Persistent dialkyl(silyl)stannylium ions

Article abstract of DOI:10.1021/ja206338u

The synthesis of dialkyl(silyl)stannylium borates 2[B(C₆F ₅)₄] by reaction of a stable β-silyl-substituted stannylene 3 with silylarenium borates 4[B(C₆F₅) ₄] is reported. The stannylium borates 2[B(C₆F ₅)₄] are characterized by NMR spectroscopy and single-crystal X-ray diffraction, supported by the results of quantum mechanical calculations. The accumulated experimental and theoretical data indicate that the stannylium ions 2 are not stabilized by β-silyl hyperconjugation and that intramolecular C-H/Sn⁺ interactions are not important.

Full text of DOI:10.1021/ja206338u

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pubs.acs.org/JACS
Persistent Dialkyl(silyl)stannylium Ions
Annemarie Sch€afer,* Wolfgang Saak, Detlev Haase, and Thomas M€uller*
Institut f€ur Reine und Angewandte Chemie, Carl von Ossietzky Universit€at Oldenburg, Carl-von-Ossietzky-Strasse 9-11,
D-26111 Oldenburg, Federal Republic of Germany
S Supporting Information
b
Scheme 1. Possible Synthetic Pathways to Organometallic
Group 14 Cations (E = Si, Ge, Sn, Pb)
ABSTRACT: The synthesis of dialkyl(silyl)stannylium bo-
rates 2[B(C6F5)4] by reaction of a stable β-silyl-substituted
stannylene 3 with silylarenium borates 4[B(C6F5)4] is
reported. The stannylium borates 2[B(C6F5)4] are charac-
terized by NMR spectroscopy and single-crystal X-ray diffrac-
tion, supported by the results of quantum mechanical
calculations. The accumulated experimental and theoretical
data indicate that the stannylium ions 2 are not stabilized by
β-silyl hyperconjugation and that intramolecular CꢀH/Sn+
interactions are not important.
Scheme 2. Synthesis of Stannylium Ions 2 (a, R = Et; b,
R = iPr)
arbocations are omnipresent intermediates in organic chem-
C
istry, and recently their higher homologues R3E+ (1), in
particular silylium ions (E = Si), have gained increasing interest in
synthetic organic main-group chemistry and catalysis.1ꢀ5 To
successfully exploit the extreme electrophilicity of the R3E+
species (E = Si, Ge, Sn, Pb), for example in bond activation
processes,3ꢀ5 novel approaches for their synthesis are required.
In addition, for any attempt to control the high reactivity of these
species, a solid understanding of the bonding situation is
mandatory. The conventional approach to organometallic group
14 element cations 1 is EꢀX bond cleavage from neutral,
tetracoordinated compounds (Scheme 1, path A) either by direct
bond heterolysis or in some cases by homolytic scission of EꢀE
bonds to give tricoordinated radicals, R3E•, and their subsequent
oxidation to give the corresponding cations, R3E+.6 An alternative
preparative approach, which circumvents the severe synthetic
problems that arise from the high steric demands made on the
substituents for stabilizing the resulting cations 1, is the addition
of a cationic species to heavy carbenes (Scheme 1, path B).7
Recently, we were able to demonstrate that in the case of
silaimidazolium ions, this procedure is superior to the conven-
tional cation synthesis from tetravalent precursors via bond
cleavage reactions.7d This approach should be particularly fruitful
for the synthesis of stannylium ions, R3Sn+ (1, E = Sn), in which
case stable stannylenes are available as starting materials. In
addition, we exploited this reaction for the synthesis of a Sn2+/
arene complex.8 Here we report on the synthesis and complete
characterization of dialkyl(silyl)stannylium ions 2 by reaction
between silylated arenium ion salts 4 and a stable stannylene 3.
As starting material, dialkyl-substituted stannylene 3, synthe-
sized previously by Kira, Sakurai, and co-workers, was chosen.9
Stannylene 3 exists as a monomer both in solution and in the
solid state and is therefore perfectly suited for our study. Reaction
of stannylene 3 with 1 equiv of the toluenium tetraarylborates
4[B(C6F5)4] in toluene gave brown-beige products, which were
identified by NMR spectroscopy as the [B(C6F5)4] salts of
stannylium ions 2 (Scheme 2).
Although the obtained borates 2[B(C6F5)4] were stable for
weeks in the solid state at ꢀ18 °C, at room temperature
decomposition occurred rapidly, giving several unidentified tin-
containing products, as indicated by 119Sn NMR spectroscopy.
The identity of stannylium ions 2 is confirmed by NMR
spectroscopy. For each cation, only one 119Sn signal is detected
in toluene-d8 solution. The respective resonances are located in a
spectral region (δ 119Sn = 1412 (2a), 1441 (2b))10 that is
approximately halfway between the 119Sn NMR chemical shifts
that were found for tris-arylstannylium ions (δ 119Sn (Ar3Sn+) =
697ꢀ806)11 and that detected for trisilyl-substituted stannylium
ion (tBu2MeSi)3Sn+ 5 (δ 119Sn for 5 = 2653).12 The established
29Si NMR chemical shifts of the R-silicon atom Si(1) (δ 29Si =
40.0 (2a) and 70.3 (2b)) agree with data reported for the silyl-
substituted stannylium ion 5 (δ 29Si = 64.9).12 The direct linkage
1
between the silicon and tin atom is indicated by the J(SnSi)
coupling constants detected for cations 2 (1J(SiSn) = 50 (2a) and
46 Hz (2b)), although these values are low for 1J(SiSn) coupling
constants (i.e., in (Me3Sn)4Si, 1J(SnSi) = 220 Hz).13 The
increase in the coordination number during the reaction from
stannylene 3 to give the stannylium ions 2 is accompanied by a
considerable high-field shift Δδ119Sn of the 119Sn resonance (Δδ
119Sn = ꢀ911 (2a) and ꢀ882 (2b)). The 13C and 1H NMR data
Received: July 13, 2011
Published: August 12, 2011
r
2011 American Chemical Society
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|

Journal of the American Chemical Society
COMMUNICATION
Table 1. Comparison of Experimentally Obtained and
Computed (Italic) Structural Parameters of Cations 2
SnꢀC(1)/
SnꢀC(10)
[pm]
C(1)ꢀSi/
SnꢀSi(1)
[pm]
C(1)ꢀSnꢀC(10)
C(10)ꢀSi
compd
[deg]
[pm]
2aa
264.53(5)
268.25
214.88(9)/
214.43(13)
215.91/
91.934(42)
91.70
189.62ꢀ
191.23
2ab
2bc
2bb
3d
190.93ꢀ
191.81
216.36
267.89(3)
271.69
215.68(13)/
216.53(9)
217.36/
91.813(42)
91.05
190.86ꢀ
193.02
Figure 1. Ellipsoid presentation of the molecular structure of cation 2a
in the crystal (at 50% probability). View a) perpendicular to the
C(1)SnC(10) plane (hydrogen atoms are omitted) and b) along the
Si(1)Sn0vector indicating the close spatial proximity of the H(CH3) and
H(CH3 ) to the tin atom (Et groups at Si(1) and all hydrogen atoms but
those attached to methyl groups close to the tin atom are omitted).
191.43ꢀ
193.24
217.62
221.8(7)/
222.3(7)
224.86
86.7(2)
85.95
186.9ꢀ
189.8
3b
189.00/
189.00
are also in agreement with the proposed structures.14 Structural
evidence for the formation of stannylium ions 2 comes from
X-ray diffraction (XRD) analysis of the salts 2[B(C6F5)4].
Suitable crystals for single-crystal XRD analysis of these com-
pounds were grown from toluene at ꢀ18 °C. The solid-state
structures14 of the salts revealed in both cases that stannylium
ions 2 are clearly separated from the borate anion; in particular,
no fluorine atom of the borate anion approaches the positively
charged tin atom at a distance smaller than 500 pm. The borate
2b[B(C6F5)4] crystallizes with half a molecule of the solvent
toluene in the asymmetric unit. However, no direct interaction
between the positively charged tin atom and the toluene mole-
cule is indicated by any structural parameter. The crystal
structure of the borate 2a[B(C6F5)4] indicates some positional
disorder of one of the ethyl groups at the silicon atom, which was
resolved by using a split model with a 6:4 occupation. The tin
atoms in stannylium ions 2 adopt trigonal planar coordination
environments (see Figure 1 for the molecular structure of cation
2a) and enforce for the stannacyclopentane rings half-chair
conformations. Planarity around the tin atoms is indicated in
both cases by the summation of the CꢀSnꢀC and the two
CꢀSnꢀSi bond angles R to 360°. The Si(1)ꢀSn bond lengths in
stannylium ions 2 are comparable to reported SiꢀSn bonds in
cation 5 (264.53 (2a) and 267.89 pm (2b) vs 268.63 pm (5)).12
The SnꢀC(1) and SnꢀC(10) bond lengths (214.43ꢀ216.53
pm) also do not differ significantly from those found for related
stannyl cations (i.e., SnꢀC(sp3): 216 pm in nBu3Sn[CB11Me12];
217.0 pm in stannyl cation 6).15,16 These SnC bonds are,
however, significantly shorter (by 5.8ꢀ7.9 pm) than those
in the starting stannylene 3,9 in agreement with a signifi-
cantly increased contribution of the 5s orbital to the SnꢀC
bond in the tricoordinated cation. Experimentally deter-
mined and calculated bonds and angles are summarized in
Table 1.
a From single-crystal XRD of 2a[B(C6F5)4]. b Optimized molecular
structure obtained using MPW1K/SDD(Sn)6-311+G(d,p)(C,H,Si).
c From single-crystal XRD of 2b[B(C6F5)4] 0.5C7H8. d Taken from ref 9.
3
Figure 2. Contour plots of the calculated Laplacian of the electron
2
density, r F(r), of stannylium ion 2a, (a) in the C(1)ꢀSnꢀSi(1) plane
0
and (b) in the Snꢀbcp(SnH)ꢀH(CH3 ) plane. The bond paths are
shown by solid (F(bcp) > 0.025 e bohrꢀ5) or dashed (F(bcp) > 0.025 e
bohrꢀ5) black lines, and the corresponding bcp's are shown as green circles.
2
Red contours indicate regions of local charge accumulation (r F(r) < 0),
2
and blue contours indicate regions of local charge depletion (r F(r) > 0).
in the molecular structures of the cations which deserve partic-
ular attention. (i) The SnꢀSi and SnꢀC bond lengths (see
Table 1) are all in the expected range;17 however, the SiꢀC
bonds in the β-position to the positively charged Sn atom are
relatively long (189.62ꢀ193.02 pm in 2a,b vs 186.0 pm for
regular SiꢀC(sp3)).17 These SiꢀC bonds are even somewhat
longer than those in stannylene 3 (by 2.0 (2a/3) or 3.5 pm (2b/
3), see Table 1). These structural effects are small though, and in
view of the high steric congestion in both cations, we conclude
that they are too small to firmly indicate β-silyl stabilization of the
stannylium ions 2a and 2b. (ii) Due to the half-chair conforma-
tion of the stannacyclopentane ring in cations 2, one hydrogen
atom of the methyl groups of each of the pseudo-axial-orientated
trimethylsilyl groups is placed above and below the C(1)ꢀ
SnꢀC(10) plane, in close proximity to the tin atom (2a, H(C-
(Me))ꢀSn 280.7 pm, H(C(Me0)ꢀSn 255.7 pm, see Figure 1b;
2b, H(C(Me))ꢀSn 284.1 pm, H(C(Me0)ꢀSn 267.8 pm, see
Supporting Information). These distances clearly exceed every
These structural features clearly identify 2a and 2b as trico-
ordinated stannylium ions. There are, however, two peculiarities
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Journal of the American Chemical Society
COMMUNICATION
Table 2. Properties of the Electron Density at the Bond Critical Points of SnꢀX Bonds (X = C, Si, H) in Cations 2 and Model Tin
Compounds According to QTAIM Analysis
2
bond type
compd
F(r) [e bohrꢀ3
]
r F(r) [e bohrꢀ5
]
atomic distance [pm]
SnꢀC
Me4Sn
0.0996
+0.1450
216.5
2a
0.0995/0.0988
0.0962/0.0976
0.0693
+0.1401/ +0.1369
+0.1268/ +0.1331
ꢀ0.0358
214.9/214.4
215.7/216.5
261.1
2b
SnꢀSi
SnꢀH
(Me3Si)4Sn
2a
0.0646
ꢀ0.0587
264.5
2b
0.0615
ꢀ0.0521
267.9
Me3SnH
0.1071
+0.1463
172.1
[H3SnꢀHꢀCH3]+
2a
0.0330
+0.0676
212.7
0.0154
+0.0433
255.7
reported SnꢀH bond length,17 but they are significantly smaller
than the sum of the van der Waals radii (327 pm)18 and suggest a
higher coordination of the tin atom and possibly involvement of
the central tin atom and the distant hydrogen atoms of the
methyl groups in multiple-center bonding. This question was
addressed by applying density functional theory at the MPW1K
level.14,19 In addition, the results of the DFT calculations were
used to confirm the structure of cations 2 in solution and to
investigate the bonding in stannylium ions 2. The results of the
computations predict molecular structures for both cations 2
having all important structural features very close to those
determined experimentally (see Table 1). The stannylium ions
2 also persist in arene solution, as indicated by the close
accordance between computed 29Si NMR chemical shifts for
Si(1) and the experimental determined values (2a, δ 29Siexp
(Si(1)) = 40.0, δ 29Sicalc(Si(1)) = 41; 2b, δ 29Siexp(Si(1)) = 70.3,
central tin atom and the distant methyl protons. Therefore,
QTAIM analysis provides no resilient indication for significant
multiple-center bonding between these hydrogen atoms and
the central tin atom.22
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, NMR
b
spectra, and crystal structure data for 2[B(C6F5)4], and compu-
tational details, including NMR chemical shift calculations and
Cartesian coordinates. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
thomas.mueller@uni-oldenburg.de
δ
29Sicalc(Si(1)) = 73). In view of the principal problems associ-
ated with the computation of tin NMR properties of tin
compounds,6c,11a,20 the calculated 119Sn NMR chemical shifts
for cations 2a,b are also in full agreement with the experimental
data (2a, δ 119Sncalc = 1605; 2b, δ 119Sncalc = 1590).14 The
newly formed SnꢀSi linkages in 2a,b are relatively strong,
despite the high steric strain imposed by the large substituents.
That is, the Gibbs free energy for redissociation into stannylene
and silylium ion is substantial (G298 = 137 (2a) or 99 kJ molꢀ1
(2b)). According to an analysis based on the quantum theory of
atoms in molecules (QTAIM),21 the Sn+ꢀSi bond in cations 2
is predominately covalent, as indicated by substantial electron
density, F(r), computed at the bond critical point (bcp) and the
’ ACKNOWLEDGMENT
This study was supported by the DFG (Mu-1440/7-1). The
Center for Scientific Computing at Carl von Ossietzky University
is thanked for computer time.
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2
negative Laplacian of the electron density, r F(r), at this point,
which indicates charge accumulation (see Figure 2a and
Table 2). In contrast, the Sn+ꢀC bond has large ionic con-
2
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characteristics of electron density at the bcp's of the SnꢀC and
Sn+ꢀSi bonds in cations 2 are very similar to those computed
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0
between the hydrogen atoms (H(CH3) and H(CH3 )) of the
pseudo-axial-oriented trimethylsilyl groups and the central tin
atom, despite their relative spatial proximity. The only excep-
tion is the Sn/H(CH3) pair in 2a, with the smallest separation
(255.7 pm), for which a weak bcp is found (see Figure 2b).
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at this bcp in 2a with those of SnH bonds of standard
compounds such as Me3SnH (two-electron, two-center (2e-
2c) bond) and the [H3SnꢀHꢀCH3]+ cation (2e-3c bond, see
Table 2) suggests only a very small interaction between the
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Journal of the American Chemical Society
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