Supramolecular grafting and stabilization of manganese complex on kryptofix23 modified Fe3O4@carbon nanosphere: as highly efficient, reusable, and clean nanocatalyst for xanthene’s unusual coupling

Grafting and stabilization of metal complexes for increasing catalytic activity have remained an enormous challenge in the catalytic arena. However, designing a catalytic system with complementary properties including high surface area, high loading, and easy separation offers a promising route for efficient utilization of magnetic material for various applications. Herein, a novel core–shell nanosphere catalyst (Fe3O4@C/KP23@MnCl2) was successfully synthesized with a magnetite core encapsulated in a carbon shell. It was modified using CPTES (3-chloropropyltriethoxysilane) and Kryptofix23 (KP23) ligand in the carbon surface for conversion to Fe3O4@C/KP23 support. The Manganese complex was coordinated onto the KP23 ligand decorated Fe3O4@C to improve the unusual coupling xanthenes in ethanol. It was found proposed nanocatalyst is a greener, recyclable, and more suitable option for large-scale application and provides some new insights into organic transformation.


Introduction
For many years, the procurement of heterogeneous catalysts has been a main field of sub-nanotechnology and remains so nowadays [1][2][3]. The approach to preparation must be examined in activity and selectivity, which both depend on the atom arrangement at a scale smaller than 0.02 nm. Sufficient access to reactants to the surface must be prepared. Catalysts mostly are used in the form of cylinders or pellets prepared by extraction, pressing, or other techniques [4]. Currently, the synthesis and application of functionalized magnetic particles have caused high fondness [5,6]. Because of their wide range of applications, including magnetic resonance imaging [7], targeted drug delivery [8], environmental remediation [9,10] and catalysis [11,12]; magnetic nanoparticles are a great topic for research. The magnetic separation technique is a promising method for solid-liquid phase separation [13].
Magnetic materials are always synthesized based on a magnetic magnetite (Fe 3 O 4 ) or maghemite (Fe 2 O 3 ) core, which is afterward improved with other compounds to form various configurations [14,15]. Due to their magnetic properties, a magnetic field is applied easily to separate the sorbent from the solution, which makes the separation process simple, fast, and highly effective. As a result, magnetic materials have been used widely and successfully in the pretreatment of samples, specifically in biochemistry at an early period, such as drug delivery, cell image, and biomolecule detection [16][17][18][19][20].
Fe 3 O 4 nanoparticles could easily be synthesized and covered in a high surface area. Meanwhile, scientists tried to functionalize Fe 3 O 4 nano-particles with some compounds, such as chitosan [21], humic acid [22], and gum Arabic [23] to improve their selectivity, adsorption capacity, and stability. These functionalized magnetic nanoparticles were found to be cost-efficient, chemically stable, and environment friendly compared to the Fe 3 O 4 nano-particles.

3
In this work, we prepared to functionalize Fe 3 O 4 with glucose. Xanthenes; as a fascinating type of oxygen-containing compound [24], has attracted a great notice for their vast range of medical and biological properties [25] such as antiviral [26], antibacterial [27], anti-inflammatory operations [28], and their wide applications in photodynamic therapy [29] and in material science [30][31][32]. These various applications have led to the great interest of synthetic chemists in developing and improving their synthesis methods [33]. In that way, the progress of nanocatalysts in various organic syntheses has received much attention [34][35][36][37][38][39][40][41][42][43]. Especially, heterogeneous nanocatalysts are suitable for the synthesis of these heterocyclic compounds because of their ability to recovery and easy separation by an outer magnet, centrifugation techniques, or several filtrations [44][45][46][47].

Instrumentation and materials
All reagents were obtained commercially from Sigma-Aldrich, Merck (Germany), and Fluka (Switzerland), they were used without further purification. Fourier transform infrared (FT-IR) spectra were recorded with a Spectrum two (PerkinElmer company) IR-640 spectrometer (Urmia University, Urmia, Iran). Melting points were measured on an Electrothermal 9100 apparatus. Homogeneous stirring was done by ultrasonic (Ultrasonic Homogenizer-model APU500 Advanced Equipment Engineering Company Adeeco, (Urmia University, Urmia, Iran). The crystalline phases of the nanoparticles were recognized by XRD measurements (X Pert Pro, Panalytical company, Daypetronic Company, Tehran, Iran). The field-emission scanning electron microscopy (FE-SEM) images and energy dispersive X-Ray Analysis (EDX) were studied by Sigma VP, Zeiss, Germany (Daypetronic Company, Tehran, Iran). Transition electron microscopy (TEM) was obtained from EM10C-100 kV, Zeiss, Germany (Daypetronic Company, Tehran, Iran). Raman spectroscopy was recorded by Senterra Raman Spectrometer, Burker Company, Germany (Daypetronic Company, Tehran, Iran). The vibration sample magnetometer (VSM) was obtained by Lbkfb modeh-Meghnatis Daghigh Kavir Company (Azad University of Mahabad, Mahabad, Iran). The TGA analysis was studied using an STA PT1000 TG-DSC (STA Simultaneous Thermal Analysis) (Daypetronic Company, Tehran, Iran) and the BET analysis for measuring the specific surface area was entered. For the measurement of the exact amount of Mn, ICP-EOS was employed by VISTA-PRO (Varian, Australia). Thin liner chromatography (TLC, type 60) was used for monitoring the reaction completion.

Synthesis of Fe 3 O 4 @C
Firstly, Fe 3 O 4 nanoparticles were prepared corresponding to the route in the literature [48]. Accordingly, FeCl 3 .6H 2 O (1.5 g, 5.55 mmol), Polyvinylpyrrolidone (PVP, 1 g), and sodium acetate (2 g) were dissolved in ethylene glycol (30 mL) and stirred vigorously for 2 h. After transferring the mixture to an autoclave (50 mL capacity) was sealed, and heated for 10 h at 200 °C. Then, the black sediment was collected by an external magnet after washing with distilled water and ethanol several times. Encapsulation of prepared Fe 3 O 4 nanoparticles with carbon shell was done with an improvement of literature [49]. Typically, 0.1 g of the synthesized Fe 3 O 4 was dispersed under ultrasonication in 30 mL of deionized water containing 2.0 g glucose. After transferring the mixture to an autoclave with a 40 mL capacity was heated for 12 h, 200 °C. Then, the black solid product was gathered using an external magnet from the mixture and washed several times with water. Finally, it was dried at 60 °C for 24 h in a vacuum.

Preparation of Fe 3 O 4 @C/KP23@MnCl 2
In a 50 mL round-bottom flask equipped with a magnetic stirrer, a reflux condenser, and gas inlet and outlet, Fe 3 O 4 /C (0.3 g), (3-chloropropyl) triethoxy silane (2 mL, 2 mmol), and n-hexane (20 mL) were added, respectively. The whole system was kept under an inert atmosphere and stirred for 24 h and the product was dried at room temperature (yield: 88%). The final product was modified with Krypto-fix32 to achieve a unique surface functionality for loading Mn 2+ (yield: 98%) and eventually, Fe 3 O 4 @C/KP23 was loaded with Mn 2+ to reach an organometallic-based heterogeneous catalyst Fe 3 O 4 @C/KP23@MnCl 2 . Scheme 1 represents a simplified detail of the catalyst preparation process.

Catalyst characterizations
The synthesis procedure of Fe 3 O 4 @C/KP23@MnCl 2 is outlined in Scheme 1. Initially, the surface of Fe 3 O 4 nanoparticles was modified by glucose as a carbon source via hydrothermal and further KP23 ligand was immobilized on their surface. Subsequently, the Mn (II) complex has been stabilized on Fe 3 O 4 @C/KP23. Finally, obtained catalyst were characterized by Fourier transform infrared (FT-IR), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), vibration sample magnetometer (VSM), Thermogravimetric analysis (TGA), the Brunauer-Emmett-Teller (BET) technique, ICP-EOS, and Raman.

Raman spectrum
Raman spectroscopy is a sensitive and local technique which usually used to assemble phase and crystallinity information of nanomaterials. The Raman spectra of the Fe 3 O 4 @C/ KP23@MnCl 2 sample are shown in Fig. 1b. According to the broadness of peaks, the nanostructured crystalline powders are clearly visible. The main peak at around 691 cm −1 , easily represents the magnetite phase [50]. Raman mode at 265 cm −1 is corresponding to Mn(II)-Cl mode of vibration [51].

XRD measurements
The XRD pattern of the prepared nanocatalyst was shown in Fig. 1d. The peaks of 2θ at 30.2°, 35.5°, 57.1°, and 62.7° connotative the crystalline of Fe 3 O 4 nanoparticles. The XRD pattern of Fe 3 O 4 @C/KP23 is almost the same in Fig. 1d (red and blue curves, respectively), except that a weak peak at 2θ = 71° in Fig. 1d (blue curve), which is related to Mn 2+ [52].

Field emission scanning electron microscopy (FE-SEM) analysis & EDS
The FE-SEM images of the samples are shown in Fig. 2a-c. In Fig. 2a, which shows the surface morphology of the Fe 3 O 4 @C, a rather smooth and monotonous structure of nanoparticles is observable. By grafting the ligand to the Fe 3 O 4 @C (Fig. 2b), the morphology has been bigger and porous structure, which get more suitable for MnCl 2 to penetrate into prepared nanoparticles pores and make a regular and monotonous surface with a crumpled structure (Fig. 2c). The EDS spectra of the Fe 3 O 4 @C/KP23@MnCl 2 are shown in Fig. 2g. Extracted image shows the strong peaks that revealed the presence and the mass percent of Fe, C, and O elements. Also, the EDS data corroborates the atomic ratio of N, Mn and Cl which are related to Kryptofix and the coordinated manganese complex (Fig. 2g)

Transmission electron microscopy (TEM)
TEM images of the nanocatalyst are presented in Fig. 2d-f. In Fig. 2d, distributed monotonous and spherical surface of Fe 3 O 4 @C. The image is shown in Fig. 2e, which just confirmed the nanostructure of Fe 3 O 4 @C/ KP23 by the results of SEM images. As observed in Fig. 2f, MnCl 2 was successfully loaded on the prepared nanoparticles.

The Brunauer-Emmett-Teller (BET) technique
The nitrogen adsorption-desorption isotherm of provided Fe 3 O 4 @C/KP23@MnCl 2 nanocatalyst is shown in Fig. 3. According to the isotherm, the specific plane area was 7.453 m 2 g −1 . Also, the hole size distributions of the nanocatalyst were determined by using the BJH technique (Barrett-Joyner-Halenda) which was 2.455 nm. This specified that the synthesized Fe 3 O 4 @C/KP23@MnCl 2 catalyst is mesoporous (2 < Dv < 50 nm).

Thermogravimetric analysis (TGA)
The TGA thermograms of the Fe 3 O 4 @C and Fe 3 O 4 @C/ KP23@MnCl 2 nanocatalyst are shown in Fig. 4 (curves  a and b, respectively). According to the curves, the small amount of weight loss below 200 °C is related to the removal of the solvent's molecules. There is also a slight weight loss starting from 100 °C, which can be attributed to the evaporation of the sample moisture. The wide range of weight loss within 200-750 °C is correlated with the decomposition of the organic functional group which is grafted on the framework.

Unusual coupling of xanthenes by catalytic amounts of Fe 3 O 4 @C/KP23@MnCl 2
The starting compounds of xanthenes 1 were obtained based on reported works in the literature (Scheme 2) [53][54][55]. As a new heterogeneous catalyst, we examined the catalytic activity of the new nanocatalyst of Fe 3 O 4 @C/KP23@MnCl 2 in the unusual coupling reaction of xanthenes (Scheme 3).
To reach this goal, the synthesis of 3,3,3′,3′-tetramethyl-2,3,3′,4′-tetrahydro-1H,1′H-[4,9′-bixanthene]-1,1′(2′H,9′H)dione (2a) and its derivatives [24] was studied to achieve the ideal conditions. According to our previously reported work [24], this synthesis was carried out by Mn 2+ for 96 h. In the present work, we optimized the condition of this coupling reaction by using the prepared nanocatalyst and reduced the time of the process to under 24 h (Scheme 3). After the successful synthesis of 2a, the synthesis of the other derivatives was done and the results are summarized in Table 1.
The proposed reaction mechanism for the formation of 2a is shown in Scheme 4. It seems that after the formation of complex 3a, in the presence of Mn 2+ loaded on nanocatalyst, it cleaved to intermediates 4 and A. Then the Michael addition of A with the β-position of the second intermediate A Finally, intermediate C converts into 2a by loss of a proton. In Scheme 4, the nucleophilic attack of fragment A was enforced by the lone pair on the oxygen atom. There is some evidence that the bis-dimedone derivative fragmented into two parts on the attack of a nucleophilic carbon on a methine carbon [24] All attempts to isolate and characterize 4 failed.

The nanocatalyst recyclability
The reusability is a significant advantage of heterogeneous catalysts. The reusability of Fe 3 O 4 @C/KP23@MnCl 2 was studied by performing the coupling reaction for fiverun under the optimal situation and after complication of each run, the nanocatalyst was trapped and brought out by an external magnet, washed several times with EtOH, and reused for next run. The percentage of reaction progress is shown in Fig. 5. As shown in Fig. 6, the prepared nanocatalyst has been used five times with no significant reduction of catalyst property. According to the ICP-OES analysis, the magnesium amount on the catalyst after five runs was 3.1%. The SEM images (Fig. 7) were applied to study and investigate the change in five-times reusing Fe 3 O 4 @C/ KP23@MnCl 2 . Also, the EDS data demonstrate the existence of Fe, C, O, Si, and Mn in the recovered Fe 3 O 4 @C/ KP23@MnCl 2 (Fig. 8). Through these analyses, the least changes in nanocatalyst are clear, and the functionalized nanocatalyst can be recycled and reused five times without obvious and remarkable changes in the structure or function.

Conclusion
In this work, the heterogeneous nanocatalyst Fe 3 O 4 @C/ KP23@MnCl 2 was synthesised by coordinating MnCl 2 on Fe 3 O 4 @C/KP23 nanoparticles and characterised by FT-IR, FE-SEM, EDS, XRD, TEM, VSM, BET, TGA, ICP-OES and Raman techniques. This novel nanocatalyst showed high catalytic activity and could be recycled and reused five times with the least amount of quality reduction. The activity of this nanocatalyst in unusual xanthene coupling reactions was investigated and reduced the time of reaction to less than 24 h. To summarize, the synthesized nanocatalyst is novel, effective, and useful for various organic reactions with high selectivity and easy accessibility.