Electrochemistry of the quasi-reversible bis[2-ethyl-2-hydroxybutanoato(2–)]-oxochromate-(V) and -(IV) and bis[2-hydroxy-2-methylbutanoato(2–)]oxochromate-(V) and -(IV) redox couples and the crystal and molecular structure of sodium bis[2-ethyl-2-hydroxybutanoato(2–)]oxochromate(V)sesquihydrate

Abstract

The Cr^V–Cr^IV redox couples of the complexes [Cr(ehba)₂O]^1–/2–[ehba = 2-ethyl-2- hydroxy-butanoate(2–)] and [Cr(hmba)₂O]^1–/2;–[hmba = 2-hydroxy-2-methylbutanoate(2–)]— exhibit quasi-reversible redox behaviour in both dimethyl sulphoxide (dmso) and dichloromethane. Solutions of the chromium(v) complex in dmso are stable for a year or more at room temperature, if they are stored in the dark. The chromium(IV) complex is much more reactive, but still has a lifetime on the minutes time-scale in dmso or CH₂Cl₂ under ambient conditions. The X-ray structure of Na[Cr(ehba)₂O]·l.5H₂O has been determined. It crystallises in the monoclinic space group C2/c, a= 23.733(6), b= 12.424(3), c= 19.678(6)Å, β= 108.30(3)°, and Z= 12. The chromium complex has a distorted trigonal bipyramid of oxygen donor atoms, with the two carboxylate oxygens occupying the axial sites. The crystallographic analysis indicates that the reasons for the unusual stability of this complex and the reversibility of the Cr^V–Cr^IV redox couple lie, at least partially, in the steric protection that is caused by the ethyl substituents. Another important factor that leads to the high chemical stability of Cr^V in aprotic solvents, such as dmso and CH₂Cl₂, is that it is a weak oxidant in these solvents.