Synthesis of Stable Pentacoordinate Silicon(IV)-NHC Adducts: An Entry to Anionic N-Heterocyclic Carbene Ligands

Article abstract of DOI:10.1021/acs.organomet.7b00838

This work features the previously undescribed interactions of Martin's spirosilane with different types of N-heterocyclic carbenes (NHCs). The level of interaction proved to be strongly dependent on the size of the Lewis base and could vary from the formation of isolable classical Lewis adducts to abnormal Lewis adducts, as evidenced by X-ray diffraction structure analyses and NMR studies. It has been found that abnormal adducts could be used as precursors for the synthesis of anionic NHCs bearing a weakly coordinating siliconate component. Complexation of these new types of carbenes with gold(I) and copper(I) has been efficiently accomplished. DFT calculations performed on the siliconate-based anionic NHC ligands revealed a high-lying HOMO and therefore a strong σ-donor character.

Full text of DOI:10.1021/acs.organomet.7b00838

Communication
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis of Stable Pentacoordinate Silicon(IV)−NHC Adducts: An
Entry to Anionic N‑Heterocyclic Carbene Ligands
Fabrizio Medici, Geoffrey Gontard, Etienne Derat, Gilles Lemiere,* and Louis Fensterbank*
̀
́ ́
Institut Parisien de Chimie Moleculaire, UMR 8232 Sorbonne Universites UPMC Paris06-CNRS, 4 place Jussieu, 75005 Paris, France
*
S Supporting Information
ABSTRACT: This work features the previously undescribed interactions of Martin’s spirosilane with different types of N-
heterocyclic carbenes (NHCs). The level of interaction proved to be strongly dependent on the size of the Lewis base and could
vary from the formation of isolable classical Lewis adducts to abnormal Lewis adducts, as evidenced by X-ray diffraction structure
analyses and NMR studies. It has been found that abnormal adducts could be used as precursors for the synthesis of anionic
NHCs bearing a weakly coordinating siliconate component. Complexation of these new types of carbenes with gold(I) and
copper(I) has been efficiently accomplished. DFT calculations performed on the siliconate-based anionic NHC ligands revealed a
high-lying HOMO and therefore a strong σ-donor character.
11
he concept of Lewis acidity−basicity appears very early in
the usual training of chemistry students. Intellectually
charged nucleophiles. Interestingly, NHCs have never been
studied in this context, which aroused our curiosity.
T
simple and essentially important in all reactivity developments, it
still enjoys important discoveries by playing with the nature of
the partners or by involving it in unusual settings. When they are
adequately substituted, tetravalent silicon derivatives can exert
Lewis acidity, which has triggered versatile synthetic develop-
We first started our study by examining the reactivity of the
smallest dimethyl NHC toward spirosilane 1. Thus, after
deprotonation of dimethylimidazolium and subsequent addition
of 1, full conversion of both starting partners was observed to
afford the Lewis adduct 3a in 84% yield after purification
1
ments in organic chemistry. Among the possible Lewis bases
(
Scheme 1, eq 1). The resulting adduct 3a proved to be very
involved in donor−acceptor complexes, N-heterocyclic carbenes
2
stable and inert to moisture, as it could resist an aqueous workup,
which attests to the strength of the interaction. The
(
NHCs) have been employed with p-block elements in order to
3
stabilize their low-valence states or to discover new reactivities.
Owing to the particular ability of silicon to reach hyper-
coordination, NHCs have also been used with tetravalent
silicon(IV) reagents in various chemical transformations
involving putative transient pentacoordinate silicon(IV) inter-
mediates. Tetravalent halosilanes have received particular
4
Scheme 1. Interaction between Martin’s Spirosilane 1 and
a
NHCs 2a,b: Normal Adducts
5
attention in this area, since they display sufficient electron
deficiency to allow the isolation of relatively stable pentacoordi-
6
,7
nate NHC adducts. As an extension, NHC-SiCl compounds
4
have judiciously served as precursors for the synthesis of NHC-
stabilized silicon(0) species through potassium graphite
8
reductions. However, pentacoordinated Si-NHC adducts
9
featuring nonhalogenated silane partners are very scarce. We
therefore considered the possibility to introduce Martin’s
spirosilane 1 in this forum, since it is an easily accessible
10
derivative with a well-established Lewis acidic character.
a
Mes = 2,4,6-trimethylphenyl.
Nucleophilic attack on the tetrahedral silicon atom of 1 triggers
strain release, and synergic stabilization provided by the very
electronegative bis-trifluoroalkoxy ligands leads in high yields to
various trigonal-bipyramidal pentacoordinate adducts with
Received: November 21, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00838
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
2
9
16,17
pentacoordinate nature of the silicon atom was confirmed by Si
NMR spectroscopy, displaying a chemical shift of −83 ppm (δ
was isolated.
The same behavior was observed with carbene
2d bearing t-Bu groups, and abnormal adduct 4d was obtained as
the only product in only 8 h at room temperature. It is worth
mentioning that these zwitterionic adducts are quite robust, since
they could be purified by chromatography on silica gel.
The X-ray crystallographic structure of 4c featured a large O−
Si−O angle of 189.2° and a short Si−C
7
.5 ppm for 1), which is in accordance with other known
12
pentacoordinate derivatives and thus excluding the formation
of adducts with other valences. Solid 3a could be recrystallized,
and an X-ray diffraction (XRD) analysis was performed,
confirming the structure and showing as salient features a large
O−Si−O angle of 181.9° (113.5° for 1), an elongated Si−O
distance of 1.91 Å
NHC
(Figure 2).
bond (1.78 Å vs 1.65 Å for 1), and a Si−C
(Figure 1).
distance of 1.94 Å
NHC
Figure 2. X-ray crystal structure of 4c (except for the imidazolium ring,
hydrogens have been omitted for clarity; CCDC 1523670).
Figure 1. X-ray crystal structures of 3a (left, CCDC 1523668) and 3b
Abnormal adducts 4c,d are neutral species that could be
(
right, CCDC 1523669).
regarded as imidazolium rings functionalized by a weakly
18
coordinating anion. At this stage, we anticipated the possibility
19
This structure represents one of the first well-defined neutral
to use these species as precursors of anionic NHCs. Such
NHCs bearing weakly coordinating anions are emerging as a new
class of promising ligands, since they can deeply modify the
pentacoordinate silicon(IV) adducts formed between an NHC
and a nonhalogenated silane derivative. As expected, bringing
steric bulk to the NHC had some consequences. When the
preformed IMes 2b was mixed with silane 1, the complete
formation of the Lewis adduct 3b was still observed (Scheme 1,
eq 2), but it appeared to be less stable than 3a since this adduct is
hydrolyzed after a few hours in contact with water. XRD analysis
confirmed a similar pentacoordinate silicon(IV) structure but
with very slightly looser features in comparison to 3a since the
20
physical and chemical properties of metal complexes. To the
best of our knowledge, no NHC with a silicon-based anion has
21
been reported so far. To our delight, deprotonation of the
16b
abnormal adduct 4d was cleanly achieved using n-butyllithium
as a base, affording the isolable lithium−carbene complex 5d
(Scheme 3). It is noteworthy that similar anionic NHCs bearing a
O−Si−O angle is now 183.7° and the Si−C
distance is 1.95 Å
Figure 1). These data reflect the greater steric congestion with
Scheme 3. Deprotonation of Neutral Imidazolium 4d and
Synthesis of Gold Complex 6d
NHC
(
the bulkier NHC. Interestingly, a remarkable “through space”
1
3
C−F coupling of 3.3 Hz between the trifluoromethyl groups
1
3
and the aromatic o-methyls was observed by C NMR
1
4
spectroscopy, suggesting a likely stabilizing interaction of the
adduct.
The use of IDipp and ItBu carbenes 2c,d, respectively,
dramatically changed the scenario (Scheme 2). While B(C F )
6
5 3
1
5
is known to form a normal Lewis adduct with IDipp (2c), no
such formation was noticeable when the same NHC was
confronted with Lewis acidic silane 1. The abnormal adduct 4c
was instead smoothly obtained in 24 h at room temperature and
Scheme 2. Interaction between Martin’s Spirosilane 1 and
IDipp (2c) or ItBu (2d): Formation of Abnormal Adducts
tris(pentafluorophenyl)borate have been previously prepared by
22
19d
Tamm and co-workers using an alternative indirect route
due to difficulties to deprotonate the corresponding borate
abnormal adducts. This striking difference clearly points out the
robustness of the siliconate moiety, especially toward strong
nucleophilic reagents. We next turned our attention to the
complexation of the NHC-siliconates toward transition metals.
Gratifyingly, the lithium complex 5d reacted smoothly with
(
triphenylphosphine)gold(I) chloride. The anionic nature of the
ligand and the presence of the lithium cation seem to favor the
B
DOI: 10.1021/acs.organomet.7b00838
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
chloride dissociation, providing the neutral gold complex 6d, as
confirmed by XRD analysis.
In a similar approach, we could efficiently synthesize the
neutral gold complex 6c and the three-coordinate Cu(I)-NHC
complex 7c from the abnormal adduct 4c bearing diisopropyl-
phenyl groups (Scheme 4).
Scheme 4. Formation of Gold(I) Complex 6c and Copper(I)
Complex 7c
Figure 4. Vizualisation of the HOMO of IDipp-Si− with Me N+
4
calculated at the B3LYP-D3/def2-SV(P) level of theory and plotted at
an isosurface value of 0.05 au.
+
We choose Me N , since it is a noncoordinating cation, and
4
placed it close to the siliconate moiety to avoid spurious
interactions with the carbene. It shows that the siliconate moiety,
while influencing the carbene, does not modify the ability of this
type of divalent carbon to be a σ donor. Frontier orbital energy
levels for two classical carbenes (IDipp and CAAC) and our
newly developed anionic siliconate NHC (IDipp-Si ) have also
been calculated and are presented in Scheme 5, along with the
Lithium tetrafluoroborate salt is eliminated during the reaction
to afford the neutral copper(I) complex 7c, stabilized by a
bispyridine and an anionic NHC ligand (Scheme 5). The X-ray
crystal structures of the neutral gold(I) complex 6c and
copper(I) complex 7c are shown in Figure 3.
−
corresponding singlet−triplet gap. In comparison to IDipp 2c,
−
the IDipp-Si carbene displays a much higher lying HOMO
(
−4.87 vs −5.72 eV), which indicates a stronger σ-donor
character.
Scheme 5. Frontier Molecular Orbital Comparison of
According to the DFT studies, the HOMO of IDipp-Si−
−
+
Carbenes IDipp (2c), IDipp-Si Me N , and CAAC at the
4
carbene is even higher than that calculated for CAAC-Dipp
B3LYP-D3/def2-SV(P) Level
(
−5.10 eV), known to be a very strong σ-donor NHC. However,
−
the singlet−triplet gap of IDipp-Si ressembles more that of
IDipp than that of CAAC.
In conclusion, this study describes the sterically dependent
interactions of Martin’s spirosilane with different NHCs,
allowing for the isolation of different types of novel neutral
pentacoordinate silicon(IV)−NHC adducts to zwitterionic
abnormal adducts in the case of bulky Lewis bases. The resulting
functionalized imidazolium rings were used as precursors of
unprecedented siliconate-based anionic NHCs. Gold(I) and
copper(I) complexes bearing these new anionic ligands were
cleanly obtained and fully characterized by X-ray diffraction.
Finally, DFT calculations revealed a strong σ-donor character
that suggests promising applications for further developments in
metal catalysis, which may also involve the stereogenicity
brought by the siliconate moiety.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00838.
Full experimental details and characterization data (PDF)
Cartesian coordinates of the calculated structures (PDF)
Figure 3. X-ray crystal structure of Au(I) complex 6c (CCDC 1575949)
left and Cu(I) complex 7c (CCDC 1575951).
Accession Codes
CCDC 1523668−1523670, 1575949, and 1575951 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.uk,
or by contacting The Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033.
Figure 4 displays the HOMO of the anionic NHC-Si IDipp-
−
Si , the deprotonated form of abnormal adduct 4c, calculated at
+
the B3LYP-D3/def2-SV(P) level of theory, with Me N as a
4
countercation to compensate the charges and make it possible to
23
compare neutral and charged carbenes.
C
DOI: 10.1021/acs.organomet.7b00838
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
(
9) (a) Xiong, Y.; Yao, S.; Driess, M. Z. Naturforsch., B: J. Chem. Sci.
AUTHOR INFORMATION
Corresponding Authors
E-mail for G.L.: gilles.lemiere@upmc.fr.
E-mail for L.F.: louis.fensterbank@upmc.fr.
■
2
013, 68, 445. Bonnette, F.; Kato, T.; Destarac, M.; Destarac, M.;
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*
8
(
*
(
ORCID
Etienne Derat: 0000-0002-8637-2707
Louis Fensterbank: 0000-0003-0001-7120
Notes
(
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Chhun, C.; Chamoreau, L.-M.; Krim, L.; Zins, E.-L.; Goddard, J.-P.;
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Miyake, H.; Kawashima, T. Organometallics 2014, 33, 2358−2362.
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(
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ACKNOWLEDGMENTS
■
We thank the UPMC, CNRS and PSL* (Ph.D. grant to F.M.).
We are grateful to Jean-Philippe Goddard for preliminary studies
and Lise-Marie Chamoreau for XRD analysis.
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́
́
2
́
́
(
4
̈
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D
DOI: 10.1021/acs.organomet.7b00838
Organometallics XXXX, XXX, XXX−XXX