Factors influencing the photocatalytic hydroamination of alkynes with anilines catalyzed by supported gold nanoparticles under visible light irradiation

Abstract

The addition of an N–H bond to C≡C triple bonds in the hydroamination of alkynes is of great importance in synthetic chemistry. We found that visible light irradiation can efficiently drive the direct hydroamination of alkynes using supported gold nanoparticles (AuNPs) due to its Localized Surface Plasmon Resonance (LSPR). Au NPs on nitrogen-doped TiO₂ could successfully induce hydroamination with a very high conversion (90%) and excellent product selectivity (91% imine product). Herein, we demonstrate that for such a photocatalytic process, the intensity and wavelength of the incident light, as well as the reaction temperature have a significant impact on the performance of the catalyst. The photocatalytic activity increases with increasing light intensity and reaction temperature. This means that the AuNP photocatalysts couple both the input light energy and thermal energy to catalyze the reaction. The influence of AuNPs and the surface properties of the support materials on the photocatalytic performance are also further discussed.

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Introduction

In recent years, photocatalysis has attracted growing interest with the raised awareness of energy conservation and environmental impacts.^1,2 In the field of heterogeneous photocatalysis, researchers have almost exclusively focused on titania (TiO₂)-based photocatalysts to address these factors.^3,4 However, there are several intrinsic problems impeding the application of semiconductor photocatalysts in this context. First, only ultraviolet (UV) irradiation can be used due to the wide band gap of TiO₂ (e.g. anatase ∼3.2 eV), which utilizes only 4% of the available solar energy.⁵ Second, there is an inverse relationship between the catalyzed reaction rates and higher operating temperatures. This means that it is not possible to efficiently utilize infrared irradiation, which also accounts for a large fraction of the available solar energy spectrum, to increase the efficiency of the photocatalytic process. Gold nanoparticles (AuNPs) on the other hand can absorb visible light (ca. 40% of the available solar energy) due to Localized Surface Plasmon Resonance (LSPR), as well as UV light due to interband electron transitions (from 5d to 6sp).^6–10 LSPR arises from the collective oscillation of conduction electrons in the NPs, which resonate with the electromagnetic field of the incident light. The conduction electrons of AuNPs absorb energy from both visible and UV light irradiation. These energized electrons can return to the ground state by locally heating the NP lattice or can relax through interactions with molecules bound to the NP surface, thereby inducing reactivity in the bound species.^11–13 It has also been determined that increasing the temperature benefits the catalytic performance of the NPs. Thus, these NPs could effectively couple thermal and photonic energy absorption to drive chemical reactions under visible light irradiation.^14,15 Consequently, these NPs represent a new class of photocatalysts capable of driving catalytic reactions for the production of important organic chemicals.

The hydroamination of alkynes, or the direct addition of an N–H bond to a C≡C bond, is of great significance in synthetic chemistry. This reaction offers an atom-economical route for the synthesis of a range of organic nitrogen-containing molecules and their derivatives and provides a convenient route for the synthesis of numerous important fine chemicals and intermediates.^16–18 Imines, for example, represent one such class of compounds,^19–21 as they are widely used in the Strecker synthesis,¹⁹ the synthesis of nitrogen heterocycles²⁰ and the synthesis of secondary and tertiary amines.²¹

Developing efficient and green processes for this addition has attracted intense interest, particularly for the intermolecular process. Many catalysts have been reported as being useful for this transformation, including lanthanide complexes,²² early transition metal complexes (such as Ti²³ and Zr²⁴), as well as the complexes of late transition metals such as Ru,^25,26 Pd²⁷ and Ag.²⁸ More than a decade ago, a gold-based complex was also identified as an active catalyst for this process.^16,29 The hydroamination of alkynes involves nucleophilic attack on the C≡C by an amine, and the activation energy of this reaction is very high due to the electrostatic repulsion between the N lone pair and the π system of alkyne.³⁰ Intermolecular hydroamination is the most challenging, as it is even more kinetically and thermodynamically problematic than the intramolecular process. Higher temperatures (generally above 100 °C) are typically necessary to achieve high yields in the heterogeneous process. However, at high temperatures, the reaction equilibrium shifts towards lower conversions as the reaction entropy (ΔS₀) of the amine addition is negative,^30,31 which creates a significant challenge. The intermolecular hydroamination of alkynes at 70 °C using a homogeneous catalyst of gold, (Ph₃P)AuCH₃, which addresses some of these issues, has been reported.³² However, this reaction proceeds efficiently only in the presence of certain tungstate polyacid catalysts, and recycling the catalyst in homogeneous reactions is difficult. Recently, Corma and co-workers³³ reported that catalysts of supported AuNPs were active for the hydroamination of alkynes. High conversions and a moderate selectivity for imines (55%) could be achieved after 22 h reaction at 100 °C. High selectivity, however, could only be achieved in the presence of activated molecular sieves (zeolite 4A).

Very recently, we determined that visible light can successfully drive hydroamination reactions at ambient temperatures when AuNPs on metal oxide supports were applied as catalysts.³⁴

Aniline and its derivatives weakly bind to gold surfaces through the nitrogen lone pair.^35,36 The aniline molecules absorbed on the surface of AuNPs become activated under the irradiation of visible light as the NPs gain light energy.^37–39 Such a photocatalytic process is distinctly different from conventional light driven catalysis using semiconductor catalysts or composite catalysts of semiconductor and metal NPs. In this study, the influence of the intensity and wavelength of the incident light, the reaction temperature and the surface properties of the supports as well as other parameters are investigated in detail.

Experimental

Synthesis of TiO₂ nanofibre supports

All chemicals were used as purchased from Sigma Aldrich (except P25, which was from Degussa, Germany) without further purification. Supports such as CeO₂ (50 nm BET), ZrO₂ (<100 nm TEM), Fe₂O₃ (<5 micron) and Al₂O₃ (∼100 mesh) were used as purchased from Sigma Aldrich. The nanofiber supports of TiO₂-based materials, including TiO₂(B), TiO₂(anatase) and nitrogen-doped TiO₂(B) nanofibres, were synthesized through various post treatments of hydrogen titanate (H₂Ti₃O₇) fibers.^43,44 In general, 6 g of anatase particles (∼325 mesh from Aldrich) was mixed with 80 mL of 10 M NaOH. The suspensions were sonicated in an ultrasonic bath for 0.5 h and then transferred into an autoclave with a PTFE container inside. The autoclave was maintained at a hydrothermal temperature of 180 °C for 48 h. The precipitate (sodium titanate nanofibers) was recovered, washed with distilled water (to remove excess NaOH), exchanged with H⁺ (using a 0.1 M HCl solution) to produce H₂Ti₃O₇ nanofibers and then washed again with distilled water until pH ∼ 7 was reached. The H₂Ti₃O₇ nanofiber product was dried at 80 °C for 12 h. TiO₂(B) nanofibers were obtained by calcining the hydrogen titanate fibers at 723 K for 3 h. TiO₂(anatase) nanofibres used as a catalyst support were synthesized by the hydrothermal reaction of H-titanate with dilute (0.05 M) HNO₃ solution for 62 h and then by calcining at 723 K for 3 h. Nitrogen-doped TiO₂(B) nanofibers (possessing a trace amount of anatase) were obtained by calcining H-titanate nanofibers at 550 °C in ammonia gas flow for 3 h and were denoted as TiO₂-N or TiO₂(B)-N.

Au loading

AuNPs were loaded onto different supports by a reduction method. Briefly, 2.5 g of support powder was dispersed into 100 mL of a given concentration of HAuCl₄ solution. 20 mL of 0.53 M lysine was added under magnetic stirring, and the stirring was continued for 30 min. To this suspension, 10 mL of 0.35 M NaBH₄ solution was then added slowly, followed by the addition of 10 mL of 0.3 M hydrochloric acid. The mixture was allowed to stand for 24 h. Finally, the solid that formed from this process was separated, washed with deionized water and ethanol and dried at 70 °C. The prepared catalysts of AuNPs on different supports were denoted as Au/TiO₂(anatase), Au/CeO₂, and Au/Al₂O₃.

Characterization

Transmission electron microscopy (TEM) images were taken with a Philips CM200 transmission electron microscope employing an accelerating voltage of 200 kV. The UV-vis diffuse reflectance spectra of the samples were obtained on a Cary 5000 UV-Vis-NIR Spectrophotometer. The Raman spectra of the samples were obtained on an inVia Renishaw microscope Raman and the excitation source was a He–Ne laser (785 nm). FTIR measurements were conducted on a NEXUS 870 FT-IR (SMART ENDURANCE), Thermo Nicolet, and the IES measurements were recoded on a Digilab FTS-60A spectrometer equipped with a TGS detector, which was modified by replacing the IR source with an emission cell (the principles of the emission experiment have been published elsewhere).⁴⁶

Photocatalytic activity test

The hydroamination of alkynes was undertaken in an argon atmosphere at 40 °C unless otherwise specified. In a typical reaction, 2 mmol alkyne and 2 mmol amine were dissolved into 2 mL toluene, into which 0.1 g photocatalyst was added. The flask was then filled with argon for 1 min to eliminate air. Afterwards, the reaction mixture was illuminated with visible light (500 W Halogen Lamp, light intensity was measured to be 0.43 W cm⁻²) for 25 h under magnetic stirring if not specified. Due to the low energy consumption of LEDs,⁴⁷ the photocatalytic hydroamination of 4-phenyl-1-butyne for Au/TiO₂(B) was also conducted using LED as the light source while keeping all the other reaction conditions unchanged (the conversion achieved 45% after 6 h of reaction). Fans or an air conditioner were used to maintain the temperature at 40 °C during the reaction course. A thermometer was used to monitor the reaction temperature during the photocatalytic process. The liquid products were filtered and analyzed using an Agilent HP-6890 GC with a HP-5 column. A typical example of a GC-MS analysis is shown in Fig. S1.†

Results and discussion

Photocatalytic hydroamination of alkynes with amines

The addition of the N–H bond across different types of C≡C triple bonds was investigated using Au/TiO₂-N (AuNPs on nitrogen-doped TiO₂). The results in Table 1 illustrate the excellent performance of the Au/TiO₂-N photocatalyst for this synthetic process. A conversion of 90% 4-phenyl-1-butyne with a selectivity of 91% for the target imine product was observed (entry 1). A high conversion and product selectivity were still achieved when electron withdrawing or donating groups were bonded to the aromatic ring of the reactants (entries 5–11). The reaction was also effective with an aliphatic alkyne (entry 4) and a secondary amine (entry 15). Of the alkynes, terminal alkynes gave the highest yields (entries 1–3). Compared with the amines with electron donating groups (entries 10 and 11), higher yields were obtained for the amines with electron withdrawing groups (entries 8 and 9). However, a poor conversion for the aliphatic aminohexane was obtained (entry 12). This may be due to the increased basicity of aliphatic amines, which can deactivate the catalysts.^33,29j A negligible reaction or no reaction occurred for non-terminal triple bonds (entries 13 and 14), which may be due to the increased steric hindrance by the groups attached to the C≡C bond.^18,32

Table 1 Photocatalytic hydroamination of alkynes with amines on Au/TiO₂-N under visible light^a

Entry	Alkynes (Conv.^b)	Amines	Product (Sel.^c)	TON^d
a Reaction conditions: 0.38 × 10^−2 mmol of AuNPs, 0.5 mmol alkynes, 0.5 mmol anilines, 0.5 mL of toluene as the solvent, reacted under visible light at 40 °C for 25 h in an argon atmosphere. b Determined by GC and GC-MS analysis. c The selectivity of imine was estimated from GC results. d Turnover number (TON) was calculated using the mole content of gold. e Internal alkynes or amines.
1				118
2				113
3				104
4				66
5				105
6				74
7				72
8				122
9				111
10				93
11				62
12				1
13^e				7
14^e				0
15^e				42

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Influence of incident light and reaction temperature on the catalytic process

In this study, the hydroamination of 4-phenyl-1-butyne with aniline was investigated. We observed a positive dependence of the reaction rate on the light intensity (Fig. 1a), whereby the rate increases with increasing light intensity. Alkyne hydroamination was also examined at different reaction temperatures and under a constant light intensity. As shown in Fig. 1b, at higher operating temperatures, higher conversions were achieved. This result is different from what is usually achieved with semiconductor photocatalysts, where a negative relationship between the reaction rate and temperature is usually observed, and this is another characteristic of photocatalytic transformations on plasmonic metal nanostructures. The enhanced performance of AuNPs at higher reaction temperatures can be explained as follows: the increase of temperature leads to a re-distribution of the conducting electrons of AuNPs and the population of excited electrons at high energy levels increases. These thermally excited electrons can still gain further energy through light irradiation via the LSPR effect, therefore becoming excited to even higher energy levels. Conduction electrons at higher energy levels have a greater ability to activate the reactant molecules adsorbed on the NP surface to overcome the activation barrier, inducing the reaction. Notably, only a limited reaction occurs in the absence of light, even at elevated temperatures (the reaction rate for the dark reaction at 70 °C is still lower than that for the reaction at 50 °C under the visible light of 0.63 W cm⁻²). This means that the supported AuNPs effectively couple thermal and photonic energy to drive chemical transformation, making them superior photocatalysts to conventional semiconductor photocatalysts.

Fig. 1 (a) The influence of light intensity on the reaction rates of photocatalytic hydroamination of phenylacetylene with aniline at different temperatures. The light intensity increases from 0.16, 0.29, 0.41, 0.51 to 0.63 W cm⁻². (b) The influence of reaction temperature on the reaction rates of photocatalytic hydroamination at different visible light intensities. The reaction temperature increases from 303, 313, 323, 333 to 343 K. Using AuNPs on N-doped titania nanofibre (Au/TiO₂-N) as photocatalysts and the reaction time is 4 h.

Since the photocatalytic reaction rates increase with operating temperature, the apparent activation energy can be calculated from the reaction kinetics at several different reaction temperatures. The difference between the apparent activation energy of the reaction under light irradiation and that of the reaction in the dark is the contribution the visible light irradiation makes to reduce the apparent activation energy of the reaction.

The kinetics experiments were conducted at temperatures between 38 °C and 65 °C under visible light irradiation (photocatalytic hydroamination) and in the dark (catalytic hydroamination). The activation energies of the photocatalytic process and catalytic process were then derived from the results of the reaction kinetics. As illustrated in Fig. 2, the values of the rate constant k were calculated from the slope of the kinetic plots at each temperature. Then, the Arrhenius plot of ln k versus 1/T was obtained. The apparent activation energies were estimated using the Arrhenius equation, being 49 kJ mol⁻¹ for the photocatalytic hydroamination and 59 kJ mol⁻¹ in the dark. The activation energy in the dark is slightly lower compared with the activation energy calculated by Shanbhag et al. for the hydroamination of phenylacetylene with 2,4-xylidine (65 kJ mol⁻¹) using copper(II) ion exchanged AlSBA-15 in the temperature ranges of 80–120 °C.⁴⁰ Visible light irradiation of the AuNPs resulted in an activation energy reduction of 10 kJ mol⁻¹ for this reaction.

Fig. 2 Arrhenius plot of ln k versus 1/T and the contribution of visible light (0.43 W cm⁻², 400–800 nm) on the reduction of activation energy (inset).

The dependence of the catalyzed reaction on the light wavelength was also investigated. Several optical low pass filters were used to block light below specific cut-off wavelengths thus, tuning the wavelength range of the irradiation. The dependence of the catalytic performance on the wavelength range for the hydroamination reaction is illustrated in Fig. 3. Visible light (400–800 nm) below specific wavelengths was blocked by glass filters, while the irradiation intensity was maintained constant by tuning the power input. The conversion obtained under the irradiation with a wavelength between 490 nm and 550 nm was then obtained by subtracting the conversion with a 550 nm glass filter (550–800 nm) from that with a 490 nm glass filter (490–800 nm). The calculated results were plotted against irradiation wavelength, giving the action spectrum of this reaction. An action spectrum is an effective tool for determining whether a reaction occurs via a thermocatalytic or photoinduced process, further identifying the species responsible for light absorption and for triggering the photochemical process.⁴¹

Fig. 3 The action spectra of photocatalytic activity versus visible light wavelength. LSPR (Localized Surface Plasmon Resonance) peak is the absorption band of AuNPs with a diameter 2–10 nm. (a) Au/TiO₂-N (reaction time is 10 h). (b) Au/TiO₂(B) (reaction time is 25 h). Glass filters were used to cut off the light with a wavelength shorter than a certain wavelength while keeping the light intensity and other reaction conditions unchanged. The conversion in a short range of wavelengths was obtained by subtraction (e.g. the conversion between 490 nm and 550 nm is derived by subtracting the conversion for filtered wavelengths below 490 nm from that below 550 nm).

As can be seen in Fig. 3, the LSPR band in the UV-vis spectrum of the supported AuNPs correlates well with the conversion of alkynes. Such an action spectrum is the third distinct signature of AuNPs photocatalysis. The best performance was observed with a wavelength between 490 nm and 550 nm, where the LSPR absorption of AuNPs peaked. The harmony between the photocatalytic activity and LSPR absorption of the AuNPs demonstrates that the photocatalytic activity stems mainly from AuNPs, namely, the light harvesting component is AuNPs at least for long wavelengths. An action spectrum analysis demonstrates that AuNPs are the core component for the photocatalytic hydroamination under visible light irradiation.⁴⁷

Role of AuNPs and the supports (mechanism study)

In the absence of AuNPs (that is: using the inorganic supports alone as the catalysts), no reaction was observed under the same reaction conditions. Moreover, as demonstrated by the action spectrum, the AuNPs play a critical role in the photocatalytic hydroamination reaction. Given that the photocatalyzed reaction takes place on the surface of AuNPs and the particle size of the NPs has an important influence on the light absorption of the NPs, we expect that the specific surface area and particle size of AuNPs would have a significant impact on their catalytic performance. Photocatalysts with different Au content on TiO₂-N were analysed. The size distribution of these catalysts was examined using transmission electron microscopy (TEM), and it was determined that most (>95%) of the AuNPs occur in the size range between 2 nm and 10 nm. The total surface area of the AuNPs in one gram of the photocatalysts was then estimated by assuming that the NPs are spherical (details of the calculation method are given in the ESI†). Finally, the photocatalytic performance of the catalysts for the hydroamination of 4-phenyl-1-butyne with aniline was correlated with the specific surface area and size of the AuNPs used. As shown in Fig. 4, the conversion increases from 67% to 90% as the surface area of NPs increases from 0.7 m² to 2.0 m². The conversion increasing as a function of the surface area of the AuNPs implies that at least one reactant was activated on the nanoparticle surface. The particle size of the AuNPs also influences the conversion, as it was observed that the conversion difference is only 3% for a size difference of 1–2 nm, while it is 20% for the size difference ≥5 nm (Fig. S2†). The larger AuNPs have a smaller specific surface area, which can result in decreased performance, as the reaction takes place on the AuNP surface. On the other hand, although the smaller AuNPs have a larger specific surface area, they exhibit a weaker LSPR effect.³⁷ The sizes of the AuNPs used in the present study are likely to lie in the range where the two size-related effects offset each other.

Fig. 4 The increase of the conversion with the increase of total surface area of AuNPs per gram of the photocatalyst. The scale bars in the micrographs from left to right are 50 nm, 50 nm and 100 nm, respectively.

As stated in our previous study, amines are most likely to be activated on AuNPs and accompanied by electron transfer to the AuNPs, as proved by Surface Enhanced Raman Spectroscopy (SERS).³⁴ Furthermore, electron transfer from aniline to AuNPs could be further supported by comparison of the oxidation potential of aniline (∼0.6 V) with the position of Au⁺, as illustrated in Scheme 1. In addition, the possibility that trace leached Au species in solution are responsible for the observed catalysis in the homogeneous phase is excluded as the reaction stopped immediately after filtering the photocatalyst out. The conversion remained unchanged for further reaction without the catalyst, even prolonging the reaction time to 48 h.

Scheme 1 Proposed mechanism for the photocatalytic hydroamination of alkynes on supported AuNPs.

The performance of the catalysts is strongly influenced by the nature of the supports employed. We found that TiO₂ supports showed higher activity, especially nitrogen-doped TiO₂. As we proved via FTIR emission (IES) and FTIR (attenuated total reflectance, ATR) spectroscopy, there is stronger interaction between nitrogen-doped TiO₂ and alkynes. The stronger interaction is probably due to the Ti³⁺ sites. No further interaction was observed when Au NPs were loaded onto the TiO₂ surface.³⁴ When the anilines are activated on the AuNPs and the alkynes on the support, the electrostatic repulsion between the N lone pair and the π system of alkyne is avoided. Herein, we further investigated the properties of the supports. As illustrated in Table S1,† compared to the AuNP catalysts supported on TiO₂(B), the conversion dropped when TiO₂(anatase) was used as the support. The lower activity of AuNPs on TiO₂(anatase) could be ascribed to the higher ability of the anatase phase to adsorb and activate oxygen,⁴³ which may impede the adsorption and activation of reactants involved in the hydroamination. This argument is supported by the observation that when the experiment was conducted with Au/TiO₂(B) under an oxygen atmosphere, while keeping all of the other reaction conditions unchanged, only a negligible conversion was observed, even at a higher temperature (70 °C). The negative effect of oxygen on the reaction could also explain the negligible activity of AuNPs on CeO₂, which has a significant capacity to activate and store oxygen. Notably, the AuNPs on supports such as ZrO₂ and Al₂O₃, which are relatively inert to oxygen adsorption and activation, exhibited substantial activity.

Another possible explanation for the higher activity of the catalyst on TiO₂(B) compared with that on the anatase phase for this reaction is the lower conduction band position of TiO₂(B),⁴⁴ which favours the migration of high energy electrons from light-excited AuNPs to the TiO₂(B) support. Such an electron transfer could benefit both the activation of the aniline on AuNPs and the triple bonds of alkynes on the support surface. The poor performance of AuNPs on the supports (e.g. zeolite) may be caused by the fact that the electron transfer process is inhibited. Evidence for the involvement of radicals or radical-ions generated by electron transfer during the reaction is provided by the fact that no products were formed when the radical scavenger 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO) was added to the system. This result is consistent with the known excited state quenching and redox chemistry of TEMPO, with it either quenching or capturing the initially transferring electrons or electrons from intermediate radicals or acting as an electron donor.⁴⁵ Herein, the function of TEMPO may not be blocking active coordination species since even using a molar ratio of TEMPO to alkyne of 0.0025 stopped the formation of the products.

Impact of the solvent on the photocatalytic process

Solvents with high polarity are capable of complexing to Au surfaces.¹¹ It is believed that solvents interacting strongly with the catalyst may inhibit activation of the reactants. Therefore, in this study, solvents with differing polarities such as benzene, tetrahydrofuran, acetonitrile and dimethyl formamide were investigated. As shown in Fig. 5, solvents with higher polarity (e.g. tetrahydrofuran and dimethyl formamide) have a negative impact on the reaction. This is in agreement with claims in the literature that polar solvents can compete with the adsorption of aniline and alkyne on the surface of catalyst.⁴² Thus, the best activity was achieved when toluene was employed as the solvent.

Fig. 5 The influence of solvent polarity on the hydroamination of 4-phenyl-1-butyne. The X-axis shows the solvents used arranged in order from low to high polarity: toluene, benzene, tetrahydrofuran, acetonitrile and dimethyl formamide.

Reusability of supported AuNPs as new photocatalysts for hydroamination reactions

The reuse of the photocatalyst was also examined, and it was found that the catalyst was easily recycled by washing with ethanol and drying at 70 °C. The conversion of alkynes was essentially unchanged when the photocatalyst were reused three times (Fig. 6).

Fig. 6 Reusability of the photocatalysts (Au/TiO₂-N) for the hydroamination of phenylacetylene with aniline.

Conclusions

In conclusion, supported AuNPs exhibit excellent catalytic activity for the hydroamination reaction of alkynes by amines under visible light irradiation at ambient temperature. The action spectrum, wherein the LSPR band of supported AuNPs correlates well with the conversion of alkynes, demonstrates that photocatalytic hydroamination is initiated by AuNPs under visible light irradiation. The reaction rate increases with both light intensity as well as reaction temperature, as the AuNPs can couple both input light energy and heat energy to drive the reaction. The support materials can influence the catalytic performance, and the AuNPs supported on nitrogen-doped titania were found to be the most effective catalyst. The support is likely to be involved in the electron transfer steps of the hydroamination reaction. This approach reveals a new class of useful catalytic processes with the potential to utilize solar energy for a cleaner, lower environmental impact synthesis of fine chemicals for organic chemistry.

Acknowledgements

The authors gratefully acknowledge financial support from the Australia Research Council (ARC DP110104990 and CE0561607).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: GC-MS of products, calculation method, size distribution from TEM, and photocatalytic hydroamination results on different supports. See DOI: 10.1039/c6ra01518b

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