Self-assembled Pd3L2 cages having flexible tri-imidazole donors

Article abstract of DOI:10.1016/j.poly.2019.03.014

Four new M₃L₂ molecular cages (CA1–CA4) have been synthesized in excellent yield via coordination driven self –assembly of flexible tri-imidazole donors based on triazine (L1) and tri-phenyl benzene (L2) core with 90° cis-blocked met

Full text of DOI:10.1016/j.poly.2019.03.014

Accepted Manuscript
Self-assembled Pd L cages having flexible tri-imidazole donors
3
2
Atul Kumar, Ennio Zangrando, Partha Sarathi Mukherjee
PII:
S0277-5387(19)30180-9
https://doi.org/10.1016/j.poly.2019.03.014
POLY 13815
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
31 January 2019
4 March 2019
9 March 2019
Please cite this article as: A. Kumar, E. Zangrando, P.S. Mukherjee, Self-assembled Pd L cages having flexible tri-
3
2
imidazole donors, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.03.014
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Self-assembled Pd
3
2
L cages having flexible tri-imidazole donors
a
b
a
Atul Kumar, Ennio Zangrando, Partha Sarathi Mukherjee *
a
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-
5
60 012, India.
b
Department of Chemical and Pharmaceutical Sciences, University of Trieste, 34127 Italy.
*
Corresponding author: P. S. Mukherjee, E-mail: psm@iisc.ac.in; Tel.: +91-80-22933352;
Fax: +91-80-23601552.
Abstract
Four new M
3
2
L molecular cages (CA1 – CA4) have been synthesized in excellent yield via
coordination driven self –assembly of flexible tri-imidazole donors based on triazine (L1) and
tri-phenyl benzene (L2) core with 90º cis-blocked metal acceptors of Pd(II). Two of them CA1
and CA2 are highly soluble in water. All the cages were characterized by spectroscopic studies
and ESI-MS; CA3 and CA4 also by X-ray diffraction. Selective formation of two separate
cages CA3 and CA4 was noticed instead of a hetero-ligand cage, when the metal acceptor
(
dppf)Pd(OTf) was treated with a mixture of ligands L1 and L2.
2
Keywords: palladium(II), cage compounds, self-assembly, supramolecular chemistry, X-ray
structure determination
1
. Introduction
Inspired from biological systems, where small molecules interact and self-organize to form
well organized functional systems,(1-12) chemists have developed metal−ligand coordination
driven self-assembly an as alternative protocol that helps to design large molecular structures
with diverse shapes and sizes in facile manner.(13-17) Over last few decades, coordination driven
self-assembly established itself as a well-defined methodology in constructing molecular
architecture from two dimensional 2-D polygons to three dimensional 3-D cages, prism,
polyhedral species, tubes and capsules which possesses diverse functionality.(18-53) Non-
covalent cages are preferred over its covalent analogues as they avoid multistep synthesis
which is obligatory in latter one. (54,55) Declining use of templates and enthralling features of an
imidazole donor over a pyridyl donor has prompted synthetic chemists to design functionalized
supramolecules without a template based on polyimidazole ligands.(56-87)

Designing of supramolecular coordination complexes requires rigid molecular building blocks
and correct orientation of the coordination site/s on metal acceptor.(31-33,38) However, building
molecular architectures based on flexible donors is a risky task due to diverse orientation of
the donors around the metal centres that leads to the formation of undesired structures. Herein,
we design two flexible tri-imidazole ligands (L1 and L2) based on electron deficient triazine
and moderate electron rich triphenyl benzene cores, respectively. Both L1 and L2 are similar
in size and metal binding site. Template-free synthesis of water-soluble M
3
2
L bifacial
molecular cages CA1 and CA2 was achieved through two-component self-assembly of triazine
and triphenyl benzene functionalized tri-imidazole donor L1 and L2 respectively, with cis
blocked 90º acceptor cis-(tmen)Pd(NO
In a similar way, CA3 and CA4 were prepared by two-component self-assembly of L1 and L2
separately with cis-(dppf)Pd(OTf) . Multicomponent self-assembly of cis-(dppf)Pd(OTf) with
3
2
) [tmen = N,N,N’N’-tetramethylethane-1,2-diamine].
2
2
L1 and L2 resulted in the formation of a mixture CA3 and CA4 instead of any multicomponent
cage comprising of both the ligands.
2
2
. Experimental
.1 Material and methods
All the reagents were purchased from different commercial sources and used without further
purification. NMR (1D and 2D) spectra were recorded on Bruker 400 MHz spectrometer and
1
the chemical shifts (δ) in the H NMR spectra are reported in ppm relative to tetramethylsilane
(
Me
deuteration of the solvents CDCl
Electrospray ionization mass spectrometry (ESI-MS) experiments were carried out in Agilent
538 Ultra-High Definition (UHD) Accurate Mass Q-TOF spectrometer using standard
spectroscopic-grade solvents.
4
Si) as internal standard (0.0 ppm) or proton resonance resulting from incomplete
3
(7.26 ppm), CD CN (1.94 ppm) and D O (4.79 ppm).
3
2
6
2
2
4
.2 Synthesis of the ligands
.2.1 Synthesis of 2,4,6-tris(4-(bromomethyl)phenyl)-1,3,5-triazine (1A)
-Bromomethylbenzonitrile (2 g, 10.2 mmol) was added to trifluoromethanesulfonic acid (2.6
o
mL, 10.2 mmol) in small portion at 0 C. After addition, the mixture was warmed to room
temperature and stirred for further 24 h, then poured into ice, neutralized by ammonium
hydroxide. The solid phase was collected by filtration to afford white powder (1.82 g, 91%).

1
m.p. 195 °C. H NMR (400 MHz, CDCl
3
) δ: 8.72 (6H, d, J = 8 Hz), 7.60 (6H, d, J = 8 Hz),
.59 (6H, s). 13C NMR (100 MHz, CDCl
) δ (ppm): 171.5, 142.7, 136.5, 129.9, 129.8,
3.1. FTIR (KBr), ν (cm ): 2922, 2852, 1584, 1518, 1415, 1371, 1226, 1178, 1018, 866, 814,
4
3
6
3
-1
+
+
06. ESI MS (m/z): Calcd. For [M+Na] 609.81, found 609.80 for [M+Na] ,
2
.2.2 Synthesis of 2,4,6-tris(4-((1H-imidazol-1-yl)methyl)phenyl)-1,3,5-triazine (L1)
Imidazole (0.38 g, 5.61 mmol) and potassium hydroxide KOH (0.57 g, 10.2 mmol) were taken
in 50 mL round bottom flask, with addition of 25 mL of acetonitrile and the reaction mixture
was stirred for 2 h at room temperature. Then compound 1A (1 g, 1.7 mmol) was added and
stirring was continued for further 12 h. Next the solvent was evaporated and washed several
times with water to give desired product as white solid. (0.84 g, 90%). m.p. >250 °C. δ: 8.72
(
6H, d, J = 8 Hz), 7.62 (3H, s), 7.33 (6H, d, J = 8 Hz), 7.14 (3H, s), 6.95 (3H, s), 5.25 (6H, s).
13C NMR (100 MHz, CDCl
) δ (ppm): 171.1, 140.8, 137.5, 136.0, 130.0, 129.6, 127.45, 119.3,
0.5. FTIR (KBr), ν (cm ): 3392, 3106, 1584, 1524, 1420, 1368, 1290, 1232, 1178, 1020, 814,
3
-
1
5
7
+
+
52, 656, 600. ESI MS (m/z): Calcd. For [M+H] 550.24, found 550.24 for [M+H] , For
+
+
[
M+Na] 572.24, found 572.26 for [M+Na] .
2
.2.3 Synthesis of 4,4''-dimethyl-5'-(p-tolyl)-1,1':3',1''-terphenyl (2A)
4
-Methylacetophenoe (2 g, 14.9 mmol) and p-toluenesulfonic acid (PTSA) (0.25 g, 1.5 mmol)
were taken in 50 mL flask and heated under nitrogen atmosphere for 16 h at 146 ℃. The
reaction mixture was allowed to warm to room temperature and then excess PTSA was
neutralized with sodium hydrogen carbonate. Then the reaction mixture was extracted with
dichloromethane. Crude product was purified by column chromatography using silica 60 – 120
1
(
hexane/ethyl acetate : 98/2) (white solid, yield =1.38 g, 80%). m.p. 176 °C. H NMR (400
13
MHz, CDCl
3
) δ: 7.74 (3H, s), 7.60 (6H, d, J = 8 Hz), 7.29 (6H, d, J = 8 Hz), 2.42 (9H, s). C
) δ (ppm): 142.7, 138.9, 137.7, 130.0, 127.7, 125.1, 21.6. FTIR (KBr),
ν (cm ): 3022, 2961, 2921, 2865, 1512, 1450, 1366, 1186, 1113, 1060, 1039, 1019, 809, 734,
NMR (100 MHz, CDCl
3
-1
+
+
7
20. ESI MS (m/z): Calcd. For [M+Na] 371.18, found 371.19 for [M+Na] .
2
.2.4 Synthesis of 2.2.3 Synthesis of 4,4''-dimethyl-5'-(p-tolyl)-1,1':3',1''-terphenyl (2B)
Compound 2A (1 g, 2.86 mmol), N-bromosuccinimide (1.68 g, 9.46 mmol), benzoyl peroxide
69 mg, 0.28 mmol) and CCl (30 mL) were taken in 100 mL round bottom flask and heated to
(
4
reflux for 14 h. The reaction mixture was then cooled to ambient temperature and filtered to
remove by-product. Crude product was purified by column chromatography using silica 60 –

1
1
20 (hexane/ethyl acetate: 90/10) as white solid product. (1.49 g, 89%). m.p. 185 °C. H NMR
400 MHz, CDCl ) δ: 7.75 (3H, s), 7.66 (6H, d, J = 8 Hz), 7.51 (6H, d, J = 8 Hz), 4.58 (6H,
s). C NMR (100 MHz, CDCl ) δ (ppm): 142.2, 141.4, 137.7, 130.1, 128.2, 125.7, 33.8. FTIR
KBr), ν (cm ): 3108, 1646, 1588, 1506, 1438, 1388, 1282, 1076, 1028, 910, 820, 744, 658.
(
3
13
3
-1
(
+
+
ESI MS (m/z): Calcd. For [M+Na] 606.91, found 606.92 for [M+Na] .
2
.2.5 Synthesis of 1,1'-((5'-(4-((1H-imidazol-1-yl)methyl)phenyl)-[1,1':3',1''-terphenyl]-4,4''-
diyl)bis(methylene))bis(1H-imidazole) (L2)
Imidazole (0.38 g, 5.64 mmol) and potassium hydroxide KOH (0.575 g, 10.25 mmol) were
taken in a 50 mL round bottom flask with addition of 25 mL of acetonitrile and the reaction
mixture was stirred for 2 h at room temperature. Then compound 2B (1 g, 1.71 mmol) was
added and stirring continued for further 12 h. Subsequently, solvent was evaporated and
washed several times with water to give desired product as white solid. (0.8 g, 85%). m.p. 190
1
°
C. H NMR (400 MHz, CDCl
3
) δ: 7.71(3H, s), 7.64 (6H, d, J = 8 Hz), 7.58 (3H, s), 7.25 (6H,
1
3
d, J = 8 Hz), 7.10 (3H, s), 6.93 (3H, s), 5.17 (6H, s). C NMR (100 MHz, CDCl ) δ (ppm):
3
-1
1
3
42.2, 141.3, 137.8, 136.1, 130.3, 128.36, 128.35, 125.7, 119.7, 50.9. FTIR (KBr), ν (cm ):
110, 1686, 1652, 1594, 1508, 1448, 1394, 1282, 1230, 1078, 1024, 910, 820. ESI MS (m/z):
+
+
Calcd. For [M+Na] 569.25, found 569.26 for [M+Na] .
2
2
.3 Synthesis of the cages
.3.1 General procedure for the synthesis of CA1 to CA2
M1 (5 mg, 3 equivalents) was dissolved in H O (1 mL) and the yellow clear solution was added
2
to solid ligand L (2 equivalents) and heated at 60ºC with stirring for 12 h. The solution turned
colourless with the consumption of the donor. Upon completion of the reaction, the mixture
was filtered, concentrated under reduced pressure, and the pure form of cage CA was obtained
as an off-white powder by trituration with acetone.
2
.3.1.1 Synthesis of CA1
1
Isolated yield: 8.92 mg (92%). m.p. >250 °C. H NMR (400 MHz, D
2
O) δ: 8.46 (6H), 7.31
(6H), 6.75 (18H), 6.29 (18H), 5.02 (12H), 2.70 – 2.78 (48H). ESI MS (m/z): 651.05 [(CA1) -
3
+
2+
3
NO
3
] , 1007.58 [(CA1) - 2NO ] .
3
2
.3.1.2 Synthesis of CA1_PF6
M.p. 210 °C. CA1 (10.0 mg, 0.0023 mmol) in 1mL H
2
O was treated with excess KPF (8.00
6
mg, 0.046 mmol) and stirred at room temperature for 6 h. Then solid precipitate was filtered

and washed several times with H
2
O and dried under reduce pressure to get the product as white
1
powder. Isolated yield: 11.2 mg (91%). H NMR (400 MHz, CD
3
CN) δ: 8.34 (6H), 7.86 (12H,
d, J = 8 Hz), 7.43 (6H), 7.32 (18H), 6.89 (6H, d, J = 8 Hz), 3.01 (12H), 2.73 (36H).
2
.3.1.3 Synthesis of CA2
1
Isolated yield: 8.7 mg (89%). m.p. >250 °C. H NMR (400 MHz, D
2
O) δ: 8.38 (6H), 7.21
3
+
(
12H), 6.67 (30H), 3.03 (12H), 2.65 – 2.68 (36H). ESI MS (m/z) 649.73 [(CA2) - 3NO ] ,
3
2
+
1
005.60 [(CA2) - 2NO
3
] .
2
.3.2 General procedure for the synthesis of CA3 and CA4
Cis-protected phosphine blocked acceptor M2 (10.0 mg, 3 equivalents) and ligand L (2
equivalents) were dissolved in CH NO followed by subsequent stirring at room temperature
for 24 h. The resulting clear solution was triturated with diethyl ether to obtain pure product.
3
2
2
.3.2.1 Synthesis of CA3
1
Isolated yield: 11.2 mg (81%). m.p. 160 °C H NMR (400 MHz, CD
3
CN) δ: 7.65 – 7.92 (78H),
3
1
1
6
.69 – 6.97 (24H), 4.95 (12H), 4.66 (24H). P{ H} NMR (CD
3
CN, 121.4 MHz) δ: 32.37 (s).
5
+
4+
3+
ESI MS (m/z): 645.94 [(CA3) – 5OTf] , 844.69 [(CA3) – 4OTf] ,1175.94 [(CA3) – 3OTf] ,
2+
1
838.45 [(CA3) – 2OTf] .
2
.3.2.2 Synthesis of CA4
1
Isolated yield: 11.0 mg (80%). m.p. 157 °C. H NMR (400 MHz, CD CN) δ: 7.59 – 7.90
3
31
1
(
78H), 6.71 – 6.96 (30H), 4.82 (12H), 4.65 (24H). P{ H} NMR (CD
3
CN, 121.4 MHz) δ:
4+
2+
3
2.24 (s). ESI MS (m/z): 844.25 [(CA4) – 4OTf] ,1837.57 [(CA4) – 2OTf] .
2
.4 Single crystal X-ray crystallography
Single crystal X-ray data of CA3 and CA4 were collected on X-ray diffraction beamline XRD1
of the ELETTRA Synchrotron, Trieste (Italy), using the rotating crystal method with a
monochromatic wavelength of 0.7000 Å, on a Dectris Pilatus 2M detector. Measurements were
performed at 100(2) K using a nitrogen stream cryo-cooler. The structure was solved by
intrinsic phasing method with ShelXT(88) and refined by the full-matrix least-squares
2
.(89)
method based on F with all observed reflections using the Olex2 program.
All non-
hydrogen atoms were refined with an isotropic displacement coefficients. The hydrogen atoms
bonded to carbon were included in geometric positions and given thermal parameters
equivalent to 1.2 times those of the atom to which they were attached. In addition, the

structure contains a huge void of disordered solvent molecules therefore, solvent mask
incorporated in the program was applied to account for embedded solvent molecules.(90)
Crystallographic data and refinement parameter are given in Table 1.
Table 1: Crystallographic Data and Refinement Parameters of Cage
CA3
CA4
empirical formula
C
168
H138Fe
3
N
18
P
6
Pd
3
C
174
H144Fe
3
N
12
P
6
Pd
3
Fw
3677.81
100(2)
3373.71
100(2)
hexagonal
R-3
T (K)
crystal system
space group
a/Å
triclinic
P-1
23.615(5)
23.634(5)
23.775(5)
61.87(3)
63.99(3)
74.92(3)
10493(5)
2
67.121(10)
67.121(10)
30.088(6)
90
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
90
120
117393(41)
18
Z
-3
ρ
calcd (g cm )
1.164
0.859
-1

(Mo K) (mm )
0.571
0.436
λ/Å
0.700
3740
0.700
F (000)
31032.0
48128
23000
collected reflns
unique reflns
38086
24599

GOF (F2)
1.05
1.171
a
R
1
0.1021
0.1068
0.3600
b
wR
2
0.3123
aR
=Σ││F
│−│F
││/Σ│F
│, wR
b
=[Σ{w(F 2
–
1
2
o
c
o
2
o
2
2 2 1/2
c o
F ) }/Σ{w(F ) }] .
3
. Result and discussion
Ligand L1 was synthesized in two steps (Scheme S1, supporting information) by
cyclotrimerization of 4-(bromomethyl)benzonitrile, using catalytic amount of trifluoro-
methane sulfonic acid, followed by allylic coupling in presence of base. While ligand L2 was
synthesized in three steps (Scheme S2) first by cyclotrimerization of 4-methylacetophenone
followed by allylic bromination and then allylic coupling in presence of base. Both ligands
1
13
were fully characterized by H and C NMR, ESI-MS and IR spectroscopy.
self-assembled molecular cages CA1 and CA2 were obtained by two-component self-
assembly. Synthesis of CA1 and CA2 was carried out by adding an aqueous solution of cis-
tmen)Pd(NO (M1) into the solid tritopic donor L1 or L2 in 3:2 molar ratio followed by
subsequent stirring at 60ºC for 12 h. The resulting clear solution was triturated with acetone to
obtain [3+2] self-assembled M molecular cage CA as an off-white precipitate in pure form.
CA3 and CA4 were synthesized by mixing the acceptor cis-Pd(dppf)(OTf) {dppf=1,1’-
M
L
3 2
(
)
3 2
L
3 2
2
Bis(dipheylphosphino) ferrocene} (M2) and ligand L1 or L2 in 3:2 molar ratio followed by
subsequent stirring at room temperature for 24 h. The resulting clear solution was triturated
with diethyl ether to obtain pure self-assembled molecular cage CA M
3
2
L .
1
Complexes CA1 and CA2 were characterize by H NMR spectroscopy, DOSY, COSY and
1
31
ESI-MS, while CA3 and CA4 were characterized by H and P NMR spectroscopy, DOSY,
COSY, ESI-MS and single crystal XRD diffraction.
1H NMR spectroscopic analysis of CA1 in D
O (Figure1) exhibited sharp distinct peaks with
2
noticeable downfield shift as compared to ligand L1, which is expected because of the
II
coordination of ligand L1 to Pd . Diffusion-ordered NMR spectroscopy (DOSY) corroborated
the formation of a single species by the appearance of a clear single band for complex CA1 at
logD = -10.40 and CA2 at -10.38. ESI-MS spectra showed peaks at m/z = 651.05 for [CA1-
3
+
]2+ (Figure 2), 649.73 for [CA2-3NO
]3+and at 1005.60
3
NO
3
] , 1007.58 for cage [CA1-2NO
3
3
2
+
for [CA2-2NO
3
] (Figure S15) with isotopic patterns consistent with the charge states. Several

attempts were made to crystallize CA1 and CA2 by vapour diffusion and slow evaporation, but
any attempt failed. Therefore we change the acceptor from cis-(tmen)Pd(NO to cis-
Pd(dppf)(OTf) . Formation of single product in this case was confirmed from the appearance
)
3 2
2
3
1
of a single peak in P NMR, which showed up-field shift [32.37 ppm for CA3 and 32.24 ppm
for CA4] compared to corresponding acceptor peak (Figures S9 and S11). Formation of single
1
M
3
L
2
system for CA3 and CA4 is also supported from H, COSY and DOSY NMR (Figures
S9-S12). Finally, diffraction quality single crystals were obtained by slow vapour diffusion of
diethyl ether into nitromethane solutions of complexes CA3 and CA4 at room temperature for
four days.
6
+
[
M L ]
3 2
Scheme 1 Schematic representation of the formation of 3D molecular cages.

1
Figure 1: Partial H NMR spectra of a) ligand L1 recorded in CDCl
recorded in D O.
3
, and b) complex CA1
2

Figure 2: ESI-MS spectrum of (a) cage CA1 recorded in H O. Inset: isotopic distributions for
2
3
+
2+
the fragments (b) [CA1-3NO
3
] and (c) [CA1-2NO ] .
3
3
.1 Crystal structure details
Complex CA3 (Figure 3a) crystallizes in triclinic system with space group P1
894555). The asymmetric unit consists of three independent Pd(II) ions and two ligands L1.
̅
(CCDC:
1
The coordination environment of each Pd centre is almost square planar where Pd-N bond
distances range from 2.05 - 2.09 Å. The closest distance between two central triazine is 3.14 Å
Bent angle of flexible CH
2
group between benzene and imidazole varies from 107.34º to
13.85º. Complex CA4 (Figure 3b) crystallizes in trigonal system with space group R3 (CCDC:
894704) Its asymmetric unit also consists of three independent Pd(II) ions and two ligands
1
1
̅
L2. The closest distance between two central benzene cores is 3.44 Å. Bent angle of flexible
CH group between benzene and imidazole varies from 107.44 to 113.65°. Distances of 3.14
and 3.44 Å between two triazine (CA3) and two benzene cores (CA4), respectively, fall in the
range of π-π interactions, which strongly favoured the self-assembly of complexes as M
system. Flexibility of CH group, as evident from bent angle measured in crystal structure of
both cages, helps in the formation of M cages. Space-filled model of the cages suggests a
2
L
3 2
2
L
3 2
highly reduced cavity size, which makes it difficult for any guest encapsulation.
Figure 3. Single crystal XRD structures of the cages (a) CA3; and (b) CA4. Colour codes: red
–
Pd, yellow-P, blue-N, grey-C, orange - Fe.
3
.2 Self-sorting experiment of CA3 and CA4

As both the ligands L1 and L2 have similar geometry and form cages of almost equivalent
shape and size with complementary building unit, we were curious to know whether (in
presence of L1 and L2) the same metal acceptor would lead to the formation of a
multicomponent cage comprising both the ligands. As L1 has an electron deficient central core
(
triazine) and L2 has a relatively electron rich core, we assumed that multicomponent cage
formation would be straightforward. For this experiment, we have used cis-Pd(dppf)(OTf) as
metal acceptor as it is easier to determine the number of products formed by simply P NMR.
To investigate this, reaction of cis-Pd(dppf)(OTf) with L1 and L2 in 3:1:1 ratio respectively
in CH NO was done by stirring at room temperature for 24 h. Final product was isolated and
characterized.
2
3
1
2
3
2
3
1
Appearance of two peaks at 32.37 and 32.34 ppm in P NMR indicated the formation of a
mixture of CA3 and CA4 (Figure 4) instead of the expected multicomponent cage (Scheme 2).
1H NMR (Figure S14) also indicates the formation of two products as two different CH
appear at 4.96 and 4.84 which correspond to CA3 and CA4, respectively. Also, from ESI-MS
Figure S19) two overlapping isotopic distribution patterns are observed at m/z = 1176 for
charge +3, which suggests the formation of mixture of cages CA3 and CA4. The formation of
complexes as M is favoured by strong π-π interaction between two central cores of triazine
peaks
2
(
L
3 2
and benzene of CA3 and CA4, respectively. The formation of two different self-assembly in
self-sorting suggests the π-π interaction is more favourable between two central triazine in case
of CA3 and between two central benzene in case of CA4 than π-π interaction between benzene
and triazine.

Scheme 2 Self-sorting reaction between cis-Pd(dppf)(OTf)
2
with L1 and L2 in CH
3
NO shows
2
the formation of two different self-assembly
3
1
Figure 4 Partial P NMR spectra of a) cage CA3, b) cage CA4 and c) self-sorting reaction of
CA3 and CA4 recorded in CD CN.
3

4
. Conclusion
In this article we report the synthesis of bifacial M
3
2
L nanocage via coordination-driven self-
assembly without using any template. All cages were fully characterized by spectroscopic
techniques and we were successful in obtaining single crystals of CA3 and CA4. The formation
of M
3
2
L self-assembly in all cases is favoured by a strong π-π interaction between two electron
deficient cores of ligands. All cages show a much reduced cavity size, which makes them
unfavourable for any guest encapsulation. Both types of cage with ligand L1 and L2 are highly
stable and do not lose their identity as evident from self-sorting reaction.
Acknowledgments
P.S.M thanks the Council of Scientific and Industrial Research (CSIR-India), India, for
financial support. Authors sincerely thank Dr. Prodip Howlader for his kind help in structure
analysis.
Appendix A. Supplementary Information
1
13
Schemes of synthesis of the ligands, H NMR, C NMR and ESI-MS of two cages. CCDC
numbers [CCDC: 1894555(CA3); 1894704(CA4)] carry the crystallographic details of the
structures.
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Self-assembled Pd
3
2
L cages having flexible tri-imidazole donors
Atul Kumar, Ennio Zangrando, Partha Sarathi Mukherjee*

Four new Pd
3
2
L self-assembled molecular cages consisting of flexible tri-imidazole donors
have been synthesized and characterized. Two of such cages are soluble in water, while the
other two are soluble in organic solvents.