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

Abstract The reaction of the nitroxy radical 2,2,6,6-tetramethylpiperidine- N -oxyl (TEMPO) with Br 2 has been investigated with CCl 4 and hexane to obtain TEMPO-Br salts: 2,2,6,6-tetramethylpiperidine-1-oxopiperidine tribromide [TEMPO] [ Br 3 − ] ( 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 Cmc2 1 , a = 10.5596(4) A, b = 14.0464(4) A, c = 9.4202(5), and with asymmetric unit of Z = 4. Crystals II belong to Pnna a = 11.9860(3) A, b = 23.6720(9) A, c = 7.7051(3) A while III belongs to C mc 2 1 with a = 10.2686(3) A, b = 10.7661(3) A, c = 10.0274(2) A; the asymmetric unit of II and III was Z = 8 and Z = 4, respectively. The crystal structure of I shows the Br 3 − 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.