ELECTROCHEMISTRY IN LIQUID SO2 .12. ELECTROCHEMICAL AND PULSE-RADIOLYSIS STUDIES ON OXIDATION OF ZN(BPY)3(2+), CD(BPY)3(2+), AND 2,2'-BIPYRIDINE

The electrochemical oxidation of Zn(bpy)32+, Cd(bpy)32+, and 2,2'-bipyridine (bpy) in liquid sulfur dioxide was studied by cyclic voltammetry (CV) and spectroelectrochemical techniques. Both Zn(bpy)32+ and Cd(bpy)32+ are oxidized in three closely spaced one-electron reversible steps. The standard potentials for these processes referenced vs the ferrocenium/ferrocene couple are as follows: Zn, 2.33 (E-degrees-3/2), 2.43 (E-degrees-4/3), and 2.52 V (E-degrees-5/4); Cd, 2.31 (E-degrees-3/2), 2.39 (E-degrees-4/3), and 2.47 V (E-degrees-5/4). Additional multielecton irreversible oxidations were also seen for both complexes at potentials closer to the anodic background limit. The reversible processes were attributed to consecutive oxidations of the three bpy ligands to corresponding radical cations. The blue-colored Zn(bpy)2(bpy.+)3+ was generated in a spectroelectrochemical cell and molar absorptivities (epsilon(391 nm) = 8790, epsilon(378 nm) = 4780, epsilon(762 nm) = 2570 M-1 cm-1) were determined using the generation/decay data. As opposed to the Zn(bpy)2(bpy.+)3+ spectrum, only one UV band centered around 340 nm was found in the transient spectrum of free bpy.+ (lifetime 1.2 mus) generated by electron pulse radiolysis of bpy in dichloromethane. The difference in properties is attributed to coordinated bpy.+ being a pi-type radical while the free bpy.+ is probably a sigma type radical. The Zn(bpy)2(bpy.+)3+ ion dimerizes with a rate constant, k2, of 70 M-1 s-1 at -70-degrees-C. The Cd(bpy)2(bpy.+)3+ ion was less stable.