Face-driven corner-linked octahedral nanocages: M6L8 cages formed by C-3-symmetric triangular facial ligands linked via C-4-symmetric square tetratopic Pd-II ions at truncated octahedron corners

Abstract

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.