Pressure-Induced Hydration of 1,4-Diazabicyclo[2.2.2]octane Hydroiodide (dabcoHI)

Ambient-pressure crystallization of 1,4-diazabicyclo[2.2.2]octane hydroiodine (dabcoHI, [C6H13N2]+I–) leads to unsolvated dabcoHI polymorphs built of NH+···N bonded polycationic chains. High-pressure crystallization from aqueous solution leads to hydration already at 0.50 GPa. Single-crystals of two different monohydrates, α-dabcoHI·H2O at 0.50 GPa and β-dabcoHI·H2O at 1.40 GPa, have been in situ grown in a diamond-anvil cell in isothermal and isochoric conditions and their structures determined by X-ray diffraction. The NH+···N bonded chains, characteristic of unsolvated dabcoHI polymorphs, in α-hydrate coexist with NH+···N, NH+···O, and N···HO bonded cyclic aggregates. In the β-hydrate all NH+···N bonds are replaced by OH···N, N···OH, and bifurcated NH+···O/I– bonds, which is the first hydrogen bond involving NH+ and I– ions in all dabcoHI hydrogen bonded complexes investigated so far. High-pressure dehydration has been achieved above 0.50 GPa by crystallizing the sample above 350 K, consistently with the positive slope of the boundary between anhydrated and hydrated dabcoHI phases in the p/T diagram. The structural mechanism of pressure-promoted hydration combining varied intermolecular interactions, H-bonds, and more efficient crystal packing has been discussed. It has been shown that pressure can be an efficient means for modifying solvation of compounds in general.