Chang Kuan, Chen Jingguang G, Lu Qi, Cheng Mu-Jeng
Department of Chemical Engineering, Tsinghua University , Beijing, China.
Department of Chemical Engineering, Columbia University , New York, New York 10027, United States.
J Phys Chem A. 2017 Apr 6;121(13):2674-2682. doi: 10.1021/acs.jpca.7b01153. Epub 2017 Mar 27.
There is increasing interest in using N-heterocyclic carbenes (NHCs) as surface ligands to stabilize transition-metal nanoparticles (NPs) and to replace thiols for the preparation of self-assembled monolayers (SAMs) on gold surfaces. This type of surface decoration is advantageous because it leads to improved catalytic activity of NPs and increased stability of SAM, as shown by recent experiments. In this work, we used quantum mechanics combined with periodic surface models to study the adsorption of NHCs on the Au(111) surface. We found that NHCs prefer to bind to the top site with adsorption energies (ΔEs) varying from 1.69 to 2.34 eV, depending on the type of NHC, and the inclusion of solvents in the calculations leads to insignificant variation in the calculated ΔEs. Three types of NHCs were found to bind to Au(111) more tightly and therefore should be better stabilizers than those commonly used. Importantly, by analyzing electronic structures using the Bader charge and energy decomposition analysis, we find that during adsorption NHC acts as an electron donor, transferring its electron density from the lone pair orbital at the carbene center to the empty d orbital of Au with negligible π-back-donation. This binding pattern is very different from that of CO, a ligand commonly used in organometallics, where both interactions are equally important. This leads to the identification of the protonation energies of NHCs as a descriptor for predicting ΔEs, providing a convenient method for computational high-throughput screening for better NHC-type surface ligands.
人们越来越关注使用N-杂环卡宾（NHCs）作为表面配体来稳定过渡金属纳米颗粒（NPs），并在金表面取代硫醇以制备自组装单分子层（SAMs）。正如最近的实验所示，这种类型的表面修饰具有优势，因为它能提高NPs的催化活性并增强SAMs的稳定性。在这项工作中，我们使用量子力学结合周期性表面模型来研究NHCs在Au(111)表面的吸附。我们发现，根据NHC的类型，NHCs倾向于以1.69至2.34 eV的吸附能（ΔEs）结合到顶位，并且在计算中包含溶剂会导致计算出的ΔEs变化不显著。发现三种类型的NHCs与Au(111)结合更紧密，因此应该比常用的那些是更好的稳定剂。重要的是，通过使用巴德电荷和能量分解分析来分析电子结构，我们发现吸附过程中NHC充当电子供体，将其电子密度从卡宾中心的孤对轨道转移到Au的空d轨道，且π-反馈作用可忽略不计。这种结合模式与有机金属化学中常用的配体CO非常不同，在CO中两种相互作用同样重要。这导致将NHCs的质子化能确定为预测ΔEs的描述符，为计算高通量筛选更好的NHC型表面配体提供了一种便捷方法。
