Synthesis, Characterization, and Crystal Structures of New Dications Bearing the −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+·I3 -·1/2I4 - (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 ICl 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 I3 - in III. However, on the basis of the Se−Cl bond length of 2.778(5) Å, III can also be considered as formed by the [(C5H8N2Se−)2Cl]+ cation, with I3 - as counterion. Similarly to III, compound IV, which was obtained by reacting 4 with IBr, can be considered as formed by [(C5H10N2Se−)2Br]+ 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 I3 - and half I4 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) Å. 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-IR spectra and should contain a contribution of the ν(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 P21, with a = 9.849(6) Å, b = 11.298(5) Å, c = 7.862(6) Å, β = 106.44(2)°, Z = 2, and R = 0.0362; II is monoclinic, space group P21, with a = 8.063(6) Å, b = 11.535(5) Å, c = 10.280(5) Å, β = 107.13(2)°, Z = 2, and R = 0.0429, III is monoclinic, space group P21/n, with a = 10.431(7) Å, b = 18.073(5) Å, c = 11.223(6) Å, β = 100.76(2)°, Z = 4, and R = 0.0490; IV is monoclinic, space group P21/n, with a = 10.298(5) Å, b = 18.428(7) Å, c = 11.475(6) Å, β = 104.10(4)°, Z = 4,