Synthesis of Electrocatalytic [2Fe-2S]-Metallopolymers and Organic/Inorganic Hybrid Materials

Abstract

This dissertation is composed of five chapters, detailing advances within the area of synthesis, characterization, and electrochemical analysis of polymer-supported [FeFe]-hydrogenase enzyme’s active site mimics, highlights the releavance of the work to the fields of polymer science, electrochemistry, and energy storage that are enabled through the development of novel [2Fe-2S] complexes and metallopolymers as discussed in the following chapters. The first chapter is a review summarizing the development of novel polymer-supported [2Fe-2S] catalyst systems and briefly highlighting non-polymeric supports for electrocatalytic and photocatalytic hydrogen evolution reactions. [FeFe]-hydrogenases are the best known naturally occurring metalloenzymes for hydrogen generation, and small molecule [2Fe-2S]-containing mimetics of the active site (H-cluster) of these metalloenzymes have been synthesized for years. These small [2Fe-2S] complexes have not reached the same capacity of the enzymes for hydrogen production thus far. Recently, modern polymer chemistry has been utilized to construct an outer coordination sphere around the [2Fe-2S] clusters to provide site isolation, water solubility, and improved catalytic activity. In this chapter, we survey the various macromolecular motifs and the catalytic properties of these supported [2Fe-2S] materials. The most recent catalysts that incorporate a single [2Fe-2S] complex, termed single-site [2Fe-2S] metallopolymers, exhibit superior activity for H2 production. The second chapter focuses on incorporation of a [2Fe-2S] catalytic site into metallopolymers using atom transfer radical polymerization (ATRP). Electrocatalytic [FeFe]-hydrogenase mimics for the hydrogen evolution reaction (HER) generally suffer from low activity, high overpotential, aggregation, oxygen sensitivity, and low solubility in water. Using ATRP, we have prepared a new class of [FeFe]-metallopolymers with precise molar mass, composition, and low polydispersity. The synthetic methodology introduced here allows for facile variation of polymer composition to optimize the [FeFe] solubility, activity, and long-term chemical and aerobic stability. We find that water-soluble functional metallopolymers perform electrocatalytic hydrogen production in neutral water with loadings as low as 2 ppm and operate at rates one order of magnitude faster than hydrogenases (ca. 250,000 s-1) and with low overpotential requirement. Furthermore, unlike the hydrogenases, these systems are insensitive to oxygen during catalysis with turnover numbers on the order of 40,000 under both anaerobic and aerobic conditions. The third chapter expands upon the methodology for the tunability of [2Fe-2S] metallopolymers by changing the monomer type and their compositions. 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 ATRP. This study illustrates the critical role of the polymer composition on enhancing hydrogen evolution and aerobic stability by comparing the catalytic activity of PDMAEMA-g-[2Fe-2S] with a non-ionic 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/cm2, while POEGMA-g-[2Fe-2S] fails to reach 1.0 mA/cm2 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. The fourth chapter focuses on another [2Fe-2S] metalloinitiator with an aliphatic bridgehead and single initiator site for polymerization. Small molecule biomimetics inspired by the active site of the [FeFe]-hydrogenase enzymes have shown promising electrocatalytic activity for hydrogen (H2) generation. However, most of the active site mimics based on [2Fe-2S] clusters are not water soluble which hence limits the use of these electrocatalysts to organic media. Polymer-supported [2Fe-2S] systems, in particular, single-site metallopolymer catalysts, have shown drastic improvements for electrocatalytic H2 generation in aqueous milieu. [2Fe-2S] complexes functionalized within well-defined macromolecular supports via covalent bonding have demonstrated water solubility, enhanced site-isolation, and improved chemical stability during catalysis. In this chapter, we demonstrate the synthesis of a new propanedithiolate (pdt)-[2Fe-2S] complex bearing a single α-bromoester moiety for the use in ATRP as a novel metalloinitiator to prepare water-soluble poly(2-(dimethylamino)ethyl methacrylate) grafted (PDMAEMA-g-[2Fe-2S]) metallopolymers. Using this approach, metallopolymers with controllable molecular weights (Mn = 5-40 kg/mol) and low polydispersity (Mw/Mn = 1.09-1.36) were prepared, which allowed for the first-time observation of the effect of the metallopolymers' chain length on the electrocatalytic activity. The ability to control the composition and molecular weight of these metallopolymers enabled macromolecular engineering via ATRP of these materials to determine optimal structural features of metallopolymer catalysts for H2 production. The fifth chapter starts with the comparison between free-radical and controlled radical polymerizations and then details the synthesis and characterization of a new [2Fe-2S] complex for polymer-growth via reversible addition-fragmentation chain-transfer (RAFT) polymerization. Although ATRP allows polymerization of many commecially available monomers, RAFT polymerization can give easy access to direct polymerization of different monomers such as (meth)acrylic acid, 2-carboxyethyl acrylate, acryl amides, 1-glycerol methacrylate and eliminate any metal contamination in contrast to Cu-cataylzed polymerization. A new [2Fe-2S]-based RAFT metallo-agent was synthesized for the first-time to design metallopolymers for electrocatalytic hydrogen evolution reaction (HER). In this chapter, [2Fe-2S]-metallopolymers synthesized via RAFT polymerization and studied for electrochemical H2 evolution are discussed.