Macromolecular Engineering of the Outer Coordination Sphere of [2Fe-2S] Metallopolymers to Enhance Catalytic Activity for H2 Production

Small-molecule catalysts inspired by the active sites of [FeFe]-hydrogenase enzymes have long struggled to achieve fast rates of hydrogen evolution, long-term stability, water solubility, and oxygen compatibility. We profoundly improved on these deficiencies by grafting polymers from a metalloinitiator containing a [2Fe-2S] moiety to form water-soluble poly­(2-dimethylamino)­ethyl methacrylate metallopolymers (PDMAEMA-g-[2Fe-2S]) using atom transfer radical polymerization (ATRP). This study illustrates the critical role of the polymer composition in enhancing hydrogen evolution and aerobic stability by comparing the catalytic activity of PDMAEMA-g-[2Fe-2S] with a nonionic water-soluble metallopolymer based on poly­(oligo­(ethylene glycol) methacrylate) prepared via ATRP (POEGMA-g-[2Fe-2S]) with the same [2Fe-2S] metalloinitiator. Additionally, the tunability of catalyst activity is demonstrated by the synthesis of metallocopolymers incorporating the 2-(dimethylamino)­ethyl methacrylate (DMAEMA) and oligo­(ethylene glycol) methacrylate (OEGMA) monomers. Electrochemical investigations into these metallo­(co)­polymers show that PDMAEMA-g-[2Fe-2S] retains complete aerobic stability with catalytic current densities in excess of 20 mA·cm–2, while POEGMA-g-[2Fe-2S] fails to reach 1 mA·cm–2 current density even with the application of high overpotentials (η > 0.8 V) and loses all activity in the presence of oxygen. Random copolymers of the two monomers polymerized with the same [2Fe-2S] initiator showed intermediate activity in terms of current density, overpotential, and aerobic stability.