cadmium II complexes of streptonigrin semiquinone

Semiquinone metal complexes derived from the antitumor antibiotic streptonigrin have been prepared for the first time. Ž . Ž . 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 Ž . 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 Ž . Ž . 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 Ž . 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 Ž . 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 113 Ž . parameters have been confirmed by deuteration. Complexation studies using isotopically enriched Cd II revealed 113 Ž . hyperfine coupling of the unpaired electron of the p-semiquinone and the nuclear spin of Cd 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 Ž . 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. q 1997 Elsevier Science B.V. All rights reserved.