| 研究実績の概要 |
We have had excellent success in the preparation of NHC-stabilized nanoclusters thus far and preliminary results on their activation and use in catalysis. The use of NHCs to displace phosphines on known phosphine-stabilized clusters has been accomplished during this funding period. We began by preparing phosphine-stabilized undecagold clusters and treating these with benzimidazolium hydrogen carbonate to give predominantly a single NHC-containing cluster. The structure of cluster was predicted by mass spectrometric analysis carried out in the Tsukuda lab and DFT work from collaborator Hakkinen. Eventually this was confirmed by a single crystal X-ray diffraction study carried out. We examined the behavior of the clusters in the electrocatalytic reduction of CO2 to CO. With revised synthetic methods in hand, we have also carried out C-H oxidation studies with NHC-stabilized clusters. Initially we used a simple substrate, benzyl alcohol, and looked for the absolute activity in terms of conversion to product and also selectivity for initial C-H oxidation to benzaldehyde vs. over oxidation to the corresponding benzyl ester. Our preliminary results with these gold clusters have shown that these give improved yields and selectivities compared with the corresponding all phosphine clusters.
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| 今後の研究の推進方策 |
In the next FY, we will prepare gold clusters stabilized by various N-heterocyclic carbenes and test their catalytic activity in C-H oxidation. We will begin with the cluster which showed the greatest stability, activity and selectivity in our previous electrocatalytic reactions. This cluster will be examined on its own for activity and selectivity in the oxidation of benzyl alcohol. It will also be tested under conditions where phosphine dissociation is known to occur, and it will be activated by treatment with reagents that are known to abstract halides in order to generate open coordination sites on gold. Once studies with benzyl alcohol are concluded, we will move to study more challenging C-H bonds, including unactivated systems such as cyclohexane. Once conditions are developed for this substrate, we will move to more complex substrates to delineate the selectivity of the catalyst for different types of C-H bonds including primary vs. secondary vs. tertiary and the effect of activating groups. We will also test recently published Au clusters that are supported entirely by NHCs. In these cases, we will examine the effect of metal-free halide abstracting agents such as [Et3OBF4] since these may be necessary to free up sites on the Au nanocluster catalysts.
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