Crown compounds for anions. Spectroscopic and theoretical studies of complexation of borohydride anions with cyclic trimeric perfluoro-o-phenylenemercury

It has been shown by IR and NMR spectroscopy that cyclic trimeric perfluoro-o-phenylenemercury (o-C6F4Hg)3 (1) is capable of binding borohydride anions in THF and 1,2-dichloroethane solutions to form complexes {[(o-C6F4Hg)3](BH4)2}2− (2) and {[(o-C6F4Hg)3]2(BH4)}− (3). According to the IR data, complex 2 contains both terminal and coordinated B–H bonds while all four B–H bonds of the BH4− ion are coordinated with the mercury atoms in complex 3. The use of NMR spectroscopy provided the identification of one more complex of 1 with BH4−, which can be formulated as {[(o-C6F4Hg)3](BH4)}− (4). The stability constants of complexes 2 and 3 have been determined by IR spectroscopy. Quantum-chemical calculations of 2–4 by the AM1 method suggest that the complexes have the unusual bipyramidal, sandwich and half-sandwich structures, respectively. In contrast to 1, the interaction of (C6F5)2Hg with BH4− anions in THF gives a single complex {[(C6F5)2Hg](BH4)}− (5) independently of the reagent ratio. This complex is much less stable than 2 and 3.