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 Delta M = 1 and 2 features, and is different from that observed in the solid state. The zero-field splitting is consistent with a distance of similar to 3.4 Angstrom 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]. 2H(2)O, [Cu(bpy)(H2O)(2)](NO3)(2) [Cu(bpy)(2)OSO3]. 4H(2)O and [Cu(bpy)(2)ONO2]NO3. H2O, after dissolution. (C) 2000 Elsevier Science S.A. All rights reserved.