Two novel 18-metallacrown-6 complexes: Synthesis,structural characterization and bioactivity

Two new macrocyclic hexanuclear metal(III) 18-metallacrowns-6, [Mn₆(acshz)₆(H₂O)₆] · 18H₂O (1) and [Fe₆(acshz)₆(CH₃OH)₆] · 6CH₃OH · 6H₂O (2), have been synthesized and characterized, where acshz³⁻ is N-acetyl-5-chlorosalicylhydrazidate. These crystal structures contain neutral 18-membered metallacrown rings consisting of six metal(III) ions and six acshz³⁻ ligands. The ring is formed by the succession of six structural moieties of the type [M(III)–N–N] through hydrazide N–N groups bridging the ring metal ions. The ligand enforces the metal ions to form the stereochemistry of a propeller configuration with alternate Λ/Δ or Δ/Λ forms. The largest diameters of the hexanuclear rings are about 6.97 Å at the entrance and 9.53 Å at the centre of the cavity for 1; and 7.94 and 10.24 Å for 2, respectively. The solution integrity and stability of the metallacrowns were confirmed using electrospray ionization ESI-MS and UV–Vis spectroscopy in methanol. Antibacterial screening data indicate the formation of the metallacrown 1 reduces the antimicrobial activity of the ligand H₃acshz hugely, while metallacrown 2 has strong antimicrobial activity against Bacillus subtilis (B. subtilis).