Donor-acceptor stabilized silaformyl chloride

Article abstract of DOI:10.1039/c2cc32887a

Formyl chloride (H(Cl)CO) is unstable at room temperature and decomposes to HCl and CO. Silicon analogue of formyl chloride, silaformyl chloride IPr·SiH(Cl)O·B(C₆F₅)₃ (3) (IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazol-2-yl

Full text of DOI:10.1039/c2cc32887a

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Donor–acceptor stabilized silaformyl chloridewz
Rajendra S. Ghadwal,* Ramachandran Azhakar, Herbert W. Roesky,* Kevin Propper,
a
a
a
a
¨
Birger Dittrich,*a Catharina Goedeckeb and Gernot Frenking*b
Received 23rd April 2012, Accepted 2nd July 2012
DOI: 10.1039/c2cc32887a
Q
Formyl chloride (H(Cl)C O) is unstable at room temperature
and decomposes to HCl and CO. Silicon analogue of formyl
Lewis base stabilization of highly reactive species with low-valent
main group elements using N-heterocyclic carbenes (NHCs) is
an exciting area of research activity in NHC chemistry.18–20
We have exploited this feature not only to stabilize highly
reactive dichlorosilylene as monomeric IPrꢀSiCl2 (1) (IPr =
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) but also
since NHC acts as a reducing agent for HSiCl3.19 Starting
from 1, some exciting stable silicon compounds have been
isolated,20,21 which were supposed to exist as intermediates in
many chemical transformations. Very recently a zwitterionic
stable silicon analogue of an acid anhydride was isolated
supported by donor–acceptor ligands.22
Q
chloride, silaformyl chloride IPrꢀSiH(Cl) OꢀB(C6F5)3 (3)
(IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazol-2-ylidene), was
stabilized by Lewis donor–acceptor ligands. Compound 3 is not
only the first stable acyclic silacarbonyl compound but also the
first silacarbonyl halide reported so far.
The last three decades have witnessed considerable progress in the
chemistry of compounds with multiple bonds with heavier group
14 elements.1–11 This is not only because of the experimental
curiosity but also because they possess frontier orbitals with small
energy separation and can activate small molecules and therefore
mimic transition metals.2
Syntheses of stable silicon compounds have been motivated
almost exclusively by their relationship with organic congeners.1–5
Formaldehyde (H2CQO) is the simplest carbonyl compound and
its mono-chlorinated derivative, formyl chloride A (Scheme 1), is
a useful formylating agent.23 Interestingly, formyl chloride A is
unstable at room temperature and decomposes to HCl and CO.24
Isolation of a stable silicon analogue B of formyl chloride
with a SiQO bond is evidently more challenging due to the
instability of A.24
Compounds with double bonds between group 14 and group
16 elements are of special interest.6–11 Ketones (R2CQO) are
ubiquitous in organic chemistry, biochemistry, and material
science but their silicon analogues are still scarce.12–15 In
contrast to the carbonyl systems, derivatives with a SiQO bond
show an extraordinarily high tendency to oligomerize with no
activation barrier.16 This is due to the high polarity of the SiQO
bond and weak pp–pp interaction. Therefore, silicon analogues
of ketones remained elusive until recently.12–15 Intramolecular
coordination of group 14 elements with nitrogen donor ligands
allows the stabilization of such reactive species. We have
isolated the first N-donor supported germathiocarboxylic acid
(LGe(QS)OH) (L = CH(MeCNAr)2, Ar = 2,6-Pri2C6H3)
analogue with a monoanionic chelating b-diketiminate (L)
ligand.17 Driess and co-workers reported some interesting silicon
heterocycles featuring SiQO bonds supported by donor–acceptor
ligands.12–15 However, to the best of our knowledge no silicon
analogue of an acyclic- as well as a halo-carbonyl compound
has been reported so far.
Herein we report on a stable silaformyl chloride IPrꢀ
SiH(Cl)QOꢀB(C6F5)3 (3), which was prepared by the reaction of
IPrꢀSiCl2 (1) with H2OꢀB(C6F5)3 in the presence of NHC as a HCl
scavenger (Scheme 2). The mechanism for the formation of 3 is at
present not known. Insertion of silylene 1 into the O–H bond of
H2OꢀB(C6F5)3 to form 2 is plausible, which under the elimina-
tion of HCl yields a stable silaformyl chloride 3 (Scheme 2).25a
Compound 3 features a significant double bond character
between Si and O atoms, however contribution of the resonating
structure 30 cannot be ruled out. Compound 3 is the first halo-
as well as acyclic-silacarbonyl compound reported so far.
Colorless crystals of 3 were obtained from a toluene solution
in 26% yield.25a Compound 3 is soluble in common organic
solvents and stable under an inert atmosphere for appreciable
time. The molecular structure of 3 was established by single
crystal X-ray measurement.
a Institut fur Anorganische Chemie, Georg-August-Universitat
¨
¨
Go¨ttingen, Tammannstrasse 4, 37077 Go¨ttingen, Germany.
E-mail: rghadwal@uni-goettingen.de, hroesky@gwdg.de,
bdittri@gwdg.de
b Fachbereich Chemie, Philipps-Universitat Marburg,
¨
Hans-Meerwein-Straße, 35032 Marburg, Germany.
E-mail: frenking@chemie.uni-marburg.de
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
procedures, X-ray crystallographic and computational information of
compound 3. CCDC 846252 (3) and 846253 (4). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2cc32887a
Scheme 1 Formyl chloride (A) and its silicon analogue (B).
c
8186 Chem. Commun., 2012, 48, 8186–8188
This journal is The Royal Society of Chemistry 2012

length (1.568(15) A) is smaller than the Si–O single bond
(1.64 A) but longer than the SiQO bond calculated for
H2SiQO (1.53 A).27 This may be due to the higher coordination
at the silicon.13–16 The Si1–O1 bond length is in agreement with
those found in compounds with four-coordinate silicon.12–15
NBO analysis for compound 3 as well as for free SiH(Cl)O
suggests a strongly polarized Si+–Oꢁ bond as illustrated by 30
(Scheme 2). As expected for Si(IV) compounds,19,20 the Si1–Cl1
(2.049(8) A) and Si1–C1 (1.911(2) A) bonds in 3 are shorter
than those of Si(II) compound 1. The O1–Si1–C1 features the
bond angle of 110.90(9)1.
We carried out DFT calculations at the BP86/TZVPP-SVP
level (see ESIz) for compound 3 and associated donor and
acceptor moieties. The calculated structures are shown in
Fig. S4 (see ESIz). The optimized geometry of 3 is very similar
to the experimental structure shown in Fig. 1, where the
calculated bond lengths and angles are also given. The theoretical
distances are always somewhat longer than the experimental values,
but otherwise the agreement between theory and experiment
is quite good. The calculated SiQO bond in free SiH(Cl)O
is shorter (1.545 A) than in 3 (1.568 A). The NBO analysis
suggests that the SiQO double bond in free SiH(Cl)O is
strongly polarized towards oxygen. The s bond is 79% at O
and 21% at Si while the p bond is 81% at O and 19% at Si.
The polarization of the SiQO bond in 3 toward the oxygen
end is even larger. In the compound 3, the s bond is 85% at O
and 15% at Si while the p bond is 96% at O and only 4% at
Si. Thus, the p electrons are better described as Si+–Oꢁ
(as depicted for 30). This is in agreement with the atomic
partial charges which are q(Si) = + 1.56 and q(O) = ꢁ1.04
in free SiH(Cl)O while they are q(Si) = + 1.58 and q(O) =
ꢁ1.03 in 3.
Scheme 2 Synthesis of silaformyl chloride (3) from dichlorosilylene
(1) (Ar = 2,6-diisopropylphenyl).
1
The H NMR spectrum of 3 shows resonances for the IPr
ligand, which are quite different from those found in 1.
A broad resonance at d 5.55 ppm is assigned to SiH proton and
1
further supported by H-29Si HSQC (HSQC = Heteronuclear
Single Quantum Coherence) experiment. 11B, 13C, and 19F NMR
spectra of 3 exhibit resonances as expected for compound 3.
The 29Si{1H} NMR spectrum of 3 shows a singlet at d ꢁ49.78 ppm
consistent with the values reported for compounds with a
four-coordinate26 silicon atom. The 29Si NMR resonance for 3
shows a significant upfield shift relative to dichlorosilylene 1,
which is comparable to those of reported silicon compounds
with an SiQO bond.12
Compound 3 crystallizes in the monoclinic space group
P2(1)/c with two molecules in the asymmetric unit.25b Presence
of the centrosymmetric space group indicates that both
enantiomers (R and S) are present in the structure. The silicon
atom in 3 is four-coordinate and features a distorted tetrahedral
environment (Fig. 1). All coordination sites are occupied with
four different atoms; one hydrogen, one chlorine, one oxygen,
and one carbon atom of the NHC ligand. The Si1–O1 bond
We calculated the strength of the donor–acceptor bonds in
IPr - SiH(Cl)O - B(C6F5)3 3 yielding the three fragments
IPr, SiH(Cl)O and B(C6F5)3 as separate moieties. The theore-
tically predicted values are shown in Table 1. The overall
bond dissociation energy (BDE) of 3 is rather high. Separate
calculations of the two donor–acceptor bonds show that
the IPr - SiH(Cl)O interactions in 3 are much stronger
than the SiH(Cl)O - B(C6F5)3 donation. The former bond
is about twice as strong as the latter. The calculated data also
show that the two donor–acceptor bonds in 3 mutually
enhance each other. The BDEs of the free acid–base pairs
IPr - SiH(Cl)O and SiH(Cl)O - B(C6F5)3 are a bit smaller
than in 3. The inclusion of dispersion interactions in the
calculations gives larger values for the bond energies but the
trends remain the same.
Table 1 Calculated bond dissociation energies DE and free energies
DG298 at BP86/TZVPP//BP86/SVP of the calculated compounds
[kcal mol–1]. The dispersion corrected values at BP86-D3/TZVPP//
BP86/SVP are given in parentheses
Fig. 1 Representation of the molecular structure of 3 with 50%
probability level for the displacement ellipsoids; isopropyl groups
and H atoms (except the one on the Si atom as located in the difference
Fourier map) were omitted for clarity. Only the non-disordered of the
two molecules in the asymmetric unit25b and the ipso-C atoms of the
C6F5 groups are shown. Selected bond lengths [A] and angles [1] with
calculated values at BP86/SVP given in brackets: Si1–O1 1.568(15) [1.609],
Si1–C1 1.911(2) [1.949], Si1–Cl1, 2.049(8) [2.087], O1–B1 1.492(3) [1.523];
O1–Si1–C1 110.90(9) [107.4], O1–Si1–Cl1 112.65(6) [115.7], C1–Si1–Cl1
103.08(7) [105.5].
Reaction
DE
DG
3 - IPr + SiH(Cl)O + B(C6F5)3
3 - IPrꢀSiH(Cl)O + B(C6F5)3
3 - IPr + SiH(Cl)OꢀB(C6F5)3
IPrꢀSiH(Cl)O - IPr + SiH(Cl)O
SiH(Cl)OꢀB(C6F5)3 -
70.6 (106.3)
23.2 (44.2)
51.1 (77.9)
47.4 (62.1)
19.5 (28.4)
36.8 (72.5)
6.5 (27.5)
32.2 (59.0)
30.2 (44.9)
4.6 (13.5)
SiH(Cl)O + B(C6F5)3
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8186–8188 8187

The NBO atomic charges suggest that the SiH(Cl)O moiety
in 3 is essentially neutral, the calculated partial charge is
ꢁ0.01 e. Thus, the extent of charge exchange and charge
acceptance at the SiH(Cl)O moiety in 3 does not correlate with
the associated energy contributions, where IPr - SiH(Cl)O
donation is stronger than SiH(Cl)O - B(C6F5)3 donation.
In conclusion we have synthesized and characterized a
remarkably stable silaformyl chloride IPrꢀSiH(Cl)QOꢀ
B(C6F5)3 (3), whose carbon homologue formyl chloride is
unstable at room temperature. Compound 3 is the first acyclic
silacarbonyl compound reported so far. Moreover, 3 represents the
first stable halo-silacarbonyl compound. Theoretical calculations
support the experimental results.
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ESI.z Attempts to isolate Lewis acid–base adduct of siliconmon-
oxide IPrꢀSiQOꢀB(C6F5)3, using 3 and IPr and also by the reaction
of 1 with H2OꢀB(C6F5)3 in the presence of two equivalents of IPr
were not successful. In both cases formation of 4 and free IPr was
observed. (b) The asymmetric unit of 3 contains two molecules. Only
one molecule of 3 was non-disordered. The second molecule of 3
(80% occupancy) was disordered and superposed to a molecule of
the imidazolium-borate salt 4 with 20% occupancy (see ESIz for
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This journal is The Royal Society of Chemistry 2012