Coordinative versatility of 2,3-bis(2-pyridyl)-5,8-dimethoxyquinoxaline (L) to different metal ions:: syntheses, crystal structures and properties of [Cu(I)L]22+ and [ML]2+ (M =Cu(II), Ni(II), Zn(II) and Co(II))

The complexes of Cu(I), Cu(II), Ni(II), Zn(II) and Co(II) with a new polypyridyl ligand, 2.3-bis(2-pyridyl)-5.8-dimethoxyquinoxaline (L), have been synthesized and characterized. The crystal structures of these complexes have been elucidated by X-ray diffraction analyses and three types of coordination modes for L were found to exist in them. In the dinuclear complex [Cu(I)L(CH3CN)](2) .(ClO4)(2) (1). L acts as a tridentate ligand with two Cu(I) centers bridged by two L ligands to form a box-like dimeric structure, in which each Cu(I) ion is penta-coordinated with three nitrogen atoms and a methoxyl oxygen atom of two L ligands, and an acetonitrile. In [Cu(II)L(NO3)(2)]. CH3CN 2, the Cu(II) center is coordinated to the two nitrogen atoms of the two pyridine rings of L which acts as a bidentate ligand. The structures of [Ni(II)L(NO3)(H2O)(2)].2CH(3)CN . NO3 (3). [Zn(II)L(NO3)(2)(H2O)] . 2CH(3)CN (4) and [CO(II)LCl2(H2O)] (5) are similar to each other in which L acts as a tridentate ligand by using its half side, and the metal centers are coordinated to a methoxyl oxygen atom and two bipyridine nitrogen atoms of L in the same side. The formation of infinite quasi-one-dimensional chains (1, 4 and 5) or a quasi-two-dimensional sheet (2) assisted by the intra- or intermolecular face-to-face aryl stacking interactions and hydrogen bonds may have stabilized the crystals of these complexes. Luminescence studies showed that 1 exhibits broad, structureless emissions at 420 nm in the solid state and at 450 tim in frozen alcohol frozen glasses at 77 K. Cyclic voltammetric studies of 1 show the presence of an irreversible metal-centered reduction wave at approximately -0.973 V versus Fc(+/0)A and a quasi-reversible ligand-centered reduction couple at approximately -1.996 V versus Fc(+/0). The solution behaviors of these complexes have been further studied by UV-Vis and ESR techniques. (C) 2002 Elsevier Science B.V. All rights reserved.