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, [Cu^II₆(L)₄(NO₃)(CH₃OH)₂](NO₃)₃·4CH₃OH, [(CH₃)₂NH₂][Cu^ICu^II₅(L)₄](SO₄)₂·4H₂O, and [Cu^I₂Cu^II₄(L)₄](NO₃)₂·2CH₃OH. The monovalent Cu^II₆ salt is an insulator, but the mixed-valent Cu^II₅-Cu^I and Cu^II₄-Cu^I₂ salts are semiconductors. Magnetic exchange interactions up to J_NN = -158 cm^-1 dominate the susceptibilities and lead to ground-state spin S_T = 1 (Cu^II₆), 1/2 (Cu^II₅-Cu^I), and 0 (Cu^II₄-Cu^I₂) 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.