Further investigation on the nitration of BODIPY with cupric nitrate: crystal structures of 4,4-difluoro-1,3,5,7,8-pentamethyl-2-nitro-4-bora-3a,4a-diaza-s-indacene, 4,4-difluoro-3-nitro-8-phenyl-4-bora-3a,4a-diaza-s-indacene, and 3-chloro-6-ethyl-5,7,8-trimethyl-2-nitro-4,4-diphenyl-4-bora-3a,4a-diaza-s-indacene

The treatment of non-fully substituted 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) with cupric nitrate leads to the introduction of a nitro group at different positions of the BODIPY core, depending on the substitution pattern. This methodology complements the treatment of fully substituted BODIPY with cupric nitrate that was previously reported. In all three structures, the fused ring system is in a very flattened ‘V-shape’ and the central six-membered ring adopts a flattened sofa conformation.


Reactions between non-fully substituted BODIPY and cupric nitrate
We report herein that treatment of BODIPY, where at least one of the R 1 -R 7 is H, with cupric nitrate leads to the nitration of the BODIPY core (see Scheme below).

Structural commentary
The molecular structures of 5a, 5b and 5d are shown in Figs. 1, 2 and 3, respectively. In all three structures the fused ring system is in a very flattened 'V-shape' with the two outer five- The molecular structure of 5d with displacement ellipsoids drawn at the 30% probability level. Neither the H atoms not the minor component of disorder are shown.

Figure 2
The molecular structure of 5b with displacement ellipsoids drawn at the 30% probability level. H atoms are not shown.

Figure 5
Part of the crystal structure of 5b with weak C-HÁ Á ÁO and C-HÁ Á ÁF hydrogen bonds shown as dashed lines. Only H atoms involved in hydrogen bonds are shown.

Figure 6
Part of the crystal structure of 5d with weak C-HÁ Á ÁO hydrogen bonds shown as dashed lines. Only H atoms involved in hydrogen bonds are shown. Both components of disorder are shown.

Database survey
A survey of the Cambridge Structural Database (V5.38, last update May 2017; Groom et al., 2016) revealed that the crystal structure of 4,4-difluoro-1,3,5,7,8-pentamethyl-4-bora-3a,4adiaza-s-indacene has been determined at three different temperatures viz. JEHFUX at 295 K (Picou et al., 1990) JEHFUX01 at 200 K (Choi et al. 2014) and JEHFUX02 at 100 K (Wang et al., 2014). This structure corresponds to compound 5a without the nitro substituent and in all three equivalent literature structures, the atoms of the fused-ring system lie on a crystallographic mirror plane and hence the fused-ring system is exactly planar. In the compound corresponding to 5b without the nitro substituent, viz. 4,4-difluoro-8phenyl-4-bora-3a,4a-diaza-s-indacene (VAWDED, Kee et al., 2005), the molecule is bisected by a crystallographic twofold rotation axis through the central B and C atoms of the sixmembered ring and the six-membered ring is essentially planar. To date, compound 5d is the only crystal structure with a 4-bora-3a,4a-diaza-s-indacene core which is substituted by two phenyl rings at boron and a Cl atom in the 3-position.

Synthesis and crystallization
1 H, 13 C, 11 B, and 19 F NMR spectra were recorded at 400.2, 100.6, 128.4, and 376.6 MHz, respectively, with a Bruker AV400 spectrometer; J values are given in Hz. Chemical shifts are given in ppm. High-resolution mass spectra were measured with a ThermoFisher high resolution Double Focusing magnetic sector mass spectrometer.
Chemicals were purchased from Aldrich or TCI America and used without further purification unless stated otherwise. Triethylamine was dried by heating under reflux in the presence of calcium hydride and distilled in an atmosphere of nitrogen. Silica gel (SiliCycle, >230 mesh) was used for flash chromatography. Thin layer chromatography was performed on SiliCycle SiliaPlate F-254 TLC plates, with the following system: ethylacetate-hexane (3:7 v/v).

General procedure for the treatment of 4a-e with cupric nitrate
To a solution of BODIPY (100 mg) in anhydrous CH 2 Cl 2 (20 mL), a solution of Cu(NO 3 ) 2 Á3H 2 O (5 mol. equiv.) in anhydrous MeCN (10 mL) was added. The reaction mixture was stirred at room temperature and the reaction progress was monitored by TLC. Upon complete consumption of starting materials, the products were evaporated under reduced pressure. The residue was redissolved in CH 2 Cl 2 (20 mL) and extracted with water (320 mL). The organic layer was collected, dried (MgSO 4 ), and evaporated under reduced pressure. The residue was purified by column chromatography on silica gel. The appropriate fractions, eluted with CH 2 Cl 2hexane, were combined and evaporated under reduced pressure to give the nitro BODIPY.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. In all three compounds, the H atoms were placed in calculated positions and included in the refinement in a riding-model approximation with U iso (H) = 1.2U eq (C) or 1.5U eq (C methyl ). In compound 5d the atoms of the nitro group were refined as disordered over two sets of sites with occupancies 0.618 (12) and 0.382 (12).
In the refinement, restraints were applied to the bond distances of the nitro group so that those in the minor component of disorder were similar to those in the major component. The refinement of the minor component of disorder was also restrained to be approximately planar. These restraints were achieved using the SADI and FLAT commands in SHELXL (Sheldrick, 2015b).  4,4-difluoro-1,3,5,7,8-pentamethyl-2-nitro-4-bora-3a,4a-diaza-sindacene, 4,4-difluoro-3-nitro-8-phenyl-4-bora-3a,4a-diaza-s-indacene, and 3chloro-6-ethyl-5,7,8-trimethyl-2-nitro-4,4-diphenyl-4-bora-3a,4a- Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq F1 0.86032 (11) 0.70745 (10) 0.51598 (7) 0.0232 (2)      Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.