Introduction

Currently, from the sustainable chemistry standpoint, decreasing environmental contamination and rising energy deficiency, are progressively concerned1. Tandem reactions which contain two or more consecutive independent reactions implemented in a one-pot manner with no isolation and purification of the intermediates, diminish the number of reaction steps, the solvent and reagents usage and consequently decrease the waste production and energy consumption.

Development of simple methods for the synthesis of required organic intermediates or final products from readily available reagents constitutes a key challenge in the synthetic organic chemistry2,3. Alcohols are considered as the most environmentally benign compounds, because of the readily availability, low cost, chance of being produced from renewable biomass resources, less poisoning, and ease of treatment, storage and transportation4. Therefore, the synthesis of organic compounds directly from alcohols by tandem organic process (TOP) has been known as an environmentally friendly chemical synthetic method5.

Although traditional thermal routs still have a significant role in current chemistry, a number of subsequent problems like waste byproducts formation due to some undesirable thermal side reactions urge to move towards using a renewable energy source6,7. In this regard, mimicking the nature in the consumption of sunlight to yield nutrient and oxygen in the plants, leads chemists to employ this available, free-charge and renewable energy source for the activation of catalytic transformations8. The examples of using solar energy as the activator in the photocatalytic transformations involve the removal of drugs9, organic dyes and heavy metal ions from wastewater by photo-degradation or -reduction10, inhibition of air contaminants, hydrogen generation by water splitting as well as chemical synthesis11. Primary studies in this field involved the use of photocatalysts with wide band gap like titanium or zinc oxide12,13. However, the big band gap and the fast recombination of electron–hole pairs cause low photocatalytic productivity and poor practicality. Moreover, the structure of traditional semiconductors, which are not matched, restricted their extra progress in the field of photocatalysis. Therefore, the discovering of efficient photocatalysts to substitute the conventional ones is highly demanded14.

Metal–organic frameworks (MOFs), a hybrid material with a three-dimensional crystalline structure, are discovering as a distinctive type of heterogeneous photocatalysts, due to their admirable photoexcitation and charge transition mechanism15,16,17,18. The inherent light absorption of MOFs can be tuned by changing either the metal ions as network nodes or the conjugation amount in the organic ligands12,19,20,21. In recent years, the use of MOFs for light-initiated oxidation reactions such as oxidation of saturated carbon-hydrogen bonds, hydroxylation of benzene, and selective oxidation of sulfides has been extensively duccumented22,23,24. Furthermore, due to the individual structure of MOFs which contains diverse catalytic sites such as metal nodes, ligands and catalytic species encapsulated in the holes, they have been used as multifunctional catalysts in tandem reactions such as epoxidation-ring opening, Sonogashira Hagiwara-click and olefination-hydrogenation reactions25,26,27. However, the use of MOF-based materials as multifunctional catalysts under light irradiation for the TOP reactions starting from alcohols are in the infancy stages28,29,30,31,32,33,34.

In continue of our studies on the introducing new heterogeneous catalysts for the organic transformations35,36,37, herein, in this paper, we have applied a MOF modified with cobalt-complex38 [Co-isatin-Schiff-base-MIL-101(Fe)] as a new heterogeneous multifunctional bio-photocatalyst for the synthesis of benz-imidazoles/-oxazoles/-thiazoles, and benzylidene malononitrile by TOP starting from readily available alcohols. The photocatalytic antibacterial activity of Co-isatin-Schiff-base-MIL-101(Fe) against E. coli, S. aureus and S. pyogenes was also studied under sunlight illumination.

Experimental

General information

Chemicals were purchased from Merck Company. The reactions were monitored by TLC using silica-gel polygram SILG/UV254 plates. X-ray photoelectron spectroscopy (XPS) was done by a VG-Microtech Multilab 3000 spectrometer, equipped with an Al anode. The deconvolution of spectra was done by Gaussian Lorentzian curves. X-ray powder diffraction (XRD) was carried out on an Xpert Pro Panalytical diffractometer incorporating Cu Kα radiation (λ = 0.154 Å). TEM images were obtained by a TEM microscope (Philips CM30). Field emission scanning electron microscopy (FESEM) images were obtained using a FESEM (model Mira 3-XMU). UV–Vis diffuse reflectance spectroscopy (UV–Vis-DRS, Shimadzu Co., Japan), FT-IR spectra were recorded on a Nicolet-Impact 400D spectrometer in the range of 400–4000 cm−1. The cobalt catalyst percentage was measured using an OPTIMA 7300DV ICP analyzer. EPR data were achieved on a Bruker EMX (freq. 9.80 GHz, time 80 ms, modulation freq. 100 kHz). Experiments were performed under sunlight irradiation between 8.00 a.m. and 4.00 p.m. (July to August 2021, at ~ 30 °C).

Synthesis of Fe-MIL-101-isatin-Schiff-base

MIL-101(Fe)-NH2 (0.13 g)33 was added to a solution of isatin (0.147 g, 1 mmol) in ethanol (20 mL) and then refluxed for 1 day. The brown solid was centrifuged, washed with EtOH (3 × 10 mL) and dried at 60 °C in a vacuum oven to give the brown isatin-Schiff-base-MIL-101(Fe).

Synthesis of Co-isatin-Schiff-base-MIL-101(Fe)

Co(OAc)2⋅4H2O (0.05 g, 0.2 mmol) was added to a mixture of isatin-Schiff-base-MIL-101(Fe) (0.1 g) in ethanol (15 mL) and stirred under reflux conditions for 24 h. Then, the solid was centrifuged, washed with EtOH (3 × 10 mL) and dried in a vacuum oven. The total amount of cobalt per gram of Fe-MIL-101-isatin-Schiff-base-Co was found to be 2.24 mmol by ICP analysis.

Typical method for photocatalytic oxidation of benzyl alcohol in the presence of Co-isatin-Schiff-base-MIL-101(Fe)

A mixture of catalyst (1 mol%), benzyl alcohol (1 mmol), and EtOH (6 mL) was stirred in a glass tube under sunlight irradiation and air bubbling at ~ 30 °C. The reaction progress followed by TLC. After an appropriate time (Table 3), the catalyst was isolated by centrifugation and washed with ethanol. The solvent of the combined organic phase was evaporated under reduced pressure. The pure product was obtained by column chromatography.

General route for the synthesis of benz-imidazoles/oxazoles/thiazoles via Tandem photo-oxidation-condensation reaction from benzyl alcohols and o-phenylenediamines catalyzed by Co-isatin-Schiff-base-MIL-101(Fe)

A mixture of Co-isatin-Schiff-base-MIL-101(Fe) (1.2 mol %, 5.3 mg), benzyl alcohol (1 mmol) and o-phenylenediamine/o-aminophenol/o-aminothiophenol (1 mmol) in ethanol (6 mL) was stirred at ~ 30 °C, while exposed to the sunlight, and air was bubbled into the reaction mixture (1 mL min−1). The reaction progress followed by TLC. After an appropriate time (Table 5), the catalyst was isolated by centrifugation, washed with ethanol (3 × 5 mL), and then reused in subsequent runs. The yellow product was obtained after addition of ice water to the remaining solution. The precipitated product was filtered and recrystallized in ethanol to provide the pure product.

Tandem photo-oxidation/Knoevenagel condensation reaction catalyzed by Co-isatin-Schiff-base-MIL-101(Fe)

Benzyl alcohol (1 mmol) and malononitrile (1 mmol) were added to a glass tube containing Co-isatin-Schiff-base-MIL-101(Fe) (1.5 mol%) and EtOH (6 mL) equipped with a stirring bar. The reaction mixture was stirred at ∼ 30 °C with mild air flow under sunlight irradiation. After appropriate times (Table 8), as monitored by TLC (elution: n-hexane/ethyl acetate: 10:1), the catalyst was separated by centrifugation. The solvent of the remaining solution was evaporated under vacuum to produce the crude product. The pure product was obtained by ethanol–water recrystallization.

Results and discussion

Synthesis and characterization of Co-isatin-Schiff-base-MIL-101(Fe)

Co-isatin-Schiff-base-MIL-101(Fe)38 was synthesized via post modification method (PSM) of MIL-101(Fe)-NH2 (Fig. 1).

Figure 1
figure 1

Synthetic rout for the preparation of Co-isatin-Schiff-base-MIL-101(Fe).

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Figure 2 shows the PXRD pattern of the as-synthesized MIL-101(Fe)–NH2 and the Co-isatin-Schiff-base-MIL-101(Fe). The well-defined diffraction peaks at 2θ of 9.2, 10.5, and 16.6º have revealed the high crystallinity of Fe-MIL-101-NH2. The XRD pattern of Fe-MIL-101-isatin-Schiff-base-Co is shown diffraction peaks at 2θ of 9.2, 10.6, and 16.6º. The slight changes observed in the XRD of the functionalized sample compared to the original MOF are probably due to the creation of some defects in the catalyst structure during the functionalization reaction.

Figure 2
figure 2

PXRD patterns of (a) MIL-101(Fe)–NH2 and (b) Co-isatin-Schiff-base-MIL-101(Fe).

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To further investigation of the presence and types of the elements expected for the Co-isatin-Schiff-base-MIL-101(Fe), X-Ray photoelectron spectroscopy (XPS) was performed as depicted in Fig. S1. C 1s spectrum represents four peaks corresponded to C=C (284.4 eV), C=N (285.4 eV), C–N (287.5 eV) and C=O (289.2 eV) (Fig. S1a)39. The N 1s spectrum (Fig. S1b) consists of peaks at 398.5 and 400.6 eV, which are related to C=N and N–H bonds, respectively40. Two peaks at 711.3 eV and 724.9 eV (Fig. S1c) for Fe 2p3/2 and Fe 2p1/2, respectively, confirm the existence of Fe+3 in the MOF41. Presence of two pairs of distinct peaks in the high resolution XPS of Co 2p (Fig. S1d) at 781.3 (Co 2p3/2) and 797.3 eV (Co 2p1/2), indicates the existence of Co2+ in Co-isatin-Schiff-base-MIL-101(Fe)42,43.

Morphology of Co-isatin-Schiff-base-MIL-101(Fe) and MIL-101(Fe)–NH2 were established by field emission scanning electron microscopy and transmission electron microscopy (FESEM and TEM). As shown in Fig. 3, the morphology of MIL-101(Fe)–NH2 remains nearly intact after PSM.

Figure 3
figure 3

FESEM images of (a) MIL-101(Fe)–NH2 and (b) Co-isatin-Schiff-base-MIL-101(Fe); TEM images of (c) MIL-101(Fe)–NH2 and (d) Co-isatin-Schiff-base-MIL-101(Fe).

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Based on EDX spectrum, elements including carbon, oxygen, nitrogen, iron and cobalt present in Co-isatin-Schiff-base-MIL-101(Fe) (Fig. 4). Investigation of EDX-mapping indicates a uniform distribution of these elements in the catalyst (Fig. 5).

Figure 4
figure 4

EDX spectrum of Co-isatin-Schiff-base-MIL-101(Fe).

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Figure 5
figure 5

EDX mapping images of Co-isatin-Schiff-base-MIL-101(Fe).

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Study of the FT-IR spectra of the as-synthesized MIL-101(Fe)–NH2 and Co-isatin-Schiff-base-MIL-101(Fe) (Fig. 6a,c) ensured the successful construction of the targeting MOF. In the spectrum of MIL-101(Fe)–NH2, the peaks at 1382–1430 (C=C) and 1501–1600 (C=O) cm−1 are attributed to the 2-aminoterephthalate linkers (Fig. 6a). After PSM of MIL-101(Fe)–NH2, some new peaks at around 1700 (C=O), 1628 (C=N) and 1332 (C–N) cm−1 are formed, which could be attributed to the isatin-Schiff-base (Fig. 6b). Complex formation of isatin-Schiff-base in the isatin-Schiff-base-MIL-101(Fe) with cobalt is associated with the shift of vibration frequencies of C=O and C=N groups to the lower amounts (Fig. 6c).

Figure 6
figure 6

FT-IR spectra of (a) MIL-101(Fe)–NH2, (b) isatin-Schiff-base-MIL-101(Fe) and (c) Co-isatin-Schiff-base-MIL-101(Fe).

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Comparison of the N2 adsorption–desorption of Co-isatin-Schiff-base-MIL-101(Fe) with MIL-101(Fe)–NH2 (Fig. S2a,b) showed that the surface areas and total pore volumes are considerably reduced in Co-isatin-Schiff-base-MIL-101(Fe) (Table 1). The pore size distribution of the two samples is nearly identical (Fig. S2c,d). These results indicated the successful incorporation of Co (II) complexes into MIL-101(Fe)-NH2 with uniform distribution.

Table 1 Textural properties of pure MIL-101(Fe)–NH2 and Co-isatin-Schiff-base-MIL-101(Fe).
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The UV–Vis DRS spectrum of Co-isatin-Schiff-base-MIL-101(Fe) shows a broad absorption band in the visible light region (250–800 nm) compared with MIL-101(Fe)–NH2 and isatin-Schiff-base-MIL-101(Fe) (Fig. 7). These results showed that Co-isatin-Schiff-base-MIL-101(Fe) can be excited in the visible region, due to the existence of Fe–O clusters in conjunction with cobalt Schiff-base, which facilities the π–π transitions. The DRS analysis showed a significant decrease in the band gap energy (Eg) of MIL-101(Fe)–NH2 from 2.2344 to 2.03 eV after modification by isatin and 1.7 eV after complex formation with cobalt (Fig. 8).

Figure 7
figure 7

UV–visible absorption spectra of (a) MIL-101(Fe)-NH2, (b) isatin-Schiff-base-MIL-101(Fe) and (c) Co-isatin-Schiff-base-MIL-101(Fe).

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Figure 8
figure 8

Band gap energy of (a) MIL-101(Fe)-NH2, (b) isatin-Schiff-base-MIL-101(Fe) and (c) Co-isatin-Schiff-base-MIL-101(Fe).

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Synthesis of benz-imidazoles/-oxazoles/-thiazoles from alcohols via TOP catalyzed by Co-isatin-Schiff-base-MIL-101(Fe)

2-Substituted benz-imidazoles/-oxazoles/-thiazoles as significant types of heterocycles, have been broadly documented as structural blocks in the preparation of biological and pharmaceutical compounds (Fig. 9)45,46,47.

Figure 9
figure 9

Examples of biologically and pharmaceutically relevant benz-imidazoles/-oxazoles/-thiazoles.

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The methods for the synthesis of these heterocyclic compounds consist of the reaction of o-phenylenediamine with a variety of reagents. Acids, aldehydes, esters, anhydrides, amides, acyl halides, and nitriles are the common reagents that have been introduced in these methods48,49,50. However, their synthesis directly from readily available alcohols by TOP is more desirable. In these reactions, aldehydes play as highly active reaction intermediates to undergo the subsequent reactions such as condensation with o-phenylene diamine, o-aminophenol, and/or o-aminothiophenol. Numerous materials have been reported to activate these reactions such as porous catalysts, ionic liquids, metal oxides, graphene oxide and MOFs51,52,53,54,55,56,57. The reported methods associated with drawbacks such as requiring long reaction time, high temperature, low yields of the products, use of expensive/toxic metals, unrecoverable catalyst and poisonous solvents that may cause environmental pollution58,59,60.

Therefore, the development of highly efficient alternate methods which proceed under economic and environmental-friendly conditions with energy-saving organic synthetic strategies is desirable. In this paper, we have studied the application of Co-isatin-Schiff-base-MIL-101(Fe) as a heterogeneous multifunctional bio-photocatalyst for the synthesis of benz-imidazoles/oxazoles/thiazoles via TOP starting from alcohols. Based on our knowledge, this is the first report of using a bio-photocatalyst for the synthesis of these target molecules.

Initially, the catalyst efficiency for the alcohol photo-oxidation was investigated by selecting benzyl alcohol as a typical compound and the effect of different catalyst loading, solvents, light source and the oxidizing agent was tested for the oxidation of this compound (Table 2). Satisfied results were obtained when 1 mol% of Co-isatin-Schiff-base-MIL-101(Fe) was used in ethanol under sunlight irradiation and air bubbling. The effect of solvents with different polarities on the progress of the reaction was investigated (Table 2, entries 4–10). Low yield of the product under solvent-free conditions (Table 2, entry 10) showed the importance of the solvent for transferring the substrates into the active sites of the catalyst. The lowest yield was obtained in water (Table 2, entry 7) among other organic solvents and the highest yield was observed in ethanol (Table 2, entry 4). The increase in the efficiency of the catalytic process in ethanol can be due to the greater solubility of substances in ethanol and subsequently the easier transfer of substances to the MOF pores and as a result faster interaction with the active catalytic sites. Solvents such as DMF and THF can be coordinated to the active sites of the MOF catalyst and so decrease its catalytic efficiency. The model reaction proceeded with low yields in the presence of H2O2 or TBHP (Table 2, entries 16 and 17) probably due to the restricted access to the active sites of the catalyst and their different mechanism pathways. To find the effect of the light on the progress of the reaction, the model reaction was also studied under dark reaction conditions and benzaldehyde was obtained in an insignificant amount after 8 h (Table 2, entry 13). When a similar reaction was performed under N2 atmosphere, any conversion of benzyl alcohol was not observed (Table 2, entry 18).

Table 2 Optimization of photo-oxidation of benzyl alcohol to benzaldehyde.
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To examine the generality of this photo-oxidation method, the reaction of a variety of alcohols was performed using optimized reaction conditions. As shown in Table 3, selective photo-oxidation of primary and secondary benzyl alcohols occurred and the corresponding carbonyl compounds were obtained in 78–90% yields without any overoxidation (entries 1–8). Furfuryl alcohol, as a famous challenging heteroaromatic alcohol and cinnamyl alcohol, containing a conjugated double bond, underwent selective photo-oxidation to furfural and cinnamaldehyde, respectively (Table 3, entries 9 and 10). Aliphatic alcohols exhibited less activity using the present photo-oxidation method (Table 3, entries 11–14).

Table 3 Photo-oxidation of alcohols catalyzed by Co-isatin-Schiff-base-MIL-101(Fe) in the presence of air under sunlight irradiation.
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With successful photo-oxidation of alcohols, in the next part, the efficiency of Co-isatin-Schiff-base-MIL-101(Fe) was investigated in the synthesis of benzimidazoles from alcohols via TOP. The reaction of benzyl alcohol and o-phenylendiamine was chosen as a typical reaction to discover the best catalyst loading and the appropriate solvent (Table 4). The best result was achieved using 1.2 mol% of the catalyst in ethanol (Table 4, entry 6).

Table 4 Optimization of the reaction conditions for the synthesis of benzimidazoles from alcohols via TOP catalyzed by Co-isatin-Schiff-base-MIL-101(Fe).
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The reactions of a number of benzyl alcohols containing electron-withdrawing or -releasing groups and o-substituted anilines catalyzed by Co-isatin-Schiff-base-MIL-101(Fe) were performed under the optimum reaction conditions (Table 5). As depicted in Table 5, benzyl alcohols and o-substituted anilines (o-phenylene diamine, o-aminophenol, and/or o-aminothiophenol) underwent the TOP reaction with high efficiency to give benz-imidazoles/-oxazoles/-thiazoles, respectively, in good to high yields. The functional groups (methyl, nitro, and chloride) in the benzyl alcohol stayed with no change in the TOP reaction.

Table 5 Photo-induced synthesis of different benz-imidazoles/-oxazoles/-thiazoles catalyzed by Co-isatin-Schiff-base-MIL-101(Fe) from alcohols via TOP.
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To understand the role of Co-isatin-Schiff-base-MIL-101(Fe) in this reaction, the photocatalytic efficiency of cobalt-free precursors of the catalyst [MIL-101(Fe)–NH2 and isatin-Schiff-base-MIL-101(Fe)] and Co-isatin-Schiff-base were examined in the model reaction for the synthesis of benzimidazole under the optimized reaction conditions (Table 6, entries 1–4). Comparison of the obtained results with those of Co-isatin-Schiff-base-MIL-101(Fe) showed a significant decrease in the catalytic efficiency of cobalt-free precursors and Co-isatin-Schiff-base. Similar reactions in the presence of FeCl3⋅6H2O or Co(OAc)2⋅4H2O did not show any significant progress (Table 6, entries 5 and 6). These experiments obviously revealed that the photocatalytic efficiency of Co-isatin-Schiff-base-MIL-101(Fe) is related primarily to the synergetic effect of Fe–O cluster and Co-Schiff-base. The presence of the synergetic effect could be clarified by a significant decrease in the band gap energy and an increase in the fluorescence emission of MIL-101(Fe) after functionalization with cobalt Schiff-base complex according to DRS analysis (Fig. 8) and fluorescence spectrophotometry (Fig. 10), respectively. In order to indicate the importance of sunlight irradiation and also air in the progress of the reaction, two more experiments were carried out. Under dark conditions, only a trace amount of benzimidazole was obtained (Table 6, entry 7) and benzyl alcohol remained intact in the presence of N2 atmosphere (Table 6, entry 8). Additionally, when benzaldehyde was used instead of benzyl alcohol as the staring material, the desired product was produced in shorter time (Table 6, entry 9). The effect of sunlight irradiation and also air in the progress of the reaction starting from benzaldehyde was studied. Under dark conditions or in the presence of N2 atmosphere, a low yield of benzimidazole was obtained (Table 6, entries 10 and 11). These experiments clearly indicate that the catalytic oxidation reaction takes place in two key steps: (1) selective photo-oxidation of benzyl alcohol to benzaldehyde and (2) dehydrogenation reaction, which takes place after condensation reaction of benzaldehyde with o-phenylendiamine.

Table 6 Effect of different catalytic species or conditions on the synthesis of benzimidazole.
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Figure 10
figure 10

Emission spectra of (a) MIL-101(Fe)–NH2 and (b) Co-isatin-Schiff-base-MIL-101(Fe).

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The electron paramagnetic resonance (EPR) measurements were recorded to detect reactive oxygen species (ROS) formed during the photocatalytic step. By using 2,2,6,6-tetramethylpiperidine (TEMP), as the singlet oxygen (1O2) detection agent, a strong 1:1:1 triplet signal was depicted (Fig. 11a), proving 1O2 formation over Co-isatin-Schiff-base-MIL-101(Fe) under the optimum conditions. When 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a superoxide radical anion (O2⋅−) detection agent was used, strong signals appeared (Fig. 11b), which indicates that O2⋅− species were also formed during the photocatalytic reaction.

Figure 11
figure 11

(a) EPR spectra for 1O2 detection by TEMP and (b) EPR detection of O2⋅– by DMPO.

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Based on our observation and also the postulated mechanisms in the literature28,29,34,62, a mechanism is proposed in Fig. 12. When Co-isatin-Schiff-base-MIL-101(Fe) is irradiated, the singlet excited state of MOF and the holes were produced. The excited electrons in the valance band reduces O2 to form O2⋅– by single electron transfer (SET). Moreover, singlet excited state of MOF can be converted to triplet excited state by intersystem crossing. The produced triplet excited state activates oxygen to 1O2 through energy transfer process. The holes (h+) oxidize benzyl alcohol into benzyl alcohol⋅+, which further reacted with formed O2⋅− and 1O2 to produce benzaldehyde. In the next step, the Lewis acidic iron and cobalt sites in Co-isatin-Schiff-base-MIL-101(Fe) catalyze the condensation of the as-formed aldehyde and o-phenylenediamine to generate the cyclic compound, which is more oxidized to benzimidazole. To determine the active species involved in the catalytic cycle, the photocatalytic reaction was individually investigated using some famous scavengers. In the presence of p-benzoquinone (BQ), NaN3 and ammonium oxalate, the photocatalytic efficiency of the MOF was decreased obviously. The above results indicate that O2⋅–, 1O2 and h+ are the principal oxidants in the photocatalytic procedure.

Figure 12
figure 12

Proposed mechanism for the synthesis of 2-phenylbenzimidazole from o-phenylenediamine and alcohol over Co-isatin-Schiff-base-MIL-101(Fe).

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The reusability of Co-isatin-Schiff-base-MIL-101(Fe) was studied in the model reaction of the synthesis of benzimidazole from alcohol via TOP. For this purpose, after completion of the reaction, ethanol (10 mL) was added to the mixture and the reaction mixture was centrifuged to separate the catalyst. The recovered catalyst after washing with ethanol was reused in five consecutive runs in the model reaction with insignificant decrease in the catalytic activity (Fig. 13). The FT-IR spectrum, and TEM image of the reused catalyst indicate that the structure of the catalyst preserved after five times recycling (Fig. 14).

Figure 13
figure 13

Reusability of the Co-isatin-Schiff-base-MIL-101(Fe) during synthesis of benzimidazole.

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Figure 14
figure 14

TEM image and FT-IR spectrum of Co-isatin-Schiff-base-MIL-101(Fe) after five times reuse in the synthesis of benzimidazole.

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A classical hot-filtration examination was employed to explore whether Co-isatin-Schiff-base-MIL-101(Fe) works as a heterogeneous bio-photocatalyst or releases cobalt ions. Towards this point, synthesis of benzimidazole directly from alcohol was investigated under the optimized reaction conditions. In half time of the reaction (4 h), the reaction mixture was centrifuged to separate the catalyst. Then the reaction was allowed to be continued for an additional 4 h. Any progress in the reaction was not observed by increasing the reaction time (Fig. 15b). The nature of the catalytic mode (homogeneous or heterogeneous) was surveyed using a poisoning test. For this purpose, the model reaction of the synthesis of benzimidazole was performed in the presence of S8 (0.05 g) to kidnap the possible released cobalt species. No changes in the reaction course compared to normal one (Fig. 15a) were identified, which revealed the absence of the leached homogeneous cobalt particles in the solution (Fig. 15c).

Figure 15
figure 15

Synthesis of benzimidazole starting from alcohol using Co-isatin-Schiff-base-MIL-101(Fe) as a photocatalyst: (a) normal reaction, (b) employing a hot filtration protocol and (c) using S8 as a poison for the catalyst.

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Tandem photo-oxidation/Knoevenagel condensation reaction catalyzed by Co-isatin-Schiff-base-MIL-101(Fe)

The results of benzimidazole synthesis from alcohols via TOP, encouraged us to investigate the photocatalytic performance of Co-isatin-Schiff-base-MIL-101(Fe) in the tandem photo-oxidation/Knoevenagel condensation reaction of alcohols and malononitrile. For this purpose, the reaction of benzyl alcohol and malononitrile under sunlight irradiation at ~ 30 °C was selected as a model reaction to optimize the reaction conditions such as the amount of the catalyst and solvent (Table 7). 1.5 mol% of Co-isatin-Schiff-base-MIL-101(Fe) was the most effective amount of the catalyst (Table 7, entry 3). Some solvents such as EtOH, THF, toluene, H2O, CH3CN and also solvent-free conditions were tested (Table 7, entries 3 and 5–10) and found that the reaction is efficiently promoted in EtOH compared to other solvents (Table 7, entry 3). To show the effect of the irradiation on the progress of the reaction, a similar reaction in the absence of any light and air was also studied and any conversion of the starting material was not detected (Table 7, entry 11).

Table 7 Tandem one-pot photo-oxidation/Knoevenagel condensation reaction of alcohols catalyzed by Co-isatin-Schiff-base-MIL-101(Fe).
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Having the optimized reaction conditions, the photo-oxidation/Knoevenagel condensation reaction of various alcohols were examined (Table 8). As the results of Table 8 show, benzyl alcohols containing different substituents underwent the photo-oxidation/Knoevenagel condensation reaction with malononitrile and produced the desired products in good to high yields (Table 8, entries 1–8). The reaction of ethyl cyanoacetate as a compound with active methylene and alcohol proceeded well under the same reaction conditions (Table 8, entries 9 and 10).

Table 8 TOP photo-oxidation/Knoevenagel condensation reaction of alcohols catalyzed by Co-isatin-Schiff-base-MIL-101(Fe).
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Furthermore, we have found that furfuryl alcohol (Fig. 16) was suitable substrate for photo-oxidation/Knoevenagel reaction under the optimal conditions, and produced the desired Knoevenagel condensation product in 75% yield.

Figure 16
figure 16

Synthesis of (2-furylmethylene) malononitrile from furfuryl alcohol.

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To find the role of Co-isatin-Schiff-base-MIL-101(Fe) in the photo-oxidation/Knoevenagel reaction, the model reaction was studied in the presence of cobalt-free precursors of the catalyst [MIL-101(Fe)–NH2 and isatin-Schiff-base-MIL-101(Fe)] or Co-isatin-Schiff-base (Table 9, entries 1–4). A significant decrease in the catalytic efficiency of cobalt-free precursors and Co-isatin-Schiff-base was observed compared with Co-isatin-Schiff-base-MIL-101(Fe). The importance of sunlight irradiation and air in the reaction was also investigated. The product was obtained only in a trace amount under dark conditions (Table 9, entry 5) and alcohol remained intact in the presence of N2 atmosphere (Table 9, entry 6).

Table 9 Effect of different catalytic species or conditions on tandem one-pot photo-oxidation/Knoevenagel.
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When benzaldehyde was used instead of benzyl alcohol as the starting material, the target.product was produced more rapidly (Table 9, Entries 7–9). These experiments clearly indicate that the reaction using Co-isatin-Schiff-base-MIL-101(Fe) actually takes place in two key steps: step (1) photo-oxidation of benzyl alcohol to benzaldehyde and step (2) Knoevenagel condensation reaction between the in situ formed benzaldehyde and malononitrile. Additional control experiments reveal that Co-isatin-Schiff-base-MIL-101(Fe) act as a photosensitizer which contains Fe (III) and Co (II) counterparts as typical Lewis acidic sites and worked well synergistically.

Hence, according to the control experiments, our observation above and literature survey32,33,34, a possible mechanism for the TOP photo-oxidation/Knoevenagel condensation reaction can be proposed (Fig. 17). The generated hole (h+) rapidly oxidizes the benzyl alcohol to create radical cation species (A) as the intermediates, which can consequently react with ROS (O2⋅− and 1O2) to produce the corresponding aldehyde (B). Next, Co-isatin-Schiff-base-MIL-101(Fe) including the Lewis acidic sites (Fe3+ and Co2+) activates the in situ-produced aldehyde and malononitrile for a nucleophilic addition via Knoevenagel coupling reaction toward the formation of the final product (C).

Figure 17
figure 17

Proposed mechanism for the TOP photo-oxidation/Knoevenagel condensation reaction of alcohols catalyzed by Co-isatin-Schiff-base-MIL-101(Fe).

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Finally, the photocatalytic efficiency of the present catalyst was compared with the MOF-based photocatalysts in the benzimidazole synthesis and Knoevenagel condensation starting from alcohols via TOP reported in the literature (Table 10). As represented in Table 10, Co-isatin-Schiff-base-MIL-101(Fe) exhibits the most effective photocatalytic activity for both benzimidazole synthesis and Knoevenagel condensation under sunlight irradiation. The reported methods suffer from drawbacks such as using toxic solvents, high temperature, requiring UV irradiation, prolong reaction time, high catalyst loading and expensive metals. It is worth to mention that because of the high energy of UV photons, which causes the chemical degradation or side reactions, there is a limitation in using UV light irradiation.

Table 10 Comparison of photocatalytic efficiency of Co-isatin-Schiff-base-MIL-101(Fe) with some reported MOFs-based photocatalysts for benzimidazole and benzylidenemalononitrile synthesis via one-pot TOP started from alcohols.
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Photocatalytic antibacterial efficiency of Co-isatin-Schiff-base-MIL-101(Fe)

Bacterial pollution intimidates the nutrition, water and community health. Antibiotics are the most famous antibacterial agents for medical treatment of infections caused by bacteria65,66. Using a great amount of antibiotics makes bacteria to be resistant towards medicines and so would be lethal for the patients67,68. Therefore, introduction of novel and efficient antibacterial agents, which are different from antibiotics is demanded69. Nowadays, due to the harmless, efficient, and extensive disinfection influence of semiconductors, they are widely used as antibacterial agents in photocatalytic process. During this process, in the semiconductors, electrons are excited by visible light irradiation and some holes are generated. Then reactive oxygen radicals (HO⋅, O2⋅−) are formed by the reaction of electrons and holes with water and oxygen, which destruct cell walls of bacteria and thus disable the microorganisms70,71. MOFs as semiconductors are known as a new generation of antibacterial agents. They have enormous surface area, highly-distributed active sites, tunable porous size, and high biocompatibility and biodegradability72,73.

Herein, the photocatalytic antibacterial activity of Co-isatin-Schiff-base-MIL-101(Fe) against E. coli, S. aureus and S. pyogenes was studied under sunlight illumination. The results showed that almost all bacteria were inactivated after 2 h irradiation in the presence of MOF as a bio-photo-antibacterial agent, with the antibacterial efficiency of 99.57%, 99.85% and 99.48%, for E. coli, S. aureus and S. pyogenes, respectively. The negligible decreasing in the bacterial density treated with photocatalyst in the dark or without Co-isatin-Schiff-base-MIL-101(Fe) at light conditions indicates that both light irradiation and photocatalyst are necessary for the efficient antibacterial activity. Moreover, minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) values for E. coli and S. aureus are the same and are higher in the case of S. pyogenes (Table 11).

Table 11. The antibacterial test results of Co-isatin-Schiff-base-MIL-101(Fe) for different bacterial strains: E. coli, S. aureus and S. pyogenes under sunlight irradiation.
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Conclusion

In conclusion, a metal–organic framework functionalized with cobalt-complex [Co-isatin-Schiff-base-MIL-101(Fe)] as a heterogeneous multifunctional bio-photocatalyst for the synthesis of benz-imidazoles/-oxazoles/-thiazoles, and benzylidene malononitrile by tandem oxidation process starting from readily available alcohols is introduced. In this method, photo-oxidation of alcohols and cyclocondensation of the in-situ formed aldehydes with o-substituted aniline (o-phenylenediamine/o-aminophenol/o-aminothiophenol) or malononitrile affords benz-imidazoles/-oxazoles/-thiazoles, or benzylidene malononitrile, respectively, in one-pot operation in good to high yields. Co-isatin-Schiff-base-MIL-101(Fe) acts both as a bio-photocatalyst to produce reactive 1O2, O2⋅−, and a Lewis acid to catalyze the reaction of the in-situ formed aldehydes with o-substituted anilines or malononitrile. A significant decrease in the band gap energy and an increase in the characteristic emission of MIL-101(Fe) after functionalization with cobalt Schiff-base according to DRS analysis and fluorescence spectroscopy, respectively, indicate that the photocatalytic efficiency of Co-isatin-Schiff-base-MIL-101(Fe) is related primarily to the synergetic effect of Fe–O cluster and Co-Schiff-base. EPR results obviously point out that Co-isatin-Schiff-base-MIL-101(Fe) is capable of generating 1O2 and O2⋅− as active oxygen species under visible light irradiation. The use of sunlight and air as abundant and cheap sources without needing any specific chemical oxidizing agent, using low loading of a reusable bio-photocatalyst in ethanol as an environmentally friendly solvent, and easy separation of the obtained products are the advantages of this method. Moreover, Co-isatin-Schiff-base-MIL-101(Fe) exhibits outstanding sunlight-induced photocatalytic antibacterial activity for the inactivation of three kinds of bacteria. Based on our knowledge, this is the first report of using a bio-photocatalyst for the synthesis of the target molecules.