PALLADIUM(II) COMPLEXES OF N-SULFONYL-ASPARAGINE AND GLUTAMINE - EVIDENCE FOR METAL COORDINATION OF THE DEPROTONATED AMIDE NITROGEN OF THE SIDE-CHAIN

The binary and ternary (bpy) palladium(II) complexes of the N-sulfonyl derivatives of asparagine and glutamine were studied by polarography and electronic spectra as a function of pH and metal-to-ligand molar r atio. Binary [Pd(L-NO)] and [Pd(L-NO)(2)](2-) complexes are formed bel ow pH 4, in which the N-sulfonyl-aminoacids act as bidentate N,O-chela te ligands through the deprotonated sulfonamide nitrogen and the carbo xylate oxygen. No other binary species are detected at higher pH value s. Both ligands form the ternary complex [Pd(L-NO)(bpy)] below pH 8.5. At higher pH values, the electrochemical and spectral data indicate t hat the ligands undergo an acid-base equilibrium with an apparent pK(a ) value of about 10, which most probably corresponds to the deprotonat ion and metal coordination of the amide nitrogen of the side-chain. Th e overall stability constants for the binary and ternary complexes wer e determined polarographically and discussed in comparison with those for the homologous species formed by N-sulfonyl-aminoacids with a non- coordinating side-chain. (C) 1998 Elsevier Science S.A. All rights res erved.