| Benzil dihydrazone (BDH) is important in building novel Schiff bases due to the simplicity in syntheses, the coordination versatility of the -C=N-N=C-fragments and the modification diversity of the structures. However, in contrast to the di-Schiff-base ligand (1:2 condensate of BDH and aldehydes), mono-Schiff-base derivatives (1:1 condensate) from BDH have been much less investigated. It is known that the two-NH2 (or-C=O) groups in BDH-at least in principle-should have the same reactivity. Then the condensation reaction of BDH can give mono-and di-Schiff-bases with equal probability, mainly depending on the molar ratio of the starting materials. But only di-Schiff-base derivatives were formed and reported upon condensation from BDH. Why did this happen? Can these reactions be controlled to form mono-Schiff-bases? These findings led us to speculate on possible chemical mechanisms for these condensations, and experiments to establish some model compounds, involving mono-Schiff-base and di-Schiff-base, resulting in the work reported here. A thorough investigation of explaining the driving forces of these reactions has heretofore not appeared in the literature.Experimentally, we designed three series of Schiff bases (HCF, HDF and MF series) derived from BDH. By controlling the reaction conditions, both mono-and di-Schiff bases were obtained. Except BDHHDF, all other five mon-or di-Schiff bases are new compounds. And the structures of these compounds are confirmed by elemental analysis, NMR, X-ray and IR. The crystals of five Schiff bases are explained detailedly (in which four are determined by single crystal X-ray diffraction, the last is determined by powder X-ray diffraction). It is found that they have similar crystal structures. For example, all molecules are double helix; two arms of the double helix are anti-conformation, and planar. Torsion angles are between 72-97°. Hydrogen bonds, π-π stacking interactions as well as weak C-H...πinteractions help to link the molecule units to form 3-D packing structures. Additionally, the UV- and fluorescence spectra are determined. Also, the mechanism of occurrence of UV-spectra is explored by theoretical investigation.Theoretically, the factors determining the end product of mono-or di-Schiff bases are investigated. The work contains:1) optimization of the structures (including gas-phase and liquid-phase); 2) calculating the enthalpy changes (△H) and free energies changes (△G) and the HOMO-LUMO energy gaps (△E); 3) analyzing the relationships between △H, △G, △E and the yields as well as the reaction conditions. It is found that △H and △G mainly affect yields, while energy gap between mono- and di-Schiff bases (△△H, △△G) and the differences in stabilities between them mainly determine the production of mono-or di-Schiff bases. When the values of △△H, △△G and stabilities are close, the reaction conditions are the determinative factor that affecting the product; when the above gap is high, the effect of the reaction conditions is decreased. Accordingly, it is conferred that difference between △H, △G and stabilities of mono-, di-Schiff bases is increased; the effect of reaction conditions (mainly reaction molar ratio) on the product will fade. That is, only mono-or di-Schiff bases can be synthesized whatever the reaction conditions are changed.We have gained some results in experimental and theoretical study, but we know there is much work to do. For example, experimentally, the study on coordination and biological activities of newly synthesized Schiff bases has not been developed; theoretically, effect of the activation energy on the product of mono-or di-Schiff bases has not been carried out. We will perform these studies lately. |
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