Synthesis of uranium-ligand multiple bonds by cleavage of a trityl protecting group

Article abstract of DOI:10.1021/ja411423a

Addition of KSCPh₃ to [U(NR₂)₃] (R = SiMe₃) in tetrahydrofuran, followed by addition of 18-crown-6, results in formation of the U(IV) sulfide, [K(18-crown-6)][U(S)(NR ₂)₃] (1) and Gomberg's dimer. Similarly, addition of KOCPh₃ to [U(NR₂)₃] in tetrahydrofuran, followed by addition of 18-crown-6, results in formation of the U(IV) oxide, [K(18-crown-6)][U(O)(NR₂)₃] (3). Also observed in this transformation are the triphenylmethyl anion, [K(18-crown-6)(THF) ₂][CPh₃] (5), and the U(IV) alkoxide, [U(OCPh ₃)(NR₂)₃] (4).

Full text of DOI:10.1021/ja411423a

Communication
pubs.acs.org/JACS
Synthesis of Uranium−Ligand Multiple Bonds by Cleavage of a Trityl
Protecting Group
Danil E. Smiles, Guang Wu, and Trevor W. Hayton*
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States
S
* Supporting Information
spectrum of the reaction mixture displays a broad resonance at
ABSTRACT: Addition of KSCPh3 to [U(NR2)3] (R =
−2.48 ppm assignable to a new uranium(IV) sulfide complex,
SiMe3) in tetrahydrofuran, followed by addition of 18-
crown-6, results in formation of the U(IV) sulfide, [K(18-
crown-6)][U(S)(NR2)3] (1) and Gomberg’s dimer.
Similarly, addition of KOCPh3 to [U(NR2)3] in tetrahy-
drofuran, followed by addition of 18-crown-6, results in
formation of the U(IV) oxide, [K(18-crown-6)][U(O)-
(NR2)3] (3). Also observed in this transformation are the
triphenylmethyl anion, [K(18-crown-6)(THF)2][CPh3]
(5), and the U(IV) alkoxide, [U(OCPh3)(NR2)3] (4).
in addition to resonances at 5.21, 5.98, and 6.23 ppm,
assignable to the allylic and vinylic protons of Gomberg’s
dimer (Figure S1 in Supporting Information [SI]).33 On a
preparative scale, reaction of KSCPh3 with [U(NR2)3],
followed by addition of 18-crown-6, affords [K(18-crown-
6)][U(S)(NR2)3] (1) as yellow-orange blocks in 48% yield
after crystallization from diethyl ether (Scheme 1). Similarly,
Scheme 1
here is significant interest in the synthesis and study of
Tcomplexes with actinide−ligand multiple bonds, due in
part to the need for a better understanding of covalency and f-
orbital participation in actinide−ligand bonding.1−6 Recently,
significant progress has been made toward the synthesis and
characterization of f-element oxos,7−10 imidos,11−15 and
nitridos.16−19 Despite this progress, however, there still exist
several unanswered challenges in the synthesis of actinide−
ligand multiple bonds. For example, alkylidene and phosphido
complexes of the actinides remain unknown, despite several
attempts toward their isolation.20,21 In addition, only a few
terminal chalcogenido (E = S, Se, Te) complexes are known,
and their isolated yields are often low.22−24 This suggests that
new methods for the installation of these functional groups are
needed to permit the further development of this field.
The triphenylmethyl (trityl) moiety is a common protecting
group for alcohols, thiols, and amines in organic synthesis.25
Selective removal of the trityl group is possible using a variety
of conditions, including reductive cleavage with Li/naphthalene
in THF.26,27 Yet, while relatively common in organic chemistry,
there are only a few instances of the trityl protecting group
being used as a leaving group in inorganic synthesis.28 For
example, Kitajima and co-workers reported the formation of a
bimetallic Cu(II) disulfide, [Tp′Cu]2(μ-η2:η2-S2) (Tp′ =
HB(3,5-iPr2pz)3), along with formation of Gomberg’s
dimer29,30 via thermal C−S bond homolysis in a Cu(I)
trityl−thiolate complex, [Tp′Cu(SCPh3)].31 Riordan and co-
workers reported the formation of a Ni(II) μ-η2:η2 disulfide by
a similar procedure.32
utilization of 2,2,2-cryptand in place of 18-crown-6 affords
[K(2,2,2-cryptand)][U(S)(NR2)3] (2) as yellow-orange nee-
dles in 45% yield after crystallization (Scheme 1). Notably, in a
related transformation Arnold and co-workers showed that
oxidation of [U(NR2)3] with ClCPh3 also resulted in
Gomberg’s dimer formation, along with the U(IV) chloride,
[U(Cl)(NR2)3].34
The connectivities of complexes 1 and 2 were verified by X-
ray crystallography (Figures 1 and S27; see SI for complete
structural details of complex 2). Complex 1 crystallizes with
two molecules in the asymmetric unit, one of which is omitted
for clarity. In both complexes, the [U(S)(NR2)3]− anion
features a pseudotetrahedral geometry, similar to that observed
previously for this moiety.23 Also of note, the sulfide ligand in 1
Inspired by the results of the Kitajima and Riordan groups,
we explored the use of trityl as a leaving group for actinide−
ligand multiple bond formation. Thus, addition of 1 equiv of
KSCPh3 to a cold (−25 °C) solution of [U(NR2)3] (R =
SiMe3) in tetrahydrofuran-d8 results in an immediate color
Received: November 8, 2013
Published: December 18, 2013
1
change from dark purple to vibrant orange. The H NMR
© 2013 American Chemical Society
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[U(OCPh3)(NR2)3] (4), was also observed in the reaction
mixture, as evidenced by the aryl C−H resonances at 7.44, 8.57,
and 17.28 ppm. Finally, the red precipitate was identified as
[K(18-crown-6)(THF)2][Ph3C] (5) by X-ray crystallography
(Scheme 1). Its identity was further supported by a comparison
of its 1H NMR spectrum with previously reported spectral data
for the trityl anion,40 and by its independent synthesis, via the
reduction of triphenylmethane with KC8 in the presence of 18-
crown-6 (Scheme S5, SI).
The terminal oxo complex, [K(18-crown-6)][U(O)(NR2)3]
(3), can be independently synthesized by reduction of the
previously reported U(V) oxo [U(O)(NR2)3]7 with KC8,
followed by the addition of 18-crown-6. Crystallization from
diethyl ether affords 3 as pale-purple blocks in 50% yield
(Scheme S3, SI). Complex 3 can also be formed by reaction of
[U(O)(NR2)3] with 5, concomitant with formation of
Gomberg’s dimer, as revealed by 1H NMR spectroscopy
(Figure S21, SI).
Figure 1. Solid-state molecular structure of 1 (left) and 3 (right), with
50% probability ellipsoids. One molecule of 1 and hydrogen atoms are
omitted for clarity. 1: U1−S1 = 2.4463(6) Å, S1−K1 = 3.0684(8) Å;
3: U1−O1 = 1.890(5) Å, O1−K1 = 2.640(5) Å.
1
is coordinated by the K+ ion of the [K(18-crown-6)]+ moiety,
whereas complex 2 exists as a discrete cation/anion pair. The
U−S bond lengths in 1 (2.4463(6) and 2.4513(6) Å) and 2
(2.4423(16) Å) are identical by the 3σ criterion, and are slightly
shorter than those reported for the U(IV) terminal sulfides,
[Ph3PCH3][U(S)(NR2)3] (U−S = 2.4805(5) Å)23 and [Na-
(18-crown-6)][U(Cp*)2(StBu)(S)] (U−S = 2.462(2) and
2.477(2) Å).24 The S−K distances in 1 (3.0684(8) and
3.1551(8) Å) are long and comparable to the S−Na interaction
in [Na(18-crown-6)][U(Cp*)2(StBu)(S)] (S−Na = 3.135(4)
Å).24 The S−K interaction in 1 is likely quite weak, a
hypothesis that is supported by the nearly identical U−S bond
lengths in complexes 1 and 2.
The H NMR spectrum of 3 in benzene-d6 exhibits two
broad resonances at −4.91 and 16.15 ppm, assignable to the
methyl groups of the silylamide ligands and the methylene
groups of the 18-crown-6 moiety, respectively (Figure S9, SI).
Complex 3 is isostructural with complex 1 in the solid state; as
with 1, complex 3 features a dative interaction between the
chalcogenido ligand and the [K(18-crown-6)]+ moiety. The
U−O bond length in 3 (1.890(5) Å) is statistically equivalent
to that of the previously reported [Cp*2Co][U(O)(NR2)3]
(U−O = 1.878(5) Å),23 which features the identical [U(O)-
(NR2)3]− anion. Finally, the E1−K1 distance in 3 (2.640(5) Å)
is shorter than that of 1, consistent with the smaller ionic radii
of O2− vs S2−.
1
The U(IV) alkoxide, [U(OCPh3)(NR2)3] (4), can also be
independently synthesized via reaction of [U(I)(NR2)3]7 with 1
equiv of KOCPh3. This complex can be isolated as pale-purple
plates in 38% yield after crystallization from a concentrated
The H NMR spectrum of 1 in benzene-d6 exhibits two
broad resonances at −2.02 and −1.11 ppm, assignable to the
methyl groups of the silylamide ligands and the methylene
groups of the 18-crown-6 moiety, respectively (Figure S2, SI).
The latter resonance is paramagnetically shifted, suggesting that
the [K(18-crown-6)]+ cation is in close contact with the
[U(S)(NR2)3]− anion in this solvent. Upon dissolution of 1 in
tetrahydrofuran-d8, the resonance assignable to the 18-crown-6
moiety shifts to 1.46 ppm, closer to that of free 18-crown-6
(3.57 ppm in tetrahydrofuran-d8),35 suggesting the formation of
better separated cation/anion pairs in this donating solvent
1
diethyl ether solution (Scheme S4, SI). Its H NMR spectrum
in benzene-d6 consists of four resonances at −4.85, 7.74, 8.56,
and 17.22 ppm, in a 54:3:6:6 ratio, respectively, corresponding
to the methyl groups of the silylamide ligands and the p-, m-,
and o-aryl protons of the trityl−alkoxide ligand (Figure S10,
SI). Importantly, the resonances for the aryl protons match
those seen in the reaction between [U(NR2)3] and KOCPh3,
and confirm the presence of 4 in that transformation.
1
(Figure S3, SI). The H NMR spectrum of 2 in pyridine-d5
In order to gain further mechanistic insight into the
formation of complexes 3, 4, and 5, we monitored the reaction
of [U(NR2)3] with KOCPh3 in THF-d8, in the presence of 18-
exhibits a broad resonance at −2.33 ppm, assignable to the
methyl groups of the silylamide ligands, and three resonances at
2.25, 3.25, and 3.29 ppm, assignable to the three proton
environments of the 2,2,2-cryptand moiety (Figure S5, SI). In
addition, the UV−vis/NIR spectra of 1 and 2 are consistent
with the presence of U(IV) centers (Figures S37 and S38,
SI).7,23,36−39
1
1
crown-6, by H NMR spectroscopy. A H NMR spectrum of
this solution, after standing at −25 °C for 30 min, reveals the
formation of a new species that we have tentatively assigned as
the U(III) alkoxide, [K(18-crown-6)][U(OCPh3)(NR2)3] (6).
The presence of 6 is supported by resonances at −8.37, 7.57,
8.33, and 17.02, in a 54:6:3:6 ratio (Figure S7, SI), which
correspond to the methyl groups of the silylamide ligands and
the m-, p-, and o-aryl protons of the trityl−alkoxide ligand,
respectively. Upon warming this solution to 25 °C, the
resonances assigned to 6 disappear (Figure S8, SI), while
those assigned to 3 and 4 grow in intensity. To account for
these observations, we propose that 6 undergoes a
disproportionation reaction, instead of C−O bond homolysis,
wherein one molecule of the U(III) alkoxide reduces the
triphenylmethyl fragment of a second molecule. The first
molecule is thereby oxidized, accounting for the formation of 4,
while reduction of the triphenylmethyl group on the second
After successful demonstration of terminal sulfide formation
via trityl radical elimination, we explored the viability of this
method for the synthesis of the analogous terminal oxide. Thus,
addition of 1 equiv of KOCPh3 to a cold (−25 °C) solution of
[U(NR2)3] in benzene-d6, in the presence of 18-crown-6,
results in an immediate color change to deep red, concomitant
1
with the deposition of a red solid. Surprisingly, the H NMR
spectrum of this reaction mixture revealed no evidence for
formation of Gomberg’s dimer (Figure S6, SI). However, the
spectrum did reveal formation of a U(IV) terminal oxo, [K(18-
crown-6)][U(O)(NR2)3] (3), as evidenced by the broad
resonance at −4.87 ppm, assignable to the methyl groups of
the silylamide ligands. In addition, the U(IV) alkoxide,
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Communication
molecule results in heterolytic cleavage of the C−O bond and
formation of both 3 and 5 (Scheme S1, SI). If true, this
mechanism suggests that the trityl group of the [OCPh3]−
ligand could be selectively removed by reduction with an
external reducing agent. Gratifyingly, reduction of 4 with KC8,
in the presence of 18-crown-6, results in the formation of both
3 and 5, which are isolated in 36% and 52% yield, respectively
(eq 1) (Figures S19 and S20, SI). To our knowledge, this is the
(NH-2,6-iPr2C6H3)5],43 consistent with the presence of a
primary amide ligand. The long U−Ntrityl bond length also rules
out the presence of an imido ligand, as U(IV)−Nimido bond
lengths are typically much shorter (1.95−2.05 Å)20,21
Undoubtedly, trityl release during the formation of
complexes 1, 2, and 3 is driven, in part, by the highly reducing
U(III/IV) redox potential. However, we suggest that the C−E
bond dissociation enthalpies and the strength of the new U−E
bond being formed also play a role in determining the reaction
outcome. While BDE data is not readily available for
triphenylmethyl−heteroatom bonds, C−E BDE data is known
for the benzyl derivatives, PhCH2EH (E = S, 60.4 kcal/mol; O,
81 kcal/mol; NH, 74.0 kcal/mol).44,45 Accordingly, the
spontaneous C−S bond cleavage observed in the KSCPh3
reaction can be explained by the relatively weak C−S bond.
The weak C−S bond should also result in a low kinetic barrier
for trityl release. In contrast, the stronger C−E bonds for the
oxygen and nitrogen trityl precursors render trityl radical
release less favorable, both thermodynamically and kinetically.
In the case of the oxygen analogue, however, an alternate
pathway for trityl release is operative (e.g., disproportionation),
no doubt because the highly oxophilic nature of uranium
greatly favors the formation of U−O multiple bonds,46 whereas
in the case of nitrogen, the weaker U−N multiple bond does
not outweigh the energy required to break the C−N bond in
[Ph3CNH]−.
In summary, we have demonstrated the syntheses of a
terminal oxo and a terminal sulfido complex of uranium by
release of the trityl protecting group. Importantly, the trityl
group can be cleaved from the heteroatom by two different
mechanisms: spontaneous loss of trityl radical or reductive
cleavage of the trityl group and loss of the trityl anion.
Intriguingly, KSCPh3 is acting as a 1e− oxidant in its reaction
with [U(NR2)3], converting the U(III) precursor into a U(IV)
terminal sulfide. This is significant because most chalcogen
atom transfer reagents are 2e− oxidants, suggesting that with a
trityl-based chalcogen source, any 1e− metal redox couple could
be harnessed to synthesize a terminal chalcogenide ligand.
Accordingly, this protocol could have wide synthetic utility in
inorganic chemistry, as most first row transition metals, and all
of the lanthanides, are resistant to 2e− redox chemistry.
first example of reductive deprotection of trityl to form an oxo
ligand and demonstrates that traditional organic deprotection
protocols25 can be applied to inorganic synthesis.
Alternately, the formation of 3, 4, and 5 from the reaction of
[U(NR2)3] and KOCPh3 could arise via formation of a U(V)
oxo intermediate, [U(O)(NR2)3]. In this scenario, complex 6
undergoes a heterolytic C−O bond cleavage, resulting in the
formation of 5 and [U(O)(NR2)3]. This U(V) species could
then be reduced by another molecule of 6, resulting in
concomitant formation of 3 and 4 (Scheme S2, SI). However,
we suggest that this pathway is not operative, as the reaction of
[U(O)(NR2)3] with 5, the microscopic reverse of the first step,
results in the formation of 3 and Gomberg’s dimer (Figure S21,
SI). Given the absence of any evidence for the presence of
[U(O)(NR2)3] or Gomberg’s dimer in the reaction mixture, we
suggest that the first pathway is more likely.
We also endeavored to synthesize a U(IV) imido complex
using the trityl deprotection protocol. Thus, reaction of
[U(NR2)3] with 1 equiv of LiNHCPh3,41 in the presence of
2 equiv of 12-crown-4, in tetrahydrofuran results in formation
of a dark red−brown solution. Crystallization from diethyl
ether affords [Li(12-crown-4)2][U(NHCPh3)(NR2)3] (7) as a
dark red−brown microcrystalline solid in 42% yield (eq 2).
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, crystallographic details (as CIF files)
and spectral data for complexes 1−5, and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
Unlike the analogous reactions with the sulfur and oxygen
derivatives, no evidence for the formation of Gomberg’s dimer
or the trityl anion was observed in the reaction mixture (Figure
AUTHOR INFORMATION
■
1
S14, SI). The H NMR spectrum of 7 features five distinct
Corresponding Author
resonances at −7.41, 3.64 6.22, 7.36, and 12.23 ppm,
corresponding to the methyl groups of the silyamide ligands,
the methylene groups of the 12-crown-4 moiety, and the p-, m-,
and o-aryl protons of the trityl group, respectively (Figure S15,
SI). While the NH resonance of the [NHCPh3]− ligand was not
hayton@chem.ucsb.edu
Notes
The authors declare no competing financial interest.
1
definitively identified in the H NMR spectrum, the UV−vis/
ACKNOWLEDGMENTS
■
NIR spectrum of 7 is consistent with the presence of a U(III)
center (Figure S41, SI).42 In the solid state, complex 7
crystallizes as a discrete cation/anion pair. The U−Ntrityl bond
length (2.342(4) Å) and U−N−C angle (151.2(3)°) are similar
to those observed in the related U(III) amide, [K(THF)2]2[U-
This work was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Chemical Sciences, Bio-
sciences, and Geosciences Division under Contract No. DE-
FG02-09ER16067.
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