C(2)-H isotopic exchange in coordinated imidazoles revisited. The case of the [Co(NH3)5ImH]3+ ion
The temperature dependence of the acid ionization constants of [Co(NH3)5ImH]3+ in H2O (I = 1.0 M (NaClO4)): pKa (°C) = 10.10 ± 0.04 (25.0), 9.92 ± 0.03 (30.0), 9.82 ± 0.02 (35.0), 9.62 ± 0.03 (40.0), and [Co(ND3)5ImD]3+ in D2O (I = 0.35 M (NaClO4)): pKa (°C) = 10.58 ± 0.06 (25.0), 9.46 ± 0.08 (60.0) is reported. Observed first-order rate constants for H/D exchange at C-2 in [Co(ND3)5ImD]3+ over the pD range 8.08-11.20 (60.0°C, I = 0.35 M (NaClO4)) follow an equation of the form: kobs = kODKw/(aD+ + Ka)γ±, with kOD (0.27 ± 0.06 M-1 s-1) corresponding to the rate constant for OD--catalyzed abstraction of H-2 in [Co(ND3)5ImD]3+, and Ka ((2.8 ± 0.7) × 10-10 M, pKa = 9.55 ± 0.13) to the acid ionization constant of this species. No evidence was found for a pathway to H/D exchange in the imidazolate moiety of [Co(ND3)5Im]2+.Key words: kinetics, H/D-exchange, imidazole, metal complex.