Unexpected solid-solid reaction upon preparation of KBr pellets and its exploitation in supramolecular cation complexation.

Pressing solid [CoIII(eta 5-C5H4COOH)(eta 5-C5H4COO)] with KBr to prepare samples for IR spectroscopy leads to a profound solid state rearrangement with formation of the supramolecular complex [CoIII(eta 5-C5H4COOH)(eta 5-C5H4COO)]2.K+Br-, which can also be obtained from solution crystallization. Similar solid-solid supramolecular complexation has been observed with K[PF6] and [NH4][PF6].