Reduction of cobalt and iron phthalocyanines and the role of the reduced species in catalyzed photoreduction of CO2

The role of cobalt and iron phthalocyanines in catalytic CO2 reduction has been studied. Chemical, photochemical, and radiolytic reductions of the met al phthalocyanines (Pc) have been carried out in organic solvents, and redu ction of their tetrasulfonated derivatives (TSPc) in aqueous solutions. (Co Pc)-Pc-II and (FePc)-Pc-II are readily reduced to [(CoPc)-Pc-I](-) and [F-I 'Pc](-), which do not react with CO2. Reduction of [(CoPc)-Pc-I](-) yields a product which is characterized as the radical anion, [(CoPc.-)-Pc-I](2-), on the basis of its absorption spectra in the visible and IR regions. This species is stable under dry anaerobic conditions and reacts rapidly with C O2. Catalytic formation of CO and formate is confirmed by photochemical exp eriments in DMF and acetonitrile solutions containing triethylamine (TEA) a s a reductive quencher. The photochemical yields are greatly enhanced by th e addition of p-terphenyl (TP). The radical anion, TP.-, formed from the re ductive quenching of the singlet excited state with TEA, reduces the phthal ocyanines very rapidly. The rate constants for reduction of (CoPc)-Pc-II, [ (CoPc)-Pc-I](-), and [(FePc)-Pc-I](-) by TP.-, determined by pulse radiolys is in DMF solutions, are nearly diffusion-controlled. The mono-reduced spec ies formed from [(CoPc)-Pc-I](-) is unstable under the pulse radiolysis con ditions but is longer-lived under the flash photolysis conditions. The inte raction of this species with CO2 is either too weak or too slow to detect i n the current experiments, where a competing reaction with protons predomin ates.