X-Ray crystallographic and spectroscopic properties of eight Schiff bases as evidence of the proton transfer reaction. Role of the intermolecular hydrogen bond

Article abstract of DOI:10.1039/c0nj00179a

A spectroscopic study of several ortho-hydroxy Schiff bases was carried out, and the corresponding crystal structures were analyzed in order to identify their characteristic hydrogen bonding patterns. The X-ray analysis showed that the enol (O-H...N) tautomer is the most stable in compounds 1-3 whereas the keto (N-H...O) form is preferred in compounds 4-7. The specific intermolecular O-H...O hydrogen bonding interactions that control the supramolecular arrangement of each tautomer are discussed. Additionally, a complete characterization of the polycrystalline samples was attained using solid-state NMR and IR experiments. Solution VT NMR and UV-visible experiments were also used to obtain valuable insights about the nature and stability of the tautomers. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011.

Full text of DOI:10.1039/c0nj00179a

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
NJC
Cite this: New J. Chem., 2011, 35, 156–164
www.rsc.org/njc
PAPER
X-Ray crystallographic and spectroscopic properties of eight Schiff bases
as evidence of the proton transfer reaction. Role of the intermolecular
hydrogen bondw
Oscar Domı
Armando Ariza, Norberto Farfan and Rosa Santillan*
nguez,a Braulio Rodrıguez-Molina,a Mario Rodrıguez,b
´ ´ ´
a
c
a
´
Received (in Gainesville, FL, USA) 6th March 2010, Accepted 4th September 2010
DOI: 10.1039/c0nj00179a
A spectroscopic study of several ortho-hydroxy Schiff bases was carried out, and the
corresponding crystal structures were analyzed in order to identify their characteristic hydrogen
bonding patterns. The X-ray analysis showed that the enol (O–Hꢀ ꢀ ꢀN) tautomer is the most stable
in compounds 1–3 whereas the keto (N–Hꢀ ꢀ ꢀO) form is preferred in compounds 4–7. The specific
intermolecular O–Hꢀ ꢀ ꢀO hydrogen bonding interactions that control the supramolecular
arrangement of each tautomer are discussed. Additionally, a complete characterization of the
polycrystalline samples was attained using solid-state NMR and IR experiments. Solution VT
NMR and UV-visible experiments were also used to obtain valuable insights about the nature
and stability of the tautomers.
Introduction
Schiff bases are important organic compounds of successful
application in several areas, such as biological chemistry1
materials science2 and organic synthesis.3 A tautomeric
Scheme 1 Tautomeric equilibrium for arylimine derivatives.
equilibrium between the enol O–Hꢀ ꢀ ꢀN and keto N–Hꢀ ꢀ ꢀO
forms is commonly present in derivatives of aromatic-o-
hydroxyaldehydes. A reversible intramolecular proton transfer
driven by electrostatic differences between the oxygen and
nitrogen atoms of the salicylideneimine fragment promotes
the delocalization of the aromatic p-electron system leading to
a quinoid form, which is also related with its canonical
zwitterionic form4 (Scheme 1).
properties of the molecules,5 making these compounds
potential candidates for optical switches and storage devices.6
Although extensive studies of several Schiff bases have been
carried out, most of them are related with the synthesis and
spectroscopic characterization of the tautomers,7 or merely
focused on simple structural description of the crystal
structures.8 Moreover, although the tautomeric equilibrum
of the Schiff bases in solution has been widely explored,9
studies concerning the factors that determine the formation
of the tautomers in the solid-state are scarce.10
Furthermore, it has been described that this tautomerism
can be suitably controlled in the solid state by using light or
temperature, depending on the photo- or thermochromic
A preliminary work by Ogawa et al. provided the first X-ray
evidence of the effects of the intermolecular contacts on
the stabilization of the keto tautomers in salicylideneimine
derivatives.12 Subsequently, he showed that the proton
transfer reaction in nonpolar solvents might be controlled by
the aggregation state of the molecules at low temperatures13
(Scheme 2).
a
´
Departamento de Quımica, Centro de Investigacion y de Estudios
Avanzados del IPN, CINVESTAV, Apdo., Postal 14-740, Mexico,
´
´
´
D. F., 07000, Mexico. E-mail: rsantill@cinvestav.mx;
Fax: +52 555-747-3389; Tel: +52 555-747-3725
b
c
´
Centro de Investigaciones en Optica, CIO, Apdo., Postal 1-948,
´
37000 Leon Gto., Mexico
Facultad de Quımica, Departamento de Quımica Organica,
´
´
´
´
´ ´ ´
Universidad Nacional Autonoma de Mexico, Mexico, D.F., 04510,
´
Mexico
w Electronic supplementary information (ESI) available: NMR
characterization and experimental procedures for the preparation of
all compounds, figure with high temperature VT NMR data of
compound 5 in DMSO-d6, table with a complete list of intra- and
intermolecular contacts, additional figure for the crystal packing of
compound 7. CCDC reference numbers 712147 (1), 712151 (2), 712153
(3), 712149 (4) 712150 (5), 712154 (6), 712155 (7) and 712152 (8). For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0nj00179a
Scheme 2 Intramolecular cyclic dimer that stabilizes keto tautomers
in solution.
c
156 New J. Chem., 2011, 35, 156–164
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

View Online
Scheme 4 Intermolecular interactions observed in imine derivatives.
aggregates, etc.)10a,14 or in the solid phase (structural and
electronic effects, hydrogen-bonding, etc.).15
Results and discussion
Scheme
3 Molecular structures of the Schiff bases studied.
Compounds are depicted as observed in X-ray diffraction. For
Characterization of compounds 1–8 in solution
compound 8 the enol tautomer is depicted.
Compounds 1–8 were prepared from the condensation
reaction in methanol of different aminoalcohols and the
corresponding salicylaldehyde. An important aspect of these
compounds is the introduction of a complementary hydroxyl
group, pertaining to the aminoalcohol unit labeled (O2–H2),
in addition to the corresponding OH-salicylidene group
marked as (O1–H1) that may confer additional stability to
the supramolecular structures of all derivatives by establishing
complementary interactions. In derivatives 2 and 8, the
hydrogen-bonding interactions among neighboring molecules
are additionally strengthened by minimization of the steric
repulsions.15
In a previous paper, we reviewed the stabilizing effect of
intermolecular contacts of keto forms of imine derivatives
from salicylaldehyde and substituted anilines.11 In this work,
we extended the investigation to eight ortho-hydroxy Schiff
bases to unveil the influence (electronic and/or structural) of
the substituents on the tautomeric structure from a crystallo-
graphic perspective (Scheme 3). In the selection of substituents
were considered weak or good electron donors as tert-butyl and
diethylamine groups, and electron withdrawing substituents
such as, –NO2, and other groups like –Cl and –Br, to explore
the acidity of the salicylidene fragment in the proton transfer
process. Furthermore, large groups as diphenyl or t-butyl
groups were introduced to evaluate the influence of the steric
effect on the tautomerism.
The use of absorption spectroscopy (UV-vis) for the
identification of tautomeric species in solution16 has permitted
to propose that the formation of intermolecular hydrogen-
bonded aggregates at low temperature stabilizes the keto
forms in nonpolar solvents13 (Scheme 2), even though the
structure of these aggregates has not been clearly determined.
In order to characterize the behavior of compounds 1–8 in
solution, we measured the absorption spectra in different
solvents at room temperature. Fig. 1 contains the spectra of
compounds 4 and 5.
Besides the intramolecular hydrogen bonds present in the
salicylidene fragment, the X-ray diffraction analysis revealed
the existence of five specific intermolecular hydrogen-bonding
patterns due to well aligned O–Hꢀ ꢀ ꢀO hydrogen bonds: zigzag
chains (compound 1, Scheme 4a), tetrameric rings (imine 2,
Scheme 4b), linear chains (compounds 3–4, Scheme 4c), helical
chains (derivative 5, Scheme 4d), cyclic dimeric rings
(compounds 6–7, Scheme 4e) or more complex arrangements
such as those observed in compound 8 (Fig. 5).
In polar-protic solvents (methanol and ethanol) the keto
tautomer (lmax = 400 nm) was considerably formed in
compound 4, although the enol species (lmax = 320 nm)
remained preferentially stabilized.21 Conversely, the intensity
of the absorption bands indicated that the keto tautomer was
slightly favored in methanol and ethanol for the derivative 5
even though a significant amount of the enol form was
detected. In addition, in aprotic solvents as toluene and
dimethylformamide, both compounds were predominantly
stabilized in their enol form (lmax = 320 nm) with a band
Complementary UV-visible and solution NMR experiments
supplied valuable insights about the tautomerization of these
compounds and proved that the main attained form in
solution is not directly related with that obtained in
the solid-state, where the tautomeric stabilization is
completely dependent on the prevailing contributions in the
liquid phase (dipoles moments, solvation effects, molecular
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 156–164 157

View Online
Fig. 1 UV-Vis spectra of compounds (a) 4 and (b) 5 in solvents of
different polarity at room temperature. The bands at 320 and 400 nm
are attributed to the enol and keto forms, respectively.
of much less intensity corresponding to the keto form
(lmax = 400 nm) also observed.16b,c
The spectral data showed that the keto form is moderately
enhanced by protic-polar solvents and much less favored in
aprotic ones. This evidence suggests that the characterization
of the Schiff bases 1–8 in CDCl3 solution (see ESIw)
corresponds to an averaged structure (zwitterionic)7a from
contributions of their enol and the keto species established
in function of the nature of the solvent,14a–c,17 which seems a
recurrent phenomenon in the solution NMR characterization
of related compounds.7a
Fig. 2 1H VT NMR data for the OH signals in different solvents:
(a) compound 4 and (b) compound 5.
We investigated further the equilibrium in solution taking
advantage of the structural simplicity of compounds 4–5,
where a zwitterionic character is assumed in solution. It is
the 1H and 13C spectra of compound 5 in CDCl3 at
room temperature showed signals at 16.75 and 166.9 ppm
for the salicylidene OH and C2 atoms, respectively, which
1
well known that the H-chemical shift of the salicylidene OH
are characteristic of derivatives with
a marked keto
signal of the Schiff bases is strongly dependent on their
tautomeric structures18 with highly shifted signals being re-
lated to the keto character. In this series of compounds the
salicylidene OH-chemical shift in CDCl3 was in the range from
13.18 to 16.75 ppm. Accordingly, 13C NMR spectra of all
compounds showed the resonances corresponding to the
salicylidene C2 atoms ranging from 160.5 to 173.7 ppm at
room temperature. Therefore, we performed a solution VT
NMR study of these compounds to investigate the effect of the
solvent over the proton transfer process as a function
of temperature10a,19 in the temperature range from 200
to 400 K.
character.14c,19c For this reason, compound 5 also was studied
by VT NMR experiments using toluene-d8 and DMSO-d6. As
expected, above room temperature the tautomeric equilibrium
shifts towards the enol form. A shift of the salicylidene OH
signal to low fequencies was observed as the temperature was
increased (Fig. 2b), in accordance with the weakening effect of
the temperature on the intramolecular hydrogen-bonding.22
Unfortunately, heating the imine 5 above 344 K in DMSO-d6
promotes a continuous hydrolysis that reaches a 15%
ratio of hydrolyzed product at 394 K (see ESIw). The
effect of the temperature in pyridine-d5 was also tested but
the results showed no clear relation with the OH-chemical
shift, due to the competition between the pyridine (solvent)
and the imine nitrogen atom (compound 5) for the mobile
proton.
With regard to compound 4 (Fig. 2a), the salicylidene OH
signal reached the maximum chemical shift at the lowest
temperature, indicating that the intramolecular hydrogen
bond (O–Hꢀ ꢀ ꢀN / N–Hꢀ ꢀ ꢀO) is evidently strengthened.20
Conversely, this signal was shifted to lower frequencies at
room temperature. These data reveals that the zwitterionic
character of 4 is increasingly shifted toward its keto form as
the temperature is decreased.10a,19 Evidently, the solvent also
contributes to the stabilization of the keto forms,9c since the
highest OH-chemical shift was observed in the solvent with the
highest polarity (DMF-d7) as compared to CD2Cl2. This effect
is based on the better stabilization of the keto form through
electrostatic interactions (Scheme 1) as has been demonstrated
by experimental and theoretical studies.10a,b,21
Characterization in the solid-state of compounds 1–8
The IR analysis of crystalline compounds 1–8 obtained from
methanol gave the first indication of the tautomeric form
present. Characteristic bands were assigned in accordance
with the literature data.23 Compounds 1–3 showed a sharp
band around 3441–3316 cmꢁ1 due to the n(OH) vibration,
whereas compounds 4–7 present a broad band assignable to
the n(NH) vibration around 3214–3053 cmꢁ1. These data
indicate the keto character of compounds 4–7 and the enol
form in compounds 1–3. Bands for the CQN bonds are in the
range of 1627 to 1614 cmꢁ1 in enol tautomers, while band for
As we have previously pointed out, the salicylideneimine
compounds in solution are generally best described as their
averaged tautomeric forms at room temperature. However,
CQO bonds in the keto species range from 1657 to 1639 cmꢁ1
.
c
158 New J. Chem., 2011, 35, 156–164
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

View Online
crystals for X-ray diffraction24 of all compounds (1–8) were
In the case of compound 8 the IR spectrum suggested the
presence of both tautomeric species showing broad bands in
all regions of the spectrum. Two bands above 3212 cmꢁ1 may
correspond to O–Hꢀ ꢀ ꢀN, N–Hꢀ ꢀ ꢀO and/or O–Hꢀ ꢀ ꢀO inter-
actions, and a very broad band at 1600 cmꢁ1 can be attributed
to the concomitant CQO and CQN bonds of keto and enol
tautomers. X-Ray experiments confirmed the presence of both
forms in the crystal of compound 8, as described in a further
section.
grown by slow evaporation of the methanolic solutions
at room temperature. The most relevant crystallographic
parameters are shown in Tables 1 and 2.
Compounds
1 and 5 crystallize in the monoclinic
space group P21/c containing eight and four molecules
per unit cell, respectively. Compounds 3 and 4 crystallize in
the orthorhombic space groups P212121 and Pca21,
respectively, with four molecules per unit cell. Finally,
compounds 2, 6 and 7 crystallize in the triclinic system, with
X-Ray diffraction studies of compounds 1–7
ꢀ
four molecules per unit cell in the imine 2 (P1), and two
crystallographically independent molecules in the imines 6
and 7 (P1).
After the initial experiments in solution, a detailed analysis of
the molecular and packing motifs was carried out. Suitable
Table 1 Selected crystal and refinement data for compounds 1–4
Crystal dataa
1
2d
3
4
Formula
MW/g molꢁ1
Crystal system
Space group
a/A
b/A
c/A
a (1)
C
22H35NO2
C22H29NO2
339.46
Triclinic
C21H19NO2
317.37
Orthorhombic
P212121
6.1478(2)
7.5477(2)
36.0798(1)
90
C14H19NO2
233.30
Orthorhombic
Pca21
9.0270(18)
6.2300(12)
22.198(4)
90
90
90
1248.4(4)
4
1.241
4269
1399 (0.0462)
1051
0.0532
0.1413
0.28
345.51
Monoclinic
P21/c
14.5103(2)
28.8821(4)
10.2616(1)
90
92.184(4)
90
4297.39(9)
8
1.068
32630
9747 (0.0601)
5426
0.0634
0.1910
0.25
ꢀ
P1
10.2518(2)
10.4845(2)
19.6987(4)
95.9376(7)
104.1553(8)
92.2975(8)
2037.31(7)
4
1.107
23875
9270 (0.0575)
5174
0.0719
b (1)
90
90
g (1)
V/A3
1674.17(7)
4
1.259
4795
1958 (0.0476)
1312
Z
rc/g cmꢁ3
Collected Refl.
Ind. Ref. (Rint
Observed Ref.
R[I > 2s(I)]b
Rw (all data)c
Drmax/e Aꢁ3
Drmin/e Aꢁ3
)
0.0499
0.1488
0.20
ꢁ0.19
0.2251
0.28
ꢁ0.26
ꢁ0.39
ꢁ0.28
P
P
P
P
a
b
2
2
c
2
d
l
Mo-Ka = 0.7103 A. R = (Fo ꢁ Fc2)/ Fo
.
Rw = [ w(Fo ꢁ Fc2)2/ w(Fo2)2]1/2
.
disordered over two positions, with SOFs for the major components of 76% and 52%, respectively.
The OH-benzyl and 5-tbutyl groups of molecule 2 are
Table 2 Crystallographic data for compounds 5–8
Crystal dataa
5d
6
7
8e
Formula
MW/g molꢁ1
Crystal system
Space group
a/A
b/A
c/A
a (1)
C10H13NO2
179.21
Monoclinic
P21/c
10.0724(2)
8.2670(17)
11.7803(2)
90
111.75(3)
90
911.0(3)
4
1.307
3178
1596 (0.0346)
1186
0.0350
0.1049
0.11
C16H16N2O4
300.31
Triclinic
P1
6.1083(2)
10.3445(4)
12.3439(5)
93.254(2)
100.417(2)
92.940(2)
764.38(5)
2
1.305
7278
3435 (0.0517)
2447
0.0613
C16H16BrNO2
334.21
Triclinic
P1
7.2044(2)
9.3402(2)
12.3377(3)
69.1986(10)
80.3396(9)
75.9081(9)
749.70(3)
2
1.480
12177
5691 (0.0305)
4738
0.0432
C18H21ClN2O2
332.82
Triclinic
ꢀ
P1
9.5103(2)
12.8864(4)
15.2106(4)
97.8703(11)
107.113(1)
100.4799(14)
1715.51(8)
4
1.289
14135
7334 (0.0393)
4472
0.0858
b (1)
g (1)
V/A3
Z
rc/g cmꢁ3
Collected Refl.
Ind. Ref. (Rint
Observed Ref.
R[I > 2s(I)]b
Rw (all data)c
Drmax/e Aꢁ3
Drmin/e Aꢁ3
)
0.1787
0.25
0.1158
0.80
0.2786
0.48
ꢁ0.17
ꢁ0.23
ꢁ0.48 ꢁ0.49
The methyl group is equally disordered over two
P
P
P
P
a
b
2
2
c
2
d
lMo-Ka = 0.7103 A. R = (Fo ꢁ Fc2)/ Fo
.
positions (SOF = 50%). The diethylamino group of the molecule 8 is disordered over two positions, SOF = 57% for the major component.
Rw = [ w(Fo ꢁ Fc2)2/ w(Fo2)2]1/2
.
e
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 156–164 159

View Online
Table 3 Selected bond distances (A) for compounds 1–7a
C2–C3 C3–C4 C4–C5 C5–C6 C6–C7
C2–C7
1a* 1.408(3) 1.388(3) 1.401(3) 1.374(3) 1.395(3) 1.403(2)
1b* 1.409(3) 1.390(2) 1.393(3) 1.375(3) 1.396(2) 1.401(3)
2a* 1.411(3) 1.384(3) 1.402(3) 1.376(3) 1.399(3) 1.403(3)
2b* 1.404(3) 1.383(3) 1.402(3) 1.369(3) 1.398(3) 1.400(3)
3*
1.386(5) 1.377(5) 1.396(5) 1.362(5) 1.396(5) 1.408(5)
1.416(5) 1.366(6) 1.397(6) 1.361(6) 1.405(6) 1.428(5)
1.415(2) 1.368(2) 1.390(2) 1.367(2) 1.412(2) 1.429(2)
4+
5+
Fig. 3 Crystal packing of enol tautomers 1 (a), and 2 (b). Ellipsoids
6a+ 1.430(6) 1.346(6) 1.395(6) 1.380(5) 1.398(5) 1.441(5)
6b+ 1.437(6) 1.358(6) 1.407(5) 1.366(5) 1.401(5) 1.443(5)
7a+ 1.449(8) 1.355(7) 1.417(6) 1.364(6) 1.388(6) 1.441(6)
7b+ 1.443(7) 1.376(7) 1.410(7) 1.369(6) 1.390(7) 1.427(7)
are drawn at 50% probability.
Analysis of the supramolecular arrangements for compounds
1–7
a
Two bond distances values are given when two crystallographically
independent molecules are present in the asymmetric unit. Parameters
correspond to (*) the enol tautomer and (+) the keto tautomer.
An exhaustive analysis of the structural parameters of
compounds 1–7 indicated that the supramolecular structure
in each tautomer is governed by one out of two intermolecular
O–Hꢀ ꢀ ꢀO hydrogen bonds. The O2–H2ꢀ ꢀ ꢀO2 interaction,
observed in the enol tautomers, associates closer molecules
through aminoalcohol fragments (the aminoalcohol-
aminoalcohol interaction), and the O2–H2ꢀ ꢀ ꢀO1 interaction,
present in keto tautomers, connects one aminoalcohol
fragment with the ortho-hydroxy group of neighboring
molecules through aminoalcohol-salicylidene interactions.
Undoubtedly, the intermolecular hydrogen bonds contribute
to the stabilization of the keto tautomers, which are less
favored as isolated molecules.10a,31
In all cases, the analysis of the Fourier maps confirmed the
results suggested by the IR experiments. In compounds 1–3 the
H1 atom was covalently bonded to the phenol moiety and in
compounds 4–7 the analysis indicated the formation of keto
tautomers, with the H1 atom completely transferred to the
nitrogen atom forming an enamine group.
In aromatic Schiff bases, the C2–O1 and the C8–N9 bond
distances are the main structural parameters to distinguish
between tautomers. For enol tautomers 1–3, the C2–O1 bond
distances were 1.349(2)/1.354(2) A in 1a/1b, 1.354(2)/1.360(2)
in 2a/2b, and 1.352(4) A in 3. While the C8QN9 bond
distances are 1.268(2)/1.282(2) A in 1a/1b, 1.281(2)/1.279(2) A
in 2a/2b and 1.276(4) A in 3. All in agreement with the
reported bond distances of related phenol tautomers25 and
close to the averaged value for a C–O single bond (1.36 A)
and CQN double bonds (1.28 A).26 Conversely, the crystal
structures of compounds 4–7 clearly showed a keto structure27
characterized by the shortening of the C2–O1 bond distances,
1.294(5) A in 4, 1.314(2) A in 5, 1.289(5)/1.268(5) A in 6a/6b
and 1.282(7)/1.286(7) A in 7a/7b.
A detailed examination of the supramolecular arrangements
in enol tautomers 1–2 showed that only the aminoalcohol-
aminoalcohol interaction (O2–H2ꢀ ꢀ ꢀO2) was involved. In
compound 1, the expansion of this interaction generates
infinite chains C(2) of molecules along the c crystallographic
axis (Fig. 3a), with Oꢀ ꢀ ꢀO distances of 3.003(2)/3.039(2) A in
1a/1b molecules. In the case of compound 2, this interaction
promoted by the benzylic hydroxy group, led to the formation
of a tetrameric ring R44(8) (Fig. 3b), showing Oꢀ ꢀ ꢀO distances
of 2.49(3)/2.78(2) A to 2a/2b molecules.
Moreover, the C–C bond lengths of the salicylidene ring
(C2–C7) in enol tautomers 1–3 showed similar values, whereas
in compounds 4–7, the bond distances reveal an alternating
shortening and lengthening of the C–C bonds (Table 3). These
results are in complete agreement with the assigned
tautomeric forms.
A shared characteristic of enol derivatives 1–2 is the
3,5-substitution by bulky tert-butyl groups that prevent the
participation of the salicylidene fragment in intermolecular
arrangements.
As a general trend, the ketonic salicylidene O1 atom
commonly adopts the role of hydrogen-acceptor in the
hydrogen-bonding geometry of the keto or zwitterionic
tautomers, which is closely related to the high electrostatic
charge of the heteroatoms (N and O)4,10a (Scheme 1).
Accordingly, in compounds 3–7, the only hydrogen-
bonding interaction in their supramolecular structures is the
aminoalcohol-salicylidene (O2–H2ꢀ ꢀ ꢀO1) interaction. By
expanding this interaction, infinite chains C(9) of molecules
are generated along the a crystallographic axis in 3 (Fig. 4a)
and along the b axis in 4 (Fig. 4b) and 5 (Fig. 4c). Additionally,
an analogous interaction promotes the formation of a dimeric
cycle R22(18) between two crystallographically independent
molecules in imine 6 (Fig. 4d) and 7 (Fig. 4e). In compound 3,
the participation of the enolic salicylidene O1 atom in the
crystal packing was attributed to the acquired conformation in
the fragment of the aminoalcohol by the presence of the bulky
diphenyl unit.
Analysis of intramolecular interactions in compounds 1–7
The compounds herein reported showed two types of intra-
molecular hydrogen bonds, O1–H1ꢀ ꢀ ꢀN9 in enol and
N9–H9ꢀ ꢀ ꢀO1 in the keto form. These interactions account
for the formation of closely planar pseudoaromatic chelates
S(6).28 Further examination of the Oꢀ ꢀ ꢀN lengths in these
intramolecular six-membered cycles for compounds 1–8 did
not reveal a clear relationship with the tautomeric nature
(A complete series of hydrogen-bonding parameters is listed
in Table 4 of the ESIw). Regarding this, the well-known
phenomenon called resonance-assisted hydrogen bonds
(RAHBs)29 may explain the pronounced stability of the keto
tautomers in terms of p-bond cooperativity effects of the
conjugated system involved.30
c
160 New J. Chem., 2011, 35, 156–164
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

View Online
Fig. 5 ORTEP drawing of the crystal structure of 8 showing one-half
of the centrosymmetric arrangement where the enol and the keto
tautomers co-crystallized in the unit cell.
values of 1.335(4) A and 1.285(4) A in the enol 8A and 1.303(3) A
and 1.328(3) A in the keto form 8B, respectively. Similar
bond distances were observed by Dong et al. in the
cocrystallization of enol and keto tautomers from aza
compounds.33
The crystal packing of derivative 8 (Fig. 5) is characterized
by
a centrosymmetric arrangement including the co-
crystallization of both tautomers. The O(1B) atom of the keto
species plays the role of triple hydrogen-bond acceptor and
two adjacent OH-benzylic groups from enol-O(2A), keto-O(2B)
and the N(9B) atoms are the corresponding donors. Two keto
molecules form an expanded dimeric array (not shown)
R22(20) by means of O2B–H2Bꢀ ꢀ ꢀO1B0 interaction
(dO2Bꢀ ꢀ ꢀO1B = 2.702(4) A). Furthermore, two neighboring
OH-benzylic groups from enol molecules interact with
keto-O(1B) atoms through O2A–H2Aꢀ ꢀ ꢀO1B contacts
(dO2Aꢀ ꢀ ꢀO1B = 2.735(4) A).
As an additional evidence supporting the coexistence of the
two tautomeric forms in the crystal of 8, its 13C CPMAS
spectrum was obtained from the same samples studied by
X-ray crystallography. Solid-state 13C CPMAS NMR is a
powerful technique capable of distinguishing between crystallo-
graphic entities such as polymorphs,34 solvates and hydrates,35
providing additional information, such as the number of
molecules in the asymmetric unit.14,36
Fig. 4 Crystal packing of enol tautomer 3 (a), keto tautomers 4 (b),
5 (c), 6 (d) and 7 (e). Ellipsoids are drawn at 50% probability.
It is worth to notice that weaker C–Hꢀ ꢀ ꢀO hydrogen
bonds32 complement the packing of the tautomers (with
exception of 3); however, the number in the case of the keto
tautomers is larger than the one present in enol forms. In
compound 4, the C10–H10Bꢀ ꢀ ꢀO2 contacts form a C(4) chain
of molecules that developed perpendicularly to the C(9) chain
established by the O2–H2ꢀ ꢀ ꢀO1 interactions (Fig. 4b). In
compound 5, an expanded intramolecular ring R24(20) is
formed by C6–H6ꢀ ꢀ ꢀO1 interactions (Fig. 4c). Finally, inter-
molecular dimers were found in the packing of keto tautomer
7 linked through the C14B–H14Bꢀ ꢀ ꢀO2A contacts, forming
chains of dimers C(13)[R22(18)] along the c crystallographic
axis (Fig. 4e).
In the case of the structurally related compounds 6–7, the
electronwithdrawing nature of the para-substituted group
(NO2 or Br) in the salicylidene fragment displaces the
equilibrium exclusively to the keto form by completing proton
transfer from the O1 to N9 atom. As expected, intermolecular
interactions O2–H2ꢀ ꢀ ꢀO1 complemented the stabilization of
these keto tautomers. The Oꢀ ꢀ ꢀO distances are 2.728(5)/
2.725(4) A in 6a/6b and 2.721(5)/2.719(5) A in 7a/7b. Lastly,
two additional Brꢀ ꢀ ꢀO contacts complete the sphere of
coordination of the salicylidene O1 atom of compound 7
(ESIw).
As noted in Fig. 6, a set of doublets of equal intensity for
each expected signal confirmed the presence of two molecules
per asymmetric unit. The 13C CPMAS dipolar dephasing
experiment (to distinguish quaternary carbon atoms by
suppressing any CH and CH2 signals) complemented the
tentative assignment of the enol and keto tautomers signals.
Two aromatic resonances at d 177.7 and d 163.6 that remained
with equal intensity after the NQS experiment were assigned to
CQO and C–OH carbon atoms, respectively, in agreement
with known shifts for the keto and enol forms.
An analogous analysis of the solid state spectra of
compounds 5–6 (see ESIw) further support our carbonyl
assignments. The signal at d = 179.5 for compound 6
corresponds to the two crystallographically independent
carbonyl groups found in the asymmetric unit. Additionally,
a single peak assigned to CQO at d = 178.7 was observed in
the spectrum of derivative 5 where one crystallographically
independent keto molecule was present per asymmetric unit. It
therefore became clear that the stabilization of the keto forms
X-Ray diffraction and solid-state NMR of compound 8
The X-ray diffraction analysis24 of compound 8 showed that
the enol (labeled as 8A) and the keto (labeled as 8B) tautomers
cocrystallized in the unit cell in a 1 : 1 ratio (Fig. 5). The crystal
structure has two intramolecular cycles S(6) that result from
the O1A–H1Aꢀ ꢀ ꢀN9A [dO1ꢀ ꢀ ꢀN9
=
2.591(4) A] and
N9B–H9Bꢀ ꢀ ꢀO1B [dO1ꢀ ꢀ ꢀN9
=
2.623(3) A] interactions.
Moreover, the C2–O1 and C8–N9 bond distances attained
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 156–164 161

View Online
and Heteronuclear Chemical Shift Correlation 1H-13C
HETCOR were recorded on JEOL eclipse +400 and
ECA +500 spectrometers. Chemical shifts (ppm) are relative
to (CH3)4Si for 1H and 13C. Mass spectra were recorded on an
Agilent G1969A APCI Atmospheric Pressure-Chemical
Ionization time-of-flight spectrometer. UV-Vis spectra were
recorded on a Perkin Elmer Lambda 900 spectrophotometer.
Solid-state NMR experiments
Crystalline samples of selected compounds obtained from
methanolic solutions were previously ground with a mortar
and a pestle at room temperature in order to get homogeneity,
and they were packed in a 4 mm wide ZrO2 rotor with a KelF
cap using 80–90 mg of each compound. 13C CPMAS solid
state NMR spectra of selected compounds were obtained on a
Bruker AVANCE II300 spectrometer operating at 13C
1
frequency of 75.47 MHz with H broadband decoupler using
Fig. 6 Below: solid-state 13C CPMAS spectra of compound 8.
Above: 13C CPMAS dipolar dephasing experiment showing only
quaternary and methyl carbons.
a 4 mm broadband probe. A spinning frequency of 10 kHz at
the magic angle was found to be successful for the removal of
spinning sidebands using an optimized cross polarization
contact time of
1 ms. Dipolar dephasing experiments
in this aggregate is enhanced by the multiple O2–H2ꢀ ꢀ ꢀO1
hydrogen bonds, which are common interactions in the
formation of the keto species.
(13C CPMAS non-quaternary suppression) were carried out
1
with a delay of 25 ms before turning the H decoupler on.
Single crystal X-ray structure determinations
Conclusions
All diffraction data were measured using an Enraf Nonius
Kappa-CCD diffractometer with graphite-monochromated
lMo-Ka = 0.71073 A. Frames were collected at T = 293 K
via o/j rotation. Direct methods SHELXS-86 and SIR-2004
were used for structure solution and SHELXL-97 program
package for refinement and data output.24 C–H hydrogen
atoms were placed in geometrically calculated positions using
a riding model. O–H and N–H hydrogen atoms have been
localized by difference Fourier maps and their bond distances
and isotropic temperature factors have been refined freely
(In compound 3, one restraint was necessary to fix a long
O1–H1 bond distance [1.16(4) A] with a DFIX 0.84 instruction).
Three and four restraints were automatically inserted by
SHELXL to fix the origin of the lattice in the refinement of
compounds 6 and 7, respectively. Fifteen restraints were
required to make the C–N and C–C bond distances of the
diethylamino groups identical (one of them disordered over
two positions) of the two crystallographically independent
molecules of compound 8. As indicated in the literature,37 in
compounds 3, 4 and 6 the Friedel opposites were merged
before final refinement (MERG 4). The absolute configuration
of 3 and 6 was chosen on the basis of the known configuration
of the starting materials, while the configuration of 4 was
arbitrarily selected. The Flack parameter for compound 7
refined satisfactorily to 0.028(9). Figures were created with
ORTEP-3 for windows version 2.02. Verification of hydrogen-
bonding interactions in the crystal lattice was carried out with
PLATON program under the WINGX interface.
The synthesis and the crystallographic study at room
temperature of eight Schiff bases is reported. In the solid-state,
each tautomer was characterized by different intermolecular
O–Hꢀ ꢀ ꢀO hydrogen bonds. The O2–H2ꢀ ꢀ ꢀO2 interactions
(aminoalcohol-aminoalcohol) were principally related to the
enol forms in compounds 1 and 2, and the O2–H2ꢀ ꢀ ꢀO1
interactions (aminoalcohol-salicylidene) were fundamentally
associated with the keto species in compounds 4–7. Although,
the enol tautomer was also observed in the case of derivative 3,
voluminous substituents (i.e. diphenyl unit in the side of the
aminoalcohol) prevent the formation of the expected
O2–H2ꢀ ꢀ ꢀO2 bonds and force the formation of O2–H2ꢀ ꢀ ꢀO1
interactions. Additionally, the co-existence in the solid-state of
both the keto and enol tautomers in 8 was supported by X-ray
diffraction and solid-state NMR experiments. Solution
UV-Vis experiments and solution VT NMR studies clearly
revealed that the predominance for one tautomer is strongly
dependent on the solvent investigated and the temperature
used. Keto forms are preferred in protic solvents whereas enol
tautomers were observed in aprotic ones. In spite of that, the
preferential formation of a particular tautomer in the solid
state is modulated through a combination of the steric factors
of the substituents modifying the consequent capability of the
molecules to interact with each other through appropriate
intermolecular O–Hꢀ ꢀ ꢀO hydrogen bonds.
Experimental
Acknowledgements
Melting points were obtained with an Electrothermal
9200 apparatus and are uncorrected. Infrared spectra were
measured on a FTIR Varian spectrophotometer ATR. 1H and
13C spectra as well as Correlation Spectroscopy 1H-1H COSY
The authors thank CONACyT (project No. 83519) and
UNAM (PAPIIT IN-214010) for financial support and the
scholarship to O. Domınguez and B. Rodrıguez-Molina.
´ ´
c
162 New J. Chem., 2011, 35, 156–164
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

View Online
Thanks are given to Consejo Superior de la Investigacio
Cientıfica en Espana for the Cambridge Crystallographic Data
Base license, to M. L. Rodrıguez and V. Gonzalez for NMR
spectra, G. Cuellar for MS, M. A. Leyva for X-ray diffraction
and Dr E. Garcıa-Baez for valuable discussion.
´
n
11 M. Rodrı
K. Nakatani, E. V. Garcı
Supramol. Chem., 2007, 19, 641–653.
´
guez, R. Santillan, Y. Lo
´
pez, N. Farfa
´
n, V. Barba,
nez,
´
a-Baez and I. I. Padilla-Martı
´
´
´
´
12 K. Ogawa, Y. Kasahara, Y. Ohtani and J. Harada, J. Am. Chem.
Soc., 1998, 120, 7107–7108.
13 (a) K. Ogawa, J. Harada, T. Fujirawa and S. J. Yoshida, J. Phys.
Chem. A, 2001, 105, 3425–3427; (b) T. Fujiwara, J. Harada and
K. Ogawa, J. Phys. Chem. A, 2009, 113, 1822–1826.
´
´
´
14 (a) L. Antonov, W. M. F. Fabian, D. Nedeltcheva and
F. S. Kamounah, J. Chem. Soc., Perkin Trans. 2, 2000,
1173–1179; (b) M. Kluba, P. Lipkowski and A. Filarowski, Chem.
Phys. Lett., 2008, 463, 426–430; (c) T. Dziembowska, M. Szafran,
A. Katrusiak and Z. Rozwadowski, J. Mol. Struct., 2009, 929,
32–42.
15 (a) A. Koll and P. Wolschann, Monatsh. Chem., 1996, 127,
475–486; (b) A. Filarowski, T. Glowiak and A. Koll, J. Mol.
Struct., 1999, 484, 75–89; (c) A. Filarowski, A. Koll and
T. Glowiak, Monatsh. Chem., 1999, 130, 1097–1108.
Notes and references
1 (a) Y. Gat and M. Sheves, J. Am. Chem. Soc., 1993, 115,
3772–3773; (b) S. Ren, R. Wang, K. Komatsu, P. Bonaz-Krause,
Y. Zyrianov, C.-E. McKenna, C. Csipke, Z. A. Tokes and
E. J. Lien, J. Med. Chem., 2002, 45, 410–419.
2 (a) G. K. Patra and I. Goldberg, Cryst. Growth Des., 2003, 3,
´
321–329; (b) M. Sliwa, S. Letard, I. Malfant, M. Nierlich,
P. G. Lacroix, T. Asahi, H. Masuhara, P. Yu and K. Nakatani,
Chem. Mater., 2005, 17, 4727–4735; (c) M. Sliwa, A. Spangenberg,
´
I. Malfant, P. G. Lacroix, R. Metivier, R. B. Pansu and
K. Nakatani, Chem. Mater., 2008, 20, 4062–4068; (d) E.-Q. Gao,
Y.-F. Yue, S.-Q. Bai, Z. He and C.-H. Yan, Cryst. Growth Des.,
2005, 5, 1119–1124.
16 (a) M. Miura, J. Harada and K. Ogawa, J. Phys. Chem. A, 2007,
111, 9854–9858; (b) R. Herzfeld and P. Nagy, Curr. Org. Chem.,
2001, 5, 373–394; (c) S. H. Alarcon, A. C. Olivieri, R. M. Cravero,
G. Labadie and M. J. Gonza
8, 713–720.
17 (a) J. Jansky and A. Koll, Struct. Chem., 2004, 15, 353–361; (b) W.
´
les-Sierra, J. Phys. Org. Chem., 1995,
´
3 (a) C. D. Meyer, C. S. Joiner and J. F. Stoddart, Chem. Soc. Rev.,
2007, 36, 1705–1723; (b) M.-D. Zhou, J. Zhao, J. Li, S. Yue,
C.-N. Bao, J. Mink, S. L. Zang and F. E. Kuhn, Chem.–Eur. J.,
M. F. Fabian, L. Antonov, D. Nedeltcheva, F. S. Kamounah and
P. J. Taylor, J. Phys. Chem. A, 2004, 108, 7603–7612.
¨
18 (a) A. Filarowski, A. Koll, M. Rospenk and I. Krol-Starzomska,
J. Phys. Chem. A, 2005, 109, 4464–4473; (b) N. Wachter-Jurcsak
and C. A. Detmer, Org. Lett., 1999, 1, 795–798.
2007, 13, 158–166; (c) R. M. Moreno, M. Rosol and A. Moyano,
Tetrahedron: Asymmetry, 2006, 17, 1089–1103.
4 (a) V. Jeseentharani, J. Selvakumar, A. Dayalan, B. Varghese and
K.-S. Nagaraja, J. Mol. Struct., 2010, 966, 122–128; (b) K. Ogawa,
J. Harada, I. Tamura and Y. Noda, Chem. Lett., 2000, 528–529;
(c) S. Bilge, Z. Kilic¸ , Z. Hayvali, T. Hokelek and S. Safran,
¨
J. Chem. Sci., 2009, 121, 989–1011.
5 (a) J. L. Scott and K. Tanaka, Cryst. Growth Des., 2005, 5,
1209–1213; (b) J. Harada, H. Uekusa and Y. Ohashi, J. Am. Chem.
Soc., 1999, 121, 5809–5810; (c) E. Hadjoudis and I.-M. Mavridis,
Chem. Soc. Rev., 2004, 33, 579–588.
6 (a) H. Tian and S. Yang, Chem. Soc. Rev., 2004, 33, 85–97;
(b) F. M. Raymo and S. Giordano, Org. Lett., 2001, 3,
3475–3478; (c) L. Giordano, T. M. Jovin, M. Irie and
E. A. Jares-Erijman, J. Am. Chem. Soc., 2002, 124, 7481–7489;
(d) S. J. Lim, J. W. Seo and S. Y. Park, J. Am. Chem. Soc., 2006,
19 (a) I. Krol-Starzomska, A. Filarowski, M. Rospenk, A. Koll and
´
S. Melikova, J. Phys. Chem. A., 2004, 108, 2131–2138;
(b) T. Dziembowska, Z. Rozwadowski, A. Filarowski and
P. E. Hansen, Magn. Reson. Chem., 2001, 39, S67–S80.
20 (a) P. Gilli, V. Bertolasi, V. Ferretti and G. Gilli, J. Am. Chem.
Soc., 2000, 122, 10405–10417; (b) P. Gilli, V. Bertolasi, L. Pretto,
L. Antonov and G. Gilli, J. Am. Chem. Soc., 2005, 127, 4943–4953.
21 A. Filarowski and I. Majerz, J. Phys. Chem. A, 2008, 112,
3119–3126.
22 (a) N. S. Golubev, S. N. Smirnov, P. M. Tolstoy, S. Sharif,
M. D. Toney, G. S. Denisov and H. H. Limbach, J. Mol. Struct.,
2007, 844–845, 319–227; (b) X. C. Tang, M. J. Pikal and
L. S. Taylor, Pharm. Res., 2002, 19, 484–490.
23 (a) J. W. J. Ledbetter, J. Phys. Chem., 1977, 81, 54–59;
(b) G. Wojciechowski, P. Przybylski, W. Schilf, B. Kamienski
´
128, 14542–14547; (e) J. L. Maldonado, Y. Po
´
nce-de-Le
guez, M. A. Meneses-Nava,
a, R. Santillan and N. Farfan, J. Phys. D:
´
on,
´
G. Ramos-Ortiz, M. Rodrı
´
and B. Brzezinski, J. Mol. Struct., 2003, 649, 197–205;
(c) V. P. Rybalkin, A. D. Dubonosov, E. N. Shepelenko,
L. L. Popova, M. I. Makarova, A. V. Tsukanov, B. A.
Bren’ and V. I. Minkin, Russ. J. Org. Chem., 2002, 38,
1326–1330.
O. Barbosa-Garcı
´
´
Appl. Phys., 2009, 42, 075102; (f) H. Fukuda, K. Amimoto,
H. Koyama and T. Kawato, Org. Biomol. Chem., 2003, 1,
1578–1583.
7 (a) P.-E. Hansen, Z. Rozwadowski and T. Dziembowska, Curr.
Org. Chem., 2009, 13, 194–215; (b) R. M. Claramunt, C. Lopez,
´
24 Structure solution: (a) G. M. Sheldrick, SHELXS-86—Program
for crystal structure solution, University of Gottingen, Germany,
¨
M.-D. Santa Marıa, D. Sanz and J. Elguero, Prog. Nucl. Magn.
´
1986; (b) M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini,
G. L. Cascarano, L. De Caro, C. Giacovazzo, G. Polidori and
R. Spagna, J. Appl. Crystallogr., 2005, 38, 381–388; (c) Data
Refinement: G. M. Sheldrick, SHELXL-97—Program for the
Reson. Spectrosc., 2006, 49, 169–206; (c) Y.-M. Hijji, B. Barare,
A.-P. Kennedy and R. Butcher, Sens. Actuators, B, 2009, 136,
297–302; (d) A. Perona, D. Sanz, R. M. Claramunt and J. Elguero,
Molecules, 2006, 11, 453–463; (e) J. H. Chong, M. Sauer,
B. O. Patrick and M. J. MacLachlan, Org. Lett., 2003, 5,
3823–3826; (f) E. Ito, H. Oji, T. Araki, K. Oichi, H. Ishii,
Y. Ouchi, T. Ohta, N. Kosugi, Y. Maruyama, T. Naito,
¨
refinement of crystal structure, University of Gottingen, Germany,
1997; (d) ORTEP-Graphics: L. Farrugia, J. Appl. Crystallogr.,
1997, 30, 565; (e) PLATON-Hydrogen bonding interactions:
A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7–13; (f) WINGX-
interface: L. Farrugia, J. Appl. Crystallogr., 1999, 32, 837–838.
T. Inabe and K. Seki, J. Am. Chem. Soc., 1997, 119, 6336–6344.
. Albayrak, M. Odabaoglu and O. Buyukgungor,
¨
8 (a) A. Ozek, C
¸
¨
¨
¨
¨
˙
25 (a) V. T. Kasumov, H. Turkmen, I. Uc¸ ar, A. Bulut and N. Yaylı,
¨
Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2007, 63,
o177–180; (b) C. C. Ersanli, M. Odabaoglu, C. Albayrak and
A. Erdonmez, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2004,
60, o264–266.
9 (a) C.-J. Chang, T.-L. Shieh and H. G. Floss, J. Med. Chem., 1977,
20, 176–178; (b) W. Schilf, B. Kamiensky and T. Dziembowska,
Spectrochim. Acta, Part A, 2008, 70, 60–68; (b) A. Takashiro and
E. Yasuaki, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2006,
62, o4315–4317.
¸
¨
26 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen
and R. Taylor, Typical Interatomic Distances: Organic Compounds
in International Tables of Crystallography, IUCR, 1992, vol. C,
pp. 790–811.
27 (a) X.-X. Sun, S.-L. Ma, H. B. Huang and C.-M. Qi, Acta
Crystallogr., Sect. C: Cryst. Struct. Commun., 2007, 63, o87–88;
´
(b) B. Kaitner and G. Pavlovic, Acta Crystallogr., Sect. C: Cryst.
´
J. Mol. Struct., 2002, 602–603, 41–47; (c) S. Sharif, G. S. Denisov,
M. D. Toney and H.-H. Limbach, J. Am. Chem. Soc., 2006, 128,
3375–3387; (d) Z. Rozwadowski and T. Dziembowska, Magn.
Reson. Chem., 1999, 37, 274–278.
10 (a) K. Ogawa and J. Harada, J. Mol. Struct., 2003, 647, 211–216;
(b) Filarowski, A. Kochel, M. Kluba and F. S. Kamounah,
J. Phys. Org. Chem., 2008, 21, 939–944; (c) M. Sauer, C. Yeung,
J. H. Chong, B. O. Patrick and M. J. MacLachlan, J. Org. Chem.,
2006, 71, 775–788.
Struct. Commun., 1996, 52, 2573–2575.
28 For hydrogen-bond motifs notations see: J. Bernstein, R. E. Davis,
L. Shimoni and N.-L. Chang, Angew. Chem., Int. Ed. Engl., 1995,
34, 1555–1573.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 156–164 163

View Online
29 (a) P. Gilli, V. Bertolasi, L. Pretto, V. Ferreti and G. Gilli, J. Am.
Chem. Soc., 2004, 126, 3845–3855; (b) V. Bertolasi, P. Gilli,
V. Ferretti and G. Gilli, J. Am. Chem. Soc., 1991, 113, 4917–4925.
30 V. Bertolasi, L. Pretto, G. Gilli and P. Gilli, Acta Crystallogr.,
Sect. B: Struct. Sci., 2006, 62, 850–863.
31 (a) S. D. Chatziefthimiou, Y. G. Lazarou, E. Hadjoudis,
T. Dziembowska and I. M. Mavridis, J. Phys. Chem. B, 2006,
110, 23701–23709; (b) K. Pyta, P. Przybylski, W. Schilf,
B. Ko"odziej, A. Szady-Che"mieniecka, E. Grech and
B. Brzezinski, J. Mol. Struct., 2010, 967, 140–166.
33 Z.-H. Wu, J.-P. Ma, X.-W. Wu, R.-Q. Huang and Y.-B. Dong, Acta
Crystallogr., Sect. C: Cryst. Struct. Commun., 2009, 65, o128–130.
34 (a) R. K. Harris, J. Pharm. Pharmacol., 2007, 59, 225–239;
(b) M. R. Caira, Top. Curr. Chem., 1998, 198, 163–208.
35 (a) S. M. Reutzel-Edens, J. K. Bush, P. A. Magee,
G. A. Stephenson and S. R. Byrn, Cryst. Growth Des., 2003, 3,
897–907; (b) G. A. Stephenson, J. G. Stowell, P. H. Toma,
R. R. Pfeiffer and S. R. Byrn, J. Pharm. Sci., 1997, 86, 1239–1244.
36 R. K. Harris, S. A. Joyce, C. J. Pickard, S. Cadars and L. Emsley,
Phys. Chem. Chem. Phys., 2006, 8, 137–143.
32 (a) T. Steiner, Crystallogr. Rev., 1996, 6, 1–57; (b) G. R. Desiraju,
Acc. Chem. Res., 1991, 24, 290–296; (c) P. A. Wood, F. H. Allen
and E. Pidcock, CrystEngComm, 2009, 11, 1563–1571.
37 (a) H. D. Flack and G. Bernardinelli, J. Appl. Crystallogr., 2000,
´
33, 1143–1148; (b) A. Bia"osnka and Z. Ciunik, Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun., 2007, 63, o120–o122.
c
164 New J. Chem., 2011, 35, 156–164
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011