Borane-mediated silylation of a metal-oxo ligand

Article abstract of DOI:10.1021/ic2008649

The addition of 1 equiv of HSiPh₃ 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/ic2008649

COMMUNICATION
pubs.acs.org/IC
Borane-Mediated Silylation of a MetalꢀOxo Ligand
David D. Schnaars, Guang Wu, and Trevor W. Hayton*
Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, California 93106, United States
S Supporting Information
b
space group P21/n as the toluene solvate 1 4.5C7H8, and its
3
ABSTRACT: The addition of 1 equiv of HSiPh3 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 the formation of U(OSiPh3)(OB{C6F5}3)(Aracnac)2 (1),
via silylation of an oxo ligand and reduction of the uranium
center. The addition of 1 equiv of Cp2Co to 1 results in a
reduction to uranium(IV) and the formation of [Cp2Co]-
[U(OSiPh3)(OB{C6F5}3)(Aracnac)2] (2) in 78% yield.
Complexes 1 and 2 have been characterized by X-ray
crystallography, while the solution-phase redox properties
of 1 have been measured with cyclic voltammetry.
solid-state molecular structure is shown in Figure 1. Complex 1
exhibits an octahedral geometry similar to that of the parent
uranium(VI) complex. However, one uranyl oxo ligand has been
converted into a triphenylsilyloxide group, while the other oxo
ligand has been coordinated by a molecule of B(C6F5)3. The
UꢀO(silyloxide) bond length [U1ꢀO2 = 2.034(9) Å] is sub-
stantially longer than the UdO bond in uranyl (1.76 Å) but is
comparable to the previously reported UV-silyloxide distances.11,17
The UꢀO(borane) bond length in 1 [U1ꢀO1 = 1.941(8) Å] is
slightly shorter than the UꢀO(SiPh3) bond but is somewhat
longer than the UꢀO bonds reported for the uranyl(VI)-
B(C6F5)3 adducts UO(OB{C6F5}3)(Aracnac)2 [1.890(4) Å] and
UO(OB{C6F5}3)(NCN)2 [NCN = (Me3SiN)CPh(NSiMe3)]
[1.898(3) Å].19,20 Interestingly, the UꢀO(acnac) bond lengths
in 1 [2.188(8) and 2.140(8) Å] are shorter than those of the
starting material [2.255(5) Å for UO2(Aracnac)2].21 This is
somewhat unexpected, given the larger size of the U5þ ion.
Silane addition across an M = E (E = O, S, NR) linkage, while
rare, has been observed previously.22ꢀ26 For example, Toste and
co-workers observed the hydrosilylation of ReO2I(PPh3)2 by the
addition of a SiꢀH bond across a RedO group.27ꢀ30 In this
example, the RedO bond exhibits a bond order of 2.5,30 a factor
that may contribute to its relatively facile silylation. In contrast,
the UdO bond in uranyl exhibits a bond order of 3 and an
extremely strong UꢀO interaction (604 kJ/mol),10 which may
explain why addition of B(C6F5)3 is required to observe oxo
silylation.
(C6F5)3-mediated hydrosilylation reactions are known for a
B
variety of organic substrates including ketones,1ꢀ3 aldehydes,4
imines,5 alkenes,6 and even CO2.7 These transformations proceed
by the initial formation of a weak boraneꢀsilane adduct,
[R3SiH B(C6F5)3], which is thought to activate the silicon
3 3 3
center toward nucleophilic attack.2,3,8 The rapid development of
this methodology over the last 10 years suggests that many further
applications are possible. In this regard, we have discovered a
unique B(C6F5)3-mediated silylation of an inorganic functional
group, namely, the recalcitrant uranyl moiety (UO22þ).9
The controlled functionalization and/or substitution of the
uranyl oxo ligands has proven to be a difficult endeavor,9,10 and
only a few examples are known. For instance, the formation of a
uranyl-derived uranium(V) silyloxide, [UO(OSiMe3)(THF)-
Fe2I2L], was achieved by deprotonation of UO2(THF)(H2L)
(L = “Pacman” = polypyrrolic macrocycle).11ꢀ14 During the
reaction, UO2(THF)(K2L) is formed as an intermediate species,
which is then reduced and silylated by HN(SiMe3)2 to generate
the final product. Additionally, oxo ligand metalation, concomi-
tant with uranyl reduction, can be achieved by the reaction of
UO2(THF)(H2L) with a variety of lithium reagents15 or rare-
earth silylamides.16 In a similar transformation, we observed the
reductive silylation of UO2(Racnac)2 (Racnac = RNC(Ph)CHC-
(Ph)O; R = 3,5-tBu2C6H3, tBu) by reaction with excess Me3SiI.17,18
Interestingly, the addition of silanes, such as HSiPh3, to UO2-
(Racnac)2 under the same experimental conditions results in no
reaction. This was surprising, considering that R3SiH is anticipated
to be a substantially better reducing agent than Me3SiI. Given this,
we turned our attention to the B(C6F5)3-mediated hydrosilylation
methodology.
The 1H NMR spectrum of 1 in CD2Cl2 is characterized by the
t
presence of two Bu resonances at ꢀ0.24 and ꢀ0.48 ppm,
assignable to the 3,5-tBu2C6H3 aryl substituent. The observation
of two resonances likely arises from hindered rotation about the
NꢀCipso bond, which results in the generation of two distinct
chemical environments. This hindered rotation is probably due
to the bulky functional groups attached to each functionalized
oxo ligand. The 19F{1H} NMR spectrum consists of three
resonances at ꢀ73.65, ꢀ96.73, and ꢀ100.45 ppm in a 2:1:2 ratio,
corresponding to the o-, p-, and m-fluorine atoms of the C6F5
groups, respectively. Interestingly, a second, less-intense, set
1
of resonances is also observed in the H NMR spectrum of 1,
which we have assigned to a minor isomer. The two isomers are
observed in a 5.5:1 ratio. The minor isomer is characterized
t
by four inequivalent Bu resonances, at ꢀ0.54, ꢀ0.85, ꢀ1.00,
and ꢀ1.24 ppm, in a 1:1:1:1 ratio, respectively. Given the
t
observation of four Bu resonances for the minor isomer, we
The addition of 1 equiv of HSiPh3 to UO2(Aracnac)2 (Ar = 3,
5-tBu2C6H3),19 in the presence of 1 equiv of B(C6F5)3, leads to
the formation of a deep-red solution, from which U(OSiPh3)-
(OB{C6F5}3)(Aracnac)2 (1) can be isolated as a red-brown solid
in 78% yield (Scheme 1). Complex 1 crystallizes in the monoclinic
tentatively suggest that this isomer is characterized by a cis
configuration of the silyloxide and borate ligands because this
Received: April 24, 2011
Published: May 09, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
COMMUNICATION
Scheme 1. Synthesis of 1 and 2
Figure 2. Room temperature cyclic voltammogram for 1 in CH2Cl2
(0.1 M [NBu4][PF6] as the supporting electrolyte). Scan rate: 200 mV/s.
in the monoclinic space group P21/n as the hexane solvate
2 2C6H14 (Figure S1 in the Supporting Information). In the solid
3
state, 2 exists as a discrete cation/anion pair. The ligand arrange-
ment around the uranium center is similar to that exhibited by
complex 1; i.e., two Aracnac ligands define the equatorial plane, and
the twooxo-derived ligands occupy the axial positions, resulting in
an overall octahedral geometry. The O1ꢀU1ꢀO2 bond angle is
175.3(3)°, and the UꢀO(borane) and UꢀO(silyloxide) bond
lengths are U1ꢀO1 = 2.056(8) Å and U1ꢀO2 = 2.173(8) Å,
respectively (Table S2 in the Supporting Information). These
distances are slightly longer than those observed in 1, consistent
with the presence of the larger U4þ ion.
Figure 1. Solid-state structure of 1 4.5C7H8 with 50% probability
3
ellipsoids. The phenyl rings on the Aracnac backbone have been removed
for clarity. Bond lengths and angles are listed in Table S2 in the
Supporting Information.
As was observed for 1, the 1H and 19F NMR spectra of 2 are
also consistent with the presence of major and minor isomers in
solution, in a 3:1 ratio. Upon cooling to ꢀ60 °C, the ratio of
major and minor isomers changes to 9:1, respectively. In CD2Cl2
at ꢀ60 °C, the major isomer is characterized by two broad
resonances at ꢀ3.18 and ꢀ11.44 ppm, assignable to two unique
tBu environments, while the minor isomer is characterized by
four broad resonances at 0.45, ꢀ2.16, ꢀ2.33, and ꢀ2.75 ppm,
assignable to four inequivalent tBu environments. Also observed
is a sharp singlet at 6.64 ppm, revealing the incorporation of
[Cp2Co]þ. The room temperature 19F NMR spectrum of 2 is
also consistent with the presence of both the major and minor
isomers in solution.
arrangement most easily explains the observation of four unique
tBu environments. Finally, the near-IR spectrum for 1 is similar to
those of other uranium(V) complexes,17,31ꢀ33 supporting the
presence of a 5f1 ion.
We have examined the solution-phase redox properties of
1 using cyclic voltammetry. Its cyclic voltammogram reveals a
reversible reduction feature at E1/2 = ꢀ0.72 V (vs Fc/Fcþ),
which we attribute to the UV/UIV redox couple (Figure 2). This
feature is 0.63 V higher than that observed for [Cp*2Co]-
[U(OB{C6F5}3)2(Aracnac)2], which exhibits a UV/UIV redox
potential at E1/2 = ꢀ1.21 V (vs Fc/Fcþ).33 We attribute the less
negative reduction potential in 1 to the replacement of a neutral
B(C6F5)3 substituent with the cationic Ph3Siþ fragment. In
addition, complex 1 exhibits an irreversible oxidation feature at
0.96 V (vs Fc/Fcþ, 200 mV/s). The irreversibility of this
feature, coupled with its relatively high potential, suggests that
conversion of 1 back to uranyl by SiꢀO and BꢀO bond
cleavage will not be easily achievable. Consistent with this
hypothesis, the attempted oxidation of complex 1 with I2 results
in no reaction, while oxidation with AgOTf results in partial
decomposition, but no evidence for the formation of UO2-
(Aracnac)2 is observed.
As anticipated, the near-IR spectrum for 2 is indicative of the
presence of a 5f2 ion.34,35 We have also examined the chemical
reversibility of the reduction from 1 to 2. Thus, the addition of
1 equiv of AgOTf to 2 in CD2Cl2 cleanly regenerates 1, as
determined by 1H and 19F NMR spectroscopies (Figures
S20ꢀS22 in the Supporting Information).
To gain further insight into the formation of 1, we monitored
the reaction of UO2(Aracnac)2, HSiPh3, and B(C6F5)3 in C6D6
1
1
by H NMR spectroscopy. The H NMR spectrum of the
reaction mixture reveals the clean formation of complex 1 along
with the presence of H2, as evidenced by a sharp singlet at
4.46 ppm (Figures S28ꢀS30 in the Supporting Information).36
To confirm the origin of H2, we followed the reaction between
UO2(Aracnac)2, DSiPh3,37,38 and B(C6F5)3 in C6H6 by 2H NMR
spectroscopy. In agreement with the protio experiment, the 2H
NMR spectrum of the reaction mixture reveals a sharp resonance
at 4.45 ppm, consistent with the formation of D2 (Figures
S25ꢀS27 in the Supporting Information) and demonstrating
In agreement with the cyclic voltammetrydata, the addition of 1
equiv of Cp2Co to an Et2O suspension of 1 results in reduction
and formation of a brown powder. Recrystallization of this solid
from a mixture of CH2Cl2 and hexane affords the uranium(IV)
complex [Cp2Co][U(OSiPh3)(OB{C6F5}3)(Aracnac)2] (2) as a
brown-red solid in 78% yield (Scheme 1). Complex 2 crystallizes
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Inorganic Chemistry
COMMUNICATION
that silane is the source of the H2. However, there are also two
minor resonances present in the spectrum, at 12.10 and
0.76 ppm, revealing that some scrambling of the H label is
occurring over the course of the reaction. Additionally, we can
rule out reduction of the Aracnac ligand by hydrogen addition.
A comparison of the metrical parameters of both 1 and 2 with
UO2(Aracnac)2 and [Cp*2Co][UO2(Aracnac)2]19 reveals little
change in the NꢀC, CꢀC, and CꢀO bond lengths of the Aracnac
backbone (Figure S35 in the Supporting Information).
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Given the importance of uranyl redox chemistry in the
environmental behavior of uranium, the mechanism of silyla-
tion is of considerable interest. In our system, we believe that
the initial formation of a boraneꢀsilane adduct, [Ph3-
SiH
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support this hypothesis. First, no reaction is observed between
UO2(Aracnac)2 and Ph3SiH alone, demonstrating the need for
B(C6F5)3 in the reduction. Second, the reaction of HSiiPr3
with UO2(Aracnac)2 in the presence of B(C6F5)3 results in the
formation of the previously characterized adduct UO(OB-
{C6F5}3)(Aracnac)219 as the only uranium-containing product
(Figures S31ꢀS32 in the Supporting Information). This is
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In summary, we have demonstrated that facile silylation of
the uranyl oxo ligands, concomitant with reduction to uranium-
(V), can be achieved via borane-mediated silylation of UO2-
(Aracnac)2. This uranium(V) complex can be further reduced
with Cp2Co to uranium(IV). However, oxidation back to
2þ
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observed, likely owing to the strong SiꢀO and BꢀO bonds in 1.
While borane-mediated silylation has been extensively used in
organic synthesis, it has not, to our knowledge, been previously
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, NMR
b
spectroscopy, and X-ray crystallography details. This material is
available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: hayton@chem.ucsb.edu.
’ ACKNOWLEDGMENT
We thank the University of California at Santa Barbara and the
Department of Energy Basic EnergySciences programfor financial
support of this work.
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’ NOTE ADDED AFTER ASAP PUBLICATION
This paper was published on May 9, 2011. One sentence was
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