Mononuclear Oxidovanadium(IV) Complexes with BIAN Ligands: Synthesis and Catalytic Activity in the Oxidation of Hydrocarbons and Alcohols with Peroxides

: Reactions of VCl 3 with 1,2-Bis[(4-methylphenyl)imino]acenaphthene (4-Me-C 6 H 4 -bian) or 1,2-Bis[(2-methylphenyl)imino]acenaphthene (2-Me-C 6 H 4 -bian) in air lead to the formation of [VOCl 2 (R-bian)(H 2 O)] (R = 4-Me-C 6 H 4 ( 1 ), 2-Me-C 6 H 4 ( 2 )). Thes complexes were characterized by IR and EPR spectroscopy as well as elemental analysis. Complexes 1 and 2 have high catalytic activity in the oxidation of hydrocarbons with hydrogen peroxide and alcohols with tert -butyl hydroperoxide in acetonitrile at 50 ◦ C. The product yields are up to 40% for cyclohexane. Of particular importance is the addition of 2-pyrazinecarboxylic acid (PCA) as a co-catalyst. Oxidation proceeds mainly with the participation of free hydroxyl radicals, as evidenced by taking into account the regio- and bond-selectivity in the oxidation of n-heptane and methylcyclohexane, as well as the dependence of the reaction rate on the initial concentration of cyclohexane. Author Contributions: I.S.F.: Investigation, Writing-original draft. Synthesis and description of vanadium complex compounds; M.I.G.: Investigation. Synthesis of ovanadium complex compounds; L.S.S.: Investigation, Writing-original draft. Conducting and describing catalytic experiments on oxidation catalyzed by vanadium compounds; N.S.I.: Investigation. Conducting catalytic experiments; A.Y.K.: Investigation. DFT calculations. Conducting DFT calculations for EPR; V.A.N.: Investigation. Recording and description of EPR spectra; Y.N.K.: Investigation. Proposal and discussion of the mechanism for vanadium catalysed alkane oxidation.; A.L.G.: Conceptualization, Writing-review & editing. Writing and editing the ﬁrst draft of the article; G.B.S.: Conceptualization, Writing-review & editing. Writing and editing the ﬁrst draft of the article. Communication with the editor of the journal.


Synthesis of Complexes 1 and 2
For the synthesis of complexes 1 and 2, an approach was applied that included the use of vanadium trichloride as a starting compound. During the reaction, vanadium(III) was oxidized in air to form the {VO} 2+ fragment. Previously, we successfully used this approach to obtain a series of oxidovanadium(IV) complexes with redox-active ligands [62,67,69]. Complexes 1 and 2 were synthesized by a similar method (Scheme 1), by refluxing vanadium trichloride with R-bian (R = 4-Me-C 6 H 4 -bian or 2-Me-C 6 H 4 ) in acetonitrile for 10 h. Fine crystalline powders of complexes 1 and 2 were obtained by recrystallization from a mixture of methylene chloride and hexane in 57% to 49% yields, respectively. Complex 1 is more soluble in most organic solvents than complex 2.

Synthesis of Complexes 1 and 2
For the synthesis of complexes 1 and 2, an approach was applied that includ use of vanadium trichloride as a starting compound. During the reaction, vanadiu was oxidized in air to form the {VO} 2+ fragment. Previously, we successfully use approach to obtain a series of oxidovanadium(IV) complexes with redox-active li [62,67,69]. Complexes 1 and 2 were synthesized by a similar method (Scheme 1), fluxing vanadium trichloride with R-bian (R = 4-Me-C6H4-bian or 2-Me-C6H4) in ac trile for 10 h. Fine crystalline powders of complexes 1 and 2 were obtained by rec lization from a mixture of methylene chloride and hexane in 57% to 49% yields, r tively. Complex 1 is more soluble in most organic solvents than complex 2.
. Scheme 1. Сomplex synthesis reaction. Our attempts to obtain single crystals suitable for X-ray structural analysis for both complexes failed. Therefore, indirect methods were used to determine the composition and structure: elemental analysis, and IR, UV-vis, and EPR spectroscopies. Elemental analysis data are in good agreement with the proposed formula. The IR spectra of these complexes showed broad vibration bands of the OH group from the coordinated H 2 O in the range of 3600-3100 cm −1 . CH vibrations of the methyl group at 3057-2870 cm −1 for 1 and 3055-2869 cm −1 for 2 were found. Vibration bands of CC and CN groups of the R-bian ligand appeared in the region of 1661-1018 cm −1 for 1 and 1662-1045 cm −1 for 2. Very strong bands at 983 cm −1 for 1 and 989 cm −1 for 2 were assigned to the VO group [70,71]. The vibrations at 890-818 cm −1 for 1 and 870-831 cm −1 for 2 can be attributed to the linear chain V = O . . . V = O [69].
The electronic absorption spectra of solutions 1 and 2 in acetonitrile revealed strong absorption in the region of 260-410 nm, which can be attributed to charge transfer bands (involving ligand and metal), as well as a low-intensity band at 497 nm for 1 and 489 nm for 2, which is typical for d-d transitions.

EPR Spectroscopy Studies
The EPR spectra of 1 and 2 in dichloromethane were recorded at 77 K (Figures 1 and 2). In both cases spectra in solution revealed an eight-line isotropic signal characteristic of V IV (d 1 ) complexes. The spectrum of 1 turned out to be a superposition of two spectra with very close parameters, related to different forms. The ratio between these species was 10:1. The simulation analysis for the strong spectrum (spectrum b) showed the following EPR parameters: g 1 = 1.96, g 2 = g 3 = 1.98, A 1 = 17.15 mT, A 2 = A 3 = 6.1 mT, and for the weak EPR spectrum (spectrum c): g 1 = 1.952, g 2 = g 3 = 1.982, A 1 = 17.45 mT, A 2 = A 3 = 7.5 mT. Our attempts to obtain single crystals suitable for X-ray structural analysis for both complexes failed. Therefore, indirect methods were used to determine the composition and structure: elemental analysis, and IR, UV-vis, and EPR spectroscopies. Elemental analysis data are in good agreement with the proposed formula. The IR spectra of these complexes showed broad vibration bands of the OH group from the coordinated H2O in the range of 3600-3100 cm −1 . CH vibrations of the methyl group at 3057-2870 cm −1 for 1 and 3055-2869 cm −1 for 2 were found. Vibration bands of СС and СN groups of the R-bian ligand appeared in the region of 1661-1018 cm −1 for 1 and 1662-1045 cm −1 for 2. Very strong bands at 983 cm −1 for 1 and 989 cm −1 for 2 were assigned to the VO group [70,71]. The vibrations at 890-818 cm −1 for 1 and 870-831 cm −1 for 2 can be attributed to the linear chain V = O … V = O [69].
The electronic absorption spectra of solutions 1 and 2 in acetonitrile revealed strong absorption in the region of 260-410 nm, which can be attributed to charge transfer bands (involving ligand and metal), as well as a low-intensity band at 497 nm for 1 and 489 nm for 2, which is typical for d-d transitions.

EPR Spectroscopy Studies
The EPR spectra of 1 and 2 in dichloromethane were recorded at 77 K (Figures 1 and  2). In both cases spectra in solution revealed an eight-line isotropic signal characteristic of V IV (d 1 ) complexes. The spectrum of 1 turned out to be a superposition of two spectra with very close parameters, related to different forms. The ratio between these species was 10:1. The simulation analysis for the strong spectrum (spectrum b) showed the following EPR parameters: g1 = 1.96, g2 = g3 = 1.98, A1 = 17.15 mT, A2 = A3 = 6.1 mT, and for the weak EPR spectrum (spectrum c): g1 = 1.952, g2 = g3 = 1.982, A1 = 17.45 mT, A2 = A3 = 7.5 mT. The simulation analysis for the spectrum of 2 gave the following EPR parameters: g1 = 1.96, g2 = g3 = 1.98, A1 = 17.15 mT, A2 = A3 = 6.1 mT. These parameters coincided with those for the strong signal in spectrum of 1 (spectrum b), which indicated an identical coordination environment around vanadium. The weak EPR signal with different parameters found in the spectrum of 1 probably belongs to a complex in which vanadium has a different coordination environment. This could be a complex with coordinated acetonitrile [VOCl2(CH3CN)(4-Me-C6H4-bian)], which was formed as a by-product at the stage of synthesis in acetonitrile. In general, the EPR parameters for 1 and 2 are typical for oxidovanadium(IV) complexes [62,72]. The simulation analysis for the spectrum of 2 gave the following EPR parameters: g 1 = 1.96, g 2 = g 3 = 1.98, A 1 = 17.15 mT, A 2 = A 3 = 6.1 mT. These parameters coincided with those for the strong signal in spectrum of 1 (spectrum b), which indicated an identical coordination environment around vanadium. The weak EPR signal with different parameters found in the spectrum of 1 probably belongs to a complex in which vanadium has a different coordination environment. This could be a complex with coordinated acetonitrile [VOCl 2 (CH 3 CN)(4-Me-C 6 H 4 -bian)], which was formed as a by-product at the stage of synthesis in acetonitrile. In general, the EPR parameters for 1 and 2 are typical for oxidovanadium(IV) complexes [62,72]. To confirm the composition and structure of the complexes, DFT calculations were carried out. The proposed geometries of the complexes were optimized, and the g-factors and hyperfine interaction (HFI) A tensors were calculated for both complexes.

Oxidation of Alkanes
We have found that compounds 1 and 2 catalyze the oxidation of alkanes with H2O2 in acetonitrile in the presence of 2-pyrazinecarboxylic acid (PCA). Accumulation of cyclohexanol and cyclohexanone in oxidation of cyclohexane with hydrogen peroxide catalyzed by compound 1 and 2 is demonstrated by Figures 3-6. The data obtained in the oxidation of cyclohexane for both complexes showed that in the presence of 2-pyrazinecarboxylic acid, the reactions proceeded much faster, which is consistent with our previous studies of oxidative processes using vanadium complexes as the catalysts [25]. A co-catalyst in these reactions was 2-pyrazinecarboxylic acid . Figure 3 shows the accumulation of products of cyclohexane oxidation with hydrogen peroxide using complex 1 as a catalyst in the absence of 2-pyrazinecarboxylic acid (PCA). Figure 4 shows the accumulation of cyclohexane oxidation products when PCA was added.
Complex 2 was studied in more detail. The reduction of the reaction solution with PPh3 gave rise to a higher concentration of cyclohexanol and a decrease in cyclohexanone concentration (Figure 4) (compare Graphs A and B). These changes indicate (the so-called Shul'pin method [24,62,73]), that alkyl hydroperoxide is formed in the course of the oxidation. Alkyl hydroperoxides are transformed in the GC injector into a mixture of the To confirm the composition and structure of the complexes, DFT calculations were carried out. The proposed geometries of the complexes were optimized (Supplementary Tables S1 and S2), and the g-factors and hyperfine interaction (HFI) A tensors were calculated for both complexes.

Oxidation of Alkanes
We have found that compounds 1 and 2 catalyze the oxidation of alkanes with H 2 O 2 in acetonitrile in the presence of 2-pyrazinecarboxylic acid (PCA). Accumulation of cyclohexanol and cyclohexanone in oxidation of cyclohexane with hydrogen peroxide catalyzed by compound 1 and 2 is demonstrated by Figures 3-6. The data obtained in the oxidation of cyclohexane for both complexes showed that in the presence of 2-pyrazinecarboxylic acid, the reactions proceeded much faster, which is consistent with our previous studies of oxidative processes using vanadium complexes as the catalysts [25]. A co-catalyst in these reactions was 2-pyrazinecarboxylic acid. Figure 3 shows the accumulation of products of cyclohexane oxidation with hydrogen peroxide using complex 1 as a catalyst in the absence of 2-pyrazinecarboxylic acid (PCA). Figure 4 shows the accumulation of cyclohexane oxidation products when PCA was added.
Complex 2 was studied in more detail. The reduction of the reaction solution with PPh 3 gave rise to a higher concentration of cyclohexanol and a decrease in cyclohexanone concentration (Figure 4) (compare Graphs A and B). These changes indicate (the so-called Shul'pin method [24,62,73]), that alkyl hydroperoxide is formed in the course of the oxidation. Alkyl hydroperoxides are transformed in the GC injector into a mixture of the corresponding ketone and alcohol. Due to this, we quantitatively reduced the reaction samples with PPh 3 to obtain the corresponding alcohol. Shul'pin's method allows us to calculate the real concentrations not only of the hydroperoxide but of the alcohols and ketones present in the solution at a given moment.
Catalysts 2022, 12, x 5 of 17 corresponding ketone and alcohol. Due to this, we quantitatively reduced the reaction samples with PPh3 to obtain the corresponding alcohol. Shul'pin's method allows us to calculate the real concentrations not only of the hydroperoxide but of the alcohols and ketones present in the solution at a given moment.  corresponding ketone and alcohol. Due to this, we quantitatively reduced the reaction samples with PPh3 to obtain the corresponding alcohol. Shul'pin's method allows us to calculate the real concentrations not only of the hydroperoxide but of the alcohols and ketones present in the solution at a given moment.     The curve of dependence of the initial oxidation rate by complex 2 in the case of catalysis approaches a plateau at a cyclohexane concentration of >0.4 M (Figures 7 and 8). The rate at [CyH] 0 = 0.1 M is approximately equal to half of the maximum rate.     We studied the parameters of selectivity in the oxidation of n-heptane (regioselectivity) and methylcyclohexane (bond selectivity) catalyzed by complexes 1 and 2. Data on the selectivity of oxidation (see above) show that the oxidizing species in the studied catalytic system are hydroxyl radicals. This is also confirmed by the data presented below on the reactivity of the oxidizing species. In the system under study, the decomposition of hydroxyl radicals can occur when they interact with the ligand (L) bound to vanadium with 2-pyrazinecarboxylic acid present in the system PCA + OH • →decomposition (2) or with acetonitrile The interaction of the hydroxyl radical with hydrogen peroxide leads to the formation of the HO2 • radical, which most likely reduces the vanadium ion Data on the selectivity of oxidation (see above) show that the oxidizing species in the studied catalytic system are hydroxyl radicals. This is also confirmed by the data presented below on the reactivity of the oxidizing species. In the system under study, the decomposition of hydroxyl radicals can occur when they interact with the ligand (L) bound to vanadium L + OH • →decomposition (1) with 2-pyrazinecarboxylic acid present in the system PCA + OH • →decomposition (2) or with acetonitrile CH 3 CN + OH • →decomposition (3) The interaction of the hydroxyl radical with hydrogen peroxide leads to the formation of the HO 2 • radical, which most likely reduces the vanadium ion The reduced vanadium ion is oxidized by peroxide and the hydroxyl radical is regenerated V(4 + ) + H 2 O 2 →V(5 + ) + OH − + OH • (6) Thus, the interaction of OH • with H 2 O 2 does not in fact lead to the destruction of the hydroxyl radical. Taking the maximum possible value of 10 10 M −1 s −1 for the rate constants of the interaction of OH • with L and PCA, we obtained for the rate constants of the pseudo-first order of the decomposition of OH • with L and PCA under our experimental conditions the values k 1 [L] = 5 × 10 6 s −1 , k 2 [PCA] = 2 × 10 7 s −1 . A similar characteristic for OH • decomposition with acetonitrile is k 3 [CH 3 CN] no less than 5 × 10 7 s −1 . Thus, the latter reaction is the main one in the absence of a substrate; the proportion of the reaction involving PCA does not exceed 30% in the total hydroxyl decomposition channel, and the contribution of the reaction involving L is negligibly small. Therefore, the dependence of the initial rate of formation of oxygenation products on the concentration of cyclohexane (Figure 7) reflects the competition of acetonitrile and PCA (reactions (2) and (3)) with the introduced cyclohexane (reaction (7)) for the hydroxyl radical Let us assume that the rate of generation of hydroxyl radicals by the catalytic system under study under the conditions of Figure 7 is W i . Let us further assume that steps (2), (3), and (7) are limiting the decomposition of OH • with PCA, acetonitrile, and cyclohexane, and the OH • concentration is quasi-stationary. Then we can write It follows from (7) Assuming that reaction (7) is the limiting one in the sequence of transformations leading to the formation of cyclohexyl hydroperoxide, we obtain the following equation for the initial rate of its formation To analyze the data in Figure 5, we transform the Equation (9 From the above estimates for k 2 [PCA] 0 and k 3 [CH 3 CN] 0 under the experimental conditions presented in Figure 7, and relation (12), it follows that 0.08 < k 3 [CH 3 CN] 0 )/k 7 < 0.11 (13) The estimated value of k 3 [CH 3 CN] 0 )/k 7 is close to the values obtained earlier for other catalytic systems in which the formation of hydroxyl radicals has been established [74,75]. Thus, data on the reactivity of an intermediate species of an oxidizing nature that arises during the catalytic decomposition of hydrogen peroxide in the presence of the catalyst under study, as well as data on the regioselectivity of the oxidation of linear alkanes, indicate that the detected intermediate species is a hydroxyl radical.
The dependence of the initial rate of ROOH formation on temperature is shown in Figure 9. It follows from the presented data that the effective activation energy of the process leading to the formation of ROOH equals 18 ± 2 kcal/mol. This value is close to the values obtained for the activation energy of alkane oxidation reactions in other previously published works [24,76,77].
arises during the catalytic decomposition of hydrogen peroxide in the presence of the catalyst under study, as well as data on the regioselectivity of the oxidation of linear alkanes, indicate that the detected intermediate species is a hydroxyl radical.
The dependence of the initial rate of ROOH formation on temperature is shown in Figure 9. It follows from the presented data that the effective activation energy of the process leading to the formation of ROOH equals 18 ± 2 kcal/mol. This value is close to the values obtained for the activation energy of alkane oxidation reactions in other previously published works [24,76,77].
These data as well as the character of dependence of the initial cyclohexane oxidation rate on the initial hydrocarbon concentration indicate that the reaction occurs with the participation of hydroxyl radicals, and that alkyl hydroperoxides are formed as the primary products. Comparative parameters of the oxidation of cyclohexane catalyzed by complexes 1 and 2 and other previously published vanadium complexes are presented in Table 2, and the differences among them are noticeable-the maximum yields of oxidation products are obtained in less time, since the reaction rate is higher, apparently due to the influence of ligands.  These data as well as the character of dependence of the initial cyclohexane oxidation rate on the initial hydrocarbon concentration indicate that the reaction occurs with the participation of hydroxyl radicals, and that alkyl hydroperoxides are formed as the primary products.
Comparative parameters of the oxidation of cyclohexane catalyzed by complexes 1 and 2 and other previously published vanadium complexes are presented in Table 2, and the differences among them are noticeable-the maximum yields of oxidation products are obtained in less time, since the reaction rate is higher, apparently due to the influence of ligands.

Physical Measurements
Elemental C, H, and N analyses were performed with a EuroEA3000 Eurovector analyzer. The IR spectra were recorded in the 4000-400 cm -1 range with a Perkin-Elmer System 2000 FTIR spectrometer, with samples in KBr pellets and Nujol. EPR spectra were recorded in the X band at 77 and 300 K on an E-109 Varian spectrometer equipped with an analog-to-digital signal converter. To analyze and simulate EPR spectra, EasySpin (Matlab software package) was used [79]. A UV-2501 PC spectrometer was used for UV-vis spectroscopic study. The UV-vis spectra were recorded in a quartz cuvette of 2 mm optical layer at room temperature.

DFT Calculations
The spin-unrestricted DFT calculations were performed using the ADF 2021 program package [80,81]. The optimized geometries were obtained with the generalized gradient approximation (GGA) functional BP86 [82,83] and the triple-zeta basis sets (with one polarization function) TZP [84]. The g-and A-tensors were calculated with hybrid PBE0 functional (25% HF exchange) [85,86] and TZP basis sets for all atoms except V, for which the larger basis set TZ2P-J (triple-zeta with two polarization and several extra tight, mainly 1s, functions) was used [84]. The EPR parameters were derived as second derivative properties with spin-orbit coupling and external magnetic field taken as perturbation [87,88]. In all calculations, the scalar relativistic effects were accounted for by the zeroth-order regular approximation (ZORA) formalism [89,90]. Solvent effects (CH 2 Cl 2 ) were considered using the conductor-like screening model (COSMO) of solvation [91] as implemented on ADF 2021 [80,81].

Catalytic Studies
Total volume of the reaction solution was 5 mL. (CAUTION: the combination of air or molecular oxygen and H 2 O 2 with organic compounds at elevated temperatures may be explosive!) Cylindrical glass vessels with vigorous stirring of the reaction mixture were used for the oxidation of alkanes with hydrogen peroxide or tert-butyl hydroperoxide (for alcohols), typically carried out in air in thermostated solution. Initially, a portion of 50% aqueous solution of hydrogen peroxide was added to the solution of the catalyst, co-catalyst (PCA), and substrate in acetonitrile. The aliquots of the reaction solution were analyzed by GC (3700, fused silica capillary column FFAP/OV-101 20/80 w/w, 30 m × 0.2 mm × 0.3 µm; argon as a carrier gas. Attribution of peaks was made by comparison with chromatograms of authentic samples). Usually samples were analyzed twice, i.e., before and after the addition portion by portion of the excess of solid PPh 3 .This method was proposed and used previously by one of us [92,93].

Conclusions
New oxidovanadium(IV) complexes 1 and 2 were synthesized by reacting vanadium trichloride with BIAN-type ligands (4-Me-C 6 H 4 -bian and 2-Me-C 6 H 4 -bian) in 57% and 49% yields, respectively. These compounds were characterized by elemental analysis and IR and EPR spectroscopy. Compounds 1 and 2 are a powerful catalyst for the efficient oxidation of alkanes with peroxides. Data on the selectivity of oxidation and the nature of the dependence of the initial rate of cyclohexane oxidation on the initial concentration of hydrocarbon, as well as kinetic studies, indicate that the reaction proceeds with the participation of hydroxyl radicals and alkyl hydroperoxides are formed as the primary products.