Low valence states of iron complexes of aliphatic and mixed aliphatic-aromatic diimine ligands

Abstract

Electrochemical reductions of iron(II) diimine complexes [FeL 3 2+, where L = CH 3NC(R)C(R′) NCH 3, aliphatic diimine series with R,R′ = H,H; H,CH 3 and CH 3,CH 3 and L = C 5H 4NC(R)N(R′), mixed diimine series with R,R′ = H,CH 3 and CH 3] were investigated through polarography and cyclic voltammetry in acetonitrile, with tetraethylammonium perchlorate supporting electrolyte (0.2 M) as a function of temperature. In the 0 to −2.4 V vs. Ag/AgCl potential range two to four polarographic waves were observed for the aliphatic series. The first two waves can be described as one-electron reversible reduction processes. They indicate that low valence states iron(I) and iron(0) are stabilized in acetonitrile. In the mixed ligand series three one-electron reversible reduction waves were observed, indicating that in addition to the low valence states stabilized in the aliphatic diimine series the formal reduction state Fe(–I) is also stabilized. The stabilization of the low oxidation states is due to the electron acceptor properties of the diimine ligands, inherent to the presence of the chromophoric iron diimine group. The half-wave potential data and the stabilization of the low valence states point to the importance of analyzing both σ and π effects. The molecular electronegativity values for the series of iron diimine complexes investigated evidences a synergistic interaction between the metal-ligand σ and π bonds. Diffusion coefficients, temperature effects on the heterogeneous electron transfer step, and electrocapillary curves were obtained for these complexes. No evidence for adsorption of the complexes on mercury electrodes was found for the one-electron reversible steps. When comparing polarographic data with those obtained on platinum disk working electrodes employed in the cyclic voltammetric experiments, we observed that for the symmetric aliphatic diimine ligands the observed cathodic currents are larger than expected on the basis of the previously calculated diffusion coefficients. In addition, the reduction waves are shifted 0.14 V to more negative potentials. The symmetric aliphatic diimine complexes exhibit adsorption of the electroactive species on the surface of the platinum electrodes in this potential range.