Tuning the Reactivity of Cofacial Porphyrin Prisms for Oxygen Reduction Using Modular Building Blocks

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 ks (∼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.