Anion-π interactions in salts with polyhalide anions: Trapping of I _4~(2-)

摘要

The directionality of interaction of electron-deficient π systems with spherical anions (e.g,. halides) can be controlled by secondary effects like NH or CH hydrogen bonding. In this study a series of pentafluorophenyl-substituted salts with polyhalide anions is investigated. The compounds are obtained by aerobic oxidation of the corresponding halide upon crystallization. Solid-state structures reveal that in bromide 2, directing NH-anion interactions position the bromide ion in an η~1-type fashion over but not in the center of the aromatic ring. The same directing forces are effective in corresponding tribromide salt 3. In the crystal, the bromide ion is paneled by four electron-deficient aromatic ring systems. In addition, compounds 4 and 6, which have triiodide and the rare tetraiodide dianion as anions, are described. Computational studies reveal that the latter is highly unstable. In the present case it is stabilized by the crystal lattice, for example, by interaction with electron-deficient π systems. Sheltered and tranquil: Br_3~-, I_3~-, and I_4~(2-) are incorporated into different ammonium and phosphonium salts, in which they show, for example, anion-π interactions with the electron-deficient C _6F_5 groups of the cations. Theoretical calculations indicate that I_4~(2-) is highly unstable, but is stabilized in the obtained crystals. A bis-phosphonium salt incorporating I~- and I_4~(2-) (C gray, F green, H light blue, I purple, P orange) is shown here.