Structural transformation of pyridinium iodide proceeding in the solid state

Co-grinding of 1-(2-oxo-2-phenylethyl)-pyridinium iodide (Py+center dot I-, 1) with 0.5 equivalent of I-2 affords the mixture of 1 and Py+center dot I-3(-) (3) soon after grinding. The resultant powder gradually converts to a double salt, Py+center dot(I-)(0.5)center dot(I-3(-))(0.5) (2), by leaving it at ambient temperature for one month. To clarify the mechanism of this "self-assembly" behavior in the solid state, mechanochemical reactions at various 1 : I-2 ratios (1 : I-2 1 : 0.1, 1 : 0.2, 1 : 0.4, 1 : 0.6, 1 : 0.8, 1 : 1, and 1 : 2) have been examined. Comparison of these studies indicates that cations and anions spontaneously diffuse and rearrange in the solid state following the anion change from I- to I-3(-).