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 is...