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