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 · 6H2O with dpq under different reaction conditions led to the isolation of three polymorphic copper(II) complexes [Cu(dpq)2(H2O)](ClO4)2 · 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 · 6H2O with dpq in methanol, has a distorted trigonal bipyramidal structure with τ = 0.55. The reaction between Cu(ClO4)2 · 6H2O 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 · 6H2O 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 π–π 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(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.

Graphical abstractFirst row transition metal complexes of dipyrido[3,2-f:2′,3′-h]-quinoxaline have been synthesized and their structural, spectroscopic and electrochemical behaviour have been studied thoroughly.Figure optionsDownload full-size imageDownload as PowerPoint slide