Reversible chair ↔ skew conformational interconversion of 1,3,2-dioxaphosphorinane ring in the solid state.

β-NH4[(MeO)2cDHAP] (where cDHAP is cyclic dihydroxyacetone phosphate) has been investigated by X-ray crystallography in the temperature range 350–100 K. Three reversible single-crystal-to-single-crystal phase transitions have been examined and four phases (high-, room-, medium- and low-temperature phase, HTP, RTP, MTP and LTP, respectively) have been structurally determined: HTP (at 350 K, P21/a, Z = 24), RTP (290 K, P \overline{1}, Z = 12), MTP (205 K, P21/a, Z = 4) and LTP (100 K, P \overline{1}, Z = 24). Various puckering modes of the 1,3,2-dioxaphosphorinane ring of the organic cyclic phosphate anion have been revealed in the crystal: chair and skew. The chair ↔ skew ring flipping, which has been shown to take place during the phase transitions, results in various populations of skew and chair conformers in different phases. The flexibility in the coordination geometry of ammonium cations is considered to assist in the conformational flexibility of the dioxaphosphorinane ring.