Survey on the Phase Transitions and Their Effect on the Ion-Exchange and on the Proton-Conduction Properties of a Flexible and Robust Zr Phosphonate Coordination Polymer

The flexible zirconium tetraphosphonate coordination
polymer with formula Zr­(O3PCH2)2N-C6H10-N­(O3CH2P)2X2–xH2+x·nH2O (X = H, Li, Na, K, 0
< x < 1, 4 < n < 7.5)
(1) possesses an open framework structure with 1D cavities
decorated with polar and acids PO and POH groups. 1 has been fully protonated by adding HCl and then subjected
to several acid–base ion-exchange reactions with alkaline metals
hydroxides. 1 is a very robust coordination polymer because
it can be regenerated in H- form using strong acid solutions and ri-exchanged
several times without hydrolysis and loss of crystallinity. The flexibility
of 1 has been also studied by means of TDXD (temperature
dependent X-ray diffraction) evidencing remarkable phase transformations
that lead to a different disposition of the water molecules. These
transformations also influence the accessibility of the cations on
the P–OH groups placed inside the channels and thus the ion-exchange
properties. The dependence of the proton conductivity properties on
these phase transitions has been also investigated and discussed.