Intramolecular hydrolysis of glycinamide and glycine dipeptides coordinated to cobalt(III). [Part I]: Hg²⁺, HOCl, and base hydrolysis of cis-[Co(en)₂Br(glyNHR)]²⁺ (R = H, CH₂CO₂C₃H₇, CH₂CO₂⁻) and properties of cis- and trans-[Co(en)₂(OH₂/OH)(glyNHR)]³⁺/²⁺ ions

Boreham, C.J., Buckingham, D.A., and Keene, F. R. (1979) Intramolecular hydrolysis of glycinamide and glycine dipeptides coordinated to cobalt(III). [Part I]: Hg²⁺, HOCl, and base hydrolysis of cis-[Co(en)₂Br(glyNHR)]²⁺ (R = H, CH₂CO₂C₃H₇, CH₂CO₂⁻) and properties of cis- and trans-[Co(en)₂(OH₂/OH)(glyNHR)]³⁺/²⁺ ions. Inorganic Chemistry, 18 (1). pp. 28-38.

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Abstract

The Hg²⁺ -catalyzed removal of Br⁻ from cis-[Co(en)₂Br(glyNHR)]²⁺ results in the immediate formation of [Co(en)₂(glyNHR)]³⁺ containing the chelated amide or dipeptide residue; full retention of configuration about the Co(III) center obtains and no intermediate aqua complex is formed. HOCl-catalyzed oxidation of cis-[Co(en)₂(OH₂)(glyglyOC₃H₇)]³⁺ results in the formation of some cis-[Co(en)₂(OH₂)(glyglyOC₃H₇)]³⁺ as well as [Co(en)₂(glyglyOC₃H₇)]³⁺, both with full retention of the configuration about the metal. Base hydrolysis (pH 9-14) results in the formation of some trans-[Co(en)₂(OH)(glyNHR)]²⁺ (~6.2%) as well as cis-[Co(en)₂(OH)(glyNHR)]²⁺ (R = H, 34% D, 20% DL; R = CH₂CO₂C₃H₇, 44% D, 19% DL; R = CH₂CO₂⁻, 70% D + DL + trans) and [Co(en)₂(glyNHR)]³⁺ (R = H, 20% D, 20% DL; R = CH₂CO₂C₃H₇, 20% D, 10% DL; R = CH₂CO₂⁻, 30% D + DL). The trans-[Co(en)₂(OH₂/OH)(glyNHR)]³⁺/²⁺ ions cyclize to [Co(en)₂(glyNHR)]³⁺ according to the rate expression R = k(H₂O)[[Co(en)₂(OH₂)(glyNHR)]³⁺] + k'(OH)[OH⁻])·[[Co(en)₂(OH)(glyNHR)]²⁺] where k(H₂O) = 4(±2) X 10⁻⁶ s⁻¹, k(OH) = 2.8(±0.1) X 10⁻⁴ s⁻¹, and k'(OH) = 5.6 X 10⁻² M⁻¹s⁻¹ for R = H at 25°C and μ = 1.0(NaClO₄); this process occurs directly and not via the monodentate cis-aqua or -hydroxo ions. The cis-[Co(en)₂(OH)(glyNHR)]²⁺ ions cyclize to [Co(en)₂(glyO)]²⁺ and [Co(en)₂(glyNHR)]³⁺ via a mechanism involving retention of the coordinated hydroxo oxygen atom. In acid solution the cis-[Co(en)₂(OH₂)(glyNHR)]³⁺ ion also forms [Co(en)₂(glyO)]²⁺ and [Co(en)₂(glyNHR)]³⁺, but only the former retains the oxygen atom of the coordinated water molecule.

Item ID: 23610
Item Type: Article (Refereed Research - C1)
ISSN: 1520-510X
Date Deposited: 10 Oct 2012 03:22
FoR Codes: 03 CHEMICAL SCIENCES > 0302 Inorganic Chemistry > 030207 Transition Metal Chemistry @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970103 Expanding Knowledge in the Chemical Sciences @ 100%
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