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Reactions of hexanuclear niobium and tantalum halide clusters with mercury(II) halides. I. Synthesis and structures of semiconducting compounds [M_6Br_12(H_2O)_6][HgBr_4]×12H_2O, M = Nb, Ta

The method for the preparation of new solvated clusters of the composition [M_6Br_12(H_2O)_6][HgBr_2X_2]×12H_2O, M = Nb, Ta ; X = Cl, Br, I, was established. The cubic crystals of [Nb_6Br_12(H_2O)_6][HgBr_4]×12H_2O (1) and [Ta_6Br_12(H_2O)_6][HgBr_4]×12H_2O (2) were characterized by the X-ray structure analysis. Crystal data for 1: cubic, space group Fd-3m, a = 21.0072(6) Angstrom, Z = 8, R = 0.051 (R_w = 0.066) for 451 unique reflections with I > 2s(I). Crystal data for 2: cubic, space group Fd-3m, a = 20.9698(1) Angstrom, Z = 8, R = 0.038 (R_w = 0.050) for 433 unique reflections with I > 2s(I). Clusters 1 and 2 contain octahedral cluster cation [M_6Br_12(H_2O)_6]^2+ and tetrahedrally arranged [HgBr_4]^2- anion. The M-M bond distances are 2.949(1) Angstrom for 1 and 2.9000(8) Angstrom for 2. The Hg-Br bond distances in [HgBr_4]^2- anion are 2.614(2) Angstrom in 1 and 2.622(2) Angstrom in 2. The crystal packing patterns of the isostructural clusters 1 and 2 involve the three-dimensional hydrogen bond network ; the crystalline water molecules act as donors of hydrogen to the bromine atoms of the cluster and [HgBr_4]^2- units, whereas the coordinated water molecules form hydrogen bonds to the crystalline water molecules. The compound [Nb_6Br_12(H_2O)_6][HgBr_4]×12H_2O is diamagnetic and semiconducting with the activation energy, E_a = 0.20 eV.

cluster compounds; crystal structures; magnetic properties; transport properties; semiconducting properties

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