Structural, spectroscopic and redox properties of transition metal complexes of dipyrido[3,2-f:2′,3′-h]-quinoxaline (dpq)

The chemistry of first row transition metal complexes obtained from the ligand dipyrido[3,2-f.2',3'-h]-quinoxaline (dpq) have been reported. The reaction between Cu(ClO4)(2)center dot 6H(2)O with dpq under different reaction conditions led to the isolation of three polymorphic copper(II) complexes [Cu(dpq)(2)(H2O)](ClO4)(2)center dot H2O (2), [Cu(dpq)(2)(ClO4)](ClO4) (3) and [{Cu(dpq)(2)(H2O)}{Cu(dpq)(2)(ClO4)}](ClO4)(3) (4). The bluish-green compound 2, obtained by reacting Cu(ClO4)(2)center dot 6H(2)O with dpq in methanol, has a distorted trigonal bipyramidal structure with r = 0.55. The reaction between Cu(ClO4)(2)center dot 6H(2)O and dpq in dry acetonitrile produced the blue compound 3 in which the copper(II) centre has a distorted square planar geometry. When the condensation reaction between 1, 10-phenanthroline-5,6-dione and 1,2-diaminoethane was carried out in the presence of Cu(ClO4)(2)center dot 6H(2)O in methanol, the green copper(II) complex 4 was isolated along with 1. The structure determination of 4 has established the presence of two different complex cations in the asymmetric unit and they are considered as co-crystals. In the zinc(II) compound [Zn(dpq)(2)(ClO4)(2)](5), the two perchlorates are unidentately coordinated to the metal centre, providing a distorted octahedral geometry. The quinoxaline ring in 5 is involved in intermolecular pi-pi interactions, leading to the generation of a sinusoidal chain. The proton NMR spectra, especially those of the paramagnetic complexes [Ni(dpq)(3)](ClO4)(2) (6) and [Co(dpq)(3)](ClO4)(2) (7), have been studied in detail. The electronic absorption spectra and the redox behaviour of the copper(l), copper(II), cobalt(II) and cobalt(III) complexes have been studied. The three copper(II) compounds 2-4 show identical absorption spectra and redox properties when measured in acetonitrile, although in nitromethane they show small but definite differences in their spectral and redox features. (c) 2007 Elsevier Ltd. All rights reserved.