Synthesis of Mono- and Dinuclear Vanadium Complexes and Their

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Synthesis of Mono- and Dinuclear Vanadium Complexes and Their Reactivity toward Dehydroperoxidation of Alkyl Hydroperoxides Anna-Corina Schmidt,† Marko Hermsen,†,‡ Frank Rominger,∥ Richard Dehn,§ Joaquim Henrique Teles,§ Ansgar Schaf̈ er,‡ Oliver Trapp,∥ and Thomas Schaub*,†,§ †

Catalysis Research Laboratory (CaRLa), Im Neuenheimer Feld 584, 69120 Heidelberg, Germany BASF SE, Quantum Chemistry & Molecular Simulation Catalysis, Carl-Bosch-Straße 38, 67056 Ludwigshafen, Germany § BASF SE, Synthesis & Homogeneous Catalysis, Carl-Bosch-Straße 38, 67056 Ludwigshafen, Germany ∥ Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany ‡

S Supporting Information *

ABSTRACT: Several vanadium(V) complexes with either dipicbased or Schiff base ligands were synthesized. The complexes were fully characterized by elemental analysis, IR, 1H, 13C, and 51V NMR spectroscopy, as well as mass spectrometry and X-ray diffraction. Furthermore, they were tested toward their catalytic deperoxidation behavior and a significant difference between 4heptyl hydroperoxide and cyclohexyl hydroperoxide was observed. In the case of 4-heptyl hydroperoxide, the selectivity toward the corresponding ketone was higher than with cyclohexyl hydroperoxide. DFT calculations performed on the vanadium complex showed that selective decomposition of secondary hydroperoxides with vanadium(V) to yield the corresponding ketone and water is indeed energetically feasible. The computed catalytic path, involving cleavage of the O−O bond, hydrogen transfer, release of ketone/water, and finally addition of hydroperoxide, can proceed without the generation of radical species.

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INTRODUCTION The oxidation of cyclohexane (Cy) to a mixture of the corresponding alcohol (CyOH), ketone (CyO), and hydroperoxide (CyOOH), either uncatalyzed or in the presence of transition metals like cobalt as a catalyst, has been known for decades and used on a large industrial scale.1−12 In a first step, cyclohexane is oxidized with O2 at elevated temperatures to cyclohexyl hydroperoxide in a radical chain reaction. Special care must be taken to avoid overoxidation of the products by keeping the conversion of the alkane low (∼3−10%).5−7 The formed hydroperoxide is subsequently decomposed to a mixture of cyclohexanone and cyclohexanol (KA oil, ketone - alcohol).4,5,13 Due to the radical chain mechanism of the decomposition, only mixtures of alcohol and ketone can be obtained. However, it would be desirable to have a catalyst, which would allow a selective decomposition of the hydroperoxide to cyclohexanone and water (Scheme 1). Depending on the transition metal used, the molar ratio of cyclohexanol to cyclohexanone can vary between 1.4 for V and 3.3 for Co or Mn as catalysts.5 In a further step, cyclohexanol has to be separated and dehydrogenated to the

ketone as this compound is the required starting material for the synthesis of ε-caprolactam. ε-Caprolactam is then used for the production of nylon-6.14,15 Although the total installed capacity worldwide is around 7 million tons per year of KA oil,6 very little is known about the selective decomposition of cyclohexyl hydroperoxide to cyclohexanone as the sole product.6,16 In the last several decades, a number of papers were published concerning the decomposition of secondary alkyl hydroperoxides to a mixture of the corresponding alcohols and ketones, mainly focused on the radical mechanism.6,16−29 Very recently, investigations with several vanadium complexes were published.30 Herein, the cyclohexane was oxidized by tert-butyl hydroperoxide under microwave irradiation to the ketone and alcohol in different ratios. Again, a radical mechanism was specified. When oxidizing cyclohexane in the absence of any catalyst under the conditions of the process, cyclohexyl hydroperoxide is the major product formed.6 In order to minimize the number of process steps, a selective decomposition of the hydroperoxide solely to the ketone would be a very attractive target. In this work, vanadium(V) complexes were identified as very promising homogeneous catalysts, as they form alkylperoxido

Scheme 1. Desired Selective Dehydroperoxidation Reaction

Received: October 10, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.inorgchem.6b02322 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry complexes,31−35 which, by intramolecular hydrogen transfer,36 release the corresponding alkyl ketone together with 1 equiv of water. Despite an unprecedented good selectivity for the conversion of 4-heptyl hydroperoxide to 4-heptanone at room temperature, the known competitive radical decomposition to a mixture of alcohol and ketone was still observed for cyclohexyl hydroperoxide. Unfortunately, this could not be suppressed by using different ligand systems with a high steric demand around the oxidovanadium center VVO. The first vanadium complexes (1−5, Chart 1), which were investigated in more detail, were based on the tridentate

pentenols using the same complex and tert-butyl hydroperoxide.31,43 They proposed the formation of a peroxido complex as an intermediate of the catalytic cycle and hence, activation of the alkyl hydroperoxide. In this work, the ortho and meta positions of the phenolates were further substituted (R1,R2ONO-R3,R4) by chlorine, tert-butyl or tert-amyl groups to increase the solubility of the complexes in apolar solvents. All new vanadium complexes were characterized by 1H, 13C, and 51V NMR, and IR spectroscopy as well as elemental analysis and mass spectrometry. Moreover, complexes 2, 5, 6, 7, 9b, 9c, 10c, 11c, 12a, and 12b formed suitable crystals for X-ray diffraction analysis, and their molecular structures are shown herein. The study of the catalytic behavior of these vanadium complexes toward the selective dehydroperoxidation of cyclohexyl hydroperoxide to the ketone was the major focus of this study. Additionally, the unsubstituted vanadium dipic complex 8 was used in the selective dehydroperoxidation of 4-heptyl hydroperoxide, and the different selectivities as well as the mechanism for the selective formation of ketone were investigated by DFT calculations.

Chart 1. Overview of the Synthesized Complexesa

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a

RESULTS AND DISCUSSION Synthesis and Molecular Structures. The precursor pyridine-2,6-dicarboxylic acid (dipic) vanadium complex 1 was synthesized according to a known literature procedure starting from V2O5 in water.35,37 Treatment of complex 1 with alcohols generates the corresponding alkoxido complexes [VO(dipic)(OR)] with R = Me (2), Et (3),35,44 4-heptyl (4, Hept), cyclohexyl (5, Cy). Bright yellow crystals of 5, suitable for XRD analysis, could be obtained from a concentrated dichloromethane solution (Figure 1). As it proved very difficult to eliminate traces of water from the complexes synthesized by this route, a different method was chosen using VO(OEt)3 as the precursor and carrying out the reaction in ethanol.44 By adding different alcohols to the so-formed compound 3, the corresponding alkoxido complexes 2−5 were obtained in high purity and water-free. From this procedure, bright yellow crystals of 2, suitable for XRD analysis, could be obtained from a concentrated methanol solution (Figure 1). In both structures, the vanadium(V) ion is situated in a distorted octahedron with the strong oxido ligand occupying one axial position and a labile solvent molecule (methanol or water) bound trans to it. Dipic behaves as a tridentate ligand and is coordinated in the equatorial plane together with the alkoxido ligand. The VO1 bond lengths (2MeOH: 1.581(2) Å, 5H2O: 1.583(2) Å) are in accordance with literature-known distances and significantly shorter than the dipic V−O2/3 bonds (2MeOH H2O MeOH : av.: 1.970 Å), the alkoxido V−O4 bonds (2 av.: 1.952 Å, 5 H2O 1.755(2) Å, 5 : 1.755(3) Å) and especially the very long V−O5 solvent bonds (2MeOH: 2.333(2) Å, 5H2O: 2.232(2) Å).35 In addition, the V−N bonds (2MeOH: 2.076(2) Å, 5H2O: 2.084(2) Å) are in good agreement with other reported vanadium nitrogen bonds.35,47 In order to increase the solubility of the resulting complexes in nonpolar solvents, a substituent was introduced in the para position of the pyridine of the dipic ligand as shown in Scheme 2. Thus, the ester-protected dipic was functionalized by a hydroxymethyl group via Fenton reaction in methanol, yielding HOdipicOMe (in 20% yield).45 In a next step, the corresponding ether was formed with 1-adamantanemethanol in the presence of triethylamine and triflic anhydride.46 After in situ deprotection by addition of KOH and subsequently HCl, the substituted ligand 4(((adamantan-1-yl)methoxy)methyl)pyridine-2,6-dicarboxylic acid (AdOdipicOH) was obtained in low yields (25%; see

tA = tert-amyl, tB = tert-butyl; s.u. = second unit.

pyridine-2,6-dicarboxylic acid (H2dipic) ligand.35,37 Such alkoxido complexes [VO(dipic)(OR)] (R = Et, tBu, methallyl) are known to react with tertiary alkyl hydroperoxides (R′OOH, R′ = tBu and CMe2PH) to form the vanadium(V) alkylperoxido complexes [VO(dipic)(OOR′)] (R′ = Cy, 6). These complexes can epoxidize olefins, oxidatively cleave CC double bonds, and perform allylic oxidations.35 When dissolving complex 1 in acetonitrile in the presence of CyOOH, complex 7 is formed. Additionally, in this work, the para position of the dipic ligand was further substituted by an (adamantane-1-ylmethoxy)methyl group, in order to increase the solubility of the corresponding vanadium complex [VO(AdOdipic)(OEt)] (8) in cyclohexane. The second class (9−12) consists of a tridentate Schiff base with a similar coordination mode compared to the dipic ligand but having different electronic properties, namely, N-(2hydroxyphenyl)salicylidenimine (ONO).38−40 In 1993, Butler et al. described the oxidation of bromide with hydrogen peroxides catalyzed by the vanadium(V) complex [VO(ONO)(OEt)].41,42 More recent studies by Hartung et al. showed the stereoselective synthesis of tetrahydrofurans from substituted 4B


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Inorganic Chemistry

butyl group and partly in the 6-position by a tert-butyl group (see Scheme 3). In the latter case, the amine had to be synthesized in advance by a two-step procedure starting from 2,4-di-tertbutylphenol, which was treated with nitric acid in acetic acid to form 2,4-di-tert-butyl-6-nitrophenol together with 2,4-di-tertbutyl-6-nitrophenyl acetate. The obtained orange-red oil was reduced with hydrogen (1 bar) and Pd/C at room temperature, leading to a color change from dark red to yellow and the formation of 2-amino-4,6-di-tert-butyl-phenol.48 Due to the high solubility of the Schiff bases in all tested solvents, only the less soluble ligands 4-chloro-2-((5-chloro-2-hydroxybenzylidene)amino)phenol (Cl-ono-Cl) and 4-(tert-butyl)-2-(((2-hydroxy5-(tert-amyl)phenyl)imino)methyl)phenol (tA-ono-tB) could be obtained in pure forms. (2,4-Di-tert-butyl-6-(((2-hydroxy-5(tert-amyl)phenyl)imino)methyl)phenol (tA-ono-2tB) and 2,4di-tert-butyl-6-((3,5-di-tert-butyl-2-hydroxybenzylidene)amino)phenol (2tB-ono-2tB) were used in the complex syntheses without further purification. Thus, the Schiff bases were stirred with VO(OEt)3 in ethanol at room temperature for 30 min to yield the monomeric species [VO(Cl-ono-Cl)(OEt)] (9a), [VO(tA-ono-tB)(OEt)] (10a), and [VO(2tB-ono-2tB)(OEt)] (12a) as precipitates in excellent (94%) to good (83%) and low (33%) yields, respectively.49 In the case of tA-ono-2tB as ligand, no precipitation in ethanol but in a mixture of nhexane:ethyl acetate (10:1) could be observed, leading solely to the formation of the dinuclear complex [{VO(tA-ono-2tB)}2-μO] (11c) in moderate yields (41%). When dissolving 10a in diethyl ether (complex 9a is not soluble under these conditions), a red-brown crystalline solid was formed which was characterized as the dinuclear compound [{VO(tA-ono-tB)}2-μ-O] (10c).30 Complex 12a, on the other hand, did not precipitate by forming a dinuclear complex due to the steric hindrance of the tert-butyl groups and higher solubility. The dinuclear complexes 10c, 11c could be transformed completely to the monomeric species or the alkoxido ligand could be exchanged when adding an alcohol like ethanol (9a−12a), methanol (9b, 11b, 12b), or cyclohexanol (10b). The alcohol is only weakly bound to the vanadium center. It could easily be replaced by a different alcohol or partly removed in vacuum, forming a mixture of the monomer and the dinuclear complex. This behavior was also observed by NMR spectroscopy (vide inf ra), elemental analysis, and mass spectrometry (see the SI) and has been previously reported in the literature.43,50

Figure 1. Molecular structures of [VO(dipic)(OMe)(MeOH)] (2MeOH) and [VO(dipic)(OCy)(H2O)] (5H2O). Hydrogen atoms are partly omitted for clarity. Thermal ellipsoids are at 50% probability. Color code: gray - carbon, red - oxygen, green - vanadium, blue nitrogen, white - hydrogen. Selected bond lengths [Å] for complex 2MeOH: V−O1: 1.581(2), V−O2: 1.942(2), V−O3: 1.962(2), V−O4: 1.755(2), V−O5: 2.333(2), V−N: 2.076(2); and complex 5H2O: V−O1: 1.583(2), V−O2: 1.970(2), V−O3: 1.970(2), V−O4: 1.755(3), V−O5: 2.232(2), V−N: 2.084(2).

Scheme 2). Reaction of this ligand with VO(OEt)3 in ethanol yields the corresponding complex [VO(AdOdipic)(OEt)] (8) in moderate yields (56%) as a yellow solid. The second class of ligands was synthesized from substituted 2-aminophenols and 2-hydroxybenzaldehydes in ethanol at room temperature to form the corresponding Schiff bases according to known procedures from the literature.38,39,41 For this purpose, the 2-hydroxybenzaldehyde was partly substituted in the 5-position by a chlorine or tert-butyl (tB) and the 3position by a tert-butyl group. Moreover, the 2-aminophenol was substituted in the 4-position by chlorine, a tert-amyl (tA), or tert-

Scheme 2. Synthesis of AdOdipicOH and the Corresponding Vanadium Complex [VO(AdOdipic)(OEt)] (8)

C


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Inorganic Chemistry Scheme 3. Synthesis of the Schiff Bases and the Corresponding Vanadium Complexes 9−12

Figure 2. Molecular structures of the monomeric vanadium(V) complexes [VO(Cl-ono-Cl)(OMe)] (9bMeOH), [VO(2tB-ono-2tB)(OEt)] (12a), and [VO(2tB-ono-2tB)(OMe)] (12b). Hydrogen atoms are partly omitted for clarity. Thermal ellipsoids are at 50% probability. Color code: gray - carbon, red - oxygen, green - vanadium, blue - nitrogen, white - hydrogen, pink - chlorine. Selected bond lengths [Å] for complex 9bMeOH: V−O1: 1.589(2), V− O2: 1.869(2), V−O3: 1.944(2), V−O4: 1.763(2), V−O5: 2.259(3), V−N: 2.179(3), NC: 1.280(4); complex 12a: V−O1: 1.597(11), V−O2: 1.82(2), V−O3: 1.879(2), V−O4: 1.749(9), V−N: 2.136(12), NC: 1.268(12); and complex 12b: V−O1: 1.617(18), V−O2: 1.84(2), V−O3: 1.84(2), V−O4: 1.764(18), V−N: 2.11(5), NC: 1.31(4).

In the literature, several vanadium complexes with different tridentate ligands, containing the O−N−O binding motif, are reported together with their molecular structure.43,50−56

Dark brown or yellow crystals of complexes 9−12, suitable for XRD analysis, were obtained from concentrated methanol, benzene, or chloroform solutions, respectively (Figures 2 and 3). D


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Figure 3. Molecular structures of the dinuclear vanadium(V) complexes [{VO(Cl-ono-Cl)}2-μ-O] (9c), [{VO(tA-ono-tB)}2-μ-O] (10c), and [{VO(tA-ono-2tB)}2-μ-O] (11c). Hydrogen atoms are partly omitted for clarity. Thermal ellipsoids are at 50% probability. Color code: gray - carbon, red - oxygen, green - vanadium, blue - nitrogen, white - hydrogen, pink - chlorine. Selected bond lengths [Å] and angles [deg] for complex 9c: V−O1: 1.599(11), V−O2: 1.762(13), V−O3: 1.891(14), V−O4: 1.779(6), V−N: 2.178(10), NC: 1.285(12), V−O−V: 180.0; complex 10c: V−O1/5: 1.581(4)/1.582(4), V−O2/6: 1.826(4)/1.819(4), V−O3/7: 1.861(4)/1.861(4), V1/2−O4: 1.765(4)/1.782(3), V−N1/2: 2.152(4)/2.144(5), N1/ 2C: 1.284(7)/1.253(7), V1−O−V2: 148.3(2); and complex 11c: V−O1: 1.582(8), V−O2: 1.872(8), V−O3: 1.848(7), V−O4: 1.793(4), V−N: 2.152(9), NC: 1.269(13), V−O−V: 143.7(6).

base, with one bridging oxygen atom. In contrast to 9bMeOH, the vanadium centers in these complexes possess no additional solvent molecules, leading to a square pyramidal environment with the terminal oxido group occupying the apical positions.60 Due to different substituents on the phenyl rings, the dinuclear complexes show different arrangements. Thus, complex 9c exhibits an inversion center on the bridging oxygen atom and 11c possesses C2 symmetry with a bent V−O−V angle of 143.7(6)°, whereas 10c is asymmetric with a bent V−O−V angle of 148.3(2)°. In addition, the orientation of the VO unit in each complex differs: the two terminal oxido groups in 9c are pointing perfectly anti to each other (180°), whereas, in 10c, the torsion angle changes to 145.86°, and in 11c, the VO groups point syn to each other (torsion angle 77.67°), leading to a strongly twisted structure. For complexes 10c and 11c, the orientation of the two coordinated Schiff bases differs in the solid state. In the molecular structure of complex 10c, the phenol rings with the tert-amyl group lie on the same side, whereas, in 11c, the phenol rings with the tert-amyl group lie on opposite sides. This feature can also be observed in solution by NMR spectroscopy (vide inf ra). In spite of these structural differences, the bond distances in the monomeric and dinuclear complexes do not differ significantly and are comparable to values reported in the literature.41,43,50,53,54,56 A slight difference can be observed for the V−N bonds, which are in the same range for the Schiff base

However, only few molecular structures are known with the herein presented framework and, in all of these cases, the phenyl rings were unsubstituted or substituted only by Cl, Br, NO2, or additional phenyl rings.41,57−59 Depending on the steric demand of the substituents and the solvent used for crystallization, the vanadium complexes form either monomers (9bMeOH, 12a, 12b) in coordinating solvents or dinuclear complexes (9c, 10c, 11c) in noncoordinating solvents. The monomeric complex 9bMeOH is comparable to the dipic complexes 2MeOH and 5H2O with the same octahedral coordination motif around the vanadium center. The terminal oxido group occupies one axial position, the weakly bound methanol, the position trans to it, and the Schiff base (Clono-Cl) together with the methoxido ligand form the square planar base. In contrast to 2MeOH and 5H2O, the two oxygens and the nitrogen coordinated to the vanadium do not lie in a perfect plane with the metal center, as the oxygen of the former benzaldehyde is twisted out of this plane toward the weakly bound methanol. This feature can also be observed in the molecular structures of 9c, 10c, and 11c, but less pronounced in 12a and 12b. In the latter cases, the complexes do not contain an additional solvent molecule, possessing a free coordination site trans to the VO group, probably due to the steric hindrance of the tert-butyl groups in the ortho position. The dinuclear complexes 9c, 10c, and 11c (Figure 3) consist of two units of the oxidovanadium units, coordinated by the corresponding Schiff E


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Inorganic Chemistry

In the 51V NMR spectra, the chemical shift of the signals for the vanadium depends both on the substituents of the phenolate rings and on the alkoxido ligand. For example, complex 9a (Cl, Cl, EtO−) shows a signal at −536 ppm, complex11a (tA, 2tB, EtO−) at −529 ppm, complex 11b (tA, 2tB, MeO−) at −527 ppm, and complex 12a (2tB, 2tB, EtO−) at −528 ppm. Furthermore, when exchanging the ethoxido ligand by cyclohexanolate, the signal shifts from −518 ppm (10a; tA, tB, EtO−) to −527 ppm (10b; tA, tB, CyO−). Although the two dinuclear complexes [{VO(tA-ono-tB)}2-μ-O] (10c) and [{VO(tA-ono2tB)}2-μ-O] (11c) exhibit either an inversion center or C2 symmetry, the NMR spectra are rather complicated (see the SI). Contrary to expectations, the 1H NMR spectra of the dinuclear complexes do not show 11 but 22 signals, two sets of signals with the same pattern. The ratio of the two sets, as measured by the intensity of the two signals corresponding to the imine hydrogen, changes with each batch synthesized. Therefore, it is not possible that the 22 signals arise from one single complex, which lost its symmetry. The additional signals have similar shifts than the monomeric complexes; however, the alkoxido ligand is missing. Nevertheless, when adding water to these solutions, no change in the ratio of the imine hydrogen signals was observed. Moreover, when mixing the monomer and the dinuclear species (10a/10c, 10b/10c, or 11a/11c), more than the 22 signals were observed together with the significant signal at ∼5.5 ppm for the alkoxido ligand. The same feature was observed in the 13C NMR spectra of 10c and 11c with minor differences. In the case of 10c, 34 instead of the expected 40 signals (assuming two sets of signals) were detected. Presumably, some signals like the one for the imine carbon (153.5 ppm), the methyl carbon of the tert-butyl group (31.5 ppm), or the distal carbon of the tert-amyl group (9.4 ppm) coincide. Complex 11c shows comparable behavior with 39 instead of 44 signals in the 13C NMR spectrum. Here, three signals in the aromatic region (143.7, 137.9, and 136.6 ppm), one methyl carbon of the tert-amyl group and one for the distal carbon atom of the tert-amyl group, are coincident with the corresponding signals of the second set. Finally, 51V NMR measurements additionally confirm the absence of monomeric species in solutions of 10c (see the SI) and 11c. The solution of 10c shows two upfield shifted signals at −535 and −539 ppm, which almost coincide, whereas 11c leads to one highly upfield shifted signal at −552 ppm. In contrast, the solution mixtures of the monomer and the dinuclear complex (see the SI) show the presence of one signal corresponding to the alkoxido complex and two or one additional signal corresponding to the dinuclear complex 10c or 11c, respectively (see the SI). Two sets of signals in NMR spectra were already reported in several publications and were attributed to different VVO conformers,52 isomers,57,62 diastereomers,43,51 or flexibility in the coordination geometry.63−65 As the molecular structures show (vide supra), the orientation of the VO groups and the two ligand units to each other can indeed differ, leading to two sets of signals in NMR spectra. Although Tiekink et al. observed a similar equilibrium between the monomer and the dinuclear complex, they do not report a second set of signals for the dinuclear complex in the 1H NMR spectrum.50 As their ligand system consists of a hydrazone framework, the second set of signals in NMR measurements, observed for the dinuclear complexes presented herein, might be a feature of the imine framework.43,50 IR Characterization. A set of selected IR bands for the ligands and complexes 1−12a are summarized in the SI. The IR

complexes (2.144(5)−2.179(3) Å) but longer compared to the ones in the dipic complexes (2.076(2) and 2.084(2) Å), a difference which can be attributed to the different electronic properties of the N atoms in an imine versus pyridine. NMR Characterization. The dipic alkoxido complexes 2−5 show almost identical 1H and 13C NMR spectra (see the SI). The labile bound solvent molecule is decoordinated, as proven by the chemical shifts and integration resulting in C2 symmetry of the complexes.35 One multiplet at around 6.0 ppm arises from the CH2 group bound to the coordinating oxygen of the alkoxido ligand. In the case of 2, which was measured in MeOD, no signals arising from the coordinated methanol can be measured, indicating fast exchange in the deuterated solvent. Due to the low solubility of the complexes, the very weak signal for the carboxylic carbon atoms can only be observed for complexes 2 and 3 at around 168 ppm in the 13C NMR spectra. The CH2 carbon atom gives a signal whose position is sensitive to the alkoxido ligand present. Therefore, this signal appears in a broader range of 88−103 ppm. The successful substitution of the para position of the pyridine ring can be proven by the loss of one of the signals in the aromatic region of HOdipicOMe (δ = 8.31 ppm). In the 1H and 13C NMR spectra (see the SI), the formation of the vanadium complex 8 has almost no influence on the shift of the signals attributed to the adamantyl group (δ = 2.00−1.69 ppm) but slightly on the CH2 groups (δ = 4.83 and 3.22 ppm) and the pyridine ring (δ = 8.24 ppm). However, when 8 is dissolved in deuterated methanol, the two signals of the coordinating ethoxide (δ = 6.11, 1.66 ppm) disappear and two new ones (δ = 3.59, 1.18 ppm) appear for one molecule of uncoordinated ethanol. In the 51V NMR spectra, the signals for the vanadium complexes shift significantly depending on the alkoxido ligand coordinated.32,61 Thus, complex 1 gives a signal at −531 ppm, complex 2 at −547 ppm, complex 3 at −592 ppm, complex 4 at −597 ppm, complex 5 at −590 ppm, and complex 8 (measured in MeOD) at −547 ppm. The purified ligands Cl-ono-Cl and tA-ono-tB show similar patterns in the 1H and 13C NMR spectra (see the SI). The coordination of the ligand to the vanadium leads to the formation of the monomers 9a, 10a, 10b, 11a, 11b, and 12a, which show very similar 1H NMR spectra (see the SI). It is worth mentioning that an excess (alkoxide:alcohol ∼1:1.5 to 8) of the corresponding alcohol was added to the NMR solutions, in order to avoid formation of the dinuclear species (vide inf ra). With the exception of 11b, each vanadium has one alkoxido ligand (ethoxide or cyclohexanoxide) coordinated as can be seen in the 1H spectrum by a multiplet at ∼5.5 ppm for the CH2 group, which is bound to the oxygen. In the case of 11b, the methoxido ligand can exchange rapidly when measured in deuterated methanol, and therefore, no signal can be observed as already shown for the dipic system (vide supra). The signal for the imine hydrogen is significantly shifted to ∼9.2 ppm, and the signals for the aromatic hydrogens are slightly spread between 7.7 and 6.8 ppm, compared to the respective ligand (for detailed assignment, see the SI). In the comparable carbon spectra of the monomers, the imine carbon signal is considerably shifted upfield to ∼153 ppm compared to the free ligand. Depending on the alkoxido ligand, one signal between ∼92 ppm (CyO−) and ∼80 ppm (EtO−), respectively, can be attributed to the carbon atom bound to the oxygen. In the case of 10a, 10b, 11a, 11b, a relatively upfield shifted signal (∼9.4 ppm) corresponding to the distal CH3 carbon atom of the tert-amyl group can be observed. F


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Scheme 4. Dehydroperoxidation of 4-Heptyl and Cyclohexyl Hydroperoxide under Different Conditions and with the Vanadium Complexes 5, 8, 10c, 11c, and 12a

This can possibly be due to the low solubility of the in situ formed hydroperoxido complex 6H2O, which crashed out from solution. Small bright yellow crystals, obtained directly from the reaction mixture, were suitable for XRD measurements, and the molecular structure shows the hydroperoxide coordinated to vanadium via the two oxygen atoms in an η2-fashion (see Figure 4).

spectra show characteristic bands for the complexes with Schiff bases (9−12) at ∼1290 and 1260 cm−1 for the C−O stretching of the phenolates as well as one at ∼1625 cm−1 for the imine CN bond stretching of the free ligand.56,66 This feature shifts to lower frequencies (∼1605−1612 cm−1) by complexation, indicating V−N bond formation.67,68 For the vanadium complexes, a new band at ∼976−996 cm−1 arises and is attributed to the VO stretching vibration that is in good agreement with reported values.43,49,69,70 In the case of the dinuclear complexes 10c and 11c, either two bands (996 and 986 cm−1) or one very broad band (989 cm−1) can be observed, verifying the presence of two nonequivalent vanadium centers. This feature corresponds to the behavior that was seen in the 51V NMR spectra of 10c and 11c with either two peaks or one coincident signal (vide supra). Furthermore, a significant band at ∼447 cm−1 for the dipic-based complexes and one at ∼474 cm−1 for complexes 9−12 can be attributed to the V−O stretching mode.69−71 Dehydroperoxidation of Alkyl Hydroperoxides. As the synthesis of 4-heptyl hydroperoxide is well described in the literature,72 this secondary hydroperoxide was initially used as a model compound (Scheme 4). First, several solvents were tested whereby it was found that coordinating solvents like methanol-d4 inhibited the reaction, as the competitive coordination of the alcohol is favored. Initially, the dehydroperoxidation reactions were carried out with complex 5 in dichloromethane-d2 or chloroform-d1. When the hydroperoxide and the vanadium complex are mixed in a 1:1 molar ratio, the in situ formation of a vanadium alkyl peroxido complex can be observed. The characteristic signal for the coordinated alcohol at 6.04 ppm disappears and a new one arises at 5.26 ppm corresponding to the coordinated hydroperoxido. The reaction at room temperature is very slow (∼5 days to reach full conversion) even with a high excess of hydroperoxide (1:16 molar ratio of catalyst:hydroperoxide), though the selectivity toward the ketone was quite good with a product mixture showing a 3.7:1 molar ratio of ketone to alcohol. When the reaction temperature was increased up to 60 °C, the reaction was much faster and was finished within 120 min; however, the molar ratio of products drops to 2.4:1 (ketone: alcohol). HeptOOH was used as a model compound, and therefore, complexes 8, 10c, 11c, and 12a were not tested toward its decomposition. As these results were very promising, reactions with the target compound cyclohexyl hydroperoxide were undertaken (Scheme 4). At room temperature, the reaction was very slow, and even after 29 days, ∼7% of the hydroperoxide was still unconverted. The selectivity was also poor (1:1 molar ratio of ketone:alcohol).

Figure 4. Molecular structure of the vanadium(V) complex [VO(dipic)(H2O)(η2-OOCy)] (6H2O) obtained from the reaction mixture. Hydrogen atoms are partly omitted for clarity. Thermal ellipsoids are at 50% probability. Color code: gray - carbon, red - oxygen, green vanadium, blue - nitrogen, white - hydrogen. Selected bond lengths [Å] for complex 6H2O: V−O1: 1.579(3), V−O2: 1.992(3), V−O3: 2.007(3), V−O4: 1.885(13), V−O5: 2.008(9), V−O6: 2.215(4), V−N: 2.057(3), O−C1: 1.480(10), O−O: 1.438(10), O···H: 2.742.

The V−O4 distance is significantly shorter (1.885(13) Å) compared to the V−O3 bond (2.007(3) Å), which is in the same range than the V−O dipic bond lengths. Although several molecular structures of vanadium peroxido complexes are known42,73,74 and vanadium alkyl peroxido complexes are characterized by various spectroscopic methods, only one molecular structure for a vanadium alkyl peroxido complex, namely, [VO(dipic)(H2O)(η2-OOtBu)], was published by Mimoun and co-workers.35 In this case, the tert-butyl hydroperoxide coordinates to the vanadium in a similar η2-fashion and all the bond lengths are in very good agreement with the ones found for complex 6H2O. Additionally, only two complexes are known in the literature with cyclohexyl hydroperoxide as a ligand.28 In these cases, the alkyl peroxide is bound in a κ1-fashion to a cobalt(III) ion with comparable O−O and O−CCy distances like in 6H2O. In contrast to the other molecular structures presented herein, the vanadium in complex 6H2O exhibits a pentagonal bipyramidal geometry. The terminal oxido ligand and G


Article

Inorganic Chemistry

Figure 5. Comparison of free energy pathways for cyclohexyl hydroperoxide (black) and 4-heptyl hydroperoxide (red) and the conversion of cyclohexyl hydroperoxide to cyclohexanone without (black) and with water, coordinating trans to the oxido group on the catalyst (blue). Energies are given in kJ/ mol and are relative to 6H2O, which was isolated experimentally. The energy levels following TS15/TS22/TS26 are adjusted in height due to the highly exergonic nature of the reaction.

When running the reaction with complex 1 and CyOOH in acetonitrile, yellow needles of complex 7, suitable for X-ray diffraction (see the SI), are formed, which decompose within 2 days and inhibit the catalysis reaction. In order to increase the solubility of the dipic complex not only in chlorinated solvents but also in cyclohexane, complex 8 was synthesized and tested toward dehydroperoxidation. These reactions were carried out in deuterated cyclohexane at 60 °C, and complete conversion was achieved after 180 min during which a second aqueous layer was formed. As the ligand framework seemed to be very promising to favor the formation of ketone (vide inf ra), a second set of vanadium(V) complexes with higher solubility in cyclohexane was tested. Using 10c, 11c, and 12a in deuterated cyclohexane at room temperature, complete decomposition of cyclohexyl hydroperoxide was achieved after 3 days. However, the selectivity could not be improved and remained comparable to the reactions with the dipic ligands with ketone-to-alcohol ratios of 1:0.8 (10c), 1:0.9 (11c), and 1:1.4 (12a). In the case of the dipic-based complexes, elevated temperatures led to a worse selectivity, and therefore, complexes 10c, 11c, and 12a were not tested at 60 °C. However, when running the reaction in stoichiometric ratios at 60 °C, slow decomposition of the ligand framework to the amine and aldehyde was observed by 1H NMR.30 DFT Calculations. In order to clarify the mechanism underlying the dehydroperoxidation of alkyl hydroperoxides to the corresponding ketones by a dipicolinato vanadium(V) complex, several DFT calculations were performed (for details, see the SI). Two different potential mechanisms were investigated in detail for each alkyl hydroperoxide containing complexes with and without a coordinating water molecule. In the first case, the catalytic cycle (see Figure 5) starts from

the labile bound water molecule occupy the axial positions, while the tridentate dipic ligand and the η2-bound peroxido coordinate to the equatorial positions. Due to the bent orientation of the cyclohexyl ring, the hydrogen atom bound to the secondary carbon is within a short distance of the terminal oxido group, which can lead to hydrogen transfer, forming cyclohexyl ketone as calculated by DFT (vide inf ra). When heating the reaction mixtures to 60 °C, the decomposition of the hydroperoxide is finished within 210 min; however, even in hot solutions, complex 6 precipitates and the selectivity remains low with a molar ratio of 1:0.88 (ketone: alcohol). Additionally, due to the high hydroperoxide loading, the formed water competes with the hydroperoxide for the coordination site at the vanadium. Furthermore, concurrent radical reactions cannot be excluded as the paramagnetic vanadium(IV) complex [VO(dipic)(H2O)2] was also formed during the reaction, as could be proven by blue crystals obtained from the aqueous phase. These crystals were suitable for XRD analysis and showed the same cell parameters published by Bai and co-workers.75 Additionally, plots of ln([A]/[A0]) versus reaction time at room temperature, 40 and 60 °C (see the SI) show different reaction orders. Whereas, at room temperature and 40 °C, the catalysis reactions proceed by first order, the reaction at 60 °C shows a more complicated reaction order. This suggests that, at higher temperatures, at least two different mechanisms compete with each other, leading to a decrease in selectivity. When a second alcohol (cyclooctanol) was added at 60 °C, it was additionally converted to the corresponding ketone (cyclooctanone). This hints at a two-step mechanism, with the hydroperoxide being decomposed to the alcohol first, which is then oxidized in a second step to the ketone. For further discussion, see the section on DFT calculations (vide inf ra). H


Article

Inorganic Chemistry complex 6H2O (or 20 when 4-hydroperoxy heptane is the substrate), which was isolated and characterized by X-ray crystallography in the case of the cyclohexyl hydroperoxide. Upon loss of water, the intermediate 13/21 is formed and recoordination of the peroxide to the η1 binding mode results in the “loose” alkyl peroxido complex 14/22 that is higher in energy than the starting complex 6H2O/20 (27 kJ/mol for CyOOH and 19 kJ/mol for HeptOOH). The change in coordination from the η2 to the η1 binding mode is critical to enter the six-membered transition states TS15 and TS23 in which the hydrogen, bound to the peroxide carbon, is transferred to the vanadium oxido group. Proton relays were also studied for this step (see the SI); however, they are higher in energy. Simultaneously, the weak O− O bond is cleaved, releasing the corresponding ketone and creating a new oxido group on the vanadium center together with the newly formed hydroxyl group. The barrier of this reaction with respect to the “loose” alkyl hydroperoxides 14 and 22 is 83 kJ/mol for CyOOH and 73 kJ/mol for HeptOOH, and irreversible decay into the ketone and intermediate 16 is strongly exergonic by 365 or 381 kJ/mol, respectively. The hydroxy vanadyl intermediate 16 can then add another molecule of alkyl hydroperoxide to regenerate the initial complexes 6H2O and 20 via TS17 and water. Addition of one molecule of cyclohexyl hydroperoxide to the vanadium hydroxyl complex 16 in the absence of water or alcohol can proceed through the calculated four-membered transition state TS19, which has a relatively high energetic barrier of 80 kJ/mol for CyOOH. However, this barrier can be lowered through proton relay mechanisms, and water or alcohol can act as such proton relay agents. The respective transition states are indeed lower, with a barrier of 65 kJ/mol for TS18 through water (from the solvent) and 57 kJ/mol for TS17 with cyclohexanol (from the starting vanadium complex). The overall highest energetic gap in this mechanism, namely, the hydrogen transfer in TS15/TS23, amounts to 110 kJ/mol for CyOOH and 92 kJ/mol for HeptOOH, which is around half of the homolytic bond dissociation energy of a typical O−O bond. A lower energetic gap of 101 and 86 kJ/mol is obtained for a similar mechanism in which one molecule of water remains at the catalyst during the reaction. Thus, complexes with water coordinating either cis (see Figure S64) or trans (see Figure 5) to the vanadium oxido group were calculated with the former being favorable by only 1 kJ/mol for CyOOH and 6 kJ/mol for HeptOOH. In contrast, the trans-coordinated water results in a lowering of the barrier by 9 kJ/mol for CyOOH (TS26), which can be explained by the oxido group’s strong trans effect. The DFT calculations confirm the experimental results, as higher selectivity toward decomposition to the corresponding ketone is observed in the case of heptyl hydroperoxide. The lower barrier may arise from the linear alkane’s capability to slightly better adapt to the transition state geometry, which is restricted in cyclic alkanes. In fact, when comparing the “loose” complexes 14 and 22 with the following transition states TS15 and TS23, looking at the distance between the oxido group and the hydrogen to be transferred, reveals that, for the linear alkane, the preceding intermediate 22 is closer to the transition state geometry than its cyclic counterpart. The changes in O−H distance are 0.72 Å for HeptOOH compared to 0.89 Å for CyOOH (for mechanisms with water coordinating trans, the changes are lower for both, but follow the same trend: 0.61 Å for HeptOOH, 0.76 Å for CyOOH). Additionally, a series of other mechanisms were also investigated for the dehydroperoxidation of cyclohexyl hydroperoxide to cyclohexanone (see the SI): Hydrogen transfer to the carboxyl group of the dipic ligand is unlikely to take place under

the employed reaction conditions as it has an overall barrier of 162 kJ/mol compared to 110 kJ/mol for the transfer to the vanadium oxido group. Addition of cyclohexyl hydroperoxide to the hydroxido vanadium complex 16 by protonation of the ligand opens up pathways similar to the mechanism shown in Figure S66. Protonation of the carboxyl group is in fact feasible with an activation energy of 57 kJ/mol; however, both following hydrogen transfer transition states to the oxido or hydroxido group are too high in energy (117 and 135 kJ/mol, respectively). Moreover, insertion of the α-hydrogen of CyOOH into the O− O bond is disfavored with a barrier of 202 kJ/mol. This value is considerably reduced by proton relay through cyclohexanol (130 kJ/mol), which can be attributed to formation of a much less strained six-membered transition state, but is still higher in energy than hydrogen transfer to the vanadium oxido group. The formation of the corresponding alcohol most likely involves radical formation and/or homocoupling to dialkyl peroxides. The latter requires two substrate molecules binding simultaneously to one vanadium center, which might be favored by sterically less flexible moieties, like the cyclohexyl ring. Indeed, the formation of the dialkyl peroxide is only observed for the reaction with cyclohexyl hydroperoxide. Decomposition of the peroxide toward the alcohol occurs at around 60 °C for 4HeptOOH and at room temperature for cyclohexyl hydroperoxide. This is generally not desired, and the corresponding mechanisms were thus not investigated. Previous research on homogeneous and heterogeneous hydroperoxide decomposition has described radical-based mechanisms for alcohol formation.22,76 Also, the observation of reduced blue vanadium species in the experiments vide supra hints at radical pathways for this process.30,77

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CONCLUSION Several vanadium(V) complexes with either dipic-based or Schiff base ligands were synthesized. The complexes were fully characterized by elemental analysis, IR, 1H, 13C, and 51V NMR spectroscopy, as well as mass spectrometry and X-ray diffraction. Furthermore, they were tested toward their catalytic dehydroperoxidation behavior and a significant difference between 4heptyl hydroperoxide and cyclohexyl hydroperoxide was observed. In the case of 4-heptyl hydroperoxide, selectivity toward the corresponding ketone could be improved, which can be attributed, beside others, to the higher solubility of the in situ formed vanadium(V) hydroperoxido complex. Finally, DFT calculations performed on the vanadium complex showed that selective decomposition of secondary hydroperoxides with vanadium(V) to yield the corresponding ketone and water is indeed energetically feasible. The computed catalytic path, involving cleavage of the O−O bond, hydrogen transfer, release of ketone/water, and finally addition of hydroperoxide, can completely proceed without the generation of radical species. Further investigations are needed, as the demand on polyamides will increase in the future,78 and consequently, a more rational route to cyclohexanone is desired. From an economic point of view, avoiding the formation of cyclohexanol will decrease extra separation and oxidation steps, leading to a more efficient and straightforward process. Although higher selectivity could not be achieved with cyclohexyl hydroperoxide so far, the results are promising to help in developing a well-performing catalyst, as we could show that selective dehydroperoxidation to the ketone is, in principle, possible. It is mandatory that the complex exhibits high solubility especially in cyclohexane and stability at elevated temperatures (130 °C). As DFT calculations suggest, having a I


Article

Inorganic Chemistry

extracted several times with dichloromethane, brine, and H2O. The solution was dried over MgSO4 and the solvent was evaporated at 20 °C to yield a yellowish oil. This oil was purified by column chromatography with n-hexane:diethyl ether 10:1 to yield the alkyl hydroperoxide as a colorless oil. This was stored at 0 °C to avoid any decomposition. Heptyl hydroperoxide. Yield: (0.189 g, 1.43 mmol, 16%). Cyclohexyl hydroperoxide. Yield: (0.133 g, 1.14 mmol, 13%). Ligand Synthesis. Synthesis of dipicOMe. Pyridine-2,6-dicarboxylic acid (10.0 g, 59.8 mmol) was dissolved in 220 mL of methanol. Then, 9.11 mL of concentrated H2SO4 was added within 5 min, and the clear solution was refluxed for 3 days. The solvent was evaporated, and 300 mL of dichloromethane was added. The organic phase was extracted several times with water and saturated NaOH solution and dried over MgSO4. Subsequently, the solvent was evaporated to yield dipicOMe as a white solid. Yield: 8.56 g, 43.9 mmol, 73%. Synthesis of HOdipicOMe. HOdipicOMe (1.95 g, 10 mmol, 1.1 equiv) was dissolved in 10 mL of methanol and 10 mL of concentrated H2SO4. The mixture was cooled to 10 °C and simultaneously a solution of FeSO4·7H2O (2.56 g, 9.21 mmol, 1 equiv) in 10 mL of water and H2O2 (7.14 mL, 30% in H2O) was added dropwise within 20 min. The reaction mixture was stirred for an additional 30 min, and subsequently, the solution was treated with a saturated K2CO3 solution (∼60 mL) to reach a pH of 6 to 7. The formed solid was filtered off, and the filtrate was washed several times with ethyl acetate. The solution was dried over MgSO4 and evaporated to yield a yellowish solid, which was further purified by column chromatography using n-hexane:ethyl acetate (1:1). Yield: 454 mg, 2.02 mmol, 20%. Synthesis of AdOdipicOH. Under argon, trifluoromethanesulfonic anhydride (462 μL, 2.75 mmol, 1.56 equiv) was dissolved in 2 mL of absolute dichloromethane and cooled to 0 °C. HOdipicOMe (444 mg, 1.97 mmol, 1.12 equiv) and triethylamine (246 μL, 1.76 mmol, 1 equiv) were dissolved in 15 mL of absolute dichloromethane and slowly added to the anhydride solution. The reaction mixture was stirred for 1 h at room temperature, and 1-adamantanemethanol (6.74 g, 41.6 mmol, 23.6 equiv), dissolved in 40 mL of absolute dichloromethane, was added. The solution was stirred for an additional 40 min at room temperature, and triethylamine (1.57 mL, 11.3 mmol, 6.4 equiv) was added one more time. After the reaction was stirred overnight, the solvent was evaporated and the residue was taken up in 100 mL of water. The solution was washed several times with n-hexane, and the combined organic phases were evaporated. The white solid was dissolved in 100 mL of methanol, KOH (8.5 g) was added, the mixture was stirred for 1 h, and the solvent was removed. The solid was taken up in 150 mL of sodium hydroxide solution (2 M), and 150 mL of ethyl acetate was added. The mixture was vigorously stirred for 10 min and extracted several times with ethyl acetate. Dichloromethane was added to the aqueous phase, and the pH was adjusted to 3 with a 2 M HCl solution (∼80 mL). The solution was washed several times with dichloromethane. The combined organic phases were dried over MgSO4 and the solvent was evaporated to yield a white solid. Yield: 170 mg, 0.49 mmol, 25%. Synthesis of Cl-ono-Cl. 2-Amino-4-chlorophenol (2.00 g, 13.9 mmol, 1 equiv ) and 5-chlorosalicylaldehyde (2.18 g, 13.9 mmol, 1 equiv) were dissolved in 20 mL of ethanol, and the mixture was stirred for 1 h at room temperature, leading to a bright orange precipitate. 38 Subsequently, the solution was heated to 60 °C and cooled down slowly. The microcrystals formed were filtered off and dried in vacuo. Yield: 2.24 g, 7.94 mmol, 57%. Synthesis of tA-ono-tB. 2-Amino-4-tert-amylphenol (2.00 g, 11.2 mmol, 1 equiv) and 5-tert-butyl-2-hydroxybenzaldehyde (1.99 g, 11.2 mmol, 1 equiv) were dissolved in 20 mL of ethanol, and the mixture was stirred for 1 h at room temperature, leading to a brown-orange solution.38 Subsequently, the solution was evaporated and the orange residue was dried in vacuo. Yield: 3.37 g, 9.92 mmol, 89%. Complex Synthesis. Synthesis of [VO(dipic)(OH)]2 (1). This procedure was carried out as reported by Mimoun and co-workers.35,37 Synthesis of [VO(dipic)(OMe)] (2). Crystals suitable for XRD analysis of complex 2 were obtained from a high concentrated solution of complex 3 in methanol. Isolation of the complex was not possible as the labile bound methoxido ligand was ripped off in vacuo, leading to complex 1.

terminal MO group supports direct hydrogen transfer from the secondary hydroperoxide to form the ketone in one step. On the basis of these insights, we will further work on this topic, to identify an appropriate catalyst for the selective dehydroperoxidation of cyclohexyl hydroperoxide.

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EXPERIMENTAL SECTION

General Methods. Unless otherwise noted, all reactions were carried out under air in round-bottom flasks or sealed NMR tubes. Solvents and deuterated solvents were purchased dry from Aldrich and used without further purification. NMR spectra were recorded using a Bruker 200 at CaRLa or Bruker AVANCE III 300, Bruker AVANCE III 400, Bruker AVANCE III 500, and Bruker AVANCE III 600 spectrometers at the Organisch-Chemisches Institut der Universität Heidelberg. Chemical shifts are given in ppm referenced to solvent (1H, 13 C) and relative to VOCl3 for 51V NMR. Mass spectroscopy was performed by the MS-Labor, Organisch-Chemisches Institut der Universität Heidelberg. Elemental analysis was performed by the “Mikroanalytisches Laboratorium” der Chemischen Institute der Universität Heidelberg with a Vario MIKRO cube. IR vibrational spectra were recorded from 3500 to 400 cm−1 (Varian 2000, Scimitar Series, FTS2000) as KBr pellets at room temperature. Crystals, suitable for XRD analysis, were measured on a Bruker APEX, Bruker APEX II Quazar, or STOE Stadivari diffractometer. Materials. Unless otherwise noted, reagents were purchased from commercial suppliers and used without further purification. All chemicals were used as received. The vanadium precursor VO(OEt)3 was purchased from Sigma-Aldrich and stored under argon. Computational Details. Geometries were optimized at the BP86/ def2-SV(P) level of theory.79,80 Final electronic energies were computed at the PBE0-D3(BJ)/def2-QZVPP level81 using dispersion correction with Becke−Johnson damping.82,83 Electronic structure calculations were carried out with TURBOMOLE,84 employing the resolution-ofidentity approximation85 (RI) and the corresponding auxiliary basis sets.86,87 Zero-point vibrational energies and thermochemical corrections were computed in the gas phase within the usual harmonicoscillator, rigid-rotor approximation at the BP86/def2-SV(P) level for T = 298.15 K and p = 1 bar with TURBOMOLE’s f reeh program. Symmetry numbers for the quasi-classical rotational partition sum were chosen based on the symmetry of each molecule. All positive vibrational frequencies were included in the calculation of the vibrational partition function. Free enthalpies of solvation for each compound at infinite dilution in cyclohexane were obtained at the BP86/def-TZVP level through COSMO-RS88 (conductor-like screening model for realistic solvents) using COSMOtherm89 (Version C3.0, Release 1501, revision 1744). The conformational partition function was set to one, so that only the energetically lowest conformer was considered for each species. For all structures involving 4-heptyl hydroperoxide, the linear conformation of the alkyl chain was chosen. In the case of cyclohexanol and cyclohexyl hydroperoxide, the lowest conformers were those with the respective group in the equatorial position on the ring. Alkyl Hydroperoxide Synthesis. Caution! Although no problems were encountered in handling the pure alkyl hydroperoxides even at higher temperature, they are potentially explosive as well as oxidizing and should be handled in small amounts with great care. The syntheses of the 4-heptyl and cyclohexyl hydroperoxides were carried out via a one-pot method by a modified procedure reported in the literature by Ball and co-workers.72 Under argon, the corresponding ketone (8.8 mmol, 1 equiv) was dissolved in 18 mL of absolute ethanol and p-toluenesulfonyl hydrazide (9.6 mmol, 1.1 equiv) was added. The clear solution was stirred for 4 h at room temperature, after which the solvent was evaporated. The white residue was taken up in 150 mL of absolute THF and cooled to 0 °C. To this mixture was added borane dimethylsulfide dropwise, and the mixture was stirred overnight. 100 mL absolute methanol was added to quench the solution, and the solvent was evaporated. Finally, 125 mL of absolute dioxane was added together with NaOH pellets (24 mmol, 2.7 equiv) and H2O2 (30% in H2O, 190 mL). The slurry mixture was vigorously stirred for 3−5 days (checked by TLC) at room temperature. Thereafter, the cloudy white solution was J


Article

Inorganic Chemistry Synthesis of [VO(dipic)(OEt)] (3). VO(OEt)3 (1.00 g, 4.95 mmol) and the ligand pyridine-2,6-dicarboxylic acid (827 mg, 4.95 mmol) were dissolved in 10 mL of absolute ethanol, and the mixture was stirred for 4 h at 70 °C. During cooling, yellow-orange microcrystals were formed, which were filtered off, washed with ethanol, and dried. By evaporating half of the solvent, a second batch of complex 3 could be obtained. Combined yield: 1.180 g, 3.66 mmol, 74%. Synthesis of [VO(dipic)(OHept)] (4). Complex 1 (0.50 g, 0.94 mmol) was dissolved in 10 mL of water, and 0.8 mL of 4-heptanol was added. The two layers were vigorously stirred 2 h at room temperature, and 10 mL of dichloromethane was added. The orange organic phase was separated and dried over MgSO4, and all volatiles were evaporated in vacuo to yield a bright yellow solid. Yield: 30 mg, 64.7 μmol, 7%. Synthesis of [VO(dipic)(OCy)] (5). Complex 1 (1.00 g, 1.87 mmol) was dissolved in 25 mL of water, and 0.5 mL of cyclohexanol was added. The two layers were vigorously stirred 2 h at room temperature, leading to a yellowish precipitate. The solid was filtered, washed with cyclohexane, and dried to yield a yellowish solid. Yield: 0.374 g, 1.07 mmol, 57%. Synthesis of [VO(dipic)(OOCy)(H2O)] (6H2O). Bright yellow crystals of complex 6H2O, suitable for XRD analysis, were obtained from a reaction mixture of complex 5 and cyclohexyl hydroperoxide in CD2Cl2. Isolation of the complex was not possible due to the fast decomposition. Synthesis of [{VO(dipic)(ACN)}-(μ-O)-{VO(dipic)}] (7). Crystals of complex 7, suitable for XRD analysis, were obtained from a reaction mixture of complex 1 and cyclohexyl hydroperoxide in acetonitrile. Synthesis of [VO(AdOdipic)(OEt)] (8). AdOdipicOH (27.7 mg, 0.08 mmol, 1 equiv) and VO(OEt)3 (14.2 μL, 0.08 mmol, 1 equiv) were dissolved in 1 mL of absolute ethanol, and the mixture was stirred at room temperature for 1 h within the solution gets turbid. The mixture was stored in the fridge overnight, leading to more precipitation. The solid was filtered off, washed with cold ethanol, and dried to yield complex 8 as a brownish solid. Yield: 20.5 mg, 0.05 mmol, 56%. Synthesis of [VO(Cl-ono-Cl)(OEt)] (9a). VO(OEt)3 (358 mg, 1.77 mmol, 1 equiv) was dissolved in 5 mL of absolute ethanol, and the ligand Cl-ono-Cl (500 mg, 1.77 mmol, 1 equiv) was added. The mixture was stirred for 30 min at 60 °C and cooled to room temperature. The formed precipitate was filtered, washed with ethanol, and dried, leading to a brown solid. Yield: 649 mg, 1.68 mmol, 94%. Synthesis of [VO(Cl-ono-Cl)(OMe)] (9b). Crystals of complex 9b, suitable for XRD analysis, were obtained from a saturated solution of complex 9a in methanol. Suitable amounts of complex 9b for further characterization could not be obtained due to the low solubility of complex 9a. Synthesis of [{VO(Cl-ono-Cl)}2-μ-O] (9c). Crystals of complex 9c, suitable for XRD analysis, were obtained from a saturated solution of complex 9a in chloroform. Suitable amounts of complex 9c for further characterization could not be obtained due to the low solubility of complex 9a. Synthesis of [VO(tA-ono-tB)(OEt)] (10a). VO(OEt)3 (298 mg, 1.47 mmol, 1 equiv) was dissolved in 5 mL of absolute dichloromethane, and the ligand tA-ono-tB (500 mg, 1.47 mmol, 1 equiv) was added. The mixture was stirred for 30 min at room temperature, and after that, the solvent was evaporated, yielding a black powder. This was taken up in diethyl ether, leading to a black brown precipitate. The solid was filtered off, washed with diethyl ether, and dissolved in ethanol. Subsequently, the solvent was removed in vacuo to obtain complex 10a. Yield: 549 mg, 1.22 mmol, 83%. Synthesis of [VO(tA-ono-tB)(OCy)] (10b). Complex 10c (50.0 mg, 60.6 μmol, 1 equiv) was dissolved in 3 mL of dichloromethane, and cyclohexanol (13 μL, 0.12 mmol, 2 equiv) was added. The solution was stirred for 1 h at room temperature; after that, the solvent was evaporated in vacuo to yield an orange-brown residue. Yield: 13 mg, 21.5 μmol, 36%. Synthesis of [{VO(tA-ono-tB)}2-μ-O] (10c). Complex 10a (200 mg, 0.45 mmol) was taken up in 3 mL of diethyl ether, and the mixture was stirred for 10 min at room temperature to obtain a brown precipitate. The solution was cooled to 0 °C, and the solid was filtered off, washed with cold diethyl ether, and dried in vacuo. Yield: 107 mg, 0.13 mmol, 58%.

Synthesis of [VO(tA-ono-2tB)(OEt)] (11a). Complex 11a was synthesized in NMR scale. Therefore, complex 11c was dissolved in CDCl3 and 2 equiv of absolute ethanol was added, leading to no color change. For IR characterization, the solvent was evaporated in vacuo. Synthesis of [VO(tA-ono-2tB)(OMe)] (11b). Complex 11b was synthesized in NMR scale. Therefore, complex 11c was dissolved in MeOD. Isolation of the complex was not possible as the labile bound methoxido ligand was ripped off in vacuo, leading to complex 11c. Synthesis of [{VO(tA-ono-2tB)}2-μ-O] (11c). The ligand was synthesized in advance according to the procedure of Cl-ono-Cl and was used without further purification due to its high solubility.38 Therefore, 2-amino-4-tert-amylphenol (1.00 g, 5.58 mmol, 1 equiv) and 3,5-di-tert-butyl-2-hydroxybenzaldehyde (1.31 g, 5.58 mmol, 1 equiv) were dissolved in 25 mL of absolute ethanol, and the mixture was stirred for 12 h at 60 °C, leading to a brown-orange solution. The solvent was evaporated in vacuo, and the orange residue was taken up in nhexane:ethyl acetate (30:1) and filtered over a Celite pad. Additionally, the solvent was evaporated in vacuo and the solid was dried in vacuo for further use. VO(OEt)3 (232 mg, 1.15 mmol) was dissolved in 5 mL of absolute dichloromethane, and the ligand tA-ono-2tB (500 mg, 1.26 mmol) was added. The mixture was stirred for 30 min at room temperature, and afterward, the solvent was evaporated in vacuo, yielding a black-brown powder. This was taken up in a mixture of nhexane:ethyl acetate (10:1), leading to a red brown precipitate, which was filtered off, washed with the same mixture, and dried in vacuo. Yield: 443 mg, 0.47 mmol, 41%. Synthesis of [VO(2tB-ono-2tB)(OEt)] (12a). The ligand was synthesized in advance according to the procedure of Cl-ono-Cl and was used without further purification due to its high solubility.38 2-Ditert-butylphenol (5.00 g, 24.2 mmol, 1 equiv) was dissolved in 45 mL of acetic acid and cooled to 5 °C. Slowly, nitric acid (65% in H2O, 3.10 g, 30.0 mmol, 1.24 equiv) was added, and the suspension was stirred at 5 °C for 40 min. Thereafter, 42 mL of iced water was added and the solution was extracted several times with ethyl acetate. The combined organic phases were dried over MgSO4 and the solvent was evaporated to yield an orange-red oil (6.08 g). A 2 L round-bottom flask was charged with 500 mL of absolute ethanol, the oil, and 1.00 g of Pd/C. The atmosphere was slightly evacuated and filled with hydrogen (1 bar), which led to an immediate color change from dark-red to yellow. The suspension was stirred for 12 h at room temperature and, after that, was filtered over a Celite pad. After the solvent was evaporated, the residue was taken up in dichloromethane, dried over MgSO4, and evaporated to yield a dark-red solid (3.89 g). 2-Amino-4,6-di-tert-butylphenol (0.95 g, 4.27 mmol, 1 equiv) and 3,5-di-tert-butyl-2-hydroxybenzaldehyde (1.00 g, 4.27 mmol, 1 equiv) were dissolved in 10 mL of absolute ethanol, and the mixture was stirred for 12 h at room temperature, leading to a bloodred solution. The solvent was evaporated, and the dark-red residue was taken up in n-hexane:ethyl acetate (10:1) and filtered over a Celite pad. Additionally, the solvent was evaporated and the solid was dried in vacuo for further use. Crystals of complex 12a, suitable for XRD analysis, were obtained from a highly concentrated chloroform solution. VO(OEt)3 (200 mg, 0.99 mmol, 1 equiv) was dissolved in 5 mL of absolute ethanol, and the ligand 2tB-ono-2tB (476 mg) was added. The mixture was stirred for 30 min at 60 °C and slowly cooled to room temperature. Thereby, a golden precipitate is formed, which was filtered, washed with ethanol, and dried in vacuo. Yield: 181 mg, 0.33 mmol, 33% (based on VO(OEt)3). Synthesis of [VO(2tB-ono-2tB)(OMe)] (12b). Crystals of complex 12b, suitable for XRD analysis, were obtained from a saturated solution of complex 12a in methanol. Dehydroperoxidation of Alkyl Hydroperoxides. Each reaction was performed several times, and herein, the results are presented as average examples. All reactions were carried out in deuterated solvents used as purchased and under air in sealed NMR tubes. Ratios of ketone to alcohol and the conversion of CyOOH were measured by integration of the corresponding characteristic signals in 1H NMR spectra. Complex 5 and HeptOOH. Complex 5 (1 equiv) and 4-heptyl hydroperoxide (16 equiv) were dissolved in 1 mL of CD2Cl2, leading to a clear pale yellow solution. The reaction was followed by NMR measurements. After 5 days at room temperature, 58% of the K


Inorganic Chemistry

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hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 3.4 to 1. Complex 5 and HeptOOH. Complex 5 (1 equiv) and 4-heptyl hydroperoxide (1 equiv) were dissolved in 1 mL of CD2Cl2, leading to a clear yellow solution. The reaction was followed by NMR measurements. After 2 days at room temperature, 100% of the hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 1.22 to 1. Complex 5 and HeptOOH. Complex 5 (1 equiv) and 4-heptyl hydroperoxide (13 equiv) were dissolved in 1 mL of CD2Cl2, leading to a clear pale yellow solution. The reaction was followed by NMR measurements. After 6 days at room temperature, 61% of the hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 3.7 to 1. Complex 5 and HeptOOH. Complex 5 (1 equiv), 4-heptyl hydroperoxide (37 equiv), and naphthalene as internal standard were dissolved in 1 mL of CDCl3 and heated to 60 °C, leading to a clear pale yellow solution. The reaction was followed by NMR measurements. After 120 min at 60 °C, 100% of the hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 2.4 to 1. Complex 5 and CyOOH. Complex 5 (1 equiv) and cyclohexyl hydroperoxide (11 equiv) were dissolved in 1 mL of CD2Cl2, leading to a dusty pale yellow solution. The reaction was followed by NMR measurements. After 29 days at room temperature, 93% of the hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 0.9 to 1. Additionally, bright yellow crystals precipitated during the reaction. Crystals, suitable for XRD analysis, could be obtained from such a reaction mixture and turned out to be complex 6. Complex 5 and CyOOH. Complex 5 (1 equiv), cyclohexyl hydroperoxide (94 equiv), naphthalene as internal standard, and cyclooctanol (19 equiv) were dissolved in 1 mL of CDCl3 and heated to 60 °C, leading to a pale yellow solution. The reaction was followed by NMR measurements. After 240 min at 60 °C, most of the hydroperoxide was decomposed to the corresponding ketone and alcohol. Additionally, almost all cyclooctanol was converted to cyclooctanone. Complex 8 and CyOOH. Complex 8 (1 equiv), cyclohexyl hydroperoxide (154 equiv), and naphthalene as internal standard were dissolved in 1 mL of deuterated cyclohexane and heated to 60 °C, leading to a pale yellow solution. The reaction was followed by NMR measurements. After 180 min at 60 °C, 100% of the hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 0.8 to 1. Additionally, a second orange layer was formed during the reaction. Complex 5 and CyOOH. Complex 5 (1 equiv), cyclohexyl hydroperoxide (88 equiv), and naphthalene as internal standard were dissolved in 1 mL of CDCl3 and heated to 60 °C, leading to a dusty pale yellow solution. The reaction was followed by NMR measurements. After 210 min at 60 °C, 93% of the hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 1.1 to 1. Complex 10c and CyOOH. Complex 10c (1 equiv), cyclohexyl hydroperoxide (224 equiv), and naphthalene as internal standard were dissolved in 1 mL of deuterated cyclohexane, leading to a pale yellow solution. The reaction was followed by NMR measurements. After 3 days at room temperature, 80% of the hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 1.3 to 1. Additionally, a second orange layer was formed during the reaction. Complex 11c and CyOOH. Complex 11c (1 equiv), cyclohexyl hydroperoxide (351 equiv), and naphthalene as internal standard were dissolved in 1 mL of deuterated cyclohexane, leading to a pale orange solution. The reaction was followed by NMR measurements. After 3 days at room temperature, 71% of the hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 1.1 to 1. Additionally, a second orange layer was formed during the reaction. Complex 12a and CyOOH. Complex 12a (1 equiv), cyclohexyl hydroperoxide (433 equiv), and naphthalene as internal standard were dissolved in 1 mL of deuterated cyclohexane, leading to a yellow solution. The reaction was followed by NMR measurements. After 3 days at room temperature, 77% of the hydroperoxide was decomposed to the corresponding ketone and alcohol with a ratio of 0.7 to 1. Additionally, a second orange layer was formed during the reaction.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02322. Detailed experimental procedures and analyses; IR and NMR spectra; crystallographic data of complexes 2MeOH, 5H2O, 6H2O, 7, 9bMeOH, 9c, 10c, 11c, 12a, 12b; computational details (PDF) Crystallographic data of complexes 2MeOH, 5H2O, 6H2O, 7, 9bMeOH, 9c, 10c, and 11c, 12a, 12b (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Oliver Trapp: 0000-0002-3594-5181 Thomas Schaub: 0000-0003-2332-0376 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS CaRLa (Catalysis Research Laboratory) is cofinanced by the Ruprecht-Karls-Universität Heidelberg (Heidelberg University) and BASF SE.

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REFERENCES

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