Electrical Conductivity of Copper Hexamers Tuned by their Ground-State Valences.

A new design concept has been realized for the construction of molecular conductors, whereby the building unit contains a core reservoir of carriers made up of metal ions with controllable valence states and shelled by flat organic ligands having an extended π-system to promote supramolecular electronic communication. Therefore, reacting the conjugated multidentate ligand 5,5'-pyridyl-3,3'-bi-1 H-pyrazole with different copper salts solvothermally led to three interesting hexameric salts having different ground-state valences, [CuII6(L)4(NO3)(CH3OH)2](NO3)3·4CH3OH, [(CH3)2NH2][CuICuII5(L)4](SO4)2·4H2O, and [CuI2CuII4(L)4](NO3)2·2CH3OH. The monovalent CuII6 salt is an insulator, but the mixed-valent CuII5-CuI and CuII4-CuI2 salts are semiconductors. Magnetic exchange interactions up to JNN = -158 cm-1 dominate the susceptibilities and lead to ground-state spin ST = 1 (CuII6), 1/2 (CuII5-CuI), and 0 (CuII4-CuI2) at 40 K. Cyclic voltammetry shows the stepwise one-electron oxidation-reduction through all the possible valence states. The theoretical calculations of the electronic and band structures of the three compounds substantiate the experimentally observed physical properties.