The Cu(II)-2,2′-bipyridine system revisited

The system formed by Cu(II) and 2,2′-bipyridine (bpy) has been revisited by use of EPR and electronic absorption spectroscopies. The mono-chelated [Cu(bpy)]2+, bis-chelated trans-[Cu(bpy)2]2+ with pseudo-tetrahedral geometry due to the steric hindrance of the 3,3′ hydrogen atoms and tris-chelated [Cu(bpy)3]2+ species exist in aqueous solution. In basic media hydrolysis yields [Cu(bpy)OH]+ and [Cu(bpy)(OH)2]. The dimer [Cu(bpy)(OH)]2 2+ is favoured by increasing metal ion concentration in the pH range 8–10. The EPR spectrum of the dimeric species shows well-resolved ΔM=1 and 2 features, and is different from that observed in the solid state. The zero-field splitting is consistent with a distance of ∼3.4 Å between metallic centres. The behaviour of the complex system is in agreement with potentiometric data reported in the literature. In the EtOH–H2O mixture the bis-chelated complex adopts a five-coordinate cis structure exhibiting a rhombic EPR spectrum and, in basic solution, the [Cu(bpy)2(OH)]+ species is formed. The attribution of the solution species has been confirmed by the analysis of solid compounds, [Cu(bpy)SO4]·2H2O, [Cu(bpy)(H2O)2](NO3)2, [Cu(bpy)2OSO3]·4H2O and [Cu(bpy)2ONO2]NO3·H2O, after dissolution.