The intersection of allenylidenes and mesomeric betaines. 1-Methylpyridinium-2-acetylide and its palladium complexes

The anion of 2-ethynyl-1-methylpyridinium salts, which polymerized rapidly, can be represented as the mesomeric betaine 1-methylpyridinium-2-acetylide or as the cumulene-type structure 1-methylpyridin-2-allenylidene. Palladium complexes have been prepared starting from 2-ethynylpyridine and Pd(PPh3)4 or PdCl2(PPh3)2 followed by methylation of the resulting complexes to the title compounds. Results of calculations and spectroscopic measurements are discussed with respect to the zwitterionic or allenylidene resonance forms.