Self-assembled palladium(ii) "click" cages: Synthesis, structural modification and stability

Article abstract of DOI:10.1039/c1dt10551e

Readily synthesised and functionalised di-1,2,3-triazole "click" ligands are shown to self-assemble into coordinatively saturated, quadruply stranded helical [Pd₂L₄](BF₄)₄ cages with Pd(ii) ions. The cages have been fully characterised by elemental analysis, HR-ESMS, IR, ¹H, ¹³C and DOSY NMR, DFT calculations, and in one case by X-ray crystallography. By exploiting the CuAAC "click" reaction we were able to rapidly generate a small family of di-1,2,3-triazole ligands with different core spacer units and peripheral substituents and examine how these structural modifications affected the formation of the [Pd₂L₄](BF₄)₄ cages. The use of both flexible (1,3-propyl) and rigid (1,3-phenyl) core spacer units led to the formation of discrete [Pd₂L₄](BF ₄)₄ cage complexes. However, when the spacer unit of the di-1,2,3-triazole ligand was a 1,4-substituted-phenyl group steric interactions led to the formation of an oligomeric/polymeric species. By keeping the 1,3-phenyl core spacer constant the effect of altering the "click" ligands' peripheral substituents was also examined. It was shown that ligands with alkyl, phenyl, electron-rich and electron-poor benzyl substituents all quantitatively formed [Pd₂L₄](BF₄)₄ cage complexes. The results suggest that a wide range of functionalised palladium(ii) "click" cages could be rapidly generated. These novel molecules may potentially find uses in catalysis, molecular recognition and drug delivery.

Full text of DOI:10.1039/c1dt10551e

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Self-Assembly in Inorganic Chemistry
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Metal ion directed self-assembly of sensors for ions, molecules and biomolecules
Jim A. Thomas
Dalton Trans., 2011, DOI: 10.1039/C1DT10876J
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Self-assembly between dicarboxylate ions and a binuclear europium complex: formation of stable
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James A. Tilney, Thomas Just Sørensen, Benjamin P. Burton-Pye and Stephen Faulkner
Dalton Trans., 2011, DOI: 10.1039/C1DT11103E
Structural and metallo selectivity in the assembly of [2 × 2] grid-type metallosupramolecular
species: Mechanisms and kinetic control
Artur R. Stefankiewicz, Jack Harrowfield, Augustin Madalan, Kari Rissanen, Alexandre N. Sobolev
and Jean-Marie Lehn
Dalton Trans., 2011, DOI: 10.1039/C1DT11226K
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PAPER
Self-assembled palladium(II) “click” cages: synthesis, structural modification
and stability†
Synøve Ø. Scott,a Emma L. Gavey,a Samuel J. Lind,a Keith C. Gordona,b and James D. Crowley*a
Received 1st April 2011, Accepted 8th June 2011
DOI: 10.1039/c1dt10551e
Readily synthesised and functionalised di-1,2,3-triazole “click” ligands are shown to self-assemble into
coordinatively saturated, quadruply stranded helical [Pd2L4](BF4)4 cages with Pd(II) ions. The cages
have been fully characterised by elemental analysis, HR-ESMS, IR, 1H, 13C and DOSY NMR, DFT
calculations, and in one case by X-ray crystallography. By exploiting the CuAAC “click” reaction we
were able to rapidly generate a small family of di-1,2,3-triazole ligands with different core spacer units
and peripheral substituents and examine how these structural modifications affected the formation of
the [Pd2L4](BF4)4 cages. The use of both flexible (1,3-propyl) and rigid (1,3-phenyl) core spacer units led
to the formation of discrete [Pd2L4](BF4)4 cage complexes. However, when the spacer unit of the
di-1,2,3-triazole ligand was a 1,4-substituted-phenyl group steric interactions led to the formation of an
oligomeric/polymeric species. By keeping the 1,3-phenyl core spacer constant the effect of altering the
“click” ligands’ peripheral substituents was also examined. It was shown that ligands with alkyl,
phenyl, electron-rich and electron-poor benzyl substituents all quantitatively formed [Pd2L4](BF4)4 cage
complexes. The results suggest that a wide range of functionalised palladium(II) “click” cages could be
rapidly generated. These novel molecules may potentially find uses in catalysis, molecular recognition
and drug delivery.
not readily allow the incorporation of “useful” functional groups.
Introduction
As such there is still a need for mild, modular synthetic methods
Inspired by nature, the use of self-assembly to generate functional
that allow for easy functionalisation of metallosupramolecular
metallosupramolecular systems from relatively “simple” building
systems. One such method is the Cu(I)-catalyzed 1,3-cycloaddition
blocks (metal ions and ligands) has received considerable attention
of organic azides with terminal alkynes (the CuAAC “click”
in recent years.1 The sustained interest in this area is driven by the
reaction) and there has recently been an explosion of interest
potential applications of these metallosupramolecular architec-
in the use of the CuAAC reaction to generate functionalised
ligand architectures containing 1,2,3-triazole units.12 This interest
is driven by the desire to use 1,2,3-triazoles as readily synthe-
sised and functionalised surrogates for the ubiquitous pyridine
ligands. While the 1,2,3-triazole unit can potentially coordinate
through both the N213 and N314 nitrogens a number of studies
have shown that simple 1,4-substituted-1,2,3-triazole ligands can
interact with metals ions in a monodentate fashion,15,16 suggesting
that these “click” ligands could potentially be used to replace
pyridine donor units within supramolecular polydentate ligand
architectures.17
tures. Systems have been exploited for the molecular recognition
and encapsulation2 of a wide variety of molecules including
highly reactive species,3 biomolecules,4 aromatic systems,5 anions,6
drugs,7 and environmental pollutants.8 Other metallosupramolec-
ular architectures have shown promise as molecular reaction
flasks,9 catalysts10 and even stimuli responsive11 machine-like
systems have been developed. While there are now a wide variety of
ligand scaffolds available for the generation of metallosupramolec-
ular systems many of them require time consuming and expensive
multi-step syntheses or are generated under conditions that do
The palladium–pyridine metal–ligand interaction is frequently
exploited in the construction of metallosupramolecular macro-
cyclic and cage architectures and most commonly dipyridyl ligands
are used as the connecting unit in the assembly process.18 As such,
we set out to show that readily synthesised and functionalised
di-1,2,3-triazole ligands could be used in place of these more com-
monly utilised dipyridyl ligands in the self-assembly of palladium
containing metallosupramolecular systems. In preliminary work,
we have previously demonstrated that the “click” ligands, 1a and
aDepartment of Chemistry, University of Otago, PO Box 56, Dunedin, New
Zealand. E-mail: jcrowley@chemistry.otago.ac.nz; Fax: +64 3 479 7906;
Tel: +64 3 479 7731
bMacDiarmid Institute for Advanced Materials and Nanotechnology, New
Zealand
† Electronic supplementary information (ESI) available: Experimental
procedures, 1H NMR, DOSY and ESMS spectra, 1H NMR stacked plots,
MMFF and DFT calculations and crystallographic data. CCDC reference
numbers 819051 and 819052. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10551e
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12117–12124 | 12117
©

1b, self-assemble into M2L2 metallomacrocycles19 with Ag(I) ions
and M2L4 helical molecular cages20 with Pd(II). Herein we extend
that work by using the CuAAC reaction to rapidy and safely
generate a family of di-1,4-substituted-1,2,3-triazole ligands with
different central and peripheral substituents and examine the effect
of these structural modifications on the formation of the Pd2L4
helical cages.
Simply mixing either 1,6-heptadiyne, 1,4-diethynylbenzene or
1,3-diethynylbenzene with the appropriate benzylic bromide or
chloride, NaN3, CuSO4·5H2O, Na2CO3 and ascorbic acid in
DMF/H2O (4 : 1) at room temperature followed by stirring for
20 h (Scheme 1) provided, after work up, the desired ligands (1a–d,
2a, 3a) in excellent yields (79–94%). The n-hexyl substituted ligand,
1f, was obtained in 86% yield by reacting 1,3-diethynylbenzene
with 1-azidohexane under the same conditions as above. The
1-azidohexane was safely formed in situ from 1-bromohexane
and NaN3 using microwave irradiation (125 ◦C, 250 W,
45 min) and reacted immediately. The phenyl substituted ligand,
1e, was synthesised in 45% yield from 1,3-diethynylbenzene, phenyl
boronic acid and NaN3 using a modified version of the one-pot
CuAAC method developed by Gao.22
Results and discussion
Ligand synthesis and characterisation
The synthesis of the di-1,2,3-triazole ligands (1a–f, 2a and
3a) was safely achieved without the need to isolate the po-
tentially explosive azide intermediates by employing the pre-
viously described one-pot procedures outlined in Scheme 1.21
The di-1,2,3-triazole ligands (1a–f, 2a and 3a) have been fully
characterised by elemental analysis, electrospray mass spectrome-
try (ESMS), IR, 1H and 13C NMR (ESI†) and in one case (1d) by
X-ray crystallography (Fig. 1, Table 1, ESI†). The X-ray crystal
structure of 1d unambiguously confirmed the bond connectivity
Scheme 1 (i) RBr or RCl, NaN3, CuSO4·5H2O, ascorbic acid, Na2CO3,
DMF/H2O (4 : 1), RT, 20 h; (ii) (a) C6H13Br, NaN3, NaI, DMF/H2O (4 : 1),
MW, 250 W, 125 ◦C, 45 min, (b) CuSO4·5H2O, ascorbic acid, Na2CO3,
DMF/H2O (4 : 1), RT, 20 h; (iii) (a) phenyl boronic acid, NaN3, CuCl,
MeOH, reflux, 6 h; (b) CuSO4·5H2O, H2O, sodium ascorbate, RT, 20 h.
Isolated yields after chromatography.
Fig. 1 A labeled ORTEP diagram showing the molecular structure of the
di-1,2,3-triazole ligand 1d. The thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (A) and angles ( ) for 1d; N1 N2
1.343(3), N2 N3 1.311(3), N4 N5 1.318(3), N5 N6 1.343(3), N1 N2 N3
107.3(2), N4 N5 N6 106.9(2).
◦
˚
Table 1 Crystallographic data for 1d and 4c
Compound
1d
4c
CCDC 819052
C26H24N6O2
452.51
Monoclinic, Pc
20.910(6)
5.727(4)
9.4091(12)
90
101.318(33)
90
1104.8(9)
2
0.68 ¥ 0.43 ¥ 0.15 mm, colourless, needle
1.360
0.090
31245
4074 (0.0481)
4074/2/309
1.116
0.0315, 0.0757
0.0368, 0.0864
0.129 and -0.163
CCDC 819051
C112H96B4F16N24O16·Pd2
2594.17
Monoclinic, C2/c
27.9204(17)
22.5838(14)
24.610(3)
Formula
Formula weight
Crystal system, Space group
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
122.689(3)
90
13060.2(20)
4
0.32 ¥ 0.16 ¥ 0.14 mm, colourless, block
1.494
3
˚
V/A
Z
Cryst. size, color, habit
r
calc/mg mm-3
m/mm-1
0.376
43487
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
)
7217 (0.0687)
7217/12/826
1.044
0.0515, 0.1495
0.0632, 0.1566
0.850 and -1.147
Final R1 and wR2 indexes [I > 2s(I)]
Final R1 and wR2 indexes [all data]
-3
˚
Largest difference in peak and hole/e A
12118 | Dalton Trans., 2011, 40, 12117–12124
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©

and illustrates, as expected, that 1,4-substituted triazole units are
formed.
(1 equiv.) in acetonitrile led to quantitative formation of the cage
complex 4a.
The 1H NMR spectrum (CD3CN, 298 K) of the cage complex,
4a shows one set of sharp signals (Fig. 2b and ESI†). All the
Formation of palladium(II) “click” cage complexes
1
proton resonances (except Ha) in the H NMR spectra of 4a are
With a small family of di-1,2,3-triazole ligands available, we
initially examined the effect of the central spacer unit on Pd cage
formation (Scheme 2). The di-benzyl substituted ligands 1a (1,3-
phenyl spacer), 2a (1,3-propyl spacer) and 3a (1,4-phenyl spacer)
with central spacer units of different geometries and flexibilities
were reacted with [Pd(CH3CN)4](BF4)2 in CD3CN or d6-DMSO
solution at RT for 1 h, then the resulting reaction mixtures were
examined using in situ 1H NMR and ESMS. As outlined in
our preliminary communication,20 simply stirring the 1,3-phenyl
linked ditriazole ligand, 1a (2 equiv.) with [Pd(CH3CN)4](BF4)2
shifted upfield relative to the “free” ligand 1a, characteristic of
face-to-face p–p stacking interactions (Fig. 2a and 2b). Conversely,
the proton resonance of Ha is shifted downfield compared to
the corresponding resonance in the “free” ligand due to the
electron withdrawing effect of the palladium(II) ions, indicative
of the formation of a cage architecture in which Ha is in
close proximity to the metal ions. The ESMS spectrum of 4a
shows a series of isotopically resolved peaks consistent with the
formulation [Pd2(1a)4-n(BF4)]n+ (n = 1–4) and is almost devoid of
fragmentation, illustrating the remarkable stability of the cage
architecture. X-ray crystallography confirmed unambiguously
that 4a is a coordinatively saturated, quadruply stranded helical
cage.20
The in situ 1H NMR spectrum of the palladium complex,
5a, formed by reacting the flexible 1,3-propyl linked ditriazole
ligand, 2a (2 equiv.) with [Pd(CH3CN)4](BF4)2 (1 equiv.) in CD3CN
indicates the clean quantitative formation of a single species. The
triazole proton on the ligand undergoes a large downfield shift
indicative of complexation to Pd(II) ions (ESI†). However, the
proton resonances due to the benzyl substituents of the ligands
are shifted upfield suggesting that some kind of face-to-face p–p
stacking interaction is present. The ESMS spectrum of palladium
complex, 5a, exhibits peaks at m/z = 1906 [Pd2(2a)4(BF4)3]+, 909
[Pd2(2a)4(BF4)2]2+, and 411 [Pd2(2a)4]4+ which all correspond to
ions of the intact molecular cage structure. Additionally, there are
a number of large fragment ions, m/z = 841 [Pd(2a)2(H2O)(H)]+,
359 [2a+H]+ suggesting that the flexible cage, 5a is less stable than
4a under the conditions of the ESMS experiment.
In stark contrast to 4a and 5a, the in situ 1H NMR and ESMS
spectra of the palladium complex, 6a, formed by reacting the
ligand, 3a (2 equiv.) with [Pd(CH3CN)4](BF4)2 (1 equiv.) in d6-
DMSO shows no evidence of cage formation. The 1H NMR
spectrum (d6-DMSO, 298 K) of 6a shows one set of broad signals
(ESI†), which are only slightly shifted from the corresponding
values of the free ligand. The ESMS spectrum of 6a is dominated
by three peaks at m/z = 714 [Pd2(3a-H)3(H2O)(Na)(BF4)2]2+, 497
[Pd(3a-H)]+, and 393 [3a+H]+ none of which are due to the intact
molecular cage structure, in fact no ions corresponding to the
molecular cage can be detected. In combination these results
indicate that the palladium(II) complex 6a is an oligomeric or
polymeric species rather than a discrete molecular cage.
The stability of the isomeric “cages”, 4a and 6a, was further
examined computationally using DFT (ESI†). Consistent with the
experimental findings, the energy minimised structure of 4a was
found to be a quadruply stranded helical cage, whereas the “cage”
6a formed a lantern shaped species whose calculated enthalpy of
formation (DHform) was ~300 kJ mol-1 higher than the isomeric 4a
helical cage. Space-filling molecular models (ESI†) of 6a indicate
that the reason for this large energy difference is the presence of
a severe steric interaction between the 1,4-phenyl spacer units of
the 2a ligands within the cage structure. This steric clash would be
relieved by oligomerization.
We next examined whether altering the di-1,2,3-triazole ligands
peripheral substituents had an effect on cage formation. As
Scheme 2 (i) [Pd(CH3CN)4](BF4)2, CD3CN or d6-DMSO, 1 h, RT; or
[Pd(CH3CN)4](BF4)2, CD3CN, 16 h, 70 ◦C.
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©

1
Fig. 2 Partial in situ H NMR spectra (400 MHz, CD3CN, 298 K) of a) the ligand 1a, b) the dipalladium(II) cage complex 4a, c) the dipalladium(II)
cage complex 4b, d) the dipalladium(II) cage complex 4c, e) the dipalladium(II) cage complex 4d, f) the dipalladium(II) cage complex 4e, g) the reaction
mixture of 1f and [Pd(CH3CN)4](BF4)2 after 1 h at RT and h) the dipalladium(II) cage complex 4f obtained after heating (g) for 16 h at 70 ◦C.
the preceding experiments indicated that the 1,3-phenyl spacer
unit provided the most stable Pd-cages, this core unit was kept
constant and the peripheral substituents were altered to give
the small family of ligands 1b–1f, which were then treated with
[Pd(CH3CN)4](BF4)2 in an attempt to assemble the cages 4b–4f
species were present (Fig. 2g and the ESI†). This was a somewhat
surprising result as space-filling molecular models (SPARTAN 08,
MMFF) indicate that there are no obvious steric impediments to
the formation of any of the cages 4a, 4e or 4f (Fig. 3a–c). Van
Koten and co-workers16 have previously shown that the binding
strength of 1,4-substituted-1,2,3-triazole “click” ligands can be
readily tuned by altering the peripheral substituents and that
alkyl substituted 1,2,3-triazoles form the most stable metal–ligand
complexes. We postulated that the higher metal–ligand binding
strength of the n-hexyl substituted ligand 1f led to slower metal–
ligand exchange kinetics, therefore heating the reaction mixture
should increase the rate of the ligand exchange processes and allow
the formation of the thermodynamic cage product. Indeed simply
heating the reaction mixture at 70 ◦C for 16 h led to the quantitative
formation of a single species whose 1H NMR spectrum was very
similar to the other palladium(II) cages, indicating the formation of
the n-hexyl substituted cage 4f (Fig. 2h). The energies of formation
(DHform) for the alkyl (4f), benzyl (4a) and phenyl (4e) substituted
cages were also calculated computationally using DFT (ESI†).
Consistent with the experimental findings and chemical intuition
the alkyl substituted cage (4f) was found to be 97 kJ mol-1 more
stable than the benzyl substituted cage (4a) which was in turn
19 kJ mol-1 more stable than the phenyl cage (4e) (ESI†).
1
(Scheme 2). H NMR and positive ion ESMS experiments were
used to probe the structures of the resulting complexes. As for
the parent cage, 4a, simply mixing the 1,3-phenyl linked ditriazole
ligands, 1b–e (2 equiv.) with [Pd(CH3CN)4](BF4)2 (1 equiv.) in
CD3CN led to quantitative formation of the cage complexes 4b–e
in less than 1 h at RT, as evidenced by the in situ 1H NMR spectra
1
(Fig. 2c–f). The H NMR spectra (CD3CN, 298 K) of the cage
complexes, 4b–e show one set of sharp signals (Fig. 2c–f and ESI†),
with the characteristic downfield shift of the Ha proton resonance
indicative of cage formation. Most of the other proton resonances
1
in the H NMR spectra of 4b–e are shifted upfield relative to
the “free” ligand due the face-to-face p–p stacking interactions,
providing strong evidence that each of the different dipalladium(II)
complexes adopts a helical cage structure in solution and this is
further supported by DFT calculations vide infra.
In contrast to what was observed for 1a–e, mixing the ligand
1f (2 equiv.) with [Pd(CH3CN)4](BF4)2 (1 equiv.), under identical
conditions to those discussed before, did not lead to quantitative
1
formation of the cage complex 4f. After 1 h at RT the in situ H
The ESMS spectra of the complexes 4b–f are similar to that
observed for the parent compound, 4a. They each show a series
NMR spectrum of the reaction mixture indicated that a number of
12120 | Dalton Trans., 2011, 40, 12117–12124
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©

Fig. 3 Space-filling molecular models (SPARTAN 08, MMFF) of a) the benzyl substituted palladium(II) cage complex 4a, b) the phenyl substituted
palladium(II) cage complex 4e, and c) the hexyl substituted palladium(II) cage complex 4f. (Spartan ’08 for Windows, Wavefunction, Irvine, CA).
Table 2 Diffusion coefficients obtained from 1H DOSY spectra (CD3CN,
much larger than the free ligands, providing strong evidence for the
selective formation of the larger molecular cage species in CD3CN
solution.24
298 K)
Diffusion Coefficient
(10-10 m2 s-1)
Palladium
Complex
Diffusion Coefficient
(10-10 m2 s-1)
The palladium “click” cages, 4a–f, 5a were isolated as micro-
crystalline solids by slow vapour diffusion of diethyl ether into
the acetonitrile reaction mixtures and their purity confirmed by
elemental analysis. In the case of 4c this isolation procedure
provided crystals of a quality suitable for X-ray crystallographic
analysis allowing the molecular structure of 4c to be determined.
Similar to the parent palladium(II) cage, 4a,20 the X-ray crystal
structure of 4c illustrates that four 1c ligands and two Pd(II) ions
form a coordinatively saturated, quadruply stranded helical cage
with the formula [Pd2(1c)4](BF4)4 (Fig. 4). 4c crystallises in the
monoclinic space group C2/c and the asymmetric unit contains
a helical cage and 16 disordered CH3CN solvates. As with the
parent cage the Pd(II) ions of 4c are found to coordinate to the
Ligand
1a
1b
1c
1d
1e
1f
6.95
6.51
5.92
6.42
7.15
7.66
7.14
4a
4b
4c
4d
4e
4f
3.51
3.25
3.26
3.36
3.76
3.38
4.03
2a
5a
of isotopically resolved peaks consistent with the formulation
[Pd2(L)4-n(BF4)]n+ (n = 1–4) and are essentially devoid of frag-
mentation, further illustrating the remarkable stability of these
helical cages (ESI†).
Diffusion-ordered NMR spectroscopy (DOSY)23 provided ad-
ditional support for the selective formation of the cages in CD3CN
N3 nitrogens of the triazole ligands (Pd–N bond lengths range
25
˚
from 2.005–2.016 A) in a distorted square-planar coordination
environment (N–Pd–N bond angles vary from 89.4 to 178.9◦, Fig.
4). The 1,2,3-triazole donor units are rotated approximately 45◦
out of the square plane and this twisting generates the overall
helical structure with a helical pitch of 90◦. The helical cage is
additionally stabilised by a number of face-to-face p–p stacking
1
solution. H DOSY spectra (CD3CN, 298 K) were obtained for
the ligands 1a–f and 2a and the palladium cages 4a–f and 5a
(Table 2, ESI†) and each of the proton signals in the individual
spectra show the same diffusion coefficients (D) indicating that
there is only one species present in solution. The ligands (1a–
1f and 2a) showed similar diffusion coefficients ranging from
6.42–7.66 ¥ 10-10 m2 s-1 consistent with their similar molecular
size. The diffusion coefficients of the palladium cages (4a–f
and 5a) ranged from 3.25–4.03 ¥ 10-10 m2 s-1 indicating that
all of the cages are similar in size. The Dcage/Dligand ratio is
approximately 0.50 : 1 suggesting that palladium(II) complexes are
˚
interactions (the centroid-centroid distance 3.745 A) between the
central 1,3-disubstituted phenyl spacer units and 1,2,3-triazole
groups on adjacent ligands. Additionally, there are edge-to-face
p–p interactions between benzyl groups on adjacent ligands
˚
(the carbon-centroid distance 3.722 A) which potentially further
stabilise the cage structure. The helical twisting flattens the cage
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12117–12124 | 12121
©

Chemistry, University of Otago, for providing financial assistance.
Dr Mervyn Thomas is thanked for his assistance collecting DOSY
NMR data. SØS thanks the University of Otago for providing a
summer scholarship (2010–2011).
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Fig. 4 a) A labeled tube representation (side view) of the molecular
structure of the cage 4c; b) a top view of the molecular structure of 4c. For
clarity, each of the four 1c ligands are represented in a different colour.
-
The hydrogen atoms, solvent molecules and B◦F4 anions are omitted for
˚
clarity. Selected bond lengths (A) and angles ( ) for 4c; Pd1 N3 2.005(3),
Pd1 N9 2.008(6), Pd2 N4 2.016(6), Pd2 N10 2.010(3), Pd1 Pd2 4.9219(7),
N3 Pd1 N9 90.1(2), N4 Pd1 N10 89.4(2), N3 Pd1 N3¢ 178.9(2), N9 Pd1 N9¢
169.9(2), N4 Pd1 N4¢ 169.3(2), N10 Pd1 N10¢ 177.5(2) Symmetry Code;
-x, y, 1/2-z.
structure and results in a small cage cavity with the dimension 3.5
˚
¥ 3.8 ¥ 4.9 A, which while slightly larger than the parent, 4a cage,
appears to be too small to incorporate guest molecules.
We have shown that di-1,4-substituted-1,2,3-triazole “click”
ligands can be used as pyridine surrogates to self-assemble
coordinatively saturated, quadruply stranded helicate cages with
Pd(II) ions. These [Pd2(L)4](BF4)4 cages are stable in solution and
the solid state as evidenced by 1H NMR, DOSY, ESMS and X-ray
crystallographic experiments. The mild, modular “click” method
used to form the di-1,4-disubstituted-1,2,3-triazole ligands should
allow the ready synthesis of complex functionalised ligand ar-
chitectures which could be exploited for the generation of novel
metallosupramolecular cages with unique properties.
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Acknowledgements
We thank Prof. Lyall Hanton, Assoc. Prof. Allan Blackman and
Dr David McMorran for useful discussions and the Department of
12122 | Dalton Trans., 2011, 40, 12117–12124
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