Novel aminonaphthoquinone Mannich bases derived from lawsone and their copper(II) complexes: Synthesis, characterization and antibacterial activity

Abstract

A series of novel Mannich bases (HL1-HL13) derived from 2-hydroxy-1,4-naphthoquinone (lawsone), substituted benzaldehydes [C6H2R1R2R3C(O)H] and various primary amines (NH2R4, R4 = n-butyl, benzyl, allyl, 2-furfuryl), and their Cu2+ complexes, [Cu(L1)2]-[Cu(L13)2], have been synthesized and fully characterized by analytical and spectroscopic methods. The structures of complexes 1 (R1 = R2 = R3 = H; R4 = Bu), 2 (R1 = R3 = H; R2 = NO2; R4= Bu) and 7 (R1 = OH; R2 = R3 = H; R4= Bu) were determined by single crystal X-ray diffraction studies. All complexes crystallize in centrosymmetric space groups, with a copper atom in the inversion centre. Two L- coordinate through the naphthalen-2-olate oxygen and secondary amine-N atoms, forming six-membered chelate rings around the copper atom in a trans-N2O2 environment; spectroscopic data confirm that the other complexes exhibit similar molecular arrangement. The antimicrobial activity of all compounds has been tested on seven different strains of bacteria: Bacillus cereus, Bacillus subtilis, Escherichia coli, Enterococcus faecalis, Klebsiella pneumoniae, Pseudomonas aeruginosa and Staphylococcus aureus. In general, Mannich bases were more active than complexes, HL11 (R1 = OH; R2 =H; R3 = Me; R4= Bn) and HL13 (R1 = OH; R2 = H; R3 = Br; R4= Bn) being the most potent inhibitors. The MIC for the most active compound HL11 against S. Coli was 20 μmol L-1 (8 μg mL-1), better than Chloramphenicol (90 μmol L-1) and well below most values reported for other naphthoquinones. © 2009 Sociedade Brasileira de Química.