The stable pentamethylcyclopentadienyl cation

Article abstract of DOI:10.1002/1521-3773(20020415)41:8<1429::AID-ANIE1429>3.0.CO;2-J

More than 100 years after the cyclopentadienyl anion appeared in the literature, the first cyclopentadienyl cation, the pentamethyl derivative C₅Me₅⁺, has now been prepared in a single step as the tetrakis(pentafluoropheny

Full text of DOI:10.1002/1521-3773(20020415)41:8<1429::AID-ANIE1429>3.0.CO;2-J

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[9] J. Manna, K. D. John, M. D. Hopkins, Adv. Organomet. Chem. 1995,
38, 79.
rule for p electrons, along with benzene (2) and the tropylium
cation (3).[2 4] In contrast, the cyclopentadienyl cation, C5H5
‡
[10] X-raystructure analysis of 6: Crystal grown from CH2Cl2/hexane;
crystal dimensions 0.33  0.33  0.10 mm3; C18H24O2P2W (Mr ˆ
518.16); monoclinic, space group P21/c, a ˆ 9.256(2), b ˆ 12.571(2),
c ˆ 17.942(6) ä, b ˆ 103.15(4)8, Vˆ 2032.9(9) ä3, Z ˆ 4, 1 ˆ
1.693 MgmÀ3, m(MoKa) ˆ 5.845 mmÀ1, l ˆ 0.71073 ä (MoKa radiation,
Nonius CAD4 single crystal diffractometer). Data collection at
293(2) K, w-2q scans, 2.0 < q < 25.97. A total of 3984 unique reflec-
tions were collected, of which 2645 were observed with I > 2s(I).
Crystal structure was solved by direct methods (SHELXS-97).[11a] An
anisotropic least-squares refinement was carried out with SHELXL-
97.[11b] The final cycle of full-matrix least-squares refinement based on
3984 reflections and 213 parameters converged to a final value of R1
(F 2 > 2s(F 2)) ˆ 0.0405, wR2 (F 2 > 2s(F 2)) ˆ 0.1071. Residual electron
density0.76/ À 1.30 eäÀ3. CCDC-1777731 contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (‡44)1223-336-033; or deposit@ccdc.cam.
ac.uk).
‡
(4, Cp ) has languished experimentallyas an elusive charge
7]
variant. It shares with cyclobutadiene[5 (5) and the cyclo-
heptatrienyl anion[8] (6), among others, the characteristics of
possessing 4n p electrons and thus potentiallybeing antiar-
omatic.
No simple cyclopentadienyl cation has been structurally
characterized. Several studies have reported electron spin
resonance (ESR) spectra,[9] and some studies have implied the
species as an intermediate.[10] These investigations variously
looked at the pentachloro, pentaphenyl, and pentamethyl
derivatives as well as the unsubstituted molecule. In general,
the observed cations were relativelyunstable, possessed
triplet multiplicity, and needed protective environment.
We now report the preparation of the pentamethylcyclo-
[11] a) G. M. Sheldrick, SHELXL-97: Program for the Refinement of
Crystal Structures, University of Gˆttingen, Germany, 1997; b) G. M.
Sheldrick, SHELXL-97: Program for the Solution of Crystal Struc-
tures, Universityof Gˆttingen, Germany, 1997.
[12] M. C. Lukehart, Fundamental Transition Metal Organometallic Chem-
istry, Brooks/Cole, Belmont CA, 1985, p. 77.
[13] G. Consiglio, R. M. Waymouth, Chem. Rev. 1989, 89, 257.
[14] For proposed migrations of this kind, see: D. Cui, N. Hashimoto, S.
Ikeda, Y. Sato, J. Org. Chem. 1995, 60, 5752.
[15] H. tom Dieck, H. Friedel, J. Organomet. Chem. 1968, 14, 375.
[16] N. S. Nudelman, C. Carro, Synlett 1999, 12, 1942.
‡
‡
pentadienyl cation C5Me5 (Cp* ) as the tetrakis(pentafluoro-
phenyl)borate (TPFPBÀ) salt. The crystalline material ob-
tained is stable for weeks at room temperature and can be left
exposed to the open atmosphere without serious decomposi-
tion. We have solved the X-raystructure and obtained NMR
spectra in the solid state and in solution. This material maybe
obtained in one step at room temperature byhdyride
abstraction from commercially available pentamethylcyclo-
[17] M. Ishikura, M. Kamada, I. Oda, T. Ohta, M. Terashima, J.
Heterocyclic. Chem. 1987, 24, 377.
[18] M. Almeida, M. Beller, G. Wang, J. B‰ckvall, Chem. Eur. J. 1996, 2,
1533.
[19] N. B. Lorrette, W. L. Howard, J. Org. Chem. 1961, 26, 4857.
‡
pentadiene [Eq. (1)].[11, 12] The trityl cation (Ph3C , with the
The Stable Pentamethylcyclopentadienyl
Cation**
Joseph B. Lambert,* Lijun Lin, and VitalyRassolov
The cyclopentadienyl anion, C5H5 (1, CpÀ), was first
À
prepared one hundred years ago.[1] In due course it became a
anion TPFPBÀ) is converted into triphenylmethane, whereas
pentamethylcyclopentadiene is converted into the corre-
sponding cation. Crystals of the product began forming
immediatelyand spontaneousl.y The overall yield is nearly
quantitative, and yields of crystals have reached 40%. The
reaction has been carried out in several solvents (benzene,
toluene, dichloromethane) and with alternative (silyl) leaving
groups.
classic example of aromaticity, exemplifying the H¸ckel 4n‡2
[*] Prof. J. B. Lambert, L. Lin
Department of Chemistry
Northwestern University
Evanston, IL 60208-3113 (USA)
Fax : (‡1)847-491-7713
E-mail: jlambert@northwestern.edu
Prof. V. Rassolov
Department of Chemistry
Universityof South Carolina
Columbia, SC 29208 (USA)
The remarkable stabilityof this material maybe attributed
to a number of factors. First, the methyl groups clearly are
critical, as analogous experiments with the unsubstituted
system were unsuccessful. The methyl group serves as an
electron donor and also mayplaya steric role, as described
subsequently. The second key factor is the choice of the
counteranion. Manyanions of low nucleophilicitynow are
[**] This work was supported bythe U.S. National Science Foundation
(Grant No. CHE-0091162). We thank Charlotte L. Stern for perform-
ing the crystal-structure analysis, Yuyang Wu for assistance in
obtaining solid-state NMR data, Min Zhao and Stoyan Smoukov for
providing ESR data, Alice L. Rodriguez for molecular modeling
graphics, and John A. Pople and Mark A. Ratner for important
discussions.
À
available.[13] We previouslyutilized TPFPB in the prepara-
tion of the first silylium cation[14] and employthis same anion
in the present study. Finally, choice of solvent also is critical, as
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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solvents with higher nucleophilicitythan those of arenes or
halocarbons react with the cation.
longer than a single bond between unconjugated sp2 carbon
atoms (typically ca. 1.48 ä).
Several valence bond structures maybe considered. The
fullydelocalized form 8, which would suffer from antiaroma-
ticity, is eliminated because of the observed irregularity of the
pentagon. Structure 9 requires a localized cation at C1 and
The crystal structure[15] of the cation with its anion is
depicted in Figure 1. As the edge view (Figure 2a) shows, the
molecule is modestlynonplanar (the five internal ring
dihedral angles are 4.1(6), 6.6(6), 4.6(5), 6.9(6), and 0.5(5)8).
Three carbon atoms (C1, C2, C3) and their attached methyl
groups form a nearlyplanar substructure closelyresembling a
1,2,3-trimethylallyl group. Atoms C4 and C5, respectively, are
below and above this plane, and their attached methyl groups
protrude appreciablyfrom the plane, the CH 3-C4-C5-CH3
dihedral angle being 106.9(6)8.
[3]
À
localized double bonds, whereas the observed C3 C4 and
À
C1 C5 lengths are closer to single bonds. Structure 10 best
describes the C1-C2-C3 (allyl-like) portion. The observed
À
C4 C5 bond is closer to a single bond and is poorlydepicted
byall the structures.
The best reported calculations on the pentamethyl deriv-
ative[16] indicate strong bond alternation resembling 9, with a
À
nearlysingle C4 C5 bond (1.56 ä) but no pyramidalization at
these carbon atoms. Our own calculations on the pentamethyl
derivative[17,18] at higher level, however, favor structure 10,
with lengths of 1.38 and 1.39 ä (observed 1.406(6) and
À
À
1.394(6) ä) for the bonds (C1 C2, C2 C3) in the allyl
portion, 1.52 and 1.53 ä (observed 1.500(6) and 1.481(6) ä)
À
À
for the next adjacent bonds (C1 C5 and C3 C4), and 1.36 ä
À
(observed 1.510(6) ä) for the bond (C4 C5) opposite the allyl
portion. All calculations[16,17] have indicated a planar struc-
ture. Thus the major differences between our calculations and
À
observations are an observed lengthening of C4 C5 by0.15 ä
and pyramidalization of C4 and C5.[19]
The solid-state 13C NMR spectrum reflects the nonequiva-
lence of the five ring carbon atoms in the crystal. As a result,
all ten carbon atoms in the molecule give distinct or nearly
distinct peaks. There are five methyl resonances in the region
d ˆ 7 22. The cationic region contains two equal peaks at
d ˆ 243 and 250, corresponding to C1 and C3. The central
carbon atom (C2) in the allyl-like fragment gives a sharp
signal at d ˆ 153, indicative of the absence of charge at the
nodal position of the allyl group. The resonances for C1-C2-
C3 closely resemble those for the 1,3-dimethylallyl cation
(d ˆ 236 and 147).[20] Finally, a peak at d ˆ 60 is intermediate
between the normal alkane and alkene regions and fits well
for the pyramidalized carbon atoms, C4 and C5. Higher
symmetry is indicated in solution, as the C1/C3 resonances
appear at the approximate average positions of those in the
solid.
The unsaturated region of the 13C NMR spectrum is
best attributed to the allyl fragment of 10 (two carbon
atoms bearing positive charge, plus one on the node).
Structure 9 has onlyone carbon atom bearing positive charge.
Thus NMR spectroscopic analysis confirms the conclusions
from the crystal structure that 10 best describes the pentam-
ethylcyclopentadienyl cation.
Figure 1. The crystal structure of pentamethylcyclopentadienyl tetrakis-
(pentafluorophenyl)borate. There is no covalent bonding between the
cation on the left and the anion on the right.
Figure 2. a) Edge view of the pentamethylcyclopentadienyl cation, sight-
ing down the C4-C5 bond on the left. The nearlyplanar allyl portion is on
the right. b) Top view.
The view from the top (Figure 2b) shows an irregular
pentagon. The irregularityis quantified in structure 7 (the
methyl groups are omitted from the following structures for
clarity). The bond lengths in the allyl-like portion across the
To probe the spin multiplicityof this cation, we examined
pure crystals and diluted powder by ESR spectroscopy. No
significant signals were obtained at either 77 or 298 K. A
triplet state thus is unlikely. The sharpness of the NMR signals
both in the solid and in solution stronglysupports the absence
of unpaired electron spins, which should have broadened the
signals. The pentamethylcyclopentadienyl cation therefore
appears to be a stable singlet molecule. Our calculations[17]
found the planar singlet state of the pentamethyl derivative to
top of the drawing are relativelyshort, that is, approximately
1.40 ä (benzene-like), as expected for charge delocalization.
The other bond lengths are somewhat shorter than a single
bond between sp3 carbon atoms (typically ca. 1.54 ä) and
À1
be lower by1.5 kcalmol than the symmetrical (D5h) triplet
state.
1430
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[11] This is an example of the Bartlett Condon Schneider reaction,
wherebyhdyride is transferred intermolecularlyfrom one carbon
atom to another and charge is shifted between molecules. The
equilibrium is driven to the right either because the products are more
stable than the starting materials or because the cationic product is
removed byprecipitation.
[12] P. D. Bartlett, F. E. Condon, A. Schneider, J. Am. Chem. Soc. 1944, 66,
1531 1539.
[13] S. H. Strauss, Chem. Rev. 1993, 93, 927 942.
Crystallography, NMR spectroscopy, and theory support a
structure most closelyresembling 10. The pyramidality of C4
and C5 and the Me-C4-C5-Me dihedral angle in the crystal
(1078), however, indicate significant deviations from a double
À
bond. The estimated barrier to torsion around the C4 C5
bond is quite small, perhaps under 100 calmolÀ1. Thus in both
calculation and observation, this formallydouble bond is very
unusual. The pyramidality seems to imply a fourth coordina-
tion, yet there is no fourth atom within the sum of the ionic
radii. The closest atom to C4 is F6 (3.092 ä) from the
counteranion (Figure 1), and the closest atom to C5 is F14
(3.394 ä) from the second anion in the asymmetric unit.
Fluorine atoms at 3.1 3.4 ä distance can provide onlya very
small perturbation to pyramidalize C4 and C5 and lengthen
[14] J. B. Lambert, Y. Zhao, H. Wu, W. C. Tse, B. Kuhlmann, J. Am. Chem.
Soc. 1999, 121, 5001 5008.
[15] Measurements were made on a Bruker SMART-NT CCD area
detector with graphite monochromated MoKa radiation. Cell constants
corresponded to a primitive cell with dimensions a ˆ 13.216(7), b ˆ
13.689(7), c ˆ 17.409(9) ä, Vˆ 3149.7(24) ä3, b ˆ 89.978(9)8. For
Z ˆ 4 and Mr ˆ 1628.54 for C34H15F20B, the calculated densityis
1.72 gcmÀ3. The systematic absences of h0l (l Æ 2n) and 0k0 (k Æ 2n)
uniquelydetermined the space group to be P21/c. Data were collected
at À120 Æ 18C to a maximum of 2q value of 46.78. Of the 20700
reflections collected, 4803 were unique (Rint ˆ 0.099). Equivalent
reflections were merged. A Gaussian face-indexed absorption correc-
tion was applied. The data were corrected for Lorentz and polar-
ization effects. The structure was solved with direct methods and
expanded byusing Fourier techniques. The non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included in idealized
positions but not refined. The final cycle of full-matrix least-squares
refinement on F 2 was based on 4569 observed reflections and 496
variable parameters and converged with an unweighted agreement
factor of R1 ˆ 0.042. The maximum and minimum peaks on the final
À
C4 C5. In the solid state, there is a fluorine atom on the anion
close to a hydrogen atom on each of the methyl groups on C4
and C5 (H12 ¥¥¥¥ F20 2.70 ä, H14 ¥¥¥¥ F18 2.50 ä). These
distances would be about 1 ä shorter if the ring substituents
moved into the plane of the ring. Crystal packing between the
anion and the cation pyramidalizes C4 and C5, a distortion
permitted bythe weak p bonding. These are noncovalent,
nonbonded interactions. The resulting deformations are a
tradeoff between coulombic attractions and
nonbonded repulsions.
In summary, we have observed that the
pentamethylcyclopentadienyl cation is a sta-
ble singlet with a largelylocalized electronic
difference Fourier map corresponded to 0.79 and À0.78 eÀ äÀ3
,
respectively. CCDC-178042 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ,
UK; fax: (‡44)1223-336-033; or deposit@ccdc.cam.ac.uk).
structure best described by 11.
[16] H. Jiao, P. von R. Schleyer, Y. Mo, M. A. McAllister, T. T. Tidwell, J.
Am Chem. Soc. 1997, 119, 7075 7083; B. Reindl, P. von R. Schleyer, J.
Comput. Chem. 1998, 19, 1402 1420.
Received: January3, 2002 [Z18473]
[17] Geometryoptimization and energycalculations were carried out at
the CASSCF(4.5)/6-31G* level of theorywith the GAMES pro-
gram.[18] Inclusion of nondynamic correlation effects allows mixing of
electronic configurations for 9 and 10 and favors geometry 10 with a
singlet ground state. The MP2 calculations used byprevious authors [15]
[1] J. Thiele, Chem. Ber. 1901, 34, 68 71.
¬
[2] No single localized (™Kekule∫) structure is correct for 4n ‡ 2 mole-
cules, so the circle within the ring is used to depict electron
¬
delocalization. The 4n molecules are drawn as localized, Kekule
for geometryoptimization are based on
a single determinant
structures.
reference and, therefore, do not permit such mixing. These calcu-
lations favor molecular structure 9 rather than 10 and the triplet rather
than the singlet.
[3] P. J. Garratt, Aromaticity, Wiley, New York, 1986; V. I. Minkin, M. N.
Glukhovtsev, B. Y. Simkin, Aromaticity and Antiaromaticity, Wiley,
New York, 1994.
[18] M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S.
Gordon, J. H. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. J. Su,
T. L. Windus, M. Dupuis, J. A. Montgomery, J. Comput. Chem. 1993,
14, 1347 1363.
[4] R. Breslow, Acc. Chem. Res. 1973, 12, 393 398; K. B. Wiberg, Chem.
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[5] L. Watts, J. D. Fitzpatrick, R. Pettit, J. Am. Chem. Soc. 1965, 87, 3253
3254.
[6] G. Maier, H.-O. Kalinowski, K. Euler, Angew. Chem. 1982, 94, 706;
Angew. Chem. Int. Ed. Engl. 1982, 21, 693 694; H. Irngartinger, M.
Nixdorf, Angew. Chem. 1983, 95, 415; Angew. Chem. Int. Ed. Engl.
1983, 22, 403 404.
[7] M. Orendt, B. R. Arnold, J. G. Radziszewski, J. C. Facelli, K. D.
Malsch, H. Strub, D. M. Grant, J. Michl, J. Am. Chem. Soc. 1988, 110,
2648 2650; D. J. Cram, M. E. Tanner, R. Thomas, Angew. Chem.
1991, 103, 1048; Angew. Chem. Int. Ed. Engl. 1991, 30, 1024 1027.
[8] H. J. Dauben, Jr., M. R. Rifi, J. Am. Chem. Soc. 1963, 85, 3041 3042;
W. v. E. Doering, P. P. Gaspar, J. Am. Chem. Soc. 1963, 85, 3043; S. W.
Staley, A. W. Orvedal, J. Am. Chem. Soc. 1974, 96, 1618 1620.
[9] R. Breslow, H. W. Chang, R. Hill, E. Wasserman, J. Am. Chem. Soc.
1967, 89, 1112 1119; M. Saunders, R. Berger, A. Jaffe, J. M. McBride,
J. O×Neill, R. Breslow, J. M. Hoffman, Jr., C. Perchonock, E. Wasser-
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[10] P. J¸tzi, A. Mix, Chem. Ber. 1992, 125, 951 954; G. A. Dushenko, I. E.
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A. D. Allen, M. Sumonja, T. T. Tidwell, J. Am. Chem. Soc. 1997, 119,
2371 2375.
À
[19] The C4 C5 bond is clearlya double bond in calculation and must be
considered formallyto be a double bond in observation to account for
all the p electrons. Because of pyramidalization, however, its length is
closer to that of a single bond.
[20] G. A. Olah, P. R. Clifford, Y. Halpern, R. G. Johanson, J. Am. Chem.
Soc. 1971, 93, 4219 4222.
Angew. Chem. Int. Ed. 2002, 41, No. 8
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