Synthesis of new viologen macrocycles with intramolecular charge transfer

Article abstract of DOI:10.1016/j.tetlet.2006.01.073

The syntheses of three new macrocycles composed of an acceptor (viologen) and a donor (dioxoaryl) are reported. These compounds exhibit intramolecular charge transfer and their X-ray analyses revealed that the aromatic systems are situated in parallel pla

Full text of DOI:10.1016/j.tetlet.2006.01.073

Tetrahedron Letters 47 (2006) 1953–1956
Synthesis of new viologen macrocycles with intramolecular
charge transfer
a
*
*
´
´
Elena Pıa, Rosa Toba, Marcos Chas, Carlos Peinador and Jose M Quintela
Departamento de Quı´mica Fundamental, Facultad de Ciencias, Universidade da Corun˜a, Campus A Zapateira s/n, 15071
A Corun˜a, Spain
Received 30 November 2005; revised 12 January 2006; accepted 16 January 2006
Available online 3 February 2006
Abstract—The syntheses of three new macrocycles composed of an acceptor (viologen) and a donor (dioxoaryl) are reported. These
compounds exhibit intramolecular charge transfer and their X-ray analyses revealed that the aromatic systems are situated in par-
˚
allel planes with a mean interplanar distance of 3.32 A.
ꢀ 2006 Elsevier Ltd. All rights reserved.
4,40-Bipyridinium dication salts (usually called violo-
gens) are a well-known electron-deficient components
in many supramolecular structures. Their electrochemi-
cal behavior, synthetic availability, and electronic prop-
erties make them ideal components for the synthesis of
many supramolecular assemblies.1 Viologen acceptor
character has been widely used in many charge-transfer
(CT) complexes, particularly with polycyclic arenes2
including rotaxanes and catenanes.3 It is well known
that the CT complex between a p electron-donor and
viologen as acceptor requires that the donor and the
acceptor units pack closely in a suitable geometry. The
optimal interplanar distance between donor and accep-
In this letter, we report the synthesis of new cyclophanes
with intramolecular charge-transfer composed of an
acceptor (viologen) connected to a donor (dioxoaryl)
by a polyether chain.
Scheme 1 outlines the synthesis of macrocycles 3a–cÆ
2PF6. Reaction of diols 1a–c12 with mesyl chloride
afforded derivatives 2a–c in high yields. Nucleophilic
displacement of mesyl groups with 4,40-bipyridine in
high dilution conditions gave macrocycles 3a–cÆ2PF6 in
moderate yields after counter anion exchange. Com-
pound 3aÆ2PF6 was obtained as a light yellow powder,
3bÆ2PF6 as an orange solid, and macrocycle 3cÆ2PF6 as
a dark violet powder. All these compounds were well
characterized by 1H NMR, 13C NMR, MS, UV–vis,
and elemental analyses.13
4
˚
tor parts is about 3.4 A. The CT complex formation
is associated with the appearance of a characteristic
broad band in the UV–vis spectrum.5
Intramolecular charge-transfer (ICT) compounds are
composed of donor and acceptor parts linked by a r
or p bond bridge. Topics of interest for ICT compounds
include molecular electronic devices,6 organic metals,7
chromophores for dyes8 and nonlinear optics,9 excited-
state energy transfer processes (including synthetic
light-harvesting systems).10 Surprisingly, ICT com-
pounds incorporating viologen units have remained rel-
atively rare.11
O
OR
N
O
N
O
O
b, c
n
n
α
X
β
O
O
O
OR
a
n
n
X
O
3a-c·2PF6
1a-c: R = H
2a-c: R = Ms
1a-3a·2PF6 n = 1, X =
1b-3b·2PF6 n = 2, X =
1c-3c·2PF6 n = 1, X =
Keywords: Intramolecular charge transfer; Cyclophanes; Bipyridinium;
Viologens.
*
Scheme 1. Reagents and conditions: (a) CH3SO2Cl, NEt3, CH2Cl2, rt,
3 h; (b) 4,40-bipyridine, KI, CH3CN, 70 ꢁC, 4 d; (c) NH4PF6, H2O.
Corresponding authors. Tel.: +34 981 167000; fax: +34 981
167065; e-mail addresses: capeveqo@udc.es; jqqoqf@udc.es
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.01.073

1954
E. Pı´a et al. / Tetrahedron Letters 47 (2006) 1953–1956
Figure 1. Absorption spectra of 3a–cÆ2PF6 measured in acetonitrile
solutions at room temperature.
The electron-accepting power of the viologen moiety in
combination with the electron donor aromatic groups
gives rise to intramolecular charge-transfer absorptions.
The longest-wavelength absorption maximum of the
macrocycles 3a–cÆ2PF6 appears at k = 450, 403, and
481 nm, respectively (Fig. 1). The hypsochromic shift
observed between 3a and 3b is probably due to the fact
that in the charge-transfer state of the cyclophanes both
the bipyridinium and the hydroquinone rings should
bear positive charges, resulting in a repulsion of the
donor and acceptor, which becomes more pronounced,
the shorter the polyether chains. Linear correlations (see
Supplementary data) between absorption and concen-
tration are observed, indicating the absence of inter-
molecular interactions.
Figure 2. Crystal structures of 3aÆ2PF6 (top), 3bÆ2PF6 (middle),
3cÆ2PF6 (bottom). Solvent molecules and PF6 anions are omitted for
clarity.
aromatic systems are situated in parallel planes with a
˚
mean interplanar distance of 3.34 and 3.32 A, respec-
tively (Table 1). The plane described by the hydroquinol
residues extends to include not only the phenoxymethyl-
ene units as expected but also the next carbon atom in
each direction along the polyether chains. The bipyridi-
nium units are distorted from the normal planar geom-
etry, involving both twisting (h = 20ꢁ and 18ꢁ) and
bowing (s = 17ꢁ and 18ꢁ) of their aromatic rings. Com-
pounds 3a,cÆ2PF6 present a C2 axis passing through the
centroids of the bipyridinium and dioxoaryl ring systems
because the aromatic donor component is centered with
respect to the central C–C bond within the bipyridinium.
On irradiation of the bipyridinium protons of macro-
cycle 3aÆ2PF6, in addition to enhancement of the neigh-
boring protons, transannular NOEs arising from the
dioxoaryl protons were also observed. The intensities
of the transannular NOEs were about 15% of those of
the normal NOEs of the neighboring bipyridinium pro-
tons. The enhancements of the dioxoaryl hydrogen sig-
nals were generally more pronounced on irradiation of
the inner bipyridinium hydrogen atoms b than on irradi-
ation of the outer bipyridinium hydrogen atoms a. Mac-
rocycle 3cÆ2PF6 also shows transannular NOEs but there
are not significant differences between a and b protons.
These NOEs evidently demonstrate approaches of the
donor and acceptor units in the cyclophanes 3b,cÆ2PF6
for at least several hundreds of milliseconds.
The cyclophanes 3a,cÆ2PF6 pack in the crystal to form
p-stacked structures, the dioxoaryl ring system of one
˚
Table 1. Distances (A) and angles (ꢁ) characterizing the geometries of
macrocycles 3a–cÆ2PF6
c
Compound
db
/
hd
se
3aÆ2PF6
3bÆ2PF6
3cÆ2PF6
3.34
3.32
3.32
20
23
18
2.5
4.2
1.2
17
0
18
a
a For 3b the angle h was defined as the angle between the hydroquinone
and the pyridine planes, and d is the mean interplanar distance
between dioxoaryl and pyridinium units.
b Mean interplanar distance between dioxoaryl and bipyridinium units.
c The torsional angle about the central C–C bond within the
bipyridinium.
d The angle between mean dioxoaryl and bipyridinium planes.
e The bowing of the bipyridine residues is expressed by the angle s,
subtended by the two N+–CH2 bonds emanating from the bipyridi-
nium rings.
The molecular structures of 3a–cÆ2PF6 were determined
by single-crystal X-ray analysis (Fig. 2).14 Single crystals
were obtained by diffusion of isopropyl ether into a solu-
tion of 3a–cÆ2PF6 in acetonitrile. The X-ray analysis of
cyclophanes with the short polyether chain (n = 1)
3a,cÆ2PF6 revealed the expected structure in which the

E. Pı´a et al. / Tetrahedron Letters 47 (2006) 1953–1956
1955
mentary data associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2006.01.073.
References and notes
1. Monk, P. The Viologens: Physicochemical Properties,
Synthesis and Applications of the Salts of 4,40-Bipyridine;
Wiley: New York, 1998.
2. Yoon, K. B. Chem. Rev. 1993, 93, 321; Nishikiori, S.-I.;
Yoshikawa, H.; Sano, Y.; Iwamoto, T. Acc. Chem. Res.
2005, 38, 227.
3. Balzani, V.; Credi, A.; Mattersteig, G.; Matthews, O. A.;
Raymo, F. M.; Stoddart, J. F.; Venturi, M.; White, A. J.
P.; Williams, D. J. J. Org. Chem. 2000, 65, 1924.
4. Yoon, K. B.; Kochi, J. K. J. Am. Chem. Soc. 1989, 111,
1128.
5. (a) Nakahara, A.; Wang, J. H. J. Phys. Chem. 1963, 67,
496; (b) Strong, R. L. In Intermolecular Forces; Pullman,
B., Ed.; D. Reidel: Dordrecht, 1981; Morokuma, K. Acc.
Chem. Res. 1977, 10, 294.
6. (a) Bloor, D. In Introduction to Molecular Electronics;
Petty, M. C., Bryce, M. R., Bloor, D., Eds.; Oxford
University Press: Oxford, 1995; p 1; (b) Metzger, R. M.
Mater. Sci. Eng. 1995, C3, 277; (c) Metzger, R. M. Chem.
Rev. 2003, 103, 3803.
Figure 3. A perspective view of the two independent molecules found
in the crystal structure of 3cÆ2PF6 showing mean interplanar distances
d = 3.32 A and l = 3.26 A.
˚
˚
molecule being aligned parallel with the bipyridinium
units of the next (Fig. 3). The mean interplanar separa-
˚
tion of the extended p-systems is l = 3.33 A for 3aÆ2PF6
˚
and 3.26 A for 3cÆ2PF6.
7. Khodorkovsky, V.; Becker, J. Y. In Organic Conductors:
Fundamentals and Applications; Farges, J. P., Ed.; Marcel
Dekker: New York, 1994; p 75.
8. Gordon, P. F.; Gregory, P. Organic Chemistry in Colour;
Springer: Berlin, 1983.
9. Reviews: (a) Marder, S. R.; Kippelen, B.; Jen, A. K.-Y.;
Peyghambarian, N. Nature 1997, 388, 845; (b) Verbiest,
T.; Houbrechts, S.; Kauranen, M.; Clays, K.; Persoons, A.
J. Mater. Chem. 1997, 7, 2175.
10. (a) Imahori, H.; Sasaka, Y. Adv. Mater. 1997, 9, 537; (b)
Li, F.; Gentemann, S.; Kalsbeck, W. A.; Seth, J.; Lindsey,
J. S.; Holten, D.; Bocian, D. F. J. Mater. Chem. 1997, 7,
The solid-state geometry (Fig. 2) of the macrocycle
3bÆ2PF6 departs from the C2 symmetric arrangement
observed for the other two macrocycles. In this case,
the dioxoaryl ring system is displaced with respect to
the molecular C2 axis, such that it is positioned preferen-
tially over one of the pyridinium rings of the bipyridi-
nium unit. The mean interplanar separation between
the hydroquinone ring and this pyridinium ring is
˚
3.32 A. In contrast to 3a,cÆ2PF6, there is no bowing of
the bipyridine residue, probably due to the longer poly-
ether chains that connect both aromatic systems. More-
over, there are no additional p-stacking interactions that
extend beyond the molecule.
1245; (c) Vollmer, M. S.; Wurthner, F.; Effenberger, F.;
¨
Emele, P.; Meyer, D. U.; Stumpfig, T.; Port, H.; Wolf, H.
¨
C. Chem. Eur. J. 1998, 4, 260.
11. (a) Simonsen, K. B.; Zong, K.; Rogers, R. D.; Cava, M.
P.; Becher, J. J. Org. Chem. 1997, 62, 679; (b) Simonsen,
K. B.; Thorup, N.; Cava, M. P.; Becher, J. Chem.
Commun. 1998, 901; (c) Bauer, H.; Stier, F.; Petry, C.;
Knorr, A.; Stadler, C.; Staab, H. A. Eur. J. Org. Chem.
2001, 3255; (d) Bryce, M. R. Adv. Mater. 1999, 11, 11.
12. (a) Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.;
Delgado, M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer,
A. E.; Philp, D.; Pietraszkiewicz, M.; Prodi, L.; Redding-
ton, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.;
Vicent, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114,
193; (b) Amabilino, D. B.; Anelli, P. L.; Ashton, P. R.;
In summary, we synthesized new viologen cyclophanes
composed by an acceptor part (bipyridinium) and a
donor (dioxoaryl) part. The appearance of a characteristic
broad band in the UV–vis spectra confirms a charge-
transfer interaction. The crystal structures of the three
compounds revealed the intramolecular proximity of
the bipyridinium and dioxoaryl moieties; furthermore,
in 3a and 3c this proximity is also intermolecular with
˚
mean interplanar distances of 3.33 and 3.26 A,
respectively.
´
´
Brown, G. R.; Cordova, E.; Godınez, L. A.; Hayes, W.;
Kaifer, A. E.; Philp, D.; Slawin, A. M. Z.; Spencer, N.;
Stoddart, J. F.; Tolley, M. S.; Williams, D. J. J. Am.
Chem. Soc. 1995, 117, 11142.
Acknowledgements
This research was supported by Xunta de Galicia (PGI-
13. General procedure for 3a–c. To a solution of 4,40-
bipyridine (0.219 g, 1.40 mmol) in CH3CN (200 ml) a
solution of 2a–c (1.54 mmol) in CH3CN (10 ml) and a
catalytic amount of KI were added. The reaction was
stirred at 70 ꢁC for 72 h. After cooling, the solvent was
evaporated in vacuo and the residue was triturated with
ether (250 ml). The solid was purified by flash chroma-
tography (acetone/1.5 M NH4Cl/MeOH 5:4:1). The prod-
uct-containing fractions were combined, and the solvents
were removed in vacuo. The residue was dissolved in water
(20 ml), and a saturated solution of NH4PF6 was added.
The precipitate was filtered and washed with water to yield
´
DIT04PXIC10307PN) and Ministerio de Educacion y
Cultura (BQU2003-00754). R.T. would like to thank
´
the Ministerio de Educacion y Cultura, for a predoc-
toral fellowship.
Supplementary data
UV–vis dilution experiments in acetonitrile solutions for
3a,cÆ2PF6 can be found as supplementary data. Supple-

1956
E. Pı´a et al. / Tetrahedron Letters 47 (2006) 1953–1956
3a–cÆ2PF6. Compound 3a: yield: 32%. 1H NMR
(300 MHz, CD3CN): d = 3.60 (m, 4H), 3.79 (m, 4H),
4.10 (m, 4H), 4.74 (m, 4H), 6.39 (s, 4H), 8.15 (d,
J = 6.9 Hz, 4H), 8.85 (d, J = 6.9 Hz, 4H) ppm. 13C
NMR (CD3CN): d = 63.1, 68.4, 69.8, 70.6, 115.4, 126.7,
146.7, 148.9, 152.8 ppm. MS (FAB): m/z = 553
radiation (k = 0.71073 A). The structures were solved by
direct methods (SHELX-97) and refined by full-matrix least-
squares on F2. Compound 3a: C24H28F12N2O4P2,
MW = 698.13, crystal size: 0.30 · 0.29 · 0.28 mm3, crystal
system: monoclinic, space group: P21/c, cell parameters:
˚
˚
˚
˚
a = 15.206(5) A, b = 15.435(5) A, c = 13.436(5) A, V =
3150.4(19) A , Z = 4, Dc = 1.559 mg/m , F0 0 0 = 1512,
R and wR2 are 0.0559 [I > 2r(I)] and 0.1515 (all
data). Compound 3b: C28H36F12N3O4P2, MW = 786.53,
crystal size: 0.37 · 0.36 · 0.09 mm3, crystal system: tri-
clinic, space group: P1, cell parameters: a = 11.100(5) A,
(MꢀPF6ꢀ)+, 408 (Mꢀ2PF6)+, 204 (Mꢀ2PF6
)
ꢀ 2+. Anal.
3
3
˚
Calcd for C24H28F12N2O4P2 (698.13): C, 41.27; H, 4.04;
N, 4.01. Found C, 41.33; H, 4.26; N, 3.92. Compound 3b:
yield: 22%. 1H NMR (300 MHz, CD3CN): d = 3.6–3.9 (m,
10H), 4.01 (m, 5H), 4.11 (m, 5H), 4.80 (m, 4H), 6.79 (s,
4H), 7.79 (d, J = 6.9 Hz, 4H), 8.95 (d, J = 6.9 Hz, 4H)
ppm. 13C NMR (CD3CN): d = 62.1, 68.9, 69.1, 70.2, 70.4,
70.8, 116.3, 127.1, 147.2, 150.6, 153.9 ppm. MS (FAB):
m/z = 787 (MH)+, 641 (MꢀPF6ꢀ)+, 496 (Mꢀ2PF6ꢀ)+,
˚
3
˚
˚
˚
b = 11.830(5) A, c = 13.894(5) A, V = 1650.5(12) A , Z =
2, Dc = 1.583 mg/m3, R and wR2 are 0.0437 [I > 2r(I)]
and 0.1089 (all data). Compound 3c: C28H36N2O6P2F12,
MW = 786.19, crystal size: 0.47 · 0.41 · 0.20 mm3, crystal
system: orthorhombic, space group: Pna21, cell para-
248 (Mꢀ2PF6
)
ꢀ 2+. C28H36N2O6P2F12 (786.52); calcd: C,
˚ ˚ ˚
meters: a = 27.557(5) A, b = 6.731(5) A, c = 15.909(5) A,
3 3
42.76; H, 4.61; N, 3.56; enc: C, 42.53; H, 4.34; N, 3.80.
Compound 3c: yield: 30%. 1H NMR (300 MHz, CD3CN):
d = 4.01 (s, 8H), 4.18 (m, 4H), 4.73 (m, 4H), 6.69 (d,
J = 7.5 Hz, 2H), 7.29 (m, 4H), 7.55 (d, J = 6.9 Hz, 4H),
8.74 (d, J = 6.9 Hz, 4H) ppm. 13C NMR (CD3CN):
d = 62.8, 68.3, 69.7, 70.3, 105.7, 114.9, 125.9, 126.5,
146.4, 148.3, 154.3 ppm. MS (ES): m/z = 603 (MꢀPF6ꢀ)+,
458 (Mꢀ2PF6ꢀ)+. C28H30F12N2O4P2 (748.48); calcd: C,
44.93; H, 4.04; N, 3.74; enc: C, 45.06; H, 4.12; N, 4.01.
˚
V = 2951(2) A , Z = 4, Dc = 1.685 mg/m , F0 0 0 = 1528,
R and wR2 are 0.0376 [I > 2r(I)] and 0.0966 (all data).
Crystallographic data (excluding structure factors) for
3a–c have been deposited with the Cambridge Crystallo-
graphic Data Center as supplementary publication
numbers 3a: CCDC 291340, 3b: CCDC 291341, 3c:
CCDC 291342. Copies of the data may be obtained, free
of charge, on application to CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (fax: +44 (0) 1223 33603 or
e-mail: deposit@ccdc.cam.ac.uk).
14. X-ray data for compounds 3a–c were collected at 293(2) K
on a Bruker Smart-CCD1000 diffractometer with Mo-Ka