| In the present study, reactions of hydroxyl radical with 2,2'-bipyridine (bpy) and complexes of iron(II) and cobalt(III) containing 2,2'-bipyridine and/or cyanide as ligands have been investigated by pulse radiolysis and also by ab initio SCF MO theoretical techniques for 2,2'-bipyridine and pyridines. In the pulse radiolysis experiments, the nascent products of hydroxyl radical reactions with these compounds have been characterized through their spectral and kinetic properties. Similar studies have also been made for the corresponding reactions of e(,aq) and H atom. All these reactions occur at near diffusion controlled rates to give transient products having absorption in the ultraviolet, visible and, in some cases, near-IR region. Based on the spectral and kinetic results for the hydroxyl radical reactions, the primary reactions of OH are considered to take place by addition mechanisms in the cases of 2,2'-bipyridine, Fe(bpy)(,3) ('2+), Fe(DMbpy)(,3) ('2+) and Co(bpy)(,3) ('3+). With Fe(bpy)(,2)(CN)(,2) and Fe(bpy)(CN)(,4) ('2-), both addition and charge transfer processes occur. Subsequent to the primary reactions, the transients decay by two or more time-separated processes that generally obey first-order kinetics. For Co(bpy)(,3) ('3+), reduction of the metal centre is observed to give Co(aq)('2+). Free bipyridine and hydroxybipyridine species are detected as final products from the reactions of both Co(bpy)(,3) ('3+) and of Fe(bpy)(,3) ('2+). Iron(III) products are found for Fe(bpy)(,2)(CN)(,2) and Fe(bpy)(CN)(,4) ('2-) with no apparent loss of the ligands. The present study indicates that hydroxyl radical reactions with 2,2'-bipyridine can be considerably altered by complexation with metal ions such as iron(II) and cobalt(III), and the factors associated with this are discussed.; In the second part of this work, ab initio SCF MO calculations have been performed for the reactions of OH with pyridine, pyridinium ion and 2,2'-bipyridine. Based on the calculated total energies for the various hydroxy radical products, the relative stability of OH addition products are found to be for pyridine, meta-C > N >> para-C > ortho-C; for pyridinium ion, meta-C >> para-C > ortho-C > N, and for 2,2'-bipyridine, C(,5) > C(,6) > C(,3) > C(,4) > N. These are in general agreement with experimental results, as are the findings for the assumed reactions paths that indicate the greater reactivity of pyridine relative to pyridinium ion. The results also indicate that the extent of delocalization of the odd electron in the OH adducts plays a major role in site selection. |
