Change of electrical conductivities between hydrated and dehydrated samples of honeycomb sheet structures with mixed oxidation state paddlewheel dirhodium complexes and halide ions

Abstract

A series of mixed oxidation state compounds, [{Rh₂(acam)₄}₃(μ₃-X)₂]·nH₂O (Hacam = acetamide; X = Cl, n = 4 (1·4H₂O); X = Br, n = 10 (2·10H₂O); X = I, n = 10 (3·10H₂O)) and [{Rh₂(pram)₄}₃(μ₃-X)₂]·6H₂O (Hpram = propionamide; X = Cl (4·6H₂O), Br (5·6H₂O), I (6·6H₂O)) were synthesized and their X-ray structures were determined. In the crystal structure of all of these complexes, dirhodium complexes and halide ions construct 2-D honeycomb sheet arrangements in which the walls consist of Rh₂ units and halide ions lie at the corners. Complexes 1·4H₂O, 4·6H₂O, 5·6H₂O and 6·6H₂O have three independent Rh₂ units, in which there are two Rh₂⁵⁺ and one Rh₂⁴⁺. In these structures, the water molecules hydrogen bond to O atoms and from the N atoms of the amidate ligands. The number of hydrogen bonds from water molecules to the Rh₂⁴⁺ unit is greater than that to the Rh₂⁵⁺ units. This suggests that there exists pinning of the oxidation states by water molecules. In the structures of 2·10H₂O and 3·10H₂O, all of the Rh₂ units are crystallographically equivalent. In these structures, eight of the 10 water molecules form a honeycomb-like network between the {Rh₂(acam)₄}₃X₂ honeycomb sheets. The former four structures show very low electrical conductivities of ca. 10⁻⁸ S cm⁻¹ (room temperature, pellets) and the latter structures have the higher values of ca. 10⁻⁴ S cm⁻¹. In the former complexes, improvement of the values to 10⁻⁶ S cm⁻¹ was observed, caused by loss of pinning water.