Browsing by Author "Nguyen, Duc Van"

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    New salt-like dodecahydro-closo-dodecaborates and efforts for the partial hydroxylation of [B12H12]2– anions
    (2009) Nguyen, Duc Van; Schleid, Thomas (Prof. Dr.)
    In this thesis, the synthesis, characterization and crystal structure determination of ionic salts with dodecahydro-closo-dodecaborate ([B12H12]2–) and hydroxylated closo-[B12H12–n(OH)n]2– anions (n = 1 - 4, 12) was described. The crystal structures of a variety of salts containing the dodecahydro-closo-dodecaborate anion with different metal cations have been solved. The counter-cation in these salts may exhibit a spherical or non-spherical coordination environment. The spherical one may possess the connection to the [B12H12]2– anions directly (e. g. in Tl2[B12H12]), or absolutely not at all (e. g. in Ni(H2O)6[B12H12] · 6 H2O). The non-spherical one can be found in Pb(H2O)3[B12H12] · 3 H2O and Bi(H2O)2(OH)[B12H12]. The crystal structure analysis revealed that closo-[B12H12]2– anions always exhibit a point symmetry lower than Ih (–5–32/m) as a consequence of the lack of a fivefold rotation axis in the three-dimensional periodic crystalline state. The partially hydroxylated closo-[B12H12–n(OH)n]2– (n = 1 - 4) anions are distorted with respect to their parent closo-[B12H12]2– anion and exhibit B–B and B–O bond lengths that are markedly deviated. The evidence of hydroxyl-substituted derivatives is proven by 11B{1H}-NMR measurements in aqueous solution with a decrease of the anionic point symmetry. For the perhydroxylation of the closo-[B12H12]2– anion four crystallographically different polymorphs of Rb2[B12(OH)12] · 2 H2O can be obtained. Especially, a surprisingly straightforward reaction between Cs2[B12H12] and H2O2 yields both diamagnetic Cs2[B12(OH)12] · 2 H2O and paramagnetic, air-stable Cs[B12(OH)12] as parts of a new structurally characterized oligoborane cluster redox system with potential for further functionalization through the hydroxyl groups. With this preparation process not only the perhydroxylation, but also a one-electron oxidation of the boron cage took place, most probably via in-situ produced ·OH radicals. In comparison to other similar paramagnetic species (hypocloso-[B12(OCH2Ph)12]·– and hypocloso-[B12Me12]·–), the hypocloso-[B12(OH)12]·– anion in solid Cs[B12(OH)12] is less distorted, even if the removal of one electron from the boron cage is weaker compensated by only pi back-donation effects.