REDOX POTENTIALS OF CHROMIUM(V) (IV), CHROMIUM(V)/(III), AND CHROMIUM(IV)/(III) COMPLEXES WITH 2-ETHYL-2-HYDROXYBUTANOATO(2-/1-) LIGANDS/

The formal reduction potentials of [Cr-V/IV(O)L(2)](-/2-) and [Cr-V(O) L(2)](-)/[Cr-IV(O)LH)](-) (L = ehba = 2-ethyl-2-hydroxybutanoato(2-)) are 0.44 and 0.65 V, respectively, from cyclic voltammetric measuremen ts. Potentiometric titrations using the [Fe(CN)(6)](4-) reduction of [ Cr-V(O)L(2)](-) yielded a formal potential of 0.84 V for the [Cr-V(O)L (2)](-)/[Cr(III)L(2)(H2O)(2)](-) redox couple. By using the values of Cr-V/IV and Cr-V/III couples, formal potentials for the [Cr-IV(O)L(2)] (2-)/[Cr(III)L(2)(H2O)(2)](-) and [Cr-IV(O)L(LH)](-)/[Cr(III)L(2)(H2O) (2)]- couples were calculated to be 1.24 and 1.03 V, respectively. Mos t of these potential data differ markedly from those estimated by Ghos h and Gould (J. Am. Chem. Sec. 1993, 115, 3167-3173; J. Chem. Sec., Ch em. Commun. 1992, 195-196) for the same complexes. The spectra of the Cr(TV) complexes were also examined by reduction of [Cr(O)L(2)](-) wit h pulse radiolysis. The structures of the Cr(IV) complexes, [Cr(O)L(2) ](2-), [Cr(O)(L)(LH)](-), and [Cr(O)(LH)(2)], have been assigned on th e basis of X-ray crystallography for isostructural V(IV) complexes. Fi nally, 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 potential s, along with the reinterpretation of literature data, will aid in und erstanding the redox chemistry involved in Cr(VI/V) oxidations of orga nic substrates and Cr(VI)-induced cancers.