Effects of Position and Electronic Nature of Substituents on Cobalt-Porphyrin-Catalyzed Hydrogen Evolution Reaction

Abstract

Cobalt(II) porphyrins bearing ortho/para-amino and ortho-nitro groups at meso-phenyl rings have been prepared and employed for catalytic hydrogen generation. Electrochemical and catalytic studies show that position and electronic nature of the substituents strongly affect catalytic activity and overpotential of catalysis. Our study reveals that the complex with ortho-aminophenyl substituents on porphyrin core displays higher activity toward H-2 evolution with rate constant of 1.1 x 10(5) M-1 s(-1) at onset potential close to thermodynamic reduction potential of trifluoroacetic acid (TFA), and 89% efficiency. On the other hand, the reactions involving cobalt porphyrins with para-aminophenyl or ortho-nitrophenyl groups showed lower or no activity under the same experimental conditions, implying the significant role of pendant ortho-amino groups in accelerating the intramolecular proton transfer and the proton-hydride interactions to thermodynamically favor H-2 evolution.