Molecular Planarity and Crystal Structures of N-Salicylideneaminopyrazine Derivatives

Schiff bases (also called azomethines) are considered to be important organic compounds containing acyclic and cyclic imine C=N bonds, which have various applications in photoluminescence materials,1 optical materials and devices,2 organic light-emitting diodes,3 oxidation hair dyes4 and color print materials.5 N-Salicylideneaniline (SA), which is one of the Schiff bases, and its structural analogues are known to show photochromism in the crystalline state upon UV light irradiation.6 Interestingly, SA crystals including non-planar molecules with dihedral angle between two aromatic rings greater than 30° are photochromic, and those including planar molecules with a dihedral angle of less than 20° are non-photochromic.7,8 N-Salicylidene aminopyrazine (SAPz) is one of the SA derivatives that include a pyrazine (Pz) (Fig. 1). SAPz derivative molecules would prefer a planar conformation by taking one of two possible Pz orientations, which avoids the intramolecular H to H steric repulsion between Pz C–H and imine C–H. On the other hand, SA derivatives have always suffered from such steric repulsions, so the conformation may not be necessarily planar. In order to demonstrate that SAPz crystals have planar molecules, but no photochromic property, we synthesized two SAPz derivatives of 2SAPZ (1) and 3,5-Br-2SAPZ (2), and analyzed their crystal structures and photochromic properties. The synthesis schemes of compounds 1 and 2 are as follows: A mixture of salicylaldehyde derivative (10 mmol) and 2-aminopyrazine (10 mmol) was heated at 393 K for 2 h without a plug. The resulting melt was cooled, and then red crude title compounds were precipitated. Single crystals were obtained by recrystallizations from chloroform and methanol, respectively. Single crystals X-ray diffraction data were collected at 293 K on a R-AXIS RAPID imaging plate area detector (RIGAKU) using graphite-monochromated Mo-Kα radiation from a rotating anode source. Scaling and absorption corrections were performed using ABSCOR.9 Crystal data and refinement details of 1 and 2 are given in Table 1. The initial structures were determined by using a dual space method with SHELXT-2014/4, and refined by full-matrix least-squares on Fo with SHELXL-2018/1.10,11 All hydrogen atoms were found in the difference Fourier map; however, they were placed by geometrical calculations and treated using a riding model with Uiso(H) = 1.2 × Ueq(C) or 1.5 × Ueq(O). ORTEP drawings are shown in Fig. 2. The hydrogen-bond lengths and angles in the crystal of 1 and 2 are summarized in Tables S1 and S2 (Supporting Information), respectively. Solid-state diffuse reflectance spectra were measured at 298 K with a JASCO V-560 spectrometer. Analytical samples were prepared as a mixture of the 2SAPz crystals (10 mg) and barium sulfate powder (100 mg). UV irradiation was performed with a high-power UV-LED irradiator (Keyence Corporation) at a wavelength of 365 nm. The measured spectra are shown in Fig. S1 (Supporting Information). In the crystal structure of 1, an intramolecular hydrogen bond is found between O1 and N1 (2.584(3)Å). However, there are no typical intermolecular interactions that include N atoms of the Pz ring, which can act as a proton accepter.12,13 The molecules are stacked along the a axis by π–π stacking interactions (Cg–Cg: 4.541(4)Å, Fig. S1). The molecular conformation of 1 is planar with a small dihedral angle between the phenol (C8–N2–C9–C10–N3–C11) and Pz (C1–C2–C3– C4–C5–C6) ring (5.26(11)°). On the other hand, the crystal structure of 2 also shows an intramolecular hydrogen between O1 and N3 (2.587(8)Å), but there are no other typical 2018 © The Japan Society for Analytical Chemistry