3.1 Reaction Scheme of ligand
3.2 Mass spectroscopy
The mass spectrum of the Schiff base ligand was recorded and observed for the confirmation of synthesized ligand. Figure 1 shows the mass spectrum of the synthesized ligand. The molecular ion peak of the ligand was observed at 267.95 which corresponds to the molecular weight of ligand. This confirms the formation of Schiff base ligand. Figures 2, 3 and 4 shows the mass spectra of Cu(II), Mn(II) and Zn(II) complexes. The molecular ion peak for Cu(II), Mn(II) and Zn(II) complexes was observed at m/z = 593.33, 585..21 and 595.22 respectively. It indicates the co-ordination of Cu, Mn and Zn ions with the ligand.
3.3 NMR spectroscopy
The 1H and 13C-NMR spectra of Schiff base ligand (HL) were recorded in DMSO-d6. The 1H-NMR spectrum of ligand is shown in Fig. 5. The singlet peak at δ = 9.522 ppm corresponds to azomethine group which confirms the formation of imine bond in a ligand. The peaks between δ = 6.437 and 7.449 ppm were attributed to the aromatic protons present in the ligand. The peaks at δ = 3.851 ppm were due to methoxy proton present in a ligand.
Similarly, the peak seen in ligand at δ = 161.543 ppm supports the existence of the azomethine group, as shown in Fig. 6. Carbon of the methoxy group (-OCH3) was found at δ = 57.140 ppm. Between δ = 113.433 and 155.161 ppm, aromatic ring carbon signals were detected. The synthesis of the reported ligand, is therefore confirmed by 1H and 13C-NMR findings.
3.4 IR spectra
Figure 7, 8, 9 and 10 shows the FT-IR spectra of the synthesized ligand and its Cu(II), Mn(II) and Zn(II) complexes. The formation of ligand and its complexes with respective metals has been confirmed by detecting the peaks of C = N groups. In ligand, the peak was found at stretching frequency of 1631 cm− 1 indicates the formation of imine bond in the ligand. But this peak was shifted to lower/higher values in the case of metal complexes due to coordination. Therefore, the peaks of C = N groups were observed at stretching frequencies of 1601 cm− 1 to 1612 cm− 1 for Cu(II), Mn(II) and Zn(II) complexes respectively.
Meanwhile, the peak of metal-nitrogen (M-N), metal-oxygen (M-O) and metal-sulfur (M-S) bonds, has verified the formation of metal complexes. For Cu(II), Mn(II) and Zn(II), M-N bonds were detected at 416 to 476 cm− 1 respectively. The M-O bonds were observed at 588 to 609 cm− 1. The M-S bonds were 370 cm− 1 to 383 cm− 1 [33–34]. The existence of these peaks, which are completely absent in the spectrum of HL ligand as given in Table 1, further supports the formation of metal complexes.
3.5 Electronic Spectra
The electronic spectra of ligand and its metal complexes were recorded in DMSO solution at room temperature was shown in Figs. 11,12,13 and 14. The absorption bands around 331 and 375 nm were observed in the spectrum of the free Schiff base ligand, corresponds to π→π * and n→π * transitions associated with benzene rings and azomethine groups respectively. In the metal complexes π-π* and n-π* transitions were shifted to longer wavelengths as a consequence of coordination to metal in Table 2, confirming the formation of Schiff base metal complexes [35–36].
3.6 Thermal property
The thermal property of the complexes was studied by TGA and DTG studies. Figures 15, 16 and 17 shows the TGA and DTG curves of Cu(II), Mn(II) and Zn(II)complexes under nitrogen atmosphere. From the TGA curves it is clear that of Cu(II), Mn(II) and Zn(II) complexes undergo decomposition in three steps and leaving a residue as their respective metal oxides. The step wise decomposition of all the metal complexes was given in a Table 3.
The results revealed that Cu(II) complex undergo decomposition in three steps. In the first step from 33.25–173.01 ◦C with the weight loss of 10.01%. In the second step, between the range 173.01–253.56 ◦C with weight loss of 19.60% corresponds to the loss of organic moiety of a ligand and in the third step of decomposition between 253.56–673.03 ◦C with a weight loss of 29.71% corresponding to the Schiff base ligand leaving with 40.68% residue as CuO.
Similarly, the results also revealed that Mn(II) complex undergo decomposition in three steps. In the first step from 28.98–215.51 ◦C with the weight loss of 11.79%. In the second step, between the range 215.51–315.09 ◦C with weight loss of 22.13% corresponds to the loss of organic moiety of a ligand and in the third step of decomposition between 315.09–685.66 ◦C with a weight loss of 30.73% corresponding to the Schiff base ligand leaving with 35.35% residue as MnO.
Similarly, the results also revealed that Zn(II) complex undergo decomposition in three steps. In the first step from 26.85–195.24 ◦C with the weight loss of 10.93%. In the second step, between the range 195.24–299.08 ◦C with weight loss of 20.19% corresponds to the loss of organic moiety of a ligand and in the third step of decomposition between 299.08–646.54 ◦C with a weight loss of 31.13% corresponding to the Schiff base ligand leaving with 37.75% residue as ZnO.
3.7 Larvicidal bioassay
Approximately 384 larvae were used to test the toxicity of synthesized chiff base ligand and its metal complexes. The difference in the mortality among the different compounds is because of the nature of metal. Here we showed the pronounced larvicidal activity of the inorganic metal complexes, of which Cu(II) metal complex have potent larvicidal activity with survival rate ranging from (35%), followed by Schiff base ligand with approximately 50% larvicidal activity as shown in Figs. 18 and 19. Percentage of survival activity was shown in Table 4.
Mechanism of biocontrol of Trichoderma against Fusarium oxysporum
The natural method of eliminating and controlling the insects, pests and other disease-causing agents using their natural, biological enemies is called biocontrol or biological control. The agents which are employed for this are called biocontrol agents. Microbes are one of them. Biocontrol works on the principle of predation and parasitism. Bio controlling method is healthier than killing insects and pests using insecticides and pesticides. Thus, this prevents soil pollution and health issues related to insecticide poisoning, etc.
Plant diseases play a direct role in the destruction of natural resources in agriculture. In particular, soil-borne pathogens cause important losses, fungi being the most aggressive. The distribution of several phytopathogenic fungi, such as Phythium, Phytophthora, Botrytis, Rhizoctonia and Fusarium have spread with detrimental effects on crops of economic importance. Several modes of action of microbial biocontrol agents have been identified, none of which are mutually exclusive. These can involve interactions between the antagonist and pathogen directly, either associated with roots or seeds, or free in the soil. Soil borne plant pathogenic fungi cause heavy crop losses all over the world. Chemical control of such plant pathogens disturbs the environment, subverts ecology, degrades soil productivity, and mismanages water resources. Biocontrol agent (BCAs) can inhibit the growth of soil borne pathogens through various biocontrol mechanisms such as ability to grow much faster than them for space and nutrients, producing many powerful plant degrading enzymes such as lytic enzymes, proteolytic enzymes and more than 200 types of antibiotics which are highly toxic to any macro- and microorganism. The role of Trichoderma species is not only to control the growth of pathogenic microbes, but there are various other uses for Trichoderma species such as, enhance plant defense responses, stimulate colonization of rhizosphere and stimulates plant growth, root growth.
3.8 Antagonistic potential of T. harzianum against F. oxysporum in vitro
The antagonistic potential of T. harzianum against F. Oxysporum showed increased in
Inhibition of F. oxysporun, after seven days of incubation in the dual culture method. The control plates which were not inoculated with T. harzianum but containing only F. oxysporum grew much faster when compared with F. oxysporum in the dual culture plates.The petriplates which have been incorporated with different metal complexes also enhanced the growth of T. harzianum and inhibited the growth of F. oxysporum. The role of different metal complexes in enhancing the growth of T. harzianum and thereby inhibiting the growth of F. oxysporum is as given in the Table 5. Among all the compounds Mn(II) shows best which enhances the growth of Trichoderma followed by Cu(II) and Zn(II) and Schiff base ligand.