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

Abstract

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(ClO 4) 2 · 6H 2O with dpq under different reaction conditions led to the isolation of three polymorphic copper(II) complexes [Cu(dpq) 2(H 2O)](ClO 4) 2 · H 2O ( 2), [Cu(dpq) 2(ClO 4)](ClO 4) ( 3) and [{Cu(dpq) 2(H 2O)}{Cu(dpq) 2(ClO 4)}](ClO 4) 3 ( 4). The bluish-green compound 2, obtained by reacting Cu(ClO 4) 2 · 6H 2O with dpq in methanol, has a distorted trigonal bipyramidal structure with τ = 0.55. The reaction between Cu(ClO 4) 2 · 6H 2O 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(ClO 4) 2 · 6H 2O 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(ClO 4) 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 π–π interactions, leading to the generation of a sinusoidal chain. The proton NMR spectra, especially those of the paramagnetic complexes [Ni(dpq) 3](ClO 4) 2 ( 6) and [Co(dpq) 3](ClO 4) 2 ( 7), have been studied in detail. The electronic absorption spectra and the redox behaviour of the copper(I), 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.