2,2,6,6-Tetramethyl-1-oxopiperidinetribromide and two forms of 1-hydroxy-2,2,6,6-tetramethylpiperidinium bromide salt: Syntheses, crystal structures and theoretical calculations

• Solvent effect on TEMPO-Br salts crystal structure.
• Intermolecular hydrogen bond due to the –N+HOH– moiety.
• Bromide anion type estimated by UV–vis and IR analysis.
• Supramolecular hydrogen bonding in molecular packing.
• Halogen counterions interacting with TEMPO in crystal systems.

The reaction of the nitroxy radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) with Br2 has been investigated with CCl4 and hexane to obtain TEMPO-Br salts: 2,2,6,6-tetramethylpiperidine-1-oxopiperidine tribromide [TEMPO][−Br3][Br3−] (I), and the 1-hydroxy-2,2,6,6-tetramethylpiperidinium bromide salts [TEMPH+OHBr−] (II and III). The salt I was isolated in crystalline form directly from the synthesis and II and III by only changing the solvent. The crystals of I belong to the orthorhombic crystal systems with space group Cmc21, a = 10.5596(4) Å, b = 14.0464(4) Å, c = 9.4202(5), and with asymmetric unit of Z = 4. Crystals II belong to Pnna a = 11.9860(3) Å, b = 23.6720(9) Å, c = 7.7051(3) Å while III belongs to Cmc21 with a = 10.2686(3) Å, b = 10.7661(3) Å, c = 10.0274(2) Å; the asymmetric unit of II and III was Z = 8 and Z = 4, respectively. The crystal structure of I shows the −Br3Br3− ion as [Br–Br–Br]− for each molecule of TEMPO. The crystal structure of II shows a weak intermolecular hydrogen bond between –N-H⋯Br(1) and O(1)H⋯Br(2) due to the presence of the –N+HOH– moiety. In contrast, crystal III shows intermolecular hydrogen bonding between O(1)H⋯Br(1)⋯HN(1) due to the –N+HOH– moiety. The resulting compounds were characterized by FT-IR and UV–vis spectroscopy. The structural parameters have been compared with the related hydroxylaminotrichlorosilane known from the literature and with results of DFT calculations.