A Unique Mechanism for Base Catalyzed Hydrolysis of Pentaaminecobalt(III) Complexes Containing Picolyl Residues
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- 作者：Alistair J. Dickie ; David C. R. Hockless ; Anthony C. Willis ; Josephine A. McKeon ; and W. Gregory Jackson
- 刊名：Inorganic Chemistry
- 出版年：2003
- 出版时间：June 16, 2003
- 年：2003
- 卷：42
- 期：12
- 页码：3822 - 3834
- 全文大小：204K
- 年卷期：v.42,no.12(June 16, 2003)
- ISSN：1520-510X
文摘
A novel [Co(pentaamine)Cl]2+ complex having all tertiary amine or pyridine donors has been synthesized (pentaamine= 1,4-bis(2'-pyridyl)-7-methyl-1,4,7-triazacyclononane). This asym-[Co(dmpmetacn)Cl]2+ species has been completelycharacterized through 1D and 2D NMR studies, and through the X-ray structure for the ZnCl42- salt. Despite thelack of an activating NH center, remarkably its hydrolysis to [Co(pentaamine)OH]2+ is base catalyzed (kOH 0.70M-1 s-1, 25 
C, I = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to theexchange of just one deuterium in one of the two -CH2- pyridyl arms, that is approximately trans to the leavinggroup, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of asym-[Co(dmpmetacn)Cl]2+ and control experiments on H/D exchange for the product asym-[Co(dmpmetacn)OD]2+ in OD-show that each act of deprotonation at the acidic methylene leads to loss of Cl-. This is the first established caseof base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. Apronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (kH/kD =5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activationafforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism.
C, I = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to theexchange of just one deuterium in one of the two -CH2- pyridyl arms, that is approximately trans to the leavinggroup, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of asym-[Co(dmpmetacn)Cl]2+ and control experiments on H/D exchange for the product asym-[Co(dmpmetacn)OD]2+ in OD-show that each act of deprotonation at the acidic methylene leads to loss of Cl-. This is the first established caseof base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. Apronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (kH/kD =5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activationafforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism.