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Abstract:
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.
