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Detonation Nanodiamond: An Organic Platform for the Suzuki Coupling of Organic Molecules Weng Siang Yeap, Shiming Chen, and Kian Ping Loh* Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543 ReceiVed September 10, 2008. ReVised Manuscript ReceiVed October 22, 2008 Detonation nanodiamond possesses facile surface functional groups and can be chemically processed for many engineering applications. In this work, we demonstrate the functionalization of nanoscale diamond particles with aryl organics using Suzuki coupling reactions. In route one, hydrogenated nanodiamond is derivatized with aryl diazonium to form the bromophenyl-nanodiamond complex, this is subsequently reacted with phenyl boronic acid to generate the biphenyl adduct. In route two, the nanodiamond is first derivatized with boronic acid groups to form the boronic acid-nanodiamond complex, this is followed by Suzuki cross coupling with arenediazonium tetrafluroborate salts to generate the biphenyl product. Good chemoselectivity can be obtained in both routes. The efficiencies of the Suzuki coupling reaction can be further improved by performing the chemistry in a microreactor where electro-osmotic flow accelerates the mixing of reactants. Using the Suzuki coupling reactions, we can functionalize nanodiamond with trifluoroaryls and increase the solubilities of nanodiamond in ethanol and hexane. Fluorescent nanodiamond can be generated by the Suzuki coupling of pyrene to nanodiamond.
1. Introduction The applications of nanodiamond (ND) in drug delivery,1 bioactive surface coatings,2 and biolabeling3,4 have become a focus point of interests owing to the remarkable properties of the nanodiamond. Nanodiamond particles which have been ion beam irradiated exhibit nonphotobleachable fluorescence3 originating from the nitrogen-vacancy center. Dean Ho investigated nanodiamond films using ATP MTT and DNA fragmentation assays and confirmed the nonapoptotic and noncytotoxic properties of these films, suggesting that nanodiamond can serve as potential drug vehicle platform with translational relevance.2 Although the production methods of nanodiamond via detonation of TNT-hexogene mixtures have been discovered decades ago, the widespread application of nanodiamond was restricted at that time due to the difficulty in processing tightly aggregated nanodiamond. The pioneering work of Liu,5 Ozawa,6,7 and Gogotsi8 showed that by using a combination of thermal oxidation and acid treatment, deagglomerated primary nanodiamond particles in stable suspensions could be obtained. These deagglomerated nanodiamond particles are invariably terminated by hydroxyl and carboxylic functional groups, making them readily amenable to conventional solution functionalization chemistry.9 The high density of functional groups on nanodiamond results in very high extraction efficiencies for proteins and nucleic acids, rendering them highly useful for extracting materials in * Author to whom correspondence should be addressed, chmlohkp@ nus.edu.sg. (1) Huang, H.; Pierstoff, E.; Osawa, E.; Ho, D. Nano Lett. 2007, 7, 3305. (2) Huang, H. J.; Pierstoff, E.; Osawa, E.; Ho, D. ACS. Nano 2008, 2, 203. (3) Yu, S. J.; Kang, M. W.; Chang, H. C.; Chen, K. M.; Yu, Y. C. J. Am. Chem. Soc. 2005, 127, 17604. (4) Chung, P. H.; Perevedentseva, E.; Tu, J. S.; Chang, C. C.; Cheng, C. L. Diamond Relat. Mater. 2006, 15, 622. (5) Liu, Y.; Gu, Z.; Margrave, J. L.; Khabashesku, V. N. Chem. Mater. 2004, 16, 3924. (6) Kruger, A.; Ozawa, M.; Kataoka, F.; Fujino, T.; Suzuki, Y.; Aleksenskii, A. E.; Vul′, A. Y.; Osawa, E. Carbon 2005, 43, 1722. (7) Ozawa, M.; Inaguma, M.; Takahashi, M.; Kataoka, F.; Kruger, A.; Osawa, E. AdV. Mater. 2007, 19, 1201. (8) Osswald, S.; Yushin, G.; Mochalin, V.; Kucheyev, S. O.; Gogotsi, Y. J. Am. Chem. Soc. 2006, 128, 11635. (9) Kruger, A. Angew. Chem., Int. Ed. 2006, 45, 6426.
unfractionated biological mixtures.10 The specific functionalization of nanodiamond with biologically active tethering groups is an active area of research. Most of the covalent functionalization chemistry developed to date concerns mainly the coupling of aliphatic groups. Kru¨ger modified nanodiamond with alkyl silane and grafted biotin on the surface and demonstrated that these could bind to enzyme-labeled streptavidin.11We showed that the functionalization of nanodiamond with alkyl chain terminating in boronic acid functional groups allows specific binding to glycoproteins with high loading capacity, i.e., 500 mg of proteins on 1 g of nanodiamond.12 To study charge transfer interactions between the nanodiamond and organic molecule which is potentially useful for chemical sensing studies, it is important to consider the functionalization of nanodiamond with aryl organic groups since the conjugated chains in these molecules facilitate charge transfer. Suzuki coupling is one of the most widely used generic methods for the C-C coupling of biaryls. In this work, we consider various chemical routes to generate nanodiamond as synthons for Suzuki coupling, where both conventional wet chemistry reactions and microreactor chemistry were applied. Importantly, we discovered that diazonium coupling chemistry occurs spontaneously on hydrogenated nanodiamond. For example, bromophenyl adduct which can be used as synthon in subsequent Suzuki coupling can be coupled readily using bromophenyl diazonium salts. In the present paper, we show that Suzuki coupling is an effective method to couple a wide range of aryl molecules on detonation diamond and we discuss the properties and surface chemistry involved.
2. Experimental Section 2.1. Nanodiamond Preparation. Detonation nanodiamond powders (as-received nanodiamond) with average primary particle size of 4.0 nm were obtained from International Technology Center (ITC, USA). As-received nanodiamond powders were heated in a tube furnace for 7 h at 425 °C. The heated nanodiamond powder was dispersed in water to form a nanodiamond suspension with a (10) Huang, L. C. L.; Chang, H. C. Langmuir 2004, 20, 5879. (11) Krueger, A.; Stegk, J.; Liang, Y. J.; Lu, Li; Jarre, G., 2008, 24, 4200. (12) Yeap, W. S.; Tan, Y. Y.; Loh, K. P Anal. Chem. 2008, 80, 4659.
10.1021/la8029787 CCC: $40.75 2009 American Chemical Society Published on Web 12/02/2008
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Figure 1. Schematic showing the coupling of diazonium salts on nanodiamond to generate the 4-nitrophenyl or 4-bromophenyl ND, which serves as a synthon for Suzuki coupling of 4-flurophenylboronic acid or 4-trifluorophenylboronic acid.
Figure 2. Schematic diagram showing Suzuki coupling of arenediazonium salts on nanodiamond particles that were pretreated with aminophenylboronic acid diazonium salts (APBA-ND), to generate biphenyl adducts terminating in either the bromo (4-bromophneyl-APBA-ND) or nitro groups (4nitrophenyl-APBA-ND). 4-Nitrophenyl-APBA-ND can be electrochemically reduced further to 4-aniline-APBA-ND.
concentration of 0.5 wt %. The particle size distribution of the resultant suspension was monitored by dynamic light scattering measurement. The heat-purified nanodiamond powders were oxidized in strong acids following the procedure of Huang and Chang.3 After these treatments, the surface of the ND was verified by FTIR to consist of OH and COOH groups. 2.2. Hydrogenation of Nanodiamond (H-ND). Hydrogenation of nanodiamond particles was carried out by microwave hydrogen
plasma treatment using 800 W microwave power and 100 sccm of hydrogen gas flow for 60 min. Hydrogenation was repeated twice to ensure complete hydrogenation. 2.3. Diazonium Coupling. Hydrogenated nanodiamond (H-ND) was suspended in 3 mL of 0.1 M HCl solution and vigorously stirred. In the case of spontaneous coupling on H-ND, 8 × 10-4 mol of 4-bromophenyldiazonium tetrafluoroborate was added. On ND that was acid-treated (oxygenated), 5 mmol of Mohrs salt (ammonium
Detonation Nanodiamond
Figure 3. (Top) Glass microcapillary reactor where Suzuki coulpling was performed in this work, (Bottom) Schematic showing electrosmotic flow in the glass microcapillary.
iron(II) sulfate hexahydrate) has to be added together with 4-nitrophenyldiazonium tetrafluoroborate in order to initiate the reaction. 2.4. APBA-Nanodiamond (APBA-ND). Nanodiamond functionalized with aminophenyl boronic acid (APBA-nanodiamond) was synthesized following a procedure developed previously.12 2.5. Suzuki Coupling. Similar to diazonium coupling, Suzuki coupling was carried out using two methods. In scheme 1 (Figure 1), about 10 mg of 4-bromophenylND was mixed with 8 × 10-4 mol of 4-fluorophenylboronic acid or 4-(trifluoromethyl)phenylboronic acid. In scheme 2 (Figure 2), nanodiamond which has been functionalized with boronic acid was used as a synthon to couple to 4-bromophenyldiazonium tetrafluoroborate or 4-nitrophenyldiazonium tetrafluoroborate. Equivalent molar amounts of sodium acetate, 5 mol % of [(C6H5)3P]4Pd, and 3 mL of solvent were added, and the resulting suspension was stirred at different temperatures
Langmuir, Vol. 25, No. 1, 2009 187 and indicated times. After completion, the reaction mixture was washed with the reaction solvent, followed by successive rinsing with dichloromethane first to remove [(C6H5)3P]4Pd, followed by rinsing with tetrahydrofuran and finally multiple rinsing with ultrapure water. The resulting products were vacuum-dried at 0.01 Torr overnight. When the Suzuki coupling was carried out in a capillary microreactor (refer to Figure 3), the reagents added were similar to the wet chemistry reactions according to either scheme 1 or scheme 2. Equilvalent molar amounts of sodium acetate, 5 mol % of [(C6H5)3P]4Pd, and 0.3 mL of phosphate buffer solution (pH 4 and pH 9) were added, and the resulting suspension was injected into the microreactor. A reaction potential of 5 kV was applied between the two electrodes. The direction of the applied potential was switched alternatively every 15 min, and the reaction was monitored for up to 1 h at room temperature. Finally, the nanodiamond derivatives were washed with plenty of ultrapure water and acetonitrile and then dried in vacuum (0.01 Torr). 2.6. Instrumentation. Absorption FTIR spectroscopy of the functionalized nanodiamond was performed with a Varian 3100 Excalibur Series FTIR spectrometer with a nominal resolution of 2 cm-1. Sixty four accumulative scans were collected. Electrochemical tests were performed using the Autolab PGSTAT30 electrochemical workstation. The three-electrode assembly consists of an Ag/AgCl (3 M KCl) reference electrode, a platinum wire counter electrode, and a glassy carbon working electrode (0.50 cm diameter) (GCE). All electrolytes were prepared with ultrapure water and purged with dry nitrogen gas for 15 min prior to experiment. Freshly prepared 0.1 M KCl with 10% methanol was used for the electrochemical reduction of the aryl nitro group. 2.7. Preparation of Nanodiamond-Ionic Liquid (ND-IL) Paste. The nanodiamond particles were mixed with ionic liquid to form a paste; this was then applied as a conductive coating on a glassy carbon electrode. Typically, about 0.0100 g of the nanodiamond was added to 100 µL of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), and the mixture was ultrasonicated for 20 min. Five microliters of this mixture was added to the polished surface of the glassy carbon electrode for performing electrochemistry.
3. Results and Discussion 3.1. Hydrogenation of Nanodiamond (H-ND). As shown in scheme 1 (Figure 1), the first step requires the use of hydrogenated nanodiamond (H-ND) to undergo coupling with diazonium salts. The hydrogenation of the acid-treated ND was performed in the hydrogen plasma reactor. After
Figure 4. FTIR spectra of nanodiamond (a) as-received, (b) oxygenated (acid treated), and (c) H terminated (H-ND).
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Figure 5. FTIR spectra of nanodiamond (ND) after immersing in diazonium salts (a) as-received ND, (b) oxygenated ND (without the addition of Mohrs salts), (c) H-terminated ND (without the addition of Mohrs salts), and (d) oxygenated ND (with the addition of Mohrs salts). (c) and (d) show successful coupling to 4-nitrophenyldiazonium salt.
hydrogen-plasma treatment, both C-H and OH groups are present on the ND surface.13 Comparing the FTIR spectra of the ND before and after acid treatment in spectra a and b of Figure 4, we found that a strong peak at 1760 cm-1 related to the sCdO of carboxylic acid appeared only after successful carboxylation (Figure 4b) with acid. After acid treatment, the weak CH/CH2/ CH3 peak at 2910 cm-1 disappeared. After two cycles of microwave hydrogen plasma treatment, it can be seen in the FTIR spectrum (Figure 4c) that the CdO peak (1760 cm-1) vanished; at the same time, the CH/CH2/CH3 peak at ∼2933 cm-1 increased in intensity which proved that the ND was successfully hydrogenated. The presence of both C-H and OH groups was similar to the bifunctional surface observed by Korolkov and co-workers.13 3.2. Diazonium Coupling on H-ND to Form NitrophenylCoupled ND (4-nitrophenylND). The diazonium coupling of the bromophenyl, nitrophenyl, or boronic ester phenyl groups was performed by simply immersing the ND into solution containing the aryldiazonium salts. Among the different types of ND used, which include hydrogenated ND (H-ND), oxygenated ND, and as-received ND, we found that the spontaneous reduction of diazonium salts was facile only on H-ND. From the FTIR spectra, as-received and oxygenated (acid treated) ND showed no reaction, the spectra were similar before and after immersion in the diazonium salts, as shown in spectra a and b of Figure 5. However in the case of the H-ND, Figure 5c shows that three new peaks appeared at 1523, 1349, and 857 cm-1, respectively; these can be assigned to -C-NO2 of 4-nitrophenyl as well as the aromatic CH of benzene. The fact that hydrogen termination of the ND is necessary before spontaneous charge transfer to diazonium salt can occur is consistent with our earlier findings on diamond thin films, where it was observed that only hydrogenterminated diamond films could undergo spontaneous diazonium salt reduction and the process was inhibited on oxygenated diamond film.14 In order for the spontaneous reduction of the diazonium salt to proceed, there must be charge transfer from (13) Korolkov, V. V.; Kulakova, B. N.; Lisichkin, G. V. Diamond Related Mater. 2007, 16, 2129. (14) Chakrapani, V; Angus, J. C.; Anderson, A. B.; Wolter, S. D.; Stoner, B. R.; Sumanasekera, G. U. Science 2007, 318.
Figure 6. Cyclic voltammograms of the reduction of aryl nitro groups on the diazonium-coupled nanodiamonds. Electrolyte: 0.1 M KCl with 10% methanol. Scan rate: 50 mV/s.
nanodiamond to the nitrophenyl diazonium salt. This is an electrochemically mediated reaction driven by the equilibration of the valence band of ND with the electrochemical potential of the reductant. The lower electron affinity of H-ND allows charge transfer to proceed from the valence band of diamond to the redox species in the solution. The charge transfer is prohibited on oxygenated ND because its higher electron affinity places the valence band beneath the electrochemical potential of the diazonium salt in the solution and no spontaneous charge transfer can occur. To overcome the inertia toward spontaneous charge transfer on oxygenated ND, a radical initiator such as Mohrs salt15 had to be added. The function of the Mohrs salt is to reduce the diazonium salt and to generate phenyl radicals. The addition of Mohrs salt resulted in the successful coupling of 4-nitrophenyldiazonium salt to the oxygenated ND, this can be judged from the FTIR spectra in Figure 5d where two new peaks at 1523 and 1349 cm-1 are assignable to the symmetric and asymmetric stretches of NO2 functional group, respectively.
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Figure 7. FTIR spectra showing the Suzuki coupling of 4-bromophenylND with 4-fluorophenylboronic acid (scheme 1): (a) before Suzuki coupling; (b) after Suzuki coupling via wet chemistry; (c) after Suzuki coupling using a microreactor, similar but coupling to 4-trifluorophenylboronic acid (scheme 1); (d) before Suzuki coupling; (e) after Suzuki coupling via wet chemistry; (f) after Suzuki coupling using microreactor.
3.3. Electrochemical Characterization of the Nitrophenyl Coupled ND. To provide evidence that the diazonium coupling process can connect functional groups to the diamond surface, the electrochemical activity of the nitrophenyl coupled nanodiamond was investigated using cyclic voltammetry (CV). The nitrophenyl coupled nanodiamond was blended with ionic liquid (butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6)) to form an electrochemically active paste which was then applied to the polished surface of the glassy carbon electrode. Electrochemical reduction of the aryl nitro group to aniline has been studied in great detail on many film surfaces. The aryl nitro group undergoes a two-step reduction producing a phenylhydroxylamine (-NHOH) as an intermediate product before it is reduced further into aniline
R-NO2 + 4H+ + 4e- f R-NHOH + H2O
(1)
R-NHOH + 2H+ + 2e- f R-NH2 + H2O
(2)
Figure 6 shows the cyclic voltammetry of the nitrophenyl coupled nanodiamond/ionic liquid; the cyclic voltammetry of untreated nanodiamond/ionic liquid and glassy carbon/ionic liquid were also performed to act as control. It can be seen clearly that two pronounced cathodic peaks were observed at Ep ) -0.90 VAg/ AgCl and Ep ) -1.07 VAg/AgCl for the nitrophenyl coupled ND; these two peaks correspond to the four-electron reduction of the aryl nitro group to phenylhydroxylamine and the two-electron reduction of phenylhydroxylamine to aniline, respectively. The anodic peaks observed at Ep ) -0.21VAg/AgCl are assigned to the (15) Zhong, Y. L.; Loh, K. P.; Midya, A.; Chen, Z. K. Chem. Mater. 2008, 20, 3137.
oxidation of phenylhydroxylamine,16 while the anodic peak at Ep ) -0.45 VAg/AgCl could be due to the direct oxidation reaction of the nanodiamond itself.17 These peaks are not present in the cyclic voltammograms of the control samples, thus verifying that they originated from the nitrophenyl groups on the nanodiamond. After the first round of electrochemical reduction, there is an obvious decrease in the redox peaks, which is consistent with the irreversible reduction of the nitro to amine groups. The result is significant because the spontaneous diazonium coupling of the nitropheny groups on nanodiamond provides a facile way of providing tethering groups on the nanodiamond for carboxylicterminated biomolecules because the nitro groups can be reduced to amine groups quite readily. 3.4. Suzuki Coupling. A classical Suzuki coupling reaction involves the cross-coupling of phenylboronic acid with bromophenyl groups. As shown in scheme 1 (Figure 1), hydrogenated nanodiamond was first treated with diazonium salt to produce either the 4-nitrophenyl or 4-bromophenyl adduct. The 4-bromophenyl nanodiamond adduct was then used as a synthon for coupling to 4-flurophenylboronic acid or 4-trifluorophenylboronic acid. An alternative to this classical reaction is Suzuki coupling to arenediazonium salts. As illustrated in scheme 2 (Figure 2), the nanodiamond was first functionalized with aminophenylboronic acid to produce the boronic acid functionalized nanodiamond. This serves as a synthon in a Suzuki cross-coupling reaction with 4-bromophenyldiazonium tetrafluoroborate or 4-nitrophenyldiazonium tetrafluoroborate. The selection of solvent for this heterogeneous Suzuki coupling reaction is not trivial. (16) Oliveira, Maria; Cristina, Fialho J. Mol. Catal. A: Chem. 2002, 182, 419. (17) Holt, Katherine B.; Ziegler, C.; Caruana, D. J.; Zang, J. B.; MillanBarrios, E. J.; Hu, J. P.; Foord, J. S. Phys. Chem. Chem. Phys. 2008, 10, 303.
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Figure 8. FTIR spectra showing the Suzuki coupling of boronic acid functionalized nanodiamond with 4-bromophenyldiazonium tetrafluoroborate (scheme 2): (a) before Suzuki coupling; (b) after Suzuki coupling via wet chemistry; (c) after Suzuki coupling using microreactor, similar but Suzuki coupling to 4-nitrophenyldiazonium tetrafluoroborate (scheme 2); (d) before Suzuki coupling; (e) after Suzuki coupling via wet chemistry; (f) after Suzuki coupling using microreactor. Table 1. Maximum Concentration (mg/L) for a Stable Suspension of Various Suzuki-Functionalized Nanodiamonds (ND) in Organic Solvents
Figure 9. Fluorescence spectra of (a) 4-bromophenyl coupled ND and (b) pyrene coupled ND. The inset shows the fluorescence from the pyrene coupled ND.
We screened a variety of technical grade solvents that can be used with the catalyst (5 mol % of [(C6H5)3P]4Pd) and found that the use of alcoholic solvents (e.g., methanol) resulted in reasonable yield while 1,4-dioxane which has been used successfully in homogeneous reactions18 resulted in very poor yields in these heterogeneous reactions. (18) Taylor, R. H.; Felpin, F. X. Org. Lett. 2007, 9, 2911.
solvent
THF
ethanol
hexane
H-ND 4-nitrophenyl-ND (scheme 1) 4-bromophenyl-ND (scheme 1) 4-fluorophenylbenzene-ND (scheme 1) 4-trifluoromethylphenylbenzeneND (scheme 1) 4-bromophenyl-APBA-ND (scheme 2) 4-nitrophenyl-APBA-ND (scheme 2)
2.0 38.8 34.5 34.5 38.3
10.0 62.0 68.0 40.0 20.0
precipitate precipitate 8.0 16.0 12.8
51.0 43.5
11.4 9.0
4.0 precipitate
To improve the reaction efficiency, the above Suzuki coupling reactions could be carried out in a microreactor (refer to Figure 3). Microreactors are microfluidic devices where chemical reactions are performed in micrometer-sized glass channels. The application of an electrical field creates an electro-osmotic force which favors migration and mixing of reactants.19 An aqueous solvent such as PBS was selected because positive and negative ions in these solvents migrate toward the anode and cathode of microreactor, respectively, setting up an electro-osmotic flow. The microreactor Suzuki coupling chemistry in this case did not work well with polar organic solvents like methanol or 1,4-dioxane. Scheme 1. Suzuki Coupling. Before Suzuki coupling, the FTIR spectra of 4-bromophenylnanodiamond showed a C-Br peak at (19) Basheer, C.; Jahir Hussain, F. S.; Lee, H. K.; Valiyaveettil, S. Tetrahedron Lett. 2007, 45, 7297.
Detonation Nanodiamond
Figure 10. Picture of (from left) 4-bromophenyl-ND, 4-fluorophenylbenzene-ND, 4-bromophenyl-APBA-ND, and 4-trifluoromethylphenylbenzene-ND dispersed in hexane.
1083.5 cm-1 in spectra a and d of Figure 7. After Suzuki coupling with 4-fluoroboronic acid, this peak disappeared and two new peaks at 1213.7 and 1156.2 cm-1 emerged, these can be assigned to aromatic C-F bonds of the 4-fluoroboronic acid (Figure 7, spectra b and c).20 In the case of 4-trifluorophenylboronic acid coupled-nanodiamond, the corresponding FTIR spectrum in Figure 7e shows sharp peaks due to aromatic C-C vibrations around 1483-1562 cm-1 and also a peak at 1234 cm-1 due to CF3.20 The microreactor-aided Suzuki coupling yielded a more complete reaction for the reactions with 4-fluoroboronic compared to 4-trifluoroboronic acid. This may be due to the greater hydrophobic character of the trifluoro-terminated diamond which resulted in aggregation in the PBS solvent, thus reducing the surface area for reaction. Suzuki coupling with 4-trifluoroboronic acid gave better yield in conventional chemical reactions with methanol as the solvent. Scheme 2. Suzuki Reactons Using Arenediazoniums Salts. Next, Suzuki coupling of the boronic acid functionalized nanodiamond with arenediazonium salts is investigated. Arenediazonium salts were reported as effective electrophiles in palladium crosscoupling reactions.18,21,22 After Suzuki coupling with 4-bromophenyldiazonium tetrafluoroborate, Figure 8b shows that the -B(OH)2 peak at 1342 cm-1 disappeared and -C-Br peak appeared. This is evident of the chemoselective reaction between the -B(OH)2 group and the diazonium partner. The reaction efficiency is higher in the case of coupling to 4-nitrophenyldiazonium compared to 4-bromophenyldiazonium. In Figure 8f, we can see the appearance of a very sharp peak at 1340 cm-1 which may be related to the -C-NO2 peak, along with the growth of aromatic C-C stretch around 1589 cm-1. 3.5. Applications of Suzuki Coupling. Coupling of Fluorescent Organic Molecules Fluorescent dyes can be coupled via Suzuki coupling onto the nanodiamond (ND). As a proof of principle, we performed Suzuki coupling of the 4-bromophenylND with pyreneboronic acid. Photoluminescence test were carried out by dispersing 1.0 mg of 4-bromophenyl-ND and pyrenefunctionalized ND in 30 mL of ethanol. Figure 9 clearly showed that pyrene functionalized nanodiamond exhibited greater fluorescence compared to 4-bromophenyl-ND. There are many commercially available boronic acid functionalized organic dyes,
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so Suzuki coupling provides great convenience for the functionalization of 4-bromophenyl-ND. ImproVed Dispersion in Ethanol and Hexane. The nanodiamond becomes more hydrophobic after undergoing Suzuki coupling with the aryl groups with fluorine functionalities. It was found that Suzuki coupling functionalized ND exhibits better resistance toward agglomeration in certain solutions. A quantitative estimate of the solubility of functionalized nanodiamonds was performed following the procedure of Liu et al.23 and the results are tabulated in Table 1. Nanodiamond functionalized with amide (4-bromophenylAPBA-ND and 4-nitrophenyl-APBA-ND) and -NO2 (4-nitrophenyl-ND and 4-nitrophenyl-APBA-ND) functional groups tends to precipitate faster than other functionalized nanodiamonds in ethanol. This is due to the propensity of the nanodiamond with these functional groups to undergo hydrogen bonding with the protic solvent used. According to Osawa et al.,7 it was difficult to disperse nanodiamond in nonpolar solvents such as n-hexane and toluene. However, in our work, the Suzuki-coupled nanodiamond (4-bromophenyl-ND, 4-fluorophenylbenzene-ND, 4-trifluoromethylphenylbenzene-ND, and 4-bromophenyl-APBA-ND in Table 1) can be dispersed in hexane and form a stable suspension, as shown in Figure 10. The size of the nanodiamond aggregate, as determined by dynamic laser scattering, is between 100 and 200 nm. The ability to functionalize the nanodiamond with a wide range of organic molecules via Suzuki coupling allows the broad applicability of nanodiamonds as useful supports in chemistry. One possibility is its application as core support in chromatographic systems which can be employed in conjunction with a wide variety of chiral organic materials to provide chiral stationary phases. In view of the stability of nanodiamond and its ease of recovery from solution, another possibility is the use of nanodiamond as a solid phase support for catalysts. Nanodiamond can also be coupled to polymers to form a hybrid material with enhanced mechanical strength. Future work may target the applications of Suzuki-functionalized nanodiamond in these areas.
4. Conclusion In this work, we have shown that nanodiamond can be used as a versatile organic platform for performing C-C cross coupling reactions. First, we demonstrated the spontaneous diazonium coupling of bromophenyl, or boronic ester phenyl, on hydrogenated nanodiamond. Nanodiamond functionalized as such can be utilized as synthons for subsequent Suzuki coupling with other aryl molecules, thus affording a facile scheme that allows the facile coupling of a wide range of aryl molecules. Diazonium coupling can also occur on oxygenated (acid treated) nanodiamond if a radical initiator such as Mohrs salt is added. As an alternative to the classical Suzuki reactions, we have also demonstrated the Suzuki coupling of arenediazonium salts on boronic acid functionalized nanodiamond. As a proof of principle, we demonstrated the usefulness of Suzuki coupling in coupling pyrene to nanodiamond to generate a fluorescent nanodiamond-pyrene complex. These reactions have general validity to a wide range of organic dyes that have boronic acid or halide functional groups. In addition, the dispersion of the nanodiamond in organic solvent can be improved by coupling hydrophobic aryl groups. Acknowledgment. The author K. P. Loh wishes to acknowledge the funding support of NUS Academic Grant: R-143-000-330-112. LA8029787
(20) Lyskawa, J.; Belanger, D. Chem. Mater. 2006, 18, 4755. (21) Sylvain, D.; Jean-Pierre, G. Tetrahedron Lett. 1997, 38, 4393. (22) Douglas, M. W.; Robert, M. S. Tetrahedron Lett. 2000, 41, 6271.
(23) Liu, Y.; Gu, Z. N.; Margrave, J. L.; Khabashesku, V. N. Chem. Mater. 2004, 16, 3924.
