Two heptacoordinated manganese(II) complexes of giant pentadentate s-triazine bis-Schiff base ligand: Synthesis, crystal structure, biological and DFT studies

A new s-triazine bis-Schiff base chelating ligand (L) and two of its heptacoordinated Mn(II) complexes were synthesized and characterized. Reaction of solid Mn(NO3)2·4H2O with methanolic solution of the ligand (L; 5) afford [MnL(MeOH)NO3]NO3.MeOH; (6). While, repeating the reaction in water/methanol solution, the complex [MnL(H2O)2](NO3)2; (7) was formed. In both complexes, the Mn(II) is coordinated with L as a pentadentate ligand via its N-atoms augmented with two axial Mn–O bonds leading to a distorted pentagonal bipyramidal configuration around the Mn(II) ion. The two-pyridine moieties at the end of the ligand arms are twisted from one another due to the short contact distance between the two hydrogen atoms at the 6 position of the pyridine moieties. As a result, the strength of the Mn–N bonds decrease ongoing from the triazine moiety towards the outside of the ligand arms. The atoms in molecules topological parameters correlated well with the Mn–N distances. The Mn–N and Mn–O bonds were analyzed using natural bond orbital calculations. Both complexes showed higher thermal stability than the free ligand. Antimicrobial studies showed that 7 and L are the best candidates as antifungal and antibacterial agents, respectively. The mechanism of biological activity depends on the interactions between the compound and cell wall.