Reductive silylation of uranyl mediated by iminosemiquinone ligands

Article abstract of DOI:10.1016/j.poly.2019.06.028

Stoichiometric silylation of the uranyl species, (^dippisq)₂UO₂THF, which features two reduced iminosemiquinone ligands, is reported. These ligand radicals facilitate the reduction of uranium 6+ to 4+, which is accompanied by silylation of the uranyl moiety with two equivalents of Me₃SiBr and release of the oxidized ligand. The intermediate, (Me₃SiO)₂UBr₂(OPPh₃)₂, is isolated prior to U–O bond cleavage by further addition of Me₃SiBr, producing UBr₄(OPPh₃)₂. U–O bond scission can also be performed in a one-pot reaction, by treating (^dippisq)₂UO₂THF with Me₂SiCl₂, forming UCl₄(OPPh₃)₂ and polymeric silyl products, (O[dbnd]SiMe₂)_n, with concomitant loss of oxidized ligand. In each case, isotopic ¹⁸O labeling experiments highlight the incorporation of the uranyl oxygen atoms into the resulting siloxanes released in the reactions.

Full text of DOI:10.1016/j.poly.2019.06.028

Accepted Manuscript
Reductive Silylation of Uranyl Mediated by Iminosemiquinone Ligands
Ezra Coughlin, Suzanne C. Bart
PII:
S0277-5387(19)30439-5
https://doi.org/10.1016/j.poly.2019.06.028
POLY 14019
DOI:
Reference:
Polyhedron
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Accepted Date:
5 April 2019
10 June 2019
11 June 2019
Please cite this article as: E. Coughlin, S.C. Bart, Reductive Silylation of Uranyl Mediated by Iminosemiquinone
Ligands, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.06.028
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Reductive Silylation of Uranyl Mediated by
Iminosemiquinone Ligands
Ezra Coughlin and Suzanne C. Bart*
H. C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana,
47907
Abstract
Stoichiometric silylation of the uranyl species, (dippisq)2UO2THF, which features two
reduced iminosemiquinone ligands, is reported. These ligand radicals facilitate the reduction of
uranium 6+ to 4+, which is accompanied by silylation of the uranyl moiety with two equivalents
of Me3SiBr and release of the oxidized ligand. The intermediate, (Me3SiO)2UBr2(OPPh3)2, is
isolated prior to U-O bond cleavage by further addition of Me3SiBr, producing UBr4(OPPh3)2.
U-O bond scission can also be performed in a one-pot reaction, by treating (dippisq)2UO2THF
with Me2SiCl2, forming UCl4(OPPh3)2 and polymeric silyl products, (O=SiMe2)n, with
concomitant loss of oxidized ligand. In each case, isotopic 18O labeling experiments highlight the
incorporation of the uranyl oxygen atoms into the resulting siloxanes released in the reactions.
Keywords
uranyl, reductive silylation, redox-active ligands, metal-oxo, oxygen-18
Introduction
Functionalization of the robust uranyl moiety, [O=U=O]2+, is a challenging goal in
actinide chemistry.1-11 The ubiquity of this ion in the environment, both naturally and from
anthropogenic sources, has prompted investigation to understand the bonding and reactivity of
these strong uranium-oxygen bonds for remediation strategies.7 Understanding how to break one
or all of the polarized U-O bonds has been the subject of much study in the last decade. In 2007,

Ephritikhine reported the reduction of uranyl salts using excess Me3SiX (X = Cl, Br, I) reagents,
which converted the hexavalent uranyl to a tetravalent halide salt, UX4(MeCN)4.12 Shortly
thereafter, P. Arnold and co-workers described the activation of uranyl supported by a large,
compartmental N8-macrocycle ligand that imparts a rigid, “Pacman-like” structure. Several
examples have been reported, including activation of this uranyl by 3d metal cations,2 reductive
silylation promoted by Lewis acidic rare earth compounds to generate singly reduced uranyl-rare
earth 5f1-4fn complexes,1 as well as reductive silylation pairing silyl amides and 3d metal
cations.3-5 Hayton and coworkers showed that pentavalent uranyl could easily be reduced to
uranium(IV) materials facilitated by cobaltocene and B(C6F5)3.9 This process was later extended
to reductive silylation, replacing cobaltocene with R3SiH (R = Et, Ph) and B(C6F5)3.13 Other
work from the Hayton team has demonstrated the utility of silylhalides in uranyl reduction using
both Ph3SiOTf14, 15 and Me3SiI16, 17 to silylate the uranium-oxygen bonds.
Our group has also contributed to the study of uranyl functionalization, which was
accomplished using redox-active ligands as an electron source, rather than a traditional external
reducing agent.18 This work has demonstrated that reducing equivalents stored in redox active
ligands serve as potent reducing agents, facilitating formation of new E-O bonds (E = Si, B, C),
obviating the need for external reductants.6, 18 We found that this chemistry is further promoted
by use of a Lewis base, which serves to activate the halosilanes prior to reaction with the U=O
multiple bond.8
In more recent studies, we reported both the borylation and acylation of [O=U=O]2+ can
be promoted by redox-active iminosemiquinone ligands in (dippisq)2UO2THF (dippisq = 4,6-di-
tert-butyl-2-[(2,6-diisopropylphenyl)imino]-semiquinone) (Figure 1).6 Characterization of this
species established that the most accurate representation of this compound is as a hexavalent

uranyl center supported by two iminosemiquinone ligand radicals, dippisq1-. These radicals serve
as the reductant in the reaction, becoming oxidized to the iminoquinone form during the E-O
bond formation (Figure 1).
Figure 1: One electron, reversible oxidation of the iminosemiquinone ligand. Blue atoms
represent neutral atoms that engage in dative bonding, red atoms engage in ionic bonding.
The final organometallic species in these cases were characterized as uranium(IV)
tetrachloride derivatives, along with either catecholboryl ether or pivalic anhydride, depending
on the functionalization reagent used. In this system, complete bond scission was noted, and
isotopic labeling experiments confirmed that the uranyl oxygen atoms are incorporated into the
organic products. We have now extended the reductive borylation and acylation observed in this
system to silylation. Herein, we report the reactivity of (dippisq)2UO2THF with halosilanes, as
well as characterization of these products using 1H NMR spectroscopy, mass spectrometry, and
18O labeling experiments to pinpoint the uranyl oxygen atoms in the organic byproducts.
Experimental
General Considerations. All air- and moisture-sensitive manipulations were performed
using standard Schlenk techniques or in an MBraun inert atmosphere drybox with an atmosphere
of purified nitrogen. The MBraun drybox was equipped with two -35 °C freezers for cooling

samples. Solvents for sensitive manipulations were dried and deoxygenated using literature
procedures with a Seca solvent purification system.19 Celite was dried by heating to 120 °C
under vacuum overnight. Benzene-d6 was purchased from Cambridge Isotope Laboratories, dried
with molecular sieves and sodium and degassed by three freeze–pump–thaw cycles. Chloroform-
d was purchased from Cambridge Isotope Laboratories, dried with molecular sieves, and
degassed by three freeze–pump–thaw cycles. (dippisq)2UO2THF and (dippisq)2U18O2THF were
prepared according to literature procedures.6 Triphenylphosphine oxide (OPPh3) was
recrystallized from a THF solution layered with n-pentane at -35 °C. Dichlorodimethylsilane and
bromotrimethylsilane were purchased from Sigma Aldrich and distilled from calcium hydride
prior to use.
1H NMR spectra were recorded on a Varian Inova 300 spectrometer operating at 299.99.
All chemical shifts are reported relative to the peak for SiMe4, using 1H (residual) chemical shifts
of the solvent (C6D6: 7.16 ppm, CDCl3: 7.26 ppm) as a secondary standard. The spectra for
paramagnetic molecules were obtained by using an acquisition time of 0.5 s, thus the peak
widths reported have an error of ±2 Hz. GC/MS data were collected using a Shimadzu GC-
2010/MS-QP2010.
Reaction of (dippisq)2UO2THF with 2 Me3SiBr and 2 OPPh3. A 20 mL scintillation vial
was charged with (dippisq)2UO2THF (63 mg, 0.057 mmol) and dissolved in 5 mL toluene,
producing a green solution. With stirring, two equivalents of OPPh3 (32 mg, 0.115 mmol) were
added and no change in color was observed. After 5 minutes, two equivalents of Me3SiBr (18
mg, 0.115 mmol) were added, immediately resulting in a color change from green to brown.
After 90 minutes, volatiles were removed in vacuo, producing a brown material. Washing with
pentane (3 × 8 mL) yields an off-white powder. The volatiles were removed in vacuo from the

pentane washings and the organics were analyzed by 1H NMR spectroscopy (C6D6, 25 °C),
revealing dippiq0. The pentane-insoluble, off-white powder was recrystallized from a concentrated
THF solution (1 mL) layered with pentane (7 mL). After 24 hours at –35 °C, the off-white
precipitate was collected and identified by 1H NMR spectroscopy (C6D6, 25 °C) to be
(Me3SiO)2UBr2(OPPh3)220 (47 mg, 0.041 mmol, 73% yield).
Reaction of (Me3SiO)2UBr2(OPPh3)2. with 2 Me3SiBr. A 20 mL scintillation vial was
charged with (Me3SiO)2UBr2(OPPh3)2 (53 mg, 0.047 mmol) and dissolved in 2 mL toluene. Two
equivalents of Me3Si-Br (14 mg, 0.094 mmol) were added, resulting in a color change from
colorless to pale yellow, with precipitation of white a powder. After 16 hours, the solution was
decanted and the white powder was washed with pentane (2 × 8 mL) and toluene (2 × 8 mL),
producing UBr4(OPPh3)221 (35 mg, 0.039 mmol, 83% yield) as identified by 1H NMR
spectroscopy (CDCl3, 25 °C). The organics from this reaction were analyzed by GC/MS,
revealing Me3SiOSiMe3.
NMR Tube Reaction of (Me3SiO)2UBr2(OPPh3)2. with 2 Me3SiBr. A J-Young NMR
tube was charged with (Me3SiO)2UBr2(OPPh3)2 (47 mg, 0.043 mmol) and dissolved in 0.5 mL
deuterated chloroform, producing a colorless solution. Two equivalents of Me3Si-Br (13 mg,
0.085 mmol) were added, resulting in a color change from colorless to pale yellow. After 5
minutes, the solution was analyzed by 1H NMR spectroscopy (CDCl3, 25 °C), revealing
21
conversion of (Me3SiO)2UBr2(OPPh3)2 to UBr4(OPPh3)2 , and production of Me3SiOSiMe3.
Reaction of (dippisq)2UO2THF with 2 Me2SiCl2 and 2 OPPh3. A 20 mL scintillation vial
was charged with (dippisq)2UO2THF (80 mg, 0.073 mmol) and dissolved in 5 mL toluene,
producing a green solution. With stirring, two equivalents of OPPh3 (40 mg, 0.145 mmol) were
added and no change in color was observed. After 5 minutes, two equivalents of Me2SiCl2 (0.018

mL, 0.145 mmol) were added, resulting in a color change from green to brown over five minutes.
After 16 hours, the brown toluene solution was decanted, and the solid material was washed with
toluene (2 × 8 mL) and pentane (3 × 8 mL), producing a white powder. This material was
identified by 1H NMR Spectroscopy (25 °C, CDCl3) to be UCl4(OPPh3)220 (55 mg, 0.059 mmol,
81% yield). An aliquot from the organics of this reaction was analyzed by GC/MS, revealing
Me3SiOSiMe3. The volatiles from the organic washes were removed in vacuo, producing a
brown powder that was analyzed 1H NMR spectroscopy (C6D6, 25 °C), revealing dippiq0.
Results and Discussion
Based on our previous work, experiments to perform silylation of the uranyl moiety were
designed using electrophilic silylation reagents.20 Initial attempts were made using simple
halotrimethylsilanes (Me3SiCl, Me3SiBr, Me3SiI) with (dippisq)2UO2THF; however, these
reagents did not provide the desired silylated products. Instead, complicated mixtures of reduced
ligand silylation products were observed. To circumvent this and to assist uranyl silylation,
triphenylphosphine oxide (OPPh3) was added to the reaction mixture. This bulky oxygen-based
ligand has been shown to activate silyl halides,8 facilitating transfer of the silylium ion to the
nucleophilic oxygen in uranyl. Triphenylphosphine oxide also serves as an excellent ligand for
the oxophilic uranium center, which we hypothesized could displace the iminoquinone ligand
and aid in U(IV) product identification.
Treating a green toluene solution of (dippisq)2UO2THF with two equivalents of OPPh3
does not produce a color change signifying no reaction; however, addition of two equivalents of
Me3SiBr to this mixture rapidly produces a brown solution (Scheme 1). Following workup, an
off-white powder was obtained in moderate yield. 1H NMR spectroscopy (25 °C, C6D6)
confirmed the product as previously characterized (Me3SiO)2UBr2(OPPh3)2,20 suggesting the

neutral iminoquinone ligands are displaced by the triphenyl phosphine oxide ligands, as
originally hypothesized. Analysis of the organic byproducts of this reaction by 1H NMR
spectroscopy (25 °C, C6D6) showed free dippiq ligand. The formation of the iminoquinone ligand
confirms that oxidation of the iminosemiquinone ligand occurs during the course of the reaction,
and that these electron equivalents serve to reduce the U(VI) center to U(IV), facilitating the
reaction. Notably, when Me3SiI is employed as the silylating reagent instead of Me3SiBr, the
silylated ring opened THF product, Me3SiOCH2CH2CH2CH2I, was observed by 1H NMR
spectroscopy. This product has been identified in previous reactions with U(IV) iodide22 and
thorium(IV) iodide23 species and suggests Me3SiI is not suitable for silylation of THF-containing
(
dippisq)2UO2THF.
Scheme 1: Silylation of (dippisq)2UO2THF with Me3SiBr. Blue atoms represent neutral
atoms that engage in dative bonding, red atoms engage in ionic bonding.
Complete uranyl U-O bond scission was attempted by treating the uranium(IV) silylated
product, (Me3SiO)2UBr2(OPPh3)2, with two additional equivalents of Me3SiBr (Scheme 2). This
procedure successfully generated the previously characterized uranium(IV) tetrahalide
derivative, UBr4(OPPh3)224 and released two equivalents of hexamethyldisiloxane
(Me3SiOSiMe3), as observed in a sealed NMR tube experiment. These reactions show stepwise,
stoichiometric U=O bond scission of (dippisq)2UO2THF was successful using this method.

Scheme 2: Silylation and U–O bond cleavage of (Me3SiO)2UBr2(OPPh3)2, with Me3SiBr.
Following complete cleavage of the uranyl U-O bond in (dippisq)2UO2THF through
reductive silylation, the path of the uranyl oxygen during this reaction was studied using
isotopically labeled (dippisq)2U18O2THF. This oxygen isotopomer allows tracking of the uranyl
oxygen throughout the course of the silylation reaction. The stepwise reaction of
(
dippisq)2U18O2THF with OPPh3 and Me3SiBr yields the expected uranium product,
(UBr4(OPPh3)2). Analysis of the organics from this reaction using gas chromatography–mass
spectrometry (GC–MS) showed that the mass composition of unlabeled Me3SiOSiMe3 features a
base peak at m/z = 147, corresponding to [M-15]+ and matching the reported spectrum for
Me3SiOSiMe3 (Figure 2).25 In comparison, analysis of the mass spectrum of Me3Si18OSiMe3
obtained from (dippisq)2U18O2THF shows a base peak at m/z = 149. The shift of [M-15]+ from 147
to 149 m/z is indicative of 18O incorporation and supports the origin of the oxygen atom in the
hexamethyldisiloxane is from the labeled U-18O bond. Isotopic incorporation is also evident in
the fragmentation pattern of Me3SiOSiMe3, with fragments containing the oxygen-18 atoms
being shifted (Figure S7). The percentage of isotopic incorporation of (dippisq)2U18O2THF can be
calculated using the abundance of the [M-15]+ mass peaks. In this case, the isotopic
incorporation is to 77% 18O in (dippisq)2U18O2THF.

Figure 2: Mass spectra (electron ionization (EI)) of Me3SiOSiMe3 (top) and Me3Si18OSiMe3
(bottom).
Based on these reductive silylation results, it was hypothesized that the
iminosemiquinone ligands of (dippisq)2UO2THF could facilitate uranyl reduction, silylation, and
U-O bond scission in the presence of OPPh3. To demonstrate this, dimethyldichlorosilane
(Me2SiCl2) was examined, as it features a strongly electrophilic silicon center, as well as two
chlorine atoms that can be transferred to the uranium to maintain charge balance. Treating

(
dippisq)2UO2THF or (dippisq)2U18O2THF with two equivalents of Me2SiCl2 in the presence of
OPPh3 resulted in a visible reaction, with a color change from green to brown noted over five
minutes (Scheme 3). Upon workup, the neutral iminoquinone ligand was observed in pentane
washes, once again confirming oxidation of the ligand had occurred during the course of the
reaction. Analysis of the isolated, white powder from the reaction by 1H NMR (CDCl3)
confirmed formation of UCl4(OPPh3)220 as the uranium product, in good yield (81%).
Scheme 3: Silylation and U–O bond cleavage of (dippisq)2UO2THF, with Me2SiCl2. Blue
atoms represent neutral atoms that engage in dative bonding, red atoms engage in ionic bonding.
GC/MS was again employed to examine the organics from this reaction in order to
confirm the fate of the uranyl oxygen atoms (Figure 3). A variety of molecules of the form
(Me2SiO)n were observed in the gas chromatogram, with n = 3 being the most abundant.
Analysis of the unlabeled (Me2SiO)3 features a base peak at m/z = 207, corresponding to [M-15]+
and matching the reported spectrum for (Me2SiO)3.26 Analysis of the spectrum of (Me2Si18O)3
derived from the labeled uranyl, (dippisq)2U18O2THF, shows a base peak at m/z = 213, signifying
the majority of the material contains three 18O-labeled oxygens. Peaks at m/z = 211, m/z = 209,
16
18
and m/z = 207 are assignable as C6H18Si3 On Om (n + m = 3) with two, one, and zero 18O-
labeled oxygens, respectively. Isotopic incorporation is also evident is the fragmentation pattern
of (Me2Si18O)3, with fragments containing a variety of oxygen-18 atoms, complicating the

spectrum (Figure S8). Once again, these data confirm that the oxygen atoms from the labeled
uranyl is being extracted by the oxophilic silicon-containing reagents.
Figure 3: Mass spectra (electron ionization (EI)) of (Me2SiO)3 (top) and (Me2Si18O)3 (bottom).
Conclusions
In summary, the reactivity of hexavalent (dippisq)2UO2THF with silanes in the presence of
OPPh3 produces uranium (IV) species of the form UX4(OPPh3)2 (X = Cl, Br). The uranyl oxygen
atoms are extracted with the oxophilic silane groups, as determined by 1H NMR spectroscopy
and 18O-labeling experiments. The reaction of (dippisq)2UO2THF with Me2SiCl2and OPPh3
produces UCl4(OPPh3)2 in a one-step reaction.

Overall, these studies contribute to the already well-established field of reductive
silylation. In these cases, external reductants are often employed to the facilitate the reduction of
uranium(VI) to uranium(IV). The work presented here shows that the reduced iminosemiquinone
ligands are potent reductants, obviating the need for external ones. Furthermore, this work
demonstrates the utility of OPPh3 for product isolation and characterization, as well as the need
for silane screening in these reactions. Future studies will be aimed at expanding the scope of
silanes suitable for these reactions and examining new redox-active ligands to facilitate reductive
silylation.
Acknowledgements
This material is based upon work supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Award Number DE-SC0008479.
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Graphical abstract
Reductive silylation of uranyl is described with chloro- and bromosilanes. The
conversion of uranium(VI) to uranium(IV) is facilitated by the redox-active, iminosemiquinone
ligand. The siloxane byproducts of these reactions are labeled with oxygen-18 and characterized
using mass spectrometry.