Coordination structures of Cu ions in the mixed-solvent system of formamide and ammonium formate

The coordination structures of the Cu ions in the mixed solvent system of HCONH₂-HCOONH₄ at room temperature and 100 °C have been investigated using the EXAFS (extended X-ray absorption fine structure) method for the Cu K-edge, together with visible absorption spectrum analyses and ion-exchange experiments. The Cu ions, which are divalent, Cu^II, at room temperature, are reduced to univalence, Cu^I, by heating with HCOO^- ions, ca. 80% of Cu^II ions are transformed into Cu^I, ions at 100 °C. In the mixed solvent system, it was found that Cu^II ions form neutral [Cu(HCOO)₂(HCONH₂)₄] octahedra, distorted by the Jahn-Teller effect, with four short Cu-O bond lengths of ca. 1.97 Å and two long Cu-O bond lengths of ca. 2.82 Å at room temperature, and Cu^I ions form cationic [Cu(HCONH₂)₃]⁺ plane triangles with three Cu-O bond lengths of ca. 1.90 Å at 100 °C. The transition of the coordination structures occurs reversibly in the mixed solvent system as a function of the temperature.