Tetradentate copper complex supported on boehmite nanoparticles as an efficient and heterogeneous reusable nanocatalyst for the synthesis of diaryl ethers

In this work boehmite nanoparticles (BNPs) were prepared through addition of aqueous solution of NaOH to solution of Al(NO3)3·9H2O. Then, the surface of BNPs was modified by (3-chloropropyl)trimethoxysilane (CPTMS) and further tetradentate ligand (MP-bis(AMP)) was anchored on its surface. At final step, a tetradentate organometallic complex of copper was stabilized on the surface of modified BNPs (Cu(II)-MP-bis(AMP)@boehmite). These obtained nanoparticles were characterized using SEM imaging, WDX, EDS, AAS and TGA analysis, BET method, FT-IR spectroscopy, and XRD pattern. In continue, the catalytic activity of Cu(II)-MP-bis(AMP)@boehmite has been used as a much efficient, reusable and hybrid of organic–inorganic nanocatalyst in the synthesis of ether derivatives through C–O coupling reaction under palladium-free and phosphine-free conditions. Cu(II)-MP-bis(AMP)@boehmite catalyst has been recovered and reused again for several times in the synthesis of ether derivatives.


Result and discussion
Surface modification of BNPs by CPTMS was performed matching to last reported method 15 . Then, MPbis(AMP) ligand was substituted with Cl of CPTMS. Subsequently copper catalyst was fixated on the surface. The schematic preparation procedure of this catalyst (Cu(II)-MP-bis(AMP)@boehmite) is outlined in Fig. 1. Cu(II)-MP-bis(AMP)@boehmite was characterized by scanning electron microscopy (SEM) imaging, wavelength dispersive X-ray spectroscopy (WDX), energy-dispersive X-ray spectroscopy (EDS), atomic absorption spectroscopy (AAS) and thermogravimetric analysis (TGA) analysis, X-ray diffraction (XRD) pattern, Fourier transform infrared spectroscopy (FT-IR) spectroscopy, N 2 adsorption-desorption isotherms method. SEM images of Cu(II)-MP-bis(AMP)@boehmite and MP-bis(AMP)@boehmite are shown in Fig. 2a,b respectively which indicate that Cu(II)-MP-bis(AMP)@boehmite has particle size in nanometer scale. As shown in Fig. 2, the SEM images of the material before the addition of Cu are similar to after the addition of Cu in term of size and morphology which shows the stability of these nanoparticles after stabilization of the copper complex.
In order to illustrate the elemental combination and distributions of catalyst, the energy-dispersive X-ray spectroscopy (EDS) and wavelength-dispersive X-ray mapping (WDX) analysis of Cu(II)-MP-bis(AMP)@boehmite have been examined, the EDS (Fig. 3) and WDX (Fig. 4) analysis of this catalyst shown the attendance of aluminum, oxygen, silica, carbon, sulfur, nitrogen, and in addition copper species in catalyst with homogeneous dispensations of all elements in the structure of Cu(II)-MP-bis(AMP)@boehmite. Also, the exact amount of copper was found to be 0.4 × 10 −3 mol g −1 by AAS analysis.
In order to determine content of organic species, which were immobilized on the surface of BNPs, TGA/ DTA analysis of Cu(II)-MP-bis(AMP)@boehmite was performed (Fig. 5). The miniature weight loss within 9% at downward temperature is related to vaporization of adsorbed solvents 49 . The organic substance including CPTMS and ligand which fixed on BNPs was decomposed at 200-500 °C that is 32% of catalyst. Last weight dissipation which is lesser than 2% a may be related to transformation of thermal crystal phase of boehmite nanoparticles 11 .
Powder XRD analysis is a great technique to determine the crystal structure of materials. Therefore, the powder XRD analysis was performed to shown the crystalline phase of Cu(II)-MP-bis(AMP)@boehmite. The experimentally obtained XRD patterns were compared to the Inorganic Crystal Structure Database (ICSD) provided which shows two series of crystal structure. The obtained results analysis from powder XRD analysis of Cu(II)-MP-bis(AMP)@boehmite is shown in Fig. 6. Also, phases list from XRD results are summarized in Table 1. As shown in Fig. 5 and Table 1, X-ray diffraction analysis of this catalyst shows two series of materials. The first of them is related to the boehmite (Aluminum Oxide Hydroxide) crystal phase, which matched with the standard pattern 01-083-1506 code of ICSD database. This pattern correspond to 2θ value positions at 14.8° (0 2 0), 28 Fig. 7 and textural properties of Cu(II)-MP-bis(AMP)@boehmite are summarized in Table 2. As shown in Table 2, surface area, pore volumes and pore diameters of this catalyst are 101.66 m 2 g −1 , 0.375 cm 3 g −1 and 4.62 nm respectively. Decreasing of surface area of Cu(II)-MP-bis(AMP)@boehmite than boehmite nanoparticles (128.8 m 2 g −1 , Ref. 29 ) is due to the linking of organic substance and copper complex. Catalytic study of Cu(II)-MP-bis(AMP)@boehmite. The catalytic activity of Cu(II)-MP-bis(AMP)@ boehmite has been investigated in the C-O coupling reaction toward the formation of diaryl ether derivatives. In the synthesis of diaryl ethers, the coupling of phenol (Ph-OH) with iodobenzene (Ph-I) using catalytic value of Cu(II)-MP-bis(AMP)@boehmite as catalyst has been chosen as a pattern reaction to found the optimize conditions. At first, the pattern reaction has been tested without Cu(II)-MP-bis(AMP)@boehmite (Table 4, entry 1) which the pattern reaction was not go proceed. Then, the pattern reaction was carried out in using variant value of catalyst which it was completed with 98% of yield when 30 mg of Cu(II)-MP-bis(AMP)@boehmite was used ( Table 4, entry 2). At second, the effect of various solvents (Table 4, entries 4-7) and bases ( Table 4, entries [8][9][10][11] were studied in the pattern reaction under wide range of temperature. As shown, DMSO solvent and KOH base at 130 ºC offered the best results for the synthesis of diaryl ether (Scheme 1).
In order to show the role of Cu(II)-MP-bis(AMP)@boehmite, the catalytic activity of Cu(II)-MP-bis(AMP)@ boehmite was compared with alone boehmite and MP-bis(AMP)@boehmite in the coupling of phenol with iodobenzene under optimized conditions ( Table 5). As shown, diphenyl ether was formed in the presence of Cu(II)-MP-bis(AMP)@boehmite with 98% of yield. While, almost no products were formed in the presence of alone boehmite or MP-bis(AMP)@boehmite.
The mentioned optimizing condition were investigated to the various aryl halide derivatives to extend catalytic scope of Cu(II)-MP-bis(AMP)@boehmite (Table 6). All aryl halide derivatives having other functional groups with electron-withdrawing or electron-donating nature were successfully coupled with phenol in superior yields in the presence of this catalyst. As shown in Table 6, aryl iodides have great reaction rate than aryl bromides, while aryl chlorides have lowest reaction rate under coupling of phenol using Cu(II)-MP-bis(AMP)@boehmite catalyst. This indicates that the C-Cl bond is stronger than the C-I bond because the carbon and chlorine orbitals are similar in size, energy, and symmetry, but the iodine and carbon orbitals have different sizes and energies. In addition, the C-I bond is longer and weaker than the C-Cl bond, which C-I bond requires less energy to break and has a faster coupling rate than the short C-Cl bond. For example, the coupling of phenol with 4-nitrobromobenzene is greater than 4-nitrochlorobenzene. This ordered was also observed at coupling of phenol with iodobenzene, bromobenzene and chlorobenzene using Cu(II)-MP-bis(AMP)@boehmite catalyst.  Table 6 for coupling of Ph-OH with Ph-I. As monitored in Table 7, biphenyl ether was synthesized in superior yields when Cu(II)-MP-bis(AMP)@boehmite employed as catalyst than other catalysts. Therefore, Cu(II)-MP-bis(AMP)@boehmite catalyst is more effective than alternative catalysts in terms of practicality, reaction rate and isolated yield. Also, in some cases, nonrecoverable homogeneous catalysts have been introduced for the formation of aromatic ethers (Table 7, entry 7). While, Cu(II)-MP-bis(AMP)@boehmite catalyst can be recycled over and over again.  www.nature.com/scientificreports/ run for 6 cycles. As shown in Fig. 9, Cu(II)-MP-bis(AMP)@boehmite catalyst can be recycled frequently at minimum to 6 times in synthesis of biphenyl ether. The copper leaching from Cu(II)-MP-bis(AMP)@boehmite in the reaction mixture was studied by AAS analysis. In order to this issue, the coupling reaction of Ph-O with Ph-I in the presence of Cu(II)-MP-bis(AMP)@ boehmite was repeated and the catalyst was recovered and collected after completion of the reaction. Then, the amount of copper in the recovered catalyst (0.32 × 10 −3 mol g −1 ) was compared with the unused catalyst (0.4 × 10 −3 mol g −1 ) by AAS analysis which indicated that copper leaching of this catalyst is negligible (less than 0.01%).

Recycling ability and
The SEM images of Cu(II)-MP-bis(AMP)@boehmite after recovered and reused are shown in Fig. 10. The particle sizes and morphology of Cu(II)-MP-bis(AMP)@boehmite were compared to the fresh catalyst. As shown, the size and morphology of recovered and reused catalyst indicated an excellent similarly to the fresh catalyst.
The heterogeneity of Cu(II)-MP-bis(AMP)@boehmite was authenticated by the hot filtration experiment. In order to this issue, the coupling reaction of Ph-O with Ph-I using Cu(II)-MP-bis(AMP)@boehmite catalyst was started and it was stopped after 30 min. In this step, 48% of biphenyl ether was formed. Then, the selected reaction was repeated and the catalyst was pick up after 30 min and the solution was permitted to proceed for 30 min again without catalyst. In this step, 51% of biphenyl ether product was obtained. it means that Cu(II)-MPbis(AMP)@boehmite catalyst have heterogeneous nature and C-O coupling reactions are take place following heterogeneous conditions.     was prepared matching to last reported method 15 . Also, MP-bis(AMP) was synthesized from condensation of 2-hydroxy benzaldehyde with 4,6-diaminopyrimidine-2-thiol 14   were stirred in DMSO at 130 °C and the progression of the reaction was seen by TLC. After performing of the reaction, the reaction mix was make cold to room temperature. Then, the mixture was dilute with water, the remaining catalyst was cleared by ordinary filtration and washout with ethyl acetate. The filtered solution was extracted with ethyl acetate and water. The solution was dried upon Na 2 SO 4 (2 g). Then the solvent was vaporized and pure ether derivatives were afforded.

Conclusion
In summary, boehmite NPs have been prepared in aqueous media and then a new Schiff base Cu-complex has been stabilized on the surface of BNPs (Cu(II)-MP-bis(AMP)@boehmite). This catalyst was evidenced using SEM imaging, WDX, EDS, AAS and TGA analysis, BET method, FT-IR spectroscopy, and XRD pattern. The yields of the obtained ethers were authenticated the good performance of Cu(II)-MP-bis(AMP)@boehmite in      www.nature.com/scientificreports/ or electron-donating nature. Excellent stability and heterogeneous nature of Cu(II)-MP-bis(AMP)@boehmite were certified by hot filtration examination.

Data availability
Data available in article Supplementary Materiall; the data that supports the findings of this study are available in the Supplementary Material of this article.