Synthesis and Structure of Uranium-Silylene Complexes

Article abstract of DOI:10.1002/chem.202000214

While carbene complexes of uranium have been known for over a decade, there are no reported examples of complexes between an actinide and a ?heavy carbene.“ Herein, we report the syntheses and structures of the first uranium-heavy tetrylene complexes: (Cp

Full text of DOI:10.1002/chem.202000214

A Journal of
Accepted Article
Title: Synthesis and Structure of Uranium–Silylene Complexes.
Authors: John Arnold, Ian Joseph Brackbill, Daniel Lussier, Michael
Boreen, Laurent Maron, and Iskander Douair
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COMMUNICATION
Synthesis and Structure of Uranium–Silylene Complexes
[a]
[b]
[a]
[a]
[b]
I. Joseph Brackbill, Iskander Douair, Daniel J. Lussier, Michael A. Boreen, Laurent Maron, and
John Arnold*[a]
Abstract: While carbene complexes of uranium have been known
for over a decade, there are no reported examples of complexes
between an actinide and a “heavy carbene.” Here we report the
such bonding interactions may possess a higher degree of
covalent character and could afford higher selectivity for trivalent
5f ions. Indeed, a recent theoretical study by Shi and co-workers
predicted a series of polarized σ bonds between uranium and
divalent heavy group 14 elements.[17] We were particularly drawn
to the amidinate-supported silylenes which have emerged in
recent years. The 2010 discovery by Roesky and co-workers
that amidinate-supported silylenes could be prepared in high
yield through dehydrochlorination pathways has caused a surge
of growth in their research and application;[18,19] these silylenes
have been bound to metals from nearly every group in the
transition metal series[20] as well as to lanthanides.[21] The
syntheses and structures of the first uranium–heavy tetrylene
t
complexes: (CpSiMe
3 3
)
U–Si[PhC(NR)
2
]R′ (R = Bu, R′ = NMe
2
1; R =
i
i
2
Pr, R′ = PhC(NPr) 2). Complex 1 features a kinetically robust
uranium–silicon bonding interaction, while the uranium–silicon bond
in 2 is easily disrupted thermally or by competing ligands in solution.
Calculations reveal polarized σ bonds, but depending on the
substituents at silicon a substantial π bonding interaction is also
present. The complexes possess relatively high bond orders which
suggests primarily covalent bonding between uranium and silicon.
These results comprise a new frontier in actinide–heavy main group
bonding.
growing application of these silylenes is largely due to the ease
t
with which the parent chlorosilylene Si[PhC(N Bu)
2
]Cl can be
derivatized to feature substituents such as alkoxy, amino,
phosphino, and even alkyl groups, thus accessing an array of
silylenes with varying steric and electronic properties.[20,22,23]
In 2009, we showed that the uranium(III) complex
Studies of the fundamental coordination chemistry of the
actinide elements lag behind those of the transition metals.[1]
Recent experimental and computational investigations into the
chemistry of the actinides incorporating new metal–element
bonds have done much to challenge our understanding of
actinide electronic structure. Despite the growing number of
examples of bonds between actinides and heavy group 13,[2–4]
group 15,[5] and group 16[6] elements, there remain few
structurally characterized examples of unsupported bonds
between actinides and group 14 elements heavier than
carbon.[7–9]
(
CpSiMe
3
)
3
U could be employed to generate species containing
[2]
actinide–group 13 bonds with aluminum(I) and gallium(I).
3 3
CpSiMe ) U has shown considerable versatility as a σ-acceptor
(
and π-donor: this species was the first molecular uranium
[24,25]
complex known to bind CO at room-temperature.
Here we
report the synthesis and characterization of the first actinide–
3 3
heavy tetrylene complexes, obtained by reaction of (CpSiMe ) U
with amidinate-supported silylenes. These complexes address a
gap in actinide–main group bonding and contribute to our
growing understanding of covalent bonding in the actinides.
In 2005, Ephritikhine and co-workers reported that
tetramethyl imidazol-2-ylidene, an “Arduengo-type”
N-
heterocyclic carbene (NHC), was capable of preferential binding
of uranium(III) complexes over isostructural cerium(III)
analogs.[ The propensity for the NHC to bind uranium with
modest selectivity over cerium was attributed to the “softness” of
the NHC ligand,[11] which has a greater capacity to interact with
the diffuse and energetically accessible 5f orbitals of uranium
than the core-like 4f orbitals of cerium.[12] Such differentiation
between trivalent 4f and 5f elements is of particular significance
to the area of nuclear waste processing and remediation.[13]
While further examples of actinide–carbene complexes have
since emerged,[14–16] species featuring bonds between actinides
and a heavy ‘tetrylene’ – a formally divalent group 14 element
heavier than carbon – have remained elusive. Since the heavier
analogs of carbon should be “softer,” we became interested in
stabilizing bonds between actinides and heavy tetrylenes, as
10]
Scheme 1. Synthesis of complexes 1 and 2.
Considering the minimal steric encumbrance of the
dimethylamido substituent, we targeted the silylene
t
Si[PhC(N Bu)
2
](NMe
2
)
as an ideal candidate for binding
(
(
CpSiMe
CpSiMe
3
)
)
3
U. Mixing room-temperature hexane solutions of
t
3
3
U and Si[PhC(N Bu)
2
](NMe
2
) yielded a dark purple
solution. Concentration and cooling to –40 °C overnight afforded
burgundy crystals of analytically pure (CpSiMe
3
)
3
U–
[
[
a]
b]
I. J. Brackbill, D. J. Lussier, M. A. Boreen, Prof. Dr. J. Arnold
Department of Chemistry, University of California, Berkeley, and the
Chemical Sciences Division, Lawrence Berkeley National
Laboratory
Berkeley, California 94720-1460 (USA)
E-mail: arnold@berkeley.edu
t
Si[PhC(N Bu)
2
](NMe
2
) (1) in 93% yield (Scheme 1). An X-ray
diffraction study on single crystals of 1 revealed a uranium–
silicon bond length of 3.1637(1) Å (Figure 1). While this distance
is longer than the sum of the covalent radii of uranium and
silicon as tabulated by Pyykkö (2.86 Å)[ or Alvarez (3.07 Å),
26]
[27]
I. Douair, Prof. Dr. L. Maron
bond lengths between f-elements and heavy main group atoms
have often exceeded the sums of the relevant covalent radii,
LPCNO, Université de Toulouse, INSA Toulouse
[3,28–
135 Avenue de Rangueil, 31077 Toulouse, France
3
0,21]
suggesting that these radii may not be the best predictor of
main group element–f element bond length.[31] Compared to its
base-free’ solid state structure,[32] the (CpSiMe
U subunit has
Supporting information for this article is given via a link at the end of
the document.
‘
3 3
)
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COMMUNICATION
structurally characterized (Scheme 1, Figure S16). Interestingly,
and in contrast to its adducts with Lewis-acidic boron, aluminum,
and transition metal compounds,[ the Si[PhC(NPr)
38]
i
2 2
] fragment
in 2 has retained one monodentate amidinate ligand, likely to
minimize steric strain.
Figure 1. Crystal structure of 1 with thermal ellipsoids shown at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond
distances and angles are tabulated in the Supporting Information (Table S1).
reorganized to minimize unfavorable steric interactions: the
trimethylsilyl (TMS) groups on the cyclopentadienyl rings of
(
3 3
CpSiMe ) U are oriented away from the fourth ligand, an
arrangement that has only been observed previously in some
[
33–35]
dimeric structures containing bridging ligands.
The bond
) fragment are
t
lengths and angles of the Si[PhC(N Bu)
2
](NMe
2
[
36]
largely unchanged upon coordination (Table S1), although the
tert-butyl groups are angled further away from the silicon center
(
C(3)–N(2)–C(10) 129.8(2), C(3)–N(3)–C(14) 129.5(3)), likely as
a result of steric interactions.
Figure 2. Solution-state NIR absorption spectra of (CpSiMe
3 3
) U (blue) and
The room-temperature 1H NMR spectrum of 1 shows
(CpSiMe ) U–Si[PhC(N Bu) ](NMe ) (1, green) in toluene, and the solid-state
t
3
3
2
2
i
t
diffuse reflectance spectrum of (CpSiMe
3 3 2 2
) U–Si[PhC(NPr) ] (2, yellow). Since
Si[PhC(N Bu)
2
](NMe
2
) resonances shifted well out of their usual
1
and 2 are in equilibrium with their starting materials in solution, absorption
1
diamagnetic region (Figure S1). Variable-temperature (VT)
NMR did not reveal free Si[PhC(N Bu)
H
spectra are reported on an arbitrary intensity scale. Furthermore, the spectra
in Figure 2 have been scaled arbitrarily to highlight similarities and differences.
t
2
](NMe
2
) upon heating to
8
4 °C, indicative of a remarkably stable and persistent bonding
U (Figure S3). The 29Si{ H} NMR
1
interaction with (CpSiMe
spectrum of shows
cyclopentadienyl-based TMS groups at –170 ppm, shifted
upfield ~5 ppm from their value in (CpSiMe U (Figure S2). The
3
)
3
The room-temperature 1H NMR spectrum of a dilute C D6
1
a
sharp resonance for the
6
solution of 2 shows only minor shifts and broadening of the
starting material peaks, indicative of dynamic behavior in
solution and an equilibrium that heavily favors the reactants side
(Figure S5). At higher concentrations, however, the shifts and
broadening effects become more dramatic (Figure S6). In the
room-temperature 29Si{ H} NMR spectrum of 2, the Cp-based
TMS resonance was observed at –164.4 ppm (Figure S7),
essentially unchanged from its value in the isolated uranium(III)
starting material (–165 ppm).[ As with complex 1, a resonance
attributable to the silylene was not observed. To probe the
temperature dependence of the solution equilibrium, we
3 3
)
resonance corresponding to the silicon center of the
t
Si[PhC(N Bu)
2
](NMe
2
) subunit, typically located around –2.6
ppm,[36] was not observed between 200 and –400 ppm due to its
proximity to the paramagnetic uranium(III) center.
1
The room-temperature electronic absorption spectrum of 1
shows high energy charge-transfer (CT) bands around 460 and
39]
5
00 nm; these transitions are red-shifted relative to the
analogous CT bands in the spectrum of isolated (CpSiMe
Figure S11). Additionally, the region between 600 and 1500 nm
3 3
) U
(
1
reveals a series of Laporte-forbidden f–f transitions that are
characteristic of uranium(III) (Figure 2).[31,37] The energies of
these transitions differ considerably from those of ‘base-free’
performed VT H NMR spectroscopic studies on 2 from 23 to –
80 °C (Figure S8). Resonances attributable to the isopropyl
i
methyl and methyne groups of Si[PhC(NPr) ] broadened and
2 2
(
CpSiMe
3
)
3
U even at low concentrations (2 mM), consistent with
shifted out of the diamagnetic region as the temperature was
lowered, indicating closer contact with the paramagnetic
uranium(III) center. However, more detailed analysis of the VT
1H NMR data was obscured by the broadness of most signals at
lower temperature, which may result from fluxionality not only
between the starting materials and 2 but also between the
mono- vs. bidentate binding modes of the amidinato ligands
the lower symmetry of 1 and suggestive of an A + B ⇌ AB
equilibrium that heavily favors the products side in solution.
Having isolated a uranium–silylene complex using the
t
sterically accessible Si[PhC(N Bu)
investigate the influence of increasing steric encumbrance on
the U–Si bond properties. A second example of a uranium–
silylene complex (2) derived from the bulkier silylene
2
](NMe
2
) silylene, we sought to
i
bound to Si[PhC(NPr) ] .
2
2
i
Si[PhC(NPr)
2
]
2
was prepared similarly in 99% yield and
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In contrast to 1, the room-temperature electronic absorption
spectrum of 2 in solution is nearly identical to the summation of
the spectra of the two starting materials (Figures S12), providing
further evidence that the solution equilibrium between the
reactants and 2 lies on the reactants side at room temperature.
The diffuse reflectance spectrum of solid 2, on the other hand,
bears an absorbance profile strikingly similar to the solution-
state NIR spectra of 1 (Figure 2), including prominent f–f
transitions at approximately 920, 980, 1020, 1080, 1220, and
1
335 nm. The close resemblance of the spectra suggests that,
while 2 readily undergoes ligand dissociation in solution, the
uranium–silylene interaction is less perturbed in the solid state
yielding a complex that is very similar electronically to 1.
Furthermore, the infra-red (IR) spectrum of 2 shows substantial
differences relative to those of the starting materials (Figure
S15); evidently, the uranium–silicon bonding interaction in 2
Figure 3. Kohn-Sham α-spin HOMO-4 of 1 showing the primary U–Si bonding
orbital.
changes the electronic and vibrational nature of the
i
Si[PhC(NPr)
2
]
2
unit, further discounting the possibility that the
BDE for 1, at 11.9 kcal/mol, is remarkably high (cf. 13.1 kcal/mol
uranium and silicon starting materials simply co-crystallized.
Magnetometric studies of 1 and 2 are ongoing. Preliminary
results on the DC magnetic susceptibility of 2 are consistent with
the trivalent oxidation state assignment for uranium (Figures
U–pyridine).[ The stability of the
2]
for the model complex Cp
3
uranium–silicon bond in 1 is further demonstrated by the low-
lying nature of the bonding orbital (HOMO–4) which contrasts
with the bonding orbital in 2 (HOMO); accordingly, the uranium–
silicon bond in 1 should be less easily perturbed. Additionally, at
the second order NBO level, a significant π backbonding
contribution (10.3 kcal/mol) is present in 1, while the
backbonding in 2 is less than half as strong (Table S3). The
substantial backbonding in 1 contrasts with the NBO analysis
S17-S18).[
40]
Notably, the room-temperature χ
M
T value of 2 is
3 3
lower than that of ‘base-free’ (CpSiMe )
U (Figure S17), which
may reflect the stronger ligand-field splitting in 2.
Accessing a third uranium–silylene complex via the widely
utilized
Si[PhC(N Bu)
reaction between Si[PhC(N Bu)
parent
chlorosilylene,
t
2
]Cl, proved impossible under our conditions:
t
performed on the theoretical complex (CpSiMe
Si(NCHMes) investigated by Shi and co-workers, which found
no U–Si π bonding character.[
3 3
) U–
2
]Cl and (CpSiMe
the uranium(IV)
and the bis(silylene), Si
3
)
3
U resulted in
2
complete
CpSiMe
conversion
UCl,[
to
chloride,
17]
41]
t
[42]
(
3
)
3
2
[PhC(N Bu)
2
]
2
,
as
1
We have begun studies into the reactivity of these
complexes with the aim of probing the nature of the U–Si
interaction. Addition of 1 atm of CO to a benzene-d solution of 1
6
resulted in an equilibrium between the CO and silylene adducts
confirmed by H NMR spectroscopy as well as unit cell checks of
the crystalline products (Figure S10). Analogous reactivity
t
toward Si[PhC(N Bu)
2
]Cl has been observed for a samarium(II)
metallocene.[
21]
The reduction from silicon(II) to silicon(I)
1
of (CpSiMe
3
)U, as monitored by H NMR spectroscopy (Figure
highlights the well-known reducing power of uranium(III)
organometallic complexes.
1
S4). The H NMR spectrum of 1 after the addition of CO closely
resembled the spectrum of isolated 1, indicating that the
equilibrium was not dominated by (CpSiMe
spectrum of 1 could be recovered by freeze-pump-thawing the
solution and replacing the CO atmosphere with dry N . Although
The electronic structures of both silylene complexes have
been investigated using density functional methods: natural
bond orbital (NBO) analyses of geometry-optimized 1 and 2
reveal polarized σ bonds between uranium and silicon (Figure 3,
Table S3). The Wiberg bond indices for 1 and 2 are relatively
high (~0.7) compared to those calculated for other donor–
acceptor complexes containing the (CpSiMe ) U fragment and
3 3
suggest a primarily covalent single bond between uranium and
silicon.[2] In both complexes, the uranium hybrid acceptor orbital
3 3
)
U–CO.[24] The
2
qualitative due to the unknown concentration of CO in solution,
t
the ability of Si[PhC(N Bu)
2
](NMe
2
) to compete with CO for
uranium binding further confirmed the kinetic persistence of the
uranium–silylene interaction of 1 in solution, as the metal–ligand
bond dissociation energy in (CpSiMe
estimated at 14.3 kcal mol . In contrast, addition of 1 atm of
3
)
3
U–CO has been
-1 [25]
is of primarily d parentage (58%) but with substantial f-orbital
contributions (12%) (Table S3). Similar f-orbital contributions
were calculated for dative bonds between U(III) and the group
CO to a benzene-d
conversion to (CpSiMe
6
solution of 2 showed essentially complete
i
3
)
3
U–CO and free Si[PhC(NPr)
2
]
2
(Figure
3 diyls AlCp* and GaCp*.[2] The silicon donor hybrids in both
S9). These ligand substitution reactions highlight the dative
bonding character of 1 and 2 while reaffirming that the U–Si
bond of 1 is less easily perturbed than that of 2.
In conclusion, we have synthesized and characterized the
first complexes containing actinide–heavy tetrylene bonds.
Complex 1 possesses a remarkably stable uranium–silicon
bonding interaction that can compete with strong π-acids such
as CO, while complex 2 shows only a weak interaction in
solution that can be stabilized by cooling or crystallization.
These bonds can be described as polarized covalent σ bonds,
1
complexes are of primarily s character (73% s, 27% p), falling in
the range of values reported for amidinato-silylene complexes of
transition metals,[43] while a substantially higher percentage of p
character was calculated for the formally X-type group 14
TIPS
3 3
ligands in ( TREN)U–SnMe and the model complex H Si–
.[8,9] Consistent with our experimental observations in the
H NMR and UV/vis/NIR spectra of 1 and 2, complex 1 is
calculated to have a BDE nearly twice that of complex 2. The
U(NH
1
2
)
3
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COMMUNICATION
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This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences Heavy Element Chemistry
Program of the U.S. Department of Energy (DOE) at LBNL
under Contract DE-AC02-05CH11231. I.J.B. acknowledges the
U.S. DOE Integrated University Program for a graduate research
fellowship. Collection and interpretation of the magnetic
susceptibility data and near-IR spectra were supported by the
National Science Foundation (NSF) under grant CHE-1800252to
Prof. Jeffrey R. Long. M.A.B. acknowledges support from NSF
GFRP No. DGE 1106400. We thank College of Chemistry's
NMR facility staff for their assistance. NMR instruments are
supported in part by National Institute of Health grant
S10OD024998. We thank Drs. S. Minasian and M. E. Garner for
helpful discussions.
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Chemistry - A European Journal
10.1002/chem.202000214
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We have a Si+Uation: Uranium–
silylene complexes are reported,
representing the first actinide–heavy
tetrylene bonding interactions.
Preliminary reactivity studies suggest
dative U–Si bonds that can be
displaced thermally or with competing
ligands. Electronic structure
I. Joseph Brackbill, Iskander Douair,
Daniel J. Lussier, Michael A. Boreen,
Laurent Maron, John Arnold*
Page No. – Page No.
Synthesis and Structure of Uranium–
Silylene Complexes
calculations reveal substantial
covalent bonding character and even
modest π-bonding.
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Title
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