Metal complexes of substituted benzimidazole to improve the photostability of rigid PVC: Synthesis, spectral and DFT studies

A series of Co2+, Ni2+, Cu2+ and Zn2+ coordination compounds of 4-(((1H-benzo[d]imidazol-2-yl)methyl)amino)phenol (BzIm), as new photo stabilizer for vinyl polymers, [Co(BzIm)(Cl)2] (1), [Ni(BzIm)(Cl)2] (2), [Cu(BzIm)(Cl)2] (3) and [Zn(BzIm)(Cl)2] (4) have been synthesized, characterized using microanalysis, fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H NMR), ultraviolet–visible (UV-Vis) spectrophotometry, mass, electron paramagnetic resonance (EPR)TG, magnetic, thermogravimetry differential thermal analysis (TG/DTA) and conductivity. These studies showed that the complexes were formed in which the ligand 4-(((1H-benzo[d]imidazol-2-yl)methyl)amino) phenol coordinated to the metal ion through benzimidazole nitrogen and the amino group of methylaminophenol. A PVC film containing the novel complexes was synthesized and used as a photo stabilizer for PVC. The results reveal the higher stabilizing potency of some of the substituted benzimidazole metal complexes compared to commercially known reference stabilizers. Based on weight loss as a criterion for photo-stabilizing efficiency the best stabilizing efficiency of the investigated complexes was given by the [Zn(BzIm)Cl2] (4), as after 8h. irradiation, its weight loss is only 2.6 % that of the blank and 3 % that of the the reference stabilizer (Tinuvin P). Additionally, the data of discoloration level of photo-degraded rigid PVC films in presence of the examined complexes stabilizers after 8h of irradiation time is light yellow in case of [Zn(BzIm)Cl2] (4), which is the palest colour relative to other investigated compounds. The photo stabilization efficiency results, based on different parameters including percent weight loss, percent gelation %, stabilizing efficiency were found to be in the following order: [Zn(BzIm)Cl2] (4) > Tinuvin P (TP) > [Ni(BzIm)Cl2] (2) > [Cu(BzIm)Cl2] (3) > [Co(BzIm)Cl2] (1). The stabilizing efficiency of these derivatives is attributed to their radical trapping potency which intervenes with the radical degradation process of PVC. The experimental data were more insighted using theoretical calculation using at DFT/B3LYP level of theory, which indicated that Zn(II) Complex (4) had the lowest HOMO-LUMO energy gap (1.162 eV) among the complexes under investigation, whereas Co(II) Complex (1) displayed the biggest energy gap (1.307 eV).