Base Hydrolysis of Pentaaminecobalt (III) Complexes: The [CoX(dien) (dapo)]n+ system. Part 3. The internal conjugate base mechanism
Abstract
Abstract Azide anation and racemization of optically pure mer‐exo(H)‐ and mer‐endo(H)‐[Co(OH)(dien)(dapo)]2+ (A and B (X = OH), resp.; dien = N‐(2‐aminoethyl)ethane‐1,2‐diamine; dapo = 1,3‐diaminopropan‐2‐ol) involve the same symmetrical pentacoordinate intermediate as the base hydrolyses of the corresponding mer‐exo(H)‐ and mer‐endo(H)‐[CoX(dien)(dapo)]2+ species A and B, respectively, where X = Cl, Br, or N3. The kinetic parameters of the anation process are fully compatible with the independently measured competition ratio. The rate data reveal that substitution of OH− is unexpectedly fast, viz. it is not consistent with the usual sequence Br− > Cl− > H2O > N > OH−. This behavior is interpreted on the basis of an internal conjugate base mechanism which involves an amino‐hydroxo/aminato‐aqua tautomerism, viz. the reaction is actually an OH− ‐catalyzed substitution of [CoH2O(dien)(dapo)]3+ where deprotonation occurs effectively at the secondary‐amine site NH of dien.
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