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 ratio. Binary [Pd(L-NO)] and [Pd(L-NO)(2)](2-) complexes are formed below pH 4, in which the N-sulfonyl-aminoacids act as bidentate N,O-chelate ligands through the deprotonated sulfonamide nitrogen and the carboxylate oxygen. No other binary species are detected at higher pH values. Both ligands form the ternary complex [Pd(L-NO)(bpy)] below pH 8.5. At higher pH values, the electrochemical and spectral data indicate that the ligands undergo an acid-base equilibrium with an apparent pK(a) value of about 10, which most probably corresponds to the deprotonation and metal coordination of the amide nitrogen of the side-chain. The overall stability constants for the binary and ternary complexes were 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 reserved.