Coordination polymers of La(III) as bunched infinite nanotubes and their conversion into an open-framework structure

Pyridine-2,6-dicarboxylic acid (pdcH2) reacts with LaCl3 x 7H2O under hydrothermal conditions followed by evaporation at room temperature to give a metal-organic framework structure of the empirical formula, [La(pdc)(H2O)4] x Cl (1), in the form of infinitely long bunched nanotubes. The chloride ions and water molecules occupy the tubular as well as the inter-tubular spaces. When La(NO3)3 x 7H2O is used in place of LaCl3 x 6H2O, a similar structure is formed with the empirical formula, [La(pdc)(H2O)4] x NO3 (2), where water molecules and the nitrate anions occupy the voids as in the case of 1. When an aqueous solution of AgNO3 is added to an aqueous solution of 1, the Cl- ions are replaced completely by NO3- ions to form 2; thus, the tubular structure is conserved. However, when AgBF4 is used in place of AgNO3, the tubular structure breaks down, and a new 3-D MOF structure, [La(pdc)(pdcH)(H2O)2] x 4H2O (3), is formed where the cavities are occupied by hexameric and dimeric water clusters. Structure 3 is also formed as the sole product when La(OAc)3 x xH2O is treated with pyridine-2,6-dicarboxylic acid following the method adopted for 1 and 2. Formation of the tubular structure depends on the molar ratio of the ligand and the metal. When higher than 1 equiv of the metal is taken, a linear coordination polymer, [La2(pdc)3(H2O)6] x 2H2O (4), is formed. This study provides the first nanotubular structure of a pure lanthanide metal.