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 (MLnLxH(2)O, M = Na, K, Cs; Ln = La, Ce, Nd; x approximate to 4: KCeL(1) center dot 4H(2)O, C2/c, a = 23.5864(2) angstrom, b = 12.1186(2) angstrom, c = 5.6613(2) angstrom, beta = 93.040(2)degrees). 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 center dot 4H(2)O, the space group changes to P2(1)/n, a = 21.8361(12) angstrom, b = 9.3519(4) angstrom, c = 5.5629(3) angstrom, beta = 96.560(4)degrees, 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) center dot 4.5H(2)O: Ln = La, P-1, a = 5.7630(13) angstrom, b = 10.213(2) angstrom, c = 11.649(2) angstrom, alpha = 84.242(2)degrees, beta = 89.051(2)degrees, gamma = 82.876(2)degrees) in which one half of the ligands coordinate via the piperazine nitrogen atoms. With the 2-methyl ligand, structure type III crystallises (LnHL(2) center dot 4H(2)O: Ln = Nd, Ce, P2(1)/c, a = 5.7540(9) angstrom, b = 14.1259(18) angstrom, c = 21.156(5) angstrom, beta = 90.14(2)degrees) due to unfavourable steric interactions of the methyl group in structure type II. (C) 2009 Elsevier Inc. All rights reserved.