Solid-State Synthesis, X-ray Structure, and Guest Inclusion in Octahedral M 6 L 4 Cages Self-Assembled by Tris-pyridyl-benzene ( TPB )

The challenges found to self-assemble crystalline octahedral M 6 L 4 metal–organic cages ( MOCs ) with the tridentate 2,4,6-tris(4-pyridyl)benzene ( TPB ) ligand and Pd 2+ are due to the dihedral angle θ ∼ 36° between the pyridines and the central benzene ring (H pyridine ···H benzene repulsion), the end-capping group, and the presence of suitable templating guests. Although it has recently been reported that the TPB M 6 L 4 cage ( Pd 6 (TPB) 4 ) can be formed in the solution state by only using the ethylenediamine (en) cis protecting group and Pd 2+, its solid-state synthesis, 3D X-ray structure, and host–guest chemistry have not been reported. Here, using mechanochemistry by neat grinding (NG) TPB and (en)-Pd(NO 3 ) 2, we report the solid-state synthesis of Pd 6 (TPB) 4, its solid-state structure, and its ability to include aromatic guests in aqueous media. In solution, the self-assembly of TPB and Pd 2+ with a templating guest (i.e., 3-phenylpropanoic acid) ( G 1 ) formed crystals of Pd 6 (TPB) 4, suitable for single-crystal X-ray diffraction (SC-XRD). Density functional theory (DFT) calculations specific to the solid state showed different high binding energy ( E b ) values for Pd 6 (TPB) 4 and the isostructural tris-pyridyl triazine ( TPT )-based MOC (Pd 6 (TPT) 4 ) . The higher stability of the Pd 6 (TPT) 4 structure with respect to Pd 6 (TPB) 4 may be related to its higher E b and density, a direct consequence of the higher degree of planarity of TPT, suggesting the importance of enthalpic/entropic contributions for the stability of these M 6 L 4 structures in the solid state. The Pd 6 (TPB) 4 cage has an electron-rich hydrophobic cavity (∼450 Å 3 ) that is stable in water and in acidic conditions and can include four G 1 in one M 6 L 4, as shown by titration experiments and using a guest model X-ray structure. In the solution state, 1 H NMR data show that G 1 guest exchange/inclusion in Pd 6 (TPB) 4 occurs in the fast exchange regime. Hollow Pd 6 (TPB) 4 is a strong candidate to be used in functional applications such as molecular recognition, guest stabilization, or unusual reactivity.