| 研究実績の概要 |
In this funding year, we have made significant advances. We developed conditions for the synthesis of gold nanoclusters functionalized by up to four N-heterocyclic carbenes. We also developed conditions for the clean preparation of Au clusters substituted by only one NHC. These clusters were extensively characterized by NMR (Nagoya) and mass spec (Tsukuda lab, Tokyo). Four different NHC ligands were examined. In all cases, the ligands were benzannulated, and they differed by the wingtip groups on nitrogen: Me, Et, iPr, Bn. In all cases, the corresponding benzimidazolium hydrogen carbonate was employed since we found that the free carbene led to breakdown/restructuring of the Au core.
The iPr substituted NHC was studied the most extensively. It was found that five equivalents of NHC precursor to Au11 starting material ([Au11(PPh3)8Cl2]Cl) were optimal, with higher equivalents leading to mixtures of products. The amount of water present in the solution was also critical, with clean monosubstitution being observed. We believe this is because of tuning the pKa of the benzimidazolium hydrogen carbonate, with the cation becoming more acidic and the anion more basic as the solution is dehydrated since there are no opportunities for hydration. In the case of the benzyl-substituted NHCs, we always observed multiple products, with up to 4 NHCs observed on the Au core in reasonable amounts. This is the first time we have observed such a high level of substitution.
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