Carboxylic acid functionalized ortho-linked oxacalix[2]benzene[2]pyrazine: Synthesis, structure, hydrogen bond and metal directed self-assembly

Article abstract of DOI:10.1039/c2dt11283c

Cyclooligomerization of 2,6-dichloropyrazine 4 and benzyl 2,3-dihydroxybenzoate 5 under microwave irradiation resulted in a racemic pair of ester functionalized ortho-linked oxacalix[2]benzene[2]pyrazine 6, which was further transformed to the correspondi

Full text of DOI:10.1039/c2dt11283c

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PAPER
Carboxylic acid functionalized ortho-linked oxacalix[2]benzene[2]pyrazine:
synthesis, structure, hydrogen bond and metal directed self-assembly†‡
Ling-Wei Kong,a Ming-Liang Ma,a Liang-Chun Wu,a Xiao-Li Zhao,b Fang Guo,c Biao Jiang*c and
Ke Wen*a,c
Received 6th July 2011, Accepted 22nd February 2012
DOI: 10.1039/c2dt11283c
Cyclooligomerization of 2,6-dichloropyrazine 4 and benzyl 2,3-dihydroxybenzoate 5 under microwave
irradiation resulted in a racemic pair of ester functionalized ortho-linked oxacalix[2]benzene[2]pyrazine 6,
which was further transformed to the corresponding racemic carboxylic acid functionalized ortho-linked
oxacalix[2]benzene[2]pyrazine 3. Both enantiomers of 3 adopt 1,3-alternate conformations with their two
carboxylic acid groups pointing to opposite directions in the solid state. Enantiomers of 3 form a step-like
one-dimensional supramolecular polymer via intermolecular hydrogen bond interactions between the
carboxylic acids for crystals obtained in methanol. No hydrogen bonds were formed between the
carboxylic acids for crystals of 3 obtained in pyridine and aqueous guanidine solutions; instead,
intermolecular hydrogen bonds between the carboxylic acid groups of 3 and pyridine, as well as
guanidinium ions were formed. Under metal-mediated self-assembly conditions, the pyrazinyl nitrogen
atoms in 3 interacted with transition metal ions, such as Ag(I), Cu(II) and Zn(II), and resulted in the
formation of four new metal-containing supramolecular complexes. Metallomacrocycles 7, 8 and 9 were
formed by reactions of 3 with Ag(I) or Cu(II) ions by bridging two ligands 3 in the equatorial region via
M–N coordination bonds. A one-dimensional coordination polymer 10 was generated by reaction
between ligand 3 and Zn(II) ions, and a cage-based structure is presented in 10 by bridging of the
cyclophane units by Zn2+ ions via Zn–N and Zn–O bonds.
in the construction of various supramolecular objects via self-
assembly. Heterocalixaromatics, structural analogues of calixar-
Introduction
Self-assembly is a widely used technology in the generation of
various supramolecular aggregates.1 Small molecular building
blocks with well-defined geometries, encoded with hydrogen
bonding, hydrophobic/hydrophilic, π–π stacking, and coordi-
nation interaction motifs, are the key to the success in obtaining
versatile supramolecular architectures.2 Macrocyclic molecules,
such as cyclodextrins,3 calix[4]arenes (including resorcin[4]-
arene),4 and cucurbiturils,5 due to their well-defined cavity struc-
tures and conformational preferences, have been intensively used
enes, have recently attracted considerable attention in the area of
supramolecular chemistry.6 Due to the intrinsic electronic nature
of the heteroatoms, which can form various degrees of conju-
gation with their neighboring aromatic units, heterocalixaro-
matics can thus exhibit unique conformational features and
versatile recognition properties.7 The incorporation of nitrogen-
containing heterocycles into oxa- and aza-calixarene derivatives
by Wang et al.,8 Katz et al.,9 Debaen et al.,10 and our group11
has made this class of molecules even more attractive in generat-
ing new metallosupramolecular objects with well-defined struc-
tures and recognition properties through metal-directed self-
assembly. In our previous reports, we have shown that ortho-
linked oxacalix[2]benzene[2]pyrazine 1 and oxacalix[2]arene[2]-
pyrazine 2 (Chart 1) could be used to assemble discrete metal-
containing molecular cages, infinite chains of coordination
cages, as well as one-dimensional coordination polymers via
coordination-driven self-assembly, and some of the resulting
coordination cages could act as host molecules to encapsulate
guest species.12 However, the studies on coordination-driven
self-assembly behaviors of heterocalixaromatics are currently
limited only to 1 and 2, mainly due to the synthetic difficulties
encountered in obtaining these macrocycles with multiple func-
tionalities, which thus hindered their use as molecular building
blocks in constructing sophisticated metallosupramolecular
aDivision of Supramolecular Chemistry and Medicinal Chemistry, Key
Laboratory of Brain Functional Genomics, MOE, East China Normal
University, 3663 Zhongshan Road N, Shanghai 200062, China
bShanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, 3663
Zhongshan Road N, Shanghai 200062, China
cCenter for Nanomedicine, Shanghai Advanced Research Institute,
Chinese Academy of Science, Shanghai 201203, China. E-mail: kwen@
brain.ecnu.edu.cn, jiangb@sari.ac.cn, wenk@sari.ac.cn; Fax: +86-21-
6260-1953; Tel: +86-21-6223-7102
†Dedicated to Professor Wen-Qi Chen on the occasion of his 80th
birthday.
‡Electronic supplementary information (ESI) available: 1H and 13C
NMR for the new synthesized ligand. HRMS (ESI) data for 1 : 1 adduct
of 3 and pyridine and complexes 7 and 8. CCDC 825937, 825938,
825939, 825940, 825941, 825942, 825943. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2dt11283c
This journal is © The Royal Society of Chemistry 2012
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objects via metal-directed self-assembly. Herein, we wish to
report our results on the synthesis and structure of an ortho-
linked oxacalix[2]benzene[2]pyrazine 3 bearing two carboxylic
acid functions (Scheme 1) and the use of 3 in assembling
various supramolecular aggregates, including hydrogen bond-
driven self-assembly of a one-dimensional step-like supramole-
cular polymer, coordination-driven self-assembly of metallo-
macrocycles and coordination polymer.
squares using SHELXS-97.13 All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were added at their geo-
metrically ideal positions and refined isotropically.
Synthesis of ester functionalized ortho-linked oxacalix[2]benzene-
[2]pyrazine 6
2,6-Dichloropyrazine 4 (488 mg, 3.28 mmol), benzyl 2,3-dihy-
droxybenzoate 5 (800 mg, 3.28 mmol) and Cs2CO3 (3.21 g,
9.84 mmol) were stirred in DMSO (20 mL) under microwave
conditions (160 °C, 30 min). The reaction mixture was poured
into 50 mL of water, extracted with ethyl acetate (3 × 20 mL).
The combined extracts were washed with water and brine, and
dried over Na2SO4. The solvent was evaporated and the crude
product was purified by chromatography on a silica gel column
Experimental section
General
All chemicals were used as received without further purification.
Chemical reactions were performed in oven-dried glassware
under an atmosphere of nitrogen. Classic column chromato-
graphy was performed using Merck 60 (70–230 mesh) silica gel.
1H and 13C NMR spectra were recorded on a Bruker Avance 500
spectrometer in the corresponding deuterized solvents. Chemical
shifts are reported in parts per million versus tetramethylsilane.
Mass spectra were recorded on a Bruker micrOTOF-Q spec-
trometer (LC/MS). Single crystal X-ray diffraction data were col-
lected on a Bruker SMART APEX 2 X-ray diffractometer
equipped with a normal focus Mo-target X-ray tube (λ =
0.71073 Å).
(petroleum ether–ethyl acetate 10 : 1). Compound
6 was
obtained as white powder. Yield: 80 mg (3.8%). 1H NMR
(500 MHz, CDCl3): δ 7.88 (s, 1H), 7.78 (s, 1H), 7.68 (d, J =
7.9, 1H), 7.35–7.32 (m, 3H), 7.17–7.16 (m, 2H), 7.05 (t, J = 8.0,
1H), 6.86 (d, J = 7.9, 1H), 5.14 (dd, J1 = 12.0, J2 = 31, 2H); 13
C
NMR (500 MHz, CDCl3): δ 163.79, 156.75, 156.44, 146.35,
144.96, 135.27, 128.55, 128.48, 128.43, 128.40, 126.09, 126.01,
125.61, 66.82; HRMS (ESI): C36H24N4O8Na+ m/z calcd
663.1486, found 663.1452.
Synthesis of acid functionalized ortho-linked oxacalix[2]benzene-
Crystal structure data collection and refinement
[2]pyrazine 3
Intensity data were collected at 173 K or room temperature
(296 K) on a diffractometer using graphite monochromated Mo
Kα radiation (λ = 0.71073 Å) (Table 1). Data reduction included
absorption corrections by the multi-scan method. The structures
were solved by direct methods and refined by full-matrix least-
Compound 6 (2 g, 3 mmol) was dissolved in methanol (200 mL)
and hydrogenated in the presence of 10% palladium on charcoal
(50 mg). The product 3 was obtained by filtering the palladium
on charcoal. Yield: 1.2 g (83%). White powder. 1H NMR
(500 MHz, DMSO-d6): δ 12.90 (s, 1H), 8.13 (s, 1H), 7.99 (s,
1H), 7.65–7.63 (m, 1H), 7.34 (dd, J1 = 1.0, J2 = 8.0, 1H),
7.27–7.24 (m, 1H); 13C NMR (500 MHz, DMSO-d6): δ 164.61,
156.59, 156.33, 145.73, 144.01, 128.29, 127.97, 126.42, 126.23,
−
126.09, 125.50; HRMS (ESI): C22H11N4O8 m/z calcd
459.0571, found 458.9823.
Synthesis of [Ag2(3)2·2ACN] 7
Compound 3 (23 mg, 0.05 mmol) and AgBF4 (29.2 mg,
0.15 mmol) were dissolved in a mixture of N-methylpyrrolidone
(480 μL) and acetonitrile (120 μL) under heating. X-ray quality
single crystals of complex 7 were obtained by slow diffusion of
Chart 1 Molecular structures of 1 and 2.
Scheme 1 Synthesis of carboxylic acid functionalized ortho-linked oxacalix[2]benzene[2]pyrazine 3 (the isomeric structures of 3 are shown in
Fig. 1).
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benzene into the above solution at room temperature in a dark
environment (10 mg, 22%). IR (KBr): 3484, 3072, 2932, 2889,
2491, 1952, 1707, 1635, 1551, 1509, 1451, 1410, 1290, 1264,
1186, 1119, 1081, 1041, 935, 854, 777, 745, 663, 635, 599, 528,
485, 456 cm−1; ESI-MS: m/z 567.1, 795.2, 1027.2, 1035.0.
Anal. calcd for C68H66N14O20Ag2B2F8: C, 45.66; H, 3.72; N,
10.96. Found: C, 45.29; H, 3.88; N, 10.34.
Synthesis of [Cu2(3)2Cl4]·2NMP 8
Compound 3 (23 mg, 0.05 mmol) and Cu(NO3)2·3H2O (36 mg,
0.15 mmol) were mixed in N-methylpyrrolidone (480 μL) and
acetonitrile (240 μL). The resulting blue–green slurry turned
clear upon addition of HCl (2%, 160 μL). X-ray quality single
crystals of complex 8 were obtained by slow diffusion of
benzene into the above solution at room temperature (20 mg,
48%). IR (KBr): 3429, 3099, 2931, 2512, 1708, 1645, 1605,
1546, 1506, 1451, 1411, 1332, 1257, 1194, 1144, 1078, 1046,
1012, 942, 856, 774, 749, 664, 530, 489 cm−1. ESI-MS: m/z
532.2, 720.2, 754.3. Anal. calcd for C54H48N10O21Cu2Cl4: C,
44.98; H, 3.36; N, 9.71. Found: C, 44.53; H, 3.39; N, 9.99.
Synthesis of [Cu2(3)2Cl4]·(3)2 9
Compound 3 (6 mg, 0.0125 mmol) and CuCl2·2H2O (6.4 mg,
0.0375 mmol) were dissolved in methanol (20 mL). X-ray
quality single crystals of complex 9 were obtained by slow evap-
oration of methanol at room temperature (4 mg, 30%). IR (KBr):
3439, 3085, 1728, 1636, 1583, 1551, 1454, 1416, 1320, 1267,
1189, 1145, 1075, 1044, 1010, 956, 852, 803, 767, 648, 513,
401 cm−1. Anal. calcd for C44H24Cl2CuN8O16: C, 50.08; H,
2.29; N, 10.62. Found: C, 49.94; H, 1.24; N, 9.86.
Synthesis of [Zn(3)2·2MeOH]n 10
Compound 3 (23 mg, 0.05 mmol) and Zn(BF4)2·6H2O (52 mg,
0.15 mmol) were dissolved in methanol (4 mL). X-ray quality
single crystals of complex 10 were obtained by slow evaporation
of methanol at room temperature (12 mg, 43%). IR (KBr): 1646,
1610, 1583, 1554, 1448, 1425, 1398, 1334, 1304, 1271, 1250,
1188, 1149, 1074, 1046, 1011, 952, 846, 767 cm−1. Anal. calcd
for C44H24N8O16Zn: C, 53.59; H, 2.45; N, 11.36. Found: C,
51.08; H, 2.05; N, 10.08.
Results and discussion
Synthesis of carboxylic acids functionalized ortho-linked
oxacalix[2]benzene[2]pyrazine 3
The synthesis of ester functionalized ortho-linked oxacalix[2]-
benzene[2]pyrazine 6 could not be isolated by straightforward
heating of 4 and 5 in DMSO in the presence of Cs2CO3 as in the
conventional synthesis of unsubstituted ortho-linked oxacalix[2]-
benzene[2]pyrazine reported previously.11a We thus examined
numerous reaction conditions, and fortunately, compound 6 was
isolated in 3.8% yield as a pair of isomers by heating the reaction
mixture of 4 and 5 in DMSO in the presence of Cs2CO3 at
160 °C for 30 min under microwave irradiation. Followed by Pd
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Fig. 2 Formation of a one-dimensional step-like polymer via intermo-
lecular hydrogen bond interactions between the carboxylic acid func-
tions of a single enantiomer 3 obtained in methanol. Color code: O
(red), N (blue), and C (gray).
corresponding centroid-to-centroid distances (4.60 Å, 4.86 Å
and 4.80 Å) between the face-to-face oriented pair of phenyl
planes of 3 obtained from methanol, pyridine and aqueous gua-
nidinium solutions, respectively. For the pair of pyrazinyl rings
in 3, different trans-annular N⋯N distances between the two
upper rim nitrogen atoms (5.74 Å, 5.25 Å, and 4.04 Å), as well
as the two lower rim nitrogen atoms (2.85 Å, 2.85 Å, and
2.88 Å) were found for 3 obtained in methanol, pyridine, and
aqueous guanidinium solution, thus creating different dihedral
angles between the pair of pyrazinyl rings (70.2°, 51.2°, and
24.3° for 3 obtained in methanol, pyridine, and aqueous guanidi-
nium solution, respectively). The two torsional angles between
the carboxylate planes and their connecting phenyl rings are
found to be different for 3 obtained in methanol (10.4° and
12.9°) and in pyridine (2.8° and 6.3°), while both torsional
angles are 37.0° between the carboxylate planes and their con-
necting phenyl rings for 3 obtained in aqueous guanidine
solution.
Fig. 1 Crystal structure of the isomeric pair of 3 obtained in methanol
(top), pyridine (middle) and aqueous guanidine solution (bottom). Color
code: O (red), N (blue), and C (gray).
catalyzed hydrogenation of 6, an isomeric pair of compound 3
was thus obtained, as shown in Scheme 1 (the two isomeric
pairs of 3 and 6 could not be distinguished in the 2-D drawing,
however, the isomeric structure of 3 can be viewed in the crystal
structure, as seen in Fig. 1; for detailed synthetic procedure, see
Experimental section).
Structure and conformation of 3
Previous studies revealed that replacing the methylene groups by
heteroatoms in the bridging positions of calix[4]arenes resulted
in heteocalix[4]aromatics with mainly 1,3-alternate conforma-
tions.8a,10b,14 An ortho-linked oxacalix[2]benzene[2]pyrazine 1
was also found to adopt a 1,3-alternate conformation.11a In order
to analyze solvent influences on the intermolecular hydrogen
bond interaction patterns between the carboxylic acid functions
of 3, single crystals of 3 of X-ray diffraction quality were grown
in methanol, pyridine and aqueous guanidine solutions. It was
found that both isomers of 3 (obtained in all three solvents)
adopt 1,3-alternate conformations, and the ratio between the two
isomers is 1 : 1 in all three cases. Compound 3 was found to co-
crystallize with pyridine and guanidinium ions (discussed in
later section), but not in the case of methanol. The structures of
the isomeric pair of 3 obtained in the above mentioned three sol-
vents are shown in Fig. 1. The bridging oxygen atoms in macro-
cycle 3 are found to be conjugated with pyrazine rings with an
average C–O bond distance of 1.36 Å, while no conjugation
exists between the bridging oxygen atoms and the phenyl planes
(average C–O bond distance: 1.39 Å). The two carboxylic acid
groups in 3 are pointing in opposite directions. Small differences
were found in dihedral angles (4.5°, 12.7° and 10.5°) and their
Molecular interactions and self-assemblies of 3
Carboxylic acids are known to form dimers via hydrogen bond
interactions in methanol.15 In the presence of organic bases, such
as pyridine and guanidinium ions, the hydrogen bonds between
the carboxylic acids are broken, instead, new hydrogen bonds (or
ion pairs) between the carboxylic acid functions and organic
bases are formed. In the present study, both isomers of com-
pound 3 are found equally presented in the single crystals grown
in the three solvent systems. Molecular crystal structure of com-
pound 3, grown in methanol, reveals a typical hydrogen-bonded
pattern for carboxylic acids. The intermolecular hydrogen bond
interactions between the carboxylic acid functions of 3 (O⋯O
distance: 2.63–2.65 Å) resulted in a one-dimensional stepped
polymeric chain-like topology, and each polymeric chain is
enantiomerically pure, as shown in Fig. 2 (also see Fig. S1‡).
No hydrogen bond interactions exist between the carboxylic acid
groups and the pyrazinyl nitrogen atoms in the crystals of 3
grown in methanol.
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Fig. 3 Intermolecular hydrogen bond interactions between the carboxylic acid groups of a pair of isomers 3 and pyridines. Color code: O (red),
N (blue), and C (gray).
Molecular crystals of 3, grown in pyridine, revealed no inter-
molecular hydrogen bonds between the carboxylic acid functions
being formed, instead, the intermolecular hydrogen bonds
between the carboxylic acid groups of 3 and pyridinyl nitrogen
atoms were observed (O⋯N distance: 2.60 Å and 2.63 Å),
1
shown in Fig. 3. H NMR spectrum of the dissolved co-crystals
of 3 and pyridine in CDCl3 shows a downfield peak at 15.4 ppm
for –COOH proton, suggesting the existence of intermolecular
hydrogen bonding interactions between the carboxylic acid
groups and pyridine in solution; ESI-MS results demonstrate the
presence of 1 : 1 adducts of 3 and pyridine in solution with a
signal at m/z 540.5266 (ESI‡).
The crystals of the guanidinium salt of 3 were obtained by
slow evaporation of its aqueous solution. Guanidinium ions and
3 formed a 2 : 1 adduct in which each guanidinium ion formed
an ion pair with one carboxylic acid function in 3 (Fig. 4 and
Fig. S2‡). Each guanidinium ion is found hydrogen-bonded to
three carboxylate groups (N⋯O distances: 2.85–2.96 Å) and a
pyrazinyl nitrogen atom (N⋯N distance: 3.03 Å) from four inde-
Fig. 4 Intermolecular hydrogen bond pattern between a guanidinium
pendent molecules of 3. The similar length of the three C–N
ion and three carboxylic acid groups, a pyrazinyl nitrogen atom from
four independent molecules of 3. Color code: O (red), N (blue), and C
bond distances in the guanidinium unit (1.31 Å–1.33 Å) indi-
cates its complete protonation. The disappearance of the acid
(gray).
protons in the 1H NMR spectrum of the guanidinium salt of 3 in
CDCl3 also demonstrates the complete deprotonation of the car-
boxylic acid functions of 3 in its aqueous guanidine solution.
the carboxylic acid groups in 3 could also participate in the inter-
actions with metal ions or solvent molecules. We thus envisioned
that under metal-directed self-assembly conditions, the inter-
actions between the cyclophane ligand 3 and metal ions, such as
Ag(I), Cu(II) and Zn(II), could potentially generate more sophisti-
cated supramolecular architectures. In the following section of
this paper, the results of metal-directed self-assembly based on 3
in N-methylpyrrolidone (NMP) and methanol are discussed.
Due to the extremely low solubility of 3 in nonpolar solvents,
the polar solvent NMP was first employed in the self-assembly
process. A discrete silver-containing molecular cage 7 was pre-
pared by dissolving AgBF4 and 3 in NMP and acetonitrile.
Single crystals of the cage complex 7 suitable for X-ray analysis
Self-assembled metallosupramolecular cages and coordination
polymers based on 3
As shown in Fig. 1, the two upper rim nitrogen atoms of the
face-to-face oriented pyrazinyl rings in 3 are directed outward
relative to the central cavity of the oxacalix[4]arene ligand, and
the adjustable dihedral angle between the two pyrazinyl rings
could make them easy to fit the coordination geometrical require-
ments adopted by different transition metal ions, leading to the
formation of coordination supramolecular cages and polymers
via metal–ligand coordinative interactions.12 On the other hand,
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Fig. 6 Crystal structure of 8 shows a Cu-containing coordination cage
self-assembled based on an isomeric pair of 3. Color code: Cu (deep
red), O (red), N (blue), C (gray) and Cl (green).
Fig. 5 Crystal structure of 7 shows the Ag-containing coordination
cage formed by an isomeric pair of ligand 3. Color code: Ag (green), O
(red), N (blue), C (gray), B (brown) and F (yellow).
of 172.5°. The equal coordination environments of the two Ag
centres in 7 indicate the highly symmetrical nature of the cage
complex. The two crystallographically equivalent Ag+ ions in
this [Ag2(3)2]-based cage are separated by 6.76 Å, which is
shorter than the Ag⋯Ag distance (7.87 Å) of the similar cage
complex [Ag2(1)2].12 In the [Ag2(1)2]-based cage, the cage void
was occupied by a hydrophobic methyl group from an aceto-
nitrile coordinated to a neighboring cage.12 However, this is not
the case in 7; the void of the present cage [Ag2(3)2] 7 is found
unfilled by any guest molecules, as its two opposite openings are
blocked by acetonitriles coordinated to its neighboring cage
complexes (Fig. S3‡). In [Ag2(3)2]-based cage 7, the eight pyra-
zinyl nitrogen atoms and the two silver centres form a plane with
a deviation of 0.021 Å. The nitrogen atoms of the two coordi-
nated acetonitriles are found located on opposite sides the plane
(0.07 Å). The dihedral angle between the two pyrazinyl rings of
ligand 3 in cage 7 is 41.8°, while the dihedral angle and the cor-
responding centroid–centroid distance between the pair of
phenyl planes of ligand 3 in cage 7 are 15.7° and 4.91 Å,
respectively. No intermolecular hydrogen bonds between the car-
boxylic acid groups (carboxylic acids dimers) of ligands 3 in 7
were formed. Instead, each carboxylic acid function in 7 is found
hydrogen bonded to a NMP molecule (Fig. 5). All attempts to
generate [Ag2(3)2]-based cage complex in methanol failed, as
crystals of ligand 3 were obtained in all attempts.
were obtained by slow diffusion of benzene into the reaction
1
mixture at ambient temperature in a dark environment. H NMR
study revealed that cage 7 does not survive in coordinating sol-
vents like DMSO-d6, acetonitrile-d3 and DMF-d7, and complex
7 was insoluble in nonpolar solvents, such as benzene-d6, CDCl3
and CD3NO2, thus preventing the study of the solution proper-
ties of complex 7 by NMR techniques. ESI-MS study was
carried out in NMP, signals at m/z 567.1, 795.2, 1027.2 and
1035.0, which can be assigned to [Ag(3)]+ or [Ag2(3)2]2+,
[Ag2(3−)·3ACN]+, [Ag(3)2]+ and [Ag2(3−)·NMP]+ (ACN =
acetonitrile, NMP = N-methylpyrrolidone), respectively, are
observed (ESI‡). These results demonstrate the presence of silver
complexes in solution, similar to silver complexes formed by
silver ions and pyrazinyl-based oxacalix[2]benzene[2]pyrazine
ligands,12 as well as pyridinyl-based cyclotriveratrylene ligands
in solution.16 The ESI-MS results suggest that the cage complex
7 was formed by these silver complex species in the crystalliza-
tion process.
Single crystal X-ray diffraction analysis unambiguously estab-
lished the cage-based structure of [Ag2(3)2·2ACN] 7. Shown in
Fig. 5, an isomeric pair of ligand 3 is bridged by two Ag+ ions
in the equatorial region, resulting in the formation of the rhom-
boidal molecular cage 7. This result suggests the existence of an
assembly process of chiral self-discrimination in the system.
Each Ag(I) ion in 7 is five coordinated and adopts a distorted tri-
gonal bipyramidal arrangement, the silver centre is occupied by
three nitrogen atoms (two from pyrazinyl nitrogen atoms of 3,
one from a coordinated acetonitrile) and two fluorine atoms
The Cu-based cage complex [Cu2(3)2Cl4]·2NMP 8 was pre-
pared by mixing Cu(NO3)2·3H2O and 3 in NMP and acetonitrile,
the resulting blue–green slurry turned clear upon addition of 2%
aqueous HCl. Single crystals of complex 8 were obtained by
slow diffusion of benzene into the reaction mixture at room
temperature. Due to the electronic effect of the paramagnetic
(from two weakly coordinated BF4 anions). The three Ag–N
Cu2+ ion, broad peaks for pyrazinyl protons are given in the H
−
1
bonds (Ag–Npy = 2.25(1) and 2.29(1) Å, and Ag–NCMe = 2.27
(1) Å) form a nonequivalent trigonal plane, with the Npy–Ag–
NMR spectrum of 8 in DMF-d7, which contrast the sharp peaks
in the H NMR spectrum of the free ligand 3 (ESI‡). ESI-MS
1
N
py and two Npy–Ag–NCMe angles of 130.3(1)°, 122.0(1)° and
shows peaks at m/z 532.2, 720.2 and 754.3 for the NMP solution
of complex 8, we could only assign the signal at 720.2 to [Cu-
(3−)·2NMP]+, and the other signals remain to be correctly
assigned to their corresponding complex species (ESI‡). The
crystal structure of 8, determined at 173 K, revealed a similar
cage-based topology to that of 7, shown in Fig. 6. The cage
107.7(1)°, respectively. The coordinated acetonitrile is pointing
outward relative to the Ag-containing cage center (Ag–Ag–
NCMe angle: 170.5°). The two weakly coordinated fluorine
atoms (Ag–F distances: 2.68 Å and 2.77 Å) occupy the opposite
apical positions of the trigonal bipyramidal with a F–Ag–F angle
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Fig. 7 Molecular structure of complex 9 shows a similar [Cu2(3′)2Cl4] cage-based structural motif similar to that of 8, but the two apical positions of
the square pyramidal Cu(II) centres are occupied by a racemic pair of ligand 3′ in 9, instead of two NMP ligands in complex 8. Selected bond angles
(°): Cl–Cu–Cl, 166.7; N–Cu–Cl, 88.0, 88.8, 90.3, 90.6°; Napi–Cu–N, 91.9, 98.3; Napi–Cu–Cl, 94.0, 99.3; Napi–Cu–Cu, 156.5. Color code: Cu (deep
red), O (red), N (blue), C (gray) and Cl (green).
complex 8 was assembled by bridging a pair of isomeric 3 via
coordinative interactions with two Cu2+ ions in the equatorial
region, suggesting an assembly process of chiral self-discrimi-
nation. Each Cu2+ ion adopts a distorted square pyramidal
arrangement, in which the Cu2+ centre is occupied by two pyra-
zinyl nitrogen atoms from the isomeric pair of cage forming
ligands 3, two chloride atoms, and an oxygen atom from a co-
ordinated solvent molecule of NMP. As a result, the two Cu–N
bonds (2.03(1) Å) and the two Cu–Cl bonds (2.27(1) Å, 2.30(1)
Å) form a distorted tetragonal plane with the four N–Cu–Cl
angles of 87.6(1)°, 88.2(1)°, 89.7(1)° and 90.7(1)°, respectively.
The coordinated NMP occupies the apical position of the tetra-
gonal pyramidal with a Cu–O bond distance of 2.20(1) Å, two
O–Cu–N angles of 92.4(1)° and 103.2(1)°, and two O–Cu–Cl
angles of 94.4(1)° and 99.6(1)°, respectively. The two coordi-
nated NMP molecules in cage complex 8 are directed outward
relative to the cage center with two equal O–Cu–Cu angles of
150.1°. The equal coordination environment of the two Cu
centres indicates the high symmetric nature of the cage 8. The
two crystallographically equivalent Cu2+ cations in cage 8 are
separated by only 4.95 Å, which is shorter than that of a similar
Cu-based cage [Cu2(1)2] (Cu⋯Cu distance: 5.66 Å),12b as well
as the Ag-based cage [Ag2(3)2] 7 (Ag⋯Ag distance: 6.76 Å). As
a result, the two coordinated pyrazinyl rings in each Cu2+ centre
in 8 are almost in one plane with a N–Cu–N angle of 164.3(1)°.
The Cl–Cu–Cl angle is 166.0(1)°. No guest molecules could fill
in the narrow cage void of 8. The dihedral angle between the
two pyrazinyl rings of ligand 3 in 8 is 30.0°, while the dihedral
angle and the corresponding centroid–centroid distance between
the pair of phenyl planes of ligand 3 in 8 are 35.5° and 5.53 Å,
respectively. Similar to complex 7, an equatorial plane is formed
by the eight pyrazinyl nitrogen atoms and the two copper atoms
with a deviation of 0.08 Å, and the two coordinated oxygen
atoms from NMPs are found located on the opposite sides of the
plane (1.24 Å). The carboxylic acid functions in 8 were found
hydrogen-bonded with either a NMP or a water molecule.
Reaction of CuCl2·2H2O and ligand 3 in methanol resulted in
a Cu-based cage complex 9, which has a ligand : Cu(II) ratio of
2 : 1. The crystals of complex 9 were insoluble, so the solution
properties of the complex could not be investigated. Due to the
quality of the single crystal, we were only able to get a dataset
that did not allow for high-quality refinement, but could be used
to confirm the basic structural motif of 9. As show in Fig. 7,
complex 9 contains a similar [Cu2(3′)2Cl4]-based (3′ = dimethyl
esters of 3) cage motif to that of 8. However, structural changes
for ligands 3 were found in complex 9, as the carboxylic acid
functions of 3 in 9 have been transformed to methyl ester groups
in methanol solution. Also different from 8, the two apically
coordinated NMP molecules in 8 were replaced by an isomeric
pair of ligands 3′ in 9. Each Cu(II) centre in 9 is coordinated by
three pyrazinyl nitrogen atoms and two chloride anions, and
adopts a distorted square pyramidal arrangement. The two Cu–N
bonds (2.05(1) Å, 2.06(1) Å) and the two Cu–Cl bonds (2.26(2)
Å, 2.28(1) Å) around each Cu(II) centre in cage [Cu2(3′)2Cl4]
form a distorted tetragonal plane, and the two pyrazine rings
around each Cu(II) centre in [Cu2(3′)2Cl4] are almost in one
plane with a N–Cu–N angle of 169.7(2)°. The Cu–N bond dis-
tances of the apically coordinated ligands 3′ are 2.34(1) Å,
which are longer than the Cu–N bond distances in the
[Cu2(3)2Cl4] cage motif (2.02–2.05 Å in 8 and 9). Only one
upper rim nitrogen atom of the apically coordinated ligand 3′
participates in the coordinative interaction with the copper
centre, while the other one remains free. The Cu⋯Cu distance in
9 is 4.43 Å, which is shorter than that in 8 (4.95 Å). Such a
short Cu⋯Cu distance resulted in a smaller dihedral angle
between the pair of pyrazinyl rings in the [Cu2(3′)2Cl4] cage in 9
(24.1°, it is 30.0° in 8). The dihedral angle and the correspond-
ing centroid–centroid distance between the pair of phenyl planes
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 5625–5633 | 5631

View Article Online
of [Cu2(3′)2Cl4] cage in 9 are 30.1° and 5.34 Å, respectively. In
the two apically coordinated ligands 3′, the dihedral angle
between the pair of pyrazine rings is 48.9°, the dihedral angle
and the corresponding centroid–centroid distance between the
pair of phenyl planes are 9.3° and 4.74 Å, respectively. Similar
to 8, the eight pyrazinyl nitrogen atoms in [Cu2(3′)2Cl4] cage in
9 and the two copper atoms form a plane with a deviation of
0.03 Å. The two apically coordinated pyrazinyl nitrogen atoms
of ligands 3′ are located on the opposite sides of the plane
(1.02 Å; ESI‡).
Coordination polymer [Zn(3)2·(MeOH)2] 10 was prepared by
dissolving Zn(BF4)2 and 3 in methanol (see Experimental
section). Single crystals of 10 suitable for X-ray diffraction
analysis were obtained by slow evaporation of its methanol sol-
1
ution. No proton shifts were observed in the H NMR spectra of
10 relative to the free ligand 3 in solvents like DMSO, methanol-
d4, acetonitrile-d3 and DMF-d7, suggesting that complex 10 did
not survive in these solvents. The extremely low solubility of 10
in benzene-d6, CDCl3 and CD3NO2, prevented the further
characterization of its solution behavior by NMR techniques.
ESI-MS data of 10 have not shown any signals related to Zn2+
complexes of ligand 3, possibly due to the polymeric nature of
complex 10. The crystal structure of 10 was determined by
single crystal X-ray techniques at 173 K. As shown in Fig. 8, the
structure of 10 reveals the formation of a one-dimensional
coordination polymer with a 2 : 1 stoichiometry between ligand
3 and Zn2+ ion, and no [Zn2(3)2] cage-based structural motif,
similar to that of 7, 8 and 9, presents in 10. However, a different
[Zn2(3)2] cage-based structural motif is present in the coordi-
nation polymer 10. Each cage was formed by bridging a pair of
enantiomerically pure ligand 3 by two Zn2+ ions via Zn–N and
Zn–O coordination bonds (Fig. 8). Viewed along the polymeric
chain, it is found that the polymeric chain is formed by the alter-
nation of two different enantiomerically pure [Zn2(3)2]-based
cages, resulting in the racemic coordination polymer. In the poly-
meric structure of 10, each ligand 3 has only one exo-nitrogen
atom participating in the coordinative interaction with a Zn2+
cation, while the other one remains free; and one carboxylic acid
function was deprotonated and connected to a second Zn2+
cation through a Zn–O bond. The free carboxylic acid function
of ligand 3 in 10 is hydrogen-bonded to a methanol molecule
(dO⋯O = 2.57 Å, Fig. 8). The Zn2+ centre is six-coordinated and
adopts a distorted octahedral coordination environment, in which
four out of the six coordination sites are occupied by indepen-
dent ligands 3 through two Zn–O (carboxylate) bonds (Zn–O
bond distances: 2.10(1) and 2.07(1) Å) and two Zn–N bonds
(Zn–N distances: 2.26(1), and 2.25(1) Å), the remaining two
coordination sites are occupied by two methanol molecules via
Zn–O bonds (Zn–O bond distances: 2.09(1) and 2.06(1) Å). As
a result, the four coordinated oxygen atoms form a distorted
tetragonal plane, and the two pyrazinyl nitrogen atoms occupy
the two apical positions of the bipyramidal with a N–Zn–N bond
angle of 177.4(1)°. The two carboxylate oxygen atoms occupy
the opposite sides of the Zn2+ centre with an O–Zn–O bond
angle of 177.9(1)°. Similarly, the two coordinated methanolic
oxygen atoms are positioned in the opposite sides of the Zn2+
centre with an O–Zn–O bond angle of 178.7(1)°. The Zn2+ ions
in the polymeric chain are separated by 6.89 Å and are arranged
with Zn–Zn–Zn angles of 169.8°. The 1,3-alternate
Fig. 8 Polymeric structure of 10 formed by the interaction of Zn2+
ions and 3 (top), two enantiomerically different [Zn2(3)2]-based cage
units in coordination polymer 10 (bottom). Color code: Zn (green), O
(red), N (blue) and C (gray).
conformation of ligand 3 is retained in the coordination polymer
10. The dihedral angle between the pair of pyrazine rings of 3 in
10 is 49.6°, while the dihedral angle and its corresponding cen-
troid–centroid distance between the pair of phenyl planes of 3 in
10 are 13.5° and 4.85 Å, respectively. The torsional angle
between the phenyl ring and the uncoordinated carboxylic acid
plane is 26.2°, while the coordinated carboxylate plane has a tor-
sional angle of 34.8° with its connecting phenyl ring. Methanol
molecules were found trapped in the crystal lattice (ESI‡).
As shown in Fig. 5–8, the 1,3-alternate conformation of
ligand 3 was preserved upon the formation of supramolecular
complexes 7, 8, 9 and 10. However, due to the different elec-
tronic structures and coordination preferences adopted by differ-
ent metal ions, the modes of interaction between those metal
ions and ligand 3 could be completely different. Therefore,
under different coordination-driven self-assembly conditions,
ligand 3 could change its coordination modes (M–N or M–O
bonds) and adjust its conformational orientation to fulfil the geo-
metric requirements adopted by those metal ions, facilitating the
formation of the resulting metallosupramolecular complexes 7,
8, 9 and 10.
5632 | Dalton Trans., 2012, 41, 5625–5633
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View Article Online
2 J. W. Steed and J. L. Atwood, Supramolecular Chemistry: A Concise
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In summary, we have successfully synthesized a carboxylic acid
functionalized ortho-linked oxacalix[2]arene[2]pyrazine 3 by
employing microwave irradiation. Compound 3 was found to
exist as a pair of isomers and each isomer adopts a 1,3-alternate
conformation with its two carboxylic acid groups pointing in
opposite directions. Crystallographic studies revealed the differ-
ent intermolecular hydrogen bond interaction patterns for crystals
of 3 grown in different solvents (methanol, pyridine and aqueous
guanidine solution). Under coordination-driven self-assembly
conditions, the interactions of racemic 3 with Ag+ and Cu2+
resulted in the formation of discrete cage-based metallosupramo-
lecules 7, 8 and 9 in a chiral self-discrimination manner by brid-
ging the pair of isomeric ligands 3 via Ag–N and Cu–N
coordination bonds in the solid state. However, in the case of
Zn2+ ions, a coordination polymer 10 was generated. In poly-
meric complex 10, dinuclear Zn-containing metallomacrocycles
(formed by bridging two enantiomerically pure ligands 3 via
Zn–N and Zn–O bonds) act as basic structural motifs of the
coordination polymer; different enantiomers of dinuclear Zn-
containing metallomacrocycles are alternated in the polymeric
chain. The Ag+ ion in cage complex 7 is found in a five coordi-
nation environment and adopts a distorted trigonal bipyramidal
geometry; the Cu2+ ions in the cage complexes 8 and 9 are also
five-coordinated, but adopt distorted square pyramidal arrange-
ments; the Zn2+ ions in coordination polymer 10 are six-coordi-
nated and adopt octahedral coordination environments. The
differing electronic structures and coordination preferences of
these metal ions are believed to play a major role in determining
the molecular geometries of the resulting supramolecular com-
plexes. The studies on the metal-mediated self-assembly based
on oxacalix[n]arene[n]hetarenes, as well as guest recognition and
inclusion, are currently ongoing in our laboratory. New results
will be reported in due course.
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Acknowledgements
The authors thank the National Natural Science Foundation of
China (21071053, 21002031, 61078071), Shanghai Commission
for Science and Technology (06Pj14034, 09ZR1422500,
10ZR1409700), Shanghai Education Development Foundation
(2008CG31) for financial support; support from the Fundamental
Research Funds for the Central Universities is also
acknowledged.
13 (a) G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Göttingen, Germany, 1997; (b) G. M. Sheldrick,
SHELXS-97, Program for solution of crystal structures, University of
Göttingen, Germany, 1997.
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This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 5625–5633 | 5633