Abstract
A hydrazone Schiff base ligand was synthesized by the condensation of 3-formyl-4-hydroxycoumarin and oxalyldihydrazide in the molar ratio 2:1. The Schiff base ligand acts as a mono-, bi-, tri- or even tetradentate ligand with metal cations in the molar ratios 1:1 or 2:1 (M:L) to yield either mono- or binuclear complexes as keto or enol isomers, where M = Co(II), Ni(II), Cu(II), VO(IV), and Fe(III). The ligand and its metal complexes were characterized by elemental analyses, IR, 1H NMR, mass, and UV–Vis spectroscopy. Furthermore, the magnetic moments were calculated from the measured electric conductivities of the complexes. According to the received data, the dihydrazone ligand contains one or two units of ONO domains and can bind to the metal ions via the azomethine nitrogen, the carbonyl oxygen atoms, and/or the phenolic oxygen atoms. Electronic spectra and the magnetic moments of all complexes show that the complexes’ geometries are either octahedral, tetrahedral, square planar, or square pyramidal. Cyclic voltammograms of the mononuclear Co(II) and Ni(II) complexes show quasi-reversible peaks. Tests against two pathogenic bacteria as Gram-positive and Gram-negative bacteria for both, the Schiff base ligand and its metal complexes were carried out. In addition, also one kind of fungi was tested. The synthesized complexes demonstrate mild antibacterial and antifungal activities against these organisms.Graphical abstract
Highlights
The chemistry of organic hydrazone compounds, which include –N–NH–CO– groups, takes the forefront position in the development of coordination chemistry of symmetrical dihydrazone transition metal complexes, as they demonstrate versatility in their coordination, a tendency to show stereochemistry [1–5] due to higher coordination numbers, an ability to act in their neutral or deprotonated forms, as they behave as keto–enol tautomers, bearing unusual coordination numbers [2, 6–9] and flexibility in assuming different conformations
A hydrazone Schiff base ligand was synthesized by the condensation of 3-formyl-4-hydroxycoumarin and oxalyldihydrazide in the molar ratio 2:1
The IR spectra show that the hydrazone Schiff base ligand behaves in its keto form in the binuclear complexes Co2(H2L)(NO3)2(H2O)2 (6), Ni2(H2L)(NO3)2(H2O)4 (7), [Cu2(H2L)(H2O)4](NO3)2 (8), and [(VO)2(H2L)(SO4)]3H2O (9), where the stretching frequency m(NH) appears at the same range in the spectrum of the ligand H4L, compared to the ones of the corresponding metal complexes, which proves that this group is not participating in the coordination
Summary
The chemistry of organic hydrazone compounds, which include –N–NH–CO– groups, takes the forefront position in the development of coordination chemistry of symmetrical dihydrazone transition metal complexes, as they demonstrate versatility in their coordination, a tendency to show stereochemistry [1–5] due to higher coordination numbers, an ability to act in their neutral or deprotonated forms, as they behave as keto–enol tautomers, bearing unusual coordination numbers [2, 6–9] and flexibility in assuming different conformations. A new band appears at 1685 and 1681 cm-1 in the spectra of the complexes 1 and 2, respectively, which suggests that the oxygen atom on one side of the ligand coordinates to the metal atom, while the other C=O group remains free.
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