Ligand preferences in ytterbium ions complexation with carboxylate-based metal-organic frameworks

Two coordination polymers of ytterbium were synthesized by employing 4,4′,4″-s-triazine-2,4,6-triyl-tribenzoic acid (H₃TATB), 4,4′,4-benzene-1,3,5-triyl-tribenzoic acid (H₃BTB), and 3,5-pyridinedicarboxylic acid (3,5-PDC) ligands and were characterized by single-crystal X-ray diffraction analysis. Reaction of ytterbium(III) chloride in the presence of H₃BTB and 3,5-PDC ligands gives preferred complexation with the 3,5-PDC ligand, producing [Yb₂(3,5-PDC)(ClO₄)₃][NH(Me)₃] (1). However, under exactly the same reaction conditions, reaction of ytterbium(III) chloride in the presence of 3,5-PDC and H₃TATB resulted in complexation with H₃TATB to form [(CH₃)₂NH₂][Yb₄(TATB)₄(HCO₂)(H₂O)₂]·3H₂O (2). The crystal structure results showed a layered structure for 1 and a metal-organic framework structure for 2. This indicates that the complexation preference of the ytterbium ion is H₃TATB ≥ 3,5-PDC ≥ H₃BTB. Conversely, the uncomplexed ligand in the metal-organic framework (2) is an auxiliary agent during the synthesis, which shows polytopic linker controls crystal properties, to form suitable crystals for single-crystal structure determination. The prepared coordination compounds were used as heterogeneous catalysts in an oxidation amidation reaction with different aldehydes and benzylamine hydrochloride.