Spectroscopic Evidence for a Covalent Sigma Au–C Bond on Au Surfaces Using ¹³C Isotope Labeling

Abstract

The Au–C linkage has been demonstrated as a robust interface for coupling thin organic films on Au surfaces. However, the nature of the Au–C interaction remains elusive up to now. Surface-enhanced Raman spectroscopy was previously used to assign a band at 412 cm –1 as a covalent sigma Au–C bond for films generated by spontaneous reduction of the 4-nitrobenzenediazonium salt on Au nanoparticles. However, this assignment is disputed based on our isotopic shift study. We now provide direct evidence for covalent Au–C bonds on the surface of Au nanoparticles using 13 C cross-polarization/magic angle spinning solid-state NMR spectroscopy combined with isotope substitution. A 13 C NMR shift at 165 ppm was identified as an aromatic carbon linked to the gold surface, while the shift at 148 ppm was attributed to C–C junctions in the arylated organic film. This demonstration of the covalent sigma Au–C bond fills the gap in metal–C bonds for organic films on surfaces, and it has great practical and theoretical significance in understanding and designing a molecular junction based on the Au–C bond.

Substrate-Modulated Synthesis of Metal–Organic Hybrids by Tunable Multiple Aryl–Metal Bonds

“…To form typical coordination bonds, as represented by M–N and M–O bonds, − organic ligands contribute lone-pair electrons to bind with the metal ions. In contrast, the combination of aromatic sigma radicals and metal atoms predominantly by sharing their unpaired electrons provides covalent aryl–metal bonds. − Despite variable structural regulations of coordination-based MOFs on single crystal surfaces, − structurally tunable metal–organic hybrids (MOHs) based on covalent aryl–metal bonds still represent a high challenge due to the stronger metal–organic interactions compared with the above coordination bonds. Since covalent aryl–metal bonds appear as the basic linkages for the assembly of polycyclic aromatic hydrocarbons, also known as emerging nanographene materials for organic electronics, − developing diverse MOHs is therefore fundamentally important for either fabricating molecular integrated circuits or understanding the electronic properties of functional 2D materials. , …”

Synthesis of γ-Pyrones and N-Methyl-4-pyridones via the Au Nanoparticle-Catalyzed Cyclization of Skipped Diynones in the Presence of Water or Aqueous Methylamine

“…Intermediate I undergoes purely intramolecular Michael-type nucleophilic attack by the enol hydroxyl (6- endo mode) on the other Au NP-activated triple bond to form intermediate II , and after protodeauration, the final γ-pyrone. Although in Scheme we provide for clarity full charges in the intermediates, in fact, they should be partial, given that the Au n –carbon σ bonds are partially polarized. , …”

Bifunctional Calix[4]arene-Coated Gold Nanoparticles for Orthogonal Conjugation

“…It is known that, upon reduction of their diazonium groups, − calix[4]­arene-tetradiazonium salts , can form multiple C-metal bonds with the surface of gold or silver nanoparticles, leading to thin and extremely robust organic layers. − When the calix[4]­arene is substituted with a reactive functional group at its small rim (e.g., carboxyl), it is possible to further post-functionalize the nanoparticles with various (bio)­molecules. , Moreover, the irreversible nature of the grafting and the similar reactivity toward surfaces of calix[4]­arene-tetradiazonium salts bearing different substituents at the small rim allow the formation of mixed layers with control over the proportions of the different calix[4]­arenes used. , We thus envisioned that the calixarene-based methodology could be used for the preparation of AuNPs bearing multiple ligands that would enable further orthogonal (bio)­conjugation. For this, the concomitant grafting of calixarenes bearing alkyne or carboxyl groups was envisaged.…”

“…22 The limitations of these two methods are particularly problematic for applications requiring multi-functional AuNPs, for example, when both a targeting ligand and a drug have to be anchored on a same particle. 23−27 It is known that, upon reduction of their diazonium groups, 28−30 calix [4]arene-tetradiazonium salts 31,32 can form multiple C-metal bonds 33 with the surface of gold or silver nanoparticles, leading to thin and extremely robust organic layers. 34−36 When the calix [4]arene is substituted with a reactive functional group at its small rim (e.g., carboxyl), it is possible to further post-functionalize the nanoparticles with various (bio)molecules.…”