f-Element Half-Sandwich Complexes: A Tetrasilylcyclobutadienyl–Uranium(IV)–Tris(tetrahydroborate) Anion Pianostool Complex

Article abstract of DOI:10.1002/anie.201913640

Despite there being numerous examples of f-element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C_5–8), cyclobutadienyl (C₄) complexes remain exceedingly rare. Here, we report that

Full text of DOI:10.1002/anie.201913640

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Accepted Article
Title: f-Element Half-Sandwich Complexes: A Tetrasilylcyclobutadienyl-
Uranium(IV)-Tris(tetrahydroborate) Anion Pianostool Complex
Authors: Stephen Liddle, Josef Boronski, Laurence Doyle, John Seed,
and Ashley Wooles
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913640
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10.1002/anie.201913640
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f-Element Half-Sandwich Complexes: A Tetrasilylcyclobutadienyl-
Uranium(IV)-Tris(tetrahydroborate) Anion Pianostool Complex
Josef T. Boronski,[a] Laurence R. Doyle,[a] John A. Seed,[a] Ashley J. Wooles,[a] and Stephen T. Liddle*[a]
Abstract: Despite there being numerous examples of f-element
compounds supported by cyclopentadienyl, arene, cycloheptatrienyl,
and cyclooctatetraenyl ligands (C5-8), cyclobutadienyl (C4) complexes
remain exceedingly rare. Here, we report that reaction of
[Li2{C4(SiMe3)4}(THF)2] (1) with [U(BH4)3(THF)2] (2) gives the
pianostool complex [U{C4(SiMe3)4}(BH4)3][Li(THF)4] (3), where use
of a borohydride and preformed C4-unit circumvents difficulties in
product isolation and closing a C4-ring at uranium. Complex 3 is an
unprecedented example of an f-element half-sandwich cyclobutadienyl
complex, and it is only the second example of an actinide-
cyclobutadienyl complex, the other being an inverse-sandwich. The U-
C distances are short (av. 2.513 Å), reflecting the formal 2- charge of
the C4-unit, and the SiMe3 groups are displaced from the C4-plane,
which we propose maximises U-C4 orbital overlap. DFT calculations
identify two quasi-degenerate U-C4 π-bonds utilising the ψ2 and ψ3
molecular orbitals of the C4-unit, but the potential δ-bond using the ψ4
orbital is vacant.
undesirable redox and acid-base side-reactions.[8] Nonetheless, the
dearth of f-element cyclobutadienyl derivatives demanded attention,
and encouraged by our synthesis of I we set out to develop this
chemistry further. We targeted a half-sandwich uranium complex,
since the continued absence of any f-element half-sandwich
cyclobutadienyl complex after seven decades of research on ηn-bonded
f-element-carbocyclic complexes was conspicuous. Furthermore, the
cyclobutadienyl ligand has long been considered an enigmatic ligand,[9]
and a half-sandwich formulation would present the opportunity to
study the U-C4 interaction in isolation, free of the inherent trans-ligand
or -metal effects in sandwich and inverse-sandwich motifs.
Herein, we report on our progress in this arena, describing how our
synthetic strategy has evolved, to ameliorate the aforementioned
synthetic challenges, in order to deliver a uranium-cyclobutadienyl
pianostool complex. This is the first example of an f-element half-
sandwich cyclobutadienyl complex, and only the second example of an
actinide-cyclobutadienyl complex. The successful synthesis and
characterisation of this half-sandwich complex underscores the
importance of using appropriate starting material combinations, and
how under-utilised reagents can succeed over more conventional
approaches. The uranium-cyclobutadienyl bonding has been probed
with quantum chemical calculations, revealing exclusively π-bonding
utilising the ψ2 and ψ3 molecular orbitals of the C4-unit, but the
potential δ-bond using the C4-unit ψ4 orbital is absent.
The discovery of ferrocene in 1951 heralded
a new age of
organometallic chemistry,[1] and this resulted in the rapid proliferation
of sandwich, half-sandwich, and inverse-sandwich ηn-bonded d-block
and f-element-carbocyclic derivatives including cyclobutadienyl (C4),
cyclopentadienyl (C5), arene (C6), cycloheptatrienyl (C7), and
cyclooctatetraenyl (C8).[2] The area is still burgeoning today due to its
continuing impact on developing our understanding of bonding theory
and its many applications in synthesis and catalysis.[3] Where f-
element-ηn-carbocyclic derivatives, first initiated in 1954,[4] are
concerned, although reports of C5-8 derivatives are numerous,[5] C4
complexes remain rare with only two reports to date.[6] In actinide
chemistry, in 2013 we reported the synthesis of the diuranium-
cyclobutadienyl inverse-sandwich complex [{U(TsXy)}2(C4Ph4)] (I,
TsXy = HC(SiMe3NC6H3-3,5-Me2)3),[6a] and in lanthanide chemistry the
synthesis of ‘ate’ cyclometallated cyclobutadienyl sandwich complexes
[M{C4(SiMe3)4}{C4(SiMe3)3(SiMe2CH2)K2(toluene)] (M = Y, Dy) was
described in 2018.[6b]
Scheme
phenyldibenz[a,c]anthracene
[LiC(Ph)C(Ph)C(Ph)C(Ph)Li].
1.
Synthesis
of
9,14-dihydro-9-
UCl4 and
from
Complex I was formed by a [2 + 2]-cycloaddition reaction that
takes over a month to complete, and it is mediated by two uranium ions
to enforce the ring-closure which, unlike transition metals, usually does
not occur at f-elements,[7,8] whereas the lanthanide examples feature
cyclometallation. These observations perhaps underscore why there is
such a paucity of f-element cyclobutadienyl complexes generally and
that synthetic progress has been slow; (i) there are few C4-ligand
transfer reagents; (ii) it is difficult to promote C4-ring formation at ions
that little enforce orbital control of reactivity; (iii) the resulting (C4R4)2-
dianions can be strongly reducing and basic, thus leading to
Given our previous work in this area,[6a] we initially examined the
potential for developing our reductive alkyne [2 + 2]-cycloaddition at
[10]
uranium. Thus, we treated UCl4
with one equivalent of acyclic
[LiC(Ph)C(Ph)C(Ph)C(Ph)Li],[11] (Scheme 1) but isolated only
elemental uranium along with concomitant formation and structural
characterisation of the hydrocarbon product 9,14-dihydro-9-
phenyldibenz[a,c]anthracene that derives from proton-transfer and
intramolecular cyclisation of the butadiene.[12,13] Analogously to I, we
deduced that the barrier to C4-ring closure at uranium is significant in
this scenario, and clearly facile phenyl-C-H activations add deleterious
complications, and we so turned our attention to using the cyclic silyl
cousin [Li2{C4(SiMe3)4}(THF)2] (1).[14] However, treatment of UCl4
with 1 resulted in thus far intractable, darkly coloured solutions that are
indicative of uncontrolled uranium reduction.[12] Thus, we switched to
a halide-free salt elimination strategy using the generally under-utilised
uranium(III)-borohydride complex [U(BH4)3(THF)2] (2),[15] since this
would be unlikely to be reduced, and we report here the first crystal
structure of 2.[12]
[a]
Mr J. T. Boronski, Dr L. R. Doyle, Dr J. A. Seed, Dr A. J. Wooles,
Prof. Dr. S. T. Liddle
Department of Chemistry and Centre for Radiochemistry Research
The University of Manchester
Oxford Road, Manchester, M13 9PL, UK
E-mail: steve.liddle@manchester.ac.uk
Supporting information for this article is given via a link at the end of
the document.
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complex incorporates uranium(III) rather than uranium(IV). The
cyclobutadienyl ring C-C distances span the range 1.452(16) to
1.488(17) Å, which is typical of metal coordinated cyclobutadienyl
ligands.[8] The four SiMe3 groups are displaced out of the C4 plane of
the cyclobutadienyl core, and all bend away from the uranium ion with
displacements ranging from 0.452 to 0.566 Å; this is comparable to the
magnitude of out of plane displacements for methyl groups in sterically
crowded tris(cyclopentadienyl) complexes,[19] but the anion component
of 3 is not obviously sterically overloaded, and we address a possible
electronic reason for this below. The U-H distances span the range
2.34(10) to 2.51(10) Å, but, as is inherent to X-ray diffraction, those
bond metrics are not determined with great precision invalidating any
detailed discussion, and so perhaps a more meaningful metric is the
U⋅⋅⋅B distances (range: 2.537(19) to 2.594(17) Å), which compare well
to the U⋅⋅⋅B distances in [U(η5-C5H5)(BH4)3] (range: 2.46(4) to 2.57(5)
Å)[17] and [U(η6-C6Me6)(BH4)3] (range: 2.49(4) to 2.69(3) Å).[18] Lastly,
the [Li(THF)4]+ cation is unremarkable so is not discussed any further.
The 1H NMR spectrum of 3 exhibits only four resonances,
spanning the range +16 to −5 ppm, suggesting a symmetrical species
on the NMR timescale with no symmetry-lowering by cyclometallation.
Scheme 2. Synthesis of complex 3.
Addition of toluene to a cold (−196 °C) 0.9:1 mixture of 1 and 2
results in the formation of a dark brown solution and the precipitation
of
elemental
uranium,
suggesting
that
uranium(III/IV)
disproportionation has occurred. Removal of solvent, washing with
SiMe4, extraction away from the elemental uranium and LiBH4 solids,
and finally recrystallisation from toluene afforded brown crystals
formulated as [U{C4(SiMe3)4}(BH4)3][Li(THF)4] (3) in 55% isolated
yield by uranium content (Scheme 2).[12]
7
The 11B{1H} and Li{1H} NMR spectra exhibit single resonances at
132.67 and 5.93 ppm, respectively, again suggesting a symmetric
species and the latter is consistent with the presence of a [Li(THF)4]+
cation.
The ATR-IR spectrum of 3 exhibits strong absorptions at 2452,
2193, 2119, and 1241 cm-1, which are characteristic of κ3-coordinated
BH4 ligands,[20] in-line with the crystallographic data. Specifically, the
highest energy band is a terminal B-H A1 stretching mode, the middle
two are a doublet (Δ 74 cm-1) bridging B-H A1 and E stretching modes
that characteristically occur within 80 cm-1 of one another, and the
lowest energy absorption is a bridge deformation E mode. An
absorption at 1307 cm-1 is tentatively assigned as a C4-ring B1 and/or E
stretching mode.[21]
The UV/Vis/NIR spectrum of 3 is dominated by a charge transfer
band across the UV and visible regions consistent with the brown
colour of 3, and the NIR region is populated with multiple weak (ε =
15-70 M-1 cm-1) absorptions that are characteristic of intra-
configurational transitions of 3H4 uranium(IV).[22]
Figure 1. Molecular structure of the anion component of 3 at 150 K.
Displacement ellipsoids are set at 40% with silyl-hydrogen atoms
and the [Li(THF)4]+ cation omitted for clarity.
The solid-state structure of
3 (Figure 1) was determined,
confirming its separated ion pair formulation. The salient features of
the uranium anion component of 3 are the coordination of one η4-
cyclobutadienyl and three κ3-tetrahydroborate ligands to uranium in a
pianostool half-sandwich structure (the hydrides were located in the
Fourier Difference Map but refined with restraints). The U-C distances
in 3 span the range 2.477(11) to 2.549(12) Å, which is ~0.12-0.18 Å
shorter than the U-C distances in I[6a] reflecting their terminal and
bridging natures,[16] respectively, and the U-Ccent distance is 2.290(15)
Å. It is instructive to compare the average U-C distances in 3 (2.513 Å)
to those of [U(η5-C5H5)(BH4)3] (2.656 Å)[17] and [U(η6-C6Me6)(BH4)3]
(2.927 Å)[18] since each complex features a {U(BH4)3} unit in a
pianostool half-sandwich structural motif; the trend of increasing U-C
distances nicely illustrates the effect on moving from formally
dianionic (C4R4)2- to monoanionic (C5R5)1- to neutral (C6R6)0 ligands,
adjusting for the increasing ring sizes and the fact that the uranium
component of 3 is a charge-rich anion and the hexamethylbenzene
Figure 2. Variable temperature SQUID magnetometry plot for 3 over
the temperature range 2-300 K. The line is a guide to the eye only.
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Figure 3. Frontier Kohn Sham Molecular Orbitals of 3'. Left to right: α-HOMO-3 (152a, −1.577 eV); α-HOMO−2 (153a, −1.560 eV); α-
LUMO+5 (161a, +1.910 eV).
To confirm the uranium(IV) formulation of 3 we recorded variable
temperature SQUID magnetometric data on a powdered sample of 3
(Figure 2). At 300 K the magnetic moment of 3 is 3.03 µB and this
decreases smoothly to a magnetic moment of 0.67 µB at 2 K which is
characteristic of uranium(IV) that is a magnetic singlet at low
temperature subject to temperature-independent paramagnetism.[23]
However, we note that the magnetic moment of 3 does not decrease as
rapidly as is usual for 3H4 uranium(IV), which has been observed
before when strongly donating, charge-rich ligands are bound to this
ion,[24] which presumably here reflects the formally dianionic and
charge-concentrated nature of the relatively small cyclobutadienyl ring.
Thus, overall the structural, spectroscopic, and magnetic
characterisation data are internally consistent and confirm the proposed
formulation of 3.
In order to gain a deeper understanding of 3, we undertook
quantum chemical unrestricted calculations on the uranium anion
component of 3, 3'. The DFT geometry optimised structure of 3' is in
good agreement with the experimental structural data, with the
computed U-C distances (av. 2.580 Å) slightly over-estimated (~0.07
Å) compared to experiment. We therefore conclude that the optimized
structure of 3' provides a qualitative model of the electronic structure
of the uranium anion component in 3.
The computed MDC-q charges for 3' of +1.26 (U), −1.49 (C4),
0.23 (B av.), −0.19 to −0.22 (hydrides), are consistent with donation of
electron density from the ligands to uranium and the formal dianionic
charge on the cyclobutadienyl ring. The uranium MDC-m spin density
is computed to be 1.96, consistent with its 5f2 formulation. The U-C
bond orders (Mayer) are computed to range from 0.47 to 0.5 and sum
up to 1.94. The U-H bond orders average 0.25, reflecting the highly
polar nature of those uranium-hydride interactions. The C-C bond
orders for the C4-ring average 1.0, presumably reflecting the π-
donation to uranium depletes these delocalised bonds from the
anticipated value of 1.5.
The α-spin, quasi-degenerate HOMO and HOMO−1 of 3' are of
essentially pure 5f-character, consistent with the uranium(IV)
formulation. HOMO−2 and −3 represent the principal quasi-degenerate
binding interactions between uranium and the cyclobutadienyl ring
(Figure 3), and are of exclusively π-bonding character deriving from
the ψ2 and ψ3 π-symmetry molecular orbitals of cyclobutadienyl and
vacant 5f/6d hybrid acceptor orbitals of uranium.[16] HOMO−2 and −3
are each composed of 70:30 C4:U character, respectively, and the latter
component in each case is composed of 67:33 5f:6d character.
Interestingly, the potential U-C δ-bonding combination involving the
ψ4 δ-symmetry molecular orbital of cyclobutadienyl is found in
LUMO+5 (Figure 3), ~3.47 eV above HOMO −2/−3, and this virtual
orbital is computed to have a 49:51 C4:U composition with the U
contribution being made up of 7:93 5f:6d character. Interestingly, the
π-δ combination gap in 3' is far larger than in I (1.395 eV),[6a] which is
consistent with the absence of any significant δ-bonding in the former
but its weak presence in the latter. This all-π-symmetric bonding in 3'
is similar that that observed in uranium-C5 complexes, but contrasts to
I,[6a] and uranium-C6/7/8 complexes where δ-bonding dominates.[16]
Lastly, HOMO−2/−3 suggest that the deviation of the silyl groups from
the C4 plane, both experimentally observed in 3 and reproduced in the
calculations on 3', may possibly be driven, in part, by less strained and
thus maximised U-C orbital overlap,[25] since the C4-ring is quite small
for uranium, and such bending of substituents out of the carbocyclic
ring plane has been noted in cyclopentadienyl chemistry.[2,3,19]
Since DFT calculations produce an orbital-based bonding picture,
we examined the bonding topology in 3' with Bader’s QTAIM method.
This reveals computed average ρ, ∇2ρ, H (energy), and ε (ellipticity)
3,-1 bond critical point values for the four U-C interactions in 3' of
0.06, 0.11, −0.02, and 0.67, which in gross terms compare well to
computed data for [U(η5-C5H5)4];[26] however, the ρ and H values for
3' are slightly larger than [U(η5-C5H5)4] which is consistent with the
cyclobutadienyl ring in the former being bonded more strongly and
covalently than the cyclopentadienyl rings in the latter.
To conclude,[27] we have prepared and characterised the first
example of an f-element half-sandwich cyclobutadienyl complex, and
indeed it is only the second example of an actinide-cyclobutadienyl
complex. Whereas synthetic endeavours with UCl4 have thus far been
intractable, using the halide-free uranium(III)-tris(tetrahydroborate),
itself an infrequently used class of complex in actinide chemistry,
proved decisively fruitful. The U-C distances in this pianostool
complex are short, consistent with the formal dianionic nature of
cyclobutadienyl, and the silyl substituents are displaced from the C4
plane in
a manner that is reminiscent of sterically crowded
cyclopentadienyl complexes. However, we suggest that the
displacement of silyl groups is not due to steric demands but possibly
to maximise orbital overlap between the size-mismatched frontier
orbitals of uranium and cyclobutadienyl. Interestingly, the two U-C4
bonding interactions are computed to be of π-bonding character with a
significant contribution from uranium, and the potential U-C4 δ-
bonding interaction is absent, being a virtual, unoccupied orbital lying
substantially above the π-bonds. This bonding pattern is similar to that
of actinide-cyclopentadienyls, but contrasts to uranium derivatives of
arenes, cycloheptatrienyl, cyclooctatetraenyl, and even an inverse-
sandwich diuranium cyclobutadienyl complex, where δ-bonding is
prevalent to varying extents. The results presented here, when placed in
the wider context of uranium-ligand bonding, demonstrate the highly
flexible nature of the chemical bonding of uranium, which can adapt its
bonding patterns to match the frontier orbitals of ligands that are
available to it.
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[20] T. J. Marks, J. R. Kolb, Chem. Rev. 1977, 77, 263.
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[22] D. P. Mills, F. Moro, J. McMaster, J. van Slageren, W. Lewis, A.
J. Blake, S. T. Liddle, Nat. Chem. 2011, 3, 454.
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Acknowledgements
We gratefully acknowledge the UK EPSRC (grants EP/M027015/1 and
EP/P001286/1), ERC (grant CoG612724), Royal Society (grant
UF110005), the National Nuclear Laboratory, and the University of
Manchester for generous funding and support.
[24] a) D. Patel, F. Moro, J. McMaster, W. Lewis, A. J. Blake, S. T.
Liddle, Angew. Chem. Int. Ed. 2011, 50, 10388; b) J. L. Brown,
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Lewis, A. J. Blake, S. T. Liddle, Angew. Chem. Int. Ed. 2013, 52,
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W. Lewis, A. J. Blake, S. T. Liddle, J. Am. Chem. Soc. 2014, 136,
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Meyer, Inorg. Chem. 2014, 53, 8418; f) B. M. Gardner, G.
Balázs, M. Scheer, F. Tuna, E. J. L. McInnes, J. McMaster, W.
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Hennig, A. C. Scheinost, J. McMaster, W. Lewis, A. J. Blake, A.
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Rookes, B. M. Gardner, G. Balázs, M. Gregson, F. Tuna, A. J.
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Keywords: uranium • cyclobutadienyl • tetrahydroborate • half-
sandwich • density functional theory
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supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre. All other data are available from
the corresponding authors on request.
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10.1002/anie.201913640
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
We report the first example of an f-
element half-sandwich cyclobutadienyl
complex, which is only the second
example of an actinide-cyclobutadienyl
complex. Calculations suggest that U-C π-
bonding dominates, with no δ-bonding
component.
Josef T. Boronski, Laurence R. Doyle, John
A. Seed, Ashley J. Wooles, and Stephen T.
Liddle*
Page No. – Page No.
f-Element Half-Sandwich Complexes: A
Tetrasilylcyclobutadienyl-Uranium(IV)-
Tris(tetrahydroborate) Anion Pianostool
Complex
This article is protected by copyright. All rights reserved.