Combined XPS and DFT investigation of the adsorption modes of methyl enol ether functionalized cyclooctyne on Si(001)

Abstract

The reaction of methyl enol ether functionalized cyclooctyne on the silicon (001) surface was investigated by means of X‐ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Three different groups of final states were identified; all of them bind on Si(001) via the strained triple bond of cyclooctyne but they differ in the configuration of the methyl enol ether group. The majority of molecules adsorbs without additional reaction of the enol ether group; the relative contribution of this configuration to the total coverage depends on substrate temperature and coverage. Further configurations include enol ether groups which reacted on the silicon surface either via ether cleavage or enol ether groups which transformed on the surface into a carbonyl group.

Click Chemistry in Ultra‐high Vacuum – Tetrazine Coupling with Methyl Enol Ether Covalently Linked to Si(001)

“…Furthermore, it is orthogonal to the strain‐promoted cycloaddition of cyclooctynes to azides [7] . Given the fact that substituted cyclooctynes can react in a chemoselective, strain promoted 2+2 cycloaddition on Si(001), [8–12] an orthogonal cycloaddition sequence under UHV‐conditions could be realized when combining both reaction schemes (Figures 1(b) and (c)).…”

“…Further evidence is obtained from the O 1s spectra shown in Figure 3, which were measured during the same experiments as the N 1s spectra shown in Figure 2. It has to be taken into account that the adsorbed MEECO on the Si(001) surface already accounts for three peaks in the O 1s spectra (Figure 3(d)), as discussed in detail in a previous work [12] . In brief, these components can be assigned to the intact ether group (534.1 eV), [16] oxygen from the ether group reacted on silicon (532 eV), [17] and a C=O group as a product of CH 2 abstraction (532.7 eV) [18] .…”

“…We carried out DFT calculations to shed light on the reaction of adsorbed MEECO with tetrazine with the main results summarized in Figure 5. We start from intact MEECO adsorbed on Si(001) as investigated before [12] . The reaction with tetrazine first leads to a physisorbed pre‐complex 2 – 3 bound by 29 kJ mol −1 at room temperature (all energies refer to Gibbs free energies, ▵ G ).…”

“… Summary of the reaction investigated: MEECO ( 1 ) reacts on the dimers of Si(001) via the strained triple bond of cyclooctyne forming 2 [12] . 1,2,4,5‐Tetrazine‐3,6‐dicarboxylate ( 3 ) reacts with 2 via 4 to the final product 5 releasing N 2 and MeOH.…”

“…[12]. MEECO adsorption on Si(001) preferentially takes place via the strained triple bond of the cyclooctyne ring; side reactions include ether cleavage and the formation of an aldehyde group [12] …”