Derivatization of 4-(dimethylamino)benzamide to dual fluorescent ionophores: Divergent spectroscopic effects dependent on N or O amide chelation

Article abstract of DOI:10.1002/1522-2675(20010919)84:9<2439::AID-HLCA2439>3.0.CO;2-S

Starting from the pentafluorophenyl ester of 4-(dimethylamino)benzoic acid, two dual fluorescent amide ligands with aza-15-crown-5 and 2-(aminomethyl)pyridine were obtained for sensing, respectively, alkali (alkaline-earth) and transition (heavy) metal cations. The crystal structure of the copper(II) complex is reported. The Cu²⁺ is coordinated through the pyridine N- and amide O-atoms of two symmetry-related ligands. The azacrown-directed Ca-chelation to the N-atom of the amide leads to a slight quenching of the two fluorescence bands. In contrast, the pyridine directed Cu¹¹-chelation to the O-atom of the amide enhances the short-wavelength emission 17-fold over the locally excited state (LE), quenching the twisted intramolecular charge-transfer (TICT) emission, and, as a result, the intensity ratio I(LE)/I(TICT) provides an accurate and sensitive measurement of the Cu¹¹ concentration. These different cation effects are dependent on which atom (N vs. O) of the amide function participates in cation coordination: while the Ca²⁺ interaction with the N-atom electron pair leads to the deconjugation of the amide N-atom from the fluorophore, Cu²⁺ interaction with the lone pair of the O-atom of the carbonyl group increases the energy of the n-π* but also of the ¹L_a transition and therefore close the channel to the TICT state.

Full text of DOI:10.1002/1522-2675(20010919)84:9<2439::AID-HLCA2439>3.0.CO;2-S

Helvetica Chimica Acta ± Vol. 84 (2001)
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Derivatization of 4-(Dimethylamino)benzamide to Dual Fluorescent
Ionophores: Divergent Spectroscopic Effects Dependent on N or O Amide
Chelation
by Jean-Pierre Malval and Rene Lapouyade*
Â
Â
Laboratoire dꢀAnalyse Chimique par Reconnaissance Moleculaire (LACReM), Ecole Nationale Superieure de
Chimie et de Physique de Bordeaux (ENSCPB), F-33607 Pessac Cedex
(Phone: ‡‡ 33556846270; e-mail: lapouyad@enscpb.u-bordeaux.fr)
Â
Dedicated to Professor Andre M. Braun on the occasion of his 60th birthday
Starting from the pentafluorophenyl ester of 4-(dimethylamino)benzoic acid, two dual fluorescent amide
ligands with aza-15-crown-5 and 2-(aminomethyl)pyridine were obtained for sensing, respectively, alkali
(alkaline-earth) and transition (heavy) metal cations. The crystal structure of the copper(II) complex is
reported. The Cu2‡ is coordinated through the pyridine N- and amide O-atoms of two symmetry-related ligands.
The azacrown-directed Ca-chelation to the N-atom of the amide leads to a slight quenching of the two
fluorescence bands. In contrast, the pyridine directed CuII-chelation to the O-atom of the amide enhances the
short-wavelength emission 17-fold over the locally excited state (LE), quenching the twisted intramolecular
charge-transfer (TICT) emission, and, as a result, the intensity ratio I(LE)/I(TICT) provides an accurate and
sensitive measurement of the CuII concentration. These different cation effects are dependent on which atom (N
vs. O) of the amide function participates in cation coordination: while the Ca2‡ interaction with the N-atom
electron pair leads to the deconjugation of the amide N-atom from the fluorophore, Cu2‡ interaction with the
lone pair of the O-atom of the carbonyl group increases the energy of the n-p* but also of the 1La transition and
therefore close the channel to the TICT state.
1. Introduction. ± The detection of ions in various media by fluorometry, a very
sensitive and nondestructive analytical method, is often realized with fluorescent
sensors [1]. Generally, fluoroionophores are modular systems where the ionophore is
either attached to the fluorophore through an isolating chain or is an integral part of the
p-electron system [2]. While, in the former case, ion complexation produces a change in
fluorescence intensity, for the later system, both spectral and intensity changes of the
fluorophore emission occur, allowing measurement of the recognized species from the
intensity ratio at two emission wavelengths with opposite ion-sensitivity responses [3].
These intrinsic probes are usually designed as donor-acceptor substituted p-conjugated
systems. In most cases, the ionophore is the electron-donor substituent of the
fluorophore and coordination of a cation leads to a pronounced blue shift of the
intramolecular charge transfer (ICT) absorption band [4] but often to only small
changes in fluorescence because of the decoordination of the cation from the
fluorophore excited state [5 ± 7]. When the excited state of the probes is weakly polar, a
new emission band, a mirror-image of the absorbance of the complex, appears [8].
There are a few examples where the cation interacts with the electron-acceptor group
of the fluorophore. As a result the ICT process is increased and there is a red shift in the
absorbance and sometimes a higher fluorescence quantum yield results [9]. Even from
electronically symmetrical probes, a cation directing and enhancing the ICT process in

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the excited state of the chromophore with a red-shifted emission has been reported
[10]. Dual emitting fluorophores, which fluoresce from two excited states of very
different charge-transfer character provide an unique opportunity to sense the
influence of cation binding on the ICT transition.
Precisely, we showed that 4-(dimethylamino)benzonitrile (DMABN) with two
fluorescing excited states, the locally excited (LE) state and the twisted intramolecular
charge transfer (TICT) state, of considerably different charge-transfer character [11],
could be derivatized to 4-azacrowns benzonitrile, (DMABN-crown), a fluoroiono-
phore that signals the presence of Ca2‡ by an increase in the relative intensity of the
emission band from the less-polar excited state [12]. Collins et al. described 4-(1,4,8,11-
tetraazacyclotetradec-1-yl) benzonitrile (DMABN-cyclam), which, in addition to the
fluorescence of the LE and TICT states, leads to an intramolecular exciplex
fluorescence, and uniquely responded to the addition of micromolar quantities of
transition-metal cations with varying changes in the triple fluorescence emission [13].
We report now the derivatization of the acceptor part of 4-(dimethylamino)benzamide
(DMABA), a compound that also gives two fluorescence bands [14], into two classes
of fluoroionophores: i) when the N-atom of the amide is part of a monoaza-15-crown-5,
the corresponding DMABA-crown5 should be a dual fluorescent sensor for s-block
alkali and alkaline-earth metal cations interacting with the fluorophore through the N-
atom of the amide function (Class A); ii) when the N-atom of the amide bears an o-
methylenepyridine group (DMABA-MP), transition metal ions, particularly CuII
cations, which have a high thermodynamic affinity for typical N,O-chelate ligands
[15], should be bound by the N-atom of the pyridine and the O-atom of the amide
group (Class B).
If the coordination of cations by these two models of molecular probes leads to
different spectroscopic effects (absorbance or/and emission), they should provide
useful optical characteristics for a sensitive test of the cations coordination sites of the
widespread amide derivatives.

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2. Results. ± DMABA-Crown5 and DMABA-MP have been easily obtained from
the pentafluorothiophenyl ester of 4-(dimethylamino)benzoic acid [16], as a common
intermediate, by reaction with monoaza-15-crown-5 and 2-(aminomethyl)pyridine,
respectively.
Excited-State Polarity of the Two Probes. The absorbance spectra were found to be
rather insensitive to the increase of solvent polarity. From a previous study of DMABA
and the N,N-dimethyl and N,N-diethyl derivatives [14], we can assign the distinctive
1
shoulder around 300 nm, in nonpolar solvents, to the Lb-type transition and the
stronger band at 280 nm to 1La-type transition which shifts to the red in polar solvents
(Fig. 1,a).
In relation to the first excited-state inversion, the fluorescence spectra of DMABA-
Crown5 and DMABA-MP show a considerable evolution from a single emission in
hexane to a major TICTemission in MeCN (Table 1 and Fig. 1,b). This is the result of a
higher dipole moment in the relaxed fluorescing state compared to the ground state. If
we assume that the contribution of the change of polarizability Dag e between ground
and excited states is negligible, an estimate of the dipole moment from fluorescence
solvatochromism can be obtained by the Lippert-Mataga equation [17][18]
nfluo
ˆ
2 mc (me mg) Df '/hcr3 ‡ const.
(1)
where h is Planckꢀs constant, c the speed of light, nfluo the fluorescence maximum in a
solvent of dielectric constant e and refractive index n, and Df ' ˆ [(e 1)/(2e ‡ 1)] 0.5
[(n2 1)(2n2 ‡ 1)] is the solvent polarity parameter. The slope of a plot of nfluo vs. Df '
gives the value of me(me mg)/13. This allows estimation of the excited-state dipole
moment (me) from the solvent cavity (Onsager) radius (1) and the ground-state dipole
moment (mg), provided that the dipole-moment direction does not change from ground
to excited states.
Table 1. UVAbsorption Maxima (labs), Fluorescence Emission Maxima (lfl), and Fluorescence Quantum Yields
(ffl) of DMABA-Crown 5 and DMABA-MP
Compound
Solventa)
labs
lfl[nm]
ffl
[nm]
LE
TICT
LE
TICT
DMABA-Crown5
Hexane
Bu2O
Et2O
276
278
279
280
282
284
291
297
298
334
343
346
350
359
332
342
351
359
±
±
0.020
0.010
0.004
0.005
0.002
0.009
0.007
0.023
0.001
±
±
390
421
484
±
366
455
474
0.004
0.006
0.008
±
0.005
0.072
0.006
THF
MeCN
Hexane
Et2O
THF
MeCN
DMABA-MP
The structures of the two probes have been drawn with HyperChem and optimized
with the AM1 method. The radius (1) of the equivalent sphere for the molecular
volume of the fluorophore is estimated to be 0.5 nm, and, from the ZINDO/S method,
we calculated the dipole moments in the ground state, mg ˆ 3.6D. With these values, the
slope of nfluo vs. Df ' leads to the dipole moments in the excited states: for MDABA-

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Fig. 1. Normalized a) absorbance and b) fluorescence spectra of DMABA-MP in hexane (±Ð), Et2O (± ± ± ±),
THF (----), butyronitrile (± - ± - ±), and MeCN (± - - ± - - ±)
Crown5, mLE ˆ 14D and mCT ˆ 24D and for DMABA-MP, mLE ˆ 11.3D and mCT ˆ 20.2D.
This indicates that full CToccurs sequentially via relaxation of the LE state to the TICT
state, and that the rate constant for this reaction increases with the polarity of the
solvent and results in a greater relative intensity of the TICTemission in polar solvents.
Cations-Complexation Effects. The addition of alkali and alkaline-earth metal ions
to DMABA-Crown5 in MeCN solution shifts the absorbance maximum to lower
energies, as a result of the cation-enhancement of photoinduced charge transfer
1
[9][10][19]. The larger shift is obtained for Ca2‡ (2820 cm ) because of its charge

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density and the closer size compatibility with aza-15-crown5 (1.98 Š for Ca2‡, 1.7 ±
2.2 Š for the estimated diameter of the azacrown5 [20]). The complexation constant
derived, according to the treatment described in [9] is log Kc ˆ 3.65 (Fig. 2,a).
Surprisingly, the fluorescence spectrum is even less modified upon Ca2‡ complex-
ation: the two bands are slightly red-shifted, and their relative intensities hardly
changed (If(TICT)/If(LE) ˆ 4) in the absence of Ca2‡, and this ratio reaches 5 when the
Fig. 2. Absorbance spectra of a) DMABA-Crown5 (1  10 5m) with increasing concentrations of Ca2‡ in MeCN,
b) DMABA-MP (4 Â 10 6m in MeCN/H2O 1:1 with Cu(ClO4)2 (3.2 Â 10 3m) after subtraction of the absorbance
corresponding to Cu(ClO4)2, showing the slow complexation

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probe is fully complexed. The fluorescence quantum yield is reduced by half by Ca2‡
complexation, because the expected increased rate of formation of the TICT state is
counterbalanced by its reduced lifetime (from 2.7 ns to 2.2 ns). On the whole, cation
complexation by the azacrown leads to a deconjugation of the N-atom of the amide and
the complexed probe presents spectroscopic characteristics similar to those of 4-
(dimethylamino)benzaldehyde [21].
DMABA-MP was designed for signalling transition metals through ligation to the
soft N-atom of pyridine and the O-atom of amide. The later interaction was expected to
lower the radiationless transition and, as a result, to enhance the fluorescence yield
[14].
We present the spectroscopic properties dependence on pH to define the useful pH
range for sensing transition metal cations. In MeCN/H2O 1:1 solution, the absorbance
and fluorescence spectra do not change with the pH in the range 5 ± 14. At lower pH,
protonation of the N-atom of the pyridine ring (pKa ˆ 3.6) leads to a small red-shift in
the absorbance (lmax shifts from 306 nm to 315 nm) and to an effective quenching of the
fluorescence arising from electron transfer to the pyridinium moiety (Fig. 3).
Fig. 3. a) Absorbance and b) fluorescence spectra of DMABA-MP in MeCN/H2O 1 :1 as a function of pH.
Insets: pKa determination from a) absorbance at l ˆ 300 nm and b) fluorescence at l ˆ 365 nm.
At neutral pH, several cations (Zn2‡, Co2‡, Ni2‡, etc.) were added without marked
changes in the absorbance spectrum, but, with Cu2‡, there is a clear blue shift of the 1La-
type transition absorbance and appearance of the weak metal-centered transition at
longer wavelengths (Fig. 2,b). The absorbance spectrum becomes stable after 70 h, at
room temperature. Similarly, the fluorescence spectra were not significantly altered by

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the different transition metal cations, except with Cu2‡, which led to a pronounced
increase in the first band (17-fold) and disappearance of the TICT emission (Fig. 4).
The change of the fluorescence intensity upon Cu2‡ addition, is time dependent, and an
approximately constant signal is obtained after 70 h (Fig. 4, Inset a). This time
evolution reveals a slow complexation, not a chemical reaction because this process is
reversible: dilution of the cation with aliquots of the probe solution restored the initial
emission.
6
Fig. 4. Fluorescence spectra of DMABA-MP (4 Â 10 M) in MeCN/H2O 1:1 with different concentrations of
Cu(ClO4)2 (Insets: a) DMABA-MP‡10 equiv. of CuII, time-dependent fluorescence enhancement; b) front-
face fluorescence from crystals of (DMABA-MP)2 ´ Cu(ClO4)2 ´ 3H2O.
Analysis according to [9] of the evolution of the fluorescence spectra as a function
of the cation concentration, at equilibrium (Fig. 4) reveals the formation of a complex
with two ligands for Cu2‡. The apparent association constant for this 1:2 stoichiometry
is log K21 ˆ 8.3 Æ 0.2.
This is the first example of cation coordination on the acceptor side of a D-A
chromophore leading to reduced charge transfer in the excited state. This property is
perhaps useful as a probe for Cu2‡ because of the enhanced fluorescence, particularly of
the ratio I(LE)/I(TICT), although the slow complexation precludes rapid analysis.
Structural Analysis of Cu2‡-Complexed DMABA-MP. To ascertain the coordination
sphere of Cu2‡ with DMABA-MP, we performed an X-ray crystal-structure analysis,
shown in Fig. 5.

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Fig. 5. X-Ray structure of (DMABA-MP)2 ´ Cu(ClO4)2 ´ 3 H2O
The cation Cu2‡ is bound to the ligand by the pyridine N(21) and the carbonyl O(17)
atoms, forming two short bonds of, respectively, 1.992(4) Š and 1.951(4) Š with a bite
angle of 92.5(2) Š (Table 3). The two ligands are in the pseudo-equatorial square plane
of the complex while a H2O molecule is located in the axial position, Cu O(3)

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(2.998(4) Š). This cation is counterbalanced by two perchlorate anions. This confor-
mation is essential for the fluorescent properties.
At the electron-donor side, the inversion angle q between the plane through the
three atoms C(7), N(9), and C(8) of the dimethylamino group and that of the phenyl
ring amounts to 0.9(1)8 typical for a completely planar sp2 N-atom. The sum of the
angles around the amino N-atom N(9) being 3608 is also in agreement with a planar sp2
N-atom. It is noteworthy that DMABN, which is the archetype of related dual
fluorescent fluorophores, presents some pyramidalization of this N-atom (q ˆ 10.88)
Table 2. Crystallographic Data, Data Collection, and Refinement Parameters for (DMABA-MP)2 ´ (CuClO4)2
´ 3 H2O
Chemical formula
Formula weight
Crystal system
Space group
Z
(C15H17N3O)2 ´ Cu(ClO4)2 ´ 3H2O
827.1
Monoclinic
C2/c
4
Cell parameters
a, b, c [Š]
16.5384(5), 16.4926(4), 16.0260(4)
b [8]
120.56(2)
3764(2)
1.460
V [Š3]
3
d [g cm
m (mm
]
1
)
0.818
Data collection
Device
T (K)
Kappa-CCD
298
Reflections collected
Observed data [I > 3s(I)]
R, wR, S
4469
2700
0.058, 0.086, 1.89
Table 3. Selected Bond Lengths [Š], Bond Angles [8], and Dihedral Angles [8] of (DMABA-MP)2 ´ Cu(ClO4)2
´ 3H2O (compared to reference bond lengths, see text)
CSDa)
N(21) Cu(1)
O(17) Cu(1)
O(30) Cu(1)
C(16) O(17)
C(16) N(18)
C(13) C(16)
1.992(4)
1.951(4)
2.998(4)
1.258(5)
1.311(5)
1.456(5)
1.23(1)
1.34(2)
1.50(1)
O(17) Cu(1) N(21)
O(17) Cu(1) O(30)
C(16) O(17) Cu(1)
92.5(2)
86.0(1)
154.7(9)
C(11) C(10) N(9) C(8)
C(12) C(13) C(16) O(17)
C(14) C(13) C(16) N(18)
C(13) C(16) N(18) C(19)
C(16) N(18) C(19) C(20)
C(19) C(20) N(21) C(22)
179.3(5)
164.0(4)
163.6(4)
179.5(0)
72.9(4)
179.8(0)
a) From Cambridge Structural Database, bond lengths of uncoordinated, structurally similar amides.

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[22] while the twist angle f, defined as the angle between the a C-atoms of the amine
substituents and the Ph mean plane, amounts only to 0.58 [23].
We compared bond lengths of the electron-acceptor substituent coordinated to
Cu2‡, with an analysis of the Cambridge Structural Database (CDS) of structures
containing a Ph attached to the carbonyl of an amide, and the N-atom was substituted
with a sp3 C-atom (N-methylbenzamide model) [24]. The coordination of Cu2‡ leads to
a lengthening of the CˆO bond (1.258 vs. 1.23 Š), a shortening of the C N bond (1.31
vs. 1.34 Š) and of the C(13) C(16) bond (1.456 vs. 1.50 Š), and the torsion angle
between the planes of the benzene and the amide group ring (168) is in the lower limit
of the 11 ± 338 range statistically observed. These effects compare well with the changes
observed in the amide bond lengths upon metal coordination; specifically, an increase
in the carbonyl C O bond of 0.03 Š and a decrease in the carbonyl C-atom to N-atom
bond length of 0.025 Š [25].
The fluorescence spectra of the complex in the solid state (Fig. 5, Inset b) shows
only one band at the same wavelength as in solution, giving a strong support for a single
structure regardless of the phase.
3. Discussion. ± Molecules containing the amide functionality are potentially useful
in a variety of applications, including as complexing agents for the selective extraction
of f-elements and of precious metals, and as improved ligands for magnetic resonance
imaging agents. In a recent review [25] the results of a systematic search of the
Cambridge Structural Database (CSD) have been analyzed to help define the structural
features of metal-amide coordination chemistry. Various experimental techniques were
used to examine amide metal interactions in these bidentate ligands but, while the
multiple fluorescence of benzanilide was thoroughly investigated [26], we are not
aware of any fluorescence study of aromatic amides in relation to a regiospecific
molecular recognition of the amide function. Nevertheless, secondary amides are used
as anion-selective receptors for optical/electrochemical sensing, through H-bonding
[27]. The amide group is also used as a linker and serves as a conformational control
element in the construction of diad and triad molecules designed for photoinduced
electron transfer.
Electron transfer between a TICT state and electron donors or acceptors have been
reported [28]. For the quenching by electron acceptors, one can consider that only the
radical anion part is involved. Comparison of the reduction potentials of benzamide
(
2.2 V) [29], which models the acceptor part of DMABA-BP in the TICT state,
pyridine ( 2.62 V) [30], and of the pyridinium group ( 1.54 V) [31] (all vs. SCE)
shows that, only for the protonated pyridine, is the condition of exothermicity for TICT
quenching by electron transfer fulfilled.
Recently, Effenberger and co-workers reported a similar photoinduced intra-
molecular charge separation between the anthryl and N-methylpyridinium moieties
[32].
Regarding the different photophysical effects observed with the two probes, they
are obviously the result of regiospecific coordination of the metal cations.
With s-block alkali and alkaline-earth metal cations, the interaction with the N-
atom of the electron-accepting substituent leads to a red shift of the absorption. It is the
result of the cation-induced enhancement of the electron-withdrawing character of the

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carbonyl group through deconjugation of the N-atom. The electronic effect of the N-
chelated amide group becomes that of a ketone. We observed a small red-shift and a
decrease of the fluorescence yield of DMABA-Crown5 because the N-chelation not
only increases the charge-transfer character of the first p-p* transition, but also lowers
the energy of the n-p* transition and accordingly increases the nonradiative
deactivation with the result of a lowering the quantum yield of fluorescence.
Generally, the binding of a transition metal with an incomplete d level (3dn, n < 10)
causes quenching of the fluorescence emission of the probe through two possible
mechanisms: electron transfer or energy transfer. Both mechanisms have been
observed with Cu2‡ (n ˆ 9). With regard to electron transfer, oxidative quenching of
an anthracene-substituted fluorophore with Cu2‡ has been observed with S-donor
ligands that stabilize Cu1‡ [33], while amidate multidentate ligands that interact
strongly with Cu3‡ have resulted in reductive quenching of the same fluorophore [34].
For dansyl sulphonamides, the quenching rate constant remains high at 77 K, suggesting
at least a contribution from the energy-transfer mechanism [35].
Sensors in which the binding of Cu2‡ causes an increase in the fluorescence emission
are very rare. We are aware of a trianthrylcryptand reported by Bharadwaj and co-
workers, where the absence of quenching is assigned to the cryptate effect [36]. The
redox activity of the metal ion is suppressed, as donor atom rearrangement upon
oxidation/reduction is energetically unfavorable. Another example was reported by
Rurack et al. [37] with fluoroionophores, where the receptor units are less efficient
‡
quenchers of the fluorophore when they are complexed with Hg2‡, Ag or Cu2‡.
With DMABA-MP, the fluorescence enhancement observed upon copper(II)
binding does not belong to any of these mechanisms. As the low quantum yield of
fluorescence of these amides has been assigned to a nonradiative channel through the
n-p* states [14], we propose that the copper coordination to the carbonyl raises the
energy of this state with the result of a slowing down of the nonradiative transitions.
Actually, copper coordination not only increases the yield of fluorescence, but, more
precisely, it is the first LE state, the less polar one, that becomes highly emitting and
does not lead anymore to the TICT state. The absorbance spectrum of the CuII complex
is shifted to the blue. This means that the interaction of Cu2‡ with the carbonyl group
decreases the acceptor strength of the amide group, in contrast with the effect of alkali
and alkaline-earth metal cations in coordination in esters. This can be tentatively
rationalized by an increased contribution of the charge-separated resonance form
involving the N-atom of the amide group as donor upon cation coordination (shorter
C
N bond). When this contribution is negligible (ester vs. amide) the spectroscopic
results are opposite. Thus alkali and alkaline-earth metal cations interacting with
oxygen atom of the carbonyl group of esters lead to a red-shift of the absorption and a
huge increase of the fluorescence quantum yield assigned to an increase of the electron
accepting character of the carbonyl group and of the n-p* state energy respectively
[19].
4. Conclusion. ± We have shown that electronic absorbance and emission of
substituted benzamides provide sensitive techniques to identify which atoms in the
amide function are bound to the metal ion. Moreover, we have obtained a selective
Cu2‡ sensor in MeCN/H2O mixtures with large fluorescence enhancements (17-fold for

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mmolar cation), which allow measurements of the Cu2‡ concentration from relative
fluorescence intensity at two wavelengths with intensity ratio change from 1 (without
cupric cation) to ca. 100 with mmolar concentration of CuII.
We thank Dr. M. Cotrait for the crystallographic data and Dr. J.-P. Desvergne for lifetime measurements.
Experimental Part
General. UV/VIS Absorbance spectra: Perkin-Elmer Lambda 2 spectrometer. Corrected stationary
fluorescence spectra: Hitachi F-4500 spectrofluorimeter; fluorescence quantum yields were determined by
comparison with quinine sulfate in 0.1n H2SO4 (ff ˆ 0.52). Fluorescence lifetimes were obtained by the time
correlated single photon counting technique (Applied Photophysics) and mathematical deconvolution [38]. IR:
n in cm 1. 1H- and 13C-NMR Spectra: Bruker instrument; d in ppm, J in Hz. MS: in m/z (%).
X-Ray crystallography was tiny, prismatic, colorless crystals grown from a soln. of DMABA-MP in MeCN/
H2O. All data were collected at 298 K with a Kappa-CCD device with Mo-Ka radiation (0.7107 Š). Complete
data sets were collected to a maximum q limit of 278. The structure was solved by direct methods with SIR92.
The lattice constants are listed with other relevant crystal data in Table 2 and selected bond lengths (Š), angles
(8), and dihedral angles (8) are reported in Table 3.
Materials. DMABA-Crown5. Pentafluorothiophenol (2.4 g, 12 mmol) and dicyclohexyl carbodiimide
(DCC) (2.5 g, 12 mmol) were added to a stirred suspension of 4-(dimethylamino)benzoic acid (1.65 g, 10 mmol)
in 20 ml of anhydrous MeCl2 at 08. The mixture was left stirring at 08 for 1 h and at r.t. for 14 h. Precipitated
dicyclohexylurea was removed by filtration and the filtrate was passed through a silica-gel column, eluting with
1
MeCl2. Evaporation of the solvent gave the ester (3.3 g, 95%). Light yellow oil. H-NMR (250 MHz, CDCl3):
3.07 (s, 6 H, Me2-N); 6.63 (d, 2 H); 7.86 (d, 2 H).
Part of this oil (1.83 g, 5 mmol) was dissolved in MeCl2 (15 ml) and a soln. of aza-5-crown-15 (1.095 g,
5 mmol) and Et3N (0.505 g, 5 mmol) in MeCl2 (5 ml) was added. After stirring for 16 h, the solvent was
evaporated and a soln. in MeCl2 of the crude product was passed through silica gel, eluting with MeCl2 followed
by MeCl2/MeOH 9 :1. Evaporation of the eluent gave the amide (0.82 g, 45%), which became solid upon
refrigeration. IR (KBr): 2900, 1625, 1510, 1450, 1350, 1200, 1125. 1H-NMR (250 MHz, CDCl3): 2.91 (s, Me2N);
2
2
3.59 (m, 16 H, CH2O); 3.71 (t, 4 H, CH2N); 6.59 (d, 2 arom. H, Jax ˆ 8.84); 7.28 (d, 2 arom. H, Ja'x' ˆ 8.83).
13C-NMR (CDCl3, 250 MHz): 40.27 (MeN); 69.83 (CH2-N); 70.33 (CH2O); 71.13 (CH2O); 111.32 (arom. C, o to
Me2N); 123.64 (arom. C CˆO); 128.64 (arom. C, o to CˆO); 151.25 (arom. C NMe2); 172.89 (CˆO). MS:
.
‡
‡
366 (2, M ), 148 (100, [M azacrown] ).
DMABA-MP. A soln. of 2-(aminomethyl)pyridine (0.648 g, 6 mmol) and Et3N (0.6 g, 6 mmol) in MeCl2
(5 ml) was added to a stirred soln. of the pentafluorophenyl ester of 4-(dimethylamino)benzoic acid (1.83 g,
5 mmol) in MeCl2 (15 ml). After stirring for 16 h, the solvent was evaporated and a soln. of the residue in MeCl2
was passed through a plug of silica gel, eluting successively with MeCl2 and MeCl2/MeOH 9 :1. Evaporation of
the eluent followed by crystallization from MeOH gave the amide as a white solid (0.79 g, 62%). 1H-NMR
(200 MHz, CDCl3): 3.02 (s, 6 H); 4.75 (d, 1 H, J ˆ 4.92); 6.68 (d, 2 H, J ˆ 9.1); 7.17 ± 7.45 (br. m, NH, 2 H of Pyr);
.
‡
7.68 (dt, 1 H, Pyr); 7.79 (d, 2 H, J ˆ 9.1); 8.56 (1 H, o to Pyr). MS: 255 (41, M ), 148 (100, [M 2-
‡
(aminomethyl)pyridine] ), 107 (28).
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Received March 14, 2001