2,6,10-Tris(dialkylamino)trioxatriangulenium ions. Synthesis, structure, and properties of exceptionally stable carbenium ions

Article abstract of DOI:10.1021/ja982550r

A general synthetic route to a novel type of triamino-substituted planar carbenium ions (5) is reported. The synthetic method is based on a facile and selective nucleophilic aromatic substitution on the tris(2,4,6- trimethoxyphenyl)carbenium ion (1) with

Full text of DOI:10.1021/ja982550r

J. Am. Chem. Soc. 1998, 120, 12255-12263
12255
2,6,10-Tris(dialkylamino)trioxatriangulenium Ions. Synthesis,
Structure, and Properties of Exceptionally Stable Carbenium Ions
Bo W. Laursen,* Frederik C. Krebs,† Merete F. Nielsen,‡ Klaus Bechgaard,
Jørn B. Christensen,‡ and Niels Harrit‡
Contribution from the Macromolecular Chemistry Group, Condensed Matter Physics and Chemistry
Department, Risø National Laboratory, DK-4000 Roskilde, Denmark
ReceiVed July 20, 1998
Abstract: A general synthetic route to a novel type of triamino-substituted planar carbenium ions (5) is reported.
The synthetic method is based on a facile and selective nucleophilic aromatic substitution on the tris(2,4,6-
trimethoxyphenyl)carbenium ion (1) with amines and gives access to a wide variety of more complex amino-
substituted carbenium ions. X-ray crystallography shows that the 2,6,10-tris(N-pyrrolidinyl)-4,8,12-
trioxatriangulenium ion (5b) is planar and forms segregated stacks of cations and PF6 anions in the solid
phase. The stability of the 2,6,10-tris(diethylamino)-4,8,12-trioxatriangulenium ion 5a is expressed as the
pKR+ value, which is determined in strongly basic nonaqueous solution on the basis of a new acidity function
C_. The pKR+ value of 5a is measured to be 19.7, which is 10 orders of magnitude higher than the values
found for the most stable carbenium ions previously reported. Electrochemical reduction of compound 5a
leads to rapid dimerization. Two consecutive one-electron oxidations are identified by cyclic voltammetry.
Introduction
Stabilized carbenium ions such as the triarylmethyl and
xanthyl cations are organic compounds of great scientific and
commercial importance. Many of them are used as textile dyes
despite their low lightfastness, whereas others serve as laser
dyes. Consequently, their photophysical properties have been
extensively studied.1
Crystal violet (CV) and the trioxatriangulenium2 cation
(TOTA) (Figure 1) are among the most stable carbenium ions.
The two carbenium ions show almost the same pKR+ value,
pKR+ ) 9.363 and 9.05,4 respectively. The pKR+ value
expresses the reactivity of the cation toward hydroxide ions.5
The high stability of the CV cation can be explained by the
strong electron-donating ability of the three dimethylamino
substituents, which by simple resonance delocalize the charge.
Good correlation between the electron-donor strength (σ+) of
the para substituents and the pKR+ value is found for substituted
triarylcarbenium ions.6-8 A similar correlation is found between
the 13C chemical shift values of the central carbon atoms, the
calculated charge density and σ+.9 Thus, among the triaryl-
Figure 1.
carbenium ions, the pKR+ value can be taken as a measure of
the charge density at the central carbon atom.
When comparing with other carbenium ions such as the tris-
(4-methoxyphenyl)carbenium ion (pKR+ ) 0.82), it becomes
obvious that the high stability of the TOTA cation does not
rely merely on the presence of the three oxygen atoms but also
on the complete planarity of the molecular framework10 which
allows substantial delocalization of the positive charge. The
reversible reaction with hydroxide ions (defining the pKR+ value)
becomes even further unfavorable due to destabilization of the
corresponding carbinol by internal strain when the central carbon
is forced into sp3 hybridization.11
We report a general synthetic scheme leading to the novel
2,6,10-tris(dialkylamino)-4,8,12-trioxatriangulenium system 5
and results from the investigations of its structure and physi-
cochemical properties.
The tris(dialkylamino)trioxatriangulenium system was de-
signed and prepared as part of our ongoing quest for highly
† Department of Chemistry, Technical University of Denmark, DK-2800
Lyngby, Denmark.
‡ Department of Chemistry-Symbion, University of Copenhagen,
Fruebjergvej 3, DK-2100 Copenhagen Ø, Denmark.
(1) Duxbury, D. F. Chem. ReV. 1993, 93, 381-433.
(2) We have chosen to use the trivial name trioxatriangulene in line with
the names used for other derivatives10,59-64 of the analogous hydrocarbon
triangulene.65 The systematic name of the TOTA system is 4,8,12-trioxa-
4,8,12,12c-tetrahydrodibenzo[cd,mn]pyrene.
(3) Goldacre, R. J.; Phillips, J. N. J. Chem. Soc. 1949, 1724-1732.
(4) Martin, J. C.; Smith, R. G. J. Am. Chem. Soc. 1964, 86, 2252-2256.
(5) Freedman, H. H. In Carbonium Ions; Olah, G. A., Schleyer, P. v. R.
Eds.; Wiley-Interscience: New York, 1973; Chapter 28.
(6) Deno, N. C.; Evans, W. L. J. Am. Chem. Soc. 1957, 79, 5804-5807.
(7) Ritchie, C. D.; Sager, W. F.; Lewis, E. S. J. Am. Chem. Soc. 1962,
84, 2349-2356.
(9) Ray, G. J.; Kurland, R. J.; Colter, A. K. Tetrahedron 1971, 27, 735-
752.
(10) Krebs, F. C.; Laursen, B. W.; Johansen, I.; Boubekeur, K.;
Bechgaard, K.; Jacobsen, C. S.; Faldt, A.; Thorup, N. Acta Crystallogr.,
Sect. B, in press.
(11) For a discussion of steric and strain effects in related systems, see
ref 5.
(8) Kulkarni, S. V.; Schure, R.; Filler, R. J. Am. Chem. Soc. 1973, 95,
1859-64.
10.1021/ja982550r CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/13/1998

12256 J. Am. Chem. Soc., Vol. 120, No. 47, 1998
Laursen et al.
Scheme 1a
Scheme 2a
-
a X ) Cl- or BF4
.
stable, planar carbenium ions with the potential to form
segregated stacks of cations and anions.
Results and Discussion
Synthesis. The synthetic procedures leading to trioxatrian-
gulenium systems are few in the literature. The only derivative
described is tri-tert-butyltrioxatriangulenium hexafluorophos-
phate,12 which was synthesized by the same method as originally
applied to the unsubstituted system,4 using 1,3-dimethoxy-5-
tert-butylbenzene as the starting material. Lofthagen et al.13-15
have prepared and used several derivatives of the nonionic 12c-
methylated triaminotrioxatriangulene system in their work with
macrocyclophanes.
The synthetic approach used in the present work is based on
a facile nucleophilic substitution of p-methoxy and p-chloro
substituents in triaryl carbenium ions.16-19 We have synthesized
the novel20 tris(2,4,6-trimethoxyphenyl)carbenium ion (1) (Scheme
1) by a method analogous to the one used by Martin and Smith
in their synthesis of tris(2,6-dimethoxyphenyl)carbinol.4 Com-
pound 1 is obtained in one step as the tetrafluoroborate or
chloride salt by addition of the corresponding acids directly to
the hydrolyzed reaction mixture containing the carbinol. This
one-step reaction was performed on a multigram scale, yielding
as much as 26 g of the crude 1 (96% yield), which was used
without further purification in the subsequent reactions.
The p-methoxy groups in compound 1 undergo aromatic
nucleophilic substitution (SNAr) with dialkylamines as outlined
in Scheme 2. By controlling the reaction conditions, it is
possible to obtain products in which one, two, or three of the
p-methoxy groups of compound 1 are substituted. During the
stepwise reaction, it is possible to replace the remaining methoxy
groups with a different dialkylamine without affecting the amino
group already introduced. This synthetic strategy permits the
preparation of a wide variety of amino-substituted carbenium
ions from one single precursor (1).
a a: R ) R′ ) R′′ ) Et. b: R ) R′ ) R′′ ) N-pyrrolidinyl. c: R
) Et, R′ ) R′′ ) N-morpholinyl. d: R ) R′ ) R′′ ) N-morpholinyl.
Two symmetrically amino-substituted carbenium ions were
obtained in one-pot reactions of compound 1 with a large excess
of diethylamine or pyrrolidine, leading to compounds 4a and
4b, respectively (Scheme 2). The observed decrease in reactiv-
ity in the subsequent SNAr reactions of 1 can be rationalized in
terms of the increasing delocalization of the positive charge as
the p-methoxy groups are exchanged by the stronger electron-
donating amino groups, whereby the electrophilicity of the
remaining unsubstituted para positions is reduced. The decreas-
ing order of reactivity observed when going from compound 1
to 3 (Scheme 2) makes selective synthesis of the partially
substituted compounds 2 and 3 possible. Thus, compound 2a
was obtained in 76% yield by the addition of 4 equiv of
diethylamine to 1 and quenching the reaction after only 5 min,
whereas compound 3a was obtained in 78% yield after 1 had
reacted with excess diethylamine for 20 h. Asymmetrically
substituted compounds can also be prepared. Thus, compound
4c was obtained when compound 2a was allowed to react with
a large excess of the strong nucleophile morpholine for 24 h.
The asymmetric triamino compound 4c was isolated in 85%
yield, and only traces of a tris(N-morpholinyl) compound 4d
were observed.21 These results clearly indicate that the intro-
duction of amino groups in compound 1 is practically irrevers-
ible under the reaction conditions applied.
Nucleophilic Substitution. The SNAr reaction with dial-
kylamines is carried out in polar solvents such as DMF or
acetonitrile at room temperature using the crude BF4 or Cl-
-
salts of compound 1. A significant decrease in reactivity
following each consecutive substitution is observed. Thus, to
substitute all three p-methoxy groups of compound 1, it is
necessary to use a large excess of amine and long reaction times.
(12) Faldt, A.; Krebs, F. C.; Thorup, N. J. Chem. Soc., Perkin Trans. 2
1997, 2219-2227.
(13) Lofthagen, M.; Siegel, J. S. J. Org. Chem. 1995, 60, 2885-2890.
(14) Lofthagen, M.; Siegel, J. S.; Chadha, R. J. Am. Chem. Soc. 1991,
113, 8785-8790.
(15) Lofthagen, M.; Siegel, J. S.; VernonClark, R.; Baldridge, K. K. J.
Org. Chem. 1992, 57, 61-69.
(16) Huszthy, P.; Lempert, K.; Simig, G.; Tama´s, J. J. Chem. Soc., Perkin
Trans. 2 1982, 1671-1674.
(17) Huszthy, P.; Lempert, K.; Simig, G.; Ve´key, K. J. Chem. Soc., Perkin
Trans. 1 1982, 3021-3025.
(18) Wada, M.; Watanabe, T.; Natsume, S.; Mishima, H.; Kirishima,
K.; Erabi, T. Bull. Chem. Soc. Jpn. 1995, 68, 3233-3240.
(19) Teruel, L.; Viadel, Ll.; Carilla, J.; Fajari, Ll.; Brillas, E.; Sane´, J.;
Rius, J.; Julia´, L. J. Org. Chem. 1996, 61, 6063-6066.
(20) During the preparation of this manuscript, Wada et al. published a
synthesis of the corresponding carbinol and its perchlorate salt; see ref 66.
The successful implementation of the SNAr reactions to these
carbenium ions, under conditions where a large excess of amine
(21) In the FABMS spectra, the intensity of the m/z ) 664 peak (4d) in
the raw product was less than 2% of the m/z ) 678 peak (4c).

Exceptionally Stable Carbenium Ions
J. Am. Chem. Soc., Vol. 120, No. 47, 1998 12257
Figure 2. ORTEP drawing of 5b-PF6, stereoview.
Scheme 3a
Scheme 4
a 5a: R1 ) R2 ) R3 ) NEt2. 5b: R1 ) R2 ) R3 ) N-pyrrolidinyl.
5c: R1 ) NEt2, R2 ) R3 ) N-morpholinyl.
position of compound 5 would offer an alternative route to these
compounds. The TOTA cation has been alkylated with alkyl-
lithium15 and alkyl copper12 reagents. In our hands, methylation
of compound 5a with methyllithium in THF gave the 12c-
methylated compound 6a in 80% yield (Scheme 4).
is present, rely on the presence of the six o-methoxy groups
that surround and shield the cationic center,22 thus preventing
a nucleophilic attack on the central carbon.
Ring Closure. The ring closure of compounds 4a-c to the
corresponding tris(dialkylamino)trioxatriangulenium ions 5a-c
was performed by heating with LiI in NMP (Scheme 3). The
use of LiI as a reagent for dealkylation of methylaryl ethers is
well known.23,24 However, to our knowledge, it has not
previously been used to promote the present type of ring closure.
LiI was selected after several attempts with other reagents. Thus,
attempts to perform the ring closure of 4a in molten pyridine
hydrochloride at 180 °C, in analogy with the preparation of
TOTA,4 resulted in a high degree of N-dealkylation along with
ring closure. The same result was obtained upon heating 4a in
polyphosphoric acid. The facile N-dealkylation of 4a and 5a
under these conditions is in accordance with the findings of
Chambers and Pearson.25 Also, a purely thermolytic, uncata-
lyzed ring closure26 was attempted by refluxing 4a in high-
boiling solvents such as DMSO and NMP. This procedure gave
the desired product (5a) but at a very low rate. Partially ring-
closed compounds were the main products even after prolonged
heating.
Molecular and Crystal Structure. All tris(dialkylamino)-
trioxatriangulenium salts investigated form needle-shaped orange
crystals. However, X-ray data of sufficient quality for structure
-
determination could only be obtained for the PF6 salt of the
tris(N-pyrrolidinyl) derivative (5b). Crystals of 5b-PF6 were
grown from dichloromethane/ethyl acetate solution by slow
evaporation. The structure was solved and refined in the space
group P2(1)/n with four molecules per unit cell. The molecular
structure of the cation shows the expected quasi-D3h symmetry
with the three nitrogens in the plane of the molecule (Figure
2). The configuration around the three nitrogens is planar, with
an average sum of the bond angles of 359.3(1)°. The length of
the bond connecting the nitrogens with the aromatic system is
only 1.344(7) Å, indicating a strong interaction and some double
bond character between the nitrogens and the trioxatriangule-
nium system. Thus, it is expected that a considerable amount
of the positive charge is localized on the three nitrogen atoms.
This charge distribution is also in accordance with the packing
of the ions, since the cations form segregated stacks surrounded
by stacks of hexafluorophosphate ions in a pseudohexagonal
pattern (Figure 3), that is, the anions are located in proximity
of the amino groups rather than the central carbon. The cations
are packed in a staggered arrangement with a repetition distance
of 8.39 Å and an interplanar distance of only 3.32 Å (Figure
4). The close packing of the cations further stresses the high
degree of charge delocalization. Comparable interplanar dis-
tances are found in the conducting fluoranthenyl radical cation
salts.27,28 The cations in 5b-PF6 are tilted 37.55° with respect
Alkylation. As previously mentioned, 12c-methylated de-
rivatives of compound 5 have been used in the preparation of
several macrocyclophanes. These compounds were synthesized
by methylation in the 12c position of the TOTA cation followed
by bromination and cine substitution.15 Alkylation at the 12c
(22) The X-ray structure of a similar carbenium ion is found in ref 67.
The low reactivity of the cationic center in analogous carbenium ions are
discussed in refs 66 and 68.
(23) Harrison, I. T. Chem. Commun. 1969, 616.
(24) Kende, A. S.; Rizzi, J. P. J. Am. Chem. Soc. 1981, 103, 4247-
4248.
(25) Chambers, R. A.; Pearson, D. E. J. Org. Chem. 1963, 28, 3144-
(27) Enkelmann, V.; Morra, B. S.; Kro¨hnke, C.; Wegner, G.; Heinze, J.
Chem. Phys. 1982, 66, 303-313.
(28) Kro¨hnke, C.; Enkelmann, V.; Wegner, G. Angew. Chem. 1980, 92,
941-942.
3147.
(26) Huszthy, P.; Lempert, K.; Simig, G. J. Chem. Soc., Perkin Trans.
2 1985, 1351-1354.

12258 J. Am. Chem. Soc., Vol. 120, No. 47, 1998
Laursen et al.
Figure 3. (a) View along the b-axis. For clarity the pyrrolidinyl groups
on one side of the stack are omitted, except in the top layer. (b)
Schematic representation of the pseudohexagonal packing of the stacks
of cations and anions. Arrows indicate the tilting direction.
Figure 5. Absorption spectra of 5a/7a in the DMSO/water/Bu4NOH
solvent system. Arrows indicate the change from 5a to 7a with
increasing mol % DMSO. An isosbestic point is observed at 296 nm,
indicating the absence of other equilibria.
Scheme 5
known as the HR function5). These studies, however, deal with
unstable carbenium ions generated in solutions more acidic than
water. As no investigations of the carbenium/carbinol equilib-
rium in strongly basic solvent systems have been reported, a
new acidity function had to be established. A solvent system
consisting of varying proportions of DMSO and water with a
constant concentration of tetrabutylammonium hydroxide (Bu4-
NOH) was chosen. This solvent system was also used by
Dolman and Stewart for the construction of an H_ acidity
function based on the deprotonation of substituted anilines and
diphenylamines.30 The H_ acidity function is an extension of
the pH scale into solutions more basic than can be obtained in
water. In the DMSO/water/Bu4NOH solvent system, the
basicity (the activity of the hydroxide ion as a proton acceptor)
is changing with the DMSO/water ratio, each ratio correspond-
ing to a specific H_ value.
Figure 4. Side view of a stack of planar carbenium ions (5b-PF6).
to the stacking axis (the b-axis). This causes a 2.55-Å
displacement of the central carbon atom relative to its nearest
neighbor (Figure 4). The packing pattern is in contrast to the
packing patterns of the TOTA salts previously investigated,10
where the anion is located near the central carbon atom. These
fundamental differences in the packing patterns illustrate the
importance of the amino groups in terms of charge distribution.
Stability of the Carbenium Ions. The affinity of the
carbenium ion toward hydroxide ions, expressed by the pKR+
value, is the most common measure of carbenium ion stability.5
The pKR+ parameter is defined in eq 1.
[R+]
The relative concentrations of 5a and the corresponding
carbinol 7a were measured in the DMSO/water/Bu4NOH system
by means of absorption spectroscopy (see Experimental Section).
Figure 5 shows the change in absorption of 5a as the molar
percentage of DMSO increases. At 56.5 mol % DMSO, the
cation is 50% converted to the carbinol.31 A logarithmic plot
of the carbinol/carbenium ratio (K) versus the H_ value in the
range of 40-70 mol % DMSO shows a linear relationship with
a slope of 1.31 (Figure 6). The linearity of the plot indicates
that the carbenium/carbinol equilibrium responds in the same
way to change in the DMSO mol % as do the amines, which
were used to construct the H_ function.30 A slope greater than
one indicates a higher susceptibility in the case of the carbenium/
pKR+ ) Hx + log
(1)
[ROH]
Hx is an acidity function characteristic for the carbenium/carbinol
equilibrium and the solvent system. In water, Hx is identical
to the normal pH scale. A pKR+ value of 9.05, as has been
reported for TOTA,4 implies that the carbenium ion is 50%
converted to its carbinol at pH ) 9.05.
To elucidate the degree of charge delocalization in the tris-
(dialkylamino)trioxatriangulenium ion (5) relative to related
compounds such as CV and TOTA, we decided to determine
the pKR+ value of 5a. However, observation of the equilibrium
between 5a and the corresponding carbinol 7a (Scheme 5) is
not possible in aqueous solution because the carbinol is not
formed in measurable quantities even in 2 M KOH.
(29) Deno, N. C.; Jaruzelski, J. J.; Schriesheim, A. J. Am. Chem. Soc.
1955, 77, 3044-3051.
(30) Dolman, D.; Stewart, R. Can. J. Chem. 1967, 45, 911-924.
(31) The carbinol (7a) was identified by NMR spectroscopy. Chemical
shift values are given in the Experimental Section.
There are several examples in the literature of pKR+ measure-
ments in nonaqueous solution by use of the C0 function29 (also

Exceptionally Stable Carbenium Ions
J. Am. Chem. Soc., Vol. 120, No. 47, 1998 12259
Figure 8. Third-order polynomium (‚‚‚) fitted to Dolman and Stewart’s
data (9). The C_ function is shown as a solid line. The position of log
K ) 0 for 5a/7a is indicated by O.
Figure 6. Linear fit to log K vs H_ for compound 5a.
Figure 7.
carbinol reaction than that observed for deprotonation of the
amines. To further investigate the relation between the carbe-
nium ion/carbinol equilibrium and the H_ function, the equi-
librium distribution between 9-(4-dimethylaminophenyl)-3,6-
bis(dimethylamino)xantenium ion (OCV, Figure 7) and its
carbinol was determined in the DMSO/water/Bu4NOH system.
In the range of 10-20 mol % DMSO, OCV shows the same
behavior as 5a with a slope of 1.30 for the linear plot of log K
vs H_. On the basis of these findings, we believe that this
relationship is general, at least up to 70 mol % DMSO. Using
the pKR+ value of OCV (determined in aqueous solution, pKR+
) 13.51), we have constructed a new acidity function C_32 to
be used in the determination of pKR+ values of highly stabilized
carbenium ions in the DMSO/water/Bu4NOH solvent system.
The C_ function was obtained by modifying Dolman and
Stewart’s H_ function, so that the slope of the straight-line plots
of log K vs mol % DMSO for 5a and OCV becomes unity (eq
2).
Figure 9. Absorption spectra of 5a-PF6 (2.85 × 10-6 M) in benzene
(s) and dichloromethane (- - -).
Applying the C_ function, the pKR+ value of 5a becomes
19.7, which is 10 orders of magnitude higher than those for the
most stable carbenium ions previously reported (CV3 and
TOTA4). It is 6 orders of magnitude higher than that for the
OCV ion, determined in this study.
It should be stressed that the validity of the C_ function is
based entirely on the assumption of proportionality between the
slopes of the H_ and C_ functions (eq 2). At the present time
only two compounds (5a and OCV) have been investigated and
found to support this hypothesis. Further validation of the C_
function requires that other carbenium ions with pKR+ values
in the range from 13 to 18 are synthesized and their pKR+
values are determined. This work is in progress.
dC_ ) 1.31 dH_
(2)
Electronic Absorption Spectra. The absorption spectrum
of compound 5a in dichloromethane (Figure 9) shows a very
intense absorption in the visible region (λmax ) 471 nm, ꢀ )
132 000 M-1 cm-1) with a shoulder at the high-energy side of
the band. The shape and intensity of this absorption resembles
the characteristic spectra of CV and related compounds.35
Relative to CV (λmax) 586 nm, ꢀ ) 115 000 M-1 cm-1),36 the
low-energy absorption of 5a is blue-shifted by approximately
4200 cm-1 and is more intense. These observations are in
agreement with previous studies of the relationship between
structure and excitation energy in triphenylmethane dyes. A
hypsochromic shift is expected as a consequence of the forced
The relationship between the two acidity functions H_ and C_
is illustrated in Figure 8. The higher susceptibility of the
carbenium/carbinol equilibrium relative to deprotonation is
analogous to the relationship between the C0 (HR) function and
the H0 acidity function.33,34 The deviations between C_ and
H_ are most likely related to the very different solvation of the
reactants and products in the two defining reactions (the
deprotonation reactions defining the H_ function and the
hydroxylation of carbenium ions defining the C_ function).
(32) The notation C_ was chosen since this acidity function expresses
the change in activities of HO- toward carbenium ions, just as the H_ acidity
function is a measure of the HO- activity toward acidic protons.
(33) Arnett, E. M.; Bushick, R. D. J. Am. Chem. Soc. 1964, 86, 1564-
1571.
(35) Lueck, H. B.; McHale, J. L.; Edwards, W. D. J. Am. Chem. Soc.
1992, 114, 2342-2348.
(36) Greve, D. R.; Schougaard, S. B.; Geisler, T.; Petersen, J. C.;
Bjørnholm, T. AdV. Mater. 1997, 9, 1113-1117.
(34) Taft, R. W. J. Am. Chem. Soc. 1960, 82, 2965-2966.

12260 J. Am. Chem. Soc., Vol. 120, No. 47, 1998
Laursen et al.
planarity of the molecule37,38 and the introduction of the electron-
donating substituents in the ortho positions of the phenyl
groups.39
propose that the radical formed by reduction of 5a dimerizes
in a fast process leading to the symmetrical center-to-center
dimer.46-48
In benzene, the absorption spectrum of 5a (Figure 9) is clearly
different because the two closely overlapping transitions in
dichloromethane become well resolved. This behavior is also
observed in the spectra of CV and related compounds,35,40-44
and has been the subject of much debate. The latest investiga-
tions35,41,44 provide strong evidence for an interpretation of the
phenomena in terms of the formation of a tight ion pair in
nonpolar solvents. In dichloromethane solution, the cation 5a
is solvated symmetrically and the molecule maintains its
fundamental D3h symmetry (D3 for CV). Therefore, the S1 state
is doubly degenerate. In nonpolar solvents such as benzene,
the formation of a tight ion pair may induce an electronic
asymmetry in the chromophore, leading to a splitting of the S1
state. If the anion in the ion pair is located in proximity of the
central carbon atom, no effect is expected, but if the anion is
located close to one of the three amino groups, as observed in
the solid state, a significant splitting of the S1 state is foreseen.35
Because electrostatic forces are decisive for the geometry of
the ion pairs, the pronounced splitting of the low-energy
absorption band of compound 5a in benzene strongly supports
the previous conclusion that the amino groups carry a substantial
part of the positive charge.
Electrochemistry. The electrochemical reduction and oxida-
tion of compound 5a was studied by cyclic voltammetry in
MeCN. Compound 5a shows two closely spaced one-electron
oxidations which at short time scales (scan rates above 100 V
s-1) become chemically reversible. The formal potentials for
the two oxidations were determined to be 1.30 and 1.37 V vs
SCE, respectively. The two consecutive oxidations correspond
to the formation of the dication radical and the trication, a
behavior resembling the reported two-electron oxidation of
crystal violet (CV) at 0.98 V vs SCE.45
The reduction of 5a proved to be chemically irreversible even
at a potential scan rate of 5000 V s-1. Thus, direct determination
of the formal potential for the reduction process was not
possible. Coulometry at constant current in MeCN under strictly
anaerobic conditions showed that the reduction was a clean 1
F process. The product obtained upon electrochemical reduction
was highly air sensitive and was reoxidized to the starting
material, 5a, on contact with the atmosphere. The reoxidation
process was followed by UV-vis spectroscopy, and complete
recovery of 5a was observed. The UV-vis spectrum of the
product of the electrochemical reduction showed no absorption
in the visible region, and the FABMS spectrum obtained using
a matrix containing trifluoroacetic acid (TFA) showed signals
corresponding to the mass of a protonated dimer and the mass
of a complex between a dimer and TFA, along with strong
signals corresponding to 5a. In accord with these results, we
Assuming that the dimerization is a simple second-order
reaction between two radicals,49 a lower limit of the second-
order rate constant for dimerization in the order of k ) 109 M-1
s-1 can be estimated50 from the absence of any reoxidation
current at 5000 V s-1 in the cyclic voltammogram of a 1 mM
solution of 5a in MeCN, that is, the rate of the dimerization
reaction is close to that of a diffusion-controlled process. On
the basis of the same mechanistic assumption, a value of k )
109 M-1 s-1 allows an estimate of the formal potential of the
reduction process to be -1.1 V vs SCE from the peak potential
measured at 1 V s-1 (see the Experimental Section).
The very fast reaction of the radical formed by reduction of
5a may be attributed to the planarity of 5a and the corresponding
radical which leaves the central carbon atom unshielded, in
contrast to the central carbon atoms in CV and other nonbridged
triarylcarbenium ions, which are shielded by the ortho hydro-
gens. An increase of the rate of reaction with water by a factor
of 104 is observed when comparing the unshielded TOTA cation
with CV.51
Conclusions
We have presented a straightforward synthetic procedure
leading to a novel class of highly stabilized carbenium ions 5.
The synthetic route is the first example of a selective, stepwise
introduction of amino substituents in the para positions of
triarylcarbenium ions, using the SNAr reaction. This strategy
may be used in the synthesis of a wide variety of complex
carbenium ions such as 2, 3, 4, and 5. It also opens an
alternative route to 12c-methylated derivatives of compound 5
(e.g., 6a).
The thermodynamic stability of the tris(diethylamino)tri-
oxatriangulenium ion (5a) has been evaluated in terms of the
pKR+ value, using a new acidity function C_ developed for the
present purpose. A pKR+ value of 19.7 has been measured for
5a, which is 10 orders of magnitude higher than the values for
the most stable carbenium ions previously reported.3,4 This
exceptional stability of the cation 5a is attributed to the favorable
conditions for charge delocalization to the amino groups
provided by the planar TOTA system. The pronounced delo-
calization of the positive charge is reflected in the X-ray
structure of the cation 5b and in the solvatochromy of 5a,
indicating that the effect is present in the solid state as well as
in solution. The features of the tris(dialkylamino)trioxatrian-
gulenium ions can be rationalized if one regards the system as
a hybrid between TOTA and CV.
Experimental Section
Synthetic Methods and Materials. All reagents used were standard
(37) Barker, C. C. In Steric Effects in Conjugated Systems; Gray, G. W.
Ed.; Butterworth Scientific Publications: London, 1958; pp 34-45.
(38) Barker, C. C.; Bride, M. H.; Stamp, A. J. Chem. Soc. 1959, 3957-
3963.
grade unless otherwise mentioned. Ether and THF were dried by
(46) Sabacky, M. J.; Johnson, C. S.; Smith, R. G.; Gutowsky, H. S.;
Martin, J. C. J. Am. Chem. Soc. 1967, 89, 2054-2058.
(47) Mu¨ller, E.; Moosmayer, A.; Rieker, A.; Scheffler, K. Tetrahedron
Lett. 1967, 3877-3880.
(48) Kessler, H.; Moosmayer, A.; Rieker, A. Tetrahedron 1969, 25, 287-
293.
(49) Detailed electrochemical studies of the mechanism of the dimer-
ization process was not pursued since the reproducibility of the voltammetric
signal was poor at both Pt and Hg electrodes, probably due to adsorption
of the dimeric product.
(39) Dewar, J. S. J. Chem. Soc. 1950, 2329-2334.
(40) a) Lewis, G. N.; Magel, T. T.; Lipkin, D. J. Am. Chem. Soc. 1942,
64, 1774-1782. (b) Lewis, G. N.; Bigeleisen, J. J. Am. Chem. Soc. 1943,
65, 2102-2106.
(41) Maruyama, Y.; Ishikawa, M.; Satozono, H. J. Am. Chem. Soc. 1996,
118, 6257-6263.
(42) Adam, F. C.; Simpson, W. T. J. Mol. Spectrosc. 1959, 3, 363-
380.
(43) Korppi-Tommola, J.; Kolehmainen, E.; Salo, E.; Yip, R. W. Chem.
Phys. Lett. 1984, 104, 373-377.
(50) From the magnitude of the dimensionless rate constant λ ) kC°RT/
(nFν) necessary to obtain a completely irreversible voltammogram in a
digital simulation of that mechanism.
(51) Diffenbach, R. A.; Sano, K.; Taft, R. W. J. Am. Chem. Soc. 1966,
88, 4747-4749.
(44) Korppi-Tommola, J.; Yip, R. W. Can. J. Chem. 1981, 59, 191-
194.
(45) Neˆmcova´, I.; Neˆmec, I. Chem. ZVesti. 1972, 26, 115-125.

Exceptionally Stable Carbenium Ions
J. Am. Chem. Soc., Vol. 120, No. 47, 1998 12261
distillation from Na/benzophenone under nitrogen. Benzene was dried
by standing over Na. DMF was purified by distillation from CaH2
under vacuum. NMR spectra were recorded on a 250 or 400 MHz
spectrometer. FABMS spectra were obtained using 1,4-dicyanobutane
as matrix. UV-vis spectra were obtained with a Perkin-Elmer Lambda
16 spectrophotometer. Melting points were determined by DSC on a
Perkin-Elmer DSC-4.
Bis(4-diethylamino-2,6-dimethoxyphenyl)-2,4,6-trimethoxyphen-
ylcarbenium Hexafluorophosphate (3a-PF6). Crude 1-BF4 (2.0 g,
3.3 mmol) was dissolved in acetonitrile (20 mL), and diethylamine
(15 mL, 144 mmol) was added. After 20 h at room temperature, the
excess diethylamine and solvent were removed in vacuo. The dark
blue solid was dissolved in ethanol (30 mL), and aqueous KPF6 solution
(100 mL, 0.2 M) was added. The precipitate was filtered off, washed
thoroughly with water, and dried over solid KOH. Recrystallization
from ethanol gave 1.90 g (78%) of dark blue crystals of the PF6 salt.
1H NMR (250 MHz, CDCl3): δ 6.09 (2H, s), 5.73 (4H, s), 3.87 (3H,
s), 3.6-3.5 (26H, m), 1.30 (12H, t, J ) 7.1 Hz). 13C NMR (250 MHz,
CDCl3): δ 164.62, 163.23, 160.35, 156.15, 155.05, 117.62, 117.39,
91.74, 88.94, 56.86, 56.42, 55.84, 45.96, 13.33. MS (FAB+): m/z 595
(M+). Anal. Calcd for C34H47N2O7PF6: C, 55.13; H, 6.35; N, 3.78.
Found (after chromatography, see above): C, 54.86; H, 6.43; N, 3.76.
UV-vis λmax (nm (log ꢀ)) (CH2Cl2): 650 (4.72), 550 (4.14) (sh), 425
(3.84), 380 (3.81).
Tris(4-diethylamino-2,6-dimethoxyphenyl)carbenium Hexafluo-
rophosphate (4a-PF6). Compound 1-BF4 (4.0 g, 6.7 mmol) was
dissolved in NMP (40 mL), and diethylamine (80 mL, 770 mmol) was
added. After 9 days at room temperature, the reaction mixture was
poured into aqueous KPF6 solution (300 mL, 0.2 M). The blue
precipitate was washed thoroughly with water and dried over solid
KOH. Recrystallization from ethanol gave 3.72 g (71%) of blue
needles. 1H NMR (250 MHz, CDCl3): δ 5.70 (6H, s), 3.50 (18H, s),
3.46 (12H, q, J ) 7.1 Hz), 1.23 (18H, t, J ) 7.1 Hz). 13C NMR (400
MHz, CDCl3): δ 163.20, 154.12, 153.83, 115.03, 88.52, 55.89, 45.05,
12.78. MS (FAB+): m/z 636 (M+). Anal. Calcd for C37H54N3O6PF6:
C, 56.73; H, 6.71; N, 5.34. Found (after chromatography, see above):
C, 56.84; H, 6.96; N, 5.37. UV-vis λmax (nm (log ꢀ)) (CH2Cl2): 643
(4.95), 340 (4.26) (sh), 310 (4.68), 262 (4.54).
Tris(4-diethylamino-2,6-dimethoxyphenyl)carbenium Chloride
(4a-Cl). Diethylamine (16 g, 220 mmol) was added to a solution of
crude 1-Cl (1.0 g, 1.8 mmol) dissolved in dry DMF (10 mL). After 4
days at room temperature, the excess diethylamine was removed in
vacuo. The remaining dark blue solution was adsorbed on silica (12
g) and washed thoroughly with ether. Purification by column chro-
matography on silica with ether/ethanol as eluent yielded 0.81 g (66%)
of compound 4a-Cl. The MS (FAB+), 1H NMR, and 13C NMR spectra
were identical with those of the PF6 salt. Anal. Calcd for C37H54N3O6-
Cl: C, 66.10; H, 8.10; N, 6.25. Found: C, 66.06; H, 8.14; N, 6.22.
Tris(4-pyrrolidinyl-2,6-dimethoxyphenyl)carbenium Tetrafluo-
roborate (4b-BF4). Crude 1-BF4 (2.0 g, 3.3 mmol) was dissolved in
acetonitrile (30 mL), and pyrrolidine (5 mL, 56 mmol) was added over
a period of 10 min. After 20 h at room temperature, the excess amine
and solvent were removed in vacuo. Recrystallization from ethanol
gave 1.83 g (77%) of blue needles. 1H NMR (250 MHz, CDCl3): δ
5.63 (6H, s), 3.55 (18H, s), 3.49 (12H, t, J ) 6.4 Hz), 2.10 (12H, s, J
) 6.4 Hz). 13C NMR (250 MHz, CDCl3): δ 163.50, 155.00, 153.75,
115.68, 89.42, 48.57, 25.67. MS (FAB+): m/z 630 (M+). Anal. Calcd
for C37H48N3O6BF4: C, 61.96; H, 6.74; N, 5.86. Found (after
chromatography, see above): C, 61.61; H, 6.85; N, 5.77.
4-Diethylamino-2,6-dimethoxyphenyl-bis(4-morpholinyl-2,6-tri-
methoxyphenyl)carbenium Tetrafluoroborate (4c-BF4). Compound
2a-BF4 (0.27 g, 0.42 mmol) was dissolved in dry DMF (7 mL), and
morpholine (7 mL, 80 mmol) was added. After 20 h at room
temperature, the reaction mixture was adsorbed on silica and washed
thoroughly with ether and dichloromethane before the blue compound
was washed out with dichloromethane/ethanol. The solvent was
removed in vacuo, and the blue solid was recrystallized from dichlo-
romethane by addition of ether to yield 270 mg (85%) of blue
microcrystals. 1H NMR (250 MHz, CDCl3): δ 5.94 (4H, s), 5.72 (2H,
s), 3.85 (8H, t, J ) 4.8 Hz), 3.62 (4H, q, J ) 7.2), 3.54 and 3.53 (18H,
two overlapping singlets), 3.31 (8H, t, J ) 4.8 Hz), 1.30 (6H, t, J )
7.1). 13C NMR (400 MHz, CDCl3): δ 165.63, 160.98, 157.89, 154.96,
154.81, 119.53, 116.30, 91.08, 88.73, 66.43, 56.22, 56.12, 47.82, 46.03,
12.94. MS (FAB+): m/z 664 (M+). Anal. Calcd for C37H50N3O8-
BF4: C, 59.13; H, 6.71; N, 5.59. Found (after chromatography, see
above): C, 58.89; H, 6.84; N, 5.54.
Elemental analysis was done at the University of Copenhagen,
Department of Chemistry, Elemental Analysis Laboratory, Univer-
sitetsparken 5, 2100 Copenhagen, Denmark. In general, standard
analysis of the carbenium salts with fluorinated anions yielded low
values in carbon. Acceptable values were obtained in most cases,
however, when a combustion catalyst (V2O5) was added. Still, for 2a-
BF4, 3a-PF6, 4a-PF6, 4b-BF4, and 4c-PF6, analysis of the recrystallized
compounds showed values in carbon, hydrogen, and nitrogen at only
93-98% of the calculated values. In each case, however, column
chromatography (silica, ether/ethanol) on small amounts produced (in
almost quantitative yields) samples showing the acceptable analytical
values cited below. No other fractions were observed from the column,
and the NMR and mass spectra were unaltered by the chromatographic
purification. Therefore, it is believed that the discrepancies were due
to the presence of small amounts of inorganic salts such as KPF6 in
the prechromatographed samples. Consequently, these were used
without further purification in the subsequent synthetic steps.
Tris(2,4,6-trimethoxyphenyl)carbenium Tetrafluoroborate (1-
BF4). A solution of phenyllithium was prepared by addition of
bromobenzene (24.5 g, 156 mmol) in dry ether (50 mL) to lithium
wire (2.30 g, 328 mmol) in dry ether (100 mL). 1,3,5-Trimethoxy-
benzene (25.1 g, 149 mmol) in dry benzene (100 mL) was added, and
the reaction mixture was stirred at room temperature under argon for
70 h. Diethyl carbonate (5.30 g, 45 mmol) in dry benzene (150 mL)
was added, and the reaction mixture was refluxed for 3 days. The
cooled reaction mixture was poured into NaOH solution (300 mL, 1
M). The phases were separated, and the water phase was extracted
with ether. The combined organic phases were dried over MgSO4 and
filtered, yielding a clear yellow solution. Addition of aqueous HBF4
solution (12 mL, 50%, approximately 100 mmol) resulted in immediate
precipitation of the deep blue carbenium salt. The dark purple
precipitate was filtered off, washed thoroughly with dry ether, and dried
over solid KOH to yield 26.0 g (96%) of the crude product.
Reprecipation by addition of water to an acetonitrile solution followed
by recrystallization from methanol gave the pure compound in 70%
overall yield. 1H NMR (250 MHz, CDCl3): δ 6.05 (6H, s), 3.99 (9H,
s), 3.59 (18H, s). 13C NMR (400 MHz, CDCl3): δ 169.70, 166.47,
163.93, 118.62, 91.53, 56.52, 56.36. MS (FAB+): m/z 513 (M+). UV-
vis λmax (nm (log ꢀ)) (CH2Cl2): 584 (4.55), 467 (3.98), 322 (3.68),
287 (4.02). Anal. Calcd for C28H33O9BF4: C, 56.01; H, 5.50.
Found: C, 55.69; H, 5.64.
Tris(2,4,6-trimethoxyphenyl)carbenium Chloride (1-Cl). This
compound was prepared analogously to 1-BF4 (starting with 15.0 g of
1,3,5-trimethoxybenzene). Instead of HBF4, gaseous HCl was intro-
duced through the hydrolyzed and dried reaction mixture. The dark
purple crystalline precipitate was filtered off, washed thoroughly with
dry ether, and dried over solid KOH to yield 10.1 g (66%) of the crude
product. The MS (FAB+), 1H NMR, and 13C NMR spectra were
identical with those of the BF4 salt.
4-Diethylamino-2,6-dimethoxyphenyl-bis(2,4,6-trimethoxyphen-
yl)carbenium Tetrafluoroborate (2a-BF4). Crude 1-BF4 (0.55 g, 0.92
mmol) was dissolved in acetonitrile (15 mL), and diethylamine (0.25
g, 3.4 mmol) was added. After 5 min, ether (150 mL) was added and
the reaction mixture was cooled on an ice bath. The precipitate was
filtered off and washed with ether, yielding 0.55 g (93%). Recrystal-
lization from methanol gave dark red crystals of compound 2a-BF4 in
76% overall yield. 1H NMR (250 MHz, CDCl3): δ 6.07 (4H, s), 5.80
(2H, s), 3.86 (6H, s), 3.75 (4H, q, J ) 7.1 Hz), 3.59 (6H, s), 3.57
(12H, s), 1.38 (6H, t, J ) 7.1 Hz). 13C NMR (250 MHz, CDCl3): δ
167.38, 163.31, 161.04, 159.89, 155.05, 123.70, 117.03, 91.42, 89.94,
56.82, 56.75, 55.81, 47.39, 13.59. MS (FAB+): m/z 554 (M+). Anal.
Calcd for C31H40NO8BF4: C, 58.04; H, 6.24; N, 2.18. Found (after
chromatography, see above): C, 57.92; H, 6.33; N, 2.14. UV-vis λmax
(nm (log ꢀ)) (CH2Cl2): 558 (4.44), 428 (3.96) (sh), 350 (3.96).
Ring Closure. All the ring-closure reactions were performed by
heating the corresponding hexamethoxy compounds with 10 equiv of

12262 J. Am. Chem. Soc., Vol. 120, No. 47, 1998
Laursen et al.
anhydrous LiI in NMP (40-50 mL/g LiI) to 170 °C for 4 h. The
synthesis of 2,6,10-tris(diethylamino)-4,8,12-trioxatriangulenium hexa-
fluorophosphate (5a-PF6) is given as a representative example.
Table 1. Crystallographic Data for
2,6,10-Tris(N-pyrrolidinyl)-4,8,12-trioxatriangulenium
Hexafluorophosphate
2,6,10-Tris(diethylamino)-4,8,12-trioxatriangulenium Hexafluo-
rophosphate (5a-PF6). Compound 4a-Cl (0.50 g, 0.74 mmol) and
LiI (1.0 g, 7.5 mmol) were dissolved in NMP (50 mL) and heated to
170 °C for 4 h under nitrogen. The reaction mixture was allowed to
cool and was poured into 250 mL of aqueous KPF6 solution (0.2 M).
The precipitate was dissolved in dichloromethane, and the solution was
washed thoroughly with aqueous KPF6 solution (0.2 M). The solvent
was removed in vacuo, and the residue was reprecipitated twice by
addition of ether to a dichloromethane solution, yielding 0.4 g of a
light brown solid. This was purified on a short column of silica with
dichloromethane/ethyl acetate (2:1) as eluent. Recrystallization from
methanol gave 250 mg (52%) of orange needles. 1H NMR (250 MHz,
CDCl3): δ 6.35 (6H, s), 3.51 (12H, q, J ) 7.1 Hz), 1.25 (18H, t, J )
7.1 Hz). 13C NMR (400 MHz, CDCl3): δ 156.92, 154.89, 131.57,
95.11, 94.99, 46.66, 12.78. MS (FAB+): m/z 498 (M+). UV-vis λmax
(nm (log ꢀ)) (CH2Cl2): 471 (5.12), 447 (4.84) (sh), 311 (4.41), 302
(4.30) (sh). Anal. Calcd for C31H36N3O3PF6: C, 57.85; H, 5.64; N,
6.53. Found: C, 57.76; H, 5.61; N, 6.49.
compound
5b-PF6
C31H30O3N3PF6
637.55
monoclinic
P2(1)/n
4
14.546(3)
8.343(2)
22.604(5)
90
95.74(3)
90
2729.4(9)
1.552
0.15 × 0.025 × 0.020
Mo KR
0.183
formula
formula wt
crystal system
space group
Z
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
F, g cm3
crystal dimensions, mm
type of radiation
µ, mm-1
T, K
120(2)
27 660
2148
0.2444
number of reflections
unique reflections (with I > 2σ)
Rint
2,6,10-Tris(diethylamino)-4,8,12-trioxatriangulenium Tetrafluo-
roborate (5a-BF4). This compound was synthesized in the same way
as the PF6 salt by using a KBF4 solution in the workup procedure.
Recrystallization was performed from dichloromethane/ethyl acetate
R(F), Rw(F2) all data
0.0881, 0.2431
1
yielding 180 mg (41%) of red-orange needles. The MS (FAB+), H
6.36 (12H, q, J ) 7.0 Hz), 1.45 (3H, s), 1.20 (18H, t, J ) 7.0 Hz). 13
C
NMR, 13C NMR, and UV-vis spectra were identical with those of the
PF6 salt. Anal. Calcd for C31H36N3O6BF4: C, 63.60; H, 6.20; N, 7.18.
Found: C, 63.36; H, 6.28; N, 7.15.
NMR (250 MHz, CDCl3): δ 154.00, 148.52, 104.70, 45.21, 32.23,
22.71, 12.99. MS (FAB+): 514 m/z (M + 1). Anal. Calcd for
C32H39N3O3: C, 74.82; H,7.60; N, 8.18. Found: C, 74.77; H, 7.60;
N, 8.26.
2,6,10-Tris(diethylamino)-4,8,12-trioxatriangulene-12c-ol (7a). A
small amount of KOH powder and DMSO-d6 (0.5 mL) was placed in
an NMR tube, and argon was introduced for 30 min before 5a-PF6
(approximately 5 mg) was added. After a further 10 min of argon
purging, the tube was sealed and the spectra of the now colorless
solution were recorded. 1H NMR (250 MHz, KOH/DMSO-d6): δ 6.23
(6H, s), 3.55 (12H, q, J ) Hz), 1.11 (18H, t, J ) Hz). 13C NMR (250
MHz, KOH/DMSO-d6): δ 154.16, 148.90, 103.04, 94.87, 48.02, 45.01,
13.18.
X-ray Crystallography. Crystals of compound 5b-PF6 were
mounted on a glass capillary using epoxy glue. Data were collected
on a Siemens SMART platform diffractometer with a CCD area
sensitive detector. Absorption corrections were made using SADABS.52
Direct methods for the structure solution and full-matrix least-squares
refinements were used. Hydrogen atoms were included in calculated
positions. Programs used were SMART, SAINT, and SHELXTL from
Siemens.53,54 The structure was checked for overlooked symmetry using
MISSYM and for voids in PLATON.55 Crystallographic data are given
in Table 1. Atomic coordinates and further crystallographic details
have been deposited with the Cambridge Crystallographic Data Center,
University Chemical Laboratory, Lensfield Road, Cambridge CB2
1EW, U.K.
Determination of pKR+ Values. Stock solutions of 5a and the 9-(4-
dimethylaminophenyl)-3,6-di(dimethylamino)xantenium ion (OCV) in
DMSO were made from the PF6 salts. OCV was synthesized by the
method described in ref 56. DMSO was purified by distillation from
CaH2 at 10 mmHg argon pressure. Dissolved oxygen was removed
from all stock solutions by purging with argon.
2,6,10-Tris(N-pyrrolidinyl)-4,8,12-trioxatriangulenium Hexafluo-
rophosphate (5b-PF6). The LiI-catalyzed ring closure was performed
using 4b-BF4 (300 mg, 0.42 mmol). The workup was performed
analogously to the synthesis of compound 5a. Instead of column
chromatography, a dichloromethane solution of the crude product was
flashed through 1 cm of silica which afterward was washed thoroughly
with dichloromethane/ether. After evaporation of the solvent, recrys-
tallization from dichloromethane gave 110 mg (41%) of red-orange
needles. In its pure form, the compound showed low solubility at room
temperature in common NMR solvents, and no 13C NMR spectra were
obtained. 1H NMR (250 MHz, pyridine-d6): δ 6.41 (6H, s), 3.22 (12H,
t, br), 1.73 (12H, t, br). MS (FAB+): m/z 492 (M+). UV-vis λmax
(nm (log ꢀ)) (CH2Cl2): 473 (5.13), 447 (4.83) (sh), 311 (4.43), 302
(4.32) (sh). Anal. Calcd for C31H30N3O3PF6: C, 58.40; H, 4.74; N,
6.59. Found: C, 58.06; H, 4.77; N, 6.44.
2-(Diethylamino)-6,10-bis(N-morpholinyl)-4,8,12-trioxatriangu-
lenium Hexafluorophosphate (5c-PF6). The LiI-catalyzed ring closure
was performed using 4c-BF4 (200 mg, 0.27 mmol). The workup was
performed analogously to the procedure used for compound 5b, except
for using dichloromethane/THF as eluent. Recrystallization from
dichloromethane/ethyl acetate yielded 40 mg (22%). In its pure form,
the compound showed low solubility at room temperature in common
NMR solvents, and no 13C NMR spectra were obtained. 1H NMR (250
MHz, pyridine-d6): δ 6.86 (4H, d, J ) 1.7 Hz), 6.55 (2H, s), 3.62
(8H, t, J ) 5.0 Hz), 3.56 (8H, t, J ) 5.0 Hz), 3.26 (4H, q, J ) 7.0 Hz),
0.96 (6H, t, J ) 7.0 Hz). MS (FAB+): m/z 526 (M+). UV-vis λmax
(nm (log ꢀ)) (CH2Cl2): 482 (4.95), 462 (4.85) (sh), 450 (4.83) (sh),
351 (3.3), 307 (4.21). Anal. Calcd for C31H32N3O5PF6: C, 55.44; H,
4.80; N, 6.26. Found: C, 55.48; H, 4.67; N, 6.21.
The pKR+ values of 5a and OCV were obtained from the relative
concentrations of the carbenium ions and the corresponding carbinols,
which were determined spectrophotometrically at wavelengths where
only the cationic compounds absorb. The total concentration of
2,6,10-Tris(diethylamino)-12c-methyl-4,8,12-trioxatriangulene (6a).
Methyllithium (5 mL, 1.5 M in ether, 7.5 mmol) was added to a
suspension of 5a (0.5 g, 0.78 mmol) in dry THF (80 mL). After 20 h
of stirring, the solution was quenched with dilute ammonium chloride
(50 mL). Ether (50 mL) was added, and the organic phase was
separated, washed with water, and dried over sodium sulfate. The
solvent was removed in vacuo, and the white/orange solid was
redissolved in ether and flashed through 1 cm of silica, which afterward
was washed thoroughly with ether. Removal of the solvent gave a
white compound which upon recrystallization from acetonitrile yielded
0.32 g (80%) of white/colorless crystals. Melting point (DSC) 20 °C/
min: 218-228 °C dec. 1H NMR (250 MHz, CDCl3): δ 6.26 (6H, s),
(52) Empirical absorption program (SADABS) written by George
Sheldrick for the Siemens SMART platform.
(53) Sheldrick, G. M. SHELXTL95; Siemens Analytical X-ray Instru-
ments, Inc.: Madison, WI, 1995.
(54) SMART and SAINT. Area-Detector Control and Integration Soft-
ware; Siemens Analytical X-ray Instruments, Inc.: Madison, WI, 1995.
(55) Spek, A. L. Acta Crystallogr. 1990, A46, C-31.
(56) Plaza, P.; Dai Hung, N.; Martin, M. M.; Meyer, Y. H.; Vogel, M.;
Rettig, W. Chem. Phys. 1992, 168, 365-373.

Exceptionally Stable Carbenium Ions
J. Am. Chem. Soc., Vol. 120, No. 47, 1998 12263
carbenium ion and carbinol was constant in all the measurements. The
equilibrium distribution K is defined in Equation 3.
Voltammetry of 5a (1 mM) was carried out in MeCN (0.1 M Bu4-
NPF6 or 0.1 M Me4NBF4) under nitrogen. Electrodes, cells, and
instrumentation were identical to what has been described previously,57
except that the working electrode was a Pt electrode, d ) 0.6 mm.
Ferrocene/ferrocenium was used as an external potential reference
(E°′(Fc/Fc+) ) 0.440 V vs SCE). The values of E°′ for the two
consecutive oxidations of 5a were determined as the midpoint potentials,
E°′(1) ) Ep(ox1) - Ep(red1) and E°′(2) ) Ep(ox2) - Ep(red2),
measured at a scan rate equal to 100 V s-1. The estimate of the formal
potential for the reduction of 5a was based on application of eq 7,
A0 - A
[ROH]
[R+]
K )
)
(3)
A
A0 is the absorbance of the pure cation at the wavelength chosen, and
A is the absorbance at the same wavelength in the current solution. In
all the solvent mixtures used, the solvatochromic effects on the
absorption spectra of the carbenium ions were investigated and found
to be negligible in relation to the pKR+ measurements. The pKR+ values
were derived from linear fits to plots of log K vs pH (in water) and C_
in the DMSO/water/Bu4NOH system. The H_ values of the individual
solutions were determined from linear fits to the data given in ref 30
for the given range of concentrations, that is, 10-20 mol % DMSO
for OCV and 40-70 mol % DMSO for 5a.
The pKR+ value of OCV was determined from the absorbance at
545 nm in a series of aqueous KCl/NaOH buffer solutions. A0 was
determined as the absorbance of OCV at 545 nm in 0.20 M KCl solution
with pH adjusted to 8. The pKR+ value of OCV was found to be 13.51.
The slope of the linear log K/pH plot was 0.98.
All the equilibrium measurements in the DMSO/water/Bu4NOH
system were performed with solutions containing 0.011 M Bu4NOH.
The molar fraction of DMSO in the individual solutions was calculated
by weighing the amount of water, Bu4NOH stock solution (in water),
DMSO, and stock solution of the carbenium salts in pure DMSO. The
accuracy of the method applied relative to the predefined H_ function
was tested by the determination of the pKa value of 4-chloro-2-
nitroaniline which was found to be 17.12, with a slope of 0.99 for the
linear log K/H_ plot. Dolman et al. report pKa ) 17.08 for this
compound.30
The equilibrium distribution of 5a/7a in the DMSO/water/Bu4NOH
system was determined in seven solutions containing from 43 to 70
mol % DMSO. The linear fit to log K vs H_ had a slope of 1.31 and
K ) 1 at 56.5 mol % DMSO (H_ ) 18.1). A0 was determined as the
absorbance of 5a at 480 nm in 50 mol % DMSO/water solution without
Bu4NOH.
RT
nF
1
-0.902 + ln
3
4 kC°RT
3 nFν
Ep - E°′ )
(7)
[
(
)]
which is valid for a simple irreversible dimerization of the primary
electrogenerated intermediate,58 from the reduction peak potential
measured at ν ) 1 V s-1 at C° ) 10-3 M and an estimated value of
the second-order rate constant, k, equal to 109 M-1 s-1
.
Coulometry was carried out in a divided cell containing ap-
proximately 30 mL catholyte, MeCN (0.1 M Me4NBF4), and 8.2 ×
10-5 mol 5a, using a platinum mesh cathode. The coulometry was
carried out using constant current, I ) 10 mA, under strictly anaerobic
conditions. The concentration of 5a during coulometry was followed
by determination of the height of the voltammetric reduction peak once
per minute (after interruption of the reduction and the stirring). The
decay in the concentration of 5a was strictly linear with charge
consumption, and linear regression gave n ) 0.97. By the end of the
coulometric experiment (95% conversion) a sample of the catholyte
was removed for UV-vis measurements of the reoxidation before the
solvent was evaporated under nitrogen and the residue stored in the
dark at -17 °C. This crude product was used in the FABMS
experiments described above.
Acknowledgment. We thank Dr. D. R. Greve, University
of Copenhagen, Chemistry Department, for supplying us with
a sample of OCV-PF6 and for fruitful discussions.
The equilibrium distribution of OCV and its carbinol in the DMSO/
water/Bu4NOH system was determined in five solutions containing from
10 to 20 mol % DMSO. The linear fit to log K vs H_ had a slope of
1.30 and K ) 1 at 12.1 mol % DMSO (H_ ) 13.42). A0 was
determined as the absorbance of OCV at 548 nm in 10 mol % DMSO/
water solution without Bu4NOH.
Supporting Information Available: Tables of fractional
coordinates, equivalent isotropic and anisotropic thermal pa-
rameters, and bond lengths and bond angles for 5b-PF6 (8
pages). See any current masthead page for ordering information
and Web access instructions.
The new acidity function C_, which was used for the determination
of the pKR+ value of 5a, was constructed by modifying Dolman and
Stewart’s H_ function.30 To represent the H_ function by a continuous
mathematical function, a third-order polynomium was fitted to the mol
% DMSO/H_ data from ref 30 in the whole range from 10 to 70 mol
% DMSO (eq 4). The C_ function was obtained by multiplying the
coefficients in eq 4 by 1.31 (eq 5) and giving the constant term k′ a
value so the new function C_ returns the right pKR+ ) 13.51 for OCV,
which was determined in water (eq 6).
JA982550R
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249.
H- ≈ aX + bX2 + cX3 + k
C- ) 1.31(aX + bX2 + cX3) + k′
(4)
(5)
13.51
1.31(aX + bx2 + cX3)
k′ )
; X ) 12.1
(6)
The C_ function depends on mol % DMSO (X) as:
C_(X) ) 11.572 + 0.1756X - 0.00189X2 + (1.925 × 10-5)X3
By use of this equation, pKR+ ) 19.7 was obtained for 5a.