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(η-iPrCHMe)(dhbq)(CoTPyP)][OTf] (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) and rate constants of overall homogeneous catalysis (k) determined from rotating ring-disk experiments (1.1 vs 0.05 h) 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.

We report a suite of coordination-driven self-assembled prisms for heterogeneous electrocatalytic oxygen reduction (ORR) differing in the molecular clips linking two porphyrin faces in a cofacial arrangement. ORR activities and selectivities of monomeric CoTPyP along with cofacial prisms Ox-Co, Oxa-Co, and Benzo-Co were probed using cyclic voltammetry and rotating ring-disk techniques. All species were immobilized as heterogeneous catalysts on glassy carbon electrodes using a Nafion ink method. The selectivities of Ox-Co, Oxa-Co, and Benzo-Co prisms towards H O as determined by RRDE were 87, 97, and 75 %, respectively. The current density of the Oxa-Co plateaus at five times that of Pt/C when normalized per Co/Pt. The high synthetic yield (79 %), competitive overpotential (η ≈800 mV) and high selectivity (%H O ≈97 %) of the Oxa-Co highlights how self-assembly can be used to address multi-electron multi-proton transformations using polynuclear catalysts.

We assembled eight cofacial porphyrin prisms using MTPyP (M = Co­(II) or Zn­(II), TPyP = 4-tetrapyridylporphyrin) and functionalized ruthenium-based “molecular clips” using coordination-driven self-assembly. Our approach allows for the rapid synthesis of these architectures in isolated yields as high as 98% for the assembly step. Structural and reactivity studies provided a deeper understanding of the role of the building blocks on the oxygen reduction reaction (ORR). Catalytic efficacy was probed by using cyclic and hydrodynamic voltammetry on heterogeneous catalyst inks in aqueous media. The reported prisms showed outstanding selectivity (>98%) for the kinetically hindered 4e–/4H+ reduction of O2 to H2O over the kinetically more accessible 2e–/2H+ reduction to H2O2. Furthermore, we have demonstrated significant cofacial enhancement in the observed catalytic rate constant k s (∼5 orders of magnitude) over the mononuclear analogue. We conclude that the steric bulk of the clip plays an important role in the structural dynamics of these prisms, which in turn modulates the ORR reactivity with respect to selectivity and kinetics.

Bodipy-based donor–acceptor dyads were evaluated using transient absorption spectroscopy to reveal the influence of beta vs. meso substitution on excited-state dynamics.