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

Abstract

Pyridine-2,6-dicarboxylic acid (pdcH₂) reacts with LaCl₃·7H₂O under hydrothermal conditions followed by evaporation at room temperature to give a metal-organic framework structure of the empirical formula, [La(pdc)(H₂O)₄]·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)₃·7H₂O is used in place of LaCl₃·6H₂O, a similar structure is formed with the empirical formula, [La(pdc)(H₂O)₄]·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]·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·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]·2H₂O (4), is formed. This study provides the first nanotubular structure of a pure lanthanide metal.