Tetracarboxyphenylporphyrin–Kaolinite Hybrid Materials as Efficient

Article pubs.acs.org/JPCC

Tetracarboxyphenylporphyrin−Kaolinite Hybrid Materials as Efficient Catalysts and Antibacterial Agents Analaura Lovo de Carvalho,† Breno F. Ferreira,† Carlos H. Gomes Martins,† Eduardo J. Nassar,† Shirley Nakagaki,‡ Guilherme Sippel Machado,‡ Vicente Rives,§ Raquel Trujillano,§ Miguel A. Vicente,*,§ Antonio Gil,∥ Sophia A. Korili,∥ Emerson H. de Faria,*,† and Katia J. Ciuffi*,† †

Grupo Sol−Gel, Universidade de Franca, Franca, SP, Brazil DQ/Universidade Federal do Paraná, Curitiba, PR, Brazil § GIR−QUESCAT, Departamento de Química Inorgánica, Universidad de Salamanca, Salamanca, Spain ∥ Departamento de Química Aplicada, Universidad Pública de Navarra, Pamplona, Spain ‡

S Supporting Information *

ABSTRACT: The preparation, characterization, and application in oxidation reactions and antibacterial activity of new biomimetic heterogeneous materials are reported. Brazilian São Simão kaolinite has been functionalized with Fe(III)-[meso-tetrakis(tetracarboxyphenyl)porphinato], [FeTCPP]. Clay functionalization with FeTCPP was confirmed by the amide bands observed in the FTIR spectrum, while UV−vis spectroscopy revealed a red-shift of the characteristic Soret band of the iron porphyrin compared to the parent iron porphyrin in solution, an evidence of FeIII-porphyrin → FeII-porphyrin reduction. Both catalysts reveal complete selectivity for the epoxide in the epoxidation of cis-cyclooctene, while in the Baeyer−Villiger (BV) oxidation of cyclohexanone reached 50% conversion (100% selectivity to ε-caprolactone). The catalysts were recovered and used as antibacterial agents against various bacteria, with activity comparable to that of commercial products such as streptomycin. These results demonstrated that smart multifunctional hybrid organic− inorganic materials that satisfy the demands of Green Chemistry with maximum usability and environmental compatibility can be designed via functionalization of kaolinite.

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H2O2. Our group was the first to report the use of these catalysts in heterogeneous systems for the Baeyer−Villiger reaction.18 Kaolinite is a class of TO (one tetrahedral−one octahedral sheets) natural layered clay,19 promising catalyst support. It can be modified with organic groups (e.g., aminopropyl and mercaptopropyl) via soft guest displacement, allowing tuning its characteristics for effective catalyst binding and the subsequent formation of hybrid or multifunctional materials.20 Catalyst leaching and alterations in the catalyst structure are thereof avoided, and reactions can be conducted in bed reactors or in continuous flow processes.21 As other clays, kaolinite has good thermal stability and low waste generation, enabling the use of mild conditions and the design of active and selective heterogeneous catalysts. Other important society concern refers to surface contamination by pathogens causing diseases in hospitals, food packaging, storage water purification systems, industrial equipment and textiles, among others.22,23 The emergence of bacteria resistant to currently used antibiotics increases the challenge of developing effective strategies for the prevention and treatment

INTRODUCTION Presently, there is a great interest in the synthesis of multifunctional organic−inorganic hybrid materials that meet the requirements of Green Chemistry, i.e., materials able to combine maximum usability with environmental compatibility.1−3 The final objective should be, for instance, to mimic the action of biological catalysts, such as cytochromes P450, able to perform the selective oxidation of several substrates under mild conditions,4 using molecular oxygen as an oxidant. This would be desirable for any industrial process. In this context, synthetic porphyrins, particularly iron porphyrins, have been successfully used as biomimetic catalysts in many oxidation reactions.5,6 However, their large-scale utilization is not yet viable because of their high cost and the difficulties inherent to homogeneous catalysis such as catalyst recovery from the reaction medium and reutilization. This problem can be overcome by heterogenization of the homogeneous catalysts, allowing for catalyst reuse, thereby reducing costs. Furthermore, enzyme site isolation1,7,8 can be mimicked, improving the selectivity.5,6,9 Therefore, new matrixes for anchoring these valuable catalysts are necessary, simultaneously fullfilling other current demands, such as the use of clean oxidants like hydrogen peroxide. Although many studies in the literature have reported on heterogenized porphyrin catalysts,9−17 only a few papers have described their efficiency and selectivity in the presence of © XXXX American Chemical Society

Received: July 31, 2014 Revised: September 24, 2014

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dx.doi.org/10.1021/jp5077356 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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of infectious diseases,24−26 and thus, the development of new antibacterial products is needed for controlling pathogenic microorganisms in exposed areas. Antimicrobial surfaces have been obtained by incorporating active agents currently under study or commercially available, such as ammonium salts,27 Nhalamines,28,29 antibiotics,30 Ag/TiO2 nanoparticles,31 or metal ions such as Ag+ and Cu2+.32 However, these products do not represent a definitive response, because they may release the active agent to the environment, decreasing the antibacterial properties of the product, while the released agents cause environmental problems and generate resistance.22 Among numerous antimicrobial agents activated by light (e.g., doped and undoped titanium oxide, methylene blue, toluidine blue), porphyrins have been extensively studied in the past decades for their biocidal potential.22,26,33 Their antimicrobial activity is due to their high binding affinity for the cellular components, membranes, DNA, and proteins; this affinity depends on porphyrin hydrophobicity, caused by the presence of the tetrapyrrole ring.34,35 Generally speaking, porphyrins are more toxic to Gram-positive bacteria such as S. aureus than to Gram-negative bacteria such as E. coli.23 This work aims at the design of smart multifunctional hybrid organic−inorganic materials that will satisfy the demands of Green Chemistry with maximum usability and environmental compatibility. The catalysts reported herein consist of a natural Brazilian kaolinite functionalized with a second-generation metalloporphyrin, namely, Fe(III)-[meso-tetrakis(tetracarboxyphenyl)porphinato], FeTCPP. Clay functionalization was achieved by successive treatments with dimethyl sulfoxide (DMSO), which was then displaced by tris(hydroxymethyl)aminomethane (TRIS). The resulting solid was grafted with FeTCPP. The obtained catalysts were completely characterized, and used in green oxidation reactions, namely, cyclooctene epoxidation as well as in the BV oxidation of cyclohexanone, using hydrogen peroxide as oxidant. The multifunctional character of the materials was confirmed by their antibacterial action against Escherichia coli, Klebsiella pneumonia, and Bacillus subtilis microorganisms.

purified kaolinite (20 g) with DMSO (100 mL) and distilled water (20 mL); the suspension thus obtained was stirred for 10 days at 60 °C.19,36−38 Then, the clay was separated by centrifugation, washed with ethanol, and dried at 50 °C. This material was designated as “Ka−DMSO”. Preparation of TRIS−Kaolinite. The second step was the substitution of DMSO by tris(hydroxymethyl)-aminomethane (TRIS). This was carried out by refluxing a mixture of Ka− DMSO (15 g) and TRIS (35 g) for 24 h. The resulting solid was washed with ethanol several times and dried at 60 °C;39,40 it was designated as Ka−TRIS. Preparation of FeTCPP and FeTCPPAc. The purity of the commercially available free-base porphyrin, [H2TCPP], where TCPP stands for the meso-tetrakis(tetracarboxyphenyl)porphinato divalent anion, was evaluated by UV−vis spectroscopy in water solution at pH 12; an absorption band was observed at λmax = 414 nm (ε = 3.4 × 105 mol−1·L·cm−1, Soret band), as well as Q bands at 517, 553, 580, and 636 nm. Iron was inserted into the free base porphyrin following a literature method,41 by refluxing 250 mg (0.316 mmol) of the protonated free-base porphyrin [H2TCPP] with 628 mg (2.53 mmol) of Fe(II) chloride in 50 mL of DMF, under an argon atmosphere. The iron porphyrin was obtained in the chloride form [FeTCPPCl] with a 96% yield, and designated here as FeTCPP. Its purity was evaluated by UV−vis spectroscopy, also in aqueous solution at pH 12: λmax = 325, 415 nm (ε = 9.5 × 104 mol−1·L·cm−1, Soret band), as well as the Q bands at 517, 553, and 636 nm. The iron porphyrin was also converted to its acyl chloride form by reaction with thionyl chloride under an inert atmosphere, using the method described by Sprintschnik et al.42 A portion of 100 mg (0.118 mmol) of FeTCPP was refluxed with 60 mL (827 mmol) of thionyl chloride at room temperature under an argon atmosphere for 3 h. Briefly, the carboxylic acid groups of the porphyrin react with thionyl chloride, giving rise to acyl chloride and sulfur dioxide. The material was denominated as FeTCPPAc. Its purity was evaluated by UV−vis spectroscopy, also in aqueous solution at pH 12: λmax = 325 and 415 nm, as well as Q bands at 517, 553, and 636 nm. Preparation of Tetracarboxyphenylporphyrin−Kaolinite Catalysts. A mixture of 52 mg of the porphyrin, FeTCPP (6.2 × 10−5 mol) or FeTCPPAc (5.7 × 10−5 mol) and Ka− TRIS (2.0 g, 7.6 × 10−3 mol) in DMF (250 mL) was submitted to reflux and stirred for 48 h under an argon atmosphere. Then, the mixtures were slowly cooled and aged overnight in the mother liquor, at room temperature. The solvent was then slowly removed in a rotary evaporator. The obtained solids, designated as “Ka−TRIS−FeTCPP” and “Ka−TRIS−FeTCPPAc”, respectively, were washed several times with ethanol and dried overnight at 60 °C. The amount of FeTCPP or FeTCPPAc leached from the support was quantified by measuring the amount of porphyrin in the successive washings by UV−vis spectroscopy. The porphyrin content in these solids, ignoring the possible variations in the degree of hydration of the solid in the several steps of the synthesis procedure, was 25.9 mg/g in both cases. The possible interactions between the iron porphyrins and the clay are summarized in Figure 1. In order to test whether FeTCPP could be anchored to the clay without carrying out all the synthesis steps described above, suspensions of FeTCPP and Ka or Ka−TRIS solids were

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EXPERIMENTAL SECTION A. Materials. The clay used in the present work was a kaolin obtained from the deposit located in the city of São Simão, state of São Paulo, Southwest of Brazil, kindly supplied by the mining company Darcy R. O. Silva & Cia (São Simão, SP, Brazil). The natural clay was purified by the dispersiondecantation method; the purified clay consisted of very pure kaolinite (see below) and was designated as “Ka” in the derivative materials. All the solvents and reagents were of high-purity commercial grade (Merck and Aldrich), and most of them were used as received. Dichloromethane (DCM) was dried over anhydrous CaCl2 for 2.5 h, filtered, and distilled over P2O5, and then stored over 4 Å molecular sieves. N,N-Dimethylformamide (DMF) was stirred over KOH at room temperature overnight, and then distilled at reduced pressure. All the oxidation reaction substrates were purified by column chromatography on basic alumina immediately prior to use. B. Preparation, Characterization, and Applications of the Catalysts. Preparation of DMSO−Kaolinite. For the preparation of the porphyrin derivatives, it is necessary to functionalize previously the clay, which was carried out by successive intercalation with DMSO (needed to swell the kaolinite) and TRIS. The first step was carried out by mixing B



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the products were quantified using calibration curves obtained with standard solutions. Yields were based on conversion of the substrate. The anhydrous solution of hydrogen peroxide (24 wt %) in ethyl acetate was prepared by azeotropic distillation as described previously by Schuchardt et al.45 An aqueous H2O2 solution was used as the starting material for the preparation of the anhydrous solution of hydrogen peroxide. The azeotropic distillation was carried out in a system where the oxygen generated by hydrogen peroxide decomposition is allowed to be released to the atmosphere.45 The aqueous solution of hydrogen peroxide was kindly supplied by Peróxidos do Brasil S.A. and was used without pretreatment. Its H2O2 content was determined by iodometric titration in a deaerated solution, as described elsewhere.46 D. Antibacterial Activity Tests. Microorganisms. Bacterial cultures used in the present studies were obtained from the American Type Culture Collection (ATCC). Three microbial strains were tested: Bacillus subtilis (6051), Klebsiella pneumoniae (13883), and Escherichia coli (25922). All strains were kept in the laboratory and cryopreserved at −86 °C. Before the experiments, the culture medium was inoculated with microorganisms under appropriate atmospheric conditions, to confirm the strain purity. Preparation of Inoculums. All the bacterial strains mentioned above were incubated at 37 ± 0.1 °C for 24 h in Brain Heart Infusion (BHI) agar. Standardization of each microorganism suspension was carried out at 625 nm by means of a spectrophotometer, to match the transmittance of 81, equivalent to 0.5 McFarland scale (1.5 × 108 CFU·mL−1). Antibacterial Assay by the Agar Diffusion (AD) Method. The antibacterial activity of the samples was determined by the agar diffusion (AD) method, using the well technique and the double-layer agar system.47 Brain Heart Infusion agar (25 mL) was poured into each sterilized Petri dish (15 × 90 mm2 diameter). Next, a 12.5 mL portion of the BHI agar (50 °C) and 2.5 mL of each test suspension were gently mixed and poured onto a previously set layer and distributed in Petri dishes homogeneously. After solidification, the seed layer was perforated with a sterilized stainless-steel cylinder (inside diameter = 4 mm), to form the wells. The latter were located 25 mm away from the plate border and 40 mm halfway from each other. Ka−TRIS− FeTCPP, distilled water (negative controls), and an aqueous solution of penicillin or streptomycin (0.1 mg/mL, positive control) were applied inside the wells. The plates were kept for 2 h at room temperature, to allow diffusion of the agents through the agar.47 Afterward, the plates were incubated at 37 °C under aerobic conditions for 48 h. At the end of the incubation period, inhibition zones formed in the media were measured in mm. The inhibitory zone was considered the shortest distance (mm) from the outside margin of the well to the initial point of microbial growth. Three replicates were accomplished for each bacterium. Statistical Analysis. Results from the biological assays carried out by the AD method were submitted to analysis by one-way ANOVA. Individual samples were tested in triplicate. The mean and standard deviation values of all the experiments were calculated from the diameter, in mm, of the inhibition halo (AD). Investigation of the Antibacterial Activity Mechanism via Kinetic Analysis between Porphyrin and Calf Thymus DNA (ctDNA). When porphyrin reacts with DNA to form a complex,

Figure 1. Schematic representation of the possible interactions between the iron porphyrins and functionalized kaolinite.

stirred for 3 h, at room temperature, and then washed with ethanol and dried at 60 °C. In both cases, total leaching of the porphyrins was observed, clearly confirming the importance of the synthesis procedure used for the preparation of the functionalized catalysts. Although these solids did not contain porphyrins, they were used for catalytic tests, being denoted as “FeTCPP/Ka” and “FeTCPP/Ka−TRIS”, respectively. C. Catalytic Performance. (Z)-Cyclooctene Oxidation with Iodosylbenzene. Iodosylbenzene (PhIO) was obtained through controlled hydrolysis of iodosylbenzene diacetate,43 and its purity was evaluated by iodometric titration.44 For the reactions, PhIO (0.023 mmol) was added to a 4.0 mL vial sealed with a Teflon-coated silicone septum containing the catalyst (10 mg), dichloroethane (DCE)/acetonitrile (ACN) 1:1 mixture (1 mL), (Z)-cyclooctene (previously purified on an alumina column), and di-n-butyl ether as an internal standard (10 μL). At the end of the reaction, the catalysts were recovered by centrifugation and washed five times with 1 mL of methanol, to ensure that any remaining oxidant would be removed. The catalyst was then dried for 3 h at 60 °C, before being reused in (Z)-cyclooctene or cyclohexanone oxidation. (Z)-Cyclooctene Oxidation Reaction with H2O2. Oxidation of (Z)-cyclooctene was carried out using anhydrous H2O2 as oxidant, at room temperature, with the catalyst:substrate:oxidant molar ratio being 1:1000:2000. Thus, 10 mg of catalyst (Ka−TRIS−FeTCPP or Ka−TRIS−FeTCPPAc), 220 μL of oxidant, 38 μL of (Z)-cyclooctene, 10 μL of di-n-butyl ether as internal standard, and the mixture DCE/ACN (1152 μL) as solvent were added into 4.0 mL vials. Baeyer−Villiger (BV) Cyclohexanone Oxidation. The BV oxidation reaction was carried out in a 4.0 mL glass reactor. Thus, 50 μL of cyclohexanone (0.48 mmol), 850 μL of benzonitrile (7.95 mmol), 5.0 μL of di-n-butyl ether (2.94 μmol), 176 μL of 60 wt % hydrogen peroxide (4.53 mmol), and 20 mg of the catalyst were added to the reactor; the mixture was heated at 60 °C and stirred for 48 h. A control reaction was conducted in the absence of the catalyst or the oxidant. The reaction products were analyzed by gas chromatography, and C



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(Servicio General de Microscopiá Electrónica, Universidad de Salamanca, Spain). The samples were coated with a thin gold layer by evaporation using a Bio-Rad ES100 SEN coating system. The reaction products were analyzed by gas chromatography carried out on a HP 6890 chromatograph equipped with a hydrogen flame ionization detector and capillary column (HPINNOWax-19091 N-133, polyethylene glycol, length = 30 m, internal diameter = 0.25 μm). The products were quantified using a calibration curve obtained with a standard solution, and the yields were based on the added PhIO or on the conversion of the substrate when hydrogen peroxide was used as an oxidant.

the rate of complex formation depends on the concentrations of free porphyrin and ctDNA, and the stability of the complex. Short displacements and also reduction of the characteristic Soret band (near 400 nm) of porphyrin evidenced the interaction with DNA.48 Thus, to confirm the reaction of FeTCPP and ctDNA (Sigma-Aldrich), the UV−vis spectrum of porphyrin was accomplished in a typical kinetic experiment followed by titration with ctDNA (10 μM in phosphate buffered saline, PBS, pH 7.4), at room temperature, in the wavelength range 300−700 nm with a 1 cm quartz cell.48 Time−Kill Curves. The Ka−Tris−FeTCPPAc sample was chosen to conduct this assay, as this solid displayed the highest antibacterial activity against B. subtilis. A tube containing 1 mL of tryptic soy broth sterile and Ka−Tris−FeTCPPCAc with a final concentration of 100 μg mL−1 was inoculated with the tested microorganism, resulting in an initial bacterial density of 7.6 × 106 CFU mL−1, and then incubated at 37 °C. Aliquots were removed (100 μL) for enumeration of viable colonies at 30 min and 2, 4, 6, 12, 18, 30, 38, and 48 h after incubation, followed by serially dilution, up to 1:10000, in tryptic soy broth sterile. The diluted samples (50 μL) were spread onto a tryptic soy agar plate, incubated at 37 °C, and viable colonies were counted after 24 h. The time−kill curves were constructed by plotting log CFU versus time using Prism software (version 5.0; GraphPad, Inc.). The assays were performed in triplicate (suspension of B. subtilis without added DA). E. Characterization Techniques. X-ray diffraction (XRD) patterns of nonoriented powder samples were obtained on a Siemens D-500 diffractometer with Ni-filtered Cu Kα radiation, at 40 kV and 30 mA, at a scanning rate of 2°/min. FT-IR spectra were recorded in the 4000−350 cm−1 range in a Perkin-Elmer 1730 infrared Fourier transform spectrometer, using the KBr pellet technique. About 1 mg of the sample and 300 mg of KBr were used to prepare the pellets. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out in a Thermal Analysis TA Instruments SDT Q 600, from 25 to 1000 °C, under an oxygen flow (30 mL/min), at a heating rate of 20 °C/min. Specific surface areas were determined by applying the BET method to the corresponding nitrogen adsorption data at −196 °C, obtained by using a Micrometrics ASAP 2020 physical adsorption analyzer. The samples were previously degassed in situ for 1 h at room temperature, at a pressure lower than 50 μm Hg. The nitrogen adsorption data were obtained using 0.2 g of the sample. Electronic spectra of the materials were recorded on a Hewlett-Packard 8453 diode array UV−vis spectrophotometer. The spectra of the solids suspended in DCM were recorded in a 2.0 mm path length cell. DCM was used as a suspension medium because it led to improved UV−vis spectra when the suspension was prepared. The spectra were preprocessed by using a wavelet filter with Dubauchie (db4) base function, to minimize noise, subtract the isotonic medium spectrum, and multiply scattering correction (MSC), to minimize Rayleigh scattering due to variation in the refractive index of the matrix. A second-degree polynomial fit was used, to minimize matrix fluorescence. Electron paramagnetic resonance (EPR) measurements of the dried powder materials were performed on a Bruker ESP 300E spectrometer operating at the X-band (ca. 9.5 GHz), at −196 °C. Scanning electron microscopy (SEM) of the materials was performed on a Zeiss DSM 960 digital scanning microscope

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RESULTS AND DISCUSSION A. Characterization of the Catalysts. Purification of kaolinite and its intercalation with DMSO were successful. A well-ordered Ka−DMSO complex with a basal spacing of 11.2 Å was obtained. DMSO was successfully substituted by TRIS, giving a well-ordered Ka−TRIS complex, with a basal spacing of 12.53 Å and structural formula Ka−(TRIS)0.567. All the processes involved in these steps have been described previously in the literature19,36,38,49−51 (Supporting Information, Figures S1, S2, and S3 and Table S1). Metalation of Tetracarboxyphenylporphyrin (FeTCPP). Figure 2 shows the UV−vis spectra of free-base H2TCPP and

Figure 2. UV−vis spectra of free base H2TCPP and FeTCPP using water as solvent (pH 12).

FeTCPP. The free-base porphyrin displays a typical Soret band at 414 nm and Q bands between 500 and 700 nm. The Q-band at 580 nm disappeared after inserting Fe(III) cations into H2TCPP, but the Soret band almost did not shift, in contrast with the phenomenon commonly observed upon porphyrin metalation.52 Indeed, Buchler et al.53 discussed that FeTCPP does not exhibit the Soret band displacement due to Fe(III) interaction with the porphyrin macrocycle. The band due to Fe(III) charge transfer about 330 nm confirmed that Fe(III) interacts with the porphyrin pyrrolic rings; the position of this band depends on the ligands in the axial positions. Reduction in the typical fluorescence of free-base TCPP upon Fe(III) insertion (results not shown) also confirmed macrocyclic metalation. Preparation of Acyl Chloride Porphyrin (FeTCPPAc). In order to optimize the reactivity of FeTCPP, its carboxylic groups were converted into acyl chlorides by reaction with D



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Figure 3. PXRD of Ka−TRIS−FeTCPP and Ka−TRIS−FeTCPPAc.

thionyl chloride.42 The resulting material was denominated FeTCPPAc (Supporting Information, Figure S4). In the presence of thionyl chloride, the four carboxylic groups were converted into acyl chlorides. Briefly, a nucleophilic attack occurred on the polarized sulfur atom of thionyl chloride, with elimination of chloride, followed by deprotonation and internal rearrangement, producing acyl chloride and sulfur dioxide.42 It was expected that the presence of electron-acceptor substituents gives the corresponding metal-oxo active species greater electrophilicity and reactivity.54,55 Substituents such as Cl provide to these second generation porphyrins steric protection of the meso carbon atoms of the porphyrin ring, preventing the rapid auto-oxidative destruction of porphyrin.54,55 Formation of the acyl chloride was confirmed by UV− vis and infrared spectroscopies. The infrared spectra of FeTCPP and FeTCPPAc (Supporting Information, Figure S5 and Table S2) displayed the typical bands of porphyrins. After the acylation reaction, all the structural bands of FeTCPP remained in the spectrum, and the bands due to the carboxylic groups are displaced or absent. Thus, the characteristic carboxylic bands observed at 2978, 2936, 2896, 2808, 2563, 1413, 1365, 1306, 1205, and 1152 cm−1 for FeTCCP shifted to 2963, 2920, 2847, 2794, 2662, and 2500 cm−1, confirming the chlorination in the carboxyphenyl groups, while the characteristic vibrations of carboxyphenyl groups at 872, 841, 802, 773, and 695 cm−1 slightly changed and the vibrations at 841 and 773 cm−1 disappear, confirming the insertion of chloride ions into the carboxy group of the porphyrin macroring. The complete assignment is given in Table S2 (Supporting Information). The weak N−H stretching vibrations above 3300 cm−l for all the ligands disappeared in the chelates, since the acidic hydrogens are replaced by the metal ion. According to Thomas and Raja,56 the replacement of these imino hydrogen atoms results in shifts of many absorption bands of the ligand to both higher and lower wavenumbers, as it was observed. The shift to higher wavenumbers was more evident for the three sharp absorption bands of the tetraphenylporphine ligands near 1000 cm−l, which were previously assigned to C−H rocking vibrations of the pyrrole ring. Another evidence of chlorination reaction was the bands observed near 1600 cm−1, where the bands at 1649 and 1607 cm−1 in FeTCPP, characteristic of the pyrrole group, shifted to 1702 and 1657 cm−1 and their intensity decreased in FeTCPPAc. The absorption bands of the metal chelates of tetraphenylporphine and its p-methoxy and/ or chloro derivatives have been reported near 1600 cm−1 and at lower wavenumbers.56

Preparation of Tetracarboxyphenylporphyrin−Kaolinite Catalysts. The X-ray powder diffraction (PXRD) diagrams (Figure 3) showed that kaolinite is slightly amorphized after interaction with FeTCPP and/or FeTCPPAc, due to the high affinity between their carboxylic and acyl chloride groups and the amine groups of the grafted matrix; the intensities of the peaks corresponding to the basal spacing decreased in both cases. In fact, the changes induced after insertion of different porphyrin molecules into Ka−TRIS are very small. As proven previously,18 the changes should not be large, because the porphyrin ring can be parallel to kaolinite layers. On the other hand, the remaining diffraction effects, namely, those not depending on the stacking of the layers in the c-direction, also changed drastically compared to the parent purified clay and the compound grafted with TRIS, indicating that insertion of FeTCPP or FeTCPPAc can alter the structure of each layer. By this reason, the relative intensities of (001) and (020) planes were compared (Supporting Information, Table S3), as their relationship is known to give good evidence of the crystallographic arrangement of several solids. DMSO−kaolinite showed a strong increase in the (001)/(020) ratio with respect to the original kaolinite, confirming the ordering reached during intercalation. Grafting with TRIS promoted the disordering of the layers reducing the crystallinity, with the subsequent decrease in this ratio. The interaction with the porphyrins produced two opposite effects: the interaction with FeTCPP induces the reorganization of kaolinite layers increasing this ratio, while the interaction with FeTCPPAc causes the disordering of the layers, and even exfoliation, with the decrease of this ratio. Exfoliation can be explained by the molecular size of the carboxyporphyrin, which largely reduced the interaction between the clay layers. The lower amount of porphyrin used in this synthesis can promote the interaction of different layers with the same complex and induce the exfoliation. The infrared spectra of the porphyrin solids and of the precursor (Supporting Information, Figure S6 and Table S4) showed that the typical bands of intra- and interlayer hydroxyl groups of kaolinite at 3618 and 3695 cm−1 did not shift nor disappear after reaction with porphyrin. The decrease of the intensity of the band due to interlayer hydroxyl groups at 3653 and 3668 cm−1 in Ka−TRIS−FeTCPP and their absence in Ka−TRIS−FeTCPPAc strongly suggested the direct interaction of carboxylic and acyl chloride groups of the porphyrins with free hydroxyls in the interlayer space of the Ka−TRIS precursor. The C−H stretching bands observed at 2937 and 2870 cm−1 for the Ka−TRIS solid did not change. The typical E



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amine band at 1462 cm−1 was observed for both catalysts. The band at 1395 cm−1 was also present for both catalysts, with a higher intensity for Ka−TRIS−FeTCPPAc; this band was assigned to secondary amide that is formed by the reaction between NH2 groups from TRIS and carboxylic or acyl chloride groups from porphyrins. TG/DTG curves of Ka−TRIS−FeTCPPAc and Ka−TRIS− FeTCPP catalysts are shown in Figure 4 and Figure S7

cell were calculated, giving the formulas Ka−TRIS− FeTCPP0.123 and Ka−TRIS−FeTCPPAc0.116. The results closely agreed with those obtained from C and N chemical analysis (Supporting Information, Table S5), which yield the formulas Ka−TRIS−(FeTCPP)0.119 and Ka−TRIS−(FeTCPPAc)0.117, respectively. With these compositions, all carboxylic groups in the iron porphyrin can be bonded to amino groups in functionalized kaolinite, even considering the possible steric hindrance, because of the large excess of kaolinite. This may also explain the differences observed in the FTIR of the solids, which may be related to different interactions and arrangements of both molecules in Ka−TRIS solid. These results indicated that the C/N molar ratio in Ka−TRIS is 3.84, close to the value in pure TRIS (4.0); the difference can be assumed to be due by the difficulty for analyzing these elements existing in the samples in low amounts.51,58 These results clearly suggested that the organic molecules did not suffer significant alteration when incorporated into the solids; they might be protonated, a process not observable by chemical analysis, but other changes did not seem to occur. After immobilization of iron porphyrin, small increases of the C/N molar ratios were observed, to 4.25 and 4.32 in Ka−TRIS−FeTCPP and Ka−TRIS−FeTCPPAc, respectively, evidencing the immobilization of iron porphyrins by bonding to Ka−TRIS. The morphological changes induced by porphyrin incorporation can be seen in Figure 5. Kaolinite showed particles

Figure 4. Thermal analysis (TG/DTG) of Ka−TRIS−FeTCPPAc catalyst.

(Supporting Information). Unfortunately, the nature of the evolved gases could not be determined. The curve for the Ka− TRIS precursor (not shown) evidenced four mass losses. The first, at 65 °C (3.7%), with an endothermic associated peak, can be assigned to the elimination of solvent and water molecules weakly adsorbed in the clay mineral surface. The second mass loss stage at 250 °C (5.3%) can be assigned to removal of intercalated TRIS molecules. The third step centered at 300 °C (7.9%) was composed by two effects centered at 330 and 430 °C, confirming the functionalization of kaolinite with TRIS molecules. This exothermic step corresponded to the decomposition of TRIS molecules inserted into the interlayer space of the clay mineral. The step centered at 474 °C (18.2%) can be attributed to kaolinite dehydroxylation, but this process was also composed of two effects probably due to the elimination of residual organic molecules bounded in the kaolinite matrix.36,38,40,42,49,51,57−59 After loading with FeTCPP or FeTCPPAc, three mass losses were recorded, and both curves were very similar, with very small differences in the central temperature of the processes. The mass lost in the step at 250 °C was smaller than that for the precursor, confirming that the interaction with FeTCPP or FeTCPPAc promoted the leaching of nongrafted species of TRIS existing in the interlayer space of kaolinite. The increase on thermal stability (higher than 300 °C), the reduction of the dehydroxylation temperature, and the presence of residual carbon trapped between the layers of kaolinite in both samples gave good evidence of the functionalization of Ka−TRIS with carboxy-porphyrin molecules. These results were similar to those achieved with materials containing other organic molecules, such as methanol, ethylene glycol, diols, and more recently polyols, amino alcohols, pyridine carboxylic acids, and alkoxides covalently grafted on the aluminol surface of kaolinite.36,38,40,42,49,51,57−59 Considering the mass loss between 140 and 400 °C, the amounts of FeTCPP or FeTCPPAc fixed on the kaolinite unit

Figure 5. SEM images (scale bar = 2 μm) of Ka−TRIS (a) and the catalysts obtained after immobilization of FeTCPP (b) or FeTCPPAc (c).

typical for this material, with a flake aspect, aggregated forming hexagonal plates and stacks.59 Upon functionalization, the hexagonal structure was mostly disintegrated into smaller particles, showing the separation of the plates. After immobilization of FeTCPP or FeTCPPAc, the aggregation of the platelets is clearly observed, as it has been reported for other kaolinite−porphyrin systems.18 The shape of the particles changed considerably in the presence of FeTCPP or FeTCPPAc; their degree of agglomeration depended on the chemical reactivity of the iron porphyrin and was more pronounced for FeTCPPAc, where the particles were larger and spongier. At the same time, FeTCPPAc induced the disorder and amorphization of the clay, due to the interaction with this large organic molecule, as already observed by PXRD. As more than one clay platelet can interact with the same porphyrin molecule, the amorphous character of the solids increased. FeTCPP and FeTCPPAc porphyrins were also analyzed via EPR, which is an important tool in the study of species with unpaired electrons.60 FeTCPP showed signals at g = 5.6 and g = 2.0 (Figure 6), characteristic of high spin Fe(III) in axial symmetry.61 However, the FeTCPPAc EPR spectrum showed only one signal at g = 2.0, which can be a result of Fe−Fe interactions between the iron porphyrin and iron chloride F



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Figure 6. EPR spectra at room temperature and −196 °C of catalysts Ka−TRIS−FeTCPP and Ka−TRIS−FeTCPPAc, and of their precursors. Microwave power = 4 mW, modulation amplitude = 4.0 G, microwave frequency = 9.260 GHz, 0.040−0.070 g sample (dry material).

remaining after washing.62 This technique confirmed the porphyrin metalation in the two studied systems. These spectra were compared to those for Ka−TRIS, Ka− TRIS−FeTCPP, and Ka−TRIS−FeTCPPAc. The EPR spectrum of Ka−TRIS−FeTCPP (Figure 6) was similar to that of Ka−TRIS, the signal at g = 5.6, characteristic of FeTCPP, might be masked by the signal of Fe existing in the kaolinite layer in the Ka−TRIS−FeTCPP spectrum. However, this cannot happen in the Ka−TRIS−FeTCPPAc spectrum, since the signal of the FeTCPPAc was absent. The similarity of the spectra was due to the higher amount of iron present in the structural composition of kaolinite (larger than 2%), greater than the amount of iron present in the porphyrin in the sample Ka−TRIS. The same occurred in the sample Ka−TRIS− FeTCPPAc. The intense signal of Fe3+ from kaolinite made EPR characterization of immobilized porphyrin difficult. Machado et al.61 found that the EPR signals from metalloporphyrins immobilized in halloysite were masked by the signals from the clay mineral. The substitution of aluminum atoms in the clay minerals, as kaolinite or halloysite, by iron(III) atoms occurred naturally in the soils, and the presence of iron(III) atoms in the clay did not affect the catalytic action of the immobilized iron porphyrins.61 These results confirmed that the iron porphyrin was located in a very confined space in the clay, promoted by several arrangements of the small platelets. The UV−vis spectra of Ka−TRIS−FeTCPP and Ka−TRIS− FeTCPPAc showed the Soret band at 422 nm, which was redshifted compared to the position for the iron porphyrins in solution, 414 nm (Figure 7). This suggested that a partial Fe(III)/Fe(II) reduction process had taken place, because of bis-coordination of the NH2 groups in the support to the Fe(III) porphyrin. Alternatively, this shift could be due to the fact that the Fe(III) porphyrin was located in a very confined space, distorting the local symmetry of the iron ions, as observed by EPR and previously reported in the case of metalloporphyrins inside zeolites or glasses.63

Figure 7. UV−vis spectra of samples Ka−TRIS−FeTCPP and Ka− TRIS−FeTCPPAc, compared to those from the porphyrins in solution.

B. Catalytic Activity. (Z)-Cyclooctene Oxidation with PhIO. To test the activity of the catalysts, oxidation of (Z)cyclooctene by PhIO, at room temperature and atmospheric pressure, was first considered. (Z)-Cyclooctene has been used as a diagnostic substrate for new catalytic systems for several reasons:64 (1) the main oxidation product is the stable cycloocteneoxide, facilitating the evaluation of the catalyst efficiency; (2) epoxides are useful intermediates in the chemical industry, since they are the starting point to prepare different products; and (3) (Z)-cyclooctene is readily oxidized, thereby decreasing the occurrence of competitive parallel reactions. PhIO is used as an oxygen source because (1) it provides high oxidant conversion rates; (2) it is relatively inert in the absence of Fe(III) porphyrins; (3) it produces stable leaving groups; (4) its reaction with Fe(III) porphyrins generates an extremely active intermediate speciesthe ferrylporphyrin π-cation radical; and (5) it is a polymeric solid oxidant that does not contain weak linkages, so it does not release free radicals, as it G



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efficiency; they favor the activity of the acyl chloride substituent in Ka−TRIS−FeTCPPAc. The acyl group in FeTCPPAc was more electrophilic than that in FeTCPP, it attracted more intensely the electrons from iron, and it can be more active for catalysis, as proposed by Franville et al.55 Indeed, replacement of OH by chloride ions increased the electron withdrawing character of this substituent, facilitating formation of the active oxidant species ferrylporphyrin π-cation radical. The enhanced activity of Ka−TRIS−FeTCPPAc can also be related to the more numerous available active sites, promoted by the exfoliation induced by porphyrin acyl chloride derivatives, as discussed above. Previous studies by Machado et al.69 showed that porphyrins immobilized on natural halloysites rendered more selective catalysts probably due to the difficult access to the active sites. On the other hand, halloysite and their derivatives showed a tubular phase structure different from that of the aminofunctionalized kaolinite matrix, with an exfoliated structure with higher available active sites. Blank tests were conducted in the absence of the Fe(III) porphyrins and in the absence of the oxidant. No product was detected, which confirmed that the Fe(III) porphyrin plays an essential role in the oxidation process. The UV−vis spectra of the supernatant solutions recorded after the oxidation reactions showed that the Fe(III) porphyrins did not leach from the support under any of the studied conditions, since the typical Fe(III) porphyrin bands were not detected. As FeTCPP is active in homogeneous catalysis, it is very important to determine if the catalytic activity of Ka−TRIS− FeTCPP and Ka−TRIS−FeTCPPAc is truly heterogeneous. Hence, the Sheldon test was first carried out;70 this test consists of filtering off the solid after the oxidation reaction, adding an additional amount of oxidant to the resulting supernatant liquid, and allowing the oxidation reaction to proceed under the same initial conditions for a further 48 h. No product was detected, indicating that the supported solids played an essential role in the reaction. Additional evidence of the heterogeneous character of the reaction was obtained by removing the solid catalyst from the reaction solution after 24 h and resuming the reaction using the filtrate; the formation of the epoxide stopped after separation of the solid catalyst. Even more, another test proposed by Sheldon was also carried out:70 the catalyst was incubated with a PhIO solution for 1 h and then filtered off; then, the substrate was added to the filtrate and proceeded with the reaction. As in the case of the previous tests, epoxide was not formed. Therefore, catalysis with both Ka−TRIS−FeTCPP and Ka−TRIS−FeTCPPAc is undoubtedly heterogeneous. The high efficiency and stability of Ka−TRIS−FeTCPP and Ka−TRIS−FeTCPPAc were proven analyzing the catalyst reuse. The solid catalysts were separated from the reaction mixture after each experiment by simple filtration and dried before use in a subsequent run. Both catalysts were reused in three consecutive runs without any significant decrease in activity. On the basis of these tests, we also confirmed that the presence of the Fe(III) porphyrin is essential for the high selectivity toward the epoxide (sole product). We obtained strong evidence that the active species accounting for oxygen transfer to the substrate is the ferrylporphyrin π-cation radical, as reported for many heterogeneous systems containing Fe(III) porphyrins.18,54 To confirm whether the high valent oxo-iron complex, FeIV(O)P+, was the only active species during (Z)-cyclooctene

happens for other oxidants such as alkylhydroperoxides (ROOH).64 Traylor et al.65 have described the mechanism for (Z)cyclooctene epoxidation. The Fe(III) porphyrin first reacts with the oxygen donor, to produce the FeIVOP+ intermediate. An electron transfer from the π bond of the alkene to the intermediate FeIVOP+ followed by cage collapse furnishes the intermediate carbocation, which may react according to three possible competitive routes: (i) nucleophilic attack of the oxygen lone pair on the positively charged intermediate, to afford an epoxide; (ii) alkyl shift; or (iii) hydride shift through a pinacol-type rearrangement carbon, which yields an aldehyde or a ketone, respectively, after oxygen−iron bond cleavage. The results of (Z)-cyclooctene oxidation by PhIO are given in Table 1, which also includes the results from homogeneous reactions. Table 1. Cycloocteneoxide Yield (%) from (Z)-Cyclooctene Oxidation by PhIO under Homogeneous or Heterogeneous Conditionsa time of reaction (h) catalyst FeTCPP FeTCPPAc Ka−TRIS−FeTCPP Ka−TRIS−FeTCPPAc a

loading (mg/g)

4

24

48

25.9 25.9

36 29 17 30

67 51 34 52

63 49 40 65

Conditions: catalyst/substrate/oxidant molar ratio = 1:3980:80.

All the studied catalysts efficiently and selectively catalyzed (Z)-cyclooctene epoxydation; aldehyde or ketone were not detected. In the case of the homogeneous reactions, the best yields were already achieved after 24 h, and slightly decreased after 48 h of reaction, while for the heterogeneous system the maximum was achieved after 48 h of reaction. Longer reaction times were expected to be needed for reaching maximum yields in the heterogeneous systems, because the matrix may delay both substrate access to and product diffusion from the active sites. As observed, the best results reached 67 and 65% epoxide yield for the homogeneous and heterogeneous systems, respectively, results which are very satisfactory. Schiavon et al.54 reported 13% epoxide yield after 24 h of reaction for the homogeneous FeTCPP catalyst by using similar reaction conditions, except for the use of DCM as solvent, suggesting that the polar character of the −COOH groups could hinder the interaction of the nonpolar Fe(III) porphyrin with the substrate. We carried out some preliminary tests with various solvents, noting that DCE provided the best result, in agreement with Traylor et al.66 and Iamamoto et al.67 According to these authors, DCE is more inert than DCM, and the latter can undergo reaction with active species to generate HCl and CO2; in other words, DCM can compete with the substrate for the active oxidant species. Additionally, Lindsay Smith et al.68 reported that a more viscous solvent, like DCE, improved the reaction efficiency in the case of cyclohexane oxidation. Although these authors noted that solvent viscosity negatively affected the cyclohexane oxidation, our results suggested that DCE facilitates the formation of the carbocation associated with epoxide production, giving good evidence that the functional groups in the phenyl ring influenced the catalyst H



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decreases in their catalytic activity. No products were detected in the absence of the Fe(III) porphyrin or of the oxidant. As in the case of the studies involving PhIO, high (Z)-cyclooctene conversion into the epoxide product suggested that the ferryl radical complex was the active species during the oxidation reaction. The reaction was also performed in the presence of hydroquinone as a radical scavenger, and the epoxide yields did not change, which allowed ruling out the radical mechanism and provided strong evidence that the active species originated via hydrogen peroxide oxidative heterolytic cleavage. Another possibility that could be used to explain the difference of yields between Ka−TRIS−FeTCPP and Ka− TRIS−FeTCPPAc is based on the concept of the proximal ligand effect, as previously observed using iron porphyrin68 and manganese porphyrin16 as catalysts in the epoxidation of olefins using iodosylbenzene as oxidant. The TRIS ligand can act as a proximal ligand favoring the iron porphyrin activation, improving the formation of the metal oxo species (intermediate active species) and the oxygen atom transfer to the substrate. In the covalent bond between Ka−TRIS and Fe(TCPPAc), the electrons from iron can be easily withdrawn promoting the formation of metal oxo species. In the specific case of Fe(TCPP), the interaction of iron and/or carboxylate from the porphyrin with the TRIS ligand cannot promote the same activation. This possibility has been taken into account when describing the possible interaction between the porphyrins and the clay in Figure 1. Baeyer−Villiger Cyclohexanone Oxidation. It has been reported that heterogeneous metalloporphyrins mimic the action of the Baeyer−Villiger (BV) monooxygenase enzymes, which catalyze the transformation of linear and cyclic ketones into their corresponding esters and lactones.18 BV reaction71 is one of the most important reactions in Organic Chemistry, being a tool to prepare pharmaceutical products for fine chemicals (antibiotics, steroids) as well as crucial industrial intermediates, and being especially interesting for the oxidation of cyclic ketones into lactones; the latter are important intermediates to produce polymers, pesticides, and herbicides, among other relevant products for the modern society.72−74 Nevertheless, the standard industrial protocol has several disadvantages, such as the use of organic peracids (which are expensive, dangerous, and potentially explosive) and the formation of the corresponding carboxylic acid salt as byproducts. Several catalysts have been evaluated for this reaction, but protocols based on clean oxidants, such as hydrogen peroxide, are still lacking.72−74 Table 2 shows that all the catalysts efficiently catalyzed cyclohexanone oxidation, selectively producing the lactone. Among the times of reaction considered, the best results were

epoxidation by PhIO catalyzed by Ka−TRIS−FeTCPP or Ka− TRIS−FeTCPPAc, we accomplished (Z)-cyclooctene oxidation reactions in the presence of hydroquinone, a well-known radical scavenger. The epoxide yields did not change in the presence of this compound, thus allowing the radical mechanism to be dismissed and confirming the mechanism proposed by Iamamoto et al.67 (Z)-Cyclooctene Oxidation with Hydrogen Peroxide. Large attention has been paid to designing heterogeneous catalysts that can function in combination with mild oxidants such as H2O2. H2O2 has important advantages over other oxidants, the main ones being the following: (1) water is the only byproduct of the reactions, (2) it contains higher active oxygen than other commercially available oxidants, and (3) most of the other oxidants originate from H2O2. Nevertheless, the use of this oxidant poses a major challenge: to avoid its destructive disproportionation and catalytic oxidation, or the presence of water in the reaction medium, which deactivates H2O2. According to Traylor et al.,65 for reaching an efficient alkene epoxidation using H2O2 as an oxidant, it is necessary to avoid secondary reactions after formation of the active intermediate species FeIV(O)P+, which can react with the peroxide forming radical species and consequently decreasing the reaction selectivity, or even destroying the catalyst by reaction of the ferryl radical with the Fe(III) porphyrin. Traylor et al.65 reported that it is possible to minimize competitive reactions using electron-deficient Fe(III) porphyrins, which favor heterolytic cleavage of the peroxide and rapid formation of FeIV(O)P+, allowing the reaction conditions to be controlled and avoiding catalyst destruction. Looking for more applicable conditions, aqueous 70% and anhydrous H2O2 were used as oxidants, using Ka−TRIS− FeTCPP and Ka−TRIS−FeTCPPAc as catalysts. Aqueous 70% H2O2 afforded very low epoxide yields, about 8%, while replacing the oxidant by the anhydrous form significantly increased the reaction efficiency (48% for Ka−TRIS−FeTCPP and 67% for Ka−TRIS−FeTCPPAc), showing that the hydrophobic substrate requires a hydrophobic environment. All the catalysts were active, efficient, and selective for (Z)cyclooctene oxidation using anhydrous H2O2; the epoxide was the sole product. As expected, homogeneous systems led to higher yields than heterogeneous systems; FeTCPPAc showed the best performance, confirming the reactivity of the acyl group and its better efficiency to generate the active ferryl species (97% after 1 h of reaction and 66% epoxide yield after 48 h for FeTCPPAc, vs 92 and 61% epoxide yield, respectively, for FeTCPP). The yields afforded by Ka−TRIS−FeTCPP and Ka−TRIS− FeTCPPAc, 48 and 67%, respectively, were similar to those obtained when using PhIO. This fact was very interesting, because hydrogen peroxide has not been widely used due to issues concerning its dismutation and the deactivation of the matrix in the presence of water. In this case, we proved that the presence of anhydrous peroxide circumvents these problems, making hydrogen peroxide a great environmentally friendly alternative. Again, the Fe(III) porphyrin containing the acyl chloride substituent showed better results, probably because it contains more available active sites due to clay exfoliation, as explained above. The lower activity of the heterogeneous catalysts compared to the homogeneous systems can be compensated by catalyst reuse; both Ka−TRIS−FeTCPP and Ka−TRIS− FeTCPPAc were reused four times, detecting only very small

Table 2. Conversion (%) of Cyclohexanone to εCaprolactone in the Baeyer−Villiger Reaction Using H2O2 (70 wt %) as Oxidant under Homogeneous or Heterogeneous Conditionsa time of reaction (h) catalyst

2

FeTCPP FeTCPPAc Ka−TRIS−FeTCPP Ka−TRIS−FeTCPPAc a

I

4

24

48

34 20 38 20

40 32 37 24

55 38 50 30

Conditions: catalyst/substrate/oxidant molar ratio = 1:810:765. dx.doi.org/10.1021/jp5077356 | J. Phys. Chem. C XXXX, XXX, XXX−XXX


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Data in Table 3 evidence that Ka−TRIS−FeTCPPAc possessed a strong antibacterial effect against the tested strains,

reached after 48 h of reaction (30−55% conversion). Interestingly, FeTCPP afforded the best conversion in both homogeneous and heterogeneous systems; that is, the yield mainly depended on the ferroporphyrin nature, with the homogeneous or heterogeneous conditions being less important. The catalysts were used in four consecutive runs without loss in terms of lactone conversion or selectivity, while no leaching or Fe(III) was detected, confirming the completely heterogeneous character of the reaction. Jeong and Park used FeTCPP under homogeneous conditions to catalyze BV oxidation of cyclohexanone by molecular oxygen, achieving 62% ε-caprolactone yield after 5 h of reaction.75 As observed in Table 2, we obtained 34% yield after 4 h of reaction time, and 55%, still lower than the Jeong and Park results, after 48 h of reaction. This is probably due to the much larger oxygen availability when using molecular oxygen than with H2O2. However, Jeong and Park used a chlorinated solvent, dichloroethane. Jeong et al. also prepared a hybrid organic−inorganic catalyst consisting of an FeTCPP-bridged periodic mesoporous organosilica by cooperative assembly with organosilane, in the presence of surfactants, by the sol−gel or microwave methods, using it in BV heterogeneous reaction, in four consecutive runs.76 Despite the high conversion and turnover frequency during ε-caprolactone conversion (100% and 411 h−1, respectively, in 5 h), the catalyst required pre-activation at 165 °C (the reaction was carried out at 40 °C), and the use of a chlorinated solvent too. Various blank reactions were carried out to obtain more information about this catalytic system, obtaining the following results:

Table 3. Mean and Standard Deviation (mm) of the Halos of Inhibition of Microbial Growth Promoted by Ka−TRIS− FeTCPPAc mean ± SDa (mm)

a

microorganisms (ATCC)

Ka−TRIS−FeTCPPAc

positive control

B. subtilis (6051) K. pneumoniae (13883) E. coli (25922)

15.00 ± 0.00 13.67 ± 1.52 13.67 ± 2.08

16.00 ± 0.00b 16.00 ± 1.00c 13.67 ± 1.15c

SD: standard deviation. bPenicillin. cStreptomycin.

especially against E. coli. The Ka−TRIS−FeTCPPAc antibacterial activity decreased in the order E. coli > B. subtilis > K. pneumoniae. The effect of Ka−TRIS−FeTCPP against the E. coli strains was comparable to that of streptomycin, while the effect of Ka−TRIS−FeTCPPAc against B. subtilis was slightly lower than that of penicillin. Blank tests were performed in the absence of the support or of the porphyrin. The support was inactive against the assayed bacteria, and in the absence of the support, FeTCPP displayed no antimicrobial activity. This clearly showed that the antimicrobial action was elicited by the synergism between the clay and FeTCPP. The time−kill assays for B. subtilis (Figure 8) showed the effective reduction in the number of viable cells after 24 h of

(i) The reaction did not take place in the absence of benzonitrile. (ii) ε-Caprolactone was not formed in the absence of Fe(III) porphyrin. (iii) ε-Caprolactone yield was low (about 2%) in the absence of Fe(III) porphyrin but in the presence of its precursor Ka−TRIS. The nitrile acts as an oxygen transfer agent, as previously observed by Llamas et al.,77 while it is clear that the Fe(III) porphyrin plays a fundamental role in this transfer. The versatile nature of these catalytic systems in the oxidation of the various substrates considered strongly evidenced that ferryl was the active species responsible for oxygen atom transfer from the oxidant to the substrates, as it happens with cytochrome P450 enzymes. C. Antimicrobial Tests. The antimicrobial activity of these catalysts was tested, in order to investigate their multifunctional nature. In addition, it should be taken into account that disposal of deactivated catalysts is becoming an important environmental problem. To use these materials after their catalytic use as antimicrobial agents (to treat contaminated soils or surgical materials) would avoid catalyst release into the environment, not to mention that porphyrins and clay components are biocompatible. The antimicrobial activity of Ka−TRIS−FeTCPPAc against the Gram-positive bacterium Bacillus subtilis and the Gramnegative bacteria Escherichia coli and Klebsiella pneumoniae was investigated. For comparison, the activity of penicillin or streptomycin, recognized antibiotics effective against these bacteria, was also evaluated.

Figure 8. Time−kill curves for B. subtilis using Ka−TRIS−FeTCPPAc as antibacterial agent.

incubation, and that total bactericidal effect occurred only after 48 h using the concentration of 100 mg/mL. Two facts are noteworthy from literature data22 on this subject: (1) porphyrins usually have more pronounced activity against Gram-positive bacteria, which have weaker cell walls than Gram-negative bacteria; (2) the antimicrobial activity of porphyrins is related to light exposure and generation of lethal highly reactive singlet oxygen (type II), which reacts with cellular components and leads to microorganism death. Mbakidi et al.22 observed excellent antimicrobial properties against Gram-positive and Gram-negative bacteria after visible light irradiation when using a cationic porphyrin covalently grafted onto cellulose. These authors attributed this activity to the photo inhibition promoted by the type II photochemical process involving singlet oxygen, which ultimately damages the J



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cell envelope. The photosensitizer does not penetrate bacterial cells, so it does not participate in this process. It has been reported that metalloporphyrins under “dark conditions” also display antimicrobial activity;34 although it is assumed that the antimicrobial activity of porphyrins preferentially occurs via photoactivation, a multifunctional character has also been proposed, in which bacterial deactivation may start by porphyrin binding to the cell DNA. Malik et al.,34 using hemeporphyrin, proposed that the antimicrobial activity took place through a DNA mutation occurring in the presence of a reducing agent and oxygen. This mechanism cannot be ruled out in our system, where the residual amine groups present in the matrix can act as a reducing agent. When Fe(III) porphyrin interacts with DNA, the Soret band becomes less intense and shifts toward the red region.48 To confirm the interaction of porphyrin with DNA, its titration with calf thymus DNA (ctDNA) was performed. The Soret band became less intense and shifted toward the red region (Figure 9), confirming the interaction between the iron porphyrin and DNA and the importance of both, the porphyrin and the matrix, to achieve the antimicrobial activity.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +34 923294574. *E-mail: [email protected]. Phone: +55 016 3711 8969. *E-mail: katia.ciuffi@unifran.edu.br. Phone: +55 016 3711 8969. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been carried out in the frame of a Spain−Brazil Interuniversity Cooperation Grant, financed by MEC (PHB2011-0164-PC) and CAPES (267/12), and a Cooperation Grant from Universidad de Salamanca and FAPESP (2013/50216-0). The Spanish group also acknowledges support from the Ministerio de Economiá y Competitividad and ERDF Funds (MAT2013-47811-C2-R), and the Brazilian group acknowledges support from Brazilian Research funding agencies FAPESP (2011/17660-8 and 2013/19523-3), CAPES, and CNPq.

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REFERENCES

(1) Gómez-Romero, P.; Sanchez, C. In Functional Hybrid Materials, 6th ed.; Gómez-Romero, P., Sanchez, C., Eds.; Wiley-VCH: Weinheim, Germany, 2004; pp 1−14. (2) Sanchez, C.; Julián, B.; Belleville, P.; Popall, M. Applications of Hybrid Organic−Inorganic Nanocomposites. J. Mater. Chem. 2005, 15, 3559−3592. (3) Sanchez, C.; Boissiere, C.; Cassaignon, S.; Chaneac, C.; Durupthy, O.; Faustini, M.; Grosso, D.; Laberty-Robert, C.; Nicole, L.; Portehault, D.; Ribot, F.; Rozes, L.; Sassoye, C. Molecular Engineering of Functional Inorganic and Hybrid Materials. Chem. Mater. 2014, 26, 221−238. (4) Ortiz de Montellano, P. R. Cytochrome P450: Structure, Mechanism, and Biochemistry, 3rd ed.; Kluwer Academic/Plenum: New York, 2005. (5) Costas, M. Selective C−H Oxidation Catalyzed by Metalloporphyrins. Coord. Chem. Rev. 2011, 255, 2912−2932. (6) Mansuy, D. A Brief History of the Contribution of Metalloporphyrin Models to Cytochrome P450 Chemistry and Oxidation Catalysis. C. R. Chim. 2007, 10, 392−413. (7) Mansuy, D. Biocatalysis and Substrate Chemodiversity: Adaptation of Aerobic Living Organisms to their Chemical Environment. Catal. Today 2008, 138, 2−8. (8) Poulos, T. L.; Finzel, B. C.; Howard, A. J. High-Resolution Crystal Structure of Cytochrome P450cam. J. Mol. Biol. 1987, 195, 687−700. (9) Mansuy, D. Activation of AlkanesThe Biomimetic Approach. Coord. Chem. Rev. 1993, 125, 129−142. (10) de Lima, O. J.; de Aguirre, D. P.; de Oliveira, D. C.; da Silva, M. A.; Mello, C.; Leite, C. A. P.; Sacco, H. C.; Ciuffi, K. J. Porphyrins Entrapped in an Alumina Matrix. J. Mater. Chem. 2001, 11, 2476− 2481. (11) Huang, G.; Liu, S.-Y.; Guo, Y.-A.; Wang, A.-P.; Luo, J.; Cai, C.C. Immobilization of Manganese Tetraphenylporphyrin on Boehmite and its Catalysis for Aerobic Oxidation of Cyclohexane. Appl. Catal., A 2009, 358, 173−179. (12) Kameyama, H.; Narumi, F.; Hattori, T.; Kameyama, H. Oxidation of Cyclohexene with Molecular Oxygen Catalyzed by Cobalt Porphyrin Complexes Immobilized on Montmorillonite. J. Mol. Catal. A: Chem. 2006, 258, 172−177. (13) Barbaro, P.; Liguori, F. Ion Exchange Resins: Catalyst Recovery and Recycle. Chem. Rev. 2009, 109, 515−529. (14) Papacidero, A. T.; Rocha, L. A.; Caetano, B. L.; Molina, E.; Sacco, H. C.; Nassar, E. J.; Martinelli, Y.; Mello, C.; Nakagaki, S.;

Figure 9. UV−vis absorption spectra of FeTCPP titrated by ctDNA.

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CONCLUSIONS Two different porphyrins (FeTCPP and FeTCPPAc) were immobilized onto kaolinite layers, as shown by PXRD, TGA, chemical analysis, FTIR spectroscopy, and SEM characterization studies. The reactive amino complex was very efficient for covalent binding of the iron porphyrins via condensation (NH2 and carboxylate or acyl chloride from porphyrins), giving rise to versatile heterogeneous hybrid catalysts. The versatile nature of the catalysts was evaluated in two oxidation reactions, (Z)-cyclooctene epoxidation and cyclohexanone Baeyer− Villiger oxidation, with excellent results. The multifunctional nature of the hybrid materials was evaluated using them as antimicrobial agents against oral pathogens Bacillus subtilis, Klebsiella pneumonia, and Escherichia coli, also giving excellent results.

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ASSOCIATED CONTENT

S Supporting Information *

Figures and tables giving additional characterization data. This material is available free of charge via the Internet at http:// pubs.acs.org. K



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