Determination of the structure and geometry of N-heterocyclic carbenes on Au(111) using high-resolution spectroscopy
Determination of the structure and geometry of N-heterocyclic carbenes on Au(111) using high-resolution spectroscopy
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DOI：
10.1039/c8sc03502d
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发表时间：
2019-01-21
期刊：
影响因子：
8.4
通讯作者：
Venkataraman, Latha
中科院分区：
文献类型：
--
作者：
Lovat, Giacomo;Doud, Evan A.;Venkataraman, Latha
N-杂环卡宾（NHC）由于其独特的电子结构而与过渡金属非常强地结合，所述电子结构具有在高度定向的sp（2）杂化轨道中具有孤对的二价碳原子。因此，它们可以在金属表面上组装成单层，与基于硫醇的对应物相比具有增强的稳定性。NHC形成这种稳健的自组装单分子膜（SAM）的效用只是最近才被认识到，许多基本问题仍然存在。本文利用高分辨X射线光电子能谱和密度泛函理论计算研究了一系列在Au（111）表面的NHC的结构和几何构型。我们发现，NHC环上的N-取代基强烈影响分子-金属相互作用，并引导分子在表面层中的取向。与以前的报道相比，我们的实验和理论结果提供了明确的证据表明，NHC与N-甲基取代基结合到欠配位的吸附原子，形成平躺的复合物。在这些自组装膜中，NHC孤对电子和欠配位的Au吸附原子之间的供体-受体相互作用是分子与表面强键合的主要原因。具有较大N-取代基的NHC通过迫使分子进入直立取向来防止这种复合物的形成。我们的工作为NHC单层的成键和几何结构提供了独特的见解;更一般地说，它为使用传统的配位化学合成策略操纵NHC与金属表面之间的相互作用提供了一条清晰的途径。
N-heterocyclic carbenes (NHCs) bind very strongly to transition metals due to their unique electronic structure featuring a divalent carbon atom with a lone pair in a highly directional sp(2)-hybridized orbital. As such, they can be assembled into monolayers on metal surfaces that have enhanced stability compared to their thiol-based counterparts. The utility of NHCs to form such robust self-assembled monolayers (SAMs) was only recently recognized and many fundamental questions remain. Here we investigate the structure and geometry of a series of NHCs on Au(111) using high-resolution X-ray photoelectron spectroscopy and density functional theory calculations. We find that the N-substituents on the NHC ring strongly affect the molecule-metal interaction and steer the orientation of molecules in the surface layer. In contrast to previous reports, our experimental and theoretical results provide unequivocal evidence that NHCs with N-methyl substituents bind to undercoordinated adatoms to form flat-lying complexes. In these SAMs, the donor-acceptor interaction between the NHC lone pair and the undercoordinated Au adatom is primarily responsible for the strong bonding of the molecules to the surface. NHCs with bulkier N-substituents prevent the formation of such complexes by forcing the molecules into an upright orientation. Our work provides unique insights into the bonding and geometry of NHC monolayers; more generally, it charts a clear path to manipulating the interaction between NHCs and metal surfaces using traditional coordination chemistry synthetic strategies.