N-Heterocyclic Silylene Coordinated Dialkyl Borenium Equivalent

Article abstract of DOI:10.1021/acs.organomet.7b00687

In an attempt to prepare the heavier analogue of NHC-stabilized borenium cation, we have prepared a silylene-coordinated borenium equivalent (1a), which can be viewed as a contact ion pair of borenium and triflate. The triflate anion coordinates to the oxophilic silicon center, leaving the boron center existing in a trigonal-planar geometry in the solid state and in solution. Nucleophilic substitution reactions of 1a all result in the release of free silylene ligand, demonstrating the borenium-ion-like reactivity of 1a. Chemical reduction of 1a leads to a neutral radical possessing a partial Si-B double bond.

Full text of DOI:10.1021/acs.organomet.7b00687

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Cite This: Organometallics XXXX, XXX, XXX-XXX
N‑Heterocyclic Silylene Coordinated Dialkyl Borenium Equivalent
Hsiu-Chen Tsai,† Ya-Fan Lin,‡ Wei-Chun Liu,† Gene-Hsiang Lee,† Shie-Ming Peng,†
and Ching-Wen Chiu*,†
†Department of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
‡Department of Fragrance and Cosmetic Science, Department of Medicinal and Applied Chemistry, Kaohsiung Medical University,
No. 100, Shih-Chuan First Road, Kaohsiung, 80708 Taiwan
S
* Supporting Information
ABSTRACT: In an attempt to prepare the heavier analogue of
NHC-stabilized borenium cation, we have prepared a silylene-
coordinated borenium equivalent (1a), which can be viewed as a
contact ion pair of borenium and triflate. The triflate anion
coordinates to the oxophilic silicon center, leaving the boron center
existing in a trigonal-planar geometry in the solid state and in
solution. Nucleophilic substitution reactions of 1a all result in the
release of free silylene ligand, demonstrating the borenium-ion-like
reactivity of 1a. Chemical reduction of 1a leads to a neutral radical
possessing a partial Si−B double bond.
orenium cation,1,2 the isoelectronic analogue of carbenium
ion, has received considerable attention in the past few
Due to the less effective π stabilization of the Si−N bond,
NHSi is markedly more π acidic than NHC. The introduction
of NHSi to a borenium cation is expected to increase the Lewis
acidity of the resulting cationic molecule. In addition, the
silylene-coordinated borenium ion ([A]+ in Scheme 2) can be
alternatively described as an α-boryl silylium ion ([B]+ in
Scheme 2), which features a π-acidic boryl group adjacent to
the highly electrophilic silylium center.37−39
B
years due to its promising application in the activation of H−
H,3−6 electrophilic borylation,7−12 and catalysis.13,14 The
introduction of positive charge significantly enhances the
electron deficiency and Lewis acidity of the tricoordinated
boron center.15,16 In addition to the well-documented amine-
and pyridine-coordinated borenium ions, phosphine,17−19 N-
heterocyclic olefin (NHO),20−22 and carbene have also been
incorporated into boron cations.23−30 Interestingly, while the
carbene-stabilized borenium ions have been proven to be
effective in hydrogen activation, the introduction of the heavier
group 14 congeners of carbene to boron cations has received
much less attention. Highly reactive diarylsilylene was found to
undergo B−E (E = B, H, Cl) bond insertions with various
boranes.31−33 Metzler and Braunschweig have also reported
similar reactions for stable N-heterocyclic silylenes (NHSi)
(Scheme 1).34−36 The coordination of stable NHSi to B(C6F5)3
yielded a Lewis acid−base adduct that gradually transformed
into a silyl borane via migration of the C6F5 group from boron
to silicon. The B−Cl bond insertion was observed when NHSi
was allowed to react with haloboranes. With the less sterically
demanding PhBCl2, further expansion of the NHSi ring to a six-
membered silaborinine ring was obtained. In attempt to expand
the ligand library of borenium cation, we decided to embark on
the synthetic challenge of the silylene-stabilized borenium
cation. Although the cationic species could not be generated
due to the oxophilicity of silicon, reactivity investigations of 1a
reveals the borenium-ion-like reaction pattern of the neutral
molecule. In addition, chemical reduction of 1a also led to the
observation of a neutral radical featuring an odd-electron π
bond between boron and silicon.
To address the relative Lewis acidity of the boron and silicon
center in the NHSi-borenium/boryl-silylium cation, hydride ion
affinity (HIA)16 and chloride ion affinity (CIA)40 of a model
compound were calculated with density functional theory
(DFT). The optimized geometry of the B-NHSi-9-borabicyclo-
[3.3.1]nonane cation ([NHSi-9BBN]+) possesses a planar
geometry with a Si−B bond distance of 204.9 pm (Figure 1).
Natural population analysis (NPA) of [NHSi-9BBN]+ showed
that the silicon center (+1.543) is actually bearing more
positive charge than the boron atom (+0.58), which is
consistent with the electropositive nature of silicon. Interest-
ingly, the relative Lewis acidities of the boron and silicon
centers of [NHSi-9BBN]+ were found to be dependent on the
nucleophile. The HIAs of −43.4 and −37.8 kcal/mol for boron-
and silicon-centered reactions suggests that [NHSi-9BBN]+
functions as a boron-based electrophile for hydride addition.
However, the CIA calculations (23.8 (B) vs 19.2 kcal/mol (Si))
support the description of [NHSi-9BBN]+ as a silicon-based
electrophile in reaction with chloride (Table S3 in the
Supporting Information). In other words, both boron and
Received: September 8, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00687
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
sharp signal at −29 ppm, which is indicative of the coordination
of triflate anion at the silicon center in solution. The molecular
structure of 1a was further verified with a single-crystal X-ray
diffraction analysis. As shown in Figure 2, the boron center
Scheme 1. Reaction between N-Heterocyclic Silylene and
Boranes
Figure 2. Molecular structure of 1a. Hydrogen atoms are omitted for
clarity. Selected bond distances (in pm) and angles (in deg): Si(1)−
O(1) 178.20(12), Si(1)−B(1) 200.92(19), Si(1)−N(2) 171.24(13),
Si(1)−N(1) 171.39(14), C(11)−B(1) 155.7(2), B(1)−C(15)
156.2(2); N(2)−Si(1)−N(1) 93.73(7) O(1)−Si(1)−B(1)
100.07(7), N(2)−Si(1)−B(1) 121.71(7), N(1)−Si(1)−B(1)
121.11(7).
features a trigonal-planar geometry with a B−Si bond distance
of 200.91(19) pm, which is comparable to the value reported
by Braunschweig.35 The coordination of triflate to the silicon
center with a Si−O distance of 178.20(12) pm confirms the
tetrahedral geometry at the silicon center.
Scheme 2. Canonical Structures of Silylene-Stabilized
Borenium Cation
In an attempt to diminish the cation−anion interaction, we
have attempted the replacement of triflate with triflimide anion.
Unlike the relatively stable, isolable 1a, 1b could only be
generated from the reaction of NHSi and 9-BBN-NTf2 in
hexane inside a J. Young NMR tube. A broad 11B NMR
resonance centered at 94 ppm reminiscent of that for 1a is
assigned to the anticipated triflimide derivative (1b). However,
no 29Si NMR signal was observed for 1b. In addition, exposure
of 1b to C6D6 led to rapid decomposition of the compound
into a complicated mixture. Thus, no further reactivity
investigation of 1b was performed.
The coordination of NHSi to the boron center was found to
be weak and reversible in 1a (Scheme 3). Upon addition of 1
equiv of MesLi to a C6D6 solution of 1a, the release of free
1
NHSi was immediately identified in the H NMR spectrum of
the reaction mixture (Figure S7 in the Supporting Informa-
Figure 1. DFT optimized structure of [NHSi-9BBN]+.
Scheme 3. Substitution Reactions of 1a
silicon atoms of [NHSi-9BBN]+ are susceptible to nucleophilic
attack, and the regioselectivity of the addition reaction is
governed by the relative bond strengths of the B−E and Si−E
bonds.
Fascinated by the theoretical predictions, we then carried out
the synthesis of the silylene-coordinated borenium cation.
Compound 1a could be readily obtained from the reaction of
NHSi and 9-BBN-OTf in C6D6. Upon addition of NHSi, the
11B NMR signal of 9-BBN-OTf shifted from 66 to 91 ppm. The
detected 11B resonance is shifted 7 ppm downfield in
comparison to that of the NHC-stabilized analogue (83.8
ppm), suggesting the exceptional electron-deficient nature of
the boron center in 1a. The 29Si spectrum of 1a contains a
B
DOI: 10.1021/acs.organomet.7b00687
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Organometallics
Communication
tion). The reaction with MesLi cleanly transformed 1a to B-
mesityl 9-BBN (2), which features a broad 11B NMR signal at
87 ppm. The introduction of neutral Lewis bases also results in
the liberation of silylene (Figure S10 in the Supporting
Information). Addition of 4-(dimethylamino)pyridine (DMAP)
to 1a led to the formation of a boronium cation coordinated to
two DMAP groups ([3][OTf]). A similar ligand replacement
reaction was observed when 1a was treated with bulky NHO to
afford the corresponding NHO-coordinated borenium cation
([4][OTf])(Figure S14 in the Supporting Information). As
indicated in the work of Rivard, the coordinating ability of
neutral ligands to borane follows the order NHC > DMAP >
NHO.41 The substitution of NHSi with NHO shows that the
coordinating ability of NHSi is the worst in the series.
The NHSi-boron interaction is surprisingly weak, such that
even an ester compound can replace the coordinated silylene
(Figure S18 in the Supporting Information). In an attempt to
test the catalytic ability of 1a in the ring-opening polymer-
ization of ε-caprolactone, 1a was mixed with 25 equiv of ε-
caprolactone in C6D6. Once again, the release of free silylene
was observed, and no polymerization of lactone was achieved.
We reasoned that the boron−silicon interaction in 1a might
be strengthened through population of the B−Si π-bonding
orbital. Although molecules featuring a BSi double bond are
rare,32,33,42 structural characterization of silaborene and its
adduct has been accomplished a few times.43,44 As shown in
Scheme 4, reduction of 1a with Cp*2Co in hexane at −78 °C
NHSi ring (N, 22%; C, 4%; H, 4%). The higher spin density on
boron supports the description of 1a as a contact ion pair of
silylene-coordinated borenium ion ([A]+). Unfortunately, such
a partial B−Si π bond is still not strong enough to sustain a
persistent radical. 5• underwent rapid decomposition at room
temperature to free silylene and 9-BBN dimer, which might be
the result of hydrogen atom abstraction of the 9-BBN radical
from the environment.
In summary, the reaction of free NHSi with 9-BBN-OTf
results in a neutral Lewis adduct that reacts like a borenium
cation. However, the weak silylene−boron interaction was
found to be problematic in keeping the silylene in the
coordination sphere of boron. Interestingly, one-electron
reduction of 1a leads to the detection of a neutral radical
that features a one-electron π bond between silicon and boron.
To avoid the dissociation of silylene, we are currently
examining molecules featuring more strongly σ donating
silylene ligands or more electron deficient boron centers.
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.7b00687.
Experimental details on the synthesis and character-
ization of 1a,b, NMR spectra of 1a,b and reactions of 1a,
and theoretical calculations (PDF)
XYZ coordinates of all optimized structures (XYZ)
Scheme 4. Chemical Reduction of 1a
Accession Codes
CCDC 1573407 contains 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.
resulted in the detection of an EPR signal centered at g =
2.0034 (Figure 3a). The signal can be simulated with hyperfine
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for C-W.C.: cwchiu@ntu.edu.tw.
ORCID
Ching-Wen Chiu: 0000-0001-7201-0943
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 3. (a) Experimental and simulated EPR spectra of 5•. (b)
Calculated spin density distribution (isovalue 0.002) on the optimized
structure of 5•.
This work was supported by the Ministry of Science and
Technology of Taiwan (Grant MOST 104-2113-M-002-018-
MY3 and 105-2119-M-002-031-MY2) and National Taiwan
University (NTU-CESRP-104R7619).
coupling constants of 15.8, 15.8, and 17.5/5.2 MHz to 1H, 14N,
and 11B/10B nuclei, respectively. The coupling of the unpaired
electron to one boron atom, two nitrogen atoms, and two
hydrogen atoms is consistent with the description of 5• as a
neutral radical having a one-electron π bond between boron
and silicon.
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