Preparation and the pH dependent reversible color change of complexes of cobalt(III) and chromium(III) with a hexafluoroacetylacetonato ligand

Abstract

Thirteen complexes of Co(III) and Cr(III) with a hexafluoroacetylacetonato (hfac) ligand were newly prepared as perchlorate salts. These are CoL(hfac) 2+ with L=(tn) 2, tren, β-2,3,2-tet, β-3,2,3-tet, (EtNH 2) 4, (n-PrNH 2) 4, (n-BuNH 2) 4, (bpy) 2, (phen) 2; and CrL(hfac) 2+ with L(NH 3) 4, (tn) 2, α-2,3,2 -tet, 3,2,3-tet. In these complexes the ligand L occupies four coordination sites and the hfac ligand acts as a bidentate. Aqueous or methanolic solutions of these complexes show a pH dependent rapid and reversible color change; orange in an acid and violet in a base. The origin of this chromism may be a covalent hydration equilibrium, where a lyate anion forms a σ bond with one of the carbonyl carbon atoms of the coordinating hfac ligand. The equilibrium constant in water is 2∼12×10 6 M −1 for Co(III) complexes and 1∼6×10 10 M −1 for Cr(III) complexes (M=mol dm −3).