Novel pillared aluminium ethylene diphosphonate displaying reversible dehydration-rehydration behavior

The novel aluminum ethylene diphosphonate Al₂(OH)₂(H₂O)₂(O₃PCH₂CH₂PO₃) has been synthesized hydrothermally. It crystallizes in the triclinic space group (P1̄; a = 8.5977(10), b = 7.0499(3), c = 9.5290(4) Å; α = 89.997(21), β = 104.321(8), γ = 114.225(11)°; V = 506.98(8) ų) with aluminophosphonate layers similar to those found in Al(OH)(H₂O)CH₃PO₃ linked via ethylene (-CH₂CH₂-) groups. The solid loses water upon heating in two steps, the first at 100 °C and the second at 360 °C. The first water loss leaves a crystalline solid with a framework structure of Al₂(OH)₂(O₃PCH₂CH₂PO₃), closely related to that of the parent (C2/c; a = 15.3723(21), b = 6.7613(8), c = 9.7374(13) Å; β = 102.392(9)°; V = 988.4438 Å3) whereas the second water loss results in major loss of crystallinity. X-ray powder diffraction has been used to determine and refine the structures of the parent and intermediate dehydrated phases. The main structural difference is the loss of one water molecule bound to aluminum, which changes the metal cation's coordination from octahedral to distorted square planar. The as-prepared and dehydrated samples have been simulated computationally. The transformation has been monitored by in-situ ²⁷Al MAS NMR spectroscopy; octahedral aluminum (δ_iso = −7.1 ppm; asymmetry parameter, η = 1.0; quadrupolar coupling constant, QCC = 5.4 MHz) is converted to 5-fold coordinated aluminum (δ_iso = 20.7 ppm; η = 0; QCC = 5.8 MHz) by heating at 130 °C for 1 h. The process is fully reversible.