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

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 (N(2)O(4)) and two amine, one aquo, and three carboxylate donors (N(2)O(3)O') 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-N(2)O(4) donor atom set, as exemplified by Cr(MIDA)(2)(-) (MIDA(2)(-) = N-methyliminodiacetate), undergo quasi-reversible Cr(III/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-N(2)O(4) 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.

Investigation of the Spectroscopic, Thermal and Electrochemical Properties of Tris-(glycinato)chromium(III)

“…reported redox potentials of E 1 °′ =−1.35 to −1.46 V and E 2 °′ =−1.19 to −1.24 V for Cr(III) complexes with cis ‐N 2 O 4 and trans ‐N 2 O 4 coordination environments. These values are somewhat higher than the value for our complex, which has an N 3 O 3 coordination environment . Similarly, Ogino et al .…”

“…The solid fac ‐[Cr(gly) 3 ] complex consists of three bidentate glycine ligands that bind through amine and carboxylate donors and undergoes reduction at E 1/2 =−0.66 to −0.69 V at different scan rates, v (10 mV/s ≤ v ≤ 1000 mV/s). Previously, Hecht et al . reported redox potentials of E 1 °′ =−1.35 to −1.46 V and E 2 °′ =−1.19 to −1.24 V for Cr(III) complexes with cis ‐N 2 O 4 and trans ‐N 2 O 4 coordination environments.…”

“…Hecht et al. investigated the electrochemical behaviour of 10 cis ‐amino carboxylateCr(III) complexes and observed reversible one‐electron reductions at formal potentials between ca. −1.4 and ca.…”

Two Rate Constant Kinetic Model for the Chromium(III)–EDTA Complexation Reaction by Numerical Simulations

“…(top) at the highest experimental value of [Cr(III)] 0 (lowest ligand/metal ratio) and at [Cr(III)] 0 = 0 (infinite ligand/metal ratio), respectively. It was reported that the 1:1 complex still retains one water molecule in its coordination sphere , and the new data suggest that the 1:2 complex retains none. As a result, it can be concluded that successive replacement of Cr(III)–H 2 O bonds by Cr(III)–EDTA bonds leads to an increase of the electronic absorption at 540 nm.…”

Electrochemistry of flavins

“…For GMC (glucose-methanol-choline)-type oxidases (e.g. glucose oxidase, pyranose oxidase and cholesterol oxidase), a conserved histidine residue at the active site 56 is able to assist in substrate oxidation as well as flavin reoxidation by O 2 . 32 Mutagenesis studies on glucose oxidase has shown that protonation of this histidine is essential for catalysing O 2 reduction.…”