Synthesis and pH-sensitive redox properties of 1,10-phenanthroline-5,6-dione complexes

Synthesis and pH-sensitive electrochemical properties of three complexes, Cu(PD)₃] · (ClO₄)₂ · 2.25CH₃CN · 6H₂O (1), Cu(PD)(DMSO)Cl₂] · DMSO · H₂O (2) and Co(PD)₃] · (ClO₄)₂ · CH₃CN · 2H₂O (3) (PD=1,10-phenanthroline-5,6-dione), are reported. Single-crystal X-ray diffraction of the complexes suggest that the structure of 1 is orthorhombic, 2 triclinic and 3 orthorhombic. The electrochemical properties of free PD and the three complexes in phosphate buffer solutions in a pH range between 2 and 9 have been investigated using cyclic voltammetry. The redox potentials of these compounds are strongly dependent on the proton concentration in the range ?0.3 V ～ 0.4 V versus SCE (saturated calomel reference electrode). The reduction behavior of PD can be described from quinone species to semiquinone anion then to the fully reduced dianion. At pH < 4, the reduction of PD proceeds via 2e^?/3H⁺ processes, while at pH > 4, the reduction of PD proceeds via 2e^?/2H⁺. For all complexes, the N–N chelate PD ligand is electrochemically active and underwent step reduction via 2e^?/2H⁺.