Competitive interactions in carboxy-functionalized pyridinium salts: crossover from O–H⋯O to O–H⋯X–M contacts

The solid state organization of bromide and tetrabromopalladate(II) derivatives of carboxy-functionalized pyridinium cations [C₅H₅N(CH₂)_nCOOH]⁺, thereafter [PYn–COOH]⁺ (n = 5, 15), was investigated by infrared spectroscopy, which revealed the active role of carboxy groups in defining the crystalline order. [PY5–COOH]₂[PdBr₄] and [PY15–COOH]₂[PdBr₄] were also characterized by single-crystal X-ray diffraction. [PY5–COOH]₂[PdBr₄] exhibits strong inter-cation O–H⋯O contacts, whereas in [PY15–COOH]₂[PdBr₄] lateral, hydrophobic interactions predominate, which allow interionic O–H⋯X–M contacts. In both cases, a large number of additional C–H⋯X–M and C–H⋯O interactions stabilize the crystal.