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) 4 Pc] 2, where R = C 12 H 25, 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 CH 2 Cl 2 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) 4 Pc] 2 in a tetraoctylphosphonium bromide (4C 8 P + Br - ) matrix at a basal plane pyrolytic graphite electrode. The half-wave potentials for reduction of Lu[(RS) 4 Pc] 2 in CH 2 Cl 2, 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 4C 8 P + 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 double-decker lutetium(III) phthalocyanine or related complex. A cast film of Lu[(RS) 4 Pc] 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) 4 Pc] 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 CH 2 Cl 2 or PhCN and this is consistent with a decreased degree of solvent or anion binding to {Lu[(RS) 4 Pc] 2 } + in the 4C 8 P + Br - matrix, as compared to what is observed in solution.