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.