Polymerized methyl imidazole silver bromide (CH 3 C 6 H 5 AgBr) 6 : Synthesis, crystal structures, and catalytic activity

: One-pot synthesis of polymerized methyl imidazole silver bromide (CH 3 C 6 H 5 AgBr) 6 Compound 1 was carried out by the reaction of methylimidazole, 1,2-dibro-moethane, and Ag 2 O in tetrahydrofuran at 60°C for 2 h. Compound 1 was characterized by elemental analysis, 1 H nuclear magnetic resonance, and single crystal X-ray diffraction. The crystal cell parameters of Compound 1 are as follows: a = 8.916(4) Å, b = 17.655(9) Å, c = 9.024(4) Å, α = 90°, β = 103.621(7)°, γ = 90°, V = 1380.6(12) Å 3 , and Z = 4. The silver atoms in Compound 1 are pentacoordinated with three bromine atoms, one silver atom, and one nitrogen atom of methyl imidazole, and polymeric methyl imidazole silver bromide was formed based on this structure. The catalytic e ﬀ ects under optimized conditions were investigated in this study, and the results showed that Compound 1 possesses a strong catalytic e ﬀ ect on the oxidation of 2-methylnaphthalene with a conversion rate of 77.15% by using hydrogen peroxide as the oxidant at 80°C for 3 h. The catalytic mechanism was explored simultaneously.


Introduction
Ag(I) complexes are of great interest due to their important structure and extensive applications as transfer reagents [1-3], drug intermediates [4], and potential catalysts [5,6].The structures of complexes vary among ligands, solvents, and reaction conditions.Coordinating anions, especially in complexes with halide anions, have led to diverse bonding motifs in the solid-state [7,8].The properties of Ag(I) complexes change greatly with different coordination environments, which affects the application of these complexes for practical use [9,10].Hitherto, various organic transformations catalyzed by Ag(I) complexes have been achieved, including diboration reactions with various terminal and internal alkenes [11], ring-opening polymerization of lactide [12], coupling of unactivated aldehydes [13], formation of propargylamines by coupling reactions [14], coupling between N-tosylhydrazone and phenylacetylene [15], Heck couplings of heteroaryl halides [16], and enantioselective and diastereoselective construction of complex heterocyclic molecules [17].
Among the processes listed above, C-H bond functionalization processes, which have been supposed to be the most effective synthetic strategies for the assembly of complex molecules, have been particularly studied.However, the activation of C-H bonds has been an enormous challenge because of their high bond dissociation energies and low carbon-hydrogen bond polarity.Some studies have demonstrated that Ag(I) complexes have high catalytic activity in C-H bond functionalization processes.Yang et al. reported an effective strategy that employs welldefined vinyl-N-triftosylhydrazones as a versatile alkylating reagent to enable direct assembly of C-H bonds from low-cost alkane feed stocks [18].Pérez and his team found that it is possible to exert regioselection in the silvercatalyzed functionalization of linear alkanes from diazo compounds [19].These findings provide new and more selective catalytic systems for the challenging functionalization of C-H bonds in a selective manner.
β-Menadione is widely used in medicine, pesticides, and additive fields as an important chemical raw material, and it is generally obtained by oxidation of 2-methylnaphthalene [20,21].The oxidation methods of 2-methylnaphthalene are generally divided into three categories according to the type of oxidant.Chromium trioxide is used as an oxidant in most industrial synthesis processes [22][23][24].However, this method only has a target product yield of 40-50% with poor selectivity and causes severe environmental pollution accompanied by the generation of a large amount of chromium-containing water [25].To solve this problem, researchers mixed trivalent and tetravalent cerium salts in a ratio of 1:2 as oxidants to obtain β-menadione, but the regeneration of salts needs to be electrolyzed, which requires enormous energy consumption [26].The air oxidation method combines air and organic matter through the gas or liquid phase and achieves catalytic oxidation in the presence of a catalyst.However, the reaction requires a high temperature, and the yield of the target product is generally low [27].As a green oxidant, hydrogen peroxide has received extensive attention and is often used to oxidize 2-methylnaphthalene.It has been reported that 60% hydrogen peroxide (H 2 O 2 ) was used as the oxidant in an acetic acid system with palladium acetate or palladium sulfate as a catalyst to obtain β-menadione, and the yield of the target product reached 50-64% [28].However, the high concentration of H 2 O 2 is prone to explosion, which limits the application of this method in industry.It has also been shown that 30% H 2 O 2 was chosen to oxidize 2-methylnaphthalene with acetone as the reaction solvent [29].The conversion rate of the raw material reached 45%, but the industrialization of this process is difficult to achieve due to the low boiling point of the solvent.Hence, it is particularly significant to find stable catalysts that can catalyze the oxidation of 2-methylnaphthalene with high yield using a suitable solvent.Based on the above research results, since insertion reactions by activating C-H bonds can react using silver complexes as catalysts, it is possible to activate C−H bonds by Ag(I) complexes to further oxidize 2-methylnaphthalene.
To verify this issue, polymerized methyl imidazole silver bromide was obtained, and the structure of the complex was characterized by elemental analysis, 1 H nuclear magnetic resonance (NMR), and single-crystal X-ray diffraction.The catalytic activity of the Ag(I)-complex in the oxidation of 2-methylnaphthalene was investigated, and the catalytic mechanism is explored in this study.To the best of our knowledge, this is the first report on the synthesis, characterization, and catalytic activities of this complex.

Experimental method 2.1 Materials and reagents
Chromatographic-grade acetonitrile was purchased from Mreda Technology Inc. (USA); methylimidazole and 1,2dibromoethane were purchased from Macklin Biochemical Technology Co., Ltd (Shanghai, China).Silver oxide was purchased from Aladdin Biochemical Technology Co., Ltd.(Shanghai, China).Other reagents used were purchased from Kelong Chemical Reagent Co. Ltd (Sichuan, China); distilled water and deionized water were obtained from a Spring-R10 water purification system (Research Scientific Instrument, Xiamen, China).
A Sartorius BSA224S-CW electronic balance (Germany) was employed to weigh the standard compounds.An SPM-20A LC system (Shimadzu Corporation, Japan), equipped with an LC-6AD binary pump, an SPD-M20A diode array detector with a wavelength range of 190-950 nm, a CTO-10AVP column oven, and Shimadzu CLASS-VP software (version 6), was used for the HPLC-DAD method.The separation of analytes was carried out on a C18-WR (255 × 4.6 mm, 5 μm) column, which was purchased from Shimadzu Corporation (Japan).A Bruker AVANCE Ⅲ HD 400 MHz NMR spectrometer with a superconducting magnet (Germany) was used to determine the NMR spectra.A Bruker Smart APEX Ⅱ single crystal X-ray diffractometer (Germany) was employed to characterize the steric structure of the complex.

Synthesis
Compound 1: Polymerized methyl imidazole silver bromide (CH 3 C 6 H 5 AgBr) 6 (1) was synthesized by the reaction of methylimidazole and 1,2-dibromoethane with Ag 2 O in THF at 60°C for 2 h.After refluxing, THF was removed under vacuum, and CH 2 Cl 2 was added to recrystallize a white crystalline solid (Table 1).Suitable crystals were selected for X-ray single crystal diffraction data for Compound 1 and were collected using a Bruker Smart APEX Ⅱ diffractometer (Germany).The crystal was kept at 296(2) K during data collection.Data reduction and analytical absorption correction were performed by the CrysAlisPro program.Intensity data were collected on a detector equipped with graphite-monochromatized MoKα radiation.The diffracted intensities were corrected for Lorentzpolarization effects and empirical absorption corrections.The structure was solved by direct methods.Non-hydrogen atoms were determined with successive difference Fourier syntheses.The hydrogen atoms were located at the calculated positions.The anisotropic thermal parameters for the non-hydrogen atoms were refined by full-matrix leastsquares techniques on F 2 .The crystal data, experimental details, and refinement results for Compound 1 are given in Table 2.

Catalytic activities 2.4.1 Determination of the analysis methodt
Based on the external standard method by HPLC-DAD analysis, the content of 2-methylnaphthalene was determined, and the catalytic performance was evaluated by its conversion rate.To obtain a high resolution of 2-methylnaphthalene and β-menadione, the mobile phase was selected as 70% acetonitrile (A) and 30% water (B).The flow rate was 0.85 mL/min with an injection volume of 10 μL at a detection wavelength of 236 nm.The standard solutions were analyzed under the above conditions, and the peak area of each standard sample was determined.The regression equations were established using five concentration levels, and the standard curve was subsequently obtained using the concentration as the abscissa and the peak area as the ordinate.

Optimization of catalytic conditions
In an acetonitrile system, Compound 1 was used for the catalytic oxidation of 2-methylnaphthalene with hydrogen peroxide.The effects of catalyst dosage, oxidation dosage, reaction temperature, and reaction time on the catalysis of the oxidation of 2-methylnaphthalene were optimized by a single-factor test (Table 3).Under the optimal catalytic conditions, the effects of the catalyst were investigated in this study.

Synthesis
New polymerized methyl imidazole silver bromide was synthesized successfully by a one-pot process.The yield of Compound 1 was 73%.The structure of Compound 1 was characterized by 1 H NMR, which was in accordance with previous studies [30][31][32].
Compound After complexation with Ag(I) metal, the chemical shift values of methyl imidazole decreased due to the electron absorption effects of the empty orbitals of silver atoms.
The elemental analysis of Compound 1 was as follows: the content of carbon was 32.67%, nitrogen was 8.49%, hydrogen was 1.83%, bromine was 24.22%, and silver was 32.69%, which verified the successful synthesis of Compound 1.

Description of the crystal structure of Compound 1
The definitive structure of Compound 1 was determined by X-ray diffraction.The crystal structure analysis showed that complex 1 is a mono-NHC of silver with bridging halides, and the geometry around the silver atom is pentacoordinated with silver and three bromine and nitrogen atoms of methyl imidazole.[39,40], suggesting that the coordination ability of silver atoms is stronger than that of cobalt and copper atoms.

Catalytic activities study 3.3.1 Validation of the HPLC-DAD method
The calibration curves of 2-methylnaphthalene had good linear regressions, and the correlated factor was 0.9990.The regression curve is as follows: Y = 19371x + 88940.The linear range was 16.45-205.60μg/mL.This result showed that the HPLC-DAD method had good accuracy for the quantification of substrate (Figures 1 and 2).

Optimization of catalytic conditions
The effects of catalyst dose, oxidant dose, reaction temperature, and reaction time on the conversion of 2-methylnaphthalene were investigated by using a single-factor trial.By comparing the conversion of 2-methylnaphthalene, the optimal reaction conditions for the catalytic oxidation by complex 1 were determined.As shown in Figure 3, when the amount of catalyst is 5 and 10% of the substrate (w/w), the conversion rates of 2-methylnaphthalene are 28.83 and 37.31%, respectively.Fortunately, the conversion rate was much improved to 53.30% by using a catalyst amount of 15%.Therefore, the amount of catalyst was chosen as 15% of the substrate (w/w).The amount of hydrogen peroxide significantly affected the conversion rates of 2-methylnaphthalene.It could not effectively oxidize 2-methylnaphthalene if the amount was too low; however, this may cause changes in catalyst stability if the concentration of H 2 O 2 is too high.Therefore, 2 mL was chosen as the  final amount of H 2 O 2 (when the mass of the substrate was 0.2 g).The reaction temperature was also a very important factor affecting the catalytic performance of Compound 1, and it affected the conversion rate of the substrate.As shown in Figure 3, we selected three different temperatures (60, 80, and 100℃) to optimize the conversion of 2-methylnaphthalene.
The results showed that the conversion rate of 2-methylnaphthalene is the largest when the reaction temperature is 80°C.Time was also optimized for the reaction.If the reaction time is too short, it will lead to a low conversion rate.However, the economic cost will be reduced when the reaction time is too long.After comprehensive consideration, the reaction time was set at 3 h.The optimal conditions for the reaction were as follows: the amount of substrate was 0.2 g, the amount of catalyst was 0.03 g, the amount of oxidant was 2.0 mL, the reaction temperature was 80°C, and the reaction time was 3 h.

Catalytic effect of Compound 1
It is particularly significant to find stable catalysts that can catalyze the oxidation of 2-methylnaphthalene with a low concentration of H 2 O 2 in a suitable solvent.In this study, the catalytic effects of complex 1 were evaluated when using 30% H 2 O 2 as the oxidant and acetonitrile with a high boiling point as the solvent.Under the optimal conditions, the conversion rate of 2-methylnaphthalene without catalyst was 32.74%, and the conversion rate reached 77.15% when using complex 1 as the catalyst, with the yield of the target product reaching 63.48%.The results showed that the Ag(I) complex promoted the conversion of 2-methylnaphthalene with an excellent catalytic effect (Scheme 1).

Catalysis mechanism of Compound 1 in the oxidation of 2-methylnaphthalene
The catalysis mechanism is shown in Scheme Intermediate A is highly electronegative, and it is not prising that complex 1 has excellent catalytic activity for the oxidation of 2-methylnaphthalene.

Conclusion
A one-pot synthesis of a polymerized complex of silver(I) from methylimidazole and 1,2-dibromoethane with Ag 2 O is reported with a single crystal structure and catalytic application.In the complex, halogen serves as a bridging atom, and the silver atom possesses a distorted trigonal bipyramid that is coordinated with three bromine atoms, one silver atom, and one nitrogen atom on a methyl imidazole.The oxidation ability of H 2 O 2 was enhanced by the properties of Compound 1, and 2-methylnaphthalene was oxidized efficiently with a conversion rate of 77.15%, and the yield of β-naphthoquinone reached 63.48%.These findings provide new catalytic systems for oxidation reactions by activating C-H bonds when using Ag(I) complexes as catalysts.

1H
NMR spectra (400 MHz) were recorded and chemical shifts (δ) are reported in ppm.

Scheme 1 :
Scheme 1: The structure and synthesis of Compound 1.

Table 1 :
Crystallographic data and structure refinement parameters for Compound 1
2. There are lone pair electrons on the oxygen atom in hydrogen peroxide, which could coordinate with the central Ag atom in Compound 1, increasing the electrophilicity of the oxygen atom and forming intermediate A. The electrophilicity of the Ag center has been increased simultaneously when translated into the metal complex intermediate A. Moreover, with the presence of imidazole rings, the coordination becomes more stable.The hydrogen atom on Compound B (2-methylnaphthalene) was attacked by the oxygen atom in intermediate A. Subsequently, the O-Ag bonds and -O-O bonds ruptured, 1,3 proton migration occurred, resulting in the formation of Compound C, and intermediate A reverted back to Compound 1. H + has been released to generate Compound D. The oxygen atom in intermediate A, which displays partial positivity, could attack C of Compound D to generate Compound E. Eventually, target product F (β-naphthoquinone) is generated through a Bayer oxidation rearrangement.