Lowering the Symmetry of Cofacial Porphyrin Prisms for Selective Oxygen Reduction Electrocatalysis

Cofacial porphyrin catalysts for the Oxygen Reduction Reaction (ORR) formed via coordination-driven self-assembly have so far been limited to designs with fourfold symmetry, where four molecular clips bridge two porphyrin sites. We have synthesized six PyPh (Py = pyridyl, Ph = phenyl) metalloporphyrin prisms (Co²⁺, Zn²⁺) bridged by molecular clips containing two Rh³⁺ centers. Four of these structures are lower symmetry, with the Py₃Ph and Py₂Ph₂ prisms containing three and two molecular clips, respectively. The Co²⁺ species were evaluated for their ORR activity. Cyclic and hydrodynamic voltammetry studies of heterogeneous catalyst inks in aqueous media revealed marked differences in selectivity from ∼5% (Py₃Ph) to ∼37% (Py₂Ph₂) for the formation of H₂O₂. The single-crystal X-ray structure of the Zn₂ Py₂Ph₂ prism shows an offset between the porphyrin faces. This structural feature may be responsible for the change in selectivity, consistent with previous studies of covalently tethered cofacial porphyrins that have shown that geometry is a critical determinant of two-electron/two-proton versus four-electron/four-proton pathways. Extraction of standard rate constants k_s for the ORR revealed a cofacial enhancement of ∼2 orders of magnitude over mononuclear Co²⁺ tetrapyridyl porphyrin. Even though all the prisms described here use the same molecular clip, the resultant structures, and thus the reactivity for the ORR, differ significantly based on the number and orientation of pyridyl donor groups on the porphyrins, highlighting how coordination-driven self-assembly can be used to rapidly tune dinuclear catalysts.