Sonosynthesis of pyranochromenes and biscoumarins catalyzed by Co3O4/NiO@GQDs@SO3H nanocomposite

Abstract Co3O4/NiO@GQDs@SO3H nanocatalyst has been used as an effective catalyst for the preparation of dihydropyrano[3,2-c]chromenes and biscoumarins under ultrasonic irradiations in ethanol. The catalyst has been characterized by FT-IR, XRD, SEM, EDS, BET, TGA, XPS and VSM. Atom economy, reusable catalyst, low catalyst loading, applicability to a wide range of substrates, high yields of products, and applying the sonochemical methodology as an efficient method and innocuous means of activation in synthetic chemistry for the preparation of medicinally privileged heterocyclic molecules are some of the substantial features of this method. The present catalytic procedure is extensible to a wide diversity of substrates for the synthesis of a variety-oriented library of pyranochromene and biscoumarins. The ultrasound approach decreases times, increases yields of products by creating the activation energy in micro surroundings. Meanwhile, this recoverable catalyst will provide a regular platform for heterogeneous catalysis, green chemistry, and environmentally benign protocols in the near future. Graphical Abstract


Introduction
Pyranochromenes and biscoumarins indicate biological activities including anticoagulant [1], anticancer [2], anti-Alzheimer [3], anti-Parkinson [4] and anti-HIV [5]. These attributes make pyranochromenes and biscoumarins notable targets in organic preparation for future consideration. A number of procedures have been developed for the preparation of pyranochromenes and biscoumarins using ruthenium(III) chloride hydrate [6], H 6 P 2 W 18 O 62 .18H 2 O [7], tetrabutylammonium bromide (TBAB) [8] and DBU [9]. Each of these methods has their own advantages but also suffer from certain disadvantages containing prolonged reaction time, tedious work-up processes, low yield, high temperature and hazardous reaction conditions. Despite the availability of these ways, there remains enough choice for a capable and reusable catalyst with high catalytic activity for the preparation of pyranochromenes and biscoumarins. Ideally, utilizing environmental and green catalysts which can be easily recycled at the end of reactions has obtained great attention in recent years. Nanocomposites have emerged as a suitable group of heterogeneous catalysts owing to their numerous applications in synthesis and catalysis [10,11]. Since, these nanocomposites are often recovered simply by easy workup, which prevents contamination of products, they may be considered as a promising safe and reusable catalysts as well as greener compared to traditional catalysts [12,13]. Graphene quantum dots (GQDs) are a novel member of carbon nanostructures that have quasispherical structures. GQDs have gained intensive attention owing to the remarkable features containing biological [14], biomedical [15], therapeutic applications [16], photocatalysts [17], surfactants [18], electrochemical biosensing [19], electrocatalytic activity [20], Li-ion battery [21], solar cells [22], photoluminescence [23,24]. bioimaging properties [25], and catalytic activity [26]. Potential applications of N-graphene quantum dots were recently reviewed on the basis of experimental and theoretical studies [27][28][29][30]. Synthesis of highly efficient nanocomposite catalysts for the synthesis of organic compounds is still an attractive challenge. To obtain larger surface area and more active sites, nanocatalysts are functionalized by active groups [31][32][33]. It has been demonstrated that the decoration of the nanocatalyst with GQDs prevents the aggregation of fine particles and thus increases the effective surface area and number of reactive sites for an efficient catalytic reaction. The chemical groups on a GQD are able to catalyze chemical reactions. The -COOH and -SO 3 H groups can serve as acid catalysts for many reactions [26][27][28][29][30][31][32][33][34]. Herein, we reported the use of Co 3 O 4 / NiO@GQDs@SO 3 H nanocomposite as a new efficient catalyst for the preparation of pyranochromenes and biscoumarins under ultrasonic irradiations in ethanol (Scheme 1). The ultrasound approach decreases times, increases yields of products by creating the activation energy in micro surroundings [35,36]. We found that Co 3 O 4 /NiO@GQDs@SO 3 H nanocomposite produce our desired compounds in high yields (86-95%) with excellent recovery and simple work-up procedure. In addition, Co 3 O 4 /NiO@GQDs@SO 3 H nanocomposite has a good recycling properties and this advantage is important from economic point of view.

Materials and characterization
Powder X-ray diffraction was taken on a Philips diffractometer of X'pert Company with monochromatized Cu Ka radiation (k ¼ 1.5406 Å). X-ray photoelectron spectroscopy (XPS) spectra were determined on an ESCA-3000 electron spectrometer.

Results and discussion
In the beginning, we prepared Co 3 O 4 /NiO nanoparticles by easy techniques. A facile hydrothermal method was used for the preparation of N-GQDs [37]. Sulfonated graphene quantum dots were prepared using chlorosulfonic acid [38].  In order to investigate the particle size and morphology of nanoparticles, SEM image of Co 3 O 4 / NiO and Co 3 O 4 /NiO@ GQDs@SO 3 H nanocomposite is indicated in Figure 2. SEM images of the Co 3 O 4 /NiO@GQDs@SO 3 H nanocomposite showed the formation of uniform particles, and the energydispersive X-ray spectrum (EDS) confirmed the presence of Co, Ni, O, S and C species in the structure of the nanocomposite (Figure 3).
Magnetic properties of nanocomposites before and after their being decorated with GQDs were tested by vibrating-sample magnetometer (VSM) (Figure 4). The lower magnetism of the as-synthesized Co 3 O 4 /NiO@GQDs@SO 3 H compared with the Co 3 O 4 /NiO was ascribed to the antiferromagnetic behavior of GQDs as a dopant. These results demonstrate that the magnetization property decreases by coating and functionalization [39,40].
FT-IR spectra of Co 3 O 4 /NiO, Co 3 O 4 /NiO@N-GQDs and Co 3 O 4 /NiO@GQDs @ SO 3 H nanocomposite are shown in Figure 5. The absorption peak at 3335 cm À1 related to the stretching vibrational absorptions of OH.   Figure 5b. The peak at approximately 1475-1580 cm À1 is attributed to C ¼ C bonds. The presence of sulfonyl group is also verified by the peaks appeared at 1215 and 1120 cm À1 . The broad peak at 3350 cm À1 related to the stretching vibrational absorptions of OH (SO 3 H) (Figure 5c).
The BET specific surface area of Co 3 O 4 /NiO and Co 3 O 4 /NiO@GQDs@SO 3 H nanocomposites was determined by the nitrogen gas adsorption-desorption isotherms ( Figure 6). The results presented that the BET specific surface area of Co 3 O 4 /NiO was        Table 2. The results of this table indicate that the excellent yields were achieved in the reaction at the presence of Co 3 O 4 / NiO@GQDs@SO 3 H nanocomposite (4 mg) under ultrasonic irradiation. Ultrasound irradiation has been employed to accelerate the chemical reactions proceed through the creation, growth, and implosive collapse of micrometer-sized bubbles in a liquid [35,36].
We also determined recycling of Co 3 O 4 / NiO@GQDs@SO 3 H nanocomposite as catalyst for the model reaction under ultrasonic irradiation in ethanol. The results showed that nanocomposite can be reused several times without noticeable loss of catalytic activity (Yields 94 to 92%).  Table 3. Our study has some advantages in compare with other mention studies including high yield of synthetic compound, reasonable time reaction and easy catalyst recovery.
In this study, Co 3 O 4 /NiO@GQDs@SO 3 H nanocomposite has been used as an effective catalyst for the preparation of dihydropyrano[3,2-c]chromenes and biscoumarins under ultrasonic irradiations in ethanol. The catalyst has been characterized by FT-IR, XRD, SEM, EDS, BET, TGA, XPS and VSM. The present catalytic procedure is extensible to a wide diversity of substrates for the synthesis of a variety-oriented library of pyranochromene and biscoumarins. The ultrasound approach decreases times, increases yields of products by creating the activation energy in micro surroundings. The results showed that nanocomposite can be reused several times without noticeable loss of catalytic activity. Thus, these results indicated that the nanocomposite was stable and could tolerate the MCR (multicomponent reactions) conditions.

Conclusion
In this study, we described the preparation of pyranochromenes and biscoumarins using Co 3 O 4 / NiO@GQDs@SO 3 H nanocomposite as a superior catalyst under ultrasonic irradiation. The current method provides obvious positive points containing environmental friendliness, significantly shorter reaction time, reusability of the catalyst, low catalyst loading and simple workup procedure.