Synthesis and properties of charge-transfer solids with cluster units [Mo6X14]2− (X = Br, I)

Abstract

The preparation of charge-transfer (CT) solids using [Mo₆X₁₄]²⁻ (X = Br, I) cluster units and several organic electron donor molecules (D) including a variety of tetrathiafulvalene (TTF) derivatives was studied. Galvanostatic electrochemical oxidation afforded single crystals of the cation radical salts of (tetrathionaphthacene)Mo₆Br₁₄(PhCN)₃ (1, PhCN: benzonitrile), (hexamethylene-TTF)₂Mo₆I₁₄(PhCN)₄ (2), (bis(ethylenedioxy)-TTF)₂Mo₆Br₁₄(PhCN)₄ (3), (tetrakis(methylthio)-TTF)₂Mo₆Br₁₄ (4), (perylene)₆Mo₆Br₁₄ (5), and powders from several TTF derivatives combined with other kinds of electron donors. In 1, the donor molecules were in the diamagnetic dicationic state and formed two-dimensional D²⁺A²⁻ (A = Mo₆Br₁₄) layers. Moreover, in 2–4, the donor molecules were in the monocationic radical state. The structural analysis of 3 indicated the presence of isolated cation radical molecules, whereas in 2 and 4, the donor cation radicals formed segregated stacks with dimerized units. Donor dimers and [Mo₆I₁₄]²⁻ cluster units in 2 formed a CsCl-related structural pattern with solvent molecules occupying the free space in the crystal. The donor dimers and [Mo₆Br₁₄]²⁻ cluster units in 4 formed infinite alternating stacks parallel to each other. The static magnetic susceptibility of 2 exhibited singlet–triplet behavior with |J|/k_B = 190 K, whereas the radical spins in 4 were strongly coupled antiferromagnetically. There were two kinds of perylene molecules in 5: one formed segregated columns with weakly dimerized units having a charge of approximately +0.5, while the other enclosed the columns with parallel molecular planes having nearly zero charge. 5 was similar to that of a dimer-type Mott insulator. The electrochemical reaction of N-(3-perylenylmethyl)-N,N-bis(2-pyridylmethyl)amine (perbpa), which is a perylene-pendant tridentate ligand, and CuCl₂ in the presence of (Bu₄N)₂Mo₆Br₁₄ in PhCN afforded a dinuclear Cu(II) complex [Cu₂(μ-Cl)₂(perbpa)₂]Mo₆Br₁₄(PhCN)₄ (6). The perylene groups in 6 were in a neutral electronic state. The temperature-dependent magnetic susceptibility measurement of 6 showed a weak ferromagnetic exchange interaction between the Cl-bridged Cu(II) cores with the S = 1 ground state (g = 2.14, J = +0.56 K) based on H = −2JS₁·S₂. The boundary between the neutral and ionic ground states of the CT solids of [Mo₆Br₁₄]²⁻ shifted to the negative side by 0.5 V and 0.3–0.6 V from those of the TTF·p-quinone and TTF·TCNQ system, respectively, with respect to the redox potential differences between the donor and acceptor species.