Synthesis, electrochemical, fluorescence and antimicrobial studies of 2-chloro-3-amino-1,4-naphthoquinone bearing mononuclear transition metal dithiocarbamate complexes [M{κ2S,S-S2C-piperazine-C2H4N(H)ClNQ}n]

A novel multifunctional ligand precursor 2-chloro-3-{2-(piperazinyl) ethyl}-amino-1,4-naphthoquinone (L) was synthesized by chemo-selective reaction of 2,3-dichloro-1,4-naphthoquinone and 2-(piperazin-1-yl) ethanamine and characterized, prior to use. A series of mononuclear transition metal dithiocarbamate complexes [M{kappa S-2, S-S2C-piperazine-C2H4N(H)ClNQ}(n)] {M = Mn(III) 1, Co(III) 2, Ni(II) 3, Cu(II) 4, Zn(II) 5; ClNQ = 2-chloro-1,4-naphthoquinone; n = 3 for 1-2 and n = 2 for 3-5} bearing pendant 2-chloro-3-amino-1,4-naphthoquinone groups were efficiently synthesized through a self-assembly process involving L, CS2 and corresponding metal acetates. 1-5 were characterized by microanalysis and standard spectroscopic methods such as ESI-MS, IR, H-1 and C-13 NMR and UV-visible absorption spectroscopy. Significantly, all compounds exhibit medium to very strong fluorescence emission bands in the visible region with concomitant Stokes shifts of approximate to 280 nm upon excitation at their respective lambda(max) values. Thermal stability of complexes 1-5 has been investigated by thermogravimetric analysis. Cyclic voltammetric study clearly reveals that L and 1-5 are electro active with respect to the dominant redox-active naphthoquinone moiety. Further, L and 1-5 were tested against six important pathogen microorganisms viz. S. aureus, B. subtilis, E. coli, P. aeruginosa, C. albicans and A. niger by using the Broth dilution method. A broad spectrum of the strongest antimicrobial activity was determined for the copper(II) complex 4 and L. The enhanced antibacterial activity of L and copper(II) complex 4 against S. aureus apparently reveals them to be a superior antibacterial agent than the reference drug ciprofloxacin.