Hydrothermal Reaction of Cu(II)/Pyrazine-2,3,5-tricarboxylic acid and Characterization of the Copper(II) Complexes

Four copper(II) complexes [Cu3(PZHD)2(2,2′-bpy)2(H2O)2]·3H2O (1), [Cu3(DHPZA)2(2,2′-bpy)2] (2), [Cu(C2O4)phen(H2O)]·H2O (3), and [Cu3(PZTC)2(2,2′-bpy)2]·2H2O (4) were synthesized by hydrothermal reactions, in which the complexes 1−3 were obtained by the in situ Cu(II)/H3PZTC reactions (PZHD3− = 2-hydroxypyrazine-3,5-dicarboxylate, 2,2′-bpy = 2,2′-bipyridine, DHPZA3− = 2,3-dihydroxypyrazine-5-carboxylate, C2O42− = oxalate, phen = 1,10-phenanthroline, and H3PZTC = pyrazine-2,3,5-tricarboxylic acid). The Cu(II)/H3PZTC hydrothermal reaction with 2,2′-bpy, without addition of NaOH, results in the formation of complex 4. The complexes 1−4 and transformations from H3PZTC to PZHD3−, DHPZA3−, and C2O42− were characterized by single-crystal X-ray diffraction and theoretical calculations. In the complexes 1, 2, and 4, the ligands PZHD3−, DPHZA3−, and PZTC3− all show pentadentate coordination to Cu(II) ion forming three different trinuclear units. The trinuclear units in 1 are assembled by hydrogen-bonding and π-π stacking to form a 3D supramolecular network. The trinuclear units in 2 acting as building blocks are connected by the carboxylate oxygen atoms forming a 2D metal-organic framework (MOF) with (4,4) topology. While the trinuclear units in 4 are linked together by the carboxylate oxygen atoms to form a novel 2D MOF containing right- and left-handed helical chains. The theoretical characterization testifies that electron transfer between OH− and Cu2+ and redox of Cu2+ and Cu+ are the most important processes involved in the in situ copper Cu(II)/H3PZTC reactions, forming complexes of 1−3.