Spatially controlled Suzuki and Heck catalytic molecular coupling

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

Is the Suzuki-Miyaura Cross-Coupling Reaction in the Presence of Pd Nanoparticles Heterogeneously or Homogeneously Catalyzed? An Interfacial Surface-Enhanced Raman Spectroscopy Study

Pd-catalyzed Suzuki-Miyaura C-C cross-coupling is very central in chemistry. The question of whether the catalysis by using Pd nanoparticles (NPs) is heterogeneous (on the Pd surface) or homogeneous (by soluble Pd released from the NP surface) remains under fundamental physicochemical debate. This work reports on the in situ characterization of the Suzuki-Miyaura cross-coupling reactions by using surface-enhanced Raman spectroscopy. We find clear evidence of heterogeneous catalysis on the Pd surface. In contrast, the soluble Pd species leaching into the solution cannot catalyze the reaction, indicating a direct contact of the aryl halides with the metal surface is a prerequisite. Accordingly, the surface ligands and charge of the Pd NPs, which determine the molecule-metal contact, are very important in the couplings. By a simple exchange of the surface ligand or a surface modification of the support material, the catalytic activity of Pd NPs is improved due to the enhanced electric attraction between the metal and the reactant molecules.

Spatially resolved acyl transfer on surface by organo-catalytic scanning probe nanolithography (o-cSPL)

Local and catalytic acyl transfer for multipatterning of surfaces.

Groundbreaking research done in the area of nanolithography makes it a versatile tool to produce nanopatterns for a broad range of chemical surface functionalization or physical modifications. We report for the first time an organocatalytic scanning probe nanolithography (o-cSPL) approach. Covalent binding of an organocatalyst on the apex of an atomic force microscope (AFM) tip gives way to a system that allows the formation of locally defined acylated-alcohol patterns on self-assembled monolayers (SAMs). With resolutions comparable to those of other cSPL methods, this first example of o-cSPL holds promise for future applications of bottom-up nanolithography set-ups employing this novel technique.

Fabrication and catalytic properties of ordered cyclopalladated diimine monolayer : investigation on catalytic mechanism

“Channel-like” self-assembled monolayers having aliphatic and aromatic diimines (denoted as Si@1DIS, Si@2DIS and Si@3DIS) immobilized on substrates and their palladacycle monolayers (Si@1DIS-Pd, Si@2DIS-Pd and Si@3DIS-Pd) were prepared and characterized. Their catalytic performances were investigated using the Suzuki coupling reaction as a model. Si@3DIS-Pd showed the highest catalytic activity in water without ligands, and better recyclability than that of Si@2DIS-Pd and Si@1DIS-Pd. The reason was the carbon in the aliphatic diimine of Si@2DIS-Pd and Si@1DIS-Pd was easily hydrolyzed because of the active hydrogen of α-C, resulting in poor recyclability. Control of the amount of catalyst could be achieved by modulating the diameter of the channel-like structure, which also affected the catalytic activity. The catalytic process and mechanism were investigated systematically and proposed based on the experimental results obtained by the water contact angle, ultraviolet spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry and atomic force spectroscopy. Changes in the morphology of monolayer surfaces during the catalytic process with or without stirring presented a clear process from order to disorder, and indicated that the reaction was a heterogeneous catalytic process occurring on the surface of the catalyst monolayer.

“Channel-like” self-assembled monolayers having aliphatic and aromatic diimines (denoted as Si@1DIS, Si@2DIS and Si@3DIS) immobilized on substrates and their palladacycle monolayers (Si@1DIS-Pd, Si@2DIS-Pd and Si@3DIS-Pd) were prepared and characterized.

The influence of carbon surface chemistry on supported palladium nanoparticles in heterogeneous reactions

The surface chemistry of nanocarbon support can tailor chemical properties of precious metal nanoparticle/nanocarbon hybrid catalyst in heterogeneous reactions. We report on modified reduced graphene oxide (rGO) support with ionic liquid-derived carbonaceous surface for palladium nanoparticle (Pd NPs) decoration and their actions in different heterogeneous reactions. The surface chemistry of support materials was characterized in detail, and the influence of which on the formation and distribution of metal particles was further investigated. Three different types of reactions including Suzuki-Miyaura coupling reaction, CO oxidation and phenol reduction were examined in terms of reactivity and selectivity. The roles of substituted nitrogen in graphitic lattice and grafted groups on the carbon surface were exploited. Nitrogen-doping can give rise to changes in electronic properties of supported metals, and the Lewis basicity of the doped nitrogen atoms can favor the adsorption of acidic reactants in phenol reduction. The grafted groups derived a negative impact to the Suzuki-Miyaura coupling reaction, due to the involvement of larger reactant molecules, despite that they could prevent significant sintering of Pd NPs in the CO oxidation.

Catalytic Scanning Probe Nanolithography (cSPL): Control of the AFM Parameters in Order to Achieve Sub-100-nm Spatially Resolved Epoxidation of Alkenes Grafted onto a Surface

Scanning probe lithography (SPL) appears to be a reliable alternative to the use of masks in traditional lithography techniques as it offers the possibility of directly producing specific chemical functionalities with nanoscale spatial control. We have recently extend the range of applications of catalytic SPL (cSPL) by introducing a homogeneous catalyst immobilized on the apex of a scanning probe. Here we investigate the importance of atomic force microscopy (AFM) physical parameters (applied force, writing speed, and interline distance) on the resultant chemical activity in this cSPL methodology through the direct topographic observation of nanostructured surfaces. Indeed, an alkene-terminated self-assembled monolayer (alkene-SAM) on a silicon wafer was locally epoxidized using a scanning probe tip with a covalently grafted manganese complex bearing the 1,4,7-triazacyclononane macrocycle as the ligand. In a post-transformation process, N-octylpiperazine was covalently grafted to the surface via a selective nucleophilic ring-opening reaction. With this procedure, we could write various patterns on the surface with high spatial control. The catalytic AFM probe thus appears to be very robust because a total area close to 500 μm(2) was patterned without any noticeable loss of catalytic activity. Finally, this methodology allowed us to reach a lower lateral line resolution down to 40 nm, thus being competitive and complementary to the other nanolithographical techniques for the nanostructuration of surfaces.

Harnessing catalysis to enhance scanning probe nanolithography

The use of scanning probes bearing catalysts to perform surface nanolithography combines the exquisite spatial precision of scanning probe microscopy with the synthetic capabilities of (bio)chemical catalysis. The ability to use these probes to direct a variety of localised chemical reactions enables the generation of nanoscale features with a high degree of chemical complexity in a "direct-write" manner. This article surveys the range of reactions that have been employed and the key factors necessary for the successful use of such catalytic scanning probes. These factors include the experimental parameters such as write speed, force applied to the probes and temperature; as well as the processes involved in the preparation of the catalysts on the probes and the surface that is to be fabricated. Where possible, the various reactions are also compared and contrasted; and future perspectives are discussed.

The recyclable cyclopalladated ferrocenylimine self-assembly catalytic film and investigation of its role in the mechanism of heterogeneous catalysis

The cyclopalladated ferrocenylimine self-assembly film, grafting onto solid substrate surfaces by covalent bond, exhibited efficiency, reusability and stability in the Suzuki–Miyaura reaction. A cycle of Pd^II to Pd⁰ and Pd⁰ to Pd^II on the surface was clearly detected and illustrated.

Building upon patterned organic monolayers produced via catalytic stamp lithography

Soft lithographic sub-100 nm chemical patterning was demonstrated on organic monolayer surfaces using poly(dimethylsiloxane)-based stamps decorated with Pd nanostructures, structures termed "catalytic stamps". Chemically reactive azide or alkene functionalities were incorporated on oxide-capped silicon surfaces and utilized for patterning via Pd-catalyzed hydrogenation or Heck reactions. The catalytic stamps were soft lithographic stamps based on PDMS with embedded nanoscale palladium catalysts, prepared via block copolymer-based templating. Nanoscale chemical patterns were readily generated on the azide or alkene precursor surfaces simply by applying the Pd catalytic stamps and the reactive molecule, the molecular ink, to the surface, thanks to the highly localized catalytic transformations induced by the patterned, immobilized solid Pd catalysts. A series of successful postfunctionalization reactions on the resulting patterned surfaces further demonstrated the utility of this approach to construct novel designs of nanoarchitectures, with potentially unique and innovative properties.

Mechanistic studies of AFM probe-driven Suzuki and Heck molecular coupling

The initiation and high resolution control of surface confined chemical reactions would be both beneficial for nanofabrication and fundamentally interesting. In this work, mechanistic aspects of spatially confined Suzuki and Heck 'catalytic nanolithography' are investigated by varying the experimental parameters and noting their effects on apparent reactivity and lithographic yield. A variety of molecular couplings can be driven by the activated scanning probe with linewidths ultimately limited by its functional apex. On ramping the speed at which the probe traverses the surface confined reagent, one reaches a limit where the catalytic chemistry becomes limiting and reaction rates can be estimated. Self-assembled monolayer (SAM) confined bromide reagents are observed to be more labile than their iodide analogues in Suzuki couplings. With Heck reactions, coupling rates are observed to be faster with the styrene surface confined than in the 'inverted configuration' with an aryl halide SAM, possibly because the accepted (solution phase) rate-determining step is bypassed in the former configuration. For both Heck and Suzuki coupling reactions, calculations indicate that catalyst turnover numbers per nanoparticle are an order of magnitude or more greater within the confines of the AFM probe-surface junction compared to solution phase reactions. A PVP matrix model is presented to account for these observations and the mechanism of catalytic nanolithography.

An Efficient and Reusable PdCl2/TBAF System for the Heck Reaction Under Ligand- and Solvent-Free Conditions

PdCl2 combined with TBAF (n-Bu4NF) were found to be efficient conditions for the Heck reactions. In the presence of PdCl2 and TBAF, a variety of aryl halides were coupled with alkenes smoothly in moderate to good yields. Moreover, the PdCl2/TBAF system could be recovered and reused several times in the reaction. Note that these reactions are conducted under solvent-free, ligand-free and reusable conditions.

Nanoscale patterning of organic monolayers by catalytic stamp lithography: scope and limitations

Developing a method to pattern organic molecules, particularly on the sub-100-nm scale, is of wide interest in current nanoscience for a broad range of technological applications. Because of the efficiency and simplicity of soft lithography, here we describe in detail the process for synthesizing and applying catalytic stamp lithography, a process that can easily produce sub-100-nm patterns on surfaces; in this work, the approach is demonstrated on silicon. Catalytic stamps were fabricated through a two-step procedure in which the nanoscale pattern of catalysts is produced via a self-assembled block-copolymer-templated synthesis of metallic nanostructures on SiO(x)/Si supports, followed by the production of the poly(dimethylsiloxane) (PDMS) stamp on top of the as-patterned metals. Simply peeling off the as-formed PDMS stamp removes the metallic nanostructures, leading to the functional stamp. Two different patterns, pseudohexagonal and linear Pt nanoarrays, were produced from a single block copolymer, PS(125000)-b-P2VP(58500), by controlling the morphology of thin-film templates through tetrahydrofuran vapor annealing. When terminal alkenes, alkynes, or aldehydes with different functionalities were used as molecular inks, these Pt nanopatterns on catalytic stamps were translated into corresponding molecular arrays on Si(111)-H and Si(100)-H(x) surfaces because catalytic hydrosilylation took place exclusively underneath patterned Pt nanostructures. With this straightforward approach, the resolution limit of conventional microcontact printing (approximately 100 nm) could be downsized to a sub-20-nm scale, while maintaining the advantages of stamp-based patterning (i.e., large-area, high-throughput capabilities and low cost).

Self-assembled monolayers of a bis(pyrazol-1-yl)pyridine-substituted thiol on Au(111)

Self-assembled monolayers (SAMs) of a bis(pyrazol-1-yl)pyridine-substituted thiol (bpp-SH) on Au (111)/mica were studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Using substrates precoated with perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA), preparation at elevated temperatures yields highly ordered layers whose structure is described by a rectangular (5 x radical3) unit cell containing one molecule. The bis(pyrazol-1-yl)pyridine (bpp) units exhibit pi-stacking along the 112 direction, and they are tilted significantly. We conclude the three imine nitrogen atoms in the bpp headgroup adopt a trans,trans arrangement.

Sequential electrochemical oxidation and site-selective growth of nanoparticles onto AFM probes

In this work, we reported an approach for the site-selective growth of nanoparticle onto the tip apex of an atomic force microscopy (AFM) probe. The silicon AFM probe was first coated with a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) through a chemical vapor deposition (CVD) method. Subsequently, COOH groups were selectively generated at the tip apex of silicon AFM probes by applying an appropriate bias voltage between the tip and a flat gold electrode. The transformation of methyl to carboxylic groups at the tip apex of the AFM probe was investigated through measuring the capillary force before and after electrochemical oxidation. To prepare the nanoparticle terminated AFM probe, the oxidized AFM probe was then immersed in an aqueous solution containing positive metal ions, for example, Ag+, to bind positive metal ions to the oxidized area (COOH terminated area), followed by chemical reduction with aqueous NaBH 4 and further development (if desired) to give a metal nanoparticle-modified AFM probe. The formation of a metal nanoparticle at the tip apex of the AFM probe was confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA).

A new route to the production and nanoscale patterning of highly smooth, ultrathin zirconium oxide films

Metal-stabilized bilayers, prepared by the self-assembly of octadecyltrichorosilane on an oxidized silicon surface followed by the Langmuir-Blodgett deposition of a monolayer of octadecylphosphonic acid, have been used to generate 1.6 nanometer thick, highly uniform, zirconium oxide films following annealing. Patterning of the thin films on the nanometre scale was achieved using nanodisplacement methodology, by careful control of an atomic force microscope (AFM) probe, which allowed the selective removal of the upper leaflet of the bilayer.