Reduction of pentavalent uranyl to U(IV) facilitated by oxo functionalization

Article abstract of DOI:10.1021/ja906880d

(Chemical Equation Presented) Addition of 2 equiv of B(C₆F ₅)₃ to [Cp*₂Co][U^VO ₂(^Aracnac)₂] (1) [^Aracnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-^tBu₂

Full text of DOI:10.1021/ja906880d

Published on Web 11/16/2009
Reduction of Pentavalent Uranyl to U(IV) Facilitated by Oxo Functionalization
David D. Schnaars, Guang Wu, and Trevor W. Hayton*
Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106
Received August 13, 2009; E-mail: hayton@chem.ucsb.edu
The reduction of uranyl (UO22+) to U(IV) has relevance to speciation
of uranium in the environment and geological storage of spent nuclear
fuel.1-3 This transformation is thought to proceed stepwise through
UO2+,4,5 a moiety that has received increased attention in recent
years.6,7 While reduction of UO22+ to UO2+ is now well-established,
the reduction of UO2+ to U(IV) is still poorly understood, in part
because reduction to U(IV) is not possible without substantial
rearrangement of the uranyl moiety or cleavage of the U-O(oxo)
bond.8 For example, reduction of UO2(OTf)2 with U(OTf)3 yields a
uranium(IV) oxo cluster, U6(µ3-O)8(µ-OTf)8(py)8, in which the uranyl
moiety is not conserved.9 Nonetheless, this is a key mechanistic
step in uranyl reduction, and its exploration would help validate
the proposed mechanisms for U(IV) formation.4,5 Herein we
report that reVersible reduction of pentavalent uranyl to U(IV)
can be achieved by functionalization of the two oxo ligands of
the uranyl moiety.
with AgOTf, which cleanly regenerates complex 2, as determined
11
1
by H and 19F NMR spectroscopies.
Scheme 1
Addition of 2 equiv of B(C6F5)3 to [Cp*2Co][UVO2(Aracnac)2]
(1)10 [Aracnac ) ArNC(Ph)CHC(Ph)O; Ar ) 3,5-tBu2C6H3] results
in the functionalization of both uranyl oxo ligands to afford
[Cp*2Co][UV{OB(C6F5)3}2(Aracnac)2] (2) in good yield (Scheme
1). Complex 2 exhibits two peaks in its 19F{1H} NMR spectrum at
-98.1 and -101.0 ppm in a 1:2 ratio, corresponding to the para
and meta fluorine atoms, respectively, indicating the incorporation
of B(C6F5)3 into the complex.11 No signal was observed for the
ortho fluorine atoms, most likely because of their proximity to the
paramagnetic uranium center. Its 1H NMR spectrum and elemental
analysis are also consistent with the proposed formulation, while
its NIR spectrum is similar to those exhibited by other U(V)
compounds,12,13 supporting the presence of a 5f1 ion. Unlike
complex 1, which exhibits limited thermal stability, 2 is stable in
CD2Cl2 for up to 40 h.
The 1H NMR spectrum of 3 exhibits large chemical shifts, which
supports its formulation as a U(IV) ion. For instance, the γ-CH
resonance of the Aracnac ligand appears as a singlet at -36.62 ppm,
while one set of o-CH resonances appears at -29.08 ppm. Its
19F{1H} NMR spectrum exhibits two extremely broad peaks at
-97.4 ppm (fwhm ) 340 Hz) and -98.5 ppm (fwhm ) 140 Hz)
that are assignable to 3. Also present are resonances at -93.2,
-103.1, and -104.5 ppm due to an unidentified minor product (see
Figure S12 in the Supporting Information). Its NIR spectrum and
magnetic susceptibility (2.37µB at 290 K) are both consistent with
the presence of a 5f2 ion,15-17 while its elemental analysis also
fits the proposed formulation.
The ability to functionalize both oxo ligands in 1 contrasts with the
reactivity of the U(VI) parent complex, UO2(Aracnac)2, which forms
only the monofunctionalized product UVIO{OB(C6F5)3}(Aracnac)2 upon
reaction with excess B(C6F5)3.10 This difference in reactivity is likely
a result of the increased electron density at the metal center in 2, which
renders the oxo ligands more nucleophilic.
The reduction of 2 to 3 is facilitated by the use of the strongly
electron-withdrawing B(C6F5)3, which lowers the redox potential
The solution redox properties of complex 2 were investigated
by cyclic voltammmetry. Its cyclic voltammogram in CH2Cl2
reveals a quasi-reversible reduction feature at E1/2 ) -1.21 V (vs
Fc/Fc+), which we attribute to the UO2+/U(IV) redox couple (see
the Supporting Information). Also observed was a reversible redox
couple for [Cp*2Co]+ (-1.81 V vs Fc/Fc+, which is comparable
to the literature value for this species14). Notably, a U(V)/U(IV)
reduction feature was not observed in the cyclic voltammogram of
the parent complex UVIO2(Aracnac)2,10 suggesting that coordination
of B(C6F5)3 to 1 moves this redox couple to a chemically accessible
value.
+
of the UO2 moiety upon coordination. It also converts the two
oxo groups of the uranyl ion into ligands that are better described
as B(C6F5)3-substituted alkoxides. As a result, cleavage of the U-O
bond is not required to access the 4+ oxidation state.
Interestingly, addition of only 1 equiv of B(C6F5)3 to 1 does not
produce the monofunctionalized U(V) complex. Instead, the
products of disproportionation, namely, 3 and UVIO2(Aracnac)2, are
1
observed in a 1:1 ratio, as determined by H NMR spectroscopy
(Scheme 1). This transformation mirrors the proposed reaction
scheme for the disproportionation of UO2 in aqueous solution4
+
and the reduction of uranyl by Geobacter sulfurreducens.5 In both
+
In line with the cyclic voltammetry results, reduction of 2
with Cp*2Co provides the unprecedented U(IV) complex
[Cp*2Co]2[UIV{OB(C6F5)3}2(Aracnac)2] (3), which can be isolated
as a green crystalline solid in good yield (Scheme 1). This
process is chemically reversible, as shown by the reaction of 3
cases, protonation of an oxo ligand in UO2 is thought to trigger
disproportionation. Addition of a second equivalent of B(C6F5)3 to
this reaction mixture results in the formation of complex 2 via
conproportionation of 3 and UVIO2(Aracnac)2. However, a small
amount of UVIO2(Aracnac)2 remains in the reaction mixture after
9
17532 J. AM. CHEM. SOC. 2009, 131, 17532–17533
10.1021/ja906880d  2009 American Chemical Society

C O M M U N I C A T I O N S
B(C6F5)3 addition. This is attributable to a competing reaction
pathway whereby B(C6F5)3 acts solely as a one-electron oxidant18
that converts 3 to 2 and leaves some UO2(Aracnac)2 unreacted (see
the Supporting Information).
Crystals of 6a were isolated from the crude reaction mixture in
low yield, allowing for structure determination by X-ray crystal-
lography (Figure S2). Its spectral properties were corroborated by
the synthesis of [NEt4][(2,4,6-Me3C6H2)NC(Ph)CHC(Ph)OB(C6F5)3]
(6b), which was independently generated by reaction of Na(Aracnac)
with 1 equiv of B(C6F5)3 and 1 equiv of [NEt4][Cl].11
The isolation of 5 and 6a from the reaction mixture suggests
that addition of B(C6F5)3 to 4 also results in disproportionation.
Consistent with this, monitoring the reaction by 1H and 19F{1H}
NMR spectroscopies revealed the formation of UVIO2(Aracnac)2
along with 6a (eq 1). However, we could not definitively assign
any resonances in these spectra to complex 5, possibly because
it is NMR-silent, and its full characterization remains to be
completed.
While we were unable to grow X-ray-quality crystals of 3, we
determined the structure of a similar oxo-functionalized complex,
namely, [Cp*2Co][UIV{OB(C6F5)3}2(Aracnac)(OEt2)] (Ar ) 2,4,6-
Me3C6H2) (5). This complex was formed in low yield by reaction
of 2 equiv of B(C6F5)3 with [Cp*2Co][UVO2(Aracnac)2] (Ar ) 2,4,6-
Me3C6H2) (4),10 the mesityl-substituted analogue of complex 1.
The solid-state molecular structure of 5 reveals a distorted
pentagonal bipyramidal geometry about uranium in which Aracnac,
Et2O, and two o-F dative interactions occupy the equatorial plane
(Figure 1). The two U-O(oxo) bond lengths, 2.029(6) and 2.025(6)
Å, are much longer than those observed in previously reported
uranyl-B(C6F5)3 adducts10,19 but similar to that observed in the
SiMe3-functionalized uranyl(V) complex [1.993(4) Å] reported by
Arnold.20,21 In addition, the O-U-O angle [153.3(2)°] deviates
significantly from linearity. This distortion likely stems from the
o-F interactions with the U(IV) center, which pull the oxygen atoms
out of the axial positions. Overall, these metrical parameters
demonstrate that 5 is no longer a uranyl complex and that its U-O
bond lengths are better compared to those of a uranium alkoxide.22
In summary, the formation of complex 3 demonstrates that
+
functionalization of UO2 with B(C6F5)3 allows for reduction to
U(IV) without cleavage of the U-O bond. The reduction is
facilitated by the coordination of the electron-withdrawing B(C6F5)3
groups to the uranyl oxo ligands, which lowers the redox potential
of the pentavalent precursor, and represents a new strategy for
2+
effecting reduction of UO2 to U(IV). Future studies will focus
on developing new methods for uranyl oxo functionalization and
selective cleavage of the U-O bond.
Acknowledgment. Financial support was provided by UC Santa
Barbara, the University of California Energy Institute, and the
UC-National Laboratory Research Program. We thank Brent C.
Melot for assistance with the SQUID measurements.
Supporting Information Available: Experimental procedures,
crystallographic details (CIF), and spectral data for all new compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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