Highly-strained cyclophanes bearing both photo- and electro-active constituents

Article abstract of DOI:10.1016/j.tetlet.2011.08.013

The synthesis of a highly-strained cyclophane comprising azobenzene and methyl viologen units was achieved by coupling 3,3′-dihydroxy-4,4′- bipyridine with azobenzoic acid in CH₂Cl₂. The molecular structure, determined by single-crystal X-ray crystallography, shows that the azobenzene NN unit adopts the trans conformation and that the bipyridinium unit is twisted. The cyclic voltammogram recorded for the target compound displays an irreversible wave at -0.37 V vs Ag/AgCl, associated with the one-electron reduction of the bipyridinium subunit. A further wave is seen at E_1/2 = -1.52 V versus Ag/AgCl and is assigned to one-electron reduction of the azobenzene group. Visible light illumination of the azobenzene chromophore in CH₃CN triggers trans to cis isomerization but the process is irreversible.

Full text of DOI:10.1016/j.tetlet.2011.08.013

Tetrahedron Letters 52 (2011) 5315–5318
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly-strained cyclophanes bearing both photo-
and electro-active constituents
a,
⇑
a
a
b
Andrew C. Benniston , Anthony Harriman , Songjie Yang , Ross W. Harrington
a
Molecular Photonics Laboratory, School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
Crystallography Centre, School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a highly-strained cyclophane comprising azobenzene and methyl viologen units was
achieved by coupling 3,3 -dihydroxy-4,4 -bipyridine with azobenzoic acid in CH Cl . The molecular struc-
2 2
Received 2 June 2011
Revised 26 July 2011
Accepted 2 August 2011
Available online 7 August 2011
0
0
ture, determined by single-crystal X-ray crystallography, shows that the azobenzene N@N unit adopts the
trans conformation and that the bipyridinium unit is twisted. The cyclic voltammogram recorded for the
target compound displays an irreversible wave at ꢀ0.37 V vs Ag/AgCl, associated with the one-electron
reduction of the bipyridinium subunit. A further wave is seen at E1/2 = ꢀ1.52 V versus Ag/AgCl and is
assigned to one-electron reduction of the azobenzene group. Visible light illumination of the azobenzene
Keywords:
0
4
,4 -Bipyridinium
chromophore in CH
3
CN triggers trans to cis isomerization but the process is irreversible.
Ó 2011 Elsevier Ltd. All rights reserved.
Azobenzene
Macrocycle
Redox center
X-ray structure
Isomerization
Functionalized cyclophanes represent a class of cyclic struc-
tures incorporating one or more units that are capable of activation
results, in fact, can be used to argue that switching of the dihedral
angle subtended between the pyridinium rings by ca. 27 will alter
1
o
by external stimulus (e.g., light, protonation,3 cation binding,
2
4
the reduction potential by some 560 mV. Such a substantial differ-
ence in reduction potential may be put to good effect in a variable
5
electrochemical oxidation ). In many cases, the cyclophane is a rel-
atively small feature in a more elaborate structure, but its activation
triggers a cascade of events that may, for example, alter the shape,
hydrophobicity, colour, and/or H-bond affinity of the superstruc-
ture. Certainly the light-driven molecular shuttling observed in
rotaxane systems is a prime example of directed large-scale struc-
1
7
resistor construct, where an alteration in dihedral angle would re-
tard (act as a gate to) electron flow in one dimension (Fig. 1). To this
end, linking together a viologen subunit and an azobenzene unit
into a single cyclophane structure seems to meet the correct design
principle for a light-activated variable resistor. If the spacer be-
tween the two units is sufficiently short, light-induced trans to cis
6
7
8
9
1
0
tural alteration. Identification of suitable molecular building
blocks capable of selective activation is critical, with the azoben-
1
1
0
12
zene subunit and viologen (4,4 -bipyridinium dication) group
representing two heavily used photo- and redox-active moieties,
respectively. Selective light activation of azobenzene is known to
facilitate N@N trans to cis isomerization, which is accompanied by
1
3
slow (seconds to hours) thermal reversion to the trans form. The
large-scale structural change that takes place is put to good use in
14
many molecular electronics applications. Likewise, the two-elec-
0
0
tron reduction of methyl viologen (N,N -dimethyl-4,4 -bipyridini-
um dication) is very well understood. In the ground state, the two
pyridinium rings of the dication are twisted around the connecting
1
5
C–C bond, but upon one-electron reduction they become planar.
Previously, we demonstrated that the reduction potentials for a ser-
ies of constrained viologen derivatives were highly dependent on
1
6
the length of the tether linking the two pyridinium rings. The
Figure 1. Pictorial representation of
molecular construct where the open gate facilitates electron flow but no current
passes when the gate is closed.
a light-activated variable resistor in a
⇑
Corresponding author.
E-mail address: a.c.benniston@ncl.ac.uk (A.C. Benniston).
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.013

5
316
A. C. Benniston et al. / Tetrahedron Letters 52 (2011) 5315–5318
isomerization should twist the bipyridinium group. The prepara-
tion and properties of the first prototype of this new class of mole-
cule, AZV, is presented here.
Scheme 1 illustrates the methodology employed for preparation
0
0
of AZV starting from the known 3,3 -dihydroxy-4,4 -bipyridine
(
2). Cyclization of 2 with the dicarboxylic acid derivative 119 in
dry CH Cl , using DCC as the coupling agent, produced two main
products after column chromatography (silica gel, CH Cl /acetone
:1 then 2:1) purification. The H NMR spectra recorded for these
18
2
2
2
2
1
4
two compounds were markedly disparate, especially with respect
to the most downfield resonances associated with the bipyridini-
um subunit. For one product, a broad singlet was observed at
d = 8.70 and a doublet at d = 8.55 (J = 3.5 Hz). In the second case,
Figure 2. X-ray molecular structure determined for AZV. Triflate ions and a solvent
molecule are omitted for clarity.
a
(
well resolved doublet (d = 8.73, J = 4.8 Hz) and
d = 8.66) were observed. The mass spectrum indicated that, in fact,
the first fraction collected by column chromatography was the
a singlet
tetra-cationic species. Multiple recrystallization failed to isolate
only one compound, while column chromatographic separation
was ruled out owing to the susceptibility of these compounds to
hydrolysis.
2
0
[
1+1] adduct (3) while the second fraction was the [2+2] product
2
1
(4).
The final step of a supposedly straightforward dialkylation of
0
the 4,4 -bipyridine group proved to be frustratingly difficult. The
Single-crystals obtained by slow vapor diffusion of Et
2
O into a
tried and tested method is to alkylate with CH
3
I followed by ion
solution of AZV in CH CN were suitable for X-ray diffraction anal-
3
2
4
exchange to convert the water-soluble iodide salt to the organic-
soluble hexafluorophosphate derivative. Under such conditions,
the attempted conversion of 3 to AZV failed, and only the N-alkyl-
ysis. The molecular structure obtained is shown in Figure 2. The
bipyridinium cation is highly twisted, the dihedral angle between
the two planes created using each ring as a reference point is
1
o
ated version of 2 was evident from the H NMR spectrum. Eventu-
67.6 . The azobenzene group is also distorted from planarity, the
ally, the source of the problem was found to be water hydrolysis of
dihedral angle between the two planes of the two aromatic rings
is 39.6 . The obvious steric strain inherent to this cyclophane can
o
the iodide salt of AZV, most likely enhanced by the close proximity
2
2
of the positive charge to the ester as discussed by Engbersen et al.
To overcome this problem, the direct alkylation of 3 using methyl
triflate in CH CN, followed by multiple recrystallization, afforded
the desired compound as a red solid, which was soluble in CH CN
be exemplified by the close distance (3.2 Å) between the two oxy-
gen atoms of the bipyridinium subunit. The energy-minimized
geometry computed by DFT calculations for the isolated molecule
differs markedly from the X-ray structure. The most notable differ-
ence relates to the reduced dihedral angle of 9.6o between the
pyridinium cations. In this case, most of the strain is taken up by
3
3
2
3
and acetone. The attempted alkylation of 4 unfortunately affor-
ded a mixture of products presumably representing the mono- to
o
the diazobenzene unit, where the dihedral angle is 27 , with the
oxygen atoms being forced into surprisingly close (4 Å) proximity.
There is a similar trend in the computed geometry for the [2+2]
cyclophane where the viologen unit tends toward planarity but
the azobenzene residue is heavily distorted. This latter structure
is interesting in as much as the four carbonyl oxygen atoms point
into the interior of the cavity in the form of two pairs, with O–O
distances of 2.4 and 4.0 Å.
Me
O
N
N
HO
O
N
O
N
(iii)
OH
OH
N
N
O
+
O
OH
O
1
2
5
Me
The electrochemical behavior of AZV was explored using cyclic
voltammetry in dry CH CN (0.2 M TBATFB as the background elec-
3
(i)
trolyte) at a glassy carbon working electrode. A typical voltammo-
gram, restricted to ꢀ1.70 V vs Ag/AgCl, is shown in Figure 3. The
first point to note is the irreversible peak seen at ꢀ0.37 V (vs Ag/
AgCl) on reductive scans which represents the one-electron reduc-
tion of the viologen unit. The position of this peak is as expected for
a slightly strained viologen derivative. However, the re-oxidation
N
N
N
N
N
N
O
N
N
O
O
O
O
O
O
+
O
O
O
O
ꢀ1
wave is not observed even at faster scan rates (62 V s ) or when
O
N
N
the reductive scan is switched at ꢀ0.7 V (vs Ag/AgCl), and this sit-
uation is most unusual for a viologen derivative. Moreover, the sec-
ond reduction wave normally observed for viologen derivatives,
which forms the neutral species and is expected at ca. ꢀ1 V (vs
Ag/AgCl), is absent for AZV, at least over the anticipated potential
range. It was noted for this first reduction that, as well as the nor-
mal increase in cathodic current, the peak potential undergoes a
gradual shift to more negative potentials on raising the scan rate
N
N
3
1
4% yield
4
1
9% yield
(
ii)
(ii)
MIXTURE OF INSEPARABLE
PRODUCTS
N
N
O
O
(
see Supplementary data). Such behavior is consistent with a fast
2
CF SO3-
3
O
chemical reaction accompanying the addition of an electron (i.e.,
an ec mechanism). The irreversible electrochemistry is not caused
O
Me
N
N
Me
0
by the 2,2 -substituents, since reversible reduction steps have been
1
6
reported for a series of dialkoxy strapped viologens, but appears
to be unique to this compound. That the first reduction peak is
associated with the viologen unit was confirmed by recording
the cyclic voltammogram for 3 under identical conditions.
AZV
Scheme 1. Reagents and Conditions: (i) CH
MeSO CF , CH CN, room temperature, (iii) MeOH, H , reflux.
2 2
Cl , DCC, room temperature, (ii)
+
3
3
3

A. C. Benniston et al. / Tetrahedron Letters 52 (2011) 5315–5318
5317
+
Fc /Fc
30
20
10
0
-
-
-
10
20
30
-
1.5
-1.0
-0.5
0.0
0.5
Potential / V
Figure 4. Computed structure for the monocation radical required to permit
electron delocalization, and showing the severe distortion at the viologen units that
leads to the banana-shaped geometry.
Figure 3. Cyclic voltammogram recorded for AZV in dry CH
3
CN (0.2 M TBATFB) at a
glassy carbon working electrode and versus an Ag/AgCl reference. Scan
rate = 50 mV sꢀ1.
tered at about 330 nm. In contrast, the absorption spectrum re-
corded for AZV in CH CN is more complex and not so easily
3
For AZV,
a
second wave is evident at
E
1/2 = ꢀ1.52 V
interpreted (Fig. 5). The low energy region comprises at least two
overlapping bands (see Supplementary data) with maxima at
(
DE = 70 mV) vs Ag/AgCl. The scan rate dependence for this latter
wave (see Supplementary data) is consistent with that expected
for a quasi-reversible step. In an attempt to assign the molecular
group responsible for this latter wave, cyclic voltammograms were
recorded for the methyl ester 5 under identical conditions (see
Supplementary data). Here, the voltammogram is dominated by a
412 nm (FWHM = 51 nm) and 457 nm (FWHM = 65 nm). This latter
⁄
band is assigned to the azobenzene-based n–
p
electronic transi-
tion by reference to the cyclophane 3. The broad band occurring
at higher energy is attributed tentatively to a charge-transfer tran-
sition between the azobenzene and viologen subunits; this partic-
ular transition is absent in 3. The relatively intense absorption
quasi-reversible wave at E1/2 = ꢀ1.05 V (
DE = 80 mV) vs Ag/AgCl
and an irreversible wave at Ered = ꢀ1.41 V vs Ag/AgCl. As noted
for other azobenzene compounds, the reverse peak for this latter
wave is located at ꢀ0.76 V vs Ag/AgCl, and becomes more pro-
nounced at higher scan rates. On the basis of the combined electro-
chemistry results, it seems most likely that the second reductive
wave seen for AZV corresponds to the reversible addition of one
electron to the azobenzene group. Steric strain imposed by the
cyclophane structure makes this step much more difficult than
for the isolated subunit.
transition identified for the azobenzene unit is also present for
⁄
AZV but overlaps somewhat with corresponding
p
,p
transitions
localized on the viologen unit and appearing at 283 nm. An impor-
⁄
tant finding from these studies is that the n,
p
transition for AZV
occurs at low energy, although overlapping with one or more
charge-transfer bands, since this favors trans to cis isomerization
under visible light excitation.
Illumination of azobenzene compounds in the liquid phase, and
sometimes in solid media, results in trans to cis isomerization until
reaching a photostationary state. This transformation is usually
accompanied by an increase in intensity of the longer-wavelength
A DFT (B3LYP, 6-311G) calculation performed on the energy-
minimized structure generated for AZV (see Supplementary data)
is consistent with the LUMO lying on the viologen moiety, as
deduced by cyclic voltammetry. Calculations further indicate that
the neutral viologen can be formed, albeit with a distorted (i.e.,
banana-shaped) geometry that will certainly push the second
reduction potential towards more negative values. As such, the
absence of a re-oxidation peak for the monocation radical must
arise because of the instability of this species. Because the poten-
tial is modest, full bond cleavage is most unlikely and instead we
attribute the accompanying chemical reaction to radical attack at
the azo centre of a second molecule. Indeed, azobenzene is known
to be susceptible to addition reactions under electrochemical con-
ditions. For AZV, we surmise that addition of the first electron gen-
erates the monocation radical whereby the radical center is not
fully delocalized but, due to steric constraints, is localized on one
pyridine ring (Fig. 4). This radical attacks an azobenzene group,
although the cyclophane arms probably prevent intramolecular
reaction.
2
6
absorption profile and a noticeable blue shift. Both observations
are explained by the elevated oscillator strength for the cis isomer
⁄
and the concomitant increase in energy between the n–
p
levels.
Visible-light irradiation of AZV in aerated (or N -purged) CH CN
2
3
led to significant changes in the absorption spectrum (see Supple-
5000
25000
20000
15000
10000
4000
3000
2000
1000
0
350
400
450
500
550
600
The absorption spectrum recorded for the control compound 5
in CH CN shows a broad, Gaussian-shaped profile with a maximum
3
at 442 nm (Full-Width at Half Maximum (FWHM) = 96 nm). This
band is characterized by a relatively small molar absorption coef-
Wavelength / nm
5
000
0
250
300
350
400
450
500
550
600
ficient at the peak maximum and bears all the recognized hall-
⁄
Wavelength / nm
marks of an n–
p
electronic transition associated with the
2
5
azobenzene unit. This band is extremely important in the context
..
Figure 5. Room temperature absorption spectrum for AZV (ꢀ) methyl ester 5 (- -)
of molecular photonic devices since it is responsible for the trans to
3
and compound 3 ( ) in dry CH CN. Insert shows the expansion of the longer-
⁄
cis isomerization. There is also a more intense
p
,p
transition cen-
wavelength absorption profiles.

5
318
A. C. Benniston et al. / Tetrahedron Letters 52 (2011) 5315–5318
mentary data). In particular, the intensity of those bands centered
at 411, 316 and 283 nm diminished gradually during photolysis,
but reached a steady-state value after some 30 min or so (see Sup-
plementary data). There is also a modest blue-shift and line-nar-
rowing for the lower-energy absorption band. After photolysis,
the absorption profile shows signs of partial recovery over a few
hours but on prolonged standing in the dark it becomes clear a
new species was formed. A similar experiment performed on 5 in
Supplementary data (a selection of cyclic voltammograms re-
corded for AZV and methyl ester 5 at different scan rates. DFT calcu-
lation results, fitted spectra and absorption spectra for AZV, 4 and 5
after irradiation with white light) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2011.08.013.
References and notes
1.
Benniston, A. C.; Mackie, P. R. In Nanostructured Materials and Nanotechnology;
3
aerated CH CN resulted in the expected blue-shift and increase
Nalwa, H. S., Ed.; Academic Press: London, 2002; pp 693–747.
in intensity to the absorption band centered at 442 nm. Rather sur-
prisingly, no major changes to the absorption spectrum occurred
after leaving the sample in the dark for several hours (see Supple-
2. Shinkai, S.; Yoshioka, A.; Nakayama, H.; Manabe, O. J. Chem. Soc., Perkin Trans. 2
990, 1905–1909.
1
3
4
.
.
Kikuchi, J.; Egami, K.; Suehiro, K.; Murakami, Y. Chem. Lett. 1992, 1685–1688.
Amabilino, D. B.; Dietrich-Buchecker, C. O.; Livoreil, A.; Pérez-García, L.;
Sauvage, J.-P.; Stoddart, J. F. J. Am. Chem. Soc. 1996, 118, 3905–3913.
mentary); after 3 days a shoulder appeared on the longer-wave-
length band. Prior work by Joshua et al.27 concluded that
5. Ashton, P. R.; Balzani, V.; Becher, J.; Credi, A.; Fyfe, M. C. T.; Mattersteig, G.;
Menzer, S.; Nielsen, M. B.; Raymo, F. M.; Stoddart, J. F.; Venturi, M.; Williams, D.
J. J. Am. Chem. Soc. 1999, 121, 3951–3957.
irradiation of 5 in 1,2-dichloroethane results in no permanent
change to the molecule, and that the observed color change arises
from trans to cis isomerization. Our findings suggest, however, that
the overall behavior is more complex. It would appear that trans to
cis isomerization of the azobenzene unit does take place upon illu-
mination of AZV, but this is followed by secondary events that lead,
in part, to decomposition of the compound.
Although alkylation products of 4 could not be isolated pure,
2 2
photoisomerization of the basic cyclophane was tested in CH Cl
and 2-MeTHF. Illumination of either solution with white light re-
sulted in no real appreciable change in the UV–visible spectrum
6. Benniston, A. C.; Harriman, A.; Lynch, V. M. J. Am. Chem. Soc. 1995, 117, 5275–
5291.
7.
Dvomikovs, V.; House, B. E.; Kaetzel, M.; Dedman, J. R.; Smithrud, D. B. J. Am.
Chem. Soc. 2003, 125, 8290–8301.
8. Jeppesen, J. O.; Nielsen, K. A.; Perkins, J.; Vignon, S. A.; Di Fabio, A.; Ballardini,
R.; Gandolfi, M. T.; Venturi, M.; Balzani, V.; Becher, J.; Stoddart, J. F. Chem. Eur. J.
2003, 9, 2982–3007.
9.
Fioravanti, G.; Haraszkiewicz, N.; Kay, E. R.; Mendoza, S. M.; Bruno, C.;
Marcaccio, M.; Wiering, P. G.; Paolucci, F.; Rudolf, P.; Brouwer, A. M.; Leigh, D.
A. J. Am. Chem. Soc. 2008, 130, 2593–2601.
10. Balzani, V.; Credi, A.; Venturi, M. Chem. Soc. Rev. 2009, 38, 1542–1550.
11. Barrett, C. J.; Mamiya, J.; Yager, K. G.; Ikeda, T. Soft Matter 2007, 3, 1249–1261.
12. Benniston, A. C. Chem. Soc. Rev. 2006, 25, 427–435.
(
see Supplementary data). It is possible that a single or concerted
13. Yager, K. G.; Barrett, C. J. J. Photochem. Photobiol. A Chem. 2006, 182, 250–261.
14. Mativetsky, J. M.; Pace, G.; Elbing, M.; Rampi, M. A.; Mayor, M.; Samori, P. J. Am.
Chem. Soc. 2008, 130, 9192–9193.
isomerization of the azobenzene groups is too difficult, or that the
thermal cis to trans isomerization rate is fast. Even so, the com-
puted cyclophane structure reveals existence of a highly oxy-
gen-rich central cavity consummate for binding metal ions. We
expect to explore this type of chemistry, especially with lantha-
nide ions.
A prototypic light-activated variable resistor has been designed
and synthesized as part of this work. A key finding is that the com-
pact azobenzene–viologen cyclophane can be assembled by way of
a simple esterification reaction, although both [1+1] and [2+2]
products appear. Stable products emerging from the subsequent
15. Bockman, T. M.; Kochi, J. K. J. Org. Chem. 1990, 55, 4127–4135.
16. Benniston, A. C.; Harriman, A.; Li, P.; Rostron, J. P.; Harrington, R. W.; Clegg, W.
Chem. Eur. J. 2007, 13, 7838–7851.
17. Zhang, C.; He, Y.; Cheng, H.-P.; Xue, Y.; Ratner, M. A.; Zhang, X.-G.; Krstic, P.
Phys. Rev. B 2006, 73, 125445-1–125455-5.
18. Benniston, A. C.; Harriman, A.; Li, P.; Rostron, J. P. Tetrahedron Lett. 2005, 46,
291–7293.
7
19. Doumit, C. J.; McPherson, G. L.; Belford, R. L.; Lanoux, S. B.; Janassen, H. B. Inorg.
Chem. 1977, 16, 565–569.
20. Data for 3: 1H NMR (300 MHz, CDCl ): d = 8.70 (2H, s), 8.55 (2H, d, J = 4.6 Hz),
3
7.87 (2H, dd, J = 7.8, 1.4 Hz), 7.68 (2H, dt, J = 7.8, 1.4 Hz), 7.54 (2H, dd, J = 8.0,
1
.2 Hz), 7.50 (2H, dt, J = 7.6, 1.4 Hz), 7.29 (2H, d, J = 5.0 Hz). 13C NMR (CDCl
3
):
d = 164.7, 153.3, 147.5, 144.8, 144.5, 136.0, 133.8, 131.7, 130.2, 125.5, 124.0,
0
N-alkylation of the 4,4 -bipyridine unit are found only for the smal-
+
1
21.8; HRMS (NSI) found: 423.1088 [M+H] , C24
21. Data for 4: 1H NMR (300 MHz, CDCl
): d = 8.73 (4H, d, J = 4.8 Hz), 8.66 (4H, s),
.64 (4H, dd, J = 7.7, 1.4 Hz), 7.49 (4H, d, J = 4.8 Hz), 7.32 (4H, dt, J = 7.7, 1.2 Hz),
.21 (4H, dt, J = 7.7, 1.2 Hz), 6.91 (4H, dd, J = 8.0, 1.2 Hz); HRMS (NSI) found:
15 8 4
H O N requires: 423.1088.
ler cyclophane, with the resultant dicationic species being sub-
jected to considerable internal strain. Part of the design rationale
for this system revolved around the desire to twist the viologen
subunit by way of light-activated trans to cis isomerization at the
corresponding azobenzene unit. Unfortunately, internal strain is
so severe that it disturbs both subunits. A direct consequence of
this effect is that the mono-cation formed on electrochemical
reduction of the viologen is unable to adopt a planar geometry
and undergoes a competing chemical reaction that relieves the ste-
ric strain. Somewhat surprisingly given this behavior, photo-in-
duced trans to cis isomerization is observed for the target
cyclophane, raising the possibility to study the electrochemical
behavior of the cis form by cyclic voltammetry. Such studies are
currently in progress and will be reported at a later date.
3
7
7
8
+
29 8 8
45.2104 [M+H] , C48H O N requires: 845.2103.
22. Engbersen, J. F. J.; Geurtsen, G.; De Bie, D. A.; Van der Plas, H. C. Tetrahedron
988, 44, 1795–1802.
3. Data for AZV: 1H NMR (300 MHz, CDCl
.09 (2H, d, J = 6.3 Hz), 7.88 (2H, dd, J = 7.7, 1.3 Hz), 7.84 (2H, dt, J = 7.1, 1.4 Hz),
1
2
3
): d = 9.09 (2H, s), 8.67 (2H, d, J = 6.4 Hz),
8
1
3
7.79 (2H, dd, J = 8.0, 1.3 Hz), 7.66 (2H, dt, J = 7.5, 1.4 Hz), 4.36 (s, 6H). C NMR
CDCl ): d = 164.3, 152.6, 147.4, 144.1, 141.6, 141.5, 135.5, 132.2, 132.1, 130.9,
(
3
1
9
124.2, 123.2, 50.2. F NMR (CDCl
3
): d = ꢀ79.3. HRMS (NSI) found: 601.0985 [M-
+
CF
3
SO
3
] , C27
H
20
F
3
N
4
O
S
7 1
requires: 601.0999. Elemental analysis found C,
45.53; H, 3.57; N, 6.84, C28H20F N O S .Et O requires C, 46.60; H, 3.67; N, 6.79.
6
4
10
2
2
For partial solvate C28
4. Selected X-ray data: C32
c = 13.8669(8); = 76.073(5) , b = 73.957(5) ,
H
20
F
6
N
4
O
10
S
2
.0.5 Et
2
O C, 45.75; H, 3.20; N, 7.11.
2
H
30
F
6
N
4
O
11
2
S ; 824.72; a = 11.6190(7), b = 11.9384(6),
o
o
o
a
c
= 71.458(5) ; V = 1727.82(17)
3
2
Å ; Z = 2; F(0 0 0) = 848;
R
indices [F > 2r] R1 = 0.0537, wR2 = 0.1463; R
indices (all data) R1 = 0.0721, wR2 = 0.1585. Crystallographic data (excluding
structure factors) for the structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication no.
CCDC 827803. Copies of the data can be obtained, free of charge, on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44-(0)1223-336033 or
e-mail: deposit@ccdc.cam.ac.Uk).
Acknowledgments
This work was supported by the Engineering and Physical Sci-
ences Research Council (EPSRC EP/G0409X/1 and EP/F03637X/1).
The EPSRC-sponsored Mass Spectrometry Service at Swansea is
also thanked for collecting the mass spectra of certain compounds.
Supplementary data
25. Lednev, I. K.; Ye, T.-Q.; Matousek, P.; Towrie, M.; Foggi, P.; Neuwahl, F. V. R.;
Umapathy, S.; Hester, R. E.; Moore, J. N. Chem. Phys. Lett. 1998, 290, 68–74.
26. Birnbaum, P. P.; Linford, J. H.;Style, D. W. G. Trans. FaradaySoc. 1953, 49, 735–744.
7. Joshua, C. P.; Rajasekharan Pillai, V. N. R. Tetrahedron Lett. 1973, 37, 3559–3562.
2