Retracted Article: Introduction of a trinuclear manganese(III) catalyst on the surface of magnetic cellulose as an eco-benign, efficient and reusable novel heterogeneous catalyst for the multi-component synthesis of new derivatives of xanthene

Abstract

In this work, the new trinuclear manganese catalyst defined as Fe₃O₄@NFC@NNSM-Mn(III) was successfully manufactured and fully characterized by different techniques, including FT-IR, XRD, TEM, SEM, EDX, VSM, and ICP analysis. There have been reports of the use of magnetic catalysts for the synthesis of xanthine derivatives. The critical potential interest in the present method include short reaction time, high yields, recyclability of the catalyst, easy workup, and the ability to sustain a variety of functional groups, which give economical as well as ecological rewards. Also, the synthesized catalyst was used as a recyclable trinuclear catalyst in alcohol oxidation reactions at 40 °C. The magnetic catalyst activity of Fe₃O₄@NFC@NNSM-Mn(III) could be attributed to the synergistic effects of the catalyst Fe₃O₄@NFC@NNS-Mn(III) with melamine. Employing a sustainable and safe low temperature, using an eco-friendly solvent, no need to use any additive, and long-term stability and magnetic recyclability of the catalyst for at least six successive runs are the advantages of the current protocol towards green chemistry. This protocol is a benign, environmentally friendly method for heterocycle synthesis.

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Introduction

In the last decades, scientists have considered new approaches, including the development of environmentally friendly chemicals in terms of academic and industrial processes. The main criteria for assisting green chemical processes are the use of benign environmental solvents and the prevention of the formation of by-products in the synthesis of reactions using heterogeneous catalysts.^1,2 A multi-component reaction (MCR) is a process in which three or more easily accessible components are combined in a single reaction vessel to produce a final product displaying all inputs' features. MCR suggests greater possibilities for molecular variety per step with minimum synthetic time and endeavor.³ Therefore, organic synthesis through multi-component reactions (MCR) is now a research area in organic chemistry. MCR reactions are a powerful tool in drug discovery processes. Multi-component reactions (MCR) play an important role in chemistry because they can synthesize drug molecules with structural diversity. These reactions show significant advantages such as reduced reaction time, high selectivity, reduction of by-products, and high efficiency.^4–6 Among the multi-component reactions referred to as biologically active heterocycles is called xanthene. The synthesis of new compounds comprising heterocyclic systems grabs an important position in the scope of synthetic organic chemistry.^7–9 xanthene and its derivatives have become an important class of organic chemistry in the last few decades because of their wide range of biological and pharmaceuticals such as anti-inflammatory,¹⁰ antibacterial,¹¹ anti-plasmodial,¹² antidepressants,¹³ antimalarial agent¹⁴ and antiviral.¹⁵ They also have basic applications in biodegradable processes such as photodynamic therapy,¹⁶ agrochemicals,¹⁷ fluorescent materials,¹⁸ laser technologies,¹⁹ and luminescent sensors.²⁰ To date, several routes and reagents have been engaged in preparing many of these organic derivatives. Many catalysts are used for preparing various xanthenes, such as PVSA,²¹ [Et₃NSO₃H]Cl,²² core/shell Fe₃O₄@GA@isinglass,¹⁴ SbCl₃/SiO₂,²³ amberlyst-15,²⁴ boric acid,²⁵ nano TiO₂,²⁶ sulfamic acid,²⁷ acetonitrile²⁸ and etc. However, many of these methods toil from certain drawbacks, including powerful acidic conditions, long reaction times, low yields of products and spicy reaction conditions, toxic solvents, and excessive and costly catalysts. Therefore, it has become important to find another route for an environment-friendly preparation of xanthene derivatives. Heterogeneous catalysis is a significant accost to environmental remediation because it decreases the cost and energy provisions of chemical operations. However, catalyst recycling and recovery from the reaction medium are essential to prevent product contamination and protect the environment. Recently, magnetic nanoparticles (Fe₃O₄) have received scientific attention due to their extraordinary performance, such as high super magnetic properties in the presence of an external magnetic field, etc.²⁹ The surface of magnetic nanoparticles has been used for biological and medical engineering due to its small size, good compatibility, low toxicity and good physical, and chemical stability such as magnetic resonance imaging, drug delivery, biosensors, cancer treatment with magnetic induced hyperthermia.³⁰ Howbeit magnetic nanoparticles can be separated from the reaction mixture by an external magnet, the removal of nanoparticles from the reaction mixture systems is very challenging. These nanoparticles can easily accumulate oxidized and agglomerate in aqueous solutions due to their petite size.³¹ These small nanoparticles can be distributed over a large area, making them difficult to be collecting. Therefore, to prevent the oxidation of magnetic nanoparticles, a new approach should be considered, such as modifying the nanoparticles' surface with polymer groups or using a mold to prevent the nanoparticles from oxidizing for a specific structure, which can be useful approaches.^32,33 In the recent past, some green plant polymers^,34 such as nanofiber cellulose, lignin, and hemicellulose, polylactic acid (PLA),^35,36 chitin^37,38 and chitosan,^39,40 were considered. Among the various materials used as a mold to prevent oxidation of magnetic nanoparticles, cellulose nanofiber can be mentioned because of its many capabilities, including low cost, easy access, biodegradability, low density, excellent mechanical properties, and high level.^41–43 Trinuclear catalyst Mn could be better alternatives for greener production of xanthene derivatives because they can provide a large surface terrain as the catalytic patronage for catalyzing organic reactions.^44,45 To the best of our knowledge, no such trinuclear catalyst based on magnetic nanofiber cellulose has been used so far to prepare these derivatives making this approach a novel one for such synthesis. In this work, we synthesized a manganese trinuclear catalyst using a mechanical method using nanofiber cellulose as a substrate and a ligand as an anchor.

During our ongoing research into the activity of manganese trinuclear catalysts finally, after a successful study of trinuclear catalysts, their application to the oxidation of primary/secondary alcohols to produce corresponding carbonyl compounds with high conversion and good selectivity has been investigated. Also inspired by these facts as part of our ongoing efforts to develop new catalytic methods for performing xanthene multi-component reactions and alcohol oxidation under more benign environmental conditions, we present here Fe₃O₄@NFC@NNSM-Mn(III). We have evaluated this magnetic ternary catalyst with various techniques and finally used it in the mentioned reactions.

Result and discussion

Synthesis and characterization of Fe₃O₄@NFC@NNSM-Mn(III)

Therefore, it is important to use an effective method that provides the xanthene simply and easily. In this research, an effective and efficient synthetic method for preparing several xanthene derivatives using tri-nuclear catalyst manganese is introduced. Fe₃O₄@NFC@NNSM-Mn(III) was synthesized, as shown in (Fig. 1). The Fe₃O₄ nanoparticles, obtained by chemical purification protocol, were coated with cellulose to obtain Fe₃O₄@NFC. The ligand was then placed on a magnetic substrate. In the next step, manganese nanoparticles were immobilized in Fe₃O₄@NFC@NNSM-Mn(III) by adsorption of Mn(OAC)₃ in Fe₃O₄@NFC@NNS. The catalysts were characterized by FT-IR, XRD, FE-SEM, EDX, TEM, TGA-DTA, VSM, and ICP-OES spectroscopy. FT-IR spectra of Fe₃O₄, Fe₃O₄@NFC, Fe₃O₄@NFC–Cl, Fe₃O₄@NFC@NN, Fe₃O₄@NFC@NNS, Fe₃O₄@NFC@NNSM-Mn(III) are shown in (Fig. 1). The FT-IR spectrum of Fe₃O₄ (Fig. 1a), exhibited a wide absorption band at about 550–600 cm⁻¹, which is related to the vibration modes of Fe–O bonds in the Fe₃O₄ crystalline lattice. The characteristic bands that appeared at 1600, and 3400 cm⁻¹ are certified to the bending and stretching vibrations of the surface hydroxyl groups and adsorbed water molecules. In addition to Fe₃O₄ peaks observed in the spectrum that approves the formation of Fe₃O₄ nanoparticles, the strong peak at 1097, 2960, 3328 cm⁻¹ shows the (C–O, C–H, O–H stretching vibrations) of cellulose nanofibers (Fig. 1b). The strong absorption bands at about 1097, 1120, 1410–1520, 2990, and 3120 cm⁻¹ could correspond to the stretching vibration frequencies of Si–O, C–N, C=C aromatic, and C–H aliphatic and aromatic bonds.

Fig. 1 (a) Fe₃O_4, (b) Fe₃O₄@NFC, (c) Fe₃O₄@NFC–Cl, (d) Fe₃O₄@NFC-NN, (e) Fe₃O₄@NFC-NNS-Mn(III), (f) Fe₃O₄@NFC-NNSM-Mn(III).

Also, the observed broadband at around 3000–3450 cm⁻¹ could attribute to the stretching vibration frequency of the NH, NH₂, and OH groups (Fig. 1c and d). As shown in (Fig. 1e), The FT-IR spectrum of compound 4 (Fig. 1d), indicates the Schiff base reaction between Fe₃O₄@NFC@NN and 5-(chloromethyl)-2-hydroxy benzaldehyde, the characteristic peaks at 753 cm⁻¹ (C–Cl stretching), 1670 cm⁻¹ (C=N stretching vibration) confirms the structure of (Fig. 1e). Complexation of manganese and melamine to Fe₃O₄@NFC@NNS causes the adsorption of chlorine bond to be lost and causes the imine bond (1670 cm⁻¹) absorption to lower wavenumbers by about 20 cm⁻¹ (1650 cm⁻¹), indicating the participation of azomethine nitrogen in bonding with metal ion and confirming the coordination of ν(C=N) stretch to the metal via a nitrogen atom. Also, absorption bands at 1100 and 1600 cm⁻¹ could be assigned to (C–N and C=N) melamine stretching, respectively (Fig. 1e). These absorption bonds support the successful preparation of the catalyst. XRD analysis was carried out for Fe₃O₄ and Fe₃O₄@NFC@NNSM-Mn(III) to understand the structural features of the stimulus (Fig. 2). Present tense Fe₃O₄, Mn₂O₃ (Mn^III), and Fe₃O₄@NFC@NNSM-Mn(III) X-ray diffraction patterns in the 2θ range between 10°.

Fig. 2 XRD spectra of (black) Fe₃O₄, (red) Mn₂O₃ and (blue) Fe₃O₄@NFC-NNSMMn(III).

The XRD pattern of Fe₃O₄ shows six characteristic diffraction peaks at 2θ = 30.3°, 35.8°, 43.6°, 54.8°, 57.3°, and 63.2° corresponding to the (220), (311), (400), (422), (511), and (440) crystallographic phases, respectively, that were entirely in agreement with the standard structure and the reported XRD pattern for magnetic (Fig. 2: Fe₃O₄).⁴⁶ Besides, diffraction peaks in Fe₃O₄ nanoparticles, a broader peak was observed in the XRD pattern of the Fe₃O₄@NFC@NNSM-Mn(III) spheres in the 2θ range of 20–30°, which is the result of the amorphous cellulose. Thus, the XRD curve of the Fe₃O₄@cellulose nanoparticles composites of the same peaks was also observed in the Fe₃O₄@NFC XRD pattern, indicating retention of the crystalline spinel ferrite core structure during the cellulose coating process.³⁶ It is the characteristic of diffraction peaks of Mn(III) nanoparticles observed in the XRD pattern in the catalyst. Weak and robust peaks at 2θ = 32.07°, 39.47°, 50.12°, 60.72°, 62.17°, and 71.02° corresponding to the (222), (400), (600), (620), (444), and (640) crystallographic phases in XRD pattern are related to Mn₂O₃.⁴⁷

EDX graph confirms the successful connection of Schiff base manganese(III) complex by the presence of all metallic and non-metallic elements involved in the catalyst like carbon, nitrogen, oxygen, silicon, iron, and manganese (Fig. 3). Besides, the best technique for verification of catalyst preparation incredibly problematic half, is inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. Loading amount of Mn on the catalyst was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) instruments due to more ensure attainment. The experiment indicated that 1.1 mmol of Mn metal per gram of the catalyst (1.1 mmol 1 g) was loaded on the catalyst framework VII. Also, the analyses give the percentage of the heavy metals as: 45.54 w%, 2.33 w%, 7.10 w% for Fe, Si, Mn respectively.

Fig. 3 EDX image of Fe₃O₄@NFC@NNSM-Mn(III).

This technique gives the amount of manganese in the fresh and reuse catalyst. The Mn content of the catalyst was measured by ICP analysis, which illustrated the attendance of 0.12 mmol Mn per 1.0 g of the Fe₃O₄@NFC@NNSM-Mn(III). The surface morphology of Fe₃O₄@NFC@NNSM-Mn(III) was further determined using SEM and TEM images. A comparison of the FE-SEM images of Fe₃O₄ with that of Fe₃O₄@NFC, Fe₃O₄@NFC@NNSM-Mn(III) clearly showed the presence of other nanoparticles in the SEM images of Fe₃O₄@NFC@NNSM-Mn(III), which could be referred to as cellulose or ligand and Mn. As it is evident in the TEM images, the mean particle sizes of Fe₃O₄@NFC@NNSM-Mn(III) were measured to be around 16–29 nm. The particles of Fe₃O₄ and NFC can also be recognized through their indicative crystal lattice fringes (Fig. 4).

Fig. 4 SEM image of (a and b) Fe₃O₄, of (c and d) Fe₃O₄@NFC, of (e and f) Fe₃O₄@NFC@NNSM-Mn(III) and (g and h) TEM image of the Fe₃O₄@NFC@NNSM-Mn(III).

The TGA analysis was conducted to determine the uncoated Fe₃O₄ NPs, cellulose (NFC), and NFC-coated Fe₃O₄ NPs, content in the Fe₃O₄@NFC@NNSM-Mn(III) nano catalyst. The results are illustrated in Fig. 5. The TGA analysis was performed to confirm the coating of organic content on the surface of the Fe₃O₄ NPs. TGA spectra of uncoated Fe₃O₄ NPs show little weight loss, which is about 10% in the range of 25–800 °C. This might be due to the loss of residual water in the uncoated Fe₃O₄ NPs.⁴⁸ On the other hand, Fig. 5b shows the TGA spectra of cellulose. The initial step at 125 °C occurs because of the loss of hydrated and constituent water. The second stage involves weight reduction that occurs at approximately 225–425 °C. This is generally associated with the natural carbon of cellulose under oxidizing conditions. Disintegration past 425 °C occurs because of oxidation of the degraded products of cellulose.⁴⁹ Comparing with the TGA curves, Fig. 5c illustrates the thermal decomposition diagrams for Fe₃O₄@NFC synthesized by the green method. The first mass reduction phase occurred at 50–120 °C, caused by the water's loss absorbed in the nanocomposite surface. The second phase occurred at 150–425 °C, which could have arisen due to the breakdown of the bonds and destruction of the cellulose structure. The third stage of weight loss occurs at temperatures above 650 °C, which can be due to the rupture of the bond between nanofiber cellulose and Fe₃O₄ nanoparticles,^50,51 which were conducted on thermal and mechanical properties of Fe₃O₄@NFC.

Fig. 5 TGA curves for (a) Fe₃O₄, (b) cellulose, (c) Fe₃O₄@NFC, and (d) Fe₃O₄@NFC@NNSM-Mn(III).

Finally, the thermal stabilities of the Fe₃O₄@NFC@NNSM-Mn(III) were examined by TGA (Fig. 5d). As introduced in Fig. 5, a total weight loss of about 47% was determined from the TGA curve of the Fe₃O₄@NFC@NNSM-Mn(III). The reduction in mass below 160 °C can be attributed to the loss of embedded water molecules. Subsequently, the main weight loss step in the temperature ranges 160–350 °C (33% reduction) is attributed to the decomposition of cellulose units through the formation of levoglucosan and other volatile compounds or another mass loss, which shows higher thermal stability than Fe₃O₄@NFC@NNSM-Mn(III) has been modified due to a reasonable reduction in the amount of oxygen-containing functional groups in cellulose.^52a

Moreover, the weight loss of about 6% in the temperature range between 300 and 700 °C may be due to the thermal crystal phase alteration from Fe₃O₄ to γ-Fe₂O₃.^52b Other stages of weight loss up to 800 °C may be due to the decomposition of organic moieties on the surface of the Fe₃O₄@NFC core–shell nanoparticles. The magnetic behavior of Fe₃O₄ and Fe₃O₄@NFC@NNSM-Mn(III) was examined by VSM analysis at room temperature (Fig. 6). As it could be perceived from the resulting magnetization curves, the saturation magnetization amount of Fe₃O₄ and Fe₃O₄@NFC@NNSM-Mn(III) were 66.50 and 24.30 emu. g⁻¹, respectively. No discover hysteresis loop in the magnetization curves of both Fe₃O₄ and Fe₃O₄@NFC@NNSM-Mn(III) illustrated the superparamagnetic feature of these compounds.

Fig. 6 VSM pattern Fe₃O₄ (a), Fe₃O₄@NFC (b) and (c) Fe₃O₄@NFC@NNSM-Mn(III).

The magnetic catalyst activity of Fe₃O₄@NFC@NNSM-Mn(III) in the xanthene reaction

To optimize the xanthene reaction parameters, we studied the effect of catalyst amount, solvent, temperature, and time were explored towards the reaction progress over the reaction of benzaldehyde with dimedone as a multi-component reaction.

Among the various solvents examined for the xanthine reaction (Table 1, entries 1–8), the ethanol solvent performed best (Table 1, entry 3). After that, the effects of different factors on the reaction progress were perused in EtOH (Table 1, entries 9–13). The result lumpish that the best yield of the product was obtained in the presence of a stimulus (Table 1, entry 3). However, the reaction did not proceed without employing any factors. The effect of the magnetic catalyst amount was next studied (Table 1, entries 9–11). As clearly understood from the results in Table 1, a premier yield of the desired product was achieved by using 0.5 mol% of the magnetic catalyst (Table 1, entry 3). A control experiment lumpish that no product was obtained when the reaction was accomplished in the absence of the catalyst (Table 1, entry 9). The effect of different temperatures was also examined on the model reaction progress (Table 1, entries 12–14) and found that the multi-component reaction proceeded surprisingly at 45 °C (Table 1, entry 3). It is noteworthy that the MCR reaction has a small efficiency without temperature conditions (entry 12). In the next experiment, time differences were separately used to investigate the effect of time on the model reaction's progress (Table 1, entries 15–17). Results indicated that 10 min is the best time for the xanthene reaction (Table 1, entry 3). To obtain better about the efficiency of the Fe₃O₄@NFC@NNSM-Mn(III) catalyst, the magnetic catalytic activity of Fe₃O₄, Fe₃O₄@NFC, Fe₃O₄@NFC@NNS-Mn(III) (mononuclear), and Fe₃O₄@NFC@NNSM-Mn(III) (trinuclear) were separately studied in the model reaction (Table 1, entries 18–20). As can be seen in Table 1, no product was gained by using Fe₃O₄ and Fe₃O₄@NFC species. However, when the reaction was carried out in the presence of Fe₃O₄@NFC@NNS-Mn(III), the result was far from satisfactory (Table 1, entry 20). These findings indicated that the enhanced magnetic catalytic activity of Fe₃O₄@NFC@NNSM-Mn(III) could be attributed to the synergistic effect of melamine and Fe₃O₄@NFC@NNS-Mn(III) towards the xanthene reaction as a high-efficiency trinuclear catalyst (see the magnetic catalytic mechanism synthesis part for more details). With these interpretations, it can be inferred that the proper reaction of melamine and Fe₃O₄@NFC@NNS-Mn(III) in the synthesis of magnetic catalyst not only increases the power of the catalyst but also multiplies the activity of the magnetic catalyst. This property expedites and facilitates the reaction process in terms of time and other reaction conditions. These observations well proved the significant influence of the trinuclear catalyst to advance the xanthene MCR reaction.

Table 1 Optimization of reaction conditions for synthesis of 3,3,3,6,6-pentamethyl-9-phenyl-3,4,5,6,7,9-hexahydro -1H-3,5-xanthene-1,8(2H)-dione

Entry	Solvent	Catalyst (mol%)	Temp. (°C)	Time (min)	Yield^a (%)
a Reaction conditions: benzaldehyde (1 mmol) and dimedone (2 mmol). The catalyst was prepared by using 0.5 mol% of catalyst as explained in the Experimental section.b Reaction was performed in the presence of Fe3O4 as the catalyst.c Reaction was performed in the presence of Fe3O4@NFC as the catalyst.d Reaction was performed in the presence of Fe3O4@NFC@NNS-Mn(III) as the catalyst.
1	EtOH : H2O	0.5	45	15	90
2	H2O	0.5	45	20	85
3	EtOH	0.5	45	10	96
4	MeOH	0.5	45	10	85
5	CH3CN	0.5	45	60	70
6	EtOAC	0.5	45	75	65
7	CHCl3	0.5	45	45	85
8	Neat	0.5	45	45	60
9	EtOH	—	45	10	Trace
10	EtOH	1	45	10	95
11	EtOH	1.5	45	10	90
12	EtOH	0.5	—	60	55
13	EtOH	0.5	55	10	90
14	EtOH	0.5	65	10	85
15	EtOH	0.5	45	20	96
16	EtOH	0.5	45	40	92
17	EtOH	0.5	45	60	85
18	EtOH	0.5^b	45	60	0
19	EtOH	0.5^c	45	60	0
20	EtOH	0.5^d	45	30	70

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The magnetic catalytic activity of Fe₃O₄@NFC@NNSM-Mn(III) in the alcohol oxidation reaction

Encouraged by the promising results obtained from the xanthene reaction, the magnetic catalytic activity of Fe₃O₄@NFC@NNSM-Mn(III) was next explored towards the alcohol oxidation reaction under solvent-free conditions (Table 3). As shown in Table 3, this magnetic catalytic method was very efficient for the oxidation reaction of different alcohol. Interestingly, it was found that the oxidation reactions for the readily available and low-cost benzyl alcohol as the highly challenging substrates, with various derivatives, were also companion with satisfactory results (Table 4, entries 1–16). One of the most powerful and useful tools for the production of aldehydes and ketones is the oxidation of alcohols, which are significant mediators in the synthesis of drugs and chemicals. To develop the application of the trinuclear catalysts, Fe₃O₄@NFC@NNSM-Mn(III) magnetic catalyst was employed for the oxidation of a series of primary and secondary alcohols. To evaluate the reaction conditions of alcohol oxidation and compare the performance of manganese catalyst, benzyl alcohol was selected as a model reaction for alcohol oxidation and the results of this reaction are summarized in Table 3. Firstly, TBHP was chosen as the oxidant, as it introduced the highest oxidative activity among all the oxidants. Noticeably, the additive also has a strong effect. On the catalytic performance in the selective oxidation of alcohols.

Table 2 One-pot three-component synthesis of various xanthene derivatives catalysed by Fe₃O₄@NFC@NNSM-Mn(III)^a

Entry	Aldehyde	R1	Product	Time (min)	Yield^b (%)	MP (°C)
						Found	Reported
a Reaction conditions: benzaldehyde (1.0 mmol), dimedone (2.0 mmol) and catalyst (0.5 mol%) and EtOH (3 ml), 45 °C.b Isolated yield.
1 (ref. 53)		H		10	98	203–204	203–205
2 (ref. 53)		H		15	95	229–230	228–230
3 (ref. 54)		H		15	92	205–206	206–207
4 (ref. 53)		H		10	95	259–261	260–261
5 (ref. 55)		H		10	97	205–206	205–206
6 (ref. 56)		Me		10	96	203–204	202–204
7 (ref. 57)		Me		10	95	169–170	168–170
8 (ref. 58)		Me		10	96	200–201	201–202
9 (ref. 59)		Me		20	93	295–297	>300
10 (ref. 60)		H		30	80	267–269	268–269
11 (ref. 61)		Me		25	85	155–157	—
12		H		15	90	168–170	New
13		Me		15	92	200–202	New
14		H		15	92	220–222	New
15		Me		20	95	250–252	New

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Table 3 Screening reaction conditions^a

Entry	Solvent	Catalyst (mol%)	Temp. (°C)	TBHP (mmol)	Yield^b (%)
a Reaction conditions: benzyl alcohol (1 mmol), 20 min.b Yields of product isolated.
1	Water	1	40	2	40
2	EtOH	1	40	2	85
3	CH3CN	1	40	2	60
4	Solvent free	1	40	2	97
5	Solvent free	—	40	2	25
6	Solvent free	0.5	40	2	65
7	Solvent free	1.5	40	2	97
8	Solvent free	2	40	2	90
9	Solvent free	1	r.t,	2	50
10	Solvent free	1	50	2	97
11	Solvent free	1	60	2	75
12	Solvent free	1	70	2	65
13	Solvent free	1	40	1	75
14	Solvent free	1	40	3	85

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Table 4 Oxidation of alcohols to corresponding aldehydes or ketones in the present of Fe₃O₄@NFC@NNSM-Mn(III) catalyst^a

Entry	Substrate	Product	Time (min)	Yield^b (%)
a Reaction conditions: alcohols (1 mmol), TBHP (2 mmol), catalyst (0.5 mol%), S. F., 40 °C.b Yields of product isolated.
1			20	97
2			20	92
3			15	90
4			15	92
5			30	88
6			35	82
7			45	80
8			45	85
9			35	95
10			35	85
11			30	90
12			60	80
13			30	95
14			20	75
15			15	97

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Additive and organic/inorganic mixed solvent can promote the smooth oxidation of alcohol at 40 °C temperature. Low to moderate yields were obtained when using Fe₃O₄, Fe₃O₄@NFC, Fe₃O₄@NFC@NNS-Mn(III), Fe₃O₄@NFC@NNSM-Mn(III) as an additive (20–97%, Fig. 7 respectively). Fe₃O₄@NFC@NNSM-Mn(III) (trinuclear) and Fe₃O₄@NFC@NNS-Mn(III) (mononuclear) catalysts only exhibited 97% and 80% yield of the product. Whereas with the same conditions Fe₃O₄@NFC@NNSM-Mn(III) catalyst, indicating that the optimization of parameters and active sites can indeed improve the overall catalytic performance of the oxidation reactions (Table 3). Finally, the Fe₃O₄@NFC@NNSM-Mn(III) catalyst improves the oxidation reaction of alcohol. Finally, the product was obtained with 97% yield at 40 °C in a solvent-free condition for 30 min. Furthermore, in the next step, solvents were optimized, which increased the yield overall. The highest yield (97%) was obtained at solvent-free conditions (Table 3, entries 1–4), which is in line with the green chemistry. Control experiments were also performed in the absence of Mn catalyst in alcohol oxidation. In this case, the product is difficult to identify (Table 3, entries 5). Increasing the amount of the catalyst up to 0.5 mol% led to improved yields, but the more increasing it had no significant effect on the product yield (entries 9–12). Then, the model reaction was carried out under solvent-free conditions at room temperature and also 40–70 °C (entries 9–12). The product yield was increased up to 40 °C and was decreased significantly to 60 °C. The reduction in the product yield could be due to the decomposition of TBHP to molecular water and tert-butanol in the presence of Fe₃O₄@NFC@NNSM-Mn(III) at evaluated temperatures. As a result, 40 °C was selected as the optimum reaction temperature. Further investigation disclosed that the number of oxidizing agents is crucial to the product yield (entries 13–14). When the oxidation of the model reaction was carried out using 1, 2, and 3 mmol of tert-butyl hydroperoxide, the product's yield increased as the concentration of tert-butyl hydroperoxide increased. However, employing more than three mmol oxidizing agents had the opposite effect on the product yield that it could be due to over-oxidation. When H₂O₂, oxone, and NaIO₄ were used instead of TBHP as the oxidant, the catalyst activity was less than the original, indicating that the choice of oxidant also has a significant impact on the oxidation of alcohol (Fig. 8). The results showed that TBHP is the best oxygen source because of its good oxidation conversion. By contrast, the Fe₃O₄@NFC@NNS-Mn(III) catalyst with a larger size in the support material exhibited better activity under the same reaction conditions (80%).

Fig. 7 Comparison of the performance of catalysts with each other. Reaction conditions: alcohols (1 mmol), TBHP (2 mmol), catalyst (0.5 mol%), S. F., 40 °C.

Fig. 8 Reaction conditions: benzyl alcohol (1 mmol), oxidant (2 mmol), catalyst (0.5 mol%), S. F., 40 °C for 20 min.

Moreover, the Fe₃O₄@NFC@NNSM-Mn(III) catalyst as a trinuclear catalyst with a larger surface area exhibited better activity than the Fe₃O₄@NFC@NNS-Mn(III) catalyst, indicating that the larger surface, the site can contribute to improved catalytic performance. After the optimized conditions were established, this catalytic system was applied to oxidate a wide variety of primary and secondary alcohols to the corresponding carbonyl compounds. As shown in Table 4, good to excellent yields were accomplished in both primary and secondary alcohols. The electron nature of the substituents on the aromatic ring of primary alcohols displayed no apparent effect on this transformation, because the products were obtained in high yields. Secondary alcohols to the corresponding carbonyl compounds. As shown in Table 4, good to excellent results were accomplished in both primary and secondary alcohols.

The stability and reusability of Fe₃O₄@NFC@NNSM-Mn(III) as a tri-nuclear catalyst manganese in the xanthene and alcohol–oxidation reactions

Recovery and reusability of the catalyst are very substantial factors, exclusively for commercial and industrial applications. These abilities, clearly contend a heterogeneous catalyst as a suitable troth towards more sustainable and green chemical developments. In this line, to investigate the catalyst recyclability, experiments were performed for the model xanthene and alcohol–oxidation reactions, under the optimized reaction conditions. To do this, after each investigation, the catalyst was separated from the reaction mixture with a magnet. Then, the isolated catalyst was washed with water and EtOH, vacuum-dried at 60 °C for 2 h, and directly used in the next run of the reaction. Residual activation was analyzed by ICP for measurement. The amount of manganese that goes from the catalyst to the solution. As shown in Fig. 9, the catalyst was recovered and reused for at least 6 consecutive runs without notable loss of activity. The xanthene and alcohol–oxidation reactions yield reached to 85% and 87% for the 6th run. To show durability and structure of the catalyst, the recovered catalyst after 6th run was subjected to some analyses. Also, metal leaching of the catalyst was measured in each cycle. As shown in Fig. 9, a few leaching was observed for Fe₃O₄@NFC@NNSM-Mn(III), whereas only 3.6% for xanthene reaction and 3.2% for alcohol oxidation reaction metal leaching was observed after the 6th run. Moreover, ICP analysis of the catalyst for each heavy metal demonstrated an insignificant change in their weight percentage than the corresponding fresh values: Fe 45.58, Si 2.35, Mn 6.9 w%. These results demonstrated insignificant changes in the percentages of the heavy metals and confirm the durability of the catalyst during recycling.

Fig. 9 Recyclability of Fe₃O₄@NFC@NNSM-Mn(III) in the model alcohol oxidation and xanthene multi-component reaction, under the optimized reaction conditions; Reaction times: 10 min (xanthene multi-component reaction) and 30 min (alcohol oxidation).

To show the durability and structure of the catalyst, the recovered catalyst after the 6th run was subjected to some analyses. The FT-IR spectrum of the recovered catalyst after five runs (Fig. 10), revealed the entire preservation of the shape, position, and relative intensity of the characteristic absorption bands. This result proved that no substantial changes occurred in the chemical structure of the present catalyst.

Fig. 10 (I) FT-IR; (II) VSM ((a) before recycling and (b) after recycling); (III) Fe-SEM analysis of Fe₃O₄@NFC@NNSM-Mn(III) after six times.

Furthermore, the VSM and FESEM analysis of the catalyst after six times of reuse showed that the design and morphology of the catalyst maintained unchanged during the recycling process (Fig. 10). These findings well suggested the commendable stability and durability of the tri-nuclear catalyst Mn(III).

Experimental

Materials and measurement

Chemicals and solvents used by Merck company have been prepared. Product identification is made by their physical and spectral properties. The FT-IR spectra are recorded by the Perkin Elmer 780. The ¹³C NMR and ¹H NMR spectra are recorded by the Bruker Avance DPX-250 in CDCl₃ solvent at room temperature and the use of TMS as an internal standard. The Electrothermal 9100 was used to measure melting temperature. The transmitting electron microscope (TEM) of the Philips CM10 model and imaging was performed with 100 kV voltage. The elements in the samples were probed by energy-dispersive X-ray (EDX) spectroscopy accessory to the Philips scanning electron microscopy, field emission scanning electron microscopy (FE-SEM) images were obtained on a Tescan MIRA3 Powder X-ray diffraction (PXRD) was conducted using an X'Pert Pro MPD diffractometer equipped with a Cu-Kα (l = 1.54060 Å) radiation source between 2y = 21 and 2y = 50.01. Magnetic isotherms were obtained at room temperature using the VSM magnetic meter, the LakeShore Cryotronics 7407 model.

Synthesis of Fe₃O₄@NFC–Cl (I–II)

Core–shell (I) Fe₃O₄@NFC nanospheres were prepared according to the formerly described method (Scheme 1).⁵² Initially, Fe₃O₄@NFC (1.5 g) was sonicated in dry toluene (15 ml) for 30 min, and then 3-chloropropyl-trimethoxy silane (1 ml) was added slowly to the mixture. The mixture was then stirred for 24 h under N₂ gas at reflux conditions. After completion of the reaction, the reaction mixture was washed with toluene and diethyl ether and separated using an external magnet, and dried at 75 °C for 8 h in an oven under vacuum.

Scheme 1 Overall flowchart for the fabrication of Fe₃O₄@NFC@NNSM-Mn(III).

Synthesis of 5-(chloromethyl)-2-hydroxybenz aldehyde (IV)

We prepared 1 according to a procedure described^62,63 that involved addition (0.125 mol) of salicylaldehyde, in small portions with stirring, to 75 ml of concentrated hydrochloric acid containing (0.225 mol) of paraformaldehyde. This reaction mixture was maintained at 70 °C with stirring for 24 h and then water (20 ml) was added to the mix, and the product was extracted into CH₂Cl₂ (20 ml). Then the reliable product was collected by suction filtration and anhydrous MgSO₄ was used for drying the organic phase. The CH₂Cl₂ was removed with a rotary evaporator and thick yellow oil was set aside overnight. Then a pale purple reliable product III was isolated (Scheme 1). MP 85–87 °C, yield: 95% of pale purple crystals. Its purity was checked using TLC with silica-gel plates.

FT-IR (KBr): [small nu, Greek, macron] = 3151 (O–H), 3033, 2975, 2865 (C–H aldehyde), 1665 (C=O), 1434 (C=C), 673 (CH₂–Cl) cm⁻¹.

¹H NMR (DMSO, 300 MHz): δ = 4.80 (s, 2H, CH₂), 7.56–7.60 (d,d, 2H, H–Ar), 7.72 (s, 1H, H–Ar), 10.06 (s, 1H, CH of aldehyde), 11.30 (s, 1H, O–H) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 47.0, 117.8, 124.8, 131.1, 136.1, 136.4, 148.4, 163.2, 197.7 ppm.

Preparation of the Fe₃O₄@NFC@NN from o-phenylenediamine functionalized Fe₃O₄@NFC–Cl (III, V)

Fe₃O₄@NFC–Cl (1.00 g) was refluxed with o-phenylenediamine (1 mmol : 0.108 g) in CH₂Cl₂ (10.0 ml) for 12 h to replace the terminal chlorine atoms. Then 5-(chloromethyl)-2-hydroxy benzaldehyde (1 mmol : 0.17 g) was added and the mixture was refluxed for 8 h. Finally, Fe₃O₄@NFC@NNS(V) was filtered off, washed three times with ethanol, and dried at 70 °C in the vacuum oven.

Loading with Mn (OAc)₃·2H₂O (VI)

Fe₃O₄@NFC@NNS (1.00 g) was mixed with Mn(OAc)₃·2H₂O (1.2 mmol : 0.32 g) in ethanol (5 ml). The mixture was stirred for 12 hours at room temperature. After completion of the reaction, then it was filtered, and the solid obtained was washed with ethanol and dried at 70 °C overnight to give Fe₃O₄@NFC@NNS-Mn. The manganese loading of Fe₃O₄@NFC@NNS-Mn(III) was determined by the inductively coupled plasma (ICP) technique.

Synthesis of Fe₃O₄@NFC@ONSM-Mn magnetic catalyst (VII)

Immobilized Fe₃O₄@NFC@NNS-Mn(III) complex (8) along with a weighted amount of melamine (molar ratio 3 : 1) were transferred to a 25 ml oven-dried round bottom flask equipped with a magnet and a condenser. A solution of triethylamine (3.0 mmol : 0.39 ml) in MeOH (10.0 ml) was added to the mentioned mixture. The mixture was refluxed in N₂ atmosphere for 12 h. The product, Fe₃O₄@NFC@NNSM-Mn(III) (9) was collected using an external magnetic field, washed with water (2 × 10 ml) and EtOH (2 × 10 ml) and dried into a vacuum oven (5 h, 60 °C). Scheme 1 shows the complete route for the preparation of VII.

Synthesis of bis-aldehyde derivatives

A mixture of 2-hydroxy benzaldehyde (10 mmol) or 3-hydroxy benzaldehyde (10 mmol) or 4-hydroxy benzaldehyde (10 mmol) and 1,4-di-bromobutane (5 mol) or 1,2-di-bromoethane (5 mmol) in NaOH (10 mmol) in ethanol solvent was refluxed for 24 hours. The reaction mixture was cooled to room temperature, and the product precipitate was separated after re-crystallization in ethanol and dried in an oven.

General experimental procedure for the synthesis of xanthene

In a round-bottomed flask, aldehyde (1 mmol), dimedone (2 mmol) and Fe₃O₄@NFC@NNSM-Mn(III) (0.5 mol%) were mixed thoroughly at EtOH (5 ml). The flask was heated at 45 °C with concomitant stirring. After completing the reaction confirmed by TLC (EtOAc : n-hexane), the solid catalyst was separated from the reaction mixture by an ordinary magnet. The solvent was evaporated, and the green products were recrystallized from ethanol, gave the pure products 88–97% yields based on the starting aldehyde (Scheme 2). The products were characterized via a comparison of their melting points with the reported ones. The spectral data for the selected compounds are as follows:

Scheme 2 Catalytic activity of the Fe₃O₄@NFC@NNSM-Mn(III) xanthene reactions.

9-Phenyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 1). ¹H NMR (DMSO, 300 MHz): δ = 1.63–1.93 (m, 4H, CH₂), 2.01–2.20 (m, 4H, CH₂), 2.37–2.48 (m, 4H, CH₂), 3.97 (s, 1H, CH), 7.13–7.20 (m, 5H, aromatic) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 21.0, 29.1, 36.3, 38.7113.1, 124.7, 127.1, 129.4, 143.9, 155.4, 199.8 ppm. Mp (°C): 203–204.

9-(4-Chlorophenyl)-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 2). ¹H NMR (DMSO, 300 MHz): δ = 2.11–2.14 (m, 4H, CH₂), 2.45–2.48 (m, 4H, CH₂), 2.97–3.04 (m, 4H, CH₂), 3.93 (s, 1H, CH), 6.97–7.09 (m, 4H, aromatic) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 21.5, 29.6, 36.9, 38.9, 113.2, 127.8, 129.7, 131.8, 143.9, 155.6, 199.5 ppm. Mp (°C): 229–230.

9-(4-Methoxyphenyl)-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 3). ¹H NMR (DMSO, 300 MHz): δ = 1.58–1.70 (m, 4H, CH₂), 2.63–2.67 (t, 4H, CH₂), 2.99–3.04 (t, 4H, CH₂), 3.67–3.71 (s, 3H, CH₃), 4.05 (s, 1H, CH), 6.70–7.13 (s, s, 4H, aromatic) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 20.9, 29.8, 35.4, 36.9, 55.3, 113.2, 113.8, 129.7, 134.6, 153.8, 157.3, 199.9 ppm. Mp (°C): 205–206.

9-(2-Nitrophenyl)-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 4). ¹H NMR (DMSO, 300 MHz): δ = 0.85–0.96 (m, 4H, CH₂), 2.52–2.56 (t, 4H, CH₂), 2.73–2.78 (t, 4H, CH₂), 3.99 (s, 1H, CH), 6.68–7.58 (m, 4H, aromatic) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 22.5, 29.3, 34.2, 36.0, 116.0, 124.5, 127.1, 129.4, 130.4, 134.6, 147.8, 154.5, 199.6 ppm. Mp (°C): 259–261.

9-(2-Hydroxyphenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 5). ¹H NMR (DMSO, 300 MHz): δ = 1.35–1.49 (m, 4H, 2CH₂), 2.01–2.09 (t, 4H, 2CH₂), 2.57–2.63 (t, 4H, 2CH₂), 4.59 (s, 1H, CH), 7.02–7.21 (m, 4H, aromatic), 10.51 (s, 1H, OH) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 20.8, 29.8, 32.3, 35.2, 111.0, 115.7, 118.3, 124.5, 127.5, 128.0, 154.7, 156.8, 196.6 ppm. Mp (°C): 259–261.

3,3,6,6-Tetramethyl-9-phenyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 6). ¹H NMR (DMSO, 300 MHz): δ = 1.11 (s, 12H, 4CH₃), 1.82 (s, 4H, 2CH₂), 2.07 (s, 4H, 2CH₂), 4.06 (s, 1H, CH), 7.02–7.09 (m, 5H, aromatic) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 28.3, 33.8, 37.8, 52.4, 110.3, 125.8, 127.8, 128.4, 144.7, 155.7, 197.7 ppm. Mp (°C): 203–204.

9-(4-Chlorophenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 7). ¹H NMR (DMSO, 300 MHz): δ = 1.25 (s, 6H, 2 × 2CH₃), 2.19 (s, 2H, 2 × CH₂), 5.51 (s, 1H, CH), 7.11–7.17 (d, 2H, 2 × 2CH aromatic) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 26.9, 30.1, 36.0, 36.5, 52.4, 114.5, 128.4, 133.1, 141.3, 157.1, 198.7 ppm. Mp (°C): 169–171.

9-(2-Hydroxyphenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 8). ¹H NMR (DMSO, 300 MHz): δ = 1.07 (s, 12H, 4CH₃), 1.71 (s, 4H, 2CH₂), 2.26 (s, 4H, 2CH₂), 1.98–2.00 (m, 2H, CH₂), 2.36 (s, 2H, CH₂), 2.40 (s, 2H, CH₂), 6.66–7.24 (m, 4H, CH-aromatic) 10.51 (s, 1H, OH) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 20.8, 27.7, 32.5, 34.2, 37.1, 48.5, 112.4, 115.7, 124.7, 126.2, 127.3, 128.8, 155.5, 157.9, 196.4 ppm. Mp (°C): 200–202.

4-(3,3,6,6-Tetramethyl-1,8-dioxo-2,3,4,5,6,7,8,9-octahydro-1H-xanthene-9-yl) benzaldehyde (Table 2, entry 9). ¹H NMR (DMSO, 300 MHz): δ = 1.25 (s, 6H, 2 × 2CH₃), 1.88 (s, 2H, 2× CH₂), 2.19 (s, 2H, 2× CH₂), 5.51 (s, 1H, CH), 7.11–7.17 (d, 2H, 2 × 2CH aromatic), 11.88 (s, 1H, COH) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 28.3, 31.7, 37.8, 52.4, 115, 127.8, 130.2, 133.1, 149, 157.1, 195.6, 206.4 ppm. Mp (°C): 295–297.

9-(1H-Indol-3-yl)-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 10). ¹H NMR (DMSO, 300 MHz): δ = 1.79–1.84 (m, 4H, 2CH₂), 1.92–1.96 (m, 4H, 2CH₂), 3.45–3.49 (t, 4H, 2CH₂), 4.89 (s, 1H, CH), 6.93–6.98 (m, 2H, aromatic), 6.99 (s, 1H, CH indole), 7.04–7.59 (d of d, 2H, aromatic), 10.80 (s, 1H, NH) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 22.3, 26.9, 34.1, 36.9, 103.9, 111.8, 118.5, 119.4, 120.9, 124.2, 126.1, 136.6, 164.7, 196.8 ppm. Mp (°C): 267–269.

3,3,6,6-Tetramethyl-9-(5-nitrofuran-2-yl)-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (Table 2, entry 11). ¹H NMR (DMSO, 300 MHz): δ = 1.14 (s, 12H, 4CH₃), 1.85 (s, 4H, 2CH₂), 2.32 (s, 4H, 2CH₂), 5.47 (s, 1H, CH), 6.25 (s, 1H, furane), 7.28 (s, 1H, furane) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 26.2, 28.0, 31.9, 35.5, 50.4, 110.3, 111.9, 115.1, 150.7, 161.5, 162.9, 196.1 ppm. Mp (°C): 155–157.

4-(4-(4-(1,8-Dioxo-2,3,4,5,6,7,8,9-octahydro-1H-xanthene-9-yl)phenoxy)butoxy)benzaldehyde (Table 2, entry 12). ¹H NMR (DMSO, 300 MHz): δ = 1.93–2.04 (m, 4H, 2CH₂), 2.33–2.67 (m, 8H, 4CH₂), 3.95–4.13 (m, 4H, 2CH₂), 4.78 (s, 1H, CH), 6.75–6.79 (m, 2H, CH aromatic), 4.78 (s, 1H, CH), 6.75–6.79 (m, 2H, Ar), 7.00–7.04 (m, 2H, Ar), 7.20–7.25 (m, 2H, Ar), 7.84–7.88 (m, 2H, Ar), 9.91 (s, 1H, COH) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 20.3, 25.9, 27.1, 30.8, 37.0, 67.1, 67.9, 114.0, 114.7, 117.0, 117.1, 129.3, 129.8, 132.0, 136.8, 157.4, 163.7, 164.1, 190.9, 196.7 ppm. Mp (°C): 168–170.

4-(2-(4-(3,3,6,6-Tetramethyl-1,8-dioxo-2,3,4,5,6,7,8,9-octahydro-1H-xanthene-9-yl)phenoxy)ethoxy)benzaldehyde (Table 2, entry 13). ¹H NMR (DMSO, 300 MHz): δ = 1.12 (s, 12H, 4CH₃), 2.21 (s, 4H, 2CH₂), 2.29 (s, 4H, 2CH₂), 3.97 (s, 1H, CH), 4.29–4.32 (m, 2H, CH₂), 4.37–4.40 (m, 2H, CH₂), 6.79–6.86 (m, 4H, aromatic), 7.04–7.07 (d, 2H, aromatic), 7.84–7.88 (m, 2H, aromatic), 9.91 (s, 1H, COH) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 27.3, 32.2, 50.7, 66.0, 66.8, 114.1, 114.9, 115.6, 115.7, 127.8, 129.4, 132.0, 137.2, 156.8, 157.0, 162.1, 163.7, 190.8, 196.5 ppm. Mp (°C): 200–202.

2-(4-(2-(1,8-Dioxo-2,3,4,5,6,7,8,9-octahydro-1H-xanthene-9-yl)phenoxy)butoxy)benzaldehyde (Table 2, entry 14). ¹H NMR (DMSO, 300 MHz): δ = 2.01–2.09 (m, 4H, 2CH₂), 2.33–2.65 (m, 8H, 4CH₂), 3.06–3.11 (m, 4H, 2CH₂), 3.80 (s, 1H, CH), 3.97–4.14 (m, 4H, 2CH₂), 6.94–7.04 (m, 3H, aromatic), 7.23–7.25 (d, 1H, aromatic), 7.42–7.45 (d, 1H, aromatic), 7.57–7.60 (t, 1H, aromatic), 7.69–7.71 (d, 1H, aromatic), 7.84–7.86 (d, 1H, aromatic), 9.91 (s, 1H, COH) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 20.4, 25.9, 27.0, 30.3, 36.9, 67.7, 68.5, 112.5, 114.0, 119.8, 121.0, 124.7, 127.9, 131.0, 132.9, 136.9, 157.7, 158.6, 161.5, 189.5, 196.5. Mp (°C): 220–222.

9,9′-((Ethane-1,2-diylbis(oxy))bis(3,1-phenylene))bis(3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione) (Table 2, entry 15). ¹H NMR (DMSO, 300 MHz): δ = 1.03 (s, 12H, 2 × 4CH₃), 2.23 (s, 4H, 2 × 2CH₂), 2.49 (s, 4H, 2 × 2CH₂), 4.28 (s, 2H, 2× CH₂), 4.77 (s, 1H, 2× CH), 6.70–7.18 (m, 4H, 2× aromatic) ppm. ¹³C NMR (DMSO, 75 MHz): δ = 27.5, 32.2, 50.7, 112.4, 114.9, 115.5, 121.4, 128.9, 145.7, 158.5, 162.3, 196.4 ppm. Mp (°C): 250–252.

The general procedure of oxidation of alcohols

In a typical experiment, Fe₃O₄@NFC@NNSM-Mn(III) (0.5 mol%) and TBHP (1 mmol) were placed in a vessel containing benzyl alcohol (1 mmol) without any organic solvents as green conditions were stirred at 40 °C for an appropriate time. Upon reaction completion, monitored by TLC, the catalyst was removed from the mixture of reaction by the magnet. The products were purified on a silica gel plate to give pure products. For the identification of the final products, their physical data were compared with those of reliable samples, because all of them have been studied in the past. Also all of these compounds are already known.

Comparative study

To better clarify the merits of the competency tri-nuclear catalytic system over the reported metal-based other catalytic systems in the xanthene reactions, the comparing efficacy results were tabulated in Table 5. It has been observed that the synthesized catalyst has almost a very good advantage over other reported catalysts which can be due to suitable reaction conditions, short reaction time, high reaction efficiency and lower catalyst load and on the other hand timely recovery of magnetic catalyst. In addition to the reusability of the catalyst, most importantly, the use of a non-toxic, benign green solvent that is environmentally friendly and does not require the use of any additives or toxic solvents supports the green chemistry approach well. Chromium derivatives containing aromatic aldehydes can be synthesized by our catalytic method. These promising results should be attributed to the co-operation of three metal verb positions: Lewis acid in Fe₃O₄@NFC@NNSM-Mn(III). Finally, a series of green metrics^72,73 such as atom economy (AE), atom efficiency (AEF), carbon efficiency (CE), reaction mass efficiency (RME), optimum efficiency (OE), process mass intensity (PMI), E-factor (E), solvent intensity (SI), and water intensity (WI) were calculated to evaluate the greenness of the one-pot three-component reaction of aldehydes, dimedone (Fig. 11–13, see ESI for detailed calculations† for detailed calculations). As it is shown in Fig. 11a and 12a, the high values of the AE, AEF, CE, RME, and OE for the synthesis of three xanthene derivatives of Table 2 (entries 5–7) and also new derivatives Table 2 (entries 11–15), illustrate well the greenness of the process. The lower the PMI, E, and SI, the more favourable is the process because of green chemistry. These values are less than 50 in the synthesis of the xanthene mentioned above's (Fig. 11b and 12b, see ESI†). Minimal amounts of WI were also obtained in these processes (Fig. 11b and 12b, see ESI†). The green organic solvent, which was used in these protocols was EtOH. Hence, it can be concluded that about the high values of RME and low values of PMI, E, SI, and WI, this one-pot three-component process is an efficient and green protocol for synthesizing xanthene. To stable, the more greenness of the current catalyst over the reported catalysts in the one-pot three-component reaction of benzaldehyde, dimedone, the current catalyst's green metrics was compared with those of two previously reported catalysts (Table 5, entries 1 and 8).^56,70

Table 5 Comparison of the reaction conditions and results for the synthesis of xanthene

Entry	Catalyst	Solvent	Time (min)	Temp. (°C)	Yield (%)	Ref.
a Trichloroisocyanuric acid.b Propyl sulfonic acid functionalized LUS-1 (Laval University silica) (LUS-Pr-SO3H).
1	[SO3H–Pyrazine–SO3H]Cl2	Solvent free	10	100	93	56
2	Fe3O4 nanoparticles	Solvent free	20	100	90	64
3	TCCA^a	Solvent free	40	110	78	65
4	[Et3N–SO3H]Cl	Solvent free	60	80	97	22
5	LUS-Pr-SO3H^b	Solvent free	15	140	90	66
6	[Cmmim][BF4]	Solvent free	120	80	94	67
7	ICl3/SiO2	Solvent free	60	75	90	68
8	NP	EtOH	6.5 h	Reflux	90	69
9	Cu(II)/NP	EtOH	120	Reflux	98	70
10	—	EtOH	24 h	Reflux	—	71
11	Fe3O4@NFC@NNSM-Mn(III)	EtOH	10	45	96	This work

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As can be perceived from the results in Fig. 13a and b (see ESI†), the higher the AEF, CE, and RME factors and the lower the PMI, E, SI, and WI factors of Fe₃O₄@NFC@NNSM-Mn(III) compared with those of NP and [SO₃H–Pyrazine–SO₃H]Cl₂, acknowledged that the Fe₃O₄@NFC@NNSM-Mn(III) is a greener and more supportable catalyst for such an Multi-component reaction (see ESI† for detailed calculations).

Mechanism studies

An acceptable mechanism for the synthesis of xanthine using the Fe₃O₄@NFC@NNSM-Mn(III) catalyst has been reported. Initially, the acid catalyst Fe₃O₄@NFC@NNSM-Mn(III) activates the carbonyl aldehyde and dimedone groups, and intermediate (I) is formed from the densified Knoevenagel densities of dimedone and aryl aldehyde by the removal of a water molecule. Then, from the increase of Michael Dimedone's next molecule to intermediate (I), intermediate (II) is formed, which is tautomeric with intermediate (III). In the end, by intramolecular looping and removal of a water molecule, product (IV) is obtained (Scheme 3).

Scheme 3 A plausible mechanism for the synthesis of xanthene while using the Fe₃O₄@NFC@NNSM-Mn(III) catalyst.

The proposed reaction mechanism for the oxidation of benzyl alcohol derivatives using TBHP in the presence of Fe₃O₄@NFC@NNSM-Mn(III) based on the available literature⁷⁴ is shown in Scheme 4. Decomposition of t-BuOOH on the surface of Fe₃O₄@NFC@NNSM-Mn(III) leads to the formation of t-BuOO˙ radical species and also increase of Mn(III) to Mn(IV).

Scheme 4 A plausible mechanism for the synthesis of oxidation alcohols while using the Fe₃O₄@NFC@NNSM-Mn(III) catalyst.

Besides, decomposition of t-BuOOH on the surface of Mn(IV) generates t-BuO˙ species. Next, t-BuOO˙ and t-BuO˙ radical species underwent hydrogen abstraction of the substrate (a) to produce intermediate (b). The reaction of intermediate (b) with t-BuOO˙ and t-BuO˙ results in (c) and (d), which ultimately have corresponding aldehyde (e) through the elimination of t-BuOOH and t-BuOH.

Conclusions

In this work, a novel magnetically trinuclear catalyst denoted as Fe₃O₄@NFC@NNSM-Mn(III) was successfully fabricated as an efficient trinuclear catalyst. To characterize the tri-nuclear catalyst, various techniques, including FT-IR, XRD, TGA, TEM, FESEM, EDX, VSM, and ICP analysis have been employed. Interestingly, Fe₃O₄@NFC@NNSM-Mn(III) was an excellent trinuclear catalyst to promote the xanthene and alcohol–oxidation reactions of a broad range of benzaldehyde and most importantly the highly challenging benzyl alcohol, which are far extensively available and cheaper. The presented catalytic system can be magnetically separated and conveniently recycled at least five times without a noticeable decrease in its activity. Moreover, low metal leaching and utilizing an eco-friendly solvent are other advantages that support the going protocol towards green chemistry. The use of the presented new trinuclear catalyst to promote another Multi-component reaction is continuing in our lab.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support of the research Council of the University of Birjand and we also thank University of Ferdowsi.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra09420j

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