PKₐ and Isomer Determinations of Cobalt(III) Imidazole and Histidine Complexes by NMR and X-ray Crystallography

The pKa\u27s for cis-Co(en)2(H2O)ImH3+, cis-Co(en)2(H2O)(N-MeIm)3+, cis-Co(en)2(H2O)(4-MeImH)3+, and Co(en)(H2O)HisH2+ are 5.85, 5.95, 5.95, and 6.20 for pKa1, respectively, at 25°C and 10.5, ..., 10.8, and 10.8 for pKa2, respectively, as determined by potentiometric titration and 1H NMR spectroscopy. pKa1 represents the water ionization and pKa2 the imidazole ionization. Changes in chemical shift for the acidic and basic forms of the coordinated imidazole were 0.6 ppm for the C(2)-H, 0.25 ppm for the C(4)-H, and about 0.16 ppm for the C(5)-H, which were similar to the changes found in Co(NH3)5ImH3+. 1H NMR titration behavior showed that the histidine was coordinated through an imidazole nitrogen, and 13C NMR spectra indicated that this is N1. The single-crystal X-ray structure determination of the coordination environment of the complex showed that the histidine is tridentate. Crystal data for [Co(HisH)(en)Cl]Cl: space group P1, Z = 2, a = 7.939 (2) Å, b = 9.247 (3) Å, c = 11.021 (4) Å, α = 118.75 (3)°, β = 97.76 (3)°, γ = 97.72 (2)°, R = 3.7% for 3533 reflections. 13C spectra also indicated the methyl group in the 4-methylimidazole complex was adjacent to the ionizable hydrogen. Changes in ligand, charge of the complex, substituents on the imidazole ring, and geometry of the complex all changed the C(2)-H chemical shift about 0.1 ppm. Ionization of coordinated water changed the C(2)-H chemical shift of coordinated imidazole less than 0.1 ppm and had less effect on the C(4)-H and C(5)-H resonances. Slow C(2) hydrogen exchange was found at pH 10.6 for cis-Co(en)2(OH)Im+. More rapid C(2)-H exchange was found for cis-Co(en)2(OH)(N-MeIm)2+ at pH 12. In both cases the rates were slower than for the analogous methylated or protonated species. © 1984 American Chemical Society