An infrared study of the co-ordination of pyridine and water to exchangeable cations in montmorillonite and saponite

Abstract

Pyridine molecules form strong hydrogen bonds with pyridinium ions in the interlayer space of montmorillonite, causing a marked perturbation of vibrations involving the NH⁺ group. These vibrations are not perturbed by interlayer benzene. Water molecules directly co-ordinated to Ca²⁺ and Mg²⁺ form strong hydrogn bonds with pyridine which readily displaces water from outer spheres of co-ordination round these ions. Preparations with a 14·7 Å layer spacing appear to contain the complex M²⁺(HOH,NC₅H₅)₆. Wider spacings (about 23 Å) are obtained when pyridine is hydrogen-bonded to both protons of co-ordinated water. Pyridine co-ordinates to Cu²⁺ both directly and also indirectly through water. Hydrogen-bonding in these complexes is stronger than in aqueous pyridine, as the acidity of co-ordinated water is increased by the polarising forces of the metal cations. The linking water molecules can be reversibly removed from Ca²⁺ and Cu²⁺ complexes but not from the Mg²⁺ complex, where residual water molecules ionise to give pyridinium ions on heating in a vacuum. This reaction is also reversible. Pyridinium ions are formed when co-ordinated pyridine is displaced by water, in amounts dependent on the basicity of the cation. Deeply coloured anhydrous complexes of Cu montmorillonite, formed in hot pyridine or on long treatment in the cold, may contain basic copper ions. Some differences between saponite and montmorillonite complexes can be related to differences in the surface density of exchange sites.

The dependence of the intensity of absorption bands on the angle of incidence of radiation on montmorillonite films gives information on the orientation of pyridine molecules in the interlayer space, and assists in the assignment of symmetry species to vibrations. The use of pyridine as an indicator of Lewis and Brönsted acidity on surfaces is discussed.