Mechanism of the Hydrosilylation Reaction of Alkenes at Porous Silicon:  Experimental and Computational Deuterium Labeling Studies

Abstract

The mechanism of the formation of Si-C bonded monolayers on silicon by reaction of 1-alkenes with hydrogen-terminated porous silicon surfaces has been studied by both experimental and computational means. We propose that monolayer formation occurs via the same radical chain process as at single-crystal surfaces: a silyl radical attacks the 1-alkene to form both the Si-C bond and a radical center on the beta-carbon atom. This carbon radical may then abstract a hydrogen atom from a neighboring Si-H bond to propagate the chain. Highly deuterated porous silicon and FTIR spectroscopy were used to provide evidence for this mechanism by identifying the IR bands associated with the C-D bond formed in the proposed propagation step. Deuterated porous silicon surfaces formed by galvanostatic etching in 48% DF/D2O:EtOD (1:1) electrolytes showed a 30% greater density of Si-D sites on the surface than Si-H sites on hydrogen-terminated porous silicon surfaces prepared in the equivalent H-electrolyte. The thermal reaction of undec-1-ene and the Lewis acid catalyzed reaction of styrene on a deuterated surface both resulted in alkylated surfaces with the same C-C and C-H vibrational features as formed in the corresponding reactions at a hydrogen-terminated surface. However, a broad band around 2100 cm(-1) was observed upon alkylating the deuterated surfaces. Ab initio and density functional theory calculations on small molecule models showed that the integrated absorbance of this band was comparable to the intensity expected for the C-D stretches predicted by the chain mechanism. The calculations also indicate that there is substantial interaction between the hydrogen atoms on the beta-carbons and the hydrogen atoms on the Si(111)-H surface. These broad 2100 cm(-1) features are therefore assigned to C-D bands arising from the involvement of surface D atoms in the hydrosilylation reactions, while the line broadening can be explained partly by interaction with neighboring surface atoms/groups.

Silicon Surface Modification and Characterization for Emergent Photovoltaic Applications Based on Energy Transfer

“…A number of reports have examined, quantified, and confirmed the kinetics related to this reaction using first-principle calculations. 149,150 Another method is hydrosilylation initiated by hydride abstraction using compounds such as triphenylcarbenium cations ( Figure 6). 151 In this case, a positively charged silicon surface is created after the cation removes H − , which in turn enables a nucleophilic attack by the CC double bond (electron-rich).…”

Chemical manipulation of multifunctional hydrocarbons on silicon surfaces

“…Different methods of initiation of this chain reaction and a variety of measurements with possible applications to molecular conductance have recently focused on this molecule and similar systems on a series of substrates [188,208,222,411, [427][428][429][430][431][432][433][434][435][436][437][438]. It is worth noting that in the case of radical initiation with TEMPO [256,420], this species can serve both as the initiator of the reaction and as a protecting group, as illustrated in Fig.…”

Interfacing porous silicon with biomolecules

“…Three different Si-H stretching vibrations as well as the vibration corresponding to SiH 2 scissoring appear in the spectrum in figure 1, with SiH 3 stretching at 2140cm -1 , SiH 2 at 2120cm -1 and SiH at 2091cm -1 [11]. The spectrum also shows a small amount of oxidation, by the peaks at 1075cm -1 that corresponds to Si-O stretching vibrations [12].…”