The Reactions of Thianthrene and Selenanthrene with AlCl3: Coordination Complexes, Radical Ions, and Investigations on the Unique Triple-Decker Molecule (Thianthrene)32+

The reactions of thianthrene (TA) and selenanthrene (SeA) with AlCl3 were studied in the melt phase and with liquid SO2 as solvent. From neat AlCl3 and TA, the colorless complex [AlCl3(TA)] was isolated as the main product and the dark red salt (TA)3[Al2Cl7]2 as the byproduct. The analogous solvent-free reaction of selenanthrene and AlCl3 takes a different course as colorless [Al(SeA)3][Al2Cl7]3 is formed in quantitative yield. With SO2 as solvent, both TA and SeA give the black radical salts (TA)2[AlCl4]2 and (SeA)2[AlCl4]2, respectively. In the structure of [AlCl3(TA)], thianthrene acts as a monodentate ligand and coordinates with one S atom to the pyramidal AlCl3 unit. The structure of the (TA)32+ ion is a stack of three almost planar TA molecules in parallel arrangement. [Al(SeA)3]3+ represents a tris-chelate complex ion with SeA acting as a bidentate ligand and both Se atoms binding to the octahedrally coordinated Al3+ ion. (SeA)2[AlCl4]2 consists of dimers of SeA+ radical ions, which are bound by weak intermolecular Se center dot center dot center dot Se bonds. Tentative reaction equations are given to explain the unexpected oxidation processes that lead to the radical ions. Quantum chemical calculations were performed on the molecular fragments and on the periodical structures. The trimeric (TA)32+ ion is in the singlet state in accordance with the magnetic properties of (TA)3[Al2Cl7]2, which show a weak temperature-independent paramagnetism. The ion is bound by long-range four-center bonds between the outer two radicals.