Liquid state NMR
The centrifuged nanoparticles were concentrated and analyzed by liquid 1H NMR. No signal corresponding to the proton of the free imidazolium was observed. However, no signal was detected in the 8 to 4 ppm region where we would normally see the signal for the protons on the aromatic backbone and in alpha of the nitrogen atoms. This is most likely due to the broadening of the signal when molecules are close to the NP surface, due to the lower rotational mobility of the ligands when attached to the surface, and their distribution on the NP’s surface. As a result, liquid NMR was inconclusive.
Solid state NMR
Attempts were also made to characterize the NPs by solid state NMR, as it had already been used in the literature to show functionalization by NHCs of metallic NPs.
In order to have a better signal, a 13C marked imidazolium at the C2 position was synthesize. 100 mg of 100% 13C marked paraformaldehyde were mixed with 400 mg of regular paraformaldehyde in order to obtain an isotope enrichment of 20%. It was then mixed in a one pot reaction with glyoxal, dodecylamine and hydrochloric acid (Scheme II.4). After evaporation of the solvent, the product, through several precipitation processes, was obtained pure as a white powder in 33% yield (2.5 g of product).
The marked imidazolium was then successfully used to synthesize nanoparticles which were centrifuged and dried. Unfortunately, technical issues made the solid-state NMR analysis impossible. Indeed, due to an absence of rotation of the rotor, no signal could be obtained. Our hypothesis is that the long alkyl chain gave a slightly oil like character to the sample that made it unsuitable for analysis.
IR analysis was also carried out on the centrifuged nanoparticles and compared to the IR spectra of 2H-AuX4 and 2H-Br (Figure II.8).
We observe a few differences when comparing the FTIR spectra of the precursors and the nanoparticles. The spectra of 2H-Br and 2H-AuX4 correspond to data available in the literature14 and are nearly identical, except for 2 bands which present a slight shift (from 1636 to 1616 cm-1 and from 773 to 757 cm-1). However, the nanoparticles spectrum is very different as a new band appears at 1410 cm-1 and bands at 1616 and 1563 cm-1 are no longer visible. Those bands are located in the ν(C=N) and ν(C=C) stretching region of the aromatic cycle of the imidazolium. These differences have already been observed in the literature15 and have been suggested to confirm the NHCs’ formation and their coordination to the gold surface. Moreover, we can stress that the ν(C-H) stretching vibration, located around 3270 cm-1, which corresponds to the imidazolium proton, is not visible on any of the 3 samples probably due to its expected very low intensity.
Figure II.8: IR spectra of 2H-Br (blue trace), 2H-AuX4 (red trace) and gold nanoparticles prepared from 2H-AuX4 (black trace).
While IR and MS gave indications that NHCs could be at the surface, the determining analysis was X-ray photoelectron spectroscopy.
X-ray photoelectron spectroscopy (XPS) was used to study the carbon-gold interaction at the nanoparticles’ surface. XPS spectra are obtained by irradiating a material (placed in a high vacuum chamber) with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the analyzed surface (up to 10 nm depth). As a result, XPS is a technique able to determine the chemical state of an analyzed surface, while giving a precise elementary composition. This technique is routinely used as a characterization tool in several fields (sensor, corrosion, catalysis). In the case of nanoparticles, XPS can be used to determine not only the integrity of the ligand on the metallic surface but also its binding mode. This makes XPS an interesting technique for the characterization of nanoparticles and could give a number of information: elementary composition of ligand and NPs, integrity of the ligand and nature of the bond. However, complete and accurate characterization of NPs sample remains challenging (peak widening, need to deposit NPs on a conductive surface).
XPS has already been used on palladium,16 platinum17 and gold18–20 nanoparticles stabilized by NHCs. The presence of the ligand was evidenced by the C1s and N1s photopeaks and the position of the latter confirmed the coordination of the carbene.18–20 However, none of these studies were able to give an elemental composition of the ligand, attesting to the difficulty that characterizing NPs to the full extent represents. Moreover, no clear evidence of a covalent carbon-metal bond was established.
XPS analysis was carried out on 2 samples: 2H-Br and gold nanoparticles prepared from 2H-AuX4 and 4 equivalents of 2H-Br without NaH. The latter will be designated by 2-AuNP. Each photopeak was carefully deconvoluted to study the possible presence of several components. Results are presented in Table II.3. For 2H-Br, the photopeak Br3d can be divided into Br3d5/2 and Br3d3/2 at 67.4 and 68.3 eV respectively. In the case of 2-AuNP, no peak is observed in the 185-210 and 65-72 eV regions which indicates an absence of bromine and chlorine in the sample and confirms the correct purification of the NPs.
For 2-AuNP, the formation of metallic gold is evidenced by the Au4f signal composed of only 2 peaks at 84.0 and 87.6 eV, which correspond to spin orbit coupling Au4f7/2 and Au4f5/2 respectively. The binding energy of Au4f7/2 and difference between the two components (3.6 eV) is characteristic of metallic gold (Au0).
Nanoparticles from imidazolium haloaurate salts and tBuNH2BH3
Now it was established that gold nanoparticles were stabilized by NHC even in the absence of a “proper” base, we decided to change the reducing agent. tBuNH2BH3, which is a milder reducing agent, was chosen. It is soluble in organic solvents, as opposed to NaBH4, which allows for homogeneous synthesis.
Unexpectedly, tests carried out in the presence of NaH yielded very different results than for the synthesis using NaBH4. Indeed, adding NaH before the reduction step by tBuNH2BH3 led solely to the formation of bulk material, this implies that the deprotonation step is not the only decisive one in the nucleation/growth mechanism of the nanoparticles.
As a result, the following tests were carried out in the absence of NaH. The effect of the ligand/Au ratio was also studied. Figure II.11 presents the TEM images and size distribution of the obtained nanoparticles for ratios of 2H-AuX4:2H-Br ranging from 1:0 (2H-AuX4 only) to 1:6. All syntheses were carried out at 55°C with 10 min of stirring. The size of the as obtained nanoparticles is strongly linked to the addition of 2H-Br. When using only 2H-AuX4, NPs are large, very polydisperse (24 ± 7 nm) and of various shapes. They get smaller, more spherical and monodisperse with an increasing quantity of 2H-Br. It appears that at least 1 equivalent of 2H-Br is necessary to obtain reasonable polydispersity (~25%). When using 6 equivalents of imidazolium, the NPs reach 5.8 ± 1.1 nm in size. As such, it seems that when using tBuNH2BH3, the ligand/Au ratio can be used to influence the obtained NPs size in a 6-12 nm range.
Nanoparticles from AuCl and unfunctionalized imidazolium salts
After seeing that the synthesis of NHC-capped NPs could be achieved without the addition of NaH to deprotonate the imidazolium, we decided to explore the synthesis further by using an even simpler gold precursor: AuCl. We hoped that the reduction of AuCl by NaBH4 in the presence of imidazolium bromide would lead to stable, NHC-capped nanoparticles.
Upon addition of AuCl to a solution of imidazolium in toluene, the solution goes from colorless to orange, thus showing an interaction between the 2 reagents and the likely formation of an imidazolium AuClBr- complex. Indeed, this type of complex have been described in the litterature28 and were obtained in quantitative yield by mixing an imidazolium bromide salt with AuClSMe2 at room temperature. Upon addition of aqueous NaBH4, the solution turns deep red and bubbles are formed, probably due to H2 release upon reduction of Au(I) and the deprotonation of the imidazolium. The obtained nanoparticles are isotropic and 3.4 ± 0.8 nm in diameter when using 1 equivalent of imidazolium per gold. Increasing to 2 and 4 equivalents of imidazolium did not have a significant effect on the size (Figure II.13).
Using tBuNH2BH3 as a reducing agent
As previously, changing the reducing agent to tBuNH2BH3 led to larger to nanoparticles (6.8 ± 1.3 nm) (Figure II.16), which were characterized by XPS. As was the case with 2H-AuX4, there is no halides or boron detected and only gold(0). There are 2 components to the nitrogen photopeak (Figure II.16). One at 400.3 eV accounting for 80% of the signal and one at 401.9 eV accounting for the remaining 20%. Once again most of the stabilization of the NPs come from NHCs with a small contribution in the higher binding energies. This contribution suggests a charged species and thus an electrostatic interaction. This also shows that once again tBuNH2BH3 is enough of a base regardless of gold precursor. The analyzed nanoparticles were synthesized with an excess of 2H-Br (6eq). Due to lack of time, no study on the influence of Au:2H-Br ratio was conducted.
Table of contents :
Table of contents
CHAPTER I: STATE OF THE ART ON NHC-STABILIZED GOLD NANOPARTICLES
I.A. Gold nanoparticles
I.B. N-heterocyclic carbenes
I.C. Gold nanoparticles stabilized by N-heterocyclic carbenes
I.C.1. Ligand exchange
I.C.2. NHC-gold complex reduction
I.C.3. Imidazolium gold complex reduction
Bibliography (Chapter I)
CHAPTER II: SYNTHESIS OF N-HETEROCYCLIC CARBENE-CAPPED GOLD NANOPARTICLES FROM IMIDAZOLIUM SALTS
II.A. Nanoparticles from imidazolium haloaurate salts and NaBH4
II.A.1. Imidazolium haloaurate salts
II.A.1.b. Crystallographic analysis
II.A.2. Nanoparticles synthesis
II.A.2.a. Synthesis with NaH
II.A.2.a.i. Effect of the ligand
II.A.2.a.ii. Effect of the ligand to gold ratio
II.A.2.b. Synthesis without NaH
II.A.2.b.i. Effect of the ligand
II.A.2.b.ii. Effect of the ligand to gold ratio
II.A.3. Surface analysis
II.A.3.a. MS analysis
II.A.3.b. NMR analysis
II.A.3.b.i. Liquid state NMR
II.A.3.b.ii. Solid state NMR
II.A.3.c. IR analysis
II.A.3.d. XPS analysis
II.B. Nanoparticles from imidazolium haloaurate salts and tBuNH2BH3
II.B.1. Synthesis of the nanoparticles
II.B.2. XPS analysis
II.C. Nanoparticles from AuCl and unfunctionalized imidazolium salts
II.C.1. Using NaBH4 as a reducing agent
II.C.2. Using tBuNH2BH3 as a reducing agent
II.D. Nanoparticle synthesis from functionalized imidazoliums
II.D.1. Water-soluble imidazolium
II.D.2. Azide functionalized imidaozlium
II.D.3. C2-functionalized imidazoliums
II.D.3.b. 2-phenyl imidazolium
II.D.3.c. C2 functionalized imidazoliums and tBuNH2BH3 as a reducing agent
CHAPTER III: SYNTHESIS OF N-HETEROCYCLIC CARBENE-CAPPED GOLD NANOPARTICLES FROM NHC-BORANES
III.A. Bibliographic introduction on N-heterocyclic carbene boranes
III.A.1. Synthesis of NHC-boranes
III.A.2. Uses of NHC-boranes in molecular chemistry
III.A.2.a. Heterolytic rupture
III.A.2.b. Homolytic rupture
III.A.3. Uses of NHC-boranes beyond molecular chemistry
III.B. NHC-BH3 synthesis
III.C. Nanoparticles synthesis from gold precursor: AuClPPh3
III.C.1. First attempts
III.C.2. Solvent screening
III.D. Nanoparticles synthesis from gold precursor HAuCl4.3H2O
III.E. Nanoparticles synthesis from gold precursor AuClSMe2
III.E.1. Reaction conditions effect
III.E.1.c. Ligand to gold ratio
III.E.2. Surface characterization
III.E.3. Mechanistic study
III.E.3.c. Theoretical chemistry
III.E.3.c.i. Hydride transfer
III.E.3.c.ii. Radical mechanism
III.E.4. Reproducibility issues
III.E.4.b. Byproduct from the NHC-BH3 synthesis
III.E.4.c. Byproduct from the imidazolium synthesis
CHAPTER IV: SYNTHESIS OF MESOIONIC CARBENE-CAPPED GOLD NANOPARTICLES FROM TRIAZOLIUM SALTS AND MIC-BH3
IV.A Mesoionic carbenes in the literature
IV.B. Gold nanoparticles stabilized by MICs from triazolium salts
IV.B.1. Synthesis of the triazolium salts precursors
IV.B.2. Gold nanoparticle synthesis from triazolium salts
IV.B.3. XPS analysis
IV.C. Gold nanoparticles stabilized by MICs from MIC-BH3
IV.C.1. MIC-BH3 in the literature
IV.C.2. Synthesis of MIC-BH3
IV.C.3. Gold nanoparticle synthesis using AuClSMe2 as a precursor
IV.C.4. Gold nanoparticle synthesis using HAuCl4.3H2O as a precursor
IV.C.5. XPS analysis
Bibliography (Chapter IV)