APPROACHES TO THE AQUEOUS CHEMISTRY OF CHROMIUM(IV) AND CHROMIUM(V) IN MACROCYCLIC AND SCHIFF-BASE COMPLEXES

The stabilization of chromium(IV) and chromium(V) in aqueous systems h as now been made through designed alteration in Franck-Condon barriers for the interconversion of various oxidation states of chromium. Equa torially coordinated macrocyclic and Schiff base ligands influence the reorganizational barriers for the conversion of Cr(III)-Cr(IV)-Cr(V)- Cr(VI) couples such that Cr(IV) and Cr(V) transients of sufficiently l ong life time can be generated. A series of macrocyclic and Schiff bas e complexes of Cr(III) has been synthesised and subjected to non-compl ementary redox reaction with Ce(IV) or electrochemical oxidation. A Cr (IV) macrocyclic complex, Cr(Me4[14]tetraene)(H2 O)2(4+), 1 has been g enerated in aqueous acidic medium, through electrochemical oxidation. The Ce(IV) oxidation of Cr(III) Schiff base complexes, diaqua[N,N'-eth ylenebis(salicylideneiminato) chromium(III)], and diaqua[N,N'-propylen ebis(salicylideneiminato) chromium(III)] proceed in two stages with 1 Cr:1 Ce(IV) redox stoichiometry. Sufficiently long-lived Cr(IV) interm ediates and Cr(V) Schiff base products have been generated. ESR eviden ce for the formation of Cr(V) Schiff base products and the mechanistic outcome of the oxidation of Cr(III) Schiff bases are discussed.