Effect of co-ligands on photoredox pathways in Cr(III) oxalate complexes

The photochemical behaviour of some mixed ligand chromium(III) complexes with amino acids, [Cr(C2O4)2(Aa)]n− (where Aa = alanine, valine, serine, cysteine, asparagine, aspartic acid) was studied. The attention was focused on the photoredox mode, which proceeded via inner- or intramolecular pathway yielding Cr(II) species and hydrated electrons, respectively. The secondary thermal processes were dependent on the O2 presence and solution pH: (i) in oxygen-free media the regeneration of substrate and photoaquation induced by the Cr(III) → Cr(II) reduction were observed, (ii) in the presence of O2 both Cr(II) and ligands were oxidized and the former was transformed not only into Cr(III) but also to Cr(VI) (provided that pH > 7). Prolonged irradiation resulted in photoreduction of Cr(VI) accompanied by photodegradation of oxalate and/or amino-acid ligands. The photoreaction modes were independent of the co-ligand nature, but the secondary reaction rates and efficiencies were sensitive both to the co-ligand nature and its side substituent. Environmental consequences of the chromium photoreduction are discussed in the paper: the parameters affecting production and consumption of Cr(VI) are analysed, and the tools of controlling the photoredox behaviour of the Cr(III) and Cr(VI) compounds are suggested.