Discovery of Novel In Situ Cascade Reactions for Fabricating Pyrazine-Derived Ligands to Construct Complexes with Intriguing Metamagnetism and Luminescence Behavior

The in situ reactions in solvothermal processes have attracted explosive attention owing to their unique potential for fabricating organic ligands which were hard to achieve by the conventional organic synthesis method and for revealing the generation mechanisms directly by capturing the key intermediate through crystallization. In this work, using formic acid and 4-pyridinemethaneamine (L⁰) as raw materials, the linear ligand 2,5-di(pyridin-4-yl)pyrazine (L) has been generalized in situ in assembling the 2D coordination polymers [Fe₂(C₂O₄)₂L₂]_n·(0.5L)_n·(0.5H₂O)_n (1) and [Zn(C₂O₄)(L)_1.5]_n (2). After adjusting the reaction time, the intermediate N-(pyridin-4-ylmethyl)formamide (L¹) was captured by crystallization of the chain complexes [Fe(L¹)₂(C₂O₄)]_n (3) and [Zn(L¹)₂(C₂O₄)]_n (4). The possible in situ reaction mechanism has been revealed to be the cascade reaction between the amidation, nucleophilic addition–elimination reaction, and the oxidative dehydrogenation reaction through the crystal data, mass spectrometry, and ¹H NMR. Magnetic studies of 1 illustrate that the spin-canted iron oxalate chains are coupled antiferromagnetically, and the antiferromagnetic interactions between the chain could be overcome by the external magnetic field and leads to metamagnetism. 2 exhibits orange fluorescence emission owing to the formation of a spatial conjugation system in the network. This work proposes an insightful synthetic route to prepare functional coordination polymers (CPs) through an in situ reaction.