An MPP⁺ analog: The 2:1 N-cyclopentyl-4-phenylpyridinium bromide-4-phenylpyridine complex

The molecular structure and cohesion in the solid state of the title addition compound between N-cyclopentyl-4-phenylpyridinium (CPPP) bromide, 4-phenylpyridine (PP), hydrogen bromide and water [bis(N-cyclopentyl-4-phenylpyridinium) 4-phenylpyridinium tribromide monohydrate, 2C₁₆H₁₈N⁺.C₁₁H₁₈N ⁺.3Br^-.H₂O], is discussed. The 4-phenylpyridine molecule, which is protonated, the bromide ions and the water molecule all lie on a mirror plane. Cohesion of the 2:1 CPPP:PP compound is achieved mainly through hydrogen bonds and salt bridges. No charge-transfer interaction between the two molecules was found. The phenyl ring is essentially coplanar with the pyridine ring in the PP molecule, while in the CPPP ion these two rings are rotated with respect to one another. The conformations of these two molecules are compared with theoretical data (conformational analysis) and with structures of phenylpyridinium analogs retrieved from the Cambridge Structural Database. As a result, this study allows the identification of stable conformers for phenylpyridinium analogs and could lead to the definition of active conformers for the neurotoxin MPP⁺ and some of its derivatives.