Base hydrolysis and mercury(II)-catalysed aquation of the chloropentamine cis-[Co(en) 2(β-alaOH)Cl] 2+ and the reactions of cis-[Co(en) 2(β-alaOMe)Cl] 2+ in basic solution

Abstract

The complexes cis-[Co(en) 2(NH 2CH 2CH 2CO 2H)Cl] 2+ and cis-[Co(en) 2(NH 2 CH 2CH 2CO 2Me)Cl] 2+ (en = 1,2-diaminoethane have been prepared. The p K of the carboxyl group of the N-coordinated amino acid is 3.76, very comparable with the value of 3.55 for the p K of NH 3 +CH 2CH 2CO 2H. Base hydrolysis of the chloride ligand in cis-[CoCl(en) 2(NH 2CH 2CH 2CO 2)] + has been studied by pH-stat over the pH range 9.0–9.6 giving k OH = 11.9 dm 3 mol −1 s −1 at I = 0.10 mol dm −3 and 25°C. Spectrophotometric monitoring of the hydrolysis gave k OH = 11.2 dm 3 mol −1 s −1 under the same conditions. The initial product of base hydrolysis is the hydroxopentamine. The Hg(II)-promoted hydrolysis of the chloropentamine follows the rate expression k obs = k aq + k Hg[Hg(II)] with k aq = 3.5 × 10 −4 s −1 and k Hg = 2.1 × 10 −2 dm 3 mol −1 at 32°C and I = 1.00 mol dm −3. Base hydrolysis of the N-coordinated ester complex was studied by pH-stat. The reaction is biphasic with k OH = 25.3 dm 3 mol −1 s −1 for the initial reaction. Values of k OH for the second reaction decrease as the pH is raised, falling in the range 9.5–6.5 dm 3 mol −1 s −1 at 25°C. Stopped-flow measurements at 490 nm indicated a single reaction with k OH = 12.6 dm 3 mol −1 s −1 assigned to chloride hydrolysis. The pH-stat behaviour can be rationalized in terms of formation of the two complexes [Co(en) 2(OH)(NH 2CH 2CH 2CO 2 −)] + and [Co(en) 2(NH 2CH 2CH 2CO 2)] 2+ and their mechanism of formation is discussed in detail.