[Sb7Se8Br2]3+ and [Sb13Se16Br2]5+ - Double and Quadruple Spiro Cubanes from Ionic Liquids

The reaction of antimony and selenium in the bromine-rich Lewis acidic ionic liquid [BMIm] Br center dot 4.7AlBr(3) (BMIm: 1-butyl3- methylimidazolium) in the presence of a small amount of NbCl5 at 160 degrees C yielded dark-red crystals of [Sb7Se8Br2][ AlX4](3). For X = Cl0.15(1)Br0.85(1), the compound is isostructural to [Sb7S8Br2][AlCl4](3) [ P2(1)2(1)2(1), alpha = 12.5132(5) angstrom, b = 17.7394(6) angstrom, c = 18.3013(6) angstrom]. For a higher chlorine content, X = Cl-0.58(1) Br-0.42(1), a slightly disordered variant with a bisected unit cell is found [P2(1)2(1)2, alpha = 12.3757(3) angstrom, b = 17.4116(5) angstrom, c = 9.0420(2) angstrom]. The [Sb7Se8Br2](3+) heteropolycation (C-2 symmetry) is a spiro double-cubane with an antimony atom on the shared corner. From this distorted octahedrally coordinated central atom, tricoordinate selenium and antimony atoms alternate in the bonding sequence. The terminal antimony atoms each bind to a bromine atom. Quantum chemical calculations confirm polar covalent Sb-Sebonding within the cubes and indicate three-center, fourelectron bonds for the six-coordinate spiro atoms. The calculated charge distribution reflects the electron-donor role of the antimony atoms. The use of a chlorine-rich ionic liquid resulted in the formation of triclinic [Sb13Se16Br2][AlX4](5) with X = Cl0.80(1)Br0.20(1) [P1, alpha = 9.0842(5) angstrom, b = 19.607(1) angstrom, c = 21.511(1) angstrom, alpha = 64.116(6)degrees, beta = 79.768(7)degrees, gamma = 88.499(7)degrees]. The cationic cluster [Sb13Se16Br2](5+) is a bromine-terminated spiro quadruple-cubane. This 31 atom concatenation of four cubes is assumed to be the largest known discrete main group polycation. A similar reaction in a chloride-free system yielded [Sb7Se8Br2][Sb13Se16Br2][AlBr4](8). In its monoclinic structure [P2/c, alpha = 27.214(5) angstrom, b = 9.383(2) angstrom, c = 22.917(4) angstrom, beta = 101.68(1)degrees], the two types of polycations alternate in layers along the a axis. In the series [Sb4+3nSe4+4nBr2]((2+n)+), these cations are the members with n = 1 and 3.