Lanthanide N,N'-piperazine-bis(methylenephosphonates) (Ln=La, Ce, Nd) that display flexible frameworks, reversible hydration and cation exchange

Hydrothermal syntheses of lanthanide bisphosphonate metal organic frameworks comprising the light lanthanides lanthanum, cerium and neodymium and N,N'-piperazine bis(methylenephosphonic acid) (H2L(1) and its 2-methyl and 2,5-dimethyl derivatives (H2L(2) and H2L(3)) gives three new structure types. At elevated starting pH (ca. 5 and above) syntheses give 'type I' materials with all metals and acids of the study (MLnLxH2O, M=Na, K, Cs; Ln=La, Ce, Nd; x∼4: KCeL(1).4H2O, C2/c, a=23.5864(2) A, b=12.1186(2) A, c=5.6613(2) A, β=93.040(2)o). The framework of structure type I shows considerable flexibility as the ligand is changed, due mainly to rotation around the -N-CH2- bond of the linker in response to steric considerations. Type I materials demonstrate cation exchange and dehydration and rehydration behaviour. Upon dehydration of KCeL.4H2O, the space group changes to P21/n, a=21.8361(12) A, b=9.3519(4) A, c=5.5629(3) A, β=96.560(4)o, as a result of a change of the piperazine ring from chair to boat configuration. When syntheses are performed at lower pH, two other structure types crystallise. With the 'non-methyl' ligand 1, type II materials result (LnL(1)H2L(1).4.5H2O: Ln=La, P-1, a=5.7630(13) A, b=10.213(2) A, c=11.649(2) A, α=84.242(2)o, β=89.051(2)o, γ=82.876(2)o) in which one half of the ligands coordinate via the piperazine nitrogen atoms. With the 2-methyl ligand, structure type III crystallises (LnHL(2).4H2O: Ln=Nd, Ce, P21/c, a=5.7540(9) A, b=14.1259(18) A, c=21.156(5) A, β=90.14(2)o) due to unfavourable steric interactions of the methyl group in structure type II. - Graphical abstract: The lanthanides La, Ce and Nd give a family of metal organic frameworks based on N,N'-piperazinebismethylenephosphonate ligands: these display reversible dehydration, structural flexibility and cation exchange.