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We report the use of polymer N -heterocyclic carbenes (NHCs) to control the microenvironment surrounding metal nanocatalysts, thereby enhancing their catalytic performance in CO 2 electroreduction. Three polymer NHC ligands were designed with different hydrophobicity: hydrophilic poly(ethylene oxide) (PEO–NHC), hydrophobic polystyrene (PS–NHC), and amphiphilic block copolymer (BCP) (PEO- b -PS–NHC). All three polymer NHCs exhibited enhanced reactivity of gold nanoparticles (AuNPs) during CO 2 electroreduction by suppressing proton reduction. Notably, the incorporation of hydrophobic PS segments in both PS–NHC and PEO- b -PS–NHC led to a twofold increase in the partial current density for CO formation, as compared to the hydrophilic PEO–NHC. While polymer ligands did not hinder ion diffusion, their hydrophobicity altered the localized hydrogen bonding structures of water. This was confirmed experimentally and theoretically through attenuated total reflectance surface-enhanced infrared absorption spectroscopy and molecular dynamics simulation, demonstrating improved CO 2 diffusion and subsequent reduction in the presence of hydrophobic polymers. Furthermore, NHCs exhibited reasonable stability under reductive conditions, preserving the structural integrity of AuNPs, unlike thiol-ended polymers. The combination of NHC binding motifs with hydrophobic polymers provides valuable insights into controlling the microenvironment of metal nanocatalysts, offering a bioinspired strategy for the design of artificial metalloenzymes.
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Do polymer ligands block the catalysis of metal nanoparticles? Unexpected importance of binding motifs in improving catalytic activity
Metal nanoparticles (NPs) tethered by synthetic polymers are of broad interest for self-assembly, nanomedicine and catalysis. The binding motifs in polymer ligands usually as the end functional groups of polymers are mostly limited to thiolates. Since the binding motif only represents a tiny fraction of many repeating units in polymers, its importance is often ignored. We herein report the uniqueness of polymeric N-heterocyclic carbene (NHC) ligands in providing oxidative stability and promoting the catalytic activity of noble metal NPs. Two “grafting to” methods were developed for polymer NHCs for pre-synthesized metal NPs in various solvents and with different sizes. Remarkably, imidazolium-terminated polystyrene can modify gold NPs (AuNPs) within 2 min while reaching a similar grafting density to polystyrene-thiol (SH) requiring 6 h modification. We demonstrate that polymer NHCs are extremely stable at high temperature in air. Interestingly, the binding motifs of polymer ligands dominate the catalytic activity of metal NPs. Polymer NHC modified metal NPs showed improved activity regardless of the surface crowdedness. In the case of AuNPs, AuNPs modified with polystyrene NHCs are approximately 5.2 times more active than citrate-capped ones and 22 times more active than those modified with polystyrene thiolates. In view of ligand-controlled catalytic properties of metal NPs, our results illustrate the importance of binding motifs that has been overlooked in the past.
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- PAR ID:
- 10294083
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 8
- Issue:
- 31
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 15900 to 15908
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The discovery of NHCs (NHC = N‐heterocyclic carbenes) as ancillary ligands in transition‐metal‐catalysis ranks as one of the most important developments in synthesis and catalysis. It is now well‐recognized that the strong σ‐donating properties of NHCs along with the ease of scaffold modification and a steric shielding of the N‐wingtip substituents around the metal center enable dramatic improvements in catalytic processes, including the discovery of reactions that are not possible using other ancillary ligands. In this context, although the classical NHCs based on imidazolylidene and imidazolinylidene ring systems are now well‐established, recently tremendous progress has been made in the development and catalytic applications of BIAN‐NHC (BIAN = bis(imino)acenaphthene) class of ligands. The enhanced reactivity of BIAN‐NHCs is a direct result of the combination of electronic and steric properties that collectively allow for a major expansion of the scope of catalytic processes that can be accomplished using NHCs. BIAN‐NHC ligands take advantage of (1) the stronger σ‐donation, (2) lower lying LUMO orbitals, (3) the presence of an extended π‐system, (4) the rigid backbone that pushes the N‐wingtip substituents closer to the metal center by buttressing effect, thus resulting in a significantly improved control of the catalytic center and enhanced air‐stability of BIAN‐NHC‐metal complexes at low oxidation state. Acenaphthoquinone as a precursor enables facile scaffold modification, including for the first time the high yielding synthesis of unsymmetrical NHCs with unique catalytic properties. Overall, this results in a highly attractive, easily accessible class of ligands that bring major advances and emerge as a leading practical alternative to classical NHCs in various aspects of catalysis, cross‐coupling and C−H activation endeavors.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0TA03906C
