Zinc complexes of amino acids and peptides .9. Coordination of cysteine and histidine derivatives to the pyrazolylborate-zinc unit

Highly substituted pyrazolylborate Ligands Tp(R,Me) were used to limit the coordination number of zinc towards cysteine- and histidine-derived coligands. Monodentate thiolate attachment (--> 1a-f) was achieved with N- and C-protected cysteine, monodentate carboxylate attachment (--> 3) with N-protected histidine. C-protected cysteine was found to form a five-membered N,S-chelate ring (2), While imidazole coordination with N- and C-protected histidine could not be achieved, cationic pyrazolylborate-zinc-coligand complexes [TpZn-L](+) (4a, b) were obtained for L = 2-methylimidazole. The new complexes were characterized by their spectra and three structure determinations.