The Electronic Effect Of Ionic-pair Substituent And Its Application

Ionic-pair substituents are composed of the combination of different anions and cations, which could also tune the reactivity like conventional organic substituents. Due to the various kinds of the combination of different cations and anions of ionic salts and the easy replacement of anions of the ionic-pair substituents, ionic-pair substituents are more convenient than the conventional substituents in tuning the reactivity.Steric, solvation effect, and the electronic effect are the reasons for this tuneability. However, the electronic effect of ionic-pair substituents has not been systematically reported yet. We choose ionic-pair monosubtituted benzenes as model molecules to investigate the electronic effect of ionic-pair substituents. Both computational and experimental methods are applied to predict the Hammett constants. The Hammett constants of 89 different ionic-pair substituents (77 anion-cationic and 12 cation-anionic) were obtained. The results show that both the anion-cationic and cation-anionic substituents are electron withdrawing groups and the different cation-anion combinations could tune the Hammett constants of the ionic-pair substituents in a wide range. Hence, the Hammett constants of ionic-pair substituents investigated in this manuscript can be used to understand the correlation between the electronic effect and structure, thereby designing more electronic effect tunable compounds in future.In addition, we present our efforts to explore the quantitative relationship between the parameters of the reactivity, e.g. reaction barrier, turnover frequency (TOF), and the Hammett constants of its anion-cationic substituents on ligand. For quaternary ammonium, pyridinium, or imidazoliumanion-cationic substituents, a linear relationship can be established between the reactivity of metal Schiff base catalysts and the Hammett constants of their anion-cationic substituents with multi-atomic anions on ligand based on the theoretical and experimental works. The quantitative electronic effect and reactivity relationships proposed in this chapter are expected to be useful in designing a more powerful catalyst with anion-cationic substituents with multi-atomic anions in large size on ligand.A series of acetylacetone metal complexes with anion-cationic substituents on the ligand Mn-[Salen-Mim][X]2 (X=Br, NO3, PF6, BF4, ClO4, BPh4, and Tf2N) were synthesized. The influence of the different ionic-pair substituents on properties of the metal complexes were characterized by the parameters of the EPR spectra. It was found that different ionic-pair substituents had an effect on the solubility of the metal complexes, g/A values, line-shape, splitting of the EPR spectra.Finally, in oreder to avoid the influence of solvation effect of conventional organic substituents, we chose chelate-based ionic liquid (IL) as example to investigate the effect of salvation of ionic-pair substituents on the properties of metal complexes. The C-D bond stretching vibrations of deuteriated dimethyl sulfoxide (DMSO-d6) and the C2-H bond stretching vibrations of 1,1,1,5,5,5-hexafluoropentane-2,4-dione (hfac) ligand in anion were chosen as probes to elucidate the solvent-solute interaction between chelate-based ionic liquids (ILs) and DMSO by vibrational spectroscopic studies. The C2-H bond stretching vibrations in hfac ligand is closely related to the ionic hydrogen bond strength between the cation and anion of chelate-based IL, solvation effect of ionic-pair substituents has significant effect on the properties of anions and cations of cheate-based ILs. The indirect effect from the interaction of adjacent S=O functional group of DMSO with the cation [Ciomim]+ and anion [Mn(hfac)3]- of the ILs leads to the blue-shift of C-D stretching vibrations of DMSO. EPR studies revealed that the crystal field of the central metal was kept when the chelate-based ILs were in different microstructure environment in the solution. That is to say, the solvation effect can only affect the ionic-pair substituents except the central metal of the complex.