Norbornyl cations of group 14 elements

Article abstract of DOI:10.1021/ja021234g

Norbornyl cations of the group 14 elements Si → Pb have been synthesized from substituted 3-cyclopentenemethyl precursors by intramolecular addition of transient cations to the C=C double bond of the 3-cyclopentenemethyl substituent (π-route to norbornyl cations). The norbornyl cations 4a (E = Si, R = Me), 4e (E = Si, R = Et), 4f (E = Si, R = Bu), 4g (E = Ge, R = Bu), 4h (E = Sn, R = Bu), and 4i (E = Pb, R = Et) have been identified by their characteristic NMR chemical shifts (4a,e,f, δ(²⁹Si) = 80-87, δ(¹³C)(CH=) = 149.6-150.6; 4g, δ(¹³C)(CH=) = 144.8; 4h, δ(¹¹⁹Sn) = 334, δ(¹³C)(CH=) = 141.5; 4i, δ(²⁰⁷Pb) = 1049, δ(¹³C)(CH=) = 138). The significant deshielding of the vinylic carbon atoms (Δδ(¹³C)) relative to those of the precursor (Δδ(¹³C) = 19.3-20.3 (4a,e,f), Δδ(¹³C) = 14.6 (4g), Δδ(¹³C) = 11.1 (4h), Δδ(¹³C) ≈ 8 (4i)) and the small J coupling constants between the element and the remote vinyl carbons in the case of 4h and 4i (J(CSn) = 26 Hz, J(CPb) = 16 Hz) give experimental evidence for the intramolecular interaction and the charge transfer between the positively charged element and the remote C=C double bond. The experimental results are supported by quantum mechanical calculations of structures, energies, and magnetic properties for the norbornyl cations 4a,b (E = Ge, R = Me), 4c (E = Sn, R = Me), 4d (E = Pb, R = Me), and 4e,f at the GIAO/B3LYP/6-311G(3d,p)//MP2/6-311G(d,p) (Si, Ge, C, H), SDD (Sn, Pb) level of theory. The calculated ²⁹Si NMR chemical shifts for the silanorbornyl cations 4a,e,f (δ(²⁹Si) = 77-93) agree well with experiment, and the calculated structures of the cations 4a-f reveal their bridged norbornyl cation nature and suggest also for the experimentally observed species 4a,e-i a formally 3 + 1 coordination for the element atom with the extra coordination provided by the C=C double bond. This places five carbon atoms in the close vicinity of the positively charged element atom. The group 14 element norbornyl cations 4a,e-i exhibit only negligible interactions with the aromatic solvent, and they are, depending on the nature of the element group, stable at room temperature in aromatic solvents for periods ranging from a few hours to days. In acetonitrile solution, the intramolecular interaction in the norbornyl cations 4a,e-h breaks down and nitrilium ions with the element in a tetrahedral environment are formed. In contrast, reaction of acetonitrile with the plumbyl cation 4i forms an acetonitrile complex, 10i, in which the norbornyl cation structure is preserved. The X-ray structure of 10i reveals a trigonal bipyramidal environment for the lead atom with the C=C double bond of the cyclopentenemethyl ligand and the nitrogen atom of the acetonitrile molecule in apical positions. Density functional calculations at the B3LYP/6-311G-(2d,p)//(B3LYP/6-31G(d) (C, H), SDD (Si, Ge, Sn, Pb)) + ΔZPVE level indicate that the thermodynamic stability of the group 14 norbornyl cations increases from Si to Pb. This results in a relative stabilization for the plumbanorbornyl cation 4d compared to tert-butyl cation of 52.7 kcal mol^-1. In contrast, the intramolecular stabilization energy E_A of the norbornyl cations 4a-d decreases, suggesting reduced interaction between the C=C double bond and the electron-deficient element center in the plumbacation compared to the silacations. This points to a reduced electrophilicity of the plumbacation compared to its predecessors.

Full text of DOI:10.1021/ja021234g

Published on Web 02/01/2003
Norbornyl Cations of Group 14 Elements
Thomas M¨uller,* Christian Bauch, Markus Ostermeier, Michael Bolte, and
Norbert Auner*
Contribution from the Institut fu¨r Anorganische Chemie der Goethe UniVersita¨t Frankfurt,
Marie Curie-Strasse 11, D-60439 Frankfurt/Main, Federal Republic of Germany
Received October 1, 2002 ; E-mail: dr.thomas.mueller@chemie.uni-frankfurt.de
Abstract: Norbornyl cations of the group 14 elements Si f Pb have been synthesized from substituted
3-cyclopentenemethyl precursors by intramolecular addition of transient cations to the CdC double bond
of the 3-cyclopentenemethyl substituent (π-route to norbornyl cations). The norbornyl cations 4a (E ) Si,
R ) Me), 4e (E ) Si, R ) Et), 4f (E ) Si, R ) Bu), 4g (E ) Ge, R ) Bu), 4h (E ) Sn, R ) Bu), and 4i
(E ) Pb, R ) Et) have been identified by their characteristic NMR chemical shifts (4a,e,f, δ(29Si) ) 80-87,
δ(13C)(CHd) ) 149.6-150.6; 4g, δ(13C)(CHd) ) 144.8; 4h, δ(119Sn) ) 334, δ(13C)(CHd) ) 141.5; 4i,
δ(207Pb) ) 1049, δ(13C)(CHd) ) 138). The significant deshielding of the vinylic carbon atoms (∆δ(13C))
relative to those of the precursor (∆δ(13C) ) 19.3-20.3 (4a,e,f), ∆δ(13C) ) 14.6 (4g), ∆δ(13C) ) 11.1 (4h),
∆δ(13C) ≈ 8 (4i)) and the small J coupling constants between the element and the remote vinyl carbons in
the case of 4h and 4i (J(CSn) ) 26 Hz, J(CPb) ) 16 Hz) give experimental evidence for the intramolecular
interaction and the charge transfer between the positively charged element and the remote CdC double
bond. The experimental results are supported by quantum mechanical calculations of structures, energies,
and magnetic properties for the norbornyl cations 4a,b (E ) Ge, R ) Me), 4c (E ) Sn, R ) Me), 4d (E )
Pb, R ) Me), and 4e,f at the GIAO/B3LYP/6-311G(3d,p)//MP2/6-311G(d,p) (Si, Ge, C, H), SDD (Sn, Pb)
level of theory. The calculated 29Si NMR chemical shifts for the silanorbornyl cations 4a,e,f
(δ(29Si) ) 77-93) agree well with experiment, and the calculated structures of the cations 4a-f reveal
their bridged norbornyl cation nature and suggest also for the experimentally observed species 4a,e-i a
formally 3 + 1 coordination for the element atom with the extra coordination provided by the CdC double
bond. This places five carbon atoms in the close vicinity of the positively charged element atom. The group
14 element norbornyl cations 4a,e-i exhibit only negligible interactions with the aromatic solvent, and they
are, depending on the nature of the element group, stable at room temperature in aromatic solvents for
periods ranging from a few hours to days. In acetonitrile solution, the intramolecular interaction in the
norbornyl cations 4a,e-h breaks down and nitrilium ions with the element in a tetrahedral environment are
formed. In contrast, reaction of acetonitrile with the plumbyl cation 4i forms an acetonitrile complex, 10i, in
which the norbornyl cation structure is preserved. The X-ray structure of 10i reveals a trigonal bipyramidal
environment for the lead atom with the CdC double bond of the cyclopentenemethyl ligand and the nitrogen
atom of the acetonitrile molecule in apical positions. Density functional calculations at the B3LYP/6-311G-
(2d,p)//(B3LYP/6-31G(d) (C, H), SDD (Si, Ge, Sn, Pb)) + ∆ZPVE level indicate that the thermodynamic
stability of the group 14 norbornyl cations increases from Si to Pb. This results in a relative stabilization for
the plumbanorbornyl cation 4d compared to tert-butyl cation of 52.7 kcal mol-1. In contrast, the intramolecular
stabilization energy EA of the norbornyl cations 4a-d decreases, suggesting reduced interaction between
the CdC double bond and the electron-deficient element center in the plumbacation compared to the
silacations. This points to a reduced electrophilicity of the plumbacation compared to its predecessors.
mechanical calculations.3a-d,4 The evidence for stannylium,
R3Sn+, and plumbylium, R3Pb+,7,8 ions with no coordination
to counteranions or solvent molecules is rather scarce. The
closest approaches to “free” stannylium ions are most probably
Lambert’s trimesitylstannylium in benzene using the tetra-
Introduction
Examples of higher homologues of the well-known trivalent
carbenium ions, R3C+,1 are extremely rare, and only recently
some specific examples of silylium, R3Si+,2-5 and germylium,
R3Ge+,3c,6 ions in the condensed phase have been synthesized,
isolated, and structurally characterized, either by X-ray structure
analysis3e,5,6 or by NMR methods supported by quantum
(2) Reviews: (a) Maerker, C.; Schleyer, P. v. R. In The chemistry of organic
silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester,
U.K., 1998; Vol. 2, p 513. (b) Lickiss, P. D. In The chemistry of organic
silicon compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester,
U.K., 1998; Vol. 2, p 557. (c) Lambert, J. B.; Kania, L.; Zhang, S. Chem.
ReV. 1995, 95, 1. (d) Reed, C. A. Acc. Chem. Res. 1998, 31, 325. (e)
Fornarini, S. In The chemistry of organic silicon compounds; Rappoport,
Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 2001; Vol. 3, p 1027.
(1) Review: (a) Lenoir, D.; Siehl, H.-U.; Subramanian, L. R. In Houben-Weyl,
Methoden der Organischen Chemie; Hanack, M., Ed.; Thieme Verlag:
Stuttgart, Germany, 1990; Vol. 19c. Persistent carbocations: (b) Olah, G.
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9
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J. AM. CHEM. SOC. 2003, 125, 2158-2168
10.1021/ja021234g CCC: $25.00 © 2003 American Chemical Society

Norbornyl Cations of Group 14 Elements
A R T I C L E S
kispentafluorophenylborate anion3b and Michl’s tributylstann-
-ylium carboranate.9 It is widely accepted that the problems
associated with the synthesis of trivalent cations of the elements
Si f Pb, R3E+, are due to the high reactivity of these species.2
Although thermodynamically more stable than equally substi-
tuted carbenium ions, the lower electronegativity of the element
compared to carbon results in a high accumulation of positive
charge and therefore in an enormous electrophilicity of the R3E+
cations. In addition, the group 14 elements Si f Pb are
significantly larger in size. The longer bonds to substituents
facilitate access by nucleophiles, and thus higher coordination
numbers prevail in the chemistry of silicon and the remainder
of the group 14 elements. For example, transient silylium ions,
which have been generated by hydride-transfer reactions,10
readily react with solvents commonly used for ionic compounds
such as amines, nitriles, ethers, and aromatic hydrocarbons to
form silylated ammonium,11 nitrilium,12 oxonium,13 and are-
nium14 ions, respectively. Therefore, the success of the synthesis
of these cations requires (i) steric protection of the highly
reactive, positively charged silicon3 and/or (ii) an efficient
lowering of the electron deficiency of the silicon by either
resonance effects or integration of the silicon in aromatic or
homoaromatic systems.4,5 Alternatively, the high electrophilicity
of the positively charged element can be modified by intramo-
lecular donation from remote donor substituents. This interaction
leads to solvent-free cations with coordination numbers for the
positively charged element >3 and to a considerable electron
transfer from the donor group to the element. Frequently used
donor substituents utilize heteroatoms with lone pairs (e.g.,
amino, hydrazino, methoxy, carboxy, phosphino, etc.)15 for the
stabilization of the cationic center. We have previously shown
that the intramolecular reaction between transient silylium ions
and CtC triple bonds or aryl groups leads to the clean formation
of silyl-substituted vinyl cations16 or silylated arenium ions.17
Wrackmeyer and co-workers synthesized and characterized the
intriguing zwitterions 1 by X-ray analysis, in which trivalent
stannylium and plumbylium ions are stabilized by intramolecular
side-on coordination to CtC triple bonds. Alternatively, these
ions can be regarded as stannyl- or plumbyl-substituted vinyl
cations. The usability of CdC double bonds for the intramo-
lecular stabilization of a transient silyl cation was demonstrated
using a cyclopentenemethyl-substituted precursor, yielding
silanorbornyl cation 219 along the so-called π-route to norbornyl
cations.20 According to quantum mechanical calculations,2a,19
silanorbornyl cation 2 adopts a symmetrically bridged structure,
with the silicon atom in a pentacoordinate bonding situation,
similar to that of the parent norbornyl cation 3.1b,c,21
(3) (a) Lambert, J. B.; Zhao, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 400.
(b) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B. J. Am.
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The success of this previous work19 prompted us to use the
3-cyclopentenemethyl substituent also for the synthesis of
intramolecularly stabilized cations of the heavier group 14
elements, 4. This series of group 14 element norbornyl cations
provides a unique opportunity to gauge the different electro-
philicities of the positively charged group 14 centers in a similar
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A R T I C L E S
Mu¨ller et al.
Scheme 1. Synthesis of Group 14 Norbornyl Cationsa
a Reagents and conditions: (a) Mg, THF; (b) HSiCl3, THF, -15 °C to room temperature; (c) RMgCl, THF; 0 °C to room temperature; (d) n-Bu2ECl2,
THF, 0 °C to room temperature; (e) (E ) Ge) LiAlH4, Et2O, room temperature; (E ) Sn) LiEt3BH, Et2O, room temperature; (f) Et3PbBr, Et2O, room
temperature; (g) TPFPB, C6D6, room temperature.
Scheme 2. Suggested Mechanism for the Formation of 4i
molecular environment. We describe herein the synthesis of
norbornyl cations of the group 14 elements under experimental
conditions that allow their NMR characterization in solution at
ambient temperature. The results of quantum mechanical
investigations on the structures, energies, and magnetic proper-
ties are reported; these not only corroborate the experimental
findings but also give a detailed description of the bonding and
charge distribution in the norbornyl cations.
and the byproduct triphenylmethane; this last compound is
1
unequivocally identified from its H and 13C NMR spectra.22
The cations 6a,e-i are transient species and undergo an intra-
molecular reaction, yielding the norbornyl cations 4a,e-i
(Scheme 1).
Results and Discussion
Experiment. The norbornyl cations 4a,e-h are synthesized
by hydride-transfer reactions between trityl cation and the
respective element hydrido compounds 5a,e-h.10 Due to the
instability of lead(IV) hydrido compounds, the plumbyl cation
4i is prepared by reaction of the triethyl-substituted plumbane
5i with trityl cation (Scheme 1). The compounds 5a,e-i are
obtained from cyclopentenemethyl chloride, 7, and the corre-
sponding element halides by the reactions summarized in
Scheme 1. The structures of compounds 5a,e-i are verified by
The preparation of the plumbyl cation 4i is rather unusual,23
and the reaction is suggested to proceed via â-hydride abstrac-
tion from the ethyl ligand of 5i (Scheme 2). Deplumbylation of
the resulting â-plumbyl-substituted carbocation 8 results in the
formation of the norbornyl cation 4i and ethene. The byproducts
1
ethene and triphenylmethane were identified by H and 13C
NMR spectroscopy.22,24 Another byproduct, the arenium ion,
[Et3Pb/C6D6]+, arises from elimination of ethene from tetra-
ethyllead, Et4Pb. The latter compound is formed from 5i by a
fast ligand disproportionation reaction.25 In our experience, even
freshly distilled samples of 5i are contaminated by traces of
tetraethyllead. Therefore, triethyllead benzenium, [Et3Pb/C6D6]+,
is an unavoidable byproduct of 4i. The identity of [Et3Pb/C6D6]+
was established by its independent synthesis from tetraethyllead
and TPFPB in benzene (see the Experimental Section).
using standard one-dimensional H, 13C, 29Si, 119Sn, and 207Pb
1
and two-dimensional 1H/13C and 1H/1H NMR techniques.
Treatment of 5a,e-i with 1 equiv of trityltetrakis(pentafluo-
rophenyl)borate (TPFPB) in benzene-d6 or toluene-d8 gives in
the first step the triorgano-substituted element cations 6a,e-i
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D. Organometallics 2001, 20, 1053. (f) Kingston, V.; Gostevskii, B.;
Kalikhman, I. Kost, D. Chem. Commun. 2001, 1272. (g) Kost, D.; Kingston,
V.; Gostevskii, B.; Ellern, A.; Stalke, D.; Walfort, B.; Kalikhman, I.
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(16) Mu¨ller, T.; Meyer, R.; Lennartz, D.; Siehl, H.-U. Angew. Chem., Int. Ed.
2000, 39, 3074.
Solutions of norbornyl cations 4a,e-i in aromatic hydrocar-
bons are stable at room temperature; however, there is a clear
dependence of the stability on the element and on the substit-
uents at the element. The dimethylsilanorbornyl cation 4a
decomposes completely within hours, while the butyl-substituted
species 4f can be handled over the same period of time without
observable disintegration. The stannanorbornyl cation 4h can
be shortly heated to 70 °C without noticeable decomposition,
and the plumbanorbornyl cation 4i is at room temperature
indefinitely stable in the absence of strong nucleophiles and
(17) Meyer, R.; Werner, K.; Mu¨ller, T. Chem.sEur. J. 2002, 8, 3203.
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(22) The following are data for triphenylmethane. 13C NMR (100 MHz, 300 K,
C6D6): δ 57.1, 126.5, 128.5, 129.6, 144.2. 1H NMR (400 MHz, 300 K,
C6D6): δ 7.09 (br, 15H), 5.41 (s, 1H).
(19) Steinberger, H.-U.; Mu¨ller, T.; Auner, N.; Maerker, C.; Schleyer, P. v. R.
Angew. Chem., Int. Ed. Engl. 1997, 36, 626.
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menium from triethylaluminum: Kim, K.-C.; Reed, C. A.; Long, G. S.;
Sen, A. J. Am. Chem. Soc. 2002, 124, 7662.
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(24) The following are data for ethene. 13C NMR (62.9 MHz, 300 K, C6D6): δ
122.3. 1H NMR (250.1 MHz, 300 K, C6D6): δ 5.60.
(25) (a) Caelingaert, G.; Soroos, H.; Shapiro, H. J. Am. Chem. Soc. 1940, 62,
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2160 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003

Norbornyl Cations of Group 14 Elements
A R T I C L E S
Table 1. NMR Data of Group 14 Element Norbornyl Cationsa
compd
E
R
δ(E)
∆δ(E)b
δ(13C)(C6/7)
∆δ(13C)(C6/7)c
δ(13C)(C4/5)
δ(13C)(C3)
δ(13C)(C2)
δ(13C)(R)
4a
Si
Me
87.2
102.1
150.6
20.3
41.4
37.4
14.5
-2.2
1J(CH) ) 170.4
87.5d
82.7
4e
Si
Et
86.4
89.4
149.8
19.5
41.6
36.5
10.4
5.3, 6.1
1J(CH) ) 173.0
83.0d
80.2
4f
Si
n-Bu
n-Bu
n-Bu
149.6
19.3
14.6
11.1
41.6
40.4
40.4
36.6
36.5
35.3
12.7
18.2
25.0
13.0, 25.4, 24.9, 12.6
18.5, 24.8, 25.8, 12.4
19.9, 27.6, 26.2, 12.4
1J(CH) ) 174.1
144.8
4g
4h
Ge
Sn
1J(CH) ) 173.2
141.5
334.0
343.0
999.0
1J(CH) ) 170.0
1J(CSn) ) 26
138.0
329.7d
1049.0
4i
Pb
Et
7.8
39.3
36.5
53.4
38.2, 12.0
1J(CH) ) 170.0
1J(CPb) ) 16
130.0
1039.0d
31.8
37.5
10a
10e
10f
10g
10h
10i
Si
Si
Si
Ge
Sn
Pb
Me
Et
n-Bu
n-Bu
n-Bu
Et
0.0
-0.6
0.4
0.4
0.3
41.4
41.0
41.0
40.2
42.5
40.9
32.6
32.1
32.0
33.2
35.1
35.6
21.8
17.8
18.7
17.5
19.5
49.1
-2.5
5.2, 3.7
12.5, 25.4, 23.8, 10.9
12.6, 25.7, 25.1, 24.5
13.3, 28.3, 26.9, 19.5
34.3, 12.0
129.5
130.7
130.6
130.7
37.5
54.4
598.0
131.7
1.5
a In benzene-d6 at room temperature versus TMS, coupling constants J in hertz. b Chemical shift difference for the element between the cations 4 and
their precursors 5. c Chemical shift difference for the vinylic carbon atoms C6/7 in the cations 4 and 10, respectively, and in the precursors 5. d In toluene-d8.
of all cations 4a,e-i (∆δ(29Si) ) 86.4-102.1, ∆δ(119Sn) ) 343,
∆δ(207Pb) ) 999), which occurs upon ionization of the
precursors 5a,e-i, indicates an accumulation of positive charge
at the element atom. The observed 29Si NMR chemical shifts
for the silyl cations 4a,e,f (δ(29Si) ) 80.2-87.2), the 119Sn NMR
chemical shift for the stannyl cation 4h (δ(119Sn) ) 334), and
the 207Pb NMR chemical shift for the plumbyl cation 4i
(δ(207Pb) ) 1049) are similar to chemical shifts found for
metalated benzenium ions (δ(29Si) ) 70-100,14b,27 δ(119Sn) )
360,8b δ(207Pb) ) 1432),28 and they are markedly smaller than
the chemical shifts reported for trivalent cations (δ(29Si) ) 225
(Mes3Si+),3a,f δ(119Sn) ) 806 (Mes3Sn+).3b
For all norbornyl cations nearly the same NMR chemical
shifts are observed in benzene-d6 solution and in toluene-d8,
that is, NMR chemical shift changes of less than 1% (see
Table 1). For silylated or stannylated arenium ions, chemical
shift differences between the benzenium and toluenium spe-
cies of 10% and more are reported ([Et3SiC6D6]+, δ ) 92.3;
[Et3SiC6D5CD3]+, δ ) 81.8;27 [Bu3SnC6D6]+, δ ) 360;8b
[Bu3SnC6D5CD3]+, δ ) 434).29 This indicates negligible
solvent-cation interactions for the norbornyl cations 4a,e-i in
aromatic solvents. For the cations 4a,e-i the saturated backbone
carbon atoms, C3-C5, show the same 13C NMR chemical shift
Figure 1. NMR spectra of plumbanorbornyl cation 4i at 300 K in benzene-
pattern, which is typical for the norbornyl cage. Only the
chemical shift of methylene carbon atom C2 is influenced by
the element substitution and varies for each cation (see Table
1). The 13C NMR chemical shifts for the vinylic carbons C6
and C7 are in the range of 138.0-150.2 and are highly diag-
nostic of the bridged structure of the ions. The marked downfield
shifts of C6/7, ∆δ(13C)(C6/7), relative to those of the precursor
compounds (∆δ(13C)(C6/7) ) 7.8-20.5) are consistent with
intramolecular coordination of the double bond to the positively
charged element and indicate charge transfer from the element
d6: (a) 52.3 MHz 207Pb NMR spectrum; (b) 63 MHz 13C NMR spectrum
(†, [Et3Pb/C6D6]+; O, [B(C6F5)4]-; b, Ph3CH). Inset: part of the 13C DEPT-
135 NMR spectrum, showing the resonance of the vinylic carbon atoms.
air. The norbornyl cation salts with [B(C6F5)4]- as counterion
have been obtained as brown-red glassy powders after removal
of the solvent. Treatment of the residue with pentane yields
brown-red (4a,e-g) or white microcrystalline (4h,i) salts in
60-95% yield.26
The norbornyl cations 4a,e-i (Table 1) were identified by
their characteristic NMR spectra (see Figure 1, for an example).
The strong downfield shift of the element NMR signal (∆δ(E))
(27) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13, 2430.
(28) [Et3Pb/C6D6]+[B(C6F5)4]- in C6D6 at room temperature; see the Experi-
mental Section.
(29) Blackwell, J. M.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc. 2002,
124, 1296.
(26) In the case of the norbornyl cation salts 4h[B(C6F5)4] and 4i[B(C6F5)4]
suitable crystals for X-ray analysis have been obtained; the crystal structure
could however not be resolved because of severe disordering of the cations.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2161

A R T I C L E S
Mu¨ller et al.
Scheme 3. Degenerate Equilibrium between Two
â-Element-Substituted Carbocations 9
Scheme 4. Formation of the Nitrilium Species 10a,e-i
to the carbon atoms C6/7.19 While the downfield shift ∆δ(13C)-
(C6/7) for the silanorbornyl cations 4a,e,f is nearly constant,
about 20, it decreases for the germanorbornyl (4g; ∆δ(13C)-
(C6/7) ) 14.6) and stannanorbornyl (4h; ∆δ(13C)(C6/7) )
11.1) cations and reaches for the plumbanorbornyl cation 4i
its minimum value of 7.8. This implies reduced electron trans-
fer from the CdC double bond to the element in the series Si
f Pb.
expected to be larger than 100 Hz (E ) Pb) or 150 Hz (E )
Sn).36 This is more than a magnitude larger than the detected
1J(EC) coupling constants in 4h and 4i (4h, 1J(SnC) ) 26 Hz;
1
4i, J(PbC) ) 16 Hz).
The addition of solvents that are stronger coordinating agents
to positively charged centers than aromatic hydrocarbons leads
to the immediate collapse of the intramolecular E/CdC interac-
tion. Thus, the addition of acetonitrile to a solution of the
norbornyl cations 4a,e-h in benzene or toluene instantaneously
gives the nitrilium species 10a,e-h, which are unequivocally
identified by their characteristic element and 13C NMR spectra
(see Scheme 4 and Table 1).12 In the nitrilium ions 10a,e-h no
noticeable interaction between the element and the CdC double
bond is present; that is, the 13C NMR chemical shifts of the
vinylic carbons are nearly identical to those of the precursor
silanes 5 (∆δ(13C) ≈ 0; see Table 1).
The small detected J coupling constants between the element
and the vinylic carbon atoms for both the stannyl cation 4h
1
and the plumbyl cation 4i (1J(SnC) ) 26 Hz, J(PbC) ) 16
Hz) demonstrate the direct bonding of the element to the CdC
double bond, since the alternative coupling path along four
bonds cannot lead to substantial scalar coupling.30,31 The
observed 1J(EC) values are by magnitudes smaller than regular
couplings along a C-E single bond (1J(SnC) ) 300-500 Hz,
1J(PbC) ) 200-300 Hz),30,31 a fact which can be rationalized
by the particular bonding situation in these cations. Similar
strongly reduced 1J(SnC) and 1J(PbC) constants have been found
for zwitterionic compounds of type 1 (1J(CPb) ) 11-30 Hz,18e
1J(CSn) ) 40-74 Hz)18a-d in which the R3E+ (E ) Sn, Pb)
units are coordinated to a CtC triple bond. Additional
independent indication for a 3 + 1 coordination of the tin atom
in 4h is provided by the relatively small 1J(CSn) of 256 Hz for
coupling to the methylene groups of the butyl substituents. This
coupling constant is clearly reduced compared to that of the
tetrahedral coordinated tin center in stannane 5h (1J(SnC) )
342 Hz) and to the pentacoordinated tin atom in stannyl cations
(i.e., in Bu3Sn(DMSO)2 1J(SnC) ≈ 465 Hz).8d,32 However, it is
similar to the corresponding coupling constants in the stanny-
lated benzenium ion [Bu3Sn/C6H6]+ (1J(SnC) ) 278 Hz)8d and
in the zwitterionic species 1a (1J(SnC) ) 233-295 Hz).18a-d
For both types of compounds the 3 + 1 coordination of the tin
atom is established.8d,18b,c
The markedly high field shifted 207Pb NMR resonance for
cation 10i (δ(207Pb) ) 598 relative to δ(207Pb) ) 1049 for 4i)
indicates also for this compound strong coordination of the
acetonitrile to the lead atom. The 13C NMR signals of the vinylic
carbons for 10i are however still at higher field than for the
starting plumbane 5i (∆δ(13C) ) 1.5; see Table 1). This small
but significant effect on δ(13C)(dCH) suggests a residual
interaction between the CdC double bond and the lead atom
and therefore pentacoordination of the lead center in 10i (see
Scheme 4). This is supported by the solid-state structure of the
salt 10i[B(C6F5)4].37 The structure of the salt reveals well-
separated cations and anions. No fluorine atom of the anion
approaches the lead atom at a smaller distance than 380 pm. In
agreement with the NMR data, obtained for 10i in solution, the
molecular structure of 10i reveals a distorted pentagonal
bipyramidal environment for the lead center (see Figure 2). The
planarity of the trigonal base is indicated by summation of the
three C-Pb-C angles (118.0°, 119.9°, and 122.1°) to 360.0°.
The coordination sphere of lead is completed by the CdC
double bond of the cyclopentenemethyl substituent and one
acetonitrile molecule which take up the two apical positions.
The steric requirements of the intramolecular interaction between
the CdC double bond and the lead atom enforces the deviations
from the ideal trigonal bipyramidal coordination. That is, (i)
The NMR spectra of the cations 4a,e-i are consistent with
a symmetrical norbornyl cation structure, and also low-temper-
ature NMR studies at -40 °C in toluene-d8 showed no kinetic
line broadening of the 13C NMR signals.33 This indicates the
absence of a conceivable degenerate equilibrium between two
â-element-substituted carbocations 9 (Scheme 3). An analysis
of the NMR data (Table 1) provides further evidence for the
static, bridged structure of the norbornyl cations and negates
1
an equilibrating system, 9: (i) The J(CH) coupling constants
of the vinylic methine carbons in the norbornyl cations 4 are
170-174 Hz. This is close to the value obtained for the parent
norbornyl cation 3 (1J(CH) ) 187.7)20b and decisively larger
than the time-averaged coupling constants Jav ≈ 150 Hz
expected for fast equilibrating cations 9.34 (ii) In contrast, the
(34) Calculated from 1J(C+-H) ) 170 Hz and 1J(C+C-H) ) 130 Hz in
secondary carbocations.35
(35) (a) Olah, G. A.; Donovan, D. J. J. Am. Chem. Soc. 1977, 99, 5026. (b)
Kalinowski, H.-O.; Berger, S.; Braun, S. 13C NMR Spektroskopie; Thi-
eme: Stuttgart, Germany, 1984.
(36) Calculated using the coupling data from n-Bu4Sn (1J(CSn) ) 314 Hz,
1
time-averaged J(EC) coupling constants for cations 9h,i are
2J(CSn) ) 20 Hz) and n-Bu4Pb (1J(CPb) ) 189 Hz, 2J(CPb) ) 27 Hz).
(37) Crystal data for 10i [B(C6F5)4] at 173 K: empirical formula C36H22BF20
-
(30) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 1985, 16, 73.
(31) Wrackmeyer, B.; Horchler, K. Annu. Rep. NMR Spectrosc. 1989, 22, 249.
(32) Nadvornik, M.; Holecek, J.; Handlir, K.; Lycka, A. J. Organomet. Chem.
1984, 275, 43.
NPb, MW ) 1066.55, orthorhombic, space group Pbca, a ) 17.1043(5)
Å, b ) 19.7079(6) Å, c ) 22.3659(8) Å, R ) 90°, â ) 90°, γ ) 90°, V
) 7539.3(4) Å3, Z ) 8, Dcalcd ) 1.879 Mg/m3. The final R factor was
0.0353 with I0 > 2σ(I0) (Rw ) 0.059 for all data, 10755 reflections). GOF
) 1.021. Cambridge database file CCDC-190293.
(33) At lower temperatures even the toluene solution of the salt becomes solid.
9
2162 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003

Norbornyl Cations of Group 14 Elements
A R T I C L E S
Figure 2. (a) Molecular structure of cation 10i in the crystal. (b)
Coordination sphere of the lead atom in cation 10i. Selected bond lengths
(pm) and bond angles (deg): Pb1-C2 222.3(4); Pb1-C8 221.9(5);
Pb1-C9 221.4(4); Pb1-C6 298.9(4); Pb1-C7 296.9(5); Pb-(center C6,
C7) 290.4; Pb1-N13 250.6(5); C6-C7 132.7(5); C6-Pb-C7 25.7(9);
C2-Pb-C8 119.88(15); C2-Pb-C9 122.10(16); C8-Pb-C9 117.99(16);
(center C6,C7)-Pb-N13 159.0; (center C6,C7)-Pb-C2 72.3;
N13-Pb-C2 86.80(14).
Figure 3. Calculated structure of cation 4a at several levels of theory (bold,
MP2/6-311G(d,p); lightface Roman, B3LYP/6-31G(d); lightface italic,
B3LYP/(6-31G(d), (C, H), SDD (Si))). Bond lengths are in picometers.
The pyramidalization angle ꢀ is in degrees.
Table 2. Calculated Stabilization Energy of Me3E+ vs tert-Butyl
Cation (Eq 1) and of Element Norbornyl Cations 4 vs Me3E+ (Eq
2) and the Intramolecular Stabilization Energy EA (kcal mol-1, at
the B3LYP/6-311G(2d,p) (C, H), SDD (E) + ∆ZPVE Level)
the axis along the center of the CdC double bond and the lead
atom is tilted by 17.7° toward the C2Pb1 axis and (ii) the
midpoint of the CdC double bond and the nitrogen atom enclose
an angle R with the central lead atom, which differs markedly
from 180° (R ) 159.0°; see Figure 2). This spatial arrangement
places six atoms at distances smaller than 300 pm around the
positively charged lead atom.
Si
Ge
Sn
Pb
eq 1
eq 2
EA
-12.0
-23.8
-19.5
-20.6
-21.5
-17.4
-25.6
-21.6
-17.0
-35.2
-17.5
-12.5
Theory. Calculations38 of the structures and energies of
norbornyl cations and related compounds were initially per-
formed using the nonlocal DFT level of theory39 and Becke’s
three-parameter hybrid functional and the LYP correlation
functional (B3LYP),40 along with the standard 6-31G(d) for C
and H. For the elements Si f Pb the Stuttgart-Dresden
pseudorelativistic effective core potential (ecp)41,42 along with
a (31/31/1) valence basis set has been used (B3LYP/6-31G(d)
(C, H), SDD (E)). Subsequent frequency calculations at the
B3LYP/6-31G(d) (C, H), SDD (E) level were carried out to
verify stationary points as minima on the potential energy
surface (PES). For the silanorbornyl cation 4a calculations using
the same method, but the all-electron 6-31G(d) basis set also
for the element, have been done for comparison. While only
minor differences in the relative energies have been found for
calculations utilizing the all-electron basis set and the ecp
calculations,43 the geometries, in particular the distances between
the elements and the CdC double bond, crucially depend on
the applied basis set and the method (see Figure 3 for an
example). Finally, relative energies were calculated at the
B3LYP/6-311G(2d,p) level with B3LYP/6-31G(d) geometries,
and those energies were further improved by addition of
unscaled zero-point vibration energy differences (∆ZPVE)
(B3LYP/6-311G(2d,p) (C, H), SDD (E)//B3LYP/6-31G(d) (C,
H), SDD (E) + ∆ZPVE). Relative energies, reaction ener-
gies, and association energies EA quoted in the text are com-
puted at this level, unless otherwise stated. 13C and 29Si NMR
chemical shifts have been calculated using the GIAO44/B3LYP
method, the 6-311G(3d,p) basis set, and geometries optimized
at MP2/6-311G(d,p).45 The electron distributions in cations 4
were analyzed using NBO theory46 and a B3LYP/6-31G(d)
density.
Thermodynamic Stabilities of Group 14 Element Cations.
From the decreasing Mulliken electronegativity of the group
14 elements (Si f Pb) it is expected that the thermodynamic
+
stability of the trivalent ER3 species advances continuously
from silylium to plumbylium ions.7a,b This trend is paralleled
by a reduced electrophilicity of the R3E+ cations, which is due
to the increasing polarizability of the element atoms.7a These
qualitative conclusions are supported by the results of density
functional calculations (B3LYP/6-311G(2d,p) (C, H), SDD (E)//
B3LYP/6-31G(d) (C, H), SDD (E)). According to the isodesmic
equation47 (eq 1) (see Table 2), the stabilization energy versus
tert-butyl cation increases significantly in the series Me3Si+ f
Me3Pb+.48 The results for the isodesmic reaction, which is
(38) All calculations were performed with Gaussian 94, Revisions C2-E2, and
Gaussian 98, Revisions A3-A9, Gaussian, Inc., Pittsburgh, PA, 1995 and
1999.
(39) (a) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
Molecules; Oxford University Press: Oxford, 1989. (b) Koch, W.;
Holthausen, M. C. A Chemist’s Guide to Density Functional Theory; Wiley-
VCH: Weinheim, Germany, 2000.
(40) (a) Becke, A. D. Phys. ReV. 1988, A 38, 3098. (b) Becke, A. D. J. Chem.
Phys. 1993, 98, 5648. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B
1988, 37, 785.
(41) Cundari, T. R.; Benson, M. T.; Lutz, M. L.; Sommerer, S. O. In ReViews
in Computational Chemistry; Lipkowitz, K. B., Boyd, D. B., Eds.; VCH:
New York, 1996; Vol. 8, p 145.
(42) Quasirelativistic effective core potentials (ECPs): (a) Bergner, A.; Dolg,
M.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431, (Si, Ge, Sn). (b)
Kuechle, W.; Dolg, M.; Stoll, H.; Preuss, H. Mol. Phys. 1991, 74, 1245,
(Pb).
(43) For example, the relative energy of 4a versus 6a is 22.7 kcal mol-1 (B3LYP/
6-31G(d) (C, H), SDD (Si)//B3LYP/6-31G(d) (C, H), SDD (Si)), 23.9 kcal
mol-1 (B3LYP/6-31G(d)//B3LYP/6-31G(d)), or 28.7 (MP2/6-311G(d,p)//
MP2/6-311G(d,p)).
(44) (a) Ditchfield R. Mol. Phys. 1974, 27, 789. (b) Wolinski, K.; Hilton; J. F.;
Pulay, P. J. Am. Chem. Soc. 1982, 104, 5667. (c) Cheeseman, J. R.; Trucks,
G. W.; Keith, T. A.; Frisch, M. J. J. Chem. Phys. 1996, 104, 5497.
(45) (a) Møller C.; Plesset M. S. Phys. ReV. 1934, 46, 618. (b) Head-Gordon,
M.; Pople, J. A.; Frisch, M. Chem. Phys. Lett. 1988, 153, 503.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2163

A R T I C L E S
Mu¨ller et al.
Table 3. Characteristic Structural Parameters of Norbornyl
Cations 4a-d (Bond Lengths, pm; Angles, deg, Calculated at
MP2/6-311G(d,p), SDD (Sn, Pb) (See Also Figure 3) and Orbital
Occupations (occ, au) According to NBO Theory
E ) Si (4a)
E) Ge (4b)
E) Sn (4c)
E) Pb (4d)
r(CdC)
r(EC6/7)
ꢀ
137.2
235.2
28.2
137.3
244.1
23.9
136.8
267.2
15.2
136.6
280.6
8.8
occ(π(CdC))
occ(p(E))
1.690
0.346
1.736
0.283
1.779
0.218
1.803
0.191
described in eq 2, show that the norbornyl cations 4a-d
Figure 4. Energetic relation between the cations 4, 6, 11, and 12 (see
Table 4).
Table 4. Calculated Relative Energies EA-ED (at B3LYP/
6-311G(2d,p) (C, H), SDD (E)//B3LYP/6-31G(d) (C, H), SDD (E) +
∆ZPVE) and Relative Free Enthalpies at 298 K,
experience an additional stabilization compared to the Me3E+
cations by intramolecular interaction with the CdC double bond
(24-18 kcal mol-1). This results for the plumbanorbornyl cation
4d in a calculated stabilization relative to tert-butyl cation of
-52.7 kcal mol-1. Remarkably, the additional stabilization of
the norbornyl cations relative to Me3E+ is reduced along the
series 4a > 4b, 4c > 4d. This trend is paralleled by the
decreasing intramolecular stabilization energy EA for 4a-d,
which is the energy difference between the isomeric cations
6a-d and the norbornyl cations 4a-d (see Table 2).49
In the calculated structures of 4a-d the intramolecular
interaction between the CdC double bond and the element is
clearly apparent (see Figure 3 and Table 3): (i) For all norbornyl
cations the element-C(6/7) distance, r(EC(6/7)), is markedly
smaller than the sum of the van der Waals radii. (ii) The Me2-
ECH2- group in 4a-d is always clearly pyramidalized toward
the CdC double bond. The degree of pyramidalization is
quantified by the angle ꢀ, which is defined as the angle between
the vector of the E-CH2 bond and the plane spanned by the
carbon atoms of the methyl groups and the element atom of
the EMe2 unit. (iii) The CdC double bond in 4a-d is pre-
dicted to be distinctively longer than calculated for the precursor
silanes (137.2-136.6 pm, 4a-d; 133.6 pm, 6(H)a-d), which
is qualitatively consistent with electron transfer from the
π-bonding orbital to the electron-deficient element atom. In
agreement with this interpretation an NBO analysis46 of the
electron density reveals clearly depleted π-orbitals of the CdC
double bond (occupation number occ(π(CdC)) ) 1.69-1.80)
and occupations for the formal empty p-orbitals at the element
atom which differ decisively from zero (occ(p(E)) ) 0.19-
0.35; see Table 3) for the norbornyl cations 4a-d. Parallel to
the attenuation of the intramolecular stabilization energy EA in
GA(298)-GD(298), at B3LYP/6-31G(d) (C, H), SDD (E) for Cations
1-4 (kcal mol-1) (See Also Figure 4)
E
EA
G (298)
A
EB
G (298)
B
EC
G (298)
C
ED
G (298)
D
Si -19.5 -18.5 -17.1 -5.7 -4.1
Ge -17.4 -16.2 -15.1 -3.5 -4.1
Sn -17.0 -15.2 -16.4 -4.3 -3.4
Pb -12.5 -10.8 -12.8 -0.9 -3.4
5.3
4.5
7.4
5.7
-7.9 -7.6
-6.4 -8.2
-4.0 -3.5
-3.1 -4.2
the series 4a f 4d, the geometric consequences arising from
this interaction also decrease. That is, (i) the pyramidalization
at the element, measured by the angle ꢀ, is reduced and (ii) the
CdC double bond is calculated to be slightly shorter for the
plumbanorbornyl cation 4d than for the silacation 4a. Further-
more, the calculated charge transfer from the CdC double bond
to the element atom decreases continuously from the silacation
4a to the plumbanorbornyl cation 4d, as shown by the calculated
orbital occupations (see Table 3).
The comparison of the calculated interaction energy EA with
the reaction energy EB to form the metalated benzenium ions
11a-c50,51 reveals that the intramolecular transformation of the
transient cations 6a-c to give the norbornyl cations 4a-c is
energetically favored over the intermolecular reaction with
benzene (see Figure 4 and Table 4). For the plumbylium ion
6d, EB is slightly larger than EA (by 0.3 kcal mol-1). Entropy
effects, however, clearly favor the formation of the norbornyl
cation 4d over the benzenium ion 11d at ambient conditions
(see Table 4), since the calculated thermal and free energy at
298K (GB(298)) to form 11d is significantly smaller than
GA(298) to form 4d. The interaction energy EC between benzene
and the norbornyl cations 4a-d,51,53 calculated for complexes
12a-d, is relatively small (-3.4 to -4.1 kcal mol-1), and at
(46) (a) NBO 4.0: Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter,
J. E.; Weinhold, F., Theoretical Chemistry Institute, University of
Wisconsin, Madison, WI, 1996. (b) Reed, A. E.; Curtiss, L. A.; Weinhold,
F. Chem. ReV. 1988, 88, 899.
(50) The basis set superposition error for 11a as estimated by a counterpoise
calculation52 at the B3LYP/6-311G(2d,p)//B3LYP/6-31G(d) level of theory
is 1.3 kcal mol-1
.
(51) This value must be regarded as a lower boundary, since the dispersion
energy term of the intermolecular forces is not covered by hybrid density
functionals as the here applied B3LYP functional. The dispersion energy
term is, however, approximately 1 order of magnitude smaller than the
here dominant ion-dipole interaction energy, and its absolute value is
usually less than 2 kcal mol-1. Hobza, P.; Sˆponer, J. Reschel, T. J. Comput.
Chem. 1995, 16, 1315.
(47) For an introduction see: Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople,
J. A. Ab initio molecular orbital theory; Wiley-Interscience: New York,
1986.
(48) Similar results have been obtained for EX3+ ions (E ) C-Pb, X ) F-I):
Frenking, G.; Fau, S.; Marchand, C. M.; Gru¨tzmacher, H. J. Am. Chem.
Soc. 1997, 119, 6648.
(49) A somewhat different trend was computed for the intermolecular interaction
(52) (a) Kestner, N. R. J. Chem. Phys. 1968, 48, 252. (b) Liu, B.; McLean, A.
D. J. Chem. Phys. 1973, 59, 4557. (b) Boys, S. F.; Bernardi, F. Mol. Phys.
1970, 19, 553.
+
between toluene and EX3 ions (E ) C-Pb, X ) H, Me, Cl): Basch, H.
Inorg. Chim. Acta 1996, 242, 191.
9
2164 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003

Norbornyl Cations of Group 14 Elements
A R T I C L E S
Table 5. Calculated NMR Chemical Shifts of Norbornyl Cations
4a-f (GIAO/B3LYP/6-311G(3d,p)//MP2/6-311G(d,p), SDD (Sn,
Pb))
calculated to be too deshielded, e.g., for 4a by 4.1-10.3 ppm.
In particular, the deviations for the vinylic carbons are large:
+8.3-14.6. However, experimental and calculated 13C NMR
chemical shift data are linearly correlated.55b This indicates that
the deviations are due to a systematic error, e.g., deficiencies
of the applied method and basis set. DFT-based methods such
as GIAO/DFT calculations are known to overestimate para-
magnetic contributions to the chemical shielding, giving overly
deshielded chemical shifts in critical cases for molecules with
small HOMO/LUMO separations.3c,56 It has been demonstrated
that the accurate calculation of the magnetic properties of
alkenes, allenes, and unsaturated carbocations requires MP2 or
even higher correlated methods, e.g., CCSD(T).57-60 The size
of the cations studied and the extent of the present investigation
however prevent the use of these highly accurate methods. The
overall agreement between the theoretical and the experimental
data (see Table 5) is however satisfying, when the reduced
accuracy of the 13C NMR chemical shift calculations is taken
into account.
δ(E)
δ(C6/7)
δ(C4/5)
δ(C3)
δ(C2)
4a
4b
4c
4d
4e
4fe
E ) Si
exptl
92.6
87.2
160.5
150.2
157.5
144.8
155.1
140.5
152.0
138.0
159.5
149.3
157.9
149.6
47.1
41.0
46.5
40.4
43.5
40.4
46.1
38.2
47.2
41.6
46.5
41.6
44.7
37.0
45.3
36.5
46.9
35.3
45.9
36.5
44.4
36.5
43.6
36.6
18.2
14.1
24.9
18.2
24.2
25.0
42.8
53.4
15.0
10.4
15.8
12.7
E ) Ge
exptlb
E ) Sn
exptlc
E ) Pb
exptld
E ) Si
exptl
81.3
82.7
76.8
80.2
E ) Si
exptl
a Relative to calculated TMS: σ(Si, TMS) ) 325.2. σ(C, TMS) ) 183.5.
b Experimental data for 4g. c Experimental data for 4h. d Experimental data
for 4i. e A geometry optimized at MP2/6-31G(d) was used.
298 K the formation of 12a-d is endergonic (GC(298) > 0).
This negates the presence of significant solvent-cation interac-
tions at room temperature and is in qualitative agreement with
the negligible solvent effects on the element NMR and 13C NMR
chemical shifts found experimentally (see Table 1).
Conclusion
Norbornyl cations 4 of the elements Si f Sn have been
synthesized in aromatic hydrocarbon solutions by hydride-
transfer reactions from cyclopentenemethyl-substituted silanes,
germanes, and stannanes (5) to trityl cation via transient trivalent
element cations. In addition, the lead congener 4i was obtained
by treatment of the tetraalkyl-substituted plumbane 5i with trityl
cation. The cations 4 have been fully characterized by NMR
spectroscopy, supported by quantum mechanical calculations
of the structures, energies, and NMR chemical shifts. The
experimental data as well as the theoretical results reveal the
bridged structures of the cations 4, with a formally pentacoor-
dinated, positively charged element atom. Alternatively, the
coordination of the element atom might be described as 3 + 1
coordination, with the unique binding side occupied by the
CdC double bond. The cations 4 exhibit only negligible
interactions with aromatic solvents and therefore can be regarded
as “free cations” in solution. Addition of the stronger coordinat-
ing solvent acetonitrile to solutions of the norbornyl cations
4a,e-h in aromatic hydrocarbons leads to a breakdown of the
intramolecular interaction between the positively charged ele-
ment and the CdC double bond and results in the formation of
nitrilium ions with a tetracoordinated element atom. Although
the interaction between the positively charged lead atom and
the CdC double bond in the plumbanorbornyl cation 4i is
strongly influenced by addition of solvents of higher donator
power, it endures, and with acetonitrile, the plumbyl cation 10i
with a trigonal bipyramidal coordinated lead center is formed
in solution and in the solid state. The thermodynamic stability
of the cations 4 and of the trimethyl-substituted element cation,
At all applied levels of theory the â-element-substituted
carbocations 9a-d are not stationary points on the potential
energy surface. The cations 9a-d collapse during the optimiza-
tion process to the symmetrical bridged norbornyl cations
4a-d. This theoretical result discards a conceivable fast
equilibrium between two â-element-substituted carbocations 9,
which is in line with the available experimental data. Further
support for the bridged norbornyl cation nature of the observed
species originates from quantum mechanical calculations of
NMR chemical shifts. This method has been extensively applied
in carbocation54 and silyl cation2a,3c,d chemistry, and the validity
of theoretical structures is frequently established by comparing
the computed NMR chemical shifts against the experimental
data. The 29Si and 13C NMR chemical shifts calculated for 4a-f
at GIAO/B3LYP/6-311G(3d,p)//MP2/6-311G(d,p) are sum-
marized along with experimental data in Table 5.55a The
agreement between the calculated 29Si NMR chemical shifts and
the experimental data for the silacations 4a,e,f is convincing;
that is, the deviation ∆δtheor(29Si) is small (∆δtheor(29Si) )
δ(29Si)(calcd) - δ(29Si)(exptl) < +6). Thus, the 29Si NMR
chemical shift calculations for the silanorbornyl cations 4a,e,f
corroborate the validity of the computed structures. Surprisingly,
the performance of this specific theoretical model for the
calculations of 13C NMR chemical shifts is much less satisfac-
tory. Although the general trend of the experimental 13C NMR
chemical shifts is faithfully reproduced by the computations,
the individual errors between computed and experimental 13C
chemical shifts are large. In general all carbon nuclei are
(53) The basis set superposition error for 12a as estimated by a counterpoise
calculation48 at the B3LYP/6-311G(2d,p)//B3LYP/6-31G(d) level of theory
(56) (a) Ottoson, C.-H.; Cremer, D. Organometallics 1996, 15, 5495. (b) For a
short review see: Webb, G. A. In Enzyclopedia of NMR Spectroscopy;
Grant, D., Webb, G. A., Eds.; Wiley: New York, 1996; p 4316.
(57) (a) Gauss, J. Chem. Phys. Lett. 1992, 191, 614. (b) Gauss, J. J. Chem.
Phys. 1993, 99, 3629. (c) Gauss, J.; Stanton, J. F. J. Chem. Phys. 1996,
104, 2574.
(58) (a) Siehl, H.-U.; Mu¨ller, T.; Gauss, J.; Buzek, P.; Schleyer, P. v. R. J. Am.
Chem. Soc. 1994, 116, 6384. (b) Stanton, J. F.; Gauss, J.; Siehl, H.-U.
Chem. Phys. Lett. 1996, 262, 183; (c) Apeloig, Y.; Mu¨ller, T. In
Dicoordinated Carbocations; Rappoport, Z., Stang, P. J., Eds.; Wiley: New
York, 1997; Chapter 2, p 9.
is 0.8 kcal mol-1
.
(54) Review: Schleyer, P. v. R.; Marker, C. Pure Appl. Chem. 1995, 67, 755.
(55) (a) At this level the calculated 29Si and 13C NMR chemical shifts for
complexes between silyl cations and alkenes are converged, as can be shown
by test calculations for [H3Si/C2H4]+ optimized at MP2/6-311G(d,p):
GIAO/B3LYP/6-311G(d,p), δ(29Si) ) -69.5, δ(13C) ) 136.3; GIAO/
B3LYP/6-311G(2d,p), δ(29Si) ) -73.1, δ(13C) ) 136.3; GIAO/B3LYP/
6-311G(2df,p), δ(29Si)
) ) 136.7; GIAO/B3LYP/6-
-71.6, δ(13C)
311G(3d,p) δ(29Si) ) -75.4, δ(13C) ) 136.8; GIAO/B3LYP/6-311G(3df,p),
δ(29Si) ) -75.4, δ(13C) ) 136.9; GIAO/B3LYP/6-311G(3df,2p), δ(29Si)
) -74.2, δ(13C) ) 136.9. (b) δtheor
1.5, 24 data points, correlation coefficient 0.996; for details see the
Supporting Information.
(
13C) ) 1.05 ( 0.02, δ(13C) ) 3.7 (
(59) Buzek, P.; Schleyer, P. v. R.; Vancik, H. Mihalic, Z.; Gauss, J. Angew.
Chem., Int. Ed. Engl. 1994, 33, 448.
(60) Sieber, S.; Schleyer, P. v. R.; Gauss, J. J. Am. Chem. Soc. 1993, 115, 6987.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2165

A R T I C L E S
Mu¨ller et al.
Me3E+, increases from Si to Pb. In contrast, the intramolecular
stabilization energy EA of the norbornyl cations decreases,
suggesting reduced interaction between the CdC double bond
and the electron-deficient element center in the plumbacations
compared to the silacations. This points to a reduced electro-
philicity of the plumbacation compared to its forerunners in
group 14. This is in agreement with the computed orbital
occupations for the cations and is experimentally supported by
the reduced downfield shift of the vinylic carbon atoms C6/7
(∆δ(13C) ≈ 20 (E ) Si), ∆δ(13C) ≈ 14 (E ) Ge), ∆δ(13C) ≈
11 (E ) Sn), ∆δ(13C) ≈ 8 (E ) Pb)) along the series sila-,
germa-, stanna-, and plumbanorbornyl cation.
diethyl ether (50 mL) is slowly added to a vigorously stirred solution
of di-n-butyldichlorostannane (10.43 g, 34.35 mmol) in diethyl ether
(80 mL). The reaction mixture is stirred for an additional day, and the
solvent is evaporated under reduced pressure (2 Pa). The residue is
extracted twice with dry pentane (20 mL). After removal of pentane
the oily product is purified by vacuum distillation (101-103 °C, 2 Pa)
to give a colorless oil (7.1 g, 59.1%). 1H NMR (250.133 MHz, CDCl3,
303 K): δ 5.68 (s, 2H, -CHdCH-), 2.8 (m, 1H, -CHCH2Sn-), 2.51
(d, 2H, -CH2CHCH2-, 2JH,H ) 15 Hz), 1.94 (d, 2H, -CH2CHCH2-,
2JH,H ) 15 Hz), 1.47 (d, 2H, -CHCH2Sn-, 3JH,H ) 6 Hz), 1.22-1.52
(m, 12H, -SnCH2CH2CH2CH3), 0.92 (t, 6H, -CH3). 13C NMR (62.896
MHz, CDCl3, 303 K): δ 130.3 (C6/7), 42.1 (C4/5), 34.6 (C3), 28.0
(-CH2CH3), 26.8 (-SnCH2CH2-), 18.2 (C2), 13.6 (-CH3), 13.5
(-SnCH2CH2-). 119Sn NMR (93.181 MHz, CDCl3, 303 K): δ 139.3
(-SnCl-).
Experimental Section
Triethyllead Bromide. At -80 °C a solution of Br2 (1.78 g, 11.26
mmol) in ethyl acetate (15 mL) is slowly added via a dropping funnel
to a stirred solution of tetraethyllead (2 mL, 10.24 mmol) and ethyl
acetate (10 mL). After addition of 1/3 of the Br2 the colorless solution
turns into a white suspension due to precipitated Et3PbBr. The addition
is stopped when the color of the reaction mixture remains yellow, which
indicates a small excess of Br2. The cold reaction mixture is decanted
off, and the white precipitate is extracted three times with 20 mL
portions of diethyl ether at room temperature. The combined ethereal
phases are evaporated and dried under reduced pressure (2 Pa) to give
1.14 g (30%) of a colorless crystalline solid. 1H NMR (250.133 MHz,
General Information. All reagents were obtained from commercial
suppliers and were used without further purification. THF was distilled
from sodium/potassium alloy/benzophenone. Benzene, benzene-d6,
toluene, and toluene-d8 were distilled from sodium. Acetonitrile was
distilled from P4O10. Chlorosilanes and -germanes were distilled from
CaH2. TPFPB was stored under vacuum for several hours prior to use.
All reactions were carried out in oven-dried glassware under inert argon
atmospheres. NMR spectra were recorded on Bruker AM-400 and DPX-
250 instruments. 1H NMR spectra were calibrated using residual
nondeuterated solvents as internal reference, 13C NMR spectra using
the central line of the solvent signal. 29Si NMR spectra were recorded
using the INEPT pulse sequence. The 29Si, 119Sn, and 207Pb NMR spectra
were calibrated using external TMS (δ(29Si) ) 0.0), (H5C2)4Sn
(δ(119Sn) ) 1.8), and (H5C2)4Pb (δ(207Pb) ) 71.0), respectively.
Chloromethyl-3-cyclopentene, 7,61 3-cyclopentenemethyldimethyl-
silane, 5a,61 and TPFPB62 were prepared as described in the literature.
3-Cyclopentenemethyldichlorosilane. At -15°C, a solution of
3-cyclopentenemethylmagnesium chloride (50.2 mmol) in diethyl ether
(30 mL) is slowly added to a vigorously stirred and cooled (-15°C)
solution of trichlorosilane (20.35 g, 150.4 mmol) in diethyl ether (125
mL). The reaction mixture is stirred overnight and filtered through a
glass frit. The residue is extracted once with dry diethyl ether (80 mL).
The filtrates are combined, and the solvent is evaporated under reduced
pressure. The oily residue is purified by vacuum distillation (40-42
3
CDCl3, 303 K): δ 1.80 (t, 9H, -CH3, JPb,H ) 177 Hz), 2.21 (q, 6H,
-CH2-). 13C NMR (62.860 MHz, CDCl3, 303 K): δ 34.3 (-CH2-,
1JPb,C ) 191.9 Hz), 13.0 (-CH3, 2JPb,C ) 43.6 Hz). 207Pb NMR (52.304
MHz, CDCl3, 303 K): δ 513 (BrPbEt3).
3-Cyclopentenemethyldiethylsilane (5e). Ethylmagnesium chloride
(50.42 mmol) in dry diethyl ether (60 mL) is added to 3-cyclopentene-
methyldichlorosilane (4.07 g, 22.5 mmol) in dry diethyl ether (50 mL).
The reaction mixture is stirred for 30 min and is quenched with water/
NH4Cl. The organic layer is separated, and the aqueous phase is washed
four times with diethyl ether (80 mL). The combined organic layers
are dried over MgSO4, and the solvent is removed. The oily residue is
purified by distillation under reduced pressure (30-32 °C, 2 Pa) to
1
yield a colorless oil (2.85 g, 75.4%). H NMR (250.133 MHz, C6D6,
303 K): δ 5.63 (s, 2H, -CHdCH-), 3.85 (m, 1H, -SiH, 1JSi,H ) 179
Hz), 2.48 (m, 2H, -CH2CHCH2-), 2.31 (m, 1H, -CH-), 1.94 (m,
1
°C, 2 Pa) to give a pale pink oil (4.07 g, 44.92%). H NMR (250.133
MHz, CDCl3, 303 K): δ 5.69 (s, 2H, -CHdCH-), 5.55 (t, 1H, -SiH,
3
2H, -CH2CHCH2-), 0.96 (t, 6H, -CH3, JH,H ) 7.9 Hz), 0.75 (dd,
3
1JSi,H ) 277 Hz, JH,SiH ) 2.2 Hz), 2.5-2.7 (m, 3H, -CH2CHCH2-),
3
3
2H, -CHCH2Si-, JH,H ) 7.3 Hz, JH,SiH ) 3.2 Hz), 0.60 (dq, 4H,
-CH2CH3, 3JH,H ) 7.9 Hz, 3JH,SiH ) 3.2 Hz). 13C NMR (62.860 MHz,
C6D6, 303 K): δ 130.3 (C6/7, 1JC,H ) 162 Hz), 42.3 (C4/5), 34.9 (C3),
18.9 (C2), 8.4 (-CH2CH3), 3.5 (-CH3). 29Si NMR (79.460 MHz, C6D6,
303 K): δ -3.7.
1.98-2.17 (m, 2H, -CH2CHCH2-), 1.44 (dd, 2H, -CH2Si-). 13C
NMR (62.860 MHz, CDCl3, 303 K): δ 129.8 (C6/7), 41.0 (C4/5), 32.3
(C3), 27.8 (C2). 29Si NMR (79.460 MHz, CDCl3, 303 K): δ 10.2.
3-Cyclopentenemethyldi-n-butylchlorogermane. At -15 °C, a
solution of 3-cyclopentenemethylmagnesium chloride (3.3 mmol) in
THF (100 mL) is slowly added to a vigorously stirred mixture of di-
n-butyldichlorogermane (0.851 g, 3.3 mmol) and THF (1 mL). The
solvent is evaporated under reduced pressure (2 Pa), and the residue is
extracted twice with dry pentane (10 mL). After removal of pentane
the oily product is purified by bulb-to-bulb distillation (85-95 °C, 2
Pa). A colorless oil (0.58 g, 58%) is obtained. 1H NMR (250.133 MHz,
CDCl3, 303 K): δ 5.66 (s, 2H, -CHdCH-), 2.55-2.59 (m, 3H,
-CH2CHCH2-), 1.99 (m, 2H, -CH2CHCH2-), 1.16-1.59 (m, 14H,
-CH2GeCH2CH2CH2-), 0.90 (t, 6H, -CH3). 13C NMR (62.896 MHz,
CDCl3, 303 K): δ 130.0 (C6/7), 41.7 (C4/5), 34.0 (C3), 26.9 (C2),
26.2, 25.8 (-GeCH2CH2CH2-), 19.4 (-GeCH2CH2-), 13.6 (-CH3).
3-Cyclopentenemethyldi-n-butylchlorostannane. At 0 °C a solu-
tion of 3-cyclopentenemethylmagnesium chloride (34.35 mmol) in
3-Cyclopentenemethyldi-n-butylsilane (5f). n-Butylmagnesium
chloride (9.84 mmol) in dry diethyl ether (25 mL) is added to 3-cyclo-
pentenemethyldichlorosilane (0.89 g, 4.92 mmol). After being re-
fluxed for 30 min the reaction mixture is quenched with water/NH4Cl.
The organic layer is separated, and the aqueous phase is washed three
times with diethyl ether (30 mL). The combined organic layers are
dried over MgSO4, and the solvent is removed by rotary evapo-
ration. Bulb-to-bulb distillation yields a colorless oil (0.79 g, 71.5%).
1H NMR (250.133 MHz, C6D6, 303 K): δ 5.64 (s, 2H, -CHdCH-),
1
3
3.91 (m, 1H, -SiH, JSi,H ) 178 Hz, JH,SiH ) 3.3 Hz), 2.5 (dd, 2H,
2
3
-CH2CHCH2-, JH,H ) 14 Hz, JH,H ) 8 Hz), 2.34 (m, 1H,
-CH-), 1.96 (dd, 2H, -CH2CHCH2-, JH,H ) 14 Hz, JH,H
2
3
)
7 Hz), 1.35 (m, 4H, -CH2CH3), 1.34 (m, 4H, -CH2CH2CH3), 0.9 (t,
3
3
6H, -CH3, JH,H ) 7 Hz), 0.84 (dd, 2H, -CHCH2Si-, JH,H
)
7 Hz, 3JH,SiH ) 3.3 Hz), 0.61 (m, 4H, -SiCH2CH2-). 13C NMR (62.860
MHz, C6D6, 303 K): δ 130.3 (C6/7, JC,H ) 162 Hz), 42.4 (C4/5),
(61) (a) Hosomi, A.; Mikami, M.; Sakurai, H. Bull. Chem. Soc. Jpn. 1983, 56,
2784. (b) Bauch, C. Master Thesis, Goethe Universita¨t Frankfurt/Main,
2000. (c) Ostermeier, M. Master Thesis, Goethe Universita¨t Frankfurt/Main,
1999.
(62) (a) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 245. (b) Chien,
J. C. W.; Tsai, W.-M.; Rausch, M. D. J. Am. Chem. Soc. 1991, 113, 8570.
1
35.0 (C3), 27.5 (-CH2CH3), 26.8 (-CH2CH2CH3), 19.7 (C2), 14.0
(-CH3), 11.8 (-CH2CH2CH2CH3). 29Si NMR (79.460 MHz, C6D6, 283
K): δ -9.2.
9
2166 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003

Norbornyl Cations of Group 14 Elements
A R T I C L E S
3-Cyclopentenemethyldi-n-butylgermane (5g). 3-Cyclopentene-
methyldi-n-butylchlorogermane (0.58 g, 1.91 mmol) is added to a
solution of LiAlH4 (0.247 g, 6.49 mmol) in diethyl ether (10 mL) and
refluxed for 5 h. The reaction mixture is quenched with water, and
Al(OH)3 is dissolved in concentrated potassium/sodium tartrate solution.
The organic layer is separated, and the aqueous phase is extracted three
times with diethyl ether (10 mL). The combined organic layers are
dried over MgSO4, and the solvent is removed. Bulb-to-bulb distillation
of the oily residue under reduced pressure (2 Pa) gives a colorless oil
2,2-Dimethyl-2-silanorbornyl Cation (4a). Red glassy powder, 286
mg, 70% yield. 1H NMR (250.133 MHz, C6D6, 298 K): δ 5.84 (s, 2H,
-CHdCH-), +2.50 to -0.29 (m, 48H, (-CH2)2CHSi(CH3)). 13C
NMR (62.896 MHz, C6D6, 289 K): δ 150.6 (C6/7, 1JC,H ) 170.4 Hz),
41.4 (C4/5), 37.4 (C3), 14.5 (C2), -2.2 (-CH3). 29Si NMR (79.460
MHz, C6D6, 298 K): δ 87.2.
2,2-Diethyl-2-silanorbornyl Cation (4e). Red oil, 253 mg, 60%
1
yield. H NMR (400.136 MHz, C6D6, 283 K): δ 5.92 (s, 2H, -CHd
3
CH-), 2.23 (t, 1H, -CH-, JH,H ) 2.4 Hz), 1.61 (dm, 2H, -CH2-
1
CHCH2-, 2JH,H ) 15.9 Hz), 1.41 (d, 2H, -CH2CHCH2-, 2JH,H ) 15.9
Hz), 0.57 (t, 6H, -CH3, 3JH,H ) 7.5 Hz), 0.35 (q, 4H, -CH2CH3, 3JH,H
(0.37 g, 72%). H NMR (250.133 MHz, CDCl3, 303 K): δ 5.67 (s,
2H, -CHdCH-), 3.99 (septet, 1H, -GeH, 3JH,H ) 3.1 Hz), 2.52 (m,
2H, -CH2CHCH2-), 2.4 (m, 1H, -CH-), 1.98 (m, 2H, -CH2-
CHCH2-), 1.37-1.38 (m, 8H, -GeCH2CH2CH2CH3), 0.99 (dd, 2H,
3
) 7.5 Hz), 0.00 (d, 2H, -CHCH2Si-, JH,H ) 2.4 Hz). 13C NMR
1
(100.614 MHz, C6D6, 300 K): δ 149.8 (C6/7, JC,H ) 173 Hz), 41.6
2
3
1
1
1
-CHCH2Ge-, JH,H ) 7.2 Hz, JH,H ) 3.2 Hz), 0.9 (t, 6H, -CH3,
(C4/5, JC,H ) 139 Hz), 36.5 (C3, JC,H ) 147 Hz), 10.4 (C2, JC,H
3JH,H ) 6.9 Hz), 0.82 (m, 4H, -GeCH2CH2-). 13C NMR (62.896 MHz,
CDCl3, 303 K): δ 130.2 (C6/7, JC,H ) 161 Hz), 42.2 (C4/5), 36.1
1
1
)137 Hz), 6.1 (-CH3, JC,H ) 139 Hz), 5.3 (-CH2CH3, JC,H ) 128
1
Hz). 29Si NMR (79.460 MHz, C6D6, 283 K): δ 82.7.
(C3), 28.9 (-CH2CH3), 26.4 (-GeCH2CH2-), 20.4 (C2), 14.0 (-CH3),
12.5 (-GeCH2CH2-).
2,2-Di-n-butyl-2-silanorbornyl Cation (4f). Red oil, 293 mg, 65%
1
yield. H NMR (250.133 MHz, C6D6, 303 K): δ 6.00 (s, 2H, -CHd
3
3-Cyclopentenemethyldi-n-butylstannane (5h). At -10 °C a
solution of LiBEt3H (6 mmol) in THF (6 mL) is slowly added to a
stirred solution of 3-cyclopentenemethyldi-n-butylchlorostannane (1.475
g, 4.22 mmol) in diethyl ether (20 mL). The reaction mixture is allowed
to warm to room temperature and is stirred for an additional 8 h. The
mixture is quenched with water, and the organic layer is separated.
The aqueous phase is extracted three times with diethyl ether (20 mL).
The combined organic layers are dried over MgSO4, and the solvent is
removed under vacuum (2 Pa). The oily residue is flash-distilled at 2
Pa using a preheated oil bath (160 °C) to give a colorless oil (1 g,
75%). 1H NMR (250.133 MHz, C6D6, 303 K): δ 5.65 (s, 2H, -CHd
CH-), 5.03 (septet, 1H, -SnH, 1JSn,H ) 1587 Hz, 3JH,H ) 2 Hz), 2.53
(m, 3H, -CH2CHCH2-), 1.94 (m, 2H, -CH2CHCH2-), 1.1 (dd, 2H,
CH-), 2.22 (m, 1H, -CH2CHCH2-, JH,H ) 3.4 Hz), 1.62 (dd, 2H,
2
3
-CH2CHCH2-, JH,H ) 16 Hz, JH,H ) 3.4 Hz), 1.40 (d, 2H, -CH2-
2
CHCH2-, JH,H ) 16 Hz), 0.83-1.07 (2m, 8H, -SiCH2CH2-), 0.77
(t, 6H, -CH3, 3JH,H ) 7.3 Hz), 0.45 (m, 4H, -CH2CH3), 0.04 (d, 2H,
-CHCH2Si-, 3JH,H ) 2.9 Hz). 13C NMR (62.860 MHz, C6D6, 303 K):
1
δ 149.6 (C6/7, JC,H ) 174 Hz), 41.6 (C4/5), 36.6 (C3), 25.4
(-SiCH2CH2-), 24.9 (-CH2CH3), 13.0 (-SiCH2CH2-), 12.7 (C2),
12.6 (-CH3). 29Si NMR (79.460 MHz, C6D6, 283 K): δ 80.2.
2,2-Di-n-butyl-2-germanorbornyl Cation (4g). Red oil, 307.6 mg,
1
65% yield. H NMR (250.133 MHz, C6D6, 303 K): δ 5.76 (s, 2H,
-CHdCH-), 2.36 (m, 1H, -CH2CHCH2-), 1.61 (dd, 2H, -CH2-
2
3
CHCH2-, JH,H ) 15.8 Hz, JH,H ) 4.4 Hz), 1.30 (d, 2H, -CH2-
CHCH2-, 2JH,H ) 15.8 Hz), 0.80-1.15 (m, 12H, -GeCH2CH2CH2-),
0.77 (t, 6H, -CH3, 3JH,H ) 7.0 Hz), 0.44 (d, 2H, -CHCH2Ge-, 3JH,H
) 2.9 Hz). 13C NMR (62.860 MHz, C6C6, 303 K): δ 144.8 (C6/7,
1JC,H ) 173.2 Hz), 40.4 (C4/5), 36.5 (C3), 25.8 (-CH2CH3), 24.8
(-GeCH2CH2-), 18.5 (-GeCH2CH2-), 18.2 (C2), 12.4 (-CH3).
2,2-Di-n-butyl-2-stannanorbornyl Cation (4h). White powder,
382.1 mg, 77% yield. 1H NMR (250.133 MHz, C6D6, 303 K): δ 5.54
(s, 2H, -CHdCH-), 2.70 (m, 1H, -CH2CHCH2-, 3JSn,H ) 183 Hz),
3
3
-CHCH2Sn-, JH,H ) 6.9 Hz, JH,SnH ) 2 Hz), 0.8-1.6 (m, 12H,
-CH2CH2CH2CH3), 0.91 (m, 6H, -CH3, JH,H ) 7.2 Hz). 13C NMR
(62.896 MHz, C6D6, 303 K): δ 130.4 (C6/7, JC,H ) 161 Hz), 43.1
3
1
(C4/5), 36.9 (C3), 30.3 (-CH2CH3), 27.4 (-SnCH2CH2-), 17.4 (C2),
13.7 (-CH3), 8.7 (-SnCH2CH2-). 119Sn NMR (93.181 MHz, C6D6,
303 K): δ -9.1 (-SnH).
3-Cyclopentenemethyltriethylplumbane (5i). A solution of 3-cy-
clopentenemethylmagnesium chloride (18.1 mmol) in diethyl ether (30
mL) is added via a Teflon tube to solid Et3PbBr (6.77 g, 18.1 mmol).
The resulting brown slurry is stirred for 1/2 h. The solvent is removed
under vacuum (2 Pa), and the residue is extracted twice with pentane
(20 mL). The combined organic layers are evaporated under reduced
pressure (2 Pa) to give a brown oil. Bulb-to-bulb distillation yields a
2
3
1.79 (dd, 2H, -CH2CHCH2-, JH,H ) 15.3 Hz, JH,H ) 4.6 Hz), 1.27
2
(d, 2H, -CH2CHCH2-, JH,H ) 15.3 Hz), 0.85-1.20 (m, 12H,
-SnCH2CH2CH2-), 0.81 (t, 6H, -CH3, 3JH,H ) 7.0 Hz), 0.68 (d, 2H,
-CHCH2Sn-, JH,SnH ) 3.0 Hz). 13C NMR (62.860 MHz, C6D6, 303
3
K): δ 141.5 (C6/7, 1JC,H ) 170 Hz, 1JSn,C ) 26 Hz), 40.4 (C4/5), 35.3
2
3
(C3), 27.6 (-SnCH2CH2-, JSn,C ) 27.9 Hz), 26.2 (-CH2CH3, JSn,C
colorless oil (3.4 g, 50%). 1H NMR (250.133 MHz, C6D6): δ 5.64 (s,
1
) 75 Hz), 25.0 (C2), 19.9 (-SnCH2CH2-, JSn,C ) 256 Hz), 12.4
3
(-CH3). 119Sn NMR (93.181 MHz, C6D6, 303 K): δ 334.
2H, -CHdCH-), 2.68 (m, 1H, -CH-, JH,H
) 7.3 Hz),
2
2.51 (dd, 2H, -CH2CHCH-, JH,H ) 14.4 Hz), 1.89 (dd, 2H,
-CH2CHCH2-, JH,H ) 14.4 Hz, JH,H ) 5.9 Hz), 1.64 (d, 2H,
2,2-Diethyl-2-plumbanorbornyl Cation (4i). Slow addition of 5i
(193 mg, 0.515 mmol) to a vigorously stirred solution of TPFPB (0.461
g, 0.5 mmol) in dry C6D6 (3 mL) results in evolution of ethene24 and
the disappearance of the red color of the solution. The reaction mixture
is allowed to separate into two layers, an upper colorless layer and a
lower light brown layer. The upper layer contains the byproduct
triphenylmethane and is removed. The lower phase is transferred via a
flexible Teflon pipe into an NMR tube. After evaporation of the solvent
in a vacuum and washing of the oily residue with 2 mL portions of
pentane, a white crystalline powder is obtained (486.9 mg, 95% yield).
1H NMR (250.133 MHz, C6D6, 303 K): δ 5.29 (s, 2H, -CHdCH-),
4.41 (m, 1H, -CH-, 3JPb,H ) 333 Hz), 2.07 (dd, 2H, -CH2CHCH2-,
2
3
3
-CHCH2Pb-, JH,H ) 7.3 Hz), 1.46 (s, 15H, -CH2CH3), 1.46 (s,
15H, -CH3). 13C NMR (62.896 MHz, C6D6, 303 K): δ 130.2 (C6/7,
1JC,H ) 161.2 Hz), 43.8 (C4/5, 3JPb,C ) 56.8 Hz), 38.4 (C3, 2JPb,C )28.6
Hz), 27.0 (C2, 1JPb,C ) 183.2 Hz), 14.0 (-CH3, 2JPb,C ) 30.5 Hz), 10.4
(-CH2CH3, JPb,C ) 188.8 Hz). 207Pb NMR (52.304 MHz, C6D6, 303
1
K): δ 50.5.
2-Norbornyl Cations 4. Slow addition of the precursor 5 (0.5 mmol)
to a vigorously stirred solution of TPFPB (461 mg (0.5 mmol) in
benzene-d6 (3 mL)) at room temperature results in a slightly exothermic
reaction. The reaction is finished when the color of the trityl cation
has disappeared. Stirring is stopped, and the reaction mixture is allowed
to separate into two layers, a dark red lower phase and a colorless to
slightly yellow upper phase. The upper layer, which contains the
byproduct triphenylmethane, is separated off, and the lower layer is
transferred via a flexible Teflon pipe into an NMR tube and is
investigated by NMR spectroscopy. Evaporation of the solvent in a
vacuum and washing of the oily residue with 2 mL portions of pentane
gives the solid salts.
3
2JH,H ) 15.5 Hz, JH,H ) 5.5 Hz), 1.80 (br, 4H, -CH2CH3), 1.62 (t,
3
3
6H, -CH3, JH,H ) 7.5 Hz), 1.51 (d, 2H, -CHCH2Pb-, JH,H ) 2.9
2
2
Hz, JPb,H ) 21 Hz), 1.35 (d, 2H, -CH2CHCH2-, JH,H ) 15.5 Hz).
13C NMR (62.860 MHz, C6D6, 303 K): δ 138.0 (C6/7, JPb,C ) 16.2
1
Hz, 1JC,H ) 170 Hz), 53.4 (C2, 1JPb,C ) 36.6 Hz), 39.3 (C4/5, 3JPb,C
)
1
2
44.2 Hz), 38.2 (-CH2CH3, JPb,C ) 36.7 Hz), 36.5 (C3, JPb,C ) 46.2
Hz), 12.0 (-CH3, JPb,C ) 41.4 Hz). 207Pb NMR (52.304 MHz, C6D6,
2
303 K): δ 1049.
9
J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003 2167

A R T I C L E S
Mu¨ller et al.
Nitrilium Ions 10. Addition of 1 equiv of dry acetonitrile (20.5 mg,
0.5 mmol) via a syringe to a solution of the norbornyl cation (0.5 mmol)
in C6D6 (1.5 mL) gave a deep red solution of the corresponding nitrilium
ion 10, which was investigated by NMR spectroscopy.
(3-Cyclopentenemethyldimethylsilyl)acetonitrilium (10a). 1H
NMR (250.133 MHz, C6D6, 303 K): δ 5.56 (s, 2H, -CHdCH-),
2.47-2.21 (m, 3H, -CH2CHCH2), 1.69 (dd, 2H, 2JH,H ) 13.2 Hz, 3JH,H
) 6.2 Hz, -CH2CHCH2-), 0.99 (br, m, 2H, -CHCH2Si), 0.37 (s,
6H, -SiCH3). 13C NMR (62.860 MHz, C6D6, 303 K): δ 130.0 (C6/7),
41.4 (C4/5), 32.6 (C3), 21.8 (C2), -2.5 (-CH3), 29Si NMR (79.460
MHz, C6D6, 283 K): δ 31.8.
(3-Cyclopentenemethyldiethylsilyl)acetonitrilium (10e). 1H
NMR (250.133 MHz, C6D6, 303 K): δ 5.57 (s, 2H, -CHdCH-), 2.32
(m, 2H, -CH2CHCH2), 2.13 (br m, 1H, -CH2CHCH2-), 1.69 (d, 2H,
-CH2CHCH2-, 2JH,H ) 14.8 Hz), 0.77 (br m, 6H, -CH3), 0.54 (br m,
4H, -CH2CH3), 0.22 (br m, 2H, -CHCH2Si-). 13C NMR (62.860
MHz, C6D6, 303 K): δ 129.5 (C6/7, 1JC,H ) 159.3 Hz), 120.5 (NCCH3),
41.0 (C4/5), 32.1 (C3), 17.8 (C2), 5.2 (-CH3), 3.7 (-CH2CH3), -0.4
(NCCH3). 29Si NMR (79.460 MHz, C6D6, 283 K): δ 37.5.
0.82 (2d, 2H, -CHCH2Sn-, 3JH,H ) 2.8 Hz). 13C NMR (62.860 MHz,
C6D6, 303 K): δ 130.7 (C6/7), 117.9 (NCCH3), 42.5 (C4/5), 35.1 (C3),
28.3 (-SnCH2CH2-, JSn,C ) 30 Hz), 26.9 (-CH2CH3, JSn,C ) 75.5
Hz), 19.5 (C2), 19.5 (-SnCH2CH2-), 13.3 (-CH3), -0.3 (NCCH3).
119Sn NMR (93.181 MHz, C6D6, 283 K): δ 54.4.
(3-Cyclopentenemethyldiethylplumbyl)acetonitrilium (10i). Sin-
gle crystals suitable for X-ray crystallography are grown in dry pentane
in a sealed ampule at 40 °C for several weeks (solvothermal conditions).
1H NMR (250.133 MHz, C6D6, 303 K): δ 5.47 (m, 2H, -CHdCH-),
1.30-2.50 (4m, (-CH2)2CHCH2PbCH2-), 1.75 (t, -CH3, 3JH,H ) 7.6
Hz). 13C NMR (62.860 MHz, C6D6, 303 K): δ 131.7 (C6/7), 117.2
(NCCH3), 49.1 (C2), 40.9 (-CH2CH3), 35.6 (C3), 34.3 (C4/5), 12.0
(-CH3), -0.9 (NCCH3). 207Pb NMR (52.304 MHz, C6D6, 303 K): δ
598.
2
3
Triethylplumbylbenzenium Ion. Slow addition of tetraethyllead
(161.5 mg, 0.5 mmol) to a vigorously stirred solution of TPFPB (0.461
g, 0.5 mmol) in dry C6D6 (3 mL) results in evolution of ethene24 and
the disappearance of the red color of the solution. The reaction mixture
is allowed to separate into two layers, an upper colorless layer and a
lower light brown layer. The upper layer contains the byproduct
triphenylmethane and is removed. The lower phase is transferred via a
flexible Teflon pipe into an NMR tube. 1H NMR (250.133 MHz, C6D6,
303 K): δ 1.82 (t, 9H, -CH3, 3JPb,H ) 173.2 Hz, 3JH,H ) 7.7 Hz), 1.71
(m, 6H, -CH2-). 13C NMR (62.860 MHz, C6D6, 303 K): δ 46.9 (br,
(3-Cyclopentenemethyldi-n-butylsilyl)acetonitrilium (10f). 1H
NMR (250.133 MHz, C6D6, 303 K): δ 5.55 (s, 2H, -CHdCH-),
0.50-2.50 (br m, -(CH2)2CHCH2SiCH2CH2-), 0.56 (m, 4H, -CH2-
3
CH3), 0.83 (t, 6H, -CH3, JH,H ) 7.3 Hz). 13C NMR (62.860 MHz,
C6D6, 303 K): δ 130.7 (C6/7), 41.0 (C4/5), 32.0 (C3), 25.4
(-SiCH2CH2-), 23.8 (-SiCH2CH2-), 18.7 (C2), 12.5 (-CH3), 10.9
(-CH2CH3), -0.9 (NCCH3). 29Si NMR (79.460 MHz, C6D6, 283 K):
δ 37.5.
2
-CH2-), 11.1 (-CH3, JPb,C ) 38.4 Hz). 207Pb NMR (52.304 MHz,
C6D6, 303 K): δ 1432 (10i).
Acknowledgment. This work was supported by the German
Israeli Foundation for Scientific Research and Development
(GIF) and by the DFG (scholarship to T.M.).
(3-Cyclopentenemethyl-di-n-butylgermyl)acetonitrilium (10g).
1H NMR (250.133 MHz, C6D6, 303 K): δ 5.42 (s, 2H, -CHdCH-),
2.25 (br m, 3H, -CH2CHCH2-), 1.59 (d, -CH2CHCH2-, 2JH,H ) 13.9
Hz), 0.80-1.30 (4m, 14H, -CHCH2GeCH2CH2CH2CH3), 0.81 (t, 6H,
Supporting Information Available: Table of computed
absolute energies, figure of δtheor(13C) vs δ(13C)(exptl), crystal-
lographic data of compound 10i, NMR spectra of cations
4a,e-h, and Cartesian coordinates for the optimized geometries
of cations 4a-f, 6a-d, 11a-d, and 12a-d (PDF) and X-ray
crystallographic data (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
3
-CH3, JH,H ) 7.1 Hz). 13C NMR (62.860 MHz, C6D6, 303 K): δ
130.6 (C6/7), 118.7 (br, NCCH3), 40.2 (C4/5), 33.2 (C3), 25.7, 25.1,
17.5 (-CH2CH2CH2CH3), 24.6 (C2), 12.6 (-CH3), -0.8 (NCCH3).
(3-Cyclopentenemethyl-di-n-butylstannyl)acetonitrilium (10h).
1H NMR (250.133 MHz, C6D6, 303 K): δ 5.44 (s, 1H, -CHdCH-),
5.69 (s, 1H, -CHdCH-), 2.72-2.81 (2m, 1H, -CH2CHCH2-), 1.94
(m, 2H, -CH2CHCH2-), 1.36 (m, -CH2CHCH2-), 0.80-1.30 (m,
3
20H, -CH2CH2CH2-), 0.87 (t, 6H, -CH3, JH,H ) 3.6 Hz), 0.77-
JA021234G
9
2168 J. AM. CHEM. SOC. VOL. 125, NO. 8, 2003