Disproportionation and structural changes of tetraarylethylene donors upon successive oxidation to cation radicals and to dications

Article abstract of DOI:10.1021/ja981095w

The stepwise (one-electron) chemical oxidation of the tetraphenylethylene donor and its substituted analogues (D) can be carded out by electron exchange with aromatic cations or antimony(V) oxidants to selectively afford the cation radical (D^+.

Full text of DOI:10.1021/ja981095w

J. Am. Chem. Soc. 1998, 120, 6931-6939
6931
Disproportionation and Structural Changes of Tetraarylethylene
Donors upon Successive Oxidation to Cation Radicals and to
Dications
R. Rathore, S. V. Lindeman, A. S. Kumar, and J. K. Kochi*
Contribution from the Chemistry Department, UniVersity of Houston, Houston, Texas 77204-5641
ReceiVed April 1, 1998
Abstract: The stepwise (one-electron) chemical oxidation of the tetraphenylethylene donor and its substituted
analogues (D) can be carried out by electron exchange with aromatic cations or antimony(V) oxidants to
selectively afford the cation radical (D+•) initially and then the dication (D2+). The ready interchange of the
latter establishes the facile disproportionation (i.e., 2D+• h D2+ + D) that was originally examined by only
transient electrochemical techniques. The successful isolations of the crystalline salts of the tetraanisylethylene
cation radical (1+•) as well as the tetraanisylethylene dication (12+) allow X-ray diffraction analysis (for the
first time) to quantify the serial changes in the molecular structure upon successive oxidations. Five structural
parameters (d, l, θ, φ, and q) are identified as quantitative measures of changes in bond (CRdCâ, CRsanisyl)
lengths, dihedral (CRdCâ)/torsional (anisyl) angles, and quinoidal (anisyl) distortion attendant upon the removal
of first one-electron and then another electron from the tetraanisylethylene framework. The linear variation
of all five parameters in Chart 3 point to a strongly coupled relaxation of tetraanisylethylene (involving
simultaneous changes of d, l, θ, φ, and q) to a severely twisted dication. Most noteworthy is the structure of
the cation radical 1+• with d, l, θ, φ, and q values that are exactly one-half those of the dication. The complex
molecular changes accompanying the transformation: D f D+• f D2+ bear directly on the donor properties
and the disproportionation processes of various tetraarylethylenes.
Introduction
(eq 2).6 Such interchanges between paramagnetic ion radicals
Tetraphenylethylene and its ring-substituted analogues are
bifunctional electron donors (or acceptors) by virtue of both
aromatic and olefinic centers present as either electron-rich (or
electron-poor) reservoirs. As such, various tetraarylethylenes
have enjoyed widespread application to reaction types as diverse
as reduction, oxidation, photocyclization, halogenation, oxy-
genation, etc.1,2 Especially noteworthy are the highly hindered
electron-rich analogues, which are known to result in intensely
colored solutions upon treatment with various electrophiles (Cl2,
Br2, I2, and ICl) as well as Brønsted and Lewis acids.3,4
Of particular relevance to tetraarylethylenes are the facile
disproportionations of the (one-electron) oxidation product, in
the form of the tetraarylethylene cation radical on one hand (eq
1)5 and the anion radical as the reduction product on the other
and diamagnetic dications are pertinent to the mechanistic
delineation of homolytic (free radical) and electrophilic (ionic)
pathways, most certainly as they apply to electron transfer and
acid catalysis, respectively.7
The thermodynamics of the redox disproportionation in eq 1
(or eq 2) derives from the corresponding (one-electron) poten-
tials, i.e., ∆G° ) F, (E°2 - E°1), where E°1 and E°2 refer to the
stepwise oxidation (or reduction) of first the tetraarylethylene
donor and then the tetraarylethylene cation radical (or anion
radical).8 It is important to note that owing to the overall
propeller shape of tetraarylethylenes, their conformation will
play an important role in determining these redox properties in
oxidation as well as reduction.9 However, little is definitively
known quantitatively about the conformational and/or structural
(5) Bard, A. J. Pure Appl. Chem. 1971, 25, 379.
(1) (a) Leigh, W. J.; Arnold, D. R. Can. J. Chem. 1981, 59, 3061. (b)
Buckles, R. E.; Hausman, E. A.; Wheeler, N. G. J. Am. Chem. Soc. 1950,
72, 2494. (c) Burchill, P. J. M.; Thorne, N. J. Chem. Soc. C 1968, 696. (d)
Bosch, E.; Kochi, J. K. J. Am. Chem. Soc. 1996, 118, 1319.
(2) (a) Garst, J. F.; Zablotny, E. R.; Cole, R. S. J. Am. Chem. Soc. 1964,
86, 2257. (b) Schultz, D. A.; Fox M. A. J. Org. Chem. 1990, 55, 1047. (c)
Parker, V. D.; Nyberg, K.; Eberson, L. J. Electroanal. Chem. 1969, 22,
150.
(3) (a) Buckles, R. E.; Erickson, R. E.; Snyder, J. D.; Person. W. B. J.
Am. Chem. Soc. 1960, 82, 2444. (b) Buck, H. M.; Lupinski, J. H.;
Oosterhoff, L. I. Mol. Phys. 1958, 1, 196. (c) Buckles, R. E.; Womer, W.
D. J. Am. Chem. Soc. 1958, 80, 5055. (d) Bock, H.; Na¨ther, C.; Havlas, Z.
J. Chem. Soc., Chem. Commun. 1995, 1111.
(6) (a) Grzeszczuk, M.; Smith, D. E. J. Electroanal. Chem. 1984, 162,
189. (b) Troll, T.; Baizer, M. M. Electrochim. Acta 1974, 19, 951. (c) Wolf,
M. O.; Fox, H. H.; Fox, M. A. J. Org. Chem. 1996, 61, 287.
(7) For a recent discussion of the mechanistic ambiguity, see: Rathore,
R.; Kochi, J. K. Acta Chem. Scand. 1998, 52, 114.
(8) (a) E°1 and E°2 refer to the reversible one-electron oxidation (or
reduction) of tetraarylethylene and its cation radical (or anion radical),
respectively. (b) When ∆G is expressed in electron volts, the Faraday
constant F is unity. (c) Any ion-pairing effects are ignored. (d) For anion
radicals, compare: Muzyka, J. L.; Fox, M. A. J. Org. Chem. 1991, 56,
4549. Fry, A. J.; Hutchins, C. S.; Chung, L. L. J. Am. Chem. Soc. 1975,
97, 591. See also Garst et al. in ref 2a.
(9) (a) Mislow, K.; Schultz, D. A.; Fox, M. A. J. Org. Chem. 1990, 55,
1047. (b) Baenziger, N. C.; Buckles, R. E.; Simpson, T. D. J. Am. Chem.
Soc. 1967, 89, 3406. See also Buck et al. in ref 4a.
(4) (a) Buck, H. M.; Bloemhoff, W.; Oosterhoff, L. J. Tetrahedron Lett.
1960, 5. (b) Valenzuela, J. A.; Bard, A. J. J. Phys. Chem. 1968, 72, 286.
S0002-7863(98)01095-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/29/1998

6932 J. Am. Chem. Soc., Vol. 120, No. 28, 1998
Rathore et al.
Figure 1. UV-vis absorption spectra of (A) the cation radical and (B) the dication from the oxidation of tetraanisylethylene 1 (s), tetratolylethylene
2 (- - -), and dianisylditolylethylene 3 (- - -) with 1 and 2 equiv of [EA+• SbCl6-], respectively, in dichloromethane at 25 °C.
changes that take place when a tetraarylethylene is converted
to its cation radical (or anion radical) and thence to the dication
(or dianion). Although X-ray crystallographic determination
is the most direct approach to molecular structure, it has not
been applied to the disproportionation, largely due to the
difficulty in growing suitable single crystals of both the cation
radical (or anion radical) and the dication (or dianion) of the
same tetraarylethylene.10
Of the various redox systems extant in the literature, we
identify the oxidation/disproportionation of tetraanisylethylene
as the potentially most viable for X-ray diffraction analysis. For
example, tetraanisylethylene (1) can be electrochemically
oxidized to its cation radical (1+•) and to its dication (12+) which
are both rather persistent in dichloromethane solutions in the
presence of 0.2 M supporting electrolyte.11 Moreover, quantita-
tive analysis of the anodic behavior affords values of the
disproportionation constant (see eq 1) in solvents of various
polarities.12 Thus our first task in this study is to confirm the
ready redox disproportionation by a wholly chemical method
(without the complications of added salt)sultimately to allow
the isolation of crystalline cation-radical and dication salts of
tetraanisylethylene for direct X-ray diffraction analysis. To
establish the scope of the structural requirements and facilitate
the ease of isolation of the one- and two-electron oxidation
products, we prepared the series of tetra- and diarylated ethylene
analogues identified in Chart 1, where An and Tol are
p-methoxy- and p-methylphenyl groups, respectively.
successive one-electron transfers from the tetraarylethylenes in
Chart 1, of which we identified three oxidants as follows.
I. Chemical Oxidation of Tetraarylethylenes to Their
Cation Radicals and Dications. A. Via Electron Exchange
with Aromatic Cation Radicals. The cation radicals of the
methano- and ethanoanthrancene derivatives MA+• and EA+•,
respectively, are selective organic oxidants with reversible
reduction potentials that differ by only 190 mV, as listed in
Chart 2.13 As such, the treatment of tetraanisylethylene (1) with
Chart 2. Aromatic Oxidants
1 equiv of MA+• in dichloromethane (under an argon atmo-
sphere at 25 °C) immediately yielded a bright blue solution,
the UV-vis spectrum (Figure 1A) of which showed the
characteristic twin absorption bands at 560 and 941 nm of
tetraanisylethylene cation radical (1+•).15 Quantitative spectral
analysis of the blue solution (log ꢀ560 ) 4.28 in Table 1)
indicated that the electron exchange in eq 3 was displaced
completely to the right. In a similar vein, the treatment of a
Chart 1. Olefinic Donors
solution of 2 equiv of MA+• at 25 °C with 1 equiv of
tetraanisylethylene immediately yielded a dark green solution
(Figure 1B) which upon quantitative spectral analysis showed
the formation of 1 equiv of tetraanisylethylene dication 12+ (λmax
) 567 nm, log ꢀmax ) 4.59) in quantitative yield (eq 4). Indeed
the distinctive pair of absorption spectra in Figure 1 indicated
that the cation radical (1+•) and the dication (12+) could be
generated cleanly via the discrete one- and two-electron
transformations in eqs 3 and 4, respectively, without admixture
of one with the other.
Results and Discussion
The requisite isolation of the various cation radicals and the
dications as crystalline salts for X-ray diffraction analysis called
for a selective (yet sufficiently powerful) reagent to effect
(10) There are no reports extant in which the cation-radical and dication
structures of the same donor are available for direct comparison.
(11) (a) Phelps, J.; Bard, A. J. J. Electroanal. Chem. 1976, 68, 313. (b)
Aalstad, B.; Parker, V. D. J. Electroanal. Chem. 1982, 136, 251.
(12) Svanholm, U.; Jensen, B. S.; Parker, V. D. J. Chem. Soc., Perkin
Trans. 2 1974, 907.
(13) Prepared as the crystalline hexachloroantimonate salt via the SbCl5
oxidation of 9,10-dimethoxy-1, 4: 5,8-dimethano-1,2,3,4,5,6,7,8-octahy-
droanthracene (MA) and 9,10-dimethoxy-1,4:5,8-diethano-1,2,3,4,5,6,7,8-
octahydroanthracene (EA).14
(14) Rathore, R.; Kochi, J. K. J. Org. Chem. 1995, 60, 4399.
(15) Compare Buck et al. in ref 4a.

Oxidation of Tetraarylethylene Donors
J. Am. Chem. Soc., Vol. 120, No. 28, 1998 6933
Table 1. Absorption (UV-Vis) Spectra of Cation Radicals and
antimony pentachloride (4.0 mmol) in anhydrous dichlo-
romethane at -50 °C immediately resulted in a dark green solu-
tion from which the crystalline dication salt readily precipitated
in quantitative yields upon the slow addition of a mixture of
anhydrous ether and hexane (eq 5).19 The microcrystalline
Dication of Tetraarylethylenes and Related Donorsa
precipitate was filtered under an argon atmosphere and dried
in vacuo. A quantitative spectrophotometric analysis of the
black solid (after dissolution of a known quantity in anhydrous
dichloromethane) showed the characteristic UV-vis absorption
spectrum of the dication salt of tetraanisylethylene which was
formed in 73% yield according to eq 5. The high purity of the
salt was further verified by iodometric titration (see Experi-
mental Section). The same procedure was also used in the
preparation of the highly pure dication salt of tetraarylethylene
3. The dark (red) crystal of the tetratolylethylene dication salt
[22+ (SbCl6-)2] for X-ray crystallography was prepared ear-
lier from SbCl5 at -78 °C (see Supporting Information Avail-
able).
C. Triethyloxonium Hexachloroantimonate for the Prepa-
ration of Crystalline Tetraanisylethylene Cation-Radical
Salt. A suspension of [Et3O+ SbCl6-] (1.5 equiv) and tetraani-
sylethylene 1 was stirred in dichloromethane at 0 °C for an hour,
during which the solution took on a dark blue coloration. The
UV-vis spectral analysis of the highly colored solution showed
the characteristic spectrum with a twin absorption band at λmax
) 560 and 941 nm (compare Figure 1) due to the tetraanisyl-
ethylene cation radical (vide supra), in essentially quantitative
yield (eq 6).19 The bright blue solution was cooled to -20 °C,
a In dichloromethane solution at 23 °C, unless otherwise indicated.
b Extinction coefficient in M-1 c See Figure 1 for other peaks. d Not
.
stable at room temperature; see text.
The stepwise oxidations of the analogous tetra-p-tolylethylene
(2) and the anisyl/tolyl hybrid (3) were only selectively effected
by MA+• at 25 °C. For example, dianisylditolylethylene (3)
reacted with 1 equiv of MA+• to produce the cation radical 3+•
(λmax ) 544 nm) in a manner much like that for tetraanisyl-
ethylene in eq 3. However, the electron exchange of tetratolyl-
ethylene (2) with 1 equiv of MA+• in dichloromethane
proceeded only to 90% conversion (together with 10% unreacted
MA+•), as judged by the quantitative spectral analysis of 2+•
(λmax ) 516 nm, log ꢀ 4.18). Although the incremental addition
of another equiv of MA+• led to the complete conversion of 2
to the cation radical 2+•, it was insufficient to effect the further
oxidation to the dication 22+. The latter was readily achieved
by 2 equiv of the somewhat stronger oxidant EA+• (vide supra).
Solutions of tetraanisylethylene and the hybrid dications 12+
and 32+ (see Figure 1B) prepared in the same manner were stable
for several hours at 25 °C.16 However, treatment of the di-
anisylethylenes 4-6 with 2 equiv of EA+• led to highly colored
solutions of the corresponding cation radicals (see Table 1)
which decomposed rapidly at room temperature.17 From spec-
tral changes such as these, it was easy to deduce the qualitative
trend in redox reactivity of tetraarylethylenes as 1 > 3 > 2 for
cation-radical formation and 1 > 3 . 2 for dication formation.
Moreover, the same monotonic trend pertained to the steady
bathochromic shifts of the absorption bands of the tetraaryl-
ethylene cation radicals as well as those of the corresponding
dications in Figure 1, A and B, respectively.
and followed by a careful layering with diethyl ether. When
the two-phase mixture was allowed to stand undisturbed at -20
°C for a prolonged period, it deposited a well-formed crop of
very dark crystals interspersed with colorless crystals of
unreacted tetraanisylethylene.20 A close scrutiny revealed a
mixture of two morphological types of essentially the same
(indistinguishably dark) color; and manual separation led to (a)
isometric prisms of the tetraanisylethylene cation radical salt
and (b) long thin plates of the tetraanisylethylene dication salt,
both structures of which were established by X-ray crystal-
lography (vide infra). Although a complete determination of
the exact stoichiometry was not possible, the (unanticipated)
codeposition of cation-radical and dication salts together with
unreacted tetraanisylethylene (from a solution consisting of only
cation radical) must have arisen from the crystallization-induced
disproportionation (eq 7).21 The treatment of dianisylditolyl-
ethylene 3 with [Et3O+ SbCl6-] by the same procedure afforded
a quantitative yield of the cation radical (3+•), as determined
by quantitative (UV-vis) spectral analysis of the dichlo-
romethane solution. Cooling the solution to 0 °C, followed by
a careful layering with toluene, led to dark crystals of the
dication salt [32+ (SbCl6-)2], the structure of which was
B. Preparative Isolation of Tetraarylethylene Dications
with Antimony Pentachloride. The strongly oxidizing anti-
mony pentachloride18 proved to be especially suitable for the
preparation and isolation of pure tetraarylethylene dications.
Thus, the treatment of tetraanisylethylene 1 (1.0 mmol) with
(16) Red solutions of tetratolylethylene dication (22+) in dichloromethane
were not persistent at room temperature. The crystalline salt [22+ (SbCl6-)2]
was prepared at -78 °C and handled without allowing the temperature to
rise (see Experimental Section).
(19) For the stoichiometry of the donor oxidation with antimony(V)
oxidants, see: Rathore, R.; Kumar, A. S.; Lindeman, S. V.; Kochi, J. K.
Submitted for publication.
(20) See Experimental Section in Supporting Information Available for
details.
(17) Note, however, that the dication 42+ was not kinetically persistent
(probably due to the presence of labile R-protons on the ethyl groups).
(18) Bard, A. J.; Ledwith, A.; Shine, H. J. AdV. Phys. Org. Chem. 1976,
13, 155.
(21) The dication actually crystallized from solution as the complex salt
[12+, SbCl6-, Sb2Cl7-] described in the Supporting Information Available.

6934 J. Am. Chem. Soc., Vol. 120, No. 28, 1998
Rathore et al.
Figure 2. Spectral changes attendant upon the reduction of 4.5 × 10-5
M dication 12+ (s) to its cation radical 1+• (- - -) by incremental addition
of 1.8 × 10-3 M 1 in dichloromethane at 25 °C.
Figure 3. The UV-vis absorption spectrum, of cyclooctatetraene
cation radical 8+• (s) generated by treatment of 0.3 mM 8 in
dichloromethane with 1 equiv of [MA+• SbCl6-] (- - -) at 25 °C.
by the oxidation of octamethylbiphenylene with MA+• as in eq
3) indicated that electron transfer was quantitative, i.e.
established by X-ray crystallography (vide infra). Curiously,
the expected crystals of the cation-radical salt [3+• SbCl6-] were
not detected. We were also unable to prepare the crystalline
salt of the tetratolylethylene cation radical using the [Et3O+
SbCl6-] procedure.22
II. Electron-Transfer Reactivity of the Tetraanisylethyl-
ene Dication (12+). The ready isolation of the crystalline salt
of tetraanisylethylene dication in eq 5 allowed its electron-
transfer reactivity toward the parent tetraanisylethylene and other
neutral donors to be examined directly, as follows.
A. Redox Conproportionation with Tetraanisylethylene
Donor. When a dark green solution of tetraanisylethylene dicat-
ion was mixed with an equimolar amount of the neutral (color-
less) donor, a dramatic color change to bright blue occurred
immediately. UV-vis spectral analysis established the simul-
taneous oxidation of 1 and reduction of 12+ in quantitative yields
(eq 8),23 and the uncluttered character of the electron transfer
C. Tetrakis-Annelated Cyclooctatetraene as Electron
Donor. The oxidation of a colorless solution of the tetrakis-
bicyclooctano-annelated cyclooctatetraene24 (8, E°ox ) 0.75 V
vs SCE) with 1 equiv of tetraanisylethylene dication (12+) in
dichloromethane yielded immediately a dark red solution, the
UV-vis spectrum of which showed a nondescript, rather con-
tinuous absorption from 500 to 1000 nm. However, spectral
subtraction of the tetraanisylethylene cation-radical absorption
(see Figure 1A) resulted in the well-defined absorption spectrum
shown in Figure 3 with λmax ) 745 nm (log ꢀ745 ) 3.66) and
442 nm (3.09) which coincided with the authentic spectrum of
the emerald green cation radical 8+• generated independently
from the oxidation of the cyclooctatetraene 8 and 1 equiv of
MA+• as in eq 3. It is interesting to note that the intense visible
was established by the pair of well-defined isosbestic points in
the UV-vis spectra (Figure 2) when a solution of 12+ was
treated with incremental amounts of 1. [Note the final spec-
trum remained unchanged upon the addition of 1 beyond 1
equiv.]
B. Electron Transfer from Octamethylbiphenylene. The
treatment of the electron-rich (colorless) donor octamethylbi-
phenylene (7, E°ox ) 0.80 V vs SCE) with an equimolar amount
of the dication 12+ in dichloromethane resulted in an immediate
color change to a dark blue solution, the UV-vis absorption
spectrum of which consisted of a broad absorption showing a
pair of major bands at λmax ) 600 and 930 nm. Spectral
subtraction of the tetraanisylethylene cation-radical (1+•) absorp-
tion in Table 1 resulted in a well-resolved absorption band with
λmax ) 602 nm, 550 (sh) and log ꢀ602 ) 4.08 of the blue
octamethylbiphenylene cation radical (7+•).7 Quantitative com-
parison of the latter with an authentic spectrum of 7+• (prepared
absorption of the cation radical 8+• derives from the one-electron
oxidation of the neutral donor 8 without significant conforma-
tional change of its pronounced “tub-shaped” structure.25
III. Direct Measurement of Cation-Radical Dispropor-
tionation. Since the conproportionation of tetraanisylethylene
(1) with its dication (12+) as described in eq 8 was the
microscopic reverse of the disproportionation of the cation
radical (1+•) previously reported by Parker and others in the
course of electrochemical (anodic) oxidation of tetraanisyleth-
ylene,12 we carefully measured the magnitudes of the dispro-
portionation constant (see Kdisp in eq 1) in three solvents of
varying polarity in the absence of added salt.
-
(22) Although Et3O+ SbCl6 effectively oxidized tetratolylethylene 2,
we were unable to isolate the crystalline salt of 2+• from the dark purple
solution.
(24) Komatsu, K.; Nishinaga, T.; Aonuma, S.; Hirosawa, C.; Takeuchi,
K.; Lindner, H. J.; Richter, J. Tetrahedron Lett. 1991, 32, 6767. For an
efficient alternative synthesis of 8, see: Rathore, R.; Lindeman, S. V.;
Kumar, A. S.; Kochi, J. K. J. Am. Chem. Soc., in press.
(25) See: Nishinaga, T.; Komatsu, K.; Sugita, N. J. Am. Chem. Soc.
1993, 115, 11642.
(23) Note, however, that the analogous 1-adamantyl-dianisylcarbinyl
cation 12+ generated from the corresponding chloride by treatment with
SbCl5 was not an effective oxidant of tetraanisylethylene (as in the
conproportionation exchange in eq 8).

Oxidation of Tetraarylethylene Donors
J. Am. Chem. Soc., Vol. 120, No. 28, 1998 6935
Table 2. Disproportionation Constant (Kdisp) for Tetraarylethylene
Cation Radicals in Different Solvents
a See Experimental Section. b Calculated from the cyclic voltam-
Figure 4. The stepwise oxidation of tetraanisylethylene 1 (6.3 × 10-5
M) with incremental additions of 0 to 1 equiv (s) and 1 to 2 equiv
(- - -) of 2.3 mM [MA+• SbCl6-] in dichloromethane at 25 °C.
metric data in Table 3 using the Nernstian expression Kdisp
)
exp(-0.039∆E°) at 298 K, where ∆E° ) E2° - E1° (mV). c Taken
from ref 12.
The relative concentrations of the cation-radical 1+• and the
dication 12+ were measured in dichloromethane by following
the (UV-vis) spectral changes attendant upon the increasing
concentrations of tetraanisylethylene cation radical (1+•) gener-
ated in situ by oxidation with incremental amounts of the
aromatic oxidant MA+• according to eq 3. Since the resulting
changes in the composite spectra (Figure 4) were rather complex
(due to the overlapping bands of 1+• and 12+), for clarity they
are best considered in two equal segments, i.e., (a) from 0 to 1
equiv (shown by the solid lines) and (b) from 1 to 2 equiv
(shown by the dashed lines) of added MA+•. In segment (a),
the concentration of cation-radical 1+• (λmax 941 nm) increased
steadily, and that of 12+ [λmax 567 nm, 654(sh)] was always
minor until 1 equiv of MA+• was added. In segment (b), the
concentration of the dication 12+ increased steadily at the
expense of 1+• until 2 equiv of MA+• was added, at which point
all the 1+• was consumed. Most importantly, the composite
spectra at the intermediate points (at which ∼1 equiv of MA+•
had been added) were identical with those obtained in Figure 2
from the conproportionation of 12+ with 1. [Note especially
the common isosbestic point at 740 nm.] Together, the spectral
changes in Figures 2 and 4 thus establish the validity of the
reversible character of the redox (disproportionation) inter-
changes, i.e.
oxidized electrochemically at a platinum electrode as 5 × 10-3
M solutions in anhydrous dichloromethane containing 0.2 M
tetra-n-butylammonium hexafluorophosphate (TBAH) as the
supporting electrolyte. The reversible cyclic voltammograms
(CV) of the tetraarylethylenes 1, 2, and 3 with two closely
coupled reversible one-electron oxidation waves were consis-
tently attained at a scan rate of ν ) 200 mV s-1; they all showed
the anodic/cathodic peak current ratio of ia/ic ) 1.0 (theoretical)
at 25 °C (see Figure 5). The internal calibration of the CV
peaks with added ferrocene then yielded the reversible oxidation
potential (E°ox) in Table 3 for the production of cation radical
via the one-electron redox couple and the dication via the two-
electron redox couple (eq 12). The values of first and second
oxidation potentials of tetraanisyl and dianisylditolylethylene
were separated by only 120 and 130 mV, respectively, whereas
tetratolylethylene 3 showed a significantly larger separation of
270 mV.
It was particularly clear from the spectral results in segment
(a) that the cation radical was always the predominant species
in dichloromethane solution, i.e., [1+•] . [12+]. Quantitative
analysis of the spectral changes in both segments (a) and (b)
afforded the consistent values of the disproportionation constant
Kdisp ) [12+][1]/[1+•]2 listed in Table 2. By contrast, the value
of Kdisp was significantly larger in acetonitrile and intermediate
in nitromethane solution.26 Most importantly, the results in
Table 2 largely confirm the values of Kdisp in dichloromethane
and acetonitrile evaluated electrochemically by Parker and co-
workers (see Table 2, column 4).12 Furthermore, the decreasing
magnitudes of Kdisp for dianisylditolylethylene 3 and tetratolyl-
ethylene 2 fall in line with their decreasing donor strengths. To
quantitatively evaluate the latter, the anodic behavior of the
tetraarylethylene donors was examined.
IV. Electrochemical Oxidation of Tetraarylethylene and
Related Donors. The various tetraarylethylenes in Chart 1 were
Figure 5. Cyclic voltammograms of 5 mM tetraarylethylenes 1, 2,
and 3 (as indicated) in dichloromethane containing 0.2 M tetra-n-
butylammonium hexafluorophosphate at a scan rate of V ) 200 mV
s-1 (25 °C).
(26) The peculiar CV behavior of tetraanisylethylene in nitromethane is
subject to further study.

6936 J. Am. Chem. Soc., Vol. 120, No. 28, 1998
Rathore et al.
Table 3. Electrochemical Oxidation Potentials of the
Tetraarylethylenes and Related Donorsa
Figure 6. Osteryoung square-wave voltammograms of 5 mM tetraani-
sylethylene 1 in dichloromethane and acetonitrile containing 0.2 M tetra-
n-butylammonium hexafluorophosphate at 25 °C.
convenience, let us first characterize the tetraanisylethylene
framework in terms of the five (principal) parameters: d, l, θ,
φ, and q that are applicable to the defining structure 9. We
a In anhydrous dichloromethane (unless otherwise indicated) contain-
ing 0.2 M tetra-n-butylammonium hexafluorophosphate at a scan rate
V ) 200 mV s-1 and 25 °C. Values in parentheses are in acetonitrile.
b Single reversible CV wave with Ecp - Eap ) 65 mV. c Quasi-reversible
CV wave at a scan rate V ) 200 mV s-1 d Single reversible CV wave
.
with Ecp - Eap ) 88 mV. e Anodic peak potential. Irreversible CV wave
at V ) 200 mV s-1
.
The anodic oxidation of the dianisylethylenes 4 and 5 also
showed two reversible CV waves that were separated by roughly
the same amount as that listed in Table 3 for tetraanisylethylene.
In other words, the mere presence of an R, â-pair of anisyl
groups was sufficient to confer roughly the same stability to
the ethylene dication (relative to the cation radical).27 The latter
was not applicable to the geminally substituted R,R-dianisyl
analogue 6 which showed the first reversible redox wave a 1.17
V vs. SCE, but the second anodic wave separated by 440 mV
was completely irreversible.28
Solvent Effect. We further investigated the electrochemical
behavior of 5 × 10-3 M solution of tetraanisylethylene in three
different solvents. Thus in dichloromethane, two partly over-
lapped reversible CV waves were observed (vide supra), whereas
in the polar acetonitrile and nitromethane only a single reversible
wave was observed.29 Such a pronounced solvent effect on the
cyclic voltammograms of 1 is pictorially demonstrated by
Osteryoung square-wave voltammograms30 in Figure 6 (see also
Table 3).
then measure the distinctive changes in the ethylenic bond length
(d), the bond to the anisyl group (l), the olefinic planarity (θ),
the anisyl coplanarity (φ), and the quinoidal distortion (q) upon
successive electron removals from tetraanisylethylene to effect
the serial transformation: 1 9 9
-e8 1+• -e8 12+ as follows.
A. Changes in Bond Lengths. Ethylenic Bond (d). The
progressive increase in the ethylenic (CR-Câ) bond length from
1.359 Å in tetraanisylethylene (1) to 1.417 Å in the cation radi-
cal (1+•) and to 1.503 Å in the dication (12+) is illustrated in
Figure 7.
Anisyl Bond (l). Such a significant increase in the (CR-
Câ) bond is accompanied by a concomitant shortening of the
bond to the anisyl group (l ) CR-C4) from 1.492 Å, to 1.460
Å and to 1.411 (1.435) Å in 1, 1+•, and 12+, respectively.
Moreover, the C-C bonds within the anisyl groups also showed
distinctive changes, and the elongation of bond length d1 at the
expense of d2 was especially notable (see Table 4).
V. Structural Changes Proceeding from Tetraanisyleth-
ylene to Its Cation Radical and Then to Its Dication. The
successful isolation of both the tetraanisylethylene cation radical
(1+•) and the dication (12+) as crystalline salts in eq 7 allowed
their molecular structures to be directly compared to the
molecular structure of the parent donor (1) by X-ray diffraction
analysis. As such, we are now able (for the first time) to
establish the precise changes in molecular structures upon the
stepwise oxidation of an electron-rich olefin donor. For
Table 4. Selected Bond Lengths in Tetraanisylethylene (1), Its
Cation Radical (1+•), and Its Dictation (12+
)
Bond
1
1+•
12+ a
Me-O
O-C1
C1-C2
C2-C3
C3-C4
C4-CR
q(%)b
1.425
1.372
1.391
1.387
1.397
1.492
1.436
1.357
1.399
1.380
1.413
1.460
1.451 (1.437)
1.330 (1.346)
1.413 (1.405)
1.362 (1.373)
1.434 (1.420)
1.411 (1.435)
80 (51)
(27) As indicated by the separation ∆ ∼ 120 mV that is the same as that
in tetraanisylethylene.
(28) At a CV scan rate as high as V ) 2 V s-1. Note also the monocation
12+ formed in eq 13 is a poor electron-transfer agent.23
(29) Although E°ox(I) and E°ox(II) for tetraanisylethylene in acetonitrile
are not distinguished in Table 3, the difference of E°ox(II) - E°ox(I) ) 10
mV has been evaluated by Phelps and Bard11a by digital simulation of the
cyclic voltammetric data.
11
37
a For the An1 group, the value for An2 in parentheses (see text).
b See footnote 33.
(30) Osteryoung, J. G.; Osteryoung, R. A. Anal. Chem. 1985, 57, 101A.

Oxidation of Tetraarylethylene Donors
J. Am. Chem. Soc., Vol. 120, No. 28, 1998 6937
Figure 7. Progressive changes in C-C bond lengths in D )
tetraanisylethylene (top) upon oxidation to its cation radical (middle)
and then to its dication (bottom). [Note that the four An groups in D
and D+• are equivalent, but only two pairs are equivalent in D2+].
Figure 9. Progressive increase in the anisyl coplanarity (φ) as
tetraanisylethylene (top) is successively oxidized to the cation radical
(middle) and to the dication (bottom). [Note the difference between φ
for An1 and An2 in D2+ is presented in a displaced perspective].
Anisyl Coplanarity (O). The conformation of an anisyl group
relative to the ethylenic (CR-Câ) bond, as described by its
torsional angle φ, progressively decreases from tetraanisyleth-
ylene (φ ) 52°) to its cation radical (φ ) 33°) and to its dication
(φ ) 19°).31 It is important to note that all four torsional angles
in tetraanisylethylene are the same (within (5°) to describe a
neutral molecule with overall D2 (local) symmetry. Although
the same is also true of the cation radical 1+•, the dication 12+
has only a decreased symmetry of C2 and exists in essentially
a bisected conformation (θ ) 62°). As a result, the geminal
anisyl groups in the dication are inequivalent, with one anisyl
group An1 more coplanar than An2 with φ ) 19 and 28°,
respectively, as illustrated in Figure 9.
C. Increase in Quinoidal Distortion (q). Bond lengths of
the anisyl group show selective (progressive) changes attendant
upon the conversion of tetraanisylethylene to its cation radical
and then to the dication, as listed in Table 4. Indeed, these
substantial alterations in the principal bond lengths within the
anisyl group (as a result of electron removal) are associated
with the increasing contribution from the quinoidal structure
10. As such, we select changes in the aromatic C3-C4 bond
Figure 8. Linear increase in the dihedral (ethylenic) angles θ in tetra-
anisylethylene (top), its cation radical (middle), and its dication (bot-
tom). The latter shows the inequivalency of An1 and An2 in the dication.
B. Changes in Dihedral and Torsional Angles. Olefinic
Planarity (θ). The perspective views along the ethylenic (CR-
Câ) bonds of tetraanisylethylene, its cation radical, and its
dication in Figure 8 underscore the increasing deviation from
planarity at the olefinic center, with the overall dihedral angle
increasing from θ ) 4° in 1, to θ ) 31° in 1+•, and to θ ) 62°
relative to the C2-C3 bond (see Table 4) as the most sensitive
indicator of the quinoidal distortion of the anisyl group. The
(31) For the anisyl group An1 and φ ) 29° for the orthogonal anisyl
group An2 (vide infra).
in 12+
.

6938 J. Am. Chem. Soc., Vol. 120, No. 28, 1998
Rathore et al.
quinoid parameter q in Table 4 increases monotonically with
each electron removal in the order32 1 < 1+• < 12+. By taking
tetraanisylethylene as the reference point, we find that all four
anisyl groups in its cation radical 1+• have essentially the same
values of q ) 32-39%. However, the situation in the dication
(12+) is quite different, and the inspection of the anisyl bond
lengths in Table 4 quickly reveal that the (relatively) coplanar
anisyl group An1 suffers substantially more quinodal distortion
(q ) 82 and 83%) than An2 (q ) 37 and 40%).
VI. Stereoelectronic Significance of the Structural Changes
of Tetraanisylethylene Upon Successive (One-Electron)
Oxidations. The profound molecular alterations of the tet-
raanisylethylene donor (1) attendant upon successive electron
removals to generate its cation radical (1+•) and then its dication
(12+) in Figures 7, 8, and 9 are most readily evaluated by a
direct comparison of the quantitative changes in the structural
parameters d, l, θ, φ, and q, as illustrated in Chart 3.33 Most
1+•) represents one-half that required for dication (i.e., 1 f 12+
is a most notable result.
)
VII. Mechanistic Implications of the Structural Changes
on Donor Strengths and Disproportionation. The linear
changes in Chart 3 of all the structural parameters attendant
upon the conversion of tetraanisylethylene to its cation radical
and then to the dication precisely parallel the energy changes
for effecting a pair of successive electron removals, as given
by reversible oxidation potentials of E°ox(I) = E°ox(II) in
acetonitrile solution (see Table 3 and Figure 6).29 As such, we
conclude that the energy changes in the cation radical and
dication are also linear in this rather polar solvent; they lead to
the disproportionation constant Kdisp of essentially unity in Table
2. However in dichloromethane, the second oxidation potential
is shifted slightly positive (Table 3), most likely due to a
significantly lower gain in the solvation energy of the dication
in the less polar solvent.34
Chart 3. Structural Parameters
In the successive transformations: 1 f 1+• f 12+, the two
halves of tetraanisylethylene undergo a significant rotation about
the CR-Câ bond, first by θ ) 31° and then to an overall θ )
62°. The latter relates to a more or less distonic dication 12+
consisting of an orthogonal pair of equivalent (positively
charged) An-C-An moieties. To quantitatively evaluate the
latter, we prepared the dianisyl monocation 12+ as a crystalline
salt via an oxidative solvolysis (eq 13).35 Indeed, X-ray dif-
fraction analysis of the cation 12+ revealed that only one anisyl
group (An1) was almost coplanar with the CR-Ad bond (φ )
18°) and suffered significant quinodal distortion (q ) 74%),35b,c
and both structural parameters were essentially those observed
in tetraanisylethylene dication (Chart 3). This structural similar-
ity of 12+ and 12+ indicates that only one anisyl group is suf-
ficient to effectively stabilize an R-cationic center, as illustrated
in structure 11. Such a conclusion also follows from the en-
hanced electron-donor properties of the hybrid dianisylditolyl-
ethylene 3, with values of E°ox(I) and E°ox(II) in Table 3, which
are essentially the same as those of tetraanisylethylene. Fur-
thermore, the latter analysis even extends to the 1,2-dianisyl-
ethylenes 4 and 5 with values of E°ox(I) and E°ox(II) which are
only slightly shifted positive (Table 3, entries 4 and 5).
Anisyl stabilization of the cationic charge (as described in
structure 11) does not extend well to the p-tolyl group since
both E°ox(I) and E°ox(II) in tetratolylethylene are significantly
more positive and the separation ∆ in Table 3 is substantially
larger than those in the dianisyl-containing 1, 3, 4, and 5 (but
not 6)36. However, a modicum of tolyl stabilization is apparent
striking is the consistent linear change (within experimental
error) that all the structural parameters experience on proceeding
from the neutral donor to the cation radical and then to the
dication, irrespective of the magnitudes of the slopes or their
algebraic sign. In other words, the structural parameters d, l,
θ, φ, and q are mutually interdependent, and they all relate to
a single change in structure, which corresponds to the simul-
taneous (i) lengthening of the ethylenic bond, (ii) tightening of
the anisyl bonding, and (iii) flattening of the anisyl coplanarity.
Since this complex change also directly relates to the increasing
quinoidal distortion of the anisyl groups, the convoluted change
can be largely envisioned as a desire for maximum delocaliza-
tion of positive charge over a pair of Vicinal anisyl groups in
the illustrative structure 11. It is particularly noteworthy that
this stereoelectronic change is also directly coupled to the
increasing orthogonality of the equivalent (An1-CR-An2) and
(An1′-Câ-An2′) moieties about the central CR-Câ bond toward
the bisected conformation. Indeed, the fact that the energy for
such a complex structural change to cation radical (i.e., 1 f
(34) Based on the potentials in Table 3 which are referenced to ferrocene
as the common internal standard with E°ox ) 0.43 and 0.37 V vs SCE in
dichloromethane and acetonitrile, respectively.
(32) The quinoidal distortion was evaluated as the difference between
bond length d1 and d2 (see 9) relative to that in various quinomethane
structures with averaged values of d1′ ) 1.444(2) Å and d2′ ) 1.354(2) Å
(by searching more than 50 compounds in the Cambridge data files); i.e.,
q(%) ) 100 (d1 - d2)/(d1′ - d2′).
(35) (a) Where Ad ) 1-adamantyl. (b) As described in the Experimental
Section, the other anisyl group (An2) in 12+ is substituted with a 3-chloro
substituent. (c) The structural parameters are: (An1) d1 ) 1.415 Å, d2 )
1.356 Å or q1 ) 74%, φ1 ) 18.2° and (An2) d1 ) 1.387 Å, d2 ) 1.368 Å
or q2 ) 22%, φ2 ) 63.7°.
(33) The structural parameters l, φ, and q for D ) tetraanisylethylene in
Chart 3 are plotted as values averaged over all four anisyl groups.
(36) Note that 1,1-dianisyl substitution as in 6 is not sufficient to stabilize
the dication (Table 3, entry 6).

Oxidation of Tetraarylethylene Donors
J. Am. Chem. Soc., Vol. 120, No. 28, 1998 6939
in the tetratolylethylene dication 22+, as indicated by the
structural parameters: d ) 1.502 Å, l ) 1.423 Å, φ ) 24.1°,
and q ) 59% in relation to those in either the tetraanisylethylene
the ethylenic donor. The linear changes in the structural
parameters (d, l, θ, φ, and q as described in Chart 3) coincide
with the incremental (linear) change in the energy requirement
for electron removal, as indicated by values of the reversible
oxidation potentials E°ox(Ia) = E°ox(II) in acetonitrile solution.
The coplanarity (φ) and quinoidal distortion (q) of the anisyl
substituents are especially useful measures of the delocalization
(and stabilization) of positive charge in order to reduce the
dication 12+ or the hybrid 32+ 37
.
The rather expanded dihedral angle θ ) 77° in tetratolyleth-
ylene dication 22+ relative to either tetraanisylethylene dication
1
2+ (θ ) 62°) or dianisylditolylethylene dication 32+ (θ ) 56°)
indicates that the delocalization of two positive charges (as in
11) is not a strong requirement for the attainment of a bisected
conformation in these dications. Although it may be tempting
to attribute the bisected conformation of tetraarylethylene dica-
tions to steric repulsion of the large aryl groups, it is noteworthy
that a variety of other ethylenic dications with different substi-
tuents all exist in twisted conformations with large dihedral (θ)
angles.38 Indeed, theoretical calculations of the simple (unen-
cumbered) ethylene dication predict a bisected structure with
structural parameters θ ) 90° and d ) 1.46 Å.39 Significant
twisting of the ethylenic bond also pertains to the ethylene cation
radical measured in the gas phase40 and confirmed by theoretical
calculations.41 Since the same elongation and twisting of the
ethylene bond have also been observed in the tetraphenylethyl-
ene dianion (d ) 1.49 Å, θ ) 56°),42 the progressive change
from a planar to a bisected conformation may simply reflect the
increasing trend for the minimization of Coulombic repulsion.43
values of E°ox and lead to increased disproportionation (Kdisp
)
in eq 1. Structural analysis also indicates that a single anisyl
substituent on an ethylenic carbon (as in various R,â-dianisyl-
ethylenes) is sufficient to confer an optimal cationic stabilization
of an ethylenic donor D in its oxidative conversion to cation
radical (D+•) and then to its dication (D2+).
Experimental Section
The olefinic and aromatic electron donors 1,1,2,2-tetrakis(4-meth-
oxyphenyl)ethylene (1),3c 1,1,2,2-tetrakis(4-methylphenyl)ethylene (2),1d
3,4-bis(4-methoxy-phenyl)-hex-3-ene (4),45 2,3-bis(4-methoxyphenyl)-
bicyclo-[2.2.2]oct-2-ene (5),46 4,4′-dimethoxybenzhydrylidene-adaman-
tane (6),1d octamethylbiphenylene (7),47 and tetrakis(bicyclo[2.2.2]-
octano)cyclooctatetraene (8)24 have been described previously. The
McMurry coupling of 4-methoxy-4′-methylbenzophenone with titanium
tetrachloride and zinc dust in tetrahydrofuran afforded 1,2-bis(4-
methylphenyl)-1,2-bis(4-methoxyphenyl)ethylene (3) in 91% yield. Bis-
(4-methoxyphenyl)-1-adamantylmethyl chloride (12) was prepared from
the reaction of Grignard reagent derived from p-bromoanisole and ethyl
1-adamantylcarboxylate to yield bis(4-methoxyphenyl)-1-adamantyl-
methyl alcohol which was in turn converted to bis(4-methoxyphenyl)-
1-adamantylmethyl chloride in excellent yield (92%), by treatment with
excess thionyl chloride. Procedures for the oxidation of tetraaryleth-
ylenes 1-3 and dianisylethylenes 4-6 to the corresponding cation-
Summary and Conclusions
The successful isolation of tetraanisylethylene (1), its cation
radical (1+•), and its dication (12+) as crystalline salts allows
X-ray diffraction analysis to establish the structural changes
upon the successive removal of one then two electrons from
(37) It is noteworthy that the structural parameters in 32+ confirm the
significant coplanarity (φ ) 18.2°) and quinoidal distortion (q ) 83%) of
the anisyl group, certainly by comparison with those of the tolyl group (φ
) 36.5° and q ) 39%). The other structural parameters for 32+ are d )
1.502 Å, θ ) 56.0°; for the anisyl groups, l ) 1.401 Å, d1 ) 1.437 Å, d2
) 1.363 Å, φ ) 18.2°, q ) 83%; and for the tolyl group, l ) 1.453 Å, d1
) 1.412 Å, d2 ) 1.377 Å, φ ) 36.5 deg, q ) 39%.
(38) (a) Bock, H.; Ruppert, K.; Merzweiler, K.; Fenske, D.; Goesmann,
H. Angew. Chem., Int. Ed., Engl. 1989, 20, 1684. (b) Elbl-Weiser, K.;
Krieger, C.; Staab, H. A. Angew. Chem., Int. Ed. Engl. 1990, 29, 211. (c)
Takanori, S.; Shiohara, H.; Monobe, M.; Sakimura, T.; Tanaka, S.;
Yamashita, Y.; Miyashi, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 455.
(d) Bock, H.; Na¨ther, C.; Havlas, Z. J. Chem. Soc., Chem. Commun. 1995,
1111. See also: (e) Baenziger, N. C.; Buckles, R. E.; Simpson, T. D. J.
Am. Chem. Soc. 1967, 89, 3405.
radical (and dication) salts with aromatic cation-radical salts [EA+•
-
SbCl6-] (λmax ) 486 nm, log ꢀ486 ) 3.66 M-1 cm-1) and [MA+• SbCl6
]
(λmax ) 518 nm, log ꢀ518 ) 3.86 M-1 cm-1) in dichloromethane,
preparative oxidation of tetraarylethylenes with [Et3O+ SbCl6-] and
antimony pentachloride for the isolation and crystallization of dication
(and cation-radical) salts, oxidation of electron-rich donors 1, 7, and 8
with tetraanisylethylene dication salt [12+ (SbCl6-)2], measurements of
cation-radical disproportionation constant, and isolation and X-ray
crystallography of single crystals of various dication salts, cation-radical
and carbenium salts (discussed above), and neutral tetraarylethylenes
are described in detail in the Supporting Information Available.
Acknowledgment. We thank the National Science Founda-
(39) (a) Lammertsma, K.; Barzaghi, M.; Olah, G. A.; Pople, J. A.; Kos,
A. J.; Schleyer, P. v. R. J. Am. Chem. Soc. 1983, 105, 5252. (b)
Lammertsma, K.; Schleyer, P. v. R.; Schwarz, H. Angew. Chem., Int. Ed.
Engl. 1989, 28, 1321 and references therein.
(40) Merer, A. J.; Schoonveld, L. Can. J. Phys. 1969, 47, 1731. Koppel,
H.; Domcke, W.; Cederbaum, L. S.; von Niessen, W. J. Chem. Phys. 1978,
69, 4252. See also: Shiotani, M.; Nagata, Y.; Sohma, J. J. Am. Chem. Soc.
1984, 106, 4604. Fujisawa, J.; Sato, S.; Shimokoshi, K. Chem. Phys. Lett.
1986, 124, 391.
(41) (a) Lunell, S.; Eriksson, L. A.; Huang, M. B. J. Mol. Struct.
(THEOCHEM) 1991, 230, 263. (b) Dewar, M. J. S.; Thiel, W. J. Am. Chem.
Soc. 1977, 99, 4899. (c) Bellville, D. J.; Bauld, N. L. J. Am. Chem. Soc.
1982, 104, 294. (d) Alvarez-Idaboy, J. R.; Eriksson, L. A.; Fa¨ngstro¨m, T.;
Lunell, S. J. Phys. Chem. 1993, 97, 12737. For twisting in other cation
radicals see: (e) Clark, T.; Nelsen, S. F. J. Am. Chem. Soc. 1988, 110,
868. (f) Takahashi, O.; Kikuchi, O. J. Mol. Struct. (Theochem.) 1994, 313,
207. (g) Gerson, F.; Lopez, J.; Krebs, A.; Wolfgang, R. Angew. Chem.,
Int. Ed. Engl. 1981, 20, 95.
(42) (a) Bock, H.; Ruppert, K.; Fenske, D. Angew. Chem., Int. Ed. Engl.
1989, 28, 1685. Compare also: (b) Walczack, M.; Stucky, G. D. J.
Organomet. Chem. 1975, 97, 313. Sekiguchi, A.; Nakanishi, T.; Kabuto,
C.; Sakurai, H. J. Am. Chem. Soc. 1989, 111, 3748. For the progressive
structural changes in tetracyanoethylene, anion radical, and dianion with
various countercations, see: Bock, H.; Ruppert, K.; Na¨ther, C.; Havlas, Z.;
Hermann, H.-F.; Arad, C.; Go¨bal, I.; John, A.; Meuret, J.; Nick, S.;
Rauschenbach, A.; Seitz, W.; Vaupel, T.; Solokui, B. Angew. Chem., Int.
Ed. Engl. 1992, 31, 550 and references therein. Note also that theoretical
calculations on ethylene dianion predict θ ) 90° and d ) 1.40 Å. Kos, A.
J.; Jemmis, E. D.; Schleyer, P. v. R.; Gleiter, R.; Fishback, V.; Pople, J. A.
J. Am. Chem. Soc. 1981, 103, 4996.
tion and Robert A. Welch Foundation for financial support.
Supporting Information Available: Materials and instru-
mentation used, synthesis of 3 and 12, the detailed procedure
for the oxidation of donors 1-6 with MA+• (and/or EA+•) and
donors 1, 7, and 8 with the dication salt (12+), the measurement
of the disproportionation constants, preparative isolation and
crystallization of the dication and cation-radical salts, as well as
the X-ray crystal structure data for 1, [1+• SbCl6-], [12+ (SbCl6
)
-
(Sb2Cl7-)‚1.7CH2Cl2], [22+ (SbCl6-)2], [32+ (SbCl6-)2], 3, and
[12+ SbCl6-] (68 pages, print/PDF). See any current masthead
page for ordering information and Web access instructions.
JA981095W
(43) We note that the loss of π-electrons from the central CR-Câ bond
in various tetraarylethylene dications leads to an essentially single C-C
bond [for 12+ (d ) 1.503 Å), 22+ (d ) 1.502 Å), and 32+ (d ) 1.499 Å)].
As such, the energy requirements for the change from a planar to the bisected
conformation (for whatever other reasons) are expected to be rather small.44
The observed deviations in the values of θ from 90° in various tetraaryl-
ethylene dications may be a result of crystal packing forces.
(44) See, e.g.: Oki, M. Top. Stereochem. 1983, 14, 1.
(45) McMurry, J. E.; Fleming, M. P. J. Am. Chem. Soc. 1974, 96, 4708.
(46) Rathore, R.; Weigand, U.; Kochi, J. K. J. Org. Chem. 1996, 61,
5246.
(47) Hart, H.; Teuerstein, A.; Babin, M. A. J. Am. Chem. Soc. 1981,
103, 903.