Supramolecular Enhancement of Electrochemical Nitrate Reduction Catalyzed by Cobalt Porphyrin Organic Cages for Ammonia Electrosynthesis in Water.

The electrochemical nitrate (NO3 - ) reduction reaction (NO3 RR) to ammonia (NH3 ) represents a sustainable approach for denitrification to balance global nitrogen cycles and an alternative to traditional thermal Haber-Bosch processes. Here, we present a supramolecular strategy for promoting NH3 production in water from NO3 RR by integrating two-dimensional (2D) molecular cobalt porphyrin (CoTPP) units into a three-dimensional (3D) porous organic cage architecture. The porphyrin box CoPB-C8 enhances electrochemical active site exposure, facilitates substrate-catalyst interactions, and improves catalyst stability, leading to turnover numbers and frequencies for NH3 production exceeding 200,000 and 56 s-1 , respectively. These values represent a 15-fold increase in NO3 RR activity and 200-mV improvement in overpotential for the 3D CoPB-C8 box structure compared to its 2D CoTPP counterpart. Synthetic tuning of peripheral alkyl substituents highlights the importance of supramolecular porosity and cavity size on electrochemical NO3 RR activity. These findings establish the incorporation of 2D molecular units into 3D confined space microenvironments as an effective supramolecular design strategy for enhancing electrocatalysis

N-heterocyclic carbene self-assembled monolayers on copper and gold : dramatic effect of wingtip groups on binding, orientation and assembly

Funding: EPSRC PhD studentship (EP/M506631/1).Self‐assembled monolayers of N‐heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N‐substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl‐ or ethyl‐substituted NHCs lie flat. Temperature‐programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes=152±10 kJ mol−1. NHCs that bind upright desorb cleanly, while flat‐lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2Cu species by extraction of a copper atom from the surface after annealing.Publisher PDFPeer reviewe

N-heterocyclic carbenes reduce and functionalize copper oxide surfaces in one pot

The Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI) are thanked for financial support of this work in terms of operating and equipment grants to CMC and JHH. AJV and MRN thank the Ontario government for OGS fellowships. AJV thanks NSERC for a Vanier Scholarship and also the Walter C. Sumner foundation for additional scholarship support.Benzimidazolium hydrogen carbonate salts have been shown to act as N-heterocyclic carbene precursors which can remove oxide from copper oxide surfaces and functionalize the resulting metallic surfaces in a single step. Both the surfaces and the etching products are fully characterized by spectroscopic methods. Analysis of surfaces before and after NHC treatment by X-ray photoelectron spectroscopy demonstrates the complete removal of copper(II) oxide. Using 13C-labelling, we determine that the products of this transformation include a cyclic urea, a ring-opened formamide and a bis-carbene copper(I) complex. These results illustrate the potential of NHCs to functionalize a much broader class of metals, including those prone to oxide treatment, greatly facilitating the preparation of NHC-based films on metals other than gold.PostprintPeer reviewe

Simple direct formation of self-assembled N-heterocyclic carbene monolayers on gold and their application in biosensing

CRL acknowledges the Engineering and Physical Sciences Research Council (UK) for the funding of his PhD studentship (EP/M506631).The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol−1) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.Publisher PDFPeer reviewe

Multifunctional Charge and Hydrogen‐Bond Effects of Second‐Sphere Imidazolium Pendants Promote Capture and Electrochemical Reduction of CO2 in Water Catalyzed by Iron Porphyrins**

Microenvironments tailored by multifunctional secondary coordination sphere groups can enhance catalytic performance at primary metal active sites in natural systems. Here, we capture this biological concept in synthetic systems by developing a family of iron porphyrins decorated with imidazolium (im) pendants for the electrochemical CO2 reduction reaction (CO2 RR), which promotes multiple synergistic effects to enhance CO2 RR and enables the disentangling of second-sphere contributions that stem from each type of interaction. Fe-ortho-im(H), which poises imidazolium units featuring both positive charge and hydrogen-bond capabilities proximal to the active iron center, increases CO2 binding affinity by 25-fold and CO2 RR activity by 2000-fold relative to the parent Fe tetraphenylporphyrin (Fe-TPP). Comparison with monofunctional analogs reveals that through-space charge effects have a greater impact on catalytic CO2 RR performance compared to hydrogen bonding in this context

Multifunctional Charge and Hydrogen-Bond Effects of Second-Sphere Imidazolium Pendants Promote Capture and Electrochemical Reduction of CO2 in Water Catalyzed by Iron Porphyrins

Microenvironments tailored by multifunctional secondary coordination sphere groups can enhance catalytic performance at primary metal active sites in natural systems. Here, we capture this biological concept in synthetic systems by developing a family of iron porphyrins decorated imidazolium (im) pendants for the electrochemical CO2 reduction reaction (CO2RR), which promotes multiple synergistic effects to enhance CO2RR and enables the disentangling of second-sphere contributions that stem from each type of interaction. Fe-ortho-im(H), which poises imidazolium units featuring both positive charge and hydrogen-bond capabilities proximal to the active iron center, increases CO2 binding affinity by 25-fold and CO2RR activity by 2,000-fold relative to the parent Fe tetraphenylporphyrin (Fe-TPP), achieving turnover frequencies (TOF) exceeding 109 s-1 with >95% Faradaic efficiency for CO product. Owing to these dual, synergistic second-sphere enhancements, this catalyst also retains high activity and selectivity for homogeneous CO2RR in aqueous media. Notably, the observed TOF value for Fe-ortho-im(H) is 14,000-fold higher than the Fe-para-im(H) positional analog, but only 40-fold higher than the Fe-ortho-im(Me) congener that retains the proximal positive charge but lacks the C2-H hydrogen-bonding moiety, revealing that through-space charge effects have a greater impact on catalytic CO2RR performance compared to hydrogen bonding in this context. This work emphasizes the use of second-sphere pendants that can promote multiple synergistic effects as a design strategy for achieving CO2 reduction catalysis in water

Supramolecular Enhancement of Electrochemical Nitrate Reduction Catalyzed by Cobalt Porphyrin Organic Cages for Ammonia Electrosynthesis in Water

The electrochemical nitrate (NO3ˉ) reduction reaction (NO3RR) to ammonia (NH3) represents a sustainable approach for denitrification to balance global nitrogen cycles and an alternative to traditional thermal Haber-Bosch processes. Here, we present a supramolecular strategy for promoting NH3 production in water from NO3RR by integrating two-dimensional (2D) molecular cobalt porphyrin (CoTPP) units into a three-dimensional (3D) porous organic cage architecture. The porphyrin box CoPB-C8 enhances electrochemical active site exposure, facilitates substrate-catalyst interactions, and improves catalyst stability, leading to turnover numbers and frequencies for NH3 production exceeding 200,000 and 56 s-1, respectively. These values represent a 15-fold increase in NO3RR activity and 200-mV improvement in overpotential for the 3D CoPB-C8 box structure compared to its 2D CoTPP counterpart. Synthetic tuning of peripheral alkyl substituents highlights the importance of supramolecular porosity and cavity size on electrochemical NO3RR activity. These findings establish the incorporation of 2D molecular units into 3D confined space microenvironments as an effective supramolecular design strategy for enhancing electrocatalysis