Silylation of the uranyl ion using B(C6F5) 3-activated Et3SiH

Article abstract of DOI:10.1021/ic201385h

Addition of 2 equiv of HSiEt₃ to UO₂( ^Aracnac)₂ (^Aracnac = ArNC(Ph)CHC(Ph)O, Ar = 3,5-^tBu₂C₆H₃) in the presence of 1 equiv of B(C₆F₅)<

Full text of DOI:10.1021/ic201385h

ARTICLE
pubs.acs.org/IC
Silylation of the Uranyl Ion Using B(C6F5)3-Activated Et3SiH
David D. Schnaars, 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: Addition of 2 equiv of HSiEt3 to UO2(Aracnac)2
(Aracnac = ArNC(Ph)CHC(Ph)O, Ar = 3,5-tBu2C6H3) in the
presence of 1 equiv of B(C6F5)3 results in formation of the
U(V) bis(silyloxide) complex [U(OSiEt3)2(Aracnac)2][HB-
(C6F5)3] (1) in 80% yield. Also produced in the reaction, as a
minor product, is U(OSiEt3)(OB{C6F5}3)(Aracnac)2 (2). Inter-
estingly, thermolysis of 1 at 85 ꢀC for 24 h also results in formation
of 2, concomitant with production of Et3SiH. Addition of 1 equiv
of Cp2Co to 1 results in formation of U(OSiEt3)2(Aracnac)2 (3)
and [Cp2Co][HB(C6F5)3] (4), which can be isolated in 61% and
71% yields, respectively. Complexes 1À3 have been characterized by X-ray crystallography, while the solution-phase redox properties of 1
have been measured with cyclic voltammetry.
’ INTRODUCTION
by reductive silylation of UO2(Aracnac)2 (Aracnac = ArNC(Ph)-
CHC(Ph)O, Ar = 3,5-tBu2C6H3) using a combination of HSiPh3
and B(C6F5)3.23 We argued that the reaction proceeded by forma-
Oxo ligand functionalization of the uranyl ion (UO22+) has
proven a challenge,1 due in part to the strength of the oxygenÀ
uranium interaction within the UO22+ fragment. For example, the
mean UÀO bond dissociation energy of uranyl (604 kJ/mol)2 is
72 kJ/mol larger than that observed for CO2 (532 kJ/mol).3 The
challenge of perturbing the uranyl moiety is well illustrated by
reaction of UO3 with neat triflic acid at 110 ꢀC.4,5 Under these
conditions, only one oxo group in UO3 is protonated, providing
UO2(OTf)2 and H2O. Even in this extremely acidic environment
the uranyl moiety remains intact, and no evidence for protonation of
the remaining oxo ligands is observed.
tion of a boraneÀsilane adduct, [Ph3SiH B(C6F5)3].29À31 The
3 3 3
activated silane was then attacked by the uranyl oxo, resulting in
SiÀO bond formation and eventual generation of UV(OSiPh3)-
(OB{C6F5}3)(Aracnac)2 and 0.5 equiv of H2. While B(C6F5)3-
mediated hydrosilylation is known for a variety of organic sub-
strates,29,32À36 to our knowledge this was the first reported example
of borane-mediated silylation of a metal oxo complex. Herein, we
report a second example of B(C6F5)3-mediated silylation of the
uranyl oxo fragment, concomitant with reduction to U(V), and
provide further insight into the mechanism of this transformation.
Despite these difficulties and motivated by the importance of
uranyl in the nuclear fuel cycle,6À11 interest in discovering chemical
strategies for functionalizing this extremely stable fragment has
grown in recent years. For example, uranyl oxo ligand metalation can
be achieved by reaction of UO2(THF)(H2L) (L = ‘Pacman’
polypyrrolic macrocycle) with a variety of lithium reagents12 or
rare earth silylamides.13 Likewise, Mazzanti and co-workers ob-
served oxo metalation in a series of uranyl(V) β-ketonate and Schiff
base complexes.14À20 In addition, several laboratories have been
investigating uranyl oxo ligand functionalization via reductive
silylation.21À28 This transformation was first observed by Ephriti-
khine and co-workers through the reaction of excess Me3SiX (X =
Cl, Br, I) with UO2I2(THF)3 in MeCN. This results in formation of
the U(IV) halide complexes UX4(MeCN)4 in good yields.24
Although not observed in the reaction mixture, the oxygen atoms
of the uranyl fragment are likely converted into Me3SiOSiMe3.22
More recently, the Arnold group demonstrated that deprotona-
tion of UO2(THF)(H2L), followed by addition of silylamine, re-
sulted in generation of a U(V) silyloxide [UO(OSiMe3)(THF)-
Fe2I2L].25À28 In a similar example recently reported by our group,23
the synthesis of U(OSiPh3)(OB{C6F5}3)(Aracnac)2 was achieved
’ RESULTS AND DISCUSSION
To expand the scope of borane-mediated silylation of uranyl,23
we explored the reactivity of the UO2(Aracnac)2/B(C6F5)3 system
with HSiEt3. Thus, addition of 2 equiv of HSiEt3 to UO2(Aracnac)2,
in the presence of B(C6F5)3, results in formation of a deep red
solution, from which [U(OSiEt3)2(Aracnac)2][HB(C6F5)3]
(1) can be isolated as a dark red crystalline material in 80% yield
(Scheme 1).
Complex 1 crystallizes in the triclinic space group P1 as a
discrete cation/anion pair (Figure 1). There are two crystallogra-
phically unique [U(OSiEt3)2(Aracnac)2]+ cations within the asym-
metric unit, each sitting on a special position. The metrical
parameters of both cations are similar, and only one will be discussed
in detail. The uranium center in 1 is coordinated by two triethylsilyl-
oxide ligands oriented is trans arrangement, while two Aracnac
ligands occupy the equatorial plane. The UÀOSi and OÀSi bond
Received: June 28, 2011
Published: August 15, 2011
r
2011 American Chemical Society
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Scheme 1
Figure 1. Solid-state structure of [U(OSiEt3)2(Aracnac)2][HB(C6F5)3] (1) with 50% probability ellipsoids for the noncarbon atoms. The phenyl rings
on the Aracnac backbone have been removed for clarity.
lengths are 2.011(4) and 1.678(4) Å, respectively, while the
UÀN and UÀOacnac bond lengths are 2.383(5) and 2.171(4) Å,
respectively (Table 1). Overall, these metrical parameters are
similar to those of the previously characterized U(V) silyloxide
U(OSiPh3)(OB{C6F5}3)(Aracnac)2.23 The metrical parameters of
the [HB(C6F5)3]À anion are also similar to those observed pre-
viously.37,38 However, it should be noted the [HB(C6F5)3]À moiety
in 1 was found to be disordered over two positions in a 65:35 ratio.
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The room-temperature 1H NMR spectrum of 1 in CD2Cl2 is
consistent with the proposed formulation and, as observed for
U(OSiPh3)(OB{C6F5}3)(Aracnac)2,23 reveals the presence of
major and minor isomers in solution in a 9:1 ratio. For example, a
À103.46 ppm in a 2:1:2 ratio, respectively, corresponding to the
ortho, para, and meta environments of the [HB(C6F5)3]À
anion.35,37À40 The solid state IR spectrum of 1 features a broad
νBH absorption at 2377 cmÀ1, further supporting the presence of
[HB(C6F5)3]À anion.41À43 Finally, the NIR spectrum of 1 is similar
to that observed for other U(V) complexes, corroborating the
presence of a 5f1 ion (Figure 2).21,23,44À46
t
resonance at À0.05 ppm is attributable to the Bu substituents
of the major isomer, while the corresponding resonances for
the minor isomer are observed as two overlapping signals at
À0.40 ppm. These latter signals separate upon cooling to 12 ꢀC,
where they are observed at À0.42 and À0.48 ppm. The observation
of two tBu environments for the minor isomer of 1 can be explained
by invoking a cis configuration of the two silyloxide groups and
assuming that rotation about the NÀCipso bond of the Aracnac
ligand is hindered (eq 1). In addition, the 19F spectrum of 1
consists of three sharp resonances at À69.80, À100.63, and
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 1À3
1
2
3
We also monitored the in situ reaction of UO2(Aracnac)2 with
1 equiv of B(C6F5)3 and 2 equiv of HSiEt3 in C6D6 over the
UÀOSi
2.011(4)
2.017(6)
1.957(6)
2.165(7)
2.198(6)
2.440(7)
2.446(7)
1.664(7)
1.525(12)
175.0(3)
168.0(4)
166.6(6)
2.129(2)
UÀOB
1
course of 21 h using H and 19F NMR spectroscopies. Under
UÀOacnac
2.171(4)
2.383(5)
1.678(4)
2.268(2)
2.484(3)
1.628(2)
these conditions, complex 1 is the major product. However, a
minor side product is also observed in the reaction mixture,
which we identified as U(OSiEt3)(OB{C6F5}3)(Aracnac)2 (2),
the triethylsilyl analogue of the previously reported U(OSiPh3)-
(OB{C6F5}3)(Aracnac)2 (Scheme 1).23 Complexes 1 and 2 are
observed in a 4:1 ratio, respectively. Complex 2 can be identified
by its characteristic CH2CH3 resonance at 3.41 ppm, which is
downfield from the corresponding resonance in 1 (Figures
UÀN
OÀSi
OÀB
—OÀUÀO
—UÀOÀSi
—UÀOÀB
180
180
161.5(3)
158.96(15)
1
S17ÀS18, Supporting Information). The H NMR spectrum
Figure 2. UVÀvis/NIR spectrum of 1 (CH2Cl2, 4.34 Â 10À3 M, orange line, right axis) and 3 (CH2Cl2, 3.13 Â 10À3 M, blue line, left axis). Solvent
absorption has been removed for clarity.
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To gather further insight into formation of 1, we monitored
the reaction between UO2(Aracnac)2, 2 equiv of DSiEt3, and
B(C6F5)3 in C6H6 by 2H NMR spectroscopy. As anticipated, the
2H NMR spectrum of the reaction mixture reveals the presence
of [DB(C6F5)3]À, which appears as a broad singlet at 4.64 ppm.40
This observation is consistent with hydride transfer from silane to
B(C6F5)3 via the [Et3SiH B(C6F5)3] adduct during the course
3 3 3
of the reaction. Also observed in the 2H NMR spectrum is a singlet
at 3.90 ppm, assignable to unreacted DSiEt3, and a resonance at 3.60
ppm, which could not be assigned (Figure S20, Supporting
Information). We do not observe a resonance attributable to D2
in this spectrum, but this could be due to overlap with the broad
[DB(C6F5)3]À signal.
Generation of both 1 and 2 during the reductive silylation of
UO2(Aracnac)2 suggests the presence of a common intermediate
in their formation, namely, UVO(OSiEt3)(Aracnac)2. The un-
functionalized oxo ligand of this proposed U(V) intermediate
could attack either [Et3SiH B(C6F5)3] or B(C6F5)3 to pro-
3 3 3
vide the observed products 1 or 2, respectively. The observed 4:1
ratio of complex 1 to 2 suggests that silylation of UO(OSiEt3)-
(Aracnac)2 with [Et3SiH B(C6F5)3] outcompetes coordina-
Figure 3. Solid-state structure of U(OSiEt3)(OB{C6F5}3)(Aracnac)2
(2) with 50% probability ellipsoids for the noncarbon atoms. The phenyl
rings on the Aracnac backbone have been removed for clarity.
3 3 3
tion by free B(C6F5)3. In contrast, when Ph3SiH is used in place
of Et3SiH, only one product is observed, namely, U(OSiPh3)-
(OB{C6F5}3)(Aracnac)2.23 To account for the effect of the silane
substituents on the reaction outcome, silylation of UVO(OSiR3)-
of this sample also reveals formation of H2, observed as a sharp
singlet at 4.46 ppm.47 Formation of H2 was also observed in the
synthesis of U(OSiPh3)(OB{C6F5}3)(Aracnac)2, where Ph3SiH
was ultimately determined to be the source of the H2.23 We also
monitored the reaction of UO2(Aracnac)2 with only 1 equiv of
HSiEt3 in the presence of 1 equiv of B(C6F5)3. Under these
conditions, complex 1 is still the dominant product but signifi-
cant amounts of unreacted UO2(Aracnac)2 remain in the reaction
mixture (Figures S23 and 24, Supporting Information).
(Aracnac)2 by [Ph3SiH B(C6F5)3] must be slower than
3 3 3
silylation by [Et3SiH B(C6F5)3], a hypothesis that fits with
3 3 3
the previously measured rate constants of hydride transfer from
Et3SiH and Ph3SiH.48,49 It should also be noted that complex 1 is
not likely the preferred thermodynamic product, as evidenced by
the conversion of 1 to 2 upon thermolysis. That said, direct
conversion of 1 to 2 is probably not an important pathway during
the room-temperature reaction of UO2(Aracnac)2 with Et3SiH
and B(C6F5)3, as complex 2 could only be formed directly
from 1 at elevated temperatures. Finally, addition of HSiEt3 and
B(C6F5)3 to 2 at room temperature does not lead to any reaction
(Figures S21 and S22, Supporting Information). As a result, the
reverse reaction (conversion of 2 into 1) can also be ruled out.
We also measured the solution-phase redox properties of 1 by
cyclic voltammetry. The resulting cyclic voltammogram reveals a
quasi-reversible redox feature at À0.37 V (vs Fc/Fc+), which we
attribute to the U(V)/U(IV) redox couple (Figure S30, Supporting
Information). This potential is 0.35 V less negative than that
Complex 2 can be made on a preparative scale by heating
equimolar amounts of UO2(Aracnac)2, HSiEt3, and B(C6F5)3 in
toluene at 85 ꢀC for 5 h. Removal of toluene in vacuo, followed by a
hexane extraction and storage at À25 ꢀC for 5 days, results in
deposition of a dark amber microcrystalline material in low yield.
Surprisingly, thermolysis of 1 in toluene-d8 at 85 ꢀC for 24 h also
results in formation of 2, concomitant with production of Et3SiH
(Scheme 1), demonstrating that 2 can be formed directly from 1
(Figures S14ÀS15, Supporting Information). However, conversion
to 2 under these conditions is not clean, and several unidentified
decomposition products are also observed in the 1H and 19F NMR
spectra. We also thermolyzed complex 1 at 50 ꢀC in toluene-d8;
however, very little conversion to 2 was observed at this temperature.
Complex 2 crystallizes in the triclinic space group P1 (Figure 3).
Its UÀOSi and UÀOB bond lengths are 2.017(6) and 1.957(6) Å,
respectively (Table 1). These parameters are identical to those
exhibited by U(OSiPh3)(OB{C6F5}3)(Aracnac)2.23 In addition, the
UÀOacnac bond lengths in 2 are 2.165(7) and 2.198(6) Å, while the
UÀN bond lengths are 2.440(7) and 2.446(7) Å. As observed for 1,
the 1Hand19FNMRspectrafor2reveal the presence of two isomers
in solution in a 5:1 ratio. For example, the 19F NMR spect-
rum in CD2Cl2 reveals resonances for the ortho, para, and
meta fluorines of the major isomer at À78.07, À97.28, and
À101.41 ppm, respectively, while the resonances for the para
and meta fluorines of the minor isomer are observed at À97.80
and À101.98 ppm. The ortho resonance ofthe minor isomer is not
observed, most likely due to overlap with the corresponding
resonance from the major isomer.
observed for U(OSiPh3)(OB{C6F5}3)(Aracnac)2 and 0.84 V
23
less negative than that seen for [Cp*2Co][U(OB{C6F5}3)2-
(Aracnac)2].46 This increase is consistent with positive charge present
in [U(OSiEt3)2(Aracnac)2]+, in contrast to the neutral charge of
U(OSiPh3)(OB{C6F5}3)(Aracnac)2 or the anionic charge of
[U(OB{C6F5}3)2(Aracnac)2]À.
Reduction of 1 with 1 equiv of Cp2Co in Et2O results in
precipitation of a yellow-green solid and formation of an orange-red
solution. Filtration of this solution, followed by crystallization from
Et2O/hexane, results in the deposition of U(OSiEt3)2(Aracnac)2 (3)
as an orange-brown crystalline solid in 61% yield (Scheme 1).
Dissolution of the yellow-green solid in THF, followed by removal
of the volatiles in vacuo, provides [Cp2Co][HB(C6F5)3] (4) as a
yellow-green powder in 71% yield.
The room-temperature 1H NMR spectrum of 3 in CD2Cl2 is
uninformative, but upon cooling to À60 ꢀC, the signals sharpen
considerably, revealing two sets of resonances that we attribute to
two isomers present in a 1.6:1 ratio. The minor isomer exhibits two
inequivalent tBu resonances located at À1.12 and À1.55 ppm, while
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’ EXPERIMENTAL SECTION
General. All reactions and subsequent manipulations were performed
under anaerobic and anhydrous conditions under an atmosphere of
nitrogen or argon. THF, hexanes, diethyl ether, and toluene were dried
using a Vacuum Atmospheres DRI-SOLV solvent purification system.
Toluene-d8, C6H6, and C6D6 were dried over activated 4 Å molecular
sieves for 24 h before use. CH2Cl2 and CD2Cl2 were dried over activated
3 Å molecular sieves for 24 h before use. UO2(Aracnac)2 and (Aracnac)H
(Ar = 3,5-tBu2C6H3) were synthesized according to previously reported
procedures.53 B(C6F5)3 was made by the published procedure, except that
pentafluoroiodobenzene was used in place of pentafluorobromobenzene.54
Cp2Co was recrystallized before use, while all other reagent were purchased
from commercial suppliers and used as received.
NMR spectra were recorded on a Varian UNITY INOVA 400 or
1
Varian UNITY INOVA 500 spectrometer. H NMR spectra were refer-
enced to external SiMe4 using the residual protio solvent peaks as internal
standards. 19F{1H} NMR spectra were referenced to external 0.05% R,R,R-
trifluorotoluene in C6D6. IR spectra were recorded on a Mattson Genesis
FTIR spectrometer, while UVÀvis/NIR experiments were performed on a
UV-3600 Shimadzu spectrophotometer. Elemental analyses were per-
formed by the Microanalytical Laboratory at UC Berkeley.
Figure 4. Solid-state structure of U(OSiEt3)2(Aracnac)2 (3) with 50%
probability ellipsoids for the noncarbon atoms. The phenyl rings on the
Aracnac backbone have been removed for clarity.
the major isomer exhibits a single tBu resonance at À4.48 ppm. In
addition, the NIR spectrum for 3 is consistent with reduction to
U(IV) and the presence of a 5f2 ion (Figure 2).50,51 The 1H NMR of
4 in CD2Cl2 exhibits a singlet at 5.66 ppm, assignable to the cyclo-
pentadienyl resonance of [Cp2Co]+, and a quartet at 3.64 ppm
(JBH = 91 Hz), corresponding to the [HB(C6F5)3]À resonance.
Complex 3 crystallizes in the monoclinic space group P21/n,
and its solid state molecular structure is shown in Figure 4. Complex 3
is isostructural with complex 1; however, there are important differ-
ences in their metrical parameters (Table 1). The UÀOSi bond
length in 3 (U1ÀO1 = 2.129(2) Å) is longer than that observed in 1
but is similar to the UÀO bonds in other U(IV) silyloxides,22 such as
[Cp2Co][U(OSiPh3)(OB{C6F5}3)(Aracnac)2] (2.173(8) Å)23 and
Cp3U(OSiPh3) (2.135(8) Å).52 Likewise, the UÀN (2.484(3) Å)
and UÀOacnac (2.268(2) Å) bond lengths in 3 are longer than those
observed in 1, consistent with the presence of the larger U4+ ion.
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), while platinum wire was used for both counter and reference
electrodes. Solutions employed during CV studies were typically 3 mM
in the uranium complex and 0.1 M in [Bu4N][PF6]. All potentials
are reported versus the [Cp2Fe]0/+ couple. For all trials, ip,a/ip,c = 1 for
the [Cp2Fe]0/+ couple, while ip,c increased linearly with the square root of
√
the scan rate (i.e., v).
[U(OSiEt3)2(Aracnac)2][HB(C6F5)3] (Ar = 3,5-tBu2C6H3) (1).
To a red dichloromethane (3 mL) solution of UO2(Aracnac)2 (167 mg,
0.153 mmol) was added HSiEt3 (49 μL, 0.31 mmol) and B(C6F5)3
(80 mg, 0.16 mmol), resulting in formation of a dark amber solution.
After stirring for 6 h, the deep red solution was filtered through a Celite
column supported on glass wool (0.5 cm  2 cm). The solution was then
concentrated in vacuo, layered with hexanes, and stored at À25 ꢀC for 24
h, producing a dark red crystalline material (225 mg, 80% yield). Anal.
Calcd for UO4N2Si2BF15C88H95: C, 57.61; H, 5.22; N, 1.53. Found: C,
’ SUMMARY
Reaction of UO2(Aracnac)2 with 2 equiv of Et3SiH in the
presence of 1 equiv of B(C6F5)3 results in silylation of both oxo
ligands and formation of [U(OSiEt3)2(Aracnac)2][HB(C6F5)3].
The presence of the borohydride anion in this material, along with
the observed 2H incorporation into the borohydride upon substitu-
tion of Et3SiH with Et3SiD, provides strong evidence for the
1
57.98; H, 5.26; N, 1.76. H NMR (CD2Cl2, 25 ꢀC, 500 MHz, trans
isomer): δ 7.32 (t, 2H, JHH = 6.8 Hz, para CH), 6.95 (t, 2H, JHH = 7.0
Hz, para CH), 6.70 (d, 4H, JHH = 6.6 Hz, meta CH), 6.64 (d, 4H, meta
CH), 6.04 (br s, 4H, ortho CH), 5.46 (br s, 12H, three overlapping
resonances attributable to the ortho CH, CH2CH3, and para N-aryl
CH), 4.85 (br s, 6H, CH2CH3), 4.06 (s, 2H, γ-CH), 3.29 (s, 18H,
CH2CH3), À0.05 (s, 36H, CMe3). The HB(C6F5)3 resonance was not
observed, most likely due to overlap with the CH2CH3 resonance at 3.29
ppm. Also, the two ortho N-aryl CH resonances were not observed,
intermediacy of a boraneÀsilane adduct, [Et3SiH B(C6F5)3],
3 3 3
during this transformation.29À31 This adduct promotes nucleophilic
attack on the silyl cation by the uranyl oxo ligand. Interestingly, a
second product is also observed in small amounts in the reaction
mixture, namely, U(OSiEt3)(OB{C6F5}3)(Aracnac)2, the product
derived from monosilylation of UO2(Aracnac)2. The observation of
two products in the reaction can be rationalized by formation of a
common intermediate, UVO(OSiEt3)(Aracnac)2, while the relative
ratio of the two products suggests that reaction of UO(OSiEt3)-
1
possibly due to paramagnetic broadening. H NMR (CD2Cl2, 25 ꢀC,
500 MHz, cis isomer): δ 9.61 (br s), 8.02 (s), 7.95 (s), 1.55 (br s), À0.40
(s, CMe3), À1.18 (br s). The remaining resonances assignable to the cis
isomer were not observed. 19F{1H} NMR (CD2Cl2, 25 ꢀC, 470 MHz):
δ À69.80 (d, 6F, JFF = 19.7 Hz, ortho CF), À100.63 (t, 3F, JFF = 20.2 Hz,
para CF), À103.46 (t, 6F, JFF = 19.0 Hz, meta CF). 1H NMR (CD2Cl2,
12 ꢀC, 500 MHz, trans isomer): δ 7.30 (t, 2H, JHH = 6.8 Hz, para CH),
6.94 (t, 2H, JHH = 7.2 Hz, para CH), 6.68 (d, 4H, JHH = 6.6 Hz, meta
CH), 6.61 (s, 4H, meta CH), 5.99 (br s, 4H, ortho CH), 5.64 (br s, 6H,
CH2CH3), 5.44 (br s, 2H, para N-aryl CH), 5.20 (br s, 4H, ortho CH),
4.98 (br s, 6H, CH2CH3), 3.91 (s, 2H, γ-CH), 3.39 (s, 18H, CH2CH3),
À0.10 (s, 36H, CMe3). The HB(C6F5)3 resonance was not observed,
most likely due to overlap with the CH2CH3 resonance at 3.39 ppm.
(Aracnac)2 with [Et3SiH B(C6F5)3] is faster than its reaction
3 3 3
with B(C6F5)3, resulting in preferential formation of [U(OSiEt3)2-
(Aracnac)2][HB(C6F5)3]. In addition, reduction of [U(OSiEt3)2-
(Aracnac)2][HB(C6F5)3] leads to generation of a neutral U(IV)
silyloxide, U(OSiEt3)2(Aracnac)2. This complex appears well poised
for selective cleavage of the UÀO bond via protonation, which, if
achieved, would represent the first well-defined, stepwise cleavage of
a uranyl-derived UÀO bond. We are currently investigating the
feasibility of this transformation.
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Also, the two ortho N-aryl CH resonances were not observed, possibly
due to paramagnetic broadening. 1H NMR (CD2Cl2, 12 ꢀC, 500 MHz,
cis isomer): δ1.54 (br s), À0.42 (s, CMe3), À0.48 (s, CMe3), À1.43 (br s).
The remaining resonances assignable to the cis isomer were not observed.
UVÀvis/NIR (CH2Cl2, 4.34 Â 10À3 M, L molÀ1 cmÀ1): 850 (sh, ε =
4.20 (s, 2H, γ-CH), 3.74 (br s, 9H, CH2CH3), 0.04 (s, 18H, CMe3), À0.40
(s, 18H, CMe3). The two ortho N-aryl CH resonances were not observed.
1H NMR (CD2Cl2, À15 ꢀC, 500 MHz, cis isomer): δ 2.01, À0.77
(s, CMe3), À0.91 (s, CMe3), À1.25 (s, CMe3), À1.38 (s, CMe3), À2.48.
The remaining resonances assignable to the cis isomer were not observed.
IR (KBr pellet, cmÀ1): 2964(s), 2940(sh m), 2921(sh m), 2874(m),
1641(m), 1596(m), 1586(m), 1559(vs), 1539(sh m), 1514(vs), 1485(sh
vs), 1471(vs), 1451(sh vs), 1440(vs), 1424(sh m), 1394(m), 1393(m),
1364(m), 1313(sh m), 1293(s), 1280(s), 1247(m), 1241(sh m), 1204(w),
1182(w), 1158(w), 1130(w), 1120(w), 1096(sh s), 1085(s), 1065(m),
1024(m), 999(m), 980(s), 936(sh w), 926(w), 920(w), 900(m), 890(m),
873(m), 870(sh m), 838(s), 817(vs), 784(m), 772(s), 763(m), 759(sh m),
750(m), 735(m), 733(sh m) 708(m), 698(s), 687(m), 683(m), 670(m),
659(w), 654(w), 633(w), 617(w), 605(w), 578(m), 547(w), 525(w),
503(w), 491(w), 480(w), 460(w), 437(w), 423(sh w), 414(w).
3
3
10), 980 (ε = 6), 1090 (ε = 3), 1210 (ε = 1), 1350 (ε = 2), 1400 (sh, ε = 3),
1430 (sh, ε = 6), 1460 (ε = 11), 1490 (ε = 8), 1640 (ε = 8), 1670 (ε = 7),
1780 (ε = 2), 2150 (ε = 8), 2170 (sh, ε = 7), 2190 nm (ε = 6). IR (KBr
pellet, cmÀ1): 3102(sh w), 3086(sh w), 3068(w), 3033(w), 2965(vs),
2940(sh s), 2911(s), 2879(s), 2377(w, νBH), 2359(w), 1660(w), 1640(m),
1594(s), 1586(s), 1556(vs), 1508(vs), 1467(vs), 1455(sh vs), 1439(vs),
1413(sh s), 1396(s), 1363(s), 1337(sh m), 1314(m), 1293(vs), 1277(s),
1244(s), 1240(sh s), 1205(w), 1182(w), 1161(w), 1127(sh m), 1114(s)
1101(s), 1078(s), 1065(s), 1024(s), 1000(m), 969(vs), 919(sh m), 900(s),
890(s), 874(m), 812(vs), 808(vs), 786(s), 771(s), 765(sh s), 748(s),
740(sh s), 712(s), 698(s), 694(sh s), 673(m), 659(m), 644(sh m), 618(w),
601(m), 581(s), 566(m), 548(m), 531(m), 503(m), 494(sh m), 462(m),
446(m), 432(m).
U(OSiEt3)2(Aracnac)2 (Ar = 3,5-tBu2C6H3) (3). To a red Et2O
(3 mL) suspension of [U(OSiEt3)2(Aracnac)2][HB(C6F5)3] (1) (192
mg, 0.105 mmol) was added an Et2O (1 mL) solution of Cp2Co (20 mg,
0.11 mmol). This resulted in formation of a red solution and a yellow-green
precipitate. The mixture was stirred for 1.5 h before being filtered through a
Celite column supported on glass wool (0.5 cm  2 cm), producing an
orange-red supernatant and yellow-green plug. The supernatant was con-
centrated in vacuo, layered with hexanes, and stored at À25 ꢀC for 48 h,
producing a dark red crystalline material (84 mg, 61% yield). Anal. Calcd for
UO4N2Si2C70H94: C, 63.61; H, 7.17; N, 2.12. Found: C, 63.53; H, 4.42; N,
1.48. 1H NMR (CD2Cl2, À60 ꢀC, 500 MHz, cis isomer): δ 14.85 (s, 2H,
para N-aryl CH), 13.36 (s, 4H, meta CH), 10.01 (s, 2H, para CH), 8.13 (s,
4H, meta CH), 7.70 (s, 2H, para CH), 6.68 (br s, 4H, ortho CH), 3.73 (s,
2H, γ-CH), 0.98 (s, 4H, ortho CH), 0.85 (s, 18H, CH2CH3), 0.16 (s, 6H,
CH2CH3), À1.12 (s, 18H, CMe3), À1.55 (s, 18H, CMe3), À9.22 (br s, 2H,
ortho N-aryl CH), À10.67 (br s, 2H, ortho N-aryl CH). One of the CH2CH3
resonances was not observed, possibly due to overlap with another resonance.
1H NMR (CD2Cl2, À60 ꢀC, 500 MHz, trans isomer): δ 53.53 (br s, 6H,
CH2CH3), 52.17 (br s, 6H, CH2CH3), 27.98 (br s, 18H, CH2CH3), 1.45 (s,
2H), À1.98 (s, 4H), À4.48 (s, 36H, CMe3), À4.61 (s, 4H), À7.96 (br s,
2H), À12.02 (br s, 4H), À21.67 (br s, 4H), À26.14 (br s, 4H), À32.14 (br
s, 2H). One resonance integrating for 2 protons was not observed. UVÀvis/
NIR (CH2Cl2, 3.13 Â 10À3 M, L molÀ1 cmÀ1): 720 (ε = 90), 760 (sh, ε =
U(OSiEt3)(OB{C6F5}3)(Aracnac)2 (Ar = 3,5-tBu2C6H3) (2). A
reaction flask was charged with a toluene (3.5 mL) solution containing
UO2(Aracnac)2 (84 mg, 0.077 mmol), HSiEt3 (12 μL, 0.075 mmol), and
B(C6F5)3 (40 mg, 0.078 mmol). The flask was sealed, and the solution
was heated at 85 ꢀC for 5.25 h. The volatiles were removed in vacuo, and
hexanes (4 mL) was added, resulting in formation of dark precipitate.
The solution was filtered through a Celite column supported on glass
wool (0.5 cm  2 cm) and concentrated in vacuo (2 mL). Storage at
À25 ꢀC for 5 days resulted in formation of a dark amber microcrystalline
material (23 mg, 18% yield), which was washed with hexane (2 Â 1 mL).
Anal. Calcd for UO4N2SiBF15C82H79 C6H14: C, 58.57; H, 5.19; N,
3
1.55. Found: C, 58.30; H, 5.05; N, 1.56. 1H NMR (C6D6, 25 ꢀC, 500
MHz, trans isomer): δ 6.93 (t, 2H, JHH = 7.2 Hz, para CH), 6.44 (t, 2H,
JHH = 7.1 Hz, para CH), 6.42 (d, 4H, JHH = 7.2 Hz, meta CH), 6.27 (s,
4H, meta CH), 6.09 (br s, 4H, ortho CH), 5.23 (br s, 6H, overlapping
ortho CH and para N-aryl CH), 4.94 (br s, 3H, CH2CH3), 4.69 (br s, 5H,
overlapping CH2CH3 and γ-CH), 3.41 (br s, 9H, CH2CH3), 0.20 (br s,
18H, CMe3), 0.02 (br s, 18H, CMe3). The two ortho N-aryl CH
resonances were not observed. 1H NMR (C6D6, 25 ꢀC, 500 MHz, cis
isomer): δ 2.16, À0.48 (s, CMe3), À0.58 (s, CMe3), À0.92 (s, CMe3),
À1.03 (s, CMe3). The remaining resonances assignable to the cis isomer
were not observed. 19F{1H} NMR (C6D6, 25 ꢀC, 470 MHz): δ À77.83
(br s, ortho CF, trans isomer), À95.81 (t, 3F, JFF = 18.5 Hz, para CF,
3
3
36), 840 (ε = 32), 940 (ε = 26), 1010 (sh, ε = 35), 1040 (ε = 40), 1100 (ε =
55), 1210 (ε = 32), 1310 (ε = 14), 1410 (sh, ε = 16), 1450 (ε = 16), 1520
(ε = 16), 1640 (ε = 10), 1670 (ε = 12), 1840 (ε = 90), 1890 (sh, ε = 65),
2160 nm (ε = 9). IR (KBr pellet, cmÀ1): 3104(w), 3083(w), 3063(w),
3029(w), 2958(vs), 2905(s), 2873(s), 2804(w), 1587(s), 1563(sh s),
1555(vs), 1494(vs), 1484(vs), 1479(vs), 1467(vs), 1457(vs), 1440(vs),
1421(s), 1389(vs), 1368(s), 1348(sh m), 1313(s), 1296(s), 1282(sh s),
1268(sh m), 1240(s), 1205(m), 1180(w), 1157(w), 1133(w), 1128(sh w),
1106(w), 1078(m), 1065(m), 1024(m), 1004(m), 984(m), 973(m),
965(m), 937(m), 931(m), 898(sh s), 885(vs), 846(sh m), 825(sh m),
803(w), 781(m), 767(sh s), 761(s), 736(s), 712(s), 694(s), 671(m),
664(sh m), 651(sh w), 643(sh w), 639(sh w), 617(w), 607(m), 575(m),
566(m), 546(w), 523(w), 495(m), 460(sh w), 444(m), 439(m).
trans isomer), À96.37 (s, 3F, para CF, cis isomer), À100.43 (d, 6F, JFF
=
18.9 Hz, meta CF, trans isomer), À100.90 (s, 6F, meta CF, cis isomer).
The ortho fluorine resonance for the cis isomer was not observed,
possibly due to overlap with the resonance for the trans isomer. 1H NMR
(CD2Cl2, 25 ꢀC, 500 MHz, trans isomer): δ 7.19 (t, 2H, JHH = 6.8 Hz,
para CH), 6.82 (t, 2H, JHH = 6.9 Hz, para CH), 6.58 (d, 4H, JHH = 6.0
Hz, meta CH), 6.54 (s, 4H, meta CH), 5.96 (br s, 4H, ortho CH), 5.19
(br s, 4H, ortho CH), 5.10 (br s, 2H, para N-aryl CH), 4.91 (br s, 3H,
CH2CH3), 4.64 (br s, 3H, CH2CH3), 4.59 (s, 2H, γ-CH), 3.35 (br s, 9H,
CH2CH3), 0.13 (br s, 18H, CCH3), À0.11 (br s, 18H, CCH3). The two
1
ortho N-aryl CH resonances were not observed. H NMR (CD2Cl2,
[Cp2Co][HB(C6F5)3] (4). The yellow-green solid generated during
the synthesis of complex 3 was dissolved in THF (2 mL) and filtered
through a Celite column supported on glass wool (0.5 cm  2 cm). The
volatiles were removed in vacuo, and the resulting solid was washed with
hexanes (3 mL) and Et2O (3 mL) and subsequently dried in vacuo
yielding [Cp2Co][HB(C6F5)3] as a yellow-green powder (54 mg, 71%
yield). 1H NMR (CD2Cl2, 25 ꢀC, 500 MHz): δ 5.66 (s, 10H, CH), 3.64
(br quartet, 1H, JBH = 90.8 Hz, BH). 19F{1H} NMR (CD2Cl2, 25 ꢀC,
470 MHz): δ À69.66 (d, 6F, JFF = 20.9 Hz, ortho CF), À100.06 (t, 3F,
JFF = 20.1 Hz, para CF), À103.08 (t, 6F, JFF = 18.9 Hz, meta CF).
NMR-Scale Synthesis of 1 Using DSiEt3. A red C6H6 solution
(1 mL) containing UO2(Aracnac)2 (23 mg, 0.021 mmol) and DSiEt3
(6.7 μL, 0.042 mmol) was sealed in a J Young NMR tube, and the 2H
25 ꢀC, 500 MHz, cis isomer): δ 10.44, 8.64, 8.17, 8.01, 7.52, 7.44, 2.02,
À0.63 (s, CMe3), À0.76 (s, CMe3), À0.85 (s, CMe3), À0.96 (s, CMe3).
The remaining resonances assignable to the cis isomer were not
observed. 19F{1H} NMR (CD2Cl2, 25 ꢀC, 470 MHz): δ À78.07 (br
s, ortho CF, trans isomer), À97.28 (t, 3F, JFF = 18.8 Hz, para CF, trans
isomer), À97.80 (s, 3F, para CF, cis isomer), À101.41 (d, 6F, JFF = 18.4
Hz, meta CF, trans isomer), À101.98 (s, 6F, meta CF, cis isomer). The
ortho CF resonance for the cis isomer is not observed. 1H NMR
(CD2Cl2, À15 ꢀC, 500 MHz, trans isomer): δ 7.14 (s, 2H, para CH),
6.77 (s, 2H, para CH), 6.44 (s, 8H, overlapping meta CH resonances),
5.77 (br s, 4H, ortho CH), 5.60 (br s, 3H, CH2CH3), 5.22 (br s, 3H,
CH2CH3), 4.99 (br s, 2H, para N-aryl CH), 4.75 (br s, 4H, ortho CH),
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Inorganic Chemistry
ARTICLE
Table 2. X-ray Crystallographic Data for Complexes 1À3
1
2
3
empirical formula
crystal habit, color
cryst size (mm)
cryst syst
UO4N2Si2BF15C88H95
UO4N2SiBF15C82H79
plate, dark red
0.20 Â 0.18 Â 0.10
triclinic
UO4N2Si2C70H94
irregular, dark red
0.40 Â 0.28 Â 0.22
monoclinic
P21/n
plate, dark orange
0.54 Â 0.34 Â 0.16
triclinic
space group
volume (Å3)
a (Å)
P-1
P-1
4238.0(10)
13.1761(18)
17.822(3)
20.054(3)
98.520(2)
102.891(2)
108.220(2)
2
3730(2)
3368.7(3)
12.6522(8)
17.5193(10)
15.3466(9)
90
12.909(4)
16.329(5)
19.010(6)
78.273(5)
87.803(5)
71.991(6)
2
b (Å)
c (Å)
R (deg)
β (deg)
97.991(2)
90
γ (deg)
Z
2
fw (g/mol)
density(calcd) (Mg/m3)
1834.68
1718.40
1321.68
1.438
1.530
1.303
abs coeff (mmÀ1
)
2.026
2.282
2.491
F000
1858
1726
1360
total no. reflns
unique reflns
32 304
28 620
26 773
15 837
13 512
6764
final R indices [I > 2σ(I)]
R1 = 0.0470
wR2 = 0.1039
1.499 and À2.673
0.959
R1 = 0.0783
wR2 = 0.1370
4.556 and À2.566
1.126
R1 = 0.0269
wR2 = 0.0661
0.808 and À0.925
0.950
largest diff. peak and hole (eÀ ÅÀ3
)
GOF
NMR spectrum was obtained. The solution was then frozen, whereupon a
C6H6 solution (1 mL) of B(C6F5)3 (10 mg, 0.021 mmol) was layered onto
the frozen plug. The tube was sealed, and the reaction was initiated by
thawing the frozen plug, resulting in a deep red solution. The reaction was
allowed to proceed for 19 h before the 2H NMR spectrum was collected
(Figures S19ÀS20, Supporting Information). 2H NMR (C6H6, 25 ꢀC, 77
MHz): δ 4.64 (br s, [DB(C6F5)3]), 3.90 (s, DSiEt3), 3.60 (s).
Paratone-N oil. Data collection was carried out on a Bruker 3-axis platform
diffractometer with SMART-1000 CCD detector. The instrument was
equipped with graphite-monochromatized Mo KR X-ray source (λ =
0.71073 Å). All data were collected at 150(2) K using an Oxford nitrogen
gas cryostream system. A hemisphere of data was collected using ω scans and
0.3ꢀ frame widths. Frame exposures of 20, 35, and 20 s were used for
complexes 1, 2, and 3, respectively. SMART55 was used to determine the cell
parameters, and the raw frame data were processed using SAINT.56 The
empirical absorption correction was applied based on Psi-scan or SADABS.
Subsequent calculations were carried out using SHELXTL.57 The structures
were solved using direct methods and difference Fourier techniques. All
hydrogen atom positions were idealized and rode on the atom of attachment.
Unless otherwise noted, the final refinement included anisotropic temperature
factors on all non-hydrogen atoms. Structure solution, refinement, graphics,
and creation of publication materials were performed using SHELXTL.
Complex 1 exhibits positional disorder of the [HB(C6F5)3]À anion. The
positional disorder was addressed by modeling the molecule in two orienta-
tions, in a 65:35 ratio. Several carbon atoms within the disordered anion were
not refined anisotropically. In addition, the CÀC and CÀF bonds of the aryl
rings were fixed with the DFIX command, while the rings were constrained
with the FLAT command. The EADP command was used to constrain C3,
C7, C10, C72B, and C73B. An idealized hydrogen atom was not assigned to
the disordered [HB(C6F5)3]À anion in 1. For complex 2, C23, C52, C44,
C66, and C70 were constrained using the EADP command. A summary of
relevant crystallographic data for complexes 1À3 is presented in Table 2.
NMR-Scale Synthesis of 1 Using HSiEt3. A C6D6 solution
(1 mL) containing UO2(Aracnac)2 (21 mg, 0.019 mmol) and HSiEt3
(6.1 μL, 0.038 mmol) was sealed in a J Young NMR tube, and the 1H
NMR spectrum was obtained. The solution was then frozen, whereupon
a C6D6 solution (1 mL) of B(C6F5)3 (10 mg, 0.021 mmol) was layered
onto the frozen plug. The tube was sealed, and the reaction was initiated
by thawing the frozen plug, resulting in a deep red solution. The reaction
1
was allowed to proceed for 21 h before the H and 19F NMR spectra
were collected (Figures S17ÀS18, Supporting Information).
NMR-Scale Synthesis of 1 Using 1 Equiv of HSiEt3. A red
C6D6 solution (0.5 mL) containing UO2(Aracnac)2 (14 mg, 0.013 mmol)
and HSiEt3 (2.1 μL, 0.013 mmol) was sealed in a J Young NMR tube, and
the 1H NMR spectrum was obtained. The solution was then frozen,
whereupon a C6D6 solution (0.5 mL) of B(C6F5)3 (7 mg, 0.013 mmol)
was layered onto the frozen plug. The tube was sealed, and the reaction was
initiated by thawing the frozen plug, resulting in a deep red solution. The
reaction was allowed to proceed for 2 h before the 1H and 19F NMR spectra
were collected (Figures S23 and S24, Supporting Information).
NMR-Scale Thermolysis of 1 in Tol-d8. Complex 1 (10 mg,
0.005 mmol) was dissolved in toluene-d8 (0.73 mL) and heated at 85 ꢀC
for 23 h, resulting in a color change to dark amber. The 1H and 19F NMR
spectra revealed a decrease in the resonances assignable to 1 and the
presence of new resonances attributable to 2 as well as HSiEt3 and other
unidentified products (Figures S14ÀS15, Supporting Information).
X-ray Crystallography. The solid state molecular structures of
complexes 1À3 were determined similarly with exceptions noted in
the following paragraph. Crystals were mounted on a glass fiber under
’ ASSOCIATED CONTENT
S
Supporting Information. Cyclic voltammetry data for
b
complex 1; crystallographic details (as CIF files); IR spectra for
complexes 1À3; 1H and 19F NMR spectra for complexes 1À4.
This material is available free of charge via the Internet at http://
pubs.acs.org.
9648
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Inorganic Chemistry
ARTICLE
’ AUTHOR INFORMATION
(29) Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000,
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Corresponding Author
*E-mail: hayton@chem.ucsb.edu.
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47, 5997–6000.
(32) Bach, P.; Albright, A.; Laali, K. K. Eur. J. Org. Chem.
’ ACKNOWLEDGMENT
2009, 1961–1966.
(33) Berkefeld, A.; Piers, W. E.; Parvez, M. J. Am. Chem. Soc. 2010,
132, 10660–10661.
We thank the University of California, Santa Barbara, and the
Department of Energy Basic Energy Sciences program for financial
support of this work.
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