Abstract
Silver salts and their complexes play crucial roles as catalysts in organic chemistry. In this study, pyridine-3-ol was employed as the raw material to synthesize the tetrahydrofuran complex of 2,4,6-trinitro-3-hydroxypyridine silver (4) through nitration, salt formation, and other reactions. The structure was characterized using nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA), and single-crystal X-ray diffraction (SC-XRD). Its thermal properties were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The results show that the crystal of compound 4 belongs to the orthorhombic system, with the space group Pna21. The unit cell parameters are as follows: a = 11.263(4) Å, b = 12.405(5) Å, c = 19.598(7) Å, α = 90°, β = 90°, γ = 90°, V = 2738.2(17) Å3, and Z = 4. The crystal density is 1.985 g·cm–3 (300 K). Compound 4 undergoes thermal decomposition at 226.4 °C.
Introduction
As a catalyst, silver salts have a wide range of applications in organic chemistry. − For example, under the catalysis of Ag(Phen)2OTf, carboxylic acid derivatives can undergo a decarboxylation bromination reaction. Additionally, silver nitrate can catalyze the substitution of boronic acid ester groups with fluorine atoms when selective fluorine sources are employed, thereby yielding fluorinated compounds. Even silver fluoride, with extremely low solubility, can effectively catalyze the [3 + 2] cycloaddition of 2-substituted allenoate with N-Ts imine via AgF-assisted P(III/V) catalysis. Among these salts, either the cost is too high or their solubility is insufficient, leading to increased reaction costs. Meanwhile, pyridine derivatives have attracted widespread attention from scientists due to their electron cloud characteristics of the pyridine ring, which make them easy to form complexes with transition metal ions. Many of these complexes have shown good applications. − Based on these reasons, we designed compound 4 (Scheme ), which contains both a pyridine ring and a tetrahydrofuran ring in its structure. The purpose of introducing the tetrahydrofuran ring is to improve the solubility of 2,4,6-trinitro-5-hydroxypyridine silver salt.
1. Synthetic Route of Compound 4 .
Results and Discussion
Synthesis
2-Nitro-3-hydroxypyridine (2) was readily synthesized by dissolving 3-hydroxypyridine in acetic acid and subsequently nitrating it with fuming nitric acid. According to the literature, intermediate 2,4,6-trinitropyridin-3-ol (3), which appears as a light yellow crystalline solid, can be efficiently obtained via cooling and filtration after the complete reaction of intermediate 2 with fuming nitric acid at 85 °C (yield: 75.6%). Under vigorous stirring, the slow addition of silver carbonate to a THF solution of compound 3 can be used to prepare compound 4. Upon completion of the reaction, the product is completely dissolved in THF. These observations suggest that complex 4 can be easily synthesized by using a straightforward synthesis method. In addition, it has been found that compound 4 has good solubility in conventional solvents such as methanol, ethanol, ethyl ether, dichloromethane, chloroform, dimethyl sulfoxide, and N,N-dimethylformamide. (The TG-DSC curve and FT-IR spectrum of compound 4 are shown in Figures and .)
1.
TG-DSC curve.
2.
FT-IR spectrum.
Thermal Performance and FT-IR Test
Under a nitrogen atmosphere (flow rate of 30 mL·min–1), the thermal decomposition behavior of compound 4 was determined using a synchronous thermal analyzer (TG-DSC) at a heating rate of 5 °C·min–1 and a temperature range of 25–500 °C. The thermal stability of compound 4 was evaluated through TG-DSC experiments. As shown in the DSC curve presented in Figure , it can be seen that at a heating rate of 5 °C·min–1, compound 4 exhibits its first exothermic peak at around 123.7 °C and the second exothermic peak at around 177.6 °C, respectively. The initial decomposition temperature was determined by drawing a tangent line between the baseline and the inflection point of the exothermic peak, yielding an onset decomposition temperature of 207.2 °C, while the thermal decomposition temperature was identified as 226.41 °C. After heating to 123.7 °C, compound 4 lost its tetrahydrofuran ring, and as the temperature continued to rise, compound 4 began to decompose.
The spectral properties of compound 4 were studied by using Fourier transform infrared (FT-IR) spectroscopy at room temperature. Figure shows the FT-IR spectrum in the wavenumber range of 400–4000 cm–1. IR (neat), ν (cm–1): 3115.75 (pyridine ring, H–C), 2960.09 (furan ring, H–C–O), 2879.14 (furan ring, H–C–C), 1579.04 (pyridine ring, CC–N), 1537.90 (pyridine ring, CC), 1539.81 (−NO2), 1487.69 (furan ring, −C–C), 1427.20 (furan ring, −C–C).
Spectroscopy
Compound 4 was characterized by nuclear magnetic resonance (NMR). Using a JNM-ECZ 600 R/S3 NMR spectrometer, we obtained 1H and 13C NMR spectra at 600 and 150.8 MHz. The chemical shifts for 1H and 13C NMR spectra were reported in ppm relative to (CH3)4Si. Unless otherwise stated, DMSO-d 6 was used as a locking solvent. The elemental analysis was performed by using a CE-440 elemental analyzer. The decomposition onset point was recorded using a differential scanning calorimeter (TA Q600, TA Q2000) at a scan rate of 5 °C min–1. The density was determined at room temperature with a gas pycnometer (Micromeritics AccuPyc 1330). The single-crystal X-ray diffraction data of compound 4 were collected using a Bruker D8 Venture IμS 3.0 diffractometer with Mo–Kα radiation (λ = 0.71073 Å). We determined the structure by a dual-space method using the SHELXT program. The model was then refined by the full-matrix least-squares method on F2 using SHELXL-2017. The CCDC number for compound 4 is 2466070. The main crystallographic data of compound 4 are listed in Table and Supporting Information.
1. Crystal Data and Structure Refinement for Compound 4 .
| Identification code | Compound 4 |
|---|---|
| Empirical formula | C18H18Ag2N8O16 |
| Formula weight | 818.14 |
| Temperature/K | 300.0 |
| Crystal system | orthorhombic |
| Space group | Pna21 |
| Hall group | P2c-2n |
| a/Å | 11.263(4) |
| b/Å | 12.405(5) |
| c/Å | 19.598(7) |
| α/° | 90 |
| β/° | 90 |
| γ/° | 90 |
| Volume/Å3 | 2738.2(17) |
| Z | 4 |
| ρcalcg/cm3 | 1.985 |
| Mu/mm–1 | 1.521 |
| F(000) | 1616.0 |
| Crystal size/mm3 | 0.11 × 0.05 × 0.03 |
| h, k, lmax | 13, 15, 24 |
| Nref | 5370 |
| Radiation | MoKα (λ = 0.71073) |
| Tmin, Tmax | 0.646, 0.745 |
| Tmin′ | 0.846 |
| 2Θ range for data collection/° | 4.884–52.734 |
| Index ranges | –13 ≤ h ≤ 11, –15 ≤ k ≤ 15, –24 ≤ l ≤ 24 |
| Reflections collected | 28341 |
| Independent reflections | 5370 [R int = 0.0519, R sigma = 0.0376] |
| Data/restraints/parameters | 5370/71/398 |
| Goodness-of-fit on F 2 | 1.026 |
| Final R indexes [I ≥ 2σ (I)] | R 1 = 0.0459,wR 2 = 0.1136 |
| Final R indexes [all data] | R 1 = 0.0778, wR 2 = 0.1358 |
| Largest diff. peak/hole/e Å–3 | 0.60/–0.47 |
| Flack parameter | 0.39(7) |
| Data completeness = 1.86/0.96 | Theta (max)= 26.367 |
| S = 1.015 | Npar = 398 |
Crystallography
The structure of compound 4 was determined using single-crystal X-ray diffraction. At 300 K, compound 4 crystallizes in the orthorhombic space group Pna21, exhibiting a density of 1.985 g·cm–3. The unit cell contains ten molecules, wherein every two molecules form a dimer (Figure ). Within the dimer, the atoms of the group O4–Ag2–O9–Ag1 form an approximate rectangle. The bond lengths are as follows: L O4–Ag2 = 2.358 Å, L Ag2–O9 = 2.541 Å, L O9–Ag1 = 2.375 Å, and L O4–Ag1 = 2.474 Å, respectively. The torsion angle of O4–Ag2–O9–Ag1 is 4.754°, indicating that these four atoms are approximately coplanar. Furthermore, the crystal structure diagram reveals that the molecular layers are stacked with an interlayer spacing of 9.182 Å (Figure ).
4.
Crystal stacking diagram of compound 4.
In the molecular crystal structure depicted in Figure , the spatial structures of the two furans also have significant differences. The dihedral angles of the two furan rings on the upper and lower sides are 32.459(26)° and 22.436(22)°, respectively, with a difference exceeding 10° between them. Conversely, the torsion angles formed by the four carbon atoms on each of the two furan rings are 2.221(28)° and 6.494(29)°, both less than 10°. This indicates that the four carbon atoms in each furan ring are approximately coplanar, with a deviation of less than 4.5° between the two rings. In addition, the distances between the oxygen atoms of the two furan rings and the corresponding silver ions are L O1–Ag1 = 2.304 Å and L O10–Ag2 = 2.359 Å, respectively. By comparing the bond lengths of O1–Ag1, O10–Ag2, Ag2–O9, and O4–Ag1, it can be found that bond L O1–Ag1 = 2.304 Å is the shortest, while bond L O4–Ag1 = 2.474 Å on the same side is the longest. This phenomenon may be attributed to their spatial interactions.
3.
Crystal structure of compound 4.
After discussion of the structure of the furan rings, the structure of the pyridine rings (R3 ring and R4 ring) is examined, as shown in Figure . The two pyridine rings (R3 ring and R4 ring) are coplanar due to the extremely small dihedral angle between them, measuring only 2.389(8)°. There are three nitro groups on both the R3 and R4 rings. On the R3 ring, the dihedral angles formed by C14–NO2, C16–NO2, and C18–NO2 with the pyridine ring R3 are 47.778(8)°, 6.127(44)°, and 13.960(38)°, respectively. These results indicate that C16–NO2 is coplanar with the pyridine ring, whereas the other two nitro groups intersect diagonally with the pyridine ring. The dihedral angles formed by the three nitro groups C14–NO2, C16–NO2, and C18–NO2 are 41.751(33)°, 16.961(56)°, and 51.431(27)°, respectively. The three nitro groups connected to the R4 ring are C1–NO2, C3–NO2, and C5–NO2. The dihedral angles formed between these three nitro groups and the pyridine ring to which they are connected are 11.182(10)°, 7.191(18)°, and 50.542(11)°, respectively. These values differ from the dihedral angles formed by the three nitro groups on the R3 ring. Additionally, the dihedral angles formed between the nitro groups themselves (C1–NO2, C3–NO2, and C5–NO2) on the R4 ring are 51.926(21)°, 18.211(23)°, and 52.811(17)°, respectively, all of which are sequentially greater than the dihedral angles formed between the three nitro groups on the R3 ring (detailed data can be found in Supporting Information).
Conclusion
Here, a novel complex (4) was obtained from 3-hydroxypyridine through nitration, salt formation, and other reactions. Its structure was characterized using nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA), and single-crystal X-ray diffraction (SC-XRD). From the crystal structure of compound 4, it can be observed that the four carbon twist angles of the tetrahydrofuran ring are less than 7°, indicating that the planes formed by these atoms are approximately parallel. Two molecules of compound 4 are coordinated together via an O–Ag bond. The four O–Ag atoms constitute an approximately rectangular structure, while other rings, including two pyridine rings and two tetrahydrofuran rings, are arranged around this rectangle. In the two molecules chelated together, the two pyridines are also coplanar with a dihedral angle of only 2.389(8)°. Compound 4 has good solubility in many conventional solvents. We hypothesize that this is due to the presence of the O–Ag group, which is typically insoluble in water, being surrounded by the molecular center. The approximately rectangular structure facilitates greater exposure of silver ions, thereby increasing their opportunities to interact with other reactants and enhancing their overall catalytic performance. Additionally, the high solubility of compound 4 in conventional solvents underscores its significant potential for application in related catalytic processes.
Experimental Section
Safety Precautions
Although we have not encountered any difficulties in preparing these new energetic materials, manipulations must be carried out using standard safety precautions. All compounds should be handled with extreme care, and eye protection and gloves must be worn. −
Synthesis of 2-Nitro-3-hydroxypyridine (2)
To ice-cold acetic acid (50 mL) was added pyridin-3-ol (5 g, 52.6 mmol), followed by the slow addition of fuming nitric acid (6 mL) to the reaction mixture. When the fuming nitric acid was added slowly through a constant-pressure dropping funnel, the system temperature naturally rose to room temperature, and the system was stirred for another 12 h. The reaction mixture was poured into 150 mL of ice water. After vacuum concentration to 30 mL, the residue was cooled with ice water and then allowed to stand. A significant amount of precipitation was observed, and the target compound 2 was obtained by filtration of the precipitate (white solid, 5.3 g, the yield was 72.5%). 1H NMR (DMSO-d 6): δ 11.45 (br, 1H), 7.95–7.95 (m, 1H), 7.56–7.60 (m, 2H). 13C NMR (DMSO-d 6): δ 147.65, 146.86, 138.76, 130.27, 128.88. Elemental analysis (%) calcd for C5H4N2O3 (140.11): C 42.87, H 2.88, O 34.26, N 20.00; Found (140.34): C 42.85, H 2.88, N 20.15.
Synthesis of 2,4,6-Trinitropyridin-3-ol (3)
To ice-cold fuming nitric acid (15 mL) was added 2-nitro-3-hydroxypyridine (2) (3.40 g, 35.7 mmol) in small portions. The compound 2 was introduced into the system prior to gradually heating it to 85 °C over 7 h. Afterward, the mixture was cooled to room temperature and subsequently poured into an ice-water bath (50 mL). The diluted mixture was condensed to 15 mL and cooled with ice water. The precipitation was filtered to obtain the target compound 2,4,6-trinitropyridin-3-ol (3) (yellow crystal, 5.30 g, the yield was 75.6%).1H NMR (D2O): δ 8.72 (s, 1H). 13C NMR (D2O): δ 156.61, 148.60, 147.01, 133.95, 121.04. Elemental analysis (%) calcd for C5H2N4O7 (230.11): C 26.10, H 0.88, O 48.67, N 24.35; Found (230.09): C 26.15, H 0.88, N 24.37.
Synthesis of Compound 4
A solution of compound 3 (920 mg, 4 mmol) in 30 mL of anhydrous THF was stirred at room temperature. Ag2CO3 (552 mg, 2 mmol) was slowly added to the water bath. Following the addition of Ag2CO3, the system was stirred for another 5 h at room temperature. The precipitate was filtered; the filtrate was subsequently collected and concentrated; and compound 4 was obtained through a final filtration step (yellow powder, 1.382 g, yield was 84.4%, the melting point: 123.7 °C). 1H NMR (DMSO-d 6): δ 8.64 (s, 1H, pyridine ring, H–C), 3.55–3.58 (m, 4H, furan ring, H–C–O), 1.71–1.73 (m, 4H, furan ring, H–C–C). 13C NMR (DMSO-d 6): δ 155.64, 151.87, 145.05, 131.01, 120.04, 67.34 (furan ring, C–O), 25.52 (furan ring, C–C). Elemental analysis (%) calcd for AgC9H9O8N4 (409.06): Ag 26.37, C 26.43, H 2.22, O 31.29, N 13.70; Found (409.17): C 26.45, H 2.23, N 13.70.
Supplementary Material
Acknowledgments
We acknowledge the funding support from The Open Fund of Dabie Mountain Laboratory (DMLOF2024013). We also want to thank the following teachers Lingyun Zheng, Peifang Liu, Dongli Xu, and Zongwen Zhang in the analysis testing center of Xinyang Normal University for all the related work in this manuscript came from their hard work.
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c06113.
The authors declare no competing financial interest.
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