Moderate and advanced intramolecular proton transfer in urea-anion hydrogen-bonded complexes

Article abstract of DOI:10.1002/chem.201100490

The study of the interactions of the three urea-based receptors AH, BH ⁺ and CH²⁺ with a variety of anions, in MeCN, has made it possible to verify the current view that hydrogen bonding is frozen proton transfer from the donor (the urea NH fragment in this case) to the acceptor (the anion X^-). The poorly acidic, neutral receptor AH establishes two equivalent hydrogen bonds N-H...X^-, with all anions, including CH₃COO^- and F^-, in which moderate proton transfer from NH to the anion takes place. The strongly acidic, dicationic receptor CH²⁺ forms, with most anions, complexes in which two inequivalent hydrogen bonds are present: one involving moderate proton transfer (N-H...X^-) and one in which advanced proton transfer has taken place, described as N^-...HX. The degree of proton advancement is directly related to the basic tendencies of the anion. The cationic receptor BH⁺ of intermediate acidic properties only forms complexes with two inequivalent hydrogen bonds (moderate+advanced proton transfer) with CH ₃COO^- and F^-, and complexes with two equivalent hydrogen bonds (moderate proton transfer) with all the other anions. Moreover, [B...HF] and [C...HF]⁺, on addition of a second F ^- ion, lose the bound HF molecule to give HF₂^-. Release of CH₃COOH, with the formation of [CH ₃COOH...CH₃COO]^-, also takes place with the [B...CH₃COOH] complex in the presence of a large excess of anion. Copyright

Full text of DOI:10.1002/chem.201100490

FULL PAPER
DOI: 10.1002/chem.201100490
Moderate and Advanced Intramolecular Proton Transfer in Urea–Anion
Hydrogen-Bonded Complexes
[
a]
[b]
[a]
[a]
Giorgio Baggi, Massimo Boiocchi, Luigi Fabbrizzi,* and Lorenzo Mosca
Abstract: The study of the interactions
of the three urea-based receptors AH,
plexes in which two inequivalent hy-
drogen bonds are present: one involv-
ing moderate proton transfer (Nꢀ
complexes with two inequivalent hy-
drogen bonds (moderate+advanced
+
2+
ꢀ
BH and CH
with a variety of
proton transfer) with CH COO and
3
ꢀ
ꢀ
anions, in MeCN, has made it possible
to verify the current view that hydro-
gen bonding is frozen proton transfer
from the donor (the urea NꢀH frag-
H···X ) and one in which advanced
F , and complexes with two equivalent
proton transfer has taken place, de-
hydrogen bonds (moderate proton
transfer) with all the other anions.
ꢀ
scribed as N ···HꢀX. The degree of
+
proton advancement is directly related
to the basic tendencies of the anion.
Moreover, [B···HF] and [C···HF] , on
ꢀ
ment in this case) to the acceptor (the
addition of a second F ion, lose the
ꢀ
+
ꢀ
anion X ). The poorly acidic, neutral
The cationic receptor BH of inter-
bound HF molecule to give HF . Re-
2
receptor AH establishes two equivalent
mediate acidic properties only forms
lease of CH COOH, with the forma-
3
ꢀ
ꢀ
hydrogen bonds NꢀH···X , with all
tion of [CH COOH···CH COO] , also
3
3
ꢀ
ꢀ
anions, including CH COO and F , in
takes place with the [B···CH COOH]
3
3
Keywords: anion binding · charge
transfer · donor–acceptor systems ·
hydrogen bonding · supramolecular
chemistry
which moderate proton transfer from
complex in the presence of a large
excess of anion.
NꢀH to the anion takes place. The
strongly acidic, dicationic receptor
2
+
CH
forms, with most anions, com-
Introduction
the substituents at the nitrogen atoms. Powerful electron-
withdrawing groups (EWG) effectively polarise the NꢀH
Urea is one of the most used hydrogen-bond donors in the
design of neutral receptors for anions. In particular, urea
offers two parallel and adjacent NꢀH fragments suitable for
groups and favour the formation of stable hydrogen-bond
complexes with anions. The interaction between each NꢀH
fragment and the anion atom receiving the hydrogen bond
can be considered as a “frozen” proton transfer from the re-
ceptor to the anion, according to a recent view of hydrogen
establishing hydrogen-bonding interactions with two oxygen
atoms of an inorganic oxoanion or a carboxylate. Since the
[1]
[7]
pioneering works by the groups of Wilcox, and Hamil-
ton, a variety of urea-based receptors displaying selective
recognition towards anions have been synthesised. Some
of these receptors contain several urea subunits incorporat-
ed in frameworks of different shapes: open chain, cyclic,
tri- and tetrapodal, spatially arranged to generate size and/
or shape selectivity. The hydrogen-bond-donor tendencies of
urea NꢀH fragments strongly depend upon the nature of
bonding. The higher the acidic properties of the receptor
[
2]
and the more basic features of the anion, the more advanced
the proton transfer and the more stable the receptor/anion
[
3]
[8]
complex.
[4]
[5]
We have previously investigated the binding tendencies of
ureas containing powerful neutral electron-withdrawing sub-
[6]
[
9]
[10]
stituents (nitrophenyl,
phthalimide
nitrobenzofurazan,
N-methyl-
[11a]
[11b]
and N-alkylnaphthalaneimide ) towards a
variety of inorganic and organic anions. We report herein an
+
2+
investigation on three urea derivatives, AH, BH and CH
[
a] Dr. G. Baggi, Prof. L. Fabbrizzi, Dr. L. Mosca
Dipartimento di Chimica
Universitꢀ di Pavia
via Taramelli 12, 27100 Pavia (Italy)
Fax : (+39)0382528544
, to evaluate the effect of the progressive substitution of the
positively charged EWG 4-N-methylpyridinium, Mepy , on
+
the anion-binding properties of the receptor.
+
In particular, the Mepy substituent is expected to exert
E-mail: luigi.fabbrizzi@unipv.it
both an electrostatic effect (through space) and, to a more
pronounced extent, an electron-withdrawing effect (through
[
b] Dr. M. Boiocchi
Centro Grandi Strumenti
Universitꢀ di Pavia
via Bassi, 27100 Pavia (Italy)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/or from the author. Supporting infor-
mation for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201100490.
Chem. Eur. J. 2011, 17, 9423 – 9439
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9423

bond), as illustrated in the resonance representation in
Scheme 1. In particular, in the limiting formula b, the formal
positive charge has been transferred onto the urea nitrogen
+
Scheme 1. Resonance formulae of a Mepy substituent linked to a urea
nitrogen atom.
+
2+
Figure 1. Absorption spectra of AH (c), BH (a) and CH (g)
in MeCN.
atom, which should significantly enhance the polarisation
and the hydrogen-bond-donor tendencies of the proximate
NꢀH fragment.
Figure 2) originates from a charge-transfer transition from a
urea nitrogen atom to the nitrogen atom of pyridine or of
the N-methylpyridinium substituent.
+
2+
The binding tendencies of AH, BH and CH towards
anions were studied through spectrophotometric and, in
1
some cases, by H NMR spectroscopic titration experiments.
By comparing the two symmetric receptors, AH and
2
+
Some complexes were isolated as crystalline salts and their
molecular structures were determined by single-crystal X-
CH , it is observed that the hn transition takes place in
1
+
2
the AH receptor at a lower wavelength than in the CH
+
ray diffraction studies. We anticipated that the Mepy sub-
analogue. Such behaviour may originate from the fact that
+
stituent induces a pronounced enhancement of the acidity of
urea NꢀH fragments to promote an advanced intra-complex
proton transfer from the NꢀH fragment to the negatively
the Mepy substituent generates a partial positive charge on
the urea nitrogen atom (formula b in Scheme 3 below),
which disfavours (i.e., makes more energetic) the N-to-O
ꢀ
charged atom of the anion X , either the oxygen atom of an
charge-transfer transition. On the other hand, the hn transi-
2
2
+
oxoanion or a halide. It is shown that these complexes can
be conventionally considered as constituted by the depro-
tonated receptor and one molecule of the acid HX. While
tion is much less energetic for CH (290 nm) than for AH
(258 nm), because the negative charge is now transferred
onto a positively charged atom, rather than on a neutral
one.
+
with the cationic receptor BH such complexes form only
ꢀ
ꢀ
+
with the more basic anions, F and CH COO , in the case of
For the asymmetric BH receptor, only one band at
3
2
+
the dication CH they form with all investigated anions, in-
282 nm is observed, which suggests that the two expected
ꢀ
ꢀ
cluding poorly basic NO3 and Br . UV/Vis spectrophotom-
etry proved to be an effective tool for monitoring the ad-
vancement of the intra-complex proton transfer.
hn transitions are rather close in energy. This may derive
2
from the fact that some positive charge is transferred from
+
the Mepy to the pyridine substituent. On the other hand,
two N-to-O charge-transfer transitions are observed. In fact,
in addition to a distinct band centred at 198 nm, due to a
+
Results and Discussion
transition from the urea nitrogen atom linked to Mepy , a
definite shoulder is observed at around 220 nm, which could
Spectral properties of receptors
+
2+
AH, BH and CH
in solu-
tion: Figure 1 reports the elec-
tronic spectra of the three in-
vestigated receptors in MeCN;
all have two main absorption
bands in the UV region ex-
plored (190–390 nm).
Both absorption bands have
to be ascribed to charge-trans-
fer transitions. In particular, the
lowest wavelength band is as-
signed to a transition from a
urea nitrogen atom to the car-
bonyl oxygen atom (hn1 in
Figure 2), whereas the highest
wavelength band (hn2 in CH
Figure 2. Charge-transfer transitions responsible for the two main absorption bands present in the spectra
shown in Figure 1: a) transitions in receptors containing the pyridine-containing moiety; b) transitions in recep-
+
+
tors containing the Mepy -containing moiety; and c) transitions in a deprotonated receptor, either BH or
2
+
.
9424
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9423 – 9439

Proton Transfer in Urea–Anion Hydrogen-Bonded Complexes
FULL PAPER
be ascribed to a transition from the urea nitrogen atom
linked the pyridine substituent.
2
+
However, receptor CH shows two additional distinctly
redshifted bands at 215 and 350 nm. They are ascribed to
the deprotonated form of the receptor, which forms in
MeCN even in the absence of basic anions, according to
acid–base Equilibrium (1):
2
þ
þ
þ
CH Ð C þ H
ð1Þ
In particular, the acidity of the NꢀH fragments is drasti-
+
cally enhanced by the presence of the two Mepy substitu-
ents. The pK value associated with the dissociation Equilib-
A
rium (1) in MeCN was determined in the following way:
2
+
First, a solution of CH was titrated with a solution of
Proton Sponge. Upon the addition of base, the band of the
2
+
non-deprotonated form of CH at 290 nm (hn ) decreased
2
to reach a minimum at 1 equivalent, whereas the band at
+
3
50 nm, pertaining to the deprotonated receptor, C , in-
creased to reach a sharp curvature, still at 1 equivalent. The
absorbance of the band at 350 nm at the equivalent point
ꢀ
1
ꢀ1
was 54000m cm , which was considered to be the limiting
+
2+
value for C . Then, several solutions of CH in MeCN of
varying concentrations, C, were prepared (with C varying
ꢀ
6
ꢀ5
from 6.34ꢂ10 to 5.07ꢂ10 m) and their spectra were mea-
sured (see Figure 3a). For each solution, the ratio of the ab-
ꢀ
1
ꢀ1
sorbance at 350 nm over 54000m cm gave the degree of
2
2+
ꢀ6
dissociation a. The plot of a versus 1/C, shown in Fig-
Figure 3. a) Spectra of solutions of CH in MeCN (6.34ꢂ10 (c),
5
ꢀ
ꢀ5
1
.27ꢂ10 (a), 2.53ꢂ10 (g) and 5.07ꢂ10ꢀ5 m (c)): on dilution,
ure 3b, was linear and its slope provided the value of KA
the band centred at 350 nm (deprotonated receptor) increased, whereas
(
pK =5.76ꢁ0.02).
A
that at 290 nm (non-deprotonated receptor) decreased. Such behaviour is
2
+
+
+
Solvent plays an important role in the deprotonation the
interpreted in terms of the acid–base equilibrium: CH QC +H ; the
2
+
ꢀ6
NꢀH fragment. Figure 4 shows the spectra CH recorded
pK
A
value (5.76; K
A
=1.753ꢂ10 ; a2=K
A
/C) can be calculated from the
linear plot in part b).
in CHCl , MeCN and DMSO. The band of the deprotonated
3
+
species CH , at about 350 nm, is distinctly observed in
MeCN and DMSO, whereas in the case of CHCl it is only
3
poorly outlined (and the presence of a slight amount of de-
protonated species could originate from the interaction with
traces of water and/or ethanol as a stabilizer).
It is suggested that the polar media, DMSO and MeCN,
+
+
stabilise both the H ion and the deprotonated receptor C ,
which has a zwitterionic nature, through solute–solvent in-
teractions; an effect that cannot be exerted by the poorly
polar CHCl3.
Anion interactions with the neutral receptor AH: The neu-
tral receptor AH interacts in MeCN with inorganic (halides,
oxoanions) and organic anions (carboxylates) to give stable
1
:1 complexes, as inferred from spectrophotometric titration
ꢀ
5
2+
Figure 4. Absorption spectra of 2ꢂ10 m solutions of CH in CHCl
(
3
experiments. Figure 5a shows, as an example, the family of
spectra taken over the course of the titration of a 2ꢂ10
c), MeCN (a) and DMSO (g).
ꢀ
5
m
solution of AH with a solution of [Bu N]Cl. Upon anion ad-
4
dition, the band centred at 257 nm, assigned to the hn2
charge-transfer transition (in Figure 2a), underwent a de-
fined redshift, which reflected the stabilisation experienced
by the excited state, detaining a positive charge on a urea ni-
trogen atom, when it interacted with the anion. Figure 5b
shows the titration profile taken at 267 nm (a wavelength
corresponding to the maximum of the redshifted band),
which corresponds to the [AH···Cl] complex. In Figure 5c,
ꢀ
the curvature of the plot at the equivalent point is shown in
detail. Best fitting of titration data over the 240–280 nm
wavelength interval, using a non-linear least-squares proce-
[12]
dure,
was obtained by assuming the formation of a 1:1
Chem. Eur. J. 2011, 17, 9423 – 9439
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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9425

L. Fabbrizzi et al.
1
Figure 6. H NMR spectra taken over the course of the titration of a
ꢀ
3
1
3 4
.22ꢂ10 m solution of AH in CD CN with [Bu N]Cl.
lence of the electrostatic repulsion exerted by the closer
anion.
Most of the investigated anions, either inorganic or organ-
ꢀ
ꢀ
ic, more basic (F , CH COO , see Figures S5 and S7 in the
3
ꢀ
ꢀ
Supporting Information) or less basic (Br , NO , Figure S6)
3
displayed similar spectral behaviour (more or less pro-
nounced redshift of the receptor band at 257 nm). Spectral
data could be fitted by assuming the formation of a 1:1 com-
plex, as described by Equilibrium (2). Table 1 shows perti-
nent logK values for equilibria of type (2).
Figure 5. a) Absorption spectra taken over the course of the titration of a
ꢀ
5
2
ꢂ10 m solution of AH in MeCN with chloride. b) Titration profile
Table 1. LogK values for Equilibrium (2) in MeCN at 258C.
based on the absorbance at 267 nm (band corresponding to the
AH···Cl] complex). c) Titration profile for the initial addition of chlo-
ꢀ
[a]
[b]
[c]
ꢀ
Anion (X )
logK
l
max [nm]
Dl [nm]
[
ꢀ
ride.
CH
PhCOO
3
COO
6.00(1)
5.84(1)
4.80(1)
3.98(1)
2.77(1)
5.19(1)
4.02(1)
3.23(1)
274
272
271
267
263
272
267
264
17
15
14
10
6
15
10
7
ꢀ
ꢀ
H
2
PO
4
ꢀ
NO
NO
2
3ꢀ
complex, according to Equilibrium (2), with an association
constant logK=4.02ꢁ0.01:
ꢀ
F
ꢀ
Cl
ꢀ
AH þ Xꢀ Ð ½AH ꢂ ꢂ ꢂ Xꢃꢀ
Br
ð2Þ
[
[
[
a] The values in parentheses are the uncertainties of the last digits.
b] Wavelength at the maximum of the hn
2
band for the complex
Detailed features on the receptor AH/chloride interaction
were obtained from a H NMR titration experiment in
ꢀ
AH···X] . [c] Redshift of the hn
2
band of AH on anion complexation.
1
CD CN. Figure 6 shows the family of spectra for the titra-
3
ꢀ3
1
tion of a 1.22ꢂ10 m solution of AH with [Bu N]Cl.
Figure 7 shows the family of H NMR spectra taken over
4
ꢀ
3
In particular, the NꢀH proton undergoes a pronounced
the course of the titration of a 1.5ꢂ10 m solution of AH in
ꢀ
downfield shift on addition of Cl , the magnitude of which
CD CN with [Bu N]CH COO.
3
4
3
is drastically reduced after the addition of 1 equiv anion.
The downfield shift reflects the electron impoverishment ex-
perienced by the NꢀH proton and the consequent deshield-
The spectral pattern is similar to that observed in the ti-
tration with chloride. The chemical shift of the NꢀH proton
ꢀ
is distinctly larger than that for Cl , which indicates a more
ing when a hydrogen-bonding interaction is established with
chloride. On the other hand, anion addition causes modest
effects on the aromatic protons of the substituents. In partic-
ular, the CꢀHa protons undergo a moderate, but definite,
intense hydrogen-bonding interaction.
ꢀ
The addition of HSO caused precipitation, whereas the
4
ꢀ
addition of I , even in a large excess, did not induce any sig-
nificant spectral change, which indicated no or very little in-
teraction, at least in the investigated concentration range
upfield shift, which results from the balance of a through-
bond effect (upfield, dominant) and a through-space, elec-
trostatic effect (downfield). In the case of CꢀHb protons, a
ꢀ
5
(200 equiv of [Bu N]I added to a 2ꢂ10 m solution of AH).
4
ꢀ
The anion affinity sequence of AH (CH COO >
3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
slight downfield effect is observed, which reflects the preva-
C H COO >F >H PO >NO2 ꢄCl >Br >NO )
is
6
5
2
4
3
9426
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Chem. Eur. J. 2011, 17, 9423 – 9439

Proton Transfer in Urea–Anion Hydrogen-Bonded Complexes
FULL PAPER
the charge-transfer band at 257 nm (Figure 8c), and 2) the
ꢀ
absorbance of the band pertaining to the [AH···X] complex
(
Figure 8d). As far as the redshift is concerned, it was ob-
served that the higher the value of Dl, the higher the associ-
ation constant. Dl expresses the stabilisation energy experi-
enced by the zwitterionic excited state of the receptor (neg-
ative charge on the pyridine nitrogen atom, positive charge
on the urea nitrogen atom, see Figure 2a), following the in-
teraction with the anion: the existence of a linear correla-
tion between logK and Dl indicates that the affinity trend
of anions is the same for both the excited and ground states
of the receptor. Figure 8d shows the existence of a roughly
linear dependence of logK values upon the molar absorptiv-
ꢀ
ities of the corresponding [AH···X] complexes. More de-
fined linear correlations are observed if oxoanions and hal-
ides are separately considered. In this context, it should be
noted that the probability of a charge-transfer transition
(
from which the intensity of the corresponding absorption
1
Figure 7. H NMR spectra taken over the course of the titration of a
1
band depends) is given by the integral symy*dt, in which y
and y* are the ground- and excited-state wave functions, m
is the dipole moment operator and the integral is over all
space. The dipole moment of the transition from the urea ni-
trogen atom to the pyridine substituent is modified upon in-
ꢀ
3
3 4 3
.22ꢂ10 m solution of AH in CD CN with [Bu N]CH COO.
that typically observed for neutral receptors not imposing
any steric constraints and essen-
tially parallels the basicity of
the anion. In particular, for ox-
oanions,
a linear correlation
was observed between logK
and the calculated partial nega-
tive charge on the anion oxygen
atoms (see Figure 8a). Such be-
haviour is consistent with the
idea of considering hydrogen
bonding as frozen proton trans-
fer from the donor (the NꢀH
fragment) to the acceptor
[7]
(
anion oxygen atom):
the
higher the partial negative
charge, the more advanced the
proton transfer and the stronger
the hydrogen-bonding interac-
tion. Such a relationship had
been previously observed for
[
9]
urea-
and squaramide-based
[13]
derivatives.
Figure 8b shows
that logK values for the forma-
tion of receptor/anion com-
plexes are also linearly correlat-
ed to pKA values of the corre-
sponding oxoacids in water,
with the notable exception of
ꢀ
NO .
2
LogK values were also found
to be linearly correlated to two
Figure 8. Linear dependence of logK from a) the partial charge on the oxygen atoms of the oxoanion, calculat-
ed by an ab initio procedure; b) logK of the corresponding oxoacid, in water; c) the redshift of the charge-
spectroscopic
) the redshift,
complex)ꢀl
parameters:
transfer band of the receptor (hn
2
, in nm), following anion interaction (circle=oxoanions, triangle=halides);
1
(
Dl=lmax
ꢀ
and d) the molar absorbance of the band pertaining to the [AH···X] complex (a=oxoanions, g=
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(receptor),
of halides).
max
Chem. Eur. J. 2011, 17, 9423 – 9439
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9427

L. Fabbrizzi et al.
teraction with an anion. In particular, the dipole is expected
to increase with the basicity of the anion and with its ten-
dency to transfer negative charge onto the urea nitrogen
atom. Thus, the linear relationship in Figure 8d mirrors that
illustrated in Figure 8a, as far as oxoanions are concerned.
investigated. Pertinent logK values for complexation equili-
bria of type (3) are reported in Table 2.
Table 2. LogK values for Equilibrium (3) in MeCN at 258C. For
ꢀ
ꢀ
CH
3
COO and F , a further stepwise equilibrium exists: [BH···X]+
ꢀ
ꢀ
X QB+HX
2
(logK
2
).
logK
6.77(1)
.81(3)
6.33(2)
–
5.61(1)
4.87(2)
4.15(1)
6.80(3)
5.81(3)
5.57(1)
4.56(1)
2.98(1)
+
Anion interactions with receptor BH : The positively
ꢀ)
[a]
[b]
[c]
Anion (X
l
max [nm]
Dl [nm]
+
charged receptor BH forms with all of the investigated
ꢀ
CH
3
COO
300
17
anions 1:1 complexes. For example, Figure 9a shows the
2
ꢀ
PhCOO
297
295
292
290
289
–
14
12
9
7
6
ꢀ
H
2
PO
4
ꢀ
NO
HSO
2
4ꢀ
ꢀ
NO
3
ꢀ
F
logK
1
–
logK
2
ꢀ
Cl
291
290
286
8
7
3
ꢀ
Br
ꢀ
I
[
[
[
a] The values in parentheses are the uncertainties of the last digits.
b] Wavelength at the maximum of the hn band for the complex
band of BH on anion complexation.
2
+
BH···X]. [c] Redshift of the hn
2
The formation of a hydrogen-bonded complex of 1:1 stoi-
1
chiometry, [BH···Cl], was confirmed by a H NMR titration
experiment. Figure 10 shows the family of spectra obtained
Figure 9. a) Absorption spectra taken over the course of the titration
ꢀ
5
+
with chloride of a 2.03ꢂ10 m solution of BH in MeCN; b) titration
profile based on the absorbances at 291 nm (!, band corresponding to
the [BH···Cl] complex) and at 283 nm (*, band corresponding to the un-
+
complexed receptor BH ).
family of spectra taken over the course of the titration of a
ꢀ
5
+
1
2
.03ꢂ10 m solution of BH in MeCN with [Bu N]Cl.
Figure 10. H NMR spectra taken over the course of the titration of a
4
ꢀ
3
+
1
.43m ꢂ 10 m solution of BH in CD
3
CN with [Bu
4
N]Cl at 258C.
Best fitting of titration data over the 250–310 nm wave-
length interval, using a non-linear least-squares proce-
[12]
dure,
complex, according to Equilibrium (3), to which a logK=
.57ꢁ0.01 corresponded.
was obtained by assuming the formation of a 1:1
ꢀ
3
+
on titration of a 1.43ꢂ10 m solution of BH CD CN with
3
5
[Bu N]Cl. The most relevant feature is concerned with the
4
behaviour of the two NꢀH protons. Both protons undergo a
þ
ꢀ
BH þ X Ð ½BH ꢂ ꢂ ꢂ Xꢃ
ð3Þ
pronounced downfield shift. The shift is greater for the Nꢀ
+
H1 proton, which belongs to the moiety with the Mepy
Similar spectroscopic patterns were observed for titrations
substituent and is, presumably, more polarised. This may
ꢀ
with all of the other anions, with the exception of F and
suggest that a stronger interaction between the NꢀH1 frag-
ꢀ
ꢀ
CH COO . H PO caused precipitation and could not be
ment and chloride is established.
3
2
4
9428
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Chem. Eur. J. 2011, 17, 9423 – 9439

Proton Transfer in Urea–Anion Hydrogen-Bonded Complexes
FULL PAPER
Definite information on bonding was obtained from X-ray
diffraction studies on a single crystal of the [BH···Cl] com-
plex, the structure of which is shown in Figure 11.
the present case, the bonding arrangement is not symmetric:
the distances involving the NꢀH1 fragment (N···O: 2.88 ꢃ),
+
linked to the Mepy substituent, is slightly, but still detecta-
bly, smaller than those referring to the NꢀH2 group ((N···O:
2
.93). Noticeably, the calculated H1···H2 distance is signifi-
cantly smaller in the chloride complex (1.99 ꢃ) than in the
triflate complex (2.09 ꢃ). This may reflect the fact that in
[
BH···Cl] the two NꢀH fragments have to converge to inter-
act with the spherical anion, whereas in [BH···CF SO ] they
3
3
can be parallel to encompass the two oxygen atoms of tri-
flate.
Acetate and fluoride displayed more intricate behaviour.
Figure 13a shows the family of UV/Vis spectra taken over
ꢀ
5
+
the course of the titration of a 2.03ꢂ10 m solution of BH
in MeCN with [Bu N]CH COO.
4
3
It is observed that, on moderate anion addition (up to
equiv), the band of the receptor at 280 nm decreases,
4
while two new bands at 300 and 335 nm simultaneously de-
velop. The decreasing band and the two increasing ones
reach a limiting value on addition of 1 equiv of acetate (see
Figure 13b). Of the two increasing bands, the band at
Figure 11. Molecular structure of the [BH···Cl] complex, as obtained
from X-ray diffraction studies. Top (a) and lateral (b) views.
3
00 nm seems to correspond to the formation of a typical
The chloride ion establishes hydrogen-bonding interac-
tions with both NꢀH fragments (Figure 10a), but N···Cl dis-
tances are distinctly smaller for NꢀH1 (2.21 ꢃ) than for Nꢀ
hydrogen-bonded complex, whereas the band at 335 nm is
that usually observed on deprotonation of BH : it is surpris-
ing that both bands grow simultaneously. Such a puzzling
spectral response can be explained by assuming the forma-
tion of a stable 1:1 complex, the structure of which has been
tentatively sketched in Scheme 2, formula a.
+
H2 (2.29 ꢃ), which confirms the formation of a stronger hy-
+
drogen bond with the NꢀH fragment linked to the Mepy
substituent. The framework of the receptor maintains a rigid
and flat arrangement (Figure 11b), which guarantees the
propagation of electronic and conjugative effects.
In particular, it is suggested that the NꢀH fragment
linked to the pyridine substituent establishes a regular hy-
drogen-bonding interaction with one oxygen atom of the
acetate and that the charge-transfer transition from the
amide nitrogen atom to the pyridine ring is responsible for
the band at 300 nm. On the other hand, complete (or very
advanced) proton transfer takes place from the other NꢀH
Crystals suitable for X-ray diffraction studies were also
+
obtained in the case of the triflate salt of the BH cation,
the molecular structure of which is shown in Figure 12.
The two oxygen atoms of the triflate anion receive hydro-
gen bonds from the NꢀH fragments, in a typical parallel
mode observed in urea complexes with oxoanions. Also in
fragment to the other oxygen atom of the acetate and a hy-
drogen-bonding interaction is then established between the
negatively charged nitrogen atom and the facing OꢀH frag-
ment. Therefore, the band at 335 nm originates from the
charge-transfer transition from the negatively charged urea
+
nitrogen atom to the Mepy substituent. In conclusion, the
1
:1 complex should be considered to result from the associa-
tion of the deprotonated zwitterionic receptor and one mol-
ecule of acetic acid: [B···CH COOH]. Moreover, it is ob-
3
served that, on addition of a large excess of acetate (up to
7
0 equiv), the spectrum undergoes significant modifications.
In particular, the intensity of the band at 337 nm increases
see the profile in Figure 13c), while that of the band at
00 nm decreases. This indicates the occurrence of a second
(
3
stepwise equilibrium. Indeed, best fitting of titration data
was obtained by assuming the occurrence of two equilibria
[
Equilibria (4) and (5)] and characterised by the following
values of the equilibrium constants: logK =6.77ꢁ0.01,
1
logK =2.81ꢁ0.03:
2
3 3
Figure 12. Molecular structure of the [BH···CF SO ]·MeCN complex, as
obtained from X-ray diffraction studies. Top (a) and lateral (b) views.
MeCN molecules not involved in any hydrogen-bonding interactions
have been omitted for clarity.
þ
ꢀ
BH þ X Ð ½B ꢂ ꢂ ꢂ HXꢃ
ð4Þ
Chem. Eur. J. 2011, 17, 9423 – 9439
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www.chemeurj.org
9429

L. Fabbrizzi et al.
Scheme 2. Hypothesised bonding mode in 1:1 complexes involving recep-
+
tor BH and acetate and fluoride ions. The occurrence of an advanced
proton transfer from a urea NꢀH fragment to the anion is suggested.
ꢀ
ꢀ
½
B ꢂ ꢂ ꢂ HXꢃ þ X Ð B þ ½HX ꢃ
ð5Þ
2
Equilibrium (4) refers to the formation of the complex in-
volving the deprotonated receptor and one molecule of
acetic acid, described by formula a in Scheme 2. Then, it is
tentatively hypothesised that in the second step, Equilibri-
um (5), the molecule of acetic acid, HX, is released by the
ꢀ
complex,
and
the
self-complex
[HX2]
(=
ꢀ
[
CH COOH···CH COO] ) forms, as well as the deprotonat-
3
3
ed form of receptor B. The formation of the acetic acid/ace-
tate complex has been observed on reaction of excess
ꢀ
[13]
CH COO with acidic anion receptors (thioureas, squara-
mides)
3
[13]
and structural features have been determined
through neutron diffraction studies on the crystalline prod-
[14]
uct.
It should to be noted that, in view of the low value of
logK , the deprotonated species B, over the course of the
2
spectrophotometric titration (analytical concentration of
ꢀ
5
(
BH)CF SO =2ꢂ10 m), forms in
a
nearly negligible
3
3
ꢀ
amount on addition of the first 4 equivalents of CH COO
3
(
see the distribution diagram in Figure 14a).
Detailed information on Equilibrium (5) and on the
1
nature of species B came from the H NMR titration experi-
ꢀ
3
ment, which was carried out on a 1.2ꢂ10 m solution of
BH)CF SO in CD CN. At this relatively high concentra-
(
3
3
3
tion level, species B is expected to form in a considerable
ꢀ
amount after the addition of the first equiv of CH COO , as
3
shown in the distribution diagram in Figure 14b.
The corresponding spectra are shown in Figure 15. The
ꢀ
NꢀH1 signal disappears on addition of CH COO due to
3
the formation of the [B···CH COOH] complex and to the
3
occurrence of fast anion-exchange processes. The NꢀH2
signal broadens and undergoes a downfield shift to become
ꢀ
sharp again on addition of 1 equiv of CH COO . Then, on
3
further acetate addition, the NꢀH2 signal broadens again
and undergoes an upfield shift. Such behaviour is consistent
with the occurrence of deprotonation of the NꢀH1 frag-
ment. In particular, on release of the bound CH COOH
3
molecule, the electrostatic effect disappears, while due to
delocalisation of the negative charge on the molecular
framework the through-bond effect dominates to induce an
upfield shift. Note that chemical shift values of the NꢀH2
Figure 13. a) Absorption spectra taken over the course of the titration of
ꢀ
5
+
ꢀ
a 2.03ꢂ10 m solution of BH in MeCN with acetate (AcO ; c=0,
a=1, g=70 equiv). b) Titration profiles based on the absorbances
at 270 (*) and 337 nm (!), indicating the formation of a complex of 1:1
proton superimpose well on the concentration profiles for
1
stoichiometry. c) Titration profile at 337 nm, indicating that, on addition
the H NMR spectroscopy experiment (see Figure 14b): the
ꢀ
of a large excess of AcO , a second equilibrium takes place.
downfield shift values (positive) fit well with the profile of
the [B···CH COOH] complex, while the upfield shift values
3
(
negative) fit the profile of the deprotonated receptor B.
9430
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Chem. Eur. J. 2011, 17, 9423 – 9439

Proton Transfer in Urea–Anion Hydrogen-Bonded Complexes
FULL PAPER
Also, the signals for CꢀHb and CꢀHd show discontinuous
behaviour: a downfield shift before the addition of 1 equiv
ꢀ
of CH COO and an upfield shift after addition. The phe-
3
nomenon is less accentuated for the CꢀHb proton, probably
because the negative charge delocalised on the moiety to
which CꢀHb belongs is neutralised by the positive charge of
+
the Mepy group. Conversely, on the other moiety, the delo-
calised negative charge can exert a more intense shielding
effect, thus inducing a more pronounced upfield shift on the
CꢀHd proton. Thus, two different investigative techniques
ꢀ
5
allowed us to explore two distinct worlds: that at the 10
m
+
concentration level, inhabited by BH and [B···CH COOH],
3
ꢀ
3
and that at the 10
m scale, also hosting B and
ꢀ
[
CH COOH···CH COO] .
3
3
Fluoride displayed the same behaviour as that of acetate.
ꢀ
The difference was that, due to the high stability of [HF ] ,
2
the most stable hydrogen-bonded self-complex was formed,
for which the highest hydrogen-bonding energy in the gas
ꢀ
1
[15]
phase has been calculated (39 kcalmol ). Deprotonation
+
ꢀ5
of BH also took place to a significant extent in the 10
solution, after the addition of 1 equiv of F . Figure 16a
shows the family of UV/Vis spectra taken over the course of
the titration of a 2.03ꢂ10 m solution of BH in MeCN
with [Bu N]F·3H O.
m
ꢀ
ꢀ
5
+
4
2
Figure 14. a) Lines: concentration profiles (% with respect to the analyti-
+
ꢀ5
cal concentration of the receptor C ACHTUNGRNENUG( BH )=2ꢂ10 m) referring to the
spectrophotometric titration experiment; symbols: absorbance at chosen
wavelength. b) Lines: concentration profiles (% with respect to the ana-
+
ꢀ3
lytical concentration of the receptor C AHCUTNGRNENUG( BH )=1.2ꢂ10 m) referring to
1
the H NMR titration experiment; symbols: chemical shift, d, of the Nꢀ
H2 proton: positive values indicate downfield shifts, negative values indi-
cate upfield shifts.
Figure 16. a) Absorption spectra taken over the course of the titration of
ꢀ
5
+
a 2.03ꢂ10 m solution of BH in MeCN with fluoride. b) Lines (left ver-
tical axis): relative abundances of the three species present at the equilib-
+
rium (receptor BH , fluoride complex [BH···F], deprotonated receptor
B); symbols: titration profiles based on the absorbances at 283 (!) and
337 nm (*).
1
Figure 15. H NMR spectra taken over the course of the titration of a
1
ꢀ
3
+
3 4 3
.2ꢂ10 m solution of BH in CD CN with [Bu N]CH COO at 258C.
Chem. Eur. J. 2011, 17, 9423 – 9439
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9431

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,
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Proton Transfer in Urea–Anion Hydrogen-Bonded Complexes
FULL PAPER
Cl proton transfer should be considered far from complete
and, in any case, distinctly less pronounced than that for the
benzoate complex.
All of the investigated anions form 1:1 complexes of simi-
lar spectral characteristics. The limiting spectra of the differ-
ent receptor/anion complexes are shown in Figure 19. The
Scheme 3. Limiting formulae describing the bonding mode in the 1:1
2
+
complexes of receptor CH with benzoate (a) and chloride (b). The oc-
currence of an advanced proton transfer from a urea NꢀH fragment to
the anion is suggested.
2
+
Figure 19. Limiting spectra of the 1:1 complexes of CH with oxoanions
ꢀ3
2+
and halides, recorded from a 2ꢂ10 m solution of CH in MeCN with
an excess of anion. The solid line represents the spectrum of the solution
+
the receptor alone, which contains 44% of the deprotonated form (C )
2
+
and 56% of the non-deprotonated one (CH ).
following features must be highlighted: 1) the higher energy
band is redshifted and the magnitude of the redshift increas-
ꢀ
ꢀ
ꢀ
es with the basicity of the anion, (AcO >BzO >NO
>
2
ꢀ
ꢀ
ꢀ
ꢀ
Cl >Br >HSO >NO ), which is behaviour usually ob-
4
3
served in hydrogen-bonded complexes with urea-based re-
+
ceptors (e.g., with AH and BH ); 2) all of the anions have a
band at 350 nm, the limiting absorbance of which seems
ꢀ
ꢀ
ꢀ
roughly related to anion basicity (AcO >BzO >NO
>
2
ꢀ
ꢀ
ꢀ
ꢀ
Br >Cl >NO >HSO ), with some notable exceptions:
Br overtakes Cl and NO overtakes HSO .
3
4
ꢀ
ꢀ
ꢀ
ꢀ
3
4
On the basis of this spectral evidence, it is suggested that
the bonding situation in the investigated receptor/anion
complexes can be tentatively illustrated by the limiting for-
mulae shown in Scheme 3. It is also suggested that the
extent of such a proton transfer is not the same for all
anions, but it is related to the intensity of the band at
350 nm: the higher the intensity, the more advanced the
proton transfer. Such a statement is based on the assump-
tion that a high absorbance derives from an especially high
dipole, which in turn corresponds to a very pronounced
proton transfer to the anion. In particular, the ratio of the
absorbance at 350 nm over the limiting absorbance of the
deprotonated receptor provides a rough estimate of the
state of advancement of the proton transfer along the
Figure 18. a) Absorption spectra taken over the course of the titration of
a 2.035ꢂ10 m solution of CH in MeCN with [Bu N]Cl. b) Titration
4
profiles based on the absorbances at 351 nm, monitoring both the disap-
pearance of the uncomplexed receptor and the formation of a 1:1 com-
plex.
ꢀ
5
2+
two distinct charge-transfer transitions take place, to which
two different NꢀH/anion interactions correspond: one in-
volving a moderate proton transfer (“normal” hydrogen-
bonding interaction, responsible for the redshift of the band
at 291 nm), the other in which a more pronounced intra-
complex proton transfer has taken place (and from which
the band at 350 nm originates). Note that the e value of the
latter band corresponds to 26% of that of the deprotonated
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
N···H···X
pathway: AcO >BzO >NO >Cl >Br >
2
ꢀ
ꢀ
ꢀ
4
HSO4 >NO . Note that HSO forms a “normal” com-
3
plex, with two equivalents hydrogen bonds, as determined
from almost no absorbance at 350 nm (see Figure S14 in the
Supporting Information). In conclusion, the appearance of
the band at 350 nm in the spectra of the various complexes,
+
2+
ꢀ
receptor C . Thus, the CH /Cl complex could be tenta-
tively described by formula b in Scheme 3, even if the N-to-
+
conventionally described by the general formula [C···HꢀX]
Chem. Eur. J. 2011, 17, 9423 – 9439
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9433

L. Fabbrizzi et al.
1
,
does not prove the presence of a definitively deprotonated
A H NMR titration experiment with the acetate ion was
ꢀ3
NꢀH fragment, but it indicates that the proton, along the
carried out on a 1.2ꢂ10 m solution of [CH]
ACHTUNGTRNENUG( CF SO ) in
3 3 2
N···H···X pathway, establishes a stronger interaction with X
than with N.
CD CN, at 258C, to investigate the occurrence, if any, of
3
2
+
CH deprotonation on this concentration scale. Pertinent
There is another distinctive spectral feature worth noting:
for all of the investigated complexes, the limiting absorbance
of the redshifted band, which pertains to the NꢀH-to-
spectra are shown in Figure 21.
+
Mepy transitions (ranging from 290 to 305 nm) decreases
with the increasing basicity of the anion, that is, the opposite
behaviour with respect to that observed with complexes of
AH. Moreover, such an absorbance shows a surprising in-
verse linear correlation with respect to the intensity of the
band at 350 nm, as illustrated in Figure 20.
1
Figure 21. H NMR spectra taken over the course of the titration of a
ꢀ
3
2+
1
3 4 3
.2ꢂ10 m solution of CH in CD CN with [Bu N]CH COO at 258C.
The disappearance of the NꢀH proton signal upon initial
ꢀ
substoichiometric additions of CH COO was observed and
3
2
+
Figure 20. Spectral features of 1:1 complexes of receptor CH
with
ascribed to a fast exchange process. On the other hand, both
ꢀ
anions (X ). A linear inverse relationship between the limiting absorban-
CꢀHa and CꢀHb protons undergo a definite upfield shift,
ces, e, of the band at 290–305 nm (charge-transfer transition from Nꢀ
ꢀ
+
even before the addition of one equivalent of CH COO ,
3
H···X to the Mepy moiety) and of the band at 350 nm (transition from
ꢀ
+
ꢀ
X) in the [C···HX] complex.
and no discontinuity was observed with the addition of an-
N ···H
other equivalent. Thus, the occurrence of CH COOH re-
3
lease from the complex and formation of the deprotonated
+
The interpretation of this behaviour is not straightfor-
ward. Because the limiting absorbance of each band pro-
C , even if probably taking place, at this concentration
1
level, cannot be assessed on the basis of the H NMR titra-
tion experiment.
This is not the case for fluoride, for which deprotonation
of CH could be assessed at the concentration level of the
+
vides a measure of the N-to-Mepy dipole, it can be derived
+
that a less intense NꢀH-to-Mepy dipole corresponds to a
ꢀ
+
2+
more intense N -to-Mepy dipole. Thus, it seems that a re-
ciprocal influence between the two proton transfer processes
operates: the more pronounced the advanced proton trans-
fer (N···HꢀX), the less pronounced the moderate proton
transfer (NꢀH···X), and vice versa, according to a concerted
spectrophotometric titration experiment. Figure 22a displays
the family of spectra recorded for the titration of a 2.035ꢂ
ꢀ
5
2+
10 m solution of CH in MeCN with [Bu N]F·3H O.
4
2
Upon addition of fluoride, the band at 291 nm decreases,
whereas that at 350 nm, pertaining to the deprotonated
form of CH , that is, C , increases. The titration profiles at
these two wavelengths, shown in Figure 22, clearly indicate
the occurrence of two stepwise equilibria, following addition
mechanism.
2
+
+
It should be noted that we were not able to determine the
logK values associated with the formation of the various
2
+
anion complexes of CH . In fact, there are several overlap-
ꢀ
ping equilibria, which include 1) complex formation, 2) the
of the first and second equivalents of F , which can be de-
scribed by Equilibria (6) and (7):
2
+
acid dissociation of CH (the pK value of which has been
A
determined through independent experiments) and 3) the
dissociation of the acid HX. In particular, a solution of
2
þ
ꢀ
þ
CH þ F Ð ½C ꢂ ꢂ ꢂ HꢀFꢃ
ð6Þ
ð7Þ
2
+
+
CH contains H ions, which can compete with the recep-
ꢀ
þ
ꢀ
þ
ꢀ
2
tor for the X anions added. Approximate values of pK in
½C ꢂ ꢂ ꢂ HꢀFꢃ þ F Ð C þ HF
A
MeCN are only available in the literature for some HX
[17]
+
acids. In any case, any attempt to fit spectrophotometric
titration data with a three-constant model failed.
For the [C···HꢀF] complex, the intensity of the band at
ꢀ
1
ꢀ1
350 nm can be estimated to be 45000m cm , correspond-
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Proton Transfer in Urea–Anion Hydrogen-Bonded Complexes
FULL PAPER
Figure 22. a) Absorption spectra taken over the course of the titration of
ꢀ
5
2+
4 2
a 2.035ꢂ10 m solution of CH in MeCN with [Bu N]F·3H O. Titration
profiles are based on b) the absorbance at 291 nm, monitoring the disap-
pearance of the uncomplexed receptor, and c) the absorbance at 351 nm,
indicating the formation of a complex of 1:1 stoichiometry.
Figure 23. a) Absorption spectra taken over the course of the titration of
ꢀ
5
2+
a 2.035ꢂ10 m solution of CH in MeCN with [Bu
4 2 4
N]H PO . b) Titra-
tion profiles based on the absorbances at 291 (*) and at 351 nm (!):
+
both bands are related to the [C···H
3
PO
4
]
complex, which forms with
ing to 83% of that of the deprotonated uncomplexed recep-
tor C . This value is second only to acetate (93%), among
the investigated anions, and indicates the occurrence of a
the addition of the first equivalent of anion and disappears on the addi-
tion of the second equivalent (with the formation of the CH··· ACHTUNGTRENNUNG( H PO ) ]
2 4 2
complex).
+
prominent intramolecular “advanced proton transfer”. No-
+
ticeably, the [C···HꢀF] complex obeys the inverse linear re-
lationship shown in Figure 20. As observed for the [B···Hꢀ
+
F] analogue, the [C···HꢀF] complex is also not stable with
respect to the addition of a second equivalent of fluoride:
the hydrogen-bonded HF molecule leaves the deprotonated
ꢀ
receptor and binds to the more appealing F ion, when a
second equivalent is added.
ꢀ
The H PO4 ion presents distinguishable behaviour, as
2
shown by the spectral features illustrated in Figure 23.
In fact, upon addition of the first equivalent of anion, the
band at 291 nm undergoes a significant redshift, while the
intensity of the band at 351 nm slightly decreases. In particu-
lar, a two-band spectrum is obtained (short dashes in Fig-
Scheme 4. Formation equilibria and hypothesised bonding modes for the
2
+
ꢀ
1:1 and 1:2 complexes of receptor CH with H PO . Advanced proton
2
4
transfer from a urea NꢀH fragment to an anion oxygen atom operates in
the 1:1 complex a. Upon addition of a second equivalent of anion, an
2
ꢀ
(
H
2
PO
4
)
2
dimer is formed, which establishes a “normal” hydrogen-
bonding interaction at the urea binding site (complex b).
+
ure 23a), which indicates the formation of a [C···H PO ]
3
4
complex, the bonding situation of which is illustrate by for-
mula a in Scheme 4.
creases and disappears. Noticeably, the titration profile at
391 nm (Figure 23b) defines the formation of a new com-
plex of stoichiometry 1:2. The limiting spectrum of this com-
plex (long dashes in Figure 23) consists of the sole redshifted
band, the intensity of which has significantly decreased.
Also, the titration profile at 291 nm in Figure 23b indicates
the occurrence of two stepwise equilibria, the structural fea-
tures of which are tentatively illustrated in Scheme 4. In par-
The ratio of the absorbances at 351 nm for the
+
[
C···H PO ] complex and for the deprotonated ligand
3
4
(
45%) demonstrates the occurrence of a significantly ad-
vanced proton transfer. However, in contrast to that ob-
served with other oxoanions, such a complex is not stable
with respect to further anion addition. In particular, upon
addition of two or more equivalents, the band at 351 nm de-
Chem. Eur. J. 2011, 17, 9423 – 9439
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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9435

L. Fabbrizzi et al.
ꢀ
4
ticular, it is suggested that the second H PO ion estab-
at 350 nm, pertaining to the deprotonated receptor, under-
goes a slight increase. The titration profile at this wave-
length (Figure 24b) shows a smooth curve and tends to
2
lishes hydrogen-bonding interactions with the urea-bound
H PO , giving rise to a coordinated dimer. The interaction
ꢀ
2
4
2
ꢀ
of (H PO )
4 2
ꢀ1
ꢀ1
dimers with ureas has been previously docu-
reach a plateau at 24000m cm , but, on further anion ad-
dition, the absorbance keeps increasing, indicating the pres-
ence of a second stepwise equilibrium. The first equilibrium
is ascribed to the formation of a 1:1 complex, involving two
inequivalent hydrogen bonds, as suggested by the presence
of two bands in the limiting spectrum (dashed line in Fig-
ure 24a). Interestingly, the complex, described by the limit-
2
[
15,16]
mented in solution and in the solid state.
In the present
ꢀ
case, the interaction with the second H PO reduces the ba-
sicity of the urea-bound phosphate, which gives back a
proton to the receptor. Thus, complex b in Scheme 4 is
formed, in which the internal H PO ion receives, on one
side, two parallel hydrogen bonds from the receptor and, on
the other side, donates two parallel hydrogen bonds to the
external H PO . The “normal” hydrogen-bonding interac-
2
4
ꢀ
2
4
+
ing formula [C···HꢀI] , fits the linear inverse relationship
ꢀ
between the absorbances of the two bands, shown in
Figure 20. The ratio of the limiting absorbance of the com-
plex at 350 nm over the limiting absorbance of the depro-
tonated receptor (=37%) is higher than those for chloride
and bromide and would indicate a more pronounced degree
of intramolecular proton transfer. On the other hand, the in-
definite increase of the absorbance of the band at 350 nm
cannot be attributed to complete deprotonation of the re-
2
4
ꢀ
tion between urea and H PO (two equivalent hydrogen
2
4
bonds) accounts for the complete disappearance of the band
at 351 nm. An alternative hypothesis might involve the two
phosphates both binding to the urea protons, forming bifur-
cated hydrogen bonds.
Finally, further distinctive behaviour is presented by
iodide. Figure 24a shows spectra recorded during the titra-
ꢀ
5
2+
ꢀ
tion of a 2.03ꢂ10 m solution of CH
Bu N]I.
in MeCN with
ceptor and formation of the [HX2] self-complex, an event
[
observed, and thermodynamically justified, for fluoride, but
not for other halides. Thus, the observed behaviour must be
specific for iodide. We suggest that the increase of the ab-
sorbance at 350 nm results from the superimposition of a
new band, originating from the charge-transfer transition
4
Upon addition of iodide, a moderate redshift of the band
of the receptor at 291 nm takes place. Moreover, the band
ꢀ
+
ꢀ
from I to the Mepy moiety in a [C···HꢀI](I ) outer-sphere
+
complex of rather low stability. The iodide-to-Mepy transi-
ꢀ
+
tion (I /Mepy !IC/MepyC), which falls in this range of wave-
lengths, was first investigated by Kosower and, in view of its
pronounced dependence upon the nature of the medium,
was used to evaluate solvent microscopic polarity (Z
[18]
value). Due to the low intensity of the outer-sphere inter-
ꢀ
action, the formation of the 1:2 complex [C···HꢀI](I ) re-
quires the addition of a large excess of iodide and, under
ꢀ
these conditions, the outer-sphere interaction of another I
ion with the second Mepy moiety cannot be excluded.
+
At this stage, one could ask why such an interaction has
not been observed when studying the interaction of iodide
+
+
ꢀ
with BH . Thus, we returned to the BH /I system and we
carried out the titration again, this time by adding an ex-
tremely large excess of anion (up to 1000 equiv). Figure 25
shows the spectra taken on titration of a 2.03m solution of
+
BH in MeCN with [Bu N]I. Indeed, on addition of a sub-
4
stantial excess of iodide, a new band develops at 335–
3
40 nm. Such a band cannot be ascribed to any partial or de-
finitive intra-complex proton transfer (observed only for
ꢀ
fluoride and acetate) and should necessarily result from I -
+
to-Mepy charge transfer. The fact that its appearance re-
quires the addition of a much larger amount of anion than
2
+
that for the CH analogue can probably be ascribed to the
ꢀ
lower stability of the [BH···I](I ) outer-sphere complex, in
which the second iodide ion interacts with a formally neutral
complex.
Finally, crystalline products, suitable for X-ray diffraction
Figure 24. a) Absorption spectra taken over the course of the titration of
ꢀ
5
2+
studies, were obtained for the salts [CH] ACHTUNGTNERUNNG( NO ) and [CH]-
3 2
a 2.035ꢂ10 m solution of CH in MeCN with [Bu
4
N]I. b) Titration pro-
AHCTUNGTRENNUNG
(CF SO ) . Figure 26 shows the molecular structure of the
file based on the absorbance at 351 nm, which monitors the formation of
3
3 2
+
ꢀ
the 1:1, [C···HꢀI] , and 1:2, [C···HꢀI](I ), complexes.
nitrate salt.
9436
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Chem. Eur. J. 2011, 17, 9423 – 9439

Proton Transfer in Urea–Anion Hydrogen-Bonded Complexes
FULL PAPER
[
10]
Scheme 5. Significant bond lengths in the deprotonated receptor 1 and
+
in the [CH···NO
3
]
complex. Distances are given in ꢃ.
ꢀ
In particular, the N ꢀC
ACHTUNGTRENNUNG
(
1.32 ꢃ), if compared with the NHꢀC
ACHTUNGTRENNUNG
2
+
two NHꢀC
AHCTUNGTR(EUNNNG aryl) distances in the nitrate complex of CH
Figure 25. Absorption spectra taken over the course of the titration of a
ꢀ
5
+
(1.37 and 1.38 ꢃ, see Scheme 5) falls in the range of stan-
dard deviations, which prevents any significant comparison.
In any case, it cannot be excluded that the solid-state struc-
ture does not correspond to the structure in solution. In par-
ticular, packing forces may favour the more symmetric mo-
lecular arrangement (two equivalent hydrogen bonds),
whereas, in a polar medium, solute–solvent interactions may
promote the formation of species with two inequivalent hy-
drogen bonds, with an asymmetric charge distribution. Note
also in the lateral view (Figure 26) that the nitrate ion is not
coplanar with the framework of the receptor. In particular,
2
4
.03ꢂ10 m solution of BH in MeCN with [Bu N]I.
ꢀ
the NO3 ion may find it convenient to be slightly rotated to
encompass, with its two facing oxygen atoms, the bite of-
fered by the two NꢀH fragments.
The molecular structure of the [CH] ACHTUNGTRNENUG( CF SO ) salt is
3 3 2
3 2
Figure 26. Molecular structure of the [CH] CAHTUNGTRENNNU(G NO ) salt, as obtained from
ꢀ
X-ray diffraction studies. Top (a) and lateral (b) views. The second NO
ion and one molecule of DMF have been omitted for clarity.
3
shown in Figure 27. The triflate ion receives two hydrogen
bonds from the urea subunits and, in contrast to what was
observed for the [BH···CF SO ] complex, N···O (2.97 and
3
3
2
.92 ꢃ) and O···H distances (2.06 and 1.87 ꢃ) are substan-
ꢀ
It is observed that one NO3 anion receives two parallel
tially different from each other. However, the presence of
two inequivalent hydrogen bonds seems to be excluded for
two main reasons: 1) the triflate ion possesses extremely low
basic tendencies and could hardly promote advanced proton
hydrogen bonds from the two urea NꢀH fragments. The two
N···O distances are similar, 2.89 and 2.81 ꢃ, and H···O dis-
tances even more alike (1.85 and 1.87 ꢃ). According to
spectral data in MeCN, the nitrate ion establishes two ineq-
uivalent hydrogen-bonding interactions and, as a conse-
quence, the positions of the two hydrogen atoms should not
be equivalent. However, the ratio of the band intensity of
the complex at 350 nm over the band intensity of the depro-
tonated receptor is only 15%, which suggests a not too pro-
nounced advancement of the intramolecular proton transfer.
Consequently, the extent of the displacement of the hydro-
gen atom along the N···H···O pathway may be of the same
order as the resolution of the X-ray diffraction experiment
and the inequivalence of the two hydrogen bonds cannot be
crystallographically detected.
On the other hand, deprotonation of a urea NꢀH frag-
ment is expected to induce a substantial rearrangement in
the framework of the receptor. For instance, in the molecu-
lar structure of the deprotonated form of the urea receptor
1
-(7-nitrobenzo
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[1,2,5]oxadiazol-4-yl)-3-(4-nitrophenyl)urea
[
10]
(
1), which has been crystallographically investigated and
3 3 2
Figure 27. Molecular structure of the [CH] ACHTUNGRTENUNGN( CF SO ) complex, as ob-
is shown in Scheme 5, the delocalisation of the electron den-
sity from N to the benzofuran moiety is well documented.
tained from X-ray diffraction studies. Top (a) and lateral (b) views. The
ꢀ
ꢀ
second molecule of CF3SO
3
has been omitted for clarity.
Chem. Eur. J. 2011, 17, 9423 – 9439
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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9437

L. Fabbrizzi et al.
transfer; in studies in MeCN, excess addition of triflate did
not induce any serious spectral modification, which accounts
for the very low stability of the 1:1 complex in solution;
ence of the long-wavelength band in the UV/Vis spectrum
that was also observed with the deprotonated, uncomplexed
receptor C . However, the advancement of proton transfer,
+
2
) in the crystalline complex salt the two NꢀC
A
H
U
G
R
N
U
G
which was monitored by the intensity of the long-wave-
length band, was not the same for all anions, but increased
with anion basicity. These complexes (of type II) could be
conventionally considered as resulting from the association
are the same (1.38 ꢃ), thus ruling out any electron delocali-
sation.
Moreover, it is observed in Figure 27b that the plane de-
fined by the two coordinated oxygen atoms and the sulfur
atom of the triflate anion is significantly inclined with re-
spect to the plane containing the urea subunit (angle: 348).
It is therefore suggested that these geometrical features (dif-
ferent hydrogen-bond lengths and inclination between
planes) result from the effort of the oxoanion to position its
oxygen atoms to better encompass the HꢀH bite of the urea
+
between the deprotonated receptor C and one molecule of
the acid HA and could be conventionally described by the
+
+
limiting formula [C···HꢀX] . The receptor BH , of inter-
mediate acidic properties, formed complexes of type II only
with acetate and fluoride and gave complexes of type I with
all remaining anions.
There exists a current interest in redefining the nature of
hydrogen bonding. Anion coordination chemistry offers a
vast playground to verify and validate new theories and
models.
moiety. In this context, it should be noted that the O···O dis-
ꢀ
tance in CF SO (2.38 ꢃ) is distinctly longer than that in
3
3
ꢀ
NO3 (2.12 ꢃ), the complex of which with CH shows a
much lower value of the angle between the planes of the
urea subunit and of nitrate (148).
Experimental Section
General methods and procedures, synthesis and characterisation of recep-
Conclusion
tors AH, [BH]X and [CH]X
nation studies on the [BH···Cl], [BH···CF
CH···CF SO ]CF SO complex salts are described in detail in the Sup-
2
(X=I, CF
3
SO
3
), crystal structure determi-
3
SO ], [CH···NO ]NO , and
3
3
3
Hydrogen bonding has been defined as a more or less ad-
[
3
3
3
3
vanced frozen proton transfer from the donor (e.g., the Nꢀ
porting Information, in which families of spectra from titration experi-
ments are also shown.
H fragment of a urea-based receptor) to the acceptor (e.g.,
ꢀ
[7]
an anion, X ). Recently, an IUPAC Committee (project:
Categorizing hydrogen bonding and other intermolecular in-
teractions) assessed that “Hydrogen bonds are involved in
proton transfer reactions (XꢀH···Y!X···HꢀY) and may be
CCDC 812466, 812467, 812468, and 812469 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
considered the partially activated precursors to such reac-
[19]
tions.” The present study, mostly on the basis of spectral
data in solution, offered the opportunity to verify such a
statement and to explore the nature of hydrogen-bonding
interactions in complexes of receptors of increasing acidity
with anions of varying basicity. The poorly acidic neutral re-
ceptor AH, containing two pyridyl substituents, of moderate
electron-withdrawing properties, established with all anions
Acknowledgements
Financial support from the Italian Ministry of University and Research
PRIN-Dispositivi Supramolecolari) is gratefully acknowledged. L.M.
thanks the Fondazione Cariplo for a fellowship.
(
ꢀ
X , including the basic acetate and fluoride, “normal” hy-
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[
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9438
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9423 – 9439

Proton Transfer in Urea–Anion Hydrogen-Bonded Complexes
FULL PAPER
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Received: February 14, 2011
Published online: July 5, 2011
Chem. Eur. J. 2011, 17, 9423 – 9439
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9439