Preparation and ESR spectroscopic characterization of the zinc(II) and cadmium(II) complexes of streptonigrin semiquinone

Semiquinone metal complexes derived from the antitumor antibiotic streptonigrin have been prepared for the first time. They were obtained by reduction of the zinc(II) and cadmium(II) complexes of the parent aminoquinone ligand with sodium borohydride, followed by air oxidation of the intermediate dihydroquinones. Alternatively, N-benzyldihydronicotinamide reduction was used to produce the same Cd(II) complex of the p-semiquinone free radical. Electron spin resonance spectroscopic studies showed that metal binding significantly changes the spin densities of the unpaired electron which is confined to the quinolinesemiquinone moiety of the complexed antibiotic. Complexation with both Cd(II) and Zn(II) perturbs the coupling constants of all atoms involved in delocalization of the unpaired electron, shifting its distribution toward the pyridine ring. The coupling constant of the pyridine ring-proton adjacent to the semiquinone ring increases from 0.31 to 0.43 G in the Cd(II) complex in methanol, while the proton meta to the pyridine nitrogen increases from 1.76 to 1.96 G. Furthermore, the coupling constant of the heterocyclic nitrogen increases from 0.46 to 0.61 G. A similar trend is noted for the Zn(II) complex as well, including the observed decrease in splitting constant of the amino nitrogen from 1.34 to 1.08 G, and perturbation of the previously equivalent amino protons from 0.89 and 0.89 to 1.09 and 0.95 G. The spectral parameters have been confirmed by deuteration. Complexation studies using isotopically enriched Cd-113(II) revealed hyperfine coupling of the unpaired electron of the p-semiquinone and the nuclear spin of Cd-113(II), indicating direct coordination between the metal and the complexing ligand. Although the metal complexes could readily be prepared in a series of different solvent systems, they appear to have substantially shorter half lives than the non complexed p-semiquinone radical (5 to 15 min vs. 2 to 3 wk in sealed ampoules). Formation of tight-binding p-semiquinone metal complexes as here described should provide useful leads for the design of related systems to study p-quinone-metal interactions for mechanistic elucidation of metal ion catalyzed quinone-dependent electron transfer reactions in biological oxidations. (C) 1997 Elsevier Science B.V.