Mercaptothiazolinyl functionalized hexapodal and tripodal receptors on benzene platform: Formation of silver ion assisted hexanuclear metallocage vs metalO-rganic polymer

Article abstract of DOI:10.1021/ic102363y

Mercaptothiazolinyl functionalized hexapodal (L¹) and tripodal (L²) receptors on the benzene platform have been synthesized easily in good yields and structurally characterized by a single-crystal X-ray crystallographic study. In the solid state, L¹ shows an orientation of six arms in 1,3,5 vs 2,4,6 facial steric gearing fashion, whereas L ² adopted C_2v symmetry where two of its thiazolinyl arms are oriented in one direction and the third arm in the another direction. Two silver complexes of L¹, 1 ([2(L¹)·6(AgClO ₄)·2(CHCl₃)·HClO⁴]) and 2 ([2(L¹)·6(AgClO₄)]), that are suitable for single-crystal X-ray studies are isolated upon the slow diffusion of a dimethylformamide solution of AgClO₄ to the solution of L¹ in chloroform and dichloromethane, respectively. Similarly, upon the slow diffusion of an acetonitrile solution of AgClO₄ to the chloroform solution of L², colorless crystals of the silver complex of L ², 3, are successfully isolated. The structural analyses of 1 and 2 show the formation of a silver ion assisted hexanuclear metallocage Ag ₆(L¹)₂ via dimeric assembly of L¹ with multiple clefts and pockets toward guests binding. In 1, two chloroform molecules sit in top and bottom pockets, whereas six perchlorate counteranions are bound in six clefts between the silver ion pillared side arms of the metallocage. Though complex 2 shows the formation of a metallocage like 1, the single crystal structural analysis depicts perchlorate counteranions bonded to the silver atoms of the metallocage. On the contrary, the silver complex of tripodal receptor L², 3, shows the formation of a metallo-organic polymeric network of L² and Ag⁺. To the best of our knowledge, this work represents the first report on the formation of an M ₆L₂ type metallosupramolecular cage topology with multiple clefts for guest binding by a semirigid hexapodal receptor.

Full text of DOI:10.1021/ic102363y

ARTICLE
pubs.acs.org/IC
Mercaptothiazolinyl Functionalized Hexapodal and Tripodal
Receptors on Benzene Platform: Formation of Silver Ion Assisted
Hexanuclear Metallocage vs MetalꢀOrganic Polymer
B. Nisar Ahamed, M. Arunachalam, and Pradyut Ghosh*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road,
Kolkata 700032, India
S Supporting Information
b
ABSTRACT: Mercaptothiazolinyl functionalized hexapodal
(L1) and tripodal (L2) receptors on the benzene platform have
been synthesized easily in good yields and structurally char-
acterized by a single-crystal X-ray crystallographic study. In the
solid state, L1 shows an orientation of six arms in 1,3,5 vs 2,4,6
facial steric gearing fashion, whereas L2 adopted C2v symmetry
where two of its thiazolinyl arms are oriented in one direction
and the third arm in the another direction. Two silver complexes
of L1, 1 ([2(L1) 6(AgClO4) 2(CHCl3) HClO4]) and 2
3
3
3
([2(L1) 6(AgClO4)]), that are suitable for single-crystal
3
X-ray studies are isolated upon the slow diffusion of a dimethyl-
formamide solution of AgClO4 to the solution of L1 in chloro-
form and dichloromethane, respectively. Similarly, upon the slow diffusion of an acetonitrile solution of AgClO4 to the chloroform
solution of L2, colorless crystals of the silver complex of L2, 3, are successfully isolated. The structural analyses of 1 and 2 show the
formation of a silver ion assisted hexanuclear metallocage Ag6(L1)2 via dimeric assembly of L1 with multiple clefts and pockets
toward guests binding. In 1, two chloroform molecules sit in top and bottom pockets, whereas six perchlorate counteranions are
bound in six clefts between the silver ion pillared side arms of the metallocage. Though complex 2 shows the formation of a
metallocage like 1, the single crystal structural analysis depicts perchlorate counteranions bonded to the silver atoms of the
metallocage. On the contrary, the silver complex of tripodal receptor L2, 3, shows the formation of a metallo-organic polymeric
network of L2 and Agþ. To the best of our knowledge, this work represents the first report on the formation of an M6L2 type
metallosupramolecular cage topology with multiple clefts for guest binding by a semirigid hexapodal receptor.
’ INTRODUCTION
compared to cavity binding.4eꢀl Many different varieties of molec-
ular cages and cage-like discrete structures have been constructed
using metallosupramolecular chemistry. The most common
topologies3d of molecular cages are M3L2,5aꢀc M4L2,5d M4L4,5e
The metal-assisted construction of discrete molecular assem-
blies is a useful strategy for the generation of different molecular
architectures.1 This area of research has enormous interest in the
chemistry community due to the assemblies’ aesthetically appeal-
ing topologies, potential applications in hostꢀguest chemistry,
and catalysis.1,2 Many examples of structurally fascinating com-
pounds have been designed and synthesized via the assembly of
multidentate ligands based on accurate control of metalꢀligand
coordinate bond formation.2 These considerations are mainly
based on the number of ligands, their relative orientation in
space, and the geometry imposed by the metal ion coordination,
resulting in a thermodynamically stable product favored by entropic
and enthalpy factors.3 The preorganization in the resulting
supramolecular structure provides a proper space, such as clefts
and pockets, specifically for a particular guest which has the
correct geometry. These metallocages have shown multiple guest
encapsulation inside the cavities and clefts through a range of
different supramolecular interactions.4 A biologically important
cleft binding mode of the supramolecular hosts is rather limited
M4L6,5f and M6L4 (M = metal, L = ligand). However, the
5f,g
M6L2 topology of a cage-like solid state structural formation over
other assemblies is rare in supramolecular chemistry.3d,6 In the
case of M6L2, the formation of metallocages arises by coordina-
tion of metal ions via the dimeric assembly of planar rigid
hexadendate receptors, mainly induced by twisting the planarity
of the ligand system.6a,b As a result, these planar hexadendate
ligands mostly favor the formation of sandwich-shaped supra-
molecular architectures. The formation of a Ag6L2 metallocage
by a macrocyclic hexacarbene ligand has also been demonstrated
in the literature.6c Significant advances in the variety of supra-
molecular metallocapsules have been made on the basis of planar
rigid2b,7 as well as flexible5a,8 1,3,5-substituted tripodal and
Received: November 27, 2010
Published: May 10, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic102363y Inorg. Chem. 2011, 50, 4772–4780
|

Inorganic Chemistry
ARTICLE
1,2,4,5-substituted tetrapodal9 receptors on a benzene platform.
Recently, we and others have utilized the conformational
flexibility10aꢀc of benzene-based semi-rigid hexa-host receptors
in order to bind cations,10d,e anions,10f,g and ion pairs10d,h via
different coordination behaviors of hexa-hosts. Herein, we de-
monstrate the formation of a hexanuclear metallocage, Ag6(L1) 2
with multiple clefts for guest binding via silver ion assisted
dimeric assembly of thiazolinyl functionalized hexapodal recep-
tor L1. Further, we also demonstrate the importance of a hexa-
host with thiazolinyl functionality toward the cage formation
with an example of a polymeric metal organic framework upon
the complexation of thiazolinyl functionalized tripodal receptor
L2 with Agþ.
Analytically pure L1 was obtained upon filtration followed by drying
under vacuum conditions. Yield: 75%. H NMR, 300 MHz (CDCl3):
1
4.58 (s, 12H), 4.22 (t, 12H), 3.42 (t, 12H). 13C NMR, 75.5 MHz (CDCl3):
49.35, 106.18, 129.42, 138.02, 139.99. HRMS (ESI), m/z calcd. for
[C30H36N6S12þNa]þ: 886.9548. Found: 886.5908. Elemental analysis
calcd for C30H36N6S12: C, 41.64; H, 4.19; N, 9.71; S, 44.46. Found: C,
41.35; H, 4.03; N, 9.74; S, 44.67. IR Data (KBr pellet, cmꢀ1): 3431.13(w),
2935.46(w), 2846.74(s), 1568.02(w), 1473.51(s), 1446.51(w), 1301.86(s),
1226.64(s), 1191.93(w), 995.2(s), 966.27(s), 919.98(s), 873.69(s),
802.33(s), 640.32(s), 526.53(s).
Synthesis of L2. 2-Thiazoline-2-thiol (0.595 g, 5 mmol) was added
into a 50 mL of dry CH3CN containing 1 g of potassium carbonate and
stirred for 10 min at RT. 1,3,5-Tris(bromomethyl)benzene (0.5 g, 1.25
mmol) was added to the reaction mixture and refluxed for 24 h. A white
precipitate was observed during the progress of the reaction that
indicated the formation of potassium bromide. Solvent was evaporated
under vacuum conditions, and the reaction mixture was poured into
250 mL of cold water and stirred for an hour. The product was crashed
out in the form of a white solid upon stirring. It was filtered, washed with
water (3 ꢁ 100 mL), and dried in vacuo to obtain L2 as an analytically
pure product. Yield: 80%. 1H NMR, 300 MHz (CDCl3): 4.45 (s, 6H),
4.28 (t, 6H), 3.44 (t, 6H) and 2.44 (s, 9H). 13C NMR, 75.5 MHz
(CDCl3): 16.24, 33.39, 35.72, 64.42, 130.69, 137.42, and 165.77. HRMS
(ESI) m/z calcd. for [C21H27N3S6]þ: 513.05. Found: 514.1843. Ele-
mental analysis calcd for C21H27N3S6: C, 49.09; H, 5.30; N, 8.18; S, 37.44.
Found: C, 48.92; H, 5.29; N, 8.20; S, 37.41. IR Data (KBr pellet, cmꢀ1):
3375.2(w), 2939.31(w), 2844.81(s), 1564.16(w), 1481.23(s), 1438.8(w),
1301.86(s), 1224.71(s), 1155.28(w), 1058.06(w), 1014.49(s), 993.27(s),
968.2(s), 921.91(s), 796.55(s), 700.11(w), 642.25(w), 530.39(s),
491.81(s), 376.09(s).
’ EXPERIMENTAL SECTION
Materials and Methods. 2-Thiazoline-2-thiol and silver per-
chlorate (AgClO4) were purchased from Aldrich and were used
directly without further purification. Potassium carbonate (K2CO3),
acetonitrile (CH3CN), methanol (CH3OH), ethanol (EtOH), chloro-
form (CHCl3), dichloromethane (CH2Cl2), dimethylformamide(DMF),
and dimethylsulfoxide (DMSO) were purchased from Cyno-chem,
India. Solvents were dried by conventional methods and distilled under
a N2 atmosphere before being used. Hexakis(bromomethyl)benzene11a
and 1,3,5- tris(bromomethyl)benzene11b were synthesized as per litera-
ture procedures.
Instrumentation. 1H NMR and 13C NMR spectra were recorded
on Bruker 300 and 75 MHz FT-NMR spectrometers, respectively, using
tetramethylsilane as an internal reference. ESI-MS measurements were
carried out on a Qtof Micro YA263 HRMS instrument. Elemental
analyses for the ligands and complexes were carried out with a 2500
series II Elemental Analyzer, Perkin-Elmer, USA. Caution! Metal per-
chlorate salts are potentially explosive under certain conditions. All due
precautions should be taken while handling perchlorate salts.
Designing Aspects. For a receptor to form self-assembled supra-
molecular architectures upon complexation with metal ions, it should
possess suitable functionalities decorated on a suitable framework.
Ligand rigidity or conformational restriction is also recommended to
reduce the likelihood of forming alternative products of lower stoichi-
ometry which are favored by entropy. Therefore, our designing princi-
ples are as follows: (1) In search of a new higher generation receptor for
the synthesis of a supramolecular metallocage of the M6L2 type, the hexa
substituted benzene moiety is selected as a suitable framework. (2)
Thioether linkages around the benzene platform are introduced to
impose a semirigid nature that should allow spatial segregation of arms
around the central benzene ring in the ababab (1, 3, 5 vs 2, 4, 6)
conformation over other conformations. (3) The thiazolinyl moiety is
chosen to act as a metal chelator via its nitrogen center. (4) The size and
shape of the cage can be controlled by the relative orientation of the
ligand and the geometry imposed by metal ions.5 To achieve a linear
coordination geometry, Agþ is chosen to maximize the space between
the ligands in a metallocage formation.5 (5) Similar functionalization on
the tripodal receptor on the benzene platform is designed for a compar-
ison of metal-assisted assembly.
Synthesis of 1 ([2(L1) 6(AgClO ) 2(CHCl3) HClO4]). To iso-
3
3
3
late single crystals of complex 1, a DMF4solution (3 mL) of AgClO4 (3.4
mg, 16.2 mmol) was slowly layered on the solution of L1 (2 mg, 2.3
mmol) in CHCl3 (3 mL). These solvent mixtures were allowed to diffuse
in the dark for a week to obtain colorless crystals of 1 (yield ∼ 10%). 1H
NMR of 1, 300 MHz (DMSO-d6): 4.56 (s, 12H), 4.22 (t, 12H), 3.42 (t,
12H). Lower solubility of 1 in DMSO-d6 prevented us from performing
13C NMR experiments. HRMS (ESI): complex 1 is dissolved in DMSO
and diluted with CH3OH prior to injection into the mass spectrometer.
Only fragmented patterns are observed. m/z calcd. for [C30H36N6-
S12þ2AgþClO4]þ: 1180.6135. Found: 1180.9083. m/z calcd. for
[C30H36N6S12Ag]þ: 973.2947. Found: 973.0341. Elemental analysis
calcd for C62H75N12S24Ag6Cl13O28: C, 22.47; H, 2.28; N, 5.07; S, 23.22.
Found: C, 22.85; H, 2.14; N, 5.12, S, 23.49. IR Data (KBr pellet, cmꢀ1):
3454.27(w), 2927.74(s), 2844.81(s), 1560.3(w), 1529.45(s), 1515.94(s),
1436.87(w), 1379.01(s), 1303.79(s), 1220.86(s), 1190.00(s), 1139.85(w),
110.92(w), 1087.78(s), 1014.49(s), 993.27(s), 964.34(s), 923.84(s),
800.4(s), 698.18(w), 630.68(s), 576.68(s), 530.39(s), 493.74(w).
Synthesis of 2 ([2(L1) 6(AgClO )]). To isolate single crystals of
3
4
complex 2, a DMF solution (3 mL) of AgClO4 (3.4 mg, 16.2 mmol) was
slowly layered on a solution of L1 (2 mg, 2.3 mmol) in CH2Cl2 (3 mL).
These solvent mixtures were allowed to diffuse in the dark over a week to
1
obtain colorless crystals of 2. Yield: ∼12%. H NMR of 2, 300 MHz
(DMSO-d6): 4.54 (s, 12H), 4.19 (t, 12H), 3.39 (t, 12H). The lower
solubility of 2 in DMSO-d6 prevented us from performing 13C NMR
experiments. HRMS (ESI): Complex 2 is dissolved in DMSO and
diluted with CH3OH prior to injection into the mass spectrometer. Only
fragmented patterns are observed. m/z calcd. for [C30H36N6S12þ2Agþ
ClO4]þ: 1180.6135. Found: 1181.0006. m/z calcd. for [C30H36N6S12-
Ag]þ: 973.2947. Found: 973.0844. Elemental analysis calcd for
C60H72N12S24Ag6Cl6O24: C, 24.23; H, 2.44; N, 5.65; S, 25.87. Found:
C, 24.15; H, 2.43; N, 5.63; S, 25.93. IR Data (KBr pellet, cmꢀ1):
3454.27(w), 3118.68(w), 2933.53(s), 2848.67(s), 1568.02(s), 1473.51(s),
1446.51(w), 1429.15(w), 1301.86(s), 1226.64(s), 1141.78(s), 110.92(s),
Synthesis of L1. 2-Thiazoline-2-thiol (0.656 g, 5.5 mmol) was
added into 50 mL of dry CH3CN containing 1 g of potassium carbonate
and stirred for 10 min at room temperature. Hexakis(bromomethyl)-
benzene11a (0.5 g, 0.786 mmol) was added to the reaction mixture and
refluxed for 48 h. During this period, a white precipitate was formed.
Then, the solvent was evaporated under vacuum conditions; the reaction
mixture was poured into 150 mL of cold water and stirred for an hour.
The formation of a white solid was observed upon stirring. The solid was
filtered and washed with a sufficient amount of water (3 ꢁ 100 mL).
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Inorganic Chemistry
ARTICLE
Table 1. Crystallographic Parameters for L1, 1, 2, and 3
parameters
L1
1
2
3
empirical formula
fw
C
30H36N6S12
C62H75Ag6Cl13N12O28S24
C60H72Ag6Cl6N12O24S24
C21H27AgClN3O4S6
721.14
865.37
3313.85
trigonal
R3
2974.66
trigonal
R3
cryst syst
space group
a (Å)
triclinic
P1
monoclinic
P2(1)/c
16.423(3)
9.4511(14)
20.270(3)
90
9.524(4)
10.240(4)
10.575(4)
71.163(9)
77.511(9)
84.535(9)
952.5(7)
1
16.4061(19)
16.4061(19)
35.098(8)
90
26.8371(17)
26.8371(17)
11.2997(8)
90
b (Å)
c (Å)
R (deg)
β (deg)
90
90
95.743(4)
90
γ (deg)
V (Å3)
120
120
8181(2)
3
7048.0(8)
3
3130.5(8)
4
Z
dcalcd (g/cm3)
cryst size (mm3)
diffractometer
F(000)
1.509
1.978
2.103
1.530
0.15 ꢁ 0.05 ꢁ 0.01
Smart CCD
450
0.08 ꢁ 0.06 ꢁ 0.05
Smart CCD
4827
0.2 ꢁ 0.18 ꢁ 0.12
Smart CCD
4428
0.18 ꢁ 0.16 ꢁ 0.14
Smart CCD
1464
μMo KR (mmꢀ1
)
0.721
1.904
2.007
1.160
T (K)
100 K
100 K
100 K
100 K
θ max
24.99
20.29
23.22
22.28
reflns collected
independent reflns
params refined
R1; WR2
8790
16534
18683
21936
3338
1772
2235
3943
217
222
1669
374
0.0427; 0.0995
1.094
0.0644; 0.1669
1.034
0.0464; 0.1090
1.035
0.0622; 0.1721
1.088
GOF (F2)
1085.85(s), 1016.42(s), 995.20(s), 966.27(s), 921.91(s), 873.69(s),
802.33(s), 702.04(w), 628.75(s), 570.89(w), 526.53(w).
of the nitrogen atoms of the thiazolinyl ring of complex 3 was
disordered over two positions (labeled N3A and N3B). The occupancy
factors of these atoms were refined using the FVAR command of the
SHELXTL program and isotropically refined. The perchlorate ion
in complex 3 was disordered over two positions, and the disorder was
modeled using the PART, SAME, SADI, and DFIX commands of the
SHELXTL program and was anisotropically refined. In the difference
Fourier map of complex 3, a number of diffused scattered peaks
with an electron density ranging from 3.79 Åꢀ3 to 1.33 Åꢀ3 were
observed, which can be attributed to disordered solvent present in
this complex. Attempts to model these peaks were unsuccessful since
residual electron density peaks obtained were diffused. PLATON/
SQUEEZE17 was used to refine the structure further.
Synthesis of 3. A CH3CN solution (5 mL) of AgClO4 (2.5 mg, 12
mmol) was slowly layered on a CHCl3 solution (3 mL) of L2 (2 mg, 3.8
mmol). Then, it was allowed to diffuse in the dark over a week to obtain
colorless crystals of 3, which are insoluble in most of the common
organic solvents. Yield: ∼27%. Crystals were dissolved in DMSO and
diluted with acetonitrile prior to injecting the mass sample. HRMS (ESI)
m/z calcd. for [C21H27N3S6Ag]þ: 621.7168. Found: 621.7838. Ele-
mental analysis calcd for C21H27N3S6AgClO4: C, 34.97; H, 3.77; N,
5.83; S, 26.68. Found: C, 34.88; H, 3.76; N, 5.81; S, 26.73. IR Data (KBr
pellet, cmꢀ1): 3525.63(w), 2920.03(s), 2850.59(s), 1625.88(w), 1542.95(s),
1438.80(s), 1309.58(s), 11263.29(s), 1230.50(s), 1091.63(w), 1035.70(s),
966.27(s), 796.55(s), 673.11(s), 621.04(s).
Crystallographic Refinement Details. The crystallographic data
and details of data collection for L1, complex 1 ([2(L1) 6(AgClO4)
’ RESULTS AND DISCUSSION
3
3
2(CHCl3) HClO4]), complex 2 ([2(L1) 6(AgClO4)]), and complex 3
Synthesis. The hexapodal receptor (L1) and tripodal receptor
(L2) are synthesized in good yield from hexakis(bromomethyl)-
benzene and 1,3,5-tris(bromomethyl)benzene, respectively, by
reacting them with 2-thiazoline-2-thiol in the presence of a base
in CH3CN, as shown in Scheme 1. The single crystals of
hexapodal receptors L1 and L2 are also obtained in good yields.
Formation of the solid products L1 and L2 was confirmed by a
comparison of the experimental XRPD patterns with those
calculated on the basis of the single crystal structures (Supporting
Information, Figure S17 and Figure S18). Efforts have been made
to isolate the complex of L1 with Agþ by adding a AgClO4
solution (methanol/ethanol/CH3CN/DMF/dimethyl sulfoxide)
to the CHCl3 as well as a CH2Cl2 solution of L1 with a metal/
ligand ratio of 7:1. In all of the cases, polymeric products
appeared immediately after mixing. Formation of an insoluble
3
3
are given in Table 1. In each case, a crystal of suitable size was selected
from the mother liquor, immersed in paratone oil, and then mounted on
the tip of a glass fiber and cemented using epoxy resin. Intensity data for all
five crystals were collected using Mo KR (λ = 0.7107 Å) radiation on a
Bruker SMART APEX diffractometer equipped with a CCD area detector
at 100 K. The data integration and reduction were processed with the
SAINT12a software. An empirical absorption correction was applied to the
collected reflections with SADABS.12b Structures were solved by direct
methods using SHELXTL13 and were refined on F2 by a full-matrix least-
squares technique using the SHELXL-9714 program package. Graphics
were generated using PLATON15 and MERCURY 2.3.16 In all cases, non-
hydrogen atoms were treated anisotropically, and all of the hydrogen
atoms attached with carbon atoms were geometrically fixed. The DANG
command was used during the SHELXTL refinement to fix all of the
atoms of the CHCl3 molecule in tetrahedral geometry in complex 1. One
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Inorganic Chemistry
ARTICLE
Scheme 1. Syntheses of L1 and L2
Figure 2. Perspective thermal ellipsoid view of L2 at 50% probability.
Color code: Red, atoms above the plane; blue, atom below the plane of
central benzene ring. Hydrogen atoms are omitted for clarity.
Figure 3. (a) Perspective thermal ellipsoid view of the single crystal
structure of metallocapsule 1 at 50% probability. (b) Partial space-filling
view of metallocapsule 1. Bond distances of Ag1ꢀN2 = 2.117(13) Å,
Ag1ꢀN12 = 2.126(12) Å, and Cg1 Cg2 = 5.311 Å. Perchlorates,
3 3 3
chloroform, and hydrogens were omitted for clarity. Color code: red,
atoms above the plane; blue, atoms below the plane of central benzene rings.
(i.e., ababab) with all thiazolinyl rings lying almost orthogonal
to the central benzene (Figure 1). Functionalization with a
mercaptothiazolinyl ring introduces a spacer and flexibility
around the benzene platform via a sulfur atom in addition to
the methylene group. In L1, nitrogen atoms of thiazolinyl rings
are oriented in such a way that all six nitrogen atoms are
directed toward the plane of the central benzene platform
(Figure 1).
Figure 1. Perspective thermal ellipsoid view of L1 at 50% probability.
Color code: Red, atoms above the plane; blue, atoms below the plane of
central benzene ring. Hydrogen atoms are omitted for clarity.
polymeric material could be due to the semi-flexible nature of the
hexadendate ligand, L1, which can bind to the metal centers in
multiple dimensions. Slow layer diffusion could crystallize the
complexes 1 ([2(L1) 6(AgClO4) 2(CHCl3) HClO4]) and 2
The single crystals of L2 are isolated in a monoclinic space
group (C2/c). Here, two of the thiazolinyl rings (shown in red
color in Figure 2) are pointing toward one side, whereas the other
arm (shown in blue color in Figure 2) is projected in the opposite
direction of the central benzene ring that generates C2v symme-
try. This is in similar agreement with the already reported crystal
structure of L2.18 Similar to L1, all of the nitrogen atoms of
thiazolinyl rings are also directed toward the plane of the benzene
platform.
3
3
3
([2(L1) 6(AgClO4)]) along with insoluble polymeric materials,
3
as observed above, although yields of the complexes are low. On
the other hand, in the case of tripodal receptor L2, the single
crystal of polymeric silver complex 3 is isolated in moderate yield
via slow layer diffusion techniques. Due to the lower yield of
complexes 1ꢀ3 and their photosensitive nature, we are unable to
perform XRPD studies for these complexes. To find the homo-
geneity of the crystals, we have collected single crystal X-ray
diffraction data by random selection of crystals from the bulk
material of crystalline complexes, and the observed data provided
the same cell parameters all the time.
The conformation (ababab) of L1 could be useful toward the
formation of an interesting metallo-supramolecular architecture
upon the proper choice of metal ion. The single crystal X-ray
analysis of the silver complex of L1, 1 ([2(L1) 6(AgClO4) 2
3
3
(CHCl3) HClO4]) (Table 1), showed the formation of a
3
Single Crystal X-Ray Structural Analysis. The single crystal
X-ray structure analysis of L1 revealed that L1 is crystallized
in the triclinic space group (Table 1) and assumed a conforma-
tion in which the alternate substituents of the benzene ring are
in 1,3,5 vs 2,4,6 facial steric gearing on the central benzene ring
cylindrical hexanuclear metallocage Ag6(L1)2 as a result of the
compact self-assembly of two molecules of L1 (Figure 3a). Thus,
we could achieve an ordered dimeric assembly of L1 connected
by six silver ions via slow diffusion techniques, though simple
mixing of L1 and Agþ salt produces instant precipitation of
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insoluble polymeric materials. In metallocage Ag6(L1)2, two
units of L1 are arranged as a discrete compact self-assembly by
linear coordination of six Agþ ions. The coordination of Agþ
toward L1 induces the rotation of all three thiazolinyl rings of one
of the planes around the benzene ring. Thus, all six nitrogen
atoms of the thiazolinyl rings (above and below the central
benzene ring) are oriented in the same direction, keeping the
conformation (ababab) the same as observed in the case of L1.
Three nitrogen atoms of the thiazolinyl ring lie above the central
benzene ring of one molecule of L1 (shown in red) and are
pillared by the coordination of Agþ to the upper thiazolinyl
nitrogen atoms of another molecule of L1 (shown in red).
Similarly, the coordination of Agþ with the three nitrogen atoms
of thiazolinyl rings lies below the benzene ring of one molecule of
L1 (shown in blue) relative to that of the other molecule to form a
Ag6(L1)2 metallocapsule, 1, with AgꢀN bond distances of
2.117(13) Å and 2.126(12) Å for Ag1ꢀN2 and Ag1ꢀN12,
respectively (Figure 3a). The two thiazolinyl rings coordinated
with Agþ by a slightly distorted linear coordination with
—N2ꢀAg1ꢀN12 of 179.0(6)ꢀ. All six silver atoms are equally
separated by 6.620 Å, and the distance between the two central
Figure 3b. Although, 1 ([2(L1) 6(AgClO4) 2(CHCl3) HC-
lO4]) provides preorganized clefts for the binding of perchlorate
ions. All six perchlorate ions are located in six clefts of 1 via
3
3
3
CꢀH O and AgꢀO interactions (Figure 4a).
3 3 3
Six ClO4ꢀ anions sit between the cleft of the Agþ bridged arms
via CꢀH O interactions with one oxygen atom of ClO4ꢀ by
3 3 3
two adjacent benzylic hydrogen atoms with a C8 O19 bond
3 3 3
distance of 3.395 Å and a C11 O19 bond distance of 3.443 Å.
Another oxygen atom of ClO4 is in a hydrogen bonding interac-
ꢀ3 3 3
tion with CꢀH protons of thiazolinyl rings with a C13 O21
3 3 3
bond distance of 3.295 Å and a AgꢀO interaction with a Ag1ꢀO22
bond distance of 2.891(14) Å in the side cleft of 1 (Figure 4b).
In addition to the six clefts formed by the Agþ pillared side
arms, 1 also possesses two cavities above and below the benzene
caps which accommodate two CHCl3 molecules, as shown in
Figure 5a. The hydrogen atom of CHCl3 in the outer cavities is in
hydrogen bonding interactions with the oxygen atoms of three
side pocket bound ClO4ꢀ’s of three other molecules of 1 with a
C26 O21 bond distance of 3.068 Å and —C26ꢀH26 O21
3 3 3
3 3 3
= 124.1ꢀ, as shown in Figure 5b.
The TGA analysis of metallocapsule 1 shows that weight loss
starts at a temperature of 140 ꢀC and is complete at ∼165.5 ꢀC
(Supporting Information, Figure S19). Total loss in this tem-
perature range is 7.24%, corresponding to the loss of two
molecules of chloroform. Whereas, the consecutive weight loss
in the temperature range 162ꢀ196 ꢀC with a total loss of 3.54%
corresponds to loss of one molecule of perchloric acid
(Supporting Information, Figure S19).
benzene caps Cg1 Cg2 is 5.311 Å (Cg1 and Cg2 are centroids
3 3 3
of two benzene caps of the metallocage). The space filling model
shows virtually no space inside the central cavity, as shown in
Similar to complex 1 ([2(L1) 6(AgClO4) 2(CHCl3) HClO4]),
3
3
3
complex 2 ([2(L1) 6(AgClO4)]) also shows the formation of a
3
structurally similar cylindrical hexanuclear Ag6(L1)2 metallocage
(Table 1) to that of 1 with a Ag1ꢀN1 bond distance of 2.131(6)
Å, a Ag1ꢀN2 distance of 2.129(7) Å, and —N1ꢀAg1ꢀN2 =
175.1(2)ꢀ (Figure 6). All six silver atoms are equally separated by
6.774 Å, which is slightly longer than that of complex 1 (6.620 Å),
and the distance between two central benzene ring centroids
Cg1 Cg2 is 5.017 Å (Figure 6a), which is slightly shorter
3 3 3
compared to 1 (5.311 Å). This suggests that complex 2 is slightly
more compressed than 1. In contrast to complex 1, complex 2
does not encompass any solvent guest at the top and bottom
pockets of the metallocage, and the binding of all six perchlorates
is governed by the direct AgꢀO bond with a Ag1ꢀO1 bond
distance of 2.628(8) Å (Figure 7a).
Figure 4. (a) Perspective view of cleft binding ClO4ꢀ in the side pocket
of 1 and (b) partial view showing the binding interactions of ClO4ꢀ with
a side pocket in 1 with bond parameters for C8ꢀH8B O19
3 3 3
(C8 O19 = 3.395 Å, —C8ꢀH8B O19 = 146.05ꢀ), C11ꢀH11B
3 3 3
3 3 3
3
O19 (C11 O19 = 3.443 Å, —C11ꢀH11B O19 = 133.34ꢀ),
3 3
3 3 3
3 3 3
C13ꢀH13B O21 (C13 O21 = 3.295 Å, —C13ꢀH13B O21 =
3 3 3
3 3 3
3 3 3
In complex 2, the bond length between Ag1 and one of the
oxygen atoms (O1) of perchlorate pointing toward the silver
132.93ꢀ), and Ag1ꢀO22 = 2.891(14) Å. CHCl3 and nonbonding
hydrogen atoms were omitted for clarity.
Figure 5. (a) Perspective view of metallocapsule 1 with cleft bound ClO4ꢀ at the side pockets and CHCl3 at the top and bottom cavities. (b) Hydrogen
bonding interactions of CHCl3 with cleft bound perchlorate anions in 1 with a C26 O21 bond distance of 3.068 Å. Color code: red, atoms above the
3 3 3
plane; blue, atoms below the plane of central benzene ring.
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Figure 6. (a) Perspective thermal ellipsoid view of metallocapsule 2 at 50% probability. (b) Partial space-filling view of metallocapsule 2. Color code:
Red, atoms above the plane; blue, atoms below the plane of central benzene ring. Bond distances: Ag1ꢀN1 = 2.131(6) Å, Ag1ꢀN2 = 2.129(7) Å,
Ag1ꢀO1 = 2.628(8) Å, and Cg1 Cg2 = 5.017 Å.
3 3 3
Figure 7. (a) Perspective view of cleft binding ClO4ꢀ in the side pocket of metallocage 2. (b) Partial view showing the binding interactions of ClO4ꢀ in
the cleft pocket of metallocage 2 with hydrogen bonding parameters C4ꢀH4A O4 (C4 O4 = 3.307 Å, —C4ꢀH4 O4 = 150.0ꢀ),
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
C7ꢀH7 O4 (C7 O4 = 3.453 Å, —C7ꢀH7A O4 = 142.54ꢀ), C9ꢀH9A O3 (C9 O3 = 3.078 Å, —C9ꢀH9A O3 = 121.53ꢀ),
3 3 3
3 3 3
3 3 3
Ag1ꢀO2 = 3.056(7) Å, and Ag1ꢀO4 = 3.170(9) Å.
atom of the cage is short with a Ag1ꢀO1 bond distance of
2.628(8) Å and a bond angle —Ag1ꢀO1ꢀCl1 = 150.39(6)ꢀ. In
the case of complex 1, one of the oxygen atoms (O20) of
perchlorate, which is sitting in the cleft of one molecule of the
metallocage, is pointed toward the silver atom of the other
molecule of the metallocage (Supporting Information, Figure
S20), having a Ag1ꢀO20 bond distance of 3.010 Å and a bond
angle (—Ag1ꢀO20ꢀCl18 = 139.75ꢀ). This suggests that the
binding of ClO4ꢀ in complex 2 is stronger with a direct AgꢀO
bond in the Ag6(L1)2 metallocage (Figure 7b) compared to
compꢀlex 1. In addition to the direct Ag1ꢀO1 bond, the six
ClO4 anions sit in the cleft of the Agþ bridged arms of 2 via
Å in the side cleft of 2, as shown in Figure 7b, similar to that of
complex 1 (Figure 4b).
The complexation of Agþ with L1 induces the slight twisting
in the orthogonal thiazolinyl substituents in metallocages 1 and
2 that causes the silver atom coordinated to the thiazolinyl ring
to lie above the central benzene ring (shown in red) in one plane.
Similarly, the silver atom coordinated to the thiazolinyl ring lies
below the central benzene ring and is in another plane, which
fulfills the requirement of helicity in the cages (Figure 8).19,20 This
twisting of thiazolinyl substituents causes the propeller orientation
of silverpillered thiazolinyl armslyingabove(shown in red) aswell
as below (shown in blue) the central benzene ring and induces the
helicity in metallocages 1 and 2.
CꢀH O interactions with one of the oxygen atoms (O4) of
ꢀ3 3 3
ClO4 by two adjacent benzylic hydrogen atoms with a
In order to understand the role of mercaptothiazolinyl sub-
stituents on the hexa-arms of the benzene platform toward the
formation of a metallocage, we also tried to isolate the complex of
tripodal L2 with Agþ. After several attempts, single crystals of a
complex of L2 with Agþ (i.e., 3) are obtained as colorless crystals
along with the insoluble polymeric materials. The single crystal
C7 O4 bond distance of 3.453 Å, and another oxygen atom
3 3 3
(O3) of ClO4ꢀ is in a hydrogen bonding interaction with CꢀH
protons of thiazolinyl rings with a C9 O3 bond distance of
3 3 3
3.078 Å, and there is a AgꢀO interaction with a Ag1ꢀO2 bond
distance of 3.056(7) Å and a Ag1ꢀO4 bond distance of 3.170(9)
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structural analysis of complex 3 revealed that it crystallized in a
monoclinic space group (P21/c; Figure 9 and Table 1).
Ag1ꢀN3A bond distance of 2.547(13) Å, and a Ag1ꢀN3B bond
distance of 2.147(12) Å. Further, the Agþ is coordinated with a
nitrogen atom (N2) of one the thiazolinyl rings of the other
molecule of L2, which is in the opposite orientation with respect
to the central benzene ring, with a Ag1ꢀN2 bond distance of
2.563(8) Å, as well as one of the oxygen atoms (O1A) of the
perchlorate anion, with a Ag1ꢀO1A bond distance of 2.661 Å,
resulting in a coordination polymer of complex 3 (Figure 9).
Of course, there are reports on the formation of a metallocage via
a tripodal receptor on the benzene platform with a methylene
spacer;5a,8 we are unable to isolate the metallocage under these
experimental conditions.
Contrary to free tripodal receptor (L2 shown in Figure 2), the
complexation of Agþ with L2 causes all three thiazolinyl rings to
orient on one side of the central benzene ring, as shown in
Figure 9a. Among the three nitrogen atoms of thiazolinyl rings of
L2, one was pointed down and the other two were projected in
the other direction of the central benzene ring (Figure 9b). Upon
complexation of L2 with Agþ, the nitrogen atoms (N1, N3A, and
N3B) of the two thiazolinyl arms pointing toward one direction
of the central benzene ring of one molecule of L2 are coordinated
with Agþ, with a Ag1ꢀN1 bond distance of 2.239(7) Å, a
In general, the complexation of a multidentate ligand with
metal ions yields polymeric materials via uncontrolled coordina-
tion. Figure 10 shows a few possible modes of complexation of L1
with Agþ. 1D-polymeric assembly (Figure 10a) can easily form if
the above arms of one molecule of L1 participate in the
coordination along with the lower arms of another molecule of
L1. Similarly, a multidimensional (mD) polymeric network
(Figure 10b) and discrete dimeric assembly (Figure 10c) are
also possible. Thus, it is essential to prevent the formation of
polymeric materials via the semirigid or flexible receptors toward
the formation of discrete M6L2 metallocages. Despite the fact
that the isolation of complexes 1 and 2 in poor yield is due to the
competitive formation of polymeric materials, we are successful
in isolating the elegant discrete dimeric assembly of metallo-
supramolecular architecture by slow diffusion techniques.
Figure 8. Perspective views of helicity in (a) metallocapsule 1 and (b)
metallocapsule 2. Color code: red, atoms above the plane; blue, atoms
below the plane of the central benzene ring.
Figure 9. (a) The partial fragment of the single crystal X-ray structure of complex 3 in thermal ellipsoid view at 50% probability. Bond distances:
Ag1ꢀN1 = 2.239(7) Å, Ag1ꢀN3A = 2.547(13) Å, Ag1ꢀN2 = 2.563(8) Å. (b) Perspective view of the coordination polymer of complex 3. Color code:
orange, carbon; blue, nitrogen; dark green, sulfur; green, chlorine; pink, silver.
Figure 10. Some of the possible assembling modes of L1 with respect to molecular conformation (ababab) by complexation with Agþ. (a) 1D-
assembly, (b) mD-polymeric network, and (c) discrete dimeric assembly of L1. Color code: Red, atoms above the plane; blue, atoms below the plane of
central benzene ring.
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Figure 11. 1H NMR (300 MHz) spectral changes of L1 with added AgClO4 in CDCl3ꢀDMSO-d6 (3:2; 25 ꢀC; [L1]0 = 4.69 mM). Equivalents of
AgClO4: (a) 0, (b) 0.5, (c) 1, (d) 1.5, (e) 2, (f) 2.5, (g) 3, (h) 3.5, (i) 4, (j) 4.5.
1H NMR Titration Experiment. In order to study the silver ion
complexation in solution, we have performed 1H NMR titration
experiments of L1 with AgClO4 in CDCl3ꢀDMSO-d6 (3:2).21
When aliquots of Agþ were added gradually to the solution of L1,
IR spectra, and PXRD spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
1
the H NMR spectrum showed very slight (0.04ꢀ0.06 ppm)
downfield shifts in all of the proton signals of L1, as shown in
Figure 11, which is indicative of silver ion complexation in
solution as well.19 No new peaks were observed for complexed
species in the NMR spectra. It may be due to the formation of
highly reversible and dynamic complexes in the solvent systems
used. Unlike slow layer diffusion crystallization techniques, in the
NMR titration experiments, L1 is immediately reacted with Agþ
ions in an irregular manner, which leads to the precipitation of
polymeric material from the NMR solution.
Corresponding Author
*E-mail: icpg@iacs.res.in.
’ ACKNOWLEDGMENT
P.G. thanks the Department of Science and Technology
(DST), India, for financial support through a Swarnajayanti
Fellowship. M.A. would like to acknowledge CSIR, India, for
SRF. The X-ray crystallography study was performed at the DST-
funded National Single Crystal X-ray Diffraction Facility at the
Department of Inorganic Chemistry, IACS.
’ CONCLUSION
’ REFERENCES
In conclusion, we have demonstrated the silver ion assisted
formation of a hexanuclear metallocage by dimeric self-assembly
of a new semirigid hexapodal receptor. Crystallization using layer
diffusion techniques is adapted to avoid complete polymer
formation by this semiflexible receptor. Though this metallocage
does not show guest encapsulation in the central cavity, the solid
state structural analysis confirmed the formation of a compact
hexanuclear helical metallocage with as many as eight binding
pockets for guests. Further development of different hexapodal
hosts with the proper choice of solvents and experimental
conditions of complexation could generate a new generation of
metallocages for different guests with selectivity. Thus, we feel
that the utilization of steric gearing of hexa-hosts for the
construction of metallocages with multiple binding sites in the
present study has great importance in hostꢀguest chemistry.
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