Base Hydrolysis of Pentaaminecobalt (III) Complexes: The [CoX(dien) (dapo)]n+ system. Part 3. The internal conjugate base mechanism

Authors

  • Peter Comba,

    Corresponding author
    1. Institut Für Anorganische Chemie, Universität Basel, CH–4056 Basel
    • Institut Für Anorganische Chemie, Universität Basel, CH–4056 Basel
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  • W. Gregory Jackson,

    Corresponding author
    1. Chemistry Department, University College (N.S.W.), Australian Defence Force Academy, Campbell, A.C.T., Australia 2600
    • Chemistry Department, University College (N.S.W.), Australian Defence Force Academy, Campbell, A.C.T., Australia 2600
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  • Werner Marty,

    1. Institut de Chimie, Université de Neuchâtel, CH–2000 Neuchâtel
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    • Deceased on September 20th, 1986.

  • Luc Zipper

    1. Institut Für Anorganische Chemie, Universität Basel, CH–4056 Basel
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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 > Nmath image > 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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