Extreme conformational constraints in π-extended tetrathiafulvalenes: Unusual topologies and redox behavior of doubly and triply bridged cyclophanes

Article abstract of DOI:10.1021/ja062358m

Doubly and triply bridged 9,10-bis(1,3-dithiol-2-ylidene)-9,10- dihydroanthracene (ex-TTF) derivatives have been synthesized. Key steps are the generation and macrocyclization reactions of ex-TTF-dithiolate reagents. The X-ray crystal structures of the do

Full text of DOI:10.1021/ja062358m

Published on Web 07/21/2006
Extreme Conformational Constraints in π-Extended
Tetrathiafulvalenes: Unusual Topologies and Redox Behavior
of Doubly and Triply Bridged Cyclophanes
Christian A. Christensen, Andrei S. Batsanov, and Martin R. Bryce*
Contribution from the Department of Chemistry, UniVersity of Durham, Durham DH1 3LE, U.K.
Received April 13, 2006; E-mail: m.r.bryce@durham.ac.uk
Abstract: Doubly and triply bridged 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (ex-TTF) deriva-
tives have been synthesized. Key steps are the generation and macrocyclization reactions of ex-TTF-
dithiolate reagents. The X-ray crystal structures of the doubly bridged cyclophanes 15 and 16 and the
triply bridged system 23 show that the saddle-like conformation of the ex-TTF framework is enhanced by
the short bridges between the dithiole rings. Unlike all previous ex-TTF derivatives (which display a single
quasi-reversible two-electron oxidation wave, D0 f D2+), cyclic voltammetry of the cyclophanes reveals
two reversible, one-electron oxidation steps (D0 f D•+ f D2+), with differences between the half-wave
1/2
potentials (E2 - E11/2) of 0.22-0.26 V. The conformational changes and gain in aromaticity which drive
the second oxidation process in unrestricted ex-TTF systems (including singly bridged cyclophanes) have
been prevented by multiple bridging. The radical cation species gives rise to a very broad, low-energy
band (λmax ) 2175 and 2040 nm for 15 and 21, respectively), assigned to an intramolecular interaction.
The steric constraints imposed by multiple bridging have become so extreme that the π-framework of 15,
16, 21, and 23 exhibits remarkable optical and redox behavior which is not characteristic of ex-TTF systems.
Chart 1
Introduction
Tetrathiafulvalene (TTF) derivatives with extended π-electron
conjugation are versatile building blocks in supramolecular and
materials chemistry.1 Of particular interest are derivatives which
incorporate a central p-quinodimethane spacer.2 For example,
the π-electron donor unit 9,10-bis(1,3-dithiol-2-ylidene)-9,10-
dihydroanthracene (1, Chart 1; herein abbreviated “ex-TTF”)
offers a unique combination of redox, charge delocalization,
and structural properties which are very different from those of
the parent TTF. Solution electrochemical studies have estab-
lished that, whereas TTF undergoes two reversible, one-electron
oxidation waves, ex-TTF system 1 undergoes the unusual
phenomenon of inverted potentials (E1 > E2ox).3,4 Solution
ox
electrochemistry reveals a single two-electron oxidation process
(1) Fre`re, P.; Skabara, P. J. Chem. Soc. ReV. 2005, 34, 69-98.
(2) (a) Mart´ın, N.; Ort´ı, E. In Handbook of AdVanced Electronic Photonic
Materials and DeVices; Nalwa, H. S., Ed., Academic Press: San Diego,
2001; Vol. 3, Chapter 6, pp 245-265. (b) Bendikov, M.; Wudl, F.;
Perepichka, D. F. Chem. ReV. 2004, 104, 4891-4945. (c) Yamashita, Y.
In TetrathiafulValene Chemistry, Fundamentals and Applications of
TetrathiafulValene; Yamada, J., Sugimoto, T., Eds.; Springer-Verlag:
Berlin, 2004; Chapter 12, pp 287-310. (d) Yamashita, Y.; Tanaka, S.;
Imaeda, K.; Inokuchi, H.; Sano, M. J. Chem. Soc., Chem. Commun. 1991,
1132-1133.
(3) (a) Hu, K.; Evans, D. H. J. Phys. Chem. 1996, 100, 3030-3036. (b) Evans,
D. H.; Hu, K. J. Chem. Soc., Faraday Trans. 1996, 92, 3983-3990. (c)
Kraiya, C.; Evans, D. H. J. Electroanal. Chem. 2004, 565, 29-35. (d)
Gruhn, N. E.; Mac´ıas-Ruvalcaba, N. A.; Evans, D. H. J. Phys. Chem. A
2006, 110, 5650-5655. (e) Mac´ıas-Ruvalcaba, N. A.; Evans, D. H. J. Phys.
Chem. B 2006, 110, 5155-5160. References 3c-e refer to 9,10-disubsti-
tuted anthracenes and emphasize that the changes in geometry which
accompany the redox process are crucial in causing inverted potentials.
(4) For pioneering work on the electrochemistry of two-electron redox systems,
see: (a) Fritsch, J. M.; Weingarten, H.; Wilson, J. D. J. Am. Chem. Soc.
1970, 92, 4038-4046. (b) Deuchert, K.; Hu¨nig, S. Angew. Chem., Int. Ed.
Engl. 1978, 17, 875-886.
to yield a thermodynamically stable dication (D0 f D2+
;
typically Eox ) 0.30-0.60 V vs Ag/AgCl, depending upon the
9
10484
J. AM. CHEM. SOC. 2006, 128, 10484-10490
10.1021/ja062358m CCC: $33.50 © 2006 American Chemical Society

Steric Constraints from Multiple Bridging in ex-TTF
A R T I C L E S
solvent and the substituents).5 This redox process, which is
electrochemically quasi-reversible and chemically reversible, is
accompanied by a remarkable conformational change,6 as shown
by X-ray crystallographic studies.7 The neutral molecules 1
possess a saddle-shaped structure comprising a concave cavity
and a strongly folded anthracenediylidene unit, imposed by steric
hindrance from the benzoannulated quinoid moiety. In contrast,
in the dications 12+, the anthracene ring system becomes planar
and aromatic, with bond lengths similar to those of anthracene
itself, and the heteroaromatic 1,3-dithiolium cations are almost
orthogonal to this plane. Theoretical studies support the
experimental evidence that the two, one-electron oxidations of
1 coalesce under the same oxidation wave, with the dication
possessing decreased intramolecular on-site Coulombic repulsion
compared to TTF.8 The radical cation of system 1 is a transient
species which has been generated by flash photolysis9 and pulse
radiolysis.10 This remarkable interplay of redox and conforma-
tional properties has led to derivatives of 1 being employed as
components of charge-transfer salts,7,11 nonlinear optical materi-
als,12 multi-stage redox assemblies,13 dimeric ex-TTFs,14 highly
charged dendrimers,15 and donor-acceptor systems for studies
of charge-separated excited states.16
observed at significantly higher potentials (+200-300 mV)
compared to their nonbridged precursors, unlike 3b, where the
longer bridge had essentially no effect on the oxidation potential.
Bridging the anthracene unit had less effect (4a, +50 mV; 4b,
unchanged) compared to their nonbridged precursors.20
The aim of the present work was to synthesize ex-TTF
derivatives which would be sterically constrained to such an
extent that they could not undergo any significant conforma-
tional change upon oxidation. We were fascinated by the
possibility that, if the central anthracenediylidene ring was forced
to retain the strongly folded (neutral) geometry upon oxidation,
and hence could not aromatize, then the single, two-electron
oxidation process (D0 f D2+) might be replaced by two
sequential, one-electron steps (D0 f D•+ f D2+) with an
observable (or even stable) radical cation. We now report the
synthesis of the first doubly and triply bridged ex-TTF cyclo-
phanes and describe the fascinating structural and electronic
consequences of this unprecedented rigidification of the π-frame-
work.
Results and Discussion
Synthesis. Our initial targets were analogues of 2 doubly
bridged across the two dithiole rings. The previous synthesis
of 2a and 2b had been achieved in low yields (22% and 10%,
respectively) by a two-fold Horner-Wadsworth-Emmons
reaction of the corresponding bis(1,3-dithiol-2-yl)phosphonate
reagent with anthraquinone.18 Cyclophanes 319 and 420 had been
obtained by a different and more efficient strategy, viz., bridging
a preformed ex-TTF system in macrolactonization reactions. For
precursors to doubly bridged analogues, we chose ex-TTF
derivatives bearing suitable functionality to undergo two in-
tramolecular cyclization reactions. Compounds 13 and 14, with
two protected thiolates and two alkyl iodide chains, seemed ideal
candidates: the short pentamethylenedithio and tetramethyl-
enedithio bridges which would form by intramolecular nucleo-
philic displacement of the iodides should ensure that only the
cis/cis isomer would form in each case; the “criss-cross” trans/
trans isomers would be too strained.
The structures and properties of aromatic molecules can be
manipulated by applying steric constraints, e.g., by incorporation
into cyclophane structures.17 The electron donor properties of
the ex-TTF system 1 have been modified in cyclophane
derivatives where the linkers are short enough to restrict the
conformational change which accompanies the oxidation to the
dication. Thus, for systems 2a,b18 and 3a,19 the characteristic
D0 f D2+ oxidation wave in the cyclic voltammogram was
(5) (a) Yamashita, Y.; Kobayashi, Y.; Miyashi, T. Angew. Chem., Int. Ed. Engl.
1989, 28, 1052-1053. (b) Moore, A. J.; Bryce, M. R. J. Chem. Soc., Perkin
Trans. 1 1991, 157-168. (c) Bryce, M. R.; Coffin, M. A.; Hursthouse, M.
B.; Karaulov, A. I.; Mu¨llen, K.; Scheich, H. Tetrahedron Lett. 1991, 32,
6029-6032. (d) Liu, S.-G.; Pe´rez, I.; Mart´ın, N.; Echegoyen, L. J. Org.
Chem. 2000, 65, 9092-9102. (e) Liu, S.-G.; Pe´rez, I.; Mart´ın, N.;
Echegoyen, L. J. Org. Chem. 2000, 65, 9092-9102.
(6) For other molecular systems where significant conformational changes are
triggered by electron transfer, see: (a) Boubekeur, K.; Lenoir, C.; Batail,
P.; Carlier, R.; Tallec, A.; Le Paillard, M.-P.; Lorcy, D.; Robert, A. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1379-1381. (b) Guerro, M.; Carlier, R.;
Boubekeur, K.; Lorcy, D.; Hapiot, P. J. Am. Chem. Soc. 2003, 125, 3159-
3167 and references therein. (c) Nygaard, S.; Laursen, B. W.; Flood, A.
H.; Hansen, C. N.; Jeppesen, J. O.; Stoddart, J. F. Chem. Commun. 2006,
144-146.
However, since no methodology existed for the synthesis of
ex-TTF derivatives bearing cyanoethyl-protected thiolates, we
modified the protocol for phosphite-mediated coupling of 1,3-
dithiole-2-chalcogenones commonly used for the synthesis of
cyanoethyl-protected TTF thiolates.21 Although the cross-
coupling reactions of anthraquinone or anthrone derivatives with
(7) Bryce, M. R.; Moore, A. J.; Hasan, M.; Ashwell, G. J.; Fraser, A. T.; Clegg,
W.; Hursthouse, M. B.; Karaulov, A. I. Angew. Chem., Int. Ed. Engl. 1990,
29, 1450-1452.
(8) Mart´ın, N.; Sa´nchez, L.; Seoane, C.; Ort´ı, E.; Viruela, P. M.; Viruela, R.
J. Org. Chem. 1998, 63, 1268-1279.
(9) Jones, A. E.; Christensen, C. A.; Perepichka, D. F.; Batsanov, A. S.; Beeby,
A.; Low, P. J.; Bryce, M. R.; Parker, A. W. Chem. Eur. J. 2001, 7, 973-
978.
(16) (a) Herranz, M. A.; Mart´ın, N.; Ramey, J.; Guldi, D. M. Chem. Commun.
2002, 2968-2968. (b) Kodis, G.; Liddell, P. A.; de la Garza, L.; Moore,
A. L.; Moore, T. A.; Gust, D. J. Mater. Chem. 2002, 12, 2100-2108. (c)
Gonza´lez, S.; Mart´ın, N.; Swartz, A.; Guldi, D. M. Org. Lett. 2003, 5,
557-560. (d) D´ıaz, M. C.; Herranz, M. A.; Illescas, B. M.; Mart´ın, N.;
Godbert, N.; Bryce, M. R.; Luo, Ch.; Swartz, A.; Anderson, G.; Guldi, D.
M. J. Org. Chem. 2003, 68, 7711-7721. (e) Perepichka, D. F.; Bryce, M.
R.; Perepichka, I. F.; Lyubchik, S. B.; Christensen, C. A.; Godbert, N.;
Batsanov, A. S.; Levillain, E.; McInnes, E. J. L.; Zhao, J. P. J. Am. Chem.
Soc. 2002, 124, 14227-14238. (f) Giacalone, F.; Segura, J. L.; Mart´ın,
N.; Guldi, D. M. J. Am. Chem. Soc. 2004, 126, 5340-5341. (g) Giacalone,
F.; Segura, J. L.; Mart´ın, N.; Ramey, G.; Guldi, D. M. Chem. Eur. J. 2005,
11, 4819-4834.
(10) (a) Mart´ın, N.; Sa´nchez, L.; Guldi, D. M. Chem. Commun. 2000, 113-
114. (b) Guldi, D. M.; Sa´nchez, L.; Mart´ın, N. J. Phys. Chem. B 2001,
105, 7139-7144.
(11) (a) Triki, S.; Ouahab, L.; Lorcy, D.; Robert, A. Acta Crystallogr. 1993,
C49, 1189-1192. (b) Batsanov, A. S.; Bryce, M. R.; Lyubchik, S. B.;
Perepichka, I. F. Acta Crystallogr. 2002, E58, o1106-o1110.
(12) (a) Otero, M.; Herranz, M. A.; Seoane, C.; Mart´ın, N.; Gar´ın, J.; Orduna,
J.; Alacala´, R.; Villacampa, B. Tetrahedron 2002, 58, 7463-7475. (b)
Insuasti, B.; Atienza, C.; Seoane, C.; Mart´ın, N.; Gar´ın, J.; Orduna, J.;
Alacala´, R.; Villacampa, B. J. Org. Chem. 2004, 69, 6986-6995.
(13) (a) Cerrada, E.; Bryce, M. R.; Moore, A. J. J. Chem. Soc., Perkin Trans.
1 1993, 537-538. (b) Marshallsay, G. J.; Bryce, M. R. J. Org. Chem. 1994,
59, 6847-6850. (c) Gautier, N.; Mercier, N.; Riou, A.; Gorgues, A.
Hudhomme, P. Tetrahedron Lett. 1998, 40, 5997-6000.
(17) (a) Diederich, F. Cyclophanes; Royal Society of Chemistry: Cambridge,
1991. (b) Vo¨gtle, F. Cyclophane Chemistry; Wiley: Chichester, 1993. (c)
For a series of multiply bridged and sterically constrained cyclophanes,
see: Song, Q.; Ho, D. M.; Pascal, R. A., Jr. J. Am. Chem. Soc. 2005, 127,
11246-11247.
(14) (a) Mart´ın, N.; Pe´rez, I.; Sa´nchez, L.; Seoane, C. J. Org. Chem. 1997, 62,
870-877. (b) Christensen, C. A.; Bryce, M. R.; Batsanov, A. S.; Becher,
J. Org. Biomol. Chem. 2003, 1, 511-522. (c) D´ıaz, M. C.; Illescas, B. M.;
Seoane, C.; Mart´ın, N. J. Org. Chem. 2004, 69, 4492-4499. (d) D´ıaz, M.
C.; Illescas, B. M.; Mart´ın, N.; Perepichka, I. F.; Bryce, M. R.; Levillain,
E.; Viruela, R.; Ort´ı, E. Chem. Eur. J. 2006, 12, 2709-2712. (e) D´ıaz, M.
C.; Illescas, B. M.; Mart´ın, N.; Stoddart, J. F.; Canales, M. A.; Jime´nez-
Barbero, J.; Sarova, G.; Guldi, D. M. Tetrahedron 2006, 62, 1998-2002.
(15) Godbert, N.; Bryce, M. R. J. Mater. Chem. 2002, 12, 27-36.
(18) Finn, T.; Bryce, M. R.; Batsanov, A. S.; Howard, J. A. K. Chem. Commun.
1999, 1835-1836.
(19) Godbert, N.; Batsanov, A. S.; Bryce, M. R.; Howard, J. A. K. J. Org. Chem.
2001, 66, 713-719.
(20) Christensen, C. A.; Batsanov, A. S.; Bryce, M. R.; Howard, J. A. K. J.
Org. Chem. 2001, 66, 3313-3320.
9
J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10485

A R T I C L E S
Scheme 1
Christensen et al.
MeOH and DMF simultaneously to DMF under very high
dilution conditions.
Although the solubility of the cyclophanes 15 and 16 in
common organic solvents was considerably reduced compared
to that of their precursors, they were unambiguously character-
1
ized by H NMR, 13C NMR, HRMS, and elemental analysis.
Especially the high symmetry of the 13C NMR spectra of 15
and 16 established that cis/cis bridging had occurred, and hence
only one isomer was formed in each case (Figure S1 in the
Supporting Information).
The high-yielding synthesis of 15 opened a way to triply
bridging the ex-TTF system by combining the methodology in
Scheme 1 with that used to synthesize 4.20 For this goal, we
needed an analogue of 15 with substituents at the 2- and
6-positions of the anthracenediylidene unit which would be
suitable for a macrolactonization reaction. Coupling of an-
thraquinone derivative 1720 with thione 6, under the same
conditions used for 8, gave anthrone derivative 18 in only 34%
yield. It was apparent that 17 is less reactive than anthraquinone
itself. This was confirmed in the second phosphite coupling:
as much as 9 equiv of the thione 10 was required to give the
ex-TTF derivative 19 in 57% yield (Scheme 2; cf. Scheme 1).
This lower reactivity of 17 and 18 could arise from the electron-
donating effect of the alkoxy substituents and/or a steric effect
of the bulky tert-butyldiphenylsilyl (TBDPS) protecting groups.
Halogen exchange gave the highly functionalized compound
20 in 88% yield. Treatment of 20 with cesium hydroxide
monohydrate (6 equiv, 20 °C, 24 h) induced the desired double
cyclization and cleaved the TBDPS protecting groups25 to afford
product 21 in 78% yield. (The 2-hydroxyethoxy substituents of
21 imparted good solubility in organic solvents, in contrast to
15.) In the final step, 21 reacted with 1,4-benzenedicarbonyl
chloride 22 in the presence of pyridine to yield the triply bridged
cyclophane 23 in 42% yield (cf. 39% for the comparable reaction
to form 4a).20
1,3-dithiole-2-thione derivatives were sluggish compared to the
self-coupling of the 1,3-dithiole-2-thiones, we found that cross-
coupling, which provided a new route to highly functionalized
ex-TTF derivatives,22 was possible by slow addition of the 1,3-
dithiole-2-thione to the anthraquinone/anthrone. Thus, slow
addition of 623 and 723 to an excess of anthraquinone 5 in triethyl
phosphite at 125 °C gave the anthrone derivatives 8 and 9 in
44% and 46% yields, respectively (Scheme 1). The second
phosphite coupling using reagent 1024 furnished the ex-TTF
derivatives 11 and 12 in 77% and 81% yields, respectively.
Halogen exchange using sodium iodide in refluxing 2-butanone
afforded the diiodide derivatives 13 and 14 in high yields. The
slow addition of compound 13 to cesium hydroxide monohy-
drate in dimethylformamide (DMF) at room temperature under
high dilution conditions21 gave the desired product 15 in 92%
yield, which is remarkably efficient for a double macrocycliza-
tion reaction. The analogous cyclization of 14 gave 16 in only
16% yield; the major product was an insoluble powder, most
likely a polymer, suggesting that the shorter tetramethylenedithio
linkers and increased ring strain in 16 disfavored the intramo-
lecular reactions. The yield of 16 was increased to 27% by
adding a solution of 14 in DMF and a solution of NaOMe in
The structure of 23 was confirmed by H NMR, 13C NMR,
1
HRMS, and elemental analysis. Due to C2 symmetry, the 13C
1
NMR spectrum was very simple and the H NMR spectrum
was similar to that of 15 combined with the high-frequency part
of the spectrum for 4a. Most diagnostic for the phenyl bridge
is the upfield shift of the phenyl protons [δ 6.91 ppm (4H), s]
due to the ring current of the dihydroanthracene ring system
(cf. 4a δ 6.89 ppm).20
X-ray Crystal Structures of 15, 16, and 23. The fascinating
architectures of 15, 16, and 23 were revealed in their X-ray
molecular structures. For 15, the structures of both unsolvated
crystals (Figure 1) and a toluene solvate (Figure S2, Supporting
Information) were solved. In the former, the ex-TTF moiety
has local C2V symmetry and both pentamethylene bridges adopt
practically planar all-trans conformations (Figure S3, Supporting
Information), but their planes are inclined differently (by 18°
and 76°) to the ex-TTF (local) mirror plane, which passes
through the C(9), C(10), C(15), and C(18) atoms, and are nearly
perpendicular to one another (dihedral angle 86°). In 15‚C7H8,
the molecule has crystallographic C2 symmetry. The entire
-S(CH2)5S- moiety of both bridges is disordered between two
orientations. In both cases, it also adopts trans conformations
(21) For a review of the synthesis and reactivity of cyanoethyl-protected TTF
thiolates, see: Simomsen, K. B.; Becher, J. Synlett 1997, 1211-1220.
(22) The development of this methodology for the synthesis of a series of new
and versatile ex-TTF building blocks will be reported elsewhere.
(23) Takimiya, K.; Imamura, K.; Shibata, Y.; Aso, Y.; Ogura, F.; Otsubo, T. J.
Org. Chem. 1997, 62, 5567-5574.
(25) It is known that cesium hydroxide monohydrate can cleave silyl protecting
groups during the deprotection of cyanoethylthio groups: Christensen, C.
A.; Bryce, M. R.; Becher, J. Synthesis 2000, 1695-1704.
(24) Svenstrup, N.; Rasmussen, K. M.; Hansen, T. K.; Becher, J. Synthesis 1994,
809-812.
9
10486 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006

Steric Constraints from Multiple Bridging in ex-TTF
A R T I C L E S
Scheme 2
around all C-C bonds and gauche conformations around C-S
bonds, the mean C5 planes being inclined to the ex-TTF mirror
plane by 76° and 88°. Thus, the whole molecule conforms
approximately to the C2V symmetry. The change of conformation
from 15 to 15‚C7H8 is a consequence of the contact with the
planar toluene molecule, and it results in a substantial increase
of molecular bending. This shows that the ex-TTF unit in 15
still possesses sufficient flexibility that its conformation can be
affected by the demands of crystal packing. In 16, both bridges
have more strained conformations, and the approximate mo-
lecular symmetry is reduced to C2, with an appreciable (chiral)
twist of the ex-TTF moiety itself.
Figure 2. X-ray molecular structure of 16.
Table 1. Selected Angles (Deg) in the Molecular Structures in the
Cyclophanes As Determined by X-ray Crystallographic Analysisa
15
15
‚C7H8
16
23
‚
(CH2Cl2)2
æ
40.2
18.4
18.3
42.9
40.8
30.2
30.9
33.0
43.4
30.6
28.7
4.8
40.6
31.9
32.4
31.7
δ1
δ2
θ
a The angles are defined in the text.
bridge (2b) reduces it to 35°. The second pentamethylenedithio
bridge in 15 adds little extra strain, but the two tetramethyl-
enedithio bridges in 16 make these peripheral moieties of the
ex-TTF unit nearly parallel. On the other hand, the folding (æ)
of the anthracene system along the C(9)‚‚‚C(10) vector does
not exceed the range observed in unconstrained ex-TTF deriva-
tives (35-45°),26 and the overall bending (θ) in 16 is enhanced
mainly by stronger folding of both dithiole rings (Table 1), as
defined by angles between planes S(1)C(15)S(2) and S(1)-
C(16)C(17)S(2) (δ1), or S(5)C(18)S(6) and S(5)C(19)C(20)S(6)
(δ2).
Figure 1. X-ray molecular structure of 15.
For 15 and 16 (Figure 2), the usual saddle-like conformation
of the ex-TTF framework is aggravated by the short bridges
between the dithiole rings. A good measure of the overall
bending is the angle (θ, Table 1) between the planar SCdCS
moieties of the dithiole rings, e.g., S(1)C(16)C(17)S(2) and
S(5)C(19)C(20)S(6) in Figure 1. In nonbridged systems, θ
ranges from 73° to 101°;26 one hexamethylenedithio bridge
(2a)18 reduces it to 46-54°, and one pentamethylenedithio
(26) (a) Bryce, M. R.; Finn, T.; Moore, A. J.; Batsanov, A. S.; Howard, J. A.
K. Eur. J. Org. Chem. 2000, 51-60. (b) Godbert, N.; Bryce, M. R.;
Dahaoui, S.; Batsanov, A. S.; Howard, J. A. K.; Hazendonk, P. Eur. J.
Org. Chem. 2001, 749-757. (c) Bryce, M. R.; Finn, T.; Batsanov, A. S.;
Kataky, R.; Howard, J. A. K.; Lyubchik, S. B. Eur. J. Org. Chem. 2002,
1199-1205.
9
J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10487

A R T I C L E S
Christensen et al.
Figure 4. UV-vis absorption spectra of 13, 15, and 16 in dichloromethane
at 20 °C.
Table 3. Cyclic Voltammetry Dataa
Figure 3. X-ray molecular structure of 23‚(CH2Cl2)2.
compound
Eoxpa/V
Eoxpc/V
∆
Ep/Vb
Table 2. UV-Vis Absorption Spectra Recorded in
Dichloromethane at 20 °C
11
12
13
14
15
16c
19
20
21
23
0.60
0.61
0.60
0.61
1.01, 1.25
0.99, 1.26
0.63
0.63
0.92, 1.13
0.98, 1.20
0.51
0.49
0.51
0.49
0.95, 1.19
0.92, 1.18
0.41
0.41
0.86, 1.08
0.92, 1.14
0.09
0.12
0.09
0.12
0.06, 0.06
0.07, 0.08
0.22
0.22
0.06, 0.05
0.06, 0.06
compound
λmax/nm (lg ꢀ)
11
12
13
14
15
16
19
20
21
23
432 (4.44), 362 (4.20)
431 (4.43), 362 (4.19)
432 (4.44), 362 (4.21)
432 (4.45), 362 (4.20)
414 (4.35), 372 (4.11), 351 (4.15), 269 (4.44)
398 (4.08), 370 (4.08), 340 (4.07), 267 (4.40)
428 (4.44), 361 (4.21)
428 (4.43), 361 (4.20)
411 (4.32), 353 (4.22)
414 (4.26), 353 (4.16)
a Compound ca. 1 × 10-3 M, electrolyte 0.1 M Bu4NPF6 in dichloro-
methane, 20 °C, vs Ag/AgCl, scan rate 100 mV s-1 b ∆Ep ) Eoxpa - Eox
.
pc
(Eox is the oxidation peak potential on the first anodic scan; Eox is the
pa
pc
coupled reduction peak potential on the cathodic scan). c Scan rate 5000
mV s-1 due to some deposition of oxidized species on the electrode at
lower scan rates.
The conformation of the ex-TTF moiety in 23‚(CH2Cl2)2
(Figure 3 and Figure S4, Supporting Information) is very similar
to that in 15‚C7H8. The folding is largely unaffected by the bis-
(lactone) bridge, as was the case for cyclophane 4a.20
Optical Absorption and Electrochemical Properties. The
UV-vis absorption data for the cyclophanes and their precursors
are collated in Table 2. Precursors 11-14 showed two bands
typical of ex-TTF derivatives9 at λmax ) 432 and 362 nm.
Incorporation of methylenedithio bridges in cyclophanes 15 and
16 leads to a blue-shift of the longest wavelength absorption
band, indicating loss of π-conjugation (see spectra of 13, 15,
and 16 in Figure 4). There is a shift from λmax ) 432 nm to
414 nm upon incorporation of the two pentamethylenedithio
bridges (cyclophane 15). Shortening of the bridges to tetra-
methylenedithio (cyclophane 16) has an even greater impact:
the extinction coefficient of the longest wavelength band is
approximately halved (relative to precursor 14; spectrum similar
to that of precursor 13 shown in Figure 4) and shifted to λmax
) 398 nm. Thus, the blue-shift and the decrease of the extinction
coefficient, especially of the longest wavelength band, are good
measures of the folding of the ex-TTF moieties. Qualitatively
similar behavior was noted for strained TTF cyclophanes.27 For
2,6-disubstituted doubly bridged cyclophane 21 the trend is the
same: the longest wavelength band is blue-shifted by 17 nm
relative to that of precursor 20. However, the third bridge in
this system [i.e., the bis(lactone) bridge in 23] makes little
Figure 5. Cyclic voltammograms of 13 and 15 vs Ag/AgCl under the
conditions stated in Table 3.
difference to the spectrum, as observed for cyclophane 4a in
comparison with its nonbridged precursors.20
Cyclic voltammetric (CV) data recorded in dichloromethane
are collated in Table 3. Figure 5 shows the CVs of 13 and 15.
As expected, all the nonbridged precursors show a single, quasi-
reversible, two-electron oxidation wave at 0.60-0.63 V. The
oxidation potentials of the doubly and triply bridged cyclophanes
15, 16, 21, and 23 are raised considerably compared to those
of their precursors, and surprisingly, 16 is slightly easier to
oxidize (by ca. 20 mV) than its less strained (longer-bridged)
analogue 15. However, most remarkably, and in striking contrast
to all other ex-TTF cyclophanes (i.e., singly bridged systems),18-20
(27) (a) Lau, J.; Blanchard, P.; Riou, A.; Jubault, M.; Cava, M. P.; Becher, J.
J. Org. Chem. 1997, 62, 4936-4942. (b) Wang, C.; Bryce, M. R.; Batsanov,
A. S.; Howard, J. A. K. Chem. Eur. J. 1997, 3, 1679-1690. (c) Otsubo,
T.; Aso, Y.; Takimiya, K. AdV. Mater. 1996, 8, 203-211.
9
10488 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006

Steric Constraints from Multiple Bridging in ex-TTF
A R T I C L E S
mixed-valence interaction between the neutral and the oxidized
dithiole rings: the ratio of the low-energy band to the other
bands in the spectrum was unaffected by the concentration of
the solution, consistent with an intramolecular interaction. The
extent of delocalization of the radical cations 15•+ and 21•+
cannot be deduced from these data. Low solubility prevented
spectroelectrochemical studies on 16.
For both 15 and 21, the radical cation species were stable on
the spectroelectrochemical time scale (>1 h), and the spectra
of the neutral species could be completely recovered upon
electrochemical reduction. Furthermore, clean isosbestic points
were seen at λ ≈ 320 and 420 nm in the formation of 21•+
(Figure 6, inset). However, further oxidation to form 152+ and
212+ caused decomposition, and the spectra of neither the cation
radicals nor the neutral species could be recovered upon
reduction. In the CV experiments, this decomposition of the
dicationic species was not observed due to the much shorter
time scale (a few seconds). The cationic species could not be
isolated as salts by electrocrystallization.
Figure 6. Spectroelectrochemistry of 21 in dichloromethane at 20 °C,
showing oxidation of the neutral species to the radical cation.
the oxidation occurs in two reVersible,28 one-electron oxidation
waves (D0 f D•+ f D2+). Hence, these are the first ex-TTF
derivatives in which the formation of the radical cation species
is observable by CV (the differences between the half-wave
Conclusions
1/2
potentials29 of the first and second oxidation processes, E2
-
E11/2, are 0.22-0.26 V). The reason must be that little or no
conformational change can accompany the oxidation of the
doubly or triply bridged cyclophanes, due to their very rigid
structures. This conformational change, with twisting of the
dithiolium rings to relieve peri-interactions and concomitant gain
in aromaticity and anthracene planarity which drives the second
oxidation process in unrestricted analogues, is prevented; thus,
in 15, 16, 21, and 23, the radical cation is stabilized with respect
to the dication. Indeed, these molecules effectively cease to
behave as ex-TTFs; the two 1,3-dithiol-2-ylidene systems have
lost much of their conjugation because of the steric constraints
in the system. The oxidation potentials of 21 are lowered by
90-120 mV compared to those of the unsubstituted analogue
15, presumably due to the electron-donating alkoxy groups. The
third bridge in 23 increases the oxidation potentials by 60-70
mV, an increase similar to that seen for 4a compared to 1 (R )
SMe).20
Extreme steric constraints have manipulated the redox
properties of the ex-TTF framework to yield a stabilized radical
cation state. The first doubly and triply bridged derivatives of
system 1 (viz., compounds 15, 16, 21, and 23) are fundamentally
interesting cyclophanes by virtue of their very unusual topology,
which is enhanced by multiple bridging, combined with their
π-electron donor properties. The optical absorption and solution
electrochemical properties establish that the π-framework has
become so constrained that 15, 16, 21, and 23 no longer exhibit
the characteristic signatures of ex-TTF systems: the two 1,3-
dithiol-2-ylidene systems have lost much of their conjugation
and behave essentially as separate redox centers. Unlike all
previous ex-TTF derivatives, the oxidation of 15, 16, 21, and
23 occurs in two reversible, one-electron oxidation waves. The
conformational changes and gain in aromaticity which drive the
second oxidation process in unrestricted ex-TTF systems
(including singly bridged cyclophanes) have been prevented by
the conformational constraints; the radical cations and dications
of 15, 16, 21, and 23 have been forced to retain the strongly
folded geometry of the neutral species, with fascinating
consequences.
Spectroelectrochemical studies on 15 (Figure S5, Supporting
Information) and 21 (Figure 6) gave similar results, with the
increased solubility of 21 facilitating these studies. Upon
oxidation of 21, the strong bands of the neutral species at λmax
Control over the redox and structural properties of the ex-
TTF system 1 has been taken to a new level. Unprecedented
opportunities now exist for utilizing the radical cation in
electron-transfer processes. Moreover, this methodology should
be versatile for different bridging groups, which will enable
limited molecular motion to occur under precise electrochemical
conditions, with possible applications in molecular machinery.32
New ex-TTF cage and sensory systems can also be envisaged.
) 353 and 411 nm were replaced by a sharp band at λmax
)
423 nm, two broad bands at λmax ) 580 and 760 nm, and a
very broad band at ca. 1300-2700 nm, with λmax ) 2040 nm,
making the spectrum of the radical cation species 21•+ most
unusual. The bands at λmax ) 580 and 760 nm are similar to
those of the radical cation of alkylthio-TTF derivatives.30 The
very broad low-energy band is reminiscent of a similar absorp-
tion (λmax ) 2300 nm) of a partly oxidized dimeric TTF
cyclophane, which was unambiguously assigned to a mixed-
valence band, resulting from intermolecular interaction.31 For
cyclophanes 15 and 21, we assign the low-energy band to a
Acknowledgment. We thank EPSRC and The Danish Re-
search Academy for funding to C.A.C. We thank Professor J.
A. K. Howard for the use of X-ray facilities and EPSRC for
funding the improvement to the X-ray instrumentation.
(28) Electrochemical reversibility in this context is defined as (i) the ratio
between the cathodic and anodic peak currents, Ic/Ia ≈ 1.0; (ii) the peak
potentials Ep, which are independent of the scan rate; and (iii) ∆Ep ≈ (59/
z) mV, where z is the number of electrons involved in the process.
Supporting Information Available: General experimental
methods, synthetic details, and characterization data for all new
(29) For a reversible redox process, the half-wave potential is defined as E1/2
(Eox + Eoxpc)/2.
)
(30) Khopdaorkovsky, V.; Shapiro, L.; Krief, P.; Shames, A.; Mabon, G.; Gorgues,
A.; Giffard, M. Chem. Commun. 2001, 2736-2737.
(32) (a) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and Machines:
A Journey into the Nanoworld; Wiley-VCH: Weinheim, 2003. (b) Easton,
C. J.; Lincoln, S. F.; Barr, L.; Onagi, H. Chem. Eur. J. 2004, 10, 3120-
3128.
(31) Spanggaard, H.; Prehn, J.; Nielsen, M. B.; Levillain, E.; Allain, M.; Becher,
J. J. Am. Chem. Soc. 2000, 122, 9486-9494.
9
J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006 10489

A R T I C L E S
Christensen et al.
compounds; copy of the 13C NMR spectrum of 15; spectra of
cyclophane 15•+ obtained in spectroelectrochemical conditions;
X-ray crystallographic file for 15, 15‚C7H8, 16, and 23‚(CH2-
Cl2)2 in CIF format and additional ORTEP drawings of their
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA062358M
9
10490 J. AM. CHEM. SOC. VOL. 128, NO. 32, 2006