Transition Metal Complexes Of 1,2,4-triazole Fungicides:Synthesis,Crystal Structures,Antifungal Activities And Theretical Investigation

1,2,4-triazole fungicides have the advantages of high efficiency, broad spectrum and low toxicity, however, the intensive use of 1,2,4-triazole fungicides has led to a rapid selection of resistance strains. In recent years, many researchers find out that the complexation of pesticides with transition metals has potential advantages including the enhancement of biological activities and environmental friendliness, reduction of mammalian toxicity. Moreover, the pesticides coordinated with transition metals could be utilized as controlled release formulations which have the capacity of extending effective period and decreasing pesticide residue. For this status, we selected the widely used 1,2,4-triazole fungicides tebuconazole L¹, triadimenol L², diniconazole L³ as ligands to construct 14 complexes with various transition metal anions. Crystal structures, antifungal activities and theoretical calculations have been systemly studied. These studies are intended to interpret the bioactivites of the complexes in the theoretical level and to screen more effective 1,2,4-triazole complex fungieides.Firstly,14 novel metal complexes based on L¹, L², L³ were synthesized by stirring at room temperature or boiling temperature, and their crystal structures are determined by X-ray diffraction. There are eight complexes based on L¹. Among them, [CuL¹SO₄·DMF]n, [CuL'4Cl₂]·2DMF·3H2O, [CuL¹2?CH₃COO?₂], [ZnL¹4?NO₃?₂] and [ZnL¹2?CH3COO?2] belong to monoclinic system, C2/c or P21/n space; [CuL¹4?ClO₄?₂], [NiL'4Cl2]-4CH3OH and belongs to triclinic system, P-1 space group. There are three complexes based on L². Among them, [CuL²4?H2O?2]·2NO3·2CH3OH and[CuL²2?CH3COO?2] belongs to triclinic system, P-1 space group; [ZnL 2Br2] belongs to orthorhombic system group, Pnam space group. There are three complexes based on L³. Among them, [CuL³4Cl₂] and [CoL³4?H₂O?₂]-2NO₃·2H₂0 belongs to triclinic system, P-1 space group; [CuL³2?CH₃COO?₂] belongs to monoclinic system, P21/c space group. The crystal structures show that all the complexes are monodentated ones and that the compositions of complexes are significantly different with variation in the anions.Secondly, the antifungal activities of the 14 obtained complexes were carried out. The bioactivities against the plant pathogenic fungi ?Gibberella nicotiancola, Botryosphaeria ribis, Botryosphaeria berengriana, Alternaria solani? were evaluated by 50% inhibiting concentration ?EC50?. The results indicate that all the complexes based on tebuconazole L¹ and triadimenol L² inhibit the four selected plant pathogenic fungi more efficiently than the their ligands, while all the complexes based on diniconazole L³ show lower antifungal activities aginsted the selected fungi except [ZnL²2Br2] aginst Gibberella nicotiancola and Alternaria solani. Since the synthesized complexes can be regarded as the molecular-level mixtures of ligand and metal cation, the synergy ratios ?SR? were calculated to investigate the extent of the interactions between ligand and metal cation. And it can be found that the complexes with higher antifungal activities than their ligands are due to the synergistic interaction between ligand and metal cation and that the complexes with lower antifungal activities than their ligands are due to the antagonistic interaction between ligand and metal cation.Finally, the geometry optimization, frequency, frontier orbitals, Mulliken charges and fukui functions were investigated by Density-Functional Theory ?DFT? method of DMol3, and the theoretical results are used to explain the antifungal activities of the obtained complexes. It shows that there should be two main reasons for the higher activities of some obtained complexes:?1? there is an additonal active site-metal cation. Meanwhile, the metal cation and the ligand inhibite the fungi by synergetic action; ?2? the transfer of the charge are responsible for the reduction of atomic polarity and the enhanced penetration of the metal complexes into the lipid membranes, resulting in the enhancement of the bioactivities. However, the reasons for the lower bioactivities of the other complexes should be that although the penetration of the metal complexes into the lipid membranes are enhanced, the metal cation and the ligand inhibite the fungi by antagonistic action, ultimately resulting in the decrease of the antifungal activities.