Synthesis and characterization of new aromatic tweezers and complex formation with tropylium ion in 1,2-dichloroethane

Article abstract of DOI:10.1002/poc.392

A series of benzene and pyridine tweezers bearing phenyl, naphthyl and anthryl receptor units was prepared and characterized. The x-ray crystal structure of the 1,3-bis(9-methanolanthracene)methylbenzene ligand (5) is reported. UV-visible and NMR spectros

Full text of DOI:10.1002/poc.392

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 551–558
DOI:10.1002/poc.392
Synthesis and characterization of new aromatic tweezers
and complex formation with tropylium ion in 1,2-
dichloroethane
Markku LaÈmsaÈ, Sari Kiviniemi, Eeva-Riitta Kettukangas, Maija Nissinen, Jouni Pursiainen Kari Rissanen2
1Department of Chemistry, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland
2Department of Chemistry, University of JyvaÈskylaÈ, P.O. Box 35, FIN-40351 JyvaÈskylaÈ, Finland
Received 27 November 2000; revised 6 April 2001; accepted 6 April 2001
ABSTRACT: A series of benzene and pyridine tweezers bearing phenyl, naphthyl and anthryl receptor units was
prepared and characterized. The x-ray crystal structure of the 1,3-bis(9-methanolanthracene)methylbenzene ligand
epoc
(5) is reported. UV–visible and NMR spectroscopy were used to investigate the host–guest chemistry of the new
ligands in complexation with tropylium tetrafluoroborate as a model aromatic cationic guest in 1,2-dichloroethane.
The appearance of coloured charge-transfer absorption bands demonstrates the complex formation with a tropylium
ion. The enlargement of aryl receptor size from phenyl and naphthyl to anthryl increases the stability of complexes.
Electron donor–acceptor interactions are an important driving force for complexation by molecular tweezers. The
results are discussed and compared with those of previously studied systems containing crown ethers and podands.
The bisanthracenes 5 and 1,3-bis(9-methanolanthracene)methylpyridine (6) are especially interesting in displaying
both complexation ability and intramolecular cycloaddition via charge-transfer excitation. Copyright  2001 John
Wiley & Sons, Ltd.
Additional material for this paper is available from the epoc website at http://www.wiley.com/epoc
KEYWORDS: charge-transfer; cleft; cycloaddition; recognition; stacking; tropylium
INTRODUCTION
Various host molecules capable of binding neutral or
ionic aromatic guests between aromatic surfaces have
been synthesized. Macrocyclic hosts bearing electron-
rich aromatic atoms such as cyclophanes, calixarenes and
homocalixcarenes have dominated. However, molecular
clefts offer an interesting approach to study molecular
recognition based on p–p and cation–p interactions.5
Usually molecular clefts or tweezers have been con-
structed of one spacer unit and two complexing receptors
forming hosts with varying degrees of rigidity depending
on design. Rebek’s baseball assemblies of two or more
cleft-like structures represent an example of the high-
lights of the development.6 Podands with central benzene
and pyridine units and rigid donor end groups (e.g. 8-
hydroxyquinoline) were studied in the late 1970s by
Vo¨gtle and Weber.7
Exploring the potentials of p–p stacking and cation–p
interactions1 in molecular recognition has been an
intensively studied area in supramolecular chemistry. In
particular, attention has been focused on the role of non-
covalent interactions in the formation of novel supramol-
ecular assemblies.2 The formation of a supramolecular
entireness demands the existence of several cooperative
non-covalent interactions, such as interactions with
participation of hydrogen (H-bonds), electrostatic,
charge-transfer, dispersion, ion-mediated and hydropho-
bic interactions.3 The incorporation of a substrate into a
host cavity forming inclusion complexes is another way
to attain supramolecular structures.4
We have observed that p–p stacking interaction plays
an important role in the complexation between benzene-
substituted crown ethers and various electron-deficient
ions such as tropylium,8 pyridinium,9 imidazolium and
pyrazolium.10 It also has an important role in the complex
formation between podands bearing aromatic end groups
and a tropylium cation.11 We describe here a class of
*Correspondence to: M. La¨msa¨, Department of Chemistry, University
of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland.
E-mail: markku.lamsa@oulu.fi
Contract/grant sponsor: TEKES, Technology Development Centre
Finland; Contract/grant number: 40581/97.
Contract/grant sponsor: Academy of Finland; Contract/grant number:
SA157506.
Contract/grant sponsor: Ministry of Education of Finland.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 551–558

¨
¨
552
M. LAMSA ET AL.
Figure 1. Ball-and-stick representation of the solid-state structure of the 5
molecular receptors that are able to bind aromatic cations
by means of charge-transfer, p–p and cation–p interac-
tions. We studied the capability of the hosts for
complexation towards aromatic molecules using a
tropylium tetrafluoroborate guest in 1,2-dichloroethane
(DCE) by UV–visible absorption spectrophotometry. In
addition, a comparison between crown ethers and
podands host structures was undertaken as it was
expected to be informative for the development of new
host molecules.
guest even though there may be a loss of degrees of
freedoms.13
The basic structure of our hosts contains three building
blocks: a spacer unit, connecting arms or bridge units and
receptor units. This simple construction concept gives us
possibilities to build up different hosts by varying only
some of the building blocks. We introduced different
aromatic ring systems for receptor unit by the Williamson
ether synthesis.
Syntheses of the phenoxy and naphthoxy derivatives
1–4 were executed in a straightforward way by refluxing
reactants and potassium carbonate as a base and acetone
or acetonitrile as a solvent. A similar procedure was
attempted in the synthesis of the tweezers 5 and 6. 9-
Hydroxymethylanthracene reacted inadequately with
1,3-bisbromomethylbenzene without forming 5 and only
a poor yield of the monosubstituted compounds was
obtained in the reaction with 1,3-bisbromomethylpy-
ridine. The tweezers 5 and 6 were prepared by using
sodium hydride in dry DMF with 39 and 78% yield,
respectively.
RESULTS AND DISCUSSION
Synthesis
A starting point for the design of tweezer structures was
our earlier complexation studies of crown ethers and
podands with organic cations.8–11 It has been shown that
the preorganization of ligand structure has great im-
portance in the complexation and it has been argued to be
the central determinant of complex stability.12 However,
if the ideal binding conformation with a rigid host is not
achieved or desired, then a flexible host that can adopt a
more favourable conformation will often better bind the
It has been debated that the spacer unit should be rigid
in order to prevent self-association of the complexing
chromophores and increase the complexation efficiency
over a flexible analogue.14 Zimmerman et al. prepared
˚
rigid molecular tweezers providing a ca 7 A interchro-
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 551–558

SYNTHESIS AND CHARACTERIZATION OF AROMATIC TWEEZERS
553
Figure 2. Absorption spectra of tropylium tetra¯uoroborate
-2.0 mM) in DCE solution at 25°C in the presence of
increasing amounts of 5: -1) 3.1; -2) 5.3; -3) 9.8; -4) 20.8;
-5) 30.3; -6) 41.7; and -7) 55.0 mM. The dashed line
represents the spectrum of 5
Figure 3. Plots obtained in experiments carried out for the
evaluation of the stability constants in the complexation of
tweezer 5 with tropylium tetra¯uoroborate -2.0 mM) in DCE
solution at 25.0°C. Inset: Benesi±Hildebrand plot
mophore distance, preventing self-association.5a How-
ever, rigid molecules with a highly preorganized
structure can still form intermolecular associations of
two clefts via chromophores which can be observed in the
crystal packing of molecular tweezers.15 The cleft
receptors prepared in this study have an inflexible central
spacer and bendable arms attached to planar aryl
receptors. The aromatic receptor units should be adequate
to undergo p–p stacking interactions with aromatic
molecules. Likewise, the enlargement of the aromatic
surface in the tweezer could improve and strengthen the
complexation with an aromatic guest. In addition, the
relatively flexible arms between the spacer unit and
aromatic receptors were expected to allow the formation
of a tweezer-like structure in the complexation event. The
x-ray structure of 1,3-bis(9-methanolanthracene)methyl-
benzene (5) in Fig. 1 is representative of the ligands
synthesized in this work.
It forms intensely coloured charge-transfer complexes in
diverse solvents, which is a great advantage when the
stability of the complexes is measured by spectro-
photometry.18,19
The complexation of the tweezers in DCE with
tropylium tetrafluoroborate was followed by absorption
changes in the UV–visible spectra. Tropylium tetra-
fluoroborate in DCE exhibits a maximum absorption at
279 nm and the pure tweezers have no absorptions at
wavelengths over 400 nm in the visible region. A
characteristic colour, from yellow to red depending on
the ligand used, developed spontaneously on addition of
tweezer to a solution of tropylium tetrafluoroborate in a
dry solvent. The electronic spectrum of the tweezers in
the presence of tropylium ion exhibited a broad
wavelength absorption at ca 300–600 nm. For example,
the addition of 5 to DCE solutions containing tropylium
salt led to the appearance of a new absorption band with
maximum at about 524 nm, not found in the spectrum of
either tropylium tetrafluoroborate or 5. The intensity of
the new band increased with incremental additions of
tweezer. The band is indicative of a charge-transfer
interaction and the formation of a molecular complex
between tropylium ion and the tweezer (Fig. 2). Typical
double-reciprocal or Benesi–Hildebrand plots for the
tweezer–tropylium ion complexes gave a linear relation-
ship for the entire range of concentrations tested,
indicating the existence of 1:1 stoichiometry of the
complexes (Fig. 3). Values for the stability constants K
and molar absorptivities of the complexes eC in DCE for
the tweezer–tropylium ion complexes were calculated by
the Rose and Drago method.8,20 The results are presented
in Table 1.
Complexation of tropylium tetra¯uoroborate in
1,2-dichloroethane
Because the tropylium ion has been shown to be a useful
guest for the investigation of p–p interactions,11 we
studied the complexation capability of the ligands
towards an organic guest with tropylium tetrafluoroborate
in DCE by UV–visible absorption spectrophotometry.
Rathore et al.16 have shown that UV–visible spectral
analyses provide quantitative information on the inter-
molecular associations of different aromatic donor–
acceptor complexes, and such measurements also
established charge-transfer absorption to be a sensitive
analytical tool for evaluating the steric inhibition of the
donor–acceptor association that is important in molecular
recognition. Tropylium itself is a monocharged organic
cation containing even numbers of electrons. It is an
aromatic, cationic and relatively stable carbenium ion.17
The hosts 1 and 2 form weak charge-transfer com-
plexes with the tropylium cation and the stability con-
stants are comparable to those for benzene–tropylium
interactions in similar systems. The 3 and 4 complexes
with tropylium have K values of 6.6 and 28 dm3 molÀ1
,
Copyright  2001 John Wiley & Sons, Ltd.
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¨ ¨
M. LAMSA ET AL.
554
Table 1.ÀS2tability constants, molar absorptivÀit3ies of complexes and free energies of binding for the interaction of tweezers [-0.5±
10) Â 10 M] with tropylium ion -2.0 Â 10 M) in 1,2-dichloroethane solution at 25°C
Tweezera
lobs (nm)
K (dm3 molÀ1
)
eC(dm3 molÀ1 cmÀ1
)
ÀDG°(kJ molÀ1
)
r2
1
600
405
430
516
520
510
425
410
470
502
3.0 Æ 0.2
2.3 Æ 0.3
2249 Æ 187
453 Æ 65
284 Æ 16
227 Æ 10
1853 Æ 17
1348 Æ 78
1060 Æ 13
662 Æ 43
1408 Æ 69
1765 Æ 31
2.7
2.1b
4.7
0.9999
0.9999
0.9999
0.9972
0.9999
0.9989
—
0.9966
0.9999
0.9995
2
3
6.6 Æ 0.4
4
27.6 Æ 2.8
41.2 Æ 0.7
57.3 Æ 4.9
273.5 Æ 13.8
7.6 Æ 1.0
8.2
5
9.2
6
10.0
13.91
5.01
4.85
11.05
2B24C8c
POD1d
POD2d
POD3d
7.1 Æ 0.4
86.2 Æ 4.6
a Crown ethers and podands are included for comparison: dibenzo-24-crown-8 (2B24C8), 1,10-diphenyl-1,4,7,10-tetraoxaundecane (POD1), 1,10-
dinaphthalene-1,4,7,10-tetraoxaundecane (POD2), 1,12-bis(anthryl-9)-2,5,8,11-tetraoxadodecane (POD3).
b 35°C.
c
Ref. 8.
d Ref. 11.
respectively. Similarly, 6 (57 dm3molÀ1) has a higher
stability constant than 5 (41 dm3molÀ1). The difference
could be caused by the nitrogen atom, known as a strong
electron donor, in the pyridine tweezer. We have shown
earlier that the number of oxygen atoms involved in the
complexation with tropylium has an impact on the
stability of the complexes formed between crown ethers
with different ring sizes and tropylium ion.8 Pyridine
nitrogen and oxygen atoms in connecting arms can accept
hydrogen bonds connecting tropylium closer to the host
molecule.
pylium and other electron-deficient organic cations.8–10
The x-ray structures of solid complexes have revealed
that organic cations are inserted into the cavity of the
macrocycle between two adjacent benzene rings.10
A
similar wrap-around phenomenon was not encountered in
the complexation of the non-cyclic counterpart of
benzene-substituted crown ethers, podands, with a
tropylium ion.11 Recently, Kla¨rner’s group studied a
rigid tweezer host in complexation experiments with
tropylium tetrafluoroborate.21 The ribbon-type concave
topology of the five benzene units shapes the molecular
tweezer that is able to bind substrate inside the
preorganized cleft cavity. The tweezer–tropylium system
It is now well documented that benzene-substituted
crown ethers are able to form stable complexes with tro-
Figure 4. 1H NMR spectrum of 6 -40 mM) and the spectra of a mixture of 6 -40 mM) and tropylium tetra¯uoroborate -1 mM) after
5 min and 7 days of mixing of solutions recorded at 200 MHz in acetonitrile-d3±chloroform-d -1:1) at 25°C. A free tropylium ion
has a singlet at 9.26 ppm
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 551–558

SYNTHESIS AND CHARACTERIZATION OF AROMATIC TWEEZERS
555
has a K value of 23 dm3 molÀ1, determined by 1H NMR
titration in 1:1 solution of CDCl3–(CD3)2CO at 21°C.
The results indicate that a preorganized rigid structure of
ligands is not the only requirement to produce stable
complexes with aromatic organic cations such as
tropylium ion. The number and type of donor atoms that
are a capable of hydrogen bonding and the area of the
aromatic surface that has an ability to undergo a p–p
stacking interaction evidently have considerable impor-
tance in tweezer–tropylium ion complexation.22
ture was left to evaporate slowly (the crystal structure of
lepidopterene is available as supplementary data at the
epoc website at http://www.wiley.com/epoc). The prime
intent was to prepare a single crystal of the complex. The
bisanthracene ligand 5 partially reacted to give lepid-
opterene25 in the solution exposed to daylight. The
influence of the tropylium ion or light on the formation of
lepidopterene was not investigated in detail in this work.
Lepidopterene is a tetracyclic benzenoid hydrocarbon
whose crystal structure has been previously deter-
mined.26 The long central C—C bonds in the dimers of
anthracene have been cited as extreme examples of bond
lengthening as a result of through-bond coupling. The
central carbon–carbon bond length in lepidopterene is
The time stability of the complexes formed between
tweezers and a tropylium ion was investigated by
measuring the stability constants for 1, 3, 5 and 6 with
different time intervals. Only insignificant changes were
observed for 1 and 3 and they were within the
determination accuracy. However, it is worth emphasiz-
ing that the absorption measurements should be carried
out immediately after mixing the donor and the acceptor
solutions together. This seems to be particularly im-
portant in the case of the bisanthracene ligands 5 and 6
that behaved differently. A well-known phenomenon of
dimerization of anthracene in solution23 was also
observed in our study. The disappearance of the original
colour of 5–tropylium and 6–tropylium systems was
monitored by UV–visible spectrophotometry at maxi-
mum charge-transfer absorbance wavelength. The deep
red colour that formed directly after mixing 5 and
tropylium DCE solutions slowly turned green. A similar
phenomenon occurred for 6. The reaction of 6 was
followed also by 200 MHz proton NMR spectroscopy.
The kinetic investigations showed that absorbance vs
time curves were single exponential measured at 524 nm
for 5 and at 502 nm for 6. The rate of cycloaddition was
calculated to be first order with respect to the concentra-
tion of ligands. The values of the observed rate constant,
kobs, were 2.5 Â 10À5 s1 (t1/2 = 7 h 45 min) for 5 and
3.7 Â 10À5 sÀ1 (t1/2 = 5 h 18 min) in DCE and 3.0 Â 10À5
sÀ1 (t1/2 = 6 h 30 min) in acetonitrile-d3–chloroform-d
(1:1) for 6 at 25°C. The concentration of tropylium
tetrafluoroborate was held constant (2 Â 10À3 M) in all
measurements. The rate constants observed were found to
be independent of the initial concentration of tweezer.
The NMR spectra in Fig. 4 indicate the formation of the
cycloaddition product of 6. After 7 days the intramol-
ecular [4p ‡ 4p] cycloadduct was formed almost com-
pletely. The rate constant results and spectroscopic
evidence indicate that charge-transfer transitions play
an important role in bringing about chemical changes in
bisanthracene–tropylium systems. Masnovi and Kochi
have shown that substituted anthracenes with electron
acceptors [e.g. tetracyanoethylene (TCNE)] in solution
afforded slowly Diels–Alder adducts on standing in the
dark.24
˚
1.651 A in the re-determined x-ray structure (pdb file as a
supplementary data at the epoc website at http://www.
wiley.com/epoc). The published structures have central
26
˚
bond lengths 1.642 and 1.645 A, respectively (for
visualization see the files tweezer 5 and lepidopterene at
the epoc website at http://www.wiley.com/epoc).
In conclusion, new non-macrocyclic receptors have
been synthesized and shown to complex with tropylium
cation in organic solvents. Electron donor–acceptor
interactions are an important driving force for complexa-
tion by molecular tweezers. The appearance of charge-
transfer absorption bands demonstrates the complex
formation with tropylium ion. Comparison of the stability
constants in Table 1 shows that varying the aryl receptor
unit from phenyl and naphthyl to anthryl can increase the
stability of the complex. Additionally, the addition of a
nitrogen donor atom into the spacer unit increases the
stability constants of the complex formed. Among these
systems, bisanthracenes are especially interesting in
displaying both complexation with a tropylium ion and
intramolecular [4p ‡ 4p] cycloaddition via charge-trans-
fer excitation. It seems appropriate to assume that the
cycloaddition of anthracene ligands observed can been
employed to modulate the binding ability and also to
prepare new molecules. Our current efforts are directed to
take advantage of the phenomena observed and we shall
report further new results in the near future.
EXPERIMENTAL
Materials and methods
The starting materials were purchased from commercial
sources and used without further purification, unless
mentioned otherwise. Acetonitrile (Fluka) and 1,2-
dichloroethane (Lab-Scan) were dried and distilled
according to literature procedures.27 Thin-layer chroma-
tography (TLC) was carried out on aluminium plates
coated with silica gel. Compounds were detected by UV
spectrophotometry. Flash column chromatography was
performed on the silica gel with the solvents specified. 1H
and 13C NMR spectra were recorded on a Bruker
It is worth adding that the small amount of solid
lepidopterene could be isolated when equimolar amounts
of 5 and tropylium tetrafluoroborate were dissolved in
acetonitrile–dichloromethane (1:1) and the solvent mix-
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 551–558

¨ ¨
M. LAMSA ET AL.
556
Advance DPX200 spectrometer. NMR chemical shifts
are reported in ppm downfield from internal tetramethyl-
silane (TMS). The coupling constants are expressed in
hertz (Hz). Electron ionization (EI) mass spectra were
obtained using a Kratos MS 80 mass spectrometer
operating with the DART data system. Electronspray
ionization (ESI) mass spectra were recorded on an LCT
time-of-flight mass spectrometer (Micromass) with an
OpenLynx 3 data system. The desolvation temperature
was 120°C and N2 was used as nebulizer and desolvation
gases. UV–visible spectra were recorded with a Phillips
PU 8740 or Shimadzu UV-1601 spectrophotometer with
matched glass or quartz cells of 10 mm pathlength. The
cell containing the solution was maintained within
Æ0.05°C (Æ0.1°C in the kinetic measurements) of a set
temperature by circulating water from a thermostated
bath through a double-walled cell insert. The temperature
inside the cell was monitored with a digital thermometer.
Elemental analyses were carried out with a Perkin-Elmer
2400 apparatus. Melting-points were determined with a
Thermopan microscope (Reichert, Vienna, Austria)
melting-point apparatus and are uncorrected.
13C NMR (CDCl3, 25°C), ꢀ 70.8, 115.2, 120.5, 121.6,
‡
130.0, 138.1, 157.3, 158.8; EIMS (70 eV), m/z 291 [M ].
1,3-Bisnaphthoxymethylbenzene 93). Recrystalliza-
tion from acetone afforded a crystalline solid in 54%
yield, m.p. 133°C (found C, 82.73; H, 5.88. C20H18O2
requires C, 82.73; H 6.25%); 1H NMR (acetone-d6,
25°C), ꢀ 7.73–7.85 (4H, aryl, m), 7.21–7.54 (28H, aryl,
m), 5.292 (4H, OCH2, s); 13C NMR (acetone-d6, 25°C), ꢀ
69.5, 107.2, 118.8, 123.5, 126.2, 126.7, 126.8, 127.0,
127.4, 128.5, 129.1, 129.3, 134.7, 137.7, 156.7; EIMS
‡
(70 eV), m/z 390 [M ].
2,6-Bisnaphthoxymethylpyridine 94). The crude pro-
duct was recrystallized from acetone, yield 71%, m.p.
142–143°C (found C, 82.54; H, 5.36; N, 3.56.
1
C19H17NO2 requires C, 82.84; H, 5.41; N, 3.58%); H
NMR (CDCl3, 25°C), ꢀ 7.67–7.85 (7H, aryl, m), 7.21–
7.55 (10H, aryl, t), 5.37 (4H, OCH2, s); 13C NMR
(CDCl3, 25°C), ꢀ 71.0, 107.8, 119.2, 120.7, 124.3, 126.8,
127.3, 128.0, 129.6, 130.0, 134.8, 138.1, 156.7, 157.1;
‡
EIMS (70 eV), m/z 391 [M ].
1,3-Bis99-methanolanthracene)methylbenzene 95).
At room temperature, to a solution of NaH (0.53 g,
11 mmol) (50% in mineral oil, washed previously with n-
pentane) in dry DMF (20 ml), 9-hydroxymethylanthra-
cene 2.08 g (10 mmol) was added dropwise with stirring
in dry DMF (10 ml). Stirring was continued for 30 min
after the end of the addition. A dry DMF solution (10 ml)
of a,a-dibromo-m-xylene, 1.32 g (5 mmol), was then
introduced drop wise. The reaction mixture was heated
for 20 h at 80°C before cooling to room temperature. The
excess of NaH was quenched by adding a few drops of
water, then the solvent was removed in vacuo and the
residue was partioned between water (30 ml) and
dichloromethane (40 ml). The organic layer was washed
twice with 1 M HCl, water and brine and dried over
MgSO4. Evaporation gave the crude product, which was
recrystallized from acetone, affording a light-yellow
solid in 39% yield, m.p. 163°C (found C, 87.89; H, 5.81.
C38H30O2 requires C, 88.00; H, 5.38%); 1H NMR
(CDCl3, 25°C), ꢀ 8.45 [1H, Ant(5), s], 8.2–8.4 (4H, aryl,
m), 7.9–8.5 (4H, aryl, m), 7.3–7.5 (12H, aryl, m), 5.48
(4H, Ant-OCH2, s), 4.71 (4H, Ph-OCH2, s); 13C NMR
(CDCl3, 25°C), ꢀ 64.8, 73.0, 125.0, 125.6, 126.8, 128.1,
128.3, 129.1, 129.2, 129.3, 129.6, 131.8, 132.1, 139.3;
Syntheses
General procedure for the synthesis of substituted
tweezers. A mixture of 1,3-bisbromomethylbenzene or
1,3-bisbromomethylpyridine (5 mmol), aromatic alcohol
(10 mmol) and K2CO3 (1.282 g, 10 mmol) was stirred
and heated at reflux in 30 ml of acetone or acetonitrile for
16–24 h under a nitrogen atmosphere, after which the
reaction mixture was allowed to cool to room tempera-
ture. The solvent was evaporated under reduced pressure
and the residue was partitioned between dichloromethane
and water. The organic layer was separated and washed
with 5% NaOH solution, water and brine and dried over
MgSO4. Evaporation gave the crude product, which was
subjected to further purification.
1,3-Bisphenoxymethylbenzene 91). The product was
recrystallized from MeOH, affording a white crystalline
solid in 75.4% yield, m.p. 75°C (lit.28 71–76°C); (found
C, 82.73; H, 5.88. C20H18O2 requires C, 82.73; H,
1
6.25%); H NMR (CDCl3, 25°C), ꢀ 7.519 (1H, phenyl),
7.402 (3H, phenyl, d, J = 1.34 Hz), 7.24–7.33 (5H, aryl,
m), 6.93–7.01 (6H, aryl, m), 5.081 (4H, OCH2, s); 13C
NMR (CDCl3, 25°C), ꢀ 70.5, 115.5, 121.7, 127.1, 127.7,
‡
EIMS (70 eV), m/z 518 [M ].
‡
129.5, 130.2, 138.2, 159.4; EIMS (70 eV), m/z 290 [M ].
1,3-Bis99-methanolanthracene)methylpyridine 96).
By the procedure described for 5 but starting with 1,3-
bisbromomethylpyridine, a yellow residue was obtained
and purified with flash chromatography (150:1 CH2Cl2–
MeOH) to afford a light-yellow solid in 78% yield, m.p.
163°C (found C, 85.18; H, 55.63; N, 2.70. C37H29NO2
requires C, 85.52; H, 5.52; N, 2.75%); 1H NMR (CDCl3,
25°C), ꢀ 8.35–8.50 (6H, aryl, m), 7.92–8.30 (4H, aryl,
2,6-Bisphenoxymethylpyridine 92). The crude product
was recrystallized from acetone, affording a 65% yield of
white crystals, m.p. 73°C (lit.29 72–74°C); (found C,
78.33; H, 5.88; N, 4.81. C19H17NO2 requires C, 78.08; H,
5.87; N, 4.79%); H NMR (CDCl3, 25°C), ꢀ 7.76 (1H,
pyridine, t, J = 7.75 Hz), 7.49 (2H, pyridine, d, J =
1
7.72 Hz), 6.96–7.04 (6H, aryl, m), 5.25 (4H, OCH2, s);
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 551–558

SYNTHESIS AND CHARACTERIZATION OF AROMATIC TWEEZERS
557
m), 7.25–7.65 (11H, aryl, m), 5.63 (4H, Ant-OCH2, s),
4.84 (4H, Py-OCH2, s); 13C NMR (CDCl3, 25°C), ꢀ 65.3,
73.6, 120.7, 124.8, 125.4, 126.6, 128.8, 129.0, 129.4,
131.5, 131.8, 137.6, 158.4; ESIMS exact mass (Micro-
tions were collected for Lorentz polarization effects and
no absorption correction was used.
The structure was solved by direct methods (SHELXS-
97)32 and refined on F2 (SHELXL-97)33 The hydrogen
atoms were calculated to their idealized positions with
isotropic thermal factors (1.2 times the C thermal
parameter) and refined as riding atoms. The weighting
‡
mass LCT, ESI‡), m/z 520.2277 ([M ‡ H] ); required,
520.2295.
2
1,3,5-Cycloheptatrienylium tetra¯uoroborate. The
tropylium salt was prepared according to literature
procedures,30 m.p. 203°C (decomp.); 1H NMR (CD3CN,
30°C), ꢀ 9.24 (7H, s).
scheme was w = 1/[ꢂ2(Fo ) ‡ (0.0191P)2], where P =
2
2
[Max(Fo , 0) ‡ 2Fc ]/3. The final residuals were R =
0.068 and wR = 0.100 for 2637 unique data with I >2ꢂ(I)
and R = 0.175, wR = 0.129 for all data and for 362
parameters.
Complexation studies. Stability constant determina-
tion by UV±visible spectroscopy. The stability constant
for 1:1 complexation was defined by a method described
in detail elsewhere.8 The absorption measurements were
made immediately after the mixing of tweezer and
tropylium solutions. The Rose–Drago equation is
The crystallographic data (excluding structure factors)
for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication No. CCDC 153263.
Acknowledgements
1
K
A À A0
CD Á CA Á ꢀ"C À "A†
A À A0
ˆ
À CA À CD ‡
ꢀ1†
"C À "A
This work was financially supported by TEKES,
Technology Development Centre Finland (Grant No.
40581/97) (M.L.), the Academy of Finland (Grant No.
SA157506) (S.K.) and the Ministry of Education of
Finland (M.N.). We thank the Mass Spectrometry
Laboratory, University of Oulu, for the mass spectra
and the Trace Element Laboratory, University of Oulu,
for the elemental analysis data.
where A0 is the absorbance of pure acceptor solution, A is
the absorbance of the acceptor–donor solution, CA and
CD are the initial concentrations of acceptor and donor,
respectively and eC and eA are the molar absorptivities of
complex and acceptor in solution, respectively. The
equation contains two unknown constants, K and eC,
which were evaluated by an iteration method with a PC
implementing a SigmaPlot Scientific Graphing System
Version 4.0 program (Jandell Scientific). The errors in K
and eC were evaluated numerically by standard deviations
of single K and eC values usually obtained from 6–10
measurements.
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