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Abstract Metal-metal contacts, though not yet widely realized, may provide exciting opportunities to serve as tunable and functional interfaces in single-molecule devices. One of the simplest components which might facilitate such binding interactions is the ferrocene group. Notably, direct bonds between the ferrocene iron center and metals such as Pd or Co have been demonstrated in molecular complexes comprising coordinating ligands attached to the cyclopentadienyl rings. Here, we demonstrate that ferrocene-based single-molecule devices with Fe-Au interfacial contact geometries form at room temperature in the absence of supporting coordinating ligands. Applying a photoredox reaction, we propose that ferrocene only functions effectively as a contact group when oxidized, binding to gold through a formal Fe³⁺center. This observation is further supported by a series of control measurements and density functional theory calculations. Our findings extend the scope of junction contact chemistries beyond those involving main group elements, lay the foundation for light switchable ferrocene-based single-molecule devices, and highlight new potential mechanistic function(s) of unsubstituted ferrocenium groups in synthetic processes. 

A new ligand comprising directly-connected disulfide-based anchors provides access to air-stable metal bis(terpyridine) complexes for the functionalization of metal surfaces. 

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. 

Abstract A single‐molecule method has been developed based on the scanning tunneling microscope (STM) to selectively couple a series of aniline derivatives and create azobenzenes. The Au‐catalyzed oxidative coupling is driven by the local electrochemical potential at the nanostructured Au STM tip. The products are detected in situ by measuring the conductance and molecular junction elongation and compared with analogous measurements of the expected azobenzene derivatives prepared ex situ. This single‐molecule approach is robust, and it can quickly and reproducibly create reactions for a variety of anilines. We further demonstrate the selective synthesis of geometric isomers and the assembly of complex molecular architectures by sequential coupling of complementary anilines, demonstrating unprecedented control over bond formation at the nanoscale.