New Oxidovanadium(IV) Complexes with 2,2 (cid:48) -bipyridine and 1,10-phenathroline Ligands: Synthesis, Structure and High Catalytic Activity in Oxidations of Alkanes and Alcohols with Peroxides

: Reactions of [VCl 3 (thf) 3 ] or VBr 3 with 2,2 (cid:48) -bipyridine (bpy) or 1,10-phenanthroline (phen) in a 1:1 molar ratio in air under solventothermal conditions has afforded polymeric oxidovanadium(IV) four complexes 1 – 4 of a general formula [VO(L)X 2 ] n (L = bpy, phen and X = Cl, Br). Monomeric complex [VO(DMF)(phen)Br 2 ] ( 4a ) has been obtained by the treatment of compound 4 with DMF. The complexes were characterized by IR spectroscopy and elemental analysis. The crystal structures of 3 and 4a were determined by an X-ray diffraction (XRD) analysis. The {VOBr 2 (bpy)} fragments in 3 form inﬁnite chains due to the V = O . . . V interactions. The vanadium atom has a distorted octahedral coordination environment. Complexes 1 – 4 have been tested as catalysts in the homogeneous oxidation of alkanes (to produce corresponding alkyl hydroperoxides which can be easily reduced to alcohols by PPh 3 ) and alcohols (to corresponding ketones) with H 2 O 2 or tert -butyl hydroperoxide in MeCN. Compound 1 exhibited the highest activity. The mechanism of alkane oxidation was established using experimental selectivity and kinetic data and theoretical DFT calculations. The mechanism is of the Fenton type involving the generation of HO • radicals. (65%). Calc. for C 12 H 8 Cl 2 N 2 45.3%; ν − 1 2


Synthesis and Characterization of Compounds 1-4a
Solventothermal conditions were applied for the synthesis of oxidovanadium(IV) complexes 1-4a. Vanadium trichloride adduct with tetrahydrofuran ([VCl 3 (thf) 3 ]) and vanadium tribromide (VBr 3 ) were used as starting compounds which were oxidized during the reaction in air to give the V = O moiety (Scheme 1). This synthetic procedure was previously applied for the synthesis of other oxidovanadium(IV) complexes with chiral dehydrophenanthroline and diazofluorene ligands as well as with the redox-active bis(N-(2,6-diisopropylphenyl)imino)acenaphthene (dpp-bian) ligand [25,63]. The reactions of the corresponding vanadium precursor and 2,2 -bipyridine (bpy) or 1,10-phenanthroline (phen) in a 1:1 molar ratio resulted in microcrystalline products in yields ranging from moderate to high.

Crystal Structures of 3 and 4a
Of the five products, 1, 2, 3, 4 and 4a, only the green crystals of 3 and 4a obtained directly from the reaction mixture were of sufficient quality for X-ray diffraction (Table 1). Compound 3 forms a 1D-polymeric structure ( Figure 1). The relevant bond distances are listed in Table 2. Vanadium is octahedrally coordinated by two nitrogen atoms of the chelating bpy ligand, as well as by two bromine atoms that lie in the plane of the bpy ligand and the oxygen atom from the vanadyl group, which is located perpendicular to the plane. The distorted octahedral environment is completed by the oxygen atom of the vanadyl group of the neighboring {VOBr2(bpy)} fragment. Thus, the {VOBr2(bpy)} fragments in 3 form infinite chains due to the V = O…V interactions ( Figure 2). Based on the data of IR spectroscopy, we assume a similar structure for the remaining compounds (1, 2 and 4). Scheme 1. The reactions carried out in the present study.

Crystal Structures of 3 and 4a
Of the five products, 1, 2, 3, 4 and 4a, only the green crystals of 3 and 4a obtained directly from the reaction mixture were of sufficient quality for X-ray diffraction (Table 1). Compound 3 forms a 1D-polymeric structure ( Figure 1). The relevant bond distances are listed in Table 2. Vanadium is octahedrally coordinated by two nitrogen atoms of the chelating bpy ligand, as well as by two bromine atoms that lie in the plane of the bpy ligand and the oxygen atom from the vanadyl group, which is located perpendicular to the plane. The distorted octahedral environment is completed by the oxygen atom of the vanadyl group of the neighboring {VOBr 2 (bpy)} fragment. Thus, the {VOBr 2 (bpy)} fragments in 3 form infinite chains due to the V = O . . . V interactions ( Figure 2). Based on the data of IR spectroscopy, we assume a similar structure for the remaining compounds (1, 2 and 4).  Table 2.         and the excision of monomeric links. Single crystals of [VO(DMF)(phen)Br2] (4a) were obtained by the treatment of 4 with DMF. An X-ray analysis of 4a shows a monomeric structure with an octahedral coordination environment around vanadium, completed with a coordinated DMF molecule ( Figure  3). The main geometric parameters of 4a are summarized in Table 2.    This activity is comparable with that found by us previously for the other vanadium complexes in the alkane oxidations with peroxides .
The oxidation reactions catalyzed by compound 1 have been studied in more detail. Figure 4 clearly demonstrates that the reduction of the reaction solution with PPh 3 gives rise to a higher concentration of cyclohexanol and a decrease of cyclohexanone concentration (compare Graphs A and B). These changes indicate (the so-called Shul'pin method [68][69][70][71][72][73][74][75][76][77][78][79][80][81][82]) that alkyl hydroperoxide is formed in the course of the oxidation. For the transfomations of cyclohexane, see a Scheme in Figure 4A.
The dependence of the initial oxidation rate W 0 on the initial concentration of catalyst 1 is shown in Figure 6. The curve of dependence of the initial oxidation rate in the case of catalysis by complex 1 is approaching a plateau at a cyclohexane concentration of >0.46 M (Figure 7). The rate at [CyH] 0 ≈ 0.2 M is approximately equal to half of the maximum rate.
The competitive oxidation of cyclohexane and acetonitrile (solvent) in the presence of complex 1 is demonstrated in Figure 7. The dependence of the oxidation rate in the case of catalyst 1 (Figure 8) is in a good agreement with the Arrhenius Equation (E a = 20 ± 2 kcal/mol) and with the value calculated in accordance with the model shown below in Scheme 2 (∆H' = 19.6 kcal/mol). The proposed model is also in agreement with the experimental data: In both cases, the first order of the oxidation relative catalyst concentration was found (see Figure 6).  The oxidation reactions catalyzed by compound 1 have been studied in more detail. Figure 4 clearly demonstrates that the reduction of the reaction solution with PPh3 gives rise to a higher concentration of cyclohexanol and a decrease of cyclohexanone concentration (compare Graphs A and B). These changes indicate (the so-called Shul'pin method [68][69][70][71][72][73][74][75][76][77][78][79][80][81][82]) that alkyl hydroperoxide is formed in the course of the oxidation. For the transfomations of cyclohexane, see a Scheme in Figure  4A.  and B). These changes indicate (the so-called Shul'pin method [68][69][70][71][72][73][74][75][76][77][78][79][80][81][82]) that alkyl hydroperoxide is formed in the course of the oxidation. For the transfomations of cyclohexane, see a Scheme in Figure  4A.
The dependence of the initial oxidation rate W0 on the initial concentration of catalyst 1 is shown in Figure 6. The curve of dependence of the initial oxidation rate in the case of catalysis by complex 1 is approaching a plateau at a cyclohexane concentration of >0.46 M (Figure 7). The rate at [CyH]0 ≈ 0.2 M is approximately equal to half of the maximum rate.  The competitive oxidation of cyclohexane and acetonitrile (solvent) in the presence of complex 1 is demonstrated in Figure 7. The dependence of the oxidation rate in the case of catalyst 1 ( Figure  8) is in a good agreement with the Arrhenius Equation (Ea = 20 ± 2 kcal/mol) and with the value calculated in accordance with the model shown below in Scheme 2 (ΔH' = 19.6 kcal/mol). The proposed model is also in agreement with the experimental data: In both cases, the first order of the oxidation relative catalyst concentration was found (see Figure 6).  10 Scheme 2. The mechanism of the HO • generation from the H2O2 catalyzed by I (Gibbs free energies are indicated in parentheses relative to Ia in kcal/mol). The bromide complexes 3 and 4 also catalyze the cyclohexane oxidation with H 2 O 2 ; the corresponding kinetic curves of the product accumulation are presented in Figure 9. The dependence of the initial oxidation rate in the reaction catalyzed by complex 3 exhibits an approach to a plateau (see an analogous dependence in Figure 7). The bromide complexes 3 and 4 also catalyze the cyclohexane oxidation with H2O2; the corresponding kinetic curves of the product accumulation are presented in Figure 9. The dependence of the initial oxidation rate in the reaction catalyzed by complex 3 exhibits an approach to a plateau (see an analogous dependence in Figure 7). We determined the selectivity parameters for the oxidation of certain alkanes (n-heptane, methylcyclohexane and cis-1,2-dimethylcyclohexane catalyzed by complexes 1-4 ( Table 3). The oxygenation of cis-1,2-dimethylcyclohexane with H2O2 catalysed by complex 3 gave corresponding isomeric tertiary alcohols in a trans/cis ratio of 0.8. These data as well as the character of dependence of the initial cyclohexane oxidation rate on the initial hydrocarbon concentration (approaching a plateau at [cyclohexane]0 > 0.3 M) indicate that the reaction occurs with the participation of hydroxyl radicals and that alkyl hydroperoxides are formed as the main primary products.

Concentration (M)
Concentration (M) We determined the selectivity parameters for the oxidation of certain alkanes (n-heptane, methylcyclohexane and cis-1,2-dimethylcyclohexane catalyzed by complexes 1-4 ( Table 3). The oxygenation of cis-1,2-dimethylcyclohexane with H 2 O 2 catalysed by complex 3 gave corresponding isomeric tertiary alcohols in a trans/cis ratio of 0.8. These data as well as the character of dependence of the initial cyclohexane oxidation rate on the initial hydrocarbon concentration (approaching a plateau at [cyclohexane] 0 > 0.3 M) indicate that the reaction occurs with the participation of hydroxyl radicals and that alkyl hydroperoxides are formed as the main primary products.  We can assume that the alkane oxidation is induced with an intermediate species generated in the catalytic decomposition of hydrogen peroxide. The dependence of the rate of cyclohexyl hydroperoxide (ROOH) formation on the initial concentration of cyclohexane (RH, see Figure 7 above) indicates a competition between cyclohexane and acetonitrile for the catalytically active species. The selectivity parameters measured in the oxidation of linear and branched alkanes (Table 3) are close to the parameters typical for the reactions of alkanes with hydroxyl radicals [83,84]. Taking this fact into account, we can consider the simplest scheme of the concurrent oxidation: Here, W i is the rate of generation of hydroxyl radicals, formed in the process of catalytic H 2 O 2 decomposition; (1) and (2) are the transformations of RH into ROOH and CH 3 CN into products and where the rate limiting steps are the interactions of HO• with RH (k 1 ) and with CH 3 CN (k 2 ).
Assuming that the concentration of hydroxyl radicals is quasi-stationary during the reaction, we can obtain the term for the initial rate of ROOH formation: In order to analyze the experimental data shown in Figure 7, let us transform Equation (3) into Equation (4) of the experimental dependence shown in Figure 7A is presented in Figure 7B. The analysis of this dependence leads to conclusions that: 1) the experimental data of Figure Figure 7, the experimental parameters are W i = 8.10 −6 M s −1 and k 2 [CH 3 CN]/k 1 = 0.11 M. The latter ratio of rate constants is equal to the value obtained by some of us earlier for other hydroxyl radical generating systems [83,84].

Theoretical Mechanistic Study
With the aim to shed light on the mechanism of alkane oxidation with H 2 O 2 catalyzed by the V(IV) complexes under study, DFT calculations have been performed for the monomeric form of 1 as a catalyst. In accordance with the experimental kinetic and selectivity data discussed above, the global reaction mechanism is radical one, involving the generation of the HO • species which then oxidize the alkane molecules R-H to give, upon the hydrogen abstraction, the corresponding alkyl radicals R • (Scheme 3). The latter react with molecular oxygen, producing the alkylperoxo radicals ROO • and then the corresponding alkyl hydroperoxide ROOH experimentally detected by GC. In accordance with the kinetic studies, the rate limiting step of the whole process is the generation of the HO • radicals. a catalyst. In accordance with the experimental kinetic and selectivity data discussed above, the global reaction mechanism is radical one, involving the generation of the HO • species which then oxidize the alkane molecules R-H to give, upon the hydrogen abstraction, the corresponding alkyl radicals R • (Scheme 3). The latter react with molecular oxygen, producing the alkylperoxo radicals ROO • and then the corresponding alkyl hydroperoxide ROOH experimentally detected by GC. In accordance with the kinetic studies, the rate limiting step of the whole process is the generation of the HO • radicals.  In the presence of hydrogen peroxide, the acetonitrile or water ligands in I can be replaced for H2O2 to give [V(=O)(bpy)(H2O2)Cl2] (II) (Scheme 2). Such a substitution is endergonic by 8.1-9.7 kcal/mol. The coordination of H2O2 to the V(IV) center tremendously activates this molecule towards the O-O homolysis. Indeed, the energy of the homolytic O-O bond cleavage in II leading to III and HO • is −9.7 kcal/mol, indicating that this process is exergonic. Meanwhile, the activation barrier for the HO • generation from II via TS1 is 11.8 kcal/mol (see Supplementary Materials). The overall activation barrier of the HO • formation relative to the most stable complex Ia is 21.5 kcal/mol in terms of ΔG ≠ and 19.6 kcal/mol in terms of ΔH ≠ . The latter value is in a good agreement with the experimentally obtained activation energy for this reaction catalyzed by complex 1 (20 ± 2 kcal/mol, see above Figure 8).
In principle, the formation of the alkyl radicals R • could occur in one step directly from II via TS2. This transition state should correspond to the simultaneous O-O and C-H bond cleavages and the O-H bond formation. However, all attempts to locate TS2 failed, leading to TS1 and C6H12 situating at the second coordination sphere. a catalyst. In accordance with the experimental kinetic and selectivity data discussed above, the global reaction mechanism is radical one, involving the generation of the HO • species which then oxidize the alkane molecules R-H to give, upon the hydrogen abstraction, the corresponding alkyl radicals R • (Scheme 3). The latter react with molecular oxygen, producing the alkylperoxo radicals ROO • and then the corresponding alkyl hydroperoxide ROOH experimentally detected by GC. In accordance with the kinetic studies, the rate limiting step of the whole process is the generation of the HO • radicals.  In the presence of hydrogen peroxide, the acetonitrile or water ligands in I can be replaced for H2O2 to give [V(=O)(bpy)(H2O2)Cl2] (II) (Scheme 2). Such a substitution is endergonic by 8.1-9.7 kcal/mol. The coordination of H2O2 to the V(IV) center tremendously activates this molecule towards the O-O homolysis. Indeed, the energy of the homolytic O-O bond cleavage in II leading to III and HO • is −9.7 kcal/mol, indicating that this process is exergonic. Meanwhile, the activation barrier for the HO • generation from II via TS1 is 11.8 kcal/mol (see Supplementary Materials). The overall activation barrier of the HO • formation relative to the most stable complex Ia is 21.5 kcal/mol in terms of ΔG ≠ and 19.6 kcal/mol in terms of ΔH ≠ . The latter value is in a good agreement with the experimentally obtained activation energy for this reaction catalyzed by complex 1 (20 ± 2 kcal/mol, see above Figure 8).
In principle, the formation of the alkyl radicals R • could occur in one step directly from II via TS2. This transition state should correspond to the simultaneous O-O and C-H bond cleavages and the O-H bond formation. However, all attempts to locate TS2 failed, leading to TS1 and C6H12 situating at the second coordination sphere. In the presence of hydrogen peroxide, the acetonitrile or water ligands in I can be replaced for H 2 O 2 to give [V(=O)(bpy)(H 2 O 2 )Cl 2 ] (II) (Scheme 2). Such a substitution is endergonic by 8.1-9.7 kcal/mol. The coordination of H 2 O 2 to the V(IV) center tremendously activates this molecule towards the O-O homolysis. Indeed, the energy of the homolytic O-O bond cleavage in II leading to III and HO • is −9.7 kcal/mol, indicating that this process is exergonic. Meanwhile, the activation barrier for the HO • generation from II via TS1 is 11.8 kcal/mol (see Supplementary Materials). The overall activation barrier of the HO • formation relative to the most stable complex Ia is 21.5 kcal/mol in terms of ∆G = and 19.6 kcal/mol in terms of ∆H = . The latter value is in a good agreement with the experimentally obtained activation energy for this reaction catalyzed by complex 1 (20 ± 2 kcal/mol, see above Figure 8).
In principle, the formation of the alkyl radicals R • could occur in one step directly from II via TS2. This transition state should correspond to the simultaneous O-O and C-H bond cleavages and the O-H bond formation. However, all attempts to locate TS2 failed, leading to TS1 and C 6 H 12 situating at the second coordination sphere.
Another plausible pathway of the HO • generation includes the preliminary proton transfer in II from the coordinated H 2 O 2 to the oxo ligand to give complex [V(OH)(bpy)(OOH)Cl 2 ] (IV). The following homolytic O-OH bond cleavage in the hydroperoxo ligand produces HO • and III. Two possible transition states for the proton transfer were found, i.e., the 4-membered cyclic TS3 and the 6-membered cyclic TS4, with H 2 O molecule playing the role of a proton shuttle. However, the calculated activation energy for such an H-transfer is too high for both these TSs (45.3 and 32.2 kcal/mol, respectively, relative to Ia). Thus, the calculations indicate that the simple Fenton mechanism is operating for the catalyst 1.

Computational Details
The full geometry optimization of all structures and transition states (TSs) has been carried out at the DFT level of theory by using the M06 functional [85] with the help of the Gaussian 09 program package [86]. No symmetry operations were applied. The geometry optimization was carried out by using a relativistic Stuttgart pseudopotential which describes 10 core electron (MDF10) and the appropriate contracted basis set (8s7p6d1f)/[6s5p3d1f] [87] for the vanadium atom and the 6-311+G** basis set for other atoms. This level of theory was successfully applied recently for the analysis of mechanisms of the V-catalyzed oxidation of alkanes with peroxides [88].
The Hessian matrix was calculated analytically for the optimized structures to prove the location of correct minima (no imaginary frequencies) or saddle points (only one imaginary frequency) and to estimate the thermodynamic parameters, with the latter calculated at 25 • C. The nature of all transition states was investigated by analysis of the vectors associated with the imaginary frequency and by the calculations of the intrinsic reaction coordinates (IRC) by using the method developed by Gonzalez and Schlegel [89][90][91].
The total energies corrected for solvent effects E s were estimated at the single-point calculations on the basis of gas-phase geometries using the polarizable continuum model in the CPCM version [92,93] with CH 3 CN as solvent. The UAKS model was applied for the molecular cavity, and dispersion, cavitation and repulsion terms were taken into account. The entropic term in CH 3 CN solution (S s ) was calculated according to the procedure described by Wertz [94] and Cooper and Ziegler [95] using Equations (5) where E s and E g are the total energies in solution and the gas phase and H g is the gas-phase enthalpy. Gibbs free energies in solution are discussed in this work if not stated otherwise.

Materials
All manipulations were carried out in air. Vanadium tribromide (VBr 3 ) was synthesized from elements as described in Reference [96]. The other reagent-grade chemicals were obtained from Aldrich and used without further purification. All solvents were distilled by standard methods before use.

Physical Measurements
An elemental analysis was performed on a Euro EA 3000 CHN elemental analyzer. The IR spectra (4000-400 cm −1 ) were recorded on a Scimitar FTS 2000 Fourier-spectrometer.

X-ray Crystallography
The crystallographic data and refinement details for 3 and 4a are given in Table 1. The diffraction data were collected on New Xcalibur (Agilent Technologies, Santa Clara, CA, United States) and Bruker Apex Duo diffractometers with MoKα radiation (λ = 0.71073). An absorption correction was done empirically using SCALE3 ABSPACK (CrysAlisPro, Agilent Technologies, Version 1.171.37.35 (release 13-08-2014 CrysAlis171 NET)) and SADABS (Bruker-AXS, 2004). The structures were solved by a dual algorithm [97] and refined by a full-matrix least-squares treatment against |F| 2 in anisotropic approximation with SHELX 2017/1 [98] in the ShelXle program [99]. The hydrogen atoms were refined in geometrically calculated positions. The main geometrical parameters are summarized in Table 2. The crystallographic data have been deposed in the Cambridge Crystallographic Data Centre under the deposition codes CCDC 1879273 (3) and 1879274 (4a).

Conclusions
New polymeric oxidovanadium(IV) complexes, [VO(L)X 2 ] n (L = bpy, phen and X = Cl, Br), have been synthesized and structurally characterized. The experimental kinetic and selectivity data and DFT calculations indicated that the simple Fenton mechanism is operating for the oxidation of alkanes catalyzed by the V(IV) complexes under study. This mechanism includes (i) the formation of the monomeric active catalytic forms [VO(L)X 2 (Solv)] (Solv = H 2 O, CH 3 CN), (ii) the substitution of the coordinated solvent for the H 2 O 2 molecule, (iii) the generation of the HO • radical upon the HO-OH bond cleavage and (iv) the oxidation of the alkane molecules by HO • via hydrogen abstraction followed by the reaction with molecular oxygen to give alkyle hydroperoxides.

Synthesis of [V IV O(bpy)Br 2 ] n (3)
A mixture of VBr 3 (150 mg, 516 µmol), 2,2 -bipyridine (81 mg, 516 µmol) and CH 3 CN (10 mL) was heated at 100 • C in a sealed Teflon container for 12 h. The slow cooling to room temperature gave a mixture of green crystals (major) and an orange powder (minor). The mixture was washed with CH 3 CN and Et 2 O and dried in a vacuum. The green crystals were mechanically separated from the orange powder. Yield: 124 mg (66%) Anal. Calc. for C 10

Oxidation of Alcohols and Hydrocarbons with Peroxides
The reactions of alcohols and hydrocarbons were usually carried out in air in thermostated Pyrex cylindrical vessels with vigorous stirring, using MeCN as the solvent. Typically, the catalyst (1-4) and the cocatalyst (acid) were introduced into the reaction mixture in the form of stock solutions in acetonitrile. The substrate (alcohol or hydrocarbon) was then added, and the reaction started when hydrogen peroxide or TBHP was introduced in one portion. (CAUTION: The combination of air or molecular oxygen and H 2 O 2 with organic compounds at elevated temperatures may be explosive). The reactions with benzene and 1-phenyethanol were analyzed by the 1 H NMR method (solutions in acetone-d 6 ; "Bruker AMX-400" instrument, 400 MHz). For the determination of the concentrations of phenol and p-quinone, the signals in the aromatic region were integrated using added 1,4-dinitrobenzene as a standard. The areas of methyl group signals were measured to quantify the oxygenates formed in the oxidations of 1-phenylethanol.
In order to determine the concentrations of all the cyclohexane oxidation products, the samples of the reaction solutions after the addition of nitromethane as a standard compound were, in some cases, analyzed twice (before and after their treatment with PPh 3 ) by GC (chromatograph-3700, fused silica capillary column FFAP/OV-101 20/80 w/w, 30 m × 0.2 mm × 0.3 µm; helium as a carrier gas) to measure the concentrations of cyclohexanol and cyclohexanone. This method (an excess of solid triphenylphosphine was added to the samples 10-15 min before the GC analysis) was proposed by one of us (Shul'pin, G.B.) earlier [4,5,46,47,[68][69][70][71][72][73][74][75][76][77][78][79][80][81][82]. The attribution of peaks was made by a comparison with the chromatograms of authentic samples. Blank experiments with cyclohexane showed that, in the absence of the catalyst, no products were formed.

Conflicts of Interest:
The authors declare no conflict of interest.