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Article

Study of Cyclohexane and Methylcyclohexane Functionalization Promoted by Manganese(III) Compounds

by
Eduardo S. Neves
,
Christiane Fernandes
and
Adolfo Horn, Jr.
*
Departamento de Química, Universidade Federal de Santa Catarina, Florianópolis 88040-900, SC, Brazil
*
Author to whom correspondence should be addressed.
Inorganics 2023, 11(3), 105; https://doi.org/10.3390/inorganics11030105
Submission received: 16 January 2023 / Revised: 20 February 2023 / Accepted: 27 February 2023 / Published: 3 March 2023
(This article belongs to the Special Issue Manganese Chemistry: From Fundamentals to Applications)
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Abstract

:
Alkane functionalization using safe and low-energy processes is of great interest to industry and academia. Aiming to contribute to the process of saturated hydrocarbon functionalization, we have studied a set of three manganese(III) complexes as catalysts for promoting the oxidation of saturated hydrocarbons (cyclohexane and methylcyclohexane) in the presence of hydrogen peroxide or trichloroisocyanuric acid (TCCA). The mononuclear manganese(III) compounds were prepared using the ligands H2LMet4 (6,6’-((1,4-diazepane-1,4-diyl)bis(methylene))bis(2,4-dimethylphenol), H2salen (2,2’-((1E,1’E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))diphenol) and H2salan (2,2’-((ethane-1,2-diylbis(azanediyl))bis(methylene))diphenol). The catalytic processes were carried out in acetonitrile at 25 and 50 °C for 24 h. The increase in the temperature was important to get a better conversion. The compounds did not promote cyclohexane oxidation in the presence of H2O2. However, they were active in the presence of TCCA, employing a ratio of 1000:333:1 equivalents of the substrate:TCCA:catalyst. The best catalytic activity was shown by the compound [Mn(salen)Cl], reaching conversions of 14.5 ± 0.3% (25 °C) and 26.3 ± 1.1% (50 °C) (yield for chlorocyclohexane) and up to 12.1 ± 0.5% (25 °C) and 29.8 ± 2.2% (50 °C) (total yield for the mixture of the products 1-chloro-4-methylcyclohexane, 3-methylcyclohexene and 1-methylcyclohexene). The interaction of the catalysts with TCCA was studied using electron paramagnetic resonance (EPR), suggesting that the catalysts [Mn(LMet4)Cl] and [Mn(salan)Cl] act via a different mechanism from that observed for [Mn(salen)Cl].

1. Introduction

The functionalization of the C–H bonds of saturated hydrocarbons in controlled, efficient, selective and environmentally friendly conditions (low temperature, low pressure, employing reagents with low toxicity) is of great interest in the scientific and technological scope [1,2,3,4,5,6]. Unlike olefins and alkynes, alkanes do not have empty low-energy or high-energy filled orbitals available that could easily participate in a chemical reaction. Moreover, the presence of only sigma bonds and the slight difference in the electronegativity between their constituents (C and H atoms) result in less polarized bonds and, therefore, lower reactivity than their unsaturated congeners [7,8,9].
Alkanes, which are constituents of oil and natural gas, are abundant and, therefore, considered low-cost commodities. The light ones have been used mainly as fuel for producing energy and carbon oxides, which limits their chemical potential. From another perspective, developing processes that lead to the functionalization of alkanes could result in more valuable molecules [10,11,12]. For example, the transformation of cyclohexane into cyclohexanol has tremendous industrial appeal. It is a feedstock for synthesizing caprolactam, the monomer for preparing polyamides, such as nylon-6 and nylon-6,6 [13,14], with extensive use in the textile and plastic industries [15]. Furthermore, cyclohexane has been extensively employed as a target molecule since it is liquid at room temperature, contains only secondary carbon atoms, does not show ring constraints and has been useful in studying the reaction mechanism [16,17,18].
Although there are new catalytic systems developed to promote this reaction [19,20], the traditional process employed occurs via the oxidation of cyclohexane under aerobic conditions (air, 0.9 Mpa) at 150–160 °C, using a homogeneous catalyst of Co(II) to form cyclohexanol/cyclohexanone (KA oil), which subsequently reacts with hydroxylamine, sulfuric acid and potassium dichromate to produce caprolactam. This monomer undergoes hydrolytic polymerization to generate nylon-6. The functionalization of cyclohexane into cyclohexanol presents a selectivity of 85% and a total conversion of only 4% [14,21,22]. Several heterogeneous and homogeneous catalytic systems have been investigated, aiming to improve the KA oil yield. Concerning the latter, biomimetic systems inspired by metalloenzymes such as methane monooxygenases [23,24,25,26,27,28,29] and cytochrome P-450 [17,30,31,32] have been reported. In such cases, peroxides (H2O2, tert-BOOH) are usually employed as oxidant agents.
The homogeneous catalytic systems studied for hydrocarbon oxidation include the Gif system and metal complexes containing amine and imine ligands [8,17,33,34,35,36,37,38,39,40] In this way, manganese compounds have attracted attention since the seminal work of Jacobsen [41,42] related to the asymmetric oxidation of alkenes employing an Mn-salen derivative as a catalyst and iodosylarene or NaClO4 as oxidant. Concerning saturated hydrocarbons, the manganese(IV) binuclear complex [Mn2L2O3][PF6]2, where L = 1,4,7-trimethyl-1,4,7-triazacyclononane, efficiently catalyzed the oxidation of alkanes in the presence of H2O2 and acetic acid or oxalic acid, where it reached a maximum yield of 46% (based on the substrate) of oxygenated products [33]. Manganese compounds containing both benzimidazole and pyridine rings were described for the oxidation of different substrates (cyclohexane, cyclopentane, cycloheptane, cyclooctane, t-butylcyclohexane and n-hexane) using H2O2 (oxidizing agent), resulting in yields of up to 89% with a ketone/alcohol ratio of up to 45:1 [34].
In contrast to the activity of olefines, compounds from the Mn(salen) family (Jacobsen catalysts) have shown a lack of activity related to the hydroxylation of alkanes in the presence of hydroperoxides [35,36,37,38]. Such behavior may be related to the catalase activity (2H2O2 → 2H2O + O2) shown by such compounds [43,44,45], which is a competitive reaction. However, they have promoted the halogenation of saturated hydrocarbon in the presence of hypochlorite [46]. Manganese porphyrins have also shown such activity: catalase in the presence of hydrogen peroxide and halogenase in the presence of hypochlorite [44,47,48,49,50,51]. For example, [MnIII(TPP)Cl], where H2TPP = meso-tetraphenylporphyrin, catalyzed the conversion of cyclohexane into chlorocyclohexane with a 54% yield (based on the oxidant) using NaClO4 as oxidant, in CH2Cl2, at 25 °C [47]. Under the same reaction conditions, structurally more rigid porphyrin [MnIII(H4TPP)Cl], where H4TPPP = 5,10,15,20-tetraquis(4-(1H-pirazole-4-il)-phenyl), also catalyzed the conversion of cyclohexane into chlorocyclohexane with a 54% yield [48].
Although the processes related to the oxygenation of saturated hydrocarbon into more valuable chemical molecules such as alcohol, ketone/aldehydes and/or carboxylic acids have been studied for a long time, the halogenation of alkanes is a less explored goal. Haloalkanes also exhibit excellent synthetic versatility as starting materials for producing molecules of higher added value, such as alcohols, alkylamines, alkylnitriles, alkylthiols, secondary amines, ethers, and upper alkynes, among others [11,52]. Such relevance may be illustrated by chlorocyclohexane as a critical intermediate in the synthesis of N-(cyclohexylthio)phthalimide, trihexyphenidyl and azocyclotin, compounds used by the rubber, pharmaceutical and pesticide industries, respectively [11,53]. In nature, natural products that have carbon–halogen bonds are produced enzymatically. Some of these compounds have therapeutic applications. Among them are vancomycin, chlortetracycline and chloramphenicol, which have been applied in treating bacterial infections [54,55,56].
Industrially, chloroalkanes are obtained by three main methods: (a) hydroxyl replacement of aliphatic alcohols by hydrochloric acid [57,58]; (b) addition of hydrochloric acid to alkenes [59,60]; and (c) alkane chlorination via molecular chlorine [52,53]. Methods (a) and (b) confer several advantages, such as using mild operating conditions with excellent selectivity for monochlorinated products. However, the high cost of the raw materials makes these two methods unattractive. Method (c) has the advantage of using low-cost, abundant raw materials and providing highly efficient conversion. Still, it is necessary to use molecular chlorine, which is toxic, corrosive, explosive and a strong oxidant, hindering its handling and operation. This reaction presents low selectivity for monochlorinated products because the involvement of chlorine radicals results in polychlorinated molecules. Furthermore, it generates hydrochloric acid (strong acid) as a by-product [52,61], failing to meet the green chemistry guidelines.
N-halo reagents such as N-halo amines, N-halo amides, N-halo saccharines and N-halo sulfonamides have excelled in organic syntheses in terms of performing oxidation reactions [62], including halogenation [63,64,65], epoxidation [66,67] and acylation [68,69]. N-halo amides are prominent among these reagents, particularly trihaloisocyanuric acids, since they show high atomic efficiency in transferring their halogen atom to the substrates. Trichloroisocyanuric acid (TCCA), tribromoisocyanuric acid (TBCA) and triiodoisocyanuric acid (TICA) can transfer 45.5%, 65.5% and 75.1% of their masses to the substrate, respectively (Figure 1) [62,70,71].
Specifically, trichloroisocyanuric acid (1,3,5-trichloro-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione; TCCA) was first reported in 1902 by Chattaway and Wadmore [72]. Its synthesis involves the reaction between cyanuric acid, NaCl and OxoneTM (potassium monopersulfate) [73,74,75]. TCCA has been described as a safe oxidant and chlorinating agent of molecules containing different functional groups, including alkenes, alkynes, heterocyclic aromatics, disulfides, amines, amides, esters and anhydrides. It is considered a green oxidant mainly because it is solid, stable, low-cost, operates under moderate reaction conditions, and has an excellent stoichiometric molar ratio, making it attractive for large-scale industrial use [70,71]. Another advantage of TCCA in organic reactions is that the by-product (cyanuric acid) usually precipitates and can be quickly recovered via filtration and reused to prepare more TCCA [73,74,75], making the process less expensive and decreasing the waste generation. Thus, TCCA is an attractive reagent from the green chemistry point of view.
Only recently, we and others have shown that TCCA can promote the halogenation of saturated hydrocarbon [63,64,65] While Combe employed a radical initiator system [65], we have shown that iron and copper complexes can promote the catalytic halogenation of alkanes [63,64]. For instance, the catalytic activity of the complexes [Fe2(BPA)2(μ-OCH3)2(Cl)2] (HBPA = N-(2-hydroxybenzyl)-N-(2-pyridilmethyl)amine) and [FeIII(HBPCINOL)(Cl)2] (HBPCINOL = N-(2-hydroxy-benzyl)-N-(2-pyridilmethyl)(3-pyridilmethyl)(3-hydroxy)propylamine) was evaluated in the cyclohexane chlorination reaction. The best result was reached with compound [FeIII(HBPCINOL)(Cl)2] (34% conversion based on the substrate, at 50 °C, 100% selective for chlorocyclohexane) [63]. A better performance was obtained with the copper compound [CuII(BPAH)(H2O)](ClO4)2 (BPAH = 1.4-bis(propanamide)homopiperazine), which showed a yield of 44.7% at 50 °C [64].
Aiming to advance in developing greener catalytic systems for saturated hydrocarbon functionalization, we are describing a study with three manganese(III) compounds employing H2O2 and TCCA as the oxidant agents and cyclohexane and methylcyclohexane as the substrates. The study was carried out at two different temperatures (25 and 50 °C), aiming to evaluate the effect of the temperature on the yield and the selectivity.

2. Results

Figure 2 shows the reaction scheme employed in the syntheses of the manganese complexes. Infrared and CHN analyses of the manganese compounds agree with the literature and the proposed compositions/structures [76,77,78]. [Mn(LMet4)Cl]: Yield: 0.43 g, 94%. Elemental analysis (CHN) found (%): C, 60.06; H, 6.60; N, 6.17. Calculated for [Mn(C23H30N2O2)Cl], MW = 456.89 g mol−1: C, 60.46; H, 6.62; N, 6.13. [Mn(salen)Cl]∙2H2O: Yield: 0.18 g, 47%. Elemental analysis (CHN) found (%): C, 48.79; H, 4.36; N, 7.11. Calculated for [Mn(C16H14N2O2)Cl]∙2H2O, MW = 356.69 g mol−1: C, 48.93; H, 4.62; N, 7.13. [Mn(salan)Cl: Yield: 0.13 g, 36%. Elemental analysis (CHN) found (%): C, 53.19; H, 4.61; N, 7.91. Calculated for [Mn(C16H18N2O2)Cl] ], MW = 360.72 g mol−1: C, 53.28; H, 5.03; N, 7.77.
The catalytic activity of the manganese compounds was initially investigated using cyclohexane as a substrate and H2O2 as an oxidant. According to the data obtained via GC–MS, no oxidation products were observed, neither at 25 nor at 50 °C.
On the other hand, the systems containing the manganese complexes showed higher effectiveness in promoting the functionalization of cyclohexane when TCCA was employed as an oxidant (Figure 3). The activity was dependent on the temperature, resulting in higher conversion at 50 °C than at 25 °C. At both temperatures, the conversion showed dependency on the structure of the complexes as follows: [Mn(salen)Cl] > [Mn(salan)Cl] > [Mn(LMet4)Cl]. The best result and selectivity were obtained with the complex [Mn(salen)Cl], reaching conversions of 14 and 26% (based on the substrate) at 25 and 50 °C, respectively, which is about 5-fold higher than the result obtained in the control reaction (cyclohexane + TCCA) at 50 °C. It is important to highlight that only chlorocyclohexane was detected in the reaction containing [Mn(salen)Cl], which indicates high selectivity. An unexpected result was observed when MnCl2∙4H2O was used as a control. Although the system containing [Mn(salen)Cl] was around 3-fold more active than MnCl2∙4H2O at 25 °C, the latter’s activity increased significantly at 50 °C, reaching 18.8%. The data obtained with MnCl2∙4H2O are similar to the compound [Mn(salan)Cl] and slightly better than [Mn(LMet4)Cl]. For the compounds [Mn(salan)Cl] and MnCl2∙4H2O, traces (~1%) of 1,2-dichlorocyclohexane were detected.
Aiming to shed light on the interaction between the manganese compounds and TCCA, EPR studies were performed. Figure 4 shows the EPR data of the manganese species alone in a CH3CN solution and in the presence of TCCA. All three complexes are EPR-silent, which agrees with the presence of Mn(III) species. The salt MnCl2∙4H2O shows the six-line pattern typical of mononuclear Mn(II) ion [79]. Distinct behaviors were observed when TCCA was added to the solutions containing the manganese compounds. A six-line pattern at g~2.0, typical of Mn(II) species [79,80], arose in the systems containing [Mn(LMet4)Cl] and [Mn(salan)Cl]. On the other hand, no spectral changes were observed for [Mn(salen)Cl] and MnCl2∙4H2O.
Due to the good cyclohexane oxidation in the presence of TCCA, the catalytic system was also evaluated on methylcyclohexane (Figure 5). The conversion reached about 12% when employing [Mn(salen)Cl] at 25 °C. Similar conversions were obtained in the presence of [Mn(LMet4)Cl] and [Mn(salan)Cl], as well as in the control reaction with MnCl2 (~6%). However, no products were detected in the reaction composed of methylcyclohexane + TCCA.
At 50 °C, the best results were obtained with the complexes [Mn(LMet4)Cl], [Mn(salen)Cl] (~30%), and MnCl2 (~29%), i.e., twice as much compared to the control reaction (methylcyclohexane + TCCA). The [Mn(salan)Cl] complex showed the lowest yield (~22%).
Using the methylcyclohexane substrate, three different products were identified at both temperatures: 1-chloro-4-methylcyclohexane, 3-methylcyclohexene and 1-methylcyclohexane (Figure 6 and SI Figures S9–S11). In all the reactions, the main product was 1-chloro-4-methylcyclohexane.

3. Discussion

All the ligands employed in this study were isolated in the solid state and characterized by means of IR and NMR (see Supporting Information). The composition and purity of the complexes were confirmed via CHN elemental analysis and agreed with the literature and the proposed structures (Figure 2). The complexes show satisfactory solubility in CH3CN, the solvent employed in the oxidation reactions.
Due to our previous good experience with iron and copper complexes in promoting cyclohexane oxidation [63,64], we decided to evaluate if a set of manganese(III) compounds could also perform such a reaction using TCCA as an oxidant. The three manganese complexes employed in this study show the same coordination environment, N2O2Cl (Figure 2). However, the ligands contain distinct features. While the well-known H2salen ligand has two phenol and two imine groups, the H2salan is its equivalent, containing two amine groups. On the other hand, the ligand H2LMet4 is similar to H2salan but has the homopiperazine backbone instead of the ethylenediamine present in the latter. The molecular structures solved by means of X-ray diffraction of the three tested compounds were described previously [76,78,81]. Furthermore, their activities in promoting olefin oxidation (cyclohexene) have also been described [41,42,76,82,83].
Using H2O2 as an oxidant, the tested compounds did not show cyclohexane oxidation under the tested conditions. It is important to highlight the development of intense positive pressure in the glass flask after the reaction, which may be attributed to the reaction 2H2O2 → O2 + 2H2O (catalase activity), as already described in relation to [Mn(salen)Cl] [43,45].
By contrast, when the catalytic activity was tested in the presence of TCCA, the formation of halogenated products was observed, mainly at 50 °C, reaching 26% (based on the substrate) in the presence of [Mn(salen)Cl], with 100% selectivity to chlorocyclohexane. Around a 20% conversion was observed in the presence of [Mn(salan)Cl] and MnCl2, for which some traces of 1,2-dichlorocyclohexane were detected (~1%). The complex [Mn(LMet4)Cl] showed the lowest activity.
The EPR studies point out some clues about the behavior of the manganese species in the presence of TCCA. For the compounds containing the amine group ([Mn(LMet4)Cl] and [Mn(salan)Cl]), the formation of mononuclear Mn(II) species was detected, indicating that these manganese complexes undergo reduction (Mn(III)→Mn(II)) in the presence of TCCA. Such results were unexpected since it is known that TCCA is an oxidant agent, which can form Cl. radical or ClO- anion species [71,73,84,85]. In the same line is the observation that the salt MnCl2∙4H2O was not oxidized in the presence of TCCA. On the other hand, no change in the spectral feature was observed for [Mn(salen)Cl]. Such distinct behavior may account for the lower activity shown by the [Mn(LMet4)Cl], [Mn(salan)Cl and MnCl2∙4H2O species (<6.0%) than the [Mn(salen)Cl] (~14%) at 25 °C.
In studies using iron and copper complexes reported by us previously [63,64], it was proposed the formation of intermediate species as M–OCl (M = Fe, Cu). Furthermore, it has been shown that compounds from the Mn(salen) family can promote the halogenation of hydrocarbon in the presence of hypochlorite [46,86]. For such a system, the proposed catalytic study suggests the formation of Mn(IV) and Mn(V) intermediates involving an Mn–OCl species. Although we have not detected the formation of Mn(IV) species in the reaction involving the [Mn(salen)Cl] compound and TCCA, it is possible to suggest that our system operates similarly to that reported in the literature [46,86], since it is known that TCCA can form ClO− in solution [73,85]. Thus, the formation of a species containing the Mn–OCl bond [46,47,49,86] may be considered an intermediate in the reaction containing [Mn(salen)Cl]. However, the formation of chlorine and nitrogen-centered radical species through the homolytic cleavage of the N–Cl bond, as suggested by Watkins [84], cannot be ruled out, mainly at 50 °C, which could account for the increase in the conversion, as observed. Similar behavior was also seen for the systems containing the iron and copper complexes [63,64].
A study was conducted to evaluate whether the catalytic systems would show some specificity related to the carbon atom type (primary, secondary, tertiary) using methylcyclohexane. As Figure 5 shows, this substrate was slightly more susceptible to oxidation than cyclohexane, achieving about a 30% (50 °C) conversion on a substrate basis. Interestingly, the functionalization occurred preferentially on the secondary carbon atom. The preference for functionalization at the tertiary carbon atom has been associated with high-valence metal-oxo species and short-lived radicals. At the same time, the lack of selectivity is connected with long-lived radicals [87]. For comparison, the system containing [Mn2L32O3][PF6]2 (L3 = 1,4,7-trimethyl-1,4,7-triazacyclononane) and Oxone® promotes oxidation preferentially at the tertiary carbon [33], for which a high-valence Mn=O species was proposed. Thus, the reaction on the secondary carbon atom points out the involvement of a long-lived radical species such as N·, Cl· [84].
As shown in Figure 6, the selectivity on methylcyclohexane was lower than on cyclohexane, resulting in a mixture of 1-chloro-4-methylcyclohexane (main product), 3-methylcyclohexene and 1-methylcyclohexene (see Figures S7–S11). No halogenation at the primary or tertiary carbon atoms was observed. The formation of double bonds suggests the scavenging of hydrogen atoms, likely captured by some species of long-lived free radicals (N·, Cl·), as proposed by Watkins, as the product of the homolytic cleavage of the N–Cl bond present in the TCCA [84]. Similar behavior concerning the dehydrogenation of a cyclic saturated hydrocarbon (norcarane) has been described in the presence of manganese porphyrin and hypochlorite [88].

4. Materials and Methods

The reagents and solvents employed were purchased from a commercial source (Sigma-Aldrich Ltd., São Paulo, SP, Brazil). The 1H NMR spectra were recorded with a Varian NMR AS 400 spectrometer operating at 400 MHz. The elemental analysis (CHN) of the complexes was performed with a PerkinElmer 2400 CHN analyzer. The infrared spectra were recorded with a Shimadzu FT-IR 8300 spectrophotometer. The solid samples were prepared in a KBr pellet and the spectra were recorded over the frequency range of 400–4000 cm−1.

4.1. Syntheses

The ligands 6,6’-((1,4-diazepane-1,4-diyl)bis(methylene))bis(2,4-dimethylphenol) (H2LMet4) [89], 2,2’-((1E,1’E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))diphenol (H2salen) [81,90], and 2,2’-((ethane-1,2-diylbis(azanediyl))bis(methylene))diphenol (H2salan) [91] were prepared as described in the literature. The catalysts [Mn(LMet4)Cl] [76], [Mn(salen)Cl] [77,92] and [Mn(salan)Cl] [78] were also synthesized according to the methodologies already described using one mmol of the ligands and one equivalent of MnCl2∙4H2O (Figure 2).

4.2. Catalytic Activity

The cyclohexane or methylcyclohexane chlorination reaction was carried out in a capped 5 mL glass vial. The reaction was performed by adding the manganese compound (MnCl2∙4H2O, [Mn(Lmet4)Cl], [Mn(salen)Cl] or [Mn(salan)Cl]) at a concentration of 7 × 10−4 mol dm−3 (0.1% catalyst), cyclohexane or methylcyclohexane at a concentration of 0.7 mol dm−3 and TCCA at a concentration of 0.233 mol dm−3 in CH3CN with a final volume of 2 cm3, resulting in a ratio of substrate:TCCA:Mn of 1000:333:1. The reactions were carried out at 25 and 50 °C for 24 h. Oxidation reactions of cyclohexane or methylcyclohexane with H2O2 at a concentration of 0.7 mol dm−3 were also carried out in a similar way using a cyclohexane:H2O2:Mn ratio of 1000:1000:1. After 24 h, a sample (1 cm3) was taken from these reactions and filtered through a silica column (glass Pasteur pipette packed with ~0.24 g of silica-gel 70–230 mesh). The filtered reactions were diluted in CH3CN (MS-grade) at a factor of 1:300 to be analyzed. The products were quantified on a Shimadzu Nexis GC-MS QP2020 NX gas chromatograph coupled to a mass spectrometer (GC–MS) equipped with an SH-Rix1 ms 30 m column. The products of the reactions were identified via the fragmentograms of the peaks compared to the equipment library. The reactions were performed in triplicate.

4.3. EPR Studies

The Electron Paramagnetic Resonance (EPR) spectra were obtained using a Bruker EMX micro-9.5/2.7/P/L system using a highly sensitive cylindrical cavity, operating in the X-band (9 GHz), with 5 mW microwave power, 5 G modulation amplitude and 100 kHz modulation frequency. The solutions of the complexes (5.6 × 10−4 mol dm−3) and TCCA (2.8 × 10−2 mol dm−3) were prepared in spectroscopic CH3CN. The spectra were obtained at 120 K. Further than the pure compounds, the analyses were conducted after mixing the compounds with TCCA. For the reactivity study, 120 μL of the solution containing TCCA was added to 600 μL of the solution containing the manganese compound of interest. The solution was transferred to an EPR tube (4 mm) and frozen in liquid nitrogen after 1 min of incubation. After the measurement, the sample was quickly thawed and kept at 25 °C for a further 14 min. The sample was frozen again at 120 K to obtain new spectra.

5. Conclusions

The compounds tested in this work showed lack of activity regarding the functionalization of cyclohexane when hydrogen peroxide was employed as oxidant. Since high pressure was developed inside the vials, it is suggested that the lack of activity is related to the decomposition of hydrogen catalyzed by the manganese compounds. On the other hand, when using TCCA as an oxidant, the manganese compounds promoted the transformation of cyclohexane into chlorocyclohexane and of methylcyclohexane into a mixture of products (1-chloro-4-methylcyclohexane, 3-methylcyclohexene and 1-methylcyclohexene), as indicated in the GC-MS analyses, mainly at 50 °C. Based on the literature [46,84,86], it is proposed the formation of an intermediate [Mn(salen)(ClO)] species, which, together with the nitrogen and chlorine radical formed by the homolytic cleavage of the N–H bond present in the TCCA molecule, can be involved in the functionalization of both substrates. In general, the compound containing the bis-imine ligand ([Mn(salen)Cl]) showed higher activity than the others with the bis-amine moiety.
Although the manganese compounds evaluated in this work were less active in the halogenation reaction of cyclohexane than the iron and copper complexes reported by us previously [63,64], this work revealed that, in the presence of TCCA, they showed higher activity in the functionalization of cyclohexane than in the presence of hydrogen peroxide, motivating the development of future studies employing other manganese compounds and this cheap, easy-to-handle and greener oxidant (TCCA).

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/inorganics11030105/s1, Figure S1: IR spectra (KBr) of the H2LMet4 ligand (black) and [Mn(LMet4)Cl] complex (red); Figure S2: IR spectra (KBr) of the H2salen ligand (black) and [Mn(salen)Cl] complex (red); Figure S3: IR spectra (KBr) of the H2salan ligand (black) and [Mn(salan)Cl] complex (red); Figure S4: 1H NMR spectrum (CDCl3, 400 MHz) of the ligand H2LMe; Figure S5: 1H NMR spectrum (CDCl3, 400 MHz) of the ligand H2salen; Figure S6: 1H NMR spectrum (CDCl3, 400 MHz) of the ligand H2salan; Figure S7: Mass fragmentogram of the signal observed in the mass spectra of the reaction of cyclohexane, TCCA and manganese compounds; Figure S8: Mass fragmentogram of the signal observed in the mass spectra for the reaction of cyclohexane, TCCA and MnCl2.4H2O or [Mn(salan)Cl], at 50 oC; Figure S9: Mass fragmentogram of the signal observed in the mass spectra for the reaction of methylcyclohexane, TCCA and manganese compounds, with the identification of the product; Figure S10: Mass fragmentogram of the signal observed in the mass spectra for the reaction of methylcyclohexane, TCCA and manganese compounds, with the identification of the product; Figure S11: Mass fragmentogram of the signal observed in the mass spectra for the reaction of methylcyclohexane, TCCA and manganese compounds, with the identification of the product.

Author Contributions

Conceptualization, A.H.J.; Investigation, E.S.N. and C.F.; Writing–original draft, E.S.N.; Writing–review & editing, C.F. and A.H.J.; Supervision, A.H.J.; Project administration, A.H.J.; Funding acquisition, C.F. and A.H.J. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior-Brazil (CAPES)-Finance Code 001 and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) through the projects 408033/2018-5 and 310542/2020-0.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Inorganics 11 00105 g001 550
Figure 1. Chemical structures of trihaloisocyanuric acids.
Figure 1. Chemical structures of trihaloisocyanuric acids.
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Figure 2. Scheme for the synthesis of Mn(III) complexes.
Figure 2. Scheme for the synthesis of Mn(III) complexes.
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Figure 3. Conversion (%) of cyclohexane in halogenated products promoted by manganese species (as indicated) using TCCA as a halogenating agent at a ratio of 1000:333:1 equivalents of the cyclohexane:TCCA:Mn. The reactions were performed for 24 h at 25 or 50 °C, and the conversions were based on the substrate.
Figure 3. Conversion (%) of cyclohexane in halogenated products promoted by manganese species (as indicated) using TCCA as a halogenating agent at a ratio of 1000:333:1 equivalents of the cyclohexane:TCCA:Mn. The reactions were performed for 24 h at 25 or 50 °C, and the conversions were based on the substrate.
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Figure 4. EPR study of the interaction between the manganese compounds and TCCA in CH3CN at 120 K and at different incubation times, as indicated. (A) MnCl2∙4H2O; (B) [Mn(LMet4)Cl]; (C) [Mn(salen)Cl]; and (D) [Mn(salan))Cl].
Figure 4. EPR study of the interaction between the manganese compounds and TCCA in CH3CN at 120 K and at different incubation times, as indicated. (A) MnCl2∙4H2O; (B) [Mn(LMet4)Cl]; (C) [Mn(salen)Cl]; and (D) [Mn(salan))Cl].
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Figure 5. Conversion (%) of methylcyclohexane in oxidized products (see Figure 6) promoted by manganese species (as indicated) using TCCA as an oxidant at a ratio 1000:333:1 equivalents of the methylcyclohexane:TCCA:Mn. The reactions were performed for 24 h at 25 or 50 °C, and the conversions were based on the substrate.
Figure 5. Conversion (%) of methylcyclohexane in oxidized products (see Figure 6) promoted by manganese species (as indicated) using TCCA as an oxidant at a ratio 1000:333:1 equivalents of the methylcyclohexane:TCCA:Mn. The reactions were performed for 24 h at 25 or 50 °C, and the conversions were based on the substrate.
Inorganics 11 00105 g005
Inorganics 11 00105 g006a 550Inorganics 11 00105 g006b 550
Figure 6. Quantification of products formed during the methylcyclohexane oxidation in the presence of different manganese species (as indicated) employing TCCA as an oxidant, at a ratio 1000:333:1 equivalents of the methylcyclohexane:TCCA:Mn, at 25 °C (A) and 50 °C (B), for 24 h. The products were identified via the fragmentation pattern of the signal obtained in the GC–MS analyses according to the equipment software/library.
Figure 6. Quantification of products formed during the methylcyclohexane oxidation in the presence of different manganese species (as indicated) employing TCCA as an oxidant, at a ratio 1000:333:1 equivalents of the methylcyclohexane:TCCA:Mn, at 25 °C (A) and 50 °C (B), for 24 h. The products were identified via the fragmentation pattern of the signal obtained in the GC–MS analyses according to the equipment software/library.
Inorganics 11 00105 g006aInorganics 11 00105 g006b
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Neves, E.S.; Fernandes, C.; Horn, A., Jr. Study of Cyclohexane and Methylcyclohexane Functionalization Promoted by Manganese(III) Compounds. Inorganics 2023, 11, 105. https://doi.org/10.3390/inorganics11030105

AMA Style

Neves ES, Fernandes C, Horn A Jr. Study of Cyclohexane and Methylcyclohexane Functionalization Promoted by Manganese(III) Compounds. Inorganics. 2023; 11(3):105. https://doi.org/10.3390/inorganics11030105

Chicago/Turabian Style

Neves, Eduardo S., Christiane Fernandes, and Adolfo Horn, Jr. 2023. "Study of Cyclohexane and Methylcyclohexane Functionalization Promoted by Manganese(III) Compounds" Inorganics 11, no. 3: 105. https://doi.org/10.3390/inorganics11030105

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