PHOTOOXIDATION OF TRIS(2,2'-BIPYRIDINE)-IRON(II) COMPLEX BY PERIODATEIN AQUEOUS-SOLUTION

Thermal and photochemical oxidations of Fe(bpy)32+ (bpy = 2,2'-bipyrid ine) by the periodate anion IO4- in acidic aqueous solution were studi ed. For the thermal process, the rate-limiting step is monomolecular d issociation of the iron(II) complex preceding the fast oxidation by IO 4-. Photo-oxidation of Fe(bpy)32+ by IO4- is reported for the first ti me. The dependence of apparent quantum yield upon IO4- concentration f avours a static mechanism, which includes outer-sphere complex [Fe(bpy )32+...IO4-] formation. The complex formation has an equilibrium const ant 38 +/- 11 mol-1 dm3 and is accompanied by an approximate 1 nm bath ochromic shift of the metal-to-ligand charge transfer band of Fe(bpy)3 2+. The complex [Fe(bpy)32+...IO4-] is believed to undergo irreversibl e intramolecular electron transfer upon excitation with the efficiency (0.45 +/- 0.1)%. In contrast with IO4-, the persulphate anion (S2O82- ) does not show any activity in the photo-oxidation of Fe(bpy)32+. Thi s surprising specificity is explained in terms of the difference betwe en the sizes and geometries of the oxidizing anions; the small IO4 pen etrates between bipyridine ligands and reaches the internal d-orbitals of Fe(bpy)32+ from where the electron is transferred, but the bulkier S2O82- has no access to the Fe d-shell.