Electrochemistry of a double-decker lutetium(III) phthalocyanine in aqueous media. The first evidence for five reductions

The double-decker lutetium(III) phthalocyanine, Lu[(RS)₄Pc]₂, where R = C₁₂H₂₅, was synthesized and electrochemically characterized under several solution conditions. The neutral complex undergoes a single reversible one-electron oxidation and up to four reversible one-electron reductions in nonaqueous solutions of CH₂Cl₂ or benzonitrile containing 0.1 M tetra-n-butylammonium perchlorate (TBAP), while one oxidation and five reductions are seen in aqueous 0.5 M KCl for a gel-like cast film membrane of Lu[(RS)₄Pc]₂ in a tetraoctylphosphonium bromide (4C₈P⁺Br^-) matrix at a basal plane pyrolytic graphite electrode. The halfwave potentials for reduction of Lu[(RS)₄Pc]₂ in CH₂Cl₂, 0.1 M TBAP are located at E_1/2 = 0.04, -1.03, -1.36 and -1.73 V, while the single oxidation is located at E_1/2 = 0.43 V vs SCE. These potentials, when measured vs Fc/Fc⁺, are negatively shifted by 0.20-0.46 V from the E_1/2 values obtained in the 0.5 M KCl solution where the electrogenerated anions of the double-decker complex are stabilized by the cations of the 4C₈P⁺Br^- matrix and five reductions are seen at E_1/2 = 0.29, -0.66, -0.94, -1.22, and -1.41 V vs SCE. The fifth reduction has been theoretically predicted but had never before been observed for any doubledecker lutetium(III) phthalocyanine or related complex. A cast film of Lu[(RS)₄Pc]₂ displays only one oxidation in the 0.5 M KCl solution (at E_1/2 = 0.78 V vs SCE), and no reductions at all are seen in the absence of a matrix. An almost identical oxidation potential of E_1/2 = 0.80 V is seen for oxidation of Lu[(RS)₄Pc]₂ in KCl when dissolved in the cationic matrix. These half-wave potentials for oxidation in the cationic film, when measured vs Fc/Fc⁺, are thus more positive by more than 0.27-0.32 V than E_1/2 values measured in CH₂Cl₂ or PhCN and this is consistent with a decreased degree of solvent or anion binding to {Lu[(RS)₄Pc]₂}⁺ in the 4C₈P⁺Br^- matrix, as compared to what is observed in solution.