Luminescent and Proton Conducting Lanthanide Coordination Networks Based On a Zwitterionic Tripodal Triphosphonate

The synthesis, structural characterization,
luminescence properties, and proton conduction performance of a new
family of isostructural cationic 2D layered compounds are reported.
These have the general formula [Ln­(H4NMP)­(H2O)2]­Cl·2H2O [Ln = La3+, Pr3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, H6NMP = nitrilotris­(methylphosphonic
acid)], and contain Cl– as the counterion. In the
case of Ce3+, a 1D derivative, [Ce2(H3NMP)2(H2O)4]·4.5H2O, isostructural with the known lanthanum compound has been isolated
by simply crystallization at room temperature. The octa-coordinated
environment of Ln3+ in 2D compounds is composed by six
oxygen atoms from three different ligands and two oxygens from each
bound water. Two of the three phosphonate groups act as both chelating
and bridging linkers, while the third phosphonate group acts solely
as a bridging moiety. The materials are stable at low relative humidity
at less at 170 °C. However, at high relative humidity transform
to other chloride-free phases, including the 1D structure. The proton
conductivity of the 1D materials varies in a wide range, the highest
values corresponding to the La derivative (σ ≈ 2 ×
10–3 S·cm–1 at RH 95% and
80 °C). A lower proton conductivity, 3 × 10–4 S·cm–1, was measured for [Gd­(H4NMP)­(H2O)2]­Cl·2H2O at 80 °C,
which remains stable under the work conditions used. Absorption and
luminescence spectra were recorded for selected [Ln­(H4NMP)­(H2O)2]­Cl·2H2O compounds. In all of
them, the observed transitions are attributed solely to f–f
transitions of the lanthanide ions present, as the H4NMP2– organic group has no measurable absorption or luminescence
properties.