A Self-Assembled Cofacial Cobalt Porphyrin Prism for Oxygen Reduction Catalysis

Herein we report the first study of the oxygen reduction reaction (ORR) catalyzed by a cofacial porphyrin scaffold accessed in high yield (overall 53%) using coordination-driven self-assembly with no chromatographic purification steps. The ORR activity was investigated using chemical and electrochemical techniques on monomeric cobalt(II) tetra(meso-4-pyridyl)-porphyrinate (CoTPyP) and its cofacial analogue [Ru-8(eta(6)-iPrC(6)H(4)Me)(8)(dhbq)(4)(CoTPyP)(2)][OTf](8) (Co Prism) (dhbq = 2,5-dihydroxy-1,4-benzoquinato, OTf = triflate) as homogeneous oxygen reduction catalysts. Co Prism is obtained in one self-assembly step that organizes six total building blocks, two CoTPyP units and four arene-Ru clips, into a cofacial motif previously demonstrated with free-base, Zn(II), and Ni(II) porphyrins. Turnover frequencies (TOFs) from chemical reduction (66 vs 6 h(-1)) and rate constants of overall homogeneous catalysis (kappa(obs)) determined from rotating ring disk experiments (1.1 vs 0.05 h(-1)) establish a cofacial enhancement upon comparison of the activities of Co Prism and CoTPyP, respectively. Cyclic voltammetry was used to initially probe the electrochemical catalytic behavior. Rotating ring disk electrode studies were completed to probe the Faradaic efficiency and obtain an estimate of the rate constant associated with the ORR.