Humidity Control of Isostructural Dehydration and Pressure-Induced Polymorphism in 1,4-Diazabicyclo[2.2.2]octane Dihydrobromide Monohydrate

1,4-Diazabicyclo[2.2.2]octane dihydrobromide (dabco2HBr, [C6H14N2]2+·2Br–) is extremely sensitive to humidity levels, by absorbing or releasing water from its structure. The gradual changes of water contents, in turn, modifies the crystal symmetries, albeit all polar at ambient pressure. The full water contents control has been achieved for the sample sealed in a high-pressure chamber. Isobaric crystallization of dabco2HBr from ethanol solution at dry conditions yields anhydrate dabco2HBr, and in open air two polymorphic hemihydrates dabco2HBr·1/2H2O and an isostructural monohydrate α-dabco2HBr·H2O; three more monohydrate dabco2HBr·H2O polymorphs have been obtained by isothermal and isochoric crystallizations in a diamond-anvil cell. All the structures have been determined in situ by single-crystal X-ray diffraction. The anhydrate dabco2HBr, its hemihydrates α and β, and monohydrates α, β, and γ all crystallize in polar space groups: Cmc21 (dabco2HBr, β-dabco2HBr·1/2H2O, β-dabco2HBr·H2O), Pbc21 (α-dabco2HBr·1/2H2O and α-dabco2HBr·H2O) and P21 (γ-dabco2HBr·H2O). However, all these crystals exhibit clear isostructural relations. Only the highest-pressure δ-dabco2HBr·H2O monohydrate is centrosymmetric, space group Pmmn. Systematic structural transformations accompanying the hydration and pressure increase have been monitored: pressure gradually increases number of H-bonds involving each of the ammonium groups, from one to three, and the H-acceptor capacity of Br– anions from zero to three.