SUBSTITUENT EFFECTS ON THE GEOMETRY OF THE PYRIDINE RING IN 1-METHYL-4-(4-R-PHENYL)-2,6-DIPHENYLPYRIDINIUM PERCHLORATES

The crystal and molecular structure of the derivatives of 1-methyl-4-(4-R-phenyl)-2,6-diphenylpyridinium perchlorates, [R = H, F, Cl, NO2, N(CH3)(2)] have been solved by X-ray diffraction techniques at room temperature with Cu K alpha radiation. Deformations of geometry in the pyridine ring are mesomeric in nature. They depend on the kind of substituent and may be explained by use of the modified Bent-Walsh rule. The charged N-atom in N+-H and N+-M derivatives of pyridine has an enhanced electronegativity which is demonstrated by changes in exocyclic bond angles at the ortho carbon atoms in the ring. The mean angles for 12 entries (CSD release April 1994) of N+-H derivatives are 118.1 degrees and 123.2 degrees, respectively, as expected From the Bent-Walsh rule. The title compounds do not exhibit these deformations, mean values for the above-mentioned angles being 119.9 degrees and 120.5 degrees. This effect is due to steric interactions between the Me group in N+ Me and neighbouring phenyl moieties which act against the electronegativity effect at C2 and C6 positions.