1718
J. Am. Chem. Soc. 1998, 120, 1718-1723
Ion Pair Properties of Lithium and Cesium Salts of Carbazole1
Alessandro Abbotto, Andrew Streitwieser,* Manolis Stratakis, and James A. Krom
Contribution from the Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460
ReceiVed August 13, 1997
Abstract: The lithium salt of carbazole, LiCb, is monomeric in THF in the concentration range (1-5) × 10-3
M. It is a contact ion pair (CIP) with an effective ion pair pK of 13.48 on a scale where fluorenyllithium
solvent separated ion pairs (SSIP) are 22.90 (per hydrogen). The cesium salt, CsCb, is a mixture of monomer
and dimer with K2 ) 300 M-1. The monomer forms a 1:1 complex, CsCb‚CbH, with Kc ) 62 M-1. The ion
pair pK of monomeric CsCb is 19.24.
Introduction
t-BuOLi/t-BuOH in THF.8 The X-ray structures of cesium and
potassium carbazide in the presence of a donating ligand were
also obtained; in both cases a dimeric aggregate was found.9
Finally, the acidity of carbazole has been reported in several
media: DMSO (pK ) 19.9,10 19.611), DME (pK ) 14.6 (Li),
18.6 (Cs)),12 and H2O (pK ) 21.1,13 17.06,14 15.1615).
The crystal structures do not relate to the state of aggregation
of the carbazole salts in solution and, in fact, information
relevant to the possible aggregation of carbazide ion pairs in
solution is lacking. We recently showed that the cesium salt
of diphenylamine has a greater tendency to aggregate than the
lithium salt and that the monomer and dimer of the cesium salt
have measurably different absorption spectra.16 In the present
study, we compare these properties for carbazole, a closely
related but more acidic amine. The aggregation states of the
lithium and cesium salts of carbazole (LiCb and CsCb,
respectively) have been studied in THF and the equilibrium
constants have been determined. We also report the corre-
sponding ion pair acidities of carbazole (CbH) for comparison
with previous results. These data further expand the lithium16,17
and cesium5,16,17b,18 ion pair acidity scales.
The anion of carbazole has been the subject of various
experimental and theoretical investigations over the past two
decades. It is well-known that in ethereal solvents the status
of carbanions and nitranions can be generally described in terms
of an equilibrium between contact ion pairs (CIP) and solvent-
separated ion pairs (SSIP), the actual position of the equilibrium
depending on many factors such as the anion structure, the
counterion, solvent, concentration, and temperature.2 Optical
absorption and emission experiments have shown that the
potassium, sodium, and lithium salts of carbazole, indole, and
4,5-iminophenanthrene exist entirely as CIP in tetrahydrofuran
(THF) and 1,2-dimethoxyethane (DME), whereas more polar
solvents such as hexamethylphosphoric triamide (HMPT) are
needed to observe SSIP.3 By comparison, the lithium salt of
the carbon analogue of carbazole, fluorene, was reported in 1965
by Hogen-Esch and Smid to be a mixture of CIP and SSIP in
THF solution.4 In contrast, the large cesium ion always prefers
the formation of CIP.5
1H, 13C, and Li NMR spectroscopy have also been used to
7
gain information concerning the ion pairing and structure of
the carbazide salts.3c,6 In particular, 7Li NMR spectroscopy has
proven to be a useful method to determine the σ-like or π-like
interaction between the cation and the anionic substrate in the
CIP. The results showed that the nitranions of carbazole and
indole are associated with the cation through their nitrogen σ
lone pairs, whereas fluorenyl anion involves several carbon sites
in coordination with lithium, thus forming a π-like complex.
Recently, the Schleyer group found that lithium carbazide
crystallizes as a dimer from a THF solution when n-BuLi is
used as a base,7 whereas a monomeric species is obtained from
Results
UV-Visible Absorption Spectra of the Anions. The
lithium and cesium salts of carbazole were obtained in THF
(8) Lambert, C.; Hampel, F.; Schleyer, P. v. R. Angew. Chem., Int. Ed.
Engl. 1992, 31, 1209.
(9) Gregory, K.; Bremer, M.; Schleyer, P. v. R.; Klusener, P. A. A.;
Brandsma, L. Angew. Chem., Int. Ed. Engl. 1989, 28, 1224.
(10) (a) Bordwell, F. G.; Drucker, G. E.; Fried, H. E. J. Org. Chem.
1981, 46, 632. (b) Bordwell, F. F. Acc. Chem. Res. 1988, 21, 456.
(11) Terekhova, M. I.; Petrov, E. S.; Rokhlina, E. M.; Kravtsov, D. N.;
Shatenshtein, A. I. Khim. Geterotsikl. Soedin. 1979, 1104.
(12) Petrov, E. S.; Terekhova, M. I.; Basmanova, V. M.; Shatenshtein,
A. I. Zh. Org. Khim. 1980, 16, 2457. The reported acidities are normalized
to the DMSO value of fluorene (22.9).
(1) Carbon Acidity. 98. For part 97 see: Streitwieser, A.; Schriver, G.
W. Heteroatom Chem. 1997, 8, 533-7.
(2) For a review, see: Ions and Ion Pairs in Organic Reactions; Szwarc,
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(16) Krom, J.; Petty, J. T.; Streitwieser, A. J. Am. Chem. Soc. 1993,
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(17) (a) Gronert, S.; Streitwieser, A. J. Am. Chem. Soc. 1986, 108, 7016.
(b) Kaufman, M. J.; Gronert, S.; Streitwieser, A. J. Am. Chem. Soc. 1988,
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(6) Cox, R. H. Can. J. Chem. 1971, 49, 1377.
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(18) (a) Streitwieser, A.; Bors, D. A.; Kaufman, M. J. J. Chem. Soc.,
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S0002-7863(97)02839-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/04/1998