Novel supramolecular charge-transfer systems based on bis(18-crown-6) stilbene and viologen analogues bearing two ammonioalkylgroups

Article abstract of DOI:10.1039/b500667h

A series of new viologen analogues bearing two ammonioalkyl groups (2-4) were synthesized in order to study their complexation with bis(18-crown-6) stilbene (1b). Electronic spectroscopy and ¹H NMR measurements show that in acetonitrile, bis(cr

Full text of DOI:10.1039/b500667h

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P A P E R
Novel supramolecular charge-transfer systems based on
bis(18-crown-6)stilbene and viologen analogues bearing two
ammonioalkyl groupsw
Sergey P. Gromov,*a Artem I. Vedernikov,a Evgeny N. Ushakov,b Natalia A. Lobova,a
Asya A. Botsmanova,a Lyudmila G. Kuz’mina,c Andrei V. Churakov,c Yuri A. Strelenko,d
Michael V. Alfimov,a Judith A. K. Howard,e Dan Johnelsf and Ulf G. Edlund*f
a Photochemistry Center, Russian Academy of Sciences, 7A Novatorov str., 119421 Moscow,
Russian Federation. E-mail: gromov@photonics.ru; Fax: þ7-095-9361255
b Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka,
142432 Moscow Region, Russian Federation
c N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninskiy prosp., 117907 Moscow, Russian Federation
d N. D. Zelinskiy Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninskiy prosp., 117913 Moscow, Russian Federation
e Chemistry Department, Durham University, South Road, Durham, UK DH1 3LE
f
˚
˚
Department of Organic Chemistry, Umea University, SE-901 87 Umea, Sweden.
E-mail: Ulf.Edlund@chem.umu.se; Fax: þ46-90-7867844
Received (in Montpellier, France) 17th January 2005, Accepted 24th March 2005
First published as an Advance Article on the web 11th May 2005
A series of new viologen analogues bearing two ammonioalkyl groups (2–4) were synthesized in order to
1
study their complexation with bis(18-crown-6)stilbene (1b). Electronic spectroscopy and H NMR
measurements show that in acetonitrile, bis(crown) stilbene 1b forms highly stable 1 : 1 and 2 : 1 charge-
transfer (CT) complexes with p-acceptors 2–4 owing to host–guest bonding. The influence of geometric and
electronic factors on the complex formation constants are discussed. The structures of the supramolecular
1
CT complexes are analyzed on the basis of H and 13C NMR data obtained in solution and in the solid
state. X-Ray diffraction data for 1b and for model tetramethoxystilbene are also reported.
bonding. It was demonstrated that the supramolecular com-
Introduction
plex between 1b and 2b can act as optical sensor for metal ions
in non-aqueous environments.5a More recently, it was shown
that under certain conditions, 1b and 2b can form a relatively
stable 2(D) : 1(A) complex.5b
Here we report the first synthesis and results of a compre-
hensive study of new bi- and termolecular complexes composed
of bis(crown) stilbene 1b and diammonium p-acceptor salts 2c,
3b (Scheme 2) and 4b (Scheme 3). The composition and the
thermodynamic stability of these supramolecular systems and a
number of model complexes in acetonitrile were determined by
electronic spectroscopy and 1H NMR titration. The structures
Crown ethers are widely utilized as efficient polydentate ligands
for metal and ammonium ions.1 Supramolecular systems com-
bining crown ethers and light-absorbing groups attract con-
siderable attention as they have a potential for use in optical
sensors for metal ions2 and in photoswitchable molecular
devices.3 Functional groups whose spectroscopic characteris-
tics are controlled by photophysical phenomena,2b such as
photoinduced internal charge transfer, photoinduced electron
transfer, excimer formation or electronic energy transfer are
used as signalling moieties in crown ether-based optical sen-
sors. The photoswitchable crown compounds are created using
functional groups that can undergo substantial structural
changes under the action of light.4
Recently,5 we proposed a new type of photosensitive crown
compound represented by a supramolecular complex between
bis(18-crown-6)stilbene (1b, Scheme 1) and 1,2-bis[1-(3-ammo-
niopropyl)-4-pyridiniumyl]ethylene
tetraperchlorate
(2b,
Scheme 2). This non-fluorescent complex shows a broad
absorption band in the visible region, indicative of charge
transfer in the ground state. Unlike usual organic donor (D)–
acceptor (A) complexes,6 this complex has a very high thermo-
dynamic stability that is provided by two-centre host–guest
w Electronic supplementary information (ESI) available: Selected bond
lengths and angles; solution and solid state 13C NMR data for 1b, 2b,
[1b ꢀ 2b] and [(1b)2 ꢀ 2b]. See http://www.rsc.org/suppdata/nj/b5/
b500667h/
Scheme 1 Structure of donors 1a and 1b.
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 8 8 1 – 8 9 4
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Scheme 2 Synthesis of acceptors 2b, c and 3b.
of the complexes in solution and in the solid state were studied
1
irreproducible or required the use of reagents difficult to
prepare and rigorous reaction conditions.8 Therefore we devel-
oped an alternative synthesis of (E)-7 with Wittig condensation
as the key step (Scheme 3).
by H and 13C NMR spectroscopy, including 2D COSY and
NOESY techniques. For 1b and 3,4,30,40-tetramethoxystilbene
(1a, Scheme 1), X-ray diffraction data are presented.
In addition, we synthesized a few reference acceptor salts,
2a–4a (Scheme 4) and 8 (Scheme 5), in order to study their
complexation with 1b. Compounds 2a–4a are readily obtained
by fusing together the appropriate heterocyclic bases with ethyl
tosylate followed by treatment with perchloric acid.9 Com-
pound 8 was prepared using the synthetic procedure developed
for 2b–4b.
Results and discussion
A. Synthesis
The synthesis of bis(crown) stilbene 1b has been previously
described in the literature and is based on the condensation of
40-formylbenzo-18-crown-6 ether in the presence of Ti(III) salts;
the yield of 1b under these conditions was 31%.7 When we
replaced the drastic cooling at the preparation step of the
tetrahydrofuran–titanium(IV) chloride complex by cooling to
0 1C, the yield of 1b increased to 40%. The reference p-donor
1a7 was prepared under the same conditions in 59% yield. The
electron-donating effect of the crown-ether fragments is mi-
micked by four methoxy groups in 1a, but the formation of
host–guest complexes is impossible. According to 1H NMR
data, stilbenes 1a, b were formed as E-isomers.
The diammonium p-acceptor salts 2b, c were synthesized by
quaternization of (E)-di(4-pyridyl)ethylene with o-bromoalk-
ylamine salts in acetonitrile followed by replacement of bro-
mide ions by perchlorate ions (Scheme 2, yields are 72% for 2b
and 29% for 2c).
Similar procedures were used to synthesize the analogues of
2b with a reduced p-acceptor strength, i.e., 3b with a central
dimethylene chain, and bis-quaternary salt 4b in which the
pyridine residues are linked by an ethylene bridge through the
b-positions. The key reactant for the synthesis of 4b, (E)-di
(3-pyridyl)ethylene [(E)-7], was unavailable, and the methods
for its preparation described in the literature were either
B. X-Ray diffraction studies
The molecular structures of 1a and 1b were determined by X-
ray diffraction analysis. Two views of the molecules are shown
in Fig. 1 and selected bond lengths and angles are listed in the
ESI.w
Both molecules are situated at symmetry centres, their
E-stilbene fragments being strictly planar. The conjugation of
p-electrons over both the benzene rings and the double bond
between them is decreased as indicated by the C(1)–C(2) and
C(1)–C(1A) bond lengths (1.467(2) and 1.338(3), 1.475(2) and
1.328(4) A for 1a and 1b, respectively) corresponding to a
localized system of C C–C bonds.
Q
The benzene rings in the stilbene systems of 1a, b exhibit
some bond length redistribution, apparently caused by electro-
nic effects of the alkoxy moieties. The C(5)–C(6) bonds
(1.414(2), 1.417(2) A) are elongated compared to most of the
other bonds in the benzene rings. Two angles O(2)–C(6)–C(5)
and O(1)–C(5)–C(6), exocyclic with respect to the benzene
rings, are reduced (114.9(1)—115.9(1)1), whereas two other
angles, O(2)–C(6)–C(7) and O(1)–C(5)–C(4), endocyclic with
respect to the benzene rings, are increased (124.9(1)–125.2(2)1).
Scheme 3 Synthesis of acceptor 4b.
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Scheme 5 Synthesis of model acceptor 8.
may be attributed to a ‘‘staircase’’ packing motif where ad-
jacent ‘‘staircases’’ are arranged in parallel:
Interestingly the methoxy groups of adjacent molecules tend
to form extended hydrophilic areas in the crystal. It is likely
that this tendency precludes the formation of a stacking
packing motif, which is more typical of conjugated organic
compounds than the ‘‘staircase’’ one.
The crystal packing of 1b is shown in Fig. 3. The molecules
are also arranged in the ‘‘staircases’’. The axes of adjacent
‘‘staircases’’ run in parallel along the b-axis. However, the
staircases are arranged in the herringbone-type packing:
Scheme 4 Model acceptors 2a, 3a, and 4a.
The O(1)ꢀ ꢀ ꢀO(2) distances (2.591(3), 2.591(1) A) are shortened
compared to twice the van der Waals radius (B2.8 A) of
oxygen. Interestingly these distances could be elongated if the
exocyclic angles were equal to the standard value (1201). These
geometric peculiarities are apparently due to both the steric
repulsion of the O(1) and O(2) atoms and the conjugation of
the lone electron pairs (LEPs) of these atoms with the p-system
of the benzene rings. Both oxygen atoms, O(1) and O(2), have a
geometry favourable for this conjugation. Indeed, the bond
angles at the O(1) and O(2) atoms in 1a, b (116.7(1)–117.1(1)1)
imply sp2 hybridization of these atoms, unlike the bond angles
at O(3)–O(6) in 1b (112.1(1)–113.0(2)1), which are typical of the
sp3 hybridization state of these oxygen atoms. The CAlk–O(2)–
C(6)–C(5) and C(8)–O(1)–C(5)–C(6) torsion angles (ꢁ167.5
and 169.21, 165.7 and ꢁ175.51 for 1a and 1b, respectively)
are close to the values most suitable for the conjugation of the
LEP in the p orbital of each of O(1) and O(2) atoms with the
benzene ring p-systems. It should be mentioned that similar
geometric features have been observed previously in benzo-18-
crown-6 butadienyl10 and in dithia-18-crown-6 styryl11 dyes.
In 1b, the oxygen atoms of the 18-crown-6 subunit form a
hexagon in which the shorter diagonals vary within 2.59–2.96
A. The least-squares mean plane through the oxygen atoms is
inclined with respect to the benzene ring plane by 39.71
(see Fig. 1).
The herringbone-type packing is a crystal packing typical of
large conjugated or aromatic molecules.12 In the crystal, the
crown ether fragments form loosely packed double hydrophilic
layers (Fig. 3). The formation of loosely packed areas by crown
ether subunits is characteristic of the crystal structures of all
the crown ether dyes we have investigated previously.
Recently,9 the X-ray structures of 2a–4a have also been
determined. The structures of the dicationic moieties in these
model acceptors are shown in Fig. 4. The dication central
fragment has a nearly planar structure in 2a and 4a but it is
non-planar in 3a. The ethylene bond lengths are equal to
1.345(4) A in 2a and 1.333(7), 1.328(4) A in 4a, whereas both
neighbouring formally single bonds are equal to 1.465(5),
1.457(5) A in 2a and 1.467(4)–1.471(5) A in 4a. These bond
length distributions suggest stronger p-conjugation between
Two layers of crystal packing of 1a are shown in Fig. 2. No
stacking packing motif of stilbene molecules from the adjacent
layers is observed here. The mutual arrangement of the layers
Fig. 1 Structure of molecules 1a and 1b in the frontal and side projections showing 50% probability of thermal anisotropic atom displacement
parameters.
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Fig. 2 Two layers of the crystal packing of 1a.
the pyridine rings in 2a as compared with that in 4a. The ethyl
substituents in 2a–4a point in opposite directions relative to the
mean plane of the unsaturated systems. This denotes that
acceptors 2–4 are pre-organized to form 2 : 1 D–A complexes.9
C. Absorption and fluorescence studies
The complexation of bis(crown) stilbene 1b with viologen
analogues 2a, b and various diammonium salts of the alkane
series, such as dodecanediammonium diperchlorate (9), in
acetonitrile has been recently studied by steady-state electronic
spectroscopy.5 Stilbene 1b was reported to form highly stable
1 : 1 complexes with diammonium compounds 2b and 9 via
two-centre host–guest bonding (Scheme 6). The 1 : 1 complex
[1b ꢀ 2b] shows an increased thermodynamic stability (Table 1),
which is attributed to additional through-space interactions
leading to charge transfer from p-electron donor 1b to
p-electron acceptor 2b. The charge-transfer (CT) interactions
in [1b ꢀ 2b] are evidenced by the presence of a broad CT band
(lmax ¼ 502 nm) in the absorption spectrum and total fluores-
cence quenching.
It was also found that stilbene 1b can react with the complex
[1b ꢀ 2b] to produce the termolecular CT complex [(1b)2 ꢀ 2b]
(Scheme 6). The latter is supposed to have a sandwich-type
layered structure in which the acceptor 2b is located between
two complexed molecules of the biscrown stilbene.
Hereinafter the thermodynamic stability of bi- and termole-
cular D–A complexes is discussed in terms of the correspond-
ing formation constants K1 and K2 associated with the
following equilibria
Fig. 4 Structures of the cations in 2a–4a (two independent molecules
for 4a) showing 50% probability of thermal anisotropic atom displace-
ment parameters.
and
K2
ꢁ!
DA þ D
DA þ A
D2A
ð2Þ
ð3Þ
ꢁ
or
K2
ꢁ!
DA2;
ꢁ
where K1 ¼ [DA]/([D][A]) is the formation constant for a
bimolecular D–A complex and K2 is the formation constant
for a termolecular D–A complex (for 2 : 1 D–A complexes,
K2 ¼ [D2A]/([DA][D]), and for 1 : 2 D–A complexes, K2
¼
[DA2]/([DA][A]).
The log K2 value for [(1b)2 ꢀ 2b] was estimated to be 3.20,5b
which corresponds to a Gibbs free energy (ꢁDG) of about 4.3
kcal molꢁ1. This is much higher than the Gibbs free energy
expected for purely CT interactions, ꢁDG o 1.5 kcal molꢁ1
K1
ꢁ!
D þ A
DA
ð1Þ
ꢁ
,
which was estimated from the log K1 value for the 1 : 1
complexation between 1b and the reference acceptor 2a. Since
the numbers of intermolecular N–Hꢀ ꢀ ꢀO bonds in complexes
[1b ꢀ 2b] and [(1b)2 ꢀ 2b] are the same, the question arises of what
the driving force of the reaction giving [(1b)2 ꢀ 2b] is. The
following hypothesis has been proposed.5b Effective two-centre
host–guest bonding between a ditopic receptor and a diammo-
nium dication requires geometric correspondence of these
molecules. In the case of 1b and 2b, this correspondence
appears far from being perfect and, therefore, complex [1b ꢀ
2b] is substantially sterically strained. The strain energy can be
released in the reaction giving complex [(1b)2 ꢀ 2b] (eqn. (2)),
thus making a considerable contribution to the reaction DG
value.
In order to test this hypothesis, we performed a comparative
study of the complexation of 1b with acceptors 2c, 3b, and 4b,
which differ from 2b in geometric features or in p-acceptor
Fig. 3 Crystal packing of molecules 1b.
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Scheme 6 Complex formation between donor 1b and diammonium acceptors 2–4.
strength. Figs. 5 and 6 show the CT absorption spectra of the
1 : 1 and 2 : 1 complexes of 1b with 2b–4b and 2c in acetonitrile
measured over the 400–700 nm range. The main characteristics
of the CT absorption bands are given in Table 1. Note that the
uncomplexed compounds do not reveal any absorption within
the given spectral range.
teristics of model acceptors 2a and 4a in the X-ray diffraction
section). With [1b ꢀ 3b], the hypsochromic shift of the lowest-
energy CT band is even greater because of the total disruption
of the p-conjugation between the pyridine rings in 3b.
The termolecular complexes [(1b)2 ꢀ 2b], [(1b)2 ꢀ 2c], [(1b)2 ꢀ
3b], and [(1b)2 ꢀ 4b] exhibit more intense CT absorption in
comparison with the corresponding bimolecular complexes.
The clear-cut CT bands of [(1b)2 ꢀ 2b] and [(1b)2 ꢀ 2c] are shifted
bathochromically relative to the corresponding bands of [1b ꢀ
2b] and [1b ꢀ 2c]. We suppose that the main factors responsible
for this shift are as follows. First, the supramolecular D–A–D
complexes in comparison with the related D–A dyads likely
have less sterically strained structures permitting shorter donor–
acceptor separation distances. Second, the termolecular com-
plexes in comparison with the related D–A dyads may have a
slightly reduced oxidation potential of the donor because one
of the two crown ether fragments in each donor component of
the D–A–D triad is free of ammonium ion (Scheme 6).
According to Mulliken’s theory, the formation of a D–A
complex involves the interaction between the HOMO of the
donor and the LUMO of the acceptor. The p-acceptors 2b and
2c have closely similar structures suggesting nearly equal
LUMO energies. However, the lowest-energy CT absorption
band of [1b ꢀ 2c] is shifted bathochromically relative to that of
[1b ꢀ 2b], which indicates stronger CT interaction between the
donor and the acceptor in [1b ꢀ 2c]. This effect presumably
arises from the shorter donor–acceptor separation distance in
[1b ꢀ 2c]. In contrast, the lowest-energy CT absorption band of
[1b ꢀ 4b] is strongly shifted to the blue region and reveals itself
as a long-wavelength shoulder of the higher-energy CT band.
This shift is attributable to the fact that acceptor 4b has a
higher LUMO energy owing to relatively weak p-conjugation
between the b-substituted pyridine rings (see also the charac-
The formation constants for the bi- and termolecular com-
plexes of 1b with 2a–c, 3b, and 4b are collected in Table 1. The
acceptors with violated p-conjugation between the pyridine
Fig. 5 CT absorption spectra of [1b ꢀ 2b] (1), [1b ꢀ 2c] (2), [1b ꢀ 3b] (3),
and [1b ꢀ 4b] (4) in MeCN.
Fig. 6 CT absorption spectra of [(1b)2 ꢀ 2b] (1), [(1b)2 ꢀ 2c] (2), [(1b)2 ꢀ
3b] (3), and [(1b)2 ꢀ 4b] (4) in MeCN.
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Table 1 Formation constants and UV/Vis absorption data for com-
plexes of stilbenes 1a, b with the p-acceptors 2a–c, 3a, b, 4a, b, and the
model compounds 8 and 9
Log K1 (log K2)a
From
UV/Vis
titrationc
From
1H NMR
titrationd
e
max/Mꢁ1
Complex
l
max/nmb
cmꢁ1
Free 1b
[1b ꢀ 9]
336
37 500
38 700
8.59e
333.5
o400
o400
502
[1b ꢀ 8]
4.64
(2.91)
[1b ꢀ (8)2]
[1b ꢀ 2b]
[1b ꢀ 2c]
[1b ꢀ 3b]
[1b ꢀ 4b]
[(1b)2 ꢀ 2b]
[(1b)2 ꢀ 2c]
[(1b)2 ꢀ 3b]
[(1b)2 ꢀ 4b]
[1b ꢀ 2a]
[1b ꢀ 3a]
[1b ꢀ 4a]
[1a ꢀ 2a]
[1a ꢀ 2b]
a
9.08e
9.42
390
340
535
8.66
o400
405 f
519
8.67
430 f
1020
900
(3.20)e
(2.73)
(2.54)
(2.18)
1.13e
(3.27)
(2.78)
(2.56)
(2.02)
1.14
555
o400
416 f
527
790 f
570
o0.5g
0.99
o400
B0.5g
B0.5g
K1 ¼ [DA]/([D][A]) is the formation constant for a 1 : 1 D–A
complex; K2 is the formation constant for a termolecular complex
(for 2 : 1 D–A complexes, K2 ¼ [D2A]/([DA][D]), and for 1 : 2 D–A
complexes, K2 ¼ [DA2]/([DA][A]); the formation constants are mea-
b
sured to within about ꢃ20%. Peak position of the lowest-energy
Scheme 7 Reactions of CT complexes with metal salts.
c
d
absorption band. Anhydrous MeCN, 22 ꢃ 2 1C. MeCN-d3, 25 ꢃ
e
f
1 1C. From ref. 5b. Estimated using LogNormal deconvolution.
g
enhancement. The quantitative analysis of the dependences
shown in Fig. 7 is complicated by the fact that 1b can form
metal complexes of different compositions (1 : 1 and 1 : 2),
which can show different fluorescent properties. However, the
patterns of these dependences imply that the ability of 1b to
bind Ba21 and Ca21 ions is much higher than its ability to bind
tetracation 2b. This may be a reason for low selectivity of
the fluorescence response of this system to Ba21 compared
Estimated by postulating the values of DdH ¼ dH(complex) – dH
(single component).
rings, i.e. 3b and 4b, form less stable bi- and termolecular
complexes with stilbene 1b than compounds 2b, c. The main
reason for this is, apparently, weaker CT interactions in
complexes involving 3b and 4b. The complex [1b ꢀ 2c] shows a
higher stability with respect to complex [1b ꢀ 2b]; however, the
log K2 value with 2c is almost three times as low as that with 2b.
Both these findings can be explained by the fact that complex
[1b ꢀ 2c] is formed with lower steric strain than [1b ꢀ 2b] owing to
more exact geometric matching between 2c and 1b for the two-
centre host–guest bonding. According to the above hypothesis,
a reduction of the steric strain in the bimolecular complex
should not only increase the thermodynamic stability of this
complex, but also decrease the log K2 value.
to Ca21
.
D. 1H NMR spectroscopy studies
According to X-ray diffraction (Fig. 1), tetramethoxystilbene
(E)-1a and bis(18-crown-6)stilbene (E)-1b exist in the crystal-
line state as symmetric syn,syn- and anti,anti-conformers,
respectively, which is probably due to the closest packing of
molecules requirements. In CD3CN solutions, the anti,anti-,
Previously, we reported that the supramolecular complex
[1b ꢀ 2b] can act as a fluorescent sensor for metal ions.5a Taking
the [1b ꢀ 2b] þ Ba(ClO4)2 system as an example, we have shown
that metal ions can displace the diammonium salt in a non-
fluorescent CT complex, which gives rise to substantial fluor-
escence from metal complexed bis(crown) stilbenez (Scheme 7).
In order to evaluate the selectivity of the fluorescence
response, we carried out a comparative study of the emission
behaviour of acetonitrile solutions of [1b ꢀ 2b] in the presence of
Mg(ClO4)2, Ca(ClO4)2, and Ba(ClO4)2. The results are pre-
sented in Fig. 7. A dilute solution of [1b ꢀ 2b] (2 ꢂ 10ꢁ6 M)
shows a very weak fluorescence, which is due to the fact that a
small fraction of molecules 1b and 2b occur in the free state.
The optical properties of this solution barely change upon the
addition of Mg(ClO4)2 up to a five-fold excess with respect to
the D–A complex. However, the addition of small amounts of
Ca(ClO4)2, or Ba(ClO4)2 induces a substantial fluorescence
ꢁ6
.
z The fluorescence quantum yield of free 1b in acetonitrile is about
0.3.5a Upon the complexation of 1b with Ba(ClO4)2, the fluorescence
quantum yield increases by B10%.
Fig. 7 Fluorescence from an acetonitrile solution of [1b 2b] (2 ꢂ 10
M) as a function of the relative concentration of metal perchlorate
added. The observation wavelength is 375 nm.
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proton signals for the a0-CH2O groups are shifted downfield
relative to those for the a-CH2O groups. Apparently, the
protons of a0-CH2O experience the deshielding influence of
the ethylene bond, like the H-2 aromatic protons.
Fig. 8 shows the changes in the proton chemical shifts
induced by complexation, DdH ¼ dH(complex) ꢁ dH(single
component), for stilbene 1b and the diammonium salts 9, 2b–
4b, and 2c in CD3CN. The DdH values were measured for
equimolar D–A mixtures with a concentration of about 1 ꢂ
10ꢁ3 M. The NMR titration showed that the reactants exist
almost entirely as 1 : 1 complexes under these conditions. For
comparison, Fig. 8 shows the changes in the proton chemical
shifts that accompany the complexation of 1b with two ethy-
lammonium ions. Complex [1b ꢀ (EtNH3)2]21 was prepared by
addition of a large excess (B2 ꢂ 10ꢁ2 M) of ethylammonium
perchlorate to a CD3CN solution of 1b (2 ꢂ 10ꢁ3 M).
The interaction of the crown ether fragments of 1b with the
EtNH13 ions in [1b ꢀ (EtNH3)2]21 through N–Hꢀ ꢀ ꢀO hydrogen
bonds brings about downfield shifts of all the proton signals of
bis(crown) stilbene (DdH ¼ 0.07–0.13 ppm). One could expect
that the complexation would shift the conformational equili-
brium for 1b to the more extended anti,anti-conformer in which
the Coulomb interaction between the complexed ammonium
ions is lower. However, this apparently does not take place,
because the distance between the H-2 and H-6 proton signals
(0.12 ppm) barely changes upon complexation.
Comparison of the DdH values for complexes [1b ꢀ
(EtNH3)2]21 and [1b ꢀ 9] shows that the polymethylene bridge
that links the two ammonium ions in complex [1b ꢀ 9] does not
influence significantly the positions of the proton signals for
complexed bis(crown) stilbene. The signals of all the methylene
protons of 9 shift upfield upon complexation. For the protons
located near the NH13 groups, this effect is not very pro-
nounced (DdH ¼ ꢁ0.18 ppm) and can be rather attributed to
a decrease in the charge on the ammonium group due to its
delocalization over the crown ether oxygen atoms. The sub-
stantial shifts of the signals of other methylene protons of 9
(DdH up to ꢁ0.58 ppm) are due to the shielding effect from the
system of conjugated p-bonds of 1b above which the methylene
bridge is located.
Scheme 8 Conformers of (E)-1b.
anti,syn-, and syn,syn-conformers of 1a, b may coexist (Scheme
8), by analogy with the crown-containing styryl and butadienyl
dyes.10,13 Unfortunately, the NOE spectra do not provide the
possibility of estimating the equilibrium content of both sym-
metric conformers of 1a, b due to substantial interaction of the
ethylene bond protons with both the H-2 and H-6 protons (see
atom numbering in Scheme 8). However, in view of the fact
that the signals for H-2 in the 1H NMR spectra are shifted
downfield relative to the H-6 signals, one can suggest that the
more compact syn,syn-form, in which H-2 falls in the deshield-
ing region of the ethylene bond, predominates (see also the 13
C
NMR section). The chemical shifts for the crown ether alipha-
tic protons of 1b are ordered as follows: the longer the distance
between the observed proton and the benzene ring, the smaller
the proton chemical shift. According to the NOE spectrum, the
Fig. 8 Changes in the proton chemical shifts, DdH ¼ dH(complex) ꢁ dH(single component), upon complexation of 1b with EtNH13 ClO4ꢁ, 9, 2b–4b,
and 2c in CD3CN at 25 1C.
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relatively strong two-centre host–guest interactions and much
weaker through-space interactions between the donor and
acceptor p-systems, resulting in charge transfer. In order to
estimate the possible contribution of the through-space inter-
actions to the Gibbs free energy of complex formation, we
studied several model systems, 1a/2a, b and 1b/2a–4a, by
1H NMR titration.
The formation of weak 1 : 1 D–A complexes was observed
for all model systems. The log K1 values for [1b ꢀ 2a] and [1b ꢀ
4a] and the changes in the proton chemical shifts upon
complete complexation, DdH, for 2a and 4a were calculated
from the titration curves using the HYPNMR program.14 For
other model systems, the degree of complex formation was too
low for exact extrapolation of the titration curves; therefore,
the log K1 values were estimated by postulating the value
DdH ¼ ꢁ0.60 ppm for the protons of the central group of the
p-acceptor. The log K1 values are listed in Table 1, and the
DdH values for 2a–4a are shown in Fig. 10.
For acceptors 2a–4a, the DdH values tend to decrease as the
distance from the centre of the molecule increases, whereas for
the corresponding diammonium compounds 2b–4b, this ten-
dency is not observed (Fig. 8). This indicates that the relative
orientation of the donor and the acceptor in model D–A
systems shows a more clear-cut X-pattern than that in supra-
molecular D–A complexes based on diammonium compounds.
This comes as no surprise, because the geometry of the
supramolecular D–A complexes is mainly determined by the
two-centre host–guest interaction, which accounts for a rela-
tive orientation of the donor and acceptor moieties other than
that favourable for the through-space interaction.
Fig. 9 Intermolecular NOE interactions detected for the supramole-
cular D–A complexes.
As expected, the coordination of the terminal ammonium
groups of the p-acceptor to the crown ether fragments of the
p-donor in supramolecular D–A complexes [1b ꢀ 2b], [1b ꢀ 2c],
[1b ꢀ 3b], and [1b ꢀ 4b], induces a downfield shift (up to 0.23 ppm)
of the signals for most of the CH2O groups. In contrast to the
complex [1b ꢀ 9], the D–A dyads show negative DdH values for
some protons of the stilbene fragment. Pronounced upfield
shifts are also observed for the signals of the central fragment
of the p-acceptors. These effects are due to the mutual shielding
of the systems of conjugated p-bonds in the donor and
acceptor components of the complex.
Fig. 9 shows the NOE interactions detected for [1b ꢀ 2b], [1b ꢀ
2c], [1b ꢀ 3b], and [1b ꢀ 4b]. All intermolecular NOE cross-peaks
had low intensities compared to the intramolecular peaks. This
is indicative of relatively long distances (3.5–5 A) between the
interacting protons of the donor and the acceptor in the D–A
complexes.
The fact that the intermolecular (through-space) interactions
between the donor and the acceptor in the supramolecular
D–A complexes are clearly seen in the 1H NMR spectra can be
used to determine the mutual spatial arrangement of the
complex components. The D–A complexes involving acceptors
with identical topologies, i.e. [1b ꢀ 2b], [1b ꢀ 2c] and [1b ꢀ 3b],
show similar distributions of the DdH values (Fig. 8) and
similar intermolecular NOE interactions (Fig. 9), suggesting
a similar relative orientation of the donor and acceptor com-
ponents. In these dyads, just a few protons of 1b (H-2, and the
protons of the ethylene bond and the a0-CH2O groups) experi-
ence significant shielding effects of the unsaturated fragments
of the acceptor. This implies that the one-plane projections of
the p-donor and p-acceptor moieties in these complexes over-
lap preferably according to the X-pattern rather than lie
strictly above each other. With this geometry, the H-2 and
H-6 protons of the pyridine rings (numbering in Scheme 2)
should fall to different extents into the shielding regions of 1b;
however, these protons show the common narrow signal,
which is attributable to exchange processes that are fast on
the 1H NMR time scale, in the D–A dyads. The same exchange
processes also affects the H-3 and H-5 protons of the pyridine
rings.
.
The tetramethoxystilbene complexes, [1a 2a] and [1a ꢀ 2b],
have lower log K1 values than [1b ꢀ 2a], despite the fact that the
oxidation potentials of stilbenes 1a and 1b are equal.15 Pre-
sumably, in addition to the weak CT interactions between the
p-systems of 1b and 2a, weak interactions of the electrostatic
nature between the crown ether oxygen atoms of 1b and the
positively charged ethylpyridinium fragments of 2a occur in the
[1b ꢀ 2a] complex.
Acceptor 4b differs from 2b, c and 3b in topology. The
presence of 1,3-disubstituted pyridine rings implies that 4b can
adopt different s-conformations by analogy with 1b. Complex
[1b ꢀ 4b] shows a specific distribution of DdH values, in parti-
cular, all aromatic protons of 1b in this complex are character-
ized by negative DdH values becoming closer together. The
number of intermolecular NOE interactions found for [1b ꢀ 4b]
is the greatest among the D–A complexes studied. These
observations seem to imply a greater overlap of the one-plane
projections of the p-donor and p-acceptor moieties in complex
[1b ꢀ 4b].
As noted in the previous Section, the interactions determin-
ing the thermodynamic stability of supramolecular D–A com-
plexes can be conventionally classified into two types, namely,
Fig. 10 Changes in the proton chemical shifts, DdH ¼ dH(complexed
form) ꢁ dH(free form), upon 1 : 1 complexation of 2a–4a with 1b.
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Fig. 12 Values of DdH ¼ dH(D/A mixture) ꢁ dH(single component)
for some protons of 2b (dashed curves) and 1b (solid curves) in CD3CN
as a function of the donor/acceptor concentration ratio.
According to UV/Vis spectroscopy data, bis(crown) stilbene
1b and diammonium p-acceptor salts 2c and 2b–4b can form
not only highly stable D–A complexes but also D–A–D triads
(Scheme 6). Additional evidence for the formation of the
1
termolecular complexes was obtained by H NMR titration.
Fig. 11 Changes in the proton chemical shifts, DdH ¼ dH(complex) ꢁ
dH(single component), upon 1 : 1 and 1 : 2 complexation between 1b
and 8.
Fig. 12 shows the values DdH ¼ dH(D/A mixture) ꢁ dH(sin-
gle component) for some protons of donor 1b and acceptor 2b
as a function of the donor/acceptor concentration ratio
(CD/CA). When CD o CA, the 1H NMR spectra of the
D/A mixtures consisted of two sets of signals, one due to the
free form of 2b and the other, to complex [1b ꢀ 2b]. The proton
signals for the free and complexed forms of the acceptor were
substantially broadened due to exchange between these forms,
slow on the 1H NMR time scale. The low rate of the exchange
processes accounts for the fact that the DdH values that refer to
the protons of the complexed donor and acceptor in Fig. 12 are
virtually independent of CD/CA when CD o CA.
We studied yet another model system incorporating bis
(crown) stilbene 1b and the ammonium derivative of 4-methyl-
pyridine 8. The 1H NMR titration data for this system
complied well with the reaction model that includes two
equilibria (eqn. (1) and eqn. (3)) and implies the formation of
complexes [1b ꢀ 8] and [1b ꢀ (8)2]. The log K1 and log K2 values
for these complexes are given in Table 1.
The doubled log K1 value for the one-centre complex [1b ꢀ 8]
(2 log K1 ¼ 9.28) is substantially greater than the value log
K1 ¼ 8.66 for the related two-centre complex [1b ꢀ 3b]. This fact
is in line with the hypothesis about substantial steric strain in
supramolecular D–A complexes of type [1b ꢀ 3b] or, at least,
does not contradict this hypothesis. The equilibrium constant
K2 for the association of 8 and [1b ꢀ 8] (eqn. (3)) is substantially
lower than the formation constant K1 for complex [1b ꢀ 8] due
to the Coulomb repulsion between two cations 8.
The growth of the CD/CA ratio in the CD 4 CA region
entails a monotonic change in the DdH value for the acceptor
protons. The titration curves for the donor protons follow a
more complex pattern. These observations are in good agree-
ment with the reaction model discussed above (eqns. (1) and
(2)), which implies the formation of a termolecular complex
[(1b)2 ꢀ 2b]. When CD 4 CA, the concentration of the free
acceptor is negligibly low, the monotonic variation of DdH
for the protons of 2b being due to the shift of the complexation
equilibrium from [1b ꢀ 2b] to [(1b)2 ꢀ 2b]. The more complex
pattern of the variation of DdH with CD/CA for the protons
of 1b is due to the fact that the donor can exist as three forms,
in particular, as the free molecule and as either of two com-
plexes, [1b ꢀ 2b] and [(1b)2 ꢀ 2b], with different ratios of the
syn,syn- to the anti,anti-conformers for each form. The lack
of separate sets of signals for different forms of the donor or
the acceptor at CD 4 CA is due to fast exchange processes
caused by relatively low stability of [(1b)2 ꢀ 2b].
Fig. 11 shows the values DdH ¼ dH(complex) ꢁ dH(single
component) for [1b ꢀ 8] and [1b ꢀ (8)2] calculated from the titra-
tion curves. These complexes, like complex [1b ꢀ (EtNH3)2]21
,
are characterized by downfield shifts of all proton signals of
bis(crown) stilbene. All the proton signals of the acceptor in
complexes [1b ꢀ 8] and [1b ꢀ (8)2] shift upfield, the DdH values for
8 being virtually independent of the composition of the com-
plex and being close to the DdH values for like protons in
complexes [1b ꢀ 2b] and [1b ꢀ 3b]. These facts indicate that in
complexes [1b ꢀ 8] and [1b ꢀ (8)2], the acceptor protons fall in the
shielding region of the stilbene p-system, but not vice versa.
This situation arises when the pyridine ring of 8 is situated
above the stilbene fragment of 1b and their planes are perpen-
dicular to each other. Apparently, among the additional
through-space interactions, contacts of the CHꢀ ꢀ ꢀp-system
type predominate in complexes of 8, resulting in the cross
arrangement of the unsaturated fragments of the donor and the
acceptor. The foregoing suggests that in supramolecular com-
plexes with two-centre binding, the diammonium compounds
2b, c, 3b, and 4b can have a twisted geometry of the central
fragment, resulting in the accumulation of steric strain, men-
tioned in the previous Section.
The dependences of DdH on CD/CA for the protons of 2b in
the CD 4 CA region were in good agreement with the
simplified reaction model that includes only one equilibrium
(eqn. (2)), which made it possible to determine the equilibrium
constant K2 and the chemical shifts for the acceptor protons in
complex [(1b)2 ꢀ 2b]. The simplified model is applicable to
this system due to the large formation constant K1 for complex
[1b ꢀ 2b].
For the systems 1b/2c, 1b/3b, and 1b/4b, the results of
titration were qualitatively similar to the results obtained for
1b/2b. The calculated values DdH ¼ dH(2 : 1 D–A com-
plex) ꢁ dH(single component) for the protons of acceptors 2b–4b
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ponents (>90%). Some supramolecular complexes were iso-
lated in the solid state (Experimental section). Compounds 1b
and 2b as well as complex [(1b)2 ꢀ 2b] were studied by solid-state
13C NMR. The solution and solid-state 13C NMR data are
presented in Table 2S (ESI).w
The solid-state 13C NMR spectrum of 2b exhibits a set of
relatively narrow signals, whose number exceeds the number of
lines in the solution 13C NMR spectrum. In solution, the
symmetrically arranged carbon atoms of the pyridine rings,
C-2 and C-6, and C-3 and C-5, are responsible for averaged
signals, one for each pair, owing to fast exchange processes. In
the solid state, these signals split. The magnitude of this
splitting is greater for the C-3 and C-5 pair, located most
closely to the C C group (5.8 ppm vs. 3.0 ppm for the C-2 and
Q
C-6 pair). A similar splitting was reported16 to take place in the
solid-state 13C NMR spectra of E-azobenzene derivatives. For
these compounds, the pair of aromatic carbon atoms closest to
the N N bond is characterized by a greater interval between
Q
the signals (12.8–16.9 ppm), the higher-field signal being
assigned to the carbon atom toward which the N N bond
Q
points. Similarly, the higher-field signal for the C-3 and C-5
pair in 2b is assigned to the C-5 atom toward which the C
bond points (see Scheme 2).
C
Q
In the solid-state 13C NMR spectrum of 1b, the number of
rather narrow signals in the aromatic region corresponds to the
number of sp2 carbon atoms in this compound and, as follows
from X-ray diffraction data, these signals refer to the anti,anti-
conformer. The signals of C-2, C-5, and C-6 fall within the
narrow interval 113.4–114.8 ppm, which complicates their
assignment. In the solution 13C NMR spectrum, the signals
of C-6 and C-2 are found at 120.0 and 109.7 ppm, respectively,
i.e., change to solution induces a significant downfield shift of
the C-6 signal ( Z 5.2 ppm) and a significant upfield shift of the
C-2 signal ( Z 3.7 ppm); the dC values for other sp2 atoms
change by 0.7–2.5 ppm. This behaviour of the C-6 and C-2
signals confirms the existence in solution of a conformer in
which the C C bond points toward the C-2 atom, i.e., the
Q
syn,syn-conformer of 1b.
Fig. 13 Changes in the proton chemical shifts, DdH ¼ dH(2 : 1 D–A
complex) ꢁ dH(single component), for acceptors 2b–4b and 2c upon
2 : 1 D–A complexation with donor 1b.
In solution, binding of 1b with 2b as a 1 : 1 complex induces
slight changes in the carbon chemical shifts for both com-
pounds, which is due to the low sensitivity of the 13C nuclei to
anisotropic effects. It is typical, however, that the signals of the
sp2 carbon atoms of 2b all shift upfield upon complexation (by
0.30–0.81 ppm). Presumably, these one-directional shift
changes arise from a slight alteration of the charge on the
p-acceptor moiety of 2b owing to through-space interactions
between this moiety and the stilbene fragment of 1b. Most of
the signals for the stilbene sp2 carbon atoms shift downfield
upon complexation (by 0.22–1.10 ppm). Exceptions are pro-
vided by the signals of the C-3 and C-4 atoms, which shift
upfield by 1.67 and 1.88 ppm, respectively. This irregularity is
likely related to the fact that the electronic structure of the
stilbene fragment in [1b ꢀ 2b] is affected not only by through-
space CT interactions but also by two-centre host–guest inter-
action involving the crown ether oxygen atoms attached to the
C-3 and C-4 aromatic atoms of 1b. The latter interaction may
decrease the O–CAr bond order due to switching of the
p-electrons of the O atoms from conjugation with the chro-
mophore system of stilbene to the formation of hydrogen
bonds with the complexed NH13 groups. The signals of the
crown ether aliphatic carbon atoms do not show any particular
trend upon complexation, indicating a conformational depen-
dence of the dC values.
and 2c are shown in Fig. 13. The constants K2 as determined by
1H NMR titration are listed in Table 1.
The results of the NMR study of termolecular D–A com-
plexes are in good agreement with the results of spectrophoto-
metric measurements. For all four systems, 1b/2b–4b and
1b/2c, 2(D) : 1(A) complexation induces strong upfield shifts
of the proton signals for the central fragment of the p-acceptor,
the values of these shifts, DdH, being much greater than the
corresponding DdH values measured for the related D–A
dyads. This observation corroborates the assumptions that ter-
molecular complexes have a sandwich-type layered D–A–D
structure and that the donor–acceptor separation distances in
these complexes are shorter than those in the related D–A
dyads. The shorter distances result from lower steric restric-
tions for the components of termolecular complexes.
E. 13C NMR spectroscopy studies
Compounds 1b and 2b as well as their mixtures were studied
using 13C NMR in CD3CN solutions containing 5% (v/v) of
deuterated water. Water was added to increase the solubility of
2b and its complexes with 1b. Qualitative 1H NMR experi-
ments have shown that a slight amount of water does not
markedly destroy complexes [1b ꢀ 2b] and [(1b)2 ꢀ 2b]. These
observations, together with high working concentrations of
compounds (ca. 0.02 M) allowed us to infer that in solutions
containing 1 : 1 and 2 : 1 mixtures of 1b and 2b, the bimolecular
and termolecular complexes, respectively, are the major com-
In the solution 13C NMR spectrum of [(1b)2 ꢀ 2b], all signals
for the sp2 carbon atoms of 2b are shifted upfield relative to the
corresponding signals for the uncomplexed form of 2b, the
magnitudes of these shifts being 2–3 times larger than in
the case of [1b ꢀ 2b]. This suggests that the total charge transfer
to the p-acceptor moiety of 2b in [(1b)2 ꢀ 2b] is larger than that
in [1b ꢀ 2b]. The signals of the stilbene sp2 carbon atoms of
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[(1b)2 ꢀ 2b] are shifted rather irregularly relative to the corre-
sponding signals for the uncomplexed form of 1b, viz., four
signals are shifted downfield, whereas the other three lie in the
higher field. It is noteworthy that the signals of the stilbene C-2
atoms for complexes [(1b)2 ꢀ 2b] and [1b ꢀ 2b] are shifted in the
opposite directions relative to the corresponding signal for the
uncomplexed form of 1b. This feature may be related to the
fact that one of the two crown ether fragments in each
bis(crown) stilbene molecule of [(1b)2 ꢀ 2b] is free of ammonium
ion, whereas in complex [1b ꢀ 2b] both crown ether fragments
are complexed.
The solid-state 13C NMR spectrum of the termolecular
complex [(1b)2 ꢀ 2b] shows a set of highly broadened and, hence,
markedly overlapped signals, which hampers their assignment
(see ESIw). The observed pronounced broadening of signals
due to 13C nuclei shows that no single structure with rigidly
fixed arrangement of components exists for [(1b)2 ꢀ 2b] in the
solid state. This implies insignificant differences between the
stabilities of complexes [(1b)2 ꢀ 2b] with different conforma-
tional compositions and different arrangements of the donor
and acceptor components.
internal reference (7.27, 1.96, 2.50, or 4.70 ppm, respectively,
for 1H; 77.00 or 118.10 ppm in CDCl3 or CD3CN, respectively,
for 13C); homonuclear 1H–1H COSY and NOESY spectra and
1
heteronuclear H–13C COSY spectra were used to assign the
proton and carbon signals. The high-resolution solid-state 13
C
magic-angle-spinning NMR spectra were recorded on a Bruker
AMX500 instrument at 125.77 MHz at ambient temperature
using cross-polarization from the protons together with proton
decoupling. The rotation speed of the 5 mm zirconium dioxide
rotors were 7 kHz. In all cases, the 13C chemical shifts were
referenced by replacement against the high-frequency line of
solid adamantane at 38.56 ppm.
IR spectra were recorded on a Bruker IFS-113V spectro-
photometer in Nujol between KBr plates. Elemental analyses
were performed at the microanalytical laboratory of the A. N.
Nesmeyanov Institute of Organoelement Compounds in Mos-
cow, Russia. Silica gel (Kieselgel 60, 0.063–0.100 mm, Merck)
and aluminium oxide (Aluminiumoxid 90, aktiv neutral, 0.063–
0.200 mm, Merck) were used for chromatography. TLC was
carried out on DC-Alufolien Aluminiumoxid 60 F254 neutral
(Typ E) and Kieselgel 60 F254 plates (Merck).
Conclusions
Preparations
Thus, we found that bis(crown) stilbene 1b and p-acceptors 2–4
containing two ammonioalkyl groups are able to form supra-
molecular complexes D–A and D–A–D. The stability of com-
plexes is ensured by two types of interaction: (1) rather strong
host–guest type complexation between the crown-ether frag-
ments of bis(crown) stilbene and the ammonium groups of the
acceptor, and (2) the weak through-space interaction between
the donor and acceptor p-systems, resulting in charge transfer.
It should be emphasized that owing to numerous hydrogen
bonds, the formation of supramolecular charge transfer com-
plexes takes place even in highly dilute solutions (below 10ꢁ6
M), as opposed to traditional complexes of this type; this
allows detailed investigation of charge and electron transfer
processes in bimolecular complexes and in trimolecular CT
complexes, which are not actually observed in the solutions.
The geometry and the p-acceptor ability of the diammonium
compounds have a substantial influence on the thermodynamic
and spectral properties of these supramolecular CT complexes.
The spatial proximity and pre-organization of the donor and
acceptor fragments of the components relative to each other in
these supramolecular systems allow the formation of CT
complexes, in particular, with very weak acceptors. These
complexes cannot be detected even at high component con-
centrations in the case of usual CT complexation in solutions.
Highly stable D–A systems based on 1b and 2–4 can be used as
selective fluorescence sensors for metal ions in non-aqueous
media.
1,12-Dodecanediammonium diperchlorate (9). This com-
pound was prepared by the addition of an excess of 70% aq.
HClO4 to 1,12-dodecanediamine in methanol followed by
quantitative precipitation with diethyl ether; white crystals,
mp >250 1C (decomp.). IR n/cmꢁ1: 3247, 3186, 3147 (NH). 1H
NMR (CD3CN, 25 1C) d: 1.31 (br s, 16 H, 8 CH2), 1.62 (m, 4H,
2 CH2CH2NH3), 2.94 (t, J ¼ 7.8 Hz, 4H, 2 CH2NH3). Anal.
calcd. for C12H30Cl2N2O8: C, 35.92; H, 7.54; N, 6.98; found: C,
35.85; H, 7.65; N, 6.94%.
3-[(1,1,1-Triphenylphosphonio)methyl]pyridinium dibromide
(6). A solution of 3-pyridylmethanol (5.41 g, 0.050 mol) in
64% aq. HBr (53 mL, 0.44 mol) was heated for 5 h at 125 1C
and thoroughly evaporated in vacuo to give 10.44 g (83%) of
3-(bromomethyl)pyridinium bromide (5) as a lacrimatory yellow-
ish solid.17 Triphenylphosphine (11.87 g, 0.045 mol) was added
to a stirred solution of 5 in anhydrous acetonitrile (100 mL).
The resulting solution was refluxed for 7 h and cooled, and the
precipitate formed was filtered off and washed with 1 : 1
mixture of benzene–CH2Cl2 (100 mL) and then with benzene
(60 mL) to give 15.18 g of 6 as a white solid. The mother
solution was evaporated in vacuo, the residue was treated with
ethyl acetate, and the insoluble substance was filtered off and
washed with ethyl acetate to give an additional 2.79 g of 6
(85% overall yield); mp 273–277 1C. 1H NMR (CDCl3, 25 1C)
d: 6.33 (d, J ¼ 15.2 Hz, 2H, CH2P), 7.72 (m, 6H, 3 H-30, 3 H-
50), 7.86 (m, 9H, 3 H-20, 3 H-40, 3 H-60), 7.93 (br m, 1H, H-5),
8.64 (br s, 1H, H-6), 8.72 (s, 1H, H-2), 8.90 (br d, J ¼ 5.9 Hz,
1H, H-4). Anal. calcd. for C24H22Br2NP: C, 55.95; H, 4.30; N,
2.72; found: C, 55.60; H, 4.32; N, 2.61%.
Experimental
General methods
3,4,30,40-Tetramethoxystilbene (1a) and bis(18-crown-6)stil-
bene (1b) were prepared according to a published procedure.7
Compounds 2a–4a were prepared as described previously.5a,9
Nicotinaldehyde, 3-pyridylmethanol, triphenylphosphine, 3-
bromopropylamine hydrobromide, 2-bromoethylamine hydro-
bromide, 1,2-di(4-pyridyl)ethylene, 1,2-di(4-pyridyl)ethane,
1,12-dodecanediamine, 4-methylpyridine, and CD3CN (water
o0.05%) were used as received (Merck or Aldrich).
Mg(ClO4)2, Ca(ClO4)2, and Ba(ClO4)2 (Aldrich) were dried
in vacuo at 240 1C.
3-[(E)-2-(3-Pyridyl)-1-ethenyl]pyridine [(E)-7]. Phosphonium
salt 6 (4.80 g, 9 mmol) was added in portions to a stirred
solution of sodium (0.54 g, 23 mmol) in methanol (40 mL) at
0 1C under argon. The resulting mixture was stirred for 5
minutes, and a solution of nicotinaldehyde (0.96 mL, 10 mmol)
in methanol (12 mL) was added over a period of 20 minutes.
The reaction mixture was stirred for 1.5 h at rt and evaporated
in vacuo. The residue was dissolved in 4% aq. HCl (110 mL)
and the solution (pH 1) was extracted with diethyl ether (4 ꢂ 50
mL), alkalized with aq. KOH up to pH 14, and extracted with
CH2Cl2 (4 ꢂ 50 mL). The CH2Cl2 extracts were evaporated
in vacuo and the residue (1.21 g) was purified by column
chromatography on aluminium oxide using a gradient benzene–
ethyl acetate mixture up to 50% of the latter and then a 9 : 1
The melting points measured with a MEL-Temp II appara-
tus in a capillary are uncorrected.
The solution 1H and 13C NMR spectra were recorded on
Bruker DRX500 and Bruker DRX400 instruments in CDCl3,
CD3CN, DMSO-d6, or D2O solutions using the solvent as an
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benzene–ethanol mixture as the eluents. Two fractions were
collected. The first contained only the Z-isomer (0.47 g) of 3-[2-
(3-pyridyl)-1-ethenyl]pyridine [(Z)-7] as a yellowish oil.8b
2 H-6). Anal. calcld for C18H30Cl4N4O16: C, 30.87; H, 4.32; N,
8.00; found: C, 31.07; H, 4.36; N, 8.04%.
1H NMR (CDCl , 25 1C) d: 6.71 (s, 2H, CH CH), 7.17 (d d,
Q
3
1-(3-Ammoniopropyl)-3-{(E)-2-[1-(3-ammoniopropyl)-3-pyri-
diniumyl]-1-ethenyl}pyridinium tetraperchlorate (4b). Obtained
as a white solid in 60% overall yield; mp 278–281 1C (de-
comp.). IR n/cmꢁ1: 3231, 3187, 3149 (NH). 1H NMR (CD3CN,
25 1C) d: 2.42 (m, 4H, 2 CH2CH2N), 3.14 (t, J ¼ 7.5 Hz, 4H,
2 CH2NH3), 4.66 (t, J ¼ 7.5 Hz, 4H, 2 CH2N), 7.66 (s, 2H,
J ¼ 7.9 Hz, J ¼ 4.8 Hz, 2H, 2H-5), 7.49 (d t, J ¼ 7.9 Hz, J ¼ 1.6
Hz, 2H, 2H-4), 8.46 (d d, J ¼ 4.8 Hz, J ¼ 1.5 Hz, 2H, H-6), 8.48
(d, J ¼ 1.6 Hz, 2H, 2H-2). The second fraction was a ca.
1 : 1 mixture (0.40 g; overall yield is 51%) of E- and Z-isomers
of the title compound.
A solution of the 1 : 1 mixture of E- and Z-isomers of 7 (0.40
g, 2.2 mmol) and some crystals of I2 in nitrobenzene (4 mL)
were refluxed for 1 h, cooled to rt, diluted with benzene (15
mL), and extracted with 10% aq. HCl (3 ꢂ 25 mL). The
aqueous extracts were washed with benzene (4 ꢂ 50 mL),
alkalized by 10% aq. NaOH to pH 14, and extracted with benzene
(4 ꢂ 50 mL). The benzene extracts were evaporated in vacuo
and the residue (0.36 g) was extracted with boiling n-heptane
(4 ꢂ 25 mL). The heptane extracts were evaporated in vacuo
to give 0.31 g (78%) of (E)-7 as a slightly yellowish solid, mp
87–89 1C. Lit. data for (E)-7: colourless solid, mp 90 1C.8b
CH CH), 8.14 (dd, J ¼ 8.0 Hz, J ¼ 6.3 Hz, 2H, 2 H-5), 8.67
Q
(d, J ¼ 6.3 Hz, 2H, 2 H-6), 8.74 (d, J ¼ 8.0 Hz, 2H, 2 H-4), 9.01
(s, 2H, 2 H-2). Anal. calcld for C18H28Cl4N4O16: C, 30.96; H,
4.04; N, 8.02; found: C, 31.14; H, 4.11; N, 8.09%.
1-(3-Ammoniopropyl)-4-methylpyridinium diperchlorate (8).
A mixture of 4-methylpyridine (0.30 mL, 3.1 mmol) and
3-bromopropylamine hydrobromide (1.02 g, 4.6 mmol) in anhy-
drous acetonitrile (20 mL) was refluxed under stirring for 4 h.
The solvent was evaporated and the residue was washed with
anhydrous ethanol (4 ꢂ 10 mL), dissolved in water (20 mL),
and filtered. The mother solution was evaporated in vacuo to
yield 0.50 g of 1-(3-ammoniopropyl)-4-methylpyridinium di-
bromide as a white solid. The dibromide was dissolved in a
mixture of methanol (5 mL) and 70% aq. HClO4 (1.0 mL) and
the target diperchlorate was precipitated by adding diethyl
ether (30 mL). The precipitate formed was filtered off and
washed with a 5 : 1 benzene–ethanol mixture (2 ꢂ 10 mL). The
crystallization was repeated using half the amount of 70% aq.
HClO4 (0.5 mL) to give 8 as a white solid (0.39 g, 36%); mp 150
Synthesis of N,N0-di(ammonioalkyl) derivatives of viologen
analogues 2b, c, 3b, 4b (general procedure)
Method (a). A mixture of (E)-1,2-di(4-pyridyl)ethylene,
(E)-1,2-di(3-pyridyl)ethylene [(E)-7] or 1,2-di(4-pyridyl)ethane
(1 mmol), and 3-bromopropylamine hydrobromide or 2-bro-
moethylamine hydrobromide (6 mmol) in anhydrous acetoni-
trile (20 mL) was refluxed under stirring for 13 h in the case of
2b–4b, or for 140 h in the case of 2c. After cooling to rt, the
precipitate formed was filtered off, washed with anhydrous
ethanol (4 ꢂ 50 mL) and chloroform (2 ꢂ 10 mL), and
dissolved in water (20 mL). The solution was filtered and the
filtrate was evaporated in vacuo to yield the corresponding
tetrabromide of N,N0-di(ammonioalkyl) derivative as a yellow
solid.
1
–152 1C. IR n/cmꢁ1: 3228, 3181, 3137 (NH). H NMR (D2O,
25 1C) d: 2.30 (m, 2H, CH2CH2N), 2.57 (s, 3H, Me), 3.03 (t,
J ¼ 7.8 Hz, 2H, CH2NH3), 4.56 (t, J ¼ 7.6 Hz, 2H, CH2N),
7.82 (d, J ¼ 6.0 Hz, 2H, H-3, H-5), 8.59 (d, J ¼ 6.0 Hz, 2H, H-2,
H-6). Anal. calcd. for C9H16Cl2N2O8: C, 30.78; H, 4.59; N,
7.98; found: C, 30.80; H, 4.64; N, 7.87%.
Method (b). The tetrabromide salt was dissolved in a
mixture of ethanol (5 mL) and the minimum quantity of water
with heating, and 70% aq. HClO4 (0.6 mL) and ethanol (5 mL)
were added to the hot solution. After cooling to 5 1C, the
precipitate formed was filtered off and washed with ethanol
(2 ꢂ 5 mL). The crystallization was repeated using half the
amount of 70% aq. HClO4 (0.3 mL) to give the corresponding
tetraperchlorate of the N,N0-di(ammonioalkyl) derivative of
the viologen analogue.
Preparation of supramolecular complexes (general procedure).
An equimolar mixture of the bis(crown) stilbene 1b and
diammonium compound 2b–4b, or 2c, or a 2.3 : 1 mixture of
1b and 2b, or a 1 : 2.1 mixture of 1b and 8 was dissolved in the
minimum quantity of acetonitrile. The solution was slowly
saturated with benzene through the gas phase at ambient
temperature up to complete precipitation (visual control, for
3–5 days) and the precipitate formed was separated by decan-
tation. The crystallization was repeated and the desired supra-
molecular complexes were obtained as fine-grained crystals in
nearly quantitative yields. The composition of the complexes
1-(3-Ammoniopropyl)-4-{(E)-2-[1-(3-ammoniopropyl)-4-pyri-
diniumyl]-1-ethenyl}pyridinium tetraperchlorate (2b). Obtained
as a white solid in 72% overall yield; mp 301–304 1C (de-
comp.). IR n/cmꢁ1: 3223, 3175, 3134 (NH). Lit. data: mp
293–295 1C (decomp.).5a
1
was confirmed by H NMR spectra and elemental analysis.
ꢁ1
.
Complex [1b 9]. White crystals, mp 214–217 1C. IR n/cm
:
3213, 3154, 3094 (NH). Anal. calcd. for C46H78Cl2N2O20: C,
52.62; H, 7.49; N, 2.67; found: C, 52.74; H, 7.47; N, 2.66%.
1-(2-Ammonioethyl)-4-{(E)-2-[1-(2-ammonioethyl)-4-pyridini-
umyl]-1-ethenyl}pyridinium tetraperchlorate (2c). Obtained as a
slightly brownish solid in 29% overall yield; mp 287–289 1C
(decomp.). IR n/cmꢁ1: 3187 (NH). 1H NMR (DMSO-d6, 30
1C) d: 3.52 (br m, 4H, 2 CH2NH3), 4.80 (t, J ¼ 5.8 Hz, 4H, 2
.
Complex [1b 2b]. Brown crystals, mp 260–263 1C. IR
n/cmꢁ1
C52H76Cl4N4O28: C, 46.37; H, 5.69; N, 4.16; found: C, 46.15;
H, 5.72; N, 3.99%.
:
3170, 3126, 3063 (NH). Anal. calcd. for
CH N), 8.15 (s, 2H, CH CH), 8.40 (d, J ¼ 6.7 Hz, 4H, 2H-3,
Q
2
2 H-5), 9.04 (d, J ¼ 6.7 Hz, 4H, 2 H-2, 2 H-6). Anal. calcld for
C16H24Cl4N4O16: C, 28.67; H, 3.61; N, 8.36; found: C, 28.57;
H, 3.67; N, 8.31%.
.
Complex [(1b)2 2b]. Deep-brown crystals, mp 235–237 1C.
IR n/cmꢁ1: 3175 (NH). Anal. calcd. for C86H124Cl4N4O40
H2O: C, 51.29; H, 6.31; N, 2.78; found: C, 51.26; H, 6.29; N,
2.72%.
ꢂ
1-(3-Ammoniopropyl)-4-{2-[1-(3-ammoniopropyl)-4-pyridiniu-
myl]ethyl}pyridinium tetraperchlorate (3b). Obtained as a white
solid in 59% overall yield; mp 270–275 1C (decomp.). IR n/
1
cmꢁ1: 3231, 3188, 3137 (NH). H NMR (D2O, 25 1C) d: 2.33
Complex [1b 2c]. Black crystals, mp 215–217 1C (decolor.)
.
(m, 4H, 2 CH2CH2N), 3.06 (t, J ¼ 7.7 Hz, 4H, 2 CH2NH3),
3.35 (s, 4H, 2 CH2), 4.60 (t, J ¼ 7.7 Hz, 4H, 2 CH2N), 7.91 (d,
J ¼ 6.5 Hz, 4H, 2 H-3, 2 H-5), 8.69 (d, J ¼ 6.5 Hz, 4H, 2 H-2,
and >260 1C (decomp.). IR n/cmꢁ1: 3170, 3127, 3050 (NH).
Anal. calcd. for C50H72Cl4N4O28: C, 45.53; H, 5.50; N, 4.25;
found: C, 45.64; H, 5.75; N, 4.11%.
892
N e w J . C h e m . , 2 0 0 5 , 2 9 , 8 8 1 – 8 9 4

View Article Online
centration of 1b was maintained at 1 ꢂ 10ꢁ3 M, and the
concentration of 8 was varied from 0 to 6 ꢂ 10ꢁ3 M. The
proton chemical shifts were measured as a function of the
donor/acceptor ratio, and the complex formation constants
were then calculated using the HYPNMR program.14
.
Complex [1b 3b]. Yellowish crystals, mp 255–260 1C (de-
comp.). IR n/cmꢁ1: 3171, 3130, 3088 (NH). Anal. calcd. for
C52H78Cl4N4O28 ꢀ 2H2O: C, 45.09; H, 5.97; N, 4.05; found: C,
45.12; H, 5.94; N, 3.82%.
.
Complex [1b 4b]. Yellow crystals, mp 185–190 1C. IR
n/cmꢁ1: 3175, 3151 (NH). Anal. calcd. for C52H76Cl4N4O28
ꢂ 1.5H2O: C, 45.46; H, 5.80; N, 4.08; found: C, 45.40; H, 5.97;
N, 3.98%.
Acknowledgements
Support from the Russian Foundation for Basic Research
(Projects 02-03-32286, 03-03-32178, and 03-03-32929), the
INTAS (Grants 2001-0267 and YSF99-4051), the President
of the Russian Federation (Grants MK-3666.2004.3 and MK-
3697.2004.3), the Russian Science Support Foundation, the
Russian Academy of Sciences, the Royal Society (L.G.K. and
A.I.V.), the Royal Society of Chemistry for the RSC Journal
Grants for International Authors (L.G.K. and A.V.C.), the
EPSRC for a Senior Research Fellowship (J.A.K.H.), and the
University of Umea in the framework of a joint project of
the University of Umea and the Photochemistry Center of the
Russian Academy of Sciences (CRDF, Grant RC0-872) is
gratefully acknowledged.
.
Complex [1b (8)2]. Yellowish crystals, mp 207–209 1C. IR
n/cmꢁ1: 3177, 3150 (NH). Anal. calcd. for C52H80Cl4N4O28: C,
46.23; H, 5.97; N, 4.15; found: C, 46.25; H, 6.03; N, 3.96%.
Crystallographic study
Crystals of 1a, b suitable for the X-ray diffraction experiment
were grown by slow evaporation of CH2Cl2–hexane solutions.
The single crystals were coated with perfluorinated oil and
mounted on a Bruker SMART-CCD diffractometer (Mo-Ka
radiation, o scan mode, 0.31 frame, 15 s per frame). The
structures were solved by direct methods and refined by full-
matrix least-squares on F2 in the anisotropic approximation for
all non-hydrogen atoms. The hydrogen atoms were located
from difference Fourier synthesis and refined in the isotropic
approximation. The SHELXS-86 and SHELXL-97 software18
were used for structure solution and refinement.
References
1
(a) Cation Binding by Macrocycles, eds. Y. Inoue and G. W.
Gokel, Marcel Dekker, New York, 1990; (b) R. M. Izatt, K.
Pawlak, J. S. Bradshaw and R. L. Bruening, Chem. Rev., 1991, 91,
1721; (c) J.-M. Lehn, Supramolecular Chemistry. Concepts and
Perspectives, VCH, Weinheim, 1995.
2
(a) H.-G. Lohr and F. Vogtle, Acc. Chem. Res., 1985, 18, 65; (b) P.
¨
¨
de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley,
C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997,
97, 1515; (c) M. V. Alfimov and S. P. Gromov, in Applied
fluorescence in chemistry, biology, and medicine, eds. W. Rettig,
B. Strehmel, S. Schrader and H. Seifert, Springer-Verlag, Berlin,
1999, pp. 161–178; (d) B. Valeur and I. Leray, Coord. Chem. Rev.,
2000, 205, 3.
(a) V. Balzani, A. Credi and M. Venturi, Molecular devices and
machines – A Journey in the Nano World, Wiley-VCH, Weinheim,
2003; (b) Molecular Switches, ed. B. L. Feringa, Wiley-VCH,
Weinheim, 2001; (c) J. D. Badjic, V. Balzani, A. Credi, S. Silvi
and J. F. Stoddart, Science, 2004, 303, 1845.
(a) S. Shinkai, in Comprehensive Supramolecular Chemistry, ed.
J.-M. Lehn, Pergamon Press, New York, 1996, vol. 1; (b) S. P.
Gromov and M. V. Alfimov, Russ. Chem. Bull., 1997, 46, 611; (c)
V. Lokshin, A. Samat and A. V. Metelitsa, Russ. Chem. Rev.,
2002, 71, 893; (d) D. G. Amirsakis, A. M. Elizarov, M. A. Garcia-
Garibay, P. T. Glink, J. F. Stoddart, A. J. P. White and D. J.
Williams, Angew. Chem. Int. Ed., 2003, 42, 1126; (e) M. V.
Alfimov, O. A. Fedorova and S. P. Gromov, J. Photochem.
Photobiol., A, 2003, 158, 183.
(a) S. P. Gromov, E. N. Ushakov, A. I. Vedernikov, N. A.
Lobova, M. V. Alfimov, Yu. A. Strelenko, J. K. Whitesell and
M. A. Fox, Org. Lett., 1999, 1, 1697; (b) E. N. Ushakov, S. P.
Gromov, A. I. Vedernikov, E. V. Malysheva, A. A. Botsmanova,
M. V. Alfimov, B. Eliasson, U. G. Edlund, J. K. Whitesell and
M. A. Fox, J. Phys. Chem. A, 2002, 106, 2020.
Crystal data for 1a. C18H20O4, M ¼ 300.34, monoclinic, a ¼
8.9484(5), b ¼ 6.7152(3), c ¼ 12.4323(7) A, b ¼ 92.500(3)1, V ¼
746.35(7) A3, T ¼ 120.0(2) K, space group P21/c (no. 14),
Z ¼ 2, l(Mo-Ka) ¼ 0.710 73 A, 2775 reflections measured,
1064 unique (Rint ¼ 0.0278) which were used in all calculations.
The final R1 and wR2 were 0.0339 and 0.0844 for I 4 2s(I),
0.0448 and 0.0892 for all data.
3
4
Crystal data for 1b. C34H48O12, M ¼ 648.72, monoclinic, a ¼
42.778(2), b ¼ 9.0073(3), c ¼ 8.6096(3) A, b ¼ 95.988(2)1, V ¼
3299.3(2) A3, T ¼ 100.0(2) K, space group C2/c (no. 15), Z ¼ 4,
l(Mo-Ka) ¼ 0.710 73 A, 8379 reflections measured, 2900
unique (Rint ¼ 0.0661) which were used in all calculations.
The final R1 and wR2 were 0.0435 and 0.0839 for I 4 2s(I),
0.0829 and 0.0949 for all data.
CCDC reference numbers 232849 and 238954.
See http://www.rsc.org/suppdata/nj/b5/b500667h/ for crys-
tallographic data in CIF or other electronic format.
5
6
UV/Vis spectroscopy
Absorption spectra were recorded on a Specord-M40 spectro-
photometer. Luminescence spectra were measured on a Shi-
madzu RF-5000 spectrofluorimeter. The complexation
equilibria were studied in anhydrous acetonitrile (water
o0.01%) at 22 ꢃ 2 1C. The complex formation constants were
measured using the procedures described previously.5b The
stability constants for the 1 : 1 D–A complexes between 1b
and 2b–4b or 2c are measured relative to the 1 : 1 complex of 1b
with 1,10-decanediammonium diperchlorate (log K1 ¼ 7.58,
ref. 5b).
(a) R. Foster, Organic Charge-Transfer Complexes, Academic
Press, New York, 1969; (b) P. Bruni and G. Tosi, Gazz. Chim.
Ital., 1997, 127, 435.
7
8
¨
G. Lindsten, O. Wennerstrom and B. Thulin, Acta Chem. Scand.,
Ser. B, 1986, 40, 545.
(a) C. Luis, M. M. Cid, R. Domingeuez, J. A. Seijas and M. C.
Villaverde, Heterocycles, 1990, 31, 1271; (b) P. Y. White and L. A.
Summers, Aust. J. Chem., 1977, 30, 1153.
L. G. Kuz’mina, A. V. Churakov, J. A. K. Howard, A. I.
Vedernikov, N. A. Lobova, A. A. Botsmanova, M. V. Alfimov
and S. P. Gromov, Crystallogr. Rep., 2005, 50, 234 [Transl. from
Kristallografiya, 2005, 50, 266 (in Russian)].
9
1H NMR titration
10 S. P. Gromov, A. I. Vedernikov, E. N. Ushakov, L. G. Kuz’mina,
A. V. Feofanov, V. G. Avakyan, A. V. Churakov, Yu. S.
Alaverdyan, E. V. Malysheva, M. V. Alfimov, J. A. K. Howard,
B. Eliasson and U. G. Edlund, Helv. Chim. Acta, 2002, 85, 60.
11 M. V. Alfimov, S. P. Gromov, Yu. V. Fedorov, O. A. Fedorova,
A. I. Vedernikov, A. V. Churakov, L. G. Kuz’mina, J. A. K.
Howard, S. Bossmann, A. Braun, M. Woerner, D. F. Sears and
J. Saltiel, J. Am. Chem. Soc., 1999, 121, 4992.
The titration experiments were performed in CD3CN solutions
at 25 ꢃ 1 1C. The concentration of acceptor (2a–4a, 2b–4b, 2c,
or 9) in solution was normally maintained at 1 ꢂ 10ꢁ3 M, and
the concentration of donor (1a or 1b) in the solution was
gradually increased, starting from nothing. The highest donor/
acceptor ratio was about 40. With the 1b/8 system, the con-
12 G. R. Desiraju, Chem. Commun., 1997, 1475.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 8 8 1 – 8 9 4
893

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13 E. N. Ushakov, S. P. Gromov, A. V. Buevich, I. I. Baskin, O. A.
Fedorova, A. I. Vedernikov, M. V. Alfimov, B. Eliasson and U.
Edlund, J. Chem. Soc., Perkin Trans. 2, 1999, 601.
14 C. Frassineti, S. Ghelli, P. Gans, A. Sabatini, M. S. Moruzzi and
A. Vacca, Anal. Biochem., 1995, 231, 374.
15 K. P. Butin, A. A. Moiseeva, S. P. Gromov, A. I. Vedernikov, A.
A. Botsmanova, E. N. Ushakov and M. V. Alfimov, J. Electro-
anal. Chem., 2003, 547, 93.
16 A. M. Chippendale, G. McGeorge, R. K. Harris and C. M.
Brennan, Magn. Reson. Chem., 1999, 37, 232.
17 A. Fisher, M. J. King and F. P. Robinson, Can. J. Chem., 1978,
56, 3068.
18 (a) G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467;
(b) G. M. Sheldrick, SHELXL-97, Program for the Refine-
ment of Crystal Structures, University of Gottingen, Germany,
¨
1997.
894
N e w J . C h e m . , 2 0 0 5 , 2 9 , 8 8 1 – 8 9 4