Tetrahedral complexes of cobalt(II) with L-histidine, histamine, imidazole, and N-acetyl-L-hystidine.

In highly alkaline aqueous solutions, cobalt(I1) forms violet, tetrahedral, bis complexes with L-histidine, histamine, and N-acetyl-L-histidine. Imidazole similarly forms a tetrakis complex. Generation of the tetrahedral complexes is dependent on deprotonation of the imidazole ring pyrrole nitrogen for the L-histidine, histamine, and imidazole complexes, and on two amide hydrogen ionizations for the 2 : 1 N-acetyl-L-histidine complex. The tetrahedral complexes all possess four nitrogen donor atoms, at least two of which are negatively charged. Approximate, average pK, values for metal ion promoted pyrrole hydrogen ionizations from 2 : 1 complexes of L-histidine with cobalt(I1) and copper(I1) are 12.5 and 11.7, respectively. Minimization of steric crowding and reduction of charge repulsion appear to be the reasons for cobalt(I1) preferring a tetrahedral environment to an octahedral one in these cases. n a recent study of some oxygenation reactions, the I properties of several cobalt(I1) complexes of amino acids in oxygen-free solution were reported. For cobalt(I1) solutions containing L-histidine, raising the pH to about 12 produces a color change from pale pink-yellow to deep violet. This change has been ascribed to the conversion of octahedral 2 : 1 histidinecobalt(I1) complexes to a 2 : 1 tetrahedral complex accompanied by the deprotonation of the pyrrole nitrogen of histidine. The tetrahedral complex contains two bidentate histidines each carrying a 2 charge.3 Previous work in this laboratory indicated that the behavior of the violet cobalt(I1) complex of histidine was complicated. In the present paper we have more fully investigated the conditions necessary for the formation of the violet histidine complex of cobalt(I1). Compounds related to histidine have also been examined in an attempt to find the structural requirements for forming the violet color. Experimental Section The equivalent weights of all the high quality ligands obtained from commercial sources were checked by titrations with standard base. All experiments involving cobalt(I1) were carried out under scrupulously oxygen-free conditions, maintained by using tank nitrogen purified by passage through vanadium(I1) chloride scrubbers to remove any oxygen.‘ Spectra were recorded on a Cary 14R spectrophotometer. Because of the extended concentration range needed in some of the experiments, molar absorbances greater than 2 (at about 560 nm) were often obtained, even in I-mm cells. In such cases, the reference compartment contained a solution similar in composition to the test solution but of lower and known absorbance. The accuracy of the molar absorbances (at about 560 nm) recorded in this way was checked by comparing the molar absorbances at about 1100 nm, which were always less than 2. Circular dichroism was measured with a Durrum-Jasco ORD-UV-5 recording spectropolarimeter with a circular dichroism attachment. Values of pH were measured using a Radiometer Type TIT-1 titrator and associated GK 2021 B combined glass-calomel electrode and T 301 temperature compensator. All experiments were conducted at 24 =k 1 a. (1) This research was supported by a grant from the National Science ( 2 ) M. S . Michailidis and R. B. Martin, J . Amer. Chem. SOC., 91, 4683 (3) C. C. McDonald and W. D. Phillips, ibid., 85, 3736 (1963). (4) L. Meites, “Poldrographic Techniques,” Interscience Publishers, Foundation.