Redox Potentials of Chromium(V)/(IV), -(V)/(III), and -(IV)/(III) Complexes with 2-Ethyl-2-hydroxybutanoato(2−/1−) Ligands

The formal reduction potentials of [CrV/IV(O)L2]-/2- and [CrV(O)L2]-/[CrIV(O)L(LH)]- (L = ehba = 2-ethyl-2-hydroxybutanoato(2−)) are 0.44 and 0.65 V, respectively, from cyclic voltammetric measurements. Potentiometric titrations using the [Fe(CN)6]4- reduction of [CrV(O)L2]- yielded a formal potential of 0.84 V for the [CrV(O)L2]-/[CrIIIL2(H2O)2]- redox couple. By using the values of CrV/IV and CrV/III couples, formal potentials for the [CrIV(O)L2]2-/[CrIIIL2(H2O)2]- and [CrIV(O)L(LH)]-/[CrIIIL2(H2O)2]- couples were calculated to be 1.24 and 1.03 V, respectively. Most of these potential data differ markedly from those estimated by Ghosh and Gould (J. Am. Chem. Soc. 1993, 115, 3167−3173; J. Chem. Soc., Chem. Commun. 1992, 195−196) for the same complexes. The spectra of the Cr(IV) complexes were also examined by reduction of [Cr(O)L2]- with pulse radiolysis. The structures of the Cr(IV) complexes, [Cr(O)L2]2-, [Cr(O)(L)(LH)]-, and [Cr(O)(LH)2], have been assigned on the basis of X-ray crystallography for isostructural V(IV) complexes. Finally, the equilibria among the Cr(V) or Cr(IV) complexes with excess ligand appear to be due to complexes involving one or two ehba ligands per chromium, as established by the isolation and characterization of the Cr(V) dimer, {[Cr(O)2(ehba)]2}2-. These new redox potentials, along with the reinterpretation of literature data, will aid in understanding the redox chemistry involved in Cr(VI/V) oxidations of organic substrates and Cr(VI)-induced cancers.