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 metal phthalocyanines (Pc) have been carried out in organic solvents, and reduction of their tetrasulfonated derivatives (TSPc) in aqueous solutions. Co^IIPc and Fe^IIPc are readily reduced to [Co^IPc]^- and [Fe^IPc]^-, which do not react with CO₂. Reduction of [Co^IPc]^- yields a product which is characterized as the radical anion, [Co^IPc^·-]^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 CO₂. Catalytic formation of CO and formate is confirmed by photochemical experiments in DMF and acetonitrile solutions containing triehylamine (TEA) as a reductive quencher. The photochemical yields are greatly enhanced by the addition of p-terphenyl (TP). The radical anion. TP^·-, formed from the reductive quenching of the singlet excited state with TEA, reduces the phthalocyanines very rapidly. The rate constants for reduction of Co^IIPc, [Co^IPc]^-, and [Fe^IPc]^- by TP^·-, determined by pulse radiolysis in DMF solutions, are nearly diffusion-controlled. The mono-reduced species formed from [Co^IPc]^- is unstable under the pulse radiolysis conditions but is longer-lived under the flash photolysis conditions. The interaction of this species with CO₂ is either too weak or too slow to detect in the current experiments, where a competing reaction with protons predominates.