Chandran Aruna, Dominique Nathaniel L, Kaur Gurkiran, Clark Vincent, Nalaoh Phattananawee, Ekowo Lilian Chinenye, Jensen Isabel M, Aloisio Mark D, Crudden Cathleen M, Arroyo-Currás Netzahualcóyotl, Jenkins David M, Camden Jon P
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA.
Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA.
Nanoscale. 2025 Feb 27;17(9):5413-5428. doi: 10.1039/d4nr04428b.
N-Heterocyclic carbenes (NHCs) are unrivaled in their ability to form persistent and tunable monolayers on noble metal surfaces, with disciplines from heterogeneous catalysis to microelectronics fabrication rapidly adopting this technology. It is currently assumed that different NHC monolayer preparation protocols yield equivalent surfaces; however, a direct comparison of the leading synthetic protocols is yet to validate this assumption. Herein, we explore the binding of NHC ligands to gold (Au) surfaces prepared using the five most common NHC deposition methods and discover significant differences in the resulting monolayer composition and structure. In this work, NHC-Au systems are prepared according to literature procedures starting from either the free carbene, the CO adduct, the bicarbonate salt, or the triflate salt. The resulting surfaces are characterized with surface-enhanced Raman spectroscopy (SERS), laser desorption/ionization mass spectrometry (LDI-MS), electrochemistry, and X-ray photoelectron spectroscopy (XPS). These data indicate that the free carbene, vacuum annealing, and solvent annealing methods form chemisorbed NHC monolayers, as expected; however, the solution phase methods without annealing yield surfaces with a fundamentally different character. Although XPS is widely used to confirm the binding of NHCs to metal surfaces, it does not capture the differences in these deposition procedures and should be used with caution. Taken together, these results reveal a significant variation of the NHC surface structure as a function of deposition procedure and provide a critical benchmark to govern the design and preparation of future NHC monolayer systems.
N-杂环卡宾（NHCs）在贵金属表面形成持久且可调谐单分子层的能力方面无与伦比，从多相催化到微电子制造等多个学科都在迅速采用这项技术。目前人们认为，不同的NHC单分子层制备方案会产生等效的表面；然而，对主要合成方案的直接比较尚未证实这一假设。在此，我们探索了NHC配体与使用五种最常见的NHC沉积方法制备的金（Au）表面的结合情况，并发现所得单分子层的组成和结构存在显著差异。在这项工作中，NHC-Au体系是根据文献方法从游离卡宾、CO加合物、碳酸氢盐或三氟甲磺酸盐开始制备的。所得表面通过表面增强拉曼光谱（SERS）、激光解吸/电离质谱（LDI-MS）、电化学和X射线光电子能谱（XPS）进行表征。这些数据表明，游离卡宾、真空退火和溶剂退火方法形成了化学吸附的NHC单分子层，正如预期的那样；然而，未经退火的溶液相方法产生的表面具有根本不同的特性。尽管XPS被广泛用于确认NHC与金属表面的结合，但它无法捕捉这些沉积过程中的差异，因此应谨慎使用。综上所述，这些结果揭示了NHC表面结构随沉积过程的显著变化，并为未来NHC单分子层系统的设计和制备提供了关键的基准。
