Dominance of Charge-Assisted Hydrogen Bonding on Short Contacts and Structures that Crystallize with <i>Z</i>′ > 1

An enantiotropic pair of polymorphs of (H₂-<i>o</i>-PDA²⁺)₄(H₅O₂⁺)(SO₄^2–)₄(HSO₄^–)·2H₂O (<b>1</b>) interconvert at 140–142 K with a concomitant change in <i>Z</i>′. The details of the structural change are studied by single-crystal X-ray and neutron diffraction which shows that both forms are close packed with the difference resulting from the linearization of hydrogen bonds at low temperature as part of an extensive charge-assisted hydrogen-bonding network. The related (H₂-<i>o</i>-PDA²⁺)₂(SO₄^2–)₂·3H₂O (<b>2</b>) has also been characterized by single-crystal neutron diffraction and exhibits a disordered “flip-flop” water chain and short nonbonded O···O contacts enforced by NH₃⁺···OSO₃^2– hydrogen-bonded bridges. A total of seven other related salts have been isolated and structurally characterized revealing a surprisingly high incidence of <i>Z</i>′ > 1 structures as a result of frustration in the charge-assisted hydrogen-bonded network