Incorporation of Boron into Uranium Metallacycles: Synthesis, Structure, and Reactivity of Boron-Containing Uranacycles Derived from Bis(alkynyl)boranes

Article abstract of DOI:10.1002/chem.202003611

The reaction of uranacyclopropene complex (C₅Me₅)₂U[η²-1,2-C₂(SiMe₃)₂] with B-aryl bis(alkynyl)borane PhB(C≡CPh)₂ led to the first six-membered uranium metallaboracycl

Full text of DOI:10.1002/chem.202003611

Accepted Article
Accepted Article
Title: Incorporate Boron into Uranium Metallacycles: Synthesis,
Structure and Reactivity of Boron-containing Uranacycles Derived
from Bis(alkynyl)boranes
Authors: Wangyang Ma, Iskander Douair, Laurent Maron, and Qing Ye
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To be cited as: Chem. Eur. J. 10.1002/chem.202003611
Link to VoR: https://doi.org/10.1002/chem.202003611
01/2020

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Incorporation of Boron into Uranium Metallacycles: Synthesis,
Structure and Reactivity of Boron-containing Uranacycles Derived
from Bis(alkynyl)boranes
Wangyang Ma, Iskander Douair, Laurent Maron* and Qing Ye*
Abstract: The reaction of uranacyclopropene complex (C5Me5)2U[η2-
1,2-C2(SiMe3)2] with B-aryl bis(alkynyl)borane PhB(C≡CPh)2 led to the
first six-membered uranium metallaboracycle, while the reaction with
B-amino bis(alkynyl)borane (Me3Si)2NB(C≡CPh)2 afforded an
unexpected uranaborabicyclo[2.2.0] complex via [2+2] cycloaddition.
The reaction with CuCl revealed the non-innocent property of the
rearranged bis(alkynyl)boron species towards oxidant. The reactions
with isocyanide DippNC: (Dipp = 2,6-iPr2-C6H3) and isocyanate
tBuNCO afforded the novel uranaborabicyclo[3.2.0] complexes. All
new complexes have been structurally characterized. DFT
calculations were performed to provide more insights into the
electronic structures and the reaction mechanism.
incorporation of three-coordinate boron into actinide
metallacycles. It is believed that one important reason is the lack
of applicable synthetic approach.
From a chemical composition point of view, it is well known
that adding another TiCp2 moiety to the titanacyclocumulene (I in
Fig. 1) leads to the dinuclear zigzag-butadiene structure (II)[12]
,
while adding a borylene unit “BR” leads to the six-membered boat-
like zirconaboracyclic structure (III)[13]. Therefore, three essential
questions need to be addressed: i) whether or not the IIU,B and
IIIU,B structures as depicted in Figure 1 could be realized upon
adding a “BR” unit to IAn; ii) their structural features; iii) influence
of 5f orbitals of uranium and empty p-orbital of boron on the
bonding mode and the reactivity. As we recently demonstrated
that the coordination of bis(alkynyl)boranes can serve as a facile
synthetic approach to metallaboracycles,[13] we decided to apply
this strategy to construct the first actinide metallaboracycles.
The incorporation of one or more d- or f-block metal centers
into the organic ring systems constitutes an important class of
organometallic molecules, namely metallacycles. In general,
giving deep insights into the electronic structure to help
understanding the bonding mode, and developing new
stoichiometric and catalytic reactions based on the metallacycles
have been the focus of research.[1] Nonetheless, although the
actinide metallacyclopentadienes were reported in 1970s by
Marks and co-workers[2,3], the actinide metallacycle chemistry has
been neglected for a long time. In recent years, the intense
interest has been re-sparked thanks in large part to the rapid
development of transition metal, in particular the group IV element
metallacycle chemistry.[4] Moreover, the resurgence is also driven
by curiosity about the influence of the 5f orbitals on the bonding,
as well as their potential use in catalysis, small molecule
activation and C−H bond activation.[5] In fact, the recent
development of actinide metallacycle chemistry has benefited
from mimicking the group IV analogues. In this context, various
actinide metallacyclopropenes,[6] metallacyclopentadienes,[7]
metallacyclocumulenes[8] and metallacyclopentyne[9] were
synthesized and have readily shown rich reaction chemistry. It
should be noted, nonetheless, that the unsaturated actinide
metallaheterocycles have been scarce.[10] Most surprisingly,
although three coordinate electron-deficient boron plays a pivotal
role in construction of unsaturated heterocycles as functional
molecular and polymeric materials, which have found wide
applications including OLEDs, anion sensing, solar cells and
electronic circuits,[11] there is no single reported example of
Figure 1. Selected d-block and f-block element metallacycles and
metallaboracycles. I: group IV metallacyclocumulenes. II: group IV dinuclear
zigzag-butadiene complexes. III: zirconium bis(alkynyl)borane complex
featuring
the boat-like
zirconaboracyclic
structure.
IAN:
actinide
metallacyclocumulenes. IIU,B and IIIU,B: uranaboracycles in this work.
Herein, we report the first examples of reaction of uranium
with bis(alkynyl)boranes, which lead to the unique fused uranium
borabicyclic complexes (IIU,B) and the first uranabora-monocyclic
complex (IIIU,B). Moreover, we presented three different reactivity
patterns of the 2-urana-5-borabicyclo[2.2.0]hexa-3,6-diene
scaffold.
[*]
[*]
Dr. W. Ma, Prof. Dr. Q. Ye
Department of Chemistry
Southern University of Science and Technology
518055 Shenzhen, China
E-mail: yeq3@sustech.edu.cn
I. Douair, Prof. Dr. L. Maron
LPCNO, CNRS & INSA, Université Paul Sabatier, Toulouse, France.
E-mail: laurent.maron@irsamc.ups-tlse.fr
Supporting information for this article is given via a link at the end of
the document.
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Scheme 1. Synthesis of 3a and 3b.
The atom connectivity of 3a was further confirmed by single-
crystal X-ray diffraction analysis (Figure 2, top). The geometry
revealed an overall six-membered uranaboracyclic structure with
the bis(alkynyl)borane ligand η5-coordinated to the uranium
center, which resembles that of the our previously reported
zirconium analogue VII.[13] The complex 3a crystallizes in the
monoclinic space group P21/c. The asymmetric unit cell contains
two nearly identical molecules of 3a. Taking one molecule for
example, the U1−C1 and U1−C4 bond lengths of 2.60(2) Å and
2.55(2) Å lie in the expected range of M(An)−C(sp2) −bond
distances (2.3~2.6 Å) in actinide metallacycles.[6-8] The C1−C2,
C2−B1, B1−C3 and C3−C4 bond lengths (1.25(3) Å, 1.51(3) Å,
1.58(4) Å and 1.22(3) Å) are comparable with that of the zirconium
analogue VII.
To provide further insight into the bonding situation in
complexes 3a, density functional theory (DFT, B3PW91)
calculations were carried out and the bonding was analyzed using
Natural Bonding Orbitals (NBO). First of all, the unpaired spin
density (see Supporting Information) is clearly located at the
uranium center and its value of 2.17 is in line with a presence of
a U(IV) system, indicating that the borane is formally doubly
reduced. The latter is also corroborated by the NBO analysis.
Indeed, two U−C bonds are found which are strongly polarized
toward C (70%). This bonding interaction implies overlap between
a sp hybrid orbital on C and a sdf on U (65% d, 25% f and 10%
s). This is due to the double reduction of the conjugated diyne,
that implies the relocalization of two lone pairs on the two external
carbon. The associated U−C Wiberg Bond Indexes (WBI) are
0.57/0.59 for two external carbon and 0.37/0.37 for the two
internal one as expected for a delocalized structure. On the other
hand, no direct U−B interaction observed but rather an indirect
one that comes from the U−C bond as found in the HOMO (Figure
3). This explains that the Wiberg Bond Index (WBI) for U−B is 0.24,
which however remains smaller than the U−C one.
As depicted in Scheme 1, as an initial attempt, we chose the
uranacyclopropene complex, (C5Me5)2U[η2-1,2-C2(SiMe3)2] 1 as
the “(C5Me5)2U(II)” synthon. The reaction of 1 with B-aryl
bis(alkynyl)borane PhB(C≡CPh)2 2a was performed in C6D6 at
ambient temperature and monitored by 1H- and 11B-NMR
spectroscopy. The 11B-NMR spectra indicated the complete
conversion of 2a (B 46.5) into a new boron-containing species
that displays a significantly downfield-shifted resonance at B 76.4.
The complex 3a could be isolated in excellent yield (95%) after
easy work-up. The 1H-NMR spectrum of 3a in C6D6 displayed two
well-separated singlets for the C5Me5 rings (H = 8.22 and 12.01)
with an integration ratio of 15:15, thus suggesting that 3b should
possess two nonequivalent C5Me5 groups in the solution phase.
Figure 3. 3D representation of the HOMO of 3a (this orbital is doubly occupied)
that displays the U−C bonds.
In order to further study the electronic effect of the B-
substituent on the U−B interaction, we performed the same
reaction with the B-amino group −N(SiMe3)2 substituted
bis(alkynyl)borane 2b (Scheme 1). Surprisingly, the reaction
resulted in the nearly quantitative formation of the unexpected
uranaborabicyclo[2.2.0] complex 3b (isolated yield 98%) at an
Figure 2. Molecular structures of complex 3a (top) and 3b (bottom) in the solid
state (ellipsoids set at 50% probability). Hydrogen atoms are omitted for clarity.
Selected bond lengths [Å] and angles [°] for 3a: U1−C1 2.60(2), U1−C2 2.67(2),
U1−C3 2.64(2), U1−C4 2.55(2), C1−C2 1.25(3), C2−B1 1.51(3), B1−C3 1.58(4),
C3−C4 1.22(3), C1−C2−B1 155(2), B1−C3−C4 156(2), C2−B1−C3 114(2). for
3b: U1−C2 2.270(7), U1−C4 2.340(7), C1−C2 1.374(10), C2−C3 1.606(10),
C3−C4 1.337(10), C3−B1 1.610(11), C1−B1 1.556(11), N1−B1 1.434(10),
C2−U1−C4 69.7(3).
o
elevated temperature (75 C). In sharp contrast to complex 3a
with a six-membered monocyclic structure, complex 3b features
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a unique fused bicyclic structure with a bora-cyclobutene four-
more stable by 8.6 kcal/mol than 3b#. This is in line with the fact
that complex 3b is obtained rather than complex 3b#.
membered ring fused to
a urana-cyclobutene ring. The
bis(alkynyl)borane 2b is isomerized upon 2e-reduction by U(II),
with the original C() atoms of the alkynyl groups being coupled.
Complex 3b is stable under N2 atmosphere, and can be
stored as solids in a N2 glove box for months without any sign of
decomposition, yet 3b is fairly air- and moisture-sensitive.
Complex 3b is easily soluble in most ordinary solvents such as
pentane, diethyl ether and toluene. The singlet signal of SiMe3
group at H 2.06 and the singlet peak of C5Me5 at H −1.53 with an
intensity ratio of 18:30 suggested a symmetric structure of 3b in
the solution phase. The 11B-NMR signal of 3b displayed a slightly
upfield-shifted resonance at 28.2 ppm with respect to that (B
31.3) of the free ligand 2b.
Single crystals suitable for X-ray diffraction analysis were
grown from saturated pentane solution of 3b. The molecular
structure of 3b is shown in Figure 2 (bottom). Complex 3b
crystallizes in the monoclinic space group P21/c. The fused
boracyclic ring is nearly planar (C1−C2−C3−C4 torsion angle:
177.5°). The U1−C2 and U1−C4 bond lengths (2.270(7) Å,
2.340(7) Å) are remarkably shorter than that of reported the
M(An)−C(sp2) distances (~2.5 Å). [6-8] The C1−C2 and C3−C4 bond
lengths (1.374(10) Å, 1.337(10) Å) are within the normal range of
C=C bonds while the C2−C3 bond lengths (1.606(10) Å) are
notably longer than the average value of a C−C single bond (~1.5
Å). The tricoordinated boron center adopts a trigonal planar
geometry. The B−C bond lengths (1.556(11) Å, 1.610(11) Å) are
as expected for a B−C single bond.
With the novel and unique fused ring structure of
uranaborabicyclo[2.2.0] complex 3b in hand, we further tested its
reactivity towards different types of inorganic/organic substrates,
including CuCl, isocyanates and isocyanides.
The reaction of 3b with 2 equiv. of CuCl was conducted in
C6D6 at 80 oC for 2 h, during which period the reaction was
monitored
by
in
situ
1H-NMR
spectroscopy
with
hexamethylbenzene as an internal standard. (Scheme 2a, See
Supporting Information for more in situ NMR monitoring details).
It clearly revealed the nearly complete consumption (>92%) of 3b
(d(C5Me5) −1.53) and the formation of (C5Me5)2UCl2 (d(C5Me5)
13.49) and the free bis(alkynyl)borane ligand 2b (d(SiMe3) 0.51).
The uranium center remains its +IV oxidation state, while the Cu(I)
in CuCl is reduced to copper black and the Cl- anions are
transferred to uranium. Apparently, the electrons transferred to
Cu(I) derive from the dianionic variant of bis(alkynyl)borane. The
loss of two electrons leads to the cleavage of the bridged C2−C3
-bond[14] and the regeneration of the original bis(alkynyl)borane
2b. Therefore, the rearranged bis(alkynyl)boron ligand in complex
3b is chemically non-innocent and acts as electron shuttle for
multielectron transfer process. Similarly, the reaction of 3a with 2
equiv. of CuCl leads to the formation of (C5Me5)2UCl2 and
regeneration of the free bis(alkynyl)borane ligand 2a.
Scheme 2. The reactivity of uranaborabicyclo[2.2.0] complex 3b with CuCl,
isocyanate tBuNCO, and isocyanide DippNC:.
Figure 4. Calculated reaction barrier on the formation pathway of complex 3b.
Alike complex 3a, DFT calculations were also carried out to
provide further insight into the bonding situation in complexes 3b.
The unpaired spin density (2.13) is consistent with a U(IV) system
(See Supporting Information). The NBO clearly shows that two
U−C polarized covalent bond are present (75% C-25% U)
involving overlap between a sp hybrid on C and a sdf on U (11%
s, 53% d and 36% f). The U−C WBI are 0.85/0.90 indicating a
slightly more covalent interaction than in complex 3a. Based on
the isolation and structure determination of complex 3a and 3b, a
possible reaction mechanism pathway to yield complex 3b was
computed at the DFT level (B3PW91). It is clearly showed that the
coordination of the bis(alkynyl)borane with low-valent
(C5Me5)2U(II) firstly give rise to the equivalent of complex 3a,
(labeled 3b#) which further undergoes an intramolecular [2+2]
cycloaddition to yield complex 3b. (Figure 4) The reaction is
kinetically facile (barrier 16.1 kcal/mol) and the complex 3b is
The reaction of complex 3b with 1 equiv. of isocyanate
tBuNCO yielded complex 4 featuring a 2-oxa-3-urana-7-aza-6-
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borabicyclo[3.2.0]hepta-1(7),4-diene scaffold as
a
brown
U1−C5 bond lengths (2.450(3) Å, 2.429(3) Å) are within the
expected range of the U−C(sp2) distances. The C1−C2 and C3−C5
bond lengths (1.390(4) Å, 1.365(3) Å) are in line with the C=C
distances, while the C2−C3 and C3−C4 separations (1.490(3) Å,
1.516(3) Å) clearly indicate their single bond character. The short
C4−N2 bond length of 1.284(3) Å is corresponding to the
exocyclic imine group.[15]
crystalline solid in high yield (86%, Scheme 2b). The X-ray single
crystal diffraction analysis of complex 4 showed a fused bicyclic
structure including a N→B dative bond (Figure 5, top). The 2b
skeleton is presumably regenerated in the first step, followed by
the [2+2+1] cycloaddition between the alkynyl, C=O of isocyanate
and the U(II) center. The U1−C1 bond length (2.373(3)Å) and the
U1−O1 bond length (2.297(2) Å) are both within the expected
range of U−C(sp2) bond and U−O s-bond. The short C1−C2
distance of 1.363(4) Å illustrated its C=C bond character, while
C2−C3 (1.507(4) Å), C3−N1 (1.311(3) Å), N1−B1 (1.667(4) Å) and
B1−C2 (1.707(4) Å) bond lengths are also in accordance with its
bora-azetine four-membered ring structure.
In summary, we have reported the synthesis of the first
uranium
metallaboracycles
upon
complexation
of
bis(alkynyl)boranes with the “(C5Me5)2U” fragment. The boron-
containing ligand is doubly reduced by the low-valent U(II) center,
leading to the dianionic species that is isoelectronic to the
bis(isonitrile) borylene.[16] Depending on the type (amino or phenyl)
of the exocyclic boron-substituent, a boat-like six-membered
uranaboracycle and a U/B-dinuclear zigzag-butadiene complex
could be prepared in excellent yields. In both cases, the
calculated unpaired spin density of ca. 2.1 is consistent with a
U(IV) system. Moreover, three reactivity patterns (i.e. non-
innocent behavior, ring-expansion, [2+2+1] cycloaddition) have
been established, suggesting that 3b can be applied as precursor
for the construction of uranaboracyclic systems of next generation.
Acknowledgements
QY gratefully acknowledge the start-up fund of SUSTech. LM is
a senior member of the Institut Universitaire de France.Humboldt
foundation and the Chinese Academy of Science are
acknowledged for support as well as CalMip for a generous grant
of computing time.
Conflict of interest
The authors declare no conflict of interest.
Keywords: bis(alkynyl)borane • uranium • boron-centered ligand
• uranacylce • boracycle
Figure 5. Molecular structures of complex 4 (top) and 5-DMAP (bottom) in the
solid state (ellipsoids set at 50% probability). Hydrogen atoms are omitted for
clarity. Selected bond lengths [Å] and angles [°] for 4: U1−C1 2.373(3), U1−O1
2.297(2), C1−C2 1.363(4), C2−C3 1.507(4), C2−B1 1.707(4), C3−N1 1.311(3),
N1−B1 1.667(4), B1−N2 1.517(4), C1−U1−O1 86.34(8). For 5-DMAP: U1−C2
2.450(3), U1−C5 2.429(3), U1−N3 2.617(2), C1−C2 1.390(4), C2−C3 1.490(3),
C3−C4 1.516(3), C4−B1 1.627(4), B1−C1 1.545(4), C3−C5 1.365(3), C4−N2
1.284(3), B1−N1 1.454(4), C2−U1−C5 61.05(8).
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10.1002/chem.202003611
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
U-B-Ring: The first
Wangyang Ma, Iskander Douair, Laurent
Maron*, Qing Ye*
bis(alkynyl)borane-supported actinide
complexes are reported. Depending
on the B-substituent, the
Page No. – Page No.
bis(alkynyl)borane ligand can either
coordinate in a 5-manner or undergo
a U(IV)-assisted intramolecular [2+2]
cycloaddition to give a highly unusual
fused bicyclic structure.
Incorporation of Boron into Uranium
Metallacycles: Synthesis, Structure
and Reactivity of Boron-containing
Uranacycles
Derived
from
Bis(alkynyl)boranes
This article is protected by copyright. All rights reserved.