[μ-Bis(5,7-dimethyl-1,8-naphthyridin-2-yl)diazene]bis[difluoridoboron(III)]

In the title compound, C20H18B2F4N6, the bis(5,7-dimethyl-1,8-naphthyridin-2-yl)diazene molecule is bisected by a symmetry centre midway between the central N atoms of the diazene group. Each of the symmetry-related halves of the molecule binds to a B atom through an N,N′-bite. Two terminal F ions complete the distorted BN2F2 tetrahedral geometry around each B atom. The BF2 plane is almost perpendicular to the boron–naphthyridine ring plane, with a dihedral angle of 87.8 (2)°. The main interactions in the crystal structure are some C—H⋯F hydrogen bonds and π–π contacts between 1,8-naphthyridine rings [centroid–centroid distance = 4.005 (1) Å].

In the title compound, C 20 H 18 B 2 F 4 N 6 , the bis(5,7-dimethyl-1,8-naphthyridin-2-yl)diazene molecule is bisected by a symmetry centre midway between the central N atoms of the diazene group. Each of the symmetry-related halves of the molecule binds to a B atom through an N,N 0 -bite. Two terminal F ions complete the distorted BN 2 F 2 tetrahedral geometry around each B atom. The BF 2 plane is almost perpendicular to the boron-naphthyridine ring plane, with a dihedral angle of 87.8 (2) . The main interactions in the crystal structure are some C-HÁ Á ÁF hydrogen bonds andcontacts between 1,8-naphthyridine rings [centroid-centroid distance = 4.005 (1) Å ].

S1. Comment
The deriatives of 1,8-naphthyridine have been widely utilized as mono-nucleating and dinucleating ligands in coordination chemistry or as antimycobacterial and antimicrobial agents (Gavrilova & Bosnich, 2004;Goswami et al., 1997;Nakatani et al., 2000). Here, we report the crystal structure of the centrosymmetric dinuclear title compound, C 20 H 18 B 2 F 4 N 6 , where the di-5,7-dimethyl-1,8-naphthyl-2,2′-diazene (ddnd) ligand is halved by a symmetry centre midway the central nitrogens in the diazene group, and where each boron atom is coordinated by a N,N′ bite of the ddnd ligand and two terminal F ions. The result is a distorted BN 2 F 2 tetrahedral geometry, with a N-B-N bite angle of 94.35 (11)°.
The compound skeleton is formed by four conjugated heterocyclic rings which are nearly coplanar; the least-squares plane through B1,N1,N2,C2->C10 has a mean deviation of 0.02 Å. The BF 2 plane is almost perpendicular to the boronnathphyridine ring plane, with a dihedral angle of 92.2 (2) °.
There are two main C-H···F hydrogen bonding interactions involving F1 as an acceptor and two C-H donor sets, C1 -H1C, C8-H8. There is also a π-π stacking interaction between adjacent parallel naphthyridyl rings: with a closest C-C distance of 3.499 (1) Å and a centroid to centroid distance of 4.005 (1) Å. Via these interactions (with H-bonding mainly in the (100) plane and π-π stacking along the [100] direction ) the compound forms a three-dimensional network structure as shown in Fig. 2.

S2. Experimental
A cold solution of 7-amino-2,4-dimethyl-1,8naphthyridine (2 g) in water (100 mL) was added dropwise to 150 ml of a 10% NaOCl solution. The mixture was stirred at 0 degree and a dark green precipitate formed. Filtration was performed a few minutes after the end of addition, and the aqueous phase and the precipitate were extracted with diethyl ether. The ether phases were gathered, dried on MgSO 4 , and evaporated. The crude product was purified by chromatography on Al 2 O 3 (eluent: acetone/hexane 1/10). Recrystallization in water yielded ddnd as green product. Yield: 70%.
The ddnd (0.34 g, 1 mmol) ligand was dissolved in 50 ml newly dry dichloromethane and then treated with triethylamine (3 ml) and boron trifluoride etherate (3 ml). After stirring for 30 min, the solution was washed with water, dried over Na 2 SO 4 , and concentrated at reduced pressure. Single crystals of (I) suitable for an X-ray study were obtained by slow evaporation of an CHCl 3 /hexane solution (50% v/v) over a period of one month.

S3. Refinement
All hydrogen atoms were generated geometrically (C-H bond lengths of methyl group fixed at 0.96 Å, C-H bond lengths of naphthyridine fixed at 0.93 Å), assigned appropriated isotropic thermal parameters, U iso (H) = 1.2U eq (C).    Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.