Stereoselectivity in divalent cobalt,nickel and zinc bis complexes of some amino(methyl or benzyl) substituted histidines

Stability constants and stereoselective effects are presented for divalent Co, Ni and Zn complexes of histidine (H-his), α-N-methyl- (H-mhis), dimethyl- (H-dimhis) and benzylhistidine (H-bhis). In the zinc-dimethylhistidine system, species Zn(dimhis), Zn(dimhis)₂ and ZnOH(dimhis) are observed. In all other cases only the 1:1 complex MA and the 1:2 complex MA₂ are found. Corresponding complexes of histidine, methylhistidine and dimethylhistidine have nearly the same constants κ₁; corresponding complexes of histidine, methylhistidine and benzylhistidine show approximatively the same stability constants κ_2,DL. Stability constants indicate that large steric hindrance between the ligands is present in M(dimhis)₂. Probably, zinc is tetrahedrally coordinated in Zn(bhis)₂; it is the only case where a (small) negative stereoselective effect (in favour of the “racemic complex” {MLL + MDD}) is found. In all other cases positive effects are found (i.e. in favour of the “meso complex” MDL), the value for bis(dimethylhistidino)nickel (1·62 kcal/mol) being the largest measured. The enhancement of the stereoselective effect per methyl group for histidine, methylhistidine and dimethylhistidine is 0·37 kcal/mol for cobalt and 0·57 kcal/mol for nickel; the enhancement for zinc is about the same of that of cobalt. The PMR spectra of cobalt complexes of dimethylhistidino and methylhistidine are interpreted with the help of deuteromethyl histidine compounds; in these ligands also the 2-CH proton of the imidazole residue is partly deuterated. The stereoselective effects in the cobalt complexes are confirmed by the PMR results. The number of signals per group of protons is doubled for Co(mhis) and a multiple for the Co(mhis)₂ complexes, which is caused by the secondary nitrogen atom becoming a new asymmetry centre. The two dimethylhistidine ligands in the meso-complexes CoDL and NiDL, probably, are no longer equivalent. Another indication of stereoselectivity in the systems cobalt and nickel-methylhistidine and cobalt-dimethylhistidine is the systematic difference of the solubility of MLL (or MDD) in regard to M(rac-A)₂, the optically active complexes being much less soluble; furthermore, they cristallize very well. Some crystallographic data on the optically active complexes are presented. On the basis of the potentiometric and PMR results it is concluded that the positive stereoselective effects are due to the stability of the C_i (all-trans) meso complex MDL being larger than that of any of the C₂ complexes of the racemic mixture and the C_i (all-cis) meso complex.