Hydrothermal in situ ligand reaction: copper(II)-mediated stepwise oxidation of 2,3,5- and 2,4,6-trimethylpyridine to pyridinecarboxylates

Abstract

Hydrothermal reactions of 2,3,5-trimethylpyridine (2,3,5-tmpy) yielded two pyridinecarboxylates, 3,5-dimethyl-2-pyridinecarboxylate (2-dmpya) and 3-methyl-2,5-pyridine dicarboxylate (2,5-mpyda) in the presence of copper nitrate. The in situ generated pyridinecarboxylates were derived from copper-mediated stepwise oxidation of 2- or/and 5-methyl groups on the pyridine by nitrate in acidic aqueous solution. The selectivity might be due to the different electronic distribution resulting from the asymmetric substitution on the aromatic ring and steric hindrance between two neighboring methyl groups. When 2,4,6-trimethylpyridine (2,4,6-tmpy) was used in the reactions, the three evenly-distributed methyl groups were oxidized simultaneously to form 2,4,6-pyridine tricarboxylate (2,4,6-pyta). Interestingly in nearly neutral solution a new ligand 3-hydroxy-2,4,6-pyridine tricarboxylate (2,4,6-Opyta) was yielded and trapped in the compound [Cu₄(2,4,6-Opyta)₂(H₂O)₆]·2H₂O (4). The in situ copper-mediated oxidation of 2,4,6-tmpy to 2,4,6-Opyta⁴⁻via 2,4,6-pyta³⁻ can be further supported by the formation of 4 from a hydrothermal reaction of 2,4,6-pyridine tricarboxylic acid with copper nitrate at 185 °C for 4 days. The new Opyta ligand was isolated as an ammonium (NH₄)(2,4,6-OpytaH₃)·H₂O and characterized by NMR, MS and X-ray crystallography. The magnetic property of 4 was investigated.