Synthesis and structural characterization of silver(I), aluminium(III) and cobalt(II) complexes with 4-isopropyltropolone (hinokitiol) showing noteworthy biological activities. Action of silver(I)-oxygen bonding complexes on the antimicrobial activities

Through two unequivalent oxygen donor atoms of the hinokitiol (Hhino; C10H12O2; 4-isopropyltropolone) ligand that showed noteworthy biological activities, the dimeric, silver(l)-oxygen bonding complex [Ag(hino)](2) 1, the monomeric aluminium(III) complex [AI(hino)(3)] . 0.5H(2)O 4 and the cobalt(II) complex "[Co(hino)(2)](2) . H2O" 6 were synthesized and characterized with elemental analysis, thermogravimetric and differential thermal analysis (TG/DTA), FTIR and solution (H-1 and C-13) NMR spectroscopy. The crystal structure of 1 was determined by Rietveld analysis based on X-ray powder diffraction (XPD) data and those of [Al(hino)(3)] . MeOH 4a and [Co(hino)(2)(EtOH)](2) 6a, being obtained as yellow block crystals and red platelet crystals, respectively, by crystallization of 4 and 6, were determined by single-crystal X-ray analysis. The antimicrobial activities of 1, 4 and 6, evaluated with minimum inhibitory concentration (MIC; mug ml(-1)), were compared with those of other metal complexes (M = Na, Li, Cs, Ca, V, Zn) with the hino(-) ligand. The antimicrobial activities observed in the alkali-metal salts strongly suggested that they were attributed to the effect of the anionic hino- species. The antimicrobial activities of 1 were significantly enhanced, whereas those of other metal complexes were suppressed, compared with those of the neutral Hhino and anionic hino- molecules. The antimicrobial activities observed in I were comparable with those of other recently found silver(l)-oxygen bonding complexes, the ligands of which had no activity. Thus, it is proposed that the antimicrobial activities of the silver(l)-oxygen bonding complexes are due to a direct interaction or complexation of the silver(l) ion with biological ligands such as protein, enzyme and membrane, and the coordinating ligands of the silver(l) complexes play the role of a carrier of the silver(l) ion to the biological system. (C) 2003 Elsevier Inc. All rights reserved.