Reduction of manganese(III) in tris(benzoyltrifluoroacetonato)Mn(III) in carbon tetrachloride in the presence of trioctylphosphine oxide

An addition of trioctylphosphine oxide (TOPO) enhanced the rate of back extraction of manganese in tris(benzoyltrifluoroacetonato)Mn(III) from carbon tetrachloride containing benzoyltrifluoroacetone (4,4,4-trifluoro-1-phenyl-1,3-butanedione, Hbfa) into acid aqueous solutions due to reduction of manganese(III) in this complex in the organic phase. It was found by spectrometry that manganese(III) in this complex in water- and air-saturated carbon tetrachloride was stable for a long time in the absence of TOPO but its reduction occurred in the presence of TOPO. The rate of reduction was first order with respect to the TOPO concentration and inverse first order with respect to the Hbfa concentration. This was explained as follows: An addition of TOPO to the metal ion in the complex promoted the dissociation of one of the bfa(-) ions in the complex and the leaving bfa(-) ion gave an electron to the manganese(III). On the other hand, a bfa(-) ion from the coexisting Hbfa in the solution should exchange with the bfa(-) in the Mn-III(bfa)(3) complex but it does not cause reduction of manganese(III) and this was assumed to compete with the addition of TOPO and to cause the inverse first order dependence of the rate of reduction on the Hbfa concentration.