Electrocatalytic Properties of Binuclear Cu(II) Fused Porphyrins for Hydrogen Evolution

Binuclear copper(II) porphyrins in which two copper(II) porphyrin macrocycles are doubly fused at the meso-beta positions are shown to be active electrocatalysts for the hydrogen evolution reaction (2H⁺ + 2e^- → H₂). Structural characterization, including use of electron paramagnetic resonance and X-ray photoelectron spectroscopies, verifies the fused species contains two copper(II) metal centers in its resting state. In comparison to the nonfused copper(II) porphyrin complex, the fused species is reduced at significantly less applied bias potentials (ΔE_1/2 ∼ 570 mV for the first reduction process). Electrochemical characterization in the presence of substrate protons confirms the production of hydrogen with near-unity Faradaic efficiency, and kinetic analysis shows the catalyst achieves a maximum turnover frequency above 2 000 000 s^-1. The enhancement in catalytic performance over analogous nonfused copper(II) porphyrins indicates extended macrocycles provide an advantageous structural motif and design element for preparing electrocatalysts that activate small molecules of consequence to renewable energy.