Positive and negative regulation of carbon nanotube catalysts through encapsulation within macrocycles

One of the most attractive applications of carbon nanomaterials is as catalysts, due to their extreme surface-to-volume ratio. The substitution of C with heteroatoms (typically B and N as p- and n-dopants) has been explored to enhance their catalytic activity. Here we show that encapsulation within weakly doping macrocycles can be used to modify the catalytic properties of the nanotubes towards the reduction of nitroarenes, either enhancing it (n-doping) or slowing it down (p-doping). This artificial regulation strategy presents a unique combination of features found in the natural regulation of enzymes: binding of the effectors (the macrocycles) is noncovalent, yet stable thanks to the mechanical link, and their effect is remote, but not allosteric, since it does not affect the structure of the active site. By careful design of the macrocycles’ structure, we expect that this strategy will contribute to overcome the major hurdles in SWNT-based catalysts: activity, aggregation, and specificity.

Synthesis and characterization of macrocycle mac-AQ: A catalytic amount of Grubb's 1 st generation catalyst was added to a solution of the corresponding linear precursor 1 in dry and degassed DCM, and the mixture was stirred at room temperature. The progress of the reaction was monitored by TLC. When the starting linear precursor was consumed, the reaction was stopped by filtration through a pad of celite. Solvent was evaporated under reduced pressure, and the crude was purified by flash chromatography (Hex:AcOEt 3:1) to obtain the product in 70% yield. 1  MINTs has been reported elsewhere 2,3 . Briefly, the nanotubes (10 mg) were suspended in 10 mL of tetrachloroethane through sonication (10 min) and mixed with 0.01 mmol of linear bisalkene U-shaped precursors of the macrocycles mac-exTTF, mac-pyr or mac-AQ, and Grubbs' second-generation catalyst at room temperature for 72 h. After this time, the suspension was filtered through a PTFE membrane of 0.2 µm pore size and the solid washed profusely with DCM. The solid was resuspended in 10 mL of DCM through sonication for 10 min and filtered through a PTFE membrane of 0.2 µm pore size again. This washing procedure was repeated three times. Samples obtained were denoted as MINT-exTTF, MINT-pyr or MINT-AQ as a function of the threading macrocycle around the nanotubes. The synthesis of the supramolecular complexes denoted as SWNT-exTTF, SWNT-pyr and SWNT-AQ was performed by the direct mixing of the adequate amounts of 6,5-SWNT and the corresponding macrocycle without catalyst to achieve the same functionalization loading of organic material over the nanotube compared to their respective MINT sample.
DFT calculations: All theoretical DFT calculations were carried out within the density functional theory (DFT) approach by using the C.01 revision of the Gaussian 09 program package. 4 Optimization and molecular orbitals calculations of MINT derivative were performed using the long-range corrected B97D density functional 5 , which are able to incorporate the dispersion effects by means of a pair-wise London-type potential. The B97D density functional has emerged as a robust and powerful density functional able to provide accurate structures in large supramolecular aggregates   Table 1, 2) and MINT-AQ (Supplementary Table 3) is defined as the energy difference between the two fully optimized monomers from the fully optimized dimer complex in the geometry of the dimer complex, where ! ! is the energy of fragment X at the geometry of Y. The basis set superposition error (BSSE) was half-corrected according to the counterpoise (CP) scheme of Boyd and Bernardi for the single-point interaction energies. 8 Otherwise, the binding energy (E bind ) (Supplementary Equation 2) was calculated taking into account the relaxation of the separate monomers and, therefore, considering the deformation energy required to transform the both moieties from their minimum-energy geometries to the geometry acquired in the assembly.
Mülliken population: The analysis of Mülliken population was carried out at CAM-B3LYP/3-21g* level of theory (Supplementary Table 4). An extra example at CAM-B3LYP/6-31g* level for the case of MINT-AQ has been included to support that a change in the basis set does not modify the sense of the charge transfer. atmosphere. Finally, 31.8 mmol of hydrazine (acting as hydrogen source 9 ) were added and the reaction was stirred magnetically and held at 85 ºC for a desired time. At regular intervals, aliquots were withdrawn from the reaction and subjected to NMR spectroscopy analysis to follow the catalytic evolution ( Supplementary Figures 19-25).
Once the reaction was complete, the crude mixture was diluted with 15 mL of DCM and the MINT-containing solid catalysts MINT-exTTF, MINT-pyr and MINT-AQ were recovered by filtration through a PTFE membrane of 0.2 µm pore-size and washed profusely with DCM. The solid was re-suspended in 10 mL of DCM through sonication for 10 min and filtered through a PTFE membrane of 0.2 µm pore size again. This washing procedure was repeated three times. After drying, the material was submitted to another catalytic run without adding in any case new catalyst precursor.
To isolate the pure products, the organic phase was washed three times with water.
Then, organic fractions were dried over anhydrous MgSO 4 and concentrated under reduced pressure. Crude product was purified by flash chromatography in silica (hexane:ethyl acetate 3:1) yielding the final product.

Analysis of X-Ray Fluorescence (TRXF):
In order to confirm that the active sites corresponded only to the nanotube walls, we conducted total reflected X-ray fluorescence (TRXF) measurements, and only ppm-level of metallic impurities were detected ( Supplementary Figures 15, 16, 17 and 18).

Characterization of the products obtained in the nitroarene reduction
Aniline.
Prepared according to the general procedure. 1

Supplementary Discussion
Analysis