Reactivity variance between stereoisomers of saturated N-heterocyclic carbenes on gold surfaces
Reactivity variance between stereoisomers of saturated N-heterocyclic carbenes on gold surfaces
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DOI：
10.1039/d3qi01541f
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发表时间：
2023-09-26
影响因子：
7
通讯作者：
Jenkins，David M.
中科院分区：
文献类型：
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作者：
Kaur，Gurkiran;Dominique，Nathaniel L.;Jenkins，David M.
控制分子表面系统的手性对于从多相催化到生物传感的应用是必不可少的。N-杂环卡宾（NHC）正迅速成为贵金属表面钝化的主要配体，但手性NHC支架的潜力尚未开发。本工作合成了两种饱和N-杂环卡宾（NHC）和一种不饱和NHC（C2 v对称）的C2对称和Cs对称立体异构体NHC-CO2加合物。这些CO2保护的NHC沉积在金膜上，并通过激光解吸电离质谱（LDI-MS）和表面增强拉曼光谱（Sers）证实了它们的存在。令人惊讶的是，C8 NHC，而不是手性C2 NHC，在与金结合时部分降解为独立合成的不饱和NHC。理论计算有助于解释这一现象，表明NHC主要平放在金表面上，这暴露了C8对称NHC上的骨架质子，使其发生H2的形式消除，而C2对称NHC仍然受到保护，不发生这种消除反应。这些结果提出了关键的问题，如何调整NHC配体的结构，以影响金表面的结合和反应。
Controlling the chirality of molecule-surface systems is essential for applications ranging from heterogeneous catalysis to biosensing. N-heterocyclic carbenes (NHCs) are quickly becoming a dominant ligand for noble metal surface passivation, but the potential of chiral NHC scaffolds remains untapped. In this work, two stereoisomers, one C2 symmetric and one Cs symmetric, of saturated N-heterocyclic carbenes (NHCs) and a structurally related unsaturated NHC (C2v symmetric) were synthesized as NHC-CO2 adducts. These CO2-protected NHCs were deposited on gold films and their presence on the films was confirmed by laser desorption ionization mass spectrometry (LDI-MS) and surface enhanced Raman spectroscopy (SERS). Surprisingly, the Cs NHC, but not the chiral C2 NHC, partially degrades to the independently synthesized unsaturated NHC upon binding to the gold. Theoretical calculations assist in explaining this phenomenon by showing that the NHCs primarily lie flat on the gold surfaces, which exposes the backbone protons on the Cs-symmetric NHC to a formal elimination of H2, while the C2-symmetric NHC remains protected from this elimination reaction. These results raise critical questions as to how the structure of NHC ligands may be tuned to influence binding and reactivity on gold surfaces.