Novel pillared-layered framework materials were synthesized by high-throughput
or microwave-assisted methodology that contain Mg2+ and the zwitterionic linker HDTMP
(hexamethylenediamine-N,N,N0,N0-tetrakis(methylenephosphonic acid)). Three compounds were
structurally characterized by X-ray powder diffraction. In the compound {Mg[(HO3PCH2)2N(CH2)6N
(CH2PO3H2)2](H2O)}n(1), obtained at 140 ºC by hydrothermal or microwave-assisted reaction,
the layers are built by isolated Mg2+ octahedra coordinated by oxygen atoms from six different
zwitterionic HDTMP ligands. Each amino-bis(methylenephosphonate) moiety links three Mg2+ ions,
bridging two of them through one phosphonate group and connecting the third polyhedron in a
monodentate fashion. In Compound 2, {Mg[(HO3PCH2)2N(CH2)6N(CH2PO3H2)2]}n, hydrothermally
synthesized at 180 C, the layers are composed of bidentate amino-bis(methylenephosphonate)
moieties connected to three Mg2+ ions, with one of the phosphonate groups acting
as a bridging ligand. Various subtle structural changes are noted for the other two
compounds. Thermodiffraction of 1 reveals that a crystalline-to-crystalline phase transformation
occurs concomitantly with its dehydration, leading to a new anhydrous phase, namely,
{Mg[(HO3PCH2)2N(CH2)6N(CH2PO3H2)2]}n(1deh). This process is fully reversible upon
equilibrating the solid at room temperature. The reported compounds can adsorb ammonia and CO2.
Compound 1 exhibits a moderate proton conductivity, ~1.5 x 10-5 S·cm-1 at 80 ºC and 95% RH, that
increases a half order of magnitude after experiencing a complete dehydration/rehydration process.
