EPR and UV-visible spectroscopic studies of copper(II) and cobalt(II) complexes of hydroxyanthraquinones

Interactions of 1-hydroxyanthraquinone (1-HAQ) and 1,4- and 1,8-dihydroxyanthraquinone (1,4-DHAQ, 1,8-DHAQ) with Co²⁺ and Cu²⁺ have been examined by UV-visible and EPR spectroscopies. These hydroxyanthraquinones form high-spin complexes with Co²⁺. It has been estimated that these Co²⁺ complexes in dichloromethane and ethanol have a tetragonal coordination structure, while in dimethlyformamide (DMF) or in mixed solvents containing DMF the complex having a tetrahedral structure is simultaneously formed. 1-HAQ forms the ML and ML₂ type complexes with Cu²⁺, their relative concentration depending on the ligand to metal mole ratio, R, in solution. 1,4-DHAQ forms also the ML₂ type complex with Cu²⁺, but the ML-type mononuclear Cu²⁺ complex was not directly detected by EPR, though 1,4-DHAQ was estimated to form a polymer-type complex with Cu²⁺ having metal-ligand ratio of ~ 1:1. For the case of 1,8-DHAQ, the formation of S = 1 M₂L₂-type dinuclear complex was observed, in addition to the formation of ML- and ML₂-type complexes. The ML₂-type Cu²⁺ complexes of 1,4- and 1,8-DHAQ were found to have two forms, the cis and trans configurations. These properties for metal complex formation of the hydroxyanthraquinones are compared with those of adriamycin, which has a similar anthraquinone structure in the molecule.