Trisilyl-substituted vinyl cations

Article abstract of DOI:10.1002/chem.200900795

A series of ss,ss-disilyl-substituted vinyl cations were prepared by intramolecular addition of transient silylium ions to C≡C triple bonds. The vinyl cations were isolated from hydrocarbon solutions as tetrakis(pentafluorophenyl) borates at room temperat

Full text of DOI:10.1002/chem.200900795

DOI: 10.1002/chem.200900795
Trisilyl-Substituted Vinyl Cations
Andrea Klaer,[a] Yvonne Syha,[b] Hamid Reza Nasiri,[c] and Thomas Mꢀller*[a]
Dedicated to Professor Yitzhak Apeloig in occasion of his 65th birthday
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Chem. Eur. J. 2009, 15, 8414 – 8423

FULL PAPER
Abstract: A series of b,b-disilyl-substi-
tuted vinyl cations were prepared by
intramolecular addition of transient
istic 13C and 29Si NMR data. The NMR
investigations revealed for b,b-disilyl-a-
germyl- and trisilyl-substituted vinyl
cations a highly dynamic structure in
which both vinylic carbon atoms under-
go an intramolecular exchange process
which is fast on the NMR time scale at
room temperature. NMR studies using
a doubly 13C-labeled vinyl cation sug-
gest as mechanism for this exchange
ꢁ
silylium ions to C C triple bonds. The
vinyl cations were isolated from hydro-
carbon solutions as tetrakis(pentafluoro-
phenyl) borates at room temperature.
The substituents directly attached to
the positively charged carbon atom
were varied from tert-butyl- to trialkyl-
silyl- to trialkylgermyl groups. The cat-
ions were identified by their character-
process a rotation of the dicarbyne
2ꢀ
ꢁ
C C unit within the triangle defined
by the three cationic silyl fragments.
Therefore the dynamic structure indi-
cated for trisilyl- or disilylgermyl-sub-
stituted vinyl cations parallels the situa-
tion found for the parent vinyl cation,
the protonated acetylene.
Keywords: ab initio calculations ·
carbocations
· density functional
calculations · Group 14 elements ·
NMR spectroscopy
Introduction
protonated acetylene 3 and that the Y-shaped vinyl cation
isomer 4 represents either only a shallow minimum on the
potential energy surface (PES) or is the transition state for a
degenerate hydrogen-interchange process in 3.[2b,9] The bar-
rier for this degenerate “merry-go-round” process is uni-
formly predicted to be very small (less than 17 kJmolꢀ1),
Vinyl cations,[1] carbenium ions of the general formula
R3C2+, usually adopt a configuration in which the positively
charged sp-hybridized carbon atom is linearly coordinated
and the second, sp2-hybridized carbon atom has trigonal
planar coordination environment. For substituted vinyl cat-
ions (R ¼ H), this classical Y-shaped structure is predicted
by quantum mechanical calculations to be that of the most
stable isomer.[2] This prediction is confirmed by the stereo-
chemical outcome of vinylic SN1-type substitution reac-
tions[3] and by the results of NMR investigations of persis-
tent vinyl cations.[4–7] Final experimental proof for the classi-
cal vinyl cation structure came from recent X-ray diffraction
investigations of salts of vinyl cations 1a and 2 with weakly
coordinating counter anions.[5]
+
which indicates a highly fluxional structure for the C2H3
ion. In agreement with these conventional theoretical meth-
ods, ab initio molecular dynamic simulations that treat the
nuclei as classical and quantum particles find a quasi-planar
+
quantum ground state of C2H3 with anisotropic delocaliza-
tion of the protons due to zero-point quantum effects.[10] Fi-
nally, advanced infrared[11–12] and millimeter wave[13–16] spec-
troscopy clearly favor a bridged structure subject to proton
tunneling and the deduced rotational constants as well as
the barrier height agree with theoretical predictions for the
hydrogen-bridged cation 3.[17]
Silylium ions, R3Si+ behave similar to protons as substitu-
ents.[18] In both types of cations positive charge is accumulat-
ed in a relatively small region in space. This high positive
charge accumulation results in a high reactivity of the cat-
ions towards all accessible nucleophiles, and in consequence
these ions are hard to observe as free entities unfettered by
any interaction in condensed media.[19] This similarity be-
tween protons and silylium ions prompted us to investigate
the structure of trisilyl-substituted and related vinyl cations
by computational and experimental methods. The straight-
forward access of cyclic b,b-disilyl-substituted vinyl cations
of the general type 1 by intramolecular addition of transient
+
Most notably, the parent C2H3 ion adopts a different
structure. All high level quantum mechanical investigations
conclude that the most stable isomer of the C2H3 ion is the
+
[a] Dipl.-Chem. A. Klaer, Prof. Dr. T. Mꢁller
Institut fꢁr Reine und Angewandte Chemie
Carl von Ossietzky Universitꢂt Oldenburg
ꢁ
silylium ions to a C C triple bond can in principle be ex-
tended to a large variety of vinyl cations which have differ-
ent substituents R directly attached to the positively charged
carbon atom Ca. Previously, we reported the synthesis of
vinyl cations 1 with cation stabilizing substituents such as
alkyl, cyclopropyl or aryl in the a-position.[6–8,20] In this
report, we extend the series of vinyl cations 1 to cations
with substituents R such as silyl and germyl which in princi-
ple have a destabilizing effect compared to alkyl groups on
the stability of a vinyl cation in the a-position. Therefore,
we synthesized and characterized trisilyl, (R=SiR3), disilyl
germyl (R=GeR3) and, for comparison, disilyl alkyl (R=
Carl von Ossietzky Strasse 9–11, 26211 Oldenburg (Germany)
E-mail: thomas.mueller@uni-oldenburg.de
[b] Dr. Y. Syha
Institut fꢁr Anorganische und Analytische Chemie
Johann Wolfgang Goethe-Universitꢂt Frankfurt/Main
Marie-Curie-Strasse 11, 60439 Frankfurt (Germany)
[c] Dr. H. R. Nasiri
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900795.
Chem. Eur. J. 2009, 15, 8414 – 8423
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T. Mꢁller et al.
tBu) substituted vinyl cations 1 in order to probe their sta-
bility, their structure and the dynamic in these vinyl cations
in dependence on the ring size and on the a-substituent R.
The experimental studies are supported and extended by
quantum mechanical calculations on structures and energies
temperature. In agreement with previous theoretical predic-
tions,[21] the silyl- and germyl-substituted vinyl cations 1b–
e,h,i,n,p are significantly less stable. In the case of the tri-
methylsilyl-substituted vinyl cations 1b,n,p the decomposi-
tion rate increases with increasing ring size. That is, the five-
membered ring cation 1n is stable at 608C in toluene while
the seven-membered ring cation 1p slowly decomposes even
at 08C into unidentified products. Substitution with more
bulky substituted silyl groups does not increase the thermal
stability of the cations significantly. For example, the tert-bu-
tyldimethylsilyl-, the triisopropylsilyl- and the tris(trimethyl-
silyl)-substituted vinyl cations 1c–e are only marginally
more stable than the trimethylsilyl-substituted cation 1b and
decompose at temperatures above 408C.
+
for vinyl cations 1 and for the parent (H3Si)3C2 cation.
Results and Discussion
Synthesis: The precursor alkynyl silanes 5 were synthesized
according to standard synthetic protocols,[6–8] which are de-
scribed in detail in the Supporting Information. As de-
scribed previously for cation 1a and related cations, treat-
ment of the precursor 5 with trityl cation in benzene or tolu-
ene gave the corresponding cyclic vinyl cations 1b–e, h, i,
m–p (see Scheme 1).[6–8] The counteranion was in all cases
NMR spectroscopic characterization: The 13C NMR spectro-
scopic parameter of the tert-butyl-substituted vinyl cations
1m and 1o (see Table 1 and Experimental Section) closely
resemble those reported previously for vinyl cation 1a.[8]
Therefore, these cations clearly adopt a classical vinyl cation
ground-state structure. The C=C+ unit of the vinyl cation is
readily identified by the low-field resonance of the positive-
ly charged Ca atom at d 13C=198.5–204.6 ppm and that at-
tributed to the trigonal Cb atom at d 13C=74.9–77.0 ppm.
The unusual small 1J(Cb,Si) coupling constant (1J(Cb,Si)=
ACTHNUTRGENNUG ACHTUNGTRENNUNG
b
Scheme 1. Synthesis of vinyl cations 1 and nitrilium ion 6 (For the com-
pound numbering see Table 1).
ꢀ
16.7 (1o), and 20.2 Hz (1m)) is typical for the C Si bond in
these cations and is a consequence of the reduced bond
order due to delocalization of electron density from the s-
b
ꢀ
tetrakis(pentafluorophenylborate) (TPFPB). Interestingly,
the polysilyl- and triphenylsilyl-substituted alkynes 5 f and
5g did not give a vinyl cation upon treatment with trityl
cation. In these cases several unidentified products were de-
tected by NMR spectroscopy. The vinyl cation salts
1·TPFPB were isolated as white or slightly brown amor-
phous or microcrystalline solids. All attempts to grow crys-
tals from the 1·TPFPB salts using different solvents such as
fluorinated or chlorinated arenes failed. Similarly, all at-
tempts to use anions such as halogenated carboranates to
Si C bond to the formally empty 2p orbital at the positive-
ly charged Ca atom.[7,8] In both tert-butyl-substituted vinyl
cations, 1m and 1o, the silicon atoms are deshielded com-
pared with 1a (d 29Si=28.9 (1a), 70.8 (1m), 38.0 (1o)). In
particular the significant low-field shift detected for the
compound 1m is remarkable, but can be attributed at least
in part to the increased ring strain in the five-membered
ring compound 1m compared with 1,3-disilahexane com-
pound 1a. Similar deshielding effects on d 29Si NMR chemi-
cal shifts have been previously described for neutral 1,3-disi-
lacyclopentanes compared with 1,3-disilacyclohexanes.[22]
The close accordance with the 13C NMR data for the a-tri-
alkylsilyl-substituted vinyl cations 1b–e, collected at temper-
atures below 08C, to those of 1a (see Table 1) clearly indi-
cates classical Y-shaped ground state structures also for
these trisilyl-substituted vinyl cations. For all four cations,
1b–e, the C=C+ unit is indicated by the low-field resonance
for the Ca atom at d 13C(Ca)=231.3–181.7 and by the rela-
tively high-field shifted signal for Cb (d 13C(Cb)=116.1–
108.6). The 29Si NMR chemical shift detected for the b-
SiMe2 groups are in the expected region for b,b-disilyl-sub-
obtain crystals from 1·ACHTNUTGRNEUNG
[CB11H6X6]ꢀ (X=Cl, Br)[8] which are
suitable for X-ray diffraction were unsuccessful. The salts
1·TPFPB form liquid clathrates of high salt concentrations
with arene solvents which faciliate the NMR spectroscopic
investigations. These clathrates remain liquid down to tem-
peratures of about ꢀ30–ꢀ408C. At lower temperatures the
clathrates become solid. No marked solvent effects on the
NMR chemical shifts can be detected as long as aromatic
hydrocarbons are used (see Table 1). Addition of solvents of
higher donicity as, for example, methylene chloride even at
temperatures as low as ꢀ508C leads to decomposition of the
vinyl cations. Addition of acetonitrile to a benzene solution
of 1b TPFPB results in the clean formation of acetonitrilium
ion 6 (see Table 1).
stituted vinyl cations (d 29Si
ACTHNUTRGNEUNG(SiMe2)=19.9-26.9, see
Table 1)[6–8] while the 29Si NMR signals for the a-trialkylsilyl
groups in cations 1b–d appear at higher field (d 29Si(Si)=
ꢀ1.1–7.7).[23] Interestingly, both types of silicon nuclei in cat-
ions 1b–e are deshielded compared with the respective pre-
cursor bissilylalkynes 5b–e. The deshielding of the b-silicon
atoms upon ionization is large (Dd 29Si=39.1–45.3)[24] and
The thermal stability of the vinyl cation salts 1·TPFPB
strongly depends on the substituent R. The tert-butyl sub-
stituent confers to the vinyl cation a high stability.[8a] As a
result solutions of cations 1a,m,o in arene solvents show no
sign of decomposition even after storage for days at room
ꢀ
can be rationalized by the occurrence of b-Si C hyperconju-
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Chem. Eur. J. 2009, 15, 8414 – 8423

Trisilyl-Substituted Vinyl Cations
FULL PAPER
Table 1. NMR chemical shifts d and experimental activation energies EA for Ca, Cb interchange in alkyl-, silyl-
temperatures, both 29Si NMR
signals appear as doublets due
to the coupling with one 13C
and germyl-substituted vinyl cations 1.[a]
[b]
Cpd
n
R
d
13Ca
d
13Cb
d
29SiMe2
d
13C
(CH2)
d
29Si(R)
EA
1a[c,d]
1b[e]
1b[c]
1b[c,i]
1c[f]
1d[f]
1e[i]
1 f
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
2
2
1
tBu
202.7
184.1
–
75.5
114.1
–
28.9
21.4
21.1
21.4
19.9
26.9
22.3
16.2; 15.2
15.9; 15.8
16.1; 16.1
15.8; 15.8
16.1; 15.9
16.0; 15.9
16.0; 15.8
nuclei (1J
and 1J(Si,Ca)
ACHTUNGTRENNUNG
A
ACHTUNGRTENUN(NG SiMe2)=12 Hz
SiMe3
SiMe3
SiMe3
SiMe2tBu
SiiPr3
ꢀ1.1
G
N
ꢀ1.2
This observed coupling pattern
is in agreement with a Y-shaped
–-
–
ꢀ1.1
181.7
209.9
231.3
116.1
109.8
108.6
6.4
ground-state
structure
for
7.7
[13C2]-1b. Above room temper-
ature the multiplicity of both
signals has changed to two trip-
lets indicating the coupling with
Si
Si
U
ꢀ95.7, ꢀ4.3
1g
SiPh3
1h[c]
1h[c,i]
1i[c]
1k
GeMe3
GeMe3
GeiPr3
SnnBu3
SnACHTUNGTRENNUNG(cy-hex)3
tBu
SiMe3
tBu
SiMe3
SiMe3
151.2
151.0
160.9
151.2
151.0
160.9
16.9
16.9
22.2
16.1; 16.1
15.8; 15.8
15.9; 15.9
about halved
1l
ACTHNUTRGNEUNG
1J(Si,C) coupling constants (1J-
1m[c]
1n[f]
1o[g]
1p[h]
6[c,i]
198.5
161.1
204.6
153.0
115.2
74.9
70.8
47.5
38.0
28.2
12.7
13.7
24.4; 17.9
24.0; 16.1
19.3; 17.6; 16.9
A
E
and
127.6[j]
77.0
ꢀ0.3
A
ACHTUNGRTEN(NNUG SiMe3)=20 Hz). Both
29Si NMR chemical shifts are
slightly temperature dependant
153.0
112.3
0.3
ꢀ19.0
33.5; ꢀ17.9
[a] In [D8]toluene. [b] Activation energies in kJmolꢀ1. [c] At T=300 K. [d] From ref. [8]. [e] At T=243 K.
[f] At T=273 K. [g] At T=263 K. [h] At T=258 K. [i] In [D6]benzene. [j] The 13C NMR signal for Cb in
(DT=80 K: Dd 29Si
ACTHNUTRGNEN(UG SiMe3)=
0.9; Dd 29Si (SiMe2)=2.3, see
[D8]toluene was detected using a 13C{1H} INEPT pulse sequence using 12 CH protons of the two SiMe2 groups
Figure 2).
2
and an estimated J
(CbH)=5 Hz.
gation and therefore localization of positive charge at the b-
silicon atoms.[6,7] The deshielding of the a-silicon atom is
smaller, but still significant (Dd 29Si=5.8–24.6) and might be
a result of the electron-withdrawing effect of the neighbor-
ing positively charged carbon atom.
The 13C NMR signals of the carbon atoms Ca and Cb in
1b–e show kinetic line broadening. That is, while at low
temperatures (T=243 K) relative narrow signals were de-
tected, the lines broaden at higher temperatures and they fi-
nally disappear in the base line at temperatures around T=
273 K. This process is fully reversible and after cooling to
T=253 K both 13C NMR signals for the C=C+ unit are
clearly visible. Up to temperatures T=373–383 K no time
averaged signal could be detected, at that temperatures,
however, decomposition of the trisilyl-substituted vinyl cat-
ions becomes an important process. NMR investigations
using [13C2]-1b·TPFPB, in which both vinylic carbon atoms
were 13C-labeled, allowed a deeper understanding of the dy-
namic processes in this cations. Two-dimensional exchange
spectroscopy (2D EXSY)[25] at T=253 K clearly indicated a
side exchange between both vinylic carbon atoms in vinyl
cation [13C2]-1b (see Figure 1). 13C{1H} NMR spectroscopic
measurements at T=243 K gave doublets at d 13C(Ca)=
184.1 and d 13C(Ca)=116.1 ppm for both vinylic carbon
Figure 1. 13C{1H} 2D EXSY phase sensitive contour map obtained at
125.705 MHz with a mixing time of 500 ms at T=253 K of [13C2]-1b
TPFPB in [D8]toluene.
We attribute this dynamic behavior of the vinyl cations
b
a
ꢁ
1b–e to a degenerate rotation of the central C
C dicar-
byne unit within the triangle of the three silicon substituents,
in which both carbon atoms are interchanged via the silyl-
bridged transition states 7b–e (see Scheme 2). The activa-
tion energies EA for this processes were extracted from line-
shape analysis in the slow-exchange regime and were deter-
mined to be EA =53.1–56.0 kJmolꢀ1 (see Table 1 and the
Supporting Information for details).
1
1
atoms with a JACHTUNGTRENNUNG(C,C) coupling constant of JACHTUTGNREN(NUGN C,C)=65 Hz.
This is significantly smaller than the coupling
constant determined for the precursor alkyne [13C2]-5b
(1J
N
The intriguing difference between the static vinyl cation
1a and the floppy molecule 1b can be qualitatively under-
stood when the relative energy of the respective transition
state structure 7 is taken into account. Alkyl-bridging in 7a
is energetically clearly less favored than bridging by the silyl
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T. Mꢁller et al.
difference between the classical Y-shaped vinyl cation
isomer 1 and the bridged species 7 decreases going from
alkyl substituents to silyl and finally to germyl groups. All
attempts to prepare stannyl-substituted vinyl cations failed,
probably due to the fast destannylation of the stannylalkyne
precursors 5k,l during the preparation of the cation.
A second factor which severely influences the barrier for
the rotation of the central C=C+ unit in cations of type 1 is
the ring size. Transition-state structures 7 with larger rings
are better suited to accommodate the carbon atoms Ca and
Cb with a formal hybridization state between sp and sp2.
Therefore, the barrier for the rotation of the central C=C+
unit in vinyl cations 1 is expected to increase with decreas-
ing lengths n of the alkyl chain. Therefore, we synthesized
the TPFPB salt of trisilyl-substituted vinyl cation 1n which
has a five-membered disilaheterocycle (n=0), and of the tri-
silyl-substituted vinyl cation 1p with a seven-membered disi-
laheterocycle (n=2) (see Scheme 2). The 13C and 29Si NMR
chemical shifts found for the five-membered ring cation 1n
clearly indicate the classical Y-shaped vinyl cation structure
(see Table 1 and Supporting Information) and the detected
Figure 2. 99.305 MHz 29Si{1H} INEPT spectra of [13C2]-1b·TPFPB in
[D8]toluene. Top: Signals due to the SiMe2 groups. Bottom: Resonances
due to the SiMe3 group.
1J
1J
G
(Cb,Si)=17.6 Hz and
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
for the [13C2]-1b and provide further evidence for its struc-
ture. In contrast to the six-membered ring cation 1b, how-
ever, the 13C NMR spectra of five-membered ring cation 1n
show at room temperature no kinetic line broadening. Even
at 708C, when the cation 1n starts to decompose no kinetic
line broadening of the 13C NMR signals of the carbon atoms
Ca and Cb could be observed. The NMR data obtained for
the trimethylsilyl-substituted seven-membered ring cation
1p (n=2) are similar to that of the trialkylgermyl-substitut-
ed vinyl cations 1h, i and to those of the silyl-substituted
vinyl cations 1b–e and are therefore in agreement with a
classical Y-shaped vinyl cation ground state structure. In
particular, the 13C NMR spectrum of the cation 1p shows
even at T=243 K only one time-averaged signal for the two
vinylic carbon atoms. At lower temperatures the [D8]toluene
solution starts to solidify. The apparent limitations, the ther-
molability of cation 1n and the relative high freezing point
of the clathrate of the salt 1p·TPFPB with [D8]toluene,
clearly prevent an accurate determination of the activation
barriers for the carbon atom interchange process in the tri-
silyl cations 1n and p. Nevertheless, these experiments can
be used to confirm qualitatively the expected relation be-
tween ring size of the cation and activation barrier.
Scheme 2. Degenerate rotation of the C=C+ unit in disilyl-substituted
vinyl cations 1 via R-bridged transition states 7.
group in 7b, therefore the barrier for the rotation of the
central C=C+ unit is high for 1a, while it is thermally acces-
sible in the case of 1b (and likewise for 1c–e). Extending
this rationalization, germyl substitution as in 1h and 1i
should further decrease this barrier. The NMR spectroscopic
parameter detected for the trialkylgermyl-substituted vinyl
cation 1h and 1i at T=303 K are very similar to those
found for the related cations 1a and 1b (see Table 1 and the
Supporting Information), with the notable exception that
for each cation for the carbon atoms Ca and Cb only one
signal at room temperature was detected (1h: d 13C=151.2;
1i: d 13C=160.9). This indicates that the degenerate rear-
rangement which interconverts the carbon atoms Ca and Cb
in 1h, i, is fast on the NMR time scale at ambient tempera-
ture. In both cases the 13C NMR signal broadens considera-
bly upon cooling, however, no coalescence and no line split-
ting could be observed before the sample becomes solid at
T=233 K. The activation energy EA for this rotation of the
C=C+ unit in the germyl-substituted vinyl cations 1h, i had
to be extracted solely from the data obtained in the fast ex-
change regime and was found to be EA =29.7 kJmolꢀ1 (1h)
and EA =28.1 kJmolꢀ1 (1i).[26] The results of the NMR inves-
tigations for vinyl cations 1a–e, h, i suggest that the energy
Computational part:[27] In order to reach a principal under-
standing of the nature of experimentally investigated tris-
(trialkylsilyl)-substituted vinyl cations 1b–e, n, p the struc-
tures of several isomers of the parent trisilyl vinyl cation
+
(H3Si)3C2 (8) were computed at the density functional
B3LYP/6-311G
AHCTUNGTRENNUNG
(2d,p)[28] and at the ab initio MP2/6-311G-
ACHTUNGTRENNUNG
(2d,p) and CCSD/cc-pvtz levels of theory.[29–32] In particular,
the bridged isomer 8a of Cs symmetry and the classical Y-
shaped vinyl cation structure 8b were investigated (see
Figure 3). Both standard methods, B3LYP and MP2, predict
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Chem. Eur. J. 2009, 15, 8414 – 8423

Trisilyl-Substituted Vinyl Cations
FULL PAPER
for each of the two isomers, 8a and 8b, very similar geome-
tries. The methods differ however partly in the description
of the nature of the stationary points (see Table 2). While
Table 2. Relative energies (kJmolꢀ1) of several isomers of trisilyl vinyl
cation 8 at different levels of theory (number of imaginary frequencies in
square brackets and size of the imaginary frequency in parenthesis).
8a[a]
8b
8c
B3LYP/6-311G
G
0; [1], (ꢀ124) ꢀ0.9; [1], (ꢀ41) ꢀ2.9; [0]
MP2/6-311G(2d,p)
G
0; [0]
21.4; [1] (ꢀ82)
CCSD/cc-pvdz
0
0
0
7.2
12.1
13.3
CCSD(T)/cc-pvtz//B3LYP[b]
CCSD(T)/cc-pvtz//MP2[b]
0.8
(2d,p);
[a] Absolute energies of 8a: ꢀ949.88425H, B3LYP/6-311G
ꢀ948.11521H, MP2/6-311G(2d,p); ꢀ948,30916H, CCSD(T)/cc-pvtz//
B3LYP/6-311G (2d,p); ꢀ948.30939, CCSD(T)/cc-pvtz//MP2/6-311G-
(2d,p). [b] A 6-311G(2d,p) basis set was used.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
G
the Y-shaped isomer 8b is found to be the transition state
for in-plane silyl migration by both methods, the silylbridged
isomer 8a is the ground state of the trisilyl vinyl cation 8
with the ab initio MP2 method, but it is a transition state at
the hybrid density functional B3LYP level of theory. At the
B3LYP level of theory the ground state of the trisilyl vinyl
cation
8 adopts a quite unsymmetrical structure 8c
(Figure 3). Qualitatively, the geometry of isomer 8c is best
described as a hybrid between the classical Y-shaped vinyl
cation geometry and the silyl-bridged structure. The most
significant structural feature of 8c is the occurrence of two
ꢀ
different b-silyl groups with clearly different Si C bond dis-
tances (205.4 and 191.7 pm) and markedly distinct bond
angles aACHTUNGTRENNUNG
(CaCbSi) (90.9 and 152.48).
The isomers 8a and 8b are the only stationary points at
the MP2 level along a reaction coordinate which allows for
the interchange of the silyl groups in cation 8. The barrier
for this process which converts two equivalent cations 8a via
the transition state 8b is 21.4 kJmolꢀ1 and a potential
energy scan along the reaction coordinate for the in-plane
migration of the silyl groups around the central carbyne
core confirms the single minimum potential at the MP2
level (see Figure 4).
Figure 3. Calculated structures of the [(H3Si)3C2]+ cation 8, obtained at
the MP2/6-311GACTHNUTRGENN(UG 2d,p) (bold), at the B3LYP/6-311GCAHTUNGTRENN(UGN 2d,p) (italic) and at
the CCSD/cc-pvdz (underlined) level of theory (Computed bond lengths
in pm).
In agreement with the larger number of stationary points
located, a similar potential energy scan at the B3LYP/6-
311GACHTUNGTRENNUNG(2d,p) level of theory suggests a more complicated re-
action coordinate for the degenerate silyl migration in trisil-
yl vinyl cation 8. Equivalent unsymmetrical ground-state
structures 8c are connected via the silyl-bridged transition
state 8a or via the nearly symmetric Y-shaped classical vinyl
cation structure 8b. These calculations reveal the “floppy”
nature of trisilyl vinyl cation 8. The located stationary points
on the PES are all very near in energy, the highest barrier at
B3LYP/6-311G
ACHTUNGTRENNUNG
(2d,p) is less than 5 kJmolꢀ1 and at MP2/6-
311G
ACHTUNGTRENNUNG
ric displacements on only small energy costs. Coupled clus-
ter single point calculations using a relatively large basis set
of triple zeta quality (CCSD(T)/cc-pvtz) were used to obtain
more reliable information about the nature of the different
isomers of 8 and their relative energy differences. Single-
point calculations were performed for all stationary points
Figure 4. Calculated reaction path for the SiH3 interchange in the
[(H3Si)3C2]+ cation 8, obtained at the MP2/6-311G
ACTHNUTRGNEUNG(2d,p) (*) and at the
B3LYP/6-311GACTHNUGRTENUNG(2d,p) (*) level of theory.
at the B3LYP and at the MP2 potential energy surface. The
coupled cluster calculations predict that the silyl-bridged
Chem. Eur. J. 2009, 15, 8414 – 8423
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8419

T. Mꢁller et al.
isomer 8a is more stable than the classical Y-shaped vinyl
cation 8b by 13.3 kJmolꢀ1 (CCSD(T)/cc-pvtz//MP2/6-311G-
A
1b·TPFPB. In contrast calculations at the MP2/6-311GACHTUNGTRENNUNG(2d,p)
level of theory predict an essentially symmetrically bridged
(2d,p), 12.1 kJmolꢀ1 at CCSD(T)/cc-pvtz//B3LYP/6-311G-
structure for cation 1b with the silicon atom Sib1 strongly
ꢁ
N
bent towards the C C triple bond (see Figure 5b). These
significant differences between the structures obtained by
both methods are even more remarkable in view of the
close agreement between both methods which is usually ob-
tained, for example for the geometry of the closely related
tert-butyl, or cyclopropyl-substituted vinyl cations 1a and
2.[8] Similar as found for the trisilyl vinyl cation 8 the mode
ACHTUNGTRENNUNG
tions predict for the trisilyl vinyl cation 8 a silyl bridged
ground state 8a with relatively small barriers for silyl migra-
tion around the central carbyne core via the classical vinyl
cation isomer 8b. This situation is reminiscent to that found
+
for the parent C2H3 cation with a hydrogen bridged ground
for bending the bond angle aACTHGNUTERNNUG
(CaCbSib1) in vinyl cation 1b is
state and very small barriers for hydrogen interchange.[9–16]
The presence of alkylidene bridges (CH2)n+2 (n=0–2) be-
tween two silicon atoms in the experimentally investigated
cations 1 is expected to stabilizes the Y-shaped vinyl cation
structure compared to its silyl-bridged isomer. Consequently,
rather weak. That is, the energy difference between both
theoretically predicted structures for vinyl cation 1b is
ACTHNUTRGNEUNG
merely 16 kJmolꢀ1 (at MP2(fu)/6-311G(2d,p); 11 kJmolꢀ1 at
B3LYP/6-311GACTHNUTRGNEUNG
(2d,p)).[33] In addition, the activation energy
for the degenerate Sib1/Sib2 exchange process via the Cs sym-
density functional calculations at the B3LYP/6-311G
N
metric transition state 9b (see Figure 5c) is rather small
level of theory predict a classical Y-shaped ground-state
structure for the tris(trialkylsilyl)-substituted vinyl cation 1b
(see Figure 5a and Table 3), in qualitative agreement with
the results of the low temperature NMR studies for [13C2]-
ACTHNGUTERNNUG
(16.0 kJmolꢀ1 at MP2/6-311G(2d,p), 1.4 kJmolꢀ1 at B3LYP/
6-311GACTHNURTGNEU(GN 2d,p)). This small barrier and the overall flat poten-
tial energy surface suggest a time-averaged Y-shaped struc-
ture for cation 1b in solution, regardless of the actual
ground-state structure. This is in qualitative agreement with
the NMR spectroscopic results that show only one signal for
the two b-silicon atoms and with the observation of a duplet
signal for the b-silicon atoms in the 29Si NMR of [13C2]-1b at
low temperatures. Further consequences that arise from the
flatness of the potential energy surface for trisilyl-substitut-
ed vinyl cations 1b-e, n, p are i) the determination of the
molecular structures by theoretical methods is very demand-
ing. As shown in Figure 5s,b standard quantum mechanical
methods such as B3LYP and MP2 using a relative large
basis set do not result in a conclusive result and higher cor-
relation methods combined with larger basis sets seems to
be appropriate;[34] ii) Steric interactions between substituents
at Sia and Sib1 will severely influence the actual structure.
Therefore, the use of less substituted model cations for the
evaluation of the influence of higher theoretical methods on
the structure prediction for cations 1 is of only little infor-
mative value.
Table 3. Calculated structural parameter of vinyl cations 1 at the B3LYP
and MP2 level of theory (bond lengths r in pm, bond angles a in [8]) cal-
culated activation barriers DEA/DG298 for the Ca/Cb interchange in these
vinyl cations (in kJmolꢀ1).
Compound
B3LYP/A[a]
MP2A[a]
1a
r
r
r
r
G
123.8
200.1
197.3
–
117.4
167.2
159.3
123.8
202.3
196.2
190.7
111.7
55.5
b
(Sib1 C )
a
DEA
DG298
1b
1h
1n
1p
r
r
r
r
124.9
218.2
191.0
191.1
78.6
aACHTUNGTRENNUNG
DEA
46.6
DG298
57.0
r
r
r
r
123.8
202.3
196.2
–
110.8
34.3
A situation quite similar to that for cation 1b is found for
the tris(trialkylsilyl)-substituted vinyl cations 1n and 1p. In
both cases MP2 and B3LYP computations result in qualita-
tive different structures with a significant more pronounced
interaction of one b-silicon atom and the positively charged
carbon atom Ca predicted by the MP2 method. This is
a
DEA
DG298
35.3
r
r
r
123.7
200.4
198.3
123.6
190.4
75.9
124.8
221.7
191.3
76.6
191.3
72.7
shown most prominently with the bond angle aACTHNUTRGNEUNG
(CaCbSib1).
The theoretical prediction of this parameter differs by 33.18
(for 1b) to 47.08 (for 1n) (see Table 3 and the Supporting
Information for more details).
By contrast, both standard methods, B3LYP and MP2 pre-
dict for the transition state 7b of the experimentally ob-
served Ca/Cb interchange in vinyl cation 1b very similar
structures (see Figure 5d). The most remarkable feature of
this transition state is that the a-trimethylsilyl group bridges
aACHTUNGTRENNUNG
r
DEA
DG298
77.8
r
r
r
124.0
202.8
196.8
109.5
190.8
26.7
124.9
226.7
190.8
70.6
191.7
17.9
aACHTUNGTRENNUNG
r
ꢁ
nearly symmetrically the C C triple bond. Therefore this
DEA
DG298
23.6
transition state structure for the Ca/Cb exchange in 1b close-
[a] Basis set A: 6-311G
G
ly resembles the ground-state structure of protonated acety-
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Trisilyl-Substituted Vinyl Cations
FULL PAPER
lene 3. The computed barrier EA for the Ca/Cb exchange in
1b is EA =55.5 kJmolꢀ1 (at B3LYP/6-311G
ACHTUNGTRENNUNG
46.6 kJmolꢀ1 at MP2/6-311G
ACHTUNGTRENNUNG
those experimentally determined for tris(trialkylsilyl)-substi-
tuted vinyl cations 1b–d (EA =53.1–56.0 kJmolꢀ1, see
Table 1). Expectedly, for the trimethylgermyl-substituted
vinyl cation 1h a significantly smaller activation energy of
EA =35.3 kJmolꢀ1 is predicted by the calculations, in agree-
ment with the experimentally determined smaller barrier for
Ca/Cb exchange process in 1h (EA =29.7 kJmolꢀ1). In con-
trast, the activation energy EA calculated for Ca/Cb inter-
change in the a-tert-butyl-substituted cation 1a is high (EA =
167.2 kJmolꢀ1) in agreement with the static structure of this
cation found in the experiment. The calculations of the acti-
vation energies for Ca/Cb exchange in the series of the a-tri-
methylsilyl-substituted vinyl cations 1b, 1n, 1p nicely con-
firms the trend found experimentally at room temperature
(see Table 3). The increasing size of the disilaheterocycle
comparing vinyl cations 1n, 1b and 1p results in decreasing
calculated activation energies EA. This in accord with the
static Y-shaped structure of 1n deduced from the NMR ex-
periments, and a Ca/Cb exchange process in cation 1b, which
is slow on the NMR time scale and with the same process
being fast on the NMR time scale in 1p.
Conclusions
A series of ten vinyl cations including trisilyl-substituted and
b,b-disilyl-a-germyl-substituted cations have been prepared
from the appropriate precursor disilane 5 by the intramolec-
ꢁ
ular addition of a transient silylium ion to the C C triple
bond. TPFPB salts of these cations are stable at room tem-
perature in the solid state and in solution of aromatic hydro-
carbons. Their identity was verified by NMR spectroscopy,
which reveals for all investigated vinyl cations a classical Y-
shaped ground state structure. In addition, the NMR spec-
troscopic results indicate for trisilyl and b,b-disilyl-a-germyl-
substituted vinyl cations 1b–e, h, i, p a highly dynamic struc-
ture. Both carbon atoms of the central CC unit in vinyl cat-
ions undergo at room temperature a degenerate exchange
process which is fast on the NMR time scale. This exchange
process can be best described as a rotation of the central di-
2ꢀ
ꢁ
carbyne unit C C within the triangle defined by the three
positively charged silyl groups or, in the case of germyl-sub-
stituted vinyl cations 1h, i, defined by silyl cations and one
germyl cation (see Scheme 3).
Figure 5. a) Computed molecular structure of vinyl cation 1b (at B3LYP/
6-311G
MP2/6-311G
the equilibration of Sib1/Sib2. d) Computed structure of the transition
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
a
b
ꢁ
state 7b for the C
C
interchange in 1b (at B3LYP/6-311G
(2d,p).
ACHTUNGTRENN(UNG 2d,p) and
Scheme 3. Degenerate exchange process of the vinyl carbon atoms in
vinyl cations 1 (n=1,2; E=Si, Ge; R=alkyl, trialkylsilyl).
underlined at MP2/6-311GAHCTNUGTRENNNUG
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T. Mꢁller et al.
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Scheme 4. Degenerate exchange process of the vinylic carbon atoms in the trinuclear ruthenium dicarbyne
complex 10.[35]
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The barrier for this process strongly depends on the ring
size and on the substituent in a-position in cations 1. Small
ring sizes and alkyl substituents increase the activation
energy and larger rings and germyl substituents facilitate the
exchange process.
The fluxional behavior of vinyl cations 1b–e, h, i, p clearly
parallels the results found recently for the cationic three-nu-
clear ruthenium complex 10[35] and for protonated acetylene
3 (see Scheme 4) with the notable difference that the
ground state configuration in both cations, 10 and 3[10,36] is
that of the bridged species and the classical Y-shaped vinyl
cation configuration is only the transition state for the rota-
2ꢀ
ꢁ
tion process of the C C unit in the triangle of the three
cationic fragments. Therefore, protonated acetylene 3, trisil-
yl-substituted vinyl cations 1b–e, p and the trinuclear ruthe-
nium dicarbyne complex 10 provide an instructive example
of an isolobal relationship in which the proton (no electron)
can be replaced by the six electron main group fragment
R3Si+ and by the sixteen electron transition metal fragment
[Ru(CO)
similar structures and dynamics.
2ACHTUNGTRENNUNG
(C5H5)]+ and molecular systems are obtained with
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Acknowledgements
Prof. Dr. Uwe Rosenthal (Leibniz-Institut fꢁr Katalyse, Universitꢂt Ro-
stock) is thanked for many helpful discussions. This work was supported
by the Carl von Ossietzky Universitꢂt Oldenburg. The Wacker AG is
thanked for generous gift of silanes. The computer centre of the Johann
Wolfgang Goethe Universitꢂt Frankfurt/Main and Center for Scientific
Computing (CSC) at the Universitꢂt Oldenburg is thanked for computer
time.
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[26] To calculate the exchange rate for 1h the frequency difference Dn
between Ca and Cb obtained for 1b was used, and for 1i the fre-
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Table 2 are calculated from the deviation of the best-line fit and ac-
count not for systematic errors, see the Supporting Information for
further details.
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Trisilyl-Substituted Vinyl Cations
FULL PAPER
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Received: March 26, 2009
Published online: June 30, 2009
Chem. Eur. J. 2009, 15, 8414 – 8423
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8423