4,4-Difluoro-2,3;5,6-bis(tetramethylene)-4-bora-3a,4a-diaza-s-indacene (LD540)

The title compound, C18H21BF2N2, is a lipophilic dye based on a BODIPY fluorophore backbone, which was developed for microscopic imaging of lipid droplets; the molecule has a planar BODIPY core [dihedral angle between the pyrrole rings = 2.3 (3)°] and two tetramethylene substituents at the 2,3- and 5,6-positions in a half-chair conformation. One of the tetramethylene substituents is disordered over two two sets of sites with site occupancies of 0.5. In the crystal, pairs of C—H⋯F interactions link the molecules into inversion dimers. Neighbouring dimers are linked by further C—H⋯F interactions, forming an infinite array. C—H⋯π and π–π [centroid–centroid distance = 4.360 (3) Å] interactions are observed between the BODIPY core and the tetramethylene substituents of neighbouring dimer pairs.

The title compound, C 18 H 21 BF 2 N 2 , is a lipophilic dye based on a BODIPY fluorophore backbone, which was developed for microscopic imaging of lipid droplets; the molecule has a planar BODIPY core [dihedral angle between the pyrrole rings = 2.3 (3) ] and two tetramethylene substituents at the 2,3-and 5,6-positions in a half-chair conformation. One of the tetramethylene substituents is disordered over two two sets of sites with site occupancies of 0.5. In the crystal, pairs of C-HÁ Á ÁF interactions link the molecules into inversion dimers. Neighbouring dimers are linked by further C-HÁ Á ÁF interactions, forming an infinite array. C-HÁ Á Á and -[centroid-centroid distance = 4.360 (3) Å ] interactions are observed between the BODIPY core and the tetramethylene substituents of neighbouring dimer pairs.   Table 1 Hydrogen-bond geometry (Å , ).

Comment
Lipid droplets are metabolically active organelles (Beller et al., 2010;Bickel et al., 2009), which function as intracellular storehouses of lipid esters found inside almost all cells. LD540 is one of the dyes that can be used for multicolor fluorescence imaging for lipid droplets in both fixed and living cells (Spandl et al., 2009). In the structure of the title compound, the BODIPY core is planar having the average dihedral angle formed between the two pyrrole rings of 2.3 (3)° (Fig. 1). The two tetramethylene substituents on either side of the BODIPY core at the 2,3-and 5,6-positions are in a half-chair conformation. Intermolecular F···H-C interactions (distance of 2.661 (3) Å) between the fluoride (F3) and methyl (C23) groups of the opposite facing molecules connect the two LD540 molecules to form a dimer (Fig.2, Table 2).

Experimental
The title compound was synthesized by a known method described by Christoph Thiele and co-workers (Spandl et al., 2009) using tetrahydropyrrole, acetylchloride and BF 3 -etherate as the starting material. For single-crystal X-ray analysis the crude product was recrystallized from dichloromethane yielding greenish red prism crystals.

Refinement
All H atoms were visible in the electron density maps, but those bonded to C were ideally positioned and allowed to ride on their parent atoms with U iso (H) of 1.2 (or 1.5 for methyl) times U eq (C). One of the tetramethylene substituent is disordered over two positions (C18-C19) having fixed site occupation factors of 0.5.  Molecular structure of the title compound with atom labels and 50% probability displacement ellipsoids for non-H atoms.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (