Ten Polymorphs of NH+···N Hydrogen-Bonded 1,4-Diazabicyclo[2.2.2]octane Complexes: Supramolecular Origin of Giant Anisotropic Dielectric Response in Polymorph V
journal contributionposted on 04.08.2010, 00:00 by Anna Olejniczak, Andrzej Katrusiak, Marek Szafrański
A subtle interplay of supramolecular aggregation, crystal symmetry, and bistable proton sites in NH+···N hydrogen bonds in 1,4-diazabicyclo[2.2.2]octane hydroiodide (dabcoHI, [C6H13N2]+I−) leads to the giant dielectric response in this compound. This unique feature disappears on increasing the temperature above 410 K or on increasing the pressure above 40 MPa at 296 K. Ten polymorphs of dabcoHI have been obtained in varied thermodynamical conditions. Phase V (space group P6̅m2) is stable at normal conditions. For five new polymorphs, the destructive phase transitions have been circumvented by growing the single crystals of specific phases in situ in their stability regions, and their space-group symmetries are Pmm2 (phase IV), Pmc21 (phase VI), Pbcm (phase VII), Cmm2 (phase VIII), and P2/c (phase IX). Of all these 10 dabcoHI phases, the giant dielectric response and anisotropic relaxor properties have been evidenced only for phases V and X. In all the determined structures of dabcoHI, linear or nearly linear NH+···N bonded chains are present, which shows that the relaxor properties of dabcoHI are not inherent to the NH+···N aggregation alone. The polymorphic structures differ mainly in the arrangement of the chains and iodide anions, in the dabco conformation, and in the location of protons, in phases IV, V, VIII, and IX disordered and in phases VI and VII ordered. Polymorph VI is highly metastable at normal conditions and only after days or weeks gradually transforms to phase V. The molecular-scale origin of the changes in dielectric and relaxor properties of dabcoHI results from the conjunction of short-range polarization induced by ordered protons in NH+···N bonds in nanoregions of phase V and the specific crystal environment of hexagonal symmetry, leading to frustration of the local crystal field. A series of hydrates and solvated dabcoHI crystals was obtained above 0.50 GPa.