CHIRALITY 13:109–117 (2001)
Circular Dichroism of Host–Guest Complexes of
Achiral Pyridino- and Phenazino-18-crown-6
Ligands with the Enantiomers of Chiral Aralkyl
Ammonium Salts
LÁSZLÓ SOMOGYI,1 ERIKA SAMU,2 PÉTER HUSZTHY,2 ARMAND LÁZÁR,3 J.G. ÁNGYÁN,4
PÉTER R. SURJÁN,3 AND MIKLÓS HOLLÓSI1*
1
Department of Organic Chemistry, Eötvös Loránd University, Budapest, Hungary
2
Institute for Organic Chemistry, Technical University of Budapest, Budapest, Hungary
3
Department of Theoretical Chemistry, Eötvös Loránd University, Budapest, Hungary
4
Laboratoire de Chimie Théorique, Université Henri Poincaré-CNRS, UMR 7565, Vandoeuvre Nancy, France
ABSTRACT
Circular dichroism (CD) spectroscopy was used for distinguishing different types of chiral interactions in host–guest complexes of achiral pyridino- and
phenazino-18-crown-6 ligands with chiral aralkyl ammonium salts. The general feature of
the CD spectra of many homochiral (e.g., (R,R)-host and (R)-guest) and heterochiral
(e.g., (R,R)-host and (S)-guest) ␣-(1-naphthyl)ethylamine hydrogenperchlorate salt
(NEA) complexes with chiral pyridino- and phenazino-18-crown-6 hosts is exciton interaction. The most interesting example is the coupling of the transitions of the chiral guest
NEA with the energetically close transitions of the achiral phenazino-18-crown-6 host 6.
The CD spectrum of the complex is predominated by exciton coupling between the 1Bb
transition of the chiral guest and the 1Bb transition of the achiral host. The redshifted
intense spectra of the complexes of (R)- or (S)-1-phenylethylamine hydrogenperchlorate
salt (PEA) with the achiral diester-pyridino-18-crown-6 host 4 are indicative of merging
the ␲ electron systems into one joint charge transfer chromophore. The appearance of
weak bands with alternating sign in the spectrum of PEA complexes of the achiral
“parent” pyridino-18-crown-6 host (1) indicates the presence of two or more conformers.
The CD spectra of the complexes of achiral phenazino-18-crown-6 host 6 with PEA are
also determined by ␲–␲ interaction. In addition to charge transfer bands, CD bands are
also induced in the long-wavelength spectral region of the achiral host. The weak ␲–␲
interaction between the achiral phenazino-18-crown-6 host and methyl phenylglycinate
hydrogenperchlorate (PGMA) or methyl phenylalaninate hydrogenperchlorate (PAMA)
does not result in a definite spectral effect in the 1La region of the spectrum of the chiral
guest, but its existence is proven by the weak CD bands induced in the long-wavelength
spectral region of the achiral host. Chirality 13:109–117, 2001. © 2001 Wiley-Liss, Inc.
KEY WORDS: supramolecular interactions; exciton coupling; charge transfer band;
␲–␲ interaction
Circular dichroism (CD) spectroscopy has been reported to be a sensitive tool for monitoring enantiomeric
recognition of aralkyl ammonium salts by chiral pyridinoand phenazino-18-crown-6 ligands.1–3 The conformation
and relative stability of diastereomeric host–guest complexes are determined by a tripod-like hydrogen bonding
involving the N atom of the heterocyclic subunit and two
alternate O atoms of the crown ethers and the three ammonium protons of the organic salt (attractive interaction),
␲–␲ stacking between the aromatic rings of the host and
the guest (attractive interaction), as well as the steric repulsion between the substituent at one of the stereogenic
centers of the host and certain hydrogens of the ammonium salt (repulsive interaction). Previous X-ray crystallo© 2001 Wiley-Liss, Inc.
graphic,4–6 1H NMR7–9 and MS studies10–12 have clearly
proved these interactions. It is a delicate balance of the
attractive and repulsive forces that determines the preference of the chiral host for one or the other enantiomer of
the guest molecule. CD spectroscopy has shown3 that,
similar to pyridino hosts, chiral phenazino hosts also form
more stable heterochiral [(R,R)-host/(S)-guest or (S,S)host/(R)-guest] than homochiral [(R,R)-host/(R)-guest or
Contract grant sponsor: OTKA (Hungary); Contract grant numbers:
T-022913, T-014942, T-025071, AKP 98-89 2,4.
*Correspondence to: Prof. Miklós Hollósi, Department of Organic Chemistry, Eötvös Loránd University, P.O. Box 32, H-1518 Budapest 112, Hungary. E-mail: [email protected]
Received for publication 5 April 2000; Accepted 4 August 2000
110
SOMOGYI ET AL.
CD and UV Measurements
CD and UV spectra of the complexes at a 1:1 molar ratio
were recorded on a Jobin-Yvon Mark VI dichrograph (calibrated with epiandrosterone) at room temperature using a
0.02 cm cell for measurements between 195 and 240 nm
and 0.1, 0.2 or 0.5 cm cells above 240 nm. Acetonitrile
(Merck, Darmstadt, Germany, for chromatography) was
used as solvent and the concentration ranged from 0.5–5
mmol × dm,−3 depending on the absorption.
Theoretical Calculations
Figure 1. Structure of pyridino- and phenazino-18-crown-6 hosts and
aralkyl ammonium perchlorate guests.
(S,S)-host/(S)-guest] complexes. (For 1H NMR and microcalorimetric determination of the stability constants of the
complexes of pyridino hosts with the enantiomers of chiral
organic ammonium salt guests, see Refs. 7–9, 13). The
effect of H-bondings on the conformation and chiroptical
properties was separated from the cooperative effect of
␲–␲ interaction and H-bondings by comparing the CD
spectra of the diastereomeric complexes to the spectrum of
the 1:1 butylammonium perchlorate (BAP) complex.1–3
Exciton couplets with oppositely signed high-amplitude
bands were general CD features of the complexes of chiral
phenazino-18-crown-6 ligands with both enantiomers of
␣-(1-naphthyl)ethylamine hydrogenperchlorate salt
(NEA),3 but couplet-like splitting in the 1La/1Bb UV spectral region was also observed for the heterochiral NEA
complexes of pyridino-18-crown-6 hosts 2 and 3 (for structures see Fig. 1).2
Herein we report the CD studies on host–guest complexes of achiral pyridino- and phenazino-18-crown-6 ligands which serve as models for analyzing chiral interactions ranging from induced to exciton chirality.
MATERIALS AND METHODS
Synthesis
(R)- and (S)-enantiomers of NEA, 1-phenylethylamine
hydrogenperchlorate salt (PEA), methyl phenylalaninate
hydrogenperchlorate salt (PAMA), and methyl phenylglycinate hydrogenperchlorate salt (PGMA) were obtained according to the procedure reported earlier.7 Pyridino-18crown-6 (1) was prepared in 32% yield by the cyclization of
2.6-pyridinedimethanol ditosylate with tetraethylene glycol
(NaH, THF) in the same way as described for its chiral
dimethyl-substituted analog [(S,S)-2]7 [mp. for 1 was: 41–
42°C (dichloromethane-pentane); lit.14 mp: 40–41°C (dichloromethane-pentane)]. (R,R)-diphenylpyridino-18crown-6 [(R,R)-3],15 diketopyridino-18-crown–6 (4),16
(S,S)-dimethyl-diketopyridino-18-crown-6 [(S,S)-5],5 phenazino-18-crown-6 (6)17 and (R,R)-dimethylphenazino-18crown-6 [(R,R)-7]17 ligands were synthesized as reported
in the literature.
Quantum chemical calculations were performed
using the ab initio Hartree-Fock method in minimal
(STO-3G), split-valence double-␨ (3-21G), double-␨ polarized (3-21G*), and valence triple-␨ (6-311G) basis sets by
the MUNGAUSS code.18 Ground state properties (Mulliken’s atomic charges) were computed at experimental (Xray) geometries.
For the complex of 1,9-dimethoxy-phenazine, a truncated model of the chiral host (R,R)-7, with the (R)-NEA
guest, separate ab initio calculations in the random phase
approximation were done using Dunning’s cc-pVDZ basis
set by the DALTON code.19 Transition moments were
evaluated for the six lowest-lying excited states. Optical
rotatory strengths from these transition moments at the
experimental (X-ray) geometry of the complex6 were calculated in the framework of the coupled oscillator model.
Technical details of these calculations as well as more extended numerical results will be published in a separate
article.
RESULTS AND DISCUSSION
Induced and Charge Transfer CD Due to ␲–␲
Interaction Between Aromatic Chromophores in PEA,
PGMA, and PAMA Complexes of Achiral Hosts
The CD spectra of the 1:1 heterochiral or homochiral
complexes of (S,S)-2, (R,R)-3, and (S,S)-5 with PEA
showed no sign of either exciton or charge transfer interaction between the aromatic chromophores.1,2 There are
no bands with significant 1La and 1Lb band intensities either
in the CD spectra of the complexes of the achiral pyridino18-crown-6 host 1 with the enantiomers of the PEA (Table
1). The appearance of weak bands with alternating sign
suggests the presence of two or more conformers due to
the enhanced flexibility of the host and the less extended ␲
electron system of the guest. (Stronger bands show up
only in the 1B band region of the spectrum.) Contrary to
this, complexes of its achiral diketo analog 4 with (R)- or
(S)-PEA show a relatively intense redshifted band at 220
nm with a long-wavelength shoulder. The sign of the CD
band is opposite to the sign of the 1La band of the guest (R)or (S)-PEA molecules.2 All these features can be explained
by close fitting of the aromatic rings and merging of the ␲
electron systems giving rise to a charge transfer chromophore (the terms charge transfer interaction, ␲–␲ interaction, and ␲–␲ stacking are used as synonyms in the literature of supramolecular chemistry).
One equivalent of (R)- or (S)-PEA had no strong influence on the CD spectrum of chiral phenazino host (R,R)-7
(Table 1). However, in the CD spectrum of the (R)- and
111
COMPLEXES OF ACHIRAL CROWN LIGANDS
TABLE 1. CD and UV/vis data in acetonitrile of crown hosts containing pyridine (1, 4, 5) or phenazine (6, 7)
chromophore and of organic ammonium salt guests, as well as CD spectra of their 1:1 complexesa
Host
1
UV
(R)-NEA
(S)-NEA
(R)-PEA
(S)-PEA
4
(S,S)-5
6
␭max [nm] (⌬␧) or (␧)
Guestb
UV c
∼206sh (0.7)
212 (+1.27)
213 (−1.12)
<190 (<−6)
203 (−1.88)
<190 (>+6)
∼203 (+2.0)
(R)-NEA
UV c
206 (−24.0)
200 (4.5)
(S)-NEA
(R)-PEA
UV c
206 (+24.5)
(S)-PEA
−
UV c
224 (−5.26)
225 (+4.72)
∼240sh
∼240sh
221.5 (+0.08)
243 (−0.11)
270 (+0.10)
221.5 (−0.14)
224 (0.68)
246 (+0.09)
270 (−0.13)
269 (0.33)
277sh (0.24)
273 (+2.08)
272 (0.87 )
280 (0.87 )
274 (−1.81)
265 (−0.45)
272 (0.39)
279sh (0.31)
265 (+0.44)
262 (−5.28)
269 (0.37 )
278sh (0.27 )
230 (−10.7)
222 (8.19)
230 (+10.9)
220 (−1.68)
223 (0.75)
205 (−22.4)
(R)-NEA
UV c
205 (−65.1)
(S)-NEA
(R)-PEA
UV c
211 (+3.75)
205 (−26.7)
(S)-PEA
BAPc
UV/visc
205 (−22.0)
205 (−18.1)
(R)-NEA
211 (40.6)
221 (+1.48)
221sh (−10.7)
222 (0.74)
222 (+2.32)
218sh (6.78)
231 (−12.0)
227 (−7.29)
220sh (−9.65)
221 (0.68)
235sh (0.40)
221sh (−6.92)
220 (−6.22)
272 (0.85)
280 (0.84)
265 (−4.10)
264 (−4.29)
272 (0.36)
280sh (0.29)
263 (−3.68)
262 (−3.33)
225 (−155)
271 (+77.1)
220 (5.53)
269 (5.22)
271 (−77.9)
(S)-NEA
210 (−40.5)
225 (+156)
(R)-PEA
205 (−3.16)
218 (+0.65)
227 (−0.40)
241 (+0.42)
254 (+0.29)
UV/visc
276 (−2.40)
269 (6.33)
217 (−0.56)
225 (+0.38)
219 (−10.7)
244 (+0.17)
277 (+1.98)
243 (−0.68)
264 (+0.95)
280 (+0.64)
269 (6.25)
(S)-PGMA
219 (+7.84)
245 (+0.35)
(R)-PAMA
217 (−2.84)
265 (−1.10)
278 (−0.83)
265 (−0.65)
278 (+0.55)
269 (6.17)
(R)-PGMA
206 (+3.25)
UV/visc
UV/visc
(S)-PAMA
282.5 (−0.47)
282 (+0.55)
295 (+1.40)
290sh (0.53)
296 (−1.20)
288 (+0.18)
288 (−0.18)
286 (3.72)
290 (+3.66)
291sh (0.51)
287 (+1.76)
288 (+1.91)
288 (+1.77)
286 (+2.19)
268 (7.15)
UV/visc
(S)-PEA
261 (0.40)
265.5 (+2.77)
265 (−2.56)
217 (+3.47)
(S)-PEA complexes of the achiral phenazino-18-crown-6
host 6, the band positions and intensities, relative to those
in the UV/vis spectra of the host and guest molecules,
clearly show merging of the ␲ electron systems. The spec-
261 (+0.97)
276 (−1.07)
293 (−2.43)
293 (+2.79)
362 (0.43)
420 (0.32)
346 (−0.31)
434 (+0.61)
368 (0.39)
428 (0.24)
346 (+0.30)
434 (−0.62)
366 (−0.30)
431 (+0.18)
367 (0.44)
428 (0.44)
357 (+0.26)
429 (−0.24)
353 (+0.19)
429 (−0.29)
368 (0.44)
429 (0.25)
353 (−0.20)
428 (+0.20)
369 (0.44)
426 (0.21)
437 (−0.12)
tra contain both charge transfer (205, 206, and 276, 277
nm) and induced CD bands (>300 nm) (Fig. 2). In contrast
to the CD spectrum of the PEA complexes of bands of the
achiral pyridino host 4, the sign of the charge transfer
112
SOMOGYI ET AL.
TABLE 1. Continued.
(R,R )-7
␭max [nm] (⌬␧) or (␧)
Guestb
Host
−
202 (−4.19)
232 (+6.80)
UV/visc
(R)-NEA
UV/visc
(S)-NEA
212 (+28.1)
225 (−101)
222 (7.00)
268 (+43.6)
284 (−6.76)
270 (6.12)
211 (−50.1)
225 (+179)
273 (−73.7)
(R)-PEA
236 (+6.22)
255 (+4.67)
UV/visc
276 (−5.97)
270 (6.50)
(S)-PEA
214 (−3.68)
233 (+7.23)
216 (−14.1)
238 (+6.14)
264 (+11.7)
279 (−3.74)
264 (+10.1)
279 (−3.82)
270 (7.30)
(S)-PGMA
221 (+13.2)
250sh (+4.57)
277 (−2.88)
(R)-PAMA
233 (+5.63)
257sh (+1.10)
275 (−6.10)
(R)-PGMA
UV/visc
UV/visc
−
−
−
−
−
−
−
−
−
−
−
−
265 (+5.22)
278 (−6.87)
269 (6.97 )
269 (7.20)
(S)-PAMA
231 (+8.21)
BAPd
232 (+5.51)
(R)-NEA
UV c
(S)-NEA
(R)-PEA
UVc
(S)-PEA
(R)-PGMA
UV c
(S)-PGMA
(R)-PAMA
UV c
(S)-PAMA
222 (−16.3)
222 (7.43)
222 (+15.5)
214 (+0.64)
210 (+0.63)
205 (0.79)
210 (−0.51)
264 (+8.69)
279 (−5.74)
264 (+3.25)
278 (−3.25)
282 (+0.52)
280 (0.53)
282 (−0.56)
261 (−0.08)
294 (4.61)
358 (+0.46)
430 (−0.86)
363 (0.45 )
423 (0.32)
368 (+1.13)
436 (−1.11)
370 (0.47 )
428 (0.28)
359 (+0.70)
437 (−2.10)
361 (+0.60)
436 (−0.84)
368 (0.47 )
430 (0.28)
359 (1.08)
436 (−1.74)
358 (+0.97)
438 (−1.49)
369 (0.49)
430 (0.27 )
362 (+0.76)
439 (−0.69)
366 (+0.60)
439 (−0.44)
369 (0.50)
430 (0.24)
363 (+0.62)
437 (−0.97)
362 (+0.72)
436 (−0.58)
312 (0.028)
256 (0.017 )
214 (−0.53)
218 (−10.1)
261 (+0.08)
262 (+0.23)
262 (0.019)
262 (−0.19)
204 (0.64)
218 (+8.33)
218 (−3.25)
206 (0.54)
218 (+3.86)
259 (−0.015)
258 (0.011)
259 (+0.017)
a
For comparison, published CD data on the complexes of NEA, PEA, and BAP with the chiral pyridino host, (S,S)-52 and the chiral phenazino host
(R,R)-73, are also enlisted together with the CD and UV parameters of the enantiomeric perchlorate salt guests.1,2
b
Perchlorates.
c
In italics: UV/vis spectral parameters (␭max [nm], ␧ × 10−4) of the hosts, (R)-guests and their complexes.
d
Butylammonium perchlorate.
bands follows the sign of the CD bands of the chiral guest
molecule. The spectra are rich in weaker bands that can be
explained by the presence of more than one conformer
(Table 1).
Practically no change, relative to the 1La band region of
the guests, was observed in the spectrum of the complexes
of PGMA and PAMA with the achiral phenazino-18-crown-6
compound 6, but relatively strong bands were measured
above ∼240 nm in the 1Bb, 1La, and 1Lb regions of the
spectrum of the host (Table 1). Centered near the ␭max
value of the 1Bb band in the UV/vis spectrum of the host,
the complexes of PGMA showed two CD bands of the
same sign, while those of PAMA two bands of opposite
sign. The intensities of the CD bands, induced in the spectra of the achiral host 6, were lower than those measured
in the CD spectrum of the related chiral crown (R,R)-7
(Table 1). As expected, the induced CD bands in the spectra of the complexes formed with the (R)- and (S)enantiomers of the guests were in mirror image relationship. (Small differences in band positions and amplitudes
of the expected mirror image spectra are due to the slightly
different optical purity of the guests.) Again, the multiplicity of bands is a sign of the presence of more than one
conformer.
COMPLEXES OF ACHIRAL CROWN LIGANDS
113
Figure 2. CD spectra in acetonitrile of the 1:1 complexes of the achiral phenazino-18-crown-6 host (6) with (R)-PEA (—䊏—) and (S)-PEA
(—䉲—), as well as CD spectra of (R)-PEA (—) and (S)-PEA (- - -).
Induced, Charge Transfer, and Exciton CD Bands in
the Spectra of the NEA Complexes of Achiral Hosts
In the mirror-image CD spectra of the complexes of the
achiral pyridino-18-crown-6 host 1 with (R)-or (S)-NEA
bands show up near to the ␭max values of the bands in the
spectra of NEA and the simplest corresponding chiral host
(S,S)-2 (Fig. 3, Table 1). Interestingly, the band at 224, 225
nm in the spectra of the complexes is much less intense
(⌬␧ = -5.26) than the 1Bb band of NEA at 222 nm (⌬␧ =
-16.3) (values are given for (R)-NEA and its complex, see
Table 1). Contrary to this, the band at 265.5 nm (⌬␧ =
+2.77) of the complex of 1 with (R)-NEA is much more
intense than the 1Lb band of the chiral host (S,S)-2 at 262
nm (⌬␧ = -0.17) or the band at 282 nm of NEA (⌬␧ +0.52 for
the R-isomer, Table 1). It is very likely that the complex is
present as a mixture of two major conformers. The population of the conformers is different because of the increased steric constraint of the conformer in which the
CH3 group of NEA is located closer to the CH2 group
attached to the pyridine ring (see also Fig. 5 and the discussion on the chiroptical properties of the NEA complexes of the achiral phenazino host 6). The two conformers are expected to have nearly mirror-image CD spectra
due to the nearly enantiomeric relationship of the interacting aromatic ring systems. The polarization directions of
the pyridine 1La and naphthalene 1Bb transitions are likely
close to perpendicular in the complexes that forbids a
strong interaction between these chromophores. This results in a sum spectrum with a decreased 1Bb band intensity of the guest. Contrary to this, the polarization directions of the 1Lb transitions are nearly parallel in the enantiomeric complexes. This may explain the strong induced
CD bands near the 1Lb transition of the achiral pyridine
host and the relatively large difference in the intensity of
the long-wavelength band measured for the conformer
mixture.
The CD spectra of the NEA complexes of the achiral
hosts 1 and 4 are significantly different. The spectra of the
complexes of 4 with bands at 206 and 230 nm (Table 1)
suggest the presence of one dominant conformer due to
the charge transfer interaction between the naphthalene
chromophore and the extended planar ␲-electron system
of the host (note the increased intensity and redshift of the
band at 230 nm, relative to position of the bands at 224 nm
and 222 nm in the spectra of the components).
In the CD spectrum of the complex of the chiral host
(S,S)-5 with (R)-NEA the strong band at 205 nm (⌬␧ =
-65.1) is a clear-cut sign of exciton interaction.2 However,
the positive band at 222 nm is extremely weak (⌬␧ = +2.32)
and another positive band below 205 nm could not be observed in the spectrum (Table 1). According to the known
X-ray structure of the heterochiral complex,4,5 there is a
torsion angle of -57° between the long axes of the rings
which lie nearly parallel. Coupling between the pyridine
1
La and naphthalene 1Bb transitions or 1Bb transitions of
both chromophores is not probable on the basis of either
the band positions and intensities or the sign of the couplet
(Table 1). In spite of the similar planar geometry but opposite sign of the angle between the long axes of the rings
in the crystal,4 the CD spectrum of the homochiral complex measured in solution2 does not show exciton coupling
(Table 1). This demonstrates the power of CD spectroscopy in revealing differences between the structures of
supramolecular complexes in the crystal and in solution.
114
SOMOGYI ET AL.
Figure 3. CD spectra in acetonitrile of the 1:1 complexes of the achiral
pyridino-18-crown-6 host (1) with (R)-NEA (—) and (S)-NEA (- - -).
The positions, signs, and high intensities of the bands in
the mirror-image CD spectra of the achiral host 6/(R)NEA or 6/(S)-NEA complexes indicate the presence of
one or two exciton couplets (Fig. 4), similar to the spectra
of the NEA complexes of the chiral host (R,R)-7 (Table 1).3
To investigate this, we performed quantum chemical rotatory strength calculations for the homochiral (R,R)-7/(R)NEA complex of known X-ray geometry.6 The result of
these calculations is presented in Figure 6, where the rotatory strengths (CD band intensities) are indicated by
simple lines placed at experimental excitation energies.
The similarity between the computed and measured spectrum is apparent (cf. Fig. 6 with the data in Table 1). According to the assignment of the computed spectrum, the
negative band at ∼225 nm and the positive one at ∼270 nm
form a real couplet, the former originating from a transition
(1Bb) on NEA as a chromophore coupled with all host
transitions, while the latter is primarily a transition on host
coupled intensively with NEA excitations. Since all the
other transitions between 200 are 250 nm must be assigned
to the guest according to the calculations, the one mentioned above is the only strong couplet we can see in the
investigated range.
The positive sign of the couplet in the CD spectrum of
the (R,R)-7/(R)-NEA complex suggests a positive torsion
angle between the long axes of the rings. (This is in agreement with the structure of the homochiral complex in the
solid state.6) Based on the same sign pattern of the CD
bands below 300 nm of the (R,R)-7/(R)-NEA and 6/(R)NEA complexes, the torsion angle between the long axes
in the rings in the complex of the achiral host 6 is expected
to also be positive (Fig. 5a). The appearance of an exciton
couplet in the spectra of the NEA complexes of 6 clearly
shows the dominance of one conformer. In the other possible orientation, characterized by a negative torsion angle
between the axes (Fig. 5b), the aromatic chromophore system is approximately in mirror-image relationship with that
in the previous case (Fig. 5a). The two conformers of each
complex differ in the position of the methyl group of the
guest relative of the macroring oxygen atoms. In the prevailing conformer of the 6/(R)-NEA complex (Fig. 5a), the
methyl group is oriented towards O5 and O8. These oxygens are located in the more flexible half of the macroring
which allows distortion of the molecule due to the repulsion between the methyl group and oxygen atoms.
Theoretical calculations were also performed on phenazine and its 1,9-dimethoxy substituted derivative modeling the achiral host molecules. The aim of these relatively
simple calculations was merely to obtain a qualitative picture about the ground state charge distributions of substituted phenazine and the NEA salt. The atomic charges
were evaluated in several basis sets, all showing the same
Figure 4. CD spectra in acetonitrile of (R)-NEA (—), (S)-NEA (- - -) and the 1:1 complexes of the achiral phenazino-18-crown-6 host (6 with (R)-NEA
(—䊏—), (S)-NEA (—䉲—). Insert: long-wavelength region of the spectra.
COMPLEXES OF ACHIRAL CROWN LIGANDS
Figure 5. Possible orientations (a and b) of the phenazine and naphtalene rings in the complex of the achiral phenazino-18-crown-6 host (6)
with (R)-NEA (Top). Torsion angles between the short or and long axes of
the rings and the corresponding polarization directions (a and b) of the 1La,
1
Bb and 1Cb transitions (Bottom)
qualitative picture. (The numbers quoted in Fig. 7 were
obtained in the 6-311G basis.)
Net atomic charges of phenazine were calculated for a
trans planar conformation in which the carbon atom and
one of the hydrogens of both methyl groups lie in the plane
of the heterocycle (Fig. 7). Calculations in several basis
sets indicate that the symmetric electron density of phenazine is strongly distorted due to the effect of the methoxy
groups: the annealing C atoms and C1 and C9 show electron deficiency, while the nitrogens and outer carbon at-
115
oms bear partial negative charge. This results in alternating, oppositely bent negative and positive charge bands
above the rings, which may serve as a template for ␲–␲
interaction between the phenazine and naphthalene ring.
Ab initio calculations were also performed on the salt
NEA, for the s-cis rotamer found in the crystal of both the
hetero- and homochiral complexes.6 Electron density of
NEA shows a relative increase of the negative charge at
atoms 1 and 8 of the naphthalene ring (Fig. 7). Comparing
the X-ray crystallographic structure6 of the homochiral
[(R,R)-(R)] and heterochiral [(R,R)-(S)] complexes of
(R,R)-7 and NEA, the fit between the oppositely charged
regions of the phenazine and naphthalene rings appears to
be better in the homochiral than in the heterochiral complex (see also Fig. 5). It is the almost complete fit between
the higher and lower electron density regions of phenazine
and the naphthalene ring which results in increased ␲–␲
interaction in the sterically more crowded homochiral complex. The increased attraction, also explaining the larger
interplanar angle (∼14.5 vs. 7.3°) between the phenazine
and naphthalene rings in the homochiral complex,6 compensates in part for the increased steric repulsion. The
overall effect of ␲–␲ interaction and steric repulsion is the
somewhat decreased stability of the homochiral complex.
X-ray crystallographic data are not available for the NEA
complexes of the achiral host 6, but the positive sign of the
couplet in the spectra of the complexes of (R)-NEA with
both the achiral host 6 and (R,R)-7 indicates the same
positive sign of the torsion angle between the axes and a
geometry of the (R)-NEA/6 complex (Fig. 5a), which is
similar to that of the homochiral (R)-NEA/(R,R)-7 complex.6 This geometry of the (R)-NEA complex of the achiral
Figure 6. Computed CD spectrum of the host 6/(R)-NEA complex. Rotatory strengths were obtained by the coupled oscillator model from ab initio
calculated transition moments, and depicted at experimental excitation energies
116
SOMOGYI ET AL.
host 6 is favored by the presumably more efficient ␲–␲
interaction in the “homochiral-like” arrangement. Because
of the lack of the methyl groups in host 6 (Fig. 5a), this
arrangement is not hindered by the repulsion of the naphthalene hydrogens and the upward-oriented methyl group
as in the complex of chiral host (R,R)-7.
A comparison of the CD spectra of the NEA complexes
of the achiral pyridino- and phenazino-hosts 1, 4, and 6
shows a striking difference in the mechanism of interaction
between the aromatic chromophores. The NEA complexes
of the achiral host 1 are likely present as a mixture of two
conformers that renders spectral analysis difficult. In the
spectra of the complexes of pyridino host 4, ␲–␲ interaction brings about bands in the 1La and 1Bb spectral regions
of the host which have the same sign as the 1Bb band in the
spectrum of the guest (Table 1). These spectra show no
exciton features contrary to those of the NEA complexes of
the achiral phenazino-host 6, which are determined by exciton coupling.
CONCLUSION
A comparison of the CD and UV/vis data on the PEA,
PGMA, PAMA, and NEA complexes of chiral and achiral
hosts (Table 1) allowed us to characterize the possible
types of interaction between the aromatic chromophores.
Exciton Interaction
1. Weak exciton interaction was detected in the CD spectra of the heterochiral NEA complexes of chiral pyridinohosts (S,S)-2, (R,R)-3, and (S,S)-5.2 The finding that an
exciton couplet was not observed for the less stable homochiral complexes and the appearance of a low intensity,
asymmetric couplet in the CD spectra of the heterochiral
pyridine complexes emphasize the importance of the geometric factor as well as the size and polarizability of the ␲
electron system of the host. The CD spectra of the complexes of the achiral host 1 with (R)- or (S)-NEA are governed by the flexibility of the host that gives rise to the
presence of a major and a minor conformer.
2. The general feature of the CD spectra of both the
homo- and heterochiral NEA complexes with chiral phenazino-hosts is exciton interaction. Exciton band splitting
can be observed even for the heterochiral complexes of
chiral phenazino hosts having larger substituents in positions 3 and 13.3
3. The most interesting exciton couplet is seen in the CD
spectra of the complexes of the achiral phenazino host 6
with (R)- or (S)-NEA (Fig. 4). The mirror-image CD spectra are predominated by exciton coupling between the 1Bb
transition of the chiral guest and the 1Bb transition of the
achiral host. Apparently, the ␲ electron systems of the
rings are sterically not close enough to merge into one
joint chromophore. From the sign of the couplet the relative position (torsion angle between the short or long axes)
of the aromatic rings in the complex of (R)- or (S)-NEA
could be concluded.
4. Exciton splitting was not seen in the CD spectra of
PEA, PGMA, or PAMA complexes of chiral phenazinohosts. This can be explained by the ␲ electron density map
of the phenazine chromophore, the presence of conformer
mixtures, or by the possible formation of host–host dimer
complexes.3
Charge Transfer Interaction and Induced CD
Figure 7. Net atomic charges, obtained on symmetry-inequivalent
heavy atoms of dimethoxy phenazine (a) and (S)-NEA (b).
1. While the complexes of the achiral pyridino-host 4
and the chiral guests PGMA or PAMA showed CD spectra
reflecting only weak interaction, (R)- and (S)-PEA resulted
in mirror-image spectra which are indicative of merging
the ␲ electron systems into one joint redshifted charge
transfer chromophore. This spectral behavior clearly differs from that of the achiral host 4/NEA complexes. In the
latter case the chiral guest induced bands with ␭max values
corresponding to the UV bands of the host. The less extended ␲ electron system of the simplest achiral pyridino
host 1 cannot anchor the NEA molecule. The low-intensity
bands in the 1Bb spectral region of the guest strongly suggest the presence of two conformers separated by a low
energy barrier.
COMPLEXES OF ACHIRAL CROWN LIGANDS
2. As in the case of the PEA complexes of the achiral
pyridino host 4, the CD spectra of the achiral phenazino
host 6/PEA complexes are also determined by ␲–␲ interaction which induces bands even in the long-wavelength
spectral region of the achiral host.
3. The weak ␲–␲ interaction between the achiral host 6
and PGMA or PAMA does not have a definite spectral
effect in the 1La region of the chiral guest but its existence
is proven by the weak CD bands induced in the longwavelength spectral region of the achiral host.
In summary, comparative studies on the chiroptical
properties of aralkyl ammonium salt complexes of chiral
and achiral pyridino- and phenazino-18-crown-6 hosts
clearly showed that the CD spectrum depends on the distance and relative orientation of the chromophores, which
are determined by the strength of ␲–␲ interaction between
the aromatic chromophores. The distance between the
rings, determined by the tripod-like H-bonding, allows exciton interaction in the NEA complexes. PEA complexes,
especially those of the achiral hosts, are characterized by
charge transfer interaction. However, effective merging of
the ␲ electron systems is prevented by the tripod Hbonding. Besides, the rotation around the N-Ca bond in the
complexes of the achiral hosts is less hindered, which results in the presence of more than one conformer and in
the appearance of an average CD spectrum.
ACKNOWLEDGMENT
The authors thank Edit Szabó for skillful assistance in
preparing the manuscript.
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