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Adjacent methyl to remote methyl isomerization of (4-methylimidazole)pentaamminecobalt(III)
Hoq, M. Fazlul,Johnson, Craig R.,Paden, Susan,Shepherd, Rex E.
, p. 2693 - 2700 (2008/10/08)
Isomers of (4-methylimidazole)pentaamminecobalt(III) have been isolated as chloride salts with the methyl group of the imidazole directed away from the five NH3 ligands (remote = R) and near the NH3 ligands (adjacent = Ad). Isomers have been assigned by a separate X-ray diffraction study. Isomers have been characterized by 1H NMR spectra showing shifts (ppm) relative to the free ligand (L) as follows: 4-CH3 Ad = 2.22, R = 2.33, L = 2.27; C2-H Ad = 7.78, R = 7.88, L = 7.65; C5-H Ad = 7.08, R = 6.73 (C4-H), L = 6.77. In Tris or pyridinium buffers two paths are found for the isomerization of Ad to R. One path (k0 = (5.83 ± 2.18) × 10-8 s-1) is attributed to the isomerization of the parent 4-methylimidazole species Ad. A second path, first order in [OD-], is attributed to the imidazolato form Ad-H (kOH = 3.19 ± 0.07 M-1 s-1). The activation parameters for the kOH path are ΔH≠ = 32.4 ± 4.1 kcal/mol and ΔS≠ = 16.8 ± 11.7 eu. A mechanism is presented, suggesting a largely dissociative-like transition state. Comparisons are made to the linkage isomerization of (NH3)5Co3+ coordinated to ONO- and N1 of the 5-methyltetrazolato ligand. While the isomerization of the 5-methyltetrazole ligand is faster by 105 relative to that for 4-methylimidazole, the anion forms favor the 4-methylimidazolato isomerization by a factor of 400 vs. the 5-methyltetrazolato case. The difference is attributed to different basicity of the lone electron pairs of these ligands The pKa's of (NH3)5CoX3+ have been found at 25.0°C and μ = 0.10 and are as follows (X, pKa): imidazole, 9.99; 2-methyhmidazole, 10.67; 4-methylimidazole (Ad), 10.46; 4-methylimidazole (R), 10.70. The acid dissociation constants of R and Ad were studied as a function of temperature, yielding values of ΔH°, ΔS° as follows: (Ad) 17.7 ± 0.5 kcal/mol, 11.2 ± 16 eu; (R) 15.4 ± 0.6 kcal/mol, 2.5 ± 1.8 eu. The values of ΔH° are within a kilocalorie of imidazole's pyrrole pKa (17.6 kcal/mol) while the values of ΔS° are more favored by charge dispersal for Ad and R by about 18 to 10 eu, respectively.
