Hydrothermal reaction of Cu(II)/pyrazine-2,3,5-tricarboxylic acid and characterization of the copper(II) complexes

Four copper(II) complexes [Cu-3(PZHD)(2)(2,2'-bpy)(2)(H2O)(2)]center dot 3H(2)O (1), [Cu-3(DHPZA)(2)(2,2'-bpy)(2)] (2), [Cu(C2O4)-phen(H2O)] center dot H2O (3), and [Cu-3(PZTC)(2)(2,2'-bpy)(2)]center dot 2H(2)O (4) were synthesized by hydrothermal reactions, in which the complexes 1-3 were obtained by the in situ Cu(II)/H(3)PZTC reactions (PZHD(3-) = 2-hydroxypyrazine-3,5-dicarboxylate, 2,2'-bpy = 2,2'-bipyridine, DHPZA(3-) = 2,3-dihydroxypyrazine-5-carboxylate, C2O42- = oxalate, phen = 1,10-phenanthroline, and H(3)PZTC = pyrazine-2,3,5-tricarboxylic acid). The Cu(II)/H(3)PZTC hydrothermal reaction with 2,2'-bpy, without addition of NaOH, results in the formation of complex 4. The complexes 1-4 and transformations from H(3)PZTC to PZHD(3-), DHPZA(3-), and C2O42- were characterized by single-crystal X-ray diffraction and theoretical calculations. In the complexes 1, 2, and 4, the ligands PZHD(3-), DPHZA(3-), and PZTC(3-) all show pentadentate coordination to Cu(II) ion forming three different trinuclear units. The trinuclear units in 1 are assembled by hydrogen-bonding and pi-pi 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)/H(3)PZTC reactions, forming complexes of 1-3.