From light to heavy alkali metal tetraphosphonates (M = Li, Na, K, Rb, Cs): cation size-induced structural diversity and water-facilitated proton conductivityElectronic supplementary information (ESI) available: CIF files for all M-HDTMP compounds (M = Li, Na, K, Rb, and Cs), Rietveld plot for Li-HDTMP-0W, powder XRD patterns, ATR-IR spectra, additional TGA traces, graphs relating coordination numbers and M-O bond distances with cation size, and impedance results. CCDC 1584815-1584820. For ESI a

A family of alkali metal-based frameworks containing the tetraphosphonate ligand hexamethylenediamine- N , N , N ′, N ′- tetrakis (methylenephosphonic acid), HDTMP, is reported. A cation size-induced structural diversity, from monodimensional solids (Li + and Na + ) through layered (K + ) to pillared-layered (Rb + and Cs + ) structures, was found. The proton conductivity properties of the Li compounds (hydrated and dehydrated) are reported and the influence of dehydration/rehydration processes in enhancing proton transfer processes is highlighted. Reversible changes in the dimensionality occurred upon full dehydration/rehydration with minor rearrangements in the framework, implying variations in the Li + -ligand connectivity but preserving the tetracoordination of the metal ion. The reversibly dehydrated-rehydrated sample displayed the highest proton conductivity (5 × 10 −3 S cm −1 at 80 °C and 95% RH), a behavior attributed to reversible formation/reformation of P-O(H)-Li bonds that, in turn, provoked changes in the acidity of acid groups and water mobility in the temperature range of impedance measurements. A family of alkali metal-based frameworks containing the tetraphosphonate linker hexamethylenediamine- N , N , N ′, N ′- tetrakis (methylenephosphonic acid), HDTMP, exhibiting rich structural diversity and water-dependent proton conductivity.