Ligand Structural Effects on the Electrochemistry of Chromium(III) Amino Carboxylate Complexes

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Abstract
The aqueous electrochemical behavior of 10 Cr(III) complexes with potentially 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‘) in which the N atoms are coordinated cis to one another undergo chemically and electrochemically reversible reduction at ca. −1.4 and ca. −1.2 V vs SCE, respectively. However, complexes with a trans-N2O4 donor atom set, as exemplified by Cr(MIDA)2- (MIDA2- = N-methyliminodiacetate), undergo quasi-reversible CrIII/II reduction at ca. −1.4 V that is followed by a sequence of reactions which establishes an electrochemical square scheme. The chemical reactions in the scheme involve displacement of a bound carboxylate group following reduction to Cr(II) and its reattachment after reoxidation to Cr(III). This mechanistic sequence is analyzed by digital simulation, and values of formal potentials, transfer coefficients, and chemical and electrochemical rate constants are reported for Cr(MIDA)2- and its N-ethyl homolog. The difference in electrochemical behavior between cis- and trans-N2O4 complexes is attributed to differences in the Jahn−Teller distortions experienced by these structures upon reduction to Cr(II). It is proposed that simultaneous N−Cr−N bond elongation, which is possible only for trans species, leads to greater strain in the facially coordinated N-alkyliminodiacetate ligand and thus increases the barrier to electron transfer and facilitates Cr−carboxylate bond cleavage after reduction.
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