Anion-directed organized assemblies of protonated pyrazole-based ionic salts

Article abstract of DOI:10.1007/s11224-012-0122-z

The reactions of 3(5)-(4-methoxyphenyl)-5(3)-phenyl-1H-pyrazole (L ¹ ) with nitric acid and 5-(4-benzyloxyphenyl)-3-(furan-2-yl)-1H- pyrazole(L ² ) with hydrochloric acid produced [HL ¹·NO₃] (Salt-1) and [HL

Full text of DOI:10.1007/s11224-012-0122-z

Struct Chem (2013) 24:705–711
DOI 10.1007/s11224-012-0122-z
ORIGINAL PAPER
Anion-directed organized assemblies of protonated
pyrazole-based ionic salts
•
Chun-yang Zheng Dun-jia Wang
•
•
Ling Fan Jing Zheng
Received: 12 August 2012 / Accepted: 3 September 2012 / Published online: 12 September 2012
Ó Springer Science+Business Media, LLC 2012
Abstract The reactions of 3(5)-(4-methoxyphenyl)-5(3)-
phenyl-1H-pyrazole (L1) with nitric acid and 5-(4-benzyl-
oxyphenyl)-3-(furan-2-yl)-1H-pyrazole(L2) with hydro-
chloric acid produced [HL1 Á NO3] (Salt-1) and [HL2 Á Cl]
(Salt-2). The structures of Salt-1 and Salt-2 were determined
by single crystal X-diffraction. In Salt-1, HL1 showed
[2 ? 2] binding of NO3- ions in the solid state to form dimer
architecture with R21(4) and R44(14) graph sets. An anion
directed one-dimensional anion-assisted helical chain with
active participation of the chloride ion and protonated pyr-
azole via N–HÁÁÁCl hydrogen bonding in Salt-2. In addition,
the protonated HL2 molecules interacted with each other
through weak C–HÁÁÁp interactions resulting in the formation
of another one-dimensional helical chain.
the use of an anionic component for the construction of
noncovalent architectures, especially helical networks, has
emerged as a frontier area of research in the supramolec-
ular chemistry arena [10, 11]. Anions are potential hydro-
gen bond acceptors, while anion-p interaction has
generated considerable attention recently [12]. The metal-
free helical networks with anion, depending on the
involvement of the anion, can broadly be divided into two
categories, anion-templated and anion-assisted networks.
For the former, the anion as such does not take part in the
helix formation, but directs the organic molecule usually
by wrapping the ligand around itself to form a helical
network, while for the latter, the anion is a part of the helix
formation via hydrogen bonds [13].
Pyrazole is an important class of heterocyclic com-
pounds; N-unsubstituted pyrazole is much more interesting
in what concerns the N–HÁÁÁN hydrogen bond network in
their crystals with several hydrogen–bonding patterns:
monomer, dimer, trimer, tetramer, hexamer, and catemer
[14–16]. At the same time, pyrazole has one set of effective
self-complimentary hydrogen bond donor (NH) and
acceptor (N) sites for the formation of hydrogen bonds and
can result in characteristic networks with another hydrogen
bond donor and acceptor [17–23]. Recently, we have
studied the solid state structures of some new substituted
pyrazoles [24, 25]; herein, we report two protonated 3,5-
diaryl-1H-pyrazole, which in the presence of acids undergo
protonation. Protonated HL1 shows [2 ? 2] binding of
NO3- ions in the solid state to form dimer architecture with
R21(4) and R44(14) graph sets, and protonated HL2 forms
one-dimensional anion-assisted helical chain via N–HÁÁÁCl
hydrogen bonding. In addition, protonated HL2 molecules
interact with each other through weak C–HÁÁÁp interactions
resulting in the formation of another one-dimensional
helical chain.
Keywords 1H-pyrazole Á [2 ? 2] Binding Á Dimer Á
Anion-assisted Á Helical chain
Introduction
The past decade has witnessed the rapid development in the
field of design and construction of various artificial helical
structures [1–4]. Except for metal-directed self-assembly
for single-, double-, triple-, or quadruple-helical coordi-
nation complexes [5–7], the helical self-assembly through
hydrogen bonds, p–p stacking, electrostatic interactions
has been extensively investigated [8, 9]. On the other hand,
C. Zheng (&) Á D. Wang Á L. Fan Á J. Zheng
Hubei Key Laboratory of Pollutant Analysis & Reuse
Technology, College of Chemistry and Environmental
Engineering, Hubei Normal University, No. 82 Cihu Road,
Huangshi 435002, People’s Republic of China
e-mail: chunyangzheng@yahoo.com.cn
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Struct Chem (2013) 24:705–711
Experimental
Materials and equipments
referenced to TMS. The elemental analysis was performed
by a Perkin Elmer 2400. The positive ESI-MS spectrum was
obtained with a Finnigan LCQ ADVANTAGE MAX.
All commercially available chemicals were of reagent grade
and were used as received without further purification.
Infrared spectrum was obtained on a Nicolet 5700 FT-IR
instrument with the compound in a KBr disk matrix. The
1H NMR spectrum was obtained with a Varian Mercury-
Plus400 instrument. The sample was dissolved in CDCl3
Synthesis of L1 and L2
3(5)-(4-Methoxyphenyl)-5(3)-phenyl-1H-pyrazole (L1) was
synthesized from 1,3-diketone according to the our pub-
lished procedures [24]. 5-(4-Benzyloxyphenyl)-3-(furan-2-
yl)-1H-pyrazole (L2) was reported by our published article
Chart 1 Preparation of [HL1 Á
NO3] (Salt-1) and [HL2 Á Cl]
(Salt-2)
Table 1 Crystal data and
Compounds
Salt-1
Salt-2
structure refinements for Salt-1
and Salt-2
Empirical formula
C16H15N3O4
C20H17ClN2O2
Formula weight
Temperature (K)
Crystal system
Space group
313.31
352.81
296(2)
296(2)
Monoclinic
Triclinic
P2(1)/c
P1
˚
a (A)
12.200(2)
9.1913(7)
˚
b (A)
7.0180(12)
31.384(2)
˚
c (A)
17.665(3)
11.9829(9)
a (°)
b (°)
c (°)
90
90
97.693(3)
90
90
90
3
˚
Volume (A )
1499.0(4)
3456.5(4)
Z
4
8
Density (calc.) (Mg m-3
)
1.388
1.356
Absorption coefficient (mm-1
)
0.102
0.237
F(000)
Crystal size (mm3)
656
1,472
0.16 9 0.12 9 0.10
2.33–25.01
0.16 9 0.12 9 0.10
2.14–28.28
Theta range for data (°)
Index ranges
-14 B 14, -8 B 8, -20 B 21
7,926
-11 B 11, -24 B 41, -15 B 15
22,875
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I [ 2r(I)]
R indices (all data)
2,557 [R(int) = 0.0418]
2,557/11/246
8,887 [R(int) = 0.0994]
11,095/1/805
1.125
1.077
R1 = 0.0766 wR2 = 0.1660
R1 = 0.0925, wR2 = 0.1742
0.340 and -0.237
R1 = 0.1076, wR2 = 0.2496
R1 = 0.1262, wR2 = 0.2640
0.653 and -0.363
Largest differential peak and
hole(e°/A-3
)
123

Struct Chem (2013) 24:705–711
707
˚
Table 2 Selected bond lengths (A) for Salt-1 and Salt-2
Salt-1
N1–N2
C9–C10
C8–N1
1.342(3)
1.389(4)
1.341(4)
C8–C9
1.384(4)
1.328(4)
C10–N2
Salt-2
N1–N2
1.325(7)
1.319(8)
1.387(9)
1.397(9)
1.334(8)
1.308(9)
1.385(9)
1.373(10)
1.358(8)
1.384(8)
N3–N4
1.354(7)
1.338(7)
1.389(10)
1.380(9)
1.336(9)
1.359(9)
1.406(9)
1.395(10)
1.319(8)
1.360(9)
N5–N6
N7–N8
C14–C15
C15–C16
C16–N2
C14–N1
C54–C55
C55–C56
C56–N6
C54–N5
C34–C35
C35–C36
C36–N4
C34–N3
C74–C75
C75–C76
C76–N8
C74–N7
Fig. 2 View of Salt-1 with 30 % probability displacement ellipsoids
[25]. L1: Colorless needles, yield 74 %. Main IR (KBr,
cm-1): 3229(s, N–H), 1608, 1566(s, C=C, C=N); 1H NMR
(CDCl3, 400): d = 3.86(s, 3H, CH3O), 6.78(s, 1H, pyrazolyl
C–H), 6.97(d, 2H, J = 8.4 Hz), 7.36-7.46(m, 3H), 7.66(d,
2H, J = 8.4 Hz), 7.43–7.75(m, 2H). Anal. Calcd (%) for
C16H14N2O: C, 76.78; H, 5.64; N, 11.19; Found C, 76.72;
H, 5.66; N, 11.13.
Preparation of [HL1 Á NO3] (Salt-1) and [HL2 Á Cl]
(Salt-2)
Salt-1 and Salt-2 were prepared by the reaction of pyrazole
with the corresponding acid (Chart 1). To the 10 mL
methanol–water solution of L1 (0.32 g, 1.0 mmol), 0.5 mL
HNO3 was added and the resulting solution was stirred for
6 h. The colorless crystals suitable for X-ray data collec-
tion were obtained by slow evaporation of the solvent at
room temperature. [HL1 Á NO3] (Salt-1): Main IR (KBr,
cm-1): 3113(br s, N–H), 1614, 1507, 1452 (m C=C, C=N).
Fig. 3 The hydrogen bond system in Salt-1, the dashed lines
represent the intermolecular hydrogen bonds
Fig. 1 Positive ion ESI-MS of
compound L1 in methane
solution
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Struct Chem (2013) 24:705–711
Anal. Calcd (%) for C16H15N3O4: C, 61.34; H, 4.83; N,
13.41; Found C, 61,29; H, 4.86; N, 13.39.
[HL2 Á Cl] (Salt-2) was prepared by the same procedure
as outlined above for using HCl. Main IR (KBr, cm-1):
3111(br s, N–H), 1608, 1502, 1461(m C=C, C=N). Anal.
Calcd (%) for C20H17N2O2Cl: C, 68.09; H, 4.86; N, 7.94;
Found C, 68.04; H, 4.91; N, 7.89.
X-ray structure determination
Fig. 4 View of compound Salt-2 with 30 % probability displacement
ellipsoids
Single crystals of Salt-1 and Salt-2 were selected and
mounted on top of the glass fibers. X-ray single crystal
diffraction measurements were carried out at 296(2) K on a
Bruker Smart 1000 CCD area diffractometer using graphite
˚
monochromatic MoKa radiation (k = 0.71073 A) for data
collection. The structures were solved by direct methods
and expanded by Fourier difference techniques with
SHELXS-97 [26]. Refinements were made by full-matrix
least squares on all F2 data using SHELXL-97 [27]. H atoms
were placed in geometrically idealized positions and con-
strained to ride on their parent atoms, with C–H distances
˚
of 0.93–0.97 A, and with Uiso(H) = 1.2 Ueq(C). All H atoms
on N atoms were placed in geometrically idealized posi-
˚
tions and as riding atoms, with N–H distances of 0.86 A,
Fig. 5 The simplified infinite helical chain of Salt-2 along the a-axis
built from Cl ions and H atoms on C atoms
Table 3 Hydrogen-bonding
geometry for Salt-1 and Salt-2
˚
D–H (A)
˚
˚
D–HÁÁÁA
HÁÁÁA (A)
DÁÁÁA (A)
D–HÁÁÁA (°)
Symmetry codes
Salt-1
N1–H1ÁÁÁO3
0.86
0.86
0.86
0.93
1.99
2.38
1.91
2.75
2.805(16)
3.093(6)
2.749(16)
3.539(4)
160
141
165
144
N1–H1ÁÁÁO4
N2–H2ÁÁÁO2
1 - x, 1 - y, -z
C13–H13ÁÁÁp (Cg3a)
Salt-2
-x, -1/2 ? y, 1/2 – z
N1–H1AÁÁÁCl1
N2–H2AÁÁÁCl4
N3–H3AÁÁÁCl3
N4–H4ÁÁÁCl1
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
2.19
2.27
2.16
2.27
2.19
2.25
2.19
2.25
2.81
2.80
2.40
2.78
2.78
2.79
2.99
2.80
2.75
2.75
3.040(6)
3.059(7)
3.012(6)
3.057(6)
3.040(5)
3.033(6)
3.044(6)
3.044(6)
3.672(4)
3.661(5)
2.724(9)
3.681(5)
3.687(5)
3.502(9)
3.872(5)
3.495(9)
3.420(8)
3.421(8)
170
152
171
153
169
151
170
154
155
155
100
163
163
134
158
133
129
130
1 ? x, y, z
2 - x, -1/2 ? y, 1 – z
1 - x, 1/2 ? y, 1 – z
1 - x, 1/2 ? y, 1 – z
N5–H5AÁÁÁCl4
N6–H6AÁÁÁCl2
N7–H7ÁÁÁCl2
N8–H8ÁÁÁCl3
1 - x, -1/2 ? y, 2 – z
1 ? x, y, z
C10–H10ÁÁÁCl1
C30–H30ÁÁÁCl3
C43–H43ÁÁÁO5
C50–H50ÁÁÁCl4
C70–H70ÁÁÁCl2
C19–H19ÁÁÁp (Cg9)
C29–H29ÁÁÁp (Cg3b)
C39–H39ÁÁÁp (Cg1)
C59–H59ÁÁÁp (Cg13)
C79–H79ÁÁÁp (Cg5)
-1 ? x, y, z
Cg3a is the centroid of atoms
C11–C16; Cg9 of atoms O6/
C57–C60, Cg3b of atoms C1–C6,
Cg1 of atoms O2/C17–C20,
Cg13 of atoms O8/77–C80, and
Cg5 of atoms O4/C37–C40
-x, -1/2 ? y, -z
1 ? x, y, z
1 ? x, y, z
-1 ? x, y, 1 ? z
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Struct Chem (2013) 24:705–711
709
Fig. 6 View of one-
dimensional helical chain
formed via N–HÁÁÁCl hydrogen
bonding with space-filling
model (Parts of C, O, H atoms
were omitted for clarity)
and Uiso(H) = 1.2 Ueq(N). The nitrate anion in Salt-1 dis-
played rotational disorder over two orientations in a
0.55:0.45 ratio. A summary of crystallographic data and
parameters is shown in Table 1 and selected bond lengths
in Table 2.
Fig. 7 Two kinds of different helices are shown in yellow and green
along the b-axis. Yellow represents the helical chain formed via
N–HÁÁÁCl (the red dashed lines) hydrogen bonding and green
represents the helical chain formed via C–HÁÁÁp (the pink dashed
lines) hydrogen bonding (parts of C, O, H atoms were omitted for
clarity) (Color figure online)
Results and discussion
even in solution. The most intense mass peak in the elec-
trospray mass spectrum of L1 appeared at m/z 500.86
corresponding to [2M]?, and the peak at m/z 251.05 was
assigned to [M ? H]? (Fig. 1).
Crystals of [HL1 Á NO3] (Salt-1) and [HL2 Á Cl] (Salt-2)
were formed by slow evaporation of a methanol solution of
the pyrazole in the presence of excess corresponding acid.
In Salt-1, upon protonation by nitric acid, the pyrazole
nitrogen is protonated as shown in Fig. 2. In pyrazolium
molecules, the C–N bond lengths are 1.328(4) and
The structures of Salt-1 and Salt-2 were confirmed by
elemental analysis, IR, and single crystal X-diffraction. In
IR spectra, the Salt-1 and Salt-2 showed N–H stretching
vibration in the region of 3,300–3,100 cm-1. In this region,
the position of the vibrations largely depended on the
extent of hydrogen bonding. The vibrations (3,113 and
3,111 cm-1) were shifted to a lower frequency (longer
wavelengths) side with band widening in Salt-1 and Salt-2
as compared to the free pyrazole L1 and L2 (3,229 and
3,215 cm-1). In addition, strong absorption bands in the
regions of 1,630–1,600 cm-1 and medium bands from
1,450 to 1,510 cm-1 were found in the free pyrazole L1,
L2; Salt-1 and Salt-2 attributed the vibration absorptions of
C=C, C=N bonds, indicating the presence of the charac-
teristic peaks for pyrazole compounds. The 1H-pyrazole
has a dimer formation tendency attributed to the dimer-
ization process by intermolecular N–HÁÁÁN hydrogen
bonds. These strong interactions render the dimer stable
˚
1.341(4) A, respectively, and are remarkably shorter than
˚
the normal C–N bond (1.47–1.50 A), but close to that of
˚
the C=N bond (1.34–1.38 A), which are the result of the
conjugation effect, indicating the significant double bond
characteristics as well as the considerable delocalization of
the electron density of the pyrazole ring. The pyrazole
ring (N1–N2/C8–C10) makes dihedral angles of 13.24(16)
and 23.84(17) ° with toluene ring and benzene ring,
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Struct Chem (2013) 24:705–711
Fig. 8 The C–HÁÁÁp hydrogen
bond in Salt-1
respectively. As shown in Fig. 3, the pyrazole nitrogen is
protonated and the resulting pyrazolium ring binds with the
anions. And, the pyrazole L1 shows [2 ? 2] binding of
hydrogen bonding. But, Pyrazole L2 molecule in the pres-
ence of acid undergoes protonation, and strong N–HÁÁÁCl
hydrogen bonding interaction disrupts the self-assembly of
the L2 molecule and leads to the formation of an infinite
anion-assisted helical chain (Fig. 6). It is important to
mention here that protonated HL2 molecules interact with
each other through weak C–HÁÁÁp interactions between the p
electron density of the furanyl ring and adjacent CH of the
furanyl ring, resulting in the formation of another helical
chain (Fig. 7). In addition, in the crystal structure of Salt-2
exist weak C–HÁÁÁCl intermolecular hydrogen bonds, and
-
NO3 ions by intermolecular N–HÁÁÁN hydrogen bonds in
the solid state to form a dimer architecture with R21(4) and
R44(14) graph set. The measured N1ÁÁÁO3 and N2ÁÁÁO2 dis-
˚
tances are 2.749(16), 2.808(16) A, respectively, showing
strong intermolecular N–HÁÁÁN bonding interactions, which
is comparable to that of the N–HÁÁÁN bonding observed in
the related pyrazole molecules, 3-methyl-5-phenylpyrazole
and 3,5-diphenylpyrazole, which formed the tetrameric
˚
structures in the solid state (2.730(4)–2.803(4) A) [18].
˚
the average bond length is 2.80 A. The chloride ion shows
For Salt-2, the asymmetric unit (triclinic, P1) contains
four independent protonated pyrazole molecules with eight
NH groups and four chloride ions. The proton gets trans-
ferred from the HCl to pyrazole and forms protonated pyr-
azole (Fig. 4). In four independent protonated pyrazole
the primary N–HÁÁÁCl as well as secondary C–HÁÁÁCl inter-
actions as reported earlier [28–30]. The crystal structures of
Salt-1 and Salt-2 both exhibit weak C–HÁÁÁp interactions
(Fig. 8), and the stability of the crystal structure can be
accounted for by these interactions.
˚
molecules, all C–N bond lengths [1.308(9)–1.384(8) A] are
almost equal, and are remarkably shorter than the normal
˚
C–N bond (1.47–1.50 A), but close to that of the C=N bond
Supplementary material
˚
(1.34–1.38 A), which is the result of the conjugation effect,
indicating the significant double bond characteristics as well
as the considerable delocalization of the electron density of
pyrazole ring. In one molecule, the central benzene ring (C8–
C13) makes dihedral angles of 29.7(3) ° and 8.6(4) ° with
rings (C1–C6) and (O2/C17–C20), respectively; the corre-
sponding angles in the other molecules are 29.1(3) ° and
8.9(4) °, 11.8(3) ° and 1.3(3) °, 13.6(3) ° and 1.7(4) °. The
pyrazolyl ring makes dihedral angles of 5.6(4) °, 6.8(4) °,
7.7(4) °, and 7.4(4) ° with four furan rings.
Crystallographic data for structure of Salt-1 and Salt-2 have
been deposited at the Cambridge Crystallographic Data
Center, CCDC Nos. 837171 and 837171 for Salt-1 and Salt-
2, respectively. Copies of this information may be obtained
free of charge from the Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (Fax: ?44 1223 336033; email:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.
ac.uk).
Acknowledgments This study was supported by the important
Foundation of the Educational Commission of Hubei Province, the
People’s Republic of China (No. Q200822002) and the Postgraduate
Program of Hubei Normal University (2008D53).
It is worth pointing out that an anion induced one-
dimensional anion-assisted helical chain with active par-
ticipation of the chloride ion and protonated pyrazole
molecule via intermolecular hydrogen bonds (Fig. 5). The
protonated HL2 molecule forms strong N–HÁÁÁCl bonds
˚
˚
˚
˚
(2.19 A, 170 °; 2.27 A, 152 °; 2.16 A, 171 °; 2.27 A,
References
˚
˚
˚
˚
153 °; 2.19 A, 169 °; 2.25 A, 151 °; 2.19 A, 170 °; 2.25 A,
154 °) with Cl ion (Table 3). Pyrazole can self-assemble
into several hydrogen–bonding patterns: monomer, dimer,
trimer, tetramer, hexamer, and catemer via N–HÁÁÁN
1. Langs DA (1988) Science 241:188
2. Ito H, Ikeda M, Hasegawa T, Furusho Y, Yashima E (2011) J Am
Chem Soc 133:3419
123

Struct Chem (2013) 24:705–711
711
´
´
3. Wu ZQ, Furusho Y, Yamada H, Yashima E (2010) Chem
Commun 46:8962
16. AGarcıa M, Cabildo P, Claramunt RM, Pinilla E, Torres MR,
Alkorta I, Elguero J (2010) Inorg Chim Acta 363:1332
17. Foces–Foces C, Alkorta I, Elguero J (2000) Acta Cryst B 56:1018
18. Arunachalam M, Ghosh P (2010) Cryst Eng Commun 12:1621
19. Basu T, Sparkes HA, Mondal R (2009) Cryst Growth Des 9:5164
20. Boldog I, Daran JC, Chernega AN, Rusanov EB, Krautscheid H,
Domasevitch KV (2009) Cryst Growth Des 9:2895
21. Maeda H, Ito Y, Kusunose Y, Nakanishi T (2007) Chem Com-
mun 11:1136
4. Yamada H, Ito H, Furusho Y, Hasegawa T, Yashima E (2008) J
Am Chem Soc 130:14008
5. Wang HB, Mudraboyina BP, Li J, Wisner JA (2010) Chem
Commun 46:7343
6. Albrecht M (2001) Chem Rev 101:3457
7. Piguet C, Bernardinelli G, Hopfgartner G (1997) Chem Rev
97:2005
8. Brel V, Huc I, Khoury RG, Lehn JM (2001) Chem Eur J 7:2810
9. Tanaka Y, Katagiri H, Furusho Y, Yashima E (2005) Angew
Chem Int Ed 44:3867
10. Hosseini MW (2003) Coord Chem Rev 240:157
11. Gale PA (2009) Chem Soc Rev 38:520
12. Schottel BL, Chifotides HT, Dunbar KR (2008) Chem Soc Rev
37:68
22. Singh UP, Kashyap S, Butcher RJ, Singh HJ, Mishra BK (2011)
Struct Chem 22:931
23. Singh UP, Kashyap S, Singh HJ, Butcher RJ (2011) Cryst Eng
Commun 13:4110
24. Wang DJ, Zheng CY, Fan L (2009) J Mol Struct 938:311
25. Zheng CY, Wang DJ, Fan L (2010) Struct Chem 21:1043
26. Sheldrick GM (1997) SHELXS-97, program for the solution of
¨
¨
13. Lankshear MD, Beer PD (2007) Acc Chem Res 40:657
crystal structures. University of Goettingen, Goettingen
27. Sheldrick GM (1997) SHELXL-97, program for the refinement of
´
14. Foces-Foces C, Echevarrıa A, Jagerovic N, Alkorta I, Elguero J,
Langer U, Klein O, Minguet-Bonvehı M, Limbach HH (2001) J
´
¨
¨
crystal structures. University of Goettingen, Goettingen
28. Kang SO, Llinares JM, Powell D, VanderVelde D, Bowman-
James K (2003) J Am Chem Soc 125:10152
Am Chem Soc 123:7898
15. Claramunt RM, Cornago P, Torres V, Pinilla E, Torres MR,
´
Samat E, Lokshin V, Vales M, Elguero J (2006) J Org Chem
71:6881
29. Chmielewski MJ, Charon M, Jurczak J (2004) Org Lett 6:3501
30. Thallapally PK, Nangia A (2001) Cryst Eng Commun 3:114
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