Disorder in the crystals of trans-4-fluoroazoxybenzene. Synthesis, spectral properties, crystal structures and DFT calculations

Two crystals of trans-4-fluoroazoxybenzene were obtained using two different methods. Oxidation of 4-fluoroazobenzene provided crystals of trans-4-fluoroazoxybenzene (I) consisting of two isomers; 51% α (ONN) and 49% β (NNO) isomer. From trans-amino-azoxybenzenes in Schiemann reaction it were obtained crystals of trans-4-fluoro-NNO-azoxybenzenes (II, β isomer) containing 4.7% of the α-isomer according to the HPLC analysis. The crystal structures of I and II were determined by the X-ray diffraction method. In the crystal I two nitrogen atoms of azoxy bridge and fluorine atom are disordered. In the crystal II, there are two independent molecules of trans-4-fluoroazoxybenzene; the molecule of β isomer and the molecule with disordered fluorine atom. The molecule of the β isomer is not planar. The torsion angles along the Ar-N(O) and the Ar-N bonds are 21.3(3) and 30.3(3)° respectively. The weak C-H⋯O and C-H⋯F hydrogen bonds were observed in both crystal networks. To aid structural data analysis, the molecular geometry of both isomers of the trans-4-fluoroazoxybenzene were optimized at the DFT B3LYP/6-311+G** level of theory, with no imaginary frequencies. The resultant equilibrium structure is planar in each calculation (CS point group symmetry). The descriptor of π-electron delocalization HOMA was calculated to estimate aromaticity of benzene rings in the trans-4-fluoroazoxybenznes.