Crystalline and Liquid Structure of Zinc Chloride Trihydrate: A Unique Ionic Liquid

The
water/ZnCl2 phase diagram in the vicinity of the
75 mol % water composition is reported, demonstrating the existence
of a congruently melting phase. Single crystals of this 3-equiv hydrate
were grown, and the crystal structure of [Zn­(OH2)6]­[ZnCl4] was determined. Synchrotron X-ray and neutron
diffraction and IR and Raman spectroscopy along with reverse Monte
Carlo modeling demonstrate that a CsCl-type packing of the molecular
ions persists into the liquid state. Consistent with the crystalline
and liquid structural data, IR spectroscopy demonstrates that the
O–H bonds of coordinated water do not exhibit strong intermolecular
hydrogen ion bonding but are significantly weakened because of the
water’s coordination to Lewis acidic zinc ions. The O–H
bond weakening makes this system a very strong hydrogen-bond donor,
whereas the ionic packing along with the nonpolar geometry of the
molecular ions makes this system a novel nonpolar, hydrogen-bonding,
ionic liquid solvent.