Gram-Scale, One-Pot Synthesis of a Cofacial Porphyrin Bridged by Ortho-xylene as a Scaffold for Dinuclear...

Herein, we report the reaction between four 1,2-dibromoxylenes and two tetra-3-pyridylporphyrins for the formation of a cofacial porphyrin core spanned by dipyridinium xylene moieties. The metal-free organic nanocage (oNC) was synthesized in one twenty-four h step at a gram-scale with a 91.5% yield. The free base oNC was subsequently metalated with cobalt(II) (Co-oNC), copper(II) (Cu-oNC), and nickel(II) (Ni-oNC) ions to furnish dinuclear complexes that were characterized by mix of mass spectrometry, NMR, EPR, electronic absorption spectroscopy, and for Co-oNC, single-crystal X-ray diffraction. Cofacial cobalt porphyrins are often active as catalysts for the Oxygen Reduction Reaction. Under heterogeneous conditions in water, Co-oNC was 83% selective for the electrocatalytic 4 e^-/4 H⁺ reduction of O₂ to H₂O, matching homogeneous experiments which revealed consistent selectivity for H₂O (88%). This oNC core offers significant advantages over prisms formed by coordination-driven self-assembly: the dipyridnium-xylene coupling can furnish over 1 g of material in a single synthesis and the tethering motif is robust, maintaining a cofacial architecture in acidic and basic solutions. We envision this approach may be generalized to other bis-bromobenzyl building blocks, providing a means to tune metal-metal separation and other structural and electronic properties.