SYNTHESIS, STRUCTURE, AND PROPERTIES OF THE NOVEL BROMOSELENATES(II) [SE3BR8]2-, [SE4BR14]2-, AND [SE5BR12]2- - CRYSTAL-STRUCTURES OF [CU(I-PROPCN)4]2[SE3BR8], [ETPH3P]2[SE4BR14], AND [N-PROP4N]2[SE5BR12]

Using the symproportionation of elemental selenium and selenium tetrabromide in polar aprotic solvents a new class of tri-, tetra-, and pentanuclear bromoselenates(II) was obtained. In 2-methylpropionitrile solution the novel ion [Se3Br8]2- was stabilized and isolated as its black solvated copper salt. The crystals of [Cu(i-PropCN)4]2[Se3Br8] are monoclinic (C 2/c, a = 20.753(6), b = 13.344(4), c = 19.149(6) angstrom, beta = 90.09(3)-degrees at 140 K, Z = 4) and contain trigonal star-shaped ions [Se3Br8]2- consisting of three edge-sharing square planar SeBr4 units. The mean terminal and mu-3-bridging Se-Br bond lengths are 2.408 angstrom and 2.911 angstrom, respectively. [EtPh3P]2[Se4Br14] was obtained from dichloromethane solutions as dark red square-shaped crystals. The X-ray structure analysis (space group P 2(1)/n, a = 11.400(3), b = 11.815(3), c = 20.720(7) angstrom, beta = 92.50(3)-degrees at 140 K, Z = 2) shows the crystals to contain tetranuclear ions [Se4Br14]2- that are built of two parallel planar [Se2Br6] units connected via two linear mu-Br-bridges. In the interesting delocalized electronic system a formal oxidation number of +1 has to be assigned to the bridging bromine. The mean terminal Se-Br bond length is 2.411 angstrom, the bridging bond lengths are 2.764 angstrom in the planar Se2Br6 units 2.594 angstrom in the linear Se-Br-Se bridges. Dark red [n-Prop4N]2[Se5Br12] was also isolated from CH2Cl2 solutions. It crystallizes in the tetragonal space group P 4(2)/mnm (a = 14.798(3), c = 11.169(2) angstrom at 140 K, Z = 2) and its structure shows the novel pentanuclear ion [Se5Br12]2-, consisting of a central square planar SeBr4 unit with two of its trans-edges being shared with additional pairs of square planar SeBr4 units. The anion can be described as two star-like Se3Br8 fragments with one sharing Se. The mean terminal and mu-3-bridging Se-Br bond lengths are 2.343 angstrom and 2.956 angstrom, respectively. The vibrational spectra of the novel anions are reported and discussed.