Isoreticular Expansion in Porous Al(III) and Ga(III) Phosphonates: Synthesis, Structure, and Properties

We report the synthesis and structural elucidation of two metal-organic frameworks (MOFs), [M(OH)(H₂BPD)]·xH₂O (M = Al³⁺, Ga³⁺), denoted as M-CAU-67, synthesized using the ditopic linker molecule N, N’ 4,4’ bipiperidine bis(methylenephosphonic acid) (H4BPD). The compounds were synthesized via hydrothermal synthesis and upscaling to the gram scale was achieved under reflux conditions, demonstrating the versatility in preparation. Single-crystal X-ray diffraction (SCXRD) revealed an isoreticular, expanded MIL-91 structure, representing a rare case of successful isoreticular expansion in porous metal phosphonates. The framework structure was confirmed by Rietveld refinement against powder X-ray diffraction (PXRD) data. Comprehensive characterization, including solid-state NMR spectroscopy (³¹P, ²⁷Al MQMAS) supports the structure model. Solvent exchange experiments, as well as variable-humidity/temperature PXRD data, revealed high flexibility with ΔV/V = 32 % between hydrated and activated states. The compounds are permanently microporous and show a complex, polarity- and size-dependent sorption behavior toward gases and vapors.