Mono- and Dinitro-BN-Naphthalenes: Formation and Characterization

Mono- and dinitro-BN-naphthalenes, i.e., 1-nitro-, 3-nitro-, 1,6-dinitro-, 3,6-dinitro-, and 1,8-dinitro-BNN, were generated in the nitration of 9,10-BN-naphthalene (BNN), a boron–nitrogen (BN) bond-embedded naphthalene, with AcONO2 and NO2BF4 in acetonitrile. The nitrated products were isolated and characterized by NMR, GC-MS, IR, and X-ray single crystallography. The effects of the nitration on the electron density and aromaticity of BNN were evaluated by B-11 NMR analysis and HOMA calculations.


Introduction
As part of a continuing program in the synthesis of nitro-substituted, boron-nitrogen (BN) bond-embedded aromatics as next-generation energetic materials, we undertook the nitration of 9,10-BN-naphthalene (BNN) with AcONO 2 and NO 2 BF 4 in acetonitrile. The nitration led to the formation of mono-and dinitro-BN-naphthalenes. To our knowledge, dinitro-BN aromatics have not been reported previously.

Introduction
As part of a continuing program in the synthesis of nitro-substituted, boron-nitrogen (BN) bond-embedded aromatics as next-generation energetic materials, we undertook the nitration of 9,10-BN-naphthalene (BNN) with AcONO2 and NO2BF4 in acetonitrile. The nitration led to the formation of mono-and dinitro-BN-naphthalenes. To our knowledge dinitro-BN aromatics have not been reported previously.

Nitration of BN-Naphthalene
As mentioned above, a successful nitration of boron-nitrogen bond-embedded aromatics critically depends on the nitration conditions. In our preliminary trial, it was found that BN-naphthalene is sensitive to certain nitration reagents, such as HNO 3 /H 2 SO 4 , N 2 O 4 /AcOH, HNO 3 /Ac 2 O/AcOH/, and HNO 3 /Ac 2 O. These acidic nitration conditions seem to decompose BNN. Therefore, neutral nitration conditions such as AcONO 2 and NO 2 BF 4 in acetonitrile were investigated in the hope that BNN would survive under the reaction conditions and produce higher nitrated BNN products.
Indeed, when AcONO 2 was generated from AgNO 3 and AcCl and the nitration was carried out in anhydrous acetonitrile, mono-and dinitro-BN-naphthalenes were formed. Using one equivalent of the nitration reagent, only mononitro-BNNs were observed, with about 50% conversion, as judged by GC-MS and NMR. By increasing the nitration reagent to 2.7 equivalents, the starting material BNN was consumed completely with the formation of a mixture of mono-and dinitro-BNNs. The procedure involved treating BNN [32] (1.7 mmol) with 2.7 equivalents of AcONO 2 generated from 2.7 equivalents of AgNO 3 and 3.1 equivalents of AcCl in anhydrous acetonitrile at 10 • C and stirring the mixture at 20 • C for 2 h. The products were isolated by silica gel column chromatography, eluting with CH 2 Cl 2 /hexane. The first product eluted was identified as the known 3-nitro-BNN (3) [31]. Further elution gave a second, previously unreported product, 1-nitro-BN-naphthalene (4). The molecular structure was confirmed by GC-MS and 1 H and 13 C NMR and further confirmed by X-ray crystallographic analysis (all spectra can be found in Supplementary Materials). The third product eluted from the column was assigned as 3,6-dinitro-BN-naphthalene (5) based on 1 H and 13 C NMR, and high-resolution mass spectroscopy (m/z = 219.0459, corresponding to dinitro-BNN: C 8 H 6 BN 3 O 4 , calculated 219.0451). Following compound 5, two other dinitro-BNNs, 1,6-(6) and 1,8-dinitro-BNN (7), were isolated. Both structures were assigned by NMR, GC-MS, and X-ray single-crystal analysis. The ratio of 3, 4, 5, 6, and 7 isolated from the nitration is 45:23:6:16:11, with a total yield of 37%.

Nitration of BN-Naphthalene
As mentioned above, a successful nitration of boron-nitrogen bond-embedded aromatics critically depends on the nitration conditions. In our preliminary trial, it was found that BN-naphthalene is sensitive to certain nitration reagents, such as HNO3/H2SO4, N2O4/AcOH, HNO3/Ac2O/AcOH/, and HNO3/Ac2O. These acidic nitration conditions seem to decompose BNN. Therefore, neutral nitration conditions such as AcONO2 and NO2BF4 in acetonitrile were investigated in the hope that BNN would survive under the reaction conditions and produce higher nitrated BNN products.
Indeed, when AcONO2 was generated from AgNO3 and AcCl and the nitration was carried out in anhydrous acetonitrile, mono-and dinitro-BN-naphthalenes were formed. Using one equivalent of the nitration reagent, only mononitro-BNNs were observed, with about 50% conversion, as judged by GC-MS and NMR. By increasing the nitration reagent to 2.7 equivalents, the starting material BNN was consumed completely with the formation of a mixture of mono-and dinitro-BNNs. The procedure involved treating BNN [32] (1.7 mmol) with 2.7 equivalents of AcONO2 generated from 2.7 equivalents of AgNO3 and 3.1 equivalents of AcCl in anhydrous acetonitrile at 10 °C and stirring the mixture at 20 °C for 2 h. The products were isolated by silica gel column chromatography, eluting with CH2Cl2/hexane. The first product eluted was identified as the known 3-nitro-BNN (3) [31]. Further elution gave a second, previously unreported product, 1-nitro-BN-naphthalene (4). The molecular structure was confirmed by GC-MS and 1 H and 13 C NMR and further confirmed by X-ray crystallographic analysis (all spectra can be found in Supplementary Materials). The third product eluted from the column was assigned as 3,6-dinitro-BN-naphthalene (5) based on 1 H and 13 C NMR, and high-resolution mass spectroscopy (m/z = 219.0459, corresponding to dinitro-BNN: C8H6BN3O4, calculated 219.0451). Following compound 5, two other dinitro-BNNs, 1,6-(6) and 1,8-dinitro-BNN (7), were isolated. Both structures were assigned by NMR, GC-MS, and X-ray single-crystal analysis. The ratio of 3, 4, 5, 6, and 7 isolated from the nitration is 45:23:6:16:11, with a total yield of 37%.
Further increasing the number of equivalents of the nitration reagent increases the amounts of dinitro-BNNs but decreases the overall yield. When the nitration reagent was increased to 6.0 equivalents, no detectable mono-and dinitro-BNNs nor trinitro-or higher nitrated BN derivatives were found.
Nitronium tetrafluoroborate (NO2BF4) is a relatively powerful nitration reagent which has been previously reported to give poly-nitro-substituted aromatics under mild conditions [33,34]. Applying the reagent to BNN and carrying out the nitration with three equivalents of NO2BF4 under similar reaction conditions to the AcONO2 nitration (anhydrous CH3CN, 10-20 °C), we isolated only 3,6-dinitro-(5) and 1,6-dinitro-BNN (6), with Further increasing the number of equivalents of the nitration reagent increases the amounts of dinitro-BNNs but decreases the overall yield. When the nitration reagent was increased to 6.0 equivalents, no detectable mono-and dinitro-BNNs nor trinitro-or higher nitrated BN derivatives were found.
To look more closely at the relative reactivities of the nitration reagents to BN-naphthalene, we carefully treated BNN with one equivalent of AcONO2 and NO2BF4, under identical reaction conditions, with the assumption that only mono-nitrated products would be formed. Indeed, AcONO2 nitration, according to NMR analysis, gave only mononitro-BNN (3) and (4) and un-reacted BNN. The ratio of 3:4:BNN was estimated to be 34:17:49 by integration of the NMR signals ( Figure 1a). Replacing AcONO2 with NO2BF4, we observed not only mono-nitrated products 3 and 4 but also dinitro-BNN (6), even though the starting material BNN was also not completely consumed. The product ratio of 3:4:6:BNN was 23:33:13:31 ( Figure 1b). Different from AcONO2 nitration, NO2BF4 nitration gave 1-nitro-BNN (4) as the major product, while AcONO2 nitration gave predominately 3-nitro-BNN (3).

The Spectral and Structural Features of Nitro-BNNs
The introduction of nitro groups to BNN results in a downfield chemical shift for all protons. Similar to carbon-carbon bond-based aromatics, nitro groups, being strongly electron-withdrawing, lower the electron density on the BNN heterocyclic rings, resulting in significant deshielding of the ring protons. However, the nitro groups do not move B-11 chemical shifts in the same manner; B-11 resonances are shifted both upfield and downfield depending on the position where the nitro group is attached. The chemical shift of BNN itself appears at 27.8 ppm (28.4 ppm in n-hexane and 27.9 ppm in AcOH [35]). The introduction of a nitro group to position-1 (4) results in a 2.2 ppm upfield shift to 25.6 ppm. A higher upfield shift of 5.2 ppm is observed when the second nitro group is introduced to position-8 (7)

The Spectral and Structural Features of Nitro-BNNs
The introduction of nitro groups to BNN results in a downfield chemical shift for all protons. Similar to carbon-carbon bond-based aromatics, nitro groups, being strongly electron-withdrawing, lower the electron density on the BNN heterocyclic rings, resulting in significant deshielding of the ring protons. However, the nitro groups do not move B-11 chemical shifts in the same manner; B-11 resonances are shifted both upfield and downfield depending on the position where the nitro group is attached. The chemical shift of BNN itself appears at 27.8 ppm (28.4 ppm in n-hexane and 27.9 ppm in AcOH [35]). The introduction of a nitro group to position-1 (4) results in a 2.2 ppm upfield shift to 25.6 ppm. A higher upfield shift of 5.2 ppm is observed when the second nitro group is introduced to position-8 (7), giving a B-11 chemical shift at 22.6 ppm. However, when the nitro group is attached to position-3 (3), the B-11 shifts downfield by 1.7 ppm to 29.5 ppm. Further introducing a nitro group to position-6 (5) leads to a further downfield shift by 2.1 ppm to 29.9 ppm (Table 1). group is attached to position-3 (3), the B-11 shifts downfield by 1.7 ppm to 29.5 ppm. Further introducing a nitro group to position-6 (5) leads to a further downfield shift by 2.1 ppm to 29.9 ppm (Table 1). The chemical shift of B-11 in homogeneous solution has a close linear relationship with the boron π-electron density [36]. Applying Michl's equation [37], the boron electron density for each nitro-BNN based on the chemical shifts was calculated (Table 1). This shows that the nitro groups attached to the carbons at position-1 and -8 increase the boron π-electron density, but at position-3 and -6, they decrease the density. Baranac-Stojanovic and Stojanovic [38] pointed out that nitro-1,2-azaborazine with a nitro group at the αposition to the boron atom is more stable than its isomer where the nitro group is in the para-position. It was reasoned that the electron-withdrawing nitro group on the carbon at the α-position to the boron atom results in the electron density being drawn to the electron-deficient boron from the neighboring nitrogen to provide a better electrostatic stabilization energy to the molecule [38].
The other important features of a BN aromatic compound are bond lengths and bond angles. These parameters are related to molecular aromaticity and stability. Therefore, we tried to prepare single crystals for all nitro-BNNs to determine the parameters, but only 4, 6, and 7 with a nitro group at position-1 provided crystals of sufficient quality for X-ray analysis. Crystals 4 and 6 are in the orthorhombic space group, with densities of 1.406 and 1.583 g/cm 3 , respectively. Compound 7 is monoclinic, with a density of 1.553 g/cm 3 (BNN [39],  = 1.215 g/cm 3 ). The crystallographic analysis confirms that the introduction of nitro groups to BNN rings does not significantly distort the structure of the molecule. The rings in 4 and 6 are completely planar. In 7, the rings have a slight torsional angle of ~7.3° centered on the boron atom. The nitro groups on 4 and 6 are co-planar with BNN rings, but the nitro groups in 7 are rotated about 39° out of the plane of the heterocyclic rings due to the steric crowding of the nitro groups.
The bond lengths of 4, 6, and 7 are summarized in Table 2, along with the reported bond lengths of the parent BNN [39,40]. All other crystal structure data can be found in the supporting information.
The B-N bond length is usually used as a measure of the relative stability of BN aromatics. Table 2 shows that the introduction of nitro groups to BNN does not significantly change the B-N bond length. When a nitro group is attached to BNN at position-1 (to 4), the B-N bond length is lengthened slightly by 0.011 Å , from 1.461 to 1.472 Å . With a further introduction of a second nitro group to position-8 (to 7), the length of the B-N bond is not lengthened but slightly shortened, changing from 1.472 to 1.460 Å . However, when the second nitro group is introduced to 4 at position-6 (to 6), the B-N bond length falls between the B-N bond lengths of 4 and 7 at 1.469 Å (Table 1). To look closely at how the bond lengths of nitro-BNNs impact the molecular aromaticity, we introduced the HOMA index [41,42] by using bond lengths collected from the X-ray crystallographic analysis. The results are depictured in Figure 2. group is attached to position-3 (3), the B-11 shifts downfield by 1.7 ppm to 29.5 ppm. Further introducing a nitro group to position-6 (5) leads to a further downfield shift by 2.1 ppm to 29.9 ppm (Table 1). The chemical shift of B-11 in homogeneous solution has a close linear relationship with the boron π-electron density [36]. Applying Michl's equation [37], the boron electron density for each nitro-BNN based on the chemical shifts was calculated (Table 1). This shows that the nitro groups attached to the carbons at position-1 and -8 increase the boron π-electron density, but at position-3 and -6, they decrease the density. Baranac-Stojanovic and Stojanovic [38] pointed out that nitro-1,2-azaborazine with a nitro group at the αposition to the boron atom is more stable than its isomer where the nitro group is in the para-position. It was reasoned that the electron-withdrawing nitro group on the carbon at the α-position to the boron atom results in the electron density being drawn to the electron-deficient boron from the neighboring nitrogen to provide a better electrostatic stabilization energy to the molecule [38].
The other important features of a BN aromatic compound are bond lengths and bond angles. These parameters are related to molecular aromaticity and stability. Therefore, we tried to prepare single crystals for all nitro-BNNs to determine the parameters, but only 4, 6, and 7 with a nitro group at position-1 provided crystals of sufficient quality for X-ray analysis. Crystals 4 and 6 are in the orthorhombic space group, with densities of 1.406 and 1.583 g/cm 3 , respectively. Compound 7 is monoclinic, with a density of 1.553 g/cm 3 (BNN [39],  = 1.215 g/cm 3 ). The crystallographic analysis confirms that the introduction of nitro groups to BNN rings does not significantly distort the structure of the molecule. The rings in 4 and 6 are completely planar. In 7, the rings have a slight torsional angle of ~7.3° centered on the boron atom. The nitro groups on 4 and 6 are co-planar with BNN rings, but the nitro groups in 7 are rotated about 39° out of the plane of the heterocyclic rings due to the steric crowding of the nitro groups.
The bond lengths of 4, 6, and 7 are summarized in Table 2, along with the reported bond lengths of the parent BNN [39,40]. All other crystal structure data can be found in the supporting information.
The B-N bond length is usually used as a measure of the relative stability of BN aromatics. Table 2 shows that the introduction of nitro groups to BNN does not significantly change the B-N bond length. When a nitro group is attached to BNN at position-1 (to 4), the B-N bond length is lengthened slightly by 0.011 Å , from 1.461 to 1.472 Å . With a further introduction of a second nitro group to position-8 (to 7), the length of the B-N bond is not lengthened but slightly shortened, changing from 1.472 to 1.460 Å . However, when the second nitro group is introduced to 4 at position-6 (to 6), the B-N bond length falls between the B-N bond lengths of 4 and 7 at 1.469 Å (Table 1). To look closely at how the bond lengths of nitro-BNNs impact the molecular aromaticity, we introduced the HOMA index [41,42] by using bond lengths collected from the X-ray crystallographic analysis. The results are depictured in Figure 2. group is attached to position-3 (3), the B-11 shifts downfield by 1.7 ppm to 29.5 ppm. Further introducing a nitro group to position-6 (5) leads to a further downfield shift by 2.1 ppm to 29.9 ppm (Table 1). The chemical shift of B-11 in homogeneous solution has a close linear relationship with the boron π-electron density [36]. Applying Michl's equation [37], the boron electron density for each nitro-BNN based on the chemical shifts was calculated (Table 1). This shows that the nitro groups attached to the carbons at position-1 and -8 increase the boron π-electron density, but at position-3 and -6, they decrease the density. Baranac-Stojanovic and Stojanovic [38] pointed out that nitro-1,2-azaborazine with a nitro group at the αposition to the boron atom is more stable than its isomer where the nitro group is in the para-position. It was reasoned that the electron-withdrawing nitro group on the carbon at the α-position to the boron atom results in the electron density being drawn to the electron-deficient boron from the neighboring nitrogen to provide a better electrostatic stabilization energy to the molecule [38].
The other important features of a BN aromatic compound are bond lengths and bond angles. These parameters are related to molecular aromaticity and stability. Therefore, we tried to prepare single crystals for all nitro-BNNs to determine the parameters, but only 4, 6, and 7 with a nitro group at position-1 provided crystals of sufficient quality for X-ray analysis. Crystals 4 and 6 are in the orthorhombic space group, with densities of 1.406 and 1.583 g/cm 3 , respectively. Compound 7 is monoclinic, with a density of 1.553 g/cm 3 (BNN [39],  = 1.215 g/cm 3 ). The crystallographic analysis confirms that the introduction of nitro groups to BNN rings does not significantly distort the structure of the molecule. The rings in 4 and 6 are completely planar. In 7, the rings have a slight torsional angle of ~7.3° centered on the boron atom. The nitro groups on 4 and 6 are co-planar with BNN rings, but the nitro groups in 7 are rotated about 39° out of the plane of the heterocyclic rings due to the steric crowding of the nitro groups.
The bond lengths of 4, 6, and 7 are summarized in Table 2, along with the reported bond lengths of the parent BNN [39,40]. All other crystal structure data can be found in the supporting information.
The B-N bond length is usually used as a measure of the relative stability of BN aromatics. Table 2 shows that the introduction of nitro groups to BNN does not significantly change the B-N bond length. When a nitro group is attached to BNN at position-1 (to 4), the B-N bond length is lengthened slightly by 0.011 Å , from 1.461 to 1.472 Å . With a further introduction of a second nitro group to position-8 (to 7), the length of the B-N bond is not lengthened but slightly shortened, changing from 1.472 to 1.460 Å . However, when the second nitro group is introduced to 4 at position-6 (to 6), the B-N bond length falls between the B-N bond lengths of 4 and 7 at 1.469 Å (Table 1). To look closely at how the bond lengths of nitro-BNNs impact the molecular aromaticity, we introduced the HOMA index [41,42] by using bond lengths collected from the X-ray crystallographic analysis. The results are depictured in Figure 2. group is attached to position-3 (3), the B-11 shifts downfield by 1.7 ppm to 29.5 ppm. Further introducing a nitro group to position-6 (5) leads to a further downfield shift by 2.1 ppm to 29.9 ppm (Table 1). The chemical shift of B-11 in homogeneous solution has a close linear relationship with the boron π-electron density [36]. Applying Michl's equation [37], the boron electron density for each nitro-BNN based on the chemical shifts was calculated (Table 1). This shows that the nitro groups attached to the carbons at position-1 and -8 increase the boron π-electron density, but at position-3 and -6, they decrease the density. Baranac-Stojanovic and Stojanovic [38] pointed out that nitro-1,2-azaborazine with a nitro group at the αposition to the boron atom is more stable than its isomer where the nitro group is in the para-position. It was reasoned that the electron-withdrawing nitro group on the carbon at the α-position to the boron atom results in the electron density being drawn to the electron-deficient boron from the neighboring nitrogen to provide a better electrostatic stabilization energy to the molecule [38].
The other important features of a BN aromatic compound are bond lengths and bond angles. These parameters are related to molecular aromaticity and stability. Therefore, we tried to prepare single crystals for all nitro-BNNs to determine the parameters, but only 4, 6, and 7 with a nitro group at position-1 provided crystals of sufficient quality for X-ray analysis. Crystals 4 and 6 are in the orthorhombic space group, with densities of 1.406 and 1.583 g/cm 3 , respectively. Compound 7 is monoclinic, with a density of 1.553 g/cm 3 (BNN [39],  = 1.215 g/cm 3 ). The crystallographic analysis confirms that the introduction of nitro groups to BNN rings does not significantly distort the structure of the molecule. The rings in 4 and 6 are completely planar. In 7, the rings have a slight torsional angle of ~7.3° centered on the boron atom. The nitro groups on 4 and 6 are co-planar with BNN rings, but the nitro groups in 7 are rotated about 39° out of the plane of the heterocyclic rings due to the steric crowding of the nitro groups.
The bond lengths of 4, 6, and 7 are summarized in Table 2, along with the reported bond lengths of the parent BNN [39,40]. All other crystal structure data can be found in the supporting information.
The B-N bond length is usually used as a measure of the relative stability of BN aromatics. Table 2 shows that the introduction of nitro groups to BNN does not significantly change the B-N bond length. When a nitro group is attached to BNN at position-1 (to 4), the B-N bond length is lengthened slightly by 0.011 Å , from 1.461 to 1.472 Å . With a further introduction of a second nitro group to position-8 (to 7), the length of the B-N bond is not lengthened but slightly shortened, changing from 1.472 to 1.460 Å . However, when the second nitro group is introduced to 4 at position-6 (to 6), the B-N bond length falls between the B-N bond lengths of 4 and 7 at 1.469 Å (Table 1). To look closely at how the bond lengths of nitro-BNNs impact the molecular aromaticity, we introduced the HOMA index [41,42] by using bond lengths collected from the X-ray crystallographic analysis. The results are depictured in Figure 2. group is attached to position-3 (3), the B-11 shifts downfield by 1.7 ppm to 29.5 ppm. Further introducing a nitro group to position-6 (5) leads to a further downfield shift by 2.1 ppm to 29.9 ppm (Table 1). The chemical shift of B-11 in homogeneous solution has a close linear relationship with the boron π-electron density [36]. Applying Michl's equation [37], the boron electron density for each nitro-BNN based on the chemical shifts was calculated (Table 1). This shows that the nitro groups attached to the carbons at position-1 and -8 increase the boron π-electron density, but at position-3 and -6, they decrease the density. Baranac-Stojanovic and Stojanovic [38] pointed out that nitro-1,2-azaborazine with a nitro group at the αposition to the boron atom is more stable than its isomer where the nitro group is in the para-position. It was reasoned that the electron-withdrawing nitro group on the carbon at the α-position to the boron atom results in the electron density being drawn to the electron-deficient boron from the neighboring nitrogen to provide a better electrostatic stabilization energy to the molecule [38].
The other important features of a BN aromatic compound are bond lengths and bond angles. These parameters are related to molecular aromaticity and stability. Therefore, we tried to prepare single crystals for all nitro-BNNs to determine the parameters, but only 4, 6, and 7 with a nitro group at position-1 provided crystals of sufficient quality for X-ray analysis. Crystals 4 and 6 are in the orthorhombic space group, with densities of 1.406 and 1.583 g/cm 3 , respectively. Compound 7 is monoclinic, with a density of 1.553 g/cm 3 (BNN [39],  = 1.215 g/cm 3 ). The crystallographic analysis confirms that the introduction of nitro groups to BNN rings does not significantly distort the structure of the molecule. The rings in 4 and 6 are completely planar. In 7, the rings have a slight torsional angle of ~7.3° centered on the boron atom. The nitro groups on 4 and 6 are co-planar with BNN rings, but the nitro groups in 7 are rotated about 39° out of the plane of the heterocyclic rings due to the steric crowding of the nitro groups.
The bond lengths of 4, 6, and 7 are summarized in Table 2, along with the reported bond lengths of the parent BNN [39,40]. All other crystal structure data can be found in the supporting information.
The B-N bond length is usually used as a measure of the relative stability of BN aromatics. Table 2 shows that the introduction of nitro groups to BNN does not significantly change the B-N bond length. When a nitro group is attached to BNN at position-1 (to 4), the B-N bond length is lengthened slightly by 0.011 Å , from 1.461 to 1.472 Å . With a further introduction of a second nitro group to position-8 (to 7), the length of the B-N bond is not lengthened but slightly shortened, changing from 1.472 to 1.460 Å . However, when the second nitro group is introduced to 4 at position-6 (to 6), the B-N bond length falls between the B-N bond lengths of 4 and 7 at 1.469 Å (Table 1). To look closely at how the bond lengths of nitro-BNNs impact the molecular aromaticity, we introduced the HOMA index [41,42] by using bond lengths collected from the X-ray crystallographic analysis. The results are depictured in Figure 2 The chemical shift of B-11 in homogeneous solution has a close linear relationship with the boron π-electron density [36]. Applying Michl's equation [37], the boron electron density for each nitro-BNN based on the chemical shifts was calculated (Table 1). This shows that the nitro groups attached to the carbons at position-1 and -8 increase the boron π-electron density, but at position-3 and -6, they decrease the density. Baranac-Stojanovic and Stojanovic [38] pointed out that nitro-1,2-azaborazine with a nitro group at the αposition to the boron atom is more stable than its isomer where the nitro group is in the para-position. It was reasoned that the electron-withdrawing nitro group on the carbon at the α-position to the boron atom results in the electron density being drawn to the electron-deficient boron from the neighboring nitrogen to provide a better electrostatic stabilization energy to the molecule [38].
The other important features of a BN aromatic compound are bond lengths and bond angles. These parameters are related to molecular aromaticity and stability. Therefore, we tried to prepare single crystals for all nitro-BNNs to determine the parameters, but only 4, 6, and 7 with a nitro group at position-1 provided crystals of sufficient quality for X-ray analysis. Crystals 4 and 6 are in the orthorhombic space group, with densities of 1.406 and 1.583 g/cm 3 , respectively. Compound 7 is monoclinic, with a density of 1.553 g/cm 3 (BNN [39], ρ = 1.215 g/cm 3 ). The crystallographic analysis confirms that the introduction of nitro groups to BNN rings does not significantly distort the structure of the molecule. The rings in 4 and 6 are completely planar. In 7, the rings have a slight torsional angle of 7.3 • centered on the boron atom. The nitro groups on 4 and 6 are co-planar with BNN rings, but the nitro groups in 7 are rotated about 39 • out of the plane of the heterocyclic rings due to the steric crowding of the nitro groups.
The bond lengths of 4, 6, and 7 are summarized in Table 2, along with the reported bond lengths of the parent BNN [39,40]. All other crystal structure data can be found in the supporting information.
The B-N bond length is usually used as a measure of the relative stability of BN aromatics. Table 2 shows that the introduction of nitro groups to BNN does not significantly change the B-N bond length. When a nitro group is attached to BNN at position-1 (to 4), the B-N bond length is lengthened slightly by 0.011 Å, from 1.461 to 1.472 Å. With a further introduction of a second nitro group to position-8 (to 7), the length of the B-N bond is not lengthened but slightly shortened, changing from 1.472 to 1.460 Å. However, when the second nitro group is introduced to 4 at position-6 (to 6), the B-N bond length falls between the B-N bond lengths of 4 and 7 at 1.469 Å (Table 1). To look closely at how the bond lengths of nitro-BNNs impact the molecular aromaticity, we introduced the HOMA index [41,42] by using bond lengths collected from the X-ray crystallographic analysis. The results are depictured in Figure 2.  1.520 a All X-ray crystallographic data can also be found at the Cambridge Crystallographic Data Center with the following codes: 4, 1977407; 6, 1977408; and 7, 1977409. b Ref. [39], the data were from X-ray crystallographic analysis. Due to the disorder of the molecule, the boron-carbon and nitrogen-carbon bond lengths are averaged. c Ref [40], the data were obtained from a microwave measurement.
According to HOMA calculations, BN-naphthalene (BNN) possesses 81% of the aromaticity of naphthalene [41]. The introduction of one nitro group to position-1 slightly changes the aromaticity of BNN (±2%). The same results were observed when the second nitro group was introduced to position-6 or -8, suggesting that dinitro-BNNs retained a significant amount of aromaticity.

Conclusions
We demonstrated the first reported formation of dinitro-substituted-BN-naphthalenes using AcONO2 and NO2BF4 as the nitrating reagents in acetonitrile. These new boron-nitrogen bond-embedded nitrocompounds were fully characterized by NMR, GC-MS, IR, and X-ray crystallographic analysis. In addition, the positional effect of nitro substitution on the electron density of the boron atom of BNN, along with a comparison of the positional effect of nitration on the aromaticity of nitrated BNN with respect to the parent BNN, was discussed.  [39], the data were from X-ray crystallographic analysis. Due to disorder of the molecule, the boron-carbon and nitrogen-carbon bond lengths are averaged. c Ref [40], the data w obtained from a microwave measurement.
According to HOMA calculations, BN-naphthalene (BNN) possesses 81% of maticity of naphthalene [41]. The introduction of one nitro group to position-1 changes the aromaticity of BNN (±2%). The same results were observed when the nitro group was introduced to position-6 or -8, suggesting that dinitro-BNNs ret significant amount of aromaticity.

Conclusions
We demonstrated the first reported formation of dinitro-substituted-BN-n lenes using AcONO2 and NO2BF4 as the nitrating reagents in acetonitrile. These n ron-nitrogen bond-embedded nitrocompounds were fully characterized by NM MS, IR, and X-ray crystallographic analysis. In addition, the positional effect of ni stitution on the electron density of the boron atom of BNN, along with a compa the positional effect of nitration on the aromaticity of nitrated BNN with respec parent BNN, was discussed. According to HOMA calculations, BN-naphthalene (BNN) possesses 81% of the aromaticity of naphthalene [41]. The introduction of one nitro group to position-1 slightly changes the aromaticity of BNN (±2%). The same results were observed when the second nitro group was introduced to position-6 or -8, suggesting that dinitro-BNNs retained a significant amount of aromaticity.

Conclusions
We demonstrated the first reported formation of dinitro-substituted-BN-naphthalenes using AcONO 2 and NO 2 BF 4 as the nitrating reagents in acetonitrile. These new boronnitrogen bond-embedded nitrocompounds were fully characterized by NMR, GC-MS, IR, and X-ray crystallographic analysis. In addition, the positional effect of nitro substitution on the electron density of the boron atom of BNN, along with a comparison of the positional effect of nitration on the aromaticity of nitrated BNN with respect to the parent BNN, was discussed.

Experimental Section
All reagents and anhydrous solvents were purchased from commercial suppliers and used without further purification except nitronium tetrafluoroborate (NO 2 BF 4 ). The purity of acetyl chloride was ≥ 99%, AgNO 3 ≥ 99%, and anhydrous CH 3 CN, ≥ 99.9%. NO 2 BF 4 was purified by washing the commercial product with anhydrous nitromethane and CH 2 Cl 2 under argon before use. NO 2 BF 4 (commercial product, ≥ 95%, 5 g) was placed into a Schlenk tube equipped with a fritted filter. Anhydrous nitromethane (10 mL) was added, and the mixture was stirred with a glass bar under argon for 10-20 s. The liquid was then drawn under argon. The solid was washed with anhydrous nitromethane (10 mL) for one more time, followed by anhydrous CH 2 Cl 2 (3 × 10 mL), and dried under slow-flowing argon for 2 h. 9,10-BN-Naphthalene was prepared from di-(3-butene-1-yl)amine using a slight modification of Dewar's method [32] and purified by column chromatography (silica gel, CH 2 Cl 2 :Hexane, 1:5, R f = 0.5), followed by recrystallization from cold pentane. 1 H and 13 C NMR spectra were acquired on either a Bruker 500 MHz spectrometer (500 and 150 MHz, respectively) or an Anasazi Instruments Eft-90 MHz spectrometer with a Varian magnet (90 and 22.5 MHz, respectively). 11 B NMR was performed on an Anasazi Instruments Eft-90 MHz spectrometer (28.1 MHz) using BF 3 .Et 2 O as the reference (0 ppm). 1 H and 13 C NMR chemical shifts were reported relative to the residual solvent as internal standard, such as CD 2 Cl 2 (5.32 ppm for proton and 53.5 ppm for C-13). Infrared spectra (thin films) were collected using a Bruker Alpha ZnSe ATR FTIR. GC-MS was performed on an Agilent 7890A gas chromatograph equipped with a 5975 mass spectrometer and an NCI detector.
X-ray crystallography: Colorless crystals were mounted on α MiteGen MicroMesh by using a small amount of Cargille Immersion oil. Data were collected on a Bruker three-circle platform diffractometer equipped with a SMART APEX II CCD detector. The crystals were irradiated by using graphite-monochromated MoKα radiation (λ = 0.71073). An Oxford Cobra low-temperature device was used to maintain the crystals at a constant 150(2) K during data collection. Data collection was performed, and the unit cell was initially refined by using APEX2 (v2010.3-0). Data reduction was performed using SAINT (v7.68 A) and XPREP (v2008/2). Corrections were applied for Lorentz, polarization, and absorption effects by using SADABS (v2008/1). The structure was solved and refined with the aid of the programs in the SHELXTL-plus (v2008/4) system of programs. The full-matrix least squares refinement on F2 included atomic coordinates and anisotropic thermal parameters for all non-hydrogen atoms.