Face-driven corner-linked octahedral nanocages: M6L8 cages formed by C3-symmetric triangular facial ligands linked via C4-symmetric square tetratopic PdII ions at truncated octahedron corners

Article abstract of DOI:10.1021/ja060051h

The face-driven corner-linked truncated octahedral nanocages, [Pd₆L₈]¹²⁺ (1, L1 = N,N-,N-tris(3-pyridinyl)-1,3,5-benzenetricarboxamide; 2, L2 = N,N-,N-tris(4-pyridinylmethyl)-1,3,5-benzenetricarboxamide), were prepared with eight C₃-symmetric tridentate ligands and six square planar tetratopic palladium(II) ions. The combination of the nitrogen donor atom at a ～120° kink position of the carboxamido pyridinyl group and the tilted pyridyl versus the facial plane of the ligands can provide the needed curvature for the formation of octahedral cages. The nitrogen atoms can coordinate to the square planar palladium(II) ions to form kinks with approximately 120° angles at the C₄-symmetric square planar corners of the truncated octahedron. Depending on the conformation of the ligand, L1, two different truncated octahedral cages of around 2.4 nm in diameters were formed. The major form of 1 with syn-conformational ligands has a cavity volume of ～1600 A³. The cage has 12 ports (3.4 × 3.5 A²) at all edges of the octahedron. The minor form of cage 1 with anti-conformational ligands has a slightly increased cavity volume (～1900 A³) and port size (3.3 × 8.0 A²). The insertion of a methylene group in L2 has not only increased the cavity volume of 2 to ～2200 A³ but also enlarged the port size to 4.1 × 8.0 A². However, an atomic force microscopy (AFM) study of cage 2 showed that the cages had a height of 1.8 ± 0.1 nm. This value is about 30% smaller than the calculated size of 2.6 nm from the crystal structure. This tip-induced decrease in height in cage 2 suggests the nonrigidity of cage 2. Copyright

Full text of DOI:10.1021/ja060051h

Published on Web 02/22/2006
Face-Driven Corner-Linked Octahedral Nanocages: M6L8 Cages Formed by
C3-Symmetric Triangular Facial Ligands Linked via C4-Symmetric Square
Tetratopic PdII Ions at Truncated Octahedron Corners
Dohyun Moon,† Sangmi Kang,† Jaejoon Park,† Kyungjae Lee,† Rohith P. John,† Hosik Won,†
Gi Hun Seong,† Yang Sun Kim,‡ Ghyung Hwa Kim,§ Hakjune Rhee,† and Myoung Soo Lah*,†
Department of Chemistry and Applied Chemistry, College of Science and Technology, Hanyang UniVersity, Ansan,
Kyunggi-Do 426-791, Korea, Microbiochip Center, Hanyang UniVersity, Ansan, Kyunggi-Do 426-791, Korea, and
Pohang Accelerator Laboratory, Pohang, Kyungbook 790-784, Korea
Received January 10, 2006; E-mail: mslah@hanyang.ac.kr
Scheme 1 a
Self-assembled high symmetry polyhedral cages are common in
biological systems. The coats of many viruses have icosahedral
structures with 60 facial protein subunits,1 and the iron storage
protein, apoferritin, has an octahedral structure with 24 facial protein
subunits.2 Following the lead of nature, attempts to prepare synthetic
cages3 for potential applications, such as storage,4 recognition,5
delivery,6 and catalysis,7 are very popular among scientists. There
is a great deal of interest in metal-induced self-assembled supramo-
lecular cages using the ability of a coordination bond to direct orien-
tation of the desired components.3c,8 Several coordination-inspired
a The combination of the nitrogen donor atom of L1 and L2 in ∼120°
kink from the amide group and the tilted pyridyl group (tilting angle φ)
high symmetry cages of trigonal pyramid,9 square pyramid,9 trigonal
bipyramid,10 trigonal prism,11 trigonal antiprism,12 square prism,11
tetrahedron,13 cuboctahedron,14 and octahedron15-17 geometries have
been reported. Self-assembly of high symmetry cages via coordination
with a directional bond utilizes two different approaches.8b The first
is edge-directed self-assembly of the linear components linked at
the corner.13b,d,14 The other is face-directed self-assembly of the fa-
cial components linked either at the edges9-11,13a or at the corners.12,13c
Here we report the use of suitably designed C3-symmetric facial
ligands (Scheme 1) and C4-symmetric metal ions as a general
strategy for the preparation of face-driven corner-linked truncated
octahedral nanocages (Scheme 2).
provides the needed curvature for out-of-plane coordination from the plane
of the C3-symmetric facial ligands.
Scheme 2 a
The cage, [Pd6L18]12+ (L1 ) N,N′,N′′-tris(3-pyridinyl)-1,3,5-
benzenetricarboxamide) 1, was prepared with eight C3-symmetric
tridentate ligands. The combination of the nitrogen donor atom at
the meta position of the carboxamido pyridinyl group and the tilted
pyridyl versus the facial plane of the ligand can provide the needed
curvature for the formation of an octahedral cage. The nitrogen
atoms can coordinate to the square planar palladium(II) ions to form
kinks with approximately 120° angles at the C4-symmetric square
planar corners of the truncated octahedron.
a (a) Cage 1: Eight C3-symmetric facial L1 ligands are connected at six
truncated octahedral corners via C4-symmetric square planar palladium(II)
ions. (b) Cage 2: Addition of a methylene group and the para positioning
of the pyridyl nitrogen in L2 ligands give similar kinds of curvature for
cage formation.
When 4 equiv of ligand L1 was treated with 3 equiv of Pd-
(NO3)2 in DMSO-d6 (0.5 mL) for 30 min, the quantitative self-
assembly of a single product was detected by 1H NMR spectroscopy
(Figure S1a). The six proton signals observed indicate that all the
ligands are located equivalently in the product and are in a C3-
symmetry environment. The coordination of the ligand to the metal
ions could be recognized by the relatively broadened and overall
downfield shifted resonance signals. Support for the formation of
the proposed octahedral cage [Pd6L18](NO3)12, 1, with a molecular
weight of 4884 Da was obtained by electrospray ionization mass
spectra (ESI-MS) (Figure S1b).
Figure 1. CPK model for the crystal structure along the C4-symmetric
axis. The major form of cage 1 (a), minor form of cage 1 (b), a dummy
ball diameter of ∼12.0 Å inside (a) and ∼13.6 Å in (b). (c) Cage 2. A
dummy ball diameter of ∼14.4 Å. Color code: green, palladium; red,
oxygen; blue, nitrogen; gray, carbon; white, hydrogen; yellow, dummy atom.
The crystal structure unambiguously demonstrates that 1 has a
[Pd6L18]12+ cage of truncated octahedral geometry (Figure 1). Eight
C3-symmetric facial L1 ligands are connected at six truncated
octahedral corners via C4-symmetric square planar palladium(II)
ions. The pyridyl donor atom of L1 is at a ∼120° kink from the
amide group, and the tilting angle, ∼57°, between the central
benzene and the pyridyl group gives the curvature needed for cage
formation for the pyridyl donors in the out-of-plane direction from
† College of Science and Technology, Hanyang University.
‡ Microbiochip Center, Hanyang University.
§ Pohang Accelerator Laboratory.
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10.1021/ja060051h CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
the plane of the C3-symmetric facial ligand. Each PdII has a square
planar geometry with Pd‚‚‚NPyr distance of 2.033(4) Å, and NPyr
host systems that can accommodate suitable small molecular guests
or chiral catalysts.
-
Pd-NPyr angles of 89.992(3) and 178.7(2)°. Two conformational
isomers in a ratio of approximately 3:2 were observed as statically
disordered structures. Depending on the conformation of the
ligands,16 two different truncated octahedral cages of around 2.4
nm in diameters were formed (Figure 1a and 1b). The major form
of 1 with a syn-conformational ligand has a cavity volume of ∼1600
Å3 (Figure 1a). The cage has 12 ports (3.4 × 3.5 Å2) at all edges
of the octahedron. The minor form of cage 1 with anti-conforma-
tional ligands has a slightly increased cavity volume (∼1900 Å3)
and port size (3.3 × 8.0 Å2) (Figure 1b). However, the ROESY
spectrum of cage 1 indicates that the major form with the ligand in
the syn-conformation is the only species in solution (Figure S3).
We could apply the same strategy for the preparation of another
O-symmetry truncated octahedral cage with similar a C3-symmetric
facial ligand, L2 (L2 ) N,N′,N′′-tris(4-pyridinylmethyl)-1,3,5-
benzenetricarboxamide), with an additional methylene group be-
tween the amide and the pyridyl group (Scheme 1). The change in
position of the pyridyl nitrogen from the meta to the para also
provides the similar 120° kink needed for cage formation. The cage
[Pd6L28](NO3)12, 2, was prepared with eight C3-symmetric tridentate
ligands with three donor atoms at the para position of the pyridyl
group (Scheme 2). When 4 equiv of ligand L2 was treated with 3
equiv of Pd(NO3)2 in DMSO-d6 (0.5 mL) for 30 min, the
quantitative self-assembly of a single product of high symmetry
Acknowledgment. The financial support of KRF (KRF-2005-
070-C00068), ITEP (M1-0213-03-0004), KOSEF (R01-2005-000-
10490-0), and CBMH is acknowledged. The authors also acknowl-
edge PAL for beam line use (2005-2063-01).
Supporting Information Available: Experimental procedures,
spectra, and an X-ray crystallographic file in CIF format for the structure
determination. This material is available free of charge via Internet at
http://pubs.acs.org.
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