Decomposition and oligomerization of 2,3-naphthyridine under high-pressure and high-temperature conditions

The chemical reaction of 2,3-naphthyridine, a nitrogen-containing aromatic compound, was investigated at pressures ranging from 0.5 to 1.5 GPa and temperatures from 473 to 573 K. A distinct decrease in the amount of residual 2,3-naphthyridine was observed in the samples recovered after reaction at ˃523 K at 0.5 and 1.0 GPa, and ˃548 K at 1.5 GPa. The formation of o-xylene and o-tolunitrile accompanied a decreasing N/C ratio of the reaction products, indicating decomposition of the aromatic ring and release of nitrogen. Precise analysis of the reaction products indicated the oligomerization of decomposed products with the residual 2,3-naphthyridine to form larger molecules up to 7mers. Nitrogen in the aromatic ring accelerated reactions to decompose the molecule and to oligomerize at lower temperatures than those typically reported for aromatic hydrocarbon oligomerization. The major reaction mechanism was similar between 0.5 and 1.5 GPa, although larger products preferentially formed in the samples at higher pressure.

the nanothread was similar to that of the starting pyridine material, indicating that most nitrogen atoms were retained during the polymerization and formation of the sp 3 bonded carbon 30 . In contrast to PAHs, the behavior of nitrogen-containing aromatic compounds under high pressure and high temperature are hardly investigated. A recent study reported that polymerization of pyridine occurs under high pressure and temperature at approximately 8 GPa and 750-800 K, a reaction temperature lower than that of benzene polymerization 34 . However, details regarding the reaction mechanism and nitrogen behavior during the reaction were not examined.
In this study, the chemistry of a nitrogen-containing aromatic compound under high-pressure and high-temperature conditions was investigated ranging from 0.5 to 1.5 GPa and from 473 to 573 K using a piston-cylinder-type high-pressure apparatus. Naphthyridines (C 8 H 6 N 2 ) consist of two condensed aromatic rings as naphthalene, while two of the CH groups have been replaced with nitrogen atoms. There are ten isomers with different positions of nitrogen atoms in the aromatic rings. Here we selected 2,3-naphthyridine as a starting material, in which two nitrogen atoms were localized in one aromatic ring forming a N-N bonding, to investigate the influence of the localized nitrogen atoms on the stability and chemical reactions of the heterocyclic aromatic compound. Precise analysis of the recovered samples was conducted and both relatively light and volatile materials and heavier oligomerized materials were detected to reveal a detailed reaction mechanism. Methods 2,3-naphthyridine (Sigma-Aldrich, purity >98%, 15-30 mg) was used as the starting material for the high-pressure and high-temperature experiments. A gold capsule (4 mm outer diameter, 3.6 mm inner diameter) was used as a sample capsule. To remove organic contaminants, the capsule was cleaned with acetone and heated in an oven at 450 °C for 3 h prior to encapsulating the sample.
The sample within the capsule was pressurized using a tungsten carbide piston-cylinder equipped with a hydraulic press. A cylinder with 4 mm inner diameter and 70 mm outer diameter was used without a pressure medium. The detail of the high-pressure apparatus is described in a Supplementary Note. The sample was first compressed to the target pressure at room temperature and subsequently heated by a band-type external heater surrounding the cylinder. The temperature was measured using a K-type thermocouple attached to the top of the cylinder. The compressed sample was recovered from the capsule in distilled dichloromethane to prevent the reaction products from escaping. The solution was initially filtered to remove the sample capsule fragments. No insoluble materials were found in the compressed samples.
The carbon and nitrogen contents of the reaction products were analyzed using an elemental analyzer (EA, Vario EL cube; Elementar Analysensysteme GmbH) after evaporating the solvent. Mass spectra of the reaction products were obtained using a gas chromatography mass spectrometer (GC/MS, JMS-K9; JEOL Co.) equipped with a 30 m × 0.25 mm I.D. capillary column, with a 0.25 μm layer of HP-5 (Agilent Technology Co.). The GC column temperature was programmed as in our previous studies 12,13 . A GC-flame ionization detector (GC/FID, GC-2014; Shimadzu) equipped with an HP-5 capillary column was used for quantitative analysis of the residual 2,3-naphthyridine and reaction products. The GC column temperature was programmed as in the GC/MS measurements. Methyl laurate (C 13 H 26 O 2 ), methyl stearate (C 19 H 38 O 2 ), and methyl triacontanate (C 31 H 62 O 2 ) were used as internal standards for GC/FID and GC/MS analyses. The mass spectra of the reaction products were analyzed using a MALDI-TOF/MS instrument (Ultraflex-III, Bruker), with an α-cyano-4-hydroxycinnamic acid (α-CHCA) matrix. In this study, peaks with >1% of the maximum intensity were considered to be reaction products, similar to the MALDI-TOF/MS study of Chanyshev et al. 19 .

Results and Discussion
In total, 19 runs were performed at 0.5, 1.0, and 1.5 GPa, changing the temperature from 473 to 573 K with a preservation time of 1-12 h (Table S1). Figure 1a shows the residual ratio of 2,3-naphthyridine at 1.5 GPa as a function of temperature and preservation time, as measured by GC/FID. Most of the 2,3-naphthyridine remained after heating at 473 K and the residual ratio was not significantly changed with preservation time. After heating at 523 K, the residual 2,3-naphthyridine decreased with preservation time, although approximately 75% of the starting material remained after heating for 12 h. In contrast, the residual 2,3-naphthyridine remarkably decreased after heating at 548 K for 4 h to approximately 1%. After heating at 573 K for 1 h, <1% of the 2,3-naphthyridine remained, indicating that the chemical reaction of 2,3-naphthyridine was significant above 548 K at 1.5 GPa. Figure 1b shows temperature dependence of the residual ratio of 2,3-naphthyridine after heating at 0.5-1.5 GPa at a fixed preservation time of 4 h. At 523 K, a significant decrease of 2,3-naphthyridine was observed in the samples at 0.5 and 1.0 GPa while approximately 70% of the 2,3-naphthyridine remained at 1.5 GPa. This suggests that the required reaction temperature increases with increasing pressure. Figure 2a shows the molar ratio of N/C in the recovered samples at 1.5 GPa. After heating at 473 K, the N/C ratio of the recovered sample was 0.240, comparable to that of the starting material (N/C = 0.244) and was unchanged with a preservation time of up to 12 h. At 523 K, the N/C ratio decreased with increasing preservation time from 1 to 4 h, while the N/C ratio remained constant at ∼0.2 between 4 and 12 h. At 548 K, the relative nitrogen content remarkably decreased resulting in an N/C ratio of 0.14 after 1 h of heating, further decreasing to 0.07 at 4 h. After heating at 573 K, the N/C ratio was ∼0.06 after 1 and 4 h. Figure 2b shows the N/C ratios at different pressures with increasing temperature after 4 h of heating. At 0.5 and 1.0 GPa, the N/C ratio significantly decreased after heating above 523 K. The N/C ratio decrease was accompanied by a decreasing residual ratio of 2,3-naphthyridine, suggesting that the chemical reaction involves nitrogen release from the aromatic ring of 2,3-naphthyridine, which is then distributed to the volatile phase(s). When the samples were recovered under ambient conditions and dissolved in dichloromethane solvent, some bubbles were observed. Although the gas phase was not directly analyzed in this study, the bubbles were likely the volatile phase(s) composed of nitrogen www.nature.com/scientificreports www.nature.com/scientificreports/ and/or ammonia gas. The N/C ratio decreased to approximately 0.05 after the reaction, showing a small amount of nitrogen was left in the recovered samples.
The recovered samples were analyzed using GC/MS to determine the molar mass and possible molecular structures of the reaction products with relatively low masses (m/z = 40-500). The products were identified based on comparison of their mass spectra with the database (NIST 02) installed in the GC/MS software. In the samples heated above 523 K regardless of pressure, o-xylene (m/z = 106) was detected (Fig. S2a). In addition, o-tolunitrile (m/z = 117) was observed in the samples heated above 523 K at 0.5 GPa and above 548 K at 1.0 and 1.5 GPa (Fig. S2b). The formation of smaller molecules suggests the decomposition of 2,3-naphthyridine to form radicals of o-xylene and o-tolunitrile. In addition, the total ion chromatogram (TIC) of the samples heated at >523 K at 1.0 and 1.5 GPa, and that of the sample heated at >473 K at 0.5 GPa show peaks corresponding to reaction products with larger m/z than that of 2,3-naphthyridine. Figure 3 shows the representative TIC of the reaction products from the samples heated at 548 K at 0.5 and 1.5 GPa for 4 h, with the major products numbered as products 1-18 and listed in Table S1. The identification processes of some of the products are shown in the Supplementary Note and mass spectra of the representative products are shown in Fig. S3.
The recovered samples from the 1.5 GPa, 473-573 K and 0.5 GPa, 473-548 K conditions were analyzed using MALDI-TOF/MS to investigate the distribution of product masses with larger mass numbers. Figure 4 shows representative MALDI-TOF/MS spectra of the sample treated at 1.5 GPa, 548 K, and heated for 4 h. Intense peaks were observed at m/z = 218, 231, 246, 306, 318, 331, 333, and 335, comparable with the mass numbers of the reaction products observed in the GC/MS analysis. Some of the peaks in the MALDI-TOF spectra were likely [M + H] + ions. The mass pattern is characterized as 5-6 series of peak clusters at intervals of 13-15 mass units. The series of clusters have mass numbers showing periodicities approximately 100 mass units apart, which is comparable to that of the xylene radical. Each cluster is likely to represent dimers, trimers, and heavier oligomers. The largest peak was observed at m/z = 780, indicating the formation of oligomers up to 7mers (Fig. 4). Intense peaks of m/z = 260 and 261 were additionally detected when the heating temperature was increased to 573 K (Fig. S4a) as well as when the pressure was decreased to 0.5 GPa (Fig. S4b). These mass numbers are consistent with twice www.nature.com/scientificreports www.nature.com/scientificreports/ the molar weight of 2,3-naphthyridine. However, the exact molecular structure could not be identified because this reaction product was not detected in GC/MS analysis. The result suggests the product was formed from further progress of the chemical reaction at higher temperatures or lower pressures.
From the GC/MS and MALDI-TOF/MS spectra, we propose a major process of dimerization and trimerization in Fig. 5a,b, respectively. Most products are considered to be formed by the oligomerization of o-xylene and o-tolunitrile radicals with a 2,3-naphthyridine. The representative molar mass of the dimers was 206, 219, 230, 232, and 245. A product with m/z = 206 was detected in MALDI-TOF/MS analysis; this was considered to be formed by the dimerization of two o-xylene radicals (reaction 1 in Fig. 5a). The product of m/z = 219 was detected in both GC/MS (product 1 in Fig. 3) and MALDI-TOF/MS analyses. This product is likely to be formed by the dimerization of o-xylene and o-tolunitrile radicals with hydrogenation (reaction 2 in Fig. 5a). Product 2 was also formed by the dimerization of o-xylene and o-tolunitrile radicals with dehydrogenation. Products 3 and 4 were the most abundant products detected in the GC/MS measurements. The possible formation mechanism of these products is the dimerization of an o-xylene radical with 2,3-naphthyridine followed by dehydrogenation ((3) in Fig. 5a). The products of m/z = 232 (products 5 and 6) were likely to be formed by the dimerization of two o-tolunitrile radicals (reaction 4 in Fig. 5a), and the product of m/z = 245 (product 9) could be formed by the dimerization of a o-tolunitrile radical with 2,3-naphthyridine followed by dehydrogenation. The possible oligomerization mechanisms for product 7-18 are also shown in Fig. 5b as the combinations of o-xylene, o-tolunitrile radicals, and a 2,3-naphthyridine. Naphthalene (C 10 H 8 ) consists of two condensed aromatic rings as well as 2,3-naphthyridine. Oligomerization of naphthalene has been reported 19,21 , where the reaction temperature (˃800 K) at ambient and high pressures was significantly higher than that of 2,3-naphthyridine. It has been proposed that the oligomerization of naphthalene and other PAHs occur concomitantly with dehydrogenation and without ring opening 19 . In shock experiments of naphthalene at up to 33.7 GPa and 1660 K, dark material comprised of amorphous carbon was predominantly observed with the formation of various methylation, phenylation, and naphthylation products 24 . During the shock experiments of naphthalene, both decomposition and oligomerization occurred, similar to the chemical www.nature.com/scientificreports www.nature.com/scientificreports/ reaction of 2,3-naphthyridine observed in this study. However, the temperature and pressure of the shock experiments were significantly higher than the values used in this study. The formation of xylene was not reported in the shock experiments of naphthalene. These results indicate that the mechanism of pressure-temperature-induced oligomerization is quite different between naphthalene and 2,3-naphthyridine. The N-N covalent bond in the heterocyclic aromatic compound is easily dissociated and the aromatic ring consisting of both nitrogen and carbon opens at lower temperatures than that of aromatic hydrocarbons. This is similar to a theoretical study that suggested any single covalent bonded compound favors the formation of monomeric molecular N 2 with a triple bond below 100 GPa 35 . In contrast, cleavage of C-C bonds in the aromatic compounds hardly occurred at the  www.nature.com/scientificreports www.nature.com/scientificreports/ pressure and temperature conditions of this study and the remaining aromatic rings oligomerized to form larger molecules.
The major decomposition and oligomerization products are similar between 0.5 GPa and 1.5 GPa, although the relative ratio of each product shows the pressure dependence. The molar yields of the major reaction products detected by GC/FID were roughly quantified using the peak area of the chromatogram relative to that of an internal standard (methyl stearate). Because suitable standards for products were not available, to quantify the reaction products we used the response factor of benz[a]anthracene, which shows similar structure to those of the major products (product 3 and 4 in Fig. 3). Figure 6a shows the ratios of the relative reaction products between 0.5 and 1.5 GPa, at 548 K. The products could be classified as dimers (m/z = 217-245) and trimers (m/z = 306-337). At 0.5 GPa, over 80% of the quantified products are dimers, while the abundance falls to approximately 60% at 1.5 GPa. The relative ratio of the trimers increases with increasing pressure suggesting the trimers preferentially form at higher pressure. The molar yields of the major reaction products are shown in Fig. 6b. The total yield of the detected reaction products decreases with increasing pressure, even though the residual ratio of 2,3-naphthyridine is quite similar between the recovered samples. The result suggests that undetected products in GC/MS are likely the heavier products, as observed in MALDI-TOF/MS analysis, and were preferentially formed at higher pressures. The results suggest heavier oligomers prefer to form with increasing pressure. Similar pressure dependence of oligomerization was also reported by a theoretical study of methane oligomerization under high-pressure and high-temperature conditions, in which heavier reaction products preferred to form at higher pressure, although both pressure and temperature were significantly higher than the present study 36 . Intermolecular distances shorten with increasing pressure and this could induce a formation of heavier oligomers at higher pressure.  Table S2). A solid line separates the products into dimers and trimers.

Conclusion
Chemical reactions of 2,3-naphthyridine were investigated at 0.5-1.5 GPa and 473-573 K using a piston-cylinder-type high-pressure apparatus. A distinct decrease in the residual 2,3-naphthyridine was observed in GC/FID analysis of the samples recovered from >523 K at 0.5/1.0 GPa and >548 K at 1.5 GPa. The recovered samples exhibited lower N/C ratios than in the original material in addition to forming o-xylene and o-tolunitrile products, as detected by GC/MS analysis. These results suggest that the decomposition of the aromatic ring occurred with nitrogen release. Various heavy oligomers (up to approximately m/z = 780) were detected by GC/ MS and MALDI/TOF-MS analyses. Most oligomers observed in GC/MS analysis contained aromatic ring(s) in their structure and the MALDI/TOF-MS spectra of the recovered samples contained products with mass numbers showing periodicities of approximately m/z = 100. This indicated that oligomerization occurred up to 7mers between o-xylene, o-tolunitrile, and the residual 2,3-naphthyridine. At higher temperatures, peaks with a mass approximately twice of that of 2,3-naphthyridine were observed, implying simple dimerization of 2,3-naphthyridine as the second step of the chemical reaction. The major reaction mechanism was similar at 0.5-1.5 GPa, although the required reaction temperature increased with increasing pressure. Quantification of the representative dimers and trimers indicated that larger products preferentially formed in the samples at higher pressure, suggesting that oligomerization is favored at higher pressure.