Pressure induced transformations of 1,4-diazabicyclo[2.2.2]octane (dabco) hydroiodide: diprotonation of dabco, its N-methylation and co-crystallization with methanol

Abstract

The non-solvated crystals of 1,4-diazabicyclo[2.2.2]octane hydroiodide (dabcoHI, [C₆H₁₃N₂]⁺I⁻) obtained at normal conditions are hexagonal and exhibit giant dielectric response and anisotropic relaxer properties. The isochoric crystallization of dabcoHI at 1.80 GPa yields non-solvated dabcoHI crystal and a [C₆H₁₄N₂]²⁺2I⁻:3CH₃OH solvate (dabco2HI:3CH₃OH) solvate, while pressure exceeding 2.30 GPa triggers the synthesis of N-methyl-1,4-diazabicyclo[2.2.2]octanium iodide, which co-crystallized with methanol (dabcoCH₃I:2CH₃OH, [C₇H₁₅N₂]⁺I⁻:2CH₃OH). Structures of both solvates were determined by single-crystal X-ray diffraction. Compared to non-solvated dabcoHI governed by NH⁺⋯N hydrogen bonds, in the two methane solvates investigated there are NH⋯O, N⋯HO, and OH⋯I hydrogen bonds, which illustrates the preference to replace the NH⁺⋯N hydrogen bonds with those involving oxygen atoms in high pressure.