Synthesis, characterization and properties of a family of lead(II)-organic frameworks based on a multi-functional ligand 2-amino-4-sulfobenzoic acid exhibiting auxiliary ligand-dependent dehydration-rehydration behaviours

A systematic investigation is reported of the use of the multi-functional ligand 2-amino-4-sulfobenzoate (asba(2-)) to construct lead(II)-organic frameworks in the presence and absence of N-donor auxiliary ligands phen, bipy and bix [phen = 1,10-phenanthroline, bipy = 2,2'-bipyridine and bix = 1,4-(methylene-benzene)bisimidazole]. Under ambient, aqueous conditions the assembly of asba(2-) with Pb(II) and phen or bipy leads to two iso-structural 2D double-layer frameworks, {[Pb2(asba)2(phen)2(H2O)]·2H2O}n (1) and {[Pb2(asba)2(bipy)2(H2O)]·2H2O}n (2). [Pb2(asba)2(bix)2(H2O)2]n (3) is obtained in the presence of the auxiliary ligand bix and possesses a 3D network built up from 2D Pb(II)-asba(2-)-bridged double-layer pillared by bix. A 2D (4,4) topological network [Pb(asba)(H2O)]n (4) is obtained in the absence of any second ligand or presence of some bistriazole bridging spacers. The coordination modes of the ligand asba(2-) in 1-4 are greatly dependent on the type of auxiliary ligand and the compounds exhibit auxiliary ligand-dependent dehydration-rehydration behaviours; 1 shows in situ rapid and reversible dehydration-rehydration behaviour in air, while the iso-structural compound 2 loses crystallinity in air and transforms into {[Pb2(asba)2(bipy)2(H2O)]·H2O}n (2A) verified by TGA, elemental analysis and powder X-ray diffraction analysis (PXRD). 3 also shows reversible dehydration-rehydration behaviour, but it takes around one week to rehydrate even after exposure to water vapor, while the dehydrated phase of 4 rehydrates to a new crystalline material. 1 and 3 fall within the category of the "recoverable collapsing" and "guest-induced re-formation" frameworks. The water molecules in 1 and 3 have some influence on their solid state fluorescent emission intensity.