NHC stabilized copper nanoparticles via reduction of a copper NHC complex

The bottom-up synthesis of plasmonic NHC@CuNPs from common starting reagents, via the formation of the synthetically accessible NHC–Cu(i)–Br complex and its reduction by NH3·BH3 is reported. The resulting NHC@CuNPs have been characterized in detail by XPS, TEM and NMR spectroscopy. The stability of NHC@CuNPs was investigated under both inert and ambient conditions using UV-Vis analysis. While the NHC@CuNPs are stable under inert conditions for an extended period of time, the NPs oxidize under air to form CuxO with concomitant release of the stabilizing NHC ligand.


Materials and Methods
All experiments, if not stated otherwise, were performed under argon atmosphere in a MBraun Unilab Pro glovebox. Commercially available reagents were used without further purification. Dry solvents (toluene and THF) were obtained from Acros/Fischer or Sigma Aldrich and stored over activated molecular sieves (3 Å) inside a glovebox. CDCl3 (99,80% D, <0.01% H2O) was obtained from Euroisotop and dried over CaCl2, redistilled and stabilized with silver wire. Thin layer chromatography was performed using silica 60 F254 plates purchased from EDM Millipore. All molecular products were fully characterized using multinuclear NMR spectroscopy and high-resolution mass spectrometry. Obtained nanomaterials were characterized using XPS, UV-Vis and NMR multinuclear spectroscopies as well as TEM.
Imidazolium salt 2 was synthesized following a procedure from literature. [1] X-ray photoelectron spectroscopy (XPS) was performed on a Nexsa Photoelectron Spectrometer (Thermo Fisher Scientific, UK) provide by the Core Facility "Interface Characterization", Faculty of Chemistry, University of Vienna. Samples were drop casted from suspensions in toluene onto freshly cleaned silicon wafer and dried for 48 h in an inert atmosphere at room temperature. The silicon wafers (~0.25 cm 2 ) were cleaned by sonication in methanol and acetone with subsequent drying under reduced pressure. Element specific high-resolution spectra for Carbon (C 1s 279-298 eV), Nitrogen (N 1s 392-410 eV) and Copper (XPS: Cu 2p 910-970 eV, Auger: 560-580 eV) were obtained after cleaning surface with Ar-clusters (1000 atoms, 6000 eV, 1 mm raster size) for 60 s followed by data collection with a step size of 0.1 eV and a pass energy of 50 eV. All measurements were performed using Al-Kα X-rays with a spot size of 400 µm. Obtained spectra were evaluated using the Advantage software package v5.9929 provided by Thermo Fisher Scientific and Origin Pro v9.7.5.184.
Mass spectrometry (MS) was performed on a Brucker amaZon speed ETD (reaction control/routine measurements) or Bruker maXis UHR-TOF (high resolution) spectrometer at Mass Spectrometry Centre, Faculty of Chemistry, University of Vienna.
UV-Vis spectroscopy was conducted on an Agilent Technologies G1103A using air-tight quartz cuvettes.
Transmission electron microscopy (TEM) was performed at the Electron Microscopy Facility at IST Austria using a Phillips CM2000 (200kV) TEM equipped with a Gaten Orius SC600 camera. Prior to imaging, samples suspended in toluene were drop casted onto carbon film on 200 mesh copper grids. The resulting pictures were processed with Gaten Micrograph software and analyzed with TVIPS EM Measure beta 0.85. The average particle diameter (d) and size distribution were determined by the analysis of obtained micrographs with ImageJ v1.53k.

Synthesis of NHC Complex 2
For the synthesis of the NHC-copper-complex 2 the procedure from Lu et al. [2] was adjusted. Therefore copper(I)oxide (0.19 mmol, 27 mg, 1 eq) was suspended in 1 mL of toluene and heated to 110 °C. 1,3-di(dodecyl)imidazole-1-ium bromide 1 (0.3 mmol, 100 mg, 1.6 eq) was added in one portion to the suspension. The reaction mixture was stirred for 24 h and filtered through silica resulting in a clear solution. The solvent was removed under reduced pressure resulting in a pale yellow, air-sensitive solid which was used without further purification. The product was obtained as offwhite solid (100 mg, 61%).

Synthesis of NHC@CuNPs
Complex 2 (0.09 mmol, 46 mg, 1 eq) was dissolved in 1 mL THF and heated to 50 °C. The reducing agent NH3·BH3 (0.1 mmol, 3 mg, 1.1 eq) was added in one portion and the mixture stirred for 24 h. The resulting nanoparticles are collected by centrifugation under inert atmosphere and re-dispersed in toluene. For the separation of unbound ligand, reducing agent and further impurities, the surfactant was decanted after 15 min of centrifugation at 12100 x g. The procedure was repeated 5 times. Final NHC@CuNPs were obtained upon redispersion in toluene as dark red liquid.

High Resolution XPS Spectra
All displayed spectra were deconvoluted using the Avantage software package (v5.9929/build 06752) provided by Thermo Fisher. Displayed baselines were generated by using the smart baseline feature of the software package. Raw data is displayed as gray symbol, generated envelope as red line and peak fits as colored areas. All high resolution XPS spectra were calibrated on the C-C contribution in C 1s spectra at 284.8 eV.

Figure S1
Cu 2p and N1 s spectra of NHC@CuNPs after air exposure.

UV-Vis Stability Study under Argon Atmosphere
Purified NHC@CuNPs in toluene were kept as stock solution under argon. The stability was assessed via UV-Vis spectroscopy by filling aliquots of the NP stock in air-tight cuvettes and recording spectra immediately as well as after 24 and 48 h. Samples were discarded after measurements.

Figure S6
TEM micrograph of NHC@CuNPs after oxidation and corresponding particle size histogram.

High Resolution Mass Spectrometry of NHC@CuNP after Oxidation
Oxidized NHC@CuNPs were centrifuged and the supernatant was analyzed by HS-MS.