Tris(dimethylamino)silylium ion: Structure and reactivity of a dimeric silaguanidinium

Article abstract of DOI:10.1039/c9cc03625c

Although several strategies for the stabilization of silylium ions have been established, "π-stabilization" with directly attached π-donor heteroatoms at silicon has not been developed yet. Hydride abstraction from (Me₂N)₃SiH generates dicationic [(Me₂N)₃Si⁺]₂ in solution and in the solid state-constituting the dimer of an elusive silaguanidinium ion. This compound can be synthesized on a gram scale and is compatible with common organic solvents. However, it readily undergoes spontaneous electrophilic silylation of electron-rich aromatic compounds or initiates a catalytic hydro-defluorination reaction.

Full text of DOI:10.1039/c9cc03625c

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Tris(dimethylamino)silylium ion: structure and reactivity of a
dimeric silaguanidinium
Nina Kramer, Hubert Wadepohl and Lutz Greb*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Although several strategies for the stabilization of silylium ions synthetic realization of threefold amino- or other heteroatom
have been established, the “π-stabilization” by directly attached π- substituted silylium ions are still missing in the literature. One
donor heteroatoms at silicon has not been developed yet. Hydride attempt by reacting (Me2N)3SiCl with AlCl3 was reported in
abstraction from (Me2N)3SiH generates dicationic [(Me2N)3Si+]2 in 1980, but it led solely to donor-acceptor complex formation
solution and in the solid state – constituting the dimer of an elusive (Figure 1b).[17] Preliminary studies on tris(dimethylamino)
silaguanidinium ion. The compound can be synthesized on gram silicon perchlorate were mentioned in a review article by
scale and is compatible with common organic solvents. However, it Lambert, but any details remained unpublished.[18] The
readily undergoes spontaneous electrophilic silylation of electron- formation of a threefold sulfur-substituted silylium ion was
rich aromatic compounds or initiates
defluorination reaction.
a
catalytic hydro- indicated by conductivity measurements, but no further
evidence was provided.[19] Structurally related, twofold amino-
substituted silylium ions were obtained in the form of
Silylium ions [R3Si+] are tricoordinated, positively charged silicon
species with enormous electrophilicity.[1] They may be
stabilized by suitable external donors and form adducts like
silylated oxonium,[2] nitrilium[3] or phosphonium[2c] ions. In less
coordinating environments, they even attract weaker donors
such as aromatic solvents,[4] weakly coordinating anions,[3b, 5]
silanes[6] or silicon halides.[7] Silylium ions can be stabilized also
by intramolecular donor coordination, as was demonstrated for
nearby arene,[8] olefine,[9] C-F,[8b, 10] Si-H, Si-F or Si-C,[11] Fe(II),[12]
chalcogen[13] or B-Cl[14] groups. As a third strategy, the
thermodynamic stabilization by direct attachment of π-donors
(π-stabilization) was suggested already in 1977.[15] Theoretical
studies revealed significant impact for such donors and
consistently identified the amino group as most efficient for π-
stabilization.[15-16] It was found, that the hydride ion and the
fluoride ion affinities of Si(NMe2)3+ (Figure 1a) in comparison to
silaimidazolium ions, but no solid state structure was
reported.[20] A formal dimeric species of a monoamino
substituted silylium ion was generated by the double
protonation of a cyclodisilazane (Figure 1c).[21]
Herein, we describe the synthesis and characterization of a
threefold amino-substituted silylium ion in its dimeric form in
solution and in the solid state (Figure 1d). It can be handled in
common organic solvents but retains some of the peculiar
reactivity known for silylium ions.
+
the parent SiH3 are reduced by 311 and 251 kJ mol-1,
respectively.[16c,
Although this resonance stabilization
16f]
amounts to only 40% in comparison to the analog carbocation
(the well-known guanidinium), it is clearly superior to the
stabilization in the commonly used alkyl/aryl-substituted
silylium ions.[16c] Despite such encouraging predictions, the
Figure 1: a) The elusive silaguanidinium, b) a previously attempted synthesis and c) the
structurally closest species to the d) dimeric silaguanidinium presented in this work.
Anorganisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im
Neuenheimer Feld 275, 69120 Heidelberg, Germany
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Two units of [
(
Me2N)3Si]+ dimerize in a head-toV-iteawilArtficalsehOinolinne,
Me2
N
Me2N
Me2N
NMe2
DOI: 10.1039/C9CC03625C
forming
a Si2N2 tetracycle, analog to the isoelectronic
Si
(Me2N)3SiH + [Ph3C][B(C6F5)4]
Si
Al2(NMe2)6.[22] The silicon atoms are coordinated by two
terminal and two bridging dimethylamino groups. The
coordination sphere around silicon is distorted-tetrahedral with
N-Si-N bond angles between 116° (terminal/terminal), 113°
(terminal/bridged) and 94° (bridged/bridged). The Si(1)-N(5)-
(Si(2) plane vs. Si(1)-N(6)-(Si(2) plane has a fold angle of 16°. The
terminal Si-N bond lengths with 1.66 Å are significantly shorter
than common Si-N single bonds (1.75 Å).[23] In contrast, the
bridging Si-N bonds with 1.87 and 1.85 Å are clearly elongated
and much longer than in the structurally related, double
protonated cyclodisilazane (1.79 Å, Figure 1 c).[21] This
observation indicated a significant stabilization of the cationic
charge at silicon due to the threefold donation of the free
electron pairs from nitrogen to silicon, raising the question if
dissociation might be detectable at elevated temperature.
Therefore, VT-NMR-spectroscopy in o-dichlorobenzene was
performed up to 120°C. However, no change in lineshape or
product composition could be observed, revealing a substantial
dimerization free energy. To support the experimental results,
the respective association enthalpy and free energy was
computed at the highly accurate DLPNO-CCSD(T)/cc-pVQZ level
of theory.[24] In vacuum, the dimerization process is not favored
(ΔH = 111 kJ mol-1, ΔG = 181 kJ mol-1), whereas upon
consideration of solvation (o-dichlorobenzene, COSMO-RS), the
process gets indeed favored decisively (ΔHsol = -104 kJ mol-1,
ΔGsol = -35 kJ mol-1). Energy decomposition analysis for the two
interacting closed shell monomeric units (Me2N)3Si+ revealed
54% orbital and 6% London dispersive attraction besides –
- Ph3CH
NMe2
2 [B(C6F5)4]-
N
o-dichlorobenzene
Me2
64% isolated yield at > 1.2 g
[1]
[B(C6F5)4]2
Scheme 1: Synthesis of compound [1][B(C6F5)4]2 by hydride abstraction.
When commercially available (Me2N)3SiH and [Ph3C][B(C6F5)4]
were mixed in o-dichlorobenzene and stored at room
temperature overnight, colorless crystals formed from solution
(Scheme 1). After removal of triphenylmethane by washing with
n-pentane, [1][B(C6F5)4]2 was isolated in 64% yield at > 1.2 g
scale. The purity of the product was verified by H-, 13C-, 11B-,
1
19F- and 29Si-NMR spectroscopy, ESI(+)-MS and elemental
analysis. ESI(+)-MS indicated a monomeric, cationic species
(
Me2N)3Si+ in the gas phase (m/z = 160.13). However, in a solution
of o-difluorobenzene, two singlets at 3.57 ppm and 2.70 ppm in
the ratio 1:2 were detected by 1H-NMR spectroscopy, revealing
two different types of Me2N-groups. Surprisingly, the 29Si-NMR
chemical shift of -30.6 ppm was shifted upfield compared to the
parent (Me2N)3SiH (-25.4 ppm), thus in a shift range typically
found for tetracoordinate silicon. The 29Si-NMR chemical shift
of the monomeric species (Me2N)3Si+ was previously calculated as
42.1 ppm.[16c] An experimental 29Si-NMR shift of -30.8 ppm (C6D6)
for [(Me2N)3Si][B(C6F5)4] was mentioned as
communication in the same manuscript. Based on chemical
shift calculations of the water adduct
Me2N)3Si+OH2
(-20.8 ppm), the authors concluded that rather the water adduct
must have been observed. However, this interpretation would not
a personal
(
despite the dicationic nature
– 40% of electrostatic
agree with the two sets of Me2N-groups found herein. X-ray
attraction.[25] These results are inverse to dimerizing neutral
aminosilanes, constituting 60% electrostatic and 40% orbital +
London dispersive contribution.[26] ETS-NOCV analysis disclosed
the orbitals that contribute mainly to this efficient
dimerization.[27] As expected, the interaction between the lone
pair at the bridging nitrogen atom and the pz(Si)/σ*(Si-N)-type
acceptor orbital at silicon plays the major role (ETS-NOCV1, Fig.
SI1). However, a second interaction was found, in which also the
terminal nitrogen lone pairs contribute as electron density
donors (ETS-NOCV2, Fig. SI1). Although the tendency for
dimerization should be diminished due to the π-stabilization
within the monomeric (Me2N)3Si+ units, the three amino groups
favor dimerization by mutually pushing their intermolecular
donor capability through intrafragment Y-conjugation
structural analysis of [1][B(C6F5)4]2 provided the answer. Single
crystals suitable for X-ray diffraction were obtained directly
from a saturated o-dichlorobenzene solution or by performing
the reaction in toluene and recrystallizing the resulting oil in
dichloromethane.
(LP(N)π*(Si-N))
and
negative
hyperconjugation
(LP(N)σ*(Si-N) at the same time.[16c, 28] Intriguingly, this might
be an additional cause for the higher intrinsic basicity of
silaguanidine in comparison to guanidine.[16c]
Having established theoretical insights, the stability and
reactivity of [1][B(C6F5)4]2 were considered. The dication
Figure 2: Molecular structure of [1]2+ (anions and hydrogen atoms are omitted for
clarity). Ellipsoids 50% probability, selected bond lengths [Å] and angles [deg]: Si(1)-N(1):
1.661(4), Si(1)-N(2): 1.656(4), Si(1)-N(5): 1.849(3), Si(1)-N(6): 1.871(4), Si(1)-Si(2):
2.7231(15), N(1)-Si(1)-N(2): 116.3(2), N(5)-Si(1)-N(6): 85.10(15), N(1)-Si(1)-N(5):
113.00(17), Si(1)-N(5)-Si(2): 94.30(15).
appeared
dichloromethane,
stable
towards
toluene,
organic
benzene,
solvents
chlorobenzene,
like
fluorobenzene, o-dichlorobenzene and o-difluorobenzene at
room temperature. The Lewis acidity of [1][B(C6F5)4]2 was
assessed by adduct formation with triethylphosphine oxide
according to Gutmann-Beckett.[29] Addition of 2 eq. Et3PO to
2 | J. Name., 2012, 00, 1-3
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1 eq. of [1][B(C6F5)4]2 in CD2Cl2 yielded a clean adduct [Et3PO- A common strategy to shift the equilibrium to theVpierwoAdrtuiccletOsnildinee
Si(NMe2)3][B(C6F5)4], with a 31P-NMR chemical shift of δ = 85.2 is the addition of proton-scavengers. Obviously, the amino
DOI: 10.1039/C9CC03625C
ppm (Δδ31P vs. free Et3PO = 34.2 ppm).[30] The induced shift groups in [1][B(C6F5)4]2 exert two favorable effects: 1) The
change Δδ31P is indeed smaller as for carbon-substituted silylation products provide a Brønsted basic side by themselves,
silylium ions (Δδ31P = 40 - 45 ppm),[31] indicating a tamed making the substitution reaction thermodynamically more
effective[32] Lewis acidity due to π-stabilization
– thus favorable. 2) The nucleophilic attack of the aromatic at silicon
spectroscopically supporting the computed diminished ion might be concerted with an intramolecular hydrogen transfer
affinities. Although the Lewis acidity within [1]2+ is decreased by process to nitrogen, thus potentially lowering the transition
both the intra- and intermolecular donation effects, it still state energy. Although catalytic variants with (Me2N)3SiH and
initiates a catalytic hydrodefluorination reaction. With 6 mol% B(C6F5)3 or [Ph3C+] as initiators remained unsuccessful thus far,
of [1][B(C6F5)4]2, 1-adamantylfluoride was fully converted to this stoichiometric methodology already provides an attractive
adamantane in presence of Et3SiH within <12 h at rt. Finally, the method for the construction of more complex silanes, given the
reaction of [1][B(C6F5)4]2 towards electron-rich aromatic multitude of functionalization methods available for the Si-N
compounds was tested at NMR scale. An immediate bond.[35]
electrophilic aromatic silylation of ferrocene in CD2Cl2 was The present study provides first insights into the stabilization of
observed under very mild conditions, providing the protonated silylium ions by directly attached π-donors (π-stabilization). It
product salt [2a][B(C6F5)4] (Figure 3a and 3b for molecular describes the formal dimer of the elusive silaguanidinium,
structure). In analogy, the reaction of [1][B(C6F5)4]2 towards N- isoelectronic to Al2(NMe2)6 or related compounds, which are
methyl indole gave the C3-silylation product [2b][B(C6F5)4] in known for several decades.[22, 36] The dicationic compound is
perfect regioselectivity. With dimethylaniline,
a highly obtained from commercial reagents, easy to handle in common
regioselective para-silylation to [2c][B(C6F5)4] was observed.[33] organic solvents, but still undergoes reactivity reminiscent to
Such spontaneous and quantitative conversions are silylium ions. Interestingly, it performs spontaneous
remarkable, since Friedel-Crafts type electrophilic aromatic electrophilic aromatic substitutions of electron-rich aromatics –
silylations usually suffer from competing protodesilylation back- potentially caused by means of ligand-element cooperativity.
reactions.[34]
a)
Conflicts of interest
There are no conflicts to declare.
1
0.5 [ ][B(C6F5)4]2
Si(HNMe2)(NMe2)2
[B(C6F5)4]
Fe
Fe
CH2Cl2, rt
2a
[
][B(C6F5)4]
Acknowledgments
Si(HNMe2)(NMe2)2
[B(C6F5)4]
We gratefully thank Prof. H.-J. Himmel for his steady support,
the FCI and DFG for funding and the BWFor/BWUniCluster for
computational resources.
1
0.5 [ ][B(C6F5)4]2
N
N
CH2Cl2, rt
2b
[
][B(C6F5)4]
References
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[B(C6F5)4]
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0.5 [ ][B(C6F5)4]2
CH2Cl2, rt
N
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4 | J. Name., 2012, 00, 1-3
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The formal dimer of the elusive
silaguanidinium is described. It undergoes spontaneous
electrophilic aromatic silylation of electron rich π-systems