C. Wild, M. Spahis, R. Blankenship
Abstract Complexes of eleven substituted pyridine N-oxide donors with stannic chloride in acetonitrile solution were investigated ultizing IR spectroscopy. Two distinct types of behaviour were noted: (1) Pyridine N-oxide donors containing electron releasing substituents generally form strong complexes with SnCl 4 and exhibit maxima in continuous variation plots at a 2:1 ligand-to-metal ratio. Little or no free ligand is detected in these solutions until this ratio is exceeded. Ligands exhibiting this behaviour include the 2-methyl-, 3-methyl-, 4-methyl-, 4-methoxy- and 4-phenylpyridine N-oxides. (2) Pyridine N-oxide donors containing electron neutral to electron with-drawing substituents generally form weaker complexes with SnCl 4 aM exhibit maxima in continuous variation plots at a 1:1 ligand-to-metal ratio. Free ligand is evident in these solutions even at ligand-to-metal ratios as low as 1:3. Formation plots for these complexes indicate the possibility that 1: 2 and 1:1 ligand to metal complexes exist in the concentration ranges studied. Ligands exhibiting this behaviour include the 4-chloro-, 4-nitro-, 3 and 4-acetyl-pyridine N-oxides and 2-pyridine methanol N-oxide. The complex of 2,6-1utidine N-oxide with SnCl 4 exhibits behaviour characteristic of the 1:1 complexes even though the substituents are electron releasing. This ligand contains the most sterically crowded N-O group in the series, however, and steric factors are invoked to explain this behavior. These results are rationalized in terms of an equilibrium model which includes the postulated existence of oxygen bridged dimers comprising a dominant species in solutions containing the 1:1 complexes. Some supporting 1 H NMR and polarographic data are also presented and discussed.