High Proton Conductivity in a Flexible, Cross-Linked, Ultramicroporous Magnesium Tetraphosphonate Hybrid Framework

Multifunctional materials, especially those combining
two or more properties of interest, are attracting immense attention
due to their potential applications. MOFs, metal organic frameworks,
can be regarded as multifunctional materials if they show another
useful property in addition to the adsorption behavior. Here, we report
a new multifunctional light hybrid, MgH6ODTMP·2H2O­(DMF)0.5 (1), which has been synthesized
using the tetraphosphonic acid H8ODTMP, octamethylenediamine-N,N,N′,N′-tetrakis­(methylenephosphonic acid), by high-throughput
methodology. Its crystal structure, solved by Patterson-function direct
methods from synchrotron powder X-ray diffraction, was characterized
by a 3D pillared open framework containing cross-linked 1D channels
filled with water and DMF. Upon H2O and DMF removal and
subsequent rehydration, MgH6ODTMP·2H2O
(2) and MgH6ODTMP·6H2O (3) can be formed. These processes take place through crystalline–quasi-amorphous–crystalline
transformations, during which the integrity of the framework is maintained.
A water adsorption study, at constant temperature, showed that this
magnesium tetraphosphonate hybrid reversibly equilibrates its lattice
water content as a function of the water partial pressure. Combination
of the structural study and gas adsorption characterization (N2, CO2, and CH4) indicates an ultramicroporous
framework. High-pressure CO2 adsorption data are also reported.
Finally, impedance data indicates that 3 has high proton
conductivity σ = 1.6 × 10–3 S cm–1 at T = 292 K at ∼100% relative
humidity with an activation energy of 0.31 eV.