Structure of sodium bis(N-methyl-iminodiacetato)iron(iii): trans-meridional N-coordination in the solid state and in solution

Abstract

The results of a detailed solid state and solution structural study of the Fe^III bis-mida complex [Fe^III(mida)₂]⁻ (mida = N-methyl-iminodiacetate) are reported. The structure of the sodium salt Na[Fe(mida)₂][NaClO₄]₂·3H₂O (1) was determined by single-crystal X-ray analysis. The complex anion in 1 contains a six-coordinate Fe^III centre bound to two tridentate mida ligands arranged in the meridional configuration, and the mer Fe^IIIN₂O₄ chromophore shows a high degree of distortion from regular octahedral symmetry. Raman- and UV/VIS/NIR spectroscopic measurements showed that no gross changes take place in the Fe^III coordination sphere upon redissolution in water. Quantum chemical calculations of all three possible configurations of the [Fe(mida)₂]⁻ complex ion in the gas phase support the finding that the mer isomer is more stable than the u-fac (cis) and s-fac (trans) isomers. Redox potential measurements of the Fe^III/II(mida) couple in dependence of pH led to the following values for the equilibrium contants: log β^III₁₀₁ = 11.98 ± 0.05, log β^III₁₀₂ = 20.49 ± 0.01, pK^III_{a1 OH} = 7.81; log β^II₁₀₁ = 6.17 ± 0.01, log β^II₁₀₂ = 11.39 ± 0.01.