Exploring Protonation Sites with Cation-Responsive Polyethylene glycol (PEG) Tethers in [FeFe]-Hydrogenase Mimics

Abstract

imics 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.