Synthesis and Crystal Structure of Tetrakis(tetramethylammonium) Bis[decabromotetraselenate(II)]‐bis[dibromodiselenate(I)], [(CH3)4N]4[(Se4Br10)2(Se2Br2)2], the Salt of a Mixed‐Valence Bromoselenate(II/I) Anion

Brown crystals of [NMe4]4[(Se4Br10)2(Se2Br2)2] (1) were obtained from the reaction of selenium and bromine in acetonitrile in the presence of tetramethylammonium bromide. The crystal structure of 1 was determined by X-ray diffraction and refined to R = 0.0297 for 8401 reflections. The crystals are monoclinic, space group P21/c with Z = 4 and a = 12.646(3) A, b = 16.499(3) A, c = 16.844(3) A, β = 101.70(3)° (123 K). In the solid-state structure, the anion of 1 is built up of two [Se4Br10]2– ions. Each shows a triangular arrangement of three planar SeBr4 units sharing a common edge through two μ3-bridging bromine atoms, and one SeBr2 molecule, which is linked to the SeII atoms of two SeBr4 units; between the Se4Br102– ions a dimerized Se2Br2 molecule (Se4Br4) is situated and one SeI atom of each Se2Br2 molecule has two weak contacts [3.3514(14) A and 3.3952(11) A] to two bromine atoms of one SeBr4 unit. Four SeI atoms of a dimerized Se2Br2 molecule are in a almost regular planar tetraangular arrangement. Contacts between the SeII atom of the SeBr2 molecule and the SeII atoms of two SeBr4 units are 3.035(1) A and 3.115(1) A, and can be interpreted as donor-acceptor type bonds with the SeII atoms of SeBr4 units as donors and the SeBr2 molecule as acceptor. The terminal SeII–Br and μ3-Br–SeII bond lengths are in the ranges 2.3376(10) to 2.4384(8) A and 2.8036(9) to 3.3183(13) A, respectively. The bond lengths in the dimerized Se2Br2 molecule are: SeI–SeI = 2.2945(8) A and 3.1398(12), SeI–Br = 2.3659(11) and 2.3689(10) A.

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