Synthesis, Characterization and Crystal Structures of New Dications, Bearing the Diseleno -Se-Se- Bridge

Starting from 1,3-dimethyl-4-imidazoline-2-selone (1), 1,2-bis(2-selenoxo-3-methyl-4-imidazolinyl-2-)ethane (3) and 1,3-dimethylimidazolidine-2-selone (4), the following six compounds, [(C5H8N2Se-)(2)](2+). 2Br(-) (I), [(C5H8N2Se-)(2)](2+). 2I(-) (II), [(C5H8N2Se-)(2)](2+). Cl-. I3(-) (III) [(C5H10N2Se-)(2)](2+). Br-. IBr2- (IV), [(C5H7N2Se-)(2)](2+). I-3(-). 1/2I(4)(-) (V) and [(C5H7N2Se-)(2)](2+). 2I(-). CH3CN (VI), in which the selenium compounds are oxidized to dications bearing the uncommon -Se-Se- bridge, have been prepared, and I-V crystallographically characterized. I and III were obtained by reacting 1 with IBr and ICI respectively, while II was obtained by reduction of previously described hypervalent selenium compound of 1 (5) bearing the I-Se-I group with elemental tellurium. These three compounds contain the same [(C5H8N2Se-)(2)](2+) dication balanced by two bromides in I, two iodides in II, and Cl- and I-3(-) in III. However, on the basis of the Se-Cl bond length of 2.778(5) Angstrom, III can also be considered as formed by the [(C5H8N2Se-)(2)Cl](+) cation, with I-3(-) as counterion. Similarly to III, compound IV, which was obtained by reacting 4 with IBr, can be considered as formed by [(C5H10N2Se-)(2)Br](+) cations and IBr2- anions. As in II, compound V has been prepared by reduction of the hypervalent selenium compound of 3 (6) bearing two I-Se-I groups with elemental tellurium. In V, the [(C5H7N2Se-)(2)](2+) cation is balanced by I-3(-) and half I-4(2-) anions. The structural data show that all the cations are very similar, with Se-Se bond lengths ranging from 2.409(2) to 2.440(2) Angstrom. FT-IR and FT-Raman spectra of I-VI allow one to identify two bands around 230 +/- 10 and 193 +/- 5 cm(-1) that are common to all compounds. These bands are generally strong in the FT-Raman and weak in the FT-LR spectra and should contain a contribution of the v(Se-Se) stretching vibration. The spectra are also in good agreement with the structural features of the polyhalide anions present in the crystals. Crystallographic data are as follows: I is monoclinic, space group P2(1), with a = 9.849(6) Angstrom, b = 11.298(5) Angstrom, c = 7.862(6) Angstrom, beta = 106.44(2)degrees, Z = 2, and R = 0.0362; II: is monoclinic, space group P2(1), with a = 8.063(6) Angstrom, b = 11.535(5) Angstrom, c = 10.280(5) Angstrom, beta = 107.13(2)degrees, Z = 2, and R = 0.0429, III is monoclinic, space group P2(1)/n, with a = 10.431(7) Angstrom, b = 18.073(5) Angstrom, c = 11.223(6) Angstrom,. beta = 100.76(2)degrees, Z = 4, and R = 0.0490; IV is monoclinic, space group P2(1)/n, with a = 10.298(5) Angstrom, b = 18.428(7) Angstrom, c = 11.475(6) Angstrom, beta = 104.10(4)degrees, Z = 4, and R = 0.0300; V is triclinic, space group <P(1)over bar>, with a = 7.456(6) Angstrom, b = 11.988(5) Angstrom, c = 12.508(5) Angstrom, alpha = 79.32(2)degrees, beta = 85.49(2)degrees, gamma = 80.62(2)degrees, Z = 2, and R = 0.0340