara, and the
Department of Energy (BES Heavy Element Program) for
financial support of this work.
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