Anion-dependent selective formation of intermolecular non-covalent bonds

The crystal structures and packings of bis(pyridiniopropyl)benzene derivatives, [1,4-(4-R-C5H4N+CH2CH2CH2)2C6H4](PF6)2 (1, R = H; 2, R = tert-Bu), were investigated by X-ray diffraction studies. In compound 1, two pyridiniopropyl chains attached at the para positions of the phenylene group are in a linear conformation, and the central phenylene ring forms an intermolecular acceptor-donor-acceptor π-π stacking with neighboring two pyridinium rings. Compound 2 also has a linear conformation similar to 1, but intermolecular π-π stackings were not observed in the crystal stackings of 2. Instead, there is found a C-H ⋯ π interaction between the central phenylene ring and one of the hydrogen atoms in the tert-Bu group linked at the 4 position of pyridinium ring. The difference of the packing modes between 1 and 2 apparently results from steric hindrance caused by bulkiness of the substituents and counter anions.