L. Fabbrizzi et al.
It is observed that, on addition of the first equivalent of
fluoride, the band for the receptor at 285 nm undergoes a
moderate redshift, typically associated with the formation of
a hydrogen-bonded complex. Moreover, a new band simul-
taneously develops at 335 nm; such a band, as mentioned
before, results from the charge-transfer transition from a de-
protonated NꢀH fragment. On addition of two or more
ꢀ
equivalents of F , the band at 335 nm keeps increasing,
while the redshifted band decreases and disappears. This be-
haviour can be accounted for by assuming the occurrence of
Equilibria (4) and (5). The bonding mode in the [B···HF]
complex should be similar to that hypothesised in the analo-
gous acetate complex and is shown in Scheme 2, formula b.
In particular, in the receptor/anion complex, (nearly) com-
plete proton transfer to the fluoride ion takes place from
+
the NꢀH fragment linked to the Mepy substituent. On fur-
ther addition of fluoride, the HF molecule finds it conven-
ient to leave the deprotonated receptor and to form a more
ꢀ
stable hydrogen-bonded complex with a second F ion: the
ꢀ
dihydrogen fluoride self-complex, HF , as described by
2
Equilibrium (5).
The following equilibrium constants were obtained for
equilibria (4) and (5) on fitting of the titration data: logK =
1
6
.80ꢁ0.03; logK =5.81ꢁ0.03. The high value of logK re-
2
2
ꢀ
flects the high stability of the HF complex. Figure 16b
2
Figure 17. a) Absorption spectra taken over the course of the titration of
a 2.035ꢂ10 m solution of CH in MeCN with [Bu N]BzO (BzO =ben-
4
zoate). b) Titration profiles based on the absorbances at 291 nm, monitor-
ing the disappearance of the uncomplexed receptor, and at 351 nm, indi-
cating the formation of a 1:1 complex.
displays the concentration profiles of the three species pres-
ent at the equilibrium, over the course of the titration: BH ,
ꢀ5
2+
ꢀ
+
[
[
B···HF] and B. It should be noted that overlapping of the
B···HF] and B species and of their spectra prevents the su-
perimposition of the absorbance at 337 nm on the concen-
tration profile of B. In any case, in the titration profile at
3
37 nm, a distinct flex is noted at 1 equivalent of added
(Figure 17b) clearly define the 1:1 stoichiometry of the pro-
cess. The limiting spectrum obtained after the addition of
one or more equivalents of benzoate (shown as a dashed
line in Figure 17a) displayed two distinct and well-defined
absorption bands. It is suggested that the first one (305 nm)
results from the charge-transfer transition from the nitrogen
anion. A Job plot based on the absorbance values at 337 nm
confirms the stoichiometry of Equilibria (4) and (5) (see Fig-
ure S15 in the Supporting Information). The fluoride-in-
duced deprotonation of a urea NꢀH fragment is typically
[
9,16]
observed with receptors containing powerful EWGs.
+
atom of one NꢀH fragment to the linked Mepy substitu-
2
+
Anion interactions with receptor CH : The doubly substi-
tuted and doubly positively charged receptor CH forms
1
ides), which present spectral features similar to those ob-
served with the 1:1 complex of CH COO with receptor
BH . The interactions of CH with benzoate and chloride
will be considered as representative examples.
ent. On the other hand, the second band (350 nm), originat-
ing from the transition from the other urea nitrogen atom,
assumed a negative charge due to NꢀH deprotonation (or,
2
+
ꢀ
:1 complexes with all anions X (either oxoanions or hal-
more precisely, to an advanced N-to-O proton transfer).
Thus, the 1:1 complex should be described by the limiting
formula a in Scheme 3 and should be considered as a com-
plex involving the deprotonated form of the receptor and
ꢀ
3
+
2+
+
Figure 17a displays the family of spectra taken over the
one molecule of benzoic acid, [C···BzOH] .
ꢀ
5
2+
course of the titration of a 2.03ꢂ10 m solution of CH
The lower energy band appears at the same wavelength as
ꢀ
+
with tetrabutylammonium benzoate (BzO ). It must be re-
membered that, at this concentration level, the receptor
exists as an equilibrium mixture of the non-deprotonated
that for the uncomplexed deprotonated receptor C , but its
limiting molar absorbance e is significantly smaller and cor-
responds to 88% of that of the deprotonated receptor.
Figure 18a shows spectra recorded over the course of the
2
+
+
form CH (56%) and of the deprotonated form C (44%).
2
+
Upon benzoate addition, the band at 291 nm, attributed
titration of CH with chloride. Anion addition induces a
2
+
2+
to the non-deprotonated receptor CH , underwent a pro-
nounced redshift (Dl=15 nm). On the other hand, the in-
tensity of the band at 350 nm (23700m cm , attributed to
the deprotonated receptor C ) increased to reach a limiting
value of 43000m cm . The corresponding titration profiles
moderate redshift of the band of receptor CH and makes
the intensity of the band at 350 nm decrease. On addition of
an excess of chloride (to 50 equiv), the latter band does not
ꢀ
1
ꢀ1
+
ꢀ1
ꢀ1
disappear, but reaches a limiting value of 14000m cm .
ꢀ
1
ꢀ1
Thus, also in this complex, the same as that with benzoate,
9432
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9423 – 9439