Approaches to the aqueous chemistry of chromium (IV) and chromium (V) in macrocyclic and Schiff base complexes

Abstract

The stabilization of chromium(IV) and chromium(V) in aqueous systems has now been made through designed alterations in Franck-Condon barriers for the interconversion of various oxidation states of chromium. Equatorially 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 long life time can be generated. A series of macrocyclic and Schiff base complexes of Cr(III) has been synthesised and subjected to non-complementary redox reaction with Ce(IV) or electrochemical oxidation. A Cr(IV) macrocyclic complex, Cr(Me4[14]tetraene)(H2O) 2 4+ , 1 has been generated in aqueous acidic medium, through electrochemical oxidation. The Ce(IV) oxidation of Cr(III) Schiff base complexes, diaqua[N,N′-ethylenebis(salicylideneiminato) chromium(III)], and diaqua[N,N′-propylenebis(salicylideneiminato) chromium(III)] proceed in two stages with 1 Cr:l Ce(IV) redox stoichiometry. Suffīciently long-lived Cr(IV) intermediates and Cr(V) Schiff base products have been generated. ESR evidence for the formation of Cr(V) Schiff base products and the mechanistic outcome of the oxidation of Cr(III) Schiff bases are discussed.