Viologen-Radical-Driven Hydrogen Evolution from Water Catalyzed by Co-NHC Catalysts: Radical Scavenging by Nitrate and Volmer-Heyrovsky-like CPET Pathway

The factors controlling the catalytic activity in photochemical hydrogen evolution reaction (HER) are studied in detail for two macrocyclic cobalt compounds bearing two N-heterocyclic carbenes and two pyridyl donors (Co-NHC1 and Co-NHC2, where Co-NHC2 has a methoxy substituent on each pyridyl ligand). The present study adopts an aqueous photosystem consisting of EDTA, [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine), and MV²⁺ (MV²⁺ = methylviologen) at pH = 5. Both catalysts are shown to promote HER in a similar efficiency (TON = 12–13 in 6 h), revealing a minor contribution of the electron-donating methoxy substituents. The catalyst degradation is shown to proceed during the photocatalysis, leading to afford [Co(edta)]⁻ (EDTA = H₄edta) as a dead-end species. The lack of any heterogeneous species was evidenced by DLS (dynamic light scattering). It was also found that nitrate involved as a counteranion in the photocatalysis components substantially inhibits the photocatalytic HER, giving rise to a large diminishment in TON from 12.7 to 7.2. The Griess test was used to confirm that NO₃^– serves as a scavenger deactivating the reduced form of MV²⁺ (i.e., MV⁺·). The detailed spectroscopic study reveals that the radical dimer (MV⁺·)₂ plays a key role in promoting the one-step two-electron process: (MV⁺·)₂ + NO₃^– + 2H⁺ → 2MV²⁺ + NO₂^– + H₂O. Experimental and DFT results also reveal that a unique double CPET (concerted proton–electron transfer) pathway is taken to evolve H₂ by the Co-NHC catalysts with substantially minimized reorganization energies: Co(II)-NHC −→−CPET$\stackrel{\mathrm{CPET}}{\to }$$\stackrel{\mathrm{CPET}}{\to }$ Co(III)(H)-NHC −→−CPET$\stackrel{\mathrm{CPET}}{\to }$$\stackrel{\mathrm{CPET}}{\to }$ Co(II)-NHC + H₂. This pathway can be viewed as related to the so-called Volmer-Heyrovsky mechanism adopted by some metals and is quite unique to the Co-NHC catalysts.