Preparation and Spectroscopic Characterization of Two Manganese(II)semiquinone Complexes

Summary.

The complexes [CTA][Mn(II)(SQ)₃] were isolated in the solid state and purified. SQ is the o-semiquinone of L-dopa or dopamine and CTA is the cetyltrimethylammonium cation. These complexes were characterized by Raman, infrared, EPR and thermogravimetry (TG) techniques. The EPR spectra of the solids presented an intense signal characteristic of the o-semiquinone radical anion with g=2.0062 and g=2.0063 for L-dopa and dopamine, respectively. Six characteristic lines around the organic radical signal confirm the presence of the Mn²⁺ ion. The most intense Raman bands were observed at $\bar{\nu}$ for dopamine and at 1356 cm⁻¹ for L-dopa and assigned to a C–O stretching with major C₁–C₂ character. The absence of an intense Raman band at ca. $\bar{\nu}=1480\,{\rm cm}^{-1}$, characterizes the ligands as an o-semiquinone radical anion. Broad bands in the $\bar{\nu}=400-750\,{\rm cm}^{-1}$ region can be assigned to deformations associated with the five-member ring chelate including the manganese ion, the oxygens, and the C₁–C₂ bonds. The more intense IR bands for the dopamine and the L-dopa-derived ligands at $\bar{\nu}=1233\ {\rm and} \ 1229\, {\rm cm}^{-1}$ are assigned to $\bar{\nu}$CO. Mass loss mechanisms for the two complexes, based on the TG results, were proposed and confirm the formula proposed.