Exploring protonation sites with cation-responsive polyethylene glycol (PEG) tethers in [FeFe]-hydrogenase mimics

Abstract

Mimics of [FeFe]-hydrogenases having two [(μ-adt)Fe₂(CO)₆] moieties linked through 1,2,3-triazole rings with polyethylene glycol (PEG) chains [(–OCH₂CH₂O–)₄ (7) and (–OCH₂CH₂O–)₅ (8)] are able to coordinate to alkali ions (Na⁺, K⁺) via the O-PEG atoms and the triazole-N3 positions. Electrocatalytic studies in trifluoroacetic acid (TFA) demonstrate that their catalytic performance is affected by the presence of Na⁺ and K⁺ salts. The addition of NaPF₆ decreases the electrocatalytic activity of 7 and 8 (about 50% reduction of the TOF values). As, in TFA, 7 and 8 could be protonated in both the triazole and adt-amino groups, the reduction in TOF values suggests that NaPF₆ inhibits the contribution of the triazolium species to the electrocatalytic process, likely due to the involvement of the triazole-N3 positions in Na⁺ binding. However, the addition of KPF₆ either does not change (7) or increases the TOF values (8). ¹H NMR titration experiments demonstrate that, despite the presence of K⁺ ions in the media, triazolium salts are formed. Therefore, the TOF values should reflect the contribution of species protonated in both the triazole and adt-amino groups to the HER process.