N-heterocyclic carbene self-assembled monolayers on copper and gold : dramatic effect of wingtip groups on binding, orientation and assembly

Funding: EPSRC PhD studentship (EP/M506631/1).Self‐assembled monolayers of N‐heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N‐substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl‐ or ethyl‐substituted NHCs lie flat. Temperature‐programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes=152±10 kJ mol−1. NHCs that bind upright desorb cleanly, while flat‐lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2Cu species by extraction of a copper atom from the surface after annealing.Publisher PDFPeer reviewe

Simple direct formation of self-assembled N-heterocyclic carbene monolayers on gold and their application in biosensing

CRL acknowledges the Engineering and Physical Sciences Research Council (UK) for the funding of his PhD studentship (EP/M506631).The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol−1) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.Publisher PDFPeer reviewe

Fabrication of a high-quality, porous, surface-confined covalent organic framework on a reactive metal surface

A major goal of heterogeneous catalysis is to optimize catalytic selectivity. Selectivity is often limited by the fact that most heterogeneous catalysts possess sites with a range of reactivities, resulting in the formation of unwanted by-products. The construction of surface-confined covalent organic frameworks (sCOFs) on catalytically active surfaces is a desirable strategy, as pores can be tailored to operate as catalytic nanoreactors. Direct modification of reactive surfaces is impractical, because the strong molecule–surface interaction precludes monomer diffusion and formation of extended architectures. Herein, we describe a protocol for the formation of a high-quality sCOF on a Pd-rich surface by first fabricating a porous sCOF through Ullmann coupling on a Au-rich bimetallic surface on Pd(111). Once the sCOF has formed, thermal processing induces a Pd-rich surface while preserving the integrity of the sCOF architecture, as evidenced by scanning tunneling microscopy and titration of Pd sites through CO adsorption

Copper adatoms mediated adsorption of benzotriazole on a gold substrate

Funding: UK Engineering and Physical Sciences Research Council (EPSRC), Grant Number(s) EP/M506631/1; FP7 Nanosciences Nanotechnologies Materials and New Production Technologies, Grant Number(s) 312284.The adsorption of organic molecules on metal surfaces is often mediated by metal adatoms, however their effect on the geometry of the resulting supramolecular structure can be difficult to determine. Herein, the role played by reactive copper atoms in the adsorption chemistry of benzotriazole (BTAH), an organic corrosion inhibitor for copper well-known for forming adatom mediated supramolecular structures, on an inert substrate, Au(111), has been investigated in an ultra-high vacuum environment using a combination of complementary surface sensitive techniques and density functional theory calculations. Pseudo-ordered and hydrogen bonded flat-lying assemblies convert into ordered -[CuBTA]n- flat-lying organometallic species of well-defined stoichiometry upon addition of copper atoms from the gas phase onto an Au(111) surface, which has been pre-dosed with BTAH. The formation and characterisation of these organometallic species are discussed in the light of the experimental results and computational modelling. The combination of complementary experimental and computational methods is crucial in order to obtain a thorough characterisation of such a complex system. The final supramolecular structure shows previously unseen BTAH phases, which are favoured by the adsorption distribution of copper adatoms on Au(111), and are stable up to 500 K.Publisher PDFPeer reviewe

On-surface condensation of low-dimensional benzotriazole–copper assemblies

The reactivity of benzotriazole with copper on a gold surface has been studied by a combination of surface sensitive methods with support from DFT (density functional theory) calculations. For some time benzotriazole has been known to enhance the corrosion resistance of copper at the monolayer level, although the exact mechanism is still a matter of discussion and disagreement in the literature. A single crystal Au(111) surface allows to evaluate the interaction of weakly physisorbed, intact benzotriazole molecules with copper atoms dosed to sub-monolayer amounts. These interactions have been characterised, in the temperature range ca. 300 – 650 K, by scanning tunnelling microscopy, high resolution electron energy loss spectroscopy and synchrotron-based X-ray photoemission spectroscopy and near-edged X-ray absorption fine structure studies. Supporting DFT calculations considered the stability of isolated, gas-phase, benzotriazole/Cu species and their corresponding spectroscopic signature at the N K absorption edge. In agreement with previous investigations, benzotriazole physisorbs on a clean Au(111) surface at room temperature forming a hydrogen-bonded network of flat-lying BTAH molecules, relatively weakly bonded to the underlying gold surface. However, in the presence of co-adsorbed copper atoms, proton removal from the molecules leads to species better described as BTA- interacting directly with Cu atoms. In these situations the molecules adopt a more upright orientation and Cu(BTA)2 and -[Cu(BTA)]n- species are formed, depending on temperature and coverage of the adsorbed species. These species are stable to relatively high temperatures, 550 – 600 K

<i>N</i>-heterocyclic carbene self-assembled monolayers on copper and gold:dramatic effect of wingtip groups on binding, orientation and assembly

Self‐assembled monolayers of N‐heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N‐substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl‐ or ethyl‐substituted NHCs lie flat. Temperature‐programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes=152±10 kJ mol−1. NHCs that bind upright desorb cleanly, while flat‐lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2Cu species by extraction of a copper atom from the surface after annealing

Metallosupramolecular assembly of Cr and p-terphenylnitrile by dissociation of metal carbonyls on Au(111)

Metal carbonyl complexes have been widely investigated for use in heterogeneous catalysis as a method for the formation of metal clusters. However their use as a metal source for metallosupramolecular assemblies has not been widely considered. In this study, the combination of Cr(CO)6 with p-terphenylnitrile on Au(111) creates ordered 1D chains which are thermally stable up to 473 K. The bond between the nitrile linkers and metal nodes undergoes a unique π-interaction which thus far has been observed only in a small set of organometallic complexes in solution. The structure and composition of the metal organic coordination network have been elucidated and characterized using scanning tunnelling microscopy, high resolution electron energy loss spectroscopy and density functional theory calculations

Data underpinning - Metallosupramolecular assembly of Cr and p-terphenylnitrile by dissociation of metal carbonyls on Au(111)

Metal carbonyl complexes have been widely investigated for use in heterogeneous catalysis as a method for the formation of metal clusters. However their use as a metal source for metallosupramolecular assemblies has not been widely considered. In this study, the combination of Cr(CO)6 with p-terphenylnitrile on Au(111) creates ordered 1D chains which are thermally stable up to 473 K. The bond between the nitrile linkers and metal nodes undergoes a unique π-interaction which thus far has been observed only in a small set of organometallic complexes in solution. The structure and composition of the metal organic coordination network have been elucidated and characterized using scanning tunnelling microscopy, high resolution electron energy loss spectroscopy and density functional theory calculations