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Imidazolium gold complex reduction

The 3rd and last synthetic method to obtain NHC stabilized AuNP described in the literature is the deprotonation and then reduction of an imidazolium gold(III) complex.
Serpell et al.56 first described this type of synthesis in 2013 where they successfully synthesized Au and Pd-NHC NPs. The starting complexes are easily obtained by biphasic anion exchange between the imidazolium salts and either HAuCl4 or K2PdCl4. In this study they compared bis n-propyl, bis n-hexyl and bis tert-butyl imidazolium salts of Au and Pd tetrahalogenate.
As with the ligand exchange procedure, Au and Pd present slightly different behaviors. Indeed, if reduced directly with NaBH4, the bishexyl imidazolium palladate complex yielded stable nanoparticles while the aurate complex yielded bulk metal. Prior deprotonation of the imidazolium cation by NaH, before addition of NaBH4, yielded NPs for both metals (Figure I.13).

Synthesis without NaH

Attempts were made to characterize, by 1H NMR, the formation of the NHC in the first step. However, after addition of NaH, no deprotonation could be observed. As a result, we questioned the necessity of the first step and tested the synthesis in the absence of NaH. It can be noted that, in their paper, Serpell et al.1 tested a synthesis without NaH starting from 1H-AuX4 and did not obtain gold nanoparticles. However, when attempting the same synthesis from the corresponding palladium precursor, they obtained nanoparticles which, they concluded, were stabilized by imidaozlium.

Effect of the ligand

When the synthesis was replicated, we did not obtain nanoparticles from 1H-AuX4 without NaH either.
However, when using only NaBH4 on 2H-AuX4 and 3H-AuX4, both gave stable nanoparticles (Figure II.5). The obtained NPs are of a larger diameter than with NaH (5.8 ± 1.0 nm and 4.9 ± 1.1 nm respectively). This means that the addition of a base is not necessary in the synthesis of nanoparticles starting from haloaurate salts, but it does have an influence on the size of the NPs. The binding mode of the ligand on the surface (covalent, electrostatic, etc.) will be further discussed in “Surface analysis” below.

Effect of the ligand to gold ratio

Once again, excess (benz)imidazolium bromide was added to each type of synthesis (Figure II.6). As was the case in the synthesis using NaH, adding 2H-Br or 3H-Br to an NaH-free synthesis yielded smaller NPs (from 5.8 ± 1.0 to 4.0 ± 0.9 nm and 4.9 ± 1.1 to 3.6 ± 0.8 nm respectively). In the case of 1H-Br, the effect is even more dramatic as adding imidazolium bromide yielded stable particles (4.1 ± 1.0 nm) when there was no NPs without it.
It can be noted that for 1 and 2, the synthesis using only NaBH4 systematically yielded larger particles than the one using NaH regardless of the presence of imidazolium bromide or not (Table II.2). On the other hand, for 3, when adding an excess of 3H-Br, the obtained nanoparticles are slightly smaller for the NaH-free synthesis. Once again, the reason for this difference in behavior is unclear.

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.

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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).

Table of contents :

Table of contents
Bibliography (Introduction)
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
I.D. Conclusion
Bibliography (Chapter I)
II.A. Nanoparticles from imidazolium haloaurate salts and NaBH4
II.A.1. Imidazolium haloaurate salts
II.A.1.a. Synthesis
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
III.D.3.a. 2-methylimidazolium
II.D.3.b. 2-phenyl imidazolium
II.D.3.c. C2 functionalized imidazoliums and tBuNH2BH3 as a reducing agent
II.E. Conclusion
Bibliography (Chapter II)
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.a. Solvent
III.E.1.b. Water
III.E.1.c. Ligand to gold ratio
III.E.1.d. Concentration
III.E.1.e. Stirring
III.E.1.f. Temperature
III.E.2. Surface characterization
III.E.2.a. MS
III.E.2.b. NMR
III.E.2.c. XPS
III.E.3. Mechanistic study
III.E.3.a. NMR
III.E.3.c. Theoretical chemistry
III.E.3.c.i. Hydride transfer
III.E.3.c.ii. Radical mechanism
III.E.3.d. EPR
III.E.3.e. Discussion
III.E.4. Reproducibility issues
III.E.4.a. Silica
III.E.4.b. Byproduct from the NHC-BH3 synthesis
III.E.4.c. Byproduct from the imidazolium synthesis
III.E.5.d. Dimer
III.F. Conclusion
Bibliography (Chapter III)
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
IV.D. Conclusion
Bibliography (Chapter IV)


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