Electrochemical deposition of N-heterocyclic carbene monolayers on metal surfaces

Abstract

N-heterocyclic carbenes (NHCs) have been widely utilized for the formation of self-assembled monolayers (SAMs) on various surfaces. The main methodologies for preparation of NHCs-based SAMs either requires inert atmosphere and strong base for deprotonation of imidazolium precursors or the use of specifically-synthesized precursors such as NHC(H)[HCO3] salts or NHC–CO2 adducts. Herein, we demonstrate an electrochemical approach for surface-anchoring of NHCs which overcomes the need for dry environment, addition of exogenous strong base or restricting synthetic steps. In the electrochemical deposition, water reduction reaction is used to generate high concentration of hydroxide ions in proximity to a metal electrode. Imidazolium cations were deprotonated by hydroxide ions, leading to carbenes formation that self-assembled on the electrode’s surface. SAMs of NO2-functionalized NHCs and dimethyl-benzimidazole were electrochemically deposited on Au films. SAMs of NHCs were also electrochemically deposited on Pt, Pd and Ag films, demonstrating the wide metal scope of this deposition technique.

Electrochemical Generation of N‐Heterocyclic Carbenes for Use in Synthesis and Catalysis

“…The electrochemical deposition of NHCs on a metal surface was first reported by Toste, Gross and coworkers (Scheme 13C). [51] The NHC is generated in situ from the corresponding imidazolium salt via deprotonation with electrogenerated hydroxide from water reduction. Scheme 11.…”

“…[46] Scheme 13. A: Electrochemical synthesis of polymeric imidazolate metal complexes; [48] B: Palladium catalyzed electrocarbonylation; [49] C: Electrochemical deposition of NHCs on metal surfaces; [51]…”

AFM-IR and s-SNOM-IR measurements of chemically addressable monolayers on Au nanoparticles

“…Attenuated total reflectance infrared (ATR-IR) spectrum of NO2-functionalized imidazolium salt, which is the precursor of nitro-functionalized N-heterocyclic carbene (NO2-NHC), showed three main peaks located at 1338, 1533 and 1608 cm -1 , correlated to the symmetric N−O, asymmetric N−O and C=C aromatic stretches, respectively (Figure S1 and Table S1). 47 Deprotonation of the imidazolium salt precursor, following previously published protocols, 45,48 enabled the self-assembly of NO2-NHCs on Sisupported Au nanoparticles (NPs) (Figure 1a). AFM-IR measurements were conducted at the center of the particle and the resulting IR spectrum (Figure 1b) featured two dominant peaks at 1330 and 1558 cm -1 , attributed to the symmetric and asymmetric stretches of the N-O bond, respectively (Table 1) 45,49 .…”

“…NH2-NHCs monolayer (Figure 1d) was attained by exposure of the NO2-NHCs coated Au NPs to reducing environment (1 atm H2, 80 °C, 10 h). [48][49][50] AFM-IR measurement was conducted at the center of the Au NP and yielded an IR spectrum with a single peak at 1647 cm -1 , assigned to N-H bend of a primary amine (Figure 1e). 45,48,51,52 s-SNOM-IR measurement was performed on the hydrogenated sample and resulted an IR spectrum with a distinct peak at 1634 cm -1 (Figure 1f), attributed to N-H signal.…”

N‐Heterocyclic Carbene Based Nanolayer for Copper Film Oxidation Mitigation

“…NHC‐nanolayer formation on copper film was induced by exposure of alkyne‐functionalized imidazolium cation to hydroxide ions that were formed near the copper electrode by electrochemical water reduction [18] . Deprotonation of the imidazolium cation led to the formation and self‐assembly of NHCs on copper surface.…”