Charge-transfer probes for molecular recognition via steric hindrance in donor-acceptor pairs

Article abstract of DOI:10.1021/ja9720319

Molecular association of various aromatic hydrocarbons (D, including sterically hindered donors) with a representative group of diverse acceptors (A = quinone, trinitrobenzene, tetracyanoethylene, tropylium, tetranitromethane, and nitrosonium) is visually apparent in solution by the spontaneous appearance of distinctive colors. Spectral (UV-vis) analyses of the colored solutions reveal their charge-transfer origin (λ(CT)) and they provide quantitative information of the intermolecular association in the form of the K(DA) and ε(CT) values for the formation and visualization, respectively, of different [D,A] complexes. Importantly, such measurements establish charge-transfer absorption to be a sensitive analytical tool for evaluating the steric inhibition of donor-acceptor association. For example, the steric differences among various hindered aromatic donors in their association with quinone are readily dramatized in their distinctive charge-transfer (color) absorptions and verified by X-ray crystallography of the charge-transfer crystals and/or QUANTA molecular modeling calculations of optimum intermolecular separations allowed by van der Waals contacts.

Full text of DOI:10.1021/ja9720319

J. Am. Chem. Soc. 1997, 119, 9393-9404
9393
Charge-Transfer Probes for Molecular Recognition
Via Steric Hindrance in Donor-Acceptor Pairs
R. Rathore, S. V. Lindeman, and J. K. Kochi*
Contribution from the Department of Chemistry, UniVersity of Houston,
Houston, Texas 77204-5641
ReceiVed June 19, 1997X
Abstract: Molecular association of various aromatic hydrocarbons (D, including sterically hindered donors) with a
representative group of diverse acceptors (A ) quinone, trinitrobenzene, tetracyanoethylene, tropylium, tetrani-
tromethane, and nitrosonium) is visually apparent in solution by the spontaneous appearance of distinctive colors.
Spectral (UV-vis) analyses of the colored solutions reveal their charge-transfer origin (λCT), and they provide
quantitative information of the intermolecular association in the form of the KDA and ꢀCT values for the formation
and visualization, respectively, of different [D,A] complexes. Importantly, such measurements establish charge-
transfer absorption to be a sensitive analytical tool for evaluating the steric inhibition of donor-acceptor association.
For example, the steric differences among various hindered aromatic donors in their association with quinone are
readily dramatized in their distinctive charge-transfer (color) absorptions and verified by X-ray crystallography of
the charge-transfer crystals and/or QUANTA molecular modeling calculations of optimum intermolecular separations
allowed by van der Waals contacts.
Introduction
charge-transfer (CT) transitions between electron donors and
electron acceptors is especially useful and easy to apply.4-6
Indeed, the ubiquitous CT absorptions are diagnostic of a very
wide spectrum of intermolecular electron donor-acceptor (DA)
interactions arising in extremely stable, isolable 1:1 complexes
on one hand,7 to highly transient complexes (with collisional
lifetimes) at the other extremum.8 From a structural point of
view, however, it is not at all clear a priori what the critical
donor-acceptor encounter (distance) must be for the relevant
charge-transfer absorptions to be in evidence.9
In order to establish the limits to which charge-transfer is
applicable as an analytical probe for intermolecular interactions,
we employ in this study four classes of aromatic donors (Chart
1), in which the essential benzenoid (donor) π-chromophore is
sterically encumbered to various degrees by increasing alkyl
bulk. Thus, the first (simply methylated) member in each class
represents the sterically most accessible donor, and all members
in each class are of comparable donor strengths.10
Molecular recognition and preassociation are conceptually
vital to catalytic stereospecificity and other contemporaneous
topics in organic chemistry like self-assembly and organization,
supramolecular (host-guest) chemistry, etc.1-3 Of the various
measures available for the quantitative evaluation of intermo-
lecular interactions in solutionsespecially weak nonbonded
onessthe appearance of new spectral bands arising from the
X Abstract published in AdVance ACS Abstracts, September 15, 1997.
(1) (a) Meissner, R. S.; Rebek, J., Jr.; de Mendoza, J. Science 1995,
270, 1485. (b) Grotzfeld, R. M.; Branda, N.; Rebek, J., Jr. Science 1996,
271, 487. (c) Wintner, E. A.; Rebek, J., Jr. Acta Chem. Scand. 1996, 50,
469 and references therein. (d) Mascal, M. Contemp. Org. Syn. 1994, 1, 1.
(e) Lawrence, D. S.; Jiang, T.; Levett, M. Chem. ReV. 1995, 95, 2229. (f)
Still, C. W. Acc. Chem. Res. 1996, 29, 155. (g) Hartshorn, C. M.; Steel, P.
J. J. Chem. Soc., Chem. Commun. 1997, 541.
(2) (a) Cram, D. J. Nature 1992, 356, 29. (b) Cram, D. J. Angew. Chem.,
Int. Ed. Engl. 1988, 27, 1009 and references therein. (c) Lehn, J.-M. Angew.
Chem., Int. Ed. Engl. 1988, 27, 89 and references therein. (d) Lehn, J.-M.
Pure Appl. Chem. 1994, 66, 1961. (e) Hunter, C. A. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1679. (f) Metzger, A.; Lynch, V. M.; Anslyn, E. V.
Angew. Chem., Int. Ed. Engl. 1997, 36, 862.
(3) (a) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35,
1154. (b) Balzani, V. Tetrahedron 1992, 48, 10433. (c) Stang, P. J.; Olenyuk,
B. Angew. Chem., Int. Ed. Engl. 1996, 35, 732. For donor-acceptor
interactions in the solid state for crystal engineering, see: Desiraju, G. R.
Angew. Chem., Int. Ed. Engl. 1995, 34, 2311.
(4) (a) Mulliken, R. S. J. Am. Chem. Soc. 1952, 74, 811. (b) Mulliken,
R. S.; Person, W. B. Molecular Complexes. A Lecture and Reprint Volume;
Wiley: New York 1969. (c) Foster, R. Organic Charge-Transfer Complexes;
Academic: New York, 1969. (d) Briegleb, G. Elektronen - Donator-
Acceptor Komplexe; Springer: Berlin, 1961.
(5) (a) The charge-transfer absorption (λCT) generally occurs in the UV-
vis region with hc/λCT ) IP - EA - ω,4 where IP is the ionization potential
of the electron donor (D), EA is the electron affinity of the electron acceptor
(A), and ω is the electrostatic energy of the ion pair [D•+, A•-]. (b) To first
approximation, the separation d in the DA complex is directly related to ω,
i.e., d ) e2/ω which is especially applicable to planar donors and acceptors.
(6) Charge-transfer interactions have been employed in the design/
synthesis of self-assembling catenanes, rotaxanes, and other molecular
machines. See: (a) Bissell, R. A.; Co´rdova, E.; Kaifer, A. E.; Stoddart, J.
F. Nature 1994, 369, 133. (b) Co´rdova, E.; Bissell, R. E.; Kaifer, A. E. J.
Org. Chem. 1995, 60, 1033. (c) Asakawa, M.; Ashton, P. R.; Boyd, S. E.;
Brown, C. L.; Gillard, R. E.; Kocian, O.; Raymo, F. M.; Stoddart, J. F.;
Tolley, M. S.; White, A. J. P.; Williams, D. J. J. Org. Chem. 1997, 62, 26.
(d) Toki, A.; Yonemura, H.; Matsuo, T. Bull. Chem. Soc. Jpn. 1993, 66,
3382.
(7) For example, the highly colored trinitrobenzene/arene complexes have
found extensive use in the purification and characterization of aromatic
hydrocarbons owing to their ready crystallization with definite stoichiometry
and sharp melting points. See: Weiss, J. J. Chem. Soc. 1942, 245. Powell,
H. M.; Huse G.; Cook, P. W. J. Chem. Soc. 1943, 153. Parini, V. P. Russ.
Chem. ReV. 1962, 31, 408.
(8) The transitory contact charge-transfer complexes have been described
by: Tamres, M.; Strong, R. L. In Molecular Association; Foster, R. F.,
Ed.; Academic: New York, 1979; Vol. 2, p 331ff.
(9) (a) There are a few cases mentioned in the literature where electron-
rich but sterically congested donors such as hexaisopropylbenzene (Arnett,
E. M.; Bollinger, J. M. J. Am. Chem. Soc. 1964, 86, 4729), hexacyclopro-
pylbenzene (Usieli, V.; Victor, R.; Sarel, S. Tetrahedron Lett. 1976, 2705),
and 1,4:5,8:9,12-triethanododecahydrotriphenylene (Komatsu, K.; Aonuma,
S.; Jimbu, Y.; Tsuji, R.; Hirosawa, C.; Takeuchi, K. J. Org. Chem. 1991,
56, 195), etc. show no charge-transfer absorptions with such commonly
used electron acceptors as tetracyanoethylene (see: Merrifield, R. E.;
Phillips, W. D. J. Am. Chem. Soc. 1950, 80, 2778 and Foster, R. in ref 4c,
p 197). However, there is no systematic study extant in the literature to
describe steric effects on donor-acceptor complexation.
(10) As evaluated by the ionization potentials (IP, gas phase) and
oxidation potentials (Eox°, solution) of benzenoid hydrocarbons, which are
mostly dependent on the number and position of carbon-centered substit-
uents, see: Howell, J. O.; Goncalves, J. M.; Amatore, C.; Klasinc, L.;
Wightman, R. M.; Kochi, J. K. J. Am. Chem. Soc. 1984, 106, 3968. For
the relevant values of IP and Eox° of the other donors in Chart 1, see Tables
2 and 3.
S0002-7863(97)02031-3 CCC: $14.00 © 1997 American Chemical Society

9394 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Rathore et al.
Chart 1
1 to the prototypical π-acceptor chloranil (CA) spontaneously
resulted in brightly colored dichloromethane solutions. The
colorations progressively evolved from yellow (mesitylene) to
orange (durene) to purple (hexamethylbenzene) in line with the
decreasing ionization potentials of the aromatic donors.10
Similar vivid colorations were also observed when the same
methylbenzenes were mixed with other electron acceptors such
as 1,3,5-trinitrobenzene (TNB), and further red-shifts in colors
occurred with the stronger π-acceptor tetracyanoethylene (TCNE)
as well as the tropylium cation (TR+) in Chart 2.
II. Quantification of Donor-Acceptor (π-π) Associa-
tions. The UV-vis spectral changes in Figure 1 typically show
the monotonic growth of the diagnostic charge-transfer absor-
bance with its visible maximum at λCT ) 520 nm upon the
incremental addition of hexamethylbenzene (HMB in Chart 1)
to a solution of chloranil (CA) in dichloromethane at 24 °C.
This well-resolved (featureless) absorption band was ascribed
to the intermolecular donor-acceptor association, i.e.
HMB + CA {K } [HMB, CA]
(1)
DA
in which the characteristic color derives from the charge-transfer
transition, as originally formulated by Mulliken.4 Such a
structural assignment was readily verified by the isolation of
dark purple crystals of the 1:1 complex in >95% yield from an
equimolar solution of hexamethylbenzene and chloranil, simply
by the very slow removal of dichloromethane in Vacuo. X-ray
crystallography of the charge-transfer crystal revealed the
hexamethylbenzene to be directly juxtaposed on the chloranil
and separated by an interplanar distance of d ) 3.51 Å, as
illustrated in the top and side perspectives AT and AS (all
hydrogens omitted for clarity).13,14 Indeed, the observed donor-
acceptor separation in the purple crystal is remarkably close to
In a complementary way, we identify two classes of sterically
graded electron acceptors in Chart 2. As such, the planar
π-acceptors are presented in the order of their increasing size
from tetracyanoethylene to 1,3,5-trinitrobenzene. Among the
σ-acceptors, the diatomic nitrosonium cation and dibromine are
the smallest and least subject to steric hindrance, especially in
comparison with the larger, tetrahedral acceptors tetrani-
tromethane and carbon tetrabromide. The relative acceptor
strengths in Chart 2 are indicated by the trend in the reduction
potentials (E°red V vs SCE).11,12
Donor-acceptor pairs in solution are quantitatively monitored
in this study by UV-vis spectral changes, and the relevant
charge-transfer interactions identified in X-ray crystal structures
and compared with molecular modeling calculations. Steric
effects are not only exploited in the structural requirements for
the intermolecular formation of 1:1 donor-acceptor complexes
but also to achieve intramolecular selectivity in biaryls and in
a tethered donor containing more than one aromatic center.
Chart 2
the calculated distance of d ) 3.57 Å by energy minimization
of the intermolecular van der Waals contacts between hexa-
methylbenzene and chloranil. The predicted structure based on
the QUANTA molecular modeling analysis15 is shown in the
space-filling representation B below.
Results and Discussion
I. Visual Detection of Intermolecular Donor-Acceptor
Associations. Exposure of the various methylbenzenes in Chart
For the quantitative analysis of the donor-acceptor associa-
tion in solution, the spectrophotometric absorbance changes in
(11) The acceptor strengths in Chart 2 can be conveniently evaluated by
the reduction potentials.4 For TCNE, Ered° ) + 0.24 V: Rehm, D.; Weller,
A. Isr. J. Chem. 1970, 8, 259. Tropylium, Ered° ) - 0.18 V: Wasielweski,
M. R.; Breslow, R. J. Am. Chem. Soc. 1976, 98, 4222. Chloranil, Ered° )
0.02 V: Mann, C. K.; Barnes, K. K. Electrochemical Reactions in
Nonaqueous Systems; Dekker: New York, 1970. Trinitrobenzene, Ered° )
-0.42: Ko¨rtu¨m, G.; Walz, H. Z. Electrochem. 1955, 59, 184.
(12) For nitrosonium, Ered° ) + 1.28 V: Lee, K. Y.; Kuchynka, D. J.;
Kochi, J. K. Inorg. Chem. 1990, 29, 4196; carbon tetrabromide, Ered° )
-0.30 V: Stackelberg, M.; Stracke, W. Z. Electrochem. 1949, 53, 118.
Also, see: Al-Ekabi, H.; Draper, A. M.; de Mayo, P. Can. J. Chem. 1989,
69, 1061. Tetranitromethane, Ered° ) 0.00 V: Altukhov, K. V.; Perekalin,
V. V. Russ. Chem. ReV. 1976, 45, 1052. Bromine, Ered° is unmeasured.

Molecular Recognition in Donor-Acceptor Pairs
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9395
Table 1. Donor-Acceptor Association of Various Methylbenzenes
with Different π-Acceptorsa
Figure 1. Spectral (UV-vis) changes attendant upon the incremental
addition of hexamethylbenzene to chloranil in dichloromethane. Inset:
Benesi-Hildebrand plot.
a In dichloromethane containing 4-6 mM acceptor and 50-500 mM
aromatic donors at 24 °C.
tropylium, and tetracyanoethylene, as listed in Tables 1 and 4.
For comparative purposes, the intensities of the CT (color)
absorptions of the various donor-acceptor complexes in Table
1 are given by the values of KDAꢀCT in column 7.18
Figure 1 were treated by the Benesi-Hildebrand procedure,16
i.e.
[CA]
ACT
1
ꢀCT
1
1
[D]
III. Complete Steric Inhibition of Donor-Acceptor (π-
π) Association. Analogous exposure of hexaethylbenzene
(HEB) to chloranil led to no coloration, and no new CT
absorption band was observed in the colorless dichloromethane
solution even in the presence of a large excess of HEB.
Furthermore, many attempts to isolate charge-transfer crystals
of the HEB complex with chloranil in various molar ratios from
dichloromethane, chloroform, acetone, ethyl acetate, etc. were
all unsuccessful, and low-temperature crystallization merely led
to phase separation of the individual (pure) components. Such
a striking difference between hexaethylbenzene and hexameth-
ylbenzene (Vide supra) was not restricted to chloranil. Thus,
all other π-acceptors including trinitrobenzene, tropylium, and
tetracyanoethylene showed distinctive CT colorations ranging
from yellow to orange to green when mixed with hexamethyl-
benzene, whereas no (or very faint) colorations were detected
with hexaethylbenzene at even higher concentrations. The UV-
vis spectral changes in Figure 2 confirmed that neither chloranil
nor trinitrobenzene participated in charge-transfer association
with hexaethylbenzene, and the smallest acceptors tetracyano-
ethylene and tropylium showed (at best) very weak CT
interactions with HEB (Table 2).19,20
The comparative charge-transfer behaviors notwithstanding,
the intrinsic electron-donor properties of HEB are even
somewhat better than those of HMB insofar as their relative
oxidation potentials of Eox° ) 1.59 and 1.62 V Vs SCE,
respectively.10 In order to clarify this anomalous variation (of
λCT Vs Eox°) between HMB and HEB, we synthesized the novel
hybrid triethylmesitylene (TEM) with an intermediate oxidation
potential of Eox° ) 1.61 V Vs SCE. In fact, the exposure of
TEM to chloranil immediately led to the characteristic purple
coloration of the charge-transfer association (λCT ) 516 nm),
and dark purple crystals of the EDA complex were readily
isolated in high yields from an equimolar mixture of triethyl-
mesitylene and chloranil. Indeed, the charge-transfer parameters
of the purple [TEM, CA] were essentially identical to those of
the HMB analogue. Further comparisons of the TEM and
)
+
(2)
K
DAꢀCT
where ACT is the molar absorbance and ꢀCT is the extinction
coefficient of the charge-transfer band at the monitoring
wavelength (generally close to λmax). For hexamethylbenzene
(D) concentrations much greater than that of chloranil, a plot
of [CA]/ACT Vs the reciprocal donor concentration was linear,
and the least-squares fit produced a correlation coefficient of
greater than 0.999 in the inset of Figure 1. From the slope
[KDAꢀCT] ]
-1 and the intercept [ꢀCT -1, the values of the associa-
tion constant and the extinction coefficient were readily extracted
as KDA ) 2.8 M-1 and ꢀ520 ) 2800 M-1 cm-1, respectively.
Such a limited magnitude of KDA for hexamethylbenzene and
chloranil indicated that the donor-acceptor interaction is
described as weak at best (∆G°DA ) -0.6 kcal mol-1), as typical
for the spontaneous formation of electron donor-acceptor
complexes of quinones (and other π-electron acceptors) with
various types of other electron donors (such as alkenes, enol
ethers, sulfides, etc.).17
The donor-acceptor interactions of the homologous meth-
ylbenzenes in Chart 1 with chloranil showed a progressive red-
shift of the charge-transfer band λCT for HMB > DUR > MES,
as listed in Table 1 (column 4). The same trend was observed
in the association constants KDA, but the magnitude of the
change was somewhat limited (see column 5). Weak but
distinctive donor-acceptor interactions of hexamethylbenzene
were also indicated by the comparison of the charge-transfer
absorptions (λCT) and the magnitudes of the formation constants
(KDA) with the other π-acceptors including trinitrobenzene,
(13) Harding, T. T.; Wallwork, S. C. Acta Crystallogr. 1955, 8, 787.
Also, see: Jones, N. D.; Marsh, R. E. Acta Crystallogr. 1962, 15, 809.
(14) Molecular structures presented hereinafter as PLUTO plots (Moth-
erwell, W. D. S.; Clegg, W. Program for Plotting Molecular and Crystal
Structures 1978, Cambridge, U. K.) were produced with the aid of an XP-
graphical package.56
(15) QUANTA (vers. 4.11) from Molecular Simulations, Inc., 16 New
England Executive Park, Burlington, MA 081803-5297. See: Experimental
Section for a brief description of the molecular modeling package.
(16) (a) Benesi, H. A.; Hildebrand, J. J. J. Am. Chem. Soc. 1949, 71,
2703. (b) Person, W. B. J. Am. Chem. Soc. 1965, 87, 167. (c) Foster, R.
Molecular Complexes; Crane, Russak & Co.: New York, 1974; Vol. 2.
(17) (a) Horner, L.; Merz, H. Ann. Chem. 1950, 89, 570. (b) Rathore,
R.; Kochi, J. K. Tetrahedron Lett, 1994, 35, 8577. (c) Bockman, T. M.;
Perrier, S.; Kochi, J. K. J. Chem. Soc., Perkin Trans. 2 1993, 595. (d)
Reichenback, G.; Santini, S.; Mazzucato, U. J. Chem. Soc., Faraday Trans.
1 1973, 49, 143.
(18) The slope of the Benesi-Hildebrand relationship as given by KDAꢀCT
is the “effective absorbance”, see: Frey, J. E.; Andrews, A. M.; Ankoviac,
G. G.; Beaman, D. N.; DuPont, L. E.; Elsner, T. E.; Lang, S. R.; Zwart, M.
A. O.; Seagle, R. E.; Torreano, L. A. J. Org. Chem. 1990, 55, 606.
(19) For the tetracyanoethylene data, see: Table 4.
(20) The values of KDAꢀCT in the table provide a numerical guide to the
visual intensity of the charge-transfer colors.18

9396 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Rathore et al.
Table 2. Donor-Acceptor Association of Sterically Hindered
Hexaalkylbenzenes with π-Acceptorsa
entries) and the homologous TMT. Energy minimization
between chloranil and HEB was predicted with the aid of
QUANTA molecular modeling calculations to occur at an
interplanar distance of d ) 4.50 Å in structure E, and the
predicted separation of chloranil and TET was d ) 4.51 Å in
structure F. As such, the difference of 0.9 Å between the
observed separation of 3.6 Å in [HMB, CA] and the predicted
a See Table 1. b Not determined. c No new absorption band. d Esti-
mated values, see Experimental Section.
HMB complexes with trinitrobenzene, tropylium, and tetracya-
noethylene in Tables 1 and 2 established the same similarities.
However, the association constants of the TEM complexes
(Table 2, column 5) were always roughly half the value of KDA
for the corresponding HMB complex (Table 1) in dichlo-
romethane. The latter was consistent with a statistical factor
of 0.5 for only half the faces available for association, as
established by the unique conformation of TEM in the chloranil
complex shown below in structure C by X-ray crystallography.
Such a tripodal arrangement of all three ethyl groups on the
opposite face of TEM is directly related to the conformation
separation of 4.5 Å in [HEB, CA] could represent a “gray”
area in which very weak, but visually (color) and spectrally
(CT) observable association may be apparent. In order to pursue
this possibility, we synthesized a series of unsymmetrical
aromatic donors designed to cover the benzenoid face only
partially.
IV. Steric Modulation of Donor-Acceptor (π-π) As-
sociations. Mesitylene (MES) yielded a bright yellow solution
when exposed to chloranil, but 1,3,5-tri-tert-butylbenzene (TTB)
under the same conditions, as expected, led to no coloration.
However, intermediate behavior was shown by the homologous
tert-butylxylene (TXY) which afforded a very pale yellow
solution with chloranil, and the coloration with di-tert-butyl-
toluene (DTT) was barely discernible. The quantitative effects
of these color (intensity) changes are given by the values of
of the ethyl groups on both faces of HEB with quasi D3d
symmetry in structure D which was previously established by
Mislow and co-workers,21 i.e.
K
DAꢀCT in Table 3 (column 6),20 which were obtained from the
spectrophotometric analysis of the charge-transfer absorptions
(Figure 3A) attendant upon the incremental additions of these
aromatic donors to chloranil in dichloromethane. The qualitative
trend of the color intensity followed the monotonic decrease in
the association constant KDA with increasing number of tert-
butyl groups.22 However, the latter had no significant effect
on the intrinsic donor strength, since the values of the oxidation
potentials Eox° tabulated in column 2 were uniformly invariant.
This observation, taken together with the constancy of the
charge-transfer transition (λCT in column 3), indicated that the
interplanar separations (d) between the chloranil and MES,
From such a comparative behavior of HMB, TEM, and HEB
with chloranil as well as the other π-acceptors TNB, TR+, and
TCNE it is easy to conclude that a group of three (1,3,5) ethyl
substituents is sufficient for the complete steric inhibition of
the face of a benzenoid (donor) chromophore for intermolecular
association by a π-acceptor. Moreover, the comparable steric
inhibition is achieved by tris-annulations at the R-carbons, as
presented in the bicyclic structure TET (see Table 2, last 3
(21) (a) Iverson, D. J.; Hunter, G.; Blount, J. F.; Damewood, Jr., J. R.;
Mislow, K. J. Am. Chem. Soc. 1981, 103, 6073. (b) See, also: Hunter, G.;
Iverson, D. J; Mislow, K.; Blount, J. F. J. Am. Chem. Soc. 1980, 102, 5942.
(c) Hunter, G.; Weakley, J. R.; Mislow, K.; Wong, M. G. J. Chem. Soc.,
Dalton Trans. 1986, 577.
(22) The sharply decreasing trend in the values of KDAꢀCT relates to the
corresponding changes in color intensities arising from the chloranil
complexes with MES, TXY, and DTT in Table 3. Such differences in color
intensity result mainly from the variations of ꢀCT since the values of KDA
are relatively invariant.

Molecular Recognition in Donor-Acceptor Pairs
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9397
Figure 2. Steric hindrance in the CT absorption bands of HMB (thick line), TEM (dashed line), and HEB (thin line) complexed with (A) chloranil,
(B) trinitrobenzene, and (C) tropylium obtained by spectral subtraction of the uncomplexed acceptor from the experimental UV-vis spectra for (A)
6 mM CA with 50 mM HMB, 50 mM TEM, and 500 mM HEB in CH2Cl2, (B) 5 mM TNB with 100 mM HMB, 100 mM TEM, and 500 mM
-
HEB in CH2Cl2, and (C) 9 or (4.5) mM TR+ BF4 with 22 mM HMB, 22 mM TEM, and (22 mM) HEB in CH3CN.
Figure 3. Partial steric hindrance in donor-acceptor association of (A) 4 mM CA with 250 mM MES, TXY, and DTT, (B) 4 mM CA with 240
mM ME, MEA, and MEA2, and (C) 1.7 mM TCNE with 102 mM PET and HEB in dichloromethane, obtained as difference spectra analogous
to those in Figure 2.
TXY, and DTT in the donor-acceptor associations were all
comparable,23 despite the decreasing strength of the interaction
(∆GDA). In order to identify the origin of the difference, we
carried out the QUANTA molecular modeling analysis of the
nonbonding interactions in all four donor-acceptor associations.
It is particularly noteworthy that energy minimization was
predicted to occur at essentially the same interplanar separation
of d ) 3.4 Å in the chloranil complex with MES, TBX, and
DTT in structures G, H, and J, respectively, but at a
significantly larger separation of d ) 4.5 Å for tri-tert-
butylbenzene in structure K. Such a donor-acceptor association
in the mono- and di-tert-butyl-substituted donors was achieved
by a small parallel shift of chloranil away from the tert-butyl
group(s).24
Among the class II donors (based on durene in Chart 1), the
tetramethyl derivative TMA showed the same “partial” steric
behavior relative to the completely hindered octamethyl ana-
logue OMA (and the bicyclic version DMA) at the other
extremum. For example, the results in Table 3 (entry 6) point
to the strongly diminished donor-acceptor interaction of TMA
relative to DUR in its association with chloranil,26 in a manner
similar to the differentiation of TXY and MES in the tert-butyl
series (class III). Similarly, the bis-annulated donors OMA and
DMA were subject to complete steric inhibition, much like tri-
tert-butylbenzene in class III and TET and TMT in class I.
The replacement of a pair of methyl groups in HMB with
methoxy groups render the class IV aromatic ethers to be the
best π-donors by virtue of the low Eox° values in Table 3.27 As
such, these methyl ethers were more tolerant to steric
encumbrancesthe bis-annulated MEA2 being much less subject
to partial steric hindrance than its counterpart DMA in class II,
Indeed, such limited lateral displacements along the aromatic
planes are not expected to be important factors in the charge-
25
transfer transitions to significantly alter the values of λCT
.

9398 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Rathore et al.
Table 3. Donor-Acceptor Association of the Class II, III, and IV
Table 4. Donor-Acceptor Association of Hexaalkylbenzenes with
Aromatic Donors with Chloranila
Tetracyanoethylenea
a From 2 mM TCNE and 10-100 mM donor in dichloromethane
at 24 °C. c ,dSee Table 2.
substantially less than those evaluated for either HMB or TEM.
In particular, the value of ꢀCT ) 300 M-1 cm-1 for the [PET,
TCNE] was sharply diminished from ꢀCT ) 5200 M-1 cm-1
for [HMB, TCNE], and such a significant difference could
result from the reduced π-orbital overlap of PET and TCNE.
Although repeated attempts to grow single crystals of [PET,
TCNE] suitable for X-ray crystallography failed, QUANTA
molecular modeling studies showed that donor-acceptor as-
sociation was possible at an interplanar separation of d ) 3.5
Å by a parallel shift along the aromatic plane, much in the
manner previously described in structures H and J.24 It is
noteworthy that with the exception of the lateral slippage of
∼2 Å, structure L for [PET, TCNE] is akin to the centrosym-
metric structure of [HMB, TCNE] with d ) 3.35 Å (established
by X-ray crystallography)29 as illustrated in the top perspective
M. If so, the donor-acceptor association observed between
the highly hindered HEB and tetracyanoethylene, albeit weak
(Table 4, entry 4),20 may be achieved by a conformational
change of a single ethyl group in hexaethylbenzene by a 180°
rotation about the aromatic-CR bond,21b so as to effect the
partial steric hindrance analogous to the [PET, TCNE] structure
in L.24 Indeed the complete absence of any donor-acceptor
association of the conformationally rigid tris-annulated donor
TET with tetracyanoethylene (Table 4, entry 5) lends a certain
credence to this possibility. Be that as it may, the enhanced
donor-acceptor association of the tetracyanoethylene with HEB
(compared to that with chloranil) was in line with its smaller
size and increased acceptor strength. In order to consider these
factors further, we next asked how molecular shape could
influence the selectivity in donor-acceptor associations by
utilizing (a) the powerful diatomic cation NO+ with Ered° )
1.28 V Vs SCE as well as the uncharged tetranitromethane
(TNM) as rather small σ-acceptors12 and (b) the substituted
biaryls and a tethered aromatic system to serve as bichro-
mophoric donors, as follows.
a From 4 mM CA and 0.01-1 M donor in dichloromethane at 24
°C. b -dSee Table 2.
as shown by a direct comparison of KDA and KDAꢀCT in entry
11 with those in entry 8 in Table 3. Repeated attempts to grow
single crystals of the weak yellow complex [MEA2,CA] for
X-ray crystallography were unsuccessful. However, from the
significantly blue-shifted value of λCT ) 452 nm in column 4,
we tentatively conclude that donor-acceptor association of the
strong donor MEA2 and chloranil probably occurs at an
intermediate interplanar separation of d greater than 3.5 Å (but
less than 4.5 Å).28
A close inspection of the charge-transfer absorption of the
small and rather powerful π-acceptor tetracyanoethylene (TCNE)
with high concentrations of the highly hindered hexaethylben-
zene revealed a weak but distinctive absorbance at λCT ) 540
nm (Table 4).20 In order to determine how hexaethylbenzene
could be subject to partial steric hindrance, we synthesized the
hybrid pentaethyltoluene (PET) to establish evidence for
donor-acceptor association with TCNE. UV-vis spectral
analysis (Figure 3c) indeed revealed a blue complex to be
formed in CH2Cl2 with values of KDA and ꢀCT which were
(23) For the relationship that connects the separation (d) with λCT and
ꢀCT, see: footnote 5.
(26) The results in Table 3 (last 3 entries) show a large decrease in the
values of KDAꢀCT despite the opposite trend in the donor properties (Eox°)
of ME, MEA, and MEA2. Such a decrease in the effective absorbance,
without large changes in KDA, is attributed to the reduced overlap of the
donor and acceptor π-orbitals24c due to the steric shielding of the dimethoxy-
substituted benzene ring by the norbornane framework.
(24) (a) The barrier to the rotation of the aryl-ethyl bond in HEB has
been estimated to be 11.8 kcal mol-1 21a
(b) The reduced overlap of the
.
donor-acceptor π-orbitals as a result of such a parallel shift is reflected in
a corresponding decrease in the ꢀCT values with increasing number of tert-
butyl groups in Table 3 (see entries 1-4). (c) According to Mulliken,4a the
charge-transfer intensity (KDAꢀCT) derives from the transition moment which
relates to the overlap integral of the donor and acceptor orbitals (see: Orgel,
L. E.; Mulliken, R. S. J. Am. Chem. Soc. 1957, 79, 4839.
(25) See: Staab, M. A.; Reibel, W. R. K.; Krieger, C. Chem. Ber. 1985,
118, 1230.
(27) See: Foster, R. in ref 4 for the relationship between the donor
strength and Eox°, as measured electrochemically (see Experimental Section).
(28) Molecular modeling calculations predict an optimum separation of
∼4 Å.
(29) Maverick, E.; Trueblood, K. N.; Bekoe, D. A. Acta Crystallogr.,
Sect. B 1978, 34, 2777.

Molecular Recognition in Donor-Acceptor Pairs
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9399
Table 5. Donor-Acceptor Association of Hexaalkylbenzenes with
Nitrosonium Tetrafluoroboratea
a In acetonitrile containing 1 mM NOBF4 and 5-10 mM donor at
24 °C.
biguous assignment of the nitrogen terminus of the slightly tilted
NO+ to be centrally situated over the benzenoid (donor)
chromophore at a nonbonded distance of 2.08 Å in structures
N and P. The donor-acceptor association, characterized as the
V. Shape Selectivity of Aromatic Donors. A. With
σ-Acceptors. Exposure of nitrosonium (NO+) tetrafluoroborate
to hexamethylbenzene in acetonitrile immediately resulted in
an intense red coloration, and the UV-vis spectral analysis of
the bright red solution revealed a well-resolved CT absorption
band (λCT ) 337 nm) with a characteristic low-energy band
extending beyond 600 nm, as shown in Figure 4A. Although
the extinction coefficient of ꢀCT ) 3100 M-1 cm-1 in Table 5
was in line with those evaluated for the HMB complexes with
the π-acceptors in Table 1, the association constant of KDA
)
31000 M-1 was more than four orders of magnitude largers
indicative of an exceptionally strong donor-acceptor association
of NO+ with HMB.30
Surprisingly, the same red color (with comparable intensity)
was observed when hexaethylbenzene was treated with NO+BF4-
under identical (concentration) conditions, and the UV-vis
spectrum in Figure 4A confirmed the mostly unaltered charge-
transfer absorption. More surprising were the results in Table
5 which showed that the highly hindered tris-annulated donors
TET and TMT enjoyed undiminished donor-acceptor associa-
tion with NO+, the measured association constants in all cases
being uniformly large, with KDA > 3 × 104 M-1. In order to
account for such an unexpected stability, we grew red crystals
very close encounter of NO+ to the benzenoid centers of both
hexaethylbenzene and TET (inside van der Waals distance), is
achieved by significant incursion within the “picket fence”
formed by three (alternating) ethyl groups in structure N and
three ethano bridges in structure P. The tight fit of NO+ within
the van der Waals cavity in the HEB complex (shown in N) is
sufficient to severely dampen its librational (crystallographic)
disorder.31 It is particularly noteworthy that such a donor-
acceptor interaction derives from the intrinsic donor properties
of TET and HEB that are akin to that in the electron-rich
HMB,33 as shown by the constant values of Eox° in Table 4.
As such, the nitrosonium association in the TET and HEB
complexes (which occurs in the teeth of the potentially repulsive
interactions with the ethyl and ethano substituents) is allowed
by the dimensions of the (van der Waals) cavity sufficient to
accommodate the diatomic acceptor.34 The three-dimensional
requirements for the nestling of NO+ is graphically illustrated
-
of the HEB and TET complexes with NO+SbCl6 for X-ray
crystallographic analysis (see Experimental Section). Indeed,
the excellent quality of both single crystals allowed an unam-
(30) Kim, E. K.; Kochi, J. K. J. Am. Chem. Soc. 1991, 113, 4962.
(31) X-ray crystallographic analysis of various arene/NO+ complexes,32
generally showed the nitrosonium to be sufficiently disordered to obscure
the distinction between N and O. On the other hand, HEB and TET
complexes showed no such disorder, and N bonding to the aromatic donor
could be readily established.
(32) (a) Brownstein, S.; Gabe, E.; Lee, F.; Tan, L. J. Chem. Soc., Chem.
Commun. 1984, 1566. (b) Brownstein, S.; Gabe, E.; Lee, F.; Piotrowski,
A. Can. J. Chem. 1986, 64, 1661. (c) Brownstein, S.; Gabe, E.; Irish, B.;
Lee, F.; Louie, B.; Piotrowski, A. Can. J. Chem. 1987, 65, 445. (d) Borodkin,
G. I.; Nagi, S. M. Gatilov, Y. V.; Mamatyuk, V. I.; Mudrakovskii, T. L.;
Shubin, V. G. Dokl. Akad. Nauk SSSR 1986, 288, 1364. (e) Kim, E. K.:
Kochi, J. K. J. Org. Chem. 1993, 58, 786.
(33) Despite the steric encumbrance in the HEB and TET complexes,
the magnitude of the separation of d ) 2.08 Å is not significantly different
from that found in the HMB complex with NO+.

9400 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Rathore et al.
Figure 4. Steric effects in charge-transfer absorptions from σ-acceptors with aromatic donors: (A) 5 mM NO+BF4- with 1 mM HMB, HEB, and
-
TET (as indicated); NO+BF4 alone (‚‚‚) in acetonitrile and (B) 2 M TNM with 5 mM HMB, HEB and TET as indicated; TNM alone (‚‚‚) in
dichloromethane.
Table 6. Donor-Acceptor Association of Hexaalkylbenzenes with
by the space-filling representations R and S. For comparison,
Tetranitromethanea
the molecular structure of the unhindered hexamethylbenzene
complex32b is presented in Q.
a In dichloromethane containing 0.2 M TNM and 5-50 mM donor
at 24 °C.
tris-annulated donors TET and TMT were not significantly
different from HEB insofar as their association with TNM. We
interpret the rather invariant values of the extinction coefficients
in Table 6 (column 5) to be consistent with optimal CT coupling
As a further elaboration of the shape-selectivity of aromatic
of the small-sized TNM irrespective of steric hindrance from
donors, we examined the donor-acceptor interaction with the
the aromatic donor. Although attempts to grow single crystals
tetrahedral σ-acceptor tetranitromethane (TNM). Thus, the
of the TNM complexes were unsuccessful, the QUANTA
exposure of a colorless solution of TNM to hexamethylbenzene
molecular modeling calculations revealed rather large donor-
in dichloromethane immediately resulted in a dark red solution,
acceptor separations of a single NO2 group of TNM to the HMB
the UV-vis spectrum of which showed an intense (unresolved)
centroid in structure T and to the tris-annulated TET in structure
CT absorption that extended to well beyond 600 nm (Figure
U.36
4B).35 The quantitative treatment of the absorbance data
The mixture of hexamethylbenzene with carbon tetrabromide
according to the Benesi-Hildebrand procedure yielded the
(colorless) resulted in a pale yellow solution which showed a
values of KDA and ꢀCT for the donor-acceptor association in
weak tailing UV-vis absorbance without a discernible absorp-
Table 6 which were substantially less than those for the NO+
tion maximum arising from the blue-shifted charge-transfer
complex. It is particularly noteworthy that the association of
band. Such an overlap of the CT absorption with the local
TNM with the hindered HEB resulted in a blue-shift of the
absorption of CBr4 precluded a quantitative evaluation of the
nondescript CT tail absorption in Figure 4B, but the quantitative
donor-acceptor association.
(UV-vis) spectral analysis indicated that the association
B. In Bichromophoric Systems. Exposure of p,p′-dimeth-
constant of KDA ) 1.5 M-1 was only slightly less than that with
ylbiphenyl (T-T) to chloranil in dichloromethane was ac-
HMB (by about a factor of 3). Furthermore, the highly hindered
companied by an immediate color change to a bright purple
(34) The van der Waals diameters of the cavities in HEB and TET were
solution. UV-vis spectral analysis of the well-resolved CT
estimated to be ∼3.8 and 3.6 Å, respectively, based on the published
structures.9,21a
(36) Although the “center” of NO2 as the acceptor moiety in TNM is
unclear, an estimate of the separation is given by calculated distances of
2.9 and 3.0 Å of the oxygen pair to the aromatic plane in T. The
corresponding distances in U are 3.3 at 3.6 Å.
(35) Sankararaman, S.; Haney, W. A.; Kochi, J. K. J. Am. Chem. Soc.
1984, 109, 5235 and 7824. Also, see: Kochi, J. K. Acta Chem. Scand.
1990, 44, 409.

Molecular Recognition in Donor-Acceptor Pairs
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9401
absorption band (λCT ) 502 nm in Table 7) indicated a very
weak association of T-T to chloranil with KDA ) 0.09 M-1
and a value of ꢀCT ) 6000 M-1 cm-1 in line with the other
chloranil complexes with methylbenzenes in Table 1. By
analogy with the structure V (top perspective) of the biphenyl/
tetracyanobenzene complex previously established by X-ray
crystallography,37 a similar centrosymmetric structure for the
for the mononuclear HMB in Table 1 (entry 1). Indeed, X-ray
crystallographic analysis of the [HMB-MEA2, CA] complex
provided unambiguous support for the predicted structure Y.
intermolecular association of the bitolyl T-T with chloranil was
indicated. In marked contrast, the unsymmetrically permeth-
ylated homologue pentamethylphenyl-p-toluene (PM-T) af-
forded a dark purple solution. The UV-vis spectral analysis
of the charge-transfer absorption (λCT ) 510 nm) indicated a
substantially larger association with KDA ) 0.99 M-1 but
significantly diminished value of ꢀCT ) 700 M-1 cm-1, which
we interpreted as a shape-selective association of chloranil with
the pentamethylphenyl moiety. Indeed, X-ray crystallography
of the purple [PM-T,CA] confirmed this spectral assignment
and revealed the interplanar separation of d ) 3.41 Å with
significant slippage along the pentamethylphenyl plane to avoid
the orthogonal tolyl moiety as shown in the side and top
perspectives WS and WT.
Summary and Conclusions
Visualization (color) and attendant charge-transfer absorption
are reliable and sensitive analytical probes for monitoring the
donor-acceptor (DA) association of the various aromatic
hydrocarbons in Chart 1 with the different types of π- and
σ-acceptors in Chart 2. Thus, the facile molecular association
of the polymethylbenzenes, including hexamethylbenzene HMB,
with all the π-acceptors is immediately apparent by the vividly
colored solutions, the UV-vis spectral analysis of which reveals
new charge-transfer absorption bands (λCT) for the quantitative
By comparison, the unsymmetrically hindered pentaethyl-
phenyl-p-toluene (PE-T) with chloranil afforded a yellow
solution. The charge-transfer absorption λCT ) 437 nm and
ꢀCT ) 70 M-1 cm-1 in Table 7 were indicative of an analogous
slipped structure for [PE-T,CA], but with the chloranil com-
plexed to the less hindered tolyl moiety.38
Table 7. Donor-Acceptor Association of Various Biphenyls and
the Tethered Aromatic Donor with Chloranila
A qualitative view of steric hindrance was also useful in
predicting the site selectivity of donor-acceptor association in
bifunctional aromatic donors that were not directly connected,
as in the bichromophoric HMB-MEA2 in which hexamethyl-
benzene (HMB) was tethered to the bis-annulated ether MEA2.
Thus the treatment of the tethered donor HMB-MEA2 with
chloranil afforded a dark purple solution in which UV-vis
spectral analysis yielded the charge-transfer parameters in Table
7 (entry 4) that were essentially identical with those obtained
(37) Pasimeni, L.; Guella, G.; Corvaja, C.; Clemente, D. A; Vicentini,
M. Mol. Cryst. Liq. Cryst. 1983, 91, 25.
(38) Note that the presence of a pair of ortho substituents in PM-T and
PE-T essentially precludes a planar biphenyl framework as in structure V.
a See Table 1.

9402 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Rathore et al.
evaluation of the association constant (KDA) as well as the CT
color (ꢀCT). In marked contrast, the homologous congener
hexaethylbenzene HEB shows neither color change nor charge-
transfer absorption when exposed to the same π-acceptors under
identical conditions. Such unfavored molecular associations of
HEB are readily ascribed to its unique conformational structure
D, in which the alternant ethyl groups sterically protect both
benzenoid faces from the close approach of all planar π-accep-
tors. Indeed, QUANTA molecular modeling calculations predict
the steric envelope around HEB to have an average van der
Waals “thickness” of 2r = 6.4 Å, which is significantly larger
than that (4.0 Å) for the lower homologue HMB. As such, we
conclude, that the difference of ∆r ) 1.2 Å represents an upper
limit for the molecular association of any of the π-acceptors
with the benzenoid chromophore.
Experimental Section
Materials. Hexaethylbenzene (Acros), hexamethylbenzene, durene,
4,4′-dimethylbiphenyl, 1,3,5-tri-tert-butylbenzene, and 3,5-di-tert-bu-
tyltoluene (Aldrich) were purified by repeated crystallization from
ethanol and heptane. Mesitylene (Aldrich) and 1-tert-butyl-3,5-
dimethylbenzene (Wiley) were purified by fractional distillation.
Tetrachloro-p-benzoquinone and tetracyanoethylene (Aldrich) were
sublimed in Vacuo and recrystallized from benzene. 1,3,5-Trinitroben-
zene was purified by crystallization from ethanol. Tropylium tetrafluo-
roborate (Aldrich) and nitrosonium tetrafluoroborate (Pfaltz and Bauer)
were purified by recrystallization from an acetonitrile/dichloromethane
mixture. Dimethoxydurene,40 9,10-dimethoxy-1,2,3,4,5,6,7,8-octahy-
dro-1,4:5,8-dimethanoanthracene,41 1,2,3,4,5,6,7,8-octahydro-1,4:5,8-
dimethanoanthracene,42 1,4-dimethoxy-2,3-dimethyl-5,6,7,8-tetrahydro-
5,8-methanonaphthalene:41 [oil, H NMR (CDCl3) δ 1.32 (d, J ) 7.2
1
Hz, 2H), 1.54 (d, J ) 8.7 Hz, 1H), 1.76 (d, J ) 8.7 Hz, 1H), 2.00 (d,
J ) 7.2 Hz, 2H), 2.20 (s, 6H), 3.64 (s, 2H), 3.82 (s, 6H); 13C NMR
(CDCl3) 12.64, 27.39, 40.89, 49.00, 61.04, 127.53, 137.64, 148.08],
1,1,4,4,5,5,8,8-octamethyl-1,2,3,4,5,6,7,8-octahydroanthracene43 [mp
222-223 °C (lit.43 mp 220-222 °C); 1H NMR (CDCl3) δ 1.52 (s, 24H),
1.90 (s, 8H), 7.44 (s, 2H); 13C NMR (CDCl3) 32.25, 34.18, 35.54],
1,1,4,4-tetramethyl-1,2,3,4,5,6,7,8-octahydroanthracene43 [mp 121-122
X-ray crystallography establishes the hexamethylbenzene
association with chloranil to be optimized at an interplanar
distance of d ) 3.51 Å in the cofacial (sandwich) structure A.
It is noteworthy that such a donor-acceptor separation corre-
sponds to the sum of van der Waals contact of hexamethyl-
benzene and chloranil,39a and this conclusion is supported by
the HMB/CA separation of r ) 3.57 Å calculated with the aid
of QUANTA molecular modeling. It is tempting to conclude
from this computational analysis that molecular associations
leading to charge-transfer absorptions derive from inner-sphere
interactions involving the intimate van der Waals contact of
the donor and acceptor chromophores.39b According to this
formulation, any steric encumbrance of either the donor or
acceptor (or both) that extends much beyond the sum of van
der Waals radii of the chromophores will lead to sharply
diminished charge-transfer absorptions. However, shape se-
lectivity can be an ameliorating factor in at least 2 ways. First,
unsymmetrical steric encumbrances in π-donors [such as those
in the mono-and di-tert-butyl derivatives TXY and DTT, as
well as in semi-annulated TMA and MEA (see Chart 1)] allow
the close cofacial (inner-sphere) approach of π-acceptors by a
small parallel shift or “slippage” along the aromatic planes, as
illustrated by structures H, L, and W. Second, small electron-
poor molecules such as σ-acceptors [in which the electron
deficiency is either largely localized at a single atom as in NO+,
CBr4, Br2, etc. or on a small group of atoms as in RNO2, O3,
ArN2+, SO2, etc.] can approach the benzenoid chromophore in
sterically encumbered donors [such as hexaethylbenzene HEB
and the multiply annulated analogues TET, TMT, OMA, DMA,
and MEA2] with van der Waals cavities of sufficient size to
allow the nestling of a single (acceptor) center as in the NO+
complexes with structures N and P or close approach of multiple
centers as in the TNM complexes with structures T and U.
°C (lit. mp43 121-123 °C); H NMR (CDCl3) δ 1.38 (s, 8H), 1.71 (s,
1
4H), 1.88 (sym m, 4H), 2.84 (sym m, 4H), 7.11 (s, 2H); 13C NMR
(CDCl3) 26.61, 29.30, 32.22, 34.05, 35.44, 127.15, 134.47, 142.31],
2,3,4,5,6-pentaethyl-4′-methylbiphenyl44 [mp 89-91 °C; 1H NMR
(CDCl3) δ 1.05 (t, J ) 7.4 Hz, 6H), 1.35 (t, J ) 7.5 Hz, 6H), 1.38 (t,
J ) 7.5 Hz, 3H), 2.44 (q, J ) 7.4 Hz, 4H), 2.52 (s, 3H), 2.82 (q, J )
7.5 Hz, 4H), 2.85 (q, J ) 7.5 Hz, 2H), 7.24 (d, J ) 7.8 Hz, 2H), 7.31
(d, J ) 7.8 Hz, 2H); 13C NMR (CDCl3) 16.00, 16.10, 21.46, 22.41,
23.79, 128.53, 130.04, 135.72, 137.62, 138.45, 139.36, 139.58, 140.29],
and 2,3,4,5,6,4′-hexamethylbiphenyl45 [mp 90-91 °C; 1H NMR
(CDCl3) δ 2.09 (s, 6H), 2.40 (s, 6H), 2.44 (s, 3H), 2.54 (s, 3H), 7.14
(d, J ) 7.8 Hz, 2H), 7.35 (d, J ) 7.8 Hz, 2H); 13C NMR (CDCl3)
16.82, 16.98, 18.59, 21.41, 129.14, 129.57, 131.00, 132.45, 134.00,
135.81, 139.99, 140.18], 1,2,3,4,5,6,7,8,9,10,11,12-dodecahydro-1,4:
5,8:9,12-triethanotriphenylene46 (TET), and 1,2,3,4,5,6,7,8,9,10,11,12-
dodecahydro-1,4:5,8:9,12-trimethanotriphenylene47 (TMT) were avail-
able from literature procedures. Synthesis of the tethered 9-[3-
(pentamethyl-phenyl)-1-propyloxy]-10-methoxy-1,2,3,4,5,6,7,8-octahydro-
1,4:5,8-dimethanoanthracene (HEB-MEA2) will be described elsewhere.
Pentaethyltoluene (PET) was prepared from hexaethylbenzene by
refluxing a mixture of hexaethylbenzene (6.15 g, 25 mmol) and acetyl
chloride (2.0 g, 25.5 mmol) in carbon disulfide (25 mL) in the presence
of anhydrous aluminum chloride for 8 h. The resulting deep brown
solution was cooled to room temperature, poured over a mixture of ice
(200 g), concentrated hydrochloric acid (25 mL), and extracted with
ether (4 × 50 mL). The combined ether extracts were washed with
water and dried over anhydrous magnesium sulfate. Removal of the
solvent and recrystallization form ethanol afforded white needles of
pentaethylacetophenone48 (6.2 g, 95%); mp 136-137 °C (lit.48 mp 136-
137 °C). A solution of pentaethylacetophenone (6.0 g, 23 mmol) in
trifluoroacetic acid (25.5 mL) and water (4.5 mL) was refluxed for 16
h and cooled to room temperature. The dark brown reaction mixture
was poured over ice and extracted with ether (4 × 50 mL). The ether
layers were washed with water and dried over anhydrous magnesium
sulfate. Evaporation of the solvent in Vacuo furnished pentaethylben-
The modulation of steric effects also allows the selective
complexation of unsymmetrically substituted biphenyls as in
the slipped structure W and in the tethered bichromophoric
donor as in structure Y. The chemical consequences of such
shape-selective complexations of (poly)chromic donors and
acceptors will be presented separately.
(40) Rathore, R.; Bosch, E.; Kochi, J. K. J. Chem. Soc., Perkin Trans 2
1994, 1157.
(41) Rathore, R.; Kochi, J. K. J. Org. Chem. 1995, 60, 4399.
(42) Halterman, R. L.; Jan. S.-T. J. Org. Chem. 1991, 56, 5253.
(43) Bruson, H. A.; Kroeger, J. W. J. Am. Chem. Soc. 1940, 62, 36.
(44) Kong, K.-C.; Cheng, C.-H. J. Chem. Soc., Chem. Commun. 1991,
423.
(45) Tamao, K.; Sumitani, K.; Kiso, Y.; Zembayashi, M.; Fujioka, A.;
Kodana, S.; Nakayima, I.; Minato, A.; Kumada, M. Bull. Chem. Soc. Jpn.
1976, 49, 1958.
(39) (a) The van der Waals radii of chloranil and benzene are 1.8 and
1.7 Å, respectively (see: Pauling, L. The Nature of the Chemical Bond,
3rd ed.; Cornell: Ithaca, NY 1960; p 257ff). We approximate the van der
Waals radius of the hexamethylbenzene moiety in structure A to be slightly
less (∼1.8 Å) than that of the methyl groups owing to the staggering of the
Cl and CH3 groups as well as the preferred conformation of the methyl
groups (as a result of restricted rotation), as shown in structure AT. (b) The
inner-sphere character is cleanly delineated in the nitrosonium/arene
complexes Q, R, and S by the (nonbonded) nitrogen-arene distance of
∼2.1 Å, which is substantially less than the sum of the van der Waals radii
of benzene and nitrosonium (3.2 Å).
(46) See: Komatsu et al. in ref 9.
(47) Gassman, P. G.; Gennick, I. J. Am. Chem. Soc. 1980, 102, 6863.
(48) Downton, P. A.; Milvaganam, B.; Frampton, C. S.; Sayer, B. G.;
McGlinchey, M. J. J. Am. Chem. Soc. 1990, 112, 27.
(49) van der Made, A. W.; van der Made, R. I. J. Org. Chem. 1993, 58,
1262.

Molecular Recognition in Donor-Acceptor Pairs
J. Am. Chem. Soc., Vol. 119, No. 40, 1997 9403
zene as an oil [GC-MS m/z 218 (M+), calcd for C16H26]. The resulting
crude pentaethylbenzene was dissolved in acetic acid (15 mL) and
treated with paraformaldehyde (0.75 g, 25 mmol) and 5 mL of a 31 wt
% HBr in acetic acid.49 The mixture was heated for 12 h at 80 °C and
then poured into 100 mL water. The crystalline precipitate thus formed
was filtered off, washed with water, and dried in Vacuo to afford
(bromomethyl)pentaethylbenzene (7.0 g, 22.5 mmol); GC-MS m/z 310,
312 (M+), calcd for C17H27Br. To a solution of crude (bromomethyl)-
pentaethylbenzene (6.22 g, 20 mmol) in anhydrous diethyl ether (100
mL) was added lithium aluminum hydride (0.76 g, 20 mmol), and the
mixture was refluxed for 2 h. The usual aqueous workup resulted an
oily residue which upon crystallization form ethanol afforded pure
pentaethyltoluene as colorless prisms (3.7 g, 80%): mp 45-46 °C; 1H
NMR (CDCl3) δ 1.44 (sym m, 15 H), 2.54 (s, 3H), 2.93 (quintet, 10H);
13C NMR (CDCl3) 14.86, 15.38, 15.91, 16.07, 22.24, 22.51, 23.16,
132.18, 137.71, 138.26, 138.62. GC-MS m/z (M+), 232, calcd for
C17H28. Anal. Calcd for C17H28: C, 87.86; H, 12.14. Found: C, 88.06;
H, 12.00. Triethylmesitylene (TEM) was prepared by dropwise
addition of a 30% solution of HBr in acetic acid (100 mL) to a stirred
mixture of paraformaldehyde (12 g, 0.4 mol) and 1,3,5-triethylbenzene
(16.2 g, 0.1 mol) in glacial acetic acid (25 mL) at 25 °C.49 The resulting
mixture was heated at 100 °C for 72 h, cooled to room temperature,
and poured over ice-water (250 mL) mixture. The precipitate thus
obtained was filtered, washed with water, and dried in Vacuo. The
crude pale brown solid (38 g) was dissolved in diethyl ether (200 mL)
and added dropwise to a suspension of lithium aluminum hydride (10
g) in diethyl ether during a 1 h period. The resulting mixture was
refluxed for 4 h and cooled to room temperature. The usual workup
as above and crystallization from ethanol afforded pure triethylmesi-
tylene (74%): mp 55-56 °C; 1H NMR (CDCl3) δ 1.22 (t, J ) 7.5 Hz,
9H), 2.37 (s, 9H), 2.79 (q, J ) 7.5 Hz, 6H); 13C NMR (CDCl3) 13.97,
15.50, 23.79, 131.43, 138.88. GC-MS m/z (M+), 204, calcd for C15H24.
Anal. Calcd for C15H24: C, 88.16; H, 11.84. Found: C, 88.37; H,
11.68.
Dichloromethane (Mallinckrodt analytical reagent) was repeatedly
stirred with fresh aliquots of concentrated sulfuric acid (∼20% by
volume) until the acid layer remained clear. After separation, it was
washed successively with water, aqueous sodium bicarbonate, water,
and aqueous sodium chloride and dried over anhydrous calcium
chloride. The dichloromethane was distilled twice from P2O5 under
an argon atmosphere and stored in a Schlenk tube equipped with a
Teflon valve fitted with Viton O-rings. Acetonitrile (Fischer) was
stirred with KMnO4 for 24 h, and the mixture was refluxed until the
liquid was colorless. The MnO2 was removed by filtration. The
acetonitrile was distilled from P2O5 under an argon atmosphere and
then refluxed over CaH2 for 6 h. After distillation from the CaH2, the
solvent was stored in a Schlenk flask under an argon atmosphere. The
UV-vis absorption spectra were recorded on a Hewlett-Packard 8450A
diode-array spectrometer. The 1H and 13C NMR spectra were recorded
on a General Electric QE-300 spectrometer, and chemical shifts are
reported in ppm units downfield from tetramethylsilane.
(in which no charge-transfer absorption band was detected), the upper
limits of the formation constants (KDA) were estimated as follows. The
detection limit of the spectrophotometer was taken as 0.01 absorbance
unit for an average charge-transfer extinction coefficient of ꢀCT ) 1000
M-1 cm-1
. We calculated the lowest concentration of the donor-
acceptor complex detectable by UV-vis spectroscopy to be [D,A] )
0.01/ꢀCT ) 1 × 10-5 M. If no DA complex was detected at a donor
concentration of [D] ) 0.1-0.5 M and an acceptor concentration of
[A] ) 4-10 mM, the upper limit for the formation constant was taken
as KDA ) [DA]/[D] [A] < 0.025-0.002 M-1 (see Tables 1 and 2).
The DA complexes with KDA < 0.025 M-1 were generally considered
to be contact charge-transfer complexes.8,51
Simulation of the Intermolecular Separations in Donor-Accep-
tor Complexes by Molecular Modeling Calculations. The most
energetically favorable separation of the donor and acceptor in isolated
[D,A] complexes was searched with the aid of molecular modeling
calculations (CHARMm program,54 vers. 22, Molecular Simulation Inc.,
1994) which included both intramolecular force field (such as covalent
bond, bond angle, and torsion angle potentials) and intermolecular
interactions (including Columbic and Lennard-Jones potential terms).
The initial molecular models of the interacting arene donors and the
various electron acceptors (see Charts 1 and 2) were constructed with
the aid of the graphics package QUANTA15 running on a Silicon
Graphics ONYX Reality workstation. The optimization of the geometry
of the individual molecules was carried out by initial energy minimiza-
tion using the Steepest Descents (SD) algorithm53 to remove any
obvious improper conformations, and this was followed by 1000 steps
of ABNR54 (Adopted Basis of Newton-Raphson) algorithm in
“CHARMm” to reach the minimum of molecular potential energy. The
charges of the individual molecules were balanced using Gasteiger’s
method,52 thereby resulting in neutral molecules. The energy mini-
mization package CHARMm allowed any number of molecules within
a single structure file to be simulated. Thus, the optimized donor and
acceptor molecules were put together in random orientation and the
intermolecular separation distance between them was varied from 3.0
to 5.0 Å, and the relative orientation of the interacting molecules was
also varied prior to the energy minimization. The energy minimization
of the various resulting aggregates was carried out in a similar way as
described for the individual molecules, i.e., using the Steepest Descents
algorithm53 to remove unfavorable steric contacts followed by 1000
steps of ABNR54 algorithm. The geometry and the separation distances
between donor and acceptors molecules of the minimized structures
were analyzed using the XP graphical package56 after the energy
tolerance was satisfied.
Isolation and X-ray Crystallography of Donor-Acceptor Com-
plexes. The chloranil complexes were crystallized from an equimolar
solution of CA and HMB, TEM, or PM-T by very slow evaporation
of dichloromethane. Since the similar treatment of the tethered HEB-
MEA2 yielded an amorphous solid, a 1:1 CH2Cl2/CCl4 mixture was
used to prepare dark purple crystals of [HMB-MEA2,CA]. For the
nitrosonium complexes, hexaethylbenzene (HEB) was added to a flask
that contained nitrosonium hexachloroantimonate, and a minimum
amount of dichloromethane was added under an argon atmosphere at
0 °C. The undissolved solid was removed by filtration, and the resulting
dark colored solution was carefully layered with anhydrous toluene
(30 mL) and stored in the refrigerator (-10 °C). After 48-72 h, dark
red crystals of HEB/NO+ complex were deposited at the dichlo-
romethane/toluene interface. A similar procedure yielded dark red
crystals of the TET/NO+ complex.
The Formation Constants of Donor-Acceptor Complexes. Gen-
eral Procedure. Typically, in a 1-cm square quartz cuvette (UV cell)
equipped with a side arm and Schlenk adapter was placed 1-4 mM
solution of chloranil (with the aid of a hypodermic syringe) under an
argon atmosphere. A known amount of arene donor was added in
increments, and the absorbance changes were measured at the absorption
maxima as well as two other wavelength close to the absorption maxima
(see Figure 1). The absorbance data were then evaluated with the aid
of the Benesi-Hildebrand correlation in eq 2.16 From the linear plot
of [CA]/ACT against [arene]-1, consisting of at least eight data points,
The X-ray crystallography of the donor-acceptor complexes was
carried out with a Siemens SMART diffractometer equipped with a
CCD detector at -150 °C. The structures were solved by direct
the slope was estimated as (KDAꢀCT) )
-1 and the intercept as (ꢀCT -1. Linear
fits obtained by least square method had a correlation coefficient of
>0.999. The runs were made in duplicate to ensure the reproducibility
of the spectra. Detailed procedures for the quantification of DA
complex formation with various electron acceptors such as tetracya-
noethylene,18 1,3,5-trinitrobenzene,11 tropylium tetrafluoroborate,50 tet-
ranitromethane,35 and nitrosonium tetrafluroborate30 have been described
previously. Estimation of the Formation Constants for DA Com-
plexes from Hindered Donors. In cases of the hindered arene donors
(51) See: Person, W. B. in ref 16b.
(52) Gasteiger, J.; Marsili, M. Tetrahedron 1980, 36, 3219.
(53) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T.
Numerical Recipes in C. The Art of Scientific Computing; Cambridge
University Press: New York, 1988; p 317.
(54) Brooks, B. R.; Bruccoleri, R. E.; Olafson, B. D.; States, D. J.;
Swaminathan, S.; Karplus, M. J. Comput. Chem. 1983, 4, 187.
(55) Sheldrick, G. M. SHELSX-86, Program for Structure Solution;
University of Go¨ttingen: Germany, 1986.
(50) Takahashi, Y.; Sankararaman, S.; Kochi, J. K. J. Am. Chem. Soc.
1989, 111, 2954.
(56) XP-Interactive Molecular Graphics (vers. 5.06); Siemens Analytical
Instruments: Madison, WI, 1996.

9404 J. Am. Chem. Soc., Vol. 119, No. 40, 1997
Rathore et al.
methods55 and refined with IBM Pentium and SGI Indigo computers.
In the various chloranil complexes, the intramolecular dimensions of
CA and the arene donors were the same as those observed in
uncomplexed molecules. However, the arene complexed to nitrosonium
cation in [HEB,NO+] and [TET,NO+] showed slight lengthening of
all aromatic CdC double bonds when compared with crystal structures
of the uncomplexed donors. The pertinent intermolecular orientations
as PLUTO plots (generated from the XP graphical package)56 are shown
in structures A, C, N, P, W, and Y, and the critical interplanar
separations (d) are included. The crystallographic data are on deposit
and can be obtained from the Director, Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge, CB2 1EZ, U.K.
and M.S.I. for permission and invaluable help with the molecular
modeling calculations and the National Science Foundation and
R. A. Welch Foundation for financial support.
Supporting Information Available: Tables of crystal
structure data for [TEM,CA], [PM-T,CA], [HMB-MEA2,CA],
[HEB,NO+SbCl6-], and [TET,NO+SbCl6-] including atomic
coordinates, anisotropic thermal parameter, bond lengths, and
bond angles and ORTEP diagrams showing thermal ellipsoids
(36 pages). See any current masthead page for ordering and
Internet access instructions.
Acknowledgment. We thank Professor B. M. Pettitt and Dr.
R. Mitra (Institute for Molecular Design, University of Houston)
JA9720319