Self-assembly and X-ray structure of a ten-component, three-dimensional metallosupramolecular cage

Article abstract of DOI:10.1039/a608081b

Reaction of 1,3,5-tris(pyrazol-1-ylmethyl)-2,4,6-triethylbenzene 7 with palladium chloride results in the self-assembly of a three-dimensional cage; an X-ray crystal structure determination shows that the cage is comprised of an octahedral arrangement of

Full text of DOI:10.1039/a608081b

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Self-assembly and X-ray structure of a ten-component, three-dimensional
metallosupramolecular cage
Chris M. Hartshorn and Peter J. Steel*
Chemistry Department, University of Canterbury, Christchurch, New Zealand
Reaction of 1,3,5-tris(pyrazol-1-ylmethyl)-2,4,6-triethyl-
benzene 7 with palladium chloride results in the self-
assembly of a three-dimensional cage; an X-ray crystal
structure determination shows that the cage is comprised of
an octahedral arrangement of six palladium atoms bridged
by a tetrahedral network of four molecules of the ligand and
contains a (disordered) guest molecule of dimethyl sulf-
oxide.
methylation11 of 1,3,5-triethylbenzene, and subsequent phase-
transfer-catalysed reaction with pyrazole.†
Reaction of 7 with palladium chloride gave immediate
precipitation of an orange product,‡ which, on dissolution in
1
Me2SO, gave a H NMR spectrum that showed significantly
different chemical shifts from those of 7, but retained the
threefold symmetry. This was difficult to explain in terms of a
square-planar palladium complex of low-nuclearity. Single
crystals of this compound were grown by diffusion of acetone
into an Me2SO solution of the solid, but these decomposed
instantaneously upon removal of the solvent. After many
attempts, a crystal was successfully tranferred to the low-
temperature stream of a four-circle diffractometer with only
minor decomposition.§
Fig. 1 shows a perspective view of the major component of
the contents of the asymmetric unit of this structure.¶ It consists
of a ten-component, three-dimensional metallosupramolecular
cage comprised of six trans-dichloropalladium units bridged by
four molecules of 7. The six palladium atoms are arranged in a
pseudo-octahedral array, while the four molecules of 7 form a
tetrahedrally disposed internal core of benzene rings of
approximately 4.7 Å internal radius. Within this core resides a
single (disordered) Me2SO molecule (not shown in Fig.1). The
potential Td symmetry is not crystallographically imposed, and
is destroyed by differing conformations of the ethyl groups;
specifically, one of the ligands has all three terminal methyl
groups directed towards the internal core, while the other three
ligands have one methyl group directed outside the core. This,
along with the presence of the Me2SO guest and intermolecular
interactions, results in a significant reduction in symmetry
The use of metal ions to control the self-assembly of
multicomponent supramolecular structures of varying archi-
tecture is a subject of much current investigation;1 particular
recent attention has focused on the formation of species with
internal cavities.2 Since 1990, several groups have reported the
construction of various molecular squares, represented sche-
matically by structure 1.3 Lehn and coworkers4 subsequently
reported the formation of a three-dimensional cylindrical
molecular box 2, comprised of six metal ions and two types of
bridging ligand. Fujita et al. have since reported the assembly of
a five-component molecular cage5 and, most recently, a ten-
component adamantanoid-type cage 3 comprised of six metals
with four identical bridging ligands.6 We now report the
formation and X-ray structure of a constitutionally similar but
topologically different M6L4 cage.
We are currently engaged in a detailed study7 of the
coordination chemistry of a series of polyheteroaryl-substituted
arenes 4, which consist of various heterocyclic rings attached
via spacer groups, X, to a central arene core, and have reported
the syntheses of a number of poly(pyrazolylmethyl)benzenes.8
We have also shown that the mesitylene-derived ligands 5 and
6 are able to encapsulate an octahedral metal atom, with
6
simultaneous h -arene coordination and tripodal chelation to
the pyrazole rings.9 We have now extented this study to
examine the complexes of these ligands with metal ions of
different coordination geometry. In an effort to pre-organise the
three pyrazole rings on the same side of the benzene ring,10 we
have now synthesised the triethyl analogue 7, by trisbromo-
M
L
M
M
M
M
M
L
L
L′
M
L
M
L
L
L
L
L
M
L
M
M
M
M
M
M
L
L′
M
1
2
3
N
N
N
R
R
N
N
X
N
n
4
5 R = H
6 R = Me
7 R = Et
R
N
Fig. 1. Perspective view of the X-ray crystal structure; hydrogen atoms and
solvate molecules not shown for clarity
Chem. Commun., 1997
541

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in the solid state. For example, the diagonally opposite
palladium atoms have separations ranging from 13.26 to 15.54
Å (Fig. 2). We believe that this cage structure is maintained in
solution, which explains the symmetry observed in the NMR
spectra and the slight broadening of the signals due to the
methyl and methylene protons.
bridging ligands, including dissymmetric and asymmetric
examples, are currently in progress.
Footnotes
† Selected data for 7: isolated yield 60%, mp 125–126 °C (light petroleum).
Found: C, 71.89; H, 7.73; N, 20.69. C24H30N6 requires C, 71.61; H, 7.51; N,
20.88%. 1H NMR (CDCl3): d 0.96 (9 H, t, CH3), 2.70 (6 H, m, 2,4,6-CH2),
5.45 (6 H, s, 1,3,5-CH2), 6.20 (3 H, t, H 4A), 6.98 (3 H, d, H5A), 7.55 (3 H,
d, H3A). 1H NMR [(CD3)2SO]: d 0.88 (9 H, t, CH3), 2.88 (6 H, m,
2,4,6-CH2), 5.47 (6 H, s, 1,3,5-CH2), 6.32 (3 H, t, H4A), 7.53 (3 H, d, H5A),
7.57 (3 H, d, H3A).
‡ Selected data: isolated yield 87%, mp > 295 °C (decomp.) Found: C,
41.09; H, 4.92; N, 11.37; Cl, 14.93. C96H120Cl12N24 Pd6·2C2H6OS·4H2O
requires C, 41.38; H, 4.86; N, 11.58; Cl, 14.66%. 1H NMR [(CH3)2SO]: d
1.23 (9 H, br t, CH3), 2.50 (6 H, br m, 2,4,6-CH2), 6.50 (6 H, br s,
1,3,5-CH2), 6.65 (3 H, t, H4A), 7.93 (3 H, d, H5A), 8.15 (3 H, d, H3A).
This molecule represents the second example of a M6L4
supramolecular adamantanoid cage, and serves to demonstrate
the generality of such assembly processes; examples are also
known of stoichiometrically inverted M4L6 compounds.12 It is
also the first X-ray structure of a free cage of this sort; in the
previous example,6 a low-precision X-ray structure of a
clathrate complex with four adamantanyl carboxylate ions was
determined. Although constitutionally similar to the previously
reported M6L4 cage, the present structure is topologically quite
different in shape. This is primarily a consequence of the
different coordination geometries of the palladium atoms in the
two compounds. In the only other example,6 the bridging
ligands were obligatorily cis-coordinated to the palladiums, due
to the presence of chelating ethylenediamine ancillary ligands;
in the present case, the ligands are trans- coordinated, which has
the effect of making the palladiums more exposed on the surface
of the cage. Furthermore, the methylene spacer groups in 7
impart greater flexibility to the ligand and, combined with the
fact that the coordinating nitrogen is adjacent to the spacer
group, allows for more compact packing within the cage; in the
earlier case6 the diagonally opposite palladiums were separated
by ca. 19 Å.
The internal core of four tetrahedrally arranged benzene rings
is reminiscent of that in the recently synthesised C36H36
spheriphane molecule which has covalently bonded ethylene
bridges linking the benzenes and a radius of the internal cavity
(as measured from the centroids of the rings) of 2.84 Å.13 In the
present case, the nature of the bridges expands this core to a
radius of approximately 4.7 Å and allows for the incorporation
of an Me2SO guest molecule. The octahedral arrangement of the
six metals is also related to that in the hexacoordinated
palladium and platinum complexes of C60, wherein the
diagonally opposite metals are separated by ca. 11.2 Å and
the internal core has a diameter of only 3.51 Å.14
§ Crystal data for (PdCl2)6(7)4·8Me2SO·8H2O: C112H184Cl12N24O16Pd6S8,
–
Mw = 3443.1, triclinic, space group P1, a = 19.648(6), b = 19.730(5),
c
= 24.995(5) Å, a = 97.94(1), b = 108.88(1), g = 95.17(2)°,
U = 8986(4) Å3, Z = 2, Dc = 1.273 g cm23, m = 0.91 mm21
,
F(000) = 3520, T = -105 °C. Cell parameters were determined by least-
squares refinement of 35 accurately centred reflections. A yellow block
(0.44 3 0.30 3 0.17 mm) was used to collect (Siemens P4s diffractometer,
Mo-Ka radiation,
w scans) 21966 independent reflections with
4 < 2q < 44°, with significant crystal decomposition during data collec-
tion. The structure was solved by a combination of Patterson and difference
Fourier methods and refined on F2, using all data, to R = 0.0692,
Rw = 0.1849 for 1654 parameters.
Atomic coordinates, bond lengths and angles, and thermal parameters
have been deposited at the Cambridge Crystallographic Data Centre
(CCDC). See Information for Authors, Issue No. 1. Any request to the
CCDC for this material should quote the full literature citation and the
reference number 182/372.
¶ In addition to the atoms shown in Fig. 1, the asymmetric unit contains a
Me2SO guest in the cavity of the cage which has the sulfur atom disordered
over two sites, and, external to the cage, seven Me2SO and eight water
solvate molecules, many of which are also disordered.
References
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In conclusion, we have shown that, upon reaction with
palladium chloride, the ligand 7 self-assembles into a ten-
component metallosupramolecular cage. Studies directed to-
wards the assembly of related structures employing other
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Fig. 2. View perpendicular to one benzene ring showing the reduction from
Td symmetry, in the solid state
Received, 29th November 1996; Com. 6/08081B
542
Chem. Commun., 1997