A Unique Mechanism for Base Catalyzed Hydrolysis of Pentaaminecobalt(III) Complexes Containing Picolyl Residues

A novel [Co(pentaamine)Cl]²⁺ complex having all tertiary amine or pyridine donors has been synthesized (pentaamine = 1,4-bis(2‘-pyridyl)-7-methyl-1,4,7-triazacyclononane). This asym-[Co(dmpmetacn)Cl]²⁺ species has been completely characterized through 1D and 2D NMR studies, and through the X-ray structure for the ZnCl₄^2- salt. Despite the lack of an activating NH center, remarkably its hydrolysis to [Co(pentaamine)OH]²⁺ is base catalyzed (k_OH 0.70 M^-1 s^-1, 25 °C, I = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to the exchange of just one deuterium in one of the two −CH₂− pyridyl arms, that is approximately trans to the leaving group, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of asym-[Co(dmpmetacn)Cl]²⁺ and control experiments on H/D exchange for the product asym-[Co(dmpmetacn)OD]²⁺ in OD^- show that each act of deprotonation at the acidic methylene leads to loss of Cl^-. This is the first established case of base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. A pronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (k_H/k_D = 5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activation afforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism.