LIGAND STRUCTURAL EFFECTS ON THE ELECTROCHEMISTRY OF CHROMIUM(III) AMINO CARBOXYLATE COMPLEXES

The aqueous electrochemical behavior of 10 Cr(III) complexes with pote ntially tri- and hexadentate amino carboxylate ligands is reported and is shown to depend on the composition and spatial arrangement of the donor atom set, Complexes with two amine and four carboxylate donors ( N2O4) and two amine, one aquo, and three carboxylate donors (N2O3O') i n which the N atoms are coordinated cis to one another undergo chemica lly and electrochemically reversible reduction at ca, -1.4 and ca, -1. 2 V vs SCE, respectively. However, complexes with a trans-N2O4 donor a tom set, as exemplified by Cr(MIDA)(2)(-) (MIDA(2-) = N-methyliminodia cetate), undergo quasi-reversible Cr-III/II reduction at ca, -1.4 V th at is followed by a sequence of reactions which establishes an electro chemical square scheme. The chemical reactions in the scheme involve d isplacement of a bound carboxylate group following reduction to Cr(II) and its reattachment after reoxidation to Cr(III). This mechanistic s equence is analyzed by digital simulation, and values of formal potent ials, transfer coefficients, and chemical and electrochemical rate con stants are reported for Cr(MIDA)(2)(-) and its N-ethyl homolog, The di fference in electrochemical behavior between cis- and trans-N2O4 compl exes is attributed to differences in the Jahn-Teller distortions exper ienced by these structures upon reduction to Cr(II). It is proposed th at simultaneous N-Cr-N bond elongation, which is possible only for tra ns species, leads to greater strain in the facially coordinated N-alky liminodiacetate ligand and thus increases the barrier to electron tran sfer and facilitates Cr-carboxylate bond cleavage after reduction.