IR spectroelectrochemistry as efficient technique for elucidation of reduction mechanism of chlorine substituted 1,10-phenanthrolines

The electrochemical behaviour of 4,7-dichloro-1,10-phenanthrolines was studied in non-aqueous solution using cyclic voltammetry, controlled potential electrolysis, in-situ UV–Vis and IR spectroelectrochemistry, and HPLC-DAD (HPLC with diode array detector) and HPLC-MS/MS techniques. The substitution of phenanthrolines at position C2 and C5 with methyl groups, chlorine and/or fluorine atoms influences the redox properties and the potential gap between the oxidation and reduction potentials. Reduction leads to the formation of a radical anion and the subsequent cleavage of halide from the molecule. Compounds containing also chlorine or fluorine atoms at position C5 were found to be the most easily reduced. The oxidation of 4,7-dichloro-1,10-phenanthrolines proceeds primarily on the conjugated aromatic rings of phenanthroline forming a short living radical cation at the first oxidation wave and a dication at the second oxidation wave. The changes of the absorption spectra during IR spectroelectrochemistry suggested the formation of an unstable species, whose dimeric structure was hypothesized. Frontier molecular orbitals calculations and theoretical calculations of IR spectra support the findings.