Observation of a Less Stable Conformer of N,N′‐Piperazine‐bis(methylenephosphonate) Ligand in a Six‐coordinated ­Samarium Phosphonate Framework

The three‐dimensional (3D) samarium phosphonate framework Sm₂(H₂L)₃]_n · 5n(H₂O) ( 1 ) H₄L = N,N′‐piperazine‐bis(methylenephosphonic acid)] was synthesized by hydrothermal reaction of Sm₂O₃ with N,N′‐piperazine‐bis(methylenephosphonic acid) hydrochloride in the presence of glutaric acid. Single‐crystal X‐ray diffraction analysis reveals that it has a 3D open framework structure with helical channels along the crystallographic c axis. The channels are filled up by discrete pentameric water clusters, which are hydrogen‐bonded to the host. Compound 1 displays two interesting structural features: (a) two of three H₂L^2– ligands adopt the less stable a,e‐cis conformation; (b) both of the Sm^III ions exhibit rather unusual octahedral coordination arrangements. In addition, the photoluminescent property was investigated.