Back to Table of Contents

45004-B10
Investigating the Chemistry of Boranes with Single-Walled Carbon Nanotubes Using FTIR and Raman Spectroscopies

Mark Ellison, Ursinus College

This year, the project has made slow but steady progress.  Work in the first year of the grant found evidence that borane complexes did not react with single-walled carbon nanotubes (SWCNTs) at 0°C or at room temperature.  However, research in fall 2007 found that, when irradiated with ultraviolet light, SWCNTs do appear to react with borane-tetrahydrofuran.  Continuation of this work in summer 2008 found that SWCNTs appear to react with borane-dimethyl sulfide but not with borane-triethylamine or borane-tert-butylamine when irradiated with ultraviolet light.  These results suggest that the more reactive borane complexes will react with SWCNTs when irradiated with ultraviolet light, but the more stable borane complexes will not.  The reaction appears to be a hydroboration reaction, which attaches a –BH₂ group and a –H group to the nanotubes.  Efforts to isolate these intermediates were difficult, presumably because of the reactivity of the –BH₂ group.  Nonetheless, one of the student researchers, through careful experimentation, did reproducibly obtain infrared spectra of SWCNTs with peaks at 2300 cm^–1 and 2900 cm^–1 attributed to B-H stretches and C-H stretches, respectively, as shown in Figure 1.

Figure 1.  FTIR spectrum of SWCNTs reacted with borane-tetrahydrofuran complex under UV irradiation.  Peaks at 2300 cm^–1 are consistent with B–H stretches, and peaks at 2950 cm^–1 are consistent with C–H stretches.

Work in spring 2008 determined that the reaction is initiated by light of wavelength less than 300 nm.  This suggests that the SWCNTs are absorbing the ultraviolet light to undergo a p→ p^* transition.  This could interrupt the aromaticity of the nanotubes' structure, increasing their reactivity.  Computational studies indicated that the hydroboration reaction is endoergonic, suggesting that it is likely not spontaneous without some energy input.  Currently, experiments are under way to elucidate the photochemical reaction mechanism.

To further test our results, in late summer 2008 and in fall 2008, SWCNTs reacted with borane-tetrahydrofuran under ultraviolet irradiation were then reacted with a sodium hydroxide/hydrogen peroxide mixture.  These conditions are those of a classic hydroboration-oxidation reaction, in which borane adds across a double bond to form –BH₂ and –H groups, and the –BH₂ group is subsequently oxidized to –OH.  Infrared spectra of the SWCNTs subjected to these steps show O–H stretches at 3400 cm^–1 and C–H stretching peaks at 2900 cm^–1, as shown in Figure 2, which are consistent with our hypothesis.  This strongly suggests that the SWCNTs are undergoing a photo-initiated hydroboration/oxidation reaction.

Figure 2.  FTIR spectrum of hydroborated and oxidized SWCNTs.  The broad peak at 3400 cm^–1 is consistent with an O–H stretch, and peaks at 2900 cm^–1 are consistent with

C–H stretches.  The peak at 2300 cm^–1 might be unreacted –BH₂ on the SWCNTs.

A second project is investigating the reductive amination of SWCNTs.  Certain borane complexes, such as a-picoline borane, have been shown to allow for the reduction of carbonyl groups, followed by an amine attachment.  Because SWCNTs can easily be oxidized to yield carboxylic acid functional groups, reductive amination could potentially be a new pathway to producing functionalized SWCNTs.  Figure 3 shows an FTIR spectrum of carboxylated SWCNTs that have undergone reductive amination with a-picoline borane and butylamine.  The peaks just below 3000 cm^–1 are consistent with the sp³-hybridized C-H stretches of butylamine molecules.  The broad peak at 3250 cm^–1 is consistent with an amide that could form during the reductive amination, although it is broader than expected.  This spectrum suggests that the reductive amination reaction might be successful and is worthy of further research.

Figure 3.  FTIR spectrum of carboxylated SWCNTs that have undergone a reductive amination.  The peak at 3250 cm^–1 might be an amide peak, and peaks at 2900 cm^–1 are

C–H stretches.  The peak at 1700 cm^–1 is unreacted carbonyl groups.

In summary, we are close to demonstrating that photochemical hydroboration/oxidation can functionalize SWCNTs.  Additionally, a reductive amination reaction involving a borane complex also appears to have promise as a means of functionalizing carboxylated SWCNTs.  Work on these two projects will continue.

Back to top