Halogen complexes of pyridines. A proton and carbon-13 nuclear magnetic resonance study

Solutions of iodine and iodine chloride with pyridine, 2,6-dimethylpyridine, and 2,4,6-trimethylpyridine were studied by ^1H and ^(13)C NMR. Support is deduced for the reported formation of three species: the charge-transfer complex B•X-I, the 1:l complex ^+B-I[X_nI_(n-1)^-], and the N-iododipyridinium salt ^+B_2I[X_nI_(n-1)^-]. The ease of formation of each species appears to be a function of pyridine basicity, the Lewis acidity of the halogen (I_2 or ICl), the solvent polarity, and the relative donor-acceptor concentrations. The complexes derived from ɑ-methyl-substituted pyridines can be distinguished by the magnitudes and directions of their ^(13)C "iodination" shifts.