Solid-State Synthesis, X-ray Structure, and Guest Inclusion in Octahedral M₆L₄ Cages Self-Assembled by...

The challenges found to self-assemble crystalline octahedral M₆L₄ metal-organic cages (MOCs) with the tridentate 2,4,6-tris(4-pyridyl)benzene (TPB) ligand and Pd²⁺ 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 M₆L₄ cage (Pd₆(TPB)₄) can be formed in the solution state by only using the ethylenediamine (en) cis protecting group and Pd²⁺, 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(NO₃)₂, we report the solid-state synthesis of Pd₆(TPB)₄, its solid-state structure, and its ability to include aromatic guests in aqueous media. In solution, the self-assembly of TPB and Pd²⁺ with a templating guest (i.e., 3-phenylpropanoic acid) (G₁) formed crystals of Pd₆(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 Pd₆(TPB)₄ and the isostructural tris-pyridyl triazine (TPT)-based MOC (Pd₆(TPT)₄). The higher stability of the Pd₆(TPT)₄ structure with respect to Pd₆(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 M₆L₄ structures in the solid state. The Pd₆(TPB)₄ cage has an electron-rich hydrophobic cavity (∼450 ų) that is stable in water and in acidic conditions and can include four G₁ in one M₆L₄, as shown by titration experiments and using a guest model X-ray structure. In the solution state, ¹H NMR data show that G₁ guest exchange/inclusion in Pd₆(TPB)₄ occurs in the fast exchange regime. Hollow Pd₆(TPB)₄ is a strong candidate to be used in functional applications such as molecular recognition, guest stabilization, or unusual reactivity.