Base Hydrolysis of Pentaaminecobalt (III) Complexes: The [CoX(dien) (dapo)]n+ system. Part 3. The internal conjugate base mechanism
✍ Scribed by Peter Comba; W. Gregory Jackson; Werner Marty; Luc Zipper
- Publisher
- John Wiley and Sons
- Year
- 1992
- Tongue
- German
- Weight
- 751 KB
- Volume
- 75
- Category
- Article
- ISSN
- 0018-019X
No coin nor oath required. For personal study only.
✦ Synopsis
Azide anation and racemiration of optically pure mer-ao(H)-and n~er-endo(H)-[Co(OH)(dien)(dapo)]~+ (A and B (X = OH), resp.; dien = N-(2-aminoethyl)ethane-l Jdiamine; dapo = I ,3-diaminopropan-2-oI) involve the same symmetrical pentacoordinate intermediate as the base hydrolyses of the corresponding mer-exo(Hj-and m~-endo(H)-[CoX(dien)(dapo)]~+ species A and B, respectively, where X = C1, Br, or N,. 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, uiz. it is not consistent with the usual sequence Br-> C1-> H,O > N j > OH-. This behavior is interpreted on the basis of an internal conjugate base mechanism which involves an amino-hydroxo/aminato-aqua tautomerism, tiz. the reaction is actually an OH--catalyzed substitution of [CoH20(dien)(dapo)13+ where deprotondtion occurs effectively at the secondary-amine site NH of dien.
Introduction. -Base hydrolysis of pentaaminecobalt(II1) is an extensively studied example of an induced substitution process [ 11. The widely accepted S,, 1 CB mechanism incorporates the formation of a conjugate base in a fast pre-equilibrium and rate-determining loss of the leaving group'). We designed and investigated a pentaaminecobalt(II1) system, mer-exo (H)-and mer-endo (H)-[CoX(dien)(dapo)]" (A and B, resp.; X = C1-,
A mer-exo(H) B mer-endo( H )
' j ' )
Deceased on September 20th, 1986. A recent study of kinetics of and competition during base hydrolysis of pentaaminecobak(II1) complexes was interpreted in terms of an IDCR mechanism [2] [3]. This is no1 fully coinpatible with our results, uiz. an I&B mechanism is only possible if the intermediate is allowed to relax stcrcochemically (see below and [MI).
,)
4,
Abbreviations: dien = N-(2-aminoethyl)ethane-1,2-diamine; dapo = 1,3-diaminopropan-2-ol.
In the timescale of the titration experiments, equilibration between epimers A and B (X = OH) would have occurred ( t l / , cu. 10 s), and pK, is, therefore, a weighted (90 :lo) average for the two forms; the two values are unlikely to be very different.
We are grateful for financial support by the Swiss National Science Foundation (grant 2.817-0.77).