One-pot synthesis and structural characterization of a Tb(III) coordination polymer based on a tripodal Schiff base ligand adopting an exo-bridging coordination mode

Abstract

A novel Tb(III) coordination polymer and discrete complexes have been synthesized by one-pot reaction utilising flexible tripodal Schiff base ligands, tris[2-(salicylideneamino)ethyl]amine (H3L) derivatives. Condensation of 5-methylsalicylaldehyde (5-MeSal) with tris(2-aminoethyl)amine (tren) in the presence of Tb(CF3SO3)3 in a 3:2:1 (=5-MeSal:tren:Tb(III)) molar ratio gives Tb1L, and the deprotonated ligand encapsulates the Tb(III) in its N4O3 cavity in endo-coordination. Here, the excess tren acts as a base to dissociate the proton of the ligand. Decreasing the amount of tren to a 3:1:1 ratio gives [Tb(H31L)2](CF3SO3)3·H2O. In this compound, the Tb(III) centre is coordinated by two H31L ligands adopting the exo-capping mode to provide three O atoms outside the cavity of H31L. Replacing Tb(CF3SO3)3 with Tb(NO3)3 in the above reaction induced a substantial structural change to coordination polymer [Tb(H31L)(NO3)3·DMF·1/6H2O]n. In this polymer, the ligand adopts an unprecedented exo-bridging coordination mode, which is the first bridging mode observed for this tripodal Schiff base ligands. Three aromatic rings of the exo-bridging ligand are close to each other by CH⋯π interactions and each O atom coordinates to three different Tb(III) ions to construct a three-dimensional polymeric connection. The 3D framework has the nano-pore structure with inner walls that consisted of alternately appearing hydrophilic and hydrophobic regions. The present study shows that the tripodal Schiff base ligand can be a novel class of flexible building block to construct the Ln(III) coordination polymer.