Isoreticular Expansion in Porous Aluminum(III) andGallium(III) Phosphonates: Synthesis, Structure, and Properties We report the synthesis, structural elucidation, and establishment of structure−property relationships for two metal−organic frameworks (MOFs), [M(OH)(H2BPD)]·xH2O (M = Al3+, Ga3+, x = 11–13), denoted as M-CAU-67, employing the ditopic linker molecule N,N′-4,4′ bipiperidine bis(methylenephosphonic acid) (H4BPD). The title compounds can be synthesized on a milligram scale under hydrothermal conditions, while gram-scale synthesis was realized under reflux conditions. Single-crystal X-ray diffraction (SCXRD) revealed an isoreticular, expanded MIL-91 type structure, representing a rare case of isoreticular expansion in porous metal phosphonates. The framework structure was confirmed by Rietveld refinement against powder X-ray diffraction (PXRD) data and further validated by solid-state NMR spectroscopy (31P MAS, 27Al 3Q MAS). Solvent-exchange experiments and variable-humidity/temperature PXRD data revealed high framework flexibility. The compounds show permanent microporosity and a complex, polarity- and size-dependent sorption behavior toward gases and vapors. 38 16 8463-8475 8463-8475 American Chemical Society (ACS)