Gold nanoclusters (AuNCs) protected or stabilized by organic molecules have attracted great interest in terms of their fundamental properties and applications. The electronic,1 optical,2 catalytic,3 electrochemical4 properties of AuNCs are significantly different from bulk gold, so-called “Quantum sized effect”. Not only the size but interface between metal core and molecules also affect to the basic properties.5-7 Thus, the precise control over the interfacial structure is important for developing new type of materials.
The physical properties of organic-inorganic hybrid materials are highly tunable by changing several parameters such as size, shape and arrangement of building block in materials.8,9 For instance, Bürgi and co-worker reported size-dependency of optical activity on amino acid-capped gold nanoclusters. It was found that the anisotropy factor nicely decrease with increase of particle size. This tendency follows the surface-to-volume ratio of the particles and may thus point toward a surface effect as the origin for optical activity (Figure 6-1-1).10
Figure 6-1-1 CD and UV-vis spectra of N-isobutyryl-L-cysteine (dashed) and N-isobutyryl-D-cysteine (solid line) capped AuNCs; a) Au22 b) Au18 c) Au15. These figures were reprinted from reference 10.
The experimental and calculation studies revealed the intrinsic optical property arose from the asymmetric arrangement of gold atom in cluster system. Wang and co-workers produced bare Au34− by laser vaporization. Thus produced Au34− adopted low-symmetrical structure which could be characterized by photoelectron spectroscopy (PES) and trapped ion electron diffraction (TIED)11. The experimental results corresponded well with the calculated structure reported by Kappes and co-workers.12 Its optical activity was realized (reveal?) by the calculation studied by Garzón and co-workers. The results showed that bare gold clusters could be intrinsicallychiral via an asymmetric arrangement of surface atoms.13
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Bürgi and co-workers further explored the asymmetric arrangement of surface gold atoms could be affected by the adsorbed chiral thiolate ligands. The ligand exchange with opposite enantiomer of thiolate ligand can cause chiral inversion.14
Figure 6-1-2 The chiral inversion induced by chiral ligand exchange.
This figure was reprinted from reference 14.
Tsukuda and co-workers reported the adsorbed chiral ligands effect on the optical activity.
They proposed the origin of chirality of prepared BINAP-stabilized AuNCs come from the distortion of surface metal atom arrangement.15 Similarly, Zhao and co-workers reported chiral diimido ligands adsorbed sigmoidal and reverse-sigmoidal gold clusters on their CD response.
The theoretical studies revealed that the strong CD responses closely relates to the gold cluster-dominated HOMO orbitals, thus the chirality arises from the contribution of ligands and the asymmetric arrangement of gold atoms.16
However, there is a severe trade-off relationship in the application on catalysis. The strong interaction between the stabilizer and the metal could give inferior catalytic activity due to the lace (loss?) of exposed surface of the metal. For example, the above-mentioned thiolate- or phosphine-ligated chiral AuNCs did not show any chemical catalytic activity because the surface of the metal is fully covered with the ligand and no more active surface was available.
Therefore, in order to develop the chiral metal surface of clusters for the application to the chiral catalyst, strong enough interaction between the chiral matrix and the metal surface with keeping sufficiently exposed surface area must be realized in good balance.
As the pioneer work on metal NCs-catalyzed asymmetric catalytic reaction, Fujihara and co-worker reported the syntheses of BINAP-PdNCs which catalyzed the asymmetric hydrosilylation of olefin under mild conditions effectively. The 89% yield with 95% ee of a single isomer product was obtained.17 Nevertheless, the mechanismin cross coupling reaction
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is still uncleared and the formation of Pd2+via oxidative addition process is a major limitation of PdNCs. Thus, the asymmetric hydrosilylation via leaching and reclusterization is possible.
There have been a couple of successful approaches via surface reaction reported previously. One is to use chiral heterogeneous support. In this case, sufficiently exposed surface must be obtained, though it would be difficult to induce the chiral environment on the metal surface. Somorjai and co-worker reported enantioselective heterogeneous AuNCs catalyst for the cyclopropanation reaction (Figure 6-1-3). The chiral self-assembled monolayer (SAM) was prepared by alkylation of proline derivatives on Br-terminated SAM. Up to 50%
enantioselectivity with high diastereoselectivity were obtained by using AuNCs@chiral-SAM.18
Figure 6-1-3 AuNCs encapsulated in chiral SAM/Mesoporous MCF-17 support catalyzed asymmetric cyclopropanation reaction. This figure was reprinted from reference 18.
Moores and co-workers reported the cellulose nanocrystal as a solid support for the PdNCs and the asymmetric hydrogenation of ketones (Figure 6-1-4). The PdNCs are important for activating the hydrogenation reactions, whereas the cellulose nanocrystal brought the enantioselectivity into the catalyst system. An ee of 65% with 100% conversions of the asymmetric hydrogenation of prochiral ketones were successfully achieved.19
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Figure 6-1-4 Enantioselective ketone hydrogenation using PdPs@CNCs as catalyst. This figure was reprinted from reference 19.
Another approach is to utilize weakly-bound ligand system. Kobayashi and co-workers reported Rh/Ag bimetallic NPs catalyzed 1,4-addition of arylboronic acid to α, -unsaturated esters. The heterogeneous metal NPs showed high activity and enantioselectivity in the presence of chiral diene as chiral modifier (Figure 6-1-5).20 Based on some reference experiments, they concluded that the reaction occurs on the surface of the cluster and not through the monometallic intermediate through the leaching process although the precise mechanism has not been yet elucidated.
Figure 6-1-5 Rh/Ag bimetallic NPs catalyzed 1,4-addition of arylboronic acid to α, -unsaturated esters. This figure was reprinted from reference 20.
In chapter 5, the author described that the strong interface interaction between PVP as stabilizing polymer and Au core if the metal core size and the polymer chain length meet good matching. The results obtained from the electronic and colloid structure studies clearly showed the generation of the anionic character on the Au surface that was highly dependent on molecular weight of PVP. The morphology effect overtook the size effect to control the catalytic activity. It is promising for emerging new chiral AuNC catalyst that not only the cluster size but polymer matrix also plays an important role on the properties of AuNCs. The most effective catalyst revealed a significantly stronger interface interaction between PVP and Au core.
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So far, there is no report on weakly coordinated stabilizing polymers stabilized metal nanoclusters on the application of asymmetric catalytic reaction. Due to weak interaction between stabilizing polymer and metal surface, the chiral induction through this interaction is extremely poor. As the author described in chapter 5, the strong interfacial interaction can be induced if the chain length of the stabilizing polymers and the core size of the metal clusters are matched well. It highly motivated the author to investigate the reaction control/selectivity of the surface reaction of the clusters by the influence from the structure and morphology of the stabilizing polymers only through the weak non-bonding interaction with Au core.
Chirality induction should be the good research topic to elucidate this hypothesis because the chiral ligands such as phosphines, commonly used for the asymmetric metal complex catalyst, cannot be used in the cluster surface reaction due to the inhibition of the catalytic activity by their strong coordination.
The choice of the chiral polymers is the most important issue for the success of this system. Biopolymers are potential choices as demonstrated by Moore’s heterogenous catalyst system.(no ref?) However, the author decided to investigate the co-polymer with chiral-vinylpyrrolidone (chiral-VP) and chiral-vinylpyrrolidone (VP) because of the following reasons.
First, physical properties of these polymers should resemble to that of PVP due to the same framework in the polymer structure, therefore the data from the PVP system in Chapter 5 can be applied directly to this system. Second, fine tunings, both on the molecular weight and ratio of the chiral monomer unit, are possible through the comprehensive polymer synthesis method.
And third, the structure of the chiral auxiliary can also be tuned. The chiral co-polymer structure was demonstrated in Scheme 6-1-1.
Scheme 6-1-1 The co-polymer of chiral-vinylpyrrolidone (chiral-VP) and vinylpyrrolidone (VP). (better to explain the meaning of “R”)
For the design of the chiral auxiliary, three positions on pyrrolidone skeleton is considered to install chirality except on the main chain (Scheme 6-1-2), where it should be difficult to install the chirality. 2-position locates at the closest point from carbonyl group, where the PVP coordinates onto the AuNCs and various synthetic methods to prepare the chiral
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2-subsituted pyrrolidone are available. However, 2-position is faced on the risk of epimerization particularly under basic conditions, which is common conditions for Au:PVP-catalyzed reactions. 3-Position is far both from the carbonyl and the main chain of the polymer, and there are few synthetic methods to prepare chiral derivatives. On the other hand, 4-position is far from the carbonyl group but possibly influential to the main chain structure of the polymer, and has few risks against the epimerization during the catalytic reaction conditions. In addition, a series of 4-substituted chiral derivatives can be synthesized from the amino acids, common
chiral resources, as described below. From the above reasons, the author decided to use 4-substituted chiral pyrrolidone moieties as chiral auxiliaries in this study.
Scheme 6-1-2 The design of the chiral auxiliary on pyrrolidone moieties.
In chapter 6, the author describes the syntheses of the chiral 4-substituted-N -vinylpyrrolidone derivatives (chiral-VP), preparation of the co-polymer with N -vinylpyroolidone (VP), and the preparation of the corresponding AuNCs protected by these copolymers. Characterization of these AuNCs were carried out and preliminary investigation of the asymmetric reactions was also tested by intramolecular hydroamination reaction as a model reaction. Although the optical activity in metal-based electronic transition (MBET) was significantly weak as compared with those observed in case of capping agent, the chiral-PVP-protected AuNCs can act as the catalyst and give high enantioselectivity toward hydroamination reaction only in the case of particular metal core size and the polymer chain length.
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6.2 Experimental Section