The catalyst has been fabricated in a stepwise manner through successive surface modifications of h-BN nanosheets, as displayed in Fig. 2. In the initial step, hydroxyl functionalized boron nitride (h-BN@OH) nanosheets were fabricated via ion-assisted liquid exfoliation approach by heating the homogeneous mixture of h-BN, sodium hydroxide (NaOH) and potassium hydroxide (KOH) using a stainless steel autoclave. Thereafter, h-BN@OH nanosheets were functionalized with amine moieties i.e. 3-aminopropyltriethoxysilane (APTES) under reflux condition in ethanol. This was done in order to generate the functional moietieson the surface of the nanosheets as amine groups are considered to be one of the most promising linkers that allow further scope of ready surface modification. Finally, h-BN@APTES@BP@Cu catalyst was synthesized by immobilizing 2-hydroxy-4-methoxybenzophenone (BP) onto the amine functionalized BN nanosheets (h-BN@APTES) via Schiff base condensation followed by metalation using copper acetate. The designed nanocatalyst was then characterized well using various physicochemical techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier transform infrared (FT-IR), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), energy-dispersive X-ray fluorescence (ED-XRF) and X-ray photoelectron spectroscopy (XPS).
Catalyst characterizations.
SEM was employed to analyse the shape and surface morphology of the nanocomposites. The morphologies of h-BN@OH, h-BN@APTES and h-BN@APTES@BP@Cu were studied as shown in Fig. 3. The SEM micrograph of exfoliated h-BN@OH exhibits lamellar like structures with smooth edges and flat surface. Further, on moving to h-BN@APTES and h-BN@APTES@BP@Cu, no morphological change is observed which suggests that the structural integrity remained unaltered even after functionalization. Besides SEM, TEM analysis of synthesized materials was also carried out which reveals the layered structure of h-BN@OH, aligned in lateral dimension. TEM micrograph also shows 4-5 stacked sheets of h-BN@OH with average thickness lying between 3-7 nm. Further, h-BN@APTES and h-BN@APTES@BP@Cu shows no significant change in morphology even after surface modification and successive metalation.
FT-IR was performed to elucidate the stepwise synthesis of surface modified BN nanosheets as depicted in Fig. S1. The spectrum of h-BN@OH represents two intense peaks observed at 1372 and 820 cm-1 that are assigned to the B−N stretching and B−N−B bending vibration. Furthermore, an additional broad peak appears at 3430 cm-1 confirms the presence of hydroxyl group on the surface of h-BN nanosheets, compared with bulk h-BN. On moving to h-BN@APTES spectrum, shift in B−N peaks towards lower wavelength is observed. Also, emergence of bands at 1040 and 1120 cm-1 corresponds to the characteristic absorption of Si−O symmetric and asymmetric mode of vibrations authenticates the existence of APTES moiety on the BN nanosheets surface through silylation process53,54. Moreover, absorption bands at 2936 and 1633 cm-1 are attributed to the CH2 and NH2 stretching vibrations of amino-propyl moiety. Furthermore, h-BN@APTES@BP@Cu spectrum exhibit a peak at 1632 cm-1 which depicts the imine group formation as a result of Schiff’s condensation between NH2 groups of h-BN@APTES and carbonyl groups of the ligand. However, C=N absorption peak is concealed under the broad band of B−N bonds55.
The crystallographic structure of the designed nanocomposites was deduced via powder XRD analysis as shown in Fig. 4a. XRD spectrum of h-BN@OH exhibits characteristic Bragg’s diffraction peak similar to the pristine h-BN powder at 2θ = 26.9°, 41.6°, 43.8°, 50.0° and 55.1° corresponding to the (002), (100), (101), (102) and (004) planes respectively. Further, similar peaks are obtained for the stepwise synthesis of h-BN@APTES@BP@Cu which unveils no significant change in the structure of nanosheets after being modified with the functionalizing agents which supports its high crystallinity. Moreover, no additional peaks corresponding to any other impurity is observed which indicates the purity of sample.
X-ray photoelectron spectroscopy was employed to investigate the surface electronic states of the developed catalyst. XPS survey spectra of h-BN@APTES@BP@Cu and core spectra of B, N, O, Si and C elements are shown in Fig. 4b and Fig. S2 respectively. As can be viewed from Fig. S2a, the appearance of two peaks located at 191.1 and 190.1 eV are attributed to the B−O and B−N bonds respectively. Hence, it can be interpreted that −OH is attached to B atoms effectively rather than N atoms. Moreover, the Si 2p spectra (Fig. S2e) reveals strong peak at 102.1 eV which is accredited to the bond formation between silicon and oxygen (B−O−Si) and thus provides a strong evidence of the completion of silylation reaction through covalent approach. Further, peak at 397.8 in the core spectrum of N 1s (Fig. S2b) is assigned to the binding energy of C−N=C, which authenticates the successful grafting of ligand onto the amine functionalized BN nanosheets via Schiff base condensation. In addition, the C 1s spectrum of h-BN@APTES@BP@Cu (Fig. S2d) represents two bands amongst which band at 284.8 eV is attributed to the binding energy of C=C bonds and 286.6 eV is assigned to the C−O−C bonds56,57. Besides, the XPS spectrum of Cu 2p core level region (Fig. 4c) displays two intense bands at 934.5 and 954.5 eV, which correspond to the binding energy of Cu(II) and an additional peak at 943.9 eV indicates the coordination interaction between copper and the ligand58.
The elemental composition of the synthesized h-BN@APTES and h-BN@APTES@BP@Cu was confirmed by EDS (Fig. S3). The well-defined peaks of B, N, O, C and Si in Fig. S3a validates the anchoring of APTES moieties onto the surface of BN nanosheets, while distinct peaks of B, C, N, O, Si and Cu in Fig. S3b corroborates the synthesis of h-BN@APTES@BP@Cu nanocatalyst. Moreover, well resolved peak of copper in the final nanocatalyst is also affirmed by ED-XRF spectroscopy (Fig. S4) which indicates successful introduction of metallic species on h-BN@APTES@BP. In addition, elemental mapping of h-BN@APTES@BP@Cu shows uniform distribution of B, N, O, C, Si and Cu elements in the final nanocatalyst (Fig. 5). Furthermore, the synthesized nanocatalyst was subjected to atomic absorption spectroscopy to analyse the copper content and the corresponding loading was found to be 0.4878 mmol g-1.
Catalytic evaluation.
The catalytic potential of newly fabricated h-BN@APTES@BP@Cu catalyst was examined for the synthesis of 5-substituted 1H-tetrazoles via [3+2] cycloaddition of azide and corresponding nitriles. To commence the investigation, benzonitrile and sodium azide were selected as test substrates. Moreover, various reaction parameters such as amount of catalyst, type of solvents, effect of time and temperature were determined to achieve an optimum reaction profile for the cycloaddition reaction with the aid of h-BN@APTES@BP@Cu (Fig. 6). A control experiment was carried out in the absence of catalyst using 1:2 ratio of test substrates (i.e. 1 mmol benzonitrile and 2 mmol sodium azide) which afforded trace amount of the desired product (Table S1, Supporting Information). In addition, various homogeneous and heterogeneous catalysts were also deployed to afford the targeted product. Amongst all the tested catalytic materials, heterogeneous h-BN@APTES@BP@Cu presented highest conversion percentage and therefore endorsed its remarkable efficacy in the desired transformation.
The influence of variation in catalyst amount was examined in the one-pot synthesis of tetrazoles. In this respect, six different sets of experiments were carried out by increasing the amount of catalyst in the range of 5 to 30 mg (Fig. 6a). The results disclosed that on increasing the amount of catalyst from 5 mg to 20 mg, an increase in conversion percentage was observed due to increase in catalytic active sites. Further increase in catalyst loading led to a decrease in conversion percentage which could be attributed to the steric hindrance caused by low dispersity of excess catalyst. Therefore, optimum amount of h-BN@APTES@BP@Cu was found to be 20 mg which resulted in maximum percentage conversion.
Choice of solvent also play a pivotal role in enhancing the catalytic efficacy of the reaction. In this context, model reaction was subjected to a series of solvents which include water, ethanol, ethylene glycol (EG), dimethyl sulfoxide (DMSO), toluene, N, N-dimethylformamide (DMF), dioxane and N-methyl-2-pyrrolidone (NMP). The results revealed that the reaction proceeded with good conversion percentage using ethanol, dioxane, NMP and DMF solvents (Fig. 6b). Moreover, reaction was also performed under solvent free conditions which showed low conversion percentage. Evidently, superior result was achieved when the reaction was conducted in ethanol. Hence, further optimizations were executed successfully using ethanol as a green solvent.
To determine the effect of time on the rate of reaction, model reaction was monitored at different time intervals ranging from 2 to 10 h. As shown in Fig. 6c, the conversion percentage is displayed as a function of time which demonstrated that maximum conversion percentage was observed when the reaction was allowed to run for 6 h. However, when the reaction proceeded further, no appreciable change was observed. Therefore, 6 h was considered as optimized time period for the cycloaddition of benzonitrile and sodium azide moieties.
In order to study the effect of temperature variance, test reaction was carried out at diverse range of temperature (40-90 oC) as presented in Fig. 6d. At 40 oC, conversion of the reactant to the desired product was found to be negligible. When temperature was increased to 60 oC, 71% of the product formation was achieved. Thereafter, an increase of 10 °C in temperature resulted in 90% conversion. The results revealed 100% conversion when temperature was raised to 80 oC. However, further rise in temperature resulted in no significant change in conversion percentage. Hence, the optimum temperature for [3+2] cycloaddition product was found to be 80 oC.
To explore the scope and applicability of this methodology, a series of benzonitriles were subjected to [3+2] cycloaddition reaction under the established ambient conditions. The concise results are summarized in Scheme 1. It was found that benzonitriles bearing both electron donating groups (entry 3e and 3f) and electron withdrawing groups (entry 3b, 3c and 3g) furnished corresponding tetrazoles in moderate to excellent conversion percentage. In particular, superior results were obtained in case of nitriles possessing electron withdrawing groups. This could be attributed to the –I effect of the substituents that makes the benzonitrile more electrophilic thereby activating the benzonitrile towards nucleophilic attack via azide ion. However, nitriles that comprise of electron donating substituents proceeded with relatively longer time period (entry 3e). Besides, the steric hindrance caused by chlorine group at ortho position resulted in lower conversion percentage (entry 3f). Moreover, alkylnitriles were also subjected to the optimized reaction conditions. Unfortunately, the desired products were not obtained (entry 3h and 3i).
aReaction conditions: Nitrile (1 mmol), sodium azide (2 mmol), h-BN@APTES@BP@Cu (20 mg) in ethanol (1 mL), 80 °C. bConversion percentages were determined via GC−MS. cTON is the number of moles of the product per mole of the catalyst.
On the basis of literature precedents, a plausible reaction pathway has been proposed to synthesize 5-substituted 1H-tetrazoles using h-BN@APTES@BP@Cu catalyst as outlined in Fig. 739. Initially, coordination of nitrogen atoms of both the nitrile and azide moieties with Cu (II) generates complex I that accelerates the [3+2] cyclization step as shown in complex II wherein subsequent nucleophilic attack of azide ion onto the nitrile group leads to the formation of complex III. Thereafter, acidic work-up protonates the complex III which results in the formation of desired tetrazole product with the release of catalyst. The structural integrity of the recovered catalyst remained unaltered even after being reused for several runs.
A leaching experiment was also performed using hot filtration method in order to certify heterogeneity of the catalyst. Thus, the test reaction was carried out under optimized reaction conditions using h-BN@APTES@BP@Cu catalyst. After passage of half of the reaction time, the catalyst was removed from the reaction mixture. The resulting supernatant was allowed to react further for appropriate period of time. GC-MS results displayed no significant increment in the conversion percentage of 5-substituted 1H-tetrazole which debarred the possibility of leaching of active metal species from its solid support. Therefore, it can be interpreted that copper remains intact with the solid material which provides strong evidence for the heterogeneous character of the catalyst.
The recyclability of h-BN@APTES@BP@Cu was examined under the optimized reaction conditions using benzonitrile and sodium azide as model substrates (Fig. 8). After completion of the reaction, the catalyst was retrieved by means of centrifugation, washed with ethyl acetate to remove residue of the reaction mixture and eventually dried well under vacuum. The recovered catalyst was then used for successive cycles by maintaining similar experimental conditions. The results authenticated that h-BN@APTES@BP@Cu could be reused efficaciously for five consecutive runs with no obvious deterioration in its catalytic activity. Further, on comparing SEM spectra of the recovered catalyst with the freshly prepared catalyst, no remarkable changes in shape and morphology was observed (Fig. S5). Additionally, XRD spectrum of the recovered catalyst showed identical Bragg’s diffraction peaks corresponding to the (002), (100), (101), (102), and (004) planes when compared with the freshly prepared catalyst (Fig. S6). Thus, it was proved that the synthesized BN supported copper catalyst exhibited good durability.
To date, various homogeneous and heterogeneous catalysts have been utilized for the one-pot synthesis of tetrazoles. As evident from the Table S2, h-BN@APTES@BP@Cu nanocatalyst showed its superiority over previously reported homogeneous and heterogeneous catalysts in terms of product yield, reaction conditions and recyclability. The previously reported homogeneous catalyst underwent decomposition immediately after the completion of reaction thereby creating separation problems and thus could not be reused for further consecutive runs. In contrast, the present catalyst could be retrieved and reused for several runs without any remarkable loss in its catalytic activity. Moreover, use of ethanol as a green solvent made this protocol economic and environmentally benign. Compared with the previously reported heterogeneous catalytic system, h-BN@APTES@BP@Cu exhibited higher yield, mild reaction conditions and good recyclability.