Tuning Cyclohexane Oxidation: Combination of Microwave Irradiation

Oct 3, 2016 - Direct selective oxidation of C–H bonds remains a challenging goal in organic synthesis.(1-3) Although several approaches have been de...
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Tuning Cyclohexane Oxidation: Combination of Microwave Irradiation and Ionic Liquid with the C‑Scorpionate [FeCl2(Tpm)] Catalyst Ana P. C. Ribeiro,† Luísa M. D. R. S. Martins,*,†,‡ Maxim L. Kuznetsov,† and Armando J. L. Pombeiro† †

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Chemical Engineering Department, Instituto Superior de Engenharia da Lisboa, Instituto Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal

‡

S Supporting Information *

ABSTRACT: For the first time, microwave (MW) irradiation was successfully applied to peroxidative cyclohexane oxidation catalyzed by a C-scorpionate complex, [FeCl2(Tpm)] (1; Tpm = hydrotris(pyrazol-1-yl)methane), providing a highly selective and fast ecofriendly procedure to produce a KA (cyclohexanol + cyclohexanone) oil mixture (up to ca. 100% selectivity) with 28% yield in 1 h at 50 °C in MeCN. Water, organics, and ionic liquids (ILs, [bmim][N(CN)2] and [bmim][BF4]) were used as solvents, and their interactions with 1 were determined by DFT calculations. The possibility to recycle and reuse 1 (up to nine consecutive cycles) without loss of activity is also observed for the IL [bmim][BF4]. Moreover, higher catalytic activities under additive-free conditions are obtained with the ILs in comparison to those with the other solvents. Tuning the alcohol/ ketone selectivity is also possible by choosing the appropriate IL solvent.

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INTRODUCTION Direct selective oxidation of C−H bonds remains a challenging goal in organic synthesis.1−3 Although several approaches have been developed1−10 to overcome safety and ecological drawbacks, namely those using catalysts based on cheap and abundant metals such as iron, the search for iron-based catalytic systems operating under more sustainable conditions still continues. Among several requirements for a suitable catalyst, its recyclability with possibility to be used in consecutive cycles is often mandatory for an economically feasible process. Moreover, high selectivity, short reaction times, and practical, easy workups are also relevant. Thus, the use of microwave (MW) radiation as an alternative energy source (that generates heat evenly throughout the reactor) is appealing, eventually enhancing product purity, selectivity, and yield as well as being more energy efficient and economical in comparison to conventional heating methods.11,12 However, the application of MW irradiation in alkane functionalization is essentially an unexplored field.13,14 Another important topic to consider in sustainable catalytic processes concerns the use of solvents. The application of room-temperature ionic liquids (ILs) in organic synthesis provides a good alternative to volatile organic solvents,15−17 but their use in alkane oxidative functionalization remains virtually unexplored.18−20 © XXXX American Chemical Society

Within our interest in the oxidation of alkanes, in particular of cyclohexane due to its industrial relevance,21,22 namely by Cscorpionate complexes (bearing tripodal ligands comparable to cyclopentadienyl on electronic and coordination grounds),23,24 the main objectives of the present study consisted of developing a selective, efficient, and sustainable protocol for the oxidation of cyclohexane to KA oil (cyclohexanone (K) and cyclohexanol (A) mixture) catalyzed by the C-scorpionate Fe(II) catalyst [FeCl2(Tpm)] (Tpm = hydrotris(pyrazol-1yl)methane),25 by (i) replacing common organic solvents by suitable ILs in order to recycle and reuse the catalyst and (ii) using MW irradiation as a reaction accelerating agent, improving the yield and selectivity of the desired products. Two ILs (Figure 1), 1-butyl-3-methylimidazolium dicyanamide ([bmim][N(CN)2], hydrophilic) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4], hydrophobic), were tested as solvents under MW irradiation (up to 6 h) in a temperature range of 30−80 °C, and their interaction with the catalyst molecule was rationalized by DFT calculations. To our knowledge, this is the first time that the successful use of ionic liquids as solvents for the oxidation of alkanes with an iron catalyst under MW irradiation has been reported. Special Issue: Hydrocarbon Chemistry: Activation and Beyond Received: August 4, 2016

A

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Under the optimized conditions, cyclohexanol (A) and cyclohexanone (K) were the only products detected by GC-MS analysis, indicating a high selectivity of the oxidation system under these conditions. The high selectivity toward such products (typically above 95%, reaching 100%) is confirmed by the mole balance between the converted substrate and those products. A maximum total (KA) yield of 33.0% was obtained after 6 h of MW irradiation at 50 °C using 0.2 mol % vs CyH of 1 in MeCN and in the presence of Hpca (entry 5 of Table 1; see optimization assays in Table S1 in the Supporting Information). However, as depicted in Figure 2, after 1 h of MW irradiation (28% total yield, entry 3 of Table 1) under the above conditions, the yield enhancement is not considerable. The yield values now obtained are significantly higher than that reported (total yield of 17%)30,31 without MW assistance. Moreover, they are achieved in a much shorter reaction time (1 h vs 6 h).30,31 Hpca, known as an effective cocatalyst for alkane oxidation with H2O2 (involving the formation of the hydroxyl radical) at room temperature or under conventional heating,10,30−33 also acts efficiently under MW irradiation (compare entries 4 and 6 of Table 1). Its effect is also much more pronounced than those observed with inorganic acids such as nitric and sulfuric acids (compare entries 3, 7, and 8 of Table 1), as reported for the non-MW-assisted catalytic system.25 The efficacy of the 1/MW/MeCN/Hpca catalytic system is dependent on the catalyst amount (Table S1 in the Supporting Information and Figure 2) and type and amount of oxidant (compare entries 10, 20, and 21 of Table S1), as well as on the reaction temperature (compare entries 8, 18, and 19 of Table S1). Decomposition of H2O2 hampered its use at higher temperatures (above 50 °C), which led to a marked decrease in the yield (entry 19, Table S1). “Overoxidation” products, such as 1,4-cyclohexanediol, detected by GC-MS for higher temperatures (80 °C), can also account for the observed lower yields of A and K.

Figure 1. Schematic structure representation of (a) 1-butyl-3methylimidazolium dicyanamide ([bmim][N(CN)2]) and (b) 1butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]).

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RESULTS AND DISCUSSION The knowledge that the complex [FeCl2(Tpm)] (1; Tpm = hydrotris(pyrazol-1-yl)methane) is one of the best Cscorpionate transition-metal catalysts for the peroxidative (with aqueous H2O2) oxidation of cyclohexane (CyH) in acetonitrile and in the presence of pyrazinecarboxylic acid (Hpca)23,25 prompted us to try to further increase the efficiency of the homogeneous catalytic system by using unconventional conditions. Thus, the catalytic activity of 1 was tested under microwave (MW) irradiation, in different solvents: water, acetonitrile, and two room-temperature ionic liquids (RTILs), 1-butyl-3methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1butyl-3-methylimidazolium dicyanamide ([bmim][N(CN)2]), in a temperature range of 30−80 °C over 0.25−6 h, according to Scheme 1 and Tables 1 and 2. Scheme 1. MW-Assisted Oxidation of Cyclohexane with Aqueous H2O2 Catalyzed by [FeCl2(Tpm)] in Different Solvents

Table 1. Selected Dataa for the MW-Assisted Oxidation of Cyclohexane (CyH) with H2O2 Catalyzed by [FeCl2(Tpm)], in MeCN and in the Presence of Hpca yield/%b entry

time/h

additive

cyclohexanone (K)

cyclohexanol (A)

total

A/Kc

total TONd

total TOF/h−1 e

1 2 3 4 5 6 7 8 9 10 11 12f 13g 14h

0.25 0.5 1 3 6 3 1 1 1 3 3 3 3 3

Hpca Hpca Hpca Hpca Hpca no additive HNO3 H2SO4 K2CO3 CBrCl3 NHPh2 Hpca Hpca no additive

0.4 3.7 7.6 11.2 12.4 2.7 4.3 2.7 1.1 1.9 1.3 14.9 0.0 2.3

8.7 13.3 20.3 20.3 20.6 2.1 2.9 3.2 3.2 2.9 1.9 10.3 0.0 1.9

9.1 17.0 27.9 31.5 33.0 4.8 7.2 5.9 4.3 4.7 3.2 25.2 0.0 4.2

21.8 3.6 2.7 1.5 0.6 0.8 0.7 1.2 2.9 1.5 1.5 0.7

46 85 141 159 165 24 36 30 22 24 16 128

184 170 141 53 28 8 36 30 22 8 5 43

0.8

21

7

Reaction conditions (unless stated otherwise): MeCN (3.0 mL), CyH (5.0 mmol), H2O2 (10.0 mmol), 10.0 μmol of 1 (0.2 mol % vs CyH), n(additive)/n(1) = 40, 50 °C, 0.25−6 h of MW irradiation. Yields and TONs were determined by GC analysis (upon treatment with PPh3).9,26−29 b Percentage molar yield ((mol of product)/(mol of cyclohexane)). cRatio between the molar concentrations of A and K. dTotal turnover number, defined as (mol of A and K)/(mol of 1). eTOF = TON/h. fWithout PPh3 treatment. gWithout 1. hUnder an N2 atmosphere. a

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Organometallics Table 2. Selected Dataa for MW-Assisted Additive-Free Oxidation of Cyclohexane (CyH) with H2O2 Catalyzed by [FeCl2(Tpm)] in ILs yield/%b entry 1 2 3 4 5 6 7 8 9f 10g 11h 12 13 14 15 16 17 18 19 20f 21g 22h

solvent

time/h

[bmim][N(CN)2]

0.25 0.5 1 3 6 1 1 1 6 6 6

[bmim][BF4]

0.25 0.5 1 3 6 1 1 1 6 6 6

additive

Hpca HNO3 K2CO3

Hpca HNO3 K2CO3

cyclohexanone (K)

cyclohexanol (A)

total

A/Kc

total TONd

total TOF/h−1e

0.8 1.1 1.8 1.7 1.7 0.9 0.6 0.5 0.2 1.6 2.9

2.5 6.3 10.6 16.9 17.7 6.3 0.5 0.9 0.5 17.3 16.5

3.3 7.4 12.4 18.6 19.4 7.2 1.1 1.4 0.7 18.9 19.4

3.1 5.7 5.9 9.9 10.4 7.0 0.8 1.8 2.5 10.8 8.2

17 37 63 94 97 36 6 7 4 95 97

67 74 63 31 16 36 6 7 0.6 16 16

1.6 4.3 7.2 12.1 12.3 0.4 2.1 5.9 0.4 12.0 12.3

0.5 1.1 2.1 4.0 4.2 0.1 0.5 1.9 0.1 3.3 3.8

2.1 5.4 9.3 16.1 16.5 0.5 2.6 7.8 0.5 15.3 16.5

0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.3 0.3 0.3 0.3

11 27 47 81 84 3 13 39 3 77 84

43 55 47 27 14 3 13 39 0.4 13 14

a Reaction conditions (unless stated otherwise): IL (3.0 mL), CyH (5.0 mmol), H2O2 (10.0 mmol), 10.0 μmol of 1 (0.2 mol % vs. CyH), n(additive)/n(1) = 40, 50 °C, 0.25−6 h of MW irradiation. Yields and TONs were determined by GC analysis (upon treatment with PPh3).9,26−29 b Percentage molar yield ((mol of product)/(mol of cyclohexane)). cRatio between the molar concentrations of A and K. dTotal turnover number, defined as (mol of A and K)/(mol of 1). eTOF = TON/h. fWithout 1. gUnder N2 atmosphere. hWithout PPh3 treatment.

Scheme 2

Figure 2. Effect of the reaction time and catalyst amount on the yields of cyclohexanol (A) and cyclohexanone (K) obtained from the MWassisted oxidation of CyH in MeCN (values from Table S1 in the Supporting Information).

severe (160 °C, 100 W MW power) than ours. Cyclohexanol, cyclohexanone, cyclohexyl hydroperoxide, and adipic acid were obtained with yields up to 17.0% and selectivities up to 35%.14 Evidence for a radical mechanism, involving the formation of cyclohexyl hydroperoxide (Scheme 1), as proposed for the nonMW-assisted system,25 arises from the application of Shul’pin’s method:7−9,26−29 the addition of PPh3 prior to the GC analysis of the products resulted in a marked increase in the amount of A (due to the reduction of the hydroperoxide by PPh3, with concomitant formation of phosphane oxide) with a decrease of K (compare entries 4 and 11 of Table 1). Moreover, addition of a radical trap (e.g., Ph2NH or CBrCl3) to the reaction mixture results in the near-suppression of the catalytic activity (see entries 16 and 17 of Table S1 in the Supporting Information). This behavior also supports the hypothesis of a free radical mechanism for the cyclohexane oxidation carried out in this study (Scheme 2).8−10,23,24,32−35

Experiments in the absence of O2 (dinitrogen atmosphere) were also performed (Table 1, entry 14). The obtained overall yield and TON values under the same reaction conditions, but in air (Table 1, entry 6), are quite similar to those obtained under dinitrogen (Table 1, entry 14), indicating only a slightly promoting effect of the air. It is noteworthy that O2 is formed even when the reaction is initiated under an inert gas atmosphere, on account of dismutation of H2O2 and of CyOO• (Scheme 2, eq 9). Our catalytic system is much more selective than the Fe(III) complexes bearing bis-2-pyridylmethylamine (BMPA) ligands and BPMA derivatives reported14 to catalyze the MW-assisted oxidation of CyH with H2O2 in acetonitrile (to our knowledge, the only ones until now) but under reaction conditions more C

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to pure cyclohexanone (at high pressure and 250 °C, in the presence of copper and chromium oxides), which is then used to make caprolactam, the monomer for producing PA 6.21,36 A selective oxidation of the cyclohexane to cyclohexanone process would therefore be an improvement in the PA 6 production. The recyclability of 1 in the used IL media (see the Experimental Section) is depicted in Figure 4. The activity and

The isolation of 1 from the reaction mixture in MeCN was not possible, thus disabling the performance of consecutive catalytic cycles. With the aim of being able to recycle and reuse the catalyst, we addressed room-temperature ionic liquids (ILs) as solvent media and studied their effect on the catalytic activity of this iron C-scorpionate complex for the above reaction. [bmim][BF4] and [bmim][N(CN)2] were found to be suitable IL solvents for [FeCl2(Tpm)] and were selected as the reaction media. To our knowledge, this is the first time that ILs have been used as solvents for the oxidation of alkanes with an iron catalyst under MW irradiation. Selected results for the catalytic systems 1/MW/H2O2/[bmim][N(CN)2] and 1/MW/H2O2/ [bmim][BF4] are presented in Table 2. A maximum total (KA) yield of 19.4 or 16.5% was obtained after 6 h of MW irradiation at 50 °C using 0.2 mol % of 1 vs CyH in [bmim][N(CN)2] or [bmim][BF4], respectively (entry 5 or 16, respectively, Table 2). Cyclohexanol (A) and cyclohexanone (K) were the only products detected by GCMS analysis under the optimized conditions, as observed for the 1/MW/MeCN/Hpca catalytic system. The oxidation of CyH with H2O2 in IL media led to total yields considerably lower in comparison to reactions run under the same conditions in MeCN and in the presence of Hpca (33%, Table 1). Interestingly, no yield enhancement due to the presence of an additive such as Hpca, HNO3, or K2CO3 (entries 6−8 and 17−19 for [bmim][N(CN)2] or [bmim][BF4], respectively, Table 2) was observed in both ILs. On the contrary, such additives had a marked inhibitory effect. Although a higher total (KA) yield is obtained for the reaction performed in MeCN and in the presence of Hpca, important advantages are observed when ILs are used as solvents, such as (i) an enhanced total (KA) yield under additive-free conditions (4.8% in MeCN vs. 18.6% in [bmim][N(CN)2] or 16.1% in [bmim][BF4]; entry 6 of Table 1 vs entry 4 or 15 of Table 2), (ii) the recycling and reuse of 1 in a considerable number of consecutive catalytic cycles (see below), and (iii) the alcohol/ketone selectivity tuning dependent on the IL. As depicted in Figure 3, high selectivity

Figure 4. Effect of the catalyst recycling on the total (A + K) yield for the MW-assisted oxidation of cyclohexane with H2O2 catalyzed by 1 in [bmim][N(CN)2] or [bmim][BF4]. Reaction conditions: IL (3.0 mL), CyH (5.0 mmol), H2O2 (10.0 mmol),10.0 μmol of 1, 3 h of MW irradiation at 50 °C. Each bar represents a run, and the respective yield value is from Table S2 in the Supporting Information.

selectivity (Table S2 in the Supporting Information) of 1 in [bmim][BF4] are maintained through a considerably high number of consecutive cycles: e.g., after the ninth cycle, 1 still retains 92% of its initial activity. Moreover, the ICP analysis of the corresponding supernatant phase revealed a leaching of 1 inferior to 1.7%. However, the reuse of 1 in [bmim][N(CN)2] was not as successful (Figure 4). In view of the hydrosolubility of [FeCl2(Tpm)] (1), the oxidation of CyH catalyzed by this complex was also tested under the above conditions but in water. Although active in this solvent (Figure 5, Table S3 in the Supporting Information), the performance of 1 in the presence of Hpca is much lower than that in acetonitrile with the same additive. In fact, the obtained

Figure 3. Effect of the reaction time and IL (solvent) on the yield of cyclohexanol (A) and cyclohexanone (K) obtained from the MWassisted oxidation of CyH in [bmim][N(CN)2] (■) and [bmim][BF4] (●).

toward cyclohexanol formation is obtained in [bmim][N(CN)2] (entries 1−5, Table 2), whereas in [bmim][BF4] the formation of cyclohexanone is clearly favored (entries 12−16, Table 2). Such an uncommon feature of our 1/MW/H2O2/IL catalytic systems can be of applied significance. For example, in the current production process of polyamide 6 (PA 6), the KA mixture obtained from cyclohexane oxidation is first converted

Figure 5. Effect of the solvent on the total (KA) yield for the MWassisted oxidation of cyclohexane with H2O2 catalyzed by 1. Yield values were obtained after 3 h MW irradiation under the conditions of Table S3 in the Supporting Information. D

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Formation of the bmim·2 associate from 1 and [bmim][N(CN)2] (optimized as an ion pair) is highly exergonic (by −26.7 kcal mol−1), indicating that the interaction of the catalyst with IL is very efficient. The concurrent interaction of 1 with IL, water, and acetonitrile was also investigated. With this aim, the structures of two complexes, i.e. [bmim][FeCl2{N(CN)2}(Tpm)](H2O) (bmim·2·H2O) (with bmim and water molecules in the second coordination sphere) and [bmim][FeCl2(H2O)(Tpm)][N(CN)2] (bmim·3·N(CN)2) (with bmim and N(CN)2 in the second coordination sphere), have been optimized. In both complexes, the outer-sphere H2O or N(CN)2 species are situated near the methine group of the tris-pyrazolyl ligand, forming several C−H···O or C−H···N hydrogen bonds. The calculated Gibbs free energy change of N(CN)2 for H2O substitution (that is, the energy of reaction bmim·2·H2O → bmim·3·N(CN)2) is slightly negative (−2.2 kcal mol−1, Table 3). However, considering that the concentration of the IL (solvent) is significantly higher than that of H2O (introduced to the system with aqueous H2O2), the predominant form in solution should be complex bmim·2. The coordination of acetonitrile (another solvent used in this work) to the pentacoordinated complex 1 is also energetically favorable (by −9.9 kcal mol−1). However, the substitution of MeCN for H2O in [FeCl2(NCMe)(Tpm)] (4) is noticeably exergonic in the gas phase (by −5.2 kcal mol−1); however, it is slightly disfavored when solvent effects are taken into account (by 1.1 kcal/mol).

KA yield (5.6%, entry 2, Table S3) in water (with Hpca) after 3 h of MW irradiation is comparable with that obtained for the same reaction time in MeCN but in the absence of Hpca (4.8%, entry 6, Table 1). When the reaction was run in water under additive-free conditions, 4.3% KA yield was achieved. Solvent-free conditions were also attempted, but the pressure generated hampered performing the catalytic assay. The interaction of the catalyst [FeCl2(Tpm)] (1) with the IL 1-butyl-3-methylimidazolium dicyanamide ([bmim][N(CN)2]) was investigated using theoretical DFT calculations at the M06/6-31+G* (Fe-ECP) level of theory. The coordination polyhedron of the equilibrium structure of complex 1 corresponds to a tetragonal pyramid with one of the donor N atoms being at the axial position (Figure 6). Thus, the iron

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CONCLUSIONS This work demonstrated that a combination of a C-scorpionate iron complex catalyst and well-adjusted reaction parameters (in particular, the use of a IL as solvent and of MW irradiation) can lead to highly selective, efficient, fast, and reusable catalytic systems for the mild oxidation of cyclohexane with hydrogen peroxide. In fact, the combination of MW irradiation with the use of [FeCl2(Tpm)] (1) as catalyst in IL solvents led to (i) a high selectivity with an enhanced total (KA) yield under additivefree conditions and short reaction times, (ii) higher catalytic activities under additive-free conditions, (iii) the possibility of recycling and reuse of 1 in a considerable number of consecutive catalytic cycles with retention of its activity and selectivity, and (iv) alcohol/ketone selectivity tuning by choice of the appropriate IL. These sustainable features of our 1/MW/ H2O2/IL catalytic systems are of synthetic significance, in particular for the production of polyamide 6. To our knowledge, this is the first report, for the oxidation of alkanes, of the successful use of ionic liquids with an iron catalyst under MW irradiation, providing a more sustainable approach to KA oil synthesis.

Figure 6. Calculated equilibrium structures.

atom has one empty coordination place in complex 1. Such a structure indicates that a solvent molecule (e.g., an anion of IL) can be easily coordinated to the metal. Indeed, the equilibrium structure of the coordinatively saturated complex [FeCl2{N(CN)2}(Tpm)]− (2) was found as a result of geometry optimization (Figure 6). The formation of this complex from 1 and N(CN)2− is energetically favorable in terms of both gasphase Gibbs free energy and SMD energy in solution (by −8.7 and −12.3 kcal mol−1, respectively, Table 3). The interaction of 1 with both the cation and anion of the ionic liquid was also considered. As a result, the equilibrium structure of the neutral associate [bmim][FeCl2{N(CN)2}(Tpm)] (bmim·2) was obtained. The bmim cation in bmim·2 is situated between the Cl− and N(CN)2− ligands, forming with them several C−H···Cl and C−H···N hydrogen bonds (Figure 6). Such a position of the bmim cation in the second coordination sphere corresponds to the most extended regions of the negative electrostatic potential in complex 2 (Figure 7).

Table 3. Calculated Free Energy Changes (in kcal mol−1) of Theoretically Considered Reactions ΔG (ΔESMD)

reaction [FeCl2(Tpm)] (1) + N(CN)2− → [FeCl2{N(CN)2}(Tpm)]− (2) [FeCl2(Tpm)] (1) + MeCN → [FeCl2(NCMe)(Tpm)] (4) [FeCl2(Tpm)] (1) + H2O → [FeCl2(H2O)(Tpm)] (3) [FeCl2(Tpm)] (1) + [bmim][N(CN)2] → [bmim][FeCl2{N(CN)2}(Tpm)] (bmim•2) [bmim][FeCl2{N(CN)2}(Tpm)](H2O) (bmim·2·H2O) → [bmim][FeCl2(H2O)(Tpm)][N(CN)2] (bmim·3·N(CN)2) [FeCl2(NCMe)(Tpm)] (4) + H2O → [FeCl2(H2O)(Tpm)] (3) + MeCN E

−8.7 −9.9 −15.1 −26.7 −2.2 −5.2

(−12.3) (−18.7) (−15.5) (−25.6) (−0.1) (1.1)

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Figure 7. Electrostatic potential distribution in complex 1. of both analyses allows us to estimate the amount of cyclohexyl hydroperoxide. In the experiments with radical traps, CBrCl3 (5.00 mmol) or NHPh2 (5.00 mmol) was added to the reaction mixture. Blank experiments were performed in the different solvent media and confirmed that no CyH oxidation products (or only traces, below 1%) were obtained in the absence of 1. Catalyst recyclability in IL media was investigated for several consecutive cycles. After completion of each run, the products were analyzed as detailed previously and the IL with the solubilized catalyst was recovered by drying it under vacuum overnight at 70 °C. Each cycle was initiated after the preceding one upon the addition of new typical portions of all other reagents. Computational Details. The full geometry optimization of the molecular structures was carried out at the DFT level of theory using M06 functional38 with the help of the Gaussian-09 program package.39 No symmetry operations were applied for any of the structures calculated. The geometry optimization was carried out using a relativistic Stuttgart pseudopotential which describes 10 core electrons and the appropriate contracted basis set (8s7p6d1f)/[6s5p3d1f] for the iron atom40 and the 6-31+G* basis set for other atoms. The Hessian matrix was calculated analytically for all optimized structures to prove the location of correct minima (no imaginary frequencies) and to estimate the thermodynamic parameters, the latter being calculated at 25 °C. The solvent effects were estimated by single-point calculations on the basis of gas-phase geometries. For the approximation of solvent effects in the solution of IL, the SMD-GIL method41 was used with the parameters ε = 11.5, n = 1.43, γ = 61.24, ∑α2H = 0.229, ∑β2H = 0.265, φ = 0.2, and ψ = 0. For the approximation of the solvent effects in water and acetonitrile solutions, the standard SMD method42 was applied.

A better understanding of how the IL properties (e.g., charge, bulkiness, ionic mobility, viscosity, and density) can affect the performance of the catalytic system seems to be worthy of further development, a topic that will be addressed in future studies. The generality of the above advantages in the field of alkane oxidation should also be tested by extending the investigation to other alkanes and catalysts.

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EXPERIMENTAL SECTION

Materials and Physical Methods. All the reagents and solvents were purchased from commercial sources and used as received. Water used in all the syntheses and catalytic studies was double-distilled. The scorpionate iron(II) complex 1 has been synthesized according to a reported procedure and characterized accordingly.25 1-Butyl-3methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3methylimidazolium dicyanamide ([bmim][N(CN)2]) were prepared by anion exchange of 1-butyl-3-methylimidazolium bromide, upon reaction with Na[BF4] or Na[N(CN)2], respectively,37 and used after drying for 24 h at 70 °C under high vacuum with stirring. 1 H and 13C NMR spectra were recorded at ambient temperature on a Bruker Avance II + 300 (UltraShield Magnet) spectrometer operating at 300.130 and 75.468 MHz for proton and carbon-13, respectively, at ambient temperature. Far-infrared spectra FIR (400− 200 cm−1) were recorded on a Vertex 70 spectrophotometer in CsI pellets. Catalytic reactions were performed in borosilicate tubes (5 mL capacity with a 10 mm internal diameter) under focused microwave irradiation (MW) in an Anton Paar Monowave 300 reactor fitted with a rotational system and an IR temperature detector. Chromatographic analyses were undertaken by using a Fisons Instruments GC 8000 series gas chromatograph with a DB-624 (J&W) capillary column (flame ionization detector) and the Jasco-Borwin v.1.50 software. The internal standard method was used to quantify the organic products. MW-Assisted Peroxidative Oxidation of Cyclohexane. Cyclohexane oxidations were carried out in air in a cylindrical Pyrex tube placed in the microwave reactor. Typically, the catalyst 1 (0.2−10.0 μmol) was added to 3 mL of solvent (CH3CN, H2O or IL), and an additive (optional) was added. Cyclohexane (0.54 mL, 5 mmol) and 30% aqueous H2O2 solution (10 mmol) were then added in that order. The reaction vessel was closed and the contents were stirred and irradiated (up to 20 W) for 0.25−6 h, at 30−80 °C. After the reaction, the mixture was cooled to room temperature and the organics were extracted with diethyl ether (5 mL). All products formed were identified by GC and retention times compared with those of commercially available products. Cycloheptanone (0.09 mL) was used as the GC internal standard. The reaction mixtures were analyzed twice by GC: with and without addition of an excess of solid triphenylphosphine, following a method developed by Shul’pin.7,26−29 The addition of PPh3 to the final organic phase reduces cyclohexyl hydroperoxide, if formed, to the corresponding alcohol and hydrogen peroxide to water. Comparison of the results

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00620. Additional selected data on catalytic oxidation of cyclohexane and DFT results (PDF) Cartesian coordinates for the calculated structures (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for L.M.D.R.S.M.: lmartins@deq isel.ipl.pt. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partially supported by the Foundation for Science and Technology (FCT) of Portugal (UID/QUI/ F

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Organometallics

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00100/2013, PTDC/QEQ-ERQ/1648/2014, and PTDC/ QEQ-QIN/3967/2014 projects). A.P.C.R. expresses gratitude to the FCT for her postdoc fellowship contract. The authors gratefully acknowledge the Portuguese NMR Network (ISTUTL Centre) for providing access to the NMR facility.

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DOI: 10.1021/acs.organomet.6b00620 Organometallics XXXX, XXX, XXX−XXX